Localization of TRPC1 channel in the sinus endothelial cells of rat spleen

Polarized Proteins in Endothelium and Their Contribution to Function

Protein localization in endothelial cells is tightly regulated to create distinct signaling domains within their tight spatial restrictions including luminal membranes, abluminal membranes, and interendothelial junctions, as well as caveolae and calcium signaling domains. Protein localization in endothelial cells is also determined in part by the vascular bed, with differences between arteries and veins and between large and small arteries. Specific protein polarity and localization is essential for endothelial cells in responding to various extracellular stimuli. In this review, we examine protein localization in the endothelium of resistance arteries, with occasional references to other vessels for contrast, and how that polarization contributes to endothelial function and ultimately whole organism physiology. We highlight the protein localization on the luminal surface, discussing important physiological receptors and the glycocalyx. The protein polarization to the abluminal membrane is especially unique in small resistance arteries with the presence of the myoendothelial junction, a signaling microdomain that regulates vasodilation, feedback to smooth muscle cells, and ultimately total peripheral resistance. We also discuss the interendothelial junction, where tight junctions, adherens junctions, and gap junctions all convene and regulate endothelial function. Finally, we address planar cell polarity, or axial polarity, and how this is regulated by mechanosensory signals like blood flow.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.

Transient receptor potential channel 1 (TRPC1) reduces calcium permeability in heteromeric channel complexes

Specific biological roles of the classical transient receptor potential channel 1 (TRPC1) are still largely elusive. To investigate the function of TRPC1 proteins in cell physiology, we studied heterologously expressed TRPC1 channels and found that recombinant TRPC1 subunits do not form functional homomeric channels. Instead, by electrophysiological analysis TRPC1 was shown to form functional heteromeric, receptor-operated channel complexes with TRPC3, -4, -5, -6, and -7 indicating that TRPC1 proteins can co-assemble with all members of the TRPC subfamily. In all TRPC1-containing heteromers, TRPC1 subunits significantly decreased calcium permeation. The exchange of select amino acids in the putative pore-forming region of TRPC1 further reduced calcium permeability, suggesting that TRPC1 subunits contribute to the channel pore. In immortalized immature gonadotropin-releasing hormone neurons endogenously expressing TRPC1, -2, -5, and -6, down-regulation of TRPC1 resulted in increased calcium permeability and elevated basal cytosolic calcium concentrations. We did not observe any involvement of TRPC1 in store-operated cation influx. Notably, TRPC1 suppressed the migration of gonadotropin-releasing hormone neurons without affecting cell proliferation. Conversely, in TRPC1 knockdown neurons, specific migratory properties like distance covered, locomotion speed, and directionality were increased. These findings suggest a novel regulatory mechanism relying on the expression of TRPC1 and the subsequent formation of heteromeric TRPC channel complexes with reduced calcium permeability, thereby fine-tuning neuronal migration.

P2Y1, P2Y6, and P2Y12 receptors in rat splenic sinus endothelial cells: an immunohistochemical and ultrastructural study

Localization of three P2X and six P2Y receptors in sinus endothelial cells of the rat spleen was examined by immunofluorescent microscopy, and ultrastructural localization of the detected receptors was examined by immunogold electron microscopy. In immunofluorescent microscopy, labeling for anti-P2Y1, P2Y6, and P2Y12 receptors was detected in endothelial cells, but P2X1, P2X2, P2X4, P2Y2, P2Y4, and P2Y13 receptors was not detected. P2Y1 and P2Y12 receptors were prominently localized in the basal parts of endothelial cells. P2Y6 receptor was not only predominantly localized in the basal parts of endothelial cells, but also in the superficial layer. Triple immunofluorescent staining for a combination of two P2Y receptors and actin filaments showed that P2Y1, P2Y6, and P2Y12 receptors were individually localized in endothelial cells. Phospholipase C-β3, phospholipase C- γ2, and inositol-1,4,5-trisphosphate receptors, related to the release of the intracellular Ca(2+) from the endoplasmic reticulum, were also predominantly localized in the basal parts of endothelial cells. In immunogold electron microscopy, labeling for P2Y1, P2Y6, and P2Y12 receptors were predominantly localized in the basal part of endothelial cells and, in addition, in the junctional membrane, basal plasma membrane, and caveolae in the basal part of endothelial cells. Labeling for phospholipase C-β3 and phospholipase C-γ2 was dominantly localized in the basal parts and in close proximity to the plasma membranes of endothelial cells. The possible functional roles of these P2Y receptors in splenic sinus endothelial cells are discussed.

