Assembly of Trp1 in a signaling complex associated with caveolin-scaffolding lipid raft domains

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.

Isolation and Analysis of Detergent-Resistant Membrane Fractions

The hypothesis that the Golgi apparatus is capable of sorting proteins and sending them to the plasma membrane through "lipid rafts," membrane lipid domains highly enriched in glycosphingolipids, sphingomyelin, ceramide, and cholesterol, was formulated by van Meer and Simons in 1988 and came to a turning point when it was suggested that lipid rafts could be isolated thanks to their resistance to solubilization by some detergents, namely Triton X-100. An incredible number of papers have described the composition and properties of detergent-resistant membrane fractions. However, the use of this method has also raised the fiercest criticisms. In this chapter, we would like to discuss the most relevant methodological aspects related to the preparation of detergent-resistant membrane fractions, and to discuss the importance of discriminating between what is present on a cell membrane and what we can prepare from cell membranes in a laboratory tube.

Microdomains Associated to Lipid Rafts

Store Operated Ca(2+) Entry (SOCE), the main Ca(2+) influx mechanism in non-excitable cells, is implicated in the immune response and has been reported to be affected in several pathologies including cancer. The basic molecular constituents of SOCE are Orai, the pore forming unit, and STIM, a multidomain protein with at least two principal functions: one is to sense the Ca(2+) content inside the lumen of the endoplasmic reticulum(ER) and the second is to activate Orai channels upon depletion of the ER. The link between Ca(2+) depletion inside the ER and Ca(2+) influx from extracellular media is through a direct association of STIM and Orai, but for this to occur, both molecules have to interact and form clusters where ER and plasma membrane (PM) are intimately apposed. In recent years a great number of components have been identified as participants in SOCE regulation, including regions of plasma membrane enriched in cholesterol and sphingolipids, the so called lipid rafts, which recruit a complex platform of specialized microdomains, which cells use to regulate spatiotemporal Ca(2+) signals.

Role of Scaffolding Proteins in the Regulation of TRPC-Dependent Calcium Entry

Plasma membrane ion channels, and in particular TRPC channels need a specific membrane environment and association with scaffolding, signaling, and cytoskeleton proteins in order to play their important functional role. The molecular composition of TRPC channels is an important factor in determining channel activation mechanisms. TRPC proteins are incorporated in macromolecular complexes including several key Ca(2 +) signaling proteins as well as proteins involved in vesicle trafficking, cytoskeletal interactions, and scaffolding. Evidence has been provided for association of TRPC with calmodulin (CaM), IP3R, PMCA, Gq/11, RhoA, and a variety of scaffolding proteins. The interaction between TRPC channels with adaptor proteins, determines their mode of regulation as well as their cellular localization and function. Adaptor proteins do not display any enzymatic activity but act as scaffold for the building of signaling complexes. The scaffolding proteins are involved in the assembling of these Ca(2+) signaling complexes, the correct sub-cellular localization of protein partners, and the regulation of the TRPC channelosome. In particular, these proteins, via their multiple protein-protein interaction motifs, can interact with various ion channels involved in the transmembrane potential, and membrane excitability. Scaffolding proteins are key components for the functional organization of TRPC channelosomes that serves as a platform regulating slow Ca(2+) entry, spatially and temporally controlled [Ca(2+)]i signals and Ca(2+) -dependent cellular functions.

TRPC6 channel translocation into phagosomal membrane augments phagosomal function

Defects in the innate immune system in the lung with attendant bacterial infections contribute to lung tissue damage, respiratory insufficiency, and ultimately death in the pathogenesis of cystic fibrosis (CF). Professional phagocytes, including alveolar macrophages (AMs), have specialized pathways that ensure efficient killing of pathogens in phagosomes. Phagosomal acidification facilitates the optimal functioning of degradative enzymes, ultimately contributing to bacterial killing. Generation of low organellar pH is primarily driven by the V-ATPases, proton pumps that use cytoplasmic ATP to load H(+) into the organelle. Critical to phagosomal acidification are various channels derived from the plasma membrane, including the anion channel cystic fibrosis transmembrane conductance regulator, which shunt the transmembrane potential generated by movement of protons. Here we show that the transient receptor potential canonical-6 (TRPC6) calcium-permeable channel in the AM also functions to shunt the transmembrane potential generated by proton pumping and is capable of restoring microbicidal function to compromised AMs in CF and enhancement of function in non-CF cells. TRPC6 channel activity is enhanced via translocation to the cell surface (and then ultimately to the phagosome during phagocytosis) in response to G-protein signaling activated by the small molecule (R)-roscovitine and its derivatives. These data show that enhancing vesicular insertion of the TRPC6 channel to the plasma membrane may represent a general mechanism for restoring phagosome activity in conditions, where it is lost or impaired.

Isoform dependent regulation of human HCN channels by cholesterol

Cholesterol has been shown to regulate numerous ion channels. HCN channels represent the molecular correlate of I_f or I_h in sinoatrial node (SAN) and neuronal cells. Previous studies have implicated a role for cholesterol in the regulation of rabbit HCN4 channels with effects on pacing in the rabbit SAN. Using electrophysiological and biochemical approaches, we examined the effect of cholesterol modulation on human HCN1, HCN2 and HCN4 isoforms. Patch-clamp experiments uncovered isoform specific differences in the effect of cholesterol on gating kinetics upon depletion by MβCD or mevastatin or enrichment using MβCD/cholesterol. Most dramatically cholesterol had isoform specific effects on mode-shifting, which has been suggested to play a key role in stabilizing firing rate and preventing arrhythmic firing in SAN cells and neurons. Mode-shifting in HCN1 channels was insensitive to cholesterol manipulation, while HCN2 and HCN4 were strongly affected. Trafficking of each isoform to the plasma membrane was also affected by cholesterol modulation differentially between isoforms, however, each isoform remained localized in lipid raft domains after cholesterol depletion. These effects may contribute to the side effects of cholesterol reducing therapies including disrupted heart rhythm and neuropathic pain, as well as the susceptibility of sinus dysfunction in patients with elevated cholesterol.

