Knockdown of the Drosophila Fused in Sarcoma (FUS) Homologue Causes Deficient Locomotive Behavior and Shortening of Motoneuron Terminal Branches

Abstract

Mutations in the fused in sarcoma/translated in liposarcoma gene (FUS/TLS, FUS) have been identified in sporadic and familial forms of amyotrophic lateral sclerosis (ALS). FUS is an RNA-binding protein that is normally localized in the nucleus, but is mislocalized to the cytoplasm in ALS, and comprises cytoplasmic inclusions in ALS-affected areas. However, it is still unknown whether the neurodegeneration that occurs in ALS is caused by the loss of FUS nuclear function, or by the gain of toxic function due to cytoplasmic FUS aggregation. Cabeza (Caz) is a Drosophila orthologue of human FUS. Here, we generated Drosophila models with Caz knockdown, and investigated their phenotypes. In wild-type Drosophila, Caz was strongly expressed in the central nervous system of larvae and adults. Caz did not colocalize with a presynaptic marker, suggesting that Caz physiologically functions in neuronal cell bodies and/or their axons. Fly models with neuron-specific Caz knockdown exhibited reduced climbing ability in adulthood and anatomical defects in presynaptic terminals of motoneurons in third instar larvae. Our results demonstrated that decreased expression of Drosophila Caz is sufficient to cause degeneration of motoneurons and locomotive disability in the absence of abnormal cytoplasmic Caz aggregates, suggesting that the pathogenic mechanism underlying FUS-related ALS should be ascribed more to the loss of physiological FUS functions in the nucleus than to the toxicity of cytoplasmic FUS aggregates. Since the Caz-knockdown Drosophila model we presented recapitulates key features of human ALS, it would be a suitable animal model for the screening of genes and chemicals that might modify the pathogenic processes that lead to the degeneration of motoneurons in ALS.

RNA Granules and Diseases: A Case Study of Stress Granules in ALS and FTLD

“…Transgenic mice homozygous for an inactive FUS allele, for example, fail to suckle and die soon after birth (Hicks et al 2000). Knock down of the FUS homologue Caz in Drosophila causes degeneration of motor neurons and abnormal locomotive behavior (Sasayama et al 2012) whereas expression of wild-type or pathogenic mutations of human FUS in Drosophila induces a similarly severe neural impairment (Chen et al 2011). Therefore, the expression level of FUS protein needs to be critically maintained for normal neuronal function.…”

A fruitful endeavor: Modeling ALS in the fruit fly

“…Pan-neuronal expression of dFUS RNAi does not affect adult lifespan, but does inhibit climbing. RNAi knockdown in motor neurons also reduces bouton number and length of synaptic branches at the NMJ (Sasayama et al, 2012). Additionally, knockdown in fly eyes causes external degeneration (Azuma et al, 2014;Shimamura et al, 2014).…”

ALS-Associated FUS Mutations Result in Compromised FUS Alternative Splicing and Autoregulation

“…Investigations in some Drosophila , C. elegans and rat models showed that expression of ALS-associated FUS mutants can lead to motor neuron dysfunction and neurodegeneration [15]–[17]. However, some Drosophila and zebrafish models support that the loss of FUS function can lead to behavioral and structural defects of motor neurons [18], [19]. Exactly how the loss of FUS nuclear function and/or the gain of cytoplasmic cytotoxicity contribute to neurodegeneration at the molecular level is still unknown.…”