Circular RNAs in Skeletal Muscle Aging and Sarcopenia: Mechanistic Evidence, Biomarker Potential, and Therapeutic Readiness.

Skeletal muscle aging arises from interacting defects in proteostasis, mitochondrial quality control, inflammatory signaling, neuromuscular integrity, regenerative capacity, and mechanical-metabolic plasticity, and may culminate in sarcopenia. Circular RNAs (circRNAs) are plausible regulators and biomarker candidates, but most evidence derives from developmental myogenesis, expression profiling, or non-aging atrophy models. This critical review organizes the literature around skeletal muscle aging and applies a four-level framework encompassing discovery, mechanistic support, in vivo causality, and functional intervention. We assess human linkage, species, treatment timing, locus of action, and endogenous or engineered modality separately. circSnd1 and circDdb1 currently provide the strongest combination of functional preclinical evidence and association with aged human skeletal muscle. circFUT10, circBBS9, circNEB, and circGLIS3 define less mature regenerative, discovery, histological, or muscle-quality domains. Non-aging atrophy studies, including circTmeff1 and engineered circmiR-29b, test whether proposed mechanisms remain active under defined catabolic stress. Human blood and serum-vesicle studies provide discovery-level associations with strength or mobility, but clinical validation of a circulating circRNA biomarker for sarcopenia remains insufficient. Therapeutic development is constrained by quantitative target credibility, systemic skeletal-muscle exposure, post-onset functional efficacy in aged organisms, safety, reversibility, and independent replication. The field should now prioritize rigorous validation over additional descriptive cataloging.