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

Expression of neuronal NO synthase α- and β-isoforms in skeletal muscle of mice

Knowledge of the primary structure of neuronal NO synthase (nNOS) in skeletal muscle is still conflicting and needs further clarification. To elucidate the expression patterns of nNOS isoforms at both mRNA and protein level, systematic reverse transcription (RT)-PCR and epitope mapping by qualitative immunoblot analysis on skeletal muscle of C57/BL6 mice were performed. The ability of the nNOS isoforms to form aggregates was characterized by native low-temperature polyacrylamide electrophoresis (LT-PAGE). The molecular analysis was focused on the rectus femoris (RF) muscle, a skeletal muscle with a nearly balanced ratio of nNOS α- and β-isoforms. RT-PCR amplificates from RF muscles showed exclusive exon-1d mRNA expression, either with or without exon-μ. Epitope mapping demonstrated the simultaneous expression of the nNOS splice variants α/μ, α/non-μ, β/μ and β/non-μ. Furthermore, immunoblotting suggests that the transition between nNOS α- and β-isoforms lies within exon-3. In LT-PAGE, three protein nNOS associated aggregates were detected in homogenates of RF muscle and tibialis anterior muscle: a 320 kDa band containing nNOS α-isoforms, while 250 and 300 kDa bands consist of nNOS β-isoforms that form homodimers or heterodimers with non-nNOS proteins.

nNOS Increases Fiber Type-Specific Angiogenesis in Skeletal Muscle of Mice in Response to Endurance Exercise

We studied the relationship between neuronal NO synthase (nNOS) expression and capillarity in the tibialis anterior (TA) muscle of mice subjected to treadmill training. The mRNA (+131%) and protein (+63%) levels of nNOS were higher (p ≤ 0.05) in the TA muscle of C57BL/6 mice undergoing treadmill training for 28 days than in those of littermates remaining sedentary, indicating an up-regulation of nNOS by endurance exercise. Both TA muscles of 16 C57BL/6 mice were subjected to gene electroporation with either the pIRES2-ZsGreen1 plasmid (control plasmid) or the pIRES2-ZsGreen1-nNOS gene-inserted plasmid (nNOS plasmid). Subsequently, one group of mice (n = 8) underwent treadmill training for seven days, while the second group of mice (n = 8) remained sedentary. At study end, 12-18% of TA muscle fibers expressed the fluorescent reporter gene ZsGreen1. Immunofluorescence for nNOS was 23% higher (p ≤ 0.05) in ZsGreen1-positive fibers than ZsGreen1-negative fibers from the nNOS-transfected TA muscle of mice subjected to treadmill training. Capillary contacts around myosin heavy-chain (MHC)-IIb immunoreactive fibers (14.2%; p ≤ 0.05) were only higher in ZsGreen1-positive fibers than ZsGreen1-negative fibers in the nNOS-plasmid-transfected TA muscles of trained mice. Our observations are in line with an angiogenic effect of quantitative increases in nNOS expression, specifically in type-IIb muscle fibers after treadmill training.

Relation of nNOS isoforms to mitochondrial density and PGC-1alpha expression in striated muscles of mice

The expression of neuronal NO synthase (nNOS) alpha- and beta-isoforms in skeletal muscle is well documented but only little information is available about their regulation/functions. Using different mouse models, we now assessed whether the expression of nNOS-isoforms in muscle fibers is related to mitochondria content/activity and regulated by peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha). Catalytic histochemistry revealed highest nNOS-concentrations to be present in type-2 oxidative muscle fibers. Differences in mitochondrial density between nNOS-KO-mice and WT-littermates established by morphometry after transmission electron microscopy were significant in the oxidative portion of the tibialis anterior muscle (TA) but not in rectus femoris muscle (RF) indicating an nNOS-dependent mitochondrial pool in TA. Quantitative immunoblotting displayed the nNOS alpha-isoform to preponderate in those striated muscles of C57BL/6-mice that comprise of many type-2 oxidative fibers, e.g. TA, while roughly even levels of the two nNOS-isoforms were expressed in those muscles that mainly consist of type-2 glycolytic fibers, e.g. RF. Differences in citrate synthase-activity in muscle homogenates between nNOS-KO-mice and WT-littermates were positively related to nNOS alpha-isoform levels. In transgenic-mice over-expressing muscular PGC-1alpha compared to WT-littermates, immunoblotting revealed a significant shift in nNOS-expression in favor of the alpha-isoform in six out of eight striated muscles (exceptions: soleus muscle and tongue) without consistent relationship to changes in the expression of mitochondrial markers. In summary, our study demonstrated the nNOS alpha-isoform expression to be related to mitochondrial content/activity and to be up-regulated by up-stream PGC-1alpha in striated muscles, particularly in those enriched with type-2 oxidative fibers implying a functional convergence of the two signaling systems in these fibers.

