Expression of regulatory and tissue-specific genes controlling regenerative potencies of the eye tissues in vertebrates

Comparative analysis of the early transformations of differentiated cells of the pigment epithelium, ciliary fold epithelium, and Muller glia in the eye of lower vertebrates and mammals during retina regeneration and cultivation was performed for the first time. Dedifferentiation and proliferation of cells and formation of progenitor multipotent cells, which are a source of retina regeneration in adult newts, were characterized using cell, molecular, and genetic markers. Neurospheres were formed during cultivation of the differentiated cells, in which progenitor multipotent cells were found that transformed into neurons of retina and brain and into glial cells. Comparative analysis of changes in the pigment epithelium cells during retina regeneration and during cultivation of differentiated cells of the pigment and ciliary epithelia and Muller glia suggests similar cell transformations at the early stages of transdifferentiation.     

Expression of regulatory and tissue-specific genes controlling regenerative potencies of eye tissues in vertebrates

Comparative analysis of the early transformations of differentiated cells of the pigment epithelium, ciliary fold epithelium, and Muller glia in the eye of lower vertebrates and mammals during retina regeneration and cultivation was performed for the first time. Dedifferentiation and proliferation of cells and formation of progenitor multipotent cells, which are a source of retina regeneration in adult newts, were characterized using cell, molecular, and genetic markers. Neurospheres were formed during cultivation of the differentiated cells, in which progenitor multipotent cells were found that transformed into neurons of retina and brain and into glial cells. Comparative analysis of changes in the pigment epithelium cells during retina regeneration and during cultivation of differentiated cells of the pigment and ciliary epithelia and Muller glia suggests similar cell transformations at the early stages of transdifferentiation.     

Multipotent and Stem Cells in the Developing, Definitive, and Regenerating Vertebrate Eye

This is a review of the experimental studies on the vertebrate retina neurogenesis. Data are provided on the distribution and localization of multipotent and stem cells in the developing, definitive, and regenerating eye. At the early stages of retina development, the neuroepithelial cells divide synchronously, thus leading to the accumulation of a certain number of the retinal rudiment cells. Synchronous divisions precede the asynchronous ones, when the differentiation of the retinal cells is initiated. The neuroepithelial cells are multipotent: the neuroblast is a source of the cells of different types, for example, neurons and glial cells. The proliferating multipotent cells are preserved in the ciliary-terminal zone of the retina of amphibians, fish, and chickens during their entire life. The differentiated pigment epithelium cells also proliferate in this area of the eye. The multipotent cells of the retinal ciliary-terminal zone and cells of the pigment epithelium in the eye periphery provide for the growth of amphibian and fish eyes during the entire life of these animals. In adult mammals, clonable and self-renewable cells were found among the pigmented differentiated cells in the ciliary folds. In a culture, the stem cells form spheroids consisting of depigmented and proliferating cells. Upon transdifferentiation, the cells of spheroids form rods, bipolar cells, and ganglion and glial cells, thus suggesting the possible regenerative potencies of the stem cells in the ciliary body of the mammalian eye. The main event of retinal regeneration in newts is the transdifferentiation of the pigment epithelium cells. The results of comparative analysis suggest that the stem cells of the ciliary body in the mammalian eye and pigment epithelium cells in lower vertebrates exhibit similar potencies and use similar mechanisms during the formation of the cells of the neural series.     

Radioautographic study of the cellular proliferation of retinal pigment epithelium in axolotls

The proliferative activity of the pigment epithelium cells in the axolotl eyes was studied using 3H-thymidine in two types experiments: after the removal of lens, iris and retina and upon the cultivation of the pigment epithelium pieces in the cavity of lens-less eye. Irrespective of the operation type, the level of proliferation of the pigment epithelium cells changed regularly with respect to the time of observation. In the intact eye, the level of proliferation of the pigment epithelium cells was not high: the index of labelled nuclei equaled 0.5%, no mitoses were found. The highest values of the index of labelled nuclei (12.6-32.1%) and of the mitotic index (0.54-1.07%) were registered on the 10-20th days after the operation. After 40 days, the indices of proliferative activity of the pigment epithelium cells approached gradually those for the intact eye. The cultivation of the pigment epithelium cells in the cavity of a lens-less eye for 50 days did not result in their transdifferentiation into retina cells. The layered retina found in 7.7% of cases after the removal of lens, iris and retina could regenerate either from the cells of the retina growth zone localized in the region of embryonic split, or due to transdifferentiation of the pigment epithelium cells.     

