alpha-, beta- and gamma-Crystallins in the regenerating lens of Notophthalmus viridescens

Lens regeneration: a historical perspective

The idea of regenerating injured body parts has captivated human imagination for centuries, and the topic still remains an area of extensive scientific research. This review focuses on the process of lens regeneration: its history, our current knowledge, and the questions that remain unanswered. By highlighting some of the milestones that have shaped our understanding of this phenomenon and the contributions of scientists who have dedicated their lives to investigating these questions, we explore how regeneration enquiry evolved into the science it is today, and how technological advances accelerated our understanding of these remarkable processes.

Expression of pluripotency factors in larval epithelia of the frog Xenopus: evidence for the presence of cornea epithelial stem cells

Understanding the biology of somatic stem cells in self renewing tissues represents an exciting field of study, especially given the potential to harness these cells for tissue regeneration and repair in treating injury and disease. The mammalian cornea contains a population of basal epithelial stem cells involved in cornea homeostasis and repair. Research has been restricted to mammalian systems and little is known about the presence or function of these stem cells in other vertebrates. Therefore, we carried out studies to characterize frog cornea epithelium. Careful examination shows that the Xenopus larval cornea epithelium consists of three distinct layers that include an outer epithelial layer and underlying basal epithelium, in addition to a deeper fibrous layer that contains the main sensory nerve trunks that give rise to numerous branches that extend into these epithelia. These nerves convey sensory and presumably also autonomic innervation to those tissues. The sensory nerves are all derived as branches of the trigeminal nerve/ganglion similar to the situation encountered in mammals, though there appear to be some potentially interesting differences, which are detailed in this paper. We show further that numerous pluripotency genes are expressed by cells in the cornea epithelium, including: sox2, p63, various oct4 homologs, c-myc, klf4 and many others. Antibody localization revealed that p63, a well known mammalian epithelial stem cell marker, was localized strictly to all cells in the basal cornea epithelium. c-myc, was visualized in a smaller subset of basal epithelial cells and adjacent stromal tissue predominately at the periphery of the cornea (limbal zone). Finally, sox2 protein was found to be present throughout all cells of both the outer and basal epithelia, but was much more intensely expressed in a distinct subset of cells that appeared to be either multinucleate or possessed multi-lobed nuclei that are normally located at the periphery of the cornea. Using a thymidine analog (EdU), we were able to label mitotically active cells, which revealed that cell proliferation takes place throughout the cornea epithelium, predominantly in the basal epithelial layer. Species of Xenopus and one other amphibian are unique in their ability to replace a missing lens from cells derived from the basal cornea epithelium. Using EdU we show, as others have previously, that proliferating cells within the cornea epithelium do contribute to the formation of these regenerated lenses. Furthermore, using qPCR we determined that representatives of various pluripotency genes (i.e., sox2, p63 and oct60) are upregulated early during the process of lens regeneration. Antibody labeling showed that the number of sox2 expressing cells increased dramatically within 4 h following lens removal and these cells were scattered throughout the basal layer of the cornea epithelium. Historically, the process of lens regeneration in Xenopus had been described as one involving transdifferentiation of cornea epithelial cells (i.e., one involving cellular dedifferentiation followed by redifferentiation). Our combined observations provide evidence that a population of stem cells exists within the Xenopus cornea. We hypothesize that the basal epithelium contains oligopotent epithelial stem cells that also represent the source of regenerated lenses in the frog. Future studies will be required to clearly identify the source of these lenses.

Lens regenerates by means of similar processes and timeline in adults and larvae of the newt Cynops pyrrhogaster

BACKGROUND

It is widely accepted that juvenile animals can regenerate faster than adults. For example, in the case of lens regeneration of the newt Cynops pyrrhogaster, larvae and adults require approximately 30 and 80 days for completion of lens regeneration, respectively. However, when we carefully observed lens regeneration in C. pyrrhogaster at the cellular level using molecular markers in the present study, we found that lens regeneration during the larval stage proceeded at similar speed and by means of similar steps to those in adults.

RESULTS

We could not find any drastic difference between regeneration at these two stages, except that the size of the eyes was very different.

CONCLUSIONS

Our observations suggested that larvae could regenerate a lens of the original size within a shorter time than adults because the larval lens was smaller than the adult lens, but the speed of regeneration was not faster in larvae. In addition, by repeatedly observing the regeneration in one individual transgenic newt that expressed fluorescence specifically in lens fiber cells in vivo and comparing the regeneration process at the embryonic, larval, and postmetamorphosis stages, we confirmed that the regeneration speed was the same at each of these stages in the same individual.

Eye on regeneration

Lens regeneration in newts is a remarkable process, whereby a lost tissue is replaced by transdifferentiation of adult tissues that only a few organisms possess. In this review, we will touch on the approaches being used to study this phenomenon, recent advances in the field of lens regeneration, similarities and differences between development and regeneration, as well as the potential role stem cells may play in understanding this process.

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.

Expression of crystallin genes in embryonic and regenerating newt lenses

The spatio-temporal expression of three crystallin genes (alphaA, betaB1 and gamma) in the developing and regenerating lenses of newt was compared by in situ hybridization in lens differentiation in normal development with during regeneration. In normal development, all crystallin transcripts were first detected at the same stage in the posterior region of the lens vesicle (McDevitt's lens development stage V) and continued during lens fiber differentiation of the posterior cells into the primary lens fiber cell differentiation (McDevitt's lens development stage VII-VIII). At later stages, the expression of the three genes was restricted to the secondary lens fibers and gradually became undetectable in primary lens fibers (McDevitt's lens development stage X). The signal for gamma-crystallin was never detected in lens epithelium at any stage, whereas signals for alphaA- and betaB1-crystallin were detected in the lens epithelium at the stage when the primary lens fiber mass was formed. During lens regeneration, signals for the three crystallins were first detected at the same stage at the ventral margin of a regenerating lens vesicle (Sato's lens regeneration stage IV). The expression patterns of three crystallin genes were similar to those in normal development (Sato's lens regeneration stage V-X). The expression pattern of the crystallin genes in normal lens development fundamentally resembles that during lens regeneration, suggesting the absence of unique expression programs of crystallin genes for lens regeneration not found in ontogeny.

