Tyro3, Axl, and Mertk receptors differentially participate in platelet activation and thrombus formation

Recent advances in TAM mechanisms in lung diseases

TYRO3, MERTK, and AXL receptor tyrosine kinases, collectively known as TAM receptors, play a vital role in maintaining lung tissue homeostasis by regulating integrity and self-renewal. These receptors activate signalling pathways that inhibit apoptosis, promote cell proliferation and differentiation, mediate cell adhesion and migration, and perform other essential biological functions. Additionally, TAM receptors are implicated in mechanisms that suppress anti-tumor immunity and confer resistance to immune checkpoint inhibitors. Disruption of the homeostatic balances can lead to pathological conditions such as lung inflammation, fibrosis, or tumors. Recent studies highlight their significant role in COVID-19-induced lung injury. However, the exact mechanisms by which TAM receptors contribute to lung diseases remain unclear. This article reviews the potential mechanisms of TAM receptor involvement in disease progression, focusing on lung inflammation, fibrosis, cancer, and COVID-19-induced lung injury. It also explores future research aspects and the therapeutic potentials of targeting TAM receptors, providing a theoretical foundation for understanding lung disease mechanisms and identifying treatment targets.

UNC9426, a Potent and Orally Bioavailable TYRO3-Specific Inhibitor

TYRO3 plays a critical role in platelet aggregation as a platelet response amplifier. Selective inhibition of TYRO3 may provide therapeutic benefits for treating thrombosis and related diseases without increasing bleeding risk. We employed a structure-based approach and discovered a novel and potent TYRO3 inhibitor UNC9426 (12) with an excellent Ambit selectivity score (S50 (1.0 μM) = 0.026) and favorable pharmacokinetic properties in mice. Treatment with UNC9426 reduced platelet aggregation without increasing bleeding time and blocked TYRO3-dependent functions in tumor cells and macrophages, implicating its utility for multiple indications.

Exploring MERTK inhibitor binding mechanisms: insights from adaptive steered molecular dynamics and free energy calculation

MERTK, a promising drug target for the treatment of human leukemia and solid tumors, and the development of its small molecule inhibitors holds significant clinical potential. However, the underlying reasons for the varying activities among these inhibitors and the specifics of their binding mechanism have not been systematically investigated. By combining conventional molecular dynamics simulations, adaptive steered molecular dynamics simulations and binding free energy calculations based on molecular mechanics Poisson-Boltzmann surface area, the interaction modes of four MERTK inhibitors and dissociation behavior are discussed in detail. The results reveal additional critical amino acids, beyond the well-known hot spot residues in the kinase hinge region, that play a pivotal role in inhibitor binding. Our findings further indicate that the binding of MERTK to its inhibitors relies not only on crucial hydrogen bonding interactions but also benefits from non-polar interactions. In addition, the analysis of hydrogen bonding within kinetic trajectories and potential of mean force explained the differences in activity between different inhibitors, providing insights for the design and optimization of subsequent MERTK-targeted small molecule inhibitors.

Thiol isomerase ERp18 enhances platelet activation and arterial thrombosis

Background

Thiol isomerases regulate the thiol-disulfide exchange of functional proteins in cells. Using genetically modified mouse models and inhibitors, we and others demonstrated that 7 thiol isomerases (ERp57, protein diisulfide isomerase, ERp72, ERp46, ERp5, TMX4, and TMX1) participate in thrombosis. There are 21 thiol isomerases in mammals, but whether other enzymes of this family also contribute to thrombosis remains unknown.

Objectives

Investigate whether and how ERp18 participates in arterial thrombosis.

Methods

ERp18 knockout mice and arterial thrombosis models were used to determine the role of ERp18 in thrombosis. Platelets from ERp18 knockout mice were used to detect aggregation, activation, spreading, and clot retraction. Finally, flow cytometry and immunoprecipitation were used to detect the binding between ERp18 and αIIbβ3.

