Membrane grease eases platelet maturation

In this issue of Blood, Valet et al¹  report a novel regulatory role of class II phosphoinositide 3-kinase (PI3K)-C2α in the morphology and remodeling of platelet membranes and its implications in platelet maturation and arterial thrombosis.

In 1906, James Homer Wright first postulated that platelets are produced in the bone marrow from megakaryocytes.²  Our megakaryocytes collectively generate and release ∼1 million platelets per second (10 billion platelets per day) into the bloodstream through a sequence of remodeling events. Megakaryocytes first undergo a series of maturation processes whereby they increase their overall size as they develop highly tortuous membrane invaginations that are called the invaginated membrane system (previously known as the demarcation membrane system).³  Megakaryocytes use this extensive membrane reservoir to form long and thin cytoplasmic extensions to produce intermediate platelet structures called proplatelets.⁴  Once released into the bloodstream, proplatelets undergo further remodeling steps that allow membrane fragmentation and generation of mature platelets. Proplatelet formation and maturation are heavily dependent on the membrane and cytoplasmic skeletal machinery, which include microtubule, actin, and spectrin.^5,6  Although microtubules and actin function as engines to power platelet formation and elongation, spectrin is necessary for the ultimate shape changes that occur when proplatelets mature into platelets. However, the signaling molecules that are regulating and triggering these processes are not well understood.

PI3Ks are one family of lipid kinases that are involved in the complex synthesis of phosphoinositides.⁷  In platelets, PI3Ks play critical signaling roles during the platelet plug formation.⁸  For instance, class I PI3K synthesize phosphatidylinositol 3,4,5 trisphosphate, which recruits downstream effector proteins that are essential for the activation of the key platelet adhesion integrin receptor, αIIbβ3. In contrast, class II and class III PI3K isoforms are thought to synthesize the phosphoinositides phosphatidylinositol 3-phosphate (PI3P) and phosphatidylinositol 3,4 bisphosphate, which are involved in the regulation of endosomal trafficking and cell migration in other cell types. However, the precise roles of class II and class III PI3K isoforms and their lipid products in platelets remain elusive.

In a recent publication, Mountford et al showed that class II PI3K-C2α is a novel regulator of the internal membrane structures of megakaryocytes and platelets.⁹  Using knockdown mouse models, the authors showed that PI3K-C2α deficiency in platelets disrupted the membrane structure of the invaginated membrane system in megakaryocytes and the open canalicular system in platelets by a mechanism that was independent of phosphoinositide production. The authors further showed that these membrane abnormalities were associated with defective platelet adhesion and stable thrombus formation. However, the mechanism by which PI3K-C2α regulates the membrane structures in megakaryocytes and in platelets was not investigated.

In this issue of Blood, Valet et al provide an important clue on how PI3K-C2α may regulate the membrane structures in platelets and in megakaryocytes.¹  Using heterozygous PI3K-C2α kinase-inactive knockin mouse models, Valet et al demonstrate that class II PI3K-C2α is essential for the synthesis of PI3P in platelets under basal but not stimulated conditions. Importantly, this distinct constitutive pool of PI3P is necessary for the remodeling of platelet membrane morphology during the maturation of proplatelets into platelets. The PI3K-C2α inactive megakaryocytes form an underdeveloped invaginated membrane system, and their platelets also develop aberrant and tortuous invaginations of the plasma membrane. Interestingly, mice with inactive PI3K-C2α accumulate barbell-shaped proplatelets in the bloodstream despite a normal platelet count. Moreover, these platelets have a more rigid plasma membrane and impaired filopodium formation upon stimulation. It is remarkable that these membrane defects were associated with a reduced recruitment of membrane skeletal proteins such as spectrin and myosin and membrane receptors such as GPIIb and GPIb. Finally, PI3K-C2α–inactive mice did not have prolonged bleeding times, but they showed delayed arterial thrombus formation when compared with control mice.

These illuminating findings by Valet et al demonstrate important biochemical and physiological roles of class II PI3K-C2α in platelets. In contrast to a recent publication where PI3K-C2α was found to be unnecessary for the intracellular pool of PI3P,⁹  Valet et al provide evidence that class II PI3K-C2α contributes to the maintenance of the basal, but not the agonist-induced, pool of PI3P in platelets. Although a sudden change in the concentration of phosphoinositides has been known to be critical for diverse cellular processes, the physiological role of the housekeeping (basal) pool of phosphoinositides within the cell membrane has been more elusive. Thus, these findings highlight the potentially important functional roles of the housekeeping pools of phosphoinositides.

This study also reveals a previously unknown physiological role of PI3K-C2α in membrane remodeling and has implications for platelet maturation as well as thrombus formation. The authors propose that PI3K-C2α generates a PI3P pool that may modulate membrane elasticity by reorganizing the membrane skeletal proteins (see figure). However, PI3P has been shown to localize predominantly on the early endosomal membranes and not on the plasma membrane. Thus, how does a phospholipid on an internal membrane reorganize events on the cellular membrane? At the very least, further analysis of the precise localization of PI3P pools in platelets will be required to understand these events.

In conclusion, the work by Valet et al sheds light on the novel role and mechanism of class II PI3K-C2α on the membrane morphology and reorganization in platelets. Further research will be essential to understand the signaling mechanisms regulating the complex process of platelet production and maturation.