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TRAM: Investigating Biochemical Mechanisms of Skeletal Muscle Atrophy


            

Apples and tomatoes are concentrated sources of ursolic acid and tomatidine, respectively. Photo Credit:  Tomatokumato.com
                                                                                                                     
            Dr. Chris Adams presented research examining one pathway of skeletal muscle atrophy (SMA). SMA (muscle wasting) is a major unsolved health problem associated with aging. Many older adults report losses of strength and mobility (nearly 20% of Americans >65 years), yet “less than 1% of atrophy-associated changes in mRNA expression have been investigated at the mechanistic level.” Dr. Adams’ lab aims to understand the relationship between select biochemical mechanisms and SMA.
            Past experiments have utilized mouse models. Mice show age-related differences in muscle mass between 6 months (young) and 22 months (old). An early experiment examined the relationship between expression of ATF4 (a protein subunit of a transcription factor expressed in stressed cells) in mouse muscle cells and fiber size. A sample of healthy, young mice had one leg injected with control serum while the other leg was injected with serum containing ATF4. Comparing muscle samples taken from both legs found significantly smaller fibers in the ATF4 leg, indicating that an increased expression of ATF4 was linked to SMA. Additional experimentation used 22 month-old mice either lacking ATF4 from birth (called ATF4 knockout) or expressing ATF4 (control). Comparisons between knockout and control mice across 22 months demonstrated control mice had significantly reduced muscle fiber size, grip strength, specific force of muscle, and capacity to perform endurance exercise.
            ATF4 is involved in the regulation of less than 2% of genes associated with SMA. Gadd45a, one realized target gene, codes for a protein that mediates muscle atrophy in stressful conditions by altering other proteins’ functions; effects of Gadd45a expression are similar to ATF4. In further experiments using mouse models (Gadd45a knockout versus controls), limbs were immobilized using casts over 3 days to produce atrophic conditions. It was concluded that Gadd45a is required for atrophy—the absence of Gadd45a was protective.
            To identify other proteins that interact with Gadd45a protein and contribute to atrophy, additional research experiments added a tag/identifier to Gadd45a. Muscle samples of 48 mice (either control or Gadd45a knockout) were examined over a 10-day period. Using the “pull-down technique” to assay samples, 209 proteins were identified as being interactive with Gadd45a protein. Further examination found the most prominent protein to be MEKK4 (a protein kinase that binds to Gadd45a during stress resulting in a conformational change and direct activation). Experimentation found that MEKK4 is required for Gadd45a-mediated signaling in the skeletal muscle and thus is required for atrophy. It was also demonstrated through additional mouse model experimentation that the activity of MEKK4 is arrested when a point mutation prevents the protein’s production. 
            Previous research demonstrated that certain identified compounds from whole foods (ursolic acid from apples and other fruits/herbs, tomatidine from tomatoes) reduce the expression of ATF4 in mice, leading to increased muscle fiber size, muscle quality, and strength. Though mouse models have been used for this lab’s mechanistic research rather than human tissues thus far, research is ongoing. Perhaps human randomized-controlled trials targeting this pathway for SMA reduction/prevention will be warranted in the future.
           

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