Dedifferentiation of iris epithelial cells

Wound healing, cellular regeneration and plasticity: the elegans way

Regeneration and wound healing are complex processes that allow organs and tissues to regain their integrity and functionality after injury. Wound healing, a key property of epithelia, involves tissue closure that in some cases leads to scar formation. Regeneration, a process rather limited in mammals, is the capacity to regrow (parts of) an organ or a tissue, after damage or amputation. What are the properties of organs and the features of tissue permitting functional regrowth and repair? What are the cellular and molecular mechanisms underlying these processes? These questions are crucial both in fundamental and applied contexts, with important medical implications. The mechanisms and cells underlying tissue repair have thus been the focus of intense investigation. The last decades have seen rapid progress in the domain and new models emerging. Here, we review the fundamental advances and the perspectives that the use of C. elegans as a model have brought to the mechanisms of wound healing and cellular plasticity, axon regeneration and transdifferentiation in vivo.

The cellular and molecular bases of vertebrate lens regeneration

Lens regeneration takes place in some vertebrates through processes of cellular dedifferentiation and transdifferentiation, processes by which certain differentiated cell types can give rise to others. This review describes the principal forms of lens regeneration that occur in vivo as well as related in vitro systems of transdifferentiation. Classic experimental studies are reviewed that define the tissue interactions that trigger these events in vivo. Recent molecular analyses have begun to identify the genes associated with these processes. These latter studies generally reveal tremendous similarities between embryonic lens development and lens regeneration. Different models are proposed to describe basic molecular pathways that define the processes of lens regeneration and transdifferentiation. Finally, studies are discussed suggesting that fibroblast growth factors play key roles in supporting the process of lens regeneration. Retinoids, such as retinoic acid, may also play important roles in this process.

Lens regeneration in mice under the influence of vitamin A

The effect of vitamin A has been studied on lens regeneration in young (7 days old) as well as adult mice. A longitudinal slit was made under local anesthesia in the cornea over the lens. The lens was extracted intact through the incision. Intraperitonial injection of vitamin A (0.05 ml of 30 IU/ml in young and 0.05 ml of 50 IU/ml in adult) was given to the operated animals. Vitamin A was found to induce lens regeneration in not only young but also in adult mice. Regenerated lenses were similar in shape, size, transparency and histological features to normal intact lenses.

Reduced macrophage phagocytic activity in Wolffian lens regeneration of the newt after nickel subsulfide administration

The inhibition of newt lens regeneration by intraocular injection of nickel subsulfide (alpha Ni3S2), a potent carcinogen, was shown in our previous studies. In the present study, we examined the effect of nickel subsulfide on macrophages involved in depigmentation in the early stages of regeneration using light and electron microscopic observations on semithin (1.5 microns) and ultrathin sections. Phagocytic activity of macrophages in ingestion of pigment granules from pigmented dorsal iris epithelial cells was much reduced in the carcinogen-treated eyes, whereas the change in their number and pattern accumulating in the iris during lens regeneration was generally the same as in untreated eyes.

Expression of pax-6 during urodele eye development and lens regeneration

Regeneration of eye tissues, such as lens, seen in some urodeles involves dedifferentiation of the dorsal pigmented epithelium and subsequent differentiation to lens cells. Such spatial regulation implies possible action of genes known to be specific for particular cell lineages and/or axis. Hox genes have been the best examples of genes for such actions. We have, therefore, investigated the possibility that such genes are expressed during lens regeneration in the newt. The pax-6 gene (a gene that contains a homeobox and a paired box) has been implicated in the development of the eye and lens determination in various species ranging from Drosophila to human and, because of these properties, could be instrumental in the regeneration of the urodele eye tissues as well. We present data showing that pax-6 transcripts are present in the developing and the regenerating eye tissues. Furthermore, expression in eye tissues, such as in retina, declines when a urodele not capable of lens regeneration (axolotl) surpasses the embryonic stages. Such a decline is not seen in adult newts capable of lens regeneration. This might indicate a vital role of pax-6 in newt lens regeneration.

