Epidermal Growth Factor Induces Tyrosine Phosphorylation, Membrane Insertion, and Activation of Transient Receptor Potential Channel 4

The role of transient receptor potential channels in metastasis

Metastasis is the hallmark of failed tumor treatment and is typically associated with death due to cancer. Transient receptor potential (TRP) channels affect changes in intracellular calcium concentrations and participate at every stage of metastasis. Further, they increase the migratory ability of tumor cells, promote angiogenesis, regulate immune function, and promote the growth of tumor cells through changes in gene expression and function. In this review, we explore the potential mechanisms of action of TRP channels, summarize their role in tumor metastasis, compile inhibitors of TRP channels relevant in tumors, and discuss current challenges in research on TRP channels involved in tumor metastasis.

Expression of canonical transient receptor potential channels in U-2 OS and MNNG-HOS osteosarcoma cell lines

In U-2 OS and MNNG-HOS osteosarcoma cells, small interfering RNA-mediated knockdown of the angiotensin-(1-7) receptor, Mas, increases cell proliferation. Whether alterations in canonical transient receptor potential channels (TRPC) expression contribute to this effect is not clear. In the present study, a basic description of TRPC subtype expression in osteosarcoma cell lines was provided. The pharmacological modulators of the angiotensin-(1-7) receptor, Mas, AVE0991 (agonist), or D-Ala⁷-Ang-(1-7) (antagonist) were applied to elucidate a possible role of Mas in the regulation of TRPC mRNA levels. The contribution of other G-protein coupled receptors (GPCR) or receptor tyrosine kinases to TRCP expression was studied by applying the selective pharmacological blockers of either PI3 kinase or MEK/Erk1/2 signaling, Ly294002 and PD98059. AVE0991 and D-Ala⁷-Ang-(1-7) exhibited no or marginal effects on TRPC mRNA expression. Ly294002 provoked a 9.6- and 5.9-fold increase in the amounts of TRPC5 mRNA in MNNG-HOS and U-2 OS cells, respectively. Additionally, Ly294002 increased TRPC6 mRNA levels; however, it had no effect on TRPCs 1, 3 and 4. Administration of PD98059 increased the amounts of TRPC6 and TRPC4 ~2-fold. In conclusion, the present study demonstrated that Mas-dependent alterations in osteosarcoma cell line proliferation were not mediated by any changes in TRPC subtype gene expression. The data shows in principle, and consistent with the literature, that the signaling pathways examined can regulate the expression of TRPCs at the mRNA level. Therefore, direct and signaling pathway-specific pharmacological targeting of TRPC subtypes may represent an option for improving the treatment of osteosarcoma.

Transient receptor potential channel regulation by growth factors

Calcium (Ca²⁺) is one of the most universal secondary messengers, owing its success to the immense concentration gradient across the plasma membrane. Dysregulation of Ca²⁺ homeostasis can result in severe cell dysfunction, thereby initiating several pathologies like tumorigenesis and fibrosis. Transient receptor potential (TRP) channels represent a superfamily of Ca²⁺-permeable ion channels that convey diverse physical and chemical stimuli into a physiological signal. Their broad expression pattern and gating promiscuity support their potential involvement in the cellular response to an altering environment. Growth factors (GF) are essential biochemical messengers that contribute to these environmental changes. Since Ca²⁺ is essential in GF signaling, altering TRP channel expression or function could be a valid strategy for GF to exert their effect onto their target. In this review, a comprehensive understanding of the current knowledge regarding the activation and/or modulation of TRP channels by GF is presented.

TRPCs: Influential Mediators in Skeletal Muscle

Ca²⁺ itself or Ca²⁺-dependent signaling pathways play fundamental roles in various cellular processes from cell growth to death. The most representative example can be found in skeletal muscle cells where a well-timed and adequate supply of Ca²⁺ is required for coordinated Ca²⁺-dependent skeletal muscle functions, such as the interactions of contractile proteins during contraction. Intracellular Ca²⁺ movements between the cytosol and sarcoplasmic reticulum (SR) are strictly regulated to maintain the appropriate Ca²⁺ supply in skeletal muscle cells. Added to intracellular Ca²⁺ movements, the contribution of extracellular Ca²⁺ entry to skeletal muscle functions and its significance have been continuously studied since the early 1990s. Here, studies on the roles of channel proteins that mediate extracellular Ca²⁺ entry into skeletal muscle cells using skeletal myoblasts, myotubes, fibers, tissue, or skeletal muscle-originated cell lines are reviewed with special attention to the proposed functions of transient receptor potential canonical proteins (TRPCs) as store-operated Ca²⁺ entry (SOCE) channels under normal conditions and the potential abnormal properties of TRPCs in muscle diseases such as Duchenne muscular dystrophy (DMD).

Post-Translational Modification and Natural Mutation of TRPC Channels

Transient Receptor Potential Canonical (TRPC) channels are homologues of Drosophila TRP channel first cloned in mammalian cells. TRPC family consists of seven members which are nonselective cation channels with a high Ca²⁺ permeability and are activated by a wide spectrum of stimuli. These channels are ubiquitously expressed in different tissues and organs in mammals and exert a variety of physiological functions. Post-translational modifications (PTMs) including phosphorylation, N-glycosylation, disulfide bond formation, ubiquitination, S-nitrosylation, S-glutathionylation, and acetylation play important roles in the modulation of channel gating, subcellular trafficking, protein-protein interaction, recycling, and protein architecture. PTMs also contribute to the polymodal activation of TRPCs and their subtle regulation in diverse physiological contexts and in pathological situations. Owing to their roles in the motor coordination and regulation of kidney podocyte structure, mutations of TRPCs have been implicated in diseases like cerebellar ataxia (moonwalker mice) and focal and segmental glomerulosclerosis (FSGS). The aim of this review is to comprehensively integrate all reported PTMs of TRPCs, to discuss their physiological/pathophysiological roles if available, and to summarize diseases linked to the natural mutations of TRPCs.

The Efemp1R345W Macular Dystrophy Mutation Causes Amplified Circadian and Photophobic Responses to Light in Mice

Purpose

The R345W mutation in EFEMP1 causes malattia leventinese, an autosomal dominant eye disease with pathogenesis similar to an early-onset age-related macular degeneration. In mice, Efemp1^R345W does not cause detectable degeneration but small subretinal deposits do accumulate. The purpose of this study was to determine whether there were abnormal responses to light at this presymptomatic stage in Efemp1^R345W mice.

Methods

Responses to light were assessed by visual water task, circadian phase shifting, and negative masking behavior. The mechanism of abnormal responses was investigated by anterior eye exam, electroretinogram, melanopsin cell quantification, and multielectrode recording of retinal ganglion cell activity.

Results

Visual acuity was not different in Efemp1^R345W mice. However, amplitudes of circadian phase shifting (P = 0.016) and negative masking (P < 0.0001) were increased in Efemp1^R345W mice. This phenotype was not explained by anterior eye defects or amplified outer retina responses. Instead, we identified increased melanopsin-generated responses to light in the ganglion cell layer of the retina (P < 0.01).

Conclusions

Efemp1^R345W increases the sensitivity to light of behavioral responses driven by detection of irradiance. An amplified response to light in melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) is consistent with this phenotype. The major concern with this effect of the malattia leventinese mutation is the potential for abnormal regulation of physiology by light to negatively affect health.

