- Email: [email protected]
A possible new role for Aβ in vascular and inflammatory dysfunction in Alzheimer's disease
Thrombosis Research 141S2 (2016) S59–S61
Contents lists available at ScienceDirect
Thrombosis Research j o u r n a l h o m e p a g e : w w w. e l s e v i e r . c o m / l o c a t e / t h r o m r e s
A possible new role for Aβ in vascular and inflammatory dysfunction in Alzheimer’s disease Daria Zamolodchikov, Sidney Strickland* Laboratory of Neurobiology and Genetics; The Rockefeller University; New York, NY 10065, USA
K E Y W O R D S
A B S T R A C T
Alzheimer’s disease beta-amyloid fibrin(ogen) factor XII contact activation system
lzheimer’s disease (AD) is often characterized by vascular pathology, a procoagulant state, and chronic A inflammation. The mechanisms behind these abnormalities in AD are not clear. Here, we review evidence for the role of the AD-associated peptide Aβ in promoting inflammation and thrombosis in AD via its interaction with the circulating proteins factor XII and fibrinogen. © 2016 Elsevier Ltd. All rights reserved.
Introduction Alzheimer’s disease (AD) is a fatal cognitive disorder with no effective therapies. The classical pathological hallmarks of AD are extracellular plaques composed of Aβ, intracellular tangles, brain atrophy, and neuronal loss. Many AD patients also present cerebrovascular abnormalities including narrowing of the vessel lumen, microvascular degeneration, and white matter lesions [1-4]. The co-existence of AD and cerebrovascular pathology prompts the question of whether these conditions are simply independent co-morbidities, both more prevalent in the aged population, or whether there is a mechanistic link between AD and vascular pathology. Accumulating epidemiological evidence indicates that AD patients have an altered coagulation profile characterized by a prothrombotic state [5], and several studies have shown that AD patients are at a higher risk for microinfarcts [6,7] and stroke [8]. A hypercoagulable state is also observed in Aβ-based mouse models of AD, which have a greater propensity to form thrombi [9,10]. The idea that this prothrombotic state contributes to AD pathogenesis is supported by studies showing that anticoagulants protect against AD development in humans [11] and that treatment with anticoagulants leads to improvements in AD pathology in humans [12-14] and mouse models [15,16]. It is also well established that AD pathology is associated with inflammation. Inflammation is observed in AD patient brains, polymorphisms in immune-related molecules are linked to AD, and epidemiology shows that nonsteroidal anti-inflammatory drugs reduce AD risk [17]. Whether inflammation is a cause or consequence of AD is still not clear [18].
* Correspondence to: Laboratory of Neurobiology and Genetics, The Rockefeller University, New York, NY 10065, USA.
E-mail address: [email protected] (Sidney Strickland).
Coagulation and inflammation, both implicated in AD, can lead to neuronal dysfunction if chronically or inappropriately activated. Chronic inflammation can promote neuronal damage via microglial, astrocytic, and complement system activation [17], and blood clots in the brain can result in blood vessel occlusion, ischemia, and neuronal degeneration. Given that coagulation and inflammation pathways are activated in AD, agents capable of initiating and/or modulating these pathways are of great interest to AD etiology. There is strong evidence that the AD-related peptide Aβ is a primary driver of both early-onset (familial) and late-onset (sporadic) disease [19,20]. Despite decades of research, the mechanism by which Aβ disrupts neuronal function in vivo is still unclear. Below, we review evidence for Aβ’s role in eliciting thrombosis and inflammation through its interaction with fibrinogen and coagulation factor XII (FXII), key mediators of thrombotic and inflammatory pathways. Ab and fibrino(ogen) in AD Cleavage of fibrinogen by thrombin leads to the formation of fibrin, the main protein component of blood clots. In vitro studies have shown that Aβ binds to both fibrinogen [21] and fibrin [22] and that clots formed in the presence of Aβ are structurally abnormal and more resistant to degradation [9,22]. Binding of Aβ to fibrinogen results in fibrinogen aggregation [21], which may contribute to the formation of structurally abnormal clots in the presence of Aβ. Another consequence of Aβ-fibrin(ogen) binding is an increase of fibrin’s resistance to degradation. This increased stability is due to thinning/tightening of the fibrin network formed in the presence of Aβ and to Aβ-mediated hindrance of plasmin(ogen)’s access to fibrin [22]. While binding of Aβ to fibrinogen during clot formation results in structurally altered fibrin, binding of Aβ to pre-formed fibrin does not alter clot structure [22], but may contribute to the stability of fibrin clots
S60
Daria Zamolodchikov, Sidney Strickland / Thrombosis Research 141S2 (2016) S59–S61
after they have formed by interfering with plasmin(ogen)-fibrin binding. In vivo support for the Aβ-fibrin(ogen) interaction comes from studies showing that Aβ often co-deposits with fibrin(ogen) in the cerebral vessel walls of AD patients and mouse models [9,23,24]. Given that fibrin clots formed in the presence of Aβ are more resistant to degradation [9, 22] and that fibrin negatively affects neuronal health via its inflammatory [25,26] and occlusive [27] properties, Aβ may contribute to AD pathology by triggering thrombosis and persistent fibrin deposition. Indeed, increased fibrin deposition is found in the brains of AD patients and mouse models [28], and clots formed in the brains of AD mice are resistant to degradation and promote inflammation [9,29]. Furthermore, vessel narrowing and occlusion caused by persistent fibrin(ogen) deposits co-localizing with Aβ contribute to impaired perfusion and reduced density of functional cortical microvessels in a mouse model of AD [24]. Fibrin deposition has been shown to negatively impact neuronal health in AD. Its accumulation in the brain parenchyma co-localizes with areas of synaptic dysfunction in AD patients and mouse models [28], and genetic or pharmacologic reduction of fibrinogen in an AD mouse model slows the progression of the disease [9] and ameliorates synaptic dysfunction and neuronal death [28]. The relevance of the Aβ-fibrinogen interaction to fibrin-mediated pathology in AD is supported by studies demonstrating that inhibiting this interaction results in restored fibrinolysis in vitro and amelioration of pro-thrombotic phenotype, microgliosis, and cognitive performance in a mouse model of AD [30]. Fibrin deposited in the vessel lumen may result in occlusion and hypoperfusion, which could contribute to the alterations in cerebral blood flow observed in AD [31]. Intravascular fibrin would also promote inflammation due to its interaction with and activation of immune cells [25], possibly contributing to the increased systemic inflammation observed in AD [32]. Parenchymal fibrin deposition, which is likely secondary to blood-brain barrier (BBB) dysfunction, may trigger local inflammation via microglial activation [33], may disrupt neuronal function by inhibiting neurite outgrowth [34], and could mediate astroglial scar formation [35], thereby contributing to neuronal dysfunction in AD.