Immunocytochemical study of extracellular matrix components during lens and neural retina regeneration in the adult newt

We have conducted an immunocytochemical study of fibronectin, laminin, heparan sulfate proteoglycans and nidogen-entactin during lens and neural retina regeneration in the adult newt from 0 to 60 days. In the normal eye, fibronectin was detected in the corneal stroma and Descemet's membrane, in dorsal and ventral irises and lens capsule but not in Bowman's membrane of the cornea. In normal neural retina, fibronectin was found in Bruch's and inner limiting membranes. Heparan sulfate proteoglycans gave a slight signal in both irises and the lens capsule. Nidogen-entactin distribution in the cornea was similar to that of fibronectin; it was absent from the stroma of both irises, and the signal was weak in the pigmented iris epithelium. Nidogen-entactin was not detected in the lens capsule and inner limiting membrane of the neural retina but was present in Bruch's membrane. During the first 15 days of lens regeneration, fibronectin and nidogen-entactin decreased but did not disappear from the pupillary margin of both irises, and no signal was obtained for laminin and heparan sulfate proteoglycans. From day 15 to day 60 fibronectin and nidogen-entactin increased in both irises and lens capsule. The signal for laminin was restricted to the lens capsule. Heparan sulfate proteoglycans gave a slight signal in both irises and in the lens capsule. During the first 25 days of neural retina regeneration, fibronectin was the first to appear in Bruch's membrane and the cell border of the new neuroepithelium and remained during the entire process. Laminin appeared after 41 days in the inner limiting and Bruch's membranes, but by day 50 it appeared as a weak signal only in the inner limiting membrane. Heparan sulfate proteoglycans were not detected at any of the regeneration stages studied. Nidogen-entactin was only detected in Bruch's membrane and around the cells and blood vessels of the new neural retina. Later it was detected in the inner limiting membrane but not in Bruch's membrane. Thus, the results obtained showed that extracellular matrix components do change during both lens and neural retina regeneration. These changes may play an important role during both regenerating processes.

Macrophage invasion and phagocytic activity during lens regeneration from the iris epithelium in newts

Following removal of the lens through the cornea, early stages of lens regeneration from the dorsal iris of the adult newt, Notophthalmus viridescens, were studied using light and electron microscopic observations on sectioned, plastic-embedded irises. Specimens were fixed in Karnovsky's fixative every 2 days from 0 to 12 and 15 days after lentectomy. Infiltration of the iris epithelium by macrophages and their phagocytosis of melanosomes and small fragments of iris epithelial cells were observed. These macrophages were characterized by coarse nuclear chromatin, numerous mitochondria, free ribosomes, granular endoplasmic reticulum, Golgi complexes, vesicles, lysosomes, and phagosomes containing ingested melanosomes. Lamellipodia of varying length projected from their surface. Most of the cells lying on or close to the posterior surface of the iris could be identified as macrophages by these criteria. During this period, there was enlargement of the intercellular spaces within the iris epithelium. The iris epithelial cells near the margin of the pupil elongated, lost their melanin pigment and some associated cytoplasm, and acquired abundant free polyribosomes to form a lens vesicle of depigmented cells.

Fibronectin distribution during cell type conversion in newt lens regeneration

The distribution of fibronectin during the cell type conversion from iris into lens that occurs in newt lens regeneration was studied by immunofluorescence. Newts were lentectomized and irises at different stages of dedifferentiation and redifferentiation were examined using as a probe a rabbit antiserum prepared to Xenopus plasma fibronectin. In the normal iris, fibronectin is predominantly located at the basal surface of the pigmented iris epithelial cells. During activation and early dedifferentiation fibronectin staining is progressively displayed at the basolateral and apical surface of the depigmenting cell, to eventually surround the surface of the dedifferentiated cells. As cells redifferentiate into lens fibers, staining for cell surface fibronectin decreases and is displayed mainly in the nascent lens capsule. Fibronectin deposition may be associated with the formation of intercellular spaces during dedifferentiation. The fibronectin-rich extracellular matrix could be important in cell reprogramming.

