α-granule biogenesis: from disease to discovery

Acidification of α-granules in megakaryocytes by vacuolar-type adenosine triphosphatase is essential for organelle biogenesis

BACKGROUND

Platelets coordinate blood coagulation at sites of vascular injury and play fundamental roles in a wide variety of (patho)physiological processes. Key to many platelet functions is the transport and secretion of proteins packaged within α-granules, organelles produced by platelet precursor megakaryocytes. Prominent among α-granule cargo are fibrinogen endocytosed from plasma and endogenously synthesized von Willebrand factor. These and other proteins are known to require acidic pH for stable packaging. Luminal acidity has been confirmed for mature α-granules isolated from platelets, but direct measurement of megakaryocyte granule acidity has not been reported.

OBJECTIVES

To determine the luminal pH of α-granules and their precursors in megakaryocytes and assess the requirement of vacuolar-type adenosine triphosphatase (V-ATPase) activity to establish and maintain the luminal acidity and integrity of these organelles.

METHODS

Cresyl violet staining was used to detect acidic granules in megakaryocytes. Endocytosis of fibrinogen tagged with the pH-sensitive fluorescent dye fluorescein isothiocyanate was used to load a subset of these organelles. Ratiometric fluorescence analysis was used to determine their luminal pH.

RESULTS

We show that most of the acidic granules detected in megakaryocytes appear to be α-granules/precursors, for which we established a median luminal pH of 5.2 (IQR, 5.0-5.5). Inhibition of megakaryocyte V-ATPase activity led to enlargement of cargo-containing compartments detected by fluorescence microscopy and electron microscopy.

CONCLUSION

These observations reveal that V-ATPase activity is required to establish and maintain a luminal acidic pH in megakaryocyte α-granules/precursors, confirming its importance for stable packaging of cargo proteins such as von Willebrand factor.

Functional characterization of Bmcap in uric acid metabolism in the silkworm

After a millennium of domestication, numerous silkworm mutants have emerged that exhibit transparent epidermis, which is caused by abnormally low levels of uric acid. We identified the Bombyx mori gene Bmcap (BMSK0003832) as the homolog of cappuccino, a subunit of the biogenesis of lysosome-related organelles complex-1 (BLOC-1) that has been extensively characterized in human, mouse, and insect species, by analyzing the amino acid sequences of putative purine metabolism genes. Using the clustered regularly interspaced palindromic repeats (CRISPR) / CRISPR-associated protein 9 system, we disrupted Bmcap, resulting in decreased uric acid levels in the silkworm epidermis and a translucent skin phenotype. In the Bmcap mutant, the purine metabolism, nitrogen metabolism, pyrimidine metabolism, and membrane system were altered compared to the wild type. Biogenesis of lysosome-related organelle complex genes play a role in the pigmentation and biogenesis of lysosome-related organelles (LROs) in platelets, melanocytes, and megakaryocytes. LROs exhibit unique morphologies and functions in various tissues and cells. Investigation of the Bmcap mutant will enhance our understanding of the uric acid metabolic pathway in silkworms, and this mutant offers a valuable silkworm model for LRO studies.

A pyruvylated and sulfated galactan from the green alga Dictyosphaeria cavernosa: Structure, anticoagulant property and inhibitory effect on zebrafish thrombosis

A pyruvylated and sulfated galactan from the green alga Dictyosphaeria cavernosa, designated PSG, was obtained by dilute alkali extraction, ion-exchange and gel filtration chromatography. The backbone of PSG was composed of 3-linked β-d-Galp units with partial sulfation on C-4 and C-6. Pyruvate ketals were linked to O-3 and O-4 of nonreducing terminal β-d-Galp, as well as O-4 and O-6 of 3-linked β-d-Galp. The branches consisting of 6-linked β-d-Galp(4SO4) and β-d-Galp(3,4-Pyr)-(1→ units were located at C-6 of 3-linked β-d-Galp unit. PSG possessed obvious anticoagulant effect in vitro as assessed by the tests of activated partial thromboplastin time and thrombin time. The assay of anticoagulant mechanism showed that PSG promoted thrombin inactivation mediated by heparin cofactor-II and antithrombin-III (ATIII), and could effectively potentiate factor Xa inactivation by ATIII. The antithrombotic activity of PSG in vivo was assessed by phenylhydrazine (PHZ)-induced zebrafish thrombotic model. The results indicated that PSG obviously reduced peripheral erythrocytes aggregation, enhanced cardiac blood flow and improved peripheral platelet circulation, and PSG possessed a marked inhibitory effect on the PHZ-induced zebrafish thrombosis. Thus, PSG is a hopeful anticoagulant and antithrombotic polysaccharide.

