ROLE OF TRANSCRIPTION FACTOR ERG IN SKELETOGENESIS
"Erg is expressed during early synovial joint development along with Gdf5 and Wnt9a and its expression dwindles over time. To test function, we created floxed Erg mice and conditionally ablated Erg in developing joints by mating with Gdf5-Cre mice. Unexpectedly, limb joint development proceeded and the Erg-deficient mice survived to adulthood. To account for absence of a major joint developmental phenotype, we asked whether Fli-1 was co-expressed and in fact it was, and may have compensated for Erg absence. To test possible postnatal roles of Erg, we subjected 2 month-old Erg-deficient mice to knee's medial collateral ligament (MCL) transection to induce experimental osteoarthritis (OA). Strikingly, the Erg-deficient mice developed serious OA-like defects far sooner than operated wild type companions. Indeed, we observed similar severe OA-like defects in aging un-operated 7-11 month-old Erg-deficient mice, while control littermates displayed mild defects. To gain insights into how Erg maintains long-term articular chondrocyte function, we focused on parathyroid hormone-related protein (PTHrP) which is also expressed in developing joints, stabilizes the chondrocyte phenotype and prevents chondrocyte hypertrophy when over-expressed in cartilage (just as transgenic Erg over-expression does). We found that Erg (as well as Fli-1) stimulates PTHrP expression and the PTHrP gene promoter contains several conserved ets binding sites needed for responsiveness. These and other data lead to our central hypothesis is that Erg is essential for the development and long-term stabilization and function of articular chondrocytes and does so in cooperation with Fli-1 and PTHrP. Our Aims are: (1) To uncover the respective roles of Erg and Fli-1 in joint formation and long-term articular cartilage stabilization; (2) To determine how Erg and Fli-1 regulate PTHrP expression; and (3) To determine if transgenic Erg expression protects joints from surgically-induced OA. The work will be carried out using diverse experimental approaches that include transgenic mouse genetics, microsurgery and cell phenotypic expression. It will produce fundamentally new data and insights into the biology and molecular biology of articular chondrocytes and will pave the way to create future repair and regeneration therapies by which function can be restored in articular chondrocytes affected by a variety of adverse conditions including osteoarthritis and natural aging."
Increased adipogenesis in cultured embryonic chondrocytes and in adult bone marrow of dominant negative erg transgenic mice.
"The Erg gene, is mainly expressed during cartilage formation. 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. Strong differential gene expression, with a decrease in chondrogenesis-related markers and an increase in adipogenesis-related gene expression [occurred] in cultured DN-Erg chondrocytes. Erg is involved in either maintaining the chondrogenic phenotype in vitro or in cell fate orientation. We compared adipocyte presence in wild-type and transgenic mice skeletons. The number of adipocytes [increased] in the bone marrow of adult DN-Erg mice even though no adipocytes were detected in embryonic cartilage or bone."
"Chondrocytes of murine embryos were isolated from the ribs of 18.5 days post-coitum (E18.5) mice "
"Erg is expressed during the earliest events of skeletal formation and is associated with precartilaginous condensation and chondrogenic differentiation"
"Erg gene is the earliest ETS member family expressed in cartilage during embryonic development followed by Fli1, Ets-2 and Pea3 in a lesser extend"
"Adpn, periplin (Plin), fatty acid binding protein 4 (Fabp4), lipoprotein lipase (Lpl), carnitine palmitoyltransferase 1a liver (Cpt-1a), acyl-Coenzyme A oxidase 2 branched chain (Acox2), angiopoietin-like 4 (Angptl4), CD36 antigen (CD36), adipose differentiation related protein (Adfp)) were associated with the ‘PPAR signalling pathway’ and were significantly upregulated in DN-Erg chondrocytes cultured for 20 days"
"Erg protein may be involved in the inhibition of the transdifferentiation of chondrocytes into adipocytes [but] it was not associated with the adipogenic process."
" Pparγ favours the differentiation of mesenchymal stem cells into adipocytes rather than osteoblasts or chondrocytes; Pparγ overexpression has been reported to promote adipogenic differentiation in growth plate chondrocytes"
Genes upregulated 10-fold in wild type chondrocytes on day 20 versus day 0 also upregulated in LSJL:
Downregulated 10-fold in wild type chondrocytes:
Upregulated Dominant negative Erg day 0 to 20 versus LSJL: