Expression of the gene for main intrinsic polypeptide (MIP): separate spatial distributions of MIP and beta-crystallin gene transcripts in rat lens development

Control of lens development by Lhx2-regulated neuroretinal FGFs

Fibroblast growth factor (FGF) signaling is an essential regulator of lens epithelial cell proliferation and survival, as well as lens fiber cell differentiation. However, the identities of these FGF factors, their source tissue and the genes that regulate their synthesis are unknown. We have found that Chx10-Cre;Lhx2lox/lox mice, which selectively lack Lhx2 expression in neuroretina from E10.5, showed an early arrest in lens fiber development along with severe microphthalmia. These mutant animals showed reduced expression of multiple neuroretina-expressed FGFs and canonical FGF-regulated genes in neuroretina. When FGF expression was genetically restored in Lhx2-deficient neuroretina of Chx10-Cre;Lhx2lox/lox mice, we observed a partial but nonetheless substantial rescue of the defects in lens cell proliferation, survival and fiber differentiation. These data demonstrate that neuroretinal expression of Lhx2 and neuroretina-derived FGF factors are crucial for lens fiber development in vivo.

Primary cultures of embryonic chick lens cells as a model system to study lens gap junctions and fiber cell differentiation

A major limitation in lens gap junction research has been the lack of experimentally tractable ex vivo systems to study the formation and regulation of fiber-type gap junctions. Although immortalized lens-derived cell lines are amenable to both gene transfection and siRNA-mediated knockdown, to our knowledge none are capable of undergoing appreciable epithelial-to-fiber differentiation. Lens central epithelial explants have the converse limitation. A key advance in the field was the development of a primary embryonic chick lens cell culture system by Drs. Sue Menko and Ross Johnson. Unlike central epithelial explants, these cultures also include cells from the peripheral (preequatorial and equatorial) epithelium, which is the most physiologically relevant population for the study of fiber-type gap junction formation. We have modified the Menko/Johnson system and refer to our cultures as dissociated cell-derived monolayer cultures (DCDMLs). We culture DCDMLs without serum to mimic the avascular lens environment and on laminin, the major matrix component of the lens capsule. Here, I review the features of the DCDML system and how we have used it to study lens gap junctions and fiber cell differentiation. Our results demonstrate the power of DCDMLs to generate new findings germane to the mammalian lens and how these cultures can be exploited to conduct experiments that would be impossible, prohibitively expensive and/or difficult to interpret using transgenic animals in vivo.

Quantitative analysis of ascorbic acid permeability of aquaporin 0 in the lens

Aquaporin 0 (AQP0) is a lens-specific protein comprising more than 30% of lens membrane protein content and is a member of the aquaporin family. Water permeates through AQP0 much more slowly than other aquaporin family members, and other compounds, such as glycerol, also permeate AQP0. In the lens, ascorbic acid (AA) is found at high concentrations, protecting the lens from photochemical events such as photo-oxidation. The aim of the present study was to clarify the function of AQP0. Mouse fibroblast L-cells stably expressing AQP0 were established and incubated in medium containing AA, and intracellular AA levels were measured by high-performance liquid chromatography (HPLC) and 2,6-dichlorophenol-indophenol (DCPIP) analysis. Intracellular AA levels in AQP0-expressing cells quickly rose and reached saturation 10 min after incubation in medium containing 1000 μM AA. In contrast, AA levels in cells slowly decreased when AA was washed out from the medium. Cells overexpressing AQP0 increased the cellular uptake of AA in a time- and concentration-dependent manner. These data suggest that AA as well as water permeates AQP0. AQP0 expression on Xenopus oocyte membranes was achieved by the injection of AQP0 cRNA into oocytes that were incubated in medium containing AA. Intracellular AA levels were then measured by HPLC. AA uptake was demonstrated in the AQP0-expressing oocytes and was shown to quickly reach saturation. Intracellular AA concentration in oocytes increased in a time- and concentration-dependent manner. The data in the present study show that AA permeates AQP0, reveal the role of AQP0 in AA permeability ex vivo, and also indicate that there is a difference between the import and export of AA via AQP0. These findings suggest that AQP0 plays an important role in controlling lens AA content.

