Adherens junctions in the ocular lens of various species: ultrastructural analysis with an improved fixation

Arvcf Dependent Adherens Junction Stability is Required to Prevent Age-Related Cortical Cataracts

The etiology of age-related cortical cataracts is not well understood but is speculated to be related to alterations in cell adhesion and/or the changing mechanical stresses occurring in the lens with time. The role of cell adhesion in maintaining lens transparency with age is difficult to assess because of the developmental and physiological roles that well-characterized adhesion proteins have in the lens. This report demonstrates that Arvcf, a member of the p120-catenin subfamily of catenins that bind to the juxtamembrane domain of cadherins, is an essential fiber cell protein that preserves lens transparency with age in mice. No major developmental defects are observed in the absence of Arvcf, however, cortical cataracts emerge in all animals examined older than 6-months of age. While opacities are not obvious in young animals, histological anomalies are observed in lenses at 4-weeks that include fiber cell separations, regions of hexagonal lattice disorganization, and absence of immunolabeled membranes. Compression analysis of whole lenses also revealed that Arvcf is required for their normal biomechanical properties. Immunofluorescent labeling of control and Arvcf-deficient lens fiber cells revealed a reduction in membrane localization of N-cadherin, β-catenin, and αN-catenin. Furthermore, super-resolution imaging demonstrated that the reduction in protein membrane localization is correlated with smaller cadherin nanoclusters. Additional characterization of lens fiber cell morphology with electron microscopy and high resolution fluorescent imaging also showed that the cellular protrusions of fiber cells are abnormally elongated with a reduction and disorganization of cadherin complex protein localization. Together, these data demonstrate that Arvcf is required to maintain transparency with age by mediating the stability of the N-cadherin protein complex in adherens junctions.

Spatially Resolved Proteomic Analysis of the Lens Extracellular Diffusion Barrier

Purpose

The presence of a physical barrier to molecular diffusion through lenticular extracellular space has been repeatedly detected. This extracellular diffusion barrier has been proposed to restrict the movement of solutes into the lens and to direct nutrients into the lens core via the sutures at both poles. The purpose of this study is to characterize the molecular components that could contribute to the formation of this barrier.

Methods

Three distinct regions in the bovine lens cortex were captured by laser capture microdissection guided by dye penetration. Proteins were digested by Lys C and trypsin. Mass spectrometry-based proteomic analysis followed by gene ontology and protein interaction network analysis was performed.

Results

Dye penetration showed that fiber cells first shrink the extracellular spaces of the broad sides followed by closure of the extracellular space between narrow sides at a normalized lens distance (r/a) of 0.9. Accompanying the closure of extracellular space of the broad sides, dramatic proteomic changes were detected, including upregulation of several cell junctional proteins. AQP0 and its interacting partners, Ezrin and Radixin, were among a few proteins that were upregulated, accompanying the closure of extracellular space of the narrow sides, suggesting a particularly important role for AQP0 in controlling the narrowing of the extracellular spaces between fiber cells. The results also provided important information related to biological processes that occur during fiber cell differentiation such as organelle degradation, cytoskeletal remodeling, and glutathione synthesis.

Conclusions

The formation of a lens extracellular diffusion barrier is accompanied by significant membrane and cytoskeletal protein remodeling.

Age-related changes in eye lens biomechanics, morphology, refractive index and transparency

Life-long eye lens function requires an appropriate gradient refractive index, biomechanical integrity and transparency. We conducted an extensive study of wild-type mouse lenses 1-30 months of age to define common age-related changes. Biomechanical testing and morphometrics revealed an increase in lens volume and stiffness with age. Lens capsule thickness and peripheral fiber cell widths increased between 2 to 4 months of age but not further, and thus, cannot account for significant age-dependent increases in lens stiffness after 4 months. In lenses from mice older than 12 months, we routinely observed cataracts due to changes in cell structure, with anterior cataracts due to incomplete suture closure and a cortical ring cataract corresponding to a zone of compaction in cortical lens fiber cells. Refractive index measurements showed a rapid growth in peak refractive index between 1 to 6 months of age, and the area of highest refractive index is correlated with increases in lens nucleus size with age. These data provide a comprehensive overview of age-related changes in murine lenses, including lens size, stiffness, nuclear fraction, refractive index, transparency, capsule thickness and cell structure. Our results suggest similarities between murine and primate lenses and provide a baseline for future lens aging studies.

Chicken GRIFIN: binding partners, developmental course of localization and activation of its lens-specific gene expression by L-Maf/Pax6

Tissue lectins appear to be involved in a broad range of physiological processes, as reflected for the members of the family of galectins by referring to them as adhesion/growth-regulatory effectors. In order to clarify the significance of galectin presence, key challenges are to define their binding partners and the profile of localization. Having identified the chicken galectin-related interfiber protein (C-GRIFIN) as lens-specific protein present in the main body of adult lens, we here report its interaction with lens proteins in ligand blotting. The assumption for pairing with α-, β- and δ-crystallins was ascertained by mass spectrometric detection of their presence in eluted fractions obtained by affinity chromatography. Biochemical and immunohistochemical monitoring revealed protein presence from about 3-day-old embryos onwards, mostly in the cytoplasm of elongated posterior cells, later in secondary lens fiber cells. On the level of gene expression, its promoter was activated by transcription factor L-Maf alone and together with Pax6 like a crystallin gene, substantiating C-GRIFIN's status as lens-specific galectin. Using this combined strategy for counterreceptor and expression profiling by bio- and histochemical methods including light, electron and fluorescence microscopy, respective monitoring in lens development can now be taken to the level of the complete galectin family.

