A Stimulus-Resolved Framework for Investigating Putative Neurotrophic Secretome–Metabolome Coupling in Aging Skeletal Muscle: CNTF and CLCF1 as Non-Equivalent Nodes

Resting myokine abundance cannot distinguish adaptive signaling from compensation, tissue injury, altered receptor availability, or non-muscle contribution. We conducted a targeted narrative review of PubMed/MEDLINE, Embase, Web of Science Core Collection, and Scopus through 30 June 2026 to examine ciliary neurotrophic factor (CNTF), cardiotrophin-like cytokine factor 1 (CLCF1), and muscle metabolism across basal, insulin-stimulated, exercise, recovery, and training states. CNTF has the better-established CNTFRα–LIFR–gp130 receptor model, and pharmacological studies link it to AMPK activation, glucose uptake, ceramide handling, and insulin responsiveness. CLCF1 depends partly on CRLF1-associated extracellular availability, whereas its receptor usage and tissue source in skeletal muscle remain incompletely resolved. One recent study provides important preclinical and exploratory human evidence for exercise-responsive CLCF1, but direct human studies pairing CNTF/CLCF1 kinetics with muscle metabolomics or isotope-resolved flux are lacking. We, therefore, present neurotrophic secretory flexibility as a testable, hypothesis-generating construct rather than a validated biological system or biomarker. Its minimum evaluation requires synchronized measurements of extracellular ligand, receptor-proximal signaling, and metabolic output within the same physiological challenge. Targeted and untargeted metabolomics, lipidomics, quality-controlled annotation, paired tissue and plasma sampling, and stable isotope tracing can determine whether putative coupling is reproducible, muscle-relevant, and altered by aging.