Differentiation of lens fibers in explanted embryonic chick lens epithelia

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.

The lens equator: a platform for molecular machinery that regulates the switch from cell proliferation to differentiation in the vertebrate lens

The vertebrate lens is a transparent, spheroidal tissue, located in the anterior region of the eye that focuses visual images on the retina. During development, surface ectoderm associated with the neural retina invaginates to form the lens vesicle. Cells in the posterior half of the lens vesicle differentiate into primary lens fiber cells, which form the lens fiber core, while cells in the anterior half maintain a proliferative state as a monolayer lens epithelium. After formation of the primary fiber core, lens epithelial cells start to differentiate into lens fiber cells at the interface between the lens epithelium and the primary lens fiber core, which is called the equator. Differentiating lens fiber cells elongate and cover the old lens fiber core, resulting in growth of the lens during development. Thus, lens fiber differentiation is spatially regulated and the equator functions as a platform that regulates the switch from cell proliferation to cell differentiation. Since the 1970s, the mechanism underlying lens fiber cell differentiation has been intensively studied, and several regulatory factors that regulate lens fiber cell differentiation have been identified. In this review, we focus on the lens equator, where these regulatory factors crosstalk and cooperate to regulate lens fiber differentiation. Normally, lens epithelial cells must pass through the equator to start lens fiber differentiation. However, there are reports that when the lens epithelium structure is collapsed, lens fiber cell differentiation occurs without passing the equator. We also discuss a possible mechanism that represses lens fiber cell differentiation in lens epithelium.

Lens fibre cell differentiation and organelle loss: many paths lead to clarity

The programmed removal of organelles from differentiating lens fibre cells contributes towards lens transparency through formation of an organelle-free zone (OFZ). Disruptions in OFZ formation are accompanied by the persistence of organelles in lens fibre cells and can contribute towards cataract. A great deal of work has gone into elucidating the nature of the mechanisms and signalling pathways involved. It is apparent that multiple, parallel and redundant pathways are involved in this process and that these pathways form interacting networks. Furthermore, it is possible that the pathways can functionally compensate for each other, for example in mouse knockout studies. This makes sense given the importance of lens clarity in an evolutionary context. Apoptosis signalling and proteolytic pathways have been implicated in both lens fibre cell differentiation and organelle loss, including the Bcl-2 and inhibitor of apoptosis families, tumour necrosis factors, p53 and its regulators (such as Mdm2) and proteolytic enzymes, including caspases, cathepsins, calpains and the ubiquitin-proteasome pathway. Ongoing approaches being used to dissect the molecular pathways involved, such as transgenics, lens-specific gene deletion and zebrafish mutants, are discussed here. Finally, some of the remaining unresolved issues and potential areas for future studies are highlighted.

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.

Association of the nuclear matrix component NuMA with the Cajal body and nuclear speckle compartments during transitions in transcriptional activity in lens cell differentiation

The transcriptional status of cells can be deduced from the staining pattern of various nuclear markers such as the Cajal body, nucleolus and nuclear speckles. In this study we have used these markers to correlate transcriptional status with cell differentiation in the lens. As a closed system with no cell loss and with each stage being spatially preserved, it is particularly well suited to such studies. To confirm that the nuclear markers in lens cells follow the same trends as in other cells, primary bovine lens epithelial cells were cultured and then treated with actinomycin D to inhibit transcription. This reduced the Cajal body markers to one or two foci per nucleus and the nucleoli became compacted as revealed by fibrillarin staining. The nuclear speckles, containing snRNPs (e.g. Sm) and the splicing factor, SC35, also became larger and more numerous while the signal for trimethylguanine (TMG) decreased suggesting a role hierarchy for the various speckle factors during transcriptional shutdown. The signal for survival of motor neurones gene product (SMN) also decreased at this point. In the lens epithelium, postmitotic cells near the equatorial region had one or two Cajal bodies per nucleus, indicating these cells had only basal levels of transcription. Sm was also present as large foci in these cells. Interestingly, both the speckles and Cajal bodies were NuMA-positive in these post-mitotic cells. At the epithelial-fibre cell transition, Cajal body number increased, while their size decreased indicative of increased transcriptional activity. Fibrillarin adopted the open floret pattern indicating increased transcriptional activity. The nuclear speckles adopted a more diffuse nucleoplasmic pattern, although some spots were still observed. All NuMA colocalisation with the Cajal bodies and nuclear speckles was lost at this stage of lens cell differentiation. Transcriptional shutdown occurs at a later stage in fibre cell differentiation, prior to programmed nuclear destruction. In the lens, both the Cajal bodies and nuclear speckles again became NuMA-positive, although separate NuMA spots were also formed during transcriptional shutdown. These data suggest the nuclear matrix is important in the concentration of Cajal body and speckle components into large, distinct spots in transcriptionally inactive nuclei and also suggest a new role for NuMA in post-mitotic cells to assist in these sub-nuclear reorganisations.