The molecular mechanisms underlying lens fiber elongation

Lens fiber cells are highly elongated cells with complex membrane morphologies that are critical for the transparency of the ocular lens. Investigations into the molecular mechanisms underlying lens fiber cell elongation were first reported in the 1960s, however, our understanding of the process is still poor nearly 50 years later. This review summarizes what is currently hypothesized about the regulation of lens fiber cell elongation along with the available experimental evidence, and how this information relates to what is known about the regulation of cell shape/elongation in other cell types, particularly neurons.

The lens actin filament cytoskeleton: Diverse structures for complex functions

The eye lens is a transparent and avascular organ in the front of the eye that is responsible for focusing light onto the retina in order to transmit a clear image. A monolayer of epithelial cells covers the anterior hemisphere of the lens, and the bulk of the lens is made up of elongated and differentiated fiber cells. Lens fiber cells are very long and thin cells that are supported by sophisticated cytoskeletal networks, including actin filaments at cell junctions and the spectrin-actin network of the membrane skeleton. In this review, we highlight the proteins that regulate diverse actin filament networks in the lens and discuss how these actin cytoskeletal structures assemble and function in epithelial and fiber cells. We then discuss methods that have been used to study actin in the lens and unanswered questions that can be addressed with novel techniques.

Breakdown of interlocking domains may contribute to formation of membranous globules and lens opacity in ephrin-A5(-/-) mice

Ephrin-A5, a ligand of the Eph family of receptor tyrosine kinases, plays a key role in lens fiber cell packing and cell-cell adhesion, with approximately 87% of ephrin-A5(-/-) mice develop nuclear cataracts. Here, we investigated the extensive formation of light-scattering globules associated with breakdown of interlocking protrusions during lens opacification in ephrin-A5(-/-) mice. Lenses from wild-type (WT) and ephrin-A5(-/-) mice between 2 and 21 weeks old were studied with light and electron microscopy, immunofluorescence labeling, freeze-fracture TEM and filipin cytochemistry for membrane cholesterol detection. Lens opacities with various densities were first observed in ephrin-A5(-/-) mice at around 60 days old. Dense cataracts in the mutant lenses were seen primarily in the nuclear region surrounded by transparent cortices from all eyes examined. We confirmed that a majority of nuclear cataracts were dislocated posteriorly and ruptured the thinner posterior lens capsule. SEM analysis indicated that numerous interlocking protrusions and wavy ridge-and-valley membrane surfaces in deep cortical and nuclear fibers did not cause lens opacity in both transparent ephrin-A5(-/-) and WT mice. In contrast, abundant isolated membranous globules of approximately 1000 nm in size were distributed randomly along the intact fiber cells during early stage of all ephrin-A5(-/-) cataracts examined. A further examination using both SEM and TEM revealed that isolated globules were generated from the disintegrated interlocking protrusions originally located along the corners of hexagonal fiber cells. Freeze-fracture TEM further revealed the association of square-array aquaporin junctions with both isolated globules and interlocking membrane domains. This study reports for the first time that disrupted interlocking protrusions are the source of numerous large membranous globules that contribute to light scattering and nuclear cataracts in the ephrin-A5(-/-) mice. Our results further suggest that dissociations of N-cadherin and adherens junctions in the associated interlocking domains may result in the formation of isolated globules and nuclear opacities in the ephrin-A5(-/-) mice.

Regional changes of AQP0-dependent square array junction and gap junction associated with cortical cataract formation in the Emory mutant mouse

The Emory mutant mouse has been widely used as an animal model for human senile cataract since it develops late-onset hereditary cataract. Here, we focus on the regional changes of aquaporin-0 (AQP0) and connexins that are associated with the cortical cataract formation in the Emory mutant mice. Emory mutant and CFW wild-type mice at age 1-16 months were used in this study. By using an established photography system with dissecting microscopy, the opacities were first detected at the anterior or posterior lens center surface in Emory mice at age 7 months, and gradually extended toward the equator during the 16 months examined. Scanning EM verified that disorganized and fragmented fiber cells were associated with the areas of opacities within approximately 200 μm from the lens surface, indicating that Emory mouse cataracts belong to the cortical cataracts. Freeze-fracture TEM further confirmed that cortical cataracts exhibited extensive wavy square array junctions, small gap junctions and globules. Immunofluorescence analysis showed that in contrast to the high labeling intensity of AQP0-loop antibody, the labeling of AQP0 C-terminus antibody was decreased considerably in superficial fibers in Emory cataracts. Similarly, a significant decrease in the labeling of the antibody against Cx50 C-terminus, but not Cx46 C-terminus, occurred in superficial and outer cortical fibers in Emory cataracts. Western blotting further revealed that the C-termini of both AQP0 and Cx50 in Emory cataracts were decreased to over 50% to that of the wild-type. Thus, this systematic study concludes that the Emory mouse cataract belongs to the cortical cataract which is due to regional breakdown of superficial fibers associated with formation of AQP0-dependent wavy square array junctions, small gap junctions and globules. The marked decreases of the C-termini of both AQP0 and Cx50 in the superficial fibers may disturb the needed interaction between these two proteins during fiber cell differentiation and thus play a role in the cortical cataract formation in Emory mutant mice.

