Normal proliferation and differentiation of Hoxc-8 transgenic chondrocytes in vitro

Site-Dependent Lineage Preference of Adipose Stem Cells

Adult stem cells have unique properties in both proliferation and differentiation preference. In this study, we hypothesized that adipose stem cells have a depot-dependent lineage preference. Four rabbits were used to provide donor-matched adipose stem cells from either subcutaneous adipose tissue (ScAT) or infrapatellar fat pad (IPFP). Proliferation and multi-lineage differentiation were evaluated in adipose stem cells from donor-matched ScAT and IPFP. RNA sequencing (RNA-seq) and proteomics were conducted to uncover potential molecular discrepancy in adipose stem cells and their corresponding matrix microenvironments. We found that stem cells from ScAT exhibited significantly higher proliferation and adipogenic capacity compared to those from donor-matched IPFP while stem cells from IPFP displayed significantly higher chondrogenic potential compared to those from donor-matched ScAT. Our findings are strongly endorsed by supportive data from transcriptome and proteomics analyses, indicating a site-dependent lineage preference of adipose stem cells.

Hox-Mediated Spatial and Temporal Coding of Stem Cells in Homeostasis and Neoplasia

Hox genes are fundamental components of embryonic patterning and morphogenesis with expression persisting into adulthood. They are also implicated in the development of diseases, particularly neoplastic transformations. The tight spatio-temporal regulation of Hox genes in concordance with embryonic patterning is an outstanding feature of these genes. In this review we have systematically analyzed Hox functions within the stem/progenitor cell compartments and asked whether their temporo-spatial topography is retained within the stem cell domain throughout development and adulthood. In brief, evidence support involvement of Hox genes at several levels along the stem cell hierarchy, including positional identity, stem cell self-renewal, and differentiation. There is also strong evidence to suggest a role for Hox genes during neoplasia. Although fundamental questions are yet to be addressed through more targeted and high- throughput approaches, existing evidence suggests a central role for Hox genes within a continuum along the developmental axes persisting into adult homeostasis and disease.

Role of Hox genes in stem cell differentiation

Hox genes are an evolutionary highly conserved gene family. They determine the anterior-posterior body axis in bilateral organisms and influence the developmental fate of cells. Embryonic stem cells are usually devoid of any Hox gene expression, but these transcription factors are activated in varying spatial and temporal patterns defining the development of various body regions. In the adult body, Hox genes are among others responsible for driving the differentiation of tissue stem cells towards their respective lineages in order to repair and maintain the correct function of tissues and organs. Due to their involvement in the embryonic and adult body, they have been suggested to be useable for improving stem cell differentiations in vitro and in vivo. In many studies Hox genes have been found as driving factors in stem cell differentiation towards adipogenesis, in lineages involved in bone and joint formation, mainly chondrogenesis and osteogenesis, in cardiovascular lineages including endothelial and smooth muscle cell differentiations, and in neurogenesis. As life expectancy is rising, the demand for tissue reconstruction continues to increase. Stem cells have become an increasingly popular choice for creating therapies in regenerative medicine due to their self-renewal and differentiation potential. Especially mesenchymal stem cells are used more and more frequently due to their easy handling and accessibility, combined with a low tumorgenicity and little ethical concerns. This review therefore intends to summarize to date known correlations between natural Hox gene expression patterns in body tissues and during the differentiation of various stem cells towards their respective lineages with a major focus on mesenchymal stem cell differentiations. This overview shall help to understand the complex interactions of Hox genes and differentiation processes all over the body as well as in vitro for further improvement of stem cell treatments in future regenerative medicine approaches.

Control of collagen production in mouse chondrocytes by using a combination of bone morphogenetic protein-2 and small interfering RNA targeting Col1a1 for hydrogel-based tissue-engineered cartilage