Microdomain-specific localization of functional ion channels in cardiomyocytes: an emerging concept of local regulation and remodelling

Cardiac excitation involves the generation of action potential by individual cells and the subsequent conduction of the action potential from cell to cell through intercellular gap junctions. Excitation of the cellular membrane results in opening of the voltage-gated L-type calcium ion (Ca2+) channels, thereby allowing a small amount of Ca2+ to enter the cell, which in turn triggers the release of a much greater amount of Ca2+ from the sarcoplasmic reticulum, the intracellular Ca2+ store, and gives rise to the systolic Ca2+ transient and contraction. These processes are highly regulated by the autonomic nervous system, which ensures the acute and reliable contractile function of the heart and the short-term modulation of this function upon changes in heart rate or workload. It has recently become evident that discrete clusters of different ion channels and regulatory receptors are present in the sarcolemma, where they form an interacting network and work together as a part of a macro-molecular signalling complex which in turn allows the specificity, reliability and accuracy of the autonomic modulation of the excitation-contraction processes by a variety of neurohormonal pathways. Disruption in subcellular targeting of ion channels and associated signalling proteins may contribute to the pathophysiology of a variety of cardiac diseases, including heart failure and certain arrhythmias. Recent methodological advances have made it possible to routinely image the topography of live cardiomyocytes, allowing the study of clustering functional ion channels and receptors as well as their coupling within a specific microdomain. In this review we highlight the emerging understanding of the functionality of distinct subcellular microdomains in cardiac myocytes (e.g. T-tubules, lipid rafts/caveolae, costameres and intercalated discs) and their functional role in the accumulation and regulation of different subcellular populations of sodium, Ca2+ and potassium ion channels and their contributions to cellular signalling and cardiac pathology.

Store-operated calcium entry: unveiling the calcium handling signalplex

Store-operated Ca(2+) entry (SOCE) is an important mechanism for Ca(2+) influx in non-excitable cells, also present in excitable cells. The activation of store-operated channels (SOCs) is finely regulated by the filling state of the intracellular agonist-sensitive Ca(2+) compartments, and both, the mechanism of sensing the Ca(2+) stores and the nature and functional properties of the SOCs, have been a matter of intense investigation and debate. The identification of STIM1 as the endoplasmic reticulum Ca(2+) sensor and both Orai1, as the pore-forming subunit of the channels mediating the Ca(2+)-selective store-operated current, and the members of the TRPC subfamily of proteins, as the channels mediating the cation-permeable SOCs, has shed new light on the underlying events. This review summarizes the initial hypothesis and the current advances on the mechanism of activation of SOCE.

TRPM3 and miR-204 establish a regulatory circuit that controls oncogenic autophagy in clear cell renal cell carcinoma

Autophagy promotes tumor growth by generating nutrients from the degradation of intracellular structures. Here we establish, using shRNAs, a dominant-negative mutant, and a pharmacologic inhibitor, mefenamic acid (MFA), that the Transient Receptor Potential Melastatin 3 (TRPM3) channel promotes the growth of clear cell renal cell carcinoma (ccRCC) and stimulates MAP1LC3A (LC3A) and MAP1LC3B (LC3B) autophagy. Increased expression of TRPM3 in RCC leads to Ca(2+) influx, activation of CAMKK2, AMPK, and ULK1, and phagophore formation. In addition, TRPM3 Ca(2+) and Zn(2+) fluxes inhibit miR-214, which directly targets LC3A and LC3B. The von Hippel-Lindau tumor suppressor (VHL) represses TRPM3 directly through miR-204 and indirectly through another miR-204 target, Caveolin 1 (CAV1).

Lipid rafts, KCa/ClCa/Ca2+ channel complexes and EGFR signaling: Novel targets to reduce tumor development by lipids?

Membrane lipid rafts are distinct plasma membrane nanodomains that are enriched with cholesterol, sphingolipids and gangliosides, with occasional presence of saturated fatty acids and phospholipids containing saturated acyl chains. It is well known that they organize receptors (such as Epithelial Growth Factor Receptor), ion channels and their downstream acting molecules to regulate intracellular signaling pathways. Among them are Ca2+ signaling pathways, which are modified in tumor cells and inhibited upon membrane raft disruption. In addition to protein components, lipids from rafts also contribute to the organization and function of Ca2+ signaling microdomains. This article aims to focus on the lipid raft KCa/ClCa/Ca2+ channel complexes that regulate Ca2+ and EGFR signaling in cancer cells, and discusses the potential modification of these complexes by lipids as a novel therapeutic approach in tumor development. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Caveolin-1 enhances rapid mucosal restitution by activating TRPC1-mediated Ca2+ signaling

Early rapid mucosal restitution occurs as a consequence of epithelial cell migration to reseal superficial wounds, a process independent of cell proliferation. Our previous studies revealed that the canonical transient receptor potential-1 (TRPC1) functions as a store-operated Ca(2+) channel (SOCs) in intestinal epithelial cells (IECs) and regulates epithelial restitution after wounding, but the exact mechanism underlying TRPC1 activation remains elusive. Caveolin-1 (Cav1) is a major component protein that is associated with caveolar lipid rafts in the plasma membrane and was recently identified as a regulator of store-operated Ca(2+) entry (SOCE). Here, we showed that Cav1 plays an important role in the regulation of mucosal restitution by activating TRPC1-mediated Ca(2+) signaling. Target deletion of Cav1 delayed gastric mucosal repair after exposure to hypertonic NaCl in mice, although it did not affect total levels of TRPC1 protein. In cultured IECs, Cav1 directly interacted with TRPC1 and formed Cav1/TRPC1 complex as measured by immunoprecipitation assays. Cav1 silencing in stable TRPC1-transfected cells by transfection with siCav1 reduced SOCE without effect on the level of resting [Ca(2+)]cyt. Inhibition of Cav1 expression by siCav1 and subsequent decrease in Ca(2+) influx repressed epithelial restitution, as indicated by a decrease in cell migration over the wounded area, whereas stable ectopic overexpression of Cav1 increased Cav1/TRPC1 complex, induced SOCE, and enhanced cell migration after wounding. These results indicate that Cav1 physically interacts with and activates TRPC1, thus stimulating TRPC1-mediated Ca(2+) signaling and rapid mucosal restitution after injury.

