Phospholipase C-independent Activation of Glycogen Synthase Kinase-3β and C-terminal Src Kinase by Gαq

A mammalian-specific Alex3/Gαq protein complex regulates mitochondrial trafficking, dendritic complexity, and neuronal survival

Mitochondrial dynamics and trafficking are essential to provide the energy required for neurotransmission and neural activity. We investigated how G protein-coupled receptors (GPCRs) and G proteins control mitochondrial dynamics and trafficking. The activation of Gαq inhibited mitochondrial trafficking in neurons through a mechanism that was independent of the canonical downstream PLCβ pathway. Mitoproteome analysis revealed that Gαq interacted with the Eutherian-specific mitochondrial protein armadillo repeat-containing X-linked protein 3 (Alex3) and the Miro1/Trak2 complex, which acts as an adaptor for motor proteins involved in mitochondrial trafficking along dendrites and axons. By generating a CNS-specific Alex3 knockout mouse line, we demonstrated that Alex3 was required for the effects of Gαq on mitochondrial trafficking and dendritic growth in neurons. Alex3-deficient mice had altered amounts of ER stress response proteins, increased neuronal death, motor neuron loss, and severe motor deficits. These data revealed a mammalian-specific Alex3/Gαq mitochondrial complex, which enables control of mitochondrial trafficking and neuronal death by GPCRs.

Glycogen Synthase Kinase 3: Ion Channels, Plasticity, and Diseases

Glycogen synthase kinase 3β (GSK3) is a multifaceted serine/threonine (S/T) kinase expressed in all eukaryotic cells. GSK3β is highly enriched in neurons in the central nervous system where it acts as a central hub for intracellular signaling downstream of receptors critical for neuronal function. Unlike other kinases, GSK3β is constitutively active, and its modulation mainly involves inhibition via upstream regulatory pathways rather than increased activation. Through an intricate converging signaling system, a fine-tuned balance of active and inactive GSK3β acts as a central point for the phosphorylation of numerous primed and unprimed substrates. Although the full range of molecular targets is still unknown, recent results show that voltage-gated ion channels are among the downstream targets of GSK3β. Here, we discuss the direct and indirect mechanisms by which GSK3β phosphorylates voltage-gated Na⁺ channels (Na_v1.2 and Na_v1.6) and voltage-gated K⁺ channels (K_v4 and K_v7) and their physiological effects on intrinsic excitability, neuronal plasticity, and behavior. We also present evidence for how unbalanced GSK3β activity can lead to maladaptive plasticity that ultimately renders neuronal circuitry more vulnerable, increasing the risk for developing neuropsychiatric disorders. In conclusion, GSK3β-dependent modulation of voltage-gated ion channels may serve as an important pharmacological target for neurotherapeutic development.

Cryo-EM structures of human bradykinin receptor-Gq proteins complexes

The type 2 bradykinin receptor (B2R) is a G protein-coupled receptor (GPCR) in the cardiovascular system, and the dysfunction of B2R leads to inflammation, hereditary angioedema, and pain. Bradykinin and kallidin are both endogenous peptide agonists of B2R, acting as vasodilators to protect the cardiovascular system. Here we determine two cryo-electron microscopy (cryo-EM) structures of human B2R-G_q in complex with bradykinin and kallidin at 3.0 Å and 2.9 Å resolution, respectively. The ligand-binding pocket accommodates S-shaped peptides, with aspartic acids and glutamates as an anion trap. The phenylalanines at the tail of the peptides induce significant conformational changes in the toggle switch W283^6.48, the conserved PIF, DRY, and NPxxY motifs, for the B2R activation. This further induces the extensive interactions of the intracellular loops ICL2/3 and helix 8 with G_q proteins. Our structures elucidate the molecular mechanisms for the ligand binding, receptor activation, and G_q proteins coupling of B2R.

Gαq activation modulates autophagy by promoting mTORC1 signaling

The mTORC1 node plays a major role in autophagy modulation. We report a role of the ubiquitous Gαq subunit, a known transducer of plasma membrane G protein-coupled receptors signaling, as a core modulator of mTORC1 and autophagy. Cells lacking Gαq/11 display higher basal autophagy, enhanced autophagy induction upon different types of nutrient stress along with a decreased mTORC1 activation status. They are also unable to reactivate mTORC1 and thus inactivate ongoing autophagy upon nutrient recovery. Conversely, stimulation of Gαq/11 promotes sustained mTORC1 pathway activation and reversion of autophagy promoted by serum or amino acids removal. Gαq is present in autophagic compartments and lysosomes and is part of the mTORC1 multi-molecular complex, contributing to its assembly and activation via its nutrient status-sensitive interaction with p62, which displays features of a Gαq effector. Gαq emerges as a central regulator of the autophagy machinery required to maintain cellular homeostasis upon nutrient fluctuations.