Ultrastructural analysis of the human lens fiber cell remodeling zone and the initiation of cellular compaction

The purpose is to determine the nature of the cellular rearrangements occurring through the remodeling zone (RZ) in human donor lenses, identified previously by confocal microscopy to be about 100 μm from the capsule. Human donor lenses were fixed with 10% formalin followed by 4% paraformaldehyde prior to processing for transmission electron microscopy. Of 27 fixed lenses, ages 22, 55 and 92 years were examined in detail. Overview electron micrographs confirmed the loss of cellular organization present in the outer cortex (80 μm thick) as the cells transitioned into the RZ. The transition occurred within a few cell layers and fiber cells in the RZ completely lost their classical hexagonal cross-sectional appearance. Cell interfaces became unusually interdigitated and irregular even though the radial cell columns were retained. Gap junctions appeared to be unaffected. After the RZ (40 μm thick), the cells were still irregular but more recognizable as fiber cells with typical interdigitations and the appearance of undulating membranes. Cell thickness was irregular after the RZ with some cells compacted, while others were not, up to the zone of full compaction in the adult nucleus. Similar dramatic cellular changes were observed within the RZ for each lens regardless of age. Because the cytoskeleton controls cell shape, dramatic cellular rearrangements that occur in the RZ most likely are due to alterations in the associations of crystallins to the lens-specific cytoskeletal beaded intermediate filaments. It is also likely that cytoskeletal attachments to membranes are altered to allow undulating membranes to develop.

Biological glass: structural determinants of eye lens transparency

The purpose of the lens is to project a sharply focused, undistorted image of the visual surround onto the neural retina. The first pre-requisite, therefore, is that the tissue should be transparent. Despite the presence of remarkably high levels of protein, the lens cytosol remains transparent as a result of short-range-order interactions between the proteins. At a cellular level, the programmed elimination of nuclei and other light-scattering organelles from cells located within the pupillary space contributes directly to tissue transparency. Scattering at the cell borders is minimized by the close apposition of lens fibre cells facilitated by a plethora of adhesive proteins, some expressed only in the lens. Similarly, refractive index matching between lens membranes and cytosol is believed to minimize scatter. Refractive index matching between the cytoplasm of adjacent cells is achieved through the formation of cellular fusions that allow the intermingling of proteins. Together, these structural adaptations serve to minimize light scatter and enable this living, cellular structure to function as 'biological glass'.

Adhesion and junction molecules in embryonic and adult lens cell differentiation

The expression of the neural cell adhesion molecule NCAM and its polysialic acid (PSA) moiety was documented during embryonic development and adult differentiation of chicken lens cells. In both the embryo and adult, NCAM is predominantly found in the epithelium and the zone of young elongating cells of the annular pad. NCAM abundance drops markedly in the cortical fibers and is further reduced in the lens nucleus. Epithelial cell NCAM is more highly poly-sialylated in the adult than in the embryonic lens. Three isoforms of NCAM at 180, 140, and 120 kDa were detected in the lens and predominantly associated with the unit membrane-enriched plasma membranes of fiber cells. The distribution of NCAM relative to MP26 and the adherence junction-associated glycoprotein N-cadherin suggests that NCAM could influence the formation of fiber cell gap junctions and adherence junctions.

The role of the lens actin cytoskeleton in fiber cell elongation and differentiation

The vertebrate ocular lens is a fascinating and unique transparent tissue that grows continuously throughout life. During the process of differentiation into fiber cells, lens epithelial cells undergo dramatic morphological changes, membrane remodeling, polarization, transcriptional activation and elimination of cellular organelles including nuclei, concomitant with migration towards the lens interior. Most of these events are presumed to be influenced in large part, by dynamic reorganization of the cellular actin cytoskeleton and by intercellular and cell: extracellular matrix interactions. In light of recent and unprecedented advancement in our understanding of the mechanistic bases underlying regulation of actin cytoskeletal dynamics and the role of the actin cytoskeleton in cell function, this review attempts to summarize current knowledge regarding the role of the cellular actin cytoskeleton, in lens fiber cell elongation and differentiation, and regulation of actin cytoskeletal organization in the lens.

