TISSUE REPAIR AND EPIMORPHIC REGENERATION: AN OVERVIEW

Myc and Tor drive growth and cell competition in the regeneration blastema of Drosophila wing imaginal discs

During the regeneration of injured or lost tissues, the regeneration blastema serves as a hub for robust growth. Drosophila imaginal discs are a genetically tractable and simple model system for the study of regeneration and organization of this regrowth. Key signals that contribute to regenerative growth in these discs, such as ROS, Wnt/Wg, JNK, p38, JAK/STAT, and the Hippo pathway, have been identified. However, a detailed exploration of the spatial organization of regrowth, the factors that directly drive this growth, and the consequences of activating drivers of regeneration has not been undertaken. Here, we find that regenerative growth in imaginal discs is controlled by the transcription factor Myc and by Tor signaling, which additively drive proliferation and translation in the regeneration blastema. The spatial organization of growth in the blastema is arranged into concentric growth zones defined by Myc expression, elevated Tor activity, and elevated translation. In addition, the increased Myc expression in the innermost zone induced Xrp1-independent cell competition-like death in the adjacent zones, revealing a delicate balance between driving growth and inducing death in the regenerating tissue.

Connecting Bone Remodeling and Regeneration: Unraveling Hormones and Signaling Pathways

Recent advancements in tissue engineering and stem cell science have positioned bone disease treatment as a promising frontier in regenerative medicine. This review explores the hormonal and signaling pathways critical to bone regeneration, with a focus on their clinical relevance. Key endocrine factors, including thyroid hormones (T3 and T4), insulin-like growth factor 1 (IGF-1), bone morphogenetic proteins (BMPs), parathyroid hormone (PTH), calcitonin, and fibroblast growth factor 23 (FGF23), play pivotal roles in bone remodeling by regulating osteoblast activity, bone resorption, and mineralization. These factors primarily act through the Wnt/β-catenin, BMP, and FGF signaling pathways, which govern bone repair and regeneration. While animal models, such as axolotls, zebrafish, and Xenopus laevis, provide valuable findings about these mechanisms, translating these findings into human applications presents challenges. This review underscores the therapeutic potential of modulating these hormonal networks to enhance bone regeneration while cautioning against possible adverse effects, such as uncontrolled tissue proliferation or metabolic imbalances. By integrating knowledge from regenerative models, this work provides a foundation for optimizing hormone-based therapies for clinical applications in bone repair and disease treatment.

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.

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.

Tail Tales: What We Have Learned About Regeneration from Xenopus Laevis Tadpoles

This review explores the regenerative capacity of Xenopus laevis, focusing on tail regeneration, as a model to uncover cellular, molecular, and developmental mechanisms underlying tissue repair. X. laevis tadpoles provide unique insights into regenerative biology due to their regeneration-competent and -incompetent stages and ability to regrow complex structures in the tail, including the spinal cord, muscle, and skin, after amputation. The review delves into the roles of key signaling pathways, such as those involving reactive oxygen species (ROS) and signaling molecules like BMPs and FGFs, in orchestrating cellular responses during regeneration. It also examines how mechanotransduction, epigenetic regulation, and metabolic shifts influence tissue restoration. Comparisons of regenerative capacity with other species shed light on the evolutionary loss of regenerative abilities and underscore X. laevis as an invaluable model for understanding the constraints of tissue repair in higher organisms. This comprehensive review synthesizes recent findings, suggesting future directions for exploring regeneration mechanisms, with potential implications for advancing regenerative medicine.

Regenerative Medicine in Gynecology

The female reproductive tract undergoes dynamic changes across the life span. Congenital abnormalities, life events, and medical interventions can negatively affect the structure and function of reproductive tract organs, resulting in lifelong sequelae. The objective of regenerative gynecology is to discover and promote endogenous mechanisms by which a healthy tissue maintains overall tissue integrity after injury, after disease, or with age. In this review, we discuss some of the key state-of-the-art cell-based and scaffolding therapies that have been applied to regenerate gynecologic tissues and organs primarily in animal and tissue culture models. We further discuss the limitations of current technologies, problems of implementation and scalability, and future outlook of the field.

PLOD2, a key factor for MRL MSC metabolism and chondroprotective properties

BACKGROUND

Initially discovered for its ability to regenerate ear holes, the Murphy Roth Large (MRL) mouse has been the subject of multiple research studies aimed at evaluating its ability to regenerate other body tissues and at deciphering the mechanisms underlying it. These enhanced abilities to regenerate, retained during adulthood, protect the MRL mouse from degenerative diseases such as osteoarthritis (OA). Here, we hypothesized that mesenchymal stromal/stem cells (MSC) derived from the regenerative MRL mouse could be involved in their regenerative potential through the release of pro-regenerative mediators.

METHOD

To address this hypothesis, we compared the secretome of MRL and BL6 MSC and identified several candidate molecules expressed at significantly higher levels by MRL MSC than by BL6 MSC. We selected one candidate, Plod2, and performed functional in vitro assays to evaluate its role on MRL MSC properties including metabolic profile, migration, and chondroprotective effects. To assess its contribution to MRL protection against OA, we used an experimental model for osteoarthritis induced by collagenase (CiOA).

