Identification of genes induced in regenerating Xenopus tadpole tails by using the differential display method

Inflammation and immunity in organ regeneration

The ability of vertebrates to regenerate amputated appendages is increasingly well-understood at the cellular level. Cells mediating an innate immune response and inflammation in the injured tissues are a prominent feature of the limb prior to formation of a regeneration blastema, with macrophage activity necessary for blastema growth and successful development of the new limb. Studies involving either anti-inflammatory or pro-inflammatory agents suggest that the local inflammation produced by injury and its timely resolution are both important for regeneration, with blastema patterning inhibited in the presence of unresolved inflammation. Various experiments with Xenopus larvae at stages where regenerative competence is declining show improved digit formation after treatment with certain immunosuppressive, anti-inflammatory, or antioxidant agents. Similar work with the larval Xenopus tail has implicated adaptive immunity with regenerative competence and suggests a requirement for regulatory T cells in regeneration, which also occurs in many systems of tissue regeneration. Recent analyses of the human nail organ indicate a capacity for local immune tolerance, suggesting roles for adaptive immunity in the capacity for mammalian appendage regeneration. New information and better understanding regarding the neuroendocrine-immune axis in the response to stressors, including amputation, suggest additional approaches useful for investigating effects of the immune system during repair and regeneration.

Acute phase response in amputated tail stumps and neural tissue-preferential expression in tail bud embryos of the Xenopus neuronal pentraxin I gene

Regeneration of lost organs involves complex processes, including host defense from infection and rebuilding of lost tissues. We previously reported that Xenopus neuronal pentraxin I (xNP1) is expressed preferentially in regenerating Xenopus laevis tadpole tails. To evaluate xNP1 function in tail regeneration, and also in tail development, we analyzed xNP1 expression in tailbud embryos and regenerating/healing tails following tail amputation in the 'regeneration' period, as well as in the 'refractory' period, when tadpoles lose their tail regenerative ability. Within 10 h after tail amputation, xNP1 was induced at the amputation site regardless of the tail regenerative ability, suggesting that xNP1 functions in acute phase responses. xNP1 was widely expressed in regenerating tails, but not in the tail buds of tailbud embryos, suggesting its possible role in the immune response/healing after an injury. xNP1 expression was also observed in neural tissues/primordia in tailbud embryos and in the spinal cord in regenerating/healing tails in both periods, implying its possible roles in neural development or function. Moreover, during the first 48 h after amputation, xNP1 expression was sustained at the spinal cord of tails in the 'regeneration' period tadpoles, but not in the 'refractory' period tadpoles, suggesting that xNP1 expression at the spinal cord correlates with regeneration. Our findings suggest that xNP1 is involved in both acute phase responses and neural development/functions, which is unique compared to mammalian pentraxins whose family members are specialized in either acute phase responses or neural functions.

Subtractive screen of potential limb regeneration related genes from Pachytriton brevipes

Regeneration capacity varies greatly among different animal species. In vertebrate, amphibian especially the Urodela, has been used as a powerful model system to study the mechanism of tissue regeneration because of the strong ability to regenerate their damaged or lost appendages. Pachytriton brevipes, a species of newt, which is widely distributed in south of China, can completely restore their damaged limbs within several months. In this study, we use modified suppression subtractive hybridization assay and dot-blot screening to identify candidate genes involved in tissue regeneration in P. brevipes. We successfully isolated 81 ESTs from a forward regeneration subtraction library. And we further verified the differential expression of four candidate genes, Rpl11, Cirbp, Ag2 and Trimx, between regenerating blastema and non-regeneration tissues by in situ hybridization. These genes were also be further characterized by phylogenetic and bioinformatic analysis. In general, we provided a comparative experimental approach to study the mechanisms of vertebrate regeneration.

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.

