Apoptosis is required during early stages of tail regeneration in Xenopus laevis

Genome-wide analysis of gene expression during Xenopus tropicalis tadpole tail regeneration

Background

The molecular mechanisms governing vertebrate appendage regeneration remain poorly understood. Uncovering these mechanisms may lead to novel therapies aimed at alleviating human disfigurement and visible loss of function following injury. Here, we explore tadpole tail regeneration in Xenopus tropicalis, a diploid frog with a sequenced genome.

Results

We found that, like the traditionally used Xenopus laevis, the Xenopus tropicalis tadpole has the capacity to regenerate its tail following amputation, including its spinal cord, muscle, and major blood vessels. We examined gene expression using the Xenopus tropicalis Affymetrix genome array during three phases of regeneration, uncovering more than 1,000 genes that are significantly modulated during tail regeneration. Target validation, using RT-qPCR followed by gene ontology (GO) analysis, revealed a dynamic regulation of genes involved in the inflammatory response, intracellular metabolism, and energy regulation. Meta-analyses of the array data and validation by RT-qPCR and in situ hybridization uncovered a subset of genes upregulated during the early and intermediate phases of regeneration that are involved in the generation of NADP/H, suggesting that these pathways may be important for proper tail regeneration.

Conclusions

The Xenopus tropicalis tadpole is a powerful model to elucidate the genetic mechanisms of vertebrate appendage regeneration. We have produced a novel and substantial microarray data set examining gene expression during vertebrate appendage regeneration.

Identification and characterization of genes required for compensatory growth in Drosophila

To maintain tissue homeostasis, some organs are able to replace dying cells with additional proliferation of surviving cells. Such proliferation can be localized (e.g., a regeneration blastema) or diffuse (compensatory growth). The relationship between such growth and the growth that occurs during development has not been characterized in detail. Drosophila melanogaster larval imaginal discs can recover from extensive damage, producing normally sized adult organs. Here we describe a system using genetic mosaics to screen for recessive mutations that impair compensatory growth. By generating clones of cells that carry a temperature-sensitive cell-lethal mutation, we conditionally ablate patches of tissue in the imaginal disc and assess the ability of the surviving sister clones to replace the lost tissue. We have used this system together with a modified whole-genome resequencing (WGS) strategy to identify several mutations that selectively compromise compensatory growth. We find specific alleles of bunched (bun) and Ribonucleoside diphosphate reductase large subunit (RnrL) reduce compensatory growth in the imaginal disc. Other genes identified in the screen, including two alleles of Topoisomerase 3-alpha (Top3α), while also required for developmental growth, appear to have an enhanced requirement during compensatory growth. Compensatory growth occurs at a higher rate than normal growth and may therefore have features in common with some types of overgrowth. Indeed, the RnrL allele identified compromises both these types of altered growth and mammalian ribonucleotide reductase and topoisomerases are targets of anticancer drugs. Finally, the approach we describe is applicable to the study of compensatory growth in diverse tissues in Drosophila.

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.

[Between homeostasis and development, which strategies to regenerate?]

The Hydra model system is well suited to decipher the mechanisms underlying adult regeneration, specifically those that were robust enough to be maintained across evolution. After mid-gastric bissection head regeneration in Hydra relies on apoptosis-induced compensatory proliferation via the release of Wnt3 by the apoptotic interstitial cells and activation of the β-catenin pathway in the surrounding cycling interstitial cells. As apoptosis-induced compensatory proliferation is also at work in Drosophila regenerating imaginal discs, Xenopus tadpole regenerating their tail and mice regenerating their skin or their liver, this mechanism might represent an evolutionarily-conserved way to launch a regenerative response. However after decapitation, the analysis of the activation of the canonical Wnt pathway in decapitated Hydra showed that apoptosis-induced compensatory proliferation does not take place in this context. Given that the proportion of interstitial stem cells is significantly higher in the middle part than in the upper part of the body column, this suggested that the route taken to regenerate a structure as complex as the head dramatically varies according to the homeostatic status of the tissue at the time of injury. From these observations, we propose a tri-modular model for animal regeneration where the first module or "wound healing module" is followed by a transient module named "inducing module of regeneration" that allows the recruitment of the third module named "re-development module", necessary for repatterning the missing structure. We claim that among these three modules, the inducing module of regeneration is the most drastically constrained by the homeostatic conditions of any given tissue or organ at the time of injury and therefore the most variable.

Mitogenic signaling from apoptotic cells in Drosophila

Apoptotic cells of Drosophila not only activate caspases, but also are able to secrete developmental signals like Hedgehog (Hh), Decapentaplegic (Dpp) and Wingless (Wg) before dying. Since Dpp and Wg are secreted in growing tissues and behave as growth factors, it was proposed that they play a role in compensatory proliferation, the process by which a growing blastema can restore normal size after massive apoptosis. We discuss recent results showing that there is normal compensatory proliferation in the absence of Dpp/Wg signaling, thus indicating it has no significant role in the process. Furthermore, we argue that Dpp/Wg signaling is not a resident feature of apoptotic cells, but a side effect of the necessary activation of the JNK pathway. Nevertheless, the ectopic JNK/Dpp/Wg signaling may have an important role in tissue regeneration. Recent work in other organisms suggests that paracrine signaling from apoptotic cells may be of general significance in wound healing and tissue regeneration in metazoans.

