Selective sorting of alpha-granule proteins

Targeting cargo to an unconventional secretory system within megakaryocytes allows the release of transgenic proteins from platelets

BACKGROUND

Platelets are essential for hemostasis and thrombosis and play vital roles during metastatic cancer progression and infection. Hallmarks of platelet function are activation, cytoskeletal rearrangements, and the degranulation of their cellular contents upon stimulation. While α-granules and dense granules are the most studied platelet secretory granules, the dense tubular system (DTS) also functions as a secretory system for vascular thiol isomerases. However, how DTS cargo is packaged and transported from megakaryocytes (MKs) to platelets is poorly understood.

OBJECTIVES

To underpin the mechanisms responsible for DTS cargo transport and leverage those for therapeutic protein packaging into platelets.

METHODS

A retroviral expression system combined with immunofluorescence confocal microscopy was employed to track protein DTS cargo protein disulfide isomerase fused to enhanced green fluorescent protein (eGFP-PDI) during platelet production. Murine bone marrow transplantation models were used to determine the release of therapeutic proteins from platelets.

RESULTS

We demonstrated that the endoplasmic reticulum retrieval motif Lys-Asp-Glu-Leu (KDEL) located at the C-terminus of protein disulfide isomerase was essential for the regular transport of eGFP-PDI-containing granules. eGFP-PDIΔKDEL, in which the retrieval signal was deleted, was aberrantly packaged, and its expression was upregulated within clathrin-coated endosomes. Finally, we found that ectopic transgenic proteins, such as tissue factor pathway inhibitor and interleukin 2, can be packaged into MKs and proplatelets by adding a KDEL retrieval sequence.

CONCLUSION

Our data corroborate the DTS as a noncanonical secretory system in platelets and demonstrate that in vitro-generated MKs and platelets may be used as a delivery system for transgenic proteins during cellular therapy.

Role and recent progress of P2Y12 receptor in cancer development

P2Y12 receptor (P2Y12R) is an adenosine-activated G protein-coupled receptor (GPCR) that plays a central role in platelet function, hemostasis, and thrombosis. P2Y12R activation can promote platelet aggregation and adhesion to cancer cells, promote tumor angiogenesis, and affect the tumor immune microenvironment (TIME) and tumor drug resistance, which is conducive to the progression of cancers. Meanwhile, P2Y12R inhibitors can inhibit this effect, suggesting that P2Y12R may be a potential therapeutic target for cancer. P2Y12R is involved in cancer development and metastasis, while P2Y12R inhibitors are effective in inhibiting cancer. However, a new study suggests that long-term use of P2Y12R inhibitors may increase the risk of cancer and the mechanism remains to be explored. In this paper, we reviewed the structural and functional characteristics of P2Y12R and its role in cancer. We explored the role of P2Y12R inhibitors in different tumors and the latest advances by summarizing the basic and clinical studies on the effects of P2Y12R inhibitors on tumors.

Role of Platelets in Rheumatic Chronic Autoimmune Inflammatory Diseases

Platelets are essential in maintaining blood homeostasis and regulating several inflammatory processes. They constantly interact with immune cells, have immunoregulatory functions, and can affect, through immunologically active substances, endothelium, leukocytes, and other immune response components. In reverse, inflammatory and immune processes can activate platelets, which might be significant in autoimmune disease progression and arising complications. Thus, considering this interplay, targeting platelet activity may represent a new approach to treatment of autoimmune diseases. This review aims to highlight the role of platelets in the pathogenic mechanisms of the most frequent chronic autoimmune inflammatory diseases to identify gaps in current knowledge and to provide potential new targets for medical interventions.

Dissecting thrombus-directed chemotaxis and random movement in neutrophil near-thrombus motion in flow chambers

BACKGROUND

Thromboinflammation is caused by mutual activation of platelets and neutrophils. The site of thromboinflammation is determined by chemoattracting agents release by endothelium, immune cells, and platelets. Impaired neutrophil chemotaxis contributes to the pathogenesis of Shwachman-Diamond syndrome (SDS). In this hereditary disorder, neutrophils are known to have aberrant chemoattractant-induced F-actin properties. Here, we aim to determine whether neutrophil chemotaxis could be analyzed using our previously developed ex vivo assay of the neutrophils crawling among the growing thrombi.

METHODS

Adult and pediatric healthy donors, alongside with pediatric patients with SDS, were recruited for the study. Thrombus formation and granulocyte movement in hirudinated whole blood were visualized by fluorescent microscopy in fibrillar collagen-coated parallel-plate flow chambers. Alternatively, fibrinogen, fibronectin, vWF, or single tumor cells immobilized on coverslips were used. A computational model of chemokine distribution in flow chamber with a virtual neutrophil moving in it was used to analyze the observed data.

RESULTS

The movement of healthy donor neutrophils predominantly occurred in the direction and vicinity of thrombi grown on collagen or around tumor cells. For SDS patients or on coatings other than collagen, the movement was characterized by randomness and significantly reduced velocities. Increase in wall shear rates to 300-500 1/s led to an increase in the proportion of rolling neutrophils. A stochastic algorithm simulating leucocyte chemotaxis movement in the calculated chemoattractant field could reproduce the experimental trajectories of moving neutrophils for 72% of cells.

CONCLUSIONS

In samples from healthy donors, but not SDS patients, neutrophils move in the direction of large, chemoattractant-releasing platelet thrombi growing on collagen.

Exploring bias in platelet P2Y1 signalling: Host defence versus haemostasis

Platelets are necessary for maintaining haemostasis. Separately, platelets are important for the propagation of inflammation during the host immune response against infection. The activation of platelets also causes inappropriate inflammation in various disease pathologies, often in the absence of changes to haemostasis. The separate functions of platelets during inflammation compared with haemostasis are therefore varied and this will be reflected in distinct pathways of activation. The activation of platelets by the nucleotide adenosine diphosphate (ADP) acting on P2Y1 and P2Y12 receptors is important for the development of platelet thrombi during haemostasis. However, P2Y1 stimulation of platelets is also important during the inflammatory response and paradoxically in scenarios where no changes to haemostasis and platelet aggregation occur. In these events, Rho-GTPase signalling, rather than the canonical phospholipase Cβ (PLCβ) signalling pathway, is necessary. We describe our current understanding of these differences, reflecting on recent advances in knowledge of P2Y1 structure, and the possibility of biased agonism occurring from activation via other endogenous nucleotides compared with ADP. Knowledge arising from these different pathways of P2Y1 stimulation of platelets during inflammation compared with haemostasis may help therapeutic control of platelet function during inflammation or infection, while preserving essential haemostasis. LINKED ARTICLES: This article is part of a themed issue on Platelet purinergic receptor and non-thrombotic disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.4/issuetoc.

