# When alternative becomes essential: The role of mitochondrial glycerol-3-phosphate dehydrogenase

## TL;DR

This study shows that mitochondrial G3P dehydrogenase is essential for energy production and redox balance in fruit flies, not just a backup pathway.

## Contribution

Demonstrates mtG3PDH's essential role in mitochondrial bioenergetics and redox homeostasis using Drosophila mutants.

## Key findings

- GPO1 mutant flies showed a 60% decrease in ATP production and 33% decrease in O2 consumption.
- GPO1 mutants produced 70% less ROS compared to controls.
- mtG3PDH is essential for mitochondrial function, not just an alternative pathway.

## Abstract

Complex I is known as the primary entry point for electrons within the mitochondrial electron transport system (ETS). However, the glycerol-3-phosphate (G3P) shuttle, composed of cytosolic and mitochondrial G3P dehydrogenase (cG3PDH and mtG3PDH, respectively), transfer reducing equivalents from the cytosol to the mitochondrial matrix. The mtG3PDH feeds electrons into the ETS via FADH2 oxidation, but with theoretically lower energy conversion efficiency than complex I. It is thus believed to be an “alternative” pathway, only supporting mitochondrial respiration when complex I fails. mtG3PDH also plays an important role in reactive oxygen species (ROS) production. To investigate the role of this understudied protein in mitochondrial bioenergetics and redox homeostasis, we generated Drosophila melanogaster mutant lines for mtG3PDH (GPO1) using a CRISPR/Cas9-based approach and determined several physiological and metabolic parameters. A drastically higher mortality rate was observed among the GPO1 flies, as well as a lethargic behavior characterized by an inability to climb. These results are in accordance with an impaired mitochondrial efficiency (ATP/O) mainly due to decreased ATP production (~60% decrease) and O2 consumption (~33% decrease), rather than elevated ROS. In fact, GPO1 flies produced ~70% less ROS than controls, likely due to the reduced direct and reverse electron transfer-related ROS production from mtG3PDH. These results support an essential role of mtG3PDH in mitochondrial bioenergetic, challenging its alternative aspect, and confirming its importance in mitochondrial redox homeostasis.