Redefining the activity of a bone-specific transcription factor: novel insights for understanding bone formation

[Dissecting the Hierarchy and Lineage of Mesenchymal Stem Cells Using Mouse Genetics as a Step toward Drug Discovery and Regenerative Medicine]

The mesenchymal stem cell (MSC) is a type of tissue stem cell. In clinical studies, cultured MSCs have shown important therapeutic effects on diseases via both the reduction of neurological defects and the regulation of immune responses. However, in vivo MSC localization, function, and properties are poorly understood; therefore, the molecular understanding of MSC hierarchy is less advanced compared to hematopoietic stem cell hierarchy. Runt-related transcription factor 2 (Runx2) is an essential transcriptional regulator of osteoblast differentiation from MSCs. Runx2 deficiency in Paired-related homeobox 1 (Prrx1)-derived cells (Runx2_Prrx1^-/- mice) results in defective intramembranous ossification. Double-positive cells for Prrx1-GFP, and stem cell antigen-1 (Sca1) (Prrx1⁺Sca1⁺ cells) in the calvaria, express Runx2 at lower levels, and are more homogeneous and primitive compared with Prrx1⁺Sca1^- cells. Our results suggest that osteoblast differentiation in vivo may begin at the Prrx1⁺Sca1⁺ MSC stage, with sequential progression to Prrx1⁺Sca1^- cells, followed by Osterix⁺Prrx1^-Sca1^- osteoblast precursors, which eventually form mature α1(I)-collagen⁺ osteoblasts. This research will enable us to better understand the in vivo molecular biology features of MSCs, leading to their therapeutic applications for tissue repair and regeneration.

Genome-wide DNase hypersensitivity, and occupancy of RUNX2 and CTCF reveal a highly dynamic gene regulome during MC3T3 pre-osteoblast differentiation

The ability to discover regulatory sequences that control bone-related genes during development has been greatly improved by massively parallel sequencing methodologies. To expand our understanding of cis-regulatory regions critical to the control of gene expression during osteoblastogenesis, we probed the presence of open chromatin states across the osteoblast genome using global DNase hypersensitivity (DHS) mapping. Our profiling of MC3T3 mouse pre-osteoblasts during differentiation has identified more than 224,000 unique DHS sites. Approximately 65% of these sites are dynamic during temporal stages of osteoblastogenesis, and a majority of them are located within non-promoter (intergenic and intronic) regions. Nearly half of all DHS sites (both constitutive and dynamic) overlap binding events of the bone-essential RUNX2 and/or the chromatin-related CTCF transcription factors. This finding reinforces the role of these regulatory proteins as essential components of the bone gene regulome. We observe a reduction in chromatin accessibility throughout the genome between pre-osteoblast and early osteoblasts. Our analysis also defined a class of differentially expressed genes that harbor DHS peaks centered within 1 kb downstream of transcriptional end sites (TES). These DHSs at the 3’-flanks of genes exhibit dynamic changes during differentiation that may impact regulation of the osteoblast genome. Taken together, the distribution of DHS regions within non-promoter locations harboring osteoblast and chromatin related transcription factor binding motifs, reflect novel cis-regulatory requirements to support temporal gene expression in differentiating osteoblasts.

Investigation of bioeffects of G protein-coupled receptor 1 on bone turnover in male mice

Maintenance of healthy bone quality and quantity requires a well-coordinated balance between bone formation by osteoblasts and bone resorption by osteoclasts. Chemerin is a novel adipokine with known functions such as regulating immunity and energy homeostasis through activation of chemokine-like receptor 1 (CMKLR1). G protein-coupled receptor 1 (GPR1) is the second mammalian chemerin receptor with similar binding affinity as CMKLR1. In male GPR1^-/- mice, a phenotype with significantly low bone mineral density was observed. We hypothesise that GPR1 might participate the process of bone remodelling. In this study, we investigated the role of GPR1 in regulating bone mass maintenance in male mice, and for the first time, revealed that GPR1^-/- male mice manifested seriously trabecular bone loss and lower serum testosterone levels compared to the wild type animals. Accordingly, the mRNA expression of biomarkers related to both osteoblast [collagen type I alpha 2 (Col1A2), osteocalcin (OCN)] and osteoclast [tartrate-resistant acid phosphatase (TRAP), Cathepsin K, NFATc1] were significantly decreased or increased in GPR1^-/- mice relative to the wild type, respectively. However, other osteogenic markers, Osterix and ALP levels, were increased. Microcomputed tomography scanning and histological analyses proved that there was a myriad of trabecular bone loss in GPR1^-/- mice. In the meantime, GPR1^-/- mice presented a significant decrease in serum testosterone level. Taken together, these findings suggested that chemerin-GPR1 signalling might be directly or indirectly communicated with testosterone synthesis on bone turnover regulation. Further detailed studies are required to unveil how chemerin-GPR1 participates in bone metabolism. The translational potential of this article: More studies and knowledge about GPR1 regulating function in bone turnover might supply a novel therapeutic target for osteoporosis in the future.