Silencing stromal interaction molecule 1 by RNA interference inhibits the proliferation and migration of endothelial progenitor cells

Knockdown of stromal interaction molecule 1 (STIM1) significantly suppresses neointima hyperplasia after vascular injury. Endothelial progenitor cells (EPCs) are the major source of cells that respond to endothelium repair and contribute to re-endothelialization by reducing neointima formation after vascular injury. We hypothesized that the effect of STIM1 on neointima hyperplasia inhibition is mediated through its effect on the biological properties of EPCs. In this study, we investigated the effects of STIM1 on the proliferation and migration of EPCs and examined the effect of STIM1 knockdown using cultured rat bone marrow-derived EPCs. STIM1 was expressed in EPCs, and knockdown of STIM1 by adenoviral delivery of small interfering RNA (siRNA) significantly suppressed the proliferation and migration of EPCs. Furthermore, STIM1 knockdown decreased store-operated channel entry 48h after transfection. Replenishment with recombinant human STIM1 reversed the effects of STIM1 knockdown. Our data suggest that the store-operated transient receptor potential canonical 1 channel is involved in regulating the biological properties of EPCs through STIM1. STIM1 is a potent regulator of cell proliferation and migration in rat EPCs and may play an important role in the biological properties of EPCs.

Intracellular trafficking of TRP channels

Thirteen years ago, it was suggested that exocytotic insertion of store-operated channels into the plasma membrane lead to increased Ca(2+) entry in non-excitable cells upon G protein-coupled or tyrosine kinase receptor stimulation. Since the discovery of the TRP channel superfamily and their involvement in receptor-induced Ca(2+) entry, many studies have shown that different members of the TRP superfamily translocate into the plasma membrane upon stimulation. While the exact molecular mechanism by which TRP channels insert into the plasma membrane is unknown, TRP-binding proteins have been shown to directly regulate this trafficking. This review summarizes recent advances related to the mechanism of TRP channel trafficking, focusing on the role of TRP-binding proteins.

TRPC1 Ca(2+)-permeable channels in animal cells

The full-length transient receptor (TRPC)1 polypeptide is composed of about 790 amino acids, and several splice variants are known. The predicted structure and topology is of an integral membrane protein composed of six transmembrane domains, and a cytoplasmic C- and N-terminal domain. The N-terminal domain includes three ankyrin repeat motifs. Antibodies which recognise TRPC1 have been developed, but it has been difficult to obtain antibodies which have high affinity and specificity for TRPC1. This has made studies of the cellular functions of TRPC1 somewhat difficult. The TRPC1 protein is widely expressed in different types of animal cells, and within a given cell is found at the plasma membrane and at intracellular sites. TRPC1 interacts with calmodulin, caveolin-1, the InsP3 receptor, Homer, phospholipase C and several other proteins. Investigations of the biological roles and mechanisms of action of TRPC1 have employed ectopic (over-expression or heterologous expression) of the polypeptide in addition to studies of endogenous TRPC1. Both approaches have encountered difficulties. TRPC1 forms heterotetramers with other TRPC polypeptides resulting in cation channels which are non-selective. TRPC1 may be: a component of the pore of store-operated Ca2+ channels (SOCs); a subsidiary protein in the pathway of activation of SOCs; activated by interaction with InsP3R; and/or activated by stretch. Further experiments are required to resolve the exact roles and mechanisms of activation of TRPC1. Cation entry through the TRPC1 channel is feed-back inhibited by Ca2+ through interaction with calmodulin, and is inhibited by Gd3+, La3+, SKF96365 and 2-APB, and by antibodies targeted to the external mouth of the TRPC1 pore. Activation of TRPC1 leads to the entry to the cytoplasmic space of substantial amounts of Na+ as well as Ca2+. A requirement for TRPC1 is implicated in numerous downstream cellular pathways. The most clearly described roles are in the regulation of growth cone turning in neurons. It is concluded that TRPC1 is a most interesting protein because of the apparent wide variety of its roles and functions and the challenges posed to those attempting to elucidate its primary intracellular functions and mechanisms of action.

The histochemistry and cell biology vade mecum: a review of 2005–2006

The procurement of new knowledge and understanding in the ever expanding discipline of cell biology continues to advance at a breakneck pace. The progress in discerning the physiology of cells and tissues in health and disease has been driven to a large extent by the continued development of new probes and imaging techniques. The recent introduction of semi-conductor quantum dots as stable, specific markers for both fluorescence light microscopy and electron microscopy, as well as a virtual treasure-trove of new fluorescent proteins, has in conjunction with newly introduced spectral imaging systems, opened vistas into the seemingly unlimited possibilities for experimental design. Although it oftentimes proves difficult to predict what the future will hold with respect to advances in disciplines such as cell biology and histochemistry, it is facile to look back on what has already occurred. In this spirit, this review will highlight some advancements made in these areas in the past 2 years.