The Sec1/Munc18 protein VPS33B forms a uniquely bidirectional complex with VPS16B

Loss of function variants of VPS33B and VIPAS39 (encoding VPS16B) are causative for arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome, where early lethality of patients indicates that VPS33B and VPS16B play essential cellular roles. VPS33B is a member of the Sec1/Munc18 (SM) protein family, and thus thought to facilitate vesicular fusion via interaction with SNARE complexes, as does its paralog VPS33A in the homotypic fusion and vacuole sorting (HOPS) complex. VPS33B and VPS16B have been shown to associate, but little is known about the composition, structure or function of the VPS33B/VPS16B complex. We show here that human VPS33B/VPS16B is a high molecular weight complex, which we expressed in yeast to obtain material for structural, composition and stability analysis. Circular dichroism data indicate VPS33B/VPS16B has a well-folded α-helical secondary structure, for which size exclusion chromatography-multi angle light scattering revealed a MW of ∼315 kDa. Quantitative immunoblotting indicated the complex has a VPS33B:VPS16B ratio of 2:3. Expression of ARC syndrome-causing VPS33B missense variants showed that L30P disrupts complex formation, but not S243F or H344D. Truncated VPS16B containing amino acids 143-316 was sufficient to form a complex with VPS33B. Small angle X-ray scattering and negative staining electron microscopy revealed a two-lobed shape for VPS33B/VPS16B. Avidin tagging indicated that each lobe contains a VPS33B molecule, and they are oriented in opposite directions. From this we propose a structure for VPS33B/VPS16B that allows the copies of VPS33B at each end to interact with separate SNARE bundles and/or SNAREpins, plus their associated membrane components. Thus our observations reveal the only known potentially bidirectional SM protein complex.

Mammalian STE20-like kinase-1/2 are activated in human platelets stimulated by collagen or thrombin and play a vital role in collagen-activated platelets

INTRODUCTION

Mammalian ste20-like kinases-1/2 (MST1/2), the core kinases of the Hippo pathway, play critical roles in the biology of hematopoietic cells via noncanonical mechanisms and contributes to megakaryocyte differentiation, polyploidization, and maturation to produce platelets. However, the role of MST1/2 in platelet functions remains unclear.

MATERIALS AND METHODS

In this study, we investigated this topic by determining platelet aggregation and through flow cytometry, ATP release assay, clot retraction assay, and immunoblotting analysis.

RESULTS

We found that MST1/2 were rapidly phosphorylated and activated upon platelet stimulation by thrombin and collagen. XMU-MP-1, a specific inhibitor of MST1/2, blocks the activation of MST1/2 in platelets. Inhibitor-pretreated platelets showed impaired platelet aggregation and dense-granule secretion mediated by collagen, thrombin, and U46619, whereas ristocetin or ADP mediated platelet aggregation was unaffected by XMU-MP-1. Although platelet-mediated clot retraction was not affected by MST1/2 inhibitors, integrin αIIbβ3 activation was significantly attenuated in XMU-MP-1-treated platelets. Moreover, MST1/2 inhibition significantly attenuated the mobilization of platelet calcium ions and the secretion of α-granules induced by convulxin.

CONCLUSIONS

This study is the first to demonstrate that MST1/2 play vital roles in human platelets and contributes to collagen-induced platelet activation and aggregation.

Platelet VPS16B is dependent on VPS33B expression, as determined in two siblings with arthrogryposis, renal dysfunction, and cholestasis syndrome

BACKGROUND

Platelet α-granule biogenesis in precursor megakaryocytes is critically dependent on VPS33B and VPS16B, as demonstrated by the platelet α-granule deficiency seen in the rare multisystem disorder arthrogryposis, renal dysfunction, and cholestasis (ARC) syndrome associated with biallelic pathogenic variants in VPS33B and VIPAS39 (encoding VPS16B). VPS33B and VPS16B are ubiquitously expressed proteins that are known to interact and play key roles in protein sorting and trafficking between subcellular locations. However, there remain significant gaps in our knowledge of the nature of these interactions in primary cells from patients with ARC syndrome.

OBJECTIVES

To use primary cells from patients with ARC syndrome to better understand the interactions and roles of VPS33B and VPS16B in platelets and precursor megakaryocytes.

PATIENTS/METHODS

The proband and his male sibling were clinically suspected to have ARC syndrome. Confirmatory genetic testing and platelet phenotyping, including electron microscopy and protein expression analysis, was performed with consent in a research setting.

RESULTS

We describe the first case of ARC syndrome identified in Costa Rica, associated with a novel homozygous nonsense VPS33B variant that is linked with loss of expression of both VPS33B and VPS16B in platelets.

CONCLUSION

These results indicate that stable expression of VPS16B in platelets, their precursor megakaryocytes, and other cells is dependent on VPS33B. We suggest that systematic evaluation of primary cells from patients with a range of VPS33B and VIPAS39 variants would help to elucidate the interactions and functions of these proteins.

PIKfyve-Dependent Phosphoinositide Dynamics in Megakaryocyte/Platelet Granule Integrity and Platelet Functions

BACKGROUND

Secretory granules are key elements for platelet functions. Their biogenesis and integrity are regulated by fine-tuned mechanisms that need to be fully characterized. Here, we investigated the role of the phosphoinositide 5-kinase PIKfyve and its lipid products, PtdIns5P (phosphatidylinositol 5 monophosphate) and PtdIns(3,5)P₂ (phosphatidylinositol (3,5) bisphosphate) in granule homeostasis in megakaryocytes and platelets.

METHODS

For that, we invalidated PIKfyve by pharmacological inhibition or gene silencing in megakaryocytic cell models (human MEG-01 cell line, human imMKCLs, mouse primary megakaryocytes) and in human platelets.

