Muscle wasting is a significant complication of many diseases including diabetes mellitus, renal and liver failure, HIV/AIDS, and cancer. Sustained loss of skeletal muscle can severely impair a patient's quality of life and often results in poor tolerance and responsiveness to disease treatments. The increased protein breakdown observed during muscle atrophy has been attributed to accelerated activity of the ubiquitin-proteasome pathway, but the precise mechanisms by which this activation stimulates muscle protein loss are poorly understood. Previous work showed that the deubiquitinating enzyme USP19 is upregulated in rat skeletal muscle in various forms of muscle wasting, including streptozotocin induced diabetes, cancer, and dexamethasone treatment. 1 To further explore the role of USP19 in muscle wasting, siRNA-mediated depletion of the enzyme was carried out in L6 myotubes. Knockdown of USP19 resulted in more rapid differentiation of myoblasts into myotubes, with a greater extent of myoblast fusion. It also produced tubes that were visibly larger than those formed by myoblasts transfected with a control siRNA. At the molecular level, silencing of USP19 increased the amount of myosin heavy chain (MHC) and tropomyosin proteins. It also increased levels of MHC transcript, suggesting that USP19 acts at the level of gene transcription or mRNA stability rather than protein degradation. USP19 may mediate its effects on muscle-specific gene expression through the myogenic transcription factor myogenin, since depletion of USP19 increased protein and mRNA levels myogenin but did not affect protein levels of the related transcription factor Myf5. Moreover, the increased tropomyosin and MHC observed upon USP19 knockdown could be abolished when myogenin was simultaneously depleted using siRNA. Collectively, these results suggest that USP19 functions to inhibit the synthesis of key muscle proteins and may therefore be a promising target for the treatment of muscle atrophy.

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Cell adhesion is essential to cell motility and tissue integrity and is regulated by the Integrin family of transmembrane receptors. Integrin binds to ligand extracellularly and provide anchor to the intracellular cytoskeleton via adhesion scaffolding proteins. In order to link cell to the surrounding matrix Integrin needs to be activated. Intracellular activation signals induce perturbations in Integrin cytoplasmic domain that are translated into a conformational change in extracellular region for high affinity ligand binding. Integrin engagement by matrix, in turn, triggers the assembly of adhesion complexes. Such early adhesions promote cytoskeletal organization with subsequent contractile activity that exerts forces against initial Integrin-matrix adhesions. In response to force, Integrin strengthens the interaction with matrix through its clustering and successive recruitment of additional adhesion components. These bidirectional regulatory loops mediated by such interactions are largely dependent on the unique function of Integrin adhesion components. / We demonstrate a novel role for the PDZ/LIM domain protein Zasp as a core component of Integrin adhesions. Specifically, Zasp colocalizes with Integrins at focal adhesion in cultured cells and myotendinous junctions in Drosophila embryos. In both cases elimination of Zasp modifies Integrin function causing consequently defects in cell spreading and muscle attachment. Zasp supports Integrin adhesion to the extracellular matrix that is required to withstand tensile forces exerted during cell spreading and muscle contraction. Furthermore, we found that the distribution of Zasp in muscle Z-lines is essential to orchestrate the cross-linking of alpha-Actinin and Actin filaments. Disruption of Zasp leads to loss of muscle cytoarchitecture, pointing to a larger role for Zasp in sarcomere assembly. Finally, we demonstrate that Zasp, in addition to alpha-Actinin, physically interacts with the Integrin- and Actin-bound cytoskeletal protein Talin. / Collectively, our results point to a dual role for Zasp as a structural scaffold. First it regulates Integrin adhesion to the extracellular matrix by interacting with the head domain of Talin at the myotendinous junctions. Second, Zasp controls sarcomere assembly by tethering the presarcomeric alpha-Actinin component to the tail domain of Talin. Zasp finding as a crucial adhesion component provides further insights on the mechanism underlying Integrin-mediated adhesion.

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