The cellular basis for animal regeneration

In silico identification and structural characterization of telomerase reverse transcriptases in parasitic platyhelminths

Telomere shortening during eukaryotic cell division can lead to severe problems such as inactivation of neighboring genes and aberrant chromosomal fusion. To protect chromosome ends from the replicative errors, most eukaryotes have evolved an enzymatic defense mechanism called telomerase, in which telomerase reverse transcriptase (TERT) plays a central role. This enzymatic activity is highly elevated in consecutively dividing somatic cells of regenerating and probably asexually reproducing, platyhelminths. Therefore, flatworms can be powerful models to investigate the biological implications of TERT in these non-embryonic developments. Current information on the protein is largely limited to a handful of representative species within the phylum Platyhelminthes. This study characterizes the structural features of TERT proteins and their encoding genes in flatworms, aiming to expand our knowledge of the telomere-protecting protein in this lower animal taxon. The platyhelminth genes exhibited exon-intron architectures that were highly divergent from their orthologs in the other lophotrochozoans, and their protein products lacked some TERT-specific domains such as the telomerase essential N-terminal and repeat addition processivity domains. Nevertheless, the unique gene and protein structures were tightly conserved among the flatworm homologs. Analysis of the tert transcripts showed that use of alternative splice acceptors or donors in a minor AT-AC intron, as well as intron retention and exon exclusion, contribute to the generation of aberrant mRNAs. The present findings demonstrate that the tert gene has undergone structural changes soon after the emergence of the platyhelminth lineage, which might have been coordinated with those of its functional counterpart, the telomerase RNA molecule.

Neuronal activation in the axolotl brain promotes tail regeneration

The axolotl retains a remarkable capacity for regenerative repair and is one of the few vertebrate species capable of regenerating its brain and spinal cord after injury. To date, studies investigating axolotl spinal cord regeneration have placed particular emphasis on understanding how cells immediately adjacent to the injury site respond to damage to promote regenerative repair. How neurons outside of this immediate injury site respond to an injury remains unknown. Here, we identify a population of dpErk+/etv1+ glutamatergic neurons in the axolotl telencephalon that are activated in response to injury and are essential for tail regeneration. Furthermore, these neurons project to the hypothalamus where they upregulate the neuropeptide neurotensin in response to injury. Together, these findings identify a unique population of neurons in the axolotl brain whose activation is necessary for successful tail regeneration, and sheds light on how neurons outside of the immediate injury site respond to an injury.

Molecular basis of cell fate plasticity - insights from the privileged cells

In the post-Yamanaka era, the rolling balls on Waddington's hilly landscape not only roll downward, but also go upward or sideways. This new-found mobility implies that the tantalizing somatic cell plasticity fueling regeneration, once only known to planarians and newts, might be sparking in the cells of mice and humans, if only we knew how to fully unlock it. The hope for ultimate regeneration was made even more tangible by the observations that partial reprogramming by the Yamanaka factors reverses many hallmarks of aging [76], even though the underlying mechanism remains unclear. We intend to revisit the milestones in the evolving understanding of cell fate plasticity and glean molecular insights from an unusual somatic cell state, the privileged cell state that reprograms in a manner defying the stochastic model. We synthesize our view of the molecular underpinning of cell fate plasticity, from which we speculate how to harness it for regeneration and rejuvenation. We propose that senescence, aging and malignancy represent distinct cell states with definable biochemical and biophysical parameters.

Pluripotent Stem Cell Plasticity is Sculpted by a Slit-Independent Robo Pathway in a Regenerative Animal

Whole-body regeneration requires adult stem cells with high plasticity to differentiate into missing cell types. Planarians possess a unique configuration of organs embedded in a vast pool of pluripotent stem cells. How stem cells integrate positional information with discrete fates remains unknown. Here, we use the planarian pharynx to define the cell fates that depend on the pioneer transcription factor FoxA. We find that Roundabout receptor RoboA suppresses aberrant pharynx cell fates by altering foxA expression, independent of the canonical ligand Slit. An RNAi screen for extracellular proteins identifies Anosmin-1 as a potential partner of RoboA. Perturbing global patterning demonstrates that roboA / anosmin-1 functions locally in the brain. By contrast, altering pharynx fate with foxA knockdown induces head-specific neurons in the pharynx, indicating a latent plasticity of stem cells. Our data links critical extracellular cues with cell fate decisions of highly plastic stem cells, ensuring the fidelity of organ regeneration.

Transforming growth factor-β-mediated regulation of atoh1-expressing neural progenitors is involved in the generation of cerebellar granule cells in larval and adult zebrafish

Granule cells in the cerebellum are the most numerous neurons in the vertebrate brain. They are derived from neural progenitor cells that express the proneural gene atoh1 (atoh1a, b, c in zebrafish) during early neurogenesis. In zebrafish, unlike in mammals, granule cells are continuously produced throughout life, from the larval stage to adulthood. Additionally, granule cells regenerate and replace damaged areas following injury in the adult cerebellum. However, the mechanisms underlying granule cell generation and their role in adult cerebellar regeneration remain largely unclear. In this study, using lineage tracing with the inducible DNA recombinase CreERT2, we found that granule cells differentiated from atoh1c-expressing neural progenitor cells and migrated to their appropriate locations in the adult stage, similar to the processes observed during early embryogenesis. Granule cells that differentiated from atoh1c-expressing neural progenitor cells in adulthood also contributed to cerebellar regeneration. Furthermore, inhibition of transforming growth factor-β (TGF-β) signaling, either via chemical inhibitors or CRISPR/Cas9, suppressed atoh1a/c expression and reduced granule cell numbers in larvae. Chemical inhibition of TGF-β signaling also suppressed neural progenitor cell proliferation, atoh1c expression, and granule cell neurogenesis in the adult cerebellum. These findings demonstrate that TGF-β signaling is essential for granule cell production from progenitor cells throughout the lifespan of zebrafish.

Physical Forces in Regeneration of Cells and Tissues

The ability to regenerate after the loss of a part is a hallmark of living systems and occurs at both the tissue and organ scales, but also within individual cells. Regeneration entails many processes that are physical and mechanical in nature, including the closure of wounds, the repositioning of material from one place to another, and the restoration of symmetry following perturbations. However, we currently know far more about the genetics and molecular signaling pathways involved in regeneration, and there is a need to investigate the role of physical forces in the process. Here, we will provide an overview of how physical forces may play a role in wound healing and regeneration, in which we compare and contrast regenerative processes at the tissue and cell scales.