Because articular cartilage does not self-repair, tissue-engineering strategies should be considered to regenerate this tissue. Autologous chondrocyte implantation is already used for treatment of focal damage of articular cartilage. Unfortunately, this technique includes a step of cell amplification, which results in dedifferentiation of chondrocytes, with expression of type I collagen, a protein characteristic of fibrotic tissues. Therefore, the risk of producing a fibrocartilage exists. The aim of this study was to propose a new strategy for authorizing the recovery of the differentiated status of the chondrocytes after their amplification on plastic. Because the bone morphogenetic protein (BMP)-2 and the transforming growth factor (TGF)-β1 are cytokines both proposed as stimulants for cartilage repair, we undertook a detailed comparative analysis of their biological effects on chondrocytes. As a cellular model, we used mouse chondrocytes after their expansion on plastic and we tested the capability of BMP-2 or TGF-β1 to drive their redifferentiation, with special attention given to the nature of the proteins synthesized by the cells. To prevent any fibrotic character of the newly synthesized extracellular matrix, we silenced type I collagen by transfecting small interfering RNA (siRNA) into the chondrocytes, before their exposure to BMP-2 or TGF-β1. Our results showed that addition of siRNA targeting the mRNA encoded by the Col1a1 gene (Col1a1 siRNA) and BMP-2 represents the most efficient combination to control the production of cartilage-characteristic collagen proteins. To go one step further toward scaffold-based cartilage engineering, Col1a1 siRNA-transfected chondrocytes were encapsulated in agarose hydrogel and cultured in vitro for 1 week. The analysis of the chondrocyte-agarose constructs by using real-time polymerase chain reaction, Western-blotting, immunohistochemistry, and electron microscopy techniques demonstrated that the BMP-2/Col1a1 siRNA combination is effective in reinitializing correct production and assembly of the cartilage-characteristic matrix in agarose hydrogel, without production of type I collagen. Because agarose is known to favor long-term expression of the chondrocyte phenotype and agarose-based hydrogels are approved for clinical trials, this strategy appears very promising to repair hyaline cartilage.

Looping mediated interaction between the promoter and 3' UTR regulates type II collagen expression in chondrocytes

Type II collagen is the major component of articular cartilage and is mainly synthesized by chondrocytes. Repeated sub-culturing of primary chondrocytes leads to reduction of type II collagen gene (Col2a1) expression, which mimics the process of chondrocyte dedifferentiation. Although the functional importance of Col2a1 expression has been extensively investigated, mechanism of transcriptional regulation during chondrocyte dedifferentiation is still unclear. In this study, we have investigated the crosstalk between cis-acting DNA element and transcription factor on Col2a1 expression in primary chondrocytes. Bioinformatic analysis revealed the potential regulatory regions in the Col2a1 genomic locus. Among them, promoter and 3′ untranslated region (UTR) showed highly accessible chromatin architecture with enriched recruitment of active chromatin markers in primary chondrocytes. 3′ UTR has a potent enhancer function which recruits Lef1 (Lymphoid enhancer binding factor 1) transcription factor, leading to juxtaposition of the 3′ UTR with the promoter through gene looping resulting in up-regulation of Col2a1 gene transcription. Knock-down of endogenous Lef1 level significantly reduced the gene looping and subsequently down-regulated Col2a1 expression. However, these regulatory loci become inaccessible due to condensed chromatin architecture as chondrocytes dedifferentiate which was accompanied by a reduction of gene looping and down-regulation of Col2a1 expression. Our results indicate that Lef1 mediated looping between promoter and 3′ UTR under the permissive chromatin architecture upregulates Col2a1 expression in primary chondrocytes.

Microarray Analysis of Defective Cartilage in Hoxc8- and Hoxd4-Transgenic Mice

OBJECTIVE

Homeobox genes of the Hox class are required for proper patterning of skeletal elements and play a role in cartilage differentiation. In transgenic mice with overexpression of Hoxc8 and Hoxd4 during cartilage development, the authors observed severe defects, namely, physical instability of cartilage, accumulation of immature chondrocytes, and decreased maturation to hypertrophy. To define the molecular basis underlying these defects, the authors performed gene expression profiling using the Affymetrix microarray platform.

RESULTS

Primary chondrocytes were isolated from Hoxc8- and Hoxd4-transgenic mouse embryo rib cartilage at 18.5 days of gestation. In both cases, differentially expressed genes were identified that have a role in cell proliferation and cell cycle regulation. A comparison between the controls for both experimental groups did not reveal significant differences, as expected. However, the repertoires of differentially expressed genes were found not to overlap between Hoxc8- and Hoxd4-transgenic cartilage. This included different Wnt genes, cell cycle, and apoptosis regulators.

