Macrophages are required for adult salamander limb regeneration. Proc Natl Acad Sci U S A. 2013;110:9415-9420. [PMID: 23690624 DOI: 10.1073/pnas.1300290110]

Two decades of heart regeneration research: Cardiomyocyte proliferation and beyond

Interest in vertebrate cardiac regeneration has exploded over the past two decades since the discovery that adult zebrafish are capable of complete heart regeneration, contrasting the limited regenerative potential typically observed in adult mammalian hearts. Undercovering the mechanisms that both support and limit cardiac regeneration across the animal kingdom may provide unique insights in how we may unlock this capacity in adult humans. In this review, we discuss key discoveries in the heart regeneration field over the last 20 years. Initially, seminal findings revealed that pre-existing cardiomyocytes are the major source of regenerated cardiac muscle, drawing interest into the intrinsic mechanisms regulating cardiomyocyte proliferation. Moreover, recent studies have identified the importance of intercellular interactions and physiological adaptations, which highlight the vast complexity of the cardiac regenerative process. Finally, we compare strategies that have been tested to increase the regenerative capacity of the adult mammalian heart. This article is categorized under: Cardiovascular Diseases > Stem Cells and Development.

Advances in the study of Müller glia reprogramming in mammals

Müller cells play an integral role in the development, maintenance, and photopic signal transmission of the retina. While lower vertebrate Müller cells can differentiate into various types of retinal neurons to support retinal repair following damage, there is limited neurogenic potential of mammalian Müller cells. Therefore, it is of great interest to harness the neurogenic potential of mammalian Müller cells to achieve self-repair of the retina. While multiple studies have endeavored to induce neuronal differentiation and proliferation of mammalian Müller cells under defined conditions, the efficiency and feasibility of these methods often fall short, rendering them inadequate for the requisites of retinal repair. As the mechanisms and methodologies of Müller cell reprogramming have been extensively explored, a summary of the reprogramming process of unlocking the neurogenic potential of Müller cells can provide insight into Müller cell fate development and facilitate their therapeutic use in retinal repair. In this review, we comprehensively summarize the progress in reprogramming mammalian Müller cells and discuss strategies for optimizing methods and enhancing efficiency based on the mechanisms of fate regulation.

Tenascin-C-enriched regeneration-specific extracellular matrix guarantees superior muscle regeneration in Ambystoma mexicanum

Severe muscle injury causes distress and difficulty in humans. Studying the high regenerative ability of the axolotls may provide hints for the development of an effective treatment for severe injuries to muscle tissue. Here, we examined the regenerative process in response to a muscle injury in axolotls. We found that axolotls are capable of complete regeneration in response to a partial muscle resection called volumetric muscle loss (VML), which mammals cannot perfectly regenerate. We investigated the mechanisms underlying this high regenerative capacity in response to VML, focusing on the migration of muscle satellite cells and the extracellular matrix (ECM) formed during VML injury. Axolotls form tenascin-C (TN-C)-enriched ECM after VML injury. This TN-C-enriched ECM promotes the satellite cell migration. We confirmed the importance of TN-C in successful axolotl muscle regeneration by creating TN-C mutant animals. Our results suggest that the maintenance of a TN-C-enriched ECM environment after muscle injury promotes the release of muscle satellite cells and supports eventually high muscle regenerative capacity. In the future, better muscle regeneration may be achieved in mammals through the maintenance of TN-C expression.

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.

Enduring questions in regenerative biology and the search for answers

The potential for basic research to uncover the inner workings of regenerative processes and produce meaningful medical therapies has inspired scientists, clinicians, and patients for hundreds of years. Decades of studies using a handful of highly regenerative model organisms have significantly advanced our knowledge of key cell types and molecular pathways involved in regeneration. However, many questions remain about how regenerative processes unfold in regeneration-competent species, how they are curtailed in non-regenerative organisms, and how they might be induced (or restored) in humans. Recent technological advances in genomics, molecular biology, computer science, bioengineering, and stem cell research hold promise to collectively provide new experimental evidence for how different organisms accomplish the process of regeneration. In theory, this new evidence should inform the design of new clinical approaches for regenerative medicine. A deeper understanding of how tissues and organs regenerate will also undoubtedly impact many adjacent scientific fields. To best apply and adapt these new technologies in ways that break long-standing barriers and answer critical questions about regeneration, we must combine the deep knowledge of developmental and evolutionary biologists with the hard-earned expertise of scientists in mechanistic and technical fields. To this end, this perspective is based on conversations from a workshop we organized at the Banbury Center, during which a diverse cross-section of the regeneration research community and experts in various technologies discussed enduring questions in regenerative biology. Here, we share the questions this group identified as significant and unanswered, i.e., known unknowns. We also describe the obstacles limiting our progress in answering these questions and how expanding the number and diversity of organisms used in regeneration research is essential for deepening our understanding of regenerative capacity. Finally, we propose that investigating these problems collaboratively across a diverse network of researchers has the potential to advance our field and produce unexpected insights into important questions in related areas of biology and medicine.

Evolutionary Insights into the Relationship of Frogs, Salamanders, and Caecilians and Their Adaptive Traits, with an Emphasis on Salamander Regeneration and Longevity

The extant amphibians have developed uncanny abilities to adapt to their environment. I compared the genes of amphibians to those of other vertebrates to investigate the genetic changes underlying their unique traits, especially salamanders' regeneration and longevity. Using the well-supported Batrachia tree, I found that salamander genomes have undergone accelerated adaptive evolution, especially for development-related genes. The group-based comparison showed that several genes are under positive selection, rapid evolution, and unexpected parallel evolution with traits shared by distantly related species, such as the tail-regenerative lizard and the longer-lived naked mole rat. The genes, such as EEF1E1, PAFAH1B1, and OGFR, may be involved in salamander regeneration, as they are involved in the apoptotic process, blastema formation, and cell proliferation, respectively. The genes PCNA and SIRT1 may be involved in extending lifespan, as they are involved in DNA repair and histone modification, respectively. Some genes, such as PCNA and OGFR, have dual roles in regeneration and aging, which suggests that these two processes are interconnected. My experiment validated the time course differential expression pattern of SERPINI1 and OGFR, two genes that have evolved in parallel in salamanders and lizards during the regeneration process of salamander limbs. In addition, I found several candidate genes responsible for frogs' frequent vocalization and caecilians' degenerative vision. This study provides much-needed insights into the processes of regeneration and aging, and the discovery of the critical genes paves the way for further functional analysis, which could open up new avenues for exploiting the genetic potential of humans and improving human well-being.

