Aquaporins Have Regional Functions in Development of Refractive Index in the Zebrafish Eye Lens

In the eye lens, cytosolic protein concentrations increase progressively from the periphery to the center, contributing to the gradient of refractive index (GRIN). Aquaporins are membrane proteins of lens fiber cells that regulate water transport and adhesion and interact with cytoskeletal proteins. This study investigates how these membrane proteins contribute to proper development of the lens GRIN.

Loss-of-function deletions of aqp0a and/or aqp0b in zebrafish were generated using CRISPR/Cas9 gene editing. Lenses of single aqp0a-/- mutants, single aqp0b-/- mutants, and double aqp0a-/-/aqp0b-/- mutants from larval to elderly adult stages were measured using x-ray Talbot interferometry at SPring8 in Japan. The three-dimensional GRIN profiles in two orthogonal cross-sectional planes of each lens were analyzed and compared with in vivo images and previous results obtained from wild-type lenses.

Single aqp0a-/- mutants tended to show asymmetric GRIN profiles, with the central plateau regions shifted anteriorly. Single aqp0b-/- mutants had smooth, symmetric GRIN profiles throughout development until spoke opacities appeared in several extremely old samples. Double aqp0a-/-/aqp0b-/- mutants showed lower magnitude GRIN profiles, as well as dips in the central plateau region.

These findings suggest that Aqp0a and Aqp0b have region-specific functions in the lens: Aqp0a is active peripherally, regulating centralization of the plateau region, and this function cannot be compensated for by Aqp0b. In the lens center, either Aqp0a or Aqp0b is required for formation of the plateau region, as well as for the GRIN to reach its maximum magnitude in mature lenses.