Novel Regulators and Developmental Pathways of Pituitary Thyrotrope Subpopulations

The pituitary gland regulates growth, metabolism, reproduction, the stress response, uterine contractions, lactation, and water retention. It secretes polypeptide hormones in response to hypothalamic input, end organ feedback, and diurnal cues. The mechanisms by which pituitary stem cells are recruited to proliferate, maintain quiescence or differentiate into specific cell types, especially thyrotropes, are not well understood. We utilized single-cell RNA sequencing in juvenile P7 mouse pituitary cells to identify transcription factors in pituitary stem and endocrine populations, with a focus on thyrotrope subpopulations. We first identified common gene regulatory networks activated in proliferating stem and proliferating endocrine cells. We ascertained Shox2 and Sox14 as candidate regulators of pars distalis and pars tuberalis thyrotropes, respectively, and subsequently found reduced hormone expression in Sox14-knockout pars tuberalis thyrotropes. We further discovered dual-identity cells expressing both POU1F1 and NR5A1 and demonstrated that approximately one-third of pars distalis thyrotropes are descended from NR5A1-expressing cells, typically considered gonadotrope progenitors. We have therefore used single-cell transcriptomics to identify novel developmental mechanisms regulating the specification and hormone expression in thyrotrope subpopulations. We performed scRNAseq of 7-day-old (P7) murine pituitary cells in two concurrently processed pools using Tshb-Cre; Rosa26-Eyfp FACS-purified pituitary cells combined with control (Cre-negative) unsorted total pituitary cells to capture all pituitary populations with increased proportion of rare Tshb-lineage populations. We also selected P7 pituitary glands to capture increased proportions of stem cells which are more abundant at young ages.