Correlation between Shh expression and DNA methylation status of the limb-specific Shh enhancer region during limb regeneration in amphibians

The link between injury-induced stress and regenerative phenomena: A cellular and genetic synopsis

Injury is an inescapable phenomenon of life that affects animals at every physiological level. Yet, some animals respond to injury by rebuilding the damaged tissues whereas others are limited to scarring. Elucidating how a tissue insult from wounding leads to a regenerative response at the genetic level is essential to make regenerative advantages translational. It has become clear that animals with regenerative abilities recycle developmental programs after injury, reactivating genes that have lied dormant throughout adulthood. The question that is critical to our understanding of regeneration is how a specific set of developmentally important genes can be reactivated only after an acute tissue insult. Here, we review how injury-induced cellular stresses such as hypoxic, oxidative, and mechanical stress may contribute to the genomic and epigenetic changes that promote regeneration in animals. This article is part of a Special Issue entitled: Stress as a fundamental theme in cell plasticity.

Proteomic analysis of fibroblastema formation in regenerating hind limbs of Xenopus laevis froglets and comparison to axolotl

Background

To gain insight into what differences might restrict the capacity for limb regeneration in Xenopus froglets, we used High Performance Liquid Chromatography (HPLC)/double mass spectrometry to characterize protein expression during fibroblastema formation in the amputated froglet hindlimb, and compared the results to those obtained previously for blastema formation in the axolotl limb.

Results

Comparison of the Xenopus fibroblastema and axolotl blastema revealed several similarities and significant differences in proteomic profiles. The most significant similarity was the strong parallel down regulation of muscle proteins and enzymes involved in carbohydrate metabolism. Regenerating Xenopus limbs differed significantly from axolotl regenerating limbs in several ways: deficiency in the inositol phosphate/diacylglycerol signaling pathway, down regulation of Wnt signaling, up regulation of extracellular matrix (ECM) proteins and proteins involved in chondrocyte differentiation, lack of expression of a key cell cycle protein, ecotropic viral integration site 5 (EVI5), that blocks mitosis in the axolotl, and the expression of several patterning proteins not seen in the axolotl that may dorsalize the fibroblastema.

Conclusions

We have characterized global protein expression during fibroblastema formation after amputation of the Xenopus froglet hindlimb and identified several differences that lead to signaling deficiency, failure to retard mitosis, premature chondrocyte differentiation, and failure of dorsoventral axial asymmetry. These differences point to possible interventions to improve blastema formation and pattern formation in the froglet limb.

Experimentally induced metamorphosis in axolotls reduces regenerative rate and fidelity

While most tetrapods are unable to regenerate severed body parts, amphibians display a remarkable ability to regenerate an array of structures. Frogs can regenerate appendages as larva, but they lose this ability around metamorphosis. In contrast, salamanders regenerate appendages as larva, juveniles, and adults. However, the extent to which fundamental traits (e.g., metamorphosis, body size, aging, etc.) restrict regenerative ability remains contentious. Here we utilize the ability of normally paedomorphic adult axolotls (Ambystoma mexicanum) to undergo induced metamorphosis by thyroxine exposure to test how metamorphosis and body size affects regeneration in age‐matched paedomorphic and metamorphic individuals. We show that body size does not affect regeneration in adult axolotls, but metamorphosis causes a twofold reduction in regeneration rate, and lead to carpal and digit malformations. Furthermore, we find evidence that metamorphic blastemal cells may take longer to traverse the cell cycle and display a lower proliferative rate. This study identifies the axolotl as a powerful system to study how metamorphosis restricts regeneration independently of developmental stage, body size, and age; and more broadly how metamorphosis affects tissue‐specific changes.

