Irregular costameres represent nitric oxide synthase-1-positive sarcolemma invaginations enriched in contracted skeletal muscle fibres

A Hypothesis for Examining Skeletal Muscle Biopsy-Derived Sarcolemmal nNOSμ as Surrogate for Enteric nNOSα Function

The pathophysiology of gastrointestinal motility disorders is controversial and largely unresolved. This provokes empiric approaches to patient management of these so-called functional gastrointestinal disorders. Preliminary evidence demonstrates that defects in neuronal nitric oxide synthase (nNOS) expression and function, the enzyme that synthesizes nitric oxide (NO), the key inhibitory neurotransmitter mediating mechano-electrical smooth muscle relaxation, is the major pathophysiological basis for sluggishness of oro-aboral transit of luminal contents. This opinion is an ansatz of the potential of skeletal muscle biopsy and examining sarcolemmal nNOSμ to provide complementary insights regarding nNOSα expression, localization, and function within enteric nerve terminals, the site of stimulated de novo NO synthesis. The main basis of this thesis is twofold: (a) the molecular similarity of the structures of nNOS α and μ, similar mechanisms of localizations to “active zones” of nitrergic synthesis, and same mechanisms of electron transfers during NO synthesis and (b) pragmatic difficulty to routinely obtain full-thickness biopsies of gastrointestinal tract, even in patients presenting with the most recalcitrant manifestations of stasis and delayed transit of luminal contents. This opinion attempts to provoke dialog whether this approach is feasible as a surrogate to predict catalytic potential of nNOSα and defects in nitrergic neurotransmission. This discussion makes an assumption that similar molecular mechanisms of nNOS defects shall be operant in both the enteric nerve terminals and the skeletal muscles. These overlaps of skeletal and gastrointestinal dysfunction are largely unknown, thus meriting that the thesis be validated in future by proof-of-principle experiments.

Costamere remodeling with muscle loading and unloading in healthy young men

Costameres are mechano-sensory sites of focal adhesion in the sarcolemma that provide a structural anchor for myofibrils. Their turnover is regulated by integrin-associated focal adhesion kinase (FAK). We hypothesized that changes in content of costamere components (beta 1 integrin, FAK, meta-vinculin, gamma-vinculin) with increased and reduced loading of human anti-gravity muscle would: (i) relate to changes in muscle size and molecular parameters of muscle size regulation [p70S6K, myosin heavy chain (MHC)1 and MHCIIA]; (ii) correspond to adjustments in activity and expression of FAK, and its negative regulator, FRNK; and (iii) reflect the temporal response to reduced and increased loading. Unloading induced a progressive decline in thickness of human vastus lateralis muscle after 8 and 34 days of bedrest (−4% and −14%, respectively; n = 9), contrasting the increase in muscle thickness after 10 and 27 days of resistance training (+5% and +13%; n = 6). Changes in muscle thickness were correlated with changes in cross-sectional area of type I muscle fibers (r = 0.66) and beta 1 integrin content (r = 0.76) at the mid-point of altered loading. Changes in meta-vinculin and FAK-pY397 content were correlated (r = 0.85) and differed, together with the changes of beta 1 integrin, MHCI, MHCII and p70S6K, between the mid- and end-point of resistance training. By contrast, costamere protein level changes did not differ between time points of bedrest. The findings emphasize the role of FAK-regulated costamere turnover in the load-dependent addition and removal of myofibrils, and argue for two phases of muscle remodeling with resistance training, which do not manifest at the macroscopic level.