Identification of a site of Hsp27 binding with Hsp27 and alpha B-crystallin as indicated by the yeast two-hybrid system

The small heat-shock proteins (sHsp), including Hsp27 and alphaB-crystallin, usually form large oligomers in cells. It has been suggested that the sHsp form oligomers by binding either a conserved C-terminal amino acid sequence or the less conserved N-terminal region. However, the site of binding has not been precisely determined. We used the yeast two-hybrid system to investigate binding of full-length rat Hsp27 or fragments of Hsp27 to full-length rat Hsp27 or alphaB-crystallin molecules. A series of cDNAs coding for fragments of Hsp27 were generated and ligated with the coding sequence for the binding domain of the yeast Gal4p transcription factor. These cDNAs were each transfected into yeast that had been transfected to express full-length rat Hsp27 or alphaB-crystallin fused with the DNA binding domain of Gal4p. Yeast cells transfected with both plasmids were assayed, by both a beta-galactosidase (beta-gal) filter assay and a quantitative liquid assay, for activation of Gal4p-driven beta-gal expression. Results indicated that the N-terminal domain of Hsp27 consisting of amino acids 1-124 did not bind to Hsp27 or alphaB-crystallin. The predominant Hsp27-Hsp27/alphaB-crystallin binding domain was the conserved C-terminal region consisting of amino acids 141-206, particularly amino acids 141-176.

Fibroblast growth factor receptors and regeneration of the eye lens

If the eye lens of the adult newt, Notophthalmus viridescens, is removed, a new lens will regenerate and only from the dorsal, not the ventral, iris. The source, pigmented epithelial cells, would normally no longer divide, but upon lentectomy they do re-enter the cell cycle and form lens. The cause for this capability is unknown, but the mitogenic Fibroblast Growth Factors and their receptors may be involved. We have demonstrated that FGF receptors are present and operative in lens regeneration, since receptor-directed mitotoxins inhibit regeneration; heterogeneity and differential density in FGF-binding and receptor localization in iris sectors is also present. We propose that the spatial distribution of FGF receptors, especially the amphibian homolog of FGFR-3, is important in initiation of regeneration of eye lens.

Eye lens regeneration and the crystallins in the adult newt, Notophthalmus viridescens

Upon lens removal, the adult Eastern Spotted newt, Notophthalmus viridescens, has the capacity to regenerate an ocular lens. Crystallins, proteins characteristic of the vertebrate lens, were studied from normal and 3-month regenerated adult newt lenses. When separated by high-performance liquid chromatography (HPLC) or Sephadex G-200SF column chromatography, the crystallins from normal and regenerated lenses were fractionated into what appear to be the classical four groups: alpha, beta High, beta Low, and gamma. Upon further examination by immunoelectrophoresis, the first peak contains both alpha and beta crystallins. This study provides evidence that most of the crystallins from the regenerated lenses share biochemical properties with those of the normal lens crystallins based on their native molecular weight, isoelectric point, and the molecular wt of their constituent polypeptides, indicating that the fidelity of gene expression in reactivated iris tissue is high. Some differences are found between normal and regenerated lens crystallins and are most obvious in the beta-crystallin region: the proportion of beta crystallins is decreased in regenerated lenses when the total proteins are fractionated by column chromatography and some of the beta-crystallin polypeptide chains found in normal lenses are missing from regenerated lenses. Iris epithelial cells are normally withdrawn from the cell cycle and are synthesizing a tissue-specific product, melanin. After lentectomy these cells dedifferentiate, redifferentiate into lens cells, and their progeny then synthesize different tissue-specific proteins, crystallins. Little is known about the specific mechanism(s) for the activation of gene expression in eukaryotes, but the regenerating lens suggests itself as a good model in which to study this biological problem.

Ontogeny of alpha A and alpha B crystallin polypeptides during Rana temporaria lens development

The ontogeny and localization of alpha A and alpha B polypeptide chains of alpha-crystallin were investigated in the developing lens of Rana temporaria, an anuran amphibian, using the indirect immunofluorescence staining method with heterologous antibodies directed against these two polypeptides. alpha A and alpha B crystallins are primary gene products and are translated by different mRNAs in mammals. Although they show about 6000 amino-acid sequence homology (Bloemendal, 1977), the alpha A cDNA of rat and mouse does not hybridize to alpha B mRNA (Dodemont et al., 1981; King and Piatigorsky, 1983). Antigenically too, alpha A and alpha B polypeptides have been shown to be different. These two polypeptides were isolated from mouse lens native alpha-crystallin by SDS-gel electrophoresis and were injected into young rabbits to raise antibodies. These antibodies were tested by immunoblotting against R. temporaria total lens soluble proteins before their use in the present investigation. Results presented here show that in the developing lens of R. temporaria, alpha A appears earlier than alpha B, suggesting a differential gene activation. In addition, these two polypeptides could not be detected either in the developing lens epithelium or in the epithelium of young froglets (2-3 weeks post-metamorphosis).