Calcium permeability increase of endplate channels in rat muscle during postnatal development

Skeletal muscle structure, physiology, and function

Contractions of skeletal muscles provide the stability and power for all body movements. Consequently, any impairment in skeletal muscle function results in some degree of instability or immobility. Factors that influence skeletal muscle structure and function are therefore of great interest scientifically and clinically. Injury, neuromuscular disease, and old age are among the factors that commonly contribute to impairments in skeletal muscle function. The goal of this chapter is to summarize the fundamentals of skeletal muscle structure and function to provide foundational knowledge for this Handbook volume. We examine the molecular interactions that provide the basis for the generation of force and movement, discuss mechanisms of the regulation of contraction at the level of myofibers, and introduce concepts of the activation and control of muscle function in vivo. Where appropriate, the chapter updates the emerging science that will increase understanding of muscle function.

Donepezil inhibits neuromuscular junctional acetylcholinesterase and enhances synaptic transmission and function in isolated skeletal muscle

BACKGROUND AND PURPOSE

Donepezil, a piperidine inhibitor of acetylcholinesterase (AChE) prescribed for treatment of Alzheimer's disease, has adverse neuromuscular effects in humans, including requirement for higher concentrations of non-depolarising neuromuscular blockers during surgery. Here, we examined the effects of donepezil on synaptic transmission at neuromuscular junctions (NMJs) in isolated nerve-muscle preparations from mice.

EXPERIMENTAL APPROACH

We measured effects of therapeutic concentrations of donepezil (10 nM to 1 μM) on AChE enzymic activity, muscle force responses to repetitive stimulation, and spontaneous and evoked endplate potentials (EPPs) recorded intracellularly from flexor digitorum brevis muscles from CD01 or C57BlWld^S mice.

KEY RESULTS

Donepezil inhibited muscle AChE with an approximate IC₅₀ of 30 nM. Tetanic stimulation in sub-micromolar concentrations of donepezil prolonged post-tetanic muscle contractions. Preliminary Fluo4-imaging indicated an association of these contractions with an increase and slow decay of intracellular Ca²⁺ transients at motor endplates. Donepezil prolonged spontaneous miniature EPP (MEPP) decay time constants by about 65% and extended evoked EPP duration almost threefold. The mean frequency of spontaneous MEPPs was unaffected but the incidence of 'giant' MEPPs (gMEPPs), some exceeding 10 mV in amplitude, was increased. Neither mean MEPP amplitude (excluding gMEPPs), mean EPP amplitude, quantal content or synaptic depression during repetitive stimulation were significantly altered by concentrations of donepezil up to 1 μM.

CONCLUSION AND IMPLICATIONS

Adverse neuromuscular signs associated with donepezil therapy, including relative insensitivity to neuromuscular blockers, are probably due to inhibition of AChE at NMJs, prolonging the action of ACh on postsynaptic nicotinic acetylcholine receptors but without substantively impairing evoked ACh release.

Early Developmental Changes of Muscle Acetylcholine Receptors Are Little Influenced by Dystrophin Absence in mdx Mouse

Dystrophin is a cytoskeletal protein contributing to the organization of the neuromuscular junction. In Duchenne muscular dystrophy, due to dystrophin absence, the distribution of endplate acetylcholine receptors (AChRs) becomes disorganized. It is still debated whether this is due to the absence of dystrophin or to the repeated damage/regeneration cycles typical of dystrophic muscle. We addressed this controversy studying the endplate in the first 3 postnatal weeks, when muscle damage in dystrophic (mdx) mice is minimal. By synaptic and extra-synaptic patch-clamp recordings in acutely dissociated mdx and wt muscle fibers, we recorded unitary events due to openings of AChR-channels containing the γ and ε subunit. We also examined AChR distribution at the endplate by immunofluorescence assays. No differences between wt and mdx fibers were found in the γ/ε switch, nor in the AChR organization at the endplates up to 21 postnatal days. Conversely, we detected a delayed appearance and disappearance of patches with high channel opening frequency in mdx fibers. Our data emphasize that the innervation-dependent γ/ε switch and AChR organization in the endplate are not affected by the absence of dystrophin, while extra-synaptic AChR cluster formation and disassembly could be differentially regulated in mdx mice.

Lithium causes differential effects on postsynaptic stability in normal and denervated neuromuscular synapses

Lithium chloride has been widely used as a therapeutic mood stabilizer. Although cumulative evidence suggests that lithium plays modulatory effects on postsynaptic receptors, the underlying mechanism by which lithium regulates synaptic transmission has not been fully elucidated. In this work, by using the advantageous neuromuscular synapse, we evaluated the effect of lithium on the stability of postsynaptic nicotinic acetylcholine receptors (nAChRs) in vivo. We found that in normally innervated neuromuscular synapses, lithium chloride significantly decreased the turnover of nAChRs by reducing their internalization. A similar response was observed in CHO-K1/A5 cells expressing the adult muscle-type nAChRs. Strikingly, in denervated neuromuscular synapses, lithium led to enhanced nAChR turnover and density by increasing the incorporation of new nAChRs. Lithium also potentiated the formation of unstable nAChR clusters in non-synaptic regions of denervated muscle fibres. We found that denervation-dependent re-expression of the foetal nAChR γ-subunit was not altered by lithium. However, while denervation inhibits the distribution of β-catenin within endplates, lithium-treated fibres retain β-catenin staining in specific foci of the synaptic region. Collectively, our data reveal that lithium treatment differentially affects the stability of postsynaptic receptors in normal and denervated neuromuscular synapses in vivo, thus providing novel insights into the regulatory effects of lithium on synaptic organization and extending its potential therapeutic use in conditions affecting the peripheral nervous system.

Maturation of a postsynaptic domain: Role of small Rho GTPases in organising nicotinic acetylcholine receptor aggregates at the vertebrate neuromuscular junction

The neuromuscular junction (NMJ) is the peripheral synapse formed between a motor axon and a skeletal muscle fibre that allows muscle contraction and the coordinated movement in many species. A main hallmark of the mature NMJ is the assembly of nicotinic acetylcholine receptor (nAChR) aggregates in the muscle postsynaptic domain, that distributes in perfect apposition to presynaptic motor terminals. To assemble its unique functional architecture, initial embryonic NMJs undergo an early postnatal maturation process characterised by the transformation of homogenous nAChR-containing plaques to elaborate and branched pretzel-like structures. In spite of a detailed morphological characterisation, the molecular mechanisms controlling the intracellular scaffolding that organises a postsynaptic domain at the mature NMJ have not been fully elucidated. In this review, we integrate evidence of key processes and molecules that have shed light on our current understanding of the NMJ maturation process. On the one hand, we consider in vitro studies revealing the potential role of podosome-like structures to define discrete low nAChR-containing regions to consolidate a plaque-to-pretzel transition at the NMJ. On the other hand, we focus on in vitro and in vivo evidence demonstrating that members of the Ras homologous (Rho) protein family of small GTPases (small Rho GTPases) play indispensable roles on NMJ maturation by regulating the stability of nAChR aggregates. We combine this evidence to propose that small Rho GTPases are key players in the assembly of podosome-like structures that drive the postsynaptic maturation of vertebrate NMJs.

