Differential tissue specific, temporal and spatial expression patterns of the Aggrecan gene is modulated by independent enhancer elements

Pre-hypertrophic chondrogenic enhancer landscape of limb and axial skeleton development

Chondrocyte differentiation controls skeleton development and stature. Here we provide a comprehensive map of chondrocyte-specific enhancers and show that they provide a mechanistic framework through which non-coding genetic variants can influence skeletal development and human stature. Working with fetal chondrocytes isolated from mice bearing a Col2a1 fluorescent regulatory sensor, we identify 780 genes and 2'704 putative enhancers specifically active in chondrocytes using a combination of RNA-seq, ATAC-seq and H3K27ac ChIP-seq. Most of these enhancers (74%) show pan-chondrogenic activity, with smaller populations being restricted to limb (18%) or trunk (8%) chondrocytes only. Notably, genetic variations overlapping these enhancers better explain height differences than those overlapping non-chondrogenic enhancers. Finally, targeted deletions of identified enhancers at the Fgfr3, Col2a1, Hhip and, Nkx3-2 loci confirm their role in regulating cognate genes. This enhancer map provides a framework for understanding how genes and non-coding variations influence bone development and diseases.

Lineage-specific differences and regulatory networks governing human chondrocyte development

To address large gaps in our understanding of the molecular regulation of articular and growth plate cartilage development in humans, we used our directed differentiation approach to generate these distinct cartilage tissues from human embryonic stem cells. The resulting transcriptomic profiles of hESC-derived articular and growth plate chondrocytes were similar to fetal epiphyseal and growth plate chondrocytes, with respect to genes both known and previously unknown to cartilage biology. With the goal to characterize the regulatory landscapes accompanying these respective transcriptomes, we mapped chromatin accessibility in hESC-derived chondrocyte lineages, and mouse embryonic chondrocytes, using ATAC-sequencing. Integration of the expression dataset with the differentially accessible genomic regions revealed lineage-specific gene regulatory networks. We validated functional interactions of two transcription factors (TFs) (RUNX2 in growth plate chondrocytes and RELA in articular chondrocytes) with their predicted genomic targets. The maps we provide thus represent a framework for probing regulatory interactions governing chondrocyte differentiation. This work constitutes a substantial step towards comprehensive and comparative molecular characterizations of distinct chondrogenic lineages and sheds new light on human cartilage development and biology.

Shoc2 controls ERK1/2-driven neural crest development by balancing components of the extracellular matrix

The extracellular signal-regulated kinase (ERK1/2) pathway is essential in embryonic development. The scaffold protein Shoc2 is a critical modulator of ERK1/2 signals, and mutations in the shoc2 gene lead to the human developmental disease known as Noonan-like syndrome with loose anagen hair (NSLH). The loss of Shoc2 and the shoc2 NSLH-causing mutations affect the tissues of neural crest (NC) origin. In this study, we utilized the zebrafish model to dissect the role of Shoc2-ERK1/2 signals in the development of NC. These studies established that the loss of Shoc2 significantly altered the expression of transcription factors regulating the specification and differentiation of NC cells. Using comparative transcriptome analysis of NC-derived cells from shoc2 CRISPR/Cas9 mutant larvae, we found that Shoc2-mediated signals regulate gene programs at several levels, including expression of genes coding for the proteins of extracellular matrix (ECM) and ECM regulators. Together, our results demonstrate that Shoc2 is an essential regulator of NC development. This study also indicates that disbalance in the turnover of the ECM may lead to the abnormalities found in NSLH patients.

