Reduced fibrillar collagen accumulation in skeletal muscle of secreted protein acidic and rich in cysteine (SPARC)-null mice.

We have previously shown that secreted protein acidic and rich in cysteine (SPARC) promotes myogenic differentiation of rat skeletal muscle progenitor cells in vitro, and in vivo small interfering RNA (siRNA)-mediated transient suppression of SPARC expression in skeletal muscle of mice causes atrophic changes of myofibers, suggesting that SPARC plays a role in the maintenance of skeletal muscle function. In order to know the effect of long-term deficiency of SPARC on skeletal muscle, we performed phenotypic analyses of skeletal muscle of SPARC-null mice. Age-associated changes of myofiber diameters were comparable between wild type (WT) and SPARC-null mice at all ages examined, indicating that the growth of myofibers is unaffected by the absence of SPARC. On the other hand, accumulation of fibrillar collagen was significantly reduced in SPARC-null mice compared to WT mice after 5 months of age without significant changes of collagen I gene expression. The results obtained in the present study suggest that SPARC plays a role to maintain the stiffness of skeletal muscle by regulating collagen accumulation.