Transient receptor potential channels contribute to pathological structural and functional remodeling after myocardial infarction

RATIONALE

The cellular and molecular basis for post-myocardial infarction (MI) structural and functional remodeling is not well understood.

OBJECTIVE

Our aim was to determine if Ca2+ influx through transient receptor potential canonical (TRPC) channels contributes to post-MI structural and functional remodeling.

METHODS AND RESULTS

TRPC1/3/4/6 channel mRNA increased after MI in mice and was associated with TRPC-mediated Ca2+ entry. Cardiac myocyte-specific expression of a dominant-negative (loss-of-function) TRPC4 channel increased basal myocyte contractility and reduced hypertrophy and cardiac structural and functional remodeling after MI while increasing survival in mice. We used adenovirus-mediated expression of TRPC3/4/6 channels in cultured adult feline myocytes to define mechanistic aspects of these TRPC-related effects. TRPC3/4/6 overexpression in adult feline myocytes induced calcineurin (Cn)-nuclear factor of activated T-cells (NFAT)-mediated hypertrophic signaling, which was reliant on caveolae targeting of TRPCs. TRPC3/4/6 expression in adult feline myocytes increased rested state contractions and increased spontaneous sarcoplasmic reticulum Ca2+ sparks mediated by enhanced phosphorylation of the ryanodine receptor. TRPC3/4/6 expression was associated with reduced contractility and response to catecholamines during steady-state pacing, likely because of enhanced sarcoplasmic reticulum Ca2+ leak.

CONCLUSIONS

Ca2+ influx through TRPC channels expressed after MI activates pathological cardiac hypertrophy and reduces contractility reserve. Blocking post-MI TRPC activity improved post-MI cardiac structure and function.

Helminth induced suppression of macrophage activation is correlated with inhibition of calcium channel activity

Helminth parasites cause persistent infections in humans and yet many infected individuals are asymptomatic. Neurocysticercosis (NCC), a disease of the central nervous system (CNS) caused by the cestode Taenia solium, has a long asymptomatic phase correlated with an absence of brain inflammation. However, the mechanisms of immune suppression remain poorly understood. Here we report that murine NCC displays a lack of cell surface maturation markers in infiltrating myeloid cells. Furthermore, soluble parasite ligands (PL) failed to induce maturation of macrophages, and inhibited TLR-induced inflammatory cytokine production. Importantly, PL treatment abolished both LPS and thapsigargin-induced store operated Ca2+ entry (SOCE). Moreover, electrophysiological recordings demonstrated PL-mediated inhibition of LPS or Tg-induced currents that were TRPC1-dependent. Concomitantly STIM1-TRPC1 complex was also impaired that was essential for SOCE and sustained Ca2+ entry. Likewise loss of SOCE due to PL further inhibited NFkB activation. Overall, our results indicate that the negative regulation of agonist induced Ca2+ signaling pathway by parasite ligands may be a novel immune suppressive mechanism to block the initiation of the inflammatory response associated with helminth infections.

The serine-threonine phosphatase calcineurin is a regulator of endothelial store-operated calcium entry

Disruption of the endothelium leads to increased permeability, allowing extravasation of macromolecules and other solutes from blood vessels. Calcium entry through a calcium-selective, store-operated calcium (SOC) channel, I soc, contributes to barrier disruption. An understanding of the mechanisms surrounding the regulation of I soc is far from complete. We show that the calcium/calmodulin-activated phosphatase calcineurin (CN) plays a role in regulation of SOC entry, possibly through the dephosphorylation of stromal interaction molecule 1 (STIM1). Phosphorylation has been implicated as a regulatory mechanism of activity for a number of canonical transient receptor potential (TRPC) and SOC channels, including I soc. Our results show that STIM1 phosphorylation increases in pulmonary artery endothelial cells (PAECs) upon activation of SOC entry. However, the phosphatases involved in STIM1 dephosphorylation are unknown. We found that a CN inhibitor (calcineurin inhibitory peptide [CIP]) increases the phosphorylation pattern of STIM1. Using a fura 2-acetoxymethyl ester approach to measure cytosolic calcium in PAECs, we found that CIP decreases SOC entry following thapsigargin treatment in PAECs. Luciferase assays indicate that thapsigargin induces activation of CN activity and confirm inhibition of CN activity by CIP in PAECs. Also, I soc is significantly attenuated in whole-cell patch-clamp studies of PAECs treated with CIP. Finally, PAECs pretreated with CIP exhibit decreased interendothelial cell gap formation in response to thapsigargin-induced SOC entry, as compared to control cells. Taken together, our data show that CN contributes to the phosphorylation status of STIM1, which is important in regulation of endothelial SOC entry and I soc activity.

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).

Src-mediated caveolin-1 phosphorylation regulates intestinal epithelial restitution by altering Ca(2+) influx after wounding

Early mucosal restitution occurs as a consequence of intestinal epithelial cell (IEC) migration to reseal superficial wounds, but its exact mechanism remains largely unknown. Caveolin-1 (Cav1), a major component associated with caveolar lipid rafts in the plasma membrane, is implicated in many aspects of cellular functions. This study determined if c-Src kinase (Src)-induced Cav1 phosphorylation promotes intestinal epithelial restitution after wounding by activating Cav1-mediated Ca(2+) signaling. Src directly interacted with Cav1, formed Cav1-Src complexes, and phosphorylated Cav1 in IECs. Inhibition of Src activity by its chemical inhibitor PP2 or suppression of the functional caveolin scaffolding domain by caveolin-scaffolding domain peptides prevented Cav1-Src interaction, reduced Cav1 phosphorylation, decreased Ca(2+) influx, and inhibited cell migration after wounding. Disruption of caveolar lipid raft microdomains by methyl-β-cyclodextrin reduced Cav1-mediated Ca(2+) influx and repressed epithelial restitution. Moreover, Src silencing prevented subcellular redistribution of phosphorylated Cav1 in migrating IECs. These results indicate that Src-induced Cav1 phosphorylation stimulates epithelial restitution by increasing Cav1-mediated Ca(2+) signaling after wounding, thus contributing to the maintenance of gut mucosal integrity under various pathological conditions.

