Shining a Light on Skeletal Muscle Regeneration: Red Photobiomodulation Boosts Myoblast Differentiation In Vitro

Although photobiomodulation (PBM) therapy (i.e., the application of light with a 600-1100 nm wavelength using laser or light-emitting diode devices, a power density of less than 100 mW/cm2, and an energy density of less than 10 J/cm2 at the target) is emerging as a significant noninvasive strategy of promoting regeneration of damaged skeletal muscle tissue, its actual benefits remain debated. In particular, operating parameters exhibiting positive effects on regenerative muscle satellite stem cells need to be clearly identified. Hence, we investigated the effects of red PBM carried out by a laser diode (635 +/- 10 nm; 0.4, 4, and 8 J/cm2; 4 mW/cm2; non-contact mode; continuous wave; single exposure) on murine myoblasts undergoing differentiation and on mature myotubes by combining morphological, biochemical, and functional analyses. Red PBM, especially with a 4 J/cm2 energy density, did not alter cell viability but successfully promoted the expression of myogenic transcription factors as myoblast determination protein 1 (MyoD) and myogenin, as well as myotube formation, mitochondrial metabolism, and biogenesis. Consistently, electrophysiological analyses of cell membrane passive properties and inward ion currents indicated the acquisition of a more differentiated phenotype in PBM-treated cells. Moreover, we found that PBM was able to enhance the release of extracellular vesicles (EVs) during cell differentiation according to a promyogenic phenotype. Red PBM treatment did not alter mature myotube viability and dimension while increasing their secretion of promyogenic EVs. Overall, this study provides experimental evidence supporting promyogenic effects of red PBM and the essential groundwork for further preclinical and clinical studies in the field of skeletal muscle regenerative medicine.