from AD patients has decreased levels of FXI zymogen [42] and its inhibitor C1 esterase inhibitor [42-44], implying chronic activation, inhibition, and clearance of FXI. Levels of FXI zymogen in AD patients are inversely correlated with plasma fibrin levels [42], suggesting that FXI activation results in chronic fibrin formation. Together, these studies indicate that Aβ is a FXII activator capable of launching both prothrombotic and inflammatory pathways, providing a novel mechanism by which these processes may be activated in AD.
Ab and contact system activation in AD
Conclusion
The contact activation system, which can launch both thrombotic and inflammatory pathways, is initiated when the plasma protein FXII is exposed to negatively charged surfaces (contact activation) [36]. Contact activated FXII (FXIIa) cleaves plasma prekallikrein to kallikrein, which then liberates the proinflammatory mediator bradykinin from high molecular weight kininogen (HK). Aβ has been shown to promote FXII-dependent plasma kallikrein activity [37, 38] and HK cleavage [37, 39] in vitro. Aβ-mediated activation of the FXII-driven contact system has also been demonstrated in vivo, with intravenous injection of Aβ42 into wild-type but not FXII knockout mice increasing plasma kallikrein activity and HK cleavage [40]. Activation of the FXII-driven contact system is also found in plasma from AD patients and mouse models [40], raising the possibility that Aβ-mediated contact system activation may play a role in disease development and progression. Moreover, HK cleavage in AD patient plasma inversely correlates with cerebrospinal fluid Aβ42 levels [40], an early biomarker of AD [41], suggesting that FXII-mediated kallikrein-kinin system activation may be an early process in disease development. In addition to activating prekallikrein, FXIIa also activates factor XI (FXI) in the intrinsic coagulation cascade, leading to thrombin generation, the conversion of fibrinogen to fibrin, and clot formation. Relevant to this pathway, Aβ can promote thrombin generation via FXII-dependent FXI activation [42], and plasma
The studies reviewed here indicate that fibrin accumulates in the AD brain and contributes to pathology, and that AD is characterized by activation of the FXII-driven contact system. While substantial evidence suggests that Aβ may be a driver of both of these processes, it is also possible that they may be driven by additional mechanisms. For example, Aβ may act on platelets to release polyphosphates or on mast cells to release heparin. Both polyanions can activate the contact system [36]. We propose the following novel mechanism contributing to neuronal dysfunction in AD (Figure 1): activation of FXII by Aβ (directly or indirectly) results in FXIIa-mediated activation of the intrinsic coagulation pathway through FXI and in kallikrein-mediated release of proinflammatory bradykinin from HK. Activation of the intrinsic coagulation pathway by Aβ leads to thrombin production and fibrin formation. Aβ further contributes to a prothrombotic state independently of FXII activation through its effects on fibrin structure stability. Increased thrombosis and elevated levels of persistent fibrin in AD, together with increased activation of the kallikrein-kinin system, may lead to vessel occlusion and inflammation in AD, compromising vascular function and contributing to cognitive decline. There is no effective treatment for AD. A link between FXII activation, persistent fibrin formation, and the pathogenesis of AD provides possible new approaches to treatment. The contact system [36] and the Aβ-fibrin(ogen) interaction [30] are attractive
Figure 1. Pathways through which Ab may influence coagulation and inflammation in the circulation and brain parenchyma. Aβ directly or indirectly activates FXII, leading to FXI activation, thrombin generation and fibrin formation via the intrinsic coagulation pathway [42]. FXII activation by Aβ also leads to plasma prekallikrein (PPK) activation and the release of proinflammatory bradykinin from high molecular weight kininogen (HMWK, HK in the text) [37-40]. Separate from its interaction with FXII, Aβ also binds to fibrin, rendering clots more resistant to degradation [9, 22]. Fibrin can occlude blood flow and is a potent proinflammatory molecule. Thus, Aβ may contribute to inflammatory and thrombotic processes in AD via activation of FXII and interaction with fibrin(ogen).
