Systemic cell cycle activation is induced following complex tissue injury in axolotl

Airway injury induces alveolar epithelial and mesenchymal responses mediated by macrophages

Acute injury in the airways or the lung activates local progenitors and stimulates changes in cell-cell interactions to restore homeostasis, but it is not appreciated how more distant niches are impacted. We utilized mouse models of airway-specific epithelial injury to examine secondary tissue-wide alveolar, immune, and mesenchymal responses. Single-cell transcriptomics and in vivo validation revealed transient, tissue-wide proliferation of alveolar type 2 (AT2) progenitor cells after club cell-specific ablation. The AT2 cell proliferative response was reliant on alveolar macrophages (AMs) via upregulation of Spp1 which encodes the secreted factor Osteopontin. A previously uncharacterized mesenchymal population we termed Mesenchymal Airway/Adventitial Niche Cell 2 (MANC2) also exhibited dynamic changes in abundance and a pro-fibrotic transcriptional signature after club cell ablation in an AM-dependent manner. Overall, these results demonstrate that acute airway damage can trigger distal lung responses including altered cell-cell interactions that may contribute to potential vulnerabilities for further dysregulation and disease.

LiverZap: a chemoptogenetic tool for global and locally restricted hepatocyte ablation to study cellular behaviours in liver regeneration

The liver restores its mass and architecture after injury. Yet, investigating morphogenetic cell behaviours and signals that repair tissue architecture at high spatiotemporal resolution remains challenging. We developed LiverZap, a tuneable chemoptogenetic liver injury model in zebrafish. LiverZap employs the formation of a binary FAP-TAP photosensitiser followed by brief near-infrared illumination inducing hepatocyte-specific death and recapitulating mammalian liver injury types. The tool enables local hepatocyte ablation and extended live imaging capturing regenerative cell behaviours, which is crucial for studying cellular interactions at the interface of healthy and damaged tissue. Applying LiverZap, we show that targeted hepatocyte ablation in a small region of interest is sufficient to trigger local liver progenitor-like cell (LPC)-mediated regeneration, challenging the current understanding of liver regeneration. Surprisingly, the LPC response is also elicited in adjacent uninjured tissue, at up to 100 µm distance to the injury. Moreover, dynamic biliary network rearrangement suggests active cell movements from uninjured tissue in response to substantial hepatocyte loss as an integral step of LPC-mediated liver regeneration. This precisely targetable liver cell ablation tool will enable the discovery of key molecular and morphogenetic regeneration paradigms.

Inter-organ communication during tissue regeneration

Tissue regeneration is not simply a local repair event occurring in isolation from the distant, uninjured parts of the body. Rather, evidence indicates that regeneration is a whole-animal process involving coordinated interactions between different organ systems. Here, we review recent studies that reveal how remote uninjured tissues and organ systems respond to and engage in regeneration. We also discuss the need for toolkits and technological advancements to uncover and dissect organ communication during regeneration.

ERK signaling waves via body-wall muscles guide planarian whole-body regeneration across long distances

Whole-body regeneration is a multifaceted process that reinstates a body to its initial three-dimension size and structure after resection injury. It is well-known that signaling waves such as calcium and extracellular signal-related kinase (ERK) signaling waves can efficiently transmit information between tissues or cells. However, the mechanisms responsible for coordinating wound responses over long distances are largely unexplored. A recent study has reported that the propagation of ERK signaling waves via longitudinal body-wall muscles play an essential role in wound response and whole-body regeneration in planarians, underscoring the significance of feedback interactions between spatially distinct tissues during whole-body regeneration over long distances. These findings not only address the central questions of regenerative biology but also have potential implications for regenerative medicine.

The Dynamic Landscapes of Circular RNAs in Axolotl, a Regenerative Medicine Model, with Implications for Early Phase of Limb Regeneration