Cyclopiazonic acid alters serotonin-induced responses in rat thoracic aorta

We previously showed that endothelin A (ETA) receptor antagonist BQ-123 partially inhibited cyclopiazonic acid (CPA)-enhanced endothelin-1 (ET-1)-induced contractions suggesting enhancement of ETA receptor internalization in caveolar structures by sarco/endoplasmic reticulum Ca+2 ATPase (SERCA) blockade. Since serotonin (5-Hydroxytryptamine, 5-HT) receptors are reported to be localized on caveolar membranes, we investigated whether SERCA inhibition affects 5-HT-induced responses and 5-HT receptor antagonism. For this purpose, vascular responses were measured in thoracic aorta segments from male Wistar albino rats using isolated tissue experiments. Data showed that CPA inhibits 5-HT- and PE-induced contractions in intact vessels while potentiating those in endothelium-denuded. Furthermore, non-selective 5-HT receptor blocker methysergide partially inhibited CPA-induced 5-HT contractions. However, α1-adrenergic receptor antagonist prazosin totally inhibited CPA-potentiated PE contractions. We suggest that SERCA inhibition results in 5-HT receptor internalization similar to ETA receptors possibly through protein kinase C activation by increased subsarcolemmal Ca2+ levels, eventually preventing 5-HT receptor antagonism.

Ca²⁺ signaling and regulation of fluid secretion in salivary gland acinar cells

Neurotransmitter stimulation of plasma membrane receptors stimulates salivary gland fluid secretion via a complex process that is determined by coordinated temporal and spatial regulation of several Ca(2+) signaling processes as well as ion flux systems. Studies over the past four decades have demonstrated that Ca(2+) is a critical factor in the control of salivary gland function. Importantly, critical components of this process have now been identified, including plasma membrane receptors, calcium channels, and regulatory proteins. The key event in activation of fluid secretion is an increase in intracellular [Ca(2+)] ([Ca(2+)]i) triggered by IP3-induced release of Ca(2+) from ER via the IP3R. This increase regulates the ion fluxes required to drive vectorial fluid secretion. IP3Rs determine the site of initiation and the pattern of [Ca(2+)]i signal in the cell. However, Ca(2+) entry into the cell is required to sustain the elevation of [Ca(2+)]i and fluid secretion. This Ca(2+) influx pathway, store-operated calcium influx pathway (SOCE), has been studied in great detail and the regulatory mechanisms as well as key molecular components have now been identified. Orai1, TRPC1, and STIM1 are critical components of SOCE and among these, Ca(2+) entry via TRPC1 is a major determinant of fluid secretion. The receptor-evoked Ca(2+) signal in salivary gland acinar cells is unique in that it starts at the apical pole and then rapidly increases across the cell. The basis for the polarized Ca(2+) signal can be ascribed to the polarized arrangement of the Ca(2+) channels, transporters, and signaling proteins. Distinct localization of these proteins in the cell suggests compartmentalization of Ca(2+) signals during regulation of fluid secretion. This chapter will discuss new concepts and findings regarding the polarization and control of Ca(2+) signals in the regulation of fluid secretion.

Lipid rafts serve as signaling platforms for mGlu1 receptor-mediated calcium signaling in association with caveolin

Background

Group I metabotropic glutamate receptors (mGlu1/5 receptors) have important roles in synaptic activity in the central nervous system. They modulate neuronal excitability by mobilizing intracellular Ca²⁺ following receptor activation. Also, accumulating evidence has indicated the association of Ca²⁺ signaling with lipid rafts. Caveolin, an adaptor protein found in a specialized subset of lipid rafts, has been reported to promote the localization of membrane proteins to lipid rafts.

Results

In the present study, we investigated the role of lipid rafts on the mGlu1α receptor-mediated Ca²⁺ signaling in association with caveolin in hippocampal primary neurons and HEK293 cells. We show that the disruption of lipid rafts using methyl-β-cyclodextrin markedly decreased mGlu1α receptor-mediated Ca²⁺ transients and lipid rafts localization of the receptor. Furthermore, transfection of mGlu1α receptor with mutated caveolin-binding domain reduced localization of the receptor to lipid rafts. Also, application of a peptide blocker of mGlu1α receptor and caveolin binding reduced the Ca²⁺ signaling and the lipid rafts localization.

Conclusions

Taken together, these results suggest that the binding of mGlu1α receptor to caveolin is crucial for its lipid rafts localization and mGlu1α receptor-mediated Ca²⁺ transients.

The role of mechanical tension on lipid raft dependent PDGF-induced TRPC6 activation

Canonical transient receptor potential channel 6 (TRPC6) can play an important role in governing how cells perceive the surrounding material environment and regulate Ca(2+) signaling. We have designed a TRPC6 reporter based on fluorescence resonance energy transfer (FRET) to visualize the TRPC6-mediated calcium entry and hence TRPC6 activity in live cells with high spatiotemporal resolutions. In mouse embryonic fibroblasts (MEFs), platelet-derived growth factor BB (PDGF) can activate the TRPC6 reporter, mediated by phospholipase C (PLC). This TRPC6 activation occurred mainly at lipid rafts regions of the plasma membrane because disruption of lipid raft/caveolae by methyl-β-cyclodextrin (MβCD) or the expression of dominant-negative caveolin-1 inhibited the TRPC6 activity. Culturing cells on soft materials or releasing the intracellular tension by ML-7 reduced this PDGF-induced activation of TRPC6 without affecting the PDGF-regulated Src or inositol 1,4,5-trisphosphate (IP3) receptor function, suggesting a specific role of mechanical tension in regulating TRPC6. We further showed that the release of intracellular tension had similar effect on the diffusion coefficients of TRPC6 and a raft marker, confirming a strong coupling between TRPC6 and lipid rafts. Therefore, our results suggest that the TRPC6 activation mainly occurs at lipid rafts, which is regulated by the mechanical cues of surrounding materials.

