- Email: [email protected]
Osteoarthritis year in review 2017: biology
Accepted Manuscript Osteoarthritis Year in Review 2017: Biology C. Thomas Appleton, MD, PhD PII:
S1063-4584(17)31256-6
DOI:
10.1016/j.joca.2017.10.008
Reference:
YJOCA 4101
To appear in:
Osteoarthritis and Cartilage
Received Date: 10 August 2017 Revised Date:
8 October 2017
Accepted Date: 10 October 2017
Please cite this article as: Appleton CT, Osteoarthritis Year in Review 2017: Biology, Osteoarthritis and Cartilage (2017), doi: 10.1016/j.joca.2017.10.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Osteoarthritis Year in Review 2017: Biology
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C. Thomas Appleton MD, PhD Western Bone and Joint Institute, Rheumatology Centre at St. Joseph’s Health Care London, Department of Medicine, Department of Physiology and Pharmacology, Schulich School of Medicine & Dentistry,
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The University of Western Ontario, London, Canada
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*Address correspondence & reprint requests to: Dr. Tom Appleton, Rheumatology Clinical Research Unit, St. Joseph’s Health Care, 268 Grosvenor St., London, Ontario, N6A 4V2, Canada, Tel: (519) 6466319, Email: [email protected]
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Keywords: osteoarthritis, biology, synovium, inflammation, bone, OA phenotypes, year in review
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Summary This Year in Review was derived from a personal selection of articles investigating biological mechanisms of osteoarthritis (OA) and presented at the OARSI World Congress on April 30, 2017.
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Selected articles were published between the March, 2016 and April, 2017 OARSI meetings. PubMed/MEDLINE searches were performed using the terms “osteoarthritis”, “cartilage”, “subchondral bone”, “synovium”, “synovitis”, and “ageing”. Biomechanical, genetic, genomic, epigenomic, biomarker,
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clinical, imaging, and tissue engineering studies were excluded since they are covered by other articles in this issue. Several new and emerging themes were identified. Incorporating new technologies such as
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designer genetic engineering, nanotechnology, and bio-selective nuclear medicine tracers into study designs helps to gain important insights into OA pathophysiology. Potentially critical differences exist between biological mechanisms of post-traumatic, age-associated, and metabolic phenotypes of OA. The concept of OA stages is highlighted, demonstrating how this may influence which biological mechanisms are at play and the need for strategic timing of treatment interventions. Not all inflammation is bad and
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fine-tuning a balance within inflammatory signaling mechanisms may be a path to regain joint homeostasis. Not only is the joint an organ system, sub-regions within each joint tissue, especially the joint lining, may play distinct roles in damage and repair. To accompany the review, the interaction
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among studies spanning multiple areas is summarized schematically.
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Introduction The field of biological mechanistic research in osteoarthritis (OA) is thriving and continues to be propelled by a massive clinical need for effective treatment of this group of closely-related, disabling joint
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diseases. This narrative review was derived from a personal selection of articles investigating biological mechanisms of OA and presented at the OARSI World Congress on April 30, 2017. Selected articles were published between the OARSI World Congress meetings (March 31, 2016 and April 27, 2017), based on
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PubMed searches using the terms “osteoarthritis”, “cartilage”, “subchondral bone”, “synovium”, “synovitis”, and “ageing”, yielding 1,507 publications. This was reduced to 435 articles after exclusion of
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biomechanical, genetic, genomic, epigenomic, biomarker, clinical, imaging, and tissue engineering studies, which are reviewed elsewhere in the Year in Review series.
As in years past, global research activity continues to be spread across various aspects of OA pathophysiology. Many of the studies reviewed address topics within one or more pillars along the continuum of OA biology including primary causes, secondary mediators, and important structure and
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symptom outcomes (Fig. 1). It bears noting that this construct represents one perspective of the OA biology continuum with particular focus on works from the past year, whereas past and future evidence may argue in favour of some areas crossing into different pillars.
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Clearly, it is beyond the scope of any single review to highlight every noteworthy study in such a broad range of topics. Instead, the list of identified articles was reviewed and grouped into common
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themes that surfaced. This method identified several new or emerging themes, heralding a change in how the field is beginning to approach key research questions that address the heterogeneity of OA. Therefore, the articles highlighted herein were selected for their ability to exemplify these new or emerging themes. Given the mechanistic nature of the research, the aggregate of the evidence has been summarized schematically in relation to the 3 primary causes of OA prevailing in this year’s literature. These include joint injury and altered biomechanics, ageing, and systemic metabolic derangement (Fig. 2).
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New applications of technology leverage new insights into pain, inflammation, and altered cell metabolism Pain is a hallmark clinical feature of OA and the primary reason for OA patients to seek medical
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attention. Since analgesia with standard therapies (e.g. non-steroidal anti-inflammatory drugs (NSAIDs)) is routinely insufficient or contraindicated, understanding mechanisms of pain caused by OA may lead to improved outcomes. Importantly though, the experience of pain is quite variable in OA patients, leading
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to the hypothesis that different types of pain may be driven by different mechanisms. For example, Hawker et al demonstrated nearly a decade ago that pain in early hip and knee OA tends to be intermittent
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and predictable, then becomes more constant and affects daily ambulation in established OA, and finally chronic aching, punctuated with unpredictable intense pain in late stage OA [1]. Thus, whether peripheral nociceptive signaling explains the entire OA pain experience was the question of a study by Miller et al, employing designer receptors exclusively activated by designer drugs (DREADD) technology in the destabilization of medial meniscus (DMM) mouse model of post-traumatic
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OA [2]. Inhibitory G-protein coupled receptors (GPCRs) are active in the central and peripheral nervous systems and targeted by many analgesics (e.g. opioids) to reduce sensory neuronal excitability [3]. Using Cre-recombinase expression driven by the sodium channel Nav1.8 promoter, Miller et al were able to
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target the expression of a designer inhibitory GPCR M4 muscarinic acetylcholine receptor selectively to dorsal root ganglion (DRG) sensory neurons. The corresponding designer drug clozapine-N-oxide (CNO)
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was then used to selectively suppress peripheral nociceptive stimuli (in principle, without altering central pain processing). They then measured evoked pain responses of knee hyperalgesia and hindpaw mechanical allodynia in the DMM mice compared to sham surgery controls. DMM OA mice develop increased hindpaw mechanical allodynia starting 4 weeks after OA induction, and this is sustained to at least 16 weeks [4]. Knee hyperalgesia was increased by DMM compared to sham surgery from 2 to 12 weeks after surgery. CNO reduced hindpaw mechanical allodynia at 8 weeks and knee hyperalgesia at 4 weeks after DMM surgery, but these effects were lost at later time points. Recently there has been
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concern about central effects of CNO, which is metabolized to clozapine in vivo and has dose-dependent effects [5]. It may not have played a role in this study, where effect was lost, but is an important issue for future studies to consider and for which to control.
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Application of the DREADD technology to precisely inhibit peripheral nociception underscores that the experience of pain in OA is a temporally evolving process, with different mechanisms likely at play at different stages of disease. Simple inhibition of peripheral pain stimuli may therefore be an
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inadequate approach, particularly as the disease progresses. Assuming the same is true in human forms of OA, a great deal of work remains to identify viable mechanistic targets in each clinical context. This has
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important implications for the development and selection of personalized analgesic therapies, reliant on valid clinical evaluation of OA stage, pain phenotypes, measurement of pain, or other factors. The described study by Miller et al. provides a powerful example how new technology enables us to answer key questions that were previously impossible to tackle experimentally. This study also highlights the importance of genetic models to tackle fundamental mechanistic questions.
