| Islet‑1 (ISL1) is a LIM‑homeodomain transcription factor of the LIM family that remains expressed in all adult pancreatic endocrine lineages and functions as a central regulator of endocrine cell differentiation, postnatal islet proliferation and β‑cell gene expression, integrating developmental signals with transcriptional control of hormone production and secretion. The protein contains two N‑terminal LIM domains that mediate protein–protein interactions with cofactors such as Ldb1, Set7/9 and PDX‑1, and a C‑terminal homeodomain that binds DNA at regulatory elements of endocrine genes, creating a modular structure that couples sequence‑specific promoter and enhancer recognition to assembly of multi‑protein transcriptional complexes. During pancreas development, ISL1 is required for the survival and differentiation of pancreatic endocrine progenitors and later acts together with the coregulator Ldb1 to control the development of α‑, β‑ and δ‑cells; genetic studies show that Ldb1 mediates much of LIM‑HD and LIM‑only factor activity in islets and that loss of Ldb1 disrupts endocrine cell development, highlighting a pathway in which ISL1–Ldb1 complexes drive lineage‑specific transcription programs. In the postnatal pancreas, ISL1 maintains β‑cell identity and function by regulating expression of key genes such as insulin, MafA, Glut2 and Pdx1; conditional Isl1 deletion in β‑cells leads to reduced insulin content, impaired glucose‑stimulated insulin secretion and progressive diabetes, demonstrating that ISL1 is essential not only for endocrine specification but also for mature β‑cell function. Mechanistically, ISL1 promotes islet cell proliferation by forming an ISL1/Set7/9/PDX‑1 complex that binds promoters of genes involved in β‑cell growth and insulin synthesis, where Set7/9 provides histone methyltransferase activity and PDX‑1 contributes pancreatic lineage specificity, and disruption of any component reduces islet proliferation and insulin gene transcription, defining a cooperative transcriptional module that links chromatin modification to endocrine expansion. In the broader islet niche, ISL1‑dependent transcription interacts with paracrine and vascular signals that shape β‑cell structure and secretory behavior, and proteomic and functional studies indicate that ISL1‑regulated networks participate in the adaptation of islets to metabolic stress, connecting transcription factor activity to changes in lipid metabolism, cytokine signaling and β‑cell resilience in type 2 diabetes. Genetic or expression alterations of ISL1 and its co‑regulators are associated with impaired β‑cell proliferation, defective insulin secretion and increased susceptibility to type 2 diabetes, and ISL1 has also been implicated in congenital heart and motor neuron development in other contexts, reflecting the reuse of LIM‑homeodomain transcription modules across excitable tissues. |
