Model systems for regeneration: salamanders

Why some hearts heal and others don't: The phylogenetic landscape of cardiac regenerative capacity

The limited ability of adult humans to replenish lost heart muscle cells after a heart attack has attracted scientists to explore natural heart regeneration capabilities in the animal kingdom. In particular, research has accelerated since the landmark discovery more than twenty years ago that zebrafish can completely regrow myocardial tissue. In this review, we survey heart regeneration studies in diverse model and non-model animals, aiming to gain insights into both the evolutionary trends in cardiac regenerative potential and the variations among closely related species. Differences in cardiomyogenesis, vasculature formation, and the communication between cardiovascular cells and other players have been investigated to understand the cellular basis, although the precise molecular and genetic causes underlying the stark differences in cardiac regenerative potential among certain close cousins remain largely unknown. By studying cardiovascular regeneration and repair in diverse organisms, we may uncover distinct mechanisms, offering new perspectives for advancing regenerative medicine.

Differentiated muscle cells of salamander Pleurodeles waltl re-enter the cell cycle

Salamander Pleurodeles waltl is an emerging animal model for developmental and regenerative biology studies. However, the exploration of skeletal muscle regeneration has been hindered by the absence of suitable in vitro cell systems for in-depth mechanism research. In this study, we established a protocol for the cultivation of muscle stem cells derived from Pleurodeles waltl for cell biology experiments. Trunk and limb muscles were minced and digested with collagenase. Cells with a high nucleoplasmic ratio were isolated from the muscle tissue. Immunofluorescence and RT-PCR analysis revealed that these proliferating cells expressed the typical muscle stem cell markers. Furthermore, these cells demonstrated effective myogenic differentiation in vitro, as evidenced by the expression of the myogenic differentiation marker protein, myosin heavy chain. Additionally, it was observed that cultured myotubes derived from these cells initiated DNA synthesis and upregulate cell cycle genes upon stimulation with a high concentration of serum. Notably, the muscle stem cells (Pw-1) maintained a steady proliferation rate even after undergoing 35 subcultures. In conclusion, this study has successfully established a method for isolating and cultivating muscle stem cells from salamanders, confirming the dedifferentiation potential of the myotubes derived from these cells. This methodology provides a valuable tool for exploring the molecular mechanisms that govern skeletal muscle regeneration.

Developmental onset of planarian whole-body regeneration depends on axis reset

Regenerative abilities vary across species and developmental stages of animal life cycles. Determining mechanisms that promote or limit regeneration in certain life cycle stages may pinpoint the most critical factors for successful regeneration and suggest strategies for reverse-engineering regenerative responses in therapeutic settings. In contrast to many mammalian systems, which typically show a loss of regenerative abilities with age, planarian flatworms remain highly regenerative throughout adulthood. The robust reproductive and regenerative capabilities of the planarian Schmidtea polychroa (S. polychroa) make them an ideal model to determine when and how regeneration competence is established during development. We report that S. polychroa gradually acquires whole-body regenerative abilities during late embryonic and early juvenile stages. Anterior fragments are capable of regenerating missing trunk and tail tissues from stage 6.5 onward. By contrast, the ability of posterior fragments to make new head tissue depends on the developmental stage, tissue composition of the amputated fragment, and axial position of the cut plane. Irradiation-sensitive cells are required, but not sufficient, for the onset of head regeneration ability. We propose that regulation of the main body axis reset, specifically the ability to remake an anterior organizing center, determines when whole-body regeneration competence arises during development. Supporting this hypothesis, knockdown of the canonical Wnt pathway effector Spol-β-catenin-1, a posterior determinant, induces precocious head regeneration under conditions that are normally head regeneration-incompetent. Our results suggest that regeneration competence emerges through interactions between irradiation-sensitive cells, the cellular source of new tissue, and developing adult tissue(s) harboring axial patterning information.

Approaches to Establish an Animal Model With Composite Bone-soft Tissue Defects for Complex Regenerative Strategies

Regenerative strategies of composite tissue defects represent a formidable clinical challenge due to the distinct regeneration pathways of each tissue type and their complex interactions. To address the limitations of single-tissue defect models, the authors firstly established an animal model with simultaneously composite bone-soft tissue defects and then observed the wound healing, body weight, and overall health after the operation. The soft tissue defects exhibited complete macroscopic closure within 21 days. Furthermore, micro-computed tomography and histological assessments at 8 weeks demonstrated that the calvarial defect was unhealed, confirming its characteristics as a critical-size defect. In the authors' model, a 5 mm defect remained unhealed at 8 weeks, suggesting that it also exhibits critical-size defect properties, though a direct comparison with the classical 8 mm model requires further validation. Quantitative evaluations revealed limited new bone formation confined primarily to the defect margins, while the central portion of the defect remained filled with fibrous tissue. This composite model not only advances authors' understanding of multitissue repair processes but also offers a valuable preclinical platform for the development and optimization of next-generation regenerative strategies for complex tissue injuries.

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.

Simulation of adult limb regeneration with lizard tail spinal cord implants reveals distinct roles of radial glia and microglia populations

Lizards are the closest relatives of humans able to suppress fibrosis and regrow multiple tissue lineages following appendage regeneration. As amniotes capable of tail, but not limb regrowth, lizards are also distinguished as the only vertebrate group that include both regenerative and non-regenerative appendages in the same animal. Lizard tail stumps naturally form blastemas - heterogenous collections of fibroblasts, adult stem cells, and immune cells that suppress scar formation and potentiate new tissue growth. Conversely, amputated lizard limbs form scars similar to those observed in human patients. Lizard blastema formation is dependent upon tail spinal cord tissue, which contains distinct populations of radial glia and microglia. Using the parthenogenetic lizard Lepidodactylus lugubris as a platform for tail-to-limb spinal cord implantations, we developed an ectopic blastema model toward defining the roles of radial glial and microglia in appendage regeneration. Removal of either population inhibits fibroblast proliferation and blastema formation, but only microglia depletion leads to enhanced fibrosis. Similarly, effects of radial glia, but not microglia, depletion on fibroblast proliferation are reversed via Hedgehog agonism. Taken together, these results indicate that lizard limbs contain all the necessary cell types and biological responses necessary for blastema formation but lack the proliferative and anti-fibrotic signals provided by tail spinal cord radial glia and microglia, respectively. Radial glia contribute Hedgehog signals that cause fibroblast proliferation but do not affect fibrosis. Conversely, microglia enhance fibroblast sensitivity to Hedgehog signaling and inhibit differentiation into fibrocytes. In summary, this study demonstrates blastema stimulation in amputated limbs of adult amniotes with application of lizard spinal cord cells and holds promise as a blueprint for limiting painful scarring and supporting new tissue growth following amputation injuries in human patients.

