Human actin-related protein 2/3 complex (Arp2/3), required for actin filament branching, has two ARPC1 component isoforms, with ARPC1B prominently expressed in blood cells. Here we show in a child with microthrombocytopenia, eosinophilia and inflammatory disease, a homozygous frameshift mutation in ARPC1B (p.Val91Trpfs*30). Platelet lysates reveal no ARPC1B protein and greatly reduced Arp2/3 complex. Missense ARPC1B mutations are identified in an unrelated patient with similar symptoms and ARPC1B deficiency. ARPC1B-deficient platelets are microthrombocytes similar to those seen in Wiskott-Aldrich syndrome that show aberrant spreading consistent with loss of Arp2/3 function. Knockout of ARPC1B in megakaryocytic cells results in decreased proplatelet formation, and as observed in platelets from patients, increased ARPC1A expression. Thus loss of ARPC1B produces a unique set of platelet abnormalities, and is associated with haematopoietic/immune symptoms affecting cell lineages where this isoform predominates. In agreement with recent experimental studies, our findings suggest that ARPC1 isoforms are not functionally interchangeable.

/pdf/loss-of-the-arp2-3-complex-component-arpc1b-causes-platelet-2burmalkho.pdf

Loss of the Arp2/3 complex component ARPC1B causes platelet abnormalities and predisposes to inflammatory disease

https://research-information.bris.ac.uk/files/73672463/BLOOD_2015_675629v1_Freson.pdf

A gain-of-function variant in DIAPH1 causes dominant macrothrombocytopenia and hearing loss

Every day, megakaryocytes produce billions of platelets that circulate for several days and eventually are cleared by the liver. The exact removal mechanism, however, remains unclear. Loss of sialic acid residues is thought to feature in the aging and clearance of platelets. Using state-of-the-art spinning disk intravital microscopy to delineate the different compartments and cells of the mouse liver, we observed rapid accumulation of desialylated platelets predominantly on Kupffer cells, with only a few on endothelial cells and none on hepatocytes. Kupffer cell depletion prevented the removal of aged platelets from circulation. Ashwell-Morell receptor (AMR) deficiency alone had little effect on platelet uptake. Macrophage galactose lectin (MGL) together with AMR mediated clearance of desialylated or cold-stored platelets by Kupffer cells. Effective clearance is critical, as mice with an aged platelet population displayed a bleeding phenotype. Our data provide evidence that the MGL of Kupffer cells plays a significant role in the removal of desialylated platelets through a collaboration with the AMR, thereby maintaining a healthy and functional platelet compartment.

/pdf/macrophage-galactose-lectin-is-critical-for-kupffer-cells-to-ojj72ficvk.pdf

Macrophage galactose lectin is critical for Kupffer cells to clear aged platelets.

In mammals, megakaryocytes (MKs) in the bone marrow (BM) produce blood platelets, required for hemostasis and thrombosis. MKs originate from hematopoietic stem cells and are thought to migrate from an endosteal niche towards the vascular sinusoids during their maturation. Through imaging of MKs in the intact BM, here we show that MKs can be found within the entire BM, without a bias towards bone-distant regions. By combining in vivo two-photon microscopy and in situ light-sheet fluorescence microscopy with computational simulations, we reveal surprisingly slow MK migration, limited intervascular space, and a vessel-biased MK pool. These data challenge the current thrombopoiesis model of MK migration and support a modified model, where MKs at sinusoids are replenished by sinusoidal precursors rather than cells from a distant periostic niche. As MKs do not need to migrate to reach the vessel, therapies to increase MK numbers might be sufficient to raise platelet counts. Megakaryocyte maturation is thought to occur as the cells migrate from a vessel-distant (endosteal) niche to the vessel within the bone. Here, the authors show that megakaryocytes represent largely sessile cells in close contact with the vasculature and homogeneously distributed in the bone marrow.

/pdf/thrombopoiesis-is-spatially-regulated-by-the-bone-marrow-fp9pp07qva.pdf

Thrombopoiesis is spatially regulated by the bone marrow vasculature

Roll, adhere, spread and contract: structural mechanics of platelet function.

Wiskott-Aldrich syndrome (WAS) is caused by mutations in the WAS gene and is characterized by immunodeficiency, eczema and microthrombocytopenia. The molecular link between WAS mutations and microthrombocytopenia is unknown. Profilin1 (Pfn1) is a key actin-regulating protein that, besides actin, interacts with phosphoinositides and multiple proline-rich proteins, including the WAS protein (WASp)/WASp-interacting protein (WIP) complex. Here we report that mice with a megakaryocyte/platelet-specific Pfn1 deficiency display microthrombocytopenia due to accelerated turnover of platelets and premature platelet release into the bone marrow. Both Pfn1-null mouse platelets and platelets isolated from WAS patients contained abnormally organized and hyperstable microtubules. These results reveal an unexpected function of Pfn1 as a regulator of microtubule organization and point to a previously unrecognized mechanism underlying the platelet formation defect in WAS patients.

/pdf/corrigendum-megakaryocyte-specific-profilin1-deficiency-50msyghfc4.pdf

Corrigendum: Megakaryocyte-specific Profilin1-deficiency alters microtubule stability and causes a Wiskott-Aldrich syndrome-like platelet defect.

In this issue of Haematologica, Lian et al. show that megakaryocyte-specific loss of arginyltransferase (ATE1)-mediated post-translational arginylation leads to enhanced clot retraction and in vivo thrombus formation in mice, due to enhanced myosin regulatory light chain (RLC) phosphorylation in

Post-translational arginylation as a novel regulator of platelet function

and sizing dependent actin turnover determines platelet formation - ADF/n-cofilin

in vivo Diverging signaling events control the pathway of GPVI down-regulation

expressed proteinases - Differentially regulated GPVI ectodomain shedding by multiple platelet

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