Type IX collagen gene expression during limb cartilage differentiation

Prospective isolation of chondroprogenitors from human iPSCs based on cell surface markers identified using a CRISPR-Cas9-generated reporter

Background

Articular cartilage shows little or no capacity for intrinsic repair, generating a critical need of regenerative therapies for joint injuries and diseases such as osteoarthritis. Human-induced pluripotent stem cells (hiPSCs) offer a promising cell source for cartilage tissue engineering and in vitro human disease modeling; however, off-target differentiation remains a challenge during hiPSC chondrogenesis. Therefore, the objective of this study was to identify cell surface markers that define the true chondroprogenitor population and use these markers to purify iPSCs as a means of improving the homogeneity and efficiency of hiPSC chondrogenic differentiation.

Methods

We used a CRISPR-Cas9-edited COL2A1-GFP knock-in reporter hiPSC line, coupled with a surface marker screen, to identify a novel chondroprogenitor population. Single-cell RNA sequencing was then used to analyze the distinct clusters within the population. An unpaired t test with Welch’s correction or an unpaired Kolmogorov-Smirnov test was performed with significance reported at a 95% confidence interval.

Results

Chondroprogenitors expressing CD146, CD166, and PDGFRβ, but not CD45, made up an average of 16.8% of the total population. Under chondrogenic culture conditions, these triple-positive chondroprogenitor cells demonstrated decreased heterogeneity as measured by single-cell RNA sequencing with fewer clusters (9 clusters in unsorted vs. 6 in sorted populations) closer together. Additionally, there was more robust and homogenous matrix production (unsorted: 1.5 ng/ng vs. sorted: 19.9 ng/ng sGAG/DNA; p < 0.001) with significantly higher chondrogenic gene expression (i.e., SOX9, COL2A1, ACAN; p < 0.05).

Conclusions

Overall, this study has identified a unique hiPSC-derived subpopulation of chondroprogenitors that are CD146⁺/CD166⁺/PDGFRβ⁺/CD45⁻ and exhibit high chondrogenic potential, providing a purified cell source for cartilage tissue engineering or disease modeling studies.

Fibronectin matrix assembly is essential for cell condensation during chondrogenesis

Mesenchymal cell condensation is the initiating event in endochondral bone formation. Cell condensation is followed by differentiation into chondrocytes, which is accompanied by induction of chondrogenic gene expression. Gene mutations involved in chondrogenesis cause chondrodysplasias and other skeletal defects. Using mesenchymal stem cells (MSCs) in an in vitro chondrogenesis assay, we found that knockdown of the diastrophic dysplasia (DTD) sulfate transporter (DTDST, also known as SLC26A2), which is required for normal cartilage development, blocked cell condensation and caused a significant reduction in fibronectin matrix. Knockdown of fibronectin with small interfering RNAs (siRNAs) also blocked condensation. Fibrillar fibronectin matrix was detected prior to cell condensation, and its levels increased during and after condensation. Inhibition of fibronectin matrix assembly by use of the functional upstream domain (FUD) of adhesin F1 from Streptococcus pyogenes prevented cell condensation by MSCs and also by the chondrogenic cell line ATDC5. Our data show that cell condensation and induction of chondrogenesis depend on fibronectin matrix assembly and DTDST, and indicate that this transporter is required earlier in chondrogenesis than previously appreciated. They also raise the possibility that certain of the skeletal defects in DTD patients might derive from the link between DTDST, fibronectin matrix and condensation.

Growth and differentiation of a long bone in limb development, repair and regeneration

Repair from traumatic bone fracture is a complex process that includes mechanisms of bone development and bone homeostasis. Thus, elucidation of the cellular/molecular basis of bone formation in skeletal development would provide valuable information on fracture repair and would lead to successful skeletal regeneration after limb amputation, which never occurs in mammals. Elucidation of the basis of epimorphic limb regeneration in amphibians would also provide insights into skeletal regeneration in mammals, since the epimorphic regeneration enables an amputated limb to re-develop the three-dimensional structure of bones. In the processes of bone development, repair and regeneration, growth of the bone is achieved through several events including not only cell proliferation but also aggregation of mesenchymal cells, enlargement of cells, deposition and accumulation of extracellular matrix, and bone remodeling.