Circular RNAs (circRNAs) are of relevance to regenerative medicine and play crucial roles in post-transcriptional and translational regulation of biological processes. circRNAs are a class of RNA molecules that are formed through a unique splicing process, resulting in a covalently closed-loop structure. Recent advancements in RNA sequencing technologies and specialized computational tools have facilitated the identification and functional characterization of circRNAs. These molecules are known to exhibit stability, developmental regulation, and specific expression patterns in different tissues and cell types across various organisms. However, our understanding of circRNA expression and putative function in model organisms for regeneration is limited. In this context, this study reports, for the first time, on the repertoire of circRNAs in axolotl, a widely used model organism for regeneration. We generated RNA-seq data from intact limb, wound, and blastema tissues of axolotl during limb regeneration. The analysis revealed the presence of 35,956 putative axolotl circRNAs, among which 5331 unique circRNAs exhibited orthology with human circRNAs. In silico data analysis underlined the potential roles of axolotl circRNAs in cell cycle, cell death, and cell senescence-related pathways during limb regeneration, suggesting the participation of circRNAs in regulation of diverse functions pertinent to regenerative medicine. These new observations help advance our understanding of the dynamic landscape of axolotl circRNAs, and by extension, inform future regenerative medicine research and innovation that harness this model organism.

Regeneration: Signaling superhighways

Regeneration requires the collective effort of multiple organ systems. A recent study of planarian whole-body regeneration finds that Erk kinase activity propagates rapidly across the entire animal through longitudinal muscle cells to coordinate animal-wide wound responses and that this signal propagation is required for regeneration.

Ultrafast distant wound response is essential for whole-body regeneration

Injury induces systemic responses, but their functions remain elusive. Mechanisms that can rapidly synchronize wound responses through long distances are also mostly unknown. Using planarian flatworms capable of whole-body regeneration, we report that injury induces extracellular signal-regulated kinase (Erk) activity waves to travel at a speed 10-100 times faster than those in other multicellular tissues. This ultrafast propagation requires longitudinal body-wall muscles, elongated cells forming dense parallel tracks running the length of the organism. The morphological properties of muscles allow them to act as superhighways for propagating and disseminating wound signals. Inhibiting Erk propagation prevents tissues distant to the wound from responding and blocks regeneration, which can be rescued by a second injury to distal tissues shortly after the first injury. Our findings provide a mechanism for long-range signal propagation in large, complex tissues to coordinate responses across cell types and highlight the function of feedback between spatially separated tissues during whole-body regeneration.

The salamander blastema within the broader context of metazoan regeneration

Throughout the animal kingdom regenerative ability varies greatly from species to species, and even tissue to tissue within the same organism. The sheer diversity of structures and mechanisms renders a thorough comparison of molecular processes truly daunting. Are "blastemas" found in organisms as distantly related as planarians and axolotls derived from the same ancestral process, or did they arise convergently and independently? Is a mouse digit tip blastema orthologous to a salamander limb blastema? In other fields, the thorough characterization of a reference model has greatly facilitated these comparisons. For example, the amphibian Spemann-Mangold organizer has served as an amazingly useful comparative template within the field of developmental biology, allowing researchers to draw analogies between distantly related species, and developmental processes which are superficially quite different. The salamander limb blastema may serve as the best starting point for a comparative analysis of regeneration, as it has been characterized by over 200 years of research and is supported by a growing arsenal of molecular tools. The anatomical and evolutionary closeness of the salamander and human limb also add value from a translational and therapeutic standpoint. Tracing the evolutionary origins of the salamander blastema, and its relatedness to other regenerative processes throughout the animal kingdom, will both enhance our basic biological understanding of regeneration and inform our selection of regenerative model systems.

Evolutionarily divergent mTOR remodels translatome for tissue regeneration

An outstanding mystery in biology is why some species, such as the axolotl, can regenerate tissues whereas mammals cannot1. Here, we demonstrate that rapid activation of protein synthesis is a unique feature of the injury response critical for limb regeneration in the axolotl (Ambystoma mexicanum). By applying polysome sequencing, we identify hundreds of transcripts, including antioxidants and ribosome components that are selectively activated at the level of translation from pre-existing messenger RNAs in response to injury. By contrast, protein synthesis is not activated in response to non-regenerative digit amputation in the mouse. We identify the mTORC1 pathway as a key upstream signal that mediates tissue regeneration and translational control in the axolotl. We discover unique expansions in mTOR protein sequence among urodele amphibians. By engineering an axolotl mTOR (axmTOR) in human cells, we show that these changes create a hypersensitive kinase that allows axolotls to maintain this pathway in a highly labile state primed for rapid activation. This change renders axolotl mTOR more sensitive to nutrient sensing, and inhibition of amino acid transport is sufficient to inhibit tissue regeneration. Together, these findings highlight the unanticipated impact of the translatome on orchestrating the early steps of wound healing in a highly regenerative species and provide a missing link in our understanding of vertebrate regenerative potential.

