Optical development in the zebrafish eye lens.

The optics of the eye is the key to a functioning visual system. The exact nature of the correlation between ocular optics and eye development is not known because of the paucity of knowledge about the growth of a key optical element, the eye lens. The sophisticated optics of the lens and its gradient of refractive index provide the superior optical quality that the eye needs and which, it is thought, has a major influence on the development of proper visual function. The nature of a gradient refractive index lens, however, renders accurate measurements of its development difficult to make and has been the reason why the influence of lens growth on visual function remains largely unknown. Novel imaging techniques have made it possible to investigate growth of the eye lens in the zebrafish. This study shows measurements using X-ray Talbot interferometry of three-dimensional gradient index profiles in eye lenses of zebrafish from late larval to adult stages. The zebrafish lens shows evidence of a gradient of refractive index from the earliest stages measured and its growth suggests an apparent coincidence between periods of rapid increase in refractive index in the lens nucleus and increased expression of a particular crystallin protein group.

BFSP1 C-terminal domains released by post-translational processing events can alter significantly the calcium regulation of AQP0 water permeability.

BFSP1 (beaded filament structural protein 1, filensin) is a cytoskeletal protein expressed in the eye lens. It binds AQP0 in vitro and its C-terminal sequences have been suggested to regulate the water channel activity of AQP0. A myristoylated fragment from the C-terminus of BFSP1 was found in AQP0 enriched fractions. Here we identify BFSP1 as a substrate for caspase-mediated cleavage at several C-terminal sites including D433. Cleavage at D433 exposes a cryptic myristoylation sequence (434-440). We confirm that this sequence is an excellent substrate for both NMT1 and 2 (N-myristoyl transferase). Thus caspase cleavage may promote formation of myristoylated fragments derived from the BFSP1 C-terminus (G434-S665). Myristoylation at G434 is not required for membrane association. Biochemical fractionation and immunogold labeling confirmed that C-terminal BFSP1 fragments containing the myristoylation sequence colocalized with AQP0 in the same plasma membrane compartments of lens fibre cells. To determine the functional significance of the association of BFSP1 G434-S665 sequences with AQP0, we measured AQP0 water permeability in Xenopus oocytes co-transfected with transcripts expressing both AQP0 and various C-terminal domain fragments of BFSP1 generated by caspase cleavage. We found that different fragments dramatically alter the response of AQP0 to different concentrations of Ca2+. The complete C-terminal fragment (G434-S665) eliminates calcium regulation altogether. Shorter fragments can enhance regulation by elevated calcium or reverse the response, indicative of the regulatory potential of BFSP1 with respect to AQP0. In particular, elimination of the myristoylation site by the mutation G434A reverses the order of water permeability sensitivity to different Ca2+ concentrations.

Calmodulin Gates Aquaporin 0 Permeability through a Positively Charged Cytoplasmic Loop.

Aquaporin 0 (AQP0), the major intrinsic protein of the eye lens, plays a vital role in maintaining lens clarity by facilitating the transport of water across lens fiber cell membranes. AQP0 reduces its osmotic water permeability constant (Pf) in response to increases in the external calcium concentration, an effect that is mediated by an interaction with the calcium-binding messenger protein, calmodulin (CaM), and phosphorylation of the CaM-binding site abolishes calcium sensitivity. Despite recent structural characterization of the AQP0-CaM complex, the mechanism by which CaM modulates AQP0 remains poorly understood. By combining atomistic molecular dynamics simulations and oocyte permeability assays, we conclude that serine phosphorylation of AQP0 does not inhibit CaM binding to the whole AQP0 protein. Instead, AQP0 phosphorylation alters calcium sensitivity by modifying the AQP0-CaM interaction interface, particularly at an arginine-rich loop that connects the fourth and fifth transmembrane helices. This previously unexplored loop, which sits outside of the canonical CaM-binding site on the AQP0 cytosolic face, mechanically couples CaM to the pore-gating residues of the second constriction site. We show that this allosteric loop is vital for CaM regulation of the channels, facilitating cooperativity between adjacent subunits and regulating factors such as serine phosphorylation. Similar allosteric interactions may also mediate CaM modulation of the properties of other CaM-regulated proteins.

The modulation of the phosphorylation status of NKCC1 in organ cultured bovine lenses: Implications for the regulation of fiber cell and overall lens volume.

