Regeneration of neural crest derivatives in the Xenopus tadpole tail

Evi5 is required for Xenopus limb and tail regeneration

Amphibians such as salamanders and the African clawed frog Xenopus are great models for regeneration studies because they can fully regenerate their lost organs. While axolotl can regenerate damaged organs throughout its lifetime, Xenopus has a limited regeneration capacity after metamorphosis. The ecotropic viral integrative factor 5 (Evi5) is of great interest because its expression is highly upregulated in the limb blastema of axolotls, but remains unchanged in the fibroblastema of post-metamorphic frogs. Yet, its role in regeneration-competent contexts in Xenopus has not been fully analyzed. Here we show that Evi5 is upregulated in Xenopus tadpoles after limb and tail amputation, as in axolotls. Down-regulation of Evi5 with morpholino antisense oligos (Mo) impairs limb development and limb blastema formation in Xenopus tadpoles. Mechanistically, we show that Evi5 knockdown significantly reduces proliferation of limb blastema cells and causes apoptosis, blocking the formation of regeneration blastema. RNA-sequencing analysis reveals that in addition to reduced PDGFα and TGFβ signaling pathways that are required for regeneration, evi5 Mo downregulates lysine demethylases Kdm6b and Kdm7a. And knockdown of Kdm6b or Kdm7a causes defective limb regeneration. Evi5 knockdown also impedes tail regeneration in Xenopus tadpoles and axolotl larvae, suggesting a conserved function of Evi5 in appendage regeneration. Thus, our results demonstrate that Evi5 plays a critical role in appendage regeneration in amphibians.

Crustacean leg regeneration restores complex microanatomy and cell diversity

Animals can regenerate complex organs, yet this process frequently results in imprecise replicas of the original structure. In the crustacean Parhyale, embryonic and regenerating legs differ in gene expression dynamics but produce apparently similar mature structures. We examine the fidelity of Parhyale leg regeneration using complementary approaches to investigate microanatomy, sensory function, cellular composition, and cell molecular profiles. We find that regeneration precisely replicates the complex microanatomy and spatial distribution of external sensory organs and restores their sensory function. Single-nuclei sequencing shows that regenerated and uninjured legs are indistinguishable in terms of cell-type composition and transcriptional profiles. This remarkable fidelity highlights the ability of organisms to achieve identical outcomes via distinct processes.

Spinal cord regeneration - the origins of progenitor cells for functional rebuilding

The spinal cord is one of the most important structures for all vertebrate animals as it connects almost all parts of the body to the brain. Injury to the mammalian spinal cord has devastating consequences, resulting in paralysis with little to no hope of recovery. In contrast, other vertebrate animals have been known for centuries to be capable of functionally regenerating large lesions in the spinal cord. Here, we will review the current knowledge of spinal cord regeneration and recent work in different proregenerative animals that has begun to shed light on the cellular and molecular mechanisms these animals use to direct cells to rebuild a complex, functional spinal cord.

Injury-induced Erk1/2 signaling tissue-specifically interacts with Ca2+ activity and is necessary for regeneration of spinal cord and skeletal muscle

The transition of stem cells from quiescence to proliferation enables tissues to self-repair. The signaling mechanisms driving these stem-cell-status decisions are still unclear. Ca²⁺ and the extracellular signal-regulated kinase (Erk1/2) are two signaling pathways that have the potential to coordinate multiple signals to promote a specific cellular response. They both play important roles during nervous system development but their roles during spinal cord and muscle regeneration are not fully deciphered. Here we show in Xenopus laevis larvae that both Ca²⁺ and Erk1/2 signaling pathways are activated after tail amputation. In response to injury, we find that Erk1/2 signaling is activated in neural and muscle stem cells and is necessary for spinal cord and skeletal muscle regeneration. Finally, we show in vivo that Erk1/2 activity is necessary for an injury-induced increase in intracellular store-dependent Ca²⁺ dynamics in skeletal muscle-associated tissues but that in spinal cord, injury increases Ca²⁺ influx-dependent Ca²⁺ activity independent of Erk1/2 signaling. This study suggests that precise temporal and tissue-specific activation of Ca²⁺ and Erk1/2 pathways is essential for regulating spinal cord and muscle regeneration.

