The lens: a model for chromatin degradation studies in terminally differentiating cells

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.

Apoptosis in lens development and pathology

The ocular lens is a distinct system to study cell death for the following reasons. First, during animal development, the ocular lens is crafted into its unique shape. The crafting processes include cell proliferation, cell migration, and apoptosis. Moreover, the lens epithelial cells differentiate into lens fiber cells through a process, which utilizes the same regulators as those in apoptosis at multiple signaling steps. In addition, introduction of exogenous wild-type or mutant genes or knock-out of the endogenous genes leads to apoptosis of the lens epithelial cells followed by absence of the ocular lens or formation of abnormal lens. Finally, both in vitro and in vivo studies have shown that treatment of adult lens with stress factors induces apoptosis of lens epithelial cells, which is followed by cataractogenesis. The present review summarizes the current knowledge on apoptosis in the ocular lens with emphasis on its role in lens development and pathology.

Nuclear degeneration in the developing lens and its regulation by TNFalpha

DNA fragmentation in lens fibre cell nuclei undergoing programmed degeneration was identified by terminal deoxynucleotidyl transferase (TdT)-mediated biotin-dUTP nick end labelling (TUNEL). Lens epithelial cells in culture were induced to differentiate into lens fibre-like clumps of cells (lentoids) by insulin and it was shown that the TUNEL method was also an effective means of labelling degenerating nuclei in lentoid cells in lens epithelial cell cultures. Using immuno-fluorescence and confocal microscopy, it was shown that TNFalpha and TNF receptor (TNFR1, and TNFR2) immunoreactivity was present in sections of chick embryo lenses. TNFalpha immunoreactivity was associated with the lens epithelium and lens fibres. TNFR1 immunoreactivity was present in lens epithelial cells, cortical lens fibres, and lens fibre cell nuclei, while TNFR2 immunoreactivity had a similar distribution to that of TNFR1, but was not associated with nuclei. Similar patterns of TNFalpha, TNFR1, and TNFR2 immunoreactivity were observed in lens epithelial cell cultures. When added to lens epithelial cell cultures, TNFalpha, at concentrations of 50 to 100 ng ml-1, and agonistic antibodies to both TNFR1 and TNFR2 significantly (P<0.05) enhanced the number of degenerating (TUNEL-positive) nuclei. On the other hand, a neutralising antibody to TNFalpha significantly (P<0. 05) reduced the number of TUNEL-positive nuclei. These results demonstrate that TUNEL is an effective means of labelling degenerating lens fibre nuclei during lens fibre and lentoid differentiation, and suggest a potential role for TNFalpha-like factors and their receptors in the degeneration of lens fibre cell nuclei during lens differentiation. We further suggest that the nuclear degeneration of lens fibre cells is analogous to the nuclear events that occur during apoptosis, and that in lens cells the nuclear degeneration is uncoupled from the plasma membrane events of apoptosis that normally lead to cell death.

Occurrence of apoptosis in serosa of Periplaneta americana l. (Blattaria: blattidae): ultrastructural and biochemical features

In 16-17-day-old embryos of Periplaneta americana, the amnion-serosa penetrates the cavity of the middle intestine, where it forms a cluster of compressed roundish cells. We demonstrated that these cells degenerate throughout apoptosis. The programmed cell death revealed by morphological and biochemical approaches showed all the apoptotic steps: chromatin fragmentation and pyknosis, cytoplasm condensation, karyorrhexis, cytoplasm cleavage. Nevertheless, some ultrastructural peculiarities (atypical heterochromatin arrangement, appearance of nuclear envelope protrusions, absence of nucleolar structures) suggest that the apoptotic expression partially depends on the biological situation (type of organism and inducing factors) in which the programmed cell death takes place. The presence of histiocytic cells internalizing cell debris, of apoptotic and non-apototic derivation, may be correlated with the importance of recycling substances useful for embryo growth.

Mitochondrial proliferation and paradoxical membrane depolarization during terminal differentiation and apoptosis in a human colon carcinoma cell line

