G protein γ (Gγ) subtype dependent targeting of GRK2 to M3 receptor by Gβγ

Spatiotemporal Optical Control of Gαq-PLCβ Interactions

Cells experience time-varying and spatially heterogeneous chemokine signals in vivo, activating cell surface proteins including G protein-coupled receptors (GPCRs). The Gαq pathway activation by GPCRs is a major signaling axis with broad physiological and pathological significance. Compared with other Gα members, GαqGTP activates many crucial effectors, including PLCβ (Phospholipase Cβ) and Rho GEFs (Rho guanine nucleotide exchange factors). PLCβ regulates many key processes, such as hematopoiesis, synaptogenesis, and cell cycle, and is therefore implicated in terminal-debilitating diseases, including cancer, epilepsy, Huntington's Disease, and Alzheimer's Disease. However, due to a lack of genetic and pharmacological tools, examining how the dynamic regulation of PLCβ signaling controls cellular physiology has been difficult. Since activated PLCβ induces several abrupt cellular changes, including cell morphology, examining how the other pathways downstream of Gq-GPCRs contribute to the overall signaling has also been difficult. Here we show the engineering, validation, and application of a highly selective and efficient optogenetic inhibitor (Opto-dHTH) to completely disrupt GαqGTP-PLCβ interactions reversibly in user-defined cellular-subcellular regions on optical command. Using this newly gained PLCβ signaling control, our data indicate that the molecular competition between RhoGEFs and PLCβ for GαqGTP determines the potency of Gq-GPCR-governed directional cell migration.

Spatiotemporal optical control of Gαq-PLCβ interactions

Cells experience time-varying and spatially heterogeneous chemokine signals in vivo, activating cell surface proteins, including G protein-coupled receptors (GPCRs). The Gαq pathway activation by GPCRs is a major signaling axis with a broad physiological and pathological significance. Compared to other Gα members, GαqGTP activates many crucial effectors, including PLCβ (Phospholipase Cβ) and Rho GEFs (Rho guanine nucleotide exchange factors). PLCβ regulates many key processes, such as hematopoiesis, synaptogenesis, and cell cycle, and is therefore implicated in terminal - debilitating diseases, including cancer, epilepsy, Huntington's Disease, and Alzheimer's Disease. However, due to a lack of genetic and pharmacological tools, examining how the dynamic regulation of PLCβ signaling controls cellular physiology has been difficult. Since activated PLCβ induces several abrupt cellular changes, including cell morphology, examining how the other pathways downstream of Gq-GPCRs contribute to the overall signaling has also been difficult. Here we show the engineering, validation, and application of a highly selective and efficient optogenetic inhibitor (Opto-dHTH) to completely disrupt GαqGTP-PLCβ interactions reversibly in user-defined cellular-subcellular regions on optical command. Using this newly gained PLCβ signaling control, our data indicate that the molecular competition between RhoGEFs and PLCβ for GαqGTP determines the potency of Gq-GPCR-governed directional cell migration.

Molecular regulation of PLCβ signaling

Phospholipase C (PLC) enzymes convert the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PIP2) into inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG). IP3 and DAG regulate numerous downstream pathways, eliciting diverse and profound cellular changes and physiological responses. In the six PLC subfamilies in higher eukaryotes, PLCβ is intensively studied due to its prominent role in regulating crucial cellular events underlying many processes including cardiovascular and neuronal signaling, and associated pathological conditions. In addition to GαqGTP, Gβγ generated upon G protein heterotrimer dissociation also regulates PLCβ activity. Here, we not only review how Gβγ directly activates PLCβ, and also extensively modulates Gαq-mediated PLCβ activity, but also provide a structure-function overview of PLC family members. Given that Gαq and PLCβ are oncogenes, and Gβγ shows unique cell-tissue-organ specific expression profiles, Gγ subtype-dependent signaling efficacies, and distinct subcellular activities, this review proposes that Gβγ is a major regulator of Gαq-dependent and independent PLCβ signaling.

A short C-terminal peptide in Gγ regulates Gβγ signaling efficacy

G protein beta-gamma (Gβγ) subunits anchor to the plasma membrane (PM) through the carboxy-terminal (CT) prenyl group in Gγ. This interaction is crucial for the PM localization and functioning of Gβγ, allowing GPCR-G protein signaling to proceed. The diverse Gγ family has 12 members, and we have recently shown that the signaling efficacies of major Gβγ effectors are Gγ-type dependent. This dependency is due to the distinct series of membrane-interacting abilities of Gγ. However, the molecular process allowing for Gβγ subunits to exhibit a discrete and diverse range of Gγ-type-dependent membrane affinities is unclear and cannot be explained using only the type of prenylation. The present work explores the unique designs of membrane-interacting CT residues in Gγ as a major source for this Gγ-type-dependent Gβγ signaling. Despite the type of prenylation, the results show signaling efficacy at the PM, and associated cell behaviors of Gβγ are governed by crucially located specific amino acids in the five to six residue preprenylation region of Gγ. The provided molecular picture of Gγ-membrane interactions may explain how cells gain Gγ-type-dependent G protein-GPCR signaling as well as how Gβγ elicits selective signaling at various subcellular compartments.

