Tubulin, Gq, and phosphatidylinositol 4,5-bisphosphate interact to regulate phospholipase Cbeta1 signaling

The impact of volatile anesthetics and propofol on phosphatidylinositol 4,5-bisphosphate signaling

Phosphatidylinositol 4,5-bisphosphate (PIP2), as well as other anionic phospholipids, play a pivotal role in various cellular processes, including ion channel regulation, receptor trafficking, and intracellular signaling pathways. The binding of volatile anesthetics and propofol to PIP2 leads to alterations in PIP2-mediated signaling causing modulation of ion channels such as ɣ-aminobutyric acid type A (GABAA) receptors, voltage-gated calcium channels, and potassium channels through various mechanisms. Additionally, the interaction between anionic phospholipids and G protein-coupled receptors plays a critical role in various anesthetic pathways, with these anesthetic-induced changes impacting PIP2 levels which cause cascading effects on receptor trafficking, including GABAA receptor internalization. This comprehensive review of various mechanisms of interaction provides insights into the intricate interplay between PIP2 signaling and anesthetic-induced changes, shedding light on the molecular mechanisms underlying anesthesia.

Neurokinin 1 receptor promotes rat airway smooth muscle cell migration in asthmatic airway remodelling by enhancing tubulin expression

Background

Airway remodelling is a major contributor to hyper-responsiveness leading to chronic asthma; however, the underlying mechanisms remain unclear. This study aimed to investigate the effects of a neurokinin 1 receptor (NK1R) antagonist (WIN62577) on the migration of airway smooth muscle cells (ASMCs) and the expression of NK1R and alpha-tubulin in airway remodelling using young rats with asthma.

Methods

Sprague-Dawley rats were randomly divided into a control group and airway remodelling group. Rats in the model group were stimulated with ovalbumin for 8 weeks. Primary ASMCs were cultured and purified from all rats, and then treated with different doses of WIN62577. The expression of NK1R and α-tubulin in ASMCs was assessed using immunofluorescence, real-time quantitative polymerase chain reaction, and western blotting. Changes in ASMC migration were detected by a transwell chamber assay.

Results

The transwell assay showed that the number of migrating ASMCs in the asthmatic airway remodelling group was significantly greater than that in the control group (P<0.01), which was inhibited by WIN62577 in a dose-dependent manner, with peak inhibition detected at 10^-8 mol/L. The mRNA and protein expression levels of NK1R and α-tubulin were significantly higher in the asthmatic airway remodelling group than in the control group (P<0.05 and P<0.01, respectively), and were significantly decreased after treatment with WIN62577 (P<0.01 and P<0.05, respectively).

Conclusions

NK1R antagonists may suppress ASMC migration in a rat model of airway remodelling by inhibiting tubulin expression, indicating a new potential target for the treatment and control of chronic asthma.

Cytokeratin-8 in Anaplastic Thyroid Carcinoma: More Than a Simple Structural Cytoskeletal Protein

Anaplastic thyroid carcinoma (ATC) is almost universally fatal. Elevated keratin-8 (KRT8) protein expression is an established diagnostic cancer biomarker in several epithelial cancers (but not ATC). Several keratins, including KRT8, have been suggested to have a role in cell biology beyond that of structural cytoskeletal proteins. Here, we provide evidence that KRT8 plays a direct role in the growth of ATCs. Genomic and transcriptomic analysis of >5000 patients demonstrates that KRT8 mutation and copy number amplification are frequently evident in epithelial-derived cancers. Carcinomas arising from diverse tissues exhibit KRT8 mRNA and protein overexpression when compared to normal tissue levels. Similarly, in a panel of patient-derived ATC cell lines and patient tumors, KRT8 expression shows a similar pattern. sh-RNA-mediated KRT8 knockdown in these cell lines increases apoptosis, whereas forced overexpression of KRT8 confers resistance to apoptosis under peroxide-induced cell stress conditions. We further show that KRT8 protein binds to annexin A2, a protein known to mediate apoptosis as well as the redox pathway.

Microtubule mechanics in the working myocyte

The mechanical role of cardiac microtubules (MTs) has been a topic of some controversy. Early studies, which relied largely on pharmacological interventions that altered the MT cytoskeleton as a whole, presented no consistent role. Recent advances in the ability to observe and manipulate specific properties of the cytoskeleton have strengthened our understanding. Direct observation of MTs in working myocytes suggests a spring-like function, one that is surprisingly tunable by post-translational modification (PTM). Specifically, detyrosination of MTs facilitates an interaction with intermediate filaments that complex with the sarcomere, altering myocyte stiffness, contractility, and mechanosignalling. Such results support a paradigm of cytoskeletal regulation based on not only polymerization, but also associations with binding partners and PTMs that divide the MT cytoskeleton into functionally distinct subsets. The evolutionary costs and benefits of tuning cytoskeletal mechanics remain an open question, one that we discuss herein. Nevertheless, mechanically distinct MT subsets provide a rich new source of therapeutic targets for a variety of phenomena in the heart.

A new non-canonical pathway of Gα(q) protein regulating mitochondrial dynamics and bioenergetics

Contrary to previous assumptions, G proteins do not permanently reside on the plasma membrane, but are constantly monitoring the cytoplasmic surfaces of the plasma membrane and endomembranes. Here, we report that the Gαq and Gα11 proteins locate at the mitochondria and play a role in a complex signaling pathway that regulates mitochondrial dynamics. Our results provide evidence for the presence of the heteromeric G protein (Gαq/11βγ) at the outer mitochondrial membrane and for Gαq at the inner membrane. Both localizations are necessary to maintain the proper equilibrium between fusion and fission; which is achieved by altering the activity of mitofusin proteins, Drp1, OPA1 and the membrane potential at both the outer and inner mitochondrial membranes. As a result of the absence of Gαq/11, there is a decrease in mitochondrial fusion rates and a decrease in overall respiratory capacity, ATP production and OXPHOS-dependent growth. These findings demonstrate that the presence of Gαq proteins at the mitochondria serves as a physiological function: stabilizing elongated mitochondria and regulating energy production in Drp1 and Opa1 dependent mechanisms. This thereby links organelle dynamics and physiology.

Tubulin, actin and heterotrimeric G proteins: coordination of signaling and structure

G proteins mediate signals from membrane G protein coupled receptors to the cell interior, evoking significant regulation of cell physiology. The cytoskeleton contributes to cell morphology, motility, division, and transport functions. This review will discuss the interplay between heterotrimeric G protein signaling and elements of the cytoskeleton. Also described and discussed will be the interplay between tubulin and G proteins that results in atypical modulation of signaling pathways and cytoskeletal dynamics. This will be extended to describe how tubulin and G proteins act in concert to influence various aspects of cellular behavior. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters.This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

TRPV1-tubulin complex: involvement of membrane tubulin in the regulation of chemotherapy-induced peripheral neuropathy

Existence of microtubule cytoskeleton at the membrane and submembranous regions, referred as 'membrane tubulin' has remained controversial for a long time. Since we reported physical and functional interaction of Transient Receptor Potential Vanilloid Sub Type 1 (TRPV1) with microtubules and linked the importance of TRPV1-tubulin complex in the context of chemotherapy-induced peripheral neuropathy, a few more reports have characterized this interaction in in vitro and in in vivo condition. However, the cross-talk between TRPs with microtubule cytoskeleton, and the complex feedback regulations are not well understood. Sequence analysis suggests that other than TRPV1, few TRPs can potentially interact with microtubules. The microtubule interaction with TRPs has evolutionary origin and has a functional significance. Biochemical evidence, Fluorescence Resonance Energy Transfer analysis along with correlation spectroscopy and fluorescence anisotropy measurements have confirmed that TRPV1 interacts with microtubules in live cell and this interaction has regulatory roles. Apart from the transport of TRPs and maintaining the cellular structure, microtubules regulate signaling and functionality of TRPs at the single channel level. Thus, TRPV1-tubulin interaction sets a stage where concept and parameters of 'membrane tubulin' can be tested in more details. In this review, I critically analyze the advancements made in biochemical, pharmacological, behavioral as well as cell-biological observations and summarize the limitations that need to be overcome in the future.

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.

