Effects on adult newt limb regeneration of partial and complete skin flaps over the amputation surface

The accessory limb model: an alternative experimental system of limb regeneration

Accessory limb model (ALM) was developed as an experimental model and functional assay for limb regeneration. The ALM provides several ways to identify pathways and test for signaling molecules that regulate limb regeneration. Here, we summarize the history of the ALM and describe the specific details involved in inducing ectopic blastemas and limbs from a skin wound on the side of the arm.

Nonthermal atmospheric pressure plasma enhances mouse limb bud survival, growth, and elongation

The enhanced differentiation of mesenchymal cells into chondrocytes or osteoblasts is of paramount importance in tissue engineering and regenerative therapies. A newly emerging body of evidence demonstrates that appendage regeneration is dependent on reactive oxygen species (ROS) production and signaling. Thus, we hypothesized that mesenchymal cell stimulation by nonthermal (NT)-plasma, which produces and induces ROS, would (1) promote skeletal cell differentiation and (2) limb autopod development. Stimulation with a single treatment of NT-plasma enhanced survival, growth, and elongation of mouse limb autopods in an in vitro organ culture system. Noticeable changes included enhanced development of digit length and definition of digit separation. These changes were coordinated with enhanced Wnt signaling in the distal apical epidermal ridge (AER) and presumptive joint regions. Autopod development continued to advance for approximately 144 h in culture, seemingly overcoming the negative culture environment usually observed in this in vitro system. Real-time quantitative polymerase chain reaction analysis confirmed the up-regulation of chondrogenic transcripts. Mechanistically, NT-plasma increased the number of ROS positive cells in the dorsal epithelium, mesenchyme, and the distal tip of each phalange behind the AER, determined using dihydrorhodamine. The importance of ROS production/signaling during development was further demonstrated by the stunting of digital outgrowth when anti-oxidants were applied. Results of this study show NT-plasma initiated and amplified ROS intracellular signaling to enhance development of the autopod. Parallels between development and regeneration suggest that the potential use of NT-plasma could extend to both tissue engineering and clinical applications to enhance fracture healing, trauma repair, and bone fusion.

A bioinformatics expert system linking functional data to anatomical outcomes in limb regeneration

Amphibians and molting arthropods have the remarkable capacity to regenerate amputated limbs, as described by an extensive literature of experimental cuts, amputations, grafts, and molecular techniques. Despite a rich history of experimental efforts, no comprehensive mechanistic model exists that can account for the pattern regulation observed in these experiments. While bioinformatics algorithms have revolutionized the study of signaling pathways, no such tools have heretofore been available to assist scientists in formulating testable models of large-scale morphogenesis that match published data in the limb regeneration field. Major barriers preventing an algorithmic approach are the lack of formal descriptions for experimental regenerative information and a repository to centralize storage and mining of functional data on limb regeneration. Establishing a new bioinformatics of shape would significantly accelerate the discovery of key insights into the mechanisms that implement complex regeneration. Here, we describe a novel mathematical ontology for limb regeneration to unambiguously encode phenotype, manipulation, and experiment data. Based on this formalism, we present the first centralized formal database of published limb regeneration experiments together with a user-friendly expert system tool to facilitate its access and mining. These resources are freely available for the community and will assist both human biologists and artificial intelligence systems to discover testable, mechanistic models of limb regeneration.

Initiating a regenerative response; cellular and molecular features of wound healing in the cnidarian Nematostella vectensis

Background

Wound healing is the first stage of a series of cellular events that are necessary to initiate a regenerative response. Defective wound healing can block regeneration even in animals with a high regenerative capacity. Understanding how signals generated during wound healing promote regeneration of lost structures is highly important, considering that virtually all animals have the ability to heal but many lack the ability to regenerate missing structures. Cnidarians are the phylogenetic sister taxa to bilaterians and are highly regenerative animals. To gain a greater understanding of how early animals generate a regenerative response, we examined the cellular and molecular components involved during wound healing in the anthozoan cnidarian Nematostella vectensis.

Results

Pharmacological inhibition of extracellular signal-regulated kinases (ERK) signaling blocks regeneration and wound healing in Nematostella. We characterized early and late wound healing events through genome-wide microarray analysis, quantitative PCR, and in situ hybridization to identify potential wound healing targets. We identified a number of genes directly related to the wound healing response in other animals (metalloproteinases, growth factors, transcription factors) and suggest that glycoproteins (mucins and uromodulin) play a key role in early wound healing events. This study also identified a novel cnidarian-specific gene, for a thiamine biosynthesis enzyme (vitamin B synthesis), that may have been incorporated into the genome by lateral gene transfer from bacteria and now functions during wound healing. Lastly, we suggest that ERK signaling is a shared element of the early wound response for animals with a high regenerative capacity.

Conclusions

This research describes the temporal events involved during Nematostella wound healing, and provides a foundation for comparative analysis with other regenerative and non-regenerative species. We have shown that the same genes that heal puncture wounds are also activated after oral-aboral bisection, indicating a clear link with the initiation of regenerative healing. This study demonstrates the strength of using a forward approach (microarray) to characterize a developmental phenomenon (wound healing) at a phylogenetically important crossroad of animal evolution (cnidarian-bilaterian ancestor). Accumulation of data on the early wound healing events across numerous systems may provide clues as to why some animals have limited regenerative abilities.

Wound epithelium function in axolotl limb regeneration

Axolotls (Ambystoma mexicanum) have the ability to regenerate amputated limbs. The amputation surface is promptly covered by wound epithelium (WE), which is significant for the initiation of limb regeneration. In the present study, we investigated the formation of functional WE by analyzing the migration of WE after amputation. In the center of the amputation surface, epithelial cells migrated from surrounding epidermis to form WE. Therefore, WE around the center of the amputation surface was composed of the cells with dorsal, ventral, anterior and posterior identities, and we tentatively called this WE with radial positional identities, "central WE". When regeneration was complete, central WE became the epidermis around the bifurcation between the first and second digits. In addition, when the artificial rotation of epidermis was performed before amputation, all examined limbs regenerated normally, and central WE formed the epidermis at the bifurcation between first and second digits, similarly to that in normal regeneration. On the basis of our observations, the most important factor for the initiation of regeneration is considered to be the discontinuity of positional identity existing in WE. It is possible that the location of bifurcation between first and second digits is specified by the positional discontinuity in WE.