The Gαq/11-provoked induction of Akr1c18 in murine luteal cells is mediated by phospholipase C

Towards the end of gestation prostaglandin F2α (PGF2α) stimulates the expression of Akr1c18 in the murine corpus luteum. Akr1c18 codes for 20α-hydroxysteroid dehydrogenase, an enzyme that precipitates parturition by catabolizing progesterone. Previous results from our laboratory have shown that this effect of PGF2α is mediated by the activation of Gα_q/11, but the downstream effector(s) of Gα_q/11 that elicit the increase in Akr1c18 expression have not been identified. The physiological effects of Gα_q/11 are mediated by its ability to interact with phospholipase Cβ, p63RhoGEF, and PKCζ. In the experiments described herein we used biochemical and pharmacological approaches, as well as adenoviral-mediated expression of a constitutively active form of Gα_q and mutants thereof, to examine the role of each of these effectors as potential mediators of the increased expression of luteal Akr1c18. By measuring the effects of PGF2α on the activation of RhoA (activated by p63RhoGEF) and the effects of activators and inhibitors of RhoA on the PGF2α-induced expression of luteal Akr1c18, we determined that RhoA is neither activated by PGF2α or involved in the PGF2α-induced expression of luteal Akr1c18. The potential involvement of PKCζ was ruled out by the inability of a mutant of a constitutively active Gα_q that prevents PKCζ binding to block the increased expression of Akr1c18. Furthermore, PGF2α does not increase the phosphorylation of ERK-5, the only known downstream target of PKCζ. On the other hand, three different mutants of a constitutively active Gα_q that prevent phospholipase C activation blocked the induction of luteal Akr1c18. We conclude that the induction of luteal Akr1c18 by Gα_q/11 is mediated by the activation of phospholipase C.

Protein Kinase C ζ Interacts with a Novel Binding Region of Gαq to Act as a Functional Effector

Heterotrimeric G proteins play an essential role in the initiation of G protein-coupled receptor (GPCR) signaling through specific interactions with a variety of cellular effectors. We have recently reported that GPCR activation promotes a direct interaction between Gαq and protein kinase C ζ (PKCζ), leading to the stimulation of the ERK5 pathway independent of the canonical effector PLCβ. We report herein that the activation-dependent Gαq/PKCζ complex involves the basic PB1-type II domain of PKCζ and a novel interaction module in Gαq different from the classical effector-binding site. Point mutations in this Gαq region completely abrogate ERK5 phosphorylation, indicating that Gαq/PKCζ association is required for the activation of the pathway. Indeed, PKCζ was demonstrated to directly bind ERK5 thus acting as a scaffold between Gαq and ERK5 upon GPCR activation. The inhibition of these protein complexes by G protein-coupled receptor kinase 2, a known Gαq modulator, led to a complete abrogation of ERK5 stimulation. Finally, we reveal that Gαq/PKCζ complexes link Gαq to apoptotic cell death pathways. Our data suggest that the interaction between this novel region in Gαq and the effector PKCζ is a key event in Gαq signaling.

Glycogen synthase kinase-3 (GSK3): regulation, actions, and diseases

Glycogen synthase kinase-3 (GSK3) may be the busiest kinase in most cells, with over 100 known substrates to deal with. How does GSK3 maintain control to selectively phosphorylate each substrate, and why was it evolutionarily favorable for GSK3 to assume such a large responsibility? GSK3 must be particularly adaptable for incorporating new substrates into its repertoire, and we discuss the distinct properties of GSK3 that may contribute to its capacity to fulfill its roles in multiple signaling pathways. The mechanisms regulating GSK3 (predominantly post-translational modifications, substrate priming, cellular trafficking, protein complexes) have been reviewed previously, so here we focus on newly identified complexities in these mechanisms, how each of these regulatory mechanism contributes to the ability of GSK3 to select which substrates to phosphorylate, and how these mechanisms may have contributed to its adaptability as new substrates evolved. The current understanding of the mechanisms regulating GSK3 is reviewed, as are emerging topics in the actions of GSK3, particularly its interactions with receptors and receptor-coupled signal transduction events, and differential actions and regulation of the two GSK3 isoforms, GSK3α and GSK3β. Another remarkable characteristic of GSK3 is its involvement in many prevalent disorders, including psychiatric and neurological diseases, inflammatory diseases, cancer, and others. We address the feasibility of targeting GSK3 therapeutically, and provide an update of its involvement in the etiology and treatment of several disorders.