MT1 melatonin receptor internalization underlies melatonin-induced morphologic changes in Chinese hamster ovary cells and these processes are dependent on Gi proteins, MEK 1/2 and microtubule modulation

Melatonin induces cellular differentiation in numerous cell types. Data show that multiple mechanisms are involved in these processes that are cell-type specific and may be receptor dependent or independent. The focus of this study was to specifically assess the role of human MT1 melatonin receptors in cellular differentiation using an MT1-Chinese hamster ovary (CHO) model; one that reproducibly produces measurable morphologic changes in response to melatonin. Using multiple approaches, we show that melatonin induces MT1-CHO cells to hyperelongate through a MEK 1/2, and ERK 1/2-dependent mechanism that is dependent upon MT1 receptor internalization, Gi protein activation, and clathrin-mediated endocytosis. Using immunoprecipitation analysis, we show that MT1 receptors form complexes with Gi(alpha) 2,3, Gq(alpha), beta-arrestin-2, MEK 1/2, and ERK 1/2 in the presence of melatonin. We also show that MEK and ERK activity that is induced by melatonin is dependent on Gi protein activation, clathrin-mediated endocytosis and is modulated by microtubules. We conclude from these studies that melatonin-induced internalization of human MT1 melatonin receptors in CHO cells is responsible for activating both MEK 1/2 and ERK 1/2 to drive these morphologic changes. These events, as mediated by melatonin, require Gi protein activation and endocytosis mediated through clathrin, to form MT1 receptor complexes with beta-arrestin-2/MEK 1/2 and ERK 1/2. The MT1-CHO model is invaluable to mapping out signaling cascades as mediated through MT1 receptors especially because it separates out MEK/ERK 1/2 activation by MT1 receptors from that of receptor tyrosine kinases.

Phospholipase C signaling involvement in macrotubule assembly and activation of the mechanism regulating protoplast volume in plasmolyzed root cells of Triticum turgidum

The role of phosphoinositide-specific phospholipase C (PI-PLC) signaling in the macrotubule-dependent protoplast volume regulation in plasmolyzed root cells of Triticum turgidum was investigated. At the onset of hyperosmotic stress, PI-PLC activation was documented. Inhibition of PI-PLC activity by U73122 blocked tubulin macrotubule formation in plasmolyzed cells and their protoplast volume regulatory mechanism. In neomycin-treated plasmolyzed cells, macrotubule formation and protoplast volume regulation were not affected. In these cells the PI-PLC pathway is down-regulated as neomycin sequesters the PI-PLC substrate, 4,5-diphosphate-phosphatidyl inositol (PtdInsP(2)). These phenomena were unaffected by R59022, an inhibitor of phosphatidic acic (PA) production via the PLC pathway. Taxol, a microtubule (MT) stabilizer, inhibited the hyperosmotic activation of PI-PLC, but oryzalin, which disorganized MTs, triggered PI-PLC activity. Taxol prevented macrotubule formation and inhibited the mechanism regulating the volume of the plasmolyzed protoplast. Neomycin partly relieved some of the taxol effects. These data suggest that PtdInspP(2) turnover via PI-PLC assists macrotubule formation and activation of the mechanism regulating the plasmolyzed protoplast volume; and the massive disorganization of MTs that is carried out at the onset of hyperosmotic treatment triggers the activation of this mechanism.

CaR activation increases TNF production by mTAL cells via a Gi-dependent mechanism

We evaluated the contribution of calcium-sensing receptor (CaR)-mediated G(i)-coupled signaling to TNF production in medullary thick ascending limb (mTAL) cells. A selective G(i) inhibitor, pertussis toxin (PTX), but not the inactive B-oligomer binding subunit, abolished CaR-mediated increases in TNF production. The inhibitory effect of PTX was partially reversed by using an adenylate cyclase inhibitor. CaR-mediated TNF production also was partially reversed by a cAMP analog, 8-Br-cAMP. IP(1) accumulation was CaR dependent and blocked by PI-PLC; partial inhibition also was observed with PTX. CaR increased calcineurin (CaN) activity by approximately threefold, and PTX prevented CaR-mediated increases in CaN activity, an nuclear factor of activated T cells (NFAT)-cis reporter construct, and a TNF promoter construct. The interaction between G(i) and PKC was determined, as we previously showed that CaR-mediated TNF production was CaN and NFAT- mediated and G(q) dependent. CaR activation increased PKC activity by twofold, an effect abolished by transient transfection with a dominant negative CaR construct, R796W, or pretreatment with PTX. Inhibition with the pan-specific PKC inhibitor GF 109203X (20 nM) abolished CaR-mediated increases in activity of CaN, an NFAT reporter, and a TNF promoter construct. Collectively, the data suggest that G(i)-coupled signaling contributes to NFAT-mediated TNF production in a CaN- and PKC-dependent manner and may be part of a CaR mechanism to regulate mTAL function. Moreover, concurrent G(q) and G(i) signaling is required for CaR-mediated TNF production in mTAL cells via a CaN/NFAT pathway that is PKC dependent. Understanding CaR-mediated signaling pathways that regulate TNF production in the mTAL is crucial to defining novel mechanisms that regulate extracellular fluid volume and salt balance.

The Importance of N-terminal Polycysteine and Polybasic Sequences for G14α and G16α Palmitoylation, Plasma Membrane Localization, and Signaling Function

Plasma membrane targeting of G protein alpha (Galpha) subunits is essential for competent receptor-to-G protein signaling. Many Galpha are tethered to the plasma membrane by covalent lipid modifications at their N terminus. Additionally, it is hypothesized that Gq family members (Gqalpha,G11alpha,G14alpha, and G16alpha) in particular utilize a polybasic sequence of amino acids in their N terminus to promote membrane attachment and protein palmitoylation. However, this hypothesis has not been tested, and nothing is known about other mechanisms that control subcellular localization and signaling properties of G14alpha and G16alpha. Here we report critical biochemical factors that mediate membrane attachment and signaling function of G14alpha and G16alpha. We find that G14alpha and G16alpha are palmitoylated at distinct polycysteine sequences in their N termini and that the polycysteine sequence along with the adjacent polybasic region are both important for G16alpha-mediated signaling at the plasma membrane. Surprisingly, the isolated N termini of G14alpha and G16alpha expressed as peptides fused to enhanced green fluorescent protein each exhibit differential requirements for palmitoylation and membrane targeting; individual cysteine residues, but not the polybasic regions, determine lipid modification and subcellular localization. However, full-length G16alpha, more so than G14alpha, displays a functional dependence on single cysteines for membrane localization and activity, and its full signaling potential depends on the integrity of the polybasic sequence. Together, these findings indicate that G14alpha and G16alpha are palmitoylated at distinct polycysteine sequences, and that the adjacent polybasic domain is not required for Galpha palmitoylation but is important for localization and functional activity of heterotrimeric G proteins.

Identification of a molecular target for the Yersinia protein kinase A

Pathogenic bacteria of the genus Yersinia employ a type III secretion system to inject bacterial effector proteins directly into the host cytosol. One of these effectors, the Yersinia serine/threonine protein kinase YpkA, is an essential virulence determinant involved in host actin cytoskeletal rearrangements and in inhibition of phagocytosis. Here we report that YpkA inhibits multiple Galphaq signaling pathways. The kinase activity of YpkA is required for Galphaq inhibition. YpkA phosphorylates Ser47, a key residue located in the highly conserved diphosphate binding loop of the GTPase fold of Galphaq. YpkA-mediated phosphorylation of Ser47 impairs guanine nucleotide binding by Galphaq. Y. pseudotuberculosis expressing wild-type YpkA, but not a catalytically inactive YpkA mutant, interferes with Galphaq-mediated signaling pathways. Identification of a YpkA-mediated phosphorylation site in Galphaq sheds light on the contribution of the kinase activity of YpkA to Yersinia pathogenesis.

Interaction network of proteins associated with abiotic stress response and development in wheat

Wheat is the most widely adapted crop to abiotic stresses and considered an excellent system to study stress tolerance in spite of its genetic complexity. Recent studies indicated that several hundred genes are either up- or down-regulated in response to stress treatment. To elucidate the function of some of these genes, an interactome of proteins associated with abiotic stress response and development in wheat was generated using the yeast two-hybrid GAL4 system and specific protein interaction assays. The interactome is comprised of 73 proteins, generating 97 interactions pairs. Twenty-one interactions were confirmed by bimolecular fluorescent complementation in Nicotiana benthamiana. A confidence-scoring system was elaborated to evaluate the significance of the interactions. The main feature of this interactome is that almost all bait proteins along with their interactors were interconnected, creating a spider web-like structure. The interactome revealed also the presence of a "cluster of proteins involved in flowering control" in three- and four-protein interaction loops. This network provides a novel insight into the complex relationships among transcription factors known to play central roles in vernalization, flower initiation and abscisic acid signaling, as well as associations that tie abiotic stress with other regulatory and signaling proteins. This analysis provides useful information in elucidating the molecular mechanism associated with abiotic stress response in plants.