Expression of βA3/A1-crystallin in the developing and adult rat eye

Crystallins are very abundant structural proteins of the lens and are also expressed in other tissues. We have previously reported a spontaneous mutation in the rat βA3/A1-crystallin gene, termed Nuc1, which has a novel, complex, ocular phenotype. The current study was undertaken to compare the expression pattern of this gene during eye development in wild type and Nuc1 rats by in situ hybridization (ISH) and immunohistochemistry (IHC). βA3/A1-crystallin expression was first detected in the eyes of both wild type and Nuc1 rats at embryonic (E) day 12.5 in the posterior portion of the lens vesicle, and remained limited to the lens fibers throughout fetal life. After birth, βA3/A1-crystallin expression was also detected in the neural retina (specifically in the astrocytes and ganglion cells) and in the retinal pigmented epithelium (RPE). This suggested that βA3/A1-crystallin is not only a structural protein of the lens, but has cellular function(s) in other ocular tissues. In summary, expression of βA3/A1-crystallin is controlled differentially in various eye tissues with lens being the site of greatest expression. Similar staining patterns, detected by ISH and IHC, in wild type and Nuc1 animals suggest that functional differences in the protein, rather than changes in mRNA/protein level of expression, likely account for developmental abnormalities in Nuc1.

Structural function of MIP/aquaporin 0 in the eye lens; genetic defects lead to congenital inherited cataracts

Aquaporin 0 (AQP0) was originally characterized as a membrane intrinsic protein, specifically expressed in the lens fibers of the ocular lens and designated MIP, for major intrinsic protein of the lens. Once the gene was cloned, an internal repeat was identified, encoding for the amino acids Asp-Pro-Ala, the NPA repeat. Shortly, the MIP gene family was emerging, with members being characterized in mammals, insects, and plants. Once Peter Agre's laboratory developed a functional assay for water channels, the MIP family became the aquaporin family and MIP became known as aquaporin 0. Besides functioning as a water channel, aquaporin 0 also plays a structural role, being required for maintaining the transparency and optical accommodation of the ocular lens. Mutations in the AQP0 gene in human and mice result in genetic cataracts; deletion of the MIP/AQP0 gene in mice results in lack of suture formation required for maintenance of the lens fiber architecture, resulting in perturbed accommodation and focus properties of the ocular lens. Crystallography studies support the notion of the double function of aquaporin 0 as a water channel (open configuration) or adhesion molecule (closed configuration) in the ocular lens fibers. The functions of MIP/AQP0, both as a water channel and an adhesive molecule in the lens fibers, contribute to the narrow intercellular space of the lens fibers that is required for lens transparency and accommodation.

Functional expression of aquaporins in embryonic, postnatal, and adult mouse lenses

Aquaporin 0 (AQP0) and AQP1 are expressed in the lens, each in a different cell type, and their functional roles are not thoroughly understood. Our previous study showed that these two AQPs function as water transporters. In order to further understand the functional significance of these two different aquaporins in the lens, we investigated their initiation and continued expression. AQP0 transcript and protein were first detected at embryonic stage (E) 11.25 in the differentiating primary fiber cells of the developing lens; its synthesis continued through the adult stage in the secondary fiber cells. Low levels of AQP1 expression were first seen in lens anterior epithelial cells at E17.5; following postnatal day (P) 6.5, the expression gradually progressed towards the equatorial epithelial cells. In the postnatal lens, the increase in membrane water permeability of epithelial cells and lens transparency coincides with the increase in AQP1 expression. AQP1 expression reaches its peak at P30 and continues through the adult stage both in the anterior and equatorial epithelial cells. The enhancement in AQP1 expression concomitant with the increase in the size of the lens suggests the progression in the establishment of the lens microcirculatory system. In vitro and in vivo studies show that both aquaporins share at least one important function, which is water transport in the lens microcirculatory system. However, the temporal expression of these two AQPs suggests an apparently unique role/s in lens development and transparency. To our knowledge, this is the first report on the expression patterns of AQP0 and AQP1 during lens development and differentiation and their relation to lens transparency.

Developmental Regulation of the Direct Interaction between the Intracellular Loop of Connexin 45.6 and the C Terminus of Major Intrinsic Protein (Aquaporin-0)

