Dannen, K. E.; Yang, J.; Bernholtz, J.; Glebov-McCloud, A.; Strack, S.; Koland, J. G.; Fisher, R. A.; Stewart, A.

Regulator of G protein Signaling 6 (RGS6), heavily implicated in neurological and neuropsychiatric disorders, is enriched in mouse and human brain. Our initial cloning effort identified 36 RGS6 mRNAs in human brain. However, we recently identified an additional RGS6 protein isoform that is larger (~69kDa) than the ubiquitously expressed ~56kDa RGS6L(+GGL) isoforms. Notably, this isoform, named RGS6B for brain-specific, is selectively expressed in the nervous system of mice and humans. Here, we report the cloning of a new RGS6-encoding mRNA, which resembles the RGS6L1(+GGL) transcript identified in our initial cloning effort but includes a highly conserved novel exon (Alternative 3, A3) that alters the reading frame of terminal exon resulting in an extension of the protein C-terminus. When expressed in cells, RGS6LA31(+GGL) co-migrates with RGS6B, and, importantly, interfering RNA targeting exon A3 results in selective depletion of RGS6B in isolated primary cortical astrocytes. RGS6B is capable of stabilizing RGS6 binding partners R7BP and G{beta}5 and, in fact, exhibits an increased protein half-life relative to RGS6L. Both RGS6L and RGS6B are downregulated in human gliomas and share the ability to kill U87MG glioblastoma cells when overexpressed indicating conservation of non-canonical cytotoxic activity between RGS6L and RGS6B species. However, RGS6B lacks the ability to counteract Gi/o-dependent suppression of cAMP signaling, indicating a lack of functional GTPase activating protein (GAP) activity. Instead, RGS6B functions in a dominant negative manner to block Gi/o regulation by RGS6L. RGSB is the first identified RGS protein member that functions to promote, rather than inhibit, G protein signaling. The discovery of the molecular identity of RGS6B will now allow for delineation of unique functions for RGS6 protein isoforms in both physiological and pathophysiological brain states.