Fig 2: Motor behavioural deficits and reduced acetylcholine receptor clusters at neuromuscular junctions (NMJs) in C9-miR fish.a Representative swimming tracks of control, C9-miR and C9-rescue fish at 6 dpf. Scale bar = 0.2 cm. b C9-miR larvae (N = 3, n = 65) displayed impaired swimming compared to controls (N = 3, n = 60; ***p < 0.0001; one-way ANOVA). Expression of the human C9orf72 long transcript in C9-miR fish (C9-rescue; N = 3, n = 53) significantly rescued the motor behavioural defects (***p < 0.0001; one-way ANOVA). c Representative images of co-immunostaining of zebrafish neuromuscular junctions with presynaptic (SV2; green) and postsynaptic (a-bungarotoxin; red) markers in 6 dpf zebrafish. Scale bar = 100 µm. d Quantification of the colocalizing presynaptic and postsynaptic markers per somite showed a significant reduction in the number of puncta in C9-miR fish at 6 dpf that can be rescued with the expression of human C9orf72 mRNA (C9-rescue) (n = 8–10; ***p < 0.0001; one-way ANOVA). Data are presented as mean ± SEM. n represents the number of fish, N represents the number of experimental repeats.

Fig 3: Sv2a interacts with C9orf72 and its protein expression is downregulated in C9-miR fish.a Volcano plot showing the log2 fold change against the -log10 p value. Proteins in blue are upregulated while proteins in red are downregulated (log2 fold change of -1.5 and 1.5, delineated by vertical lines) and are significantly dysregulated (-log10 (p value) > 1.301, delineated by horizontal dotted line) between control and C9-miR fish (N = 4). b Differentially expressed proteins with p < 0.05 in C9-miR 6 dpf larvae (N = 4). c Immunoblot analysis of RFP-immunoprecipitated proteins and lysate of HEK293T cells co-expressing RFP-tagged C9orf72 or RFP in combination with Myc-tagged SV2a or Myc-tagged SMRC8 (a known interactor of C9orf72; positive control). Myc-SV2a and Myc-SMCR8 were extracted out of solution in samples where RFP-C9orf72 was co-transfected, but not in samples where RFP was co-transfected (N = 3).

Fig 4: C9-miR zebrafish exhibited attenuated miniature endplate currents (mEPCs) at NMJs.a Recordings of mEPCs, which result from spontaneous release of a quantum, were recorded in 6 dpf control and C9-miR fish (n = 6). b Representative mEPCs. Animals with reduced C9orf72 (C9-miR) displayed mEPCs with reduced frequency (c) (n = 6–7; ***p < 0.0001; Student’s t test) and amplitude (d) (n = 7; **p < 0.001; Student’s t test). Rise time (e) (n = 7; p = 0.3947; Student’s t test) and decay time (f) constant kinetics of mEPC (n = 7; p = 0.8385; Student’s t test) were not found to be significantly different between controls and C9-miR. Data are presented as mean ± SEM. n represents the number of fish.

Fig 5: Generation of a stable zebrafish C9orf72 knockdown line.a Schematic representation of the technique used to silence C9orf72 in zebrafish. The transgene is designed to express four different micro-RNAs targeting C9orf72’s 3’UTR and triggering knockdown by both repressing C9orf72 translation and affecting its stability. b Images demonstrating proper eGFP expression in the crystallin of the transgenic fish, a marker used to identify carrier/knockdown larvae. Scale bar = 100 µm. c Bar graph shows the relative expression of the endogenous C9orf72 gene. mRNA was normalized to elf1a mRNA levels (N = 4, ***p < 0.0001, Student’s t test). d Immunoblotting of the zebrafish protein C9orf72 and beta-actin as control. e Bar graph shows the relative expression of the C9orf72 protein compared with actin between C9orf72 mutants and control line (N = 3; **p = 0.0034; Student’s t test). f Gross morphological analyses of wild-type control and C9orf72-LOF fish (C9-miR). g Kaplan–Meier survival plot over 17 days after fertilization (dpf) showing low survival of C9-miR compared to controls after 10 dpf (N = 3, n = 25). Data are presented as mean ± SEM. n represents the number of fish, N represents the number of experimental repeats.