Differential expression of myogenic regulatory genes and Msx-1 during dedifferentiation and redifferentiation of regenerating amphibian limbs

Restoring the Cell Cycle and Proliferation Competence in Terminally Differentiated Skeletal Muscle Myotubes

Terminal differentiation is an ill-defined, insufficiently characterized, nonproliferation state. Although it has been classically deemed irreversible, it is now clear that at least several terminally differentiated (TD) cell types can be brought back into the cell cycle. We are striving to uncover the molecular bases of terminal differentiation, whose fundamental understanding is a goal in itself. In addition, the field has sought to acquire the ability to make TD cells proliferate. Attaining this end would probe the very molecular mechanisms we are trying to understand. Equally important, it would be invaluable in regenerative medicine, for tissues depending on TD cells and devoid of significant self-repair capabilities. The skeletal muscle has long been used as a model system to investigate the molecular foundations of terminal differentiation. Here, we summarize more than 50 years of studies in this field.

Tgf-β superfamily and limb regeneration: Tgf-β to start and Bmp to end

Axolotls represent a popular model to study how nature solved the problem of regenerating lost appendages in tetrapods. Our work over many years focused on trying to understand how these animals can achieve such a feat and not end up with a scarred up stump. The Tgf-β superfamily represents an interesting family to target since they are involved in wound healing in adults and pattern formation during development. This family is large and comprises Tgf-β, Bmps, activins and GDFs. In this review, we present work from us and others on Tgf-β & Bmps and highlight interesting observations between these two sub-families. Tgf-β is important for the preparation phase of regeneration and Bmps for the redevelopment phase and they do not overlap with one another. We present novel data showing that the Tgf-β non-canonical pathway is also not active during redevelopment. Finally, we propose a molecular model to explain how Tgf-β and Bmps maintain distinct windows of expression during regeneration in axolotls.

Abnormal DNA methylation in pluripotent stem cells from a patient with Prader-Willi syndrome results in neuronal differentiation defects

DNA methylation is a common method of gene expression regulation, and this form of regulation occurs in the neurodevelopmental disorder Prader-Willi syndrome (PWS). Gene expression regulation via methylation is important for humans, although there is little understanding of the role of methylation in neuronal differentiation. We characterized the cellular differentiation potential of iPS cells derived from a patient with PWS with abnormal methylation (M-iPWS cells). A comparative genomic hybridization (CGH) array revealed that, unlike iPWS cells (deletion genes type), the abnormally methylated M-iPWS cells had no deletion in the15q11.2-q13 chromosome region. In addition, methylation-specific PCR showed that M-iPWS cells had strong methylation in CpG island of the small nuclear ribonucleoprotein polypeptide N (SNRPN) on both alleles. To assess the effect of abnormal methylation on cell differentiation, the M-iPWS and iPWS cells were induced to differentiate into embryoid bodies (EBs). The results suggest that iPWS and M-iPWS cells are defective at differentiation into ectoderm. Neural stem cells (NSCs) and neurons derived from M-iPWS cells had fewer NSCs and mature neurons with low expression of NSCs and neuronal markers. We conclude that expression of the downstream of genes in the PWS region regulated by methylation is involved in neuronal differentiation.

Twist3 is required for dedifferentiation during extraocular muscle regeneration in adult zebrafish

Severely damaged adult zebrafish extraocular muscles (EOMs) regenerate through dedifferentiation of residual myocytes involving a muscle-to-mesenchyme transition. Members of the Twist family of basic helix-loop-helix transcription factors (TFs) are key regulators of the epithelial-mesenchymal transition (EMT) and are also involved in craniofacial development in humans and animal models. During zebrafish embryogenesis, twist family members (twist1a, twist1b, twist2, and twist3) function to regulate craniofacial skeletal development. Because of their roles as master regulators of stem cell biology, we hypothesized that twist TFs regulate adult EOM repair and regeneration. In this study, utilizing an adult zebrafish EOM regeneration model, we demonstrate that inhibiting twist3 function using translation-blocking morpholino oligonucleotides (MOs) impairs muscle regeneration by reducing myocyte dedifferentiation and proliferation in the regenerating muscle. This supports our hypothesis that twist TFs are involved in the early steps of dedifferentiation and highlights the importance of twist3 during EOM regeneration.

Elixir of Life: Thwarting Aging With Regenerative Reprogramming

All living beings undergo systemic physiological decline after ontogeny, characterized as aging. Modern medicine has increased the life expectancy, yet this has created an aged society that has more predisposition to degenerative disorders. Therefore, novel interventions that aim to extend the healthspan in parallel to the life span are needed. Regeneration ability of living beings maintains their biological integrity and thus is the major leverage against aging. However, mammalian regeneration capacity is low and further declines during aging. Therefore, modalities that reinforce regeneration can antagonize aging. Recent advances in the field of regenerative medicine have shown that aging is not an irreversible process. Conversion of somatic cells to embryonic-like pluripotent cells demonstrated that the differentiated state and age of a cell is not fixed. Identification of the pluripotency-inducing factors subsequently ignited the idea that cellular features can be reprogrammed by defined factors that specify the desired outcome. The last decade consequently has witnessed a plethora of studies that modify cellular features including the hallmarks of aging in addition to cellular function and identity in a variety of cell types in vitro. Recently, some of these reprogramming strategies have been directly used in animal models in pursuit of rejuvenation and cell replacement. Here, we review these in vivo reprogramming efforts and discuss their potential use to extend the longevity by complementing or augmenting the regenerative capacity.