Identification and characterisation of novel tubulin-binding motifs located within the C-terminus of TRPV1

Previously, we reported that TRPV1, the vanilloid receptor, interacts with soluble alphabeta-tubulin dimers as well as microtubules via its C-terminal cytoplasmic domain. The interacting region of TRPV1, however, has not been defined. We found that the TRPV1 C-terminus preferably interacts with beta-tubulin and less with alpha-tubulin. Using a systematic deletion approach and biotinylated-peptides we identified two tubulin-binding sites present in TRPV1. These two sequence stretches are highly conserved in all known mammalian TRPV1 orthologues and partially conserved in some of the TRPV1 homologues. As these sequence stretches are not similar to any known tubulin-binding sequences, we conclude that TRPV1 interacts with tubulin and microtubule through two novel tubulin-binding motifs.

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.

Modulation of melatonin receptors and G-protein function by microtubules

Chronic melatonin exposure produces microtubule rearrangements in Chinese hamster ovary (CHO) cells expressing the human MT1 melatonin receptor while at the same time desensitizing MT1 receptors. Because microtubule rearrangements parallel MT1 receptor desensitization, we tested whether microtubules modulate receptor responsiveness. We determined whether depolymerization of microtubules by Colcemid, which prevents melatonin-induced outgrowths in MT1-expressing CHO cells, also prevents MT1 receptor desensitization by affecting G(alpha)-GTP exchange on G-proteins. In this study, we found that depolymerization of microtubules in MT1 receptor expressing cells, prevented melatonin-induced receptor desensitization reflected by an increase in the number of high potency sites when compared with melatonin-treated cells. Further examination of the mechanism(s) underlying this desensitization suggested that these effects occurred at the level of G-proteins. Depolymerization of microtubules during melatonin-induced desensitization, attenuated forskolin-induced cAMP accumulation, the opposite of which usually occurs following melatonin exposure alone. Concomitant to this attenuation in the forskolin response was a reduction in the amount of G(i alpha) protein coupled to MT1 receptors and an increase in [32P] azidoanilido GTP incorporation into G(i) proteins. These data are consistent with the findings that microtubule depolymerization did not affect MT1/G(q) coupling nor did it affect melatonin-induced phosphoinositide hydrolysis following melatonin exposure. However, interestingly, microtubule depolymerization enhanced melatonin-induced protein kinase C activation that was blocked in the presence of pertussis toxin. These data demonstrate that microtubule dynamics can modulate melatonin receptor function through their actions on G(i) proteins and impact on downstream signaling cascades.

Rapid disassembly of dynamic microtubules upon activation of the capsaicin receptor TRPV1

The transmission of pain signalling involves the cytoskeleton, but mechanistically this is poorly understood. We recently demonstrated that the capsaicin receptor TRPV1, a non-selective cation channel expressed by nociceptors that is capable of detecting multiple pain-producing stimuli, directly interacts with the tubulin cytoskeleton. We hypothesized that the tubulin cytoskeleton is a downstream effector of TRPV1 activation. Here we show that activation of TRPV1 results in the rapid disassembly of microtubules, but not of the actin or neurofilament cytoskeletons. TRPV1 activation mainly affects dynamic microtubules that contain tyrosinated tubulins, whereas stable microtubules are apparently unaffected. The C-terminal fragment of TRPV1 exerts a stabilizing effect on microtubules when over-expressed in F11 cells. These findings suggest that TRPV1 activation may contribute to cytoskeleton remodelling and so influence nociception.

Cell signalling diversity of the Gqalpha family of heterotrimeric G proteins

Many receptors for neurotransmitters and hormones rely upon members of the Gqalpha family of heterotrimeric G proteins to exert their actions on target cells. Galpha subunits of the Gq class of G proteins (Gqalpha, G11alpha, G14alpha and G15/16alpha) directly link receptors to activation of PLC-beta isoforms which, in turn, stimulate inositol lipid (i.e. calcium/PKC) signalling. Although Gqalpha family members share a capacity to activate PLC-beta, they also differ markedly in their biochemical properties and tissue distribution which predicts functional diversity. Nevertheless, established models suggest that Gqalpha family members are functionally redundant and that their cellular responses are a result of PLC-beta activation and downstream calcium/PKC signalling. Growing evidence, however, indicates that Gqalpha, G11alpha, G14alpha and G15/16alpha are functionally diverse and that many of their cellular actions are independent of inositol lipid signalling. Recent findings show that Gqalpha family members differ with regard to their linked receptors and downstream binding partners. Reported binding partners distinct from PLC-beta include novel candidate effector proteins, various regulatory proteins, and a growing list of scaffolding/adaptor proteins. Downstream of these signalling proteins, Gqalpha family members exhibit unexpected differences in the signalling pathways and the gene expression profiles they regulate. Finally, genetic studies using whole animal models demonstrate the importance of certain Gqalpha family members in cardiac, lung, brain and platelet functions among other physiological processes. Taken together, these findings demonstrate that Gqalpha, G11alpha, G14alpha and G15/16alpha regulate both overlapping and distinct signalling pathways, indicating that they are more functionally diverse than previously thought.

Identification and characterization of a Ca2+ -sensitive interaction of the vanilloid receptor TRPV1 with tubulin

The vanilloid receptor TRPV1 plays a well-established functional role in the detection of a range of chemical and thermal noxious stimuli, such as those associated with tissue inflammation and the resulting pain. TRPV1 activation results in membrane depolarization, but may also trigger intracellular Ca2+ -signalling events. In a proteomic screen for proteins associated with the C-terminal sequence of TRPV1, we identified beta-tubulin as a specific TRPV1-interacting protein. We demonstrate that the TRPV1 C-terminal tail is capable of binding tubulin dimers, as well as of binding polymerized microtubules. The interaction is Ca2+ -sensitive, and affects microtubule properties, such as microtubule sensitivity towards low temperatures and nocodazole. Our data thus provide compelling evidence for the interaction of TRPV1 with the cytoskeleton. The Ca2+ -sensitivity of this interaction suggests that the microtubule cytoskeleton at the cell membrane may be a downstream effector of TRPV1 activation.

Disruption of microtubular network attenuates histamine-induced dilation in rat mesenteric vessels

Cytoplasmic microtubules are important in many cellular homeostatic processes in the cell. They regulate cell shape and movement as well as serving as a network by which vesicles and membrane-bound organelles can travel. Lately, there have been many studies demonstrating that microtubules are involved in regulation of intracellular signaling and, therefore, affect vascular reactivity. In this study, we tested the hypothesis that microtubule disruption attenuates agonist-induced endothelium-dependent vasodilation. Isolated mesenteric arterial bed from normotensive rats was preconstricted with phenylephrine, and dose-response curves for histamine, acetylcholine (ACh), sodium nitroprusside (SNP), and pinacidil were performed before and after incubation with nocodazole or colchicine. Treatment of the vascular beds with nocodazole or colchicine significantly attenuated histamine relaxation but did not change the ACh-, SNP-, or pinacidil-induced vasorelaxation. Nocodazole did not cause an additional attenuation of the histamine-mediated dilation in mesenteric vessels in the presence of Nω-nitro-l-arginine methyl ester, high extracellular K+, or K+channel blockers. These data suggest that disruption of microtubules affects an essential endothelial component of histamine-mediated vasodilation in the mesenteric arterial bed. The mechanism(s) involved in this effect might be related to an impairment of endothelial NO synthesis, which might not be as important for the ACh as for the histamine vasodilator response in rat mesenteric vessels. These results demonstrate the importance of the microtubular system for endothelium-dependent NO-mediated smooth muscle relaxation.

