[Purification of the mitochondrial calcium uniporter from the beef heart and characterization of its properties]

A low-molecular-weight component (LMC) inducing selective transport of calcium across the bilayer lipid membrane has been isolated from mitochondria of the bovine heart by the method developed in our laboratory, which excludes the use of detergents and proteolytic enzymes. It was shown that, in the presence of 10 mM CaCl2, LMC forms conduction channels in the membrane multiples of 5 pS. The specific inhibitor of mitochondrial calcium uniporter, ruthenium red, closes Ca2(+)-induced channels formed in the membrane by LMC. In the absence of calcium or in the presence of potassium ions only, the component is incapable of forming channels of conduction. It was shown using nuclear magnetic resonance that LMC is a complex consisting of lipids, amino acids, and sugars with a molecular weight of 1-2 kDa.

Rapid functional assays of recombinant IP3 receptors

Functional assays of inositol 1,4,5-trisphosphate receptors (IP3R) currently use 45Ca2+ release methods, fluorescent Ca2+ indicators within either the ER or cytosol, or electrophysiological analyses of IP3R in the nuclear envelope or artificial bilayers. None of the methods is presently amenable to the rapid, high-throughput quantitative analyses of IP3R function needed to address the structural determinants of IP3R behavior. We use a low-affinity Ca2+ indicator (Mag-fluo-4) to measure free [Ca2+] within the ER of permeabilized DT40 cells expressing only rat type 1 IP(3)R, and establish that the indicator is capable of reliably reporting the Ca(2+) release evoked by IP3. A 96-well fluorescence plate reader equipped for automated fluid additions (FlexStation, Molecular Devices) is used to monitor IP3-evoked Ca2+ release. The method allows quick and economical functional assays of recombinant IP3R in small volumes (< or = 100 microl).

Expression, purification and crystallization of a functional core of the voltage-dependent calcium channel β subunit

Two versions of the functional core of the rabbit voltage-dependent calcium channel beta2a subunit were expressed in Escherichia coli. These proteins were purified to homogeneity and screened for crystallization. Crystallization conditions were refined using the hanging-drop vapour-diffusion method and two crystal forms were pursued. Crystal form I is represented by thick rods with tetragonal symmetry, unit-cell parameters a = b = 75, c = 165 A and a diffraction limit of 3.4 A which were obtained using ammonium sulfate as a precipitant. Crystal form II gives rise to plates with orthorhombic symmetry, unit-cell parameters a = 35, b = 75, c = 165 A and a diffraction limit of 2.3 A which were grown using polyethylene glycol 20K as a precipitant.

Identification of a putative voltage-gated Ca2+ -permeable channel (OsTPC1) involved in Ca2+ influx and regulation of growth and development in rice

Cytosolic free Ca2+ serves as an important second messenger participating in signal transduction of various environmental stresses. However, molecular bases for the plasma membrane Ca2+ influx and its regulation remain largely unknown. We here identified a gene (OsTPC1) encoding a putative voltage-gated Ca2+ channel from rice, ubiquitously expressed in mature leaves, shoots and roots as well as in cultured cells. OsTPC1 rescued the Ca2+ uptake activity and growth rate of a yeast mutant cch1. To elucidate its physiological roles, we generated transgenic rice plants and cultured cells overexpressing OsTPC1 mRNA. Furthermore, a retrotransposon (Tos17) insertional knockout mutant of OsTPC1 was isolated. OsTPC1-overexpressing cells showed hypersensitivity to excess Ca2+ but higher growth rate under Ca2+ limitation, while growth of the OsTPC1-knockout cultured cells was less sensitive to extracellular free Ca2+ concentration, suggesting that OsTPC1 has Ca2+ transport activity across the plasma membrane. OsTPC1-overexpressing plants showed reduced growth and abnormal greening of roots. Growth of Ostpc1 seedlings was comparable to the control on agar plates, while significantly reduced in adult plants. These results suggest that OsTPC1 functions as a Ca2+ -permeable channel involved in the regulation of growth and development.

Identification of human and mouse CatSper3 and CatSper4 genes: characterisation of a common interaction domain and evidence for expression in testis

BACKGROUND

CatSper1 and CatSper2 are two recently identified channel-like proteins, which show sperm specific expression patterns. Through targeted mutagenesis in the mouse, CatSper1 has been shown to be required for fertility, sperm motility and for cAMP induced Ca2+ current in sperm. Both channels resemble a single pore forming repeat from a four repeat voltage dependent Ca2+ /Na+ channel. However, neither CatSper1 or CatSper2 have been shown to function as cation channels when transfected into cells, singly or in conjunction. As the pore forming units of voltage gated cation channels form a tetramer it has been suggested that the known CatSper proteins require additional subunits and/or interaction partners to function.

RESULTS

Using in silico gene identification and prediction techniques, we have identified two further members of the CatSper family, CatSper3 and Catsper4. Each carries a single channel-forming domain with the predicted pore-loop containing the consensus sequence TxDxW. Each of the new CatSper genes has evidence for expression in the testis. Furthermore we identified coiled-coil protein-protein interaction domains in the C-terminal tails of each of the CatSper channels, implying that CatSper channels 1,2,3 and 4 may interact directly or indirectly to form a functional tetramer.

CONCLUSIONS

The topological and sequence relationship of CatSper1 and CatSper2 to the four repeat Ca2+ /Na+ channels suggested other members of this family may exist. We have identified a further two novel CatSper genes, conserved in both the human and mouse genomes. Furthermore, all four of the CatSper proteins are predicted to contain a common coiled-coil protein-protein interaction domain in their C-terminal tail. Coupled with expression data this leads to the hypothesis that the CatSper proteins form a functional hetero-tetrameric channel in sperm.

painless, a Drosophila gene essential for nociception

We describe a paradigm for nociception in Drosophila. In response to the touch of a probe heated above 38 degrees C, Drosophila larvae produce a stereotypical rolling behavior, unlike the response to an unheated probe. In a genetic screen for mutants defective in this noxious heat response, we identified the painless gene. Recordings from wild-type larval nerves identified neurons that initiated strong spiking above 38 degrees C, and this activity was absent in the painless mutant. The painless mRNA encodes a protein of the transient receptor potential ion channel family. Painless is required for both thermal and mechanical nociception, but not for sensing light touch. painless is expressed in peripheral neurons that extend multiple branched dendrites beneath the larval epidermis, similar to vertebrate pain receptors. An antibody to Painless binds to localized dendritic structures that we hypothesize are involved in nociceptive signaling.

Domain organization of the type 1 inositol 1,4,5-trisphosphate receptor as revealed by single-particle analysis

The inositol 1,4,5-trisphosphate receptor (IP(3)R) is a tetrameric intracellular Ca(2+) channel, which mediates the release of Ca(2+) from the endoplasmic reticulum in response to many different extracellular stimuli. We present a 3D structure of the type 1 IP(3)R obtained by electron microscopy and single-particle analysis that reveals its domain organization. The IP(3)R has a flower-like appearance with fourfold symmetry and is made up of three distinct domains connected by slender links. By relating the organization of the structural domains to secondary-structure predictions and biochemical data we develop a model in which structural domains are mapped onto the amino acid sequence to deduce the location of the channel region and the cytoplasmic inositol 1,4,5-trisphosphate-binding and modulatory subdomains. The structure of the IP(3)R is compared with that of other tetrameric cation channels. The channel domain is similar in size and shape to its counterparts in the ryanodine receptor and the Shaker voltage-gated K(+) channel.

Three-dimensional structure of the type 1 inositol 1,4,5-trisphosphate receptor at 24 A resolution

We report here the first three-dimensional structure of the type 1 inositol 1,4,5-trisphosphate receptor (IP(3)R). From cryo-electron microscopic images of purified receptors embedded in vitreous ice, a three-dimensional structure was determined by use of standard single particle reconstruction techniques. The structure is strikingly different from that of the ryanodine receptor at similar resolution despite molecular similarities between these two calcium release channels. The 24 A resolution structure of the IP(3)R takes the shape of an uneven dumbbell, and is approximately 170 A tall. Its larger end is bulky, with four arms protruding laterally by approximately 50 A and, in comparison with the receptor topology, probably corresponds to the cytoplasmic domain of the receptor. The lateral dimension at the height of the protruding arms is approximately 155 A. The smaller end, whose lateral dimension is approximately 100 A, has structural features indicative of the membrane-spanning domain. A central opening in this domain, which is occluded on the cytoplasmic half, outlines a pathway for calcium flow in the open state of the channel.