CONCLUSION

Overexpression of Hoxc8 and Hoxd4 transcription factors alters transcriptional profiles in chondrocytes at E18.5. The differences in repertoires of altered gene expression between the 2 transgenic conditions suggest that the molecular mechanisms underlying the cartilage defects may be different in both transgenic paradigms, despite apparently similar phenotypes.

Engineered cartilage heals skull defects

INTRODUCTION

The purposes of this study were to differentiate embryonic limb bud cells into cartilage, characterize the nodules produced, and determine their ability to heal a mouse skull defect.

METHODS

Aggregated mouse limb bud cells (E12-E12.5), cultured in a bioreactor for 3 weeks, were analyzed by histology or implanted in 6 skull defects. Six controls had no implants. The mice were scanned with microcomputed tomography weekly. At 2 and 4 weeks, a mouse from each group was killed, and the defect region was prepared for histology.

RESULTS

Chondrocytes in nodules were mainly hypertrophic. About 90% of the nodules mineralized. BrdU staining showed dividing cells in the perichondrium. Microcomputed tomography scans showed increasing minerals in implanted nodules that completely filled the defect by 6 weeks; defects in the control mice were not healed by then. At 2 and 4 weeks, the control skull sections showed only a thin bony layer over the defect. At 2 weeks, bone and cartilage filled the defects with implants, and the implants were well integrated with the adjacent cortical bone. At 4 weeks, the implant had turned almost entirely into bone.

CONCLUSIONS

Cartilage differentiated in the bioreactor and facilitated healing when implanted into a defect. Engineering cartilage to replace bone is an alternative to current methods of bone grafting.

Expression of cartilage developmental genes in Hoxc8- and Hoxd4-transgenic mice

Hox genes encode transcription factors, which regulate skeletal patterning and chondrocyte differentiation during the development of cartilage, the precursor to mature bone. Overexpression of the homeobox transcription factors Hoxc8 and Hoxd4 causes severe cartilage defects due to delay in cartilage maturation. Matrix metalloproteinases (MMPs), bone morphogenetic proteins (BMPs) and fibroblastic growth factors (FGFs) are known to play important roles in skeletal development and endochondral bone formation and remodeling. In order to investigate whether these molecules are aberrantly expressed in Hoxc8- and/or Hoxd4-transgenic cartilage, we performed quantitative RT-PCR on chondrocytes from Hox-transgenic mice. Gene expression levels of Bmp4, Fgf8, Fgf10, Mmp9, Mmp13, Nos3, Timp3, Wnt3a and Wnt5a were altered in Hoxc8-transgenic chondrocytes, and Fgfr3, Ihh, Mmp8, and Wnt3a expression levels were altered in Hoxd4-transgenic chondrocytes, respectively. Notably, Wnt3a expression was elevated in Hoxc8- and reduced in Hoxd4-transgenic cartilage. These results suggest that both transcription factors affect cartilage maturation through different molecular mechanisms, and provide the basis for future studies into the role of these genes and possible interactions in pathogenesis of cartilage defects in Hoxc8- and Hoxd4-transgenic mice.

Morpholino-mediated knockdown in primary chondrocytes implicates Hoxc8 in regulation of cell cycle progression

Numerous experiments in mutant and transgenic mice have implicated Hox transcription factors in development of the skeletal system, postulating a role for these proteins in cell proliferation of precursor cells and regulation of cell differentiation. Our own data from Hoxc8 and Hoxd4 transgenic mice suggest that Hoxc8 is involved in cell proliferation during cartilage development. In order to directly assess its role in cell proliferation of a specific skeletal cell type, the cartilage-producing chondrocyte, we performed morpholino-mediated knockdown experiments in normal primary chondrocytes. Through analysis of PCNA expression and staining for phosphorylated Histone 3, two cell cycle markers, we show that interference with Hoxc8 expression in chondrocytes reduces cell proliferation, but in the absence of apoptosis. Instead, cells with a knockdown in Hoxc8 expression appear to be delayed in their progression through the cell cycle. Our results provide evidence for prolonged duration of and delayed exit from M-phase, thus implicating a role for Hoxc8 in controlling cell cycle progression at this critical check point.