The eye lens is dependent upon a network of gap junction-mediated intercellular communication to facilitate its homeostasis and development. Three gap junction-forming proteins are expressed in the lens of which two are in lens fibers, namely connexin (Cx) 45.6 and 56. Major intrinsic protein (MIP), also known as aquaporin-0 (AQP0), is the most abundant membrane protein in lens fibers. However, its role in the lens is not clear. Our previous studies show that MIP(AQP0) associates with gap junction plaques formed by Cx45.6 and Cx56 during the early stages of embryonic chick lens development but not in late embryonic and adult lenses. We report here that MIP(AQP0) directly interacts with Cx45.6 but not with Cx56. We further identified the intracellular loop of Cx45.6 as the interacting domain for the MIP(AQP0) C terminus. Surface plasmon resonance experiments indicated that the C-terminal domain of MIP(AQP0) interacts with two binding sites within the intracellular loop region of Cx45.6 with a K(D(app)) of 7.5 and 10.3 microm, respectively. The K(D(app)) for the full-length loop region is 7.7 microm. The cleavage at the intracellular loop of Cx45.6 was observed during lens development, and the C terminus of MIP(AQP0) did not interact with the loop-cleaved form of Cx45.6. Thus, the dissociation between these two proteins that occurs in the mature fibers of late lens development is likely caused by this cleavage. Finally this interaction had no impact on Cx45.6-mediated intercellular communication, suggesting that the Cx45.6-MIP(AQP0) interaction plays a novel unidentified role in lens fibers.

Wnt signaling enhances FGF2-triggered lens fiber cell differentiation

Wnt signaling is implicated in many developmental processes, including cell fate changes. Several members of the Wnt family, as well as other molecules involved in Wnt signaling, including Frizzled receptors, LDL-related protein co-receptors, members of the Dishevelled and Dickkopf families, are known to be expressed in the lens during embryonic or postembryonic development. However, the function of Wnt signaling in lens fiber differentiation remains unknown. Here, we show that GSK-3β kinase is inactivated and thatβ-catenin accumulates during the early stages of lens fiber cell differentiation. In an explant culture system, Wnt conditioned medium (CM)induced the accumulation of β-crystallin, a marker of fiber cell differentiation, without changing cell shape. In contrast, epithelial cells stimulated with Wnt after priming with FGF elongated, accumulatedβ-crystallin, aquaporin-0, p57kip2, and altered their expression of cadherins. Treatment with lithium, which stabilizes β-catenin, induced the accumulation of β-crystallin, but explants treated with lithium after FGF priming did not elongate as they did after Wnt application. These results show that Wnts promote the morphological aspects of fiber cell differentiation in a process that requires FGF signaling, but is independent ofβ-catenin. Wnt signaling may play an important role in lens epithelial-to-fiber differentiation.

Interaction of major intrinsic protein (aquaporin-0) with fiber connexins in lens development

We observed that chick lens-fiber gap-junction-forming proteins, connexin (Cx) 45.6 and Cx56, were associated with an unknown protein, which was then identified as major intrinsic protein (MIP), also known as aquaporin-0 (AQP0), the most abundant membrane protein in lens fibers. A 1063 bp cDNA of chick MIP(AQP0) was identified that encodes a 262 amino acid protein with a predicted molecular weight of 28.1 kDa. Dual immunofluorescence and confocal microscopy of sagittal and coronal sections of the lens tissues showed that MIP(AQP0) consistently localized with gap junction plaques formed by Cx45.6 and Cx56 during the early stages of embryonic chick lens development. Immunoprecipitation combined with immunoblotting analyses revealed that MIP(AQP0) was associated with Cx45.6 and Cx56 at these developmental stages. The specificity of this interaction was further confirmed with the silver staining of the protein components of immunoprecipitates. The pull-down analysis of lens lysates revealed that C-terminus of MIP(AQP0) probably interacted with these two fiber connexins. In late embryonic and adult lenses, however, uniform co-distribution of MIP(AQP0) and fiber connexins was largely disrupted, except for the area surrounding the actively differentiating bow regions, as was revealed by immunofluorescence and immunoprecipitation experiments. The interaction of MIP(AQP0) with lens fiber connexins in differentiating lens cells but not in mature lens fibers suggests a potential role for MIP(AQP0) in the facilitation of fiber connexins for the formation of gap junctions during lens development.

Bilateral congenital cataracts result from a gain-of-function mutation in the gene for aquaporin-0 in mice

Cataract Tohoku (Cat(Tohm)) is a dominant cataract mutation that leads to severe degeneration of lens fiber cells. Linkage analysis showed that the Cat(Tohm) mutation is located on mouse chromosome 10, close to the gene for aquaporin-0 (Aqp0), which encodes a membrane protein that is expressed specifically in lens fiber cells. Sequence analysis of Aqp0 revealed a 12-bp deletion without any change in the reading frame, which resulted in a deletion of four amino acids within the second transmembrane region of the AQP0 protein. Targeted expression of the mutated Aqp0 caused lens opacity in transgenic mice, the pathological severity of which depended on the expression level of the transgene. The mutated AQP0 protein was localized to the intracellular and perinuclear spaces rather than to the plasma membranes of the lens fiber cells. The cataract phenotype of Cat(Tohm) is caused by a gain-of-function mutation in the mutated AQP0 protein and not by a loss-of-function mutation.

