Multiple functional Sp1 domains in the minimal promoter region of the neuronal nicotinic receptor alpha5 subunit gene

The alpha5 subunit is a component of the neuronal nicotinic acetylcholine receptors, which are probably involved in the activation step of the catecholamine secretion process in bovine adrenomedullary chromaffin cells. The promoter of the gene coding for this subunit was isolated, and its proximal region was characterized, revealing several GC boxes located close to the site of transcription initiation (from -111 to -40). Deletion analysis and transient transfections showed that a 266-base pair region (-111 to +155) gave rise to approximately 77 and 100% of the maximal transcriptional activity observed in chromaffin and SHSY-5Y neuroblastoma cells, respectively. Site-directed mutagenesis of five different GC motifs indicated that all of them contribute to the activity of the alpha5 gene, but in a different way, depending on the type of transfected cell. Thus, in SHSY-5Y cells, alteration of the most promoter-proximal of the GC boxes decreased alpha5 promoter activity by approximately 50%, whereas single mutations of the other GC boxes had no effect. In chromaffin cells, by contrast, modification of any of the GC boxes produced a similar decrease in promoter activity (50-69%). In both cell types, however, activity was almost abolished when four GC boxes were suppressed simultaneously. Electrophoretic mobility shift assays using nuclear extracts from either chromaffin or SHSY-5Y cells showed the specific binding of Sp1 protein to fragment -111 to -27. Binding of Sp1 to the GC boxes was also demonstrated by DNase I footprint analysis. This study suggests that the general transcription factor Sp1 plays a dominant role in alpha5 subunit expression, as has also been demonstrated previously for alpha3 and beta4 subunits. Since these three subunits have their genes tightly clustered and are expressed in chromaffin cells, probably as components of the same receptor subtype, we propose that Sp1 constitutes the key factor of a regulatory mechanism common to the three subunits.

Interaction between prostanoids and nitric oxide in the control of tubular function in rats with chronic bile duct ligation

Recent work indicates that both nitric oxide and cyclooxygenase products play an important role in the renal alterations of liver cirrhosis, although the interactions between them have not been completely established. The purpose of this study was to assess the effect of simultaneous blockade of nitric oxide synthase and cyclooxygenase in rats with chronic bile duct ligation and in control, sham-operated rats. Compared with control rats, chronic bile duct ligation rats, 23-25 days after surgery, showed a decreased mean arterial pressure, natriuresis, and kaliuresis, without differences in glomerular filtration rate, and an increased urinary nitrite excretion. Nitric oxide synthesis inhibition by administration of N(G)-nitro-L-arginine methyl ester induced, in control rats, an increase in mean arterial pressure, without significant changes in natriuresis or glomerular filtration rate. In chronic bile duct ligation rats, N(G)-nitro-L-arginine methyl ester induced an increase in mean arterial pressure, natriuresis, and kaliuresis, together with a reduction in urinary nitrite excretion and an increase in prostaglandin E2 excretion. Cyclooxygenase inhibition with indomethacin induced in both experimental groups a marked inhibition in urinary prostaglandin E2 excretion without significant changes in Na+ or K+ excretion, and a significant increase in urinary nitrite excretion in control rats. N(G)-Nitro-L-arginine methyl ester in addition to indomethacin prevented the indomethacin-induced increase in nitrite excretion and dramatically reduced sodium excretion in both experimental groups. Thus, the present study suggests that both nitric oxide and cyclooxygenase products interact in the control of urinary sodium excretion and that each system is activated in the absence of the other one.

A residue in the middle of the M2-M3 loop of the beta4 subunit specifically affects gating of neuronal nicotinic receptors

An aspartate residue in the M2-M3 loop of neuronal nicotinic receptor alpha7 subunits is a major determinant of the channel functional response. This residue is conserved in most beta4 subunits, e.g. human and rat, but not in others, e.g. bovine. We have used these differences to examine the mechanism by which this residue alters the functional properties of alpha3beta4 receptors. Having ruled out an effect on the macroscopic binding ability of the agonist, the level of receptor expression, or the single channel conductance, the results suggest that receptors lacking that residue have a deficient coupling between binding and gating.

