CaT1 knock-down strategies fail to affect CRAC channels in mucosal-type mast cells

Bovine Leukemia Virus Infection Affects Host Gene Expression Associated with DNA Mismatch Repair

Bovine leukemia virus (BLV) causes enzootic bovine leukosis, a malignant form of B-cell lymphoma, and is closely related to human T-cell leukemia viruses. We investigated whether BLV infection affects host genes associated with DNA mismatch repair (MMR). Next-generation sequencing of blood samples from five calves experimentally infected with BLV revealed the highest expression levels of seven MMR genes (EXO1, UNG, PCNA, MSH2, MSH3, MSH6, and PMS2) at the point of peak proviral loads (PVLs). Furthermore, MMR gene expression was only upregulated in cattle with higher PVLs. In particular, the expression levels of MSH2, MSH3, and UNG positively correlated with PVL in vivo. The expression levels of all seven MMR genes in pig kidney-15 cells and the levels of PMS2 and EXO1 in HeLa cells also increased tendencies after transient transfection with a BLV infectious clone. Moreover, MMR gene expression levels were significantly higher in BLV-expressing cell lines compared with those in the respective parental cell lines. Expression levels of MSH2 and EXO1 in BLV-infected cattle with lymphoma were significantly lower and higher, respectively, compared with those in infected cattle in vivo. These results reveal that BLV infection affects MMR gene expression, offering new candidate markers for lymphoma diagnosis.

CRAC inhibitors: identification and potential

BACKGROUND

Ca(2+) release-activated Ca(2+) (CRAC) channels, a subfamily of store-operated channels, play an essential role in various diseases such as immune disorders and allergic responses.

OBJECTIVE

The successful treatment of these diseases requires the identification of specific inhibitors. So far, a variety of chemical compounds blocking CRAC have been identified; however, they have all turned out to be less specific. Recently two proteins, STIM1 and ORAI1, have been identified as the essential components that fully reconstitute CRAC currents with a similar biophysical fingerprint.

METHOD

These two proteins and their activation process represent direct targets for the application of specific CRAC inhibitors.

RESULTS/CONCLUSION

For drug development, fluorescence microscopy adaptable for high-throughput screening will provide a powerful assay to mechanistically identify potential CRAC inhibitors that act on various stages within the STIM1/ORAI1 activation pathway visualized by fluorescent-tagged proteins.

Canonical transient receptor potential (TRPC) 1 acts as a negative regulator for vanilloid TRPV6-mediated Ca2+ influx

TRP proteins mostly assemble to homomeric channels but can also heteromerize, preferentially within their subfamilies. The TRPC1 protein is the most versatile member and forms various TRPC channel combinations but also unique channels with the distantly related TRPP2 and TRPV4. We show here a novel cross-family interaction between TRPC1 and TRPV6, a Ca²⁺ selective member of the vanilloid TRP subfamily. TRPV6 exhibited substantial co-localization and in vivo interaction with TRPC1 in HEK293 cells, however, no interaction was observed with TRPC3, TRPC4, or TRPC5. Ca²⁺ and Na⁺ currents of TRPV6-overexpressing HEK293 cells are significantly reduced by co-expression of TRPC1, correlating with a dramatically suppressed plasma membrane targeting of TRPV6. In line with their intracellular retention, remaining currents of TRPC1 and TRPV6 co-expression resemble in current-voltage relationship that of TRPV6. Studying the N-terminal ankyrin like repeat domain, structurally similar in the two proteins, we have found that these cytosolic segments were sufficient to mediate a direct heteromeric interaction. Moreover, the inhibitory role of TRPC1 on TRPV6 influx was also maintained by expression of only its N-terminal ankyrin-like repeat domain. Our experiments provide evidence for a functional interaction of TRPC1 with TRPV6 that negatively regulates Ca²⁺ influx in HEK293 cells.