Epigenetic basis of regeneration: analysis of genomic DNA methylation profiles in the MRL/MpJ mouse

Epigenetic regulation plays essential role in cell differentiation and dedifferentiation, which are the intrinsic processes involved in regeneration. To investigate the epigenetic basis of regeneration capacity, we choose DNA methylation as one of the most important epigenetic mechanisms and the MRL/MpJ mouse as a model of mammalian regeneration known to exhibit enhanced regeneration response in different organs. We report the comparative analysis of genomic DNA methylation profiles of the MRL/MpJ and the control C57BL/6J mouse. Methylated DNA immunoprecipitation followed by microarray analysis using the Nimblegen '3 × 720 K CpG Island Plus RefSeq Promoter' platform was applied in order to carry out genome-wide DNA methylation profiling covering 20 404 promoter regions. We identified hundreds of hypo- and hypermethylated genes and CpG islands in the heart, liver, and spleen, and 37 of them in the three tissues. Decreased inter-tissue diversification and the shift of DNA methylation balance upstream the genes distinguish the genomic methylation patterns of the MRL/MpJ mouse from the C57BL/6J. Homeobox genes and a number of other genes involved in embryonic morphogenesis are significantly overrepresented among the genes hypomethylated in the MRL/MpJ mouse. These findings indicate that epigenetic patterning might be a likely molecular basis of regeneration capability in the MRL/MpJ mouse.

Epigenetics and regeneration

During newt lens regeneration a unique transdifferentiation event occurs. In this process, dorsal iris pigmented epithelial cells transdifferentiate into lens cells. This system should provide a new insight into cellular plasticity in basic and applied research. Recently, a series of approaches to study epigenetic reprogramming during transdifferentiation have been performed. In this review, we introduce the regulation of dynamic regulation of core-histone modifications and the emergence of an oocyte-type linker histone during transdifferentiation. Finally, we show supporting evidence that there are common strategies of reprogramming between newt somatic cell in transdifferentiation and oocytes after somatic cell nuclear transfer.

Transient reduction of 5-methylcytosine and 5-hydroxymethylcytosine is associated with active DNA demethylation during regeneration of zebrafish fin

Although dedifferentiation, transformation of differentiated cells into progenitor cells, is a critical step in the regeneration of amphibians and fish, the molecular mechanisms underlying this process, including epigenetic changes, remain unclear. Dot blot assays and immunohistochemical analyses revealed that, during regeneration of zebrafish fin, the levels of 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) are transiently reduced in blastema cells and cells adjacent to the amputation plane at 30 h post-amputation (hpa), and the level of 5mC, but not 5hmC, is almost restored by 72 hpa. We observed that the dedifferentiated cells showed reduced levels of 5mC and 5hmC independent of cell proliferation by 24 hpa. Furthermore, expressions of the proposed demethylation- and DNA repair-related genes were detected during fin regeneration. Taken together, our findings illustrate that the transient reduction of 5mC and 5hmC in dedifferentiated cells is associated with active demethylation during regeneration of zebrafish fin.

Loss of cell-extracellular matrix interaction triggers retinal regeneration accompanied by Rax and Pax6 activation

The whole retina regenerates from retinal pigmented epithelial (RPE) cells by transdifferentiation in the adult newt and Xenopus laevis when it is surgically removed. We produced a transgenic animal line, in which EGFP expression is under the control of Rax pomotor. Using F1 and F2 generations, we analyzed Rax-EGFP expression during retinal regeneration in a tissue culture model. In the culture, 4 zones were distinguished as RPE cells migrating outwards from the periphery of the explant: the explant zone, epithelial zone, transition zone and differentiation zone. Expression of transcription factors such as Pax6 and Rax-EGFP was observed in different zones. Rax-EGFP expression preceded Pax6 expression, and the expression of both genes occurred in RPE cells that had lost contact with the basement membrane facing the choroid. We have developed a new culture method in which RPE tissues are embedded in Matrigel. This method has many advantages over the previous gel-overlay method to reproduce construction of 3D-retinal structures and clearly showed that RPE cells need to be detached from the choroid before entering the regeneration pathway. The present results indicate that the temporal changes in cell-cell and cell-extracellular matrix interactions regulate transdifferentiation.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

The developing Xenopus limb as a model for studies on the balance between inflammation and regeneration

