Characterization of the recombinant human neuronal nicotinic acetylcholine receptors alpha3beta2 and alpha4beta2 stably expressed in HEK293 cells

HEK293 cells were stably transfected with the cDNAs encoding full-length human neuronal nicotinic acetylcholine receptor (nAChR) subunit combinations alpha3beta2 or alpha4beta2. [(3)H]-(+/-)Epibatidine ([(3)H]-(+/-)EPI) bound to membranes from A3B2 (alpha3beta2) and A4B2.2 (alpha4beta2) cells with K(d) values of 7.5 and 33.4 pM and B(max) values of 497 and 1564 fmol/mg protein, respectively. Concentration-dependent increases in intracellular free Ca(2+) concentration were elicited by nAChR agonists with a rank order of potency of EPI>1,1-dimethyl-4-phenylpiperazinium (DMPP)>nicotine (NIC)=suberyldicholine (SUB)>cytisine (CYT)=acetylcholine (ACh) for A3B2 cells and EPI>CYT=SUB=NIC=DMPP>ACh for A4B2.2 cells. Antagonists of nAChRs blocked NIC-induced responses with a rank order of potency of d-tubocurarine (d-Tubo)=mecamylamine (MEC)>dihydro-beta-erythroidine (DHbetaE) in A3B2 cells and MEC=DHbetaE>d-Tubo in A4B2.2 cells. Whole-cell patch clamp recordings indicate that the decay rate of macroscopic ACh-induced currents is faster in A3B2 than in A4B2.2 cells and that A3B2 cells are less sensitive to ACh than A4B2.2 cells. ACh currents elicited in alpha3beta2 and alpha4beta2 human nAChRs are maximally potentiated at 20 and 2 mM external Ca(2+), respectively. Our results indicate that stably expressed alpha3beta2 and alpha4beta2 human nAChRs are pharmacologically and functionally distinct.

Structural elements in domain IV that influence biophysical and pharmacological properties of human alpha1A-containing high-voltage-activated calcium channels

We have cloned two splice variants of the human homolog of the alpha1A subunit of voltage-gated Ca2+ channels. The sequences of human alpha1A-1 and alpha1A-2 code for proteins of 2510 and 2662 amino acids, respectively. Human alpha1A-2alpha2bdeltabeta1b Ca2+ channels expressed in HEK293 cells activate rapidly (tau+10mV = 2.2 ms), deactivate rapidly (tau-90mV = 148 micros), inactivate slowly (tau+10mV = 690 ms), and have peak currents at a potential of +10 mV with 15 mM Ba2+ as charge carrier. In HEK293 cells transient expression of Ca2+ channels containing alpha1A/B(f), an alpha1A subunit containing a 112 amino acid segment of alpha1B-1 sequence in the IVS3-IVSS1 region, resulted in Ba2+ currents that were 30-fold larger compared to wild-type (wt) alpha1A-2-containing Ca2+ channels, and had inactivation kinetics similar to those of alpha1B-1-containing Ca2+ channels. Cells transiently transfected with alpha1A/B(f)alpha2bdeltabeta1b expressed higher levels of the alpha1, alpha2bdelta, and beta1b subunit polypeptides as detected by immunoblot analysis. By mutation analysis we identified two locations in domain IV within the extracellular loops S3-S4 (N1655P1656) and S5-SS1 (E1740) that influence the biophysical properties of alpha1A. alpha1AE1740R resulted in a threefold increase in current magnitude, a -10 mV shift in steady-state inactivation, and an altered Ba2+ current inactivation, but did not affect ion selectivity. The deletion mutant alpha1ADeltaNP shifted steady-state inactivation by -20 mV and increased the fast component of current inactivation twofold. The potency and rate of block by omega-Aga IVA was increased with alpha1ADeltaNP. These results demonstrate that the IVS3-S4 and IVS5-SS1 linkers play an essential role in determining multiple biophysical and pharmacological properties of alpha1A-containing Ca2+ channels.

Functional consequences of mutations in the human alpha1A calcium channel subunit linked to familial hemiplegic migraine

Mutations in alpha1A, the pore-forming subunit of P/Q-type calcium channels, are linked to several human diseases, including familial hemiplegic migraine (FHM). We introduced the four missense mutations linked to FHM into human alpha1A-2 subunits and investigated their functional consequences after expression in human embryonic kidney 293 cells. By combining single-channel and whole-cell patch-clamp recordings, we show that all four mutations affect both the biophysical properties and the density of functional channels. Mutation R192Q in the S4 segment of domain I increased the density of functional P/Q-type channels and their open probability. Mutation T666M in the pore loop of domain II decreased both the density of functional channels and their unitary conductance (from 20 to 11 pS). Mutations V714A and I1815L in the S6 segments of domains II and IV shifted the voltage range of activation toward more negative voltages, increased both the open probability and the rate of recovery from inactivation, and decreased the density of functional channels. Mutation V714A decreased the single-channel conductance to 16 pS. Strikingly, the reduction in single-channel conductance induced by mutations T666M and V714A was not observed in some patches or periods of activity, suggesting that the abnormal channel may switch on and off, perhaps depending on some unknown factor. Our data show that the FHM mutations can lead to both gain- and loss-of-function of human P/Q-type calcium channels.