RESULTS

Among the candidate molecules highly expressed by MRL MSC, we focused our attention on procollagen-lysine,2-oxoglutarate 5-dioxygenase 2 (PLOD2). Plod2 silencing induced a decrease in the glycolytic function of MRL MSC, resulting in the alteration of their migratory and chondroprotective abilities in vitro. In vivo, we showed that Plod2 silencing in MRL MSC significantly impaired their capacity to protect mouse from developing OA.

CONCLUSION

Our results demonstrate that the chondroprotective and therapeutic properties of MRL MSC in the CiOA experimental model are in part mediated by PLOD2.

Single-cell analysis of lizard blastema fibroblasts reveals phagocyte-dependent activation of Hedgehog-responsive chondrogenesis

Lizards cannot naturally regenerate limbs but are the closest known relatives of mammals capable of epimorphic tail regrowth. However, the mechanisms regulating lizard blastema formation and chondrogenesis remain unclear. Here, single-cell RNA sequencing analysis of regenerating lizard tails identifies fibroblast and phagocyte populations linked to cartilage formation. Pseudotime trajectory analyses suggest spp1+-activated fibroblasts as blastema cell sources, with subsets exhibiting sulf1 expression and chondrogenic potential. Tail blastema, but not limb, fibroblasts express sulf1 and form cartilage under Hedgehog signaling regulation. Depletion of phagocytes inhibits blastema formation, but treatment with pericytic phagocyte-conditioned media rescues blastema chondrogenesis and cartilage formation in amputated limbs. The results indicate a hierarchy of phagocyte-induced fibroblast gene activations during lizard blastema formation, culminating in sulf1+ pro-chondrogenic populations singularly responsive to Hedgehog signaling. These properties distinguish lizard blastema cells from homeostatic and injury-stimulated fibroblasts and indicate potential actionable targets for inducing regeneration in other species, including humans.

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.

Epimorphic Regeneration of Elastic Cartilage: Morphological Study into the Role of Cellular Senescence

Control over endogenous reparative mechanisms is the future of regenerative medicine. The rabbit ear defect is a rare model which allows the observation of the epimorphic regeneration of elastic cartilage. However, the mechanisms of phenotypical restoration of this highly differentiated tissue have not been studied. We modelled circular ear defects of different sizes (4, 6, and 8 mm in diameter) in 12 laboratory rabbits, and observed them during 30, 60, 90, and 120 day periods. Excised tissues were processed and analyzed by standard histological methods and special histochemical reactions for senescence associated-β-galactosidase and lectin markers. We demonstrated that larger defects caused significant elevation of senescence associated-β-galactosidase in chondrocytes. The fullness of epimorphic regeneration of elastic cartilage depended on the activation of cellular senescence and synthesis of elastic fibers. Further investigation into the role of cells with senescence-associated secretory phenotype in damaged tissues can present new targets for controlled tissue regeneration.

Cartilage regeneration in zebrafish depends on Nrg1/ErbB signaling pathway

Objective: Cartilage, as the majority of adult mammalian tissues, has limited regeneration capacity. Cartilage degradation consecutive to joint injury or aging then leads to irreversible joint damage and diseases. In contrast, several vertebrate species such as the zebrafish have the remarkable capacity to spontaneously regenerate skeletal structures after severe injuries. The objective of our study was to test the regenerative capacity of Meckel's cartilage (MC) upon mechanical injury in zebrafish and to identify the mechanisms underlying this process. Methods and Results: Cartilage regenerative capacity in zebrafish larvae was investigated after mechanical injuries of the lower jaw MC in TgBAC(col2a1a:mCherry), to visualize the loss and recovery of cartilage. Confocal analysis revealed the formation of new chondrocytes and complete regeneration of MC at 14 days post-injury (dpi) via chondrocyte cell cycle re-entry and proliferation of pre-existing MC chondrocytes near the wound. Through expression analyses, we showed an increase of nrg1 expression in the regenerating lower jaw, which also expresses Nrg1 receptors, ErbB3 and ErbB2. Pharmacological inhibition of the ErbB pathway and specific knockdown of Nrg1 affected MC regeneration indicating the pivotal role of this pathway for cartilage regeneration. Finally, addition of exogenous NRG1 in an in vitro model of osteoarthritic (OA)-like chondrocytes induced by IL1β suggests that Nrg1/ErbB pathway is functional in mammalian chondrocytes and alleviates the increased expression of catabolic markers characteristic of OA-like chondrocytes. Conclusion: Our results show that the Nrg1/ErbB pathway is required for spontaneous cartilage regeneration in zebrafish and is of interest to design new therapeutic approaches to promote cartilage regeneration in mammals.