The development-associated cleavage of lens connexin 45.6 by caspase-3-like protease is regulated by casein kinase II-mediated phosphorylation

Gap junctions are important in maintaining lens transparency and metabolic homeostasis. In this paper, we report that the gap junction-forming protein, connexin (Cx) 45.6, was specifically truncated during lens development and that the majority of the truncated fragments were located in the differentiated lens fibers. When isolated lens membranes were treated by caspase-3, the truncated fragments of Cx45.6 were reproduced, and this truncation occurred at the COOH terminus of Cx45.6. Moreover, when primary lens cells were treated with apoptosis-inducing reagents, Cx45.6 was cleaved similarly as the in vitro treatment by caspase-3, and this cleavage was blocked by a caspase-3 inhibitor. These results suggest that caspase-3 is responsible for the development-associated cleavage of Cx45.6. The cleavage site of Cx45.6 was identified between amino acid residues Glu(367) and Gly(368). We have shown previously that Ser(363) is an in vivo phosphorylated site by casein kinase II, and this specific phosphorylation leads to a rapid turnover of Cx45.6. Interestingly, we found here that when Ser(363) was phosphorylated by casein kinase II, the cleavage of Cx45.6 catalyzed by caspase-3 was inhibited. This study, for the first time, demonstrates that a connexin can be a direct target of an apoptotic protease and that cleavage by caspase-3-like protease leads to the development-associated truncation of a lens connexin. Finally, caspase-3-mediated cleavage can be regulated by casein kinase II-mediated phosphorylation, suggesting that Cx45.6 turnover and specific cleavage by caspase-3-like protease is alternatively modulated.

The tensegrity model applied to the lens: a hypothesis for the presence of the fiber cell ball and sockets

The concept of tensegrity refers to the tensional integrity of a system. Such a system mechanically stabilizes itself by balancing the opposing forces of tension and compression. One category of this model involves a 'pre-stressed' condition in which the structural elements are in a state of tensional compression. The other category includes rigid struts that can sustain the opposing forces. The lens of the eye contains epithelial cells that differentiate at the lens equator to fiber cells. With time, these fiber cells are packed into the center of the lens or the lens nucleus. Part of the process of differentiation is the loss of the cellular nucleus and organelles. During maturation of the lens fiber cells, much of the cytoskeleton is lost as a result of proteolysis. As the lens cells mature, ball and socket joints and interlocking ridges appear on the cellular surface. Applying the tensegrity model to the lens, it appears that the lens cells go from prestressed state of tensional compression to the other state that is defined by rigid connections between cells.

Casein kinase II phosphorylates lens connexin 45.6 and is involved in its degradation

Connexin (Cx) 45.6, an avian counterpart of rodent Cx50, is phosphorylated in vivo, but the sites and function of the phosphorylation have not been elucidated. Our peptide mapping experiments showed that the Ser(363) site in the carboxyl (COOH) terminus of Cx45.6 was phosphorylated and that this site is within casein kinase (CK) II consensus sequence, although showing some similarity to CKI sequence. The peptide containing Ser(363) could be phosphorylated in vitro by CKII, but not by CKI. Furthermore, CKII phosphorylated Cx45.6 in embryonic lens membrane and the fusion protein containing the COOH terminus of Cx45.6. Two-dimensional peptide mapping experiments showed that one of the Cx45.6 peptides phosphorylated in vivo migrated to the same spot as one of those phosphorylated by CKII in vitro. Furthermore, CKII activity could be detected in lens lysates. To assess the function of this phosphorylation event, exogenous wild type and mutant Cx45.6 (Ser(363) --> Ala) were expressed in lens primary cultures by retroviral infection. The mutant Cx45.6 was shown to be more stable having a longer half-life compared with wild type Cx45.6. Together, the evidence suggests that CKII is likely a kinase responsible for the Ser(363) phosphorylation, leading to the destablization and degradation of Cx45.6. The connexin degradation induced by phosphorylation has a broad functional significance in the regulation of gap junctions in vivo.

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.

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.

