Acetylcholine receptor epsilon-subunit deletion causes muscle weakness and atrophy in juvenile and adult mice

Acetylcholine receptor channel subtype directs the innervation pattern of skeletal muscle

Acetylcholine receptors (AChRs) mediate synaptic transmission at the neuromuscular junction, and structural and functional analysis has assigned distinct functions to the fetal (alpha2beta(gamma)delta) and adult types of AChR (alpha2beta(epsilon)delta). Mice lacking the epsilon-subunit gene die prematurely, showing that the adult type is essential for maintenance of neuromuscular synapses in adult muscle. It has been suggested that the fetally and neonatally expressed AChRs are crucial for muscle differentiation and for the formation of the neuromuscular synapses. Here, we show that substitution of the fetal-type AChR with an adult-type AChR preserves myoblast fusion, muscle and end-plate differentiation, whereas it substantially alters the innervation pattern of muscle by the motor nerve. Mutant mice form functional neuromuscular synapses outside the central, narrow end-plate band region in the diaphragm, with synapses scattered over a wider muscle territory. We suggest that one function of the fetal type of AChR is to ensure an orderly innervation pattern of skeletal muscle.

Synapses form in skeletal muscles lacking neuregulin receptors

The formation of the neuromuscular junction (NMJ) is directed by reciprocal interactions between motor neurons and muscle fibers. Neuregulin (NRG) and Agrin from motor nerve terminals are both implicated. Here, we demonstrate that NMJs can form in the absence of the NRG receptors ErbB2 and ErbB4 in mouse muscle. Postsynaptic differentiation is, however, induced by Agrin. We therefore conclude that NRG signaling to muscle is not required for NMJ formation. The effects of NRG signaling to muscle may be mediated indirectly through Schwann cells.

The acetylcholine receptor γ-to-ε switch occurs in individual endplates

Maturation of the neuromuscular junction is accompanied by molecular switching of acetylcholine receptor (AChR) channels from embryonic types with gamma-subunits to adult ones with epsilon-subunits after birth. As a step toward understanding the molecular mechanisms of the gamma-to-epsilon switch, we addressed the question of whether embryonic- and adult-type AChRs constitute different endplates during the transitional period. From analyses with double- or triple-staining with anti-gamma- and/or anti-epsilon-antibodies together with alpha-bungarotoxin, which binds to alpha-subunits, we demonstrated that during neonatal stages in mice, adult-type AChRs are incorporated into individual endplates expressing embryonic-AChRs and replace these embryonic-AChRs gradually. The main period of AChR transition in the mouse diaphragm was between postnatal days 5 (P5) and P7, similar to the period described previously in which endplates shift from multi-axon to single-axon innervation. This finding will help our understanding of the mechanisms of the gamma-to-epsilon switch during establishment of the neuromuscular junction.

Novel role for cyclin-dependent kinase 2 in neuregulin-induced acetylcholine receptor epsilon subunit expression in differentiated myotubes

Cyclin-dependent kinases (CDKs) are a family of evolutionarily conserved serine/threonine kinases. CDK2 acts as a checkpoint for the G(1)/S transition in the cell cycle. Despite a down-regulation of CDK2 activity in postmitotic cells, many cell types, including muscle cells, maintain abundant levels of CDK2 protein. This led us to hypothesize that CDK2 may have a function in postmitotic cells. We show here for the first time that CDK2 can be activated by neuregulin (NRG) in differentiated C2C12 myotubes. In addition, this activity is required for expression of the acetylcholine receptor (AChR) epsilon subunit. The switch from the fetal AChRgamma subunit to the adult-type AChRepsilon is required for synapse maturation and the neuromuscular junction. Inhibition of CDK2 activity with either the specific CDK2 inhibitory peptide Tat-LFG or by RNA interference abolished neuregulin-induced AChRepsilon expression. Neuregulin-induced activation of CDK2 also depended on the ErbB receptor, MAPK, and PI3K, all of which have previously been shown to be required for AChRepsilon expression. Neuregulin regulated CDK2 activity through coordinating phosphorylation of CDK2 on Thr-160, accumulation of CDK2 in the nucleus, and down-regulation of the CDK2 inhibitory protein p27 in the nucleus. In addition, we also observed a novel mechanism of regulation of CDK2 activity by a low molecular weight variant of cyclin E in response to NRG. These findings establish CDK2 as an intermediate molecule that integrates NRG-activated signals from both the MAPK and PI3K pathways to AChRepsilon expression and reveal an undiscovered physiological role for CDK2 in postmitotic cells.