Tumor-educated platelets

Beyond traditional roles in homeostasis and coagulation, growing evidence suggests that platelets also reflect malignant transformation in cancer. Platelets are present in the tumor microenvironment where they interact with cancer cells. This interaction results in direct and indirect "education" as evident by platelet alterations in adhesion molecules, glycoproteins, nucleic acids, proteins and various receptors. Subsequently, these tumor-educated platelets (TEPs) circulate throughout the body and play pivotal roles in promotion of tumor growth and dissemination. Accordingly, platelet status can be considered a unique blood-based biomarker that can potentially predict prognosis and therapeutic success. Recently, liquid biopsies including TEPs have received much attention as safe, minimally invasive and sensitive alternatives for patient management. Herein, we provide an overview of TEPs and explore their benefits and limitations in cancer.

Expanding applications of allogeneic platelets, platelet lysates, and platelet extracellular vesicles in cell therapy, regenerative medicine, and targeted drug delivery

Platelets are small anucleated blood cells primarily known for their vital hemostatic role. Allogeneic platelet concentrates (PCs) collected from healthy donors are an essential cellular product transfused by hospitals to control or prevent bleeding in patients affected by thrombocytopenia or platelet dysfunctions. Platelets fulfill additional essential functions in innate and adaptive immunity and inflammation, as well as in wound-healing and tissue-repair mechanisms. Platelets contain mitochondria, lysosomes, dense granules, and alpha-granules, which collectively are a remarkable reservoir of multiple trophic factors, enzymes, and signaling molecules. In addition, platelets are prone to release in the blood circulation a unique set of extracellular vesicles (p-EVs), which carry a rich biomolecular cargo influential in cell-cell communications. The exceptional functional roles played by platelets and p-EVs explain the recent interest in exploring the use of allogeneic PCs as source material to develop new biotherapies that could address needs in cell therapy, regenerative medicine, and targeted drug delivery. Pooled human platelet lysates (HPLs) can be produced from allogeneic PCs that have reached their expiration date and are no longer suitable for transfusion but remain valuable source materials for other applications. These HPLs can substitute for fetal bovine serum as a clinical grade xeno-free supplement of growth media used in the in vitro expansion of human cells for transplantation purposes. The use of expired allogeneic platelet concentrates has opened the way for small-pool or large-pool allogeneic HPLs and HPL-derived p-EVs as biotherapy for ocular surface disorders, wound care and, potentially, neurodegenerative diseases, osteoarthritis, and others. Additionally, allogeneic platelets are now seen as a readily available source of cells and EVs that can be exploited for targeted drug delivery vehicles. This article aims to offer an in-depth update on emerging translational applications of allogeneic platelet biotherapies while also highlighting their advantages and limitations as a clinical modality in regenerative medicine and cell therapies.

The proteins derived from platelet-rich plasma improve the endothelialization and vascularization of small diameter vascular grafts

Vascular transplantation has become an ideal substitute for heart and peripheral vascular bypass therapy and tissue-engineered vascular grafts (TEVGs) present an attractive potential solution for vascular surgery. However, small diameter (Ф < 6 mm) vascular do not have ideal TEVGs for clinical use. Platelet-rich plasma (PRP), a key source of bioactive molecules, has been confirmed to promote tissue repair and regeneration. In this study, we prepared PRP-loaded TEVGs (PRP-TEVGs) by electrospinning, investigated the characterization of TEVGs, and verified the effect of PRP-TEVGs in vivo and in vitro experiments. The results suggested that PRP-TEVGs had good biocompatibility, released growth factors stably, promoted cell proliferation and migration significantly, up-regulated the expression of endothelial NO synthase (eNOS) in functional vascular endothelial cells (VECs), and maintained the stability of the endothelial structure. In vivo experiments suggest that PRP can promote rapid endothelialization and reconstruction of TEVGs. Overall, this finding indicated that PRP could promote the rapid vascular endothelialization of small-diameter TEVGs by improving contractile vascular smooth muscle cells (VSMCs) regeneration, and maintaining the integrity and functionality of VECs.

Regulatory Effects of Curcumin on Platelets: An Update and Future Directions

The rhizomatous plant turmeric, which is frequently used as a spice and coloring ingredient, yields curcumin, a bioactive compound. Curcumin inhibits platelet activation and aggregation and improves platelet count. Platelets dysfunction results in several disorders, including inflammation, atherothrombosis, and thromboembolism. Several studies have proved the beneficial role of curcumin on platelets and hence proved it is an important candidate for the treatment of the aforementioned diseases. Moreover, curcumin is also frequently employed as an anti-inflammatory agent in conventional medicine. In arthritic patients, it has been shown to reduce the generation of pro-inflammatory eicosanoids and to reduce edema, morning stiffness, and other symptoms. Curcumin taken orally also reduced rats' acute inflammation brought on by carrageenan. Curcumin has also been proven to prevent atherosclerosis and platelet aggregation, as well as to reduce angiogenesis in adipose tissue. In the cerebral microcirculation, curcumin significantly lowered platelet and leukocyte adhesion. It largely modulated the endothelium to reduce platelet adhesion. Additionally, P-selectin expression and mice survival after cecal ligation and puncture were improved by curcumin, which also altered platelet and leukocyte adhesion and blood-brain barrier dysfunction. Through regulating many processes involved in platelet aggregation, curcuminoids collectively demonstrated detectable antiplatelet activity. Curcuminoids may therefore be able to prevent disorders linked to platelet activation as possible therapeutic agents. This review article proposes to highlight and discuss the regulatory effects of curcumin on platelets.

Platelet Reactivity and Inflammatory Phenotype Induced by Full-Length Spike SARS-CoV-2 Protein and Its RBD Domain

A state of immunothrombosis has been reported in COVID-19. Platelets actively participate in this process. However, little is known about the ability of SARS-CoV-2 virus proteins to induce platelet activity. Platelet-rich plasma (PRP) was incubated with spike full-length protein and the RBD domain in independent assays. We evaluated platelet activation through the expression of P-selectin and activation of glicoprotein IIbIIIa (GP IIbIIIa), determined by flow cytometry and the ability of the proteins to induce platelet aggregation. We determined concentrations of immunothrombotic biomarkers in PRP supernatant treated with the proteins. We determined that the spike full-length proteins and the RBD domain induced an increase in P-selectin expression and GP IIbIIIa activation (p < 0.0001). We observed that the proteins did not induce platelet aggregation, but favored a pro-aggregating state that, in response to minimal doses of collagen, could re-establish the process (p < 0.0001). On the other hand, the viral proteins stimulated the release of interleukin 6, interleukin 8, P-selectin and the soluble fraction of CD40 ligand (sCD40L), molecules that favor an inflammatory state p < 0.05. These results indicate that the spike full-length protein and its RBD domain can induce platelet activation favoring an inflammatory phenotype that might contribute to the development of an immunothrombotic state.

