Hyperglucagonemia in an animal model of insulin- deficient diabetes: what therapy can improve it?

Fabrizio Barbetti,Sergio Bernardini,Corentin Cras-Méneur,Peter Arvan,Leena Haataja,Carlo Colombo

doi:10.1186/s40842-016-0029-5

Abstract

BackgroundIntra-islet insulin contributes to alpha-cell suppression. Akita mice carry a toxic-gain-of- function Ins2 gene mutation encoding proinsulin-C(A7)Y, similar to that described in human Mutant Ins-gene induced Diabetes of Youth, which decreases intra-islet insulin. Herein, we examined Akita mice for examination of circulating insulin and circulating glucagon levels. The possibility that loss of intra-islet suppression of alpha-cells, with increased circulating glucagon, contributes to diabetes under conditions of intra-islet insulin deficiency, raises questions about effective treatments that may be available.MethodsBlood glucose, plasma insulin, C-peptide I, C-peptide II, and glucagon were measured at various times during development of diabetes in Akita mice. We also used Akita- like hProC(A7)Y-CpepGFP transgenic mice in Ins2 +/+, Ins2 +/− and Ins2 −/− genetic backgrounds (providing animals with greater or lesser defects in islet insulin production, respectively) in order to examine the relative abundance of immunostainable intra-islet glucagon-positive and insulin-positive cells. Similar measurements were made in Akita mice. Finally, the effects of treatment with insulin, exendin-4, and leptin on blood glucose were then compared in Akita mice.ResultsInterestingly, total insulin levels in the circulation were not frankly low in Akita mice, although they did not rise appropriately with the onset of hyperglycemia. By contrast, in severely diabetic Akita mice at 6 weeks of age, circulating glucagon levels were significantly elevated. Additionally, in Ins2 +/− and Ins2 −/− mice bearing the Akita-like hProC(A7)Y-CpepGFP transgene, development of diabetes correlated with an increase in the relative intra-islet abundance of immunostainable glucagon-positive cells, and a similar observation was made in Akita islets. In Akita mice, whereas a brief treatment with exendin-4 resulted in no apparent improvement in hyperglycemia, leptin treatment resulted in restoration of normoglycemia. Curiously, leptin treatment also suppressed circulating glucagon levels.ConclusionsLoss of insulin-mediated intra-islet suppression of glucagon production may be a contributor to hyperglycemia in Akita mice, and leptin treatment appears beneficial in such a circumstance. This treatment might also be considered in some human diabetes patients with diminished insulin reserve.

Highlights

The dominant-negative effect of these mutants is caused by impairment of wild-type proinsulin intracellular transport [12,13,14] and eventually, the insulin-deficient diabetes is compounded by apoptosis of beta cells that suffer from endoplasmic reticulum (ER) stress [15]
The Akita mutant proinsulin is the product of one allelic copy of the Ins2 gene, it is not surprising that C-peptide II levels in Akita males were significantly lower that that of wildtype animals even at two weeks of age — and remained low for the entire observation period (Fig. 1b)
Loss of insulin-mediated intra-islet suppression of glucagon production in Akita mice We previously reported that patients with permanent neonatal diabetes mellitus (PNDM) due to proteotoxic mutant ins-gene induced diabetes of youth (MIDY) mutations have fasting hyperglucagonemia [11], suggesting loss of insulin- mediated intra-islet suppression of glucagon production [24, 25]

Summary

Introduction

Akita mice carry a toxic-gain-of- function Ins gene mutation encoding proinsulin-C(A7)Y, similar to that described in human Mutant Ins-gene induced Diabetes of Youth, which decreases intra-islet insulin. The possibility that loss of intra-islet suppression of alpha-cells, with increased circulating glucagon, contributes to diabetes under conditions of intra-islet insulin deficiency, raises questions about effective treatments that may be available. Patients with autosomal dominant Mutant INS-gene induced Diabetes of Youth (MIDY [6, 7]) serve as an excellent model of insulin-deficient diabetes. Such mutations affect proinsulin folding and cause insulin-deficient diabetes in both humans and animal models [8,9,10,11]. Little has been reported about circulating glucagon levels in such patients, and currently, treatment options consist mainly of full insulin replacement dosing [11], which comes with concomitant risk of unintended – or even fatal – hypoglycemia

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