The Function of ASK1 in Sepsis and Stress-Induced Disorders

Apoptosis signal-regulating kinase 1 (ASK1) is a serine-threonine kinase that is ubiquitously expressed in nucleated cells and is responsible for the activation of multiple mitogen-activated protein kinases (MAPK) to regulate cell stress. Activation of ASK1 via cellular stress leads to activation of downstream signaling components, activation of transcription factors, and proinflammatory cytokine production. ASK1 is also expressed in anucleate platelets and is a key player in platelet activation as it is important for signaling. Interestingly, the mechanism of ASK1 activation is cell type-dependent. In this review we will explore how ASK1 regulates a variety of cellular processes from innate immune function to thrombosis and hemostasis. We will discuss how ASK1 influences FcγRIIA-mediated platelet reactivity and how that reactivity drives platelet clearance. Furthermore, we will explore the role of ASK1 in thromboxane (TxA2) generation, which highlights differences in the way ASK1 functions in mouse and human platelets.

Crosstalk between Platelet and Bacteria: A Therapeutic Prospect

Platelets are typically recognized for their roles in the maintenance of hemostasis and vascular wall repair to reduce blood loss. Beyond hemostasis, platelets also play a critical role in pathophysiological conditions like atherosclerosis, stroke, thrombosis, and infections. During infection, platelets interact directly and indirectly with bacteria through a wide range of cellular and molecular mechanisms. Platelet surface receptors such as GPIbα, FcγRIIA, GPIIbIIIa, and TLRs, etc. facilitate direct interaction with bacterial cells. Besides, the indirect interaction between platelet and bacteria involves host plasma proteins such as von Willebrand Factor (vWF), fibronectin, IgG, and fibrinogen. Bacterial cells induce platelet activation, aggregation, and thrombus formation in the microvasculature. The activated platelets induce the Neutrophil Extracellular Traps (NETs) formation, which further contribute to thrombosis. Thus, platelets are extensively anticipated as vital immune modulator cells during infection, which may further lead to cardiovascular complications. In this review, we cover the interaction mechanisms between platelets and bacteria that may lead to the development of thrombotic disorders. Platelet receptors and other host molecules involved in such interactions can be used to develop new therapeutic strategies to combat against infection-induced cardiovascular complications. In addition, we highlight other receptor and enzyme targets that may further reduce infection-induced platelet activation and various pathological conditions.

Cancer and Thrombosis: The Platelet Perspective

Platelets are critical to hemostatic and immunological function, and are key players in cancer progression, metastasis, and cancer-related thrombosis. Platelets interact with immune cells to stimulate anti-tumor responses and can be activated by immune cells and tumor cells. Platelet activation can lead to complex interactions between platelets and tumor cells. Platelets facilitate cancer progression and metastasis by: (1) forming aggregates with tumor cells; (2) inducing tumor growth, epithelial-mesenchymal transition, and invasion; (3) shielding circulating tumor cells from immune surveillance and killing; (4) facilitating tethering and arrest of circulating tumor cells; and (5) promoting angiogenesis and tumor cell establishment at distant sites. Tumor cell-activated platelets also predispose cancer patients to thrombotic events. Tumor cells and tumor-derived microparticles lead to thrombosis by secreting procoagulant factors, resulting in platelet activation and clotting. Platelets play a critical role in cancer progression and thrombosis, and markers of platelet-tumor cell interaction are candidates as biomarkers for cancer progression and thrombosis risk.

Platelets Mediate Host Defense against Staphylococcus aureus through Direct Bactericidal Activity and by Enhancing Macrophage Activities

Platelets are the chief effector cells in hemostasis. However, recent evidence suggests they have multiple roles in host defense against infection. Reports by us and others showed that platelets functionally contribute to protection against Staphylococcus aureus infection. In the current study, the capacity of mouse platelets to participate in host defense against S. aureus infection was determined by assessing two possibilities. First, we determined the ability of platelets to kill S. aureus directly; and, second, we tested the possibility that platelets enhance macrophage phagocytosis and intracellular killing of S. aureus In this study we report evidence in support of both mechanisms. Platelets effectively killed two different strains of S. aureus. A clinical isolate of methicillin-resistant S. aureus was killed by platelets (>40% killing in 2 h) in a thrombin-dependent manner whereas a methicillin-sensitive strain was killed to equal extent but did not require thrombin. Interestingly, thrombin-stimulated platelets also significantly enhanced peritoneal macrophage phagocytosis of both methicillin-resistant S. aureus and methicillin-sensitive S. aureus by >70%, and restricted intracellular growth by >40%. Enhancement of macrophage anti-S. aureus activities is independent of contact with platelets but is mediated through releasable products, namely IL-1β. These data confirm our hypothesis that platelets participate in host defense against S. aureus both through direct killing of S. aureus and enhancing the antimicrobial function of macrophages in protection against S. aureus infection.