Use of a purified and functional recombinant calcium-channel beta4 subunit in surface-plasmon resonance studies

Native high-voltage-gated calcium channels are multi-subunit complexes comprising a pore-forming subunit Ca(v) and at least two auxiliary subunits alpha(2)delta and beta. The beta subunit facilitates cell-surface expression of the channel and contributes significantly to its biophysical properties. In spite of its importance, detailed structural and functional studies are hampered by the limited availability of native beta subunit. Here, we report the purification of a recombinant calcium-channel beta(4) subunit from bacterial extracts by using a polyhistidine tag. The purified protein is fully functional since it binds on the alpha1 interaction domain, its main Ca(v)-binding site, and regulates the activity of P/Q calcium channel expressed in Xenopus oocytes in a similar way to the beta(4) subunit produced by cRNA injection. We took advantage of the functionality of the purified material to (i) develop an efficient surface-plasmon resonance assay of the interaction between two calcium channel subunits and (ii) measure, for the first time, the affinity of the recombinant His-beta(4) subunit for the full-length Ca(v)2.1 channel. The availability of this purified material and the development of a surface-plasmon resonance assay opens two immediate research perspectives: (i) drug screening programmes applied to the Ca(v)/beta interaction and (ii) crystallographic studies of the calcium-channel beta(4) subunit.

Activation of the inositol 1,4,5-trisphosphate receptor by the calcium storage protein chromogranin A

Secretory granules of neuroendocrine cells are inositol 1,4,5-trisphosphate (InsP(3))-sensitive Ca(2+) stores in which the Ca(2+) storage protein, chromogranin A (CGA), couples with InsP(3)-gated Ca(2+) channels (InsP(3)R) located in the granule membrane. The functional aspect of this coupling has been investigated via release studies and planar lipid bilayer experiments in the presence and absence of CGA. CGA drastically increased the release activity of the InsP(3)R by increasing the channel open probability by 9-fold and the mean open time by 12-fold. Our results show that CGA-coupled InsP(3)Rs are more sensitive to activation than uncoupled receptors. This modulation of InsP(3)R channel activity by CGA appears to be an essential component in the control of intracellular Ca(2+) concentration by secretory granules and may regulate the rate of vesicle fusion and exocytosis.

Subconductance activity induced by quinidine and quinidinium in purified cardiac sarcoplasmic reticulum calcium release channels

This study examined the effects of quinidine, quinine, and the quaternary quinidine derivative, quinidinium, on the conductance and activity of purified cardiac sarcoplasmic reticulum calcium release channels/ryanodine receptors (RyR) incorporated into planar lipid bilayers. Quinidine (50-500 microM) reduced the single-channel open probability in a voltage- and concentration-dependent manner. Reduction of channel activity was evident only at positive holding potentials where current flow is from the cytoplasmic to luminal side of the channel and when the drug was present only on the cytoplasmic face of the channel. A more pronounced effect was the appearance of a subconductance state at positive potentials. Single channel recordings and dose-response experiments revealed that at least two quinidine molecules were involved in reduction of the RyR activity. The permanently charged quinidinium compound produced nearly identical effects as quinidine when present only on cytoplasmic side of the channel, suggesting the positive-charged form of quinidine is responsible for the effects on the channel. There was no stereospecificity in the effects of quinidine because the levoisomer, 100 microM quinine, produced a similar subconductance activity of the channel. Ryanodine modification of the channel prevented subconductance activity. These findings suggest that the quinidine-induced subconductance activity may be the result of a partial occlusion of the channel pore interfering with ion conduction. Modification of the channel by ryanodine alters quinidine binding to the channel through a conformational change in protein structure.

Purification and characterization of receptors for myoinositol trisphosphate and myoinositol tetrakis phosphate from Entamoeba histolytica

The microsomal fraction from the log phase of Entamoeba histolytica cells contains Ins(1,4,5)P3 and Ins(1,3,4,5)P4 binding activity. The binding proteins/receptors for both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 were purified and found to be specific for each ligand. The molecular masses for native proteins for InsP3 and InsP4 are 138 kDa and 130 kDa respectively having subunits of 69 kDa and 64 kDa respectively. That these proteins are associated with Ca2+ release was confirmed by including these proteins separately in proteoliposomes and adding InsP3 and InsP4 in both the cases.

[Molecular structure and function of mechanosensor mechanisms: an alternative prospect for the development of new drugs]

Mechanical stress to cells induces various physiological cellular responses and pathophysiological changes in many cell types. However, the molecular mechanisms remain unclear. Clarification of the mechanosensor mechanisms on the molecular level may provide an alternative approach for the development of new drugs. In the 73rd Annual Meeting of The Japanese Pharmacological Society, the latest studies performed by the following five departments were reported and discussed: 1) Expression of putative stretch sensitive nonselective cation channels of mammal, by M. Suzuki et al. (Department of Pharmacology, Jichi Medical School); 2) Mechanosensitive ATP release in aortic endothelial cells, by M. Oike et al. (Department of Pharmacology, Graduate School of Medical Science, Kyushu University); 3) Lysophosphatidic acid acts as an endogenous modulator on mechanotransduction, by H. Ohata et al. (Department of Pharmacology, School of Pharmaceutical Sciences, Showa University); 4) Stretch-induced myosin light chain phosphorylation without force development in canine basilar artery, by K. Obara et al. (Department of Pharmacology, School of Pharmaceutical Sciences, University of Shizuoka); and 5) Transcriptional regulation of smooth muscle phenotypic modulation, by R. Nagai (Department of Cardiovascular Medicine, University of Tokyo). This article reviews these reports.

Greater diversity than previously thought of chromaffin cell Ca2+ channels, derived from mRNA identification studies

Using reverse transcription followed by PCR amplification (RT-PCR), we have identified multiple messenger RNAs encoding for the neuronal pore-forming Ca(2+) channel subunits alpha(1A) (P/Q channel), alpha(1B) (N channel), alpha(1D) (neuronal/endocrine L channel), alpha(1E) (R channel), alpha(1G-H) (T channel) and alpha(1S) (skeletal muscle L channel) in bovine chromaffin cells. mRNAs for the auxiliary beta(2), beta(3), beta(4), alpha(2)/delta and gamma(2) subunits were also identified. In agreement with these molecular data, perforated patch-clamp recordings of whole-cell Ca(2+) currents reveal the existence of functional R-type Ca(2+) channels in these cells that were previously undetected with other techniques. Our results provide a molecular frame for a much wider functional diversity of Ca(2+) channels in chromaffin cells than that previously established using pharmacological and electrophysiological approaches.

The mammalian Sec6/8 complex interacts with Ca(2+) signaling complexes and regulates their activity

The localization of various Ca(2+) transport and signaling proteins in secretory cells is highly restricted, resulting in polarized agonist-stimulated Ca(2+) waves. In the present work, we examined the possible roles of the Sec6/8 complex or the exocyst in polarized Ca(2+) signaling in pancreatic acinar cells. Immunolocalization by confocal microscopy showed that the Sec6/8 complex is excluded from tight junctions and secretory granules in these cells. The Sec6/8 complex was found in at least two cellular compartments, part of the complex showed similar, but not identical, localization with the Golgi apparatus and part of the complex associated with Ca(2+) signaling proteins next to the plasma membrane at the apical pole. Accordingly, immunoprecipitation (IP) of Sec8 did not coimmunoprecipitate betaCOP, Golgi 58K protein, or mannosidase II, all Golgi-resident proteins. By contrast, IP of Sec8 coimmunoprecipitates Sec6, type 3 inositol 1,4,5-trisphosphate receptors (IP(3)R3), and the Gbetagamma subunit of G proteins from pancreatic acinar cell extracts. Furthermore, the anti-Sec8 antibodies coimmunoprecipitate actin, Sec6, the plasma membrane Ca(2+) pump, the G protein subunits Galphaq and Gbetagamma, the beta1 isoform of phospholipase C, and the ER resident IP(3)R1 from brain microsomal extracts. Antibodies against the various signaling and Ca(2+) transport proteins coimmunoprecipitate Sec8 and the other signaling proteins. Dissociation of actin filaments in the immunoprecipitate had no effect on the interaction between Sec6 and Sec8, but released the actin and dissociated the interaction between the Sec6/8 complex and Ca(2+) signaling proteins. Hence, the interaction between the Sec6/8 and Ca(2+) signaling complexes is likely mediated by the actin cytoskeleton. The anti-Sec6 and anti-Sec8 antibodies inhibited Ca(2+) signaling at a step upstream of Ca(2+) release by IP(3). Disruption of the actin cytoskeleton with latrunculin B in intact cells resulted in partial translocation of Sec6 and Sec8 from membranes to the cytosol and interfered with propagation of agonist-evoked Ca(2+) waves. Our results suggest that the Sec6/8 complex has multiple roles in secretory cells including governing the polarized expression of Ca(2+) signaling complexes and regulation of their activity.