Active participation of cell death in development and organismal homeostasis

The precise, regional execution of programmed cell death is required for the proper patterning and sculpting of the embryonic primordium during animal development. In addition, cell death that is not directly involved in sculpting is also widely observed. The most abundant morphological form of programmed cell death in developing animals is apoptosis, and identification of the apoptotic genetic pathways has enabled the study of apoptosis' regulation and roles during development. Genetic and bio-imaging studies have permitted the study of the active roles of cell death in development and organismal homeostasis.

Atrazine exposure affects growth, body condition and liver health in Xenopus laevis tadpoles

Six studies were performed regarding the effects of atrazine, the most frequently detected pesticide in fresh water in the US, on developing Xenopus laevis tadpoles exposed 5 days post-hatch through Nieuwkoop Faber Stage 62. The levels of atrazine tested included those potentially found in puddles, vernal ponds and runoff soon after application (200 and 400 μg/L) and a low level studied by a number of other investigators (25 μg/L). One study tested 0, 25 and 200 μg/L, another tested 0, 200 and 400 μg/L, while the remaining four studies tested 0 and 400 μg/L. During all exposures, mortality, growth, metamorphosis, sex ratio, fat body (a lipid storage organ) size and liver weights, both relative to body weight, were evaluated. In selected studies, feeding behavior was recorded, livers and fat bodies were histologically evaluated, liver glycogen and lipid content were determined by image analysis, and immunohistochemical detection of activated caspase-3 in hepatocytes was performed. The NOEC was 25 μg/L. None of these exposure levels changed sex ratios nor were intersex gonads noted, however, no definitive histological evaluation of the gonads was performed. Although a marginal increase in mortality at the 200 μg/L level was noted, this was not statistically significant. Nor was there an increase in mortality at 400 μg/L versus controls. At the 400 μg/L level, tadpoles were smaller than controls by 72 h of exposure and remained smaller throughout the entire exposure. Appetite was not decreased at any exposure level. Slowed metamorphosis was noted only at 400 μg/L in two of five studies. Livers were significantly smaller in the study that tested both 200 and 400 μg/L, yet no pathological changes or differences in glycogen or lipid stores were noted. However, livers from 400 μg/L exposed tadpoles had higher numbers of activated caspase-3 immunopositive cells suggesting increased rates of apoptosis. Fat body size decreased significantly after exposure to 200 and 400 μg/L although these organs still contained some lipid and lacked any pathology. Since this was noted across all studies, it was considered the most sensitive indicator of atrazine exposure measured. The changes noted in body and organ size at 200 and 400 μg/L atrazine indicated exposure throughout development compromised the tadpoles. Significant reductions in fat body size could potentially decrease their ability to survive the stresses of metamorphosis or reduce reproductive fitness as frogs rely on stored lipids for these processes.

Gut regeneration in holothurians: a snapshot of recent developments

Visceral regeneration in sea cucumbers has been studied since early last century; however, it is only within the last 15 years that real progress has been made in understanding the cellular and molecular events involved. In the present review, we bring together these recent studies, providing readers with basic information on the anatomy and histology of the normal gut and detailing the changes in tissue organization and gene expression that occur during the regenerative process. We discuss the nature and possible sources of cells involved in the formation of the intestinal regenerate as well as the role of cell death and proliferation in this process. In addition, we compare gut formation during regeneration and during embryogenesis. Finally, we describe the molecular studies that have helped advance regenerative studies in holothurians and integrate the gene expression information with data on cellular events. Studies on visceral regeneration in these echinoderms provide a unique view that complements regeneration studies in other animal phyla, which are mainly focused on whole-animal regeneration or appendage regeneration.

Injury-induced activation of the MAPK/CREB pathway triggers apoptosis-induced compensatory proliferation in hydra head regeneration

After bisection, Hydra polyps regenerate their head from the lower half thanks to a head-organizer activity that is rapidly established at the tip. Head regeneration is also highly plastic as both the wild-type and the epithelial Hydra (that lack the interstitial cell lineage) can regenerate their head. In the wild-type context, we previously showed that after mid-gastric bisection, a large subset of the interstitial cells undergo apoptosis, inducing compensatory proliferation of the surrounding progenitors. This asymmetric process is necessary and sufficient to launch head regeneration. The apoptotic cells transiently release Wnt3, which promotes the formation of a proliferative zone by activating the beta-catenin pathway in the adjacent cycling cells. However the injury-induced signaling that triggers apoptosis is unknown. We previously reported an asymmetric immediate activation of the mitogen-activated protein kinase/ribosomal S6 kinase/cAMP response element binding protein (MAPK/RSK/CREB) pathway in head-regenerating tips after mid-gastric bisection. We show here that pharmacological inhibition of the MAPK/ERK pathway or RNAi knockdown of the RSK, CREB, CREB binding protein (CBP) genes prevents apoptosis, compensatory proliferation and blocks head regeneration. As the activation of the MAPK pathway upon injury plays an essential role in regenerating bilaterian species, these results suggest that the MAPK-dependent activation of apoptosis-induced compensatory proliferation represents an evolutionary-conserved mechanism to launch a regenerative process.

