Local subplasma membrane Ca 2+ signals detected by a tethered Ca 2+ sensor

Abstract

Accumulating evidence indicates that plasma membrane (PM) microdomains and the subjacent “junctional” sarcoplasmic/endoplasmic reticulum (jS/ER) constitute specialized Ca²⁺ signaling complexes in many cell types. We examined the possibility that some Ca²⁺ signals arising in the junctional space between the PM and jS/ER may represent cross-talk between the PM and jS/ER. The Ca²⁺ sensor protein, GCaMP2, was targeted to different PM domains by constructing genes for fusion proteins with either the α1 or α2 isoform of the Na⁺ pump catalytic (α) subunit. These fusion proteins were expressed in primary cultured mouse brain astrocytes and arterial smooth muscle cells. Immunocytochemistry demonstrated that α2(f)GCaMP2, like native Na⁺ pumps with α2-subunits, sorted to PM domains that colocalized with subjacent S/ER; α1(f)GCaMP2, like Na⁺ pumps with α1-subunits, was more uniformly distributed. The GCaMP2 moieties in both constructs were tethered just beneath the PM. Both constructs detected global Ca²⁺ signals evoked by serotonin (in arterial smooth muscle cells) and ATP, and by store-operated Ca²⁺ channel-mediated Ca²⁺ entry after S/ER unloading with cyclopiazonic acid (in Ca²⁺-free medium). When cytosolic Ca²⁺ diffusion was markedly restricted with EGTA, however, only α2(f)GCaMP2 detected the local, store-operated Ca²⁺ channel-mediated Ca²⁺ entry signal. Thus, α1 Na⁺ pumps are apparently excluded from the PM microdomains occupied by α2 Na²⁺ pumps. The jS/ER and adjacent PM may communicate by Ca²⁺ signals that are confined to the tiny junctional space between the two membranes. Similar methods may be useful for studying localized Ca²⁺ signals in other subPM microdomains and signals associated with other organelles.