The mouse hairy ears mutation exhibits an extended growth (anagen) phase in hair follicles and altered Hoxc gene expression in the ears

The mouse In(15)2Rl (hairy ears, Eh) mutation is a paracentric inversion of the distal half of chromosome 15 (Chr 15). Heterozygous Eh/+ mice display misshaped and hairy ears that have more and longer hair than the ears of their wild-type littermates. We mapped, cloned and sequenced both inversion breakpoints. No protein-coding transcript was disrupted by either breakpoint. The proximal breakpoint is located between syntrophin basic 1 (Sntb1) and hyaluronan synthase 2 (Has2), and the distal breakpoint maps between homeobox C4 (Hoxc4) and single-strand selective monofunctional uracil DNA glycosylase (Smug1), near the middle and the telomere ends of Chr 15, respectively. The inversion spans ~47 megabases. Our genetic analysis suggests that the hairy-ear phenotype is caused by the proximal breakpoint of the inversion-bearing Chr 15. Quantitative RNA analysis by real-time polymerase chain reaction for the genes flanking the breakpoint indicated no changes in expression levels except for some homeobox C (Hoxc) genes whose expression was elevated in developing and mature skin of the ears but not of other body regions. The increased hair length on the ears of Eh/+ mice was due to an extension of the anagen stage in the hair cycle, as determined by histological analysis. Our data indicate that the Eh phenotype arises from mis-expression of Hoxc genes.

Targeting the beta-catenin/TCF transcriptional complex in the treatment of multiple myeloma

Multiple myeloma (MM) is an invariably fatal form of cancer characterized by clonal proliferation of malignant plasma cells in the bone marrow. The canonical Wnt signaling pathway is activated in MM cells through constitutively active beta-catenin, a messenger molecule relevant to growth, survival, and migration of MM cells. The identification of a number of small molecular compounds, such as PKF115-584, which disrupt the interaction of the transcriptionally active beta-catenin/TCF protein complex, provides valuable new therapeutic tools to target an alternative pathway in MM independent of the proteasome. Here we evaluated the transcriptional, proteomic, signaling changes, and biological sequelae associated with the inhibition of Wnt signaling in MM by PKF115-584. The compound blocks expression of Wnt target genes and induces cytotoxicity in both patient MM cells and MM cell lines without a significant effect in normal plasma cells. In xenograft models of human MM, PKF115-584 inhibits tumor growth and prolongs survival. Taken together, these data demonstrate the efficacy of disrupting the beta-catenin/TCF transcriptional complex to exploit tumor dependence on Wnt signaling as a therapeutic approach in the treatment of MM.

NF-kappaB regulates Lef1 gene expression in chondrocytes

The relation of Wnt/beta-catenin signaling to osteoarthritis progression has been revealed with little information on the underlying molecular mechanism. In this study we found overexpression of Lef1 in cartilage tissue of osteoarthritic patients and elucidated molecular mechanism of NF-kappaB-mediated Lef1 gene regulation in chondrocytes. Treatment of IL-1beta augmented Lef1 upregulation and nuclear translocation of NF-kappaB in chondrocytes. Under IL-1beta signaling, treatment of NF-kappaB nuclear translocation inhibitor SN-50 reduced Lef1 expression. A conserved NF-kappaB-binding site between mouse and human was selected through bioinformatic analysis and mapped at the 14 kb upstream of Lef1 transcription initiation site. NF-kappaB binding to the site was confirmed by chromatin immunoprecipitation assay. Lef1 expression was synergistically upregulated by interactions of NF-kappaB with Lef1/beta-catenin in chondrocytes. Our results suggest a pivotal role of NF-kappaB in Lef1 expression in arthritic chondrocytes or cartilage degeneration.

Expression of folate pathway genes in the cartilage of Hoxd4 and Hoxc8 transgenic mice

BACKGROUND

Hox transcription factors are well known for their role in skeletal patterning in vertebrates. They regulate gene expression during the development of cartilage, the precursor to mature bone. We previously reported that overexpression of the homeobox genes Hoxc8 and Hoxd4 results in severe cartilage defects, reduced proteoglycan content, accumulation of immature chondrocytes, and decreased maturation to hypertrophy. We have also shown that Hoxd4 transgenic mice whose diets were supplemented with folate had their skeletal development restored. Since folate is required for growth and differentiation of chondrocytes, we hypothesized that the beneficial effect of folate in Hoxd4 transgenic mice might indicate a local deficiency in folate utilization, possibly caused by deregulation of genes encoding folate transport proteins or folate metabolic enzymes.