Proenkephalin-A secreted by renal proximal tubules functions as a brake in kidney regeneration

Organ regeneration necessitates precise coordination of accelerators and brakes to restore organ function. However, the mechanisms underlying this intricate molecular crosstalk remain elusive. In this study, the level of proenkephalin-A (PENK-A), expressed by renal proximal tubular epithelial cells, decreases significantly with the loss of renal proximal tubules and increased at the termination phase of zebrafish kidney regeneration. Notably, this change contrasts with the role of hydrogen peroxide (H2O2), which acts as an accelerator in kidney regeneration. Through experiments with penka mutants and pharmaceutical treatments, we demonstrate that PENK-A inhibits H2O2 production in a dose-dependent manner, suggesting its involvement in regulating the rate and termination of regeneration. Furthermore, H2O2 influences the expression of tcf21, a vital factor in the formation of renal progenitor cell aggregates, by remodeling H3K4me3 in renal cells. Overall, our findings highlight the regulatory role of PENK-A as a brake in kidney regeneration.

Epithelial-mesenchymal transition: an organizing principle of mammalian regeneration

Introduction: The MRL mouse strain is one of the few examples of a mammal capable of healing appendage wounds by regeneration, a process that begins with the formation of a blastema, a structure containing de-differentiating mesenchymal cells. HIF-1α expression in the nascent MRL wound site blastema is one of the earliest identified events and is sufficient to initiate the complete regenerative program. However, HIF-1α regulates many cellular processes modulating the expression of hundreds of genes. A later signal event is the absence of a functional G1 checkpoint, leading to G2 cell cycle arrest with increased cellular DNA but little cell division observed in the blastema. This lack of mitosis in MRL blastema cells is also a hallmark of regeneration in classical invertebrate and vertebrate regenerators such as planaria, hydra, and newt. Results and discussion: Here, we explore the cellular events occurring between HIF-1α upregulation and its regulation of the genes involved in G2 arrest (EVI-5, γH3, Wnt5a, and ROR2), and identify epithelial-mesenchymal transition (EMT) (Twist and Slug) and chromatin remodeling (EZH-2 and H3K27me3) as key intermediary processes. The locus of these cellular events is highly regionalized within the blastema, occurring in the same cells as determined by double staining by immunohistochemistry and FACS analysis, and appears as EMT and chromatin remodeling, followed by G2 arrest determined by kinetic expression studies.

Macrophage-driven cardiac inflammation and healing: insights from homeostasis and myocardial infarction

Early and prompt reperfusion therapy has markedly improved the survival rates among patients enduring myocardial infarction (MI). Nonetheless, the resulting adverse remodeling and the subsequent onset of heart failure remain formidable clinical management challenges and represent a primary cause of disability in MI patients worldwide. Macrophages play a crucial role in immune system regulation and wield a profound influence over the inflammatory repair process following MI, thereby dictating the degree of myocardial injury and the subsequent pathological remodeling. Despite numerous previous biological studies that established the classical polarization model for macrophages, classifying them as either M1 pro-inflammatory or M2 pro-reparative macrophages, this simplistic categorization falls short of meeting the precision medicine standards, hindering the translational advancement of clinical research. Recently, advances in single-cell sequencing technology have facilitated a more profound exploration of macrophage heterogeneity and plasticity, opening avenues for the development of targeted interventions to address macrophage-related factors in the aftermath of MI. In this review, we provide a summary of macrophage origins, tissue distribution, classification, and surface markers. Furthermore, we delve into the multifaceted roles of macrophages in maintaining cardiac homeostasis and regulating inflammation during the post-MI period.

Distinct mononuclear diploid cardiac subpopulation with minimal cell-cell communications persists in embryonic and adult mammalian heart

A small proportion of mononuclear diploid cardiomyocytes (MNDCMs), with regeneration potential, could persist in adult mammalian heart. However, the heterogeneity of MNDCMs and changes during development remains to be illuminated. To this end, 12 645 cardiac cells were generated from embryonic day 17.5 and postnatal days 2 and 8 mice by single-cell RNA sequencing. Three cardiac developmental paths were identified: two switching to cardiomyocytes (CM) maturation with close CM-fibroblast (FB) communications and one maintaining MNDCM status with least CM-FB communications. Proliferative MNDCMs having interactions with macrophages and non-proliferative MNDCMs (non-pMNDCMs) with minimal cell-cell communications were identified in the third path. The non-pMNDCMs possessed distinct properties: the lowest mitochondrial metabolisms, the highest glycolysis, and high expression of Myl4 and Tnni1. Single-nucleus RNA sequencing and immunohistochemical staining further proved that the Myl4+Tnni1+ MNDCMs persisted in embryonic and adult hearts. These MNDCMs were mapped to the heart by integrating the spatial and single-cell transcriptomic data. In conclusion, a novel non-pMNDCM subpopulation with minimal cell-cell communications was unveiled, highlighting the importance of microenvironment contribution to CM fate during maturation. These findings could improve the understanding of MNDCM heterogeneity and cardiac development, thus providing new clues for approaches to effective cardiac regeneration.

Functional regeneration at the blood-biomaterial interface

The use of cardiovascular implants is commonplace in clinical practice. However, reproducing the key bioactive and adaptive properties of native cardiovascular tissues with an artificial replacement is highly challenging. Exciting new treatment strategies are under development to regenerate (parts of) cardiovascular tissues directly in situ using immunomodulatory biomaterials. Direct exposure to the bloodstream and hemodynamic loads is a particular challenge, given the risk of thrombosis and adverse remodeling that it brings. However, the blood is also a source of (immune) cells and proteins that dominantly contribute to functional tissue regeneration. This review explores the potential of the blood as a source for the complete or partial in situ regeneration of cardiovascular tissues, with a particular focus on the endothelium, being the natural blood-tissue barrier. We pinpoint the current scientific challenges to enable rational engineering and testing of blood-contacting implants to leverage the regenerative potential of the blood.