Autophagy in Tissue Repair and Regeneration

Autophagy is a cellular recycling system that, through the sequestration and degradation of intracellular components regulates multiple cellular functions to maintain cellular homeostasis and survival. Dysregulation of autophagy is closely associated with the development of physiological alterations and human diseases, including the loss of regenerative capacity. Tissue regeneration is a highly complex process that relies on the coordinated interplay of several cellular processes, such as injury sensing, defense responses, cell proliferation, differentiation, migration, and cellular senescence. These processes act synergistically to repair or replace damaged tissues and restore their morphology and function. In this review, we examine the evidence supporting the involvement of the autophagy pathway in the different cellular mechanisms comprising the processes of regeneration and repair across different regenerative contexts. Additionally, we explore how modulating autophagy can enhance or accelerate regeneration and repair, highlighting autophagy as a promising therapeutic target in regenerative medicine for the development of autophagy-based treatments for human diseases.

Primary regulatory T cell activator FOXP3 is present across Amphibia

The overall structure of the immune system is highly conserved across jawed vertebrates, but characterization and description of the immune system is heavily biased toward mammals. One arm of the vertebrate immune system, the adaptive immune system, mounts pathogen-specific responses that tend to be robust and effective at clearing pathogens. This system requires selection against self-recognition and modulation of the immune response. One of the mechanisms of immune modulation is the presence of regulatory T cells that suppress other effector immune cells. Regulatory T cells and their primary activator forkhead box protein P3 (FOXP3) have been well characterized in mammalian models but unexplored in most other vertebrate taxa. Amphibians are a good focal group for the characterization of FOXP3 due to their phylogenetic position on the vertebrate tree of life, and their susceptibility to emerging pathogens. In this study, we mined available transcriptomic and genomic data to confirm the presence of FOXP3 across the amphibian tree of life. We find that FOXP3 is present in all major clades of amphibians. We also test whether selection on FOXP3 shows signatures of intensification among the three main clades of amphibians, which may reflect shifts in the stringency of natural selection on this gene. Our findings provide insights into the evolutionary history of the vertebrate immune system and confirm the conservation of vertebrate immune genes within amphibians.

Chromosome-scale genome assembly reveals how repeat elements shape non-coding RNA landscapes active during newt limb regeneration

Newts have large genomes harboring many repeat elements. How these elements shape the genome and relate to newts' unique regeneration ability remains unknown. We present here the chromosome-scale assembly of the 20.3 Gb genome of the Iberian ribbed newt, Pleurodeles waltl, with a hitherto unprecedented contiguity and completeness among giant genomes. Utilizing this assembly, we demonstrate conserved synteny as well as genetic rearrangements, such as in the major histocompatibility complex locus. We provide evidence suggesting that intronic repeat elements drive newt-specific circular RNA (circRNA) biogenesis and show their regeneration-specific expression. We also present a comprehensive in-depth annotation and chromosomal mapping of microRNAs, highlighting genomic expansion profiles as well as a distinct regulatory pattern in the regenerating limb. These data reveal links between repeat elements, non-coding RNAs, and adult regeneration and provide key resources for addressing developmental, regenerative, and evolutionary principles.

Regeneration of Lumbriculus variegatus requires post-amputation production of reactive oxygen species

Animals vary in their ability to replace body parts lost to injury, a phenomenon known as restorative regeneration. Uncovering conserved signaling steps required for regeneration may aid regenerative medicine. Reactive oxygen species (ROS) are necessary for proper regeneration in species across a wide range of taxa, but it is unknown whether ROS are essential for annelid regeneration. As annelids are a widely used and excellent model for regeneration, we sought to determine whether ROS play a role in the regeneration of the highly regenerative annelid, Lumbriculus variegatus. Using a ROS-sensitive fluorescent probe we observed ROS accumulation at the wound site within 15 min after amputation; this ROS burst lessened by 6 h post-amputation. Chemical inhibition of this ROS burst delayed regeneration, an impairment that was partially rescued with exogenous ROS. Our results suggest that similar to other animals, annelid regeneration depends upon ROS signaling, implying a phylogenetically ancient requirement for ROS in regeneration.

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.

Iberian ribbed newts

Matheson et al. introduce the Iberian ribbed newt (Pleurodeles waltl), a species of salamander that lives some of its adult life on land and some in water, requiring remarkable physiological and behavioral plasticity to adapt to these very different environments.

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.

Autophagy mediated by ROS-AKT-FoxO pathway is required for intestinal regeneration in echinoderms

Autophagy is essential for maintaining material balance and energy circulation and plays a critical role as a regulatory mechanism in tissue regeneration. However, current studies primarily describe this phenotype, with limited exploration of its molecular mechanisms. In this study, we provided the first evidence that autophagy is required for intestinal regeneration in Apostichopus japonicus and identified a previously unrecognized regulatory mechanism involved in this process. We observed that autophagy activation was significantly associated with enhanced regeneration, and its upregulation was shown to be regulated by reactive oxygen species (ROS) bursts. Mechanistically, ROS induced the dephosphorylation of Forkhead box protein O (FoxO) through AjAKT dephosphorylation. The dephosphorylated AjFoxO translocated to the nucleus, where it bound to the promoters of AjLC3 and AjATG4, inducing their transcription. This study highlights the ROS-AjAKT-AjFoxO-AjATG4/AjLC3 pathway as a novel regulatory mechanism underlying autophagy-mediated intestinal regeneration in echinoderms, providing a reference for studying regenerative processes and cytological mechanisms in economically important echinoderms.