The roles of inflammation and immune cell reactivity triggered by amputation have only recently begun to be addressed in investigations of epimorphic regeneration, although studies of tissue repair in mammals clearly show the importance of the immune system in determining the quality of the repair process. Here, we first review inflammation-related work in non-mammalian systems of epimorphic regeneration which suggests that regeneration of an amputated appendage requires continuous modulation of the local immune response, from the first hours after amputation through the period of blastema patterning. We then present data on the effects of anti-inflammatory and proinflammatory agents on regeneration of larval Xenopus hindlimbs. Treatment with the glucocorticoid beclomethasone immediately after amputation inhibits regeneration in regeneration-complete stage 53 limbs. Other anti-inflammatory agents, including the inhibitors of cyclooxygenase-2 (COX-2) activity celecoxib and diclofenac, applied similarly to larvae amputated at stage 55, when the capacity for limb regeneration is normally being lost, restore regenerative capacity. This suggests that although injury-related events sensitive to glucocorticoids are necessary for regeneration, resolution of the inflammatory response may also be required to allow the complete regenerative response and normal blastema patterning. Conversely, if resolution of inflammation is prevented by local treatment of amputated limbs with beryllium, a strong immunoadjuvant, regeneration is inhibited, and gene expression data suggest that this inhibition results from a failure of normal blastema patterning. Both positive and negative effects of immune- or inflammation-related activities occur during anuran limb regeneration and this underscores the importance of considering immune cells in studies of epimorphic regeneration.

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.

Mechanisms of disease progression in NASH: new paradigms

The incidence of nonalcoholic fatty liver disease is increasing at an astonishing rate in the US population. Although only a small proportion of these patients develop steatohepatitis (NASH), those who do have a greater likelihood of developing end-stage liver disease and complications. Research on liver fibrosis and NASH progression shows that hedgehog (Hh) is reactivated after liver injury to assist in liver repair and regeneration. When the process of tissue repair and regeneration is prolonged or when Hh ligand and related genes are aberrantly regulated and excessive, tissue repair goes awry and NASH progresses to cirrhosis and hepatocellular carcinoma.

Limited dedifferentiation provides replacement tissue during zebrafish fin regeneration

Unlike humans, some vertebrate animals are able to completely regenerate damaged appendages and other organs. For example, adult zebrafish will regenerate the complex structure of an amputated caudal fin to a degree that the original and replacement fins are indistinguishable. The blastema, a mass of cells that uniquely forms following appendage amputation in regenerating animals, is the major source of regenerated tissue. However, the cell lineage(s) that contribute to the blastema and their ultimate contribution(s) to the regenerated fin have not been definitively characterized. It has been suggested that cells near the amputation site dedifferentiate forming multipotent progenitors that populate the blastema and then give rise to multiple cell types of the regenerated fin. Other studies propose that blastema cells are non-uniform populations that remain restricted in their potential to contribute to different cell lineages. We tested these models by using inducible Cre-lox technology to generate adult zebrafish with distinct, isolated groups of genetically labeled cells within the caudal fin. We then tracked populations of several cell types over the entire course of fin regeneration in individual animals. We found no evidence for the existence of multipotent progenitors. Instead, multiple cell types, including epidermal cells, intra-ray fibroblasts, and osteoblasts, contribute to the newly regenerated tissue while remaining highly restricted with respect to their developmental identity. Our studies further demonstrate that the regenerating fin consists of many repeating blastema "units" dedicated to each fin ray. These blastemas each have an organized structure of lineage restricted, dedifferentiated cells that cooperate to regenerate the caudal fin.