Relationship between α7 nAChR and apoptosis in human lymphocytes

The presence of nicotinic receptors (nAChRs) in blood cells has been demonstrated. However, little is known about their functional roles. We have detected mRNA of alpha7 nAChR in peripheral human lymphocytes and determined that its expression is highly variable among individuals and within the same individual at different times. Upregulation of alpha7 is systematically observed after incubation of lymphocytes with nicotine or alpha-bungarotoxin. In addition, the incubation with these drugs decreases the percentage of apoptotic cells induced by the exposure to cortisol. Our results suggest that alpha7 nAChRs are involved in the modulation of cortisol-induced apoptosis.

Acetylcholine receptors direct rapsyn clusters to the neuromuscular synapse in zebrafish

Clustering of nicotinic muscle acetylcholine receptors (AChRs) requires association with intracellular rapsyn, a protein with an intrinsic ability to self-cluster. Previous studies on sofa potato (sop), an AChR null line of zebrafish, have suggested that AChRs may play an active role in subsynaptic localization of rapsyn clusters. To test this proposal directly, we identified and cloned the gene responsible for the sop phenotype and then attempted to rescue subsynaptic localization of the receptor-rapsyn complex in mutant fish. sop contains a leucine to proline mutation at position 28, near the N terminus of the zebrafish AChR delta subunit. Transient expression of mutant delta subunit in sop fish was unable to restore surface expression of muscle AChRs. In contrast, expression of wild-type delta subunit restored the ability of muscle to assemble surface receptors along with the ability of fish to swim. Most importantly, the ability of rapsyn clusters to localize effectively to subsynaptic sites also was rescued in large part. Our results point to direct involvement of the AChR molecule in restricting receptor-rapsyn clusters to the synapse.

A mouse model of AChR deficiency syndrome with a phenotype reflecting the human condition

The two subtypes of mammalian muscle nicotinic acetylcholine receptors (AChR) are generated by the substitution of the epsilon (adult) subunit for the gamma (fetal) subunit within the AChR pentamer. Null mutations of the adult AChR epsilon-subunit gene are the most common cause of the AChR deficiency syndrome. This is a disorder of neuromuscular transmission characterized by non-progressive fatigable muscle weakness present throughout life. In contrast with the human disorder, mice with AChR epsilon-subunit null mutations die between 10 and 14 weeks of age. We generated transgenic mice that constitutively express the human AChR gamma-subunit in an AChR epsilon-subunit 'knock-out' background. These mice, in which neuromuscular transmission is mediated by fetal AChR, live well into adult life but show striking similarities to human AChR deficiency syndrome. They display fatigable muscle weakness, reduced miniature endplate potentials and endplate potentials, reduced motor endplate AChR number and altered endplate morphology. Our results illustrate how species differences in the control of ion-channel gene expression may affect disease phenotype, demonstrate that expression of adult AChR subtype is not essential for long-term survival, and suggest that in patients with AChR deficiency syndrome, up-regulation of the gamma-subunit could be a beneficial therapeutic strategy.

The diversity of subunit composition in nAChRs: evolutionary origins, physiologic and pharmacologic consequences

Nicotinic acetylcholine receptors are made up of homologous subunits, which are encoded by a large multigene family. The wide number of receptor oligomers generated display variable pharmacological properties. One of the main questions underlying research in molecular pharmacology resides in the actual role of this diversity. It is generally assumed that the observed differences between the pharmacology of homologous receptors, for instance, the EC(50) for the endogenous agonist, or the kinetics of desensitization, bear some kind of physiologic relevance in vivo. Here we develop the quite challenging point of view that, at least within a given subfamily of nicotinic receptor subunits, the pharmacologic variability observed in vitro would not be directly relevant to the function of receptor proteins in vivo. In vivo responses are not expected to be sensitive to mild differences in affinities, and several examples of functional replacement of one subunit by another have been unravelled by knockout animals. The diversity of subunits might have been conserved through evolution primarily to account for the topologic diversity of subunit distribution patterns, at the cellular and subcellular levels. A quantitative variation of pharmacological properties would be tolerated within a physiologic envelope, as a consequence of a near-neutral genetic drift. Such a "gratuitous" pharmacologic diversity is nevertheless of practical interest for the design of drugs, which would specifically tackle particular receptor oligomers with a defined subunit composition among the multiple nicotinic receptors present in the organism.

