A novel mechanism for the store-operated calcium influx pathway

Calcium Channels and Ca2+ Fluctuations in Sperm Physiology

Generating new life in animals by sexual reproduction depends on adequate communication between mature and competent male and female gametes. Ion channels are instrumental in the dialogue between sperm, its environment, and the egg. The ability of sperm to swim to the egg and fertilize it is modulated by ion permeability changes induced by environmental cues and components of the egg outer layer. Ca(2+) is probably the key messenger in this information exchange. It is therefore not surprising that different Ca(2+)-permeable channels are distinctly localized in these tiny specialized cells. New approaches to measure sperm currents, intracellular Ca(2+), membrane potential, and intracellular pH with fluorescent probes, patch-clamp recordings, sequence information, and heterologous expression are revealing how sperm channels participate in fertilization. Certain sperm ion channels are turning out to be unique, making them attractive targets for contraception and for the discovery of novel signaling complexes.

Ca2+-calmodulin-dependent facilitation and Ca2+ inactivation of Ca2+ release-activated Ca2+ channels

In non-excitable cells, one major route for Ca2+ influx is through store-operated Ca2+ channels in the plasma membrane. These channels are activated by the emptying of intracellular Ca2+ stores, and in some cell types store-operated influx occurs through Ca2+ release-activated Ca2+ (CRAC) channels. Here, we report that intracellular Ca2+ modulates CRAC channel activity through both positive and negative feedback steps in RBL-1 cells. Under conditions in which cytoplasmic Ca2+ concentration can fluctuate freely, we find that store-operated Ca2+ entry is impaired either following overexpression of a dominant negative calmodulin mutant or following whole-cell dialysis with a calmodulin inhibitory peptide. The peptide had no inhibitory effect when intracellular Ca2+ was buffered strongly at low levels. Hence, Ca2+-calmodulin is not required for the activation of CRAC channels per se but is an important regulator under physiological conditions. We also find that the plasma membrane Ca2+ATPase is the dominant Ca2+ efflux pathway in these cells. Although the activity of the Ca2+ pump is regulated by calmodulin, the store-operated Ca2+ entry is more sensitive to inhibition by the calmodulin mutant than by Ca2+ extrusion. Hence, these two plasmalemmal Ca2+ transport systems may differ in their sensitivities to endogenous calmodulin. Following the activation of Ca2+ entry, the rise in intracellular Ca2+ subsequently feeds back to further inhibit Ca2+ influx. This slow inactivation can be activated by a relatively brief Ca2+ influx (30-60 s); it reverses slowly and is not altered by overexpression of the calmodulin mutant. Hence, the same messenger, intracellular Ca2+, can both facilitate and inactivate Ca2+ entry through store-operated CRAC channels and through different mechanisms.

Calmidazolium and arachidonate activate a calcium entry pathway that is distinct from store-operated calcium influx in HeLa cells

Agonists that deplete intracellular Ca2+ stores also activate Ca2+ entry, although the mechanism by which store release and Ca2+ influx are linked is unclear. A potential mechanism involves 'store-operated channels' that respond to depletion of the intracellular Ca2+ pool. Although SOCE (store-operated Ca2+ entry) has been considered to be the principal route for Ca2+ entry during hormonal stimulation of non-electrically excitable cells, recent evidence has suggested that alternative pathways activated by metabolites such as arachidonic acid are responsible for physiological Ca2+ influx. It is not clear whether such messenger-activated pathways exist in all cells, whether they are truly distinct from SOCE and which metabolites are involved. In the present study, we demonstrate that HeLa cells express two pharmacologically and mechanistically distinct Ca2+ entry pathways. One is the ubiquitous SOCE route and the other is an arachidonate-sensitive non-SOCE. We show that both these Ca2+ entry pathways can provide long-lasting Ca2+ elevations, but that the channels are not the same, based on their differential sensitivity to 2-aminoethoxydiphenyl borate, LOE-908 [(R,S)-(3,4-dihydro-6,7-dimethoxy-isochinolin-1-yl)-2-phenyl-N,N-di[2-(2,3,4-trimethoxyphenyl)ethyl]acetamid mesylate] and gadolinium. In addition, non-SOCE and not SOCE was permeable to strontium. Furthermore, unlike SOCE, the non-SOCE pathway did not require store depletion and was not sensitive to displacement of the endoplasmic reticulum from the plasma membrane using jasplakinolide or ionomycin pretreatment. These pathways did not conduct Ca2+ simultaneously due to the dominant effect of arachidonate, which rapidly curtails SOCE and promotes Ca2+ influx via non-SOCE. Although non-SOCE could be activated by exogenous application of arachidonate, the most robust method for stimulation of this pathway was application of the widely used calmodulin antagonist calmidazolium, due to its ability to activate phospholipase A2.

