Role of Trpc channels, Stim1 and Orai1 in PGF(2α)-induced calcium signaling in NRK fibroblasts

TRP channel function in platelets and megakaryocytes: basic mechanisms and pathophysiological impact

Transient receptor potential (TRP) proteins form a superfamily of cation channels that are expressed in a wide range of tissues and cell types. During the last years, great progress has been made in understanding the molecular complexity and the functions of TRP channels in diverse cellular processes, including cell proliferation, migration, adhesion and activation. The diversity of functions depends on multiple regulatory mechanisms by which TRP channels regulate Ca²⁺ entry mechanisms and intracellular Ca²⁺ dynamics, either through membrane depolarization involving cation influx or store- and receptor-operated mechanisms. Abnormal function or expression of TRP channels results in vascular pathologies, including hypertension, ischemic stroke and inflammatory disorders through effects on vascular cells, including the components of blood vessels and platelets. Moreover, some TRP family members also regulate megakaryopoiesis and platelet production, indicating a complex role of TRP channels in pathophysiological conditions. In this review, we describe potential roles of TRP channels in megakaryocytes and platelets, as well as their contribution to diseases such as thrombocytopenia, thrombosis and stroke. We also critically discuss the potential of TRP channels as possible targets for disease prevention and treatment.

Mechanisms of Nausea and Vomiting: Current Knowledge and Recent Advances in Intracellular Emetic Signaling Systems

Nausea and vomiting are common gastrointestinal complaints that can be triggered by diverse emetic stimuli through central and/or peripheral nervous systems. Both nausea and vomiting are considered as defense mechanisms when threatening toxins/drugs/bacteria/viruses/fungi enter the body either via the enteral (e.g., the gastrointestinal tract) or parenteral routes, including the blood, skin, and respiratory systems. While vomiting is the act of forceful removal of gastrointestinal contents, nausea is believed to be a subjective sensation that is more difficult to study in nonhuman species. In this review, the authors discuss the anatomical structures, neurotransmitters/mediators, and corresponding receptors, as well as intracellular emetic signaling pathways involved in the processes of nausea and vomiting in diverse animal models as well as humans. While blockade of emetic receptors in the prevention of vomiting is fairly well understood, the potential of new classes of antiemetics altering postreceptor signal transduction mechanisms is currently evolving, which is also reviewed. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide potential answers.

STIM1 a calcium sensor promotes the assembly of an ECM that contains Extracellular vesicles and factors that modulate mineralization

Osteoblasts and odontoblasts, are non-excitable cells and facilitate mass calcium transport during matrix mineralization. A sophisticated Ca²⁺ sensing mechanism is used to maintain Ca²⁺ homeostasis. STIM1 (Stromal interaction molecule 1) is a calcium sensor protein localized in the ER membrane and maintains calcium homeostasis by initiating the store-operated Ca²⁺ entry (SOCE) process, following store depletion. The role of STIM1 in dentin mineralization is yet to be elucidated. Therefore, transgenic DPSCs were generated in which overexpression or knockdown of STIM1 was achieved to study its function in matrix mineralization. Gene expression analysis and Alizarin Red staining assay demonstrated upregulation of genes involved in odontogenic differentiation and matrix mineralization with increased calcium deposition with STIM1 overexpression. Topology of the ECM examined by Field Emission Scanning Electron Microscopy (FESEM) showed the presence of large amounts of extracellular microvesicles with mineral deposits. Interestingly, silencing STIM1 resulted in fewer vesicles and less mineral deposits in the ECM. Analysis of the dentin-pulp complex of STIM1- deficient mice by micro-CT show reduced dentin thickness, malformed and highly porous alveolar bone, suggesting a cell intrinsic role for STIM1 in dentin mineralization. Confocal microscopy showed that DMP1-mediated depletion of store Ca²⁺ resulted in aggregation or "puncta-formation" of STIM1 at the plasma membrane indicative of a gating arrangement with Orai1 for Ca²⁺ influx. Together, our data provide evidence for an important role for STIM1 in dentin and alveolar bone mineralization by influencing intracellular Ca²⁺ oscillations that could provide signals for a wide array of cellular functions. STATEMENT OF SIGNIFICANCE: Calcium signaling and transport are fundamental to bone and dentin mineralization. Osteoblasts and odontoblasts transport large amounts of Ca²⁺ to the extracellular matrix. These cells maintain calcium homeostasis by spatially distributed calcium pumps and channels at the plasma membrane. STIM1 an ER Ca²⁺ sensor protein is an important component of the store-operated calcium entry (SOCE) process. In this study, we examined the role of STIM1 during the differentiation of dental pulp stem cells into functional odontoblasts and formation of mineralized dentin matrix. Stimulation of these cells with DMP1, a key regulatory protein in matrix mineralization, stimulates STIM1-mediated release of ER Ca²⁺ and SOCE activation. Silencing of STIM1 impairs signaling events, release of exosomes containing matrix proteins and matrix mineralization.