HDAC activity is required during Xenopus tail regeneration

The ability to fully restore damaged or lost organs is present in only a subset of animals. The Xenopus tadpole tail is a complex appendage, containing epidermis, muscle, nerves, spinal cord, and vasculature, which regenerates after amputation. Understanding the mechanisms of tail regeneration may lead to new insights to promote biomedical regeneration in non-regenerative tissues. Although chromatin remodeling is known to be critical for stem cell pluripotency, its role in complex organ regeneration in vivo remains largely uncharacterized. Here we show that histone deacetylase (HDAC) activity is required for the early stages of tail regeneration. HDAC1 is expressed during the 1(st) two days of regeneration. Pharmacological blockade of HDACs using Trichostatin A (TSA) increased histone acetylation levels in the amputated tail. Furthermore, treatment with TSA or another HDAC inhibitor, valproic acid, specifically inhibited regeneration. Over-expression of wild-type Mad3, a transcriptional repressor known to associate in a complex with HDACs via Sin3, inhibited regeneration. Similarly, expression of a Mad3 mutant lacking the Sin3-interacting domain that is required for HDAC binding also blocks regeneration, suggesting that HDAC and Mad3 may act together to regulate regeneration. Inhibition of HDAC function resulted in aberrant expression of Notch1 and BMP2, two genes known to be required for tail regeneration. Our results identify a novel early role for HDAC in appendage regeneration and suggest that modulation of histone acetylation is important in regenerative repair of complex appendages.

Mature and juvenile tissue models of regeneration in small fish species

The multitude of cells constituting organisms are fragile and easily damaged day by day. Therefore, maintenance of tissue morphology and function is fundamental for multicellular organisms to attain long life. For proper maintenance of tissue integrity, organisms must have mechanisms that detect the loss of tissue mass, activate the de novo production of cells, and organize those cells into functional tissues. However, these processes are only poorly understood. Here we give an overview of adult and juvenile tissue regeneration models in small fish species, such as zebrafish and medaka, and highlight recent advances at the molecular level. From these advances, we have come to realize that the epidermal and mesenchymal parts of the regenerating fish fin-that is, the wound epidermis and blastema, respectively-comprise heterogeneous populations of cells with different molecular identities that can be termed "compartments." These compartments and their mutual interactions are thought to play important roles in promoting the proper progression of tissue regeneration. We further describe the current understanding of these compartments and discuss the possible approaches to affording a better understanding of their roles and interactions during regeneration.

Limb regeneration: a new development?

Salamander limb regeneration is a classical model of tissue morphogenesis and patterning. Through recent advances in cell labeling and molecular analysis, a more precise, mechanistic understanding of this process has started to emerge. Long-standing questions include to what extent limb regeneration recapitulates the events observed in mammalian limb development and to what extent are adult- or salamander- specific aspects deployed. Historically, researchers studying limb development and limb regeneration have proposed different models of pattern formation. Here we discuss recent data on limb regeneration and limb development to argue that although patterning mechanisms are likely to be similar, cell plasticity and signaling from nerves play regeneration-specific roles.

Ezh2 regulates anteroposterior axis specification and proximodistal axis elongation in the developing limb

Specification and determination (commitment) of positional identities precedes overt pattern formation during development. In the limb bud, it is clear that the anteroposterior axis is specified at a very early stage and is prepatterned by the mutually antagonistic interaction between Gli3 and Hand2. There is also evidence that the proximodistal axis is specified early and determined progressively. Little is known about upstream regulators of these processes or how epigenetic modifiers influence axis formation. Using conditional mutagenesis at different time points, we show that the histone methyltransferase Ezh2 is an upstream regulator of anteroposterior prepattern at an early stage. Mutants exhibit posteriorised limb bud identity. During later limb bud stages, Ezh2 is essential for cell survival and proximodistal segment elongation. Ezh2 maintains the late phase of Hox gene expression and cell transposition experiments suggest that it regulates the plasticity with which cells respond to instructive positional cues.

