Loss of the small heat shock protein αA-crystallin does not lead to detectable defects in early zebrafish lens development

Abstract

Alpha crystallins are small heat shock proteins essential to normal ocular lens function. They also help maintain homeostasis in many non-ocular vertebrate tissues and their expression levels change in multiple diseases of the nervous and cardiovascular system and during cancer. The specific roles that α-crystallins may play in eye development are unclear. Studies with knockout mice suggested that only one of the two mammalian α-crystallins is required for normal early lens development. However, studies in two fish species suggested that reduction of αA-crystallin alone could inhibit normal fiber cell differentiation, cause cataract and contribute to lens degeneration. In this study we used synthetic antisense morpholino oligomers to suppress the expression of zebrafish αA-crystallin to directly test the hypothesis that, unlike mammals, the zebrafish requires αA-crystallin for normal early lens development. Despite the reduction of zebrafish αA-crystallin protein to undetectable levels by western analysis through 4 days of development we found no changes in fiber cell differentiation, lens morphology or transparency. In contrast, suppression of AQP0a expression, previously shown to cause lens cataract, produced irregularly shaped lenses, delay in fiber cell differentiation and lens opacities detectable by confocal microscopy. The normal development observed in αA-crystallin deficient zebrafish embryos may reflect similarly non-essential roles for this protein in the early stages of both zebrafish and mammalian lens development. This finding has ramifications for a growing number of researchers taking advantage of the zebrafish's transparent external embryos to study vertebrate eye development. Our demonstration that lens cataracts can be visualized in three-dimensions by confocal microscopy in a living zebrafish provides a new tool for studying the causes, development and prevention of lens opacities.

Abnormal retinal development in Cloche mutant zebrafish

“…Lenses of clo−/− embryos remained abnormal, also displaying cell‐free gaps between the lens epithelium and the developing lens fibers, particularly in the region of the lens equator (Fig. F), as observed in other cases of developmental lens abnormality (Posner et al, ). These findings indicate that clo mutant embryos fail to develop normally laminated retinas, as well as show defects in developing RPE and lens tissues.…”

Transient cerebellar alterations during development prior to obvious motor phenotype in a mouse model of spinocerebellar ataxia type 6

“…; Posner et al . ; Peixoto et al . ) suggesting that the nervous system may have multiple compensatory mechanisms for developmental alterations.…”

“…However, alterations in development without an associated detectable behavioural change are frequently observed (e.g. Erickson et al 2007;Posner et al 2013;Peixoto et al 2016) suggesting that the nervous system may have multiple compensatory mechanisms for developmental alterations.…”

A conserved role of αA-crystallin in the development of the zebrafish embryonic lens

“…Overexpression of αA-crystallin, achieved by mRNA injection, rescued the phenotype. Posner et al challenged this conclusion on the basis of morpholino injections that failed to produce visible lens abnormalities (Posner et al, 2013). However, the preliminary nature of their study, manifested by the limited number of embryos screened, calls into question their conclusions.…”

“…However, their role in embryonic zebrafish lens development and transparency is controversial (Goishi et al, 2006; Posner et al, 2013). Here, we report the results of an investigation into the roles of three α-crystallin genes, cryaa , cryaba , and cryabb , in the development of the zebrafish lens.…”