Characterizing the lens regeneration process in Pleurodeles waltl

BACKGROUND

Aging and regeneration are heavily linked processes. While it is generally accepted that regenerative capacity declines with age, some vertebrates, such as newts, can bypass the deleterious effects of aging and successfully regenerate a lens throughout their lifetime.

RESULTS

Here, we used Spectral-Domain Optical Coherence Tomography (SD-OCT) to monitor the lens regeneration process of larvae, juvenile, and adult newts. While all three life stages were able to regenerate a lens through transdifferentiation of the dorsal iris pigment epithelial cells (iPECs), an age-related change in the kinetics of the regeneration process was observed. Consistent with these findings, iPECs from older animals exhibited a delay in cell cycle re-entry. Furthermore, it was observed that clearance of the extracellular matrix (ECM) was delayed in older organisms.

CONCLUSIONS

Collectively, our results suggest that although lens regeneration capacity does not decline throughout the lifespan of newts, the intrinsic and extrinsic cellular changes associated with aging alter the kinetics of this process. By understanding how these changes affect lens regeneration in newts, we can gain important insights for restoring the age-related regeneration decline observed in most vertebrates.

Ultrafast and long-range coordination of wound responses is essential for whole-body regeneration

Injury induces systemic, global responses whose functions remain elusive. In addition, mechanisms that rapidly synchronize wound responses through long distances across the organismal scale are mostly unknown. Using planarians, which have extreme regenerative ability, we report that injury induces Erk activity to travel in a wave-like manner at an unexpected speed (∼1 mm/h), 10-100 times faster than those measured in other multicellular tissues. This ultrafast signal propagation requires longitudinal body-wall muscles, elongated cells forming dense parallel tracks running the length of the organism. Combining experiments and computational models, we show that the morphological properties of muscles allow them to minimize the number of slow intercellular signaling steps and act as bidirectional superhighways for propagating wound signals and instructing responses in other cell types. Inhibiting Erk propagation prevents cells distant to the wound from responding and blocks regeneration, which can be rescued by a second injury to distal tissues within a narrow time window after the first injury. These results suggest that rapid responses in uninjured tissues far from wounds are essential for regeneration. Our findings provide a mechanism for long-range signal propagation in large and complex tissues to coordinate cellular responses across diverse cell types, and highlights the function of feedback between spatially separated tissues during whole-body regeneration.

Limb blastema formation: How much do we know at a genetic and epigenetic level?

Regeneration of missing body parts is an incredible ability which is present in a wide number of species. However, this regenerative capability varies among different organisms. Urodeles (salamanders) are able to completely regenerate limbs after amputation through the essential process of blastema formation. The blastema is a collection of relatively undifferentiated progenitor cells that proliferate and repattern to form the internal tissues of a regenerated limb. Understanding blastema formation in salamanders may enable comparative studies with other animals, including mammals, with more limited regenerative abilities and may inspire future therapeutic approaches in humans. This review focuses on the current state of knowledge about how limb blastemas form in salamanders, highlighting both the possible roles of epigenetic controls in this process as well as limitations to scientific understanding that present opportunities for research.

Axolotl: A resourceful vertebrate model for regeneration and beyond

The regenerative capacity varies significantly among the animal kingdom. Successful regeneration program in some animals results in the functional restoration of tissues and lost structures. Among the highly regenerative animals, axolotl provides multiple experimental advantages with its many extraordinary characteristics. It has been positioned as a regeneration model organism due to its exceptional renewal capacity, including the internal organs, central nervous system, and appendages, in a scar-free manner. In addition to this unique regeneration ability, the observed low cancer incidence, its resistance to carcinogens, and the reversing effect of its cell extract on neoplasms strongly suggest its usability in cancer research. Axolotl's longevity and efficient utilization of several anti-aging mechanisms underline its potential to be employed in aging studies.