Optical dysfunction of the crystalline lens in aquaporin-0-deficient mice

Aquaporin-0 (AQP0), a water transport channel protein, is the major intrinsic protein (MIP) of lens fiber cell plasma membranes. Mice deficient in the gene for AQP0 (Aqp0, Mip) were generated from a library of gene trap embryo stem cells. Sequence analysis showed that the gene trap vector had inserted into the first exon of Aqp0, causing a null mutation as verified by RNA blotting and immunochemistry. At 3 wk of age (postnatal day 21), lenses from null mice (Aqp0(-/-)) contained polymorphic opacities, whereas lenses from heterozygous mice (Aqp0(+/-)) were transparent and did not develop frank opacities until approximately 24 wk of age. Osmotic water permeability values for Aqp0(+/-) and Aqp0(-/-) lenses were reduced to approximately 46% and approximately 20% of wild-type values, respectively, and the focusing power of Aqp0(+/-) lenses was significantly lower than that of wild type. These findings show that heterozygous loss of AQP0 is sufficient to trigger cataractogenesis in mice and suggest that this MIP is required for optimal focusing of the crystalline lens.

Requirement for TGFβ receptor signaling during terminal lens fiber differentiation

Several families of growth factors have been identified as regulators of cell fate in the developing lens. Members of the fibroblast growth factor family are potent inducers of lens fiber differentiation. Members of the transforming growth factor β (TGFβ) family, particularly bone morphogenetic proteins, have also been implicated in various stages of lens and ocular development, including lens induction and lens placode formation. However, at later stages of lens development, TGFβ family members have been shown to induce pathological changes in lens epithelial cells similar to those seen in forms of human subcapsular cataract. Previous studies have shown that type I and type II TGFβ receptors, in addition to being expressed in the epithelium, are also expressed in patterns consistent with a role in lens fiber differentiation. In this study we have investigated the consequences of disrupting TGFβ signaling during lens fiber differentiation by using the mouse αΑ-crystallin promoter to overexpress mutant (kinase deficient), dominant-negative forms of either type I or type II TGFβ receptors in the lens fibers of transgenic mice. Mice expressing these transgenes had pronounced bilateral nuclear cataracts. The phenotype was characterized by attenuated lens fiber elongation in the cortex and disruption of fiber differentiation, culminating in fiber cell apoptosis and degeneration in the lens nucleus. Inhibition of TGFβ signaling resulted in altered expression patterns of the fiber-specific proteins, α-crystallin, filensin, phakinin and MIP. In addition, in an in vitro assay of cell migration, explanted lens cells from transgenic mice showed impaired migration on laminin and a lack of actin filament assembly, compared with cells from wild-type mice. These results indicate that TGFβ signaling is a key event during fiber differentiation and is required for completion of terminal differentiation.

Normal differentiation of cultured lens cells after inhibition of gap junction-mediated intercellular communication

The cells of the vertebrate lens are linked to each other by gap junctions, clusters of intercellular channels that mediate the direct transfer of low-molecular-weight substances between the cytosols of adjoining cells. Although gap junctions are detectable in the unspecialized epithelial cells that comprise the anterior face of the organ, both their number and size are greatly increased in the secondary fiber cells that differentiate from them at the lens equator. In other organs, gap junctions have been shown to play an important role in tissue development and differentiation. It has been proposed, although not experimentally tested, that this may be true in the lens as well. To investigate the function of gap junctions in the development of the lens, we have examined the effect of the gap junction blocker 18beta-glycyrrhetinic acid (betaGA) on the differentiation of primary cultures (both dissociated cell-derived monolayers and central epithelium explants) of embryonic chick lens epithelial cells. We found that betaGA greatly reduced gap junction-mediated intercellular transfer of Lucifer yellow and biocytin throughout the 8-day culture period. betaGA did not, however, affect the differentiation of these cells into MP28-expressing secondary fibers. Furthermore, inhibition of gap junctions had no apparent effect on either of the two other types of intercellular (adherens and tight) junctions present in the lens. We conclude that the high level of gap junctional intercellular communication characteristic of the lens equator in vivo is not required for secondary fiber formation as assayed in culture. Up-regulation of gap junctions is therefore likely to be a consequence rather than a cause of lens fiber differentiation and may primarily play a role in lens physiology.

