BAC constructs in transgenic reporter mouse lines control efficient and specific LacZ expression in hypertrophic chondrocytes under the complete Col10a1 promoter

The role of hypertrophic chondrocytes in regulation of the cartilage-to-bone transition in fracture healing

Endochondral bone formation is an important pathway in fracture healing, involving the formation of a cartilaginous soft callus and the process of cartilage-to-bone transition. Failure or delay in the cartilage-to-bone transition causes an impaired bony union such as nonunion or delayed union. During the healing process, multiple types of cells including chondrocytes, osteoprogenitors, osteoblasts, and endothelial cells coexist in the callus, and inevitably crosstalk with each other. Hypertrophic chondrocytes located between soft cartilaginous callus and bony hard callus mediate the crosstalk regulating cell-matrix degradation, vascularization, osteoclast recruitment, and osteoblast differentiation in autocrine and paracrine manners. Furthermore, hypertrophic chondrocytes can become osteoprogenitors and osteoblasts, and directly contribute to woven bone formation. In this review, we focus on the roles of hypertrophic chondrocytes in fracture healing and dissect the intermingled crosstalk in fracture callus during the cartilage-to-bone transition.

Role and application of stem cells in dental regeneration: A comprehensive overview

Recently, a growing attention has been observed toward potential advantages of stem cell (SC)-based therapies in regenerative treatments. Mesenchymal stem/stromal cells (MSCs) are now considered excellent candidates for tissue replacement therapies and tissue engineering. Autologous MSCs importantly contribute to the state-of-the-art clinical strategies for SC-based alveolar bone regeneration. The donor cells and immune cells play a prominent role in determining the clinical success of MSCs therapy. In line with the promising future that stem cell therapy has shown for tissue engineering applications, dental stem cells have also attracted the attention of the relevant researchers in recent years. The current literature review aims to survey the variety and extension of SC-application in tissue-regenerative dentistry. In this regard, the relevant English written literature was searched using keywords: "tissue engineering", "stem cells", "dental stem cells", and "dentistry strategies". According to the available database, SCs application has become increasingly widespread because of its accessibility, plasticity, and high proliferative ability. Among the growing recognized niches and tissues containing higher SCs, dental tissues are evidenced to be rich sources of MSCs. According to the literature, dental SCs are mostly present in the dental pulp, periodontal ligament, and dental follicle tissues. In this regard, the present review has described the recent findings on the potential of dental stem cells to be used in tissue regeneration.

Hypertrophic chondrocyte-specific Col10a1 controlling elements in Cre recombinase transgenic studies

The type X collagen gene (COL10A1) is specifically expressed in chondrocytes undergoing hypertrophy, which is an essential late stage of endochondral ossification during the development of long bones. We have previously localized multiple murine Col10a1 promoter-enhancer elements and used these elements for transgenic studies with LacZ reporter gene or genes of interest. Here, we report two additional transgenic mouse lines in which Cre was driven by the 10 kb Col10a1 promoter/intron and the 300-bp enhancer elements respectively. Cre activity was assessed by breeding the transgenic founders onto the RosA26R genetic background and to examine its β-gal activity (blue staining) via Cre/Lox P recombination. Our results showed that, in addition to the Cre activity in hypertrophic chondrocytes, we also observed blue staining of the bone marrow and the surrounding digits when the 10 kb Col10a1 promoter/intron element was used, whereas the 300-bp enhancer element could drive Cre expression exclusively within the hypertrophic zone as demonstrated by the blue staining pattern. This is intriguing, as the 10 kb promoter covers the 300-bp enhancer element. We then further reanalyzed the LacZ transgenic mice. We did observe non-specific blue staining in 10 kb-LacZ mice but not the mice with the 300-bp enhancer. In addition, the Cre reporter construct was on a coat-color vector backbone, which enables direct visual genotyping of the transgenic mice in the FVB/N albino background. Together, our results support that the 300 bp Col10a1 enhancer provides a more efficient genetic tool to target the hypertrophic zone for studies of skeletal development and disease.

