Transient receptor potential channel activation causes a novel form of [Ca2+]i oscillations and is not involved in capacitative Ca2+ entry in glial cells

Abstract

Astrocytes express transient receptor potential channels (TRPCs), which have been implicated in Ca2+ influx triggered by intracellular Ca2+ stores depletion, a phenomenon known as capacitative Ca2+ entry. We studied the properties of capacitative Ca2+ entry in astrocytes by means of single-cell Ca2+ imaging with the aim of understanding the involvement of TRPCs in this function. We found that, in astrocytes, capacitative Ca2+ entry is not attributable to TRPC opening because the TRPC-permeable ions Sr2+ and Ba2+ do not enter astrocytes during capacitative Ca2+ entry. Instead, natively expressed oleyl-acetyl-glycerol (OAG) (a structural analog of DAG)-sensitive TRPCs, when activated, initiate oscillations of cytosolic Ca2+ concentration ([Ca2+]i) pharmacologically and molecularly consistent with TRPC3 activation. OAG-induced [Ca2+]i oscillations are not affected by inhibition of inositol trisphosphate (InsP3) production or blockade of the InsP3 receptor, therefore representing a novel form of [Ca2+]i signaling. Instead, high [Ca2+]i inhibited oscillations, by closing the OAG-sensitive channel. Also, treatment of astrocytes with antisense against TRPC3 caused a consistent decrease of the cells responding to OAG. Exogenous OAG but not endogenous DAG seems to activate TRPC3. In conclusion, in glial cells, natively expressed TRPC3s mediates a novel form of Ca2+ signaling, distinct from capacitative Ca2+ entry, which suggests a specific signaling function for this channel in glial cells.