Motor function recovery: deciphering a regenerative niche at the neuromuscular synapse

The coordinated movement of many organisms relies on efficient nerve–muscle communication at the neuromuscular junction (NMJ), a peripheral synapse composed of a presynaptic motor axon terminal, a postsynaptic muscle specialization, and non‐myelinating terminal Schwann cells. NMJ dysfunctions are caused by traumatic spinal cord or peripheral nerve injuries as well as by severe motor pathologies. Compared to the central nervous system, the peripheral nervous system displays remarkable regenerating abilities; however, this capacity is limited by the denervation time frame and depends on the establishment of permissive regenerative niches. At the injury site, detailed information is available regarding the cells, molecules, and mechanisms involved in nerve regeneration and repair. However, a regenerative niche at the final functional step of peripheral motor innervation, i.e. at the mature neuromuscular synapse, has not been deciphered. In this review, we integrate classic and recent evidence describing the cells and molecules that could orchestrate a dynamic ecosystem to accomplish successful NMJ regeneration. We propose that such a regenerative niche must ensure at least two fundamental steps for successful NMJ regeneration: the proper arrival of incoming regenerating axons to denervated postsynaptic muscle domains, and the resilience of those postsynaptic domains, in morphological and functional terms. We here describe and combine the main cellular and molecular responses involved in each of these steps as potential targets to help successful NMJ regeneration.

The Structure, Function, and Physiology of the Fetal and Adult Acetylcholine Receptor in Muscle

The neuromuscular junction (NMJ) is a highly developed synapse linking motor neuron activity with muscle contraction. A complex of molecular cascades together with the specialized NMJ architecture ensures that each action potential arriving at the motor nerve terminal is translated into an action potential in the muscle fiber. The muscle-type nicotinic acetylcholine receptor (AChR) is a key molecular component located at the postsynaptic muscle membrane responsible for the generation of the endplate potential (EPP), which usually exceeds the threshold potential necessary to activate voltage-gated sodium channels and triggers a muscle action potential. Two AChR isoforms are found in mammalian muscle. The fetal isoform is present in prenatal stages and is involved in the development of the neuromuscular system whereas the adult isoform prevails thereafter, except after denervation when the fetal form is re-expressed throughout the muscle. This review will summarize the structural and functional differences between the two isoforms and outline congenital and autoimmune myasthenic syndromes that involve the isoform specific AChR subunits.

Neuromuscular Development in Neonates and Postnatal Infants: Implications for Neuromuscular Electrical Stimulation Therapy for Dysphagia

Purpose The aim of the current study was to review neuromuscular development, summarize the current body of evidence describing the use of neuromuscular electrical stimulation (NMES) therapy in infants, and identify possible contraindications for the use of NMES in the neonate and young infant. Method After a review of the literature describing neuromuscular development, we created a timeline of the developmental processes. Key milestones were determined, and a literature search was conducted to identify potential effects of electrical stimulation on this process. Results Current evidence supporting the use of NMES in the pediatric population is limited and of poor quality. Contraindications of the use of NMES in the neonate and young infant were identified, including (a) inhibited expression of the neural cell adhesion molecule that is vital for neuromuscular development, (b) alteration of muscle fiber type metabolic profile away from intended muscle fiber type morphology, and (c) interruption of postsynaptic acetylcholine receptor synthesis during neuromuscular junction development. Conclusion The use of NMES for the treatment of dysphagia in the neonate and young infant may influence early neuromuscular development in a manner that is not currently well understood. Future research is needed to further understand the effects of NMES on the developing neuromuscular system.

Calcium influx through muscle nAChR-channels: One route, multiple roles

Although Ca²⁺ influx through muscle nAChR-channels has been described over the past 40 years, its functions remain still poorly understood. In this review we suggest possible roles of Ca²⁺ entry at all stages of muscle development, summarizing the evidence present in literature. nAChRs are expressed in myoblasts prior to fusion, and can be activated in the absence of an ACh-releasing nerve terminal, with Ca²⁺ influx likely contributing to regulate cell fusion. Upon establishment of nerve-muscle contact, Ca²⁺ influx contributes to orchestrate the signaling required for the correct formation of the neuromuscular junction. Finally, in the mature synapse, Ca²⁺ entry through postsynaptic nAChR-channels - highly Ca²⁺ permeable, in particular in humans - acts on K⁺ and Na⁺ channels to shape endplate excitability. However, when genetic defects cause excessive channel activation, Ca²⁺ influx becomes toxic and causes endplate myopathy. Throughout the review, we highlight how Ricardo Miledi has contributed to construct this whole body of knowledge, from the initial description of Ca²⁺ permeability of endplate nAChR channels, to the rationale for the treatment of endplate excitotoxic damage under pathological conditions. This article is part of a Special Issue entitled: SI: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

REVIEW ■ : Molecular Basis of Congenital Myasthenic Syndromes: Mutations in the Acetylcholine Receptor

The congenital myasthenic syndromes include end-plate (EP) acetylcholinesterase deficiency, presynaptic abnormalities affecting the evoked release or size of transmitter quanta, and acetylcholine (ACh) receptor (AChR) channelopathies stemming from a kinetic abnormality and/or deficiency of AChR. A kinetic abnor mality predicts, and AChR deficiency may predict, one or more mutations in an AChR subunit gene. These clues have led to the identification of 53 mutations in different subunits of AChR in 55 kinships of the congenital myasthenic syndromes. The mutations either increase or decrease the response to ACh, produce AChR deficiency, or both. In the slow-channel syndromes, prolonged opening episodes of AChR cause cationic overloading of the EP and an EP myopathy; the mutations occur in different subunits and different domains of the subunits and have dominant positive effects. The M1 and M2 mutations slow channel closure, increase apparent affinity for ACh, and variably enhance desensitization, and the extracellular αG153S enhances affinity for ACh, promoting reopening of the diliganded receptor. In the low-affinity fast-channel syndrome, εP121L reduces affinity for ACh and reopening of the diliganded receptor, resulting in a de creased response to ACh and shorter burst durations. Severe EP AChR deficiency results from heterozy gous or homozygous mutations that terminate translation prematurely; these are concentrated in the ε subunit, probably because substitution of the fetal γ for the adult ε subunit can rescue the phenotype from fatal null mutations in ε. Variable AChR deficiency and variable functional abnormalities stem from hetero allelic nonsense and missense mutations in AChR subunit genes. NEUROSCIENTIST 4:185-194, 1998

