Downregulated Brain-Derived Neurotrophic Factor-Induced Oxidative Stress in the Pathophysiology of Diabetic Retinopathy

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Review

Downregulated Brain-Derived Neurotrophic Factor-Induced Oxidative Stress in the Pathophysiology of Diabetic Retinopathy Tapan Behl M Pharm *, Anita Kotwani PhD Department of Pharmacology, Vallabhbhai Patel Chest Institute, University of Delhi, Delhi, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 April 2016 Received in revised form 23 July 2016 Accepted 31 August 2016

Brain-derived neurotrophic factor (BDNF), a member of neurotrophin growth factor family, physiologically mediates induction of neurogenesis and neuronal differentiation, promotes neuronal growth and survival and maintains synaptic plasticity and neuronal interconnections. Unlike the central nervous system, its secretion in the peripheral nervous system occurs in an activity-dependent manner. BDNF improves neuronal mortality, growth, differentiation and maintenance. It also provides neuroprotection against several noxious stimuli, thereby preventing neuronal damage during pathologic conditions. However, in diabetic retinopathy (a neuromicrovascular disorder involving immense neuronal degeneration), BDNF fails to provide enough neuroprotection against oxidative stress-induced retinal neuronal apoptosis. This review describes the prime reasons for the downregulation of BDNF-mediated neuroprotective actions during hyperglycemia, which renders retinal neurons vulnerable to damaging stimuli, leading to diabetic retinopathy. © 2016 Canadian Diabetes Association.

Keywords: brain-derived neurotrophic factor (BDNF) glucocorticoids high-mobility group box-1 (HMGB1) neurogenesis synaptic plasticity

r é s u m é Mots clés : facteur neurotrophique dérivé du cerveau (BDNF) glucocorticoïdes B1 du groupe de haute mobilité (HMGB1) neurogenèse plasticité synaptique

Le facteur neurotrophique dérivé du cerveau (BDNF pour brain-derived neurotrophic factor) est un facteur de croissance, membre de la famille des neurotrophines, qui médie physiologiquement l’induction de la neurogenèse et la différenciation neuronale, promeut la croissance et la survie des neurones, et maintient la plasticité synaptique et les interconnexions neuronales. Contrairement au système nerveux central, sa sécrétion dans le système nerveux périphérique apparaît d’une manière dépendante de l’activité. Le BDNF améliore la mortalité, la croissance, la différenciation et le maintien des neurones. Il offre également une neuroprotection contre plusieurs stimuli nocifs, empêchant ainsi les dommages neuronaux au cours des états pathologiques. Cependant, lors de rétinopathie diabétique (un trouble de la neuromicrovascularisation impliquant une dégénérescence considérable des neurones), le BDNF n’offre pas assez de neuroprotection contre l’apoptose des neurones rétiniens induite par le stress oxydatif. Cette revue décrit les raisons principales de la régulation à la baisse des effets neuroprotecteurs médiés par le BDNF durant l’hyperglycémie, qui rend les neurones rétiniens vulnérables aux stimuli dommageables, et qui mène à la rétinopathie diabétique. © 2016 Canadian Diabetes Association.

Introduction Brain-derived neurotrophic factor (BDNF) is a protein that belongs to a family of growth factors, called neurotrophins, whose other mammalian members include nerve growth factor (NGF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4). Neurotrophins are basically involved in regulating the development, survival and functioning of neurons. Among all neurotrophins, the physiology of BDNF is the most well defined and well characterized (1). It is encoded by the BDNF gene