Matrix metalloproteinases (MMPs) are required for wound closure and healing during larval leg regeneration in the flour beetle, Tribolium castaneum

Regenerative abilities are found ubiquitously among many metazoan taxa. To compare mechanisms underlying the initial stages of limb regeneration between insects and vertebrates, the roles of matrix metalloproteinases (MMPs) and fibroblast growth factor (FGF) signaling were investigated in the red flour beetle, Tribolium castaneum. RNA interference-mediated knockdown of MMP2 expression delayed wound healing and subsequent leg regeneration. Additionally, pairwise knockdown of MMP1/2 and MMP2/3, but not MMP1/3, resulted in inhibition of wound closure. Wound healing on the dorsal epidermis after injury was also delayed when MMPs were silenced. Our findings show that functionally redundant MMPs play key roles during limb regeneration and wound healing in Tribolium. This MMP-mediated wound healing is necessary for the subsequent formation of a blastema. In contrast, silencing of FGF receptor did not interfere with the initial stages of leg regeneration despite the alterations in tanning of the cuticle. Thus, insects and vertebrates appear to employ similar developmental processes for the initial stages of wound closure during limb regeneration, while the role of FGF in limb regeneration appears to be unique to vertebrates.

Regeneration: the ultimate example of wound healing

The outcome of wound repair in mammals is often characterized by fibrotic scaring. Vertebrates such as zebrafish, frogs, and salamanders not only heal scarlessly, but also can regenerate lost appendages. Decades of study on the process of animal regeneration has produced key insights into the mechanisms of how complex tissue is restored. By examining our current knowledge of regeneration, we can draw parallels with mammalian wound healing to identify the molecular determinants that produce such differing outcomes.

Gene expression patterns specific to the regenerating limb of the Mexican axolotl

Salamander limb regeneration is dependent upon tissue interactions that are local to the amputation site. Communication among limb epidermis, peripheral nerves, and mesenchyme coordinate cell migration, cell proliferation, and tissue patterning to generate a blastema, which will form missing limb structures. An outstanding question is how cross-talk between these tissues gives rise to the regeneration blastema. To identify genes associated with epidermis-nerve-mesenchymal interactions during limb regeneration, we examined histological and transcriptional changes during the first week following injury in the wound epidermis and subjacent cells between three injury types; 1) a flank wound on the side of the animal that will not regenerate a limb, 2) a denervated limb that will not regenerate a limb, and 3) an innervated limb that will regenerate a limb. Early, histological and transcriptional changes were similar between the injury types, presumably because a common wound-healing program is employed across anatomical locations. However, some transcripts were enriched in limbs compared to the flank and are associated with vertebrate limb development. Many of these genes were activated before blastema outgrowth and expressed in specific tissue types including the epidermis, peripheral nerve, and mesenchyme. We also identified a relatively small group of transcripts that were more highly expressed in innervated limbs versus denervated limbs. These transcripts encode for proteins involved in myelination of peripheral nerves, epidermal cell function, and proliferation of mesenchymal cells. Overall, our study identifies limb-specific and nerve-dependent genes that are upstream of regenerative growth, and thus promising candidates for the regulation of blastema formation.

The role of peripheral nerves in urodele limb regeneration

Nerve axons and the apical epidermal cap (AEC) are both essential for the formation of an accumulation blastema by amputated limbs of urodele salamanders. The AEC forms in the absence of axons, but is not maintained, and blastema formation fails. Growth stages of the blastema become nerve-independent for morphogenesis, but remain dependent on the nerve for blastema growth. Denervated growth stage blastemas form smaller than normal skeletal parts, owing to diminished mitosis, but form the full proximodistal array of skeletal elements. This difference in nerve dependency of morphogenesis and proliferation is hypothesized to be the result of a dependence of the AEC on nerves for blastema cell proliferation but not for blastema morphogenesis. Regenerating axons induce the synthesis and secretion of the anterior gradient protein (AGP) by distal Schwann cells during dedifferentiation and by the gland cells of the AEC during blastema growth stages. AGP promotes the regeneration of a denervated limb to digit stages when electroporated into the limb during dedifferentiation. Once a critical mass of blastema cells has been attained, the blastema can undergo morphogenesis in the absence of the nerve, but the regenerate will be a miniature, because the nerve is no longer inducing the AEC to carry out its AGP-mediated proliferative function. AGP expression by both Schwann cells and the AEC is induced by axons, but the nature of the inductive agent is unclear.

Mouse digit tip regeneration is mediated by fate-restricted progenitor cells

Regeneration of appendages is frequent among invertebrates as well as some vertebrates. However, in mammals this has been largely relegated to digit tip regeneration, as found in mice and humans. The regenerated structures are formed from a mound of undifferentiated cells called a blastema, found just below the site of amputation. The blastema ultimately gives rise to all of the tissues in the regenerate, excluding the epidermis, and has classically been thought of as a homogenous pool of pluripotent stem cells derived by dedifferentiation of stump tissue, although this has never been directly tested in the context of mammalian digit tip regeneration. Successful digit tip regeneration requires that the level of amputation be within the nail bed and depends on expression of Msx1. Because Msx1 is strongly expressed in the nail bed mesenchyme, it has been proposed that the Msx1-expressing cells represent a pluripotent cell population for the regenerating digit. In this report, we show that Msx1 is dynamically expressed during digit tip regeneration, and it does not mark a pluripotent stem cell population. Moreover, we show that both the ectoderm and mesoderm contain fate-restricted progenitor populations that work in concert to regenerate their own lineages within the digit tip, supporting the hypothesis that the blastema is a heterogeneous pool of progenitor cells.