Proteins Interacting with STIM1 and Store-Operated Ca2+ Entry

The endoplasmic reticulum (ER) Ca²⁺ sensor stromal interaction molecule 1 (STIM1) interacts with ORAI Ca²⁺ channels at the plasma membrane to regulate immune and muscle cell function. The conformational changes underlying STIM1 activation, translocation, and ORAI1 trapping and gating, are stringently regulated by post-translational modifications and accessory proteins. Here, we review the recent progress in the identification and characterization of ER and cytosolic proteins interacting with STIM1 to control its activation and deactivation during store-operated Ca²⁺ entry (SOCE).

Mouse transient receptor potential channel type 6 selectively regulates agonist-induced platelet function

While changes in intracellular calcium levels is a central step in platelet activation and thrombus formation, the contribution and mechanism of receptor-operated calcium entry (ROCE) via transient receptor potential channels (TRPCs) in platelets remains poorly defined. In previous studies, we have shown that TRPC6 regulates hemostasis and thrombosis, in mice. In the present studies, we employed a knockout mouse model system to characterize the role of TRPC6 in ROCE and platelet activation. It was observed that the TRPC6 deletion (Trpc6^−/−) platelets displayed impaired elevation of intracellular calcium, i.e., defective ROCE. Moreover, these platelets also exhibited defects in a host of functional responses, namely aggregation, granule secretion, and integrin αIIbβ3. Interestingly, the aforementioned defects were specific to the thromboxane receptor (TPR), as no impaired responses were observed in response to ADP or the thrombin receptor-activating peptide 4 (TRAP4). The defect in ROCE in the Trpc6^−/− was also observed with 1-oleoyl-2-acetyl-sn-glycerol (OAG). Finally, our studies also revealed that TRPC6 regulates clot retraction. Taken together, our findings demonstrate that TRPC6 directly regulates TPR-dependent ROCE and platelet function. Thus, TRPC6 may serve as a novel target for the therapeutic management of thrombotic diseases.

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TRPC6 regulates TPR-mediated/receptor-operated calcium entry.

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TRPC6 regulates TPR-dependent platelet aggregation, secretion and integrin activation.

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TRPC6 regulates clot retraction.

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TRPC6 expression levels are age-dependent in platelets.

Prostaglandins in the pathogenesis of kidney diseases

Prostaglandins (PGs) are important lipid mediators produced from arachidonic acid via the sequential catalyzation of cyclooxygenases (COXs) and specific prostaglandin synthases. There are five subtypes of PGs, namely PGE2, PGI2, PGD2, PGF2α, and thromboxane A2 (TXA2). PGs exert distinct roles by combining to a diverse family of membrane-spanning G protein-coupled prostanoid receptors. The distribution of these PGs, their specific synthases and receptors vary a lot in the kidney. This review summarized the recent findings of PGs together with the COXs and their specific synthases and receptors in regulating renal function and highlighted the insights into their roles in the pathogenesis of various kidney diseases.

Inhibition of TRPC1-Dependent Store-Operated Calcium Entry Improves Synaptic Stability and Motor Performance in a Mouse Model of Huntington's Disease

BACKGROUND

Huntington disease (HD) is a dominantly inherited neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene. We previously discovered that mutant Huntingtin sensitizes type 1 inositol 1,4,5-trisphosphate receptor (InsP3R1) to InsP3. This causes calcium leakage from the endoplasmic reticulum (ER) and a compensatory increase in neuronal store-operated calcium (nSOC) entry. We previously demonstrated that supranormal nSOC leads to synaptic loss in striatal medium spiny neurons (MSNs) in YAC128 HD mice.

