Regulated lens regeneration from isolated pigmented epithelial cells of newt iris in culture in response to FGF2/4

In Vivo and Ex Vivo View of Newt Lens Regeneration

Lens regeneration in the adult newt illustrates a unique example of naturally occurring cell transdifferentiation. During this process, iris pigmented epithelial cells (iPECs) reprogram into a lens, a tissue that is derived from a different embryonic source. Several methodologies both in vivo and in culture have been utilized over the years to observe this phenomenon. Most recently, Optical Coherence Tomography (OCT) has been identified as an effective tool to study the lens regeneration process in continuity through noninvasive, real-time imaging of the same animal. Described in this chapter are three different methodologies that can be used to observe the newt lens regeneration process both in vivo and ex vivo.

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.

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.

Intrinsic lens forming potential of mouse lens epithelial versus newt iris pigment epithelial cells in three-dimensional culture

Adult newts (Notophthalmus viridescens) are capable of complete lens regeneration that is mediated through dorsal iris pigment epithelial (IPE) cells transdifferentiation. In contrast, higher vertebrates such as mice demonstrate only limited lens regeneration in the presence of an intact lens capsule with remaining lens epithelial cells. To compare the intrinsic lens regeneration potential of newt IPE versus mouse lens epithelial cells (MLE), we have established a novel culture method that uses cell aggregation before culture in growth factor-reduced Matrigel. Dorsal newt IPE aggregates demonstrated complete lens formation within 1 to 2 weeks of Matrigel culture without basic fibroblast growth factor (bFGF) supplementation, including the establishment of a peripheral cuboidal epithelial cell layer, and the appearance of central lens fibers that were positive for αA-crystallin. In contrast, the lens-forming potential of MLE cell aggregates cultured in Matrigel was incomplete and resulted in the formation of defined-size lentoids with partial optical transparency. While the peripheral cell layers of MLE aggregates were nucleated, cells in the center of aggregates demonstrated a nonapoptotic nuclear loss over a time period of 3 weeks that was representative of lens fiber formation. Matrigel culture supplementation with bFGF resulted in higher transparent bigger-size MLE aggregates that demonstrated increased appearance of βB1-crystallin expression. Our study demonstrates that bFGF is not required for induction of newt IPE aggregate-dependent lens formation in Matrigel, while the addition of bFGF seems to be beneficial for the formation of MLE aggregate-derived lens-like structures. In conclusion, the three-dimensional aggregate culture of IPE and MLE in Matrigel allows to a higher extent than older models the indepth study of the intrinsic lens-forming potential and the corresponding identification of lentogenic factors.

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.

miRNAs in newt lens regeneration: specific control of proliferation and evidence for miRNA networking

BACKGROUND

Lens regeneration in adult newts occurs via transdifferentiation of the pigment epithelial cells (PECs) of the dorsal iris. The same source of cells from the ventral iris is not able to undergo this process. In an attempt to understand this restriction we have studied in the past expression patterns of miRNAs. Among several miRNAs we have found that mir-148 shows an up-regulation in the ventral iris, while members of the let-7 family showed down-regulation in dorsal iris during dedifferentiation.

METHODOLOGY/PRINCIPAL FINDINGS

We have performed gain- and loss-of-function experiments of mir-148 and let-7b in an attempt to delineate their function. We find that up-regulation of mir-148 caused significant decrease in the proliferation rates of ventral PECs only, while up-regulation of let-7b affected proliferation of both dorsal and ventral PECs. Neither miRNA was able to affect lens morphogenesis or induction. To further understand how this effect of miRNA up-regulation is mediated we examined global expression of miRNAs after up-regulation of mir148 and let-7b. Interestingly, we identified a novel level of mirRNA regulation, which might indicate that miRNAs are regulated as a network.

CONCLUSION/SIGNIFICANCE

The major conclusion is that different miRNAs can control proliferation in the dorsal or ventral iris possibly by a different mechanism. Of interest is that down-regulation of the let-7 family members has also been documented in other systems undergoing reprogramming, such as in stem cells or oocytes. This might indicate that reprogramming during newt regeneration shares common molecular signatures with reprogramming in stem or germ cells. On the other hand that miRNAs can regulate the levels of other miRNAs is a novel level of regulation, which might provide new insights on their function.

Tissue factor expression in newt iris coincides with thrombin activation and lens regeneration

Lens regeneration in adult salamanders occurs at the pupillary margin of the mid-dorsal iris where pigmented epithelial cells (PEC) re-enter the cell cycle and transdifferentiate into lens. It is not understood how the injury caused by removal of the lens (lentectomy) in one location is linked to initiating the response in a different spatial location (dorsal iris) and to this particular sector. We propose that the blood provides a link between the localised coagulation and signal transduction pathways that lead to regeneration. A transmembrane protein (tissue factor) is expressed in a striking patch-like domain in the dorsal iris of the newt that localises coagulation specifically to this location, but is not expressed in the axolotl, a related species that does not show thrombin activation after lentectomy and cannot regenerate its lens. Our hypothesis is that tissue factor expression localises the initiation of regeneration through the activation of thrombin and the recruitment of blood cells, leading to local growth factor release. This is the first example of gene expression in a patch of cells that prefigures the location of a regenerative response, and links the immune system with the initiation of a regenerative program.