The aneurogenic limb identifies developmental cell interactions underlying vertebrate limb regeneration

The removal of the neural tube in salamander embryos allows the development of nerve-free aneurogenic limbs. Limb regeneration is normally nerve-dependent, but the aneurogenic limb regenerates without nerves and becomes nerve-dependent after innervation. The molecular basis for these tissue interactions is unclear. Anterior Gradient (AG) protein, previously shown to rescue regeneration of denervated limbs and to act as a growth factor for cultured limb blastemal cells, is expressed throughout the larval limb epidermis and is down-regulated by innervation. In an aneurogenic limb, the level of AG protein remains high in the epidermis throughout development and regeneration, but decreases after innervation following transplantation to a normal host. Aneurogenic epidermis also shows a fivefold difference in secretory gland cells, which express AG protein. The persistently high expression of AG in the epithelial cells of an aneurogenic limb ensures that regeneration is independent of the nerve. These findings provide an explanation for this classical problem, and identify regulation of the epidermal niche by innervation as a distinctive developmental mechanism that initiates the nerve dependence of limb regeneration. The absence of this regulation during anuran limb development might suggest that it evolved in relation to limb regeneration.

Different requirement for Wnt/β-catenin signaling in limb regeneration of larval and adult Xenopus

BACKGROUND

In limb regeneration of amphibians, the early steps leading to blastema formation are critical for the success of regeneration, and the initiation of regeneration in an adult limb requires the presence of nerves. Xenopus laevis tadpoles can completely regenerate an amputated limb at the early limb bud stage, and the metamorphosed young adult also regenerates a limb by a nerve-dependent process that results in a spike-like structure. Blockage of Wnt/β-catenin signaling inhibits the initiation of tadpole limb regeneration, but it remains unclear whether limb regeneration in young adults also requires Wnt/β-catenin signaling.

METHODOLOGY/PRINCIPAL FINDINGS

We expressed heat-shock-inducible (hs) Dkk1, a Wnt antagonist, in transgenic Xenopus to block Wnt/β-catenin signaling during forelimb regeneration in young adults. hsDkk1 did not inhibit limb regeneration in any of the young adult frogs, though it suppressed Wnt-dependent expression of genes (fgf-8 and cyclin D1). When nerve supply to the limbs was partially removed, however, hsDkk1 expression blocked limb regeneration in young adult frogs. Conversely, activation of Wnt/β-catenin signaling by a GSK-3 inhibitor rescued failure of limb-spike regeneration in young adult frogs after total removal of nerve supply.

CONCLUSIONS/SIGNIFICANCE

In contrast to its essential role in tadpole limb regeneration, our results suggest that Wnt/β-catenin signaling is not absolutely essential for limb regeneration in young adults. The different requirement for Wnt/β-catenin signaling in tadpoles and young adults appears to be due to the projection of nerve axons into the limb field. Our observations suggest that nerve-derived signals and Wnt/β-catenin signaling have redundant roles in the initiation of limb regeneration. Our results demonstrate for the first time the different mechanisms of limb regeneration initiation in limb buds (tadpoles) and developed limbs (young adults) with reference to nerve-derived signals and Wnt/β-catenin signaling.

Gene expression profile of the regeneration epithelium during axolotl limb regeneration

Urodele amphibians are unique among adult vertebrates in their ability to regenerate missing limbs. The process of limb regeneration requires several key tissues including a regeneration-competent wound epidermis called the regeneration epithelium (RE). We used microarray analysis to profile gene expression of the RE in the axolotl, a Mexican salamander. A list of 125 genes and expressed sequence tags (ESTs) showed a ≥1.5-fold expression in the RE than in a wound epidermis covering a lateral cuff wound. A subset of the RE ESTs and genes were further characterized for expression level changes over the time-course of regeneration. This study provides the first large scale identification of specific gene expression in the RE.

Ex vivo generation of a functional and regenerative wound epithelium from axolotl (Ambystoma mexicanum) skin

Urodele amphibians (salamanders) are unique among adult vertebrates in their ability to regenerate structurally complete and fully functional limbs. Regeneration is a stepwise process that requires interactions between keratinocytes, nerves and fibroblasts. The formation of a wound epithelium covering the amputation site is an early and necessary event in the process but the molecular mechanisms that underlie the role of the wound epithelium in regeneration remain unclear. We have developed an ex vivo model that recapitulates many features of in vivo wound healing. The model comprises a circular explant of axolotl (Ambystoma mexicanum) limb skin with a central circular, full thickness wound. Re-epithelialization of the wound area is rapid (typically <11 h) and is dependent on metalloproteinase activity. The ex vivo wound epithelium is viable, responds to neuronal signals and is able to participate in ectopic blastema formation and limb regeneration. This ex vivo model provides a reproducible and tractable system in which to study the cellular and molecular events that underlie wound healing and regeneration.

Amphibians as research models for regenerative medicine

The ability to regenerate bone across a critical size defect would be a marked clinical advance over current methods for dealing with such structural gaps. Here, we briefly review the development of limb bones and the mandible, the regeneration of urodele limbs after amputation, and present evidence that urodele and anuran amphibians represent a valuable research model for the study of segment defect regeneration in both limb bones and mandible.

IGF signaling between blastema and wound epidermis is required for fin regeneration

In mammals, the loss of a limb is irreversible. By contrast, urodele amphibians and teleost fish are capable of nearly perfect regeneration of lost appendages. This ability depends on direct interaction between the wound epithelium and mesenchymal progenitor cells of the blastema. It has been known for decades that contact between the wound epithelium and the underlying blastema is essential for successful regeneration. However, the underlying mechanisms are poorly understood. Here, we show that upon amputation the blastema induces expression of the ligand Igf2b, which then activates IGF signaling specifically in cells of the adjacent apical epithelium. Inhibition of IGF signaling by either morpholino antisense technology, or by specific chemical inhibitors of Igf1 receptor function NVP-AEW541 and NVP-ADW742, impairs fin regeneration. At the cellular level, this block in regeneration is reflected by a lack of the distinctive basal epithelium, increased apoptosis in the wound epidermis and reduced proliferation of blastema cells. Furthermore, induction of the blastemal and wound epidermal markers cannot be supported in the absence of IGF signaling. These data provide evidence that Igf2b expressed in the blastema promotes the properties of the adjacent wound epidermis, which subsequently are necessary for blastema function. Thus, IGF signaling upregulated upon fin amputation represents a signal from the blastema to the wound epithelium, a crucial step in appendage 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.

