Store-mediated calcium entry in the regulation of phosphatidylserine exposure in blood cells from Scott patients

Abstract

SummaryScott syndrome is a bleeding disorder, characterized by impaired surface exposure of procoagulant phosphatidylserine (PS) on platelets and other blood cells, following activation with Ca2+-elevating agents. Since store-mediated Ca2+ entry (SMCE) forms an important part of the Ca2+ response in various blood cells, it has been proposed that deficiencies in Ca2+ entry may relate to the impaired PS exposure in the Scott syndrome. Here, we have tested this hypothesis by investigating the relationship between Ca2+ fluxes and PS exposure in platelets as well as B-lymphoblasts derived from the original Scott patient (M.S.), a newly identified Welsh patient (V.W.) with similar bleeding symptoms, and two control subjects. Procoagulant activity of V.W. platelets in suspension, measured after stimulation with collagen/thrombin or Ca2+-ionophore, ionomycin, resulted in 52% or 17%, respectively, compared to that of correspondingly activated control platelets. Procoagulant activity of V.W. erythrocytes treated with Ca2+-ionophore resulted in less than 6% of the activity of control erythrocytes. Single-cell Ca2+ responses of M.S. and V.W. platelets, adhering to collagen, were similar to those of platelets from control subjects, while PS exposure was reduced to 7% and 15%, respectively, compared to controls. Stimulation of non-apoptotic B-lymphoblasts derived from both patients and controls with Ca2+-ionophore or agents causing Ca2+ mobilization and SMCE, resulted in similar Ca2+ responses. However, in lymphoblasts from M.S. and V.W. Ca2+-induced PS exposure was reduced to 7% and 13% of the control lymphoblasts, respectively. We conclude that i. patient V.W. is a new case of Scott syndrome, ii. Ca2+ entry in the platelets and lymphoblasts from both Scott patients is normal, and iii. elevated [Ca2+]i as caused by SMCE is not sufficient to trigger PS exposure.

Roles of Platelet STIM1 and Orai1 in Glycoprotein VI- and Thrombin-dependent Procoagulant Activity and Thrombus Formation

“…However, the current findings do not support this because Stim1 Ϫ/Ϫ and Orai1 Ϫ/Ϫ platelets (both devoid of SOCE) almost normally expose PS upon combined collagen-and thrombin-receptor stimulation. This is in agreement with the finding that platelets from Scott syndrome patients, which have a defect in PS exposure, show unchanged Ca 2ϩ signals in response to collagen/thrombin (44). Together, this strongly argues against a role of SOCE or other Ca 2ϩ channels as phospholipid scramblase proteins.…”

Coordinated Membrane Ballooning and Procoagulant Spreading in Human Platelets

“…5,35 Our observations show that platelet ballooning is also markedly impaired in this syndrome ( Figure 1C). It is possible that TMEM16F provides the critical mechanism driving ballooning, through regulated ion influx followed by water, and that the membrane stretching lowers the activation energy required for scrambling of membrane phospholipids and movement of inner leaflet PS to the outer leaflet.…”

Cellular Mechanisms Underlying the Formation of Circulating Microparticles

“…54,59 Importantly, this mechanism of refilling empty internal stores 59 has been found to be reduced in Scott B-lymphoblasts of one Scott patient, 54 although it was normal in others. 60,61 This clearly suggests that defective SOCE cannot be considered a typical feature of Scott syndrome. Nonetheless, this apparent discrepancy might be explained by the heterogeneity of transformation by Epstein-Barr virus.…”

“…This observation is in good agreement with the finding of an unchanged intraplatelet Ca 2ϩ signaling following collagen-thrombin activation in one Scott patient. 60 STIM1 and Orai1 jointly contribute to glycoprotein VI-induced SOCE and procoagulant activity, but a different Ca 2ϩ entry mechanism provides a compensatory pathway when thrombin acts as a coagonist.…”