Agonist-independent activation of acetylcholine receptor channels by protein kinase A phosphorylation

Purinergic Tuning of the Tripartite Neuromuscular Synapse

The vertebrate neuromuscular junction (NMJ) is a specialised chemical synapse involved in the transmission of bioelectric signals between a motor neuron and a skeletal muscle fiber, leading to muscle contraction. Typically, the NMJ is a tripartite synapse comprising (a) a presynaptic region represented by the motor nerve ending, (b) a postsynaptic skeletal motor endplate area, and (c) perisynaptic Schwann cells (PSCs) that shield the motor nerve terminal. Increasing evidence points towards the role of PSCs in the maintenance and control of neuromuscular integrity, transmission, and plasticity. Acetylcholine (ACh) is the main neurotransmitter at the vertebrate skeletal NMJ, and its role is fine-tuned by co-released purinergic neuromodulators, like adenosine 5'-triphosphate (ATP) and its metabolite adenosine (ADO). Adenine nucleotides modulate transmitter release and expression of postsynaptic ACh receptors at motor synapses via the activation of P2Y and P2X receptors. Endogenously generated ADO modulates ACh release by acting via co-localised inhibitory A₁ and facilitatory A_2A receptors on motor nerve terminals, whose tonic activation depends on the neuronal firing pattern and their interplay with cholinergic receptors and neuropeptides. Thus, the concerted action of adenine nucleotides, ADO, and ACh/neuropeptide co-transmitters is paramount to adapting the neuromuscular transmission to the working load under pathological conditions, like Myasthenia gravis. Unravelling these functional complexities prompted us to review our knowledge about the way purines orchestrate neuromuscular transmission and plasticity in light of the tripartite synapse concept, emphasising the often-forgotten role of PSCs in this context.

Interplay Between Cholinergic and Adenosinergic Systems in Skeletal Muscle

Since the pioneering works of Ricardo Miledi, the neuromuscular junction represents the best example of a synapse where ACh is the neurotransmitter acting on nicotinic ACh receptors. ATP, co-released with ACh, is promptly degraded to Ado, which acts as a modulator of the cholinergic synaptic activity. Consequently, both ACh and adenosine play a crucial role in controlling the nerve-muscle communication. Apart from their role in the context of synaptic transmission, ACh and adenosine are autocrinally released by skeletal muscle cells, suggesting also a non nerve-driven function of these molecules. Indeed, the existence of cholinergic and adenosinergic systems has been widely described in many other non neuronal cell types. In this review, we will describe the two systems and their interplay in non-innervated differentiating skeletal muscle cells, and in innervated adult skeletal muscle fibers. We believe that the better comprehension of the interactions between the activity of nAChRs and adenosine could help the knowledge of skeletal muscle physiology. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Adenosine Promotes Endplate nAChR Channel Activity in Adult Mouse Skeletal Muscle Fibers via Low Affinity P1 Receptors

Adenosine is a powerful modulator of skeletal neuromuscular transmission, operating via inhibitory or facilitatory purinergic-type P1 receptors. To date, studies have been focused mainly on the effect of adenosine on presynaptic P1 receptors controlling transmitter release. In this study, using two-microelectrode voltage-clamp and single-channel patch-clamp recording techniques, we have explored potential postsynaptic targets of adenosine and their modulatory effect on nicotinic acetylcholine receptor (nAChR)-mediated synaptic responses in adult mouse skeletal muscle fibers in vitro. In the whole-mount neuromuscular junction (NMJ) preparation, adenosine (100 μM) significantly reduced the frequency of the miniature endplate currents (MEPCs) and slowed their rising and decay time. Consistent with a postsynaptic site of action, adenosine and the potent P1 receptor agonist NECA significantly increased the open probability, the frequency and the open time of single nAChR channels, recorded at the endplate region. Using specific ligands for the P1 receptor subtypes, we found that the low-affinity P1 receptor subtype A_2B was responsible for mediating the effects of adenosine on the nAChR channel openings. Our data suggest that at the adult mammalian NMJ, adenosine acts not only presynaptically to modulate acetylcholine transmitter release, but also at the postsynaptic level, to enhance the activity of nAChRs. Our findings open a new scenario in understanding of purinergic regulation of nAChR activity at the mammalian endplate region.