Molecular cloning and functional characterization of chick lens fiber connexin 45.6

The avian lens is an ideal system to study gap junctional intercellular communication in development and homeostasis. The lens is experimentally more accessible in the developing chick embryo than in other organisms, and chick lens cells differentiate well in primary cultures. However, only two members of the connexin gene family have been identified in the avian lens, whereas three are known in the mammalian system. We report here the molecular cloning and characterization of the third lens connexin, chick connexin45.6 (ChCx45.6), a protein with a predicted molecular mass of 45.6 kDa. ChCx45.6 was encoded by a single copy gene and was expressed specifically in the lens. There were two mRNA species of 6.4 kilobase (kb) and 9.4 kb in length. ChCx45.6 was a functional connexin protein, because expression in Xenopus oocyte pairs resulted in the development of high levels of conductance with a characteristic voltage sensitivity. Antisera were raised against ChCx45.6 and chick connexin56 (ChCx56), another avian lens-specific connexin, permitting the examination of the distribution of both proteins. Immunofluorescence localization showed that both ChCx45.6 and ChCx56 were abundant in lens fibers. Treatment of lens membranes with alkaline phosphatase resulted in electrophoretic mobility shifts, demonstrating that both ChCx45.6 and ChCx56 were phosphoproteins in vivo.

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.

Lens fiber differentiation correlated with activation of two different DNAases in lens embryonic cells

In order to identify the different DNAases present in the lens differentiating tissue, we have used an assay which reveals their activity directly on DNA-containing gels after SDS polyacrylamide gel electrophoresis. DNAase renaturation from nuclear embryonic lens extracts does not occur after separation in 0.1% SDS polyacrylamide gel electrophoresis in contrast to that observed with purified micrococcal nuclease. When the SDS concentration in the running buffer and separating gel is decreased to 0.075%, renaturation of lens DNAase and enzyme activities are observed. Isoelectrofocusing was carried out in a polyacrylamide gel which was overlaid with an agarose gel containing DNA, permitting the visualization of the pI of DNAase activity. The presence of several DNAase isoenzymes was demonstrated in 11-day embryonic lenses. In epithelial lens nuclei, high molecular weight (MW) isoenzymes with basic pI were predominant. In post-mitotic fiber lens nuclei, two lower MW isoenzymes with acidic pI were detected as well as high MW activity with a basic pI.

Increased sensitivity of various genes to endogenous DNase activity in terminal differentiating chick lens fibers

In the lens, epithelial cells from the equatorial zone differentiate into postmitotic elongated fibers. One aspect of this differentiation is nuclear shape transformation and DNA degradation. This process is controlled by DNase activity which in fiber nuclei increases with development. DNase activity is also present in the epithelial cell nuclei which appears to be non-functional but could be activated in vitro by exogenous addition of Ca2+. We have analyzed the possible selective action of endogenous DNase on 3 genes involved in lens terminal differentiation, namely delta-crystallin, beta-tubulin and vimentin, and on 1 gene not thought to participate in this process, ovalbumin. We have compared restriction DNA patterns of these genes in nuclei isolated from 11-day-old chick embryos and incubated in Ca2+-free medium or in fresh epithelial and fiber lens tissue at 11 and 18 days of development. During incubation in vitro of 11-day fiber nuclei, there is a net increase in the sensitivity of the delta-crystallin, beta-tubulin, ovalbumin and vimentin chromatin to the endogenous DNase. The vimentin gene appears to be more stable than the beta-tubulin and delta-crystallin genes indicating a degree of specificity of the endogenous DNase activity. In the epithelial nuclei, the lens-specific genes appear to be more stable but paradoxically there is a net degradation of the ovalbumin gene. In freshly isolated tissues the 4 genes were detected in epithelial and fiber cells at 11 and 18 days. Furthermore, in the mature fibers in which the nuclei were degenerating, the latter genes were still not completely digested.

