Amputation-induced reactive oxygen species are required for successful Xenopus tadpole tail regeneration

Homeostatic and regenerative neurogenesis in salamanders

Large-scale regeneration in the adult central nervous system is a unique capacity of salamanders among tetrapods. Salamanders can replace neuronal populations, repair damaged nerve fibers and restore tissue architecture in retina, brain and spinal cord, leading to functional recovery. The underlying mechanisms have long been difficult to study due to the paucity of available genomic tools. Recent technological progress, such as genome sequencing, transgenesis and genome editing provide new momentum for systematic interrogation of regenerative processes in the salamander central nervous system. Understanding central nervous system regeneration also entails designing the appropriate molecular, cellular, and behavioral assays. Here we outline the organization of salamander brain structures. With special focus on ependymoglial cells, we integrate cellular and molecular processes of neurogenesis during developmental and adult homeostasis as well as in various injury models. Wherever possible, we correlate developmental and regenerative neurogenesis to the acquisition and recovery of behaviors. Throughout the review we place the findings into an evolutionary context for inter-species comparisons.

Intermediate Developmental Phases During Regeneration

The initial view that regeneration can be a continuum in terms of regulatory mechanisms is gradually changing, and recent evidence points towards the presence of discrete regulatory steps and intermediate phases. Furthermore, regeneration presents an excellent example of a process generating order and pattern, i.e. a self-organization process. It is likely that the process traverses a set of intermediate phases before reaching an endpoint. Although some progress has been made in deciphering the identity of these intermediate phases, a lot more work is needed to derive a comprehensive and complete picture. Here, we discuss the intermediate developmental phases in plant regeneration and compare them with the possible intermediate developmental phases in animal regeneration.

Drosophila as a Model System to Study Cell Signaling in Organ Regeneration

Regeneration is a fascinating phenomenon that allows organisms to replace or repair damaged organs or tissues. This ability occurs to varying extents among metazoans. The rebuilding of the damaged structure depends on regenerative proliferation that must be accompanied by proper cell fate respecification and patterning. These cellular processes are regulated by the action of different signaling pathways that are activated in response to the damage. The imaginal discs of Drosophila melanogaster have the ability to regenerate and have been extensively used as a model system to study regeneration. Drosophila provides an opportunity to use powerful genetic tools to address fundamental problems about the genetic mechanisms involved in organ regeneration. Different studies in Drosophila have helped to elucidate the genes and signaling pathways that initiate regeneration, promote regenerative growth, and induce cell fate respecification. Here we review the signaling networks involved in regulating the variety of cellular responses that are required for discs regeneration.

Innate Immune Response and Off-Target Mis-splicing Are Common Morpholino-Induced Side Effects in Xenopus

Antisense morpholino oligomers (MOs) have been indispensable tools for developmental biologists to transiently knock down (KD) genes rather than to knock them out (KO). Here we report on the implications of genetic KO versus MO-mediated KD of the mesoderm-specifying Brachyury paralogs in the frog Xenopus tropicalis. While both KO and KD embryos fail to activate the same core gene regulatory network, resulting in virtually identical morphological defects, embryos injected with control or target MOs also show a systemic GC content-dependent immune response and many off-target splicing defects. Optimization of MO dosage and increasing incubation temperatures can mitigate, but not eliminate, these MO side effects, which are consistent with the high affinity measured between MO and off-target sequence in vitro. We conclude that while MOs can be useful to profile loss-of-function phenotypes at a molecular level, careful attention must be paid to their immunogenic and off-target side effects.

Melatonin: A Cutaneous Perspective on its Production, Metabolism, and Functions

Melatonin, an evolutionarily ancient derivative of serotonin with hormonal properties, is the main neuroendocrine secretory product of the pineal gland. Although melatonin is best known to regulate circadian rhythmicity and lower vertebrate skin pigmentation, the full spectrum of functional activities of this free radical-scavenging molecule, which also induces/promotes complex antioxidative and DNA repair systems, includes immunomodulatory, thermoregulatory, and antitumor properties. Because this plethora of functional melatonin properties still awaits to be fully appreciated by dermatologists, the current review synthesizes the main features that render melatonin a promising candidate for the management of several dermatoses associated with substantial oxidative damage. We also review why melatonin promises to be useful in skin cancer prevention, skin photo- and radioprotection, and as an inducer of repair mechanisms that facilitate the recovery of human skin from environmental damage. The fact that human skin and hair follicles not only express functional melatonin receptors but also engage in substantial, extrapineal melatonin synthesis further encourages one to systematically explore how the skin's melatonin system can be therapeutically targeted in future clinical dermatology and enrolled for preventive medicine strategies.

Quantitative Proteomics After Spinal Cord Injury (SCI) in a Regenerative and a Nonregenerative Stage in the Frog Xenopus laevis

The capacity to regenerate the spinal cord after an injury is a coveted trait that only a limited group of nonmammalian organisms can achieve. In Xenopus laevis, this capacity is only present during larval or tadpole stages, but is absent during postmetamorphic frog stages. This provides an excellent model for comparative studies between a regenerative and a nonregenerative stage to identify the cellular and molecular mechanisms that explain this difference in regenerative potential. Here, we used iTRAQ chemistry to obtain a quantitative proteome of the spinal cord 1 day after a transection injury in regenerative and nonregenerative stage animals, and used sham operated animals as controls. We quantified a total of 6,384 proteins, with 172 showing significant differential expression in the regenerative stage and 240 in the nonregenerative stage, with an overlap of only 14 proteins. Functional enrichment analysis revealed that although the regenerative stage downregulated synapse/vesicle and mitochondrial proteins, the nonregenerative stage upregulated lipid metabolism proteins, and downregulated ribosomal and translation control proteins. Furthermore, STRING network analysis showed that proteins belonging to these groups are highly interconnected, providing interesting candidates for future functional studies. Data are available via ProteomeXchange with identifier PXD006993.