The effect of catecholamines and adenosine on the induction of morphological alterations and depigmentation of newt iris epithelial cells in vitro

Catecholamines and adenosine have a stimulating effect on the process of dedifferentiation of cultured iris epithelial cells (IECs) from the adult newt Notophthalmus viridescens. Micromolar concentrations of adrenergic ligands such as isoproterenol, norepinephrine, and epinephrine induced marked morphological alterations culminating in the stellate configuration and depigmentation of IECs. Dopamine at 100 microM or higher induced the morphological response, while serotonin was ineffective. The morphological change was transient, requiring 80-90 min for maximum induction, and only a fraction of the cells was responsive. The response was blocked by beta-adrenergic antagonists, such as propranolol and alprenolol, but not by alpha-adrenergic blockers. Adenosine at 10 microM, or higher, also induced morphological alterations of IECs. The effect of adenosine was partially blocked by various adenosine receptor antagonists. The effect of isoproterenol and norepinephrine on the induction of morphological alterations was potentiated by adenosine. The release of melanosomes from IECs was increased in the presence of catecholamines and adenosine. Catecholamines and adenosine at 10 microM increased the intracellular levels of cAMP of dedifferentiating dorsal irides. The increase in cAMP levels induced by isoproterenol was inhibited by propranolol and the adenosine receptor antagonist 5'-deoxy-5'-methyl thioadenosine (MTA) partially blocked the effect of adenosine. Our results suggest that adrenergic hormones may be coupled to a beta-adrenergic adenylate cyclase system. The presence of an adenosine receptor is also suggested by the results. Our data strongly support previous work in which cAMP and substances related to it induced morphological alterations and depigmentation of IECs. It is proposed that catecholamines and adenosine may participate in the regulation of dedifferentiation during the transdifferentiation of IECs into lens cells.

Hyaluronic acid production and hyaluronidase activity in the newt iris during lens regeneration

The process of lens regeneration in newts involves the dedifferentiation of pigmented iris epithelial cells and their subsequent conversion into lens fibers. In vivo this cell-type conversion is restricted to the dorsal region of the iris. We have examined the patterns of hyaluronate accumulation and endogenous hyaluronidase activity in the newt iris during the course of lens regeneration in vivo. Accumulation of newly synthesized hyaluronate was estimated from the uptake of [3H]glucosamine into cetylpyridinium chloride-precipitable material that was sensitive to Streptomyces hyaluronidase. Endogenous hyaluronidase activity was determined from the quantity of reducing N-acetylhexosamine released upon incubation of iris tissue extract with exogenous hyaluronate substrate. We found that incorporation of label into hyaluronate was consistently higher in the regeneration-activated irises of lentectomized eyes than in control irises from sham-operated eyes. Hyaluronate labeling was higher in the dorsal (lens-forming) region of the iris than in ventral (non-lens-forming) iris tissue during the regeneration process. Label accumulation into hyaluronate was maximum between 10 and 15 days after lentectomy, the period of most pronounced dedifferentiation in the dorsal iris epithelium. Both normal and regenerating irises demonstrated a high level of endogenous hyaluronidase activity with a pH optimum of 3.5-4.0. Hyaluronidase activity was 1.7 to 2 times higher in dorsal iris tissue than in ventral irises both prior to lentectomy and throughout the regeneration process. We suggest that enhanced hyaluronate accumulation may facilitate the dedifferentiation of iris epithelial cells in the dorsal iris and prevent precocious withdrawal from the cell cycle. The high level of hyaluronidase activity in the dorsal iris may promote the turnover and remodeling of extracellular matrix components required for cell-type conversion.