Crystallins and their synthesis in human lens epithelial cells in tissue culture

Explants of epithelial cells from young human lenses of 5-12 months of age, obtained from patients who underwent surgery for retinopathy of prematurity, were cultured in Dulbecco's modified Eagle's medium supplemented with 20% fetal calf serum. Without exception, every piece of the anterior capsule explant showed cell outgrowth within 48-72 h and resulted in confluent monolayer culture within 2 weeks. From these monolayer cultures, two to three passages of subcultures were obtained by routinely seeding cells in a ratio of 1:4. The doubling times for these human lens epithelium (HLE) cultures during the first 4 weeks of two passages were found to be 24-36 h. In a majority of cultures through the first three passages, more than 12 population doublings were attained. However, no lentoid bodies were formed during this period. These cells were studied for the presence of crystallins and their synthesis. Using SDS-polyacrylamide gel electrophoresis, the presence of alpha- and beta-crystallins was demonstrated in HLE cells through three passages. The amount of alpha-crystallin in the first two passages amounted to nearly 13% of the total protein, but decreased significantly in the third passage. The presence of crystallins was corroborated by antibody reaction to the specific crystallins. Indirect immunofluorescence revealed the presence of actin and vimentin in these cell cultures. The synthesis of crystallins in HLE cultures was shown by the incorporation of [35S]methionine which was time dependent. The crystallin synthesis was found to decrease in third passage when the cell growth slowed down without consistent formation of confluent monolayer. These studies have demonstrated that primary cultures of HLE cells can be successfully grown from young lenses through several passages which continue to express the characteristic crystallins of the epithelial cells.

Differentiation of retrovirus-infected avian neuroretina cells

The effects of oncogenic retroviruses on the expression of differentiation markers were studied in monolayer cultures of chick and quail embryo neuroretinas. Transformation by Rous sarcoma virus (RSV) did not affect the appearance of synapses, and the expression of glutamic acid decarboxylase was stimulated by pp60v-src, the product of the src gene. Quail embryo neuroretina cells transformed by Mill Hill 2 (which contains the two oncogenes v-mil and v-myc) were induced to proliferate into permanent cultures that synthesized crystallins and produced lentoid bodies. In contrast, transformation with a temperature-sensitive mutant of RSV reversibly blocked the production of crystallins and lentoid bodies. These data show that given cellular genes can respond differently to distinct oncogenes.

Crystallin gene expression and lentoid body formation in quail embryo neuroretina cultures transformed by the oncogenic retrovirus Mill Hill 2 or Rous sarcoma virus

The lens-specific proteins alpha and delta crystallins and lentoid bodies, structures that follow a differentiation pathway similar to that of the lens, regularly appear after 4 to 5 weeks in quail embryo neuroretina monolayer cultures. We have investigated the effects of the avian oncogenic retroviruses Mill Hill 2 and Rous sarcoma virus on this process. Quail embryo neuroretina cells transformed by Mill Hill 2 virus were established into permanent cultures that synthesized alpha and delta crystallins and contained stem cells for the production of lentoid bodies. In contrast, transformation with the Rous sarcoma virus mutant tsNY-68 blocked the appearance of mRNA crystallins, but cytoplasmic alpha and delta crystallin mRNA and alpha crystallin appeared 44 h after a shift to the nonpermissive temperature. However, delta crystallins and lentoid bodies were only present after 7 days. The crystallins of transformed quail neuroretina cultures were immunologically indistinguishable from those of quail lenses and of normal quail embryo neuroretina cultures.

Crystallin gene expression and lentoid body formation in quail embryo neuroretina cultures transformed by the oncogenic retrovirus Mill Hill 2 or Rous sarcoma virus

The lens-specific proteins alpha and delta crystallins and lentoid bodies, structures that follow a differentiation pathway similar to that of the lens, regularly appear after 4 to 5 weeks in quail embryo neuroretina monolayer cultures. We have investigated the effects of the avian oncogenic retroviruses Mill Hill 2 and Rous sarcoma virus on this process. Quail embryo neuroretina cells transformed by Mill Hill 2 virus were established into permanent cultures that synthesized alpha and delta crystallins and contained stem cells for the production of lentoid bodies. In contrast, transformation with the Rous sarcoma virus mutant tsNY-68 blocked the appearance of mRNA crystallins, but cytoplasmic alpha and delta crystallin mRNA and alpha crystallin appeared 44 h after a shift to the nonpermissive temperature. However, delta crystallins and lentoid bodies were only present after 7 days. The crystallins of transformed quail neuroretina cultures were immunologically indistinguishable from those of quail lenses and of normal quail embryo neuroretina cultures.

Lens-specific promoter activity of a mouse gamma-crystallin gene

Crystallins are the major water-soluble proteins in vertebrate eye lenses. These lens-specific proteins are encoded by several gene families, and their expression is differentially regulated during lens cell differentiation. Here we show that a cloned mouse gamma-crystallin promoter is active in lens explants derived from 14-day-old chicken embryos but inactive in a variety of cells of non-lens origin. We also show that sequences required for proper utilization of this promoter are contained between nucleotide positions -392 and +47 relative to the transcription initiation site; deletion of sequences from positions -392 to -171 completely abolishes promoter activity. Since chickens do not have gamma-crystallin genes, the expression of a mouse gamma-crystallin promoter in chicken lens cells suggests that different classes of crystallin genes may be regulated by common lens tissue-specific mechanism(s) independent of species.