Tirofiban potentiates agonist-induced platelet activation and degranulation, despite effectively inhibiting aggregation

We aimed to investigate the effects of integrin αIIbβ3 inhibitor tirofiban on hallmarks of platelet activation, degranulation, and aggregation during its use to analyze activated but non-complexed platelets via flow cytometry. To do so, we used washed platelets from healthy human donors. We combined aggregometry, an assay of platelet functionality, with flow cytometry and ELISA to detect and correlate, respectively, platelet aggregation, activation, and granule release. While tirofiban effectively inhibited agonist-induced platelet aggregation (thrombin receptor-activating peptide 6 (TRAP), convulxin (CVX), U46619 and IV.3), the surface expression of P-selectin and CD63 and granule release of RANTES were significantly increased, indicating that tirofiban enhances degranulation, uncoupled from aggregation. The results show that tirofiban alters the activation phenotype of platelets, something that should be considered when using tirofiban to enable flow cytometric analysis of activated but unaggregated platelet suspensions.

Targeting transgenic proteins to alpha granules for platelet-directed gene therapy

Platelets are anucleate blood cells that are shed from megakaryocytes (MKs) into the bloodstream to maintain hemostasis and promote wound healing after vascular injury. To carry out their functions, platelets become activated and release bioactive substances from their secretory granules. As alpha granules (αGs) in resting platelets store proteins and release them only after activation, the packaging of proteins into αGs is an attractive strategy to deliver therapeutic proteins. Here, we propose an adjustable model for targeting transgenic proteins to platelet αGs using third-generation self-inactivating lentiviral vectors. The vectors express from the murine platelet factor 4 promoter (mPf4P), restricting transgene expression to the MK lineage. For the delivery and retention of expressed proteins in αGs, proteins are fused to short peptide sorting signals derived from the human cytokine RANTES or from the transmembrane protein P-selectin. We demonstrate effective targeting of GFP to αGs of murine and human in vitro-differentiated MKs and murine platelets in vivo. Furthermore, interferon-α (IFNα), as a potentially therapeutic cytokine, was successfully delivered to and stored in murine platelets in vivo, was released after activation, and inhibited virus replication in vitro. Our vectors create possibilities for numerous applications in cell therapy utilizing platelets as carriers of therapeutic proteins.

Role of β1 integrin in thrombocytopoiesis

Thrombocytopoiesis is a complex process beginning at the level of hematopoietic stem cells, which ultimately generate megakaryocytes, large marrow cells with a distinctive morphology, and then, through a process of terminal maturation, megakaryocytes shed thousands of platelets into the circulation. This process is controlled by intrinsic and extrinsic factors. Emerging data indicate that an important intrinsic control on the late stages of thrombopoiesis is exerted by integrins, a family of transmembrane receptors composed of one α and one β subunit. One β subunit expressed by megakaryocytes is the β1 integrin, the role of which in the regulation of platelet formation is beginning to be clarified. Here, we review recent data indicating that activation of β1 integrin by outside-in and inside-out signaling regulates the interaction of megakaryocytes with the endosteal niche, which triggers their maturation, while its inactivation by galactosylation determines the migration of these cells to the perivascular niche, where they complete their terminal maturation and release platelets in the bloodstream. Furthermore, β1 integrin mediates the activation of transforming growth factor β (TGF-β), a protein produced by megakaryocytes that may act in an autocrine fashion to halt their maturation and affect the composition of their surrounding extracellular matrix. These findings suggest that β1 integrin could be a therapeutic target for inherited and acquired disorders of platelet production.

Platelets as messengers of early-stage cancer

Platelets have an important role in tumor angiogenesis, growth, and metastasis. The reciprocal interaction between cancer and platelets results in changes of several platelet characteristics. It is becoming clear that analysis of these platelet features could offer a new strategy in the search for biomarkers of cancer. Here, we review the human studies in which platelet characteristics (e.g., count, volume, protein, and mRNA content) are investigated in early-stage cancer. The main focus of this paper is to evaluate which platelet features are suitable for the development of a blood test that could detect cancer in its early stages.

Megakaryocytes package contents into separate α-granules that are differentially distributed in platelets

In addition to their primary roles in hemostasis and thrombosis, platelets participate in many other physiological and pathological processes, including, but not limited to inflammation, wound healing, tumor metastasis, and angiogenesis. Among their most interesting properties is the large number of bioactive proteins stored in their α-granules, the major storage granule of platelets. We previously showed that platelets differentially package pro- and antiangiogenic proteins in distinct α-granules that undergo differential release upon platelet activation. Nevertheless, how megakaryocytes achieve differential packaging is not fully understood. In this study, we use a mouse megakaryocyte culture system and endocytosis assay to establish when and where differential packaging occurs during platelet production. Live cell microscopy of primary mouse megakaryocytes incubated with fluorescently conjugated fibrinogen and endostatin showed differential endocytosis and packaging of the labeled proteins into distinct α-granule subpopulations. Super-resolution microscopy of mouse proplatelets and human whole-blood platelet α-granules simultaneously probed for 2 different membrane proteins (VAMP-3 and VAMP-8), and multiple granular content proteins (bFGF, ENDO, TSP, VEGF) confirmed differential packaging of protein contents into α-granules. These data suggest that megakaryocytes differentially sort and package α-granule contents, which are preserved as α-granule subpopulations during proplatelet extension and platelet production.