Herbimycin A, a tyrosine kinase inhibitor, induces cellular differentiation and delayed apoptosis in Colo-205 cells, a poorly differentiated human colon carcinoma cell line. Cell cycle analysis in conjunction with end labeling of DNA fragments revealed that G2 arrest preceded apoptotic cell death. Ultrastructural examination of herbimycin-treated cells demonstrated morphologic features of epithelial differentiation, including formation of a microvillar apical membrane and lateral desmosome adhesions. A marked accumulation of mitochondria was also observed. Fluorometric analysis using the mitochondrial probes nonyl-acridine orange and JC-1 confirmed a progressive increase in mitochondrial mass. However these cells also demonstrated a progressive decline in unit mitochondrial transmembrane potential (DeltaPsim) as determined by the DeltaPsim-sensitive fluorescent probes rhodamine 123 and JC-1 analyzed for red fluorescence. In concert with these mitochondrial changes, Colo-205 cells treated with herbimycin A produced increased levels of reactive oxygen species as evidenced by oxidation of both dichlorodihydrofluorescein diacetate and dihydroethidium. Cell-free assays for apoptosis using rat-liver nuclei and extracts of Colo-205 cells at 24 h showed that apoptotic activity of Colo-205 lysates requires the early action of mitochondria. Morphological and functional mitochondrial changes were observed at early time points, preceding cleavage of poly (ADP-ribose) polymerase. These results suggest that apoptosis in differentiated Colo-205 cells involves unrestrained mitochondrial proliferation and progressive membrane dysfunction, a novel mechanism in apoptosis.

Chromatin degradation in differentiating fiber cells of the eye lens

During development, the lens of the eye becomes transparent, in part because of the elimination of nuclei and other organelles from the central lens fiber cells by an apoptotic-like mechanism. Using confocal microscopy we showed that, at the border of the organelle-free zone (OFZ), fiber cell nuclei became suddenly irregular in shape, with marginalized chromatin. Subsequently, holes appeared in the nuclear envelope and underlying laminae, and the nuclei collapsed into condensed, spherical structures. Nuclear remnants, containing DNA, histones, lamin B2, and fragments of nuclear membrane, were detected deep in the OFZ. We used in situ electrophoresis to demonstrate that fragmented DNA was present only in cells bordering the OFZ. Confocal microscopy of terminal deoxynucleotidyl transferase (TdT)-labeled lens slices confirmed that DNA fragmentation was a relatively late event in fiber differentiation, occurring after the loss of the nuclear membrane. DNA fragments with 3'-OH or 3'-PO(4) ends were not observed elsewhere in the lens under normal conditions, although they could be produced by pretreatment with DNase I or micrococcal nuclease, respectively. Dual labeling with TdT and an antibody against protein disulfide isomerase, an ER-resident protein, revealed a distinct spatial and temporal gap between the disappearance of ER and nuclear membranes and the onset of DNA degradation. Thus, fiber cell chromatin disassembly differs significantly from classical apoptosis, in both the sequence of events and the time course of the process. The fact that DNA degradation occurs only after the disappearance of mitochondrial, ER, and nuclear membranes suggests that damage to intracellular membranes may be an initiating event in nuclear breakdown.

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.

Involvement of DNase II in nuclear degeneration during lens cell differentiation

The characterization of DNase II and DNase I activity was undertaken to discriminate their different roles in physiological nuclear degradation during lens fiber cell differentiation. The activity of both nucleases determined in a new assay allows to discriminate DNase II from DNase I in the same extract. In fibers, both types of nuclease activities are found and appear higher than in epithelial cells. Specific polyclonal antibodies directed against these two nucleases reveal by Western blot analysis the presence of various DNase isoforms. DNase II like-nuclease, present in fibers, is represented by three major bands (60,23, and 18 kDa), which are not detected, at least for two of them (60 and 23 kDa), in epithelial cells. DNase I like-nuclease pattern in fiber cells shows a single 32-kDa band, while several bands can be detected in epithelial cells. Immunocytochemistry studies show both nucleases present in lens cell sections. DNase II is, as usual, in cytoplasm of epithelial cells, but it appears strikingly concentrated in the nuclei of fibers. DNase I is always concentrated in nuclei of epithelial and fiber cells. DNA degradation observed in agarose gels shows that DNase II-activating medium cleaves the DNA from fiber cells more efficiently than DNase I-activating buffer. In addition, DNase II antibody is able to prevent this degradation. These results suggest a specific involvement of DNase II in nuclear degradation during lens cell differentiation.

Apoptosis in antitumor strategies: modulation of cell cycle or differentiation

There is a strong evidence that administration of antitumor drugs triggers apoptotic death of target cells. A characteristic feature of apoptosis is active participation of the affected cell in its demise. Attempts have been made, therefore, to potentiate the cytotoxicity of a variety of agents by modulating the propensity of cells to respond by apoptosis. Several strategies to enhance apoptosis that involve modulation of the cell cycle or differentiation are discussed. Loss of control of the G1 checkpoint in tumor cells allows one to design treatments that arrest normal cells at the checkpoint and attempt to selectively kill tumor cells with S phase specific drugs. The possibility of a restoration of the apoptosis triggering function of the tumor suppressor gene p53 when the G1 checkpoint function is abolished is expected to increase tumor cells' sensitivity to S phase poisons. Because induction of apoptosis by many antitumor drugs is cell cycle phase specific, drug combinations that preferentially trigger apoptosis at different phases of the cycle, or recruitment of cells to the sensitive phase, offer another antitumor strategy. There is also evidence that apoptosis is potentiated when cell differentiation is triggered following DNA damage. This observation suggests that strategies which combine DNA damaging and differentiating drugs, under conditions where the latter are administered following DNA damage caused by the former, may be successful.