The Role of TRP Proteins in Mast Cells

Transient receptor potential (TRP) proteins form cation channels that are regulated through strikingly diverse mechanisms including multiple cell surface receptors, changes in temperature, in pH and osmolarity, in cytosolic free Ca(2+) concentration ([Ca(2+)](i)), and by phosphoinositides which makes them polymodal sensors for fine tuning of many cellular and systemic processes in the body. The 28 TRP proteins identified in mammals are classified into six subfamilies: TRPC, TRPV, TRPM, TRPA, TRPML, and TRPP. When activated, they contribute to cell depolarization and Ca(2+) entry. In mast cells, the increase of [Ca(2+)](i) is fundamental for their biological activity, and several entry pathways for Ca(2+) and other cations were described including Ca(2+) release activated Ca(2+) (CRAC) channels. Like in other non-excitable cells, TRP channels could directly contribute to Ca(2+) influx via the plasma membrane as constituents of Ca(2+) conducting channel complexes or indirectly by shifting the membrane potential and regulation of the driving force for Ca(2+) entry through independent Ca(2+) entry channels. Here, we summarize the current knowledge about the expression of individual Trp genes with the majority of the 28 members being yet identified in different mast cell models, and we highlight mechanisms how they can regulate mast cell functions. Since specific agonists or blockers are still lacking for most members of the TRP family, studies to unravel their function and activation mode still rely on experiments using genetic approaches and transgenic animals. RNAi approaches suggest a functional role for TRPC1, TRPC5, and TRPM7 in mast cell derived cell lines or primary mast cells, and studies using Trp gene knock-out mice reveal a critical role for TRPM4 in mast cell activation and for mast cell mediated cutaneous anaphylaxis, whereas a direct role of cold- and menthol-activated TRPM8 channels seems to be unlikely for the development of cold urticaria at least in mice.

Intracellular Ca Mobilization and Beta-hexosaminidase Release Are Not Influenced by 60 Hz-electromagnetic Fields (EMF) in RBL 2H3 Cells

The effects of extremely low frequency electromagnetic fields (EMF) on intracellular Ca(2+) mobilization and cellular function in RBL 2H3 cells were investigated. Exposure to EMF (60 Hz, 0.1 or 1 mT) for 4 or 16 h did not produce any cytotoxic effects in RBL 2H3 cells. Melittin, ionomycin and thapsigargin each dose-dependently increased the intracellular Ca(2+) concentration. The increase of intracellular Ca(2+) induced by these three agents was not affected by exposure to EMF (60 Hz, 1 mT) for 4 or 16 h in RBL 2H3 cells. To investigate the effect of EMF on exocytosis, we measured beta-hexosaminidase release in RBL 2H3 cells. Basal release of beta-hexosaminidase was 12.3±2.3% in RBL 2H3 cells. Exposure to EMF (60 Hz, 0.1 or 1 mT) for 4 or 16 h did not affect the basal or 1 µM melittin-induced beta-hexosaminidase release in RBL 2H3 cells. This study suggests that exposure to EMF (60 Hz, 0.1 or 1 mT), which is the limit of occupational exposure, has no influence on intracellular Ca(2+) mobilization and cellular function in RBL 2H3 cells.

Gold activates mast cells via calcium influx through multiple H2O2-sensitive pathways including L-type calcium channels

Heavy metals, including gold, induce severe contact hypersensitivity and autoimmune disorders, which develop through an initial Th2-independent process followed by a Th2-dependent process. It has been shown that mast cell activation plays a role in the Th2-independent process and that gold stimulates histamine release in vitro. However, the mechanisms of the gold-induced mast cell activation remain largely unclear. Here we report that gold directly activates mast cells in a Ca2+-dependent manner. HAuCl4 [Au(III)] at nontoxic concentrations (≤50 μM) induced substantial degranulation and leukotriene C4 secretion in an extracellular Ca2+-dependent manner. Au(III) induced a robust Ca2+ influx but not Ca2+ mobilization from internal stores. Au(III) also stimulated intracellular production of reactive oxygen species, including H2O2, and blockade of the production abolished the mediator release and Ca2+ influx. Au(III) induced Ca2+ influx through multiple store-independent Ca2+ channels, including Cav1.2 L-type Ca2+ channels (LTCCs) and 2-aminoethoxydiphenyl borate (2-APB)-sensitive Ca2+ channels. The 2-APB-sensitive channel seemed to mediate Au(III)-induced degranulation. Our results indicate that gold stimulates Ca2+ influx and mediator release in mast cells through multiple H2O2-sensitive Ca2+ channels including LTCCs and 2-APB-sensitive Ca2+ channels. These findings provide insight into the roles of these Ca2+ channels in the Th2-independent process of gold-induced immunological disorders.

