Hsp90 interactions and acylation target the G protein Galpha 12 but not Galpha 13 to lipid rafts

High-Throughput GPCRome Screen of Pollutants Reveals the Activity of Polychlorinated Biphenyls at Melatonin and Sphingosine-1-phosphate Receptors

Exposure to environmental pollutants is linked to numerous toxic outcomes, warranting concern about the effect of pollutants on human health. To assess the threat of pollutant exposure, it is essential to understand their biological activity. Unfortunately, gaps remain for many pollutants' specific biological activity and molecular targets. A superfamily of signaling proteins, G-protein-coupled receptors (GPCRs), has been shown as potential targets for pollutant activity. However, research investigating the pollutant activity at the GPCRome is scarce. This work explores pollutant activity across a library of human GPCRs by leveraging modern high-throughput screening techniques devised for drug discovery and pharmacology. We designed and implemented a pilot screen of eight pollutants at 314 human GPCRs and discovered specific polychlorinated biphenyl (PCB) activity at sphingosine-1-phosphate and melatonin receptors. The method utilizes open-source resources available to academic and governmental institutions to enable future campaigns that screen large numbers of pollutants. Thus, we present a novel high-throughput approach to assess the biological activity and specific targets of pollutants.

Impact of lipid rafts on transient receptor potential channel activities

Members of the transient receptor potential (TRP) superfamily are cation channels that are expressed in nearly every mammalian cell type and respond as cellular sensors to various environmental stimuli. Light, pressure, osmolarity, temperature, and other stimuli can induce TRP calcium conductivity and correspondingly trigger many signaling processes in cells. Disruption of TRP channel activity, as a rule, harms cellular function. Despite numerous studies, the mechanisms of TRP channel regulation are not yet sufficiently clear, in part, because TRP channels are regulated by a broad set of ligands having diverse physical and chemical features. It is now known that some TRP members are located in membrane microdomains termed lipid rafts. Moreover, interaction between specific raft-associated lipids with channels may be a key regulation mechanism. This review examines recent findings related to the roles of lipid rafts in regulation of TRP channel activity. The mechanistic events of channel interactions with the main lipid raft constituent, cholesterol, are being clarified. Better understanding of mechanisms behind such interactions would help establish the key elements of TRP channel regulation and hence allow control of cellular responses to environmental stimuli.

Intertwined associations between oxidative and nitrosative stress and endocannabinoid system pathways: Relevance for neuropsychiatric disorders

The endocannabinoid system (ECS) appears to regulate metabolic, cardiovascular, immune, gastrointestinal, lung, and reproductive system functions, as well as the central nervous system. There is also evidence that neuropsychiatric disorders are associated with ECS abnormalities as well as oxidative and nitrosative stress pathways. The goal of this mechanistic review is to investigate the mechanisms underlying the ECS's regulation of redox signalling, as well as the mechanisms by which activated oxidative and nitrosative stress pathways may impair ECS-mediated signalling. Cannabinoid receptor (CB)1 activation and upregulation of brain CB2 receptors reduce oxidative stress in the brain, resulting in less tissue damage and less neuroinflammation. Chronically high levels of oxidative stress may impair CB1 and CB2 receptor activity. CB1 activation in peripheral cells increases nitrosative stress and inducible nitric oxide (iNOS) activity, reducing mitochondrial activity. Upregulation of CB2 in the peripheral and central nervous systems may reduce iNOS, nitrosative stress, and neuroinflammation. Nitrosative stress may have an impact on CB1 and CB2-mediated signalling. Peripheral immune activation, which frequently occurs in response to nitro-oxidative stress, may result in increased expression of CB2 receptors on T and B lymphocytes, dendritic cells, and macrophages, reducing the production of inflammatory products and limiting the duration and intensity of the immune and oxidative stress response. In conclusion, high levels of oxidative and nitrosative stress may compromise or even abolish ECS-mediated redox pathway regulation. Future research in neuropsychiatric disorders like mood disorders and deficit schizophrenia should explore abnormalities in these intertwined signalling pathways.

Hsp90 facilitates acquired drug resistance of tumor cells through cholesterol modulation however independent of tumor progression

Acquired multidrug resistance of cancer cells challenges the chemotherapeutic interventions. To understand the role of molecular chaperone, Hsp90 in drug adapted tumor cells, we have used in vitro drug adapted epidermoid tumor cells as a model system. We found that chemotherapeutic drug adaptation of tumor cells is mediated by induced activities of both Hsp90 and P-glycoprotein (P-gp). Although the high-affinity conformation of Hsp90 has correlated with the enhanced drug efflux activity, we did not observe a direct interaction between P-gp and Hsp90. The enrichment of P-gp and Hsp90 at the cholesterol-rich membrane microdomains is found obligatory for enhanced drug efflux activity. Since inhibition of cholesterol biosynthesis is not interfering with the drug efflux activity, it is presumed that the net cholesterol redistribution mediated by Hsp90 regulates the enhanced drug efflux activity. Our in vitro cholesterol and Hsp90 interaction studies have furthered our presumption that Hsp90 facilitates cholesterol redistribution. The drug adapted cells though exhibited anti-proliferative and anti-tumor effects in response to 17AAG treatment, drug treatment has also enhanced the drug efflux activity. Our findings suggest that drug efflux activity and metastatic potential of tumor cells are independently regulated by Hsp90 by distinct mechanisms. We expose the limitations imposed by Hsp90 inhibitors against multidrug resistant tumor cells.

Gα12/13 signaling in metabolic diseases

As the key governors of diverse physiological processes, G protein-coupled receptors (GPCRs) have drawn attention as primary targets for several diseases, including diabetes and cardiovascular disease. Heterotrimeric G proteins converge signals from ~800 members of the GPCR family. Among the members of the G protein α family, the Gα₁₂ family members comprising Gα₁₂ and Gα₁₃ have been referred to as gep oncogenes. Gα_12/13 levels are altered in metabolic organs, including the liver and muscles, in metabolic diseases. The roles of Gα_12/13 in metabolic diseases have been investigated. In this review, we highlight findings demonstrating Gα_12/13 amplifying or dampening regulators of phenotype changes. We discuss the molecular basis of G protein biology in the context of posttranslational modifications to heterotrimeric G proteins and the cell signaling axis. We also highlight findings providing insights into the organ-specific, metabolic and pathological roles of G proteins in changes associated with specific cells, energy homeostasis, glucose metabolism, liver fibrosis and the immune and cardiovascular systems. This review summarizes the currently available knowledge on the importance of Gα_12/13 in the physiology and pathogenesis of metabolic diseases, which is presented according to the basic understanding of their metabolic actions and underlying cellular and molecular bases.

Understanding the activities of two members of a vital category of proteins called G proteins, which initiate metabolic changes when signaling molecules bind to cells, could lead to new therapies for many diseases. Researchers in South Korea and Japan, led by Sang Geon Kim at Seoul National University, review the significance of the Gα12 and Gα13 proteins in diseases characterised by significant changes in metabolism, including liver conditions and disorders of the cardiovascular and immune systems. Specific roles for the proteins have been identified by a variety of methods, including studying the effect of disabling the genes that code for them in mice. Recent insights suggest that drugs interfering with the activity of these Gα proteins might help treat many conditions in which the molecular signalling networks involving the proteins are disrupted.

Heterotrimeric Gα12/13 proteins in kidney injury and disease

Gα₁₂ and Gα₁₃ are ubiquitous members of the heterotrimeric guanine nucleotide-binding protein (G protein) family that play central and integrative roles in the regulation of signal transduction cascades within various cell types in the kidney. Gα₁₂/Gα₁₃ proteins enable the kidney to adapt to an ever-changing environment by transducing stimuli from cell surface receptors and accessory proteins to effector systems. Therefore, perturbations in Gα₁₂/Gα₁₃ levels or their activity can contribute to the pathogenesis of various renal diseases, including renal cancer. This review will highlight and discuss the complex and expanding roles of Gα₁₂/Gα₁₃ proteins on distinct renal pathologies, with emphasis on more recently reported findings. Deciphering how the different Gα₁₂/Gα₁₃ interaction networks participate in the onset and development of renal diseases may lead to the discovery of new therapeutic strategies.