Fibronectin and stem cell differentiation - lessons from chondrogenesis

The extracellular matrix (ECM) is an intricate network of proteins that surrounds cells and has a central role in establishing an environment that is conducive to tissue-specific cell functions. In the case of stem cells, this environment is the stem cell niche, where ECM signals participate in cell fate decisions. In this Commentary, we describe how changes in ECM composition and mechanical properties can affect cell shape and stem cell differentiation. Using chondrogenic differentiation as a model, we examine the changes in the ECM that occur before and during mesenchymal stem cell differentiation. In particular, we focus on the main ECM protein fibronectin, its temporal expression pattern during chondrogenic differentiation, its potential effects on functions of differentiating chondrocytes, and how its interactions with other ECM components might affect cartilage development. Finally, we discuss data that support the possibility that the fibronectin matrix has an instructive role in directing cells through the condensation, proliferation and/or differentiation stages of cartilage formation.

Articular cartilage development: a molecular perspective

In this article, development of articular cartilage and endochondral ossification is reviewed, from the perspective of both morphologic aspects of histogenesis and molecular biology, particularly with respect to key signaling molecules and extracellular matrix components most active in cartilage development. The current understanding of the roles of transforming growth factor β and associated signaling molecules, bone morphogenic proteins, and molecules of the Wnt-β catenin system in chondrogenesis are described. Articular cartilage development is a highly conserved complex biological process that is dynamic and robust in nature, which proceeds well without incident or failure in all joints of most young growing individuals.

The transcription factor AP-2ɛ regulates CXCL1 during cartilage development and in osteoarthritis

OBJECTIVE

Recently, the transcription factor AP-2ɛ was shown to be a regulator of hypertrophy in cartilage and to be differentially expressed in osteoarthritis (OA). However, the only known target gene of AP-2ɛ up to date is integrin alpha10. To better characterize the function of AP-2ɛ in cartilage we screened for additional target genes.

DESIGN

Promoter analysis, ChIP-assays and electrophoretic mobility shift assay were used to characterize the regulation of a new AP-2ɛ target gene in detail.

RESULTS

In this study, we determined the chemokine CXCL1, already known to be important in osteoarthritis (OA), as a new target gene of AP-2ɛ. We could confirm that CXCL1 is expressed in chondrocytes and significantly over-expressed in OA-chondrocytes. Transient transfection of chondrocytes with an AP-2ɛ expression construct led to a significant increase of the CXCL1 mRNA level in these cells. We identified three potential AP-2 binding sites within the CXCL1 promoter and performed luciferase assays, indicating that an AP-2 binding motif (AP-2.2) ranging from position -135 to -144 bp relative to the translation start is responsive to AP-2ɛ. This result was further addressed by site-directed mutagenesis demonstrating that activation of the CXCL1 promoter by AP-2ɛ is exclusively dependent on AP-2.2. Chromatin immunoprecipitation and electromobility shift assays confirmed the direct binding of AP-2ɛ to the CXCL1 promoter in OA-chondrocytes at this site.

CONCLUSION

These findings revealed CXCL1 as a novel target gene of AP-2ɛ in chondrocytes and support the important role of AP-2ɛ in cartilage.

