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.

Mg2+ plays a vital role in platelet function, but despite implications for life-threatening conditions such as stroke or myocardial infarction, the mechanisms controlling [Mg2+]i in megakaryocytes (MKs) and platelets are largely unknown. Transient receptor potential melastatin-like 7 channel (TRPM7) is a ubiquitous, constitutively active cation channel with a cytosolic α-kinase domain that is critical for embryonic development and cell survival. Here we report that impaired channel function of TRPM7 in MKs causes macrothrombocytopenia in mice (Trpm7fl/fl-Pf4Cre) and likely in several members of a human pedigree that, in addition, suffer from atrial fibrillation. The defect in platelet biogenesis is mainly caused by cytoskeletal alterations resulting in impaired proplatelet formation by Trpm7fl/fl-Pf4Cre MKs, which is rescued by Mg2+ supplementation or chemical inhibition of non-muscle myosin IIA heavy chain activity. Collectively, our findings reveal that TRPM7 dysfunction may cause macrothrombocytopenia in humans and mice.

Objective— Ischemic stroke, which is mainly caused by thromboembolic occlusion of brain arteries, is the second leading cause of death and disability worldwide with limited treatment options. The platelet collagen receptor glycoprotein VI (GPVI) is a key player in arterial thrombosis and a critical determinant of stroke outcome, making its signaling pathway an attractive target for pharmacological intervention. The spleen tyrosine kinase (Syk) is an essential signaling mediator downstream of not only GPVI but also other platelet and immune cell receptors. We sought to assess whether Syk might be an effective antithrombotic target. Approach and Results— We demonstrate that mice lacking Syk in platelets specifically are protected from arterial thrombus formation and ischemic stroke but display unaltered hemostasis. Furthermore, we show that mice treated with the novel, selective, and orally bioavailable Syk inhibitor BI1002494 were protected in a model of arterial thrombosis and had smaller infarct sizes and a significantly better neurological outcome 24 hours after transient middle cerebral artery occlusion, also when BI1002494 was administered therapeutically, that is, after ischemia. Conclusions— These results provide direct evidence that pharmacological Syk inhibition might provide a safe therapeutic strategy to prevent arterial thrombosis and to limit infarct progression in acute stroke.