Nonautonomous apoptosis is triggered by local cell cycle progression during epithelial replacement in Drosophila

Tissue remodeling involves collective cell movement, and cell proliferation and apoptosis are observed in both development and disease. Apoptosis and proliferation are considered to be closely correlated, but little is known about their coordinated regulation in physiological tissue remodeling in vivo. The replacement of larval abdominal epidermis with adult epithelium in Drosophila pupae is a simple model of tissue remodeling. During this process, larval epidermal cells (LECs) undergo apoptosis and are replaced by histoblasts, which are adult precursor cells. By analyzing caspase activation at the single-cell level in living pupae, we found that caspase activation in LECs is induced at the LEC/histoblast boundary, which expands as the LECs die. Manipulating histoblast proliferation at the LEC/histoblast boundary, either genetically or by UV illumination, indicated that local interactions with proliferating histoblasts triggered caspase activation in the boundary LECs. Finally, by monitoring the spatiotemporal dynamics of the S/G₂/M phase in histoblasts in vivo, we found that the transition from S/G₂ phases is necessary to induce nonautonomous LEC apoptosis at the LEC/histoblast boundary. The replacement boundary, formed as caspase activation is regulated locally by cell-cell communication, may drive the dynamic orchestration of cell replacement during tissue remodeling.

Tail regenerative capacity and iNOS immunolocalization in Xenopus laevis tadpoles

The morphology and the immuno-distribution of the inducible isoform of nitric oxide synthase (iNOS) have been examined in regenerating tails from differently aged Xenopus laevis larvae. By comparing stage-50 and stage-55/56 tadpoles, various morphological aspects and immunoreactivity to anti-iNOS antibody in terms of the number and duration of positive cells have been demonstrated in the regenerating buds. Unlike in stage-50 larvae, the extent of responses to tail amputation in older larvae is more dependent on the individual tadpole and a high percentage (70%-80%) of malformed tails has been seen. The findings indicate that the decline in the efficiency of Xenopus tail regeneration is driven by differences in the inflammatory responses and in the involvement of nitric oxide. This molecule is induced and required for normal tail regeneration, whereas in excess, it is probably associated with progressive loss in the regeneration capability.

Cell plasticity in homeostasis and regeneration

Over the past decades, genetic analyses performed in vertebrate and invertebrate organisms deciphered numerous cellular and molecular mechanisms deployed during sexual development and identified genetic circuitries largely shared among bilaterians. In contrast, the functional analysis of the mechanisms that support regenerative processes in species randomly scattered among the animal kingdom, were limited by the lack of genetic tools. Consequently, unifying principles explaining how stress and injury can lead to the reactivation of a complete developmental program with restoration of original shape and function remained beyond reach of understanding. Recent data on cell plasticity suggest that beside the classical developmental approach, the analysis of homeostasis and asexual reproduction in adult organisms provides novel entry points to dissect the regenerative potential of a given species, a given organ or a given tissue. As a clue, both tissue homeostasis and regeneration dynamics rely on the availability of stem cells and/or on the plasticity of differentiated cells to replenish the missing structure. The freshwater Hydra polyp provides us with a unique model system to study the intricate relationships between the mechanisms that regulate the maintenance of homeostasis, even in extreme conditions (starvation and overfeeding) and the reactivation of developmental programs after bisection or during budding. Interestingly head regeneration in Hydra can follow several routes according to the level of amputation, suggesting that indeed the homeostatic background dramatically influences the route taken to bridge injury and regeneration.

Transmembrane potential of GlyCl-expressing instructor cells induces a neoplastic-like conversion of melanocytes via a serotonergic pathway