Overlapping Sp1 and AP2 binding sites in a promoter element of the lens-specific MIP gene

The MIP gene, the founder of the MIP family of channel proteins, is specifically expressed in fiber cells of the ocular lens and expression is regulated temporally and spatially during development. We previously found that a DNA fragment containing 253 bp of 5'-flanking sequence and 42 bp of exon 1 of the human MIP gene contains regulatory elements responsible for lens-specific expression of the MIP gene. In this report we have analyzed the function of overlapping Sp1 and AP2 binding sites present in the MIP promoter. Using DNase I footprinting analysis we found that purified Sp1 and AP2 transcription factors interact with several domains of the human MIP promoter sequence -253/+42. Furthermore, addition of purified Sp1 to Drosophila nuclear extracts activates in vitro transcription from the MIP promoter -253/+42. This promoter activity is competed by oligonucleotides containing domains footprinted with Sp1. Using promoter-reporter gene ( CAT ) constructs we found that the sequence -39/-70 contains a cis regulatory element essential for promoter activity in transient assays in lens cells. EMSA analysis showed that lens nuclear extracts contain factors that bind to the MIP 5'-flanking sequence containing overlapping Sp1 and AP2 binding domains at positions -37/-65. Supershift experiments with lens nuclear extracts indicated that Sp3 is also able to interact with this regulatory element, suggesting that Sp1 and Sp3 may be involved in regulation of transcription of the MIP gene in the lens.

Mutations in the founder of the MIP gene family underlie cataract development in the mouse

The major intrinsic protein (MIP) of the vertebrate eye lens is the first identified member of a sequence-related family of cell-membrane proteins that appears to have evolved by gene duplication. Several members of the MIP family transport water (aquaporins), glycerol and other small molecules in microbial, plant and animal cells. Mutations in two aquaporin homologues of MIP underlie an autosomal recessive form of nephrogenic diabetes insipidus and absence of the Colton blood group antigens in humans, whereas, mutation of a third MIP-like gene underlies 'big brain' development in Drosophila. Here we show that distinct mutations in the murine Mip gene underlie one form of autosomal dominant cataract in the mouse. The cataract Fraser mutation is a transposon-induced splicing error that substitutes a long terminal repeat sequence for the carboxy-terminus of MIP. The lens opacity mutation is an amino-acid substitution that inhibits targeting of MIP to the cell-membrane. These allelic cataract mutations provide the first direct evidence that MIP plays a crucial role in the development of a transparent eye lens.

Isolation and characterization of the 5'-flanking sequence of the human ocular lens MIP gene

The MIP (major intrinsic protein) gene, a member of an ancient family of membrane channel genes, encodes the predominant fiber cell membrane protein of the ocular lens. Its specific expression in the lens fibers is temporally and spatially regulated during development. To study the regulation of expression of MIP and delineate the regulatory elements underlying its tissue specificity and ontogenic profile, we have cloned 2840 bp of the human MIP 5'-flanking sequence. The human MIP 5'-flanking sequence contains three complete Alu repetitive elements in tandem at position between nt -1699 and -2684 (nt -1699/-2684). These Alu elements appear to have had a complex evolutionary history with insertions at different times. We have fused DNA fragments containing MIP 5'-flanking sequences to the bacterial cat reporter gene encoding chloramphenicol acetyltransferase and assayed them in primary cultures of chicken lens cells. We have mapped two negative regulatory regions in the human MIP 5'-flanking sequences -1564/-1696 and -948/-1000. We demonstrated that the human MIP 5'-flanking sequence -253/+42 contains a functional promoter in lens cells but is inactive in kidney epithelial cells or mouse fibroblasts, suggesting that this sequence contains regulatory elements responsible for the lens-specific expression of MIP.

An ultrastructural analysis of the epithelial-fiber interface (EFI) in primate lenses

