Interplay between the chromatin organizer SATB1 and the transcription factor PIT1 during postnatal maturation of the pituitary gland

/Abstract The pituitary gland plays a central role in various physiological functions, including growth, lactation, stress response, reproduction, metabolism, and water balance. The pituitary lineage-specific transcription factor PIT1 specifies three pituitary lineages that produce growth hormone (GH), prolactin (PRL), and thyroid-stimulating hormone (TSH). Naturally occurring mutations in PIT1 can cause combined pituitary hormone deficiency (CPHD) in humans, leading to deficiencies in these hormones. We previously found that PIT1 associates with a cell-type- specific chromatin organizer, SATB1, and β-catenin. This association is crucial for tethering PIT1-bound enhancers to an insoluble, salt-extraction-resistant subnuclear structure, and this tethering is consequently required for PIT1-mediated gene expression. The R271W mutation in PIT1 associated with CPHD causes failure of PIT1 to associate with SATB1 and connect with the nuclear substructure to produce GH. By developing an alternative ChIP-seq protocol (ureaChIP-seq), we recently reported that in vivo, SATB1 directly binds to specialized genomic regions called base-unpairing regions (BURs, ~300 base pairs in length) genome-wide in both mouse and human genomes. By comparing SATB1 direct versus indirect chromatin association, we found that SATB1 forms a “two-tiered” type of chromatin organization, consisting of (a) SATB1- bound BURs within the insoluble subnuclear structure and (b) accessible chromatin that SATB1 indirectly interacts with, presumably through its association with other nuclear factors bound to regulatory regions. This two-tiered organization is distinct from chromatin organized into topologically associating domains (TADs) by CTCF and cohesin. Based on our findings, we hypothesize that the three-dimensional chromatin organization regulated by SATB1 enables PIT1 to establish a cell-type-specific transcriptional program in the pituitary gland. To test this hypothesis, in Aim 1, we will use a GH-producing GC cell line to determine whether BURs targeted by SATB1 provide the chromatin scaffold to support the PIT1-regulated gene regulatory network that drives proper gene expression. In Aim 2, we will employ a novel and modified auxin-inducible degron (AID2) system to reversibly deplete SATB1 protein in vivo in a cell-type-specific manner. Using the AID2 system, we will investigate SATB1’s role in postnatal pituitary gland expansion, development, and the function of PIT1+ cells in mice. We will explore whether the SATB1-mediated two-tiered chromatin organization facilitates PIT1 association with its target enhancers in vivo. We envision that the results from this proposed research, linking chromatin architecture to cell-type-specific gene expression, will significantly advance our understanding of how the pituitary gland grows, matures, and maintains its function during adulthood. The proposed studies on the novel function of SATB1 in the pituitary gland are likely to guide the development of future therapies for pituitary- related disorders. Project Number: 1R01HD121802-01 | Fiscal Year: 2026 | NIH Institute/Center: Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) | Principal Investigator: Terumi Kohwi-Shigematsu | Institution: UNIVERSITY OF CALIFORNIA, SAN FRANCISCO, SAN FRANCISCO, CA | Award Amount: $352,600 | Activity Code: R01 | Study Section: Cell Signaling and Molecular Endocrinology Study Section[CSME] View on NIH RePORTER: https://reporter.nih.gov/project-details/11347225