Cataracts, the loss of lens transparency, are the leading cause of human
blindness. The zebrafish embryo, with its transparency and relatively large eyes, is an excellent model for studying ocular disease in vivo. We found that the zebrafish cloche mutant, both the cloche(m39) and cloche(S5) alleles, which have defects in hematopoiesis and blood vessel development, also have lens
cataracts. Quantitative examination of the living zebrafish lens by confocal microscopy showed significant increases in lens reflectance. Histological analysis revealed retention of lens fiber cell nuclei owing to impeded terminal differentiation. Proteomics identified
gamma-crystallin as a
protein that was substantially diminished in cloche mutants.
Crystallins are the major structural
proteins in mouse, human and zebrafish lens. Defects in
crystallins have previously been shown in mice and humans to contribute to
cataracts. The loss of
gamma-crystallin protein in cloche was not due to lowered
mRNA levels but rather to
gamma-crystallin protein insolubility. AlphaA-
crystallin is a chaperone that protects
proteins from misfolding and becoming insoluble. The cloche lens is deficient in both alphaA-
crystallin mRNA and
protein during development from 2-5 dpf. Overexpression of exogenous alphaA-
crystallin rescued the cloche lens phenotype, including solubilization of
gamma-crystallin, increased lens transparency and induction of lens fiber cell differentiation. Taken together, these results indicate that alphaA-
crystallin expression is required for normal lens development and demonstrate that
cataract formation can be prevented in vivo. In addition, these results show that proteomics is a valuable tool for detecting
protein alterations in zebrafish.