Tail regression, apoptosis and thyroid hormone regulation of myosin heavy chain isoforms in Xenopus tadpoles

T 3 effects on myosin heavy chain gene expression were analysed in muscles undergoing different fates during metamorphosis. Muscle fate was followed by somatic gene transfer of a constitutively expressed luciferase vector. Persistent expression was found in dorsal muscle which is remodelled during metamorphosis whilst the signal disappeared in apoptosing caudal muscle. RNAse protection assay was used to follow production of myosin heavy chain isoforms: two isoforms identified as embryonic (E3 and E19) and one adult form (A7). The effects of T 3 treatment were followed over 120 h. During this time frame E3 and A7 expression patterns were found to be similar in both caudal and dorsal muscles. Most notably, at 48 h E3 expression was significantly down-regulated and production of A7 significantly upregulated in both caudal and dorsal muscle. Thus T 3-induced transitions in muscle gene expression are independent of muscle fate during amphibian metamorphosis.