Appendage regeneration in adult vertebrates and implications for regenerative medicine

Sall4 regulates downstream patterning genes during limb regeneration

Many salamanders can completely regenerate a fully functional limb. Limb regeneration is a carefully coordinated process involving several defined stages. One key event during the regeneration process is the patterning of the blastema to inform cells of what they must differentiate into. Although it is known that many genes involved in the initial development of the limb are re-used during regeneration, the exact molecular circuitry involved in this process is not fully understood. Several large-scale transcriptional profiling studies of axolotl limb regeneration have identified many transcription factors that are up-regulated after limb amputation. Sall4 is a transcription factor that has been identified to play essential roles in maintaining cells in an undifferentiated state during development and also plays a unique role in limb development. Inactivation of Sall4 during limb bud development results in defects in anterior-posterior patterning of the limb. Sall4 has been found to be up-regulated during limb regeneration in both Xenopus and salamanders, but to date it function has been untested. We confirmed that Sall4 is up-regulated during limb regeneration in the axolotl using qRT-PCR and identified that it is present in the skin cells and also in cells within the blastema. Using CRISPR technology we microinjected gRNAs specific for Sall4 complexed with cas9 protein into the blastema to specifically knockout Sall4 in blastema cells only. This resulted in limb regenerate defects, including missing digits, fusion of digit elements, and defects in the radius and ulna. This suggests that during regeneration Sall4 may play a similar role in regulating the specification of anterior-proximal skeletal elements.

Mechanisms and translational applications of regeneration in limbs: From renewable animals to humans

BACKGROUND

The regenerative capacity of organisms declines throughout evolution, and mammals lack the ability to regenerate limbs after injury. Past approaches to achieving successful restoration through pharmacological intervention, tissue engineering, and cell therapies have faced significant challenges.

OBJECTIVES

This review aims to provide an overview of the current understanding of the mechanisms behind animal limb regeneration and the successful translation of these mechanisms for human tissue regeneration.

RESULTS

Particular attention was paid to the Mexican axolotl (Ambystoma mexicanum), the only adult tetrapod capable of limb regeneration. We will explore fundamental questions surrounding limb regeneration, such as how amputation initiates regeneration, how the limb knows when to stop and which parts to regenerate, and how these findings can apply to mammalian systems.

CONCLUSIONS

Given the urgent need for regenerative therapies to treat conditions like diabetic foot ulcers and trauma survivors, this review provides valuable insights and ideas for researchers, clinicians, and biomedical engineers seeking to facilitate the regeneration process or elicit full regeneration from partial regeneration events.

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.

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

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

Advances in protein glycosylation and its role in tissue repair and regeneration

After tissue damage, a series of molecular and cellular events are initiated to promote tissue repair and regeneration to restore its original structure and function. These events include inter-cell communication, cell proliferation, cell migration, extracellular matrix differentiation, and other critical biological processes. Glycosylation is the crucial conservative and universal post-translational modification in all eukaryotic cells [1], with influential roles in intercellular recognition, regulation, signaling, immune response, cellular transformation, and disease development. Studies have shown that abnormally glycosylation of proteins is a well-recognized feature of cancer cells, and specific glycan structures are considered markers of tumor development. There are many studies on gene expression and regulation during tissue repair and regeneration. Still, there needs to be more knowledge of complex carbohydrates' effects on tissue repair and regeneration, such as glycosylation. Here, we present a review of studies investigating protein glycosylation in the tissue repair and regeneration process.

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

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

Aquaporin5 Deficiency Aggravates ROS/NLRP3 Inflammasome-Mediated Pyroptosis in the Lacrimal Glands

Purpose

The pathogenesis of the lacrimal glands (LGs) is facilitated by inflammation mediated by the NACHT, LRR, and NLRP3 inflammasomes in dry eye disease. This research aimed to explore the protective effects of Aquaporin 5 (AQP5) on LGs by inhibiting reactive oxygen species (ROS) and the NLRP3 inflammasome.

Methods

AQP5 knockout (AQP5-/-) mice were used to evaluate pathological changes in LGs. ROS generation was detected with a dichlorodihydro-fluorescein diacetate assay. Lipid metabolism was assessed by Oil Red O staining. The reversal of the mitochondrial membrane potential was detected using a JC-1 fluorescent probe kit. The effect of AQP5 on NLRP3/caspase-1/Gasdermin-D (GSDMD)-mediated pyroptosis was examined using pharmacological treatment of N-acetyl L-cysteine or MCC950.

