title: Rac1 signaling stimulates N-cadherin expression, mesenchymal condensation, and chondrogenesis.
abstract: The molecular mechanisms controlling differentiation of mesenchymal precursor cells into chondrocytes (chondrogenesis) are not completely understood. We have recently shown that the small GTPase RhoA inhibits this process. Here we demonstrate that a different Rho GTPase family member, Rac1, promotes chondrogenesis. Pharmacological inhibition of Rac1 expression in micromass culture resulted in reduced mRNA levels of the chondrogenic markers collagen II and aggrecan, and decreased accumulation of glycosaminoglycans. Expression of the essential chondrogenic transcription factors Sox9, Sox5, and Sox6 was also reduced upon inhibition of Rac1 signaling. In contrast, overexpression of Rac1 in the chondrogenic ATDC5 cell line increased mRNA transcripts of Sox9, 5, and 6, collagen II, and aggrecan. Inhibition of Rac1 resulted in a reduction in the number, size, and organization of cellular condensations and decreased expression of N-cadherin. Overexpression of Rac1 resulted in an increase in N-cadherin expression levels. Furthermore, genetic ablation of Rac1 in primary micromass cultures resulted in reduced expression of chondrogenic markers. Additionally, we provide evidence that Cdc42 also promotes chondrogenesis. Overexpression of Cdc42 in ATDC5 cells resulted in increased expression of Sox5, Sox9, and collagen II but not Sox6, aggrecan, or N-cadherin. Therefore, we demonstrate that Rac1 and Cdc42 are positive regulators of chondrogenesis, but act at least in part through different cellular and molecular mechanisms.
authors.affiliation: CIHR Group in Skeletal Development and Remodeling, Department of Physiology and Pharmacology, University of Western Ontario, London, Ontario N6A 5C1, Canada.
authors.forename: Anita
authors.initials: A
authors.lastname: Woods
authors.forename: Guoyan
authors.initials: G
authors.lastname: Wang
authors.forename: Holly
authors.initials: H
authors.lastname: Dupuis
authors.forename: Zhuhong
authors.initials: Z
authors.lastname: Shao
authors.forename: Frank
authors.initials: F
authors.lastname: Beier
mesh_terms: D000818:Animals; D015820:Cadherins / Q000378:metabolism*; D019902:Chondrocytes / Q000378:metabolism*; D020219:Chondrogenesis; D004268:DNA-Binding Proteins / Q000378:metabolism; D005786:Gene Expression Regulation; D006609:High Mobility Group Proteins / Q000378:metabolism; D037102:Lectins / Q000378:metabolism; D018878:Limb Buds / Q000378:metabolism; D008648:Mesoderm / Q000378:metabolism*; D051379:Mice; D008954:Models, Biological; D009479:Neuropeptides / Q000378:metabolism / Q000502:physiology*; D012333:RNA, Messenger / Q000378:metabolism; D055748:SOXB1 Transcription Factors; D015398:Signal Transduction; D020764:cdc42 GTP-Binding Protein / Q000378:metabolism; D020744:rac GTP-Binding Proteins / Q000378:metabolism / Q000502:physiology*; D020830:rac1 GTP-Binding Protein
chemical_list: D015820:Cadherins; D004268:DNA-Binding Proteins; D006609:High Mobility Group Proteins; D037102:Lectins; D009479:Neuropeptides; D012333:RNA, Messenger; C484602:Rac1 protein, mouse; D055748:SOXB1 Transcription Factors; C098693:Sox1 protein, mouse; D020764:cdc42 GTP-Binding Protein; D020744:rac GTP-Binding Proteins; D020830:rac1 GTP-Binding Protein