The intracellular and plasma membrane pools of PI4P control megakaryocyte maturation and proplatelet formation

Abstract

Megakaryocytes (MKs) develop from hematopoietic stem cells (HSCs) after stimulation by the cytokine thrombopoietin (TPO). During megakaryopoiesis, MKs enlarge, undergo the process of endomitosis and develop intracellular membranes (the demarcation membrane system, DMS) which serve as a source for future platelets (PLTs). During DMS formation, there is an active transport from the Golgi apparatus to the DMS for the delivery of proteins, lipids, and membranes. The most important phosphoinositide that controls anterograde transport from the Golgi apparatus to the PM is phosphatidylinositol-4-monophosphate (PI4P) controlled by the SACM1L phosphatase at the Golgi and the endoplasmic reticulum (ER). The role of SACM1L and PI4P in megakaryopoiesis has not been investigated so far. Here we show that in primary mouse MKs, SACM1L is mostly localized and condensed perinuclearly in immature MKs, while at later stages it is mostly dispersed and confines to the ER. At the same time, PI4P is mostly found at the Golgi apparatus and the plasma membrane (PM) in immature MKs while in mature MKs it is in the periphery of the cell and at the PM. The exogenous expression of wild-type, but not C389S mutant (catalytically dead) SACM1L, results in the retention of the Golgi apparatus leading to the increased number of immature MKs, as well as a decreased number of MKs that form proplatelets. The inhibition of the production of PI4P specifically at the PM (inhibiting PI4-kinase III) resulted also in a significant decrease of MKs that form proplatelets. These results indicate that both Golgi and PM pools of PI4P mediate MK maturation and proplatelet formation.