Characterization of a BAC transgenic mouse expressing Krt19-driven iCre recombinase in its digestive organs

Cytokeratin 19 (KRT19) protein is highly expressed in the epithelium of the gastrointestinal (GI) tract, hepatobiliary tissues, and pancreas of humans and mice. In the present study, we used an improved Cre (iCre) gene to enhance the efficiency of Cre expression in mammalian cells. We established a new transgenic Krt19-iCre bacterial artificial chromosome (BAC) mouse model using the BAC recombineering strategy. Site-specific iCre expression pattern was examined in embryos, adults, and elderly Krt19-iCre mice crossed with Tomato or LacZ reporter mice. Both iCre and reporter protein expressions in adult Krt19-iCre;Tomato^flox/+(Krt19-iCre Tomato reporter) mice were observed mainly in the epithelial cells of the GI tract, hepatobiliary tissues, and pancreas. However, the expression in the intrahepatic and small pancreatic duct were lower than those in the common bile and large pancreatic duct. In the Krt19-iCre; LacZ reporter embryos, β-galactosidase for the LacZ reporter was expressed in the glandular epithelial cells of the GI tract in 9.5-day embryos, 12-day embryos, and newborn mice. The reporter protein expression in Krt19-iCre-Tomato reporter mice was consistent with the KRT19 expression in human GI tissues. In conclusion, Krt19-iCre BAC transgenic mice can be used to investigate developmental and pathological conditions using the iCre-loxP system.

Characterization of the Proliferating Layer Chondrocytes of Growth Plate for Cartilage Regeneration

IMPACT STATEMENT

In recent years, cell-based therapy is a promising strategy for repairing defect cartilage. However, in vitro expansion of articular chondrocytes (ACs) for collecting enough cell numbers eventually develops cell de-differentiation. In the present study, we choose the proliferative layer chondroctytes (PLCs) of growth plate as new candidate. The novel findings include (1) the higher proliferation potential of PLCs in comparison with the ACs, (2) PLCs produced more GAG than ACs, (3) the increased in GAG matrix production, (4) and lower senescence in PLCs. From these results, we found PLCs might be suitable as cell source for cartilage regeneration.

High expression of COL10A1 is associated with poor prognosis in colorectal cancer

BACKGROUND

High expression of collagen type X alpha 1 chain (COL10A1), a member of the collagen family, had been observed in various human cancers, but the detailed function and molecular mechanism of COL10A1 were largely unclear.

AIM

The aim of this study was to investigate the expression of COL10A1 in colorectal cancer (CRC) tissues and cells and to reveal its biological function and mechanism in CRC.

MATERIALS AND METHODS

Immunohistochemistry (IHC), real-time quantitative polymerase chain reaction (QPCR) and Western blot experiments were used to determine the clinical relevance between expression levels of COL10A1 and CRC.

RESULTS

Compared with normal tissues, COL10A1 expression was significantly higher in CRC tissues. Biological functional experiments showed that overexpression of COL10A1 enhanced proliferation, migration, and invasion of CRC cells, and knockdown of COL10A1 inhibited tumorigenesis in vivo. Western blot assays showed that COL10A1 promoted the process of epithelial-mesenchymal transition (EMT). The overexpression of COL10A1 was associated with adverse prognosis in CRC by tissue microarray (TMA) analysis.

CONCLUSION

Our findings had provided evidences to support the fact that COL10A1 was abnormally up-expressed in CRC and involved in the progression of CRC and the process of EMT. Furthermore, we demonstrated that the high-level expression of COL10A1 was an independent risk factor of prognosis and overall survival in CRC patients. These suggested that COL10A1 might be a new potential target for cancer therapy in the future.

Adult Versus Pluripotent Stem Cell-Derived Mesenchymal Stem Cells: The Need for More Precise Nomenclature

The complexity of human pluripotent stem cell (hPSC) fate represents both opportunity and challenge. In theory, all somatic cell types can be differentiated from hPSCs, opening the door to many opportunities in transplant medicine. However, such clinical applications require high standards of purity and identity, that challenge many existing protocols. This underscores the need for increasing precision in the description of cell identity during hPSC differentiation. We highlight one salient example, namely, the numerous published reports of hPSC-derived mesenchymal stem cells (MSCs). We suggest that many of these reports likely represent an improper use of certain cluster of differentiation (CD) antigens in defining bone marrow-derived MSCs. Instead, most such hPSC-derived mesenchymal cells are likely a complex mixture of embryonic anlagen, primarily of diverse mesodermal and neural crest origins, making precise identification, reproducible manufacture, and uniform differentiation difficult to achieve. We describe a potential path forward that may provide more precision in nomenclature, and cells with higher purity and identity for potential therapeutic use.