Structural correlates of affinity in fetal versus adult endplate nicotinic receptors

Adult-type nicotinic acetylcholine receptors (AChRs) mediate signalling at mature neuromuscular junctions and fetal-type AChRs are necessary for proper synapse development. Each AChR has two neurotransmitter binding sites located at the interface of a principal and a complementary subunit. Although all agonist binding sites have the same core of five aromatic amino acids, the fetal site has ∼30-fold higher affinity for the neurotransmitter ACh. Here we use molecular dynamics simulations of adult versus fetal homology models to identify complementary-subunit residues near the core that influence affinity, and use single-channel electrophysiology to corroborate the results. Four residues in combination determine adult versus fetal affinity. Simulations suggest that at lower-affinity sites, one of these unsettles the core directly and the others (in loop E) increase backbone flexibility to unlock a key, complementary tryptophan from the core. Swapping only four amino acids is necessary and sufficient to exchange function between adult and fetal AChRs.

Adult and fetal nicotinic acetylcholine receptors (AChRs) have different functional requirements and affinity for ACh. Here, the authors use molecular dynamics and electrophysiology to investigate this affinity, and identify four amino acids that when swapped exchange function between adult and fetal AChRs.

Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting

The neuromuscular junction is the chemical synapse between motor neurons and skeletal muscle fibers. It is designed to reliably convert the action potential from the presynaptic motor neuron into the contraction of the postsynaptic muscle fiber. Diseases that affect the neuromuscular junction may cause failure of this conversion and result in loss of ambulation and respiration. The loss of motor input also causes muscle wasting as muscle mass is constantly adapted to contractile needs by the balancing of protein synthesis and protein degradation. Finally, neuromuscular activity and muscle mass have a major impact on metabolic properties of the organisms. This review discusses the mechanisms involved in the development and maintenance of the neuromuscular junction, the consequences of and the mechanisms involved in its dysfunction, and its role in maintaining muscle mass during aging. As life expectancy is increasing, loss of muscle mass during aging, called sarcopenia, has emerged as a field of high medical need. Interestingly, aging is also accompanied by structural changes at the neuromuscular junction, suggesting that the mechanisms involved in neuromuscular junction maintenance might be disturbed during aging. In addition, there is now evidence that behavioral paradigms and signaling pathways that are involved in longevity also affect neuromuscular junction stability and sarcopenia.

The craniosacral progression of muscle development influences the emergence of neuromuscular junction alterations in a severe murine model for spinal muscular atrophy

AIMS

As 4-day-old mice of the severe spinal muscular atrophy (SMA) model (dying at 5-8 days) display pronounced neuromuscular changes in the diaphragm but not the soleus muscle, we wanted to gain more insight into the relationship between muscle development and the emergence of pathological changes and additionally to analyse intercostal muscles which are affected in human SMA.

METHODS

Structures of muscle fibres and neuromuscular junctions (NMJs) of the diaphragm, intercostal and calf muscles of prenatal (E21) and postnatal (P0 and P4) healthy and SMA mice were analysed by light and transmission electron microscopy. NMJ innervation was studied by whole mount immunofluorescence in diaphragms of P4 mice.

RESULTS

During this period, the investigated muscles still show a significant neck-to-tail developmental gradient. The diaphragm and calf muscles are most and least advanced, respectively, with respect to muscle fibre fusion and differentiation. The number and depth of subsynaptic folds increases, and perisynaptic Schwann cells (PSCs) acquire a basal lamina on their outer surface. Subsynaptic folds are connected to an extensive network of tubules and beaded caveolae, reminiscent of the T system in adult muscle. Interestingly, intercostal muscles from P4 SMA mice show weaker pathological involvement (that is, vacuolization of PSCs and perineurial cells) than those previously described by us for the diaphragm, whereas calf muscles show no pathological changes.

CONCLUSION

SMA-related alterations appear to occur only when the muscles have reached a certain developmental maturity. Moreover, glial cells, in particular PSCs, play an important role in SMA pathogenesis.

Asymmetric transmitter binding sites of fetal muscle acetylcholine receptors shape their synaptic response

Neuromuscular acetylcholine receptors (AChRs) have two transmitter binding sites: at α-δ and either α-γ (fetal) or α-ε (adult) subunit interfaces. The γ-subunit of fetal AChRs is indispensable for the proper development of neuromuscular synapses. We estimated parameters for acetylcholine (ACh) binding and gating from single channel currents of fetal mouse AChRs expressed in tissue-cultured cells. The unliganded gating equilibrium constant is smaller and less voltage-dependent than in adult AChRs. However, the α-γ binding site has a higher affinity for ACh and provides more binding energy for gating compared with α-ε; therefore, the diliganded gating equilibrium constant at -100 mV is comparable for both receptor subtypes. The -2.2 kcal/mol extra binding energy from α-γ compared with α-δ and α-ε is accompanied by a higher resting affinity for ACh, mainly because of slower transmitter dissociation. End plate current simulations suggest that the higher affinity and increased energy from α-γ are essential for generating synaptic responses at low pulse [ACh].

Molecular mechanisms underlying maturation and maintenance of the vertebrate neuromuscular junction

The vertebrate neuromuscular junction (NMJ), a peripheral synapse formed between motoneuron and skeletal muscle, is characterized by a protracted postnatal period of maturation and life-long maintenance. In neuromuscular disorders such as congenital myasthenic syndromes (CMSs), disruptions of NMJ maturation and/or maintenance are frequently observed. In particular, defective neuromuscular transmission associated with structural and molecular abnormalities at the pre- and postsynaptic membranes, as well as at the synaptic cleft, has been reported in these patients. Here, we review recent advances in the understanding of molecular and cellular events that mediate NMJ maturation and maintenance. The underlying regulatory mechanisms, including key molecular regulators at the presynaptic nerve terminal, synaptic cleft, and postsynaptic muscle membrane, are discussed.

