The role of TGFβs and Sox9 during limb chondrogenesis
Introduction
The formation of the skeleton in vertebrates involves the differentiation of mesenchymal cells to cartilage. This process, called chondrogenesis, is a tightly regulated event involving multiple steps, including condensation of the precartilagineous mesenchyme, commitment to the chondrogenic lineage, and differentiation into chondroblasts and, eventually, into chondrocytes. All these events are regulated by the concerted action of extracellular and intracellular cues, including extracellular factors specific for cartilage differentiation (e.g. type IIa1 Collagen and Aggrecan), and more general pathways that, while not exclusively specific to this process, play an important role at various stages of chondrogenesis, for example the bone morphogenetic protein (BMP), Hedgehog, Wnt and fibroblast growth factor pathways. The intracellular events elicited by the activation of these and other pathways lead to the transcriptional regulation of chondrogenesis-specific genes (e.g. the gene for Sox9), enabling the embryonic differentiation of mesenchymal cells towards chondrocytes. Here, using the vertebrate developing limb as a paradigm, we will review recent studies on how members of the transforming growth factor β (TGFβ) superfamily and Sox9 regulate the early process of chondrogenesis.
Section snippets
Extracellular regulation of chondrogenesis by members of the TGFβ superfamily
TGFβ-related proteins form a large family of secreted molecules including, among others, TGFβs, activins and BMPs [1]. These molecules form either homodimers or heterodimers, and exert their activity through type I and type II serine/threonine kinase receptors. These receptors are grouped into a large family that includes BMP and activin receptors.
The fact that distinct ligands can bind and activate multiple receptors in a context-dependent manner [2], points towards the importance of TGFβ
Intracellular regulation of chondrogenesis by Sox9
Among several intracellular factors, Sox9, a transcription factor, is a key molecule in early chondrogenesis. Sox9 belongs to the SRY (sex-determining region on the Y chromosome) family and contains the HMG (high mobility group) box DNA binding domain. Sox9 is expressed in association with chondrogenic aggregates (Figure 1) [13, 14] and a variety of mouse genetic studies, as well as human mutations, have shown that Sox9 is a master regulator of chondrogenesis. Sox9+/- mouse embryos die
Regulation of chondrogenesis-specific expression of Sox9
Given that Sox9-expressing cells mark osteochondroprogenitors as well as progenitors in a variety of tissues, it is of great interest to elucidate how Sox9 expression is regulated. Identification of chondroprogenitor-specific regulatory elements would shed light on how chondroprogenitors emerge from undifferentiated mesenchymal cells, and might eventually help to guide therapeutic approaches to artificially engineer chondrocyte differentiation.
Several lines of evidence indicate that BMP
Sox9 post-translational regulation
Increasing evidence suggests that the levels of Sox9 protein are to be strictly controlled in order for normal chondrogenesis to occur. A Sox9 gain-of-function experiment in which the Sox9 transcript level was increased by ∼20% caused dwarfism [18•], and conversely, 50% downregulation in Sox9+/− mice and human campomelic dysplasia patients resulted in chondrodysplasia [15].
Sox9 protein levels are controlled via a self-regulated balance between degradation and synthesis of protein, maintained by
Transcriptional complex of Sox9 regulates chondrogenesis
Given that Sox9 acts as a transcriptional activator, not only its modification but also its molecular interactions may help to regulate its transcriptional activity [40]. Several cofactors have been shown to interact with Sox9 through its transactivation domain. Among them, p300/CBP, a general coactivator, has been reported to interact with and enhance the transcriptional activity of Sox9 in vitro [41]. These proteins are shown to act synergistically on artificially chromatinized DNA and elicit
Conclusions
Differentiation of limb mesenchymal cells into cartilage is a well-studied model system of vertebrate embryogenesis, in which condensation, lineage determination towards chondroblasts and subsequent differentiation into chondrocytes are regulated in time and space to allow definitive bone formation. We have reviewed our current knowledge on the role of several extracellular members of the TGFβ family of secreted factors, together with the intracellular activities modulated by Sox9 during the
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We apologize for the many original references that cannot be cited within the limitations of this review. We want to acknowledge Dr. Masanao Tsuda for discussions and May Schwartz for help in the preparation of this manuscript. JRL is supported by a grant from Instituto de Salud Carlos III Health Institute. Work in our laboratories is supported by the Cellex and G. Harold and Leila Y. Mathers Charitable Foundations, the Health Ministries of Spain and Catalonia, and the NIH.
