Aberrantly expressed fused in sarcoma (FUS) is a hallmark of FUS-related amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Wildtype FUS localises to synapses and interacts with mitochondrial proteins while mutations have been shown to cause to pathological changes affecting mitochondria, synapses and the neuromuscular junction (NMJ). This indicates a crucial physiological role for FUS in regulating synaptic and mitochondrial function that is currently poorly understood. In this paper we provide evidence that mislocalised cytoplasmic FUS causes mitochondrial and synaptic changes and that FUS plays a vital role in maintaining neuronal health in vitro and in vivo. Overexpressing mutant FUS altered synaptic numbers and neuronal complexity in both primary neurons and zebrafish models. The degree to which FUS was mislocalised led to differences in the synaptic changes which was mirrored by changes in mitochondrial numbers and transport. Furthermore, we showed that FUS co-localises with the mitochondrial tethering protein Syntaphilin (SNPH), and that mutations in FUS affect this relationship. Finally, we demonstrated mutant FUS led to changes in global protein translation. This localisation between FUS and SNPH could explain the synaptic and mitochondrial defects observed leading to global protein translation defects. Importantly, our results support the ‘gain-of-function’ hypothesis for disease pathogenesis in FUS-related ALS.

BackgroundAberrantly expressed fused in sarcoma (FUS) is a hallmark of FUS-related amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Wildtype FUS localises to synapses and interacts with mitochondrial proteins while mutations have been shown to cause to pathological changes affecting mitochondria, synapses and the neuromuscular junction (NMJ) This indicates a crucial physiological role for FUS in regulating synaptic and mitochondrial function that is currently poorly understood.MethodsNeurite complexity and synaptic density were studied in rat primary neurons expressing eGFP-FUSWT, eGFP-FUSR514G or eGFP-FUSΔNLS. To investigate synaptic and neuronal changes in vivo, a motor neuron specific zebrafish over-expression model with mosaic expression of MNX1:Gal4 and UAS: eGFP-FUSWT, UAS: eGFP-FUSR514G or UAS: eGFP-FUSΔNLS was assessed for alterations to axonal growth, branching and NMJ density. Using live imaging, mitochondrial trafficking in in vitro neuronal models expressing mutant FUS was conducted. Complementary proximity ligation assays assessed endogenous protein interactions, while overexpression of mutant FUS evaluated if mutations led to alterations in this interaction. Lastly, Puromycin assays investigated how mutant FUS caused differences in global protein translation. ResultsWe found that mutant FUS alters synaptic numbers and neuronal complexity in both primary neurons and zebrafish models. The degree to which FUS is mislocalised leads to differences in the synaptic changes which is mirrored by changes in mitochondrial numbers and transport. Furthermore, we showed that FUS interacts and localises with Syntaphilin (SNPH), and that mutations in FUS affect this relationship, which may lead to the synaptic and mitochondrial phenotypes observed. Finally, we demonstrated that in primary neurons mutant FUS-driven changes in global protein translation correlate with synaptic and mitochondrial defects shown for each respective mutation. ConclusionsWe provide evidence that mislocalised cytoplasmic FUS causes mitochondrial and synaptic changes and that FUS plays a vital role in maintaining neuronal health in vitro and in vivo. Moreover, we demonstrate a novel interaction between FUS and SNPH, which could explain the synaptic and mitochondrial defects observed leading to global protein translation defects. Importantly, our results support the ‘gain-of-function’ hypothesis for disease pathogenesis in FUS-related ALS.

Aberrantly expressed fused in sarcoma (FUS) is a hallmark of FUS-related amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Wildtype FUS localises to synapses and interacts with mitochondrial proteins while mutations have been shown to cause to pathological changes affecting mitochondria, synapses and the neuromuscular junction (NMJ). This indicates a crucial physiological role for FUS in regulating synaptic and mitochondrial function that is currently poorly understood. In this paper we provide evidence that mislocalised cytoplasmic FUS causes mitochondrial and synaptic changes and that FUS plays a vital role in maintaining neuronal health in vitro and in vivo. Overexpressing mutant FUS altered synaptic numbers and neuronal complexity in both primary neurons and zebrafish models. The degree to which FUS was mislocalised led to differences in the synaptic changes which was mirrored by changes in mitochondrial numbers and transport. Furthermore, we showed that FUS interacts with the mitochondrial tethering protein Syntaphilin (SNPH), and that mutations in FUS affect this relationship. Finally, we demonstrated mutant FUS led to changes in global protein translation. This interaction between FUS and SNPH could explain the synaptic and mitochondrial defects observed leading to global protein translation defects. Importantly, our results support the ‘gain-of-function’ hypothesis for disease pathogenesis in FUS-related ALS.