The promise of perfect adult tissue repair and regeneration in mammals: Learning from regenerative amphibians and fish

Thyroid hormone receptor- and stage-dependent transcriptome changes affect the initial period of Xenopus tropicalis tail regeneration

BACKGROUND

Thyroid hormone (T3) has an inhibitory effect on tissue/organ regeneration. It is still elusive how T3 regulates this process. It is well established that the developmental effects of T3 are primarily mediated through transcriptional regulation by thyroid hormone receptors (TRs). Here we have taken advantage of mutant tadpoles lacking both TRα and TRβ (TRDKO), the only receptor genes in vertebrates, for RNA-seq analyses to investigate the transcriptome changes underlying the initiation of tail regeneration, i.e., wound healing and blastema formation, because this crucial initial step determines the extent of the functional regeneration in the later phase of tissue regrowth.

RESULTS

We discovered that GO (gene ontology) terms related to inflammatory response, metabolic process, cell apoptosis, and epithelial cell migration were highly enriched among commonly regulated genes during wound healing at either stage 56 or 61 or with either wild type (WT) or TRDKO tadpoles, consistent with the morphological changes associated with wound healing occurring in both regenerative (WT stage 56, TRDKO stage 56, TRDKO stage 61) and nonregenerative (WT stage 61) animals. Interestingly, ECM-receptor interaction and cytokine-cytokine receptor interaction, which are essential for blastema formation and regeneration, were significantly enriched among regulated genes in the 3 regenerative groups but not the non-regenerative group at the blastema formation period. In addition, the regulated genes specific to the nonregenerative group were highly enriched with genes involved in cellular senescence. Finally, T3 treatment at stage 56, while not inducing any measurable tail resorption, inhibited tail regeneration in the wild type but not TRDKO tadpoles.

CONCLUSIONS

Our study suggests that TR-mediated, T3-induced gene regulation changed the permissive environment during the initial period of regeneration and affected the subsequent patterning/outgrowth period of the regeneration process. Specifically, T3 signaling via TRs inhibits the expression of ECM-related genes while promoting the expression of inflammation-related genes during the blastema formation period. Interestingly, our findings indicate that amputation-induced changes in DNA replication-related pathways can occur during this nonregenerative period. Further studies, particularly on the regenerative microenvironment that may depend on ECM-receptor interaction and cytokine-cytokine receptor interaction, should provide important insights on the regulation of regenerative capacity during vertebrate development.

Distinct features of the regenerating heart uncovered through comparative single-cell profiling

Adult humans respond to heart injury by forming a permanent scar, yet other vertebrates are capable of robust and complete cardiac regeneration. Despite progress towards characterizing the mechanisms of cardiac regeneration in fish and amphibians, the large evolutionary gulf between mammals and regenerating vertebrates complicates deciphering which cellular and molecular features truly enable regeneration. To better define these features, we compared cardiac injury responses in zebrafish and medaka, two fish species that share similar heart anatomy and common teleost ancestry but differ in regenerative capability. We used single-cell transcriptional profiling to create a time-resolved comparative cell atlas of injury responses in all major cardiac cell types across both species. With this approach, we identified several key features that distinguish cardiac injury response in the non-regenerating medaka heart. By comparing immune responses to injury, we found altered cell recruitment and a distinct pro-inflammatory gene program in medaka leukocytes, and an absence of the injury-induced interferon response seen in zebrafish. In addition, we found a lack of pro-regenerative signals, including nrg1 and retinoic acid, from medaka endothelial and epicardial cells. Finally, we identified alterations in the myocardial structure in medaka, where they lack primordial layer cardiomyocytes and fail to employ a cardioprotective gene program shared by regenerating vertebrates. Our findings reveal notable variation in injury response across nearly all major cardiac cell types in zebrafish and medaka, demonstrating how evolutionary divergence influences the hidden cellular features underpinning regenerative potential in these seemingly similar vertebrates.

Regenerative inflammation: When immune cells help to re-build tissues

Inflammation is an essential immune response critical for responding to infection, injury and maintenance of tissue homeostasis. Upon injury, regenerative inflammation promotes tissue repair by a timed and coordinated infiltration of diverse cell types and the secretion of growth factors, cytokines and lipids mediators. Remarkably, throughout evolution as well as mammalian development, this type of physiological inflammation is highly associated with immunosuppression. For instance, regenerative inflammation is the consequence of an in situ macrophage polarization resulting in a transition from pro-inflammatory to anti-inflammatory/pro-regenerative response. Immune cells are the first responders upon injury, infiltrating the damaged tissue and initiating a pro-inflammatory response depleting cell debris and necrotic cells. After phagocytosis, macrophages undergo multiple coordinated metabolic and transcriptional changes allowing the transition and dictating the initiation of the regenerative phase. Differences between a highly efficient, complete ad integrum tissue repair, such as, acute skeletal muscle injury, and insufficient regenerative inflammation, as the one developing in Duchenne Muscular Dystrophy (DMD), highlight the importance of a coordinated response orchestrated by immune cells. During regenerative inflammation, these cells interact with others and alter the niche, affecting the character of inflammation itself and, therefore, the progression of tissue repair. Comparing acute muscle injury and chronic inflammation in DMD, we review how the same cells and molecules in different numbers, concentration and timing contribute to very different outcomes. Thus, it is important to understand and identify the distinct functions and secreted molecules of macrophages, and potentially other immune cells, during tissue repair, and the contributors to the macrophage switch leveraging this knowledge in treating diseases.