Radial Glia and Neuronal-like Ependymal Cells Are Present within the Spinal Cord of the Trunk (Body) in the Leopard Gecko (Eublepharis macularius)

As is the case for many lizards, leopard geckos (Eublepharis macularius) can self-detach a portion of their tail to escape predation, and then regenerate a replacement complete with a spinal cord. Previous research has shown that endogenous populations of neural stem/progenitor cells (NSPCs) reside within the spinal cord of the original tail. In response to tail loss, these NSPCs are activated and contribute to regeneration. Here, we investigate whether similar populations of NSPCs are found within the spinal cord of the trunk (body). Using a long-duration 5-bromo-2′-deoxyuridine pulse-chase experiment, we determined that a population of cells within the ependymal layer are label-retaining following a 20-week chase. Tail loss does not significantly alter rates of ependymal cell proliferation within the trunk spinal cord. Ependymal cells of the trunk spinal cord express SOX2 and represent at least two distinct cell populations: radial glial-like (glial fibrillary acidic protein- and Vimentin-expressing) cells; and neuronal-like (HuCD-expressing) cells. Taken together, these data demonstrate that NSPCs of the trunk spinal cord closely resemble those of the tail and support the use of the tail spinal cord as a less invasive proxy for body spinal cord injury investigations.

Local problems need global solutions - the metabolic needs of regenerating organisms

The vast majority of species that belong to the plant or animal kingdom evolved with two main strategies to counter tissue damage - scar formation and regeneration. Whereas scar formation provides a fast and cost-effective repair to exit life-threatening conditions, complete tissue regeneration is time-consuming and requires vast resources to reinstall functionality of affected organs or structures. Local environments in wound healing are widely studied and findings have provided important biomedical applications. Less well understood are organismic physiological parameters and signaling circuits essential to maintain effective tissue repair. Here, we review accumulated evidence that positions the interplay of local and systemic changes in metabolism as essential variables modulating the injury response. We particularly emphasize the role of lipids and lipid-like molecules as significant components long overlooked. This article is protected by copyright. All rights reserved.

Beyond Genetics: The Role of Metabolism in Photoreceptor Survival, Development and Repair

Vision commences in the retina with rod and cone photoreceptors that detect and convert light to electrical signals. The irreversible loss of photoreceptors due to neurodegenerative disease leads to visual impairment and blindness. Interventions now in development include transplanting photoreceptors, committed photoreceptor precursors, or retinal pigment epithelial (RPE) cells, with the latter protecting photoreceptors from dying. However, introducing exogenous human cells in a clinical setting faces both regulatory and supply chain hurdles. Recent work has shown that abnormalities in central cell metabolism pathways are an underlying feature of most neurodegenerative disorders, including those in the retina. Reversal of key metabolic alterations to drive retinal repair thus represents a novel strategy to treat vision loss based on cell regeneration. Here, we review the connection between photoreceptor degeneration and alterations in cell metabolism, along with new insights into how metabolic reprogramming drives both retinal development and repair following damage. The potential impact of metabolic reprogramming on retinal regeneration is also discussed, specifically in the context of how metabolic switches drive both retinal development and the activation of retinal glial cells known as Müller glia. Müller glia display latent regenerative properties in teleost fish, however, their capacity to regenerate new photoreceptors has been lost in mammals. Thus, re-activating the regenerative properties of Müller glia in mammals represents an exciting new area that integrates research into developmental cues, central metabolism, disease mechanisms, and glial cell biology. In addition, we discuss this work in relation to the latest insights gleaned from other tissues (brain, muscle) and regenerative species (zebrafish).

An approach for elucidating dermal fibroblast dedifferentiation in amphibian limb regeneration

Urodele amphibians, Pleurodeles waltl and Ambystoma mexicanum, have organ-level regeneration capability, such as limb regeneration. Multipotent cells are induced by an endogenous mechanism in amphibian limb regeneration. It is well known that dermal fibroblasts receive regenerative signals and turn into multipotent cells, called blastema cells. However, the induction mechanism of the blastema cells from matured dermal cells was unknown. We previously found that BMP2, FGF2, and FGF8 (B2FF) could play sufficient roles in blastema induction in urodele amphibians. Here, we show that B2FF treatment can induce dermis-derived cells that can participate in multiple cell lineage in limb regeneration. We first established a newt dermis-derived cell line and confirmed that B2FF treatment on the newt cells provided plasticity in cellular differentiation in limb regeneration. To clarify the factors that can provide the plasticity in differentiation, we performed the interspecies comparative analysis between newt cells and mouse cells and found the Pde4b gene was upregulated by B2FF treatment only in the newt cells. Blocking PDE4B signaling by a chemical PDE4 inhibitor suppressed dermis-to-cartilage transformation and the mosaic knockout animals showed consistent results. Our results are a valuable insight into how dermal fibroblasts acquire multipotency during the early phase of limb regeneration via an endogenous program in amphibian limb regeneration.

