Bioluminescence imaging of bone formation using hairless osteocalcin-luciferase transgenic mice

Osteocalcin is a major noncollagenous protein component of bone extracellular matrix, synthesized and secreted exclusively by osteoblastic cells during the late stage of maturation. We introduced a 10 kb human osteocalcin enhancer/promoter (OC)-luciferase (Luc) construct into a hairless mouse line. Examination of tissue RNAs from these transgenic mice showed a predominant restriction of Luc mRNA expression to bone-associated tissues. Immunohistochemical staining of calvaria tissue sections revealed the localization of Luc protein to osteoblasts. Utilizing in vivo bioluminescence imaging, supplementation of 1α,25-dihydroxyvitamin D3 increased Luc activity throughout the skeleton, consistent with in vitro transient transfection studies in osteoblast-like cells. Moreover, we observed an abrupt decrease in bioluminescence activity as the mice reached puberty, and a further decrease gradually thereafter. Using a radius skeletal repair model, we observed enhanced bioluminescence at the fracture site in both young (14-22 weeks old) and aged (50-66 weeks old) mice. However, peak bioluminescence was delayed in aged mice compared with young mice, suggesting retarded osteocalcin expression with aging. Our in vivo imaging system may contribute to the therapy and prevention of various bone metabolic disorders through its effective monitoring of the bone formation process.

► In vivo bioluminescence imaging is useful in the study of bone development and regeneration. ► A hairless transgenic mouse line that expresses luciferase under the control of osteocalcin enhancer/promoter sequences was established. ► The hairless background enabled easy analysis of bone formation activity through bioluminescence in the entire mouse. ► We identified abrupt decrease in bone formation activity as the mice reached puberty and gradual further decreases with age. ► Retarded osteocalcin expression during fracture healing in aged mice, as compared with young mice, suggests age-related impairment of healing.