STIM1-dependent membrane insertion of heteromeric TRPC1-TRPC4 channels in response to muscarinic receptor stimulation

Muscarinic receptor stimulation results in activation of nonselective cation (NSC) channels in guinea pig adrenal medullary (AM) cells. The biophysical and pharmacological properties of the NSC channel suggest the involvement of heteromeric channels of TRPC1 with TRPC4 or TRPC5. This possibility was explored in PC12 cells and guinea pig AM cells. Proximity ligation assay (PLA) revealed that when exogenously expressed in PC12 cells, TRPC1 forms a heteromeric channel with TRPC4, but not with TRPC5, in a STIM1-dependent manner. The heteromeric TRPC1-TRPC4 channel was also observed in AM cells and trafficked to the cell periphery in response to muscarine stimulation. To explore whether heteromeric channels are inserted into the cell membrane, tags were attached to the extracellular domains of TRPC1 and TRPC4. PLA products developed between the tags in cells stimulated by muscarine, but not in resting cells, indicating that muscarinic stimulation results in the membrane insertion of channels. This membrane insertion required expression of full-length STIM1. We conclude that muscarinic receptor stimulation results in the insertion of heteromeric TRPC1-TRPC4 channels into the cell membrane in PC12 cells and guinea pig AM cells.

Photopharmacology and opto-chemogenetics of TRPC channels-some therapeutic visions

Non-selective cation conductances formed by transient receptor potential canonical (TRPC) proteins govern the function and fate of a wide range of human cell types. In the past decade, evidence has accumulated for a pivotal role of these channels in human diseases, raising substantial interest in their therapeutic targeting. As yet, an appreciable number of small molecules for block and modulation of recombinant TRPC conductances have been identified. However, groundbreaking progress in TRPC pharmacology towards therapeutic applications is lagging behind due to incomplete understanding of their molecular pharmacology and their exact role in disease. A major breakthrough that is expected to overcome these hurdles is the recent success in obtaining high-resolution structure information on TRPC channel complexes and the advent of TRP photopharmacology and optogenetics. Here, we summarize current concepts of enhancing the precision of therapeutic interference with TRPC signaling and TRPC-mediated pathological processes.

TRPCing around the hypothalamus

All of the canonical transient receptor potential channels (TRPC) with the exception of TRPC 2 are expressed in hypothalamic neurons and are involved in multiple homeostatic functions. Although the metabotropic glutamate receptors have been shown to be coupled to TRPC channel activation in cortical and sub-cortical brain regions, in the hypothalamus multiple amine and peptidergic G protein-coupled receptors (GPCRs) and growth factor/cytokine receptors are linked to activation of TRPC channels that are vital for reproduction, temperature regulation, arousal and energy homeostasis. In addition to the neurotransmitters, circulating hormones like insulin and leptin through their cognate receptors activate TRPC channels in POMC neurons. Many of the post-synaptic effects of the neurotransmitters and hormones are regulated in different physiological states by expression of TRPC channels in the post-synaptic neurons. Therefore, TRPC channels are key targets not only for neurotransmitters but circulating hormones in their vital role to control multiple hypothalamic functions, which is the focus of this review.

Potential Mechanisms Underlying Inflammation-Enhanced Aminoglycoside-Induced Cochleotoxicity

Aminoglycoside antibiotics remain widely used for urgent clinical treatment of life-threatening infections, despite the well-recognized risk of permanent hearing loss, i.e., cochleotoxicity. Recent studies show that aminoglycoside-induced cochleotoxicity is exacerbated by bacteriogenic-induced inflammation. This implies that those with severe bacterial infections (that induce systemic inflammation), and are treated with bactericidal aminoglycosides are at greater risk of drug-induced hearing loss than previously recognized. Incorporating this novel comorbid factor into cochleotoxicity risk prediction models will better predict which individuals are more predisposed to drug-induced hearing loss. Here, we review the cellular and/or signaling mechanisms by which host-mediated inflammatory responses to infection could enhance the trafficking of systemically administered aminoglycosides into the cochlea to enhance the degree of cochleotoxicity over that in healthy preclinical models. Once verified, these mechanisms will be potential targets for novel pharmacotherapeutics that reduce the risk of drug-induced hearing loss (and acute kidney damage) without compromising the life-saving bactericidal efficacy of aminoglycosides.

Membrane translocation of transient receptor potential ankyrin 1 induced by inflammatory cytokines in lung cancer cells

Transient receptor potential ankyrin 1 (TRPA1) is known as one of the nociceptors expressed in sensory neurons. It also plays a role in non-neural cells in inflammatory sites. However, the regulatory mechanisms for the reactivity of TRPA1 in these cells under inflammatory conditions are not clear. To clarify these mechanisms, we examined the effects of inflammatory cytokines (interleukin [IL]-1α, IL-1β and tumor necrosis factor α [TNFα]) on TRPA1 reactivity and expression in the endogenously TRPA1-expressing lung tumor cell line A549. Treatment with IL-1α, but not IL-1β or TNFα, increased the number of cells responding to allyl isothiocyanate, a TRPA1 agonist, in a dose- and time-dependent manner. The IL-1α-induced increase of TRPA1 responsiveness was inhibited by an extracellular-regulated kinase (Erk) inhibitor (PD98059) but not by inhibitors of c-Jun kinase, p38 mitogen-activated protein kinase or phosphatidylinositol-3 kinase. Phosphorylation of Erk gradually increased at 24 h after its transient induction in cells treated with IL-1α. IL-1α increased the TRPA1 levels on biotinylated cell surface proteins. These results suggest that IL-1α enhances the translocation of TRPA1 to the plasma membrane via the activation of Erk in A549. TRPA1 may have a pathophysiological role in non-neural lung cells under inflammatory conditions.

TRPC Channels and Glioma

Glioma is the most common type of brain tumors and malignant glioma is extremely lethal, with patients' 5-year survival rate less than 10%. Treatment of gliomas poses remarkable clinical challenges, not only because of their particular localization but also because glioma cells possess several malignant biological features, including highly proliferative, highly invasive, highly angiogenic, and highly metabolic aberrant. All these features make gliomas highly recurrent and drug resistant. Finding new and effective molecular drug targets for glioma is an urgent and critical task for both basic and clinical research. Recent studies have proposed a type of non-voltage-gated calcium channels, namely, canonical transient receptor potential (TRPC) channels, to be newly emerged potential drug targets for glioma. They are heavily involved in the proliferation, migration, invasion, angiogenesis, and metabolism of glioma cells. Abundant evidence from both cell models and preclinical mouse models has demonstrated that inhibition of TRPC channels shows promising anti-glioma effect. In this chapter, we will give a comprehensive review on the current progress in the studies on TRPC channels and glioma and discuss their potential clinical implication in glioma therapy.

TRPC Channel Downstream Signaling Cascades

The family of TRP channel is comprised of a large group of cation-permeable channels, displaying as signaling integrators for sensing extracellular stimulus and initiating intracellular signaling cascades. This chapter offers a brief review of the signaling molecules related to TRPC channels, the first identified mammalian TRP family. Besides the signaling molecules involved in TRPC activation, I will focus on their upstream and downstream signaling cascades and the molecules involved in their intracellular trafficking.