From injury to patterning—MAPKs and Wnt signaling in Hydra

Hydra has a regenerative capacity that is not limited to individual organs but encompasses the entire body. Various global and integrative genome, transcriptome and proteome approaches have shown that many of the signaling pathways and transcription factors present in vertebrates are already present in Cnidaria, the sister group of Bilateria, and are also activated in regeneration. It is now possible to investigate one of the central questions of regeneration biology, i.e., how does the patterning system become activated by the injury signals that initiate regeneration. This review will present the current data obtained in Hydra and draw parallels with regeneration in Bilateria. Important findings of this global analysis are that the Wnt signaling pathway has a dual function in the regeneration process. In the early phase Wnt is activated generically and in a second phase of pattern formation it is activated in a position specific manner. Thus, Wnt signaling is part of the generic injury response, in which mitogen-activated protein kinases (MAPKs) are initially activated via calcium and reactive oxygen species (ROS). The MAPKs, p38, c-Jun N-terminal kinases (JNKs) and extracellular signal-regulated kinases (ERK) are essential for Wnt activation in Hydra head and foot regenerates. Furthermore, the antagonism between the ERK signaling pathway and stress-induced MAPKs results in a balanced induction of apoptosis and mitosis. However, the early Wnt genes are activated by MAPK signaling rather than apoptosis. Early Wnt gene activity is differentially integrated with a stable, β-Catenin-based gradient along the primary body axis maintaining axial polarity and activating further Wnts in the regenerating head. Because MAPKs and Wnts are highly evolutionarily conserved, we hypothesize that this mechanism is also present in vertebrates but may be activated to different degrees at the level of early Wnt gene integration.

Identification and characterization of Deoxyribonuclease II in planarian Dugesia japonica

Deoxyribonuclease II (DNase II) has been found to regulate inflammation, autoimmunity and apoptosis in vertebrates and invertebrates. The strong capacity of degrading DNA makes DNase II play an important role in the immune process. Planarian has become one of the model references due to its strong immune system, the environment they live makes planarians face the threat of microorganisms and injury, the strong immune system can protect planarians from the threat of bacterial and infection. In this study, we found that there was DNase in the lysis buffer of planarians, then we acquired the sequence of DjDN2s (Dugesia japonica DNase2s) and confirmed the DjDN2s were conserved DNase IIs. The predicted structure showed the active sites and binding patterns of DjDN2s. Whole-mount in situ hybridization results showed DjDN2s mainly expressed in immune organs. Quantitative real-time PCR revealed that the expression of DjDN2s upregulated in varying degrees when got hurt and challenged with bacteria, and the knockdown of DjDN2s led to the slower repair of wound. The recombinant phages which take DjDN2 also had the ability to degrade DNA and clear young biofilm of Gram-negative bacteria. Collectively, DNase II of planarian might play a role in the antimicrobial response and wound-induced response.

Inducing Vertebrate Limb Regeneration: A Review of Past Advances and Future Outlook

Limb loss due to traumatic injury or amputation is a major biomedical burden. Many vertebrates exhibit the ability to form and pattern normal limbs during embryogenesis from amorphous clusters of precursor cells, hinting that this process could perhaps be activated later in life to rebuild missing or damaged limbs. Indeed, some animals, such as salamanders, are proficient regenerators of limbs throughout their life span. Thus, research over the last century has sought to stimulate regeneration in species that do not normally regenerate their appendages. Importantly, these efforts are not only a vital aspect of regenerative medicine, but also have fundamental implications for understanding evolution and the cellular control of growth and form throughout the body. Here we review major recent advances in augmenting limb regeneration, summarizing the degree of success that has been achieved to date in frog and mammalian models using genetic, biochemical, and bioelectrical interventions. While the degree of whole limb repair in rodent models has been modest to date, a number of new technologies and approaches comprise an exciting near-term road map for basic and clinical progress in regeneration.

Exploring Macrophage-Dependent Wound Regeneration During Mycobacterial Infection in Zebrafish

The molecular and cellular mechanisms associated with tissue degradation or regeneration in an infectious context are poorly defined. Herein, we explored the role of macrophages in orchestrating either tissue regeneration or degradation in zebrafish embryos pre-infected with the fish pathogen Mycobacterium marinum. Zebrafish were inoculated with different infectious doses of M. marinum prior to fin resection. While mild infection accelerated fin regeneration, moderate or severe infection delayed this process by reducing blastemal cell proliferation and impeding tissue morphogenesis. This was correlated with impaired macrophage recruitment at the wound of the larvae receiving high infectious doses. Macrophage activation characterized, in part, by a high expression level of tnfa was exacerbated in severely infected fish during the early phase of the regeneration process, leading to macrophage necrosis and their complete absence in the later phase. Our results demonstrate how a mycobacterial infection influences the macrophage response and tissue regenerative processes.

Facial Flap Necrosis From COVID-19 Face Mask Precautions

ABSTRACT

Use of facial mask coverings has been a strong Centers for Disease Control and Prevention recommendation as an essential mitigation measure in the spread of the SARS-CoV-2 novel coronavirus (COVID)-19 virus. Face mask utilization has been shown to induce changes in the skin microclimate, especially around the perioral and perinasal regions. This results in increased mask adjustments and development of friction between masks and the underlying skin. The authors report novel findings of 2 individuals with skin cancer who underwent facial reconstruction during the COVID-19 pandemic. They encountered untoward sequelae of facial flap pressure necrosis due to the use of face mask coverings. These individuals were ultimately successfully treated with local wound care. One individual experienced auricular implant extrusion and flap loss. It is critical that reconstructive surgeons be aware of potential complications and the need for potential revision surgeries due to the use of face masks, and educate their patients to properly position the protective face masks based on the type of reconstruction performed. Plastic surgeons might also reconsider reconstructive management options in light of these additional obstacles.

