The role of islet specific deletion of selenoprotein S (Seps1) on insulin secretion

© 2017 Dr. Chia-Heng WengSelenoprotein S (Seps1) is a transmembrane protein that was initially discovered in the diabetic animal model, Psammonys obesus (P. obesus) where its hepatic gene expression was found to be increased by fasting. Subsequently, an overexpression of Seps1 in pancreatic β-cell line Min6 cells was shown to protect against oxidative stress- (H2O2)-induced apoptosis. To complement this in vitro study, Seps1 transgenic mice were generated in our laboratory which displayed higher tolerance to diabetogenic agents, alloxan and streptozotocin (STZ), but with preserved insulin secretion, compared to their control littermates. Based on the findings from the literature and our studies, we hypothesised that Seps1 may be an important antioxidant enzyme in β-cell to maintain insulin secretion. As such, Seps1 deficiency in mice may lead to higher susceptibility to oxidative or endoplasmic reticulum (ER) stress, and consequently to β-cell dysfunction. The aim of this thesis was to examine this hypothesis by generating Seps1 knockout mouse models. Seps1-floxed mice have exon 3 of the Seps1 gene flanked by LoxP sites. These Seps1-floxed mice were bred with mice expressing phosphoglycerate kinase (PGK)-Cre to generate whole-body Seps1 knockout mice. Heterozygous global deletion (Seps1+/-) of Seps1 with widespread 0 - 60 % reduction in Seps1 protein levels were identified in tissues including pancreas, liver, heart, kidney and skeletal muscle (white quadriceps), whereas a 100 % reduction of Seps1 protein levels was observed in the same tissues from homozygous knockout mice (Seps1-/-). It was found that at both 8 and 16 weeks of age, Seps1+/- mice displayed increased insulin secretion compared with control littermates, indicating a possible role of Seps1 in the β-cell to secrete insulin. To directly investigate the function of Seps1 in relation to insulin secretion, Seps1 β-cell specific knockout mice were generated using rat insulin promoter (RIP)-Cre mice. β-cell homozygous deletion of Seps1 in mice (Seps1-/-β-Cell) displayed 74 % and 88 % reduction in gene and protein expression levels respectively compared with the control littermates (Seps1+/+β-Cell). Seps1-/-β-Cell mice was shown to have a slight increase in fasting plasma glucose compared with β-cell heterozygous deletion of Seps1 (Seps1+/-β-Cell). There was no statistical difference in fasting plasma insulin and body weight among the genotypes. Seps1-/-β-Cell mice were found to have reduced insulin secretion when challenged with intravenous glucose tolerance tests. Consistent with this, ex vivo glucose-stimulated insulin secretion was impaired in the islets of Seps1-/-β-Cell mice, indicating defect(s) in β-cell function. Increased levels of reactive oxygen species, glutathione peroxidase 1 (GPx1), haem oxygenase-1 (HO-1), 78 kDa glucose-regulated protein (GRP78) and C/EBP homologous protein (CHOP) were found in islets of Seps1-/-β-Cell mice. These are indicative of oxidative and ER stress. Seps1-/-β-Cell mice were challenged with 60 % high fat diet for 7 weeks. These mice displayed an increase in feeding efficiency, body weight and adiposity compared to their chow-fed counterparts. In addition, these high-fat-fed Seps1-/-β-Cell mice were capable of secreting more insulin compared to their chow-diet counterparts. This could be due to an expansion of β-cell mass to adapt to the increased demand of substrate supply. This thesis provides clues as to what the physiological functions of Seps1 are in mice. Seps1 is extremely important in maintaining pancreatic β-cell function. Mice with β-cell specific deletion of Seps1 exhibited a significant reduction in insulin secretion which was likely due to a disrupted reactive oxygen species (ROS) homeostasis and the presence of both moderate oxidative and ER stress in the β-cells. This is the first in vivo study to demonstrate the impact of Seps1 in regulating insulin release in mammalian system such as mice. Our findings provide clues on how Seps1 deletion can have an impact on oxidative and ER stress – affecting β-cell function and the pathogenesis of diabetes