Identification and characterization of alternatively spliced murine Rgs11 isoforms: genomic structure and gene analysis

Regulators of G-protein-signaling proteins: negative modulators of G-protein-coupled receptor signaling

Regulators of G-protein-signaling (RGS) proteins are a category of intracellular proteins that have an inhibitory effect on the intracellular signaling produced by G-protein-coupled receptors (GPCRs). RGS along with RGS-like proteins switch on through direct contact G-alpha subunits providing a variety of intracellular functions through intracellular signaling. RGS proteins have a common RGS domain that binds to G alpha. RGS proteins accelerate GTPase and thus enhance guanosine triphosphate hydrolysis through the alpha subunit of heterotrimeric G proteins. As a result, they inactivate the G protein and quickly turn off GPCR signaling thus terminating the resulting downstream signals. Activity and subcellular localization of RGS proteins can be changed through covalent molecular changes to the enzyme, differential gene splicing, and processing of the protein. Other roles of RGS proteins have shown them to not be solely committed to being inhibitors but behave more as modulators and integrators of signaling. RGS proteins modulate the duration and kinetics of slow calcium oscillations and rapid phototransduction and ion signaling events. In other cases, RGS proteins integrate G proteins with signaling pathways linked to such diverse cellular responses as cell growth and differentiation, cell motility, and intracellular trafficking. Human and animal studies have revealed that RGS proteins play a vital role in physiology and can be ideal targets for diseases such as those related to addiction where receptor signaling seems continuously switched on.

Retina-specific GTPase accelerator RGS11/G beta 5S/R9AP is a constitutive heterotrimer selectively targeted to mGluR6 in ON-bipolar neurons

Members of the R7 family of the regulators of G-protein signaling (R7 RGS) proteins form multi-subunit complexes that play crucial roles in processing the light responses of retinal neurons. The disruption of these complexes has been shown to lead to the loss of temporal resolution in retinal photoreceptors and deficient synaptic transmission to downstream neurons. Despite the well established role of one member of this family, RGS9-1, in controlling vertebrate phototransduction, the roles and organizational principles of other members in the retina are poorly understood. Here we investigate the composition, localization, and function of complexes containing RGS11, the closest homolog of RGS9-1. We find that RGS11 forms a novel obligatory trimeric complex with the short splice isoform of the type 5 G-protein beta subunit (G beta 5) and the RGS9 anchor protein (R9AP). The complex is expressed exclusively in the dendritic tips of ON-bipolar cells in which its localization is accomplished through a direct association with mGluR6, the glutamate receptor essential for the ON-bipolar light response. Although association with both R9AP and mGluR6 contributed to the proteolytic stabilization of the complex, postsynaptic targeting of RGS11 was not determined by its membrane anchor R9AP. Electrophysiological recordings of the light response in mouse rod ON-bipolar cells reveal that the genetic elimination of RGS11 has little effect on the deactivation of G alpha(o) in dark-adapted cells or during adaptation to background light. These results suggest that the deactivation of mGluR6 cascade during the light response may require the contribution of multiple GTPase activating proteins.

The R7 RGS protein family: multi-subunit regulators of neuronal G protein signaling

G protein-coupled receptor signaling pathways mediate the transmission of signals from the extracellular environment to the generation of cellular responses, a process that is critically important for neurons and neurotransmitter action. The ability to promptly respond to rapidly changing stimulation requires timely inactivation of G proteins, a process controlled by a family of specialized proteins known as regulators of G protein signaling (RGS). The R7 group of RGS proteins (R7 RGS) has received special attention due to their pivotal roles in the regulation of a range of crucial neuronal processes such as vision, motor control, reward behavior, and nociception in mammals. Four proteins in this group, RGS6, RGS7, RGS9, and RGS11, share a common molecular organization of three modules: (i) the catalytic RGS domain, (ii) a GGL domain that recruits G beta(5), an outlying member of the G protein beta subunit family, and (iii) a DEP/DHEX domain that mediates interactions with the membrane anchor proteins R7BP and R9AP. As heterotrimeric complexes, R7 RGS proteins not only associate with and regulate a number of G protein signaling pathway components, but have also been found to form complexes with proteins that are not traditionally associated with G protein signaling. This review summarizes our current understanding of the biology of the R7 RGS complexes including their structure/functional organization, protein-protein interactions, and physiological roles.

Full length cloning and expression analysis of splice variants of regulator of G-protein signaling RGS4 in human and murine brain

RGS4 (regulator of G protein signaling 4) protein is a GTPase-activating protein specific for Gi/o and Gq alpha subunits. It is highly expressed in brain but the mechanisms by which RGS4 expression is regulated remain unknown. RGS4 is associated with schizophrenia either through heritable genetic polymorphisms or as a co-regulated mediator of the pathology, and may play a role in other brain diseases. As a necessary step towards understanding the transcriptional regulation of RGS4, we isolated full-length splice variants of the human RGS4 and mouse Rgs4 gene using bioinformatic predictions, followed by RACE, RT-PCR, and sequencing. In human brain, we found five different isoforms RGS4-1, RGS4-2, RGS4-3, RGS4-4 and RGS4-5 of which RGS4-2, RGS4-3, RGS4-4 and RGS4-5 are novel. RGS4-1 and 2 encode a 205-amino acid protein, while RGS4-3 encodes a 302 aa protein with an N-terminal extension. RGS4-4 and RGS4-5 encode truncated proteins of 93 aa and 187 aa respectively. Our results indicate that RGS4-1, RGS4-2, RGS4-3 and RGS4-4 are translated into proteins. In contrast, the mouse brain has 3 different splice variants, Rgs4-1, Rgs4-2 and Rgs4-3 which encode the same 205 aa protein but vary in their 3'UTRs. Among the mouse isoforms, Rgs4-1 and Rgs4-3 are novel. Human RGS4 has four different transcription start sites and three different stop sites. We found differential expression of the human isoforms in dorsolateral prefrontal and visual cortex. All five RGS4 splice variants are expressed at high levels in human cortical areas although RGS4 isoforms 1, 2, and 3 are not expressed in the cerebellum. RGS4-2 is tissue-specific whereas RGS4-4 and RGS4-5 appear to be ubiquitously expressed. Our results suggest the intriguing possibility that RGS4 gene expression in the human brain is spatially and temporally regulated through differential transcription of isoforms from alternative promoters. This may have implications for the physiological role of RGS4 and in pathologies of the brain.