Appendage regeneration is context dependent at the cellular level

Species that can regrow their lost appendages have been studied with the ultimate aim of developing methods to enable human limb regeneration. These examinations highlight that appendage regeneration progresses through shared tissue stages and gene activities, leading to the assumption that appendage regeneration paradigms (e.g. tails and limbs) are the same or similar. However, recent research suggests these paradigms operate differently at the cellular level, despite sharing tissue descriptions and gene expressions. Here, collecting the findings from disparate studies, I argue appendage regeneration is context dependent at the cellular level; nonetheless, it requires (i) signalling centres, (ii) stem/progenitor cell types and (iii) a regeneration-permissive environment, and these three common cellular principles could be more suitable for cross-species/paradigm/age comparisons.

Spinal Cord Regeneration in Amphibians: A Historical Perspective

In some vertebrates, a grave injury to the central nervous system (CNS) results in functional restoration, rather than in permanent incapacitation. Understanding how these animals mount a regenerative response by activating resident CNS stem cell populations is of critical importance in regenerative biology. Amphibians are of a particular interest in the field because the regenerative ability is present throughout life in urodele species, but in anuran species it is lost during development. Studying amphibians, who transition from a regenerative to a nonregenerative state, could give insight into the loss of ability to recover from CNS damage in mammals. Here, we highlight the current knowledge of spinal cord regeneration across vertebrates and identify commonalities and differences in spinal cord regeneration between amphibians.

Dicer inactivation stimulates limb regeneration ability in Xenopus laevis

The ontogenetic decline of regeneration capacity in the anuran amphibian Xenopus makes it an excellent model for regeneration studies. However, the cause of the regeneration ability decline is not fully understood. MicroRNAs regulate animal development and have been indicated in various regeneration situations. However, little is known about the role of microRNAs during limb regeneration in Xenopus. This study investigates the effect of Dicer, an enzyme responsible for microRNA maturation, on limb development and regeneration in Xenopus. Dicer is expressed in the developing Xenopus limbs and is up-regulated after limb amputation during both regeneration-competent and regeneration-deficient stages of tadpole development. Inactivation of Dicer in early (NF stage 53) tadpole limb buds leads to shorter tibulare/fibulare formation but does not affect limb regeneration. However, in late-stage, regeneration-deficient tadpole limbs (NF stage 57), Dicer inactivation restores the regeneration blastema and stimulates limb regeneration. Thus, our results demonstrated that Xenopus limb regeneration can be stimulated by the inactivation of Dicer in nonregenerating tadpoles, indicating that microRNAs present in late-stage tadpole limbs may be involved in the ontogenetic decline of limb regeneration in Xenopus.

Spinal Cord Cells from Pre-metamorphic Stages Differentiate into Neurons and Promote Axon Growth and Regeneration after Transplantation into the Injured Spinal Cord of Non-regenerative Xenopus laevis Froglets

Mammals are unable to regenerate its spinal cord after a lesion, meanwhile, anuran amphibians are capable of spinal cord regeneration only as larvae, and during metamorphosis, this capability is lost. Sox2/3⁺ cells present in the spinal cord of regenerative larvae are required for spinal cord regeneration. Here we evaluate the effect of the transplantation of spinal cord cells from regenerative larvae into the resected spinal cord of non-regenerative stages (NR-stage). Donor cells were able to survive up to 60 days after transplantation in the injury zone. During the first 3-weeks, transplanted cells organize in neural tube-like structures formed by Sox2/3⁺ cells. This was not observed when donor cells come from non-regenerative froglets. Mature neurons expressing NeuN and Neurofilament-H were detected in the grafted tissue 4 weeks after transplantation concomitantly with the appearance of axons derived from the donor cells growing into the host spinal cord, suggesting that Sox2/3⁺ cells behave as neural stem progenitor cells. We also found that cells from regenerative animals provide a permissive environment that promotes growth and regeneration of axons coming from the host. These results suggest that Sox2/3 cells present in the spinal cord of regenerative stage (R-stage) larvae are most probably neural stem progenitor cells that are able to survive, proliferate, self-organize and differentiate into neurons in the environment of the non-regenerative host. In addition, we have established an experimental paradigm to study the biology of neural stem progenitor cells in spinal cord regeneration.