RESULTS

We unveiled that PIKfyve expression and its lipid product levels increased with megakaryocytic maturation. In megakaryocytes, PtdIns5P and PtdIns(3,5)P₂ were found in alpha and dense granule membranes with higher levels in dense granules. Pharmacological inhibition or knock-down of PIKfyve in megakaryocytes decreased PtdIns5P and PtdIns(3,5)P₂ synthesis and induced a vacuolar phenotype with a loss of alpha and dense granule identity. Permeant PtdIns5P and PtdIns(3,5)P₂ and the cation channel TRPML1 (transient receptor potential mucolipins) and TPC2 activation were able to accelerate alpha and dense granule integrity recovery following release of PIKfyve pharmacological inhibition. In platelets, PIKfyve inhibition specifically impaired the integrity of dense granules culminating in defects in their secretion, platelet aggregation, and thrombus formation.

CONCLUSIONS

These data demonstrated that PIKfyve and its lipid products PtdIns5P and PtdIns(3,5)P₂ control granule integrity both in megakaryocytes and platelets.

Current methods to analyze lysosome morphology, positioning, motility and function

Since the discovery of lysosomes more than 70 years ago, much has been learned about the functions of these organelles. Lysosomes were regarded as exclusively degradative organelles, but more recent research has shown that they play essential roles in several other cellular functions, such as nutrient sensing, intracellular signalling and metabolism. Methodological advances played a key part in generating our current knowledge about the biology of this multifaceted organelle. In this review, we cover current methods used to analyze lysosome morphology, positioning, motility and function. We highlight the principles behind these methods, the methodological strategies and their advantages and limitations. To extract accurate information and avoid misinterpretations, we discuss the best strategies to identify lysosomes and assess their characteristics and functions. With this review, we aim to stimulate an increase in the quantity and quality of research on lysosomes and further ground-breaking discoveries on an organelle that continues to surprise and excite cell biologists.

Platelets as Key Factors in Inflammation: Focus on CD40L/CD40

Platelets are anucleate cytoplasmic fragments derived from the fragmentation of medullary megakaryocytes. Activated platelets adhere to the damaged endothelium by means of glycoproteins on their surface, forming the platelet plug. Activated platelets can also secrete the contents of their granules, notably the growth factors contained in the α-granules, which are involved in platelet aggregation and maintain endothelial activation, but also contribute to vascular repair and angiogenesis. Platelets also have a major inflammatory and immune function in antibacterial defence, essentially through their Toll-like Receptors (TLRs) and Sialic acid-binding immunoglobulin-type lectin (SIGLEC). Platelet activation also contributes to the extensive release of anti- or pro-inflammatory mediators such as IL-1β, RANTES (Regulated on Activation, Normal T Expressed and Secreted) or CD154, also known as the CD40-ligand. Platelets are involved in the direct activation of immune cells, polynuclear neutrophils (PNNs) and dendritic cells via the CD40L/CD40 complex. As a general rule, all of the studies presented in this review show that platelets are capable of covering most of the stages of inflammation, primarily through the CD40L/CD40 interaction, thus confirming their own role in this pathophysiological condition.

Syntaxin 12 and COMMD3 are new factors that function with VPS33B in the biogenesis of platelet α-granules

Platelet α-granules regulate hemostasis and myriad other physiological processes, but their biogenesis is unclear. Mutations in only 3 proteins are known to cause α-granule defects and bleeding disorders in humans. Two such proteins, VPS16B and VPS33B, form a complex mediating transport of newly synthesized α-granule proteins through megakaryocyte (MK) endosomal compartments. It is unclear how the VPS16B/VPS33B complex accomplishes this function. Here we report VPS16B/VPS33B associates physically with Syntaxin 12 (Stx12), a SNARE protein that mediates vesicle fusion at endosomes. Importantly, Stx12-deficient MKs display reduced α-granule numbers and overall levels of α-granule proteins, thus revealing Stx12 as a new component of the α-granule biogenesis machinery. VPS16B/VPS33B also binds CCDC22, a component of the CCC complex working at endosome exit sites. CCDC22 competes with Stx12 for binding to VPS16B/VPS33B, suggesting a possible hand-off mechanism. Moreover, the major CCC form expressed in MKs contains COMMD3, one of 10 COMMD proteins. Deficiency of COMMD3/CCDC22 causes reduced α-granule numbers and overall levels of α-granule proteins, establishing the COMMD3/CCC complex as a new factor in α-granule biogenesis. Furthermore, P-selectin traffics through the cell surface in a COMMD3-dependent manner and depletion of COMMD3 results in lysosomal degradation of P-selectin and PF4. Stx12 and COMMD3/CCC deficiency cause less severe phenotypes than VPS16B/VPS33B deficiency, suggesting Stx12 and COMMD3/CCC assist but are less important than VPS16B/VPS33B in α-granule biogenesis. Mechanistically, our results suggest VPS16B/VPS33B coordinates the endosomal entry and exit of α-granule proteins by linking the fusogenic machinery with a ubiquitous endosomal retrieval complex that is repurposed in MKs to make α-granules.