Adenosine enhances acetylcholine receptor channel openings and intracellular calcium 'spiking' in mouse skeletal myotubes

AIMS

The autocrine activity of the embryonic isoform of the nicotinic acetylcholine receptor is crucial for the correct differentiation and trophism of skeletal muscle cells before innervation. The functional activity of extracellular adenosine and adenosine receptor subtypes expressed in differentiating myotubes is still unknown. In this study, we performed a detailed analysis of the role of adenosine receptor-mediated effects on the autocrine-mediated nicotinic acetylcholine receptor channel openings and the associated spontaneous intracellular calcium 'spikes' generated in differentiating mouse myotubes in vitro.

METHODS

Cell-attached patch-clamp recordings and intracellular calcium imaging experiments were performed in contracting myotubes derived from mouse satellite cells.

RESULTS

The endogenous extracellular adenosine and the adenosine receptor-mediated activity modulated the properties of the embryonic isoform of the nicotinic acetylcholine receptor in myotubes in vitro, by increasing the mean open time and the open probability of the ion channel, and sustaining nicotinic acetylcholine receptor-driven intracellular [Ca(2+) ]i 'spikes'. The pharmacological characterization of the adenosine receptor-mediated effects suggested a prevalent involvement of the A2B adenosine receptor subtype.

CONCLUSION

We propose that the interplay between endogenous adenosine and nicotinic acetylcholine receptors represents a potential novel strategy to improve differentiation/regeneration of skeletal muscle.

Short openings in high resolution single channel recordings of mouse nicotinic receptors

The temporal fine structure of single channel currents was studied to obtain information on how agonists open nicotinic acetylcholine receptor channels. Currents were recorded from mouse myoballs with quartz pipettes in the on-cell mode of the patch-clamp technique. With 62 kHz filter cut-off and root mean square (r.m.s.) noise levels as low as 1.45 pA at 200 mV hyperpolarization, events down to 6 micros duration could be resolved with negligible error rate. Three types of openings with mean durations of 750 micros, 89 micros and 4 micros were identified with 0.1-10 microM suberyldicholine (SubCh). The relative frequencies of the three types of openings were 84% for long, 5% for medium and 11% for short openings with 1 microM SubCh. Stability plots and single channel current amplitude comparisons suggest that the three types of openings arise from a homogenous channel population. Above 10 microM SubCh, the three types of openings could not be discerned because channel openings occurred too closely spaced and open channels were increasingly blocked. Three types of openings can be generated with a mechanistic receptor model with two unequal binding sites, short and medium openings arising from one or the other monoliganded state, and long openings from the fully liganded state of the receptor. Maximum likelihood fitting of the rate constants of this model directly to the sequence of observed open and shut times accurately predicted the main physiological properties of the receptors with 0.1 microM SubCh. However, fitting recordings with 0.1-10 microM SubCh simultaneously revealed that this model cannot reproduce the weak influence of SubCh concentration on the proportions of the three types of openings. Therefore we conclude that short and medium openings are unlikely to arise preferentially from one or the other monoliganded state of nicotinic acetylcholine receptor channels.

Sensitivity of neuronal nicotinic acetylcholine receptors to the opiate antagonists naltrexone and naloxone: receptor blockade and up-regulation

In HEK293 cells stably expressing alpha4beta2 nAChRs, naltrexone, but not naloxone, blocked alpha4beta2 nAChRs via an open-channel blocking mechanism. In primary hippocampal cultures, naltrexone inhibited alpha7 nAChRs up-regulated by nicotine, and in organotypic hippocampal cultures naltrexone caused a time-dependent up-regulation of functional alpha7 nAChRs that was detected after removal of the drug. These results indicate that naltrexone could be used as a smoking cessation aid.

Synthetic peptides of Goodpasture's antigen in antiglomerular basement membrane nephritis in rats

Goodpasture's syndrome (GPS) is an autoimmune disease characterized by pulmonary hemorrhage, glomerulonephritis and anti-glomerular basement membrane (GBM) antibodies. The alpha(3) noncollagenous domain (NC1) of type IV collagen [alpha(3)(IV)] is the pathogen. The disease is T-cell-dependent; thus linear peptides initiate the autoimmune process. Studies in a rat model of GPS, experimental autoimmune glomerulonephritis (EAG), have shown that the carboxy-terminal 36 amino acids (purportedly the pathogenic epitope) are not responsible for disease induction. More recent studies implicate the amino terminus of alpha(3)(IV)NC1. Finding the nephritogenic epitope(s) is crucial in the understanding of the disease and for treatment. Because alpha(3)(IV)NC1 contains the antigens that induce GN in rats and human beings, we hypothesized that regions of the alpha(3)(IV)NC1 other than the carboxy terminus were responsible for disease. We investigated overlapping peptides spanning the entire NC1 domain of the alpha(3)(IV) chain N-terminal to the 36-mer (Goodpasture epitope) using the EAG rat model. Most peptides elicited antibody responses exclusively to themselves but not to native GBM. T-cells from GBM-immunized rats proliferated in vitro after stimulation with peptides 6, 8, 14, and 15, 24-mer and 23-mer. Fifteen percent of peptide 8 and peptide 14 rats had mild glomerulonephritis. In none of the animals immunized with other peptides did glomerulonephritis develop. These data suggest that conformation-dependent sites, posttranslational modification, multiple epitopes, concomitant antibody formation, or other disturbances are important in the ability of alpha(3)(IV)NC1 to induce EAG in rats and may also be important in the induction of GPS in human beings.