Understanding the mechanisms that coordinate stem cell behavior within the host is a high priority for developmental biology, regenerative medicine and oncology. Endogenous ion currents and voltage gradients function alongside biochemical cues during pattern formation and tumor suppression, but it is not known whether bioelectrical signals are involved in the control of stem cell progeny in vivo. We studied Xenopus laevis neural crest, an embryonic stem cell population that gives rise to many cell types, including melanocytes, and contributes to the morphogenesis of the face, heart and other complex structures. To investigate how depolarization of transmembrane potential of cells in the neural crest's environment influences its function in vivo, we manipulated the activity of the native glycine receptor chloride channel (GlyCl). Molecular-genetic depolarization of a sparse, widely distributed set of GlyCl-expressing cells non-cell-autonomously induces a neoplastic-like phenotype in melanocytes: they overproliferate, acquire an arborized cell shape and migrate inappropriately, colonizing numerous tissues in a metalloprotease-dependent fashion. A similar effect was observed in human melanocytes in culture. Depolarization of GlyCl-expressing cells induces these drastic changes in melanocyte behavior via a serotonin-transporter-dependent increase of extracellular serotonin (5-HT). These data reveal GlyCl as a molecular marker of a sparse and heretofore unknown cell population with the ability to specifically instruct neural crest derivatives, suggest transmembrane potential as a tractable signaling modality by which somatic cells can control stem cell behavior at considerable distance, identify a new biophysical aspect of the environment that confers a neoplastic-like phenotype upon stem cell progeny, reveal a pre-neural role for serotonin and its transporter, and suggest a novel strategy for manipulating stem cell behavior.

Visceral regeneration in a sea cucumber involves extensive expression of survivin and mortalin homologs in the mesothelium

Background

The proper balance of cell division and cell death is of crucial importance for all kinds of developmental processes and for maintaining tissue homeostasis in mature tissues. Dysregulation of this balance often results in severe pathologies, such as cancer. There is a growing interest in understanding the factors that govern the interplay between cell death and proliferation under various conditions. Survivin and mortalin are genes that are known to be implicated in both mitosis and apoptosis and are often expressed in tumors.

Results

The present study takes advantage of the ability of the sea cucumber Holothuria glaberrima Selenka, 1867 (Holothuroidea, Aspidochirota) to discard its viscera and completely regrow them. This visceral regeneration involves an extensive expression of survivin and mortalin transcripts in the gut mesothelium (the outer tissue layer of the digestive tube), which coincides in time with drastic de-differentiation and a burst in cell division and apoptosis. Double labeling experiments (in situ hybridization combined with TUNEL assay or with BrdU immunohistochemistry) suggest that both genes support cell proliferation, while survivin might also be involved in suppression of the programmed cell death.

Conclusions

Visceral regeneration in the sea cucumber H. glaberrima is accompanied by elevated levels of cell division and cell death, and, moreover, involves expression of pro-cancer genes, such as survivin and mortalin, which are known to support proliferation and inhibit apoptosis. Nevertheless, once regeneration is completed and the expression pattern of both genes returns to normal, the regrown digestive tube shows no anomalies. This strongly suggests that sea cucumbers must possess some robust cancer-suppression mechanisms that allow rapid re-growth of the adult tissues without leading to runaway tumor development.

Inversin relays Frizzled-8 signals to promote proximal pronephros development

Mutations of inversin cause type II nephronophthisis, an infantile autosomal recessive disease characterized by cystic kidney disease and developmental defects. Inversin regulates Wnt signaling and is required for convergent extension movements during early embryogenesis. We now show that Inversin is essential for Xenopus pronephros formation, involving two distinct and opposing forms of cell movements. Knockdown of Inversin abrogated both proximal pronephros extension and distal tubule differentiation, phenotypes similar to that of Xenopus deficient in Frizzled-8. Exogenous Inversin rescued the pronephric defects caused by lack of Frizzled-8, indicating that Inversin acts downstream of Frizzled-8 in pronephros morphogenesis. Depletion of Inversin prevents the recruitment of Dishevelled in response to Frizzled-8 and impeded the accumulation of Dishevelled at the apical membrane of tubular epithelial cells in vivo. Thus, defective tubule morphogenesis seems to contribute to the renal pathology observed in patients with nephronophthisis type II.

Spinal cord repair in regeneration-competent vertebrates: adult teleost fish as a model system

Spinal cord injuries in mammals, including humans, have devastating long-term consequences. Despite substantial research, therapeutic approaches developed in mammalian model systems have had limited success to date. An alternative strategy in the search for treatment of spinal cord lesions is provided by regeneration-competent vertebrates. These organisms, which include fish, urodele amphibians, and certain reptiles, have a spinal cord very similar in structure to that of mammals, but are capable of spontaneous structural and functional recovery after spinal cord injury. The present review aims to provide an overview of the current status of our knowledge of spinal cord regeneration in one of these groups, teleost fish. The findings are discussed from a comparative perspective, with reference to other taxa of regeneration-competent vertebrates, as well as to mammals.

