Neuronal loss of the pentose phosphate pathway in the living nervous system is causally linked to NADPH reduction and elevated oxidative stress

Neurons are highly specialized cells that require large amounts of energy to function. Glial cells support neurons in many ways, including metabolically. In Drosophila, neuronal glycolysis has been found to be dispensable, as long as glial glycolysis is intact, a finding supporting a conservation of the astrocyte-neuron-lactate shuttle (i.e. ANLS)-hypothesis. Neurons use glia-derived lactate to fuel their highly oxidative metabolism. Nevertheless, they readily take up glucose. It has been hypothesized that neuronal glucose might be pre-dominantly metabolized through the pentose phosphate pathway (PPP) rather than glycolysis to produce reduction equivalents in the form of NADPH to cope with the oxidative stress caused by a highly oxidative metabolism and prevent oxidative damage. We show that knockdown of components of the PPP in all neurons in Drosophila induces mild neurodegeneration, which can be rescued by antioxidant feeding. To directly link a putative loss of neuronal NADPH to elevated reactive oxygen species (ROS), we generated fly lines expressing biosensors for NADPH and H2O2 and developed methods to image the sensors in Drosophila neurons. Panneuronal PPP knockdown results in reduced neuronal NADPH and elevated H2O2 levels in larval tissue. In addition, multiparametric live imaging of fully differentiated neurons in the adult Drosophila brain shows decreased NADPH levels and increased ROS stress upon PPP knockdown. Even though the phenotypic consequences of elevated ROS are mild, these data demonstrate that loss of PPP, reduced NADPH levels and increased oxidative stress are indeed functionally linked in living tissue. SIGNIFICANCE STATEMENT: The neuronal pentose phosphate pathway (PPP) has been linked to various phenotypes, including failures in long term memory formation (de Tredern et al., 2021). The PPP has long been postulated to play a neuro-protective role by providing reduction equivalents in the form of NADPH (Tang, 2019). However, studies directly linking the oxidative phase of the PPP to NADPH concentrations and subsequently reactive oxygen species (ROS) detoxification are missing. Here, we demonstrate the use of genetically encoded fluorescent metabolite indicators in Drosophila and reveal a causal link between PPP activity, NADPH and ROS concentrations. KEY POINTS: Neuronal pentose phosphate pathway (PPP) knockdown induces neurodegeneration that can be rescued by food-derived antioxidants. Neuronal PPP deficiency results in reduced neuronal NADPH levels in living tissue. Neuronal PPP deficiency results in elevated neuronal H2O2 levels in living tissue and oxidative stress.