Lizard Blastema Organoid Model Recapitulates Regenerated Tail Chondrogenesis

(1) Background: Lizard tail regeneration provides a unique model of blastema-based tissue regeneration for large-scale appendage replacement in amniotes. Green anole lizard (Anolis carolinensis) blastemas contain fibroblastic connective tissue cells (FCTCs), which respond to hedgehog signaling to create cartilage in vivo. However, an in vitro model of the blastema has not previously been achieved in culture. (2) Methods: By testing two adapted tissue dissociation protocols and two optimized media formulations, lizard tail FCTCs were pelleted in vitro and grown in a micromass blastema organoid culture. Pellets were analyzed by histology and in situ hybridization for FCTC and cartilage markers alongside staged original and regenerating lizard tails. (3) Results: Using an optimized serum-free media and a trypsin- and collagenase II-based dissociation protocol, micromass blastema organoids were formed. Organoid cultures expressed FCTC marker CDH11 and produced cartilage in response to hedgehog signaling in vitro, mimicking in vivo blastema and tail regeneration. (4) Conclusions: Lizard tail blastema regeneration can be modeled in vitro using micromass organoid culture, recapitulating in vivo FCTC marker expression patterns and chondrogenic potential.

Approaches for Regenerative Healing of Cutaneous Wound with an Emphasis on Strategies Activating the Wnt/β-Catenin Pathway

Significance: In adult mammals, spontaneous repair of a cutaneous wound occurs slowly and leaves a scar with skin adnexa deficiencies. To accelerate cutaneous wound-healing rates and avoid scar formation, current studies have focused on regenerative therapies. Recent Advances: Emerging therapeutics for regenerative wound healing often focus on the use of growth factors and stem cells. However, these therapeutic approaches have limited routine clinical use due to high costs and technical requirements. Critical Issue: Understanding the molecular mechanisms involved in the signaling pathways for cutaneous wound healing and neogenic synthesis of the skin components is important for identification of novel targets for the development of regenerative wound-healing agents. Future Directions: The Wnt/β-catenin pathway is a well-known key player for enhancement of the overall healing process involving tissue regeneration via crosstalk with other signaling pathways. Strategies that activate the Wnt/β-catenin pathway via modulation of the pathway-controlling regulatory factors could provide effective therapeutic approaches for regenerative wound healing.

brca2-mutant zebrafish exhibit context- and tissue-dependent alterations in cell phenotypes and response to injury

Cancer cells frequently co-opt molecular programs that are normally activated in specific contexts, such as embryonic development and the response to injury. Determining the impact of cancer-associated mutations on cellular phenotypes within these discrete contexts can provide new insight into how such mutations lead to dysregulated cell behaviors and subsequent cancer onset. Here we assess the impact of heritable BRCA2 mutation on embryonic development and the injury response using a zebrafish model (Danio rerio). Unlike most mouse models for BRCA2 mutation, brca2-mutant zebrafish are fully viable and thus provide a unique tool for assessing both embryonic and adult phenotypes. We find that maternally provided brca2 is critical for normal oocyte development and embryonic survival in zebrafish, suggesting that embryonic lethality associated with BRCA2 mutation is likely to reflect defects in both meiotic and embryonic developmental programs. On the other hand, we find that adult brca2-mutant zebrafish exhibit aberrant proliferation of several cell types under basal conditions and in response to injury in tissues at high risk for cancer development. These divergent effects exemplify the often-paradoxical outcomes that occur in embryos (embryonic lethality) versus adult animals (cancer predisposition) with mutations in cancer susceptibility genes such as BRCA2. The altered cell behaviors identified in brca2-mutant embryonic and adult tissues, particularly in adult tissues at high risk for cancer, indicate that the effects of BRCA2 mutation on cellular phenotypes are both context- and tissue-dependent.

FGF2 and FGF10 expression patterns in the epidermis and mesenchyme during homeotic transformation of tail into hindlimbs in frog tadpoles

Limbs are trunk quintessential in tetrapods. Their development relies on the Retinoic acid (RA) gradient in association with the Fibroblast Growth Factors (FGFs). The role of various FGFs have been probed extensively and confirmed during the induction of ectopic limbs in vertebrates. On such factual backdrops, we studied the expression patterns of FGF2 and FGF10 in the epidermis and mesenchyme by immunohistochemical localization in the regenerating tails of tadpoles of the Indian tree frog, Polypedates maculatus. These tadpoles are known to exhibit a kind of homeotic transformation of tail to limbs during regeneration, whose exact mechanism is still to be established by scientific investigations. Here in this study, we provide the first evidence of the putative involvement of FGF2 and FGF10 during such ectopic appendage development.

