The cellular and molecular bases of vertebrate lens regeneration

Insights into Bone Morphogenetic Protein-(BMP-) Signaling in Ocular Lens Biology and Pathology

Bone morphogenetic proteins (BMPs) are a diverse class of growth factors that belong to the transforming growth factor-beta (TGFβ) superfamily. Although originally discovered to possess osteogenic properties, BMPs have since been identified as critical regulators of many biological processes, including cell-fate determination, cell proliferation, differentiation and morphogenesis, throughout the body. In the ocular lens, BMPs are important in orchestrating fundamental developmental processes such as induction of lens morphogenesis, and specialized differentiation of its fiber cells. Moreover, BMPs have been reported to facilitate regeneration of the lens, as well as abrogate pathological processes such as TGFβ-induced epithelial-mesenchymal transition (EMT) and apoptosis. In this review, we summarize recent insights in this topic and discuss the complexities of BMP-signaling including the role of individual BMP ligands, receptors, extracellular antagonists and cross-talk between canonical and non-canonical BMP-signaling cascades in the lens. By understanding the molecular mechanisms underlying BMP activity, we can advance their potential therapeutic role in cataract prevention and lens regeneration.

In Vivo Imaging of Newt Lens Regeneration: Novel Insights Into the Regeneration Process

Purpose

To establish optical coherence tomography (OCT) as an in vivo imaging modality for investigating the process of newt lens regeneration.

Methods

Spectral-domain OCT was employed for in vivo imaging of the newt lens regeneration process. A total of 37 newts were lentectomized and followed by OCT imaging over the course of 60 to 80 days. Histological images were obtained at several time points to compare with the corresponding OCT images. Volume measurements were also acquired.

Results

OCT can identify the key features observed in corresponding histological images based on the scattering differences from various eye tissues, such as the cornea, intact and regenerated lens, and the iris. Lens volume measurements from three-dimensional OCT images showed that the regenerating lens size increased linearly until 60 days post-lentectomy.

Conclusions

Using OCT imaging, we were able to track the entire process of newt lens regeneration in vivo for the first time. Three-dimensional OCT images allowed us to volumetrically quantify and visualize the dynamic spatial relationships between tissues during the regeneration process. Our results establish OCT as anin vivo imaging modality to track/analyze the entire lens regeneration process from the same animal.

Translational Relevance

Lens regeneration in newts represents a unique example of vertebrate tissue plasticity. Investigating the cellular and morphological events that govern this extraordinary process in vivo will advance our understanding and shed light on developing new therapies to treat blinding disorders in higher vertebrates.

Xenopus, a Model to Study Wound Healing and Regeneration: Experimental Approaches

Xenopus has been widely used as a model organism to study wound healing and regeneration. During early development and at tadpole stages, Xenopus is a quick healer and is able to regenerate multiple complex organs-abilities that decrease with the progression of metamorphosis. This unique capacity leads us to question which mechanisms allow and direct regeneration at stages before the beginning of metamorphosis and which ones are responsible for the loss of regenerative capacities during later stages. Xenopus is an ideal model to study regeneration and has contributed to the understanding of morphological, cellular, and molecular mechanisms involved in these processes. Nevertheless, there is still much to learn. Here we provide an overview on using Xenopus as a model organism to study regeneration and introduce protocols that can be used for studying wound healing and regeneration at multiple levels, thus enhancing our understanding of these phenomena.

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.

Methods for Examining Lens Regeneration in Xenopus

Some vertebrates are able to regenerate the lens following its removal. This includes species in the genus Xenopus (i.e., X. laevis, X. tropicalis, and X. borealis), the only anurans known to undergo lens regeneration. In Xenopus the regenerated lens is derived de novo from cells located within the basal-most layer of the larval corneal epithelium, and is triggered by factors provided by the neural retina. In larval frogs the corneal epithelium is underlain by an endothelium separated from the corneal epithelium except for a small central attachment (i.e., the "stromal-attracting center"). This connection grows larger as the stroma forms and the frogs approach metamorphosis. Here we provide instructions for performing lentectomies (removal of the original lens) to study lens regeneration.

