Chicken collagen X regulatory sequences restrict transgene expression to hypertrophic cartilage in mice

The first report of the presence of collagen X in mammalian dentinal matrix

Collagen X is an extracellular matrix protein, usually found in the hypertrophic cartilage destined to be mineralized. It is intimately associated with the mineralization process of the mammalian hard tissues, and particularly, regulating the compartmentalization of matrix components. Despite the fact that the dentine of the tooth is highly mineralized, there are no previous reports to indicate the presence of collagen X in this connective tissue. Here we report, for the first time, its presence in mammalian dentine based on micromorphological and immunohistochemical data. We hypothesize that the collagen X in dentine may in the long term arrest the progression of the mineralization front towards the soft tissue components of the pulp that are not destined to be mineralized.

How to build an inducible cartilage-specific transgenic mouse

Transgenic mice are used to study the roles of specific proteins in an intact living system. Use of transgenic mice to study processes in cartilage, however, poses some challenges. First of all, many factors involved in cartilage homeostasis and disease are also crucial factors in embryogenesis. Therefore, meddling with these factors often leads to death before birth, and mice who do survive cannot be considered normal. The build-up of cartilage in these mice is altered, making it nearly impossible to truly interpret the role of a protein in adult cartilage function.An elegant way to overcome these limitations is to make transgenic mice time- and tissue-specific, there by omitting side-effects in tissues other than cartilage and during embryology. This review discusses the potential building blocks for making an inducible cartilage-specific transgenic mouse. We review which promoters can be used to gain chondrocyte-specificity - all chondrocytes or a specific subset thereof - as well as different systems that can be used to enable inducibility of a transgene.

Defective endochondral ossification-derived matrix and bone cells alter the lymphopoietic niche in collagen X mouse models

Despite the appreciated interdependence of skeletal and hematopoietic development, the cell and matrix components of the hematopoietic niche remain to be fully defined. Utilizing mice with disrupted function of collagen X (ColX), a major hypertrophic cartilage matrix protein associated with endochondral ossification, our data identified a cytokine defect in trabecular bone cells at the chondro-osseous hematopoietic niche as a cause for aberrant B lymphopoiesis in these mice. Specifically, analysis of ColX transgenic and null mouse chondro-osseous regions via micro-computed tomography revealed an altered trabecular bone environment. Additionally, cocultures with hematopoietic and chondro-osseous cell types highlighted impaired hematopoietic support by ColX transgenic and null mouse derived trabecular bone cells. Further, cytokine arrays with conditioned media from the trabecular osteoblast cocultures suggested an aberrant hematopoietic cytokine milieu within the chondro-osseous niche of the ColX deficient mice. Accordingly, B lymphopoiesis was rescued in the ColX mouse derived trabecular osteoblast cocultures with interlukin-7, stem cell factor, and stromal derived factor-1 supplementation. Moreover, B cell development was restored in vivo after injections of interlukin-7. These data support our hypothesis that endrochondrally-derived trabecular bone cells and matrix constituents provide cytokine-rich niches for hematopoiesis. Furthermore, this study contributes to the emerging concept that niche defects may underlie certain immuno-osseous and hematopoietic disorders.

FoxA family members are crucial regulators of the hypertrophic chondrocyte differentiation program

During endochondral ossification, small, immature chondrocytes enlarge to form hypertrophic chondrocytes, which express collagen X. In this work, we demonstrate that FoxA factors are induced during chondrogenesis, bind to conserved binding sites in the collagen X enhancer, and can promote the expression of a collagen X-luciferase reporter in both chondrocytes and fibroblasts. In addition, we demonstrate by both gain- and loss-of-function analyses that FoxA factors play a crucial role in driving the expression of both endogenous collagen X and other hypertrophic chondrocyte-specific genes. Mice engineered to lack expression of both FoxA2 and FoxA3 in their chondrocytes display defects in chondrocyte hypertrophy, alkaline phosphatase expression, and mineralization in their sternebrae and, in addition, exhibit postnatal dwarfism that is coupled to significantly decreased expression of both collagen X and MMP13 in their growth plates. Our findings indicate that FoxA family members are crucial regulators of the hypertrophic chondrocyte differentiation program.

