Differences in the localization of the postsynaptic nitric oxide synthase I and acetylcholinesterase suggest a heterogeneity of neuromuscular junctions in rat and mouse skeletal muscles

A link between agrin signalling and Cav3.2 at the neuromuscular junction in spinal muscular atrophy

SMN protein deficiency causes motoneuron disease spinal muscular atrophy (SMA). SMN-based therapies improve patient motor symptoms to variable degrees. An early hallmark of SMA is the perturbation of the neuromuscular junction (NMJ), a synapse between a motoneuron and muscle cell. NMJ formation depends on acetylcholine receptor (AChR) clustering triggered by agrin and its co-receptors lipoprotein receptor-related protein 4 (LRP4) and transmembrane muscle-specific kinase (MuSK) signalling pathway. We have previously shown that flunarizine improves NMJs in SMA model mice, but the mechanisms remain elusive. We show here that flunarizine promotes AChR clustering in cell-autonomous, dose- and agrin-dependent manners in C2C12 myotubes. This is associated with an increase in protein levels of LRP4, integrin-beta-1 and alpha-dystroglycan, three agrin co-receptors. Furthermore, flunarizine enhances MuSK interaction with integrin-beta-1 and phosphotyrosines. Moreover, the drug acts on the expression and splicing of Agrn and Cacna1h genes in a muscle-specific manner. We reveal that the Cacna1h encoded protein Cav3.2 closely associates in vitro with the agrin co-receptor LRP4. In vivo, it is enriched nearby NMJs during neonatal development and the drug increases this immunolabelling in SMA muscles. Thus, flunarizine modulates key players of the NMJ and identifies Ca_v3.2 as a new protein involved in the NMJ biology.

Enteric co-innervation of striated muscle fibres in human oesophagus

Oesophageal striated muscle of several mammalian species receives dual innervation from both vagal motor fibres originating in the brain stem and enteric nerve fibres originating in myenteric ganglia. The aim of this study was to investigate this so-called enteric co-innervation in the human oesophagus. Histochemical and immunohistochemical methods combined with confocal laser scanning microscopy were utilized to study innervation of 14 oesophagi obtained from body donors (age range 47-95 years). In addition, the distribution of striated and smooth muscle in longitudinal and circular layers of the tunica muscularis was studied semiquantitatively. The upper half of the oesophagus was built up of both muscle types with a predominance (>50-60%) of striated muscle, whereas the lower half consisted of smooth muscle only. The majority of motor endplates was compact and ovoid. Enteric nerve fibres on approximately 17% of motor endplates stained for neuronal nitric oxide synthase, vasoactive intestinal polypeptide, galanin and neuropeptide Y and were completely separated from vagal cholinergic nerve terminals. There was remarkable variability of co-innervation rates between striated muscle bundles with some reaching almost 50%. Myenteric neurons representing the putative source of enteric co-innervating nerve fibres, stained for all these markers, which were almost completely colocalized with NADPH-diaphorase. Our study provides evidence for enteric co-innervation of striated muscle in human oesophagus. From these and recent functional results in various rodent species, we suggest that this innervation component represents an integral part of an intramural reflex mechanism for local most likely inhibitory modulation of oesophageal motility.

Nerve-terminal and Schwann-cell response after nerve injury in the absence of nitric oxide

Dystrophic muscles show alterations in the dystrophin-glycoprotein complex and a lack of neuronal nitric oxide (NO) synthase. In mdx mice, presynaptic expression of neuronal NO synthase is decreased, suggesting that presynaptic signaling may be altered in dystrophic muscle. In this study, we examined the nerve-terminal and Schwann-cell responses after a crush lesion in control and NO-deficient mice. Seven days after nerve crush, 24% of control neuromuscular junctions (n = 200) showed ultraterminal sprouts, whereas in NO-deficient mice this frequency was 28.5% (n = 217; P > 0.05 compared to controls; chi-square test). Schwann-cell response did not change in the absence of NO, after a nerve lesion of 7-day duration. Fourteen days after the lesion, nerve terminals sprouted and Schwann cells showed an extensive network of processes away from the synaptic site in controls. In the absence of NO, there was a dramatic decrease in nerve-terminal sprouting and Schwann-cell processes failed to extend away from the endplate. These results show that NO is involved in the nerve-terminal and Schwann-cell response to nerve injury. They also suggest that presynaptic molecular signaling may be impaired in dystrophic muscles, and this could influence the innervation and survival of newly formed myofibers generated by cell-mediated therapies.

