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These protocols were developed to analyze cortical lens morphology, structural integrity of the zebrafish lens sutures in fixed and live lenses and to measure the position of the zebrafish lens nucleus along the anterior-posterior axis.
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.
The zebrafish is an excellent model for studying lens development and physiology due to the anatomical similarities to mammalian lenses, relative ease of genetic and pharmacological manipulation, speed of embryonic eye development, small size and transparency at early stages allowing for in vivo imaging. The methods described here were developed to analyze zebrafish lens morphology at embryonic and adult stages with a focus on sutural integrity, cortical membrane morphology in vitro and in vivo, and location of lens nuclear position along the anterior-posterior axis ex vivo. These methods serve as a starting point for functional studies of lens development and physiology, as well as reverse genetic screens for lens phenotypes in zebrafish.
Imaging zebrafish lens morphology
Aquaporin 0 (AQP0) is the most abundant lens membrane protein and is critical for both, lens development and clarity, due to multiple essential functions in mammals. Zebrafish have two Aqp0s (Aqp0a and Aqp0b) and we have developed methods to analyze loss of their functions in both embryonic and adult lenses. Our studies reveal that aqp0a-/- mutants develop anterior polar opacity due to instability of the anterior suture, and aqp0a/b double mutants develop nuclear opacity1. AQP0 has been shown to play roles in water transport2, adhesion3,4, cytoskeletal anchoring5 and generation of the refractive index gradient6, but these studies have largely been performed in vitro. The zebrafish provides a unique opportunity to study how loss of function, or perturbed function of Aqp0a or Aqp0b would affect morphology and function in a living lens. To assess lens cell morphology and sutural integrity during development, we modified existing in vitro immunohistochemical methods7 for use in embryonic and adult lenses, and generated transgenics to monitor this process in vivo.
Immunohistochemical analysis of plasma membrane structure and sutural integrity was performed in whole fixed embryos and adult lenses. Zebrafish lenses are extremely small (lens diameter is ~100 µm in embryos and up to 1 mm in adults) compared with their mammalian counterparts and have point sutures8, which are infrequently captured in cryosections. Thus, whole lenses are essential for analyzing sutural integrity. For in vivo analysis of anterior suture formation, and imaging of precise lens cell architecture, we generated transgenics expressing mApple specifically labeling lens membranes.
Advantages of live imaging of lens membrane transgenics include: 1) avoiding fixation artifacts, 2) studying dynamic morphological changes in time-lapse movies, and 3) enabling longitudinal studies in which earlier events can be correlated with later phenotypes. Pigmentation of the iris normally prevents clear imaging of the lens periphery. Addition of 1-phenyl 2-thiourea (PTU) before the primordia-5 (prim-5) stage9 prevents melanogenesis and eye pigmentation up to around 4 days postfertilization (dpf). However, after 4 dpf, the lens periphery is obscured in vivo, particularly at older stages. Furthermore, the density of the lens itself obscures imaging of its posterior pole. Therefore, to study morphology of the lens periphery, or the posterior suture, after 4 dpf, lenses need to be excised and fixed.
Transgenic zebrafish lines have been used to analyze embryonic lens membrane structure in vivo10. The Q01 transgenic expresses a cyan fluorescent protein fused to a membrane targeting sequence, Gap43, driven by the EF1α promoter and a hexamer of the DF4 pax6 enhancer element ubiquitously in lens fiber cells11. Q01 does have extra-lenticular expression, including amacrine cells in the retina, which adds background signal if the primary interest is the lens. We developed a novel transgenic line that expresses a membrane-tethered mApple specifically in the lens, with the aim of avoiding any extra-lenticular signal.
Lens nucleus localization
We discovered that the lens nucleus moves from an initial anterior location in larval zebrafish to a central location in the anterior-posterior axis in adult lenses. This shift in the position of the high refractive index lens nucleus is thought to be a requirement for normal development of zebrafish lens optics1. Our methods allow quantification of lens nuclear centralization in relation to the lens radius. Using this method, we showed that Aqp0a is required for lens nuclear centralization1, and this simple method can be applied to other studies of the development and physiology of the lens and its optical properties in the zebrafish model.
The animal protocols used in this study adhere to the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research and have been approved by the Institutional Animal Care and Use Committee (IACUC) of University of California, Irvine.
1. Zebrafish Husbandry and Euthanasia
2. Fixation of Embryos and Larvae
NOTE: Immunohistochemical protocols were adapted from previously published materials7.