Damage-induced reactive oxygen species regulate vimentin and dynamic collagen-based projections to mediate wound repair

Tissue injury leads to early wound-associated reactive oxygen species (ROS) production that mediate tissue regeneration. To identify mechanisms that function downstream of redox signals that modulate regeneration, a vimentin reporter of mesenchymal cells was generated by driving GFP from the vimentin promoter in zebrafish. Early redox signaling mediated vimentin reporter activity at the wound margin. Moreover, both ROS and vimentin were necessary for collagen production and reorganization into projections at the leading edge of the wound. Second harmonic generation time-lapse imaging revealed that the collagen projections were associated with dynamic epithelial extensions at the wound edge during wound repair. Perturbing collagen organization by burn wound disrupted epithelial projections and subsequent wound healing. Taken together our findings suggest that ROS and vimentin integrate early wound signals to orchestrate the formation of collagen-based projections that guide regenerative growth during efficient wound repair.

Exposure Memory and Lung Regeneration

Many acute and chronic lung diseases could benefit from improved regeneration therapy. In development and throughout life, genetically encoded exposure memory systems allow environmental exposures, diet, and infectious agents to direct subsequent phenotypic adaptation and responses. The impact of such memory systems on lung regeneration is currently unknown. This article provides a brief overview of advances in redox biology and medicine as a framework for elucidating exposure memory and delineating spatiotemporal responses in lung regeneration. New imaging and omics methods enable precise definition to advance knowledge of development and the cumulative changes in lung biochemistry, structure, and cell populations occurring from prior and ongoing exposures. Importantly, conditioning steps may be needed to reverse exposure memory and enable effective regeneration. Thus, to complement developmental biology and regenerative medicine, research programs are needed to gain systematic knowledge of how lifelong exposures impact lung biology and support transition of lung regeneration from hypothetical to practical medicine.

A transgenic reporter under control of an es1 promoter/enhancer marks wound epidermis and apical epithelial cap during tail regeneration in Xenopus laevis tadpole

Rapid wound healing and subsequent formation of the apical epithelial cap (AEC) are believed to be required for successful appendage regeneration in amphibians. Despite the significant role of AEC in limb regeneration, its role in tail regeneration and the mechanisms that regulate the wound healing and AEC formation are not well understood. We previously identified Xenopus laevis es1, which is preferentially expressed in wounded regions, including the AEC after tail regeneration. In this study we established and characterized transgenic Xenopus laevis lines harboring the enhanced green fluorescent protein (EGFP) gene under control of an es1 gene regulatory sequence (es1:egfp). The EGFP reporter expression was clearly seen in several regions of the embryo and then declined to an undetectable level in larvae, recapitulating the endogenous es1 expression. After amputation of the tadpole tail, EGFP expression was re-activated at the edge of the stump epidermis and then increased in the wound epidermis (WE) covering the amputation surface. As the stump started to regenerate, the EGFP expression became restricted to the most distal epidermal region, including the AEC. EGFP was preferentially expressed in the basal or deep cells but not in the superficial cells of the WE and AEC. We performed a small-scale pharmacological screening for chemicals that affected the expression of EGFP in the stump epidermis after tail amputation. The EGFP expression was attenuated by treatment with an inhibitor for ERK, TGF-β or reactive oxygen species (ROS) signaling. These treatments also impaired wound closure of the amputation surface, suggesting that the three signaling activities are required for es1 expression in the WE and successful wound healing after tail amputation. These findings showed that es1:egfp Xenopus laevis should be a useful tool to analyze molecular mechanisms regulating wound healing and appendage regeneration.

The blastema and epimorphic regeneration in mammals

Studying regeneration in animals where and when it occurs is inherently interesting and a challenging research topic within developmental biology. Historically, vertebrate regeneration has been investigated in animals that display enhanced regenerative abilities and we have learned much from studying organ regeneration in amphibians and fish. From an applied perspective, while regeneration biologists will undoubtedly continue to study poikilothermic animals (i.e., amphibians and fish), studies focused on homeotherms (i.e., mammals and birds) are also necessary to advance regeneration biology. Emerging mammalian models of epimorphic regeneration are poised to help link regenerative biology and regenerative medicine. The regenerating rodent digit tip, which parallels human fingertip regeneration, and the regeneration of large circular defects through the ear pinna in spiny mice and rabbits, provide tractable, experimental systems where complex tissue structures are regrown through blastema formation and morphogenesis. Using these models as examples, we detail similarities and differences between the mammalian blastema and its classical counterpart to arrive at a broad working definition of a vertebrate regeneration blastema. This comparison leads us to conclude that regenerative failure is not related to the availability of regeneration-competent progenitor cells, but is most likely a function of the cellular response to the microenvironment that forms following traumatic injury. Recent studies demonstrating that targeted modification of this microenvironment can restrict or enhance regenerative capabilities in mammals helps provide a roadmap for eventually pushing the limits of human regeneration.

Concise Review: Translating Regenerative Biology into Clinically Relevant Therapies: Are We on the Right Path?

