Bone Abstracts

Deficiency of Cyclophilin B (CyPB) causes recessively inherited Type IX osteogenesis imperfecta, a moderately severe to lethal bone dysplasia. CyPB, encoded by PPIB, is an ER-resident peptidyl-prolyl cis-trans isomerase (PPIase) that catalyzes the rate-limiting step in collagen folding, and also functions as a component of the collagen prolyl 3-hydroxylation complex. We previously demonstrated in a Ppib^−/− mouse model that CyPB PPIase activity regulates collagen lysyl hydroxylation, crosslinking and fibrillogenesis, which contribute to maintaining bone mechanical properties. The roles of CyPB in the differentiation and function of specific bone cell populations are unknown. Our screening of whole femora revealed significantly decreased expression of genes associated with osteogenesis in CyPB-deficient mice versus wild-type, including Runx2 (−30%), Rankl/Opg (−55%) and Ibsp (−23%), respectively. In differentiated Ppib^−/− calvarial osteoblasts in culture, expression of osteocyte markers was increased, including Mepe (+125%), Dmp1 (+150%) and Sost (+250%). Osteocyte lacunae from 2 month Ppib^−/− femora were also abnormal in histologic and 2D analyses of serial qBEI cross-sections, revealing significantly increased density of lacunae with a higher proportion of lacunae having smaller areas, versus wild-type. Most interestingly, Ppib^−/− osteoclasts revealed an intrinsic maturation defect. In Ppib^−/− femora, both Acp5 (Tracp) and Ctsk (Cathepsin K) transcripts associated with osteoclast numbers and activity were reduced 38–39%, consistent with reduced Rankl/Opg versus wild-type. RANKL-induced maturation of osteoclasts from Ppib^−/− femoral marrow was severely reduced, with a nearly complete absence of multinucleated cell formation, compared to wild-type. Moreover, qBEI revealed a shift of the bone mineralization density distribution towards higher mineral content, with a concominant decrease in the heterogeneity of mineralization. These data suggest that absence of CyPB alters maturation and function of both osteoblasts and osteoclasts, resulting in a decreased formation and turnover state contributing to bone hypermineralization. The mechanisms by which CyPB regulates bone cell populations are currently under investigation.