Lens opacity and photoreceptor degeneration in the zebrafish lens opaque mutant

The zebrafish lens opaque (lop) mutant was identified in a chemical mutagenesis screen. The lop mutant, which develops normally through 4 days postfertilization (dpf), exhibits several signs of lens and retinal degeneration at 7 dpf. Histology revealed disrupted lens fibers and increased numbers of nucleated cells within the mutant lens and anterior chamber. The mutant lens also exhibited aberrant epithelial cell morphologies and lacked a definitive transition zone, which suggests that secondary fiber differentiation was interrupted. In addition, the mutant exhibits severely reduced photoreceptors and a reduction in the number of horizontal cells at 7 dpf. Other retinal cell classes appeared unaffected in the mutant. Transmission electron microscopy and opsin immunohistochemistry showed that the different photoreceptor types were generated at the retinal margin, but the rods and cones failed to mature and disappeared. The mutant lens and retina also displayed increased cell proliferation based on proliferating cell nuclear antigen immunolabeling, suggesting that the lens opacity was due to unregulated cell proliferation and undifferentiated cell accumulation within the mutant lens. The lop mutant phenotype supports recent studies showing the lens has a role in regulating teleost retinal development.

ABI2-deficient mice exhibit defective cell migration, aberrant dendritic spine morphogenesis, and deficits in learning and memory

The Abl-interactor (Abi) family of adaptor proteins has been linked to signaling pathways involving the Abl tyrosine kinases and the Rac GTPase. Abi proteins localize to sites of actin polymerization in protrusive membrane structures and regulate actin dynamics in vitro. Here we demonstrate that Abi2 modulates cell morphogenesis and migration in vivo. Homozygous deletion of murine abi2 produced abnormal phenotypes in the eye and brain, the tissues with the highest Abi2 expression. In the absence of Abi2, secondary lens fiber orientation and migration were defective in the eye, without detectable defects in proliferation, differentiation, or apoptosis. These phenotypes were consistent with the localization of Abi2 at adherens junctions in the developing lens and at nascent epithelial cell adherens junctions in vitro. Downregulation of Abi expression by RNA interference impaired adherens junction formation and correlated with downregulation of the Wave actin-nucleation promoting factor. Loss of Abi2 also resulted in cell migration defects in the neocortex and hippocampus, abnormal dendritic spine morphology and density, and severe deficits in short- and long-term memory. These findings support a role for Abi2 in the regulation of cytoskeletal dynamics at adherens junctions and dendritic spines, which is critical for intercellular connectivity, cell morphogenesis, and cognitive functions.

Identification of caveolae and their signature proteins caveolin 1 and 2 in the lens

This study shows that caveolae are present in lens epithelia of rabbit and guinea pig under normal conditions. Caveolae are unique lipid membrane microdomains observed in many cell types. They are believed to play crucial roles in a variety of basic physiological functions including signal transduction, lipid and transcellular transport. Using TEM, immunocytochemistry and immunoblotting we show for the first time the existence of caveolae and the co-localization of their signature marker integral proteins, caveolin-1 and caveolin-2, in the intact lens of rabbit and guinea pig. Thin-section TEM shows that among several species studied, lens epithelia of rabbit and guinea pig exhibited a large number of caveolae. The caveolae were pear shaped, approximately 70 nm in diameter, and were found frequently along the lateral membranes of epithelial cells in the intact lens. In the intact cortical fibers, only a small number of caveolae was seen in the superficial cells. In cultured lens epithelial cells, however, caveolae were observed along all membrane surfaces, but were more abundant at the apical membrane of the cells. Immunofluorescence and immunoblot analyses confirmed the presence of caveolin-1 and caveolin-2 in the lens epithelium. In addition, caveolin-1 and caveolin-2 co-exist in the lens epithelium of both rabbit and guinea pig. HRP tracer study demonstrated that caveolae could carry out endocytosis, suggesting their involvement in molecular transport. Cultured rabbit lens epithelial cells (line N/N1003A) were used to examine the response of caveolae to methyl-beta-cyclodextrin (MBCD), a specific cholesterol-depleting drug. The lens epithelial cells were incubated in freshly prepared MEM medium plus 8% rabbit serum containing 10mm MBCD for 0 (control), 15, 30 or 60 min. Controls for MBCD treatment were cultured in MEM plus 8% rabbit serum. MBCD treatment for 30 min revealed that depletion of cholesterol abolished the majority of caveolae in cultured lens epithelial cells. This result strongly suggests that caveolae are cholesterol-rich lipid rafts that are likely to play important roles in the lens.

Microtubule configuration and membranous vesicle transport in elongating fiber cells of the rat lens

