Genome-wide screening in human growth plates during puberty in one patient suggests a role for RUNX2 in epiphyseal maturation

Brd4 is required for chondrocyte differentiation and endochondral ossification

Differentiation of multi-potent mesenchymal stromal cells (MSCs) is directed by the activities of lineage-specific transcription factors and co-factors. A subset of these proteins controls the accessibility of chromatin by recruiting histone acetyl transferases or deacetylases that regulate acetylation of the N-termini of H3 and H4 histone proteins. Bromodomain (BRD) proteins recognize these acetylation marks and recruit the RNA pol II containing transcriptional machinery. Our previous studies have shown that Brd4 is required for osteoblast differentiation in vitro. Here, we investigated the role of Brd4 on endochondral ossification in C57BL/6 mice and chondrogenic differentiation in cell culture models. Conditional loss of Brd4 in the mesenchyme (Brd4 cKO, Brd4^fl/fl: Prrx1-Cre) yields smaller mice that exhibit alteration in endochondral ossification. Importantly, abnormal growth plate morphology and delayed long bone formation is observed in juvenile Brd4 cKO mice. One week old Brd4 cKO mice have reduced proliferative and hypertrophic zones within the physis and exhibit a delay in the formation of the secondary ossification center. At the cellular level, Brd4 function is required for chondrogenic differentiation and maturation of both ATDC5 cells and immature mouse articular chondrocytes. Mechanistically, Brd4 loss suppresses Sox9 levels and reduces expression of Sox9 and Runx2 responsive endochondral genes (e.g., Col2a1, Acan, Mmp13 and Sp7/Osx). Collectively, our results indicate that Brd4 is a key epigenetic regulator required for normal chondrogenesis and endochondral ossification.

An ECHO of Cartilage: In Silico Prediction of Combinatorial Treatments to Switch Between Transient and Permanent Cartilage Phenotypes With Ex Vivo Validation

A fundamental question in cartilage biology is: what determines the switch between permanent cartilage found in the articular joints and transient hypertrophic cartilage that functions as a template for bone? This switch is observed both in a subset of OA patients that develop osteophytes, as well as in cell-based tissue engineering strategies for joint repair. A thorough understanding of the mechanisms regulating cell fate provides opportunities for treatment of cartilage disease and tissue engineering strategies. The objective of this study was to understand the mechanisms that regulate the switch between permanent and transient cartilage using a computational model of chondrocytes, ECHO. To investigate large signaling networks that regulate cell fate decisions, we developed the software tool ANIMO, Analysis of Networks with interactive Modeling. In ANIMO, we generated an activity network integrating 7 signal transduction pathways resulting in a network containing over 50 proteins with 200 interactions. We called this model ECHO, for executable chondrocyte. Previously, we showed that ECHO could be used to characterize mechanisms of cell fate decisions. ECHO was first developed based on a Boolean model of growth plate. Here, we show how the growth plate Boolean model was translated to ANIMO and how we adapted the topology and parameters to generate an articular cartilage model. In ANIMO, many combinations of overactivation/knockout were tested that result in a switch between permanent cartilage (SOX9+) and transient, hypertrophic cartilage (RUNX2+). We used model checking to prioritize combination treatments for wet-lab validation. Three combinatorial treatments were chosen and tested on metatarsals from 1-day old rat pups that were treated for 6 days. We found that a combination of IGF1 with inhibition of ERK1/2 had a positive effect on cartilage formation and growth, whereas activation of DLX5 combined with inhibition of PKA had a negative effect on cartilage formation and growth and resulted in increased cartilage hypertrophy. We show that our model describes cartilage formation, and that model checking can aid in choosing and prioritizing combinatorial treatments that interfere with normal cartilage development. Here we show that combinatorial treatments induce changes in the zonal distribution of cartilage, indication possible switches in cell fate. This indicates that simulations in ECHO aid in describing pathologies in which switches between cell fates are observed, such as OA.