Behavior of Stem-Like Cells, Precursors for Tissue Regeneration in Urodela, Under Conditions of Microgravity

We summarize data from our experiments on stem-like cell-dependent regeneration in amphibians in microgravity. Considering its deleterious effect on many tissues, we asked whether microgravity is compatible with reparative processes, specifically activation and proliferation of source cells. Experiments were conducted using tailed amphibians, which combine profound regenerative capabilities with high robustness, allowing an in vivo study of lens, retina, limb, and tail regeneration in challenging settings of spaceflight. Microgravity promoted stem-like cell proliferation to a varying extent (up to 2-fold), and it seemed to speed up source cell dedifferentiation, as well as sequential differentiation in retina, lens, and limb, leading to formation of bigger and more developed regenerates than in 1g controls. It also promoted proliferation and hypertrophy of Müller glial cells, eliciting a response similar to reactive gliosis. A significant increase in stem-like cell proliferation was mostly beneficial for regeneration and only in rare cases caused moderate tissue growth abnormalities. It is important that microgravity yielded a lasting effect even if applied before operations. We hypothesize on the potential mechanisms of gravity-dependent changes in stem-like cell behavior, including fibroblast growth factor 2 signaling pathway and heat shock proteins, which were affected in our experimental settings. Taken together, our data indicate that microgravity does not disturb the natural regenerative potential of newt stem-like cells, and, depending on the system, even stimulates their dedifferentiation, proliferation, and differentiation. We discuss these data along with publications on mammalian stem cell behavior in vitro and invertebrate regeneration in vivo in microgravity. In vivo data are very scarce and require further research using contemporary methods of cell behavior analysis to elucidate mechanisms of stem cell response to altered gravity. They are relevant for both practical applications, such as managing human reparative responses in spaceflight, and fundamental understanding of stem cell biology.

Wound-induced cell proliferation during animal regeneration

Many animal species are capable of replacing missing tissues that are lost upon injury or amputation through the process of regeneration. Although the extent of regeneration is variable across animals, that is, some animals can regenerate any missing cell type whereas some can only regenerate certain organs or tissues, regulated cell proliferation underlies the formation of new tissues in most systems. Notably, many species display an increase in proliferation within hours or days upon wounding. While different cell types proliferate in response to wounding in various animal taxa, comparative molecular data are beginning to point to shared wound-induced mechanisms that regulate cell division during regeneration. Here, we synthesize current insights about early molecular pathways of regeneration from diverse model and emerging systems by considering these species in their evolutionary contexts. Despite the great diversity of mechanisms underlying injury-induced cell proliferation across animals, and sometimes even in the same species, similar pathways for proliferation have been implicated in distantly related species (e.g., small diffusible molecules, signaling from apoptotic cells, growth factor signaling, mTOR and Hippo signaling, and Wnt and Bmp pathways). Studies that explicitly interrogate molecular and cellular regenerative mechanisms in understudied animal phyla will reveal the extent to which early pathways in the process of regeneration are conserved or independently evolved. This article is categorized under: Comparative Development and Evolution > Body Plan Evolution Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration Comparative Development and Evolution > Model Systems.

Mechanisms of urodele limb regeneration

This review explores the historical and current state of our knowledge about urodele limb regeneration. Topics discussed are (1) blastema formation by the proteolytic histolysis of limb tissues to release resident stem cells and mononucleate cells that undergo dedifferentiation, cell cycle entry and accumulation under the apical epidermal cap. (2) The origin, phenotypic memory, and positional memory of blastema cells. (3) The role played by macrophages in the early events of regeneration. (4) The role of neural and AEC factors and interaction between blastema cells in mitosis and distalization. (5) Models of pattern formation based on the results of axial reversal experiments, experiments on the regeneration of half and double half limbs, and experiments using retinoic acid to alter positional identity of blastema cells. (6) Possible mechanisms of distalization during normal and intercalary regeneration. (7) Is pattern formation is a self-organizing property of the blastema or dictated by chemical signals from adjacent tissues? (8) What is the future for regenerating a human limb?

Advances in Decoding Axolotl Limb Regeneration

Humans and other mammals are limited in their natural abilities to regenerate lost body parts. By contrast, many salamanders are highly regenerative and can spontaneously replace lost limbs even as adults. Because salamander limbs are anatomically similar to human limbs, knowing how they regenerate should provide important clues for regenerative medicine. Although interest in understanding the mechanics of this process has never wavered, until recently researchers have been vexed by seemingly impenetrable logistics of working with these creatures at a molecular level. Chief among the problems has been the very large size of salamander genomes, and not a single salamander genome has been fully sequenced to date. Recently the enormous gap in sequence information has been bridged by approaches that leverage mRNA as the starting point. Together with functional experimentation, these data are rapidly enabling researchers to finally uncover the molecular mechanisms underpinning the astonishing biological process of limb regeneration.