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.

Engineered biomaterials in stem cell-based regenerative medicine

Stem cell-based regenerative therapies, which harness the self-renewal and differentiation properties of stem cells, have been in the spotlight due to their widespread applications in treating degenerative, aging, and other, generally intractable diseases. Therapeutically effective hematopoietic stem cells, mesenchymal stem cells, embryonic stem cells, and induced pluripotent stem cells have been used in numerous basic and translational studies with exciting results. However, pre-/post-transplantation issues of poor cell survival and retention, uncontrolled differentiation, and insufficient numbers of cells engrafted into host tissues are the major challenges in stem cell-based regenerative therapies. Engineered biomaterials have adjustable biochemical and biophysical properties that significantly affect cell behaviors, such as cell engraftment, survival, migration, and differentiation outcomes, thereby enhancing the engraftment of implanted stem cells and guiding tissue regeneration. Therefore, the combination of stem cell biology with bioengineered materials is a promising strategy to improve the therapeutic outcomes of stem cell-based regenerative therapy. In this review, we summarize the advances in the modulation of behaviors of stem cells via engineered biomaterials. We then present different approaches to harnessing bioengineered materials to enhance the transplantation of stem cells. Finally, we will provide future directions in regenerative therapy using stem cells.

Distinct features of the regenerating heart uncovered through comparative single-cell profiling

Adult humans respond to heart injury by forming a permanent scar, yet other vertebrates are capable of robust and complete cardiac regeneration. Despite progress towards characterizing the mechanisms of cardiac regeneration in fish and amphibians, the large evolutionary gulf between mammals and regenerating vertebrates complicates deciphering which cellular and molecular features truly enable regeneration. To better define these features, we compared cardiac injury responses in zebrafish and medaka, two fish species that share similar heart anatomy and common teleost ancestry but differ in regenerative capability. We used single-cell transcriptional profiling to create a time-resolved comparative cell atlas of injury responses in all major cardiac cell types across both species. With this approach, we identified several key features that distinguish cardiac injury response in the non-regenerating medaka heart. By comparing immune responses to injury, we found altered cell recruitment and a distinct pro-inflammatory gene program in medaka leukocytes, and an absence of the injury-induced interferon response seen in zebrafish. In addition, we found a lack of pro-regenerative signals, including nrg1 and retinoic acid, from medaka endothelial and epicardial cells. Finally, we identified alterations in the myocardial structure in medaka, where they lack embryonic-like primordial layer cardiomyocytes, and fail to employ a cardioprotective gene program shared by regenerating vertebrates. Our findings reveal notable variation in injury response across nearly all major cardiac cell types in zebrafish and medaka, demonstrating how evolutionary divergence influences the hidden cellular features underpinning regenerative potential in these seemingly similar vertebrates.

Macrophages modulate fibrosis during newt lens regeneration

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

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.

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.

α-Gal Nanoparticles Mediated Homing of Endogenous Stem Cells for Repair and Regeneration of External and Internal Injuries by Localized Complement Activation and Macrophage Recruitment

This review discusses a novel experimental approach for the regeneration of original tissue structure by recruitment of endogenous stem-cells to injured sites following administration of α-gal nanoparticles, which harness the natural anti-Gal antibody. Anti-Gal is produced in large amounts in all humans, and it binds the multiple α-gal epitopes (Galα1-3Galβ1-4GlcNAc-R) presented on α-gal nanoparticles. In situ binding of anti-Gal to α-gal nanoparticles activates the complement system and generates complement cleavage chemotactic-peptides that rapidly recruit macrophages. Macrophages reaching anti-Gal coated α-gal nanoparticles bind them via Fc/Fc receptor interaction and polarize into M2 pro-reparative macrophages. These macrophages secrete various cytokines that orchestrate regeneration of the injured tissue, including VEGF inducing neo-vascularization and cytokines directing homing of stem-cells to injury sites. Homing of stem-cells is also directed by interaction of complement cleavage peptides with their corresponding receptors on the stem-cells. Application of α-gal nanoparticles to skin wounds of anti-Gal producing mice results in decrease in healing time by half. Furthermore, α-gal nanoparticles treated wounds restore the normal structure of the injured skin without fibrosis or scar formation. Similarly, in a mouse model of occlusion/reperfusion myocardial-infarction, near complete regeneration after intramyocardial injection of α-gal nanoparticles was demonstrated, whereas hearts injected with saline display ~20% fibrosis and scar formation of the left ventricular wall. It is suggested that recruitment of stem-cells following anti-Gal/α-gal nanoparticles interaction in injured tissues may result in induction of localized regeneration facilitated by conducive microenvironments generated by pro-reparative macrophage secretions and “cues” provided by the extracellular matrix in the injury site.

