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

The strength of Ca(2+) signaling is a hallmark of T cell activation, yet the role of Ca(2+) 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(2+) ATPases expressed in T cells, PMCA1 and PMCA4. On a transcriptional and protein level we found that PMCA4 was expressed at low levels in CD4(−)CD8(−) double negative (DN) thymocytes and was even downregulated in subsequent stages while PMCA1 was present throughout development and upregulated in CD4(+)CD8(+) double positive (DP) thymocytes. Mice with a targeted deletion of Pmca1 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(2+) 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 Pmca4 did not show any signs of altered progression through the developmental thymocyte stages, nor altered Ca(2+) homeostasis throughout this process. PMCA1 is, therefore, non-redundant in keeping cellular Ca(2+) levels low in the early thymocyte development required for the DN to DP transition.