Daria Zamolodchikov, Sidney Strickland / Thrombosis Research 141S2 (2016) S59–S61
targets for therapy. Humans deficient in FXII, and mice lacking FXII, FXI, or HK, all have normal hemostasis. However, deficiencies in the contact system protect mice from thrombogenic challenges [36,45]. If FXII activation is involved in AD pathogenesis, therapies that block the contact system might slow disease progression by reducing thrombosis and inflammation without affecting hemostasis. Furthermore, if Aβ-dependent fibrin deposition contributes to AD development and progression, targeting the Aβ-fibrinogen interaction could reduce fibrin-mediated pathology without compromising hemostasis. Identification of individuals in which these pathways are affected could also lead to better diagnostic tests and individualized therapies for AD. Conflicts of interest None. Acknowledgements This work was supported by NIH grant NS50537; the Alzheimer’s Drug Discovery, Thome Memorial Medical, Litwin, Rudin Family, Blanchette Hooker Rockefeller, and Mellam Family Foundations; the Cure Alzheimer’s Fund; John A. Herrmann; and the Nicholson Exchange program. References [1] Miyakawa T, Uehara Y, Desaki J, Kimura T, Kuramoto R. Morphological changes of microvessels in the brain with Alzheimer’s disease. The Japanese journal of psychiatry and neurology. 1988;42:819–24. [2] Kalaria RN, Ballard C. Overlap between pathology of Alzheimer disease and vascular dementia. Alzheimer disease and associated disorders. 1999;13 Suppl 3:S115–23. [3] Kalaria RN. The role of cerebral ischemia in Alzheimer’s disease. Neurobiology of aging. 2000;21:321–30. [4] Farkas E, Luiten PG. Cerebral microvascular pathology in aging and Alzheimer’s disease. Progress in neurobiology. 2001;64:575–611. [5] Cortes-Canteli M, Zamolodchikov D, Ahn HJ, Strickland S, Norris EH. Fibrinogen and altered hemostasis in Alzheimer’s disease. J Alzheimers Dis 2012;32:599–608. [6] Brundel M, de Bresser J, van Dillen JJ, Kappelle LJ, Biessels GJ. Cerebral microinfarcts: a systematic review of neuropathological studies. J Cereb Blood Flow Metab. 2012;32:425–36. [7] van Rooden S, Goos JD, van Opstal AM, Versluis MJ, Webb AG, Blauw GJ, et al. Increased number of microinfarcts in Alzheimer disease at 7-T MR imaging. Radiology. 2014;270:205–11. [8] Chi NF, Chien LN, Ku HL, Hu CJ, Chiou HY. Alzheimer disease and risk of stroke: a population-based cohort study. Neurology. 2013;80:705–11. [9] Cortes-Canteli M, Paul J, Norris EH, Bronstein R, Ahn HJ, Zamolodchikov D, et al. Fibrinogen and beta-Amyloid Association Alters Thrombosis and Fibrinolysis: a Possible Contributing Factor to Alzheimer’s Disease. Neuron. 2010;66:695–709. [10] Jarre A, Gowert NS, Donner L, Munzer P, Klier M, Borst O, et al. Pre-activated blood platelets and a pro-thrombotic phenotype in APP23 mice modeling Alzheimer’s disease. Cellular signalling. 2014;26:2040–50. [11] Barber M, Tait RC, Scott J, Rumley A, Lowe GD, Stott DJ. Dementia in subjects with atrial fibrillation: hemostatic function and the role of anticoagulation. J Thromb Haemost 2004;2:1873–8. [12] Ratner J, Rosenberg G, Kral VA, Engelsmann F. Anticoagulant therapy for senile dementia. J Am Geriatr Soc. 1972;20:556–9. [13] Walsh AC, Walsh BH, Melaney C. Senile-presenile dementia: follow-up data on an effective psychotherapy-anticoagulant regimen. J Am Geriatr Soc. 1978;26:467–70. [14] Walsh AC. Anticoagulant therapy for Alzheimer’s disease. J Neuropsychiatry Clin Neurosci. 1996;8:361–2. [15] Bergamaschini L, Rossi E, Storini C, Pizzimenti S, Distaso M, Perego C, et al. Peripheral treatment with enoxaparin, a low molecular weight heparin, reduces plaques and beta-amyloid accumulation in a mouse model of Alzheimer’s disease. J Neurosci. 2004;24:4181–6. [16] Timmer NM, van Dijk L, van der Zee CE, Kiliaan A, de Waal RM, Verbeek MM. Enoxaparin treatment administered at both early and late stages of amyloid beta deposition improves cognition of APPswe/PS1dE9 mice with differential effects on brain Abeta levels. Neurobiol Dis. 2010;40:340–7. [17] Wyss-Coray T, Rogers J. Inflammation in Alzheimer disease-a brief review of the basic science and clinical literature. Cold Spring Harb Perspect Med. 2012;2:a006346.
