SummaryThe arrangement of β cells within islets of Langerhans is critical for insulin release through the generation of rhythmic activity. A privileged role for individual β cells in orchestrating these responses has long been suspected, but not directly demonstrated. We show here that the β cell population in situ is operationally heterogeneous. Mapping of islet functional architecture revealed the presence of hub cells with pacemaker properties, which remain stable over recording periods of 2 to 3 hr. Using a dual optogenetic/photopharmacological strategy, silencing of hubs abolished coordinated islet responses to glucose, whereas specific stimulation restored communication patterns. Hubs were metabolically adapted and targeted by both pro-inflammatory and glucolipotoxic insults to induce widespread β cell dysfunction. Thus, the islet is wired by hubs, whose failure may contribute to type 2 diabetes mellitus.

A longstanding unsettled question is whether pancreatic beta cells originate from exocrine duct cells. We have now used genetic labeling to fate map embryonic and adult pancreatic duct cells. We show that Hnf1beta+ cells of the trunk compartment of the early branching pancreas are precursors of acinar, duct, and endocrine lineages. Hnf1beta+ cells subsequent form the embryonic duct epithelium, which gives rise to both ductal and endocrine lineages, but not to acinar cells. By the end of gestation, the fate of Hnf1beta+ duct cells is further restrained. We provide compelling evidence that the ductal epithelium does not make a significant contribution to acinar or endocrine cells during neonatal growth, during a 6 month observation period, or during beta cell growth triggered by ligation of the pancreatic duct or by cell-specific ablation with alloxan followed by EGF/gastrin treatment. Thus, once the ductal epithelium differentiates it has a restricted plasticity, even under regenerative settings.

Tissue-specific transcriptional regulation is central to human disease1. To identify regulatory DNA active in human pancreatic islets, we profiled chromatin by FAIRE (Formaldehyde-Assisted Isolation of Regulatory Elements)2–4 coupled with high-throughput sequencing. We identified ~80,000 open chromatin sites. Comparison of islet FAIRE-seq to five non-islet cell lines revealed ~3,300 physically linked clusters of islet-selective open chromatin sites, which typically encompassed single genes exhibiting islet-specific expression. We mapped sequence variants to open chromatin sites and found that rs7903146, a TCF7L2 intronic variant strongly associated with type 2 diabetes (T2D)5, is located in islet-selective open chromatin. We show that rs7903146 heterozygotes exhibit allelic imbalance in islet FAIRE signal, and that the variant alters enhancer activity, indicating that genetic variation at this locus acts in cis with local chromatin and regulatory changes. These findings illuminate the tissue-specific organization of cis-regulatory elements, and show that FAIRE-seq can guide identification of regulatory variants important for disease.