In previous work, we have shown the Sodium/Potassium/2 Chloride Cotransporter (NKCC1) to be a key effector of lens fiber cell volume regulation. Since others have shown that the activity of NKCC1 is regulated via its phosphorylation status, the purpose of this study was to investigate whether NKCC1 phosphorylation can be modulated in organ cultured bovine lenses, and to see how this relates to changes in lens wet weight. Western blotting was first used to confirm the expression of NKCC1, phosphorylated NKCC1 (NKCC1-P) and the regulatory kinases WNK/SPAK and phosphatases PP1/PP2A in bovine lenses at the protein level. Changes to NKCC1-P status were then assessed by organ culturing bovine lenses in either isotonic, hypertonic or hypotonic solutions in the presence or absence of the NKCC inhibitor, bumetanide, or phosphatase inhibitors okadaic acid and calyculin A. After 1-22 h of culturing, lenses were weighed, assessed for transparency and the cortical protein fractions analyzed by western blot using antibodies to detect total NKCC1 and NKCC1-P. NKCC1, NKCC1-P, SPAK, PP1 and PP2A were all detected in the membrane fraction of bovine lenses. Under hypertonic conditions, NKCC1 is phosphorylated and activated to mediate a regulatory volume increase. Finally, NKCC1-P signal increased in the presence of phosphatase inhibitors indicating that PP1/PP2A can dephosphorylate NKCC1. These results show that the phosphorylation status and hence activity of NKCC1 is dynamically regulated and that in response to hypertonic stress, NKCC1 activity is increased to effect a regulatory volume increase that limits cell shrinkage. These findings support the view that the lens dynamically regulates ion fluxes to maintain steady state lens volume, and suggest that dysfunction of this regulation maybe an initiating factor in the localized fiber cell swelling that is a characteristic of diabetic lens cataract.

KRAP is required for diffuse and punctate IP3-mediated Ca2+ liberation and determines the number of functional IP3R channels within clusters.

KRas-induced actin-interacting protein (KRAP) has been identified as crucial for the appropriate localization and functioning of the inositol trisphosphate receptors (IP3Rs) that mediate Ca2+ release from the endoplasmic reticulum. Here, we used siRNA knockdown of KRAP expression in HeLa and HEK293 cells to examine the roles of KRAP in the generation of IP3-mediated local Ca2+ puffs and global, cell-wide Ca2+ signals. High resolution Ca2+ imaging revealed that the mean amplitude of puffs was strongly reduced by KRAP knockdown, whereas the Ca2+ flux during openings of individual IP3R channels was little affected. In both control and KRAP knockdown cells the numbers of functional channels in the clusters underlying puff sites were stochastically distributed following a Poisson relationship, but the mean number of functional channels per site was reduced by about two thirds by KRAP knockdown. We conclude that KRAP is required for activity of IP3R channels at puff sites and stochastically 'licenses' the function of individual channels on a one-to-one basis, rather than determining the functioning of the puff site as a whole. In addition to puff activity ('punctate' Ca2+ release), global, cell-wide Ca2+ signals evoked by higher levels of IP3 are further composed from a discrete 'diffuse' mode of Ca2+ release. By applying fluctuation analysis to isolate the punctate component during global Ca2+ signals, we find that KRAP knockdown suppresses to similar extents punctate and diffuse Ca2+ release in wild-type cells and in HEK293 cells exclusively expressing type 1 and type 3 IP3Rs. Thus, KRAP appears essential for the functioning of the IP3Rs involved in diffuse Ca2+ release as well as the clustered IP3Rs that generate local Ca2+ puffs.

In vivo macromolecular crowding is differentially modulated by aquaporin 0 in zebrafish lens: Insights from a nanoenvironment sensor and spectral imaging.

Macromolecular crowding is crucial for cellular homeostasis. In vivo studies of macromolecular crowding and water dynamics are needed to understand their roles in cellular physiology and fate determination. Macromolecular crowding in the lens is essential for normal optics, and an understanding of its regulation will help prevent cataract and presbyopia. Here, we combine the use of the nanoenvironmental sensor [6-acetyl-2-dimethylaminonaphthalene (ACDAN)] to visualize lens macromolecular crowding with in vivo studies of aquaporin 0 zebrafish mutants that disrupt its regulation. Spectral phasor analysis of ACDAN fluorescence reveals water dipolar relaxation and demonstrates that mutations in two zebrafish aquaporin 0s, Aqp0a and Aqp0b, alter water state and macromolecular crowding in living lenses. Our results provide in vivo evidence that Aqp0a promotes fluid influx in the deeper lens cortex, whereas Aqp0b facilitates fluid efflux. This evidence reveals previously unidentified spatial regulation of macromolecular crowding and spatially distinct roles for Aqp0 in the lens.

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.

