Local overexpression of V1a-vasopressin receptor enhances regeneration in tumor necrosis factor-induced muscle atrophy.

Skeletal muscle atrophy occurs during disuse and aging, or as a consequence of chronic diseases such as cancer and diabetes. It is characterized by progressive loss of muscle tissue due to hypotrophic changes, degeneration, and an inability of the regeneration machinery to replace damaged myofibers. Tumor necrosis factor (TNF) is a proinflammatory cytokine known to mediate muscle atrophy in many chronic diseases and to inhibit skeletal muscle regeneration. In this study, we investigated the role of Arg-vasopressin-(AVP-)dependent pathways in muscles in which atrophy was induced by local overexpression of TNF. AVP is a potent myogenesis-promoting factor and is able to enhance skeletal muscle regeneration by stimulating Ca(2+)/calmodulin-dependent kinase and calcineurin signaling. We performed morphological and molecular analyses and demonstrated that local over-expression of the AVP receptor V1a enhances regeneration of atrophic muscle. By upregulating the regeneration/differentiation markers, modulating the inflammatory response, and attenuating fibrogenesis, the stimulation of AVP-dependent pathways creates a favourable environment for efficient and sustained muscle regeneration and repair even in the presence of elevated levels of TNF. This study highlights a novel in vivo role for AVP-dependent pathways, which may represent an interesting strategy to counteract muscle decline in aging or in muscular pathologies.

Skeletal muscle regeneration in mice is stimulated by local overexpression of V1a-vasopressin receptor.

Skeletal muscle has a remarkable capacity to regenerate after mechanical or pathological injury. We show that the V1a receptor (V1aR) for vasopressin, a potent myogenic-promoting factor that stimulates differentiation and hypertrophy in vitro, is expressed in mouse skeletal muscle and modulated during regeneration after experimental injury. We used gene delivery by electroporation to overexpress the myc-tagged vasopressin V1aR in specific muscles, thus sensitizing them to circulating vasopressin. The correct localization on the surface of the fibers of the recombinant product was demonstrated by confocal immunofluorescence directed against the myc tag. V1aR overexpression dramatically enhanced regeneration. When compared with mock-transfected controls, V1aR overexpressing muscles exhibited significantly accelerated activation of satellite cells and increased expression of differentiation markers. Downstream of V1aR activation, calcineurin was strongly up-regulated and stimulated the expression of IL-4, a potent mediator of myogenic cell fusion. The central role of calcineurin in mediating V1aR-dependent myogenesis was also demonstrated by using its specific inhibitor, cyclosporine A. This study identifies skeletal muscle as a physiological target of hormones of the vasopressin family and reveals a novel in vivo role for vasopressin-dependent pathways. These findings unveil several steps, along a complex signaling pathway, that may be exploited as potential targets for the therapy of diseases characterized by altered muscle homeostasis and regeneration.

Skeletal muscle is enriched in hematopoietic stem cells and not inflammatory cells in cachectic mice.

OBJECTIVE: Cachexia, a debilitating syndrome characterized by skeletal muscle wasting, is associated to many chronic diseases and diminishes the quality of life and survival of patients. Tumor-derived factors and proinflammatory cytokines, including TNF-alpha, IL-6 and IL-1 beta, mediate cachexia. In response to elevated cytokine levels, increased proteasome-mediated proteolysis and auto-phagocytosis result in muscle wasting. The histologic features of muscle cachexia are not fully elucidated. Therefore, we analysed alterations of different cell populations in cachectic muscle. METHODS: By immunohistochemical and cytological approaches, we characterized changes in the abundance of cellular populations in the musculature of a murine model of cancer cachexia (C26-bearing mice). RESULTS: Cachectic muscle displayed a decreased DNA content proportional to muscle mass wastage. A decrease in the number of nuclei occurred in the muscular but not in the stromal compartment. Cachectic muscle showed: mild modulation of myeloperoxidase activity, a neutrophil marker; reduction of macrophages in the endomysium; decrease in CD3(+) lymphocyte number. Conversely, a statistically significant enrichment in Sca-1(+) CD45(+) hematopoietic stem cells (HSCs) occurred in cachectic muscle. DISCUSSION: The elevated levels of cytokines which characterize cachexia may represent a trigger for inflammatory cell activation. However, we find that in cachexia, inflammatory cells in muscle are not increased while muscle tissue nuclei decline. Our data suggest that the inflammatory cell-mediated stress is not an etiologic component of muscle wasting in cachexia. The relative increase in HSCs in cachectic skeletal muscle suggests an attempt to maintain muscle homeostasis by recruitment and/or activation of stem cells.