Platelet Toll-Like Receptors Mediate Thromboinflammatory Responses in Patients With Essential Thrombocythemia

Essential thrombocythemia (ET) is comprised among chronic myeloproliferative neoplasms (MPN) and is caused by driver mutations in JAK2, CALR, and MPL, which lead to megakaryocyte proliferation and prominent thrombocytosis. Thrombosis remains the main cause of morbidity in ET and is driven by the interplay between blood cells, the endothelium, the clotting cascade, and host-derived inflammatory mediators. Platelet activation plays a key role in the thrombotic predisposition, although the underlying mechanisms remain poorly defined. In addition to their role in hemostasis, platelets participate in innate immunity and inflammation owing to the expression of toll-like receptors (TLR), which recognize inflammatory signals, triggering platelet functional responses. Considering the impact of inflammation on ET procoagulant state, we assessed the contribution of TLR2 and TLR4 to platelet hemostatic and inflammatory properties in ET patients, by using Pam3CSK4 and lipopolysaccharide (LPS) as specific TLR2 and TLR4 ligands, respectively. TLR2 ligation induced increased surface translocation of α-granule-derived P-selectin and CD40L, which mediate platelet interaction with leukocytes and endothelial cells, respectively, and higher levels of dense granule-derived CD63 in patients, whereas PAC-1 binding was not increased and LPS had no effect on these platelet responses. Platelet-neutrophil aggregate formation was elevated in ET at baseline and after stimulation of both TLR2 and TLR4. In addition, ET patients displayed higher TLR2- and TLR4-triggered platelet secretion of the chemokine RANTES (CCL5), whereas von Willebrand factor release was not enhanced, revealing a differential releasate pattern for α-granule-stored inflammatory molecules. TLR-mediated hyperresponsiveness contrasted with impaired or preserved responses to classic platelet hemostatic agonists, such as TRAP-6 and thrombin. TLR2 and TLR4 expression on the platelet surface was normal, whereas phosphorylation of downstream effector ERK1/2 was higher in patients at baseline and after incubation with Pam3CSK4, which may partly explain the enhanced TLR2 response. In conclusion, exacerbated response to TLR stimulation may promote platelet activation in ET, boosting platelet/leukocyte/endothelial interactions and secretion of inflammatory mediators, overall reinforcing the thromboinflammatory state. These findings highlight the role of platelets as inflammatory sentinels in MPN prothrombotic scenario and provide additional evidence for the close intertwining between thrombosis and inflammation in this setting.

Hemostasis stimulates lymphangiogenesis through release and activation of VEGFC

Hemostasis associated with tissue injury is followed by wound healing, a complex process by which damaged cellular material is removed and tissue repaired. Angiogenic responses are a central aspect of wound healing, including the growth of new lymphatic vessels by which immune cells, protein, and fluid are transported out of the wound area. The concept that hemostatic responses might be linked to wound healing responses is an old one, but demonstrating such a link in vivo and defining specific molecular mechanisms by which the 2 processes are connected has been difficult. In the present study, we demonstrate that the lymphangiogenic factors vascular endothelial growth factor C (VEGFC) and VEGFD are cleaved by thrombin and plasmin, serine proteases generated during hemostasis and wound healing. Using a new tail-wounding assay to test the relationship between clot formation and lymphangiogenesis in mice, we find that platelets accelerate lymphatic growth after injury in vivo. Genetic studies reveal that platelet enhancement of lymphatic growth after wounding is dependent on the release of VEGFC, but not VEGFD, a finding consistent with high expression of VEGFC in both platelets and avian thrombocytes. Analysis of lymphangiogenesis after full-thickness skin excision, a wound model that is not associated with significant clot formation, also revealed an essential role for VEGFC, but not VEGFD. These studies define a concrete molecular and cellular link between hemostasis and lymphangiogenesis during wound healing and reveal that VEGFC, the dominant lymphangiogenic factor during embryonic development, continues to play a dominant role in lymphatic growth in mature animals.

Exogenous carbon monoxide suppresses LPS-Induced platelet SNAREs complex assembly and α-granule exocytosis via integrin αIIbβ3-Mediated PKCθ/Munc18a pathway

Activation of coagulation occurs in sepsis and contributes to the development of thrombosis. Platelet α-granule exocytosis plays an important role in septic coagulation abnormalities. The present study aimed to investigate the effects and the underlying mechanisms of exogenous carbon monoxide, carbon monoxide-releasing molecules II (CORM-2)-liberated CO, on suppressing platelet α-granule exocytosis in sepsis. It was shown that CORM-2 weakened α-granule membrane fusion with platelet plasma membrane and attenuated α-granule contents exocytosis in LPS-Induced platelet. Further studies revealed that CORM-2 suppressed the expression of integrin α_IIbβ₃ in platelets stimulated by LPS. This was accompanied by a decrease in production and phosphorylation of PKCθ and Munc18a, SNARE complex assembly and subsequently platelet α-granule exocytosis. Taken together, we suggested that the potential mechanism of suppressive effect of CORM-2 on LPS-induced platelet SNAREs complex assembly and α-Granule Exocytosis might involve integrin α_IIbβ₃-mediated PKCθ/Munc18a pathway activation.

Platelets and platelet extracellular vesicles in hemostasis and sepsis

Platelets, cell fragments traditionally thought of as important only for hemostasis, substantially and dynamically contribute to the immune system's response to infection. In addition, there is increasing evidence that externally active platelet entities, including platelet granules and platelet extracellular vesicles (PEVs), play a role not only in hemostasis, but also in inflammatory actions previously ascribed to platelets themselves. Given the functions of platelets and PEVs during inflammation and infection, their role in sepsis is being investigated. Sepsis is a condition marked by the dysregulation of the body's normal activation of the immune system in response to a pathogen. The mechanisms for controlling infection locally become detrimental to the host if they are applied systemically. Similar to cells traditionally ascribed to the immune system, including neutrophils, lymphocytes, and macrophages, platelets are instrumental in helping a host clear an infection, but are also implicated in the uncontrolled amplification of the immune response that leads to sepsis. Clearly, the function of platelets is more complicated than its simple structure and primary role in hemostasis initially suggest. This review provides an overview of platelet and platelet extracellular vesicle structure and function, highlighting the complex role platelets and PEVs play in the body in the context of infection and sepsis.

Leukocyte-rich PRP versus leukocyte-poor PRP - The role of monocyte/macrophage function in the healing cascade

The mechanism of action of Platelet Rich Plasma (PRP) is thought to be related to the biomolecules present in α-granules. However, for the healing process to occur, an inflammatory phase is also deemed necessary. Leukocytes present in the inflammatory phase release both pro- and anti-inflammatory molecules. The latter may play an important role in the process of "inflammatory regeneration". Thus, we propose that in the context of healing, both platelets and leukocytes play an important role, specifically due to the macrophage's plasticity to switch from the M1 to M2 fraction. Therefore, we propose that PRP products derived from the buffy coat may be more beneficial than detrimental from a standpoint of the regenerative potential of PRP.