Structure and functional characterization of a novel human low-voltage activated calcium channel

We have isolated and characterized overlapping cDNAs encoding a novel, voltage-gated Ca2+ channel alpha1 subunit, alpha1H, from a human medullary thyroid carcinoma cell line. The alpha1H subunit is structurally similar to previously described alpha1 subunits. Northern blot analysis indicates that alpha1H mRNA is expressed throughout the brain, primarily in the amygdala, caudate nucleus, and putamen, as well as in several nonneuronal tissues, with relatively high levels in the liver, kidney, and heart. Ba2+ currents recorded from human embryonic kidney 293 cells transiently expressing alpha1H activated at relatively hyperpolarized potentials (-50 mV), rapidly inactivated (tau = 17 ms), and slowly deactivated. Similar results were observed in Xenopus oocytes expressing alpha1H. Single-channel measurements in human embryonic kidney 293 cells revealed a single-channel conductance of approximately 9 pS. These channels are blocked by Ni2+ (IC50 = 6.6 microM) and the T-type channel antagonists mibefradil (approximately 50% block at 1 microM) and amiloride (IC50 = 167 microM). Thus, alpha1H-containing channels exhibit biophysical and pharmacological properties characteristic of low voltage-activated, or T-type, Ca2+ channels.

The human N-methyl-D-aspartate receptor 2C subunit: genomic analysis, distribution in human brain, and functional expression

cDNAs encoding four isoforms of the human NMDA receptor (NMDAR) NMDAR2C (hNR2C-1, -2, -3, and -4) have been isolated and characterized. The overall identity of the deduced amino acid sequences of human and rat NR2C-1 is 89.0%. The sequences of the rat and human carboxyl termini (Gly925-Val1,236) are encoded by different exons and are only 71.5% homologous. In situ hybridization in human brain revealed the expression of the NR2C mRNA in the pontine reticular formation and lack of expression in substantia nigra pars compacta in contrast to the distribution pattern observed previously in rodent brain. The pharmacological properties of hNR1A/2C were determined by measuring agonist-induced inward currents in Xenopus oocytes and compared with those of other human NMDAR subtypes. Glycine, glutamate, and NMDA each discriminated between hNR1A/2C-1 and at least one of hNR1A/2A, hNR1A/2B, or hNR1A/2D subtypes. Among the antagonists tested, CGS 19755 did not significantly discriminate between any of the four subtypes, whereas 5,7-dichlorokynurenic acid distinguished between hNR1A/2C and hNR1A/2D. Immunoblot analysis of membranes isolated from HEK293 cells transiently transfected with cDNAs encoding hNR1A and each of the four NR2C isoforms indicated the formation of heteromeric complexes between hNR1A and all four hNR2C isoforms. HEK293 cells expressing hNR1A/ 2C-3 or hNR1A/2C-4 did not display agonist responses. In contrast, we observed an agonist-induced elevation of intracellular free calcium and whole-cell currents in cells expressing hNR1A/2C-1 or hNR1A/2C-2. There were no detectable differences in the macroscopic biophysical properties of hNR1A/2C-1 or hNR1A/2C-2.

The expression of voltage-dependent calcium channel beta subunits in human hippocampus

The beta subunits of voltage-dependent calcium channels (VDCC) modulate the electrophysiology and cell surface expression of pore-forming alpha1 subunits. In the present study we have investigated the distribution of beta1,beta2,beta3 and beta4 in the human hippocampus using in situ hybridization (ISH) and immunohistochemistry. ISH studies showed a similar distribution of expression of beta1,beta2 and beta3 subunit mRNAs, including labelling of the dentate granule cell layer, all CA pyramidal regions, and the subiculum. Relatively low levels of expression of beta1 and beta2 subunit mRNAs correlated with low protein expression in the immunocytochemical (ICC) studies. There was a relative lack of beta4 expression by both ISH and ICC in the CA1 region, compared with high levels of expression in the subiculum. Immunostaining for beta1 and beta2 subunits was weak and relatively homogeneous throughout the hippocampus. The beta3 and beta4 subunits appeared to be more discretely localized. In general, beta3-immunoreactivity was moderate both in cell bodies, and as diffuse staining in the surrounding neuropil. Strongest staining was observed in mossy fibres and their terminal region in the CA3 stratum lucidum. In contrast, beta4-immunoreactivity in the neuropil showed intense dendritic localisation. Unlike the other subunits, beta4-immunoreactivity was absent from CA1 pyramidal neurones but was present in a small population of interneurone-like cells. The localisation of beta3 and beta4 may represent presynaptic and postsynaptic compartments in some populations of hippocampal neurones. Comparison of beta subunit distribution with previously published data on alpha1 subunits indicates certain neuronal groups and subcellular compartments in which the subunit composition of native pre- and postsynaptic VDCC can be predicted.

The potential of subtype-selective neuronal nicotinic acetylcholine receptor agonists as therapeutic agents

Neuronal nicotinic acetylcholine receptors (NAChRs) are pentameric ligand-gated ion channel receptors which exist as different functional subunit combinations which apparently subserve different physiological functions as indicated by molecular biological and pharmacological techniques. It is possible to design and synthesize novel compounds that have greater selective affinities and efficacies than nicotine for different NAChRs, which should translate into different behavioral profiles and therapeutic potentials. Examples of NAChR agonists studied are nicotine, SIB-1508Y, SIB-1553A and epibatidine. These compounds have different degrees of selectivity for human recombinant NAChRs, different neurotransmitter release profiles in vitro and in vivo and differential behavioral profiles. Preclinical studies suggest that SIB-1508Y is a candidate for the treatment of the motor and cognitive deficits of Parkinson's disease, whereas SIB-1553A appears to have potential as a candidate for the treatment of Alzheimer's disease. Epibatidine has a strong analgesic profile, however the ratio between pharmacological activity and undesirable effects is so low that it is difficult to envisage the use of this compound therapeutically. Nicotine has a broad profile of pharmacological activity, for instance demonstrating activity in models for cognition and analgesia. As for epibatidine, the adverse effects of nicotine severely limits its therapeutic use in humans. The discovery of subtype-selective NAChR agonists such as SIB-1508Y and SIB-1553A provides a new class of neuropsychopharmacological agents with better therapeutic ratios than nonspecific agents such as nicotine.