Aerobic glycolysis is important for zebrafish larval wound closure and tail regeneration

The underlying mechanisms of appendage regeneration remain largely unknown and uncovering these mechanisms in capable organisms has far-reaching implications for potential treatments in humans. Recent studies implicate a requirement for metabolic reprogramming reminiscent of the Warburg effect during successful appendage and organ regeneration. As changes are thus predicted to be highly dynamic, methods permitting direct, real-time visualisation of metabolites at the tissue and organismal level would offer a significant advance in defining the influence of metabolism on regeneration and healing. We sought to examine whether glycolytic activity was altered during larval fin regeneration, utilising the genetically encoded biosensor, Laconic, enabling the spatiotemporal assessment of lactate levels in living zebrafish. We present evidence for a rapid increase in lactate levels within min following injury, with a role of aerobic glycolysis in actomyosin contraction and wound closure. We also find a second wave of lactate production, associated with overall larval tail regeneration. Chemical inhibition of glycolysis attenuates both the contraction of the wound and regrowth of tissue following tail amputation, suggesting aerobic glycolysis is necessary at two distinct stages of regeneration.

A Morphological and Histological Investigation of Imperfect Lungfish Fin Regeneration

Regeneration, the replacement of body parts in a living animal, has excited scientists for centuries and our knowledge of vertebrate appendage regeneration has increased significantly over the past decades. While the ability of amniotes to regenerate body parts is very limited, members of other vertebrate clades have been shown to have rather high regenerative capacities. Among tetrapods (four-limbed vertebrates), only salamanders show unparalleled capacities of epimorphic tissue regeneration including replacement of organ and body parts in an apparently perfect fashion. The closest living relatives of Tetrapoda, the lungfish, show regenerative abilities that are comparable to those of salamanders and recent studies suggest that these high regenerative capacities may indeed be ancestral for bony fish (osteichthyans) including tetrapods. While great progress has been made in recent years in understanding the cellular and molecular mechanisms deployed during appendage regeneration, comparatively few studies have investigated gross morphological and histological features of regenerated fins and limbs. Likewise, rather little is known about how fin regeneration compares morphologically to salamander limb regeneration. In this study, we investigated the morphology and histology of regenerated fins in all three modern lungfish families. Data from histological serial sections, 3D reconstructions, and x-ray microtomography scans were analyzed to assess morphological features, quality and pathologies in lungfish fin regenerates. We found several anomalies resulting from imperfect regeneration in regenerated fins in all investigated lungfish species, including fusion of skeletal elements, additional or fewer elements, and distal branching. The similarity of patterns in regeneration abnormalities compared to salamander limb regeneration lends further support to the hypothesis that high regenerative capacities are plesiomorphic for sarcopterygians.

MicroRNA roles in regeneration: Multiple lessons from zebrafish

MicroRNAs (miRNAs) are small noncoding RNAs with pivotal roles in the control of gene expression. By comparing the miRNA profiles of uninjured vs. regenerating tissues and structures, several studies have found that miRNAs are potentially involved in the regenerative process. By inducing miRNA overexpression or inhibition, elegant experiments have directed regenerative responses validating relevant miRNA-to-target interactions. The zebrafish (Danio rerio) has been the epicenter of regenerative research because of its exceptional capability to self-repair damaged tissues and body structures. In this review, we discuss recent discoveries that have improved our understanding of the impact of gene regulation mediated by miRNAs in the context of the regeneration of fins, heart, retina, and nervous tissue in zebrafish. We compiled what is known about the miRNA control of regeneration in these tissues and investigated the links among up-regulated and down-regulated miRNAs, their putative or validated targets, and the regenerative process. Finally, we briefly discuss the forthcoming prospects, highlighting directions and the potential for further development of this field.

Pro-regenerative Dialogue Between Macrophages and Mesenchymal Stem/Stromal Cells in Osteoarthritis

Osteoarthritis (OA), the most common degenerative and inflammatory joint disorder, is multifaceted. Indeed, OA characteristics include cartilage degradation, osteophytes formation, subchondral bone changes, and synovium inflammation. The difficulty in discovering new efficient treatments for OA patients up to now comes from the adoption of monotherapy approaches targeting either joint tissue repair/catabolism or inflammation to address the diverse components of OA. When satisfactory, these approaches only provide short-term beneficial effects, since they only result in the repair and not the full structural and functional reconstitution of the damaged tissues. In the present review, we will briefly discuss the current therapeutic approaches used to repair the damaged OA cartilage. We will highlight the results obtained with cell-based products in clinical trials and demonstrate how the current strategies result in articular cartilage repair showing restricted early-stage clinical improvements. In order to identify novel therapeutic targets and provide to OA patients long-term clinical benefits, herein, we will review the basis of the regenerative process. We will focus on macrophages and their ambivalent roles in OA development and tissue regeneration, and review the therapeutic strategies to target the macrophage response and favor regeneration in OA.