Use of Xenopus Frogs to Study Renal Development/Repair

The Xenopus genus includes several members of aquatic frogs native to Africa but is perhaps best known for the species Xenopus laevis and Xenopus tropicalis. These species were popularized as model organisms from as early as the 1800s and have been instrumental in expanding several biological fields including cell biology, environmental toxicology, regenerative biology, and developmental biology. In fact, much of what we know about the formation and maturation of the vertebrate renal system has been acquired by examining the intricate genetic and morphological patterns that epitomize nephrogenesis in Xenopus. From these numerous reports, we have learned that the process of kidney development is as unique among organs as it is conserved among vertebrates. While development of most organs involves increases in size at a single location, development of the kidney occurs through a series of three increasingly complex nephric structures that are temporally distinct from one another and which occupy discrete spatial locales within the body. These three renal systems all serve to provide homeostatic, osmoregulatory, and excretory functions in animals. Importantly, the kidneys in amphibians, such as Xenopus, are less complex and more easily accessed than those in mammals, and thus tadpoles and frogs provide useful models for understanding our own kidney development. Several descriptive and mechanistic studies conducted with the Xenopus model system have allowed us to elucidate the cellular and molecular mediators of renal patterning and have also laid the foundation for our current understanding of kidney repair mechanisms in vertebrates. While some species-specific responses to renal injury have been observed, we still recognize the advantage of the Xenopus system due to its distinctive similarity to mammalian wound healing, reparative, and regenerative responses. In addition, the first evidence of renal regeneration in an amphibian system was recently demonstrated in Xenopus laevis. As genetic and molecular tools continue to advance, our appreciation for and utilization of this amphibian model organism can only intensify and will certainly provide ample opportunities to further our understanding of renal development and repair.

Could we also be regenerative superheroes, like salamanders?

Development of methods to reawaken the semi-dormant regenerative potential that lies within adult human tissues would hold promise for the restoration of diseased or damaged organs and tissues. While most of the regeneration potential is suppressed in many vertebrates, including humans, during adult life, urodele amphibians (salamanders) retain their regenerative ability throughout adulthood. Studies in newts and axolotls, two salamander models, have provided significant knowledge about adult limb regeneration. In this review, we present a comparative analysis of salamander and mammalian regeneration and discuss how evolutionarily altered properties of the regenerative environment can be exploited to restore full regenerative potential in the human body.

Human regeneration: An achievable goal or a dream?

The main objective of regenerative medicine is to replenish cells or tissues or even to restore different body parts that are lost or damaged due to disease, injury and aging. Several avenues have been explored over many decades to address the fascinating problem of regeneration at the cell, tissue and organ levels. Here we discuss some of the primary approaches adopted by researchers in the context of enhancing the regenerating ability of mammals. Natural regeneration can occur in different animal species, and the underlying mechanism is highly relevant to regenerative medicine-based intervention. Significant progress has been achieved in understanding the endogenous regeneration in urodeles and fishes with the hope that they could help to reach our goal of designing future strategies for human regeneration.

Dedifferentiation, Redifferentiation, and Transdifferentiation of Striated Muscles During Regeneration and Development

In some rare and striking cases, striated muscle fibers of the skeleton or body wall, which consist of terminally differentiated syncytia with complex ultrastructures, were found to be capable of dedifferentiating and fragmenting into mononucleate cells. Examples of such events will be discussed in which the dedifferentiated cells reenter the cell cycle, proliferate, and rebuilt damaged muscle fibers during limb regeneration or transdifferentiate to generate new types of muscles during normal development.

Characterization of Proliferating Neural Progenitors after Spinal Cord Injury in Adult Zebrafish

Zebrafish can repair their injured brain and spinal cord after injury unlike adult mammalian central nervous system. Any injury to zebrafish spinal cord would lead to increased proliferation and neurogenesis. There are presences of proliferating progenitors from which both neuronal and glial loss can be reversed by appropriately generating new neurons and glia. We have demonstrated the presence of multiple progenitors, which are different types of proliferating populations like Sox2⁺ neural progenitor, A2B5⁺ astrocyte/ glial progenitor, NG2⁺ oligodendrocyte progenitor, radial glia and Schwann cell like progenitor. We analyzed the expression levels of two common markers of dedifferentiation like msx-b and vimentin during regeneration along with some of the pluripotency associated factors to explore the possible role of these two processes. Among the several key factors related to pluripotency, pou5f1 and sox2 are upregulated during regeneration and associated with activation of neural progenitor cells. Uncovering the molecular mechanism for endogenous regeneration of adult zebrafish spinal cord would give us more clues on important targets for future therapeutic approach in mammalian spinal cord repair and regeneration.

Acquisition of germ plasm accelerates vertebrate evolution

Primordial germ cell (PGC) specification occurs either by induction from pluripotent cells (epigenesis) or by a cell-autonomous mechanism mediated by germ plasm (preformation). Among vertebrates, epigenesis is basal, whereas germ plasm has evolved convergently across lineages and is associated with greater speciation. We compared protein-coding sequences of vertebrate species that employ preformation with their sister taxa that use epigenesis and demonstrate that genes evolve more rapidly in species containing germ plasm. Furthermore, differences in rates of evolution appear to cause phylogenetic incongruence in protein-coding sequence comparisons between vertebrate taxa. Our results support the hypothesis that germ plasm liberates constraints on somatic development and that enhanced evolvability drives the evolution of germ plasm.

Molecular regulation of stem cell quiescence

Subsets of mammalian adult stem cells reside in the quiescent state for prolonged periods of time. This state, which is reversible, has long been viewed as dormant and with minimal basal activity. Recent advances in adult stem cell isolation have provided insights into the epigenetic, transcriptional and post-transcriptional control of quiescence and suggest that quiescence is an actively maintained state in which signalling pathways are involved in maintaining a poised state that allows rapid activation. Deciphering the molecular mechanisms regulating adult stem cell quiescence will increase our understanding of tissue regeneration mechanisms and how they are dysregulated in pathological conditions and in ageing.