Results

The mice lacking ERp18 exhibited a prolonged tail bleeding time and decreased platelet thrombus formation in FeCl3-induced carotid arterial injury and laser-induced cremaster artery injury models. ERp18 deficiency inhibited platelet aggregation, adenosine triphosphate release, integrin αIIbβ3 activation, P-selectin expression, platelet adhesion, as well as clot retraction. Flow cytometry and coimmunoprecipitation analyses revealed that ERp18 binds to the platelet surface via interaction with integrin αIIbβ3. Moreover, the ERp18 protein promoted the binding of integrin αIIbβ3 to fibrinogen and platelet aggregation. Furthermore, the recombinant ERp18 protein exhibited reductase activity and cleaved integrin αIIbβ3 disulfides.

Conclusion

ERp18 participates in platelet activation and thrombosis. Its function is, at least in part, through the regulation of integrin αIIbβ3 function. This finding expands our understanding of the role of thiol isomerases in the redox regulation of thrombosis and platelet function.

Immunological role of Gas6/TAM signaling in hemostasis and thrombosis

The immune system is an emerging regulator of hemostasis and thrombosis. The concept of immunothrombosis redefines the relationship between coagulation and immunomodulation, and the Gas6/Tyro3-Axl-MerTK (TAM) signaling pathway builds the bridge across them. During coagulation, Gas6/TAM signaling pathway not only activates platelets, but also promotes thrombosis through endothelial cells and vascular smooth muscle cells involved in inflammatory responses. Thrombosis appears to be a common result of a Gas6/TAM signaling pathway-mediated immune dysregulation. TAM TK and its ligands have been found to be involved in coagulation through the PI3K/AKT or JAK/STAT pathway in various systemic diseases, providing new perspectives in the understanding of immunothrombosis. Gas6/TAM signaling pathway serves as a breakthrough target for novel therapeutic strategies to improve disease management. Many preclinical and clinical studies of TAM receptor inhibitors are in process, confirming the pivotal role of Gas6/TAM signaling pathway in immunothrombosis. Therapeutics targeting the TAM receptor show potential both in anticoagulation management and immunotherapy. Here, we review the immunological functions of the Gas6/TAM signaling pathway in coagulation and its multiple mechanisms in diseases identified to date, and discuss the new clinical strategies that may generated by these roles.

AXL receptor tyrosine kinase modulates gonadotropin-releasing hormone receptor signaling

BACKGROUND

Gonadotropin-releasing hormone (GnRH) receptors are essential for reproduction and are expressed in numerous urogenital, reproductive, and non-reproductive cancers. In addition to canonical G protein-coupled receptor signaling, GnRH receptors functionally interact with several receptor tyrosine kinases. AXL is a receptor tyrosine kinase expressed in numerous tissues as well as multiple tumors. Here we tested the hypothesis that AXL, along with its endogenous ligand Gas6, impacts GnRH receptor signaling.

METHODS

We used clonal murine pituitary αT3-1 and LβT2 gonadotrope cell lines to examine the effect of AXL activation on GnRH receptor-dependent signaling outcomes. ELISA and immunofluorescence were used to observe AXL and GnRH receptor expression in αT3-1 and LβT2 cells, as well as in murine and human pituitary sections. We also used ELISA to measure changes in ERK phosphorylation, pro-MMP9 production, and release of LHβ. Digital droplet PCR was used to measure the abundance of Egr-1 transcripts. A transwell migration assay was used to measure αT3-1 and LβT2 migration responses to GnRH and AXL.

RESULTS

We observed AXL, along with the GnRH receptor, expression in αT3-1 and LβT2 gonadotrope cell lines, as well as in murine and human pituitary sections. Consistent with a potentiating role of AXL, Gas6 enhanced GnRH-dependent ERK phosphorylation in αT3-1 and LβT2 cells. Further, and consistent with enhanced post-transcriptional GnRH receptor responses, we found that Gas6 increased the abundance of Egr-1 transcripts. Suggesting functional significance, in LβT2 cells, Gas6/AXL signaling stimulated LHβ production and enhanced GnRH receptor-dependent generation of pro-MMP9 protein and promoted cell migration.