Integrated regulation of chondrogenic differentiation in mesenchymal stem cells and differentiation of cancer cells

Chondrogenesis is the formation of chondrocytes and cartilage tissues and starts with mesenchymal stem cell (MSC) recruitment and migration, condensation of progenitors, chondrocyte differentiation, and maturation. The chondrogenic differentiation of MSCs depends on co-regulation of many exogenous and endogenous factors including specific microenvironmental signals, non-coding RNAs, physical factors existed in culture condition, etc. Cancer stem cells (CSCs) exhibit self-renewal capacity, pluripotency and cellular plasticity, which have the potential to differentiate into post-mitotic and benign cells. Accumulating evidence has shown that CSCs can be induced to differentiate into various benign cells including adipocytes, fibrocytes, osteoblast, and so on. Retinoic acid has been widely used in the treatment of acute promyelocytic leukemia. Previous study confirmed that polyploid giant cancer cells, a type of cancer stem-like cells, could differentiate into adipocytes, osteocytes, and chondrocytes. In this review, we will summarize signaling pathways and cytokines in chondrogenic differentiation of MSCs. Understanding the molecular mechanism of chondrogenic differentiation of CSCs and cancer cells may provide new strategies for cancer treatment.

Hyaline cartilage differentiation of fibroblasts in regeneration and regenerative medicine

Amputation injuries in mammals are typically non-regenerative; however, joint regeneration is stimulated by BMP9 treatment, indicating the presence of latent articular chondrocyte progenitor cells. BMP9 induces a battery of chondrogenic genes in vivo, and a similar response is observed in cultures of amputation wound cells. Extended cultures of BMP9-treated cells results in differentiation of hyaline cartilage, and single cell RNAseq analysis identified wound fibroblasts as BMP9 responsive. This culture model was used to identify a BMP9-responsive adult fibroblast cell line and a culture strategy was developed to engineer hyaline cartilage for engraftment into an acutely damaged joint. Transplanted hyaline cartilage survived engraftment and maintained a hyaline cartilage phenotype, but did not form mature articular cartilage. In addition, individual hypertrophic chondrocytes were identified in some samples, indicating that the acute joint injury site can promote osteogenic progression of engrafted hyaline cartilage. The findings identify fibroblasts as a cell source for engineering articular cartilage and establish a novel experimental strategy that bridges the gap between regeneration biology and regenerative medicine.

Summary:In vivo articular cartilage regeneration serves as a model to develop novel approaches for engineering cartilage to repair damaged joints and identifies fibroblasts as a BMP9-inducible chondroprogenitor.

A highly porous type II collagen containing scaffold for the treatment of cartilage defects enhances MSC chondrogenesis and early cartilaginous matrix deposition

The combination of type II collagen (CII) and hyaluronic acid (HyA) resulted in the development of a CII-containing scaffold with improved chondrogenic benefits for simple and effective “off-the-shelf” application for enhanced cartilage repair.

Joint disease-specificity at the regulatory base-pair level

Given the pleiotropic nature of coding sequences and that many loci exhibit multiple disease associations, it is within non-coding sequence that disease-specificity likely exists. Here, we focus on joint disorders, finding among replicated loci, that GDF5 exhibits over twenty distinct associations, and we identify causal variants for two of its strongest associations, hip dysplasia and knee osteoarthritis. By mapping regulatory regions in joint chondrocytes, we pinpoint two variants (rs4911178; rs6060369), on the same risk haplotype, which reside in anatomical site-specific enhancers. We show that both variants have clinical relevance, impacting disease by altering morphology. By modeling each variant in humanized mice, we observe joint-specific response, correlating with GDF5 expression. Thus, we uncouple separate regulatory variants on a common risk haplotype that cause joint-specific disease. By broadening our perspective, we finally find that patterns of modularity at GDF5 are also found at over three-quarters of loci with multiple GWAS disease associations.