Acute infection with an avirulent strain of Toxoplasma gondii causes decreasing and atrophy of nitrergic myenteric neurons of rats

In the enteric nervous system (ENS), nitrergic neurons produce and use nitric oxide (NO) as an inhibitory motor neurotransmitter in response to parasitic infections, including those caused by Toxoplasma gondii. However, damage to the host caused by NO has been reported by various authors, and the role of NO in protection or cytotoxicity continues to be extensively studied. In this study, nitrergic neurons were investigated in the myenteric plexus of the jejunum and the distal colon of rats infected with 500 oocysts of the M7741 strain of T. gondii. Ten rats were randomly assigned into a control group (CG) and infected group (IG; received 500 sporulated oocysts of T. gondii orally). After 24h, the rats were euthanized, and samples of the jejunum and distal colon were obtained and processed for NADPH-diaphorase histochemical analysis. Quantitative and morphometric analysis of the nitrergic neurons in whole mounts containing the myenteric plexus was performed. There was a numeric reduction of nitrergic neurons per mm² in both jejunum and distal colon. The remaining nitrergic neurons suffered atrophy in the areas of the cell body and nucleus, which resulted in a decrease in cytoplasm. Thus, we conclude that an avirulent strain of T. gondii in a short time causes neuroplastic changes in the small and large intestine of rats.

Glucose uptake during contraction in isolated skeletal muscles from neuronal nitric oxide synthase μ knockout mice

Inhibition of nitric oxide synthase (NOS) significantly attenuates the increase in skeletal muscle glucose uptake during contraction/exercise, and a greater attenuation is observed in individuals with Type 2 diabetes compared with healthy individuals. Therefore, NO appears to play an important role in mediating muscle glucose uptake during contraction. In this study, we investigated the involvement of neuronal NOSμ (nNOSμ), the main NOS isoform activated during contraction, on skeletal muscle glucose uptake during ex vivo contraction. Extensor digitorum longus muscles were isolated from nNOSμ(-/-) and nNOSμ(+/+) mice. Muscles were contracted ex vivo in a temperature-controlled (30°C) organ bath with or without the presence of the NOS inhibitor N(G)-monomethyl-l-arginine (L-NMMA) and the NOS substrate L-arginine. Glucose uptake was determined by radioactive tracers. Skeletal muscle glucose uptake increased approximately fourfold during contraction in muscles from both nNOSμ(-/-) and nNOSμ(+/+) mice. L-NMMA significantly attenuated the increase in muscle glucose uptake during contraction in both genotypes. This attenuation was reversed by L-arginine, suggesting that L-NMMA attenuated the increase in muscle glucose uptake during contraction by inhibiting NOS and not via a nonspecific effect of the inhibitor. Low levels of NOS activity (~4%) were detected in muscles from nNOSμ(-/-) mice, and there was no evidence of compensation from other NOS isoform or AMP-activated protein kinase which is also involved in mediating muscle glucose uptake during contraction. These results indicate that NO regulates skeletal muscle glucose uptake during ex vivo contraction independently of nNOSμ.

Phenotype of capillaries in skeletal muscle of nNOS-knockout mice

Because neuronal nitric oxide synthase (nNOS) has a well-known impact on arteriolar blood flow in skeletal muscle, we compared the ultrastructure and the hemodynamics of/in the ensuing capillaries in the extensor digitorum longus (EDL) muscle of male nNOS-knockout (KO) mice and wild-type (WT) littermates. The capillary-to-fiber (C/F) ratio (-9.1%) was lower (P ≤ 0.05) in the nNOS-KO mice than in the WT mice, whereas the mean cross-sectional fiber area (-7.8%) and the capillary density (-3.1%) varied only nonsignificantly (P > 0.05). Morphometrical estimation of the area occupied by the capillaries as well as the volume and surface densities of the subcellular compartments differed nonsignificantly (P > 0.05) between the two strains. Intravital microscopy revealed neither the capillary diameter (+3% in nNOS-KO mice vs. WT mice) nor the mean velocity of red blood cells in EDL muscle (+25% in nNOS-KO mice vs. WT mice) to significantly vary (P > 0.05) between the two strains. The calculated shear stress in the capillaries was likewise nonsignificantly different (3.8 ± 2.2 dyn/cm² in nNOS-KO mice and 2.1 ± 2.2 dyn/cm² in WT mice; P > 0.05). The mRNA levels of vascular endothelial growth factor (VEGF)-A were lower in the EDL muscle of nNOS-KO mice than in the WT littermates (-37%; P ≤ 0.05), whereas mRNA levels of VEGF receptor-2 (VEGFR-2) (-11%), hypoxia inducible factor-1α (+9%), fibroblast growth factor-2 (-14%), and thrombospondin-1 (-10%) differed nonsignificantly (P > 0.05). Our findings support the contention that VEGF-A mRNA expression and C/F-ratio but not the ultrastructure or the hemodynamics of/in capillaries in skeletal muscle at basal conditions depend on the expression of nNOS.