Identification of two novel mutations in Shh long-range regulator associated with familial pre-axial polydactyly

Pre-axial polydactyly type II (PPDII, MIM #174500), Werner mesomelic syndrome (MIM %188770) and Haas polysyndactyly (MIM #186200) are a group of closely related conditions caused by mutations in a long-range Sonic hedgehog (SHH, MIM *600725) regulator called ZRS. To date, 19 point mutations, 10 duplications and 1 triplication of the ZRS associated with those pre-axial polydactylies have been reported in humans, mice, cats and chickens. Some of these have been shown to cause ectopic expression of Shh in the limb bud in mice, leading to a polydactylous phenotype, but the precise mechanism by which ZRS mutations generate this phenotype remains unknown. We present two PPDII families with fully penetrant point mutations in ultra-conserved predicted binding sites for transcription factors SOX9 and PAX3, two possible candidates for regulating SHH expression. Screening for point mutations or copy-number variation of the ZRS, high-resolution array-CGH, and screening of other conserved non-coding sequences (CNS) surrounding SHH in a third family are negative. This is the sixth PPDII pedigree with possible linkage to 7q36 that presents with no detectable ZRS mutation. We hypothesize that another nearby regulatory sequence, or an undetected position effect between ZRS and SHH, could be responsible for negative familial cases linked to 7q36.

Epigenetic reprogramming during tissue regeneration

Epigenetic control of gene regulation is fundamental to the maintenance of cellular identities during all stages of metazoan life. Tissue regeneration involves cellular reprogramming processes, like dedifferentiation, re-differentiation, and trans-differentiation. Hence, in these processes epigenetic maintenance of gene expression programs requires a resetting through mechanisms that we are only beginning to understand. Here we summarize the current status of these studies, in particular regarding the role of epigenetic mechanisms of cellular reprogramming during tissue regeneration.

Regenerating the epigenome

The ability of some organisms to regenerate parts of their body has fascinated scientists for decades. The process of regeneration depends on the potential of certain cells to proliferate and contribute to the formation of new tissue. Organisms have evolved two strategies by which to achieve this: the maintenance of adult stem cells and the induction of stem-cell properties in differentiated cells. In both cases, cells must undergo extensive epigenetic reprogramming to attain the specialized functions of the new tissue. Ultimately, the regenerative capacity of a tissue might depend on the plasticity of the cellular epigenome, which determines the ability of the cell to respond to injury-related signals. Understanding this epigenetic plasticity will allow the development of strategies to stimulate the regeneration of damaged tissues and organs in humans.

Hedgehog signaling in the liver

Reactivation of Hedgehog (Hh), a morphogenic signaling pathway that controls progenitor cell fate and tissue construction during embryogenesis occurs during many types of liver injury in adult. The net effects of activating the Hedgehog pathway include expansion of liver progenitor populations to promote liver regeneration, but also hepatic accumulation of inflammatory cells, liver fibrogenesis, and vascular remodeling. All of these latter responses are known to be involved in the pathogenesis of cirrhosis. In addition, Hh signaling may play a role in primary liver cancers, such as cholangiocarcinoma and hepatocellular carcinoma. Study of Hedgehog signaling in liver cells is in its infancy. Additional research in this area is justified given growing experimental and clinical data supporting a role for the pathway in regulating outcomes of liver injury.

Frequent deregulations in the hedgehog signaling network and cross-talks with the epidermal growth factor receptor pathway involved in cancer progression and targeted therapies

The hedgehog (Hh)/glioma-associated oncogene (GLI) signaling network is among the most important and fascinating signal transduction systems that provide critical functions in the regulation of many developmental and physiological processes. The coordinated spatiotemporal interplay of the Hh ligands and other growth factors is necessary for the stringent control of the behavior of diverse types of tissue-resident stem/progenitor cells and their progenies. The activation of the Hh cascade might promote the tissue regeneration and repair after severe injury in numerous organs, insulin production in pancreatic beta-cells, and neovascularization. Consequently, the stimulation of the Hh pathway constitutes a potential therapeutic strategy to treat diverse human disorders, including severe tissue injuries; diabetes mellitus; and brain, skin, and cardiovascular disorders. In counterbalance, a deregulation of the Hh signaling network might lead to major tissular disorders and the development of a wide variety of aggressive and metastatic cancers. The target gene products induced through the persistent Hh activation can contribute to the self-renewal, survival, migration, and metastasis of cancer stem/progenitor cells and their progenies. Moreover, the pivotal role mediated through the Hh/GLI cascade during cancer progression also implicates the cooperation with other oncogenic products, such as mutated K-RAS and complex cross-talk with different growth factor pathways, including tyrosine kinase receptors, such as epidermal growth factor receptor (EGFR), Wnt/beta-catenin, and transforming growth factor-beta (TGF-beta)/TGF-beta receptors. Therefore, the molecular targeting of distinct deregulated gene products, including Hh and EGFR signaling components and other signaling elements that are frequently deregulated in highly tumorigenic cancer-initiating cells and their progenies, might constitute a potential therapeutic strategy to eradicate the total cancer cell mass. Of clinical interest is that these multitargeted approaches offer great promise as adjuvant treatments for improving the current antihormonal therapies, radiotherapies, and/or chemotherapies against locally advanced and metastatic cancers, thereby preventing disease relapse and the death of patients with cancer.