Diverse Evolutionary Origins and Mechanisms of Lens Regeneration

In this review, we compare and contrast the three different forms of vertebrate lens regeneration: Wolffian lens regeneration, cornea-lens regeneration, and lens regeneration from lens epithelial cells. An examination of the diverse cellular origins of these lenses, their unique phylogenetic distribution, and the underlying molecular mechanisms, suggests that these different forms of lens regeneration evolved independently and utilize neither conserved nor convergent mechanisms to regulate these processes.

The lens regenerative competency of limbal vs. central regions of mature Xenopus cornea epithelium

The frog, Xenopus laevis, is capable of completely regenerating a lens from the cornea epithelium. Because this ability appears to be limited to the larval stages of Xenopus, virtually all the work to understand the mechanisms regulating this process has been limited to pre-metamorphic tadpoles. It has been reported that the post-metamorphic cornea is competent to regenerate under experimental conditions, despite the fact that the in vivo capacity to regenerate is lost; however, that work didn't examine the regenerative potential of different regions of the cornea. A new model suggests that cornea-lens regeneration in Xenopus may be driven by oligopotent stem cells, and not by transdifferentiation of mature cornea cells. We investigated the regenerative potential of the limbal region in post-metamorphic cornea, where the stem cells of the cornea are thought to reside. Using EdU (5-Ethynyl-2'-deoxyuridine), we identified long-term label retaining cells in the basal cells of peripheral post-metamorphic Xenopus cornea, consistent with slow-cycling stem cells of the limbus that have been described in other vertebrates. Using this data to identify putative stem cells of the limbal region in Xenopus, we tested the regenerative competency of limbal regions and central cornea. These regions showed a similarly high ability for the cells of the basal epithelium to express lens proteins when cultured in proximity to larval retina. Thus, the regenerative competency in the post-metamorphic cornea is not restricted to stem cells of the limbal region, but also occurs in the transit amplifying cells throughout the basal layer of the cornea epithelium.

Lens regeneration from the cornea requires suppression of Wnt/β-catenin signaling

The frog, Xenopus laevis, possesses a high capacity to regenerate various larval tissues, including the lens, which is capable of complete regeneration from the cornea epithelium. However, the molecular signaling mechanisms of cornea-lens regeneration are not fully understood. Previous work has implicated the involvement of the Wnt signaling pathway, but molecular studies have been very limited. Iris-derived lens regeneration in the newt (Wolffian lens regeneration) has shown a necessity for active Wnt signaling in order to regenerate a new lens. Here we provide evidence that the Wnt signaling pathway plays a different role in the context of cornea-lens regeneration in Xenopus. We examined the expression of frizzled receptors and wnt ligands in the frog cornea epithelium. Numerous frizzled receptors (fzd1, fzd2, fzd3, fzd4, fzd6, fzd7, fzd8, and fzd10) and wnt ligands (wnt2b.a, wnt3a, wnt4, wnt5a, wnt5b, wnt6, wnt7b, wnt10a, wnt11, and wnt11b) are expressed in the cornea epithelium, demonstrating that this tissue is transcribing many of the ligands and receptors of the Wnt signaling pathway. When compared to flank epithelium, which is lens regeneration incompetent, only wnt11 and wnt11b are different (present only in the cornea epithelium), identifying them as potential regulators of cornea-lens regeneration. To detect changes in canonical Wnt/β-catenin signaling occurring within the cornea epithelium, axin2 expression was measured over the course of regeneration. axin2 is a well-established reporter of active Wnt/β-catenin signaling, and its expression shows a significant decrease at 24 h post-lentectomy. This decrease recovers to normal endogenous levels by 48 h. To test whether this signaling decrease was necessary for lens regeneration to occur, regenerating eyes were treated with either 6-bromoindirubin-3'-oxime (BIO) or 1-azakenpaullone - both activators of Wnt signaling - resulting in a significant reduction in the percentage of cases with successful regeneration. In contrast, inhibition of Wnt signaling using either the small molecule IWR-1, treatment with recombinant human Dickkopf-1 (rhDKK1) protein, or transgenic expression of Xenopus DKK1, did not significantly affect the percentage of successful regeneration. Together, these results suggest a model where Wnt/β-catenin signaling is active in the cornea epithelium and needs to be suppressed during early lens regeneration in order for these cornea cells to give rise to a new lentoid. While this finding differs from what has been described in the newt, it closely resembles the role of Wnt signaling during the initial formation of the lens placode from the surface ectoderm during early embryogenesis.

