Cell elongation in the cultured embryonic chick lens epithelium with and without protein synthesis. Involvement of microtubules

Insights into the biochemical and biophysical mechanisms mediating the longevity of the transparent optics of the eye lens

In the human eye, a transparent cornea and lens combine to form the "refracton" to focus images on the retina. This requires the refracton to have a high refractive index "n," mediated largely by extracellular collagen fibrils in the corneal stroma and the highly concentrated crystallin proteins in the cytoplasm of the lens fiber cells. Transparency is a result of short-range order in the spatial arrangement of corneal collagen fibrils and lens crystallins, generated in part by post-translational modifications (PTMs). However, while corneal collagen is remodeled continuously and replaced, lens crystallins are very long-lived and are not replaced and so accumulate PTMs over a lifetime. Eventually, a tipping point is reached when protein aggregation results in increased light scatter, inevitably leading to the iconic protein condensation-based disease, age-related cataract (ARC). Cataracts account for 50% of vision impairment worldwide, affecting far more people than other well-known protein aggregation-based diseases. However, because accumulation of crystallin PTMs begins before birth and long before ARC presents, we postulate that the lens protein PTMs contribute to a "cataractogenic load" that not only increases with age but also has protective effects on optical function by stabilizing lens crystallins until a tipping point is reached. In this review, we highlight decades of experimental findings that support the potential for PTMs to be protective during normal development. We hypothesize that ARC is preventable by protecting the biochemical and biophysical properties of lens proteins needed to maintain transparency, refraction, and optical function.

Genome-wide DNA methylation profiles may reveal new possible epigenetic pathogenesis of sporadic congenital cataract

Aim: To investigate the possible epigenetic pathogenesis of sporadic congenital cataract. Materials & methods: We conducted whole genome bisulfite sequencing on peripheral blood from sporadic binocular or monocular congenital cataract patients and cataract-free participants. Results: We found massive differentially methylated regions within the whole genomes between any two groups. Meanwhile, we identified five genes (ACTN4, ACTG1, TUBA1A, TUBA1C, TUBB4B) for the binocular and control groups and TUBA1A for the monocular and control groups as the core differentially methylated region-related genes. The proteins encoded by these core genes are involved in building cytoskeleton and intercellular junctions. Conclusion: Changes in the methylation levels of core genes may disturb the function of cytoskeleton and intercellular junctions, eventually leading to sporadic congenital cataract.

Microtubules: Evolving roles and critical cellular interactions

Microtubules are cytoskeletal elements known as drivers of directed cell migration, vesicle and organelle trafficking, and mitosis. In this review, we discuss new research in the lens that has shed light into further roles for stable microtubules in the process of development and morphogenesis. In the lens, as well as other systems, distinct roles for characteristically dynamic microtubules and stabilized populations are coming to light. Understanding the mechanisms of microtubule stabilization and the associated microtubule post-translational modifications is an evolving field of study. Appropriate cellular homeostasis relies on not only one cytoskeletal element, but also rather an interaction between cytoskeletal proteins as well as other cellular regulators. Microtubules are key integrators with actin and intermediate filaments, as well as cell–cell junctional proteins and other cellular regulators including myosin and RhoGTPases to maintain this balance.

Impact statement

The role of microtubules in cellular functioning is constantly expanding. In this review, we examine new and exciting fields of discovery for microtubule’s involvement in morphogenesis, highlight our evolving understanding of differential roles for stabilized versus dynamic subpopulations, and further understanding of microtubules as a cellular integrator.

Functional role for stable microtubules in lens fiber cell elongation

The process of tissue morphogenesis, especially for tissues reliant on the establishment of a specific cytoarchitecture for their functionality, depends a balanced interplay between cytoskeletal elements and their interactions with cell adhesion molecules. The microtubule cytoskeleton, which has many roles in the cell, is a determinant of directional cell migration, a process that underlies many aspects of development. We investigated the role of microtubules in development of the lens, a tissue where cell elongation underlies morphogenesis. Our studies with the microtubule depolymerizing agent nocodazole revealed an essential function for the acetylated population of stable microtubules in the elongation of lens fiber cells, which was linked to their regulation of the activation state of myosin. Suppressing myosin activation with the inhibitor blebbistatin could attenuate the loss of acetylated microtubules by nocodazole and rescue the effect of this microtubule depolymerization agent on both fiber cell elongation and lens integrity. Our results also suggest that acetylated microtubules impact lens morphogenesis through their interaction with N-cadherin junctions, with which they specifically associate in the region where lens fiber cell elongate. Disruption of the stable microtubule network increased N-cadherin junctional organization along lateral borders of differentiating lens fiber cells, which was prevented by suppression of myosin activity. These results reveal a role for the stable microtubule population in lens fiber cell elongation, acting in tandem with N-cadherin cell-cell junctions and the actomyosin network, giving insight into the cooperative role these systems play in tissue morphogenesis.

