Aging-Associated Humoral Remodeling of Skeletal Muscle: Identification and Therapeutic Restoration of a Protective Circulating Factor

Abstract

Skeletal muscle aging is increasingly recognized as a multidimensional biological process characterized not only by loss of tissue mass, but also by progressive deterioration in muscle quality, regenerative competence, metabolic flexibility, and functional reserve. This dissertation was based on the concept that such decline may be driven, at least in part, by aging-associated remodeling of the circulating humoral milieu, including depletion of protective factors that normally support skeletal muscle maintenance. On this basis, the present work aimed to identify an age-declining protective circulating factor relevant to skeletal muscle aging and to determine whether therapeutic restoration of this factor could preserve skeletal muscle integrity across both natural aging and metabolically stressed conditions. In Chapter I, an integrative discovery framework combining public skeletal muscle transcriptomics, secretome-informed filtering, machine-learning–based prioritization, and human plasma proteomic validation identified P55789 (UniProt identifier) as a biologically credible age-declining circulating candidate. In older adults, circulating P55789 was reduced in sarcopenia and was associated with adverse frailty- and performance-related phenotypes, supporting its translational relevance in human muscle aging. Molecular characterization further provided biological plausibility for P55789 through its established links to mitochondrial redox homeostasis and cellular stress adaptation. Functional studies then showed that recombinant human P55789 induced a coordinated muscle-supportive transcriptional program in differentiated C2C12 myotubes and promoted myogenic differentiation, myoblast fusion, and anabolic protein synthesis. On the basis of this experimentally validated activity profile, P55789 was functionally designated as AMPEX in this dissertation. In aged mice, chronic recombinant human AMPEX administration improved muscle-related physical performance, preserved lean mass and hindlimb muscle weights, maintained myofiber architecture, reduced fibrotic remodeling, and did not produce overt biochemical evidence of systemic toxicity. These findings establish proof of concept that restoration of a protective age-declining circulating factor can preserve skeletal muscle integrity during aging. In Chapter II, the restorative logic of Chapter I was extended to a more complex translational context in which aging, obesity-associated metabolic stress, and pharmacologically induced body-composition remodeling converge. HFHS-fed aged mice were used to model an obesogenic and functionally compromised state, and the effects of recombinant human AMPEX were evaluated during intermittent tirzepatide treatment. AMPEX did not materially enhance the overall magnitude of tirzepatide-induced body- weight reduction or further suppress food intake. Instead, its major effect was to improve the biological quality PhD/MD 20232078 of weight loss by preserving lean mass, maintaining muscle-related physical performance, and supporting anatomical preservation of skeletal muscle while allowing adipose reduction to proceed. Thus, in the setting of obesity treatment, AMPEX functioned not as an additional weight-loss agent, but as a lean tissue– preserving adjunct during tirzepatide-associated body-composition remodeling. Taken together, this dissertation supports a unified model in which skeletal muscle aging is shaped not only by tissue-intrinsic degeneration, but also by progressive erosion of beneficial humoral support. Within this framework, P55789/AMPEX emerges as a protective circulating factor whose restoration preserves skeletal muscle resilience across distinct aging-related conditions. More broadly, the work establishes a translational strategy that links discovery, human validation, mechanistic testing, and therapeutic restoration in the study of skeletal muscle aging, and suggests that future interventions should target not only the magnitude of tissue change, but also the biological quality of muscle preservation.