Pleckstrin homology domains of phospholipase C-gamma1 directly interact with beta-tubulin for activation of phospholipase C-gamma1 and reciprocal modulation of beta-tubulin function in microtubule assembly

Phosphoinositide-specific phospholipase C-gamma1 (PLC-gamma1) has two pleckstrin homology (PH) domains, an N-terminal domain and a split PH domain. Here we show that pull down of NIH3T3 cell extracts with PLC-gamma1 PH domain-glutathione S-transferase fusion proteins, followed by matrix-assisted laser desorption ionization-time of flight-mass spectrometry, identified beta-tubulin as a binding protein of both PLC-gamma1 PH domains. Tubulin is a main component of microtubules and mitotic spindle fibers, which are composed of alpha- and beta-tubulin heterodimers in all eukaryotic cells. PLC-gamma1 and beta-tubulin colocalized in the perinuclear region in COS-7 cells and cotranslocated to the plasma membrane upon agonist stimulation. Membrane-targeted translocation of depolymerized tubulin by agonist stimulation was also supported by immunoprecipitation analyses. The phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolyzing activity of PLC-gamma1 was substantially increased in the presence of purified tubulin in vitro, whereas the activity was not promoted by bovine serum albumin, suggesting that beta-tubulin activates PLC-gamma1. Furthermore, indirect immunofluorescent microscopy showed that PLC-gamma1 was highly concentrated in mitotic spindle fibers, suggesting that PLC-gamma1 is involved in spindle fiber formation. The effect of PLC-gamma1 in microtubule formation was assessed by overexpression and silencing PLC-gamma1 in COS-7 cells, which resulted in altered microtubule dynamics in vivo. Cells overexpressing PLC-gamma1 showed higher microtubule densities than controls, whereas PLC-gamma1 silencing with small interfering RNAs led to decreased microtubule network densities as compared with control cells. Taken together, our results suggest that PLC-gamma1 and beta-tubulin transmodulate each other, i.e. that PLC-gamma1 modulates microtubule assembly by beta-tubulin, and beta-tubulin promotes PLC-gamma1 activity.

Clathrin-mediated endocytosis of m3 muscarinic receptors. Roles for Gbetagamma and tubulin

Receptors as well as some G protein subunits internalize after agonist stimulation. It is not clear whether Galpha(q) or Gbetagamma undergo such regulated translocation. Recent studies demonstrate that m3 muscarinic receptor activation in SK-N-SH neuroblastoma cells causes recruitment of tubulin to the plasma membrane. This subsequently transactivates Galpha(q) and activates phospholipase Cbeta1. Interaction of tubulin-GDP with Gbetagamma at the offset of phospholipase Cbeta1 signaling appears involved in translocation of tubulin and Gbetagamma to vesicle-like structures in the cytosol (Popova, J. S., and Rasenick, M. M. (2003) J. Biol. Chem. 278, 34299-34308). The relationship of this internalization to the clathrin-mediated endocytosis of the activated m3 muscarinic receptors or Galpha(q) involvement in this process has not been clarified. To test this, SK-N-SH cells were treated with carbachol, and localization of Galpha(q), Gbetagamma, tubulin, clathrin, and m3 receptors were analyzed by both cellular imaging and biochemical techniques. Upon agonist stimulation both tubulin and clathrin translocated to the plasma membrane and co-localized with receptors, Galpha(q) and Gbetagamma. Fifteen minutes later receptors, Gbetagamma and tubulin, but not Galpha(q), internalized with the clathrin-coated vesicles. Coimmunoprecipitation of m3 receptors with Gbetagamma, tubulin, and clathrin from the cytosol of carbachol-treated cells was readily observed. These data suggested that Gbetagamma subunits might organize the formation of a multiprotein complex linking m3 receptors to tubulin since they interacted with both proteins. Such protein assemblies might explain the dynamin-dependent but beta-arrestin-independent endocytosis of m3 muscarinic receptors since tubulin interaction with dynamin might guide or insert the complex into clathrin-coated pits. This novel mechanism of internalization might prove important for other beta-arrestin-independent endocytic pathways. It also suggests cross-regulation between G protein-mediated signaling and the dynamics of the microtubule cytoskeleton.

Association of adenylate cyclase with an actin-like protein in the human myometrium

Regulation of muscle contraction by second messengers such as cAMP and regulation of the adenylate cyclase enzyme by the cytoskeleton have been previously described. However, the physical association of both effector and structural elements is still unknown. In this context, we have co-purified a human myometrial adenylate cyclase with an actin-like protein in a two-step purification protocol. The adenylate cyclase catalytic unit was solubilized with Lubrol-PX, submitted to anionic exchange chromatography and purified about 7-fold. The eluate was applied to a forskolin-agarose column obtaining an adenylate cyclase extract enriched 257-fold (enzymatic activity of 1390 pmol/30 min per mg protein) that co-eluted with a 74.6-kDa protein that possessed the 18-27 amino-acid fragment from the N-terminal region of human actin. Under non-reducing conditions, the apparent molecular weight of this protein decreased to 54 kDa, which has been previously described for arthrin. These results provide the first demonstration of the physical association of human myometrial adenylate cyclase with a cytoskeleton-related protein, supporting the hypothesis that adenylate cyclase is regulated by mechanical stimuli.

Tubulin as a regulator of G-protein signaling

Tubulin is known to form high-affinity complexes with certain G proteins. The formation of such complexes allows tubulin to activate Galpha and fosters a system whereby elements of the cytoskeleton can influence G-protein signaling. This article describes the interaction between tubulin and G proteins and discusses methods for examining this interaction.

Active surface transport of metabotropic glutamate receptors through binding to microtubules and actin flow

Receptors for neurotransmitters are concentrated and stabilized at given sites such as synapses through interactions with scaffolding proteins and cytoskeletal elements. The transport of receptors first involves directed vesicular trafficking of intracellularly stored receptors followed by their targeting to the plasma membrane. Once expressed at the cell surface, receptors are thought to reach their final location by random Brownian diffusion in the plasma membrane plane. Here, we investigate whether the metabotropic glutamate receptor mGluR5 can also be transported actively on the cell surface. We used single particle tracking to follow mGluR5 movement in real time at the surface of neuronal growth cones or fibroblast lamellipodia, both of which bear a particularly active cytoskeleton. We found that after a certain lag time mGluR5 undergoes directed rearward transport, which depends on actin flow. On actin depolymerization, directed movement was suppressed, but receptors still bound to a rigid structure. By contrast, receptor transport and immobilization was fully suppressed by microtubule depolymerization but favored by microtubule stabilization. Furthermore, mGluR5 could be immunoprecipitated with tubulin from rat brains, confirming the ability of mGluR5 to bind to microtubules. We propose that mGluR5 can be transported on the cell surface through actin-mediated retrograde transport of microtubules. This process may play a role in receptor targeting and organization during synapse formation or during glutamate-mediated growth cone chemotaxis.

Alteration in calcium channel properties is responsible for the neurotoxic action of a familial frontotemporal dementia tau mutation

Tau, a microtubule binding protein, is not only a major component of neurofibrillary tangles in Alzheimer's disease, but also a causative gene for hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). We show here that an FTDP-17 tau mutation (V337M) in SH-SY5Y cells reduces microtubule polymerization, increases voltage-dependent calcium current (ICa) density, and decreases ICa rundown. The reduced rundown of ICa by V337M was significantly inhibited by nifedipine (L-type Ca channel blocker), whereas omega-conotoxin GVIA (N-type Ca channel blocker) showed smaller effects, indicating that tau mutations affect L-type calcium channel activity. The depolarization-induced increase in intracellular calcium was also significantly augmented by the V337M tau mutation. Treatment with a microtubule polymerizing agent (taxol), an adenylyl cyclase inhibitor, or a protein kinase A (PKA) inhibitor, counteracted the effects of mutant tau on ICa. Taxol also attenuated the Ca2+ response to depolarization in cells expressing mutant tau. Apoptosis in SH-SY5Y cells induced by serum deprivation was exacerbated by the V337M mutation, and nifedipine, taxol, and a PKA inhibitor significantly protected cells against apoptosis. Our results indicate that a tau mutation which decreases its microtubule-binding ability augments calcium influx by depolymerizing microtubules and activating adenylyl cyclase and PKA.

