“SOX9 is a transcriptional activator required for chondrogenesis, and SOX5 and SOX6 are closely related DNA-binding proteins that critically enhance its function. We use here genome-wide approaches to gain novel insights into the full spectrum of the target genes and modes of action of this chondrogenic trio. Using the RCS cell line as a faithful model for proliferating/early prehypertrophic growth plate chondrocytes, we uncover that SOX6 and SOX9 bind thousands of genomic sites, frequently and most efficiently near each other. SOX9 recognizes pairs of inverted SOX motifs, whereas SOX6 favors pairs of tandem SOX motifs. The SOX proteins primarily target enhancers. While binding to a small fraction of typical enhancers, they bind multiple sites on almost all super-enhancers (SEs) present in RCS cells. These SEs are predominantly linked to cartilage-specific genes. The SOX proteins effectively work together to activate these SEs and are required for in vivo expression of their associated genes. These genes encode key regulatory factors, including the SOX trio proteins, and all essential cartilage extracellular matrix components. Chst11, Fgfr3, Runx2 and Runx3 are among many other newly identified SOX trio targets. SOX9 and SOX5/SOX6 thus cooperate genome-wide, primarily through SEs, to implement the growth plate chondrocyte differentiation program.”
“Once settled in skeletogenic sites, progenitor cells coalesce into precartilaginous condensations and activate the chondrocyte differentiation program. Early-stage chondrocytes proliferate and build an abundant, cartilage-specific extracellular matrix. They express such genes as Col2a1 (encoding collagen type II) and Acan (proteoglycan aggrecan). Elongation of cartilage anlagen occurs in growth plates, structures in which chondrocytes proceed through terminal maturation steps in a precise spatial and temporal manner. They proliferate and produce cartilage matrix while aligning into longitudinal columns. They then cease proliferation, become prehypertrophic and express novel markers, such as Ihh (Indian hedgehog). As they become hypertrophic, they turn off most early markers and activate unique ones, including Col10a1 (collagen type X). They eventually die or switch to the osteoblast fate to participate in endochondral ossification. Chondrocytes forming permanent cartilage never proceed through these growth plate maturation steps. They keep expressing pancartilaginous early-chondrocyte markers and also express tissue-specific markers. For instance, superficial articular chondrocytes distinctively express Prg4 (lubricin).”
“Expression of the three SOX genes culminates in growth plate proliferating and prehypertrophic chondrocytes, and is abruptly turned off when chondrocytes undergo hypertrophy. SOX9 is required for chondrogenesis.”
“SOX9 is required to turn on and maintain chondrocyte-specific genes and that SOX5/6 strikingly augment SOX9’s transcriptional activity by securing SOX9 binding to DNA”
“The phenotype of RCS[rat chondrosarcoma] cells thus faithfully matches that of growth plate chondrocytes at the proliferating/early prehypertrophic stage. “<-For more on this read the study.
“RCS cells are bona fide growth plate proliferating/prehypertrophic chondrocytes “<-Since chondrosarcoma can occur in older individuals this is a very promising statement for the creation of neo-growth plates.