Inductive characteristics of proteins secreted by retinal cells

The studies of the development of eye rudiments and formation of adult eye tissues have always been among priorities in developmental biology and then in developmental genetics, which is associated with the peculiarities of the development and structure of the eye. In the late 80s, it was established by the group of developmental factors of the Institute of Gene Biology of RAS that many differentiated tissues are able to produce proteins causing homologous differentiations in polypotent cells of early gastrula ectoderm. The aim of our present study was isolation of proteins secreted by mammalian and fish retinal cells and determination of their inductive properties in early gastrula ectoderm of Xenopus laevis. The sets of proteins secreted by retina induce tissues homologous to the inducer, that is, neural tissue, brain, retina, pigmented epithelium, and also lenses and ear vesicles. The retinal inductive proteins retain their homologous inductive capacity after lyophilization. Biological testing shows that a total mixture of proteins secreted by retinal cells induces in polypotent gastrula ectoderm of X. laevis a narrower spectrum of tissues than the fractions obtained from this mixture. The above-outlined results obtained in thecourse of investigations of inductive peculiarities of retina and its fractions help in the elucidation of questions concerning embryonic induction and factors determining it, as well as questions concerning the maintenance of tissue specifity and regenerative capacity of the tissue studied.     

Concentration dependence of inductive activity in the mixture of lens epithelium proteins

As shown elsewhere, the mixture of proteins secreted by lens epithelium cells in the process of microcultivation can selectively induce eye and forebrain tissues in the early gastrula ectoderm (Zemchikhina et al., 2000, 2003). In the present work, the dependence of inductive activity of this protein mixture on its concentration in culture solution has been studied. The test-system was the early gastrula ectoderm of Xenopus laevis frogs. The results of the experiments revealed no direct dependence of the spectrum of induced tissues on the concentration of the protein mixture. At a concentration of 0.5 mg/ml, brain appeared being accompanied by retina, pigmented epithelium, and lentoids, while at 0.031 mg/ml a perfect lens developed along with brain, retina and pigmented epithelium. At 0.125 mg/ml not only brain with accompanying structures but also muscle fibers were equally differentiated. These data suggest a new approach to the problem of dependence of the character of induction on the concentration of inducing factors, and they enable us to suppose that this dependence is not realized as a simple concentration dependence but may de determined by some adaptive, yet not elucidation processes.     

The role of cell death during morphogenesis of the mammalian eye

Serial sections of embryonic rat eyes were stained with hematoxylin and eosin, quantified (by counting pycnotic and viable nuclei), reproduced by camera lucida on wax plates, and moulded into reconstructions in order to study the normal progression of cellular death during morphogenesis. At least nine distinct necrotic loci (A through I) can be distinguished. Immediately following contact between the retina and surface ectoderm (day 11) degenerating cells were observed in (A) the ventral extent of the optic vesicle, beginning in the mid-retinal primordium and continuing ventrally in the optic stalk, (B) in the rostral optic stalk base, and (C) in the surface ectoderm encircling the early lens placode. No degeneration was observed in the dorsal half of the presumptive retina, in the entire pigment epithelium, or in the lens placode proper. During day 11.5 the lens placode thickens and forms a degenerating locus (D) in its ventral portion opposite the underlying pycnotic zone in the retina (A). During day 12 the ventral pycnotic zone (A) divides into two subunits (A₁ and A₂). Invagination of the lens displaces its marginal and ventral components (C and D) so that they come to occupy the lens pore area and presumptive corneal epithelium. Simultaneous invagination of the retinal rudiment juxtaposes the pigment epithelium which concurrently forms a necrotic area (E) adjacent ventrally to that in the retina (A₁). Degeneration appears in the caudal optic stalk (I). The density of viable cells decreases adjacent to pycnotic areas in the retina and pigment epithelium and increases within these death centers. During day 13 the optic fissure forms within the subunits of the ventral pycnotic zone (A₁ and A₂). Degenerations are seen in the dorsal optic stalk (F) and in the walls of the optic fissure (G and H). Throughout these stages necrosis appears only in those portions of the eye rudiment where invagination is either retarded or completely absent. In part, these observations suggest that cell death serves (1) to retard or inhibit invagination within death centers, (2) to integrate the series of invaginations which mould the dorsal optic cup and optic fissure, (3) to assist formation of the pigment epithelium monolayer, and (4) to orient the lens vesicle within the eye cup. The spatio-temporal relationship between necrotic loci suggests that pycnotic cells in the retina may influence their production in the lens and pigment epithelium. Preliminary observations on the mouse, pig, and human substantiate those on the rat.     