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

Purpose: 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. Methods: 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. Results: 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. Conclusions: 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.

Assessment of Zebrafish Lens Nucleus Localization and Sutural Integrity.

The zebrafish is uniquely suited to genetic manipulation and in vivo imaging, making it an increasingly popular model for reverse genetic studies and for generation of transgenics for in vivo imaging. These unique capabilities make the zebrafish an ideal platform to study ocular lens development and physiology. Our recent findings that an Aquaporin-0, Aqp0a, is required for stability of the anterior lens suture, as well as for the shift of the lens nucleus to the lens center with age led us to develop tools especially suited to analyzing the properties of zebrafish lenses. Here we outline detailed methods for lens dissection that can be applied to both larval and adult lenses, to prepare them for histological analysis, immunohistochemistry and imaging. We focus on analysis of lens suture integrity and cortical cell morphology and compare data generated from dissected lenses with data obtained from in vivo imaging of lens morphology made possible by a novel transgenic zebrafish line with a genetically encoded fluorescent marker. Analysis of dissected lenses perpendicular to their optical axis allows quantification of the relative position of the lens nucleus along the anterior-posterior axis. Movement of the lens nucleus from an initial anterior position to the center is required for normal lens optics in adult zebrafish. Thus, a quantitative measure of lens nuclear position directly correlates with its optical properties.

Aqp0a Regulates Suture Stability in the Zebrafish Lens.

Purpose: To investigate the roles of Aquaporin 0a (Aqp0a) and Aqp0b in zebrafish lens development and transparency. Methods: CRISPR/Cas9 gene editing was used to generate loss-of-function deletions in zebrafish aqp0a and/or aqp0b. Wild type (WT), single mutant, and double mutant lenses were analyzed from embryonic to adult stages. Lens transparency, morphology, and growth were assessed. Immunohistochemistry was used to map protein localization as well as to assess tissue organization and distribution of cell nuclei. Results: aqp0a-/- and/or aqp0b-/- cause embryonic cataracts with variable penetrance. While lenses of single mutants of either gene recover transparency in juveniles, double mutants consistently form dense cataracts that persist in adults, indicating partially redundant functions. Double mutants also reveal redundant Aqp0 functions in lens growth. The nucleus of WT lenses moves from the anterior pole to the lens center with age. In aqp0a-/- mutants, the nucleus fails to centralize as it does in WT or aqp0b-/- lenses, and in double mutant lenses there is no consistent lens nuclear position. In addition, the anterior sutures of aqp0a-/-, but not aqp0b-/- mutants, are unstable resulting in failure of suture maintenance at older stages and anterior polar opacity. Conclusions. Zebrafish Aqp0s have partially redundant functions, but only Aqp0a promotes suture stability, which directs the lens nucleus to centralize, failure of which results in anterior polar opacity. These studies support the hypothesis that the two Aqp0s subfunctionalized during fish evolution and that Aqp0-dependent maintenance of the anterior suture is essential for lens transparency.

Identification of the WNK-SPAK/OSR1 signaling pathway in rodent and human lenses.

PURPOSE: To identify whether the kinases that regulate the activity of cation chloride cotransporters (CCC) in other tissues are also expressed in rat and human lenses. METHODS: The expression of with-no-lysine kinase (WNK 1, 3, 4), oxidative stress response kinase 1 (OSR1), and Ste20-like proline alanine rich kinase (SPAK) were determined at either the transcript or protein levels in the rat and human lenses by reverse-transcriptase PCR and/or Western blotting, respectively. Selected kinases were regionally and subcellularly characterized in rat and human lenses. The transparency, wet weight, and tissue morphology of lenses extracted from SPAK knock-out animals was compared with wild-type lenses. RESULTS: WNK 1, 3, 4, SPAK, and OSR1 were identified at the transcript level in rat lenses and WNK1, 4, SPAK, and OSR1 expression confirmed at the protein level in both rat and human lenses. SPAK and OSR1 were found to associate with membranes as peripheral proteins and exhibited distinct subcellular and region-specific expression profiles throughout the lens. No significant difference in the wet weight of SPAK knock-out lenses was detected relative to wild-type lenses. However, SPAK knock-out lenses showed an increased susceptibility to opacification. CONCLUSIONS: Our results show that the WNK 1, 3, 4, OSR1, and SPAK signaling system known to play a role in regulating the phosphorylation status, and hence activity of the CCCs in other tissues, is also present in the rat and human lenses. The increased susceptibility of SPAK lenses to opacification suggests that disruption of this signaling pathway may compromise the ability of the lens to control its volume, and its ability to maintain its transparency.