Endoplasmic Reticulum Stress Mediates Axon Initial Segment Shortening: Implications for Diabetic Brain Complications

Endoplasmic reticulum (ER) stress and activation of the three unfolded protein response pathways, in particular the protein kinase RNA-like ER kinase (PERK) pathway, contribute to the pathophysiology of various neurodegenerative conditions including type 2 diabetes mellitus (T2DM). T2DM is an increasingly prevalent metabolic disorder affecting millions. Even with strict glucose control, patients with T2DM frequently experience mild cognitive impairment and exhibit a significantly increased risk of developing dementia. We previously demonstrated that impaired cognitive flexibility is associated with shortening of axon initial segment (AIS) length in the prefrontal cortex in the T2DM model db/db mice. The AIS plays the crucial roles of regulation of action potential initiation and neuronal output. Even subtle shortening of AIS length can reduce excitability of neurons. In this study, we hypothesized that ER stress mediates AIS shortening in diabetic conditions. Utilizing primary mouse cortical cultures, we show that sodium 4-phenylbutyrate, a well-documented ER stress inhibitor, prevents AIS shortening and PERK activation induced by the T2DM factor methylglyoxal. Exposure of cortical cultures to an established ER stress inducer tunicamycin caused dose-dependent reduction of AIS length in the generalized population of the neurons without affecting neuronal viability. Co-exposure to a PERK-specific inhibitor GSK2606414 prevented AIS shortening induced by tunicamycin. These results demonstrate ER stress is sufficient and necessary for AIS shortening in vitro. Our findings identify ER stress and AIS shortening as potential therapeutic targets in T2DM-related cognitive impairment.