A constitutively expressed fluorescent ubiquitination-based cell-cycle indicator (FUCCI) in axolotls for studying tissue regeneration

Regulation of cell cycle progression is essential for cell proliferation during regeneration following injury. After appendage amputation, the axolotl (Ambystoma mexicanum) regenerates missing structures through an accumulation of proliferating cells known as the blastema. To study cell division during blastema growth, we generated a transgenic line of axolotls that ubiquitously expresses a bicistronic version of the fluorescent ubiquitination-based cell-cycle indicator (FUCCI). We demonstrate near-ubiquitous FUCCI expression in developing and adult tissues, and validate these expression patterns with DNA synthesis and mitosis phase markers. We demonstrate the utility of FUCCI for live and whole-mount imaging, showing the predominantly local contribution of cells during limb and tail regeneration. We also show that spinal cord amputation results in increased proliferation at least 5 mm from the site of injury. Finally, we use multimodal staining to provide cell type information for cycling cells by combining fluorescence in situ hybridization, EdU click-chemistry and immunohistochemistry on a single FUCCI tissue section. This new line of animals will be useful for studying cell cycle dynamics using in situ endpoint assays and in vivo imaging in developing and regenerating animals.

Single cell biology-a Keystone Symposia report

Single cell biology has the potential to elucidate many critical biological processes and diseases, from development and regeneration to cancer. Single cell analyses are uncovering the molecular diversity of cells, revealing a clearer picture of the variation among and between different cell types. New techniques are beginning to unravel how differences in cell state-transcriptional, epigenetic, and other characteristics-can lead to different cell fates among genetically identical cells, which underlies complex processes such as embryonic development, drug resistance, response to injury, and cellular reprogramming. Single cell technologies also pose significant challenges relating to processing and analyzing vast amounts of data collected. To realize the potential of single cell technologies, new computational approaches are needed. On March 17-19, 2021, experts in single cell biology met virtually for the Keystone eSymposium "Single Cell Biology" to discuss advances both in single cell applications and technologies.

The cellular and signaling dynamics of salamander limb regeneration

Limb amputation in salamanders yields a wound response that ultimately leads to replacement of the missing part. This unique-among-tetrapod trait involves the migration and recruitment of multiple cell types including epithelium, immune cells, axonal growth cones, and connective tissue cells to build the blastema which contains the proliferating stem and progenitor cells to rebuild the limb tissues. A number of the signaling and cell biological events have been defined. They point to the intimate coordination of physical events such as osmotic pressure, cell migration, and cell-cell communication with changes in cell identity such as dedifferentiation into embryonic-like epithelial and mesenchymal cells.

Salamander Insights Into Ageing and Rejuvenation

Exhibiting extreme regenerative abilities which extend to complex organs and entire limbs, salamanders have long served as research models for understanding the basis of vertebrate regeneration. Yet these organisms display additional noteworthy traits, namely extraordinary longevity, indefinite regenerative potential and apparent lack of traditional signs of age-related decay or “negligible senescence.” Here, I examine existing studies addressing these features, highlight outstanding questions, and argue that salamanders constitute valuable models for addressing the nature of organismal senescence and the interplay between regeneration and ageing.

Kynurenine Metabolism in the Fat Body Non-autonomously Regulates Imaginal Disc Repair in Drosophila

Tissue interactions are critical for maintaining homeostasis; however, little is known about how remote tissue regulates regeneration. Previously, we established a genetic dual system that induces cell ablation in Drosophila larval imaginal discs and simultaneously manipulates genes in non-damaged tissues. Using humoral metabolome analysis and a genetic damage system, we found that the Tryptophan (Trp)-Kynurenine (Kyn) pathway in the fat body is required for disc repair. Genetic manipulation of Trp-Kyn metabolism in the fat body impaired disc regeneration without affecting wing development. In particular, the fat body-derived humoral kynurenic acid (KynA) was required for disc repair. The impairment of S-adenosylmethionine (SAM) synthesis from methionine (Met) in the fat body hampers the maintenance of KynA levels in hemolymph at the early stage of disc repair, suggesting a connection between Met-SAM and Trp-Kyn metabolisms. Our data indicate KynA from the fat body acts as a permissive metabolite for tissue repair and regeneration.