Blunted IgE-mediated activation of mast cells in mice lacking the serum- and glucocorticoid-inducible kinase SGK3

Previous studies have shown that pharmacological inhibition of the phosphoinositol-3 (PI3) kinase disrupts the activation of mast cells. Through phosphoinositide-dependent kinase PDK1, PI3 kinase activates the serum- and glucocorticoid-inducible kinase 3 (SGK3). The present study explored the role of SGK3 in mast cell function. Mast cells were isolated and cultured from bone marrow (BMMCs) of gene-targeted mice lacking SGK3 (sgk3(-/-)) and their wild-type littermates (sgk3(+/+)). BMMC numbers in the ear conch were similar in both genotypes. Stimulation with IgE and cognate antigen triggered the release of intracellular Ca(2+) and entry of extracellular Ca(2+). Influx of extracellular Ca(2+) but not Ca(2+) release from intracellular stores was significantly blunted in sgk3(-/-) BMMCs compared with sgk3(+/+) BMMCs. Antigen stimulation further led to a rapid increase of a K(+)-selective conductance in sgk3(+/+) BMMCs, an effect again blunted in sgk3(-/-) BMMCs. In contrast, the Ca(2+) ionophore ionomycin activated K(+) currents to a similar extent in sgk3(-/-) and in sgk3(+/+) BMMCs. β-Hexosaminidase release, triggered by antigen stimulation, was also significantly decreased in sgk3(-/-) BMMCs. IgE-dependent anaphylaxis measured as a sharp decrease in body temperature upon injection of DNP-HSA antigen was again significantly blunted in sgk3(-/-) compared with sgk3(+/+) mice. Serum histamine levels measured 30 min after induction of an anaphylactic reaction were significantly lower in sgk3(-/-) than in sgk3(+/+) mice. In conclusion, both in vitro and in vivo function of BMMCs are impaired in gene targeted mice lacking SGK3. Thus SGK3 is critical for proper mast cell function.

CRAC channelopathies

Store-operated Ca2+ entry (SOCE) is an important Ca2+ influx pathway in many non-excitable and some excitable cells. It is regulated by the filling state of intracellular Ca2+ stores, notably the endoplasmic reticulum (ER). Reduction in [Ca2+]ER results in activation of plasma membrane Ca2+ channels that mediate sustained Ca2+ influx which is required for many cell functions as well as refilling of Ca2+ stores. The Ca2+ release activated Ca2+ (CRAC) channel is the best characterized SOC channel with well-defined electrophysiological properties. In recent years, the molecular components of the CRAC channel, long mysterious, have been defined. ORAI1 (or CRACM1) acts as the pore-forming subunit of the CRAC channel in the plasma membrane. Stromal interaction molecule (STIM) 1 is localized in the ER, senses [Ca2+]ER, and activates the CRAC channel upon store depletion by binding to ORAI1. Both proteins are widely expressed in many tissues in both human and mouse consistent with the widespread prevalence of SOCE and CRAC channel currents in many cells types. CRAC channelopathies in human patients with mutations in STIM1 and ORAI1 are characterized by abolished CRAC channel currents, lack of SOCE and-clinically-immunodeficiency, congenital myopathy, and anhydrotic ectodermal dysplasia. This article reviews the role of ORAI and STIM proteins for SOCE and CRAC channel function in a variety of cell types and tissues and compares the phenotypes of ORAI1 and STIM1-deficient human patients and mice with targeted deletion of Orai and Stim genes.

L-type Ca2+ channels: a new player in the regulation of Ca2+ signaling, cell activation and cell survival in immune cells

Ca(2+) is a highly versatile intracellular second messenger in many cell types, and regulates many complicated cellular processes, including cell activation, proliferation and apoptosis. Influx of Ca(2+) from the extracellular fluid is required for sustained elevation of the cytosolic Ca(2+) concentration and full activation of Ca(2+)-dependent processes. It is widely accepted that Ca(2+) release-activated Ca(2+) channels are the major routes of Ca(2+) influx in electrically non-excitable cells, including hematopoietic cells, whereas voltage-gated Ca(2+) channels such as L-type Ca(2+) channels (LTCCs) serve as the principal routes of Ca(2+) entry into electrically excitable cells such as neurons and myocytes. However, recent pharmacological and molecular genetic studies have revealed the existence of functional LTCCs and/or LTCC-like channels in a variety of immune cells including mast cells. In this article, we review recent advances in our understanding of Ca(2+) signaling in immune cells with a special interest in mast cells. We highlight roles for LTCCs in antigen receptor-mediated mast cell activation and survival.