Control of cell mechanics by RhoA and calcium fluxes during epithelial scattering

Epithelial tissues use adherens junctions to maintain tight interactions and coordinate cellular activities. Adherens junctions are remodeled during epithelial morphogenesis, including instances of epithelial-mesenchymal transition, or EMT, wherein individual cells detach from the tissue and migrate as individual cells. EMT has been recapitulated by growth factor induction of epithelial scattering in cell culture. In culture systems, cells undergo a highly reproducible series of cell morphology changes, most notably cell spreading followed by cellular compaction and cell migration. These morphology changes are accompanied by striking actin rearrangements. The current evidence suggests that global changes in actomyosin-based cellular contractility, first a loss of contractility during spreading and its activation during cell compaction, are the main drivers of epithelial scattering. In this review, we focus on how spreading and contractility might be controlled during epithelial scattering. While we propose a central role for RhoA, which is well known to control cellular contractility in multiple systems and whose role in epithelial scattering is well accepted, we suggest potential roles for additional cellular systems whose role in epithelial cell biology has been less well documented. In particular, we propose critical roles for vesicle recycling, calcium channels, and calcium-dependent kinases.

Lung Circulation

The circulation of the lung is unique both in volume and function. For example, it is the only organ with two circulations: the pulmonary circulation, the main function of which is gas exchange, and the bronchial circulation, a systemic vascular supply that provides oxygenated blood to the walls of the conducting airways, pulmonary arteries and veins. The pulmonary circulation accommodates the entire cardiac output, maintaining high blood flow at low intravascular arterial pressure. As compared with the systemic circulation, pulmonary arteries have thinner walls with much less vascular smooth muscle and a relative lack of basal tone. Factors controlling pulmonary blood flow include vascular structure, gravity, mechanical effects of breathing, and the influence of neural and humoral factors. Pulmonary vascular tone is also altered by hypoxia, which causes pulmonary vasoconstriction. If the hypoxic stimulus persists for a prolonged period, contraction is accompanied by remodeling of the vasculature, resulting in pulmonary hypertension. In addition, genetic and environmental factors can also confer susceptibility to development of pulmonary hypertension. Under normal conditions, the endothelium forms a tight barrier, actively regulating interstitial fluid homeostasis. Infection and inflammation compromise normal barrier homeostasis, resulting in increased permeability and edema formation. This article focuses on reviewing the basics of the lung circulation (pulmonary and bronchial), normal development and transition at birth and vasoregulation. Mechanisms contributing to pathological conditions in the pulmonary circulation, in particular when barrier function is disrupted and during development of pulmonary hypertension, will also be discussed.

Recent advances in therapeutic strategies that focus on the regulation of ion channel expression

A number of different ion channel types are involved in cell signaling networks, and homeostatic regulatory mechanisms contribute to the control of ion channel expression. Profiling of global gene expression using microarray technology has recently provided novel insights into the molecular mechanisms underlying the homeostatic and pathological control of ion channel expression. It has demonstrated that the dysregulation of ion channel expression is associated with the pathogenesis of neural, cardiovascular, and immune diseases as well as cancers. In addition to the transcriptional, translational, and post-translational regulation of ion channels, potentially important evidence on the mechanisms controlling ion channel expression has recently been accumulated. The regulation of alternative pre-mRNA splicing is therefore a novel therapeutic strategy for the treatment of dominant-negative splicing disorders. Epigenetic modification plays a key role in various pathological conditions through the regulation of pluripotency genes. Inhibitors of pre-mRNA splicing and histone deacetyalase/methyltransferase have potential as potent therapeutic drugs for cancers and autoimmune and inflammatory diseases. Moreover, membrane-anchoring proteins, lysosomal and proteasomal degradation-related molecules, auxiliary subunits, and pharmacological agents alter the protein folding, membrane trafficking, and post-translational modifications of ion channels, and are linked to expression-defect channelopathies. In this review, we focused on recent insights into the transcriptional, spliceosomal, epigenetic, and proteasomal regulation of ion channel expression: Ca(2+) channels (TRPC/TRPV/TRPM/TRPA/Orai), K(+) channels (voltage-gated, KV/Ca(2+)-activated, KCa/two-pore domain, K2P/inward-rectifier, Kir), and Ca(2+)-activated Cl(-) channels (TMEM16A/TMEM16B). Furthermore, this review highlights expression of these ion channels in expression-defect channelopathies.

Complementary roles of KCa3.1 channels and β1-integrin during alveolar epithelial repair

BACKGROUND

Extensive alveolar epithelial injury and remodelling is a common feature of acute lung injury and acute respiratory distress syndrome (ARDS) and it has been established that epithelial regeneration, and secondary lung oedema resorption, is crucial for ARDS resolution. Much evidence indicates that K(+) channels are regulating epithelial repair processes; however, involvement of the KCa3.1 channels in alveolar repair has never been investigated before.

RESULTS

Wound-healing assays demonstrated that the repair rates were increased in primary rat alveolar cell monolayers grown on a fibronectin matrix compared to non-coated supports, whereas an anti-β1-integrin antibody reduced it. KCa3.1 inhibition/silencing impaired the fibronectin-stimulated wound-healing rates, as well as cell migration and proliferation, but had no effect in the absence of coating. We then evaluated a putative relationship between KCa3.1 channel and the migratory machinery protein β1-integrin, which is activated by fibronectin. Co-immunoprecipitation and immunofluorescence experiments indicated a link between the two proteins and revealed their cellular co-distribution. In addition, we demonstrated that KCa3.1 channel and β1-integrin membrane expressions were increased on a fibronectin matrix. We also showed increased intracellular calcium concentrations as well as enhanced expression of TRPC4, a voltage-independent calcium channel belonging to the large TRP channel family, on a fibronectin matrix. Finally, wound-healing assays showed additive effects of KCa3.1 and TRPC4 inhibitors on alveolar epithelial repair.

CONCLUSION

Taken together, our data demonstrate for the first time complementary roles of KCa3.1 and TRPC4 channels with extracellular matrix and β1-integrin in the regulation of alveolar repair processes.

The Roles of Rasd1 small G proteins and leptin in the activation of TRPC4 transient receptor potential channels

TRPC4 is important regulators of electrical excitability in gastrointestinal myocytes, pancreatic β-cells and neurons. Much is known regarding the assembly and function of these channels including TRPC1 as a homotetramer or a heteromultimer and the roles that their interacting proteins play in controlling these events. Further, they are one of the best-studied targets of G protein-coupled receptors and growth factors in general and Gαi/o and Gαq protein coupled receptor or epidermal growth factor and leptin in particular. However, our understanding of the roles of small G proteins and leptin on TRPC4 channels is still rudimentary. We discuss potential roles for Rasd1 small G protein and leptin in channel activation in addition to their known role in cellular signaling.

Dexamethasone activates transient receptor potential canonical 4 (TRPC4) channels via Rasd1 small GTPase pathway

Canonical transient receptor potential 4 (TRPC4) channels are calcium-permeable, nonselective cation channels that are widely distributed in mammalian cells. It is generally speculated that TRPC4 channels are activated by Gq/11-PLC pathway or directly activated by Gi/o proteins. Although many mechanistic studies regarding TRPC4 have dealt with heterotrimeric G proteins, here, we first report the functional relationship between TRPC4 and small GTPase, Rasd1. Rasd1 selectively activated TRPC4 channels, and it was the only Ras protein among Ras protein family that can activate TRPC4 channels. For this to occur, it was found that certain population of functional Gαi1 and Gαi3 proteins are essential. Meanwhile, dexamethasone, a synthetic glucocorticoid and anti-inflammatory drug was known to increase messenger RNA (mRNA) level of Rasd1 in pancreatic β-cells. We have found that dexamethasone triggers TRPC4-like cationic current in INS-1 cells via increasing protein expression level of Rasd1. This relationship among dexamethasone, Rasd1, and TRPC4 could suggest a new therapeutic agent for hospitalized diabetes mellitus (DM) patients with prolonged dexamethasone prescription.