TRPC1

The TRPC1 ion channel was the first mammalian TRP channel to be cloned. In humans, it is encoded by the TRPC1 gene located in chromosome 3. The protein is predicted to consist of six transmembrane segments with the N- and C-termini located in the cytoplasm. The extracellular loop connecting transmembrane segments 5 and 6 participates in the formation of the ionic pore region. Inside the cell, TRPC1 is present in the endoplasmic reticulum, plasma membrane, intracellular vesicles, and primary cilium, an antenna-like sensory organelle functioning as a signaling platform. In human and rodent tissues, it shows an almost ubiquitous expression. TRPC1 interacts with a diverse group of proteins including ion channel subunits, receptors, and cytosolic proteins to mediate its effect on Ca(2+) signaling. It primarily functions as a cation nonselective channel within pathways controlling Ca(2+) entry in response to cell surface receptor activation. Through these pathways, it affects basic cell functions, such as proliferation and survival, differentiation, secretion, and cell migration, as well as cell type-specific functions such as chemotropic turning of neuronal growth cones and myoblast fusion. The biological role of TRPC1 has been studied in genetically engineered mice where the Trpc1 gene has been experimentally ablated. Although these mice live to adulthood, they show defects in several organs and tissues, such as the cardiovascular, central nervous, skeletal and muscular, and immune systems. Genetic and functional studies have implicated TRPC1 in diabetic nephropathy, Parkinson's disease, Huntington's disease, Duchenne muscular dystrophy, cancer, seizures, and Darier-White skin disease.

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.

TRPC1 protects dopaminergic SH-SY5Y cells from MPP+, salsolinol, and N-methyl-(R)-salsolinol-induced cytotoxicity

Neurotoxins and alterations in Ca2+ homeostasis have been associated with Parkinson's disease (PD), but the role of store-operated Ca2+ entry channels is not well understood. Previous studies have shown the neurotoxicity of salsolinol and 1-methyl-4-phenylpyridinium ion on SH-SY5Y cells and cytoprotection induced by transient receptor potential protein 1 (TRPC1). In the present study, N-methyl-(R)-salsolinol was tested for its cellular toxicity and effects on TRPC1 expression. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assays, DAPI (4',6-diamidino-2-phenylindole), fluorescein isothiocyanate-Annexin-V/propidium iodide, western blot analysis, and JC-1 labeling revealed that the three indicated drugs could induce caspase-dependent, mitochondrial-mediated apoptosis. Exposure of SH-SY5Y cells to the indicated drugs resulted in a significant decrease in thapsigargin-mediated Ca2+ influx and TRPC1 expression. Immunocytochemistry experiments revealed that neurotoxins treatment induced TRPC1 translocation to the cytoplasm. Taken together, our results indicate that treatment with neurotoxins may alter Ca2+ homeostasis and induce mitochondrial-mediated caspase-dependent cytotoxicity, an important characteristic of PD.

Defective interactions of protein partner with ion channels and transporters as alternative mechanisms of membrane channelopathies

The past twenty years have revealed the existence of numerous ion channel mutations resulting in human pathology. Ion channels provide the basis of diverse cellular functions, ranging from hormone secretion, excitation-contraction coupling, cell signaling, immune response, and trans-epithelial transport. Therefore, the regulation of biophysical properties of channels is vital in human physiology. Only within the last decade has the role of non-ion channel components come to light in regard to ion channel spatial, temporal, and biophysical regulation in physiology. A growing number of auxiliary components have been determined to play elemental roles in excitable cell physiology, with dysfunction resulting in disorders and related manifestations. This review focuses on the broad implications of such dysfunction, focusing on disease-causing mutations that alter interactions between ion channels and auxiliary ion channel components in a diverse set of human excitable cell disease. 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

Chronic administration of anticholinergics in rats induces a shift from muscarinic to purinergic transmission in the bladder wall

BACKGROUND

First-line pharmacotherapy for overactive bladder consists of anticholinergics. However, patient compliance is exceptionally low, which may be due to progressive loss of effectiveness.

OBJECTIVE

To decipher the involved molecular mechanisms and to evaluate the effects of chronic systemic administration of anticholinergics on bladder function and on muscarinic and purinergic receptors expression in rats.

DESIGN, SETTING, AND PARTICIPANTS

Female Wistar rats were implanted with an osmotic pump that chronically administered vehicle (Vehc), 0.36 mg/kg per day oxybutynin (Oxyc), or 0.19 mg/kg per day fesoterodine (Fesoc) for 28 d.

INTERVENTIONS

For cystometry experiments, a small catheter was implanted in the bladder.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

Urologic phenotype was evaluated by the analysis of the micturition pattern and urodynamics. Expression of muscarinic and purinergic receptors was assessed by Western blot analysis of detrusor membrane protein. Functional responses to carbachol and adenosine triphosphate (ATP) were evaluated using muscle-strip contractility experiments.

RESULTS AND LIMITATIONS

The number of voided spots was transiently decreased in Oxyc rats. In Oxyc rats, the effect of an acute high dose of oxybutynin (1mg/kg intraperitoneally [IP]) on the intermicturition interval was abolished. Expression experiments revealed a decrease of muscarinic acetylcholine receptors M2 (mAChR2) and M3 (mAChR3), whereas the purinergic receptor P2X, ligand-gated ion channel, 1 (P2X1) was enhanced in Oxyc and Fesoc rats compared to Vehc rats. In concordance with the modification of the expression pattern in Oxyc rats, the force generated by carbachol and ATP in muscle-strip contractility experiments was, respectively, lower and higher. Urodynamics revealed that the effects of systemic administration of the purinergic blocker pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (50mg/kg IP) were enhanced in Oxyc rats. As rat bladder physiology is different from that of humans, it is difficult to directly extrapolate our findings to human patients.

