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

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.