Key result
Transgenic overexpression of gamma-cytoplasmic actin restored total actin levels but did not extend the life span of alpha-skeletal actin-null mice, indicating it cannot fully substitute for alpha-actin.
Sarcomeric thin filaments can accommodate substantial incorporation of gamma-(cyto) actin without functional consequences, but it cannot fully substitute for alpha-(sk) actin to prevent lethality.
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Gamma-actin cannot rescue alpha-actin null lethality despite restoring levels; leaves open isoform-specific sarcomere roles in muscle disease models.
We generated transgenic mice that overexpressed gamma-(cyto) actin 2000-fold above wild-type levels in skeletal muscle. gamma-(cyto) actin comprised 40% of total actin in transgenic skeletal muscle, with a concomitant 40% decrease in alpha-actin. Surprisingly, transgenic muscle was histologically and ultrastructurally identical to wild-type muscle despite near-stoichiometric incorporation of gamma-(cyto) actin into sarcomeric thin filaments. Furthermore, several parameters of muscle physiological performance in the transgenic animals were not different from wild type. Given these surprising results, we tested whether overexpression of gamma-(cyto) actin could rescue the early postnatal lethality in alpha-(sk) actin-null mice (Acta1(-/-)). By quantitative Western blot analysis, we found total actin levels were decreased by 35% in Acta1(-/-) muscle. Although transgenic overexpression of gamma-(cyto) actin on the Acta1(-/-) background restored total actin levels to wild type, resulting in thin filaments composed of 60% gamma-(cyto) actin and a 40% mixture of cardiac and vascular actin, the life span of transgenic Acta1(-/-) mice was not extended. These results indicate that sarcomeric thin filaments can accommodate substantial incorporation of gamma-(cyto) actin without functional consequences, yet gamma-(cyto) actin cannot fully substitute for alpha-(sk) actin.
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Jaeger et al. (2009) studied alpha-(sk) actin-null (Acta1(-/-)). Transgenic overexpression of gamma-(cyto) actin vs. Wild-type mice and Acta1(-/-) mice without transgene was evaluated on Life span / early postnatal lethality. Transgenic overexpression of gamma-cytoplasmic actin restored total actin levels but did not extend the life span of alpha-skeletal actin-null mice, indicating it cannot fully substitute for alpha-actin.
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