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.

Dual pathways to endochondral osteoblasts: a novel chondrocyte-derived osteoprogenitor cell identified in hypertrophic cartilage

According to the general understanding, the chondrocyte lineage terminates with the elimination of late hypertrophic cells by apoptosis in the growth plate. However, recent cell tracking studies have shown that murine hypertrophic chondrocytes can survive beyond “terminal” differentiation and give rise to a progeny of osteoblasts participating in endochondral bone formation. The question how chondrocytes convert into osteoblasts, however, remained open. Following the cell fate of hypertrophic chondrocytes by genetic lineage tracing using BACCol10;Cre induced YFP-reporter gene expression we show that a progeny of Col10Cre-reporter labelled osteoprogenitor cells and osteoblasts appears in the primary spongiosa and participates – depending on the developmental stage – substantially in trabecular, endosteal, and cortical bone formation. YFP⁺ trabecular and endosteal cells isolated by FACS expressed Col1a1, osteocalcin and runx2, thus confirming their osteogenic phenotype. In searching for transitory cells between hypertrophic chondrocytes and trabecular osteoblasts we identified by confocal microscopy a novel, small YFP⁺Osx⁺ cell type with mitotic activity in the lower hypertrophic zone at the chondro-osseous junction. When isolated from growth plates by fractional enzymatic digestion, these cells termed CDOP (chondrocyte-derived osteoprogenitor) cells expressed bone typical genes and differentiated into osteoblasts in vitro. We propose the Col10Cre-labeled CDOP cells mark the initiation point of a second pathway giving rise to endochondral osteoblasts, alternative to perichondrium derived osteoprogenitor cells. These findings add to current concepts of chondrocyte-osteocyte lineages and give new insight into the complex cartilage-bone transition process in the growth plate.

Generation and characterization of an endothelin-2 iCre mouse

A novel transgenic mouse line that expresses codon-improved Cre recombinase (iCre) under regulation of the Endothelin-2 gene (edn2) promoter was developed for the conditional deletion of genes in Endothelin-2 lineage cells and for the spatial and temporal localization of Endothelin-2 expression. Endothelin-2 (EDN2, ET-2, previously VIC) is a transcriptionally regulated 21 amino acid peptide implicated in vascular homeostasis, and more recently in female reproduction, gastrointestinal function, immunology, and cancer pathogenesis that acts through membrane receptors and G-protein signaling. A cassette (edn2-iCre) was constructed that contained iCre, a polyadenylation sequence, and a neomycin selection marker in front of the endogenous start codon of the edn2 gene in a mouse genome BAC clone. The cassette was introduced into the C57BL/6 genome by pronuclear injection, and two lines of edn2-iCre positive mice were produced. The edn2-iCre mice were bred with ROSA26-lacZ and Ai9 reporter mice to visualize areas of functional iCre expression. Strong expression was seen in the periovulatory ovary, stomach and small intestine, and colon. Uniquely, we report punctate expression in the corneal epithelium, the liver, the lung, the pituitary, the uterus, and the heart. In the embryo, expression is localized in developing hair follicles and the dermis. Therefore, edn2-iCre mice will serve as a novel line for conditional gene deletion in these tissues.