Effects of Post-translational Modifications on Membrane Localization and Signaling of Prostanoid GPCR-G Protein Complexes and the Role of Hypoxia

G protein-coupled receptors (GPCRs) play a pivotal role in the adaptive responses to cellular stresses such as hypoxia. In addition to influencing cellular gene expression profiles, hypoxic microenvironments can perturb membrane protein localization, altering GPCR effector scaffolding and altering downstream signaling. Studies using proteomics approaches have revealed significant regulation of GPCR and G proteins by their state of post-translational modification. The aim of this review is to examine the effects of post-translational modifications on membrane localization and signaling of GPCR-G protein complexes, with an emphasis on vascular prostanoid receptors, and to highlight what is known about the effect of cellular hypoxia on these mechanisms. Understanding post-translational modifications of protein targets will help to define GPCR targets in treatment of disease, and to inform research into mechanisms of hypoxic cellular responses.

The role of heat shock protein 90 in regulating pain, opioid signaling, and opioid antinociception

Heat shock protein 90 (Hsp90) is one of the central signal transduction regulators of the cell. Via client interactions with hundreds of proteins, including receptors, receptor regulatory kinases, and downstream signaling regulators, Hsp90 has a crucial and wide-ranging impact on signaling in response to numerous drugs with impacts on resultant physiology and behavior. Despite this importance, however, Hsp90 has barely been studied in the context of pain and the opioid receptor system, leaving open the possibility that Hsp90 could be manipulated to improve pain therapeutic outcomes, a current area of massive medical need. In this review, we will highlight the known roles of Hsp90 in directly regulating the initiation and maintenance of the pain state. We will also explore how Hsp90 regulates signaling and antinociceptive responses to opioid analgesic drugs, with a special emphasis on ERK MAPK signaling. Understanding this new and growing area will improve our understanding of how Hsp90 regulates signaling and physiology, and also may provide new ways to treat pain, and perhaps reduce the severe impact of the ongoing opioid addiction and overdose crisis.

The Role of Heat Shock Proteins in Regulating Receptor Signal Transduction

Heat shock proteins (Hsp) are a class of stress-inducible proteins that mainly act as molecular protein chaperones. This chaperone activity is diverse, including assisting in nascent protein folding and regulating client protein location and translocation within the cell. The main proteins within the Hsp family, particularly Hsp70 and Hsp90, also have a highly diverse and numerous set of protein clients, which when combined with the high expression levels of Hsp proteins (2%-6% of total protein content) establishes these molecules as "central regulators" of cell protein physiology. Among the client proteins, Hsps regulate numerous signal-transduction and receptor-regulatory kinases, and indeed directly regulate some receptors themselves. This also makes the Hsps, particularly Hsp90, central regulators of signal-transduction machinery, with important impacts on endogenous and drug ligand responses. Among these roles, Hsp90 in particular acts to maintain mature signaling kinases in a metastable conformation permissive for signaling activation. In this review, we will focus on the roles of the Hsps, with a special focus on Hsp90, in regulating receptor signaling and subsequent physiologic responses. We will also explore potential means to manipulate Hsp function to improve receptor-targeted therapies. Overall, Hsps are important regulators of receptor signaling that are receiving increasing interest and exploration, particularly as Hsp90 inhibitors progress toward clinical approval for the treatment of cancer. Understanding the complex interplay of Hsp regulation of receptor signaling may provide important avenues to improve patient treatment.

The G protein α chaperone Ric-8 as a potential therapeutic target

Resistance to inhibitors of cholinesterase (Ric-8)A and Ric-8B are essential genes that encode positive regulators of heterotrimeric G protein α subunits. Controversy persists surrounding the precise way(s) that Ric-8 proteins affect G protein biology and signaling. Ric-8 proteins chaperone nucleotide-free Gα-subunit states during biosynthetic protein folding prior to G protein heterotrimer assembly. In organisms spanning the evolutionary window of Ric-8 expression, experimental perturbation of Ric-8 genes results in reduced functional abundances of G proteins because G protein α subunits are misfolded and degraded rapidly. Ric-8 proteins also act as Gα-subunit guanine nucleotide exchange factors (GEFs) in vitro. However, Ric-8 GEF activity could strictly be an in vitro phenomenon stemming from the ability of Ric-8 to induce partial Gα unfolding, thereby enhancing GDP release. Ric-8 GEF activity clearly differs from the GEF activity of G protein-coupled receptors (GPCRs). G protein βγ is inhibitory to Ric-8 action but obligate for receptors. It remains an open question whether Ric-8 has dual functions in cells and regulates G proteins as both a molecular chaperone and GEF. Clearly, Ric-8 has a profound influence on heterotrimeric G protein function. For this reason, we propose that Ric-8 proteins are as yet untested therapeutic targets in which pharmacological inhibition of the Ric-8/Gα protein-protein interface could serve to attenuate the effects of disease-causing G proteins (constitutively active mutants) and/or GPCR signaling. This minireview will chronicle the understanding of Ric-8 function, provide a comparative discussion of the Ric-8 molecular chaperoning and GEF activities, and support the case for why Ric-8 proteins should be considered potential targets for development of new therapies.

Cellular mechanisms of the 5-HT7 receptor-mediated signaling

Serotonin (5-hydroxytryptamine or 5-HT) is an important neurotransmitter regulating a wide range of physiological and pathological functions via activation of heterogeneously expressed 5-HT receptors. The 5-HT₇ receptor is one of the most recently described members of the 5-HT receptor family. Functionally, 5-HT₇ receptor is associated with a number of physiological and pathological responses, including serotonin-induced phase shifting of the circadian rhythm, control of memory as well as locomotor and exploratory activity. A large body of evidence indicates involvement of the 5-HT₇ receptor in anxiety and depression, and recent studies suggest that 5-HT₇ receptor can be highly relevant for the treatment of major depressive disorders. The 5-HT₇ receptor is coupled to the stimulatory G_s-protein, and receptor stimulation results in activation of adenylyl cyclase (AC) leading to a rise of cAMP concentration. In addition, this receptor is coupled to the G₁₂-protein to activate small GTPases of the Rho family. This review focuses on molecular mechanisms responsible for the 5-HT₇ receptor-mediated signaling. We provide detailed overview of signaling cascades controlled and regulated by the 5-HT₇ receptor and discuss the functional impact of 5-HT₇ receptor for the regulation of different cellular and subcellular processes.

Gα12 structural determinants of Hsp90 interaction are necessary for serum response element-mediated transcriptional activation

The G12/13 class of heterotrimeric G proteins, comprising the α-subunits Gα12 and Gα13, regulates multiple aspects of cellular behavior, including proliferation and cytoskeletal rearrangements. Although guanine nucleotide exchange factors for the monomeric G protein Rho (RhoGEFs) are well characterized as effectors of this G protein class, a variety of other downstream targets has been reported. To identify Gα12 determinants that mediate specific protein interactions, we used a structural and evolutionary comparison between the G12/13, Gs, Gi, and Gq classes to identify "class-distinctive" residues in Gα12 and Gα13. Mutation of these residues in Gα12 to their deduced ancestral forms revealed a subset necessary for activation of serum response element (SRE)-mediated transcription, a G12/13-stimulated pathway implicated in cell proliferative signaling. Unexpectedly, this subset of Gα12 mutants showed impaired binding to heat-shock protein 90 (Hsp90) while retaining binding to RhoGEFs. Corresponding mutants of Gα13 exhibited robust SRE activation, suggesting a Gα12-specific mechanism, and inhibition of Hsp90 by geldanamycin or small interfering RNA-mediated lowering of Hsp90 levels resulted in greater downregulation of Gα12 than Gα13 signaling in SRE activation experiments. Furthermore, the Drosophila G12/13 homolog Concertina was unable to signal to SRE in mammalian cells, and Gα12:Concertina chimeras revealed Gα12-specific determinants of SRE activation within the switch regions and a C-terminal region. These findings identify Gα12 determinants of SRE activation, implicate Gα12:Hsp90 interaction in this signaling mechanism, and illuminate structural features that arose during evolution of Gα12 and Gα13 to allow bifurcated mechanisms of signaling to a common cell proliferative pathway.

