smpx-deficient zebrafish embryos: a tool for studying ciliopathies and deafness

Firmly established as a model organism for embryonic developmental studies, zebrafish is now emerging as an effective system to elucidate the fundamental aspects of several human diseases. Despite the significant evolutionary distance, gene functions and pathways are often highly conserved between zebrafish and humans. Previous work has identified nonsense mutations, associated with hearing deficiency, in the small muscle protein, X-linked (SMPX) gene and a loss-of-function mechanism underlying this form of impairment has been proposed. SMPX encodes a cytoskeleton-associated protein that has been suggested being responsive to mechanical stress. The presence of Smpx in hair cells of the murine cochlea reinforces the thesis of its importance in ear functioning. In this study we first provided an overview of the expression of smpx during zebrafish embryonic development. We showed that from the 4-somite stage, zebrafish smpx was first expressed in the ciliated organ Kupffer’s vesicle (KV), responsible for the left-right organs asymmetry, and then, as in mammals, expressed in the heart and in the inner ear, among other territories. Knock-down of smpx in zebrafish embryos resulted in the reduced number of cilia in KV and, consequently, the left-right patterning of internal organs resulted compromised, mimicking human heterotaxy, a disorder of laterality. Moreover, smpx knock-down resulted in the marked decrease of the number of cilia in the ear macular regions, providing a possible explanation for the hearing loss in SMPXmutated patients. In conclusion, this study contributes to the comprehension of the link between SMPX and human hearing loss, delivers a novel heterotaxy-candidate gene, and finally provides a robust animal tool for the future elucidation of the mechanisms behind both deafness/hearing loss and heterotaxy.