Loss of alpha Ba-crystallin, but not alpha A-crystallin, increases age-related cataract in the zebrafish lens

The vertebrate eye lens is an unusual organ in that most of its cells lack
nuclei and the ability to replace aging protein. The small heat shock
protein α-crystallins evolved to become key components of this lens,
possibly because of their ability to prevent aggregation of aging protein
that would otherwise lead to lens opacity. Most vertebrates express two
α-crystallins, αA- and αB-crystallin, and mutations in each are linked to
human cataract. In a mouse knockout model, only the loss of αA-crystallin
led to early-stage lens cataract. We have used the zebrafish as a model
system to investigate the role of α-crystallins during lens development.
Interestingly, while zebrafish express one lens-specific αA-crystallin
gene (cryaa), they express two αB-crystallin genes, with one evolving lens
specificity (cryaba) and the other retaining the broad expression of its
mammalian ortholog (cryabb). In this study, we used individual mutant
zebrafish lines for all three α-crystallin genes to determine the impact
of their loss on age-related cataract. Surprisingly, unlike mouse knockout
models, we found that the loss of the αBa-crystallin gene cryaba led to an
increase in lens opacity compared to cryaa null fish at 24 months of age.
Loss of αA-crystallin did not increase the prevalence of cataract. We also
used single-cell RNA-Seq and RT-qPCR data to show a shift in the lens
expression of zebrafish α-crystallins between 5 and 10 days post
fertilization (dpf), with 5 and 6 dpf lenses expressing cryaa almost
exclusively, and expression of cryaba and cryabb becoming more prominent
after 10 dpf. These data show that cryaa is the primary α-crystallin
during early lens development, while the protective role for cryaba
becomes more important during lens aging. This study is the first to
quantify cataract prevalence in wild-type aging zebrafish, showing that
lens opacities develop in approximately 25% of fish by 18 months of age.
None of the three α-crystallin mutants showed a compensatory increase in
the expression of the remaining two crystallins, or in the abundant
βB1-crystallin. Overall, these findings indicate an ontogenetic shift in
the functional importance of individual α-crystallins during zebrafish
lens development. Our finding that the lens-specific zebrafish
αBa-crystallin plays the leading role in preventing age-related cataract
adds a new twist to our understanding of vertebrate lens evolution.