This study examines the microtubule configuration and its close association with the Golgi complex and Golgi-derived membranous vesicles in elongating fiber cells of the rat lens. Since fiber cells elongate tremendously during lens differentiation, we hypothesize that a microtubule-based motor system exists in the elongating fiber cells for transporting important membrane proteins and organelles to the target regions for cell growth. The newly synthesized membrane proteins are known to be transported from the trans-Golgi network in the form of vesicles to the target plasma membrane. By thin-section TEM, we observed a large number of vesicles of various sizes and shapes randomly distributed throughout the cytoplasm of elongating fiber cells. Both Golgi complex and vesicles exhibited characteristic normal structural features seen in other cell types and thus represented real vesicular organelles in the fiber cells. A large number of microtubules were regularly arranged into bundles parallel to the long axis of fiber cells as examined in both longitudinal and cross-section views. Many of these microtubules were closely associated or in intimate contact with the Golgi complex and vesicles in elongating fiber cells. The microtubule polarity assay revealed that microtubules exhibited a unidirectional polarity for the entire length of fiber cells as examined in both anterior and posterior cortical fiber segments. Namely, the minus end of microtubules was towards the anterior lens pole while the plus end was headed towards the posterior pole. This suggests that multiple molecular motors such as kinesin and dynein are needed for carrying the vesicles to both lens poles, since conventional kinesin is known to transport vesicular organelles towards the plus end whereas cytoplasmic dynein carries them towards the minus end of microtubules. By immunoblot analysis, we indeed detected the presence of both kinesin (120 kD) and dynein (70 kD) in homogenate prepared from lens cortical fibers. Moreover, immunogold TEM demonstrated that the aquaporin 0 (formally MIP26) antibody was localized on the membranous vesicles as well as plasma membranes of the cortical fiber cells. This study suggests that a microtubule-based motor system exists in the lens and plays an important role in transporting membrane proteins such as aquaporin 0 in the vesicles during fiber cell differentiation and elongation.

Association of clathrin, AP-2 adaptor and actin cytoskeleton with developing interlocking membrane domains of lens fibre cells

Interlocking membrane domains are specialized membrane interdigitations in the form of ball-and-sockets and protrusions between lens fibre cells of all species. They are believed to play a key role in maintaining fibre-fibre stability and are therefore, important for normal lens function. Here we report the specific association of the clathrin/AP-2 adaptor complex and the branching F-actin network with the development of interlocking domains in rats and several other species. By thin-section electron microscopy we consistently observed a layer of distinct coating (approximately 25-nm thick) on the concave membrane surface of small and intermediate-sized developing interlocking domains. These membrane coats remarkably resembled the clathrin-coat of endocytic vesicles in which clathrin and the AP-2 adaptor are involved in the induction of coated pit formation during receptor-mediated endocytosis. We hypothesize that the clathrin/AP-2 complex is directly involved in the induction of interlocking domains in fibre cells. By immunoconfocal microscopy, co-labelling of a dotted-pattern of clathrin and AP-2 adaptor antibodies was seen along the cortical fibre cells. Immunoblot analysis further confirmed that clathrin and AP-2 adaptor antibodies specifically stained a polypeptide band of 180 and 106kD, respectively, in the membrane fractions prepared separately from the outer and inner cortical fibres where interlocking domains are abundant but endocytic vesicles are absent. Immunoelectron microscopy showed that the clathrin antibody was localized along the interlocking membrane. In addition, branching actin filament networks were frequently observed within the cytoplasmic compartment of developing interlocking domains by TEM, in consistent with the findings by fluorescence and immunogold labelling of the F-actin antibody in the domains. These results demonstrate for the first time that the clathrin/AP-2 complex plays a new role for the formation of interlocking domains in lens fibre cells. Branching actin networks and possibly other cytoskeletal components are also associated with the development and maintenance of these interlocking domains. The coordinated 'pulling and pushing' actions generated by the clathrin/AP-2 complex and branching actin networks during interlocking domain formation are discussed.

Pharmacologic prevention of posterior capsule opacification: in vitro effects of preservative-free lidocaine 1% on lens epithelial cells

PURPOSE

To assess the in vitro effectiveness of preservative-free lidocaine 1% in removing lens epithelial cells (LECs) from the anterior capsule and to evaluate the effect of lidocaine on the LECs.

SETTING

Center for Research on Ocular Therapeutics and Biodevices, Storm Eye Institute, Medical University of South Carolina, Charleston, South Carolina, USA.

METHODS

Eight rabbits (16 eyes) were used in the study. After the rabbits were killed, the eyes were enucleated and divided into 2 groups. In Group 1 (n = 8 eyes), LECs were exposed to preservative-free lidocaine 1% or balanced salt solution (BSS) for 1, 2, or 5 minutes. The anterior capsules were then stained with trypan blue and alizarin red. Photomicrographs of each capsule were taken and analyzed for LEC damage. In Group 2 (n = 8 eyes), hydrodissection was performed with 1 of the agents, followed by phacoemulsification and cortical cleanup. The LEC attachment to the anterior capsule was evaluated by histopathology.

RESULTS

Anterior capsule fragments irrigated with BSS showed no LEC nuclear staining; ie, no direct toxic effect. In those irrigated with preservative-free lidocaine 1%, the LECs showed mild toxicity; some cells showed blue nuclear staining. After hydrodissection with lidocaine, the capsules were almost free of LECs; after hydrodissection with BSS, the capsules showed a normal layer of LECs attached to the anterior capsule.

CONCLUSIONS

Preservative-free lidocaine 1% may help diminish the amount of live LECs by facilitating cortical cleanup, by loosening the desmosomal area of cell-cell adhesion with decreased cellular adherence, or by a direct toxic effect. The use of this agent may help prevent posterior capsule opacification.