Partial nicotinic receptor blockade unmasks a modulatory role of nitric oxide on urethral striated neuromuscular transmission

The objective of this study was to investigate the possible modulatory role of endogenous nitric oxide (NO) production on the urethral striated muscle (USM) function in the sheep urethra. Significant NO synthase (NOS) activity was measured in both the particulate and cytosolic fractions of USM homogenates. NOS activity was calcium-dependent and showed greater inhibition by NOS inhibitors selective of the neural NOS isoform (nNOS). nNOS immunoreactivity was present in intramural nerves as well as in the sarcolemma of some striated fibers, being denser at the neuromuscular junction (NMJ). Double immunolabeling showed co-localization of nNOS with both alpha-bungarotoxin and choline acetyltransferase, at the USM endplates. For the first time, functional data support a role of NO on the USM contractility "in vitro," which became evident following partial nicotinic receptor inactivation with low concentrations of D-tubocurarine. Only under D-tubocurarine (0.25 microM) treatment, different NOS inhibitors, specially N(G)-propyl-L-arginine, as well as the guanylate cyclase inhibitor ODQ, all showed a significant enhancing effect on contractions induced by electrical field stimulation of intrinsic somatic nerves. These data suggest that local production of NO at the urethral NMJ may modulate release and/or action of acetylcholine on motor endplates by cyclic GMP-mediated effects. This modulatory action could be especially relevant when neuromuscular transmission at the USM is impaired.

NG-Nitro-L-arginine Methyl Ester-Induced Potentiaton of the Effect of Aminophylline on Rat Diaphragm: the Role of Extracellular Calcium

The role of extracellular calcium in the interaction between intracellular cAMP and nitric oxide (NO)/cGMP on the contractility of rat diaphragm pretreated with cumulative concentrations of aminophylline (0.36 - 3.60 mM) was investigated. In a Ca2+-free medium, NG-nitro-L-arginine methyl ester (L-NAME) (1 and 3 mM) depressed tension developed (Td) and also aminophylline-induced potentiation of Td in a concentration-dependent manner. Verapamil (2.5 microM) or nicardipine (20 microM) significantly antagonized the potentiating effect of L-NAME on Td in a calcium-containing medium. However, in the presence of verapamil or nicardipine, L-NAME still produced statistically significant potentiation of the cumulative concentrations of aminophylline (0.36 - 3.60 mM), given in the second series.

NG-nitro-L-arginine methyl ester potentiates the effect of aminophylline on the isolated rat hemidiaphragm

The effects of different concentrations of N(G)-nitro-L-arginine methyl ester (L-NAME) (0.3, 1, 3, and 10 mM), a non-selective inhibitor of NOS, on the effect of aminophylline on the isometric contraction of the isolated rat hemidiaphragm were investigated. The muscle contractions were induced by direct subtetanic electrical stimulation. Aminophylline (0.36 - 3.60 mM) produced a typical concentration-dependent increase in both parameters of the isometric contraction: tension developed (Td) and the maximum rate of rise of tension (dT/dt max). The second series of additions of aminophylline produced a more pronounced effect. L-NAME (0.3, 1, 3, and 10 mM, 30 min of incubation without stimulation) itself did not change Td and dT/dt max. However, L-NAME (1, 3, and 10 mM) produced a statistically significant potentiation of the effect of aminophylline on Td and dT/dt max.

Nitric oxide synthase in the external urethral sphincter of the sheep: immunohistochemical and functional study

PURPOSE

We studied the distribution of neuronal nitric oxide synthase (nNOS) and the effects of nitric oxide (NO) modulating drugs on contractile function of the external urethral sphincter of lambs. Gender differences were evaluated.

MATERIALS AND METHODS

Longitudinal and transverse sections of the external urethral sphincter from 10 female and 10 male lambs were studied using reduced nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry and nNOS immunocytochemistry. Isometric contractile responses to electrical field stimulation were recorded from external urethral sphincter preparations from 47 female and 45 male lambs and the effects of NO modulating drugs were evaluated.

RESULTS

We detected nNOS in the sarcolemma of some but not all striated fibers, where nNOS seems to be concentrated at the neuromuscular junction. In addition, nNOS was present in nerve fibers and intramural ganglia. The density of innervation decreased toward the distal part of the external urethral sphincter and was higher in male preparations. No significant functional effects of the NOS inhibitor NG-nitro-L-arginine (10 mM.) or the NO donors diethylamine and spermine NONOate (Sigma Chemical Co., St. Louis, Missouri) (5 mM. each) on external urethral sphincter isometric contractility were found in either gender.

CONCLUSIONS

Despite the evidence for nNOS at the sarcolemma and nerve fibers of the external urethral sphincter the physiological relevance of these immunohistochemical findings remains to be determined.