GC- and E-box motifs as regulatory elements in the proximal promoter region of the neuronal nicotinic receptor alpha7 subunit gene

The alpha7 subunit is a component of alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors expressed in bovine adrenomedullary chromaffin cells. The proximal promoter of the gene coding for this subunit contains several GC-boxes and one E-box. Deletion analysis and transient transfections showed that a 120-base pair region (-77 to +43) including all of these elements gave rise to approximately 70 and 95% of the maximal transcriptional activity observed in chromaffin and SHSY-5Y neuroblastoma cells, respectively. Site-directed mutagenesis of the different elements indicated that both GC and E motifs contribute to the activity of the alpha7 gene in a very prominent way. Using electrophoretic mobility shift assays, the upstream stimulatory factor (USF) was shown to be a component of the complexes that interacted with the E-box when nuclear extracts from chromaffin and SHSY-5Y cells were used. Binding of the early growth response gene transcription factor (Egr-1) to three different GC-boxes was also demonstrated by shift assays and DNase I footprint analysis. Likewise, alpha7 promoter activity increased by up to 5-fold when alpha7 constructs and an Egr-1 expression vector were cotransfected into chromaffin cell cultures. Mutagenesis of individual GC-boxes had little effect on Egr-1 activation. By contrast, pairwise suppression of GC-boxes abolished activation, especially when the most promoter-proximal of the Egr-1 sites was removed. Taken together, these studies indicate that the alpha7 gene is likely to be a target for multiple signaling pathways, in which various regulatory elements are involved.

Increased nitric oxide synthesis and inducible nitric oxide synthase expression in patients with alcoholic and non-alcoholic liver cirrhosis

1. The synthesis and release of nitric oxide may play a role in the pathogenesis of peripheral vasodilatation and hyperdynamic circulation observed in liver cirrhosis. In this work, we analysed the synthesis of nitric oxide by the lympho-mononuclear cells of peripheral blood from patients with chronic alcoholic and non-alcoholic liver disease and we identified the isoform of nitric oxide synthase involved in the increased nitric oxide synthesis. 2. Patients were classified following clinical and histological criteria in non-alcoholic cirrhotic, alcoholic cirrhotic and non-cirrhotic chronic liver disease. We studied clinical and analytical characteristics, haemodynamic parameters and endotoxin levels in these patients. 3. Cirrhotic patients showed an increase of cardiac output and a decrease of peripheral vascular resistance. These patients had higher levels of plasma endotoxin than those observed in the control group. N omega-Nitro-L-arginine methyl ester (L-NAME)-inhibitable nitrite production from mononuclear lymphocyte cells was higher in patients than in the control group, the highest levels being in non-alcoholic cirrhotic patients, and the lowest levels in patients with non-cirrhotic alcoholic liver disease. 4. Immunocytochemistry studies revealed a positive immunoreactivity for the inducible isoform of nitric oxide synthase in lympho-mononuclear cells that was more evident in non-alcoholic than in alcoholic cirrhotic patients. By Northern blot, inducible nitric oxide synthase mRNA expression was observed only in lymphomononuclear cells from non-alcoholic cirrhotic patients. 5. Our patients show a correlation between nitric oxide synthesis, endotoxin levels and haemodynamic parameters. 6. These findings indicate that lympho-mononuclear cell stimulation may play a role in elevated nitric oxide production in hepatic cirrhosis. Thus, this increased nitric oxide synthesis could be implicated in the pathogenesis of the haemodynamic disturbances frequently found in cirrhotic patients. This increase seems to be induced, at least in part, by activation of an inducible isoform of nitric oxide synthase.