Spinal cord self-repair during tail regeneration in Polypedates maculatus and putative role of FGF1 as a neurotrophic factor

Spinal cord injury could be fatal in man and often results in irreversible medical conditions affecting mobility. However, anuran amphibians win over such pathological condition by the virtue of regeneration abilities. The tail of anuran tadpoles therefore allures researchers to study spinal cord injury and self- repair process. In the present study, we inflicted injury to the spinal cord by means of surgical transection of the tail and investigated the self-repair activity in the tadpoles of the Indian tree frog Polypedates maculatus. We also demonstrate for the first time by immunofluorescence localization the expression pattern of Fibroblast Growth Factor1 (FGF1) during spinal cord regeneration which has not been documented earlier in anurans. FGF1, bearer of the mitogenic and neurotrophic properties seems to be expressed by progenitor cells that facilitate regeneration. Spinal cord during tail regeneration in P. maculatus attains functional recovery within a span of 2 weeks thus enabling the organism to survive in an aquatic medium till metamorphosis. Moreover, during the course of spinal cord regeneration in the regenerating tail, melanocytes showed an interesting behaviour as these neural crest derivatives were missing near the early regenerates until their reappearance where they were positioned in close proximity with the regenerated spinal cord as in the normal tail.

Animal regeneration: ancestral character or evolutionary novelty?

An old question about regeneration is whether it is an ancestral character which is a general property of living matter, or whether it represents a set of specific adaptations to the different circumstances faced by different types of animal. In this review, some recent results on regeneration are assessed to see if they can throw any new light on this question. Evidence in favour of an ancestral character comes from the role of Wnt and bone morphogenetic protein signalling in controlling the pattern of whole-body regeneration in acoels, which are a basal group of bilaterian animals. On the other hand, there is some evidence for adaptive acquisition or maintenance of the regeneration of appendages based on the occurrence of severe non-lethal predation, the existence of some novel genes in regenerating organisms, and differences at the molecular level between apparently similar forms of regeneration. It is tentatively concluded that whole-body regeneration is an ancestral character although has been lost from most animal lineages. Appendage regeneration is more likely to represent a derived character resulting from many specific adaptations.

Turning One Cell Type into Another

The nature of cells in early embryos may be respecified simply by exposure to inducing factors. In later stage embryos, determined cell populations do not respond to inducing factors but may be respecified by other stimuli, especially the introduction of specific transcription factors. Fully differentiated cell types are hard to respecify by any method, but some degree of success can be achieved using selected combinations of transcription factors, and this may have clinical significance in the future.

The regeneration blastema of lizards: an amniote model for the study of appendage replacement

Although amniotes (reptiles, including birds, and mammals) are capable of replacing certain tissues, complete appendage regeneration is rare. Perhaps the most striking example is the lizard tail. Tail loss initiates a spontaneous epimorphic (blastema‐mediated) regenerative program, resulting in a fully functional but structurally non‐identical replacement. Here we review lizard tail regeneration with a particular focus on the blastema. In many lizards, the original tail has evolved a series of fracture planes, anatomical modifications that permit the tail to be self‐detached or autotomized. Following tail loss, the wound site is covered by a specialized wound epithelium under which the blastema develops. An outgrowth of the spinal cord, the ependymal tube, plays a key role in governing growth (and likely patterning) of the regenerate tail. In some species (e.g., geckos), the blastema forms as an apical aggregation of proliferating cells, similar to that of urodeles and teleosts. For other species (e.g., anoles) the identification of a proliferative blastema is less obvious, suggesting an unexpected diversity in regenerative mechanisms among tail‐regenerating lizards.

Cyp19a1 (aromatase) expression in the Xenopus brain at different developmental stages

Cytochrome P450 aromatase (P450arom; aromatase) is a microsomal enzyme involved in the production of endogeneous sex steroids by converting testosterone into oestradiol. Aromatase is the product of the cyp19a1 gene and plays a crucial role in the sexual differentiation of the brain and in the regulation of reproductive functions. In the brain of mammals and birds, expression of cyp19a1 has been demonstrated in neuronal populations of the telencephalon and diencephalon. By contrast, a wealth of evidence established that, in teleost fishes, aromatase expression in the brain is restricted to radial glial cells. The present study investigated the precise neuroanatomical distribution of cyp19a1 mRNA during brain development in Xenopus laevis (late embryonic to juvenile stages). For this purpose, we used in situ hybridisation alone or combined with the detection of a proliferative (proliferating cell nuclear antigen), glial (brain lipid binding protein, Vimentin) or neuronal (acetylated tubulin; HuC/D; NeuroβTubulin) markers. We provide evidence that cyp19a1 expression in the brain is initiated from the very early larval stage and remains strongly detected until the juvenile and adult stages. At all stages analysed, we found the highest expression of cyp19a1 in the preoptic area and the hypothalamus compared to the rest of the brain. In these two brain regions, cyp19a1-positive cells were never detected in the ventricular layers. Indeed, no co-labelling could be observed with radial glial (brain lipid binding protein, Vimentin) or dividing progenitors (proliferating cell nuclear antigen) markers. By contrast, cyp19a1-positive cells perfectly matched with the distribution of post-mitotic neurones as shown by the use of specific markers (HuC/D, acetylated tubulin and NeuroβTubulin). These data suggest that, similar to that found in other tetrapods, aromatase in the brain of amphibians is found in post-mitotic neurones and not in radial glia as reported in teleosts.

