Abstract
In monolayer culture, primary articular chondrocytes have an intrinsic tendency to lose their phenotype during expansion. The molecular events underlying this chondrocyte dedifferentiation are still largely unknown. Several transcription factors are important for chondrocyte differentiation. The Ets transcription factor family may be involved in skeletal development. One family member, the Erg gene, is mainly expressed during cartilage formation. To further investigate the potential role of Erg in the maintenance of the chondrocyte phenotype, we isolated and cultured chondrocytes from the rib cartilage of embryos of transgenic mice that express a dominant negative form of Erg (DN-Erg) during cartilage formation. DN-Erg expression in chondrocytes cultured for up to 20 days did not affect the early dedifferentiation usually observed in cultured chondrocytes. However, lipid droplets accumulated in DN-Erg chondrocytes, suggesting adipocyte emergence. Transcriptomic analysis using a DNA microarray, validated by quantitative RT-PCR, revealed strong differential gene expression, with a decrease in chondrogenesis-related markers and an increase in adipogenesis-related gene expression in cultured DN-Erg chondrocytes. These results indicate that Erg is involved in either maintaining the chondrogenic phenotype in vitro or in cell fate orientation. Along with the in vitro studies, we compared adipocyte presence in wild-type and transgenic mice skeletons. Histological investigations revealed an increase in the number of adipocytes in the bone marrow of adult DN-Erg mice even though no adipocytes were detected in embryonic cartilage or bone. These findings suggest that the Ets transcription factor family may contribute to the homeostatic balance in skeleton cell plasticity.
Highlights
Chondrocytes, osteoblasts, fibroblasts, adipocytes and skeletal myoblasts are highly specific cell types derived from multipotent mesenchymal stem cells (MSC) through a specific differentiation pathway [1,2,3,4,5]
These morphological and cytochemical observations showed that monolayer-cultured dominant negative form of Ets-related gene (Erg) (DN-Erg) chondrocytes display an adipocytelike phenotype, suggesting that DN-Erg transgene expression favours an adipocyte-like phenotype in monolayer culture over time
We showed that Erg is an important component in the maintenance of the chondrogenic phenotype in vitro, and in the healthy ageing of the skeleton
Summary
Chondrocytes, osteoblasts, fibroblasts, adipocytes and skeletal myoblasts are highly specific cell types derived from multipotent mesenchymal stem cells (MSC) through a specific differentiation pathway [1,2,3,4,5]. Chondrogenesis is a tightly regulated process that is initiated by the condensation of committed MSCs, followed by differentiation into chondrocytes and the expression of cartilage-specific markers [11,12,13]. In skeletal formation, the Erg gene is the earliest ETS member family expressed in cartilage during embryonic development followed by Fli, Ets-2 and Pea in a lesser extend [24,26]. This family of transcriptional regulators shares a highly conserved 85 amino-acid DNA-binding domain (ETS domain) that recognises DNA over an 11 bp sequence centred around a consensus core sequence, 59-GGAA/T-39 [27]
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