[Role of endogenous nitric oxide (NO) in pulmonary vascular adaptation to one-lung ventilation during thoracic surgery]

To determine whether endogenous production of nitric oxide (NO) increases during one-lung lung ventilation (OLV) and to observe its possible influence on pulmonary vascular resistance (PVR). The experimental group consisted of 19 patients undergoing chest surgery with OLV, with monitoring by Swan-Ganz catheter. Hemodynamic parameters were measured and peripheral venous blood samples were taken at the start of surgery and 10 minutes after starting OLV. The control group consisted of 13 patients with comparable clinical characteristics undergoing chest surgery with conventional lung ventilation. In this group samples of peripheral venous blood were taken at the start of surgery and at a moment comparable to the sampling of the experimental group. NO was assessed by quantitative colorimetry as the concentration of nitrites in venous blood. Nitrite concentration in the experimental group was higher during OLV (12.7 +/- 10.1 muMol/ml) than initially (7.1 +/- 5.4 muMol/ml) (p < 0.05). Nitrite concentration was similar at the two moments sampled in the control group (8.7 +/- 2.8 muMol/ml at the start of surgery and 8.1 +/- 3.2 muMol/ml in the second sample taken). Mean percent increase in nitrite concentration in the experimental group was 93.7 +/- 117%; mean percent change in the control group was 3.9 +/- 23% (p < 0.001). PVR during OLV (134 +/- 45 din.s.cm-5) is lower than at the start of surgery (163 +/- 37 din.s.cm-5, p < 0.05). During OLV endogenous production of NO, measured as nitrite concentration in plasma, increases. PVR decreases, possibly because of the increase in nitrite concentration.

Differential expression of alpha-bungarotoxin-sensitive neuronal nicotinic receptors in adrenergic chromaffin cells: a role for transcription factor Egr-1

Adrenomedullary chromaffin cells express at least two subtypes of acetylcholine nicotinic receptors, which differ in their sensitivity to the snake toxin alpha-bungarotoxin. One subtype is involved in the activation step of the catecholamine secretion process and is not blocked by the toxin. The other is alpha-bungarotoxin-sensitive, and its functional role has not yet been defined. The alpha7 subunit is a component of this subtype. Autoradiography of bovine adrenal gland slices with alpha-bungarotoxin indicates that these receptors are restricted to medullary areas adjacent to the adrenal cortex and colocalize with the enzyme phenylethanolamine N-methyl transferase (PNMT), which confers the adrenergic phenotype to chromaffin cells. Transcripts corresponding to the alpha7 subunit also are localized exclusively to adrenergic cells. To identify possible transcriptional regulatory elements of the alpha7 subunit gene involved in the restricted expression of nicotinic receptors, we isolated and characterized its 5' flanking region, revealing putative binding sites for the immediate early gene transcription factor Egr-1, which is known to activate PNMT expression. In reporter gene transfection experiments, Egr-1 increased alpha7 promoter activity by up to sevenfold. Activation was abolished when the most promoter-proximal of the Egr-1 sites was mutated, whereas modification of a close upstream site produced a partial decrease of the Egr-1 response. Because Egr-1 was found to be expressed exclusively in adrenergic cells, we suggest that this transcription factor may be part of a common mechanism involved in the induction of the adrenergic phenotype and the differential expression of alpha-bungarotoxin-sensitive nicotinic receptors in the adrenal gland.

Elevated glomerular and blood mononuclear lymphocyte nitric oxide production in rats with chronic bile duct ligation: role of inducible nitric oxide synthase activation

