NBCe1 is a novel regulator of pancreatic β‐cell function in diabetes

Abstract

Dysregulation of glucose homeostasis in Type 2 diabetes mellitus (T2DM) is attributed to pancreatic β‐cell failure in maintaining adequate glucose‐stimulated insulin secretion (GSIS) to compensate for diminished insulin action. Recent single cell transcriptomics studies of human pancreatic islets have identified Slc4a4 (encoding an electrogenic Na+‐nHCO3− cotransporter, NBCe1) as a gene repressed in healthy non‐diabetic β‐cells, but expressed in human T2DM β‐cells. However the role of Slc4a4/NBCe1 in pancreatic β‐cell function remains unknown. Thus, in the current study we set out to test the hypothesis that mis‐expression of Slc4a4 in β‐cells contributes to loss of GSIS and impaired glucose homeostasis in T2DM. Subsequently, we first assessed Slc4a4 mRNA expression in isolated human islets (Control vs. T2DM) by qPCR and quantified NBCe1 protein expression by immunofluorescence in autopsy‐derived human pancreas tissue (Control vs. T2DM). We observed a 4‐fold induction in Slc4a4 mRNA expression in T2DM islets compared to non‐diabetic controls (p=0.004 vs. Control) and significantly increased NBCe1 co‐localization with insulin‐positive β‐cells of T2DM patients compared to controls (p<0.001 vs. Control). In addition, we also demonstrated that chronic inhibition of NBCe1 activity using the channel specific inhibitor (S0859) significantly enhanced GSIS in islets isolated from T2DM patients (~1.5‐fold increase vs. untreated, p=0.006). To delineate the role NBCe1 plays on β‐cell function in vivo, we generated β‐cell specific knockout (β‐Slc4a4−/−) mice by crossing Slc4a4fl/fl (G. Shull, UC) and Ins2‐Cre mouse models. We confirmed >50% reduction in Slc4a4 mRNA expression in β‐Slc4a4−/− islets and complete ablation of NBCe1 protein expression. In order to model the environmental stress that leads to metabolic defects common to T2DM, we exposed β‐Slc4a4−/− mice and control littermates to either standard chow diet or 60% high fat diet (HFD) for 8 weeks ad libitum. Under chow‐fed conditions, β‐Slc4a4−/− mice displayed normal glucose tolerance (p>0.05 vs. β‐Slc4a4+/+ Chow). Interestingly, β‐Slc4a4−/− mice exposed to HFD exhibited enhanced glucose tolerance and GSIS (p<0.05 vs. β‐Slc4a4+/+ HFD) and thus were protected from development of T2DM phenotype characteristics of HFD treatment. Finally, to understand the mechanism by which Slc4a4 deletion enhanced β‐cell function, we performed RNA‐sequencing on islets isolated from β‐Slc4a4−/− and β‐Slc4a4+/+ mice exposed to chow diet or HFD. Notably, gene‐set enrichment analysis of Kyoto Encyclopedia of Genes and Genomes (KEGG) defined biological pathways revealed that Slc4a4 deletion enriched genes associated with the cell cycle (p<0.001 vs. β‐Slc4a4+/+ Chow; p<0.001 vs. β‐Slc4a4+/+ HFD), while suppressing genes associated with apoptosis (p=0.002 vs. β‐Slc4a4+/+ Chow; p=0.004 vs. β‐Slc4a4+/+ HFD) in the presence of both chow diet and HFD. These result suggest that mis‐expression of Slc4a4 plays a novel role in the induction of β‐cell failure in T2DM. Future studies will delineate exact molecular mechanisms underlying NBCe1‐mediated regulation of β‐cell function in health and under conditions of metabolic stress.Support or Funding InformationMayo Clinic Center of Biomedical DiscoveryThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.