The "Stars and Stripes" Metaphor for Animal Regeneration-Elucidating Two Fundamental Strategies along a Continuum

A number of challenges have hindered the development of a unified theory for metazoan regeneration. To describe the full range of complex regeneration phenomena in Animalia, we suggest that metazoans that regenerate missing body parts exhibit biological attributes that are tailored along a morpho-spatial regeneration continuum, illustrated in its polar scenarios by the USA “stars and stripes” flag. Type 1 organisms (“T1, ‘stars’”) are typical colonial organisms (but contain unitary taxa) that are able to regenerate “whole new stars”, namely, whole bodies and colonial modules, through systemic induction and sometimes multiple regeneration foci (hollow regeneration spheres, resembling the blastula) that compete for dominance. They regenerate soma and germ constituents with pluripotent adult stem cells and exhibit somatic-embryogenesis mode of ontogeny. Type 2 organisms (“T2, ‘stripes’”) are capable of limited regeneration of somatic constituents via fate-restricted stem cells, and regenerate through centralized inductions that lead to a single regeneration front. T2 organisms are unitary and use preformistic mode of ontogeny. T1 and T2 organisms also differ in interpretation of what constitutes positional information. T2 organisms also execute alternative, less effective, regeneration designs (i.e., scar formation). We assigned 15 characteristics that distinguish between T1/T2 strategies: those involving specific regeneration features and those operating on biological features at the whole-organism level. Two model organisms are discussed, representing the two strategies of T1/T2 along the regeneration continuum, the Botrylloides whole body regeneration (T1) and the mouse digit-tip regeneration (T2) phenomena. The above working hypothesis also postulates that regeneration is a primeval attribute of metazoans. As specified, the “stars and stripes” paradigm allows various combinations of the biological features assigned to T1 and T2 regeneration strategies. It does not consider any concentration gradient or thresholds and does not refer to the “epimorphosis” and “morphallaxis” terms, regeneration types across phyla or across body plans. The “stars and stripes” paradigm also ignores, at this stage of analysis, cases of regeneration loss that may obscure biological trajectories. The main advantage of the “stars and stripes” paradigm is that it allows us to compare T1/T2 regeneration, as well as other modes of regeneration, through critical determining characteristics.

Multi-tissue microarray analysis identifies a molecular signature of regeneration

The inability to functionally repair tissues that are lost as a consequence of disease or injury remains a significant challenge for regenerative medicine. The molecular and cellular processes involved in complete restoration of tissue architecture and function are expected to be complex and remain largely unknown. Unlike humans, certain salamanders can completely regenerate injured tissues and lost appendages without scar formation. A parsimonious hypothesis would predict that all of these regenerative activities are regulated, at least in part, by a common set of genes. To test this hypothesis and identify genes that might control conserved regenerative processes, we performed a comprehensive microarray analysis of the early regenerative response in five regeneration-competent tissues from the newt Notophthalmus viridescens. Consistent with this hypothesis, we established a molecular signature for regeneration that consists of common genes or gene family members that exhibit dynamic differential regulation during regeneration in multiple tissue types. These genes include members of the matrix metalloproteinase family and its regulators, extracellular matrix components, genes involved in controlling cytoskeleton dynamics, and a variety of immune response factors. Gene Ontology term enrichment analysis validated and supported their functional activities in conserved regenerative processes. Surprisingly, dendrogram clustering and RadViz classification also revealed that each regenerative tissue had its own unique temporal expression profile, pointing to an inherent tissue-specific regenerative gene program. These new findings demand a reconsideration of how we conceptualize regenerative processes and how we devise new strategies for regenerative medicine.

DNAJB6 governs a novel regulatory loop determining Wnt/β-catenin signalling activity

DKK1 (dickkopf 1 homologue) is a secreted inhibitor of the Wnt signalling pathway and a critical modulator of tumour promotion and the tumour microenvironment. However, mechanisms regulating DKK1 expression are understudied. DNAJB6 {DnaJ [HSP40 (heat-shock protein 40 kDa)] homologue, subfamily B, member 6} is an HSP40 family member whose expression is compromised during progression of breast cancer and melanoma. Inhibition of the Wnt/β-catenin signalling pathway by up-regulation of DKK1 is one of the key mechanisms by which DNAJB6 suppresses tumour metastasis and EMT (epithelial-mesenchymal transition). Analysis of the DKK1 promoter to define the cis-site responsible for its up-regulation by DNAJB6 revealed the presence of two binding sites for a transcriptional repressor, MSX1 (muscle segment homeobox 1). Our investigations showed that MSX1 binds the DKK1 promoter and inhibits DKK1 transcription. Interestingly, silencing DNAJB6 resulted in up-regulation of MSX1 concomitant with increased stabilization of β-catenin. ChIP (chromatin immunoprecipitation) studies revealed that β-catenin binds the MSX1 promoter and stabilization of β-catenin elevates MSX1 transcription, indicating that β-catenin works as a transcription co-activator for MSX1. Functionally, exogenous expression of MSX1 in DNAJB6-expressing cells promotes the mesenchymal phenotype by suppression of DKK1. Thus we have identified a novel regulatory mechanism of DNAJB6-mediated DKK1 transcriptional up-regulation that can influence EMT. DKK1 is a feedback regulator of β-catenin levels and thus our studies also define an additional negative control of this β-catenin/DKK1 feedback loop by MSX1, which may potentially contribute to excessive stabilization of β-catenin.