Recent work indicates that nitric oxide (NO) plays an important role in the systemic and renal alterations of cirrhosis. In the present study, we have evaluated whether the inducible NO synthase (iNOS) isoform participates in the enhanced renal and systemic NO production of a rat model of cirrhosis. In vitro and in vivo experiments were performed in rats subjected to chronic bile duct ligation (BDL) and in sham-operated (SO) animals. Plasma nitrite (3.1 +/- 0.1 micromol/L in SO and 6.6 +/- 0.2 micromol/L in BDL), glomerular nitrite production (6.4 +/- 0.1 vs. 9.8 +/- 0.1 nmol/24h/7,000 glomeruli, respectively), and mononuclear lymphocyte cells nitrite production (0.3 +/- 0.04 vs. 0.6 +/- 0.12 nmol/10(6) cells, respectively) were all significantly higher in BDL than in SO. Moreover, mononuclear lymphocytes and glomeruli from BDL rats showed an increased expression of macrophage-type iNOS, detected by Western blot. Kidneys from BDL animals also showed an increased calcium-independent NO synthase activity, compared with those from SO rats. Constitutive endothelial-type NO synthase expression in glomeruli or the activity of calcium-dependent NO synthase in whole kidney did not show differences between BDL and SO rats. In cultured mesangial cells from normal rats, the addition of plasma from BDL but not of plasma from SO significantly stimulated (35%) nitrite production and increased the expression of macrophage-type iNOS. In addition, administration of aminoguanidine (AG), a preferential iNOS inhibitor, elevated dose-dependently mean arterial pressure in both groups, but this increase was greater in BDL (26.5 +/- 4.4 mm hg) than in SO (13.6 +/- 2.6). In BDL, AG also increased sodium and water excretion and glomerular filtration rate. In contrast, there were only small nonsignificant changes in SO animals. Therefore, these results indicate that the expression, activity and production of NO in kidneys, glomeruli, and mononuclear lymphocyte cells is elevated in BDL rats, and this is partly because of a plasma-derived substance(s), which stimulates iNOS formation. The amelioration of the arterial hypotension and the associated reduced excretory levels of these cirrhotic animals by aminoguanidine further support the involvement of the inducible NO synthase isoform in the renal alterations observed in BDL animals.

Role of the putative transmembrane segment M3 in gating of neuronal nicotinic receptors

The involvement of some structural domains in the gating of the neuronal nicotinic acetylcholine receptor (AChR) was studied by expressing functional alpha7/alpha3 chimeric subunits in Xenopus oocytes. Substitution of the M3 transmembrane segment in the alpha7 subunit modifies the kinetic properties of the chimeric AChRs as follows: (a) a 6-fold reduction in the maximal current evoked by nicotinic agonists, (b) a 10-fold decrease in the macroscopic desensitization rate, (c) an increase of almost 1 order of magnitude in the apparent affinity for acetylcholine and nicotine, and (d) a decrease in the affinity for alpha-bungarotoxin. Computer simulations showed that the first three effects could be accounted for by a simple kinetic model in which chimeric AChRs presented a smaller ratio of the gating rates, beta/alpha, and a slightly slower desensitization rate. It is concluded that the M3 domain influences the gating of neuronal AChRs.

Neuronal nicotinic acetylcholine receptors on bovine chromaffin cells: cloning, expression, and genomic organization of receptor subunits

Neuronal nicotinic acetylcholine receptors from bovine adrenomedullary chromaffin cells play a primary role in triggering catecholamine secretion. In the present study, their constituent subunits were characterized. In addition to the alpha 3 subunit, which we have previously cloned, the presence of alpha 5 and beta 4 but not of beta 2 subunits was detected by reverse transcription-PCR analysis of mRNA from adrenal medulla. In situ hybridization indicated that alpha 3, alpha 5, and beta 4 subunits are coexpressed in all chromaffin cells. The primary structure of alpha 5 and beta 4 subunits was determined and functional receptors were obtained upon coinjection of subunit cRNAs into Xenopus oocytes. In contrast to other beta 4-containing nicotinic receptors, the ones formed by the bovine beta 4 subunit are insensitive to the agonist cytisine. Finally, we characterized the intergenic region of alpha 3 and alpha 5 subunits, which together with the beta 4 subunit, form a gene cluster in rats and chickens. RNase assays and the existence of overlapping cDNAs indicate that, in the bovine genome, the alpha 3 and alpha 5 genes overlap at their 3' ends. This fact is probably due to inefficient transcription termination, as a result of weak polyadenylation signals.