CONCLUSIONS

Altogether, these data describe a novel role for AXL as a modulator of GnRH receptor signaling. Video Abstract.

Receptor tyrosine kinases Tyro3, Axl, and Mertk differentially contribute to antibody-induced arthritis

Tyro3, Axl, and Mertk (abbreviated TAMs) comprise a family of homologous type 1 receptor tyrosine kinases (RTKs) that have been implicated as inhibitory receptors that dampen inflammation, but their roles in the pathogenesis of rheumatoid arthritis remains understudied. Here, to investigate TAMs in an inflammatory arthritis model, antibody-induced arthritis in single TAM-deficient mice (Tyro3- KO, Axl-KO, Mertk-KO) was induced by K/BxN serum injection. Subsequently, joint inflammation and cytokine levels, as well as the expression of Fcγ Rs and complement receptors were assessed in WT and TAM-deficient mice. Compared with littermate control mice, Axl-/- and Mertk-/- mice developed more severe antibody-induced arthritis, while in contrast, Tyro3-/- mice showed diminished joint inflammation. Concomitantly, the levels of cytokines in joints of Axl-/- and Mertk-/- mice were also significantly increased, while cytokines in the Tyro3-/- joint tissues were decreased. At the molecular and cellular level, TAMs showed distinct expression patterns, whereby monocytes expressed Axl and Mertk, but no Tyro3, while neutrophils expressed Axl and Tyro3 but little Mertk. Moreover, expression of Fcγ receptors and C5aR showed different patterns with TAMs expression, whereby FcγRIV was higher in monocytes of Axl-/- and Mertk-/- mice compared to wild-type mice, while Tyro3-/- neutrophils showed lower expression levels of FcγRI, FcγRIII and FcγRIV. Finally, expression of C5aR was increased in Mertk-/- monocytes, and was decreased in Tyro3-/- neutrophils. These data indicate that Axl, Mertk and Tyro3 have distinct functions in antibody-induced arthritis, due in part to the differential regulation of cytokines production, as well as expression of FcγRs and C5aR. Video Abstract.

AXL/Gas6 signaling mechanisms in the hypothalamic-pituitary-gonadal axis

AXL is a receptor tyrosine kinase commonly associated with a variety of human cancers. Along with its ligand Gas6 (growth arrest-specific protein 6), AXL is emerging as an important regulator of neuroendocrine development and function. AXL signaling in response to Gas6 binding impacts neuroendocrine structure and function at the level of the brain, pituitary, and gonads. During development, AXL has been identified as an upstream inhibitor of gonadotropin receptor hormone (GnRH) production and also plays a key role in the migration of GnRH neurons from the olfactory placode to the forebrain. AXL is implicated in reproductive diseases including some forms of idiopathic hypogonadotropic hypogonadism and evidence suggests that AXL is required for normal spermatogenesis. Here, we highlight research describing AXL/Gas6 signaling mechanisms with a focus on the molecular pathways related to neuroendocrine function in health and disease. In doing so, we aim to present a concise account of known AXL/Gas6 signaling mechanisms to identify current knowledge gaps and inspire future research.

Jedi-1/MEGF12-mediated phagocytosis controls the pro-neurogenic properties of microglia in the ventricular-subventricular zone

Microglia are the primary phagocytes in the central nervous system and are responsible for clearing dead cells generated during development or disease. The phagocytic process shapes the phenotype of the microglia, which affects the local environment. A unique population of microglia reside in the ventricular-subventricular zone (V-SVZ) of neonatal mice, but how they influence this neurogenic niche is not well-understood. Here, we demonstrate that phagocytosis creates a pro-neurogenic microglial phenotype in the V-SVZ and that these microglia phagocytose apoptotic cells via the engulfment receptor Jedi-1. Deletion of Jedi-1 decreases apoptotic cell clearance, triggering the development of a neuroinflammatory phenotype, reminiscent of neurodegenerative and-age-associated microglia, that reduces neural precursor proliferation via elevated interleukin (IL)-1β signaling; inhibition of IL-1 receptor rescues precursor proliferation in vivo. Together, these results reveal a critical role for Jedi-1 in connecting microglial phagocytic activity to a phenotype that promotes neurogenesis in the developing V-SVZ.