CONCLUSIONS

Chronic administration of anticholinergics in rats induces receptor loss of efficiency and a shift from muscarinic to purinergic transmission.

Cyclic AMP directs inositol (1,4,5)-trisphosphate-evoked Ca2+ signalling to different intracellular Ca2+ stores

Cholesterol depletion reversibly abolishes carbachol-evoked Ca(2+) release from inositol (1,4,5)-trisphosphate (IP3)-sensitive stores, without affecting the distribution of IP3 receptors (IP3R) or endoplasmic reticulum, IP3 formation or responses to photolysis of caged IP3. Receptors that stimulate cAMP formation do not alone evoke Ca(2+) signals, but they potentiate those evoked by carbachol. We show that these potentiated signals are entirely unaffected by cholesterol depletion and that, within individual cells, different IP3-sensitive Ca(2+) stores are released by carbachol alone and by carbachol combined with receptors that stimulate cAMP formation. We suggest that muscarinic acetylcholine receptors in lipid rafts deliver IP3 at high concentration to associated IP3R, stimulating them to release Ca(2+). Muscarinic receptors outside rafts are less closely associated with IP3R and provide insufficient local IP3 to activate IP3R directly. These IP3R, probably type 2 IP3R within a discrete Ca(2+) store, are activated only when their sensitivity is increased by cAMP. Sensitization of IP3R by cAMP extends the effective range of signalling by phospholipase C, allowing muscarinic receptors that are otherwise ineffective to recruit additional IP3-sensitive Ca(2+) stores.

Stiffened lipid platforms at molecular force foci

How mechanical forces are sensed remains largely mysterious. The forces that gate prokaryotic and several eukaryotic channels were found to come from the lipid membrane. Our survey of animal cells found that membrane force foci all have cholesterol-gathering proteins and are reinforced with cholesterol. This result is evident in overt force sensors at the tips of stereocilia for vertebrate hearing and the touch receptor of Caenorhabditis elegans and mammalian neurons. For less specialized cells, cadherins sustain the force between neighboring cells and integrins between cells and matrix. These tension bearers also pass through and bind to a cholesterol-enriched platform before anchoring to cytoskeleton through other proteins. Cholesterol, in alliance with sphingomyelin and specialized proteins, enforces a more ordered structure in the bilayer. Such a stiffened platform can suppress mechanical noise, redirect, rescale, and confine force. We speculate that such platforms may be dynamic. The applied force may allow disordered-phase lipids to enter the platform-staging channel opening in the thinner mobile neighborhood. The platform may also contain specialized protein/lipid subdomains enclosing mechanosensitive channels to open with localized tension. Such a dynamic stage can mechanically operate structurally disparate channels or enzymes without having to tie them directly to cadherin, integrin, or other protein tethers.

Regulation of store-operated calcium entry by FK506-binding immunophilins

Calcium entry from the extracellular space into cells is an important signaling mechanism in both physiological and pathophysiological functions. In non-excitable cells, store-operated calcium (SOC) entry represents a principal mode of calcium entry. Activation of SOC entry in pulmonary artery endothelial cells leads to the formation of inter-endothelial cell gaps and subsequent endothelial barrier disruption. Regulation of endothelial SOC entry is poorly understood. In this work, we identify two large molecular weight immunophilins, FKBP51 and FKBP52, as novel regulators of SOC entry in endothelial cells. Using cell fractionation studies and immunocytochemistry we determined that a fraction of these largely cytosolic proteins localize to the plasma membrane where SOC entry channels are found. That FKBP51 and FKBP52 associate with SOC entry channel protein complexes was supported by co-precipitation of the immunophilins with TRPC4, a subunit of the calcium-selective, SOC entry channel ISOC. Dexamethasone-induced upregulation of FKBP51 expression in pulmonary artery endothelial cells reduced global SOC entry as well as ISOC. Similar results were observed when FKBP51 was over-expressed in an inducible HEK293 cell line. On the other hand, when FKBP52 was over-expressed SOC entry was enhanced. When expression of FKBP52 was inhibited, SOC entry was decreased. Collectively, our observations support regulatory roles for these large molecular weight immunophilins in which FKBP51 inhibits, whereas FKBP52 enhances, SOC entry in endothelial cells.

Cytoskeletal and scaffolding proteins as structural and functional determinants of TRP channels

Transient receptor potential (TRP) channels are six transmembrane-spanning proteins, with variable selectivity for cations, that play a relevant role in intracellular Ca(2+) homeostasis. There is a large body of evidence that shows association of TRP channels with the actin cytoskeleton or even the microtubules and demonstrating the functional importance of this interaction for TRP channel function. Conversely, cation currents through TRP channels have also been found to modulate cytoskeleton rearrangements. The interplay between TRP channels and the cytoskeleton has been demonstrated to be essential for full activation of a variety of cellular functions. Furthermore, TRP channels have been reported to take part of macromolecular complexes including different signal transduction proteins. Scaffolding proteins play a relevant role in the association of TRP proteins with other signaling molecules into specific microdomains. Especially relevant are the roles of the Homer family members for the regulation of TRPC channel gating in mammals and INAD in the modulation of Drosophila TRP channels. 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.

Contribution and regulation of TRPC channels in store-operated Ca2+ entry

Store-operated calcium entry (SOCE) is activated in response to depletion of the endoplasmic reticulum-Ca(2+) stores following stimulation of plasma membrane receptors that couple to PIP2 hydrolysis and IP3 generation. Search for the molecular components of SOCE channels led to the identification of mammalian transient receptor potential canonical (TRPC) family of calcium-permeable channels (TRPC1-TRPC7), which are all activated in response to stimuli that result in PIP2 hydrolysis. While several TRPCs, including TRPC1, TRPC3, and TRPC4, have been implicated in SOCE, the data are most consistent for TRPC1. Extensive studies in cell lines and knockout mouse models have established the contribution of TRPC1 to SOCE. Furthermore, there is a critical functional interaction between TRPC1 and the key components of SOCE, STIM1, and Orai1, which determines the activation of TRPC1. Orai1-mediated Ca(2+) entry is required for recruitment of TRPC1 and its insertion into surface membranes while STIM1 gates the channel. Notably, TRPC1 and Orai1 generate distinct patterns of Ca(2+) signals in cells that are decoded for the regulation of specific cellular functions. Thus, SOCE appears to be a complex process that depends on temporal and spatial coordination of several distinct steps mediated by proteins in different cellular compartments. Emerging data suggest that, in many cell types, the net Ca(2+) entry measured in response to store depletion is the result of the coordinated regulation of different calcium-permeable ion channels. Orai1 and STIM1 are central players in this process, and by mediating recruitment or activation of other Ca(2+) channels, Orai1-CRAC function can elicit rapid changes in global and local [Ca(2+)]i signals in cells. It is most likely that the type of channels and the [Ca(2+)]i signature that are generated by this process reflect the physiological function of the cell that is regulated by Ca(2+).