Up-regulation of the peroxiredoxin-6 related metabolism of reactive oxygen species in skeletal muscle of mice lacking neuronal nitric oxide synthase

Although neuronal nitric oxide synthase (nNOS) plays a substantial role in skeletal muscle physiology, nNOS-knockout mice manifest an only mild phenotypic malfunction in this tissue. To identify proteins that might be involved in adaptive responses in skeletal muscle of knockout mice lacking nNOS, 2D-PAGE with silver-staining and subsequent tandem mass spectrometry (LC-MS/MS) was performed using extracts of extensor digitorum longus muscle (EDL) derived from nNOS-knockout mice in comparison to C57Bl/6 control mice. Six proteins were significantly (P < or = 0.05) more highly expressed in EDL of nNOS-knockout mice than in that of C57 control mice, all of which are involved in the metabolism of reactive oxygen species (ROS). These included prohibitin (2.0-fold increase), peroxiredoxin-3 (1.9-fold increase), Cu(2+)/Zn(2+)-dependent superoxide dismutase (SOD; 1.9-fold increase), heat shock protein beta-1 (HSP25; 1.7-fold increase) and nucleoside diphosphate kinase B (2.6-fold increase). A significantly higher expression (4.1-fold increase) and a pI shift from 6.5 to 5.9 of peroxiredoxin-6 in the EDL of nNOS-knockout mice were confirmed by quantitative immunoblotting. The concentrations of the mRNA encoding five of these proteins (the exception being prohibitin) were likewise significantly (P < or = 0.05) higher in the EDL of nNOS-knockout mice. A higher intrinsic hydrogen peroxidase activity (P < or = 0.05) was demonstrated in EDL of nNOS-knockout mice than C57 control mice, which was related to the presence of peroxiredoxin-6. The treatment of mice with the chemical NOS inhibitor L-NAME for 3 days induced a significant 3.4-fold up-regulation of peroxiredoxin-6 in the EDL of C57 control mice (P < or = 0.05), but did not alter its expression in EDL of nNOS-knockout mice. ESR spectrometry demonstrated the levels of superoxide to be 2.5-times higher (P < or = 0.05) in EDL of nNOS-knockout mice than in C57 control mice while an in vitro assay based on the emission of 2,7-dichlorofluorescein fluorescence disclosed the concentration of ROS to be similar in both strains of mice. We suggest that the up-regulation of proteins that are implicated in the metabolism of ROS, particularly of peroxiredoxin-6, within skeletal muscles of nNOS-knockout mice functionally compensates for the absence of nNOS in scavenging of superoxide.

15-Lipoxygenase-1 Prevents Vascular Endothelial Growth Factor A– and Placental Growth Factor–Induced Angiogenic Effects in Rabbit Skeletal Muscles via Reduction in Growth Factor mRNA Levels, NO Bioactivity, and Downregulation of VEGF Receptor 2 Expression

Human 15-lipoxygenase-1 (15-LO-1) is an oxidizing enzyme capable of producing reactive lipid hydroperoxides. 15-LO-1 and its products have been suggested to be involved in many pathological conditions, such as inflammation, atherogenesis, and carcinogenesis. We used adenovirus-mediated gene transfers to study the effects of 15-LO-1 on vascular endothelial growth factor (VEGF)-A 165 – and placental growth factor (PlGF)-induced angiogenesis in rabbit skeletal muscles. 15-LO-1 significantly decreased all angiogenic effects induced by these growth factors, including capillary perfusion, vascular permeability, vasodilatation, and an increase in capillary number. The effects are attributable to the reduction in the amount of VEGF-A 165 and PlGF transcripts by 15-LO-1, resulting in reduced protein expression. The most likely mediator of the VEGF family–induced capillary vasodilatation is nitric oxide (NO), which is produced by NO synthases. Endothelial NO synthase protein expression and NO synthase activity were significantly induced by VEGF-A 165 , and these inductions were reduced by 15-LO-1. VEGF-A 165 induces its angiogenic effects primarily via vascular endothelial growth factor receptor (VEGFR)2, and also PlGF mediates angiogenic signaling via VEGFR2, even though it binds to VEGFR1. VEGFR2 expression is induced by peroxisome proliferator-activating receptor γ. We showed by quantitative RT-PCR and immunohistochemistry that expression of endogenous rabbit peroxisome proliferator-activating receptor γ and VEGFR2 were significantly increased in the growth factor–transduced muscles, but these inductions were efficiently prevented by 15-LO-1. In conclusion, the results suggest that expression of 15-LO-1 has an efficient antiangiogenic effect in vivo via reduction in growth factor mRNA levels, NO bioactivity, and VEGFR2 expression.