Study of adhesive proteins from neural tissues of the 8-day old chicken embryonic eye

Two groups of proteins were isolated from the retina and pigment epithelium of eight-day-old chick embryos. Experiments with suspension cultures of retinal cells demonstrated that only the retinal extracts and the fraction of its acidic proteins can stimulate cell aggregation in vitro. Analysis by the method of high-performance liquid chromatography showed that fractions of acidic and basic retinal proteins, which markedly differ in their electric charge and biological activity, have similar composition. To study the effect of these proteins on the morphological and functional state of pigment epithelium in vitro, a new experimental model is proposed, with the posterior segment of the newt (Pleurodeles waltl) eye used as a test tissue. The fraction of basic proteins isolated from the chick embryonic pigment epithelium stabilized cell differentiation in the newt pigment epithelium. The analyzed proteins proved to be biologically active at extremely low doses, corresponding to 10(-12) M solutions.     

Distribution of differentiation potentials and the conditions for their realization in the amphibian neuroectoderm

The X. laevis neuroectoderm (NE) at the mid and late gastrula stages is capable to form mesoderm in vitro after its separation from mesoderm. This capacity is inherent in posterior 2/3 of NE underlied by axial mesoderm in the embryo and forming deuterencephalic and trunk regions of the brain in the normal development. The archencephalic 1/3 of NE of the late gastrula, underlied in the embryo by prechordal plate, is capable of differentiation into archencephalic regions of the brain, rather than into mesoderm. For the typical differentiation of archencephalic NE to be realized, it should be surrounded by the outer ectoderm layer. In the absence of the latter, the whole explant develops into retina and brain only. Inside the closed explants, ectomesenchyme and melanophores arise and the eye material is subdivided into retina and pigmented epithelium. The archencephalic NE, dissociated to individual cells and wrapped into epidermis, forms much more ectomesenchyme and melanophores than the usual NE explants.     

Elimination of egg pigment from developing ocular tissues in the frog Rana pipiens

The method by which egg pigment is eliminated from the developing retina, corneal epithelium and lens in Rana pipiens was studied with light and electron microscopy. The retina expells egg pigment into the space between the retina and pigment epithelium. This pigment is then engulfed by the pigment epithelial cells. The corneal epithelium eliminates egg pigment directly to the outside via the free surface of the epithelial cells. Egg pigment accumulates in a few cells in the lens. These cells probably degenerate and are extruded. These ectodermal derivatives in the eye are free of egg pigment long before ectodermal derivatives in other parts of the embryo lose their pigment. The early elimination of egg pigment from ocular tissues may related to the fact that these tissues must be transparent in order that light may pass freely to the photoreceptors.     

Cultivation of the retinal pigment epithelium in the cavity of the lentectomized eye of newts

A study was made of proliferative activity and transdifferentiation of the cells of retinal pigment epithelium (RPE) cultivated in the cavity of the lensectomized eye of adult newt. Implantation of the newt RPE together with vascular membrane and scleral coat resulted in the regeneration of retina. In this process the character of changes in the proliferative activity of RPE and differentiation of retinal cells were the same as in the regeneration of retina in situ. RPE implanted with the vascular membrane alone, despite a high level of proliferation during the first ten days of cultivation, no differentiated retina was formed. Possible causes of these differences are discussed, and the comparison is made of the data obtained with those on RPE cultivation in vitro. After lens removal, with RPE implants present in the eye cavity, in addition to the regenerated lens, 2-3 extra lenses and retina were formed from the cells of the inner layer of the recipient's dorsal iris. Also some cases were revealed of lens formation from the cells of ventral iris. With a complete detachment of the recipient's retina (an after-effect of transplantation) a second differentiated retina regenerated in situ from the recipient's RPE cells.     