* Address for correspondence: Tapan Behl, M. Pharm, Department of Pharmacology, Vallabhbhai Patel Chest Institute, Delhi University, North Campus, Vijay Nagar Marg, New Delhi, Delhi 110007, India. E-mail address: [email protected] 1499-2671 © 2016 Canadian Diabetes Association. http://dx.doi.org/10.1016/j.jcjd.2016.08.228

and is known to control vital physiologies of the vertebral nervous system, encompassing mainly the monitoring of differentiation and growth of neurons as well as their maintenance. Moreover, subsequent studies have revealed more complex functions of BDNF in the nervous system, including regulation of synaptic plasticity, dendritic arborization, mediation of long-term potentiation and establishment of neuronal circuits to modulate complex behaviours. Also, its involvement in other pivotal roles, such as in hippocampaldependent cognitive functions and the control of hyperactiveness, has also been demonstrated. Owing to the vehemence of BDNFmediated functions, any deviation from its normal physiology leads to psychiatric disorders (such as bipolar disorder, schizophrenia, anxiety and cognitive dysfunctions), stroke, spinal cord injuries and several disorders concerning neuronal degeneration (2,3). (Figure 1)

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Figure 1. Hyperglycemia-induced downregulation of protective effects of brain-derived neurotrophic factor (BDNF) against oxidative stress-induced retinal neurodegeneration and progression of diabetic retinopathy.

Diabetic retinopathy, earlier considered merely a microvascular complication, is now widely recognized as a neuromicrovascular complication, with damage to the neuronal system that plays an integral part in its pathophysiology. BDNF is acknowledged for its role in neurogenesis and in protecting neurons from several damaging stimuli. This review describes the effects of various hyperglycemia-induced alterations on the neuroprotective mechanisms offered by BDNF, which render the latter incapable of preventing retinal neurodegeneration during diabetic retinopathy.

Retinal Anatomy, Neuronal Damage and Diabetic Retinopathy Diabetic retinopathy is a severe sight-threatening complication of diabetes mellitus, and it accounts for a large number of cases of acquired, yet potentially avoidable, blindness. The principal mechanism of its pathogenesis appears to be alterations in the microvasculature of retina as the result of hyperglycemia. The elevated concentration of blood glucose is a harbinger of numerous molecular changes. These lead to various responses that result in neuropathy and microangiopathy. Retinal detachment is the ultimate reason for the irreversible blindness associated with the advanced stages of diabetic retinopathy. Retinal detachment is caused by the destruction of the anatomic structure of the retina as well as the hyperosmolar pathologic conditions created in the retina during hyperglycemia. Recent studies conducted to explore the role of retinal neurodegeneration in the pathophysiology of diabetic retinopathy have revealed that oxidative stress-induced destruction of retinal neurons leads to disruption in normal retinal physiology and, thus, compromises vision. The anatomic structure of the retina basically comprises neuronal tissue containing several layers of different types of cells, namely,

ganglionic cells, amacrine cells, Müller glial cells, bipolar cells, horizontal cells and the photoreceptor layer of rods and cones (4). Lipids are widely found to be associated with the retina. Studies employing matrix-assisted laser desorption/ionization (MALDI) imaging mass spectroscopy have shown the presence of plasmalogen phosphatidylethanolamine lipid-containing docosahexaenoic acid in the inner retina and docosahexaenoic acid-containing glycerophosphatidylcholine and other phosphatidylethanolamine lipids in the photoreceptor cells (5). These whole units collectively make up the retinal screen, which is sandwiched between the posterior wall of the vitreous chamber and the retinal pigment epithelium. The inner limiting membrane formed by the Müller glial cells adjoins the posterior chamber, whereas the photoreceptors are present at the other extreme end, adjoining the retinal pigment epithelium. The retinal pigment epithelium comprises the outer bloodretinal barrier and interacts with the choroidal blood supply for the exchange of oxygen and vital nutrients, besides serving several other functions (6). Because neurons are the basic structural units of the retina, any damage to these cells would directly hamper retinal physiology. Inflammation, oxidative stress and hypoxia-ischemia are the 3 major hyperglycemia-induced pathologic conditions that account for retinal neuronal damage. The causes of all 3 mentioned pathologic states are interlinked and ultimately lead to similar damaging outcomes. Hyperglycemia-induced mitochondrial dysfunction may be regarded as an initial trigger for the induction of these conditions. The human body responds to excessive blood glucose primarily by upregulating 2 major glucose-metabolizing pathways— glycolysis and the citric acid cycle. In both these pathways, reduced forms of nicotinamide adenine dinucleotide and flavin adenine dinucleotide (NADH and FADH 2 , respectively) are formed as