Expression of alpha 7 neuronal nicotinic receptors during postnatal development of the rate cerebellum

Several lines of evidence suggest that alpha-bungarotoxin-sensitive neuronal nicotinic acetylcholine receptors may play a developmental role by modulating plasticity in neuronal circuits. The alpha 7 subunit, a main component of these receptors, is expressed in most regions of the brain, including the cerebellum, where it is present almost exclusively in Purkinje cells and deep cerebellar nuclei. Purkinje cells constitute the only efferent pathway of the cerebellum and their development involves complex interactions, which have been extensively studied. They therefore provide a potentially useful model for analysis of development plasticity which could be influenced by alpha 7 neuronal nicotinic receptors. In the present study a previously characterized monoclonal antibody (mAb 307) has been used to determine the temporal pattern of expression of the alpha 7 subunit in the developing rat cerebellum. No detectable alpha 7 immunoreactivity is found between P0 and P2. Between P3 and P5, however, the Purkinje cell layer shows moderate immunolabeling. alpha 7 expression in this layer increases rapidly between P8 and P15. This increase in alpha 7 staining, which overlaps in time with important developmental and synaptogenic events, is not uniform throughout the cerebellar cortex. Thus, between P3 and P5 all Purkinje cells are weakly labeled, while at later stages (P8-P15) immunolabeling becomes more intense, but at the same time, disappears from Purkinje cells in rostral lobules. In addition, a very well defined pattern for discontinuous or columnar labeling is detected in regions of the Purkinje cell layer where alpha 7 subunits were being expressed. Finally, at P20, alpha 7 subunit labeling is found again in all Purkinje cells, although with lower intensity. These results suggest that alpha 7 receptor expression is developmentally regulated, with a time course that parallels the final differentiation of Purkinje cells. In addition, the heterogeneous spatial distribution of alpha 7-containing nicotinic receptors indicates that, during cerebellar maturation, these cells may receive different signals that modulate receptor gene expression in a very specific way.

Acetylcholine receptor subunit homomer formation requires compatibility between amino acid residues of the M1 and M2 transmembrane segments

The neuronal nicotinic acetylcholine receptor (nAChR) subunits alpha3 and alpha7 have different assembly behavior when expressed in heterologous expression systems: alpha3 subunits require other subunits to assemble functional nAChRs, whereas alpha7 subunits can produce homomeric nAChRs. A previous analysis of alpha7/alpha3 chimeric constructs identified a domain comprising the first putative membrane-spanning segment, M1, as essential to homomeric assembly. The present study dissected further this domain, identifying three amino acid residues, which are located at the most intracellular third of the M1 transmembrane segment, as important in the assembly of homomers. Moreover, formation of homooligomeric complexes seems to require a compatible accommodation between this region and certain residues of the second transmembrane segment, M2. Thus, compatibility between defined domains of the M1 and M2 transmembrane segments appears as a determinant factor governing homomer association of nAChR subunits.

A single residue in the M2-M3 loop is a major determinant of coupling between binding and gating in neuronal nicotinic receptors

Binding of agonists to nicotinic acetylcholine receptors generates a sequence of changes that activate a cation-selective conductance. By measuring electrophysiological responses in chimeric alpha7/alpha3 receptors expressed in Xenopus oocytes, we have showed the involvement of the M2-M3 loop in coupling agonist binding to the channel gate. An aspartate residue therein, Asp-266 in the alpha7 subunit, was identified by site-directed mutagenesis as crucial, since mutants at this position exhibited very poor functional responses to three different nicotinic agonists. We have extended this investigation to another neuronal nicotinic receptor (alpha3/beta4), and found that a homologous residue in the beta4 subunit, Asp-268, played a similar role in coupling. These findings are consistent with a hypothesis that the aspartate residue in the M2-M3 loop, which is conserved in all homomer-forming alpha-type subunits and all neuronal beta-type subunits that combine to form functional receptors, is a major determinant of information transmission from binding site to channel gate in all neuronal nicotinic receptors.

alpha-Bungarotoxin-sensitive nicotinic receptors on bovine chromaffin cells: molecular cloning, functional expression and alternative splicing of the alpha 7 subunit