The role of macrophage TAM receptor family in the acute-to-chronic progression of liver disease: From friend to foe?

Hepatic macrophages, the key cellular components of the liver, emerge as essential players in liver inflammation, tissue repair and subsequent fibrosis, as well as tumorigenesis. Recently, the TAM receptor tyrosine kinase family, consisting of Tyro3, Axl and MerTK, was found to be a pivotal modulator of macrophages. Activation of macrophage TAM receptor signalling promotes the efferocytosis of apoptotic cells and skews the polarization of macrophages. After briefly reviewing the mechanisms of TAM receptor signalling in macrophage polarization, we focus on their role in liver diseases from acute injury to chronic inflammation, fibrosis and then to tumorigenesis. Notably, macrophage TAM receptor signalling seems to be a two-edged sword for liver diseases. On one hand, the activation of TAM receptor signalling inhibits inflammation and facilitates tissue repair during acute liver injury. On the other hand, continuous activation of the signalling contributes to the process of chronic inflammation into fibrosis and tumorigenesis by evoking hepatic stellate cells and inhibiting anti-tumour immunity. Therefore, targeting macrophage TAM receptors and clarifying its downstream pathways will be exciting prospects for the precaution and treatment of liver diseases, particularly at different stages or statuses.

Dissecting the Role of AXL in Cancer Immune Escape and Resistance to Immune Checkpoint Inhibition

The development and implementation of Immune Checkpoint Inhibitors (ICI) in clinical oncology have significantly improved the survival of a subset of cancer patients with metastatic disease previously considered uniformly lethal. However, the low response rates and the low number of patients with durable clinical responses remain major concerns and underscore the limited understanding of mechanisms regulating anti-tumor immunity and tumor immune resistance. There is an urgent unmet need for novel approaches to enhance the efficacy of ICI in the clinic, and for predictive tools that can accurately predict ICI responders based on the composition of their tumor microenvironment. The receptor tyrosine kinase (RTK) AXL has been associated with poor prognosis in numerous malignancies and the emergence of therapy resistance. AXL is a member of the TYRO3-AXL-MERTK (TAM) kinase family. Upon binding to its ligand GAS6, AXL regulates cell signaling cascades and cellular communication between various components of the tumor microenvironment, including cancer cells, endothelial cells, and immune cells. Converging evidence points to AXL as an attractive molecular target to overcome therapy resistance and immunosuppression, supported by the potential of AXL inhibitors to improve ICI efficacy. Here, we review the current literature on the prominent role of AXL in regulating cancer progression, with particular attention to its effects on anti-tumor immune response and resistance to ICI. We discuss future directions with the aim to understand better the complex role of AXL and TAM receptors in cancer and the potential value of this knowledge and targeted inhibition for the benefit of cancer patients.

White matter tract conductivity is resistant to wide variations in paranodal structure and myelin thickness accompanying the loss of Tyro3: an experimental and simulated analysis

Myelination within the central nervous system (CNS) is crucial for the conduction of action potentials by neurons. Variation in compact myelin morphology and the structure of the paranode are hypothesised to have significant impact on the speed of action potentials. There are, however, limited experimental data investigating the impact of changes in myelin structure upon conductivity in the central nervous system. We have used a genetic model in which myelin thickness is reduced to investigate the effect of myelin alterations upon action potential velocity. A detailed examination of the myelin ultrastructure of mice in which the receptor tyrosine kinase Tyro3 has been deleted showed that, in addition to thinner myelin, these mice have significantly disrupted paranodes. Despite these alterations to myelin and paranodal structure, we did not identify a reduction in conductivity in either the corpus callosum or the optic nerve. Exploration of these results using a mathematical model of neuronal conductivity predicts that the absence of Tyro3 would lead to reduced conductivity in single fibres, but would not affect the compound action potential of multiple myelinated neurons as seen in neuronal tracts. Our data highlight the importance of experimental assessment of conductivity and suggests that simple assessment of structural changes to myelin is a poor predictor of neural functional outcomes.