New aspects of the location of neuronal nitric oxide synthase in the skeletal muscle: A light and electron microscopic study

The action of nitric oxide (NO) synthesized by NO synthases (NOS) is spatially restricted. Hence, the intracellular location of NOS might play an important role for the functional interactions of NO with its target molecules. In the skeletal muscle the neuronal NOS (nNOS) is considered to be the predominant isoform expressed as a muscle specific elongated splice variant. There are only a few and highly discrepant reports of the subcellular distribution of nNOS, which prompted us to re-examine the distribution of nNOS in the skeletal muscle of rat and mouse applying immunocytochemistry and NADPH-diaphorase (NADPH-d) histochemistry. Light microscopically, the sarcolemma, areas beneath the sarcolemma, areas around the nuclei, and the cross striation were labeled by antibodies and by the NADPH-d reaction as well. Ultrastructurally, nNOS visualized immunocytochemically or by the histochemical BSPT-reaction, was associated discretely with extrajunctional portions of the sarcolemma. Both reaction products were additionally observed in the vicinity of endoplasmic reticulum and mitochondria, or associated with their outer membranes. In the neuromuscular junction (NMJ)-region NOS was localized to the cytoplasm of nerve terminals and terminal Schwann cells. In contrast to the commonly accepted assumption, the enzyme was found in association with the presynaptic, and not with the postsynaptic membrane. Cytosolic NADPH-d was exhibited especially between mitochondria accumulated in the postsynaptic region of the NMJ. Surprisingly, in nNOS-/--mice the skeletal muscle showed patterns of significant nNOS-immunoreactivity and NADPH-d activity possibly due to alternative nNOS-splice isoforms, which might be up-regulated to compensate for decreased NO formation.

Absence of dystrophin in mice reduces NO-dependent vascular function and vascular density: total recovery after a treatment with the aminoglycoside gentamicin

OBJECTIVE

Mutations in the dystrophin gene causing Duchenne's muscular dystrophy (DMD) lead to premature stop codons. In mice lacking dystrophin (mdx mice), a model for DMD, these mutations can be suppressed by aminoglycosides such as gentamicin. Dystrophin plays a role in flow (shear stress)-mediated endothelium-dependent dilation (FMD) in arteries. We investigated the effect of gentamicin on vascular contractile and dilatory functions, vascular structure, and density in mdx mice.

METHODS AND RESULTS

Isolated mice carotid and mesenteric resistance arteries were mounted in arteriographs allowing continuous diameter measurements. Mdx mice showed lower nitric oxide (NO)-dependent FMD and endothelial NO synthase (eNOS) expression as well as decreased vascular density in gracilis and cardiac muscles compared with control mice. Treatment with gentamycin restored these parameters. In contrast, smooth muscle-dependent contractions as well as endothelium-dependent or -independent dilation were not affected by dystrophin deficiency or by gentamicin treatment.

CONCLUSIONS

Dystrophin deficiency induces a selective defect in flow-dependent mechanotransduction, thus attenuating FMD and eNOS expression, and may contribute to low arteriolar density. These findings open important perspectives regarding the mechanism involved in the pathophysiology of genetic diseases related to premature stop codons such as DMD.

The specificity of the histochemical NADPH diaphorase reaction for nitric oxide synthase-1 in skeletal muscles is increased in the presence of urea

Nitric oxide synthase-1 (NOS-1) can be demonstrated in the sarcolemma region of myofibers in rodent skeletal muscles with the use of NADPH diaphorase histochemistry. Since other, especially intrafibrar enzymes also exhibit NADPH diaphorase activity, we tried to increase the specificity of the histochemical reaction for NOS-1. A qualitative and quantitative analysis was performed on cryostat sections of fast-twitch oxidative myofiber-rich tongue and fast-twitch glycolytic myofibers-rich tibialis anterior muscle derived from C57 mice and NOS-1 deficient knockout mice. All myofibers of both C57 mice and NOS-1 knockout mice contained significant intrafibrar NADPH diaphorase activity which was inhibited to almost background levels when 2 M urea was added to the incubation medium. On the other hand, myofibers of C57 mice but not of NOS-1-deficient knockout mice exhibited NADPH diaphorase activity in their sarcolemma region which was only weakly reduced in the presence of 2 M urea as was demonstrated by image analysis. Quantitative data on the activity of NADPH diaphorase(s) were obtained in situ by photometric analysis of formazan extracted from cryostat sections. The catalytic activity in tongue and tibialis anterior muscle was reduced in presence of 2 M urea to approximately 27% in C57 mice and to 7-17% in NOS-1 knockout mice, respectively. An in vitro NADPH diaphorase assay performed on homogenates of skeletal muscles also revealed an inhibitory effect of 2 M urea in both mouse strains and, additionally, indicated an upregulation of NADPH diaphorase activity in NOS-1 knockout mice. Finally, an immunodepletion analysis demonstrated that NOS-1 comprises 38% of the total NADPH diaphorase activity in tongue and approximately 59% in tibialis anterior muscle in C57 mice. In conclusion, we recommend the addition of 2 M urea to the incubation medium to increase the specificity of the NADPH diaphorase reaction to localise NOS-1 with the use of catalytic histochemistry.

