Blockade of Annexin A2 Prevents Early Microvasculopathy in Murine Models of Diabetic Retinopathy.

Purpose: We examined the role of annexin A2 (A2) in the development of diabetic retinal vasculopathy by testing the effect of Anxa2 gene deletion as well as administration of anti-A2 antibodies on pericyte dropout and retinal neovascularization in diabetic Akita mice, and in mice subjected to oxygen-induced retinopathy. Methods: We analyzed diabetic Ins2AKITA mice with or without global deletion of Anxa2, as well as Ins2AKITA mice that received intravitreal anti-A2 IgG or control antibody at 2, 4, and 6 months, for retinal pericyte dropout at 7 months of age. In addition, we assessed the effect of intravitreal anti-A2 on oxygen-induced retinopathy (OIR) in neonatal mice by quantifying retinal neovascular and vaso-obliterative area, and by enumeration of neovascular tufts. Results: Both deletion of the Anxa2 gene and immunologic blockade of A2 prevented pericyte depletion in retinas of diabetic Ins2AKITA mice. Blockade of A2 also reduced vaso-obliteration and neovascularization in the OIR model of vascular proliferation. This effect was amplified when a combination of antivascular endothelial growth factor (VEGF) and anti-A2 antibodies was used. Conclusions: Therapeutic approaches that target A2, alone or in combination with anti-VEGF therapy, are effective in mice, and may also curtail the progression of retinal vascular disease in humans with diabetes.

Annexin A2 Loss After Cardiopulmonary Bypass and Development of Acute Postoperative Respiratory Dysfunction in Children.

The primary objective of this study was to determine whether expression of the multifunctional and adherens junction-regulating protein, annexin A2 (A2), is altered following cardiopulmonary bypass (CPB). A secondary objective was to determine whether depletion of A2 is associated with post-CPB organ dysfunction in children. DESIGN: In a prospective, observational study conducted over a 1-year period in children undergoing cardiac surgery requiring CPB, we analyzed A2 expression in peripheral blood mononuclear cells at different time points. We then assessed the relationship of A2 expression with organ function at each time point in the early postoperative period. SETTING: Twenty-three-bed mixed PICU in a tertiary academic center. PARTICIPANTS: Patients 1 month to 18 years old undergoing cardiac surgery requiring CPB. MEAN OUTCOME MEASUREMENTS AND RESULTS: We analyzed A2 expression in 22 enrolled subjects (n = 9, 1-23 mo old; n = 13, 2-18 yr old) and found a proteolysis-mediated decline in intact A2 immediately after bypass (p = 0.0009), reaching a median of 4% of baseline at 6 hours after bypass (p < 0.0001), and recovery by postoperative day 1. The degree of A2 depletion immediately after bypass in 1-23-month-olds correlated strongly with the extent of organ dysfunction, as measured by PICU admission Vasoactive-Ventilation-Renal (p = 0.004) and PEdiatric Logistic Organ Dysfunction-2 (p = 0.039) scores on postoperative day 1. A2 depletion immediately after bypass also correlated with more protracted requirement for both respiratory support (p = 0.007) and invasive ventilation (p = 0.013) in the 1-23-month-olds. CONCLUSIONS AND RELEVANCE: The degree of depletion of A2 following CPB correlates with more severe organ dysfunction, especially acute respiratory compromise in children under 2 years. These findings suggest that loss of A2 may contribute to pulmonary microvascular leak in young children following CPB.

Lipofuscin causes atypical necroptosis through lysosomal membrane permeabilization.

Lipofuscin granules enclose mixtures of cross-linked proteins and lipids in proportions that depend on the tissue analyzed. Retinal lipofuscin is unique in that it contains mostly lipids with very little proteins. However, retinal lipofuscin also presents biological and physicochemical characteristics indistinguishable from conventional granules, including indigestibility, tendency to cause lysosome swelling that results in rupture or defective functions, and ability to trigger NLRP3 inflammation, a symptom of low-level disruption of lysosomes. In addition, like conventional lipofuscins, it appears as an autofluorescent pigment, considered toxic waste, and a biomarker of aging. Ocular lipofuscin accumulates in the retinal pigment epithelium (RPE), whereby it interferes with the support of the neuroretina. RPE cell death is the primary cause of blindness in the most prevalent incurable genetic and age-related human disorders, Stargardt disease and age-related macular degeneration (AMD), respectively. Although retinal lipofuscin is directly linked to the cell death of the RPE in Stargardt, the extent to which it contributes to AMD is a matter of debate. Nonetheless, the number of AMD clinical trials that target lipofuscin formation speaks for the potential relevance for AMD as well. Here, we show that retinal lipofuscin triggers an atypical necroptotic cascade, amenable to pharmacological intervention. This pathway is distinct from canonic necroptosis and is instead dependent on the destabilization of lysosomes. We also provide evidence that necroptosis is activated in aged human retinas with AMD. Overall, this cytotoxicity mechanism may offer therapeutic targets and markers for genetic and age-related diseases associated with lipofuscin buildups.