Programmed cell death in development

Although cell death has long been recognized to be a significant element in the process of embryonic morphogenesis, its relationships to differentiation and its mechanisms are only now becoming apparent. This new appreciation has come about not only through advances in the understanding of cell death in parallel immunological and pathological situations, but also through progress in developmental genetics which has revealed the roles played by death in the cell lineages of invertebrate embryos. In this review, we discuss programmed cell death as it is understood in developmental situations, and its relationship to apoptosis. We describe the morphological and biochemical features of apoptosis, and some methods for its detection in tissues. The occurrence of programmed cell death during invertebrate development is reviewed, as well as selected examples in vertebrate development. In particular, we discuss cell death in the early vertebrate embryo, in limb development, and in the nervous system.

DNA strand breakage during physiological apoptosis of the embryonic chick lens: free 3' OH end single strand breaks do not accumulate even in the presence of a cation-independent deoxyribonuclease

Epithelial cells from the lens equator differentiate into elongated fiber cells. In the final steps of differentiation, the chromatin appears quite condensed and chromatin breakdown into nucleosomes occurs. DNA breaks due to an endodeoxyribonuclease activity corresponding to at least two polypeptides of 30 and 40 kDa have been identified. To identify the nature and the developmental appearance of initial breaks, nick translation reaction was followed both biochemically and in situ in fiber and epithelial cells from chick embryonic lenses. There is no accumulation of single-strand breaks (SSB) with 3'OH ends in lens fiber cells during embryonic development. Such damage can be increased in these cells by treatment with DNAase I indicating the absence of an inhibitor of the nick translation reaction in fiber cells. However, there are indications of the presence of DNA breaks with blocked termini when the phosphatase activity of nuclease P1 is used. The presence of breaks is also indicated by the large amounts of (ADP-ribose)n found in lens fibers particularly at 11 days of embryonic development (E11) as ADP-ribosyl transferase binds to and is activated by DNA strand breaks. Incubation of lens cells in vitro, which causes nucleosomal fragmentation only in fiber cells, produces SSB with 3'OH ends in both epithelia and fibers. Incubation for short periods, observed in experiments in situ, induces SSB first in the central fiber nuclei, which are late in differentiation. This may indicate that these SSB play a physiological role. Long incubations produce larger numbers of SSB in epithelia than fibers. The SSB in the fibers may have been converted into double-strand breaks (D SB), seen as nucleosomal fragments, and therefore no longer act as substrates for nick translation. The nuclease activity responsible for SSB production is independent of divalent cations and could be implicated in lens terminal differentiation.

Expression of tumor necrosis factor-alpha (TNF alpha)-cross-reactive proteins during early chick embryo development

We have investigated the expression of tumor necrosis factor-alpha (TNF alpha)-cross-reactive proteins during the early development of the chick embryo from day 1 to day 6 (H-H stages 5-29) using a polyclonal antibody and two monoclonal antibodies to recombinant mouse TNF alpha. We have confirmed the cross-reactivity of the antibodies with chicken tissue in Western blotting studies. Proteins of 50 kDa and 70 kDa, showing anti-TNF alpha cross-reactivity, have been identified during early chick development. In addition, both monoclonal antibodies recognize a 120 kDa protein. These molecules probably represent cytosolic or transmembrane TNF-alpha-like proteins, similar to those previously identified on the surface of cytotoxic T-lymphocytes. We show by ultrastructural cytochemistry that immunoreactivity can be detected at the surfaces of some cells, suggesting that at least some of the antigen is membrane-associated. The proteins are shown to have a widespread tissue distribution during this period of development. Immunoreactivity is first detected in the gastrulating embryo, in the mesoderm and the endoderm. By day 2, expression is confined to the ectoderm and the endoderm, while at day 3 expression appears in the myotome, the notochord, and in nervous tissue. At day 4 the distribution of reactivity is more extensive and includes the notochord, the sclerotome, and the myotome, while the cranial and spinal nerves also become intensely immunoreactive. Also at this stage, neural tube reactivity becomes localized to the marginal neuroepithelial zone, and the lens fibers become positive. This distribution of staining then persists until 6 days of development. We hypothesize that the expression of TNF alpha-cross-reactive proteins in early development could be indicative of a role for them in programmed cell death (apoptosis) during differentiation of the notochord, the lens, and the nervous system, and in tissue remodeling.