G beta gamma mediates the interplay between tubulin dimers and microtubules in the modulation of Gq signaling

Agonist stimulation causes tubulin association with the plasma membrane and activation of PLC beta 1 through direct interaction with, and transactivation of, G alpha q. Here we demonstrate that G beta gamma interaction with tubulin down-regulates this signaling pathway. Purified G beta gamma, alone or with phosphatidylinositol 4,5-bisphosphate (PIP2), inhibited carbachol-evoked membrane recruitment of tubulin and G alpha q transactivation by tubulin. Polymerization of microtubules elicited by G beta gamma overrode tubulin translocation to the membrane in response to carbachol stimulation. G beta gamma sequestration of tubulin reduced the inhibition of PLC beta 1 observed at high tubulin concentration. G beta 1 gamma 2 interacted preferentially with tubulin-GDP, whereas G alpha q was transactivated by tubulin-GTP. Prenylation of the gamma 2 polypeptide was required for G beta gamma/tubulin interaction. Both confocal microscopy and coimmunoprecipitation studies revealed the spatiotemporal pattern of G beta gamma/tubulin interaction during carbachol stimulation of neuroblastoma SK-N-SH cells. In resting cells G beta gamma localized predominantly at the cell membrane, whereas tubulin was found in well defined microtubules in the cytosol. Within 2 min of agonist exposure, a subset of tubulin translocated to the plasma membrane and colocalized with G beta. Fifteen min post-carbachol addition, tubulin and G beta colocalized in vesicle-like structures in the cytosol. G beta/tubulin colocalization increased after pretreatment of cells with the microtubule-depolymerizing agent, colchicine, and was inhibited by taxol. Taxol also inhibited carbachol-induced PIP2 hydrolysis. It is suggested that G beta gamma/tubulin interaction mediates internalization of membrane-associated tubulin at the offset of PLC beta 1 signaling. Newly cytosolic G beta gamma/tubulin complexes might promote microtubule polymerization attenuating further tubulin association with the plasma membrane. Thus G protein-coupled receptors might evoke G alpha and G beta gamma to orchestrate regulation of phospholipase signaling by tubulin dimers and control of cell shape by microtubules.

Calmodulin is a phospholipase C-beta interacting protein

Phospholipase C-beta 3 (PLC beta 3) is an important effector enzyme in G protein-coupled signaling pathways. Activation of PLC beta 3 by G alpha and G beta gamma subunits has been fairly well characterized, but little is known about other protein interactions that may also regulate PLC beta 3 function. A yeast two-hybrid screen of a mouse brain cDNA library with the amino terminus of PLC beta 3 has yielded potential PLC beta 3 interacting proteins including calmodulin (CaM). Physical interaction between CaM and PLC beta 3 is supported by a positive secondary screen in yeast and the identification of a CaM binding site in the amino terminus of PLC beta 3. Co-precipitation of in vitro translated and transcribed amino- and carboxyl-terminal PLC beta 3 revealed CaM binding at a putative amino-terminal binding site. Direct physical interaction of PLC beta 3 and PLC beta 1 isoforms with CaM is supported by pull-down of both isoenzymes with CaM-Sepharose beads from 1321N1 cell lysates. CaM inhibitors reduced M1-muscarinic receptor stimulation of inositol phospholipid hydrolysis in 1321N1 astrocytoma cells consistent with a physiologic role for CaM in modulation of PLC beta activity. There was no effect of CaM kinase II inhibitors, KN-93 and KN-62, on M1-muscarinic receptor stimulation of inositol phosphate hydrolysis, consistent with a direct interaction between PLC beta isoforms and CaM.

G protein signaling and the molecular basis of antidepressant action

Over the past four decades, a variety of interventions have been used for the treatment of clinical depression and other affective disorders. Several distinct pharmacological compounds show therapeutic efficacy. There are three major classes of antidepressant drugs: monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), and tricyclic compounds. There are also a variety of atypical antidepressant drugs, which defy ready classification. Finally, there is electroconvulsive therapy, ECT. All require chronic (2-3 weeks) treatment to achieve a clinical response. To date, no truly inclusive hypothesis concerning a mechanism of action for these diverse therapies has been formed. This review is intended to give an overview of research concerning G protein signaling and the molecular basis of antidepressant action. In it, the authors attempt to discuss progress that has been made in this arena as well as the possibility that some point (or points) along a G protein signaling cascade represent a molecular target for antidepressant therapy that might lead toward a unifying hypothesis for depression. This review is not designed to address the clinical studies. Furthermore, as it is a relatively short paper, citations to the literature are necessarily selective. The authors apologize in advance to authors whose work we have failed to cite.

Heterotrimeric G-proteins associate with microtubules during differentiation in PC12 pheochromocytoma cells

Tubulin modifies G-protein signaling and heterotrimeric G-proteins regulate microtubule assembly. Here we report an interplay among G-protein-coupled receptor and receptor tyrosine kinase (such as nerve growth factor-NGF) signaling systems in PC12 pheochromocytoma cells that resulted in a translocation of Galpha(s), Galpha(i1), and Galpha(o) from cell bodies to cellular processes where they appear to localize with tubulin-containing structures. This relocation appeared to depend on the integrity of microtubules, as it was blocked and reversed by nocodazole. Latrunculin, which promotes actin filament depolymerization, had no effect. Both deconvolution microscopy and immunoprecipitation showed a significant increase of Galpha association with microtubules that was coincident with the extension of "neurites." There were distinctions among the Galpha subtypes, with Galpha(s) showing the most profound NGF-induced colocalization with tubulin. Translocation of Galpha was blocked by agents that inhibit the MAP kinases required for neuronal differentiation, suggesting that G-protein relocation is triggered by the intracellular signals for differentiation. Consistent with this, Galpha in Neuro-2A cells, which spontaneously differentiate, showed a similar translocation coincident with differentiation. Thus, diverse signals that promote neuronal differentiation and changes in cell morphology may use specific G-proteins to evoke cytoskeletal rearrangement.

Interruption of signal transduction between G protein and PKC-epsilon underlies the impaired myocardial response to ischemic preconditioning in postinfarct remodeled hearts

We have recently shown that the protective mechanism of ischemic preconditioning (PC) is impaired in the myocardium that survived infarction and underwent postinfarct ventricular remodeling. In this study, we examined the hypothesis that failure of PC to activate PKC-epsilon underlies the refractoriness of the remodeling heart to PC. Circumflex coronary arteries were ligated in rabbits to induce infarction and subsequent ventricular remodeling, and only sham operations were performed in controls. Hearts were isolated before (i.e. 4 days later) or after (i.e. 2 weeks later) remodeling of the left ventricle and used for isolated buffer-perfused heart experiments. Myocardial infarction was induced in isolated hearts by 30 min global ischemia/2 h reperfusion, and its size was measured by tetrazolium staining. Using separate groups of hearts, tissue biopsies were taken before and after PC, and PKC translocation was assessed by Western blotting. Areas infarcted in vivo by coronary ligation (CL) were excluded from subsequent infarct size/PKC analyses. In the hearts 4 days after CL, PC with 2 cycles of 5 min ischemia/5 min reperfusion induced PKC-epsilon translocation from cytosol to particulate fractions and limited infarct size to 40% of control value. In the hearts remodeled 2 weeks after CL, PC failed to induce PKC-epsilon translocation and infarct size limitation. In this group, PKC activity and hemodynamic responses to adenosine were similar to those in sham-operated controls. When remodeling after CL was prevented by valsartan infusion (10 mg/kg/day), an angiotensin II type 1 (AT1) receptor blocker, PC could induce both infarct limitation and PKC-epsilon translocation. The present results suggest that persistent activation of AT1 receptors during remodeling disturbed the PC signaling between G proteins and PKC-epsilon, which underlies the refractoriness of the remodeled myocardium to PC.

A specific domain of Gialpha required for the transactivation of Gialpha by tubulin is implicated in the organization of cellular microtubules

G(s)alpha, G(i)alpha(1), and G(q)alpha subunits bind tubulin with high affinity, whereas transducin (G(t)alpha) does not. The interaction between tubulin and Galpha, which also involves the direct transfer of GTP from tubulin to Galpha (transactivation), is not yet fully understood. This study, using chimeras of G(i)alpha and G(t)alpha, showed that the G(i)alpha (215-295) segment converted G(t)alpha to bind to tubulin and this chimera (chimera 1) could be transactivated by tubulin. Insertion of G(t)alpha (237-270) into chimera 1 to form chimera 2 resulted in a protein that, like G(t)alpha, did not bind tubulin. Thus, it was thought that the G(i)alpha (237-270) domain was essential to modulate the binding of G(i)alpha(1) to tubulin. Surprisingly, when domain (237-270) of G(i)alpha was replaced by G(t)alpha (237-270) to form chimera 3, the chimera bound to tubulin with a similar affinity (K(D) congruent with 120 nm) as wild-type G(i)alpha(1). However, even though chimera 3 displayed normal GTP binding, it was not transactivated by GTP-tubulin. Furthermore, when these chimeras were expressed in COS-1 cells, cellular processes in cells overexpressing G(i)alpha(1) or chimera 1 were more abundant and longer than those in native cells. Galpha was seen throughout the length of the process. Morphology of cells expressing chimera 2 was identical to controls. Consistent with the role of Chimera 3 as a "dominant negative" Galpha, cells transfected with chimera 3 had only few truncated processes. This study demonstrates that although G(i)alpha (237-270) is not obligatory for the binding of G(i)alpha to tubulin, it is crucial for the transactivation of Galpha by tubulin. These results also suggest that the transactivation of Galpha by tubulin may play an important role in modulating microtubule organization and cell morphology.