S61
[18] Wyss-Coray T. Inflammation in Alzheimer disease: driving force, bystander or beneficial response? Nat Med. 2006;12:1005–15. [19] Hardy J, Selkoe DJ. The amyloid hypothesis of Alzheimer’s disease: progress and problems on the road to therapeutics. Science. 2002;297:353–6. [20] Tanzi RE, Bertram L. Twenty years of the Alzheimer’s disease amyloid hypothesis: a genetic perspective. Cell. 2005;120:545–55. [21] Ahn HJ, Zamolodchikov D, Cortes-Canteli M, Norris EH, Glickman JF, Strickland S. Alzheimer’s disease peptide beta-amyloid interacts with fibrinogen and induces its oligomerization. Proc Natl Acad Sci U S A. 2010;107:21812–7. [22] Zamolodchikov D, Strickland S. Abeta delays fibrin clot lysis by altering fibrin structure and attenuating plasminogen binding to fibrin. Blood. 2012;119:3342–51. [23] Hultman K, Strickland S, Norris EH. The APOE varepsilon4/varepsilon4 genotype potentiates vascular fibrin(ogen) deposition in amyloid-laden vessels in the brains of Alzheimer’s disease patients. J Cereb Blood Flow Metab. 2013;33:1251–8. [24] Klohs J, Baltes C, Princz-Kranz F, Ratering D, Nitsch RM, Knuesel I, et al. Contrast-enhanced magnetic resonance microangiography reveals remodeling of the cerebral microvasculature in transgenic ArcAbeta mice. J Neurosci. 2012;32:1705–13. [25] Adams RA, Passino M, Sachs BD, Nuriel T, Akassoglou K. Fibrin mechanisms and functions in nervous system pathology. Mol Interv. 2004;4:163–76. [26] Davalos D, Akassoglou K. Fibrinogen as a key regulator of inflammation in disease. Sem Immunopathol. 2012;34:43–62. [27] Nishimura N, Schaffer CB. Big effects from tiny vessels: imaging the impact of microvascular clots and hemorrhages on the brain. Stroke. 2013;44:S90–2. [28] Cortes-Canteli M, Mattei L, Richards AT, Norris EH, Strickland S. Fibrin deposited in the Alzheimer’s disease brain promotes neuronal degeneration. Neurobiol Aging. 2015;36:608–17. [29] Paul J, Strickland S, Melchor JP. Fibrin deposition accelerates neurovascular damage and neuroinflammation in mouse models of Alzheimer’s disease. J Exp Med. 2007;204:1999–2008. [30] Ahn HJ, Glickman JF, Poon KL, Zamolodchikov D, Jno-Charles OC, Norris EH, et al. A novel Abeta-fibrinogen interaction inhibitor rescues altered thrombosis and cognitive decline in Alzheimer’s disease mice. J Exp Med. 2014;211:1049–62. [31] Pimentel-Coelho PM, Rivest S. The early contribution of cerebrovascular factors to the pathogenesis of Alzheimer’s disease. Eur J Neurosci. 2012;35:1917–37. [32] Takeda S, Sato N, Morishita R. Systemic inflammation, blood-brain barrier vulnerability and cognitive/non-cognitive symptoms in Alzheimer disease: relevance to pathogenesis and therapy. Front Aging Neurosci. 2014;6:171. [33] Adams RA, Bauer J, Flick MJ, Sikorski SL, Nuriel T, Lassmann H, et al. The fibrin-derived gamma377-395 peptide inhibits microglia activation and suppresses relapsing paralysis in central nervous system autoimmune disease. J Exp Med. 2007;204:571–82. [34] Schachtrup C, Lu P, Jones LL, Lee JK, Lu J, Sachs BD, et al. Fibrinogen inhibits neurite outgrowth via beta 3 integrin-mediated phosphorylation of the EGF receptor. Proc Natl Acad Sci U S A. 2007;104:11814–9. [35] Schachtrup C, Ryu JK, Helmrick MJ, Vagena E, Galanakis DK, Degen JL, et al. Fibrinogen triggers astrocyte scar formation by promoting the availability of active TGF-beta after vascular damage. J Neurosci. 2010;30:5843–54. [36] Renné T, Schmaier AH, Nickel KF, Blomback M, Maas C. In vivo roles of factor XII. Blood. 2012;120:4296–303. [37] Shibayama Y, Joseph K, Nakazawa Y, Ghebreihiwet B, Peerschke EI, Kaplan AP. Zinc-dependent activation of the plasma kinin-forming cascade by aggregated beta amyloid protein. Clin Immunol. 1999;90:89–99. [38] Maas C, Govers-Riemslag JW, Bouma B, Schiks B, Hazenberg BP, Lokhorst HM, et al. Misfolded proteins activate factor XII in humans, leading to kallikrein formation without initiating coagulation. J Clin Invest. 2008;118:3208–18. [39] Bergamaschini L, Donarini C, Gobbo G, Parnetti L, Gallai V. Activation of complement and contact system in Alzheimer’s disease. Mech Ageing Dev. 2001;122:1971–83. [40] Zamolodchikov D, Chen ZL, Conti BA, Renne T, Strickland S. Activation of the factor XII-driven contact system in Alzheimer’s disease patient and mouse model plasma. Proc Natl Acad Sci U S A. 2015;112:4068–73. [41] Musiek ES, Holtzman DM. Origins of Alzheimer’s disease: reconciling cerebrospinal fluid biomarker and neuropathology data regarding the temporal sequence of amyloid-beta and tau involvement. Curr Opin Neurol. 2012;25:715–20. [42] Zamolodchikov D, Renne T, Strickland S. The Alzheimer’s disease peptide Abeta promotes thrombin generation through activation of coagulation factor XII. J Thromb Haemost 2015 [Epub ahead of print]. [43] Cutler P, Akuffo EL, Bodnar WM, Briggs DM, Davis JB, Debouck CM, et al. Proteomic identification and early validation of complement 1 inhibitor and pigment epithelium-derived factor: Two novel biomarkers of Alzheimer’s disease in human plasma. Proteomics Clinical applications. 2008;2:467–77. [44] Muenchhoff J, Poljak A, Song F, Raftery M, Brodaty H, Duncan M, et al. Plasma protein profiling of mild cognitive impairment and Alzheimer’s disease across two independent cohorts. J Alzheimers Dis. 2015;43:1355–73. [45] Merkulov S, Zhang WM, Komar AA, Schmaier AH, Barnes E, Zhou Y, et al. Deletion of murine kininogen gene 1 (mKng1) causes loss of plasma kininogen and delays thrombosis. Blood. 2008;111:1274–81.