Effect of self-gating on action potential firing at neuromuscular junction

The neuromuscular junction (NMJ) is the place where the axon terminal of motoneuron connects the 'endplate' of a muscle fiber. During this transduction a large depolarization (endplate potential) caused by the nerve impulse opens a large number of voltage-sensitive sodium channels at the post-junctional terminal. As a result, action potentials are generated and propagated along the muscle fiber causing contraction. This work shows simulated results of the voltage-dependent sodium channels' firing behavior at the NMJ using a mathematical model. It is found that the firing behavior of the sodium channels change basing on their activation and inactivation kinetics which are highly influenced by the self-gating behavior of the sodium conductances. The simulation results showed that self-gating of sodium channels increase conduction efficiency at the NMJ and decrease threshold for firing.

Skeletal muscle IP3R1 receptors amplify physiological and pathological synaptic calcium signals

Ca(2+) release from internal stores is critical for mediating both normal and pathological intracellular Ca(2+) signaling. Recent studies suggest that the inositol 1,4,5-triphosphate (IP(3)) receptor mediates Ca(2+) release from internal stores upon cholinergic activation of the neuromuscular junction (NMJ) in both physiological and pathological conditions. Here, we report that the type I IP(3) receptor (IP(3)R(1))-mediated Ca(2+) release plays a crucial role in synaptic gene expression, development, and neuromuscular transmission, as well as mediating degeneration during excessive cholinergic activation. We found that IP(3)R(1)-mediated Ca(2+) release plays a key role in early development of the NMJ, homeostatic regulation of neuromuscular transmission, and synaptic gene expression. Reducing IP(3)R(1)-mediated Ca(2+) release via siRNA knockdown or IP(3)R blockers in C2C12 cells decreased calpain activity and prevented agonist-induced acetylcholine receptor (AChR) cluster dispersal. In fully developed NMJ in adult muscle, IP(3)R(1) knockdown or blockade effectively increased synaptic strength at presynaptic and postsynaptic sites by increasing both quantal release and expression of AChR subunits and other NMJ-specific genes in a pattern resembling muscle denervation. Moreover, in two mouse models of cholinergic overactivity and NMJ Ca(2+) overload, anti-cholinesterase toxicity and the slow-channel myasthenic syndrome (SCS), IP(3)R(1) knockdown eliminated NMJ Ca(2+) overload, pathological activation of calpain and caspase proteases, and markers of DNA damage at subsynaptic nuclei, and improved both neuromuscular transmission and clinical measures of motor function. Thus, blockade or genetic silencing of muscle IP(3)R(1) may be an effective and well tolerated therapeutic strategy in SCS and other conditions of excitotoxicity or Ca(2+) overload.

Physiological characterization of human muscle acetylcholine receptors from ALS patients

Amyotrophic lateral sclerosis (ALS) is characterized by progressive degeneration of motor neurons leading to muscle paralysis. Research in transgenic mice suggests that the muscle actively contributes to the disease onset, but such studies are difficult to pursue in humans and in vitro models would represent a good starting point. In this work we show that tiny amounts of muscle from ALS or from control denervated muscle, obtained by needle biopsy, are amenable to functional characterization by two different technical approaches: "microtransplantation" of muscle membranes into Xenopus oocytes and culture of myogenic satellite cells. Acetylcholine (ACh)-evoked currents and unitary events were characterized in oocytes and multinucleated myotubes. We found that ALS acetylcholine receptors (AChRs) retain their native physiological characteristics, being activated by ACh and nicotine and blocked by α-bungarotoxin (α-BuTX), d-tubocurarine (dTC), and galantamine. The reversal potential of ACh-evoked currents and the unitary channel behavior were also typical of normal muscle AChRs. Interestingly, in oocytes injected with muscle membranes derived from ALS patients, the AChRs showed a significant decrease in ACh affinity, compared with denervated controls. Finally, riluzole, the only drug currently used against ALS, reduced, in a dose-dependent manner, the ACh-evoked currents, indicating that its action remains to be fully characterized. The two methods described here will be important tools for elucidating the role of muscle in ALS pathogenesis and for developing drugs to counter the effects of this disease.

Modulation of the Ca(2+) permeability of human endplate acetylcholine receptor-channel

In slow-channel congenital myasthenic syndrome, point mutations of the endplate acetylcholine receptor (AChR) prolong channel openings, leading to excessive Ca(2+) entry with ensuing endplate degeneration and myasthenic symptoms. The Ca(2+) permeability of the human endplate AChR-channel is quite high, and is further increased by two slow-channel mutations in its ɛ subunit, worsening the pathological cascade. To gain further support to the hypothesis that the ɛ subunit plays a crucial role in controlling Ca(2+) permeability of endplate AChR-channel, in this work we measured the fractional Ca(2+) current (P(f), i.e., the percentage of the total current carried by Ca(2+) ions) of a panel of AChR carrying slow-channel mutations in the α, β and ɛ subunits detected in patients (α(N217K), α(S226Y), α(C418W), β(V266A), β(V266M), ɛ(I257F), ɛ(V265A) and ɛ(L269F)). We confirm that only mutations in the ɛ subunit altered Ca(2+) permeability of AChR-channels, with ɛ(L269F) increasing P(f) (10% vs. 7% of wild type AChR) and ɛ(I257F) decreasing it (to 4.6%). We also found that, for ɛ(L269F)-AChR, the Ca(2+) permeability and ACh-induced cell death can be normalized by clinically relevant concentrations of salbutamol or verapamil, providing the first evidence that the Ca(2+) permeability of AChR-channels can be modulated and this treatment may provide protection against excitotoxic insults.

Potency of nondepolarizing muscle relaxants on muscle-type acetylcholine receptors in denervated mouse skeletal muscle

AIM

to investigate the changing resistance to nondepolarizing muscle relaxants (NDMRs) during the first month after denervation.

METHODS

the denervated and innervated skeletal muscle cells were examined on days 1, 4, 7, 14, 21, and 28 after denervation. Individual denervated and innervated cells were prepared from the flexor digitorum brevis of the surgically denervated and contralateral hind feet, respectively. Nicotinic acetylcholine receptors (nAChRs) in the cells were activated with 30 micromol/L acetylcholine, either alone or in combination with various concentrations of vecuronium. Currents were recorded using a whole-cell patch-clamp technique.

RESULTS

the concentrations of vecuronium resulting in half-maximal inhibitory responses (IC(50)) increased 1.2- (P>0.05), 1.7-, 3.7-, 2.5-, 1.9-, and 1.8-fold (P<0.05) at Days 1, 4, 7, 14, 21, and 28 after denervation, respectively, compared to the innervated control. Resistance to vecuronium appeared at Day 4, peaked at Day 7, and declined at Day 14 after denervation. Nevertheless, IC(50) values at Day 28 remained significantly higher than those for the innervated control, suggesting that the resistance to vecuronium had not disappeared at Day 28.