Despite approaches in regenerative medicine using stem cells, bio‐engineered scaffolds, and targeted drug delivery to enhance human tissue repair, clinicians remain unable to regenerate large‐scale, multi‐tissue defects in situ. The study of regenerative biology using mammalian models of complex tissue regeneration offers an opportunity to discover key factors that stimulate a regenerative rather than fibrotic response to injury. For example, although primates and rodents can regenerate their distal digit tips, they heal more proximal amputations with scar tissue. Rabbits and African spiny mice re‐grow tissue to fill large musculoskeletal defects through their ear pinna, while other mammals fail to regenerate identical defects and instead heal ear holes through fibrotic repair. This Review explores the utility of these comparative healing models using the spiny mouse ear pinna and the mouse digit tip to consider how mechanistic insight into reparative regeneration might serve to advance regenerative medicine. Specifically, we consider how inflammation and immunity, extracellular matrix composition, and controlled cell proliferation intersect to establish a pro‐regenerative microenvironment in response to injuries. Understanding how some mammals naturally regenerate complex tissue can provide a blueprint for how we might manipulate the injury microenvironment to enhance regenerative abilities in humans. Stem Cells Translational Medicine2018;7:220–231

Fibroblast growth factors: key players in regeneration and tissue repair

Tissue injury initiates a complex repair process, which in some organisms can lead to the complete regeneration of a tissue. In mammals, however, the repair of most organs is imperfect and results in scar formation. Both regeneration and repair are orchestrated by a highly coordinated interplay of different growth factors and cytokines. Among the key players are the fibroblast growth factors (FGFs), which control the migration, proliferation, differentiation and survival of different cell types. In addition, FGFs influence the expression of other factors involved in the regenerative response. Here, we summarize current knowledge on the roles of endogenous FGFs in regeneration and repair in different organisms and in different tissues and organs. Gaining a better understanding of these FGF activities is important for appropriate modulation of FGF signaling after injury to prevent impaired healing and to promote organ regeneration in humans.

Regeneration Genetics

Understanding how and why animals regenerate complex tissues has the potential to transform regenerative medicine. Here we present an overview of genetic approaches that have recently been applied to dissect mechanisms of regeneration. We describe new advances that relate to central objectives of regeneration biologists researching different tissues and species, focusing mainly on vertebrates. These objectives include defining the cellular sources and key cell behaviors in regenerating tissue, elucidating molecular triggers and brakes for regeneration, and defining the earliest events that control the presence of these molecular factors.

Xenopus laevis as a Model Organism for the Study of Spinal Cord Formation, Development, Function and Regeneration

The spinal cord is the first central nervous system structure to develop during vertebrate embryogenesis, underscoring its importance to the organism. Because of its early formation, accessibility to the developing spinal cord in amniotes is challenging, often invasive and the experimental approaches amenable to model systems like mammals are limited. In contrast, amphibians, in general and the African-clawed frog Xenopus laevis, in particular, offer model systems in which the formation of the spinal cord, the differentiation of spinal neurons and glia and the establishment of spinal neuron and neuromuscular synapses can be easily investigated with minimal perturbations to the whole organism. The significant advances on gene editing and microscopy along with the recent completion of the Xenopus laevis genome sequencing have reinvigorated the use of this classic model species to elucidate the mechanisms of spinal cord formation, development, function and regeneration.

Trends in tissue repair and regeneration

The 6th EMBO conference on the Molecular and Cellular Basis of Regeneration and Tissue Repair took place in Paestum (Italy) on the 17th-21st September, 2016. The 160 scientists who attended discussed the importance of cellular and tissue plasticity, biophysical aspects of regeneration, the diverse roles of injury-induced immune responses, strategies to reactivate regeneration in mammals, links between regeneration and ageing, and the impact of non-mammalian models on regenerative medicine.

Image-Based Measurement of H2O2 Reaction-Diffusion in Wounded Zebrafish Larvae

Epithelial injury induces rapid recruitment of antimicrobial leukocytes to the wound site. In zebrafish larvae, activation of the epithelial NADPH oxidase Duox at the wound margin is required early during this response. Before injury, leukocytes are near the vascular region, that is, ∼100-300 μm away from the injury site. How Duox establishes long-range signaling to leukocytes is unclear. We conceived that extracellular hydrogen peroxide (H₂O₂) generated by Duox diffuses through the tissue to directly regulate chemotactic signaling in these cells. But before it can oxidize cellular proteins, H₂O₂ must get past the antioxidant barriers that protect the cellular proteome. To test whether, or on which length scales this occurs during physiological wound signaling, we developed a computational method based on reaction-diffusion principles that infers H₂O₂ degradation rates from intravital H₂O₂-biosensor imaging data. Our results indicate that at high tissue H₂O₂ levels the peroxiredoxin-thioredoxin antioxidant chain becomes overwhelmed, and H₂O₂ degradation stalls or ceases. Although the wound H₂O₂ gradient reaches deep into the tissue, it likely overcomes antioxidant barriers only within ∼30 μm of the wound margin. Thus, Duox-mediated long-range signaling may require other spatial relay mechanisms besides extracellular H₂O₂ diffusion.

Nerves, H2O2 and Shh: Three players in the game of regeneration

The tight control of reactive oxygen species (ROS) levels is required during regeneration. H₂O₂ in particular assumes clear signalling functions at different steps in this process. Injured nerves induce high levels of H₂O₂ through the activation of the Hedgehog (Shh) pathway, providing an environment that promotes cell plasticity, progenitor recruitment and blastema formation. In turn, high H₂O₂ levels contribute to growing axon attraction. Once re-innervation is completed, nerves subsequently downregulate H₂O₂ levels to their original state. A similar regulatory loop between H₂O₂ levels and nerves also exists during development. This suggests that redox signalling is a major actor in cell plasticity.

New tools for redox biology: From imaging to manipulation

Redox reactions play a key role in maintaining essential biological processes. Deviations in redox pathways result in the development of various pathologies at cellular and organismal levels. Until recently, studies on transformations in the intracellular redox state have been significantly hampered in living systems. The genetically encoded indicators, based on fluorescent proteins, have provided new opportunities in biomedical research. The existing indicators already enable monitoring of cellular redox parameters in different processes including embryogenesis, aging, inflammation, tissue regeneration, and pathogenesis of various diseases. In this review, we summarize information about all genetically encoded redox indicators developed to date. We provide the description of each indicator and discuss its advantages and limitations, as well as points that need to be considered when choosing an indicator for a particular experiment. One chapter is devoted to the important discoveries that have been made by using genetically encoded redox indicators.