Electrophysiologic characteristics of the Ca-permeable channels, ECaC and CaT, in the kidney

To investigate the molecular mechanism of Ca transport in the kidney, we have isolated Ca-permeable channels, rECaC (rat ECaC) and mCaT (mouse CaT1), from rodent kidney, which are recently reported as Ca-transporting proteins. RT-PCR suggested the presence of CaT1 in medullary tubules. It showed 67% homology with rECaC constructing a family. Whole cellular currents in Chinese hamster ovary (CHO) cells were measured by patch clamp. Expression of both proteins exhibited a similar large cation current, a high permeability to Ca, a time-dependent rapid inactivation, and a "run-down." When the pipet contained EGTA, the inactivation and the run-down did not occur. Addition of db-cAMP activated and following rp-cAMPS recovered the mCaT-induced current significantly, whereas no influence was observed in the rECaC-induced one. We conclude that ECaC and CaT are a molecular family of ion channel with similar characteristics, contributing Ca transport in the kidney.

Isolation and characterization of a calcium channel gene, Cacna1f, the murine orthologue of the gene for incomplete X-linked congenital stationary night blindness

The mutant L-type calcium channel alpha(1)-subunit gene, CACNA1F, was recently identified as the gene responsible for incomplete X-linked congenital stationary night blindness. The 6070-bp mRNA transcript is predicted to encode a 1977-amino-acid pore-forming protein with cytoplasmic amino- and carboxyl-termini separated by four homologous repeat domains, each consisting of six transmembrane segments. CACNA1F has been shown to be preferentially expressed in the retina, indicative of a specific functional role in visual processing. We have established the complete sequence of the murine orthologue of CACNA1F, namely Cacna1f. The total length of the mRNA transcript of the murine gene was established to be 6080 bp with an open reading frame that translates into a 1985-amino-acid protein. Cacna1f is highly homologous to the human sequence, with 90% identity at the amino acid level and almost perfect conservation between the functional domains. Furthermore, as in the human gene, the 3' end of the Cacna1f gene maps within 5 kb of the 5' end of the mouse synaptophysin gene in a region orthologous to Xp11.23. Using in situ hybridization, Cacna1f was found to be expressed in the inner and outer nuclear layers and the ganglion cell layer of the retina.

Inositol 1,4,5-trisphosphate receptor/Ca2+ channel modulatory role of chromogranin A, a Ca2+ storage protein of secretory granules

The secretory granules of neuroendocrine cells, which contain large amounts of Ca(2+) and chromogranins, have been demonstrated to release Ca(2+) in response to inositol 1,4,5-trisphosphate (IP(3)), indicating the IP(3)-sensitive intracellular Ca(2+) store role of secretory granules. In our previous study, chromogranin A (CGA) was shown to interact with several secretory granule membrane proteins, including the IP(3) receptor (IP(3)R), at the intravesicular pH 5.5 (Yoo, S. H. (1994) J. Biol. Chem. 269, 12001-12006). To examine the functional aspect of this coupling, we measured the IP(3)-mediated Ca(2+) release property of the IP(3)R reconstituted into liposomes in the presence and absence of CGA. Presence of CGA in the IP(3)R-reconstituted liposome significantly enhanced the IP(3)-mediated Ca(2+) release from the liposomes. Moreover, the number of IP(3) bound to the reconstituted IP(3)R increased. The fluorescence energy transfer and IP(3)R Trp fluorescence quenching studies indicated that the structure of reconstituted IP(3)R becomes more ordered and exposed in the presence of CGA, suggesting that the coupled CGA in the liposome caused structural changes of the IP(3)R, changing it to a structure that is better suited to IP(3) binding and subsequent Ca(2+) release. These results appear to underscore the physiological significance of IP(3)R-CGA coupling in the secretory granules.

Heterooligomer of type 1 and type 2 inositol 1, 4, 5-trisphosphate receptor expressed in rat liver membrane fraction exists as tetrameric complex

Functional IP(3)-sensitive intracellular Ca(2+) release channel is considered to be a tetramer of IP(3)R. Heterooligomeric complexes composed of distinct types of IP(3)R have been reported, however, crucial evidences for them being tetramer have not appeared. Here we report that the heterooligomer composed of IP(3)R1 and IP(3)R2 also exists as tetramer. Cross-linked heterooligomer was immunoprecipitated with IP(3)R1-specific antibody and detected by agarose-PAGE/Western blot analysis with IP(3)R2-specific antibody. Tetramer, trimer, dimer, and possibly monomer were detected. The trimer, dimer, and monomer were likely to be originated from the tetramer, since: (1) the immunoprecipitating antibody (IP(3)R1-specific) does not recognize IP(3)R2, therefore IP(3)R2 monomer itself could not have been immunoprecipitated; and (2) tetramer was the major native product of IP(3)R complex containing type 2 isoform in liver membrane fraction. Thus we conclude tetramer is the native form of heterooligomer composed of IP(3)R1 and IP(3)R2.

Molecular cloning and characterization of TRPC5 (HTRP5), the human homologue of a mouse brain receptor-activated capacitative Ca2+ entry channel

A novel human gene, TRPC5, was cloned from the region of Xq23 that contains loci for nonsyndromic mental retardation (MRX47 and MRX35) and two genes, DCX and HPAK3, implicated in two X-linked disorders (LISX and MRX30). Within a single YAC, we have determined the order cen-HPAK3(5'-3')-DCX(3'-5')-DXS7012E-TRPC5(3'-5' )-ter. TRPC5 encodes a 974-residue novel human protein (111.5 kDa predicted mass) and displays 99% homology with mouse TRP5, (MGD-approved symbol Trrp5) a novel member of a family of receptor-activated Ca2+ channels. It contains eight transmembrane domains, including a putative pore region. A transcript larger than 9.5 kb is observed only in fetal and adult human brain, with a relatively higher level in the adult human cerebellum. We devised an efficient method, Incorporation PCR SSCP (IPS), for detection of gene alterations. Five single-nucleotide variations in the TRPC5 gene were identified in males with mental retardation. However, these were found to be polymorphic variants. Exclusive expression of the TRPC5 gene in developing and adult brain suggests a possible role during development and provides a candidate gene for instances of mental retardation and other developmental defects.

Syntaxin 1 interacts with the L(D) subtype of voltage-gated Ca(2+) channels in pancreatic beta cells

Interaction of syntaxin 1 with the alpha(1D) subunit of the voltage-gated L type Ca(2+) channel was investigated in the pancreatic beta cell. Coexpression of the enhanced green fluorescent protein-linked alpha(1D) subunit with the enhanced blue fluorescent protein-linked syntaxin 1 and Western blot analysis together with subcellular fractionation demonstrated that the alpha(1D) subunit and syntaxin 1 were colocalized in the plasma membrane. Furthermore, the alpha(1D) subunit was coimmunoprecipitated efficiently by a polyclonal antibody against syntaxin 1. Syntaxin 1 also played a central role in the modulation of L type Ca(2+) channel activity because there was a faster Ca(2+) current run-down in cells incubated with antisyntaxin 1 compared with controls. In parallel, antisyntaxin 1 markedly reduced insulin release in both intact and permeabilized cells, subsequent to depolarization with K(+) or exposure to high Ca(2+). Exchanging Ca(2+) for Ba(2+) abolished the effect of antisyntaxin 1 on both Ca(2+) channel activity and insulin exocytosis. Moreover, antisyntaxin 1 had no significant effects on Ca(2+)-independent insulin release trigged by hypertonic stimulation. This suggests that there is a structure-function relationship between the alpha(1D) subunit of the L type Ca(2+) channel and the exocytotic machinery in the pancreatic beta cell.

