Nitric oxide in skeletal muscle: role on mitochondrial biogenesis and function

Abstract

Nitric oxide (NO) has been implicated in several cellular processes as a signaling molecule and also as a source of reactive nitrogen species (RNS). NO is produced by three isoenzymes called nitric oxide synthases (NOS), all present in skeletal muscle. While neuronal NOS (nNOS) and endothelial NOS (eNOS) are isoforms constitutively expressed, inducible NOS (iNOS) is mainly expressed during inflammatory responses. Recent studies have demonstrated that NO is also involved in the mitochondrial biogenesis pathway, having PGC-1α as the main signaling molecule. Increased NO synthesis has been demonstrated in the sarcolemma of skeletal muscle fiber and NO can also reversibly inhibit cytochrome c oxidase (Complex IV of the respiratory chain). Investigation on cultured skeletal myotubes treated with NO donors, NO precursors or NOS inhibitors have also showed a bimodal effect of NO that depends on the concentration used. The present review will discuss the new insights on NO roles on mitochondrial biogenesis and function in skeletal muscle. We will also focus on potential therapeutic strategies based on NO precursors or analogs to treat patients with myopathies and mitochondrial deficiency.

Figure 1

Schematic diagram illustrating the organization of human nitric oxide synthases. (a) Schematic figure showing the dimeric conformation of nitric oxide synthases (NOS) with both subunits attached at the oxygenase domains (green); (b) Main structure differences between the three types of NOS isoforms. PDZ domain is typically present in neuronal NOS (nNOS), the presence of myristoylation (Myr) and palmitoylation (Palm) sites are specific to endothelial NOS (eNOS). All isoforms the oxygenase domain contains binding sites for l-arginine (Arg), Heme and tetrahydrobiopterin (BH4) while the reductase domain binds to calmodulin (CAM), FMN, FAD and NADPH. iNOS = induced NOS.

Figure 2

The influence of NO on mitochondrial respiratory chain. The main site of inhibition of respiratory chain by NO is at complex IV (cytochrome c oxidase) by competition with oxygen. NO can also inhibits the electron transport chain at complex I (NADH dehydrogenase) and III (ubiquinol cytochrome c oxido reductase). The disturbance in the electron transport chain favors the formation of superoxide anions (O₂⁻). The reaction between superoxide anions and NO, results in formation of peroxynitrite (ONOO⁻) inducing macromolecular damage and cell death.

Figure 3

Schematic diagram illustrating the major NO pathways in the activation of mitochondrial biogenesis. Intracellular calcium release activates calcium/calmodulin kinase II (CaMK) triggering sequential activation of NOS and guanylate cyclase (GC) to generate cyclic GMP, which in turn activates protein kinase A (PKA). PKA phosphorylates CREB1 allowing its nuclear translocation and activation of the PGC-1 gene (peroxisome proliferator-activated receptor gamma co-activator 1), a co-activator for NRF-1 (nuclear respiratory factor-1), a transcription factors for mitochondrial biogenesis. The NOS-dependent induction of mitochondrial biogenesis also involves activation of AMP-activated kinase (AMPK), allowing phosphorylation of PGC-1.