Yap: a novel regulator of satellite cell fate with regenerative medicine potential

Stem cells are required to maintain and regenerate our organs. Stem cells are also a treatment option and could potentially be used to replace dead insulin-secreting cells in the pancreas of patients with diabetes. Another strategy is to develop drugs for example to stimulate the production of additional stem cells by the body itself. This could potentially be used to increase the number of stem cells in aged human muscle, where it is reduced roughly by half. The aim of the proposed project is to investigate for the first time a protein termed Yap in satellite cells which are the stem cells of our muscles. We hypothesize that Yap is important for controlling the number of muscle stem cells and their repair activity. To test our hypothesis we will either remove or overactivate Yap in muscle stem cells and then investigate how this affects the numbers and repair activity of muscle stem cells. We then aim to identify the parts on our chromosomes to which Yap binds to in order to investigate how Yap exerts its function in muscle stem cells. Taken together, these experiments will inform us whether Yap is a suitable drug target to modulate the numbers and repair activity of muscle stem cells. If it was then Yap-targetting treatments could be used in the management for example in regulating the behaviour of stem cells in patients with muscular dystrophy and in the elderly.

Satellite cells are somatic skeletal muscle stem cells. Recent experiments demonstrate that satellite cell grafts can be used to improve function in dystrophic muscles and ameliorate muscle ageing in rodents. Thus satellite cell based therapies are now a much more realistic strategy for the treatment of human disease and ageing than several years ago. Satellite cells could potentially also be targeted in situ for example to increase the decreased satellite cell numbers in sarcopenic muscle. Our preliminary data suggest that the Hippo pathway and its core member Yap are expressed in muscle cells including satellite cells. Our functional studies so far suggest that Yap can potentially be targeted to expand satellite cells or to manipulate asymmetric cell division. The aim of the proposed project is to study the function of Yap in satellite cells in vivo. To test whether Yap is required for satellite cell identity, activation and proliferation, we wish to breed a transgenic mouse with a switch that will allow us to knock out Yap selectively in satellite cells. We then wish to study whether postnatal growth, muscle regeneration after injury and muscle hypertrophy are affected as satellite cells play a key role in these processes. Next we plan to transplant transgenic satellite cells into mdx-nude recipient mice and then induce the overexpression of constitutively active hYAP1 S127A in the satellite cell grafts. This gold standard experiment of satellite cell biology will inform us whether Yap can be targeted to activate and/or expand satellite cells and whether Yap affects asymmetric cell division and thus the numbers of satellite cells that go on to differentiate or self-renew. We will also see how this affects the expression of dystrophin in the dystrophic recipients. Finally, we plan to use a method known as ChIP-Seq (i.e. chromatin immunoprecipitation followed by next generation sequencing) to identify binding sequence motifs to which Yap co-activated transcription factors bind in satellite cells and the genes that are directly regulated by Yap. This last experiment will give us insight into how Yap achieves its effects. This is not only important to understand the function of Yap in satellite cells but potentially also in other somatic stem cells, where Yap has already been identified as a key regulator. Taken together, these experiments will inform us about the function of Yap in satellite cells in vivo and about potential clinical applications.