The monomeric G proteins AGS1 and Rhes selectively influence Gαi-dependent signaling to modulate N-type (Ca_V2.2) calcium channels

Abstract

Activator of G protein Signaling 1 (AGS1) and Ras homologue enriched in striatum (Rhes) define a new group of Ras-like monomeric G proteins whose signaling properties and physiological roles are just beginning to be understood. Previous results suggest that AGS1 and Rhes exhibit distinct preferences for heterotrimeric G proteins, with AGS1 selectively influencing Galphai and Rhes selectively influencing Galphas. Here, we demonstrate that AGS1 and Rhes trigger nearly identical modulation of N-type Ca(2+) channels (Ca(V)2.2) by selectively altering Galphai-dependent signaling. Whole-cell currents were recorded from HEK293 cells expressing Ca(V)2.2 and Galphai- or Galphas-coupled receptors. AGS1 and Rhes reduced basal current densities and triggered tonic voltage-dependent (VD) inhibition of Ca(V)2.2. Additionally, each protein attenuated agonist-initiated channel inhibition through Galphai-coupled receptors without reducing channel inhibition through a Galphas-coupled receptor. The above effects of AGS1 and Rhes were blocked by pertussis toxin (PTX) or by expression of a Gbetagamma-sequestering peptide (masGRK3ct). Transfection with HRas, KRas2, Rap1A-G12V, Rap2B, Rheb2, or Gem failed to duplicate the effects of AGS1 and Rhes on Ca(V)2.2. Our data provide the first demonstration that AGS1 and Rhes exhibit similar if not identical signaling properties since both trigger tonic Gbetagamma signaling and both attenuate receptor-initiated signaling by the Gbetagamma subunits of PTX-sensitive G proteins. These results are consistent with the possibility that AGS1 and Rhes modulate Ca(2+) influx through Ca(V)2.2 channels under more physiological conditions and thereby influence Ca(2+)-dependent events such as neurosecretion.

Activators of G Protein Signaling Exhibit Broad Functionality and Define a Distinct Core Signaling Triad

“…Both AGS1 and Rhes increased the basal activity of N-type calcium channels via apparent activation of Gaibg, and both proteins antagonized the channel activation elicited by activation of a GPCR coupled to pertussis toxin-sensitive G proteins (Thapliyal et al, 2008). The inhibition of receptor-mediated events by AGS1 is similar to that observed for AGS1 in the regulation of potassium channels by M 2 -muscarinic receptors in Xenopus oocytes (Takesono et al, 2002).…”

“…Group I AGS proteins, which include AGS1 (Dexras1, RASD1); resistance to inhibitors of cholinesterase (Ric)-8A and -8B; Ga-interacting, vesicleassociated protein (GIV)/Girdin/Akt-phosphorylation enhancer, act as nonreceptor guanine nucleotide exchange factors (GEFs) for Ga and/or Gabg (Cismowski et al, , 2000Tall et al, 2003;Tall and Gilman, 2005;Garcia-Marcos et al, 2009;Chan et al, 2011aChan et al, , 2013Tall, 2013). Ras homolog enriched in striatum (Rhes) or RASD2, which exhibits 66% amino acid similarity to AGS1, interacts with Gai, and regulates G protein signaling, may be included in this group of AGS proteins, although it has not yet been shown to exhibit GEF activity in experiments with purified proteins (Falk et al, 1999;Vargiu et al, 2004;Thapliyal et al, 2008;Harrison and He, 2011). The Saccharomyces cerevisiae protein Arr4 also acts as a GEF for the yeast G protein GPA1 (Lee and Dohlman, 2008).…”

Ras dexamethasone-induced protein 1 is a modulator of hormone secretion in the volume overloaded heart

“…Constitutively active RASD1 has been found to inhibit spontaneous and, to a greater extent, cAMP-stimulated secretion of human growth hormone in AtT-20 corticotroph cells (11), indicating that RASD1 may act at the level of the dense core secretory granule pathway. RASD1 was also shown to attenuate agonist-dependent Ca v 2.2 channel inhibition through G i -coupled receptors in HEK293 cells, an effect that was blocked by PTX (33). Hence, RASD1 appears to modulate ion influx and thereby potentially influencing Ca 2ϩ -dependent events such as hormone secretion.…”

G protein inhibition of CaV2 calcium channels

“…29 Coexpression of AGS1 or the related Rhes protein with Ca V 2.2 in HEK293 cells induced tonic inhibition due to liberation of Gβγ from Gi/o-type heterotrimers. 179 Thus it is possible that such signaling can occur, but this remains to be demonstrated in native channels under a physiological (or pathophysiological) context.…”