Characterization of human recombinant neuronal nicotinic acetylcholine receptor subunit combinations alpha2beta4, alpha3beta4 and alpha4beta4 stably expressed in HEK293 cells

Human embryonic kidney (HEK293) cells were transfected with cDNA encoding the human beta4 neuronal nicotinic acetylcholine (ACh) receptor subunit in pairwise combination with human alpha2, alpha3 or alpha4 subunits. Cell lines A2B4, A3B4.2 and A4B4 were identified that stably express mRNA and protein corresponding to alpha2 and beta4, to alpha3 and beta4 and to alpha4 and beta4 subunits, respectively. Specific binding of [3H]epibatidine was detected in A2B4, A3B4.2 and A4B4 cells with Kd (mean +/- S.D. in pM) values of 42 +/- 10, 230 +/- 12 and 187 +/- 29 and with Bmax (fmol/mg protein) values of 1104 +/- 338, 2010 +/- 184 and 3683 +/- 1450, respectively. Whole-cell patch-clamp recordings in each cell line demonstrated that (-)nicotine (Nic), ACh, cytisine (Cyt) and 1, 1-dimethyl-4-phenylpiperazinium iodide (DMPP) elicit transient inward currents. The current-voltage (I-V) relation of these currents showed strong inward rectification. Pharmacological characterization of agonist-induced elevations of intracellular free Ca++ concentration revealed a distinct rank order of agonist potency for each subunit combination as follows: alpha2beta4, (+)epibatidine (Epi) > Cyt > suberyldicholine (Sub) = Nic = DMPP; alpha3beta4, Epi > DMPP = Cyt = Nic = Sub; alpha4beta4, Epi > Cyt = Sub > Nic > DMPP. The noncompetitive antagonists mecamylamine and d-tubocurarine did not display subtype selectivity. In contrast, the Kb value for the competitive antagonist dihydro-beta-erythroidine (DHbetaE) was highest at alpha3beta4 compared with alpha2beta4 or alpha4beta4 receptors. These data illustrate that the A2B4, A3B4.2 and A4B4 stable cell lines are powerful tools for examining the functional and pharmacological properties of human alpha2beta4, alpha3beta4 and alpha4beta4 neuronal nicotinic receptors.

Rat group I metabotropic glutamate receptors inhibit neuronal Ca2+ channels via multiple signal transduction pathways in HEK 293 cells

We have shown previously that metabotropic glutamate receptors with group I-like pharmacology couple to N-type and P/Q-type calcium channels in acutely isolated cortical neurons using G proteins most likely belonging to the Gi/Go subclass. To better understand the potential mechanisms forming the basis for group I mGluR modulation of voltage-gated calcium channels in the CNS, we have examined the ability of specific mGluRs to couple to neuronal N-type (alpha1B-1/alpha2delta/beta1b) and P/Q-type (alpha1A-2/alpha2delta/beta1b) voltage-gated calcium channels in an HEK 293 heterologous expression system. Using the whole cell patch-clamp technique where intracellular calcium is buffered to low levels, we have shown that group I receptors inhibit both N-type and P/Q-type calcium channels in a voltage-dependent fashion. Similar to our observations in cortical neurons, this voltage-dependent inhibition is mediated almost entirely by N-ethylmaleimide (NEM)-sensitive heterotrimeric G proteins, strongly suggesting that these receptors can use Gi/Go-like G proteins to couple to N-type and P/Q-type calcium channels. However, inconsistent with the apparent NEM sensitivity of group I modulation of calcium channels, modulation of N-type channels in group I mGluR-expressing cells was only partially sensitive to pertussis toxin (PTX), indicating the potential involvement of both PTX-sensitive and -resistant G proteins. The PTX-resistant modulation was voltage dependent and entirely resistant to NEM and cholera toxin. A time course of treatment with PTX revealed that this toxin caused group I receptors to slowly shift from using a primarily NEM-sensitive G protein to using a NEM-resistant form. The PTX-induced switch from NEM-sensitive to -resistant modulation was also dependent on protein synthesis, indicating some reliance on active cellular processes. In addition to these voltage-dependent pathways, perforated patch recordings on group I mGluR-expressing cells indicate that another slowly developing, calcium-dependent form of modulation for N-type channels may be seen when intracellular calcium is not highly buffered. We conclude that group I mGluRs can modulate neuronal Ca2+ channels using a variety of signal transduction pathways and propose that the relative contributions of different pathways may exemplify the diversity of responses mediated by these receptors in the CNS.

Relative contributions of G protein, channel, and receptor to voltage-dependent inhibition of neuronal N-type and P/Q-type calcium channels in HEK 293 cell lines

The voltage-dependent modulation of neuronal voltage-gated calcium channels by heterotrimeric G protein-coupled receptors potentially provides a means for activity-dependent modulation of synaptic efficacy. Recent attention has focused upon the molecular mechanisms by which such G proteins influence the biophysical properties of calcium channels. We have used an HEK 293-based heterologous system which stably expresses human neuronal calcium channels to address the relative contributions of receptor, G protein, and channel to voltage-dependent inhibition. We find that the receptor and channel subtype only insignificantly influence the time it takes to re-establish modulation following voltage-dependent relief of inhibition. In contrast, the G protein subtype mediating inhibition appears to play a significant part in this process. These results emphasize the importance of G protein subtype in the modulation of neuronal calcium channels.

