Adult skeletal muscle stem cell regulation by Bone morphogenetic protein signaling – BMP-MYOSTEM

Adult skeletal muscle possesses the ability to fully regenerate and this depends on the presence of quiescent muscle stem cells, so-called satellite cells that lie in juxtaposition with each myofiber and which become activated upon injury and form progenitors to replace degenerated muscle fibers. Understanding signaling cues that regulate satellite cell function is a pivotal step towards the development of stem cell therapies as well as understanding the pathophysiological basis of muscle wasting disorders with failing regeneration potential.

We here hypothesize, based on preliminary findings, that Bone morphogenetic proteins (BMPs), diffusible morphogens of the Transforming growth factor-ß (TGF-ß) superfamily of signaling molecules, and their antagonists are essential factors that regulate satellite cell quiescence, quiescence exit and subsequent steps that permit muscle regeneration and renewal of the stem cell pool. Preliminary data further suggest that the BMP antagonist Gremlin, which is regulated by the transcription factor Six1, modulates BMP signaling and the myogenic lineage progression of satellite cells. In analogy to our previous findings in differentiated skel-etal muscle, we further hypothesize that BMPs, via activating their downstream target pSmad1/5/8, protect satellite cells from the inhibiting TGF-ß/Myostatin/pSmad2/3 signaling by competing for the available co-Smad4.

Here we will determine the cellular and molecular mechanism of how BMPs and antagonistic pathways control adult satellite cell function. We will question the role of different components along the BMP/TGF-ß signaling cascade by using a panel of conditional gain- and loss-of-function experiments targeted to satellite cells which allow manipulating the expression of BMP-ligands, -antagonists, -transmembrane receptor and BMP/TGF-ß induced Smad transcriptional factors. These transgenic mouse models will elucidate the impact of BMP signaling on the behaviour of satellite cells, their ability to maintain or exit quiescence, to become activated, to self-renew and to regenerate muscle following injury. We will identify BMP target genes by analysing the transcriptome response following satellite cell specific manipulation of BMP signaling, which will be confirmed by ChIP analysis and subsequent functional analysis of ascertained BMP targets. We will determine the exact role of the upstream regulatory Six/Eya/Dach complex on BMP signaling by transgenic loss-of-function experiments. We will also analyse, how BMPs and other TGF-ßs intersect to modulate respective signaling cascades along the myogenic lineage progression.

The aims/tasks of the proposed project are therefore:
1. To determine the cellular mechanism of BMP responsiveness in quiescent satellite cells
2. To understand the role of BMP signaling during muscle regeneration
3. To determine the autocrine role of BMP/TGF-ß signals on satellite cell behavior
4. To determine the molecular mechanism of BMP signaling in satellite cells

Expected results will decipher how BMP signaling regulates satellite cells in their stem cell niche and during regeneration. These findings will allow to project on future strategies aiming at modulating satellite cell behaviour in order to increase the regenerative capacity of skeletal muscle.