Age-related changes in the response of intestinal cells to parathyroid hormone

The concept of the role(s) of parathyroid hormone (PTH), has expanded from that on acting on the classical target tissues, bone and kidney, to the intestine where its actions are of regulatory and developmental importance: regulation of intracellular calcium through modulation of second messengers and, activation of mitogenic cascades leading to cell proliferation. Several causes have been postulated to modify the hormone response in intestinal cells with ageing, among them, alterations of PTH receptor (PTHR1) binding sites, reduced expression of G proteins and hormone signal transduction changes. The current review summarizes the actual knowledge regarding the molecular and biochemical basis of age-impaired PTH receptor-mediated signaling in intestinal cells. A fundamental understanding of why PTH functions are impaired with age will enhance our understanding of its importance in intestinal cell physiology.

Calcium-calcineurin signaling in the regulation of cardiac hypertrophy

Cardiac hypertrophy is a leading predicator of progressive heart disease that often leads to heart failure and a loss of cardiac contractile performance associated with profound alterations in intracellular calcium handling. Recent investigation has centered on identifying the molecular signaling pathways that regulate cardiac myocyte hypertrophy, as well as the mechanisms whereby alterations in calcium handling are associated with progressive heart failure. One potential focal regulator of cardiomyocyte hypertrophy that also responds to altered calcium handling is the calmodulin-activated serine/threonine protein phosphatase calcineurin (PP2B). Once activated by increases in calcium, calcineurin mediates the hypertrophic response through its downstream transcriptional effector nuclear factor of activated T cells (NFAT), which is directly dephosphorylated by calcineurin resulting in nuclear translocation. While previous studies have convincingly demonstrated the sufficiency of calcineurin to mediate cardiac hypertrophy and progressive heart failure, its necessity remains an area of ongoing investigation. Here we weigh an increasing body of literature that suggests a causal link between calcineurin signaling and the cardiac hypertrophic response and heart failure through the use of pharmacologic inhibitors (cyclosporine A and FK506) and genetic approaches. We will also discuss the manner in which calcineurin-NFAT signaling is negatively regulated in the heart through a diverse array of kinases and inhibitory proteins. Finally, we will discuss emerging theories as to the mechanisms whereby alterations in intracellular calcium handling might stimulate calcineurin within the context of a contractile cell continually experiencing calcium flux.

Stimulation of beta-adrenoceptors inhibits store-operated channel currents via a cAMP-dependent protein kinase mechanism in rabbit portal vein myocytes

Previously we have described the properties of store-operated channel currents (SOCs) in freshly dispersed rabbit portal vein smooth muscle cells. In addition to Ca(2+) store depletion these SOCs could also be activated by alpha-adrenoceptor stimulation and diacylglycerol (DAG) via a protein kinase C (PKC)-dependent mechanism. In the present study we have investigated the effect of beta-adrenoceptor stimulation on SOCs in rabbit portal vein myocytes. With whole-cell recording the selective beta-adrenoceptor agonist isoprenaline reduced the current evoked by cyclopiazonic acid (CPA, sarcoplasmic/endoplasmic reticulum ATPase inhibitor) by over 85%. With cell-attached patch recording, bath application of isoprenaline produced a pronounced inhibition of SOC activity evoked by either CPA or the acetoxymethyl ester form of BAPTA (BAPTA-AM). SOC activity evoked by CPA, the DAG analogue, 1-oleoyl-acetyl-sn-glycerol (OAG) or the phorbol ester, phorbol-12,13-dibutyrate (PDBu) was also markedly inhibited by the adenylate cyclase activator, forskolin, and the cell-permeable non-hydrolysable analogue of cyclic adenosine monophosphate (cAMP), 8-Br-cAMP. With inside-out patches, bath application of PDBu evoked channel currents with similar properties to SOCs which were inhibited by over 90% by a catalytic subunit of protein kinase A (PKA) and by 8-Br-cAMP. Moreover bath application of PKA inhibitors, H-89, KT5720 and an inhibitory peptide to quiescent cell-attached or inside-out patches, activated channel currents with similar properties to SOCs. These data suggest that in rabbit portal vein myocytes, stimulation of beta-adrenoceptors inhibits SOC activity via a cAMP-dependent protein kinase signal transduction cascade. In addition it is concluded that constitutive PKA activity has a profound inhibitory effect on SOC activity in this vascular preparation.

