OC5.4 - ECE_3532 - Constitutive active expression of ZFP36L1 in anterior pituitary SOX2+ stem cells results in pituitary hypoplasia and a failure of terminal differentiation via a non-cell autonomous mechanism
SOX2+ pituitary stem cells (PSCs) are a critical source of signalling cues during pituitary development, yet the molecular orchestrators of this paracrine environment remain poorly defined. In this study, we identify ZFP36L1 as a key regulator of secreted factor expression within the PSC niche and demonstrate that its precise control is essential for terminal differentiation during pituitary organogenesis. Using a Sox2-CreERT2 model to induce constitutive active ZFP36L1 (ZFP36L1-CA) in PSCs from E12.5, we observed severe pituitary hypoplasia and a profound lack of terminal differentiation across POU1F1+ and NR5A1+ hormone-producing lineages by E18.5. To determine the nature of this developmental arrest, we performed mosaic lineage tracing via tamoxifen titration. Notably, wild-type cells neighbouring ZFP36L1-CA cells also failed to undergo hormone commitment, establishing a non-cell autonomous defect and suggesting a disruption of the extrinsic signalling environment. This requirement for regulated ZFP36L1 expression was further validated using a Hesx1-Cre model, where early developmental ZFP36L1-CA expression resulted in dwarfism, perinatal lethality, and failure of POU1F1+ and NR5A1+ endocrine cell commitment. Crucially, this differentiation defect was rescued when Hesx1-Cre mutant pituitaries were co-cultured with normal PSCs, providing functional confirmation that the phenotype is driven by the loss of essential stem-cell-derived paracrine signals. Mechanistic interrogation through scRNA-seq and mRNA in situ hybridisation identified secreted factors regulated by ZFP36L1 that co-localise with PSCs in vivo. ELISAs confirmed that these factors are actively secreted by PSCs and are significantly downregulated upon ZFP36L1-CA expression. Collectively, these findings establish that ZFP36L1 regulation in PSCs is indispensable for pituitary organogenesis. These experiments identify specific paracrine mediators required for terminal differentiation in the anterior pituitary, offering new molecular targets for efficient regenerative approaches.