Differential distribution of G-protein beta-subunits in brain: an immunocytochemical analysis

Non-canonical G protein signaling

The original paradigm of classical - also referred to as canonical - cellular signal transduction of heterotrimeric G proteins (G protein) is defined by a hierarchical, orthograde interaction of three players: the agonist-activated G protein-coupled receptor (GPCR), which activates the transducing G protein, that in turn regulates its intracellular effectors. This receptor-transducer-effector concept was extended by the identification of regulators and adapters such as the regulators of G protein signaling (RGS), receptor kinases like βARK, or GPCR-interacting arrestin adapters that are integrated into this canonical signaling process at different levels to enable fine-tuning. Finally, the identification of atypical signaling mechanisms of classical regulators, together with the discovery of novel modulators, added a new and fascinating dimension to the cellular G protein signal transduction. This heterogeneous group of accessory G protein modulators was coined "activators of G protein signaling" (AGS) proteins and plays distinct roles in canonical and non-canonical G protein signaling pathways. AGS proteins contribute to the control of essential cellular functions such as cell development and division, intracellular transport processes, secretion, autophagy or cell movements. As such, they are involved in numerous biological processes that are crucial for diseases, like diabetes mellitus, cancer, and stroke, which represent major health burdens. Although the identification of a large number of non-canonical G protein signaling pathways has broadened the spectrum of this cellular communication system, their underlying mechanisms, functions, and biological effects are poorly understood. In this review, we highlight and discuss atypical G protein-dependent signaling mechanisms with a focus on inhibitory G proteins (Gi) involved in canonical and non-canonical signal transduction, review recent developments and open questions, address the potential of new approaches for targeted pharmacological interventions.

Cellular and Subcellular Localization of the RGS7/Gβ5/R7BP Complex in the Cerebellar Cortex

A member of regulator of G-protein signaling family, RGS7, is an essential modulator of signaling through GABA_B receptors. RGS7 functions as a macromolecular complex with type 5 G protein β (Gβ5) and R7 binding protein (R7BP) to control the localization and function of the resultant heterotrimeric complexes. Here, we used co-immunoprecipitation, in situ hybridization, histoblot and immunohistochemical techniques at the light and electron microscopic level to advance understanding of RGS7-Gβ5-R7BP complexes in the central nervous system, focusing on distinct neuronal populations in the cerebellar cortex. Histoblot analysis showed that RGS7, Gβ5 and R7BP proteins were widely expressed in the brain, with mostly an overlapping pattern and showing a high expression level in the molecular layer of the cerebellar cortex. Co-immunoprecipitation experiments established that the RGS7/Gβ5 forms complexes with R7BP in the cerebellum. At the cellular level, RGS7 and R7BP mRNAs were expressed at the highest level in Purkinje cells (PCs) and Golgi cells, and at low levels in granule cells. Immunohistochemistry confirmed that labeling for RGS7, Gβ5 and R7BP were present in the three neuronal populations and concentrated in dendrites and spines. At the electron microscopic level, immunolabeling for RGS7, Gβ5 and R7BP proteins was found both at postsynaptic and presynaptic sites and showed similar distribution patterns. Immunoreactivity for the three proteins was mostly localized along the extrasynaptic plasma membrane of dendritic shafts and spines of PCs and to a lesser extent, in axon terminals (AT) establishing excitatory synapses. Quantitative analysis of immunogold particles for RGS7, Gβ5 and R7BP revealed that they are non-uniformly distributed along the surface of PCs, and show enrichment around excitatory synapses on dendritic spines. We further report that deletion of R7BP in mice reduced the targeting of both RGS7 and Gβ5 to the plasma membrane. Altogether, these data support the existence of macromolecular complexes composed of RGS7-Gβ5-R7BP in PCs. The location at post- and pre-synaptic sites in PCs spines-parallel fiber synapses suggests their involvement in the modulation of glutamatergic neurotransmission in the cerebellar cortex.