Employing BAC-reporter constructs in the sea anemone Nematostella vectensis

Changes in the expression and function of genes drive evolutionary change. Comparing how genes are regulated in different species is therefore becoming an important part of evo-devo studies. A key tool for investigating the regulation of genes is represented by bacterial artificial chromosomes (BAC)-reporter constructs. BACs are large insert libraries, often >100 kb, which thus capture the genomic sequences surrounding a gene of interest, including all, or nearly all, of the elements underpinning regulation. Recombinant BACs, containing a reporter gene in place of the endogenous coding sequence of genes, can be utilized to drive the expression of reporter genes under the regulatory control of the gene of interest while still embedded within its genomic context. Systematic deletions within the BAC-reporter construct can be used to identify the minimal reporter in an unbiased way, avoiding the risk of overlooking regulatory elements that may be many kilobases away from the transcription start-site. Nematostella vectensis (Edwardsiidae, Anthozoa, Cnidaria) has become an important model in regenerative biology, ecology, and especially in studies of evo-devo and gene-regulatory networks due to its interesting phylogenetic position and amenability to molecular techniques. The increasing interest in this rising model system also led to a demand for methods that can be used to study the regulation of genes in Nematostella. Here, we present our progress in employing BAC-reporter constructs to visualize gene-expression in Nematostella. Using a new Nematostella-specific recombination cassette, we made nine different BAC-reporter constructs. Although five BAC recombinants gave variable effects, three constructs, namely Nv-bra:eGFP::L10 BAC, Nv-dpp:eGFP::L10 BAC, and Nv-grm:eGFP::L10 BAC delivered promising results. We show that these three constructs express the reporter gene eGFP in 10.4-17.2% of all analyzed larvae, out of which 26.2-41.9% express GFP in a mosaic fashion within the expected domain. In addition to the expression within the known domains, we also observed cases of misexpression of eGFP and examples that could represent actual expression outside the described domain. Furthermore, we deep-sequenced and assembled five different BACs containing Nv-chordin, Nv-foxa, Nv-dpp, Nv-wnta, and Nv-wnt1, to improve assembly around these genes. The use of BAC-reporter constructs will foster cis-regulatory analyses in Nematostella and thus help to improve our understanding of the regulatory network in this cnidarian system. Ultimately, this will advance the comparison of gene-regulation across species and lead to a much better understanding of evolutionary changes and novelties.

Defining cell-matrix combination products in the era of pluripotency

Human pluripotent stem (hPS) cells provide an attractive opportunity for the manufacture of a wide array of therapeutic cell types. The challenges to commercialization include the thousand-fold diversity of cell types emerging from hPS cells and the associated difficulties in validating processes to reliably generate cells with precise identity and purity. Improved methods of controlling the dosage and migration of hPS-derived cells in solid tissues are also needed. To directly address these issues, we clonally expanded proliferating lineages of cells that were intermediate in regard to their state of differentiation between hPS and terminally differentiated cells. These cells called monoclonal embryonic progenitors (hEP), are expandable mortal lineages with diverse site-specific homeobox gene expression and multipotentiality. In this review, we discuss methods of generating combination products wherein the fate space of precisely identified monoclonal hEP cells is mapped by differentiating the cells in vitro in HyStem-3D bead arrays in the presence of diverse growth factors. This combination of discovery processes has the potential to translate directly into cell-matrix formulations that can be used to generate pre-clinical data leading to human clinical trials and potentially new medical therapies.

Seven diverse human embryonic stem cell-derived chondrogenic clonal embryonic progenitor cell lines display site-specific cell fates

AIM

The transcriptomes of seven diverse clonal human embryonic progenitor cell lines with chondrogenic potential were compared with that of bone marrow-derived mesenchymal stem cells (MSCs).

MATERIALS & METHODS

The cell lines 4D20.8, 7PEND24, 7SMOO32, E15, MEL2, SK11 and SM30 were compared with MSCs using immunohistochemical methods, gene expression microarrays and quantitative real-time PCR.

RESULTS

In the undifferentiated progenitor state, each line displayed unique combinations of site-specific markers, including AJAP1, ALDH1A2, BMP5, BARX1, HAND2, HOXB2, LHX1, LHX8, PITX1, TBX15 and ZIC2, but none of the lines expressed the MSC marker CD74. The lines showed diverse responses when differentiated in the presence of combinations of TGF-β3, BMP2, 4, 6 and 7 and GDF5, with the lines 4D20.8, SK11, SM30 and MEL2 showing osteogenic markers in some differentiation conditions. The line 7PEND24 showed evidence of regenerating articular cartilage and, in some conditions, markers of tendon differentiation.

CONCLUSION

The scalability of site-specific clonal human embryonic stem cell-derived embryonic progenitor cell lines may provide novel models for the study of differentiation and methods for preparing purified and identified cells types for use in therapy.