Early postnatal death and motor disorders in mice congenitally deficient in calnexin expression

Calnexin is a ubiquitously expressed type I membrane protein which is exclusively localized in the endoplasmic reticulum (ER). In mammalian cells, calnexin functions as a chaperone molecule and plays a key role in glycoprotein folding and quality control within the ER by interacting with folding intermediates via their monoglucosylated glycans. In order to gain more insight into the physiological roles of calnexin, we have generated calnexin gene-deficient mice. Despite its profound involvement in protein folding, calnexin is not essential for mammalian-cell viability in vivo: calnexin gene knockout mice were carried to full term, although 50% died within 48 h and the majority of the remaining mice had to be sacrificed within 4 weeks, with only a very few mice surviving to 3 months. Calnexin gene-deficient mice were smaller than their littermates and showed very obvious motor disorders, associated with a dramatic loss of large myelinated nerve fibers. Thus, the critical contribution of calnexin to mammalian physiology is tissue specific.

Spontaneous muscle action potentials fail to develop without fetal-type acetylcholine receptors

In mammals, two combinations of muscle nicotinic acetylcholine receptors (AChRs) are used: alpha2betagammadelta (gamma-AChR) or alpha2betaepsilondelta (epsilon-AChR). After birth, gamma-AChRs are replaced by epsilon-AChRs (gamma/epsilon-switch). The two receptors have different conductances and open times. During perinatal period, the long open time gamma-AChRs generate random myofiber action potentials from uniquantal miniature end-plate potentials (mEPPs). epsilon-AChRs are suitable for strong adult muscle activities. Since the effect of the gamma/epsilon-switch on neuromuscular development was unclear, despite the many differences in channel characteristics, we carried out this study to generate gamma-subunit-deficient mice. Homozygotes born alive survived for 2 days in a stable condition, and were able to move their forelimbs. Endplate AChRs included epsilon-subunits, and muscle fibers had multiple neuromuscular junctions. Both pre- and postsynapses were abnormal and spontaneous action potentials generated from mEPPs were totally absent. Results suggest a requirement for gamma-AChRs in mediating synaptically-induced action potential activity critical for neuromuscular development.

Clustering of nicotinic acetylcholine receptors: from the neuromuscular junction to interneuronal synapses

Fast and accurate synaptic transmission requires high-density accumulation of neurotransmitter receptors in the postsynaptic membrane. During development of the neuromuscular junction, clustering of acetylcholine receptors (AChR) is one of the first signs of postsynaptic specialization and is induced by nerve-released agrin. Recent studies have revealed that different mechanisms regulate assembly vs stabilization of AChR clusters and of the postsynaptic apparatus. MuSK, a receptor tyrosine kinase and component of the agrin receptor, and rapsyn, an AChR-associated anchoring protein, play crucial roles in the postsynaptic assembly. Once formed, AChR clusters and the postsynaptic membrane are stabilized by components of the dystrophin/utrophin glycoprotein complex, some of which also direct aspects of synaptic maturation such as formation of postjunctional folds. Nicotinic receptors are also expressed across the peripheral and central nervous system (PNS/CNS). These receptors are localized not only at the pre- but also at the postsynaptic sites where they carry out major synaptic transmission. In neurons, they are found as clusters at synaptic or extrasynaptic sites, suggesting that different mechanisms might underlie this specific localization of nicotinic receptors. This review summarizes the current knowledge about formation and stabilization of the postsynaptic apparatus at the neuromuscular junction and extends this to explore the synaptic structures of interneuronal cholinergic synapses.

