REAPPEARANCE OF THE MINOR ALPHA-SARCOMERIC ACTINS IN POSTNATAL MUSCLE

The postnatal expression profiles of alpha-sarcomeric actin transcript s and protein are quantified in mouse striated muscles from birth to p ostnatal clay 56 by Northern and Western blot analyses. alpha-Cardiac actin (alpha-CA) transcripts transiently increase between 12 and 21 da ys after birth in the quadriceps muscle, reaching similar to 90% that found in the adult mouse heart. Although alpha-CA is the alpha-sarcome ric actin isoform expressed in the immature fiber, the expression prof iles of other contractile protein isoforms indicate that this postnata l period is not reflective of an immature phenotype. alpha-Skeletal ac tin (alpha-SA) transcripts accumulate to similar to 32% of the total a lpha-sarcomeric actin transcripts in the adult heart. Our study shows that 1) there is a simultaneous reappearance of alpha-CA and alpha-SA in postnatal skeletal and heart muscles, respectively, and 2) the cont ractile protein gene expression profile characteristic of adult skelet al muscle is not achieved until after 42 days postnatal in the mouse. We propose there is a previously uncharacterized period of postnatal s triated muscle maturation marked by the reappearance of the minor alph a-sarcomeric actins.