Lysophosphatidic acid triggers calcium entry through a non-store-operated pathway in human neutrophils

Lysophosphatidic acid (LPA) is a bioactive lipid, which is structurally similar to sphingosine 1-phosphate (S1P) and which can mobilize Ca2+ in multiple cell types. We recently showed that S1P induces Ca2+ entry directly through store-operated Ca2+ entry (SOCE) channels in human polymorphonuclear neutrophils (PMN). We therefore examined the mechanisms by which LPA induces intracellular Ca2+ mobilization in PMN. External application of low micromolar LPA caused dose-dependent Ca2+ influx without releasing Ca2+ stores, whereas G-protein-coupled (GPC) LPA receptors respond to nanomolar LPA. Additive Ca2+ influx by LPA compared with 100 nM ionomycin-induced Ca2+ influx suggests that LPA-induced Ca2+ influx does not pass through SOCE channels. Ca2+ influx was resistant to inhibition of Gi/o by pertussis toxin, of phospholipase C by U73122, and of G12/13/Rho by Y27632, all demonstrating GPC receptor independence. This Ca2+ influx was inhibited by Gd3+, La3+, Zn2+, or MRS1845 but not by Ni2+ or the sphingosine kinase inhibitor dimethylsphingosine. In addition, we found that LPA has no effect on neutrophil chemotaxis; however, it has stimulatory effects on neutrophil respiratory burst in a dose-response manner. These findings suggest that LPA-induced Ca2+ influx in PMN occurs through a mechanism other than SOCE channels, independent of Ca2+ store-depletion and S1P synthesis, and that the characteristics of LPA-induced Ca2+ influx are similar to those of S1P-induced influx in terms of sensitivity to inorganic inhibitors. Unlike S1P, LPA has stimulatory effects on neutrophil respiratory burst.

Lysophosphatidic acid is a mediator of Trp-Lys-Tyr-Met-Val-d-Met-induced calcium influx

Intracellular calcium (Ca(2+)) homeostasis is very strictly regulated, and the activation of G-protein-coupled receptor (GPCR) can cause two different calcium changes, intracellular calcium release, and calcium influx. In this study, we investigated the possible role of lysophosphatidic acid (LPA) on GPCR-induced Ca(2+) signaling. The addition of exogenous LPA induced dramatic Ca(2+) influx but not intracellular Ca(2+) release in U937 cells. LPA-induced Ca(2+) influx was not affected by pertussis toxin and phospholipase C inhibitor (U73122), ruling out the involvement of pertussis toxin-sensitive G-proteins, and phospholipase C. Stimulation of U937 cells with Trp-Lys-Tyr-Met-Val-D-Met (WKYMVm), which binds to formyl peptide receptor like 1, enhanced phospholipase A(2) and phospholipase D activation, indicating LPA formation. The inhibition of LPA synthesis by phospholipase A(2)-specific inhibitor (MAFP) or n-butanol significantly inhibited WKYMVm-induced Ca(2+) influx, suggesting a crucial role for LPA in the process. Taken together, we suggest that LPA mediates WKYMVm-induced Ca(2+) influx.