Human p8 is a HMG-I/Y-like protein with DNA binding activity enhanced by phosphorylation

We have studied the biochemical features, the conformational preferences in solution, and the DNA binding properties of human p8 (hp8), a nucleoprotein whose expression is affected during acute pancreatitis. Biochemical studies show that hp8 has properties of the high mobility group proteins, HMG-I/Y. Structural studies have been carried out by using circular dichroism (near- and far-ultraviolet), Fourier transform infrared, and NMR spectroscopies. All the biophysical probes indicate that hp8 is monomeric (up to 1 mm concentration) and partially unfolded in solution. The protein seems to bind DNA weakly, as shown by electrophoretic gel shift studies. On the other hand, hp8 is a substrate for protein kinase A (PKA). The phosphorylated hp8 (PKAhp8) has a higher content of secondary structure than the nonphosphorylated protein, as concluded by Fourier transform infrared studies. PKAhp8 binds DNA strongly, as shown by the changes in circular dichroism spectra, and gel shift analysis. Thus, although there is not a high sequence homology with HMG-I/Y proteins, hp8 can be considered as a HMG-I/Y-like protein.

Kinetic, mechanistic, and structural aspects of unliganded gating of acetylcholine receptor channels: a single-channel study of second transmembrane segment 12' mutants

The spontaneous activity of adult mouse muscle acetylcholine receptor channels, transiently expressed in HEK-293 cells, was studied with the patch-clamp technique. To increase the frequency of unliganded openings, mutations at the 12' position of the second transmembrane segment were engineered. Our results indicate that: (a) in both wild type and mutants, a C <--> O kinetic scheme provides a good description of spontaneous gating. In the case of some mutant constructs, however, additional states were needed to improve the fit to the data. Similar additional states were also needed in one of six patches containing wild-type acetylcholine receptor channels; (b) the delta12' residue makes a more pronounced contribution to unliganded gating than the homologous residues of the alpha, beta, and straightepsilon subunits; (c) combinations of second transmembrane segment 12' mutations in the four different subunits appear to have cumulative effects; (d) the volume of the side chain at delta12' is relevant because residues larger than the wild-type Ser increase spontaneous gating; (e) the voltage dependence of the unliganded gating equilibrium constant is the same as that of diliganded gating, but the voltage dependences of the opening and closing rate constants are opposite (this indicates that the reaction pathway connecting the closed and open states of the receptor changes upon ligation); (f) engineering binding-site mutations that decrease diliganded gating (alphaY93F, alphaY190W, and alphaD200N) reduces spontaneous activity as well (this suggests that even in the absence of ligand the opening of the channel is accompanied by a conformational change at the binding sites); and (g) the diliganded gating equilibrium constant is also increased by the 12' mutations. Such increase is independent of the particular ligand used as the agonist, which suggests that these mutations affect mostly the isomerization step, having little, if any, effect on the ligand-affinity ratio.

Building a bilayer model of the neuromuscular synapse

Progress over the past 10 years has made it possible to construct a simple model of neurotransmitter release. Currently, some models use artificially formed vesicles to represent synaptic vesicles and a planar lipid bilayer as a presynaptic membrane. Fusion of vesicles with the bilayer is via channel proteins in the vesicle membrane and an osmotic gradient. In this paper; a framework is presented for the successful construction of a more complete model of synaptic transmission. This model includes real synaptic vesicles that fuse with a planar bilayer. The bilayer contains acetylcholine receptor (AChR) channels which function as autoreceptors in the membrane. Vesicle fusion is initiated following a Ca2+ flux through voltage-gated Ca2+ channels. Key steps in the plan are validated by mathematical modeling. Specifically, the probability that a reconstituted AChR channel opens following the release of ACh from a fusing vesicle, is calculated as a function of time, quantal content, and number of reconstituted AChRs. Experimentally obtainable parameters for construction of a working synapse are given. The inevitable construction of a full working model will mean that the minimal structures necessary for synaptic transmission are identified. This will open the door in determining regulatory and modulatory factors of transmitter release.