The Role of Macrophages During Zebrafish Injury and Tissue Regeneration Under Infectious and Non-Infectious Conditions

The future of regenerative medicine relies on our understanding of the mechanistic processes that underlie tissue regeneration, highlighting the need for suitable animal models. For many years, zebrafish has been exploited as an adequate model in the field due to their very high regenerative capabilities. In this organism, regeneration of several tissues, including the caudal fin, is dependent on a robust epimorphic regenerative process, typified by the formation of a blastema, consisting of highly proliferative cells that can regenerate and completely grow the lost limb within a few days. Recent studies have also emphasized the crucial role of distinct macrophage subpopulations in tissue regeneration, contributing to the early phases of inflammation and promoting tissue repair and regeneration in late stages once inflammation is resolved. However, while most studies were conducted under non-infectious conditions, this situation does not necessarily reflect all the complexities of the interactions associated with injury often involving entry of pathogenic microorganisms. There is emerging evidence that the presence of infectious pathogens can largely influence and modulate the host immune response and the regenerative processes, which is sometimes more representative of the true complexities underlying regenerative mechanics. Herein, we present the current knowledge regarding the paths involved in the repair of non-infected and infected wounds using the zebrafish model.

The Macrophage Response Is Driven by Mesenchymal Stem Cell-Mediated Metabolic Reprogramming

Mesenchymal stem cells (MSCs) are multipotent adult stromal cells widely studied for their regenerative and immunomodulatory properties. They are capable of modulating macrophage plasticity depending on various microenvironmental signals. Current studies have shown that metabolic changes can also affect macrophage fate and function. Indeed, changes in the environment prompt phenotype change. Therefore, in this review, we will discuss how MSCs orchestrate macrophage’s metabolic plasticity and the impact on their function. An improved understanding of the crosstalk between macrophages and MSCs will improve our knowledge of MSC’s therapeutic potential in the context of inflammatory diseases, cancer, and tissue repair processes in which macrophages are pivotal.

Salamanders: The molecular basis of tissue regeneration and its relevance to human disease

Salamanders are recognized for their ability to regenerate a broad range of tissues. They have also have been used for hundreds of years for classical developmental biology studies because of their large accessible embryos. The range of tissues these animals can regenerate is fascinating, from full limbs to parts of the brain or heart, a potential that is missing in humans. Many promising research efforts are working to decipher the molecular blueprints shared across the organisms that naturally have the capacity to regenerate different tissues and organs. Salamanders are an excellent example of a vertebrate that can functionally regenerate a wide range of tissue types. In this review, we outline some of the significant insights that have been made that are aiding in understanding the cellular and molecular mechanisms of tissue regeneration in salamanders and discuss why salamanders are a worthy model in which to study regenerative biology and how this may benefit research fields like regenerative medicine to develop therapies for humans in the future.

Transcriptomic and proteomic analysis of Hemidactylus frenatus during initial stages of tail regeneration

Epimorphic regeneration of appendages is a complex and complete phenomenon found in selected animals. Hemidactylus frenatus, house gecko has the remarkable ability to regenerate the tail tissue upon autotomy involving epimorphic regeneration mechanism. This study has identified and evaluated the molecular changes at gene and protein level during the initial stages, i.e., during the wound healing and repair mechanism initiation stage of tail regeneration. Based on next generation transcriptomics and De novo analysis the transcriptome library of the gecko tail tissue was generated. A total of 254 genes and 128 proteins were found to be associated with the regeneration of gecko tail tissue upon amputation at 1, 2 and 5-day post amputation (dpa) against control, 0-dpa through differential transcriptomic and proteomic analysis. To authenticate the expression analysis, 50 genes were further validated involving RTPCR. 327 genes/proteins identified and mapped from the study showed association for Protein kinase A signaling, Telomerase BAG2 signaling, paxillin signaling, VEGF signaling network pathways based on network pathway analysis. This study empanelled list of transcriptome, proteome and the list of genes/proteins associated with the tail regeneration.

Unlocking mammalian regeneration through hypoxia inducible factor one alpha signaling

Historically, the field of regenerative medicine has aimed to heal damaged tissue through the use of biomaterials scaffolds or delivery of foreign progenitor cells. Despite 30 years of research, however, translation and commercialization of these techniques has been limited. To enable mammalian regeneration, a more practical approach may instead be to develop therapies that evoke endogenous processes reminiscent of those seen in innate regenerators. Recently, investigations into tadpole tail regrowth, zebrafish limb restoration, and the super-healing Murphy Roths Large (MRL) mouse strain, have identified ancient oxygen-sensing pathways as a possible target to achieve this goal. Specifically, upregulation of the transcription factor, hypoxia-inducible factor one alpha (HIF-1α) has been shown to modulate cell metabolism and plasticity, as well as inflammation and tissue remodeling, possibly priming injuries for regeneration. Since HIF-1α signaling is conserved across species, environmental or pharmacological manipulation of oxygen-dependent pathways may elicit a regenerative response in non-healing mammals. In this review, we will explore the emerging role of HIF-1α in mammalian healing and regeneration, as well as attempts to modulate protein stability through hyperbaric oxygen treatment, intermittent hypoxia therapy, and pharmacological targeting. We believe that these therapies could breathe new life into the field of regenerative medicine.