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.

Immunomodulatory contribution of mast cells to the regenerative biomaterial microenvironment

Bioactive immunomodulatory biomaterials have shown promise for influencing the immune response to promote tissue repair and regeneration. Macrophages and T cells have been associated with this response; however, other immune cell types have been traditionally overlooked. In this study, we investigated the role of mast cells in the regulation of the immune response to decellularized biomaterial scaffolds using a subcutaneous implant model. In mast cell-deficient mice, there was dysregulation of the expected M1 to M2 macrophage transition typically induced by the biomaterial scaffold. Polarization progression deviated in a sex-specific manner with an early transition to an M2 profile in female mice, while the male response was unable to properly transition past a pro-inflammatory M1 state. Both were reversed with adoptive mast cell transfer. Further investigation of the later-stage immune response in male mice determined a greater sustained pro-inflammatory gene expression profile, including the IL-1 cytokine family, IL-6, alarmins, and chemokines. These results highlight mast cells as another important cell type that influences the immune response to pro-regenerative biomaterials.

Matrisomal components involved in regenerative wound healing in axolotl and Acomys: implications for biomaterial development

Achieving regeneration in humans has been a long-standing goal of many researchers. Whereas amphibians like the axolotl (Ambystoma mexicanum) are capable of regenerating whole organs and even limbs, most mammals heal their wounds via fibrotic scarring. Recently, the African spiny mouse (Acomys sp.) has been shown to be injury resistant and capable of regenerating several tissue types. A major focal point of research with Acomys has been the identification of drivers of regeneration. In this search, the matrisome components related to the extracellular matrix (ECM) are often overlooked. In this review, we compare Acomys and axolotl skin wound healing and blastema-mediated regeneration by examining their wound healing responses and comparing the expression pattern of matrisome genes, including glycosaminoglycan (GAG) related genes. The goal of this review is to identify matrisome genes that are upregulated during regeneration and could be potential candidates for inclusion in pro-regenerative biomaterials. Research papers describing transcriptomic or proteomic coverage of either skin regeneration or blastema formation in Acomys and axolotl were selected. Matrisome and GAG related genes were extracted from each dataset and the resulting lists of genes were compared. In our analysis, we found several genes that were consistently upregulated, suggesting possible involvement in regenerative processes. Most of the components have been implicated in regulation of cell behavior, extracellular matrix remodeling and wound healing. Incorporation of such pro-regenerative factors into biomaterials may help to shift pro-fibrotic processes to regenerative responses in treated wounds.

A system for bioelectronic delivery of treatment directed toward wound healing

The development of wearable bioelectronic systems is a promising approach for optimal delivery of therapeutic treatments. These systems can provide continuous delivery of ions, charged biomolecules, and an electric field for various medical applications. However, rapid prototyping of wearable bioelectronic systems for controlled delivery of specific treatments with a scalable fabrication process is challenging. We present a wearable bioelectronic system comprised of a polydimethylsiloxane (PDMS) device cast in customizable 3D printed molds and a printed circuit board (PCB), which employs commercially available engineering components and tools throughout design and fabrication. The system, featuring solution-filled reservoirs, embedded electrodes, and hydrogel-filled capillary tubing, is assembled modularly. The PDMS and PCB both contain matching through-holes designed to hold metallic contact posts coated with silver epoxy, allowing for mechanical and electrical integration. This assembly scheme allows us to interchange subsystem components, such as various PCB designs and reservoir solutions. We present three PCB designs: a wired version and two battery-powered versions with and without onboard memory. The wired design uses an external voltage controller for device actuation. The battery-powered PCB design uses a microcontroller unit to enable pre-programmed applied voltages and deep sleep mode to prolong battery run time. Finally, the battery-powered PCB with onboard memory is developed to record delivered currents, which enables us to verify treatment dose delivered. To demonstrate the functionality of the platform, the devices are used to deliver H[Formula: see text] in vivo using mouse models and fluoxetine ex vivo using a simulated wound environment. Immunohistochemistry staining shows an improvement of 35.86% in the M1/M2 ratio of H[Formula: see text]-treated wounds compared with control wounds, indicating the potential of the platform to improve wound healing.

Conserved and tissue-specific immune responses to biologic scaffold implantation

Upon implantation into a patient, any biomaterial induces a cascade of immune responses that influences the outcome of that device. This cascade depends upon several factors, including the composition of the material itself and the location in which the material is implanted. There is still significant uncertainty around the role of different tissue microenvironments in the immune response to biomaterials and how that may alter downstream scaffold remodeling and integration. In this study, we present a study evaluating the immune response to decellularized extracellular matrix materials within the intraperitoneal cavity, the subcutaneous space, and in a traumatic skeletal muscle injury microenvironment. All different locations induced robust cellular recruitment, specifically of macrophages and eosinophils. The latter was most prominent in the subcutaneous space. Intraperitoneal implants uniquely recruited B cells that may alter downstream reactivity as adaptive immunity has been strongly implicated in the outcome of scaffold remodeling. These data suggest that the location of tissue implants should be taken together with the composition of the material itself when designing devices for downline therapeutics.

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.

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.

Senescence during early differentiation reduced the chondrogenic differentiation capacity of mesenchymal progenitor cells

Introduction: Mesenchymal stromal/progenitor cells (MSCs) are promising for cartilage cell-based therapies due to their chondrogenic differentiation capacity. However, MSCs can become senescent during in vitro expansion, a state characterized by stable cell cycle arrest, metabolic alterations, and substantial changes in the gene expression and secretory profile of the cell. In this study, we aimed to investigate how senescence and the senescence-associated secretory phenotype (SASP) affect chondrogenic differentiation of MSCs. Methods: To study the effect of senescence, we exposed MSCs to gamma irradiation during expansion or during chondrogenic differentiation (the pellet culture). Western blot analysis was used to evaluate MSCs response to the chondrogenic inductor TGF-β. Results: When senescence was induced during expansion or at day 7 of chondrogenic differentiation, we observed a significant reduction in the cartilage matrix. Interestingly, when senescence was induced at day 14 of differentiation, chondrogenesis was not significantly altered. Moreover, exposing chondrogenic pellets to the medium conditioned by senescent pellets had no significant effect on the expression of anabolic or catabolic cartilage markers, suggesting a neglectable paracrine effect of senescence on cartilage generation in our model. Finally, we show that senescent MSCs showed lower phosphorylated SMAD2 levels after TGFβ1 stimulation than control MSCs. Conclusion: Overall, these results suggest that the occurrence of senescence in MSCs during expansion or early differentiation could be detrimental for cartilage tissue engineering.