Kisspeptin and Gonadotropin-Releasing Hormone Neuronal Excitability: Molecular Mechanisms Driven by 17β-Estradiol

Kisspeptin is a neuropeptide that signals via a Gαq-coupled receptor, GPR54, in gonadotropin-releasing hormone (GnRH) neurons and is essential for pubertal maturation and fertility. Kisspeptin depolarizes and excites GnRH neurons primarily through the activation of canonical transient receptor potential (TRPC) channels and the inhibition of K+ channels. The gonadal steroid 17β-estradiol (E2) upregulates not only kisspeptin (Kiss1) mRNA but also increases the excitability of the rostral forebrain Kiss1 neurons. In addition, a primary postsynaptic action of E2 on GnRH neurons is to upregulate the expression of channel transcripts that orchestrate the downstream signaling of kisspeptin in GnRH neurons. These include not only TRPC4 channels but also low-voltage-activated T-type calcium channels and high-voltage-activated L-, N- and R-type calcium channel transcripts. Moreover, E2 has direct membrane-initiated actions to alter the excitability of GnRH neurons by enhancing ATP-sensitive potassium channel activity, which is critical for maintaining GnRH neurons in a hyperpolarized state for the recruitment of T-type calcium channels that are important for burst firing. Therefore, E2 modulates the excitability of GnRH neurons as well as of Kiss1 neurons by altering the expression and/or function of ion channels; moreover, kisspeptin provides critical excitatory input to GnRH neurons to facilitate burst firing activity and peptide release.

Identification of a membrane-targeting domain of the transient receptor potential canonical (TRPC)4 channel unrelated to its formation of a tetrameric structure

Canonical transient receptor potential (TRPC) channels are Ca(2+)-permeable nonselective cation channels that are activated by a wide variety of stimuli, including G protein-coupled receptors (GPCRs). The TRPC4 channel is expressed in a punctate distribution in the membrane. To identify the regulating region of the channel trafficking to the membrane, we generated deletion mutants of the TRPC4 channel. We determined that when either region that was downstream of the 20 amino acids of the N terminus or the 700-730 amino acids was deleted, the mutants were retained in the endoplasmic reticulum. By coexpression of the wild-type TRPC4 with deletion mutants, we found that the 23-29 amino acids of the N terminus regulate a membrane trafficking. Additionally, by the fluorescence resonance energy transfer (FRET) method, we found that the regions downstream of the 99 amino acid region of the N terminus and upstream of the 730 amino acid region in the C terminus produce assembly of the TRPC4 tetramers. We inferred the candidate proteins that regulate or interact with the 23-29 domain of TRPC4.

In pursuit of small molecule chemistry for calcium-permeable non-selective TRPC channels -- mirage or pot of gold?

The primary purpose of this review is to address the progress towards small molecule modulators of human Transient Receptor Potential Canonical proteins (TRPC1, TRPC3, TRPC4, TRPC5, TRPC6 and TRPC7). These proteins generate channels for calcium and sodium ion entry. They are relevant to many mammalian cell types including acinar gland cells, adipocytes, astrocytes, cardiac myocytes, cochlea hair cells, endothelial cells, epithelial cells, fibroblasts, hepatocytes, keratinocytes, leukocytes, mast cells, mesangial cells, neurones, osteoblasts, osteoclasts, platelets, podocytes, smooth muscle cells, skeletal muscle and tumour cells. There are broad-ranging positive roles of the channels in cell adhesion, migration, proliferation, survival and turning, vascular permeability, hypertrophy, wound-healing, hypo-adiponectinaemia, angiogenesis, neointimal hyperplasia, oedema, thrombosis, muscle endurance, lung hyper-responsiveness, glomerular filtration, gastrointestinal motility, pancreatitis, seizure, innate fear, motor coordination, saliva secretion, mast cell degranulation, cancer cell drug resistance, survival after myocardial infarction, efferocytosis, hypo-matrix metalloproteinase, vasoconstriction and vasodilatation. Known small molecule stimulators of the channels include hyperforin, genistein and rosiglitazone, but there is more progress with inhibitors, some of which have promising potency and selectivity. The inhibitors include 2-aminoethoxydiphenyl borate, 2-aminoquinolines, 2-aminothiazoles, fatty acids, isothiourea derivatives, naphthalene sulfonamides, N-phenylanthranilic acids, phenylethylimidazoles, piperazine/piperidine analogues, polyphenols, pyrazoles and steroids. A few of these agents are starting to be useful as tools for determining the physiological and pathophysiological functions of TRPC channels. We suggest that the pursuit of small molecule modulators for TRPC channels is important but that it requires substantial additional effort and investment before we can reap the rewards of highly potent and selective pharmacological modulators.

Transient receptor potential channels as drug targets: from the science of basic research to the art of medicine

The large Trp gene family encodes transient receptor potential (TRP) proteins that form novel cation-selective ion channels. In mammals, 28 Trp channel genes have been identified. TRP proteins exhibit diverse permeation and gating properties and are involved in a plethora of physiologic functions with a strong impact on cellular sensing and signaling pathways. Indeed, mutations in human genes encoding TRP channels, the so-called "TRP channelopathies," are responsible for a number of hereditary diseases that affect the musculoskeletal, cardiovascular, genitourinary, and nervous systems. This review gives an overview of the functional properties of mammalian TRP channels, describes their roles in acquired and hereditary diseases, and discusses their potential as drug targets for therapeutic intervention.

Trafficking mechanisms and regulation of TRPC channels

TRPC channels are Ca(2+)-permeable cation channels which are regulated downstream from receptor-coupled PIP2 hydrolysis. These channels contribute to a wide variety of cellular functions. Loss or gain of channel function has been associated with dysfunction and aberrant physiology. TRPC channel functions are influenced by their physical and functional interactions with numerous proteins that determine their regulation, scaffolding, trafficking, as well as their effects on the downstream cellular processes. Such interactions also compartmentalize the Ca(2+) signals arising from TRPC channels. A large number of studies demonstrate that trafficking is a critical mode by which plasma membrane localization and surface expression of TRPC channels are regulated. This review will provide an overview of intracellular trafficking pathways as well as discuss the current state of knowledge regarding the mechanisms and components involved in trafficking of the seven members of the TRPC family (TRPC1-TRPC7).

Cross-talk between reproduction and energy homeostasis: central impact of estrogens, leptin and kisspeptin signaling

The central nervous system receives hormonal cues (e.g., estrogens and leptin, among others) that influence reproduction and energy homeostasis. 17β-estradiol (E2) is known to regulate gonadotropin-releasing hormone (GnRH) secretion via classical steroid signaling and rapid non-classical membrane-initiated signaling. Because GnRH neurons are void of leptin receptors, the actions of leptin on these neurons must be indirect. Although it is clear that the arcuate nucleus of the hypothalamus is the primary site of overlap between these two systems, it is still unclear which neural network(s) participate in the cross-talk of E2 and leptin, two hormones essential for reproductive function and metabolism. Herein we review the progress made in understanding the interactions between reproduction and energy homeostasis by focusing on the advances made to understand the cellular signaling of E2 and leptin on three neural networks: kisspeptin, pro-opiomelanocortin (POMC) and neuropeptide Y (NPY). Although critical in mediating the actions of E2 and leptin, considerable work still remains to uncover how these neural networks interact in vivo.