A Deep Dive into the Pathology of Gray Platelet Syndrome: New Insights on Immune Dysregulation

The gray platelet syndrome (GPS) is a rare platelet disorder, characterized by impaired alpha-granule biogenesis in megakaryocytes and platelets due to NBEAL2 mutations. Typical clinical features include macrothrombocytopenia, bleeding and elevated vitamin B12 levels, while bone marrow fibrosis and splenomegaly may develop during disease progression. Recently, the involvement of other blood lineages has been highlighted, revealing the role of NBEAL2 outside the megakaryocyte-platelet axis. Low leukocyte counts, decreased neutrophil granulation and impaired neutrophil extracellular trap formation represent prominent findings in GPS patients, reflecting deranged innate immunity and associated with an increased susceptibility to infection. In addition, low numbers and impaired degranulation of NK cells have been demonstrated in animal models. Autoimmune diseases involving different organs and a spectrum of autoantibodies are present in a substantial proportion of GPS patients, expanding the syndromic spectrum of this disorder and pointing to dysregulation of the adaptive immune response. Low-grade inflammation, as evidenced by elevation of liver-derived acute-phase reactants, is another previously unrecognized feature of GPS which may contribute to disease manifestations. This review will focus on the mechanisms underlying the pathogenesis of blood cell abnormalities in human GPS patients and NBEAL2-null animal models, providing insight into the effects of NBEAL2 in hemostasis, inflammation and immunity.

Inherited Platelet Disorders

Bleeding disorders due to platelet dysfunction are a common hematologic complication affecting patients, and typically present with mucocutaneous bleeding or hemorrhage. An inherited platelet disorder should be suspected in individuals with a suggestive family history and no identified secondary causes of bleeding. Genetic defects have been described at all levels of platelet activation, including receptor binding, signaling, granule release, cytoskeletal remodeling, and platelet hematopoiesis. Management of these disorders is typically supportive, with an emphasis on awareness, patient education, and anticipatory guidance to prevent future episodes of bleeding.

The endoplasmic reticulum protein SEC22B interacts with NBEAL2 and is required for megakaryocyte α-granule biogenesis

Studies of inherited platelet disorders have provided many insights into platelet development and function. Loss of function of neurobeachin-like 2 (NBEAL2) causes gray platelet syndrome (GPS), where the absence of platelet α-granules indicates NBEAL2 is required for their production by precursor megakaryocytes. The endoplasmic reticulum is a dynamic network that interacts with numerous intracellular vesicles and organelles and plays key roles in their development. The megakaryocyte endoplasmic reticulum is extensive, and in this study we investigated its role in the biogenesis of α-granules by focusing on the membrane-resident trafficking protein SEC22B. Coimmunoprecipitation (co-IP) experiments using tagged proteins expressed in human HEK293 and megakaryocytic immortalized megakaryocyte progenitor (imMKCL) cells established binding of NBEAL2 with SEC22B, and demonstrated that NBEAL2 can simultaneously bind SEC22B and P-selectin. NBEAL2-SEC22B binding was also observed for endogenous proteins in human megakaryocytes using co-IP, and immunofluorescence microscopy detected substantial overlap. SEC22B binding was localized to a region of NBEAL2 spanning amino acids 1798 to 1903, where 2 GPS-associated missense variants have been reported: E1833K and R1839C. NBEAL2 containing either variant did not bind SEC22B coexpressed in HEK293 cells. CRISPR/Cas9-mediated knockout of SEC22B in imMKCL cells resulted in decreased NBEAL2, but not vice versa. Loss of either SEC22B or NBEAL2 expression resulted in failure of α-granule production and reduced granule proteins in imMKCL cells. We conclude that SEC22B is required for α-granule biogenesis in megakaryocytes, and that interactions with SEC22B and P-selectin facilitate the essential role of NBEAL2 in granule development and cargo stability.

Functional Roles of the von Willebrand Factor Propeptide

The primary polypeptide sequence of von Willebrand factor (VWF) includes an N-terminal 741-amino acid VWF propeptide (VWFpp). In cells expressing VWF, the VWFpp performs two critical functions. In the Golgi, VWFpp mediates the intermolecular disulfide linkages that generate high-molecular-weight VWF multimers. Subsequently, the VWFpp, which is proteolytically cleaved from mature VWF by furin, functions to generate the endothelial storage organelles (Weibel-Palade bodies) in which VWF and a distinct collection of proteins are stored, and from where they undergo regulated secretion from the endothelium. The VWFpp is secreted from endothelial cells as dimers and circulates in plasma with at least some of the dimers associating with a noncovalent manner with the D'D3 domain of mature VWF. The VWFpp has a half-life of 2 to 3 hours in plasma, but to date no extracellular function has been determined for the molecule. Nevertheless, its large size and several biologically interesting structural features (two sets of vicinal cysteines and an RGD sequence) suggest that there may be roles that the VWFpp plays in hemostasis or associated physiological processes such as angiogenesis or wound repair.

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.

Platelets in aging and cancer-"double-edged sword"

Platelets control hemostasis and play a key role in inflammation and immunity. However, platelet function may change during aging, and a role for these versatile cells in many age-related pathological processes is emerging. In addition to a well-known role in cardiovascular disease, platelet activity is now thought to contribute to cancer cell metastasis and tumor-associated venous thromboembolism (VTE) development. Worldwide, the great majority of all patients with cardiovascular disease and some with cancer receive anti-platelet therapy to reduce the risk of thrombosis. However, not only do thrombotic diseases remain a leading cause of morbidity and mortality, cancer, especially metastasis, is still the second cause of death worldwide. Understanding how platelets change during aging and how they may contribute to aging-related diseases such as cancer may contribute to steps taken along the road towards a "healthy aging" strategy. Here, we review the changes that occur in platelets during aging, and investigate how these versatile blood components contribute to cancer progression.