TRPC channels and store-operated Ca(2+) entry

Store-operated calcium entry (SOCE) is a major mechanism for Ca(2+) influx. Since SOCE was first proposed two decades ago many techniques have been used in attempting to identify the nature of store-operated Ca(2+) (SOC) channels. The first identified and best-characterised store-operated current is I(CRAC), but a number of other currents activated by Ca(2+) store depletion have also been described. TRPC proteins have long been proposed as SOC channel candidates; however, whether any of the TRPCs function as SOC channels remains controversial. This review attempts to provide an overview of the arguments in favour and against the role of TRPC proteins in the store-operated mechanisms of agonist-activated Ca(2+) entry.

Mechanisms and regulation of epithelial Ca2+ absorption in health and disease

Ca2+ is essential for numerous physiological functions in our bodies. Therefore, its homeostasis is finely maintained through the coordination of intestinal absorption, renal reabsorption, and bone resorption. The Ca2+-selective epithelial channels TRPV5 and TRPV6 have been identified, and their physiological roles have been revealed: TRPV5 is important in final renal Ca2+ reabsorption, and TRPV6 has a key role in intestinal Ca2+ absorption. The TRPV5 knockout mice exhibit renal leak hypercalciuria and accordingly upregulate their intestinal TRPV6 expression to compensate for their negative Ca2+ balance. In contrast, despite their severe negative Ca2+ balance, TRPV6-null mice do not display any compensatory mechanism, thus resulting in secondary hyperparathyroidism. These results indicate that the genes for TRPV5 and TRPV6 are differentially regulated in human diseases associated with disturbed Ca2+ balance such as hypercalciuria, osteoporosis, and vitamin D-resistant rickets.

The first ankyrin-like repeat is the minimum indispensable key structure for functional assembly of homo- and heteromeric TRPC4/TRPC5 channels

The closely related TRPC4 and TRPC5 proteins, members of the canonical transient receptor potential (TRPC) family, assemble into either homo- or heterotetrameric, non-selective cation-channels. To elucidate domains that mediate channel complex formation, we evaluated dominant negative effects of N- or C-terminal TRPC4/5 fragments on respective currents of full-length proteins overexpressed in HEK293 cells with whole-cell electrophysiology. Confocal Förster Resonance Energy Transfer (FRET) measurements enabled to probe the interaction potential of these CFP/YFP-labelled fragments in vivo. Only N-terminal fragments that included the first ankyrin-like repeat potently down-regulated TRPC4/TRPC5 currents, while fragments including either the second ankyrin-like repeat and the coiled-coil domain or the C-terminus remained ineffective. Total internal reflection fluorescence (TIRF) microscopy data suggested that the dominant negative N-terminal fragments led to a predominantly intracellular localisation of coexpressed TRPC5 proteins. FRET measurements clearly revealed that only fragments including the first ankyrin-like repeat were able to multimerise. Moreover a TRPC5 mutant that lacked the first ankyrin-like repeat was unable to homo-multimerise, failed to interact with wild-type TRPC5 and resulted in non-functional channels.

Assembly domains in TRP channels

The large family of mammalian TRP (transient receptor potential) ion channels encompasses diverse sensory functions. TRP proteins consist of six transmembrane domains, with a pore-loop motif between the fifth and sixth domains and cytosolic N- and C-termini. The intracellular strands not only interact with various proteins and lipids, but also include essential multimerization regions. This review summarizes the current knowledge of the intrinsic assembly domains that assure tetrameric TRP channel formation.