Structural stabilization of botulinum neurotoxins by tyrosine phosphorylation

Tyrosine phosphorylation of botulinum neurotoxins augments their proteolytic activity and thermal stability, suggesting a substantial modification of the global protein conformation. We used Fourier-transform infrared (FTIR) spectroscopy to study changes of secondary structure and thermostability of tyrosine phosphorylated botulinum neurotoxins A (BoNT A) and E (BoNT E). Changes in the conformationally-sensitive amide I band upon phosphorylation indicated an increase of the alpha-helical content with a concomitant decrease of less ordered structures such as turns and random coils, and without changes in beta-sheet content. These changes in secondary structure were accompanied by an increase in the residual amide II absorbance band remaining upon H-D exchange, consistent with a tighter packing of the phosphorylated proteins. FTIR and differential scanning calorimetry (DSC) analyses of the denaturation process show that phosphorylated neurotoxins denature at temperatures higher than those required by non-phosphorylated species. These findings indicate that tyrosine phosphorylation induced a transition to higher order and that the more compact structure presumably imparts to the phosphorylated neurotoxins the higher catalytic activity and thermostability.

Anatomy of the antigenic structure of a large membrane autoantigen, the muscle-type nicotinic acetylcholine receptor

The neuromuscular junction nicotinic acetylcholine receptor (AChR), a pentameric membrane glycoprotein, is the autoantigen involved in the autoimmune disease myasthenia gravis (MG). In animals immunized with intact AChR and in human MG, the anti-AChR antibody response is polyclonal. However, a small extracellular region of the AChR alpha-subunit, the main immunogenic region (MIR), seems to be a major target for anti-AChR antibodies. A major loop containing overlapping epitopes for several anti-MIR monoclonal antibodies (mAbs) lies within residues alpha 67-76 at the extreme synaptic end of each alpha-subunit: however, anti-MIR mAbs are functionally and structurally quite heterogeneous. Anti-MIR mAbs do not affect channel gating, but are very effective in the passive transfer of MG to animals; in contrast, their Fab or Fv fragments protect the AChR from the pathogenic effects of the intact antibodies. Antibodies against the cytoplasmic region of the AChR can be elicited by immunization with denatured AChR and the precise epitopes of many such mAbs have been identified; however, it is unlikely that such antibodies are present in significant amounts in human MG. Antibodies to other extracellular epitopes on all AChR subunits are present in both experimental and human MG; these include antibodies to the acetylcholine-binding site which affect AChR function in various ways and also induce acute experimental MG. Finally, anti-AChR antibodies cross-reactive with non-AChR antigens exist, suggesting that MG may result from molecular mimicry. Despite extensive studies, many gaps remain in our understanding of the antigenic structure of the AChR; especially in relation to human MG. A thorough understanding of the antigenic structure of the AChR is required for an in-depth understanding, and for possible specific immunotherapy, of MG.

Calcium-dependent desensitizing function of the postsynaptic neuronal-type nicotinic acetylcholine receptors at the neuromuscular junction

Several subunits that commonly have been regarded as neuronal-type nicotinic acetylcholine receptor (nAChR) subtypes, have been found in the postjunctional endplate membrane of adult skeletal muscle fibres. The postsynaptic function of these neuronal-type nAChR subtypes at the neuromuscular junction has been investigated by using aequorin luminescence and fluorescence confocal imaging. A biphasic elevation of intracellular Ca2+ is elicited by prolonged nicotinic action at the mouse muscle endplates. The fast and slow Ca2+ components are operated by a postsynaptic muscle- and colocalized neuronal-type nAChR, respectively. Neuromuscular functions may be regulated by a dual nAChR system to maintain the normal postsynaptic excitability. Certain neuronal-type nAChR may be endowed with the same functional role in the central nervous system also.