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.

A dimensionless ordered pull-through model of the mammalian lens epithelium evidences scaling across species and explains the age-dependent changes in cell density in the human lens

We present a mathematical (ordered pull-through; OPT) model of the cell-density profile for the mammalian lens epithelium together with new experimental data. The model is based upon dimensionless parameters, an important criterion for inter-species comparisons where lens sizes can vary greatly (e.g. bovine (approx. 18 mm); mouse (approx. 2 mm)) and confirms that mammalian lenses scale with size. The validated model includes two parameters: β/α, which is the ratio of the proliferation rate in the peripheral and in the central region of the lens; and γ_GZ, a dimensionless pull-through parameter that accounts for the cell transition and exit from the epithelium into the lens body. Best-fit values were determined for mouse, rat, rabbit, bovine and human lens epithelia. The OPT model accounts for the peak in cell density at the periphery of the lens epithelium, a region where cell proliferation is concentrated and reaches a maximum coincident with the germinative zone. The β/α ratio correlates with the measured FGF-2 gradient, a morphogen critical to lens cell survival, proliferation and differentiation. As proliferation declines with age, the OPT model predicted age-dependent changes in cell-density profiles, which we observed in mouse and human lenses.

Abnormal expression of collagen IV in lens activates unfolded protein response resulting in cataract

Human diseases caused by mutations in extracellular matrix genes are often associated with an increased risk of cataract and lens capsular rupture. However, the underlying mechanisms of cataract pathogenesis in these conditions are still unknown. Using two different mouse models, we show that the accumulation of collagen chains in the secretory pathway activates the stress signaling pathway termed unfolded protein response (UPR). Transgenic mice expressing ectopic Col4a3 and Col4a4 genes in the lens exhibited activation of IRE1, ATF6, and PERK associated with expansion of the endoplasmic reticulum and attenuation of general protein translation. The expression of the transgenes had adverse effects on lens fiber cell differentiation and eventually induced cell death in a group of transgenic fiber cells. In Col4a1(+/Deltaex40) mutant mice, the accumulation of mutant chains also caused low levels of UPR activation. However, cell death was not induced in mutant lenses, suggesting that low levels of UPR activation are not proapoptotic. Collectively, the results provide in vivo evidence for a role of UPR in cataract formation in response to accumulation of terminally unfolded proteins in the endoplasmic reticulum.

A novel form of epithelial wound healing of the embryonic epidermis

The embryonic epidermis of amniotes is a two-cell layer sheet with a periderm positioned superficial to the basal cell layer which, itself, attaches apically to the basal surface of the periderm and basally to the basal lamina. The presence of the periderm is essential to maintain the basal layer as a two-dimensional monolayer. Wounds to the epidermis that remove selectively just the periderm are healed by a stacking of the basal layer cells that constitute the wound bed. Basal cell stacking involves the desertion of the basal lamina by many of the cells so as to increase their contact area with other basal layer cells. This suggests that a preferential adhesion to the planar basal lamina is not important for the monolayered organization of the basal layer but, instead, association with inner surface of the planar periderm is the principal process that maintains the basal layer as a monolayer. The conversion of the basal layer from monolayer to multilayer during wound healing diminishes its planar area, resulting in movement of the wound borders toward the center of the wound. This is a novel scenario for wound healing.