TRPV2

Transient receptor potential vanilloid type 2, TRPV2, is a calcium-permeable cation channel belonging to the TRPV channel family. This channel is activated by heat (>52 °C), various ligands, and mechanical stresses. In most of the cells, a large portion of TRPV2 is located in the endoplasmic reticulum under unstimulated conditions. Upon stimulation of the cells with phosphatidylinositol 3-kinase-activating ligands, TRPV2 is translocated to the plasma membrane and functions as a cation channel. Mechanical stress may also induce translocation of TRPV2 to the plasma membrane. The expression of TRPV2 is high in some types of cells including neurons, neuroendocrine cells, immune cells involved in innate immunity, and certain types of cancer cells. TRPV2 may modulate various cellular functions in these cells.

Physiological functions and regulation of TRPC channels

The TRP-canonical (TRPC) subfamily, which consists of seven members (TRPC1-TRPC7), are Ca(2+)-permeable cation channels that are activated in response to receptor-mediated PIP2 hydrolysis via store-dependent and store-independent mechanisms. These channels are involved in a variety of physiological functions in different cell types and tissues. Of these, TRPC6 has been linked to a channelopathy resulting in human disease. Two key players of the store-dependent regulatory pathway, STIM1 and Orai1, interact with some TRPC channels to gate and regulate channel activity. The Ca(2+) influx mediated by TRPC channels generates distinct intracellular Ca(2+) signals that regulate downstream signaling events and consequent cell functions. This requires localization of TRPC channels in specific plasma membrane microdomains and precise regulation of channel function which is coordinated by various scaffolding, trafficking, and regulatory proteins.

TRP channels in reproductive (neuro)endocrinology

Transient receptor potential (TRP) ion channels have been detected in neurons that are part of the neural network controlling reproductive physiology and behavior. In this chapter we will primarily take a look at the classical/canonical TRP (TRPC) channels but will also examine some other members of the TRP channel superfamily in reproductive (neuro)endocrinology. The referenced data suggest that different TRP proteins could play functional roles at different levels of the reproductive pathway. Still, our understanding of TRP channel involvement in (neuro)endocrinology is quite limited. Due to their mechanism of activation and complex regulation, these channels are however ideally suited to be part of the transduction machinery of hormone-secreting cells.

TRPC4- and TRPC4-containing channels

TRPC4 proteins comprise six transmembrane domains, a putative pore-forming region, and an intracellularly located amino- and carboxy-terminus. Among eleven splice variants identified so far, TRPC4α and TRPC4β are the most abundantly expressed and functionally characterized. TRPC4 is expressed in various organs and cell types including the soma and dendrites of numerous types of neurons; the cardiovascular system including endothelial, smooth muscle, and cardiac cells; myometrial and skeletal muscle cells; kidney; and immune cells such as mast cells. Both recombinant and native TRPC4-containing channels differ tremendously in their permeability and other biophysical properties, pharmacological modulation, and mode of activation depending on the cellular environment. They vary from inwardly rectifying store-operated channels with a high Ca(2+) selectivity to non-store-operated channels predominantly carrying Na(+) and activated by Gαq- and/or Gαi-coupled receptors with a complex U-shaped current-voltage relationship. Thus, individual TRPC4-containing channels contribute to agonist-induced Ca(2+) entry directly or indirectly via depolarization and activation of voltage-gated Ca(2+) channels. The differences in channel properties may arise from variations in the composition of the channel complexes, in the specific regulatory pathways in the corresponding cell system, and/or in the expression pattern of interaction partners which comprise other TRPC proteins to form heteromultimeric channels. Additional interaction partners of TRPC4 that can mediate the activity of TRPC4-containing channels include (1) scaffolding proteins (e.g., NHERF) that may mediate interactions with signaling molecules in or in close vicinity to the plasma membrane such as Gα proteins or phospholipase C and with the cytoskeleton, (2) proteins in specific membrane microdomains (e.g., caveolin-1), or (3) proteins on cellular organelles (e.g., Stim1). The diversity of TRPC4-containing channels hampers the development of specific agonists or antagonists, but recently, ML204 was identified as a blocker of both recombinant and endogenous TRPC4-containing channels with an IC50 in the lower micromolar range that lacks activity on most voltage-gated channels and other TRPs except TRPC5 and TRPC3. Lanthanides are specific activators of heterologously expressed TRPC4- and TRPC5-containing channels but can block individual native TRPC4-containing channels. The biological relevance of TRPC4-containing channels was demonstrated by knockdown of TRPC4 expression in numerous native systems including gene expression, cell differentiation and proliferation, formation of myotubes, and axonal regeneration. Studies of TRPC4 single and TRPC compound knockout mice uncovered their role for the regulation of vascular tone, endothelial permeability, gastrointestinal contractility and motility, neurotransmitter release, and social exploratory behavior as well as for excitotoxicity and epileptogenesis. Recently, a single-nucleotide polymorphism (SNP) in the Trpc4 gene was associated with a reduced risk for experience of myocardial infarction.

Kisspeptin activation of TRPC4 channels in female GnRH neurons requires PIP2 depletion and cSrc kinase activation

Kisspeptin signaling via its Gαq-coupled receptor GPR54 plays a crucial role in modulating GnRH neuronal excitability, which controls pituitary gonadotropins secretion and ultimately reproduction. Kisspeptin potently depolarizes GnRH neurons primarily through the activation of canonical transient receptor potential (TRPC) channels, but the intracellular signaling cascade has not been elucidated. Presently, we have established that kisspeptin activation of TRPC channels requires multiple membrane and intracellular signaling molecules. First, phosphatidylinositol-4,5-bisphosphate (PIP(2)) hydrolysis by phospholipase Cβ is required because whole-cell dialysis of Dioctanoylglycerol-PIP(2) (DiC8-PIP(2)) inhibited the kisspeptin activation of TRPC channels, and the phosphatidylinositol 4-kinase inhibitor wortmannin, which attenuates PIP(2) synthesis, prolonged TRPC channel activation. Using single cell RT-PCR, we identified that the mRNA for the PIP(2)-interacting TRPC channel subunit, TRPC4α, is expressed in GnRH neurons. Depletion of intracellular Ca(2+) stores by thapsigargin and inositol 1,4,5-trisphosphate had no effect, indicating that the TRPC channels are not store-operated. Neither removing extracellular Ca(2+) nor buffering intracellular Ca(2+) with EGTA or BAPTA had any effect on the kisspeptin activation of the TRPC channels. However, the Ca(2+) channel blocker Ni(2+) inhibited the kisspeptin-induced inward current. Moreover, inhibition of protein kinase C by bisindolylmaleimide-I or calphostin C had no effect, but activation of protein kinase C by phorbol 12,13-dibutyrate occluded the kisspeptin-activated current. Finally, inhibition of the cytoplasmic tyrosine kinase cSrc by genistein or the pyrazolo-pyrimidine PP2 blocked the activation of TRPC channels by kisspeptin. Therefore, TRPC channels in GnRH neurons are receptor-operated, and kisspeptin activates TRPC channels through PIP(2) depletion and cSrc tyrosine kinase activation, which is a novel signaling pathway for peptidergic excitation of GnRH neurons.