OBJECTIVE

We sought to identify calcium channels supporting supranormal nSOC in HD MSNs and to validate these channels as potential therapeutic targets for HD.

METHODS

Cortico-striatal cultures were established from wild type and YAC128 HD mice and the density of MSN spines was quantified. The expression of candidate nSOC components was suppressed by RNAi knockdown and by CRISPR/Cas9 knockout. TRPC1 knockout mice were crossed with YAC128 HD mice for evaluation of motor performance in a beamwalk assay.

RESULTS

RNAi-mediated knockdown of TRPC1, TRPC6, Orai1, or Orai2, but not other TRPC isoforms or Orai3, rescued the density of YAC128 MSN spines. Knockdown of stromal interaction molecule 1 (STIM1), an ER calcium sensor and nSOC activator, also rescued YAC128 MSN spines. Knockdown of the same targets suppressed supranormal nSOC in YAC128 MSN spines. These channel subunits co-immunoprecipitated with STIM1 and STIM2 in synaptosomal lysates from mouse striata. Crossing YAC128 mice with TRPC1 knockout mice improved motor performance and rescued MSN spines in vitro and in vivo, indicating that inhibition of TRPC1 may serve as a neuroprotective strategy for HD treatment.

CONCLUSIONS

TRPC1 channels constitute a potential therapeutic target for treatment of HD.

TRPC1, Orai1, and STIM1 in SOCE: Friends in tight spaces

Store-operated calcium entry (SOCE) is a ubiquitous Ca2+ entry pathway that is activated in response to depletion of ER-Ca2+ stores and critically controls the regulation of physiological functions in miscellaneous cell types. The transient receptor potential canonical 1 (TRPC1) is the first member of the TRPC channel subfamily to be identified as a molecular component of SOCE. While TRPC1 has been shown to contribute to SOCE and regulate various functions in many cells, none of the reported TRPC1-mediated currents resembled ICRAC, the highly Ca2+-selective store-dependent current first identified in lymphocytes and mast cells. Almost a decade after the cloning of TRPC1 two proteins were identified as the primary components of the CRAC channel. The first, STIM1, is an ER-Ca2+ sensor protein involved in activating SOCE. The second, Orai1 is the pore-forming component of the CRAC channel. Co-expression of STIM1 and Orai1 generated robust ICRAC. Importantly, STIM1 was shown to also activate TRPC1 via its C-terminal polybasic domain, which is distinct from its Orai1-activating domain, SOAR. In addition, TRPC1 function critically depends on Orai1-mediated Ca2+ entry which triggers recruitment of TRPC1 into the plasma membrane where it is then activated by STIM1. TRPC1 and Orai1 form discrete STIM1-gated channels that generate distinct Ca2+ signals and regulate specific cellular functions. Surface expression of TRPC1 can be modulated by trafficking of the channel to and from the plasma membrane, resulting in changes to the phenotype of TRPC1-mediated current and [Ca2+]i signals. Thus, TRPC1 is activated downstream of Orai1 and modifies the initial [Ca2+]i signal generated by Orai1 following store depletion. This review will summarize the important findings that underlie the current concepts for activation and regulation of TRPC1, as well as its impact on cell function.

Enhanced store-operated Ca2+ influx and ORAI1 expression in ventricular fibroblasts from human failing heart

