Investigating the cellular mechanisms underlying Yap-dependent choroid fissure closure in zebrafish.

Coloboma is a clinical condition in which failure of choroid fissure closure during development leads to defects in the mature eye. Coloboma is one of the main causes of visual impairments in humans and it can be both syndromic and isolated. Mutations in the YAP gene, a key regulator of tissue and organ shape and size, cause coloboma both in humans and zebrafish by largely unknown mechanisms. In zebrafish, Yap and its homologue Taz have been shown to be critical in the specification of the retina pigment epithelium (RPE). This study investigates potential cellular mechanisms of Yap-mediated choroid fissure closure using the yapnl13 zebrafish coloboma mutant, focusing on the RPE. Firstly, we investigated the specificity of yapnl13 phenotype. Although yap is broadly expressed in the zebrafish embryo, the phenotype of yapnl13 is specific to the eye. Using RT-PCR and qRT-PCR experiments we assessed and excluded the presence of eye specific alternative yap isoforms that could correlate with the specificity of the phenotype. Secondly, we aimed to understand whether the yapnl13 mutation affected the specification and proliferation of the RPE. Confocal time-lapse experiments revealed that both specification and proliferation of the RPE are not affected in yapnl13 zebrafish mutant. Finally, we aimed to understand if the yapnl13 mutation impairs mechanotransduction in the developing RPE. Yap is a key effector in mechanotransduction and is critical for the maintenance of cell and tissue mechano-homeostasis. Immunohistochemistry experiments coupled with morphological analysis of the RPE cells suggested an impairment in actomyosin contractility in the yapnl13 mutant. Decreasing cell contractility, using both chemical and genetic manipulation of myosin, increases the frequency of coloboma in yapnl13 mutants whereas increasing cell contractility rescues the phenotype. This work highlights the role of Yap in generating the tissue tension required for optic cup fusion. We suggest that the yapnl13 mutation results in an RPE-specific downregulation of cell contractility which affects the correct folding of the optic cup resulting in failure of choroid fissure closure