Reduction of neutrophil extracellular traps accelerates inflammatory resolution and increases bone formation on titanium implants

Neutrophils are the most abundant immune cells in the blood and the first cells to be recruited to the biomaterial implantation site. Neutrophils are fundamental in recruiting mononuclear leukocytes to mount an immune response at the injury site. Neutrophils exert significant pro-inflammatory effects through the release of cytokines and chemokines, degranulation and release of myeloperoxidase (MPO) and neutrophil elastase (NE), and the production of large DNA-based networks called neutrophil extracellular traps (NETs). Neutrophils are initially recruited and activated by cytokines and pathogen- and damage-associated molecular patterns, but little is known about how the physicochemical composition of the biomaterial affects their activation. This study aimed to understand how ablating neutrophil mediators (MPO, NE, NETs) affected macrophage phenotype in vitro and osseointegration in vivo. We discovered that NET formation is a crucial mediator of pro-inflammatory macrophage activation, and inhibition of NET formation significantly suppresses macrophage pro-inflammatory phenotype. Furthermore, reducing NET formation accelerated the inflammatory phase of healing and produced greater bone formation around the implanted biomaterial, suggesting that NETs are essential regulators of biomaterial integration. Our findings emphasize the importance of the neutrophil response to implanted biomaterials and highlight innate immune cells' regulation and amplification signaling during the initiation and resolution of the inflammatory phase of biomaterial integration. STATEMENT OF SIGNIFICANCE: Neutrophils are the most abundant immune cells in blood and are the first to be recruited to the injury/implantation site where they exert significant pro-inflammatory effects. This study aimed to understand how ablating neutrophil mediators affected macrophage phenotype in vitro and bone apposition in vivo. We found that NET formation is a crucial mediator of pro-inflammatory macrophage activation. Reducing NET formation accelerated the inflammatory phase of healing and produced greater appositional bone formation around the implanted biomaterial, suggesting that NETs are essential regulators of biomaterial integration.

Spatial transcriptomics reveals asymmetric cellular responses to injury in the regenerating spiny mouse (Acomys) ear

In contrast to other mammals, the spiny mouse (Acomys) regenerates skin and ear tissue, which includes hair follicles, glands, and cartilage, in a scar-free manner. Ear punch regeneration is asymmetric with only the proximal wound side participating in regeneration. Here, we show that cues originating from the proximal side are required for normal regeneration and use spatially resolved transcriptomics (tomo-seq) to understand the molecular and cellular events underlying this process. Analyzing gene expression across the ear and comparing expression modules between proximal and distal wound sides, we identify asymmetric gene expression patterns and pinpoint regenerative processes in space and time. Moreover, using a comparative approach with nonregenerative rodents (Mus, Meriones), we strengthen a hypothesis in which particularities in the injury-induced immune response may be one of the crucial determinants for why spiny mice regenerate whereas their relatives do not. Our data are available in SpinyMine, an easy-to-use and expandable web-based tool for exploring Acomys regeneration-associated gene expression.

Macrophage-mediated PDGF Activation Correlates With Regenerative Outcomes Following Musculoskeletal Trauma

OBJECTIVE

Our objective was to identify macrophage subpopulations and gene signatures associated with regenerative or fibrotic healing across different musculoskeletal injury types.

BACKGROUND

Subpopulations of macrophages are hypothesized to fine tune the immune response after damage, promoting either normal regenerative, or aberrant fibrotic healing.

METHODS

Mouse single-cell RNA sequencing data before and after injury were assembled from models of musculoskeletal injury, including regenerative and fibrotic mouse volumetric muscle loss (VML), regenerative digit tip amputation, and fibrotic heterotopic ossification. R packages Harmony , MacSpectrum , and Seurat were used for data integration, analysis, and visualizations.

RESULTS

There was a substantial overlap between macrophages from the regenerative VML (2 mm injury) and regenerative bone models, as well as a separate overlap between the fibrotic VML (3 mm injury) and fibrotic bone (heterotopic ossification) models. We identified 2 fibrotic-like (FL 1 and FL 2) along with 3 regenerative-like (RL 1, RL 2, and RL 3) subpopulations of macrophages, each of which was transcriptionally distinct. We found that regenerative and fibrotic conditions had similar compositions of proinflammatory and anti-inflammatory macrophages, suggesting that macrophage polarization state did not correlate with healing outcomes. Receptor/ligand analysis of macrophage-to-mesenchymal progenitor cell crosstalk showed enhanced transforming growth factor β in fibrotic conditions and enhanced platelet-derived growth factor signaling in regenerative conditions.

CONCLUSION

Characterization of macrophage subtypes could be used to predict fibrotic responses following injury and provide a therapeutic target to tune the healing microenvironment towards more regenerative conditions.