Nitric oxide synthase in muscular dystrophies: a re-evaluation

Duchenne and Becker muscular dystrophies (DMD and BMD) are associated with decreased total nitric oxide (NO). However, mechanisms leading to NO deficiency with consequent muscle-cell degeneration remain unknown. To address this issue, we examined skeletal muscles of DMD and BMD patients for co-expression of NO synthase (NOS) with nitrotyrosine and transcription factor CREB, as well as with enzymes engaged in NO signaling. Employing immunocytochemical labeling, Western blotting and RT-PCR, we found that, in contrast to the most commonly accepted view, neuronal NOS was not restricted to the sarcolemma and that muscles of DMD and BMD patients retained all three NOS isoforms with an up-regulation of the inducible NOS isoform, CREB and nitrotyrosine. We suggest that enhanced nitrotyrosine immunostaining in muscle fibers as well as in the vasculature of DMD and BMD specimens reflects massive oxidative stress, resulting in withdrawal of NO from its regular physiological course via the scavenging actions of superoxides.

Constitutive Coexpression of Nitric Oxide Synthase-1 and Soluble Guanylyl Cyclase in Myoepithelial Cells of Mammary Glands in Mice

An impact of nitric oxide (NO) on lactation and milk secretion in mammary glands has previously been documented, but the underlying molecular mechanisms for this modulatory effect remain unclear. Therefore, we investigated the expression patterns of NO synthase (NOS)-1, NOS-3 and the NO receptor soluble guanylyl cyclase (sGC) in mammary glands of lactating and non-lactating female C57/Bl6 mice. RT-PCR demonstrated the existence of NOS-1-mRNA and NOS-3-mRNA in both lactating and resting mammary tissue. Immunoblots loaded with equal amounts of homogenate proteins from lactating and resting mammary tissues revealed comparable intensities of NOS-1 and sGC bands. Performing catalytic NADPH diaphorase histochemistry and immunohistochemistry, NOS-1 was only detected in myoepithelial cells (MEC), while sGC was localized in alveolar epithelial cells (lactocytes) and MEC in both lactating and non-lactating mammary glands. The non-modulated co-expression of both enzymes suggests that NOS-1 and sGC contribute to the constitutive regulation of tone in MEC.

Compartmentalization of NO signaling cascade in skeletal muscles

Skeletal muscle functions regulated by NO are now firmly established. However, the literature on the compartmentalization of NO signaling in myocytes is highly controversial. To address this issue, we examined localization of enzymes engaged in L-arginine-NO-cGMP signaling in the rat quadriceps muscle. Employing immunocytochemical labeling complemented with tyramide signal amplification and electron microscopy, we found NO synthase expressed not only in the sarcolemma, but also along contractile fibers, in the sarcoplasmic reticulum and mitochondria. The expression pattern of NO synthase in myocytes showed striking parallels with the enzymes engaged in L-arginine-NO-cGMP signaling (arginase, phosphodiesterase, and soluble guanylyl cyclase). Our findings are indicative of an autocrine fashion of NO signaling in skeletal muscles at both cellular and subcellular levels, and challenge the notion that the NO generation is restricted to the sarcolemma.