Potentiation of developing synapses by postsynaptic release of neurotrophin-4

The hypothesis that synaptic functions can be regulated by neurotrophins secreted from the postsynaptic cell was examined in Xenopus nerve-muscle cultures. Neuromuscular synapses formed on myocytes overexpressing neurotrophin-4 (M+ synapses) exhibited a higher level of spontaneous synaptic activity and enhanced evoked synaptic transmission as compared to those formed on normal control myocytes (M- synapses). The NT-4 effects involve a potentiation of presynaptic transmitter secretion as well as a lengthening of the mean burst duration of postsynaptic low conductance acetylcholine channels. Repetitive stimulation of either the presynaptic neuron or the postsynaptic myocyte led to a potentiation of synaptic transmission at M+ synapses. All potentiation effects of NT-4 overexpression were abolished by the extracellular presence of TrkB-IgG but not by the presence of TrkA-IgG, indicating that postsynaptic secretion of NT-4 was responsible for the synaptic modification.

Tyrosine phosphorylation modulates the activity of clostridial neurotoxins

Clostridial neurotoxins' metalloprotease domain selectively cleaves proteins implicated in the process of synaptic vesicle fusion with the plasma membrane and, accordingly, blocks neurotransmitter release into the synaptic cleft. Here we investigate the potential modulation of these neurotoxins by intracellular cascades triggered by environmental signals, which in turn may alter its activity on target substrates. We report that the nonreceptor tyrosine kinase Src phosphorylates botulinum neurotoxins A, B, and E and tetanus neurotoxin. Protein tyrosine phosphorylation of serotypes A and E dramatically increases both their catalytic activity and thermal stability, while dephosphorylation reverses the effect. This suggests that the biologically significant form of the neurotoxins inside neurons is phosphorylated. Indeed, in PC12 cells in which tyrosine kinases such as Src and PYK2 are highly abundant, stimulation by membrane depolarization in presence of extracellular calcium induces rapid and selective tyrosine phosphorylation of internalized light chain, the metalloprotease domain, of botulinum toxin A. These findings provide a conceptual framework to connect intracellular signaling pathways involving tyrosine kinases, G-proteins, phosphoinositides, and calcium with the action of botulinum neurotoxins in abrogating vesicle fusion and neurosecretion.

Enhancement by calcitonin gene-related peptide of non-contractile Ca2(+)-induced nicotinic receptor desensitization at the mouse neuromuscular junction

1. Nicotinic acetylcholine receptor (AChR)-operated non-contractile Ca2+ mobilization (unaccompanied by muscle contraction) depressed contractile Ca2+ mobilization (accompanied by muscle contraction) in mouse diaphragm muscles. In the process of nicotinic AChR desensitization, the enhancing role of calcitonin gene-related peptide (CGRP) on the non-contractile Ca2(+)-induced depression of contractile Ca2+ mobilization was investigated by measurement of Ca2(+)-aequorin luminescence in the presence of neostigmine (0.1 microM). 2. When the phrenic nerve was stimulated with paired pulses at intervals of 150, 300, 600, 1000 and 2000 ms, contractile Ca2+ transients were elicited during the generation of non-contractile Ca2+ mobilization. The amplitude of the contractile Ca2 transients elicited by the second pulse (S2) was depressed at the shorter pulse intervals, but not at the longer pulse intervals. 3. The extent of depression of S2 was enhanced when the duration of non-contractile Ca2+ mobilization was prolonged by CGRP (10 nM). However, CGRP failed to enhance the depression of S2 when non-contractile Ca2+ mobilization was not observed at the low external Ca2+ concentration (1.3 mM). 4. The enhancing effect by CGRP on the depression of S2 was counteracted by staurosporine (3 nM), a protein kinase-C inhibitor, despite prolongation of the duration of non-contractile Ca2+ mobilization. 5. When H-89 (1 microM), a protein kinase-A inhibitor, completely blocked non-contractile Ca2+ mobilization, the depression of S2 was diminished. The prolongation of the duration of non-contractile Ca2+ mobilization by AA373 (300 microM), a protein kinase-A activator, enhanced the depression of S2. The enhancing effect was observed neither with CGRP nor with AA373, in the presence of H-89 (0.1 microM). 6. These findings suggest that the CGRP mobilizes non-contractile Ca2+ through activation of protein kinase-A, which in turn may activate protein kinase-C, then enhance the desensitization of postsynaptic nicotinic AChRs at the neuromuscular junction.

Spontaneous opening of the acetylcholine receptor channel in developing muscle cells from normal and dystrophic mice

Single-channel activity was recorded from cell-attached patches on skeletal muscle cells isolated from wild-type mice and from mice carrying the dy or mdx mutations. Spontaneous openings of the nicotinic acetylcholine receptor channel (nAChR) were detected in virtually all recordings from either dy/dy or dy/+ myotubes, but only infrequently from wild-type or mdx myotubes. Spontaneous openings were also present in most recordings from undifferentiated myoblasts from all of the mouse strains studied. The biophysical properties of the spontaneous activity were similar to those of the embryonic form of the nAChR in the presence of acetylcholine (ACh). Examination of the single-channel currents evoked by low concentrations of ACh showed a reduced sensitivity to the agonist in the dystrophic dy and mdx myotubes, but not in wild-type myotubes. The results suggest that alterations in nAChR function are associated with the pathogenesis of muscular dystrophy in the dy mouse.

