Project description:The connection between different tissues is vital for the development and function of all organs and systems. In the musculoskeletal system, the attachment of elastic tendons to stiff bones poses a mechanical challenge that is solved by the formation of a transitional tissue, which allows the transfer of muscle forces to the skeleton without tearing. Here, we show that tendon-to-bone attachment cells are bi-fated, activating a mixture of chondrocyte and tenocyte transcriptomes, which is regulated by sharing regulatory elements with these cells and by Krüppel-like factors transcription factors (KLF). To uncover the molecular identity of attachment cells, we first applied high-throughput bulk and single-cell RNA sequencing to murine humeral attachment cells. The results, which were validated by in situ and single-molecule in situ hybridization, reveal that together with the expression of unique sets of genes, the attachment cells express hundreds of chondrogenic and tenogenic genes. In search for the underlying mechanism, we performed ATAC sequencing and found that attachment cells share a significant fraction of accessible intergenic chromatin areas with either tenocytes or chondrocytes. Epigenomic analysis further revealed transcriptional enhancer signatures for the majority of these regions. We then examined a subset of these regions using transgenic mouse enhancer reporter assay. Results verified the shared activity of some of these enhancers, supporting the possibility that the transcriptome of attachment cells is regulated by enhancers with shared activities in tenocytes or chondrocytes. Finally, integrative chromatin and motif analyses, as well as the transcriptome data, indicated that KLFs are regulators of attachment cells. Indeed, blocking the expression of Klf2 and Klf4 in the developing limb mesenchyme led to abnormal differentiation of attachment cells, establishing these factors as key regulators of the fate of these cells. In summary, our findings show how the molecular identity of bi-fated attachment cells enables the formation of the unique transitional tissue that connects tendon to bone. More broadly, we show how mixing the transcriptomes of two cell types through shared enhancers and a dedicated set of transcription factors can lead to the formation of a new cell fate that connects them.
Project description:"The connection between different tissues is vital for the development and function of all organs and systems. In the musculoskeletal system, the attachment of elastic tendons to stiff bones poses a mechanical challenge that is solved by the formation of a transitional tissue, which allows the transfer of muscle forces to the skeleton without tearing. Here, we show that tendon-to-bone attachment cells are bi-fated, activating a mixture of chondrocyte and tenocyte transcriptomes, which is regulated by sharing regulatory elements with these cells and by Krüppel-like factors transcription factors (KLF). To uncover the molecular identity of attachment cells, we first applied high-throughput bulk and single-cell RNA sequencing to murine humeral attachment cells. The results, which were validated by in situ and single-molecule in situ hybridization, reveal that together with the expression of unique sets of genes, the attachment cells express hundreds of chondrogenic and tenogenic genes. In search for the underlying mechanism, we performed ATAC sequencing and found that attachment cells share a significant fraction of accessible intergenic chromatin areas with either tenocytes or chondrocytes. Epigenomic analysis further revealed transcriptional enhancer signatures for the majority of these regions. We then examined a subset of these regions using transgenic mouse enhancer reporter assay. Results verified the shared activity of some of these enhancers, supporting the possibility that the transcriptome of attachment cells is regulated by enhancers with shared activities in tenocytes or chondrocytes. Finally, integrative chromatin and motif analyses, as well as the transcriptome data, indicated that KLFs are regulators of attachment cells. Indeed, blocking the expression of Klf2 and Klf4 in the developing limb mesenchyme led to abnormal differentiation of attachment cells, establishing these factors as key regulators of the fate of these cells. In summary, our findings show how the molecular identity of bi-fated attachment cells enables the formation of the unique transitional tissue that connects tendon to bone. More broadly, we show how mixing the transcriptomes of two cell types through shared enhancers and a dedicated set of transcription factors can lead to the formation of a new cell fate that connects them."