The purpose of this study was to conduct a comprehensive ultrastructural analysis of the epithelial-fiber interface (EFI) in normal adult primate (Macaque nemestrina and fascicularis; 6-9 years old, n = 10) lenses. Scanning electron microscopy (SEM) was used to initially characterize the gross size, shape and three-dimensional organization of central zone (cz) epithelial cells and the anterior ends of elongating fibers beneath these cells. This fiducial information was essential to properly orient lens pieces in freeze fracture specimen carriers for the production of replicas with unambiguously identifiable EFI. Transmission electron microscopy (TEM) of replicas and thin-sectioned material were used to ultrastructurally analyse the cz EFI. TEM thin-sectioned material was also used to ultrastructurally analyse the pregerminative (pgz), germinative (gz) and transitional zone (tz) EFI. Correlative SEM and TEM of cz EFI components revealed that the apical membrane of both epithelial and elongating fiber cells were irregularly polygonal in shape, and aligned in parallel as smooth, concave-convex surfaces. However, whereas epithelial cell apical surfaces had minimal size variation, elongating fibers were larger and considerably variable in size. Quantitative analysis of > 10000 micron2 cz elongating fiber apical surfaces failed to detect any gap junctions defined in freeze fracture replicas as complementary aggregates of transmembrane proteins (connexons) conjoined across a narrowed extracellular space. However, a comparable frequency of vesicular events was noted in this region as quantified previously in adult and embryonic chick lens. Correlative TEM analysis > 1500 linear micrometers of thin-sectioned EFI from this region confirmed the presence of epithelial-epithelial gap junctions, elongating fiber-elongating fiber gap junctions, and an extreme paucity of epithelial-elongating fiber gap junctions. In contrast, TEM analysis of > 1000 linear micrometers of thin-sectioned pgz, gz and tz EFI, confirmed the presence of epithelial-epithelial gap junctions, elongating fiber-elongating fiber gap junctions, numerous epithelial-elongating fiber adherens junctions and a few epithelial-elongating fiber gap junctions. Thus, the results of this and previous quantitative morphological and physiological studies (electronic and dye coupling) demonstrate that there is limited coupling between cz epithelial cells and underlying elongating fibers. Furthermore, the absence of gap junctional plaques in cz EFI freeze-fracture replicas and either pentalaminar or septalaminar profiles in correlative thin-sections, suggests that this limited coupling could be mediated via isolated gap junction channels. However, the results of this and previous quantitative studies further show that a greater degree of coupling exists across the pgz, gz and tz regions of the EFI and that this coupling is likely to be mediated by gap junction plaques. Finally, this and other studies continue to demonstrate that transcytotic processes play a role in lens physiology at the EFI.

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)

A progressive early onset cataract gene maps to human chromosome 17q24

Cerulean cataract is an autosomal dominant, early onset, progressive cataract characterized by blue or white opacifications in the nucleus and cortex of the lens. A large four-generation pedigree in which cerulean cataract segregates was studied for linkage analysis. A genome wide search was undertaken after the plausible candidate genes were excluded and the cerulean cataract phenotype was mapped to chromosome 17q24. The three markers closest to the disease gene are D17S802 (Z)(theta) = 9.46 at (theta) = 0.085), D17S836 (Z(theta) = 5.26 at (theta) = 0.031) and AFMa238yb5 (Z(theta) = 7.11 at (theta) = 0.032). Multipoint linkage analyses yielded a maximum lod score of Z(theta) = 13.71, placing the cerulean cataract gene between D17S802 and D17S836 at (theta) = 0.048 and (theta) = 0.013, respectively.

p53-dependent apoptosis produced by Rb-deficiency in the developing mouse lens

The retinoblastoma tumour-suppressor gene (RB) has been implicated in negative growth regulation, induction of differentiation, and inhibition of cellular transformation. Homozygous inactivation of the Rb gene in the mouse leads to mid-gestational lethality with defects in erythropoiesis and neurogenesis. Here we describe the effects of the Rb-deficient state on the development of the ocular lens. The regional compartmentalization of growth, differentiation and apoptosis in the developing lens provides an ideal system to examine more closely the relationships of these processes in vivo. We demonstrate that loss of Rb function is associated with unchecked proliferation, impaired expression of differentiation markers, and inappropriate apoptosis in lens fibre cells. In addition, we show that ectopic apoptosis in Rb-deficient lenses is dependent on p53, because embryos doubly null for Rb and p53 show a nearly complete suppression of this effect. This developmental system provides a framework for understanding the consequences of the frequent mutation of both RB and p53 in human cancer.

Differentiation and angiogenic growth factor message in two mammalian lens epithelial cell lines

Lens epithelial cells in culture can sometimes be induced to form spheroid aggregates termed lentoid bodies, composed of cells exhibiting various characteristics of the more highly differentiated lens fiber cells. However, lentoid bodies are often slow to form, and the ability to produce them declines with serial subculture. It was therefore of interest to establish and/or characterize lens epithelial cell lines capable of forming lentoid bodies. The differentiation state was assessed in lentoid bodies formed by each of two lens epithelial cell lines, the transformed alpha TN4 cell line from mouse and the nontransformed N/N1135A cell line from rabbit. Lentoid and monolayer cultures of each cell line were examined for transcripts of the lens-specific alpha A-crystallin ("alpha A"), gamma D-crystallin ("gamma D"; formerly gamma 1-crystallin) and MP26 genes. alpha TN4 lentoid bodies contained 2.5 times the alpha A RNA found in monolayer cells, but lacked detectable gamma D and MP26 RNA. None of the three markers were detected in either lentoid or monolayer N/N1135A cultures grown under the conditions described. Lentoid body formation alone, therefore, does not indicate the extent of differentiation occurring. At least some of the changes in cell adhesion occurring during lentoid body formation involve laminin-like and fibronectin-like interactions, and are reminiscent of those observed during embryonic lens formation. Finally, vascular endothelial growth factor mRNA was absent from the lens but present in alpha TN4 cells, suggesting a mechanism whereby the lens tumors of the founder mouse became vascularized.