Isolation and functional characterization of the 5'-upstream region of mouse P/Q-type Ca2+ channel alpha1A subunit gene

The omega-agatoxin-IVA-sensitive P/Q-type Ca2+ channel is predominantly expressed in the nervous system. To dissect the molecular mechanisms underlying the neuron-specific expression of the P/Q-type channel, we have isolated and characterized the 5'-upstream region of the mouse alpha1A subunit gene. A transcription start site appeared to exist at -269 bp upstream from the start codon as found by 5' RACE analysis. The proximal promoter of the alpha1A subunit gene lacks a typical TATA box, but contains several transcription factor binding sequences, including two Sp1 sites. When linked to a placental alkaline phosphatase (PLAP) reporter gene to examine the promoter activity, the 6.3-kb (-6,273 to +269) 5'-upstream region, but not a smaller 3.0-kb construct (-3, 021 to +269), was able to drive the reporter gene in neuron-like PC12 cells. In contrast, neither of these constructs enhanced the PLAP expression in fibroblast NIH3T3 cells. The sequence between 6.3 and 3.0 kb of the 5'-upstream region did not show promoter activity in either of the cell lines, but enhanced TK promoter activity in PC12 cells, though not in NIH3T3 cells. These results suggest that neuron-specific elements of the alpha1A subunit gene are likely to be located in the distal upstream regions (-6,273 to -3,021) of the 5'-upstream sequence.

N-type calcium channel/syntaxin/SNAP-25 complex probed by antibodies to II-III intracellular loop of the alpha1B subunit

Neuronal voltage-dependent calcium channels are integral components of cellular excitation and neurosecretion. In addition to mediating the entry of calcium across the plasma membrane, both N-type and P/Q-type voltage-dependent calcium channels have been shown to form stable complexes with synaptic vesicle and presynaptic membrane proteins, indicating a structural role for the voltage-dependent calcium channels in secretion. Recently, detailed structural analyses of N-type calcium channels have identified residues amino acids 718-963 as the site in the rat alpha1B subunit that mediates binding to syntaxin, synaptosome-associated protein of 25,000 mol. wt and synaptotagmin [Sheng et al. (1996) Nature 379, 451-454]. The purpose of this study was to employ site-directed antibodies to target domains within and outside of the interaction site on the rat alpha1B to probe potential binding sites for syntaxin/SNAP-25/synaptotagmin. Our results demonstrate that both antibodies employed in this study have access to their epitopes on the alpha1B as evidenced by equivalent immunoprecipitation of native [125I]omega-conotoxin GVIA-labeled alpha1B protein from CHAPS-solubilized preparations. The N-type voltage-dependent calcium channel immunoprecipitated by Ab CW14, the antibody directed to a domain outside of the synprint site, is associated with syntaxin and SNAP-25 with the recovery of these proteins, increasing in parallel to the recovery of alpha1B. However, when we used the antibody raised to an epitope within the synprint site (Ab CW8) to immunoprecipitate N-type calcium channels, the alpha1B was depleted of more than 65% of syntaxin and 80% of SNAP-25 when compared to the recovery of these proteins using Ab CW14. This is the first report of a defined epitope on the alpha1B subunit II-III loop (amino acids 863-875) whose perturbation by a site-directed antibody influences the dissociation of SNAP-25 and syntaxin.

Interactions between proteins implicated in exocytosis and voltage-gated calcium channels

Neurotransmitter release from synaptic vesicles is triggered by voltage-gated calcium influx through P/Q-type or N-type calcium channels. Purification of N-type channels from rat brain synaptosomes initially suggested molecular interactions between calcium channels and two key proteins implicated in exocytosis: synaptotagmin I and syntaxin 1. Co-immunoprecipitation experiments were consistent with the hypothesis that both N- and P/Q-type calcium channels, but not L-type channels, are associated with the 7S complex containing syntaxin 1, SNAP-25, VAMP and synaptotagmin I or II. Immunofluorescence confocal microscopy at the frog neuromuscular junction confirmed that calcium channels, syntaxin 1 and SNAP-25 are co-localized at active zones of the presynaptic plasma membrane where transmitter release occurs. Experiments with recombinant proteins were performed to map synaptic protein interaction sites on the alpha 1A subunit, which forms the pore of the P/Q-type calcium channel. In vitro-translated 35S-synaptotagmin I bound to a site located on the cytoplasmic loop linking homologous domains II and III of the alpha 1A subunit. This direct link would target synaptotagmin, a putative calcium sensor for exocytosis, to a microdomain of calcium influx close to the channel mouth. Cysteine string proteins (CSPs) contain a J-domain characteristic of molecular chaperones that cooperate with Hsp70. They are located on synaptic vesicles and thought to be involved in modulating the activity of presynaptic calcium channels. CSPs were found to bind to the same domain of the calcium channel as synaptotagmin, and also to associate with VAMP. CSPs may act as molecular chaperones in association with Hsp70 to direct assembly or dissociation of multiprotein complexes at the calcium channel.

Properties of a new calcium-permeable single channel from tracheal microsomes

After the incorporation of the tracheal microsomal membrane into bilayer lipid membrane (BLM), a new single channel permeable for calcium was observed. Using the BLM conditions, 53 mM Ca2+ in trans solution versus 200 nM Ca2+ in cis solution, the single calcium channel current at 0 mV was 1.4-2.1 pA and conductance was 62-75 pS. The channel Ca2+/K+ permeability ratio was 4.8. The open probability (P-open) was in the range of 0.7-0.97. The P-open, measured at -10 mV to +30 mV (trans-cis), was not voltage dependent. The channel was neither inhibited by 10-20 microM ruthenium red, a specific blocker of ryanodine calcium release channel, nor by 10-50 microM heparin, a specific blocker of IP3 receptor calcium release channel, and its activity was not influenced by addition of 0.1 mM MgATP. We suggest that the observed new channel is permeable for calcium, and it is neither identical with the known type 1 or 2 ryanodine calcium release channel, nor type 1 or 2 IP3 receptor calcium release channel.

Effective diameter and structural organization of reconstituted calcium channels from the Characeae algae Nitellopsis

Using hydrophilic non-electrolytes, we determined the effective diameter of calcium channels from the Characeae alga Nitellopsis, reconstituted in a planar lipid membrane. It is suggested that a "single" reconstituted channel is a cluster consisting of several protochannels, in which the channel-forming molecules are stabilized by calcium ions. It is shown that the channels assembled in a cluster with the common conductivity of 350 pS (in 0.1 M KCl) have a funnel-like structure. The diameter of the smaller section is 0.96 +/- 0.20 nm, the diameter of the wider vestibule is 1.78 +/- 0.30 nm. Similarity of the structural organization of the channels investigated to the channels formed by annexin proteins is discussed.

Voltage-dependent anion channel proteins in synaptosomes of the torpedo electric organ: immunolocalization, purification, and characterization

In this study, we purified and characterized the voltage-dependent anion channel (VDAC) from the Torpedo electric organ. Using immunogold labeling, VDAC was colocalized with the voltage-gated Ca2+ channel in the synaptic plasma membrane. By immunoblot analysis, five protein bands in synaptosomes isolated from the Torpedo electric organ cross reacted with two monoclonal anti-VDAC antibody. No more than about 7 to 10% mitochondrial contains could be detected in any synaptosomal membrane preparation tested. This was estimated by comparing the specific activity in mitochondria and synaptosomes of succinate-cytochrome-c oxidoreductase and antimycin-insensitive NADH-cytochrome-c oxidoreductase activities; mitochondrial inner and outer membrane marker enzymes, respectively. [14C]DCCD (dicyclohexylcarbodiimide), which specifically label mitochondrial VDAC, labeled four 30-35 kDa protein bands that were found to interact with the anti-VDAC antibody. The distribution of the Torpedo VDAC protein bands was different among membranes isolated from various tissues. VDAC was purified from synaptosomes and a separation between two of the proteins was obtained. The two purified proteins were characterized by their single channel activity and partial amino acid sequences. Upon reconstitution into a planar lipid bilayer, the purified VDACs showed voltage-dependent channel activity with properties similar to those of purified mitochondrial VDAC. Amino acid sequence of four peptides, derived from VDAC band II, exhibited high homology to sequences present in human VDACI (98%), VDAC2 (91.8%), and VDAC3 (90%), while another peptide, derived from VDAC band III, showed lower homology to either VDAC1 (88.4%) or VDAC2 (79%). Two more peptides show high homology to the sequence present in mouse brain VDAC3 (100 and 78%). In addition, we demonstrate the translocation of ATP into synaptosomes, which is inhibited by DCCD and by the anion transport inhibitor DIDS. The possible function of VDAC in the synaptic plasma membrane is discussed.