Skeletal muscle regeneration in Xenopus tadpoles and zebrafish larvae

Background

Mammals are not able to restore lost appendages, while many amphibians are. One important question about epimorphic regeneration is related to the origin of the new tissues and whether they come from mature cells via dedifferentiation and/or from stem cells. Several studies in urodele amphibians (salamanders) indicate that, after limb or tail amputation, the multinucleated muscle fibres do dedifferentiate by fragmentation and proliferation, thereby contributing to the regenerate. In Xenopus laevis tadpoles, however, it was shown that muscle fibres do not contribute directly to the tail regenerate. We set out to study whether dedifferentiation was present during muscle regeneration of the tadpole limb and zebrafish larval tail, mainly by cell tracing and histological observations.

Results

Cell tracing and histological observations indicate that zebrafish tail muscle do not dedifferentiate during regeneration. Technical limitations did not allow us to trace tadpole limb cells, nevertheless we observed no signs of dedifferentiation histologically. However, ultrastructural and gene expression analysis of regenerating muscle in tadpole tail revealed an unexpected dedifferentiation phenotype. Further histological studies showed that dedifferentiating tail fibres did not enter the cell cycle and in vivo cell tracing revealed no evidences of muscle fibre fragmentation. In addition, our results indicate that this incomplete dedifferentiation was initiated by the retraction of muscle fibres.

Conclusions

Our results show that complete skeletal muscle dedifferentiation is less common than expected in lower vertebrates. In addition, the discovery of incomplete dedifferentiation in muscle fibres of the tadpole tail stresses the importance of coupling histological studies with in vivo cell tracing experiments to better understand the regenerative mechanisms.

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.

Differentiated skeletal cells contribute to blastema formation during zebrafish fin regeneration

The origin of cells that generate the blastema following appendage amputation has been a long-standing question in epimorphic regeneration studies. The blastema is thought to originate from either stem (or progenitor) cells or differentiated cells of various tissues that undergo dedifferentiation. Here, we investigate the origin of cells that contribute to the regeneration of zebrafish caudal fin skeletal elements. We provide evidence that the process of lepidotrichia (bony rays) regeneration is initiated as early as 24 hours post-amputation and that differentiated scleroblasts acquire a proliferative state, detach from the lepidotrichia surface, migrate distally, integrate into the blastema and dedifferentiate. These findings provide novel insights into the origin of cells in epimorphic appendage regeneration in zebrafish and suggest conservation of regeneration mechanisms between fish and amphibians.

Limb regeneration: a new development?

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

The yeast two hybrid system in a screen for proteins interacting with axolotl (Ambystoma mexicanum) Msx1 during early limb regeneration

Urodele amphibians are exceptional in their ability to regenerate complex body structures such as limbs. Limb regeneration depends on a process called dedifferentiation. Under an inductive wound epidermis terminally differentiated cells transform to pluripotent progenitor cells that coordinately proliferate and eventually redifferentiate to form the new appendage. Recent studies have developed molecular models integrating a set of genes that might have important functions in the control of regenerative cellular plasticity. Among them is Msx1, which induced dedifferentiation in mammalian myotubes in vitro. Herein, we screened for interaction partners of axolotl Msx1 using a yeast two hybrid system. A two hybrid cDNA library of 5-day-old wound epidermis and underlying tissue containing more than 2×10⁶ cDNAs was constructed and used in the screen. 34 resulting cDNA clones were isolated and sequenced. We then compared sequences of the isolated clones to annotated EST contigs of the Salamander EST database (BLASTn) to identify presumptive orthologs. We subsequently searched all no-hit clone sequences against non redundant NCBI sequence databases using BLASTx. It is the first time, that the yeast two hybrid system was adapted to the axolotl animal model and successfully used in a screen for proteins interacting with Msx1 in the context of amphibian limb regeneration.

High resolution three-dimensional imaging: Evidence for cell cycle reentry in regenerating skeletal muscle

Newts and other urodele amphibians can replace lost structures including limbs, providing a vertebrate model for the study of regeneration of complex tissues. The composite of different cell and tissue types in the limb, however, presents a challenge for their imaging in three-dimensions (3D) at cellular level resolution. To observe myofibers in vivo without distortion, we developed a streamlined protocol whereby 80 μm thick cryosections are mounted on slides, processed for immunohistochemistry, imaged using confocal microscopy and z-stacks rendered in 3D. This methodology enabled precise in situ rendering of regenerating muscle, demonstrating cell cycle reentry of nuclei within the myofiber syncytium. The high resolution imaging of muscle or comparable tissue types as intact 3D entities in the context of extracellular and intracellular molecules allows for the determination of signaling and cell response pathways, making this method useful for studies that attempt to characterize rare physiological events in vivo.

Molecular physiology of cardiac regeneration

Heart disease is the leading cause of death in the industrialized world. This is partially attributed to the inability of cardiomyocytes to divide in a significant manner, and therefore the heart responds to injury through scar formation. One of the challenges of modern medicine is to develop novel therapeutic strategies to facilitate regeneration of cardiac muscle in the diseased heart. Numerous methods have been studied and a wide variety of cell types have been considered. To date, bone marrow stem cells, endogenous populations of cardiac stem cells, embryonic stem cells, and induced pluripotent stem cells have been investigated for their ability to regenerate infarcted myocardium, although stem cell transplantation has produced ambiguous results in human clinical trials. Several studies support another approach that seems very appealing: enhancing the limited endogenous regenerative capacity of the heart. The recent advances in stem cell and regenerative biology are giving rise to the view that cardiac regeneration, although not quite ready for clinical treatment, may translate into therapeutic reality in the not too distant future.