Inherited Platelet Disorders: Diagnosis and Management

Inherited platelet disorders are rare but they can have considerable clinical impacts, and studies of their causes have advanced understanding of platelet formation and function. Effective hemostasis requires adequate circulating numbers of functional platelets. Quantitative, qualitative and combined platelet disorders with a bleeding phenotype have been linked to defects in platelet cytoskeletal elements, cell surface receptors, signal transduction pathways, secretory granules and other aspects. Inherited platelet disorders have variable clinical presentations, and diagnosis and management is often challenging. Evaluation begins with detailed patient and family histories, including a bleeding score. The physical exam identifies potential syndromic features of inherited platelet disorders and rules out other causes. Laboratory investigations include a complete blood count, blood film, coagulation testing and Von Willebrand factor assessment. A suspected platelet function disorder is further assessed by platelet aggregation, flow cytometry, platelet dense granule release and/or content, and genetic testing. The management of platelet function disorders aims to minimize the risk of bleeding and achieve adequate hemostasis when needed. Although not universal, platelet transfusion remains a crucial component in the management of many inherited platelet disorders.

Recent advances in inherited platelet disorders

PURPOSE OF REVIEW

The increasing use of high throughput sequencing and genomic analysis has facilitated the discovery of new causes of inherited platelet disorders. Studies of these disorders and their respective mouse models have been central to understanding their biology, and also in revealing new aspects of platelet function and production. This review covers recent contributions to the identification of genes, proteins and variants associated with inherited platelet defects, and highlights how these studies have provided insights into platelet development and function.

RECENT FINDINGS

Novel genes recently implicated in human platelet dysfunction include the galactose metabolism enzyme UDP-galactose-4-epimerase in macrothrombocytopenia, and erythropoietin-producing hepatoma-amplified sequence receptor transmembrane tyrosine kinase EPHB2 in a severe bleeding disorder with deficiencies in platelet agonist response and granule secretion. Recent studies of disease-associated variants established or clarified roles in platelet function and/or production for the membrane receptor G6b-B, the FYN-binding protein FYB1/ADAP, the RAS guanyl-releasing protein RASGRP2/CalDAG-GEFI and the receptor-like protein tyrosine phosphatase PTPRJ/CD148. Studies of genes associated with platelet disorders advanced understanding of the cellular roles of neurobeachin-like 2, as well as several genes influenced by the transcription regulator RUNT-related transcription factor 1 (RUNX1), including NOTCH4.

SUMMARY

The molecular bases of many hereditary platelet disorders have been elucidated by the application of recent advances in cell imaging and manipulation, genomics and protein function analysis. These techniques have also aided the detection of new disorders, and enabled studies of disease-associated genes and variants to enhance understanding of platelet development and function.

Hemostasis vs. homeostasis: Platelets are essential for preserving vascular barrier function in the absence of injury or inflammation

Platelets are best known for their vasoprotective responses to injury and inflammation. Here, we have asked whether they also support vascular integrity when neither injury nor inflammation is present. Changes in vascular barrier function in dermal and meningeal vessels were measured in real time in mouse models using the differential extravasation of fluorescent tracers as a biomarker. Severe thrombocytopenia produced by two distinct methods caused increased extravasation of 40-kDa dextran from capillaries and postcapillary venules but had no effect on extravasation of 70-kDa dextran or albumin. This reduction in barrier function required more than 4 h to emerge after thrombocytopenia was established, reverting to normal as the platelet count recovered. Barrier dysfunction was also observed in mice that lacked platelet-dense granules, dense granule secretion machinery, glycoprotein (GP) VI, or the GPVI signaling effector phospholipase C (PLC) γ2. It did not occur in mice lacking α-granules, C type lectin receptor-2 (CLEC-2), or protease activated receptor 4 (PAR4). Notably, although both meningeal and dermal vessels were affected, intracerebral vessels, which are known for their tighter junctions between endothelial cells, were not. Collectively, these observations 1) highlight a role for platelets in maintaining vascular homeostasis in the absence of injury or inflammation, 2) provide a sensitive biomarker for detecting changes in platelet-dependent barrier function, 3) identify which platelet processes are required, and 4) suggest that the absence of competent platelets causes changes in the vessel wall itself, accounting for the time required for dysfunction to emerge.

Mechanism of platelet α-granule biogenesis: study of cargo transport and the VPS33B-VPS16B complex in a model system

Platelet α-granules play important roles in platelet function. They contain hundreds of proteins that are synthesized by the megakaryocyte or taken up by endocytosis. The trafficking pathways that mediate platelet α-granule biogenesis are incompletely understood, especially with regard to cargo synthesized by the megakaryocyte. Vacuolar-protein sorting 33B (VPS33B) and VPS16B are essential proteins for α-granule biogenesis, but they are largely uncharacterized. Here, we adapted a powerful method to directly map the pathway followed by newly synthesized cargo proteins to reach α-granules. Using this method, we revealed the recycling endosome as a key intermediate compartment in α-granule biogenesis. We then used CRISPR/Cas9 gene editing to knock out VPS33B in pluripotent stem cell-derived immortalized megakaryocyte cells (imMKCLs). Consistent with the observations in platelets from patients with VPS33B mutation, VPS33B-knockout (KO) imMKCLs have drastically reduced levels of α-granule proteins platelet factor 4, von Willebrand factor, and P-selectin. VPS33B and VPS16B form a distinct and small complex in imMKCLs with the same hydrodynamic radius as the recombinant VPS33B-VPS16B heterodimer purified from bacteria. Mechanistically, the VPS33B-VPS16B complex ensures the correct trafficking of α-granule proteins. VPS33B deficiency results in α-granule cargo degradation in lysosomes. VPS16B steady-state levels are significantly lower in VPS33B-KO imMKCLs, suggesting that VPS16B is destabilized in the absence of its partner. Exogenous expression of green fluorescent protein-VPS33B in VPS33B-KO imMKCLs reconstitutes the complex, which localizes to the recycling endosome, further defining this compartment as a key intermediate in α-granule biogenesis. These results advance our understanding of platelet α-granule biogenesis and open new avenues for the study of these organelles.