Transient receptor potential channels and caveolin-1: good friends in tight spaces

Caveolae formation has raised the concept of energy efficiency to new heights. The ultimate purpose of caveolae formation is to colocalize signaling proteins with membrane microdomains in order to facilitate their interaction and improve signal transduction efficiency. Although we know that the main structural protein of caveolae is caveolin, how caveolin interacts with membrane proteins to facilitate their integration into lipid raft domains is unclear. A caveolin-scaffolding domain (CSD) on caveolin itself can associate with membrane proteins such as G proteins and endothelial nitric oxide synthase. In this issue, Kwiatek et al. (p. 1174) report that the TRPC1 channel protein contains a C-terminal CSD-consensus binding sequence that allows for its physical and functional interaction with caveolin-1 in the caveolae of human pulmonary artery endothelial cells (PAEC). Competitive interaction with a CSD-conjugated peptide attenuates thrombin- and thapsigargin-induced Ca2+ influx via store-operated TRPC1 channels. Their data suggest that caveolin-1 can directly regulate TRPC1 function, extending its already ascribed role as a structural protein.

Recent developments in vascular endothelial cell transient receptor potential channels

Among the 28 identified and unique mammalian TRP (transient receptor potential) channel isoforms, at least 19 are expressed in vascular endothelial cells. These channels appear to participate in a diverse range of vascular functions, including control of vascular tone, regulation of vascular permeability, mechanosensing, secretion, angiogenesis, endothelial cell proliferation, and endothelial cell apoptosis and death. Malfunction of these channels may result in disorders of the human cardiovascular system. All TRP channels, except for TRPM4 and TRPM5, are cation channels that allow Ca2+ influx. However, there is a daunting diversity in the mode of activation and regulation in each case. Specific TRP channels may be activated by different stimuli such as vasoactive agents, oxidative stress, mechanical stimuli, and heat. TRP channels may then transform these stimuli into changes in the cytosolic Ca2+, which are eventually coupled to various vascular responses. Evidence has been provided to suggest the involvement of at least the following TRP channels in vascular function: TRPC1, TRPC4, TRPC6, and TRPV1 in the control of vascular permeability; TRPC4, TRPV1, and TRPV4 in the regulation of vascular tone; TRPC4 in hypoxia-induced vascular remodeling; and TRPC3, TRPC4, and TRPM2 in oxidative stress-induced responses. However, in spite of the large body of data available, the functional role of many endothelial TRP channels is still poorly understood. Elucidating the mechanisms regulating the different endothelial TRP channels, and the associated development of drugs selectively to target the different isoforms, as a means to treat cardiovascular disease should, therefore, be a high priority.

Recent progress in histochemistry and cell biology: the state of the art 2005

Advances in the field of histochemistry, a multidisciplinary area including the detection, localization and functional characterization of molecules in single cells and complex tissues, often drives the attainment of new knowledge in the broadly defined discipline of cell biology. These two disciplines, histochemistry and cell biology, have been joined in this journal to facilitate the flow of information with celerity from technical advancement in histochemical procedures, to their utilization in experimental models. This review summarizes advancements in these fields during the past year.

Distribution of adherens junction mediated by VE-cadherin complex in rat spleen sinus endothelial cells

The splenic sinus endothelium regulates the passage of blood cells through the splenic cord. The goal of the present study was to assess the localization of vascular endothelial (VE)-cadherin, beta-catenin, and p120-catenin in the sinus endothelial cells of rat spleen and to characterize the presence and distribution of adherens junction formation mediated by the cadherin-catenin complex. Immunofluorescent microscopy of tissue cryosections demonstrated that VE-cadherin, beta-catenin, and p120-catenin were localized in the junctional regions of adjacent endothelial cells. Double-staining immunofluorescent microscopy for VE-cadherin and beta-catenin revealed colocalization at junctional regions. Transmission electron microscopy of thin sections of sinus endothelial cells treated with Triton X-100 clearly showed adherens junctions within the plasma membrane. Adherens junctions were located at various levels in the lateral membranes of adjacent endothelial cells regardless of the presence or absence of underlying ring fibers. Immunogold electron microscopy revealed VE-cadherin, beta-catenin, and p120-catenin in the juxtaposed junctional membranes of adjacent sinus endothelial cells. Double-staining immunogold microscopy for VE-cadherin and beta-catenin and for VE-cadherin and p120-catenin demonstrated colocalization to the junctional membranes of adjacent endothelial cells. Immunolabeling was evident at various levels in the lateral junctional membranes and was intermittently observed in the sinus endothelium. These data suggest that adherens junctions, whose formation appears to be mediated by VE-cadherin-catenin complexes, probably regulate the passage of blood cells through the spleen.