Results

AQP5 loss significantly increased ROS generation, lipid metabolism disorders, TUNEL-positive cells, and reversal of the mitochondrial membrane potential in the AQP5-/- LGs. NLRP3 upregulation, increased caspase-1 and GSDMD activity, and enhanced IL-1β release were detected in the AQP5-/- mouse LGs and primary LG epithelial cells. MCC950 significantly suppressed NLRP3 inflammasome-related pyroptosis induced by AQP5 deficiency in LGs and primary LG epithelial cells. Furthermore, we discovered that prestimulating the AQP5-/- primary LG epithelial cells with N-acetyl L-cysteine decreased NLRP3 expression, caspase-1 and GSDMD activity levels, and IL-1β release.

Conclusions

Our results revealed that AQP5 loss promoted NLRP3 inflammasome activation through ROS generation. Inhibiting the ROS or NLRP3 inflammasome significantly alleviated the damage and pyroptosis of AQP5-deficient LG epithelial cells, which could provide new insights into dry eye disease.

Oxaloacetate enhances and accelerates regeneration in young mice by promoting proliferation and mineralization

Cell metabolism coordinates the biochemical reactions that produce carbon and ATP in order for the cell to proliferate, differentiate, and respond to environmental changes. Cell type determines metabolic demand, so proliferating skeletal progenitors and differentiated osteoblasts exhibit different levels of cell metabolism. Limb regeneration is an energetically demanding process that involves multiple types of tissues and cell functions over time. Dysregulation of cell metabolism in aged mice results in impaired regeneration, a defect that can be rescued in part by the administration of oxaloacetate (OAA). A better understanding of how cell metabolism regulates regeneration in general, and how these changes can be modulated to benefit potential regenerative strategies in the future is needed. Here we sought to better understand the effects of OAA on young mice and determine whether the same mechanism could be tapped to improve regeneration without an aged-defect. We also asked which dosing time periods were most impactful for promoting regenerative outcomes, and whether these effects were sustained after dosing was stopped. Consistent with our findings in aged mice we found that OAA enhanced regeneration by accelerating bone growth, even beyond control measures, by increasing trabecular thickness, decreasing trabecular spacing, and improving the patterning by decreasing the taper, making the regenerated bone more like an unamputated digit. Our data suggests that the decrease in spacing, an improvement over aged mice, may be due to a decrease in hypoxia-driven vasculature. Our findings suggest that OAA, and similar metabolites, may be a strong tool to promote regenerative strategies and investigate the mechanisms that link cell metabolism and regeneration.

Prolyl-hydroxylase inhibitor-induced regeneration of alveolar bone and soft tissue in a mouse model of periodontitis through metabolic reprogramming

Bone injuries and fractures reliably heal through a process of regeneration with restoration to original structure and function when the gap between adjacent sides of a fracture site is small. However, when there is significant volumetric loss of bone, bone regeneration usually does not occur. In the present studies, we explore a particular case of volumetric bone loss in a mouse model of human periodontal disease (PD) in which alveolar bone surrounding teeth is permanently lost and not replaced. This model employs the placement a ligature around the upper second molar for 10 days leading to inflammation and bone breakdown and faithfully replicates the bacterially-induced inflammatory etiology of human PD to induce bone degeneration. After ligature removal, mice are treated with a timed-release formulation of a small molecule inhibitor of prolylhydroxylases (PHDi; 1,4-DPCA) previously shown to induce epimorphic regeneration of soft tissue in non-regenerating mice. This PHDi induces high expression of HIF-1α and is able to shift the metabolic state from OXPHOS to aerobic glycolysis, an energetic state used by stem cells and embryonic tissue. This regenerative response was completely blocked by siHIF1a. In these studies, we show that timed-release 1,4-DPCA rapidly and completely restores PD-affected bone and soft tissue with normal anatomic fidelity and with increased stem cell markers due to site-specific stem cell migration and/or de-differentiation of local tissue, periodontal ligament (PDL) cell proliferation, and increased vascularization. In-vitro studies using gingival tissue show that 1,4-DPCA indeed induces de-differentiation and the expression of stem cell markers but does not exclude the role of migrating stem cells. Evidence of metabolic reprogramming is seen by the expression of not only HIF-1a, its gene targets, and resultant de-differentiation markers, but also the metabolic genes Glut-1, Gapdh, Pdk1, Pgk1 and Ldh-a in jaw periodontal tissue.