Protein palmitoylation in signal transduction of hematopoietic cells

The covalent attachment of palmitate to proteins can alter protein-lipid and protein-protein interactions thereby influencing protein function. Palmitoylation is a reversible post-translational modification. Thus, like protein phosphorylation, protein palmitoylation can function in activation-dependent signaling pathways. This review will provide an overview of the mechanisms and regulation of protein palmitoylation and focus on the role of palmitoylation in signal transduction pathways of lymphocytes and platelets.

Yeast one-hybrid gγ recruitment system for identification of protein lipidation motifs

Fatty acids and isoprenoids can be covalently attached to a variety of proteins. These lipid modifications regulate protein structure, localization and function. Here, we describe a yeast one-hybrid approach based on the Gγ recruitment system that is useful for identifying sequence motifs those influence lipid modification to recruit proteins to the plasma membrane. Our approach facilitates the isolation of yeast cells expressing lipid-modified proteins via a simple and easy growth selection assay utilizing G-protein signaling that induces diploid formation. In the current study, we selected the N-terminal sequence of Gα subunits as a model case to investigate dual lipid modification, i.e., myristoylation and palmitoylation, a modification that is widely conserved from yeast to higher eukaryotes. Our results suggest that both lipid modifications are required for restoration of G-protein signaling. Although we could not differentiate between myristoylation and palmitoylation, N-terminal position 7 and 8 play some critical role. Moreover, we tested the preference for specific amino-acid residues at position 7 and 8 using library-based screening. This new approach will be useful to explore protein-lipid associations and to determine the corresponding sequence motifs.

Ric-8 regulation of heterotrimeric G proteins

Resistance to inhibitors of cholinesterase 8 proteins (Ric-8A and Ric-8B) collectively bind the four classes of heterotrimeric G protein α subunits. Ric-8A and Ric-8B act as non-receptor guanine nucleotide exchange factors (GEFs) toward the Gα subunits that each binds in vitro and seemingly regulate diverse G protein signaling systems in cells. Combined evidence from worm, fly and mammalian systems has shown that Ric-8 proteins are required to maintain proper cellular abundances of G proteins. Ric-8 proteins support G protein levels by serving as molecular chaperones that promote Gα subunit biosynthesis. In this review, the evidence that Ric-8 proteins act as non-receptor GEF activators of G proteins in signal transduction contexts will be weighed against the evidence supporting the molecular chaperoning function of Ric-8 in promoting G protein abundance. I will conclude by suggesting that Ric-8 proteins may act in either capacity in specific contexts. The field awaits additional experimentation to delineate the putative multi-functionality of Ric-8 towards G proteins in cells.

Inverted formin 2 regulates actin dynamics by antagonizing Rho/diaphanous-related formin signaling

Mutations in inverted formin 2 INF2 are a common cause of familial FSGS. INF2 interacts with diaphanous-related formins (mDia) and antagonizes mDia-mediated actin polymerization in response to active Rho signaling, suggesting that dysregulation of these pathways may mediate the development of INF2-related FSGS. However, the precise mechanisms by which INF2 regulates actin-dependent podocyte behavior remain largely unknown. Here, we investigated the possible role of INF2 in both lamellipodia-associated actin dynamics and actin-dependent slit diaphragm (SD) protein trafficking by manipulating the expression of INF2 and the activity of Rho/mDia signaling in cultured podocytes. Activation of mDia in the absence of INF2 led to defective formation of lamellipodia and abnormal SD trafficking. Effects of mutations disrupting the INF2-mDia interaction suggested the specificity of the mDia-antagonizing effect of INF2 in maintaining the lamellipodium. Furthermore, we found that SD trafficking requires INF2 interaction with lipid raft components. In summary, INF2 regulates lamellipodial actin dynamics and the trafficking of slit diaphragm proteins by opposing Rho/mDia-mediated actin polymerization. Thus, in podocytes, INF2 appears to be an important modulator of actin-dependent behaviors that are under the control of Rho/mDia signaling.

Molecular chaperoning function of Ric-8 is to fold nascent heterotrimeric G protein α subunits

We have shown that resistance to inhibitors of cholinesterase 8 (Ric-8) proteins regulate an early step of heterotrimeric G protein α (Gα) subunit biosynthesis. Here, mammalian and plant cell-free translation systems were used to study Ric-8A action during Gα subunit translation and protein folding. Gα translation rates and overall produced protein amounts were equivalent in mock and Ric-8A-immunodepleted rabbit reticulocyte lysate (RRL). GDP-AlF4(-)-bound Gαi, Gαq, Gα13, and Gαs produced in mock-depleted RRL had characteristic resistance to limited trypsinolysis, showing that these G proteins were folded properly. Gαi, Gαq, and Gα13, but not Gαs produced from Ric-8A-depleted RRL were not protected from trypsinization and therefore not folded correctly. Addition of recombinant Ric-8A to the Ric-8A-depleted RRL enhanced GDP-AlF4(-)-bound Gα subunit trypsin protection. Dramatic results were obtained in wheat germ extract (WGE) that has no endogenous Ric-8 component. WGE-translated Gαq was gel filtered and found to be an aggregate. Ric-8A supplementation of WGE allowed production of Gαq that gel filtered as a ∼100 kDa Ric-8A:Gαq heterodimer. Addition of GTPγS to Ric-8A-supplemented WGE Gαq translation resulted in dissociation of the Ric-8A:Gαq heterodimer and production of functional Gαq-GTPγS monomer. Excess Gβγ supplementation of WGE did not support functional Gαq production. The molecular chaperoning function of Ric-8 is to participate in the folding of nascent G protein α subunits.

Methods for the study of dopamine receptors within lipid rafts of kidney cells

There is increasing evidence that G protein-coupled receptor (GPCR) signaling is regulated in lipid raft microdomains. GPCRs and GPCR-signaling molecules, including G proteins and protein kinases, have been reported to compartmentalize in these microdomains. Dopamine D(1)-like receptors (D(1)R and D(5)R) belong to a family of GPCRs that are important in the regulation of renal function. These receptors are not only localized and regulated in caveolae that contains caveolin-1 but are also distributed in non--caveolar lipid rafts which do not contain caveolin-1. This chapter describes detergent- and non-detergent-based methods to obtain lipid raft fractions from renal proximal tubule cells.