Single-cell RNA sequencing of the holothurian regenerating intestine reveals the pluripotency of the coelomic epithelium

In holothurians, the regenerative process following evisceration involves the development of a 'rudiment' or 'anlage' at the injured end of the mesentery. This regenerating anlage plays a pivotal role in the formation of a new intestine. Despite its significance, our understanding of the molecular characteristics inherent to the constituent cells of this structure has remained limited. To address this gap, we employed state-of-the-art scRNA-seq and hybridization chain reaction fluorescent in situ hybridization analyses to discern the distinct cellular populations associated with the regeneration anlage. Through this approach, we successfully identified 13 distinct cell clusters. Among these, two clusters exhibit characteristics consistent with putative mesenchymal cells, while another four show features akin to coelomocyte cell populations. The remaining seven cell clusters collectively form a large group encompassing the coelomic epithelium of the regenerating anlage and mesentery. Within this large group of clusters, we recognized previously documented cell populations such as muscle precursors, neuroepithelial cells, and actively proliferating cells. Strikingly, our analysis provides data for identifying at least four other cellular populations that we define as the precursor cells of the growing anlage. Consequently, our findings strengthen the hypothesis that the coelomic epithelium of the anlage is a pluripotent tissue that gives rise to diverse cell types of the regenerating intestinal organ. Moreover, our results provide the initial view into the transcriptomic analysis of cell populations responsible for the amazing regenerative capabilities of echinoderms.

Notch Is Required for Neural Progenitor Proliferation During Embryonic Eye Regrowth

The ability of an organism to regrow tissues is regulated by various signaling pathways. One such pathway that has been studied widely both in the context of regeneration and development is the Notch signaling pathway. Notch is required for the development of the eye and regeneration of tissues in multiple organisms, but it is unknown if Notch plays a role in the regulation of Xenopus laevis embryonic eye regrowth. We found that Notch1 is required for eye regrowth and regulates retinal progenitor cell proliferation. Chemical and molecular inhibition of Notch1 significantly decreased eye regrowth by reducing retinal progenitor cell proliferation without affecting retinal differentiation. Temporal inhibition studies showed that Notch function is required during the first day of regrowth. Interestingly, Notch1 loss-of-function phenocopied the effects of the inhibition of the proton pump, vacuolar-type ATPase (V-ATPase), where retinal proliferation but not differentiation was blocked during eye regrowth. Overexpression of a form of activated Notch1, the Notch intracellular domain (NICD) rescued the loss of eye regrowth due to V-ATPase inhibition. These findings highlight the importance of the Notch signaling pathway in eye regeneration and its role in inducing retinal progenitor cell proliferation in response to injury.

Systemic coordination of whole-body tissue remodeling during local regeneration in sea anemones

The complexity of regeneration extends beyond local wound responses, eliciting systemic processes across the entire organism. However, the functional relevance and coordination of distant molecular processes remain unclear. In the cnidarian Nematostella vectensis, we show that local regeneration triggers a systemic homeostatic response, leading to coordinated whole-body remodeling. Leveraging spatial transcriptomics, endogenous protein tagging, and live imaging, we comprehensively dissect this systemic response at the organismal scale. We identify proteolysis as a critical process driven by both local and systemic upregulation of metalloproteases. We show that metalloproteinase expression levels and activity scale with the extent of tissue loss. This proportional response drives long-range tissue and extracellular matrix movement. Our findings demonstrate the adaptive nature of the systematic response in regeneration, enabling the organism to maintain shape homeostasis while coping with a wide range of injuries.

Functional significance of earthworm clitellum in regulating the various biological aspects of cell survival and regeneration

Earthworms are a highly abundant species in nature, with nearly 7000 different species being discovered. Despite the similarities in morphology among earthworm species, their regeneration capabilities vary based on the clitellum. The clitellum plays a crucial role in the clitellum-dependent worms, as it is involved in the processes of regeneration and reproduction in earthworms. The fascinating characteristic of the clitellum, which serves as a hub for stem cells in clitellum-dependent worms, plays a crucial role in various biological processes that require further exploration. This review focuses on the overall physiological functions and uncovers the lesser-known roles of the clitellum that have been documented in various research articles. In recent times, numerous studies have been conducted using the earthworm model to explore various areas. In that regard, the clitellum's different roles in regulating and controlling stem cells, the regeneration process, regulation of organogenesis, stress response, aging, autotomy, and various features have been briefly discussed. Ultimately, we emphasized the unique and versatile role of the clitellum in the animal model, making it an ideal choice for studying development, regeneration, stem cells, organogenesis, toxicology, autotomy, and aging response.

Stem cells (neoblasts) and positional information jointly dominate regeneration in planarians

Regeneration is the ability to accurately regrow missing body parts. The unparalleled regenerative capacity and incredible tissue plasticity of planarians, both resulting from the presence of abundant adult stem cells referred to as neoblasts, offer a unique opportunity to investigate the cellular and molecular principles underlying regeneration. Neoblasts are capable of self-renewal and differentiation into the desired cell types for correct replacement of lost parts after tissue damage. Positional information in muscle cells governs the polarity and patterning of the body plan during homeostasis and regeneration. For planarians, removal of neoblasts disables the regenerative feats and disruption of positional information results in the regeneration of inappropriate missing body regions, only the combination of neoblasts and positional information enables regeneration. Here, I summarize the current state of the field in neoblast lineage potential, subclasses and specification, and in the roles of positional information for proper tissue turnover and regeneration in planarians.

Tokay gecko tail regeneration involves temporally collinear expression of HOXC genes and early expression of satellite cell markers

BACKGROUND

Regeneration is the replacement of lost or damaged tissue with a functional copy. In axolotls and zebrafish, regeneration involves stem cells produced by de-differentiation. These cells form a growth zone which expresses developmental patterning genes at its apex. This system resembles an embryonic developmental field where cells undergo pattern formation. Some lizards, including geckos, can regenerate their tails, but it is unclear whether they show a "development-like" regeneration pathway.

RESULTS

Using the tokay gecko (Gekko gecko) model species, we examined seven stages of tail regeneration, and three stages of embryonic tail bud development, using transcriptomics, single-cell sequencing, and in situ hybridization. We find no apical growth zone in the regenerating tail. The transcriptomes of the regenerating vs. embryonic tails are quite different with respect to developmental patterning genes. Posterior HOXC genes were activated in a temporally collinear sequence in the regenerating tail. The major precursor populations were stromal cells (regenerating tail) vs. pluripotent stem cells (embryonic tail). Segmented skeletal muscles were regenerated with no expression of classical segmentation genes, but with the early activation of satellite cell markers.

CONCLUSIONS

Our study suggests that tail regeneration in the tokay gecko-unlike tail development-might rely on the activation of resident stem cells, guided by pre-existing positional information.