The Drosophila Duox maturation factor is a key component of a positive feedback loop that sustains regeneration signaling

Regenerating tissue must initiate the signaling that drives regenerative growth, and sustain that signaling long enough for regeneration to complete. How these key signals are sustained is unclear. To gain a comprehensive view of the changes in gene expression that occur during regeneration, we performed whole-genome mRNAseq of actively regenerating tissue from damaged Drosophila wing imaginal discs. We used genetic tools to ablate the wing primordium to induce regeneration, and carried out transcriptional profiling of the regeneration blastema by fluorescently labeling and sorting the blastema cells, thus identifying differentially expressed genes. Importantly, by using genetic mutants of several of these differentially expressed genes we have confirmed that they have roles in regeneration. Using this approach, we show that high expression of the gene moladietz (mol), which encodes the Duox-maturation factor NIP, is required during regeneration to produce reactive oxygen species (ROS), which in turn sustain JNK signaling during regeneration. We also show that JNK signaling upregulates mol expression, thereby activating a positive feedback signal that ensures the prolonged JNK activation required for regenerative growth. Thus, by whole-genome transcriptional profiling of regenerating tissue we have identified a positive feedback loop that regulates the extent of regenerative growth.

Regenerating tissue must initiate the signaling that drives regenerative growth, and then sustain that signaling long enough for regeneration to complete. Drosophila imaginal discs, the epithelial structures in the larva that will form the adult animal during metamorphosis, have been an important model system for tissue repair and regeneration for over 60 years. Here we show that damage-induced JNK signaling leads to the upregulation of a gene called moladietz, which encodes a co-factor for an enzyme, NADPH dual oxidase (Duox), that generates reactive oxygen species (ROS), a key tissue-damage signal. High expression of moladietz induces continuous production of ROS in the regenerating tissue. The sustained production of ROS then continues to activate JNK signaling throughout the course of regeneration, ensuring maximal tissue regrowth.

ROS in gastrointestinal inflammation: Rescue Or Sabotage?

The intestine is composed of many distinct cell types that respond to commensal microbiota or pathogens with immune tolerance and proinflammatory signals respectively. ROS produced by mucosa-resident cells or by newly recruited innate immune cells are essential for antimicrobial responses and regulation of signalling pathways including processes involved in wound healing. Impaired ROS production due to inactivating patient variants in genes encoding NADPH oxidases as ROS source has been associated with Crohn's disease and pancolitis, whereas overproduction of ROS due to up-regulation of oxidases or altered mitochondrial function was linked to ileitis and ulcerative colitis. Here, we discuss recent advances in our understanding of how maintaining a redox balance is crucial to preserve gut homeostasis.

LINKED ARTICLES

This article is part of a themed section on Redox Biology and Oxidative Stress in Health and Disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.12/issuetoc.

Transcriptional dynamics of tail regeneration in Xenopus tropicalis

In contrast to humans, many amphibians are able to rapidly and completely regenerate complex tissues, including entire appendages. Following tail amputation, Xenopus tropicalis tadpoles quickly regenerate muscle, spinal cord, cartilage, vasculature and skin, all properly patterned in three dimensions. To better understand the molecular basis of this regenerative competence, we performed a transcriptional analysis of the first 72 h of tail regeneration using RNA-Seq. Our analysis refines the windows during which many key biological signaling processes act in regeneration, including embryonic patterning signals, immune responses, bioelectrical signaling and apoptosis. Our work provides a deep database for researchers interested in appendage regeneration, and points to new avenues for further study.

Macrophages are necessary for epimorphic regeneration in African spiny mice

How the immune system affects tissue regeneration is not well understood. In this study, we used an emerging mammalian model of epimorphic regeneration, the African spiny mouse, to examine cell-based inflammation and tested the hypothesis that macrophages are necessary for regeneration. By directly comparing inflammatory cell activation in a 4 mm ear injury during regeneration (Acomys cahirinus) and scarring (Mus musculus), we found that both species exhibited an acute inflammatory response, with scarring characterized by stronger myeloperoxidase activity. In contrast, ROS production was stronger and more persistent during regeneration. By depleting macrophages during injury, we demonstrate a functional requirement for these cells to stimulate regeneration. Importantly, the spatial distribution of activated macrophage subtypes was unique during regeneration with pro-inflammatory macrophages failing to infiltrate the regeneration blastema. Together, our results demonstrate an essential role for inflammatory cells to regulate a regenerative response.

Cap-n-Collar Promotes Tissue Regeneration by Regulating ROS and JNK Signaling in the Drosophila melanogaster Wing Imaginal Disc

Regeneration is a complex process that requires an organism to recognize and repair tissue damage, as well as grow and pattern new tissue. Here, we describe a genetic screen to identify novel regulators of regeneration. We ablated the Drosophila melanogaster larval wing primordium by inducing apoptosis in a spatially and temporally controlled manner and allowed the tissue to regenerate and repattern. To identify genes that regulate regeneration, we carried out a dominant-modifier screen by assessing the amount and quality of regeneration in adult wings heterozygous for isogenic deficiencies. We have identified 31 regions on the right arm of the third chromosome that modify the regenerative response. Interestingly, we observed several distinct phenotypes: mutants that regenerated poorly, mutants that regenerated faster or better than wild-type, and mutants that regenerated imperfectly and had patterning defects. We mapped one deficiency region to cap-n-collar (cnc), the Drosophila Nrf2 ortholog, which is required for regeneration. Cnc regulates reactive oxygen species levels in the regenerating epithelium, and affects c-Jun N-terminal protein kinase (JNK) signaling, growth, debris localization, and pupariation timing. Here, we present the results of our screen and propose a model wherein Cnc regulates regeneration by maintaining an optimal level of reactive oxygen species to promote JNK signaling.

