Aims/hypothesis Endoplasmic reticulum (ER) stress has been implicated in glucose-induced beta cell dysfunction. However, its causal role has not been established in vivo. Our objective was to determine the causal role of ER stress and its link to oxidative stress in glucose-induced beta cell dysfunction in vivo. Methods Healthy Wistar rats were infused i.v. with glucose for 48 h to achieve 20 mmol/l hyperglycaemia with or without the co-infusion of the superoxide dismutase mimetic tempol (TPO), or the chemical chaperones 4-phenylbutyrate (PBA) or tauroursodeoxycholic acid (TUDCA). This was followed by assessment of beta cell function and measurement of ER stress markers and superoxide in islets.Results Glucose infusion for 48 h increased mitochondrial superoxide and ER stress markers and impaired beta cell function. Co-infusion of TPO, which we previously found to reduce mitochondrial superoxide and prevent glucoseinduced beta cell dysfunction, reduced ER stress markers.Similar to findings with TPO, co-infusion of PBA, which decreases mitochondrial superoxide, prevented glucoseinduced beta cell dysfunction in isolated islets. TUDCA was also effective. Also similar to findings with TPO, PBA prevented beta cell dysfunction during hyperglycaemic clamps in vivo and after hyperglycaemia (15 mmol/l) for 96 h. Conclusions/interpretation Here, we causally implicate ER stress in hyperglycaemia-induced beta cell dysfunction in vivo. We show that: (1) there is a positive feedback cycle between oxidative stress and ER stress in glucose-induced beta cell dysfunction, which involves mitochondrial superoxide; and (2) this cycle can be interrupted by superoxide dismutase mimetics as well as chemical chaperones, which are of potential interest to preserve beta cell function in type 2 diabetes.

Lafora disease is a fatal genetic disorder with neurotoxic deposits of malformed insoluble glycogen. In humans it is caused by genetic variants in the EPM2A or NHLRC1 genes. There is a known sequence variant in Miniature Wirehaired Dachshunds but not in different dog breeds, including in Beagle dogs in which the disease is relatively commonly reported. This case report describes the causative defect in two affected Beagles, namely the same massive expansion as in Miniature Wirehaired Dachshunds of a 12-nucleotide repeat sequence that is unique to the canine NHLRC1 gene. This is the first sequence variant described in Beagles with Lafora disease, and so far the only Lafora disease genetic variant in dogs.

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