Tosetti et al. investigated the modulation of heterotrimeric guanosine 5′-triphosphate (GTP)-binding protein (G protein)-mediated inhibition of voltage-dependent Ca2+ channels in dorsal root ganglion cells (DRGs) and discovered two Ca2+-dependent mechanisms that could preferentially enhance Ca2+ signaling in particularly active neurons. Signaling by G proteins is terminated by the GTPase activity of the α subunit. Regulators of G protein signaling (RGS) accelerate this process; the mechanisms governing RGS activity, however, remain poorly understood. In chick DRGs, γ-aminobutyric acid (GABA) acts through Gαo to inhibit Ca2+ channels. Tosetti et al. overexpressed variants of RGS3 and examined their effects on GABA-mediated inhibition of whole-cell Ca2+ currents. RGS3s produced a rapid attenuation--or desensitization--of GABA-mediated inhibition of Ca2+ currents. GABA-mediated inhibition of Ba2+ flux through Ca2+ channels was not affected, and an RGS3 mutant lacking an EF-hand (a Ca2+-binding domain) was ineffective. A gel mobility-shift assay indicated that Ca2+, but not Ba2+, bound intact RGS3. RGS3ss, an alternately spliced variant that lacked the EF hand did not elicit rapid desensitization, but did cause a slower form of Ca2+-dependent desensitization. Pharmacological analysis indicated that this slow desensitization depended on Ca2+ indirectly through calmodulin, rather than directly through Ca2+ binding. Thus, the two RGS variants are subject to different modes of Ca2+-dependent regulation. The authors suggest that the two isoforms could provide flexibility in Ca2+-dependent regulation of G protein-mediated inhibition of Ca2+ channels, and that Ca2+ regulation of RGS could provide a positive feedback mechanism to selectively enhance Ca2+ influx into particularly active neurons.
P. Tosetti, N. Pathak, M. H. Jacob, K. Dunlap, RGS3 mediates a calcium-dependent termination of G protein signaling in sensory neurons. Proc. Natl. Acad. Sci. U.S.A. 100, 7337-7342 (2003). [Abstract] [Full Text]