Involvement of lipid rafts in multiple signal transductions mediated by two isoforms of thromboxane A₂ receptor: dependency on receptor isoforms and downstream signaling types

Lipid rafts, microdomains in the plasma membrane, are known to be involved in G protein-coupled receptor signal transduction; however, their involvement in thromboxane A(2) receptor (TP) signaling remains to be clarified. We examined whether two isoforms of TP, TPα and TPβ, utilize lipid rafts for multiple G protein signal transduction. Sucrose density gradient centrifugation followed by western blotting of HEK cells expressing TPα or TPβ revealed the localization of both TPα and TPβ in lipid rafts. Furthermore, methyl-β-cyclodextrin, which destroys lipid raft structure by depleting cholesterol, influenced G protein signaling elicited by TPα and TPβ to varying degrees. Phosphatidylinositol hydrolysis and cAMP accumulation induced by TPα or TPβ stimulation was markedly inhibited by methyl-β-cyclodextrin. In contrast, treatment with methyl-β-cyclodextrin partially inhibited RhoA activation induced by TPα stimulation, but failed to affect TPβ stimulation. Furthermore, the inhibitory action of methyl-β-cyclodextrin on cAMP accumulation was specific to TPα and TPβ, because methyl-β-cyclodextrin enhanced forskolin and β-adrenergic stimulation-induced cAMP accumulation. These results indicate that TP isoforms depend on lipid rafts during G(q) and G(s) signaling, while G(13) signaling mediated by TP isoforms does not. Moreover, TPα seems to be more lipid raft-dependent with respect to RhoA activation than TPβ. These results indicate that the two isoforms of the TP mediate multiple signal transductions with varying degrees of lipid raft dependency. Moreover, our results provide a deeper understanding of the function of lipid rafts in G protein signaling and the physiological meaning of TP isoforms.

Rnd1 and Rnd3 targeting to lipid raft is required for p190 RhoGAP activation

The N-terminal region is the lipid raft–targeting determinant. A novel regulatory mechanism is found by which Rnd proteins function as RhoA antagonists, as is a striking mechanism by which differential membrane targeting governs activities of G proteins having similarities in effector interaction.

The Rnd proteins Rnd1, Rnd2, and Rnd3/RhoE are well known as key regulators of the actin cytoskeleton in various cell types, but they comprise a distinct subgroup of the Rho family in that they are GTP bound and constitutively active. Functional differences of the Rnd proteins in RhoA inhibition signaling have been reported in various cell types. Rnd1 and Rnd3 antagonize RhoA signaling by activating p190 RhoGAP, whereas Rnd2 does not. However, all the members of the Rnd family have been reported to bind directly to p190 RhoGAP and equally induce activation of p190 RhoGAP in vitro, and there is no evidence that accounts for the functional difference of the Rnd proteins in RhoA inhibition signaling. Here we report the role of the N-terminal region in signaling. Rnd1 and Rnd3, but not Rnd2, have a KERRA (Lys-Glu-Arg-Arg-Ala) sequence of amino acids in their N-terminus, which functions as the lipid raft-targeting determinant. The sequence mediates the lipid raft targeting of p190 RhoGAP correlated with its activation. Overall, our results demonstrate a novel regulatory mechanism by which differential membrane targeting governs activities of Rnd proteins to function as RhoA antagonists.

Non-canonical signaling and localizations of heterotrimeric G proteins

Heterotrimeric G proteins typically transduce signals from G protein-coupled receptors (GPCRs) to effector proteins. In the conventional G protein signaling paradigm, the G protein is located at the cytoplasmic surface of the plasma membrane, where, after activation by an agonist-bound GPCR, the GTP-bound Gα and free Gβγ bind to and regulate a number of well-studied effectors, including adenylyl cyclase, phospholipase Cβ, RhoGEFs and ion channels. However, research over the past decade or more has established that G proteins serve non-canonical roles in the cell, whereby they regulate novel effectors, undergo activation independently of a GPCR, and/or function at subcellular locations other than the plasma membrane. This review will highlight some of these non-canonical aspects of G protein signaling, focusing on direct interactions of G protein subunits with cytoskeletal and cell adhesion proteins, the role of G proteins in cell division, and G protein signaling at diverse organelles.

Signalling mechanisms of RhoGTPase regulation by the heterotrimeric G proteins G12 and G13

G protein-mediated signal transduction can transduce signals from a large variety of extracellular stimuli into cells and is the most widely used mechanism for cell communication at the membrane. The RhoGTPase family has been well established as key regulators of cell growth, differentiation and cell shape changes. Among G protein-mediated signal transduction, G12/13-mediated signalling is one mechanism to regulate RhoGTPase activity in response to extracellular stimuli. The alpha subunits of G12 or G13 have been shown to interact with members of the RH domain containing guanine nucleotide exchange factors for Rho (RH-RhoGEF) family of proteins to directly connect G protein-mediated signalling and RhoGTPase signalling. The G12/13-RH-RhoGEF signalling mechanism is well conserved over species and is involved in critical steps for cell physiology and disease conditions, including embryonic development, oncogenesis and cancer metastasis. In this review, we will summarize current progress on this important signalling mechanism.

Association of membrane rafts and postsynaptic density: proteomics, biochemical, and ultrastructural analyses

J. Neurochem. (2011) 119, 64-77.

ABSTRACT

Postsynaptic membrane rafts are believed to play important roles in synaptic signaling, plasticity, and maintenance. However, their molecular identities remain elusive. Further, how they interact with the well-established signaling specialization, the postsynaptic density (PSD), is poorly understood. We previously detected a number of conventional PSD proteins in detergent-resistant membranes (DRMs). Here, we have performed liquid chromatography coupled with tandem mass spectrometry (LC/MS/MS) analyses on postsynaptic membrane rafts and PSDs. Our comparative analysis identified an extensive overlap of protein components in the two structures. This overlapping could be explained, at least partly, by a physical association of the two structures. Meanwhile, a significant number of proteins displayed biased distributions to either rafts or PSDs, suggesting distinct roles for the two postsynaptic specializations. Using biochemical and electron microscopic methods, we directly detected membrane raft-PSD complexes. In vitro reconstitution experiments indicated that the formation of raft-PSD complexes was not because of the artificial reconstruction of once-solubilized membrane components and PSD structures, supporting that these complexes occurred in vivo. Taking together, our results provide evidence that postsynaptic membrane rafts and PSDs may be physically associated. Such association could be important in postsynaptic signal integration, synaptic function, and maintenance.

Anandamide exerts its antiproliferative actions on cholangiocarcinoma by activation of the GPR55 receptor

Cholangiocarcinomas are devastating cancers of biliary origin with limited treatment options. It has previously been shown that the endocannabinoid anandamide exerts antiproliferative effects on cholangiocarcinoma independent of any known cannabinoid receptors, and by the stabilization of lipid rafts, thereby allowing the recruitment and activation of the Fas death receptor complex. Recently, GPR55 was identified as a putative cannabinoid receptor; therefore, the role of GPR55 in the antiproliferative effects of anandamide was evaluated. GPR55 is expressed in all cholangiocarcinoma cells and liver biopsy samples to a similar level as in non-malignant cholangiocytes. Treatment with either anandamide or the GPR55 agonist, O-1602, reduced cholangiocarcinoma cell proliferation in vitro and in vivo. Furthermore, knocking down the expression of GPR55 prevented the antiproliferative effects of anandamide. Coupled to these effects was an increase in JNK activity. The antiproliferative effects of anandamide could be blocked by pretreatment with a JNK inhibitor and the lipid raft disruptors β-methylcyclodextrin and fillipin III. Activation of GPR55 by anandamide or O-1602 increased the amount of Fas in the lipid raft fractions, which could be blocked by pretreatment with the JNK inhibitor. These data represent the first evidence that GPR55 activation by anandamide can lead to the recruitment and activation of the Fas death receptor complex and that targeting GPR55 activation may be a viable option for the development of therapeutic strategies to treat cholangiocarcinoma.

Contribution of the P2Y12 receptor-mediated pathway to platelet hyperreactivity in hypercholesterolemia

BACKGROUND

In hypercholesterolemia, platelets demonstrate increased reactivity and promote the development of cardiovascular disease.

OBJECTIVE

This study was carried out to investigate the contribution of the ADP receptor P2Y12-mediated pathway to platelet hyperreactivity due to hypercholesterolemia.