COL10A1 expression is elevated in diverse solid tumor types and is associated with tumor vasculature

AIM

The identification of molecular markers that are upregulated in multiple tumor types could lead to novel diagnostic and therapeutic strategies. The authors screened a panel of RNAs prepared from diverse tumors and tumor cell lines, and compared them with normal tissues and cultured somatic cell types, in order to identify candidate genes expressed in a broad spectrum of tumor types.

MATERIALS & METHODS

Gene expression microarray analysis was carried out on 128 individual tumor samples representing over 20 tumor types, 85 samples representing 31 diverse normal tissue types, 68 tumor cell lines and 97 diverse normal primary cell cultures. Genes were ranked for elevated expression across a large number and variety of tumors relative to normal tissues.

RESULTS & CONCLUSION

COL10A1 was identified as a gene with restricted expression in most normal tissues and elevated expression in many diverse tumor types. By contrast, COL10A1 expression was undetectable in the 68 tumor cell lines surveyed in this study. Immunofluorescence studies localized collagen, type X, α-1 (collagen X) staining to tumor vasculature in breast tumors, whereas the vasculature of normal breast tissue was either collagen X-negative or had markedly lower levels. The tumor microenvironment-specific expression of collagen X, together with its localization in the vasculature, may facilitate its use as a novel target for the diagnosis and treatment of diverse solid tumor types.

A human embryonic stem cell-derived clonal progenitor cell line with chondrogenic potential and markers of craniofacial mesenchyme

AIMS

We screened 100 diverse human embryonic stem-derived progenitor cell lines to identify novel lines with chondrogenic potential.

MATERIALS & METHODS

The 4D20.8 cell line was compared with mesenchymal stem cells and dental pulp stem cells by assessing osteochondral markers using immunohistochemical methods, gene expression microarrays, quantitative real-time PCR and in vivo repair of rat articular condyles.

RESULTS

4D20.8 expressed the site-specific gene markers LHX8 and BARX1 and robustly upregulated chondrocyte markers upon differentiation. Differentiated 4D20.8 cells expressed relatively low levels of COL10A1 and lacked IHH and CD74 expression. Transplantation of 4D20.8 cells into experimentally induced defects in the femoral condyle of athymic rats resulted in cartilage and bone differentiation approximating that of the original tissue architecture. Relatively high COL2A1 and minimal COL10A1 expression occurred during differentiation in HyStem-C hydrogel with TGF-β3 and GDF-5.

CONCLUSION

Human embryonic stem cell-derived embryonic progenitor cell lines may provide a novel means of generating purified site-specific osteochondral progenitor cell lines that are useful in research and therapy.

Generation and characterization of Col10a1-mcherry reporter mice

We report here on the generation of a new fluorescent protein reporter transgenic mouse line, Col10a1-mCherry, which can be used as a tool to study chondrocyte biology and pathology. Collagen, Type X, alpha 1 (Col10a1) is highly expressed in hypertrophic chondrocytes and commonly used as a gene marker for this cell population. The Col10a1-mCherry reporter line was generated using a bacterial recombination strategy with the mouse BAC clone RP23-192A7. To aid in the characterization of this animal model, we intercrossed Col10a1-mCherry mice with Collagen, Type II, alpha 1 (Col2a1) enhanced cyan fluorescent protein (ECFP) reporter mice and characterized the expression of both chondrocyte reporters during embryonic skeletal development from days E10.5 to E17.5. Additionally, at postnatal day 0, Col10a1-mCherry reporter expression was compared to endogenous Col10a1 mRNA expression in long bones and revealed that mCherry fluorescence extended past the Col10a1 expression domain. However, in situ hybridization for mCherry was consistent with the zone of Col10a1 mRNA expression, indicating that the persistent detection of mCherry fluorescence was a result of the long protein half life of mCherry in conjunction with a very rapid phase of skeletal growth and not due to aberrant transcriptional regulation. Taking advantage of the continued fluorescence of hypertrophic chondrocytes at the chondro-osseus junction, we intercrossed Col10a1-mCherry mice with two different Collagen, Type 1, alpha 1, (Col1a1) osteoblast reporter mice, pOBCol3.6-Topaz and pOBCol2.3-Emerald to investigate the possibility that hypertrophic chondrocytes transdifferentiate into osteoblasts. Evaluation of long bones at birth suggests that residual hypertrophic chondrocytes and osteoblasts in the trabecular zone exist as two completely distinct cell populations. genesis 49:410-418, 2011.

