SUN-546 ISL1 and LDB1 Transcriptional Complex Regulators of Pancreatic Islet Alpha-Cell Function

Abstract Disclosure: I. Deeba: None. S. Poole: None. Y. Liu: None. T. Mitchell: None. C. Hunter: None. Diabetes was traditionally considered a dysglycemia centered on the insulin-secreting β-cell but is now recognized to also involve significant contributions from dysregulated glucagon-producing α-cells within the pancreatic islets. However, little is known of the key transcriptional regulators, including transcription factors (TFs) and interacting co-regulators, driving α-cell development and function. In mice, a pancreas-wide knockout (KO) of the LIM-Homeodomain TF Islet-1 (ISL1) resulted in β-cell dysfunction, characterized by glucose intolerance and decreased insulin secretion. Interestingly, α-cells were also affected, as evidenced by significant reductions in glucagon (Gcg) and Arx mRNA levels and pancreatic GCG content. Furthermore, we demonstrated that pancreatic ISL1 activity involves interactions with the scaffolding co-regulator, LDB1. However, nothing is known of the comparative α-cell-specific target genes and functions of these protein partners. To investigate the interaction between ISL1 and LDB1 in α-cells, we conducted co-IP and proximity ligation assays (PLA) using mouse α-cell lines (αTC1-6) and human islets, which revealed that low-glucose conditions enhance ISL1:LDB1 complex formation. These observations led us to hypothesize that ISL1 and LDB1 complexes regulate transcription of key target genes driving α-cell function. To assess comparative DNA occupancy, we performed ChIP and observed significant ISL1 and LDB1 binding in conserved Gcg promoter and intronic domains. siRNA-mediated Isl1 or Ldb1 knockdown followed by RNA analysis revealed a similar differential expression of α-cell mRNAs (e.g., Gcg, IL6ra, Sox8, Cox5a, Atp5k, Atp5e), again supporting roles in α-cell function. Given the disruption of key mitochondrial genes, we conducted mitochondrial stress test (MST) and ATP production rate assays using the Seahorse XF 96 Pro Analyzer. We found that silencing Ldb1 expression in αTC1-6 cells reduced mitochondrial function and ATP production rates. To compare ISL1 and LDB1 importance in vivo, we developed two novel mouse α-cell specific ISL1 or LDB1 KO models using Gcg-Cre, termed Isl1Δα and Ldb1Δα, respectively. ISL1 and LDB1 loss was confirmed in α-cells by immunofluorescence. Postnatal Isl1Δα and Ldb1Δα mice exhibited significant fasting hypoglycemia and hypoglucagonemia. Additionally, Ldb1Δα mice showed a significant decrease in GCG+ cell numbers compared to controls. These findings underscore the crucial roles of these factors in regulating GCG production and secretion. We are currently developing inducible α-cell-specific KO mice to explore the function of ISL1 and LDB1 in adult α-cells. Overall, this research has advanced our understanding of α-cell biology, paving the way for the development of cell-based therapeutic strategies for diabetes treatment. Presentation: Sunday, July 13, 2025