Biosynthetic Activity Differs Between Islet Cell Types and in Beta Cells Is Modulated by Glucose and Not by Secretion.

David Cottet-Dumoulin,Vanessa Lavallard,Géraldine Parnaud,Kevin Bellofatto,Ekaterine Berishvili,Thierry Berney,Charles H Wassmer,Fanny Lebreton,Domenico Bosco

doi:10.1210/endocr/bqaa239

Abstract

A correct biosynthetic activity is thought to be essential for the long-term function and survival of islet cells in culture and possibly also after islet transplantation. Compared to the secretory activity, biosynthetic activity has been poorly studied in pancreatic islet cells. Here we aimed to assess biosynthetic activity at the single cell level to investigate if protein synthesis is dependent on secretagogues and increased as a consequence of hormonal secretion. Biosynthetic activity in rat islet cells was studied at the single cell level using O-propargyl-puromycin (OPP) that incorporates into newly translated proteins and chemically ligates to a fluorescent dye by “click” reaction. Heterogeneous biosynthetic activity was observed between the four islet cell types, with delta cells showing the higher relative protein biosynthesis. Beta cells protein biosynthesis was increased in response to glucose while 3-isobutyl-1-methylxanthine and phorbol-12-myristate-13-acetate, 2 drugs known to stimulate insulin secretion, had no similar effect on protein biosynthesis. However, after several hours of secretion, protein biosynthesis remained high even when cells were challenged to basal conditions. These results suggest that mechanisms regulating secretion and biosynthesis in islet cells are different, with glucose directly triggering beta cells protein biosynthesis, independently of insulin secretion. Furthermore, this OPP labeling approach is a promising method to identify newly synthesized proteins under various physiological and pathological conditions.

Highlights

Islets of Langerhans are small clusters of endocrine cells scattered in the pancreas and composed of 4 major cell types: beta cells, alpha cells, delta cells and pancreatic polypeptide cells
Less attention has been paid to biosynthetic activity of islet cells, even if it is evident that this activity is essential in maintaining a correct function of pancreatic islet cells and could be affected in diabetes
A linear correlation between fluorescence intensity and protein biosynthesis has not been demonstrated, but there is no doubt that increasing fluorescence intensity means increasing biosynthesis and OPP labeling can be considered as a semiquantitative approach to evaluate biosynthesis at the single cell level

Summary

Introduction

Islets of Langerhans are small clusters of endocrine cells scattered in the pancreas and composed of 4 major cell types: beta cells, alpha cells, delta cells and pancreatic polypeptide cells. After a secretion phase, cells must replenish their hormone stores and renew some enzymes, receptors, and any other proteins consumed during the secretion phase This is made possible by an increase of protein synthesis activity, which can be regulated both at the level of transcription and of translation. Translational control of existing messenger ribonucleic acids, compared to transcriptional regulation, allows for faster changes in cellular protein concentration and is a more likely actor for the recovery of proteins from islet cells after secretion [17,18] It is known for a long time that glucose increases insulin biosynthesis of beta cells [19,20,21,22]. This approach allows quantification of biosynthesis activity in every individual islet cell and enabled to study changes of protein biosynthesis in response to secretagogues and to compare the different islet cell types

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