Expression of secreted Wnt pathway components reveals unexpected complexity of the planarian amputation response

Regeneration is widespread throughout the animal kingdom, but our molecular understanding of this process in adult animals remains poorly understood. Wnt/β-catenin signaling plays crucial roles throughout animal life from early development to adulthood. In intact and regenerating planarians, the regulation of Wnt/β-catenin signaling functions to maintain and specify anterior/posterior (A/P) identity. Here, we explore the expression kinetics and RNAi phenotypes for secreted members of the Wnt signaling pathway in the planarian Schmidtea mediterranea. Smed-wnt and sFRP expression during regeneration is surprisingly dynamic and reveals fundamental aspects of planarian biology that have been previously unappreciated. We show that after amputation, a wounding response precedes rapid re-organization of the A/P axis. Furthermore, cells throughout the body plan can mount this response and reassess their new A/P location in the complete absence of stem cells. While initial stages of the amputation response are stem cell independent, tissue remodeling and the integration of a new A/P address with anatomy are stem cell dependent. We also show that WNT5 functions in a reciprocal manner with SLIT to pattern the planarian mediolateral axis, while WNT11-2 patterns the posterior midline. Moreover, we perform an extensive phylogenetic analysis on the Smed-wnt genes using a method that combines and integrates both sequence and structural alignments, enabling us to place all nine genes into Wnt subfamilies for the first time.

Apoptosis, stem cells, and tissue regeneration

Most metazoans have at least some ability to regenerate damaged cells and tissues, although the regenerative capacity varies depending on the species, organ, or developmental stage. Cell replacement and regeneration occur in two contexts: renewal of spent cells during tissue homeostasis (homeostatic growth), and in response to external injury, wounding, or amputation (epimorphic regeneration). Model organisms that display remarkable regenerative capacity include amphibians, planarians, Hydra, and the vertebrate liver. In addition, several mammalian organs--including the skin, gut, kidney, muscle, and even the human nervous system--have some ability to replace spent or damaged cells. Although the regenerative response is complex, it typically involves the induction of new cell proliferation through formation of a blastema, followed by cell specification, differentiation, and patterning. Stem cells and undifferentiated progenitor cells play an important role in both tissue homeostasis and tissue regeneration. Stem cells are typically quiescent or passing slowly through the cell cycle in adult tissues, but they can be activated in response to cell loss and wounding. A series of studies, mostly performed in Drosophila as well as in Hydra, Xenopus, and mouse, has revealed an unexpected role of apoptotic caspases in the production of mitogenic signals that stimulate the proliferation of stem and progenitor cells to aid in tissue regeneration. This Review summarizes some of the key findings and discusses links to stem cell biology and cancer.

Induction of vertebrate regeneration by a transient sodium current

Amphibians such as frogs can restore lost organs during development, including the lens and tail. To design biomedical therapies for organ repair, it is necessary to develop a detailed understanding of natural regeneration. Recently, ion transport has been implicated as a functional regulator of regeneration. Whereas voltage-gated sodium channels play a well known and important role in propagating action potentials in excitable cells, we have identified a novel role in regeneration for the ion transport function mediated by the voltage-gated sodium channel, Na(V)1.2. A local, early increase in intracellular sodium is required for initiating regeneration following Xenopus laevis tail amputation, and molecular and pharmacological inhibition of sodium transport causes regenerative failure. Na(V)1.2 is absent under nonregenerative conditions, but misexpression of human Na(V)1.5 can rescue regeneration during these states. Remarkably, pharmacological induction of a transient sodium current is capable of restoring regeneration even after the formation of a nonregenerative wound epithelium, confirming that it is the regulation of sodium transport that is critical for regeneration. Our studies reveal a previously undetected competency window in which cells retain their intrinsic regenerative program, identify a novel endogenous role for Na(V) in regeneration, and show that modulation of sodium transport represents an exciting new approach to organ repair.

Inhibition of caspase-3-mediated apoptosis improves spinal cord repair in a regeneration-competent vertebrate system

Teleost fish exhibit an excellent potential for structural and functional recovery after CNS lesions. The function of apoptosis in the process of regeneration remains controversial. While some studies have identified this type of cell death as essential for successful regeneration, other investigations have suggested some degree of functional improvement after inhibition of apoptosis. In the present study, we examined whether inhibition of apoptosis immediately after injury can improve spinal cord regeneration. As a model system, we used Apteronotus leptorhynchus, a regeneration-competent weakly electric fish. To inhibit apoptosis, we employed 2,2'-methylenebis (1,3-cyclohexanedione) (M50054), a compound that prevents caspase-3 activation. Administration of this apoptosis inhibitor led to a significant reduction in the numbers of apoptotic cells at 24 h, 5 days, and 30 days after the lesion. Using triple immunolabeling, we identified a significant reduction in the level of apoptosis at 5 and 30 days after the lesion among the following cellular categories: cells generated shortly after the lesion, existing neurons, and newly differentiated neurons. This reduced rate of apoptosis led to an increase in the relative number of differentiating and surviving neurons at both 5 and 30 days post-injury, compared to the control groups. Functional regeneration, as indicated by the recovery rate of the amplitude of the electric organ discharge (EOD), was significantly improved within the first 20 days after the lesion in the fish treated with M50054. Our data provide the first evidence that modulation of caspase-3 activation can significantly improve neuroregeneration and functional recovery in a regeneration-competent organism.

