A novel mechanism of thrombocytopenia by PS exposure through TMEM16F in sphingomyelin synthase 1 deficiency

Abstract

Sphingomyelin synthase 1 (SMS1) contributes to the generation of membrane sphingomyelin (SM) and affects SM-mediated physiological functions. Here, we describe the hematologic phenotypes, such as reduced circulating platelets and dysfunctional hemostasis, in SMS1-deficient (SMS1-KO) mice. SMS1-KO mice display pathologic manifestations related to idiopathic thrombocytopenia (ITP), including relatively high amounts of peripheral blood reticulated platelets, enhanced megakaryopoiesis in the bone marrow and spleen, and splenomegaly. Deficiency of SMS1, but not SMS2, prevented SM production and enhanced phosphatidylserine (PS) externalization on the plasma membranes of platelets and megakaryocytes. Consequently, SMS1-KO platelets were excessively cleared by macrophages in the spleen. Multimer formation in the plasma membrane of TMEM16F, a known calcium (Ca2+)-activated nonselective ion channel and Ca2+-dependent PS scramblase, was enhanced; the result was PS externalization to outer leaflets through increased Ca2+ influx in immortalized mouse embryonic fibroblasts established from SMS1-KO mice (SMS1-KO tMEFs), as seen with SMS1-KO platelets. Thus, SMS1 deficiency changed the TMEM16F distribution on the membrane microdomain, regulating Ca2+ influx-dependent PS exposure. SMS1-KO tMEFs in which TMEM16F was knocked out by using the CRISPR/Cas9 system lacked both the Ca2+ influx and excess PS exposure seen in SMS1-KO tMEFs. Therefore, SM depletion on platelet membrane microdomains due to SMS1 deficiency enhanced PS externalization via a Ca2+ influx through TMEM16F activation, leading to elevated platelet clearance and causing hemostasis dysfunction through thrombocytopenia. Our current findings show that the SM-rich microdomain generated by SMS1 is a potent regulator of thrombocytopenia through TMEM16F, suggesting that its dysfunction may be a novel additional mechanism of ITP.

Apoptosis in megakaryocytes: Safeguard and threat for thrombopoiesis

“…Moreover, the mice exhibited highly abnormal platelets, with a doubling of mean platelet volume (MPV) and a significantly higher platelet distribution width (PDW). Although it has also been shown that Bcl-xl deficiency induces platelet apoptosis and thus affects platelet lifespan, mathematical modeling of productivity further supports the concept that shortened platelet lifespan cannot explain the severity of thrombocytopenia ( 150 ). Thus, the study further found that the mice with MK-specific deletion of Bcl-xl had a slightly increased number of MK progenitors and MKs.…”

Alterations to Sphingomyelin Metabolism Affect Hemostasis and Thrombosis

“…SMS2 –/– mice, as in ASMase −/− mice, had normal levels of platelets. 63 Therefore, the possibility that potential differences in platelet number could be the reason for observed differences in hemostasis in ASMase −/− and SMS2 –/– mice following the vascular injury is unlikely. However, our current observation with SMS2 −/− mice differs from the earlier data of Guo et al 64 who reported significantly prolonged bleeding time in SMS2 −/− mice.…”

SARS-CoV-2 Spike protein activates TMEM16F-mediated platelet procoagulant activity

“…Decreased platelet levels due to clearance in the spleen are reported in other conditions of PS overexposure ( 64 ), including mice with reduced levels of Bcl-xL ( 64 ). Recent evidence links this feature specifically to TMEM16F and extracellular Ca 2+ influx, as mice with sphingomyelin synthase one deficiency show marked thrombocytopenia due to increased PS exposure consequent to excessive TMEM16F activation ( 65 ). Based on these observations, we speculate that thrombocytopenia during severe COVID-19 may be consequent to abnormal platelet activation through Spike-mediated TMEM16F stimulation.…”