Mutation of light-dependent phosphorylation sites of the Drosophila transient receptor potential-like (TRPL) ion channel affects its subcellular localization and stability

The Drosophila phototransduction cascade terminates in the opening of the ion channel transient receptor potential (TRP) and TRP-like (TRPL). Contrary to TRP, TRPL undergoes light-dependent subcellular trafficking between rhabdomeric photoreceptor membranes and an intracellular storage compartment, resulting in long term light adaptation. Here, we identified in vivo phosphorylation sites of TRPL that affect TRPL stability and localization. Quantitative mass spectrometry revealed a light-dependent change in the TRPL phosphorylation pattern. Mutation of eight C-terminal phosphorylation sites neither affected multimerization of the channels nor the electrophysiological response of flies expressing the mutated channels. However, these mutations resulted in mislocalization and enhanced degradation of TRPL after prolonged dark-adaptation. Mutation of subsets of the eight C-terminal phosphorylation sites also led to a reduction of TRPL content and partial mislocalization in the dark. This suggests that a light-dependent switch in the phosphorylation pattern of the TRPL channel mediates stable expression of TRPL in the rhabdomeres upon prolonged dark-adaptation.

mRNA expression of ion channels in GnRH neurons: subtype-specific regulation by 17β-estradiol

Burst firing of neurons optimizes neurotransmitter release. GnRH neurons exhibit burst firing activity and T-type calcium channels, which are vital for burst firing activity, are regulated by 17β-estradiol (E2) in GnRH neurons. To further elucidate ion channel expression and E2 regulation during positive and negative feedback on GnRH neurosecretion, we used single cell RT-PCR and real-time qPCR to quantify channel mRNA expression in GnRH neurons. GFP-GnRH neurons expressed numerous ion channels important for burst firing activity. E2-treatment sufficient to induce an LH surge increased mRNA expression of HCN1 channels, which underlie the pacemaker current, the calcium-permeable Ca(V)1.3, Ca(V)2.2, Ca(V)2.3 channels, and TRPC4 channels, which mediate the kisspeptin excitatory response. E2 also decreased mRNA expression of SK3 channels underlying the medium AHP current. Therefore, E2 exerts fundamental changes in ion channel expression in GnRH neurons, to prime them to respond to incoming stimuli with increased excitability at the time of the surge.

Canonical transient receptor channel 5 (TRPC5) and TRPC1/4 contribute to seizure and excitotoxicity by distinct cellular mechanisms

Seizures are the manifestation of highly synchronized burst firing of a large population of cortical neurons. Epileptiform bursts with an underlying plateau potential in neurons are a cellular correlate of seizures. Emerging evidence suggests that the plateau potential is mediated by neuronal canonical transient receptor potential (TRPC) channels composed of members of the TRPC1/4/5 subgroup. We previously showed that TRPC1/4 double-knockout (DKO) mice lack epileptiform bursting in lateral septal neurons and exhibit reduced seizure-induced neuronal cell death, but surprisingly have unaltered pilocarpine-induced seizures. Here, we report that TRPC5 knockout (KO) mice exhibit both significantly reduced seizures and minimal seizure-induced neuronal cell death in the hippocampus. Interestingly, epileptiform bursting induced by agonists for metabotropic glutamate receptors in the hippocampal CA1 area is unaltered in TRPC5 KO mice, but is abolished in TRPC1 KO and TRPC1/4 DKO mice. In contrast, long-term potentiation is greatly reduced in TRPC5 KO mice, but is normal in TRPC1 KO and TRPC1/4 DKO mice. The distinct changes from these knockouts suggest that TRPC5 and TRPC1/4 contribute to seizure and excitotoxicity by distinct cellular mechanisms. Furthermore, the reduced seizure and excitotoxicity and normal spatial learning exhibited in TRPC5 KO mice suggest that TRPC5 is a promising novel molecular target for new therapy.

Vascular remodeling in pulmonary hypertension

Pulmonary hypertension is a complex, progressive condition arising from a variety of genetic and pathogenic causes. Patients present with a spectrum of histologic and pathophysiological features, likely reflecting the diversity in underlying pathogenesis. It is widely recognized that structural alterations in the vascular wall contribute to all forms of pulmonary hypertension. Features characteristic of the remodeled vasculature in patients with pulmonary hypertension include increased stiffening of the elastic proximal pulmonary arteries, thickening of the intimal and/or medial layer of muscular arteries, development of vaso-occlusive lesions, and the appearance of cells expressing smooth muscle-specific markers in normally non-muscular small diameter vessels, resulting from proliferation and migration of pulmonary arterial smooth muscle cells and cellular transdifferentiation. The development of several animal models of pulmonary hypertension has provided the means to explore the mechanistic underpinnings of pulmonary vascular remodeling, although none of the experimental models currently used entirely replicates the pulmonary arterial hypertension observed in patients. Herein, we provide an overview of the histological abnormalities observed in humans with pulmonary hypertension and in preclinical models and discuss insights gained regarding several key signaling pathways contributing to the remodeling process. In particular, we will focus on the roles of ion homeostasis, endothelin-1, serotonin, bone morphogenetic proteins, Rho kinase, and hypoxia-inducible factor 1 in pulmonary arterial smooth muscle and endothelial cells, highlighting areas of cross-talk between these pathways and potentials for therapeutic targeting.

Mass spectrometric analysis of novel phosphorylation sites in the TRPC4β channel

RATIONALE

The transient receptor potential canonical (TRPC) channel 4β is a non-selective cation channel that is regulated by intracellular Ca(2+) and G protein-coupled receptors. Tyrosine phosphorylation of TRPC4β is important in mediating the activity and membrane expression of this channel protein. However, studies of TRPC4β Ser/Thr phosphorylation are lacking.

METHODS

To investigate the phosphorylation sites involved in regulating the diverse functions of TRPC4β in mammalian cells, we used nano-liquid chromatography/tandem mass spectrometry to identify key phosphorylation sites in TRPC4β that was immunopurified from HEK293 cells with monoclonal anti-TRPC4β antibody.

RESULTS

We identified four phosphorylation sites in the C-terminus of TRPC4β, none of which had been previously reported. Our data show that TRPC4β in mammalian cells is highly phosphorylated under basal conditions at multiple sites, and that a mass spectrometric proteomic technique combined with antibody-based affinity purification is an effective approach to define the phosphorylation sites of TRPC4β channels in mammalian cells.

CONCLUSIONS

These novel phosphorylation sites on TRPC4β may play a potential role in the phosphorylation-mediated regulation of TRPC4β channel activity and function in mammalian cells.

The roles of G proteins in the activation of TRPC4 and TRPC5 transient receptor potential channels

TRPC4 and TRPC5 channels are important regulators of electrical excitability in both gastrointestinal myocytes and neurons. Much is known regarding the assembly and function of these channels including TRPC1 as a homotetramer or a heteromultimer and the roles that their interacting proteins play in controlling these events. Further, they are one of the best-studied targets of G protein-coupled receptors and growth factors in general and Gαq protein coupled receptor or epidermal growth factor in particular. However, our understanding of the roles of Gαi/o proteins on TRPC4/5 channels is still rudimentary. We discuss potential roles for Gαi/o proteins in channel activation in addition to their known role in cellular signaling.