Endothelial NOS is main mediator for shear stress-dependent angiogenesis in skeletal muscle after prazosin administration

The increase of wall shear stress in capillaries by oral administration of the α1-adrenergic receptor antagonist prazosin induces angiogenesis in skeletal muscles. Because endothelial nitric oxide synthase (eNOS) is upregulated in response to elevated wall shear stress, we investigated the relevance of eNOS for prazosin-induced angiogenesis in skeletal muscles. Prazosin and/or the NOS inhibitor Nω-nitro-l-arginine methyl ester (l-NAME) were given to C57BL/6 wild-type mice and eNOS-knockout mice for 14 days. The capillary-to-fiber (C/F) ratio and capillary density (CD; no. of capillaries/mm2) were determined in frozen sections from extensor digitorum longus (EDL) muscles of these mice. Immunoblotting was performed to quantify eNOS expression in endothelial cells isolated from skeletal muscles, whereas VEGF (after precipitation with heparin-agarose) and neuronal NOS (nNOS) concentrations were determined in EDL solubilizates. In EDL muscles of C57BL/6 mice treated for 14 days, the C/F ratio was 28% higher after prazosin administration and 11% higher after prazosin and l-NAME feeding, whereas the CD increased by 21 and 13%, respectively. The C/F ratio was highest after day 4 of prazosin treatment and decreased gradually to almost constant values after day 8. Prazosin administration led to elevation of eNOS expression. VEGF levels were lowest at day 4, whereas nNOS values decreased after day 8. In EDL muscles of eNOS-knockout mice, no significant changes in C/F ratio, CD, or VEGF and nNOS expression were observed in response to prazosin administration. Our data suggest that the presence of eNOS is essential for prazosin-induced angiogenesis in skeletal muscle, albeit other signaling molecules might partially compensate for or contribute to this angiogenic activity. Furthermore, subsequent remodeling of the capillary system accompanied by sequential downregulation of VEGF and nNOS in skeletal muscle fibers characterizes shear stress-dependent angiogenesis.

Localization of NOS-1 in the sarcolemma region of a subpopulation of atrial cardiomyocytes including myoendocrine cells and NOS-3 in vascular and endocardial endothelial cells of the rat heart

Cellular localization patterns of NOS isoforms 1 and 3 (nNOS and eNOS, respectively) in the mammalian heart under basal (non-stimulated) working conditions are still a matter of discussion. Therefore, this issue was reinvestigated in rats using RT-PCR, Western blotting, catalytic histochemistry, immunohistochemistry and image analysis. Tongue and extensor digitorum longus muscles served as positive controls for NOS-1 and NOS-3. RT-PCR revealed NOS-1 mRNA and NOS-3 mRNA in atria and ventricles. Western blotting showed NOS-1 protein in atria and NOS-3 protein in the walls of both heart chambers. Localization of the activity of urea-resistant (and therefore specific) NADPH diaphorase (NADPH-D) and NOS-1 immunohistochemistry showed that NOS-1 is present in the sarcolemma region of a subpopulation of atrial cardiomyocytes but not in working and impulse-conducting cardiomyocytes of atria and ventricles. Atrial natriuretic peptide (ANP) immunohistochemistry revealed that a minority of the NOS-1-expressing atrial cardiomyocytes are myoendocrine cells. eNOS immunostaining was present in endothelial cells of capillaries of the conducting and working myocardium and endocardial cells. Image analysis of the activity of urea-resistant NOS diaphorase showed that NOS-1 activity is lower in the sarcolemma region of atrial cardiomyocytes than in that of tongue and extensor digitorum longus myofibers. These data suggest that, in the non-stimulated rat heart. NOS-1 is expressed in a subpopulation of atrial cardiomyocytes including myoendocrine cells, and that NOS-3 is expressed in the vascular and endocardial endothelium.

Vascular endothelial growth factor is expressed in endothelial cells isolated from skeletal muscles of nitric oxide synthase knockout mice during prazosin-induced angiogenesis

In skeletal muscles, angiogenesis can be induced by increases in wall shear stress. To identify molecules involved in the angiogenic process, a method based on the use of BS-1 lectin-coated magnetic beads was developed to isolate a cellular fraction enriched in microvascular endothelial cells which are directly exposed to wall shear stress. Using such cellular fractions from skeletal muscles of C57 mice in which angiogenesis was induced by administration with the alpha(1)-adrenergic antagonist prazosin, we found the concentration of vascular endothelial growth factor (VEGF) increased in correlation to the duration of the prazosin stimulus. In contrast, the angiopoietin-2/tie-2 system was not changed even after 4days of prazosin treatment. In neuronal nitric oxide synthase (nNOS) knockout mice, the VEGF concentration was also elevated after prazosin treatment but remained almost unchanged in endothelial nitric oxide synthase (eNOS) knockout mice. However, eNOS (and not nNOS) knockout mice expressed higher levels of VEGF under non-stimulated conditions as compared to C57 mice. These results suggest that VEGF produced in endothelial cells is involved in angiogenesis in skeletal muscles of mice responding to the administration of systemic vasodilators. NO derived from eNOS and nNOS may be an important regulator of the angiogenic response in skeletal muscles in vivo.

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.