Plasma membrane calcium ATPase isoforms in astrocytes

The plasma membrane Ca²⁺-ATPase (PMCA) is an essential component of the machinery responsible for cellular Ca²⁺ homeostasis. Together with the Na⁺/Ca²⁺ exchanger, the plasma membrane Ca²⁺-ATPase (PMCA) is responsible for the extrusion of Ca²⁺ from the cytosol. Although both PMCAs and Na⁺/Ca²⁺ exchangers are present in high amounts in the brain, it is thought that only the latter localize to glia. This study investigates whether PMCAs are also present in astrocytes and thus are components of Ca²⁺ signalling in this cell type. Membrane proteins and mRNA were isolated from primary cultures of rat cortical astrocytes and C6 glioma cells. PMCA isoforms were investigated with isoform specific antibodies and the splice variant pattern was studied in RT-PCR experiments using specific oligonucleotides. The PMCA1, 2, and 4 isoforms were detected in rat cortical astrocytes, whereas only PMCA1 and 2 were found in C6 cells. While neurons express both the CI and CII splice variants, only the splice variant CI of PMCA1, 2, and 4 was detected in astrocytes. Thus, the PMCA pump is present in mammalian glial cells. These results also show that the amounts of PMCA1 and 4 isoforms in astrocytes are comparable to those found in neurons. In contrast, astrocytes contain smaller amounts of PMCA2. Furthermore, PMCA2 and PMCA4 underwent an evident time dependent up-regulation in astrocytes cultured in vitro.