The Role of Apoptosis Repressor with Caspase Recruitment Domain in Aging Mouse Skeletal Muscle Morphology and Apoptotic Signaling
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Abstract Aging is associated with a loss of skeletal muscle mass, known as sarcopenia, which results in numerous degenerative alterations and decreased strength. One of the major mechanisms influencing muscle wasting in aged skeletal muscle is through augmented apoptotic signaling. Altered apoptotic signaling can result in cell dysfunction, and degradation of muscle contractile proteins and loss of myonuclei; ultimately contributing to muscle atrophy and contractile dysfunction. Apoptosis repressor with caspase recruitment domain (ARC) is an anti- apoptotic protein that is highly expressed in terminally differentiated tissue (heart and skeletal muscle) which can regulate several apoptotic pathways. Interestingly, ARC knockout (KO) mice display morphological and phenotypic differences, as well as altered protein expression of a number of key apoptotic regulatory factors in skeletal muscle compared to wild-type (WT) mice. Currently, the influence of ARC in aging skeletal muscle has not been studied. Therefore, the current work examined the role of ARC protein on age-related muscle wasting, and apoptosis by utilizing an ARC-deficient mouse (along with age-matched WT controls) model at several time- points throughout the lifespan (18-week, 1 year, and 2 years). Slow (oxidative) and fast (glycolytic) muscle was used to compare differences between ages and genotypes. Soleus weight, CSA, type I and IIA CSA, and total fiber number decreased in the 2 year animals, with a fiber shift towards a slower MHC expression. Contractility measurements revealed a higher rate of contraction and relaxation in the 2 year animals. No differences were found in pro-apoptotic proteins, and caspase and calpain activity; however, four anti-apoptotic proteins were increased in the 2 year animals. However, the Bax:Bcl-2 ratio, and ARC expression decreased in the 2 year animals. Subfractionation analysis in the RQ revealed increased cytosolic SMAC in the 2 year animals and increased mitochondrial Bax in the 18 week animals. Furthermore, the 2 year KO vs. WT mice had an increased release of mitochondrial housed pro-apoptotic proteins and an increased Bax:Bcl-2 ratio. Additionally, ARC KO mice display a decreased total fiber number. Plantaris weight, CSA, type IIA, IIX, and IIB CSA, and total fiber number decreased in the 2 year animals, with a fiber shift towards a faster MHC expression. Contractility measurements revealed lower contraction and relaxation rates in the 2 year animals. Similar to the soleus, three anti-apoptotic proteins increased, and the Bax:Bcl-2 ratio decreased in the 2 year animals, with no differences in caspase and calpain activity. In the WQ cytosolic AIF decreased in the 18 week animals, whereas cytosolic SMAC increased in the aged animals. The effects influenced exclusively by genotype were a decreased plantaris weight, and an increased Bcl-XL expression in the KO group. Overall, these results indicate that aged mice display increased muscle atrophy despite an increase in anti-apoptotic protein expression, as well as altered force characteristics. With aged ARC KO mice displaying several altered protein expressions. This work provides a better understanding of the role of cell death signaling and ARC protein in the skeletal muscle morphological and functional alterations observed during the aging process.