Spatiotemporal distribution of zonulae adherens and associated actin bundles in both epithelium and fiber cells during chicken lens development

Zonulae adherens and associated actin bundles (ZA/AB) are believed to play a major role in epithelial folding and invagination during morphogenesis of neural tube and other vesicular structures. The lens morphogenesis is associated with the formation of the lens vesicle in which ZA/AB would be needed during the formation process. However, the existence of ZA/AB in the lens has never been established. In this study we report for the first time the existence of ZA/AB in both lens epithelium and fiber cells during embryonic development of chicken lens from E4 to E20. Light microscopy revealed contacts between the lens epithelium and primary fiber cells, and between the lens epithelium and secondary fiber cells at E4 and E11, respectively. Thin-section electron microscopy consistently revealed ZA/AB near both the apical ends of lens epithelial cells and primary fiber cells at E4. This arrangement manifests as a parallel pair of belt-like ZA/AB along the epithelium-fiber interface. In semi-tangential sections, a continuous belt-like ZA/AB was also evidenced in individual epithelial cells and fiber cells. Furthermore, the same ZA/AB arrangement was observed near both the apical ends of epithelial cells and secondary fiber cells at E11. Besides ZA/AB, macular-type fasciae adherens were distributed regularly between epithelial cells, between primary fibers, between secondary fibers, and between epithelium and both primary and secondary fibers. Immunofluorescence strongly and preferentially labeled N-cadherin at both the apical ends of lens epithelium and primary or secondary fibers at the corresponding ages, suggesting a direct association with the zonulae adherens. Also punctate N-cadherin labeling was commonly seen along various regions of primary and secondary fiber cells at different ages, and to a larger extent in the mature fibers of older lenses. This study suggests that: (1) ZA/AB located at the apices of lens epithelial cells may play a crucial role in the early stages of lens morphogenesis (e.g. lens vesicle formation); (2) ZA/AB of primary and secondary fiber cells originate from the epithelial cells during their elongation and differentiation; (3) owing to the restricted distribution of ZA/AB, abundant fasciae adherens are needed to maintain the structural stability of the epithelium and fiber cells during development and maturation; and (4) N-cadherin is the principle adhesion protein for both the zonulae adherens and fasciae adherens in the lens.

Structure, development and function of cytoskeletal elements in non-neuronal cells of the human eye

The cytoskeleton, of which the main components in the human eye are actin microfilaments, intermediate filaments and microtubules with their associated proteins, is essential for the normal growth, maturation, differentiation, integrity and function of its cells. These components interact with intra- and extracellular environment and each other, and their profile frequently changes during development, according to physiologic demands, and in various diseases. The ocular cytoskeleton is unique in many ways. A special pair of cytokeratins, CK 3 and 12, has apparently evolved only for the purposes of the corneal epithelium. However, other cytokeratins such as CK 4, 5, 14, and 19 are also important for the normal ocular surface epithelia, and other types may be acquired in keratinizing diseases. The intraocular tissues, which have a relatively simple cytoskeleton consisting mainly of vimentin and simple epithelial CK 8 and 18, differ in many details from extraocular ones. The iris and lens epithelium characteristically lack cytokeratins in adults, and the intraocular muscles all have a cytoskeletal profile of their own. The dilator of the iris contains vimentin, desmin and cytokeratins, being an example of triple intermediate filament expression, but the ciliary muscle lacks cytokeratin and the sphincter of the iris is devoid even of vimentin. Conversion from extraocular-type cytoskeletal profile occurs during fetal life. It seems that posttranslational modification of cytokeratins in the eye may also differ from that of extraocular tissues. So far, it has not been possible to reconcile the cytoskeletal profile of intraocular tissues with their specific functional demands, but many theories have been put forward. Systematic search for cytoskeletal elements has also revealed novel cell populations in the human eye. These include transitional cells of the cornea that may represent stem cells on migration, myofibroblasts of the scleral spur and juxtacanalicular tissue that may modulate aqueous outflow, and subepithelial matrix cells of the ciliary body and myofibroblasts of the choroid that may both participate in accommodation. In contrast to the structure and development of the ocular cytoskeleton, changes that take place in ocular disease have not been analysed systematically. Nevertheless, potentially meaningful changes have already been observed in corneal dystrophies (Meesmann's dystrophy, posterior polymorphous dystrophy and iridocorneal endothelial syndrome), degenerations (pterygium) and inflammatory diseases (Pseudomonas keratitis), in opacification of the lens (anterior subcapsular and secondary cataract), in diseases characterized by proliferation of the retinal pigment epithelium (macular degeneration and proliferative vitreoretinopathy), and in intraocular tumours (uveal melanoma). In particular, upregulation of alpha-smooth muscle actin seems to be a relatively general response typical of spreading and migrating corneal stromal and lens epithelial cells, trabecular cells and retinal pigment epithelial cells.