Single-cell RNA sequencing of the holothurian regenerating intestine reveals the pluripotency of the coelomic epithelium

In holothurians, the regenerative process following evisceration involves the development of a "rudiment" or "anlage" at the injured end of the mesentery. This regenerating anlage plays a pivotal role in the formation of a new intestine. Despite its significance, our understanding of the molecular characteristics inherent to the constituent cells of this structure has remained limited. To address this gap, we employed state-of-the-art scRNA-seq and HCR-FISH analyses to discern the distinct cellular populations associated with the regeneration anlage. Through this approach, we successfully identified thirteen distinct cell clusters. Among these, two clusters exhibit characteristics consistent with putative mesenchymal cells, while another four show features akin to coelomocyte cell populations. The remaining seven cell clusters collectively form a large group encompassing the coelomic epithelium of the regenerating anlage and mesentery. Within this large group of clusters, we recognized previously documented cell populations such as muscle precursors, neuroepithelial cells and actively proliferating cells. Strikingly, our analysis provides data for identifying at least four other cellular populations that we define as the precursor cells of the growing anlage. Consequently, our findings strengthen the hypothesis that the coelomic epithelium of the anlage is a pluripotent tissue that gives rise to diverse cell types of the regenerating intestinal organ. Moreover, our results provide the initial view into the transcriptomic analysis of cell populations responsible for the amazing regenerative capabilities of echinoderms.

Regeneration, Regengrow and Tissue Repair in Animals: Evolution Indicates That No Regeneration Occurs in Terrestrial Environments but Only Recovery Healing

The present, brief review paper summarizes previous studies on a new interpretation of the presence and absence of regeneration in invertebrates and vertebrates. Broad regeneration is considered exclusive of aquatic or amphibious animals with larval stages and metamorphosis, where also a patterning process is activated for whole-body regeneration or for epimorphosis. In contrast, terrestrial invertebrates and vertebrates can only repair injury or the loss of body parts through a variable "recovery healing" of tissues, regengrow or scarring. This loss of regeneration likely derives from the change in genomes during land adaptation, which included the elimination of larval stages and intense metamorphosis. The terrestrial conditions are incompatible with the formation of embryonic organs that are necessary for broad regeneration. In fact, no embryonic organ can survive desiccation, intense UV or ROS exposition on land, and rapid reparative processes without embryonic patterning, such as recovery healing and scarring, have replaced broad regeneration in terrestrial species. The loss of regeneration in land animals likely depends on the alteration of developmental gene pathways sustaining regeneration that occurred in progenitor marine animals. Terrestrial larval stages, like those present in insects among arthropods, only metamorphose using small body regions indicated as imaginal disks, a terrestrial adaptation, not from a large restructuring process like in aquatic-related animals. These invertebrates can reform body appendages only during molting, a process indicated as regengrow, not regeneration. Most amniotes only repair injuries through scarring or a variable recovery healing, occasionally through regengrow, the contemporaneous healing in conjunction with somatic growth, forming sometimes new heteromorphic organs.

Mitochondrial dynamics govern whole-body regeneration through stem cell pluripotency and mitonuclear balance

Tissue regeneration is a complex process involving large changes in cell proliferation, fate determination, and differentiation. Mitochondrial dynamics and metabolism play a crucial role in development and wound repair, but their function in large-scale regeneration remains poorly understood. Planarians offer an excellent model to investigate this process due to their remarkable regenerative abilities. In this study, we examine mitochondrial dynamics during planarian regeneration. We find that knockdown of the mitochondrial fusion gene, opa1, impairs both tissue regeneration and stem cell pluripotency. Interestingly, the regeneration defects caused by opa1 knockdown are rescued by simultaneous knockdown of the mitochondrial fission gene, drp1, which partially restores mitochondrial dynamics. Furthermore, we discover that Mitolow stem cells exhibit an enrichment of pluripotency due to their fate choices at earlier stages. Transcriptomic analysis reveals the delicate mitonuclear balance in metabolism and mitochondrial proteins in regeneration, controlled by mitochondrial dynamics. These findings highlight the importance of maintaining mitochondrial dynamics in large-scale tissue regeneration and suggest the potential for manipulating these dynamics to enhance stem cell functionality and regenerative processes.

Notch signaling regulates limb regeneration through Hes1 and HeyL in the Chinese mitten crab

Tissue regeneration is an efficient strategy developed by animals to compensate for damaged tissues, involving various types of progenitor cells. Deciphering the signal network that modulates the activity of these progenitors during regeneration is crucial for understanding the differences in regenerative capacities across species. In this study, we evaluated the expression profile and phenotypic function of Notch signaling during limb regeneration in arthropod Chinese mitten crabs. The expression of key components of the Notch signaling pathway was upregulated at 7-day post-autotomy (7 DPA), and declined later at 18-day post-autotomy (18 DPA). To assess the role of Notch, we injected dsRNA targeting the Notch gene into the automized area and evaluated the regeneration efficiency. Our results indicated that blocking Notch signaling led to regenerative defects, manifested by delays in the wound closure and blastema emergence processes. Furthermore, the expression of Notch target genes, Hes1 and HeyL, was significantly reduced following Notch knockdown by dsRNA. Knockdown of Hes1 specifically impaired the proliferation and expression of neural progenitor cell markers, without affecting myogenic cells. In contrast, blockage of HeyL inhibited the proliferation and expression of markers in both activated neurogenic and myogenic progenitor cells, while up-regulating markers of quiescent neural progenitor cells. These findings suggest that Notch signaling plays an important role in limb regeneration of E. sinensis by activating downstream effectors Hes1 and HeyL, regulating neurogenesis and myogenesis through distinct mechanisms.

Animals as Architects: Building the Future of Technology-Supported Rehabilitation with Biomimetic Principles

Rehabilitation science has evolved significantly with the integration of technology-supported interventions, offering objective assessments, personalized programs, and real-time feedback for patients. Despite these advances, challenges remain in fully addressing the complexities of human recovery through the rehabilitation process. Over the last few years, there has been a growing interest in the application of biomimetics to inspire technological innovation. This review explores the application of biomimetic principles in rehabilitation technologies, focusing on the use of animal models to help the design of assistive devices such as robotic exoskeletons, prosthetics, and wearable sensors. Animal locomotion studies have, for example, inspired energy-efficient exoskeletons that mimic natural gait, while insights from neural plasticity research in species like zebrafish and axolotls are advancing regenerative medicine and rehabilitation techniques. Sensory systems in animals, such as the lateral line in fish, have also led to the development of wearable sensors that provide real-time feedback for motor learning. By integrating biomimetic approaches, rehabilitation technologies can better adapt to patient needs, ultimately improving functional outcomes. As the field advances, challenges related to translating animal research to human applications, ethical considerations, and technical barriers must be addressed to unlock the full potential of biomimetic rehabilitation.

