Transgenic Expression of <i>Bmp3b</i> in Mesenchymal Progenitors Mitigates Age-Related Muscle Mass Loss and Neuromuscular Junction Degeneration

Skeletal muscle is a vital organ for a healthy life, but its mass and function decline with aging, resulting in a condition termed sarcopenia. The etiology of sarcopenia remains unclear. We recently demonstrated that interstitial mesenchymal progenitors are essential for homeostatic muscle maintenance, and a diminished expression of the mesenchymal-specific gene Bmp3b is associated with sarcopenia. Here, we assessed the protective function of Bmp3b against sarcopenia by generating conditional transgenic (Tg) mice that enable a forced expression of Bmp3b specifically in mesenchymal progenitors. The mice were grown until they reached the geriatric stage, and the age-related muscle phenotypes were examined. The Tg mice had significantly heavier muscles compared to control mice, and the type IIB myofiber cross-sectional areas were preserved in Tg mice. The composition of the myofiber types did not differ between the genotypes. The Tg mice showed a decreasing trend of fibrosis, but the degree of fat infiltration was as low as that in the control mice. Finally, we observed the preservation of innervated neuromuscular junctions (NMJs) in the Tg muscle in contrast to the control muscle, where the NMJ degeneration was conspicuous. Thus, our results indicate that the transgenic expression of Bmp3b in mesenchymal progenitors alleviates age-related muscle deterioration. Collectively, this study strengthens the beneficial role of mesenchymal Bmp3b against sarcopenia and suggests that preserving the youthfulness of mesenchymal progenitors may be an effective means of combating sarcopenia