Muscles are barely required for the patterning and cell dynamics in axolotl limb regeneration

Regeneration of a complex appendage structure such as limb requires upstream and downstream coordination of multiple types of cells. Given type of cell may sit at higher upstream position to control the activities of other cells. Muscles are one of the major cell masses in limbs. However, the subtle functional relationship between muscle and other cells in vertebrate complex tissue regeneration are still not well established. Here, we use Pax7 mutant axolotls, in which the limb muscle is developmentally lost, to investigate limb regeneration in the absence of skeletal muscle. We find that the pattern of regenerated limbs is relative normal in Pax7 mutants compared to the controls, but the joint is malformed in the Pax7 mutants. Lack of muscles do not affect the early regeneration responses, specifically the recruitment of macrophages to the wound, as well as the proliferation of fibroblasts, another major population in limbs. Furthermore, using single cell RNA-sequencing, we show that, other than muscle lineage that is mostly missing in Pax7 mutants, the composition and the status of other cell types in completely regenerated limbs of Pax7 mutants are similar to that in the controls. Our study reveals skeletal muscle is barely required for the guidance of other cells, as well the patterning in complex tissue regeneration in axolotls, and provides refined views of the roles of muscle cell in vertebrate appendage regeneration.

Evaluating differences in Young's Modulus of regenerated and uninjured mouse digit bone through microCT density-based calculation and nanoindentation testing

The mouse digit tip amputation model is an excellent model of bone regeneration, but its size and shape present an obstacle for biomechanical testing. As a result, assessing the structural quality of the regenerated bone in this model has focused on mineral density and bone architecture analysis. Here we describe an image-processing based method for assessment of mechanical properties in the regenerated digit by using micro-computed tomography mineral density data to calculate spatially discrete Young's modulus values throughout the entire distal third phalange. Further, we validate this method through comparison to nanoindentation-measured values for Young's modulus. Application to a set of regenerated and unamputated digits shows that regenerated bone has a lower Young's modulus compared to the uninjured digit, with a similar trend for experimental hardness values. Importantly, this method heightens the utility of the digit regeneration model, allows for more impactful treatment evaluation using the model, and introduces an analysis platform that can be used for other bones that do not conform to a standard long-bone model.

Injury-induced MAPK activation triggers body axis formation in Hydra by default Wnt signaling

Hydra's almost unlimited regenerative potential is based on Wnt signaling, but so far it is unknown how the injury stimulus is transmitted to discrete patterning fates in head and foot regenerates. We previously identified mitogen-activated protein kinases (MAPKs) among the earliest injury response molecules in Hydra head regeneration. Here, we show that three MAPKs-p38, c-Jun N-terminal kinases (JNKs), and extracellular signal-regulated kinases (ERKs)-are essential to initiate regeneration in Hydra, independent of the wound position. Their activation occurs in response to any injury and requires calcium and reactive oxygen species (ROS) signaling. Phosphorylated MAPKs hereby exhibit cross talk with mutual antagonism between the ERK pathway and stress-induced MAPKs, orchestrating a balance between cell survival and apoptosis. Importantly, Wnt3 and Wnt9/10c, which are induced by MAPK signaling, can partially rescue regeneration in tissues treated with MAPK inhibitors. Also, foot regenerates can be reverted to form head tissue by a pharmacological increase of β-catenin signaling or the application of recombinant Wnts. We propose a model in which a β-catenin-based stable gradient of head-forming capacity along the primary body axis, by differentially integrating an indiscriminate injury response, determines the fate of the regenerating tissue. Hereby, Wnt signaling acquires sustained activation in the head regenerate, while it is transient in the presumptive foot tissue. Given the high level of evolutionary conservation of MAPKs and Wnts, we assume that this mechanism is deeply embedded in our genome.