Regulation of postsynaptic responses by calcitonin gene related peptide and ATP at developing neuromuscular junctions

Neuronal factors co-released with neurotransmitters may play an important role in synapse development and function. Calcitonin gene related peptide (CGRP) and adenosine 5'-triphosphate (ATP), two principal neuromodulators present in the motor nerve terminals, were studied for their roles and mechanisms during early development of neuromuscular synapses in Xenopus nerve--muscle co-cultures. CGRP treatment increased the decay time and amplitude of spontaneous synaptic currents (SSCs) recorded from innervated myocytes, without affecting SSC frequency, suggesting a postsynaptic mechanism. ATP also increased the SSC amplitude and decay time. In addition, ATP was shown to potentiate the responses of isolated myocytes to iontophoretically applied acetylcholine (ACh). Single-channel recording from isolate myocytes showed that both CGRP and ATP specifically increased the open time of embryonic-type, low-conductance ACh channels. Pharmacological experiments suggest that the CGRP actions were mediated by cAMP-dependent protein kinase (PKA), while ATP exerted its effects by binding to P2 purinoceptors and thereby activating protein kinase C (PKC). Moreover, the effects of CGRP and ATP on ACh channel activity were restricted to immature myocytes. Taken together, these results suggest that endogenous CGRP and ATP co-released with ACh from the nerve terminal may promote synaptic development by potentiating postsynaptic ACh channel activity during the early phase of synaptogenesis.

Spontaneous, ligand-independent activity of the cGMP-gated ion channels in cone photoreceptors of fish

1. We studied the electrical conductance of membrane patches detached from the outer segment of single cone photoreceptors isolated from striped bass retina. 2. Only a single class of ion channels exists in the plasma membrane of the cone outer segments; they are gated by cytoplasmic cGMP and select cations over anions, but distinguish poorly among cations. In the absence of added cGMP and of divalent cations, however, membrane patches detached from the outer segments exhibit a small conductance that ideally selects cations over anions, but distinguishes poorly between Na+ and Li+. 3. The cGMP-independent conductance does not arise from the effect of residual cGMP that may remain associated with the detached membrane, because treatment of the patch with cGMP-specific phosphodiesterase does not affect this conductance. 4. The cGMP-independent conductance is pharmacologically indistinguishable from that activated by cGMP. Ca2+ and L-cis-diltiazem block both conductances at comparable concentrations and with similar quantitative characteristics. 5. We analysed the noise of Ca(2+)- or L-cis-diltiazem-dependent macroscopic currents both in the presence and in the absence of cGMP. In the presence of cGMP, the power density spectrum of the noise is well fitted by the sum of two Lorentzian components. The same function with similar corner frequencies fits the noise of the cGMP-independent currents. However, the total power in the current fluctuations is smaller in the absence of cGMP than in its presence; also, the ratio of the zero frequency asymptotes of the low over the high frequency components, S1(0)/Sh(0), is larger in the absence of cGMP than in its presence.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of the Goodpasture antigen by type A protein kinases

Collagen IV is the major component of basement membranes. The human alpha 3 chain of collagen IV contains an antigenic domain called the Goodpasture antigen that is the target for the circulating immunopathogenic antibodies present in patients with Goodpasture syndrome. Characteristically, the gene region encoding the Goodpasture antigen generates multiple alternative products that retain the antigen amino-terminal region with a five-residue motif (KRGDS). The serine therein appears to be the major in vitro cAMP-dependent protein kinase phosphorylation site in the isolated antigen and can be phosphorylated in vitro by two protein kinases of approximately 50 and 41 kDa associated with human kidney plasma membrane, suggesting that it can also be phosphorylated in vivo. Consistent with this, the Goodpasture antigen is isolated from human kidney in phosphorylated and non-phosphorylated forms and only the non-phosphorylated form is susceptible to phosphorylation in vitro. Since this motif is exclusive to the human alpha 3(IV) chain and includes the RGD cell adhesion motif, its phosphorylation might play a role in pathogenesis and influence cell attachment to basement membrane.

