Identification of genetic and chemical modulators of zebrafish mechanosensory hair cell death

Abstract

Inner ear sensory hair cell death is observed in the majority of hearing and balance disorders, affecting the health of more than 600 million people worldwide. While normal aging is the single greatest contributor, exposure to environmental toxins and therapeutic drugs such as aminoglycoside antibiotics and antineoplastic agents are significant contributors. Genetic variation contributes markedly to differences in normal disease progression during aging and in susceptibility to ototoxic agents. Using the lateral line system of larval zebrafish, we developed an in vivo drug toxicity interaction screen to uncover genetic modulators of antibiotic-induced hair cell death and to identify compounds that confer protection. We have identified 5 mutations that modulate aminoglycoside susceptibility. Further characterization and identification of one protective mutant, sentinel (snl), revealed a novel conserved vertebrate gene. A similar screen identified a new class of drug-like small molecules, benzothiophene carboxamides, that prevent aminoglycoside-induced hair cell death in zebrafish and in mammals. Testing for interaction with the sentinel mutation suggests that the gene and compounds may operate in different pathways. The combination of chemical screening with traditional genetic approaches is a new strategy for identifying drugs and drug targets to attenuate hearing and balance disorders.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Figure 1. Screening for modifiers of aminoglycoside toxicity.

(A) Neuromast from a control animal pretreated with 0.5% DMSO and stained with rapidly with FM 1-43FX (red) and the nuclear label Yo-Pro-1 (green). (B) Negative control pretreated with 0.05% DMSO for 1 hour followed by 200 µM neomycin treatment for 30 min. Hair cells are stained with FM 1-43FX (red) and Yo-Pro-1 (green). Hair cell loss, nuclear condensation and cytoplasmic shrinking are observed. (C) Dose-response function showing decreased hair cell labeling with DASPEI, a mitochondrial potentiometric dye, as a function of increasing neomycin concentration for wildtype zebrafish (N = 25–37 total fish per group, from triplicate experiments). Bars are SEM. Screens for increased or decreased susceptibility to hair cell loss were performed by treatment with either low, 25 µM, or high, 200 µM, neomycin doses, respectively, as highlighted by the orange arrows. (D) Neuromast pretreated with PROTO-2, a compound identified to provide protection against 200 µM neomycin exposure. (E,F) Show the structure for the identified compounds, PROTO-1 (E) and PROTO-2 (F), respectively.

Figure 2. Ranges of protection for PROTO-1 and PROTO-2.

Hair cells were vitally stained with FM1-43 and Yo-Pro-1, treated with PROTO-1 or PROT0-2 for 1 hour at various concentrations of compounds, then exposed to neomycin for 30 minutes, allowed 1 hr recovery in normal media. Graphs show mean hair cell counts for the SO1, SO2, OC1, and O1 neuromasts (+SEM) as percent of control (mock-treated, no neomycin exposure). Missing error bars indicate that was less than symbol size. (A,B) Neomycin dose-response curve showing effects of 10 µM PROTO-1 ((A), closed squares) and PROTO-2 ((B), closed squares) pretreatment in comparison to controls (without PROTO-1 or –2). (C,D) Profile of each compound at increasing doses without aminoglycoside and after 200 µM neomycin exposure. N = 10–20 fish per group.

Figure 3. Mutations that confer protection against neomycin exposure.

Larvae are labeled with DASPEI after 30 min exposure to 200 µM neomycin and 1 hr recovery in normal media. (A) Wildtype animal shows retention of hair cells in neuromasts after mock-treatment. (B) Wildtype animal shows loss of hair cells after aminoglycoside treatment. (C) persephone mutant animal shows robust protection of neuromasts against neomycin treatment. No morphological defects are observed. (D) sentinel mutant animal shows protection, along with sinusoidal body curvature. Bar = 200 µm.

Figure 4. Hair cell retention after neomycin treatment in wildtype and mutant animals.

Histograms show the fraction of animals with different levels of DASPEI staining. For each animal, 10 specific neuromasts are evaluated and assigned a score of 2 (normal staining), 1 (reduced staining), or 0 (no staining) for a maximum total score of 0–20. For each group, the distribution of animals given each DASPEI staining score is displayed as a percentage of the total number of animals to illustrate the phenotypic variation within the group; 40–80 animals were tested for each group. (A) Distribution of wildtype fish after mock treatment without neomycin (green bars) or after exposure to 200 µM neomycin (blue bars). (B–F) Distribution of progeny from crosses between heterozygous mutant carriers treated with 200 µM neomycin, showing both wildtype and mutant phenotypes. (B) persephone. (C) merovingian. (D) sentinel. Animals with sinusoidal bodies (later shown to be homozygous mutants) are represented by orange bars, and animals with wildtype body shape (wildtype or heterozygous siblings) are represented by blue bars. (E) bane. (F) trainman.