Contents lists available at ScienceDirect
Thrombosis Research j o u r n a l h o m e p a g e : w w w. e l s e v i e r . c o m / l o c a t e / t h r o m r e s
A possible new role for Aβ in vascular and inflammatory dysfunction in Alzheimer’s disease Daria Zamolodchikov, Sidney Strickland* Laboratory of Neurobiology and Genetics; The Rockefeller University; New York, NY 10065, USA
K E Y W O R D S
A B S T R A C T
Alzheimer’s disease beta-amyloid fibrin(ogen) factor XII contact activation system
lzheimer’s disease (AD) is often characterized by vascular pathology, a procoagulant state, and chronic A inflammation. The mechanisms behind these abnormalities in AD are not clear. Here, we review evidence for the role of the AD-associated peptide Aβ in promoting inflammation and thrombosis in AD via its interaction with the circulating proteins factor XII and fibrinogen. © 2016 Elsevier Ltd. All rights reserved.
Introduction Alzheimer’s disease (AD) is a fatal cognitive disorder with no effective therapies. The classical pathological hallmarks of AD are extracellular plaques composed of Aβ, intracellular tangles, brain atrophy, and neuronal loss. Many AD patients also present cerebrovascular abnormalities including narrowing of the vessel lumen, microvascular degeneration, and white matter lesions [1-4]. The co-existence of AD and cerebrovascular pathology prompts the question of whether these conditions are simply independent co-morbidities, both more prevalent in the aged population, or whether there is a mechanistic link between AD and vascular pathology. Accumulating epidemiological evidence indicates that AD patients have an altered coagulation profile characterized by a prothrombotic state [5], and several studies have shown that AD patients are at a higher risk for microinfarcts [6,7] and stroke [8]. A hypercoagulable state is also observed in Aβ-based mouse models of AD, which have a greater propensity to form thrombi [9,10]. The idea that this prothrombotic state contributes to AD pathogenesis is supported by studies showing that anticoagulants protect against AD development in humans [11] and that treatment with anticoagulants leads to improvements in AD pathology in humans [12-14] and mouse models [15,16]. It is also well established that AD pathology is associated with inflammation. Inflammation is observed in AD patient brains, polymorphisms in immune-related molecules are linked to AD, and epidemiology shows that nonsteroidal anti-inflammatory drugs reduce AD risk [17]. Whether inflammation is a cause or consequence of AD is still not clear [18].
* Correspondence to: Laboratory of Neurobiology and Genetics, The Rockefeller University, New York, NY 10065, USA.
E-mail address: [email protected] (Sidney Strickland).
Coagulation and inflammation, both implicated in AD, can lead to neuronal dysfunction if chronically or inappropriately activated. Chronic inflammation can promote neuronal damage via microglial, astrocytic, and complement system activation [17], and blood clots in the brain can result in blood vessel occlusion, ischemia, and neuronal degeneration. Given that coagulation and inflammation pathways are activated in AD, agents capable of initiating and/or modulating these pathways are of great interest to AD etiology. There is strong evidence that the AD-related peptide Aβ is a primary driver of both early-onset (familial) and late-onset (sporadic) disease [19,20]. Despite decades of research, the mechanism by which Aβ disrupts neuronal function in vivo is still unclear. Below, we review evidence for Aβ’s role in eliciting thrombosis and inflammation through its interaction with fibrinogen and coagulation factor XII (FXII), key mediators of thrombotic and inflammatory pathways. Ab and fibrino(ogen) in AD Cleavage of fibrinogen by thrombin leads to the formation of fibrin, the main protein component of blood clots. In vitro studies have shown that Aβ binds to both fibrinogen [21] and fibrin [22] and that clots formed in the presence of Aβ are structurally abnormal and more resistant to degradation [9,22]. Binding of Aβ to fibrinogen results in fibrinogen aggregation [21], which may contribute to the formation of structurally abnormal clots in the presence of Aβ. Another consequence of Aβ-fibrin(ogen) binding is an increase of fibrin’s resistance to degradation. This increased stability is due to thinning/tightening of the fibrin network formed in the presence of Aβ and to Aβ-mediated hindrance of plasmin(ogen)’s access to fibrin [22]. While binding of Aβ to fibrinogen during clot formation results in structurally altered fibrin, binding of Aβ to pre-formed fibrin does not alter clot structure [22], but may contribute to the stability of fibrin clots
S60
Daria Zamolodchikov, Sidney Strickland / Thrombosis Research 141S2 (2016) S59–S61
after they have formed by interfering with plasmin(ogen)-fibrin binding. In vivo support for the Aβ-fibrin(ogen) interaction comes from studies showing that Aβ often co-deposits with fibrin(ogen) in the cerebral vessel walls of AD patients and mouse models [9,23,24]. Given that fibrin clots formed in the presence of Aβ are more resistant to degradation [9, 22] and that fibrin negatively affects neuronal health via its inflammatory [25,26] and occlusive [27] properties, Aβ may contribute to AD pathology by triggering thrombosis and persistent fibrin deposition. Indeed, increased fibrin deposition is found in the brains of AD patients and mouse models [28], and clots formed in the brains of AD mice are resistant to degradation and promote inflammation [9,29]. Furthermore, vessel narrowing and occlusion caused by persistent fibrin(ogen) deposits co-localizing with Aβ contribute to impaired perfusion and reduced density of functional cortical microvessels in a mouse model of AD [24]. Fibrin deposition has been shown to negatively impact neuronal health in AD. Its accumulation in the brain parenchyma co-localizes with areas of synaptic dysfunction in AD patients and mouse models [28], and genetic or pharmacologic reduction of fibrinogen in an AD mouse model slows the progression of the disease [9] and ameliorates synaptic dysfunction and neuronal death [28]. The relevance of the Aβ-fibrinogen interaction to fibrin-mediated pathology in AD is supported by studies demonstrating that inhibiting this interaction results in restored fibrinolysis in vitro and amelioration of pro-thrombotic phenotype, microgliosis, and cognitive performance in a mouse model of AD [30]. Fibrin deposited in the vessel lumen may result in occlusion and hypoperfusion, which could contribute to the alterations in cerebral blood flow observed in AD [31]. Intravascular fibrin would also promote inflammation due to its interaction with and activation of immune cells [25], possibly contributing to the increased systemic inflammation observed in AD [32]. Parenchymal fibrin deposition, which is likely secondary to blood-brain barrier (BBB) dysfunction, may trigger local inflammation via microglial activation [33], may disrupt neuronal function by inhibiting neurite outgrowth [34], and could mediate astroglial scar formation [35], thereby contributing to neuronal dysfunction in AD.