METHODS

Low-density lipoprotein receptor-deficient mice and C57Bl/6 wild-type mice were fed on normal chow and high-fat (Western or Paigen) diets for 8 weeks to generate differently elevated cholesterol levels. P2Y12 receptor-induced functional responses via G(i) signaling were studied ex vivo when washed murine platelets were activated by 2MeSADP and PAR4 agonist AYPGKF in the presence and absence of indomethacin. Platelet aggregation and secretion, α(IIb)β(3) receptor activation and the phosphorylation of extracellular signal-regulated protein kinase (ERK) and Akt were analyzed.

RESULTS

Plasma cholesterol levels ranged from 69 ± 10 to 1011 ± 185 mg dL(-1) depending on diet in mice with different genotypes. Agonist-dependent aggregation, dense and α-granule secretion and JON/A binding were gradually and significantly (P < 0.05) augmented at low agonist concentration in correlation with the increasing plasma cholesterol levels, even if elevated thromboxane generation was blocked. These functional responses were induced via increased levels of G(i) -mediated ERK and Akt phosphorylation in hypercholesterolemic mice vs. normocholesterolemic animals. In addition, blocking of the P2Y12 receptor by AR-C69931MX (Cangrelor) resulted in strongly reduced platelet aggregation in mice with elevated cholesterol levels compared with normocholesterolemic controls.

CONCLUSIONS

These data revealed that the P2Y12 receptor pathway was substantially involved in platelet hyperreactivity associated with mild and severe hypercholesterolemia.

Alterations of GABA(A) and glutamate receptor subunits and heat shock protein in rat hippocampus following traumatic brain injury and in posttraumatic epilepsy

Traumatic brain injury (TBI) can result in the development of posttraumatic epilepsy (PTE). Recently, we reported differential alterations in tonic and phasic GABA(A) receptor (GABA(A)R) currents in hippocampal dentate granule cells 90 days after controlled cortical impact (CCI) (Mtchedlishvili et al., 2010). In the present study, we investigated long-term changes in the protein expression of GABA(A)R α1, α4, γ2, and δ subunits, NMDA (NR2B) and AMPA (GluR1) receptor subunits, and heat shock proteins (HSP70 and HSP90) in the hippocampus of Sprague-Dawley rats evaluated by Western blotting in controls, CCI-injured animals without PTE (CCI group), and CCI-injured animals with PTE (PTE group). No differences were found among all three groups for α1 and α4 subunits. Significant reduction of γ2 protein was observed in the PTE group compared to control. CCI caused a 194% and 127% increase of δ protein in the CCI group compared to control (p<0.0001), and PTE (p<0.0001) groups, respectively. NR2B protein was increased in CCI and PTE groups compared to control (p=0.0001, and p=0.011, respectively). GluR1 protein was significantly decreased in CCI and PTE groups compared to control (p=0.003, and p=0.001, respectively), and in the PTE group compared to the CCI group (p=0.036). HSP70 was increased in CCI and PTE groups compared to control (p=0.014, and p=0.005, respectively); no changes were found in HSP90 expression. These results provide for the first time evidence of long-term alterations of GABA(A) and glutamate receptor subunits and a HSP following CCI.

Soft X-ray Laser Microscopy of Lipid Rafts towards GPCR-Based Drug Discovery Using Time-Resolved FRET Spectroscopy

Many signaling molecules involved in G protein-mediated signal transduction, which are present in the lipid rafts and believed to be controlled spatially and temporally, influence the potency and efficacy of neurotransmitter receptors and transporters. This has focus interest on lipid rafts and the notion that these microdomains acts as a kind of signaling platform and thus have an important role in the expression of membrane receptor-mediated signal transduction, cancer, immune responses, neurotransmission, viral infections and various other phenomena due to specific and efficient signaling according to extracellular stimuli. However, the real structure of lipid rafts has not been observed so far due to its small size and a lack of sufficiently sophisticated observation systems. A soft X-ray microscope using a coherent soft X-ray laser in the water window region (2.3–4.4 nm) should prove to be a most powerful tool to observe the dynamic structure of lipid rafts of several tens of nanometers in size in living cells. We have developed for the X-ray microscope a new compact soft X-ray laser using strongly induced plasma high harmonic resonance. We have also developed a time-resolved highly sensitive fluorescence resonance energy transfer (FRET) system and confirmed protein-protein interactions coupled with ligands. The simultaneous use of these new tools for observation of localization of G-protein coupled receptors (GPCRs) in rafts has become an important and optimum tool system to analyze the dynamics of signal transduction through rafts as signaling platform. New technology to visualize rafts is expected to lead to the understanding of those dynamics and innovative development of drug discovery that targets GPCRs localized in lipid rafts.

Identification of polycystin-1 and Gα12 binding regions necessary for regulation of apoptosis

Most patients with autosomal dominant polycystic kidney disease (ADPKD) harbor mutations in PKD1, the gene for polycystin-1 (PC1), a transmembrane protein with a cytoplasmic C-terminus that interacts with numerous signaling molecules, including Gα12. The functions of PC1 and the mechanisms of cyst development leading to renal failure are complex. Recently, we reported that PC1 expression levels modulate activity of Gα12-stimulated apoptosis (Yu et al., J. Biol. Chem. 2010 285(14):10243-51). Herein, a mutational analysis of Gα12 and PC1 was undertaken to identify regions required for their interaction and ability to modulate apoptosis. A set of Gα12 mutations with systematic replacement of six amino acids with NAAIRS was tested for binding to the PC1 C-terminus in GST pulldowns. Additionally, a series of deletions within the PC1 C-terminus was examined for binding to Gα12. We identified 3 NAAIRS substitutions in Gα12 that completely abrogated binding, and identified a previously described 74 amino acid Gαi/o binding domain in the PC1 C-terminus as necessary for Gα12 interaction. The functional consequences of uncoupling PC1/Gα12 binding were studied in apoptosis assays utilizing HEK293 cells with inducible PC1 overexpression. Gα12 mutants deficient in PC1 binding were refractory to PC1 inhibition of Gα12-stimulated apoptosis. Likewise, deletion of the Gα12-interacting sequence from the PC1 cytoplasmic domain abrogated its inhibition of Gα12-stimulated apoptosis. Based on the crystal structure of Gα12, the PC1 interaction sites are likely to reside on exposed regions within the G protein helical domain. These structural details should facilitate the design of reagents to uncouple PC1/Gα12 signaling in ADPKD.

An analysis of the Caenorhabditis elegans lipid raft proteome using geLC-MS/MS

Lipid rafts are microdomains of the phospholipid bilayer, proposed to form semi-stable "islands" that act as a platform for several important cellular processes; major classes of raft-resident proteins include signalling proteins and glycosylphosphatidylinositol (GPI)-anchored proteins. Proteomic studies into lipid rafts have been mainly carried out in mammalian cell lines and single cell organisms. The nematode Caenorhabditis elegans, the model organism with a well-defined developmental profile, is ideally suited for the study of this subcellular locale in a complex developmental context. A study of the lipid raft proteome of C. elegans is presented here. A total of 44 proteins were identified from the lipid raft fraction using geLC-MS/MS, of which 40 have been determined to be likely raft proteins after analysis of predicted functions. Prediction of GPI-anchoring of the proteins found 21 to be potentially modified in this way, two of which were experimentally confirmed to be GPI-anchored. This work is the first reported study of the lipid raft proteome in C. elegans. The results show that raft proteins, including numerous GPI-anchored proteins, may have a variety of potentially important roles within the nematode, and will hopefully lead to C. elegans becoming a useful model for the study of lipid rafts.