Retina regeneration: lessons from vertebrates

Unlike mammals, vertebrates such as fishes and frogs exhibit remarkable tissue regeneration including the central nervous system. Retina being part of the central nervous system has attracted the interest of several research groups to explore its regenerative ability in different vertebrate models including mice. Fishes and frogs completely restore the size, shape and tissue structure of an injured retina. Several studies have unraveled molecular mechanisms underlying retina regeneration. In teleosts, soon after injury, the Müller glial cells of the retina reprogram to form a proliferating population of Müller glia-derived progenitor cells capable of differentiating into various neural cell types and Müller glia. In amphibians, the transdifferentiation of retinal pigment epithelium and differentiation of ciliary marginal zone cells contribute to retina regeneration. In chicks and mice, supplementation with external growth factors or genetic modifications cause a partial regenerative response in the damaged retina. The initiation of retina regeneration is achieved through sequential orchestration of gene expression through controlled modulations in the genetic and epigenetic landscape of the progenitor cells. Several developmental biology pathways are turned on during the Müller glia reprogramming, retinal pigment epithelium transdifferentiation and ciliary marginal zone differentiation. Further, several tumorigenic pathways and gene expression events also contribute to the complete regeneration cascade of events. In this review, we address the various retinal injury paradigms and subsequent gene expression events governed in different vertebrate species. Further, we compared how vertebrates such as teleost fishes and amphibians can achieve excellent regenerative responses in the retina compared with their mammalian counterparts.

Hybrid Biomaterial Initiates Refractory Wound Healing via Inducing Transiently Heightened Inflammatory Responses

Inflammation plays a crucial role in triggering regeneration, while inadequate or chronic inflammation hinders the regenerative process, resulting in refractory wounds. Inspired by the ideal regeneration mode in lower vertebrates and the human oral mucosa, realigning dysregulated inflammation to a heightened and acute response provides a promising option for refractory wound therapy. Neutrophils play important roles in inflammation initiation and resolution. Here, a hybrid biomaterial is used to stimulate transiently heightened inflammatory responses by precise tempospatial regulation of neutrophil recruitment and apoptosis. The hybrid biomaterial (Gel@fMLP/SiO2 -FasL) is constructed by loading of formyl-met-leu-phe (fMLP) and FasL-conjugated silica nanoparticles (SiO2 -FasL) into a pH-responsive hydrogel matrix. This composition enables burst release of fMLP to rapidly recruit neutrophils for heightened inflammation initiation. After neutrophils act to produce acids, the pH-responsive hydrogel degrades to expose SiO2 -FasL, which induces activated neutrophils apoptosis via FasL-Fas signaling triggering timely inflammation resolution. Apoptotic neutrophils are subsequently cleared by macrophages, and this efferocytosis activates key signalings to promote macrophage anti-inflammatory phenotypic transformation to drive regeneration. Ultimately, Gel@fMLP/SiO2 -FasL successfully promotes tissue regeneration by manipulating inflammation in critical-sized calvarial bone defects and diabetic cutaneous wound models. This work provides a new strategy for refractory wound therapy via inducing transiently heightened inflammatory responses.

Incomplete Recovery of Zebrafish Retina Following Cryoinjury

Zebrafish show an extraordinary potential for regeneration in several organs from fins to central nervous system. Most impressively, the outcome of an injury results in a near perfect regeneration and a full functional recovery. Indeed, among the various injury paradigms previously tested in the field of zebrafish retina regeneration, a perfect layered structure is observed after one month of recovery in most of the reported cases. In this study, we applied cryoinjury to the zebrafish eye. We show that retina exposed to this treatment for one second undergoes an acute damage affecting all retinal cell types, followed by a phase of limited tissue remodeling and regrowth. Surprisingly, zebrafish developed a persistent retinal dysplasia observable through 300 days post-injury. There is no indication of fibrosis during the regeneration period, contrary to the regeneration process after cryoinjury to the zebrafish cardiac ventricle. RNA sequencing analysis of injured retinas at different time points has uncovered enriched processes and a number of potential candidate genes. By means of this simple, time and cost-effective technique, we propose a zebrafish injury model that displays a unique inability to completely recover following focal retinal damage; an outcome that is unreported to our knowledge. Furthermore, RNA sequencing proved to be useful in identifying pathways, which may play a crucial role not only in the regeneration of the retina, but in the first initial step of regeneration, degeneration. We propose that this model may prove useful in comparative and translational studies to examine critical pathways for successful regeneration.