A Novel Microchip Technique for Quickly Identifying Nanogranules in an Aqueous Solution by Transmission Electron Microscopy: Imaging of Platelet Granules

Ultrastructural observation of biological specimens or nanogranules usually requires the use of electron microscopy. Electron microscopy takes a lot of time, requires many steps, and uses many chemicals, which may affect the native state of biological specimens. A novel microchip (K-kit) was used as a specimen kit for in situ imaging of human platelet granules in an aqueous solution using a transmission electron microscope. This microchip enabled us to observe the native human platelet granules very quickly and easily. The protocols included blood collection, platelet purification, platelet granule isolation, sample loading into this microchip, and then observation by a transmission electron microscope. In addition, these granules could still remain in aqueous solution, and only a very small amount of the sample was required for observation and analysis. We used this microchip to identify the native platelet granules by negative staining. Furthermore, we used this microchip to perform immunoelectron microscopy and successfully label α-granules of platelets with the anti-P-selectin antibody. These results demonstrate that the novel microchip can provide researchers with faster and better choices when using a transmission electron microscope to examine nanogranules of biological specimens in aqueous conditions.

The road to lysosome-related organelles: Insights from Hermansky-Pudlak syndrome and other rare diseases

Lysosome-related organelles (LROs) comprise a diverse group of cell type-specific, membrane-bound subcellular organelles that derive at least in part from the endolysosomal system but that have unique contents, morphologies and functions to support specific physiological roles. They include: melanosomes that provide pigment to our eyes and skin; alpha and dense granules in platelets, and lytic granules in cytotoxic T cells and natural killer cells, which release effectors to regulate hemostasis and immunity; and distinct classes of lamellar bodies in lung epithelial cells and keratinocytes that support lung plasticity and skin lubrication. The formation, maturation and/or secretion of subsets of LROs are dysfunctional or entirely absent in a number of hereditary syndromic disorders, including in particular the Hermansky-Pudlak syndromes. This review provides a comprehensive overview of LROs in humans and model organisms and presents our current understanding of how the products of genes that are defective in heritable diseases impact their formation, motility and ultimate secretion.

Inherited thrombocytopenias: history, advances and perspectives

Over the last 100 years the role of platelets in hemostatic events and their production by megakaryocytes have gradually been defined. Progressively, thrombocytopenia was recognized as a cause of bleeding, first through an acquired immune disorder; then, since 1948, when Bernard-Soulier syndrome was first described, inherited thrombocytopenia became a fascinating example of Mendelian disease. The platelet count is often severely decreased and platelet size variable; associated platelet function defects frequently aggravate bleeding. Macrothrombocytopenia with variable proportions of enlarged platelets is common. The number of circulating platelets will depend on platelet production, consumption and lifespan. The bulk of macrothrombocytopenias arise from defects in megakaryopoiesis with causal variants in transcription factor genes giving rise to altered stem cell differentiation and changes in early megakaryocyte development and maturation. Genes encoding surface receptors, cytoskeletal and signaling proteins also feature prominently and Sanger sequencing associated with careful phenotyping has allowed their early classification. It quickly became apparent that many inherited thrombocytopenias are syndromic while others are linked to an increased risk of hematologic malignancies. In the last decade, the application of next-generation sequencing, including whole exome sequencing, and the use of gene platforms for rapid testing have greatly accelerated the discovery of causal genes and extended the list of variants in more common disorders. Genes linked to an increased platelet turnover and apoptosis have also been identified. The current challenges are now to use next-generation sequencing in first-step screening and to define bleeding risk and treatment better.

Platelet immunophenotyping in health and inherited bleeding disorders, a review and practical hints

Inherited platelet function disorders are rare hemorrhagic diseases. The gold standard for their exploration is optical aggregometry; however, investigations by flow cytometry (FCM) are being increasingly used. In this review, the physiology of platelets is first recalled, setting the stage for the compartments of platelets that can be apprehended by specific and appropriate labeling. As this requires some pre-analytical precautions and specific analytical settings, a second part focuses on these characteristic aspects, based on literature and on the authors' experience in the field, for qualitative or quantitative explorations. Membrane labeling with antibodies to CD42a or CD41, respectively, useful to assess the genetic-related defects of Glanzmann thrombocytopenia and Bernard Soulier syndrome are then described. Platelet degranulation disorders are detailed in the next section, as they can be explored, upon platelet activation, by measuring the expression of surface P-Selectin (CD62P) or CD63. Mepacrin uptake and release after activation is another test allowing to explore the function of dense granules. Finally, the flip-flop anomaly related to Scott syndrome is depicted. Tables summarizing possible FCM assays, and characteristic histograms are provided as reference for flow laboratories interested in developing platelet exploration.

