PERSISTENT INHIBITION OF MITOCHONDRIAL BIOGENESIS IN DYSTROPHIC MICE: IDENTIFICATION OF NITRIC OXIDE-DEPENDENT SALVAGE PATHWAY
Creators: name: PAMBIANCO, SARAH
Publisher: name: Universit� degli Studi di Milano
Year: 2014

description: Muscular dystrophies are a group of genetic diseases showing muscle degeneration characterized by progressive skeletal muscle weakness, defects in muscle proteins fiber necrosis, and progressive substitution of fibers with connective and adipose tissue. The therapeutic protocols currently in use, based on corticosteroid administration, provide some delay in the progression of the disease, but they are associated with severe side effects. The genetic approaches (exon skipping and antisense oligonucleotides) currently being investigated show some degree of success, however they are directed to specific subsets of population and cannot restore fully the damage already caused by the disease to the muscle. Several studies had demonstrate that the pathophysiology of muscular dystrophies correlates with an altered synthesis of nitric oxide (NO), in fact neuronal nitric oxide synthase (nNOS) is absent from the sarcolemma and relocated to the cytosol, with total muscle NOS activity being thus reduced. During the years, nitric oxide donors were identified as good candidate molecules for Duchenne Muscular dystrophy therapy and recently, our group found that a NO donor, molsidomine, is able to slow disease progression and to restore the functional capacity of damaged muscle, significantly enhancing spontaneous and forced motor activities1. Nitric oxide regulates some mitochondrial functions, such as morphology and complexes activity; moreover mitochondrial dysfunction has long been suspected to be an important pathogenetic feature in muscular dystrophies even if their role is not fully understood. The aim of this project is to analyze the mitochondrial profile of alpha-Sarcoglican-null (a-SG) mice, a mouse model for Limb Girdle muscular dystrophy 2D (LGMD 2D) and to evaluate a possible effect of molsidomine on mitochondrial function. Our long term aim is to define approaches that limit muscle wasting, with a dual finality, on the one hand to ameliorate the dystrophic symptoms per se, on the other to increase the efficiency of cell/gene therapies, in a combined therapy for the disease. To this end elucidation of novel possible targets is necessary and this is the final goal of this project. We find out a severe reduction in mitochondrial content in both tibialis anterior and diaphragm accounting for a lower OxPhos capacity of these muscles. The respiratory rates relative to mitochondrial DNA suggest that mitochondrial content is the major determinant of the lower oxidative capacity of a-SG null muscles. The low mitochondrial content in dystophyc mice is due to a persistent inhibition of the mitochondrial biogenesis pathway. Unexpectedly, the treatment with the NO-donor molsidomine is not able to restore mitochondrial content in a-SG-/- mice, but it is able to improve significantly their oxidative capacity, triggering a therapeutic fiber switch and stimulating fatty acid oxidation rather than improving mitochondrial function per se. Molsidomine promotes in fact an important deacetylation and activation of peroxisome proliferator-activated receptor ? coactivator 1-gene ? (PGC-1?), the principal trascriptional co-activator involved in muscle fiber type determination. Deacetylation of PGC-1? occurs through a nitric oxide-dependent AMP activated protein kinase (APMK) activation leading to an increase expression and activity of the deacetylase Sirt1. Altogether these results highlight for the first time a defective mitochondrial biogenesis in LGMD 2D impairing mitochondrial metabolism and define the increase in OxPhos capacity associated with fiber switch as rescue mechanism with a mechanism independent on mitochondrial biogenesis but focused on lipid metabolism.