Three-dimensional reconstruction of cells in the living lens: the relationship between cell length and volume

During terminal differentiation, lens fiber cells increase in length by 100-1000 fold. The mechanism of cellular elongation is not well understood but previous measurements on differentiating embryonic fiber cells indicated that cell volume increases in direct proportion to cell length. Fiber cells are tightly packed within the lens. Under these circumstances, a cellular volume increase might be transduced into an increase in length. Such considerations have led to the suggestion that volume increase may be the driving force for cell elongation. In the present study, we tested this model using a strain of mice in which a GFP transgene is expressed sporadically in the lens. We employed a combination of confocal microscopy, image deconvolution, and volume-rendering techniques to visualize the structure of individual GFP-expressing cells within the living lens. We examined their morphology at various stages of differentiation; from the central epithelium to young fiber cells. At each stage, cell length and volume were determined. In contrast to earlier studies, our data indicated that increases in cell length were not matched by a proportional increase in volume. Rather, the initial phase of elongation (in which cells increased in length from <10 to >150 microm) appeared to result largely from a change in cell shape. The present observations suggest that the driving force for elongation of primary and secondary fiber cells may differ fundamentally.

Decreased gap junctional communication in neurobiotin microinjected lens epithelial cells after taxol treatment

The aim of the study was to examine gap-junction-mediated intercellular communication after experimentally induced aggregations of microtubules in cultured bovine lens epithelial cells. Intercellular communication between lens cells appears to be crucial for normal lens homeostasis. However, investigations on the maintenance of direct ion and metabolite exchange via gap junctions and its quantified dependency of cytoskeletal microtubules have not been available under conditions leading to bundling of microtubules. Thus, metabolic coupling of neighboring lens epithelial cells was quantified following microinjections of neurobiotin into single cells under various conditions. In controls, intensive gap-junction-mediated intercellular communication could be documented by dye-spreading of microinjected neurobiotin. In contrast, taxol treatment for 1-3 days impaired, but did not completely block gap-junction-mediated intercellular communication. After depletion of taxol, a complete recovery of intercellular communication was achieved. In addition, confocal laser scanning microscopy and rapid-freeze deep-etch electron microscopy revealed a displacement of actin-filaments from the perinuclear cytoplasm, accompanied by an abnormal aggregation of microtubules after taxol treatment, including impeded translocation of connexin 43 from the cytoplasm into the plasma membrane. Incubation of cells with nocodazole destroyed the microtubule network, accompanied by a clear reduction of plasma-membrane-integrated connexin 43 and significant impairment of dye spreading. Thus, in lens epithelial cells intercellular communication at gap junctions made by connexin 43 depends on the integrity of the microtubule network through the translocation of connexins to the plasma membrane.

Growth factor regulation of lens development

Lens arises from ectoderm situated next to the optic vesicles. By thickening and invaginating, the ectoderm forms the lens vesicle. Growth factors are key regulators of cell fate and behavior. Current evidence indicates that FGFs and BMPs are required to induce lens differentiation from ectoderm. In the lens vesicle, posterior cells elongate to form the primary fibers whereas anterior cells differentiate into epithelial cells. The divergent fates of these embryonic cells give the lens its distinctive polarity. There is now compelling evidence that, at least in mammals, FGF is required to initiate fiber differentiation and that progression of this complex process depends on the synchronized and integrated action of a number of distinct growth factor-induced signaling pathways. It is also proposed that an antero-posterior gradient of FGF stimulation in the mammalian eye ensures that the lens attains and maintains its polarity and growth patterns. Less is known about differentiation of the lens epithelium; however, recent studies point to a role for Wnt signaling. Multiple Wnts and their receptors are expressed in the lens epithelium, and mice with impaired Wnt signaling have a deficient epithelium. Recent studies also indicate that other families of molecules, that can modulate growth factor signaling, have a role in regulating the ordered growth and differentiation of the lens.

Cellular and molecular features of lens differentiation: a review of recent advances

In this paper, the more recent literature pertaining to differentiation in the developing vertebrate lens is reviewed in relation to previous work. The literature reviewed reveals that the developing lens has been, and will continue to be, a useful model system for the examination of many fundamental processes occurring during embryonic development. Areas of lens development reviewed here include: the induction and early embryology of the lens; lens cell culture techniques; the role of growth factors and cytokines; the involvement of gap junctions in lens cell-cell communication; the role of cell adhesion molecules, integrins, and the extracellular matrix; the role of the cytoskeleton; the processes of programmed cell death (apoptosis) and lens fibre cell denucleation; the involvement of Pax and Homeobox genes; and crystallin gene regulation. Finally, some speculation is provided as to possible directions for further research in lens development.