The GSK3 signaling pathway is activated by cocaine and is critical for cocaine conditioned reward in mice

The Akt - GSK3 signaling pathway has been recently implicated in psychostimulant-induced behavioral and cellular effects. Here, the ability of cocaine to regulate the activity of Akt and GSK3 was investigated by measuring the phosphorylation states of the two kinases. The anatomical specificity of the response was determined, as was the contributions of dopamine and NMDA receptors to the actions of cocaine. As GSK3 activity was found to be increased by cocaine, subsequent experiments investigated the importance of GSK3 activation in cocaine conditioned reward. Adult male CD-1 mice were injected with cocaine or saline, and levels of phosphorylated Akt and GSK3α/β were measured 30 minutes later. Acute administration of cocaine significantly decreased the phosphorylation of Akt-Thr308 (pAkt-Thr308) and GSK3β in the caudate putamen and nucleus accumbens core, without altering pAkt-Ser473 and pGSK3α. To investigate the role of dopamine and NMDA receptors in the regulation of Akt and GSK3 by cocaine, specific receptor antagonists were administered prior to cocaine. Blockade of dopamine D2 receptors with eticlopride prevented the reduction of pAkt-Thr308 produced by cocaine, whereas antagonists at dopamine D1, dopamine D2 or glutamatergic NMDA receptors each blocked cocaine-induced reductions in pGSK3β. The potential importance of GSK3 activity in the rewarding actions of cocaine was determined using a cocaine conditioned place preference procedure. Administration of the selective GSK3 inhibitor, SB 216763, prior to cocaine conditioning sessions blocked the development of cocaine place preference. In contrast, SB 216763 did not alter the acquisition of a contextual fear conditioning response, demonstrating that SB 216763 did not globally inhibit contextual learning processes. The results of this study indicate that phosphorylation of GSK3β is reduced, hence GSK3β activity is increased following acute cocaine, an effect that is contingent upon both dopaminergic and glutamatergic receptors. Further, GSK3 activity is required for the development of cocaine conditioned reward.

Gαq signalling: the new and the old

In the last few years the interactome of Gαq has expanded considerably, contributing to improve our understanding of the cellular and physiological events controlled by this G alpha subunit. The availability of high-resolution crystal structures has led the identification of an effector-binding region within the surface of Gαq that is able to recognise a variety of effector proteins. Consequently, it has been possible to ascribe different Gαq functions to specific cellular players and to identify important processes that are triggered independently of the canonical activation of phospholipase Cβ (PLCβ), the first identified Gαq effector. Novel effectors include p63RhoGEF, that provides a link between G protein-coupled receptors and RhoA activation, phosphatidylinositol 3-kinase (PI3K), implicated in the regulation of the Akt pathway, or the cold-activated TRPM8 channel, which is directly inhibited upon Gαq binding. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has also been described as a novel PLCβ-independent signalling axis that relies upon the interaction between this G protein and two novel effectors (PKCζ and MEK5). Additionally, the association of Gαq with different regulatory proteins can modulate its effector coupling ability and, therefore, its signalling potential. Regulators include accessory proteins that facilitate effector activation or, alternatively, inhibitory proteins that downregulate effector binding or promote signal termination. Moreover, Gαq is known to interact with several components of the cytoskeleton as well as with important organisers of membrane microdomains, which suggests that efficient signalling complexes might be confined to specific subcellular environments. Overall, the complex interaction network of Gαq underlies an ever-expanding functional diversity that puts forward this G alpha subunit as a major player in the control of physiological functions and in the development of different pathological situations.

Arsenic trioxide suppressed mantle cell lymphoma by downregulation of cyclin D1

Mantle cell lymphoma (MCL) is aggressive with poor prognosis. Due to t(11;14)(q13;q32), cyclin D1 is overexpressed. The in vitro activities of arsenic trioxide (As2O3) in MCL were investigated. In MCL lines Jeko-1 and Granta-519, As2O3 induced dose-dependent and time-dependent increases in apoptosis accompanied by cyclin D1 suppression. Downregulation of cyclin D1 resulted in decreased retinoblastoma protein phosphorylation, which led to repressed G1 progression to S/G2 phases. As2O3 did not affect cyclin D1 gene transcription. Instead, As2O3 activated glycogen synthase kinase-3beta (by tyrosine-216 phosphorylation) and IkappaB kinase alpha/beta (by serine-176/180 phosphorylation), both of which phosphorylated cyclin D1 at threonine-286, leading to its poly-ubiquitination and degradation in the proteasome. These observations were recapitulated partly in primary MCL samples obtained from patients refractory to conventional treatment. Our findings suggested that As2O3 might be clinically useful in MCL.