Cartilage-Specific Cre Recombinase Transgenes/Alleles in the Mouse

Cartilage is a specialized skeletal tissue with a unique extracellular matrix elaborated by its resident cells, chondrocytes. The tissue presents in several forms, including growth plate and articular cartilage, wherein chondrocytes follow a differential differentiation program and have different fates. The induction of gene modifications in cartilage specifically relies on mouse transgenes and knockin alleles taking advantages of transcriptional elements primarily active in chondrocytes at a specific differentiation stage or in a specific cartilage type. These transgenes/alleles have been widely used to study the roles of specific genes in cartilage development, adult homeostasis, and pathology. As cartilage formation is critical for postnatal life, the inactivation or significant alteration of key cartilaginous genes is often neonatally lethal and therefore hampers postnatal studies. Gold standard approaches to induce postnatal chondrocyte-specific gene modifications include the Cre-loxP and Tet-ON/OFF systems. Selecting the appropriate promoter/enhancer sequences to drive Cre expression is of crucial importance and determines the specificity of conditional gain- or loss-of-function models. In this chapter, we discuss a series of transgenes and knockin alleles that have been developed for gene manipulation in cartilage and we compare their expression patterns and efficiencies.

Post-traumatic osteoarthritis development is not modified by postnatal chondrocyte deletion of Ccn2

CCN2 is a matricellular protein involved in several crucial biological processes. In particular, CCN2 is involved in cartilage development and in osteoarthritis. Ccn2 null mice exhibit a range of skeletal dysmorphisms, highlighting its importance in regulating matrix formation during development; however, its role in adult cartilage remains unclear. The aim of this study was to determine the role of CCN2 in postnatal chondrocytes in models of post-traumatic osteoarthritis (PTOA). Ccn2 deletion was induced in articular chondrocytes of male transgenic mice at 8 weeks of age. PTOA was induced in knees either surgically or non-invasively by repetitive mechanical loading at 10 weeks of age. Knee joints were harvested, scanned with micro-computed tomography and processed for histology. Sections were stained with Toluidine Blue and scored using the Osteoarthritis Research Society International (OARSI) grading system. In the non-invasive model, cartilage lesions were present in the lateral femur, but no significant differences were observed between wild-type (WT) and Ccn2 knockout (KO) mice 6 weeks post-loading. In the surgical model, severe cartilage degeneration was observed in the medial compartments, but no significant differences were observed between WT and Ccn2 KO mice at 2, 4 and 8 weeks post-surgery. We conclude that Ccn2 deletion in chondrocytes does not modify the development of PTOA in mice, suggesting that chondrocyte expression of CCN2 in adults is not a crucial factor in protecting cartilage from the degeneration associated with PTOA.This article has an associated First Person interview with the first author of the paper.

Histone ChIP-Seq identifies differential enhancer usage during chondrogenesis as critical for defining cell-type specificity

Epigenetic mechanisms are known to regulate gene expression during chondrogenesis. In this study, we have characterized the epigenome during the in vitro differentiation of human mesenchymal stem cells (hMSCs) into chondrocytes. Chromatin immunoprecipitation followed by next‐generation sequencing (ChIP‐seq) was used to assess a range of N‐terminal posttranscriptional modifications (marks) to histone H3 lysines (H3K4me3, H3K4me1, H3K27ac, H3K27me3, and H3K36me3) in both hMSCs and differentiated chondrocytes. Chromatin states were characterized using histone ChIP‐seq and cis‐regulatory elements were identified in chondrocytes. Chondrocyte enhancers were associated with chondrogenesis‐related gene ontology (GO) terms. In silico analysis and integration of DNA methylation data with chondrogenesis chromatin states revealed that enhancers marked by histone marks H3K4me1 and H3K27ac were de‐methylated during in vitro chondrogenesis. Similarity analysis between hMSC and chondrocyte chromatin states defined in this study with epigenomes of cell‐types defined by the Roadmap Epigenomics project revealed that enhancers are more distinct between cell‐types compared to other chromatin states. Motif analysis revealed that the transcription factor SOX9 is enriched in chondrocyte enhancers. Luciferase reporter assays confirmed that chondrocyte enhancers characterized in this study exhibited enhancer activity which may be modulated by DNA methylation and SOX9 overexpression. Altogether, these integrated data illustrate the cross‐talk between different epigenetic mechanisms during chondrocyte differentiation.