Vertebrates that regenerate as models for guiding stem cels

There are several animal model organisms that have the ability to regenerate severe injuries by stimulating local cells to restore damaged and lost organs and appendages. In this chapter, we will describe how various vertebrate animals regenerate different structures (central nervous system, heart and appendages) as well as detail specific cellular and molecular features concerning the regeneration of these structures.

Determinative role of Wnt signals in dorsal iris-derived lens regeneration in newt eye

We have previously shown that lens regeneration from the pigmented epithelium of the dorsal iris in the adult newt eye proceeds in two steps after lens removal or intraocular FGF2 injection. The FGF2-dependent proliferation of iris pigmented epithelium and activation of early lens genes that occur over the entire circumference of the iris comprise the first step, while subsequent dorsally confined lens development marks the second step. Here, we investigated the expression of Wnt and Wnt receptor Frizzled genes in lens-regenerating iris tissues. Wnt2b and Frizzled4 were activated only in the dorsal half of the iris in synchrony with the occurrence of the second step, whereas Wnt5a and Frizzled2 were activated in both halves throughout the period of the first and second steps. Cultured explants of the iris-derived pigmented epithelium in the presence of FGF2 underwent dorsal-specific lens development fully recapitulating the in vivo lens regeneration process. Under these conditions, Wnt inhibitors Dkk1, which specifically inhibits the canonical signal pathway, and/or sFRP1 repressed the lens development, while exogenous Wnt3a, which generally activates the canonical pathway like Wnt2b, stimulated lens development from the dorsal iris epithelium and even caused lens development from the ventral iris epithelium, albeit at a reduced rate. Wnt5a did not elicit lens development from the ventral epithelium. These observations indicate that dorsal-specific activation of Wnt2b determines the dorsally limited development of lens from the iris pigmented epithelium.

Signaling during lens regeneration

The newt is one of the few organisms that is able to undergo lens regeneration as an adult. This review will examine the signaling pathways that are involved in this amazing phenomenon. In addition to outlining the current research involved in elucidating the key signaling molecules in lens regeneration, we will also highlight some of the similarities and differences between lens regeneration and development.

Tissue interaction between the retinal pigment epithelium and the choroid triggers retinal regeneration of the newt Cynops pyrrhogaster

Complete retinal regeneration in adult animals occurs only in certain urodele amphibians, in which the retinal pigmented epithelial cells (RPE) undergo transdifferentiation to produce all cell types constituting the neural retina. A similar mechanism also appears to be involved in retinal regeneration in the embryonic stage of some other species, but the nature of this mechanism has not yet been elucidated. The organ culture model of retinal regeneration is a useful experimental system and we previously reported RPE transdifferentiation of the newt under this condition. Here, we show that cultured RPE cells proliferate and differentiate into neurons when cultured with the choroid attached to the RPE, but they did not exhibit any morphological changes when cultured alone following removal of the choroid. This finding indicates that the tissue interactions between the RPE and the choroid are essential for the former to proliferate. This tissue interaction appears to be mediated by diffusible factors, because the choroid could affect RPE cells even when the two tissues were separated by a membrane filter. RPE transdifferentiation under the organotypic culture condition was abolished by a MEK (ERK kinase) inhibitor, U0126, but was partially suppressed by an FGF receptor inhibitor, SU5402, suggesting that FGF signaling pathway has a central role in the transdifferentiation. While IGF-1 alone had no effect on isolated RPE, combination of FGF-2 and IGF-1 stimulated RPE cell transdifferentiation similar to the results obtained in organ-cultured RPE and choroid. RT-PCR revealed that gene expression of both FGF-2 and IGF-1 is up-regulated following removal of the retina. Thus, we show for the first time that the choroid plays an essential role in newt retinal regeneration, opening a new avenue for understanding the molecular mechanisms underlying retinal regeneration.

FGF2 triggers iris-derived lens regeneration in newt eye

Lens regeneration in newts occurs exclusively from the dorsal aspect of the iris pigment epithelium. Although the phenomenon has been a paradigm of experimental tissue regeneration, little is understood about how it is initiated and restricted to the dorsal iris. Here we show among various growth factors injected in an intact eye, a single injection of FGF2 specifically caused morphological changes of the iris characteristic of lens regeneration, induced expression of transcription factor genes Pax6, Sox2 and MafB, as well as endogenous Fgf2 in both dorsal and ventral halves, and provoked second lens development only from the dorsal iris. FGF2 protein accumulated in the iris tissue after the lens was removed, and injection of a soluble form of FGF receptor titrating FGF2 inhibited all reactions observed after the lens removal or after administration of FGF2. These results indicate that FGF2 and/or related molecules trigger lens regeneration from the dorsal iris in the newt. The observations also indicate that the absence of lens regeneration from the ventral iris is due to a block in a later phase of lens developmental pathway.

Eye regeneration at the molecular age

Eye tissues such as the lens and the retina possess remarkable regenerative abilities. In amphibians, a complete lens can be regenerated after lentectomy. The process is a classic example of transdifferentiation of one cell type to another. Likewise, retina can be regenerated, but the strategy used to replace the damaged retina differs, depending on the animal system and the age of the animal. Retina can be regenerated by transdifferentiation or by the use of stem cells. In this review, we present a synthesis on the regenerative capacity of eye tissues in different animals with emphasis on the strategy and the molecules involved. In addition, we stress the place of this field at the molecular age and the importance of the recent technologic advances.

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.