TRP channels in cancer

The progression of cells from a normal differentiated state in which rates of proliferation and apoptosis are balanced to a tumorigenic and metastatic state involves the accumulation of mutations in multiple key signalling proteins and the evolution and clonal selection of more aggressive cell phenotypes. These events are associated with changes in the expression of numerous other proteins. This process of tumorigenesis involves the altered expression of one or more TRP proteins, depending on the nature of the cancer. The most clearly described changes are those involving TRPM8, TRPV6 and TRPM1. Expression of TRPM8 is substantially increased in androgen-dependent prostate cancer cells, but is decreased in androgen independent and metastatic prostate cancer. TRPM8 expression is regulated, in part, by androgens, most likely through androgen response elements in the TRPM8 promoter region. TRPM8 channels are involved in the regulation of cell proliferation and apoptosis. Expression of TRPV6 is also increased in prostate cancer and in a number of other cancers. In contrast to TRPM8, expression of TRPV6 is not directly regulated by androgens. TRPM1 is highly expressed in early stage melanomas but its expression declines with increases in the degree of aggressiveness of the melanoma. The expression of TRPV1, TRPC1, TRPC6, TRPM4, and TRPM5 is also increased in some cancers. The level of expression of TRPM8 and TRPV6 in prostate cancer, and of TRPM1 in melanomas, potentially provides a good prognostic marker for predicting the course of the cancer in individuals. The Drosophila melanogaster, TRPL, and the TRPV1 and TRPM8 proteins, have been used to try to develop strategies to selectively kill cancer cells by activating Ca(2+) and Na(+) entry, producing a sustained increase in the cytoplasmic concentration of these ions, and subsequent cell death by apoptosis and necrosis. TRPV1 is expressed in neurones involved in sensing cancer pain, and is a potential target for pharmacological inhibition of cancer pain in bone metastases, pancreatic cancer and most likely in other cancers. Further studies are required to assess which other TRP proteins are associated with the development and progression of cancer, what roles TRP proteins play in this process, and to develop further knowledge of TRP proteins as targets for pharmaceutical intervention and targeting in cancer.

Ion channels in death and differentiation of prostate cancer cells

Plasma membrane ion channels contribute to virtually all basic cellular processes, including such crucial ones for maintaining tissue homeostasis as proliferation, differentiation, and apoptosis. Enhanced proliferation, aberrant differentiation, and impaired ability to die are the prime reasons for abnormal tissue growth, which can eventually turn into uncontrolled expansion and invasion, characteristic of cancer. Prostate cancer (PCa) cells express a variety of plasma membrane ion channels. By providing the influx of essential signaling ions, perturbing intracellular ion concentrations, regulating cell volume, and maintaining membrane potential, PCa cells are critically involved in proliferation, differentiation, and apoptosis. PCa cells of varying metastatic ability can be distinguished by their ion channel characteristics. Increased malignancy and invasiveness of androgen-independent PCa cells is generally associated with the shift to a 'more excitable' phenotype of their plasma membrane. This shift is manifested by the appearance of voltage-gated Na(+) and Ca(2+) channels which contribute to their enhanced apoptotic resistance together with downregulated store-operated Ca(2+) influx, altered expression of different K(+) channels and members of the Transient Receptor Potential (TRP) channel family, and strengthened capability for maintaining volume constancy. The present review examines channel types expressed by PCa cells and their involvement in metastatic behaviors.

Link between TRPV channels and mast cell function

Mast cells are tissue-resident immune effector cells. They respond to diverse stimuli by releasing potent biological mediators into the surrounding tissue, and initiating inflammatory responses that promote wound healing and infection clearance. In addition to stimulation via immunological routes, mast cells also respond to polybasic secretagogues and physical stimuli. Each mechanism for mast cell activation relies on the influx of calcium through specific ion channels in the plasma membrane. Recent reports suggest that several calcium-permeant cation channels of the TRPV family are expressed in mast cells. TRPV channels are a family of sensors that receive and react to chemical messengers and physical environmental cues, including thermal, osmotic, and mechanical stimuli. The central premise of this review is that TRPVs transduce physiological and pathophysiological cues that are functionally coupled to calcium signaling and mediator release in mast cells. Inappropriate mast cell activation is at the core of numerous inflammatory pathologies, rendering the mast cell TRPV channels potentially important therapeutic targets.