Inhibition of Ca2+-independent phospholipase A2 results in insufficient insulin secretion and impaired glucose tolerance

Islet Ca2+-independent phospholipase A2 (iPLA2) is postulated to mediate insulin secretion by releasing arachidonic acid in response to insulin secretagogues. However, the significance of iPLA2 signaling in insulin secretion in vivo remains unexplored. Here we investigated the physiological role of iPLA2 in beta-cell lines, isolated islets, and mice. We showed that small interfering RNA-specific silencing of iPLA2 expression in INS-1 cells significantly reduced insulin-secretory responses of INS-1 cells to glucose. Immunohistochemical analysis revealed that mouse islet cells expressed significantly higher levels of iPLA2 than pancreatic exocrine acinar cells. Bromoenol lactone (BEL), a selective inhibitor of iPLA2, inhibited glucose-stimulated insulin secretion from isolated mouse islets; this inhibition was overcome by exogenous arachidonic acid. We also showed that iv BEL administration to mice resulted in sustained hyperglycemia and reduced insulin levels during glucose tolerance tests. Clamp experiments demonstrated that the impaired glucose tolerance was due to insufficient insulin secretion rather than decreased insulin sensitivity. Short-term administration of BEL to mice had no effect on fasting glucose levels and caused no apparent pathological changes of islets in pancreas sections. These results unambiguously demonstrate that iPLA2 signaling plays an important role in glucose-stimulated insulin secretion under physiological conditions.

Non-selective cationic channels of smooth muscle and the mammalian homologues of Drosophila TRP

Throughout the body there are smooth muscle cells controlling a myriad of tubes and reservoirs. The cells show enormous diversity and complexity compounded by a plasticity that is critical in physiology and disease. Over the past quarter of a century we have seen that smooth muscle cells contain--as part of a gamut of ion-handling mechanisms--a family of cationic channels with significant permeability to calcium, potassium and sodium. Several of these channels are sensors of calcium store depletion, G-protein-coupled receptor activation, membrane stretch, intracellular Ca2+, pH, phospholipid signals and other factors. Progress in understanding the channels has, however, been hampered by a paucity of specific pharmacological agents and difficulty in identifying the underlying genes. In this review we summarize current knowledge of these smooth muscle cationic channels and evaluate the hypothesis that the underlying genes are homologues of Drosophila TRP (transient receptor potential). Direct evidence exists for roles of TRPC1, TRPC4/5, TRPC6, TRPV2, TRPP1 and TRPP2, and more are likely to be added soon. Some of these TRP proteins respond to a multiplicity of activation signals--promiscuity of gating that could enable a variety of context-dependent functions. We would seem to be witnessing the first phase of the molecular delineation of these cationic channels, something that should prove a leap forward for strategies aimed at developing new selective pharmacological agents and understanding the activation mechanisms and functions of these channels in physiological systems.

Male mice that do not express group VIA phospholipase A2 produce spermatozoa with impaired motility and have greatly reduced fertility

The Group VIA Phospholipase A(2) (iPLA(2)beta) is the first recognized cytosolic Ca(2+)-independent PLA(2) and has been proposed to participate in arachidonic acid (20:4) incorporation into glycerophosphocholine lipids, cell proliferation, exocytosis, apoptosis, and other processes. To study iPLA(2)beta functions, we disrupted its gene by homologous recombination to generate mice that do not express iPLA(2)beta. Heterozygous iPLA(2)beta(+/-) breeding pairs yield a Mendelian 1:2:1 ratio of iPLA(2)beta(+/+), iPLA(2)beta(+/-), and iPLA(2)beta(-/-) pups and a 1:1 male:female gender distribution of iPLA(2)beta(-/-) pups. Several tissues of wild-type mice express iPLA(2)beta mRNA, immunoreactive protein, and activity, and testes express the highest levels. Testes or other tissues of iPLA(2)beta(-/-) mice express no iPLA(2)beta mRNA or protein, but iPLA(2)beta(-/-) testes are not deficient in 20:4-containing glycerophosphocholine lipids, indicating that iPLA(2)beta does not play an obligatory role in formation of such lipids in that tissue. Spermatozoa from iPLA(2)beta(-/-) mice have reduced motility and impaired ability to fertilize mouse oocytes in vitro and in vivo, and inhibiting iPLA(2)beta with a bromoenol lactone suicide substrate reduces motility of wild-type spermatozoa in a time- and concentration-dependent manner. Mating iPLA(2)beta(-/-) male mice with iPLA(2)beta(+/+), iPLA(2)beta(+/-), or iPLA(2)beta(-/-) female mice yields only about 7% of the number of pups produced by mating pairs with an iPLA(2)beta(+/+) or iPLA(2)beta(+/-) male, but iPLA(2)beta(-/-) female mice have nearly normal fertility. These findings indicate that iPLA(2)beta plays an important functional role in spermatozoa, suggest a target for developing male contraceptive drugs, and complement reports that disruption of the Group IVA PLA(2) (cPLA(2)alpha) gene impairs female reproductive ability.

