Na/Ca Exchanger and PMCA Localization in Neurons and Astrocytes
Immunocytochemistry reveals that the Na/Ca exchanger (NCX) in neuronal somata and astrocytes is confined to plasma membrane (PM) microdomains that overlie sub-PM (junctional) endoplasmic reticulum (jER). By contrast, the PM Ca2+ pump (PMCA) is more uniformly distributed in the PM. At presynaptic nerve terminals, the NCX distribution is consistent with that observed in the neuronal somata, but the PMCA is clustered at the active zones. Thus, the PMCA, with high affinity for Ca2+ (Kd?100 nM), may keep active zone Ca2+ very low and thereby reprime the vesicular release mechanism following activity. NCX, with lower affinity for Ca2+ (Kd?1,000 nM), on the other hand, may extrude Ca2+ that has diffused away from the active zones and been temporarily sequestered in the endoplasmic reticulum. The PL microdomains that contain the NCX also contain Na+ pump high ouabain affinity ?2 (astrocytes) or ?3 (neurons) subunit isoforms (IC50?5-50 nM ouabain). In constrast, the ?1 isoform (low ouabain affinity in rodents; IC50 >10,000 nM), like the PMCA, is more uniformly distributed in these cells. The sub-PM endoplasmic reticulum in neurons (and probably glia and other cell types as well) and the adjacent PM form junctions that resemble cardiac muscle dyads. We suggest that the PM microdomains containing NCX and ?2/?3 Na+ pumps, the underlying jER, and the intervening tiny volume of cytosol (<10?18 l) form functional units (PLasmERosomes); diffusion of Na+ and Ca2+ between these cytosolic compartments and bulk cytosol may be markedly restricted. The activity of the Na+ pumps with ?2/?3 subunits may thus regulate NCX activity and jER Ca2+ content. This view is supported by studies in mice with genetically reduced (by ?50%) ?2 Na+ pumps
na/ca exchanger pmca localization neurons astrocytes