Development and validation of a cross-species C-telopeptide ELISA for bone resorption: application in ovariectomized mice and Pacific Bluefin Tuna

C-telopeptide (CTX) fragments of type I collagen are widely used as markers of bone resorption in mammals. This study evaluated the performance of two enzyme-linked immunosorbent assays (ELISAs) using antibodies against different CTX epitopes-CTP-18 and CTP-8-in ovariectomized (OVX) mice, and examined the cross-species applicability of the CTP-18 ELISA in Pacific bluefin tuna (Thunnus orientalis). In OVX mice, both serum and urinary CTP-18 levels significantly increased compared to sham-operated controls (P<0.05), and were moderately correlated with deoxypyridinoline (DPD) levels and inversely with bone mineral density. In contrast, CTP-8 was less responsive. The CTP-18 antibody cross-reacted with mouse, rat, human, and tuna sera, and showed good dilution linearity in tuna samples. Tuna serum CTP-18 levels were positively correlated with DPD (r=0.82, P<0.05). These findings suggest that the CTP-18-based ELISA is a reliable tool for monitoring bone resorption not only in rodents but also in fish, providing a new approach for assessing skeletal turnover in aquaculture species.