The amphibian immune system

Amphibians are at the forefront of bridging the evolutionary gap between mammals and more ancient, jawed vertebrates. Currently, several diseases have targeted amphibians and understanding their immune system has importance beyond their use as a research model. The immune system of the African clawed frog, Xenopus laevis, and that of mammals is well conserved. We know that several features of the adaptive and innate immune system are very similar for both, including the existence of B cells, T cells and innate-like T cells. In particular, the study of the immune system at early stages of development is benefitted by studying X. laevis tadpoles. The tadpoles mainly rely on innate immune mechanisms including pre-set or innate-like T cells until after metamorphosis. In this review we lay out what is known about the innate and adaptive immune system of X. laevis including the lymphoid organs as well as how other amphibian immune systems are similar or different. Furthermore, we will describe how the amphibian immune system responds to some viral, bacterial and fungal insults. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

Newt A1 cell-derived extracellular vesicles promote mammalian nerve growth

Newts have the extraordinary ability to fully regenerate lost or damaged cardiac, neural and retinal tissues, and even amputated limbs. In contrast, mammals lack these broad regenerative capabilities. While the molecular basis of newts' regenerative ability is the subject of active study, the underlying paracrine signaling factors involved remain largely uncharacterized. Extracellular vesicles (EVs) play an important role in cell-to-cell communication via EV cargo-mediated regulation of gene expression patterns within the recipient cells. Here, we report that newt myogenic precursor (A1) cells secrete EVs (A1EVs) that contain messenger RNAs associated with early embryonic development, neuronal differentiation, and cell survival. Exposure of rat primary superior cervical ganglion (SCG) neurons to A1EVs increased neurite outgrowth, facilitated by increases in mitochondrial respiration. Canonical pathway analysis pinpointed activation of NGF/ERK5 signaling in SCG neurons exposed to A1EV, which was validated experimentally. Thus, newt EVs drive neurite growth and complexity in mammalian primary neurons.

Clodronate Liposome-Mediated Phagocytic Hemocyte Depletion Affects the Regeneration of the Cephalic Tentacle of the Invasive Snail, Pomacea canaliculata

After amputation, granular hemocytes infiltrate the blastema of regenerating cephalic tentacles of the freshwater snail Pomacea canaliculata. Here, the circulating phagocytic hemocytes were chemically depleted by injecting the snails with clodronate liposomes, and the effects on the cephalic tentacle regeneration onset and on Pc-Hemocyanin, Pc-transglutaminase (Pc-TG) and Pc-Allograft Inflammatory Factor-1 (Pc-AIF-1) gene expressions were investigated. Flow cytometry analysis demonstrated that clodronate liposomes targeted large circulating hemocytes, resulting in a transient decrease in their number. Corresponding with the phagocyte depletion, tentacle regeneration onset was halted, and it resumed at the expected pace when clodronate liposome effects were no longer visible. In addition to the regeneration progress, the expressions of Pc-Hemocyanin, Pc-TG, and Pc-AIF-1, which are markers of hemocyte-mediated functions like oxygen transport and immunity, clotting, and inflammation, were modified. After the injection of clodronate liposomes, a specific computer-assisted image analysis protocol still evidenced the presence of granular hemocytes in the tentacle blastema. This is consistent with reports indicating the large and agranular hemocyte population as the most represented among the professional phagocytes of P. canaliculata and with the hypothesis that different hemocyte morphologies could exert diverse biological functions, as it has been observed in other invertebrates.

Macrophage Migration Inhibitory Factor on Apoptotic Extracellular Vesicles Regulates Compensatory Proliferation

Apoptotic cells can signal to neighboring cells to stimulate proliferation and compensate for cell loss to maintain tissue homeostasis. While apoptotic cell-derived extracellular vesicles (AEVs) can transmit instructional cues to mediate communication with neighboring cells, the molecular mechanisms that induce cell division are not well understood. Here we show that macrophage migration inhibitory factor (MIF)-containing AEVs regulate compensatory proliferation via ERK signaling in epithelial stem cells of larval zebrafish. Time-lapse imaging showed efferocytosis of AEVs from dying epithelial stem cells by healthy neighboring stem cells. Proteomic and ultrastructure analysis of purified AEVs identified MIF localization on the AEV surface. Pharmacological inhibition or genetic mutation of MIF, or its cognate receptor CD74, decreased levels of phosphorylated ERK and compensatory proliferation in the neighboring epithelial stem cells. Disruption of MIF activity also caused decreased numbers of macrophages patrolling near AEVs, while depletion of the macrophage lineage resulted in a reduced proliferative response by the epithelial stem cells. We propose that AEVs carrying MIF directly stimulate epithelial stem cell repopulation and guide macrophages to cell non-autonomously induce localized proliferation to sustain overall cell numbers during tissue maintenance.

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.

Are There Hopeful Therapeutic Strategies to Regenerate the Infarcted Hearts?

Ischemic heart disease remains the primary cause of morbidity and mortality worldwide. Despite significant advancements in pharmacological and revascularization techniques in the late 20th century, heart failure prevalence after myocardial infarction has gradually increased over the last 2 decades. After ischemic injury, pathological remodeling results in cardiomyocytes (CMs) loss and fibrosis, which leads to impaired heart function. Unfortunately, there are no clinical therapies to regenerate CMs to date, and the adult heart's limited turnover rate of CMs hinders its ability to self-regenerate. In this review, we present novel therapeutic strategies to regenerate injured myocardium, including (1) reconstruction of cardiac niche microenvironment, (2) recruitment of functional CMs by promoting their proliferation or differentiation, and (3) organizing 3-dimensional tissue construct beyond the CMs. Additionally, we highlight recent mechanistic insights that govern these strategies and identify current challenges in translating these approaches to human patients.

Physiology and diseases of tissue-resident macrophages

Embryo-derived tissue-resident macrophages are the first representatives of the haematopoietic lineage to emerge in metazoans. In mammals, resident macrophages originate from early yolk sac progenitors and are specified into tissue-specific subsets during organogenesis-establishing stable spatial and functional relationships with specialized tissue cells-and persist in adults. Resident macrophages are an integral part of tissues together with specialized cells: for instance, microglia reside with neurons in brain, osteoclasts reside with osteoblasts in bone, and fat-associated macrophages reside with white adipocytes in adipose tissue. This ancillary cell type, which is developmentally and functionally distinct from haematopoietic stem cell and monocyte-derived macrophages, senses and integrates local and systemic information to provide specialized tissue cells with the growth factors, nutrient recycling and waste removal that are critical for tissue growth, homeostasis and repair. Resident macrophages contribute to organogenesis, promote tissue regeneration following damage and contribute to tissue metabolism and defence against infectious disease. A correlate is that genetic or environment-driven resident macrophage dysfunction is a cause of degenerative, metabolic and possibly inflammatory and tumoural diseases. In this Review, we aim to provide a conceptual outline of our current understanding of macrophage physiology and its importance in human diseases, which may inform and serve the design of future studies.