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Even though OA is not an autoimmune arthritis, evidence of inflammatory processes in synovium, chondrocytes, and synovial fluid have long been recognized [6]. How OA-related inflammatory mechanisms differ from autoimmune arthritis (e.g. rheumatoid arthritis) is important to
technology
using
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understand, and may also be aided by technology. Kraus et al adapted an in vivo nuclear medicine etarfolatide
(technetium-99-labeled
EC20,
which
specifically
binds
the
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glycosphosphatidylinositol-anchored folate receptor β), to detect activated (but not resting) macrophages with single photon emission computed tomography (SPECT) imaging in 25 patients with symptomatic knee OA [7]. In this cross-sectional study, activated macrophages were detected in 76% of patients’ knees, and significantly correlated with knee pain severity (R = 0.60), radiographic joint space narrowing (R = 0.68), and the presence of osteophytes (R = 0.66). Beyond implicating macrophages in knee OA symptoms and disease progression, this non-invasive (but relatively expensive) technology may be used
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to assess multiple anatomical locations where OA occurs, and for longitudinal studies in humans and animal models of OA. Disturbed cellular metabolism has been demonstrated to be an important secondary mediator of
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OA, most recently in respect of reduced chondrocyte autophagy, especially with ageing [8]. Several studies have also suggested a role for mitochondrial dysfunction in OA pathogenesis [9-11]. This year, Kim et al applied a nanotechnology to destroy lysosomes, uncovering an important inter-organelle
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regulatory network among mitochondria, lysosomes, and peroxisomes that was previously unknown in chondrocytes [12]. They found that the fission protein Fis1 is reduced in human OA chondrocytes and
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reduction of Fis1 by siRNA resulted in peroxisome dysfunction, and was associated with impaired mitochondrial function. Interestingly, Fis1 suppression also impaired chondrocyte lysosomal autophagy through the disruption of multiple lysosome-associated miRNA species. The application of LAMP-Au nanorods to destroy lysosomes by coupling gold nanoparticles to a LAMP peptide, which in turn are incorporated into lysosomal membranes, confirmed this effect, impaired pexophagy (selective
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degradation of peroxisomes), and increased chondrocyte apoptosis. Thus, the interconnectedness of peroxisome, mitochondria, and lysosome function is potentially mediated by Fis1, and beckons deeper investigation for potential exploitation as a chondrocyte-preserving therapy. An additional question that
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remains is what causes the reduction in Fis1 in OA chondrocytes? Together, these examples demonstrate the creativity of researchers in the OA field to adapt and
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develop technologies to gain important insights into OA pathophysiology. Phenotyping shows distinct pathophysiology For several years, multiple initiatives, reviews, and editorials have recognized the heterogeneity
of OA at clinical, physiologic, anatomic, and molecular levels [13-15]. Calls for attention to be paid to adequately phenotyping patients, as well as in mechanistic studies, are starting to be answered. However, much work remains to address this important issue. The development of generally-accepted definitions for OA phenotypes across the taxonomy [16], while daunting to conceive, is undoubtedly a key milestone
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in this mission. Notwithstanding, evidence from 2016-2017 included several examples underscoring the importance of respecting OA phenotypes from the perspective of biological mechanisms. Rowe and colleagues examined the effects of macrophage migration inhibitory factor (MIF)
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deletion in the context of two different phenotypes of experimental OA [17]. MIF recruits neutrophils, activates macrophages and other innate immune processes, and has defined roles in rheumatoid arthritis and lupus. MIF is also found in the serum and synovial fluid of OA patients [18, 19]. Homozygous
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deletion of the Mif gene in male mice aged 12 and 22 months resulted in significant protection against age-related OA joint damage and osteophytes, but did not protect against DMM-induced post-traumatic
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OA in 12-week old male mice with the same genetic (C57BL/6) background. The lack of effect was confirmed with anti-MIF antibody treatment in DMM mice.
Age is another factor that is also likely to influence the mechanisms driving OA. Illustrating this point, Usmani and colleagues studied the effects of genetic deletion of transforming growth factor alpha (Tgfa), which we have shown promotes post-traumatic OA in a post-traumatic rat model [20], and is
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associated with hip and knee OA in humans [21, 22]. By inducing post-traumatic (DMM) OA in young (10 weeks) and older (6 months) mice [23],they showed that loss of Tgfa was protective against posttraumatic OA in young mice, but not in middle-aged mice. Skeletal maturity alters the mechanical
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properties of bone and cartilage, which may partly explain these effects. This further underscores that OA needs to be studied systematically in different contexts to understand how age or other factors influence
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the mechanisms driving OA. These works are important as they support the notion that different phenotypes of OA should not be lumped together in studies and may require different therapeutic approaches.
Whether metabolic derangement, a priori, is a primary cause of human OA is still a matter of
debate [24]. While clinical evidence is hampered by associative data or confounded by the intertwinement of joint loading and obesity with metabolic syndrome [25], animal models demonstrate that diet-induced obesity [26] or hypercholesterolemia [27] can induce OA-like damage in knee joints of mice and rats.
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Cholesterol-induced joint disease was a common theme among phenotypes this year. Farnaghi et al reported that diet-induced hypercholesterolemia in rats and ApoE-deficient mice (genetic deletion) develop premature/worse spontaneous (metabolic) knee OA that is not only due to ageing [28].
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Hypercholesterolemia also worsened post-traumatic OA induced by DMM in mice or meniscectomy in outbred rats in the same study. Hypercholesterolemia-induced OA and hypercholesterolemia-aggravated post-traumatic OA were both rescued/improved by atorvastatin therapy. And, the importance of
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mitochondrial dysfunction was highlighted again since the addition of a mitochondria-specific antioxidant (Mito-Tempo) significantly improved cartilage damage induced by high cholesterol diets in rats and
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ApoE-deficient mice.
Oxidized low-density lipoprotein (LDL), increased by inflammation and oxidative stress, can bind the lectin-like oxidized-LDL receptor LOX-1 and increase the production of reactive oxygen species in articular chondrocytes [29]. Now, Hashimoto and colleagues provide evidence that loss of LOX-1 is protective against OA-related cartilage damage in the context of age-related and post-traumatic (DMM)
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OA in mice [30, 31]. Together with the Farnaghi study, this series of papers places metabolic derangement (manifested by hypercholesterolemia), downstream mediators of oxidized LDL, and mitochondrial reactive oxygen species back in the spotlight as potentially important modifiers of OA risk.
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The impact can be seen in the context of experimental metabolic OA on its own, as well by aggravating post-traumatic and age-associated phenotypes.
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Another important link of metabolic derangement with OA is IL-6. While IL-6 inhibition is an
effective therapy for rheumatoid arthritis, serum elevations in IL-6 is also associated with obesity, the metabolic syndrome, and radiographic progression of knee OA based on data from the Chingford study [32]. Systemic inhibition of IL-6 with a monoclonal antibody to the IL-6 receptor reduced cartilage lesions and synovitis in the DMM model of post-traumatic OA in mice [33]. Latourte et al further showed that this is likely mediated by Stat3 signaling, known to be downstream of IL-6R activation. Blockade of Stat3 with the small molecule inhibitor ‘Stattic’ also reduced osteophyte formation in DMM mice. IL-6
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has been shown to be increased in OA synovial fluid, and also shortly after joint injury [34], and to increase chondrocyte production of matrix metalloproteinases (MMP-1, -3, -13) and aggrecanases (ADAMTS-4 and -5) [35]. Another new study from Nasi et al implicates IL-6 again, but this time in the
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context of OA related to hydroxyapatite crystal formation. Basic calcium phosphate (BCP), especially hydroxyapatite, is found in the synovial fluid of approximately 50% of OA cases [36], and induces joint disease when injected into mouse knees [37]. Nasi demonstrated that BCP induces IL-6 production by
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human chondrocytes and, reciprocally, IL-6 induced calcium crystal formation by the upregulation of calcium-regulating genes. These effects could by blocked by inhibiting the IL-6R in vitro with a
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monoclonal antibody. Furthermore, BCP crystals were increased in a post-traumatic model of OA in mice (by surgical meniscectomy). The effects of IL-6 induction by BCP appear to form a positive feedback loop leading to cartilage damage, but what remains unclear is how BCP crystals activate the chondrocytes. The authors appear to infer this is due to IL-6R activation directly, but studies investigating this interaction are needed.