Post-amputation reactive oxygen species production is necessary for axolotls limb regeneration

Introduction: Reactive oxygen species (ROS) represent molecules of great interest in the field of regenerative biology since several animal models require their production to promote and favor tissue, organ, and appendage regeneration. Recently, it has been shown that the production of ROS such as hydrogen peroxide (H₂O₂) is required for tail regeneration in Ambystoma mexicanum. However, to date, it is unknown whether ROS production is necessary for limb regeneration in this animal model. Methods: forelimbs of juvenile animals were amputated proximally and the dynamics of ROS production was determined using 2′7- dichlorofluorescein diacetate (DCFDA) during the regeneration process. Inhibition of ROS production was performed using the NADPH oxidase inhibitor apocynin. Subsequently, a rescue assay was performed using exogenous hydrogen peroxide (H₂O₂). The effect of these treatments on the size and skeletal structures of the regenerated limb was evaluated by staining with alcian blue and alizarin red, as well as the effect on blastema formation, cell proliferation, immune cell recruitment, and expression of genes related to proximal-distal identity. Results: our results show that inhibition of post-amputation limb ROS production in the A. mexicanum salamander model results in the regeneration of a miniature limb with a significant reduction in the size of skeletal elements such as the ulna, radius, and overall autopod. Additionally, other effects such as decrease in the number of carpals, defective joint morphology, and failure of integrity between the regenerated structure and the remaining tissue were identified. In addition, this treatment affected blastema formation and induced a reduction in the levels of cell proliferation in this structure, as well as a reduction in the number of CD45⁺ and CD11b + immune system cells. On the other hand, blocking ROS production affected the expression of proximo-distal identity genes such as Aldha1a1, Rarβ, Prod1, Meis1, Hoxa13, and other genes such as Agr2 and Yap1 in early/mid blastema. Of great interest, the failure in blastema formation, skeletal alterations, as well as the expression of the genes evaluated were rescued by the application of exogenous H₂O₂, suggesting that ROS/H₂O₂ production is necessary from the early stages for proper regeneration and patterning of the limb.

Regulatory T cells regulate blastemal proliferation during zebrafish caudal fin regeneration

The role of T cells in appendage regeneration remains unclear. In this study, we revealed an important role for regulatory T cells (Tregs), a subset of T cells that regulate tolerance and tissue repair, in the epimorphic regeneration of zebrafish caudal fin tissue. Upon amputation, fin tissue-resident Tregs infiltrate into the blastema, a population of progenitor cells that produce new fin tissues. Conditional genetic ablation of Tregs attenuates blastemal cell proliferation during fin regeneration. Blastema-infiltrating Tregs upregulate the expression of igf2a and igf2b, and pharmacological activation of IGF signaling restores blastemal proliferation in Treg-ablated zebrafish. These findings further extend our understandings of Treg function in tissue regeneration and repair.

Distinct gene expression dynamics in developing and regenerating crustacean limbs

Regenerating animals have the ability to reproduce body parts that were originally made in the embryo and subsequently lost due to injury. Understanding whether regeneration mirrors development is an open question in most regenerative species. Here, we take a transcriptomics approach to examine whether leg regeneration shows similar temporal patterns of gene expression as leg development in the embryo, in the crustacean Parhyale hawaiensis. We find that leg development in the embryo shows stereotypic temporal patterns of gene expression. In contrast, the dynamics of gene expression during leg regeneration show a higher degree of variation related to the physiology of individual animals. A major driver of this variation is the molting cycle. We dissect the transcriptional signals of individual physiology and regeneration to obtain clearer temporal signals marking distinct phases of leg regeneration. Comparing the transcriptional dynamics of development and regeneration we find that, although the two processes use similar sets of genes, the temporal patterns in which these genes are deployed are different and cannot be systematically aligned.

Cryopreservation of Animals and Cryonics: Current Technical Progress, Difficulties and Possible Research Directions

The basis of cryonics or medical cryopreservation is to safely store a legally dead subject until a time in the future when technology and medicine will permit reanimation after eliminating the disease or cause of death. Death has been debunked as an event occurring after cardiac arrest to a process where interjecting its progression can allow for reversal when feasible. Cryonics technology artificially halts further damages and injury by restoring respiration and blood circulation, and rapidly reducing temperature. The body can then be preserved at this extremely low temperature until the need for reanimation. Presently, the area has attracted numerous scientific contributions and advancement but the practice is still flooded with challenges. This paper presents the current progression in cryonics research. We also discuss obstacles to success in the field, and identify the possible solutions and future research directions.

