TRANSIENT EXPRESSION OF 3,5,3'-TRIIODOTHYRONINE NUCLEAR RECEPTORS IN RAT OLIGODENDROCYTES - IN-VIVO AND IN-VITRO IMMUNOCYTOCHEMICAL STUDIES

It is generally accepted that the action of thyroid hormones is mediat ed through specific nuclear receptors. Recent studies have demonstrate d the homology of the thyroid receptor with the cellular product of th e oncogen v-erbA. So far, two genes have been identified and classifie d as alpha and beta subtypes. In this study, the expression of nuclear triiodothyronine (T-3) receptors (NT(3)Rs) was examined in secondary cultures containing 85-90% oligodendrocytes (OL) prepared from newborn rat brain primary cultures enriched in OL. These cultures, which are able to produce myelin membranes, were examined by double immunolabell ing with a monoclonal antibody (2B3) raised against purified rat liver NT(3)Rs and with antibodies against two maturation markers of OL: an early marker, galactocerebroside (GC), and myelin basic protein (MBP), which is expressed later than GC. 2B3 recognized three nuclear protei ns with the same molecular weights as beta 1, alpha 1, and alpha 2 sub types with different capacities for binding T-3. In 5-day-old OL secon dary cultures (25 days, total time in culture), 2B3-NT(3)R immunoreact ivity was located in 77% of morphologically immature OL (GC)(+) cells, whereas only 44% of morphologically mature OL were immunoreactive. On ly 35% of the MBP(+) cells co-expressed NT(3)Rs. In the corpus callosu m of developing rat brain, at all ages studied from 7-60 days postnata l, the total absence of NT(3)Rs in dark OL (morphologically mature), c onfirmed by ultrastructural immunocytochemistry, indicates an even mor e dramatic decrease during maturation. Furthermore, the percentage of medium OL (less mature) stained by 2B3 is reduced by approximately hal f in 60- compared to 20-day-old rat brain. It is of interest to note t hat the in vitro observation with maturation markers mirrors the in vi vo decrease of NT(3)R expression during development. It is interesting that NT(3)Rs are absent in vivo before the critical period of active myelination. These data indicate the presence of a nuclear T-3 binding protein in the nuclei of OL at the time of myelination both in vitro and in vivo. The transient expression of these NT(3)Rs during active m yelination argues in favour of a direct effect of thyroid hormones on OL. (C) 1994 Wiley-Liss, Inc.