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Encouragingly, multiple phenotypes of OA have been independently investigated this year. Emerging as primary phenotypes, at least in animal model studies, are post-traumatic OA, metabolic OA, and age-associated OA. BCP crystals may be a separate phenotype or more generally a secondary
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mediator relevant to different OA phenotypes. Interestingly, there appear to be some mechanisms that are unique to certain OA phenotypes, as in the case of MIF and age-associated OA, whereas other
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mechanisms such as hypercholesterolemia exert effects either independently or in concert with other OA phenotypes.
Timing of treatment interventions matters In the Miller study of nociceptive pain inhibition described above [2], analgesia was effective in
early post-traumatic OA but not at later stages. This is a great example of the importance of doing temporal studies in animal models. The notion also offers us a viable explanation for why studies that are
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effective in animal models don’t often translate successfully into human studies, because patients are often at much later stages of OA disease than were modeled in preclinical work. A few years ago, we discovered early upregulation of the chemokine CCL2 in the cartilage of a
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post-traumatic model of OA in rats, and that its receptor CCR2 could be successfully targeted pharmacologically to reduce joint damage [20]. In contrast, genetic deletion of its receptor CCR2 in mice reduced pain behaviour without reducing structural damage [4]. This year’s report by Longobardi et al
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may explain the discrepancy as it relates to timing [38]. Transient therapy with the CCR2 small molecule inhibitor RS504393 resulted in joint protection only if treatment was started contemporaneously with, or
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at 4 weeks after, post-traumatic OA induction surgery. In contrast, delay of treatment to 8 weeks or sustained therapy for at least 8 weeks resulted in loss of the protective effect. This is an interesting observation and supports the effects seen in genetic Ccr2 ablation studies. First, CCR2 is expressed by inflammatory and resident joint cell types, including monocytes, macrophages, chondrocytes, and neuronal cells [39]. Early introduction of the chemokine inhibitor may result in reduced inflammatory cell
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infiltration or inflammatory responses by resident joint cell types, which might be expected to be protective. However, sustained treatment may prevent any inflammatory response, without which normal healing may not be initiated. This is speculative, but the observation that pulsatile treatment at the right
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time is more effective than sustained treatment is novel and could point to an alternative strategy for OA treatment depending on the target.
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Sirtuins are deacetylases with important roles in DNA repair. SIRT1 promotes chondrocyte
survival and reduced inflammatory responses through multiple mechanisms, including deacetylation of p65/RelA, which may be enhanced by treatment with resveratrol [40]. Li et al recently demonstrated that temporal loss of SIRT1 corresponds to disease stage, as determined by cartilage histopathologic Mankin scoring of samples obtained at knee arthroplasty [41]. Later stages of OA with loss of expression of chondrocyte SIRT1 corresponded to higher levels of p53 and p53 acetylation, relative to age-matched controls (trauma patients without OA). In addition, Elayyan and colleagues provided evidence for a
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mechanistic link between SIRT1 and control of MMP-13 gene expression mediated by the LEF1 transcription factor in human OA chondrocytes [42]. Overexpression of SIRT1 resulted in suppression of MMP-13 and LEF1 as well as their induction by IL-1β. SIRT1 was also linked to circadian rhythm by
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Yang et al [43], showing that siRNA-mediated knockdown or pharmacologic inhibition of SIRT1 in chondrocytes results in lower levels of transcripts for the clock gene Bmal1. It was already known that loss of Bmal1 increases cartilage damage and disrupted circadian rhythm in chondrocytes [44], as
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highlighted in last year’s Year in Review [45]. Interestingly, similar methods used to knock down Bmal1 expression reciprocally reduced SIRT1 expression. Loss of either SIRT1 or Bmal1 potentiated higher
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levels of MMP-3, -13, and ADAMTS-5 expression in response to IL-1β stimulation of chondrocytes. In light of the evidence that reduction of SIRT1 occurs in early to moderate OA cartilage, if SIRT1 can be targeted in OA it is likely best done at an early stage of disease.
Moreover, together these studies lend further support to the importance of considering the timing and duration of any intervention in the context of the specific pathophysiology at play in each
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experimental design. OA continues to challenge in its complexity and heterogeneity. Tuning protective and pathogenic inflammatory pathways: implications for treatment approaches Rela/p65 is a key subunit required for NF-kB signaling, with important roles in regulating the
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expression of Sox9 [46] and Adamts5 [47] in chondrocytes. Thus, Rela may confer anabolic and catabolic
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functions to chondrocytes. However, NF-kB signaling is a common final pathway of many inflammatory mechanisms thought to play a deleterious role in OA cartilage pathophysiology. Given the wide array of functional roles of NF-kB signaling in maintaining chondrocyte homeostasis, Kobayashi et al conducted an in vivo study investigating the effects of Rela using tamoxifen-inducible chondrocyte-specific (Col2a1Cre) Rela gene deletion in adult mice [48]. Surprisingly, homozygous deletion of Rela resulted in acceleration of OA-related joint damage, mostly mediated by increased chondrocyte apoptosis, despite the dramatic reduction in inflammatory cytokine expression. In contrast, haploinsufficiency of Rela protected against DMM-induced post-traumatic OA development and was associated with marked reduction in
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Hif2a and Adamts5 expression. Glycosaminoglycan release from cartilage explants stimulated by IL-1β ex-vivo was also greatly reduced, but the increase in apoptosis that occurred with homozygous Rela deletion was prevented. Thus, a balance of just enough Rela to prevent apoptosis but reducing Rela just
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enough to prevent catabolic responses in chondrocytes was protective. Similar results were seen with a low dose of an IKK small molecule inhibitor (BMS-345541), which resulted in similar partial inhibition of NF-kB signaling. Interestingly, similar effects were seen in both the DMM and age-associated OA
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mouse models in this study.
Similarly, two studies investigating the role of macrophages found that macrophages mediate the
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pro-inflammatory and catabolic effects of alarmins, while they are also required for mitigating an inflammatory response to oxidized-LDL injections into the joint. van den Bosch et al demonstrated that the alarmins S100A8 and S100A9 are mainly produced in the joint by synovial macrophages (not by fibroblasts) [49]. Moreover, synovial M1-like macrophages were the primary cell type to respond to stimulation with the alarmins by producing IL-1, -6, -8, and TNF-α. In contrast, de Munter et al
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repeatedly injected oxidized LDL into murine knee joints, which resulted in synovial upregulation of TGF-b activity but no inflammatory response [50]. However, depletion of macrophages from healthy mouse knee joints followed by repeated oxidized LDL injection caused the upregulation of inflammatory
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mediators and synovial thickening, despite the absence of synovial macrophages. These studies provide a more nuanced view of inflammatory signaling in roles in OA and
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cartilage homeostasis. Not all inflammation is bad, and indeed, targeting inflammation may be beneficial as long as it is not at the expense of impaired chondrocyte homeostasis, such as in the case of NF-kB signaling mediated by Rela. Similarly, M1-like macrophages may be responsible for the production of inflammatory mediators detected in response to alarmin signaling in OA joints, but have an important role to play in the homeostasis of the healthy joint. Overall, inflammation is a complex issue and the reductionist concept of ‘good’ and ‘bad’ inflammation, while useful for illustrative purposes, is also an oversimplification. The fact that mechanisms of chronic OA-related inflammation (e.g. signaling
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pathways and cell types influencing inflammation) are also required for tissue reparative processes belies the challenges inherent in developing anti-inflammatory treatments for OA. Clearly, a context-specific understanding of inflammation is required.