The expression of voltage-dependent calcium channel beta subunits in human cerebellum

The beta subunits of voltage-dependent calcium channels, exert marked regulatory effects on the biophysical and pharmacological properties of this diverse group of ion channels. However, little is known about the comparative neuronal expression of the four classes of beta genes in the CNS. In the current investigation we have closely mapped the distribution of beta1, beta2, beta3 and beta4 subunits in the human cerebellum by both in situ messenger RNA hybridization and protein immunohistochemistry. To our knowledge, these studies represent the first experiments in any species in which the detailed localization of each beta protein has been comparatively mapped in a neuroanatomically-based investigation. The data indicate that all four classes of beta subunits are found in the cerebellum and suggest that in certain neuronal populations they may each be expressed within the same cell. Novel immunohistochemical results further exemplify that the beta voltage-dependent calcium channel subunits are regionally distributed in a highly specific manner and studies of Purkinje cells indicate that this may occur at the subcellular level. Preliminary indication of the subunit composition of certain native voltage-dependent calcium channels is suggested by the observation that the distribution of the beta3 subunit in the cerebellar cortex is identical to that of alpha(1E). Our cumulative data are consistent with the emerging view that different native alpha1/beta subunit associations occur in the CNS.

Assignment of human genes for beta 2 and beta 4 subunits of voltage-dependent Ca2+ channels to chromosomes 10p12 and 2q22-q23

We have used human beta 2 and beta 4 cDNA probes to map the genes encoding two isoforms of the regulatory beta subunit of voltage-activated Ca2+ channels, viz. CACNB2 (beta 2) and CACNB4 (beta 4), to human chromosomes 10p12 and 2q22-q23, respectively, by fluorescence in situ hybridization. The gene encoding the beta 2 protein, first described as a Lambert-Eaton myasthenic syndrome (LEMS) antigen in humans, is found close to a region that undergoes chromosome rearrangements in small cell lung cancer, which occurs in association with LEMS. CACNB2 (beta 2) and CACNB4 (beta 4) genes are members of the ion-channel gene superfamily and it should now be possible to examine their loci by linkage analysis of ion-channel-related disorders. To date, no such disease-related gene has been assigned to 10p12 and 2q22-q23.

Preparation and purification of antibodies specific to human neuronal voltage-dependent calcium channel subunits

Neuronal voltage-dependent calcium channels (VDCCs) each comprising of alpha 1, alpha 2 delta, and beta subunits, are one mechanism by which excitable cells regulate the flux of calcium ions across the cell membrane following depolarisation Studies have shown the expression of several alpha 1 and beta subtypes within neuronal tissue. The comparative distribution of these in normal human brain is largely unknown. The aim of this work is to prepare antibodies directed specifically to selected subunits of human neuronal VDCCs for use in biochemical and mapping studies of calcium channel subtypes in the brain. Previous studies have defined DNA sequences specific for each subunit Comparison of these sequences allows the selection of unique amino acid sequences for use as immunogens which are prepared as glutathione-S-transferase (GST) fusion proteins in E. coli. Polyclonal antibodies raised against these fusion proteins are purified by Protein A chromatography, followed by immunoaffinity chromatography and extensive adsorptions using the appropriate fusion protein-GST Sepharose 4B columns. The resultant antibodies are analysed for specificity against the fusion proteins by ELISA, and by immunofluorescence and Western immunoblot analysis of recombinant HEK293 cells stably transfected with cDNAs encoding alpha 1, alpha 2 delta and beta subunits.

Functional coupling of rat group II metabotropic glutamate receptors to an omega-conotoxin GVIA-sensitive calcium channel in human embryonic kidney 293 cells

Metabotropic glutamate receptors are G protein-coupled receptors that perform a variety of modulatory roles in the central and peripheral nervous systems. The development of receptor subtype-specific agonists/antagonists has lagged far behind the isolation and characterization of receptor cDNAs. Further more, the coupling of specific metabotropic receptors to the various neuronal-specific effector molecules, such as voltage gated Ca2+ channels, has not been well studied. It was recently demonstrated that a rat group II metabotropic receptor (rm-GluR2) is capable of coupling to endogenous N-type Ca2+ channels when heterologously expressed in adult rat sympathetic ganglia neurons. To eventually understand the molecular aspects of metabotropic receptor modulation of the N-type Ca2+ channel, we have transiently expressed both group II receptors in a human embryonic kidney 293 cell line (G1A1) that stably expresses the human alpha 1B-1, alpha 2b, and beta 1-3 Ca2+ channel subunits. rmGluR2 and rmGluR3 modulate the omega-conotoxin GVIA-sensitive Ba2+ currents in G1A1 cells using a voltage-dependent mechanism via an endogenous pertussis toxin-sensitive G protein. Cell-attached "macropatch" recordings demonstrate that modulation by rmGluR2 and rmGluR3 is membrane delimited. This is the first report of Ca2+ channel modulation mediated by rmGluR3. In addition, an extensive pharmacological comparison between rmGluR2 and rmGluR3 reveals that these group II receptors interact with agonists and antagonists in unique ways.

