Bone Abstracts

The co-chaperone protein, Bcl-2-associated athanogene 1 (BAG-1) is expressed ubiquitously in bone including cells of the osteoblast-lineage and, plays an important role in cell proliferation, apoptosis and differentiation by regulating signalling and transcription. The study aims to elucidate the function of BAG-1 in osteoblast development by examining differences in osteogenic differentiation of bone marrow stromal cells (BMSCs) from Bag-1^+/− (heterozygous) and wild-type mice.

BMSCs isolated from femora and tibiae of 14-week-old Bag-1^+/− and wild-type mice were cultured for 28 days in basal and osteogenic (100 ng/ml rhBMP-2) media. Cells were harvested for analysis of proliferation by DNA assay, apoptosis by TUNEL staining, expression of differentiation stage-specific osteogenic genes by qPCR, Alkaline phosphatase (ALP) specific activity and Osteocalcin (OCN) production by ELISA.

BMSCs from Bag-1^+/− female mice failed to undergo osteogenic differentiation in response to BMP-2, unlike BMSCs from wild-type female mice that responded to BMP-2 by significantly upregulating ALP and OCN expression in day 28 cultures. Interestingly, in osteogenic cultures, BMSCs from Bag-1^+/− female mice proliferated at a significantly higher rate throughout 28 days of culture in comparison to their wild-type counterparts. In contrast, BMSCs from male Bag-1^+/− mice exhibited robust osteogenic differentiation, comparable to the osteogenic response by BMSCs from male wild-type mice. In osteogenic cultures, BMSCs from Bag-1^+/− male mice proliferated at a significantly higher rate than their wild-type counterparts between days 1 and 14, while proliferation of BMSCs from both groups decreased between days 14 and 28 of culture. In both female and male mice, no differences in apoptosis were observed between the wild-type and heterozygous groups.

Thus, in female mice heterozygous for Bag-1, proliferation of BMSCs was enhanced at the expense of osteogenic differentiation. These studies indicate an important role for BAG-1 in osteoblast development and the need to understand the role of interacting factors modulating gender differences.