The non-haemostatic role of platelets in systemic lupus erythematosus

Dysregulation of lymphocyte function, accumulation of autoantibodies and defective clearance of circulating immune complexes and apoptotic cells are hallmarks of systemic lupus erythematosus (SLE). Moreover, it is now evident that an intricate interplay between the adaptive and innate immune systems contributes to the pathogenesis of SLE, ultimately resulting in chronic inflammation and organ damage. Platelets circulate in the blood and are chiefly recognized for their role in the prevention of bleeding and promotion of haemostasis; however, accumulating evidence points to a role for platelets in both adaptive and innate immunity. Through a broad repertoire of receptors, platelets respond promptly to immune complexes, complement and damage-associated molecular patterns, and represent a major reservoir of immunomodulatory molecules in the circulation. Furthermore, evidence suggests that platelets are activated in patients with SLE, and that they could contribute to the circulatory autoantigenic load through the release of microparticles and mitochondrial antigens. Herein, we highlight how platelets contribute to the immune response and review evidence implicating platelets in the pathogenesis of SLE.

Platelets in Skin Autoimmune Diseases

Systemic lupus erythematosus (SLE), systemic sclerosis (SSc), and small vessel vasculitis are three autoimmune diseases frequently manifested in the skin. They share common pathogenic features, including production of autoantibodies, loss of tolerance to self-antigens, tissue necrosis and fibrosis, vasculopathy and activation of the coagulation system. Platelets occupy a central part within the coagulation cascade and are well-recognized for their hemostatic role. However, recent cumulative evidence implicates their additional and multifaceted immunoregulatory functions. Platelets express immune receptors and they store growth factors, cytokines, and chemokines in their granules enabling a significant contribution to inflammation. A plethora of activating triggers such as damage associated molecular patterns (DAMPs) released from damaged endothelial cells, immune complexes, or complement effector molecules can mediate platelet activation. Activated platelets further foster an inflammatory environment and the crosstalk with the endothelium and leukocytes by the release of immunoactive molecules and microparticles. Further insight into the pathogenic implications of platelet activation will pave the way for new therapeutic strategies targeting autoimmune diseases. In this review, we discuss the inflammatory functions of platelets and their mechanistic contribution to the pathophysiology of SSc, ANCA associated small vessel vasculitis and other autoimmune diseases affecting the skin.

Platelets and Platelet-Derived Extracellular Vesicles in Liver Physiology and Disease

Beyond their role in hemostasis, platelets are proposed as key mediators of several physiological and pathophysiological processes of the liver, such as liver regeneration, toxic or viral acute liver injury, liver fibrosis, and carcinogenesis. The effects of platelets on the liver involve interactions with sinusoidal endothelial cells and the release of platelet‐contained molecules following platelet activation. Platelets are the major source of circulating extracellular vesicles, which are suggested to play key roles in platelet interactions with endothelial cells in several clinical disorders. In the present review, we discuss the implications of platelet‐derived extracellular vesicles in physiological and pathophysiological processes of the liver.

TREM-like transcript 1: a more sensitive marker of platelet activation than P-selectin in humans and mice

Platelet activation in vitro results in a more rapid and greater upregulation of TLT-1 surface expression compared with P-selectin. TLT-1 is more rapidly translocated to the surface of activated platelets than P-selectin during thrombus formation in vivo.

Tunable activation of therapeutic platelet-rich plasma by pulse electric field: Differential effects on clot formation, growth factor release, and platelet morphology

BACKGROUND

Activation of platelet-rich plasma (PRP) by pulse electric field (PEF) releases growth factors which promote wound healing (e.g., PDGF, VEGF for granulation, EGF for epithelialization).

AIMS

To determine after PEF activation of therapeutic PRP: 1) platelet gel strength; 2) profile of released growth factors; 3) alpha- and T-granule release; 4) platelet morphology.

METHODS

Concentrated normal donor PRP was activated by 5 μsec (long) monopolar pulse, ~4000 V/cm (PEF A) or 150 nsec (short) bipolar pulse, ~3000 V/cm (PEF B) in the presence of 2.5 mM (low) or 20 mM (high) added CaCl2. Clot formation was evaluated by thromboelastography (TEG). Surface exposure of alpha granule (P-selectin) and T-granule (TLR9 and protein disulfide isomerase [PDI]) markers were assessed by flow cytometry. Factors in supernatants of activated PRP were measured by ELISA. Platelet morphology was evaluated by transmission electron microscopy (TEM).

RESULTS

Time to initial clot formation was shorter with thrombin (<1 min) than with PEF A and B (4.4-8.7 min) but clot strength (elastic modulus, derived from TEG maximum amplitude) was greater with PEF B than with either thrombin or PEF A (p<0.05). Supernatants of PRP activated with PEF A had higher EGF levels than supernatants from all other conditions. In contrast, levels of PF4, PDGF, and VEGF in supernatants were not significantly different after PEF A, PEF B, and thrombin activation. T-granule markers (TLR9 and PDI) were higher after thrombin than after PEF A or B with low or high CaCl2. By TEM, platelets in PEF-treated samples retained a subset of granules suggesting regulated granule release.

CONCLUSION

Pulse length and polarity can be modulated to produce therapeutic platelet gels as strong or stronger than those produced by thrombin, and this is tunable to produce growth factor profiles enhanced in specific factors important for different stages of wound healing.

Mouse Platelet Ral GTPases Control P-Selectin Surface Expression, Regulating Platelet-Leukocyte Interaction

OBJECTIVE

RalA and RalB GTPases are important regulators of cell growth, cancer metastasis, and granule secretion. The purpose of this study was to determine the role of Ral GTPases in platelets with the use of platelet-specific gene-knockout mouse models.

APPROACH AND RESULTS

This study shows that platelets from double knockout mice, in which both GTPases have been deleted, show markedly diminished (≈85% reduction) P-selectin translocation to the surface membrane, suggesting a critical role in α-granule secretion. Surprisingly, however, there were only minor effects on stimulated release of soluble α- and δ-granule content, with no alteration in granule count, morphology, or content. In addition, their expression was not essential for platelet aggregation or thrombus formation. However, absence of surface P-selectin caused a marked reduction (≈70%) in platelet-leukocyte interactions in blood from RalAB double knockout mice, suggesting a role for platelet Rals in platelet-mediated inflammation.

CONCLUSIONS

Platelet Ral GTPases primarily control P-selectin surface expression, in turn regulating platelet-leukocyte interaction. Ral GTPases could therefore be important novel targets for the selective control of platelet-mediated immune cell recruitment and inflammatory disease.