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Topography of the joint organ system uncovers roles for joint lining cells
Significant progress was made to understanding the role of the superficial zone (SZ) of articular cartilage in joint homeostasis and disease. First, Li et al used R26-Confetti multicolour reporter mice
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crossed with tamoxifen-inducible joint lining-cell-specific (PRG4-Cre) mice to label and follow SZ cells from P5 through 6 months of age [51]. Fluorescent-tagged cells initially moved down, formed
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chondrocyte clusters, then populated the entire depth of cartilage, suggesting that articular cartilage grows by appositional growth in this particular context. While this implies that PRG4-expressing superficial zone cells may play a role in tissue homeostasis and repair, it is important to note that this was determined in a developmental context, which may be different in adult mice. In contrast, lineage tracing by Decker et al using similar methods [52] found that cartilage growth is more likely achieved by expansion in cell
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volume and rearrangement of cells into columnar alignment. At least in developmental tissues, PRG4+ synovial cells were highly responsive to cartilage injury and mobilized to the site of tissue repair, while local chondrocytes remained relatively fixed in situ. Both of these studies highlight the potential of joint
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lining tissues to contribute to joint tissue homeostasis and repair. Fibulins are known to have functions in maintenance of basement membrane organization and
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elastic fiber formation, but were introduced to the OA field in the context of SZ cells in early 2017 by Hasegaswa and colleagues [53]. Fibulin 3 expression was limited to SZ and other joint lining cells in homeostatic conditions, but expression in these cell populations declines with age or post-traumatic OA in DMM mice. Genetic deletion of fibulin 3 exacerbated age-related and post-traumatic OA and was associated with lower expression of PRG4. siRNA driven suppression of fibulin 3 increased apoptosis of SZ cells, but also promoted chondrogenesis in bone marrow-derived mesenchymal stem cells. In contrast, fibulin 3 overexpression prevented chondrogenesis. Interestingly, it appears that fibulin 3 may be
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important for maintaining a pool of progenitor cells in the SZ and joint lining, through maintenance of lubricin (or other PRG4 gene product expression), but can be lost with injury or ageing. This potentially would render cartilage susceptible to OA due to loss of an endogenous repair cell pool. On the other hand,
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reduction in fibulin 3 may be a necessary step to release joint lining cells from a progenitor status and enter into chondrogenesis, putatively for joint repair. This is an area that warrants further investigation. Further implications for cells in the SZ in the development of OA were uncovered by Jeon et al
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[54]. This work was prompted by the discovery that senescent cells accumulate during adulthood, leading to shortened the lifespan and age-related functional decline of organs in mice. Deletion of senescent cells
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leads to increased lifespan and improved organ function using a pharmacologic inducer of apoptosis in senescent cells expressing the INK-ATTAC transgene [55]. Senescent cells have a senescence-associated secretory phenotype (SASP), which includes the production of IL-6 (linking back to other studies this year demonstrating the pathologic role of IL-6 in OA) and the alarmin HMGB1 [56]. Jeon et al used p163MR bioluminescent reporter mice to study the role of senescent cells in the development of two OA
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phenotypes, leveraging the feature that senescent cells in these mice are sensitive to the senolytic effects of ganciclovir and UBX0101. Senescent cells accumulated in the SZ and joint lining cells of mice with ageing and after anterior cruciate ligament transection (ACLT) induction of post-traumatic OA.
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Pharmacologic deletion of senescent cells with two different inhibitors led to improved anabolic and catabolic gene expression profiles of cartilage and reduced joint damage scores in both mouse models of
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OA. However, the protective effects on joint damage were less pronounced when ACLT was performed in aged mice (19 months old), although pain behaviour was significantly reduced with senolytic agent treatment. Notwithstanding, this study clearly highlights the importance of senescent cells accumulating in the SZ and joint lining of knee joints in OA models and should be examined further using additional models and human OA tissues.
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Together, these studies demonstrate that the topography of the joint matters greatly. In addition to other joint tissues, even different zones of the cartilage, as one example, may have differential roles to play.
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Conclusion
As with any review, particular articles of merit were not included in this review. Highlighted studies were selected based on their representative nature of particular themes that emerged during the
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literature search and exploration, and several of the selected studies could have fit well into multiple theme areas. Much of the work on biological mechanisms of OA has focused on the secondary mediators
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of disease. Emerging themes this year demonstrated the importance of embracing new technologies to provide key insights into the nature of different forms of OA. Additional biological outcomes research is warranted, in particular for underrepresented areas including pain and pain sensitization, bone marrow lesions, and impaired joint function. Biological mechanisms related to OA prevention is another area that would benefit from additional resource dedication.
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It is particularly exciting to see an increased focus on investigating biological mechanisms in the context of different OA phenotypes. And although this review focused on biological mechanisms and therefore is heavily influenced by preclinical studies, much work on clinical OA phenotypes is also
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ongoing. From animal models research, three particular phenotypes seem to have emerged, including post-traumatic, age-associated, and systemic metabolic forms of OA. As a field, it is important that we
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continue to expand this list, such as with BCP, genetic, inflammatory, or other candidate OA phenotypes, but also into anatomic (e.g. hand, hip, etc) and physiologic domains. Encompassing these perspectives will be highly informative, since it is likely that OA will require a personalized medicine approach to diagnosis and staging in order to select the most appropriate therapy for each person, at the right time. As soon as possible, it is also vital that biomechanistic studies align closely with the phenotypes of OA identified in clinical studies. Such an effort will require representative members from each area of strength in the OA field (highlighted by the themes of the OA Year in Review series) coming together to
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devise a schema and common language that we may all utilize as a common approach to this complex and
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heterogeneous disease.
Author Contributions
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Dr. Appleton performed the literature search, designed the theme selection, wrote and edited the work.
Conflict of Interest
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No direct financial or other conflicts of interest related to this work. Dr. Appleton has received
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consultancy honoraria from Novartis and meeting sponsorship from Pfizer.
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Akagi R, Akatsu Y, Fisch KM, Alvarez-Garcia O, Teramura T, Muramatsu Y, et al. Dysregulated circadian rhythm pathway in human osteoarthritis: NR1D1 and BMAL1 suppression alters TGF-β signaling in chondrocytes. Osteoarthritis and cartilage 2016. Blaney Davidson EN, van Caam AP, van der Kraan PM. Osteoarthritis year in review 2016: biology. Osteoarthritis and cartilage 2017; 25: 175-180. Ushita M, Saito T, Ikeda T, Yano F, Higashikawa A, Ogata N, et al. Transcriptional induction of SOX9 by NF-κB family member RelA in chondrogenic cells. Osteoarthritis and Cartilage 2009; 17: 1065-1075. Kobayashi H, Hirata M, Saito T, Itoh S, Chung U-iI, Kawaguchi H. Transcriptional induction of ADAMTS5 protein by nuclear factor-κB (NF-κB) family member RelA/p65 in chondrocytes during osteoarthritis development. The Journal of biological chemistry 2013; 288: 28620-28629. Kobayashi H, Chang SH, Mori D, Itoh S, Hirata M, Hosaka Y, et al. Biphasic regulation of chondrocytes by Rela through induction of anti-apoptotic and catabolic target genes. Nature communications 2016; 7: 13336. van den Bosch MH, Blom AB, Schelbergen RF, Koenders MI, van de Loo FA, van den Berg WB, et al. Alarmin S100A9 Induces Proinflammatory and Catabolic Effects Predominantly in the M1 Macrophages of Human Osteoarthritic Synovium. The Journal of rheumatology 2016; 43: 1874-1884. de Munter W, Geven EJ, Blom AB, Walgreen B, Helsen MM, Joosten LA, et al. Synovial macrophages promote TGF-β signaling and protect against influx of S100A8/S100A9producing cells after intra-articular injections of oxidized low-density lipoproteins. Osteoarthritis and cartilage 2017; 25: 118-127. Li L, Newton PT, Bouderlique T, Sejnohova M, Zikmund T, Kozhemyakina E, et al. Superficial cells are self-renewing chondrocyte progenitors, which form the articular cartilage in juvenile mice. FASEB journal : official publication of the Federation of American Societies for Experimental Biology 2017; 31: 1067-1084. Decker RS, Um H-B, Dyment NA, Cottingham N, Usami Y, Enomoto-Iwamoto M, et al. Cell origin, volume and arrangement are drivers of articular cartilage formation, morphogenesis and response to injury in mouse limbs. Developmental Biology 2017. Hasegawa A, Yonezawa T, Taniguchi N, Otabe K, Akasaki Y, Matsukawa T, et al. Role of Fibulin 3 in Aging-Related Joint Changes and Osteoarthritis Pathogenesis in Human and Mouse Knee Cartilage. Arthritis & rheumatology (Hoboken, N.J.) 2017; 69: 576-585. Jeon O, Kim C, Laberge R-M, Demaria M, Rathod S, Vasserot AP, et al. Local clearance of senescent cells attenuates the development of post-traumatic osteoarthritis and creates a pro-regenerative environment. Nature Medicine 2017. Baker DJ, Childs BG, Durik M, Wijers ME, Sieben CJ, Zhong J, et al. Naturally occurring p16(Ink4a)-positive cells shorten healthy lifespan. Nature 2016; 530: 184-189. Salama R, Sadaie M, Hoare M, Narita M. Cellular senescence and its effector programs. Genes & development 2014; 28: 99-114.