FGF suppresses apoptosis and induces differentiation of fibre cells in the mouse lens

To determine whether fibroblast growth factor (FGF) has a role in lens development, we have generated transgenic mice expressing a dominant-negative form of the murine FGF receptor-1 (FGFRDN) in the lens. Using the fibre cell-specific alpha A-crystallin promoter to express the FGFRDN, we have asked whether FGF is required for fibre cell differentiation. The transgenic mice display diminished differentiation of fibre cells as indicated by their reduced elongation. In addition, transgenic lenses have an unusual refractile anomaly that morphological and biochemical data show results from the apoptosis of fibre cells in the central region of the lens. These results show that lens fibre cells are dependent on FGF for their survival and differentiation, and demonstrate that growth factor deprivation in vivo can lead to apoptosis.

Methylisocyanate and actin polymerization: the in vitro effects of carbamylation

Uremia has been implicated in cataractogenesis due to protein carbamylation by cyanate derived from urea. The present study was designed to directly identify the effects of carbamylation on actin polymerization and the possible contribution to cataract formation. The susceptibility of actin to carbamylation is expected because of the 19 lysines distributed along its length. The lysines of actin were selectively carbamylated by methylisocyanate (MIC) at pH 8.0 and 4 degrees C and actin polymerization assayed by high-shear viscometry, fluorescence and transmission electron microscopy. Our results provide evidence that non-enzymatic carbamylation of the lysine residues prevents the polymerization of actin. In addition, this carbamylated actin inhibited the polymerization of nascent, unmodified actin. High-shear viscosity measurements demonstrated decreased initial apparent rates and decreased steady-states (final specific viscosities) of polymerization. Fluorescence measurements showed decreased relative intensities of fluorescence versus control and confirmed the inhibitory effects of carbamylation by MIC on the steady state of F-actin. Transmission electron microscopy (TEM) showed the presence of disorganized filaments when carbamylated actin was added to polymerizing unmodified actin. Our results suggest that carbamylation of actin can cause a loss of ordered filament structure and shape of the lens fiber cell, thus predisposing it to cataract development.

Regulation of lens cell growth and polarity by an embryo-specific growth factor and by inhibitors of lens cell proliferation and differentiation

We used a double-label method, which monitors the rate at which cells enter S-phase of the cell cycle, to identify factors that control the growth of chicken embryo lens epithelial cells in vivo. With this assay, we identified a mitogen for lens epithelial cells in the anterior segment of the embryonic eye. When the anterior chamber was opened briefly, by tearing the cornea or displacing the lens, the growth-promoting activity was lost. None of the purified growth factors tested replaced this growth activity, including EGF, bFGF, PDGF, IGF-1, IGF-2, TGF beta and mixtures of these factors. However, chicken embryo serum or plasma did cause chicken embryo lens epithelial cells to progress through the cell cycle. The activity in serum was destroyed by heat and protease treatment. It was most active in serum from 10-day embryos, decreased with subsequent development and was undetectable from 2 days after hatching through adulthood. When embryo serum or plasma was mixed with vitreous humor or IGF-1, agents that induce lens fiber cell formation, cell elongation was prevented. In contrast to the mitogenic activity in serum, this inhibitor of differentiation was insensitive to trypsin treatment. We also identified an activity in vitreous humor that inhibited the growth-promoting agent in embryo serum. Plasma proteins readily enter the anterior chamber of the eye of chicken embryos.(ABSTRACT TRUNCATED AT 250 WORDS)

Ocular teratology

The principles of teratology are described, and animal models for research in abnormal ocular development and clinical studies of human teratogens are surveyed. A review is made of presumed ocular teratogenic agents: radiation; external environmental teratogens; maternal conditions such as infections, diabetes, and epilepsy; alcohol and drugs such as thalidomide, retinoic acid, and coumarin anticoagulants; and other agents, such as cigarettes.