Identification of the Adult Hematopoietic Liver as the Primary Reservoir for the Recruitment of Pro-regenerative Macrophages Required for Salamander Limb Regeneration

The lack of scar-free healing and regeneration in many adult human tissues imposes severe limitations on the recovery of function after injury. In stark contrast, salamanders can functionally repair a range of clinically relevant tissues throughout adult life. The impressive ability to regenerate whole limbs after amputation, or regenerate following cardiac injury, is critically dependent on the recruitment of (myeloid) macrophage white blood cells to the site of injury. Amputation in the absence of macrophages results in regeneration failure and scar tissue induction. Identifying the exact hematopoietic source or reservoir of myeloid cells supporting regeneration is a necessary step in characterizing differences in macrophage phenotypes regulating scarring or regeneration across species. Mammalian wounds are dominated by splenic-derived monocytes that originate in the bone marrow and differentiate into macrophages within the wound. Unlike mammals, adult axolotls do not have functional bone marrow but instead utilize liver and spleen tissues as major sites for adult hematopoiesis. To interrogate leukocyte identity, tissue origins, and modes of recruitment, we established several transgenic axolotl hematopoietic tissue transplant models and flow cytometry protocols to study cell migration and identify the source of pro-regenerative macrophages. We identified that although bidirectional trafficking of leukocytes can occur between spleen and liver tissues, the liver is the major source of leukocytes recruited to regenerating limbs. Recruitment of leukocytes and limb regeneration occurs in the absence of the spleen, thus confirming the dependence of liver-derived myeloid cells in regeneration and that splenic maturation is dispensable for the education of pro-regenerative macrophages. This work provides an important foundation for understanding the hematopoietic origins and education of myeloid cells recruited to, and essential for, adult tissue regeneration.

Epithelial proliferation and cell cycle dysregulation in kidney injury and disease

Various cellular insults and injury to renal epithelial cells stimulate repair mechanisms to adapt and restore the organ homeostasis. Renal tubular epithelial cells are endowed with regenerative capacity, which allows for a restoration of nephron function after acute kidney injury. However, recent evidence indicates that the repair is often incomplete, leading to maladaptive responses that promote the progression to chronic kidney disease. The dysregulated cell cycle and proliferation is also a key feature of renal tubular epithelial cells in polycystic kidney disease and HIV-associated nephropathy. Therefore, in this review, we provide an overview of cell cycle regulation and the consequences of dysregulated cell proliferation in acute kidney injury, polycystic kidney disease, and HIV-associated nephropathy. An increased understanding of these processes may help define better targets for kidney repair and combat chronic kidney disease progression.

Anatomical and histological analyses reveal that tail repair is coupled with regrowth in wild-caught, juvenile American alligators (Alligator mississippiensis)

Reptiles are the only amniotes that maintain the capacity to regenerate appendages. This study presents the first anatomical and histological evidence of tail repair with regrowth in an archosaur, the American alligator. The regrown alligator tails constituted approximately 6–18% of the total body length and were morphologically distinct from original tail segments. Gross dissection, radiographs, and magnetic resonance imaging revealed that caudal vertebrae were replaced by a ventrally-positioned, unsegmented endoskeleton. This contrasts with lepidosaurs, where the regenerated tail is radially organized around a central endoskeleton. Furthermore, the regrown alligator tail lacked skeletal muscle and instead consisted of fibrous connective tissue composed of type I and type III collagen fibers. The overproduction of connective tissue shares features with mammalian wound healing or fibrosis. The lack of skeletal muscle contrasts with lizards, but shares similarities with regenerated tails in the tuatara and regenerated limbs in Xenopus adult frogs, which have a cartilaginous endoskeleton surrounded by connective tissue, but lack skeletal muscle. Overall, this study of wild-caught, juvenile American alligator tails identifies a distinct pattern of wound repair in mammals while exhibiting features in common with regeneration in lepidosaurs and amphibia.

Beyond Adult Stem Cells: Dedifferentiation as a Unifying Mechanism Underlying Regeneration in Invertebrate Deuterostomes

The diversity of regenerative phenomena seen in adult metazoans, as well as their underlying mechanistic bases, are still far from being comprehensively understood. Reviewing both ultrastructural and molecular data, the present work aims to showcase the increasing relevance of invertebrate deuterostomes, i.e., echinoderms, hemichordates, cephalochordates and tunicates, as invaluable models to study cellular aspects of adult regeneration. Our comparative approach suggests a fundamental contribution of local dedifferentiation -rather than mobilization of resident undifferentiated stem cells- as an important cellular mechanism contributing to regeneration in these groups. Thus, elucidating the cellular origins, recruitment and fate of cells, as well as the molecular signals underpinning tissue regrowth in regeneration-competent deuterostomes, will provide the foundation for future research in tackling the relatively limited regenerative abilities of vertebrates, with clear applications in regenerative medicine.

[Mechanisms of the wound-healing action of native collagen type I in ischemic model full-thickness skin wounds on the example - medical devices Collost «(part I)]

Active collagen type I successfully used in regenerative medicine. However, despite the large amount of material of cellular and molecular mechanisms underlying skin repair, the molecular mechanisms of wound healing with use collagen type I, not studied enough.

PURPOSE OF THE STUDY

To study the mechanism of the native collagen type I wound-healing action of native type I collagen on the example of the medical device Collost (7% gel) in a model of the rats difficult-to-heal skin wounds.

MATERIAL AND METHODS

Male rats in population SD (72 individuals) surgically formed an ischemic dorsal skin flap (3×10 cm) with two full-thickness skin wounds 6 mm in diameter.The trained animals divided into 2 groups: in the experimental group, medical device Collost (gel) applied once after the operation, in the control group - a standard medical device for comparison. The dynamics of wound healing assessed, the number of M2 macrophages, myofibroblasts, vascularization and expression of the main markers of the repair process in the wound tissues and time points for assessment were: after 3, 7 and 14 days after operation using macroscopic, immunohistochemical, and molecular methods.