Dynamic changes after murine digit amputation: the MRL mouse digit shows waves of tissue remodeling, growth, and apoptosis

Digit regrowth following amputation injury proximal to the first phalangeal joint is not a property of mammalian wound healing. However, the regenerative potential observed in the MRL mouse invites a reexamination of this rule. In this study, healing was assessed in three mouse strains after amputation midway through the second phalangeal bone. Three distinct outcomes were observed though evidence for regrowth was observed only in the MRL mouse. Here, a blastema-like structure was seen along with apparent chondrogenesis, consistent with a histological profile of a regenerative response to injury. Analysis of trichrome staining and basement membrane changes, proliferation and apoptosis indicated that these processes contributed to the formation of new digit tissue. On the other hand, SW and B6 digits did not show evidence of growth with little mesenchymal BrdU incorporation or phosphorylation of H3.

Comparative aspects of animal regeneration

Most but not all phyla include examples of species that are able to regenerate large sections of the body plan. The mechanisms underlying regeneration on this scale are currently being studied in a variety of contexts in both vertebrates and invertebrates. Regeneration generally involves the formation of a wound epithelium after transection or injury, followed by the generation of regenerative progenitor cells and morphogenesis to give the regenerate. Common mechanisms may exist in relation to each of these aspects. For example, the initial proliferation of progenitor cells often depends on the nerve supply, whereas morphogenesis reflects the generation of positional disparity between adjacent cells-the principle of intercalation. These mechanisms are reviewed here across a range of contexts. We also consider the evolutionary origins of regeneration and how regeneration may relate to both agametic reproduction and to ontogeny.

Molecular basis for the nerve dependence of limb regeneration in an adult vertebrate

The limb blastemal cells of an adult salamander regenerate the structures distal to the level of amputation, and the surface protein Prod 1 is a critical determinant of their proximodistal identity. The anterior gradient protein family member nAG is a secreted ligand for Prod 1 and a growth factor for cultured newt blastemal cells. nAG is sequentially expressed after amputation in the regenerating nerve and the wound epidermis-the key tissues of the stem cell niche-and its expression in both locations is abrogated by denervation. The local expression of nAG after electroporation is sufficient to rescue a denervated blastema and regenerate the distal structures. Our analysis brings together the positional identity of the blastema and the classical nerve dependence of limb regeneration.

Initiation of limb regeneration: the critical steps for regenerative capacity

While urodele amphibians (newts and salamanders) can regenerate limbs as adults, other tetrapods (reptiles, birds and mammals) cannot and just undergo wound healing. In adult mammals such as mice and humans, the wound heals and a scar is formed after injury, while wound healing is completed without scarring in an embryonic mouse. Completion of regeneration and wound healing takes a long time in regenerative and non-regenerative limbs, respectively. However, it is the early steps that are critical for determining the extent of regenerative response after limb amputation, ranging from wound healing with scar formation, scar-free wound healing, hypomorphic limb regeneration to complete limb regeneration. In addition to the accumulation of information on gene expression during limb regeneration, functional analysis of signaling molecules has recently shown important roles of fibroblast growth factor (FGF), Wnt/beta-catenin and bone morphogenic protein (BMP)/Msx signaling. Here, the routine steps of wound healing/limb regeneration and signaling molecules specifically involved in limb regeneration are summarized. Regeneration of embryonic mouse digit tips and anuran amphibian (Xenopus) limbs shows intermediate regenerative responses between the two extremes, those of adult mammals (least regenerative) and urodele amphibians (more regenerative), providing a range of models to study the various abilities of limbs to regenerate.

Transgenic Xenopus with prx1 limb enhancer reveals crucial contribution of MEK/ERK and PI3K/AKT pathways in blastema formation during limb regeneration

Understanding the mechanisms that control amphibian limb regeneration should allow us to decipher the critical differences between amphibians and humans, which have the limited ability of organ regeneration. However, many issues at the cellular and molecular levels still remain unresolved. We have generated a transgenic Xenopus laevis line that expresses green fluorescent protein (GFP) under the control of mouse prx1 limb enhancer, which directs reporter gene expression in limb mesenchyme in mice, and found that GFP accumulated in blastemal mesenchymal cells of the transgenic froglets after limb amputation. Thus, this transgenic line should provide a new approach to gain insights into the cellular dynamics and signaling pathways involved in limb blastema formation. We have also developed a culture system for forelimb explants of froglets and found that treatment with inhibitors of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated protein kinase (ERK) kinase 1/2 (MEK1/2) and phosphatidylinositol 3-kinase (PI3K) repressed GFP expression. These effects were partially reversible, and down-regulation of GFP was associated with inhibition of cell-cycle progression and induction of ectopic apoptosis. In addition, we found that ERK1/2 and AKT, downstream mediators of MEK1/2 and PI3K pathways, were activated in amputated forelimb stumps. These results demonstrate that MEK/ERK and PI3K/AKT pathways regulate limb blastema formation in the X. laevis froglet.

Timing the commitment to a wound-healing response of denervated limb stumps in the adult newt, Notophthalmus viridescens

Adult newt limbs that are denervated 1 day after amputation undergo a wound-healing response and, although they become reinnervated, will not regenerate unless reinjured. Experiments were designed to determine when denervated limb stumps of adult newts become committed to a wound-healing response. In Experiment I, limbs were amputated and denervated 1 day after amputation. On days 7, 14, 21, 28, and 35 days after the initial amputation and denervation, stumps were reamputated to remove the distal tip. This design varied the time the distal stump was devoid of nerves before reamputation. None of the limbs reamputated at 7 days regenerated. About half of the limbs reamputated at 14 days regenerated and almost all of those reamputated at days 21, 28, and 35 regenerated. In Experiment II, limbs were denervated and then amputated on day 7 or 14, at two different levels. Limbs with a short stump became innervated earlier and regeneration occurred more frequently at both levels of amputation. The results of these experiments show that denervated limb stumps become committed to a wound-healing response between days 7 and 14 after amputation/denervation. If sufficient nerves arrive before day 7, a regeneration response is initiated. If the stump is denervated for 14 days or longer, commitment to wound healing occurs and ingrowing nerves cannot initiate a regeneration response.

