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Abstract: Sialidase Neu2 is an exoglycosidase that removes terminal sialic acids from glycolipids and glycoproteins. In this study, we investigated Neu2 expression during muscle hypertrophy and atrophy. Neu2 mRNA and enzymatic activity were significantly increased in hypertrophic myofibers. A rise in Neu2 activity was observed after constitutive activation of AKT or Igf-1 treatment as well as in myoblasts treated with vasopressin or trichostatin, an inhibitor of histone deacetylases. In contrast, myofiber atrophy obtained by dexamethasone treatment or starvation triggered a significant loss of Neu2 activity and was paralleled by downregulation of Neu2 transcript levels. Overall, we may conclude that Neu2 enzymatic activity is causally linked to proper muscle differentiation and growth. [Copyright &y& Elsevier]

Abstract: Background: The sialidase Neu2 is a cytosolic enzyme which is fully expressed in mature muscle myofibers. Methods: To investigate Neu2 expression during muscle atrophy, we employed an in vitro model consisting of terminally differentiated C2C12 myotubes exposed to different pro-atrophic stimuli that triggered catabolic pathways involved in proteasome activation or autophagy. Results: Neu2 expression was unchanged in myotubes treated with TNF-alpha, a cytokine known to activate the proteasome. However, Neu2 transcript levels and enzymatic activity were downregulated in starved or dexamethasone-treated myotubes that showed proteosomal activation and several hallmarks of macroautophagy, such as formation of autophagosomes, the accumulation of LC3 dots and bulk degradation of long-lived proteins. Neu2 activity and protein levels were rescued upon cotreatment with the lysosomotropic agent NH4Cl, the autophagy inhibitor 3-methyladenine or cathepsin inhibitors, as well as by insulin administration, but were unaffected upon pharmacological inhibition of the proteasome. Moreover, HA- or GST-Neu2 recombinant fusion proteins were rapidly degraded in vitro by purified cathepsin L and B. Overall, we may conclude that Neu2 is degraded by lysosomal enzymes in myotubes undergoing autophagy-mediated atrophy. General significance: This study demonstrates that Neu2 enzyme degradation occurs in atrophic myotubes via macroautophagy and independently of proteasome activation. [Copyright &y& Elsevier]

Abstract: Caveolin-3 (Cav-3) is a muscle-specific membrane protein crucial for myoblast differentiation, as loss of the protein due to mutations within the gene causes an autosomal dominant form of limb girdle muscular dystrophy 1-c. Here we show that along with p38 activity the PI3-kinase/AKT/mTOR pathway is required for proper Cav-3 up-regulation during muscle differentiation and hypertrophy, as confirmed by the marked increase of Cav-3 expression in hypertrophied C2C12 cells transfected with an activated form of AKT. Accordingly, Cav-3 expression was further increased during hypertrophy of L6C5 myoblasts treated with Arg8-vasopressin and in hypertrophic muscles of MLC/mIGF-1 transgenic mice. In contrast, Cav-3 expression was down-regulated in C2C12 myotubes exposed to atrophic stimuli such as starvation or treatment with dexamethasone. This study clearly suggests that Cav-3 expression is causally linked to the maturation of muscle phenotype and it is tightly regulated by hypertrophic and atrophic stimuli. [Copyright &y& Elsevier]