Molecular profiles, sources and lineage restrictions of stem cells in an annelid regeneration model

Regeneration of missing body parts can be observed in diverse animal phyla, but it remains unclear to which extent these capacities rely on shared or divergent principles. Research into this question requires detailed knowledge about the involved molecular and cellular principles in suitable reference models. By combining single-cell RNA sequencing and mosaic transgenesis in the marine annelid Platynereis dumerilii, we map cellular profiles and lineage restrictions during posterior regeneration. Our data reveal cell-type specific injury responses, re-expression of positional identity factors, and the re-emergence of stem cell signatures in multiple cell populations. Epidermis and mesodermal coelomic tissue produce distinct putative posterior stem cells (PSCs) in the emerging blastema. A novel mosaic transgenesis strategy reveals both developmental compartments and lineage restrictions during regenerative growth. Our work supports the notion that posterior regeneration involves dedifferentiation, and reveals molecular and mechanistic parallels between annelid and vertebrate regeneration.

Progressive modifications during evolution involving epigenetic changes have determined loss of regeneration mainly in terrestrial animals: A hypothesis

In order to address a biological explanation for the different regenerative abilities present among animals, a new evolutionary speculation is presented. It is hypothesized that epigenetic mechanisms have lowered or erased regeneration during the evolution of terrestrial invertebrates and vertebrates. The hypothesis indicates that a broad regeneration can only occur in marine or freshwater conditions, and that life on land does not allow for high regeneration. This is due to the physical, chemical and microbial conditions present in the terrestrial environment with respect to those of the aquatic environment. The present speculation provides examples of hypothetic evolutionary animal lineages that colonized the land, such as parasitic annelids, terrestrial mollusks, arthropods and amniotes. These are the animals where regeneration is limited or absent and their injuries are only repaired through limited healing or scarring. It is submitted that this loss derived from changes in the developmental gene pathways sustaining regeneration in the aquatic environment but that cannot be expressed on land. Once regeneration was erased in terrestrial species, re-adaptation to freshwater niches could not reactivate the previously altered gene pathways that determined regeneration. Therefore a broad regeneration was no longer possible or became limited and heteromorphic in the derived, extant animals. Only in few cases extensive healing abilities or regengrow, a healing process where regeneration overlaps with somatic growth, have evolved among arthropods and amniotes. The present paper is an extension of previous speculations trying to explain in biological terms the different regenerative abilities present among metazoans.

Variations in cell plasticity and proliferation underlie distinct modes of regeneration along the antero-posterior axis in the annelid Platynereis

The capacity to regenerate lost tissues varies significantly among animals. Some phyla, such as the annelids, display substantial regenerating abilities, although little is known about the cellular mechanisms underlying the process. To precisely determine the origin, plasticity and fate of the cells participating in blastema formation and posterior end regeneration after amputation in the annelid Platynereis dumerilii, we developed specific tools to track different cell populations. Using these tools, we find that regeneration is partly promoted by a population of proliferative gut cells whose regenerative potential varies as a function of their position along the antero-posterior axis of the worm. Gut progenitors from anterior differentiated tissues are lineage restricted, whereas gut progenitors from the less differentiated and more proliferative posterior tissues are much more plastic. However, they are unable to regenerate the stem cells responsible for the growth of the worms. Those stem cells are of local origin, deriving from the cells present in the segment abutting the amputation plane, as are most of the blastema cells. Our results favour a hybrid and flexible cellular model for posterior regeneration in Platynereis relying on different degrees of cell plasticity.

Satellite glial cell manipulation prior to axotomy enhances developing dorsal root ganglion central branch regrowth into the spinal cord

The central and peripheral nervous systems (CNS and PNS, respectively) exhibit remarkable diversity in the capacity to regenerate following neuronal injury with PNS injuries being much more likely to regenerate than those that occur in the CNS. Glial responses to damage greatly influence the likelihood of regeneration by either promoting or inhibiting axonal regrowth over time. However, despite our understanding of how some glial lineages participate in nerve degeneration and regeneration, less is known about the contributions of peripheral satellite glial cells (SGC) to regeneration failure following central axon branch injury of dorsal root ganglia (DRG) sensory neurons. Here, using in vivo, time-lapse imaging in larval zebrafish coupled with laser axotomy, we investigate the role of SGCs in axonal regeneration. In our studies we show that SGCs respond to injury by relocating their nuclei to the injury site during the same period that DRG neurons produce new central branch neurites. Laser ablation of SGCs prior to axon injury results in more neurite growth attempts and ultimately a higher rate of successful central axon regrowth, implicating SGCs as inhibitors of regeneration. We also demonstrate that this SGC response is mediated in part by ErbB signaling, as chemical inhibition of this receptor results in reduced SGC motility and enhanced central axon regrowth. These findings provide new insights into SGC-neuron interactions under injury conditions and how these interactions influence nervous system repair.

Characterization of regeneration initiating cells during Xenopus laevis tail regeneration

BACKGROUND

Embryos are regeneration and wound healing masters. They rapidly close wounds and scarlessly remodel and regenerate injured tissue. Regeneration has been extensively studied in many animal models using new tools such as single-cell analysis. However, until now, they have been based primarily on experiments assessing from 1 day post injury.

RESULTS

In this paper, we reveal that critical steps initiating regeneration occur within hours after injury. We discovered the regeneration initiating cells (RICs) using single-cell and spatial transcriptomics of the regenerating Xenopus laevis tail. RICs are formed transiently from the basal epidermal cells, and their expression signature suggests they are important for modifying the surrounding extracellular matrix thus regulating development. The absence or deregulation of RICs leads to excessive extracellular matrix deposition and defective regeneration.

CONCLUSION

RICs represent a newly discovered transient cell state involved in the initiation of the regeneration process.

Clinical, mechanistic, and therapeutic landscape of cutaneous fibrosis

When dysregulated, skin fibrosis can lead to a multitude of pathologies. We provide a framework for understanding the wide clinical spectrum, mechanisms, and management of cutaneous fibrosis encompassing a variety of matrix disorders, fibrohistiocytic neoplasms, injury-induced scarring, and autoimmune scleroses. Underlying such entities are common mechanistic pathways that leverage morphogenic signaling, immune activation, and mechanotransduction to modulate fibroblast function. In light of the limited array of available treatments for cutaneous fibrosis, scientific insights have opened new therapeutic and investigative avenues for conditions that still lack effective interventions.

