# Mitochondrial-specific perturbation of Drosophila RNase Z in neurons leads to motor impairments, disrupted learning and neurodegeneration

## TL;DR

Mutations in the RNase Z gene in fruit fly neurons cause neurological issues, including motor and learning impairments, and suggest mitochondrial dysfunction is key to the disease.

## Contribution

This study experimentally confirms that mitochondrial RNase Z mutations cause neuropathology and establishes the organelle-specific role of RNase Z in disease.

## Key findings

- Neuronal RNase Z knockout in flies causes morphological neuron defects, motor impairments, and learning deficits.
- Mitochondrial-specific RNase Z mutation is sufficient to induce neurodegeneration and ROS elevation.
- Rescue experiments confirm that mitochondrial RNase Z activity is critical for preventing neuropathology.

## Abstract

Clinical studies have linked a rare form of neurological disorder to the highly conserved RNase Z gene, which encodes an endoribonuclease responsible for the processing of nuclear and mitochondrial primary tRNA transcripts. Patients harboring mutant variants of this gene exhibit a spectrum of neurological dysfunction; however, no studies to date have established the causality of RNase Z-linked neuropathology. We employed CRISPR/Cas9 technology to create flies with a neuron-specific knockout of the RNase Z gene, which is rescued with transgenes encoding a wild-type or a mutant copy of RNase Z. Neuronal activity of RNase Z is vital, as mutants display striking morphological abnormalities in central and peripheral neurons, along with attenuated motor circuit function and associative learning performance. Neuron-specific mutations of RNase Z also led to mitochondrial fragmentation and elevated ROS production. By employing the rescue transgene encoding RNase Z devoid of a mitochondrial targeting signal (MTS), we segregated the mitochondrial activity of RNase Z from that in other compartments, allowing us to assess organelle-specific contributions to disease etiology and progression. We found that mutating mitochondrial RNase Z was sufficient to induce the neuropathology in flies, as they recapitulate the salient phenotypes observed in the pan-neuronal mutants. Collectively, our study validates the pathogenicity of mutant RNase Z and establishes mitochondrial-specific contributions to neuropathology.

Over a third of the world’s population is affected by neurological conditions, making disorders of the nervous system a leading cause of disability. Many neurological disorders have a genetic component, and recent clinical research has linked mutations in genes that process transfer RNA molecules to nervous system dysfunction. One such gene encodes a highly conserved enzyme, RNase Z, involved in the maturation of nuclear and mitochondrial transfer RNAs. Mutations in this gene have been identified in patients with neurological symptoms, but until now, no direct experimental evidence has confirmed its role in causing disease. Using the fruit fly model, we found that flies lacking functional RNase Z in neurons show severe defects in brain structure and behavior, including impaired movement and memory. Importantly, we show that neuronal mitochondria are dysfunctional and that mutating mitochondrial RNase Z is sufficient to produce neurophenotypes. The versatility of the fly model allowed us to study the requirement of this enzyme at an organelle-specific level and provide substantial evidence supporting prior claims classifying RNase Z-linked pathology as a mitochondrial disease. Our findings offer greater insight into how RNase Z mutations contribute to neuropathology in humans and further guide therapeutic development.