An optimised protocol for platelet-rich plasma preparation to improve its angiogenic and regenerative properties

Although platelet-rich plasma (PRP) is used as a source of growth factors in regenerative medicine, its effectiveness remains controversial, partially due to the absence of PRP preparation protocols based on the regenerative role of platelets. Here, we aimed to optimise the protocol by analysing PRP angiogenic and regenerative properties. Three optimising strategies were evaluated: dilution, 4 °C pre-incubation, and plasma cryoprecipitate supplementation. Following coagulation, PRP releasates (PRPr) were used to induce angiogenesis in vitro (HMEC-1 proliferation, migration, and tubule formation) and in vivo (chorioallantoic membrane), as well as regeneration of excisional wounds on mouse skin. Washed platelet releasates induced greater angiogenesis than PRPr due to the anti-angiogenic effect of plasma, which was decreased by diluting PRPr with saline. Angiogenesis was also improved by both PRP pre-incubation at 4 °C and cryoprecipitate supplementation. A combination of optimising variables exerted an additive effect, thereby increasing the angiogenic activity of PRPr from healthy donors and diabetic patients. Optimised PRPr induced faster and more efficient mouse skin wound repair compared to that induced by non-optimised PRPr. Acetylsalicylic acid inhibited angiogenesis and tissue regeneration mediated by PRPr; this inhibition was reversed following optimisation. Our findings indicate that PRP pre-incubation at 4 °C, PRPr dilution, and cryoprecipitate supplementation improve the angiogenic and regenerative properties of PRP compared to the obtained by current methods.

Integrin β3/Akt signaling contributes to platelet-induced hemangioendothelioma growth

Hemangioendothelioma (HE) is a type of angiomatous lesions that features endothelial cell proliferation. Understanding the mechanisms orchestrating HE angiogenesis can provide therapeutic insights. It has been shown that platelets can support normal and malignant endothelial cells during angiogenesis. Using the mouse endothelial-derived EOMA cell line as a model of HE, we explored the regulatory effect of platelets. We found that platelets stimulated EOMA proliferation but did not mitigate apoptosis. Furthermore, direct platelet-EOMA cell contact was required and the proliferation was mediated via integrin β3/Akt signaling in EOMA cells. SiRNA knockdown of integrin β3 and inhibition of Akt activity significantly abolished platelet-induced EOMA cell proliferation in vitro and tumor development in vivo. These results provide a new mechanism by which platelets support HE progression and suggest integrin β3 as a potential target to treat HE.

Resveratrol suppresses pulmonary tumor metastasis by inhibiting platelet-mediated angiogenic responses

BACKGROUND

To explore the impact of Resveratrol (RSV) on the angiogenic potential of activated platelets and to elucidate the underlying mechanism.

METHODS

Vascular endothelial growth factor concentrations were measured by enzyme-linked immunosorbent assay. Capillary tube formation assay was used to examine the impact of RSV on the angiogenic potential of activated platelets. The levels of cyclic adenosine monophosphate and cyclic guanosine monophosphate (cGMP) in the supernatant were evaluated using corresponding enzyme-linked immunosorbent assay kits. Immunoblotting assays were used to determine the expression of vasodilator-stimulated phosphoprotein and Akt phosphorylation. A pulmonary metastasis experiment with male nude mice model was performed to test the effect of RSV on pulmonary metastasis and angiogenesis in vivo.

RESULTS

RSV inhibited platelets-mediated angiogenic responses induced by adenosine diphosphate (ADP)ADP through increased cGMP generation and cGMP-mediated vasodilator-stimulated phosphoprotein phosphorylation along with reduced intracellular Ca²⁺ mobilization. In addition, RSV attenuated the platelet secretion and angiogenic responses induced by A549 cells in vitro and suppressed A549 lung cancer metastasis and angiogenesis in nude mice.

CONCLUSIONS

RSV is a potential therapeutic drug for the prevention of tumor metastasis by interrupting the platelet-tumor cell amplification loop.

The transcription factor GATA1 regulates NBEAL2 expression through a long-distance enhancer

Gray platelet syndrome is named after the gray appearance of platelets due to the absence of α-granules. It is caused by recessive mutations in NBEAL2, resulting in macrothrombocytopenia and myelofibrosis. Though using the term gray platelets for GATA1 deficiency has been debated, a reduced number of α-granules has been described for macrothrombocytopenia due to GATA1 mutations. We compared platelet size and number of α-granules for two NBEAL2 and two GATA1-deficient patients and found reduced numbers of α-granules for all, with the defect being more pronounced for NBEAL2 deficiency. We further hypothesized that the granule defect for GATA1 is due to a defective control of NBEAL2 expression. Remarkably, platelets from two patients, and Gata1-deficient mice, expressed almost no NBEAL2. The differentiation of GATA1 patient-derived CD34⁺ stem cells to megakaryocytes showed defective proplatelet and α-granule formation with strongly reduced NBEAL2 protein and ribonucleic acid expression. Chromatin immunoprecipitation sequencing revealed 5 GATA binding sites in a regulatory region 31 kb upstream of NBEAL2 covered by a H3K4Me1 mark indicative of an enhancer locus. Luciferase reporter constructs containing this region confirmed its enhancer activity in K562 cells, and mutagenesis of the GATA1 binding sites resulted in significantly reduced enhancer activity. Moreover, DNA binding studies showed that GATA1 and GATA2 physically interact with this enhancer region. GATA1 depletion using small interfering ribonucleic acid in K562 cells also resulted in reduced NBEAL2 expression. In conclusion, we herein show a long-distance regulatory region with GATA1 binding sites as being a strong enhancer for NBEAL2 expression.

Normal Wound Healing and Tumor Angiogenesis as a Game of Competitive Inhibition

Both normal wound healing and tumor angiogenesis are mitigated by the sequential, carefully orchestrated release of growth stimulators and inhibitors. These regulators are released from platelet clots formed at the sites of activated endothelium in a temporally and spatially controlled manner, and the order of their release depends on their affinity to glycosaminoglycans (GAG) such as heparan sulfate (HS) within the extracellular matrix, and platelet open canallicular system. The formation of vessel sprouts, triggered by angiogenesis regulating factors with lowest affinities for heparan sulfate (e.g. VEGF), is followed by vessel-stabilizing PDGF-B or bFGF with medium affinity for HS, and by inhibitors such as PF-4 and TSP-1 with the highest affinities for HS. The invasive wound-like edge of growing tumors has an overabundance of angiogenesis stimulators, and we propose that their abundance out-competes angiogenesis inhibitors, effectively preventing inhibition of angiogenesis and vessel maturation. We evaluate this hypothesis using an experimentally motivated agent-based model, and propose a general theoretical framework for understanding mechanistic similarities and differences between the processes of normal wound healing and pathological angiogenesis from the point of view of competitive inhibition.