Retinal stem/progenitor cells in the ciliary marginal zone complete retinal regeneration: a study of retinal regeneration in a novel animal model

Our research group has extensively studied retinal regeneration in adult Xenopus laevis. However, X. laevis does not represent a suitable model for multigenerational genetics and genomic approaches. Instead, Xenopus tropicalis is considered as the ideal model for these studies, although little is known about retinal regeneration in X. tropicalis. In the present study, we showed that a complete retina regenerates at approximately 30 days after whole retinal removal. The regenerating retina was derived from the stem/progenitor cells in the ciliary marginal zone (CMZ), indicating a novel mode of vertebrate retinal regeneration, which has not been previously reported. In a previous study, we showed that in X. laevis, retinal regeneration occurs primarily through the transdifferentiation of retinal pigmented epithelial (RPE) cells. RPE cells migrate to the retinal vascular membrane and reform a new epithelium, which then differentiates into the retina. In X. tropicalis, RPE cells also migrated to the vascular membrane, but transdifferentiation was not evident. Using two tissue culture models of RPE tissues, it was shown that in X. laevis RPE culture neuronal differentiation and reconstruction of the retinal three-dimensional (3-D) structure were clearly observed, while in X. tropicalis RPE culture neither ßIII tubulin-positive cells nor 3-D retinal structure were seen. These results indicate that the two Xenopus species are excellent models to clarify the cellular and molecular mechanisms of retinal regeneration, as these animals have contrasting modes of regeneration; one mode primarily involves RPE cells and the other mode involves stem/progenitor cells in the CMZ.

Zebrafish cardiac injury and regeneration models: a noninvasive and invasive in vivo model of cardiac regeneration

Despite current treatment regimens, heart failure still remains one of the leading causes of morbidity and mortality in the world due to failure to adequately replace lost ventricular myocardium from ischemia-induced infarct. Although adult mammalian ventricular cardiomyocytes have a limited capacity to divide, this proliferation is insufficient to overcome the significant loss of myocardium from ventricular injury. However, lower vertebrates, such as the zebrafish and newt, have the remarkable capacity to fully regenerate their hearts after severe injury. Thus, there is great interest in studying these animal model systems to discover new regenerative approaches that might be applied to injured mammalian hearts. To this end, the zebrafish has been utilized more recently to gain additional mechanistic insight into cardiac regeneration because of its genetic tractability. Here, we describe two cardiac injury methods, a mechanical and a genetic injury model, for studying cardiac regeneration in the zebrafish.

Transgenic analysis of signaling pathways required for Xenopus tadpole spinal cord and muscle regeneration

The Xenopus tadpole has the capacity fully to regenerate its tail after amputation. Previously, we have established that this regeneration process requires the operation of several signaling pathways including the bone morphogenic protein, Wnt, and Fgf pathways. Here, we have addressed the signaling requirements for spinal cord and muscle regeneration in a tissue-specific manner. Two methods were used namely grafts of transgenic spinal cord to a wild type host, and the use of the Tet-on conditional transgenic system to express inhibitors in the individual tissues. For the grafting experiments, the tail was amputated through the graft, which contained a temperature inducible inhibitor of the Wnt-β-catenin pathway. For the Tet-on experiments, treatment with doxycycline was used to induce cell autonomous inhibitors of the Wnt-β-catenin or the Fgf pathway in either spinal cord or muscle. The results show that both spinal cord and muscle regeneration depend on both the Wnt-β-catenin and the Fgf pathways. This experimental design also enables us to observe the effect of inhibition of regeneration of one tissue on the regeneration of the others. Regardless of the method of inhibition, we find that reduction of spinal cord regeneration reduces regeneration of other parts of the tail, including the myotomal muscles. In contrast, reduction of muscle regeneration has no effect on the regeneration of the spinal cord. In common with other regeneration systems, this indicates that soluble factors from the spinal cord are needed to promote the regeneration of the other tissues in the tail.

