XBP1 (X-Box–Binding Protein-1)–Dependent O-GlcNAcylation Is Neuroprotective in Ischemic Stroke in Young Mice and Its Impairment in Aged Mice Is Rescued by Thiamet-G

Abstract

Background and Purpose Impaired protein homeostasis induced by endoplasmic reticulum (ER) dysfunction is a key feature of a variety of age-related brain diseases including stroke. To restore ER function impaired by stress, the unfolded protein response (UPR) is activated. A key UPR pro-survival pathway is controlled by the ER stress sensor inositol-requiring enzyme-1 (IRE1), downstream X-box binding protein-1 (XBP1), and O-linked b-N-acetylglucosamine (O-GlcNAc) modification of proteins (O-GlcNAcylation). Stroke impairs ER function, which activates UPR. The rationale of this study was to explore the potentials of the IRE1/XBP1/O-GlcNAc axis as a target for neuroprotection in ischemic stroke. Methods Mice with Xbp1 loss- and gain-of-function in neurons were generated. Stroke was induced by transient or permanent occlusion of the middle cerebral artery (MCAO) in young and aged mice. Thiamet-G was used to increase O-GlcNAcylation. Results Deletion of Xbp1 worsened outcome after transient and permanent MCAO. After stroke, O-GlcNAcylation was activated in neurons of the stroke penumbra in young mice, which was largely Xbp1-dependent. This activation of O-GlcNAcylation was impaired in aged mice. Pharmacologic increase of O-GlcNAcylation before or after stroke improved outcome in both young and aged mice. Conclusions Our study indicates a critical role for the IRE1/XBP1 UPR branch in stroke outcome. O-GlcNAcylation is a pro-survival pathway that is activated in the stroke penumbra in young mice but impaired in aged mice. Boosting pro-survival pathways to counterbalance the age-related decline in the brain’s self-healing capacity could be a promising strategy to improve ischemic stroke outcome in aged brains.

Aging Is Associated With Impaired Activation of Protein Homeostasis‐Related Pathways After Cardiac Arrest in Mice

“…These findings suggest that aging is associated with a decline in the capacity of cells to activate these posttranslational modifications, particularly SUMOylation and O‐GlcNAcylation. Rapid activation of both modifications has been reported in various postischemic organs 6, 8, 27, 28, 29, 30. Notably, in the present study, these were the only posttranslational modifications that showed impaired activation in both brain and kidney of aged mice.…”

“…Thus, it is reasonable to propose that at advanced age, cells are more sensitive to a severe form of stress that impairs proteostasis. Indeed, such findings have been recently reported after myocardial ischemia and stroke 8, 18. For CA, however, the role of aging‐related decline in the capacity of cells to restore proteostasis impaired by I/R stress has not yet been considered a potential contributor to worse outcome in elderly individuals.…”

“…We previously reported that activation of O‐GlcNAcylation is impaired in the aged brain in both ischemic stroke and forebrain ischemia models, a finding that is thought to have important implications in understanding worse outcome in aged brain after ischemia 7, 8. Therefore, we decided to systematically compare activation of proteostasis‐related pathways in young versus aged mice.…”

“…When prosurvival pathways dominate, outcomes improve. Experimental ischemic stroke studies have shown that the capacity of cells to activate prosurvival pathways markedly declines with increasing age 8. Notably, I/R stress‐induced activation of prosurvival pathways that maintain and restore cellular protein homeostasis (proteostasis) is particularly impaired at advanced age 8, 9.…”

“…Experimental ischemic stroke studies have shown that the capacity of cells to activate prosurvival pathways markedly declines with increasing age 8. Notably, I/R stress‐induced activation of prosurvival pathways that maintain and restore cellular protein homeostasis (proteostasis) is particularly impaired at advanced age 8, 9. On resuscitation after CA, all organs are challenged by I/R stress, and, thus, we hypothesized that aging is associated with impairment of proteostasis‐related pathways activated after CA and resuscitation.…”