CONCLUSION

The NDMR doses required to achieve satisfactory clinical effects differ at different times after muscle denervation.

Mechanism of verapamil action on wild-type and slow-channel mutant human muscle acetylcholine receptor

Verapamil, a Ca(2+) channel blocker widely used in clinical practice, also affects the properties of frog and mouse muscle acetylcholine receptor (AChR). Here, we examine the mechanism of action of verapamil on human wild-type and slow-channel mutant muscle AChRs harboring in any subunit a valine-to-alanine mutation of 13' residue of the pore-lining M2 transmembrane segment. Verapamil, after a pre-treatment of 0.5-10 s, accelerated the decay of whole-cell or macroscopic outside-out currents within milliseconds of ACh application even at clinically attainable doses. Recordings of unitary events in the cell-attached and outside-out configurations showed that verapamil does not alter single-channel conductance, but reduces channel open probability, by prolonging the dwell time into the closed state for wild-type and all mutant AChR. The duration of channel openings decreased only for the epsilonV265A-AChR, by shortening the longest exponential component of the open-time distribution. These results provide a rationale for the therapeutic use of verapamil in the slow-channel syndrome and emphasize the major role played by epsilon subunit in controlling the functional properties of human muscle AChR, as revealed by the peculiar alterations imparted by mutations in this subunit.

Presence of mRNA of muscle nicotinic acetylcholine receptor subunits and an epsilon-subunit splice variant in the mouse brain

Transcripts encoding for alpha1, beta1, delta, gamma and epsilon (and its splice variant epsilon(s)) subunits of the muscle-type nicotinic acetylcholine receptor (nAChR) were assessed using reverse transcription followed by polymerase chain reaction (RT-PCR) assays, with RNA extracted from the mouse skeletal muscle (diaphragm) and brain regions (cortex, hippocampus and cerebellum). The presence of alpha1, beta1, delta, gamma, epsilon and epsilon(s) transcripts was confirmed in the diaphragm muscle, used as positive control. mRNAs coding for muscle alpha1, beta1, delta, epsilon, epsilon(s), but not gamma subunits, were detected in adult mouse brain regions. An epsilon-subunit sequence variant, named epsilon(t), was also detected in all brain regions examined, but not in skeletal muscle. This new epsilon-subunit splice variant lacks a 115 bp cassette corresponding to exon 8 in the first intracellular transmembrane domain of the subunit, leading to a truncated protein. The data provide evidence for the presence of muscle-type nAChR subunits in the mouse central nervous system.

Postsynaptic development of the neuromuscular junction in mice lacking the gamma-subunit of muscle nicotinic acetylcholine receptor

The mammalian muscle nicotinic acetylcholine receptor (AChR) is composed of five membrane-spanning subunits and its composition differs between embryonic and adult muscles. In embryonic muscles, it is composed of two alpha-, one beta-, one delta-, and one gamma-subunit; the gamma-subunit is later replaced by the epsilon-subunit during postnatal development. This unique temporal expression pattern of the gamma-subunit suggests it may play specific roles in embryonic muscles. To address this issue, we examined the formation and function of the neuromuscular junction in mouse embryos deficient in the gamma-subunit. At embryonic day 15.5, AChR clusters were absent and the spontaneous miniature endplate potentials were undetectable in the mutant muscles. However, electrical stimulation of the nerves triggered muscle contraction and elicited postsynaptic endplate potential (EPP) in the mutant muscles, although the magnitude of the muscle contraction and the amplitudes of the EPPs were smaller in the mutant compared to the wild-type muscles. Reintroducing a wild-type gamma-subunit into the mutant myotubes restored the formation of AChR clusters in vitro. Together, these results have demonstrated that functional AChRs were present in the mutant muscle membrane, but at reduced levels. Thus, in the absence of the gamma-subunit, a combination of alpha, beta, and delta subunits may assemble into functional receptors in vivo. These results also suggest that the gamma-subunit maybe involved in interacting with rapsyn, a cytoplasmic protein required for AChR clustering.

Muscle-wide secretion of a miniaturized form of neural agrin rescues focal neuromuscular innervation in agrin mutant mice

Agrin and its receptor MuSK are required for the formation of the postsynaptic apparatus at the neuromuscular junction (NMJ). In the current model the local deposition of agrin by the nerve and the resulting local activation of MuSK are responsible for creating and maintaining the postsynaptic apparatus including clusters of acetylcholine receptors (AChRs). Concomitantly, the release of acetylcholine (ACh) and the resulting depolarization disperses those postsynaptic structures that are not apposed by the nerve and thus not stabilized by agrin-MuSK signaling. Here we show that a miniaturized form of agrin, consisting of the laminin-binding and MuSK-activating domains, is sufficient to fully restore NMJs in agrin mutant mice when expressed by developing muscle. Although miniagrin is expressed uniformly throughout muscle fibers and induces ectopic AChR clusters, the size and the number of those AChR clusters contacted by the motor nerve increase during development. We provide experimental evidence that this is due to ACh, because the AChR agonist carbachol stabilizes AChR clusters in organotypic cultures of embryonic diaphragms. In summary, our results show that agrin function in NMJ development requires only two small domains, and that this function does not depend on the local deposition of agrin at synapses. Finally, they suggest a novel local function of ACh in stabilizing postsynaptic structures.

Acetylcholine receptor gamma-subunits mRNA isoforms expressed in denervated rat muscle

The fetal acetylcholine (ACh) receptor, composed of the alphabetagammadelta subunits, is expressed in fetal, neonatal, and denervated muscle. Single-channel recording has revealed three kinetically distinct classes in neonatal and denervated muscle, suggesting that at least three forms of the gamma-subunit are required. To account for the kinetic classes observed, we compared the messenger ribonucleic acid (mRNA) forms expressed in neonatal and denervated muscle using reverse transcriptase polymerase chain reaction, cloning, and RNAse protection assays. We found five novel forms arising from alternative splicing, which we named gamma5-gamma9. The forms gamma5, gamma6, and gamma7 lack exon 4 and 63-, 89-, and 136 bp of exon 5, respectively. A gamma8 form lacks exons 3 and 4 and 19 bp of exon 5. The last, gamma9, lacks exons 3, 4, and 5. Results indicate that gamma4 predominates in fetal muscle and gamma7 in denervated adult muscle. Some of the gamma-subunit mRNAs found may generate the receptors observed in muscle.

