Chapter Seven - Roles and regulation of SOX transcription factors in skeletogenesis

https://doi.org/10.1016/bs.ctdb.2019.01.007Get rights and content

Abstract

SOX transcription factors participate in the specification, differentiation and activities of many cell types in development and beyond. The 20 mammalian family members are distributed into eight groups based on sequence identity, and while co-expressed same-group proteins often have redundant functions, different-group proteins typically have distinct functions. More than a handful of SOX proteins have pivotal roles in skeletogenesis. Heterozygous mutations in their genes cause human diseases, in which skeletal dysmorphism is a major feature, such as campomelic dysplasia (SOX9), or a minor feature, such as LAMSHF syndrome (SOX5) and Coffin-Siris-like syndromes (SOX4 and SOX11). Loss- and gain-of-function experiments in animal models have revealed that SOX4 and SOX11 (SOXC group) promote skeletal progenitor survival and control skeleton patterning and growth; SOX8 (SOXE group) delays the differentiation of osteoblast progenitors; SOX9 (SOXE group) is essential for chondrocyte fate maintenance and differentiation, and works in cooperation with SOX5 and SOX6 (SOXD group) and other types of transcription factors. These and other SOX proteins have also been proposed, mainly through in vitro experiments, to have key roles in other aspects of skeletogenesis, such as SOX2 in osteoblast stem cell self-renewal. We here review current knowledge of well-established and proposed skeletogenic roles of SOX proteins, their transcriptional and non-transcriptional actions, and their modes of regulation at the gene, RNA and protein levels. We also discuss gaps in knowledge and directions for future research to further decipher mechanisms underlying skeletogenesis in health and diseases and identify treatment options for skeletal malformation and degeneration diseases.

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.).

References (101)

  • A.R. Lourenco et al.

    SOX4: Joining the master regulators of epithelial-to-mesenchymal transition?

    Trends in Cancer

    (2017)
  • N.P. Mandalos et al.

    Sox2: To crest or not to crest?

    Seminars in Cell & Developmental Biology

    (2017)
  • L.J. Ng et al.

    SOX9 binds DNA, activates transcription, and coexpresses with type II collagen during chondrogenesis in the mouse

    Developmental Biology

    (1997)
  • S. Ohba et al.

    Distinct transcriptional programs underlie Sox9 regulation of the mammalian chondrocyte

    Cell Reports

    (2015)
  • N. Phochanukul et al.

    No backbone but lots of Sox: Invertebrate sox genes

    The International Journal of Biochemistry & Cell Biology

    (2010)
  • S. Piera-Velazquez et al.

    Regulation of the human SOX9 promoter by Sp1 and CREB

    Experimental Cell Research

    (2007)
  • A. Sarkar et al.

    The sox family of transcription factors: Versatile regulators of stem and progenitor cell fate

    Cell Stem Cell

    (2013)
  • G.E. Schepers et al.

    Twenty pairs of sox: Extent, homology, and nomenclature of the mouse and human sox transcription factor gene families

    Developmental Cell

    (2002)
  • P. Smits et al.

    The transcription factors L-Sox5 and Sox6 are essential for cartilage formation

    Developmental Cell

    (2001)
  • L. Topol et al.

    Sox9 inhibits Wnt signaling by promoting beta-catenin phosphorylation in the nucleus

    The Journal of Biological Chemistry

    (2009)
  • A. Zawerton et al.

    De novo SOX4 variants cause a neurodevelopmental diseases with mild dysmorphism

    American Journal of Human Genetics

    (2019)
  • H. Akiyama et al.

    The transcription factor Sox9 has essential roles in successive steps of the chondrocyte differentiation pathway and is required for expression of Sox5 and Sox6

    Genes & Development

    (2002)
  • H. Akiyama et al.

    Osteo-chondroprogenitor cells are derived from Sox9 expressing precursors

    Proceedings of the National Academy of Sciences of the United States of America

    (2005)
  • H. Akiyama et al.

    Interactions between Sox9 and beta-catenin control chondrocyte differentiation

    Genes & Development

    (2004)
  • R. Amarilio et al.

    HIF1alpha regulation of Sox9 is necessary to maintain differentiation of hypoxic prechondrogenic cells during early skeletogenesis

    Development

    (2007)
  • M.J. Barter et al.