Retinoic acid regulation by CYP26 in vertebrate lens regeneration

Xenopus laevis is among the few species that are capable of fully regenerating a lost lens de novo. This occurs upon removal of the lens, when secreted factors from the retina are permitted to reach the cornea epithelium and trigger it to form a new lens. Although many studies have investigated the retinal factors that initiate lens regeneration, relatively little is known about what factors support this process and make the cornea competent to form a lens. We presently investigate the role of Retinoic acid (RA) signaling in lens regeneration in Xenopus. RA is a highly important morphogen during vertebrate development, including the development of various eye tissues, and has been previously implicated in several regenerative processes as well. For instance, Wolffian lens regeneration in the newt requires active RA signaling. In contrast, we provide evidence here that lens regeneration in Xenopus actually depends on the attenuation of RA signaling, which is regulated by the RA-degrading enzyme CYP26. Using RT-PCR we examined the expression of RA synthesis and metabolism related genes within ocular tissues. We found expression of aldh1a1, aldh1a2, and aldh1a3, as well as cyp26a1 and cyp26b1 in both normal and regenerating corneal tissue. On the other hand, cyp26c1 does not appear to be expressed in either control or regenerating corneas, but it is expressed in the lens. Additionally in the lens, we found expression of aldh1a1 and aldh1a2, but not aldh1a3. Using an inhibitor of CYP26, and separately using exogenous retinoids, as well as RA signaling inhibitors, we demonstrate that CYP26 activity is necessary for lens regeneration to occur. We also find using phosphorylated Histone H3 labeling that CYP26 antagonism reduces cell proliferation in the cornea, and using qPCR we find that exogenous retinoids alter the expression of putative corneal stem cell markers. Furthermore, the Xenopus cornea is composed of an outer layer and inner basal epithelium, as well as a deeper fibrillar layer sparsely populated with cells. We employed antibody staining to visualize the localization of CYP26A, CYP26B, and RALDH1 within these corneal layers. Immunohistochemical staining of these enzymes revealed that all 3 proteins are expressed in both the outer and basal layers. CYP26A appears to be unique in also being present in the deeper fibrillar layer, which may contain cornea stem cells. This study reveals a clear molecular difference between newt and Xenopus lens regeneration, and it implicates CYP26 in the latter regenerative process.

Cell signaling pathways in vertebrate lens regeneration

Certain vertebrates are capable of regenerating parts of the eye, including the lens. Depending on the species, two principal forms of in vivo lens regeneration have been described wherein the new lens arises from either the pigmented epithelium of the dorsal iris or the cornea epithelium. These forms of lens regeneration are triggered by retinal factors present in the eye. Studies have begun to illuminate the nature of the signals that support lens regeneration. This review describes evidence for the involvement of specific signaling pathways in lens regeneration, including the FGF, retinoic acid, TGF-beta, Wnt, and Hedgehog pathways.

Lens and retina regeneration: new perspectives from model organisms

Comparative studies of lens and retina regeneration have been conducted within a wide variety of animals over the last 100 years. Although amphibians, fish, birds and mammals have all been noted to possess lens- or retina-regenerative properties at specific developmental stages, lens or retina regeneration in adult animals is limited to lower vertebrates. The present review covers the newest perspectives on lens and retina regeneration from these different model organisms with a focus on future trends in regeneration research.

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.