Postnatal induction and localization of R7BP, a membrane-anchoring protein for regulator of G protein signaling 7 family-Gbeta5 complexes in brain

Members of the regulator of G protein signaling 7 (RGS7) (R7) family and Gbeta5 form obligate heterodimers that are expressed predominantly in the nervous system. R7-Gbeta5 heterodimers are GTPase-activating proteins (GAPs) specific for Gi/o-class Galpha subunits, which mediate phototransduction in retina and the action of many modulatory G protein-coupled receptors (GPCRs) in brain. Here we have focused on the R7-family binding protein (R7BP), a recently identified palmitoylated protein that can bind R7-Gbeta5 complexes and is hypothesized to control the intracellular localization and function of the resultant heterotrimeric complexes. We show that: 1) R7-Gbeta5 complexes are obligate binding partners for R7BP in brain because they co-immunoprecipitate and exhibit similar expression patterns. Furthermore, R7BP and R7 protein accumulation in vivo requires Gbeta5. 2) Expression of R7BP in Neuro2A cells at levels approximating those in brain recruits endogenous RGS7-Gbeta5 complexes to the plasma membrane. 3) R7BP immunoreactivity in brain concentrates in neuronal soma, dendrites, spines or unmyelinated axons, and is absent or low in glia, myelinated axons, or axon terminals. 4) RGS7-Gbeta5-R7BP complexes in brain extracts associate inefficiently with detergent-resistant lipid raft fractions with or without G protein activation. 5) R7BP and Gbeta5 protein levels are upregulated strikingly during the first 2-3 weeks of postnatal brain development. Accordingly, we suggest that R7-Gbeta5-R7BP complexes in the mouse or rat could regulate signaling by modulatory Gi/o-coupled GPCRs in the developing and adult nervous systems.

A Golgi-localized hexose transporter is involved in heterotrimeric G protein-mediated early development in Arabidopsis

Signal transduction involving heterotrimeric G proteins is universal among fungi, animals, and plants. In plants and fungi, the best understood function for the G protein complex is its modulation of cell proliferation and one of several important signals that are known to modulate the rate at which these cells proliferate is D-glucose. Arabidopsis thaliana seedlings lacking the beta subunit (AGB1) of the G protein complex have altered cell division in the hypocotyl and are D-glucose hypersensitive. With the aim to discover new elements in G protein signaling, we screened for gain-of-function suppressors of altered cell proliferation during early development in the agb1-2 mutant background. One agb1-2-dependent suppressor, designated sgb1-1(D) for suppressor of G protein beta1 (agb1-2), restored to wild type the altered cell division in the hypocotyl and sugar hypersensitivity of the agb1-2 mutant. Consistent with AGB1 localization, SGB1 is found at the highest steady-state level in tissues with active cell division, and this level increases in hypocotyls when grown on D-glucose and sucrose. SGB1 is shown here to be a Golgi-localized hexose transporter and acts genetically with AGB1 in early seedling development.

Functional G-protein heterotrimers are associated with vesicles of putative glutamatergic terminals: implications for regulation of transmitter uptake

Changes in the vesicular transmitter content modulate synaptic strength and may contribute to synaptic plasticity. Several transporters mediating transmitter uptake into small synaptic vesicles (SSVs) have been identified but their regulation is largely unknown. Here we show by quantitative immunoelectron microscopy that the heterotrimeric G-protein subunits Galphao(2), Galpha(q/11), Gbeta(2), and Ggamma(7) are associated with vesicle-containing areas in terminals of cerebellar parallel fibers. These terminals also contain the vesicular glutamate transporter 1 (VGLUT1). In contrast, SSVs of climbing fiber terminals that contain VGLUT2 express one of the Gbeta-subunits Gbeta(1), Gbeta(3), or Gbeta(4), Ggamma(7), and one Galpha-subunit, probably Galphao(2). Glutamate uptake into cerebellar SSVs was inhibited by more than 50% by GMppNp, an activator of G proteins. Thus, vesicle populations with different subtypes of vesicular glutamate transporters contain functional G proteins with distinct subunit profiles. Heterotrimeric G proteins may play an important role in the control of vesicular filling.

