Experimental Eye Research 78 (2004) 161–172 www.elsevier.com/locate/yexer Review Lens and retina regeneration: transdifferentiation, stem cells and clinical applications Panagiotis A. Tsonisa,*, Katia Del Rio-Tsonisb aUniversity of Dayton, Laboratory of Molecular Biology, Department of Biology, Dayton, OH 45469 2320, USA bDepartment of Zoology, Miami University, Oxford, OH 45056, USA Received 17 July 2003; accepted in revised form 24 October 2003 Abstract In this review we present a synthesis on the potential of vertebrate eye tissue regeneration, such as lens and retina. Particular emphasis is given to two different strategies used for regeneration, transdifferentiation and stem cells. Similarities and differences between these two strategies are outlined and it is proposed that both strategies might follow common pathways. Furthermore, we elaborate on specific clinical applications as the outcome of regeneration-based research q 2003 Elsevier Ltd. All rights reserved. Keywords: eye; lens; retina; regeneration; transdifferentiation; stem cells; cataracts; retinal diseases An old Greek proverb says that when you have something clear evolutionary advantage (tail regeneration in lizards) precious you should guard it as you do your eyes. Vision, and some with no obvious evolutionary advantage (i.e. lens among all the other senses, provides the link to the outside regeneration in newts). In recent years, however, intense world which is extremely important for survival of species research, especially on stem cells, has shown that the body and is much valued by humans. So it should not come as a has more remarkable reparative capabilities than previously surprise that nature must have devised back-up strategies to thought. The same we believe is true with repair of eye loss or damage of the eye tissues. Why are then, among tissues and in this review we intend to popularize this view. vertebrates, regenerative abilities of the lens and retina so Before we examine the regeneration process and mechan- pronounced only in some amphibia? Why is regeneration of isms involved in lens and retina, let us take a note of the two the lens or retina an advantage to some salamanders and not major strategies that animals use to repair damaged tissues. to the rest of the vertebrates? Thinking along these lines we Regeneration occurs by two strategies. One strategy uses are dealing with an evolutionary paradox. differentiated cells neighbouring the damaged site. These When it comes to evolution, regeneration of body parts cells restore the damaged tissue by proliferation or by must have been an advantage, especially in asexually transdifferentiation. Transdifferentiation is the process by reproduced animals (Tsonis, 2000; Brockes et al., 2001). In which cells are able to dedifferentiate (lose the character- istics of their origin) and subsequently redifferentiate. This many cases regeneration in asexual animals is very similar strategy is used in many cases, such as liver, pancreas and is to their mode of reproduction. As species became more characteristic of epimorphic regeneration as well (Tsonis, advanced and reproduction became sexual, regenerative 2000, 2002). As we will see transdifferentiation is the capabilities diminished. Several species, however, have strategy used in lens regeneration. The other strategy is by retained remarkable regenerative capabilities, some with stem cells. See later section for a discussion on the two regeneration strategies. In retina regeneration, however, * Corresponding author. Dr Panagiotis A. Tsonis, University of Dayton, Laboratory of Molecular Biology, Department of Biology, Dayton, OH both strategies can be used. As we will see depending on 45469 2320, USA. species, transdifferentiation or progenitor cells can be E-mail address: [email protected] (P.A. Tsonis). recruited to populate damaged retina. 0014-4835/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. DOI:10.1016/j.exer.2003.10.022 162 P.A. Tsonis, K. Del Rio-Tsonis / Experimental Eye Research 78 (2004) 161–172 1. Lens regeneration As it was mentioned above, lens regeneration was first observed in adult newts (Colluci, 1891; Wolff, 1895). These animals have been the major experimental material for the study of lens regeneration. Lens regeneration is also possible in certain frogs, but the process differs considerably from the one in newts. In these frogs, lens regeneration is possible only during the pre-metamorphic stages of development (see below). Adult frogs are not capable of regeneration. The only adult animals with that capability are some urodeles. In Section 1.1, we will examine the two animal models and compare the mechanisms in both. In mammals such regenerative properties are absent. Regen- eration, however, can