References (46)
- et al.
Mechanisms of TGF-β signaling from cell membrane to the nucleus
Cell
(2003) - et al.
Specificity and versatility in tgf-β signaling through Smads
Annu Rev Cell Dev Biol
(2005) - et al.
Control of digit formation by activin signalling
Development
(1999) - et al.
Regulation by members of the transforming growth factor beta superfamily of the digital and interdigital fates of the autopodial limb mesoderm
Cell Tissue Res
(1999) - et al.
A new role for BMP5 during limb development acting through the synergic activation of Smad and MAPK pathways
Dev Biol
(2004) - et al.
In vivo evidence that BMP signaling is necessary for apoptosis in the mouse limb
Dev Biol
(2002) - et al.
Programmed cell death in the embryonic vertebrate limb
Semin Cell Dev Biol
(2005) - et al.
Bone morphogenetic protein signaling is required for maintenance of differentiated phenotype, control of proliferation, and hypertrophy in chondrocytes
J Cell Biol
(1998) - et al.
The type I BMP receptor BMPRIB is required for chondrogenesis in the mouse limb
Development
(2000) - et al.
Combinatorial signaling through BMP receptor IB and GDF5: shaping of the distal mouse limb and the genetics of distal limb diversity
Development
(2000)
BMP receptor type IA in limb bud mesenchyme regulates distal outgrowth and patterning
Dev Biol
Bmpr1a and Bmpr1b have overlapping functions and are essential for chondrogenesis in vivo
Proc Natl Acad Sci USA
The Sry-related gene Sox9 is expressed during chondrogenesis in mouse embryos
Nat Genet
Expression of the chicken Sox9 gene marks the onset of cartilage differentiation
Ann N Y Acad Sci
Haploinsufficiency of Sox9 results in defective cartilage primordia and premature skeletal mineralization
Proc Natl Acad Sci USA
Deletion of long-range regulatory elements upstream of SOX9 causes campomelic dysplasia
Proc Natl Acad Sci USA
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 Dev
Interactions between Sox9 and β-catenin control chondrocyte differentiation
Genes Dev
Osteo-chondroprogenitor cells are derived from Sox9 expressing precursors
Proc Natl Acad Sci USA
Regulation of skeletal development by the Runx family of transcription factors
J Cell Biochem
Hierarchy revealed in the specification of three skeletal fates by Sox9 and Runx2
Dev Biol
Canonical Wnt signaling in differentiated osteoblasts controls osteoclast differentiation
Dev Cell
Canonical Wnt/β-catenin signaling prevents osteoblasts from differentiating into chondrocytes
Dev Cell
Cited by (139)
Comparison of chondro-inductivity between collagen and hyaluronic acid hydrogel based on chemical/physical microenvironment
2021, International Journal of Biological MacromoleculesGenerating adipose stem cell-laden hyaluronic acid-based scaffolds using 3D bioprinting via the double crosslinked strategy for chondrogenesis
2021, Materials Science and Engineering CEngineering the cellular mechanical microenvironment to regulate stem cell chondrogenesis: Insights from a microgel model
2020, Acta BiomaterialiaCitation Excerpt :In Gel-HA (L), “TGF-β/Smad signaling pathway” and “Hippo signaling pathway” are activated whilst “Integrins/YAP/ TAZ signaling pathway” is inhibited. Specifically, the up-regulated TGF-β/Smad2/3 and BMPs/ Smad1/5 genes further activate downstream Sox9 and Sox5/6 in TGF-β/Smad signaling pathway, resulting in the up-regulation of hyaline cartilage-related genes like Acan, COMP and Col2a1 [59,86-88]. Comparatively, the up-regulated Scrimbble and Nf2 genes activate Hippo signaling pathway [60,61,89] and then the up-regulation of MST1/2 and LATS1/2, which causes phosphorylation and degradation of YAP/TAZ in the cytoplasm [90].
An Osteocartilaginous 3D Printing Implant Using a Biocompatible Polymer and Pre-Differentiated Mesenchymal Stem Cells in Sheep
2023, Applied Sciences (Switzerland)