Runx Expression in Normal and Osteoarthritic Cartilage: Possible Functions of Runx Proteins in Chondrocytes: A Dissertation

Abstract

The Runx family of transcription factors supports cell fate determination, cell cycle regulation, global protein synthesis control, and genetic as well as epigenetic regulation of target genes. Runx1, which is essential for hematopoiesis; Runx2, which is required for osteoblast differentiation; and Runx3, which is involved in neurologic and gut development; are expressed in the growth plate during chondrocyte maturation, and in the chondrocytes of permanent cartilage structures. While Runx2 is known to control genes that contribute to chondrocyte hypertrophy, the functions of Runx1 and Runx3 during chondrogenesis and in cartilage tissue have been less well studied. The goals of this project were to characterize expression of Runx proteins in articular cartilage and differentiating chondrocytes and to determine the contribution of Runx1 to osteoarthritis (OA). Here, the expression pattern of Runx1 and Runx2 was characterized in normal bovine articular cartilage. Runx2 is expressed at higher levels in deep zone chondrocytes, while Runx1 is primarily expressed in superficial zone chondrocytes, which is the single cell layer that lines the surface of articular cartilage. Based on this finding, the hypothesis was tested that Runx1 is involved in osteoarthritis, which is a disease characterized by degradation of articular cartilage and changes in chondrocytes. These studies showed that Runx1 is upregulated in articular cartilage explants in response to mechanical compression. Runx1 was also expressed in chondrocytes found at the periphery of OA lesions in the articular cartilage of mice that underwent an OA-inducing surgery. Runx1 was also upregulated in cartilage explants of human osteoarthritic knees, and IHC data showed that Runx1 is mainly expressed in chondrocyte “clones” characteristic of OA. To ascertain the potential function of the upregulation of Runx1 in these cartilage stress conditions and disease states, the hypothesis was tested that Runx1 is upregulated in very specific chondrocyte populations in response to the cartilage damage in osteoarthritis. These studies addressed the properties of these cells that related to functions in cell growth and differentiation. In both the surface layer of normal articular cartilage, and in OA cartilage, Runx1 expression by IF co-localized with markers of mesenchymal progenitor cells, as well as markers of proliferation Ki-67 and PCNA. This finding indicated that Runx1 is found in a population of cells that represent a proliferative population of mesenchymal progenitor cells in osteoarthritis. To further address Runx1 function and identify downstream targets of Runx proteins, a promoter analysis of genes that are known to be either downregulated or upregulated during chondrocyte maturation was done. These studies found that many of these genes have 1 or more Runx binding sites within 2kb of their transcription start site, indicating that they are potential downstream Runx target genes. Lastly, some preliminary experiments were done to characterize novel roles of Runx proteins in the chondrocyte. Runx proteins have been shown to epigenetically regulate their target genes by remaining bound to them throughout mitosis, “poising” them for transcription upon exit from mitosis. The hypothesis that Runx proteins also function by remaining bound to their target genes throughout mitosis in chondrocytes was tested. It was demonstrated by immunofluorescense imaging of Runx proteins on metaphase chromosomes of ATDC5 cells, that Runx2 remains bound to chromosomes during mitosis. Cell proliferation and hypertrophy are both linked to increases in protein synthesis. Runx factors, which regulate rates of global protein synthesis, are expressed in both proliferating and hypertrophic chondrocytes. Thus, it was hypothesized that Runx proteins regulate rates of global protein synthesis during chondrocyte maturation. These studies showed that the overexpression of Runx proteins in a chondrocyte cell line (ATDC5) did not affect protein synthesis rates or levels of protein synthesis machinery. Additionally, Runx proteins did not affect proliferation rates in this chondrocyte cell line.