3. Dissection of Larval and Adult Zebrafish Lenses
4. Fixation of Dissected Lenses
5. Lens Immunohistochemistry
6. Analysis of Zebrafish Anterior Lens Sutures Using a Transgenic Line In Vivo
7. Analysis of Zebrafish Anterior Lens Sutures Using a Transgenic Line In Vivo
8. Lens Cryosectioning and Immunohistochemistry
9. Imaging
10. Measurement of Lens Nucleus Localization in the Anterior-posterior Axis
Adult zebrafish eye anatomy closely resembles that of mammals (Figure 1A). Despite some differences between zebrafish and mammalian eyes, such as having a ciliary zone instead of a ciliary body17, differences in optical properties18, and differences in morphogenesis during embryonic development19, the zebrafish eye is an excellent model for studying eye development and understanding ophth...
Analysis of zebrafish lens morphology is the initial step in understanding phenotypes in mutants, or the effects of pharmacological interventions aimed at studying biology of the ocular lens. We outline methods to analyze lens sutures, cortical fiber cell morphology and aspects of the lens nucleus. These approaches are a combination of in vitro and in vivo (compared in Table 1). The in vitro methods allow for greater detail of the outer cortical cell morphology, as well as acce...
We have no disclosures.
We would like to acknowledge our funding source: NIH R01 EY05661 to J.E.H, Ines Gehring for assisting with generating the aqp0a/b double mutants and zebrafish husbandry, Dr Daniel Dranow for discussions leading to generation of the transgenics, Dr Bruce Blumberg and Dr Ken Cho’s labs for use of their dissecting microscopes, and Dr Megan Smith for help with statistical analysis.
Name | Company | Catalog Number | Comments |
1-phenyl-2-thiourea (PTU) | Sigma | P7629 | CAUTION – very toxic |
4% Paraformaldehyde aqueous solution | Electron Microscopy Sciences | RT 157-4 | CAUTION – health hazard, combustible |
Confocal microscope | Nikon | Eclipse Ti-E | |
Cryostat | Leica | CM3050S | Objective and chamber temperature set to -21˚C |
DAPI | Invitrogen | D1306 | CAUTION – irritating to eyes and skin |
Dimethyl Sulfoxide (DMSO) | Fisher Scientific | D128 | CAUTION – combustible, penetrates skin |
Disposable base mold | VWR Scientific | 15154-631 | |
Disposable Pasteur glass pipets | Fisherbrand | 13-678-20A | |
Dumont # 5 forceps | Dumont & Fils | Keep forceps sharpened | |
Ethyl 3-aminobenzoate methanesulfonate salt (Tricaine) | Sigma-Aldrich | A5040 | CAUTION - toxic |
Glass bottom microwell dish (35mm petri dish, 14mm microwell, #1.5 coverglass) | MatTek Corporation | P35G-1.5-14-C | |
Glycerol | Sigma | G2025 | |
huβB1cry:mAppleCAAX DNA construct | Addgene | ID:122451 | |
ImageJ | Wayne Rasband, NIH | v1.51n | |
Low Melt Agarose (LMA) | Apex | 902-36-6 | |
NIS-Elements | Nikon | V 4.5 | |
NIS-Elements AR software | Nikon | ||
Olympus with a model 2.1.1.183 controller | Olympus Corp | DP70 | |
Olympus microscope | Olympus Corp | SZX12 | |
Phalloidin-Alexa Flour 546 | Thermo Fisher | A22283 | |
Phenol Red indicator (1% w/v) | Ricca Chemical Company | 5725-16 | |
Phosphate buffered saline (PBS) | Fisher Scientific | BP399 | |
Photoshop | Adobe | CS5 v12.0 | |
Photoshop software | Adobe | CS5 v12.0 | |
Plan Apo 60x/1.2 WD objective | Nikon | ||
Power source | Wild Heerbrugg | MTr 22 | Or equivalent power source |
Slide warmer model No. 26020FS | Fisher Scientific | 12-594 | |
Sodium Hydroxide beads | Fisher Scientific | S612-3 | CAUTION - corrosive/irritating to eyes and skin, target organ - respiratory system, corrosive to metals |
Superfrost/Plus microscope slide | Fisher Scientific | 12-550-15 | |
Sylgard 184 silicone Dow Corning | World Prevision Instruments | SYLG184 | |
Tissue-Tek O.C.T. Compound | Sakura Finetek | 4583 | |
Triton X-100 BioXtra | Sigma | T9284 | CAUTION – Toxic, hazardous to aquatic environment, corrosive |
Vannas micro-dissection scissors | Ted Pella Inc | 1346 | Sharp/sharp straight tips |
Vectashield antifade mouting medium | Vector laboratories | H-1000 | |
Wheat Germ Agglutinin (WGA)-Alexa Flour-594 | Life Technologies | W11262 |
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