Mammalian organ regeneration in spiny mice

Fibrosis-driven solid organ failure is a major world-wide health burden with few therapeutic options. Spiny mice (genus: Acomys) are terrestrial mammals that regenerate severe skin wounds without fibrotic scars to evade predators. Recent studies have shown that spiny mice also regenerate acute ischemic and traumatic injuries to kidney, heart, spinal cord, and skeletal muscle. A common feature of this evolved wound healing response is a lack of formation of fibrotic scar tissue that degrades organ function, inhibits regeneration, and leads to organ failure. Complex tissue regeneration is an extremely rare property among mammalian species. In this article, we discuss the evidence that Acomys represents an emerging model organism that offers a unique opportunity for the biomedical community to investigate and clinically translate molecular mechanisms of scarless wound healing and regeneration of organ function in a mammalian species.

Platelet-derived chemokines promote skeletal muscle regeneration by guiding neutrophil recruitment to injured muscles

Skeletal muscle regeneration involves coordinated interactions between different cell types. Injection of platelet-rich plasma is circumstantially considered an aid to muscle repair but whether platelets promote regeneration beyond their role in hemostasis remains unexplored. Here, we find that signaling via platelet-released chemokines is an early event necessary for muscle repair in mice. Platelet depletion reduces the levels of the platelet-secreted neutrophil chemoattractants CXCL5 and CXCL7/PPBP. Consequently, early-phase neutrophil infiltration to injured muscles is impaired whereas later inflammation is exacerbated. Consistent with this model, neutrophil infiltration to injured muscles is compromised in male mice with Cxcl7-knockout platelets. Moreover, neo-angiogenesis and the re-establishment of myofiber size and muscle strength occurs optimally in control mice post-injury but not in Cxcl7ko mice and in neutrophil-depleted mice. Altogether, these findings indicate that platelet-secreted CXCL7 promotes regeneration by recruiting neutrophils to injured muscles, and that this signaling axis could be utilized therapeutically to boost muscle regeneration.

The role of metabolic reprogramming of tumor-associated macrophages in shaping the immunosuppressive tumor microenvironment

Macrophages are potent immune effector cells in innate immunity and exert dual-effects in the tumor microenvironment (TME). Tumor-associated macrophages (TAMs) make up a significant portion of TME immune cells. Similar to M1/M2 macrophages, TAMs are also highly plastic, and their functions are regulated by cytokines, chemokines and other factors in the TME. The metabolic changes in TAMs are significantly associated with polarization towards a protumour or antitumour phenotype. The metabolites generated via TAM metabolic reprogramming in turn promote tumor progression and immune tolerance. In this review, we explore the metabolic reprogramming of TAMs in terms of energy, amino acid and fatty acid metabolism and the potential roles of these changes in immune suppression.

Immune-Epithelial Cross Talk in Regeneration and Repair

The epithelial tissues that line our body, such as the skin and gut, have remarkable regenerative prowess and continually renew throughout our lifetimes. Owing to their barrier function, these tissues have also evolved sophisticated repair mechanisms to swiftly heal and limit the penetration of harmful agents following injury. Researchers now appreciate that epithelial regeneration and repair are not autonomous processes but rely on a dynamic cross talk with immunity. A wealth of clinical and experimental data point to the functional coupling of reparative and inflammatory responses as two sides of the same coin. Here we bring to the fore the immunological signals that underlie homeostatic epithelial regeneration and restitution following damage. We review our current understanding of how immune cells contribute to distinct phases of repair. When unchecked, immune-mediated repair programs are co-opted to fuel epithelial pathologies such as cancer, psoriasis, and inflammatory bowel diseases. Thus, understanding the reparative functions of immunity may advance therapeutic innovation in regenerative medicine and epithelial inflammatory diseases.

Emerging Therapeutic Approaches to Target the Dark Side of Senescent Cells: New Hopes to Treat Aging as a Disease and to Delay Age-Related Pathologies

Life expectancy has drastically increased over the last few decades worldwide, with important social and medical burdens and costs. To stay healthy longer and to avoid chronic disease have become essential issues. Organismal aging is a complex process that involves progressive destruction of tissue functionality and loss of regenerative capacity. One of the most important aging hallmarks is cellular senescence, which is a stable state of cell cycle arrest that occurs in response to cumulated cell stresses and damages. Cellular senescence is a physiological mechanism that has both beneficial and detrimental consequences. Senescence limits tumorigenesis, lifelong tissue damage, and is involved in different biological processes, such as morphogenesis, regeneration, and wound healing. However, in the elderly, senescent cells increasingly accumulate in several organs and secrete a combination of senescence associated factors, contributing to the development of various age-related diseases, including cancer. Several studies have revealed major molecular pathways controlling the senescent phenotype, as well as the ones regulating its interactions with the immune system. Attenuating the senescence-associated secretory phenotype (SASP) or eliminating senescent cells have emerged as attractive strategies aiming to reverse or delay the onset of aging diseases. Here, we review current senotherapies designed to suppress the deleterious effect of SASP by senomorphics or to selectively kill senescent cells by "senolytics" or by immune system-based approaches. These recent investigations are promising as radical new controls of aging pathologies and associated multimorbidities.

Unravelling the limb regeneration mechanisms of Polypedates maculatus, a sub-tropical frog, by transcriptomics

BACKGROUND

Regeneration studies help to understand the strategies that replace a lost or damaged organ and provide insights into approaches followed in regenerative medicine and engineering. Amphibians regenerate their limbs effortlessly and are indispensable models to study limb regeneration. Xenopus and axolotl are the key models for studying limb regeneration but recent studies on non-model amphibians have revealed species specific differences in regeneration mechanisms.