Differential analysis of genome-wide methylation and gene expression in mesenchymal stem cells of patients with fractures and osteoarthritis

Insufficient activity of the bone-forming osteoblasts leads to low bone mass and predisposes to fragility fractures. The functional capacity of human mesenchymal stem cells (hMSCs), the precursors of osteoblasts, may be compromised in elderly individuals, in relation with the epigenetic changes associated with aging. However, the role of hMSCs in the pathogenesis of osteoporosis is still unclear. Therefore, we aimed to characterize the genome-wide methylation and gene expression signatures and the differentiation capacity of hMSCs from patients with hip fractures. We obtained hMSCs from the femoral heads of women undergoing hip replacement due to hip fractures and controls with hip osteoarthritis. DNA methylation was explored with the Infinium 450K bead array. Transcriptome analysis was done by RNA sequencing. The genomic analyses revealed that most differentially methylated loci were situated in genomic regions with enhancer activity, distant from gene bodies and promoters. These regions were associated with differentially expressed genes enriched in pathways related to hMSC growth and osteoblast differentiation. hMSCs from patients with fractures showed enhanced proliferation and upregulation of the osteogenic drivers RUNX2/OSX. Also, they showed some signs of accelerated methylation aging. When cultured in osteogenic medium, hMSCs from patients with fractures showed an impaired differentiation capacity, with reduced alkaline phosphatase activity and poor accumulation of a mineralized matrix. Our results point to 2 areas of potential interest for discovering new therapeutic targets for low bone mass disorders and bone regeneration: the mechanisms stimulating MSCs proliferation after fracture and those impairing their terminal differentiation.

Selective Distal Enhancer Control of the Mmp13 Gene Identified through Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) Genomic Deletions

Matrix metalloproteinase 13 (Mmp13, collagenase-3) plays an essential role in bone metabolism and mineral homeostasis. It is regulated by numerous factors, including BMP-2, parathyroid hormone, and 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3), through transcription factors such as Runt-related transcription factor 2 (RUNX2), CCAAT/enhancer-binding protein β (C/EBPβ), OSX, and vitamin D receptor (VDR). During osteoblast maturation, the basal expression of Mmp13 and its sensitivity to 1,25(OH)2D3 are strikingly increased. In this report, ChIP-sequencing analysis in mouse preosteoblasts revealed that the Mmp13 gene was probably regulated by three major enhancers located -10, -20, and -30 kb upstream of the gene promoter, occupied by activated VDR and prebound C/EBPβ and RUNX2, respectively. Initially, bacterial artificial chromosome clone recombineering and traditional mutagenesis defined binding sites for VDR and RUNX2. We then employed a CRISPR/Cas9 gene editing approach to delete the -10 and -30 kb Mmp13 enhancers, a region proximal to the promoter, and VDR or RUNX2. VDR-mediated up-regulation of Mmp13 transcription was completely abrogated upon removal of the -10 kb enhancer, resulting in a 1,25(OH)2D3-directed repression of Mmp13. Deletion of either the -30 kb enhancer or RUNX2 resulted in a complete loss of basal transcript activity and a ChIP-identified destabilization of the chromatin enhancer environment and factor binding. Whereas enhancer deletions only affected Mmp13 expression, the RUNX2 deletion led to changes in gene expression, a reduction in cellular proliferation, and an inability to differentiate. We conclude that the Mmp13 gene is regulated via at least three specific distal enhancers that display independent activities yet are able to integrate response from multiple signaling pathways in a model of activation and suppression.