Volume regulation in lens epithelial cells and differentiating lens fiber cells

Previous studies from our laboratory have led us to conclude that lens cell elongation is caused by an increase in cell volume. This volume increase results from an increase in the potassium content of the cells due to decreased potassium efflux. In contrast, an increase in the volume of most cells triggers a regulatory volume decrease (RVD) that is usually mediated by increased potassium efflux. For this reason, chicken embryo lens epithelial cells were tested to see whether they were capable of typical cell volume regulation. Changes in cell volume during lens fiber differentiation were first estimated by 3H2O water uptake. Cell water increased in proportion to cell length in elongating lens cells. Treatment of epithelial cells cultured in basal medium with dilute or concentrated medium, or with medium containing 50 mM sucrose, resulted in typical volume regulatory responses. Cells lost or gained volume in response to osmotic stress, then returned to their previous volume. In addition, the elongation and increase in cell volume that accompanies lens fiber cell differentiation occurred normally in either hypo- or hypertonic media. This observation showed that the activation of mechanisms to compensate for osmotic stress did not interfere with the increase in volume that accompanies elongation. The ability of elongating cells to volume regulate was also tested. Lens epithelial cells were stimulated to elongate by exposure to embryonic vitreous humor, then challenged with hypotonic medium. These elongating cells regulated their volume as effectively as unstimulated cells. Therefore, cells that were increasing their volume due to reduced potassium efflux could adjust their volume in response to osmotic stress, presumably by increasing potassium efflux. This suggests that the changes in potassium efflux that occur during differentiation and RVD are regulated by distinct mechanisms.

Increase in aldose reductase mRNA in dog lens epithelial cells under hypertonic conditions

Aldose reductase (AR) mRNA levels increase when dog lens epithelial cells are exposed to hypertonic conditions. Hybridization of mRNA to an AR cDNA, using Northern and slot blots, showed that AR mRNA is elevated at least fourfold when primary dog lens epithelial cells are grown in media (300 mosmol kg-1) supplemented with 150 mM NaCl (600 mosmol kg-1 final). A time course showed an increase in AR mRNA of approximately twofold by 24 hr with a maximum increase of between four- and eightfold by 48 hr. AR mRNA remained elevated for the duration of the experiment, 8 days. The addition of Tolrestat, an inhibitor of aldose reductase, had no effect on the increased level of AR mRNA in these hypertonically stressed cells. Cells grown in media supplemented with 250 mM sorbitol also showed a substantial increase in AR mRNA. These data indicate, as in other cell types, the lens, a target tissue of diabetes, responds to hypersomotic stress with an induction of AR expression and suggests that AR may play a role in intracellular osmotic regulation.

Overexpression of the vimentin gene in transgenic mice inhibits normal lens cell differentiation

To investigate the role of the intermediate filament protein vimentin in the normal differentiation and morphogenesis of the eye lens fiber cells, we generated transgenic mice bearing multiple copies of the chicken vimentin gene. In most cases, the vimentin transgene was overexpressed in the lenses of these animals, reaching up to 10 times the endogenous levels. This high expression of vimentin interfered very strongly with the normal differentiation of the lens fibers. The normal fiber cell denucleation and elongation processes were impaired and the animals developed pronounced cataracts, followed by extensive lens degeneration. The age of appearance and extent of these abnormalities in the different transgenic lines were directly related to the vimentin level. Electron microscopic analysis revealed that the accumulated transgenic protein forms normal intermediate filaments.

NCAM in the differentiation of embryonic lens tissue

The role of the neural cell adhesion molecule (NCAM)2 in ocular lens differentiation was investigated in chicken embryos. Changes in expression of NCAM were documented by immunohistology of frozen sections. This analysis revealed that NCAM diminished during lens fiber differentiation, in contrast to the gap junction-associated protein MP26 which became more abundant. The form of NCAM expressed was determined by Western blot analysis of proteins extracted from the different regions of the Embryonic Day 6 lenses. All regions expressed NCAM with an apparent molecular weight of 140 kDa and relatively low levels of polysialylation. The function of NCAM in lens differentiation was investigated using antibodies that inhibit NCAM-mediated adhesion. Two parameters that change during maturation of the lens epithelial cells were monitored: the thickness of the tissue, indicating the length of lens cells, and the particle arrangement of gap junctions, reflecting the state of junctional differentiation. When epithelial cell explants of Embryonic Day 6 lenses were cultured for 5 days, the cells elongated and displayed an increase in the loose, random intramembranous particle arrangements characteristic of maturing lens fiber gap junctions. When the explants were cultured in the presence of anti-NCAM Fabs, the epithelia were thinner than in matched controls and had particle arrangements characteristic of a less mature state. The expression of NCAM during lens differentiation and the effects of attenuating NCAM function suggest that adhesion mediated by NCAM is an essential event in lens cell differentiation.

