O-GlcNAcylation regulates mitochondrial remodeling and quality control mechanisms in Alzheimer’s disease

Abstract

Impaired mitochondria homeostasis is linked to numerous human pathologies including neurodegeneration. In the nervous system, O-GlcNAcylation, a single sugar post-translational modification, is highly enriched and required for neuronal function. O-GlcNAc is regulated by O- GlcNAcase (OGA) and O-GlcNAc transferase (OGT), which respectively remove and add the modification. O-GlcNAc is considered as a therapeutic target against tauopathy for Alzheimer’s disease (AD). However, mitochondrial homeostasis is highly impaired in AD, and our understanding on how O-GlcNAc regulates mitochondrial homeostasis remains limited. We hypothesize that O-GlcNAc regulates mitochondrial homeostasis at multiple levels, mitochondrial remolding, mitochondrial retrograde signaling, and mitochondrial turn over. To examine the effect of the long term OGA inhibition on brain mitochondrial proteome, we used multiplexed quantitative mass spectrometry on mouse treated with OGA inhibitor, Thiamet-G (TMG). We revealed that TMG influences the mitochondrial proteome in a gender specific manner. Mitochondrial electron transport chain (ETC) proteins were significantly decreased in TMG treated male mouse brains compared to saline with almost no change in female. Also, antioxidant proteins and ROS production were significantly altered in both genders treated with TMG in opposite directions, elevation in antioxidant and reduction in ROS were detected in male, while female have reduced antioxidant and elevated ROS. These results indicates that OGA inhibition has a profound effect on cellular energetics, and ROS production in a gender specific manner. We next investigated whether O-GlcNAc regulates of mitochondrial interrogate stress response (ISRmt). Mitochondrial dysfunction triggers ISRmt, which in turns increases the translation of transcription factor Activating Transcription Factor 4 (ATF4). We showed that TMG elevates the expression of A TF4 and its downstream targets upon mitochondrial stress. Importantly, ATF4 occupancy increases at the ATF5 promoter site in brains isolated from TMG treated mice suggesting that O-GlcNAc is regulating ATF4 targeted gene expression. Together, these results indicate that O-GlcNAc regulates the ISRmt through regulating ATF4. Furthermore, we studied whether O-GlcNAc regulates one of the downstream targets of ISRmt, mitophagy, the process of recycling damaged mitochondria. Abnormalities in mitophagy is shown in many studies of AD. We showed that sustained elevation in O-GlcNAcylation via pharmacologically inhibiting OGA increases mitochondrial level of PINK-1 (PTEN-induced kinase-1) and autophagy-related protein light chain 3 (LC3). However, decreasing cellular O- GlcNAcylation by knocking down or knocking out OGT decreases both PINK1 and LC3. Moreover, we detected O-GlcNAc on PINK-1. Collectively, these data demonstrate that O- GlcNAc plays a crucial role in activation of mitophagy. Altogether, these studies provide new evidence supporting the role of O-GlcNAc as a critical regulator of mitochondrial homeostasis. However, we showed that the effect of OGA inhibition on both ISRmt and mitophagy is limited in AD models. Thus, these results indicates that OGA inhibitors might not restore mitochondrial homeostasis in AD. Therefore, administering OGA inhibitors to patients with AD needs further evaluation, especially highlighting gender- specific mitochondrial response to long term OGA inhibition.