Identification of Cell Types that Express Dio3 Deiodinase, a Thyroid Hormone-Inactivating Enzyme, Using a Dio3-CreERT2 Reporter System

Background: Thyroid hormone promotes development, growth and metabolism. The level of thyroid hormone ligand (triiodothyronine, T3) in tissues depends not only on circulating levels but also upon tight regulation by activating and inactivating deiodinases within tissues. Type 3 deiodinase (Dio3) inactivates T3 and its precursor thyroxine (T4) and mediates many functions including in neurodevelopmental, sensory and reproductive systems. Dio3 is subject to genomic imprinting. Despite its critical functions, Dio3 is often expressed transiently and at low levels in restricted cell populations making it difficult to detect in natural tissues. Methods: To visualize Dio3 expression at cellular resolution, we derived a Dio3-CreERt2 knockin allele that expresses tamoxifen-dependent Cre recombinase from the endogenous Dio3 gene. When crossed with Ai6 reporter mice, Dio3-CreERt2-positive cells display fluorescent signals. When tamoxifen-treated at neonatal ages, Dio3-CreERt2 recapitulates endogenous Dio3 expression as previously reported in brain: in the bed nucleus of the stria terminalis and preoptic nuclei. In addition, we uncovered several positive cell groups in the hypothalamus, brain stem, pituitary and other tissues. Drastic differences were observed for Dio3-CreERt2 as a paternally versus maternally inherited allele, revealing imprinting effect in specific cell types. Dio3-CreERT2 activity is enhanced by T3 administration, in accordance with Dio3 as a T3-indicible gene. Conclusion: The Dio3-CreERT2 model sensitively reveals Dio3-expressing cell types in tissues. The model is useful for studying expression patterns, imprinting and lineage tracing of Dio3-positive cells during development and homeostatic challenges.