Phosphatidylinositol 3-monophosphate: A novel actor in thrombopoiesis and thrombosis

Phosphoinositides are lipid second messengers regulating in time and place the formation of protein complexes involved in the control of intracellular signaling, vesicular trafficking, and cytoskeleton/membrane dynamics. One of these lipids, phosphatidylinositol 3 monophosphate (PtdIns3P), is present in small amounts in mammalian cells and is involved in the control of endocytic/endosomal trafficking and in autophagy. Its metabolism is finely regulated by specific kinases and phosphatases including class II phosphoinositide 3-kinases (PI3KC2s) and the class III PI3K, Vps34. Recently, PtdIns3P has emerged as an important regulator of megakaryocyte/platelet structure and functions. Here, we summarize the current knowledge in the role of different pools of PtdIns3P regulated by class II and III PI3Ks in platelet production and thrombosis. Potential new antithrombotic therapeutic perspectives based on the use of inhibitors targeting specifically PtdIns3P-metabolizing enzymes will also be discussed. Finally, we provide report of new research in this area presented at the International Society of Thrombosis and Haemostasis 2019 Annual Congress.

FERARI is required for Rab11-dependent endocytic recycling

Endosomal transport is essential for cellular organization and compartmentalization and cell-cell communication. Sorting endosomes provide a crossroads for various trafficking pathways and determine recycling, secretion or degradation of proteins. The organization of these processes requires membrane-tethering factors to coordinate Rab GTPase function with membrane fusion. Here, we report a conserved tethering platform that acts in the Rab11 recycling pathways at sorting endosomes, which we name factors for endosome recycling and Rab interactions (FERARI). The Rab-binding module of FERARI consists of Rab11FIP5 and rabenosyn-5/RABS-5, while the SNARE-interacting module comprises VPS45 and VIPAS39. Unexpectedly, the membrane fission protein EHD1 is also a FERARI component. Thus, FERARI appears to combine fusion activity through the SM protein VPS45 with pinching activity through EHD1 on SNX-1-positive endosomal membranes. We propose that coordination of fusion and pinching through a kiss-and-run mechanism drives cargo at endosomes into recycling pathways.

NBEAL2 (Neurobeachin-Like 2) Is Required for Retention of Cargo Proteins by α-Granules During Their Production by Megakaryocytes

Objective- Human and mouse megakaryocytes lacking NBEAL2 (neurobeachin-like 2) produce platelets where α-granules lack protein cargo. This cargo is mostly megakaryocyte-synthesized, but some proteins, including FGN (fibrinogen), are endocytosed. In this study, we examined the trafficking of both types of cargo within primary megakaryocytes cultured from normal and NBEAL2-null mice, to determine the role of NBEAL2 in α-granule maturation. We also examined the interaction of NBEAL2 with the granule-associated protein P-selectin in human megakaryocytes and platelets. Approach and Results- Fluorescence microscopy was used to compare uptake of labeled FGN by normal and NBEAL2-null mouse megakaryocytes, which was similar in both. NBEAL2-null cells, however, showed decreased FGN retention, and studies with biotinylated protein showed rapid loss rather than increased degradation. Intracellular tracking via fluorescence microscopy revealed that in normal megakaryocytes, endocytosed FGN sequentially associated with compartments expressing RAB5 (Ras-related protein in brain 5), RAB7 (Ras-related protein in brain 7), and P-selectin, where it was retained. A similar initial pattern was observed in NBEAL2-null megakaryocytes, but then FGN passed from the P-selectin compartment to RAB11 (Ras-related protein in brain 11)-associated endosomes before release. Megakaryocyte-synthesized VWF (Von Willebrand factor) was observed to follow the same route out of NBEAL2-null cells. Immunofluorescence microscopy revealed intracellular colocalization of NBEAL2 with P-selectin in human megakaryocytes, proplatelets, and platelets. Native NBEAL2 and P-selectin were coimmunoprecipitated from platelets and megakaryocytes. Conclusions- NBEAL2 is not required for FGN uptake by megakaryocytes. NBEAL2 is required for the retention of both endocytosed and megakaryocyte-synthesized proteins by maturing α-granules, and possibly by platelet-borne granules. This function may involve interaction of NBEAL2 with P-selectin.

Regulation of Innate Immune Responses by Platelets

The role of platelets has been extensively studied in the context of coagulation and vascular integrity. Their hemostatic imbalance can lead to known conditions as atherosclerotic plaques, thrombosis, and ischemia. Nevertheless, the knowledge regarding the regulation of different cell types by platelets has been growing exponentially in the past years. Among these biological systems, the innate immune response is remarkably affected by the crosstalk with platelets. This interaction can come from the formation of platelet-leukocyte aggregates, signaling by direct contact between membrane surface molecules or by the stimulation of immune cells by soluble factors and active microparticles secreted by platelets. These ubiquitous blood components are able to sense and react to danger signals, guiding leukocytes to an injury site and providing a scaffold for the formation of extracellular traps for efficient microbial killing and clearance. Using several different mechanisms, platelets have an important task as they regulate the release of different cytokines and chemokines upon sterile or infectious damage, the expression of cell markers and regulation of cell death and survival. Therefore, platelets are more than clotting agents, but critical players within the fine inflammatory equilibrium for the host. In this review, we present pointers to a better understanding about how platelets control and modulate innate immune cells, as well as a summary of the outcome of this interaction, providing an important step for therapeutic opportunities and guidance for future research on infectious and autoimmune diseases.