Direct and progressive differentiation of human embryonic stem cells into the chondrogenic lineage

Treatment of common and debilitating degenerative cartilage diseases particularly osteoarthritis is a clinical challenge because of the limited capacity of the tissue for self-repair. Because of their unlimited capacity for self-renewal and ability to differentiate into multiple lineages, human embryonic stem cells (hESCs) are a potentially powerful tool for repair of cartilage defects. The primary objective of the present study was to develop culture systems and conditions that enable hESCs to directly and uniformly differentiate into the chondrogenic lineage without prior embryoid body (EB) formation, since the inherent cellular heterogeneity of EBs hinders obtaining homogeneous populations of chondrogenic cells that can be used for cartilage repair. To this end, we have subjected undifferentiated pluripotent hESCs to the high density micromass culture conditions we have extensively used to direct the differentiation of embryonic limb bud mesenchymal cells into chondrocytes. We report that micromass cultures of pluripotent hESCs undergo direct, rapid, progressive, and substantially uniform chondrogenic differentiation in the presence of BMP2 or a combination of BMP2 and TGF-beta1, signaling molecules that act in concert to regulate chondrogenesis in the developing limb. The gene expression profiles of hESC-derived cultures harvested at various times during the progression of their differentiation has enabled us to identify cultures comprising cells in different phases of the chondrogenic lineage ranging from cultures just entering the lineage to well differentiated chondrocytes. Thus, we are poised to compare the abilities of hESC-derived progenitors in different phases of the chondrogenic lineage for cartilage repair.

Studies on the role of Cux1 in regulation of the onset of joint formation in the developing limb

Joint formation, the onset of which is characterized by the segmentation of continuous skeletal rudiments into two or more separate elements, is a fundamental aspect of limb pattern formation, playing a critical role in determining the size, shape, and number of individual skeletal elements. Joint formation is initiated by conversion of differentiated chondrocytes at sites of presumptive joints into densely packed nonchondrogenic cells of the joint interzone. This conversion is accompanied by loss of Alcian blue-staining cartilage matrix and downregulation of cartilage-specific gene expression. Here, we report that Cux1, which encodes a transcription factor containing a homeodomain and other DNA-binding motifs, is highly expressed at all of the discrete sites of incipient joint formation in the developing limb concomitant with conversion of differentiated chondrocytes into interzone tissue. Moreover, differentiated limb chondrocytes in micromass cultures infected with a Cux1 retroviral expression vector are converted into nonchondrogenic cells which exhibit loss of Alcian blue cartilage matrix and downregulation of cartilage-specific gene expression as occurs at the onset of normal joint formation. These results suggest that Cux1 is involved in regulating the onset of joint formation by facilitating conversion of chondrocytes into nonchondrogenic cells of the interzone.

Roles of insulin-like growth factor-I (IGF-I) and IGF-I binding protein-2 (IGFBP2) and -5 (IGFBP5) in developing chick limbs