Actin filament bundles in cortical fiber cells of the rat lens

The distribution and organization of actin filament bundles were studied in cortical fiber cells of rat lenses at various ages (3 days to 2.5 months old), using thin-section electron microscopy, immunocytochemistry and immunoblotting. Electron microscopy showed that actin bundles were regularly found along cortical fiber cell membranes of the lens at all ages studied. The actin bundles were commonly arranged in three distinct units, one bundle in each fiber cell, located at the intersections where three hexagonal fiber cells meet as seen in cross sections. These actin bundles were approximately 150 nm in diameter and were composed of 7-nm small filaments. They were aligned parallel to the long axis of fiber cells as judged by both cross and longitudinal sections. The outside border of each bundle was always surrounded by a zone of 10-nm intermediate filaments which have the same orientation as that of the actin bundles. In longitudinal sections, elongated actin bundles were always parallel to the cell membranes. A number of individual actin bundles sometimes were found to form a chain with periodic short intervals. In addition, actin bundles were frequently associated with adherens junctions near the intersections and other regions of fiber cell membranes. By immunoelectron microscopy, we demonstrated that these filament bundles indeed contained actins. By rhodamine-phalloidin labelling, we found that labeled actin bundles appeared as large, distinct dots at the corners of hexagonal fiber cells in all ages studied. In addition, non-bundle F-actins were labeled preferentially along the cell membranes of the short sides of hexagonal fiber cells. This resulted in a unique zigzag pattern of actin labeling commonly seen in the cortical fiber cells of a mature rat lens. Finally, we showed that alpha-actinin was associated with the actin bundles in the fiber cells by immunofluorescent double labeling and immunoblotting. It is suggested that this unique arrangement of actin bundles in fiber cells may provide a stabilizing structure for forming a sharp angle at each corner of fiber cells, thereby the hexagonal shape of the cells can be maintained.

Phorbol esters affect cyclic nucleotide-mediated responses in cultured chick lens annular pad cells

Cultured chick lens annular pad cells were treated with a lipid soluble cAMP analog, the phorbol ester TPA or a combination of the two compounds in order to assess their effects on mitotic activity, cell spreading and the accumulation of differentiation marker proteins. Both 8b-cAMP and TPA were individually able to inhibit mitotic activity in cells cultured in the presence of 5% serum. The combination of the two produced a greater degree of mitotic inhibition. Both compounds were also able to inhibit cellular spreading onto laminin coated surfaces. Opposite effects on the accumulation of differentiation marker proteins were observed for the two compounds. While 8b-cAMP increased levels of marker proteins, TPA or the combination of TPA and 8b-cAMP reduced levels of marker proteins. These data indicate that crosstalk between two distinct signal transduction systems in the lens is able to influence cell behaviors implicated in the development of secondary or posterior subcapsular cataract. In addition, these data demonstrate that both positive and negative regulatory influences affect the accumulation of differentiated characteristics.

The ultrastructure of epithelial and fiber cells in the crystalline lens

Crystalline lenses are often simply described as inside-out stratified epithelial-like organs composed of uniform (hexagonal cross-section profiles) crescent-like cells, arranged end-to-end in concentric shells around a polar axis. In this manner, as light is transmitted through lenses, their highly ordered architecture contributes to transparency by effectively transforming the multicellular organ into a series of coaxial refractive surfaces. This review will attempt to demonstrate that such a description seriously understates the structural complexity that produces lenses of variable optical quality in different species as a function of development, growth, and age. Embryological development of the lens occurs in a similar manner in all species. However, the growth patterns and effects of aging on lens fibers varies significantly among species. The terminally differentiated fiber cells of all lenses are generally hexagonal in cross section and crescent shaped along their length. But, while the fibers of all lenses are arranged in both highly ordered radial cell columns and concentric growth shells, only avian lens fibers are meridian-like, extending from pole to pole. In all other species, two types of fibers defined by different shapes are continuously formed throughout life. The majority of fibers are s-shaped, with ends that do not extend to the poles. Rather, the ends of these fibers are arranged as latitudinal arc lengths within and between growth shells. The overlap of the ends of specifically defined groups of such fibers constitutes the lens suture branches. The location, number, and extent of suture branches within and between growth shells are important considerations in lens function because the shapes of fiber ends, unlike that along fiber length, are very irregular. Consequently, as light is transmitted through sutures, spherical aberration (i.e., focal length variation) is increased. The degree of focal length variability depends on the arrangement of suture branches within and between growth shells, and this architecture varies significantly between species. The lifelong production of additional fibers at the circumference of the lens, culminating in new growth shells, neither proceeds equally around the lens equator, nor features identical fibers formed around the equator. Suture formation commences in the inferonasal quadrant, and continues sequentially in the superotemporal, inferotemporal, and finally the superonasal quadrants. During this process, lens growth produces fibers of specifically defined length and shape as a function of their equatorial location.(ABSTRACT TRUNCATED AT 250 WORDS)

The zones of discontinuity in the human lens: development and distribution with age

Statistical analysis of Scheimpflug images from the crystalline lenses of 100 emmetropic human subjects ranging in age from 18 to 70 yr confirms that specific zones of discontinuity are a function of lens development and growth. At and beyond the age of 40 yr, as many as four sharply demarcated and complementary zones are seen within the anterior and posterior lens cortex. The locations of the inner edges of the anterior cortical zones of discontinuity were characterized relative to the central sulcus of the lens. Consecutively from the central sulcus, the distances were 1.094, 1.415, 1.695, and 1.994 (+/- 0.11 mm). Since nuclear thickness in the adult lens is age-independent and the rate of cortical growth has been characterized, the location of the inner margins of the zones are indicative of the age at which they originated; these ages were 4 (+/- 1 yr), 9 (+/- 2 yr), 19 (+/- 4yr), and 46 (+/- 10 yr). All of the zones become broader along the outer margin and more dense upon aging, with specific zones appearing to merge in older presbyopic lenses. While lens fetal nuclear transparency decreases with age, it does not feature zones of discontinuity; instead, symmetrically amorphous regions appear centrally in the anterior and posterior nucleus. This demonstration of the onset of specific zones of discontinuity in emmetropic individuals, at defined periods of lens growth that are synchronous the production of successively more complex lens sutures, strongly suggests a causal relationship between lens sutures and the zones of discontinuity.