Cytodifferentiation and tissue phenotype change during transformation of embryonic lens epithelium to mesenchyme-like cells in vitro

A number of adult and embryonic epithelia, when suspended within native type I collagen gels, give rise to elongate bipolar cells that migrate freely within the three-dimensional matrix. The morphology of these newly formed mesenchyme-like cells is indistinguishable from "true" mesenchymal cells at the light and ultrastructural level. In this report, we extend previous observations on the transformation of embryonic avian lens epithelium to mesenchyme-like cells. Lens epithelia, dissected from 12-day chick embryos, were cultured either within a collagen matrix or on a two-dimensional surface. Cells derived from explants on the surface of type I collagen express the epithelial phenotype. The cells form new basal lamina, continue to express delta-crystallin protein and secrete both type IV collagen and laminin. In contrast, epithelia suspended within collagen gels lose epithelial morphology, phenotype, and cytodifferentiation. The newly formed mesenchyme-like cells lack the ability to synthesize lens-specific delta-crystallin protein, type IV collagen, and laminin. They do, however, express type I collagen de novo, a characteristic of mesenchymal cells. The changes in cytodifferentiation and tissue phenotype which occur during the transformation are stable under the conditions studied here. When mesenchyme-like cells are removed from the gel and replated onto two-dimensional surfaces, they remain bipolar, will invade collagen matrices, and are unable to synthesize delta-crystallin protein.

Nuclear endogenous Ca2+-dependent endodeoxyribonuclease in differentiating chick embryonic lens fibers

During terminal differentiation of lens epithelial cells into fiber cells, nuclei become pycnotic and DNA degradation occurs. We investigated the putative role in this process of an endogenous DNAase. After incubation of isolated nuclei of both cell types at 37 degrees C, DNAase activity was revealed by DNA size analysis on 0.3-1% neutral and alkaline agarose, one- and two-dimensional gels. This DNAase activity is more prominent in lens fiber nuclei than in epithelial nuclei at all the embryonic stages probably because of a preexisting higher concentration of divalent cations in the former. This activity is calcium or magnesium dependent in both types of nuclei.

Lens-specific promoter activity of a mouse gamma-crystallin gene

Crystallins are the major water-soluble proteins in vertebrate eye lenses. These lens-specific proteins are encoded by several gene families, and their expression is differentially regulated during lens cell differentiation. Here we show that a cloned mouse gamma-crystallin promoter is active in lens explants derived from 14-day-old chicken embryos but inactive in a variety of cells of non-lens origin. We also show that sequences required for proper utilization of this promoter are contained between nucleotide positions -392 and +47 relative to the transcription initiation site; deletion of sequences from positions -392 to -171 completely abolishes promoter activity. Since chickens do not have gamma-crystallin genes, the expression of a mouse gamma-crystallin promoter in chicken lens cells suggests that different classes of crystallin genes may be regulated by common lens tissue-specific mechanism(s) independent of species.

DNA repeat size in chick embryonic lens epithelium, lens fiber, brain and liver cell nuclei

The DNA repeat size is determined by micrococcal nuclease digestion kinetics and subsequent electrophoresis of the products among various chick embryonic tissues. The repeat size is found to be not significantly different from 193 to 197 bp, for brain and liver at 11 days and for lens epithelium and fiber at different embryonic stages. However, the pattern of micrococcal digestion seems to reveal an overall chromatin modification as a function of development in the lens fibers.

Lens fiber organization in four avian species: a scanning electron microscopic study