RESULTS

It has been established that the mechanism of action of native collagen type I is associated with the acceleration of the appearance of «progenitorous» M2-macrophages in the wound tissues, decrease in the severity of inflammation or reduction in the duration of the inflammatory stage of the repair process, change in the expression spectrum of number of growth factors, an acceleration of neovasculogenesis.

CONCLUSION

In this work, on the modern experimental model shown regenerative efficiency of a medical device based on collagen type I and described the molecular and cellular processes of wound healing when using it It has been shown that the acceleration of wound healing processes occurs when using a medical device based on native collagen type 1, it is also accompanied by a better aesthetic closure of the damaged skin area.

Cellular and molecular mechanisms that regulate mammalian digit tip regeneration

Digit tip regeneration is one of the few examples of true multi-tissue regeneration in an adult mammal. The key step in this process is the formation of the blastema, a transient proliferating cell mass that generates the different cell types of the digit to replicate the original structure. Failure to form the blastema results in a lack of regeneration and has been postulated to be the reason why mammalian limbs cannot regrow following amputation. Understanding how the blastema forms and functions will help us to determine what is required for mammalian regeneration to occur and will provide insights into potential therapies for mammalian tissue regeneration and repair. This review summarizes the cellular and molecular mechanisms that influence murine blastema formation and govern digit tip regeneration.

LIN28B/let-7 control the ability of neonatal murine auditory supporting cells to generate hair cells through mTOR signaling

Mechano-sensory hair cells within the inner ear cochlea are essential for the detection of sound. In mammals, cochlear hair cells are only produced during development and their loss, due to disease or trauma, is a leading cause of deafness. In the immature cochlea, prior to the onset of hearing, hair cell loss stimulates neighboring supporting cells to act as hair cell progenitors and produce new hair cells. However, for reasons unknown, such regenerative capacity (plasticity) is lost once supporting cells undergo maturation. Here, we demonstrate that the RNA binding protein LIN28B plays an important role in the production of hair cells by supporting cells and provide evidence that the developmental drop in supporting cell plasticity in the mammalian cochlea is, at least in part, a product of declining LIN28B-mammalian target of rapamycin (mTOR) activity. Employing murine cochlear organoid and explant cultures to model mitotic and nonmitotic mechanisms of hair cell generation, we show that loss of LIN28B function, due to its conditional deletion, or due to overexpression of the antagonistic miRNA let-7g, suppressed Akt-mTOR complex 1 (mTORC1) activity and renders young, immature supporting cells incapable of generating hair cells. Conversely, we found that LIN28B overexpression increased Akt-mTORC1 activity and allowed supporting cells that were undergoing maturation to de-differentiate into progenitor-like cells and to produce hair cells via mitotic and nonmitotic mechanisms. Finally, using the mTORC1 inhibitor rapamycin, we demonstrate that LIN28B promotes supporting cell plasticity in an mTORC1-dependent manner.

The myeloid lineage is required for the emergence of a regeneration-permissive environment following Xenopus tail amputation

Regeneration-competent vertebrates are considered to suppress inflammation faster than non-regenerating ones. Hence, understanding the cellular mechanisms affected by immune cells and inflammation can help develop strategies to promote tissue repair and regeneration. Here, we took advantage of naturally occurring tail regeneration-competent and -incompetent developmental stages of Xenopus tadpoles. We first establish the essential role of the myeloid lineage for tail regeneration in the regeneration-competent tadpoles. We then reveal that upon tail amputation there is a myeloid lineage-dependent change in amputation-induced apoptosis levels, which in turn promotes tissue remodelling, and ultimately leads to the relocalization of the regeneration-organizing cells responsible for progenitor proliferation. These cellular mechanisms failed to be executed in regeneration-incompetent tadpoles. We demonstrate that regeneration incompetency is characterized by inflammatory myeloid cells whereas regeneration competency is associated with reparative myeloid cells. Moreover, treatment of regeneration-incompetent tadpoles with immune-suppressing drugs restores myeloid lineage-controlled cellular mechanisms. Collectively, our work reveals the effects of differential activation of the myeloid lineage on the creation of a regeneration-permissive environment and could be further exploited to devise strategies for regenerative medicine purposes.

Summary:Xenopus tail regeneration requires a hierarchy of cellular events initiated by the myeloid lineage and culminating in the mobilization of regeneration-organizing cells.