Chromaffin cells from the bovine adrenal medulla express alpha-bungarotoxin-sensitive acetylcholine receptors whose subunit composition is unknown. Northern blot analysis showed that the alpha 7 subunit, a main component of these alpha-bungarotoxin-sensitive acetylcholine receptors in avian and rat brain, is expressed in chromaffin cells. The cDNA of this bovine alpha 7 subunit was cloned by polymerase chain reaction amplification of adrenal medulla RNA for detailed characterization of structure and function. The protein-coding region revealed 92% amino acid sequence identity to rat alpha 7 and 89% to chicken alpha 7 subunits. The alpha-bungarotoxin affinity of alpha 7 homomers expressed in Xenopus oocytes was similar to that observed previously with native chromaffin alpha-bungarotoxin-sensitive acetylcholine receptors. Cross-linking and sucrose gradient experiments suggested that, like the muscular and neuronal acetylcholine receptors; the alpha 7 receptor has a pentameric structure. Upon activation with nicotinic agonists the alpha 7 receptor exhibited rapidly desensitizing cation currents that were blocked by nicotinic antagonists and showed inward rectification. The amplification of adrenal medulla RNA by reverse transcription-polymerase chain reaction methods revealed an alternatively spliced isoform of the bovine alpha 7 subunit, where the exon that codes for the M2 transmembrane segment was skipped during mRNA processing. Oocyte expression of this isoform does not yield functional channels. However, this alternative mRNA exhibits dose-dependent inhibition of alpha 7 homomer expression when coinjected with the undeleted isoform.

Adoption of beta structure by the inactivating "ball" peptide of the Shaker B potassium channel

The conformation of the inactivating peptide of the Shaker B K+ channel (ShB peptide) and that of a noninactivating mutant (ShBL7E peptide) have been studied. Under all experimental conditions explored, the mutant peptide remains in a predominantly nonordered conformation. On the contrary, the inactivating ShB peptide has a great tendency to adopt a highly stable beta structure, particularly when challenged "in vitro" by anionic phospholipid vesicles. Because the putative peptide binding elements at the inner mouth of the channel comprise a ring of anionic residues and a hydrophobic pocket, we hypothesize that the conformational restrictions imposed on the ShB peptide by its interaction with the anionic lipid vesicles could partly imitate those imposed by the above ion channel elements. Thus, we propose that adoption of beta structure by the inactivating peptide may also occur during channel inactivation. Moreover, the difficulties encountered by the noninactivating ShBL7E peptide mutant to adopt beta structure and the observation that trypsin hydrolysis of the ShB peptide prevent both structure formation and channel inactivation lend further support to the hypothesis that adoption of beta structure by the inactivating peptide in a hydrophobic environment is important in determining channel blockade.

Role of two acetylcholine receptor subunit domains in homomer formation and intersubunit recognition, as revealed by alpha 3 and alpha 7 subunit chimeras

Differential expression of subunit genes from the nicotinic acetylcholine receptor (AChR) superfamily yields distinct receptor subtypes. As each AChR subtype has a specific subunit composition and many subunit combinations appear not to be expressed, each subunit must contain some information leading to proper assembly. The neuronal AChR subunits alpha 3 and alpha 7 are expressed in bovine chromaffin cells, probably as constituents of two different AChR subtypes. These subunits have different assembly behavior when expressed in heterologous expression systems: alpha 7 subunits are able to produce homomeric AChRs, whereas alpha 3 subunits require other "structural" subunits for functional expression of AChRs. This feature allows the dissection of the requirements for subunit interactions during AChR formation. Analysis of alpha 7/alpha 3 chimeric constructs identified two regions essential to homomeric assembly and intersubunit recognition: an N-terminal extracellular region, controlling the initial association between subunits, and a second domain within a region comprising the first putative transmembrane segment, M1, and the cytoplasmic loop coupling it to the pore-forming segment, M2, involved in the subsequent interaction and stabilization of the oligomeric complex.