Balancing central control and sensory feedback produces adaptable and robust locomotor patterns in a spiking, neuromechanical model of the salamander spinal cord

This study introduces a novel neuromechanical model employing a detailed spiking neural network to explore the role of axial proprioceptive sensory feedback, namely stretch feedback, in salamander locomotion. Unlike previous studies that often oversimplified the dynamics of the locomotor networks, our model includes detailed simulations of the classes of neurons that are considered responsible for generating movement patterns. The locomotor circuits, modeled as a spiking neural network of adaptive leaky integrate-and-fire neurons, are coupled to a three-dimensional mechanical model of a salamander with realistic physical parameters and simulated muscles. In open-loop simulations (i.e., without sensory feedback), the model replicates locomotor patterns observed in-vitro and in-vivo for swimming and trotting gaits. Additionally, a modular descending reticulospinal drive to the central pattern generation network allows to accurately control the activation, frequency and phase relationship of the different sections of the limb and axial circuits. In closed-loop swimming simulations (i.e. including axial stretch feedback), systematic evaluations reveal that intermediate values of feedback strength increase the tail beat frequency and reduce the intersegmental phase lag, contributing to a more coordinated, faster and energy-efficient locomotion. Interestingly, the result is conserved across different feedback topologies (ascending or descending, excitatory or inhibitory), suggesting that it may be an inherent property of axial proprioception. Moreover, intermediate feedback strengths expand the stability region of the network, enhancing its tolerance to a wider range of descending drives, internal parameters' modifications and noise levels. Conversely, high values of feedback strength lead to a loss of controllability of the network and a degradation of its locomotor performance. Overall, this study highlights the beneficial role of proprioception in generating, modulating and stabilizing locomotion patterns, provided that it does not excessively override centrally-generated locomotor rhythms. This work also underscores the critical role of detailed, biologically-realistic neural networks to improve our understanding of vertebrate locomotion.

An adult myogenic cell line of the Japanese fire-bellied newt Cynops pyrrhogaster

Adult myogenic cell lines are useful to study muscle development, repair and regeneration. In newts, which are known for their high regenerative capacity, myogenic cell lines have not been established in species other than the Eastern newt Notophthalmus viridescens. In this study, we established another myogenic cell line, named CpM01, from the skeletal muscle of the forearm of the adult Japanese fire-bellied newt Cynops pyrrhogaster. CpM01 maintained high proliferative ability even after numerous passages, and could be induced to differentiate into myotubes by changing the culture medium. CpM01 expressed myogenic regulatory factors (MRFs) such as Myf5, MRF4 and myogenin. Changes in the immunorectivities of MRFs during differentiation of CpM01 into myotubes were consistent with those during new muscle generation in limb regeneration. In newts, myogenic cells have two origins, muscle fibers or satellite cells. CpM01 expressed Pax7, suggesting the origin might be satellite cells. scRNA-seq analysis deeply characterized CpM01 and demonstrated that the expression patterns of myogenic genes (Pax3, Pax7, myocyte-specific enhancer factor 2 A, and genes encoding MRFs) in CpM01 are related to progress of the cell cycle. CpM01 can be a useful tool for future studies of limb muscle regeneration in adult newts.

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.

Key Proteins for Regeneration in A. mexicanum: Transcriptomic Insights From Aged and Juvenile Limbs

The axolotl, known for its remarkable regenerative abilities, is an excellent model for studying regenerative therapies. Nevertheless, the precise molecular mechanisms governing its regenerative potential remain uncertain. In this study, we collected samples from axolotls of different ages, including 8-year-old individuals and 8-month-old juveniles, obtaining their blastemas 10 days after amputation. Subsequently, we conducted a transcriptomic analysis comparing our samples to a set of previously published experiments. Our analysis unveiled a distinctive transcriptional response in the blastema, characterized by differential gene expression associated with processes such as bone and tissue remodeling, transcriptional regulation, angiogenesis, and intercellular communication. To gain deeper insights, we compared these findings with those from aged axolotls that showed no signs of regeneration 10 days after amputation. We identified four genes-FSTL1, ADAMTS17, GPX7, and CTHRC1-that showed higher expression in regenerating tissue compared to aged axolotls. Further scrutiny, including structural and homology analysis, revealed that these genes are conserved across vertebrate species. Our discoveries point to a group of proteins relevant to tissue regeneration, with their conservation in vertebrates suggesting critical roles in development. These findings also propose a novel gene set involved in axolotl regeneration, laying a promising foundation for future investigations across vertebrates.

Development of a ZRS Reporter System for the Newt (Cynops pyrrhogaster) During Terrestrial Limb Regeneration

BACKGROUND

Newts, a type of urodele amphibian, offer remarkable insights into regenerative medicine due to their extraordinary tissue regeneration capabilities-a challenging feat in humans. During limb regeneration of adult newts, fascinating cellular and molecular processes are revealed, including scarless healing, de-differentiation of mature cells, and regeneration of limbs and digits. Sonic hedgehog (Shh), crucial for vertebrate limb development, is regulated by the zone of polarizing activity regulatory sequence (ZRS) in the limb bud zone of polarizing activity (ZPA). The metamorphosed (terrestrial) newt can reactivate Shh during regeneration, facilitating proper limb patterning. Cell types capable of regulating the ZRS in metamorphosed newts remain unknown. The identification of such cell types provides invaluable insight into novel regenerative mechanisms.

OBJECTIVE

In this study, we developed the first newt ZRS reporter.

METHODS

We isolated and characterized the newt ZRS enhancer (nZRS), identifying conserved DNA binding sites. Several binding sites with medical relevance were conserved in the newt ZRS. In functional analysis, we developed a system composed of a transgenic nZRS reporter newt and a new newt anti-Shh antibody, which allowed Shh monitoring during limb regeneration.

RESULTS

We identified a group of Schwann cells capable of ZRS reporter and Shh protein expression during terrestrial limb regeneration.

CONCLUSIONS

This system provides a valuable in vivo approach for future genetic studies of patterning during limb regeneration.