Appendage Regeneration in Vertebrates: What Makes This Possible?

The ability to regenerate amputated or injured tissues and organs is a fascinating property shared by several invertebrates and, interestingly, some vertebrates. The mechanism of evolutionary loss of regeneration in mammals is not understood, yet from the biomedical and clinical point of view, it would be very beneficial to be able, at least partially, to restore that capability. The current availability of new experimental tools, facilitating the comparative study of models with high regenerative ability, provides a powerful instrument to unveil what is needed for a successful regeneration. The present review provides an updated overview of multiple aspects of appendage regeneration in three vertebrates: lizard, salamander, and zebrafish. The deep investigation of this process points to common mechanisms, including the relevance of Wnt/β-catenin and FGF signaling for the restoration of a functional appendage. We discuss the formation and cellular origin of the blastema and the identification of epigenetic and cellular changes and molecular pathways shared by vertebrates capable of regeneration. Understanding the similarities, being aware of the differences of the processes, during lizard, salamander, and zebrafish regeneration can provide a useful guide for supporting effective regenerative strategies in mammals.

Mitochondrial microRNAs: A Putative Role in Tissue Regeneration

The most famous role of mitochondria is to generate ATP through oxidative phosphorylation, a metabolic pathway that involves a chain of four protein complexes (the electron transport chain, ETC) that generates a proton-motive force that in turn drives the ATP synthesis by the Complex V (ATP synthase). An impressive number of more than 1000 mitochondrial proteins have been discovered. Since mitochondrial proteins have a dual genetic origin, it is predicted that ~99% of these proteins are nuclear-encoded and are synthesized in the cytoplasmatic compartment, being further imported through mitochondrial membrane transporters. The lasting 1% of mitochondrial proteins are encoded by the mitochondrial genome and synthesized by the mitochondrial ribosome (mitoribosome). As a result, an appropriate regulation of mitochondrial protein synthesis is absolutely required to achieve and maintain normal mitochondrial function. Regarding miRNAs in mitochondria, it is well-recognized nowadays that several cellular mechanisms involving mitochondria are regulated by many genetic players that originate from either nuclear- or mitochondrial-encoded small noncoding RNAs (sncRNAs). Growing evidence collected from whole genome and transcriptome sequencing highlight the role of distinct members of this class, from short interfering RNAs (siRNAs) to miRNAs and long noncoding RNAs (lncRNAs). Some of the mechanisms that have been shown to be modulated are the expression of mitochondrial proteins itself, as well as the more complex coordination of mitochondrial structure and dynamics with its function. We devote particular attention to the role of mitochondrial miRNAs and to their role in the modulation of several molecular processes that could ultimately contribute to tissue regeneration accomplishment.

Uterine Scarring Leads to Adverse Pregnant Consequences by Impairing the Endometrium Response to Steroids

Uterine surgical scarring is an increasing risk factor for adverse pregnant consequences that threaten fetal-maternal health. The detailed molecular features of scar implantation remain largely unknown. We aim to study the pathologic features of uterine surgical scarring and the mechanisms of compromised pregnancy outcomes of scar implantation. We generated a mouse model of uterine surgical scarring with a uterine incision penetrating the myometrium to endometrium to examine the pathologic changes and transcriptome profiles of uterine scarring at various postsurgery (PS) time points, as well as features of the feto-maternal interface during scar implantation. We found that uterine surgical scar recovery was consistently poor at PS3 until PS90, as shown by a reduced number of endometrial glands, inhibition of myometrial smooth muscle cell growth but excessive collagen fiber deposition, and massive leukocyte infiltration. Transcriptome annotation indicated significant chronic inflammation at the scarring site. At the peri-implantation and postimplantation stages, abnormal expression of various steroid-responsive genes at the scarring site was in parallel with lumen epithelial cell hyperplasia, inappropriate luminal closure, and disorientation of the implanted embryo, restricted stromal cell proliferation, and defective decidualization. High embryonic lethality (around 70%) before E10.5 was observed, and the small amount of survival embryos at E10.5 exhibited restricted growth and aberrant placenta defects including overinvasion of trophoblast cells into the decidua and insufficient fetal blood vessel branching in the labyrinth. The findings indicate that chronic inflammation and compromised responses to steroids in uterine scar tissues are the pivotal molecular basis for adverse pregnancy consequences of scar implantation.

Interactions at engineered graft-tissue interfaces: A review

The interactions at the graft-tissue interfaces are critical for the results of engraftments post-implantation. To improve the success rate of the implantations, as well as the quality of the patients' life, understanding the possible reactions between artificial materials and the host tissues is helpful in designing new generations of material-based grafts aiming at inducing specific responses from surrounding tissues for their own reparation and regeneration. To help researchers understand the complicated interactions that occur after implantations and to promote the development of better-designed grafts with improved biocompatibility and patient responses, in this review, the topics will be discussed from the basic reactions that occur chronologically at the graft-tissue interfaces after implantations to the existing and potential applications of the mechanisms of such reactions in designing of grafts. It offers a chance to bring up-to-date advances in the field and new strategies of controlling the graft-tissue interfaces.