Insulin-like growth factor-I (IGF-I) and the IGF-I binding proteins (IGFBPs) which modulate IGF-I action have been implicated in the development of the vertebrate limbs and skeleton. We have examined the distribution of IGF-I, IGFBP2 and IGFBP5 in developing chick limb buds and have investigated their functional roles and relationships during chick limb development. IGF-I and IGFBP2 are co-expressed throughout the lateral plate from which limbs form, although IGFBP2, unlike IGF-I, does not promote formation of rudimentary limb buds from non-limb-forming flank regions in vitro. During limb outgrowth, IGF-I is present in non-AER limb ectoderm, but little IGF-I is present in the AER itself, suggesting that restriction of endogenous IGF-I activity may be required for proper AER function. Consistent with this possibility, the ectoderm of mutant limbless and wingless wing buds, which fail to form an AER, continues to express IGF-I. We also found that the AER contains abundant IGFBP2 but that IGFBP2 is not present in limb subridge mesoderm. In contrast, IGFBP2 is present in the distal mesoderm of mutant limbless or wingless limb buds, which fail to grow out. This suggests that attenuation of IGFBP2 expression is controlled by the AER and that cessation of IGFBP2 expression may be necessary for the proliferation and suppression of differentiation of subridge mesoderm that is required for limb outgrowth to occur. Consistent with this possibility, we found that exogenous IGFBP2 inhibits the anti-differentiative activity of the AER in vitro. We also found that regions of cell death in the limb contain abundant IGF-I-immunoreactive cells, consistent with a role for IGF-I in apoptosis. During skeletogenesis, IGF-I and IGFBP2 are co-localized to the condensing central core of the limb, implicating these factors as potential regulators of the onset of chondrogenic differentiation. Intriguingly, we found that IGF-I and IGFBP2 have opposing effects on chondrogenesis, as IGF-I stimulates but IGFBP2 inhibits accumulation of cartilage matrix by micromass cultures in vitro. Long [R(3)] IGF-I, an analog of IGF-I that cannot bind IGFBPs, is more effective than IGF-I in stimulating matrix accumulation, consistent with a negative role for IGFBP2 in chondrogenesis. As the chondrocytes of the limb mature, IGF-I is present only in terminal hypertrophic chondrocytes, which undergo programmed cell death, while IGFBP2 becomes localized to prehypertrophic and hypertrophic chondrocytes, suggesting involvement in chondrocyte maturation. Consistent with this possibility, we found that exogenous IGFBP2 induces precocious expression of Indian hedgehog, a marker of prehypertrophy, in maturing chondrocytes in vitro. IGF-I and IGFBP2 are also present in the osteoblasts, clasts and nascent matrix of the long bones, consistent with roles in endochondral bone formation. Unlike in rodent limbs, IGFBP5 is not expressed by chick limb ectoderm or AER. IGFBP5 expression is highly localized to developing limb musculature and, later, to the developing skeletal elements where it is expressed by osteoblast precursers and osteoblasts. The results of this study suggest potential novel roles for IGF-I and IGFBP2 in several aspects of limb development including limb outgrowth and AER activity, programmed cell death, chondrogenesis and chondrocyte maturation.

Cellular interactions and signaling in cartilage development

The long bones of the developing skeleton, such as those of the limb, arise from the process of endochondral ossification, where cartilage serves as the initial anlage element and is later replaced by bone. One of the earliest events of embryonic limb development is cellular condensation, whereby pre-cartilage mesenchymal cells aggregate as a result of specific cell-cell interactions, a requisite step in the chondrogenic pathway. In this review an extensive examination of historical and recent literature pertaining to limb development and mesenchymal condensation has been undertaken. Topics reviewed include limb initiation and axial induction, mesenchymal condensation and its regulation by various adhesion molecules, and regulation of chondrocyte differentiation and limb patterning. The complexity of limb development is exemplified by the involvement of multiple growth factors and morphogens such as Wnts, transforming growth factor-beta and fibroblast growth factors, as well as condensation events mediated by both cell-cell (neural cadherin and neural cell adhesion molecule) and cell-matrix adhesion (fibronectin, proteoglycans and collagens), as well as numerous intracellular signaling pathways transduced by integrins, mitogen activated protein kinases, protein kinase C, lipid metabolites and cyclic adenosine monophosphate. Furthermore, information pertaining to limb patterning and the functional importance of Hox genes and various other signaling molecules such as radical fringe, engrailed, Sox-9, and the Hedgehog family is reviewed. The exquisite three-dimensional structure of the vertebrate limb represents the culmination of these highly orchestrated and strictly regulated events. Understanding the development of cartilage should provide insights into mechanisms underlying the biology of both normal and pathologic (e.g. osteoarthritis) adult cartilage.

Syndecan-3 in limb skeletal development

Syndecan-3 is a member of a family of heparan sulfate proteoglycans that function as extracellular matrix receptors and as co-receptors for growth factors and signalling molecules. A variety of studies indicate that syndecan-3 is involved in several aspects of limb morphogenesis and skeletal development. Syndecan-3 participates in limb outgrowth and proliferation in response to the apical ectodermal ridge; mediates cell-matrix and/or cell-cell interactions involved in regulating the onset of chondrogenesis; may be involved in regulating the onset of osteogenesis and joint formation and, plays a role in regulating the proliferation of epiphyseal chondrocytes during endochondral ossification.