NMDA receptor-dependent synaptic activation of TRPC channels in olfactory bulb granule cells

Canonical transient receptor potential (TRPC) channels are widely expressed throughout the nervous system including the olfactory bulb where their function is largely unknown. Here, we describe their contribution to central synaptic processing at the reciprocal mitral and tufted cell-granule cell microcircuit, the most abundant synapse of the mammalian olfactory bulb. Suprathreshold activation of the synapse causes sodium action potentials in mouse granule cells and a subsequent long-lasting depolarization (LLD) linked to a global dendritic postsynaptic calcium signal recorded with two-photon laser-scanning microscopy. These signals are not observed after action potentials evoked by current injection in the same cells. The LLD persists in the presence of group I metabotropic glutamate receptor antagonists but is entirely absent from granule cells deficient for the NMDA receptor subunit NR1. Moreover, both depolarization and Ca²⁺ rise are sensitive to the blockade of NMDA receptors. The LLD and the accompanying Ca²⁺ rise are also absent in granule cells from mice deficient for both TRPC channel subtypes 1 and 4, whereas the deletion of either TRPC1 or TRPC4 results in only a partial reduction of the LLD. Recordings from mitral cells in the absence of both subunits reveal a reduction of asynchronous neurotransmitter release from the granule cells during recurrent inhibition. We conclude that TRPC1 and TRPC4 can be activated downstream of NMDA receptor activation and contribute to slow synaptic transmission in the olfactory bulb, including the calcium dynamics required for asynchronous release from the granule cell spine.

Gs cascade regulates canonical transient receptor potential 5 (TRPC5) through cAMP mediated intracellular Ca2+ release and ion channel trafficking

Canonical transient receptor potential (TRPC) channels are Ca(2+)-permeable, non-selective cation channels those are widely expressed in mammalian cells. Various molecules have been found to regulate TRPC both in vivo and in vitro, but it is unclear how heterotrimeric G proteins transmit external stimuli to regulate the activity of TRPC5. Here, we demonstrated that TRPC5 was potentiated by the Gα(s) regulatory pathway. Whole-cell TRPC5 current was significantly increased by β-adrenergic receptor agonist, isoproterenol (ISO, 246±36%, n=6), an activator of the adenylate cyclase, forskolin (FSK, 273±6%, n=5), or a membrane permeable cAMP analogue, 8-Br-cAMP (251±63%, n=7). In addition, robust Ca(2+) transient induced by isoproterenol was observed utilizing a Ca(2+) imaging technique. When intracellular [Ca(2+)](i) was buffered to 50nM, cAMP-induced potentiation was attenuated. We also found that the Ca(2+) release is mediated by IP(3) since intracellular IP(3) infusion attenuated the potentiation of TRPC5 by Gα(s) cascade. Finally, we identified that the membrane localization of TRPC5 was significantly increased by ISO (155±17%, n=3), FSK (172±39%, n=3) or 8-Br-cAMP (216±59%, n=3). In conclusion, these results suggest that the Gα(s)-cAMP pathway potentiates the activity of TRPC5 via facilitating intracellular Ca(2+) dynamics and increasing channel trafficking to the plasma membrane.

Hypoxia. 4. Hypoxia and ion channel function

The ability to sense and respond to oxygen deprivation is required for survival; thus, understanding the mechanisms by which changes in oxygen are linked to cell viability and function is of great importance. Ion channels play a critical role in regulating cell function in a wide variety of biological processes, including neuronal transmission, control of ventilation, cardiac contractility, and control of vasomotor tone. Since the 1988 discovery of oxygen-sensitive potassium channels in chemoreceptors, the effect of hypoxia on an assortment of ion channels has been studied in an array of cell types. In this review, we describe the effects of both acute and sustained hypoxia (continuous and intermittent) on mammalian ion channels in several tissues, the mode of action, and their contribution to diverse cellular processes.

Tyrosine phosphorylation-dependent activation of TRPC6 regulated by PLC-γ1 and nephrin: effect of mutations associated with focal segmental glomerulosclerosis

The surface expression and channel activation of transient receptor potential canonical 6 (TRPC6) were regulated by tyrosine phosphorylation and resultant binding with stimulatory PLC-γ1 and inhibitory nephrin. Disease-causing mutations made the TRPC6s insensitive to nephrin suppression, suggesting that the cell-type–specific regulation of TRPC6 might be involved in the pathogenesis.

Transient receptor potential canonicals (TRPCs) play important roles in the regulation of intracellular calcium concentration. Mutations in the TRPC6 gene are found in patients with focal segmental glomerulosclerosis (FSGS), a proteinuric disease characterized by dysregulated function of renal glomerular epithelial cells (podocytes). There is as yet no clear picture for the activation mechanism of TRPC6 at the molecular basis, however, and the association between its channel activity and pathogenesis remains unclear. We demonstrate here that tyrosine phosphorylation of TRPC6 induces a complex formation with phospholipase C (PLC)-γ1, which is prerequisite for TRPC6 surface expression. Furthermore, nephrin, an adhesion protein between the foot processes of podocytes, binds to phosphorylated TRPC6 via its cytoplasmic domain, competitively inhibiting TRPC6–PLC-γ1 complex formation, TRPC6 surface localization, and TRPC6 activation. Importantly, FSGS-associated mutations render the mutated TRPC6s insensitive to nephrin suppression, thereby promoting their surface expression and channel activation. These results delineate the mechanism of TRPC6 activation regulated by tyrosine phosphorylation, and imply the cell type–specific regulation, which correlates the FSGS mutations with deregulated TRPC6 channel activity.

A gain-of-function SNP in TRPC4 cation channel protects against myocardial infarction

AIMS

The TRPC4 non-selective cation channel is widely expressed in the endothelium, where it generates Ca(2+) signals that participate in the endothelium-mediated vasodilatory response. This study sought to identify single-nucleotide polymorphisms (SNPs) in the TRPC4 gene that are associated with myocardial infarction (MI).

METHODS AND RESULTS

Our candidate-gene association studies identified a missense SNP (TRPC4-I957V) associated with a reduced risk of MI in diabetic patients [odds ratio (OR) = 0.61; confidence interval (CI), 0.40-0.95, P= 0.02]. TRPC4 was also associated with MI in the Wellcome Trust Case-Control Consortium's genome-wide data: an intronic SNP (rs7319926) within the same linkage disequilibrium block as TRPC4-I957V showed an OR of 0.86 (CI, 0.81-0.94; P =10(-4)). Functional studies of the missense SNP were carried out in HEK293 and CHO cells expressing wild-type or mutant channels. Patch-clamp studies and measurement of intracellular [Ca(2+)] in response to muscarinic agonists and direct G-protein activation showed increased channel activity in TRPC4-I957V-transfected cells compared with TRPC4-WT. Site-directed mutagenesis and molecular modelling of TRPC4-I957V suggested that the gain of function was due to the presence of a less bulky Val-957. This permits a firmer interaction between the TRPC4 and the catalytic site of the tyrosine kinase that phosphorylates TRPC4 at Tyr-959 and facilitates channel insertion into the plasma membrane.