ETV2/ER71 regulates hematopoietic regeneration by promoting hematopoietic stem cell proliferation

Xu et al. show that Etv2 is required for hematopoietic stem and progenitor cell (HSPC) proliferation and expansion after bone marrow transplantation and hematopoietic injury. c-Kit functions downstream of Etv2 in mediating HSPC proliferation and expansion.

Recent studies have established that hematopoietic stem cells (HSCs) are quiescent in homeostatic conditions but undergo extensive cell cycle and expansion upon bone marrow (BM) transplantation or hematopoietic injury. The molecular basis for HSC activation and expansion is not completely understood. In this study, we found that key developmentally critical genes controlling hematopoietic stem and progenitor cell (HSPC) generation were up-regulated in HSPCs upon hematopoietic injury. In particular, we found that the ETS transcription factor Ets variant 2 (Etv2; also known as Er71) was up-regulated by reactive oxygen species in HSPCs and was necessary in a cell-autonomous manner for HSPC expansion and regeneration after BM transplantation and hematopoietic injury. We found c-Kit to be downstream of ETV2. As such, lentiviral c-Kit expression rescued Etv2-deficient HSPC proliferation defects in vitro and in short-term BM transplantation in vivo. These findings demonstrate that Etv2 is an important regulator of hematopoietic regeneration.

Hydrogen peroxide as a central redox signaling molecule in physiological oxidative stress: Oxidative eustress

Hydrogen peroxide emerged as major redox metabolite operative in redox sensing, signaling and redox regulation. Generation, transport and capture of H2O2 in biological settings as well as their biological consequences can now be addressed. The present overview focuses on recent progress on metabolic sources and sinks of H2O2 and on the role of H2O2 in redox signaling under physiological conditions (1-10nM), denoted as oxidative eustress. Higher concentrations lead to adaptive stress responses via master switches such as Nrf2/Keap1 or NF-κB. Supraphysiological concentrations of H2O2 (>100nM) lead to damage of biomolecules, denoted as oxidative distress. Three questions are addressed: How can H2O2 be assayed in the biological setting? What are the metabolic sources and sinks of H2O2? What is the role of H2O2 in redox signaling and oxidative stress?

Killers creating new life: caspases drive apoptosis-induced proliferation in tissue repair and disease

Apoptosis is a carefully orchestrated and tightly controlled form of cell death, conserved across metazoans. As the executioners of apoptotic cell death, cysteine-dependent aspartate-directed proteases (caspases) are critical drivers of this cellular disassembly. Early studies of genetically programmed cell death demonstrated that the selective activation of caspases induces apoptosis and the precise elimination of excess cells, thereby sculpting structures and refining tissues. However, over the past decade there has been a fundamental shift in our understanding of the roles of caspases during cell death-a shift precipitated by the revelation that apoptotic cells actively engage with their surrounding environment throughout the death process, and caspases can trigger a myriad of signals, some of which drive concurrent cell proliferation regenerating damaged structures and building up lost tissues. This caspase-driven compensatory proliferation is referred to as apoptosis-induced proliferation (AiP). Diverse mechanisms of AiP have been found across species, ranging from planaria to mammals. In this review, we summarize the current knowledge of AiP and we highlight recent advances in the field including the involvement of reactive oxygen species and macrophage-like immune cells in one form of AiP, novel regulatory mechanisms affecting caspases during AiP, and emerging clinical data demonstrating the critical importance of AiP in cancer.

Glutathione peroxidase 4 inhibits Wnt/β-catenin signaling and regulates dorsal organizer formation in zebrafish embryos

The Wnt/β-catenin signaling pathway plays pivotal roles in axis formation during embryogenesis and in adult tissue homeostasis. Glutathione peroxidase 4 (GPX4) is a selenoenzyme and participates in the reduction of peroxides. Its synthesis depends on the availability of the element selenium. However, the roles of GPX4 in vertebrate embryonic development and underlying mechanisms are largely unknown. Here, we show that maternal loss of zebrafish gpx4b promotes embryonic dorsal organizer formation, whereas overexpression of gpx4b inhibits the development of the dorsal organizer. Depletion of human GPX4 and zebrafish gpx4b (GPX4/gpx4b) increases, while GPX4/gpx4b overexpression decreases, Wnt/β-catenin signaling in vivo and in vitro Functional and epistatic studies showed that GPX4 functions at the Tcf/Lef level, independently of selenocysteine activation. Mechanistically, GPX4 interacts with Tcf/Lefs and inhibits Wnt activity by preventing the binding of Tcf/Lefs to the promoters of Wnt target genes, resulting in inhibitory action in the presence of Wnt/β-catenin signaling. Our findings unravel GPX4 as a suppressor of Wnt/β-catenin signals, suggesting a possible relationship between the Wnt/β-catenin pathway and selenium via the association of Tcf/Lef family proteins with GPX4.

Gene Expression Profiling in the Injured Spinal Cord of Trachemys scripta elegans: An Amniote with Self-Repair Capabilities

Slider turtles are the only known amniotes with self-repair mechanisms of the spinal cord that lead to substantial functional recovery. Their strategic phylogenetic position makes them a relevant model to investigate the peculiar genetic programs that allow anatomical reconnection in some vertebrate groups but are absent in others. Here, we analyze the gene expression profile of the response to spinal cord injury (SCI) in the turtle Trachemys scripta elegans. We found that this response comprises more than 1000 genes affecting diverse functions: reaction to ischemic insult, extracellular matrix re-organization, cell proliferation and death, immune response, and inflammation. Genes related to synapses and cholesterol biosynthesis are down-regulated. The analysis of the evolutionary distribution of these genes shows that almost all are present in most vertebrates. Additionally, we failed to find genes that were exclusive of regenerating taxa. The comparison of expression patterns among species shows that the response to SCI in the turtle is more similar to that of mice and non-regenerative Xenopus than to Xenopus during its regenerative stage. This observation, along with the lack of conserved “regeneration genes” and the current accepted phylogenetic placement of turtles (sister group of crocodilians and birds), indicates that the ability of spinal cord self-repair of turtles does not represent the retention of an ancestral vertebrate character. Instead, our results suggest that turtles developed this capability from a non-regenerative ancestor (i.e., a lineage specific innovation) that was achieved by re-organizing gene expression patterns on an essentially non-regenerative genetic background. Among the genes activated by SCI exclusively in turtles, those related to anoxia tolerance, extracellular matrix remodeling, and axonal regrowth are good candidates to underlie functional recovery.