MID1 and MID2 are required for Xenopus neural tube closure through the regulation of microtubule organization

Closure of the neural tube requires both the change and maintenance of cell shape. The change occurs mainly through two coordinated morphogenetic events: cell elongation and apical constriction. How cytoskeletal elements, including microtubules, are regulated in this process in vivo is largely unknown. Here, we show that neural tube closure in Xenopus depends on orthologs of two proteins: MID1, which is responsible for Opitz G/BBB syndrome in humans, and its paralog MID2. Depletion of the Xenopus MIDs (xMIDs) by morpholino-mediated knockdown disrupted epithelial morphology in the neural plate, leading to neural tube defects. In the xMID-depleted neural plate, the normal epithelial organization was perturbed without affecting neural fate. Furthermore, the xMID knockdown destabilized and caused the disorganization of microtubules, which are normally apicobasally polarized, accounting for the abnormal phenotypes. We also found that the xMIDs and their interacting protein Mig12 were coordinately required for microtubule stabilization during remodeling of the neural plate. Finally, we showed that the xMIDs are required for the formation of multiple epithelial organs. We propose that similar MID-governed mechanisms underlie the normal morphogenesis of epithelial tissues and organs, including the tissues affected in patients with Opitz G/BBB syndrome.

The effects of DNA methyltransferase inhibitors and histone deacetylase inhibitors on digit regeneration in mice

METHOD

We injected two drugs that modify the epigenome, the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine (5-aza-dC) and the histone deacetylase inhibitor trichostatin A (TSA), alone or in combination, into C57Bl/6 mice subjected to amputation through the mid-second phalanx of the third digit. Wound-site tissue was collected.

RESULTS

We observed increased staining of the stem cell markers Rex1 (Zfp42) and stem cell antigen-1 at digit amputation sites from drug-treated mice. Samples from 5-aza-dC plus TSA and TSA treated mice also showed increased proliferating cell nuclear antigen staining, a measure of cell proliferation. Drug treatments increased Msx1, but not Cyp26a1 or ALDH1a2 (RALDH2) mRNA.

CONCLUSION

5-aza-dC and TSA treatments stimulated cell proliferation at the amputation site, possibly via increased expression of genes involved in digit development and regeneration.

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.

Analysis of hoxa11 and hoxa13 expression during patternless limb regeneration in Xenopus

During limb regeneration, anuran tadpoles and urodele amphibians generate pattern-organizing, multipotent, mesenchymal blastema cells, which give rise to a replica of the lost limb including patterning in three dimensions. To facilitate the regeneration of nonregenerative limbs in other vertebrates, it is important to elucidate the molecular differences between blastema cells that can regenerate the pattern of limbs and those that cannot. In Xenopus froglet (soon after metamorphosis), an amputated limb generates blastema cells that do not produce proper patterning, resulting in a patternless regenerate, a spike, regardless of the amputation level. We found that re-expression of hoxa11 and hoxa13 in the froglet blastema is initiated although the subsequent proximal-distal patterning, including separation of the hoxa11 and hoxa13 expression domains, is disrupted. We also observed an absence of EphA4 gene expression in the froglet blastema and a failure of position-dependent cell sorting, which correlated with the altered hoxa11 and hoxa13 expression. Quantitative analysis of hoxa11 and hoxa13 expression revealed that hoxa13 transcript levels were reduced in the froglet blastema compared with the tadpole blastema. Moreover, the expression of sox9, an important regulator of chondrogenic differentiation, was detected earlier in patternless blastemas than in tadpole blastemas. These results suggest that appropriate spatial, temporal, and quantitative gene expression is necessary for pattern regeneration by blastema cells.