Pathogenic point mutations in a transmembrane domain of the epsilon subunit increase the Ca2+ permeability of the human endplate ACh receptor

The epsilon subunit of the human endplate ACh receptor (AChR) is a key determinant of the large fraction of the ACh-evoked current carried by Ca2+ ions (P(f)). Consequently, missense mutations in the epsilon subunit are potential targets for altering the P(f) of human AChR. In this paper we investigate the effects of two pathogenic point mutations in the M2 transmembrane segment AChR epsilon subunit, epsilonT264P and epsilonV259F, that cause slow-channel syndromes (SCS). When expressed in GH4C1 cells, the mutant receptors subunits raise Ca2+ permeability of the receptors approximately 1.5 and approximately 2-fold above that of wild-type, to attain P(f) values of 11.8% (epsilonT264P) and 15.4% (epsilonV259F). The latter value exceeds most P(f) values reported to date for ligand-gated ion channels. Consistent with these findings, the biionic Ca2+ permeability ratio (P(Ca)/P(Cs)) of the mutant AChRs is also increased. Upon repetitive stimulation with ACh, the mutant receptors show an enhanced current run-down compared with wild-type, leading to a strong reduction of their function. We propose that the enhanced Ca2+ permeability of the mutant receptors overrides the protective effect of desensitization and, together with the prolonged opening events of the AChR channel, is an important determinant of the excitotoxic endplate damage in the SCS.

The human adult subtype ACh receptor channel has high Ca2+ permeability and predisposes to endplate Ca2+ overloading

Slow-channel congenital myasthenic syndrome, caused by mutations in subunits of the endplate ACh receptor (AChR), results in prolonged synaptic currents and excitotoxic injury of the postsynaptic region by Ca2+ overloading. The Ca2+ overloading could be due entirely to the prolonged openings of the AChR channel or could be abetted by enhanced Ca2+ permeability of the mutant channels. We therefore measured the fractional Ca2+ current, defined as the percentage of the total ACh-evoked current carried by Ca2+ ions (Pf), for AChRs harbouring the alphaG153S or the alphaV249F slow-channel mutation, and for wild-type human AChRs in which Pf has not yet been determined. Experiments were performed in transiently transfected GH4C1 cells and human myotubes with simultaneous recording of ACh-evoked whole-cell currents and fura-2 fluorescence signals. We found that the Pf of the wild-type human endplate AChR was unexpectedly high (Pf approximately 7%), but neither the alphaV249F nor the alphaG153S mutation altered Pf. Fetal human AChRs containing either the wild-type or the mutated alpha subunit had a much lower Pf (2-3%). We conclude that the Ca2+ permeability of human endplate AChRs is higher than that reported for any other human nicotinic AChR, with the exception of alpha7-containing AChRs (Pf > 10%); and that neither the alphaG153S nor the alphaV249F mutations affect the Pf of fetal or adult endplate AChRs. However, the intrinsically high Ca2+ permeability of human AChRs probably predisposes to development of the endplate myopathy when opening events of the AChR channel are prolonged by altered AChR-channel kinetics.

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.

Ca2+ permeability of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are expressed in muscle cells and neurons, as well as in an increasing number of other cell types. The nAChR channels are permeable to cations, including Ca(2+). Ca(2+) entry through nAChR channels has been shown to modulate several Ca(2+)-dependent cellular processes, such as neurotransmitter release, synaptic plasticity, and cell motility. The value of Ca(2+) permeability associated to a particular nAChR subtype thus represents an important indication for its physiological role. This review summarizes the quantitative data on Ca(2+) permeability obtained from several nAChR subtypes in native and heterologous systems. Different experimental approaches are compared, and the structural determinants of Ca(2+) permeability are discussed.

IP3 receptors and associated Ca2+ signals localize to satellite cells and to components of the neuromuscular junction in skeletal muscle

Recently, we described an inositol 1,4,5-trisphosphate (IP3) signaling system in cultured rodent skeletal muscle, triggered by high K+ and affecting gene transcription (Powell et al., 2001). Now, in a study of adult rodent skeletal muscle, using immunocytology and confocal microscopy, we have found a high level of IP3 receptor (IP3R) staining in satellite cells, which have been shown recently to contribute to nuclei in adult fibers after muscle exercise. These IP3R staining cells are positively identified as satellite cells by their position, morphology and staining with satellite-cell-specific antibodies such as desmin and neural cell adhesion molecule. IP3Rs are also localized to postsynaptic components of the neuromuscular junction (NMJ), in areas surrounding the nuclei of the motor end plate, and in perisynaptic Schwann cells, and localized close to nicotinic acetylcholine receptors of the endplate gutters. Ca2+ imaging experiments show calcium release at the motor endplate upon K+ depolarization precisely in these IP3R-rich regions. We suggest that electrical activity stimulates IP3-associated Ca2+ signals that may be involved in gene regulation in satellite cells and in elements of the NMJ, contributing both to muscle fiber growth and stabilization of the NMJ.

IP3 receptors and associated Ca2+ signals localize to satellite cells and to components of the neuromuscular junction in skeletal muscle

Recently, we described an inositol 1,4,5-trisphosphate (IP3) signaling system in cultured rodent skeletal muscle, triggered by high K+ and affecting gene transcription (Powell et al., 2001). Now, in a study of adult rodent skeletal muscle, using immunocytology and confocal microscopy, we have found a high level of IP3 receptor (IP3R) staining in satellite cells, which have been shown recently to contribute to nuclei in adult fibers after muscle exercise. These IP3R staining cells are positively identified as satellite cells by their position, morphology and staining with satellite-cell-specific antibodies such as desmin and neural cell adhesion molecule. IP3Rs are also localized to postsynaptic components of the neuromuscular junction (NMJ), in areas surrounding the nuclei of the motor end plate, and in perisynaptic Schwann cells, and localized close to nicotinic acetylcholine receptors of the endplate gutters. Ca2+ imaging experiments show calcium release at the motor endplate upon K+ depolarization precisely in these IP3R-rich regions. We suggest that electrical activity stimulates IP3-associated Ca2+ signals that may be involved in gene regulation in satellite cells and in elements of the NMJ, contributing both to muscle fiber growth and stabilization of the NMJ.