Single Cell Sequencing Analysis of Lizard Phagocytic Cell Populations and Their Role in Tail Regeneration

Lizards are the closest relatives of mammals capable of tail regeneration, but the specific determinants of amniote regenerative capabilities are currently unknown. Macrophages are phagocytic immune cells that play a critical role in wound healing and tissue regeneration in a wide range of species. We hypothesize that macrophages regulate the process of lizard tail regeneration, and that comparisons with mammalian cell populations will yield insight into the role phagocytes play in determining an organism's regenerative potential. Single cell RNA sequencing (scRNAseq) was used to profile lizard immune cells and compare with mouse counterparts to contrast cell types between the two species. Treatment with clodronate liposomes effectively inhibited lizard tail stump tissue ablation and subsequent regeneration, and scRNAseq was used to profile changes in lizard immune cell populations resulting from tail amputation as well as identifying specific cell types affected by clodronate treatment. ScRNAseq analysis of lizard bone marrow, peripheral blood, and tissue-resident phagocyte cell populations was used to trace marker progression during macrophage differentiation and activation. These results indicated that lizard macrophages are recruited to tail amputation injuries faster than mouse populations and express high levels of matrix metalloproteinases (MMPs). In turn, treatment with MMP inhibitors inhibited lizard tail regeneration. These results provide single cell sequencing data sets for evaluating and comparing lizard and mammalian immune cell populations, and identifying macrophage populations that are critical regulators of lizard tail regrowth.

A single-cell view of tissue regeneration in plants

With the development of advanced molecular-genetic and computational technologies it becomes possible to tackle individual cells within a regenerating tissue, to define morphogenetic and cellular changes in space and time by live imaging, to acquire transcriptome status with single-cell RNA sequencing (ScRNA-seq), and to delineate the candidate mechanisms by iterative computational and experimental approaches. Here, we review recent findings and current knowledge on tissue regeneration in plants, focusing on four evolutionarily conserved scenarios that a cell may embark on to facilitate the regeneration of a plant tissue structure lost by injury, namely cell death, division, dedifferentiation, and transdifferentiation. Understanding of these scenarios at single-cell resolution, singularly and in combination, could provide an unprecedented view of tissue regeneration in plants.

Evolution and Regulation of Limb Regeneration in Arthropods

Regeneration has fascinated both scientists and non-scientists for centuries. Many organisms can regenerate, and arthropod limbs are no exception although their ability to regenerate is a product shaped by natural and sexual selection. Recent studies have begun to uncover cellular and molecular processes underlying limb regeneration in several arthropod species. Here we argue that an evo-devo approach to the study of arthropod limb regeneration is needed to understand aspects of limb regeneration that are conserved and divergent. In particular, we argue that limbs of different species are comprised of cells at distinct stages of differentiation at the time of limb loss and therefore provide insights into regeneration involving both stem cell-like cells/precursor cells and differentiated cells. In addition, we review recent studies that demonstrate how limb regeneration impacts the development of the whole organism and argue that studies on the link between local tissue damage and the rest of the body should provide insights into the integrative nature of development. Molecular studies on limb regeneration are only beginning to take off, but comparative studies on the mechanisms of limb regeneration across various taxa should not only yield interesting insights into development but also answer how this remarkable ability evolved across arthropods and beyond.

Differences in neural stem cell identity and differentiation capacity drive divergent regenerative outcomes in lizards and salamanders

While lizards and salamanders both exhibit the ability to regenerate amputated tails, the outcomes achieved by each are markedly different. Salamanders, such as Ambystoma mexicanum, regenerate nearly identical copies of original tails. Regenerated lizard tails, however, exhibit important morphological differences compared with originals. Some of these differences concern dorsoventral patterning of regenerated skeletal and spinal cord tissues; regenerated salamander tail tissues exhibit dorsoventral patterning, while regrown lizard tissues do not. Additionally, regenerated lizard tails lack characteristically roof plate-associated structures, such as dorsal root ganglia. We hypothesized that differences in neural stem cells (NSCs) found in the ependyma of regenerated spinal cords account for these divergent regenerative outcomes. Through a combination of immunofluorescent staining, RT-PCR, hedgehog regulation, and transcriptome analysis, we analyzed NSC-dependent tail regeneration. Both salamander and lizard Sox2⁺ NSCs form neurospheres in culture. While salamander neurospheres exhibit default roof plate identity, lizard neurospheres exhibit default floor plate. Hedgehog signaling regulates dorsalization/ventralization of salamander, but not lizard, NSCs. Examination of NSC differentiation potential in vitro showed that salamander NSCs are capable of neural differentiation into multiple lineages, whereas lizard NSCs are not, which was confirmed by in vivo spinal cord transplantations. Finally, salamander NSCs xenogeneically transplanted into regenerating lizard tail spinal cords were influenced by native lizard NSC hedgehog signals, which favored salamander NSC floor plate differentiation. These findings suggest that NSCs in regenerated lizard and salamander spinal cords are distinct cell populations, and these differences contribute to the vastly different outcomes observed in tail regeneration.