Secreted phosphoprotein 1 upstream invasive network construction and analysis of lung adenocarcinoma compared with human normal adjacent tissues by integrative biocomputation

The aim of this study is to set up single molecular secreted phosphoprotein 1 (SPP1) upstream invasive network of lung adenocarcinoma. This paper proposed an integrated method based on linear programming and a decomposition procedure with integrated analysis of the significant function cluster using Kappa statistics and fuzzy heuristic clustering. Our study proved that only modules appearing in lung adenocarcinoma include cytokine module (CXCL13, GREM1_2 inhibition), cell adhesion module (COL11A1_2 activation; CDH3 inhibition), and receptor binding module (NMU activation; CXCL13, GREM1_2 inhibition), which increase the invasion of cancer cell. We compared skeletal development, signal, biological regulation, sequence variant modules between human normal adjacent tissues and lung adenocarcinoma. SPP1 skeletal development module appears in human normal adjacent tissues (COL11A1_1 activation; COL10A1 inhibition), whereas in lung adenocarcinoma (COL11A1_2, COL1A2 activation); signal module appears in human normal adjacent tissues (COL11A1_1, CXCL13, MMP11, SPINK1 activation; COL10A1, COL3A1 inhibition), whereas in lung adenocarcinoma (COL11A1_2, COL1A2, MMP12 activation; CDH3, CXCL13, GREM1_2, MMP11, SPINK1 inhibition); biological regulation module appears in human normal adjacent tissues (CXCL13, MKI67, PYCR1 activation; NEK2, SPDEF, TOP2A_2, TOX3_1 inhibition), whereas in lung adenocarcinoma (HMGB3, MKI67, NMU, PYCR1, TOX3_2 activation; CXCL13, SPDEF, TOP2A_2 inhibition); sequence variant module appears in human normal adjacent tissues (COL11A1_1, MKI67, MMP11 activation; ASPM, COL10A1, COL3A1, NEK2, TMPRSS4, TOP2A_2 inhibition), whereas in lung adenocarcinoma (COL11A1_2, COL1A2, HMMR, MKI67, MMP12 activation; ABCC3, ASPM, CDH3, MMP11, TOP2A_2 inhibition). It can be deduced that modules above in human normal adjacent tissues reflect the invasive inhibition of normal cells, whereas in lung adenocarcinoma increase the invasion of cancer cell. Our study of SPP1 upstream invasive network may be useful to identify novel and potentially targets for prognosis and therapy of lung adenocarcinoma.

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

SOX9 is a transcription factor of the SRY family that regulates sex determination, cartilage development and numerous other developmental events. In the foetal growth plate, Sox9 is highly expressed in chondrocytes of the proliferating and prehypertrophic zone but declines abruptly in the hypertrophic zone, suggesting that Sox9 downregulation in hypertrophic chondrocytes might be a necessary step to initiate cartilage-bone transition in the growth plate. In order to test this hypothesis, we generated transgenic mice misexpressing Sox9 in hypertrophic chondrocytes under the control of a BAC-Col10a1 promoter. The transgenic offspring showed an almost complete lack of bone marrow in newborns, owing to strongly retarded vascular invasion into hypertrophic cartilage and impaired cartilage resorption, resulting in delayed endochondral bone formation associated with reduced bone growth. In situ hybridization analysis revealed high levels of Sox9 misexpression in hypertrophic chondrocytes but deficiencies of Vegfa, Mmp13, RANKL and osteopontin expression in the non-resorbed hypertrophic cartilage, indicating that Sox9 misexpression in hypertrophic chondrocytes inhibits their terminal differentiation. Searching for the molecular mechanism of SOX9-induced inhibition of cartilage vascularization, we discovered that SOX9 is able to directly suppress Vegfa expression by binding to SRY sites in the Vegfa gene. Postnatally, bone marrow formation and cartilage resorption in transgenic offspring are resumed by massive invasion of capillaries through the cortical bone shaft, similar to secondary ossification. These findings imply that downregulation of Sox9 in the hypertrophic zone of the normal growth plate is essential for allowing vascular invasion, bone marrow formation and endochondral ossification.