SNF8, a member of the ESCRT-II complex, interacts with TRPC6 and enhances its channel activity

Background

Transient receptor potential canonical (TRPC) channels are non-selective cation channels involved in receptor-mediated calcium signaling in diverse cells and tissues. The canonical transient receptor potential 6 (TRPC6) has been implicated in several pathological processes, including focal segmental glomerulosclerosis (FSGS), cardiac hypertrophy, and pulmonary hypertension. The two large cytoplasmic segments of the cation channel play a critical role in the proper regulation of channel activity, and are involved in several protein-protein interactions.

Results

Here we report that SNF8, a component of the endosomal sorting complex for transport-II (ESCRT-II) complex, interacts with TRPC6. The interaction was initially observed in a yeast two-hybrid screen using the amino-terminal cytoplasmic domain of TRPC6 as bait, and confirmed by co-immunoprecipitation from eukaryotic cell extracts. The amino-terminal 107 amino acids are necessary and sufficient for the interaction. Overexpression of SNF8 enhances both wild-type and gain-of-function mutant TRPC6-mediated whole-cell currents in HEK293T cells. Furthermore, activation of NFAT-mediated transcription by gain-of-function mutants is enhanced by overexpression of SNF8, and partially inhibited by RNAi mediated knockdown of SNF8. Although the ESCRT-II complex functions in the endocytosis and lysosomal degradation of transmembrane proteins, SNF8 overexpression does not alter the amount of TRPC6 present on the cell surface.

Conclusion

SNF8 is novel binding partner of TRPC6, binding to the amino-terminal cytoplasmic domain of the channel. Modulating SNF8 expression levels alters the TRPC6 channel current and can modulate activation of NFAT-mediated transcription downstream of gain-of-function mutant TRPC6. Taken together, these results identify SNF8 as a novel regulator of TRPC6.

TRPC1 protein forms only one type of native store-operated channels in HEK293 cells

TRPC1 is a major component of store-operated calcium entry in many cell types. In our previous studies, three types of endogenous store-operated calcium channels have been described in HEK293 cells, but it remained unknown which of these channels are composed of TRPC1 proteins. Here, this issue has been addressed by performing single-channel analysis in HEK293 cells transfected with anti-TRPC1 siRNA (siTPRC1) or a TPRC1-encoding plasmid. The results show that thapsigargin-or agonist-induced calcium influx is significantly attenuated in siTRPC1-transfected HEK293 cells. TRPC1 knockdown by siRNA results in the disappearance of store-operated I(max) channels, while the properties of I(min) and I(NS) channels are unaffected. In HEK293 cells with overexpressed TRPC1 protein, the unitary current-voltage relationship of exogenous TRPC1 channels is almost linear, with a slope conductance of about 17 pS. The extrapolated reversal potential of expressed TRPC1 channels is +30 mV. Therefore, the main electrophysiological and regulatory properties of expressed TRPC1 and native I(max) channels are identical. Moreover, TRPC1 overexpression in HEK293 cells results in an increased number of store-operated I(max) channels. All these data allow us to conclude that TRPC1 protein forms native store-operated I(max) channels but is not an essential subunit for other store-operated channel types in HEK293 cells.

Distant cytosolic residues mediate a two-way molecular switch that controls the modulation of inwardly rectifying potassium (Kir) channels by cholesterol and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2))

BACKGROUND

Cholesterol modulates inwardly rectifying potassium (Kir) channels.

RESULTS

A two-way molecular cytosolic switch controls channel modulation by cholesterol and PI(4,5)P(2).

CONCLUSION

Cholesterol and PI(4,5)P(2) induce a common gating pathway of Kir2.1 despite their opposite impact on channel function.

SIGNIFICANCE

These findings provide insights into structure-function relationship of ion channels and contribute to understanding of the mechanisms underlying their regulation by lipids. Inwardly rectifying potassium (Kir) channels play an important role in setting the resting membrane potential and modulating membrane excitability. An emerging feature of several Kir channels is that they are regulated by cholesterol. However, the mechanism by which cholesterol affects channel function is unclear. Here we show that mutations of two distant Kir2.1 cytosolic residues, Leu-222 and Asn-251, form a two-way molecular switch that controls channel modulation by cholesterol and affects critical hydrogen bonding. Notably, these two residues are linked by a residue chain that continues from Asn-251 to connect adjacent subunits. Furthermore, our data indicate that the same switch also regulates the sensitivity of the channels to phosphatidylinositol 4,5-bisphosphate, a phosphoinositide that is required for activation of Kir channels. Thus, although cholesterol and phosphatidylinositol 4,5-bisphosphate do not interact with the same region of Kir2.1, these different modulators induce a common gating pathway of the channel.