Protein kinase C enhances recombinant bovine alpha 1 beta 1 gamma 2L GABAA receptor whole-cell currents expressed in L929 fibroblasts

The beta 1 and gamma 2L subunits of the gamma-aminobutyric acid type A receptor (GABAR) contain phosphorylation sites for PKC. To determine the effect of PKC on GABAR function, whole-cell recordings were obtained from mouse fibroblasts expressing recombinant alpha 1 beta 1 gamma 2L receptors, and catalytically active PKC (PKM) was applied via the recording pipette. The first experiment was a population study. Intracellular application of PKM increased GABAR currents, and the enhancement was antagonized by coapplication of the PKC inhibitory peptide. No acceleration or deceleration of GABAR desensitization was observed. The second experiment was a reimpalement study in which paired recordings were made successively from individual cells. Enhancement of GABAR currents by PKM was again obtained. PKM increased GABAR currents at high (> 10 microM) but not at low (< 10 microM) GABA concentrations, resulting in increases in both EC50 and maximal GABAR current. Thus, PKC phosphorylation enhanced recombinant alpha 1 beta 1 gamma 2L GABAR current by increasing maximal current without increasing the affinity of GABA for the GABARs.

Pharmacology and biophysical properties of alpha 7 and alpha 7-alpha 8 alpha-bungarotoxin receptor subtypes immunopurified from the chick optic lobe

Two chick optic lobe alpha-bungarotoxin receptor subtypes (alpha 7 and alpha 7-alpha 8) were immunopurified using polyclonal antibodies raised against synthetic peptides of chick alpha 7 and alpha 8 alpha-bungarotoxin receptor subunits. The alpha 7 subtype contained the M(r) 57,000 alpha 7 subunit, and represented 60-70% of the alpha-bungarotoxin receptors; the alpha 7-alpha 8 subtype contained the M(r) 57,000 alpha 7 and alpha 8 subunits, and represented only 20-25% of the receptors. Both subtypes also had an additional M(r) 52,000 subunit. The affinity of these subtypes for alpha-bungarotoxin as well as antagonists was similar. However, the alpha 7-alpha 8 subtype displayed consistently higher affinities for agonists. When reconstituted in planar lipid bilayers, the alpha 7-alpha 8 subtype displayed several conductance states of 10-50 pS; the alpha 7 subtype had only one conductance state of 45 pS. The alpha 7-alpha 8 subtype was activated by lower agonist concentrations than the alpha 7 subtype. When expressed in Xenopus oocytes, the alpha 8 subunit formed functional homomeric receptors that desensitized rapidly. These channels were blocked by alpha-bungarotoxin and displayed a higher affinity for agonists than the alpha 7 homomeric receptor. Taken together, these data indicate that at least two alpha-bungarotoxin subtypes are present in the chick optic lobe. They operate as ligand-gated channels and display different agonist sensitivities and kinetics/conductance properties.

Potentiation of acetylcholine responses in Xenopus embryonic muscle cells by dibutyryl cAMP

Application of membrane-permeable analogues of adenosine 3',5'-(cyclic)monophosphate (cAMP) or forskolin to embryonic muscle cells of 1-day-old Xenopus cultures altered the response of the myocyte to iontophoretically applied acetylcholine (ACh). The initial amplitude and the decay time of the ACh-induced currents were elevated, and the rate of ACh-induced channel desensitization was increased. Single-channel recordings showed that cAMP analogues increased the mean open time of the low-conductance ACh channels, without affecting the single-channel conductance. Interestingly, this effect on ACh channels disappeared in myocytes of 3-day-old cultures, suggesting developmental changes in the susceptibility of the ACh channel to modulation. A possible involvement of cAMP in modulating the synaptic activity of early developing synapses was further indicated by prolonged decay times of spontaneous synaptic currents following treatment with dibutyryl-cAMP (Bt2cAMP). Factors released from the nerve terminals, if they activate the muscle adenylate cyclase system, could thus enhance the synaptic response during synaptogenesis.