Estradiol and testosterone have opposite effects on microtubule polymerization

We have reported earlier the purification of tubulin from a plasmalemmal-microsomal fraction derived from rat hippocampus using an estradiol (E(2)) affinity column and the specific binding of tubulin to both E(2) and testosterone (T). To further investigate the effect of E(2) and T on the function of this protein, changes in microtubule polymerization as a result of exposure to the steroids were examined in this study, using both pure tubulin and rat hippocampal primary cell cultures. First, pure tubulin was incubated with or without steroids for 30 min on ice followed by polymerization at 37 degrees C. The numbers of microtubules formed were counted from electron microscopic pictures. The results showed that at 30 min of polymerization, 10 nM, 30 nM and 30 microM of E(2) inhibited microtubule assembly by -70%, -94%, and -92%, respectively (p < 0.01), while T at the same three concentrations stimulated it by +83%, +66%, and +121%, respectively (p < 0.05). The inhibitory effect of E(2) and the stimulatory effect of T were observed at 15, 30 and 60 min of the polymerization process. Next, primary cell cultures from 17-day rat fetus hippocampal tissues were treated with the steroids and polymerized microtubules (Triton X-100 resistant) were examined by immunocytochemistry. The results demonstrated that 60 min of E(2) treatment (10 nM) decreased the intensity of the immunolabeling of polymerized microtubules. The effect of T at nM concentration was not significant though it increased the immunolabeling at microM concentration. Of great significance was a remarkable inhibition by T of the well-established depolymerization effect of colchicine in both the pure tubulin assay and the cell culture model, while E(2) was not effective. In an effort to pursue the possible mechanism(s) of the effect of E(2) and T on microtubule formation, we found that T only inhibited the microtubule depolymerization process without affecting the rate of polymerization. In contrast, E(2) modifies only the polymerization process without altering the depolymerization. Overall, these data indicate that E(2) and T may be considered as novel regulators of microtubule dynamics and thereby controlling cytoskeleton function in cells.

Microtubule-disruption-induced and chemotactic-peptide-induced migration of human neutrophils: implications for differential sets of signalling pathways

Neutrophil granulocytes rely on a functional actin network for directed migration. Microtubule disassembly does not impair receptor-linked chemotaxis, instead it induces development of polarity and chemokinesis in neutrophils concomitant with polarized distribution of alpha-actinin and F-actin. Cells stimulated with colchicine, which disassembles microtubules, migrate with a speed comparable to cells exposed to chemotactic peptide. We investigated signalling pathways involved in colchicine-induced neutrophil polarization and migration. Colchicine-induced development of polarity was insensitive to treatment with pertussis toxin, in contrast to chemotactic-peptide-induced shape changes, which were completely abolished by this treatment. Thus, colchicine does not appear to act via activating heterotrimeric G(i) proteins. Colchicine does also not seem to act via phosphatidylinositol 3-kinase, as it failed to induce phosphorylation of its downstream target Akt and the potent phosphatidylinositol 3-kinase inhibitor wortmannin failed to inhibit colchicine-induced shape changes. By contrast, wortmannin significantly reduced chemotactic-peptide-induced shape changes. However, the Rho-kinase inhibitor Y-27632 (10 micro M) inhibited colchicine-induced development of polarity by 95+/-3% (n=5) and chemokinesis by 76+/-9% (n=3), which suggests that the Rho-Rho-kinase pathway has a crucial role in polarity and migration. Indeed, treatment of cells with colchicine induced a significant increase in membrane-bound Rho-kinase II, which is indicative of activation of this protein. This membrane translocation could be prevented by taxol, which stabilizes microtubules. Colchicine also induced a marked increase in myosin light chain phosphorylation, which could be suppressed by Y-27632 and by taxol. In summary, we provide evidence that microtubule disassembly induces in neutrophils a selective activation of Rho-kinase, bypassing activation of heterotrimeric Gi proteins and phosphatidylinositol 3-kinase. This process is sufficient for induction of chemokinesis and mediates increased phosphorylation of myosin light chain and accumulation of F-actin and alpha-actinin in the leading edge.

Identification of sites of phosphorylation by G-protein-coupled receptor kinase 2 in beta-tubulin

G-protein-coupled receptor kinase 2 (GRK2) is known to specifically phosphorylate the agonist-bound forms of G-protein-coupled receptors (GPCRs). This strict specificity is due at least partly to activation of GRK2 by agonist-bound GPCRs, in which basic residues in intracellular regions adjacent to transmembrane segments are thought to be involved. Tubulin was found to be phosphorylated by GRK2, but it remains unknown if tubulin can also serve as both a substrate and an activator for GRK2. Purified tubulin, phosphorylated by GRK2, was subjected to biochemical analysis, and the phosphorylation sites in beta-tubulin were determined to be Thr409 and Ser420. In addition, the Ser444 in beta III-tubulin was also indicated to be phosphorylated by GRK2. The phosphorylation sites in tubulin for GRK2 reside in the C-terminal domain of beta-tubulin, which is on the outer surface of microtubules. Pretreatment of tubulin with protein phosphatase type-2A (PP2A) resulted in a twofold increase in the phosphorylation of tubulin by GRK2. These results suggest that tubulin is phosphorylated in situ probably by GRK2 and that the phosphorylation may affect the interaction of microtubules with microtubule-associated proteins. A GST fusion protein of a C-terminal region of beta I-tubulin (393-445 residues), containing 19 acidic residues but only one basic residue, was found to be a good substrate for GRK2, like full-length beta-tubulin. These results, together with the finding that GRK2 may phosphorylate synuclein and phosducin in their acidic domains, indicate that some proteins with very acidic regions but without basic activation domains could serve as substrates for GRK2.

Similar ultrastructural distribution of the 5-HT(2A) serotonin receptor and microtubule-associated protein MAP1A in cortical dendrites of adult rat

As visualized by light and electron microscopic immunocytochemistry, the distribution of the neuronal serotonin-2A (5-HT(2A)) receptor is mainly intracellular throughout adult rat brain. This localization is particularly striking in the pyramidal cells of cerebral cortex, the dendrites of which are intensely immunoreactive, but without any labeling of their spines. In view of recent yeast two-hybrid and biochemical results suggesting an association of 5-HT(2A) receptors with the cytoskeletal microtubule-associated protein MAP1A, the respective subcellular distributions of the receptors and of MAP1A were compared by quantitative electron microscopic immunocytochemistry in dendrites of adult rat frontoparietal cortex. Counts of silver-intensified immunogold particles revealed a higher density of 5-HT(2A) receptors in smaller rather than larger dendrites, and an apportionment between pre-defined compartments representing the plasma membrane and the cytoplasm that was proportional to the relative surface area of these compartments. MAP1A immunoreactivity also predominated in smaller versus larger dendrites, but with a slightly lower proportion of labeling in the plasma membrane versus cytoplasmic compartment. The co-localization of 5-HT(2A) receptors and MAP1A protein in the same dendrites could be demonstrated in double immunolabeling experiments. These results confirmed the predominantly somato-dendritic, intracellular localization of 5-HT(2A) receptors in cerebral cortex, showed their higher concentration in distal as opposed to proximal dendrites, and suggested their potential association to the cytoskeleton in cortical neurons in vivo. Such a distribution of 5-HT(2A) receptors reinforces our earlier hypothesis that 5-HT(2A) receptors participate in intraneuronal signaling processes involving the cytoskeleton, and raises the possibility that their activation could be dependent upon that of another co-localized, plasma membrane-bound, 5-HT receptor.

