Heme oxygenase-2 is present in the sarcolemma region of skeletal muscle fibers and is non-continuously co-localized with nitric oxide synthase-1

Caveolae compartmentalization of heme oxygenase-1 in endothelial cells

The heme oxygenase (HO) and nitric oxide synthase (NOS) enzymes generate the gaseous signaling molecules carbon monoxide (CO) and nitric oxide, respectively. Constitutive NOSs localize to caveolae, and their activities are modulated by caveolin-1. Nothing is known of the localization of the inducible heme oxygenase-1 (HO-1) in plasma membrane caveolae. Thus, we examined the distribution and subcellular localization of HO-1, biliverdin reductase (BVR), and NADPH:cytochrome P450 reductase (NPR) in pulmonary artery endothelial cells. Each of these proteins localized in part to plasma membrane caveolae in endothelial cells. Inducers of HO-1 or overexpression of HO-1 increased the content of this protein in a detergent-resistant fraction containing caveolin-1. Inducible HO activity appeared in plasma membrane, cytosol, and isolated caveolae. In addition, caveolae contained endogenous BVR activity, supporting the same compartmentalization of both enzymes. Caveolin-1 physically interacted with HO-1, as shown by coimmunoprecipitation studies. HO activity dramatically increased in cells expressing caveolin-1 antisense transcripts, suggesting a negative regulatory role for caveolin-1. Conversely, caveolin-1 expression attenuated LPS-inducible HO activity. Since their initial characterization in 1969, HO enzymes have been described as endoplasmic reticulum-associated proteins. We demonstrate for the first time the localization of heme degradation enzymes to plasma membrane caveolae, and present novel evidence that caveolin-1 interacts with and modulates HO activity.

Kinetic proofreading by the cavity system of myoglobin: protection from poisoning

Throughout its matrix of atoms, myoglobin has a network of cavities that are inhabited for short lengths of time by ligands released by photolysis from the myoglobin heme. The purpose or effect of this cavity network is not clear. A recently published kinetic scheme that fits data from many native and mutant myoglobin oxygen photolysis experiments can be modified easily into a kinetic scheme that includes kinetic proofreading. Proofreading would provide protection against contaminants and, specifically, might help protect the cell from carbon monoxide poisoning. Here we present a two-part model: (1) myoglobin represented by a kinetic description, which includes proofreading reactions associated with the cavities, and (2) a reaction-diffusion description of a myocyte model in which the part 1 myoglobin acts as a mobile buffer in simultaneous carbon monoxide and oxygen gradients. The non-equilibrium nature of part 2 should promote the proofreading function of part 1. A simulation using the model demonstrates that the cavity system can in principle proofread, reducing mitochondrial enzyme contamination.

A major role for carbon monoxide as an endogenous hyperpolarizing factor in the gastrointestinal tract

Carbon monoxide (CO) is proposed as a physiological messenger. CO activates cGMP and has a direct effect on potassium channels. Both actions of CO lead to hyperpolarization of a cell's resting membrane potential, suggesting that CO may function as a hyperpolarizing factor, although direct evidence is still lacking. Here we take advantage of the known membrane potential gradient that exists in the muscle layers of the gastrointestinal tract to determine whether CO is an endogenous hyperpolarizing factor. We find that heme oxygenase-2-null mice have depolarized smooth muscle cells and that the membrane potential gradient in the gut is abolished. Exogenous CO hyperpolarizes the membrane potential. Regions of the canine gastrointestinal tract that are more hyperpolarized generate more CO and have higher heme oxygenase activity than more depolarized regions. Our results suggest that CO is a critical hyperpolarizing factor required for the maintenance of intestinal smooth muscle membrane potential and gradient.

Role of heme oxygenases in sepsis-induced diaphragmatic contractile dysfunction and oxidative stress

Heme oxygenases (HOs), essential enzymes for heme metabolism, play an important role in the defense against oxidative stress. In this study, we evaluated the expression and functional significance of HO-1 and HO-2 in the ventilatory muscles of normal rats and rats injected with bacterial lipopolysaccharide (LPS). Both HO-1 and HO-2 proteins were detected inside ventilatory and limb muscle fibers of normal rats. Diaphragmatic HO-1 and HO-2 expressions rose significantly within 1 and 12 h of LPS injection, respectively. Inhibition of the activity of inducible nitric oxide synthase (iNOS) in rats and absence of this isoform in iNOS(-/-) mice did alter sepsis-induced regulation of muscle HOs. Systemic inhibition of HO activity with chromium mesoporphyrin IX enhanced muscle protein oxidation and hydroxynonenal formation in both normal and septic rats. Moreover, in vitro diaphragmatic force generation declined substantially in response to HO inhibition both in normal and septic rats. We conclude that both HO-1 and HO-2 proteins play an important role in the regulation of muscle contractility and in the defense against sepsis-induced oxidative stress.

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.