RESULTS

The present study describes the de novo transcriptome of intact limbs and three-day post-amputation blastemas of tadpoles and froglets of the Asian tree frog Polypedates maculatus, a non-model amphibian species commonly found in India. Differential gene expression analysis between early tadpole and froglet limb blastemas discovered species-specific novel regulators of limb regeneration. The present study reports upregulation of proteoglycans, such as epiphycan, chondroadherin, hyaluronan and proteoglycan link protein 1, collagens 2,5,6, 9 and 11, several tumour suppressors and methyltransferases in the P. maculatus tadpole blastemas. Differential gene expression analysis between tadpole and froglet limbs revealed that in addition to the expression of larval-specific haemoglobin and glycoproteins, an upregulation of cysteine and serine protease inhibitors and downregulation of serine proteases, antioxidants, collagenases and inflammatory genes in the tadpole limbs were essential for creating an environment that would support regeneration. Dermal myeloid cells were GAG+, EPYC+, INMT+, LEF1+ and SALL4+ and seemed to migrate from the unamputated regions of the tadpole limb to the blastema. On the other hand, the myeloid cells of the froglet limb blastemas were few and probably contributed to sustained inflammation resulting in healing.

CONCLUSIONS

Studies on non-model amphibians give insights into alternate tactics for limb regeneration which can help devise a plethora of methods in regenerative medicine and engineering.

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.

Proximal ear hole injury heals by limited regeneration during the early postnatal phase in mice

Ear pinna is a particular feature of mammals that shows several repair responses depending on age. Two millimeter hole made in the pinna of middle-aged female mice heals due to partial reconstitution of new tissues (limited regeneration), whereas a hole punched in the ear of young mice forms a scar tissue. In these studies, the injury is made in the center of the ear pinna, but little is known about the type of reparative response along the proximodistal polarity of the ear. This study evaluated the effect of pinna polarity, age, and sex in the ear hole-repairing response in Balb/c mice. Proximal injuries were repaired more efficiently by limited regeneration than wounds made in the middle region. Non-injured ear histological analysis revealed a higher presence of muscle, adipose tissue, cartilage, and larger blood vessels in the proximal ear area, which could influence ear hole closure by limited regeneration. To evaluate the healing response during ear growth, we punched a standard hole in the proximal area of the ear on postnatal day 21 and 8-month-old mice (adults). Thirty-five days after the wound, both groups reached the same wound closure, despite the greater proportional size of holes made in the younger mice. Ear growth also improved ear hole closure in male mice. These results suggest that ear growth accelerates hole closure, providing an example of enhanced regenerative abilities in growing structures. Finally, hole closure kinetics in the growing ear indicated an early re-differentiation phase exhibited at 14 days post-wound. In conclusion, ear topography and growth positively influenced the healing response to ear holes, making it a tractable model to study in mammals.

Macrophages and Bone Remodeling

Bone remodeling in the adult skeleton facilitates the removal and replacement of damaged and old bone to maintain bone quality. Tight coordination of bone resorption and bone formation during remodeling crucially maintains skeletal mass. Increasing evidence suggests that many cell types beyond osteoclasts and osteoblasts support bone remodeling, including macrophages and other myeloid lineage cells. Herein, we discuss the origin and functions for macrophages in the bone microenvironment, tissue resident macrophages, osteomacs, as well as newly identified osteomorphs that result from osteoclast fission. We also touch on the role of macrophages during inflammatory bone resorption. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

A population of stem cells with strong regenerative potential discovered in deer antlers

The annual regrowth of deer antlers provides a valuable model for studying organ regeneration in mammals. We describe a single-cell atlas of antler regrowth. The earliest-stage antler initiators were mesenchymal cells that express the paired related homeobox 1 gene (PRRX1+ mesenchymal cells). We also identified a population of "antler blastema progenitor cells" (ABPCs) that developed from the PRRX1+ mesenchymal cells and directed the antler regeneration process. Cross-species comparisons identified ABPCs in several mammalian blastema. In vivo and in vitro ABPCs displayed strong self-renewal ability and could generate osteochondral lineage cells. Last, we observed a spatially well-structured pattern of cellular and gene expression in antler growth center during the peak growth stage, revealing the cellular mechanisms involved in rapid antler elongation.

Chitosan nanoparticle loaded by epidermal growth factor as a potential protein carrier for wound healing: In vitro and in vivo studies

Epidermal growth factor (EGF) can be efficiently used in wound healing process; but the main obstacle of its clinical use is its susceptibility to proteolysis and maintaining its effective concentration in the site of action. In this study, chitosan nanoparticles containing EGF is formulated using a simple method to increase its stability in physiological pH as well as protect its biological activity and effectiveness in wound healing process. Nanoparticles with different ratios of chitosan/EGF were prepared and evaluated in vitro and in vivo. Obtained results showed nanoparticles with 2:1 ratio of chitosan/EGF were able to release 80% of encapsulated protein after 12 h. Cell proliferation study demonstrated that prepared nanoparticles could protect EGF functionality in physiological pH. In vivo results showed that nanoparticles with 2:1 ratio of chitosan/EGF could significantly accelerate the wound closure-rate, re-epithelialisation and collagen deposition. In conclusion, the designed nanoparticles in optimal ratio can be considered as a potential vehicle for EGF delivery to wounds with the aim of improving healing process.

Integrative biology of injury in animals

Mechanical injury is a prevalent challenge in the lives of animals with myriad potential consequences for organisms, including reduced fitness and death. Research on animal injury has focused on many aspects, including the frequency and severity of wounding in wild populations, the short- and long-term consequences of injury at different biological scales, and the variation in the response to injury within or among individuals, species, ontogenies, and environmental contexts. However, relevant research is scattered across diverse biological subdisciplines, and the study of the effects of injury has lacked synthesis and coherence. Furthermore, the depth of knowledge across injury biology is highly uneven in terms of scope and taxonomic coverage: much injury research is biomedical in focus, using mammalian model systems and investigating cellular and molecular processes, while research at organismal and higher scales, research that is explicitly comparative, and research on invertebrate and non-mammalian vertebrate species is less common and often less well integrated into the core body of knowledge about injury. The current state of injury research presents an opportunity to unify conceptually work focusing on a range of relevant questions, to synthesize progress to date, and to identify fruitful avenues for future research. The central aim of this review is to synthesize research concerning the broad range of effects of mechanical injury in animals. We organize reviewed work by four broad and loosely defined levels of biological organization: molecular and cellular effects, physiological and organismal effects, behavioural effects, and ecological and evolutionary effects of injury. Throughout, we highlight the diversity of injury consequences within and among taxonomic groups while emphasizing the gaps in taxonomic coverage, causal understanding, and biological endpoints considered. We additionally discuss the importance of integrating knowledge within and across biological levels, including how initial, localized responses to injury can lead to long-term consequences at the scale of the individual animal and beyond. We also suggest important avenues for future injury biology research, including distinguishing better between related yet distinct injury phenomena, expanding the subjects of injury research to include a greater variety of species, and testing how intrinsic and extrinsic conditions affect the scope and sensitivity of injury responses. It is our hope that this review will not only strengthen understanding of animal injury but will contribute to building a foundation for a more cohesive field of 'injury biology'.