Comparative structure of human neuronal alpha 2-alpha 7 and beta 2-beta 4 nicotinic acetylcholine receptor subunits and functional expression of the alpha 2, alpha 3, alpha 4, alpha 7, beta 2, and beta 4 subunits

cDNA clones encoding human neuronal nicotinic acetylcholine receptor alpha 2, alpha 3, alpha 4, alpha 5, alpha 6, alpha 7, beta 2, beta 3, and beta 4 subunits were isolated from brainstem, hippocampus, prefrontal cortex, substantia nigra, thalamus, and IMR32 libraries. Human alpha 2 and alpha 6 and full-length beta 3 and beta 4 clones have not been previously reported. Deduced amino acid sequences of the alpha 2, alpha 6, beta 3, and beta 4 predicted mature peptides are 503 residues (56.9 kDa), 464 residues (53.7 kDa), 440 residues (50.8 kDa), and 477 residues (54.1 kDa), respectively. These sequences show 84 (alpha 2), 87 (alpha 6), 89 (beta 3), and 84% (beta 4) identity to the corresponding rat sequences. The amino termini of the human alpha 2 and beta 3 mature peptides contain 23 and six additional residues, respectively, compared to those of rat alpha 2 and beta 3. Recombinant receptors were expressed in Xenopus laevis oocytes injected with in vitro transcripts encoding either alpha 7 alone or alpha 2, alpha 3, or alpha 4 in pairwise combination with beta 2 or beta 4. Inward currents were elicited by the application of acetylcholine (1-100 microM) and other agonists; these responses were blocked 65-97% by application of 10 microM d-tubocurare, confirming functional expression of human nicotinic receptors.

Distribution of alpha 1A, alpha 1B and alpha 1E voltage-dependent calcium channel subunits in the human hippocampus and parahippocampal gyrus

The distribution of voltage-dependent calcium channel subunits in the central nervous system may provide information about the function of these channels. The present study examined the distribution of three alpha-1 subunits, alpha 1A, alpha 1B and alpha 1E, in the normal human hippocampal formation and parahippocampal gyrus using the techniques of in situ hybridization and immunocytochemistry. All three subunit mRNAs appeared to be similarly localized, with high levels of expression in the dentate granule and CA pyramidal layer. At the protein level, alpha 1A, alpha 1B and alpha 1E subunits were differentially localized. In general, alpha 1A-immunoreactivity was most intense in cell bodies and dendritic processes, including dentate granule cells, CA3 pyramidal cells and entorhinal cortex pre-alpha and pri-alpha cells. The alpha 1B antibody exhibited relatively weak staining of cell bodies but stronger staining of neuropil, especially in certain regions of high synaptic density such as the polymorphic layer of the dentate gyrus and the stratum lucidum and radiatum of the CA regions. The alpha 1E staining pattern shared features in common with both alpha 1A and alpha 1B, with strong immunoreactivity in dentate granule, CA3 pyramidal and entorhinal cortex pri-alpha cells, as well as staining of the CA3 stratum lucidum. These findings suggest regions in which particular subunits may be involved in synaptic communication. For example, comparison of alpha 1B and alpha 1E staining in the CA3 stratum lucidum with calbindin-immuno-reactivity suggested that these two calcium channels subunits may be localized presynaptically in mossy fibre terminals and therefore may be involved in neurotransmitter release from these terminals.

Chromosomal localization of the human genes for alpha 1A, alpha 1B, and alpha 1E voltage-dependent Ca2+ channel subunits

The alpha 1 subunit genes encoding voltage-dependent Ca2+ channels are members of a gene family. We have used human brain cDNA probes to localize the neuronal isoform genes CACNL1A4 (alpha 1A), CACNL1A5 (alpha 1B), and CACNL1A6 (alpha 1E) to 19p13, 9q34, and 1q25-q31, respectively, using fluorescence in situ hybridization on human chromosomes. These genes are particularly interesting gene candidates in the pathogenesis of neuronal disorders. Although genetic disorders have been linked to loci 9q34 and 19p13, no genetic disease related to Ca2+ signaling defects has yet been linked to these loci.

Characteristics of a human N-type calcium channel expressed in HEK293 cells

The human alpha 1B-1 alpha 2b beta 1-2 Ca2+ channel was stably expressed in HEK293 cells producing a human brain N-type voltage-dependent calcium channel (VDCC). Whole cell voltage-clamp electrophysiology and fura-2 based microfluorimetry have been used to study its characteristics. Calcium currents (ICa) recorded in transfected HEK293 cells were activated at potentials more depolarized than -20 mV with peak currents occurring at approx + 10 mV in 5 mM extracellular CaCl2. ICa and associated rises in intracellular free calcium concentrations ([Ca2+]i) were sensitive to changes in both the [Ca2+]o and holding potential. Steady-state inactivation was half maximal at a holding potential of -60 mV. Ba2+ was a more effective charge carrier than Ca2+ through the alpha 1B-1 alpha 2b beta 1-2 Ca2+ channel and combinations of both Ba2+ and Ca2+ as charge carriers resulted in the anomalous mole fraction effect. Ca2+ influx into transfected HEK293 cells was irreversibly inhibited by omega-conotoxin-GVIA (omega-CgTx-GVIA; 10 nM-1 microM) and omega-conotoxin-MVIIA; 100 nM-1 microM) whereas 1 microM) whereas no reductions were seen with agents which block P or L-type Ca2+ channels. The inorganic ions, gadolinium (Gd3+), cadmium (Cd2+) and nickel (Ni2+) reduced the ICa under voltage-clamp conditions in a concentration-dependent manner. The order of potency of the three ions was Gd3+ > Cd2+ > Ni2+. These experiments suggest that the cloned and expressed alpha 1B-1 alpha 2b beta 1-2 Ca2+ channel subunits form channels in HEK293 cells that exhibit properties consistent with the activity of the native-N-type VDCC previously described in neurons.