Myopathy-dependent changes in activity of ATPase, SDH and GPDH and NOS expression in the different fibre types of hamster muscles

Proximal (vastus lateralis) and distal (gastrocnemius) muscles of 100-day-old normal and myopathic BIO TO-2 hamsters were analysed to study the effects of myopathy on the different muscle fibre types: SO (slow oxidative), FOG (fast oxidative glycolytic) and FG (fast glycolytic). Cytophotometric measurements of enzyme activities (myofibrillic adenosine triphosphatase, succinate dehydrogenase and glycerol-3-phosphate dehydrogenase), Western blot analysis of nitric oxide synthase (NOS) I, II, III isoforms and NOS II immunohistochemistry were performed. The following alterations were found in myopathic muscle fibres: all fibre types of both proximal and distal myopathic muscles showed decreased myofibrillic adenosine triphosphatase activity indicating depressed contractility. This was associated with depressed oxidative activity of the muscle fibres. A shift to more glycolytic metabolism was observed, mainly in FG fibres of proximal muscle. We found an increased NOS II expression in both myopathic muscle types investigated. It means that increased NO production inhibits force generation in myopathic muscle. NOS II immunoreactivity was found mainly in the cytoplasm of FG fibres. NOS I and NOS III expression was not significantly effected by this form of myopathy. Our findings demonstrate that muscle fibres of proximal and distal skeletal muscles of 100-day-old cardiomyopathic BIO TO-2 hamsters are altered with respect to contractility, metabolism and NOS II expression. FG fibres of the proximal muscle were effected most strongly.

Role of nitric oxide and nitric oxide synthases in experimental models of denervation and reinnervation

Nitric oxide (NO) is a short-living free molecule synthesized by three different isoforms of nitric oxide synthases (NOS)-neuronal NOS, endothelial NOS, and inducible NOS-associated with neuromuscular transmission, muscle contractility, mitochondrial respiration, and carbohydrate metabolism in skeletal muscle. Neuronal NOS is constitutively expressed at the muscle fiber sarcolemma linked to the dystrophin-glycoprotein complex and concentrated at the neuromuscular endplate. There is increasing evidence that altered expression of neuronal NOS plays a role in muscle fiber damage in neuromuscular diseases such as dystrophinopathies and denervating disorders. Although there have been some previous conflicting results on the neuronal NOS expression pattern in denervated muscle fibers, it is now well established that denervation is associated with a down-regulation and disappearance of sarcolemmal neuronal NOS at synaptic/extrasynaptic or both sites. As NO has been shown to induce collapse and growth arrest on neuronal growth cones, down-regulation of sarcolemmal neuronal NOS may contribute to axonal regeneration and attraction to muscle fibers aiming at the formation of new motor endplates providing reinnervation and reconstitution of NOS expression. As NO serves as a retrograde messenger, it may trigger structural downstream events responsible for neuromuscular synaptogenesis and preventing polyneural innervation. Nevertheless, decreased NO production in denervation reduces the cytoprotective scavenger function of NO for superoxide anions promoting oxidative stress that is likely to be involved in muscle fiber damage and death. However, the multifaced role of NOS and NO under physiological and pathological conditions remains poorly understood on the basis of the current knowledge.

Safety factor at the neuromuscular junction

Reliable transmission of activity from nerve to muscle is necessary for the normal function of the body. The term 'safety factor' refers to the ability of neuromuscular transmission to remain effective under various physiological conditions and stresses. This is a result of the amount of transmitter released per nerve impulse being greater than that required to trigger an action potential in the muscle fibre. The safety factor is a measure of this excess of released transmitter. In this review we discuss the practical difficulties involved in estimating the safety factor in vitro. We then consider the factors that influence the safety factor in vivo. While presynaptic transmitter release may be modulated on a moment to moment basis, the postsynaptic features that determine the effect of released transmitter are not so readily altered to meet changing demands. Different strategies are used by different species to ensure reliable neuromuscular transmission. Some, like frogs, rely on releasing a large amount of transmitter while others, like man, rely on elaborate postsynaptic specialisations to enhance the response to transmitter. In normal adult mammals, the safety factor is generally 3-5. Both pre- and postsynaptic components change during development and may show plasticity in response to injury or disease. Thus, both acquired autoimmune and inherited congenital diseases of the neuromuscular junction (NMJ) can significantly reduce, or even transiently increase, safety factor.