    The long non-coding RNA ROCR contributes to SOX9 expression and chondrogenic differentiation of human mesenchymal stem cells

    Development

    (2017)
  • U. Basu-Roy et al.

    The transcription factor Sox2 is required for osteoblast self-renewal

    Cell Death and Differentiation

    (2010)
  • S. Benko et al.

    Highly conserved non-coding elements on either side of SOX9 associated with pierre robin sequence

    Nature Genetics

    (2009)
  • M. Bergsland et al.

    Sequentially acting Sox transcription factors in neural lineage development

    Genes & Development

    (2011)
  • P. Bhattaram et al.

    Inflammatory cytokines stabilize SOXC transcription factors to mediate the transformation of fibroblast-like synoviocytes in arthritic disease

    Arthritis & Rhematology

    (2018)
  • P. Bhattaram et al.

    SOXC proteins amplify canonical WNT signaling to secure nonchondrocytic fates in skeletogenesis

    The Journal of Cell Biology

    (2014)
  • P. Bhattaram et al.

    Organogenesis relies on SoxC transcription factors for the survival of neural and mesenchymal progenitors

    Nature Communications

    (2010)
  • W. Bi et al.

    Sox9 is required for cartilage formation

    Nature Genetics

    (1999)
  • E. Budd et al.

    MiR-146b is down-regulated during the chondrogenic differentiation of human bone marrow derived skeletal stem cells and up-regulated in osteoarthritis

    Scientific Reports

    (2017)
  • Z.Z. Chen et al.

    LncSox4 promotes the self-renewal of liver tumour-initiating cells through Stat3-mediated Sox4 expression

    Nature Communications

    (2016)
  • S. Corbani et al.

    Mild campomelic dysplasia: Report on a case and review

    Molecular Syndromology

    (2011)
  • P. Dy et al.

    The three SoxC proteins—Sox4, Sox11 and Sox12—exhibit overlapping expression patterns and molecular properties

    Nucleic Acids Research

    (2008)
  • A.T. Egunsola et al.

    Loss of DDRGK1 modulates SOX9 ubiquitination in spondyloepimetaphyseal dysplasia

    The Journal of Clinical Investigation

    (2017)
  • M. Franke et al.

    Formation of new chromatin domains determines pathogenicity of genomic duplications

    Nature

    (2016)
  • K. Giese et al.

    DNA-binding properties of the HMG domain of the lymphoid-specific transcriptional regulator LEF-1

    Genes & Development

    (1991)
  • N. Gonen et al.

    Sex reversal following deletion of a single distal enhancer of Sox9

    Science

    (2018)
  • C.T. Gordon et al.

    Long-range regulation at the SOX9 locus in development and disease

    Journal of Medical Genetics

    (2009)
  • M.D. Hall et al.

    Mesoderm-specific Stat3 deletion affects expression of Sox9 yielding Sox9-dependent phenotypes

    PLoS Genetics

    (2017)
  • K. Hata et al.

    Transcriptional network controlling endochondral ossification

    Journal of Bone Metabolism

    (2017)
  • T. Hattori et al.

    SOX9 is a major negative regulator of cartilage vascularization, bone marrow formation and endochondral ossification

    Development

    (2010)
  • X. He et al.

    AP-1 family members act with Sox9 to promote chondrocyte hypertrophy

    Development

    (2016)
  • A. Hempel et al.

    Deletions and de novo mutations of SOX11 are associated with a neurodevelopmental disorder with features of coffin-siris syndrome

    Journal of Medical Genetics

    (2016)
  • S.P. Henry et al.

    The postnatal role of Sox9 in cartilage

    Journal of Bone and Mineral Research

    (2012)
  • M. Hoser et al.

    Sox12 deletion in the mouse reveals nonreciprocal redundancy with the related Sox4 and Sox11 transcription factors

    Molecular and Cellular Biology

    (2008)
  • D.P. Hu et al.

    Cartilage to bone transformation during fracture healing is coordinated by the invading vasculature and induction of the core pluripotency genes

    Development

    (2017)
  • Cited by (74)

    View all citing articles on Scopus
    View full text