Advances in understanding tissue regenerative capacity and mechanisms in animals

Questions about how and why tissue regeneration occurs have captured the attention of countless biologists, biomedical engineers and clinicians. Regenerative capacity differs greatly across organs and organisms, and a range of model systems that use different regenerative strategies and that offer different technical advantages have been studied to understand regeneration. Making use of this range of systems and approaches, recent advances have allowed progress to be made in understanding several key issues that are common to natural regenerative events. These issues include: the determination of regenerative capacity; the importance of stem cells, dedifferentiation and transdifferentiation; how regenerative signals are initiated and targeted; and the mechanisms that control regenerative proliferation and patterning.

The Hydra model: disclosing an apoptosis-driven generator of Wnt-based regeneration

The Hydra model system is well suited for the eludication of the mechanisms underlying regeneration in the adult, and an understanding of the core mechanisms is likely to cast light on pathways conserved in other species. Recent detailed analyses of the activation of the Wnt-beta-catenin pathway in bisected Hydra shows that the route taken to regenerate a structure as complex as the head varies dramatically according to the level of the amputation. When decapitation induces direct re-development due to Wnt3 signaling from epithelial cells, head regeneration after mid-gastric section relies first on Wnt3 signaling from interstitial cells, that undergo apoptosis-induced compensatory proliferation, and subsequently on activation of Wnt3 signaling in the epithelial cells. The relative distribution between stem cells and head progenitor cells is strikingly different in these two contexts, indicating that the pre-amputation homeostatic conditions define and constrain the route that bridges wound-healing to the re-development program of the missing structure.

Lack of p21 expression links cell cycle control and appendage regeneration in mice

Animals capable of regenerating multiple tissue types, organs, and appendages after injury are common yet sporadic and include some sponge, hydra, planarian, and salamander (i.e., newt and axolotl) species, but notably such regenerative capacity is rare in mammals. The adult MRL mouse strain is a rare exception to the rule that mammals do not regenerate appendage tissue. Certain commonalities, such as blastema formation and basement membrane breakdown at the wound site, suggest that MRL mice may share other features with classical regenerators. As reported here, MRL fibroblast-like cells have a distinct cell-cycle (G2/M accumulation) phenotype and a heightened basal and wound site DNA damage/repair response that is also common to classical regenerators and mammalian embryonic stem cells. Additionally, a neutral and alkaline comet assay displayed a persistent level of intrinsic DNA damage in cells derived from the MRL mouse. Similar to mouse ES cells, the p53-target p21 was not expressed in MRL ear fibroblasts. Because the p53/p21 axis plays a central role in the DNA damage response and cell cycle control, we directly tested the hypothesis that p21 down-regulation could functionally induce a regenerative response in an appendage of an otherwise nonregenerating mouse strain. Using the ear hole closure phenotype, a genetically mapped and reliable quantitative indicator of regeneration in the MRL mouse, we show that the unrelated Cdkn1a(tmi/Tyj)/J p21(-/-) mouse (unlike the B6129SF2/J WT control) closes ear holes similar to MRL mice, providing a firm link between cell cycle checkpoint control and tissue regeneration.

Apoptotic cells activate the "phoenix rising" pathway to promote wound healing and tissue regeneration

The ability to regenerate damaged tissues is a common characteristic of multicellular organisms. We report a role for apoptotic cell death in promoting wound healing and tissue regeneration in mice. Apoptotic cells released growth signals that stimulated the proliferation of progenitor or stem cells. Key players in this process were caspases 3 and 7, proteases activated during the execution phase of apoptosis that contribute to cell death. Mice lacking either of these caspases were deficient in skin wound healing and in liver regeneration. Prostaglandin E(2), a promoter of stem or progenitor cell proliferation and tissue regeneration, acted downstream of the caspases. We propose to call the pathway by which executioner caspases in apoptotic cells promote wound healing and tissue regeneration in multicellular organisms the "phoenix rising" pathway.

Apoptotic cells provide an unexpected source of Wnt3 signaling to drive hydra head regeneration

Decapitated Hydra regenerate their heads via morphallaxis, i.e., without significant contributions made by cell proliferation or interstitial stem cells. Indeed, Hydra depleted of interstitial stem cells regenerate robustly, and Wnt3 from epithelial cells triggers head regeneration. However, we find a different mechanism controlling regeneration after midgastric bisection in animals equipped with both epithelial and interstitial cell lineages. In this context, we see rapid induction of apoptosis and Wnt3 secretion among interstitial cells at the head- (but not foot-) regenerating site. Apoptosis is both necessary and sufficient to induce Wnt3 production and head regeneration, even at ectopic sites. Further, we identify a zone of proliferation beneath the apoptotic zone, reminiscent of proliferative blastemas in regenerating limbs and of compensatory proliferation induced by dying cells in Drosophila imaginal discs. We propose that different types of injuries induce distinct cellular modes of Hydra head regeneration, which nonetheless converge on a central effector, Wnt3.