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intermediates that further take part in the mitochondrial electron transport chain reaction as electron-donating substrates, resulting in the production of superoxide radicals. During hyperglycemia, both these pathways become overexpressed, so they eventually lead to overproduction of superoxide radicals, resulting in oxidative stress. The presence of immense lipid content, excessive exposure to sunlight and rich oxygen supply make the retina highly vulnerable to oxidative stress-induced damage, which results mainly in DNA damage and lipid peroxidation (7–9). The former accounts mainly for neuronal apoptosis, whereas the latter is responsible for inducing a subsequent inflammatory response by mediating the production of toxic lipid aldehyde species, namely 4-hydroxy-trans2-nonenal, acrolein and malondialdehyde, which further activate several intermediates involved in redox signalling, ultimately leading to the activation of nuclear factor-kappa B (NF-κB). NF-κB, a nuclear transcription factor, leads to the production of several proinflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α) and various cytokines via its downstream signalling, thereby inducing a robust inflammatory response and aggravating neuronal damage (10). In addition, hyperglycemia-induced oxidative stress is eminently involved in inducing retinal hypoxia and ischemia. During hyperglycemia, oxidative stress downregulates the expression of an enzyme, dimethylarginine-dimethylaminohydrolase, which is responsible for the metabolism of asymmetric dimethylarginine, a potent inhibitor of endothelial nitric oxide synthase. Hence, oxidative stress-induced upregulation of assymmetric dimethylarginine and subsequent downregulation of endothelial nitric oxide synthase result in decreased production of nitric oxide. Moreover, the small amount of nitric oxide produced by other isoforms of nitric oxide synthase or by other pathways is utilized in a combination reaction with superoxide radicals (produced during hyperglycemiainduced mitochondrial dysfunction) to produce peroxynitrite, a highly reactive species, thus further increasing oxidative stressinduced damage, besides rendering the body devoid of nitric oxide, a potent vasodilator. In the absence of nitric oxide, the physiologic vasodilation of the retinal blood vessels cannot be maintained, leading to relative vasoconstriction and subsequent states of retinal hypoxia and ischemia. This adversely affects the nutritional fulfillment of neurons, resulting in their apoptosis (11,12). Hence, hyperglycemia-induced various pathologic conditions mediate neuronal apoptosis, thereby leading to the progression of diabetic retinopathy.

Neuroprotective Roles of BDNF BDNF is physiologically involved in maturation of neurons from the neural progenitor and stem cells as well as providing them protection against various noxious stimuli, thereby enhancing their survival rates. These physiologic roles are described in detail in the following sections.