Altered matrix at the chondro-osseous junction leads to defects in lymphopoiesis

The collagen X transgenic and null (ColX-Tg/KO) mice have revealed a link between endochondral ossification (EO) and hematopoiesis, and thus serve as model systems to study hematopoietic niches. The altered collagen X function in ColX-Tg/KO mice resulted not only in skeletal defects, which included changes in growth plate ultrastructure, altered localization of heparan sulfate proteoglycans (HSPG), and reduced trabecular bone, but also in hematopoietic defects, which included reduced B lymphocyte numbers throughout life without associated increases in B cell apoptosis. Consequently, the ColX-Tg/KO mice exhibited diminished in vitro and in vivo immune responses. Moreover, reduced expression of several hematopoietic and B lymphopoietic cytokines were measured from ColX-KO-derived hypertrophic chondrocyte and trabecular osteoblast cultures. Together, these data expand the current hematopoietic niche model by including the EO-derived extracellular matrix, for example, the collagen X/HSPG network, as well as the EO-derived hypertrophic chondrocytes and trabecular osteoblasts as hematopoietic signal mediating cells.

Congenic mice confirm that collagen X is required for proper hematopoietic development

The link between endochondral skeletal development and hematopoiesis in the marrow was established in the collagen X transgenic (Tg) and null (KO) mice. Disrupted function of collagen X, a major hypertrophic cartilage matrix protein, resulted in skeletal and hematopoietic defects in endochondrally derived tissues. Manifestation of the disease phenotype was variable, ranging from perinatal lethality in a subset of mice, to altered lymphopoiesis and impaired immunity in the surviving mice. To exclude contribution of strain specific modifiers to this variable manifestation of the skeleto-hematopoietic phenotype, C57Bl/6 and DBA/2J collagen X congenic lines were established. Comparable disease manifestations confirmed that the skeleto-hematopoietic alterations are an inherent outcome of disrupted collagen X function. Further, colony forming cell assays, complete blood count analysis, serum antibody ELISA, and organ outgrowth studies established altered lymphopoiesis in all collagen X Tg and KO mice and implicated opportunistic infection as a contributor to the severe disease phenotype. These data support a model where endochondral ossification-specific collagen X contributes to the establishment of a hematopoietic niche at the chondro-osseous junction.

Heparan sulfate proteoglycans: a GAGgle of skeletal-hematopoietic regulators

This review summarizes our current understanding of the presence and function of heparan sulfate proteoglycans (HSPGs) in skeletal development and hematopoiesis. Although proteoglycans (PGs) comprise a large and diverse group of cell surface and matrix molecules, we chose to focus on HSPGs owing to their many proposed functions in skeletogenesis and hematopoiesis. Specifically, we discuss how HSPGs play predominant roles in establishing and regulating niches during skeleto-hematopoietic development by participating in distinct developmental processes such as patterning, compartmentalization, growth, differentiation, and maintenance of tissues. Special emphasis is placed on our novel hypothesis that mechanistically links endochondral skeletogenesis to the establishment of the hematopoietic stem cell (HSC) niche in the marrow. HSPGs may contribute to these developmental processes through their unique abilities to establish and mediate morphogen, growth factor, and cytokine gradients; facilitate signaling; provide structural stability to tissues; and act as molecular filters and barriers.

Altered endochondral ossification in collagen X mouse models leads to impaired immune responses

Disruption of collagen X function in hypertrophic cartilage undergoing endochondral ossification was previously linked to altered hematopoiesis in collagen X transgenic (Tg) and null (KO) mice (Jacenko et al., [2002] Am J Pathol 160:2019-2034). Mice displayed altered growth plates, diminished trabecular bone, and marrow hypoplasia with an aberrant lymphocyte profile throughout life. This study identifies altered B220+, CD4+, and CD8+ lymphocyte numbers, as well as CD4+/fox3P+ T regulatory cells in the collagen X mice. Additionally, diminished in vitro splenocyte responses to mitogens and an inability of mice to survive a challenge with Toxoplasma gondii, confirm impaired immune responses. In concert, ELISA and protein arrays identify aberrant levels of inflammatory, chemo-attractant, and matrix binding cytokines in collagen X mouse sera. These data link the disruption of collagen X function in the chondro-osseous junction to an altered hematopoietic stem cell niche in the marrow, resulting in impaired immune function.