Gene expression profiles of lens regeneration and development in Xenopus laevis

Seven hundred and thirty-four unique genes were recovered from a cDNA library enriched for genes up-regulated during the process of lens regeneration in the frog Xenopus laevis. The sequences represent transcription factors, proteins involved in RNA synthesis/processing, components of prominent cell signaling pathways, genes involved in protein processing, transport, and degradation (e.g., the ubiquitin/proteasome pathway), matrix metalloproteases (MMPs), as well as many other proteins. The findings implicate specific signal transduction pathways in the process of lens regeneration, including the FGF, TGF-beta, MAPK, Retinoic acid, Wnt, and hedgehog signaling pathways, which are known to play important roles in eye/lens development and regeneration in various systems. In situ hybridization revealed that the majority of genes recovered are expressed during embryogenesis, including in eye tissues. Several novel genes specifically expressed in lenses were identified. The suite of genes was compared to those up-regulated in other regenerating tissues/organisms, and a small degree of overlap was detected.

Suppression of the immune response potentiates tadpole tail regeneration during the refractory period

Regenerative ability varies depending on animal species and developmental stage, but the factors that determine this variability remain unclear. Although Xenopus laevis tadpole tails possess high regenerative ability, this is transiently lost during the ;refractory period'. Here, we show that tail amputation evokes different immune responses in wound tail stumps between the ;refractory' and ;regeneration' periods: there was delayed or prolonged expression of some immune-related genes in the refractory period, whereas there was no obvious or transient expression of other immune-related genes in the regeneration periods. In addition, immune suppression induced by either immunosuppressant treatment or immune cell depletion by knockdown of PU.1 significantly restored regenerative ability during the refractory period. These findings indicate that immune responses have a crucial role in determining regenerative ability in Xenopus tadpole tails.

Tail regeneration in the Xenopus tadpole

The tail of the Xenopus tadpole contains major axial structures, including a spinal cord, notochord and myotomes, and regenerates within 2 weeks following amputation. The tail regeneration in Xenopus can provide insights into the molecular basis of the regeneration mechanism. The regenerated tail has some differences from the normal tail, including an immature spinal cord and incomplete segmentation of the muscle masses. Lineage analyses have suggested that the tail tissues are reconstructed with lineage-restricted stem cells derived from their own tissues in clear contrast to urodele regeneration, in which multipotent blastema cells derived from differentiated cells play a major role. Comprehensive gene expression analyses resulted in the identification of a panel of genes involved in sequential steps of the regeneration. Manipulation of genes' activities suggested that the tail regeneration is regulated through several major signaling pathways.

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.

Global analysis of gene expression in Xenopus hindlimbs during stage-dependent complete and incomplete regeneration

Xenopus laevis tadpoles are capable of limb regeneration after amputation, in a process that initially involves the formation of a blastema. However, Xenopus has full regenerative capacity only through premetamorphic stages. We have used the Affymetrix Xenopus laevis Genome Genechip microarray to perform a large-scale screen of gene expression in the regeneration-complete, stage 53 (st53), and regeneration-incomplete, stage 57 (st57), hindlimbs at 1 and 5 days postamputation. Through an exhaustive reannotation of the Genechip and a variety of comparative bioinformatic analyses, we have identified genes that are differentially expressed between the regeneration-complete and -incomplete stages, detected the transcriptional changes associated with the regenerating blastema, and compared these results with those of other regeneration researchers. We focus particular attention on striking transcriptional activity observed in genes associated with patterning, stress response, and inflammation. Overall, this work provides the most comprehensive views yet of a regenerating limb and different transcriptional compositions of regeneration-competent and deficient tissues.

Cells of cutaneous immunity in Xenopus: studies during larval development and limb regeneration

The anuran Xenopus laevis is an experimental model for vertebrate development, immunology, and regenerative biology. Using histochemistry and immunohistochemistry (IHC) we examined embryonic, larval, and postmetamorphic Xenopus skin for the presence of dendritic cells (DCs), Langerhans cells (LCs), and dendritic epidermal T cells (DETCs), all components of cutaneous immunity that have been implicated in skin repair and regeneration. Cells expressing three markers for dendritic and Langerhans cells (formalin-resistant ATPase activity, major histocompatibility complex [MHC] class II antigens, and vimentin) and having morphology like that of these cells first appeared during late embryonic stages, becoming abundant by prometamorphosis. Cells positive for these markers were also numerous in the wound epithelia of regenerating hindlimbs at both early and late larval stages. Cells tentatively identified as DETCs were found, beginning at early larval stages, using IHC with antibodies against heterologous CD3epsilon chain and T-cell receptor delta. Further characterization and work with the putative DCs, LCs, and DETCs demonstrated here will allow not only greater understanding of the amphibian immune system, but also further elucidation of regenerative growth and scarring.