Interactions between presynaptic calcium channels and proteins implicated in synaptic vesicle trafficking and exocytosis

Monoclonal antibodies were generated by immunizing mice with chick brain synaptic membranes and screening for immunoprecipitation of solubilized omega conotoxin GVIA receptors (N-type calcium channels). Antibodies against two synaptic proteins (p35--syntaxin 1 and p58--synaptotagmin) were produced and used to purify and characterize a ternary complex containing N-type channels associated with these two proteins. These results provided the first evidence for a specific interaction between presynaptic calcium channels and SNARE proteins involved in synaptic vesicle docking and calcium-dependent exocytosis. Immunoprecipitation experiments supported the conclusion that syntaxin 1/SNAP-25/VAMP/synaptotagmin I or II complexes associate with N-type, P/Q-type, but not L-type calcium channels from rat brain nerve terminals. Immunofluorescent confocal microscopy at the frog neuromuscular junction was consistent with the co-localization of syntaxin 1, SNAP-25, and calcium channels, all of which are predominantly expressed at active zones of the presynaptic plasma membrane facing post-synaptic folds rich in acetylcholine receptors. The interaction of proteins implicated in calcium-dependent exocytosis with presynaptic calcium channels may locate the sensor(s) that trigger vesicle fusion within a microdomain of calcium entry.

Purification and reconstitution of an outwardly rectified Cl- channel from tracheal epithelia

We reported the identification of three outwardly rectified Cl- channel (ORCC) candidate proteins (115, 85, and 52 kDa) from bovine tracheal epithelia. We have raised polyclonal antibodies against these isolated proteins. Incorporation into planar lipid bilayers of material partly purified from bovine tracheal apical membranes with one of these antibodies as a ligand (anti-p115) resulted in the incorporation of an ORCC identical in biophysical characteristics to one we previously described. We developed a new purification procedure to increase the yield and purity of this polypeptide. The purification scheme that gave the best results in terms of overall protein yield and purity was a combination of anion- and cation-exchange chromatography followed by immunopurification. By use of this purification scheme, 7 microg of the 115-kDa protein were purified from 20 mg of tracheal apical membrane proteins. Incorporation of this highly purified material into planar lipid bilayers revealed a DIDS-inhibitable channel with the following properties: linear conductance of 87 +/- 9 pS in symmetrical Cl- solutions, halide selectivity sequence of I- > Cl- > Br-, and lack of sensitivity to protein kinase A, Ca2+, or dithiothreitol. Using anti-Galphai antibodies to precipitate Galphai protein(s) from the partly purified preparations, we demonstrated that the loss of rectification of the ORCC was due to uncoupling of Galphai protein(s) from the ORCC protein and that the 115-kDa polypeptide is an ORCC.

Localisation of inositol trisphosphate and ryanodine receptors during mouse spermatogenesis: possible functional implications

During spermatogenesis the activity of intracellular Ca(2+)-release channels is likely to play an important role in different specific cellular functions. Accordingly, messenger RNAs for the three inositol 1,4,5-trisphosphate receptor (IP3R) subtypes were found to be present throughout spermatogenesis. Immunocytochemical analysis revealed distinct distribution patterns of the mature IP3Rs during sperm differentiation. At early stages, IP3Rs are distributed throughout the cytoplasm, and as differentiation proceeds they become selectively localised to the Golgi complex. Consistently, spermatogonia underwent large intracellular Ca2+ release in response to thapsigargin (TG), while smaller responses were detected in late spermatocytes and spermatids. The distribution of IP3Rs and the larger Ca(2+)-release responses found in spermatogonia, suggest that IP3Rs may be involved in cell proliferation at this stage. This notion is supported by our observations in a spermatogenic cell line that depletion of intracellular Ca2+ pools using TG inhibits cell division, and that incubation with an IP3R-I antisense oligonucleotide completely inhibited proliferation. Furthermore, the three genes encoding ryanodine receptor proteins (RyRs) are expressed at all stages of spermatogenesis. However, immunocytochemical studies with specific antibodies against each of the RyR subtypes detected types 1 and 3 in spermatogenic cells and only type 3 in mature sperm. In contrast to IP3Rs, RyRs remain scattered in the cytoplasm throughout differentiation. Functional responses to caffeine and ryanodine were absent in spermatogenic cells and in mature sperm. These findings suggest that IP3Rs have significantly more important roles in spermatogenesis than RyRs, and that one of these roles is crucial for cell proliferation.

Isolation of the [3H]gabapentin-binding protein/alpha 2 delta Ca2+ channel subunit from porcine brain: development of a radioligand binding assay for alpha 2 delta subunits using [3H]leucine

The novel antiepileptic agent gabapentin (Neurontin) binds with high affinity to the alpha 2 delta subunit of a voltage-dependent Ca2+ channel. We report here a simple purification scheme for detergent-solubilized alpha 2 delta subunits from porcine brain. This involves sequential chromatography on Q-Sepharose, Cu(2+)-charged iminodiacetic acid-Sepharose, wheat germ lectin-agarose, and Mono Q. The purified protein was essentially homogeneous by SDS-polyacrylamide gel electrophoresis with a subunit Mr of 145,000. Using [3H] gabapentin as the radiolabeled tracer and (S)-3-isobutyl gamma-aminobutyric acid to define nonspecific binding, the overall purification factor was 2760-fold and the apparent yield 26.6%. We also developed and validated a novel binding assay for alpha 2 delta Ca2+ channel subunits using the ligand pair L-[3H]leucine/L-isoleucine. Even in binding assays of crude brain membrane fractions, [3H]leucine proved to be remarkably stable and specific for the alpha 2 delta Ca2+ channel subunit. [3H]Leucine offers several advantages over custom-labeled [3H]gabapentin: it has a higher specific activity, is relatively inexpensive, and is available from commercial sources.

Inositol triphosphate receptors in sea urchin sperm

Inositol 1,4,5-triphosphate (Ins(1,4,5)P3) is a second messenger that regulates Ca2+ channels in many important cell signalling pathways. In sea urchin sperm the outer investment of the egg triggers the acrosome reaction (AR) that involves Ins(1,4,5)P3 production and the opening of two Ca2+ channels. Here we have sought to identify a high-affinity Ins(1,4,5)P3 receptor in Strongylocentrotus purpuratus sperm. An Ins(1,4,5)P3 binding component was affinity-purified 12-fold from sperm extracts. It displayed similar characteristics to the Ins(1,4,5)P3 receptor from other sources: pH-dependent high affinity for Ins(1,4,5)P3 (KD = 261 nM), a tau1/2 of association and dissociation of 50 and 40 s, respectively, specificity (IC50 > 5 microM for Ins(1)P1, Ins(1,4)P2 and Ins(1,3,4,5)P4), and pharmacological sensitivity (10 and 100 microg heparin/ml inhibited 75% and 100% binding respectively). An antibody against the carboxy-terminal of the type I Ins(1,4,5)P3 receptor of somatic cells recognised a plasma membrane component in the sperm head and less intensely in the flagella. This antibody also recognised a 240 kDa band from isolated head plasma membranes, and weakly in flagellar membrane. This IP3 receptor-like protein may mediate the sustained uptake of Ca2+ through the second Ca2+ channel opened during the AR.

Functional interactions between cytoplasmic domains of the skeletal muscle Ca2+ release channel

The skeletal muscle Ca2+ release channel (RYR1), which plays a critical role in excitation-contraction coupling, is a homotetramer with a subunit molecular mass of 565 kDa. Oxidation of the channel increases its activity and produces intersubunit cross-links within the RYR1 tetramer (Aghdasi, B., Zhang, J., Wu, Y., Reid, M. B., and Hamilton, S. L. (1997) J. Biol. Chem. 272, 3739-3748). Alkylation of hyperreactive sulfhydryls on RYR1 with N-ethylmaleimide (NEM) inhibits channel function and blocks the intersubunit cross-linking. We used calpain and tryptic cleavage, two-dimensional SDS-polyacrylamide gel electrophoresis, N-terminal sequencing, sequence-specific antibody Western blotting, and [14C]NEM labeling to identify the domains involved in these effects. Our data are consistent with a model in which 1) diamide, an oxidizing agent, simultaneously produces an intermolecular cross-link between adjacent subunits within the RYR1 tetramer and an intramolecular cross-link within a single subunit; 2) all of the cysteines involved in both cross-links are in either the region between amino acids approximately 2100 and 2843 or the region between amino acids 2844 and 4685; 3) oxidation exposes a new calpain cleavage site in the central domain of the RYR1 (in the region around amino acid 2100); 4) sulfhydryls that react most rapidly with NEM are located in the N-terminal domain (between amino acids 426 and 1396); 5) alkylation of the N-terminal cysteines completely inhibits the formation of both inter- and intrasubunit cross-links. In summary, we present evidence for interactions between the N-terminal region and the putatively cytoplasmic central domains of RYR1 that appear to influence subunit-subunit interactions and channel activity.

