Osteocalcin Beyond Bone: Molecular Mechanisms, Endocrine Networks, and Translational Perspectives Across Metabolism, Neurobiology, and Chronic Disease.

Osteocalcin (OCN) is increasingly recognized as a multifunctional hormone whose actions extend far beyond its traditional role as a marker of bone turnover. This review provides an integrated examination of the molecular, endocrine, and translational dimensions of osteocalcin biology, with emphasis on its bioactive undercarboxylated form (ucOCN), which links skeletal remodeling to systemic physiological processes. The structural determinants, biosynthetic pathways, and vitamin K-dependent carboxylation mechanisms underlying OCN isoform diversity are summarized, together with analytical limitations arising from assay variability and differences between N-MID and ucOCN-specific measurements. Mechanistic evidence demonstrates that ucOCN signals through GPRC6A and GPR158 to modulate insulin secretion, muscle glucose uptake, adipokine production, testosterone synthesis, neurocognitive function, hepatic lipid metabolism, and acute stress response. These receptor-level pathways position osteocalcin as a central regulator at the intersection of bone metabolism and whole-body homeostasis. The review synthesizes data across major clinical contexts, including metabolic syndrome, type 2 diabetes (T2DM), non-alcoholic fatty liver disease (NAFLD), chronic kidney disease-mineral and bone disorder (CKD-MBD), cardiovascular dysfunction, and neurodegeneration, highlighting the modifying influence of vitamin K status, circadian rhythms, renal clearance, and local tissue microenvironments. The need for biomarker standardization, methodological harmonization, and receptor-targeted translational strategies is emphasized, alongside emerging therapeutic concepts involving vitamin K supplementation and exercise-induced activation of OCN. Collectively, the evidence reframes osteocalcin as a versatile endocrine mediator at the interface of bone physiology, systemic metabolic regulation, and disease mechanisms.