Cloning and expression of three K+ channel cDNAs from Xenopus muscle

Embryonic Xenopus muscle cells grown in culture express voltage-gated K+ currents with inactivating and non-inactivating kinetics. Here we report the cloning of three K+ channel cDNAs, designated XKv1.2', XKv1.4 and XKv1.10, from muscle which may underlie these currents. XKv1.2' cDNA appears to be an allelic variant of the XKv1.2 previously cloned from Xenopus. The second cDNA encodes a homologue of Kv1.4 that has not been previously cloned from Xenopus. The predicted XKv1.4 protein shows 73% overall similarity to mouse and chick Kv1.4, but shows significant divergence in the region corresponding to the chain of the inactivating 'ball and chain' domain. The third K+ channel cDNA isolated from Xenopus muscle is a novel Kv1 isoform designated XKv1.10. The predicted protein shares about 70% similarity with other members of the Kv1 subfamily, and about 40% with members of the Kv2, Kv3 and Kv4 subfamilies. XKv1.4 mRNA appears as early as stage 10.5 in whole embryos and is prominent in muscle throughout development from stage 14 to adult. XKv1.2' mRNA is detected by stage 11.5 in whole embryos, but remains at low levels in embryonic skeletal muscle (stages 14 and 21), and is absent from adult muscle. XKv1.10 mRNA is first detected at stage 21 in whole embryos, and is present in muscle from this stage onwards. All three transcripts are present in spinal cord at stage 21. The results support the notion that channels encoded by XKv1.4 contribute to the inactivating K+ current observed in embryonic muscle cells in culture.

Immunolocalisation of neuronal nitric oxide synthase at the neuromuscular junction of MDX mice: a confocal microscopy study

The distribution of nitric oxide synthase at the neuromuscular junction (NMJ) of normal, denervated and mdx mice was studied using a specific antibody against the neuronal isoform of nitric oxide synthase (nNOS). Fluorescence confocal microscopy demonstrated that nNOS immunoreactivity was localised mainly in the sarcolemma and presynaptic region covering acetylcholine receptor branches. The expression of presynaptic nNOS was greatly reduced in dystrophin-deficient muscles. In normal denervated muscles, nNOS was still present in the presynaptic region and there were no qualitative changes in the expression of this protein. These results suggest that the presynaptic distribution of nNOS is associated with terminal Schwann cells. The relationship between nNOS and the presynaptic components of the neuromuscular junction may open new perspectives for improving our understanding of the pathogenesis of dystrophic muscles.

Nitric oxide synthase isoforms I, III and protein kinase-Ctheta in skeletal muscle fibres of normal and streptozotocin-induced diabetic rats with and without Ginkgo biloba extract treatment

The expression of nitric oxide synthase (NOS) isoforms I, III and protein kinase-Ctheta (PKCtheta) in rat vastus lateralis muscle was demonstrated immunohistochemically and then correlated to the physiological metabolic fibre types: SO (slow-oxidative), FOGI, FOGII (fast-oxidative glycolytic; I more glycolytic, II more oxidative), and FG (fast-glycolytic). NOS expression in muscles from different experimental groups (normal and diabetic rats, with and without Ginkgo biloba extract treatment) was assayed by Western blotting. Generally, NOS I and PKCtheta were co-expressed in fibres with predominantly oxidative metabolism (SO, FOGII). This suggests an interplay of PKCtheta and NOS I in nitric oxide production by oxidative fibres. NOS III was more highly expressed in fibres with predominantly glycolytic metabolism (FOGI, FG). A somewhat lower NOS I immunoreactivity was also found in NOS III positive fibres suggesting that NOS III and NOS I are co-expressed in these fibres. Western blotting revealed that NOS I as well as NOS III expression in the vastus lateralis muscle was down-regulated in diabetes and increased after Ginkgo biloba extract treatment. These effects may be associated with a diminished glucose uptake by myocytes of diabetic musclesand with an improved muscle function after Ginkgo biloba treatment.

Effect of nitric oxide and NO synthase inhibition on nonquantal acetylcholine release in the rat diaphragm

After anticholinesterase treatment, the postsynaptic muscle membrane is depolarized by about 5 mV due to nonquantal release of acetylcholine (ACh) from the motor nerve terminal. This can be demonstrated by the hyperpolarization produced by the addition of curare (H-effect). The magnitude of the H-effect was decreased significantly to 3 mV when the nitric oxide (NO) donors, sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP) were applied to the muscle, or when NO production was elevated by adding L-arginine, but not D-arginine, as a substrate. The H-effect was increased to 8-9 mV by inhibition of NO synthase by L-nitroarginine methylester (L-NAME), or by guanylyl cyclase inhibition by methylene blue and 1H-[1,2,4]oxidiazolo[4,3-a]quinoxalin-1-one (ODQ). ODQ increased the H-effect to 7.3 +/- 0.2 mV and diminished the SNP-induced decrease of the H-effect when applied together with SNP. The effects of NO donors and L-arginine were eliminated by adding reduced haemoglobin, an extracellular NO scavenger. The present results, together with earlier evidence for the presence of NO synthase in muscle fibres, indicate that nonquantal release of ACh is modulated by NO production in the postsynaptic cell.