Identification and functional reconstitution of the type 2 inositol 1,4,5-trisphosphate receptor from ventricular cardiac myocytes

The inositol 1,4,5-trisphosphate receptor (InsP3R) is an intracellular Ca2+ release channel that mediates the rise in cytoplasmic calcium in response to receptor-activated production of InsP3. The InsP3R-mediated signaling pathway appears to be ubiquitous and is involved in many cellular processes including cell division, smooth muscle contraction, and neuronal signaling. Different regions of the heart also express InsP3 receptors. We report here that acutely dissociated ventricular myocytes from ferret and rat hearts express significant levels of InsP3R as indicated by immunoblotting with a receptor consensus antibody. InsP3 binding experiments (KD = 23.6 nM and Bmax = 0.46 pmol/mg) suggest the myocytes contain the high affinity type 2 InsP3 receptor. Exhaustive mRNA screening by polymerase chain reaction, RNase protection, and subsequent DNA sequencing positively identify the InsP3R as type 2. The type 2 receptor from ferret heart was then incorporated into planar lipid bilayers and formed Ca2+-selective, InsP3-activated, heparin-blocked ion channels. We conclude that the predominant InsP3 receptor isoform expressed in cardiac myocytes is type 2 and that it forms a functional InsP3-gated Ca2+ channel when reconstituted in planar lipid bilayers.

Conducting and voltage-dependent behaviors of the native and purified SR Ca2+-release channels from the canine diaphragm

The ryanodine-sensitive Ca2+-release channel of the canine diaphragm sarcoplasmic reticulum (SR) was characterized using biochemical assays and the planar lipid bilayer technique. Diaphragm SR membranes have a [3H]ryanodine-binding capacity (Bmax) of 1.2 pmol/mg protein and a binding affinity (K(D)) of 6.3 nM. The conductance of the native channel was 330 pS in 50 mM/250 mM trans/cis CsCH3SO3 and was reduced to 71 pS by 10 mM Ca2+ trans. The Ca2+-release channel was purified as a 400 kDa protein on SDS-PAGE and displayed a conductance of 715 pS in 200 mM KCl. The native and purified Ca2+ channels were activated by micromolar Ca2+ and ATP and inhibited by Mg2+, ryanodine and ruthenium red. Although diaphragm muscle contraction was shown to depend on extracellular Ca2+ like cardiac muscles, we provide evidence that the diaphragm SR Ca2+-release channel may be classified as a skeletal ryanodine receptor isoform. First, the IC50 for [3H]ryanodine binding was in the same range as estimated for skeletal SR, with 20 nM. Second, the channel was maximally activated by 10-30 microM cytoplasmic Ca2+ and inhibited at higher concentrations. Third, ryanodine binding to the diaphragm SR was less sensitive to Ca2+ than cardiac SR, with EC50, values of 50 and 1 microM, respectively. Finally, Ca2+-release activity and [3H]ryanodine binding capacity of the diaphragm and skeletal SR were similarly more sensitive to Mg2+ than cardiac SR. Together, these results suggest a predominantly skeletal-type of excitation-contraction coupling in the diaphragm.

Coassembly of TRP and TRPL produces a distinct store-operated conductance

The Drosophila retinal-specific protein, TRP (transient receptor potential), is the founding member of a family of store-operated channels (SOCs) conserved from C. elegans to humans. In vitro studies indicate that TRP is a SOC, but that the related retinal protein, TRPL, is constitutively active. In the current work, we report that coexpression of TRP and TRPL leads to a store-operated, outwardly rectifying current distinct from that owing to either TRP or TRPL alone. TRP and TRPL interact directly, indicating that the TRP-TRPL-dependent current is mediated by heteromultimeric association between the two subunits. We propose that the light-activated current in photoreceptor cells is produced by a combination of TRP homo- and TRP-TRPL heteromultimers.

Isolation and characterization of the primary structure of testis-specific L-type calcium channel: implications for contraception

Therapeutic administration of calcium channel-blocking medications has been correlated with reduced mannose receptor expression and iatrogenic human male infertility. In this report, we investigate whether the pharmacological activity of dihydropyridines, which block calcium influx through voltage-dependent calcium channels, contributes to the production of an infertile state. An influx of extracellular calcium is an absolute requirement for the initiation of a progesterone-stimulated acrosome reaction by human spermatozoa. To determine whether dihydropyridines could inhibit progesterone-induced acrosome loss, we have studied a protein expressed in rat and human spermatozoa which is related both antigenically and by cDNA sequence to the alpha 1 subunit of the rat cardiac muscle voltage-dependent calcium channel, which forms the pore of the channel. Using reverse transcription-polymerase chain reaction, we have isolated a 2169 base clone from rat testis mRNA whose sequence was largely identical to that of the alpha 1 subunit of the rat cardiac muscle calcium channel, but had an 84 base change, attributable to splicing and alternate exon usage. This change inserts a peptide cassette encoding an amphipathic membrane-spanning helix that constitutes part of the ionic pore of the skeletal muscle calcium channel regulating the kinetics of activation of the calcium channel and may serve as an intramembrane dihydropyridine binding site. In parallel, human spermatozoa from fertile donors were exposed to nifedipine in vitro. Nifedipine inhibited progesterone-stimulated calcium influx and subsequent acrosome reactions in human spermatozoa at concentrations effective in excitable cells, but required a prolonged time to do so. In contrast, progesterone ligand binding was unaffected by nifedipine treatment. These data demonstrate that human spermatozoa express an L-type calcium channel which is responsive to nifedipine. Assuming sperm calcium transport pathways are highly conserved, the slow kinetics by which the blockade of the human sperm channel was obtained can be correlated with alterations in channel activation and conductance associated with isoform diversity generated by alternate splicing as observed in the rat. These data provide unequivocal evidence for the presence of functional L-type voltage-dependent calcium channels in rat and human spermatozoa. The data also define an altered binding site for calcium entry antagonists in this channel and offer a unique target for the design of new male contraceptive agents.

Spermine modulation of specific [3H]-gabapentin binding to the detergent-solubilized porcine cerebral cortex alpha 2 delta calcium channel subunit

1. Recent studies have identified the [3H]-gabapentin-binding protein, purified from porcine cerebral cortical membranes, as the alpha 2 delta subunit of voltage-sensitive calcium channels (Gee et al., 1996). The present study investigates the influence of the polyamine spermine on specific [3H]-gabapentin binding to detergent-solubilized porcine cerebral cortical membranes. 2. Spermine, spermidine, 1,10 diaminodecane, Mg2+ and Zn2+, all divalent cations, displaced [3H]-gabapentin binding to detergent-solubilized membranes in a concentration-dependent manner with a maximal inhibition of 65-75%. Radioligand binding studies showed that spermine did not directly interact with the [3H]-gabapentin-binding site. Spermine inhibited [3H]-gabapentin binding by interacting with a polyamine-sensitive allosteric site on the membrane protein. The steep concentration-dependence of spermine inhibition of [3H]-gabapentin binding may suggest multi-site co-operativity. 3. Prolonged dialysis of cerebral cortical membranes and Tween 20-solubilized membranes resulted in a > 2.0 fold increase in [3H]-gabapentin binding. The increase in binding was due to the removal of a heat stable, low molecular weight (< 12,000Da) endogenous molecule which influences [3H]-gabapentin binding competitively. 4. Dialysis of detergent-solubilized cerebral cortical membranes also resulted in a decrease in the maximum inhibition of [3H]-gabapentin binding by spermine. Since the rates of the increase in [3H]-gabapentin binding and the loss of the ability of spermine to inhibit [3H]-gabapentin binding on dialysis were different it was inferred that a second endogenous ligand was removed during dialysis. 5. During initial steps of purification of the [3H]-gabapentin-binding protein there was a decrease in the maximum inhibition of [3H]-gabapentin binding by spermine. The loss of the second endogenous molecule during initial purification would reasonably explain the reduction in inhibition of binding by spermine. However, spermine stimulation of [3H]-gabapentin binding to material that eluted from the gel-filtration column later in the purification scheme does not appear to be due to removal of a dialysable endogenous factor or to the dissociation of other calcium channel subunit(s). 6. Adding back dialysate, before or after boiling, to detergent solubilized membranes resulted in a dose-dependent restoration of the inhibition of [3H]-gabapentin binding and of the maximal inhibition [3H]-gabapentin binding by spermine. This result is consistent with the re-addition of two endogenous heat stable ligands. 7. The findings that [3H]-gabapentin binding to the pure alpha 2 delta subunit was stimulated by spermine indicates that the alpha 2 delta subunit of voltage-sensitive calcium channels bears a modulatory spermine site. Such a spermine site has not been identified before. Spermine stimulation of [3H]-gabapentin binding to the purified protein was reversed to inhibition after adding back dialysate. Thus the inhibitory spermine effect in membranes is also probably due to one or more modulatory sites on the alpha 2 delta subunit.