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Figure Legends
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Figure 1. Topic areas encompassed by studies published on biological mechanisms of OA between 2016-2017. Topics are organized in proposed pillars across a continuum of OA biology. The figure is not meant to be complete, but represents topics that were featured or commonly reported in the literature
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based on the year in review. Figure 2. Schematic summary of conclusions from mechanistic studies of OA biology between 20162017. Themes are shown in relation to the most significantly associated primary cause (risk factor) for
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OA based on the context of studies included. Primary causes include injury and altered biomechanics, ageing, and systemic metabolic derangement (e.g. hypercholesterolemia). Green indicators represent a
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positive association (protective against OA). Red indicators represent a negative association (promoting OA). Arrows indicate a positive relationship (stimulatory effect), whereas blunt-ended arrows indicate an
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inhibitory effect. Based on reported evidence from the 2016-2017 literature.
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S1063-4584(17)31256-6
DOI:
10.1016/j.joca.2017.10.008
Reference:
YJOCA 4101
To appear in:
Osteoarthritis and Cartilage
Received Date: 10 August 2017 Revised Date:
8 October 2017
Accepted Date: 10 October 2017
Please cite this article as: Appleton CT, Osteoarthritis Year in Review 2017: Biology, Osteoarthritis and Cartilage (2017), doi: 10.1016/j.joca.2017.10.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Osteoarthritis Year in Review 2017: Biology
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C. Thomas Appleton MD, PhD Western Bone and Joint Institute, Rheumatology Centre at St. Joseph’s Health Care London, Department of Medicine, Department of Physiology and Pharmacology, Schulich School of Medicine & Dentistry,
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The University of Western Ontario, London, Canada
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*Address correspondence & reprint requests to: Dr. Tom Appleton, Rheumatology Clinical Research Unit, St. Joseph’s Health Care, 268 Grosvenor St., London, Ontario, N6A 4V2, Canada, Tel: (519) 6466319, Email: [email protected]
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Keywords: osteoarthritis, biology, synovium, inflammation, bone, OA phenotypes, year in review
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Summary This Year in Review was derived from a personal selection of articles investigating biological mechanisms of osteoarthritis (OA) and presented at the OARSI World Congress on April 30, 2017.
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Selected articles were published between the March, 2016 and April, 2017 OARSI meetings. PubMed/MEDLINE searches were performed using the terms “osteoarthritis”, “cartilage”, “subchondral bone”, “synovium”, “synovitis”, and “ageing”. Biomechanical, genetic, genomic, epigenomic, biomarker,
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clinical, imaging, and tissue engineering studies were excluded since they are covered by other articles in this issue. Several new and emerging themes were identified. Incorporating new technologies such as
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designer genetic engineering, nanotechnology, and bio-selective nuclear medicine tracers into study designs helps to gain important insights into OA pathophysiology. Potentially critical differences exist between biological mechanisms of post-traumatic, age-associated, and metabolic phenotypes of OA. The concept of OA stages is highlighted, demonstrating how this may influence which biological mechanisms are at play and the need for strategic timing of treatment interventions. Not all inflammation is bad and
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fine-tuning a balance within inflammatory signaling mechanisms may be a path to regain joint homeostasis. Not only is the joint an organ system, sub-regions within each joint tissue, especially the joint lining, may play distinct roles in damage and repair. To accompany the review, the interaction
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among studies spanning multiple areas is summarized schematically.
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Introduction The field of biological mechanistic research in osteoarthritis (OA) is thriving and continues to be propelled by a massive clinical need for effective treatment of this group of closely-related, disabling joint
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diseases. This narrative review was derived from a personal selection of articles investigating biological mechanisms of OA and presented at the OARSI World Congress on April 30, 2017. Selected articles were published between the OARSI World Congress meetings (March 31, 2016 and April 27, 2017), based on
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PubMed searches using the terms “osteoarthritis”, “cartilage”, “subchondral bone”, “synovium”, “synovitis”, and “ageing”, yielding 1,507 publications. This was reduced to 435 articles after exclusion of
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biomechanical, genetic, genomic, epigenomic, biomarker, clinical, imaging, and tissue engineering studies, which are reviewed elsewhere in the Year in Review series.
As in years past, global research activity continues to be spread across various aspects of OA pathophysiology. Many of the studies reviewed address topics within one or more pillars along the continuum of OA biology including primary causes, secondary mediators, and important structure and
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symptom outcomes (Fig. 1). It bears noting that this construct represents one perspective of the OA biology continuum with particular focus on works from the past year, whereas past and future evidence may argue in favour of some areas crossing into different pillars.
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Clearly, it is beyond the scope of any single review to highlight every noteworthy study in such a broad range of topics. Instead, the list of identified articles was reviewed and grouped into common
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themes that surfaced. This method identified several new or emerging themes, heralding a change in how the field is beginning to approach key research questions that address the heterogeneity of OA. Therefore, the articles highlighted herein were selected for their ability to exemplify these new or emerging themes. Given the mechanistic nature of the research, the aggregate of the evidence has been summarized schematically in relation to the 3 primary causes of OA prevailing in this year’s literature. These include joint injury and altered biomechanics, ageing, and systemic metabolic derangement (Fig. 2).
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New applications of technology leverage new insights into pain, inflammation, and altered cell metabolism Pain is a hallmark clinical feature of OA and the primary reason for OA patients to seek medical
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attention. Since analgesia with standard therapies (e.g. non-steroidal anti-inflammatory drugs (NSAIDs)) is routinely insufficient or contraindicated, understanding mechanisms of pain caused by OA may lead to improved outcomes. Importantly though, the experience of pain is quite variable in OA patients, leading
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to the hypothesis that different types of pain may be driven by different mechanisms. For example, Hawker et al demonstrated nearly a decade ago that pain in early hip and knee OA tends to be intermittent
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and predictable, then becomes more constant and affects daily ambulation in established OA, and finally chronic aching, punctuated with unpredictable intense pain in late stage OA [1]. Thus, whether peripheral nociceptive signaling explains the entire OA pain experience was the question of a study by Miller et al, employing designer receptors exclusively activated by designer drugs (DREADD) technology in the destabilization of medial meniscus (DMM) mouse model of post-traumatic
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OA [2]. Inhibitory G-protein coupled receptors (GPCRs) are active in the central and peripheral nervous systems and targeted by many analgesics (e.g. opioids) to reduce sensory neuronal excitability [3]. Using Cre-recombinase expression driven by the sodium channel Nav1.8 promoter, Miller et al were able to
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target the expression of a designer inhibitory GPCR M4 muscarinic acetylcholine receptor selectively to dorsal root ganglion (DRG) sensory neurons. The corresponding designer drug clozapine-N-oxide (CNO)
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was then used to selectively suppress peripheral nociceptive stimuli (in principle, without altering central pain processing). They then measured evoked pain responses of knee hyperalgesia and hindpaw mechanical allodynia in the DMM mice compared to sham surgery controls. DMM OA mice develop increased hindpaw mechanical allodynia starting 4 weeks after OA induction, and this is sustained to at least 16 weeks [4]. Knee hyperalgesia was increased by DMM compared to sham surgery from 2 to 12 weeks after surgery. CNO reduced hindpaw mechanical allodynia at 8 weeks and knee hyperalgesia at 4 weeks after DMM surgery, but these effects were lost at later time points. Recently there has been
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concern about central effects of CNO, which is metabolized to clozapine in vivo and has dose-dependent effects [5]. It may not have played a role in this study, where effect was lost, but is an important issue for future studies to consider and for which to control.