Mertk: An emerging target in cancer biology and immuno-oncology

Mertk, a type I Receptor Tyrosine Kinase (RTK) and member of the TAM (Tyro3, Axl, and Mertk) family of homologous tyrosine kinases, has important roles in signal transduction both homeostatically on normal cells as well as patho-physiologically on both tumor-associated macrophages and malignant cells by its overexpression in a wide array of cancers. The main ligands of Mertk are Vitamin K-modified endogenous proteins Gas6 and Protein S (ProS1), heterobifunctional modular proteins that bind Mertk via two carboxyl-terminal laminin-like globular (LG) domains, and an N-terminal Gla domain that binds anionic phospholipids, whereby externalized phosphatidylserine (PS) on stressed viable and caspase-activated apoptotic cells is most emblematic. Recent studies indicate that Vitamin K-dependent γ-carboxylation on the N-terminal Gla domain of Gas6 and Protein S is necessary for PS binding and Mertk activation, implying that Mertk is preferentially active in tissues where there is high externalized PS, such as the tumor microenvironment (TME) and acute virally infected tissues. Once stimulated, activated Mertk can provide a survival advantage for cancer cells as well as drive compensatory proliferation. On monocytes and tumor-associated macrophages, Mertk promotes efferocytosis and acts as an inhibitory receptor that impairs host anti-tumor immunity, functioning akin to a myeloid checkpoint inhibitor. In recent years, inhibition of Mertk has been implicated in a dual role to enhance the sensitivity of cancer cells to cytotoxic agents along with improving host anti-tumor immunity with anti-PD-1/PD-L1 immunotherapy. Here, we examine the rationale of Mertk-targeted immunotherapies, the current and potential therapeutic strategies, the clinical status of Mertk-specific therapies, and potential challenges and obstacles for Mertk-focused therapies.

MERTK Inhibition: Potential as a Treatment Strategy in EGFR Tyrosine Kinase Inhibitor-Resistant Non-Small Cell Lung Cancer

Epidermal growth factor tyrosine kinase inhibitors (EGFR-TKIs) are currently the most effective treatment for non-small cell lung cancer (NSCLC) patients, who carry primary EGFR mutations. However, the patients eventually develop drug resistance to EGFR-TKIs after approximately one year. In addition to the acquisition of the EGFR T790M mutation, the activation of alternative receptor-mediated signaling pathways is a common mechanism for conferring the insensitivity of EGFR-TKI in NSCLC. Upregulation of the Mer receptor tyrosine kinase (MERTK), which is a member of the Tyro3-Axl-MERTK (TAM) family, is associated with a poor prognosis of many cancers. The binding of specific ligands, such as Gas6 and PROS1, to MERTK activates phosphoinositide 3-kinase (PI3K)/Akt and mitogen-activated protein kinase (MAPK) cascades, which are the signaling pathways shared by EGFR. Therefore, the inhibition of MERTK can be considered a new therapeutic strategy for overcoming the resistance of NSCLC to EGFR-targeted agents. Although several small molecules and monoclonal antibodies targeting the TAM family are being developed and have been described to enhance the chemosensitivity and converse the resistance of EGFR-TKI, few have specifically been developed as MERTK inhibitors. The further development and investigation of biomarkers which can accurately predict MERTK activity and the response to MERTK inhibitors and MERTK-specific drugs are vitally important for obtaining appropriate patient stratification and increased benefits in clinical applications.

The role of phospho-tyrosine signaling in platelet biology and hemostasis

Platelets are small enucleated cell fragments specialized in the control of hemostasis, but also playing a role in angiogenesis, inflammation and immunity. This plasticity demands a broad range of physiological processes. Platelet functions are mediated through a variety of receptors, the concerted action of which must be tightly regulated, in order to allow specific and timely responses to different stimuli. Protein phosphorylation is one of the main key regulatory mechanisms by which extracellular signals are conveyed. Despite the importance of platelets in health and disease, the molecular pathways underlying the activation of these cells are still under investigation. Here, we review current literature on signaling platelet biology and in particular emphasize the newly emerging role of phosphatases in these processes.