Conversion of iris epithelial cells as a model of differentiation control

It has been shown that, upon lentectomy or in culture, iris epithelial cells (IECs) of adult newts become converted into lens cells, and this conversion is the basic event of lens regeneration in newts. Whether in situ or in cell culture, the conversion requires the passage of a specific number of cell cycles. The progeny of IECs which fails to traverse this cell-cycle number redifferentiates as IECs in situ. The passage through cell cycles of IECs is associated with progressive alterations of cytoplasm and cell surface, during which the original state of differentiation disappears (dedifferentiation). It is speculated that the altered state of cells caused by proliferation leads to the appearance of factors which interact with the genome and switch the gene activation pattern to that of the lens cell. In this model, developmental controls are geared to the cell-cycle progression and not directly to the activation of lens-characteristic genes. A number of points are raised which speak against the long-held idea that a factor from neural retina induces lens differentiation in IECs. It is proposed that the retinal factor plays the role of growth factor which is essential in the conversion in situ, but not required in the conversion in cell culture. The proposed model is compared with reprogramming of differentiation of some cell lines by cytidine analogs and with ontogenic systems of differentiation control.

Availability time of tritium-labeled DNA precursors in newt eyes following intraperitoneal injection of 3H-thymidine

Following intraperitoneal injection of 3H-thymidine into host newts, iris together with a regenerating lens was transplanted from a donor eye into a lentectomized host eye at frequent intervals for 20 hours and then every 1 or 2 days for 14 days. The eyes were fixed 2 hours and 1 or 2 days after implantation and autoradiographs prepared. Following fixation 2 hours after operation, incorporation of 3H-thymidine into DNA, as evidenced by grain counts over nuclei, fell rapidly for 3.5 hours after injection and was no longer apparent after 4.5 hours. However, almost one-half of the implants were lightly labeled when they remained in the host eyes for 1 or 2 days beginning from 1 to 14 days after isotope injection. When these implanted, regenerating lenses were left in the host eyes for longer periods of time, then a light label was found over nuclei in most of the implants remaining in the eye for 3 to 24 days. When 3H-thymidine was injected from 1 to 3 days after extirpation of both lens and neural retina, before DNA synthesis had been initiated in the pigmented retinal epithelium or iris, there were numerous cases of labeled nuclei among depigmenting cells of the pigmented retinal epithelium which was regenerating a new neural retina from 2 to 25 days after isotope injection. Depigmenting cells of the dorsal iris and regenerating lens were similarly labeled. These results provide evidence for the continued availability of small amounts of tritiated DNA-precursor molecules which can be incorporated in DNA of proliferating cells long after the initial injection of 3H-thymidine.

Development of retinal amacrine cells in the mouse embryo: evidence for two modes of formation

Developing amacrine and ganglion cells have been graphically reconstructed from a series of 567 consecutive thin sections of the E17 mouse retina on the first day when an obvious inner plexiform layer (IPL) is present and 2 days later than for our previous study of amacrine cell formation at E15 (Hinds and Hinds ('78). At E17 amacrine cells of the neuroblastic layer (normally placed amacrine cells), unlike those at E15, appear to develop directly from ventricular cells; intermediate elements are bipolar-shaped cells with terminal arborization in the IPL. On the other hand, the development of displaced amacrine cells and some normally placed amacrine cells at E17 appears to closely resemble that described for all amacrine cells at E15: derivation from "ganglion cells" by loss of the axon and transformation of the cell. Three lines of evidence support this conclusion. (1) Cells have been found that resemble ganglion cells except that they have only an apparent axon remnant and have somata restricted to the IPL and the immediately adjacent portion of the ganglion cell layer (GCL); amacrine cells transitional between these cells and the smaller and darker, normally placed amacrine cells also occur in the IPL. (2) Axons of two ganglion cells have been found which appeared to be in the process of breaking up and degenerating. (3) The fraction of anaxonic cells with somata in the GCL (two out of 79, or 3%) or in the GCL plus IPL (ten out of 88 or 11%) is too small to account easily for the large fraction (probably at least 45%) of displaced amacrine cells found in the adult, even with conservative assumptions (P less than 0.05). A mathematical model suggests that approximately 40% of the ganglion cells present at E17 will lose their axon, and of these around half will migrate to the neuroblastic layer, while the other half will become displaced amacrine cells. The results suggest a natural explanation for the recent finding that wide field amacrine cells are found with somata on both sides of the IPL, while narrow field amacrine cells are never displaced: the former may be derived from ganglion cells by loss of the axon, while the latter may be formed directly from ventricular cells.