The vesicular monoamine content regulates VMAT2 activity through Galphaq in mouse platelets. Evidence for autoregulation of vesicular transmitter uptake

Variations in the neurotransmitter content of secretory vesicles enable neurons to adapt to network changes. Vesicular content may be modulated by vesicle-associated Go(2), which down-regulates the activity of the vesicular monoamine transmitter transporters VMAT1 in neuroendocrine cells and VMAT2 in neurons. Blood platelets resemble serotonergic neurons with respect to transmitter storage and release. In streptolysin O-permeabilized platelets, VMAT2 activity is also down-regulated by the G protein activator guanosine 5'-(beta(i)gamma-imido)triphosphate (GMppNp). Using serotonin-depleted platelets from peripheral tryptophan hydroxylase knockout (Tph1-/-) mice, we show here that the vesicular filling initiates the G protein-mediated down-regulation of VMAT2 activity. GMppNp did not influence VMAT2 activity in naive platelets from Tph1-/- mice. GMppNp-mediated inhibition could be reconstituted, however, when preloading Tph1-/- platelets with serotonin or noradrenaline. Galpha(q) mediates the down-regulation of VMAT2 activity as revealed from uptake studies performed with platelets from Galpha(q) deletion mutants. Serotonergic, noradrenergic, as well as thromboxane A(2) receptors are not directly involved in the down-regulation of VMAT2 activity. It is concluded that in platelets the vesicle itself regulates transmitter transporter activity via its content and vesicle-associated Galpha(q).

Subunit composition and functional properties of G-protein heterotrimers on rat chromaffin granules

Heterotrimeric G-proteins at the plasma membrane serve as switches between heptahelical receptors and intracellular signal cascades. Likewise endomembrane associated G-proteins may transduce signals from intracellular compartments provided they consist of a functional trimer. Using quantitative immunoelectron microscopy we found heterotrimeric G-protein subunits Galpha2, Galpha(q/11), Gbeta2 and Gbeta5 to reside on secretory granules in chromaffin cells of rat adrenal glands. Thus rat chromaffin granules are equipped with functional G-proteins that consist of a specific alpha-, beta- and probably gamma-subunit combination. Serotonin uptake into a crude rat chromaffin granule preparation was inhibited by activated Galphao2 (10 nM) to nearly the same extent as by GMppNp (50 microM) whereas GDPbetaS was ineffective. The data support the idea that vesicular G-proteins directly regulate the transmitter content of secretory vesicles. In this respect Galphao2 appears to be the main regulator of vesicular momoamine transporter activity.

Receptor-G protein gamma specificity: gamma11 shows unique potency for A(1) adenosine and 5-HT(1A) receptors

G protein coupled receptors activate signal transducing guanine nucleotide-binding proteins (G proteins), which consist of an alpha subunit and a betagamma dimer. Whole cell studies have reported that receptors signal through specific betagamma subtypes. Membrane reconstitution studies with the adenosine A(1) and alpha(2A) adrenergic receptors have reached a similar conclusion. We aimed to test the generality of this finding by comparing the gamma subtype specificity for four G(i)-coupled receptors: alpha(2A) adrenergic; A1 adenosine (A(1)-R); 5-hydroxytryptamine(1A) (5-HT(1A)-R); mu opioid. Membranes were reconstituted with Galpha(i)(1) and five gamma subtypes (dimerized to beta1). Using a sensitive alpha-betagamma binding assay, we show that all recombinant betagamma (except beta1gamma1) had comparable affinity for alpha(i)(1). Using high affinity agonist binding as a measure of receptor-G protein coupling, betagamma-containing gamma11 was the most potent for A(1)-R and 5-HT(1A)-R (p < 0.05, one way ANOVA) while gamma7 was most potent for the other two receptors. gamma11 was 3-8-fold more potent for the A(1)-R than were the other gamma subtypes. Also, gamma11 was 2-8-fold more potent for A(1)-R than at the other receptors, suggesting a unique coupling specificity of the A(1)-R for gamma11. In contrast, the discrimination by receptors for the other betagamma subtypes (beta1 and gamma1, gamma2, gamma7, and gamma10) was limited (2-3-fold). Thus the exquisite betagamma specificity of individual receptors reported in whole cell studies may depend on in vivo mechanisms beyond direct receptor recognition of betagamma subtypes.