Excessive cardiac fibrosis, characterized by increased collagen-rich extracellular matrix (ECM) deposition, is a major predisposing factor for mechanical and electrical dysfunction in heart failure (HF). The human ventricular fibroblast (hVF) remodeling mechanisms that cause excessive collagen deposition in HF are unclear, although reports suggest a role for intracellular free Ca²⁺ in fibrosis. Therefore, we determined the association of differences in cellular Ca²⁺ dynamics and collagen secretion/deposition between hVFs from failing and normal (control) hearts. Histology of left ventricle sections (Masson trichrome) confirmed excessive fibrosis in HF versus normal. In vitro, hVFs from HF showed increased secretion/deposition of soluble collagen in 48 h of culture compared with control [85.9±7.4 µg/10⁶ cells vs 58.5±8.8 µg/10⁶ cells, P<0.05; (Sircol™ assay)]. However, collagen gene expressions (COL1A1 and COL1A2; RT-PCR) were not different. Ca²⁺ imaging (fluo-3) of isolated hVFs showed no difference in the thapsigargin-induced intracellular Ca²⁺ release capacity (control 16±1.4% vs HF 17±1.1%); however, Ca²⁺ influx via store-operated Ca²⁺ entry/Ca²⁺ release-activated channels (SOCE/CRAC) was significantly (P≤0.05) greater in HF-hVFs (47±3%) compared with non-failing (35±5%). Immunoblotting for I_CRAC channel components showed increased ORAI1 expression in HF-hVFs compared with normal without any difference in STIM1 expression. The Pearson's correlation coefficient for co-localization of STIM1/ORAI1 was significantly (P<0.01) greater in HF (0.5±0.01) than control (0.4±0.01) hVFs. The increase in collagen secretion of HF versus control hVFs was eliminated by incubation of hVFs with YM58483 (10 µM), a selective I_CRAC inhibitor, for 48 h (66.78±5.87 µg/10⁶ cells vs 55.81±7.09 µg/10⁶ cells, P=0.27). In conclusion, hVFs from HF have increased collagen secretion capacity versus non-failing hearts and this is related to increase in Ca²⁺ entry via SOCE and enhanced expression of ORAI, the pore-forming subunit. Therapeutic inhibition of SOCE may reduce the progression of cardiac fibrosis/HF.

Summary: The excessive collagen secretory phenotype found in failing human hearts is associated with ventricular fibroblast remodeling, caused by an elevated influx of intracellular calcium via SOC channels.

Assembly of ER-PM Junctions: A Critical Determinant in the Regulation of SOCE and TRPC1

Store-operated calcium entry (SOCE), a unique plasma membrane Ca²⁺ entry mechanism, is activated when ER-[Ca²⁺] is decreased. SOCE is mediated via the primary channel, Orai1, as well as others such as TRPC1. STIM1 and STIM2 are ER-Ca²⁺ sensor proteins that regulate Orai1 and TRPC1. SOCE requires assembly of STIM proteins with the plasma membrane channels which occurs within distinct regions in the cell that have been termed as endoplasmic reticulum (ER)-plasma membrane (PM) junctions. The PM and ER are in close proximity to each other within this region, which allows STIM1 in the ER to interact with and activate either Orai1 or TRPC1 in the plasma membrane. Activation and regulation of SOCE involves dynamic assembly of various components that are involved in mediating Ca²⁺ entry as well as those that determine the formation and stabilization of the junctions. These components include proteins in the cytosol, ER and PM, as well as lipids in the PM. Recent studies have also suggested that SOCE and its components are compartmentalized within ER-PM junctions and that this process might require remodeling of the plasma membrane lipids and reorganization of structural and scaffolding proteins. Such compartmentalization leads to the generation of spatially- and temporally-controlled Ca²⁺signals that are critical for regulating many downstream cellular functions.

STIM-TRP Pathways and Microdomain Organization: Ca2+ Influx Channels: The Orai-STIM1-TRPC Complexes

Ca²⁺ influx by plasma membrane Ca²⁺ channels is the crucial component of the receptor-evoked Ca²⁺ signal. The two main Ca²⁺ influx channels of non-excitable cells are the Orai and TRPC families of Ca²⁺ channels. These channels are activated in response to cell stimulation and Ca²⁺ release from the endoplasmic reticulum (ER). The protein that conveys the Ca²⁺ content of the ER to the plasma membrane is the ER Ca²⁺ sensor STIM1. STIM1 activates the Orai channels and is obligatory for channel opening. TRPC channels can function in two modes, as STIM1-dependent and STIM1-independent. When activated by STIM1, both channel types function at the ER/PM (plasma membrane) junctions. This chapter describes the properties and regulation of the channels by STIM1, with emphasis how and when TRPC channels function as STIM1-dependent and STIM1-independent modes and their unique Ca²⁺-dependent physiological functions that are not shared with the Orai channels.

