Differential expression of thyroid hormone receptor isoforms is strikingly related to cardiac and skeletal muscle phenotype during postnatal development.

The genomic actions of thyroid hormones (THs) are mediated by receptors (TRs) that are encoded by two protooncogenes, c-erbA-alpha and c-erbA-beta. The precise functions of the TR isoforms are unclear and this study focuses on the potential roles of the TRalpha and TRbeta isoforms in mammalian striated muscles postnatally. The porcine TRalpha1, TRalpha2 and TRbeta1 cDNAs were first cloned, sequenced and characterised by Northern blotting. A quantitative analysis of TR isoform expression was then undertaken, using RNase protection analysis with novel riboprobes designed to detect relative expression levels of TRalpha1, TRalpha2, TRbeta1 and TRbeta2, in functionally distinct muscles from 7-week-old pigs kept under controlled conditions of nutrition and thermal environment. We found a striking muscle-specific pattern of TRalpha isoform distribution: in heart the mRNA level of TRalpha2 (non-TH binding) was markedly greater (P<0.01) than that of TRalpha1 (TH binding); in longissimus dorsi the opposite pattern of expression occurred (TRalpha1>TRalpha2, P<0.001); in soleus, diaphragm and rhomboideus there were no differences between the two isoforms. The overall abundance of TRbeta was very much lower than that of TRalpha, and TRbeta1 was expressed at a higher level than TRbeta2 in all muscles. Together with recent data from TR gene inactivation studies and the established role of TH in determining myosin heavy chain isoform expression and muscle phenotype, these results suggest a role for differential expression of TR isoforms in acquisition and maintenance of optimal cardiac and skeletal muscle function.