Epithelial-mesenchymal transition in tissue repair and degeneration

Epithelial-mesenchymal transitions (EMTs) are the epitome of cell plasticity in embryonic development and cancer; during EMT, epithelial cells undergo dramatic phenotypic changes and become able to migrate to form different tissues or give rise to metastases, respectively. The importance of EMTs in other contexts, such as tissue repair and fibrosis in the adult, has become increasingly recognized and studied. In this Review, we discuss the function of EMT in the adult after tissue damage and compare features of embryonic and adult EMT. Whereas sustained EMT leads to adult tissue degeneration, fibrosis and organ failure, its transient activation, which confers phenotypic and functional plasticity on somatic cells, promotes tissue repair after damage. Understanding the mechanisms and temporal regulation of different EMTs provides insight into how some tissues heal and has the potential to open new therapeutic avenues to promote repair or regeneration of tissue damage that is currently irreversible. We also discuss therapeutic strategies that modulate EMT that hold clinical promise in ameliorating fibrosis, and how precise EMT activation could be harnessed to enhance tissue repair.

SoxC and MmpReg promote blastema formation in whole-body regeneration of fragmenting potworms Enchytraeus japonensis

Regeneration in many animals involves the formation of a blastema, which differentiates and organizes into the appropriate missing body parts. Although the mechanisms underlying blastema formation are often fundamental to regeneration biology, information on the cellular and molecular basis of blastema formation remains limited. Here, we focus on a fragmenting potworm (Enchytraeus japonensis), which can regenerate its whole body from small fragments. We find soxC and mmpReg as upregulated genes in the blastema. RNAi of soxC and mmpReg reduce the number of blastema cells, indicating that soxC and mmpReg promote blastema formation. Expression analyses show that soxC-expressing cells appear to gradually accumulate in blastema and constitute a large part of the blastema. Additionally, similar expression dynamics of SoxC orthologue genes in frog (Xenopus laevis) are found in the regeneration blastema of tadpole tail. Our findings provide insights into the cellular and molecular mechanisms underlying blastema formation across species.

Mechanisms of regeneration: to what extent do they recapitulate development?

One of the enduring debates in regeneration biology is the degree to which regeneration mirrors development. Recent technical advances, such as single-cell transcriptomics and the broad applicability of CRISPR systems, coupled with new model organisms in research, have led to the exploration of this longstanding concept from a broader perspective. In this Review, I outline the historical parallels between development and regeneration before focusing on recent research that highlights how dissecting the divergence between these processes can uncover previously unreported biological mechanisms. Finally, I discuss how these advances position regeneration as a more dynamic and variable process with expanded possibilities for morphogenesis compared with development. Collectively, these insights into mechanisms that orchestrate morphogenesis may reshape our understanding of the evolution of regeneration, reveal hidden biology activated by injury, and offer non-developmental strategies for restoring lost or damaged organs and tissues.

Peptide REF1 is a local wound signal promoting plant regeneration

Plants frequently encounter wounding and have evolved an extraordinary regenerative capacity to heal the wounds. However, the wound signal that triggers regenerative responses has not been identified. Here, through characterization of a tomato mutant defective in both wound-induced defense and regeneration, we demonstrate that in tomato, a plant elicitor peptide (Pep), REGENERATION FACTOR1 (REF1), acts as a systemin-independent local wound signal that primarily regulates local defense responses and regenerative responses in response to wounding. We further identified PEPR1/2 ORTHOLOG RECEPTOR-LIKE KINASE1 (PORK1) as the receptor perceiving REF1 signal for plant regeneration. REF1-PORK1-mediated signaling promotes regeneration via activating WOUND-INDUCED DEDIFFERENTIATION 1 (WIND1), a master regulator of wound-induced cellular reprogramming in plants. Thus, REF1-PORK1 signaling represents a conserved phytocytokine pathway to initiate, amplify, and stabilize a signaling cascade that orchestrates wound-triggered organ regeneration. Application of REF1 provides a simple method to boost the regeneration and transformation efficiency of recalcitrant crops.

Tail and Spinal Cord Regeneration in Urodelean Amphibians

Urodelean amphibians can regenerate the tail and the spinal cord (SC) and maintain this ability throughout their life. This clearly distinguishes these animals from mammals. The phenomenon of tail and SC regeneration is based on the capability of cells involved in regeneration to dedifferentiate, enter the cell cycle, and change their (or return to the pre-existing) phenotype during de novo organ formation. The second critical aspect of the successful tail and SC regeneration is the mutual molecular regulation by tissues, of which the SC and the apical wound epidermis are the leaders. Molecular regulatory systems include signaling pathways components, inflammatory factors, ECM molecules, ROS, hormones, neurotransmitters, HSPs, transcriptional and epigenetic factors, etc. The control, carried out by regulatory networks on the feedback principle, recruits the mechanisms used in embryogenesis and accompanies all stages of organ regeneration, from the moment of damage to the completion of morphogenesis and patterning of all its structures. The late regeneration stages and the effects of external factors on them have been poorly studied. A new model for addressing this issue is herein proposed. The data summarized in the review contribute to understanding a wide range of fundamentally important issues in the regenerative biology of tissues and organs in vertebrates including humans.

Shake it off: exploring drivers and outcomes of autotomy in marine invertebrates

Autotomy refers to self-amputation where the loss of a limb or organ is generally said to be (1) in response to stressful external stimuli; (2) voluntary and nervously mediated; (3) supported by adaptive features that increase efficiency and simultaneously mediate the cost; and (4) morphologically delineated by a predictable breakage plane. It is estimated that this phenomenon has evolved independently nine different times across the animal kingdom, appearing in many different taxa, including vertebrate and invertebrate as well as aquatic and terrestrial animals. Marine invertebrates use this behaviour in a diversity of manners that have yet to be globally reviewed and critically examined. Here, published data from marine invertebrate taxa were used to explore instances of injury as an evolutionary driver of autotomy. Findings suggest that phyla (e.g. Echinodermata and Arthropoda) possibly experiencing high rates of injury (tissue damage or loss) are more likely to be able to perform autotomy. Additionally, this review looks at various morphological, physiological and environmental conditions that have either driven the evolution or maintained the behaviour of autotomy in marine invertebrates. Finally, the use of autotomic abilities in the development of more sustainable and less ecologically invasive fisheries is explored.