Respective contributions of single and compound granule fusion to secretion by activated platelets

Although granule secretion is pivotal in many platelet responses, the fusion routes of α and δ granule release remain uncertain. We used a 3D reconstruction approach based on electron microscopy to visualize the spatial organization of granules in unstimulated and activated platelets. Two modes of exocytosis were identified: a single mode that leads to release of the contents of individual granules and a compound mode that leads to the formation of granule-to-granule fusion, resulting in the formation of large multigranular compartments. Both modes occur during the course of platelet secretion. Single fusion events are more visible at lower levels of stimulation and early time points, whereas large multigranular compartments are present at higher levels of agonist and at later time points. Although α granules released their contents through both modes of exocytosis, δ granules underwent only single exocytosis. To define the underlying molecular mechanisms, we examined platelets from vesicle-associated membrane protein 8 (VAMP8) null mice. After weak stimulation, compound exocytosis was abolished and single exocytosis decreased in VAMP8 null platelets. Higher concentrations of thrombin bypassed the VAMP8 requirement, indicating that this isoform is a key but not a required factor for single and/or compound exocytosis. Concerning the biological relevance of our findings, compound exocytosis was observed in thrombi formed after severe laser injury of the vessel wall with thrombin generation. After superficial injury without thrombin generation, no multigranular compartments were detected. Our studies suggest that platelets use both modes of membrane fusion to control the extent of agonist-induced exocytosis.

Are the changes in the peripheral brain-derived neurotrophic factor levels due to platelet activation?

Brain-derived neurotrophic factor (BDNF) plays an important role in central nervous system development, neurogenesis and neuronal plasticity. BDNF is also expressed in several non-neuronal tissues, and it could play an important role in other processes, such as cancer, angiogenesis, etc. Platelets are the major source of peripheral BDNF. However, platelets also contain high amounts of serotonin; they express specific surface receptors during activation, and a multitude of pro-inflammatory and immunomodulatory bioactive compounds are secreted from the granules. Until recently, there was insufficient knowledge regarding the relationship between BDNF and platelets. Recent studies showed that BDNF is present in two distinct pools in platelets, in α-granules and in the cytoplasm, and only the BDNF in the granules is secreted following stimulation, representing 30% of the total BDNF in platelets. BDNF has an important role in the pathophysiology of depression. Low levels of serum BDNF have been described in patients with major depressive disorder, and BDNF levels increased with chronic antidepressant treatment. Interestingly, there is an association between depression and platelet function. This review analyzed studies that evaluated the relationship between BDNF and platelet activation and the effect of treatments on both parameters. Only a few studies consider this possible confounding factor, and it could be very important in diseases such as depression, which show changes in both parameters.

Assessment of canine autologous platelet-rich plasma produced with a commercial centrifugation and platelet recovery kit

OBJECTIVES

To characterize the cellular composition (platelets, erythrocytes, and leukocytes) and confirm reproducibility of platelet enrichment, as well as determine the platelet activation status in the final product of a commercial platelet-rich plasma kit using canine blood.

METHODS

Venous blood from 20 sedated client-owned dogs was used to prepare platelet-rich plasma (PRP) from a commercial kit. Complete blood counts were performed to determine erythrocyte, leukocyte, and platelet numbers in both whole blood (WB) and resultant PRP. The WB and PRP samples from jugular (fast collection) and cephalic (slow collection) venipuncture were also compared. P-selectin externalization was measured in WB and PRP samples from 15 of 20 dogs.

RESULTS

This commercial kit produced an average percent recovery in platelets of 64.7 ± 17.4; erythrocytes of 3.7 ± 0.8, and leukocytes of 31.6 ± 10.0. Neutrophil, monocyte, and lymphocyte percent recovery was 19.6 ± 7.2, 44.89 ± 19.8, and 57.5 ± 10.6, respectively. The recovery of platelets from jugular venipuncture (59.7 ± 13.6%) was lower than from cephalic recovery (68.8 ± 19.1%). The mean percent P-Selectin externalization for WB, PRP, and PRP with thrombin was 25.5 ± 30.9, 4.5 ± 6.4, and 90.6 ± 4.4 respectively.

CLINICAL SIGNIFICANCE

Cellular reproducibility of this kit was confirmed and platelets were concentrated within autologous serum. Additionally, measurements of P-selectin externalization showed that platelets are inactive in PRP unless stimulated to degranulate.

The angiogenic responses induced by release of angiogenic proteins from tumor cell-activated platelets are regulated by distinct molecular pathways

There is mounting evidence that tumor angiogenesis can be regulated by platelets (Plts), which serve as major sources and delivery vehicles of many proangiogenic cytokines including transforming growth factor-β and vascular endothelial growth factor. Although considerable progress has been made in understanding the role for Plt secretion in tumor angiogenesis, very little is known about the precise mechanisms underlying cancer cell induction of Plt granule release. Here, we demonstrated that nonsmall cell lung cancer (NSCLC) cells directly induced Plt secretion of several angiogenic regulatory cytokines that promoted angiogenesis in concert. Moreover, we discovered that these Plt-derived angiogenesis modulators were regulated by different molecular pathways and could be largely inhibited by combination of multiple signaling inhibitors. Our present studies indicated that manipulation of Plt secretion of angiogenic cytokines without compromising hemostatic functions could provide a novel option for management of tumor angiogenesis and metastasis in NSCLC patients with thrombocytosis.

Platelet-rich plasma stimulated by pulse electric fields: Platelet activation, procoagulant markers, growth factor release and cell proliferation

Therapeutic use of activated platelet-rich plasma (PRP) has been explored for wound healing, hemostasis and antimicrobial wound applications. Pulse electric field (PEF) stimulation may provide more consistent platelet activation and avoid complications associated with the addition of bovine thrombin, the current state of the art ex vivo activator of therapeutic PRP. The aim of this study was to compare the ability of PEF, bovine thrombin and thrombin receptor activating peptide (TRAP) to activate human PRP, release growth factors and induce cell proliferation in vitro. Human PRP was prepared in the Harvest SmartPreP2 System and treated with vehicle, PEF, bovine thrombin, TRAP or Triton X-100. Platelet activation and procoagulant markers and microparticle generation were measured by flow cytometry. Released growth factors were measured by ELISA. The releasates were tested for their ability to stimulate proliferation of human epithelial cells in culture. PEF produced more platelet-derived microparticles, P-selectin-positive particles and procoagulant annexin V-positive particles than bovine thrombin or TRAP. These differences were associated with higher levels of released epidermal growth factor after PEF than after bovine thrombin or TRAP but similar levels of platelet-derived, vascular-endothelial, and basic fibroblast growth factors, and platelet factor 4. Supernatant from PEF-treated platelets significantly increased cell proliferation compared to plasma. In conclusion, PEF treatment of fresh PRP results in generation of microparticles, exposure of prothrombotic platelet surfaces, differential release of growth factors compared to bovine thrombin and TRAP and significant cell proliferation. These results, together with PEF's inherent advantages, suggest that PEF may be a superior alternative to bovine thrombin activation of PRP for therapeutic applications.

