FUS causes synaptic hyperexcitability in Drosophila dendritic arborization neurons

Abstract

Mutations in the nuclear localization signal of the RNA binding protein FUS cause both Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). These mutations result in a loss of FUS from the nucleus and the formation of FUS-containing cytoplasmic aggregates in patients. To better understand the role of cytoplasmic FUS mislocalization in the pathogenesis of ALS, we identified a population of cholinergic neurons in Drosophila that recapitulate these pathologic hallmarks. Expression of mutant FUS or the Drosophila homolog, Cabeza (Caz), in class IV dendritic arborization neurons results in cytoplasmic mislocalization and axonal transport to presynaptic terminals. Interestingly, overexpression of FUS or Caz causes the progressive loss of neuronal projections, reduction of synaptic mitochondria, and the appearance of large calcium transients within the synapse. Additionally, we find that overexpression of mutant but not wild type FUS results in a reduction in presynaptic Synaptotagmin, an integral component of the neurotransmitter release machinery, and mutant Caz specifically disrupts axonal transport and induces hyperexcitability. These results suggest that FUS/Caz overexpression disrupts neuronal function through multiple mechanisms, and that ALS-causing mutations impair the transport of synaptic vesicle proteins and induce hyperexcitability.

Review: Modelling the pathology and behaviour of frontotemporal dementia

“…In the majority of Drosophila , zebrafish and rodent models, overexpression of FUS whether wildtype or containing ALS‐linked mutation leads to motor phenotypes , also observed in knockin models , suggesting that this is the dominant phenotype in response to increased FUS. However, in some models, nonmotor phenotypes have been seen.…”

“…The only study of the other FET proteins is a knockout EWS mouse model which shows early post-natal lethality [190] similar to FUS. In the majority of Drosophila, zebrafish and rodent models, overexpression of FUS whether wildtype or containing ALS-linked mutation leads to motor phenotypes [162,[191][192][193][194][195][196][197][198][199], also observed in knockin models [200][201][202][203], suggesting that this is the dominant phenotype in response to increased FUS. However, in some models, nonmotor phenotypes have been seen.…”

C9orf72-associated arginine-rich dipeptide repeats induce RNA-dependent accumulation of Staufen in nucleus

“…To identify potential RNA-binding proteins (RBPs) that show increased nuclear localization in C9-ALS/FTD Drosophila model, we screened in C4 da neurons, which was recently used to model ALS in flies [19], for RBPs whose accumulation in the nucleus was increased compared to the controls. For the screen, we used as C9-ALS/FTD Drosophila model the flies that expressed PR repeat proteins (V5-PR36) and not GR repeat proteins, because recent studies showed that between the two arginine-rich DPRs, PR is more closely associated with nucleocytoplasmic transport defects [10, 20, 21].…”

Drosophila as a Model for Assessing the Function of RNA-Binding Proteins during Neurogenesis and Neurological Disease

“…Overexpression of Fus or Caz forms results in progressive loss of dendrite and axon projections. Moreover, the ALS-causing forms of FUS/Caz disrupted transport of synaptic machinery in axons and resulted in neuronal hyperexcitability, which has previously been reported among ALS patients [ 21 , 22 ]. Thus, da neurons may serve as a particularly useful model for studying FUS cytoplasmic aggregation and its consequences on neuronal pathophysiology.…”

“…For example, knockdown of the FUS Drosophila ortholog cabeza (caz) results in locomotion defects in adults, as well as NMJ branch length defects in larvae [ 20 ]. Nonetheless, many of these models, although they recapitulate various aspects of ALS pathophysiology, do not result in the redistribution of FUS into cytoplasmic inclusions [ 21 ]. Recently, Drosophila sensory dendrite arborization (da) neurons were identified as a useful model for studying cytoplasmic FUS aggregates and their consequences, since overexpression of mutant forms of FUS or Caz results in the formation of cytoplasmic FUS/Caz aggregates, as well as neuronal phenotypes within these sensory neurons.…”