Crystal Structure of the Kinase Domain of MerTK in Complex with AZD7762 Provides Clues for Structure-Based Drug Development

Aberrant tyrosine-protein kinase Mer (MerTK) expression triggers prosurvival signaling and contributes to cell survival, invasive motility, and chemoresistance in many kinds of cancers. In addition, recent reports suggested that MerTK could be a primary target for abnormal platelet aggregation. Consequently, MerTK inhibitors may promote cancer cell death, sensitize cells to chemotherapy, and act as new antiplatelet agents. We screened an inhouse chemical library to discover novel small-molecule MerTK inhibitors, and identified AZD7762, which is known as a checkpoint-kinase (Chk) inhibitor. The inhibition of MerTK by AZD7762 was validated using an in vitro homogeneous time-resolved fluorescence (HTRF) assay and through monitoring the decrease in phosphorylated MerTK in two lung cancer cell lines. We also determined the crystal structure of the MerTK:AZD7762 complex and revealed the binding mode of AZD7762 to MerTK. Structural information from the MerTK:AZD7762 complex and its comparison with other MerTK:inhibitor structures gave us new insights for optimizing the development of inhibitors targeting MerTK.

Targeting TAM to Tame Pancreatic Cancer

Pancreatic cancer is expected to become the second leading cause of cancer-related death within the next few years. Current therapeutic strategies have limited effectiveness and therefore there is an urgency to develop novel effective therapies. The receptor tyrosine kinase subfamily TAM (Tyro3, Axl, MerTK) is directly implicated in the pathogenesis of the metastatic, chemoresistant, and immunosuppressive phenotype in pancreatic cancer. TAM inhibitors are promising investigational therapies for pancreatic cancer due to their potential to target multiple aspects of pancreatic cancer biology. Specifically, recent mechanistic investigations and therapeutic combinations in the preclinical setting suggest that TAM inhibition with chemotherapy, targeted therapy, and immunotherapy should be evaluated clinically.

CD14+CD16- monocytes are the main precursors of osteoclasts in rheumatoid arthritis via expressing Tyro3TK

Background

Monocytes as precursors of osteoclasts in rheumatoid arthritis (RA) are well demonstrated, while monocyte subsets in osteoclast formation are still controversial. Tyro3 tyrosine kinase (Tyro3TK) is a member of the receptor tyrosine kinase family involved in immune homeostasis, the role of which in osteoclast differentiation was reported recently. This study aimed to compare the osteoclastic capacity of CD14⁺CD16⁺ and CD14⁺CD16⁻ monocytes in RA and determine the potential involvement of Tyro3TK in their osteoclastogenesis.

Methods

Osteoclasts were induced from CD14⁺CD16⁺ and CD14⁺CD16⁻ monocyte subsets isolated from healthy control (HC) and RA patients in vitro and evaluated by tartrate-resistant acid phosphatase (TRAP) staining. Then, the expression of Tyro3TK on CD14⁺CD16⁺ and CD14⁺CD16⁻ monocyte subsets in the peripheral blood of RA, osteoarthritis (OA) patients, and HC were evaluated by flow cytometry and qPCR, and their correlation with RA patient clinical and immunological features was analyzed. The role of Tyro3TK in CD14⁺CD16⁻ monocyte-mediated osteoclastogenesis was further investigated by osteoclast differentiation assay with Tyro3TK blockade.

Results

The results revealed that CD14⁺CD16⁻ monocytes were the primary source of osteoclasts. Compared with HC and OA patients, the expression of Tyro3TK on CD14⁺CD16⁻ monocytes in RA patients was significantly upregulated and positively correlated with the disease manifestations, such as IgM level, tender joint count, and the disease activity score. Moreover, anti-Tyro3TK antibody could inhibit Gas6-mediated osteoclast differentiation from CD14⁺CD16⁻ monocytes in a dose-dependent manner.