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.

Limb regeneration in higher vertebrates: developing a roadmap

We review what is known about amphibian limb regeneration from the prospective of developing strategies for the induction of regeneration in adult mammals. Prominent in urodele amphibian limb regeneration is the formation of a blastema of undifferentiated cells that goes on to reform the limb. The blastema shares many properties with the developing limb bud; thus, the outgrowth phase of regeneration can be thought of as cells going through development again, i.e., redevelopment. Getting to a redevelopment phase in mammals would be a major breakthrough given our extensive understanding of limb development. The formation of the blastema itself represents a transition phase in which limb cells respond to injury by dedifferentiating to become embryonic limb progenitor cells that can undergo redevelopment. During this phase, rapid wound closure is followed by the dedifferentiation of limb cells to form the blastema. Thus, the regeneration process can be divided into a wound-healing/dedifferentiation phase and a redevelopment phase, and we propose that the interface between the wound-healing response and gaining access to developmentally regulated programs (dedifferentiation) lies at the heart of the regeneration problem in mammals. In urodele amphibians, dedifferentiation can occur in all of the tissues of the limb; however, numerous studies lead us to focus on the epidermis, the dermis, and muscle as key regulators of regeneration. Among higher vertebrates, the digit tip in mammals, including humans, is regeneration-competent and offers a unique mammalian model for regeneration. Recent genetic studies in mice identify the Msx1 gene as playing a critical role in the injury response leading to digit tip regeneration. The results from regeneration studies ranging from amphibians to mammals can be integrated to develop a roadmap for mammalian regeneration that has as its focus understanding the phenomenon of dedifferentiation.

Expression profiles of elastase1 (NvElastaseI) and secretory leukocyte protease inhibitor (NvSLPI) during forelimb regeneration in adult Notophthalmus viridescens suggest a role in epithelial remodeling and delamination

Extracellular proteases and their inhibitors may regulate a number of important processes involved in forelimb regeneration in the adult newt, including epithelial remodeling, breakdown of extracellular matrix, and dedifferentiation. We have identified a newt homologue of human ElastaseI (NvElastaseI) and its potential inhibitor, SLPI (NvSLPI), and evaluated their spatial and temporal expression during limb regeneration. NvElastaseI is upregulated early in regeneration and is associated with subdermal and wound epithelial cells, suggesting an involvement in wound healing and the generation of the wound epithelium. Up until 15 days post-amputation, NvElastaseI is also scattered throughout the developing blastema and may have a role in the dedifferentiation of stump tissues. NvSLPI is found at the interface between the intact skin and the wound epithelium, and may limit NvElastaseI activity. NvSLPI is also expressed in dermal glands, and is likely involved in anti-microbial activity or function. Quite apart from regeneration, complementary patterns of expression of NvElastaseI and NvSLPI are associated with newt epithelial sloughing.

Appendage regeneration in adult vertebrates and implications for regenerative medicine

The regeneration of complex structures in adult salamanders depends on mechanisms that offer pointers for regenerative medicine. These include the plasticity of differentiated cells and the retention in regenerative cells of local cues such as positional identity. Limb regeneration proceeds by the local formation of a blastema, a growth zone of mesenchymal stem cells on the stump. The blastema can regenerate autonomously as a self-organizing system over variable linear dimensions. Here we consider the prospects for limb regeneration in mammals from this viewpoint.

Normal newt limb regeneration requires matrix metalloproteinase function

Newts regenerate lost limbs through a complex process involving dedifferentiation, migration, proliferation, and redifferentiation of cells proximal to the amputation plane. To identify the genes controlling these cellular events, we performed a differential display analysis between regenerating and nonregenerating limbs from the newt Notophthalmus viridescens. This analysis, coupled with a direct cloning approach, identified a previously unknown Notophthalmus collagenase gene (nCol) and three known matrix metalloproteinase (MMP) genes, MMP3/10a, MMP3/10b, and MMP9, all of which are upregulated within hours of limb amputation. MMP3/10b exhibits the highest and most ubiquitous expression and appears to account for the majority of the proteolytic activity in the limb as measured by gel zymography. By testing purified recombinant MMP proteins against potential substrates, we show that nCol is a true collagenase, MMP9 is a gelatinase, MMP3/10a is a stromelysin, and MMP3/10b has an unusually broad substrate profile, acting both as a stromelysin and noncanonical collagenase. Exposure of regenerating limbs to the synthetic MMP inhibitor GM6001 produces either dwarfed, malformed limb regenerates or limb stumps with distal scars. These data suggest that MMPs are required for normal newt limb regeneration and that MMPs function, in part, to prevent scar formation during the regenerative process.

Regeneration of tubular complex is promoted by a free space

OBJECTIVES

Regeneration of the pancreas is initiated by the tubular complexes that consist of a cluster of epithelia surrounded by the mesenchymal cells. They have the potential to become pancreatic lobes, but their growth stops before the complete regeneration of the organ. To elucidate the possibility that we could promote the regeneration of the pancreas, the potential for growth or differentiation of tubular complex was analyzed.

METHODS

The intact lobes were growing around the silk knot after ligation of the pancreas in adult mice. To develop this reaction to a quantitative assay, tubular complexes were induced on the silk strings in the pancreas and were growing into a free space under the silicon cover. The proliferation and differentiation of new lobes with or without the space were analyzed.

RESULTS

The number of tubular complexes, which express PDX-1, was increased 5.4 times by the space effect. The proliferating cell nuclear antigen labeling index of acinar cells was 1.7 times stimulated, but that of tubular complex was not changed. The amputated pancreas recovered 49.5% of the resected part under the silicon cover; however, it remained the same weight without the cover.