Single-cell resolution of intestinal regeneration in pythons without crypts illuminates conserved vertebrate regenerative mechanisms

Canonical models of intestinal regeneration emphasize the critical role of the crypt stem cell niche to generate enterocytes that migrate to villus ends. Burmese pythons possess extreme intestinal regenerative capacity yet lack crypts, thus providing opportunities to identify noncanonical but potentially conserved mechanisms that expand our understanding of regenerative capacity in vertebrates, including humans. Here, we leverage single-nucleus RNA sequencing of fasted and postprandial python small intestine to identify the signaling pathways and cell-cell interactions underlying the python's regenerative response. We find that python intestinal regeneration entails the activation of multiple conserved mechanisms of growth and stress response, including core lipid metabolism pathways and the unfolded protein response in intestinal enterocytes. Our single-cell resolution highlights extensive heterogeneity in mesenchymal cell population signaling and intercellular communication that directs major tissue restructuring and the shift out of a dormant fasted state by activating both embryonic developmental and wound healing pathways. We also identify distinct roles of BEST4+ enterocytes in coordinating key regenerative transitions via NOTCH signaling. Python intestinal regeneration shares key signaling features and molecules with mammalian gastric bypass, indicating that conserved regenerative programs are common to both. Our findings provide different insights into cooperative and conserved regenerative programs and intercellular interactions in vertebrates independent of crypts which have been otherwise obscured in model species where temporal phases of generative growth are limited to embryonic development or recovery from injury.

Recent advances in neurotechnology-based biohybrid robots

Biohybrid robots retain the innate biological characteristics and behavioral traits of animals, making them valuable in applications such as disaster relief, exploration of unknown terrains, and medical care. This review aims to comprehensively discuss the evolution of biohybrid robots, their key technologies and applications, and the challenges they face. By analyzing studies conducted on terrestrial, aquatic, and aerial biohybrid robots, we gain a deeper understanding of how these technologies have made significant progress in simulating natural organisms, improving mechanical performance, and intelligent control. Additionally, we address challenges associated with the application of electrical stimulation technology, the precision of neural signal monitoring, and the ethical considerations for biohybrid robots. We highlight the importance of future research focusing on developing more sophisticated and biocompatible control methods while prioritizing animal welfare. We believe that exploring multimodal monitoring and stimulation technologies holds the potential to enhance the performance of biohybrid robots. These efforts are expected to pave the way for biohybrid robotics technology to introduce greater innovation and well-being to human society in the future.

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.

The mRNA and microRNA Landscape of the Blastema Niche in Regenerating Newt Limbs

Newts are excellent vertebrate models for investigating tissue regeneration due to their remarkable regenerative capabilities. To investigate the mRNA and microRNAs (miRNAs) profiles within the blastema niche of regenerating newt limbs, we amputated the limbs of Chinese fire belly newts (Cynops orientalis) and conducted comprehensive analyses of the transcriptome and microRNA profiles at five distinct time points post-amputation (0 hours, 1 day, 5 days 10 days and 20 days). We identified 24 significantly differentially expressed (DE) genes and 20 significantly DE miRNAs. Utilizing weighted gene co-expression network analysis (WGCNA) and gene ontology (GO) enrichment analysis, we identified four genes likely to playing crucial roles in the early stages of limb regeneration: Cemip, Rhou, Gpd2 and Pcna. Moreover, mRNA-miRNA integration analysis uncovered seven human miRNAs (miR-19b-1, miR-19b-2, miR-21-5p, miR-127-5p, miR-150-5p, miR-194-5p, and miR-210-5p) may regulate the expression of these four key genes. The temporal expression patterns of these key genes and miRNAs further validated the robustness of the identified mRNA-miRNA landscape. Our study successfully identified candidate key genes and elucidated a portion of the genetic regulatory mechanisms involved in newt limb regeneration. These findings offer valuable insights for further exploration of the intricate processes of tissue regeneration.

The Phoenix of stem cells: pluripotent cells in adult tissues and peripheral blood

Pluripotent stem cells are defined as cells that can generate cells of lineages from all three germ layers, ectoderm, mesoderm, and endoderm. On the contrary, unipotent and multipotent stem cells develop into one or more cell types respectively, but their differentiation is limited to the cells present in the tissue of origin or, at most, from the same germ layer. Multipotent and unipotent stem cells have been isolated from a variety of adult tissues, Instead, the presence in adult tissues of pluripotent stem cells is a very debated issue. In the early embryos, all cells are pluripotent. In mammalians, after birth, pluripotent cells are maintained in the bone-marrow and possibly in gonads. In fact, pluripotent cells were isolated from marrow aspirates and cord blood and from cultured bone-marrow stromal cells (MSCs). Only in few cases, pluripotent cells were isolated from other tissues. In addition to have the potential to differentiate toward lineages derived from all three germ layers, the isolated pluripotent cells shared other properties, including the expression of cell surface stage specific embryonic antigen (SSEA) and of transcription factors active in the early embryos, but they were variously described and named. However, it is likely that they are part of the same cell population and that observed diversities were the results of different isolation and expansion strategies. Adult pluripotent stem cells are quiescent and self-renew at very low rate. They are maintained in that state under the influence of the "niche" inside which they are located. Any tissue damage causes the release in the blood of inflammatory cytokines and molecules that activate the stem cells and their mobilization and homing in the injured tissue. The inflammatory response could also determine the dedifferentiation of mature cells and their reversion to a progenitor stage and at the same time stimulate the progenitors to proliferate and differentiate to replace the damaged cells. In this review we rate articles reporting isolation and characterization of tissue resident pluripotent cells. In the attempt to reconcile observations made by different authors, we propose a unifying picture that could represent a starting point for future experiments.

Evolution, Diversity, and Conservation of Herpetofauna

Amphibians and reptiles play a critical role in the evolution of Tetrapoda, showcasing significant diversity in terms of their genetics, species, morphology, life history traits, and evolutionary functions [...].

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.