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Application of the DREADD technology to precisely inhibit peripheral nociception underscores that the experience of pain in OA is a temporally evolving process, with different mechanisms likely at play at different stages of disease. Simple inhibition of peripheral pain stimuli may therefore be an
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inadequate approach, particularly as the disease progresses. Assuming the same is true in human forms of OA, a great deal of work remains to identify viable mechanistic targets in each clinical context. This has
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important implications for the development and selection of personalized analgesic therapies, reliant on valid clinical evaluation of OA stage, pain phenotypes, measurement of pain, or other factors. The described study by Miller et al. provides a powerful example how new technology enables us to answer key questions that were previously impossible to tackle experimentally. This study also highlights the importance of genetic models to tackle fundamental mechanistic questions.
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Even though OA is not an autoimmune arthritis, evidence of inflammatory processes in synovium, chondrocytes, and synovial fluid have long been recognized [6]. How OA-related inflammatory mechanisms differ from autoimmune arthritis (e.g. rheumatoid arthritis) is important to
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using
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understand, and may also be aided by technology. Kraus et al adapted an in vivo nuclear medicine etarfolatide
(technetium-99-labeled
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which
specifically
binds
the
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glycosphosphatidylinositol-anchored folate receptor β), to detect activated (but not resting) macrophages with single photon emission computed tomography (SPECT) imaging in 25 patients with symptomatic knee OA [7]. In this cross-sectional study, activated macrophages were detected in 76% of patients’ knees, and significantly correlated with knee pain severity (R = 0.60), radiographic joint space narrowing (R = 0.68), and the presence of osteophytes (R = 0.66). Beyond implicating macrophages in knee OA symptoms and disease progression, this non-invasive (but relatively expensive) technology may be used
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to assess multiple anatomical locations where OA occurs, and for longitudinal studies in humans and animal models of OA. Disturbed cellular metabolism has been demonstrated to be an important secondary mediator of
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OA, most recently in respect of reduced chondrocyte autophagy, especially with ageing [8]. Several studies have also suggested a role for mitochondrial dysfunction in OA pathogenesis [9-11]. This year, Kim et al applied a nanotechnology to destroy lysosomes, uncovering an important inter-organelle
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regulatory network among mitochondria, lysosomes, and peroxisomes that was previously unknown in chondrocytes [12]. They found that the fission protein Fis1 is reduced in human OA chondrocytes and
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reduction of Fis1 by siRNA resulted in peroxisome dysfunction, and was associated with impaired mitochondrial function. Interestingly, Fis1 suppression also impaired chondrocyte lysosomal autophagy through the disruption of multiple lysosome-associated miRNA species. The application of LAMP-Au nanorods to destroy lysosomes by coupling gold nanoparticles to a LAMP peptide, which in turn are incorporated into lysosomal membranes, confirmed this effect, impaired pexophagy (selective
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degradation of peroxisomes), and increased chondrocyte apoptosis. Thus, the interconnectedness of peroxisome, mitochondria, and lysosome function is potentially mediated by Fis1, and beckons deeper investigation for potential exploitation as a chondrocyte-preserving therapy. An additional question that
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remains is what causes the reduction in Fis1 in OA chondrocytes? Together, these examples demonstrate the creativity of researchers in the OA field to adapt and
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develop technologies to gain important insights into OA pathophysiology. Phenotyping shows distinct pathophysiology For several years, multiple initiatives, reviews, and editorials have recognized the heterogeneity
of OA at clinical, physiologic, anatomic, and molecular levels [13-15]. Calls for attention to be paid to adequately phenotyping patients, as well as in mechanistic studies, are starting to be answered. However, much work remains to address this important issue. The development of generally-accepted definitions for OA phenotypes across the taxonomy [16], while daunting to conceive, is undoubtedly a key milestone
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in this mission. Notwithstanding, evidence from 2016-2017 included several examples underscoring the importance of respecting OA phenotypes from the perspective of biological mechanisms. Rowe and colleagues examined the effects of macrophage migration inhibitory factor (MIF)
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deletion in the context of two different phenotypes of experimental OA [17]. MIF recruits neutrophils, activates macrophages and other innate immune processes, and has defined roles in rheumatoid arthritis and lupus. MIF is also found in the serum and synovial fluid of OA patients [18, 19]. Homozygous
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deletion of the Mif gene in male mice aged 12 and 22 months resulted in significant protection against age-related OA joint damage and osteophytes, but did not protect against DMM-induced post-traumatic
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OA in 12-week old male mice with the same genetic (C57BL/6) background. The lack of effect was confirmed with anti-MIF antibody treatment in DMM mice.
Age is another factor that is also likely to influence the mechanisms driving OA. Illustrating this point, Usmani and colleagues studied the effects of genetic deletion of transforming growth factor alpha (Tgfa), which we have shown promotes post-traumatic OA in a post-traumatic rat model [20], and is
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associated with hip and knee OA in humans [21, 22]. By inducing post-traumatic (DMM) OA in young (10 weeks) and older (6 months) mice [23],they showed that loss of Tgfa was protective against posttraumatic OA in young mice, but not in middle-aged mice. Skeletal maturity alters the mechanical
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properties of bone and cartilage, which may partly explain these effects. This further underscores that OA needs to be studied systematically in different contexts to understand how age or other factors influence
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the mechanisms driving OA. These works are important as they support the notion that different phenotypes of OA should not be lumped together in studies and may require different therapeutic approaches.
Whether metabolic derangement, a priori, is a primary cause of human OA is still a matter of
debate [24]. While clinical evidence is hampered by associative data or confounded by the intertwinement of joint loading and obesity with metabolic syndrome [25], animal models demonstrate that diet-induced obesity [26] or hypercholesterolemia [27] can induce OA-like damage in knee joints of mice and rats.
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Cholesterol-induced joint disease was a common theme among phenotypes this year. Farnaghi et al reported that diet-induced hypercholesterolemia in rats and ApoE-deficient mice (genetic deletion) develop premature/worse spontaneous (metabolic) knee OA that is not only due to ageing [28].
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Hypercholesterolemia also worsened post-traumatic OA induced by DMM in mice or meniscectomy in outbred rats in the same study. Hypercholesterolemia-induced OA and hypercholesterolemia-aggravated post-traumatic OA were both rescued/improved by atorvastatin therapy. And, the importance of
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mitochondrial dysfunction was highlighted again since the addition of a mitochondria-specific antioxidant (Mito-Tempo) significantly improved cartilage damage induced by high cholesterol diets in rats and
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ApoE-deficient mice.
Oxidized low-density lipoprotein (LDL), increased by inflammation and oxidative stress, can bind the lectin-like oxidized-LDL receptor LOX-1 and increase the production of reactive oxygen species in articular chondrocytes [29]. Now, Hashimoto and colleagues provide evidence that loss of LOX-1 is protective against OA-related cartilage damage in the context of age-related and post-traumatic (DMM)
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OA in mice [30, 31]. Together with the Farnaghi study, this series of papers places metabolic derangement (manifested by hypercholesterolemia), downstream mediators of oxidized LDL, and mitochondrial reactive oxygen species back in the spotlight as potentially important modifiers of OA risk.
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The impact can be seen in the context of experimental metabolic OA on its own, as well by aggravating post-traumatic and age-associated phenotypes.