Enhancement by calcitonin gene-related peptide of nicotinic receptor-operated noncontractile Ca2+ mobilization at the mouse neuromuscular junction

1. The involvement of calcitonin gene-related peptide (CGRP) in the mechanism of nicotinic acetylcholine receptor-operated noncontractile Ca2+ mobilization (not accompanied by twitch tension) was investigated by measuring Ca(2+)-aequorin luminescence at the neuromuscular junction of mouse diaphragm muscle treated with neostigmine. 2. Noncontractile Ca2+ transients were enhanced by 4-aminopyridine (100 microM), a K+ channel blocker, and inhibited by botulinum toxin (1-100 micrograms, i.p.) and hexamethonium (10-100 microM), a neuronal nicotinic receptor antagonist. 3. Noncontractile Ca2+ transients were diminished by CGRP8-37 (10-20 microM), a CGRP antagonist. CGRP (0.3-10 nM) prolonged the duration of noncontractile Ca2+ transients. The effect of CGRP was suppressed by CGRP8-37 (0.1 microM). 4. Noncontractile Ca2+ transients were inhibited by H-89 (0.1-1 microM), a protein kinase-A inhibitor. The catalytic subunit of protein kinase-A and AA373 (300 microM), a protein kinase-A activator, prolonged the duration of noncontractile transients. The prolongations either by CGRP or by AA373 were not observed in the presence of H-89 (0.1 microM). 5. Contractile (accompanied by twitch tension) but not noncontractile Ca2+ transients were decreased by 12-O-tetradecanoyl phorbol 13-acetate (TPA, 0.3-1 microM), a protein kinase-C activator. Phospholipase A2 increased only contractile Ca2+ transients. Calmodulin-related agents affected neither type of Ca2+ transients. 6. These results provide the first evidence that nicotinic acetylcholine receptor-operated noncontractile Ca2+ mobilization is promoted by nerve-released CGRP activating protein kinase-A, and is dependent on the accumulated amounts of acetylcholine at the neuromuscular junction where desensitization might readily develop.

Calcitonin gene-related peptide potentiates synaptic responses at developing neuromuscular junction

Protein phosphorylation is important in synaptic transmission and plasticity. At the neuromuscular junction, phosphorylation of acetylcholine (ACh) receptor-channels increases the rate of agonist-induced channel desensitization. In contrast, potentiation of ACh channel activity through protein phosphorylation has not been described. We report here that calcitonin gene-related peptide (CGRP), a neuropeptide present at presynaptic motor nerve terminals, enhances the postsynaptic response at developing neuromuscular junctions by increasing the burst duration of embryonic ACh channels. The effect of CGRP on these ACh channels is mimicked by dibutyryl-cyclic AMP and by cAMP-dependent protein kinase (PKA) and prevented by a specific peptide inhibitor of PKA. Moreover, postsynaptic inhibition of PKA reduced the amplitude and decay time of spontaneous synaptic currents, suggesting that endogenous CGRP may act as a potentiating factor during the early phase of synaptogenesis.

Design, synthesis and functional characterization of a pentameric channel protein that mimics the presumed pore structure of the nicotinic cholinergic receptor

Nicotinic cholinergic receptors are membrane proteins composed of five subunits organized around a central aqueous pore. A pentameric channel protein, T5M2 delta, that emulates the presumed pore-forming structure of this receptor was generated by assembling five helix-forming peptide modules at the lysine epsilon-amino groups of the 11-residue template [K*AK*KK*PGK*EK*G], where * indicates attachment sites. Helical modules represent the sequence of the M2 segment of the Torpedo californica acetylcholine receptor (AChR) delta subunit; M2 segments are considered involved in pore-lining. Purified T5M2 delta migrates in SDS-PAGE with an apparent M(r) approximately 14,000, concordant with a protein of 126 residues. T5M2 delta forms cation-selective channels when reconstituted in planar lipid bilayers. The single channel conductance in symmetric 0.5 M KCl is 40 pS. This value approximates the 45 pS single channel conductance characteristic of authentic purified Torpedo AChR, recorded under otherwise identical conditions. These results, together with conformational energy calculations, support the notion that a bundle of five amphipathic alpha-helices is a plausible structural motif underlying the inner bundle that forms the pore of the pentameric AChR channel.

The functional architecture of the acetylcholine nicotinic receptor explored by affinity labelling and site-directed mutagenesis

The scientific community will remember Peter Läuger as an exceptional man combining a generous personality and a sharp and skilful mind. He was able to attract by his views the interest of a large spectrum of biologists concerned by the mechanism of ion translocation through membranes. Yet, he was not a man with a single technique or theory. Using an authentically multidisciplinary approach, his ambition was to ‘understand transmembrane transport at the microscopic level, to capture its dynamics in the course of defined physiological processes’ (1987). According to him, ‘new concepts in the molecular physics of proteins’ had to be imagined, and ‘the traditional static picture of proteins has been replaced by the notions that proteins represent dynamic structures, subjected to conformational fluctuations covering a very wide time-range’ (1987).