Annelids as models of germ cell and gonad regeneration

Germ cells (reproductive cells and their progenitors) give rise to the next generation in sexually reproducing organisms. The loss or removal of germ cells often leads to sterility in established research organisms such as the fruit fly, nematodes, frog, and mouse. The failure to regenerate germ cells in these organisms reinforced the dogma of germline-soma barrier in which germ cells are set-aside during embryogenesis and cannot be replaced by somatic cells. However, in stark contrast, many animals including segmented worms (annelids), hydrozoans, planaria, sea stars, sea urchins, and tunicates can regenerate germ cells. Here I review germ cell and gonad regeneration in annelids, a rich history of research that dates back to the early 20th century in this highly regenerative group. Examples include annelids from across the annelid phylogeny, across developmental stages, and reproductive strategies. Adult annelids regenerate germ cells as a part of regeneration, grafting, and asexual reproduction. Annelids can also recover germ cells after ablation of germ cell progenitors in the embryos. I present a framework to investigate cellular sources of germ cell regeneration in annelids, and discuss the literature that supports different possibilities within this framework, where germ-soma separation may or may not be preserved. With contemporary genetic-lineage tracing and bioinformatics tools, and several genetically enabled annelid models, we are at the brink of answering the big questions that puzzled many for over more than a century.

Temporal dynamics of apoptosis-induced proliferation in pupal wing development: implications for regenerative ability

BACKGROUND

The ability of animals to regenerate damaged tissue is a complex process that involves various cellular mechanisms. As animals age, they lose their regenerative abilities, making it essential to understand the underlying mechanisms that limit regenerative ability during aging. Drosophila melanogaster wing imaginal discs are epithelial structures that can regenerate after tissue injury. While significant research has focused on investigating regenerative responses during larval stages our comprehension of the regenerative potential of pupal wings and the underlying mechanisms contributing to the decline of regenerative responses remains limited.

RESULTS

Here, we explore the temporal dynamics during pupal development of the proliferative response triggered by the induction of cell death, a typical regenerative response. Our results indicate that the apoptosis-induced proliferative response can continue until 34 h after puparium formation (APF), beyond this point cell death alone is not sufficient to induce a regenerative response. Under normal circumstances, cell proliferation ceases around 24 h APF. Interestingly, the failure of reinitiating the cell cycle beyond this time point is not attributed to an incapacity to activate the JNK pathway. Instead, our results suggest that the function of the ecdysone-responsive transcription factor E93 is involved in limiting the apoptosis-induced proliferative response during pupal development.

CONCLUSIONS

Our study shows that apoptosis can prolong the proliferative period of cells in the wing during pupal development as late as 34 h APF, at least 10 h longer than during normal development. After this time point, the regenerative response is diminished, a process mediated in part by the ecdysone-responsive transcription factor E93.

Extracellular vesicles promote proliferation in an animal model of regeneration

Extracellular vesicles (EVs) are secreted nanoparticles composed of a lipid bilayer that carry lipid, protein, and nucleic acid cargo between cells as a mode of intercellular communication. Although EVs can promote tissue repair in mammals, their roles in animals with greater regenerative capacity are not well understood. Planarian flatworms are capable of whole body regeneration due to pluripotent somatic stem cells called neoblasts that proliferate in response to injury. Here, using transmission electron microscopy, nanoparticle tracking analysis, and protein content examination, we showed that EVs enriched from the tissues of the planarian Schmidtea mediterranea had similar morphology and size as other eukaryotic EVs, and that these EVs carried orthologs of the conserved EV biogenesis regulators ALIX and TSG101. PKH67-labeled EVs were taken up more quickly by S/G2 neoblasts than G1 neoblasts/early progeny and differentiated cells. When injected into living planarians, EVs from regenerating tissue fragments enhanced upregulation of neoblast-associated transcripts. In addition, EV injection increased the number of F-ara-EdU-labelled cells by 49% as compared to buffer injection only. Our findings demonstrate that regenerating planarians produce EVs that promote stem cell proliferation, and suggest the planarian as an amenable in vivo model for the study of EV function during regeneration.

ERK-activated CK-2 triggers blastema formation during appendage regeneration

Appendage regeneration relies on the formation of blastema, a heterogeneous cellular structure formed at the injury site. However, little is known about the early injury-activated signaling pathways that trigger blastema formation during appendage regeneration. Here, we provide compelling evidence that the extracellular signal-regulated kinase (ERK)-activated casein kinase 2 (CK-2), which has not been previously implicated in appendage regeneration, triggers blastema formation during leg regeneration in the American cockroach, Periplaneta americana. After amputation, CK-2 undergoes rapid activation through ERK-induced phosphorylation within blastema cells. RNAi knockdown of CK-2 severely impairs blastema formation by repressing cell proliferation through down-regulating mitosis-related genes. Evolutionarily, the regenerative role of CK-2 is conserved in zebrafish caudal fin regeneration via promoting blastema cell proliferation. Together, we find and demonstrate that the ERK-activated CK-2 triggers blastema formation in both cockroach and zebrafish, helping explore initiation factors during appendage regeneration.

Quantitative Live Imaging of Zebrafish Scale Regeneration: From Adult Fish to Signaling Patterns and Tissue Flows

In regeneration, a damaged body part grows back to its original form. Understanding the mechanisms and physical principles underlying this process has been limited by the difficulties of visualizing cell signals and behaviors in regeneration. Zebrafish scales are emerging as a model system to investigate morphogenesis during vertebrate regeneration using quantitative live imaging. Scales are millimeter-sized dermal bone disks forming a skeletal armor on the body of the fish. The scale bone is deposited by an adjacent monolayer of osteoblasts that, after scale loss, regenerates in about 2 weeks. This intriguing regenerative process is accessible to live confocal microscopy, quantifications, and mathematical modeling. Here, I describe methods to image scale regeneration live, tissue-wide and at sub-cellular resolution. Furthermore, I describe methods to process the resulting images and quantify cell, tissue, and signal dynamics.

Xenopus retinal ganglion cell axon extension is unaffected by 5-HT 1B/D receptor activation during visual system development

Activating 5-HT 1B/D receptors with the agonist Zolmitriptan was previously shown to facilitate Xenopus retinal ganglion cell (RGC) axon extension from ectopic eye primordia transplanted to the ventral fin. To determine if 5-HT 1B/D receptor activation influenced entopic RGC axonal outgrowth toward the optic tectum during typical visual system development, we reared embryos in 50 μΜ Zolmitriptan then visualized optic tracts with anterograde HRP labeling. Zolmitriptan did not significantly alter entopic RGC extension in the contralateral brain. Consequently, RGC axon extension in ectopic but not entopic locations is influenced by altering serotonergic signaling .

