OR16-05 Single-Cell Sequencing Identifies Novel Regulators of Thyrotrope Populations and POU1F1-Independent Thyroid-Stimulating Hormone Expression

We implemented single-cell RNA sequencing (scRNAseq) technology as a discovery tool to identify factors enriched in differentiated thyrotropes. Thyroid-stimulating hormone (TSH) is produced in the pars distalis of the anterior pituitary (AP) and primarily acts on the thyroid gland to regulate metabolism through T3/T4. However, TSH is also produced by cells in the pars tuberalis (PT), which is comprised of a thin layer of cells that extends rostrally from the pars distalis along the pituitary stalk to the median eminence in the hypothalamus. TSH produced by PT thyrotropes acts on hypothalamic tanycytes to regulate seasonal reproduction. PT thyrotropes likely descend from rostral tip thyrotropes that arise at e12.5 of mouse development, which transcribe the TSH beta subunit (Tshb) without detectable expression of the transcription factor POU1F1. POU1F1 is required for Tshb transcription in thyrotropes of the adenohypophysis, and it acts synergistically with GATA2 to drive cell fate. The molecular mechanisms driving Tshb expression independently of Pou1f1 in PT thyrotropes are unclear. Thyrotropes are the least abundant endocrine cell-type in the pituitary gland. We used genetic labeling and fluorescence-activated cell sorting (FACS) to enrich for thyrotropes for single-cell sequencing. We performed scRNAseq on 7-day-old GFP-positive pituitary cells from Tshb-Cre; R26-LSL-eYFP and intact whole pituitaries, recovering more than 15,000 cells altogether. We observe two distinct populations of cells expressing Tshb. The larger thyrotrope population has approximately twenty fold higher levels of Tshb and five fold higher Cga transcripts than the smaller population, and they are also distinguished by expression of Pou1f1, TSH-releasing hormone receptor (Trhr), and deiodinase 2 (Dio2), consistent with expectations for AP thyrotropes. The smaller thyrotrope population does not express Pou1f1, but those cells are characterized by expression of TSH receptor (Tshr) and melatonin receptor 1A (Mtnr1a), consistent with expectations for PT thyrotropes. They express mildly increased levels of Eya3 and Six1, although these genes are expressed in other cell-types including AP thyrotropes, stem cells, and gonadotropes. They have two-fold higher levels of Gata2 transcripts and uniquely express the transcription factor Sox14. SOX14 is a SoxB2 family transcription factor that counteracts the transcriptional activity of SoxB1 family members, such as Sox2. In conclusion, our scRNAseq has identified novel markers of PT thyrotropes and unveils novel insights into the similarities and differences in the development and function of pituitary thyrotrope subpopulations.