Subtype-dependent regulation of Gβγ signalling

G protein-coupled receptors (GPCRs) transmit information to the cell interior by transducing external signals to heterotrimeric G protein subunits, Gα and Gβγ subunits, localized on the inner leaflet of the plasma membrane. Though the initial focus was mainly on Gα-mediated events, Gβγ subunits were later identified as major contributors to GPCR-G protein signalling. A broad functional array of Gβγ signalling has recently been attributed to Gβ and Gγ subtype diversity, comprising 5 Gβ and 12 Gγ subtypes, respectively. In addition to displaying selectivity towards each other to form the Gβγ dimer, numerous studies have identified preferences of distinct Gβγ combinations for specific GPCRs, Gα subtypes and effector molecules. Importantly, Gβ and Gγ subtype-dependent regulation of downstream effectors, representing a diverse range of signalling pathways and physiological functions have been found. Here, we review the literature on the repercussions of Gβ and Gγ subtype diversity on direct and indirect regulation of GPCR/G protein signalling events and their physiological outcomes. Our discussion additionally provides perspective in understanding the intricacies underlying molecular regulation of subtype-specific roles of Gβγ signalling and associated diseases.

Dissociation of the G protein βγ from the Gq-PLCβ complex partially attenuates PIP2 hydrolysis

Phospholipase C β (PLCβ), which is activated by the Gq family of heterotrimeric G proteins, hydrolyzes the inner membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2), generating diacylglycerol and inositol 1,4,5-triphosphate (IP3). Because Gq and PLCβ regulate many crucial cellular processes and have been identified as major disease drivers, activation and termination of PLCβ signaling by the Gαq subunit have been extensively studied. Gq-coupled receptor activation induces intense and transient PIP2 hydrolysis, which subsequently recovers to a low-intensity steady-state equilibrium. However, the molecular underpinnings of this equilibrium remain unclear. Here, we explored the influence of signaling crosstalk between Gq and Gi/o pathways on PIP2 metabolism in living cells using single-cell and optogenetic approaches to spatially and temporally constrain signaling. Our data suggest that the Gβγ complex is a component of the highly efficient lipase Gαq_GTP-PLCβ-Gβγ. We found that over time, Gβγ dissociates from this lipase complex, leaving the less-efficient Gαq_GTP-PLCβ lipase complex and allowing the significant partial recovery of PIP2 levels. Our findings also indicate that the subtype of the Gγ subunit in Gβγ fine-tunes the lipase activity of Gq-PLCβ, in which cells expressing Gγ with higher plasma membrane interaction show lower PIP2 recovery. Given that Gγ shows cell- and tissue-specific subtype expression, our findings suggest the existence of tissue-specific distinct Gq-PLCβ signaling paradigms. Furthermore, these results also outline a molecular process that likely safeguards cells from excessive Gq signaling.

Optical approaches for single-cell and subcellular analysis of GPCR-G protein signaling

G protein-coupled receptors (GPCRs), G proteins, and their signaling associates are major signal transducers that control the majority of cellular signaling and regulate key biological functions including immune, neurological, cardiovascular, and metabolic processes. These pathways are targeted by over one-third of drugs on the market; however, the current understanding of their function is limited and primarily derived from cell-destructive approaches providing an ensemble of static, multi-cell information about the status and composition of molecules. Spatiotemporal behavior of molecules involved is crucial to understanding in vivo cell behaviors both in health and disease, and the advent of genetically encoded fluorescence proteins and small fluorophore-based biosensors has facilitated the mapping of dynamic signaling in cells with subcellular acuity. Since we and others have developed optogenetic methods to regulate GPCR-G protein signaling in single cells and subcellular regions using dedicated wavelengths, the desire to develop and adopt optogenetically amenable assays to measure signaling has motivated us to take a broader look at the available optical tools and approaches compatible with measuring single-cell and subcellular GPCR-G protein signaling. Here we review such key optical approaches enabling the examination of GPCR, G protein, secondary messenger, and downstream molecules such as kinase and lipid signaling in living cells. The methods reviewed employ both fluorescence and bioluminescence detection. We not only further elaborate the underlying principles of these sensors but also discuss the experimental criteria and limitations to be considered during their use in single-cell and subcellular signal mapping.

G protein αq exerts expression level-dependent distinct signaling paradigms

G protein αq-coupled receptors (Gq-GPCRs) primarily signal through GαqGTP mediated phospholipase Cβ (PLCβ) stimulation and the subsequent hydrolysis of phosphatidylinositol 4, 5 bisphosphate (PIP2). Though Gq-heterotrimer activation results in both GαqGTP and Gβγ, unlike Gi/o-receptors, it is unclear if Gq-coupled receptors employ Gβγ as a major signal transducer. Compared to Gi/o- and Gs-coupled receptors, we observed that most cell types exhibit a limited free Gβγ generation upon Gq-pathway and Gαq/11 heterotrimer activation. We show that cells transfected with Gαq or endogenously expressing more than average-levels of Gαq/11 compared to Gαs and Gαi exhibit a distinct signaling regime primarily characterized by recovery-resistant PIP2 hydrolysis. Interestingly, the elevated Gq-expression is also associated with enhanced free Gβγ generation and signaling. Furthermore, the gene GNAQ, which encodes for Gαq, has recently been identified as a cancer driver gene. We also show that GNAQ is overexpressed in tumor samples of patients with Kidney Chromophobe (KICH) and Kidney renal papillary (KIRP) cell carcinomas in a matched tumor-normal sample analysis, which demonstrates the clinical significance of Gαq expression. Overall, our data indicates that cells usually express low Gαq levels, likely safeguarding cells from excessive calcium as wells as from Gβγ signaling.