Transducing bioelectric signals into epigenetic pathways during tadpole tail regeneration

One important component of the cell-cell communication that occurs during regenerative patterning is bioelectrical signaling. In particular, the regeneration of the tail in Xenopus laevis tadpoles both requires, and can be initiated at non-regenerative stages by, specific regulation of bioelectrical signaling (alteration in resting membrane potential and a subsequent change in sodium content of blastemal cells). Although standing gradients of transmembrane voltage and ion concentration can provide positional guidance and other morphogenetic cues, these biophysical parameters must be transduced into transcriptional responses within cells. A number of mechanisms have been described for linking slow voltage changes to gene expression, but recent data on the importance of epigenetic regulation for regeneration suggest a novel hypothesis: that sodium/butyrate transporters link ion flows to influx of small molecules needed to modify chromatin state. Here, we briefly review the data on bioelectricity in tadpole tail regeneration, present a technique for convenient alteration of transmembrane potential in vivo that does not require transgenes, show augmentation of regeneration in vivo by manipulation of voltage, and present new data in the Xenopus tail consistent with the hypothesis that the monocarboxlyate transporter SLC5A8 may link regeneration-relevant epigenetic modification with upstream changes in ion content.

Reconstitution of the central and peripheral nervous system during salamander tail regeneration

We show that after tail amputation in Ambystoma mexicanum (Axolotl) the correct number and spacing of dorsal root ganglia are regenerated. By transplantation of spinal cord tissue and nonclonal neurospheres, we show that the central spinal cord represents a source of peripheral nervous system cells. Interestingly, melanophores migrate from preexisting precursors in the skin. Finally, we demonstrate that implantation of a clonally derived spinal cord neurosphere can result in reconstitution of all examined cell types in the regenerating central spinal cord, suggesting derivation of a cell with spinal cord stem cell properties.

Spinal cord regeneration in Xenopus tadpoles proceeds through activation of Sox2-positive cells

BACKGROUND

In contrast to mammals, amphibians, such as adult urodeles (for example, newts) and anuran larvae (for example, Xenopus) can regenerate their spinal cord after injury. However, the cellular and molecular mechanisms involved in this process are still poorly understood.

RESULTS

Here, we report that tail amputation results in a global increase of Sox2 levels and proliferation of Sox2(+) cells. Overexpression of a dominant negative form of Sox2 diminished proliferation of spinal cord resident cells affecting tail regeneration after amputation, suggesting that spinal cord regeneration is crucial for the whole process. After spinal cord transection, Sox2(+) cells are found in the ablation gap forming aggregates. Furthermore, Sox2 levels correlated with regenerative capabilities during metamorphosis, observing a decrease in Sox2 levels at non-regenerative stages.

CONCLUSIONS

Sox2(+) cells contribute to the regeneration of spinal cord after tail amputation and transection. Sox2 levels decreases during metamorphosis concomitantly with the lost of regenerative capabilities. Our results lead to a working hypothesis in which spinal cord damage activates proliferation and/or migration of Sox2(+) cells, thus allowing regeneration of the spinal cord after tail amputation or reconstitution of the ependymal epithelium after spinal cord transection.

Fig4 expression in the rodent nervous system and its potential role in preventing abnormal lysosomal accumulation

The phosphatase FIG4 regulates the concentration of phosphatidylinositol 3,5-diphosphate (PI3,5P2), a molecule critical for endosomal/lysosomal membrane trafficking and neuron function. We investigated Fig4 expression in the developing CNS of mice and rats using Western blot, real-time polymerase chain reaction, and morphological techniques in situ and in vitro and after spinal cord injury. Fig4 was expressed at a high levels throughout development in myelinating cells, particularly Schwann cells, and dorsal root ganglia sensory neurons. Fig4 protein and mRNA in CNS neurons were markedly diminished in adult versus embryonal animals. Spinal cord hemisection induced upregulation of Fig4 in adult spinal cord tissues that was associated with accumulation of lysosomes in neurons and glia. This accumulation appeared similar to the abnormal lysosomal storage observed in dorsal root ganglia of young fig4-null mice. The results suggest that Fig4 is involved in normal neural development and the maintenance of peripheral nervous system myelin. We speculate that adequate levels of Fig4 may be required to prevent neurons and glia from excessive lysosomal accumulation after injury and in neurodegeneration.