CONCLUSION

We provide evidence for the association of a TRPC4 SNP with MI in population-based genetic studies. The higher Ca(2+) signals generated by TRPC4-I957V may ultimately facilitate the generation of endothelium- and nitric oxide-dependent vasorelaxation, thereby explaining its protective effect at the vasculature.

Determinants of TRPV4 activity following selective activation by small molecule agonist GSK1016790A

TRPV4 (Transient Receptor Potential Vanilloid 4) channels are activated by a wide range of stimuli, including hypotonic stress, non-noxious heat and mechanical stress and some small molecule agonists (e.g. phorbol ester 4α-PDD). GSK1016790A (GSK101) is a recently discovered specific small molecule agonist of TRPV4. Its effects on physical determinants of TRPV4 activity were evaluated in HeLa cells transiently transfected with TRPV4 (HeLa-TRPV4). GSK101 (10 nM) causes a TRPV4 specific Ca(2+) influx in HeLa-TRPV4 cells, but not in control transfected cells, which can be inhibited by ruthenium red and Ca(2+)-free medium more significantly at the early stage of the activation rather than the late stage, reflecting apparent partial desensitization. Western blot analysis showed that GSK101 activation did not induce an increase in TRPV4 expression at the plasma membrane, but caused an immediate and sustained downregulation of TRPV4 on the plasma membrane in HeLa-TRPV4 cells. Patch clamp analysis also revealed an early partial desensitization of the channel which was Ca(2+)-independent. FRET analysis of TRPV4 subunit assembly demonstrated that the GSK101-induced TRPV4 channel activation/desensitization was not due to alterations in homotetrameric channel formation on the plasma membrane. It is concluded that GSK101 specifically activates TRPV4 channels, leading to a rapid partial desensitization and downregulation of the channel expression on the plasma membrane. TRPV4 subunit assembly appears to occur during trafficking from the ER/Golgi to the plasma membrane and is not altered by agonist stimulation.

Regulation of TRP signalling by ion channel translocation between cell compartments

The TRP (transient receptor potential) family of ion channels is a heterogeneous family of calcium permeable cation channels that is subdivided into seven subfamilies: TRPC ("Canonical"), TRPV ("Vanilloid"), TRPM ("Melastatin"), TRPA ("Ankyrin"), TRPN ("NOMPC"), TRPP ("Polycystin"), and TRPML ("Mucolipin"). TRP-mediated ion currents across the cell membrane are determined by the single channel conductance, by the fraction of activated channels, and by the total amount of TRP channels present at the plasma membrane. In many cases, the amount of TRP channels at the plasma membrane is altered in response to physiological stimuli by translocation of channels to and from the plasma membrane. Regulated translocation has been described for channels of the TRPC, TRPV, TRPM, and TRPA family and is achieved by vesicular transport of these channels along cellular exocytosis and endocytosis pathways. This review summarizes the stimuli and signalling cascades involved in the translocation of TRP channels and highlights interactions of TRP channels with proteins of the endocytosis and exocytosis machineries.

TRP channels and neural persistent activity

One of the integrative properties of the nervous system is its capability to, by transient motor commands or brief sensory stimuli, evoke persistent neuronal changes, mainly as a sustained, tonic action potential firing. This neural activity, named persistent activity, is found in a good number of brain regions and is thought to be a neural substrate for short-term storage and accumulation of sensory or motor information [1]. Examples of this persistent neural activity have been reported in prefrontal [2] and entorhinal [3] cortices, as part of the neural mechanisms involved in short-term working memory [4]. Interestingly, the general organization of the motor systems assumes the presence of bursts of short-lasting motor commands encoding movement characteristics such as velocity, duration, and amplitude, followed by a maintained tonic firing encoding the position at which the moving appendage should be maintained [5, 6]. Generation of qualitatively similar sustained discharges have also been found in spinal and supraspinal regions in relation to pain processing [7, 8]. Thus, persistent neural activity seems to be necessary for both behavioral (positions of fixation) and cognitive (working memory) processes. Persistent firing mechanisms have been proposed to involve the participation of a non-specific cationic current (CAN current) mainly mediated by activation of TRPC channels. Because the function and generation of persistent activity is still poorly understood, here we aimed to review and discuss the putative role of TRP-like channels on its generation and/or maintenance.

Nerve growth factor-regulated emergence of functional delta-opioid receptors

Sorting of intracellular G-protein-coupled receptors (GPCRs) either to lysosomes for degradation or to plasma membrane for surface insertion and functional expression is a key process regulating signaling strength of GPCRs across the plasma membrane in adult mammalian cells. However, little is known about the molecular mechanisms governing the dynamic process of receptor sorting to the plasma membrane for functional expression under normal and pathological conditions. In this study, we demonstrate that delta-opioid receptor (DOPr), a GPCR constitutively targeted to intracellular compartments, is driven to the surface membrane of central synaptic terminals and becomes functional by the neurotrophin nerve growth factor (NGF) in native brainstem neurons. The NGF-triggered DOPr translocation is predominantly mediated by the signaling pathway involving the tyrosine receptor kinase A, Ca(2+)-mobilizing phospholipase C, and Ca(2+)/calmodulin-dependent protein kinase II. Importantly, it requires interactions with the cytoplasmic sorting protein NHERF-1 (Na(+)/H(+) exchange regulatory factor-1) and N-ethyl-maleimide-sensitive factor-regulated exocytosis. In addition, this NGF-mediated mechanism is likely responsible for the emergence of functional DOPr induced by chronic opioids. Thus, NGF may function as a key molecular switch that redirects the sorting of intracellularly targeted DOPr to plasma membrane, resulting in new functional DOPr on central synapses under chronic opioid conditions.

Phosphorylation of yeast phosphatidylserine synthase by protein kinase A: identification of Ser46 and Ser47 as major sites of phosphorylation

The CHO1-encoded phosphatidylserine synthase from Saccharomyces cerevisiae is phosphorylated and inhibited by protein kinase A in vitro. CHO1 alleles bearing Ser(46) --> Ala and/or Ser(47) --> Ala mutations were constructed and expressed in a cho1Delta mutant lacking phosphatidylserine synthase. In vitro, the S46A/S47A mutation reduced the total amount of phosphorylation by 90% and abolished the inhibitory effect protein kinase A had on phosphatidylserine synthase activity. The enzyme phosphorylation by protein kinase A, which was time- and dose-dependent and dependent on the concentration of ATP, caused a electrophoretic mobility shift from a 27-kDa form to a 30-kDa form. The two electrophoretic forms of phosphatidylserine synthase were present in exponential phase cells, whereas only the 27-kDa form was present in stationary phase cells. In vivo labeling with (32)P(i) and immune complex analysis showed that the 30-kDa form was predominantly phosphorylated when compared with the 27-kDa form. However, the S46A/S47A mutations abolished the protein kinase A-mediated electrophoretic mobility shift. The S46A/S47A mutations also caused a 55% reduction in the total amount of phosphatidylserine synthase in exponential phase cells and a 66% reduction in the amount of enzyme in stationary phase cells. In phospholipid composition analysis, cells expressing the S46A/S47A mutant enzyme exhibited a 57% decrease in phosphatidylserine and a 40% increase in phosphatidylinositol. These results indicate that phosphatidylserine synthase is phosphorylated on Ser(46) and Ser(47) by protein kinase A, which results in a higher amount of enzyme for the net effect of stimulating the synthesis of phosphatidylserine.