TRP channels: sensors and transducers of gasotransmitter signals

The transient receptor potential (trp) gene superfamily encodes cation channels that act as multimodal sensors for a wide variety of stimuli from outside and inside the cell. Upon sensing, they transduce electrical and Ca²⁺ signals via their cation channel activities. These functional features of TRP channels allow the body to react and adapt to different forms of environmental changes. Indeed, members of one class of TRP channels have emerged as sensors of gaseous messenger molecules that control various cellular processes. Nitric oxide (NO), a vasoactive gaseous molecule, regulates TRP channels directly via cysteine (Cys) S-nitrosylation or indirectly via cyclic GMP (cGMP)/protein kinase G (PKG)-dependent phosphorylation. Recent studies have revealed that changes in the availability of molecular oxygen (O₂) also control the activation of TRP channels. Anoxia induced by O₂-glucose deprivation and severe hypoxia (1% O₂) activates TRPM7 and TRPC6, respectively, whereas TRPA1 has recently been identified as a novel sensor of hyperoxia and mild hypoxia (15% O₂) in vagal and sensory neurons. TRPA1 also detects other gaseous molecules such as hydrogen sulfide (H₂S) and carbon dioxide (CO₂). In this review, we focus on how signaling by gaseous molecules is sensed and integrated by TRP channels.

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.

Cholesterol enhances neuron susceptibility to apoptotic stimuli via cAMP/PKA/CREB-dependent up-regulation of Kv2.1

Cholesterol is a major component of membrane lipid rafts. It is more abundant in the brain than in other tissues and plays a critical role in maintaining brain function. We report here that a significant enhancement in apoptosis in rat cerebellar granule neurons (CGNs) was observed upon incubation with 5mM K(+) /serum free (LK-S) medium. Cholesterol enrichment further potentiated CGN apoptosis incubated under LK-S medium. On the contrary, cholesterol depletion using methyl-beta-cyclodextrin protected the CGNs from apoptosis induced by LK-S treatment. Cholesterol enrichment, however, did not induce apoptosis in CGNs that have been incubated with 25mM K(+) /serum medium. Mechanistically, increased I(K) currents and DNA fragmentation were found in CGNs incubated in LK-S, which was further potentiated in the presence of cholesterol. Cholesterol-treated CGNs also exhibited increased cAMP levels and up-regulation of Kv2.1 expression. Increased levels of activated form of PKA and phospho-CREB further supported activation of the cAMP/PKA pathway upon treatment of CGNs with cholesterol-containing LK-S medium. Conversely, inhibition of PKA or small G protein Gs abolished the increase in I(K) current and the potentiation of Kv2.1 expression, leading to reduced susceptibility of CGNs to LK-S and cholesterol-induced apoptosis. Our results demonstrate that the elevation of membrane cholesterol enhances CGN susceptibility to apoptotic stimuli via cAMP/PKA/CREB-dependent up-regulation of Kv2.1. Our data provide new evidence for the role of cholesterol in eliciting neuronal cell death.

Integration of transient receptor potential canonical channels with lipids

Transient receptor potential canonical (TRPC) channels are the canonical (C) subset of the TRP proteins, which are widely expressed in mammalian cells. They are thought to be primarily involved in determining calcium and sodium entry and have wide-ranging functions that include regulation of cell proliferation, motility and contraction. The channels are modulated by a multiplicity of factors, putatively existing as integrators in the plasma membrane. This review considers the sensitivities of TRPC channels to lipids that include diacylglycerols, phosphatidylinositol bisphosphate, lysophospholipids, oxidized phospholipids, arachidonic acid and its metabolites, sphingosine-1-phosphate, cholesterol and some steroidal derivatives and other lipid factors such as gangliosides. Promiscuous and selective lipid sensing have been detected. There appear to be close working relationships with lipids of the phospholipase C and A₂ enzyme systems, which may enable integration with receptor signalling and membrane stretch. There are differences in the properties of each TRPC channel that are further complicated by TRPC heteromultimerization. The lipids modulate activity of the channels or insertion in the plasma membrane. Lipid microenvironments and intermediate sensing proteins have been described that include caveolae, G protein signalling, SEC14-like and spectrin-type domains 1 (SESTD1) and podocin. The data suggest that lipid sensing is an important aspect of TRPC channel biology enabling integration with other signalling systems.

Tumour necrosis factor α-induced adipose-related protein (TIARP): co-localization with caveolin-1

We previously identified TIARP (TNF(alpha)-induced adipose-related protein, where TNF(alpha) stands for tumour necrosis factor alpha), a novel plasma-membrane protein that is induced during 3T3-L1 preadipocytes differentiation by TNF(alpha). Whereas the biological function of TIARP is currently unknown, its protein sequence is reminiscent of transporter protein and/or NAD(P)/NAD(P)H-dependent oxidoreductase activities. We hypothesized that TIARP could be associated with the 3T3-L1 adipocyte plasma-membrane caveolae domains that contain many proteins involved in cellular trafficking and signalling processes. Studies by confocal microscopy showed that TIARP and caveolin-1, a major protein of caveolae, co-localized as patches at the plasma membrane. Immunoblot analysis of cell extracts indicated that TIARP was completely detergent-extractible from membranes, whereas caveolin-1 was present as both detergent-extractible and -insoluble pools. Since TIARP is compartmentalized with caveolin-1 within caveolae domains, we suggest this protein to be part of a signalling complex in association with caveolin-1 and regulatory proteins.

The TR (i)P to Ca²⁺ signaling just got STIMy: an update on STIM1 activated TRPC channels

Calcium is a ubiquitous signaling molecule, indispensable for cellular metabolism of organisms from unicellular life forms to higher eukaryotes. The biological function of most eukaryotic cells is uniquely regulated by changes in cytosolic calcium, which is largely achieved by the universal phenomenon of store-operated calcium entry (SOCE). The canonical TRPs and Orai channels have been described as the molecular components of the store-operated calcium channels (SOCC). Importantly, the ER calcium-sensor STIM1 has been shown to initiate SOCE via gating of SOCC. Since the discovery of STIM1, as the critical regulator of SOCE, there has been a flurry of observations suggesting its obligatory role in regulating TRPC and Orai channel function. Considerable effort has been made to identify the molecular details as how STIM1 activates SOCC. In this context, findings as of yet has substantially enriched our understanding on, the modus operandi of SOCE, the distinct cellular locales that organize STIM1-SOCC complexes, and the physiological outcomes entailing STIM1-activated SOCE. In this review we discuss TRPC channels and provide an update on their functional regulation by STIM1.