Studying regeneration in Xenopus

For most Xenopus embryos, life is very short. The majority of research labs working with this model organism study the processes of early vertebrate patterning and morphogenesis. And quite rightly too, since over the last two decades labs across the world have provided the fate maps, animal cap assays, expression patterns, and functional screens that put Xenopus firmly on the map as a developmental model organism. Xenopus, however, still has a lot more to offer. A new wave of interest in later developmental events has followed the development of transgenic technology, which has opened up opportunities for studying events that occur after stage 40. In this chapter, I will give a brief descriptive background of some of the different types of regeneration studied in Xenopus, and provide protocols and morphological scoring information with the aim of facilitating progress in understanding regeneration in this model system. Additionally, some particularly elegant recent examples are used to highlight the advantages of Xenopus as a model for regeneration and the future opportunities that this offers.

Brain regeneration in physiology and pathology: the immune signature driving therapeutic plasticity of neural stem cells

Regenerative processes occurring under physiological (maintenance) and pathological (reparative) conditions are a fundamental part of life and vary greatly among different species, individuals, and tissues. Physiological regeneration occurs naturally as a consequence of normal cell erosion, or as an inevitable outcome of any biological process aiming at the restoration of homeostasis. Reparative regeneration occurs as a consequence of tissue damage. Although the central nervous system (CNS) has been considered for years as a "perennial" tissue, it has recently become clear that both physiological and reparative regeneration occur also within the CNS to sustain tissue homeostasis and repair. Proliferation and differentiation of neural stem/progenitor cells (NPCs) residing within the healthy CNS, or surviving injury, are considered crucial in sustaining these processes. Thus a large number of experimental stem cell-based transplantation systems for CNS repair have recently been established. The results suggest that transplanted NPCs promote tissue repair not only via cell replacement but also through their local contribution to changes in the diseased tissue milieu. This review focuses on the remarkable plasticity of endogenous and exogenous (transplanted) NPCs in promoting repair. Special attention will be given to the cross-talk existing between NPCs and CNS-resident microglia as well as CNS-infiltrating immune cells from the circulation, as a crucial event sustaining NPC-mediated neuroprotection. Finally, we will propose the concept of the context-dependent potency of transplanted NPCs (therapeutic plasticity) to exert multiple therapeutic actions, such as cell replacement, neurotrophic support, and immunomodulation, in CNS repair.

Long-distance signals are required for morphogenesis of the regenerating Xenopus tadpole tail, as shown by femtosecond-laser ablation

Background

With the goal of learning to induce regeneration in human beings as a treatment for tissue loss, research is being conducted into the molecular and physiological details of the regeneration process. The tail of Xenopus laevis tadpoles has recently emerged as an important model for these studies; we explored the role of the spinal cord during tadpole tail regeneration.

Methods and Results

Using ultrafast lasers to ablate cells, and Geometric Morphometrics to quantitatively analyze regenerate morphology, we explored the influence of different cell populations. For at least twenty-four hours after amputation (hpa), laser-induced damage to the dorsal midline affected the morphology of the regenerated tail; damage induced 48 hpa or later did not. Targeting different positions along the anterior-posterior (AP) axis caused different shape changes in the regenerate. Interestingly, damaging two positions affected regenerate morphology in a qualitatively different way than did damaging either position alone. Quantitative comparison of regenerate shapes provided strong evidence against a gradient and for the existence of position-specific morphogenetic information along the entire AP axis.

Conclusions

We infer that there is a conduit of morphology-influencing information that requires a continuous dorsal midline, particularly an undamaged spinal cord. Contrary to expectation, this information is not in a gradient and it is not localized to the regeneration bud. We present a model of morphogenetic information flow from tissue undamaged by amputation and conclude that studies of information coming from far outside the amputation plane and regeneration bud will be critical for understanding regeneration and for translating fundamental understanding into biomedical approaches.

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.