Tubulin and microtubules in cochlear hair cells: comparative immunocytochemistry and ultrastructure

The distribution of tubulin has been investigated in surface preparations of the guinea pig organ of Corti using indirect immunofluorescence microscopy. Two different monoclonal antibodies to tubulin produce similar distinct patterns of labelling in hair cells. Labelling is greater in inner hair cells than outer hair cells. It occurs in rings around the cell apex, and in a meshwork below and channels through, the cuticular plate. In outer hair cells from the apical region of the cochlea, labelling occurs around the location of a basalward protrusion of the cuticular plate. These patterns correlate with the location of microtubules observed using transmission electron microscopy. A large patch of labelling occurs on the strial side of the cell corresponding to the largest channel through the cuticular plate and the kinociliary basal body. Strands of labelling are seen running parallel to the long axis of the cell between the subcuticular and synaptic region. Many more of these strands are seen in the inner hair cell than the outer hair cell and may correspond to tracks of microtubules transporting neurotransmitter vesicles or other organelles. In outer hair cells, intense labelling and many microtubules are seen in the subnuclear region. The possible roles of the different microtubule arrangements are discussed.

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.

Decreased membrane permeability to potassium is responsible for the cell volume increase that drives lens fiber cell elongation

Differentiation of lens epithelial cells into fiber cells involves an increase in cell volume which previously was proposed to be the direct cause of the extensive cell elongation which accompanies fiber formation. In this study we have continued to investigate the mechanism underlying cell elongation by defining the minimum nutrient and ion requirements of elongating cells, measuring potassium and sodium fluxes in stimulated and unstimulated lens epithelia, and determining the effects of several pharmacological agents on elongation and ion transport. We have shown that elongation will occur in a basic salt/glucose solution with Insulin-like growth factor I/somatomedin-C stimulation. Neither sodium nor any metabolite appears to be the osmotically active species which drives the increase in cell volume. However, potassium efflux rate coefficient was 47% lower in differentiating cells than in unstimulated cells, whereas potassium uptake rates and ouabain effects were similar. Cells did not elongate in potassium-free medium nor in the presence of several drugs which prevent the accumulation of intracellular potassium or hinder osmotic water flux. Unstimulated cells elongated, however, with the application of quinidine, a drug known to block potassium channels. We propose that stimulation of lens epithelial cells with an insulin-like growth factor signals the closure of a certain population of potassium channels. As a result, potassium efflux from the differentiating cells slows while active potassium uptake continues at a constant rate. Potassium then accumulates within the cell causing water influx, an increase in cell volume, and cell elongation.

Immunocytochemical localization of the 27K beta-crystallin polypeptide in the mouse lens during development using a specific monoclonal antibody: implications for cataract formation in the Philly mouse

The Philly mouse develops a hereditary cataract about 5 weeks after birth. Although the causative agent is not known, data suggest that there is a correlation between cataract formation and the selective absence of a 27 kilodalton (27K) beta-crystallin lens polypeptide. The ontogeny of the 27K beta-crystallin polypeptide was examined in normal mice in order to evaluate its role in normal development and determine what impact its absence may have on the Philly mouse lens. A monoclonal antibody was used with the PAP method to immunocytochemically localize the 27K polypeptide in lenses of normal mice during development. beta-Crystallins detected with polyclonal antisera were found in differentiated fiber cells throughout the lens. In contrast, the 27K beta-crystallin polypeptide detected with a specific monoclonal antibody was not found in the fiber cells of the inner part of the lens (nucleus), but was specifically localized in the fiber cells of the outer part of the lens called the cortex. The polypeptide was found only in elongating and differentiated fiber cells and not in mitotically active epithelial cells. Although a minor component of the 2-day-old lens, the 27K polypeptide comprised a large portion of the 16-day-old lens including the anterior and posterior poles. These data show that the 27K polypeptide is a minor component of the embryonic lens, but becomes a major contributor to the postnatal lens. The 27K beta-crystallin lens polypeptide is abundant in the fiber cells of the normal postnatal mouse lens. The absence of the 27K polypeptide in the Philly mouse may contribute to the observed failure of fiber cells to differentiate in the Philly mouse after birth or may be deleterious in some other manner to normal lens development. The selective absence of the 27K beta-crystallin polypeptide, a defect which precedes cataract formation in the Philly mouse, is intriguing since it suggests a relationship between this major lens polypeptide and lens clarity.