TRPV6

The ion channel TRPV6 is likely to function as an epithelial calcium channel in organs with high calcium transport requirements such as the intestine, kidney, and placenta. Transcriptional regulation of TRPV6 messenger RNA (mRNA) is controlled by 1,25-dihydroxyvitamin D, which is the active hormonal form of vitamin D3, and by additional calcium-dependent and vitamin D3-independent mechanisms. Under physiological conditions, the conductance of the channel itself is highly calcium-selective and underlies complex inactivation mechanisms triggered by intracellular calcium and magnesium ions. There is growing evidence that transcriptional regulation of TRPV6 in certain tissues undergoing malignant transformation, such as prostate cancer, is linked to cancer progression.

TRPC channels: integrators of multiple cellular signals

TRPC channels are ubiquitously expressed among cell types and mediate signals in response to phospholipase C (PLC)-coupled receptors. TRPC channels function as integrators of multiple signals resulting from receptor-induced PLC activation, which catalyzes the breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) to produce inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DAG). InsP3 depletes Ca2+ stores and TRPC3 channels can be activated by store-depletion. InsP3 also activates the InsP3 receptor, which may undergo direct interactions with the TRPC3 channel, perhaps mediating store-dependence. The other PLC product, DAG, has a direct non-PKC-dependent activating role on TRPC3 channels likely by direct binding. DAG also has profound effects on the TRPC3 channel through PKC. Thus PKC is a powerful inhibitor of most TRPC channels and DAG is a dual regulator of the TRPC3 channel. PLC-mediated DAG results in rapid channel opening followed later by a slower DAG-induced PKC-mediated deactivation of the channel. The decreased level of PIP2 from PLC activation also has an important modifying action on TRPC3 channels. Thus, the TRPC3 channel and PLCgamma form an intermolecular PH domain that has high specificity for binding PIP2. This interaction allows the channel to be retained within the plasma membrane, a further operational control factor for TRPC3. As nonselective cation channels, TRPC channel opening results in the entry of both Na+ and Ca2+ ions. Thus, while they may mediate Ca2+ entry signals, TRPC channels are also powerful modifiers of membrane potential.

Dynamic but not constitutive association of calmodulin with rat TRPV6 channels enables fine tuning of Ca2+-dependent inactivation

The Ca(2+)-selective TRPV6 as well as the L-type Ca(2+) channel are regulated by the Ca(2+)-binding protein calmodulin (CaM). Here, we investigated the interaction of CaM with rat (r)TRPV6 in response to alterations of intracellular Ca(2+), employing Ca(2+)-imaging and patch-clamp techniques. Additionally, confocal Förster resonance energy transfer (FRET) microscopy on living cells was utilized as a key method to visualize in vivo protein-protein interactions essential for CaM regulation of rTRPV6 activity. The effects of overexpressed CaM or its Ca(2+)-insensitive mutant (CaM(MUT)) was probed on various rTRPV6 mutants and fragments in an attempt to elucidate the molecular mechanism of Ca(2+)/CaM-dependent regulation and to pinpoint the physiologically relevant rTRPV6-CaM interaction site. A significant reduction of rTRPV6 activity, as well as an increase in current inactivation, were observed when CaM was overexpressed in addition to endogenous CaM. The Ca(2+)-insensitive CaM(MUT), however, failed to affect rTRPV6-derived currents. Accordingly, live cell confocal FRET microscopy revealed a robust interaction for CaM but not CaM(MUT) with rTRPV6, suggesting a strict Ca(2+) dependence for their association. Indeed, interaction of rTRPV6 or its C terminus with CaM increased with rising intracellular Ca(2+) levels, as observed by dynamic FRET measurements. An rTRPV6Delta(695-727) mutant with the very C-terminal end deleted, yielded Ca(2+) currents with a markedly reduced inactivation in accordance with a lack of CaM interaction as substantiated by FRET microscopy. These results, in contrast with those for CaM-dependent L-type Ca(2+) channel inactivation, demonstrate a dynamic association of CaM with the very C-terminal end of rTRPV6 (aa 695-727), and this enables acceleration of the rate of rTRPV6 current inactivation with increasing intracellular CaM concentrations.