Dedifferentiation of iris epithelium during lens regeneration in newt larvae

Early stages in lens regeneration from the pigmented epithelium of the dorsal iris were studied in larval Notophthalmus viridescens by means of transmission electron microscopy. Normal iris epithelium is composed of two layers of low cuboidal cells. packed with melanosomes and surrounded by a basal lamina. Scattered desmosomes attach adjacent cells. Following lens removal, the intercellular spaces enlarge and the epithelial cells increase in size. Some irregular microvilli from these cells extent into the intercellular spaces. Macrophages invade the iris epithelium and phagocytize melanosomes discharged from the pigmented cells. These invading macrophages have numerous microprojections and are often separated from the surface by a very thin layer of iris epithelial cell cytoplasm. In the iris cells, nucleoli become more prominent and granular, polyribosomes increase greatly in number, melanosomes gradually disappear, mitochondria become more numerous, and mitotic activity is greatly augmented. Fine cell processes of adjacent interdigitate near the external surface, where numerous micropinocytotic vesicles can be seen. Over the external surface, the basal lamina may be disrupted or duplicated in places where pseudopodia project from iris cells or a macrophage has entered an intercellular space. It is lacking on the lumenal surface. Sloughed membranes are often found in these intercellular spaces.

Lens regeneration from the pupillary margin of the eyecup in newt embryos

Early stages in lens regeneration from the dorsal margin of the pupil, following removal of the young lens from the embryonic eyecup, were studied with transmission electron microscopy in Notophthalmus viridescens. At the stage of operation, the eyecup cells have an undifferentiated, embryonic appearance with numerous free ribosomes and scattered mitochondria. In normal embryonic eyes containing a lens, the iris epithelium differentiates from the edge of the optic cup by growth and flattening of the cells to form a two-layered cuboidal epithelium. There is extensive melanogenesis in both inner and outer layers. In lentectomized eyes, this flattening does not occur and melanogenesis takes place mainly in the outer layer of epithelium. Mitoses are abundant and a vesicle of unpigmented columnar cells, enclosed by a basal lamina, develops from the dorsal pupillary margin of the eyecup. Macrophages are present and phagocytize some of the melanin but less than during dedifferentiation of pigmented iris epithelium. During this regeneration of a lens vesicle, there is an increase in the number of polyribosomes, granular endoplasmic reticulum, and mitochondria in preparation for synthesis of the lens proteins. Micropinocytosis occurs at the cells surfaces.

Intracellular levels of adenosine 3':5'-cyclic monophosphate in the dorsal iris of the adult newt, during lens regeneration

The intracellular levels of adenosine 3':5'-cyclic monophosphate (cAMP) were measured in the dorsal iris of the adult newt, during the first 20 days of lens regeneration. It was found that by day 2 after lens removal there is a significant drop in the levels of cAMP. After day 2 the levels of the nucleotide increase and by day 3 they are higher than those detected on day 0. The levels of cAMP remain high up to day 8. From day 8 to day 9 there is a second drop. From day 9 to day 20 the levels of cAMP did not differ significantly from the value obtained for day 0, except for days 10, 12, and 15. The period of high levels of cAMP coincides with the period of depigmentation of iris epithelial cells, the key event of lens regeneration.

Telolysomes in cultured iris epithelial cells and in the TVI cell-line

Iris epithelial cells of adult newts, which are fully differntiated melanocytes and non-dividing, become dedifferentiated and converted into lens cells when put in culture. A recent study shows that this dedifferentiation is based on an autophagic process which is associated with proliferation and mainly affects melanosomes. The present report shows that in primary culture of iris epithelial cells after the majority of melanosomes have disappeared, myelinoid bodies, which are interpreted to be telolysosomes of autophagic nature, appear in high frequencies. This suggest that in these cells autophagy persists after the loss of melanosomes. A possible connection of this type of autophagy with the differentation of lens fiber which occurs in this culture is discussed. In the TVI cell line which is believed to be derived from the same cell type, but devoid of melanosomes, similar myelinoid bodies are a characteristic cell component, suggesting that the tendency for autophagy is inherited in theis cell line.