Aberrant expression of the gene for lens major intrinsic protein in the CAT mouse

Immunocytochemistry fails to detect expression of the lens major intrinsic protein (MIP) in 16-day embryos of the congenitally cataractous mouse, CAT, which inherits a dominant mutation assigned to the distal end of mouse chromosome 10. In situ hybridisation, however, detects MIP mRNA in CAT embryo lens fibre cells at a level approximating 60% of that detected in embryonic lens fibres of the non-cataractous MF1 mouse. Northern blot hybridisation reveals that the most abundant MIP mRNA transcript in the adult CAT lens is truncated when compared to that in the adult MF1 lens. The results are consistent with a cataractogenic mutation in the mouse gene for MIP (Mip) which has also been mapped to the distal end of mouse chromosome 10.

Genomic cloning, complete nucleotide sequence, and structure of the human gene encoding the major intrinsic protein (MIP) of the lens

Major intrinsic protein (MIP, also called MP26) is the predominant fiber cell membrane protein of the ocular lens. MIP has been suggested to play a role in cell-cell communication in the lens. Its expression is tissue-specific and developmentally regulated. We have isolated and characterized the human gene encoding MIP and report here its genomic structure and entire nucleotide sequence. The gene is 3.6 kb, contains four exons separated by introns ranging in size from 0.4 to 1.6 kb, and is present in single copy per haploid human genome. Primer extension of human lens RNA indicates that transcription of the gene initiates from a single site 26 nt downstream from the TATA box. Three complete Alu repetitive elements are found in tandem in the 5'-flanking region of the gene, and a single complete Alu sequence is present in the third intron. The interspecies comparisons of the MIP gene coding sequence and homologies to other members of a putative transmembrane channel protein superfamily are also discussed.

The role of fibroblast growth factor in eye lens development

In this review we have presented evidence that FGF plays an important role in inducing events in lens morphogenesis and growth. Our studies show that FGF stimulates lens epithelial cells in explants to proliferate, migrate, and differentiate into fibers at low, medium, and high concentrations, respectively. This has some important implications for understanding the behavior of lens cells in the eye. The fact that aFGF is detected in the equatorial region of the lens where cells are actively proliferating, possibly migrating, and differentiating into fibers suggests that these events may be under autocrine control in vivo, at least to some extent. Because FGF is also present in the ciliary and iridial region of retina and in the vitreous, paracrine control may also be involved. Cell proliferation, fiber differentiation, and (possibly) cell migration occur in characteristic spatial patterns that are related to distinct compartments of the lens. We suggest that cells in the germinative zone receive only a low level of FGF stimulation arising from the cells themselves and possibly also from the ciliary and iridial regions of the retina but, whatever the source, this is only sufficient to stimulate proliferation. Lens epithelial cells that migrate or are displaced into the transitional zone below the lens equator receive some FGF from these sources but in addition receive a strong stimulus from the high level of FGF in the vitreous; thus, fiber differentiation is induced. Cells at the junction between these two zones may receive an intermediate level of FGF stimulation, sufficient to induce cell migration. In essence, we are proposing that, in the eye, FGF acts as a lens morphogen in the sense that different levels of FGF stimulation elicit different lens cell responses. Hence its characteristic distribution in the eye establishes lens polarity and growth patterns. Since FGF has an inductive effect on lens cells from mature age animals, we also propose that this specific distribution of FGF in the eye is also important for maintenance of a normal lens throughout life. Finally the synergistic effects of insulin/IGF on the FGF-induced responses highlight the importance of considering the distribution of members of the insulin/IGF family of molecules in vivo. Mechanisms that control levels of both the FGF and insulin/IGF families of factors in the eye are probably of crucial importance in the formation and maintenance of a normal lens.