Greasing their way: lipid modifications determine protein association with membrane rafts

Increasing evidence suggests that biological membranes can be laterally subdivided into domains enriched in specific lipid and protein components and that these domains may be involved in the regulation of a number of vital cellular processes. An example is membrane rafts, which are lipid-mediated domains dependent on preferential association between sterols and sphingolipids and inclusive of a specific subset of membrane proteins. While the lipid and protein composition of rafts has been extensively characterized, the structural details determining protein partitioning to these domains remain unresolved. Here, we review evidence suggesting that post-translation modification by saturated lipids recruits both peripheral and transmembrane proteins to rafts, while short, unsaturated, and/or branched hydrocarbon chains prevent raft association. The most widely studied group of raft-associated proteins are glycophosphatidylinositol-anchored proteins (GPI-AP), and we review a variety of evidence supporting raft-association of these saturated lipid-anchored extracellular peripheral proteins. For transmembrane and intracellular peripheral proteins, S-acylation with saturated fatty acids mediates raft partitioning, and the dynamic nature of this modification presents an exciting possibility of enzymatically regulated raft association. The other common lipid modifications, that is, prenylation and myristoylation, are discussed in light of their likely role in targeting proteins to nonraft membrane regions. Finally, although the association between raft affinity and lipid modification is well-characterized, we discuss several open questions regarding regulation and remodeling of these post-translational modifications as well as their role in transbilayer coupling of membrane domains.

Lipid raft association and cholesterol sensitivity of P2X1-4 receptors for ATP: chimeras and point mutants identify intracellular amino-terminal residues involved in lipid regulation of P2X1 receptors

Cholesterol-rich lipid rafts act as signaling microdomains and can regulate receptor function. We have shown in HEK293 cells recombinant P2X1-4 receptors (ATP-gated ion channels) are expressed in lipid rafts. Localization to flotillin-rich lipid rafts was reduced by the detergent Triton X-100. This sensitivity to Triton X-100 was concentration- and subunit-dependent, demonstrating differential association of P2X1-4 receptors with lipid rafts. The importance of raft association to ATP-evoked P2X receptor responses was determined in patch clamp studies. The cholesterol-depleting agents methyl-β-cyclodextrin or filipin disrupt lipid rafts and reduced P2X1 receptor currents by >90%. In contrast, ATP-evoked P2X2-4 receptor currents were unaffected by lipid raft disruption. To determine the molecular basis of cholesterol sensitivity, we generated chimeric receptors replacing portions of the cholesterol-sensitive P2X1 receptor with the corresponding region from the insensitive P2X2 receptor. These chimeras identified the importance of the intracellular amino-terminal region between the conserved protein kinase C site and the first transmembrane segment for the sensitivity to cholesterol depletion. Mutation of any of the variant residues between P2X1 and P2X2 receptors in this region in the P2X1 receptor (residues 20-23 and 27-29) to cysteine removed cholesterol sensitivity. Cholesterol depletion did not change the ATP sensitivity or cell surface expression of P2X1 receptors. This suggests that cholesterol is normally needed to facilitate the opening/gating of ATP-bound P2X1 receptor channels, and mutations in the pre-first transmembrane segment region remove this requirement.

Heterotrimeric G protein signaling outside the realm of seven transmembrane domain receptors

Heterotrimeric G proteins, consisting of the guanine nucleotide-binding Galpha subunits with GTPase activity and the closely associated Gbeta and Ggamma subunits, are important signaling components for receptors with seven transmembrane domains (7TMRs). These receptors, also termed G protein-coupled receptors (GPCRs), act as guanine nucleotide exchange factors upon agonist stimulation. There is now accumulating evidence for noncanonical functions of heterotrimeric G proteins independent of 7TMR coupling. Galpha proteins belonging to all 4 subfamilies, including G(s), G(i), G(q), and G(12) are found to play important roles in receptor tyrosine kinase signaling, regulation of oxidant production, development, and cell migration, through physical and functional interaction with proteins other than 7TMRs. Association of Galpha with non-7TMR proteins also facilitates presentation of these G proteins to specific cellular microdomains. This Minireview aims to summarize our current understanding of the noncanonical roles of Galpha proteins in cell signaling and to discuss unresolved issues including regulation of Galpha activation by proteins other than the 7TMRs.

Scavenger receptor BI modulates platelet reactivity and thrombosis in dyslipidemia

Hypercholesterolemia is associated with increased platelet sensitivity to agonists and a prothrombotic phenotype. Mechanisms of platelet hypersensitivity are poorly understood; however, increased platelet cholesterol levels associated with hypercholesterolemia were proposed as leading to hypersensitivity. Scavenger receptor class B type I (SR-BI) in the liver controls plasma high-density lipoprotein (HDL) levels, and SR-BI-deficient mice display a profound dyslipoproteinemia. SR-BI is also expressed on platelets, and recent studies have suggested a role for SR-BI in platelet function; however, its role in hemostasis is unknown. Our present studies demonstrated that non-bone marrow-derived SR-BI deficiency and the dyslipidemia associated with it lead to platelet hyperreactivity that was mechanistically linked to increased platelet cholesterol content. Platelet-specific deficiency of SR-BI, on the other hand, was associated with resistance to hyperreactivity induced by increased platelet cholesterol content. Intravital thrombosis studies demonstrated that platelet SR-BI deficiency protected mice from prothrombotic phenotype in 2 types of dyslipidemia associated with increased platelet cholesterol content. These novel findings demonstrate that SR-BI plays dual roles in thrombosis and may contribute to acute cardiovascular events in vivo in hypercholesterolemia.

Structure, function, and localization of Gβ5-RGS complexes

Members of the R7 subfamily of regulator of G protein signaling (RGS) proteins (RGS6, 7, 9, and 11) exist as heterodimers with the G protein beta subunit Gβ5. These protein complexes are only found in neurons and are defined by the presence of three domains: DEP/DHEX, Gβ5/GGL, and RGS. This article summarizes published work in the following areas: (1) the functional significance of structural organization of Gβ5-R7 complexes, (2) regional distribution of Gβ5-R7 in the nervous system and regulation of R7 family expression, (3) subcellular localization of Gβ5-R7 complexes, and (4) novel binding partners of Gβ5-R7 proteins. The review points out some contradictions between observations made by different research groups and highlights the importance of using alternative experimental approaches to obtain conclusive information about Gβ5-R7 function in vivo.

Regulation and physiological functions of G12/13-mediated signaling pathways

Accumulating data indicate that G12 subfamily (Galpha12/13)-mediated signaling pathways play pivotal roles in a variety of physiological processes, while aberrant regulation of this pathway has been identified in various human diseases. It has been demonstrated that Galpha12/13-mediated signals form networks with other signaling proteins at various levels, from cell surface receptors to transcription factors, to regulate cellular responses. Galpha12/13 have slow rates of nucleotide exchange and GTP hydrolysis, and specifically target RhoGEFs containing an amino-terminal RGS homology domain (RH-RhoGEFs), which uniquely function both as a GAP and an effector for Galpha12/13. In this review, we will focus on the mechanisms regulating the Galpha12/13 signaling system, particularly the Galpha12/13-RH-RhoGEF-Rho pathway, which can regulate a wide variety of cellular functions from migration to transformation.

Schizosaccharomyces pombe Hsp90/Git10 is required for glucose/cAMP signaling

The fission yeast Schizosaccharomyces pombe senses environmental glucose through a cAMP-signaling pathway. Elevated cAMP levels activate protein kinase A (PKA) to inhibit transcription of genes involved in sexual development and gluconeogenesis, including the fbp1(+) gene, which encodes fructose-1,6-bisphosphatase. Glucose-mediated activation of PKA requires the function of nine glucose-insensitive transcription (git) genes, encoding adenylate cyclase, the PKA catalytic subunit, and seven "upstream" proteins required for glucose-triggered adenylate cyclase activation. We describe the cloning and characterization of the git10(+) gene, which is identical to swo1(+) and encodes the S. pombe Hsp90 chaperone protein. Glucose repression of fbp1(+) transcription is impaired by both git10(-) and swo1(-) mutant alleles of the hsp90(+) gene, as well as by chemical inhibition of Hsp90 activity and temperature stress to wild-type cells. Unlike the swo1(-) mutant alleles, the git10-201 allele supports cell growth at 37 degrees , while severely reducing glucose repression of an fbp1-lacZ reporter, suggesting a separation-of-function defect. Sequence analyses of three swo1(-) alleles and the one git10(-) allele indicate that swo1(-) mutations alter core functional domains of Hsp90, while the git10(-) mutation affects the Hsp90 central domain involved in client protein binding. These results suggest that Hsp90 plays a specific role in the S. pombe glucose/cAMP pathway.