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

The Xenopus tadpole is able to regenerate its tail, including skin, muscle, notochord, spinal cord and neurons and blood vessels. This process requires rapid tissue growth and morphogenesis. Here we show that a focus of apoptotic cells appears in the regeneration bud within 12 h of amputation. Surprisingly, when caspase-3 activity is specifically inhibited, regeneration is abolished. This is true of tails both before and after the refractory period. Programmed cell death is only required during the first 24 h after amputation, as later inhibition has no effect on regeneration. Inhibition of caspase-dependent apoptosis results in a failure to induce proliferation in the growth zone, a mispatterning of axons in the regenerate, and the appearance of ectopic otoliths in the neural tube, in the context of otherwise normal continued development of the larva. Larvae amputated during the refractory stage exhibit a much broader domain of caspase-3-positive cells, suggesting a window for the amount of apoptosis that is compatible with normal regeneration. These data reveal novel roles for apoptosis in development and indicate that a degree of apoptosis is an early and obligate component of normal tail regeneration, suggesting the possibility of the existence of endogenous inhibitory cells that must be destroyed by programmed cell death for regeneration to occur.

Early regeneration genes: Building a molecular profile for shared expression in cornea-lens transdifferentiation and hindlimb regeneration in Xenopus laevis

Recent studies in Xenopus laevis have begun to compare gene expression during regeneration with that of the original development of specific structures (e.g., the hindlimb and lens), while other studies have sought differences in gene expression between regeneration-competent and regeneration-incompetent stages. To determine whether there are any similarities between the regeneration of different structures, we have used a differential screen to seek shared early gene expression between hindlimb regeneration and cornea-lens transdifferentiation in the Xenopus tadpole. We have isolated 13 clones representing genes whose expression is up-regulated within the first few days of both regenerating processes and which are not demonstrably up-regulated in the context of basic wound healing. Furthermore, all of these genes also show prominent late embryonic expression. The expression patterns and putative identities of all 13 genes are presented, and a model is considered that allows us to characterize and profile important changes in gene expression, which might be shared among various regenerating and developmental systems.

Identification of cDNAs associated with late dedifferentiation in adult newt forelimb regeneration

Epimorphic limb regeneration in the adult newt involves the dedifferentiation of differentiated cells to yield a pluripotent blastemal cell. These mesenchymal-like cells proliferate and subsequently respond to patterning and differentiation cues to form a new limb. Understanding the dedifferentiation process requires the selective identification of dedifferentiating cells within the heterogeneous population of cells in the regenerate. In this study, representational differences analysis was used to produce an enriched population of dedifferentiation-associated cDNA fragments. Fifty-nine unique cDNA fragments were identified, sequenced, and analyzed using bioinformatics tools and databases. Some of these clones demonstrate significant similarity to known genes in other species. Other clones can be linked by homology to pathways previously implicated in the dedifferentiation process. These data will form the basis for further analyses to elucidate the role of candidate genes in the dedifferentiation process during newt forelimb regeneration.

Regeneration in MRL mice: further genetic loci controlling the ear hole closure trait using MRL and M.m. Castaneus mice

The MRL mouse has been shown to display an epimorphic regenerative response after ear hole punching leading to complete closure within 30 days and cartilage regrowth. The regenerative capacity of the MRL has also been seen after a severe cryoinjury to the heart leads to complete healing without scarring and functional myocardium. The wound healing ear hole closure response that occurs in MRL mice has been shown to be genetically controlled. We have previously identified 11 quantitative trait loci (QTL) that govern healing in an intercross of (MRL x C57BL/6 J) mice. However, it is desirable to use another poorly healing mouse strain to elucidate the full range of genetic factors that affect this important process. In the current study, we have used an inbred subspecies of the mouse, M. castaneus, and have confirmed a number of loci identified previously. In addition, we report three new healing QTL. Furthermore, in this strain combination, we note a strong sexual dimorphism also observed in the MRL x C57BL/6 cross, both in the healing trait and in the QTL that control it.