Project leader: GRAZIELLA MESSINA (graziella.messina@unimi.it)

Project leader: GRAZIELLA MESSINA (graziella.messina@unimi.it)

Location: Department of Biosciences, University of Milan, Italy

RESEARCH PROJECT SUMMARY

Skeletal muscle is the tissue responsible for posture, locomotion and diaphragmatic breathing. The molecular mechanisms regulating muscle differentiation and maturation are quite well characterized ^1-4. Interestingly, skeletal myogenesis, like hematopoiesis, occurs in successive, distinct though overlapping developmental stages that involve different cell populations and expression of different genes. Skeletal muscle is, in fact, a heterogeneous tissue composed of individual muscle fibres, diversified in size, shape and contractile protein content, to fulfil the different functional needs of the vertebrate body. This heterogeneity derives and depends at least in part upon distinct classes of myogenic progenitors, i.e. embryonic and fetal myoblasts and satellite cells. In particular embryonic and fetal myoblasts control the differentiation of the pre-natal musculature, whereas satellite cells (SCs) are responsible for post-natal muscle growth and regeneration following muscle damage or injury ⁵. In particular, embryonic myoblasts fuse into multinucleate muscle fibres and characterize “embryonic” or primary myogenesis necessary to establish the basic muscle pattern. The second wave of myogenesis takes place between E14.5 and E17.5 and involves the fusion of fetal myoblasts either with each other to form secondary fibres (initially smaller and surrounding primary fibres) or, at a minor extent, with primary fibres. A genome wide expression analysis carried on purified embryonic and fetal myoblasts ⁶ identified many differentially expressed genes, clearly revealing that embryonic and fetal myoblasts are intrinsically different populations of myoblasts with distinct genetic programs. We have demonstrated the pivotal role of the transcription factor Nuclear Factor IX, Nfix, in driving the transcriptional switch from embryonic to fetal myogenesis ⁷. Interestingly, satellite cells express high levels of Nfix and we have very interesting results on the role of Nfix even during post-natal muscle regeneration ⁸.

Since we demonstrated that Nfix is able to directly bind the promoters of different genes crucial and/or involved in proper skeletal muscle development and regeneration, this project will aim to perform a genome-wide ChiP-seq analysis for Nfix in order to identify other possible targets of Nfix in skeletal muscle and therefore to discover other and new pathways in which Nfix may play new functions.

References

1. Davis, R.L., Cheng, P.F., Lassar, A.B. & Weintraub, H. The MyoD DNA binding domain contains a recognition code for muscle-specific gene activation. Cell 60, 733-746 (1990).

2. Weintraub, H. The MyoD family and myogenesis: redundancy, networks, and thresholds. Cell 75, 1241-1244 (1993).

3. Relaix, F., Rocancourt, D., Mansouri, A. & Buckingham, M. A Pax3/Pax7-dependent population of skeletal muscle progenitor cells. Nature 435, 948-953 (2005).

4. Hutcheson, D.A., Zhao, J., Merrell, A., Haldar, M. & Kardon, G. Embryonic and fetal limb myogenic cells are derived from developmentally distinct progenitors and have different requirements for beta-catenin. Genes & development 23, 997-1013 (2009).

5. Biressi, S., Molinaro, M. & Cossu, G. Cellular heterogeneity during vertebrate skeletal muscle development. Developmental biology 308, 281-293 (2007).

6. Biressi, S. et al. Intrinsic phenotypic diversity of embryonic and fetal myoblasts is revealed by genome-wide gene expression analysis on purified cells. Developmental biology 304, 633-651 (2007).

7. Messina, G. et al. Nfix regulates fetal-specific transcription in developing skeletal muscle. Cell 140, 554-566

8. Giuliana Rossi, Stefania Antonini, Chiara Vezzali, Giulio Cossu and Graziella Messina The transcription factor Nfix regulates the proper timing of muscle regeneration and the progression of Muscular Dystrophy, in preparation