Connective tissue is an irreplaceable component of the musculoskeletal system; however it has attracted far less attention than other tissue types. Odd-skipped is a drosophila pair-rule gene involved in embryonic segmentation. In mammals, two odd-skipped paralogs exist, Osr1 and Osr2. Odd-skipped genes encode zinc-finger transcription factors expressed in several organs of the developing embryo. For Osr1, functions in heart and kidney development have been described, while Osr2 has been linked to secondary palate formation and tooth development. We have analysed the expression of Osr1 and Osr2 in the chick limb and found both genes to be highly specific for irregular connective tissue. We have recently shown that Osr1 and Osr2 are both involved in the differentiation of irregular connective tissue fibroblasts and are potent inhibitors of mesenchymal differentiation along other cell lineages (Stricker et al. 2012).

Expression of Osr1 and Osr2: left panel: in-situ hybridisation for Osr1 and Osr2 showing widespread expression of both genes in the chicken limb at Hamburger-Hamilton stage 32. The right panel shows a two-colour ISH for Osr1 or Osr2 together with the myogenic factor MyoD. This shows expression of both Osr genes in chicken muscle connective tissue. Modified from Stricker et al. 2012.

A global analysis of Osr1 target genes in comparison to other connective tissue-expressed transcription factors via combining RNA-Sequencing and ChIP-Sequencing highlighted paracrine signalling molecules as well as extracellular matrix molecules as pivotal downstream targets. Intriguingly, different transcription factors from different connective tissue sub-types showed a remarkable overlap in extracellular matrix-related gene expression, defining a core connective tissue signature. Each transcription factor, on the other hand, regulated specific matrix targets, indicating a transcription factor-based matrix code during limb development, instructive for tissue patterning (Orgeur et al. 2018).

Transcriptional network of five connective tissue-expressed transcription factors reflecting common and individual target genes coding extracellular matrix components. From Orgeur et al. 2018.

Going from chicken to mouse as model system we found a conserved expression pattern of Osr1 in muscle connective tissue during limb development. Loss of Osr1 function impaired connective tissue-mediated limb muscle patterning. This involved an altered expression of extracellular matrix molecules, which we assume to play a key role in instructing myogenic progenitors and maintaining the myogenic pool.

Osr1, expressed in muscle connective tissue, is necessary for correct myogenesis in the mouse limb. Whole hindlimb muscle reconstruction shows defects in several muscles (highlighted in pseudocoloration) in Osr1 mutants. Adapted from (Vallecillo-Garcia et al. 2017).

Furthermore we showed that Osr1-expressing mesenchymal cells in the embryonic limb bud are a developmental source of the so-called fibro-adipogenic progenitors, FAPs, an adult muscle-resident mesenchymal stem cell-like population (Vallecillo-Garcia et al. 2017).

Adult muscle-resident mouse FAPs do not express Osr1, as opposed to their developmental counterparts. Intriguingly, upon acute injury, FAPs re-activate Osr1 expression, making Osr1 a suitable marker for FAPs during muscle injury regeneration (Stumm et al. 2018). At present we analyse the function of Osr1 in adult FAPs during muscle regeneration.

Re-expression of Osr1 in injured muscle shown by LacZ staining from an Osr1-LacZ allele. Adapted from (Stumm et al. 2018)

This project is a part of the international graduate school for Myology, “MyoGrad”

Collaboration partners: 

• Delphine Duprez, UMR7622, CNRS, Université Pierre et Marie Curie, Laboratoire de Biologie du Développement, Campus de Jussieu, Paris, France 
• David Sassoon, Giovanna Marazzi; Stem Cells and Regenerative Medicine,ICAN Institute of Cardiometabolism and Nutrition, UMRS 1166, INSERM/Sorbonne University (Université Pierre et Marie Curie Paris VI), Paris, France
• Chiara Mozzetta, Dept. of Biology and Biotechnology "Charles Darwin", Sapienza - University of Rome, P.le A. Moro 5, 00185 Rome, Italy
• Tim J. Schulz, Deutsches Institut für Ernährungsforschung, Arthur-Scheunert-Allee 114-116, 14558 Nuthetal