CONCLUSION

The proliferation and differentiation of tubular complex are promoted by a free space.

Vascular supplies differ in regenerating and nonregenerating amputated rodent digits

Bone regenerates following amputation through the level of the nail, but bone is capped following amputation through more proximal levels. Because osteogenesis requires an ample blood supply, we postulated that a restricted vascular supply might be correlated with restricted regenerative ability at proximal levels. More than 40 rats and mice were injected with ink or resin to visualize vascular supplies of intact, regenerating, and nonregenerating rat and mouse digits. Ink-injected specimens were viewed as histological sections or cleared whole mounts. Partially digested resin casts were viewed using scanning electron microscopy. Contrary to our hypothesis, prior to amputation, proximal sites are more vascular than distal sites. At both proximal and distal levels, endosteal and periosteal vascular systems are evident. However, in proximal phalanges, additional subcutaneous and dermal layers encircle the bone. Beneath the distal nail, these layers are absent, and a single layer of vessels provides both periosteal and cutaneous supplies. After amputation at both levels, new vessels sprout profusely in osteogenic areas of both endosteum and periosteum. However, at proximal levels, the additional hypodermal and dermal vessels contribute to a vascular plexus that, paradoxically, may impair bone regrowth by contributing to the formation of dermal scar rather than bone.

Forelimb spike regeneration inXenopus laevis: Testing for adaptiveness

Experiments were designed to test adaptability of forelimb spike regenerates in Xenopus laevis froglets. The results show that when amputation is at the radius/ulna level, regeneration occurs in 100% of the cases and a single spike of cartilage is the result. The spike regenerates originating from radius/ulna level amputations can be used for feeding and froglet growth is only minimally compromised by the spike. The spike grows in length as the froglet body grows and thus is in homeostasis with the body. The spike develops nuptial pad tissue in reproductively mature males and is occasionally molted, indicating responsiveness to gonadal and thyroid hormones. Finally, and most important, the spike can be used for amplexus and successful mating. In contrast, spikes originating from humerus level amputations were considerably shorter and regeneration from that limb level was less frequent. When amputation was at the body wall regeneration did not occur.

Amphibian regeneration and stem cells

Larval and adult urodeles and anuran tadpoles readily regenerate their limbs via a process of histolysis and dedifferentiation of mature cells local to the amputation surface that accumulate under the wound epithelium as a blastema of stem cells. These stem cells require growth and trophic factors from the apical epidermal cap (AEC) and the nerves that re-innervate the blastema for their survival and proliferation. Members of the fibroblast growth factor (FGF) family synthesized by both AEC and nerves, and glial growth factor, substance P, and transferrin of nerves are suspected survival and proliferation factors. Stem cells derived from fibroblasts and muscle cells can transdifferentiate into other cell types during regeneration. The regeneration blastema is a self-organizing system based on positional information inherited from parent limb cells. Retinoids, which act through nuclear receptors, have been used in conjunction with assays for cell adhesivity to show that positional identity of blastema cells is encoded in the cell surface. These molecules are involved in the cell-cell signaling network that re-establishes the original structural pattern of the limb. Other systems of interest that regenerate by histolysis and dedifferentiation of pigmented epithelial cells are the neural retina and lens. Members of the FGF family are also important to the regeneration of these structures. The mechanism of amphibian regeneration by dedifferentiation is of importance to the development of a regenerative medicine, since understanding this mechanism may offer insights into how we might chemically induce the regeneration of mammalian tissues.

Intercalary and supernumerary regeneration in the limbs of the frog, Xenopus laevis

Anuran amphibians, such as Xenopus laevis, can regenerate their limbs only when they are young tadpoles, whereas urodele amphibians have a regenerative ability throughout their lives. It is still unclear whether anuran and urodele use the same mechanism during regeneration. In the present study, we analyzed intercalary and supernumerary regeneration in Xenopus. In contrast to urodele blastema that induces intercalary regeneration along the proximodistal (PD) axis, intercalation did not occur in the Xenopus limb bud when the presumptive zeugopodium (fibula and tibia) was removed. However, when the limb bud tip (presumptive autopodium) was transplanted to the presumptive stylopodium (femur) with a 180-degree rotation at stage 52, the complete zeugopodium was regenerated. These results were similar to the results of urodele mature limbs, suggesting that Xenopus limb buds are equivalent to the urodele mature limbs but not to the urodele blastemas. We hypothesized that the ability for intercalation depends on the expression pattern of fibroblast growth factor (fgf)-8, because the expression of fgf-8 in the urodele spreads over the whole blastema and is close enough to activate the growth of the stump. To test this hypothesis, an FGF-8-soaked bead was implanted at the boundary between the stump and tip of a Xenopus limb bud. Intercalary regeneration was induced at stages 52 and 53. These results suggest that the Xenopus limb bud possesses the potential for intercalation, but endogenous FGF-8 in the apical ectodermal ridge (AER) does not induce intercalation to the stump because of the long distance between the AER and stump.

Regeneration: if they can do it, why can't we?

The therapeutic potential of stem cells and nuclear cloning has led to renewed interest in classical models of regeneration. This longstanding problem is undergoing a renaissance spurred by the availability of new techniques that finally allow analysis on the cellular and molecular level.

Regeneration of the urodele limb: a review

Urodele amphibians have been widely used for studies of limb regeneration. In this article, we review studies on blastema cell proliferation and propose a model of blastemal self-organization and patterning. The model is based on local cell interactions that intercalate positional identities within circumferential and proximodistal boundaries that outline the regenerate. The positional identities created by the intercalation process appear to be reflected in the molecular composition of the cell surface. Transcription factors and signaling molecules involved in patterning are discussed within the context of the boundary/intercalation model.