Characterization of a cytosolic and a luminal Ca2+ binding site in the type I inositol 1,4,5-trisphosphate receptor

To study the Ca2+ regulation of the inositol 1,4,5-trisphosphate receptor (InsP3R) at the molecular level, we expressed various cytosolic and luminal regions of the mouse type I InsP3R as glutathione S-transferase fusion proteins. 45Ca2+ and ruthenium red overlay studies and Stains-all spectra and staining revealed both a cytosolic and a luminal Ca2+ binding site. The luminal Ca2+ binding site was mapped to the nonconserved acidic subregion of the luminal loop between amino acids 2463 and 2528. A K0.5 of 0.3 microM and a Hill coefficient of 1.1 were determined by 45Ca2+ overlay by quantification of 45Ca2+ binding on blots. The cytosolic Ca2+ binding site was localized in a region just preceding the transmembrane domain M1. The Ca2+ binding was mapped to a 23-amino acid stretch between amino acids 2124 and 2146. This cytosolic region showed a single high affinity site for Ca2+, with a K0.5 of 0. 8 microM and a Hill coefficient of 1.0. Neither of the identified Ca2+ binding regions contained an EF-hand motif. We conclude that the type I InsP3R has at least two quite distinct types of Ca2+ binding sites, which are localized in different structural regions of the protein.

cAMP-dependent phosphorylation of the cardiac-type alpha 1 subunit of the voltage-dependent Ca2+ channel in a murine pancreatic beta-cell line

Two voltage-dependent calcium channels (VDCCs) have been reported in pancreatic islets: the beta-cell/endocrine-brain and cardiac subtypes. The cardiac-type alpha 1 subunit was isolated from cultured beta TC3 cells, a murine pancreatic beta-cell line, by immunoprecipitation with a specific polyclonal antibody. We have examined the effects of 1-isobutyl-3-methylxanthine (IBMX) and forskolin, agonists that elevate cAMP in these cells, on the phosphorylation of this subunit in intact beta TC3 cells using a sensitive back-phosphorylation technique. This technique allows quantitative detection of protein phosphorylation that is specifically stimulated by cAMP. The stimulation of intact beta TC3 cells with forskolin or IBMX resulted in the phosphorylation of the cardiac-type alpha 1 subunit as evidenced by a 40-60% decrease in the ability of the 257-kDa form to serve as a substrate in the in vitro back-phosphorylation reaction with [gamma-32P]ATP and the catalytic subunit of cAMP-dependent protein kinase (PKA). The effects of forskolin were time- and concentration-dependent. The concentration-dependency of forskolin-induced phosphorylation of the cardiac-type alpha 1 subunit and the potentiation of glucose-induced insulin secretion were highly correlated, a finding that is consistent with a role for such phosphorylation in mediating at least some of the effects of cAMP on secretion.

Re-evaluation of the P/Q Ca2+ channel components of Ba2+ currents in bovine chromaffin cells superfused with solutions containing low and high Ba2+ concentrations

This study was undertaken to reassess the set of voltage-dependent Ca2+ channel subtypes expressed by bovine adrenal chromaffin cells maintained in primary cultures. Previous views on the pharmacology of such channels had to be revised in the light of the novel data which arose from the use in this study of low and high micromolar concentrations of omega-agatoxin IVA, and low (2 mM) and high (10 mM) concentrations of the charge carrier Ba2+. Whole-cell Ba2+ currents (IBa) through Ca2+ channels were elicited in voltage-clamped chromaffin cells, with a holding potential of -80 mV and depolarising pulses to 0 mV. Mean peak IBa was 425 pA in 2 mM Ba2+ (59 cells) and 787 pA in 10 mM Ba2+ (42 cells). In 2 mM Ba2+, omega-conotoxin MVIIC (3 microM) inhibited IBa by 79%; in 10 mM Ba2+, the blockade developed much more slowly and reached only 44%. A low concentration of omega-agatoxin IVA (20 nM) inhibited IBa by 9%; 2 microM inhibited IBa by 60%. This blockade was similar in low and high Ba2+ concentrations. After giving furnidipine (3 microM) and omega-conotoxin GVIA (1 microM), 2 microM omega-agatoxin IVA inhibited the remaining current (about 40-45%); this blockade was independent of the Ba2+ concentration. The current could be fully blocked by the cocktail furnidipine/omega-conotoxin GVIA/high omega-agatoxin IVA, both in low and high Ba2+ concentrations. The large Q-type channel component of IBa is blocked by micromolar concentrations of omega-agatoxin IVA and omega-conotoxin MVIIC. While solutions with a high Ba2+ concentration strongly delayed the development of blockade by omega-conotoxin MVIIC, the blockade by high concentrations of omega-agatoxin IVA was equally effective in solutions with a low or a high Ba2+ concentration. Hence, the use of appropriate Ba2+ and toxin concentrations in this study reveals that P-type Ca2+ channels are poorly expressed in bovine chromaffin cells; in contrast, a robust component of the current depends on Q-type Ca2+ channels. An R-type residual current is not present in these cells.

Divergent effects of ruthenium red and ryanodine on Ca2+/calmodulin-dependent phosphorylation of the Ca2+ release channel (ryanodine receptor) in cardiac sarcoplasmic reticulum

In cardiac muscle, a Ca2+/calmodulin-dependent protein kinase (CaM kinase) associated with the sarcoplasmic reticulum (SR) is known to phosphorylate the membrane proteins phospholamban, Ca(2+)-ATPase, and Ca(2+)-release channel (ryanodine receptor). Phosphorylation of phospholamban and Ca(2+)-ATPase is recognized to stimulate Ca2+ sequestration by the SR but the functional consequence of Ca2+ channel phosphorylation has not been clearly established. In this study, we investigated the effects of the SR Ca(2+)-release inhibitor, ruthenium red (RR), and the SR Ca(2+)-release activator, ryanodine (at submicromolar concentrations), on CaM kinase-mediated phosphorylation of the Ca(2+)-cycling proteins in rabbit cardiac SR. Incubation of SR with RR (5-30 microM) for 3 min at 37 degrees C resulted in marked (up to 85%) inhibition of Ca2+ channel phosphorylation (50% inhibition with 15 +/- 2 microM RR) by the endogenous membrane-associated CaM kinase. Phosphorylation of the Ca2+ channel by exogenously added multifunctional alpha CaM kinase II was also inhibited similarly by RR. Phosphorylation of the Ca(2+)-ATPase by endogenous and exogenous CaM kinase was inhibited only modestly (25-30%) by RR, and phospholamban phosphorylation was unaffected by RR. The magnitude of RR-induced inhibition of Ca2+ channel phosphorylation did not differ appreciably at saturating or subsaturating concentrations of Ca2+ or calmodulin, and in the absence or presence of protein phosphatase inhibitors. In contrast to the effects of RR, low concentrations of ryanodine (0.25-1 microM) caused significant stimulation (up to approximately 50%) of Ca2+ channel phosphorylation but had no effect on Ca(2+)-ATPase and phospholamban phosphorylation. These findings suggest that interaction of RR with the ryanodine receptor induces a "nonphosphorylatable state" of the Ca(2+)-release channel, likely through a conformational change involving occlusion of the CaM kinase phosphorylation site. On the other hand, ryanodine binding to the receptor may serve to maintain an open, "phosphorylatable state" of the channel.