Distinct stem-like cell populations facilitate functional regeneration of the Cladonema medusa tentacle

Blastema formation is a crucial process that provides a cellular source for regenerating tissues and organs. While bilaterians have diversified blastema formation methods, its mechanisms in non-bilaterians remain poorly understood. Cnidarian jellyfish, or medusae, represent early-branching metazoans that exhibit complex morphology and possess defined appendage structures highlighted by tentacles with stinging cells (nematocytes). Here, we investigate the mechanisms of tentacle regeneration, using the hydrozoan jellyfish Cladonema pacificum. We show that proliferative cells accumulate at the tentacle amputation site and form a blastema composed of cells with stem cell morphology. Nucleoside pulse-chase experiments indicate that most repair-specific proliferative cells (RSPCs) in the blastema are distinct from resident stem cells. We further demonstrate that resident stem cells control nematogenesis and tentacle elongation during both homeostasis and regeneration as homeostatic stem cells, while RSPCs preferentially differentiate into epithelial cells in the newly formed tentacle, analogous to lineage-restricted stem/progenitor cells observed in salamander limbs. Taken together, our findings propose a regeneration mechanism that utilizes both resident homeostatic stem cells (RHSCs) and RSPCs, which in conjunction efficiently enable functional appendage regeneration, and provide novel insight into the diversification of blastema formation across animal evolution.

Evolutionary dynamics of whole-body regeneration across planarian flatworms

Regenerative abilities vary dramatically across animals. Even amongst planarian flatworms, well-known for complete regeneration from tiny body fragments, some species have restricted regeneration abilities while others are almost entirely regeneration incompetent. Here, we assemble a diverse live collection of 40 planarian species to probe the evolution of head regeneration in the group. Combining quantification of species-specific head-regeneration abilities with a comprehensive transcriptome-based phylogeny reconstruction, we show multiple independent transitions between robust whole-body regeneration and restricted regeneration in freshwater species. RNA-mediated genetic interference inhibition of canonical Wnt signalling in RNA-mediated genetic interference-sensitive species bypassed all head-regeneration defects, suggesting that the Wnt pathway is linked to the emergence of planarian regeneration defects. Our finding that Wnt signalling has multiple roles in the reproductive system of the model species Schmidtea mediterranea raises the possibility that a trade-off between egg-laying, asexual reproduction by fission/regeneration and Wnt signalling drives regenerative trait evolution. Although quantitative comparisons of Wnt signalling levels, yolk content and reproductive strategy across our species collection remained inconclusive, they revealed divergent Wnt signalling roles in the reproductive system of planarians. Altogether, our study establishes planarians as a model taxon for comparative regeneration research and presents a framework for the mechanistic evolution of regenerative abilities.

Perish in the Attempt: Regulated Cell Death in Regenerative and Nonregenerative Tissue

Significance: A cell plays its roles throughout its life span, even during its demise. Regulated cell death (RCD) is one of the key topics in modern biomedical studies. It is considered the main approach for removing stressed and/or damaged cells. Research during the past two decades revealed more roles of RCD, such as coordinating tissue development and driving compensatory proliferation during tissue repair. Recent Advances: Compensatory proliferation, initially identified in primitive organisms during the regeneration of lost tissue, is an evolutionarily conserved process that also functions in mammals. Among various types of RCD, apoptosis is considered the top candidate to induce compensatory proliferation in damaged tissue. Critical Issues: The roles of apoptosis in the recovery of nonregenerative tissue are still vague. The roles of other types of RCD, such as necroptosis and ferroptosis, have not been well characterized in the context of tissue regeneration. Future Directions: In this review article, we attempt to summarize the recent insights on the role of RCD in tissue repair. We focus on apoptosis, with expansion to ferroptosis and necroptosis, in primitive organisms with significant regenerative capacity as well as common mammalian research models. After gathering hints from regenerative tissue, in the second half of the review, we take a notoriously nonregenerative tissue, the myocardium, as an example to discuss the role of RCD in terminally differentiated quiescent cells. Antioxid. Redox Signal. 39, 1053-1069.

Cellular and Molecular Triggers of Retinal Regeneration in Amphibians

Understanding the mechanisms triggering the initiation of retinal regeneration in amphibians may advance the quest for prevention and treatment options for degenerating human retina diseases. Natural retinal regeneration in amphibians requires two cell sources, namely retinal pigment epithelium (RPE) and ciliary marginal zone. The disruption of RPE interaction with photoreceptors through surgery or injury triggers local and systemic responses for retinal protection. In mammals, disease-induced damage to the retina results in the shutdown of the function, cellular or oxidative stress, pronounced immune response, cell death and retinal degeneration. In contrast to retinal pathology in mammals, regenerative responses in amphibians have taxon-specific features ensuring efficient regeneration. These include rapid hemostasis, the recruitment of cells and factors of endogenous defense systems, activities of the immature immune system, high cell viability, and the efficiency of the extracellular matrix, cytoskeleton, and cell surface remodeling. These reactions are controlled by specific signaling pathways, transcription factors, and the epigenome, which are insufficiently studied. This review provides a summary of the mechanisms initiating retinal regeneration in amphibians and reveals its features collectively directed at recruiting universal responses to trauma to activate the cell sources of retinal regeneration. This study of the integrated molecular network of these processes is a prospect for future research in demand biomedicine.

The Chordate Origins of Heart Regeneration

The human heart is infamous for not healing after infarction in adults, prompting biomedical interest in species that can regenerate damaged hearts. In such animals as zebrafish and neonatal mice, cardiac repair relies on remaining heart tissue supporting cardiomyocyte proliferation. Natural de novo cardiogenesis in post-embryonic stages thus remains elusive. Here we show that the tunicate Ciona, an ascidian among the closest living relatives to the vertebrates, can survive complete chemogenetic ablation of the heart and loss of cardiac function, and recover both cardiac tissue and contractility. As in vertebrates, Ciona heart regeneration relies on Bone Morphogenetic Protein (BMP) signaling-dependent proliferation of cardiomyocytes, providing insights into the evolutionary origins of regenerative cardiogenesis in chordates. Remarkably, prospective lineage tracing by photoconversion of the fluorescent protein Kaede suggested that new cardiomyocytes can emerge from endodermal lineages in post-metamorphic animals, providing an unprecedented case of regenerative de novo cardiogenesis. Finally, while embryos cannot compensate for early losses of the cardiogenic lineage, forming heartless juveniles, developing animals gain their regenerative ability during metamorphosis, uncovering a fundamental transition between deterministic embryogenesis and regulative post-embryonic development.

Comparison of Blastema Formation after Injury in Two Cephalopod Species

Regeneration is the ability to functionally replace significant amounts of lost tissue or whole appendages like arms, limbs or tentacles. The amount of tissue that can be regenerated varies among species, but regeneration is found in both invertebrate and vertebrate animals. Cephalopods have been broadly reported in the literature to regenerate their arms. There are over 800 species of Cephalopod; however, regeneration has only been documented in the literature in a few species (1). Here we compare arm regeneration in two species of cephalopod, the Octopus bimaculoides and the hummingbird bobtail squid Euprymna berryi.