Injury of Adult Zebrafish Expressing Acvr1lQ204D Does Not Result in Heterotopic Ossification

Fibrodysplasia Ossificans Progressiva (FOP) is a rare, autosomal dominant genetic disorder in humans characterized by the gradual ossification of fibrous tissues, including skeletal muscle, tendons, and ligaments. In humans, mutations in the Type I BMP/TGFβ family member receptor gene, ACVR1, are associated with FOP. Zebrafish acvr1l, previously known as alk8, is the functional ortholog of human ACVR1. We previously created and characterized the first adult zebrafish model for FOP by generating animals harboring heat shock-inducible mCherry-tagged constitutively active Acvr1l (Q204D). Since injury is a known trigger for heterotopic ossification (HO) development in human FOP patients, in this study, we investigated several injury models in Acvr1l^Q204D-expressing zebrafish and the subsequent formation of HO. We performed studies of Activin A injection, cardiotoxin (CTX) injection, and caudal fin clip injury. We found that none of these methods resulted in HO formation at the site of injury. However, some of the cardiotoxin-injected and caudal fin-clipped animals did exhibit HO at distant sites, including the body cavity and along the spine. We describe these results in the context of new and exciting reports on FOP, and discuss future studies to better understand the etiology and progression of this disease.

Bone regeneration after traumatic skull injury in Xenopus tropicalis

The main purpose of regenerative biology is to improve human health by exploiting cellular and molecular mechanisms favoring tissue repair. In recent years, non-mammalian vertebrates have emerged as powerful model organisms to tackle the problem of tissue regeneration. Here, we analyze the process of bone repair in metamorphosing Xenopus tropicalis tadpoles subjected to traumatic skull injury. Five days after skull perforation, a dense and highly vascularized mesenchymal is apparent over the injury site. Using an in vivo bone staining procedure based on independent pulses of Alizarin red and Calcein green, we show that the deposition of new bone matrix completely closes the wound in 15 days. The absence of cartilage implies that bone repair follows an intramembranous ossification route. Collagen second harmonic imaging reveals that while a well-organized lamellar type of bone is deposited during development, a woven type of bone is produced during the early-phase of the regeneration process. Osteoblasts lying against the regenerating bone robustly express fibrillar collagen 1a1, SPARC and Dlx5. These analyses establish Xenopus tropicalis as a new model system to improve traumatic skull injury recovery.

TISSUE REPAIR AND EPIMORPHIC REGENERATION: AN OVERVIEW

PURPOSE OF THE REVIEW

This manuscript discusses wound healing as a component of epimorphic regeneration and the role of the immune system in this process.

RECENT FINDINGS

Epimorphic regeneration involves formation of a blastema, a mass of undifferentiated cells capable of giving rise to the regenerated tissues. The apical epithelial cap plays an important role in blastemal formation.

SUMMARY

True regeneration is rarely observed in mammals. With the exception of transgenic strains, tissue repair in mammals usually leads to non-functional fibrotic tissue formation. In contrast, a number of lower order species including planarians, salamanders, and reptiles, have the ability to overcome the burden of scarring and tissue loss through complex adaptations that allow them to regenerate various anatomic structures through epimorphic regeneration. Blastemal cells have been suggested to originate via various mechanisms including de-differentiation, transdifferentiation, migration of pre-existing adult stem cell niches, and combinations of these.

Regeneration in distantly related species: common strategies and pathways

While almost all animals are able to at least partially replace some lost parts, regeneration abilities vary considerably across species. Here we study gene expression patterns in distantly related species to investigate conserved regeneration strategies. To this end, we collect from the literature transcriptomic data obtained during the regeneration of three species (Hydra magnipapillata, Schmidtea mediterranea, and Apostichopus japonicus), and compare them with gene expression during regeneration in vertebrates and mammals. This allows us to identify a common set of differentially expressed genes and relevant shared pathways that are conserved across species during the early stage of the regeneration process. We also find a set of differentially expressed genes that in mammals are associated to the presence of macrophages and to the epithelial–mesenchymal transition. This suggests that features of the sophisticated wound healing strategy of mammals are already observable in earlier emerging metazoans.

All animals capable of regenerating a lost body part, from an organ or a limb to the whole organism, use a common set of genes. This is the striking discovery of a team of researchers from the Center for Complexity and Biosystems of the University of Milan, led by Caterina La Porta. They analyzed the genetic activity in regenerating tissues from widely different species—from hydra to mice. They found that some of the genes active at the beginning of the regeneration process are the same in very different species, including mammals which have lost this function during evolution, except for the restoration of the liver. The discovery of this shared genetic signature is of great importance to understand how regeneration evolved and could be useful for future regeneration therapies.

Stem cells, niches and scaffolds: Applications to burns and wound care

The importance of skin to survival, and the devastating physical and psychological consequences of scarring following reparative healing of extensive or difficult to heal human wounds, cannot be disputed. We discuss the significant challenges faced by patients and healthcare providers alike in treating these wounds. New state of the art technologies have provided remarkable insights into the role of skin stem and progenitor cells and their niches in maintaining skin homeostasis and in reparative wound healing. Based on this knowledge, we examine different approaches to repair extensive burn injury and chronic wounds, including full and split thickness skin grafts, temporising matrices and scaffolds, and composite cultured skin products. Notable developments include next generation skin substitutes to replace split thickness skin autografts and next generation gene editing coupled with cell therapies to treat genodermatoses. Further refinements are predicted with the advent of bioprinting technologies, and newly defined biomaterials and autologous cell sources that can be engineered to more accurately replicate human skin architecture, function and cosmesis. These advances will undoubtedly improve quality of life for patients with extensive burns and difficult to heal wounds.

