OR28-2 The Ldb1 Transcriptional Co-regulator Is Required for Establishment, Development, and Survival of the Pancreatic Endocrine Lineage

Pancreatic islets are comprised of hormone-secreting cell types that are vital regulators of glucose homeostasis. Specifically, the pancreatic β-cell is indispensable for glucose metabolism and its dysfunction is central to diabetes mellitus. β-cell development is regulated by transcription factor (TF) cascades, mediating differentiation of embryonic progenitors into mature insulin-producing β-cells. Our prior studies showed that the pancreatic TF, Islet-1 (Isl1), interacts with a broadly-expressed transcriptional co-regulator, Ldb1, to regulate β-cell maturation during late embryogenesis and postnatal function (i.e embryonic day (E) 18.5 onward). However, Ldb1 is also expressed early in development, before Isl1 is present (i.e. as early as E10.5). The earlier Ldb1(+)cell types include Pdx1(+)multipotent progenitor cells (MPCs) and early endocrine progenitors expressing the TF Neurogenin-3 (Ngn3). MPC progeny will populate the entire pancreas (endocrine and exocrine), while Ngn3(+)endocrine progenitors (Isl1(-)) are fated to become the islet cells. Our hypothesis is that Ldb1 has Isl1-independent roles in maintaining progenitor identity and survival in these requisite populations during early pancreatic development. To test this, we generated a whole pancreas knockout of Ldb1, termed Ldb1(∆panc), and observed severe developmental and postnatal phenotypes. At E13.5, Ldb1(∆panc)mice exhibited disorganized progenitor pools, suggesting early defects in endocrine identity. At E15.5 Ldb1(∆panc)mice had a significant reduction of Ngn3(+)endocrine progenitors, and Pdx1(HI)/insulin immunoreactivity as marks of presumptive β-cells. Ldb1(∆panc)mice died by postnatal day 7 with severe hyperglycemia and hypoinsulinemia due to a drastic islet hormone(+)cell reduction. Interestingly, total pancreatic mass remained unchanged in Ldb1(∆panc)neonates, suggesting that Ldb1 impacts are islet-specific in the pancreas. Considering these observations, we generated a new model of Ldb1 loss specifically in Ngn3(+)islet progenitors, termed Ldb1(∆endo). We confirmed loss of Ldb1 in endocrine clusters and observed postnatal hyperglycemia, with a reduction of islet cells in Ldb1(∆endo)neonates, similar to Ldb1(∆panc)mice. Chromatin immunoprecipitation experiments highlight that Ldb1 likely imparts control on Pdx1 levels through occupation of the Pdx1 Area I-II regulatory domains. We are now further assessing the developmental phenotype, specifically examining markers of proliferation, apoptosis, and altered cell identity through lineage tracing in the Ldb1(∆endo)model. Our work provides insight into how islet cells arise and the factors required for endocrine progenitors to adopt islet cell fates. This knowledge will not only enhance our understanding of this vital cell population but can contribute to novel cell-based therapeutics for helping the growing diabetic population. Unless otherwise noted, all abstracts presented at ENDO are embargoed until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. Abstracts presented at a news conference are embargoed until the date and time of the news conference. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.