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networks of GABA transmission upon binding to tropomyosin receptor kinase B (TrkB) receptors. The BDNF-TrkB complex translocates from dendrites to the cell bodies of interneurons and initiates a cascade of downstream signalling, leading to activation of transcriptional programs. During this activation, the BDNF-TrkB complex mediates the Ras-ERK-CREB-dependent signalling pathway and upregulates the transcription of the glutamic acid decarboxylase 65 (GAD65) gene, thereby inducing the expression of the enzyme responsible for the presynaptic synthesis of GABA. Earlier studies have reported beneficial effects of GABAA receptor-mediated inhibitory transmission on the augmentation of progenitor neuronal differentiation, so BDNF-mediated upregulation of GABA synthesis and expression might be considered 1 of the mechanisms by which the former induces neurogenesis (14–16). Another hypothesis regarding this context is associated with the involvement of Ca 2+ / calmodulin-dependent protein kinase II in BDNF-mediated responses. It states that BDNF induces the activation of Ca2+/calmodulindependent protein kinase II and initiates a cascade of intracellular signalling involving subsequent activation of the p38 subfamily of mitogen-activated protein kinases (MAPK) and its downstream target, MAPK-activated protein kinase 2 (MAPKAPK-2). It is speculated that MAPKAPK-2 is possibly the kinase activated during this pathway and that it phosphorylates the key positive regulatory site of cAMPresponse element-binding (CREB) protein, leading to its activation (17). Numerous studies have further demonstrated that CREB is the prime mediator of BDNF responses, including synaptic plasticity and neuronal survival. In an experiment conducted on subventricular zone-derived neuroblasts, CREB phosphorylation was shown to be essential for neuronal differentiation and survival of the newly generated cells. The stated observation was further proved in the same study when inhibition of CREB phosphorylation led to downregulation of the earlier demonstrated responses, hence validating the role of BDNF-induced signalling in neuronal differentiation and survival (18). Also, the establishment of an organized neuronal network is highly essential for the development of a healthy nervous system. BDNF plays a pivotal role in the development of synaptic connections and neuronal circuits. As described earlier, because BDNF is potentially involved the upregulating intracellular cAMP signalling, it leads to the activation of several other cAMPdependent pathways, including phosphatidylinositol-3 (PI3) kinase, mitogen-activated protein (MAP) kinase and phospholipase C-gamma (PLC-gamma) pathways, which play important roles in neuronal growth and development. BDNF also possesses an intrinsic ability to induce localized structural and functional alterations in the axonal and dendritic terminals, thereby modulating neuron shape as well as physiology during nerve development (19). Hence, BDNF plays a vital influential role in governing the neuronal differentiation and development and, thus, is an essential factor concerning neuronal functioning.

BDNF Prevents Neurons From Oxidative Stress-Induced Damage and Apoptosis BDNF Enhances Neuronal Differentiation and Development Research concerning differing physiologic systems have demonstrated beneficial actions of BDNF in increasing the number of neurons as well as the survival factor of neurons. Both in vivo and in vitro studies have manifested the above-stated results (13). Although these BDNF-mediated effects have been known for a decade, the exact molecular mechanism behind them is still unknown. However, several hypotheses have been proposed to justify them. One of them includes mediation of BDNF effects via gammaaminobutyric acid (GABA) -ergic transmission. According to a study conducted of cultured hippocampal neurons, BDNF considerably enhances the secretions of GABA and promotes the inhibitory

Based on the previous discussion of the role of oxidative stress in inducing neuronal damage, it is clear that free radicals are highly noxious stimuli for neurons (and all other types of cells) and eventually lead to their apoptosis. Studies describing the role of BDNF in augmenting the survival of neurons aroused the speculations of its possible involvement in protecting neurons from such noxious stimuli as well. Indeed, the speculations were proven true by several subsequent studies that demonstrated decreased levels of free radicals in the body upon exogenous BDNF treatment. BDNF inhibits oxidative stress-induced microglial activation and, hence, prevents microglia-mediated downregulation of antioxidant enzymes. Also, decrease in BDNF-mediated intracellular cAMP signalling is