Annexin A2 in Fibrinolysis, Inflammation and Fibrosis.

As a cell surface tissue plasminogen activator (tPA)-plasminogen receptor, the annexin A2 (A2) complex facilitates plasmin generation on the endothelial cell surface, and is an established regulator of hemostasis. Whereas A2 is overexpressed in hemorrhagic disease such as acute promyelocytic leukemia, its underexpression or impairment may result in thrombosis, as in antiphospholipid syndrome, venous thromboembolism, or atherosclerosis. Within immune response cells, A2 orchestrates membrane repair, vesicle fusion, and cytoskeletal organization, thus playing a critical role in inflammatory response and tissue injury. Dysregulation of A2 is evident in multiple human disorders, and may contribute to the pathogenesis of various inflammatory disorders. The fibrinolytic system, moreover, is central to wound healing through its ability to remodel the provisional matrix and promote angiogenesis. A2 dysfunction may also promote tissue fibrogenesis and end-organ fibrosis.

Reduced expression of annexin A2 is associated with impaired cell surface fibrinolysis and venous thromboembolism.

Reduced plasma fibrinolysis has been identified as a potential risk factor for venous thromboembolism (VTE), but the role of cell surface fibrinolysis in VTE is unknown. The annexin A2/S100A10 complex serves as a coreceptor for plasminogen and tissue plasminogen activator (tPA), augmenting plasmin generation by 60-fold on the endothelial cell surface. Several studies in both mice and humans support the concept that A2 regulates fibrin homeostasis and intravascular thrombosis in vivo. Here, we examined A2 protein expression and function in 115 adult subjects with VTE and 87 healthy controls. Using peripheral blood mononuclear cells as a surrogate for endothelial cells, we found a 41% mean decrease in cell surface tPA-dependent fibrinolytic activity in subjects who had a positive personal and family history of VTE but tested negative for known inherited thrombophilias (ITs). A2 protein was reduced on average by 70% and messenger RNA levels by 30%, but neither decrease correlated with anticoagulant therapy. Neither cell A2 protein nor cell surface plasmin generation correlated with plasma-based clot lysis times, suggesting that the plasma and cell surface fibrinolytic systems operate independently of one another. These data suggest that reduced expression of annexin A2 protein is associated with cell surface hypofibrinolysis and may represent a novel risk factor for IT.

Annexin A2 in Inflammation and Host Defense.

Annexin A2 (AnxA2) is a multifunctional calcium2+ (Ca2+) and phospholipid-binding protein that is expressed in a wide spectrum of cells, including those participating in the inflammatory response. In acute inflammation, the interaction of AnxA2 with actin and adherens junction VE-cadherins underlies its role in regulating vascular integrity. In addition, its contribution to endosomal membrane repair impacts several aspects of inflammatory regulation, including lysosome repair, which regulates inflammasome activation, and autophagosome biogenesis, which is essential for macroautophagy. On the other hand, AnxA2 may be co-opted to promote adhesion, entry, and propagation of bacteria or viruses into host cells. In the later stages of acute inflammation, AnxA2 contributes to the initiation of angiogenesis, which promotes tissue repair, but, when dysregulated, may also accompany chronic inflammation. AnxA2 is overexpressed in malignancies, such as breast cancer and glioblastoma, and likely contributes to cancer progression in the context of an inflammatory microenvironment. We conclude that annexin AnxA2 normally fulfills a spectrum of anti-inflammatory functions in the setting of both acute and chronic inflammation but may contribute to disease states in settings of disordered homeostasis.

Annexin A2 is a Robo4 ligand that modulates ARF6 activation-associated cerebral trans-endothelial permeability.