Metabolomics and Lipidomics Reveal the Effect of Hepatic Vps33b Deficiency on Bile Acids and Lipids Metabolism

Vascular protein sorting-associated protein 33B (VPS33B) plays important roles in hepatic polarity, which directly maintains the functional structure of the liver. It has reported that VPS33B has close association with arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome. Unfortunately, no further studies were conducted to reveal the role of Vps33b in the homeostasis of bile acids. In the current study, hepatic Vps33b-depleted male mice were used to investigate the metabolomics and lipidomics profiles of hepatic Vps33b deficiency based on ultrahigh-performance liquid chromatography coupled with an electrospray ionization high-resolution mass spectrometry (UHPLC-ESI-HRMS) system. Hepatic Vps33b-depleted male mice displayed cholestasis and slight liver damage with increased serum levels of ALT, AST, ALP and T-Bili compared to wild-type mice. Targeted metabolomics analysis of bile acids revealed that increased taurine-conjugated bile acids accumulated in the serum of hepatic Vps33b-depleted mice, while unconjugated bile acids were prone to decrease, accompanied by the regulation of bile acid homeostasis-related genes. In addition, lipid profiles were significantly altered with the lack of Vps33b in the liver. A variety of lipids, such as triglycerides and sphingomyelins, were significantly decreased in the liver and increased in the serum of hepatic Vps33b-depleted mice compared to those in wild-type mice. Our study demonstrated that Vps33b influences the progress of liver metabolism both in bile acid circulation and lipid metabolism, which is involved in the progression of liver cholestasis in mice.

The Birth and Death of Platelets in Health and Disease

Blood platelets are involved in a wide range of physiological responses and pathological processes. Recent studies have considerably advanced our understanding of the mechanisms of platelet production and clearance, revealing new connections between the birth and death of these tiny, abundant cells. Key insights have also been gained into how physiological challenges such as inflammation, infection, and chemotherapy can affect megakaryocytes, the cells that produce platelets.

Sorting machineries: how platelet-dense granules differ from α-granules

Platelets respond to vascular injury via surface receptor stimulation and signaling events to trigger aggregation, procoagulant activation, and granule secretion during hemostasis, thrombosis, and vascular remodeling. Platelets contain three major types of secretory granules including dense granules (or δ-granules, DGs), α-granules (AGs), and lysosomes. The contents of platelet granules are specific. Platelet DGs store polyphosphate and small molecules such as ADP, ATP, Ca²⁺, and serotonin, while AGs package most of the proteins that platelets release. The platelet DGs and AGs are regarded as being budded from the endosomes and the trans-Golgi network (TGN), respectively, and then matured from multivesicular bodies (MVBs). However, the sorting machineries between DGs and AGs are different. Inherited platelet disorders are associated with deficiency of DGs and AGs, leading to bleeding diathesis in patients with Hermansky-Pudlak syndrome (HPS), gray platelet syndrome (GPS), and arthrogryposis, renal dysfunction, and cholestasis syndrome (ARC). Here, we reviewed the current understanding about how DGs differ from AGs in structure, biogenesis, and function. In particular, we focus on the sorting machineries that are involved in the formation of these two types of granules to provide insights into their diverse biological functions.

Congenital Disorders of Platelet Function and Number

Mucocutaneous bleeding symptoms and/or persistent thrombocytopenia occur in individuals with congenital disorders of platelet function and number. Apart from bleeding, these disorders are often associated with additional hematologic and clinical manifestations, including auditory, immunologic, and oncologic disease. Autosomal recessive, dominant, and X-linked inheritance patterns have been demonstrated. Precise delineation of the molecular cause of the platelet disorder can aid the pediatrician in the detection and prevention of specific disorder-associated manifestations and guide appropriate treatment and anticipatory care for the patient and family.

The life cycle of platelet granules

Platelet granules are unique among secretory vesicles in both their content and their life cycle. Platelets contain three major granule types—dense granules, α-granules, and lysosomes—although other granule types have been reported. Dense granules and α-granules are the most well-studied and the most physiologically important. Platelet granules are formed in large, multilobulated cells, termed megakaryocytes, prior to transport into platelets. The biogenesis of dense granules and α-granules involves common but also distinct pathways. Both are formed from the trans-Golgi network and early endosomes and mature in multivesicular bodies, but the formation of dense granules requires trafficking machinery different from that of α-granules. Following formation in the megakaryocyte body, both granule types are transported through and mature in long proplatelet extensions prior to the release of nascent platelets into the bloodstream. Granules remain stored in circulating platelets until platelet activation triggers the exocytosis of their contents. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, located on both the granules and target membranes, provide the mechanical energy that enables membrane fusion during both granulogenesis and exocytosis. The function of these core fusion engines is controlled by SNARE regulators, which direct the site, timing, and extent to which these SNAREs interact and consequently the resulting membrane fusion. In this review, we assess new developments in the study of platelet granules, from their generation to their exocytosis.