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Trp-Kyn pathway in Drosophila larval fat body is remotely required for disc repair

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The fat body-derived humoral KynA is required for disc repair

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SAM synthesis in the fat body affects KynA levels in hemolymph during disc repair

Functional Characterization of the Lin28/let-7 Circuit During Forelimb Regeneration in Ambystoma mexicanum and Its Influence on Metabolic Reprogramming

The axolotl (Ambystoma mexicanum) is a caudate amphibian, which has an extraordinary ability to restore a wide variety of damaged structures by a process denominated epimorphosis. While the origin and potentiality of progenitor cells that take part during epimorphic regeneration are known to some extent, the metabolic changes experienced and their associated implications, remain unexplored. However, a circuit with a potential role as a modulator of cellular metabolism along regeneration is that formed by Lin28/let-7. In this study, we report two Lin28 paralogs and eight mature let-7 microRNAs encoded in the axolotl genome. Particularly, in the proliferative blastema stage amxLin28B is more abundant in the nuclei of blastemal cells, while the microRNAs amx-let-7c and amx-let-7a are most downregulated. Functional inhibition of Lin28 factors increase the levels of most mature let-7 microRNAs, consistent with an increment of intermediary metabolites of the Krebs cycle, and phenotypic alterations in the outgrowth of the blastema. In summary, we describe the primary components of the Lin28/let-7 circuit and their function during axolotl regeneration, acting upstream of metabolic reprogramming events.

Conservation analysis of core cell cycle regulators and their transcriptional behavior during limb regeneration in Ambystoma mexicanum

Ambystoma mexicanum (axolotl) has been one of the major experimental models for the study of regeneration during the past 100 years. Axolotl limb regeneration takes place through a multi-stage and complex developmental process called epimorphosis that involves diverse events of cell reprogramming. Such events start with dedifferentiation of somatic cells and the proliferation of quiescent stem cells to generate a population of proliferative cells called blastema. Once the blastema reaches a mature stage, cells undergo progressive differentiation into the diverse cell lineages that will form the new limb. Such pivotal cell reprogramming phenomena depend on the fine-tuned regulation of the cell cycle in each regeneration stage, where cell populations display specific proliferative capacities and differentiation status. The axolotl genome has been fully sequenced and released recently, and diverse RNA-seq approaches have also been generated, enabling the identification and conservatory analysis of core cell cycle regulators in this species. We report here our results from such analyses and present the transcriptional behavior of key regulatory factors during axolotl limb regeneration. We also found conserved protein interactions between axolotl Cyclin Dependent Kinases 2, 4 and 6 and Cyclins type D and E. Canonical CYC-CDK interactions that play major roles in modulating cell cycle progression in eukaryotes.

mTOR Signaling at the Crossroad between Metazoan Regeneration and Human Diseases

A major challenge in medical research resides in controlling the molecular processes of tissue regeneration, as organ and structure damage are central to several human diseases. A survey of the literature reveals that mTOR (mechanistic/mammalian target of rapamycin) is involved in a wide range of regeneration mechanisms in the animal kingdom. More particularly, cellular processes such as growth, proliferation, and differentiation are controlled by mTOR. In addition, autophagy, stem cell maintenance or the newly described intermediate quiescence state, Galert, imply upstream monitoring by the mTOR pathway. In this review, we report the role of mTOR signaling in reparative regenerations in different tissues and body parts (e.g., axon, skeletal muscle, liver, epithelia, appendages, kidney, and whole-body), and highlight how the mTOR kinase can be viewed as a therapeutic target to boost organ repair. Studies in this area have focused on modulating the mTOR pathway in various animal models to elucidate its contribution to regeneration. The diversity of metazoan species used to identify the implication of this pathway might then serve applied medicine (in better understanding what is required for efficient treatments in human diseases) but also evolutionary biology. Indeed, species-specific differences in mTOR modulation can contain the keys to appreciate why certain regeneration processes have been lost or conserved in the animal kingdom.

von Willebrand factor D and EGF domains is an evolutionarily conserved and required feature of blastemas capable of multitissue appendage regeneration

