Ontogeny and regulation of variant thyroid hormone receptor isoforms in developing rat testis

High affinity-low capacity nuclear triiodothyronine (T 3) receptors (TR(s)), identified as a product of c-erbA(α) proto-oncogene, are expressed in prepubertal rat Sertoli cell. At this age, exogenous T 3 treatment as well as hypothyroidism affects Sertoli cell functions. We examined the ontogenetic expression pattern of TR(s) in the rat testis. Northern analysis confirms that TR(s) are expressed at high level from fetal development until prepubertal period. RNase protection analysis demonstrates that TR(α2), the variant isoform of TR(α1), is constitutively expressed at all ages, while TR(α3) is absent in the adult gonad. While TR(α1) and TR(α2) expression declines during development, Rev-erbA(α) (Rev), the antisense mRNA encoded by the same c-erbA(α) genomic locus, increases beginning 5 days after birth and maximizing in adulthood. TR(α1), TR(α2), and Rev mRNA(s) do not appear to be directly regulated by thyroid hormone in testis; however, short-term neonatal hypothyroidism leads to the expression of TR(α1) and its variant in adult testis, which is absent in control coeval animals. Thus, during development of rat testis, the levels of messages of genes encoded in the c-erbA(α) genomic locus have different ontogenetic control. The ontogenetic profile of TR(α1) and its variant isoforms within the seminiferous epithelium suggests that these receptors are involved in the differentiation of the male gonad. (J. Endocrinol. Invest. 22: 843-848, 1999) (C)1999 Editrice Kurtis.