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Another important link of metabolic derangement with OA is IL-6. While IL-6 inhibition is an
effective therapy for rheumatoid arthritis, serum elevations in IL-6 is also associated with obesity, the metabolic syndrome, and radiographic progression of knee OA based on data from the Chingford study [32]. Systemic inhibition of IL-6 with a monoclonal antibody to the IL-6 receptor reduced cartilage lesions and synovitis in the DMM model of post-traumatic OA in mice [33]. Latourte et al further showed that this is likely mediated by Stat3 signaling, known to be downstream of IL-6R activation. Blockade of Stat3 with the small molecule inhibitor ‘Stattic’ also reduced osteophyte formation in DMM mice. IL-6
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has been shown to be increased in OA synovial fluid, and also shortly after joint injury [34], and to increase chondrocyte production of matrix metalloproteinases (MMP-1, -3, -13) and aggrecanases (ADAMTS-4 and -5) [35]. Another new study from Nasi et al implicates IL-6 again, but this time in the
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context of OA related to hydroxyapatite crystal formation. Basic calcium phosphate (BCP), especially hydroxyapatite, is found in the synovial fluid of approximately 50% of OA cases [36], and induces joint disease when injected into mouse knees [37]. Nasi demonstrated that BCP induces IL-6 production by
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human chondrocytes and, reciprocally, IL-6 induced calcium crystal formation by the upregulation of calcium-regulating genes. These effects could by blocked by inhibiting the IL-6R in vitro with a
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monoclonal antibody. Furthermore, BCP crystals were increased in a post-traumatic model of OA in mice (by surgical meniscectomy). The effects of IL-6 induction by BCP appear to form a positive feedback loop leading to cartilage damage, but what remains unclear is how BCP crystals activate the chondrocytes. The authors appear to infer this is due to IL-6R activation directly, but studies investigating this interaction are needed.
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Encouragingly, multiple phenotypes of OA have been independently investigated this year. Emerging as primary phenotypes, at least in animal model studies, are post-traumatic OA, metabolic OA, and age-associated OA. BCP crystals may be a separate phenotype or more generally a secondary
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mediator relevant to different OA phenotypes. Interestingly, there appear to be some mechanisms that are unique to certain OA phenotypes, as in the case of MIF and age-associated OA, whereas other
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mechanisms such as hypercholesterolemia exert effects either independently or in concert with other OA phenotypes.
Timing of treatment interventions matters In the Miller study of nociceptive pain inhibition described above [2], analgesia was effective in
early post-traumatic OA but not at later stages. This is a great example of the importance of doing temporal studies in animal models. The notion also offers us a viable explanation for why studies that are
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effective in animal models don’t often translate successfully into human studies, because patients are often at much later stages of OA disease than were modeled in preclinical work. A few years ago, we discovered early upregulation of the chemokine CCL2 in the cartilage of a
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post-traumatic model of OA in rats, and that its receptor CCR2 could be successfully targeted pharmacologically to reduce joint damage [20]. In contrast, genetic deletion of its receptor CCR2 in mice reduced pain behaviour without reducing structural damage [4]. This year’s report by Longobardi et al
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may explain the discrepancy as it relates to timing [38]. Transient therapy with the CCR2 small molecule inhibitor RS504393 resulted in joint protection only if treatment was started contemporaneously with, or
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at 4 weeks after, post-traumatic OA induction surgery. In contrast, delay of treatment to 8 weeks or sustained therapy for at least 8 weeks resulted in loss of the protective effect. This is an interesting observation and supports the effects seen in genetic Ccr2 ablation studies. First, CCR2 is expressed by inflammatory and resident joint cell types, including monocytes, macrophages, chondrocytes, and neuronal cells [39]. Early introduction of the chemokine inhibitor may result in reduced inflammatory cell
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infiltration or inflammatory responses by resident joint cell types, which might be expected to be protective. However, sustained treatment may prevent any inflammatory response, without which normal healing may not be initiated. This is speculative, but the observation that pulsatile treatment at the right
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time is more effective than sustained treatment is novel and could point to an alternative strategy for OA treatment depending on the target.
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Sirtuins are deacetylases with important roles in DNA repair. SIRT1 promotes chondrocyte
survival and reduced inflammatory responses through multiple mechanisms, including deacetylation of p65/RelA, which may be enhanced by treatment with resveratrol [40]. Li et al recently demonstrated that temporal loss of SIRT1 corresponds to disease stage, as determined by cartilage histopathologic Mankin scoring of samples obtained at knee arthroplasty [41]. Later stages of OA with loss of expression of chondrocyte SIRT1 corresponded to higher levels of p53 and p53 acetylation, relative to age-matched controls (trauma patients without OA). In addition, Elayyan and colleagues provided evidence for a
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mechanistic link between SIRT1 and control of MMP-13 gene expression mediated by the LEF1 transcription factor in human OA chondrocytes [42]. Overexpression of SIRT1 resulted in suppression of MMP-13 and LEF1 as well as their induction by IL-1β. SIRT1 was also linked to circadian rhythm by
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Yang et al [43], showing that siRNA-mediated knockdown or pharmacologic inhibition of SIRT1 in chondrocytes results in lower levels of transcripts for the clock gene Bmal1. It was already known that loss of Bmal1 increases cartilage damage and disrupted circadian rhythm in chondrocytes [44], as
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highlighted in last year’s Year in Review [45]. Interestingly, similar methods used to knock down Bmal1 expression reciprocally reduced SIRT1 expression. Loss of either SIRT1 or Bmal1 potentiated higher
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levels of MMP-3, -13, and ADAMTS-5 expression in response to IL-1β stimulation of chondrocytes. In light of the evidence that reduction of SIRT1 occurs in early to moderate OA cartilage, if SIRT1 can be targeted in OA it is likely best done at an early stage of disease.
Moreover, together these studies lend further support to the importance of considering the timing and duration of any intervention in the context of the specific pathophysiology at play in each
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experimental design. OA continues to challenge in its complexity and heterogeneity. Tuning protective and pathogenic inflammatory pathways: implications for treatment approaches Rela/p65 is a key subunit required for NF-kB signaling, with important roles in regulating the
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expression of Sox9 [46] and Adamts5 [47] in chondrocytes. Thus, Rela may confer anabolic and catabolic
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functions to chondrocytes. However, NF-kB signaling is a common final pathway of many inflammatory mechanisms thought to play a deleterious role in OA cartilage pathophysiology. Given the wide array of functional roles of NF-kB signaling in maintaining chondrocyte homeostasis, Kobayashi et al conducted an in vivo study investigating the effects of Rela using tamoxifen-inducible chondrocyte-specific (Col2a1Cre) Rela gene deletion in adult mice [48]. Surprisingly, homozygous deletion of Rela resulted in acceleration of OA-related joint damage, mostly mediated by increased chondrocyte apoptosis, despite the dramatic reduction in inflammatory cytokine expression. In contrast, haploinsufficiency of Rela protected against DMM-induced post-traumatic OA development and was associated with marked reduction in
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Hif2a and Adamts5 expression. Glycosaminoglycan release from cartilage explants stimulated by IL-1β ex-vivo was also greatly reduced, but the increase in apoptosis that occurred with homozygous Rela deletion was prevented. Thus, a balance of just enough Rela to prevent apoptosis but reducing Rela just
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enough to prevent catabolic responses in chondrocytes was protective. Similar results were seen with a low dose of an IKK small molecule inhibitor (BMS-345541), which resulted in similar partial inhibition of NF-kB signaling. Interestingly, similar effects were seen in both the DMM and age-associated OA
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mouse models in this study.
