How many Ca2+ATPase isoforms are expressed in a cell type?: A growing family of membrane proteins illustrated by studies in platelets

Ca2+ signaling plays a key role in normal and abnormal platelet functions. Understanding platelet Ca2+ signaling requires the knowledge of proteins involved in this process. Among these proteins are Ca2+ ATPases or Ca2+ pumps that deplete the cytosol of Ca2+ ions. Here, we will particularly focus on two Ca2+ pump families: the plasma membrane Ca(2+)ATPases (PMCAs) that extrude cytosolic Ca2+ towards the extracellular medium and the sarco/endoplasmic reticulum Ca(2+)ATPases (SERCAs) that pump Ca2+ into the endoplasmic reticulum ( ER). In the present review, we will summarize data on platelet Ca(2+)ATPases including their identification and biogenesis. First of all, we will present the Ca(2+)ATPase genes and their isoforms expressed in platelets. We will especially focus on a member of the SERCA family, SERCA3, recently found to give rise to a number of species-specific isoforms. Next, we will describe the differences in Ca(2+)ATPase patterns observed in human and rat platelets. Last, we will analyze how the expression of Ca(2+)ATPase isoforms changes during megakaryocytic maturation and show that megakaryocytopoiesis is associated with a profound reorganization of the expression and/or activity of Ca(2+)ATPases. Taken together, these data provide new aspects of investigations to better understand normal and abnormal platelet Ca2+ signaling.