Gene regulation could be attributed to TCF3 and other key transcription factors in the muscle of pubertal heifers

Puberty is a whole-body event, driven by the hypothalamic integration of peripheral signals such as leptin or IGF-1. In the process of puberty, reproductive development is simultaneous to growth, including muscle growth. To enhance our understanding of muscle function related to puberty, we performed transcriptome analyses of muscle samples from six pre- and six post-pubertal Brahman heifers (Bos indicus). Our aims were to perform differential expression analyses and co-expression analyses to derive a regulatory gene network associate with puberty. As a result, we identified 431 differentially expressed (DEx) transcripts (genes and non-coding RNAs) when comparing pre- to post-pubertal average gene expression. The DEx transcripts were compared with all expressed transcripts in our samples (over 14,000 transcripts) for functional enrichment analyses. The DEx transcripts were associated with "extracellular region," "inflammatory response" and "hormone activity" (adjusted p < .05). Inflammatory response for muscle regeneration is a necessary aspect of muscle growth, which is accelerated during puberty. The term "hormone activity" may signal genes that respond to progesterone signalling in the muscle, as the presence of this hormone is an important difference between pre- and post-pubertal heifers in our experimental design. The DEx transcript with the highest average expression difference was a mitochondrial gene, ENSBTAG00000043574 that might be another important link between energy metabolism and puberty. In the derived co-expression gene network, we identified six hub genes: CDC5L, MYC, TCF3, RUNX2, ATF2 and CREB1. In the same network, 48 key regulators of DEx transcripts were identified, using a regulatory impact factor metric. The hub gene TCF3 was also a key regulator. The majority of the key regulators (22 genes) are members of the zinc finger family, which has been implicated in bovine puberty in other tissues. In conclusion, we described how puberty may affect muscle gene expression in cattle.

Molecular characterization of physis tissue by RNA sequencing

The physis is a well-established and anatomically distinct cartilaginous structure that is crucial for normal long-bone development and growth. Abnormalities in physis function are linked to growth plate disorders and other pediatric musculoskeletal diseases. Understanding the molecular pathways operative in the physis may permit development of regenerative therapies to complement surgically-based procedures that are the current standard of care for growth plate disorders. Here, we performed next generation RNA sequencing on mRNA isolated from human physis and other skeletal tissues (e.g., articular cartilage and bone; n = 7 for each tissue). We observed statistically significant enrichment of gene sets in the physis when compared to the other musculoskeletal tissues. Further analysis of these upregulated genes identified physis-specific networks of extracellular matrix proteins including collagens (COL2A1, COL6A1, COL9A1, COL14A1, COL16A1) and matrilins (MATN1, MATN2, MATN3), and signaling proteins in the WNT pathway (WNT10B, FZD1, FZD10, DKK2) or the FGF pathway (FGF10, FGFR4). Our results provide further insight into the gene expression networks that contribute to the physis' unique structural composition and regulatory signaling networks. Physis-specific expression profiles may guide ongoing initiatives in tissue engineering and cell-based therapies for treatment of growth plate disorders and growth modulation therapies. Furthermore, our findings provide new leads for therapeutic drug discovery that would permit future intervention through pharmacological rather than surgical strategies.

Fetal mesenchymal stromal cells differentiating towards chondrocytes acquire a gene expression profile resembling human growth plate cartilage

We used human fetal bone marrow-derived mesenchymal stromal cells (hfMSCs) differentiating towards chondrocytes as an alternative model for the human growth plate (GP). Our aims were to study gene expression patterns associated with chondrogenic differentiation to assess whether chondrocytes derived from hfMSCs are a suitable model for studying the development and maturation of the GP. hfMSCs efficiently formed hyaline cartilage in a pellet culture in the presence of TGFβ3 and BMP6. Microarray and principal component analysis were applied to study gene expression profiles during chondrogenic differentiation. A set of 232 genes was found to correlate with in vitro cartilage formation. Several identified genes are known to be involved in cartilage formation and validate the robustness of the differentiating hfMSC model. KEGG pathway analysis using the 232 genes revealed 9 significant signaling pathways correlated with cartilage formation. To determine the progression of growth plate cartilage formation, we compared the gene expression profile of differentiating hfMSCs with previously established expression profiles of epiphyseal GP cartilage. As differentiation towards chondrocytes proceeds, hfMSCs gradually obtain a gene expression profile resembling epiphyseal GP cartilage. We visualized the differences in gene expression profiles as protein interaction clusters and identified many protein clusters that are activated during the early chondrogenic differentiation of hfMSCs showing the potential of this system to study GP development.