Regeneration in Mice of Injured Skin, Heart, and Spinal Cord by α-Gal Nanoparticles Recapitulates Regeneration in Amphibians

The healing of skin wounds, myocardial, and spinal cord injuries in salamander, newt, and axolotl amphibians, and in mouse neonates, results in scar-free regeneration, whereas injuries in adult mice heal by fibrosis and scar formation. Although both types of healing are mediated by macrophages, regeneration in these amphibians and in mouse neonates also involves innate activation of the complement system. These differences suggest that localized complement activation in adult mouse injuries might induce regeneration instead of the default fibrosis and scar formation. Localized complement activation is feasible by antigen/antibody interaction between biodegradable nanoparticles presenting α-gal epitopes (α-gal nanoparticles) and the natural anti-Gal antibody which is abundant in humans. Administration of α-gal nanoparticles into injuries of anti-Gal-producing adult mice results in localized complement activation which induces rapid and extensive macrophage recruitment. These macrophages bind anti-Gal-coated α-gal nanoparticles and polarize into M2 pro-regenerative macrophages that orchestrate accelerated scar-free regeneration of skin wounds and regeneration of myocardium injured by myocardial infarction (MI). Furthermore, injection of α-gal nanoparticles into spinal cord injuries of anti-Gal-producing adult mice induces recruitment of M2 macrophages, that mediate extensive angiogenesis and axonal sprouting, which reconnects between proximal and distal severed axons. Thus, α-gal nanoparticle treatment in adult mice mimics physiologic regeneration in amphibians. These studies further suggest that α-gal nanoparticles may be of significance in the treatment of human injuries.

DNA Damage, Genome Stability, and Adaptation: A Question of Chance or Necessity?

DNA damage causes the mutations that are the principal source of genetic variation. DNA damage detection and repair mechanisms therefore play a determining role in generating the genetic diversity on which natural selection acts. Speciation, it is commonly assumed, occurs at a rate set by the level of standing allelic diversity in a population. The process of speciation is driven by a combination of two evolutionary forces: genetic drift and ecological selection. Genetic drift takes place under the conditions of relaxed selection, and results in a balance between the rates of mutation and the rates of genetic substitution. These two processes, drift and selection, are necessarily mediated by a variety of mechanisms guaranteeing genome stability in any given species. One of the outstanding questions in evolutionary biology concerns the origin of the widely varying phylogenetic distribution of biodiversity across the Tree of Life and how the forces of drift and selection contribute to shaping that distribution. The following examines some of the molecular mechanisms underlying genome stability and the adaptive radiations that are associated with biodiversity and the widely varying species richness and evenness in the different eukaryotic lineages.

TMT-Based Quantitative Proteomic Analysis Reveals the Key Role of Cell Proliferation and Apoptosis in Intestine Regeneration of Apostichopus japonicus

Sea cucumbers are widely known for their powerful regenerative abilities, which allow them to regenerate a complete digestive tract within a relatively short time following injury or autotomy. Recently, even though the histological changes and cellular events in the processes of intestinal regeneration have been extensively studied, the molecular machinery behind this faculty remains unclear. In this study, tandem mass tag (TMT)-based quantitation was utilized to investigate protein abundance changes during the process of intestine regeneration. Approximately 538, 445, 397, 1012, and 966 differential proteins (DEPs) were detected (p < 0.05) between the normal and 2, 7, 12, 20, and 28 dpe stages, respectively. These DEPs also mainly focus on pathways of cell proliferation and apoptosis, which were further validated by 5-Ethynyl-2'-deoxyuridine (EdU) or Tunel-based flow cytometry assay. These findings provide a reference for a comprehensive understanding of the regulatory mechanisms of various stages of intestinal regeneration and provide a foundation for subsequent research on changes in cell fate in echinoderms.

FGF-stimulated tendon cells embrace a chondrogenic fate with BMP7 in newt tissue culture

Newts can regenerate functional elbow joints after amputation at the joint level. Previous studies have suggested the potential contribution of cells from residual tendon tissues to joint cartilage regeneration. A serum-free tissue culture system for tendons was established to explore cell dynamics during joint regeneration. Culturing isolated tendons in this system, stimulated by regeneration-related factors, such as fibroblast growth factor (FGF) and platelet-derived growth factor, led to robust cell migration and proliferation. Moreover, cells proliferating in an FGF-rich environment differentiated into Sox9-positive chondrocytes upon BMP7 introduction. These findings suggest that FGF-stimulated cells from tendons may aid in joint cartilage regeneration during functional elbow joint regeneration in newts.

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.

The salamander limb: a perfect model to understand imperfect integration during skeletal regeneration

Limb regeneration in salamanders is achieved by a complex coordination of various biological processes and requires the proper integration of new tissue with old. Among the tissues found inside the limb, the skeleton is the most prominent component, which serves as a scaffold and provides support for locomotion in the animal. Throughout the years, researchers have studied the regeneration of the appendicular skeleton in salamanders both after limb amputation and as a result of fracture healing. The final outcome has been widely seen as a faithful re-establishment of the skeletal elements, characterised by a seamless integration into the mature tissue. The process of skeletal integration, however, is not well understood, and several works have recently provided evidence of commonly occurring flawed regenerates. In this Review, we take the reader on a journey through the course of bone formation and regeneration in salamanders, laying down a foundation for critically examining the mechanisms behind skeletal integration. Integration is a phenomenon that could be influenced at various steps of regeneration, and hence, we assess the current knowledge in the field and discuss how early events, such as tissue histolysis and patterning, influence the faithful regeneration of the appendicular skeleton.

Transcriptomic analysis reveals the early body wall regeneration mechanism of the sea cucumber Holothuria leucospilota after artificially induced transverse fission

BACKGROUND

Sea cucumbers exhibit a remarkable ability to regenerate damaged or lost tissues and organs, making them an outstanding model system for investigating processes and mechanisms of regeneration. They can also reproduce asexually by transverse fission, whereby the anterior and posterior bodies can regenerate independently. Despite the recent focus on intestinal regeneration, the molecular mechanisms underlying body wall regeneration in sea cucumbers still remain unclear.

RESULTS

In this study, transverse fission was induced in the tropical sea cucumber, Holothuria leucospilota, through constrainment using rubber bands. Histological examination revealed the degradation and loosening of collagen fibers on day-3, followed by increased density but disorganization of the connective tissue on day-7 of regeneration. An Illumina transcriptome analysis was performed on the H. leucospilota at 0-, 3- and 7-days after artificially induced fission. The differential expression genes were classified and enriched by GO terms and KEGG database, respectively. An upregulation of genes associated with extracellular matrix remodeling was observed, while a downregulation of pluripotency factors Myc, Klf2 and Oct1 was detected, although Sox2 showed an upregulation in expression. In addition, this study also identified progressively declining expression of transcription factors in the Wnt, Hippo, TGF-β, and MAPK signaling pathways. Moreover, changes in genes related to development, stress response, apoptosis, and cytoskeleton formation were observed. The localization of the related genes was further confirmed through in situ hybridization.