GPx1 knockdown suppresses chondrogenic differentiation of ATDC5 cells through induction of reductive stress

Glutathione peroxidase 1 (GPx1) is a selenium (Se)-containing protein and is induced in cartilage formation. GPx1 eliminates reactive oxygen species (ROS), which are required for chondrogenic induction. The physiological properties of GPx1 in cartilage and the redox mechanisms involved are not known. The effects of GPx1 on chondrogenic differentiation of ATDC5 cells were examined through short hairpin RNA-mediated gene silencing. The results demonstrated that GPx1 knockdown impaired gene expression of sex determining region Y-box 9, collagen II (Col II), and aggrecan. GPx1 knockdown suppressed the accumulation of cartilage glycosaminoglycans (GAGs) and the proliferation of chondrocyte. GPx1 knockdown also induced cell apoptosis. However, cell sensitivity toward exogenous oxidative stress was not increased after GPx1 knockdown. Unexpectedly, GPx1 knockdown not only induced oxidative stress characterized by the increased production of ROS but also caused reductive stress indicated by an elevation of glutathione (GSH)/oxidized GSH (GSSG) ratio. Furthermore, GPx1 knockdown-mediated reductive and oxidative stress could be antagonized by a thiol-oxidizing agent diamide and a thiol-containing compound N-acetylcysteine (NAC), respectively. Moreover, NAC attenuated GPx1 knockdown-induced cell apoptosis, while diamide prevented GPx1 knockdown-suppressed chondrocyte proliferation. Finally, diamide but not NAC could rescue GPx1 knockdown-mediated impaired chondrogenic differentiation. In summary, GPx1 is essential for chondrogenic induction in ATDC5 cells mainly through modulation of intracellular GSH/GSSG ratio, rather than an antioxidant enzyme to detoxify ROS. In addition, GPx1 knockdown-induced impaired chondrogenesis may participate in the pathogenesis of the endemic osteoarthropathy due to Se deficiency. These observations offer novel insights for the development of therapeutic target during cartilage degeneration.

IKKα regulates human keratinocyte migration through surveillance of the redox environment

Although the functions of H₂O₂ in epidermal wound repair are conserved throughout evolution, the underlying signaling mechanisms are largely unknown. In this study we used human keratinocytes (HEK001) to investigate H₂O₂-dependent wound repair mechanisms. Scratch wounding led to H₂O₂ production in two or three cell layers at the wound margin within ∼30 min and subsequent cysteine modification of proteins via sulfenylation. Intriguingly, exogenous H₂O₂ treatment resulted in preferential sulfenylation of keratinocytes that adopted a migratory phenotype and detached from neighboring cells, suggesting that one of the primary functions of H₂O₂ is to stimulate signaling factors involved in cell migration. Based on previous findings that revealed epidermal growth factor receptor (EGFR) involvement in H₂O₂-dependent cell migration, we analyzed oxidation of a candidate upstream target, the inhibitor of κB kinase α (IKKα; encoded by CHUK), as a mechanism of action. We show that IKKα is sulfenylated at a conserved cysteine residue in the kinase domain, which correlates with de-repression of EGF promoter activity and increased EGF expression. Thus, this indicates that IKKα promotes migration through dynamic interactions with the EGF promoter depending on the redox state within cells.

Summary: This study provides a newly identified mechanism by which H₂O₂-dependent oxidation of the inhibitor of κB kinase α and de-repression of epidermal growth factor promoter activity stimulates keratinocyte migration.

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.

Inflammation and immunity in organ regeneration

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

Early bioelectric activities mediate redox-modulated regeneration

Reactive oxygen species (ROS) and electric currents modulate regeneration; however, the interplay between biochemical and biophysical signals during regeneration remains poorly understood. We investigate the interactions between redox and bioelectric activities during tail regeneration in Xenopus laevis tadpoles. We show that inhibition of NADPH oxidase-mediated production of ROS, or scavenging or blocking their diffusion into cells, impairs regeneration and consistently regulates the dynamics of membrane potential, transepithelial potential (TEP) and electric current densities (J_I) during regeneration. Depletion of ROS mimics the altered TEP and J_I observed in the non-regenerative refractory period. Short-term application of hydrogen peroxide (H₂O₂) rescues (from depleted ROS) and induces (from the refractory period) regeneration, TEP increase and J_I reversal. H₂O₂ is therefore necessary for and sufficient to induce regeneration and to regulate TEP and J_I Epistasis assays show that voltage-gated Na⁺ channels act downstream of H₂O₂ to modulate regeneration. Altogether, these results suggest a novel mechanism for regeneration via redox-bioelectric orchestration.

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

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

The cellular and molecular mechanisms of tissue repair and regeneration as revealed by studies in Xenopus

Survival of any living organism critically depends on its ability to repair and regenerate damaged tissues and/or organs during its lifetime following injury, disease, or aging. Various animal models from invertebrates to vertebrates have been used to investigate the molecular and cellular mechanisms of wound healing and tissue regeneration. It is hoped that such studies will form the framework for identifying novel clinical treatments that will improve the healing and regenerative capacity of humans. Amongst these models, Xenopus stands out as a particularly versatile and powerful system. This review summarizes recent findings using this model, which have provided fundamental knowledge of the mechanisms responsible for efficient and perfect tissue repair and regeneration.