Macrophages are necessary for epimorphic regeneration in African spiny mice

How the immune system affects tissue regeneration is not well understood. In this study, we used an emerging mammalian model of epimorphic regeneration, the African spiny mouse, to examine cell-based inflammation and tested the hypothesis that macrophages are necessary for regeneration. By directly comparing inflammatory cell activation in a 4 mm ear injury during regeneration (Acomys cahirinus) and scarring (Mus musculus), we found that both species exhibited an acute inflammatory response, with scarring characterized by stronger myeloperoxidase activity. In contrast, ROS production was stronger and more persistent during regeneration. By depleting macrophages during injury, we demonstrate a functional requirement for these cells to stimulate regeneration. Importantly, the spatial distribution of activated macrophage subtypes was unique during regeneration with pro-inflammatory macrophages failing to infiltrate the regeneration blastema. Together, our results demonstrate an essential role for inflammatory cells to regulate a regenerative response.

Evolutionary origin of endochondral ossification: the transdifferentiation hypothesis

The vertebrate endoskeleton results from the piecemeal assembly of bone and cartilage as well as additional types of calcified extracellular matrices produced by seemingly hybrid cell types of intermediate phenotypes between osteoblasts and chondrocytes. Hence, shedding light on the emergence and subsequent diversification of skeletal tissues represents a major challenge in vertebrate evolutionary developmental biology. A 150-year-old debate in the field was recently solved by lineage tracing experiments demonstrating that, during mouse endochondral bone development, a subset of chondrocytes evades apoptosis and transdifferentiates into osteoblasts at the chondro-osseous junction. Here, we interpret these new data from a broad phylogenetic perspective, integrating fossil, histological, cellular, and genetic evidence from a wide range of vertebrates. We propose a testable scenario according to which transdifferentiation played a fundamental role in the emergence of endochondral ossification, an osteichthyan-specific evolutionary novelty. The remarkable skeletal cell plasticity might be contingent on the similar architectures of the osteoblastic and chondrocytic gene regulatory networks, thereby providing a unifying mechanism underlying both complete transdifferentiation as well as partial cell type conversions observed in intermediate skeletal tissues such as the chondroid bone or globuli ossei.

Inflammation and immunity in organ regeneration

The ability of vertebrates to regenerate amputated appendages is increasingly well-understood at the cellular level. Cells mediating an innate immune response and inflammation in the injured tissues are a prominent feature of the limb prior to formation of a regeneration blastema, with macrophage activity necessary for blastema growth and successful development of the new limb. Studies involving either anti-inflammatory or pro-inflammatory agents suggest that the local inflammation produced by injury and its timely resolution are both important for regeneration, with blastema patterning inhibited in the presence of unresolved inflammation. Various experiments with Xenopus larvae at stages where regenerative competence is declining show improved digit formation after treatment with certain immunosuppressive, anti-inflammatory, or antioxidant agents. Similar work with the larval Xenopus tail has implicated adaptive immunity with regenerative competence and suggests a requirement for regulatory T cells in regeneration, which also occurs in many systems of tissue regeneration. Recent analyses of the human nail organ indicate a capacity for local immune tolerance, suggesting roles for adaptive immunity in the capacity for mammalian appendage regeneration. New information and better understanding regarding the neuroendocrine-immune axis in the response to stressors, including amputation, suggest additional approaches useful for investigating effects of the immune system during repair and regeneration.

The cellular and molecular mechanisms of tissue repair and regeneration as revealed by studies in Xenopus

Survival of any living organism critically depends on its ability to repair and regenerate damaged tissues and/or organs during its lifetime following injury, disease, or aging. Various animal models from invertebrates to vertebrates have been used to investigate the molecular and cellular mechanisms of wound healing and tissue regeneration. It is hoped that such studies will form the framework for identifying novel clinical treatments that will improve the healing and regenerative capacity of humans. Amongst these models, Xenopus stands out as a particularly versatile and powerful system. This review summarizes recent findings using this model, which have provided fundamental knowledge of the mechanisms responsible for efficient and perfect tissue repair and regeneration.