Dedifferentiated follicular granulosa cells derived from pig ovary can transdifferentiate into osteoblasts

Transdifferentiation is the conversion of cells from one differentiated cell type into another. How functionally differentiated cells already committed to a specific cell lineage can transdifferentiate into other cell types is a key question in cell biology and regenerative medicine. In the present study we show that porcine ovarian follicular GCs (granulosa cells) can transdifferentiate into osteoblasts in vitro and in vivo. Pure GCs isolated and cultured in Dulbecco's modified Eagle's medium supplemented with 20% FBS (fetal bovine serum) proliferated and dedifferentiated into fibroblast-like cells. We referred to these cells as DFOG (dedifferentiated follicular granulosa) cells. Microarray analysis showed that DFOG cells lost expression of GC-specific marker genes, but gained the expression of osteogenic marker genes during dedifferentiation. After osteogenic induction, DFOG cells underwent terminal osteoblast differentiation and matrix mineralization in vitro. Furthermore, when DFOG cells were transplanted subcutaneously into SCID mice, these cells formed ectopic osteoid tissue. These results indicate that DFOG cells derived from GCs can differentiate into osteoblasts in vitro and in vivo. We suggest that GCs provide a useful model for studying the mechanisms of transdifferentiation into other cell lineages in functionally differentiated cells.

FGF signaling is required for lens regeneration in Xenopus laevis

In species of the frog genus Xenopus, lens regeneration occurs through a process of transdifferentiation, in which cornea epithelial cells presumably undergo dedifferentiation and subsequently redifferentiate to form a new lens. Experimental studies have shown that the retina provides the key signal required to trigger this process once the original lens is removed. A previous study showed that addition of an exogenous fibroblast growth factor (i.e., FGF1 protein) could initiate transdifferentiation of cornea epithelial cells in culture. To determine the role of FGF signaling in X. laevis lens regeneration, we have examined the presence of specific FGFs and their receptors (FGFRs) during this process and evaluated the necessity of FGFR signaling. Reverse transcriptase-polymerase chain reaction analyses reveal that a number of FGF family members are expressed in cornea epithelium and retinal tissues both before and during the process of lens regeneration. Of these, FGF1, FGF8, and FGF9 are expressed principally in retinal tissue and not in the cornea epithelium. Hence, these ligands could represent key signaling factors originating from the retina that trigger regeneration. The results of experiments using an in vitro eye culture system and an FGFR inhibitor (SU5402) suggest that FGFR signaling is required for lens regeneration in Xenopus.

The G-protein-coupled receptor, GPR84, is important for eye development in Xenopus laevis

G-protein-coupled receptors (GPCRs) represent diverse, multifamily groups of cell signaling receptors involved in many cellular processes. We identified Xenopus laevis GPR84 as a member of the A18 subfamily of GPCRs. During development, GPR84 is detected in the embryonic lens placode, differentiating lens fiber cells, retina, and cornea. Anti-sense morpholino oligonucleotide-mediated knockdown and RNA rescue experiments demonstrate GPR84's importance in lens, cornea, and retinal development. Examination of cell proliferation using an antibody against histone H3 S10P reveals significant increases in the lens and retina following GPR84 knockdown. Additionally, there was also an increase in apoptosis in the retina and lens, as revealed by TUNEL assay. Reciprocal transplantation of the presumptive lens ectoderm between uninjected controls and morpholino-injected embryos demonstrates that GPR84 is necessary in the retina for proper development of the retina, as well as other eye tissues including the lens and cornea.

Molecular and cellular aspects of amphibian lens regeneration

Lens regeneration among vertebrates is basically restricted to some amphibians. The most notable cases are the ones that occur in premetamorphic frogs and in adult newts. Frogs and newts regenerate their lens in very different ways. In frogs the lens is regenerated by transdifferentiation of the cornea and is limited only to a time before metamorphosis. On the other hand, regeneration in newts is mediated by transdifferentiation of the pigment epithelial cells of the dorsal iris and is possible in adult animals as well. Thus, the study of both systems could provide important information about the process. Molecular tools have been developed in frogs and recently also in newts. Thus, the process has been studied at the molecular and cellular levels. A synthesis describing both systems was long due. In this review we describe the process in both Xenopus and the newt. The known molecular mechanisms are described and compared.