The early wound signals

Wounding of tissue barriers, such as epithelia, disrupts homeostasis and allows infection. Within minutes, animals detect injury and respond to it by recruitment of phagocytes and barrier breach closure. The signals that activate these first events are scarcely known. Commonly considered are cytoplasmic factors released into the extracellular space by lysing cells (Damage Associated Molecular Patterns, DAMPs). DAMPs activate inflammatory gene transcription through pattern recognition receptors. But the promptness of wound responses is difficult to explain by transcriptional mechanisms alone. This review highlights the emerging role of nonlytic stress signals in the rapid detection of wounds.

The Influence of the Gut Microbiota on Host Physiology: In Pursuit of Mechanisms

The results generated from the NIH funded Human Microbiome Project (HMP) are necessarily tied to the overall mission of the agency, which is to foster scientific discoveries as a basis for protecting and improving health. The investment in the HMP phase 1 accomplished many of its goals including the preliminary characterization of the human microbiome and the identification of links between microbiome diversity and disease states. Going forward, the next step in these studies must involve the identification of the functional molecular elements that mediate the positive influence of a eubiotic microbiome on health and disease. This review will focus on recent advances describing mechanistic events in the intestine elicited by the microbiome. These include symbiotic bacteria-induced activation of redox-dependent cell signaling, the bacterial production of short chain fatty acids and ensuing cellular responses, and the secretion of bacteriocins by bacteria that have anti-microbial activities against potential pathogens.

The African clawed frog Xenopus laevis: A model organism to study regeneration of the central nervous system

While an injury to the central nervous system (CNS) in humans and mammals is irreversible, amphibians and teleost fish have the capacity to fully regenerate after severe injury to the CNS. Xenopus laevis has a high potential to regenerate the brain and spinal cord during larval stages (47-54), and loses this capacity during metamorphosis. The optic nerve has the capacity to regenerate throughout the frog's lifespan. Here, we review CNS regeneration in frogs, with a focus in X. laevis, but also provide some information about X. tropicalis and other frogs. We start with an overview of the anatomy of the Xenopus CNS, including the main supraspinal tracts that emerge from the brain stem, which play a key role in motor control and are highly conserved across vertebrates. We follow with the advantages of using Xenopus, a classical laboratory model organism, with increasing availability of genetic tools like transgenesis and genome editing, and genomic sequences for both X. laevis and X. tropicalis. Most importantly, Xenopus provides the possibility to perform intra-species comparative experiments between regenerative and non-regenerative stages that allow the identification of which factors are permissive for neural regeneration, and/or which are inhibitory. We aim to provide sufficient evidence supporting how useful Xenopus can be to obtain insights into our understanding of CNS regeneration, which, complemented with studies in mammalian vertebrate model systems, can provide a collaborative road towards finding novel therapeutic approaches for injuries to the CNS.

Metabolomic approach for identifying and visualizing molecular tissue markers in tadpoles of Xenopus tropicalis by mass spectrometry imaging

In developmental and cell biology it is crucial to evaluate the dynamic profiles of metabolites. An emerging frog model system using Xenopus tropicalis, whose genome sequence and inbred strains are available, is now ready for metabolomics investigation in amphibians. In this study we applied matrix-assisted laser desorption/ionization (MALDI)-mass spectrometry imaging (MSI) analysis to identify and visualize metabolomic molecular markers in tadpoles of Xenopus tropicalis. We detected tissue-specific peaks and visualized their distribution in tissues, and distinguished 19 tissues and their specific peaks. We identified, for the first time, some of their molecular localizations via tandem mass spectrometric analysis: hydrocortisone in artery, L-DOPA in rhombencephalon, taurine in eye, corticosterone in gill, heme in heart, inosine monophosphate and carnosine in muscle, dopamine in nerves, and phosphatidylethanolamine (16:0/20:4) in pharynx. This is the first MALDI-MSI study of X. tropicalis tadpoles, as in small tadpoles it is hard to distinguish and dissect the various organs. Furthermore, until now there has been no data about the metabolomic profile of each organ. Our results suggest that MALDI-MSI is potentially a powerful tool for examining the dynamics of metabolomics in metamorphosis as well as conformational changes due to metabolic changes.

Summary: We applied matrix-assisted laser desorption/ionization−mass spectrometry imaging analyses to identify and visualize metabolomic molecular markers in tadpoles of Xenopus tropicalis. We found new molecular markers in various tissues and cells.

Hydrogen peroxide and central redox theory for aerobic life: A tribute to Helmut Sies: Scout, trailblazer, and redox pioneer

When Rafael Radi and I wrote about Helmut Sies for the Redox Pioneer series, I was disappointed that the Editor restricted us to the use of "Pioneer" in the title. My view is that Helmut was always ahead of the pioneers: He was a scout discovering paths for exploration and a trailblazer developing strategies and methods for discovery. I have known him for nearly 40 years and greatly enjoyed his collegiality as well as brilliance in scientific scholarship. He made monumental contributions to 20th century physiological chemistry beginning with his first measurement of H2O2 in rat liver. While continuous H2O2 production is dogma today, the concept of H2O2 production in mammalian tissues was largely buried for half a century. He continued this leadership in research on oxidative stress, GSH, selenium, and singlet oxygen, during the timeframe when physiological chemistry and biochemistry transitioned to contemporary 21st century systems biology. His impact has been extensive in medical and health sciences, especially in nutrition, aging, toxicology and cancer. I briefly summarize my interactions with Helmut, stressing our work together on the redox code, a set of principles to link mitochondrial respiration, bioenergetics, H2O2 metabolism, redox signaling and redox proteomics into central redox theory.