Cellular heredity, its alteration and organ restoration

Patterns of cell heredity in vertebrates and possibility of its alteration, i. e. artificial tissue metaplasia, are considered. These problems are compared with the well studied phenomenon of metaplasia in eyes of the newt in which the removal of some parts of the eye leads to natural metaplasia, an initial step for restoration of eye parts. A brief analysis of sequence of inductive processes in development shows that by the end of the period of induction and the onset of terminal differentiation the maximum concentration of specific inducing agents in induced rudiments can be expected. This suggestion was confirmed by the experiments of specific assimilatory induction in gastrula ectoderm and artificial conversion of pigmented epithelium in retina and lens tissues. On the basis of the data reported and in comparison with the theories of intragenomic regulation, a new hypothesis of cell heredity is put forward. The basic idea of this hypothesis is that the regulatory genes can switch on the genes responsible for the synthesis of terminal proteins via inducing proteins; the latter can simultaneously programm the function of regulatory genes, initiating their own synthesis. Due to such a feedback mechanism forming during development, stable cell types arise. Their inheritance can be altered by the introduction of new inducing agents in parallel with the elimination of conditions stabilizing cell differentiation. Possible ways for application of artificial metaplasia for restoration of eye defects in medical practice are considered.     

Isolation and effects of inducing factors secreted by the lens epithelium cells

Our previous studies showed that, unlike tissue extracts, the cells of living organs secrete substances capable of inducing the same organ rudiments in the early gastrula ectoderm (EGE). In this work, the molecular nature of these substances was studied. The porcine lens epithelium was chosen for the initial analysis. When cultivated, this epithelium secreted a mixture of proteins, which were separated by gel-filtration. Both the total protein mixture and its individual fractions were tested for their inducing capacity using the early gastrula ectoderm of Rana temporaria. Unexpected results were obtained, which indicated that (a) the mixture of native proteins secreted by lens epithelium has a selective inducing capacity differing from those of individual fractions isolated from this mixture and (b) each fraction has a specific effect, but all of them cause the induction of neural tissue or sensory organs. These results (obtained for the first time) suggest that the inducing capacity of individual protein fractions is wider than that of the total protein mixture secreted by lens epithelium. This fact raises a question concerning the relationships between the mechanisms underlying the corresponding inducing effects.     

Epithelia-mesenchyme interaction plays an essential role in transdifferentiation of retinal pigment epithelium of silver mutant quail: localization of FGF and related molecules and aberrant migration pattern of neural crest cells during eye rudiment formation

Homozygotes of the quail silver mutation, which have plumage color changes, also display a unique phenotype in the eye: during early embryonic development, the retinal pigment epithelium (RPE) spontaneously transdifferentiates into neural retinal tissue. Mitf is considered to be the responsible gene and to function similarly to the mouse microphthalmia mutation, and tissue interaction between RPE and surrounding mesenchymal tissue in organ culture has been shown to be essential for the initiation of the transdifferentiation process in which fibroblast growth factor (FGF) signaling is involved. The immunohistochemical results of the present study show that laminin and heparan sulfate proteoglycan, both acting as cofactors for FGF binding, are localized in the area of transdifferentiation of silver embryos much more abundantly than in wild-type embryos. More intense immunohistochemical staining with FGF-1 antibody, but not with FGF-2 antibody, is also found in the neural retina, RPE, and choroidal tissue of silver embryos than in wild-type embryos. HNK-1 immunohistochemistry revealed that clusters of HNK-1-positive cells (presumptive migrating neural crest cells) are frequently located around the developing eyes and in the posterior region of the silver embryonic eye. Finally, chick-quail chimerical eyes were made by grafting silver quail optic vesicles to chicken host embryos: in most cases, no transdifferentiation occurs in the silver RPE, but in a few cases, transdifferentiation occurs where silver quail cells predominate in the choroid tissue. These observations together with our previous in vitro study indicate that the silver mutation affects not only RPE cells but also cephalic neural crest cells, which migrate to the eye rudiment, and that these crest cells play an essential role in the transdifferentiation of RPE, possibly by modifying the FGF signaling pathway. The precise molecular mechanism involved in RPE-neural crest cell interaction is still unknown, and the quail silver mutation is considered to be a good experimental model for studying the role of neural crest cells in vertebrate eye development.     