Gene expression profiles of lens regeneration and development in Xenopus laevis

Seven hundred and thirty-four unique genes were recovered from a cDNA library enriched for genes up-regulated during the process of lens regeneration in the frog Xenopus laevis. The sequences represent transcription factors, proteins involved in RNA synthesis/processing, components of prominent cell signaling pathways, genes involved in protein processing, transport, and degradation (e.g., the ubiquitin/proteasome pathway), matrix metalloproteases (MMPs), as well as many other proteins. The findings implicate specific signal transduction pathways in the process of lens regeneration, including the FGF, TGF-beta, MAPK, Retinoic acid, Wnt, and hedgehog signaling pathways, which are known to play important roles in eye/lens development and regeneration in various systems. In situ hybridization revealed that the majority of genes recovered are expressed during embryogenesis, including in eye tissues. Several novel genes specifically expressed in lenses were identified. The suite of genes was compared to those up-regulated in other regenerating tissues/organisms, and a small degree of overlap was detected.

Retinoic acid signaling in mammalian eye development

Retinoic acid (RA) is a biologically active metabolite of vitamin A (retinol) that serves as a signaling molecule during a number of developmental and physiological processes. RA signaling plays multiple roles during embryonic eye development. RA signaling is initially required for reciprocal interactions between the optic vesicle and invaginating lens placode. RA signaling promotes normal development of the ventral retina and optic nerve through its activities in the neural crest cell-derived periocular mesenchyme. RA coordinates these processes by regulating biological activities of a family of non-steroid hormone receptors, RARalpha/beta/gamma, and RXRalpha/beta/gamma. These DNA-binding transcription factors recognize DNA as RAR/RXR heterodimers and recruit multiprotein transcriptional co-repressor complexes. RA-binding to RAR receptors induces a conformational change in the receptor, followed by the replacement of co-repressor with co-activator complexes. Inactivation of RARalpha/beta/gamma receptors in the periocular mesenchyme abrogates anterior eye segment formation. This review summarizes recent genetic studies of RA signaling and progress in understanding the molecular mechanism of transcriptional co-activators that function with RAR/RXR.

Skeletal elements in the vertebrate eye and adnexa: morphological and developmental perspectives

Although poorly appreciated, the vertebrate eye and adnexa are relatively common sites for skeletogenesis. In many taxa, the skeleton contributes to internal reinforcement in addition to the external housing of the eye (e.g., the circumorbital bones and eyelids). Eyeball elements such as scleral cartilage and scleral ossicles are present within a broad diversity of vertebrates, albeit not therian mammals, and have been used as important models for the study of condensations and epithelial-mesenchymal interactions. In contrast, other elements invested within the eye or its close surroundings remain largely unexplored. The onset and mode of development of these skeletal elements are often variable (early versus late; involving chondrogenesis, osteogenesis, or both), and most (if not all) of these elements appear to share a common neural crest origin. This review discusses the development and distribution of the skeletal elements within and associated with the developing eye and comments on homology of the elements where these are questionable.

Clonally cultured differentiated pigment cells can dedifferentiate and generate multipotent progenitors with self-renewing potential

The differentiation of a given cell should be irreversible in order to ensure cell-type-specific function and stability of resident tissue. However, under stimulation in vitro or during regeneration, differentiated cells may recover properties of immature cells. Yet the mechanisms whereby differentiated cells can change fate or reverse to precursor cells are poorly understood. We show here that neural crest (NC)-derived pigment cells that have differentiated in quail embryo, when isolated from the skin and clonally cultured in vitro, are able to generate glial and myofibroblastic cells. The phenotypic reprogramming involves dedifferentiation of dividing pigment cells into cells that re-express NC early marker genes Sox10, FoxD3, Pax3 and Slug. Single melanocytes generate multipotent progenitors able to self-renew along serial subcloning, thus exhibiting stem cell properties. The presence of endothelin 3 promotes the emergence and maintenance of multipotent progenitors in melanocyte progeny. These multipotent cells are heterogeneous with respect to marker identity, including pigmented cells and dedifferentiated cells that have reacquired expression of the early NC marker HNK1. These data provide evidence that, when removed from their niche and subjected to appropriate culture conditions, pigment cells are phenotypically unstable and can reverse to their NC-like ancestors endowed with self-renewal capacity.