from AD patients has decreased levels of FXI zymogen [42] and its inhibitor C1 esterase inhibitor [42-44], implying chronic activation, inhibition, and clearance of FXI. Levels of FXI zymogen in AD patients are inversely correlated with plasma fibrin levels [42], suggesting that FXI activation results in chronic fibrin formation. Together, these studies indicate that Aβ is a FXII activator capable of launching both prothrombotic and inflammatory pathways, providing a novel mechanism by which these processes may be activated in AD.
Ab and contact system activation in AD
Conclusion
The contact activation system, which can launch both thrombotic and inflammatory pathways, is initiated when the plasma protein FXII is exposed to negatively charged surfaces (contact activation) [36]. Contact activated FXII (FXIIa) cleaves plasma prekallikrein to kallikrein, which then liberates the proinflammatory mediator bradykinin from high molecular weight kininogen (HK). Aβ has been shown to promote FXII-dependent plasma kallikrein activity [37, 38] and HK cleavage [37, 39] in vitro. Aβ-mediated activation of the FXII-driven contact system has also been demonstrated in vivo, with intravenous injection of Aβ42 into wild-type but not FXII knockout mice increasing plasma kallikrein activity and HK cleavage [40]. Activation of the FXII-driven contact system is also found in plasma from AD patients and mouse models [40], raising the possibility that Aβ-mediated contact system activation may play a role in disease development and progression. Moreover, HK cleavage in AD patient plasma inversely correlates with cerebrospinal fluid Aβ42 levels [40], an early biomarker of AD [41], suggesting that FXII-mediated kallikrein-kinin system activation may be an early process in disease development. In addition to activating prekallikrein, FXIIa also activates factor XI (FXI) in the intrinsic coagulation cascade, leading to thrombin generation, the conversion of fibrinogen to fibrin, and clot formation. Relevant to this pathway, Aβ can promote thrombin generation via FXII-dependent FXI activation [42], and plasma
The studies reviewed here indicate that fibrin accumulates in the AD brain and contributes to pathology, and that AD is characterized by activation of the FXII-driven contact system. While substantial evidence suggests that Aβ may be a driver of both of these processes, it is also possible that they may be driven by additional mechanisms. For example, Aβ may act on platelets to release polyphosphates or on mast cells to release heparin. Both polyanions can activate the contact system [36]. We propose the following novel mechanism contributing to neuronal dysfunction in AD (Figure 1): activation of FXII by Aβ (directly or indirectly) results in FXIIa-mediated activation of the intrinsic coagulation pathway through FXI and in kallikrein-mediated release of proinflammatory bradykinin from HK. Activation of the intrinsic coagulation pathway by Aβ leads to thrombin production and fibrin formation. Aβ further contributes to a prothrombotic state independently of FXII activation through its effects on fibrin structure stability. Increased thrombosis and elevated levels of persistent fibrin in AD, together with increased activation of the kallikrein-kinin system, may lead to vessel occlusion and inflammation in AD, compromising vascular function and contributing to cognitive decline. There is no effective treatment for AD. A link between FXII activation, persistent fibrin formation, and the pathogenesis of AD provides possible new approaches to treatment. The contact system [36] and the Aβ-fibrin(ogen) interaction [30] are attractive
Figure 1. Pathways through which Ab may influence coagulation and inflammation in the circulation and brain parenchyma. Aβ directly or indirectly activates FXII, leading to FXI activation, thrombin generation and fibrin formation via the intrinsic coagulation pathway [42]. FXII activation by Aβ also leads to plasma prekallikrein (PPK) activation and the release of proinflammatory bradykinin from high molecular weight kininogen (HMWK, HK in the text) [37-40]. Separate from its interaction with FXII, Aβ also binds to fibrin, rendering clots more resistant to degradation [9, 22]. Fibrin can occlude blood flow and is a potent proinflammatory molecule. Thus, Aβ may contribute to inflammatory and thrombotic processes in AD via activation of FXII and interaction with fibrin(ogen).