G alpha12 is targeted to the mitochondria and affects mitochondrial morphology and motility

G alpha12 constitutes, along with G alpha13, one of the four families of alpha subunits of heterotrimeric G proteins. We found that the N terminus of G alpha12, but not those of other G alpha subunits, contains a predicted mitochondrial targeting sequence. Using confocal microscopy and cell fractionation, we demonstrated that up to 40% of endogenous G alpha12 in human umbilical vein endothelial cells colocalize with mitochondrial markers. N-terminal sequence of G alpha12 fused to GFP efficiently targeted the fusion protein to mitochondria. G alpha12 with mutated mitochondrial targeting sequence was still located in mitochondria, suggesting the existence of additional mechanisms for mitochondrial localization. Lysophosphatidic acid, one of the known stimuli transduced by G alpha12/13, inhibited mitochondrial motility, while depletion of endogenous G alpha12 increased mitochondrial motility. G alpha12Q229L variants uncoupled from RhoGEFs (but not fully functional activated G alpha12Q229L) induced transformation of the mitochondrial network into punctate mitochondria and resulted in a loss of mitochondrial membrane potential. All examined G alpha12Q229L variants reduced phosphorylation of Bcl-2 at Ser-70, while only mutants unable to bind RhoGEFs also decreased cellular levels of Bcl-2. These G alpha12 mutants were also more efficient Hsp90 interactors. These findings are the first demonstration of a heterotrimeric G protein alpha subunit specifically targeted to mitochondria and involved in the control of mitochondrial morphology and dynamics.

Galpha12 regulates protein interactions within the MDCK cell tight junction and inhibits tight-junction assembly

The polarized functions of epithelia require an intact tight junction (TJ) to restrict paracellular movement and to separate membrane proteins into specific domains. TJs contain scaffolding, integral membrane and signaling proteins, but the mechanisms that regulate TJs and their assembly are not well defined. Galpha12 (GNA12) binds the TJ protein ZO-1 (TJP1), and Galpha12 activates Src to increase paracellular permeability via unknown mechanisms. Herein, we identify Src as a component of the TJ and find that recruitment of Hsp90 to activated Galpha12 is necessary for signaling. TJ integrity is disrupted by Galpha12-stimulated Src phosphorylation of ZO-1 and ZO-2 (TJP2); this phosphorylation leads to dissociation of occludin and claudin 1 from the ZO-1 protein complex. Inhibiting Hsp90 with geldanamycin blocks Galpha12-stimulated Src activation and phosphorylation, but does not affect protein levels or the Galpha12-ZO-1 interaction. Using the calcium-switch model of TJ assembly and GST-TPR (GST-fused TPR domain of PP5) pull-downs of activated Galpha12, we demonstrate that switching to normal calcium medium activates endogenous Galpha12 during TJ assembly. Thrombin increases permeability and delays TJ assembly by activating Galpha12, but not Galpha13, signaling pathways. These findings reveal an important role for Galpha12, Src and Hsp90 in regulating the TJ in established epithelia and during TJ assembly.

Membranes: a meeting point for lipids, proteins and therapies

Membranes constitute a meeting point for lipids and proteins. Not only do they define the entity of cells and cytosolic organelles but they also display a wide variety of important functions previously ascribed to the activity of proteins alone. Indeed, lipids have commonly been considered a mere support for the transient or permanent association of membrane proteins, while acting as a selective cell/organelle barrier. However, mounting evidence demonstrates that lipids themselves regulate the location and activity of many membrane proteins, as well as defining membrane microdomains that serve as spatio-temporal platforms for interacting signalling proteins. Membrane lipids are crucial in the fission and fusion of lipid bilayers and they also act as sensors to control environmental or physiological conditions. Lipids and lipid structures participate directly as messengers or regulators of signal transduction. Moreover, their alteration has been associated with the development of numerous diseases. Proteins can interact with membranes through lipid co-/post-translational modifications, and electrostatic and hydrophobic interactions, van der Waals forces and hydrogen bonding are all involved in the associations among membrane proteins and lipids. The present study reviews these interactions from the molecular and biomedical point of view, and the effects of their modulation on the physiological activity of cells, the aetiology of human diseases and the design of clinical drugs. In fact, the influence of lipids on protein function is reflected in the possibility to use these molecular species as targets for therapies against cancer, obesity, neurodegenerative disorders, cardiovascular pathologies and other diseases, using a new approach called membrane-lipid therapy.

Selective activation of human atrial Galpha12 and Galpha13 by Galphaq-coupled angiotensin and endothelin receptors

Galphaq-coupled receptors such as alpha1-adrenergic, angiotensin, and endothelin receptors, play key roles in cardiac physiology. These receptors have also been shown to couple to G proteins of the G12 family, including Galpha12 and Galpha13. In this report, we determined whether these G proteins interact with endothelin, angiotensin, and alpha1-adrenergic receptors in the human heart. We find that these receptors activate cardiac Galpha12 and Galpha13 differentially; endothelin receptors activate only Galpha12 (to 218 +/- 22% of unstimulated levels), angiotensin receptors activate only Galpha13 (to 236 +/- 49% of unstimulated levels), and alpha1-adrenergic receptors activate neither Galpha12 (123 +/- 18% of unstimulated levels) nor Galpha13 (113 +/- 12% of unstimulated levels). Consistent with these data, translocation of guanine nucleotide exchange factor p115RhoGEF, which responds to Galpha13, occurs only after stimulation of angiotensin receptors (shifting from 73 +/- 12% to 41 +/- 10% cytosolic). These differences in the activation of Galpha12 and Galpha13 by Galphaq-coupled receptors may underlie reported differences in the functions of these receptors.

Salt-induced changes in the plasma membrane proteome of the halotolerant alga Dunaliella salina as revealed by blue native gel electrophoresis and nano-LC-MS/MS analysis

The halotolerant alga Dunaliella salina is a recognized model photosynthetic organism for studying plant adaptation to high salinity. The adaptation mechanisms involve major changes in the proteome composition associated with energy metabolism and carbon and iron acquisition. To clarify the molecular basis for the remarkable resistance to high salt, we performed a comprehensive proteomics analysis of the plasma membrane. Plasma membrane proteins were recognized by tagging intact cells with a membrane-impermeable biotin derivative. Proteins were resolved by two-dimensional blue native/SDS-PAGE and identified by nano-LC-MS/MS. Of 55 identified proteins, about 60% were integral membrane or membrane-associated proteins. We identified novel surface coat proteins, lipid-metabolizing enzymes, a new family of membrane proteins of unknown function, ion transporters, small GTP-binding proteins, and heat shock proteins. The abundance of 20 protein spots increased and that of two protein spots decreased under high salt. The major salt-regulated proteins were implicated in protein and membrane structure stabilization and within signal transduction pathways. The migration profiles of native protein complexes on blue native gels revealed oligomerization or co-migration of major surface-exposed proteins, which may indicate mechanisms of stabilization at high salinity.

