Sorting Nexin 24 is required for α-granule biogenesis and cargo delivery in megakaryocytes

Germline defects affecting the DNA-binding domain of the transcription factor FLI1 are associated with a bleeding disorder that is characterised by the presence of large, fused α-granules in platelets. We investigated whether the genes showing abnormal expression in FLI1-deficient platelets could be involved in platelet α-granule biogenesis by undertaking transcriptome analysis of control platelets and platelets harbouring a DNA-binding variant of FLI1. Our analysis identified 2276 transcripts that were differentially expressed in FLI1- deficient platelets. Functional annotation clustering of the coding transcripts revealed significant enrichment for gene annotations relating to protein transport, and identified Sorting nexin 24 (SNX24) as a candidate for further investigation. Using an iPSC-derived megakaryocyte model, SNX24 expression was found to be increased during the early stages of megakaryocyte differentiation and downregulated during proplatelet formation, indicating tight regulatory control during megakaryopoiesis. CRISPR-Cas9 mediated knockout (KO) of SNX24 led to decreased expression of immature megakaryocyte markers, CD41 and CD61, and increased expression of the mature megakaryocyte marker CD42b (p=0.0001), without affecting megakaryocyte polyploidisation, or proplatelet formation. Electron microscopic analysis revealed an increase in empty membrane-bound organelles in SNX24 KO megakaryocytes, a reduction in α-granules and an absence of immature and mature multivesicular bodies, consistent with a defect in the intermediate stage of α-granule maturation. Co-localisation studies showed that SNX24 associates with each compartment of α-granule maturation. Reduced expression of CD62P and VWF was observed in SNX24 KO megakaryocytes. We conclude that SNX24 is required for α-granule biogenesis and intracellular trafficking of α-granule cargo within megakaryocytes.

Interferon alpha-induced SAMHD1 regulates human cultured megakaryocyte apoptosis and proplatelet formation

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Don't you forget about me(gakaryocytes)

Platelets, small, anucleate cell fragments, derive from large precursor cells, megakaryocytes (MKs), that reside in the bone marrow. MKs emerge from hematopoietic stem cells in a complex differentiation process that involves cytoplasmic maturation, including the formation of the demarcation membrane system, and polyploidization. The main function of MKs is the generation of platelets, which predominantly occurs through the release of long, microtubule-rich proplatelets into vessel sinusoids. However, the idea of a one-dimensional role of MKs as platelet precursors is currently being questioned due to advances in high resolution microscopy and single-cell Omics. On the one hand, recent findings suggest that proplatelet formation from bone marrow-derived MKs is not the only mechanism of platelet production, but that it might also occur through budding of the plasma membrane and in distant organs like lung or liver. On the other hand, novel evidence suggests that MKs do not only maintain physiological platelet levels but further contribute to bone marrow homeostasis through the release of extracellular vesicles or cytokines such as transforming growth factor β1 or platelet factor 4. The notion of multitasking MKs was reinforced in recent studies using single cell RNA sequencing approaches on MKs derived from adult and fetal bone marrow and lungs, leading to the identification of different MK subsets that appear to exhibit immunomodulatory or secretory roles. In the following, novel insights into the mechanisms leading to proplatelet formation in vitro and in vivo will be reviewed and the hypothesis of megakaryocytes as immunoregulatory cells will be critically discussed.

Expanding the genetic spectrum of TUBB1-related thrombocytopenia

β1-tubulin plays a major role in proplatelet formation and platelet shape maintenance, and pathogenic variants in TUBB1 lead to thrombocytopenia and platelet anisocytosis (TUBB1-RT). To date, the reported number of pedigrees with TUBB1-RT and of rare TUBB1 variants with experimental demonstration of pathogenicity is limited. Here, we report 9 unrelated families presenting with thrombocytopenia carrying six β1-tubulin variants: p.Cys12Leufs12*, p.Thr107Pro, p.Gln423*, p.Arg359Trp, p.Gly109Glu, and p.Gly269Asp, the last of which novel. Segregation studies showed incomplete penetrance of these variants for platelet traits. Indeed, most carriers showed macrothrombocytopenia, some only increased platelet size and a minority no abnormalities. Moreover, only homozygous carriers of the p.Gly109Glu variant, displayed macrothrombocytopenia, highlighting the importance of allele burden in the phenotypic expression of TUBB1-RT. The p.Arg359Trp, p.Gly269Asp and p.Gly109Glu variants deranged β1-tubulin incorporation into the microtubular marginal ring in platelets, while had negligible effect on platelet activation, secretion or spreading, suggesting that β1-tubulin is dispensable for these processes. Transfection of TUBB1 missense variants in CHO cells altered β1-tubulin incorporation into the microtubular network. In addition, TUBB1 variants markedly impaired proplatelet formation from peripheral blood CD34+ cell-derived megakaryocytes. Our study, using in vitro modeling, molecular characterization, and clinical investigations provides a deeper insight into the pathogenicity of rare TUBB1 variants. These novel data expand the genetic spectrum of TUBB1-RT and highlight a remarkable heterogeneity in its clinical presentation, indicating that allelic burden or combination with other genetic or environmental factors modulate the phenotypic impact of rare TUBB1 variants.