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

To identify candidate gene(s) involved in the tail regeneration of Xenopus laevis tadpoles, we used the differential display method to isolate four genes (clones 1, 2, 13a, and 13b) whose expression is induced in regenerating tadpole tails. Among them, clones 13a and 13b were found to encode the Xenopus homologues of the alpha1 chain of type XVIII collagen and neuronal pentraxin I, respectively. Expression of clone 2 and neuronal pentraxin I genes increased dramatically in the blastema 3 days after amputation, whereas that for the clone 1 and type XVIII collagen genes was induced gradually after amputation. In situ hybridization revealed that the neuronal pentraxin I gene is expressed specifically in the regenerating tail epidermis but not in the normal tail epidermis or the most distal margin of the tail blastema, suggesting that it has a tissue-inductive role in tail regeneration. Expression of the four genes was induced in the limb and in the tail blastema, suggesting that they are involved in the regeneration of both organs. Finally, expression of clone 2 and neuronal pentraxin I genes was scarce during embryonic stages in comparison to the tail blastema, suggesting that their main functions are in organ regeneration. Our results demonstrate unique features of spatial and temporal gene expression patterns during Xenopus tadpole tail regeneration.

Skin flaps inhibit both the current of injury at the amputation surface and regeneration of that limb in newts

For over two decades, we have been investigating a strong (ca. 20-100 microA/cm2), outwardly directed electric current driven through the limb stump for the first few days following amputation in regenerating salamanders. This current is driven through the stump in a proximal/distal direction by the amiloride-sensitive transcutaneous voltage of the intact skin of the stump. Limb regeneration can be manipulated by several technique that manipulate this physiology, demonstrating that the ionic current is necessary, but not sufficient, for normal regeneration of the amphibian limb. Here, we demonstrate that a full thickness graft of skin covering the forelimb stump of newts strikingly inhibits the regeneration of the limb, and that this procedure is also highly correlated to a suppression of peak outwardly directed stump currents in those animals that fail to regenerate.

Expression and biological effect of urodele fibroblast growth factor 1: relationship to limb regeneration

Fibroblast growth factors (FGFs) have been previously implicated in urodele limb regeneration. Here, we examined expression of FGF-1 by blastema cells and neurons and investigated its involvement in wound epithelial formation and function and in the trophic effect of nerves. Neurons innervating the limb and blastema cells in vivo and in vitro expressed the FGF-1 gene. The peptide was present in blastemas in vivo. Wound epithelium thickened when recombinant newt FGF-1 was provided on heparin-coated beads, demonstrating that the FGF-1 was biologically active and that the wound epithelium is a possible target tissue of FGF. FGF-1 did not stimulate accessory limb formation. FGF-1 was as effective as 10% fetal bovine serum in maintaining proliferative activity of blastema cells in vitro but was unable to maintain growth of denervated, nerve-dependent stage blastemas when provided on beads or by injection. FGF-1 had a strong stimulating effect on blastema cell accumulation and proliferation of limbs inserted into the body cavity that were devoid of an apical epithelial cap (AEC). These results show that FGF-1 can signal wound epithelium cap formation and/or function and can stimulate mesenchyme accumulation/proliferation in the absence of the AEC but that FGF-1 is not directly involved in the neural effect on blastema growth.

Cell proliferation during blastema formation in the regenerating teleost fin

Epimorphic regeneration in teleost fins occurs through the establishment of a balanced growth state in which a blastema gives rise to all the mesenchymal cells, whereas definite areas of the epidermis proliferate leading to its extension, thus, allowing the enlargement of the whole structure. This type of regeneration involves specific mechanisms that temporally and spatially regulate cell proliferation. To understand how the blastema is formed and how this growth situation is set up, we investigated cell proliferation patterns in the regenerating fin of the goldfish Carassius auratus from the time of amputation to that of blastema formation by using proliferating cell nuclear antigen immunostaining and bromodeoxyuridine labeling. Wound closure and apical epidermal cap formation took place by epidermal migration and re-arrangement, without the contribution of cell proliferation. As soon as the apical cap had formed, the epidermis started to proliferate at its lateral surfaces, in which all layers maintained cycling for the duration of the studied process. The distal epidermal cap, on the contrary, presented very few cycling cells, and its cytoarchitecture was indicative of continuous remodeling due to ray growth. The basal layer of this epidermal cap showed a typical morphology and remained nonproliferative whilst in contact with the proliferating blastema. Proliferation in the mesenchymal compartment of the ray started far from the amputation plane. Subsequently, cycling cells approached that location, until they formed the blastema in contact with the apical epidermal cap. Differences observed between the epidermis and mesenchyma, regarding activation of the cell cycle and the establishment of proliferative patterns, suggest that differential mechanisms regulate cell proliferation in each of these compartments during the initial stages of regeneration.

Fibroblast growth factors in regenerating limbs of Ambystoma: cloning and semi-quantitative RT-PCR expression studies

Urodele amphibians (newts and salamanders) have the ability to regenerate amputated limbs throughout their life span. Because fibroblast growth factors (Fgfs) play important roles in developing limbs, we initiated studies to investigate these growth factors in regenerating limbs. Partial cDNAs of Fgf4, 8, and 10 were cloned from both the Mexican axolotl, Ambystoma mexicanum, and locally collected spotted salamander, Ambystoma maculatum, two salamanders well recognized for their regenerative capabilities. cDNAs from the two Ambystoma species were virtually identical, ranging from 97-100% nucleotide identity. Axolotl Fgf4, 8, and 10 showed nucleotide sequence identity with chick Fgf4, 8, and 10 of 79%, 83%, and 72%, respectively. RT-PCR showed that these growth factors are expressed in regenerating axolotl limbs as well as in developing salamander larvae at the three-digit forelimb stage. Fgf8 and 10 are upregulated during regeneration and thus may be involved in distal signaling similar to that of the developing chick limb. Fgf4, however, was undetectable by RT-PCR in the distal tips of regenerates, suggesting that it does not play the same role in limb regeneration that it does in limb development. We also investigated the role these Fgfs may have in the nerve-dependence of regeneration. They were expressed similarly in aneurogenic and innervated limbs, suggesting that they are not the neurotrophic factors responsible for nerve-dependence. Denervation prevented Fgf8 and 10 upregulation, suggesting Fgf pathways are downstream of nerve-dependence. These data highlight important similarities and differences in Fgf expression between limb development and limb regeneration. J. Exp. Zool. 290:529-540, 2001.

