Ca 2+ Homeostasis by Plasma Membrane Ca 2+ ATPase (PMCA) 1 Is Essential for the Development of DP Thymocytes.

The strength of Ca signaling is a hallmark of T cell activation, yet the role of Ca homeostasis in developing T cells before expressing a mature T cell receptor is poorly understood. We aimed to unveil specific functions of the two plasma membrane Ca ATPases expressed in T cells, PMCA1 and PMCA4. On a transcriptional and protein level we found that PMCA4 was expressed at low levels in CD4CD8 double negative (DN) thymocytes and was even downregulated in subsequent stages while PMCA1 was present throughout development and upregulated in CD4CD8 double positive (DP) thymocytes. Mice with a targeted deletion of in DN3 thymocytes had an almost complete block of DP thymocyte development with an accumulation of DN4 thymocytes but severely reduced numbers of CD8 immature single positive (ISP) thymocytes. The DN4 thymocytes of these mice showed strongly elevated basal cytosolic Ca levels and a pre-mature CD5 expression, but in contrast to the DP thymocytes they were only mildly prone to apoptosis. Surprisingly, mice with a germline deletion of did not show any signs of altered progression through the developmental thymocyte stages, nor altered Ca homeostasis throughout this process. PMCA1 is, therefore, non-redundant in keeping cellular Ca levels low in the early thymocyte development required for the DN to DP transition.