Structure and functional characterization of neuronal alpha 1E calcium channel subtypes

We have cloned overlapping cDNAs encoding alpha 1E Ca2+ channel subunits from mouse and human brain. We observed that these alpha 1E transcripts were widely distributed in the central nervous system. We also demonstrated the existence of two variants of the human alpha 1E subunit. Comparison of the sequence of these alpha 1E subunits to those from other species suggests that at least four alternatively spliced variants of alpha 1E exist. Expression of human alpha 1E in HEK293 cells and Xenopus oocytes produced high voltage-activated Ca2+ currents that inactivated rapidly (tau approximately 20 ms at 0 mV). The size of the currents obtained were enhanced approximately 40-fold by co-expression with human neuronal alpha 2 and beta Ca2+ channel subunits. alpha 1E currents were insensitive to the drugs and toxins previously used to define other classes of voltage-activated Ca2+ channels. Thus, alpha 1E-mediated Ca2+ channels appear to be a pharmacologically distinct class of voltage-activated Ca2+ channels.

Human neuronal voltage-dependent calcium channels: studies on subunit structure and role in channel assembly

Voltage-dependent calcium (Ca2+) channels, expressed in the CNS, appear to be multimeric complexes comprised of at least alpha 1, alpha 2 and beta subunits. Previously, we cloned and expressed human neuronal alpha 1, alpha 2 and beta subunits to study recombinant channel complexes that display properties of those expressed in vivo. The alpha 1B-mediated channel subtype binds omega-conotoxin (CgTx) GVIA with high affinity and exhibits properties of N-type voltage-dependent Ca2+ channels. Here we describe several alpha 2 and beta splice variants and report results on the expression of omega-CgTx GVIA binding sites, assembly of the subunit complex and biophysical function of alpha 1B-mediated channel complexes containing some of these splice variants. We optimized recombinant expression in human embryonic kidney (HEK) 293 cells of alpha 1B alpha 2b beta 1 subunit complexes by controlling the expression levels of subunit mRNAs and monitored cell surface expression by binding of omega-CgTx GVIA to the alpha 1B subunit. Co-expression of either alpha 2b or beta 1 subunits with an alpha 1B subunit increased expression of binding sites while the most efficient expression was achieved when both alpha 2b and beta 1 subunits were co-expressed with an alpha 1B subunit. The presence of alpha 2b affects the affinity of omega-CgTx GVIA binding and barium (Ba2+) current magnitudes, although it does not appear to alter kinetic properties of the Ba2+ current. This is the first evidence of an alpha 2 subunit modulating the binding affinity of a cell-surface Ca2+ channel ligand. Our results demonstrate that alpha 1, alpha 2 and beta subunits together contribute to the efficient assembly and functional expression of voltage-dependent Ca2+ channel complexes.

Structure and functional expression of an omega-conotoxin-sensitive human N-type calcium channel

N-type calcium channels are omega-conotoxin (omega-CgTx)-sensitive, voltage-dependent ion channels involved in the control of neurotransmitter release from neurons. Multiple subtypes of voltage-dependent calcium channel complexes exist, and it is the alpha 1 subunit of the complex that forms the pore through which calcium enters the cell. The primary structures of human neuronal calcium channel alpha 1B subunits were deduced by the characterization of overlapping complementary DNAs. Two forms (alpha 1B-1 and alpha 1B-2) were identified in human neuroblastoma (IMR32) cells and in the central nervous system, but not in skeletal muscle or aorta tissues. The alpha 1B-1 subunit directs the recombinant expression of N-type calcium channel activity when it is transiently co-expressed with human neuronal beta 2 and alpha 2b subunits in mammalian HEK293 cells. The recombinant channel was irreversibly blocked by omega-CgTx but was insensitive to dihydropyridines. The alpha 1B-1 alpha 2b beta 2-transfected cells displayed a single class of saturable, high-affinity (dissociation constant = 55 pM) omega-CgTx binding sites. Co-expression of the beta 2 subunit was necessary for N-type channel activity, whereas the alpha 2b subunit appeared to modulate the expression of the channel. The heterogeneity of alpha 1B subunits, along with the heterogeneity of alpha 2 and beta subunits, is consistent with multiple, biophysically distinct N-type calcium channels.

Structure and functional expression of alpha 1, alpha 2, and beta subunits of a novel human neuronal calcium channel subtype

The primary structures of human neuronal alpha 1, alpha 2, and beta subunits of a voltage-dependent Ca2+ channel were deduced by characterizing cDNAs. The alpha 1 subunit (alpha 1D) directs the recombinant expression of a dihydropyridine-sensitive L-type Ca2+ channel when coexpressed with the beta (beta 2) and the alpha 2 (alpha 2b) subunits in Xenopus oocytes. The recombinant channel is also reversibly blocked by 10-15 microM omega-conotoxin. Expression of the alpha 1D subunit alone, or coexpression with the alpha 2b subunit, did not elicit functional Ca2+ channel activity. Thus, the beta 2 subunit appears to serve an obligatory function, whereas the alpha 2b subunit appears to play an accessory role that potentiates expression of the channel. The primary transcripts encoding the alpha 1D, alpha 2, and beta subunits are differentially processed. At least two forms of neuronal alpha 1D were identified. Different forms of alpha 2 and beta transcripts were also identified in CNS, skeletal muscle, and aorta tissues.

Structural characterization of the dihydropyridine-sensitive calcium channel alpha 2-subunit and the associated delta peptides

Upon disulfide bond reduction, the alpha 2-subunit of the dihydropyridine-sensitive Ca2+ channel undergoes a characteristic mobility shift on sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis with the concurrent appearance of the three delta peptides delta 1 (25,000 Da), delta 2 (22,000 Da), and delta 3 (17,000 Da). Densitometric scanning of Coomassie Blue-stained gels shows a stoichiometric ration of 1.0:0.31.47:0.08 for the alpha 2-subunit and the delta peptides 1, 2, and 3, respectively. Characterization of the delta peptides using antibodies, photoincorporation of a hydrophobic probe, and lectin staining shows tham to be antigenically similar hydrophobic glycoproteins. Amino-terminal sequence analysis of the delta peptides reveals three identical sequences that match the predicted amino acid sequence of the alpha 2-subunit starting at Ala935. Enzymatic deglycosylation of the reduced alpha 2.delta complex produces individual core peptides of 105,000 and 17,000 Da, respectively. Treatment of skeletal muscle membranes with high pH in the presence of reducing agents is able to extract the larger amino-terminal peptide but not the smaller carboxyl (delta) peptide, consistent with a single transmembrane domain in the carboxyl (delta) region. The data support a model of the alpha 2-subunit in which the propeptide is processed into two chains that remain attached through disulfide linkages.

