Auto-Adhesion Potential of Extraocular Aqp0 during Teleost Development

Differences in a Single Extracellular Residue Underlie Adhesive Functions of Two Zebrafish Aqp0s

Aquaporin 0 (AQP0) is the most abundant lens membrane protein, and loss of function in human and animal models leads to cataract formation. AQP0 has several functions in the lens including water transport and adhesion. Since lens optics rely on strict tissue architecture achieved by compact cell-to-cell adhesion between lens fiber cells, understanding how AQP0 contributes to adhesion would shed light on normal lens physiology and pathophysiology. We show in an in vitro adhesion assay that one of two closely related zebrafish Aqp0s, Aqp0b, has strong auto-adhesive properties while Aqp0a does not. The difference appears to be largely due to a single amino acid difference at residue 110 in the extracellular C-loop, which is T in Aqp0a and N in Aqp0b. Similarly, P110 is the key residue required for adhesion in mammalian AQP0, highlighting the importance of residue 110 in AQP0 cell-to-cell adhesion in vertebrate lenses as well as the divergence of adhesive and water permeability functions in zebrafish duplicates.

Positively charged amino acid residues in the extracellular loops A and C of lens aquaporin 0 interact with the negative charges in the plasma membrane to facilitate cell-to-cell adhesion

This investigation was undertaken to find out whether the positive charges in the Extracellular Loops A (ELA) and C (ELC) of Aquaporin 0 (AQP0) are involved in lens fiber cell-to-cell adhesion (CTCA), and the possible mechanism of CTCA. AQP0 ELA or ELC was substituted with the corresponding AQP1 loop via Polymerase Chain Reaction. Positively charged arginine (R) and histidine (H) of mouse AQP0 ELA and ELC were substituted individually with glutamine (Q) to create R33Q, H40Q, R113Q and H122Q by mutagenesis. cRNA expression, immunostaining, Förster Resonance Energy Transfer (FRET) studies and protein analyses showed localization of all mutants except AQP0-AQP1ELC chimera (AQP0 ELC substituted with AQP1 ELC) at the plasma membrane. Osmotic Swelling Assay revealed comparable water permeability (P_f) among AQP0-AQP1ELA, R33Q, R113Q, and WT. CTCA assay demonstrated a significant reduction in adhesion in all mutants compared to the WT (14-73%) suggesting the importance of the conserved positively charged residues of ELA and ELC for adhesion. Studies involving AQP0-transfected L-cells, and lipid vesicles indicated that CTCA was due to the electrostatic interaction between the positively charged amino acids of AQP0 extracellular loops and the negative charges of the plasma membrane. Schematic models are provided to illustrate the mechanism.

The cellular localization and redistribution of multiple aquaporin paralogs in the spermatic duct epithelium of a maturing marine teleost

Aquaporin-mediated fluid transport in the mammalian efferent duct and epididymis is believed to play a role in sperm maturation and concentration. In fish, such as the marine teleost gilthead seabream (Sparus aurata), the control of fluid homeostasis in the spermatic duct seems also to be crucial for male fertility, but no information exists on the expression and distribution of aquaporins. In this study, reverse transcriptase-polymerase chain reaction and immunoblotting analyses, employing available and newly raised paralog-specific antibodies for seabream aquaporins, indicate that up to nine functional aquaporins, Aqp0a, -1aa, -1ab, -3a, -4a, -7, -8bb, -9b and -10b, are expressed in the spermatic duct. Immunolocalization of the channels in the resting spermatic duct reveals that Aqp0a, -1aa, -4a, -7 and -10b are expressed in the monolayered luminal epithelium, Aqp8b and -9b in smooth muscle fibers, and Aqp1ab and -3a in different interstitial lamina cells. In the epithelial cells, Aqp0a and -1aa are localized in the short apical microvilli, and Aqp4a and -10b show apical and basolateral staining, whereas Aqp7 is solely detected in vesicular compartments. Upon spermiation, an elongation of the epithelial cells sterocilia, as well as the folding of the epithelium, is observed. At this stage, single- and double-immunostaining, using two aquaporin paralogs or the Na⁺ /K⁺ -ATPase membrane marker, indicate that Aqp1ab, -3a, -7, -8bb and -9b staining remains unchanged, whereas in epithelial cells Aqp1aa translation is supressed, Aqp4a internalizes, and Aqp0a and -10b accumulate in the apical, lateral and basal plasma membrane. These findings uncover a cell type- and region-specific distribution of multiple aquaporins in the piscine spermatic duct, which shares conserved features of the mammalian system. The data therefore suggest that aquaporins may play different roles in the regulation of fluid homeostasis and sperm maturation in the male reproductive tract of fish.