The cellular basis of epithelial morphogenesis

Epithelial tissues are ubiquitous in metazoan organisms, performing many different functions and assuming a variety of shapes. This diversity of form and function is ultimately dependent on the behavior of the cells within the epithelia. For example, it is intercellular adhesion and the control of paracellular permeability by cell junctions that permit epithelia to form barriers and act as selective filters. It is cellular polarity that enables absorptive epithelia to extract materials from a particular side of the sheet; it is the collective contributions of cell proliferation, cellular translocation, and changes in cell shape that sculpt epithelia from simple sheets into folds, pouches and tubes. Clearly, a complete understanding of epithelial morphogenesis is inextricably entwined with questions of cell behavior in general, such as how any cell adheres, moves, and maintains its shape. The study of epithelial systems has lent considerable insight into these problems and should continue to do so, just as examination of the behavior and architecture of nonepithelial cells will undoubtedly clarify many aspects of the cellular events underlying epithelial morphogenesis. Although the action of individual cells ultimately shapes epithelial, coordination of that action is necessary for the development of a coherent tissue. Attention must therefore be given to integrative mechanisms in epithelial morphogenesis. How do the many cells in an epithelial sheet act in virtual unison during folding? What defines the boundaries of epithelial invaginations? How does an individual cell detect its position within, and thereby know its role in the morphogenesis of, the epithelial whole of which it is a part? At the most elementary level, epithelial cells interact via their physical attachments to one other. Even such rudimentary communication affects cell shape, movement, and possibly proliferation and also plays a part in the maintenance of epithelial polarity. Additional signals pass among epithelial cells by a number of other mechanisms as well, most notably electrical coupling. However, many questions remain regarding the quality and quantity of what is communicated between epithelial cells. Accordingly, elucidating the means by which supracellular order is maintained in epithelial tissues may still be regarded as the major problem in the study of epithelial morphogenesis.

Vinculin in cultured bovine lens-forming cells

The distribution and organization of vinculin in bovine lens-forming cells was studied in short- and long-term cultures by using monospecific antibodies and immunofluorescence microscopy. In the epithelioid cells, originating from the epithelial cell layer of the anterior capsule, vinculin was located at contact sites of the adjacent cells. In the elongated cells, representing the cortical fiber cells, vinculin could be seen exclusively as numerous patches at the substratum-facing side of the cells. In these cells, no lateral foci of vinculin, corresponding to cell-cell contact sites, were discernible. The results show that cells originating from different parts of the bovine lens display divergent vinculin locations, probably reflecting altered cytoskeletal organization and cell-cell interactions during differentiation.

Transmethylation of phosphatidylethanolamine: an initial event in embryonic chicken lens fiber cell differentiation

Agents that induce differentiation of lens epithelial cells into lens fiber cells in vitro transiently stimulate the transmethylation of phosphatidylethanolamine. Inhibition of transmethylation by 3-deazaadenosine results in a corresponding inhibition of the cell elongation that characterizes lens fiber formation, suggesting that phospholipid methylation plays an essential role in the differentiation of these cells.

Microfilament assembly during lens cell elongation in vitro

Bovine epitheloid lens cells can be kept in culture for almost one year. During subculturing a gradual process of cell elongation is observed. Using several techniques, including immunofluorescence with antibodies against actin, electron microscopy, two-dimensional gel electrophoresis, drug treatment and measurement of actin polymerization by DNase I inhibition, it is shown that microfilament assembly parallels the process of cell elongation.

Nocodazole inhibition of the vasopressin-induced water permeability increase in toad urinary bladder

Nocodazole is a synthetic antitumor drug that binds rapidly to tubulin. When this drug is applied to toad bladder prior to vasopressin stimulation it inhibits the vasopressin response. A maximum inhibition (68%) is reached with a dose level of 10 micrograms/ml applied one-half hour prior to vasopressin stimulation (20 mU/ml). This compares with an inhibition of 50% seen with a 3-h exposure of the tissue to colchicine (0.1 mM) prior to stimulation with vasopressin. Application of nocodazole (1 microgram/ml) 3 min after hormonal stimulation shows no inhibition of the response at one-half hour past stimulation. These data support the view that microtubules are involved in the vasopressin-induced increase in water permeability in toad bladder and also indicate that this involvement is limited to the period prior to or directly after stimulation.