Daria Zamolodchikov, Sidney Strickland / Thrombosis Research 141S2 (2016) S59–S61
targets for therapy. Humans deficient in FXII, and mice lacking FXII, FXI, or HK, all have normal hemostasis. However, deficiencies in the contact system protect mice from thrombogenic challenges [36,45]. If FXII activation is involved in AD pathogenesis, therapies that block the contact system might slow disease progression by reducing thrombosis and inflammation without affecting hemostasis. Furthermore, if Aβ-dependent fibrin deposition contributes to AD development and progression, targeting the Aβ-fibrinogen interaction could reduce fibrin-mediated pathology without compromising hemostasis. Identification of individuals in which these pathways are affected could also lead to better diagnostic tests and individualized therapies for AD. Conflicts of interest None. Acknowledgements This work was supported by NIH grant NS50537; the Alzheimer’s Drug Discovery, Thome Memorial Medical, Litwin, Rudin Family, Blanchette Hooker Rockefeller, and Mellam Family Foundations; the Cure Alzheimer’s Fund; John A. Herrmann; and the Nicholson Exchange program. References [1] Miyakawa T, Uehara Y, Desaki J, Kimura T, Kuramoto R. Morphological changes of microvessels in the brain with Alzheimer’s disease. The Japanese journal of psychiatry and neurology. 1988;42:819–24. [2] Kalaria RN, Ballard C. Overlap between pathology of Alzheimer disease and vascular dementia. Alzheimer disease and associated disorders. 1999;13 Suppl 3:S115–23. [3] Kalaria RN. The role of cerebral ischemia in Alzheimer’s disease. Neurobiology of aging. 2000;21:321–30. [4] Farkas E, Luiten PG. Cerebral microvascular pathology in aging and Alzheimer’s disease. Progress in neurobiology. 2001;64:575–611. [5] Cortes-Canteli M, Zamolodchikov D, Ahn HJ, Strickland S, Norris EH. Fibrinogen and altered hemostasis in Alzheimer’s disease. J Alzheimers Dis 2012;32:599–608. [6] Brundel M, de Bresser J, van Dillen JJ, Kappelle LJ, Biessels GJ. Cerebral microinfarcts: a systematic review of neuropathological studies. J Cereb Blood Flow Metab. 2012;32:425–36. [7] van Rooden S, Goos JD, van Opstal AM, Versluis MJ, Webb AG, Blauw GJ, et al. Increased number of microinfarcts in Alzheimer disease at 7-T MR imaging. Radiology. 2014;270:205–11. [8] Chi NF, Chien LN, Ku HL, Hu CJ, Chiou HY. Alzheimer disease and risk of stroke: a population-based cohort study. Neurology. 2013;80:705–11. [9] Cortes-Canteli M, Paul J, Norris EH, Bronstein R, Ahn HJ, Zamolodchikov D, et al. Fibrinogen and beta-Amyloid Association Alters Thrombosis and Fibrinolysis: a Possible Contributing Factor to Alzheimer’s Disease. Neuron. 2010;66:695–709. [10] Jarre A, Gowert NS, Donner L, Munzer P, Klier M, Borst O, et al. Pre-activated blood platelets and a pro-thrombotic phenotype in APP23 mice modeling Alzheimer’s disease. Cellular signalling. 2014;26:2040–50. [11] Barber M, Tait RC, Scott J, Rumley A, Lowe GD, Stott DJ. Dementia in subjects with atrial fibrillation: hemostatic function and the role of anticoagulation. J Thromb Haemost 2004;2:1873–8. [12] Ratner J, Rosenberg G, Kral VA, Engelsmann F. Anticoagulant therapy for senile dementia. J Am Geriatr Soc. 1972;20:556–9. [13] Walsh AC, Walsh BH, Melaney C. Senile-presenile dementia: follow-up data on an effective psychotherapy-anticoagulant regimen. J Am Geriatr Soc. 1978;26:467–70. [14] Walsh AC. Anticoagulant therapy for Alzheimer’s disease. J Neuropsychiatry Clin Neurosci. 1996;8:361–2. [15] Bergamaschini L, Rossi E, Storini C, Pizzimenti S, Distaso M, Perego C, et al. Peripheral treatment with enoxaparin, a low molecular weight heparin, reduces plaques and beta-amyloid accumulation in a mouse model of Alzheimer’s disease. J Neurosci. 2004;24:4181–6. [16] Timmer NM, van Dijk L, van der Zee CE, Kiliaan A, de Waal RM, Verbeek MM. Enoxaparin treatment administered at both early and late stages of amyloid beta deposition improves cognition of APPswe/PS1dE9 mice with differential effects on brain Abeta levels. Neurobiol Dis. 2010;40:340–7. [17] Wyss-Coray T, Rogers J. Inflammation in Alzheimer disease-a brief review of the basic science and clinical literature. Cold Spring Harb Perspect Med. 2012;2:a006346.
