Chapter Seven - Roles and regulation of SOX transcription factors in skeletogenesis
Introduction
The vertebrate skeleton is an edifice of many structures varying in composition, size, shape and anatomical position. Its development involves the specification and coordinated actions of highly specialized progenitor and differentiated cells (Berendsen & Olsen, 2015). Progenitors arise from the cranial neural crest, paraxial mesoderm and lateral plate mesoderm. Upon migrating to their destined locations, they form skeletogenic mesenchymal condensations. They then engage in multi-step differentiation programs to become chondrocytes, osteoblasts, synovial fibroblasts or tenocytes, which build the skeleton and ensure its growth and maturation. Subsets of progenitors persist throughout development within and around skeletal structures to produce new waves of differentiating cells and participate in intense patterning and differentiation cross talk with them. All cells' phenotypes rely on the implementation of specific genetic programs, and thus on proper expression and utilization of unique sets of transcription factors. The discovery three decades ago that forced expression of the transcription factor MYOD was sufficient to convert mesenchymal cells into myoblasts led to the proposition that each cell type would be governed by a single master transcription factor. Since then, it has been well proven that transcription factors work in sets rather than solo and that many families of transcription factors participate in cell type-specific functions. Each family is characterized by a unique DNA-binding domain, which typically recognizes a precise DNA sequence. Most transcription factors also feature domains that confer specific transcriptional activities. Pioneer transcription factors physically interact with naive chromatin and recruit chromatin-modifying enzymes to displace nucleosomes and poise gene loci for transcriptional activation (Iwafuchi-Doi & Zaret, 2016). Transactivators bind specific DNA sequences at open enhancers or promoters and recruit co-activators that contact the basal transcription machinery to effect transcription. Transrepressors, in contrast, recruit co-repressors to inhibit the basal transcriptional machinery. Architectural factors promote the assembly of enhanceosomes (protein complexes bound to enhancers) by binding DNA near other factors and facilitating their physical and functional interactions.
The family of SOX proteins are key members of cell type-specific transcription factor sets in many lineages (Kamachi & Kondoh, 2013). We here review current knowledge and gaps in knowledge regarding skeletogenic SOX proteins. We introduce them in the context of their family, describe human diseases due to mutations in their genes, and review their roles and modes of regulation. We conclude with suggestions for future research on the SOX family to deepen understanding of skeletogenesis and related diseases.
Section snippets
Shared and distinctive features of SOX proteins
SOX proteins belong to the super-family of HMG (high-mobility-group) domain-containing proteins, as do the TCF/LEF WNT signaling targets and mediators (Kamachi & Kondoh, 2013). The HMG domain comprises three α-helices that bind DNA in the minor groove and force a 30–100° DNA bent (Fig. 1A). The latter property allows LEF1 to promote the assembly of enhanceosomes and might therefore be a property of SOX proteins too (Giese, Amsterdam, & Grosschedl, 1991).
SRY was the first SOX protein to be
Skeletal dysmorphism due to SOX mutations
Mutations in 10 SOX genes, including skeletogenic ones, are known to date to cause a human developmental syndrome. The diseases are rare and most often due to de novo heterozygous mutations that result in gene or protein inactivation and thus reflect gene haploinsufficiency.
Mutations affecting SOX9 cause Campomelic Dysplasia (CD), a severe skeletal malformation syndrome associated with XY sex reversal (Unger, Scherer, & Superti-Furga, 2008). Features include limb (melic) bending (campo),
SOX genes and the control of skeletal progenitors
Embryogenesis involves a cellular hierarchy, whereby embryonic pluripotent stem (ES) cells give rise to progenitor cells with progressively more restricted lineage potential. ES cell programming, self-renewal, and activity are governed by a quartet made of the transcription factors SOX2, OCT3/4 (POU-domain protein), KLF4 (zinc-finger protein), and c-Myc (a basic-helix-loop-helix protein) (Sarkar & Hochedlinger, 2013). The quartet has both pioneer and transactivation functions. The proteins bind
Roles of SOX genes in chondrogenesis
SOX9 and SOX5/SOX6 have long been known to be essential for chondrogenesis (Hata, Takahata, Murakami, & Nishimura, 2017; Kozhemyakina, Lassar, & Zelzer, 2015; Lefebvre & Dvir-Ginzberg, 2017). Yet, several outstanding questions on their specific actions were answered only recently and others remain unanswered. Their genes are active in the chondrocyte lineage from the progenitor mesenchymal stage until the prehypertrophic stage in growth plates or throughout adulthood in permanent cartilages (Dy
Roles of SOX genes in osteogenesis
Osteoblasts form bone and osteoclasts resorb bone. Their coordinated actions help ensure proper bone development and adult homeostasis. While no SOX gene is known to be expressed and critical in osteoclasts, several SOX genes are expressed in the osteoblast lineage and may functionally interact with master regulators, namely, RUNX2 (RUNT-domain protein) and OSX/SP7 (zinc-finger protein) (Liu & Lee, 2013).
As described earlier, SOX2 maintains a population of osteoblasts with stem cell properties
Regulation of SOX genes and RNAs in skeletal cells
Each SOX gene is expressed in a discrete number of cell types. This pattern is specific to each one, and likely involves complex regulatory mechanisms.
Sox2 expression is upregulated downstream of fibroblast growth factor signaling in calvarium osteoblast progenitors in vitro (Basu-Roy et al., 2010). This result is consistent with the importance of FGF signaling in the development of skull and other bones (Ornitz & Marie, 2015), but it remains to be validated in vivo.
Disease-causing genomic
Post-translational regulation of SOX proteins in skeletal cells
Various types of post-translational modifications have been shown to affect SOX protein stability, intracellular localization, or activity, but few, reviewed below, have been validated in skeletogenesis in vivo to this date.
PKA (cAMP-dependent protein kinase A) increases SOX9 activity in vitro by phosphorylating the protein at Ser64 (upstream of the dimerization domain) and Ser181 (C-terminal to the HMG domain) (Huang, Zhou, Lefebvre, & de Crombrugghe, 2000). The latter event occurs in growth
Conclusions and perspectives
The efforts of many research teams over the last three decades have uncovered or postulated central roles, distinct and complementary, for several SOX family members in pivotal cell fate determination and differentiation events in skeletogenesis. The SOXC proteins—SOX4 and SOX11—control progenitor cell survival, undifferentiated status, and ability to cross talk with other cells to properly pattern, grow and mature skeletal structures. SOX8 maintains osteoblast progenitors at the proliferating,
Acknowledgments
We thank B. Olsen for advice on the manuscript. Work in the Lefebvre lab was supported by the NIH/NIAMS Grants AR68308 and AR72649 (to V.L.).
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