Phosphatidylinositol 4,5-bisphosphate modifies tubulin participation in phospholipase Cbeta1 signaling

Tubulin forms the microtubule and regulates certain G-protein-mediated signaling pathways. Both functions rely on the GTP-binding properties of tubulin. Signal transduction through Galpha(q)-regulated phospholipase Cbeta1 (PLCbeta1) is activated by tubulin through a direct transfer of GTP from tubulin to Galpha(q). However, at high tubulin concentrations, inhibition of PLCbeta1 is observed. This report demonstrates that tubulin inhibits PLCbeta1 by binding the PLCbeta1 substrate phosphatidylinositol 4,5-bisphosphate (PIP2). Tubulin binding of PIP2 was specific, because PIP2 but not phosphatidylinositol 3,4,5-trisphosphate, phosphatidylinositol 3-phosphate, phosphatidylinositol, phosphatidylcholine, phosphatidylethanolamine, or inositol 1,4,5-trisphosphate inhibited microtubule assembly. PIP2 did not affect GTP binding or GTP hydrolysis by tubulin. Muscarinic agonists promoted microtubule depolymerization and translocation of tubulin to the plasma membrane. PIP2 augmented this process in both Sf9 cells, containing a recombinant PLCbeta1 pathway, and SK-N-SH neuroblastoma cells. Colocalization of tubulin and PIP2 at the plasma membrane was demonstrated with confocal laser immunofluorescence microscopy. Although tubulin bound to both Galpha(q) and PLCbeta1, PIP2 facilitated the interaction between tubulin and PLCbeta1 but not that between tubulin and Galpha(q). However, PIP2 did augment formation of tubulin--Galpha(q)-PLCbeta1 complexes. Subsequent to potentiating PLCbeta1 activation, sustained agonist-independent membrane binding of tubulin at PIP2- and PLCbeta1-rich sites appeared to inhibit Galpha(q) coupling to PLCbeta1. Furthermore, colchicine increased membrane-associated tubulin and also inhibited PLCbeta1 activity in SK-N-SH cells. Thus, tubulin, depending on local membrane concentration, may serve as a positive or negative regulator of phosphoinositide hydrolysis. Rapid changes in membrane lipid composition or in the cytoskeleton might modify neuronal signaling through such a mechanism.

Microtubule-dependent regulation of vasomotor tone requires Rho-kinase

Changes in microtubule polymerization state have been shown to affect many cellular events, including the contractile properties of smooth muscle. We have previously shown that depolymerization of microtubules causes significant vasoconstriction in arterioles. This vasoconstriction does not require the endothelium or an increase in vascular smooth muscle Ca2+. Consequently, we hypothesized that a Ca2+-sensitizing mechanism may be involved in the constrictor response. The purpose of these experiments was to further elucidate cell signaling pathways responsible for vasoconstriction following microtubule disruption. Rat skeletal muscle arterioles were isolated, cannulated and pressurized without intraluminal flow. All arterioles used for experiments developed spontaneous, myogenic tone (54% of passive diameter). Microtubule depolymerization with colcemid or vinblastine caused arterioles to constrict by an additional 20% from resting basal diameter. In addition, arterioles treated with colcemid showed significantly enhanced responsiveness to norepinephrine and reduced responsiveness to adenosine. To investigate a role for Rho-kinase, vessels were incubated with inhibitors of the Rho-kinase pathway - Y-27632 or C3 exoenzyme. Inhibition of Rho-kinase significantly inhibited the constriction associated with colcemid-induced microtubule depolymerization. Inhibition of Rho-kinase also abolished the increased responsiveness to norepinephrine whereas adenosine responsiveness continued to be reduced. By comparison, inhibition of the tyrosine kinase, Src, with PP2 did not have any effect on the colcemid-induced changes in vascular tone or reactivity. These data indicate that the vasoconstriction and enhanced norepinephrine reactivity associated with microtubule disruption involves a Ca2+-sensitization process that is mediated by the Rho-kinase pathway.

Evidence for a direct negative coupling between dopamine-D2 receptors and PLC by heterotrimeric Gi1/2 proteins in rat anterior pituitary cell membranes

Dopamine (DA) is known to inhibit basal and hormone TRH- or angiotensin II (AngII)-stimulated PRL secretion and inositol phosphate accumulation in rat pituitary cells in primary culture. This inhibition persists when cells are incubated in a calcium-free medium (a condition in which DA could not inhibit PLC activities by blocking calcium influx) and is abolished by a Pertussis toxin treatment. These data suggest that DA receptor could be negatively coupled to PLC by a direct mechanism involving a Pertussis toxin-sensitive G protein. To demonstrate this hypothesis, we measured PLC activities on crude plasma membranes obtained from rat pituitary cells in primary culture grown in the presence of tritiated myo-inositol. We showed that 1) DA and quinpirole or RU24926 (specific D2 agonists) inhibited both basal and TRH- or AngII-stimulated membrane PLC activities. 2) Such inhibitions were completely prevented by sulpiride (specific D2 antagonist). 3) Heterotrimeric Gi1/2 proteins coupled the DA receptors to PLC because DA inhibitions were completely reversed by preincubation either with Pertussis toxin or with a specific G(alpha)i1/(alpha)i2 antibody. Such data are in favor of the existence of a direct negative coupling between DA-D2 receptor and PLC on a native physiological plasma membrane model.

Interaction between metabotropic glutamate receptor 7 and alpha tubulin

Metabotropic glutamate receptors (mGluRs) mediate a variety of responses to glutamate in the central nervous system. A primary role for group-III mGluRs is to inhibit neurotransmitter release from presynaptic terminals, but the molecular mechanisms that regulate presynaptic trafficking and activity of group-III mGluRs are not well understood. Here, we describe the interaction of mGluR7, a group-III mGluR and presynaptic autoreceptor, with the cytoskeletal protein, alpha tubulin. The mGluR7 carboxy terminal (CT) region was expressed as a GST fusion protein and incubated with rat brain extract to purify potential mGluR7-interacting proteins. These studies yielded a single prominent mGluR7 CT-associated protein of 55 kDa, which subsequent microsequencing analysis revealed to be alpha tubulin. Coimmunoprecipitation assays confirmed that full-length mGluR7 and alpha tubulin interact in rat brain as well as in BHK cells stably expressing mGluR7a, a splice variant of mGluR7. In addition, protein overlay experiments showed that the CT domain of mGluR7a binds specifically to purified tubulin and calmodulin, but not to bovine serum albumin. Further pull-down studies revealed that another splice variant mGluR7b also interacts with alpha tubulin, indicating that the binding region is not localized to the splice-variant regions of either mGluR7a (900-915) or mGluR7b (900-923). Indeed, deletion mutagenesis experiments revealed that the alpha tubulin-binding site is located within amino acids 873-892 of the mGluR7 CT domain, a region known to be important for regulation of mGluR7 trafficking. Interestingly, activation of mGluR7a in cells results in an immediate and significant decrease in alpha tubulin binding. These data suggest that the mGluR7/alpha tubulin interaction may provide a mechanism to control access of the CT domain to regulatory molecules, or alternatively, that this interaction may lead to morphological changes in the presynaptic membrane in response to receptor activation.

Real-time visualization of a fluorescent G(alpha)(s): dissociation of the activated G protein from plasma membrane

To study behavior of activated G(alpha)(s) in living cells, green fluorescent protein (GFP) was inserted within the internal amino acid sequence of G(alpha)(s) to generate a G(alpha)(s)-GFP fusion protein. The fusion protein maintained a bright green fluorescence and was identified by immunoblotting with antibodies against G(alpha)(s) or GFP. The cellular distribution of G(alpha)(s)-GFP was similar to that of endogenous G(alpha)(s). G(alpha)(s)-GFP was tightly coupled to the beta adrenergic receptor to activate the G(alpha)(s) effector, adenylyl cyclase. Activation of G(alpha)(s)-GFP by cholera toxin caused a gradual displacement of the fusion protein from the plasma membrane throughout the cytoplasm in living cells. Unlike the slow release of G(alpha)(s)-GFP from the membrane induced by cholera toxin, the beta-adrenergic agonist isoproterenol caused a rapid partial release of the fusion protein into the cytoplasm. At 1 min after treatment with isoproterenol, the extent of G(alpha)(s)-GFP release from plasma membrane sites was maximal; however, insertion of G(alpha)(s)-GFP at other membrane sites occurred during the same time period. Translocation of G(alpha)(s)-GFP fusion protein induced by isoproterenol suggested that the internalization of G(alpha)(s) might play a role in signal transduction by interacting with effector molecules and cytoskeletal elements at multiple cellular sites.