STIM-TRP Pathways and Microdomain Organization: Contribution of TRPC1 in Store-Operated Ca2+ Entry: Impact on Ca2+ Signaling and Cell Function

Store-operated calcium entry (SOCE) is a ubiquitous Ca²⁺ entry pathway that is activated in response to depletion of ER-Ca²⁺ stores and critically controls the regulation of physiological functions in a wide variety of cell types. The transient receptor potential canonical (TRPC) channels (TRPCs 1-7), which are activated by stimuli leading to PIP₂ hydrolysis, were first identified as molecular components of SOCE channels. While TRPC1 was associated with SOCE and regulation of function in several cell types, none of the TRPC members displayed I _CRAC, the store-operated current identified in lymphocytes and mast cells. Intensive search finally led to the identification of Orai1 and STIM1 as the primary components of the CRAC channel. Orai1 was established as the pore-forming channel protein and STIM1 as the ER-Ca²⁺ sensor protein involved in activation of Orai1. STIM1 also activates TRPC1 via a distinct domain in its C-terminus. However, TRPC1 function depends on Orai1-mediated Ca²⁺ entry, which triggers recruitment of TRPC1 into the plasma membrane where it is activated by STIM1. TRPC1 and Orai1 form distinct store-operated Ca²⁺ channels that regulate specific cellular functions. It is now clearly established that regulation of TRPC1 trafficking can change plasma membrane levels of the channel, the phenotype of the store-operated Ca²⁺ current, as well as pattern of SOCE-mediated [Ca²⁺]_i signals. Thus, TRPC1 is activated downstream of Orai1 and modifies the initial [Ca²⁺]_i signal generated by Orai1. This review will highlight current concepts of the activation and regulation of TRPC1 channels and its impact on cell function.

Pathophysiological Significance of Store-Operated Calcium Entry in Megakaryocyte Function: Opening New Paths for Understanding the Role of Calcium in Thrombopoiesis

Store-Operated Calcium Entry (SOCE) is a universal calcium (Ca²⁺) influx mechanism expressed by several different cell types. It is now known that Stromal Interaction Molecule (STIM), the Ca²⁺ sensor of the intracellular compartments, together with Orai and Transient Receptor Potential Canonical (TRPC), the subunits of Ca²⁺ permeable channels on the plasma membrane, cooperate in regulating multiple cellular functions as diverse as proliferation, differentiation, migration, gene expression, and many others, depending on the cell type. In particular, a growing body of evidences suggests that a tight control of SOCE expression and function is achieved by megakaryocytes along their route from hematopoietic stem cells to platelet production. This review attempts to provide an overview about the SOCE dynamics in megakaryocyte development, with a focus on most recent findings related to its involvement in physiological and pathological thrombopoiesis.

Blockade of Orai1 Store-Operated Calcium Entry Protects against Renal Fibrosis

Evidence supports an important role of Ca²⁺ release-activated Ca²⁺ channel protein 1 (Orai1)-mediated Ca²⁺ entry in the development of renal fibrosis, a common pathologic feature of CKDs that lead to ESRD, but the molecular mechanisms remain unclear. We determined the role of Orai1 calcium channel in renal fibrosis induced by high-fat diet and by unilateral ureteral obstruction. Mouse kidneys with fibrosis had higher levels of Orai1 protein expression than did kidneys without fibrosis. In vivo knockdown of Orai1 with adenovirus harboring Orai1-short hairpin RNA or inhibition of Orai1 with SKF96365 dramatically prevented renal fibrosis and significantly decreased protein expression of fibronectin, α‑smooth muscle actin, and TGF‑β1 in the kidney cortex of ApoE^-/- mice on a high-fat diet and in the obstructed kidneys of mice with unilateral ureteral obstruction. Compared with kidney biopsy specimens of patients with glomerular minimal change disease, those of patients with fibrotic nephropathy had higher expression levels of Orai1. In cultured human proximal tubule epithelial cells (HK2), knockdown of Orai1 Ca²⁺ channel with adenovirus-Orai1-short hairpin RNA markedly inhibited TGF-β1-induced intracellular Ca²⁺ influx and phosphorylation of smad2/3. Knockdown or blockade of the Orai1 Ca²⁺ channel in HK2 cells also prevented epithelial-to-mesenchymal transition induced by TGF‑β1. In conclusion, blockade of the Orai1 Ca²⁺ channel prevented progression of renal fibrosis in mice, likely by suppressing smad2/3 phosphorylation and TGF-β1-induced epithelial-to-mesenchymal transition. These results render the Orai1 Ca²⁺ channel a potential therapeutic target against renal fibrosis.

