Genetic and epigenetic control of adult muscle fiber type – MYOLINC
In adult skeletal muscles, myofibers are classified based on their contraction velocity, a property that is directly related to the isoform of the myosin heavy chain (Myh) gene expressed within each individual myofiber. There are four main myofiber types which express either slow (Myh7) or fast (Myh1, Myh2, Myh4) forms of the Myh gene. Important to this application, the three fast Myh genes are organized as a single locus of ~300 kb where the expression of each gene is regulated in a highly restricted spatio-temporal manner, much like Globin genes within the ß-globin locus, allowing a single fast Myh gene to be expressed in hundreds of myonuclei of an individual fiber. Previous work from the Maire lab has identified an enhancer for the fast Myh locus, and shown that Six family of transcription factors bind to it to positively regulate the expression of the fast Myh genes as well as a novel gene coding for a long intergenic non-coding RNAs (linc-RNA) – termed linc-Myh. In addition to directly activating fast Myh4 gene expression, linc-Myh also acts in trans to suppress the expression of the slow fiber type genes and ensure efficient fast fiber-type specialization.
There is a gap in our knowledge with respect to the mechanisms controlling the spatiotemporal expression of the fast Myh genes, and whether the identified enhancer may control 3D chromatin landscape of the locus to direct and restrict the transcription of each fast Myh during development and in the adult. In this project we will test the hypothesis that the identified enhancer is responsible for organizing the alternate DNA conformations within the locus to ensure that only a single Myh gene is expressed in the muscle fiber.
We currently lack an understanding of how linc-Myh locks fast myofiber specialization. We hypothesize that linc-Myh targets epigenetic enzymes to specific genomic loci through a direct tethering to DNA sequences. This hypothesis will be tested using genomic and proteomic approaches to identify nuclear proteins associated with linc-Myh and to characterize the DNA sequences and associated genes bound by linc-Myh-protein complexes.
We currently have an incomplete view of the epigenetics in slow/oxidative/Myh7 and fast subtypes myofibres. Furthermore, myofibers epigenetics marks may be dynamic during development, in trained or untrained animals, during aging and in pathophysiological conditions.
Genesis of slow and fast myofibre subtypes during perinatal development and development of associated slow and fast fatigable or resistant motoneurons is presently not understood.
In the present proposal we will address some fundamental biological questions associated with adult muscle homeostasis focusing on the myofiber compartment:
1°) Does the central Six1-bound enhancer at the Myh fast locus control its 3D chromatin architecture?
2°) How are adaptive answers in slow and fast subtype myofibers translated at the epigenome level? More specifically, we will test the consequences of a Six1 null environment (which is observed in slower myofibers) on the epigenetic landscape of adult and old mice.
3°) How does linc-Myh trigger both down-regulation of slow muscle genes and up-regulation of fast type muscle genes? Knowledge of linc-Myh mode of action will help understanding how myofiber specialization and plasticity is finely controlled.
These aims will be achieved by genetic approaches using knockout models for the SIX genes, by analysis of BAC transgenic mice, by the creation of a CrispR/Cas9 Myh-enhancer KO, and by using genomics and proteomics approaches.
This proposal will further our understanding of muscle homeostasis. This has important implication for its own sake, and may also have direct applications to human well-being; it is known that during ageing, in DMD patients, in cancer cachexia fast glycolytic myofibers are more fragile than slow oxidative fibers, but the basis of this higher fragility are not completely understood.
Monsieur Pascal MAIRE (INSTITUT NATIONAL DE LA SANTE ET DE LA RECHERCHE MEDICALE)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
IMRB Institut Mondor de Recherche Biomedicale - Institut National de La Santé et de la Recherche Medicale
INSERM INSTITUT NATIONAL DE LA SANTE ET DE LA RECHERCHE MEDICALE
Help of the ANR 546,090 euros
Beginning and duration of the scientific project: January 2017 - 48 Months