Correlation of COL10A1 induction during chondrogenesis of mesenchymal stem cells with demethylation of two CpG sites in the COL10A1 promoter

OBJECTIVE

Human articular chondrocytes do not express COL10A1 and do not undergo hypertrophy except in close vicinity to subchondral bone. In contrast, chondrocytes produced in vitro from mesenchymal stem cells (MSCs) show premature COL10A1 expression and cannot form stable ectopic cartilage transplants, which indicates that they may be phenotypically unstable and not suitable for treatment of articular cartilage lesions. CpG methylation established during natural development may play a role in suppression of COL10A1 expression and hypertrophy in human articular chondrocytes. This study was undertaken to compare gene methylation patterns and expression of COL10A1 and COL2A1 in chondrocyte and MSC populations, in order to determine whether failed genomic methylation patterns correlate with an unstable chondrocyte phenotype after chondrogenesis of MSCs.

METHODS

COL10A1 and COL2A1 regulatory gene regions were computationally searched for CpG-rich regions. CpG methylation of genomic DNA from human articular chondrocytes, MSCs, and MSC-derived chondrocytes was analyzed by Combined Bisulfite Restriction Analysis and by sequencing of polymerase chain reaction fragments amplified from bisulfite-treated genomic DNA.

RESULTS

The CpG island around the transcription start site of COL2A1 was unmethylated in all cell groups independent of COL2A1 expression, while 9 tested CpG sites in the sparse CpG promoter of COL10A1 were consistently methylated in human articular chondrocytes. Induction of COL10A1 expression during chondrogenesis of MSCs correlated with demethylation of 2 CpG sites in the COL10A1 promoter.

CONCLUSION

Our findings indicate that methylation-based COL10A1 gene silencing is established in cartilage tissue and human articular chondrocytes. Altered methylation levels at 2 CpG sites of COL10A1 in MSCs and their demethylation during chondrogenesis may facilitate induction of COL10A1 as observed during in vitro chondrogenesis of MSCs.

GADD45beta enhances Col10a1 transcription via the MTK1/MKK3/6/p38 axis and activation of C/EBPbeta-TAD4 in terminally differentiating chondrocytes

GADD45beta (growth arrest- and DNA damage-inducible) interacts with upstream regulators of the JNK and p38 stress response kinases. Previously, we reported that the hypertrophic zone of the Gadd45beta(-/-) mouse embryonic growth plate is compressed, and expression of type X collagen (Col10a1) and matrix metalloproteinase 13 (Mmp13) genes is decreased. Herein, we report that GADD45beta enhances activity of the proximal Col10a1 promoter, which contains evolutionarily conserved AP-1, cAMP-response element, and C/EBP half-sites, in synergism with C/EBP family members, whereas the MMP13 promoter responds to GADD45beta together with AP-1, ATF, or C/EBP family members. C/EBPbeta expression also predominantly co-localizes with GADD45beta in the embryonic growth plate. Moreover, GADD45beta enhances C/EBPbeta activation via MTK1, MKK3, and MKK6, and dominant-negative p38alphaapf, but not JNKapf, disrupts the combined trans-activating effect of GADD45beta and C/EBPbeta on the Col10a1 promoter. Importantly, GADD45beta knockdown prevents p38 phosphorylation while decreasing Col10a1 mRNA levels but does not affect C/EBPbeta binding to the Col10a1 promoter in vivo, indicating that GADD45beta influences the transactivation function of DNA-bound C/EBPbeta. In support of this conclusion, we show that the evolutionarily conserved TAD4 domain of C/EBPbeta is the target of the GADD45beta-dependent signaling. Collectively, we have uncovered a novel molecular mechanism linking GADD45beta via the MTK1/MKK3/6/p38 axis to C/EBPbeta-TAD4 activation of Col10a1 transcription in terminally differentiating chondrocytes.