Historical Overview of Store-Operated Ca(2+) Entry

Calcium influx is an essential mechanism for the activation of cellular functions both in excitable and non-excitable cells. In non-excitable cells, activation of phospholipase C by occupation of G protein-coupled receptors leads to the generation of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), which, in turn, initiate two Ca(2+) entry pathways: Ca(2+) release from intracellular Ca(2+) stores, signaled by IP3, leads to the activation of store-operated Ca(2+) entry (SOCE); on the other hand, DAG activates a distinct second messenger-operated pathway. SOCE is regulated by the filling state of the intracellular calcium stores. The search for the molecular components of SOCE has identified the stromal interaction molecule 1 (STIM1) as the Ca(2+) sensor in the endoplasmic reticulum and Orai1 as a store-operated channel (SOC) subunit. Furthermore, a number of reports have revealed that several members of the TRPC family of channels also take part of the SOC macromolecular complex. This introductory chapter summarizes the early pieces of evidence that led to the concept of SOCE and the components of the store-operated signaling pathway.

Roles of store-operated Ca2+ channels in regulating cell cycling and migration of human cardiac c-kit+ progenitor cells

Cardiac c-kit(+) progenitor cells are important for maintaining cardiac homeostasis and can potentially contribute to myocardial repair. However, cellular physiology of human cardiac c-kit(+) progenitor cells is not well understood. The present study investigates the functional store-operated Ca(2+) entry (SOCE) channels and the potential role in regulating cell cycling and migration using confocal microscopy, RT-PCR, Western blot, coimmunoprecipitation, cell proliferation, and migration assays. We found that SOCE channels mediated Ca(2+) influx, and TRPC1, STIM1, and Orai1 were involved in the formation of SOCE channels in human cardiac c-kit(+) progenitor cells. Silencing TRPC1, STIM1, or Orai1 with the corresponding siRNA significantly reduced the Ca(2+) signaling through SOCE channels, decreased cell proliferation and migration, and reduced expression of cyclin D1, cyclin E, and/or p-Akt. Our results demonstrate the novel information that Ca(2+) signaling through SOCE channels regulates cell cycling and migration via activating cyclin D1, cyclin E, and/or p-Akt in human cardiac c-kit(+) cells.

Evidence of a Role for Fibroblast Transient Receptor Potential Canonical 3 Ca2+ Channel in Renal Fibrosis

Transient receptor potential canonical (TRPC) Ca(2+)-permeant channels, especially TRPC3, are increasingly implicated in cardiorenal diseases. We studied the possible role of fibroblast TRPC3 in the development of renal fibrosis. In vitro, a macromolecular complex formed by TRPC1/TRPC3/TRPC6 existed in isolated cultured rat renal fibroblasts. However, specific blockade of TRPC3 with the pharmacologic inhibitor pyr3 was sufficient to inhibit both angiotensin II- and 1-oleoyl-2-acetyl-sn-glycerol-induced Ca(2+) entry in these cells, which was detected by fura-2 Ca(2+) imaging. TRPC3 blockade or Ca(2+) removal inhibited fibroblast proliferation and myofibroblast differentiation by suppressing the phosphorylation of extracellular signal-regulated kinase (ERK1/2). In addition, pyr3 inhibited fibrosis and inflammation-associated markers in a noncytotoxic manner. Furthermore, TRPC3 knockdown by siRNA confirmed these pharmacologic findings. In adult male Wistar rats or wild-type mice subjected to unilateral ureteral obstruction, TRPC3 expression increased in the fibroblasts of obstructed kidneys and was associated with increased Ca(2+) entry, ERK1/2 phosphorylation, and fibroblast proliferation. Both TRPC3 blockade in rats and TRPC3 knockout in mice inhibited ERK1/2 phosphorylation and fibroblast activation as well as myofibroblast differentiation and extracellular matrix remodeling in obstructed kidneys, thus ameliorating tubulointerstitial damage and renal fibrosis. In conclusion, TRPC3 channels are present in renal fibroblasts and control fibroblast proliferation, differentiation, and activation through Ca(2+)-mediated ERK signaling. TRPC3 channels might constitute important therapeutic targets for improving renal remodeling in kidney disease.