Blood-brain barrier (BBB) disruption in neurological disorders remains an intractable problem with limited therapeutic options. Here, we investigate whether the endothelial cell membrane protein annexin A2 (ANXA2) may play a role in reducing trans-endothelial permeability and maintaining cerebrovascular integrity after injury. Compared with wild-type mice, the expression of cerebral endothelial junctional proteins was reduced in E15.5 and adult ANXA2 knockout mice, along with increased leakage of small molecule tracers. In human brain endothelial cells that were damaged by hypoxia plus IL-1ß, treatment with recombinant ANXA2 (rA2) rescued the expression of junctional proteins and decreased trans-endothelial permeability. These protective effects were mediated in part by interactions with F-actin and VE-cadherin, and the ability of rA2 to modulate signaling via the roundabout guidance receptor 4 (Robo4)-paxillin-ADP-ribosylation factor 6 (ARF6) pathway. Taken together, these observations suggest that ANXA2 may be associated with the maintenance of endothelial tightness after cerebrovascular injury. ANXA2-mediated pathways should be further explored as potential therapeutic targets for protecting the BBB in neurological disorders.

Annexin A2 supports pulmonary microvascular integrity by linking vascular endothelial cadherin and protein tyrosine phosphatases.

Relative or absolute hypoxia activates signaling pathways that alter gene expression and stabilize the pulmonary microvasculature. Alveolar hypoxia occurs in disorders ranging from altitude sickness to airway obstruction, apnea, and atelectasis. Here, we report that the phospholipid-binding protein, annexin A2 (ANXA2) functions to maintain vascular integrity in the face of alveolar hypoxia. We demonstrate that microvascular endothelial cells (ECs) from Anxa2-/- mice display reduced barrier function and excessive Src-related tyrosine phosphorylation of the adherens junction protein vascular endothelial cadherin (VEC). Moreover, unlike Anxa2+/+ controls, Anxa2-/- mice develop pulmonary edema and neutrophil infiltration in the lung parenchyma in response to subacute alveolar hypoxia. Mice deficient in the ANXA2-binding partner, S100A10, failed to demonstrate hypoxia-induced pulmonary edema under the same conditions. Further analyses reveal that ANXA2 forms a complex with VEC and its phosphatases, EC-specific protein tyrosine phosphatase (VE-PTP) and Src homology phosphatase 2 (SHP2), both of which are implicated in vascular integrity. In the absence of ANXA2, VEC is hyperphosphorylated at tyrosine 731 in response to vascular endothelial growth factor, which likely contributes to hypoxia-induced extravasation of fluid and leukocytes. We conclude that ANXA2 contributes to pulmonary microvascular integrity by enabling VEC-related phosphatase activity, thereby preventing vascular leak during alveolar hypoxia.

Intracellular targeting of annexin A2 inhibits tumor cell adhesion, migration, and in vivo grafting.

Cytoskeletal-associated proteins play an active role in coordinating the adhesion and migration machinery in cancer progression. To identify functional protein networks and potential inhibitors, we screened an internalizing phage (iPhage) display library in tumor cells, and selected LGRFYAASG as a cytosol-targeting peptide. By affinity purification and mass spectrometry, intracellular annexin A2 was identified as the corresponding binding protein. Consistently, annexin A2 and a cell-internalizing, penetratin-fused version of the selected peptide (LGRFYAASG-pen) co-localized and specifically accumulated in the cytoplasm at the cell edges and cell-cell contacts. Functionally, tumor cells incubated with LGRFYAASG-pen showed disruption of filamentous actin, focal adhesions and caveolae-mediated membrane trafficking, resulting in impaired cell adhesion and migration in vitro. These effects were paralleled by a decrease in the phosphorylation of both focal adhesion kinase (Fak) and protein kinase B (Akt). Likewise, tumor cells pretreated with LGRFYAASG-pen exhibited an impaired capacity to colonize the lungs in vivo in several mouse models. Together, our findings demonstrate an unrecognized functional link between intracellular annexin A2 and tumor cell adhesion, migration and in vivo grafting. Moreover, this work uncovers a new peptide motif that binds to and inhibits intracellular annexin A2 as a candidate therapeutic lead for potential translation into clinical applications.

The annexin A2 system and angiogenesis.