Localization of the cis-enhancer element for mouse type X collagen expression in hypertrophic chondrocytes in vivo

The type X collagen gene (Col10a1) is a specific molecular marker of hypertrophic chondrocytes during endochondral bone formation. Mutations in human COL10A1 and altered chondrocyte hypertrophy have been associated with multiple skeletal disorders. However, until recently, the cis-enhancer element that specifies Col10a1 expression in hypertrophic chondrocytes in vivo has remained unidentified. Previously, we and others have shown that the Col10a1 distal promoter (-4.4 to -3.8 kb) may harbor a critical enhancer that mediates its tissue specificity in transgenic mice studies. Here, we report further localization of the cis-enhancer element within this Col10a1 distal promoter by using a similar transgenic mouse approach. We identify a 150-bp Col10a1 promoter element (-4296 to -4147 bp) that is sufficient to direct its tissue-specific expression in vivo. In silico analysis identified several putative transcription factor binding sites including two potential activator protein-1 (AP-1) sites within its 5'- and 3'-ends (-4276 to -4243 and -4166 to -4152 bp), respectively. Interestingly, transgenic mice using a reporter construct deleted for these two AP-1 elements still showed tissue-specific reporter activity. EMSAs using oligonucleotide probes derived from this region and MCT cell nuclear extracts identified DNA/protein complexes that were enriched from cells stimulated to hypertrophy. Moreover, these elements mediated increased reporter activity on transfection into MCT cells. These data define a 90-bp cis-enhancer required for tissue-specific Col10a1 expression in vivo and putative DNA/protein complexes that contribute to the regulation of chondrocyte hypertrophy. This work will enable us to identify candidate transcription factors essential both for skeletal development and for the pathogenesis of skeletal disorders.

Specific expression of Cre recombinase in hypertrophic cartilage under the control of a BAC-Col10a1 promoter

Previously we have shown that insertion of a LacZ reporter gene into the Col10a1 gene in the context of a bacterial artificial chromosome (BAC) drives strong and specific expression of LacZ in hypertrophic cartilage of transgenic mice [Gebhard S., Hattori T., Bauer E., Bosl M.R., Schlund B., Poschl E., Adam N., de Crombrugghe B., von der Mark K., 2007 Histochem. Cell Biol. 19 127:183-194]. BAC constructs in transgenic reporter mouse lines control efficient and specific LacZ expression in hypertrophic chondrocytes under the complete Col10a1 promoter. Here we report on the generation of Col10a1-specific Cre deleter mice using a BAC recombineering technique based on homologous recombination in E. coli. Sixteen BAC-Col10-Cre transgenic lines were generated containing between 1 and 5 copies of the BAC-Col10-Cre gene. All lines tested so far expressed Cre specifically in hypertrophic chondrocytes of E16.5 and P1 growth plates of long bones, ribs, vertebrae and sternum as examined by crossing with ROSA26 reporter mice. Cre activity was detected as early as E13.5 when hypertrophic cartilage develops in the diaphysis of femur and humerus. The data confirm that expression of Cre under the control of the complete BAC-Col10a1 promoter occurs with high efficiency and specificity in hypertrophic chondrocytes. The BAC-Col10-Cre lines should thus provide a valuable tool to get further insight into the role of genes involved in endochondral ossification by allowing their specific deletion in the hypertrophic zone of the growth plate.

Genomic identification of regulatory elements by evolutionary sequence comparison and functional analysis

Despite remarkable recent advances in genomics that have enabled us to identify most of the genes in the human genome, comparable efforts to define transcriptional cis-regulatory elements that control gene expression are lagging behind. The difficulty of this task stems from two equally important problems: our knowledge of how regulatory elements are encoded in genomes remains elementary, and there is a vast genomic search space for regulatory elements, since most of mammalian genomes are noncoding. Comparative genomic approaches are having a remarkable impact on the study of transcriptional regulation in eukaryotes and currently represent the most efficient and reliable methods of predicting noncoding sequences likely to control the patterns of gene expression. By subjecting eukaryotic genomic sequences to computational comparisons and subsequent experimentation, we are inching our way toward a more comprehensive catalog of common regulatory motifs that lie behind fundamental biological processes. We are still far from comprehending how the transcriptional regulatory code is encrypted in the human genome and providing an initial global view of regulatory gene networks, but collectively, the continued development of comparative and experimental approaches will rapidly expand our knowledge of the transcriptional regulome.

Recent progress in histochemistry

The progress in discerning the structure and function of cells and tissues in health and disease has been achieved to a large extent by the continued development of new reagents for histochemistry, the improvement of existing techniques and new imaging techniques. This review will highlight some advancements made in these fields.