Effects of locomotor training on hindlimb regeneration in the urodele amphibian Pleurodeles waltlii

1. The effects of locomotor training on hindlimb regeneration were studied in the urodele amphibian Pleurodeles waltlii. 2. After amputation of one hindlimb at mid-femur, adult animals were subjected to regular training sessions (1 h daily, 5 days a week, over 8 months) of terrestrial stepping. 3. Eight months post-amputation, trained animals exhibited regenerated limbs of reduced size as compared to animals kept in their aquaria. Histological data showed an abnormal regeneration of both the femur and distal structures (e.g. digit muscles, metatarsi and phalanges) while medial structures (e.g. tibia and fibula) were totally re-formed. The study of the electromyographical activity in regenerated limbs during stepping and that of their reflex responsiveness to electrical stimulation showed that both motor and sensory innervations were functional in the limb stump of trained animals. 4. The regenerative capacity of the abnormal stumps was preserved since following a second amputation a quite normal hindlimb was regenerated in 3 months, provided the re-amputated animals were not trained to terrestrial stepping. 5. The stress due to handling, change in locomotor medium (aquatic vs. terrestrial) and the friction of the wound epidermis with the ground were not involved in the disruption of limb regeneration. 6. The locomotor pattern, the reflex responsiveness and the muscle fibre composition were similar in supernumerary forelimbs grafted on the back and in normal forelimbs. However, the supernumerary forelimbs regenerated normally even in animals subjected to locomotor training while the hindlimb did not. It is concluded that the disrupting effects of locomotor training on limb regeneration were localized to the the limb directly involved in locomotion. 7. The mechanisms underlying abnormal limb regeneration in animals subjected to locomotor training are discussed.

FGF-10 stimulates limb regeneration ability in Xenopus laevis

By reciprocal transplantation experiments with regenerative and nonregenerative Xenopus limbs, we recently demonstrated that the regenerative capacity of a Xenopus limb depends on mesenchymal tissue and we suggested that fgf-10 is likely to be involved in this capacity (Yokoyama et al., 2000, Dev. Biol. 219, 18-29). However, the data obtained in that study are not conclusive evidence that FGF-10 is responsible for the regenerative capacity. We therefore investigated the role of FGF-10 in regenerative capacity by directly introducing FGF-10 protein into nonregenerative Xenopus limb stumps. Exogenously applied FGF-10 successfully stimulated the regenerative capacity, resulting in the reinduction of all gene expressions (including shh, msx-1, and fgf-10) that we examined and the regeneration of well-patterned limb structures. We report here for the first time that a certain molecule activates the regenerative capacity of Xenopus limb, and this finding suggests that FGF-10 could be a key molecule in possible regeneration of nonregenerative limbs in higher vertebrates.

Cloning of a cDNA encoding cathepsin D from salamander, Hynobius leechii, and its expression in the limb regenerates

Cathepsin D is a major lysosomal aspartic proteinase participating in the degradation or modification of intra- and extracellular matrix molecules, and its activity is known to increase in the process of tissue reorganization during the early phase of salamander limb regeneration. Here, we report the cloning of a salamander cathepsin D cDNA from Hynobius leechii and its expression profile in normal and retinoic acid (RA) treated limb regenerates. The gene expression of cathepsin D increased notably during the dedifferentiation stage and decreased gradually thereafter. Furthermore, RA that enhances dedifferentiation of regenerating salamander limb caused the elevation of cathepsin D expression both in terms of level and duration. These results suggest that cathepsin D plays important role(s) in the dedifferentiation process, and enhancement of cathepsin D expression might be closely related to RA-evoked pattern duplication.

Roles for Fgf signaling during zebrafish fin regeneration

Following amputation of a urodele limb or teleost fin, the formation of a blastema is a crucial step in facilitating subsequent regeneration. Using the zebrafish caudal fin regeneration model, we have examined the hypothesis that fibroblast growth factors (Fgfs) initiate blastema formation from fin mesenchyme. We find that fibroblast growth factor receptor 1 (fgfr1) is expressed in mesenchymal cells underlying the wound epidermis during blastema formation and in distal blastemal tissue during regenerative outgrowth. fgfr1 transcripts colocalize with those of msxb and msxc, putative markers for undifferentiated, proliferating cells. A zebrafish Fgf member, designated wfgf, is expressed in the regeneration epidermis during outgrowth. Furthermore, we show that a specific inhibitor of Fgfr1 applied immediately following fin amputation blocks blastema formation, without obvious effects on wound healing. This inhibitor blocks the proliferation of blastemal cells and the onset of msx gene transcription. Inhibition of Fgf signaling during ongoing fin regeneration prevents further outgrowth while downregulating the established expression of blastemal msx genes and epidermal sonic hedgehog. Our findings indicate that zebrafish fin blastema formation and regenerative outgrowth require Fgf signaling.

Mesenchyme with fgf-10 expression is responsible for regenerative capacity in Xenopus limb buds

A young tadpole of an anuran amphibian can completely regenerate an amputated limb, and it exhibits an ontogenetic decline in the ability to regenerate its limbs. However, whether mesenchymal or epidermal tissue is responsible for this decrease of the capacity remains unclear. Moreover, little is known about the molecular interactions between these two tissues during regeneration. The results of this study showed that fgf-10 expression in the limb mesenchymal cells clearly corresponds to the regenerative capacity and that fgf-10 and fgf-8 are synergistically reexpressed in regenerating blastemas. However, neither fgf-10 nor fgf-8 is reexpressed after amputation of a nonregenerative limb. Nevertheless, nonregenerative epidermal tissue can reexpress fgf-8 under the influence of regenerative mesenchyme, as was demonstrated by experiments using a recombinant limb composed of regenerative limb mesenchyme and nonregenerative limb epidermis. Taken together, our data demonstrate that the regenerative capacity depends on mesenchymal tissue and suggest that fgf-10 is likely to be involved in this capacity.