Amphetamine stimulates Wnt3 increases in rat nucleus accumbens

Dopaminergic neurotransmission is thought to be involved in reward-related incentive learning and addictive behaviour. Amphetamine will alter glycogen synthase kinase-3β (GSK-3β) activity by increasing dopamine transporter efflux rates. We investigated the hypothesis that Wnt signalling will be altered in rat nucleus accumbens within 15 min of injection of amphetamine compared with saline. We isolated RNA from the nucleus accumbens and used reverse transcriptase-PCR to screen for altered Wnt expression. We found that amphetamine had no effect on Wnt5a or Wnt7a expression but increased Wnt3. We then measured protein expression of Wnt3, phosphorylated lipoprotein-related peptide 6, GSK-3β phosphorylated at serine-9 and tyrosine-216 and total β-catenin. We found that amphetamine increased Wnt3 protein expression, increased pLRP6 (threonine-1572) levels, increased β-catenin levels, increased GSK-3β phosphorylation at serine-9, consistent with inhibition of GSK-3β activity, and diminished GSK-3β phosphorylation at tyrosine-216. Our data support the hypothesis that proximate Wnt signalling is rapidly activated by amphetamine in the adult rat nucleus accumbens.

Glucose dependence of glycogen synthase activity regulation by GSK3 and MEK/ERK inhibitors and angiotensin-(1-7) action on these pathways in cultured human myotubes

Glycogen synthase (GS) is activated by glucose/glycogen depletion in skeletal muscle cells, but the contributing signaling pathways, including the chief GS regulator GSK3, have not been fully defined. The MEK/ERK pathway is known to regulate GSK3 and respond to glucose. The aim of this study was to elucidate the GSK3 and MEK/ERK pathway contribution to GS activation by glucose deprivation in cultured human myotubes. Moreover, we tested the glucose-dependence of GSK3 and MEK/ERK effects on GS and angiotensin (1-7) actions on these pathways. We show that glucose deprivation activated GS, but did not change phospho-GS (Ser640/1), GSK3β activity or activity-activating phosphorylation of ERK1/2. We then treated glucose-replete and -depleted cells with SB415286, U0126, LY294 and rapamycin to inhibit GSK3, MEK1/2, PI3K and mTOR, respectively. SB415286 activated GS and decreased the relative phospho-GS (Ser640/1) level, more in glucose-depleted than -replete cells. U0126 activated GS and reduced the phospho-GS (Ser640/1) content significantly in glucose-depleted cells, while GSK3β activity tended to increase. LY294 inactivated GS in glucose-depleted cells only, without affecting relative phospho-GS (Ser640/1) level. Rapamycin had no effect on GS activation. Angiotensin-(1-7) raised phospho-ERK1/2 but not phospho-GSK3β (Ser9) content, while it inactivated GS and increased GS phosphorylation on Ser640/1, in glucose-replete cells. In glucose-depleted cells, angiotensin-(1-7) effects on ERK1/2 and GS were reverted, while relative phospho-GSK3β (Ser9) content decreased. In conclusion, activation of GS by glucose deprivation is not due to GS Ser640/1 dephosphorylation, GSK3β or ERK1/2 regulation in cultured myotubes. However, glucose depletion enhances GS activation/Ser640/1 dephosphorylation due to both GSK3 and MEK/ERK inhibition. Angiotensin-(1-7) inactivates GS in glucose-replete cells in association with ERK1/2 activation, not with GSK3 regulation, and glucose deprivation reverts both hormone effects. Thus, the ERK1/2 pathway negatively regulates GS activity in myotubes, without involving GSK3 regulation, and as a function of the presence of glucose.