The three-dimensional organization of the eye lenses of the chicken, the canary, the song-thrush and the kestrel was studied using light and scanning electron microscopy. The lenses of birds are characterized by the presence of two distinct compartments: the annular pad and the main lens body, separated by a cavum lenticuli. The annular pad fibers had a hexagonal circumference all contained a round nucleus and except for the canary were smooth-surfaced and lacking anchoring devices. In the canary, however, the annular pad fibers were studded with edge protrusions and ball-and-socket junctions. The semicircular main lens body fibers of all four species were studded with ball-and-socket junctions and edge protrusions. In contrast with mammals these anchoring devices were present throughout the lens up to the embryonal nucleus. Superficially the main lens body fibers were extremely flat. Additionally membrane elevations and depressions and globular elements were found on these central fibers in three species, the kestrel being the exception. At the transition between annular pad and main lens body the fibers turned their course and the nuclei became oval and disappeared in the deeper aspect of the main lens body. The cavum lenticuli was filled with globules tied off from the annular pad fibers. It seems attractive to assume that the presence of a separated annular pad, a cavum lenticuli filled with globular elements, the extreme flatness of the superficial central fibers and the studding of these central fibers with anchoring devices up to the embryonal nucleus are morphological expressions of the mouldability of the bird's eye lenses and consequently would explain their efficient accommodative mechanism including formation of a lenticonus. The presence of nuclei in the annular pad fibers and their typical change at the transitional zone between annular pad and main lens body are suggestive for a two-phased differentiation in bird's lens fibers: differentiation of the germinative epithelial cells to annular pad fibers which migrate to the main lens body after which they differentiate further to main lens body fibers.

Detection in situ of gamma-ray-induced DNA strand breaks in single cells: enzymatic labelling of free 3'-OH ends

We report a procedure allowing the detection and counting of free 3'-OH DNA strand extremities in single cells in situ. Terminal transferase (TdT) catalysed the incorporation of 3H-dGMP into fixed nuclei of human colonic adenocarcinoma cells (HT29), using free 3'-OH ends as initiator. Radioactivity was detected by autoradiography and determined quantitatively with a rapid image-processing system for grain counting. The initiator activity for TdT increases with the dose of gamma-rays in the dose range 2.5-20 Gy.

Chicken embryo lens cultures mimic differentiation in the lens

Embryonic chicken lenses, which had been disrupted by trypsin, were grown in culture. These cultures mimic lens development as it occurred in vivo, forming lens-like structures known as lentoids. Using a variety of techniques including electron microscopic analysis, autoradiography, immunofluorescence, and polyacrylamide gel electrophoresis, it was shown that the lentoid cells had many characteristics in common with the differentiated cells of the intact lens, the elongated fiber cells. These characteristics included a shut off of DNA synthesis, a loss of cell organelles, an increase in cell volume, an increase in delta-crystallin protein, and the development of extensive intercellular junctions. The cultures began as a simple epithelial monolayer but then underwent extensive morphogenesis as they differentiated. This morphogenesis involved three distinctive morphological types which appeared in sequence as an epithelial monolayer of polygonal shaped cells with pavement packing, elongated cells oriented end to end, and the multilayered, multicellular lentoids. These distinct morphological stages of differentiation in culture mimic morphogenesis as it occurs in the lens.

Changes in the DNA breakage and crystallin synthesis of embryonic chicken lenses cultured in a tryptophan-deficient medium

In vitro experiments were performed in order to understand the biochemistry of tryptophan-deficient cataract. Eleven-day-old embryonic chick lenses were cultured in vitro for 3 hr, one and three days in a tryptophan-deficient medium. DNA breakage was followed on sucrose gradient and water-soluble protein synthesis was analysed by SDS-PAGE coupled with fluorography. A medium lacking tryptophan delays the DNA degradation and decreases the synthesis of all soluble proteins including the crystallins.

Phosphatidylcholine and phosphatidylethanolamine metabolism during lens fiber cell formation

Phosphatidylinositol is metabolized with a half-life of about 5 h in lens epithelial cells of 6-day-old embryonic chickens. When these cells differentiate to form lens fiber cells, however, phosphatidylinositol turnover virtually ceases. The present study was undertaken to determine whether there is a similar change in the metabolism of phosphatidylcholine and phosphatidylethanolamine. [32P]Orthophosphate was injected into 6-day-old chicken embryos, and the incorporation of label into phosphatidylcholine and phosphatidylethanolamine was followed for 48 h. The specific activities of the precursors phosphorylcholine and phosphorylethanolamine were also measured during this time. The data were then analysed by means of a simple kinetic model to determine the rate of synthesis and the half-life of each phospholipid. The results showed that phosphatidylcholine is synthesized at a rate of about 1.2 X 10(-20) mol/s per cell in the lens epithelial cells, and 6.4 X 10(-20) mol/s per cell in the fiber cells. Phosphatidylethanolamine is synthesized at approximately 0.9 X 10(-2)) mol/s per cell in the epithelial cells, and 4.0 X 10(-20) mol/s per cell in the fiber cells. Both phospholipids are stable in both the epithelial cells and in the fiber cells, with half-lives of 48 h or greater. Thus, although phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol all experience an increase in synthesis following lens fiber formation, the previously observed decrease in phosphatidylinositol turnover accompanying differentiation is a specific effect.

