Figure 1A. Simplified schematic representation of the posttranslational membrane insertion pathway of tail-anchored (TA) proteins. B. The ER marker ER-tdTomato (magenta) is co-localized with ectopically expressed Wrb-EGFP (green) in inner saccular HCs of 5-dpf zebrafishlarvae ('control' refers to either wild-type or +/pwi fish that were picked in a phenotypic screen and did not exert any abnormal behavioral phenotype). While the left HC solely expressed ER-tdTomato, the neighboring HC expressed both, ER-tdTomato and Wrb-GFP. Colocalization between both proteins occurs in areas exhibiting white pixels. Scale bars: 5 µm C. Projection of confocal sections of control inner ear HCs immunolabelled for otoferlin (magenta) and expressing an EGFP-tagged truncated Wrb fragment containing only the cytosolic coiled-coiled domains (Wrbcc-EGFP, green). Wrbcc-EGFP distribution was diffuse and found throughout the HC. D. Projection of confocal sections of inner ear HCs of 5-dpf control (D) and Wrb-deficient pwi mutant fish (wrb pwi/pwi; D'), showing strongly reduced otoferlin immunofluorescence in the mutant HCs. The color look-up table used represents higher pixel intensities with warmer colors. E. A representative transgenically-rescued pwi-mutant HC (white arrow), expressing Wrb-GFP, exhibits a strongly increased otoferlin signal (magenta) in direct comparison to the neighboring non-rescued mutant HCs. (E') Same image as in E but intensity-coded for otoferlin fluorescence. F. A representative control HC expressing Wrbcc-EGFP (white arrow), immunolabelled for otoferlin (magenta). The Wrbcc-EGFP-expressing HC shows significantly less otoferlin (F'), suggesting a dominant negative effect of Wrbcc in otoferlin biogenesis. The transfected HC is encircled with a dashed line. G. Quantification otoferlin downregulation of Wrbcc-EGFP shown in F. Otoferlin immunofluorescence intensity of Wrbcc-EGFP transfected inner ear HCs n = 22, from three 5-dpf control larva) showed significant decrease of ~43% compared to the adjacent Wrbcc-EGFP negative wild-type HCs (n = 22, from the same larva). Fluorescent intensity values were normalized with the average value of Wrbcc-EGFP negative HCs. **p < 0.01 H. Quantification of otoferlin immunohistochemistry data shown in (D) from hair cells of control (n = 88 HCs from three 5-dpf larva), wrb pwi/pwi (n = 80 HCs from 3 sibling larva) normalized against the mean intensity value of the control group and Wrb-GFP transfected wrb pwi/pwi zebrafish inner ears. Mutant HCs showed ~82% reduction in otoferlin fluorescence intensity when compared with control HCs imaged under the same conditions. Otoferlin fluorescent intensity of Wrb-EGFP transfected mutant HCs (n = 24, from three, 5-dpf larva) showed significant increases compared to adjacent Wrb-EGFP negative mutant HCs (n = 24 from the same larva). Fluorescent intensity values were normalized with the average value of Wrb-EGFP negative HCs. ***p<0.001 I-I'. Acoustic startle reflex measurements of (I) live, intact, 5-dpf control (n = 20) and wrb pwi/pwi (n = 18) zebrafishlarva, where almost 100% of the larva responded to five successive acoustic stimuli at 3 seconds intervals. In wrb pwi/pwi animals, only 39% of larva responded to the first stimulus, thereafter, the number of responding larva decreased rapidly to less than 6%. (I') Acoustic startle reflexes of 5-dpf zebrafish larva after injection with either EGFP capped mRNA (control, n = 16, or wrb pwi/pwi n = 17), Wrb-EGFP capped mRNA (control, n = 20, or wrb pwi/pwi n = 20) or two different concentrations of otoferlin mRNA (1x: control, n = 13; wrb pwi/pwi n = 11; 2x: control, n = 16; wrb pwi/pwi n = 6). In contrast to EGFP mRNA injection, Wrb-EGFP as well as otoferlin mRNAs could partially rescue the acoustic startle reflex in the mutants, while not displaying detrimental effects on overall zebrafish morphology and development. Please note the dose-dependent effect of otoferlin mRNA injection. Similar results as in 1x EGFP mRNA were also obtained from injection of 2X EGFP mRNA as control (data not shown). Measurements from two (2x otoferlin) or three (all other) different experiments were compiled in the graph.