Resveratrol supplementation affects bone acquisition and osteoporosis: Pre-clinical evidence toward translational diet therapy

Osteoporosis is a major public health issue that is expected to rise as the global population ages. Resveratrol (RES) is a plant polyphenol with various anti-aging properties. RES treatment of bone cells results in protective effects, but dose translation from in vitro studies to clinically relevant doses is limited since bioavailability is not taken into account. The aims of this review is to evaluate in vivo evidence for a role of RES supplementation in promoting bone health to reduced osteoporosis risk and potential mechanisms of action. Due to multiple actions on both osteoblasts and osteoclasts, RES has potential to attenuate bone loss resulting from different etiologies and pathologies. Several animal models have investigated the bone protective effects of RES supplementation. Ovariectomized rodent models of rapid bone loss due to estrogen-deficiency reported that RES supplementation improved bone mass and trabecular bone without stimulating other estrogen-sensitive tissues. RES supplementation prior to age-related bone loss was beneficial. The hindlimb unloaded rat model used to investigate bone loss due to mechanical unloading showed RES supplementation attenuated bone loss in old rats, but had inconsistent bone effects in mature rats. In growing rodents, RES increased longitudinal bone growth, but had no other effects on bone. In the absence of human clinical trials, evidence for a role of RES on bone heath relies on evidence generated by animal studies. A better understanding of efficacy, safety, and molecular mechanisms of RES on bone will contribute to the determination of dietary recommendations and therapies to reduce osteoporosis. This article is part of a Special Issue entitled: Resveratol: Challenges in translating pre-clinical findings to improved patient outcomes.

The RUNX2 cistrome in osteoblasts: characterization, down-regulation following differentiation, and relationship to gene expression

RUNX2 is a transcription factor that is first expressed in early osteoblast-lineage cells and represents a primary determinant of osteoblastogenesis. While numerous target genes are regulated by RUNX2, little is known of sites on the genome occupied by RUNX2 or of the gene networks that are controlled by these sites. To explore this, we conducted a genome-wide analysis of the RUNX2 cistrome in both pre-osteoblastic MC3T3-E1 cells (POB) and their mature osteoblast progeny (OB), characterized the two cistromes and assessed their relationship to changes in gene expression. We found that although RUNX2 was widely bound to the genome in POB cells, this binding profile was reduced upon differentiation to OBs. Numerous sites were lost upon differentiation, new sites were also gained; many sites remained common to both cell states. Additional features were identified as well including location relative to potential target genes, abundance with respect to single genes, the frequent presence of a consensus TGTGGT RUNX2 binding motif, co-occupancy by C/EBPβ and the presence of a typical epigenetic histone enhancer signature. This signature was changed quantitatively following differentiation. While RUNX2 binding sites were associated extensively with adjacent genes, the distal nature of the majority of these sites prevented assessment of whether they represented direct targets of RUNX2 action. Changes in gene expression, however, revealed an abundance of genes that contained RUNX2 binding sites and were regulated in concert. These studies establish a basis for further analysis of the role of RUNX2 activity and its function during osteoblast lineage maturation.

Epigenetic landscape during osteoblastogenesis defines a differentiation-dependent Runx2 promoter region

Runx2 is a developmentally regulated gene in vertebrates and is essential for bone formation and skeletal homeostasis. The induction of runx2-P1 isoform transcripts is a hallmark of early osteoblastogenesis. Although previous in vitro studies have defined a minimal Runx2-P1 promoter sequence with well-characterized functional elements, several lines of evidence suggest that transcription of the Runx2-P1 isoform relies on elements that extend beyond the previously defined P1 promoter boundaries. In this study, we examined Runx2-P1 transcriptional regulation in a cellular in vivo context during early osteoblastogenesis of MC3T3-E1 cultures and BMSCs induced towards the bone lineage by multi-layered analysis of the Runx2-P1 gene promoter using the following methodologies: 1) sequence homology among several mammalian species, 2) DNaseI hypersensitivity coupled with massively parallel sequencing (DNase-seq), and 3) chromatin immunoprecipitation of activating histone modifications coupled with massively parallel sequencing (ChIP-seq). These epigenetic features have allowed the demarcation of boundaries that redefine the minimal Runx2-P1 promoter to include a 336-bp sequence that mediates responsiveness to osteoblast differentiation. We also find that an additional level of control is contributed by a regulatory region in the 5'-UTR of Runx2-P1.