O-linked N-acetylglucosamine transferase (OGT) regulates pancreatic α-cell function in mice

Ahmad Essawy,Seokwon Jo,Megan Beetch,Amber Lockridge,Eric Gustafson,Emilyn U Alejandro

doi:10.1016/j.jbc.2021.100297

Ahmad Essawy, Seokwon Jo + Show 4 more

Open Access

https://doi.org/10.1016/j.jbc.2021.100297

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Abstract

The nutrient sensor O-GlcNAc transferase (OGT) catalyzes posttranslational addition of O-GlcNAc onto target proteins, influencing signaling pathways in response to cellular nutrient levels. OGT is highly expressed in pancreatic glucagon-secreting cells (α-cells), which secrete glucagon in response to hypoglycemia. The objective of this study was to determine whether OGT is necessary for the regulation of α-cell mass and function in vivo. We utilized genetic manipulation to produce two α-cell specific OGT-knockout models: a constitutive glucagon-Cre (αOGTKO) and an inducible glucagon-Cre (i-αOGTKO), which effectively delete OGT in α-cells. Using approaches including immunoblotting, immunofluorescent imaging, and metabolic phenotyping in vivo, we provide the first insight on the role of O-GlcNAcylation in α-cell mass and function. αOGTKO mice demonstrated normal glucose tolerance and insulin sensitivity but displayed significantly lower glucagon levels during both fed and fasted states. αOGTKO mice exhibited significantly lower α-cell glucagon content and α-cell mass at 6 months of age. In fasting, αOGTKO mice showed impaired pyruvate stimulated gluconeogenesis in vivo and reduced glucagon secretion in vitro. i-αOGTKO mice showed similarly reduced blood glucagon levels, defective in vitro glucagon secretion, and normal α-cell mass. Interestingly, both αOGTKO and i-αOGTKO mice had no deficiency in maintaining blood glucose homeostasis under fed or fasting conditions, despite impairment in α-cell mass and function, and glucagon content. In conclusion, these studies provide a first look at the role of OGT signaling in the α-cell, its effect on α-cell mass, and its importance in regulating glucagon secretion in hypoglycemic conditions.

Highlights

Levels, dysregulation of glucagon levels in T1D and T2D contributes to the pathology of these diseases [4, 5]
We reported that loss of O-GlcNAcylation in the β-cell leads to cell failure and diabetes in mice due to increased endoplasmic reticulum (ER) stress and apoptosis [14]
RL2 is a specific O-GlcNAc antibody that has been validated in our lab [18] and others [19, 20]. These data show that O-linked GlcNAc transferase (OGT) and OGA are expressed in α-cells, and the role of O-GlcNAcylation in α-cell physiology should be tested in vivo

Summary

Introduction

Levels, dysregulation of glucagon levels in T1D and T2D contributes to the pathology of these diseases [4, 5]. Independent of its role in cell survival, OGT regulates insulin secretion at basal [14] and in obesity conditions in part through SERCA2 [17]. We hypothesize that OGT plays a key role in the maintenance of α-cell mass and proper function of secreting glucagon in response to hypoglycemia. It is unknown how nutrient-driven posttranslational OGlcNAcylation of proteins impacts pancreas α-cell mass and function. Through the characterization of mice lacking α-cell OGT, the only enzyme capable of adding O-GlcNAc modification onto proteins, we show that OGlcNAcylation is necessary for the maintenance of α-cell mass and regulation of glucagon secretion

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Conclusion