Caveolin-3 and nitric oxide synthase I in healthy and diseased skeletal muscle

Recently, it has been shown for mouse skeletal muscle that caveolin-3 is localized in the sarcolemma and cofractionates with the original dystrophin complex (DC). In order to find out whether caveolin-3 is a further component of the recently established and enlarged nitric oxide synthase (NOS) I-DC and whether members of this complex interact with and are potentially regulated by caveolin-3, mammalian and non-mammalian healthy and diseased (dystrophic) skeletal muscles were investigated using caveolin-3, NOS I, DC components and myosin immunohistochemistry as well as NOS I-associated diaphorase histochemistry. In healthy mammalian skeletal muscle, caveolin-3 was colocalized with the DC components in all extra- and intrafusal fibers. By contrast, NOS I was absent in type I extrafusal fibers of certain species. In patients with Duchenne muscular dystrophy and mdx mice the components of the NOS I-DC were not detected in all extra- and intrafusal fiber types, while caveolin-3 was found unchanged. In healthy non-mammalian skeletal muscle, i.e. of birds, reptiles and fishes, caveolin-3 immunoreactivity was lacking in the sarcolemma as was alpha-sarcoglycan; the other NOS I-DC components were either present or absent. In conclusion, although caveolin-3 is localized in the sarcolemma of mammalian myofibers, there are differences in the microarchitecture of the components of the DC complex and of caveolin-3 which does not appear to be linked with the NOS I-DC. Potential regulatory interactions between caveolin-3 and NOS I may nevertheless exist in those fibers where both molecules are colocalized. The absence of caveolin-3 and alpha-sarcoglycan immunoreactivities in non-mammalian myofibers may suggest that the functions of these proteins are subserved by other components of NOS I-DC complex.

Nitric oxide synthase (NOS) I during postnatal development in rat and mouse skeletal muscle

Previous studies on adult rat and mouse skeletal muscles have shown the spatial association of nitric oxide synthase (NOS) I to the dystrophin complex (DC) in the sarcolemma of type II fibers and, in combination with the NMDA receptor-1 (NMDAR-1), an accumulation of the enzyme at the neuromuscular junctions (NMJ) of this fiber type. Using immunohistochemistry, enzyme histochemistry and alpha-bungarotoxin labeling we report here temporal relationships of NOS I, members of the DC, other components of the cortical cytoskeleton in the junctional and non-junctional sarcolemma as well as of molecules involved in NMJ transmission of either type I or II myofibers especially in head and neck muscles during postnatal rat and mouse development. Fiber typing was performed by specific anti-myosin antibodies. Beginning with postnatal day (PD) 1 in both fiber types dystrophin, dystrophin-associated glycoproteins (DAG), beta-dystroglycan, alpha-sarcoglycan (adhalin) and spectrin were present in the junctional and extrajunctional sarcolemma, while utrophin, acetylcholinesterase, alpha-bungarotoxin labeled acetylcholine receptors were concentrated in the NMJ of both fiber types. NOS I activity and immunoreactivity were only found in the NMJ region of type II fibers, where NMDAR-1 appeared around PD 15. Primarily in the tongue there was no strict correlation between muscle fiber type and NOS I behaviour during early postnatal development, and muscle fibers not reactive for myosin antibodies against both fiber types were negative or positive for NOS I but always positive for the other molecules either in both the junctional and extrajunctional sarcolemma or in the NMJ only; later all muscle fibers of the tongue were of type II and NOS I-positive. Maturation of enzyme activities, immunoreactivities and AChR intensity depended on the respective muscle and can last until PD 50; in the tongue and neck muscles they appeared to increase approximately until PD 20 or 25. In conclusion, in type II fibers of rat and mouse skeletal muscle all molecules with the exception of NMDAR-1 and relevant for NOS I targeting and positioning as well as function inside and outside the NMJ are already present at birth, but their concentrations and/or activities increase postnatally, and the adult situation appears to be reached between the third and seventh week of postnatal life. Therefore, initial interactions between NOS I and the other molecules necessary for the formation of the NOS I-DC in and on the way to the sarcolemma presumably take place before birth.