Roxadustat Does Not Affect Platelet Production, Activation, and Thrombosis Formation

Objective: Roxadustat is a new medication for the treatment of renal anemia. EPO (erythropoietin)—the current treatment standard—has been reported to enhance platelet activation and production. However, to date, the effect of roxadustat on platelets is unclear. To address this deficiency, herein, we have evaluated the effect of roxadustat on platelet production and function. Approach and Results: We performed several mouse platelet functional assays in the presence/absence of in vitro and in vivo roxadustat treatment. Both healthy and 5/6 nephrectomized mice were utilized. The effect of roxadustat on platelet function of healthy volunteers and chronic kidney disease patients was also evaluated. For platelet production, megakaryocyte maturation and proplatelet formation were assayed in vitro. Peripheral platelet and bone marrow megakaryocyte counts were also determined. We found that roxadustat could not stimulate washed platelets directly, and platelet aggregation, spreading, clot retraction, and P-selectin/JON/A exposure were similar with or without in vitro or in vivo roxadustat treatment among both healthy and 5/6 nephrectomized mice. In vivo mouse thrombosis models were additionally performed, and no differences were detected between the vehicle and roxadustat treatment groups. EPO, which was considered a positive control in the present study, promoted platelet function and production as reported previously. Megakaryocyte maturation and proplatelet formation were also not significantly different between control mice and those treated with roxadustat. After receiving roxadustat for 14 days, no difference in the peripheral platelet count was observed in the mice. Conclusions: Administration of roxadustat has no significant impact on platelet production and function.

Dual role of EZH2 on megakaryocyte differentiation

EZH2, the enzymatic component of PRC2, has been identified as a key factor in hematopoiesis. EZH2 loss of function mutations have been found in myeloproliferative neoplasms, more particularly in myelofibrosis, but the precise function of EZH2 in megakaryopoiesis is not fully delineated. Here, we show that EZH2 inhibition by small molecules and shRNA induces MK commitment by accelerating lineage marker acquisition without change in proliferation. Later in differentiation, EZH2 inhibition blocks proliferation, polyploidization and decreases proplatelet formation. EZH2 inhibitors similarly reduce MK polyploidization and proplatelet formation in vitro and platelet level in vivo in a JAK2V617F background. In transcriptome profiling, the defect in proplatelet formation was associated with an aberrant actin cytoskeleton regulation pathway, whereas polyploidization was associated with an inhibition of expression of genes involved in DNA replication and repair, and an upregulation of CDK inhibitors, more particularly CDKN1A and CDKN2D. The knockdown of CDKN1A and at a lesser extend of CDKN2D could partially rescue the percentage of polyploid MKs. Moreover, H3K27me3 and EZH2 ChIP assays revealed that only CDKN1A is a direct EZH2 target while CDKN2D expression is not directly regulated by EZH2 suggesting that EZH2 controls MK polyploidization directly through CDKN1A and indirectly through CDKN2D.

The (Patho)Biology of SRC Kinase in Platelets and Megakaryocytes

Proto-oncogene tyrosine-protein kinase SRC (SRC), as other members of the SRC family kinases (SFK), plays an important role in regulating signal transduction by different cell surface receptors after changes in the cellular environment. Here, we reviewed the role of SRC in platelets and megakaryocytes (MK). In platelets, inactive closed SRC is coupled to the β subunit of integrin αIIbβ3 while upon fibrinogen binding during platelet activation, αIIbβ3-mediated outside-in signaling is initiated by activation of SRC. Active open SRC now further stimulates many downstream effectors via tyrosine phosphorylation of enzymes, adaptors, and especially cytoskeletal components. Functional platelet studies using SRC knockout mice or broad spectrum SFK inhibitors pointed out that SRC mediates their spreading on fibrinogen. On the other hand, an activating pathological SRC missense variant E527K in humans that causes bleeding inhibits collagen-induced platelet activation while stimulating platelet spreading. The role of SRC in megakaryopoiesis is much less studied. SRC knockout mice have a normal platelet count though studies with SFK inhibitors point out that SRC could interfere with MK polyploidization and proplatelet formation but these inhibitors are not specific. Patients with the SRC E527K variant have thrombocytopenia due to hyperactive SRC that inhibits proplatelet formation after increased spreading of MK on fibrinogen and enhanced formation of podosomes. Studies in humans have contributed significantly to our understanding of SRC signaling in platelets and MK.