Primary structure of the gamma subunit of the DHP-sensitive calcium channel from skeletal muscle

Affinity-purified, polyclonal antibodies to the gamma subunit of the dihydropyridine (DHP)-sensitive, voltage-dependent calcium channel have been used to isolate complementary DNAs to the rabbit skeletal muscle protein from an expression library. The deduced primary structure indicates that the gamma subunit is a 25,058-dalton protein that contains four transmembrane domains and two N-linked glycosylation sites, consistent with biochemical analyses showing that the gamma subunit is a glycosylated hydrophobic protein. Nucleic acid hybridization studies indicate that there is a 1200-nucleotide transcript in skeletal muscle but not in brain or heart. The gamma subunit may play a role in assembly, modulation, or the structure of the skeletal muscle calcium channel.

Sequence and expression of mRNAs encoding the alpha 1 and alpha 2 subunits of a DHP-sensitive calcium channel

Complementary DNAs were isolated and used to deduce the primary structures of the alpha 1 and alpha 2 subunits of the dihydropyridine-sensitive, voltage-dependent calcium channel from rabbit skeletal muscle. The alpha 1 subunit, which contains putative binding sites for calcium antagonists, is a hydrophobic protein with a sequence that is consistent with multiple transmembrane domains and shows structural and sequence homology with other voltage-dependent ion channels. In contrast, the alpha 2 subunit is a hydrophilic protein without homology to other known protein sequences. Nucleic acid hybridization studies suggest that the alpha 1 and alpha 2 subunit mRNAs are expressed differentially in a tissue-specific manner and that there is a family of genes encoding additional calcium channel subtypes.

Isolation of alcohol oxidase and two other methanol regulatable genes from the yeast Pichia pastoris

The oxidation of methanol follows a well-defined pathway and is similar for several methylotrophic yeasts. The use of methanol as the sole carbon source for the growth of Pichia pastoris stimulates the expression of a family of genes. Three methanol-responsive genes have been isolated; cDNA copies have been made from mRNAs of these genes, and the protein products from in vitro translations have been examined. The identification of alcohol oxidase as one of the cloned, methanol-regulated genes has been made by enzymatic, immunological, and sequence analyses. Methanol-regulated expression of each of these three isolated genes can be demonstrated to occur at the level of transcription. Finally, DNA subfragments of two of the methanol-responsive genomic clones from P. pastoris have been isolated and tentatively identified as containing the control regions involved in methanol regulation.

Large heterogeneous nuclear ribonucleic acid has three times as many 5' caps as polyadenylic acid segments, and most caps do not enter polyribosomes

The rate of synthesis in Chinese hamster cells of 5' cap structures, m7 GpppNmp, in large (greater than 700 bases) heterogeneous nuclear ribonucleic acid (RNA) molecules is two to three times faster than the synthesis of 3'-terminal polyadenylic acid segments. As judged by presence of caps, newly synthesized polysomal messenger RNA, exclusive of messenger RNA the size of histone messenger RNA, is more than 90% in the polyadenylated category. It appears, therefore, that between half and two-thirds of the long capped heterogeneous nuclear RNA molecules do not contribute a capped polysomal derivative to the cytoplasm. There are capped, nonpolysomal, non-polyadenylated molecules with a rapid turnover rate that fractionate with the cytoplasm. These metabolically unstable molecules either could represent leakage into the cytoplasm during fractionation or could truly spend a brief time in the cytoplasm before decay.

Chinese hamster polyadenylated messenger ribonucleic acid: relationship to non-polyadenylated sequences and relative conservation during messenger ribonucleic acid processing

We have further analyzed the metabolism of specific messenger ribonucleic acid (mRNA) sequences within the cytoplasmic and nuclear RNA of Chinese hamster ovary (CHO) cells by using a set of previously constructed complementary deoxyribonucleic acid (DNA) clones (Harpold et al., Cell 17:1025-1035, 1979) as specific molecular probes in a variety of RNA:DNA hybridization experiments. The majority of the labeled mRNA complementary to each of the nine clones was found in the polyribosomes, with some variation between individual sequences. The great majority of each specific mRNA labeled for 3 h or less was in the polyadenylated [poly(A)+] fraction. However, the amount of each sequence increased in the non-poly(A)+ [poly(A)-] fraction after very long label times, suggesting the derivation of the poly(A)- RNA from the poly(A)+ RNA. Eight of the nine mRNA's have cytoplasmic half-lives ranging from 8 to 14 h, whereas one of the mRNA's, the scarcest in the group, has a somewhat shorter half-life of approximately 3 h. The proportion of each of the specific long-lived mRNA's within the total labeled mRNA increased as a function of labeling time, indicating that a large fraction, probably greater than 50%, of the initially labeled poly(A)+ mRNA in CHO cells has a half-life of less than 3 h. A quantitative analysis of the kinetics of labeling of specific nuclear and cytoplasmic sequences indicated that a significant fraction of the mRNA sequences transcribed from genes containing these nine CHO sequences were successfully processed into mRNA. However, two of the CHO mRNA sequences were only partially conserved during nuclear processing to yield mRNA. These studies demonstrated that events at two post-transcriptional levels, differential nuclear processing efficiency of different primary transcripts and cytoplasmic stability of different mRNA's, can be involved in the determination of the cytoplasmic concentrations of different mRNA's.