Heart of newt: a recipe for regeneration

The field of regenerative medicine holds tremendous promise for the treatment of chronic diseases. While the adult mammalian heart has limited regenerative capacity, previous studies have focused on cellular therapeutic strategies in an attempt to modulate cardiac regeneration. An alternative strategy relies on the modulation of endogenous stem/progenitor cells or signaling pathways to promote cardiac regeneration. Several organisms, including the newt, have an incomparable capacity for the regeneration of differentiated tissues. An enhanced understanding of the signals, pathways, and factors that mediate the regenerative response in these organisms may be useful in modulating the regenerative response of mammalian organs including the injured adult heart.

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

METHOD

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

RESULTS

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

CONCLUSION

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

Stem cell options for kidney disease

Chronic kidney disease (CKD) is increasing at the rate of 6-8% per annum in the US alone. At present, dialysis and transplantation remain the only treatment options. However, there is hope that stem cells and regenerative medicine may provide additional regenerative options for kidney disease. Such new treatments might involve induction of repair using endogenous or exogenous stem cells or the reprogramming of the organ to reinitiate development. This review addresses the current state of understanding with respect to the ability of non-renal stem cell sources to influence renal repair, the existence of endogenous renal stem cells and the biology of normal renal repair in response to damage. It also examines the remaining challenges and asks the question of whether there is one solution for all forms of renal disease.

Stem cells and plasticity of skeletal muscle cell differentiation: potential application to cell therapy for degenerative muscular diseases

Degenerative muscular diseases, such as muscular dystrophies, have been the representative targets of regenerative cell therapy. Although satellite cells play central roles in skeletal muscle regeneration that intrinsically occurs after muscle injury, their application to cell therapy is confronted by difficulties. Other stem cells expected to be applicable to cell therapy include muscle-resident stem cells and nonmuscle-resident stem cells. Moreover, dedifferentiated cells of skeletal muscle might provide unique system for cell therapy. Terminally differentiated myotubes have plasticity of differentiation and dedifferentiate under certain experimental conditions, including the expression of SV40 large T antigen or the homeobox gene Msx1. The dedifferentiated cells exhibit multipotency to transdifferentiate into multiple mesenchymal origin cells. In addition, fibroblasts or undifferentiated myoblasts treated with a drug acquire multipotency. These cells may open new doors in cell therapy.

Analysis of the expression and function of Wnt-5a and Wnt-5b in developing and regenerating axolotl (Ambystoma mexicanum) limbs

Urodele amphibians are unique adult vertebrates because they are able to regenerate body parts after amputation. Studies of urodele limb regeneration, the key model system for vertebrate regeneration, have led to an understanding of the origin of blastema cells and the importance of positional interactions between blastema cells in the control of growth and pattern formation. Progress is now being made in the identification of the signaling pathways that regulate dedifferentiation, blastema morphogenesis, growth and pattern formation. Members of the Wnt family of secreted proteins are expressed in developing and regenerating limbs, and have the potential to control growth, pattern formation and differentiation. We have studied the expression of two non-canonical Wnt genes, Wnt-5a and Wnt-5b. We report that they are expressed in equivalent patterns during limb development and limb regeneration in the axolotl (Ambystoma mexicanum), and during limb development in other tetrapods, implying conservation of function. Our analysis of the effects of ectopic Wnt-5a expression is consistent with the hypothesis that canonical Wnt signaling functions during the early stages of regeneration to control the dedifferentiation of stump cells giving rise to the regeneration-competent cells of the blastema.

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.

Rapid accumulation of nucleostemin in nucleolus during newt regeneration

In newt regeneration, differentiated cells can revert to stem cell-like cells in which the proliferative ability and multipotentiality are restored after dedifferentiation. However, the molecular events that occur during the dedifferentiation still remain obscure. Nucleostemin has been identified in mammals as a nucleolar protein specific to stem cells and cancer cells. In this study, a newt nucleostemin homologue was cloned and its regulation was analyzed. During lens regeneration, the expression of nucleostemin was activated and nucleostemin rapidly accumulated in the nucleoli of dedifferentiating pigmented epithelial cells 2 days before cell cycle reentry. During limb regeneration, nucleostemin also accumulated in the nucleoli of degenerating multinucleate muscle fibers before blastema formation. These findings suggest that nucleostemin plays a role in the dedifferentiation of newt cells and can provide crucial clues for addressing the molecular events at early steps of cellular dedifferentiation in newts.

Re-programming of newt cardiomyocytes is induced by tissue regeneration

Newt hearts are able to repair substantial cardiac injuries without functional impairment, whereas mammalian hearts cannot regenerate. The cellular and molecular mechanisms that control the regenerative capacity of the newt heart are unknown. Here, we show that the ability of newt cardiomyocytes to regenerate cardiac injuries correlates with their ability to transdifferentiate into different cell types. Mechanical injury of the heart led to a severe reduction of sarcomeric proteins in the myocardium, indicating a partial de-differentiation of adult newt cardiomyocytes during regeneration. Newt cardiomyocytes implanted into regenerating limbs lost their cardiac phenotype and acquired skeletal muscle or chondrocyte fates. Reprogramming of cardiomyocytes depended on contact with the limb blastema because cardiomyocytes implanted into intact, non-regenerating limbs or cultured in vitro retained their original identity. We reason that signals from the limb blastema led to de-differentiation of cardiomyocytes, cell proliferation and re-differentiation into specialized cells and propose that the ability of cardiomyocytes to transdifferentiate into different cell types reflects the cellular program that enables heart regeneration.

