A Model of Toxic Neuropathy inDrosophilaReveals a Role for MORN4 in Promoting Axonal Degeneration

Abstract

Axonal degeneration is a molecular self-destruction cascade initiated following traumatic, toxic, and metabolic insults. Its mechanism underlies a number of disorders including hereditary and diabetic neuropathies and the neurotoxic side effects of chemotherapy drugs. Molecules that promote axonal degeneration could represent potential targets for therapy. To identify such molecules, we designed a screening platform based on intoxication ofDrosophilalarvae with paclitaxel (taxol), a chemotherapeutic agent that causes neuropathy in cancer patients. InDrosophila, taxol treatment causes swelling, fragmentation, and loss of axons in larval peripheral nerves. This axonal loss is not due to apoptosis of neurons. Taxol-induced axonal degeneration inDrosophilashares molecular execution mechanisms with vertebrates, including inhibition by both NMNAT (nicotinamide mononucleotide adenylyltransferase) expression and loss ofwallenda/DLK (dual leucine zipper kinase). In a pilot RNAi-based screen we found that knockdown ofretinophilin(rtp), which encodes a MORN (membrane occupation and recognition nexus) repeat-containing protein, protects axons from degeneration in the presence of taxol. Loss-of-function mutants ofrtpreplicate this axonal protection. Knockdown ofrtpalso delays axonal degeneration in severed olfactory axons. We demonstrate that the mouse ortholog ofrtp, MORN4, promotes axonal degeneration in mouse sensory axons following axotomy, illustrating conservation of function. Hence, this new model can identify evolutionarily conserved genes that promote axonal degeneration, and so could identify candidate therapeutic targets for a wide-range of axonopathies.