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directly correlated with upregulated apoptotic signalling in cells, therefore confirming the protective role of BDNF and its downstream cAMP-activated targets in keeping a check on neuronal apoptosis (20). Other studies intended to describe the rationale behind BDNF-mediated downregulation of oxidative stress further manifested a direct influence of BDNF on antioxidant expressions of glutathione. Moreover, the same study also described the downregulating effects of BDNF on the levels of malondialdehyde, the end product of lipid peroxidation, thereby signifying a decrease in the oxidative stress-induced damage to cellular membranes (21). The molecular mechanism behind BDNF-mediated neuroprotection from oxidative stress-induced damage is also speculated to be mediated by the cAMP response element (CRE) through the CREB/CRE transcriptional pathway. This finding is validated by the exhibition of increased expression of free radicals and enhanced vulnerability of neurons to oxidative stress-induced damage and neuronal toxicity upon negative regulation of the CREB/CRE transcriptional pathway, thus confirming its involvement in neuroprotection (22). Further research into the involvement of this pathway have led to the demonstration of even more clearly defined molecular signalling. According to these studies, the phosphorylation and activation of CREB by protein kinase B, also known as Akt (which itself is activated upon BDNF-mediated TrkB receptor activation), stimulates the transcription of several target genes. Among these targets, the transcription of antiapoptotic factors such Bcl-2 is considered of prime importance for inhibiting oxidative stress-induced as well as ischemia-induced apoptosis. Hence, it can be stated that BDNF upregulates the production of various antiapoptotic proteins and, therefore, inhibits neuronal apoptosis (23). BDNF also inhibits cell cycle reentry following N-methyl-D-aspartate- or hydrogen peroxideinduced oxidative stress, thereby preventing the activation of the retinoblastoma protein and E2F1 transcription factor. Both these factors are critically involved in mediating oxidative stressinduced cell senescence and subsequent apoptosis, so their inhibition by BDNF demonstrates another pathway by which the latter exhibits its neuroprotective effects. Another study demonstrated that inhibition of phosphatidylinositol 3-kinase diminishes the BDNFmediated neuroprotective response, hence justifying the importance of this pathway in reducing oxidative stress. However, there is controversy regarding the involvement of the Ras-MAPK pathway, with some studies suggesting that activation of MAPK-ERK1/2 leads to exhibition of oxidative stress-induced neuronal damages, whereas others suggest that this pathway is beneficial in mediating neuroprotective effects against oxidative stress (24–26). Nevertheless, from the above discussion, it is quite clear that BDNF is involved in the mediation of several pathways that ultimately favour neuroprotection against oxidative stress-induced damage and neuronal apoptosis. Effects of Hyperglycemia-Induced Alterations on BDNF Although BDNF provides neuroprotection during physiologic as well as several pathologic conditions, studies have observed a downregulation in its expression during diabetes, thereby rendering neurons vulnerable to the damages produced by numerous hyperglycemia-induced alterations, such as inflammation and oxidative stress. Retinal neurons are also strongly affected by the same, hence aggravating the progression of diabetic retinopathy. The prime reasons accounting for BDNF downregulation during diabetes are discussed below. Hyperglycemia-mediated increases in glucocorticoid levels downregulate BDNF Although hyperglycemia induces numerous alterations, most studies focus on the aftermaths of these alterations in the

vascular components, primarily endothelial cells; however, certain alterations also affect the nervous system. One such alteration includes hyperglycemia-induced dysfunction in the hypothalamicpituitary-adrenal (HPA) axis, which is physiologically responsible for the regulation of hormonal homeostasis in the body. Several studies have demonstrated the involvement of HPA axis dysfunction in diabetes-induced cognitive impairments; however, its eventual role in the pathogenesis of such disorders is not fully illustrated. Some studies involving diabetic animal models and concerning functional associations of the HPA axis with cognitive dysfunctions have shown that diabetes induces hippocampus-dependent memory impairment, synaptic plasticity and neurogenesis. Hyperglycemiamediated upregulation in the levels of glucocorticoids was described as the justification for the above observations. Although it had been demonstrated by earlier studies that increased levels of glucocorticoids were involved in the pathophysiology of diabetes mellitus, and the pathways of their involvement were fully elucidated, it was probably the first time that the reverse was being reported; however, the exact molecular mechanisms that mediate hyperglycemiastimulated increased levels of glucocorticoids are not yet known. Restoration of the upregulated expression of glucocorticoids, primarily corticosterone (an adrenal hormone) greatly reestablished the altered physiology of nervous system, thereby validating the role of hyperglycemia-induced dysfunctions of the HPA axis in such disorders (27–29). As already described, BDNF is critically involved in the regulation of synaptic plasticity and neurogenesis, so the results of these experiments led subsequently to speculations about a possible relationship between hyperglycemia-induced dysfunction of the HPA axis, upregulated glucocorticoid levels and altered expression of BDNF during diabetes and its complications, here focusing primarily on diabetic retinopathy. Along with the presence of an overactivated HPA axis, increased secretions of glucocorticoids and downregulated levels of BDNF have been reported separately during the presentation of diabetic retinopathy by several studies, thereby further substantiating the speculated relationship among the mentioned eventualities (30,31). Indeed, a relationship between upregulated levels of corticosteroids and BDNF was established by succeeding studies, confirming that pathologically induced increased concentrations of glucocorticoids downregulate the expression of BDNF (32). As well, several alterations are induced in BDNFmediated pathways during upregulated expressions of glucocorticoids. Although it has been demonstrated by some studies that glucocorticoids interact with the receptor for BDNF (TrkB), the responses elicited by such interactions are very different from those expressed following BDNF binding. The altered responses include considerable reduction in BDNF-mediated synaptic function, induction of neuronal apoptosis, hampering of synaptic maturation, inhibition of BDNF-induced corticotrophin-releasing hormone release, among several others (33,34). Corticotrophin-releasing hormone is known to play an insulinotropic role; hence, its inhibition by glucocorticoid-mediated responses further aggravates hyperglycemia, intensifying the pathologic conditions responsible for BDNF downregulation (35). Also, overexpression of corticosteroids suppresses BDNF-mediated MAPK-ERK signalling, thereby downregulating the neuroprotective effects mediated by BDNF via this pathway. It also inhibits the activation of PLC-gamma pathway and, subsequently, leads to decreased glutamate release. This results in alterations in the normal physiology of neuronal development and the regulation of synaptic transmissions, hence causing neuronal dysfunction. The molecular mechanisms behind these responses are said to be mediated primarily via glucocorticoidinduced activation of the glucocorticoid receptor and subsequent transcription of several target genes. However, the involvement of several other receptors, such as the TrkB receptor, T-cell receptor, membrane-associated G-protein coupled receptors (and the

