CYR61, PRODUCT OF A GROWTH FACTOR-INDUCIBLE IMMEDIATE-EARLY GENE, REGULATES CHONDROGENESIS IN MOUSE LIMB BUD MESENCHYMAL CELLS

Chondrogenesis during embryonic skeletal development involves the cond ensation of mesenchymal cells followed by their differentiation into c hondrocytes. We describe herein a previously unrecognized regulator of mammalian chondrogenesis encoded by a murine growth factor-inducible immediate-early gene, cyr61. The Cyr61 protein is a secreted, heparin- binding protein (379 amino acids with 38 conserved cysteines) that pro motes cell adhesion, migration, and proliferation. The expression patt ern of the cyr61 gene during embryogenesis is tissue specific and temp orally regulated. Most notably, cyr61 is transiently expressed in mese nchymal cells of both mesodermal and neuroectodermal origins undergoin g chondrogenesis, suggesting that Cyr61 may play a role in the develop ment of the embryonic skeleton. In this communication, we demonstrate that the Cyr61 protein promotes chondrogenesis in micromass cultures o f limb bud mesenchymal cells in vitro and is likely to play a similar role in vivo based on the following observations: (1) Cyr61 is present in the embryonic limb mesenchyme during chondrogenesis in vivo and in vitro; (2) purified recombinant Cyr61. protein added exogenously to m icromass cultures promotes chondrogenesis as judged by precocious expr ession of type II collagen, increased [S-35]sulfate incorporation, and larger Alcian blue-staining cartilage nodules; (3) Cyr61 enhances cel l-cell aggregation, an initial step in chondrogenesis, and promotes ch ondrogenic differentiation in cultures plated at subthreshold cell den sities that are otherwise unable to support differentiation; and (4) n eutralization of the endogenous Cyr61 with specific antibodies inhibit s chondrogenesis. Taken together, these results identify Cyr61 as a no vel player in chondrogenesis that contributes to the development of th e mammalian embryonic skeleton. (C) 1997 Academic Press.