Gbeta gamma isoforms selectively rescue plasma membrane localization and palmitoylation of mutant Galphas and Galphaq

Mutation of Galpha(q) or Galpha(s) N-terminal contact sites for Gbetagamma resulted in alpha subunits that failed to localize at the plasma membrane or undergo palmitoylation when expressed in HEK293 cells. We now show that overexpression of specific betagamma subunits can recover plasma membrane localization and palmitoylation of the betagamma-binding-deficient mutants of alpha(s) or alpha(q). Thus, the betagamma-binding-defective alpha is completely dependent on co-expression of exogenous betagamma for proper membrane localization. In this report, we examined the ability of beta(1-5) in combination with gamma(2) or gamma(3) to promote proper localization and palmitoylation of mutant alpha(s) or alpha(q). Immunofluorescence localization, cellular fractionation, and palmitate labeling revealed distinct subtype-specific differences in betagamma interactions with alpha subunits. These studies demonstrate that 1) alpha and betagamma reciprocally promote the plasma membrane targeting of the other subunit; 2) beta(5), when co-expressed with gamma(2) or gamma(3), fails to localize to the plasma membrane or promote plasma membrane localization of mutant alpha(s) or alpha(q); 3) beta(3) is deficient in promoting plasma membrane localization of mutant alpha(s) and alpha(q), whereas beta(4) is deficient in promoting plasma membrane localization of mutant alpha(q); 4) both palmitoylation and interactions with betagamma are required for plasma membrane localization of alpha.

New thoughts on the role of the beta-gamma subunit in G-protein signal transduction

Heterotrimeric G proteins are involved in numerous biological processes, where they mediate signal transduction from agonist-bound G-protein-coupled receptors to a variety of intracellular effector molecules and ion channels. G proteins consist of two signaling moieties: a GTP-bound alpha subunit and a beta-gamma heterodimer. The beta-gamma dimer, recently credited as a significant modulator of G-protein-mediated cellular responses, is postulated to be a major determinant of signaling fidelity between G-protein-coupled receptors and downstream effectors. In this review we have focused on the role of beta-gamma signaling and have included examples to demonstrate the heterogeneity in the heterodimer composition and its implications in signaling fidelity. We also present an overview of some of the effectors regulated by beta-gamma and draw attention to the fact that, although G proteins and their associated receptors play an instrumental role in development, there is rather limited information on beta-gamma signaling in embryogenesis.

Gbeta 5gamma 2 is a highly selective activator of phospholipid-dependent enzymes

In this study, Gbeta specificity in the regulation of Gbetagamma-sensitive phosphoinositide 3-kinases (PI3Ks) and phospholipase Cbeta (PLCbeta) isozymes was examined. Recombinant mammalian Gbeta(1-3)gamma(2) complexes purified from Sf9 membranes stimulated PI3Kgamma lipid kinase activity with similar potency (10-30 nm) and efficacy, whereas transducin Gbetagamma was less potent. Functionally active Gbeta(5)gamma(2) dimers were purified from Sf9 cell membranes following coexpression of Gbeta(5) and Ggamma(2-His). This preparation as well as Gbeta(1)gamma(2-His) supported pertussis toxin-mediated ADP-ribosylation of Galpha(i1). Gbeta(1)gamma(2-His) stimulated PI3Kgamma lipid and protein kinase activities at nanomolar concentrations, whereas Gbeta(5)gamma(2-His) had no effect. Accordingly, Gbeta(1)gamma(2-His), but not Gbeta(5)gamma(2-His), significantly stimulated the lipid kinase activity of PI3Kbeta in the presence or absence of tyrosine-phosphorylated peptides derived from the p85-binding domain of the platelet derived-growth factor receptor. Conversely, both preparations were able to stimulate PLCbeta(2) and PLCbeta(1). However, Gbeta(1)gamma(2-His), but not Gbeta(5)gamma(2-His), activated PLCbeta(3). Experimental evidence suggests that the mechanism of Gbeta(5)-dependent effector selectivity may differ between PI3K and PLCbeta. In conclusion, these data indicate that Gbeta subunits are able to discriminate among effectors independently of Galpha due to selective protein-protein interaction.

