Essential role of class II PI3K-C2α in platelet membrane morphology

Abstract

Key Points• PI3K-C2a controls platelet membrane structure and remodeling.• PI3K-C2a is a key regulator of a basal housekeeping PI3P pool in platelets.The physiologic roles of the class II phosphoinositide 3-kinases (PI3Ks) and their contributions to phosphatidylinositol 3-monophosphate (PI3P) and PI(3,4)P 2 production remain elusive. Here we report that mice heterozygous for a constitutively kinase-dead PI3K-C2a display aberrant platelet morphology with an elevated number of barbell-shaped proplatelets, a recently discovered intermediate stage in the final process of platelet production. Platelets with heterozygous PI3K-C2a inactivation have critical defects in a-granules and membrane structure that are associated with modifications in megakaryocytes. These platelets are more rigid and unable to form filopodia after stimulation. Heterozygous PI3K-C2a inactivation in platelets led to a significant reduction in the basal pool of PI3P and a mislocalization of several membrane skeleton proteins known to control the interactions between the plasma membrane and cytoskeleton. These alterations had repercussions on the performance of platelet responses with delay in the time of arterial occlusion in an in vivo model of thrombosis and defect in thrombus formation in an ex vivo blood flow system. These data uncover a key role for PI3K-C2a activity in the generation of a basal housekeeping PI3P pool and in the control of membrane remodeling, critical for megakaryocytopoiesis and normal platelet production and function. (Blood. 2015;126(9):1128-1137

The PI 3‐kinase PI3KC2α regulates mouse platelet membrane structure and function independently of membrane lipid composition

“…We and others have previously shown that platelets from PI3KC2a-deficient mice exhibit an increased surface area occupied by the channels of the OCS in TEM images [6,8]. We showed that PI3KC2a deficiency leads to an ultrastructural change in platelets that is limited to the internal membrane (OCS) of these cells.…”

“…As a result, the significant reduction in platelet deposition in this system using blood from PI3KC2a mice may suggest that PI3KC2a is important for the earliest stages of platelet adhesion, where membrane tethers, presumably derived from the OCS, are pulled from platelets without detectable cell activation via an interaction between von Willebrand factor and the platelet adhesion receptor GPIba [17]. The surprising lack of effect of PI3KC2a deficiency in standard assays of agonist-induced in vitro platelet function [6,8] supports this hypothesis. Alternatively, PI3KC2a may be involved in the subsequent propagation of platelet aggregates, since incoming platelets that tether to the surface of forming thrombi are discoid and demonstrate low levels of activation markers and minimal calcium flux, in sharp contrast to the initial core of the thrombus where platelets are generally fully activated [12,18].…”

“…PI3KC2a in particular regulates the thrombotic function of mouse platelets by a unique and as-yet-undefined mechanism that may involve the structure of the cell membrane. A subsequent study confirmed this role for PI3KC2a in platelet structure and function in a distinct mouse model involving heterozygosity of a kinase-inactivating point mutation in the PI3KC2a active site [8]. Here, transmission electron microscopy (TEM) revealed that PI3KC2a-deficient platelets exhibit dilatations of the channels of the OCS.…”

“…It has previously been shown that PI3KC2a-deficiency reduces the basal levels of PI(3)P in platelets [8]. It has previously been shown that PI3KC2a-deficiency reduces the basal levels of PI(3)P in platelets [8].…”

“…There are eight mammalian PI3K isoforms, divided into three classes based on structural and functional similarities: four class I, three class II, and one class III PI3K. More recently, mouse genetic models have been used to uncover the importance of the sole class III PI3K, Vps34 [4,5], and one of the class II PI3Ks, PI3KC2a [6][7][8], in platelet function. More recently, mouse genetic models have been used to uncover the importance of the sole class III PI3K, Vps34 [4,5], and one of the class II PI3Ks, PI3KC2a [6][7][8], in platelet function.…”