Project 3.  Modulation of atrophy pathways in muscular dystrophy therapeutics.

H. Lee Sweeney, PhD.  University of Pennsylvania.

The major goals of this project are to slow the loss of function that occurs in skeletal muscular dystrophy as a result of either increased protein breakdown or apopotosis. This functional loss is accelerated during disuse, leading to even greater levels of injury upon reloading.  Of military relevance is the fact that disuse atrophy is a major problem during extended bed rest due to hospitalization, or due to limb immobilization associated with casting. It also is one of the major hurdles that must be overcome for extended human exposure to microgravity (i.e. long periods in the space environment). We will take advantage of the ability of recombinant adeno-associated virus (AAV) to effect gene transfer in skeletal muscle, as well as the availability of existing targeted protease inhibitors. This work will lay the foundations for both gene transfer and pharmacological therapies to preserve muscle function during in muscular dystrophy and during periods of unloading.

Loss of muscle occurs both in muscular dystrophy and in disuse atrophy. In both cases, accelerated protein breakdown plays a major role. In disuse atrophy, and likely in some forms of muscular dystrophy, there is a significant apoptotic component to the turnover of myonuclei. In all forms of muscular dystrophy, the rate of muscle loss is accelerated when the mass falls to a point that disuse atrophy is triggered. The accelerated muscle breakdown with disuse and in dystrophy involves a number of proteolytic pathways that provide possible therapeutic targets in many forms of muscular dystrophy. The goal would be to slow the turnover of muscle, which would maintain an increased muscle mass (and spare satellite cells in muscular dystrophy), leading to more functional muscle. Such treatments would not only benefit the muscular dystrophies, but would lead to attenuation of disuse atrophy following injury or during a period of muscle unloading.

We propose two hypotheses that drive our technical objectives, as follows:

Hypothesis: Protein breakdown pathways are greatly up regulated in dystrophic muscle due to a combination of increased intracellular calcium load and the extracellular inflammatory response. Many of these same pathways are activated during disuse atrophy in normal muscles. Targeting specific classes of proteases individually and in combination can slow this turnover, resulting in stronger muscles and lessen the demand on muscle repair.

Hypothesis: Disuse atrophy and some forms of muscular dystrophy may involve a significant component of apoptosis. Disuse atrophy of muscle greatly accelerates loss of muscle in dystrophy when muscle function begins to fail. This atrophy is initiated following unloading of cytoskeletal-extracellular matrix connections and is mediated via activation of apoptotic pathways and increased protein breakdown, in combination with decreased drive through load-dependent growth pathways. By blocking apoptotic pathways alone, and in combination with inhibiting muscle breakdown, we should be able to attenuate disuse atrophy.