C-terminal modulator controls Ca2+-dependent gating of Cav1.4 L-type Ca2+ channels

Tonic neurotransmitter release at sensory cell ribbon synapses is mediated by calcium (Ca2+) influx through L-type voltage-gated Ca2+ channels. This tonic release requires the channels to inactivate slower than in other tissues. Ca(v)1.4 L-type voltage-gated Ca2+ channels (LTCCs) are found at high densities in photoreceptor terminals, and alpha1 subunit mutations cause human congenital stationary night blindness type-2 (CSNB2). Ca(v)1.4 voltage-dependent inactivation is slow and Ca2+-dependent inactivation (CDI) is absent. We show that removal of the last 55 or 122 (C122) C-terminal amino acid residues of the human alpha1 subunit restores calmodulin-dependent CDI and shifts voltage of half-maximal activation to more negative potentials. The C terminus must therefore form part of a mechanism that prevents calmodulin-dependent CDI of Ca(v)1.4 and controls voltage-dependent activation. Fluorescence resonance energy transfer experiments in living cells revealed binding of C122 to C-terminal motifs mediating CDI in other Ca2+ channels. The absence of this modulatory mechanism in the CSNB2 truncation mutant K1591X underlines its importance for normal retinal function in humans.

A severe defect in CRAC Ca2+ channel activation and altered K+ channel gating in T cells from immunodeficient patients

Engagement of the TCR triggers sustained Ca²⁺ entry through Ca²⁺ release-activated Ca²⁺ (CRAC) channels, which helps drive gene expression underlying the T cell response to pathogens. The identity and activation mechanism of CRAC channels at a molecular level are unknown. We have analyzed ion channel expression and function in T cells from SCID patients which display 1–2% of the normal level of Ca²⁺ influx and severely impaired T cell activation. The lack of Ca²⁺ influx is not due to deficient regulation of Ca²⁺ stores or expression of several genes implicated in controlling Ca²⁺ entry in lymphocytes (kcna3/Kv1.3, kcnn4/IKCa1, trpc1, trpc3, trpv6, stim1). Instead, electrophysiologic measurements show that the influx defect is due to a nearly complete absence of functional CRAC channels. The lack of CRAC channel activity is correlated with diminished voltage sensitivity and slowed activation kinetics of the voltage-dependent Kv1.3 channel. These results demonstrate that CRAC channels provide the major, if not sole, pathway for Ca²⁺ entry activated by the TCR in human T cells. They also offer evidence for a functional link between CRAC and Kv1.3 channels, and establish a model system for molecular genetic studies of the CRAC channel.

Knocking down transport: applications of RNA interference in the study of drug transport proteins

RNA interference (RNAi) is a gene silencing process mediated by double-stranded RNA (dsRNA). The silencing process is comprised of an initiation step, in which small interfering RNA (siRNA) is introduced to the cell, and an effector step, which involves degrading mRNA molecules of the target gene. RNA interference has been observed in most organisms from plants to vertebrates. As a gene silencing approach, RNAi has proven to be extremely useful in characterizing gene function and developing new tools in cancer therapy and drug delivery. The development of RNAi-related technologies is an emerging area in biomedical research. In this review, recent progress in the application of RNAi to the study of transport proteins is summarized and evaluated; the advantages, disadvantages and future directions of RNAi technology are discussed.

Structure-function analysis of TRPV channels

In recent years many new members of the family of TRP ion channels have been identified. These channels are classified into several subgroups and participate in many sensory and physiological functions. TRPV channels are important for the perception of pain, temperature sensing, osmotic regulation, and maintenance of calcium homeostasis, and much recent research concerns the identification of protein domains involved in mediating specific channel functions. Recent literature on TRPV channel subunit composition, protein domains required for subunit assembly, trafficking, and regulation will be reviewed and discussed.