Effects of activation of hedgehog signaling on patterning, growth, and differentiation in Xenopus froglet limb regeneration

Regenerating limbs of urodele amphibians and Xenopus tadpole are reconstructed along proximal-distal, anterior-posterior (AP), and dorsal-ventral axes. In contrast, a regenerated limb of the Xenopus froglet does not have digits, and only a simple cartilaginous structure referred to as a "spike" is formed. This suggests that repatterning along the AP axis is absent in the froglet blastema. Previous studies have shown that Shh and its target genes are not expressed in the froglet blastema. In this study, we activated Hedgehog signaling in the froglet blastema and found that target genes of Shh were inducible in the mesenchyme of limb blastema. Furthermore, we found that activation of the signaling had effects on blastema cell proliferation and chondrogenesis and resulted in the formation of multiple cartilaginous structures. These findings indicate that activation of signaling that is absent in the froglet blastema is effective for improvement of limb regeneration ability in the Xenopus froglet.

A histone demethylase is necessary for regeneration in zebrafish

Urodele amphibians and teleost fish regenerate amputated body parts via a process called epimorphic regeneration. A hallmark of this phenomenon is the reactivation of silenced developmental regulatory genes that previously functioned during embryonic patterning. We demonstrate that histone modifications silence promoters of numerous genes involved in zebrafish caudal fin regeneration. Silenced developmental regulatory genes contain bivalent me(3)K4/me(3)K27 H3 histone modifications created by the concerted action of Polycomb (PcG) and Trithorax histone methyltransferases. During regeneration, this silent, bivalent chromatin is converted to an active state by loss of repressive me(3)K27 H3 modifications, occurring at numerous genes that appear to function during regeneration. Loss-of-function studies demonstrate a requirement for a me(3)K27 H3 demethylase during fin regeneration. These results indicate that histone modifications at discreet genomic positions may serve as a crucial regulatory event in the initiation of fin regeneration.

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.

Microarray and cDNA sequence analysis of transcription during nerve-dependent limb regeneration

Background

Microarray analysis and 454 cDNA sequencing were used to investigate a centuries-old problem in regenerative biology: the basis of nerve-dependent limb regeneration in salamanders. Innervated (NR) and denervated (DL) forelimbs of Mexican axolotls were amputated and transcripts were sampled after 0, 5, and 14 days of regeneration.

Results

Considerable similarity was observed between NR and DL transcriptional programs at 5 and 14 days post amputation (dpa). Genes with extracellular functions that are critical to wound healing were upregulated while muscle-specific genes were downregulated. Thus, many processes that are regulated during early limb regeneration do not depend upon nerve-derived factors. The majority of the transcriptional differences between NR and DL limbs were correlated with blastema formation; cell numbers increased in NR limbs after 5 dpa and this yielded distinct transcriptional signatures of cell proliferation in NR limbs at 14 dpa. These transcriptional signatures were not observed in DL limbs. Instead, gene expression changes within DL limbs suggest more diverse and protracted wound-healing responses. 454 cDNA sequencing complemented the microarray analysis by providing deeper sampling of transcriptional programs and associated biological processes. Assembly of new 454 cDNA sequences with existing expressed sequence tag (EST) contigs from the Ambystoma EST database more than doubled (3935 to 9411) the number of non-redundant human-A. mexicanum orthologous sequences.

Conclusion

Many new candidate gene sequences were discovered for the first time and these will greatly enable future studies of wound healing, epigenetics, genome stability, and nerve-dependent blastema formation and outgrowth using the axolotl model.

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.