The platelet Fc receptor, FcγRIIa

Human platelets express FcγRIIa, the low-affinity receptor for the constant fragment (Fc) of immunoglobulin (Ig) G that is also found on neutrophils, monocytes, and macrophages. Engagement of this receptor on platelets by immune complexes triggers intracellular signaling events that lead to platelet activation and aggregation. Importantly these events occur in vivo, particularly in response to pathological immune complexes, and engagement of this receptor on platelets has been causally linked to disease pathology. In this review, we will highlight some of the key features of this receptor in the context of the platelet surface, and examine the functions of platelet FcγRIIa in normal hemostasis and in response to injury and infection. This review will also highlight pathological consequences of engagement of this receptor in platelet-based autoimmune disorders. Finally, we present some new data investigating whether levels of the extracellular ligand-binding region of platelet glycoprotein VI which is rapidly shed upon engagement of platelet FcγRIIa by autoantibodies, can report on the presence of pathological anti-heparin/platelet factor 4 immune complexes and thus identify patients with pathological autoantibodies who are at the greatest risk of developing life-threatening thrombosis in the setting of heparin-induced thrombocytopenia.

Distinct pathways regulate Syk protein activation downstream of immune tyrosine activation motif (ITAM) and hemITAM receptors in platelets

Tyrosine kinase pathways are known to play an important role in the activation of platelets. In particular, the GPVI and CLEC-2 receptors are known to activate Syk upon tyrosine phosphorylation of an immune tyrosine activation motif (ITAM) and hemITAM, respectively. However, unlike GPVI, the CLEC-2 receptor contains only one tyrosine motif in the intracellular domain. The mechanisms by which this receptor activates Syk are not completely understood. In this study, we identified a novel signaling mechanism in CLEC-2-mediated Syk activation. CLEC-2-mediated, but not GPVI-mediated, platelet activation and Syk phosphorylation were abolished by inhibition of PI3K, which demonstrates that PI3K regulates Syk downstream of CLEC-2. Ibrutinib, a Tec family kinase inhibitor, also completely abolished CLEC-2-mediated aggregation and Syk phosphorylation in human and murine platelets. Furthermore, embryos lacking both Btk and Tec exhibited cutaneous edema associated with blood-filled vessels in a typical lymphatic pattern similar to CLEC-2 or Syk-deficient embryos. Thus, our data show, for the first time, that PI3K and Tec family kinases play a crucial role in the regulation of platelet activation and Syk phosphorylation downstream of the CLEC-2 receptor.

Primary haemostasis: newer insights

At the same time as biophysical and omics approaches are drilling deeper into the molecular details of platelets and other blood cells, as well as their receptors and mechanisms of regulation, there is also an increasing awareness of the functional overlap between human vascular systems. Together, these studies are redefining the intricate networks linking haemostasis and thrombosis with inflammation, infectious disease, cancer/metastasis and other vascular pathophysiology. The focus of this state-of-the-art review is some of the newer advances relevant to primary haemostasis. Of particular interest, platelet-specific primary adhesion-signalling receptors and associated activation pathways control platelet function in flowing blood and provide molecular links to other systems. Platelet glycoprotein (GP)Ibα of the GPIb-IX-V complex and GPVI not only initiate platelet aggregation and thrombus formation by primary interactions with von Willebrand factor and collagen, respectively, but are also involved in coagulation, leucocyte engagement, bacterial or viral interactions, and are relevant as potential risk markers in a range of human diseases. Understanding these systems in unprecedented detail promises significant advances in evaluation of individual risk, in new diagnostic or therapeutic possibilities and in monitoring the response to drugs or other treatment.