Alternative translation initiation of human regulators of G-protein signaling-2 yields a set of functionally distinct proteins

The regulator of G-protein signaling (RGS2) contains a characteristic RGS domain flanked by short amino and carboxyl terminal sequences. The RGS domain mediates inhibition of Galpha(q) and Galpha(i) signaling, whereas the amino terminal domain (NTD) directs interaction with adenylyl cyclases, G-protein-coupled receptors, and other signaling partners. Here, we identify a set of novel RGS2 protein products that differ with respect to their amino terminal architecture and functional characteristics. An RGS2 expression reporter cassette revealed four distinct open reading frames (ORFs) that can be expressed from the RGS2 NTD. We hypothesized that alternative translation initiation from four AUG codons corresponding to amino acid positions 1, 5, 16, and 33 could produce the observed RGS2 expression profile. Selective disruption of each AUG confirmed that alternate sites of translation initiation accounted for each of the observed products. Proteins derived from ORFs 1 to 4 showed no difference in Galpha(q) inhibitory potential or recruitment from the nucleus in response to Galpha(q) signaling. By contrast, RGS2 products initiating from methionines at positions 16 (ORF3) and 33 (ORF4) were impaired as inhibitors of type V adenylyl cyclase (ACV) compared with full-length RGS2. We predicted that regulation of the RGS2 expression profile would allow cells to adapt to changing signaling conditions. Consistent with this model, activation of Galpha(s)/ACV but not Galpha(q) signaling increased the relative abundance of the full-length RGS2 protein, suggesting that alternative translation initiation of RGS2 is part of a novel negative feedback control pathway for adenylyl cyclase signaling.

Effector antagonism by the regulators of G protein signalling (RGS) proteins causes desensitization of mu-opioid receptors in the CNS

RATIONALE

In cell culture systems, agonists can promote the phosphorylation and internalization of receptors coupled to G proteins (GPCR), leading to their desensitization. However, in the CNS opioid agonists promote a profound desensitization of their analgesic effects without diminishing the presence of their receptors in the neuronal membrane. Recent studies have indicated that CNS proteins of the RGS family, specific regulators of G protein signalling, may be involved in mu-opioid receptor desensitization in vivo.

OBJECTIVE

In this work we review the role played by RGS proteins in the intensity and duration of the effects of mu-opioid receptor agonists, and how they influence the delayed tolerance that develops in response to specific doses of opioids.

RESULTS

RGS proteins are GTPase-activating proteins (GAP) that accelerate the hydrolysis of GalphaGTP to terminate signalling at effectors. The GAP activity of RGS-R4 and RGS-Rz proteins restricts the amplitude of opioid analgesia, and the efficient deactivation of GalphazGTP subunits by RGS-Rz proteins prevents mu receptor desensitization. However, RGS-R7 proteins antagonize effectors by binding to and sequestering mu receptor-activated Galphai/o/z subunits. Thus, they reduce the pool of receptor-regulated G proteins and hence, the effects of agonists. The delayed tolerance observed following morphine administration correlates with the transfer of Galpha subunits from mu receptors to RGS-R7 proteins and the subsequent stabilization of this association.

CONCLUSION

In the CNS, the RGS proteins control the activity of mu opioid receptors through GAP-dependent (RGS-R4 and RGS-Rz) as well as by GAP-independent mechanisms (RGS-R7). As a result, they can both antagonize effectors and desensitize receptors under certain circumstances.

Regulators of G-protein signalling: multifunctional proteins with impact on signalling in the cardiovascular system

Regulator of G-protein signalling (RGS) proteins form a superfamily of at least 25 proteins, which are highly diverse in structure, expression patterns, and function. They share a 120 amino acid homology domain (RGS domain), which exhibits GTPase accelerating activity for alpha-subunits of heterotrimeric G-proteins, and thus, are negative regulators of G-protein-mediated signalling. Based on the organisation of the Rgs genes, structural similarities, and differences in functions, they can be divided into at least six subfamilies of RGS proteins and three more families of RGS-like proteins. Many of these proteins regulate signalling processes within cells, not only via interaction with G-protein alpha-subunits, but are G-protein-regulated effectors, Gbetagamma scavenger, or scaffolding proteins in signal transduction complexes as well. The expression of at least 16 different RGS proteins in the mammalian or human myocardium have been described. A subgroup of at least eight was detected in a single atrial myocyte. The exact functions of these proteins remain mostly elusive, but RGS proteins such as RGS4 are involved in the regulation of G(i)-protein betagamma-subunit-gated K(+) channels. An up-regulation of RGS4 expression has been consistently found in human heart failure and some animal models. Evidence is increasing that the enhanced RGS4 expression counter-regulates the G(q/11)-induced signalling caused by hypertrophic stimuli. In the vascular system, RGS5 seems to be an important signalling regulator. It is expressed in vascular endothelial cells, but not in cultured smooth muscle cells. Its down-regulation, both in a model of capillary morphogenesis and in an animal model of stroke, render it a candidate gene, which may be involved in the regulation of capillary growth, angiogenesis, and in the pathophysiology of stroke.