Chromatin accessibility differences between alpha, beta, and delta cells identifies common and cell type-specific enhancers

High throughput sequencing has enabled the interrogation of the transcriptomic landscape of glucagon-secreting alpha cells, insulin-secreting beta cells, and somatostatin-secreting delta cells. These approaches have furthered our understanding of expression patterns that define healthy or diseased islet cell types and helped explicate some of the intricacies between major islet cell crosstalk and glucose regulation. All three endocrine cell types derive from a common pancreatic progenitor, yet alpha and beta cells have partially opposing functions, and delta cells modulate and control insulin and glucagon release. While gene signatures that define and maintain cellular identity have been widely explored, the underlying epigenetic components are incompletely characterized and understood. Chromatin accessibility and remodeling is a dynamic attribute that plays a critical role to determine and maintain cellular identity. Here, we compare and contrast the chromatin landscape between mouse alpha, beta, and delta cells using ATAC-Seq to evaluate the significant differences in chromatin accessibility. The similarities and differences in chromatin accessibility between these related islet endocrine cells help define their fate in support of their distinct functional roles. We identify patterns that suggest that both alpha and delta cells are poised, but repressed, from becoming beta-like. We also identify patterns in differentially enriched chromatin that have transcription factor motifs preferentially associated with different regions of the genome. Finally, we identify and visualize both novel and previously discovered common endocrine- and cell specific- enhancer regions across differentially enriched chromatin.

High co-expression of the ghrelin and LEAP2 receptor GHSR with pancreatic polypeptide in mouse and human islets

Islets represent an important site of direct action of the hormone ghrelin, with expression of the ghrelin receptor (growth hormone secretagogue receptor; GHSR) having been localized variably to alpha-cells, beta-cells, and/or somatostatin (SST)-secreting delta-cells. To our knowledge, GHSR expression by pancreatic polypeptide (PP)-expressing gamma-cells has not been specifically investigated. Here, histochemical analyses of Ghsr-IRES-Cre X Cre-dependent ROSA26-YFP reporter mice showed 85% of GHSR-expressing islet cells co-express PP, 50% co-express SST, and 47% co-express PP + SST. Analysis of single-cell transcriptomic data from mouse pancreas revealed 95% of Ghsr-expressing cells co-express Ppy, 100% co-express Sst, and 95% co-express Ppy + Sst. This expression was restricted to gamma-cell and delta-cell clusters. Analysis of several single-cell human pancreatic transcriptome datasets revealed 59% of GHSR-expressing cells co-express PPY, 95% co-express SST, and 57% co-express PPY + SST. This expression was prominent in delta-cell and beta-cell clusters, also occurring in other clusters including gamma-cells and alpha-cells. GHSR expression levels were upregulated by type 2 diabetes mellitus in beta-cells. In mice, plasma PP positively correlated with fat mass and with plasma levels of the endogenous GHSR antagonist/inverse agonist LEAP2. Plasma PP also elevated upon LEAP2 and synthetic GHSR antagonist administration. These data suggest that in addition to delta-cells, beta-cells, and alpha-cells, PP-expressing pancreatic cells likely represent important direct targets for LEAP2 and/or ghrelin in both mice and humans.

Physically interacting beta-delta pairs in the regenerating pancreas revealed by single-cell sequencing

The endocrine pancreas is able to regenerate in response to insult, including by driving beta-cells into the cell division cycle. Until recently, communication between neighboring cells in islets of Langerhans was overlooked by single-cell genomic technologies, which require rigorous tissue dissociation into single cells. Here, we utilize sorting of physically interacting cells (PICs) with single-cell RNA-sequencing to systematically map cellular interactions in the regenerating endocrine pancreas. The cellular landscape of the regenerated pancreas features regeneration-associated endocrine populations.We explore the unexpected heterogeneity of beta-cells in regeneration, including an interaction-specific program between paired beta and delta-cells. Our analysis suggests that the particular cluster of beta-cells that pair with delta-cells benefits from stress protection, implying that the interaction between beta and delta-cells safeguards against regeneration-associated challenges.

Innervation modulates the functional connectivity between pancreatic endocrine cells

Direct modulation of pancreatic endocrine cell activity by autonomic innervation has been debated. To resolve this question, we established an in vivo imaging model which also allows chronic and acute neuromodulation. Starting at a stage when zebrafish islet architecture is reminiscent of that in adult rodents, we imaged calcium dynamics simultaneously in multiple islet cell types. We first find that activity coupling between beta cells increases upon glucose exposure. Surprisingly, glucose exposure also increases alpha-alpha, alpha-beta and beta-delta coordination. We further show that both chronic and acute loss of nerve activity diminish activity coupling, as observed upon gap junction depletion. Notably, chronic loss of innervation severely disrupts delta cell activity, suggesting that delta cells receive innervation which coordinates its output. Overall, these data show that innervation plays a vital role in the establishment and maintenance of homotypic and heterotypic cellular connectivity in pancreatic islets, a process critical for islet function.

CHRONIC USE OF PROTON PUMP INHIBITORS AND THE QUANTITY OF G, D, AND ECL CELLS IN THE STOMACH

ABSTRACT Background: Acid inhibition from chronic proton pump inhibitor use and a possible increase in gastrin can lead to changes in the regulation of hydrochloric acid production. However, it has not known whether such chronic use changes the presence of gastrin, delta, and enterochromaffin-like cells in the stomach or the relationship between gastrin and delta cells. Aim: To analyze the number of gastrin-producing gastrin cells, somatostatin-producing cells, and histamine-producing cells in patients who were chronic users of proton pump inhibitor, with or without related Helicobacter pylori infection. Methods: Biopsies from 105 patients, including 81 chronic proton pump inhibitor users (experimental group) and 24 controls, were processed immunohistochemically and subjected to counting of gastrin, delta, and enterochromaffin-like cells in high-magnification microscopic fields and in 10 glands. Results: Gastrin cell, delta cell, and enterochromaffin-like cells counts were similar across the groups and appeared to be unaffected by Helicobacter pylori infection. The ratio between gastrin cells and delta cells was higher in the chronic users of proton pump inhibitor group than in controls. Conclusion: Chronic users of proton pump inhibitor does not affect gastrin cell, delta cell, and enterochromaffin-like cell counts significantly, but may alter the ratio between gastrin cells and delta cells.

Single-cell analyses identify distinct and intermediate states of zebrafish pancreatic islet development

Pancreatic endocrine islets are vital for glucose homeostasis. However, the islet developmental trajectory and its regulatory network are not well understood. To define the features of these specification and differentiation processes, we isolated individual islet cells from TgBAC(neurod1:EGFP) transgenic zebrafish and analyzed islet developmental dynamics across four different embryonic stages using a single-cell RNA-seq strategy. We identified proliferative endocrine progenitors, which could be further categorized by different cell cycle phases with the G1/S subpopulation displaying a distinct differentiation potential. We identified endocrine precursors, a heterogeneous intermediate-state population consisting of lineage-primed alpha, beta and delta cells that were characterized by the expression of lineage-specific transcription factors and relatively low expression of terminally differentiation markers. The terminally differentiated alpha, beta, and delta cells displayed stage-dependent differentiation states, which were related to their functional maturation. Our data unveiled distinct states, events and molecular features during the islet developmental transition, and provided resources to comprehensively understand the lineage hierarchy of islet development at the single-cell level.