Defects in Synapse Structure and Function in a Fly Model of FUS-Related ALS

Abstract

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease that leads invariably to fatal paralysis. Although most cases of ALS are sporadic, about 10% are familial. One gene associated with familial ALS encodes the DNA/RNA binding protein Fused in Sarcoma (FUS). There exists a Drosophila model of ALS, in which human FUS with ALS-causing mutations is expressed in motor neurons. These flies exhibit motor neuron degeneration, larval locomotor defects and early death. Similar phenotypes are observed in flies null for the gene Cabeza (Caz), the fly homolog of FUS. We have examined evoked and spontaneous synaptic transmission at the larval neuromuscular junction, larval motor neuron cell body excitability, and presynaptic active zone structure in these fly models of ALS. The amplitude of evoked synaptic currents is decreased by more than 80% in larvae in which human mutant FUS (R521C) is expressed in motor neurons. A similar decrease in evoked synaptic transmission is seen in Caz1 null flies. Furthermore, the frequency of spontaneous miniature synaptic currents is decreased dramatically in FUS-R521C expressing flies. In marked contrast, recordings from motor neuron cell bodies demonstrate that both wild type and mutant FUS expressing neurons can fire normal action potentials, and the voltage-dependent inward and outward currents in the cell bodies are indistinguishable in wild type and mutant FUS motor neurons. Although confocal microscopic analysis of the larval neuromuscular junction does not reveal gross abnormalities, examination of synapses using super-resolution STED microscopy suggests that presynaptic active zones are aberrantly organized in larvae in which FUS-R521C is expressed in the motor neurons. The results are consistent with the idea that defects in synaptic structure and function precede, and may contribute to, the later motor neuron degeneration that is characteristic of ALS.