Cornea-lens transdifferentiation in the anuran, Xenopus tropicalis

Previously, the only anuran amphibian known to regenerate the lens of the eye was Xenopus laevis. This occurs during larval stages through transdifferentiation of the outer cornea epithelium under control of factors presumably secreted by the neural retina. This study demonstrates that a distantly related species, X. tropicalis, is also able to regenerate lenses through this process. A transgenic line of X. tropicalis was used to examine the process of cornea-lens transdifferentiation in which green fluorescent protein (GFP) is expressed in differentiated lens cells under the control of the Xenopus gamma1-crystallin promoter element. Unlike X. laevis, the process of cornea-lens transdifferentiation typically occurs at a very low frequency in X. tropicalis due to the rapid rate at which the inner cornea endothelium heals to recover the pupillary opening. The inner cornea endothelium serves as a key physical barrier that normally prevents retinal signals from reaching the outer cornea epithelium. If this barrier is circumvented by implanting outer cornea epithelium of transgenic tadpoles directly into the vitreous chamber of non-transgenic X. tropicalis larval eyes, a higher percentage of cases formed lenses expressing GFP. Lenses were also formed if these tissues were implanted into X. laevis larval eyes, suggesting the same or similar inducing factors are present in both species. When pericorneal ectoderm and posteriolateral flank ectoderm were implanted into the vitreous chamber, only in rare cases did pericorneal ectoderm form lens cells. Thus, unlike the case in X. laevis, competence to respond to the inducing factors is tightly restricted to the cornea epithelium in X. tropicalis. As controls, all these tissues were implanted into the space located between the inner and outer corneas. None of these implants, including outer cornea epithelium, exhibited GFP expression. Thus, the essential inductive factors are normally contained within the vitreous chamber. One explanation why this type of lens regeneration is not seen in some other anurans could be due to the rapid rate at which the inner cornea endothelium heals to recover the pupillary opening once the original lens is removed. These findings are discussed in terms of the evolution of this developmental process within the anurans.     

Compatibility of chick embryo eye anlagen with the ectoderm of the early amphibian gastrula in vitro

Eye vesicles were isolated from the early chick embryos (stage 9+ after Hamburger and Hamilton, 1951) and combined with the Rana temporaria early gastrula ectoderm (EGE) in vitro. The tissues were jointly incubated in medium 199 diluted twice with deionized water at 22 +/- 1 degree for 7-8 days or the eye vesicles were removed from the EGE ectoderm within 16-18 h. At the joint long-term incubation of these tissues, a toxic effect of the chick embryonic tissues on the EGE cells was noted. In none of the experiments, the inducing effect of the eye vesicle on the EGE was found. Similar data were obtained when the EGE was jointly cultivated with the brain (stage 9-10) and retina (stage 15) of chick embryos. The brain of the chick embryos at stage 15 exerted a weak neuralizing effect on the EGE. In the control experiments, the eye vesicles explanted with the chick embryonic ectoderm remained viable till the end of cultivation but no lentoids formed in the ectoderm. The absence of lens-inducing effect at the joint cultivation of the chick embryonic eye vesicles with the EGE is considered as a result of disturbance of the synthesis or secretion of the corresponding agents rather than a sequence of the species "incompatibility" of the inductor and reacting tissue. Hence, the use of "xenogenic" tissue recombinants is not justified when analyzing the lens-inducing activity of the eye vesicles.     