A biochemical hallmark of apoptosis: internucleosomal degradation of the genome

Apoptosis, or programmed cell death, is an endogenous cellular process whereby an external signal activates a metabolic pathway that results in cell death. This form of cell death appears to be a common feature in many biological processes where cell deletion is a mechanism for altering tissue structure and function. Historically, apoptosis has been studied using histological techniques; however, more recent interest has focused on analyzing this process at the biochemical level. A biochemical hallmark of apoptosis is a characteristic form of DNA degradation in which the genome is cleaved at internucleosomal sites, generating a 'ladder' of DNA fragments when analyzed by agarose gel electrophoresis. A number of assay systems have been developed to study this nuclease activity. For example, nuclease activity has been analyzed by measuring the release of endogenous DNA from apoptotic cells, by flow cytometric analysis of apoptotic cells and by analyzing in situ apoptotic nuclease activity in polyacrylamide gels containing DNA. Use of these assay systems has enabled investigators to study the signal transduction pathways that mediate apoptosis and to characterize the endonuclease itself. Future biochemical studies in this field will focus on isolating the genes and gene products that mediate apoptosis.

Nuclear breakdown during terminal differentiation of primary lens fibres in mice: a transmission electron microscopic study

The pre and post-natal development of wild type mouse lenses was studied by transmission electron microscopy, with special emphasis on denucleation of primary lens fibres. Denucleation of primary fibres is characterized by nuclear accumulation of small granules, most likely nucleosomes, which are condensed to osmiophilic bodies in the nucleus and in the cytoplasm. The osmiophilic bodies are laid down in apposition to the fibre membrane and are invaded by vesicles and granules, which probably contain proteolytic enzymes. Part of the breakdown products are extruded into the extracellular space, transported to the anterior and posterior poles where they might be finally digested or discarded from the lens. The morphology of the denucleation process of primary fibres is different from the gradual fading of nuclei in secondary fibres as described by Kuwabara and Imaizumi (1974: Invest. Ophthalmol. Vis. Sci. 13, 973-81).

Decrease of DNA per cell during development of the lens in chickens

Developmental changes in the amount and conformation of DNA in chicken lens were studied. For this, DNA in situ in lens fiber cell nuclei of chickens was examined by microfluorometry with Hoechst 33258 (Hoe) fluorochrome. On 1 M NaCl-aided Hoe staining, by which the amount of DNA can be determined accurately, the fluorescence intensity of lens fiber cells was found to decrease with no change in that of the lens epithelial cells during development. On the contrary, on normal NaCl-free Hoe staining the fluorescence intensity of the lens cells was found to increase gradually during development. These results suggest that during development the amount of DNA in lens fiber cells decreases in association with some change in its conformation.

Cell proliferation, cell death and aging

An integrated view of the processes which most likely play a critical role in the aging process at the cellular level is proposed. Cells are continuously exposed to a variety of internal and external stressors, potentially dangerous for the maintenance of the functional integrity of the cell (UV and gamma radiation, heat, oxygen free radicals, glucose, bacteria, viruses). In the course of evolution a number of mechanisms [DNA repair, production of heat shock and other stress proteins, enzymatic and non-enzymatic antioxidant defence systems, poly(ADP-ribose) polymerase activation] have emerged which allow the cell to cope with such a variety of potentially harmful agents. These mechanisms are in fact interconnected and constitute a network of cellular defence systems. It is suggested that they play a physiological role, being involved in the control of gene expression. A failure of these mechanisms does not allow the cell to maintain homeostasis and has profound consequences as far as two of the major programs of the cell are concerned, i.e. cell proliferation and cell death. Recent data suggesting that these are two physiologically active phenomena tightly linked and regulated are examined. Thus, activation of cell cycle related genes and active inhibition of suicide genes appear to be a part of an integrated process. Conversely, deprivation of growth factors seems able to induce an active process of programmed cell death characterized by Ca++,Mg+(+)-dependent endonuclease activity and DNA fragmentation (apoptosis). Similar phenomena have been shown to accompany the terminal differentiation process in several cellular systems. The understanding of the factors which favour or prevent cell death (a phenomenon which has been recognized as one of the most important in fetal development and morphogenesis) will help to unravel and eventually to manipulate the aging process. In an evolutionary perspective, cell senescence appears to be the price paid to avoid unlimited capability of proliferation, i.e. cell transformation and cancer.