"Fast" plasma membrane calcium pump PMCA2a concentrates in GABAergic terminals in the adult rat brain.

Ca2+ signaling plays a central role in neurons. To regulate different functions independently within spatially restricted zones, neurons compartmentalize their Ca2+ signals within microdomains. Thus, within a single neuron characteristic ensembles of Ca2+ channels, transporters, and binding proteins are found at distinct locations within the soma, axon, and dendrites (Bootman et al., 2001; Rizzuto, 2001). Growing evidence shows that the plasma membrane Ca2+-ATPase (PMCA) family of calcium pumps plays an important role in shaping Ca2+ dynamics in these restricted cellular compartments (Strehler et al., 2007a). An intriguing feature of the PMCA family is its diversity. In mammals, four PMCA isoforms are encoded by separate genes. From these four genes, more than 20 distinct PMCAs are generated through alternative splicing (Strehler and Zacharias, 2001; Di Leva et al., 2008). Alternative splicing affects two major regions of the protein: the first intracellular loop (site A), and the C-terminal tail (site C), which includes sites for phosphorylation by protein kinases A and C. Calmodulin binding to the C-tail regulates the activity of the pump. In its absence, the C-tail acts as an autoinhibitory domain of the pump (Penniston et al., 1988). When calmodulin binds, the C-tail dissociates from the catalytic core, thus activating the pump. The main splice variants generated at the C-terminal site (termed “a” and “b”) differ in their primary amino acid sequence due to a change in reading frame. Functionally, the most notable consequence of C-terminal splicing is a difference in calmodulin and kinase regulation, leading to different kinetics of calmodulin and Ca2+ activation of these pumps (Penniston and Enyedi, 1998). The large number of functionally different PMCAs raises the possibility that specific PMCA variants are optimized for different kinds of [Ca2+] transients (Domi et al., 2007). Different PMCAs may help shape slow tonic Ca2+ in some cells and provide rapid Ca2+ clearance in others (Strehler et al., 2007b). Consistent with this idea, PMCA isoforms show developmental-, tissue-, and cell-specific patterns (Kip et al., 2006). PMCA isoforms are differentially expressed among different regions of the adult brain, and among different classes of neurons within a single brain region (Burette et al., 2003). Moreover, neurons can restrict PMCA expression to specific subcellular compartments. For example, we recently demonstrated that in cerebellar Purkinje cells, PMCA2 (the main PMCA isoform in the brain) concentrates in dendritic spines (Burette and Weinberg, 2007). Analogously, individual splice variants might be selectively distributed. Such organization could be functionally important, since different C-terminal splice variants of individual PMCA isoforms vary in their basal activity, kinetics, and regulation by modulatory proteins, but little empirical evidence for splice variant specificity is yet available. Here, we used immunohistochemistry to investigate the spatial distribution of the “a” variant of PMCA2 (PMCA2a) in the adult rat brain, testing whether this splice variant is targeted to specific neuronal compartments. We found that PMCA2a concentrates in parvalbumin-positive inhibitory terminals throughout the brain. The only major exception to this targeting pattern was in the cerebellar cortex, where PMCA2a concentrates postsynaptically in the spines of Purkinje cells. This exquisitely specific targeting reinforces the idea that the multiplicity of PMCA splice variants may allow neurons to use the right calcium pump at the right place.