Epithelia suspended in collagen gels can lose polarity and express characteristics of migrating mesenchymal cells

This study of epithelial-mesenchymal transformation and epithelial cell polarity in vitro reveals that environmental conditions can have a profound effect on the epithelial phenotype, cell shape, and polarity as expressed by the presence of apical and basal surfaces. A number of different adult and embryonic epithelia were suspended within native collagen gels. Under these conditions, cells elongate, detach from the explants, and migrate as individual cells within the three-dimensional lattice, a previously unknown property of well-differentiated epithelia. Epithelial cells from adult and embryonic anterior lens were studied in detail. Elongated cells derived from the apical surface develop pseudopodia and filopodia characteristic of migratory cells and acquire a morphology and ultrastructure virtually indistinguishable from that of mesenchymal cells in vivo. It is concluded from these experiments that the three-dimensional collagen gel can promote dissociation, migration, and acquisition of secretory organelles by differentiated epithelial cells, and can abolish the apical-basal cell polarity characteristic of the original epithelium.

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.

DNA polymerase, DNA ligase, and thymidine kinase activity in chicken lens, related to DNA X-ray lesion repair

The activity of DNA polymerases alpha, beta, and gamma; DNA ligases I and II; and thymidine kinase in chicken lenses is determined. These enzymes are present in embryonic intact lenses freshly isolated at 6 days and 11 days of development and in lenses isolated at 11 days of development and cultured for three days. They are also found in both epithelium and fibers when separated at 10 days of embryonic development and in the epithelium of 141/2-month-old hen lenses. In the anucleate mature hen lens fibers, the only detected enzyme is thymidine kinase. Previous results showed that repair of X-ray irradiated DNA was total in 11-day-old embryonic lenses while repair was not detected at six days. On the other hand, repair was very much impaired when the 11-day-old lenses were cultured for three days. Enzymic analyses suggest that the defect in DNA repair and the spontaneous DNA degradation observed in chick lenses at certain embryonic stages, are not due to the absence of any of the above enzymic activities. Alternative explanations are discussed.

Evolution of the distribution, proliferation and ultraviolet repair capacity of rat lens epithelial cells as a function of maturation and aging

The symmetrical organization of lens epithelium was a determining factor in this study whose purpose was to investigate the growth of this tissue during development, maturation and aging. By direct observation it was possible to score all the nuclei, to determine four different zones of cellular density and to monitor them during the whole lifespan. The central zone is mainly quiescent and the proliferation at the periphery accounts for the low steady growth rate of the tissue. With aging there is a continuous decrease in the mitotic index and a cell size enlargement. These cells were able to perform unscheduled DNA synthesis in vitro after ultraviolet irradiation. There is an age-related decrease in DNA repair, but most of it occurs during development (until 59 weeks of age) and then remains constant (59-173 weeks). These results mean that in a differentiated pure cell population aging is not directly related to decline in unscheduled DNA synthesis.

Analysis of DNA double- and single-strand breaks by two dimensional electrophoresis: action of micrococcal nuclease on chromatin and DNA, and degradation in vivo of lens fiber chromatin

We describe a novel system for two dimensional electrophoresis at neutral and alkaline pH for determining the double-stranded and single-stranded lengths of DNA. With this system we analysed the mode of micrococcal nuclease digestion of DNA in cellular and SV40 viral chromatin and of supercoiled SV40 DNA. The enzyme reaction occurred in two steps : the enzyme first introduced single-strand breaks, then converted these to double-strand breaks by an adjacent cleavage on the opposite strand. Digestion of cellular chromatin DNA occurred by a similar mechanism. Chromatin fragments produced by limited micrococcal nuclease action contained many single-strand breaks, which may be important when this method is used to prepare chromatin fragments for biochemical and biophysical studies. Nucleosome monomer to tetramer produced at later stages of digestion contained few if any single-strand breaks.