Disparate dysfunction of skeletal muscles located near and distant from burn site in the rat

This study tested the hypotheses that burn-induced change in muscle function varies at sites local and distant from burn and is related to changes in expression of acetylcholine receptors (AChRs) and muscle mass. In anesthetized rats, approximately 4% burn was inflicted over the tibialis anterior muscle of one limb. The contralateral leg served as control. In another study, a approximately 45% body surface area burn was produced on the trunk; controls were body sham-burned rats. The evoked twitch tensions of tibialis anterior muscles in both legs were measured together with AChR proteins and their transcripts. Compared with the contralateral leg, absolute tensions in the burned leg declined at days 1, 4, and 7 without loss of muscle mass so that tension per unit wet muscle mass (specific tension) decreased; at day 14, the tension decreased with muscle atrophy so that specific tension was unchanged. Membrane AChRs and/or the immature subunit transcript, AChRgamma messenger ribonucleic acid (mRNA) increased at days 4, 7, and 14, and both were inversely related to evoked tension (r =.43, P <.01 and r =.61, P <.0001, respectively). There was a direct correlation between AChR and AChRgamma mRNA (r =.82, P <.001), suggesting that the upregulated AChRs may contain the immature gamma-subunit isoform. After approximately 45% body burn, AChRs and mRNA did not change and the evoked tensions did not decline, but there was relative loss of muscle mass at days 7 and 14 so that specific tension increased. Burn trauma initially causes weakness of muscles directly under the burn, and this weakness may be partially related to increased expression of immature AChRs and later to muscle atrophy.

Electrical activity and postsynapse formation in adult muscle: gamma-AChRs are not required

Skeletal muscle fibers will not accept hyperinnervation by foreign motor axons unless they are paralyzed, suggesting that paralysis makes them receptive to innervation, e.g., by upregulating extrasynaptic expression of gamma-AChRs and/or of the agrin receptor MuSK. To examine the involvement of these parameters in paralysis-mediated synapse induction, ectopic expression of agrin, a factor from motor neurons controlling neuromuscular synapse formation, was made dependent on the administration of doxycycline in innervated adult muscle fibers. In response to doxycycline-induced agrin secretion, adult fibers did form ectopic postsynaptic specializations, even when they were electrically active, lacked fetal AChRs, and expressed normal low levels of MuSK. These data demonstrate that paralysis and changes associated with it are not required for agrin-induced postsynapse formation. They suggest that paralyzed muscle induces synapse formation via the release of factors that make motor neurites contact muscle fibers and secrete agrin.

Different functions of fetal and adult AChR subtypes for the formation and maintenance of neuromuscular synapses revealed in epsilon-subunit-deficient mice

Mice deficient in epsilon-subunits of the acetylcholine receptor (AChR) channel die prematurely due to severe AChR deficiency that leads to the progressive reduction in AChR density at the neuromuscular endplate [Witzemann, V., Schwarz, H., Koenen, M., Berberich, C., Villarroel, A., Wernig, A., Brenner, H.R. & Sakmann, B. (1996) Proc. Natl Acad. Sci. USA, 93, 13286-13291]. The mice may serve as a model for studying AChR-related myasthenic diseases. The postnatal development of the subsynaptic apparatus takes place in the absence of the adult type, epsilon-subunit-containing receptors which normally replace the fetal gamma-subunit-containing receptors. During later development the secondary folds of the postsynaptic membrane disappear concomitant with the decrease in AChR density, so that the flattened-out membrane with its remaining nicotinic receptors is in close proximity to the subsynaptic cytoplasmatic compartment and the subsynaptic myonuclei. The decrease in AChR concentration is correlated with a decrease of postsynaptic rapsyn, but has less effect on agrin, a neuronally released aggregating factor for AChRs. Thus, despite the presence of agrin at the synapse, AChR expression is not maintained at the level required to stabilize normal synaptic structure comprising secondary postsynaptic membrane folds. Collectively the results suggest that the postnatal switch from the global, activity-sensitive gamma-subunit gene transcription to the synapse-specific, activity-independent epsilon-subunit gene transcription is not required for the formation and differentiation of synapses but is essential for the maintenance of the highly organized structure of the neuromuscular endplate.