Cell death and tissue remodeling in planarian regeneration

Many long-lived organisms, including humans, can regenerate some adult tissues lost to physical injury or disease. Much of the previous research on mechanisms of regeneration has focused on adult stem cells, which give rise to new tissue necessary for the replacement of missing body parts. Here we report that apoptosis of differentiated cells complements stem cell division during regeneration in the planarian Schmidtea mediterranea. Specifically, we developed a whole-mount TUNEL assay that allowed us to document two dramatic increases in the rate of apoptosis following amputation-an initial localized response near the wound site and a subsequent systemic response that varies in magnitude depending on the type of fragment examined. The latter cell death response can be induced in uninjured organs, occurs in the absence of planarian stem cells, and can also be triggered by prolonged starvation. Taken together, our results implicate apoptosis in the restoration of proper anatomical scale and proportion through remodeling of existing tissues. We also report results from initial mechanistic studies of apoptosis in planarians, which revealed that a S. mediterranea homolog of the antiapoptotic gene BCL2 is required for cell survival in adult animals. We propose that apoptosis is a central mechanism working in concert with stem cell division to restore anatomical form and function during metazoan regeneration.

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.

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.

Bioelectric mechanisms in regeneration: Unique aspects and future perspectives

Regenerative biology has focused largely on chemical factors and transcriptional networks. However, endogenous ion flows serve as key epigenetic regulators of cell behavior. Bioelectric signaling involves feedback loops, long-range communication, polarity, and information transfer over multiple size scales. Understanding the roles of endogenous voltage gradients, ion flows, and electric fields will contribute to the basic understanding of numerous morphogenetic processes and the means by which they can robustly restore pattern after perturbation. By learning to modulate the bioelectrical signals that control cell proliferation, migration, and differentiation, we gain a powerful set of new techniques with which to manipulate growth and patterning in biomedical contexts. This chapter reviews the unique properties of bioelectric signaling, surveys molecular strategies and reagents for its investigation, and discusses the opportunities made available for regenerative medicine.

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.

RASSF7 is a member of a new family of RAS association domain-containing proteins and is required for completing mitosis

Mitosis is a fundamental feature of all cellular organisms. It must be tightly regulated to allow normal tissue growth and to prevent cancer formation. Here, we identify a new protein that is required for mitosis. We show that the Ras association (RA) domain-containing protein, RASSF7, is part of an evolutionarily conserved group of four proteins. These are RASSF7, RASSF8, and two new RASSF proteins P-CIP1/RASSF9 and RASSF10. We call this group the N-terminal RASSF family. We analyzed the function of Xenopus RASSF7. RASSF7 was found to be expressed in several embryonic tissues including the skin, eyes, and neural tube. Knocking down its function led to cells failing to form a mitotic spindle and arresting in mitosis. This caused nuclear breakdown, apoptosis, and a striking loss of tissue architecture in the neural tube. Consistent with a role in spindle formation, RASSF7 protein was found to localize to the centrosome. This localization occurred in a microtubule-dependent manner, demonstrating that there is a mutually dependant relationship between RASSF7 localization and spindle formation. Thus RASSF7, the first member of the N-terminal RASSF family to be functionally analyzed, is a centrosome-associated protein required to form a spindle and complete mitosis in the neural tube.

Perturbation of organogenesis by the herbicide atrazine in the amphibian Xenopus laevis

BACKGROUND

Exposure to anthropogenic chemicals during development can disrupt the morphogenesis of organ systems. Use of the herbicide atrazine has been debated in recent years because of its implicated, but poorly characterized, effects on vertebrates. Previous studies primarily examined the effects of atrazine exposure during metamorphosis or early developmental stages of amphibians.

OBJECTIVES

We sought to identify and characterize the susceptibility during the often-overlooked developmental stage of organ morphogenesis.

METHODS

We used a static renewal experimental treatment to investigate the effects of 10, 25, and 35 mg/L atrazine from early organ morphogenesis through the onset of tadpole feeding in the aquatic amphibian model system, Xenopus laevis. We quantified malformations of the body axis, heart, and intestine, as well as apoptosis in the midbrain and pronephric kidney.

RESULTS

We found a significant dose-dependent increase in the percentage of atrazine-exposed tadpoles with malformations of multiple tissues including the main body axis, circulatory system, kidney, and digestive system. Incidence of apoptotic cells also increased in the both midbrain and kidney of atrazine-exposed tadpoles.

CONCLUSIONS

Our results demonstrate that acute atrazine exposure (10-35 mg/L for < or = 48 hr) during early organ morphogenesis disrupts proper organ development in an amphibian model system. The concurrent atrazine-induced apoptosis in the pronephric kidney and midbrain begins to elucidate a mechanism by which atrazine may disrupt developmental processes in nontarget organisms.