be surgically manipulated. In rabbits if the lens is removed, but the capsule stays behind and rather intact, remaining lens epithelial cells differentiate and fill the capsule, thus reconstructing the lens. Lens regeneration in the adult newt begins with proliferation and dedifferentiation of dorsal iris pigment epithelial cells (PECs). By dedifferentiation we mean the loss of characteristics that define the PECs, such as pigmentation (Eguchi, 1963). Dedifferentiation initiates molecular events, such as cell cycle re-entry, which is necessary for cell proliferation and the subsequent regen- eration of the lens. So far, the fastest known event that occurs after lentectomy is thrombin activation in the dorsal iris. Such activation cannot be seen in the ventral iris or in the irises of other salamanders that are incapable of lens regeneration (Imokawa and Brockes, 2003). At about 10 days post-lentectomy, a lens vesicle is formed from the depigmented dorsal PECs (Fig. 1(A)). Around 12–16 days post-lentectomy, the internal layer of the lens vesicle thickens and synthesis of crystallins begins (Fig. 1(B)). This marks the beginning of primary lens Fig. 1. Lens regeneration in newts via transdifferentiation of the PECs fiber differentiation. During days 15 –19, proliferation and from the dorsal iris (di). (A) Ten-days post-lentectomy. Note an early lens depigmentation of PECs slows down. In the internal layer, vesicle (arrow) formed by dedifferentiation of the PECs from the dorsal the lens fibre complex is formed and in the margin of the iris. (B) Fifteen-days post-lentectomy. The cells at the posterior part of the vesicle (arrow) elongate to form lens fibres. (C) Twenty-days post- external layers non-dividing secondary lens fibres appear. lentectomy. A well differentiated lens with lens fibres (lf) covered by the By 18–20 days the PECs have stopped proliferating, and the lens epithelium (le). lens fibres continue to accumulate crystallins (Fig. 1(C)). Lens regeneration is considered complete by day 25–30 Among other amphibians frogs can regenerate their lost (Eguchi, 1963, 1964; Reyer, 1977; Yamada, 1977; Tsonis, lens, but in contrast to the newt, regeneration occurs via 1999, 2000). Lens regeneration, therefore, is a clear case of transdifferentiation of the inner layer of the outer cornea. transdifferentiation. A very interesting restriction is that the Another important difference is that lens regeneration in ventral iris, which is seemingly comprised by the same frogs is possible only during premetamorphic stages and PECs is not capable of regenerating a lens. The process of ceases after metamorphosis (Freeman, 1963; Filoni et al., transdifferentiation has been proven beyond any doubt in 1997; Henry and Elkins, 2001). Also, in Xenopus laevis, the this system. These processes can also be observed when capacity seems to depend on factors that are provided by the single PEC cells are placed in culture (Eguchi et al., 1974; retina (Filoni et al., 1982). When a piece of outer cornea is Kodama and Eguchi, 1995; Tsonis et al., 2001). The implanted in the vitreous chamber, even in the presence of restrictions that we see in the in vivo newt model do not the host lens, transdifferentiation can occur. It is possible apply for the in vitro models. PECs from the whole eye that the rapid closure of the inner cornea after metamor- (including from the ventral iris) and from any species, phosis is an inhibitor to regeneration (Reeve and Wild, including aged humans are capable of transdifferentiating to 1978; Filoni et al., 1997). The stages during lens lens cells under certain conditions (Tsonis et al., 2001). regeneration from the cornea are very similar to the ones P.A. Tsonis, K. Del Rio-Tsonis / Experimental Eye Research 78 (2004) 161–172 163 Fig. 2. Lens regeneration in pre-metamorphic Xenopus via transdifferentiation of the cornea. (A) Stage 2 (early). (B) Stage 2 (late, 3 days) representing the vesicle formation. (C) Stage 4, 6 days. Differentiation of lens fibres has started (red staining with anti-crystallin antibody). (D) Stage 4, 8 days. Definite differentiation of lens fiber. (E) Stage 5, 10 days. The lens has increased in size and has positioned by the dorsal and ventral iris. (Courtesy: Dr Stafano Cannata). seen from the dorsal iris in newts. A vesicle is first formed embryogenesis, and can induce lens morphogenesis, are also and then gradually crystallins and lens fibres accumulate. activated during lens regeneration as well (Del Rio-Tsonis During the final stages the lens is positioned along the dorsal et al., 1995, 1997, 1999; Mizuno et al., 1999; Schaefer et al., and the ventral iris (Fig. 2). It is interesting to note here that 1999). It remains though to be seen whether or not these FGF-1 seems to be very