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subsequent activation of their downstream intracellular signalling pathways), tyrosine kinase proteins and direct cytosolic signalling mechanisms have also been demonstrated in glucocorticoidsmediated responses (36). Hence, from the above discussion, it is clear that hyperglycemia-induced upregulation of glucocorticoids is highly responsible for downregulating the levels of BDNF and their neuroprotective actions, thereby rendering retinal neurons vulnerable to the prevalent pathologic conditions, such as inflammation and oxidative stress-induced damages, thus accounting for the progression of diabetic retinopathy. Hyperglycemia-induced upregulation of HMGB1 reduces BDNF levels Another, lesser known hyperglycemia-induced pathologic alteration includes induction of the high mobility group box-1 (HMGB1) protein, an atrocious proinflammatory cytokine. It is responsible for downregulating the expression of BDNF, hence inhibiting neuroprotective mechanisms as well as, itself, inducing several pathways leading to neurodegeneration (37). Unlike hyperglycemiainduced upregulation of corticosteroids, several molecular mechanisms have been established to explain this induction. Several studies have demonstrated the roles of several hyperglycemiainduced conditions, such as inflammation, oxidative stress and ischemia/reperfusion injury, in the activation of HMGB1. During inflammation, activated microglia are the most prominent source of production of this cytokine. Neurons, degenerated after being affected by other noxious stimuli, also serve as precursors for its release. Hence, it is concluded that activated inflammatory cells, such as microglia, monocytes and macrophages and necrotic cells, are strongly accountable for the upregulation of HMGB1 during hyperglycemia (38,39). HMGB1 is secreted by activated microglia and, in turn, further activates more microglia and produces proinflammatory effects. Microglia are inflammatory cells of the nervous system, so their activation is responsible for the secretion of several proinflammatory mediators, including glutamate, proteases, leukotrienes, interleukin1-beta (IL-1β), IL-3, IL-6, TNF-α, vascular endothelial growth factor (VEGF), lymphotoxin, macrophage inflammatory protein 1 (MIP-1) and matrix metalloproteinases (MMPs). In addition, HMGB1 directly induces the gene expression of nicotinamide adenine dinucleotide phosphate oxidase (via the Toll-like receptor-4 signalling pathway) and inducible nitric oxide synthase, henceforth increasing the production of superoxide radical and nitric oxide, respectively (38,40,41). All the above-mentioned mediators are critically involved in the pathophysiology of diabetic retinopathy. TNF-α upregulates the expressions of several cellular adhesion molecules, such as intracellular adhesion molecule-1 and vascular cell adhesion molecule-1, which primarily initiate a cascade of events, leading to leukostasis. Increasing interactions of leukocytes with the retinal endothelium causes endothelial dysfunction, ultimately leading to breakdown of the inner blood-retinal barrier and, hence, increasing vascular permeability, which causes macular edema, a characteristic event of diabetic retinopathy (42,43). In addition, TNF-α, along with IL-1β, is also proven to be involved in upregulating the release of glutamate (an excitatory neurotransmitter) from activated microglia by inducing glutaminase 1, an enzyme responsible for the conversion of glutamine to glutamate. Excessive release of glutamate into the extracellular space during diabetic retinopathy overactivates the ionotropic glutamate receptors, primarily alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate and N-methyl-D-aspartame receptors, resulting in rampant calcium influx. This overactivity due to superfluous calcium leads to excitotoxicity and subsequent postsynaptic neuronal cell death, resulting in retinal neurodegeneration (41,42,44). VEGF and MMPs upregulate angiogenic processes leading to retinal neovascularization, hence upregulating the progression of