Beta- and gamma-crystallin synthesis in rat lens epithelium explanted with neural retinal

The main aim of this study was to determine whether the neural retina brought about the qualitative changes in protein synthesis associated with lens fibre differentiation. Explants of lens epithelium from newborn rats which only contain alpha-crystallin in detectable amounts were cultured alone or on pieces of neural retina. In the latter case the explants increased in size and contained alpha-, beta-, and gamma-crystallins. This size increase and change in crystallin composition is characteristic of lens fibre differentiation from epithelial cells. To determine if this was a specific property of the neural retina the epithelium was grown on other tissues. Corneal stroma and cartilage brought about some differentiation in the presence of foetal calf serum but were not as effective as the neural retina. Dermis had no effect on differentiation of the lens epithelium. One interpretation of these results is that the neural retina simply provides a good substratum on which the epithelium can grow and differentiate. However, epithelial explants grown in retina-conditioned medium also differentiate. Therefore the explant response must be a result of the modification of the medium by the neural retina.

Rat lens epithelial cells in vitro. II. Changes of protein patterns during aging and transformation

Serially subcultured rat lens epithelial cells grow in different stages, which can be classified according to morphology, chromosome numbers and population kinetics. A lens-specific gamma-crystallin appears in the diploid stage, when elongated cell types are observed. One of the beta-crystallin bands (pH 5.7) disappears during aging in higher passage numbers of the diploid stage B. A weak band in the beta-crystallin region (pH 6.4), which is present in all stages, becomes very intensive in aneuploid cells of stage D, which exhibit a fibroblast-like morphology.

Rat lens epithelial cells in vitro. I. Observations on aging, differentiation and culture alterations

Lens epithelial cells from rats aged 5 days were grown in long-term cultures. These cells age, differentiate and transform spontaneously. Morphological observations indicate five different stages (A-E). The epithelial character is lost after the first two passages. Elongated cells appearing afterwards are considered as cells that have started differentiation to fiberlike cells. Big flattened cells are considered as senescent cells that have lost theie proliferative capacity. Data from population kinetics also reflect these five stages. Chromosome analysis shows that three of the five stages are no longer diploid. Two alternative modes of spontaneous transformation are possible. The proliferative capacity of rat lens epithelial cells is higher than that of rat embryonic fibroblast systems.

Lentropin: a factor in vitreous humor which promotes lens fiber cell differentiation

An activity has been identified in chicken vitreous humor which stimulates embryonic chicken lens epithelial cells to elongate and specialize for lens crystallin synthesis. The activity is heat-labile and is destroyed by treatment with trypsin or agents that reduce disulfides. Gel filtration and ultrafiltration analyses indicate that it has an apparent molecular weight of approximately 60,000. Its properties differ from those of an activity present in serum which also can promote lens fiber cell formation in vitro. We call this material "lentropin" and suggest that it is responsible for stimulating lens fiber cell formation in vivo and, consequently, plays an important role in determining the shape and polarity of the lens.

Lens epithelial cell elongation in the absence of microtubules: evidence for a new effect of colchicine

Embryonic chick lens epithelial cells cultured in serum-supplemented medium elongated in the absence of microtubules after treatment with the antimicrotubule drug nocodazole. Colchicine, at concentrations lower than those that dissociate microtubules, blocks cell elongation and the associated increase in cell volume. These results indicate that an increase in cell volume, not microtubules, is responsible for lens cell elongation and suggest a previously undescribed effect of colchicine on cell volume regulation.

Contractile proteins in retinal endothelium and other non-muscle tissues of the eye

The present study is the first investigation to demonstrate, by employing the combined approach of immunologically and electron microscope methods, the presence of actin-like contractile proteins in the mammalian retina, the corneal epithelium and endothelium, the iris, and the ciliary body, and to confirm their presence in lens epithelium. This is also the first report to demonstrate by these methods the presence of microfilaments and intermediate filaments in retinal vascular endothelium. Since we have shown that actin filaments are especially abundant in immature retinal endothelial cells, the question of their function arises, and we have discussed their possible relevance to the closure of immature retinal vessels when exposed to hyperoxia.