Lentoid Bodies in the Avian Retina

In-vitro studies suggest that, in avian retina, lentoid bodies arise from Müller cells or developing neuroretina. This report describes lentoid bodies in adult avian retinas in association with retinal trauma or degeneration. Retinal lentoids were identified in four birds (three owls and one parrot) in the course of routine diagnostic histopathology. Sections were stained with periodic acid-Schiff for the purposes of descriptive histology, and immunolabelled for a Müller cell marker (glial fibrillary acidic protein; GFAP) and a lens-specific marker (crystallin alpha-A). Intraretinal lentoids of varying size were identified, the constituent cells resembling bladder cells similar to those seen in cataracts. The process of lentoid formation followed a consistent pattern, characterized by progressive Müller cell hypertrophy in damaged areas, culminating in lentoid formation. GFAP immunoreactivity was strongest in Müller cells in the early stages of hypertrophy and receded as Müller cell hypertrophy advanced and lentoids developed. In contrast to GFAP immunoreactivity, crystalline alpha-A labelling increased in distribution and intensity as Müller hypertrophy became more prominent and lentoids were formed. This represents the first report of intraretinal lentoids in birds in vivo. The immunohistochemical data suggest that they arise from Müller cells. Association of lentoids with retinal damage supports the assertion that they arise following disruption of normal cell-cell communication.

Reversibility of the Differentiated State: Regeneration in Amphibians

In contrast to mammals, some fish and amphibians have retained the ability to regenerate complex body structures or organs, such as the limb, tail, eye lens, or even parts of the heart. One major difference in the response to injury is the appearance of a mesenchymal growth zone or blastema in these regenerative species instead of the scarring seen in mammals. This blastema is thought to largely derive from the dedifferentiation of various functional cell types, such as skeletal muscle, dermis, and cartilage. In the case of multinucleated skeletal muscle fibers, cell cycle reentry into S-phase as well as fragmentation into mononucleated progenitors is observed both in vitro and in vivo.

The role of Pax-6 in lens regeneration

Pax-6 is a master regulator of eye development and is expressed in the dorsal and ventral iris during newt lens regeneration. We show that expression of Pax-6 during newt lens regeneration coincides with cell proliferation. By knocking down expression of Pax-6 via treatment with morpholinos, we found that proliferation of iris pigment epithelial cells was dramatically reduced both in vitro and in vivo, and, as a result, lens regeneration was significantly retarded. However, induction of dedifferentiation in the dorsal iris was not inhibited. Pax-6 knockdown early in lens regeneration resulted in inhibition of crystallin expression and retardation of lens fiber induction. Once crystallin expression and differentiation of lens fibers has ensued, however, loss of function of Pax-6 did not affect crystallin expression and lens fiber maintenance, even though the effects on proliferation persisted. These results conclusively show that Pax-6 is associated with distinct early events during lens regeneration, namely control of cell proliferation and subsequent lens fiber differentiation.

Effects of exogenous FGF-1 treatment on regeneration of the lens and the neural retina in the newt, Notophthalmus viridescens

