The structural basis of molecular adaptation

Expression levels of fast-twitch (SERCA1), slow-twitch (SERCA2a) and "housekeeping" (SERCA2b) isoforms of the sarcoplasmic reticulum Ca(2+)-transport ATPase were monitored during regeneration of rat soleus muscles following necrosis induced by the toxin notexin at the tissue level by Western blot analysis and at the cellular level by immunocytochemical analysis. Due to necrosis, levels of muscle-specific SERCA1 and SERCA2a isoforms dropped to low levels on the third day after injection of the toxin. Subsequently, during regeneration both isoforms recovered but with a different time course. Expression of the fast type SERCA1 increased first. This type showed its most pronounced increase between day 3 and 10. Expression of the slow type SERCA2a was biphasic. After an increase to approximately one third of the control value on days 5-10, it showed its main increase up to the control level between day 10 and 21. Expression levels of the house-keeping SERCA2b isoform remained relatively constant throughout the 4 weeks of regeneration. Between day 10 and 28, when new innervation is established, SERCA2a expression spread gradually over almost all fibers whereas the number of SERCA1-expressing fibers decreased and only a limited number of fibers co-expressed SERCA1 and SERCA2a. At 4 weeks of regeneration, expression of the fast isoform was found only in 12% of the fibers, whereas the slow form was found in 98% of the fibers. In the contralateral untreated soleus muscles, 26% SERCA1-positive and 81% SERCA2a-positive fibers were observed. Immunocytochemical analysis showed that SERCA1 and SERCA2a were co-expressed with fast and slow myosin isoforms in fibers of normal muscles but in regenerated muscle only slow myosin and slow SERCA isoforms correlated. The results show that during regeneration levels of fast and slow SERCA proteins change in a similar way as their mRNAs do. However, in regenerated soleus, unlike in normal muscle, expression of slow SERCA is coregulated only with the slow myosin isoform. This finding is in agreement with the fact that the number of slow type fibers is increased in regenerated soleus.