Step-wise specification of retinal stem cells during normal embryogenesis

The specification of embryonic cells to produce the retina begins at early embryonic stages as a multi-step process that gradually restricts fate potentials. First, a subset of embryonic cells becomes competent to form retina by their lack of expression of endo-mesoderm-specifying genes. From these cells, a more restricted subset is biased to form retina by virtue of their close proximity to sources of bone morphogenetic protein antagonists during neural induction. During gastrulation, the definitive RSCs (retinal stem cells) are specified as the eye field by interactions with underlying mesoderm and the expression of a network of retina-specifying genes. As the eye field is transformed into the optic vesicle and optic cup, a heterogeneous population of RPCs (retinal progenitor cells) forms to give rise to the different domains of the retina: the optic stalk, retinal pigmented epithelium and neural retina. Further diversity of RPCs appears to occur under the influences of cell-cell interactions, cytokines and combinations of regulatory genes, leading to the differentiation of a multitude of different retinal cell types. This review examines what is known about each sequential step in retinal specification during normal vertebrate development, and how that knowledge will be important to understand how RSCs might be manipulated for regenerative therapies to treat retinal diseases.     

Effect of heterogeneous inductors on the ectoderm of the early gastrula in Rana temporaria in vitro. 5. Biochemical analysis of induction-active extracts from chick embryo retina and brain

The water extracts from the retina and brain of 7-8-day old chick embryos were centrifuged at 20,000 g; sediments were discarded and supernatants were additionally centrifuged at 110,000 g. The inductive activity of supernatants (20,000 and 110,000 g) and sediments (110,000 g) was estimated in vitro on the Rana temporaria early gastrula ectoderm. The neutralizing activity was related exclusively to the soluble fractions of the extracts from the chick embryo retina and brain. The lens-inducing activity appeared to be characteristic of both the supernatants and the microsome fractions of these extracts. A comparative biochemical analysis of the extracts (isoelectrofocusing, electrophoresis in the presence of sodium dodecylsulfate, electroblotting) has shown that the chick embryo retina and brain are similar by the spectrum and properties of peptides. It is suggested that the similarity of the extracts inducing effect on the early gastrula ectoderm is due to the presence of the same proteins (peptides) in the retina and brain. Peptides with a positive immunohistochemical reaction to vimentin and peptides of neurofilaments were found in trace quantities in the retina and brain extracts by means of immunoelectroblotting.     

Dominant inhibition of lens placode formation in mice

The lens in the vertebrate eye has been shown to be critical for proper differentiation of the surrounding ocular tissues including the cornea, iris and ciliary body. In mice, previous investigators have assayed the consequences of molecular ablation of the lens. However, in these studies, lens ablation was initiated (and completed) after the cornea, retina, iris and ciliary body had initiated their differentiation programs thereby precluding analysis of the early role of the lens in fate determination of these tissues. In the present study, we have ablated the lens precursor cells of the surface ectoderm by generation of transgenic mice that express an attenuated version of diphtheria toxin (Tox176) linked to a modified Pax6 promoter that is active in the lens ectodermal precursors. In these mice, lens precursor cells fail to express Sox2, Prox1 and αA-crystallin and die before the formation of a lens placode. The Tox176 mice also showed profound alterations in the corneal differentiation program. The corneal epithelium displayed histological features of the skin, and expressed markers of skin differentiation such as Keratin 1 and 10 instead of Keratin 12, a marker of corneal epithelial differentiation. In the Tox176 mice, in the absence of the lens, extensive folding of the retina was seen. However, differentiation of the major cell types in the retina including the ganglion, amacrine, bipolar and horizontal cells was not affected. Unexpectedly, ectopic placement of the retinal pigmented epithelium was seen between the folds of the retina. Initial specification of the presumptive ciliary body and iris at the anterior margins of the retina was not altered in the Tox176 mice but their subsequent differentiation was blocked. Lacrimal and Harderian glands, which are derived from the Pax6-expressing surface ectodermal precursors, also failed to differentiate. These results suggest that, in mice, specification of the retina, ciliary body and iris occurs at the very outset of eye development and independent of the lens. In addition, our results also suggest that the lens cells of the surface ectoderm may be critical for the proper differentiation of the corneal epithelium.