Unusual development of light-reflecting pigment cells in intact and regenerating tail in the periodic albino mutant of Xenopus laevis

Unusual light-reflecting pigment cells, “white pigment cells”, specifically appear in the periodic albino mutant (a^p/a^p) of Xenopus laevis and localize in the same place where melanophores normally differentiate in the wild-type. The mechanism responsible for the development of unusual pigment cells is unclear. In this study, white pigment cells in the periodic albino were compared with melanophores in the wild-type, using a cell culture system and a tail-regenerating system. Observations of both intact and cultured cells demonstrate that white pigment cells are unique in (1) showing characteristics of melanophore precursors at various stages of development, (2) accumulating reflecting platelets characteristic of iridophores, and (3) exhibiting pigment dispersion in response to α-melanocyte stimulating hormone (α-MSH) in the same way that melanophores do. When a tadpole tail is amputated, a functionally competent new tail is regenerated. White pigment cells appear in the mutant regenerating tail, whereas melanophores differentiate in the wild-type regenerating tail. White pigment cells in the mutant regenerating tail are essentially similar to melanophores in the wild-type regenerating tail with respect to their localization, number, and response to α-MSH. In addition to white pigment cells, iridophores which are never present in the intact tadpole tail appear specifically in the somites near the amputation level in the mutant regenerating tail. Iridophores are distinct from white pigment cells in size, shape, blue light-induced fluorescence, and response to α-MSH. These findings strongly suggest that white pigment cells in the mutant arise from melanophore precursors and accumulate reflecting platelets characteristic of iridophores.

Improved cre reporter transgenic Xenopus

We have produced and characterized improved transgenic reporter lines for detection of Cre recombinase activity during Xenopus development. Improvements include choice of fluorophores, which make these Cre reporter lines generally suitable for lineage tracing studies. We also include data for several new parameters affecting survival and transgenesis efficiency using the recently developed meganuclease method of frog transgenesis. These transgenic frogs express cyan fluorescent protein (CFP) under control of the ubiquitous promoter CMV, where CFP is replaced by DsRed2 (a red fluorescent protein) in the presence of Cre. Three independent, high expression, Cre-sensitive lines have been identified that maintain robust fluorophore expression across generations and lack DsRed2 expression in the absence of Cre. A novel use of these lines is to indelibly mark embryonic blastomeres by Cre mRNA injection for permanent fate mapping. Similarly, transgenically expressed Cre under control of tissue-specific promoters will allow detailed analysis of cell lineage relationships throughout embryogenesis, metamorphosis, and adulthood.

Considering the evolution of regeneration in the central nervous system

For many years the mammalian CNS has been seen as an organ that is unable to regenerate. However, it was also long known that lower vertebrate species are capable of impressive regeneration of CNS structures. How did this situation arise through evolution? Increasing cellular and molecular understanding of regeneration in different animal species coupled with studies of adult neurogenesis in mammals is providing a basis for addressing this question. Here we compare CNS regeneration among vertebrates and speculate on how this ability may have emerged or been restricted.

Beyond early development: Xenopus as an emerging model for the study of regenerative mechanisms

While Xenopus is a well-known model system for early vertebrate development, in recent years, it has also emerged as a leading model for regeneration research. As an anuran amphibian, Xenopus laevis can regenerate the larval tail and limb by means of the formation of a proliferating blastema, the lens of the eye by transdifferentiation of nearby tissues, and also exhibits a partial regeneration of the postmetamorphic froglet forelimb. With the availability of inducible transgenic techniques for Xenopus, recent experiments are beginning to address the functional role of genes in the process of regeneration. The use of soluble inhibitors has also been very successful in this model. Using the more traditional advantages of Xenopus, others are providing important lineage data on the origin of the cells that make up the tissues of the regenerate. Finally, transcriptome analyses of regenerating tissues seek to identify the genes and cellular processes that enable successful regeneration. Developmental Dynamics 238:1226-1248, 2009. (c) 2009 Wiley-Liss, Inc.

Use of adenovirus for ectopic gene expression in Xenopus

We show that replication defective adenovirus can be used for localized overexpression of a chosen gene in Xenopus tadpoles. Xenopus contains two homologs of the Coxsackie and Adenovirus Receptor (xCAR1 and 2), both of which can confer sensitivity for adenovirus infection. xCAR1 mRNA is present from the late gastrula stage and xCAR2 throughout development, both being widely expressed in the embryo and tadpole. Consistent with the expression of the receptors, adenovirus will infect a wide range of Xenopus tissues cultured in vitro. It will also infect early embryos when injected into the blastocoel or archenteron cavities. Furthermore, adenovirus can be delivered by localized injection to tadpoles and will infect a patch of cells around the injection site. The expression of green fluorescent protein in infected cells persists for several weeks. This new gene delivery method complements the others that are already available. Developmental Dynamics 238:1412-1421, 2009. (c) 2009 Wiley-Liss, Inc.