The formation of new blood vessels from pre-existing vasculature, the process known as angiogenesis, is highly regulated by pro- and anti-angiogenic signaling molecules including growth factors and proteases. As an endothelial cell-surface co-receptor for plasminogen and tissue plasminogen activator, the annexin A2 (ANXA2) complex accelerates plasmin generation and facilitates fibrinolysis. Plasmin can subsequently activate a downstream proteolytic cascade involving multiple matrix metalloproteinases. Thus, in addition to maintaining blood vessel patency, the ANXA2 complex can also promote angiogenesis via its pro-fibrinolytic activity. The generation of ANXA2-deficient mice allowed us to first observe the pro-angiogenic role of ANXA2 in vivo. Further investigations have provided additional details regarding the mechanism for ANXA2 regulation of retinal and corneal angiogenesis. Other studies have reported that ANXA2 supports angiogenesis in specific tumor-related settings. Here, we summarize results from in vivo studies that illustrate the pro-angiogenic role of ANXA2, and discuss the critical questions that may lead to an advanced understanding of the molecular mechanisms for ANXA2-mediated angiogenesis. Finally, highlights from studies on ANXA2-interacting agents offer potential therapeutic opportunities for the application of ANXA2-centered pharmaceuticals in angiogenesis-related disorders.

The Membrane Phospholipid Binding Protein Annexin A2 Promotes Phagocytosis and Nonlytic Exocytosis of Cryptococcus neoformans and Impacts Survival in Fungal Infection.

Cryptococcus neoformans is a fungal pathogen with a unique intracellular pathogenic strategy that includes nonlytic exocytosis, a phenomenon whereby fungal cells are expunged from macrophages without lysing the host cell. The exact mechanism and specific proteins involved in this process have yet to be completely defined. Using murine macrophages deficient in the membrane phospholipid binding protein, annexin A2 (ANXA2), we observed a significant decrease in both phagocytosis of yeast cells and the frequency of nonlytic exocytosis. Cryptococcal cells isolated from Anxa2-deficient (Anxa2(-/-)) bone marrow-derived macrophages and lung parenchyma displayed significantly larger capsules than those isolated from wild-type macrophages and tissues. Concomitantly, we observed significant differences in the amount of reactive oxygen species produced between Anxa2(-/-) and Anxa2(+/+) macrophages. Despite comparable fungal burden, Anxa2(-/-) mice died more rapidly than wild-type mice when infected with C. neoformans, and Anxa2(-/-) mice exhibited enhanced inflammatory responses, suggesting that the reduced survival reflected greater immune-mediated damage. Together, these findings suggest a role for ANXA2 in the control of cryptococcal infection, macrophage function, and fungal morphology.

Prohibitin/annexin 2 interaction regulates fatty acid transport in adipose tissue.

We have previously identified prohibitin (PHB) and annexin A2 (ANX2) as proteins interacting on the surface of vascular endothelial cells in white adipose tissue (WAT) of humans and mice. Here, we demonstrate that ANX2 and PHB also interact in adipocytes. Mice lacking ANX2 have normal WAT vascularization, adipogenesis, and glucose metabolism but display WAT hypotrophy due to reduced fatty acid uptake by WAT endothelium and adipocytes. By using cell culture systems in which ANX2/PHB binding is disrupted either genetically or through treatment with a blocking peptide, we show that fatty acid transport efficiency relies on this protein complex. We also provide evidence that the interaction between ANX2 and PHB mediates fatty acid transport from the endothelium into adipocytes. Moreover, we demonstrate that ANX2 and PHB form a complex with the fatty acid transporter CD36. Finally, we show that the colocalization of PHB and CD36 on adipocyte surface is induced by extracellular fatty acids. Together, our results suggest that an unrecognized biochemical interaction between ANX2 and PHB regulates CD36-mediated fatty acid transport in WAT, thus revealing a new potential pathway for intervention in metabolic diseases.

Alterations of Cholesterol Metabolism in Inflammation-Induced Atherogenesis.

Vascular inflammation is central to the pathogenesis of the atherosclerotic lesion. In the setting of hypercholesterolemia, vascular inflammation accelerates the accumulation of cholesterol within arterial smooth muscle cells, macrophages, and other immune cells. In disorders such as obesity, diabetes, and thrombosis, a myriad of interactions between sterol metabolites and inflammatory mediators exacerbate cholesterol deposition in the vessel wall, leading to the well-known consequences of stroke, transient ischemic attack, myocardial infarction, and peripheral vascular insufficiency. This review highlights emerging concepts in the regulation of cholesterol synthesis, the lipolytic enzymes involved in cholesterol utilization, and the therapies that successfully modulate vascular inflammation. In addition, developments relating to the role of inflammasomes in the management of cholesterol-mediated inflammation are discussed.

The Biology of Annexin A2: From Vascular Fibrinolysis to Innate Immunity.