The complex G protein-coupled receptor kinase 2 (GRK2) interactome unveils new physiopathological targets

GRK2 is a ubiquitous member of the G protein-coupled receptor kinase (GRK) family that appears to play a central, integrative role in signal transduction cascades. GRKs participate together with arrestins in the regulation of G protein-coupled receptors (GPCR), a family of hundreds of membrane proteins of key physiological and pharmacological importance, by triggering receptor desensitization from G proteins and GPCR internalization, and also by helping assemble macromolecular signalosomes in the receptor environment acting as agonist-regulated adaptor scaffolds, thus contributing to signal propagation. In addition, emerging evidence indicates that GRK2 can phosphorylate a growing number of non-GPCR substrates and associate with a variety of proteins related to signal transduction, thus suggesting that this kinase could also have diverse 'effector' functions. We discuss herein the increasing complexity of such GRK2 'interactome', with emphasis on the recently reported roles of this kinase in cell migration and cell cycle progression and on the functional impact of the altered GRK2 levels observed in several relevant cardiovascular, inflammatory or tumour pathologies. Deciphering how the different networks of potential GRK2 functional interactions are orchestrated in a stimulus, cell type or context-specific way is critical to unveil the contribution of GRK2 to basic cellular processes, to understand how alterations in GRK2 levels or functionality may participate in the onset or development of several cardiovascular, tumour or inflammatory diseases, and to assess the feasibility of new therapeutic strategies based on the modulation of the activity, levels or specific interactions of GRK2.

G alpha(q) acts as an adaptor protein in protein kinase C zeta (PKCzeta)-mediated ERK5 activation by G protein-coupled receptors (GPCR)

G(q)-coupled G protein-coupled receptors (GPCR) mediate the actions of a variety of messengers that are key regulators of different cellular functions. These receptors can regulate a highly interconnected network of biochemical routes that control the activity of several members of the mitogen-activated protein kinase (MAPK) family. The ERK5 MAPK has been shown to be activated by G(q)-coupled GPCR via unknown mechanisms. We find that the atypical protein kinase C (PKCzeta), previously reported to interact with the ERK5 activator MEK5 and to be involved in epidermal growth factor-mediated ERK5 stimulation, plays a crucial role in the activation of the ERK5 pathway by G(q)-coupled GPCR. Stimulation of ERK5 by G(q)-coupled GPCR is abolished upon pharmacological inhibition of PKCzeta as well as in embryonic fibroblasts obtained from PKCzeta-deficient mice. Both PKCzeta and MEK5 associate to G alpha(q) upon activation of GPCR, thus forming a ternary complex that seems essential for the activation of ERK5. These data put forward a novel function of G alpha(q) as a scaffold protein involved in the modulation of the ERK5 cascade by GPCR that could be relevant in G(q)-mediated physiological functions.

Learning transcriptional regulatory networks from high throughput gene expression data using continuous three-way mutual information

Background

Probability based statistical learning methods such as mutual information and Bayesian networks have emerged as a major category of tools for reverse engineering mechanistic relationships from quantitative biological data. In this work we introduce a new statistical learning strategy, MI3 that addresses three common issues in previous methods simultaneously: (1) handling of continuous variables, (2) detection of more complex three-way relationships and (3) better differentiation of causal versus confounding relationships. With these improvements, we provide a more realistic representation of the underlying biological system.

Results

We test the MI3 algorithm using both synthetic and experimental data. In the synthetic data experiment, MI3 achieved an absolute sensitivity/precision of 0.77/0.83 and a relative sensitivity/precision both of 0.99. In addition, MI3 significantly outperformed the control methods, including Bayesian networks, classical two-way mutual information and a discrete version of MI3. We then used MI3 and control methods to infer a regulatory network centered at the MYC transcription factor from a published microarray dataset. Models selected by MI3 were numerically and biologically distinct from those selected by control methods. Unlike control methods, MI3 effectively differentiated true causal models from confounding models. MI3 recovered major MYC cofactors, and revealed major mechanisms involved in MYC dependent transcriptional regulation, which are strongly supported by literature. The MI3 network showed that limited sets of regulatory mechanisms are employed repeatedly to control the expression of large number of genes.

Conclusion

Overall, our work demonstrates that MI3 outperforms the frequently used control methods, and provides a powerful method for inferring mechanistic relationships underlying biological and other complex systems. The MI3 method is implemented in R in the "mi3" package, available under the GNU GPL from and from the R package archive CRAN.