S61
[18] Wyss-Coray T. Inflammation in Alzheimer disease: driving force, bystander or beneficial response? Nat Med. 2006;12:1005–15. [19] Hardy J, Selkoe DJ. The amyloid hypothesis of Alzheimer’s disease: progress and problems on the road to therapeutics. Science. 2002;297:353–6. [20] Tanzi RE, Bertram L. Twenty years of the Alzheimer’s disease amyloid hypothesis: a genetic perspective. Cell. 2005;120:545–55. [21] Ahn HJ, Zamolodchikov D, Cortes-Canteli M, Norris EH, Glickman JF, Strickland S. Alzheimer’s disease peptide beta-amyloid interacts with fibrinogen and induces its oligomerization. Proc Natl Acad Sci U S A. 2010;107:21812–7. [22] Zamolodchikov D, Strickland S. Abeta delays fibrin clot lysis by altering fibrin structure and attenuating plasminogen binding to fibrin. Blood. 2012;119:3342–51. [23] Hultman K, Strickland S, Norris EH. The APOE varepsilon4/varepsilon4 genotype potentiates vascular fibrin(ogen) deposition in amyloid-laden vessels in the brains of Alzheimer’s disease patients. J Cereb Blood Flow Metab. 2013;33:1251–8. [24] Klohs J, Baltes C, Princz-Kranz F, Ratering D, Nitsch RM, Knuesel I, et al. Contrast-enhanced magnetic resonance microangiography reveals remodeling of the cerebral microvasculature in transgenic ArcAbeta mice. J Neurosci. 2012;32:1705–13. [25] Adams RA, Passino M, Sachs BD, Nuriel T, Akassoglou K. Fibrin mechanisms and functions in nervous system pathology. Mol Interv. 2004;4:163–76. [26] Davalos D, Akassoglou K. Fibrinogen as a key regulator of inflammation in disease. Sem Immunopathol. 2012;34:43–62. [27] Nishimura N, Schaffer CB. Big effects from tiny vessels: imaging the impact of microvascular clots and hemorrhages on the brain. Stroke. 2013;44:S90–2. [28] Cortes-Canteli M, Mattei L, Richards AT, Norris EH, Strickland S. Fibrin deposited in the Alzheimer’s disease brain promotes neuronal degeneration. Neurobiol Aging. 2015;36:608–17. [29] Paul J, Strickland S, Melchor JP. Fibrin deposition accelerates neurovascular damage and neuroinflammation in mouse models of Alzheimer’s disease. J Exp Med. 2007;204:1999–2008. [30] Ahn HJ, Glickman JF, Poon KL, Zamolodchikov D, Jno-Charles OC, Norris EH, et al. A novel Abeta-fibrinogen interaction inhibitor rescues altered thrombosis and cognitive decline in Alzheimer’s disease mice. J Exp Med. 2014;211:1049–62. [31] Pimentel-Coelho PM, Rivest S. The early contribution of cerebrovascular factors to the pathogenesis of Alzheimer’s disease. Eur J Neurosci. 2012;35:1917–37. [32] Takeda S, Sato N, Morishita R. Systemic inflammation, blood-brain barrier vulnerability and cognitive/non-cognitive symptoms in Alzheimer disease: relevance to pathogenesis and therapy. Front Aging Neurosci. 2014;6:171. [33] Adams RA, Bauer J, Flick MJ, Sikorski SL, Nuriel T, Lassmann H, et al. The fibrin-derived gamma377-395 peptide inhibits microglia activation and suppresses relapsing paralysis in central nervous system autoimmune disease. J Exp Med. 2007;204:571–82. [34] Schachtrup C, Lu P, Jones LL, Lee JK, Lu J, Sachs BD, et al. Fibrinogen inhibits neurite outgrowth via beta 3 integrin-mediated phosphorylation of the EGF receptor. Proc Natl Acad Sci U S A. 2007;104:11814–9. [35] Schachtrup C, Ryu JK, Helmrick MJ, Vagena E, Galanakis DK, Degen JL, et al. Fibrinogen triggers astrocyte scar formation by promoting the availability of active TGF-beta after vascular damage. J Neurosci. 2010;30:5843–54. [36] Renné T, Schmaier AH, Nickel KF, Blomback M, Maas C. In vivo roles of factor XII. Blood. 2012;120:4296–303. [37] Shibayama Y, Joseph K, Nakazawa Y, Ghebreihiwet B, Peerschke EI, Kaplan AP. Zinc-dependent activation of the plasma kinin-forming cascade by aggregated beta amyloid protein. Clin Immunol. 1999;90:89–99. [38] Maas C, Govers-Riemslag JW, Bouma B, Schiks B, Hazenberg BP, Lokhorst HM, et al. Misfolded proteins activate factor XII in humans, leading to kallikrein formation without initiating coagulation. J Clin Invest. 2008;118:3208–18. [39] Bergamaschini L, Donarini C, Gobbo G, Parnetti L, Gallai V. Activation of complement and contact system in Alzheimer’s disease. Mech Ageing Dev. 2001;122:1971–83. [40] Zamolodchikov D, Chen ZL, Conti BA, Renne T, Strickland S. Activation of the factor XII-driven contact system in Alzheimer’s disease patient and mouse model plasma. Proc Natl Acad Sci U S A. 2015;112:4068–73. [41] Musiek ES, Holtzman DM. Origins of Alzheimer’s disease: reconciling cerebrospinal fluid biomarker and neuropathology data regarding the temporal sequence of amyloid-beta and tau involvement. Curr Opin Neurol. 2012;25:715–20. [42] Zamolodchikov D, Renne T, Strickland S. The Alzheimer’s disease peptide Abeta promotes thrombin generation through activation of coagulation factor XII. J Thromb Haemost 2015 [Epub ahead of print]. [43] Cutler P, Akuffo EL, Bodnar WM, Briggs DM, Davis JB, Debouck CM, et al. Proteomic identification and early validation of complement 1 inhibitor and pigment epithelium-derived factor: Two novel biomarkers of Alzheimer’s disease in human plasma. Proteomics Clinical applications. 2008;2:467–77. [44] Muenchhoff J, Poljak A, Song F, Raftery M, Brodaty H, Duncan M, et al. Plasma protein profiling of mild cognitive impairment and Alzheimer’s disease across two independent cohorts. J Alzheimers Dis. 2015;43:1355–73. [45] Merkulov S, Zhang WM, Komar AA, Schmaier AH, Barnes E, Zhou Y, et al. Deletion of murine kininogen gene 1 (mKng1) causes loss of plasma kininogen and delays thrombosis. Blood. 2008;111:1274–81.