Nicotinic acetylcholine receptor knockout mice as animal models for studying receptor function

Nicotinic acetylcholine receptors are pentameric ligand-gated ion channels, which are involved in a wide range of neuronal functions. During the past decade, a large number of nicotinic acetylcholine receptor subunits have been cloned and showed a discreet yet overlapping distribution pattern. Recently, several groups have produced mutant mice lacking specific nicotinic acetylcholine receptor subunits. In this review, we focus on how the study of these knockout mouse models has advanced our understanding of the role individual nicotinic acetylcholine receptor subunits play in the function and composition of endogenous receptors and the diverse pharmacological actions of nicotine in the mammalian nervous system.

Novel functional epsilon-subunit polypeptide generated by a single nucleotide deletion in acetylcholine receptor deficiency congenital myasthenic syndrome

Acetylcholine receptor (AChR) deficiency is a recessively inherited congenital myasthenic syndrome in which fatigable muscle weakness results from impaired neuromuscular transmission caused by reduced AChR numbers. In mature muscle, AChRs consist of alpha2 betadelta together with the adult-specific epsilon subunit. We have identified a deletion of the first nucleotide in exon 12 of the AChR epsilon-subunit gene (epsilon1267delG) and demonstrate its recessive inheritance segregates with disease in 6 unrelated cases of AChR deficiency. In addition, we found that both healthy and AChR-deficient muscle contain a population of AChR epsilon-subunit mRNA transcripts that retain intron 11. We investigated the possible consequences of combining this mutation with the alternative mRNA species through AChR expression studies in human embryonic kidney cells and Xenopus oocytes. Epsilon1267delG generates a polypeptide that lacks M4 and is not detected in surface AChR, whereas retention of intron 11 in the RNA transcript restores the reading frame, conserves M4, and generates a polypeptide that is incorporated into functional surface AChR, although at a reduced level, consistent with the disease phenotype. Our results indicate that for some AChR deficiency mutations located between M3 and M4, the retention of intron 11 in the epsilon-subunit mRNA transcripts may rescue adult AChR function.

Roles of rapsyn and agrin in interaction of postsynaptic proteins with acetylcholine receptors

At the neuromuscular junction, aggregates of acetylcholine receptors (AChRs) are anchored in the muscle membrane by association with rapsyn and other postsynaptic proteins. We have investigated the interactions between the AChR and these proteins in cultured C2 myotubes before and after treatment with agrin, a nerve-derived protein that induces AChRs to cluster. When AChRs were isolated from detergent extracts of untreated C2 myotubes, they were associated with rapsyn and, to a lesser degree, with utrophin, beta-dystroglycan, MuSK, and src-related kinases, but not with syntrophin. Treatment with agrin increased the association of AChRs with MuSK, a receptor tyrosine kinase that forms part of the agrin receptor complex, without affecting other interactions. Analysis of rapsyn-deficient myotubes, which do not form protein clusters in response to agrin, revealed that rapsyn is required for association of the AChR with utrophin and beta-dystroglycan, and for the agrin-induced increase in association with MuSK, but not for constitutive interactions with MuSK and src-related kinases. In rapsyn -/- myotubes, agrin caused normal tyrosine phosphorylation of AChR-associated and total MuSK, whereas phosphorylation of the AChR beta subunit, both constitutive and agrin-induced, was strongly reduced. These results show first that aneural myotubes contain preassembled AChR protein complexes that may function in the assembly of the postsynaptic apparatus, and second that rapsyn, in addition to its role in AChR phosphorylation, mediates selected protein interactions with the AChR and serves as a link between the AChR and the dystrophin/utrophin glycoprotein complex.

Roles of rapsyn and agrin in interaction of postsynaptic proteins with acetylcholine receptors