The nitric oxide synthase-1 and nitric oxide synthase-3/nitric oxide signalling systems in the heart of wild type mice and mouse mutants

Recently, we have shown that nitric oxide synthase-1 (NOS-1) and thus its product NO are present in the sarcolemma region of a subpopulation of atrial cardiomyocytes in the rat heart. In order to find out whether this newly discovered sarcolemma-associated NOS/NO system represents a general signalling mechanism in the murine rodent heart and whether its properties are comparable to those in skeletal muscle fibres, immunohistochemical and catalytic histochemical methods (including image analysis) were applied to the heart and extensor digitorum longus (EDL) and tongue muscles of wild type and mutant mice. In different strains of wild type mice and NOS-3 knockouts, urea-resistant (and therefore specific) NOS NADPH diaphorase histochemistry and NOS-1 immunohistochemistry revealed that NOS-1 activity and protein were present in the sarcolemma region of a subpopulation of atrial and ventricular working cardiomyocytes, but not in those of the impulse conducting system. Using image analysis, NOS-1 showed similar activities in the sarcolemma region of cardiomyocytes and in EDL type I myofibres. In mdx and NOS-1 knockout mice, NOS-1 was absent from the sarcolemma region of atrial and ventricular cardiomyocytes and of EDL and tongue muscle fibres, whereas NOS-1 was present in the hearts of NOS-3 knockouts. Atrial natriuretic peptide immunohistochemistry identified part of the atrial NOS-1-expressing cardiomyocytes as myoendocrine cells. In mdx mice as well as in NOS-1 - and NOS-3-deficient animals, the peptide was found in greater abundance than in wild type mice. These data suggest that NOS-1 is expressed in a subpopulation of working cardiomyocytes in the murine rodent heart, that the myoendocrine cells may be negatively modulated by NOS-1 - and NOS-3-produced NO, and that the anchoring mechanisms for NOS-1 in these cells (i.e. their confinement to the sarcolemma region) are comparable to those in skeletal muscle fibres.

Association of soluble guanylate cyclase with the sarcolemma of mammalian skeletal muscle fibers

Previous investigations have shown that NO-producing nitric oxide synthase (NOS)-1 and CO-generating heme oxygenase (HO-2) are associated with the sarcolemma of skeletal muscle fibers in many mammalian species. Despite numerous roles ascribed to NO and possibly also CO in skeletal muscle, a specific receptor for both gases has hitherto not been found in myofibers. Therefore, in the present work the appearance of the alpha1, beta1 and beta2 subunits of soluble guanylate cyclase (sGC), the most commonly known receptor for NO and potentially also CO, was analysed in mammalian skeletal muscles using immunoblotting and immunohistochemistry. Immunoblotting with an antibody against the beta1 subunit of sGC revealed a band of 70 kDa corresponding to the molecular weight of this protein. Immunohistochemistry with antibodies against the alpha1, beta1 and beta2 sGC subunits showed that the larger part of positivity was present in the sarcolemma region of skeletal muscle fibers and colocalized with NOS-1 mainly in type II myofibers and with HO-2 in type I and type II myofibers. For the first time, sarcolemmal association of sGC and its colocalization with NOS-1 generating the sGC-activator NO and with HO-2 producing the potential sGC upregulator CO have been demonstrated in the present study. These results enable a better understanding of the role of NO and CO in myofibers and suggest a so far unknown molecular mechanism for the interaction of sGC with the sarcolemma.