The Role of 5-HT on Proplatelet Formation and Thrombopoietin Production

Background: Our previous work confirmed that serotonin (5-HT) promotes the proliferation of hemopoietic stem cells and megakaryocytes (Yang M et al, Stem Cells, 2007; 2014). However, the mechanisms remain indefinite. Methods: Q-PCR, Flow Cytometry, Western Blot, or Immunofluorescence microscope were used in the receptor and TPO study. MTT/CCK-8, Proplatelet assay, and Flow Cytometry were also used in cell proliferation and apoptosis study. The relationship between 5-HT and TPO was studied in a traumatic stress mice model. Results: In-vitro study, there was a stimulating effect of 5-HT on proplatelet formation in human bone marrow megakaryocytes. Human BM MK progenitors cultured in serum-free medium with either 5-HT (200nM) or TPO (100 ng/ml) had more proplatelet bearing MKs than the control group (5-HT (12.3 ± 5.0)% vs. Control (6.2 ± 3.5)%, P=0.025; TPO (15.6 ± 2.5)% vs. Control, P=0.04; n=4). The 5-HT treatment group showed more mature and more in the final stage MK cells as compared to the TPO group. 5-HT2A, 2B, 2C receptors were detected in the surface of megakaryocytes. The effect of 5-HT on proplatelet formation in MK cells was via 5-HT2 receptors and this effect was reduced by 5-HT2 receptor inhibitor ketanserin. 5-HT acted on cytoskeleton reorganization in MKs via 5-HT2 receptors and ERK1/2 pathway. Using an immunofluorescence microscope with F-actin specific binder rhodamine-phalloidin staining, the polymerized actin level was lower in the control group than the 5-HT group and actin distributed diffusely throughout the cytoplasm. In contrast, the polymerization actin level was higher in the 5-HT group. Adding ketanserin and ERK1/2 inhibitor PD98059 to 5-HT treatment, the fluorescence intensity was correspondingly reduced. Our data also demonstrated that ERK1/2 was activated in MKs treated with 5-HT for 30 minutes. In a traumatic stress mice model, both of 5-HT and TPO were increased, but the increasing of TPO is posterior to 5-HT. After added LX1606, the synthesis inhibitor of 5-HT, 5-HT was reduced markedly, as well as TPO. The expression of TPO mRNA and the production of TPO protein were increased as compared with the control in this model. Conclusions: This study suggests that 5-HT promotes thrombopoiesis from two aspects: one is the direct effect on megakaryocytes. 5-HT could promote the proplatelet formation from megakaryocytes. The second is the indirect effect by promoting the production of TPO, which is a paracrine secretion to influence thrombopoiesis. Disclosures No relevant conflicts of interest to declare.

PAK1 Regulates MEC-17 Acetyltransferase Activity and Microtubule Acetylation during Proplatelet Extension

Mature megakaryocytes extend long processes called proplatelets from which platelets are released in the blood stream. The Rho GTPases Cdc42 and Rac as well as their downstream target, p21-activated kinase 2 (PAK2), have been demonstrated to be important for platelet formation. Here we address the role, during platelet formation, of PAK1, another target of the Rho GTPases. PAK1 decorates the bundled microtubules (MTs) of megakaryocyte proplatelets. Using a validated cell model which recapitulates proplatelet formation, elongation and platelet release, we show that lack of PAK1 activity increases the number of proplatelets but restrains their elongation. Moreover, in the absence of PAK1 activity, cells have hyperacetylated MTs and lose their MT network integrity. Using inhibitors of the tubulin deacetylase HDAC6, we demonstrate that abnormally high levels of MT acetylation are not sufficient to increase the number of proplatelets but cause loss of MT integrity. Taken together with our previous demonstration that MT acetylation is required for proplatelet formation, our data reveal that MT acetylation levels need to be tightly regulated during proplatelet formation. We identify PAK1 as a direct regulator of the MT acetylation levels during this process as we found that PAK1 phosphorylates the MT acetyltransferase MEC-17 and inhibits its activity.