METHODS

We assayed the prevalence of transcripts for 22 folate transport proteins and metabolizing enzymes, here collectively referred to as folate pathway genes. Quantitative real-time PCR was performed on cDNA samples derived from RNA isolated from primary chondrocytes of individual rib cartilages from Hoxd4 and Hoxc8 transgenic mice, respectively.

RESULTS

This study shows that the Hox transgenes produce overexpression of Hoxd4 and Hoxc8 in primary chondrocytes from perinatal transgenic mice. However, no differences were found in expression levels of the folate pathway genes in transgenic cells compared to littermate controls.

CONCLUSIONS

Our results provide evidence that folate pathway genes are only indirect targets of Hox transgene overexpression in our transgenic animals. These expression studies provide a baseline for future studies into the role of folate metabolism in chondrocyte differentiation.

Microarray analyses of gene expression during chondrocyte differentiation identifies novel regulators of hypertrophy

Ordered chondrocyte differentiation and maturation is required for normal skeletal development, but the intracellular pathways regulating this process remain largely unclear. We used Affymetrix microarrays to examine temporal gene expression patterns during chondrogenic differentiation in a mouse micromass culture system. Robust normalization of the data identified 3300 differentially expressed probe sets, which corresponds to 1772, 481, and 249 probe sets exhibiting minimum 2-, 5-, and 10-fold changes over the time period, respectively. GeneOntology annotations for molecular function show changes in the expression of molecules involved in transcriptional regulation and signal transduction among others. The expression of identified markers was confirmed by RT-PCR, and cluster analysis revealed groups of coexpressed transcripts. One gene that was up-regulated at later stages of chondrocyte differentiation was Rgs2. Overexpression of Rgs2 in the chondrogenic cell line ATDC5 resulted in accelerated hypertrophic differentiation, thus providing functional validation of microarray data. Collectively, these analyses provide novel information on the temporal expression of molecules regulating endochondral bone development.

Hypophosphatemia leads to rickets by impairing caspase-mediated apoptosis of hypertrophic chondrocytes

Rickets is seen in association with vitamin D deficiency and in several genetic disorders associated with abnormal mineral ion homeostasis. Studies in vitamin D receptor (VDR)-null mice have demonstrated that expansion of the late hypertrophic chondrocyte layer, characteristic of rickets, is secondary to impaired apoptosis of these cells. The observation that normalization of mineral ion homeostasis in the VDR-null mice prevents rachitic changes suggests that rickets is secondary to hypocalcemia, hypophosphatemia, or hyperparathyroidism, rather than impaired VDR action. To determine which of these abnormalities is responsible for impaired chondrocyte apoptosis and subsequent rachitic changes, two additional models were examined: diet-induced hypophosphatemia/hypercalcemia and hypophosphatemia secondary to mutations in the Phex gene. The former model is associated with suppressed parathyroid hormone levels as a consequence of hypercalcemia. The latter model demonstrates normal calcium and parathyroid hormone levels, but 1,25-dihydroxyvitamin D levels that are inappropriately low for the degree of hypophosphatemia. Our studies demonstrate that normal phosphorus levels are required for growth plate maturation and implicate a critical role for phosphate-regulated apoptosis of hypertrophic chondrocytes via activation of the caspase-9-mediated mitochondrial pathway.

Folate modulates Hox gene-controlled skeletal phenotypes

Hox genes are well-known regulators of pattern formation and cell differentiation in the developing vertebrate skeleton. Although skeletal variations are not uncommon in humans few mutations in human HOX genes have been described. If such mutations are compatible with life, there may be physiological modifiers for the manifestation of Hox gene-controlled phenotypes, masking underlying mutations. Here we present evidence that the essential nutrient folate modulates genetically induced skeletal defects in Hoxd4 transgenic mice. We also show that chondrocytes require folate for growth and differentiation and that they express folate transport genes, providing evidence for a direct effect of folate on skeletal cells. To our knowledge, this is the first report of nutritional influence on Hox gene-controlled phenotypes, and implicates gene-environment interactions as important modifiers of Hox gene function. Taken together, our results demonstrate a beneficial effect of folate on skeletal development that may also be relevant to disorders and variations of the human skeleton.