Project description:The mechanical challenge of attaching elastic tendons to stiff bones is solved by the formation of a unique transitional tissue. Here, we show that murine tendon-to-bone attachment cells are bi-fated, activating a mixture of chondrocyte and tenocyte transcriptomes, under regulation of shared regulatory elements and Krüppel-like factors (KLFs) transcription factors. High-throughput bulk and single-cell RNA sequencing of humeral attachment cells revealed expression of hundreds of chondrogenic and tenogenic genes, which was validated by in situ hybridization and single-molecule ISH. ATAC sequencing showed that attachment cells share accessible intergenic chromatin areas with either tenocytes or chondrocytes. Epigenomic analysis revealed enhancer signatures for most of these regions. Transgenic mouse enhancer reporter assays verified the shared activity of some of these enhancers. Finally, integrative chromatin and motif analyses and transcriptomic data implicated KLFs as regulators of attachment cells. Indeed, blocking expression of both Klf2 and Klf4 in developing limb mesenchyme impaired their differentiation.
Project description:Tendon from young and old donors was used for RNA-Seq analysis. The aim of the study was to identify differentially expressed tendon transcripts in ageing in order to to characterize molecular mechanisms associated with age-related changes in tendon.
Project description:We have undertaken a screen of mouse limb tendon cells in order to identify molecular pathways involved in tendon development. Mouse limb tendon cells were isolated based on Scleraxis (Scx) expression at different stages of development: E11.5, E12.5 and E14.5 Microarray comparisons were carried out between tendon progenitor and differentiated stages. Forelimbs from E11.5, E12.5 and E14.5 Scx-GFP embryos were collected and dissociated with trypsin to obtain cell suspensions. Scx-positive tendon cells were isolated by FACS. RNA was extracted and Fragmented biotin-labelled cRNA samples were hybridized on Affymetrix Gene Chip Mouse Genome 430 2.0 arrays.
Project description:Adhesion formation after flexor tendon repair remains a clinical problem. Early postoperative motion after tendon repair has been demonstrated to reduce adhesion formation while increasing tendon strength. It is hypothesized that during mobilization, tendon cells experience mechanical shear forces that alter their biology in a fashion that reduces scar formation but also activates key genes involved in tendon healing. To test this hypothesis, primary intrinsic tenocyte cultures were established from flexor tendons of 20 Sprague-Dawley rats and sheared at 50 rpm (0.41 Pa) using a cone viscometer for 6 and 12 hours. Total RNA was harvested and compared with time-matched unsheared controls using cDNA microarrays and Northern blot analysis. Microarray analysis demonstrated that mechanical shear stress induced an overall "antifibrotic" expression pattern with decreased transcription of collagen type I and collagen type III. Shear stress down-regulated profibrotic molecules in the platelet-derived growth factor, insulin-like growth factor, and fibroblast growth factor signaling pathways. In addition, shear stress induced an overall decrease in transforming growth factor (TGF)-beta signaling pathway molecules with down-regulation of TGF-beta2, TGF-beta3, TGF-RI, and TGF-RII expression. Moreover, sheared tendon cells increased expression of matrix metalloproteinases and decreased expression of tissue inhibitors of metalloproteinase, an expression pattern consistent with an antifibrotic increase in extracellular matrix degradation. However, up-regulation of genes implicated in tendon healing, specifically, vascular endothelial growth factor-A and several bone morphogenetic proteins. Interestingly, the known mechanoresponsive gene, TGF-beta1, also implicated in tendon healing, was differentially up-regulated by shear stress. Northern blot validation of our results for TGF-beta1, TGF-beta2, TGF-beta3, and collagen type I demonstrated direct correlation with microarray data. Groups of assays that are related as part of a time series. Computed
Project description:Little is understood about the roles of tendon cells during flexor tendon healing. To better understand tendon cell functions, the Scx-Cre mouse was crossed to the DTR mouse model to facilitate scleraxis lineage cell depletion prior to acute flexor tendon injury and repair. WT (cre-) and experimental (cre+) mice underwent complete transection and repair of the flexor digitorum longus tendon. Repaired tendons were harvested at 14 and 28 days post-repair for bulk RNA-Seq analysis to examine possible mechanisms driving differential healing due to Scx lineage cell depletion.
Project description:To identify if genes is regulated by time of day in human tendon, RNAseq analysis was performed on biopsies taken from patella tendon 12 hours apart in young healthy subjects (9 AM and 9 PM).