Reversibility of the Differentiated State: Regeneration in Amphibians

In contrast to mammals, some fish and amphibians have retained the ability to regenerate complex body structures or organs, such as the limb, tail, eye lens, or even parts of the heart. One major difference in the response to injury is the appearance of a mesenchymal growth zone or blastema in these regenerative species instead of the scarring seen in mammals. This blastema is thought to largely derive from the dedifferentiation of various functional cell types, such as skeletal muscle, dermis, and cartilage. In the case of multinucleated skeletal muscle fibers, cell cycle reentry into S-phase as well as fragmentation into mononucleated progenitors is observed both in vitro and in vivo.

Cellular plasticity in vertebrate regeneration

Within the animal kingdom, there are several examples of organisms with remarkable regenerative abilities. Among vertebrates, newts appear to be the most adept at replacing lost structures and injured organs and can regenerate their limbs, tails, spinal cords, jaws, retinas, lenses, optic nerves, intestines, and heart ventricles. This regenerative ability is dependent on the induction of an unusual degree of cellular plasticity near the site of injury. Mature cells lose their differentiated characteristics and revert to proliferating progenitor cells that will later redifferentiate to replace the lost or injured tissues. This degree of cellular plasticity appears to be restricted to those vertebrates with the most remarkable regenerative abilities and is not observed in mammals. However, in the last several years, there have been a few studies suggesting that certain factors present in newt tissues can induce a dedifferentiation response in mammalian cells. These results suggest that the knowledge gained from studying the molecular basis of cellular plasticity in newts and other regeneration-competent model organisms might one day be used to enhance the regenerative potential in mammals.

Inhibition of zebrafish fin regeneration using in vivo electroporation of morpholinos againstfgfr1 andmsxb

Increased interest in using zebrafish as a model organism has led to a resurgence of fin regeneration studies. This has allowed for the identification of a large number of gene families, including signaling molecules and transcription factors, which are expressed during regeneration. However, in cases where no specific inhibitor is available for the gene product of interest, determination of a functional role for these genes has been difficult. Here we demonstrate that in vivo electroporation of morpholino oligonucleotides is a feasible approach for protein knock-down during fin regeneration. Morpholino oligonucleotides against fgfr1 and msxb were utilized and knock-down of both proteins resulted in reduced fin outgrowth. Importantly, Fgfr1 knock-down phenocopied outgrowth inhibition obtained with an Fgfr1 inhibitor. Furthermore, this method provided direct evidence for a functional role for msxb in caudal fin regeneration. Finally, knock-down of Fgfr1, but not Msxb, affected the blastemal expression of msxc, suggesting this technique can be used to determine epistasis in genetic pathways affecting regeneration. Thus, this convenient reverse genetic approach allows researchers to quickly (1) assess the function of genes known to be expressed during fin regeneration, (2) screen genes for functional relevance during fin regeneration, and (3) assign genes to the molecular pathways underlying fin regeneration.

Macroarray-based analysis of tail regeneration in Xenopus laevis larvae

Xenopus larvae possess a remarkable ability to regenerate their tails after they have been severed. To gain an understanding of the molecular mechanisms underlying tail regeneration, we performed a cDNA macroarray-based analysis of gene expression. A Xenopus cDNA macroarray representing 42,240 independent clones was differentially hybridized with probes synthesized from the total RNA of normal and regenerating tails. Temporal expression analysis revealed that the up-regulated genes could be grouped into early or late responding genes. A comparative expression analysis revealed that most genes showed similar expression patterns between tail development and regeneration. However, some genes showed regeneration-specific expression. Finally, we identified 48 up-regulated genes that fell into several categories based on their putative functions. These categories reflect the various processes that take place during regeneration, such as inflammation response, wound healing, cell proliferation, cell differentiation, and control of cell structure. Thus, we have identified a panel of genes that appear to be involved in the process of regeneration.

Ontogenetic decline of regenerative ability and the stimulation of human regeneration

Although we cannot regenerate our limbs today, it is likely that when we were embryos we could regenerate many of our tissues, including our limbs. Like other vertebrates, our impressive regenerative abilities were lost during embryogenesis, leaving us with a relatively limited ability to repair tissue damage. In contrast, adult salamanders can reactivate the embryonic regeneration response, and thus they provide the opportunity to discover the principles and mechanisms of tissue and organ regeneration. One important lesson we have learned from salamanders is that regeneration occurs in two steps. While the second step shares the mechanisms of growth control and pattern formation with limb development, the first step is unique and leads to the formation of a regeneration blastema. A second lesson is that connective tissue fibroblasts control regeneration, and that the unique regenerative ability of salamanders (the first step of regeneration) is a consequence of the ability of fibroblasts to dedifferentiate and give rise to blastema cells. Since we all developed limbs as embryos, we all possess the genetic program for making a limb (the second step of regeneration). Therefore, the challenge for inducing limb regeneration in humans is to discover how to induce fibroblast dedifferentiation.