The molecular characterization of the brain protein 44-like (Brp44l) gene of Gekko japonicus and its expression changes in spinal cord after tail amputation

The cDNA encoding brain protein 44-like (Brp44l) was identified from the brain and spinal cord cDNA library of Gekko japonicus. It contains a 324 bp open reading frame corresponding to a deduced protein of 107 amino acids. The molecular weight of the BRP44L protein is 12.2664 kDa and its theoretical pI is 9.67. At amino acid level, the BRP44L from gecko shares 85-89% similarity with those from other species. Northern blotting showed that Brp44l was ubiquitously expressed in the major organs of Gekko japonicus including brain, spinal cord, heart, lung, liver, and kidney, suggesting a fundamental biological role of this gene. In situ hybridization (ISH) revealed that positive hybridization signals were present in gray matter of spinal cord. The change of Brp44l expression in spinal cord after tail amputation was examined by reverse transcription polymerase chain reaction (RT-PCR). The expression level of this gene increased in the spinal cord after tail amputation, and reached the highest value at 1 week after tail amputation, which indicated that the expression level of Brp44l might be associated with the process of spinal cord injury and regeneration.

Advances in signaling in vertebrate regeneration as a prelude to regenerative medicine

While all animals have evolved strategies to respond to injury and disease, their ability to functionally recover from loss of or damage to organs or appendages varies widely damage to skeletal muscle, but, unlike amphibians and fish, they fail to regenerate heart, lens, retina, or appendages. The relatively young field of regenerative medicine strives to develop therapies aimed at improving regenerative processes in humans and is predicated on >40 years of success with bone marrow transplants. Further progress will be accelerated by implementing knowledge about the molecular mechanisms that regulate regenerative processes in model organisms that naturally possess the ability to regenerate organs and/or appendages. In this review we summarize the current knowledge about the signaling pathways that regulate regeneration of amphibian and fish appendages, fish heart, and mammalian liver and skeletal muscle. While the cellular mechanisms and the cell types involved in regeneration of these systems vary widely, it is evident that shared signals are involved in tissue regeneration. Signals provided by the immune system appear to act as triggers of many regenerative processes. Subsequently, pathways that are best known for their importance in regulating embryonic development, in particular fibroblast growth factor (FGF) and Wnt/beta-catenin signaling (as well as others), are required for progenitor cell formation or activation and for cell proliferation and specification leading to tissue regrowth. Experimental activation of these pathways or interference with signals that inhibit regenerative processes can augment or even trigger regeneration in certain contexts.

Large-scale biophysics: ion flows and regeneration

Regeneration requires exquisite orchestration of growth and morphogenesis. A powerful but still largely mysterious system of biophysical signals functions during regeneration, embryonic development and neoplasm. Ion transporters generate pH and voltage gradients, as well as ion fluxes, regulating proliferation, differentiation and migration. Endogenous bioelectrical signals are implicated in the control of wound healing, limb development, left-right patterning and spinal cord regeneration. Recent advances in molecular biology and imaging technology have allowed unprecedented insight into the sources and downstream consequences of ion flows. In complement to the current focus on molecular genetics and stem cell biology, artificial modulation of bioelectrical signals in somatic tissues is a powerful modality that might result in profound advances in understanding and augmentation of regenerative capacity.

H+ pump-dependent changes in membrane voltage are an early mechanism necessary and sufficient to induce Xenopus tail regeneration

In many systems, ion flows and long-term endogenous voltage gradients regulate patterning events, but molecular details remain mysterious. To establish a mechanistic link between biophysical events and regeneration, we investigated the role of ion transport during Xenopus tail regeneration. We show that activity of the V-ATPase H(+) pump is required for regeneration but not wound healing or tail development. The V-ATPase is specifically upregulated in existing wound cells by 6 hours post-amputation. Pharmacological or molecular genetic loss of V-ATPase function and the consequent strong depolarization abrogates regeneration without inducing apoptosis. Uncut tails are normally mostly polarized, with discrete populations of depolarized cells throughout. After amputation, the normal regeneration bud is depolarized, but by 24 hours post-amputation becomes rapidly repolarized by the activity of the V-ATPase, and an island of depolarized cells appears just anterior to the regeneration bud. Tail buds in a non-regenerative ;refractory' state instead remain highly depolarized relative to uncut or regenerating tails. Depolarization caused by V-ATPase loss-of-function results in a drastic reduction of cell proliferation in the bud, a profound mispatterning of neural components, and a failure to regenerate. Crucially, induction of H(+) flux is sufficient to rescue axonal patterning and tail outgrowth in otherwise non-regenerative conditions. These data provide the first detailed mechanistic synthesis of bioelectrical, molecular and cell-biological events underlying the regeneration of a complex vertebrate structure that includes spinal cord, and suggest a model of the biophysical and molecular steps underlying tail regeneration. Control of H(+) flows represents a very important new modality that, together with traditional biochemical approaches, may eventually allow augmentation of regeneration for therapeutic applications.