Microtubule disruption modulates the Rho-kinase pathway in vascular smooth muscle

Microtubules constitute one of the main cytoskeletal components in eukaryotic cells. Recent studies have shown that microtubule disruption induced significant vasoconstriction or enhanced agonist-induced contraction in vascular smooth muscle. However, the underlying mechanisms are not clear. We hypothesize that microtubule disruption may affect contractile signaling in vascular smooth muscle and lead to the enhanced contraction. The present study demonstrates that both colchicine and nocodazole induced a small but sustained contraction (4-6% P0) in rat aortic rings. This microtubule disruption-induced contraction was abolished by co-treatment with either HA 1077 or Y-27632, both of which are relatively specific Rho-kinase inhibitors. However, co-treatment with ML-9, an inhibitor of myosin light chain kinase, (MLCK) did not have a significant effect on the colchicine-induced contraction. The enhanced KCl-induced contraction due to treatment with colchicine was also blocked by inhibition of Rho-kinase, but not by inhibition of MLCK. These results indicate that microtubule disruption modulates contractile signaling in vascular smooth muscle, mainly through the Rho-kinase pathway, but not MLCK. Interestingly, the colchicine-enhanced, phenylephrine-induced contraction was not completely blocked by inhibition of Rho-kinase suggesting that other signaling pathways might also be involved.

Estradiol, in the CNS, targets several physiologically relevant membrane-associated proteins

We will describe the identity and function of two unexpected estrogen binding proteins from rat brain cell membranes in search for the putative membrane estrogen receptor (mER). An E-6-BSA column retained a distinctive 37-kDa protein that showed 100% homology with glyceraldehyde-3-phosphate dehydrogenase (GAPDH). A P-3-BSA column also retained the same protein but with less affinity. E-6-BSA bound to GAPDH with an IC50 of 50 nM, whereas the IC50 for P-3-BSA was about 500 nM. A dose of 10 nM 17beta-estradiol stimulated the catalysis of GAPDH, whereas progesterone at 100 nM inhibited it. Other steroids were ineffective. We examined if GAPDH activity would change during the rat estrous cycle, and what would be the effect of ovariectomy and estrogen treatment. The hippocampus and cerebellum were collected and GAPDH catalysis in both cytosolic and plasmalemmal-microsomal fractions was tested. The highest activity was found in Proestrus morning and the lowest in Estrus in both fractions. After ovariectomy (3 weeks) the hippocampus membrane fraction showed significantly reduced activity compared to that of Diestrus. An injection of estradiol in ovariectomized rats (10 microg/rat, s.c.) increased GAPDH activity in the hippocampus membrane fractions close to 60% from that of ovariectomized oil-treated controls 24 h after treatment maintaining similar levels by 48 h. No changes were detected in the preparations from the cerebellum of the same rats. The other protein retained by E-BSA columns was a 55-kDa protein identified as beta-tubulin. Two other proteins were also co-purified from the rat hippocampus: a 37-kDa (GAPDH) and a 45-kDa (actin). A purified brain tubulin (Cytoskeleton) was also retained with high affinity by the E-6-BSA, but with less affinity by an E-17-BSA column and not retained by either BSA, P-3-BSA or C-21-BSA columns. E-6-[125I]BSA bound with high affinity to tubulin (1 microg) and 17beta-estradiol completely displaced the binding at 10(-7) M. 17alpha-estradiol was ineffective and neither progesterone, corticosterone, DES nor 2-methoxyestradiol (2-ME) was able to displace the ligand. The T-3-[125I]BSA also bound to tubulin. But it seems to interact with another binding site, because colchicine at 10(-5) M completely eliminated the binding of T-3-[125 I]BSA to tubulin but did not displace the E-6-BSA site. Taxol competed off both ligands but only by 50%. None of the two ligands bound actin. These novel findings add new information to be considered in the intracellular actions of estradiol, particularly in the remodeling and functions of the cytoskeleton.

The association of heterotrimeric GTP-binding protein (Go) with microtubules

The heterotrimeric GTP-binding regulatory proteins (G proteins) play an important role in the regulation of membrane signal transduction. Recently, we identified the association of Go protein with mitotic spindles. Here we have investigated the relationship between Go protein and microtubules. We used temperature-dependent reversible assembly and taxol methods to purify microtubules from bovine brains. Goalpha and Gbeta proteins were identified in the microtubular fraction by both methods. The Goalpha subunit in the microtubular fraction could be ADP ribosylated by pertussis toxin. Co-immunoprecipitation data also revealed that Go protein can interact with microtubules. Exogenous Go protein could be incorporated into the assembled microtubular fraction, and 5 microg/ml (60 nM) of Go protein inhibited 40% of microtubule assembly. Western blot analysis of Goalpha-1 and Goalpha-2 in microtubular fractions showed that only Goalpha-1 is associated with microtubules. We conclude that the Goalpha-1betagamma proteins are associated with microtubules and may play some role in regulating the assembly and disassembly of microtubules.

Microtubule disruption by colchicine reversibly enhances calcium signaling in intact rat cardiac myocytes

Using the whole-cell patch-clamp configuration in rat ventricular myocytes, we recently reported that microtubule disruption increases calcium current (I(Ca)) and [Ca(2+)](i) transient and accelerates their kinetics by adenylyl cyclase activation. In the present report, we further analyzed the effects of microtubule disruption by 1 micromol/L colchicine on Ca(2+) signaling in cardiac myocytes with intact sarcolemma. In quiescent intact cells, it is possible to investigate ryanodine receptor (RyR) activity by analyzing the characteristics of spontaneous Ca(2+) sparks. Colchicine treatment decreased Ca(2+) spark amplitude (F/F(0): 1.78+/-0.01, n=983, versus 1.64+/-0.01, n=1660, recorded in control versus colchicine-treated cells; P<0.0001) without modifying the sarcoplasmic reticulum Ca(2+) load and enhanced their time to peak (in ms: 6.85+/-0.09, n=1185, versus 7.33+/-0.13, n=1647; P<0.0001). Microtubule disruption also induced the appearance of Ca(2+) sparks in doublets. These alterations may reflect RyR phosphorylation. To further investigate Ca(2+) signaling in cardiac myocytes with intact sarcolemma, we analyzed [Ca(2+)](i) transient evoked by field stimulation. Cells were loaded with the fluorescence Ca(2+) indicator, Fluo-3 cell permeant, and stimulated at 1 HZ: [Ca(2+)](i) transient amplitude was greater and its decay was accelerated in colchicine-treated, field-stimulated myocytes. This effect is reversible. When colchicine-treated myocytes were placed in a colchicine-free solution for 30 minutes, tubulin was repolymerized into microtubules, as shown by immunofluorescence, and the increase in [Ca(2+)](i) transient was reversed. In summary, we demonstrate that microtubule disruption by colchicine reversibly modulates Ca(2+) signaling in cardiac cells with intact sarcolemma.

Differences in Ca(2+) signaling underlie age-specific effects of secretagogues on colonic Cl(-) transport

Taurodeoxycholic acid (TDC) stimulates Cl(-) transport in adult (AD), but not weanling (WN) and newborn (NB), rabbit colonic epithelial cells (colonocytes). The present study demonstrates that stimuli like neurotensin (NT) are also age specific and identifies the age-dependent signaling step. Bile acid actions are segment and bile acid specific. Thus although TDC and taurochenodeoxycholate stimulate Cl(-) transport in AD distal but not proximal colon, taurocholate has no effect in either segment. TDC increases intracellular Ca(2+) concentration ([Ca(2+)](i)) in AD, but not in WN and NB, colonocytes. In AD cells, TDC (5 min) action on Cl(-) transport needs intra- but not extracellular Ca(2+). NT, histamine, and bethanechol increase Cl(-) transport and [Ca(2+)](i) in AD, but not WN, distal colonocytes. However, A-23187 increased [Ca(2+)](i) and Cl(-) transport in all age groups, suggesting that Ca(2+)-sensitive Cl(-) transport is present from birth. Study of the proximal steps in Ca(2+) signaling revealed that NT, but not TDC, activates a GTP-binding protein, Galpha(q), in AD and WN cells. In addition, although WN and AD colonocytes had similar levels of phosphatidylinositol 4,5-bisphosphate, NT and TDC increased 1,4,5-inositol trisphosphate content only in AD cells. Nonresponsiveness of WN cells to Ca(2+)-dependent stimuli, therefore, is due to the absence of measurable phospholipase C activity. Thus delays in Ca(2+) signaling afford a crucial protective mechanism to meet the changing demands of the developing colon.