Lung Beractant Increases Free Cytosolic Levels of Ca2+ in Human Lung Fibroblasts

Beractant, a natural surfactant, induces an antifibrogenic phenotype and apoptosis in normal human lung fibroblasts (NHLF). As intracellular Ca2+ signalling has been related to programmed cell death, we aimed to assess the effect of beractant on intracellular Ca2+ concentration ([Ca2+]i) in NHLF in vitro. Cultured NHLF were loaded with Fura-2 AM (3 μM) and Ca2+ signals were recorded by microfluorimetric techniques. Beractant causes a concentration-dependent increase in [Ca2+]i with a EC50 of 0.82 μg/ml. The application of beractant, at a concentration of 500 μg/ml, which has been shown to exert an apoptotic effect in human fibroblasts, elicited different patterns of Ca2+ signals in NHLF: a) a single Ca2+ spike which could be followed by b) Ca2+ oscillations, c) a sustained Ca2+ plateau or d) a sustained plateau overlapped by Ca2+ oscillations. The amplitude and pattern of Ca2+ transients evoked by beractant were dependent on the resting [Ca2+]i. Pharmacological manipulation revealed that beractant activates a Ca2+ signal through Ca2+ release from intracellular stores mediated by phospholipase Cβ (PLCβ), Ca2+ release from inositol 1,4,5-trisphosphate receptors (IP3Rs) and Ca2+ influx via a store-operated pathway. Moreover, beractant-induced Ca2+ release was abolished by preventing membrane depolarization upon removal of extracellular Na+ and Ca2+. Finally, the inhibition of store-operated channels prevented beractant-induced NHLF apoptosis and downregulation of α1(I) procollagen expression. Therefore, beractant utilizes SOCE to exert its pro-apoptotic and antifibrinogenic effect on NHLF.

A critical role for the transient receptor potential channel type 6 in human platelet activation

While calcium signaling is known to play vital roles in platelet function, the mechanisms underlying its receptor-operated calcium entry component (ROCE) remain poorly understood. It has been proposed, but never proven in platelets, that the canonical transient receptor potential channel-6 (TRPC6) mediates ROCE. Nonetheless, we have previously shown that the mouse TRPC6 regulates hemostasis, thrombogenesis by regulating platelet aggregation. In the present studies, we used a pharmacological approach to characterize the role of TRPC6 in human platelet biology. Thus, interestingly, we observed that a TRPC6 inhibitor exerted significant inhibitory effects on human platelet aggregation in a thromboxane receptor (TPR)-selective manner; no additional inhibition was observed in the presence of the calcium chelator BAPTA. This inhibitor also significantly inhibited human platelet secretion (dense and alpha granules), integrin IIb-IIIa, Akt and ERK phosphorylation, again, in a TPR-selective manner; no effects were observed in response to ADP receptor stimulation. Furthermore, there was a causal relationship between these inhibitory effects, and the capacity of the TRPC6 inhibitor to abrogate elevation in intracellular calcium, that was again found to be TPR-specific. This effect was not found to be due to antagonism of TPR, as the TRPC6 inhibitor did not displace the radiolabeled antagonist [³H]SQ29,548 from its binding sites. Finally, our studies also revealed that TRPC6 regulates human clot retraction, as well as physiological hemostasis and thrombus formation, in mice. Taken together, our findings demonstrate, for the first time, that TRPC6 directly regulates TPR-dependent ROCE and platelet function. Moreover, these data highlight TRPC6 as a novel promising therapeutic strategy for managing thrombotic disorders.

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.

Broad-spectrum antiemetic potential of the L-type calcium channel antagonist nifedipine and evidence for its additive antiemetic interaction with the 5-HT(3) receptor antagonist palonosetron in the least shrew (Cryptotis parva)