Similarly, two studies investigating the role of macrophages found that macrophages mediate the
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pro-inflammatory and catabolic effects of alarmins, while they are also required for mitigating an inflammatory response to oxidized-LDL injections into the joint. van den Bosch et al demonstrated that the alarmins S100A8 and S100A9 are mainly produced in the joint by synovial macrophages (not by fibroblasts) [49]. Moreover, synovial M1-like macrophages were the primary cell type to respond to stimulation with the alarmins by producing IL-1, -6, -8, and TNF-α. In contrast, de Munter et al
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repeatedly injected oxidized LDL into murine knee joints, which resulted in synovial upregulation of TGF-b activity but no inflammatory response [50]. However, depletion of macrophages from healthy mouse knee joints followed by repeated oxidized LDL injection caused the upregulation of inflammatory
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mediators and synovial thickening, despite the absence of synovial macrophages. These studies provide a more nuanced view of inflammatory signaling in roles in OA and
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cartilage homeostasis. Not all inflammation is bad, and indeed, targeting inflammation may be beneficial as long as it is not at the expense of impaired chondrocyte homeostasis, such as in the case of NF-kB signaling mediated by Rela. Similarly, M1-like macrophages may be responsible for the production of inflammatory mediators detected in response to alarmin signaling in OA joints, but have an important role to play in the homeostasis of the healthy joint. Overall, inflammation is a complex issue and the reductionist concept of ‘good’ and ‘bad’ inflammation, while useful for illustrative purposes, is also an oversimplification. The fact that mechanisms of chronic OA-related inflammation (e.g. signaling
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pathways and cell types influencing inflammation) are also required for tissue reparative processes belies the challenges inherent in developing anti-inflammatory treatments for OA. Clearly, a context-specific understanding of inflammation is required.
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Topography of the joint organ system uncovers roles for joint lining cells
Significant progress was made to understanding the role of the superficial zone (SZ) of articular cartilage in joint homeostasis and disease. First, Li et al used R26-Confetti multicolour reporter mice
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crossed with tamoxifen-inducible joint lining-cell-specific (PRG4-Cre) mice to label and follow SZ cells from P5 through 6 months of age [51]. Fluorescent-tagged cells initially moved down, formed
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chondrocyte clusters, then populated the entire depth of cartilage, suggesting that articular cartilage grows by appositional growth in this particular context. While this implies that PRG4-expressing superficial zone cells may play a role in tissue homeostasis and repair, it is important to note that this was determined in a developmental context, which may be different in adult mice. In contrast, lineage tracing by Decker et al using similar methods [52] found that cartilage growth is more likely achieved by expansion in cell
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volume and rearrangement of cells into columnar alignment. At least in developmental tissues, PRG4+ synovial cells were highly responsive to cartilage injury and mobilized to the site of tissue repair, while local chondrocytes remained relatively fixed in situ. Both of these studies highlight the potential of joint
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lining tissues to contribute to joint tissue homeostasis and repair. Fibulins are known to have functions in maintenance of basement membrane organization and
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elastic fiber formation, but were introduced to the OA field in the context of SZ cells in early 2017 by Hasegaswa and colleagues [53]. Fibulin 3 expression was limited to SZ and other joint lining cells in homeostatic conditions, but expression in these cell populations declines with age or post-traumatic OA in DMM mice. Genetic deletion of fibulin 3 exacerbated age-related and post-traumatic OA and was associated with lower expression of PRG4. siRNA driven suppression of fibulin 3 increased apoptosis of SZ cells, but also promoted chondrogenesis in bone marrow-derived mesenchymal stem cells. In contrast, fibulin 3 overexpression prevented chondrogenesis. Interestingly, it appears that fibulin 3 may be
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important for maintaining a pool of progenitor cells in the SZ and joint lining, through maintenance of lubricin (or other PRG4 gene product expression), but can be lost with injury or ageing. This potentially would render cartilage susceptible to OA due to loss of an endogenous repair cell pool. On the other hand,
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reduction in fibulin 3 may be a necessary step to release joint lining cells from a progenitor status and enter into chondrogenesis, putatively for joint repair. This is an area that warrants further investigation. Further implications for cells in the SZ in the development of OA were uncovered by Jeon et al
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[54]. This work was prompted by the discovery that senescent cells accumulate during adulthood, leading to shortened the lifespan and age-related functional decline of organs in mice. Deletion of senescent cells
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leads to increased lifespan and improved organ function using a pharmacologic inducer of apoptosis in senescent cells expressing the INK-ATTAC transgene [55]. Senescent cells have a senescence-associated secretory phenotype (SASP), which includes the production of IL-6 (linking back to other studies this year demonstrating the pathologic role of IL-6 in OA) and the alarmin HMGB1 [56]. Jeon et al used p163MR bioluminescent reporter mice to study the role of senescent cells in the development of two OA
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phenotypes, leveraging the feature that senescent cells in these mice are sensitive to the senolytic effects of ganciclovir and UBX0101. Senescent cells accumulated in the SZ and joint lining cells of mice with ageing and after anterior cruciate ligament transection (ACLT) induction of post-traumatic OA.
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Pharmacologic deletion of senescent cells with two different inhibitors led to improved anabolic and catabolic gene expression profiles of cartilage and reduced joint damage scores in both mouse models of
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OA. However, the protective effects on joint damage were less pronounced when ACLT was performed in aged mice (19 months old), although pain behaviour was significantly reduced with senolytic agent treatment. Notwithstanding, this study clearly highlights the importance of senescent cells accumulating in the SZ and joint lining of knee joints in OA models and should be examined further using additional models and human OA tissues.
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Together, these studies demonstrate that the topography of the joint matters greatly. In addition to other joint tissues, even different zones of the cartilage, as one example, may have differential roles to play.
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Conclusion
As with any review, particular articles of merit were not included in this review. Highlighted studies were selected based on their representative nature of particular themes that emerged during the
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literature search and exploration, and several of the selected studies could have fit well into multiple theme areas. Much of the work on biological mechanisms of OA has focused on the secondary mediators
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of disease. Emerging themes this year demonstrated the importance of embracing new technologies to provide key insights into the nature of different forms of OA. Additional biological outcomes research is warranted, in particular for underrepresented areas including pain and pain sensitization, bone marrow lesions, and impaired joint function. Biological mechanisms related to OA prevention is another area that would benefit from additional resource dedication.
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It is particularly exciting to see an increased focus on investigating biological mechanisms in the context of different OA phenotypes. And although this review focused on biological mechanisms and therefore is heavily influenced by preclinical studies, much work on clinical OA phenotypes is also
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ongoing. From animal models research, three particular phenotypes seem to have emerged, including post-traumatic, age-associated, and systemic metabolic forms of OA. As a field, it is important that we
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continue to expand this list, such as with BCP, genetic, inflammatory, or other candidate OA phenotypes, but also into anatomic (e.g. hand, hip, etc) and physiologic domains. Encompassing these perspectives will be highly informative, since it is likely that OA will require a personalized medicine approach to diagnosis and staging in order to select the most appropriate therapy for each person, at the right time. As soon as possible, it is also vital that biomechanistic studies align closely with the phenotypes of OA identified in clinical studies. Such an effort will require representative members from each area of strength in the OA field (highlighted by the themes of the OA Year in Review series) coming together to
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devise a schema and common language that we may all utilize as a common approach to this complex and
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heterogeneous disease.
Author Contributions
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Dr. Appleton performed the literature search, designed the theme selection, wrote and edited the work.
Conflict of Interest
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No direct financial or other conflicts of interest related to this work. Dr. Appleton has received
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consultancy honoraria from Novartis and meeting sponsorship from Pfizer.
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Figure Legends
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Figure 1. Topic areas encompassed by studies published on biological mechanisms of OA between 2016-2017. Topics are organized in proposed pillars across a continuum of OA biology. The figure is not meant to be complete, but represents topics that were featured or commonly reported in the literature
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based on the year in review. Figure 2. Schematic summary of conclusions from mechanistic studies of OA biology between 20162017. Themes are shown in relation to the most significantly associated primary cause (risk factor) for
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OA based on the context of studies included. Primary causes include injury and altered biomechanics, ageing, and systemic metabolic derangement (e.g. hypercholesterolemia). Green indicators represent a
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positive association (protective against OA). Red indicators represent a negative association (promoting OA). Arrows indicate a positive relationship (stimulatory effect), whereas blunt-ended arrows indicate an
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inhibitory effect. Based on reported evidence from the 2016-2017 literature.
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