Chondrogenesis in a cell-polymer-bioreactor system

Chondrogenesis was studied under controlled in vitro conditions using a cell-polymer-bioreactor system. Bovine calf articular chondrocytes were seeded onto biodegradable polymer scaffolds and cultured in rotating bioreactor vessels. Concomitant increases in the amounts of glycosaminoglycan (GAG) and type II collagen resulted in cell-polymer constructs with continuous cartilaginous matrix over their entire cross sections (6.7 mm diameter x 5 mm thick) after 40 days of cultivation. As compared to natural calf cartilage, constructs had comparable cellularities, 68% as much GAG and 33% as much type II collagen per gram wet weight. The progression of chondrogenesis in chondrocyte-polymer constructs was similar to that suggested previously for precursor cells in vitro and developing limbs in vivo. In particular, the polymer scaffold provided a three-dimensional structure that could be seeded with chondrocytes at high cell densities in order to establish cell-to-cell contacts and initiate cartilage tissue development, whereas the bioreactor vessel provided a permissive microenvironment for chondrogenesis. This work demonstrates the promise of using tissue engineered constructs for in vitro studies of cell interactions and differentiation.

Inhibition of in vitro limb cartilage differentiation by syndecan-3 antibodies

The transmembrane heparan sulfate proteoglycan syndecan-3 is transiently expressed in high amounts during the cellular condensation process that characterizes the onset of limb cartilage differentiation. During condensation, limb mesenchymal cells become closely juxtaposed and undergo cell-cell and cell-matrix interactions that are necessary to trigger cartilage differentiation and cartilage-specific gene expression. To test directly the possible involvement of syndecan-3 in regulating the onset of limb chondrogenesis, we examined the effect of polyclonal antibodies against a syndecan-3 fusion protein on the chondrogenic differentiation of chick limb mesenchymal cells in micromass culture. Syndecan-3 antiserum elicits a dose-dependent inhibition of the accumulation of Alcian blue-stainable cartilage matrix by high density limb mesenchymal cell micromass cultures (2 x 10(5) cells/10 microliters) and a corresponding reduction in steady-state levels of mRNAs for cartilage-characteristic type II collagen and the core protein of the cartilage proteoglycan aggrecan. In preimmune serum-treated control cultures proliferating cells are limited to the periphery of areas of cartilage matrix deposition, whereas large numbers of proliferating cells are uniformly distributed throughout the undifferentiated cultures supplemented with syndecan-3 antiserum. Limb mesenchymal cells cultured at lower densities (1 x 10(5) cells/10 microliters) in the presence of preimmune serum form extensive precartilage condensations characterized by the close juxtaposition of rounded cells by day 2 of culture. In contrast, in the presence of syndecan-3 antiserum, the cells fail to aggregate but rather remain flattened and spatially separated from one another, suggeting that syndecan-3 antibodies impair the formation of precartilage condensations. These results indicate that syndecan-3 plays an important role in regulating the onset of limb chondrogenesis, perhaps by mediating the cell-cell and cell-matrix interactions required for condensation and subsequent cartilage differentiation.

The expression pattern of the Distal-less homeobox-containing gene Dlx-5 in the developing chick limb bud suggests its involvement in apical ectodermal ridge activity, pattern formation, and cartilage differentiation