At the neuromuscular junction, aggregates of acetylcholine receptors (AChRs) are anchored in the muscle membrane by association with rapsyn and other postsynaptic proteins. We have investigated the interactions between the AChR and these proteins in cultured C2 myotubes before and after treatment with agrin, a nerve-derived protein that induces AChRs to cluster. When AChRs were isolated from detergent extracts of untreated C2 myotubes, they were associated with rapsyn and, to a lesser degree, with utrophin, beta-dystroglycan, MuSK, and src-related kinases, but not with syntrophin. Treatment with agrin increased the association of AChRs with MuSK, a receptor tyrosine kinase that forms part of the agrin receptor complex, without affecting other interactions. Analysis of rapsyn-deficient myotubes, which do not form protein clusters in response to agrin, revealed that rapsyn is required for association of the AChR with utrophin and beta-dystroglycan, and for the agrin-induced increase in association with MuSK, but not for constitutive interactions with MuSK and src-related kinases. In rapsyn -/- myotubes, agrin caused normal tyrosine phosphorylation of AChR-associated and total MuSK, whereas phosphorylation of the AChR beta subunit, both constitutive and agrin-induced, was strongly reduced. These results show first that aneural myotubes contain preassembled AChR protein complexes that may function in the assembly of the postsynaptic apparatus, and second that rapsyn, in addition to its role in AChR phosphorylation, mediates selected protein interactions with the AChR and serves as a link between the AChR and the dystrophin/utrophin glycoprotein complex.

Positive- and negative-regulation of Ca2+ entry through ACh receptor channels by phosphorylation

During development of mammalian neuromuscular junctions, the expression of ACh receptors (AChRs) switches from an embryonic form to an adult form, which provides a higher Ca2+ permeable channel. The lack of this switch can prevent normal development of neuromuscular junctions. In non-mammalian species, however, the switch appears not to occur. The results of the present study show that Ca2+ entry through non-mammalian AChR channels was controlled by differential phosphorylation rather than a subunit switch. This finding provides support for the hypothesis that localized Ca2+ influx through AChR channels has a critical role in the formation and maintenance of neuromuscular junctions.

Maturation of neuromuscular transmission during early development in zebrafish

We have examined the rapid development of synaptic transmission at the neuromuscular junction (NMJ) in zebrafish embryos and larvae by patch-clamp recording of spontaneous miniature endplate currents (mEPCs) and single acetylcholine receptor (AChR) channels. Embryonic (24-36 h) mEPCs recorded in vivo were small in amplitude (<50 pA). The rate of mEPCs increased in larvae (3.5-fold increase measured by 6 days), and these mEPCs were mostly of larger amplitude (10-fold on average) with (</=5-fold) faster kinetics. Intracellular labeling with Lucifer yellow indicated extensive coupling between muscle cells in both embryos and larvae (</=10 days). Blocking acetylcholinesterase (AChE) with eserine had no effect on mEPC kinetics in embryos at 1 day and only partially slowed (by approximately 1/2) the decay rate in larvae at 6 days. In acutely dissociated muscle cells, we observed the same two types of AChR with conductances of 45 and 60 pS and with similar, brief (<0.5 ms) mean open times in both embryos and larvae. We conclude that AChR properties are set early during development at these early stages; functional maturation of the NMJ is only partly shaped by expression of AChE and may also depend on postsynaptic AChR clustering and presynaptic maturation.

ERK MAP kinase activation is required for acetylcholine receptor inducing activity-induced increase in all five acetylcholine receptor subunit mRNAs as well as synapse-specific expression of acetylcholine receptor epsilon-transgene

The AChR is a pentamer of four different subunits in a stoichiometry of alpha2betagammadelta in embryonic and alpha2betaepsilondelta in adult animals. Transcription of AChR subunit genes is most active in synaptic nuclei in adult skeletal muscle cells, and is regulated by neural factors such as ARIA. We report here that ARIA up-regulated specifically the expression of all five AChR subunits in C2C12 cells. The mRNA level of erbB2, erbB3, rapsyn, MuSK, SHP-2 and beta-actin remained unchanged in response to ARIA stimulation in C2C12 cells. The ARIA-induced increase in AChR subunit expression in C2C12 cells was inhibited by the erbB kinase inhibitor tyrphostin AG1478 and the MEK inhibitor PD98059, but not by the PI3 kinase inhibitor wortmannin, suggesting an important role of the erbB protein tyrosine kinases and MAP kinase in the regulation of the expression of the five different AChR subunits. To determine the signaling pathways in vivo, we studied the expression of reporter genes driven by the epsilon-promoter in injected muscles. The in vivo expression of the epsilon-transgene was inhibited by co-expression of dominant negative mutants of key components in the MAP kinase pathway including ras, raf and MEK, but not the dominant negative mutant of PI3 kinase. These results suggest that ERK MAP kinase activation is required for ARIA-induced increase in all five AChR subunit mRNAs as well as synapse-specific expression of AChR epsilon-transgene.