The known, unknown, and unknown unknowns of cell-cell communication in planarian regeneration

Planarians represent the most primitive bilateral triploblastic animals. Most planarian species exhibit mechanisms for whole-body regeneration, exemplified by the regeneration of their cephalic ganglion after complete excision. Given their robust whole-body regeneration capacity, planarians have been model organisms in regenerative research for more than 240 years. Advancements in research tools and techniques have progressively elucidated the mechanisms underlying planarian regeneration. Accurate cell-cell communication is recognized as a fundamental requirement for regeneration. In recent decades, mechanisms associated with such communication have been revealed at the cellular level. Notably, stem cells (neoblasts) have been identified as the source of all new cells during planarian homeostasis and regeneration. The interplay between neoblasts and somatic cells affects the identities and proportions of various tissues during homeostasis and regeneration. Here, this review outlines key discoveries regarding communication between stem cell compartments and other cell types in planarians, as well as the impact of communication on planarian regeneration. Additionally, this review discusses the challenges and potential directions of future planarian research, emphasizing the sustained impact of this field on our understanding of animal regeneration.

aristaless-like homeobox-3 is wound induced and promotes a low-Wnt environment required for planarian head regeneration

The planarian Schmidtea mediterranea is a well-established model of adult regeneration, which is dependent on a large population of adult stem cells called neoblasts. Upon amputation, planarians undergo transcriptional wounding programs and coordinated stem cell proliferation to give rise to missing tissues. Interestingly, the Wnt signaling pathway is key to guiding what tissues are regenerated, yet less known are the transcriptional regulators that ensure proper activation and timing of signaling pathway components. Here, we have identified an aristaless-like homeobox transcription factor, alx-3, that is enriched in a population of putative neural-fated progenitor cells at homeostasis, and is also upregulated in stem cells and muscle cells at anterior-facing wounds upon amputation. Knockdown of alx-3 results in failure of head regeneration and patterning defects in amputated tail fragments. alx-3 is required for the expression of several early wound-induced genes, including the Wnt inhibitor notum, which is required to establish anterior polarity during regeneration. Together, these findings reveal a role for alx-3 as an early wound-response transcriptional regulator in both muscle cells and stem cells that is required for anterior regeneration by promoting a low-Wnt environment.

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.

Senescence-induced cellular reprogramming drives cnidarian whole-body regeneration

Cell fate stability is essential to maintaining "law and order" in complex animals. However, high stability comes at the cost of reduced plasticity and, by extension, poor regenerative ability. This evolutionary trade-off has resulted in most modern animals being rather simple and regenerative or complex and non-regenerative. The mechanisms mediating cellular plasticity and allowing for regeneration remain unknown. We show that signals emitted by senescent cells can destabilize the differentiated state of neighboring somatic cells, reprogramming them into stem cells that are capable of driving whole-body regeneration in the cnidarian Hydractinia symbiolongicarpus. Pharmacological or genetic inhibition of senescence prevents reprogramming and regeneration. Conversely, induction of transient ectopic senescence in a regenerative context results in supernumerary stem cells and faster regeneration. We propose that senescence signaling is an ancient mechanism mediating cellular plasticity. Understanding the senescence environment that promotes cellular reprogramming could provide an avenue to enhance regeneration.

Regeneration among animals: An evolutionary hypothesis related to aquatic versus terrestrial environment

The present hypothesis tries to explain animal regeneration in relation to the life cycles and environment of different animals. Regeneration is a basic phenomenon present since the origin of life in the sea, as testimonial in lower or more complex extant marine animals. Aquatic animals that evolved an indirect development, forming larvae and transiting into the adult stage through metamorphosis, use gene networks present in their genome for these transformations. In case of injury or organ loss as adults, they can re-utilize most or part of the gene networks previously activated during larval growth and metamorphosis. In contrast, terrestrial animals that evolved life cycles with the elimination of larvae and metamorphosis for the adaptation to land conditions lost some of the genes implicated in these post-developmental processes and consequently also the ability to regenerate. Few arthropods and lizards are capable to form hydrated regenerative blastemas with a similar consistence of embryonic tissues. The present hypothesis submits that regeneration cannot be activated in the dry land environment and consequently was largely or completely abolished in terrestrial animals. After injury or organ loss, nematodes, most arthropods and terrestrial vertebrates can only form scars or a limited healing or regengrow in juveniles. This is a process where somatic growth is superimposed to wound healing so that the apparent regeneration derives from the combination from both processes. When full growth is terminated these terrestrial animals can only heal by scarring.

Stem cells: The cell that does it all

How do animals replace all their worn-out cells to maintain their tissues? A new study shows that, in the cnidarian Hydractinia symbiolongicarpus, a single adult stem cell is sufficient to generate the entire repertoire of somatic and germ line cells.

Regulation of chromatin organization during animal regeneration

Activation of regeneration upon tissue damages requires the activation of many developmental genes responsible for cell proliferation, migration, differentiation, and tissue patterning. Ample evidence revealed that the regulation of chromatin organization functions as a crucial mechanism for establishing and maintaining cellular identity through precise control of gene transcription. The alteration of chromatin organization can lead to changes in chromatin accessibility and/or enhancer-promoter interactions. Like embryogenesis, each stage of tissue regeneration is accompanied by dynamic changes of chromatin organization in regeneration-responsive cells. In the past decade, many studies have been conducted to investigate the contribution of chromatin organization during regeneration in various tissues, organs, and organisms. A collection of chromatin regulators were demonstrated to play critical roles in regeneration. In this review, we will summarize the progress in the understanding of chromatin organization during regeneration in different research organisms and discuss potential common mechanisms responsible for the activation of regeneration response program.

An ERK-dependent molecular switch antagonizes fibrosis and promotes regeneration in spiny mice (Acomys)

Although most mammals heal injured tissues and organs with scarring, spiny mice (Acomys) naturally regenerate skin and complex musculoskeletal tissues. Now, the core signaling pathways driving mammalian tissue regeneration are poorly characterized. Here, we show that, while immediate extracellular signal-regulated kinase (ERK) activation is a shared feature of scarring (Mus) and regenerating (Acomys) injuries, ERK activity is only sustained at high levels during complex tissue regeneration. Following ERK inhibition, ear punch regeneration in Acomys shifted toward fibrotic repair. Using single-cell RNA sequencing, we identified ERK-responsive cell types. Loss- and gain-of-function experiments prompted us to uncover fibroblast growth factor and ErbB signaling as upstream ERK regulators of regeneration. The ectopic activation of ERK in scar-prone injuries induced a pro-regenerative response, including cell proliferation, extracellular matrix remodeling, and hair follicle neogenesis. Our data detail an important distinction in ERK activity between regenerating and poorly regenerating adult mammals and open avenues to redirect fibrotic repair toward regenerative healing.