Production of mRNA in Chinese hamster cells: relationship of the rate of synthesis to the cytoplasmic concentration of nine specific mRNA sequences

We constructed cloned DNA sequences complementary to unselected mRNAs [poly(A)+ cytoplasmic RNA] from Chinese hamster ovary cells and used them in RNA:DNA hybridization experiments. Each cloned DNA hybridized a single mRNA from 1.3-3.5 kb in length. The relative rates of labeling (transcription rates) of nuclear RNA complementary to each individual DNA segment varied approximately 10 fold. The relative cytoplasmic concentration of the same specific RNA sequences in the mRNA after an equilibrium labeling of the cells varied approximately 100 fold. In addition, we estimated the sizes of the nuclear RNA precursor molecules to these cytoplasmic mRNAs. Four main conclusions arise from these studies. First, the primary RNA transcripts, which range in size from 2.4-13.5 kb, are 2-6 times larger than the mRNAs; second, each cloned DNA segment is complementary to only one species of mRNA; third, for the RNA complementary to at least three of the nine cloned DNA segments, the relative cytoplasmic content is considerably different from the relative rate of nuclear RNA synthesis, suggesting the post-transcriptional events are involved in the determination of the cytoplasmic concentrations of some mammalian mRNAs; and fourth, the fraction of total nonribosomal nuclear RNA complementary to the nine cloned DNA segments is in most cases 10 fold less than the fraction of cytoplasmic mRNA complementary to the same cloned DNA segments, suggesting the synthesis of many hnRNA molecules that are qualitatively different from those which eventually contribute mRNA to the cytoplasm.

The synthesis and processing of a nuclear RNA precursor to rat pregrowth hormone messenger RNA

A recombinant DNA plasmid, pBR322-GH1, which contains about 80% of the sequences of rat pregrowth hormone (pGH) mRNA, allowed an analysis of nuclear RNA from GH3 cells for possible precursors of cytoplasmic pGH mRNA. A single 20-22S RNA SPECIES ABOUT 2-3 TIMes larger than pGH mRNA was detected in nuclear RNA from GH3 cells labeled for 5 min. with 3H-uridine. After longer label times a 12S RNA indistinguishable in size from cytoplasmic 12S pGH mRNA became the predominant labeled RNA complementary to the plasmid pBR322-GH1. Both of these nuclear RNA species contained poly (A). Kinetic analysis of the labeling of nuclear and cytoplasmic pGH mRNA sequences showed that the 20S and 12S nuclear RNA molecules were labeled before significant labeling of cytoplasmic pGH mRNA was detected, and also indicated that there is complete conservation of nuclear pGH mRNA sequences in the production of cytoplasmic pGH mRNA. These results indicate that cytoplasmic pGH mRNA is generated by nuclear processing of a larger nuclear RNA molecule.

Construction and identification by positive hybridization-translation of a bacterial plasmid containing a rat growth hormone structural gene sequence

The construction, identification, and use of a recombinant DNA clone containing a growth hormone structural gene sequence is described. A cDNA copy of partially purified pregrowth hormone mRNA from cultured rat pituitary tumor (GC) cells was employed in the construction of a hybrid plasmid, designated pBR322-GH1. The cloned DNA sequence was positively identified by a hybridization-translation procedure which should be applicable to any cloned structural gene sequence. This procedure involved hybridization of cytoplasmic poly(A)-containing RNA from GC cells to the cloned DNA immobilized on nitrocellulose filters, followed by elution of the hybridized RNA and translation in a mRNA-depleted rabbit reticulocyte lysate system. Physical and immunological criteria were employed to show that the translation products were enriched for pregrowth hormone. Hybridization to excess plasmid DNA of [3H]uridine-labeled, size fractionated GC cell cytoplasmic RNA was used to show that all growth hormone-specific RNA sequences are the same size as functional pregrowth hormone mRNA.

The evolution of nonrepetitive DNA in sea urchins

Molecular hybridization of nuclear DNAs has been employed to study the evolution of nonrepetitive DNA sequences in four species of sea urchin. The data indicate that the extent of homology between the nonrepetitive DNA sequences of S. purpuratus, S. droebachiensis, S. franciscanus, and L. pictus confirms the phylogenetic relationship established through palaeontological evidence. The average rate of divergence of nonrepetitive DNA sequences was found to be approximately 0.22% per million years. In addition, a small fraction (approximately 14%) of the nonrepetitive DNA sequences is highly conserved between S. purpuratus and L. pictus after 120-200 million years divergence. This study may provide a basis for the use of interspecific hybrid embryos of these organisms to investigate the evolution and importance of certain DNA sequences in early developmental processes leading to cell differentiation.

The evolution of repetitive DNA sequences in sea urchins

Molecular hybridization of nuclear DNAs has been employed to study the evolution of the repetitive DNA sequences in four species of sea urchin. The data show that relative to S. purpuratus there has been approximately 0.1% sequence divergence per million years in the repetitive DNA sequences of S. droebachiensis, S. franciscanus, and L. pictus. These results confirm that repetitive DNA sequences are strongly conserved during evolution. However, comparison of the extent of base pair mismatch in the repetitive DNA heteroduplexes formed at Cot 20 with those formed at Cot 200 during the hybridization of S. purpuratus and L. pictus DNAs reveals that highly repetitive sequences of sea urchins may diverge more rapidly than do the more moderately repetitive sequences.