Myasthenic syndromes in Turkish kinships due to mutations in the acetylcholine receptor

We report and functionally characterize five new mutations of the acetylcholine receptor (AChR) in 11 Turkish patients with recessive congenital myasthenic syndromes (CMS) belonging to six families. All mutations are in the epsilon-subunit gene. Parental consanguinity is present in three families. The disease cosegregates with homozygous mutations in five families and with two different heteroallelic mutations in one family. Four mutations are frameshifting, predicting truncation of the epsilon subunit, and one occurs at a splice donor site. Expression of each frameshifting mutation and the likely transcripts of the splice-site mutation in human embryonic kidney 293 cells shows that each mutation is a null mutation. The findings support the notion that loss-of-function mutations of the acetylcholine receptor causing CMS are concentrated in the epsilon subunit, and that such mutations are a frequent cause of CMS.

Deficient development and maintenance of postsynaptic specializations in mutant mice lacking an ‘adult’ acetylcholine receptor subunit

At many synapses, ‘fetal’ neurotransmitter receptor subunits are replaced by ‘adult’ subunits as development proceeds. To assess the significance of such transitions, we deleted the gene encoding the adult acetylcholine receptor (AChR) epsilon subunit, which replaces its fetal counterpart, the gamma subunit, at the skeletal neuromuscular junction during early postnatal life. Several aspects of postnatal maturation, including synapse elimination, proceeded normally in the absence of the adult AChR, but structural development of the endplate was compromised. Later, inadequate compensation by the gamma subunit led to severely reduced AChR density in mutant endplates relative to controls. This decreased density led to a profound reorganization of AChR-associated components of the postsynaptic membrane and cytoskeleton. Together, these results suggest novel roles for AChRs in assembly of the postsynaptic apparatus.

Pathological mutations of nicotinic receptors and nicotine-based therapies for brain disorders

Nicotinic acetylcholine receptors are allosteric ligand-gated ion channels present in muscle and brain. Recent studies suggest that mutations altering their functional properties may produce congenital myasthenia and familial frontal lobe epilepsy. Current research also indicates that although nicotinic ligands often possess addictive properties, they could serve as therapeutic agents for Alzheimer's disease and Tourette's syndrome, as well as for schizophrenia.

Congenital myasthenic syndromes due to heteroallelic nonsense/missense mutations in the acetylcholine receptor epsilon subunit gene: identification and functional characterization of six new mutations

We describe and functionally characterize six mutations of the acetylcholine receptor (AChR) epsilon subunit gene in three congenital myasthenic syndrome patients. Endplate studies demonstrated severe endplate AChR deficiency, dispersed endplate regions and well preserved junctional folds in all three patients. Electrophysiologic studies were consistent with expression of the fetal gamma-AChR at the endplates in one patient, prolongation of some channel events in another and gamma-AChR expression as well as some shorter than normal channel events in still another. Genetic analysis revealed two recessive and heteroallelic epsilon subunit gene mutations in each patient. One mutation in each (epsilonC190T [epsilon R64X], epsilon 127ins5 and epsilon 553del 7) generates a nonsense codon that predicts truncation of the epsilon subunit in its N-terminal, extracellular domain; and one mutation in each generates a missense codon (epsilon R147L, epsilon P245L and epsilon R311W). None of the mutations was detected in 100 controls. Expression studies in HEK cells indicate that the three nonsense mutations are null mutations and that surface expression of AChRs harboring the missense mutations is significantly reduced. Kinetic analysis of AChRs harboring the missense mutations show that epsilon R147L is kinetically benign, epsilon P245L prolongs burst open duration 2-fold by slowing the rate of channel closing and epsilon R311W shortens burst duration 2-fold by slowing the rate of channel opening and speeding the rate of ACh dissociation. The modest changes in activation kinetics are probably overshadowed by reduced expression of the missense mutations. The consequences of the endplate AChR deficiency are mitigated by persistent expression of gamma-AChR, changes in the release of transmitter quanta and appearance of multiple endplate regions on the muscle fiber.