Sex differences in the acetylcholine receptor kinetics of postnatal and denervated rat muscle

Single-channel recording from visualised endplates in freshly dissociated muscles from postnatal and denervated rat muscle revealed the presence of a low conductance, fetal type of acetylcholine receptor. Kinetic analysis showed a main component in the burst durations with a mean of 10.8 +/- 2.7 ms (n = 29). Receptors from female rats had an additional 27.3 +/- 5.5 ms (n = 5) kinetic component which was found in one-third of the 15 female endplates. Recordings from male and female denervated muscles gave more homogeneous kinetics with single time constants of 7.2 +/- 1.3 and 7.4 +/- 1.3 ms, respectively. It is concluded that the acetylcholine receptor channels present during early development are different from those of denervated muscle.

Assembly and clustering of acetylcholine receptors containing GFP-tagged epsilon or gamma subunits: selective targeting to the neuromuscular junction in vivo

Acetylcholine receptor (AChR) gamma and epsilon subunits were tagged by green fluorescent protein (GFP) to analyse assembly and targeting in live muscle fibers at the neuromuscular junction. N- or C-terminal fusion polypeptides showed no fluorescence upon transfection of HEK cells. When GFP was inserted into the cytoplasmic loop connecting putative transmembrane regions M3 and M4, the gamma/GFP and epsilon/GFP subunits were fluorescent and formed together with the alpha, beta, and delta subunits GFP-tagged AChR complexes that were integrated into the plasma membrane. As the AChR were also clustered by rapsyn, the results indicate that the cytoplasmatic domains of the gamma and epsilon subunits may not be required for assembly and rapsyn-dependent clustering. The gamma/GFP and epsilon/GFP subunit-containing receptors were expressed in X. laevis oocytes and have affinities for acetylcholine similar to that of the wild-type receptors. Direct gene transfer into single muscle fibers reveals that gamma/GFP or epsilon/GFP polypeptides are expressed at the site of injection and are transported within the endoplasmatic reticulum. When reaching subsynaptic regions, both gamma/GFP or epsilon/GFP subunits compete with endogenous epsilon subunits to assemble GFP-tagged receptors, which are selectively targeted to the postsynaptic membrane.

Zinc permeates mouse muscle ACh receptor channels expressed in BOSC 23 cells and affects channel function

1. The influx of Zn2+ through the channels of fetal and adult mouse muscle nicotinic acetylcholine receptors (gamma- and epsilon-AChRs) and its effects on receptor function were studied in transiently transfected human BOSC 23 cells, by combining patch-clamp recordings with digital fluorescence microscopy. 2. ACh-induced whole-cell currents were reversibly reduced by external ZnCl2, with half-maximal inhibitory concentrations of 3 and 1 mM for gamma- and epsilon-AChRs, respectively. 3. Both gamma- and epsilon-AChR channels were permeable to Zn2+, as shown by fluorescence measurements using Zn2+-sensitive dyes. The fractional current carried by Zn2+ (Pf,Zn; 0.5 mM Zn2+ in Ca2+- and Mg2+-free medium) through gamma- and epsilon">-AChR channels was 1.7 and 4 %, respectively. 4. Pf,Zn increased with the concentration of ZnCl2, but was little affected by physiological concentrations of Ca2+ and Mg2+ in the external medium. 5. The conductance of ACh-evoked unitary events, measured by cell-attached or outside-out recordings, decreased when the patched membrane was exposed to ZnCl2 (1 or 3 mM). Simultaneous application of ACh and Zn2+ to the extra-patch membrane lengthened channel open duration (tau op) by 50%. No obvious increment of tau op was observed following exposure of inside-out patches to Zn2+. 6. The possible physiological relevance of zinc-induced modulation of AChR channels is discussed.

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.

Nerve terminals form but fail to mature when postsynaptic differentiation is blocked: in vivo analysis using mammalian nerve-muscle chimeras

To better understand the role of the postsynaptic cell in the differentiation of presynaptic terminals, we transplanted muscles that lacked postsynaptic differentiation from mutant mice into normal adult immunocompatible hosts and attached the host nerve to the grafts. Host motor axons innervated wild-type grafted muscle fibers and established normal appearing chimeric neuromuscular junctions. By repeated in vivo imaging, we found that these synapses were stably maintained. Results were different when nerves entered transplanted muscles derived from mice lacking muscle-specific receptor tyrosine kinase (MuSK) or rapsyn, muscle-specific components required for postsynaptic differentiation. Initial steps in presynaptic differentiation (e.g., formation of rudimentary arbors and vesicle clustering at terminals) occurred when wild-type neurites contacted MuSK- or rapsyn deficient muscle fibers, either in vivo or in vitro. However, wild-type terminals contacting MuSK or rapsyn mutant muscle fibers were unable to mature, even when the chimeras were maintained for up to 7 months. Moreover, in contrast to the stability of wild-type synapses, wild-type nerve terminals in mutant muscles underwent continuous remodeling. These results suggest that postsynaptic cells supply two types of signals to motor axons: ones that initiate presynaptic differentiation and others that stabilize the immature contacts so that they can mature. Normal postsynaptic differentiation appears to be dispensable for initial stages of presynaptic differentiation but required for presynaptic maturation.

Nerve terminals form but fail to mature when postsynaptic differentiation is blocked: in vivo analysis using mammalian nerve-muscle chimeras

To better understand the role of the postsynaptic cell in the differentiation of presynaptic terminals, we transplanted muscles that lacked postsynaptic differentiation from mutant mice into normal adult immunocompatible hosts and attached the host nerve to the grafts. Host motor axons innervated wild-type grafted muscle fibers and established normal appearing chimeric neuromuscular junctions. By repeated in vivo imaging, we found that these synapses were stably maintained. Results were different when nerves entered transplanted muscles derived from mice lacking muscle-specific receptor tyrosine kinase (MuSK) or rapsyn, muscle-specific components required for postsynaptic differentiation. Initial steps in presynaptic differentiation (e.g., formation of rudimentary arbors and vesicle clustering at terminals) occurred when wild-type neurites contacted MuSK- or rapsyn deficient muscle fibers, either in vivo or in vitro. However, wild-type terminals contacting MuSK or rapsyn mutant muscle fibers were unable to mature, even when the chimeras were maintained for up to 7 months. Moreover, in contrast to the stability of wild-type synapses, wild-type nerve terminals in mutant muscles underwent continuous remodeling. These results suggest that postsynaptic cells supply two types of signals to motor axons: ones that initiate presynaptic differentiation and others that stabilize the immature contacts so that they can mature. Normal postsynaptic differentiation appears to be dispensable for initial stages of presynaptic differentiation but required for presynaptic maturation.

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.

