Gßγ subunits colocalize with RNA polymerase II and regulate transcription in cardiac fibroblasts.

Gßγ subunits mediate many different signaling processes in various compartments of the cell, including the nucleus. To gain insight into the functions of nuclear Gßγ signaling, we investigated the functional role of Gßγ signaling in the regulation of GPCR-mediated gene expression in primary rat neonatal cardiac fibroblasts. We identified a novel, negative, regulatory role for the Gß1γ dimer in the fibrotic response. Depletion of Gß1 led to derepression of the fibrotic response at the mRNA and protein levels under basal conditions and an enhanced fibrotic response after sustained stimulation of the angiotensin II type I receptor. Our genome-wide chromatin immunoprecipitation experiments revealed that Gß1 colocalized and interacted with RNA polymerase II on fibrotic genes in an angiotensin II-dependent manner. Additionally, blocking transcription with inhibitors of Cdk9 prevented association of Gßγ with transcription complexes. Together, our findings suggest that Gß1γ is a novel transcriptional regulator of the fibrotic response that may act to restrict fibrosis to conditions of sustained fibrotic signaling. Our work expands the role for Gßγ signaling in cardiac fibrosis and may have broad implications for the role of nuclear Gßγ signaling in other cell types.

Gßγ subunits-Different spaces, different faces.

Gßγ subunits play key roles in modulation of canonical effectors in G protein-coupled receptor (GPCR)-dependent signalling at the cell surface. However, a number of recent studies of Gßγ function have revealed that they regulate a large number of molecules at distinct subcellular sites. These novel, non-canonical Gßγ roles have reshaped our understanding of how important Gßγ signalling is compared to our original notion of Gßγ subunits as simple negative regulators of Gα subunits. Gßγ dimers have now been identified as regulators of transcription, anterograde and retrograde trafficking and modulators of second messenger molecule generation in intracellular organelles. Here, we review some recent advances in our understanding of these novel non-canonical roles of Gßγ.

The expanding roles of Gßγ subunits in G protein-coupled receptor signaling and drug action.

Gßγ subunits from heterotrimeric G proteins perform a vast array of functions in cells with respect to signaling, often independently as well as in concert with Gα subunits. However, the eponymous term "Gßγ" does not do justice to the fact that 5 Gß and 12 Gγ isoforms have evolved in mammals to serve much broader roles beyond their canonical roles in cellular signaling. We explore the phylogenetic diversity of Gßγ subunits with a view toward understanding these expanded roles in different cellular organelles. We suggest that the particular content of distinct Gßγ subunits regulates cellular activity, and that the granularity of individual Gß and Gγ action is only beginning to be understood. Given the therapeutic potential of targeting Gßγ action, this larger view serves as a prelude to more specific development of drugs aimed at individual isoforms.

Gß4γ1 as a modulator of M3 muscarinic receptor signalling and novel roles of Gß1 subunits in the modulation of cellular signalling.

Much is known about the how Gßγ subunits regulate effectors in response to G protein-coupled receptor stimulation. However, there is still a lot we don't know about how specific combinations of Gß and Gγ are wired into different signalling pathways. Here, using an siRNA screen for different Gß and Gγ subunits, we examined an endogenous M3 muscarinic receptor signalling pathway in HEK 293 cells. We observed that Gß(4) subunits were critical for calcium signalling and a downstream surrogate measured as ERK1/2 MAP kinase activity. A number of Gγ subunits could partner with Gß(4) but the best coupling was seen via Gß(4)γ(1). Intriguingly, knocking down Gß(1) actually increased signalling through the M3-mAChR most likely via an increase in Gß(4) levels. We noted that Gß(1) occupies the promoter of Gß(4) and may participate in maturation of its mRNA. This highlights a new role for Gßγ signalling beyond their canonical roles in cellular signalling.

Kir3 channel ontogeny - the role of Gßγ subunits in channel assembly and trafficking.

The role of Gßγ subunits in Kir3 channel gating is well characterized. Here, we have studied the role of Gßγ dimers during their initial contact with Kir3 channels, prior to their insertion into the plasma membrane. We show that distinct Gßγ subunits play an important role in orchestrating and fine-tuning parts of the Kir3 channel life cycle. Gß1γ2, apart from its role in channel opening that it shares with other Gßγ subunit combinations, may play a unique role in protecting maturing channels from degradation as they transit to the cell surface. Taken together, our data suggest that Gß1γ2 prolongs the lifetime of the Kir3.1/Kir3.2 heterotetramer, although further studies would be required to shed more light on these early Gßγ effects on Kir3 maturation and trafficking.