Multiple structural types of gap junctions in mouse lens

Gap junctions in the epithelium and superficial fiber cells from young mice were examined in lenses prepared by rapid-freezing, and processed for freeze-substitution and freeze-fracture electron microscopy. There appeared to be three structural types of gap junction: one type between epithelial cells and two types between fiber cells. Epithelial gap junctions seen by freeze-substitution were approximately 20 nm thick and consistently associated with layers of dense material lying along both cytoplasmic surfaces. Fiber gap junctions, in contrast, were 15–16 nm (type 1) or 17–18 nm thick (type 2), and had little associated cytoplasmic material. Type 1 fiber gap junctions were extensive in flat expanses of cell membrane and had a thin, discontinuous central lamina, whereas type 2 fiber gap junctions were associated with the ball-and-socket domains and exhibited a dense, continuous central lamina. Both types of fiber gap junction had a diffuse arrangement of junctional intramembrane particles, whereas particles and pits of epithelial gap junctions were in a tight, hexagonal configuration. The type 2 fiber gap junctions, however, had a larger particle size (approximately 9 nm) than the type 1 (approximately 7.5 nm). In addition, a large number of junctional particles typified the E-faces of both fiber types but not the epithelial type of gap junction. Gap junctions between fiber and epithelial cells had structural features of type 1 fiber gap junctions.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of murine Caraparu Bunyavirus liver infection and immunomodulator-mediated antiviral protection

A rapid, peripheral disease model utilizing the Bunyavirus, Caraparu, was established in mice for the evaluation of antiviral therapy with immunomodulators. 4-6-week-old B6C3F1 female mice, inoculated intraperitoneally with virus, developed coagulative liver necrosis and died between 4-6 days after infection. This Caraparu disease model was relatively resistant to treatment with immunomodulators, such as ABMP, Ampligen, alpha-interferon (IFN-alpha) or beta-interferon (IFN-beta). However, a significant increase in median survival time (MST) was consistently observed upon treatment with gamma-interferon (IFN-gamma). The nucleoside analog--ribavirin--was highly effective against Caraparu virus in repeated treatment schedules begun on either day -1, day 0, or day +1 of infection. Ribavirin gave little protection when initiation of treatment was delayed until day +2. However, combined treatment with IFN-gamma, starting on day 0 and ribavirin starting on day +2, significantly reduced mortality.

Nature and localization of avian lens glycogen by electron microscopy and Raman spectroscopy

Electron microscopy confirms the presence of a high concentration of glycogen particles in the lens nuclear region of birds of flying habit such as the ring-neck dove and pigeon. This observation is consistent with Raman spectroscopy. The glycogen particles in the dove lens, which are approximately 35 nm in diameter, are classified as beta type particles. Although this type has been previously characterized by high rates of glycogen turnover in other tissues, its localization in the lens nucleus indicates that it may serve a structural function rather than as a storage depot of carbohydrate in the lens. In a comparative electron microscopy study, glycogen particles were not observed in the chicken lens.

N-cadherin detected in the membrane fraction of lens fiber cells

N-cadherin was identified as a glycoprotein present in the fiber cell membranes of frog, chick, bovine, rabbit and human lenses. The molecular size of N-cadherin varies with the species. Homogenization of the chick lens in the presence of Ca2+ resulted in a decrease in the concentration of N-cadherin. This suggests that the lens contains a Ca2(+)-activated protease which can act on N-cadherin.

Visualization of crystallin droplets associated with cold cataract formation in young intact rat lens

The structure, distribution, and nature of scattering elements associated with cold cataract formation in the young rat lens were studied in situ using light and electron microscopy and ImmunoGold electron microscopy. A large accumulation of spherical droplets, ranging from approximately 1.5 microns to 10 microns in diameter, were found in the lens nucleus in cold cataracts induced at 22 degrees C or 4 degrees C in TC-199 culture medium. Many droplets of all sizes were associated with the cell membranes. A cooled and then rewarmed lens was found to lose its opacity and subsequently no droplets were observed, indicating that there was a good correlation between the onset of opacification and the formation of droplets. Electron microscopy showed that droplets were composed of homogeneous electron-dense aggregates without limiting membranes. ImmunoGold study revealed that alpha-, beta-, and gamma-crystallins were all present within each droplet. This study demonstrates that extensive accumulation of the crystallin droplets in the lens nucleus is the contributing factor for the light scattering and opacification of the cold cataract in the young intact rat lens.