Characterization of the Gja1Jrt/+ Skeletal Phenotype and the Cellular and Molecular Effects of the G60S Connexin 43 Mutation in the Long Bone Microenvironment

Gja1Jrt/+ mice carry a mutation in one allele of the gap junction protein, alpha 1 gene (Gja1), encoding for a dominant negative G60S Connexin 43 (Cx43) mutant protein. Similar to other Cx43 mutant mouse models described, a reduction in Cx43 gap junction formation and/or function resulted in mice with early onset osteopenia. Here we show that in contrast to other Cx43 mutants, the G60S Cx43 mutation activates the osteoblast lineage, with higher bone marrow stromal osteoprogenitor numbers and increased appendicular skeleton osteoblast activity. The data presented in this thesis are the first to describe a Cx43 mutation in which osteopenia is caused by activation of osteoclasts secondary to activation of osteoblast lineage cells, which occurs not only through increased membrane-bound receptor activator of nuclear factor kappa-B ligand (mbRANKL) but production of an abnormal resorption-stimulating bone matrix. Gja1Jrt/+ is also the only Cx43 mutant mouse model described to date with early and progressive bone marrow atrophy, with a significant increase in bone marrow adiposity and adipocyte-specific gene expression in Gja1Jrt/+ mice compared to wild type littermates. We analyzed the mechanism by which the G60S Cx43 mutation activates the osteoblast and adipocyte lineage and show that increased bone morphogenic protein 2 and 4 (BMP2/4) production and signaling increase osteoblast-specific marker expression and peroxisome proliferator-activated receptor gamma (Pparg2) gene expression in Gja1Jrt/+ osteoblasts and bone marrow stromal cells. Taken together, this thesis provides new insight into the role of Cx43 and the effects of the G60S mutation on bone formation and homeostasis, and on the differentiation and activity of bone cell lineages.Ph.D