Conclusions

These findings indicate that elevated Tyro3TK on CD14⁺CD16⁻ monocytes serves as a critical signal for osteoclast differentiation in RA.

Mouse laser injury models: variations on a theme

A confluence of technological advances in genetic manipulation and molecular-based fluorescence imaging has led to the widespread adoption of laser injury models to study hemostasis and thrombosis in mice. In all animal models of hemostasis and thrombosis, detailing the nature of experimentally induced vascular injury is paramount in enabling appropriate interpretation of experimental results. A careful appraisal of the literature shows that direct laser-induced injury can result in variable degrees of vascular damage. This review will compare and contrast models of laser injury utilized in the field, with an emphasis on the mechanism and extent of injury, the use of laser injury in different vascular beds and the molecular mechanisms regulating the response to injury. All of these topics will be discussed in the context of how distinct applications of laser injury models may be viewed as representing thrombosis and/or hemostasis.

Modulation of Immune Responses by Platelet-Derived ADAM10

Platelets have a crucial function in maintaining hemostasis. However, beyond their role in coagulation and thrombus formation, platelets have been implicated to affect various pathophysiological conditions such as infectious diseases, autoimmune disorders, and cancer. It is well-established that platelets aid local cancer growth by providing growth factors or contributing to cancer angiogenesis. In addition, they promote metastasis, among others by facilitation of tumor cell-extravasation and epithelial-to-mesenchymal-like transition as well as protecting metastasizing cancer cells from immunosurveillance. A variety of membrane-bound and soluble platelet-derived factors are involved in these processes, and many aspects of platelet biology in both health and disease are regulated by platelet-associated metalloproteinases and their inhibitors. Platelets synthesize (i) members of the matrix metalloproteinase (MMP) family and also inhibitors of MMPs such as members of the "tissue inhibitor of metalloproteinases" (TIMP) family as well as (ii) members of the "a disintegrin and metalloproteinase" (ADAM) family including ADAM10. Notably, platelet-associated metalloproteinase activity not only influences functions of platelets themselves: platelets can also induce expression and/or release of metalloproteinases e.g., in leukocytes or cancer cells, and ADAMs are emerging as important components by which platelets directly affect other cell types and function. This review outlines the function of metalloproteinases in platelet biology with a focus on ADAM10 and discusses the role of platelet-derived metalloproteinases in the interaction of platelets with components of the immune system and/or cancer cells.

Scavenger receptor A1 attenuates aortic dissection via promoting efferocytosis in macrophages

Macrophage class A1 scavenger receptor (SR-A1) is a pattern recognition receptor with an anti-inflammatory feature in cardiovascular diseases. However, its role in acute aortic dissection (AD) is not known yet. Using an aortic dissection model in SR-A1-deficient mice and their wild type littermates, we found that SR-A1 deficiency aggravated beta-aminopropionitrile monofumarate induced thoracic aortic dilation, false lumen formation, extracellular matrix degradation, vascular inflammation and accumulation of apoptotic cells. These pathological changes were associated with an impaired macrophage efferocytosis mediated by tyrosine-protein kinase receptor Tyro3 in vitro and in vivo. SR-A1 could directly interact with Tyro3 and was required for Tyro3 phosphorylation to activate its downstream PI3K/Akt signaling pathway. Importantly, co-culture of SR-A1^-/- macrophages with apoptotic Jurkat cells resulted in less devoured apoptotic cells accompanied by swelling mitochondria and damaged ATP generation, following poor IL-10 and robust TNF-α production. Deficiency of SR-A1 did not influence phagolysosome formation during the efferocytosis. Lentiviral overexpression of Tyro3 in SR-A1^-/- macrophages induced restorative phagocytosis in vitro. Administration of Tyro3 agonist protein S could restore SR-A1^-/- macrophages phagocytosis in vitro and in vivo. These findings suggest that SR-A1-Tyro3 axis in macrophages mitigate AD damage by promoting efferocytosis and inhibiting inflammation.