Satellite Cell Biology and IGF-1 Pathways in Duchenne Muscular Dystrophy

The preservation of muscle function in the muscular dystrophies is the product of the relative success of muscle remodeling (myofiber homeostasis; see Project 2), and muscle regenerative pathways. It is widely believe that muscle regenerative capacity is gradually lost in Duchenne dystrophy, and this "failure of regeneration" is thought to underlie the progressive nature of the disease. One looks to the muscle satellite cells as the source of regenerative capacity, and there is a literature, albeit limited one, on loss of satellite cells and muscle regenerative potential as a function of age both in Duchenne dystrophy, and normal aging adults. To the contrary, we have used an experimental system of isolated muscle fibers to show that there is no significant reduction in number of satellite cells with age in the normal or dystrophic (mdx) mouse. We have shown a progressive reduction in the proliferative capacity of satellite cells during aging, but again this is similar in dystrophic and normal mice. Preliminary experiments indicate that this loss of proliferative capacity may be ameliorated by treatment with IGF-1. This, together with the demonstration of beneficial effects of the IGF-1 transgene on muscle pathology in the mdx mouse, suggests that this class of cytokine may help to slow the progress of the dystrophinopathies by enhancing activation of dormant satellite cells. To validate and extend our models of the underpinnings of "failed regeneration", we propose a series of aims directed towards defining the subpopulations of satellite cells by immunophenotyping (Aim 1), and testing the capacity of these different subpopulations to regenerate muscle in our irradiated, degen/regen host model (Aim 2). The subpopulations will then be tested for response to IGF-1, where we hypothesize that there will be downstream effects on their ability to generate muscle in vivo and in vitro (Aims 1 and 2). Aims 1 and 2 will also investigate the effect of age and dystrophin-deficiency on the specific subpopulations of satellite cells, and will create a "transcriptome" database of these cells that will be matched with their regenerative potential (Core C). Aim 3 is focused on defining transcriptional pathways downstream of IGF-1 treatment, using in vitro time series data, as has been extensively practiced by Core C (Zhao et al. 2002; 2003; 2004; Almon et al. 2003). Again, the effects of age and dystrophy will be studied as variables to determine any blunting or differences in response pathways. The proposed project combines the cell biology and satellite cell expertise of Dr. Partridge, with the mRNA profiling expertise of Dr. Hoffman and Core C, and should result in a resource that can be queried by the larger scientific community, as we have shown for other data sets (see Core C).