Mechanism of angiotensin II-induced superoxide production in cells reconstituted with angiotensin type 1 receptor and the components of NADPH oxidase

The mechanism of angiotensin II (Ang II)-induced superoxide production was investigated with HEK293 or Chinese hamster ovary cells reconstituted with the angiotensin type 1 receptor (AT(1)R) and NADPH oxidase (either Nox1 or Nox2) along with a pair of adaptor subunits (either NOXO1 with NOXA1 or p47(phox) with p67(phox)). Ang II enhanced the activity of both Nox1 and Nox2 supported by either adaptor pair, with more effective activation of Nox1 in the presence of NOXO1 and NOXA1 and of Nox2 in the presence of p47(phox) and p67(phox). Expression of several AT(1)R mutants showed that interaction of the receptor with G proteins but not that with beta-arrestin or with other proteins (Jak2, phospholipase C-gamma1, SH2 domain-containing phosphatase 2) that bind to the COOH-terminal region of AT(1)R, was necessary for Ang II-induced superoxide production. The effects of constitutively active alpha subunits of G proteins and of various pharmacological agents implicated signaling by a pathway comprising AT(1)R, Galpha(q/11), phospholipase C-beta, and protein kinase C as largely, but not exclusively, responsible for Ang II-induced activation of Nox1 and Nox2 in the reconstituted cells. A contribution of Galpha(12/13), phospholipase D, and phosphatidyl-inositol 3-kinase to Ang II-induced superoxide generation was also suggested, whereas Src and the epidermal growth factor receptor did not appear to participate in this effect of Ang II. In reconstituted cells stimulated with Ang II, Nox2 exhibited a more sensitive response than Nox1 to the perturbation of protein kinase C, phosphatidylinositol 3-kinase, or the small GTPase Rac1.

G alpha(q/11)-coupled P2Y2 nucleotide receptor inhibits human keratinocyte spreading and migration

Reepithelialization is a critical step in wound healing. It is initiated by keratinocyte migration at the wound edges. After wounding, extracellular nucleotides are released by keratinocytes and other skin cells. Here, we report that activation of P2Y2 nucleotide receptor by ATP/UTP inhibits keratinocyte cell spreading and induces lamellipodium withdrawal. Kymography analysis demonstrates that these effects correlate with a durable decrease of lamellipodium dynamics. P2Y2 receptor activation also induces a dramatic dismantling of the actin network, the loss of alpha3 integrin expression at the cell periphery, and the dissolution of focal contacts as indicated by the alteration of alpha(v) integrins and focal contact protein distribution. In addition, activation of P2Y2R prevents growth factor-induced phosphorylation of Erk(1,2) and Akt/PkB. The use of a specific pharmacological inhibitor (YM-254890), the depletion of G alpha(q/11) by siRNA, or the expression of a constitutively active G alpha(q/11) mutant (Q209L) show that activation of G alpha(q/11) is responsible for these ATP/UTP-induced effects. Finally, we report that ATP delays growth factor-induced wound healing of keratinocyte monolayers. Collectively, these findings provide evidence for a unique and important role for extracellular nucleotides as efficient autocrine/paracrine regulators of keratinocyte shape and migration during wound healing.

Targeting glycogen synthase kinase-3 as an approach to develop novel mood-stabilising medications

Historically, success in the pharmacological treatment of bipolar disorder has arisen either from serendipitous findings or from studies with drugs (antipsychotics and anticonvulsants) developed for other indications (schizophrenia and epilepsy, respectively). Lithium has been in widespread clinical use in the treatment of bipolar disorder for > 30 years. Development of lithium-mimetic compounds has the potential to result in a more specific medication, with fewer side effects and a less narrow dose range. However, novel medications based upon a known mechanism of action of this drug are yet to be developed. Increasing evidence suggests that a next-generation lithium compound may derive from knowledge of a direct target of lithium, glycogen synthase kinase-3 (GSK-3). GSK-3 is an intracellular enzyme implicated as a critical component in many neuronal signalling pathways. However, despite the large body of preclinical data discussed in this review, definitive validation of GSK-3 as therapeutically relevant target of lithium will require clinical trials with novel GSK-3 inhibitors. A number of recent reports suggest that it is possible to develop selective, small-molecule GSK-3 inhibitors.

Galphaq binds to p110alpha/p85alpha phosphoinositide 3-kinase and displaces Ras

Several studies have reported that activation of G(q)-coupled receptors inhibits PI3K (phosphoinositide 3-kinase) signalling. In the present study, we used purified proteins to demonstrate that Galpha(q) directly inhibits p110alpha/p85alpha PI3K in a GTP-dependent manner. Activated Galpha(q) binds to the p110alpha/p85alpha PI3K with an apparent affinity that is seven times stronger than that for Galpha(q).GDP as measured by fluorescence spectroscopy. In contrast, Galpha(q) did not bind to the p110gamma PI3K. Fluorescence spectroscopy experiments also showed that Galpha(q) competes with Ras, a PI3K activator, for binding to p110alpha/p85alpha. Interestingly, co-precipitation studies using deletion mutants showed that Galpha(q) binds to the p85-binding domain of p110alpha and not to the Ras-binding domain. Expression of constitutively active Galpha(q)Q209L in cells inhibited Ras activation of the PI3K/Akt pathway but had no effect on Ras/Raf/MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase] signalling. These results suggest that activation of G(q)-coupled receptors leads to increased binding of Galpha(q).GTP to some isoforms of PI3K, which might explain why these receptors inhibit this signalling pathway in certain cell types.