Identification of a promoter element within the zebrafish colXalpha1 gene responsive to runx2 isoforms Osf2/Cbfa1 and til-1 but not to pebp2alphaA2

Type X collagen is a short chain collagen specifically expressed by hypertrophic chondrocytes during endochondral ossification. We report here the functional analysis of the zebrafish (Danio rerio) collagen Xalpha1 gene (colXalpha1) promoter with the identification of a region responsive to two isoforms of the runt domain transcription factor runx2. Furthermore, we provide evidence for the presence of dual promoter usage in zebrafish, a finding that should be important to further understanding of the regulation of its restricted tissue distribution and spatial-temporal expression during early development. The zebrafish colXalpha1 gene structure is comparable to that recently identified by comparative genomics in takifugu and shows homology with corresponding mammalian genes, indicating that its general architecture has been maintained throughout vertebrate evolution. Our data suggest that, as in mammals, runx2 plays a role in the development of the osteogenic lineage, supporting zebrafish as a model for studies of bone and cartilage development.

A highly conserved enhancer in mammalian type X collagen genes drives high levels of tissue-specific expression in hypertrophic cartilage in vitro and in vivo

Previously we have identified a cis-acting regulatory domain in the human type X collagen gene upstream of the transcription start site which acts as a strong enhancer in hypertrophic, but not in resting chondrocytes. Here we show that this enhancer is highly conserved also in the murine and bovine Col10a1 genes, but not found in the known promoter sequences of chicken Col10a1. It contains a functionally active AP-1 site (TPA Responsive Element, TRE) which is essential for the high transcriptional activity of the COL10A1 enhancer in transiently transfected hypertrophic chondrocytes. Gel-shift experiments with nuclear extracts of hypertrophic chondrocytes revealed FosB and Fra-1 as candidates regulating AP-1 factors binding to the TRE site. In fact, coexpression of FosB and Fra-1 in reporter gene assays greatly stimulated transcriptional activity of enhancer bearing reporter genes. Quantitative analysis of AP-1 factor mRNA levels in distinct fractions of fetal bovine epiphyseal chondrocytes by real-time PCR confirmed significant levels of FosB and Fra-1 mRNA besides other AP-1 factors in hypertrophic chondrocytes. A key role of the enhancer element in regulating tissue-specific expression of the Col10a1 gene was shown by establishing transgenic mouse lines with a reporter gene containing a 4.6 kb murine Col10a1 promoter fragment which included the enhancer, exon 1, part of exon 2 and the first intron. Reporter gene expression was seen exclusively in hypertrophic cartilages in the growth plates of long bones, ribs, vertebrae, sternum and mandibles of 17.5-18.5 dpc embryos, confirming that the 4.6 kb promoter is able to drive specific expression of Col10a1 in hypertrophic cartilage. These established transgenic lines should facilitate the genetic analysis of regulatory pathways of chondrocyte maturation and Col10a1 gene expression in the future.

SP3/SP1 transcription activity regulates specific expression of collagen type X in hypertrophic chondrocytes

Previously, we have shown that two non-canonical specificity protein (SP)-binding sites within the proximal promoter (nucleotide (nt) -139 to +5) of the chicken Col10a1 gene are involved in conferring tissue-specific expression of type X collagen to hypertrophic chondrocytes. In the present study, we examined the role of SP3/SP1 transcription factors in the regulation of the Col10a1 promoter. The SP3/SP1 ratio is higher in hypertrophic versus non-hypertrophic chondrocytes, due to the significant decrease in SP1 in hypertrophic cells detected by real-time PCR and Western blot analyses. Functional analyses by transfection-mediated overexpression of SP1 and SP3 suggest that SP1 inhibits the Col10a1 promoter. This effect is negated by an interaction with SP3 in hypertrophic chondrocytes. Additionally, mutation analysis showed that the 40-bp intervening sequence (nt -115 to -75) is required for expression of the Col10a1 gene. In this sequence, a binding site for Dlx5/6 transcription factors (nt -99 to -87) retards a protein specific for hypertrophic chondrocytes in electrophoretic mobility shift assay. Endogenous levels of Dlx5 are 3-fold higher in hypertrophic versus non-hypertrophic cells by real-time PCR analysis, and overexpression of Dlx5 in non-hypertrophic chondrocytes activates the proximal Col10a1 promoter 3-fold. These results indicate that the SP3/SP1 ratio and Dlx5 are important regulators of the proximal Col10a1 promoter in hypertrophic cartilage and suggest that interactions between SP3 and SP1 regulate expression of different types of collagen during chondrocyte differentiation.