Here we report the isolation from a chick limb bud cDNA library of a cDNA that contains the full coding sequence of chicken Dlx-5, a member of the Distal-less (Dlx) family of homeobox-containing genes that encode homeodomains highly similar to that of the Drosophila Distal-less gene, a gene that is required for limb development in the Drosophila embryo. The expression pattern of Dlx-5 in the developing chick limb bud suggests that it may be involved in several aspects of limb morphogenesis. Dlx-5 is expressed in the apical ectodermal ridge (AER) which directs the outgrowth and patterning of underlying limb mesoderm. During early limb development Dlx-5 is also expressed in the mesoderm at the anterior margin of the limb bud and in a discrete group of mesodermal cells at the mid-proximal posterior margin that corresponds to the posterior necrotic zone. These mesodermal domains of Dlx-5 expression roughly correspond to the anterior and posterior boundaries of the progress zone, the group of highly proliferating undifferentiated mesodermal cells underneath the AER that will give rise to the skeletal elements of the limb and associated structures. The AER and anterior and posterior mesodermal domains of Dlx-5 expression are regions in which the homeobox-containing gene Msx-2 is also highly expressed, suggesting that Dlx-5 and Msx-2 might be involved in regulatory networks that control AER activity and demarcate the progress zone. In addition, Dlx-5 is expressed in high amounts by the differentiating cartilaginous skeletal elements of the limb, suggesting it may be involved in regulating the onset of limb cartilage differentiation.

The membranous skeleton: the role of cell condensations in vertebrate skeletogenesis

Elements of the vertebrate skeleton are initiated as cell condensations, collectively termed the 'membranous skeleton' whether cartilages or bones by Grüneberg (1963). Condensations, which were identified as the basic cellular units in a recent model of morphological change in development and evolution (Atchley and Hall 1991) are reviewed in this paper. Condensations are initiated either by increased mitotic activity or by aggregation of cells towards a centre. Prechondrogenic (limb bud) and preosteogenic (scleral ossicle) condensations are discussed and contrasted. Both types of skeletogenic condensations arise following epithelial-mesenchymal interactions; condensations are identified as the first cellular product of such tissue interactions. Molecular characteristics of condensations are discussed, including peanut agglutinin lectin, which is used to visualize prechondrogenic condensations, and hyaluronan, hyaladherins, heparan sulphate proteoglycan, chondroitin sulphate proteoglycan, versican, tenascin, syndecan, N-CAM, alkaline phosphatase, retinoic acid and homeo-box-containing genes. The importance for the initiation of chondrogenesis or osteogenesis of upper and lower limits to condensation size and the numbers of cells in a condensation are discussed, as illustrated by in vitro studies and by mutant embryos, including Talpid3 in the chick and Brachypod, Congenital hydrocephalus and Phocomelia in the mouse. Evidence that genes specific to the skeletal type are selectively activated at condensation is discussed, as is a recent model involving TGF-beta and fibronectin in condensation formation. Condensations emerge as a pivotal stage in initiation of the vertebrate skeleton in embryonic development and in the modification of skeletal morphology during evolution.

Syndecan 3: a member of the syndecan family of membrane-intercalated proteoglycans that is expressed in high amounts at the onset of chicken limb cartilage differentiation

A partial cDNA that encodes a newly discovered member of the syndecan family of integral membrane proteoglycans, which we have termed syndecan 3, has been isolated from an embryonic chicken limb bud cDNA library. Syndecan 3 is distinct from but structurally related to syndecan and fibroglycan, two previously characterized members of this family of membrane-intercalated proteoglycans. Syndecan 3 contains a cytoplasmic domain potentially associated with the cytoskeleton that is 85% identical in amino acid sequence to the cytoplasmic domain of syndecan. Syndecan 3 also possesses a hydrophobic transmembrane domain and an extracellular domain containing several clustered potential glycosaminoglycan attachment sites. Like syndecan, the ectodomain of syndecan 3 has a single dibasic protease-susceptible site adjacent to the transmembrane domain, which might be involved in shedding the ectodomain from the cell surface. A striking feature of syndecan 3 is an extensive (182 amino acid) threonine, serine, and proline (T+S+P)-rich domain that closely resembles T+S+P-rich regions in several mucin-like proteins in which O-linked oligosaccharides are bound to the threonine and serine residues. Syndecan 3 is expressed in high amounts during a critical phase of chicken limb chondrogenesis in which limb mesenchymal cells condense, round up, and interact with one another before depositing a cartilage matrix. The multiple functional domains of syndecan 3 provide potential sites for mediating the adhesive cell-matrix interactions and cytoskeletal reorganization involved in this critical condensation process.