CONCLUSION

The early regeneration of H. leucospilota body wall is associated with the degradation and subsequent reconstruction of the extracellular matrix. Pluripotency factors participate in the regenerative process. Multiple transcription factors involved in regulating cell proliferation were found to be gradually downregulated, indicating reduced cell proliferation. Moreover, genes related to development, stress response, apoptosis, and cell cytoskeleton formation were also involved in this process. Overall, this study provides new insights into the mechanisms of whole-body regeneration and uncover potential cross-species regenerative-related genes.

Transcriptomic analysis of spinal cord regeneration after injury in Cynops orientalis

Cynops orientalis (C. orientalis) has a pronounced ability to regenerate its spinal cord after injury. Thus, exploring the molecular mechanism of this process could provide new approaches for promoting mammalian spinal cord regeneration. In this study, we established a model of spinal cord thoracic transection injury in C. orientalis, which is an endemic species in China. We performed RNA sequencing of the contused axolotl spinal cord at two early time points after spinal cord injury - during the very acute stage (4 days) and the subacute stage (7 days) - and identified differentially expressed genes; additionally, we performed Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses, at each time point. Transcriptome sequencing showed that 13,059 genes were differentially expressed during C. orientalis spinal cord regeneration compared with uninjured animals, among which 4273 were continuously down-regulated and 1564 were continuously up-regulated. Down-regulated genes were most enriched in the Gene Ontology term "multicellular organismal process" and in the ribosome pathway at 10 days following spinal cord injury. We found that multiple genes associated with energy metabolism were down-regulated and multiple genes associated with the lysosome were up-regulated after spinal cord injury, indicating the importance of low metabolic activity during wound healing. Immune response-associated pathways were activated during the early acute phase (4 days), while the expression of extracellular matrix proteins such as glycosaminoglycan and collagen, as well as tight junction proteins, was lower at 10 days post-spinal cord injury than 4 days post-spinal cord injury. However, compared with 4 days post-injury, at 10 days post-injury neuroactive ligand-receptor interactions were no longer down-regulated, up-regulated differentially expressed genes were enriched in pathways associated with cancer and the cell cycle, and SHH, VIM, and Sox2 were prominently up-regulated. Immunofluorescence staining showed that glial fibrillary acidic protein was up-regulated in axolotl ependymoglial cells after injury, similar to what is observed in mammalian astrocytes after spinal cord injury, even though axolotls do not form a glial scar during regeneration. We suggest that low intracellular energy production could slow the rapid amplification of ependymoglial cells, thereby inhibiting reactive gliosis, at early stages after spinal cord injury. Extracellular matrix degradation slows cellular responses, represses the expression of neurogenic genes, and reactivates a transcriptional program similar to that of embryonic neuroepithelial cells. These ependymoglial cells act as neural stem cells: they migrate and proliferate to repair the lesion and then differentiate to replace lost glial cells and neurons. This provides the regenerative microenvironment that allows axon growth after injury.

Molecular Signatures Integral to Natural Reprogramming in the Pigment Epithelium Cells after Retinal Detachment in Pleurodeles waltl

The reprogramming of retinal pigment epithelium (RPE) cells into retinal cells (transdifferentiation) lies in the bases of retinal regeneration in several Urodela. The identification of the key genes involved in this process helps with looking for approaches to the prevention and treatment of RPE-related degenerative diseases of the human retina. The purpose of our study was to examine the transcriptome changes at initial stages of RPE cell reprogramming in adult newt Pleurodeles waltl. RPE was isolated from the eye samples of day 0, 4, and 7 after experimental surgical detachment of the neural retina and was used for a de novo transcriptome assembly through the RNA-Seq method. A total of 1019 transcripts corresponding to the differently expressed genes have been revealed in silico: the 83 increased the expression at an early stage, and 168 increased the expression at a late stage of RPE reprogramming. We have identified up-regulation of classical early response genes, chaperones and co-chaperones, genes involved in the regulation of protein biosynthesis, suppressors of oncogenes, and EMT-related genes. We revealed the growth in the proportion of down-regulated ribosomal and translation-associated genes. Our findings contribute to revealing the molecular mechanism of RPE reprogramming in Urodela.

Macrophages modulate fibrosis during newt lens regeneration

Background

Previous studies indicated that macrophages play a role during lens regeneration in newts, but their function has not been tested experimentally.

Methods

Here we generated a transgenic newt reporter line in which macrophages can be visualized in vivo. Using this new tool, we analyzed the location of macrophages during lens regeneration. We uncovered early gene expression changes using bulk RNAseq in two newt species, Notophthalmus viridescens and Pleurodeles waltl. Next, we used clodronate liposomes to deplete macrophages, which inhibited lens regeneration in both newt species.

Results

Macrophage depletion induced the formation of scar-like tissue, an increased and sustained inflammatory response, an early decrease in iris pigment epithelial cell (iPEC) proliferation and a late increase in apoptosis. Some of these phenotypes persisted for at least 100 days and could be rescued by exogenous FGF2. Re-injury alleviated the effects of macrophage depletion and re-started the regeneration process.

Conclusions

Together, our findings highlight the importance of macrophages in facilitating a pro-regenerative environment in the newt eye, helping to resolve fibrosis, modulating the overall inflammatory landscape and maintaining the proper balance of early proliferation and late apoptosis.

Arthritis Foundation/HSS Workshop on Hip Osteoarthritis, Part 2: Detecting Hips at Risk: Early Biomechanical and Structural Mechanisms

Far more publications are available for osteoarthritis of the knee than of the hip. Recognizing this research gap, the Arthritis Foundation (AF), in partnership with the Hospital for Special Surgery (HSS), convened an in-person meeting of thought leaders to review the state of the science of and clinical approaches to hip osteoarthritis. This article summarizes the recommendations gleaned from 5 presentations given in the "early hip osteoarthritis" session of the 2023 Hip Osteoarthritis Clinical Studies Conference, which took place on February 17 and 18, 2023, in New York City. It also summarizes the workgroup recommendations from a small-group discussion on clinical research gaps.