Autophagy in dedifferentiating newt iris epithelial cells in vitro

The ultrastructure of dedifferentiating iris epithelial cells of adult newts was studied in vitro, under the conditions which allow subsequent conversion into lens cells. The dense population of melanosomes which characterize the normal differentiated cells are progressively lost before the cells convert into lens cells. The loss of melanosomes is accompanied by the appearance of a layer of material of intermediate density surrounding the melanosomes. Subsequently vacuoles of various sizes appear in which melanosomes, ribosomes, and multivesicular bodies are sequestered. Further observations suggest that these affected organelles are destined for exocytosis. The cytochemical test for acid phosphatase is positive in the immediate vicinity of melanosomes, in the melanosome clusters, and in the multivesicular bodies. On the other hand, normal iris epithelial cells in situ are negative to the acid phosphatase test. The results are interpreted to indicate that autophagy and exocytosis of melanosomes and other organelles occur during the dedifferentiation of iris epithelial cells.

Interaction between mesenchymal cells and the posterior iris epithelium in chicken embryos

The iris anlage of 3--10 day old chicken embryos was studied by both light and electron microscopy. Serial semithin sections showed that some of the mesenchymal cells overlying the eye cup moved into the primitive eye cavity by the 3rd day of incubation. On the 4th day some of these cells came into close contact with the basement membrane of the dorsal iris epithelium. The bases of the epithelial cells were flat at this stage. Towards the 10th day they formed cytoplasmic processes which did not penetrate the basement membrane. Fine mesenchymal cytoplasmic processes and a large number of extracellular fibrils developed in the epithelial--mesenchymal interface. The fine mesenchymal processes came into close contact with the basement membrane of the posterior iris epithelium but did not penetrate it. Collagen-like material was observed within the cisternae of the rough ER of the mesenchymal cells at certain stages of development. Both, the mesenchymal cells and the collagen fibrils adjacent to the posterior iris layer disappeared by the 10th day when the entire iris epithelium was completely pigmented. The possible origin of the collagen fibrils and the differentiation of the posterior iris epithelium are discussed.

An established cell line from the newt Notophthalmus viridescens

An established cell line, derived from the dorsal iris of the eastern North American newt, Notophthalmus viridescens, is described. The cells display an epithelial-like behaviour in culture, grow relatively slowly, possess considerably larger chromosomes than mammals and are heteroploid, although some near-diploid cells are present in the culture. The line is characterized by a strong tendency to overlapping and aggregation in spite of its origin from adult tissue.

Differentiation of lens-like structures from newt iris epithelial cells in vitro

Dissociated cells of pigmented iris epithelium from adult newts grew intensively in monolayer cultures after a lag of two to three weeks. During the lag period, depigmentation occurred in many cells. When cultures became confluent five to six weeks after seeding, many tiny lens-like structures (30-70 per plate) differentiated from dense foci of amelanotic epithelial cells. These lens-like structures appeared in all cultures originated from cells of ventral as well as dorsal iris. The identification of these structures as lens was established by both immunological and ultrastructural techniques.

Induction of the stellate configuration in cultured iris epithelial cells by adenosine and compounds related to adenosine 3':5'-cyclic monophosphate

Adenosine, dibutyryl cyclic AMP, monobutyryl cyclic AMP, cyclic AMP, and 5'-AMP have a remarkable morphogenetic effect on cultured iris epithelial cells obtained from adult newt. They alter the broad undulating membrane of the cell into branching strands of cytoplasm, a configuration that has been named "stellate." Theophylline, a phosphodiesterase inhibitor also induces the stellate configuration. This transient morphological alteration is detectable by 30 min and becomes maximal 80 min after treatment. In the continued presence of the effective compounds the altered cells return to their normal shape, although the recovery period is variable. The morphological alteration of iris epithelial cells in vitro observed in the present experiment is reminiscent of that which occurs during the dedifferentiation phase in lens regeneration in vivo. These observations suggest that induction of the stellate configuration is relevant to the mechanism of dedifferentiation of newt iris epithelial cells during Wolffian lens regeneration.