The neuronal monoamine transporter VMAT2 is regulated by the trimeric GTPase Go(2)

Monoamines such as noradrenaline and serotonin are stored in secretory vesicles and released by exocytosis. Two related monoamine transporters, VMAT1 and VMAT2, mediate vesicular transmitter uptake. Previously we have reported that in the rat pheochromocytoma cell line PC 12 VMAT1, localized to peptide-containing secretory granules, is controlled by the heterotrimeric G-protein Go(2). We now show that in BON cells, a human serotonergic neuroendocrine cell line derived from a pancreatic tumor expressing both transporters on large, dense-core vesicles, VMAT2 is even more sensitive to G-protein regulation than VMAT1. The activity of both transporters is only downregulated by Galphao(2), whereas comparable concentrations of Galphao(1) are without effect. In serotonergic raphe neurons in primary culture VMAT2 is also downregulated by pertussis toxin-sensitive Go(2). By electron microscopic analysis from prefrontal cortex we show that VMAT2 and Galphao(2) associate preferentially to locally recycling small synaptic vesicles in serotonergic terminals. In addition, Go(2)-dependent modulation of VMAT2 also works when using a crude synaptic vesicle preparation from this brain area. We conclude that regulation of monoamine uptake by the heterotrimeric G proteins is a general feature of monoaminergic neurons that controls the content of both large, dense-core and small synaptic vesicles.

Signal transduction by a nondissociable heterotrimeric yeast G protein

Many signal transduction pathways involve heterotrimeric G proteins. The accepted model for activation of heterotrimeric G proteins states that the protein dissociates to the free G(alpha) (GTP)-bound subunit and free G(betagamma) dimer. On GTP hydrolysis, G(alpha) (GDP) then reassociates with G(betagamma) [Gilman, A. G. (1987) Annu. Rev. Biochem. 56, 615-649]. We reexamined this hypothesis, by using the mating G protein of the yeast Saccharomyces cerevisiae encoded by the genes GPA1, STE4, and STE18. In the absence of mating pheromone, the G(alpha) (Gpa1) subunit represses the mating pathway. On activation by binding of pheromone to a serpentine receptor, the G(betagamma) (Ste4, Ste18) dimer transmits the signal to a mitogen-activated protein kinase cascade, leading to gene activation, arrest in the G(1) stage of the cell cycle, production of shmoos (mating projections), and cell fusion. We found that a Ste4-Gpa1 fusion protein transmitted the pheromone signal and activated the mating pathway as effectively as when Ste4 (G(beta)) and Gpa1 (G(alpha)) were coexpressed as separate proteins. Hence, dissociation of this G protein is not required for its activation. Rather, a conformational change in the heterotrimeric complex is likely to be involved in signal transduction.

Signal transduction by a nondissociable heterotrimeric yeast G protein

Many signal transduction pathways involve heterotrimeric G proteins. The accepted model for activation of heterotrimeric G proteins states that the protein dissociates to the free G(alpha) (GTP)-bound subunit and free G(betagamma) dimer. On GTP hydrolysis, G(alpha) (GDP) then reassociates with G(betagamma) [Gilman, A. G. (1987) Annu. Rev. Biochem. 56, 615-649]. We reexamined this hypothesis, by using the mating G protein of the yeast Saccharomyces cerevisiae encoded by the genes GPA1, STE4, and STE18. In the absence of mating pheromone, the G(alpha) (Gpa1) subunit represses the mating pathway. On activation by binding of pheromone to a serpentine receptor, the G(betagamma) (Ste4, Ste18) dimer transmits the signal to a mitogen-activated protein kinase cascade, leading to gene activation, arrest in the G(1) stage of the cell cycle, production of shmoos (mating projections), and cell fusion. We found that a Ste4-Gpa1 fusion protein transmitted the pheromone signal and activated the mating pathway as effectively as when Ste4 (G(beta)) and Gpa1 (G(alpha)) were coexpressed as separate proteins. Hence, dissociation of this G protein is not required for its activation. Rather, a conformational change in the heterotrimeric complex is likely to be involved in signal transduction.