Annexin A2 is a multicompartmental protein that orchestrates a spectrum of dynamic membrane-related events. At cell surfaces, A2 forms the (A2•S100A10)2 complex which accelerates tissue plasminogen activator-dependent activation of the fibrinolytic protease, plasmin. Anti-A2 antibodies are associated with clinical thrombosis in antiphospholipid syndrome, whereas overexpression of A2 promotes hyperfibrinolytic bleeding in acute promyelocytic leukemia. A2 is upregulated in hypoxic tissues, and mice deficient in A2 are resistant to hypoxia-related retinal neovascularization in a model of diabetic retinopathy. Within the cell, A2 regulates membrane fusion processes involved in the secretion of pre-packaged, ultra-large molecules. In stimulated dendritic cells, A2 maintains lysosomal membrane integrity, thereby modulating inflammasome activation and cytokine secretion. Together, these findings suggest an emerging, multifaceted role for annexin A2 in human health and disease. The author's work has been inspired by numerous colleagues and mentors, and by the author's grandfather, and former ACCA member, Dr. J. Burns Amberson.

Semaphorin 3D autocrine signaling mediates the metastatic role of annexin A2 in pancreatic cancer.

Most patients with pancreatic ductal adenocarcinoma (PDA) present with metastatic disease at the time of diagnosis or will recur with metastases after surgical treatment. Semaphorin-plexin signaling mediates the migration of neuronal axons during development and of blood vessels during angiogenesis. The expression of the gene encoding semaphorin 3D (Sema3D) is increased in PDA tumors, and the presence of antibodies against the pleiotropic protein annexin A2 (AnxA2) in the sera of some patients after surgical resection of PDA is associated with longer recurrence-free survival. By knocking out AnxA2 in a transgenic mouse model of PDA (KPC) that recapitulates the progression of human PDA from premalignancy to metastatic disease, we found that AnxA2 promoted metastases in vivo. The expression of AnxA2 promoted the secretion of Sema3D from PDA cells, which coimmunoprecipitated with the co-receptor plexin D1 (PlxnD1) on PDA cells. Mouse PDA cells in which SEMA3D was knocked down or ANXA2-null PDA cells exhibited decreased invasive and metastatic potential in culture and in mice. However, restoring Sema3D in AnxA2-null cells did not entirely rescue metastatic behavior in culture and in vivo, suggesting that AnxA2 mediates additional prometastatic mechanisms. Patients with primary PDA tumors that have abundant Sema3D have widely metastatic disease and decreased survival compared to patients with tumors that have relatively low Sema3D abundance. Thus, AnxA2 and Sema3D may be new therapeutic targets and prognostic markers of metastatic PDA.

Annexin A2 promotes phagophore assembly by enhancing Atg16L⁺ vesicle biogenesis and homotypic fusion.

Plasma membrane budding of Atg-16L-positive vesicles represents a very early event in the generation of the phagophore and in the process of macroautophagy. Here we show that the membrane curvature-inducing protein annexin A2 contributes to the formation of these vesicles and their fusion to form phagophores. Ultrastructural, proteomic and FACS analyses of Atg16L-positive vesicles reveal that 30% of Atg16L-positive vesicles are also annexin A2-positive. Lipidomic analysis of annexin A2-deficient mouse cells indicates that this protein plays a role in recruiting phosphatidylserine and phosphatidylinositides to Atg16L-positive vesicles. Absence of annexin A2 reduces both vesicle formation and homotypic Atg16L vesicle fusion. Ultimately, a reduction in LC3 flux and dampening of macroautophagy are observed in dendritic cells from Anxa2(-/-) mice. Together, our analyses highlight the importance of annexin A2 in vesiculation of a population of Atg16L-positive structures from the plasma membrane, and in their homotypic fusion to form phagophore structures.

Fibrinolysis and the control of blood coagulation.

Fibrin plays an essential role in hemostasis as both the primary product of the coagulation cascade and the ultimate substrate for fibrinolysis. Fibrinolysis efficiency is greatly influenced by clot structure, fibrinogen isoforms and polymorphisms, the rate of thrombin generation, the reactivity of thrombus-associated cells such as platelets, and the overall biochemical environment. Regulation of the fibrinolytic system, like that of the coagulation cascade, is accomplished by a wide array of cofactors, receptors, and inhibitors. Fibrinolytic activity can be generated either on the surface of a fibrin-containing thrombus, or on cells that express profibrinolytic receptors. In a widening spectrum of clinical disorders, acquired and congenital defects in fibrinolysis contribute to disease morbidity, and new assays of global fibrinolysis now have potential predictive value in multiple clinical settings. Here, we summarize the basic elements of the fibrinolytic system, points of interaction with the coagulation pathway, and some recent clinical advances.