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proliferative diabetic retinopathy (43). Leukotrienes, MIP-1 and other interleukins intensify the inflammatory response by activating other inflammatory mediators while eliciting their own proinflammatory responses. The superoxide radical, besides itself damaging cellular DNA material and causing neuronal apoptosis, further reacts with nitric oxide to form peroxynitrite, a highly reactive species, subsequently increasing oxidative stress-induced damage and causing neuronal degeneration. Moreover, reactive oxygen species, such as the superoxide radical and peroxynitrite, have also been demonstrated to stimulate the release of HMGB1 (41). Hence, from the above discussion, it is clear that the activation and responses of HMGB1 work in a cyclic manner in which the stimuli leading to the secretion of HMGB1 (such as inflammation and oxidative stress) are themselves upregulated during the downstream signalling of activated HMGB1. The most prominent pathogenic roles of HMGB1 are mediated via activation of receptors for advanced glycation end products, which are further involved in upregulating oxidative stress and inducing the activation of a redox-sensitive nuclear transcription factor, namely, nuclear factor-kappa B (NF-κB) (37). NF-κB, after being activated, translocates to the nucleus and initiates the transcription of several target genes, including primarily cytokines, such as TNF-α and interleukins, further increasing inflammatory responses and retinal damage. Also, the proinflammatory responses of NF-κB can directly induce neuronal dysfunction (44). Hence, due to HMGB1induced downregulation of the secretion and expression of BDNF, the neuroprotective mechanisms become highly compromised. This exposes the retinal neurons to the subsequent damage caused by HMGB1, as described above. Thus, hyperglycemia-induced HMGB1mediated downregulation of BDNF greatly accounts for retinal neurodegeneration and ultimately leads to the progression of diabetic retinopathy.

Conclusions BDNF is an essential neurotrophin required for the generation, proper growth and maintenance of neurons. Because of the various beneficial effects of its physiology, it is quite widely used to prevent neuronal degradation in several disorders. However, the same neuroprotective effects of BDNF are not observed during diabetic retinopathy. This is the result of the downregulation of this factor due to several pathologic alterations induced by hyperglycemia. In the absence of BDNF, the various hyperglycemia-induced damaging stimuli (such as inflammation and oxidative stress-induced apoptosis) cause extensive retinal neuronal degeneration, leading to the progression of retinal neurovascular complications. Restoring the levels of BDNF, probably by inhibiting the hyperglycemia-induced pathologic factors or by administrating exogenous BDNF, might prevent retinal neurodegeneration, hence retarding or inhibiting the progression of this complication.

Acknowledgements No commercial, financial or similar relationships or corporate appointments of the authors or members of their families to products or companies mentioned in or related to the subject matter of this article.

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