Cisplatin-like chemotherapeutics cause vomiting via release of multiple neurotransmitters (dopamine, serotonin (5-HT), or substance P (SP)) from the gastrointestinal enterochromaffin cells and/or the brainstem via a calcium dependent process. Diverse channels in the plasma membrane allow extracellular Ca(2+) entry into cells for the transmitter release process. Agonists of 5-HT3 receptors increase calcium influx through both 5-HT3 receptors and L-type Ca(2+) channels. We envisaged that L-type calcium agonists such as FPL 64176 should cause vomiting and corresponding antagonists such as nifedipine would behave as broad-spectrum antiemetics. Administration of FPL 64176 did cause vomiting in the least shrew in a dose-dependent fashion. Nifedipine and the 5-HT3 receptor antagonist palonosetron, potently suppressed FPL 64176-induced vomiting, while a combination of ineffective doses of these antagonists was more efficacious. Subsequently, we investigated the broad-spectrum antiemetic potential of nifedipine against diverse emetogens including agonists of serotonergic 5-HT3- (e.g. 5-HT or 2-Me-5-HT), SP tachykinin NK1- (GR73632), dopamine D2- (apomorphine or quinpirole), and cholinergic M1- (McN-A-343) receptors, as well as the non-specific emetogen, cisplatin. Nifedipine by itself suppressed vomiting in a potent and dose-dependent manner caused by the above emetogens except cisplatin. Moreover, low doses of nifedipine potentiated the antiemetic efficacy of non-effective or semi-effective doses of palonosetron against vomiting caused by either 2-Me-5-HT or cisplatin. Thus, our findings demonstrate that activation of L-type calcium channels causes vomiting, whereas blockade of these ion channels by nifedipine-like antagonists not only provides broad-spectrum antiemetic activity but can also potentiate the antiemetic efficacy of well-established antiemetics such as palonosetron. L-type calcium channel antagonists should also provide antiemetic activity against drug-induced vomiting as well as other emetogens including bacterial and viral proteins.

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.

Histidine triad nucleotide-binding protein 1 (HINT1) regulates Ca(2+) signaling in mouse fibroblasts and neuronal cells via store-operated Ca(2+) entry pathway

Recent findings indicate that histidine triad nucleotide-binding protein 1 (HINT1) is implicated in the pathophysiology of certain psychiatric disorders and also exhibits tumor suppressor properties. However, the authentic functions of HINT1 in cellular physiology and especially its role in Ca(2+) signaling remain unclear. Here, we studied Ca(2+) signaling in cultured embryonic fibroblasts derived from wild-type control and HINT1 knockout (KO) mice. The resting cytosolic Ca(2+) level (measured with fura-2) was not altered in fibroblasts lacking HINT1. The stored Ca(2+) evaluated by measuring peak amplitude of ATP (10 μM)-induced Ca(2+) transients in Ca(2+)-free medium was significantly larger in HINT1 KO fibroblasts than in wild-type cells. Ca(2+) influx after external Ca(2+) restoration, likely via store- and receptor-operated channels (SOCs and ROCs, respectively), was greatly (by 2-fold) reduced in HINT1 KO fibroblasts. This correlated with a downregulated expression of Orai1 and stromal interacting molecule 1 (STIM1), essential components of store-operated Ca(2+) entry pathway. Expression of canonical transient receptor potential (TRPC)3 and TRPC6, which function as ROCs, was not altered in HINT1 KO fibroblasts. Immunoblots also revealed that Orai1 was downregulated by twofold in brain lysates of HINT1 KO mice compared with the wild-type littermates. Importantly, silencer RNA knockdown of HINT1 in Neuro-2A cells markedly downregulated Orai1 and STIM1 protein expression and significantly (by 2.5-fold) reduced ATP-induced Ca(2+) influx, while ATP-evoked Ca(2+) release was not changed. Thus the study demonstrates a novel function of HINT1 that involves the regulation of SOC-mediated Ca(2+) entry pathway (Orai1 and STIM1), essential for regulation of cellular Ca(2+) homeostasis.

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+).

HIV Assembly and Budding: Ca(2+) Signaling and Non-ESCRT Proteins Set the Stage

More than a decade has elapsed since the link between the endosomal sorting complex required for transport (ESCRT) machinery and HIV-1 protein trafficking and budding was first identified. L domains in HIV-1 Gag mediate recruitment of ESCRT which function in bud abscission releasing the viral particle from the host cell. Beyond virus budding, the ESCRT machinery is also involved in the endocytic pathway, cytokinesis, and autophagy. In the past few years, the number of non-ESCRT host proteins shown to be required in the assembly process has also grown. In this paper, we highlight the role of recently identified cellular factors that link ESCRT machinery to calcium signaling machinery and we suggest that this liaison contributes to setting the stage for productive ESCRT recruitment and mediation of abscission. Parallel paradigms for non-ESCRT roles in virus budding and cytokinesis will be discussed.