Red fluorescent Xenopus laevis: a new tool for grafting analysis

BACKGROUND

Fluorescent proteins such as the green fluorescent protein (GFP) have widely been used in transgenic animals as reporter genes. Their use in transgenic Xenopus tadpoles is especially of interest, because large numbers of living animals can easily be screened. To track more than one event in the same animal, fluorescent markers that clearly differ in their emission spectrum are needed.

RESULTS

We established the transgenic Xenopus laevis strain tom3 that expresses ubiquitously red fluorescence from the tdTomato gene through all larval stages and in the adult animal. This new tool was applied to track transplanted blastemas obtained after tail amputation. The blastema can regenerate ectopic tails marked by red fluorescence in the host animal. Surprisingly, we also found contribution of the host animal to form the regenerate.

CONCLUSION

We have established a useful new tool to label grafts in Xenopus transplantation experiments.

A chemical genomic approach identifies matrix metalloproteinases as playing an essential and specific role in Xenopus melanophore migration

To dissect the function of matrix metalloproteinases (MMPs) involved in cellular migration in vivo, we undertook both a forward chemical genomic screen and a functional approach to discover modulators of melanophore (pigment cell) migration in Xenopus laevis. We identified the 8-quinolinol derivative NSC 84093 as affecting melanophore migration in the developing embryo and have shown it to act as a MMP inhibitor. Potential targets of NSC 84093 investigated include MMP-14 and MMP-2. MMP-14 is expressed in migrating neural crest cells from which melanophores are derived. MMP-2 is expressed at the relevant time of development and in a pattern that suggests it contributes to melanophore migration. Morpholino-mediated knockdown of both MMPs demonstrates they play a key role in melanophore migration and partially phenocopy the effect of NSC 84093.

Conserved and novel gene expression between regeneration and asexual fission in Nematostella vectensis

Due to work in model systems (e.g., flies and mice), the molecular mechanisms of embryogenesis are known in exquisite detail. However, these organisms are incapable of asexual reproduction and possess limited regenerative abilities. Thus, the mechanisms of alternate developmental trajectories and their relation to embryonic mechanisms remain understudied. Because these developmental trajectories are present in a diverse group of animal phyla spanning the metazoan phylogeny, including cnidarians, annelids, and echinoderms, they are likely to have played a major role in animal evolution. The starlet sea anemone Nematostella vectensis, an emerging model system, undergoes larval development, asexual fission, and complete bi-directional regeneration in the field and laboratory. In order to investigate to what extent embryonic patterning mechanisms are utilized during alternate developmental trajectories, we examined expression of developmental regulatory genes during regeneration and fission. When compared to previously reported embryonic expression patterns, we found that all genes displayed some level of expression consistent with embryogenesis. However, five of seven genes investigated also displayed striking differences in gene expression between one or more developmental trajectory. These results demonstrate that alternate developmental trajectories utilize distinct molecular mechanisms upstream of major developmental regulatory genes such as fox, otx, and Hox-like.

Tail regeneration in Xenopus laevis as a model for understanding tissue repair

Augmentation of regenerative ability is a powerful strategy being pursued for the biomedical management of traumatic injury, cancer, and degeneration. While considerable attention has been focused on embryonic stem cells, it is clear that much remains to be learned about how somatic cells may be controlled in the adult organism. The tadpole of the frog Xenopus laevis is a powerful model system within which fundamental mechanisms of regeneration are being addressed. The tadpole tail contains spinal cord, muscle, vasculature, and other terminally differentiated cell types and can fully regenerate itself through tissue renewal--a process that is most relevant to mammalian healing. Recent insight into this process has uncovered fascinating molecular details of how a complex appendage senses injury and rapidly repairs the necessary morphology. Here, we review what is known about the chemical and bioelectric signals underlying this process and draw analogies to evolutionarily conserved pathways in other patterning systems. The understanding of this process is not only of fundamental interest for the evolutionary and cell biology of morphogenesis, but will also generate information that is crucial to the development of regenerative therapies for human tissues and organs.

The spark of life: electricity and regeneration

A recent paper shows that a proton pump, the vacuolar-type adenosine triphosphatase (V-ATPase), is necessary for Xenopus tadpole tail regeneration. The V-ATPase is synthesized in distal cells of the regeneration bud as an early response to amputation, and its role in regeneration is specifically to pump H+ out of cells. Although the downstream effects of proton movement are not known, they may be mediated through changes in membrane potential. This work could reopen a consideration of the role of electric fields in regeneration, an issue that has caused much controversy in the past.