Regenerative ability varies tremendously across species. A common feature of regeneration of appendages such as limbs, fins, antlers, and tails is the formation of a blastema—a transient structure that houses a pool of progenitor cells that can regenerate the missing tissue. We have identified the expression of von Willebrand factor D and EGF domains (vwde) as a common feature of blastemas capable of regenerating limbs and fins in a variety of highly regenerative species, including axolotl (Ambystoma mexicanum), lungfish (Lepidosiren paradoxa), and Polpyterus (Polypterus senegalus). Further, vwde expression is tightly linked to the ability to regenerate appendages in Xenopus laevis. Functional experiments demonstrate a requirement for vwde in regeneration and indicate that Vwde is a potent growth factor in the blastema. These data identify a key role for vwde in regenerating blastemas and underscore the power of an evolutionarily informed approach for identifying conserved genetic components of regeneration.

vwde expression is a common feature of blastemas capable of fin and limb regeneration.

vwde expression is tightly tied to regeneration‐competency.

vwde is required for axolotl limb regeneration, with transient knockdown resulting in severe endpoint phenotypes.

Midkine is a dual regulator of wound epidermis development and inflammation during the initiation of limb regeneration

Formation of a specialized wound epidermis is required to initiate salamander limb regeneration. Yet little is known about the roles of the early wound epidermis during the initiation of regeneration and the mechanisms governing its development into the apical epithelial cap (AEC), a signaling structure necessary for outgrowth and patterning of the regenerate. Here, we elucidate the functions of the early wound epidermis, and further reveal midkine (mk) as a dual regulator of both AEC development and inflammation during the initiation of axolotl limb regeneration. Through loss- and gain-of-function experiments, we demonstrate that mk acts as both a critical survival signal to control the expansion and function of the early wound epidermis and an anti-inflammatory cytokine to resolve early injury-induced inflammation. Altogether, these findings unveil one of the first identified regulators of AEC development and provide fundamental insights into early wound epidermis function, development, and the initiation of limb regeneration.

Parallels between wound healing, epimorphic regeneration and solid tumors

Striking similarities between wound healing, epimorphic regeneration and the progression of solid tumors have been uncovered by recent studies. In this Review, we discuss systemic effects of tumorigenesis that are now being appreciated in epimorphic regeneration, including genetic, cellular and metabolic heterogeneity, changes in circulating factors, and the complex roles of immune cells and immune modulation at systemic and local levels. We suggest that certain mechanisms enabling regeneration may be co-opted by cancer to promote growth at primary and metastatic sites. Finally, we advocate that working with a unified approach could complement research in both fields.

Common themes in tetrapod appendage regeneration: a cellular perspective

Complete and perfect regeneration of appendages is a process that has fascinated and perplexed biologists for centuries. Some tetrapods possess amazing regenerative abilities, but the regenerative abilities of others are exceedingly limited. The reasons underlying these differences have largely remained mysterious. A great deal has been learned about the morphological events that accompany successful appendage regeneration, and a handful of experimental manipulations can be reliably applied to block the process. However, only in the last decade has the goal of attaining a thorough molecular and cellular biological understanding of appendage regeneration in tetrapods become within reach. Advances in molecular and genetic tools for interrogating these remarkable events are now allowing for unprecedented access to the fundamental biology at work in appendage regeneration in a variety of species. This information will be critical for integrating the large body of detailed observations from previous centuries with a modern understanding of how cells sense and respond to severe injury and loss of body parts. Understanding commonalities between regenerative modes across diverse species is likely to illuminate the most important aspects of complex tissue regeneration.

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.

Cross-limb communication during Xenopus hind-limb regenerative response: non-local bioelectric injury signals

Regeneration of damaged body-parts requires coordination of size, shape, location, and orientation of tissue with the rest of the body. It is not currently known how far injury sites communicate with the remaining soma during repair, or what information may emanate from the injury site to other regions. We examined the bioelectric properties (resting potential gradients in the epidermis) of Xenopus froglets undergoing hind-limb amputation and observed that the contralateral (un-damaged) limb exhibits apparent depolarization signals immediately after the opposite hind-limb is amputated. The pattern of depolarization matches that of the amputated limb and is correlated to the position and type of injury, revealing that information about damage is available to remote body tissues and is detectable non-invasively in vivo by monitoring of the bioelectric state. These data extend knowledge about the electrophysiology of regenerative response, identify a novel communication process via long-range spread of injury signaling, a phenomenon which we call bioelectric injury mirroring (BIM), and suggests revisions to regenerative medicine and diagnostic strategies focused entirely on the wound site and to the use of contralateral limbs as controls.