Nerve-dependent and -independent events in blastema formation during Xenopus froglet limb regeneration

Blastema formation, the initial stage of epimorphic limb regeneration in amphibians, is an essential process to produce regenerates. In our study on nerve dependency of blastema formation, we used forelimb of Xenopus laevis froglets as a system and applied some histological and molecular approaches in order to determine early events during blastema formation. We also investigated the lateral wound healing in comparison to blastema formation in limb regeneration. Our study confirmed at the molecular level that there are nerve-dependent and -independent events during blastema formation after limb amputation, Tbx5 and Prx1, reliable markers of initiation of limb regeneration, that start to be expressed independently of nerve supply, although their expressions cannot be maintained without nerve supply. We also found that cell proliferation activity, cell survival and expression of Fgf8, Fgf10 and Msx1 in the blastema were affected by denervation, suggesting that these events specific for blastema outgrowth are controlled by the nerve supply. Wound healing, which is thought to be categorized into tissue regeneration, shares some nerve-independent events with epimorphic limb regeneration, although the healing process results in simple restoration of wounded tissue. Overall, our results demonstrate that dedifferentiated blastemal cells formed at the initial phase of limb regeneration must enter the nerve-dependent epimorphic phase for further processes, including blastema outgrowth, and that failure of entry results in a simple redifferentiation as tissue regeneration.

Msx1 expression in the adult mouse brain: characterization of populations of beta-galactosidase-positive cells in the hippocampus and fimbria

We have analyzed Msx1 expression in the mature mouse brain using in situ hybridization and beta-galactosidase activity in Msx1(nLacZ) mice. The study revealed that Msx1 is strongly expressed in the circumventricular organs, such as the subcommissural organ and choroid plexus, and in some epithelia, such as that of the dorsal, but not the ventral part of the third ventricle. Immunohistochemical analysis revealed that the Msx1-expressing cells of the hippocampus and fimbria are astrocytes, oligodendrocytes or immature oligodendrocytes. In contrast, no co-expression was detected in these structures using several neuronal markers. These results were confirmed, using transmission electron microscopy, by the presence of 5-bromo-3-indolyl-beta-D-galactopyranosideprecipitates in astrocytes and oligodendrocytes in both sites. Moreover, using an anti-glial fibrillary acidic protein antibody (GFAP), our study reveals two populations of astrocytes in the adult hippocampus and other areas, such as the fimbria, namely Msx1+/GFAP+ and Msx1-/GFAP+. Beta-galactosidase activity was also observed in endothelial cells of hippocampal fissure blood vessels. We also observed co-localization of polysialic acid neural cell adhesion molecule, a marker of the polysialylated form of the neural cell adhesion molecule, in Msx1-expressing cells in the fimbria. These cells may be precursors of glial cells and originate from the epithelium of the fimbria. The present study indicates, in the mature mouse brain, that Msx1 may be linked to secretory activity in circumventricular organs, and to glial proliferation and differentiation in the hippocampus and fimbria, and presumably also in other cerebral areas. We suggest that Msx1 could be associated with brain homeostasis and blood-brain barrier function.

Exploring the mechanisms regulating regeneration of deer antlers

Deer antlers are the only mammalian appendages capable of repeated rounds of regeneration; every year they are shed and regrow from a blastema into large branched structures of cartilage and bone that are used for fighting and display. Longitudinal growth is by a process of modified endochondral ossification and in some species this can exceed 2 cm per day, representing the fastest rate of organ growth in the animal kingdom. However, despite their value as a unique model of mammalian regeneration the underlying mechanisms remain poorly understood. We review what is currently known about the local and systemic regulation of antler regeneration and some of the many unsolved questions of antler physiology are discussed. Molecules that we have identified as having potentially important local roles in antlers include parathyroid hormone-related peptide and retinoic acid (RA). Both are present in the blastema and in the rapidly growing antler where they regulate the differentiation of chondrocytes, osteoblasts and osteoclasts in vitro. Recent studies have shown that blockade of RA signalling can alter cellular differentiation in the blastema in vivo. The trigger that regulates the expression of these local signals is likely to be changing levels of sex steroids because the process of antler regeneration is linked to the reproductive cycle. The natural assumption has been that the most important hormone is testosterone, however, at a cellular level oestrogen may be a more significant regulator. Our data suggest that exogenous oestrogen acts as a 'brake', inhibiting the proliferation of progenitor cells in the antler tip while stimulating their differentiation, thus inhibiting continued growth. Deciphering the mechanism(s) by which sex steroids regulate cell-cycle progression and cellular differentiation in antlers may help to address why regeneration is limited in other mammalian tissues.

Growth and apoptosis during larval forelimb development and adult forelimb regeneration in the newt ( Notophthalmus viridescens)

Many of the genes involved in the initial development of the limb in higher vertebrates are also expressed during regeneration of the limb in urodeles such as Notophthalmus viridescens. These similarities have led researchers to conclude that the regeneration process is a recapitulation of development, and that patterning of the regenerate mimics pattern formation in development. However, the developing limb and the regenerating limb do not look similar. In developing urodele forelimbs, digits appear sequentially as outgrowths from the limb palette. In regeneration, all the digits appear at once. In this work, we address the issue of whether regeneration and development are similar by examining growth and apoptosis patterns. In contrast to higher vertebrates, forelimb development in the newt, N. viridescens, does not use interdigital apoptosis as the method of digit separation. During adult forelimb regeneration, apoptosis seems to play an important role in wound healing and again during cartilage to bone turnover in the advanced digits and radius/ulna. However, similar to forelimb development, demarcation of the digits in adult forelimb regeneration does not involve interdigital apoptosis. Outgrowth, rather than regression of the interdigital mesenchyme, leads to the individualization of forelimb digits in both newt development and regeneration.