Immunocytochemical localization of the alpha 7 subunit of the nicotinic acetylcholine receptor in the rat central nervous system

Previous molecular cloning studies have revealed that alpha-bungarotoxin binding proteins present in the brain are members of the neuronal nicotinic acetylcholine receptor gene family. The alpha 7 subunit is structurally related to the agonist binding subunits present in the central and peripheral nervous systems and, when expressed in Xenopus oocytes, forms functional channels blockable by alpha-bungarotoxin. In the present study, three different monoclonal antibodies raised against the alpha 7 subunit were used to map its distribution throughout the central nervous system of the rat. Immunohistochemical localization revealed that the alpha 7 subunit is expressed in most regions of the brain, being, overall, well correlated with previous "in situ" localization of alpha 7 transcripts and alpha-bungarotoxin autoradiographic binding studies. Particularly strong immunoreactivity was observed in several sensory and motor nuclei of the brainstem as well as the red nucleus. At the cellular level, alpha 7 immunostaining was usually found both in somata and dendrites, whereas axonal and terminal labeling was not observed. The widespread distribution of the alpha 7 subunit polypeptide is consistent with immunoprecipitation data demonstrating that it is a component of the predominant subtype of brain alpha-bungarotoxin-sensitive nicotinic receptors.

A delayed rectifier potassium channel cloned from bovine adrenal medulla. Functional analysis after expression in Xenopus oocytes and in a neuroblastoma cell line

Using a cDNA library from bovine adrenal medulla, and, subsequently, a bovine genomic library, we have isolated the gene coding for a non inactivating potassium channel. This gene encodes a 597-amino acid protein which we have called BAK5 as its sequence is very similar to members of Kv1.5 potassium channel family. Neuroblastoma cells (Neuro-2a cell line) were stably transfected with BAK5 DNA. Protein expression was under the control of a heat-shock promoter. Transfected cells showed a current highly selective for potassium, insensitive to tetraethylammonium but reversibly blocked by 4-aminopyridine. Oocytes injected with BAK5 mRNA also expressed a potassium current with the same characteristics.

Muscarinic receptor subtypes in bovine adrenal medulla

Catecholamine secretion in the bovine adrenal medulla is evoked largely by nicotinic receptor activation. However, bovine adrenal medulla also contain muscarinic receptors that mediate several cell responses. To understand the physiological role of muscarinic receptors in the bovine adrenal medulla it is important to identify the pharmacological subtypes present in this tissue. For this, we analyzed the abilities of different selective muscarinic antagonists in displacing the binding of the non-selective antagonist [3H] quinuclidinyl benzylate to an enriched plasma membrane fraction prepared from bovine adrenal medulla. All the selective antagonists bind at least two bindings sites with different affinities. The binding profile of the sites with high proportion is similar to the M2 subtype and those present in low proportion have a M1 profile. However, some variation in the proportion of the sites for the different ligands suggest the presence of the third pharmacological subtype (M3). We conclude that the sites in high proportion (60-80%) correspond to M2 muscarinic subtypes, and the rest is constituted by M1 plus M3 subtypes. The presence of multiplicity of subtypes in the adrenal medulla membranes suggests a diversity of functions of muscarinic receptors in the adrenal gland.

Molecular cloning and permanent expression in a neuroblastoma cell line of a fast inactivating potassium channel from bovine adrenal medulla

Using a cDNA library from bovine adrenal medulla, we have isolated cDNAs coding for a potassium channel. These cDNAs encode a 660-amino acid protein that has a molecular weight of 73,288 kDa and no amino-terminal signal peptide. We have called it BAK4. Analysis of its sequence reveals close similarity (94% homology) with a recently described potassium channel from rat brain (RCK4) and heart (RHK1). Neuroblastoma cells (Neuro-2a cell line) were stably transfected with BAK4 DNA. Expression of the DNA was under the control of a heat-shock promoter. Several clones, that were isolated by neomycin resistance selection, had integrated the plasmid DNA in a stable form. Upon heat induction, these cells produced BAK4 RNA and a potassium outward current, not present in control non-transfected cells. The current, which was transient and decayed markedly during the duration of 200 ms-pulses, can be described as a Ik(A) potassium current. The expression of these types of channels in brain (RCK4,RHK1), heart (RHK1) and adrenal medulla (BAK4) suggest their possible implication in important functions for the cell.