Inhibition of a background potassium channel by Gq protein alpha-subunits

Two-pore-domain K(+) channels provide neuronal background currents that establish resting membrane potential and input resistance; their modulation provides a prevalent mechanism for regulating cellular excitability. The so-called TASK channel subunits (TASK-1 and TASK-3) are widely expressed, and they are robustly inhibited by receptors that signal through Galphaq family proteins. Here, we manipulated G protein expression and membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) levels in intact and cell-free systems to provide electrophysiological and biochemical evidence that inhibition of TASK channels by Galphaq-linked receptors proceeds unabated in the absence of phospholipase C (PLC) activity, and instead involves association of activated Galphaq subunits with the channels. Receptor-mediated inhibition of TASK channels was faster and less sensitive to a PLCbeta1-ct minigene construct than inhibition of PIP(2)-sensitive Kir3.4(S143T) homomeric channels that is known to be dependent on PLC. TASK channels were strongly inhibited by constitutively active Galphaq, even by a mutated version that is deficient in PLC activation. Receptor-mediated TASK channel inhibition required exogenous Galphaq expression in fibroblasts derived from Galphaq/11 knockout mice, but proceeded unabated in a cell line in which PIP(2) levels were reduced by regulated overexpression of a lipid phosphatase. Direct application of activated Galphaq, but not other G protein subunits, inhibited TASK channels in excised patches, and constitutively active Galphaq subunits were selectively coimmunoprecipitated with TASK channels. These data indicate that receptor-mediated TASK channel inhibition is independent of PIP(2) depletion, and they suggest a mechanism whereby channel modulation by Galphaq occurs through direct interaction with the ion channel or a closely associated intermediary.

GSK-3 is activated by the tyrosine kinase Pyk2 during LPA1-mediated neurite retraction

Glycogen synthase kinase-3 (GSK-3) is a multifunctional serine/threonine kinase that is usually inactivated by serine phosphorylation in response to extracellular cues. However, GSK-3 can also be activated by tyrosine phosphorylation, but little is known about the upstream signaling events and tyrosine kinase(s) involved. Here we describe a G protein signaling pathway leading to GSK-3 activation during lysophosphatidic acid (LPA)-induced neurite retraction. Using neuronal cells expressing the LPA(1) receptor, we show that LPA(1) mediates tyrosine phosphorylation and activation of GSK-3 with subsequent phosphorylation of the microtubule-associated protein tau via the G(i)-linked PIP(2) hydrolysis-Ca(2+) mobilization pathway. LPA concomitantly activates the Ca(2+)-dependent tyrosine kinase Pyk2, which is detected in a complex with GSK-3beta. Inactivation or knockdown of Pyk2 inhibits LPA-induced (but not basal) tyrosine phosphorylation of GSK-3 and partially inhibits LPA-induced neurite retraction, similar to what is observed following GSK-3 inhibition. Thus, Pyk2 mediates LPA(1)-induced activation of GSK-3 and subsequent phosphorylation of microtubule-associated proteins. Pyk2-mediated GSK-3 activation is initiated by PIP(2) hydrolysis and may serve to destabilize microtubules during actomyosin-driven neurite retraction.

A transgenic mouse model of heart failure using inducible Galpha q

Receptors coupled to Galpha q play a key role in the development of heart failure. Studies using genetically modified mice suggest that Galpha q mediates a hypertrophic response in cardiac myocytes. Galpha q signaling in these models is modified during early growth and development, whereas most heart failure in humans occurs after cardiac damage sustained during adulthood. To determine the phenotype of animals that express increased Galpha q signaling only as adults, we generated transgenic mice that express a silent Galpha q protein (Galpha qQ209L-hbER) in cardiac myocytes that can be activated by tamoxifen. Following drug treatment to activate Galpha q Q209L-hbER, these mice rapidly develop a dilated cardiomyopathy and heart failure. This phenotype does not appear to involve myocyte hypertrophy but is associated with dephosphorylation of phospholamban (PLB), decreased sarcoplasmic reticulum Ca2+-ATPase activity, and a decrease in L-type Ca2+ current density. Changes in Ca2+ handling and decreased cardiac contractility are apparent 1 week after Galpha qQ209L-hbER activation. In contrast, transgenic mice that express an inducible Galpha q mutant that cannot activate phospholipase Cbeta (PLCbeta) do not develop heart failure or changes in PLB phosphorylation, but do show decreased L-type Ca2+ current density. These results demonstrate that activation of Galpha q in cardiac myocytes of adult mice causes a dilated cardiomyopathy that requires the activation of PLCbeta. However, increased PLCbeta signaling is not required for all of the Galpha q-induced cardiac abnormalities.

