Browsing by Author "Thomas, David D."

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    Impaired muscle relaxation and mitochondrial fission associated with genetic ablation of cytoplasmic actin isoforms
    (Wiley, 2018) O'Rourke, Allison R.; Lindsay, Angus; Tarpey, Michael D.; Yuen, Samantha; McCourt, Preston; Nelson, D'anna M.; Perrin, Benjamin J.; Thomas, David D.; Spangenburg, Espen E.; Lowe, Dawn A.; Ervasti, James M.; Biology, School of Science
    While α-actin isoforms predominate in adult striated muscle, skeletal muscle-specific knockouts (KOs) of nonmuscle cytoplasmic βcyto- or γcyto-actin each cause a mild, but progressive myopathy effected by an unknown mechanism. Using transmission electron microscopy, we identified morphological abnormalities in both the mitochondria and the sarcoplasmic reticulum (SR) in aged muscle-specific βcyto- and γcyto-actin KO mice. We found βcyto- and γcyto-actin proteins to be enriched in isolated mitochondrial-associated membrane preparations, which represent the interface between mitochondria and sarco-endoplasmic reticulum important in signaling and mitochondrial dynamics. We also measured significantly elongated and interconnected mitochondrial morphologies associated with a significant decrease in mitochondrial fission events in primary mouse embryonic fibroblasts lacking βcyto- and/or γcyto-actin. Interestingly, mitochondrial respiration in muscle was not measurably affected as oxygen consumption was similar in skeletal muscle fibers from 12 month-old muscle-specific βcyto- and γcyto-actin KO mice. Instead, we found that the maximal rate of relaxation after isometric contraction was significantly slowed in muscles of 12-month-old βcyto- and γcyto-actin muscle-specific KO mice. Our data suggest that impaired Ca2+ re-uptake may presage development of the observed SR morphological changes in aged mice while providing a potential pathological mechanism for the observed myopathy.
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    Sarcoplasmic Reticulum from Horse Gluteal Muscle Is Poised for Enhanced Calcium Transport
    (MDPI, 2021-11-23) Autry, Joseph M.; Svensson, Bengt; Carlson, Samuel F.; Chen, Zhenhui; Cornea, Razvan L.; Thomas, David D.; Valberg, Stephanie J.; Medicine, School of Medicine
    We have analyzed the enzymatic activity of the sarcoplasmic reticulum (SR) Ca2+-transporting ATPase (SERCA) from the horse gluteal muscle. Horses are bred for peak athletic performance yet exhibit a high incidence of exertional rhabdomyolysis, with elevated levels of cytosolic Ca2+ proposed as a correlative linkage. We recently reported an improved protocol for isolating SR vesicles from horse muscle; these horse SR vesicles contain an abundant level of SERCA and only trace-levels of sarcolipin (SLN), the inhibitory peptide subunit of SERCA in mammalian fast-twitch skeletal muscle. Here, we report that the in vitro Ca2+ transport rate of horse SR vesicles is 2.3 ± 0.7-fold greater than rabbit SR vesicles, which express close to equimolar levels of SERCA and SLN. This suggests that horse myofibers exhibit an enhanced SR Ca2+ transport rate and increased luminal Ca2+ stores in vivo. Using the densitometry of Coomassie-stained SDS-PAGE gels, we determined that horse SR vesicles express an abundant level of the luminal SR Ca2+ storage protein calsequestrin (CASQ), with a CASQ-to-SERCA ratio about double that in rabbit SR vesicles. Thus, we propose that SR Ca2+ cycling in horse myofibers is enhanced by a reduced SLN inhibition of SERCA and by an abundant expression of CASQ. Together, these results suggest that horse muscle contractility and susceptibility to exertional rhabdomyolysis are promoted by enhanced SR Ca2+ uptake and luminal Ca2+ storage.