Primary structure of an agonist binding subunit of the nicotinic acetylcholine receptor from bovine adrenal chromaffin cells

Activation by acetylcholine of a nicotinic acetylcholine receptor on the membrane of bovine chromaffin cells leads to membrane depolarization and to the subsequent triggering of catecholamine secretion. It is evident that acetylcholine receptors play a central role in the initial phase of the secretion process and, therefore, an extensive characterization of their molecular components and properties is of fundamental interest. With this intention, we have screened bovine adrenal medullary cDNA libraries with a probe coding for a fragment of the rat muscle acetylcholine receptor alpha subunit. Several cDNA clones were isolated. The longest cDNA had an open reading frame encoding a 495-amino acid protein with a molecular weight of 56,911. The deduced primary structure contains features that indicate that the encoded protein is an alpha or acetylcholine binding subunit, and, in fact, it manifests significant sequence similarity to previously cloned alpha subunits. Sequence identity is particularly high with the alpha 3 subunit, which is expressed in the rat pheochromocytoma PC12 cell line and in several brain areas, and, consequently, it is considered a component of a neuronal acetylcholine receptor. Accordingly, the present results suggest that the agonist binding subunit of the nicotinic acetylcholine receptor from bovine chromaffin cells is an alpha 3-type subunit, corroborating previous immunological and pharmacological evidence for the presence of a neuronal nicotinic receptor in chromaffin cells.

Primary structure and functional expression of the alpha-, beta-, gamma-, delta- and epsilon-subunits of the acetylcholine receptor from rat muscle

The isolation and characterization of five clones carrying sequences of the alpha-, beta-, gamma-, delta- and epsilon-subunit precursors of the rat muscle acetylcholine receptor (AChR) are described. The deduced amino acid sequences indicate that these polypeptides contain 457-519 amino acids and reveal the structural characteristics common to subunits of ligand-gated ion channels. The pattern of subunit-specific mRNA levels in rat muscle shows characteristic changes during development and following denervation, suggesting that innervation of muscle reduces the expression of the alpha-, beta- and delta-subunit mRNAs, suppresses the expression of the gamma-subunit mRNA, and induces expression of epsilon-subunit mRNA. Subunit-specific cRNAs generated in vitro were injected into Xenopus laevis oocytes, resulting in the assembly of two functionally different AChR channel subtypes. The AChR gamma, composed of the alpha-, beta-, gamma- and delta-subunits, has functional properties similar to those of the native AChRs in fetal muscle. The AChR epsilon, composed of alpha-, beta-, delta- and epsilon-subunits, corresponds to the end-plate channel of the adult muscle. Thus in rat skeletal muscle the motor nerve regulates the expression of two functionally different AChR subtypes with different molecular composition by the differential expression of subunit-specific mRNAs.

Assembly of an adult type acetylcholine receptor in a mouse cell line transfected with rat muscle epsilon-subunit DNA

The mouse muscle cell line BC3H-1 expresses an acetylcholine receptor (AChR) composed of alpha-, beta-, gamma- and delta-subunits. The functional characteristics of this AChR are comparable to the non-synaptic AChR subtype in mouse muscle. To investigate the role of the epsilon-subunit, which is believed to replace the gamma-subunit in forming the adult AChR subtype, BC3H-1 cells were stably transfected with cDNA encoding the rat muscle AChR epsilon-subunit. Expression of this cDNA was under the control of a heat shock promoter, and the plasmid carried the neomycin resistance gene for selection. Several clones were isolated that had integrated the plasmid DNA in a stable form and produced epsilon-subunit specific RNA after heat induction. Single-channel current recording from cells which contained abundant epsilon-subunit mRNA identified a novel AChR channel having a larger conductance than the native AChR in these cells. These results suggest that the rat muscle epsilon-subunit may assemble with mouse muscle alpha-, beta- and delta-subunits to form a mouse-rat hybrid AChR with properties similar to that of end-plate channels in the mature mammalian neuromuscular synapse. The novel AChR channel appears in the surface membrane within a few hours following the rise in epsilon-subunit mRNA. Thus, the notion that replacement of the gamma-subunit by the epsilon-subunit during development is the result of the postnatal rise in the level of epsilon-subunit specific mRNA is further supported.