NAD Depletion Mediates Cytotoxicity in Human Neurons With Autophagy Deficiency

Abstract

Autophagy is a homeostatic process critical for cellular survival, and its malfunction is implicated in myriad human diseases including neurodegeneration. Loss of autophagy contributes to cytotoxicity and tissue degeneration, but the mechanistic understanding of this phenomenon remains elusive. Here we generated autophagy-deficient (ATG5^–/–) human embryonic stem cells (hESCs), from which we established human neuronal platform to investigate how loss of autophagy affects neuronal survival. ATG5^–/– neurons exhibited basal cytotoxicity accompanied by metabolic defects. Depletion of nicotinamide adenine dinucleotide (NAD) due to hyperactivation of NAD-consuming enzymes was found to trigger cell death via mitochondrial depolarisation in ATG5^–/– neurons. Boosting intracellular NAD levels improved cell viability by restoring mitochondrial membrane potential in ATG5^–/– neurons. Our findings elucidate a mechanistic link between autophagy deficiency and neuronal cell death that could be targeted for therapeutic interventions in neurodegenerative and lysosomal storage diseases associated with autophagic defect.

Keywords: autophagy, cell death, cell survival, human embryonic stem cell, neuron, mitochondria, NAD, NADases, L-tryptophan, nicotinamide