Galpha q inhibits cardiac L-type Ca2+ channels through phosphatidylinositol 3-kinase

Cardiac myocyte contractility is initiated by Ca2+ entry through the voltage-dependent L-type Ca2+ channel (LTCC). To study the effect of Galpha q on the cardiac LTCC, we utilized two transgenic mouse lines that selectively express inducible Galpha q-estrogen receptor hormone-binding domain fusion proteins (Galpha qQ209L-hbER or Galpha qQ209L-AA-hbER) in cardiac myocytes. Both of these proteins inhibit phosphatidylinositol (PI) 3-kinase (PI3K) signaling, but Galpha qQ209L-AA-hbER cannot activate the canonical Galpha q effector phospholipase Cbeta (PLCbeta). L-type Ca2+ current (I(Ca,L)) density measured by whole-cell patch clamping was reduced by more than 50% in myocytes from both Galpha q animals as compared with wild-type cells, suggesting that inhibition of the LTCC by Galpha q does not require PLCbeta. To investigate the role of PI3K in this inhibitory effect, I(Ca,L) was measured in the presence of various phosphoinositides infused through the patch pipette. Infusion of PI 3,4,5-trisphosphate (PI(3,4,5)P3) into wild-type myocytes did not affect I(Ca,L), but it fully restored I(Ca,L) density in both Galpha q transgenic myocytes to wild-type levels. By contrast, PI 4,5-bisphosphate (PI(4,5)P2) or PI 3,5-bisphosphate had no effect. Infusion with p110beta/p85alpha or p110gamma PI3K in the presence of PI(4,5)P2 also restored I(Ca,L) density to wild-type levels. Last, infusion of either PTEN, a PI(3,4,5)P3 phosphatase, or the pleckstrin homology domain of Grp1, which sequesters PI(3,4,5)P3, reduced the peak I(Ca,L) density in wild-type myocytes by approximately 30%. Taken together, these results strongly suggest that the inhibitory effect of Galpha q on the cardiac LTCC is mediated by inhibition of PI3K.

Differential Effects of Gqα, G14α, and G15α on Vascular Smooth Muscle Cell Survival and Gene Expression Profiles

Gqalpha family members (Gqalpha, G11alpha, G14alpha, and G15/16alpha) stimulate phospholipase Cbeta (PLCbeta) and inositol lipid signaling but differ markedly in amino acid sequence and tissue distribution predicting unappreciated functional diversity. To examine functional differences, we compared the signaling properties of Gqalpha, G14alpha, and G15alpha and their cellular responses in vascular smooth muscle cells (VSMC). Constitutively active forms of Gqalpha, G14alpha, or G15alpha elicit markedly different responses when introduced to VSMC. Whereas each Galpha stimulated PLCbeta to similar extents when expressed at equal protein levels, Gqalpha and G14alpha but not G15alpha initiated profound cell death within 48 h. This response was the result of activation of apoptotic pathways, because Gqalpha and G14alpha, but not G15alpha, stimulated caspase-3 activation and did not alter phospho-Akt, a regulator of cell survival pathways. Gqalpha and G14alpha stimulate nuclear factor of activated T cell (NFAT) activation in VSMC, but Galpha-induced cell death seems independent of PKC, InsP(3)/Ca(2+), and NFAT, in that pharmacological inhibitors of these pathways did not block cell death. Gene expression analysis indicates that Gqalpha, G14alpha, and G15alpha each elicit markedly different profiles of altered gene sets in VSMC after 24 h. Whereas all three Galpha stimulated changes (> or =2-fold) in 50 shared mRNA, Gqalpha and G14alpha (but not G15alpha) stimulated changes in 221 shared mRNA, many of which are reported to be pro-apoptotic and/or involved with TNF-alpha signaling. We were surprised to find that each Galpha also stimulated changes in nonoverlapping Galpha-specific gene sets. These findings demonstrate that Gqalpha family members activate both overlapping and distinct signaling pathways and are more functionally diverse than previously thought.