SOX9 and the many facets of its regulation in the chondrocyte lineage

Abstract

SOX9 is a pivotal transcription factor in developing and adult cartilage. Its gene is expressed from the multipotent skeletal progenitor stage and is active throughout chondrocyte differentiation. While it is repressed in hypertrophic chondrocytes in cartilage growth plates, it remains expressed throughout life in permanent chondrocytes of healthy articular cartilage. SOX9 is required for chondrogenesis: it secures chondrocyte lineage commitment, promotes cell survival, and transcriptionally activates the genes for many cartilage-specific structural components and regulatory factors. Since heterozygous mutations within and around SOX9 were shown to cause the severe skeletal malformation syndrome called campomelic dysplasia, researchers around the world have worked assiduously to decipher the many facets of SOX9 actions and regulation in chondrogenesis. The more we learn, the more we realize the complexity of the molecular networks in which SOX9 fulfills its functions and is regulated at the levels of its gene, RNA, and protein, and the more we measure the many gaps remaining in knowledge. At the same time, new technologies keep giving us more means to push further the frontiers of knowledge. Research efforts must be pursued to fill these gaps and to better understand and treat many types of cartilage diseases in which SOX9 has or could have a critical role. These diseases include chondrodysplasias and cartilage degeneration diseases, namely osteoarthritis, a prevalent and still incurable joint disease. We here review the current state of knowledge of SOX9 actions and regulation in the chondrocyte lineage, and propose new directions for future fundamental and translational research projects.

Keywords: Cartilage; SOX9; chondrocyte; posttranscriptional regulation; posttranslational modification; transcriptional regulation.

Figure 1. SOX9 actions in the chondrocyte lineage

Schematic of the progressive differentiation of multipotent skeletogenic cells into chondrocytes and related skeletal cell types. Blue color is used to highlight the expression domain of SOX9.

Figure 2. SOX9 molecular actions in chondrocytes

(A) Schematic of the genetic locus of an arbitrary cartilage-specific gene. A super-enhancer is presented as a cluster of three enhancers located upstream and downstream of the gene. Each enhancer is bound by a complex of proteins that include SOX9 and co-factors, namely SOX5/SOX6, P300, WWP2 and other yet-to-be identified proteins. Synergy between the enhancers and the basal transcription machinery, based at the promoter, results in high and cartilage-specific expression of the gene. (B) Schematic of the binding of SOX9 to DNA. Two molecules of SOX9 homodimerize upon binding in the minor groove of a DNA sequence featuring two SOX recognition sites facing each other and separated by 3 or 4 nucleotides.

Figure 3. SOX9 genetic locus, disease-causing chromosomal alterations, and enhancers

(A) Schematic of the human chromosomal segment containing SOX9 and its flanking upstream and downstream domains. The coding genes that are SOX9’s neighbors are shown (KCNJ2 and SLC39A11), as well as non-coding genes (RefSeq and GeneBank, CCDS, Rfam and tRNA, as listed on the UCSC genome browser). (B) Schematic of the domains upstream of SOX9 that are prone to chromosomal alterations causing PRS, ACD and CD disease. The shaded background illustrates that other domains are also subject to disease-causing alterations. (C) Location of tissue-specific enhancers identified upstream and downstream of SOX9. The position of each enhancer is indicated with an arrow. The main domain of activity of each enhancer is indicated and represented with colors.

Figure 4. Identification microRNA binding sites in the human SOX9 mRNA

(A) A schematic of the 3′ end of the SOX9 coding sequence (black rectangle) and entire 3′ untranslated region (3′UTR; blue line) is shown above a conservation plot for 100 vertebrate genomes downloaded from the UCSC genome browser (www.genome.ucsc.edu). The chromosome coordinates of the 3′UTR are indicated. The position of recognition sites for six microRNAs is shown with arrows. (B) Alignment of the miRNA binding sites and flanking sequences in eight vertebrate genomes.

Figure 5. SOX9 protein domain organization and posttranslational regulation

Schematic of the 509-residue-long human SOX9 protein. Functional and highly conserved domains are shown with various colors and their residue boundaries are given in grey. Posttranslational modifications and nuclear trafficking domains are represented as indicated.