ECM-Focused Proteomic Analysis of Ear Punch Regeneration in Acomys Cahirinus

In mammals, significant injury is generally followed by the formation of a fibrotic scar which provides structural integrity but fails to functionally restore damaged tissue. Spiny mice of the genus Acomys represent the first example of full skin autotomy in mammals. Acomys cahirinus has evolved extremely weak skin as a strategy to avoid predation and is able to repeatedly regenerate healthy tissue without scar after severe skin injury or full-thickness ear punches. Extracellular matrix (ECM) composition is a critical regulator of wound repair and scar formation and previous studies have suggested that alterations in its expression may be responsible for the differences in regenerative capacity observed between Mus musculus and A. cahirinus , yet analysis of this critical tissue component has been limited in previous studies by its insolubility and resistance to extraction. Here, we utilize a 2-step ECM-optimized extraction to perform proteomic analysis of tissue composition during wound repair after full-thickness ear punches in A. cahirinus and M. musculus from weeks 1 to 4 post-injury. We observe changes in a wide range of ECM proteins which have been previously implicated in wound regeneration and scar formation, including collagens, coagulation and provisional matrix proteins, and matricryptic signaling peptides. We additionally report differences in crosslinking enzyme activity and ECM protein solubility between Mus and Acomys. Furthermore, we observed rapid and sustained increases in CD206, a marker of pro-regenerative M2 macrophages, in Acomys, whereas little or no increase in CD206 was detected in Mus. Together, these findings contribute to a comprehensive understanding of tissue cues which drive the regenerative capacity of Acomys and identify a number of potential targets for future pro-regenerative therapies.

Multi-species atlas resolves an axolotl limb development and regeneration paradox

Humans and other tetrapods are considered to require apical-ectodermal-ridge (AER) cells for limb development, and AER-like cells are suggested to be re-formed to initiate limb regeneration. Paradoxically, the presence of AER in the axolotl, a primary model organism for regeneration, remains controversial. Here, by leveraging a single-cell transcriptomics-based multi-species atlas, composed of axolotl, human, mouse, chicken, and frog cells, we first establish that axolotls contain cells with AER characteristics. Further analyses and spatial transcriptomics reveal that axolotl limbs do not fully re-form AER cells during regeneration. Moreover, the axolotl mesoderm displays part of the AER machinery, revealing a program for limb (re)growth. These results clarify the debate about the axolotl AER and the extent to which the limb developmental program is recapitulated during regeneration.

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.

A scATAC-seq atlas of chromatin accessibility in axolotl brain regions

Axolotl (Ambystoma mexicanum) is an excellent model for investigating regeneration, the interaction between regenerative and developmental processes, comparative genomics, and evolution. The brain, which serves as the material basis of consciousness, learning, memory, and behavior, is the most complex and advanced organ in axolotl. The modulation of transcription factors is a crucial aspect in determining the function of diverse regions within the brain. There is, however, no comprehensive understanding of the gene regulatory network of axolotl brain regions. Here, we utilized single-cell ATAC sequencing to generate the chromatin accessibility landscapes of 81,199 cells from the olfactory bulb, telencephalon, diencephalon and mesencephalon, hypothalamus and pituitary, and the rhombencephalon. Based on these data, we identified key transcription factors specific to distinct cell types and compared cell type functions across brain regions. Our results provide a foundation for comprehensive analysis of gene regulatory programs, which are valuable for future studies of axolotl brain development, regeneration, and evolution, as well as on the mechanisms underlying cell-type diversity in vertebrate brains.

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.

Differentially expressed chaperone genes reveal a stress response required for unidirectional regeneration in the basal chordate Ciona

BACKGROUND

Unidirectional regeneration in the basal chordate Ciona intestinalis involves the proliferation of adult stem cells residing in the branchial sac vasculature and the migration of progenitor cells to the site of distal injury. However, after the Ciona body is bisected, regeneration occurs in the proximal but not in the distal fragments, even if the latter include a part of the branchial sac with stem cells. A transcriptome was sequenced and assembled from the isolated branchial sacs of regenerating animals, and the information was used to provide insights into the absence of regeneration in distal body fragments.

RESULTS

We identified 1149 differentially expressed genes, which were separated into two major modules by weighted gene correlation network analysis, one consisting of mostly upregulated genes correlated with regeneration and the other consisting of only downregulated genes associated with metabolism and homeostatic processes. The hsp70, dnaJb4, and bag3 genes were among the highest upregulated genes and were predicted to interact in an HSP70 chaperone system. The upregulation of HSP70 chaperone genes was verified and their expression confirmed in BS vasculature cells previously identified as stem and progenitor cells. siRNA-mediated gene knockdown showed that hsp70 and dnaJb4, but not bag3, are required for progenitor cell targeting and distal regeneration. However, neither hsp70 nor dnaJb4 were strongly expressed in the branchial sac vasculature of distal fragments, implying the absence of a stress response. Heat shock treatment of distal body fragments activated hsp70 and dnaJb4 expression indicative of a stress response, induced cell proliferation in branchial sac vasculature cells, and promoted distal regeneration.

CONCLUSIONS

The chaperone system genes hsp70, dnaJb4, and bag3 are significantly upregulated in the branchial sac vasculature following distal injury, defining a stress response that is essential for regeneration. The stress response is absent from distal fragments, but can be induced by a heat shock, which activates cell division in the branchial sac vasculature and promotes distal regeneration. This study demonstrates the importance of a stress response for stem cell activation and regeneration in a basal chordate, which may have implications for understanding the limited regenerative activities in other animals, including vertebrates.

Macrophages modulate fibrosis during newt lens regeneration

Previous studies indicated that macrophages play a role during lens regeneration in newts, but their function has not been tested experimentally. Here we generated a transgenic newt reporter line in which macrophages can be visualized in vivo. Using this new tool, we analyzed the location of macrophages during lens regeneration. We uncovered early gene expression changes using bulk RNAseq in two newt species, Notophthalmus viridescens and Pleurodeles waltl. Next, we used clodronate liposomes to deplete macrophages, which inhibited lens regeneration in both newt species. Macrophage depletion induced the formation of scar-like tissue, an increased and sustained inflammatory response, an early decrease in iris pigment epithelial cell (iPEC) proliferation and a late increase in apoptosis. Some of these phenotypes persisted for at least 100 days and could be rescued by exogenous FGF2. Re-injury alleviated the effects of macrophage depletion and re-started the regeneration process. Together, our findings highlight the importance of macrophages in facilitating a pro-regenerative environment in the newt eye, helping to resolve fibrosis, modulating the overall inflammatory landscape and maintaining the proper balance of early proliferation and late apoptosis.

