Dlx3b/4b is required for early-born but not later-forming sensory hair cells during zebrafish inner ear development

Mutant dlx3b disturbs normal tooth mineralization and bone formation in zebrafish

Background

Tricho-dento-osseous (TDO) syndrome is an autosomal dominant disorder characterized by anomalies in hair, teeth and bone (OMIM190320). Various mutations of Distal-Less 3 (DLX3) gene are found to be responsible for human TDO. The aim of this study was to investigate effects of DLX3 on tooth and bone development using a zebrafish model.

Methods

The dlx3b mutant zebrafish lines were established using the gene targeting tool transcription activator-like effector nuclease (TALEN). Micro-computed tomography was used to render the three-dimensional skeletal structures of mutant fishes. The pharyngeal bone along with connected teeth was isolated and stained by Alizarine Red S, then observed under stereomicroscope. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) were used to examine the tooth surface morphology and mineral composition. Quantitative real-time PCR was used to analyze gene expression.

Results

A moderate curvature of the spine toward the dorsal side was found at the early larval stages, appearing in 86 out of 100 larvae in dlx3b^-/- group as compared to 3 out of 99 in the dlx3b^+/+ group. At the adult stage, three of the thirty dlx3b^-/- homozygotes exhibited prominent abnormal curvature in the spine. SEM revealed morphological surface changes in pharyngeal teeth enameloid, accompanied by a decrease in the mineral content detected by EDS. Furthermore, specific secretory calcium-binding phosphoprotein (SCPP) genes, including odam, scpp9, spp1, scpp1, and scpp5 were significantly downregulated in dlx3b mutants.

Conclusion

The findings of this study suggest that dlx3b is critical for enamel mineralization and bone formation in zebrafish. Moreover, the discovery of the downregulation of SCPP genes in dlx3b mutants sheds new light on the molecular mechanisms underlying TDO syndrome.

An Assay for Systematically Quantifying the Vestibulo-Ocular Reflex to Assess Vestibular Function in Zebrafish Larvae

Zebrafish (Danio rerio) larvae are widely used to study otic functions because they possess all five typical vertebrate senses including hearing and balance. Powerful genetic tools and the transparent body of the embryo and larva also make zebrafish a unique vertebrate model to study otic development. Due to its small larval size and moisture requirement during experiments, accurately acquiring the vestibulo-ocular reflex (VOR) of zebrafish larva is challenging. In this report, a new VOR testing device has been developed for quantifying linear VOR (LVOR) in zebrafish larva, evoked by the head motion about the earth horizontal axis. The system has a newly designed larva-shaped chamber, by which live fish can be steadily held without anesthesia, and the system is more compact and easier to use than its predecessors. To demonstrate the efficacy of the system, the LVORs in wild-type (WT), dlx3b and dlx4b morphant zebrafish larvae were measured and the results showed that LVOR amplitudes were consistent with the morphological changes of otoliths induced by morpholino oligonucleotides (MO). Our study represents an important advance to obtain VOR and predict the vestibular conditions in zebrafish.

CRISPR/Cas9 disease models in zebrafish and Xenopus: The genetic renaissance of fish and frogs

The speed by which clinical genomics is currently identifying novel potentially pathogenic variants is outperforming the speed by which these can be functionally (genotype-phenotype) annotated in animal disease models. However, over the past few years the emergence of CRISPR/Cas9 as a straight-forward genome editing technology has revolutionized disease modeling in vertebrate non-mammalian model organisms such as zebrafish, medaka and Xenopus. It is now finally possible, by CRISPR/Cas9, to rapidly establish clinically relevant disease models in these organisms. Interestingly, these can provide both cost-effective genotype-phenotype correlations for gene-(variants) and genomic rearrangements obtained from clinical practice, as well as be exploited to perform translational research to improve prospects of disease afflicted patients. In this review, we show an extensive overview of these new CRISPR/Cas9-mediated disease models and provide future prospects that will allow increasingly accurate modeling of human disease in zebrafish, medaka and Xenopus.