An animal model of Miller Fisher syndrome: Mitochondrial hydrogen peroxide is produced by the autoimmune attack of nerve terminals and activates Schwann cells

The neuromuscular junction is a tripartite synapse composed of the presynaptic nerve terminal, the muscle and perisynaptic Schwann cells. Its functionality is essential for the execution of body movements and is compromised in a number of disorders, including Miller Fisher syndrome, a variant of Guillain-Barré syndrome: this autoimmune peripheral neuropathy is triggered by autoantibodies specific for the polysialogangliosides GQ1b and GT1a present in motor axon terminals, including those innervating ocular muscles, and in sensory neurons. Their binding to the presynaptic membrane activates the complement cascade, leading to a nerve degeneration that resembles that caused by some animal presynaptic neurotoxins. Here we have studied the intra- and inter-cellular signaling triggered by the binding and complement activation of a mouse monoclonal anti-GQ1b/GT1a antibody to primary cultures of spinal cord motor neurons and cerebellar granular neurons. We found that a membrane attack complex is rapidly assembled following antibody binding, leading to calcium accumulation, which affects mitochondrial functionality. Consequently, using fluorescent probes specific for mitochondrial hydrogen peroxide, we found that this reactive oxygen species is rapidly produced by mitochondria of damaged neurons, and that it triggers the activation of the MAP kinase pathway in Schwann cells. These results throw light on the molecular and cellular pathogenesis of Miller Fisher syndrome, and may well be relevant to other pathologies of the motor axon terminals, including some subtypes of the Guillain Barré syndrome.

The beneficial role of extracellular reactive oxygen species in apoptosis-induced compensatory proliferation

Apoptosis-induced proliferation (AiP) maintains tissue homeostasis following massive stress-induced cell death. During this phenomenon, dying cells induce proliferation of the surviving cells to compensate for the tissue loss, and thus restore organ size. Along with wound healing and tissue regeneration, AiP also contributes to tumor repopulation following radiation or chemotherapy. There are several models of AiP. Using an “undead” AiP model that causes hyperplastic overgrowth of Drosophila epithelial tissue, we recently demonstrated that extracellular reactive oxygen species (eROS) are produced by undead epithelial cells, and are necessary for inducing AiP and overgrowth. Furthermore, hemocytes, the Drosophila blood cells, are seen adjacent to the undead epithelial tissue, and may secrete the TNF ortholog Eiger that signals through the TNF receptor to active Jun-N-terminal kinase (JNK) in the undead tissue and induce proliferation. We propose that undead epithelial tissue triggers an inflammatory response that resembles recruitment of macrophages to human epithelial tumors, and that these tumor-associated macrophages release signals for proliferation and tumor growth of the epithelium. This Extra View article summarizes these recent findings with a focus on the role of eROS for promoting regeneration and inflammation-induced tumorigenesis.

Developmental Drift and the Role of Wnt Signaling in Aging

Population aging is a public health problem affecting the majority of the developed world. As populations age, the incidence of degenerative diseases increases exponentially, leading to large increases in public spending on healthcare. Here we summarize recent findings on the developmental drift theory of aging, and the links that have been established between aging and the Wnt signaling pathways. We focus on insights derived from model organisms connecting the evolutionary basis of aging and the link to developmental programming.

Endogenous Mechanisms of Cardiac Regeneration

Zebrafish possess a remarkable capacity for cardiac regeneration throughout their lifetime, providing a model for investigating endogenous cellular and molecular mechanisms regulating myocardial regeneration. By contrast, adult mammals have an extremely limited capacity for cardiac regeneration, contributing to mortality and morbidity from cardiac diseases such as myocardial infarction and heart failure. However, the viewpoint of the mammalian heart as a postmitotic organ was recently revised based on findings that the mammalian heart contains multiple undifferentiated cell types with cardiogenic potential as well as a robust regenerative capacity during a short period early in life. Although it occurs at an extremely low level, continuous cardiomyocyte turnover has been detected in adult mouse and human hearts, which could potentially be enhanced to restore lost myocardium in damaged human hearts. This review summarizes and discusses recent advances in the understanding of endogenous mechanisms of cardiac regeneration.

Cell migration during heart regeneration in zebrafish

Zebrafish possess the remarkable ability to regenerate injured hearts as adults, which contrasts the very limited ability in mammals. Although very limited, mammalian hearts do in fact have measurable levels of cardiomyocyte regeneration. Therefore, elucidating mechanisms of zebrafish heart regeneration would provide information of naturally occurring regeneration to potentially apply to mammalian studies, in addition to addressing this biologically interesting phenomenon in itself. Studies over the past 13 years have identified processes and mechanisms of heart regeneration in zebrafish. After heart injury, pre-existing cardiomyocytes dedifferentiate, enter the cell cycle, and repair the injured myocardium. This process requires interaction with epicardial cells, endocardial cells, and vascular endothelial cells. Epicardial cells envelope the heart, while endocardial cells make up the inner lining of the heart. They provide paracrine signals to cardiomyocytes to regenerate the injured myocardium, which is vascularized during heart regeneration. In addition, accumulating results suggest that local migration of these major cardiac cell types have roles in heart regeneration. In this review, we summarize the characteristics of various heart injury methods used in the research community and regeneration of the major cardiac cell types. Then, we discuss local migration of these cardiac cell types and immune cells during heart regeneration. Developmental Dynamics 245:774-787, 2016. © 2016 Wiley Periodicals, Inc.

Zebrafish fin and heart: what's special about regeneration?

Many organs regenerate well in adult zebrafish, but most research has been directed toward fin and heart regeneration. Cells have been found to remain generally lineage-restricted during regeneration, and proliferative regenerative progenitors can be formed by dedifferentiation from differentiated cells. Recent studies begin to shed light on the molecular underpinnings of differences between development and regeneration. Retinoic acid, BMP and NF-κB signaling are emerging as regulators of cellular dedifferentiation. Reactive oxygen species promote regeneration, and the dynamics of ROS signaling might help explain differences between wound healing and regeneration. Finally, the heart has been added to those organs that require a nerve supply to regenerate, and a trade-off between regeneration and tumor suppression has been proposed to help explain why mammals regenerate poorly.