Chasing the recipe for a pro-regenerative immune system

Identification of the key ingredients and essential processes required to achieve perfect tissue regeneration in humans has so far remained elusive. Injury in vertebrates induces an obligatory wound response that will precede or overlap any regeneration specific program or scarring outcome. This process shapes the cellular and molecular landscape of the tissue, influencing the success of endogenous repair pathways or for potential clinical intervention. The involvement of immune cells is also required for aspects of development extending beyond the initial inflammatory phase of wounding. It has now become clear from amphibian, fish and mammalian models of tissue injury that the type of immune response and the profile of immune cells attending the site of injury can act as the gatekeepers that determine wound repair quality. The heterogeneity among innate and adaptive immune cell populations, along with the developmental origin of these cells, form key ingredients affecting the potential for downstream repair and the suppression of fibrosis. Cell-to-cell interactions between immune cells, such as macrophages and T cells, with stem cells and mesenchymal cells are critically important for shaping this process and these exchanges, are in turn influenced by the type of injury, tissue location and developmental stage of the organism. Developmentally, mouse cardiac regeneration is restricted to early stages of postnatal life where the balance of innate to adaptive immune cells may be poised towards regeneration. In the injured adult mouse liver, specific macrophage subsets improve repair while other bone marrow derived cells can exacerbate injury. Other studies using genetically diverse mice have shown enhanced regeneration in certain strains, restricted to specific tissues. This enhanced repair is linked with expression of genes such as Insulin-like Growth Factor- 1 (IGF-1) and activin (Act 1), that both play important roles in shaping the immune system. Immune cells are now appreciated to have powerful influences on critical cell types required for regeneration success. The winning recipe for tissue regeneration is likely to be found ultimately by identifying the genetic elements and specific cell populations that limit or allow intrinsic potential. This will be essential for developing therapeutic strategies for tissue regeneration in humans.

Working with zebrafish at postembryonic stages

As the processes of embryogenesis become increasingly well understood, there is growing interest in the development that occurs at later, postembryonic stages. Postembryonic development holds tremendous potential for discoveries of both fundamental and translational importance. Zebrafish, which are small, rapidly and externally developing, and which boast a wealth of genetic resources, are an outstanding model of vertebrate postembryonic development. Nonetheless, there are specific challenges posed by working with zebrafish at these stages, and this chapter is meant to serve as a primer for those working with larval and juvenile zebrafish. Since accurate staging is critical for high-quality results and experimental reproducibility, we outline best practices for reporting postembryonic developmental progress. Emphasizing the importance of accurate staging, we present new data showing that rates of growth and size-stage relationships can differ even between wild-type strains. Finally, since rapid and uniform development is particularly critical when working at postembryonic stages, we briefly describe methods that we use to achieve high rates of growth and developmental uniformity through postembryonic stages in both wild-type and growth-compromised zebrafish.

Xenopus: An in vivo model for imaging the inflammatory response following injury and bacterial infection

A major goal in regenerative medicine is to identify therapies to facilitate our body׳s innate abilities to repair and regenerate following injury, disease or aging. In the past decade it has become apparent that the innate immune system is able to affect the speed and quality of the regenerative response through mechanisms that are not entirely clear. For this reason there has been a resurgent interest in investigating the role of inflammation during tissue repair and regeneration. Remarkably, there have only been a handful of such studies using organisms with high regenerative capacity. Here we perform a study of the inflammatory response following injury in Xenopus larvae, which are able to achieve scarless wound healing and to regenerate appendages, as a preamble into understanding the role that inflammation plays during tissue repair and regeneration in this organism. We characterized the morphology and migratory behavior of granulocytes and macrophages following sterile and infected wounding regimes, using various transgenic lines that labeled different types of myeloid lineages, including granulocytes and macrophages. Using this approach we found that the inflammatory response following injury and infection in Xenopus larvae is very similar to that seen in humans, suggesting that this model provides an easily tractable and medically relevant system to investigate inflammation following injury and infection in vivo.

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Xenopus larvae is an ideal model to study injury-induced inflammation in vivo.

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Xenopus larvae provides an easily tractable model of human inflammation.

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Xenopus larvae provides a powerful model for investigating cell migration in vivo.

Regeneration of Xenopus laevis spinal cord requires Sox2/3 expressing cells

Spinal cord regeneration is very inefficient in humans, causing paraplegia and quadriplegia. Studying model organisms that can regenerate the spinal cord in response to injury could be useful for understanding the cellular and molecular mechanisms that explain why this process fails in humans. Here, we use Xenopus laevis as a model organism to study spinal cord repair. Histological and functional analyses showed that larvae at pre-metamorphic stages restore anatomical continuity of the spinal cord and recover swimming after complete spinal cord transection. These regenerative capabilities decrease with onset of metamorphosis. The ability to study regenerative and non-regenerative stages in Xenopus laevis makes it a unique model system to study regeneration. We studied the response of Sox2(/)3 expressing cells to spinal cord injury and their function in the regenerative process. We found that cells expressing Sox2 and/or Sox3 are present in the ventricular zone of regenerative animals and decrease in non-regenerative froglets. Bromodeoxyuridine (BrdU) experiments and in vivo time-lapse imaging studies using green fluorescent protein (GFP) expression driven by the Sox3 promoter showed a rapid, transient and massive proliferation of Sox2(/)3(+) cells in response to injury in the regenerative stages. The in vivo imaging also demonstrated that Sox2(/)3(+) neural progenitor cells generate neurons in response to injury. In contrast, these cells showed a delayed and very limited response in non-regenerative froglets. Sox2 knockdown and overexpression of a dominant negative form of Sox2 disrupts locomotor and anatomical-histological recovery. We also found that neurogenesis markers increase in response to injury in regenerative but not in non-regenerative animals. We conclude that Sox2 is necessary for spinal cord regeneration and suggest a model whereby spinal cord injury activates proliferation of Sox2/3 expressing cells and their differentiation into neurons, a mechanism that is lost in non-regenerative froglets.