Biologic functions of the G12 subfamily of heterotrimeric g proteins: growth, migration, and metastasis

The G12 subfamily of heterotrimeric G proteins has been the subject of intense scientific interest for more than 15 years. During this period, studies have revealed more than 20 potential G12-interacting proteins and numerous signaling axes emanating from the G12 proteins, Galpha12 and Galpha13. In addition, more recent studies have begun to illuminate the various and sundry functions that the G12 subfamily plays in biology. In this review, we summarize the diverse range of proteins that have been identified as Galpha12 and/or Galpha13 interactors and describe ongoing studies designed to dissect the biological roles of specific Galpha-effector protein interactions. Further, we describe and discuss the expanding role of G12 proteins in the biology of cells, focusing on the distinct properties of this subfamily in regulating cell proliferation, cell migration, and metastatic invasion.

Domains necessary for Galpha12 binding and stimulation of protein phosphatase-2A (PP2A): Is Galpha12 a novel regulatory subunit of PP2A?

Many cellular signaling pathways share regulation by protein phosphatase-2A (PP2A), a widely expressed serine/threonine phosphatase, and the heterotrimeric G protein Galpha(12). PP2A activity is altered in carcinogenesis and in some neurodegenerative diseases. We have identified binding of Galpha(12) with the Aalpha subunit of PP2A, a trimeric enzyme composed of A (scaffolding), B (regulatory), and C (catalytic) subunits and demonstrated that Galpha(12) stimulated phosphatase activity (J Biol Chem 279: 54983-54986, 2004). We now show in substrate-velocity analysis using purified PP2A that V(max) was stimulated 3- to 4-fold by glutathione transferase (GST)-Galpha(12) with little effect on K(m) values. To identify the binding domains mediating the Aalpha-Galpha(12) interaction, an extensive mutational analysis was performed. Well-characterized mutations of Aalpha were expressed in vitro and tested for binding to GST-Galpha(12) in pull-down assays. Galpha(12) binds to Aalpha along repeats 7 to 10, and PP2A B subunits are not necessary for binding. To identify where Aalpha binds to Galpha(12), a series of 61 Galpha(12) mutants were engineered to contain the sequence Asn-Ala-Ala-Ile-Arg-Ser (NAAIRS) in place of 6 consecutive amino acids. Mutant Galpha(12) proteins were individually expressed in human embryonic kidney cells and analyzed for interaction with GST or GST-Aalpha in pull-down assays. The Aalpha binding sites were localized to regions near the N and C termini of Galpha(12). The expression of constitutively activated Galpha(12) (QLalpha(12)) in Madin Darby canine kidney cells stimulated PP2A activity as determined by decreased phosphorylation of tyrosine 307 on the catalytic subunit. Based on crystal structures of Galpha(12) and PP2A Aalpha, a model describing the binding surfaces and potential mechanisms of Galpha(12)-mediated PP2A activation is presented.

Inhibition of cell death by a novel 16.2 kD heat shock protein predominantly via Hsp90 mediated lipid rafts stabilization and Akt activation pathway

AlphaB-crystallin homology, heat stress induction and chaperone activity suggested that a previously encloned gene product is a novel small heat shock protein (Hsp16.2). Suppression of Hsp16.2 by siRNA sensitized cells to hydrogen peroxide or taxol induced cell-death. Over-expressing of Hsp16.2 protected cells against stress stimuli by inhibiting cytochrome c release from the mitochondria, nuclear translocation of AIF and endonuclease G, and caspase 3 activation. Recombinant Hsp16.2 protected mitochondrial membrane potential against calcium induced collapse in vitro indicating that Hsp16.2 stabilizes mitochondrial membrane systems. Hsp16.2 formed self-aggregates and bound to Hsp90. Inhibition of Hsp90 by geldanamycin diminished the cytoprotective effect of Hsp16.2 indicating that this effect was Hsp90-mediated. Hsp16.2 over-expression increased lipid rafts formation as demonstrated by increased cell surface labeling with fluorescent cholera toxin B, and increased Akt phosphorylation. The inhibition of PI-3-kinase-Akt pathway by LY-294002 or wortmannin significantly decreased the protective effect of the Hsp16.2. These data indicate that the over-expression of Hsp16.2 inhibits cell death via the stabilization of mitochondrial membrane system, activation of Hsp90, stabilization of lipid rafts and by the activation of PI-3-kinase-Akt cytoprotective pathway.

[Role of lipid rafts in trimeric G protein-mediated signal transduction]

Lipid rafts and caveolae are microdomains in the cell membranes, which contain cholesterol, glycolipids, and sphingomyelin. While caveolae are relatively stable because caveolin, an integral protein, supports the structure, lipid rafts are considered to be unstable, being dynamically produced and degraded. Recent studies have reported that lipid rafts contain many signaling molecules, such as glycosylphosphatidylinositol-anchored proteins, acylated proteins, G-protein-coupled receptors (GPCRs), trimeric and small G-proteins and their effectors, suggesting that the lipid rafts have an important role in receptor-mediated signal transduction. Therefore drugs that modify the composition of lipid rafts might influence the efficacy of cellular signal transduction. In this review, we demonstrate the role of lipid rafts in GPCR-G-protein signaling and also present our recent results showing that the wasp toxin mastoparan modifies G(q/11)-mediated phospholipase C activation through the interaction with gangliosides in lipid rafts.

Lipid raft microdomains and neurotransmitter signalling

Lipid rafts are specialized structures on the plasma membrane that have an altered lipid composition as well as links to the cytoskeleton. It has been proposed that these structures are membrane domains in which neurotransmitter signalling might occur through a clustering of receptors and components of receptor-activated signalling cascades. The localization of these proteins in lipid rafts, which is affected by the cytoskeleton, also influences the potency and efficacy of neurotransmitter receptors and transporters. The effect of lipid rafts on neurotransmitter signalling has also been implicated in neurological and psychiatric diseases.

P2X7 and phospholipid signalling: The search of the “missing link” in epithelial cells

The purinergic receptor P2X(7) is widely expressed in epithelial cells. This receptor shares in common with the other P2X receptors the ability to form a non-selective cation channel. On the other hand, the COOH terminus of P2X(7) seems to allow this receptor to couple to a spectrum of downstream effectors responsible for the regulation of cell death and pore formation among other functions. However, the coupling of P2X(7) to these downstream effectors, as well as the identity of possible adapters directly interacting with the receptor, remains poorly understood. Here we review the ability of P2X(7) to activate phospholipid signalling pathways in epithelial cells and propose this step as a possible link between the receptor and other downstream effectors. The P2X(7) ability to control the cellular levels of several lipid messengers (PA, AA, DAG, ceramide, etc.) through the modulation of phospholipases (C, A(2), D) and neutral sphingomyelinase is described. These pathways are sometimes regulated independently of the channel function of the receptor. Recent data concerning P2X(7) localization in lipid rafts is also discussed in relation to the coupling to these pathways and dissociation from channel function.

Association with HSP90 Inhibits Cbl-Mediated Down-regulation of Mutant Epidermal Growth Factor Receptors

Activating mutations in the epidermal growth factor receptor (EGFR), localized in the activation loop within the kinase domain, have been discovered in non-small cell lung cancers (NSCLC). Most of these mutants are exquisitely sensitive to EGFR tyrosine kinase inhibitors, suggesting that they generate receptor dependence in the cancers that express them. 32D cells stably expressing EGFR-L861Q and EGFR-L858R but not wild-type EGFR exhibited ligand-independent receptor phosphorylation and viability. Ligand-induced receptor down-regulation (LIRD) was impaired in mutant-expressing cells. The EGFR mutants were constitutively associated with the E3 ubiquitin ligase Cbl but did not associate with the adaptor protein CIN85 on the addition of ligand. Inhibition of HSP90 activity with geldanamycin restored Cbl function as indicated by receptor ubiquitination and LIRD. These results suggest that EGFR mutants form defective endocytic complexes. In addition, HSP90 plays a role in maintaining the functional conformation of EGFR mutants and protecting activated receptors from LIRD.