Dio2-CreErt2 Knockin Model to Identify T3-Generating Cells That Express Type 2 Deiodinase in Tissues

Background: Thyroid hormone promotes many developmental and homeostatic functions. Apart from adequate circulating levels, the concentration of the active hormone T3 within tissues may be amplified by type 2 deiodinase (Dio2) by conversion from the precursor T4. Dio2 is critical in auditory development, bone maturation, brain function and control of the hypothalamic-pituitary-thyroid axis. Despite its crucial role, an obstacle to studying Dio2 is that the protein has a short half-life, is at low levels and is often transiently expressed, making it difficult to identify Dio2 in tissues at cellular resolution. Methods: We derived a Dio2-CreERt2 knockin mouse that expresses tamoxifen-dependent Cre recombinase from the endogenous Dio2 gene. When crossed onto Ai6 reporter mice, following tamoxifen treatment, Dio2-CreERt2 expression is detected as fluorescent signal in specific cells in brain regions, pituitary, and other tissues. We showed previously that Dio2 is essential for hearing with rising expression levels in the cochlea prior to onset of hearing. The Dio2-CreERt2 model identified positive cell types in the cochlear spiral ligament, septal divisions and modiolus around the sensory epithelium. Dio2-positive fibrocytes were adjacent to and extended projections around blood capillary networks, the source of T4 supply. Transcriptome analysis of isolated positive cells revealed bone lineage-related origins for many of these cells. Conclusion: The Dio2-CreERt2 model detects Dio2 expression sensitively at cellular resolution. In the cochlea, Dio2-positive cell types reside in vascularized support tissues, suggesting combined endocrine and paracrine-like control of the T3 supply. Analysis of cell origins suggests novel interactions between endocrine and skeletal systems in promoting T3 action required for hearing.