Immunochemical comparison of the major intrinsic protein of eye-lens fibre cell membranes in mice with hereditary cataracts

Expression of the major intrinsic protein (MIP) of eye-lens fibre cell membranes was compared in normal (DBA), cataractous (CAT, LOP, NCT) and chimaeric (CBA-LOP) mice at different stages of development using immunofluorescence microscopy and immunoblotting techniques. MIP of apparent molecular mass 26 kDa was detected in extracts of adult DBA, LOP and CBA-LOP lenses, but only low molecular mass (less than 26 kDa) immunoreactive proteins were detected in similar extracts from adult CAT and NCT lenses. The corresponding MIP distribution patterns confirmed the highly organised fibre-cell histology in embryonic DBA and adult CBA-LOP lenses and also highlighted the severe fibre-cell degeneration in the LOP lens. In contrast, however, no immunoreactive MIP was detected in situ in embryonic CAT and NCT lenses. These results suggest that a structural alteration of MIP occurs during embryonic lens development in the cataractous CAT (dominant) and NCT (recessive) mutant mice.

Restriction fragment length polymorphisms associated with the gene for the major intrinsic protein of eye-lens fibre cell membranes in mice with hereditary cataracts

Cloned cDNAs coding for eye-lens fibre cell-membrane proteins, MIP and MP70, were used to detect restriction fragment length polymorphisms (RFLPs) in genomic DNA from inbred mice with autosomally inherited cataracts. Whereas distinct RFLPs associated with the MIP gene were identified in the Cba Cat and Nct mutants, no such genetic variation was associated with the MP70 gene. RFLPs associated with the mouse MIP gene may provide informative DNA markers in gene linkage studies of murine hereditary cataracts.

The effects of insulin and basic fibroblast growth factor on fibre differentiation in rat lens epithelial explants

We have shown that fibroblast growth factors (FGFs) induce epithelial cells throughout lens explants to progressively divide, migrate and differentiate into fibres as the concentration of FGF is increased. We now report the effects of insulin and insulin-like growth factor-1 on rat lens epithelial explants, alone or in combination with the basic form of FGF. Fibre cell-specific beta- and gamma-crystallins were localised in explants by immunofluorescence or determined by ELISAs. For insulin, high doses induced limited beta-crystallin accumulation, much less than for FGF and mostly restricted to the explant periphery. When insulin was included with a low concentration of FGF, fibre differentiation was substantially enhanced. Both beta- and gamma-crystallin accumulation were affected synergistically, the effect being greater for gamma- than for beta-crystallin, and epithelial cells in both the central and peripheral region of the explant participated in the synergistic response. Insulin-like growth factor-1 at a concentration of 50 ng/ml mimicked the effects of insulin.

Structural analysis of lens epithelial explants induced to differentiate into fibres by fibroblast growth factor (FGF)

Recently we identified fibroblast growth factor (FGF), which is present in significant amounts in neural retinas, as a potent inducer of lens fibre differentiation in our epithelial explant cultures. Fibre differentiation was assessed by synthesis of fibre specific, proteins, beta- and gamma-crystallins, and by cell elongation. However, to establish whether FGF induced the dramatic structural changes characteristic of fibre differentiation we carried out an ultrastructural analysis. In this study epithelial explants exposed to either the acidic or basic form of FGF were shown to undergo the structural changes characteristic of fibre differentiation in the intact lens. These include: (i) cell elongation, (ii) a reduction in cytoplasmic organelles, (iii) the formation of specialized cell-cell junctions, including finger-like processes and fingerprints, ball and socket junctions, tongue-like flaps and imprints, and gap junctions, and (iv) nuclear pyknosis. This shows that FGF faithfully reproduces structural events associated with fibre differentiation as well as the molecular events reported previously, thus providing further evidence that FGF in the eye is important for the control of normal lens fibre differentiation.

Induction of lens fibre differentiation by acidic and basic fibroblast growth factor (FGF)

Explants of epithelial cells from newborn rat lenses undergo changes characteristic of fibre differentiation when cultured with neural retina or retina-conditioned medium. Here we show that similar changes occur when acidic and basic fibroblast growth factor (FGF) are used instead of retina-conditioned medium. When cultured without FGF, epithelial explants contained negligible amounts of beta-crystallin, a lens protein found only in fibre cells. However, at saturating concentrations of FGF, about 20 micrograms beta-crystallin was produced per explant in 5 days. The response was dose-dependent, half maximal response requiring 55 and 290 ng/ml of basic and acidic FGF, respectively. FGF also stimulated cell proliferation and cell migration. All three responses to basic FGF were blocked by an antibody specific for basic FGF. The concentration of FGF required to produce a maximal response was lower for cell proliferation and migration than for beta-crystallin accumulation. The results suggest a possible role for FGF in the control of events in lens development.