Immunocytochemical delineation of thyroid hormone receptor β2-like immunoreactivity in the rat central nervous system

Abstract

The thyroid hormone receptors (TR) are nuclear proteins that include TRα and TRβ subtypes, each encoded by a separate gene. Both TRα and TRβ give rise to several isoforms of which three. TRα1, TRβ1, and TRβ2 bind T3 and mediate the action of thyroid hormone. Although TRβ2 was initially thought to be confined to the anterior pituitary, we recently observed small quantities of TRβ2 messenger RNA (mRNA) by polymerase chain reaction analysis of discrete hvpothalamic regions. To further examine the distribution of TRβ2 in the brain, we performed immunocytochemical studies using a highly specific antiserum to TRβ2, raised against a unique amino acid sequence (TRβ2[131-145]) that is not present in the other known TRs. This antiserum immunoprecipitated TRβ2 but not TRβ1 or TRβ1. Immunoreactive TRβ2 was widely distributed throughout the brain and primarily localized to the cell nucleus. Particularly intense immunostaining was present in the cerebral cortex, cerebellum, and hypothalamus, including regions where TRβ2 mRNA had not previously been identified. In addition, immunoprecipitation of nuclear extracts with anti-TRβ2, reduced total T3 binding capacity by approximately 20%, suggesting that immunoreactive TRβ2 comprises a substantial portion of the total content of nuclear thyroid hormone binding proteins. These studies demonstrate that immunoreactive TRβ2 is more widely represented in the central nervous system than previously suspected and may play an important role in mediating the action of T3 in many different regions of the brain. The finding of Tβ2-like material could be due to a disproportionately high ratio of the TRβ2 translation product and its mRNA in certain regions of the brain, or could indicate the existence of a novel TRβ2-related protein that is important for T3 binding.