Characterisation of Ppy-lineage cells clarifies the functional heterogeneity of pancreatic beta cells in mice

AIMS/HYPOTHESIS

Pancreatic polypeptide (PP) cells, which secrete PP (encoded by the Ppy gene), are a minor population of pancreatic endocrine cells. Although it has been reported that the loss of beta cell identity might be associated with beta-to-PP cell-fate conversion, at present, little is known regarding the characteristics of Ppy-lineage cells.

METHODS

We used Ppy-Cre driver mice and a PP-specific monoclonal antibody to investigate the association between Ppy-lineage cells and beta cells. The molecular profiles of endocrine cells were investigated by single-cell transcriptome analysis and the glucose responsiveness of beta cells was assessed by Ca²⁺ imaging. Diabetic conditions were experimentally induced in mice by either streptozotocin or diphtheria toxin.

RESULTS

Ppy-lineage cells were found to contribute to the four major types of endocrine cells, including beta cells. Ppy-lineage beta cells are a minor subpopulation, accounting for 12-15% of total beta cells, and are mostly (81.2%) localised at the islet periphery. Unbiased single-cell analysis with a Ppy-lineage tracer demonstrated that beta cells are composed of seven clusters, which are categorised into two groups (i.e. Ppy-lineage and non-Ppy-lineage beta cells). These subpopulations of beta cells demonstrated distinct characteristics regarding their functionality and gene expression profiles. Ppy-lineage beta cells had a reduced glucose-stimulated Ca²⁺ signalling response and were increased in number in experimental diabetes models.

CONCLUSIONS/INTERPRETATION

Our results indicate that an unexpected degree of beta cell heterogeneity is defined by Ppy gene activation, providing valuable insight into the homeostatic regulation of pancreatic islets and future therapeutic strategies against diabetes.

DATA AVAILABILITY

The single-cell RNA sequence (scRNA-seq) analysis datasets generated in this study have been deposited in the Gene Expression Omnibus (GEO) under the accession number GSE166164 ( www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE166164 ).

Functional analysis of islet cells in vitro, in situ, and in vivo

The islet of Langerhans contains at least five types of endocrine cells producing distinct hormones. In response to nutrient or neuronal stimulation, islet endocrine cells release biochemicals including peptide hormones to regulate metabolism and to control glucose homeostasis. It is now recognized that malfunction of islet cells, notably insufficient insulin release of β-cells and hypersecretion of glucagon from α-cells, represents a causal event leading to hyperglycemia and frank diabetes, a disease that is increasing at an alarming rate to reach an epidemic level worldwide. Understanding the mechanisms regulating stimulus-secretion coupling and investigating how islet β-cells maintain a robust secretory activity are important topics in islet biology and diabetes research. To facilitate such studies, a number of biological systems and assay platforms have been developed for the functional analysis of islet cells. These technologies have enabled detailed analyses of individual islets at the cellular level, either in vitro, in situ, or in vivo.

Targeting insulin-producing beta cells for regenerative therapy

Pancreatic beta cells differ in terms of glucose responsiveness, insulin secretion and proliferative capacity; however, the molecular pathways that regulate this cellular heterogeneity are unknown. We have identified the Wnt-planar cell polarity (PCP) effector Flattop (FLTP) as a biomarker that identifies mature beta cells in the islets of Langerhans. Interestingly, three-dimensional architecture and Wnt-PCP ligands are sufficient to trigger mouse and human beta cell maturation. These results highlight the fact that novel biomarkers shed light on the long-standing mystery of beta cell heterogeneity and identify the Wnt-PCP pathway as triggering beta cell maturation. Understanding heterogeneity in the islets of Langerhans might allow targeting of beta cell subpopulations for regenerative therapy and provide building principles for stem cell-derived islets. This review summarises a presentation given at the 'Can we make a better beta cell?' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Amin Ardestani and Kathrin Maedler, DOI: 10.1007/s00125-016-3892-9 , and by Harry Heimberg and colleagues, DOI: 10.1007/s00125-016-3879-6 ) and a commentary by the Session Chair, Shanta Persaud (DOI: 10.1007/s00125-016-3870-2 ).