RhoA/ROCK Signaling Suppresses Hypertrophic Chondrocyte Differentiation

Microarray analyses of gene expression during chondrocyte differentiation identifies novel regulators of hypertrophy

Ordered chondrocyte differentiation and maturation is required for normal skeletal development, but the intracellular pathways regulating this process remain largely unclear. We used Affymetrix microarrays to examine temporal gene expression patterns during chondrogenic differentiation in a mouse micromass culture system. Robust normalization of the data identified 3300 differentially expressed probe sets, which corresponds to 1772, 481, and 249 probe sets exhibiting minimum 2-, 5-, and 10-fold changes over the time period, respectively. GeneOntology annotations for molecular function show changes in the expression of molecules involved in transcriptional regulation and signal transduction among others. The expression of identified markers was confirmed by RT-PCR, and cluster analysis revealed groups of coexpressed transcripts. One gene that was up-regulated at later stages of chondrocyte differentiation was Rgs2. Overexpression of Rgs2 in the chondrogenic cell line ATDC5 resulted in accelerated hypertrophic differentiation, thus providing functional validation of microarray data. Collectively, these analyses provide novel information on the temporal expression of molecules regulating endochondral bone development.

Rac1/Cdc42 and RhoA GTPases antagonistically regulate chondrocyte proliferation, hypertrophy, and apoptosis

The intracellular signaling pathways controlling chondrocyte physiology are largely unknown. Here we show that the small GTPases, Rac1 and Cdc42, accelerate the rate of chondrocyte differentiation and apoptosis, thereby antagonizing the activity of RhoA. These results identify Rac1 and Cdc42 pathways as novel regulators of cartilage development.

INTRODUCTION

Proliferation, hypertrophic differentiation, and ultimate apoptosis of chondrocytes regulate endochondral bone growth and development, but the intracellular signaling pathways controlling chondrocyte biology are incompletely understood. In this study, we investigated the role of the small GTPases Rac1 and Cdc42 in chondrocytes.

MATERIALS AND METHODS

Rac1 and Cdc42 expression during chondrogenic differentiation was assessed by RT-PCR and Western blotting. Effects of Rac1 and Cdc42 on parameters of chondrocyte biology were studied using transient transfections into primary mouse chondrocytes and stable transfections of the chondrogenic cell line ATDC5. Luciferase assays, RT-PCR, cell proliferation, alkaline phosphatases assays, staining procedures, TUNEL assays, and caspase activity assays were performed to study the chondrocyte response to overexpression of Rac1 and Cdc42 proteins. Activation of the p38 pathway was analyzed using Western blotting with phospho-specific antibodies, and mitogen-activated protein (MAP) kinase pathways were inhibited using pharmacological approaches.

RESULTS AND CONCLUSIONS

Rac1 and Cdc42 activities are required for maximal activity of the collagen X promoter, a hypertrophic marker, in primary chondrocytes, suggesting essential roles of these GTPases in chondrocyte hypertrophy. Overexpression of Rac1 or Cdc42 in chondrogenic ATDC5 cells results in reductions in cell numbers and marked acceleration of hypertrophic differentiation, thus opposing the effects of the related GTPase RhoA. Rac1 and Cdc42 also induce accelerated chondrocyte apoptosis, as shown by TUNEL and caspase activity assays and changes in cell morphology and actin organization. Rac1 and Cdc42 overexpression results in activation of the p38 MAP kinase pathway in ATDC5 cells, and pharmacological inhibition of p38 signaling blocks the effects of Rac1 and Cdc42 overexpression on hypertrophy and apoptosis. Our results therefore suggest that Rac1 and Cdc42 signaling accelerates progression through the chondrocyte life cycle in a p38-dependent fashion and antagonizes RhoA signaling pathways in chondrocyte proliferation, hypertrophy, and apoptosis.

RhoA/ROCK signaling regulates Sox9 expression and actin organization during chondrogenesis

Endochondral ossification is initiated by the differentiation of mesenchymal precursor cells to chondrocytes (chondrogenesis). This process is characterized by a strong interdependence of cell shape, cytoskeletal organization, and the onset of chondrogenic gene expression, but the molecular mechanisms mediating these interactions are not known. Here we investigated the role of the RhoA/ROCK pathway, a well characterized regulator of cytoskeletal organization, in chondrogenesis. We show that pharmacological inhibition of ROCK signaling by Y27632 resulted in increased glycosaminoglycan synthesis and elevated expression of the chondrogenic transcription factor Sox9, whereas overexpression of RhoA in the chondrogenic cell line ATDC5 had the opposite effects. Suppression of Sox9 expression by ROCK signaling was achieved through repression of Sox9 promoter activity. These molecular changes were accompanied by reorganization of the actin cytoskeleton, where RhoA/ROCK signaling suppressed cortical actin organization, a hallmark of differentiated chondrocytes. This led us to analyze the regulation of Sox9 expression by drugs affecting cytoskeletal dynamics. Both inhibition of actin polymerization by cytochalasin D and stabilization of existing actin filaments by jasplakinolide resulted in increased Sox9 mRNA levels, whereas inhibition of microtubule polymerization by colchicine completely blocked Sox9 expression. In conclusion, our data suggest that RhoA/ROCK signaling suppresses chondrogenesis through the control of Sox9 expression and actin organization.