Experiments were designed to compare the effects of recombinant newt fibroblast growth factor-1 (rnFGF-1) and recombinant human glial growth factor (rhGGF) on lens and retina regeneration in the eyes of adult newts. Both eyes were retinectomized and lentectomized. Beginning 3 days after the operation, one eye was given either 0.1 microg of rnFGF-1 or 0.1 microg of rhGGF in 1 microl of phosphate-buffered saline (PBS) per injection, three per week. Contralateral operated eyes served as controls and were treated with PBS alone or were not injected. In eyes that were not injected, injected with PBS alone, or with PBS containing rhGGF, regeneration of both the retina and the lens proceeded normally as described in the literature. In these control eyes, the entire retinal pigmented epithelium (RPE) depigmented/dedifferentiated and a retina rudiment formed from which a new retina regenerated by the end of the experiment at day 41 post-operation. Likewise, only a small area of dorsal iris depigmented/dedifferentiated and formed a lens vesicle from which a lens subsequently regenerated. The vitreous remained relatively free of loose cells. In eyes given rnFGF-1, the RPE depigmented/dedifferentiated and formed what appeared to be a retina rudiment but a new retina did not regenerate. Instead, vesicles were seen associated with the retina rudiment. In eyes given rnFGF-1, both the dorsal iris and ventral iris depigmented/dedifferentiated and lens regeneration occurred but the new lenses had abnormal fiber cells and the lens epithelium was very thin or absent. In addition, ectopic lenses usually regenerated in rnFGF-1-treated eyes. An abundance of loose cells were present in the vitreous of rnFGF-1-treated eyes associated largely with the RPE and the dorsal and ventral irises. The results are consistent with the view that the timely expression of FGFs is involved in the depigmentation/dedifferentiation of the RPE and dorsal iris and is necessary for proper regeneration of the lens and neural retina. Continued presence of FGF results in continued and excessive dedifferentiation, resulting in the lack of retina regeneration and abnormal lens regeneration.

Transcription factor snail modulates hormone expression in established endocrine pancreatic cell lines

The development of differentiated cells from undifferentiated progenitor cells is one of the central tenets of developmental biology. However, under conditions of tissue morphogenesis, regeneration, and cancer, this process of development is reversed and fully differentiated cells transition to an undifferentiated phenotype. Here we present evidence that the zinc-finger transcription factor Snail modulates this transition in differentiated pancreatic endocrine cell lines. During passage and growth of these cell lines, Snail expression is induced in a subset of cells within the culture, concomitant with a decrease in insulin and/or glucagon expression. As the cells cluster and exit the cell division cycle, nuclear levels of Snail are reduced and hormone expression is resumed. Snail represses proinsulin and proglucagon gene transcription, and reduction of Snail levels by small interfering RNA treatment increases proinsulin gene expression. We propose that Snail modulates the dynamic balance between differentiated and dedifferentiated cells allowing their migration and proliferation. These findings may be relevant to providing approaches for the enhancement of beta-cell growth in individuals with diabetes mellitus.

Identity Deception: Not a Crime for a Stem Cell

Stem cell transdifferentiation in the adult organism is the most common and questioned mechanism of growth and repair. Recent data suggest that adult stem cells are capable of generating mature cells beyond their own tissue boundaries, a process called developmental plasticity. To date, the most versatile cell discovered is the bone marrow progenitor cell.

Early regeneration genes: Building a molecular profile for shared expression in cornea-lens transdifferentiation and hindlimb regeneration in Xenopus laevis

Recent studies in Xenopus laevis have begun to compare gene expression during regeneration with that of the original development of specific structures (e.g., the hindlimb and lens), while other studies have sought differences in gene expression between regeneration-competent and regeneration-incompetent stages. To determine whether there are any similarities between the regeneration of different structures, we have used a differential screen to seek shared early gene expression between hindlimb regeneration and cornea-lens transdifferentiation in the Xenopus tadpole. We have isolated 13 clones representing genes whose expression is up-regulated within the first few days of both regenerating processes and which are not demonstrably up-regulated in the context of basic wound healing. Furthermore, all of these genes also show prominent late embryonic expression. The expression patterns and putative identities of all 13 genes are presented, and a model is considered that allows us to characterize and profile important changes in gene expression, which might be shared among various regenerating and developmental systems.

Plasticity and Reprogramming of Differentiated Cells in Amphibian Regeneration: Partial Purification of a Serum Factor that Triggers Cell Cycle Re-Entry in Differentiated Muscle Cells

The reversal of cellular differentiation to form proliferating progenitor cells is a critical aspect of regenerative ability in the urodele amphibians. This process has been studied using skeletal muscle during limb or tail regeneration, or dorsal iris epithelium during lens regeneration. An unknown activity in serum triggers cell cycle re-entry from the differentiated state. Here we describe the biochemical properties and fractionation of this serum factor. The factor is a glycoprotein that associates with large molecular weight complexes. The purification and molecular identification of the serum factor represents an important avenue in understanding regenerative ability and dedifferentiation capacity on a molecular basis.