Bacteria-induced intracellular signalling in platelets

Multiple studies have now shown that various species of bacteria can stimulate platelets; many in a strain and donor-dependent manner. The signalling pathways underlying this platelet activation has been the subject of scrutiny for the last decade. The best-delineated pathway is that in response to Streptococcal species, such as Streptococcus sanguinis (S. sanguinis), Streptococcus gordonii (S. gordonii) and Streptococcus oralis (S. oralis), where a pathway is initiated by the engagement of the low affinity IgG receptor, FcγRIIA. This leads to and involves the tyrosine kinase Syk, the adaptor protein Linker of Activated T Cells (LAT) and subsequently both phospholipase Cγ2 (PLCγ2) and phosphatidylinositol-3-kinase (PI-3-K). Finally, this leads to the expression of the αIIbβ3 integrin, the synthesis and release of thromboxane A2 (T × A2) and the exocytosis of PF4, each of which plays a crucial role in secondary signalling and full platelet activation. Roles for other signalling pathways in Streptococcal-induced platelet activation are less clear, although an ADP-mediated inhibition of adenylyl cyclase, a glycoprotein Ib/IX/V-mediated pathway and perhaps a complement-induced pathway have each been proposed. Platelet activation by Porphyromonas gingivalis (P. gingivalis) at least partially shares the FcγRIIA/Syk/PLCγ2/PI-3-K mechanism utilised by Streptococcal species. However, it has also been suggested that P. gingivalis activates platelets by two additional methods; stimulation of the protease-activated receptors leading to activation of phospholipase Cβ (PLCβ), and the engagement of Toll-like receptors 2 and 4 by released lipopolysaccharide leading to an ill-defined pathway which may involve PI-3-K. Consequently, it appears that bacteria can stimulate platelets by eliciting multiple signalling pathways some of which are common, and some unique, to individual species.

von Willebrand factor biosynthesis, secretion, and clearance: connecting the far ends

To understand the placement of a certain protein in a physiological system and the pathogenesis of related disorders, it is not only of interest to determine its function but also important to describe the sequential steps in its life cycle, from synthesis to secretion and ultimately its clearance. von Willebrand factor (VWF) is a particularly intriguing case in this regard because of its important auxiliary roles (both intra- and extracellular) that implicate a wide range of other proteins: its presence is required for the formation and regulated release of endothelial storage organelles, the Weibel-Palade bodies (WPBs), whereas VWF is also a key determinant in the clearance of coagulation factor VIII. Thus, understanding the molecular and cellular basis of the VWF life cycle will help us gain insight into the pathogenesis of von Willebrand disease, design alternative treatment options to prolong the factor VIII half-life, and delineate the role of VWF and coresidents of the WPBs in the prothrombotic and proinflammatory response of endothelial cells. In this review, an update on our current knowledge on VWF biosynthesis, secretion, and clearance is provided and we will discuss how they can be affected by the presence of protein defects.

Platelet activation using electric pulse stimulation: growth factor profile and clinical implications

BACKGROUND

Autologous platelet gel therapy using platelet-rich plasma has emerged as a promising alternative for chronic wound healing, hemostasis, and wound infection control. A critical step for this therapeutic approach is platelet activation, typically performed using bovine thrombin (BT) and calcium chloride. However, exposure of humans to BT can stimulate antibody formation, potentially resulting in severe hemorrhagic or thrombotic complications. Electric pulse stimulation using nanosecond PEFs (pulse electric fields) is an alternative, nonbiochemical platelet activation method, thereby avoiding exposure to xenogeneic thrombin and associated risks.

METHODS

In this study, we identified specific requirements for a clinically relevant activator instrument by dynamically measuring current, voltage, and electric impedance for platelet-rich plasma samples. From these samples, we investigated the profile of growth factors released from human platelets with electric pulse stimulation versus BT, specifically platelet-derived growth factor, transforming growth factor β, and epidermal growth factor, using commercial enzyme-linked immunosorbent assay kits.

RESULTS

Electric pulse stimulation triggers growth factor release from platelet α-granules at the same or higher level compared with BT.

CONCLUSION

Electric pulse stimulation is a fast, inexpensive, easy-to-use platelet activation method for autologous platelet gel therapy.

Platelet hyaluronidase-2: an enzyme that translocates to the surface upon activation to function in extracellular matrix degradation

Following injury, platelets rapidly interact with the exposed extracellular matrix (ECM) of the vessel wall and the surrounding tissues. Hyaluronan (HA) is a major glycosaminoglycan component of the ECM and plays a significant role in regulating inflammation. We have recently reported that human platelets degrade HA from the surfaces of activated endothelial cells into fragments capable of inducing immune responses by monocytes. We also showed that human platelets contain the enzyme hyaluronidase-2 (HYAL2), one of two major hyaluronidases that digest HA in somatic tissues. The deposition of HA increases in inflamed tissues in several inflammatory diseases, including inflammatory bowel disease (IBD). We therefore wanted to define the mechanism by which platelets degrade HA in the inflamed tissues. In this study, we show that human platelets degrade the proinflammatory matrix HA through the activity of HYAL2 and that platelet activation causes the immediate translocation of HYAL2 from a distinct population of α-granules to platelet surfaces where it exerts its catalytic activity. Finally, we show that patients with IBD have lower platelet HYAL2 levels and activity than healthy controls.

Of von Willebrand factor and platelets

Hemostasis and pathological thrombus formation are dynamic processes that require multiple adhesive receptor-ligand interactions, with blood platelets at the heart of such events. Many studies have contributed to shed light on the importance of von Willebrand factor (VWF) interaction with its platelet receptors, glycoprotein (GP) Ib-IX-V and αIIbβ3 integrin, in promoting primary platelet adhesion and aggregation following vessel injury. This review will recapitulate our current knowledge on the subject from the rheological aspect to the spatio-temporal development of thrombus formation. We will also discuss the signaling events generated by VWF/GPIb-IX-V interaction, leading to platelet activation. Additionally, we will review the growing body of evidence gathered from the recent development of pathological mouse models suggesting that VWF binding to GPIb-IX-V is a promising target in arterial and venous pathological thrombosis. Finally, the pathological aspects of VWF and its impact on platelets will be addressed.