Dataset Information

Stress amplification during development of the tendon-to-bone attachment.

ABSTRACT: Mechanical stress is necessary to sustain the mineral content of bone in adults. However, in a developing neonatal mouse, the mineralization of soft tissues progresses despite greatly reduced average mechanical stresses. In adults, these reduced loads would likely lead to bone loss. Although biochemical factors may partly explain these different responses, it is unclear how mineralization is initiated in low load environments. We present here the effect of morphometric data and initial modeling supporting a hypothesis that mechanical factors across several length scales amplify stresses, and we suggest that these stresses are of a level adequate to contribute to mechanical signaling for initiation of mineralization at the developing tendon-to-bone enthesis. A mineral gradient is evident across the insertion from the onset of mineralization. This grading maintains a constant size from early postnatal time points to adulthood. At the tissue level, this grading contributes to reduced stresses in an adult animal and to a minor elevation of stresses in a neonatal animal. At the cellular level, stress concentrations around mineralizing chondrocytes are enhanced in neonatal animals compared with adult animals. The enhancement of stresses around cells at early time points may serve to amplify and transduce low loads in order to initiate mineralization.

SUBMITTER: Liu Y

PROVIDER: S-EPMC4074273 | biostudies-literature | 2014 Oct

REPOSITORIES: biostudies-literature

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Stress amplification during development of the tendon-to-bone attachment.

Liu Yanxin Y Schwartz Andrea G AG Birman Victor V Thomopoulos Stavros S Genin Guy M GM

Biomechanics and modeling in mechanobiology 20131227 5

Mechanical stress is necessary to sustain the mineral content of bone in adults. However, in a developing neonatal mouse, the mineralization of soft tissues progresses despite greatly reduced average mechanical stresses. In adults, these reduced loads would likely lead to bone loss. Although biochemical factors may partly explain these different responses, it is unclear how mineralization is initiated in low load environments. We present here the effect of morphometric data and initial modeling ...[more]

PMID: 24370852

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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."

Dataset Information

Stress amplification during development of the tendon-to-bone attachment.

Publications

Stress amplification during development of the tendon-to-bone attachment.

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