Guang Zhou

The transcriptional cofactor Jab1 controls cell proliferation, apoptosis, and differentiation in diverse developmental processes by regulating the activity of various transcription factors. To determine the role of Jab1 during early limb development, we developed a novel Jab1 ^flox/flox ; Prx1‐Cre c onditional K nock o ut ( cKO ) mutant mouse model in which Jab1 was deleted in the osteochondral progenitor cells of the limb buds. Jab1 cKO mutant mice displayed drastically shortened limbs at birth. The short‐limb defect became apparent in Jab1 cKO mutants at E15.5 and increasingly worsened thereafter. By E18.5, Jab1 cKO mutant mice exhibited significantly shorter limbs with: very few hypertrophic chondrocytes, disorganized chondrocyte columns, much smaller primary ossification centers, and significantly increased apoptosis. Real‐time RT‐PCR analysis showed decreased expression of Sox9 , Col2a1 , Ihh , and Col10a1 in Jab1 cKO mutant long bones, indicating impaired chondrogenesis. Furthermore, in a micromass culture model of early limb mesenchyme cells, alcian blue staining showed a significant decrease in chondrogenesis in Jab1 cKO limb bud cells. The expression of Sox9 and its downstream targets Col2a1 and Aggrecan , as well as BMP signaling downstream targets, Noggin , Id1 , and Ihh , were significantly decreased in Jab1 cKO micromass cultures. Moreover, over‐expression of SOX9 in Jab1 cKO micromass cultures partially restored Col2a1 and Aggrecan expression. Jab1 ‐deficient micromass cultures also exhibited decreased BMP signaling response and reduced BMP‐specific reporter activity ex vivo. In summary, our study demonstrates that Jab1 is an essential regulator of early embryonic limb development in vivo, likely in part by co‐activating Sox9 and BMP signaling. J. Cell. Physiol. 229: 1607–1617, 2014.

Osteoporosis is a common skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture. We previously demonstrated that Col1a1-SOX9 transgenic (TG) mice, in which SOX9 specifically expresses in osteoblasts driven by a 2.3-kb Col1a1 promoter, display osteopenia during the early postnatal stage. In this study, to further analyze the osteopenia phenotype and especially the effect of the osteoblast-specific expression of SOX9 on bone mechanical properties, we performed bone geometry and mechanical property analysis of long bones from Col1a1-SOX9 TG mice and wild-type littermates (WT) at different time points. Interestingly, after body weight adjustment, TG mice had similar whole-bone strength as WT mice but significantly thinner cortical bone, lower elastic modulus, and higher moment of inertia. Thus, osteoblast-specific SOX9 expression results in altered bone structure and material properties. Furthermore, the expression levels of Pcna, Col1a1, osteocalcin, and the Opg/Rankl ratio in TG mice were significantly lower until 4 months of age compared with WT mice, suggesting that TG mice have dysregulated bone homeostasis. Finally, bone marrow stromal cells (MSCs) isolated from TG mice display enhanced adipocyte differentiation and decreased osteoblast differentiation in vitro, suggesting that osteoblast-specific expression of SOX9 can lead to altered mesenchymal stem cell differentiation potentials. In conclusion, our study implies that SOX9 activity has to be tightly regulated in the adult skeleton to ensure optimal bone quality.