Cycloheximide effect on DNA degradation and delta-crystallin synthesis in terminally differentiating lens cells

Low concentrations of a protein synthesis inhibitor, cycloheximide, were added throughout the process of in vitro differentiation of 11-day old embryonic chick lens cells. We found with low concentrations of cycloheximide (0.01 to 0.03 microgram/ml, 3 days of culture), that there was an almost complete delay of DNA degradation as observed on alkaline sucrose gradient. Identical concentrations and exposure time had no blocking effect on increased delta-crystallin synthesis as detected by immunoprecipitation and electrophoresis. Higher concentrations of cycloheximide (0.1 to 1 microgram/ml) showed a marked effect on DNA size and a net inhibition on delta-crystallin synthesis. Thus a selective effect of low doses of cycloheximide was observed on terminal differentiation suggesting that there was not a relationship between DNA degradation and delta-crystallin synthesis in these short term experiments. The investigations of minor proteins could be of interest as they may have a crucial role in intact nuclei cataracts.

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.

Longevity, stability and DNA repair

The functional capacity of a cell, tissue, organ, or organism is dependent upon its ability to maintain the stability of its unit components. The higher the differentiated state of the system, the greater the amount of stability required to maintain that state as a function of time. Stability can be achieved via either redundancy or repair. Redundancy while easily achievable in biological systems is both costly and limited by thermodynamic considerations. Repair, in its general sense, has no such limitations. Repair at the cellular and macromolecular level is multiple in its forms and varies as a function of species, tissue, and stage of the cell cycle. The repair of DNA damage is a dynamic process with many components and subcomponents, each interacting with one another in order to achieve a balance between individual stability and evolutionary diversity. Thus, between internal and external factors which damage DNA and the subsequent expression of alterations in the functional stability of DNA lie the multi-functional pathways which attempt to maintain DNA fidelity. A strong correlation between ulta-violet light induced excision or pre-replication repair, as measured by autoradiogrphy and maximum species lifespan has been reported within different strains of the same species, between related species (e.g. Mus musculus and Peromyscus leucopus), between five orders of mammals, and most recently within members of the primate family. As has been demonstrated by the authors and others, differences in excision repair between species and tissues may relate to the turning off of portions of the repair processes during embryogenesis. Regardless of why such correlations exist or the nature of their mechanisms, it is naive to either assert or deny a causal role for DNA repair in longevity assurance systems. For example, while species-related differences in DNA repair may reflect the turning off of such repair processes during fetal development this does not mean that rates of accumulation of DNA damage are not altered by such changes. Indeed, such a phenomena might well explain the rapid evolution of lifespan within the primates without a concurrent input of new genes.

Gel electrophoretic analysis of histones in lens epithelium, lens fiber, liver, brain, and erythrocytes of late chick embryos

Histones from 19-day-old chick embryo lens epithelium, lens fibers, liver, brain, and erythrocytes were electrophoresed in polyacrylamide gels using buffers containing sodium dodecylsulfate, acetic acid urea, or mixtures of Triton X-100 acetic acid urea. In the last two buffer systems, histone bands were characterized by their apparent molecular weights determined by electrophoresis in the second dimension in sodium dodecylsulfate containing polyacrylamide gels. From the densitograms of the stained gels, the relative proportion of protein in different histone bands was estimated. With the exception of the erythrocyte-specific histone H5, all histones from different tissues examined at any of the gel systems migrated with the same mobilities. In lens epithelium and lens fibers, all histones were present in identical proportions. As compared to liver and brain, the total amount of histone Hl was significantly lower in lens cells and erythrocytes, possibly reflecting differences between the differentiated states. However, no tissue-specific differences were found in the relative distribution of histone Hl I and Hl II among lens epithelium, lens fiber, liver and, brain, but a threefold higher Hl I : Hl II ratio (0.5--0.7) was found in erythrocytes.

DNA repair in lens cells during chick embryo development

When chick lens epithelium is cultured in vitro, differentiation into lens fiber cells is accompanied by DNA degradation. This phenomenom of terminal differentiation was studied in the epithelium from embryos at the 6th and 11th days of development. DNA size and the ability of the cells to repair DNA damage induced by X-rays were analysed in alkaline sucrose gradients. In the 6-day epithelium a rapid degradation and complete lack of DNA repair were recorded. Similar observations have been made in previous studies on the 11-day sample, but here degradation is progressive and occurs after a lag of several days. In the younger epithelium, internal irradiation by [3H]thymidine also had a drastic effect resembling that caused by X-rays. In order to assess the process of differentiation in our experimental system the synthesis of delta- and alpha-crystallins was monitored. Stage-related modifications in the rates of synthesis were recorded. The results confirm that the DNA repair system is impaired during terminal differentiation. The differences observed between the two stages may reflect either a developmental modification in DNA repair mechanisms or a change in the relative proportions of differentiating cells. An hypothesis is proposed in support of the latter case.