Functionally heterogenous ryanodine receptors in avian cerebellum

The functional heterogeneity of the ryanodine receptor (RyR) channels in avian cerebellum was defined. Heavy endoplasmic reticulum microsomes had significant levels of ryanodine and inositol 1,4,5-trisphosphate binding. Scatchard analysis and kinetic studies indicated the existence of at least two distinct ryanodine binding sites. Ryanodine binding was calcium-dependent but was not significantly enhanced by caffeine. Incorporation of microsomes into planar lipid bilayers revealed ion channels with pharmacological features (calcium, magnesium, ATP, and caffeine sensitivity) similar to the RyR channels found in mammalian striated muscle. Despite a wide range of unitary conductances (220-500 picosiemens, symmetrical cesium methanesulfonate), ryanodine locked both channels into a characteristic slow gating subconductance state, positively identifying them as RyR channels. Two populations of avian RyR channels were functionally distinguished by single channel calcium sensitivity. One population was defined by a bell-shaped calcium sensitivity analogous to the skeletal muscle RyR isoform (type I). The calcium sensitivity of the second RyR population was sigmoidal and analogous to the cardiac muscle RyR isoform (type II). These data show that there are at least two functionally distinct RyR channel populations in avian cerebellum. This leads to the possibility that these functionally distinct RyR channels are involved in different intracellular calcium signaling pathways.

Inositol 1,4,5-trisphosphate-mediated Ca2+ release from platelet internal membranes is regulated by differential phosphorylation

Platelets are activated by an increase in cytosolic Ca2+, and a portion of this increase is derived from inositol 1,4,5-trisphosphate (InsP3)-mediated Ca2+ release from internal stores via the InsP3 receptor. Cytosolic cAMP inhibits platelet activation, and experiments were designed to determine if cAMP-dependent phosphorylation affects the rate of InsP3-mediated Ca2+ release. Western blotting of platelet internal membranes with anti-InsP3 receptor and anti-actin binding protein antibodies revealed that the platelet contains type 1 InsP3 receptor and that it is distinct from actin binding protein. The platelet InsP3 receptor was shown to be phosphorylated by endogenous, membrane-bound kinases as well as by exogenous protein kinase A. Prior phosphorylation of the insP3 receptor by endogenous kinases inhibited additional protein kinase A-dependent phosphorylation by 60%. Furthermore, endogenous phosphorylation resulted in a 2-fold increase in the InsP3-mediated Ca2+ release rate relative to dephosphorylated controls. Following endogenous phosphorylation, additional phosphorylation by protein kinase A returned the Ca2+ release rate to control values, while protein kinase A-dependent phosphorylation of dephosphorylated membranes did not affect the release rate. These results suggest that the InsP3 receptor within intact platelets is phosphorylated by endogenous kinases which results in a high InsP3-mediated Ca2+ release rate, and that increases in cAMP result in additional phosphorylation that inhibits Ca2+ release, thus contributing to inhibition of platelet activation.

Differential distribution of ryanodine receptor type 3 (RyR3) gene product in mammalian skeletal muscles

Activation of intracellular Ca(2+)-release channels/ryanodine receptors (RyRs) is a fundamental step in the regulation of muscle contraction. In mammalian skeletal muscle, Ca(2+)-release channels containing the type 1 isoform of RyR (RyR1) open to release Ca2+ from the sarcoplasmic reticulum (SR) upon stimulation by the voltage-activated dihydropyridine receptor on the T-tubule/plasma membrane. In addition to RyR1, low levels of the mRNA of the RyR3 isoform have been recently detected in mammalian skeletal muscles. Here we report data on the distribution of the RyR3 gene product in mammalian skeletal muscles. Western-blot analysis of SR of individual muscles indicated that, at variance with the even distribution of the RyR1 isoform, the RyR3 content varies among different muscles, with relatively higher amounts being detected in diaphragm and soleus, and lower levels in abdominal muscles and tibialis anterior. In these muscles RyR3 was localized in the terminal cisternae of the SR. No detectable levels of RyR3 were observed in the extensor digitorum longus. Preferential high content of RyR3 in the diaphragm muscle was observed in several mammalian species. In situ hybridization analysis demonstrated that RyR3 transcripts are not restricted to a specific subset of skeletal-muscle fibres. Differential utilization of the RyR3 isoform in skeletal muscle may be relevant to the modulation of Ca2+ release with respect to specific muscle-contraction properties.

Isolation and characterization of vascular smooth muscle inositol 1,4,5-trisphosphate receptor

myo-Inositol 1,4,5-trisphosphate (InsP3) receptor of porcine aorta was purified to near homogeneity and its biochemical properties were compared with those of cerebellar InsP3 receptor of the same animal species. The aortic InsP3 receptor consisted of equal amounts of two polypeptides with slightly differing molecular masses of around 240 kDa and was found to possess a single population of InsP3-binding site (Kd of 1.2 nM). The InsP3 receptor purified from porcine cerebellum was also comprised of two polypeptides. However, the molecular mass was slightly but definitely larger, being 250 kDa, and the amounts of the two polypeptides were not equal. The aortic InsP3 receptor cross-reacted with polyclonal antibody specific to type 1 InsP3 receptor as did the cerebellar InsP3 receptor. The aortic InsP3 receptor bound to calmodulin-Sepharose in a Ca(2+)-dependent manner, while the cerebellar InsP3 receptor did not. Reverse transcriptase-PCR analysis revealed two splicing variants of the type 1 InsP3 receptor in porcine aortic smooth muscle distinct from those of the type 1 InsP3 receptor of porcine cerebellum. The possible relevance of this difference to difference in calmodulin-binding property was discussed.

Electrophysiological effects of ryanodine derivatives on the sheep cardiac sarcoplasmic reticulum calcium-release channel

We have examined the effects of a number of derivatives of ryanodine on K+ conduction in the Ca2+ release channel purified from sheep cardiac sarcoplasmic reticulum (SR). In a fashion comparable to that of ryanodine, the addition of nanomolar to micromolar quantities to the cytoplasmic face (the exact amount depending on the derivative) causes the channel to enter a state of reduced conductance that has a high open probability. However, the amplitude of that reduced conductance state varies between the different derivatives. In symmetrical 210 mM K+, ryanodine leads to a conductance state with an amplitude of 56.8 +/- 0.5% of control, ryanodol leads to a level of 69.4 +/- 0.6%, ester A ryanodine modifies to one of 61.5 +/- 1.4%, 9,21-dehydroryanodine to one of 58.3 +/- 0.3%, 9 beta,21beta-epoxyryanodine to one of 56.8 +/- 0.8%, 9-hydroxy-21-azidoryanodine to one of 56.3 +/- 0.4%, 10-pyrroleryanodol to one of 52.2 +/- 1.0%, 3-epiryanodine to one of 42.9 +/- 0.7%, CBZ glycyl ryanodine to one of 29.4 +/- 1.0%, 21-p-nitrobenzoyl-amino-9-hydroxyryanodine to one of 26.1 +/- 0.5%, beta-alanyl ryanodine to one of 14.3 +/- 0.5%, and guanidino-propionyl ryanodine to one of 5.8 +/- 0.1% (chord conductance at +60 mV, +/- SEM). For the majority of the derivatives the effect is irreversible within the lifetime of a single-channel experiment (up to 1 h). However, for four of the derivatives, typified by ryanodol, the effect is reversible, with dwell times in the substate lasting tens of seconds to minutes. The effect caused by ryanodol is dependent on transmembrane voltage, with modification more likely to occur and lasting longer at +60 than at -60 mV holding potential. The addition of concentrations of ryanodol insufficient to cause modification does not lead to an increase in single-channel open probability, such as has been reported for ryanodine. At concentrations of > or = 500 mu M, ryanodine after initial rapid modification of the channel leads to irreversible closure, generally within a minute. In contrast, comparable concentrations of beta-alanyl ryanodine do not cause such a phenomenon after modification, even after prolonged periods of recording (>5 min). The implications of these results for the site(s) of interaction with the channel protein and mechanism of the action of ryanodine are discussed. Changes in the structure of ryanodine can lead to specific changes in the electrophysiological consequences of the interaction of the alkaloid with the sheep cardiac SR Ca2+ release channel.