Human neuronal threonine-for-leucine-248 alpha 7 mutant nicotinic acetylcholine receptors are highly Ca2+ permeable

A cDNA coding for the human neuronal nicotinic alpha7 receptor subunit with Leu-248 mutated to threonine was expressed in Xenopus oocytes. When activated by acetylcholine (AcCho), the receptors expressed generated currents that had low desensitization, linear current-voltage relation, and high apparent affinity for both AcCho and nicotine. These characteristics are similar to those already described for the chick threonine-for-leucine-247 alpha7 nicotinic AcCho receptor (nAcChoR) mutant (L247Talpha7). These properties were all substantially maintained when the human L248Talpha7 mutant was transiently expressed in human Bosc 23 cells. Simultaneous whole-cell clamp and fluorescence measurements with the Ca(2+) indicator dye Fura-2 showed that nicotine induced a Ca(2+) influx in standard 2 mM Ca(2+) solution. The average fractional Ca(2+) current flowing through L248Talpha7 nAcChoRs was 6.7%, which is larger than that flowing through muscle alpha(beta)epsilon(delta) nAcChoRs (4.1%). The relative Ca(2+) permeability, determined in oocytes in the absence of Cl(-), was measured from the shift in reversal potential caused by increasing the external Ca(2+) concentration from 1 to 10 mM. The human wild-type alpha7 nAcChoR was found to be more permeable than the L248Talpha7 mutant to Ca(2+). Our findings indicate that the Ca(2+) permeability of the homomeric alpha7 nAcChoR is larger than that of the heteromeric neuronal nicotinic receptors studied to date and is possibly similar to that of the N-methyl-D-aspartate subtype of brain glutamate receptors.

Human neuronal threonine-for-leucine-248 7 mutant nicotinic acetylcholine receptors are highly Ca2+ permeable

A cDNA coding for the human neuronal nicotinic alpha7 receptor subunit with Leu-248 mutated to threonine was expressed in Xenopus oocytes. When activated by acetylcholine (AcCho), the receptors expressed generated currents that had low desensitization, linear current-voltage relation, and high apparent affinity for both AcCho and nicotine. These characteristics are similar to those already described for the chick threonine-for-leucine-247 alpha7 nicotinic AcCho receptor (nAcChoR) mutant (L247Talpha7). These properties were all substantially maintained when the human L248Talpha7 mutant was transiently expressed in human Bosc 23 cells. Simultaneous whole-cell clamp and fluorescence measurements with the Ca(2+) indicator dye Fura-2 showed that nicotine induced a Ca(2+) influx in standard 2 mM Ca(2+) solution. The average fractional Ca(2+) current flowing through L248Talpha7 nAcChoRs was 6.7%, which is larger than that flowing through muscle alpha(beta)epsilon(delta) nAcChoRs (4.1%). The relative Ca(2+) permeability, determined in oocytes in the absence of Cl(-), was measured from the shift in reversal potential caused by increasing the external Ca(2+) concentration from 1 to 10 mM. The human wild-type alpha7 nAcChoR was found to be more permeable than the L248Talpha7 mutant to Ca(2+). Our findings indicate that the Ca(2+) permeability of the homomeric alpha7 nAcChoR is larger than that of the heteromeric neuronal nicotinic receptors studied to date and is possibly similar to that of the N-methyl-D-aspartate subtype of brain glutamate receptors.

A molecular link between inward rectification and calcium permeability of neuronal nicotinic acetylcholine alpha3beta4 and alpha4beta2 receptors

Many nicotinic acetylcholine receptors (nAChRs) expressed by central neurons are located at presynaptic nerve terminals. These receptors have high calcium permeability and exhibit strong inward rectification, two important physiological features that enable them to facilitate transmitter release. Previously, we showed that intracellular polyamines act as gating molecules to block neuronal nAChRs in a voltage-dependent manner, leading to inward rectification. Our goal is to identify the structural determinants that underlie the block by intracellular polyamines and govern calcium permeability of neuronal nAChRs. We hypothesize that two ring-like collections of negatively charged amino acids (cytoplasmic and intermediate rings) near the intracellular mouth of the pore mediate the interaction with intracellular polyamines and also influence calcium permeability. Using site-directed mutagenesis and electrophysiology on alpha(4)beta(2) and alpha(3)beta(4) receptors expressed in Xenopus oocytes, we observed that removing the five negative charges of the cytoplasmic ring had little effect on either inward rectification or calcium permeability. However, partial removal of negative charges of the intermediate ring diminished the high-affinity, voltage-dependent interaction between intracellular polyamines and the receptor, abolishing inward rectification. In addition, these nonrectifying mutant receptors showed a drastic reduction in calcium permeability. Our results indicate that the negatively charged glutamic acid residues at the intermediate ring form both a high-affinity binding site for intracellular polyamines and a selectivity filter for inflowing calcium ions; that is, a common site links inward rectification and calcium permeability of neuronal nAChRs. Physiologically, this molecular mechanism provides insight into how presynaptic nAChRs act to influence transmitter release.

A molecular link between inward rectification and calcium permeability of neuronal nicotinic acetylcholine alpha3beta4 and alpha4beta2 receptors

Many nicotinic acetylcholine receptors (nAChRs) expressed by central neurons are located at presynaptic nerve terminals. These receptors have high calcium permeability and exhibit strong inward rectification, two important physiological features that enable them to facilitate transmitter release. Previously, we showed that intracellular polyamines act as gating molecules to block neuronal nAChRs in a voltage-dependent manner, leading to inward rectification. Our goal is to identify the structural determinants that underlie the block by intracellular polyamines and govern calcium permeability of neuronal nAChRs. We hypothesize that two ring-like collections of negatively charged amino acids (cytoplasmic and intermediate rings) near the intracellular mouth of the pore mediate the interaction with intracellular polyamines and also influence calcium permeability. Using site-directed mutagenesis and electrophysiology on alpha(4)beta(2) and alpha(3)beta(4) receptors expressed in Xenopus oocytes, we observed that removing the five negative charges of the cytoplasmic ring had little effect on either inward rectification or calcium permeability. However, partial removal of negative charges of the intermediate ring diminished the high-affinity, voltage-dependent interaction between intracellular polyamines and the receptor, abolishing inward rectification. In addition, these nonrectifying mutant receptors showed a drastic reduction in calcium permeability. Our results indicate that the negatively charged glutamic acid residues at the intermediate ring form both a high-affinity binding site for intracellular polyamines and a selectivity filter for inflowing calcium ions; that is, a common site links inward rectification and calcium permeability of neuronal nAChRs. Physiologically, this molecular mechanism provides insight into how presynaptic nAChRs act to influence transmitter release.