Store-operated channels: diversity and activation mechanisms

This perspective addresses two questions: How many store-operated channels (SOCs) are there, and how many mechanisms can account for SOC activation by depleted stores? Accumulating evidence suggests that the SOC family is not limited to the calcium-selective SOC that is responsible for ICRAC (Ca2+-SOC), but includes poorly selective cation SOCs (cat-SOCs) that may satisfy physiological needs in diverse excitable and nonexcitable cells. A growing number of studies in different cell types support the idea that all the members of SOC family (Ca2+-SOC and cat-SOC) may be activated by depletion of the stores through the same mechanism, which is mediated by calcium influx factor (CIF) and calcium-independent phospholipase A2 (iPLA2). A conformational coupling model is also discussed. To account for the most recent findings, we propose that two distinct classes of calcium-conducting channels may exist in plasma membrane, which respond to different signals: SOCs, which are activated by depletion of calcium stores through the CIF-iPLA2 mechanism [no inositol triphosphate (IP3) needed]; and IP3 receptor-operated channels (IP3ROCs), which are activated by IP3 receptor through a direct coupling mechanism (no store depletion is needed). This model, with two separate mechanisms linked to different channels, may resolve many conflicting findings and interpretations and may give a new perspective on the diversity of calcium influx pathways.

Toward a consensus on the operation of receptor-induced calcium entry signals

Receptor-induced Ca2+ signals involve both Ca2+ release from intracellular stores and extracellular Ca2+ entry across the plasma membrane. The channels mediating Ca2+ entry and the mechanisms controlling their function remain largely a mystery. Here we critically assess current views on the Ca2+ entry process and consider certain modifications to the widely held hypothesis that Ca2+ store emptying is the fundamental trigger for receptor-induced Ca2+ entry channels. Under physiological conditions, receptor-induced store depletion may be quite limited. A number of distinct channel activities appear to mediate receptor-induced Ca2+ entry, and their activation is observed to occur through quite diverse coupling processes.

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.

A role for SNAP-25 but not VAMPs in store-mediated Ca2+ entry in human platelets

Store-mediated Ca2+ entry (SMCE) is a major mechanism for Ca2+ influx in non-excitable cells. Recently, a conformational coupling mechanism allowing coupling between transient receptor potential channels (TRPCs) and IP3 receptors has been proposed to activate SMCE. Here we have investigated the role of two soluble N-ethylmaleimide-sensitive-factor attachment protein receptors (SNAREs), which are involved in membrane trafficking and docking, in SMCE in human platelets. We found that the synaptosome-associated protein (SNAP-25) and the vesicle-associated membrane proteins (VAMP) coimmunoprecipitate with hTRPC1 in platelets. Treatment with botulinum toxin (BoNT) E or with tetanus toxin (TeTx), induced cleavage and inactivation of SNAP-25 and VAMPs, respectively. BoNTs significantly reduced thapsigargin- (TG) and agonist-evoked SMCE. Treatment with BoNTs once SMCE had been activated decreased Ca2+ entry, indicating that SNAP-25 is required for the activation and maintenance of SMCE. In contrast, treatment with TeTx had no effect on either the activation or the maintenance of SMCE in platelets. Finally, treatment with BoNT E impaired the coupling between naturally expressed hTRPC1 and IP3 receptor type II in platelets. From these findings we suggest SNAP-25 has a role in SMCE in human platelets.