Distinctive role of inflammation in tissue repair and regeneration

Inflammation is an essential host defense mechanism in response to microbial infection and tissue injury. In addition to its well-established role in infection, inflammation is actively involved in the repair of damaged tissues and restoration of homeostatic conditions after tissue injury. The intensity of the inflammatory response and types of cells involved in inflammation have a significant impact on the quality of tissue repair. Numerous immune cell subtypes participate in tissue repair and regeneration. In particular, immune cell-derived secretants, including cytokines and growth factors, can actively modulate the proliferation of resident stem cells or progenitor cells to facilitate tissue regeneration. These findings highlight the importance of inflammation during tissue repair and regeneration; however, the precise role of immune cells in tissue regeneration remains unclear. In this review, we summarize the current knowledge on the contribution of specific immune cell types to tissue repair and regeneration. We also discuss how inflammation affects the final outcome of tissue regeneration.

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

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

Failure of digit tip regeneration in the absence of Lmx1b suggests Lmx1b functions disparate from dorsoventral polarity

Mammalian digit tip regeneration is linked to the presence of nail tissue, but a nail-explicit model is missing. Here, we report that nail-less double-ventral digits of ΔLARM1/2 mutants that lack limb-specific Lmx1b enhancers fail to regenerate. To separate the nail's effect from the lack of dorsoventral (DV) polarity, we also interrogate double-dorsal double-nail digits and show that they regenerate. Thus, DV polarity is not a prerequisite for regeneration, and the nail requirement is supported. Transcriptomic comparison between wild-type and non-regenerative ΔLARM1/2 mutant blastemas reveals differential upregulation of vascularization and connective tissue functional signatures in wild type versus upregulation of inflammation in the mutant. These results, together with the finding of Lmx1b expression in the postnatal dorsal dermis underneath the nail and uniformly in the regenerative blastema, open the possibility of additional Lmx1b roles in digit tip regeneration, in addition to the indirect effect of mediating the formation of the nail.

Reimagining Cancer: Moving from the Cellular to the Tissue Level

The current universally accepted explanation of cancer origin and behavior, the somatic mutation theory, is cell-centered and rooted in perturbation of gene function independent of the external environmental context. However, tumors consist of various epithelial and stromal cell populations temporally and spatially organized into an integrated neoplastic community, and they can have properties similar to normal tissues. Accordingly, we review specific normal cellular and tissue traits and behaviors with adaptive temporal and spatial self-organization that result in ordered patterns and structures. A few recent theories have described these tissue-level cancer behaviors, invoking a conceptual shift from the cellular level and highlighting the need for methodologic approaches based on the analysis of complex systems. We propose extending the analytical approach of regulatory networks to the tissue level and introduce the concept of "cancer attractors." These concepts require reevaluation of cancer imaging and investigational approaches and challenge the traditional reductionist approach of cancer molecular biology.

Macrophages and Intervertebral Disc Degeneration

The intervertebral disc (IVD) aids in motion and acts to absorb energy transmitted to the spine. With little inherent regenerative capacity, degeneration of the intervertebral disc results in intervertebral disc disease, which contributes to low back pain and significant disability in many individuals. Increasing evidence suggests that IVD degeneration is a disease of the whole joint that is associated with significant inflammation. Moreover, studies show elevated macrophage accumulation within the IVD with increasing levels of disease severity; however, we still need to understand the roles, be they causative or consequential, of macrophages during the degenerative process. In this narrative review, we discuss hallmarks of IVD degeneration, showcase evidence of macrophage involvement during disc degeneration, and explore burgeoning research aimed at understanding the molecular pathways regulating macrophage functions during intervertebral disc degeneration.

Wound healing activity of aqueous dispersion of fullerene C60 produced by "green technology"

In addition to exhibited antioxidant and anti-inflammatory activity, fullerene C60 is a promising wound healing agent. An important stage in the production of fullerene-based ointments is the stability of the aqueous fullerene dispersion (AFD) with minimum size of colloidal fullerene aggregates and sufficiently high concentration. To achieve these parameters tangential flow filtration of fullerene C60 was used ("green technology"). As estimated by small-angle neutron scattering and dynamic light scattering purified AFDs with narrow-size distribution nanoclusters have a size of 6 nm and are assembled into agglomerates which reach a size of 150 nm. The ability of the AFD to exhibit regenerative activity was studied using the animal wound model. This study shows for the first time that the fullerene-based composition stimulates the healing of wounds of various origins. We assume that the mechanism of the AFD wound-healing activity is associated with the aryl hydrocarbon receptor and macrophages activity.

Isolation and Characterization of Peritoneal Macrophages from Salamanders

Salamanders have been used as research models for centuries. While they exhibit a wide range of biological features not seen in mammals, none has captivated scientists like their ability to regenerate. Interestingly, axolotl macrophages have emerged as an essential cell population for tissue regeneration. Whether the same is true in other salamanders such as newt species Notophthalmus viridescens, Cynops pyrrhogaster, or Pleurodeles waltl remains to be seen. Unfortunately, regardless of the species, molecular tools to study macrophage function in salamanders are lacking. We propose that the readily available, terminally differentiated peritoneal macrophages from newts or axolotls could be used to validate molecular reagents in the study of macrophage function during tissue regeneration in salamanders.

Now that We Got There, What Next?

As seen in the protocols in this book, the opportunities to pursue work at the cellular and molecular work in salamanders have considerably broadened over the last years. The availability of genomic information and genome editing, and the possibility to image tissues live and other methods enhance the spectrum of biological questions accessible to all researchers. Here I provide a personal perspective on what I consider exciting future questions open for investigation.