Figure 5. Dose dependent protection of sentinel mutants to neomycin.

Hair cell loss as determined by DASPEI staining of progeny of sentinel heterozygous parents with wildtype body shape (blue) or sinusoidal body shape (red) are compared to wildtype *AB fish (green). Error bars are ±1 S.D. Mutants show robust, but partial, protection following 30 min neomycin exposure and one hour recovery.

Figure 6. The sentinel mutation creates a stop codon in a novel vertebrate gene.

(A) A schematic of chromosome 23 region illustrates fraction of recombinant chromosomes among informative meioses for genetic markers defining the sentinel linked region (orange box). (B) Colored bars represent the genomic structures of the snl orthologs from zebrafish (green), mouse (red), and human (blue). Black boxes denote exons, with dotted lines connecting orthologous regions between species, and colored bars represent introns. Divergent exons encoding 5′ UTR are shown as colored boxes. Three coding exons present only in the mammalian orthologs are noted with black arrows. Red rings highlight exons absent in human ortholog. The black arrowhead indicates the seven amino acids within exon 8 of zebrafish absent in the mammalian orthologs. A red asterisk marks the stop codon present in the sentinel allele within exon 14. (C), Phylogenetic tree of predicted proteins from sentinel orthologs. (D) cDNA sequence of wildtype zebrafish encoding tryptophan at amino acid 491 and of sentinel mutant bearing a stop codon. (E) Schematic of the predicted Sentinel protein including a C2 domain (yellow box) and a highly charged region (green box) with glutamine-rich basic clusters (blue boxes) flanking a lysine-rich acidic cluster (pink).

Figure 7. Epistasis analysis of sentinel and protective compounds.

Neomycin dose-response relationship showing effects of 10 µM PROTO-1 against 100 µM or 200 µM neomycin exposure in wildtype and sentinel larvae. For each group, hair cells were pre-labeled with FM1-43FX. Animals were pretreated with PROTO-1 for 1 hour (or mock-treated), treated with neomycin and PROTO-1 for 1 hour, euthanized, and fixed. Hair cells of four neuromasts (left and right) were counted and the average number of hair cells per neuromast was determined. Number of hair cells in control animals (no PROTO-1, no neomycin) are shown with black bars, animals treated with only 100 µM or 200 µM neomycin are shown by solid colored bars, and animals treated with PROTO-1 and neomycin are shown by hatched colored bars. Error bars show 1 S.E.M. PROTO-1 and sentinel mutants show similar protection, and there is a small, statistically significant effect of the combined treatment of the mutation plus PROTO-1.

Figure 8. sentinel mutation does not affect transduction-dependent dye or aminoglycoside uptake.

(A–D) Uptake of FM1-43FX after 45 sec exposure in wildtype (A,C) and sentinel mutants (B,D). Nuclei are labeled with Yo-Pro-1 (A-D). Confocal images of apical (A,B) and basal (C,D) optical sections through the hair cells. (E,F) Gentamicin-conjugated Texas Red uptake in wildtype (E) and sentinel mutant (F) animals after rapid 45 sec exposure.

Figure 9. sentinel mutation and PROTO-1 do not protect against cisplatin.

Hair cell survival was quantified using the vital dye DASPEI, and in each case DASPEI scores were normalized to those from wildtype, untreated fish. Fish (n≥12 fish per treatment group) were treated in cisplatin for 4 hours, then allowed to recover for 3 hours prior to DASPEI assessment. (A) Hair cell responses in wild-type versus sentinel mutants. No difference in the dose-response relationship was observed between wildtype fish (green), homozygous sentinel mutants (red, sinusoidal body), and sentinel siblings (blue, including heterozygous and homozygous wildtype sibling, straight body). (B) Response of cisplatin-treated hair cells from wildtype fish in the presence of the potentially protective compound PROTO-1. There is no difference between dose-response curves with (red) and without (green) PROTO-1. Error bars represent ±1 S.D.

Figure 10. Protective compounds reduce neomycin toxicity in adult mouse utricle cultures.

(A,B) Extrastriolar utricular hair cells stained with antibodies against calmodulin and calbindin after 4 mM neomycin exposure. An increased number of hair cells remain after PROTO-2 pretreatment (B) compared to control (A). (C,D) Neomycin dose-response curve showing effect of 10 µM PROTO-2 pretreatment on striolar (C) and extrastriolar (D) utricular hair cells. Counts were made at high magnification in areas of 900 µM2, converted to density, and averaged over the three sampled areas of each region for each utricle. Ten utricles were analyzed for each treatment group. Data were normalized relative to mock-treated controls (no PROTO drug, no neomycin).