A store-operated nonselective cation channel in human lymphocytes

1. Agonist interaction with phospholipase C-linked receptors at the plasma membrane can elicit both Ca2+ and Na+ influxes in lymphocytes. While Ca2+ influx is mediated by Ca2+ release-activated Ca2+ (CRAC) channels, the pathway responsible for Na+ influx is largely unknown. 2. We show that thapsigargin, ionomycin, ADP-ribose and IP3 activated a nonselective cation channel in lymphocytes that had a slightly outwardly rectifying I-V relationship, and a single channel conductance of 23.1 pS. We termed this channel a Ca2+ release-activated nonselective cation (CRANC) channel. 3. On activation in cell-attached configuration, switching to an inside-out configuration abolished CRANC channel activity. 4. Transfection of Jurkat T cells with antisense oligonucleotides for LTRPC2 reduced capacitative Ca2+ entry. 5. These results suggest that CRANC channels are responsible for the Na+ influx as well as a portion of the Ca2+ influx in lymphocytes induced by store depletion, that sustained activation of CRANC channels requires some property of the environment of a cell depleted of its Ca2+ stores; and that LTRPC2 protein is a likely component of the CRANC channel.

STIM1, an essential and conserved component of store-operated Ca2+ channel function

Store-operated Ca2+ (SOC) channels regulate many cellular processes, but the underlying molecular components are not well defined. Using an RNA interference (RNAi)-based screen to identify genes that alter thapsigargin (TG)-dependent Ca2+ entry, we discovered a required and conserved role of Stim in SOC influx. RNAi-mediated knockdown of Stim in Drosophila S2 cells significantly reduced TG-dependent Ca2+ entry. Patch-clamp recording revealed nearly complete suppression of the Drosophila Ca2+ release-activated Ca2+ (CRAC) current that has biophysical characteristics similar to CRAC current in human T cells. Similarly, knockdown of the human homologue STIM1 significantly reduced CRAC channel activity in Jurkat T cells. RNAi-mediated knockdown of STIM1 inhibited TG- or agonist-dependent Ca2+ entry in HEK293 or SH-SY5Y cells. Conversely, overexpression of STIM1 in HEK293 cells modestly enhanced TG-induced Ca2+ entry. We propose that STIM1, a ubiquitously expressed protein that is conserved from Drosophila to mammalian cells, plays an essential role in SOC influx and may be a common component of SOC and CRAC channels.

Store-Operated Ca2+ Entry Channels: Still Elusive!

Depletion of intracellular Ca2+ stores is believed to trigger Ca2+ entry through a Ca2+- permeable channel in the plasma membrane called the store-operated Ca2+ channel (SOC). This type of Ca2+ entry is thought to play a pivotal role in a plethora of cell functions ranging from gene expression to sensory signal transduction. However, the molecular nature of these channels is still elusive. Many molecular candidates have been described as SOCs, but the candidacy of each has been countered by reports indicating that they are not SOCs. Most of the suggested candidates are members of the recently discovered superfamily of transient receptor potential cation channels (TRPCs). However, no TRP-family channel has yet been incontestably identified as an SOC. Even for the electrophysiologically best-described SOC, the highly Ca2+-permeable channel Ca2+ release-activated Ca2+ channel (CRAC), an acceptable candidate has yet to emerge. This perspective summarizes problems in identifying SOCs, suggests approaches to solving these problems, and discusses concerns over the current tendency to focus exclusively on the hypothesis that TRPCs comprise SOCs.

Two types of store-operated Ca2+ channels with different activation modes and molecular origin in LNCaP human prostate cancer epithelial cells

The one or more coupling mechanisms of store-operated channels (SOCs) to endoplasmic reticulum (ER) Ca2+ store depletion as well as the molecular identity of SOCs per se still remain a mystery. Here, we demonstrate the co-existence of two populations of molecular distinct endogenous SOCs in LNCaP prostate cancer epithelial cells, which are preferentially activated by either active inositol 1,4,5-trisphosphate (IP3)-mediated or passive thapsigargin-facilitated store depletion and have different ER store content sensitivity. The first population, called SOC(CC) (for "conformational coupling"), is characterized by preferential IP3 receptor-dependent mode of activation, as judged from sensitivity to cytoskeleton modifications, and dominant contribution of transient receptor potential (TRP) TRPC1 within it. The second one, called SOC(CIF) (for "calcium influx factor"), depends on Ca(2+)-independent phospholipase A2 for activation with probable CIF involvement and is mostly represented by TRPC4. The previously identified SOC constituent in LNCaP cells, TRPV6, seems to play equal role in both SOC populations. These results provide new insight into the nature of SOCs and their representation in the single cell type as well as permit reconciliation of current SOC activation hypotheses.