Senescent cells enhance newt limb regeneration by promoting muscle dedifferentiation

Salamanders are able to regenerate their entire limbs throughout lifespan, through a process that involves significant modulation of cellular plasticity. Limb regeneration is accompanied by the endogenous induction of cellular senescence, a state of irreversible cell cycle arrest associated with profound non-cell-autonomous consequences. While traditionally associated with detrimental physiological effects, here, we show that senescent cells can enhance newt limb regeneration. Through a lineage tracing approach, we demonstrate that exogenously derived senescent cells promote dedifferentiation of mature muscle tissue to generate regenerative progenitors. In a paradigm of newt myotube dedifferentiation, we uncover that senescent cells promote myotube cell cycle re-entry and reversal of muscle identity via secreted factors. Transcriptomic profiling and loss of function approaches identify the FGF-ERK signalling axis as a critical mediator of senescence-induced muscle dedifferentiation. While chronic senescence constrains muscle regeneration in physiological mammalian contexts, we thus highlight a beneficial role for cellular senescence as an important modulator of dedifferentiation, a key mechanism for regeneration of complex structures.

Ageing as a software design flaw

Ageing is inherent to all human beings, yet why we age remains a hotly contested topic. Most mechanistic explanations of ageing posit that ageing is caused by the accumulation of one or more forms of molecular damage. Here, I propose that we age not because of inevitable damage to the hardware but rather because of intrinsic design flaws in the software, defined as the DNA code that orchestrates how a single cell develops into an adult organism. As the developmental software runs, its sequence of events is reflected in shifting cellular epigenetic states. Overall, I suggest that to understand ageing we need to decode our software and the flow of epigenetic information throughout the life course.

A small noncoding RNA links ribosome recovery and translation control to dedifferentiation during salamander limb regeneration

Building a blastema from the stump is a key step of salamander limb regeneration. Stump-derived cells temporarily suspend their identity as they contribute to the blastema by a process generally referred to as dedifferentiation. Here, we provide evidence for a mechanism that involves an active inhibition of protein synthesis during blastema formation and growth. Relieving this inhibition results in a higher number of cycling cells and enhances the pace of limb regeneration. By small RNA profiling and fate mapping of skeletal muscle progeny as a cellular model for dedifferentiation, we find that the downregulation of miR-10b-5p is critical for rebooting the translation machinery. miR-10b-5p targets ribosomal mRNAs, and its artificial upregulation causes decreased blastema cell proliferation, reduction in transcripts that encode ribosomal subunits, diminished nascent protein synthesis, and retardation of limb regeneration. Taken together, our data identify a link between miRNA regulation, ribosome biogenesis, and protein synthesis during newt limb regeneration.

Gradual specialization of phagocytic ameboid cells may have impaired regenerative capacities in metazoan lineages

Animal regeneration is a fascinating field of research that has captured the attention of many generations of scientists. Among the cellular mechanisms underlying tissue and organ regeneration, we highlight the role of phagocytic ameboid cells (PACs). Beyond their ability to engulf nutritional particles, microbes, and apoptotic cells, their involvement in regeneration has been widely documented. It has been extensively described that, at least in part, animal regenerative mechanisms rely on PACs that serve as a hub for a range of critical physiological functions, both in health and disease. Considering the phylogenetics of PAC evolution, and the loss and gain of nutritional, immunological, and regenerative potential across Metazoa, we aim to discuss when and how phagocytic activity was first co-opted to regenerative tissue repair. We propose that the gradual specialization of PACs during metazoan derivation may have contributed to the loss of regenerative potential in animals, with critical impacts on potential translational strategies for regenerative medicine.

Thyroid hormone receptor knockout prevents the loss of Xenopus tail regeneration capacity at metamorphic climax

BACKGROUND

Animal regeneration is the natural process of replacing or restoring damaged or missing cells, tissues, organs, and even entire body to full function. Studies in mammals have revealed that many organs lose regenerative ability soon after birth when thyroid hormone (T3) level is high. This suggests that T3 play an important role in organ regeneration. Intriguingly, plasma T3 level peaks during amphibian metamorphosis, which is very similar to postembryonic development in humans. In addition, many organs, such as heart and tail, also lose their regenerative ability during metamorphosis. These make frogs as a good model to address how the organs gradually lose their regenerative ability during development and what roles T3 may play in this. Early tail regeneration studies have been done mainly in the tetraploid Xenopus laevis (X. laevis), which is difficult for gene knockout studies. Here we use the highly related but diploid anuran X. tropicalis to investigate the role of T3 signaling in tail regeneration with gene knockout approaches.

RESULTS

We discovered that X. tropicalis tadpoles could regenerate their tail from premetamorphic stages up to the climax stage 59 then lose regenerative capacity as tail resorption begins, just like what observed for X. laevis. To test the hypothesis that T3-induced metamorphic program inhibits tail regeneration, we used TR double knockout (TRDKO) tadpoles lacking both TRα and TRβ, the only two receptor genes in vertebrates, for tail regeneration studies. Our results showed that TRs were not necessary for tail regeneration at all stages. However, unlike wild type tadpoles, TRDKO tadpoles retained regenerative capacity at the climax stages 60/61, likely in part by increasing apoptosis at the early regenerative period and enhancing subsequent cell proliferation. In addition, TRDKO animals had higher levels of amputation-induced expression of many genes implicated to be important for tail regeneration, compared to the non-regenerative wild type tadpoles at stage 61. Finally, the high level of apoptosis in the remaining uncut portion of the tail as wild type tadpoles undergo tail resorption after stage 61 appeared to also contribute to the loss of regenerative ability.

CONCLUSIONS

Our findings for the first time revealed an evolutionary conservation in the loss of tail regeneration capacity at metamorphic climax between X. laevis and X. tropicalis. Our studies with molecular and genetic approaches demonstrated that TR-mediated, T3-induced gene regulation program is responsible not only for tail resorption but also for the loss of tail regeneration capacity. Further studies by using the model should uncover how T3 modulates the regenerative outcome and offer potential new avenues for regenerative medicines toward human patients.