Type 2 diabetes is associated with complications in the CNS; however, the pathophysiology is not well understood. Here we report the first evaluation of the genome-wide effects of type 2 diabetes in the hippocampus of db+ and db/db mice at 8 and 24 weeks, using microarrays to assess the alterations in gene expression in the hippocampus. The most significantly enriched terms in the 24 week downregulated DEGs were related to mitochondria, HSPs, the ER and sterol biosynthesis. The ER-enriched cluster included 42 genes. Literature mining revealed that four of our DEGs were in the top 25 genes related to ER stress and directly involved in the UPR. Thus, we validated the biological significance of these four downregulated genes—Hspa5, Hsp90b1, Xbp1 and Ddit3.
are ER molecular chaperones that interact with misfolded proteins and are associated with cell survival [8
]. The downregulation of these chaperones may leave cells particularly vulnerable to further stress [36
]. The expression levels of Hspa5
decline in the ageing hippocampus [14
]. Thus, one limitation in our study is the inability to determine whether the down-regulation is a direct result of the obesity or diabetes, or is associated with brain ageing, which can be accelerated by diabetes [38
]. Further studies are required to fully elucidate the mechanisms contributing to ER stress in the hippocampus during diabetes.
, a transcription factor and master regulator of ER folding capacity, is transcriptionally upregulated during the typical UPR to acute stress [9
]. In response to this stress, an intron is excised from the unspliced variant of Xbp1
mRNA yielding spliced Xbp1
]. Spliced Xbp1
mRNA encodes XBP1s, a transcriptional activator of Hspa5
, whereas unspliced Xbp1
mRNA encodes XBP1u, an inhibitor of the UPR and chaperone induction [11
]. XBP1u shuttles between the nucleus and cytoplasm and may function as a negative feedback regulator of XBP1s [39
Chronic stressors, such as diabetes or obesity, lead to years of persistent ER stress and UPR activation and cells must adapt to survive [10
]. Adaptation to ER stress is a complex cell-type dependent mechanism that involves maintenance of cellular function and avoidance of apoptosis. This adaptation may involve partial activation or suppression of one of more branches of the UPR [9
mRNA declines after prolonged ER stress and is not activated in cells that have adapted to this stress [41
]. Furthermore, Ddit3
, a mediator of apoptosis, is rapidly increased at the initial onset of stress, but is quickly reduced owing to the instability of the mRNA and protein during adaptation [42
]. Thus, the downregulation in the gene (Hspa5
and mRNA (Hspa5
) expression levels, and protein levels (HSPA5, HSP90B1, XBP1s and XBP1u) provide evidence that hippocampal cells adapt to ER stress during diabetes.
XBP1s plays a role in inhibiting insulin signalling and contributes to the development of peripheral insulin resistance [18
]. Treatment with small molecules that act as chaperones for proper protein folding results in the resolution of ER stress, normalisation of blood glucose and restoration of insulin sensitivity in the liver of ob/ob
]. Similarly, overexpression of an ER chaperone, GRP150, results in improvements in glucose tolerance and insulin sensitivity in db/db
]. Therefore, ER stress undoubtedly plays a role in insulin resistance. Insulin resistance is associated with cognitive decline in rodents and in epidemiological studies [44
]. Pharmacological inhibition of ER stress using chemical chaperones not only improves insulin sensitivity but also protects against cognitive deficits in mouse models of diabetes [19
]. Thus, ER stress may contribute to cognitive deficits.
Collectively, our data suggest that hippocampal cells adapt to type 2 diabetes-induced prolonged ER stress with partial suppression of Xbp1
(). The decrease in the ratio of XBP1s to XBP1u, which indicates that there are higher levels of XBP1u compared with XBP1s, may be responsible for the suppression of the induction of Hspa5
and contribute to the development of insulin resistance, which we have demonstrated in the hippocampi of db/db
]. Although this study does not reveal the mechanism underlying the connection between ER stress and cognitive decline, it does provide a foundation for further exploration of the role of adaptation to hippocampal ER stress in vivo in neuronal insulin resistance and cognitive deficits. Our results provide the first evidence of ER stress in the hippocampus of a murine model of type 2 diabetes.
Fig. 4 Representation of our proposed model of adaptation to ER stress in the diabetic hippocampus. An intron of the unspliced (u) variant of Xbp1 mRNA is excised during the response to ER stress, yielding spliced (s) Xbp1 mRNA. Xbp1s mRNA encodes the protein (more ...)