Nicotinamide Riboside and Metformin Ameliorate Mitophagy Defect in Induced Pluripotent Stem Cell-Derived Astrocytes With POLG Mutations.

Anbin Chen,Jian Wang,Xingang Li,Gareth John Sullivan,Atefeh Kianian,Evandro Fei Fang,Laurence A Bindoff,Kristina Xiao Liang,Yu Hong,Cecilie Katrin Kristiansen

doi:10.3389/fcell.2021.737304

Abstract

Mitophagy specifically recognizes and removes damaged or superfluous mitochondria to maintain mitochondrial homeostasis and proper neuronal function. Defective mitophagy and the resulting accumulation of damaged mitochondria occur in several neurodegenerative diseases. Previously, we showed mitochondrial dysfunction in astrocytes with POLG mutations, and here, we examined how POLG mutations affect mitophagy in astrocytes and how this can be ameliorated pharmacologically. Using induced pluripotent stem cell (iPSC)-derived astrocytes carrying POLG mutations, we found downregulation of mitophagy/autophagy-related genes using RNA sequencing-based KEGG metabolic pathway analysis. We confirmed a deficit in mitochondrial autophagosome formation under exogenous stress conditions and downregulation of the mitophagy receptor p62, reduced lipidation of LC3B-II, and decreased expression of lysosome protein lysosomal-associated membrane protein 2A (LAMP2A). These changes were regulated by the PINK1/Parkin pathway and AKT/mTOR/AMPK/ULK1 signaling pathways. Importantly, we found that double treatment with nicotinamide riboside (NR) and metformin rescued mitophagy defects and mitochondrial dysfunction in POLG-mutant astrocytes. Our findings reveal that impaired mitophagy is involved in the observed mitochondrial dysfunction caused by POLG mutations in astrocytes, potentially contributing to the phenotype in POLG-related diseases. This study also demonstrates the therapeutic potential of NR and metformin in these incurable mitochondrial diseases.

Highlights

Mitochondrial DNA polymerase γ replicates the mitochondrial genome and the holoenzyme consists of a catalytic subunit and a dimeric form of its accessory subunit (Graziewicz et al, 2006)
Guided by unsupervised transcriptomic analysis, which indicated a major dysregulation of mitophagy/autophagy, we found that POLG mutations were clearly associated with impaired mitophagy in astrocytes
We previously demonstrated the downregulation of the PINK1/Parkin pathway and showed that POLG mutations drive a lower level of ATP in astrocytes (Liang et al, 2020a)

Summary

Introduction

Mitochondrial DNA (mtDNA) polymerase γ (pol γ) replicates the mitochondrial genome and the holoenzyme consists of a catalytic subunit (encoded by POLG) and a dimeric form of its accessory subunit (encoded by POLG2) (Graziewicz et al, 2006). Mutations in POLG lead to neuronal loss conditioned by abnormal mtDNA homeostasis comprising both depletion and a time-dependent increase in mtDNA damage, along with loss of complex I (Tzoulis et al, 2014). We have demonstrated that these findings could be replicated using induced pluripotent stem cell (iPSC)-derived neural stem cells (NSCs) in which we found evidence of increased reactive oxygen species (ROS) production and cellular senescence (Liang et al, 2020b). In iPSC-derived neurons, we showed mitochondrial dysfunction and mtDNA depletion in dopaminergic neurons from POLG patients (Liang et al, 2021)

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