Spatial transcriptomics reveals metabolic changes underly age-dependent declines in digit regeneration

De novo limb regeneration after amputation is restricted in mammals to the distal digit tip. Central to this regenerative process is the blastema, a heterogeneous population of lineage-restricted, dedifferentiated cells that ultimately orchestrates regeneration of the amputated bone and surrounding soft tissue. To investigate skeletal regeneration, we made use of spatial transcriptomics to characterize the transcriptional profile specifically within the blastema. Using this technique, we generated a gene signature with high specificity for the blastema in both our spatial data, as well as other previously published single-cell RNA-sequencing transcriptomic studies. To elucidate potential mechanisms distinguishing regenerative from non-regenerative healing, we applied spatial transcriptomics to an aging model. Consistent with other forms of repair, our digit amputation mouse model showed a significant impairment in regeneration in aged mice. Contrasting young and aged mice, spatial analysis revealed a metabolic shift in aged blastema associated with an increased bioenergetic requirement. This enhanced metabolic turnover was associated with increased hypoxia and angiogenic signaling, leading to excessive vascularization and altered regenerated bone architecture in aged mice. Administration of the metabolite oxaloacetate decreased the oxygen consumption rate of the aged blastema and increased WNT signaling, leading to enhanced in vivo bone regeneration. Thus, targeting cell metabolism may be a promising strategy to mitigate aging-induced declines in tissue regeneration.

Development of high-throughput lacrimal gland organoid platforms for drug discovery in dry eye disease

Dysfunction and damage of the lacrimal gland (LG) results in ocular discomfort and dry eye disease (DED). Current therapies for DED do not fully replenish the necessary lubrication to rescue optimal vision. New drug discovery for DED has been limited perhaps because in vitro models cannot mimic the biology of the native LG. The existing platforms for LG organoid culture are scarce and still not ready for consistency and scale up production towards drug screening. The magnetic three-dimensional (3D) bioprinting (M3DB) is a novel system for 3D in vitro biofabrication of cellularized tissues using magnetic nanoparticles to bring cells together. M3DB provides a scalable platform for consistent handling of spheroid-like cell cultures facilitating consistent biofabrication of organoids. Previously, we successfully generated innervated secretory epithelial organoids from human dental pulp stem cells with M3DB and found that this platform is feasible for epithelial organoid bioprinting. Research targeting LG organogenesis, drug discovery for DED has extensively used mouse models. However, certain inter-species differences between mouse and human must be considered. Porcine LG appear to have more similarities to human LG than the mouse counterparts. We have conducted preliminary studies with the M3DB for fabricating LG organoids from primary cells isolated from murine and porcine LG, and found that this platform provides robust LG organoids for future potential high-throughput analysis and drug discovery. The LG organoid holds promise to be a functional model of tearing, a platform for drug screening, and may offer clinical applications for DED.

Age-Dependent Changes in Bone Architecture, Patterning, and Biomechanics During Skeletal Regeneration

Mouse digit amputation provides a useful model of bone growth after injury, in that the injury promotes intramembranous bone formation in an adult animal. The digit tip is composed of skin, nerves, blood vessels, bones, and tendons, all of which regenerate after digit tip amputation, making it a powerful model for multi-tissue regeneration. Bone integrity relies upon a balanced remodeling between bone resorption and formation, which, when disrupted, results in changes to bone architecture and biomechanics, particularly during aging. In this study, we used recently developed techniques to evaluate bone patterning differences between young and aged regenerated bone. This analysis suggests that aged mice have altered trabecular spacing and patterning and increased mineral density of the regenerated bone. To further characterize the biomechanics of regenerated bone, we measured elasticity using a micro-computed tomography image-processing method combined with nanoindentation. This analysis suggests that the regenerated bone demonstrates decreased elasticity compared with the uninjured bone, but there is no significant difference in elasticity between aged and young regenerated bone. These data highlight distinct architectural and biomechanical differences in regenerated bone in both young and aged mice and provide a new analysis tool for the digit amputation model to aid in evaluating the outcomes for potential therapeutic treatments to promote 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.

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

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

A new approach to analyzing regenerated bone quality in the mouse digit amputation model using semi-automatic processing of microCT data

Bone regeneration is a critical area of research impacting treatment of diseases such as osteoporosis, age-related decline, and orthopaedic implants. A crucial question in bone regeneration is that of bone architectural quality, or how "good" is the regenerated bone tissue structurally? Current methods address typical long bone architecture, however there exists a need for improved ability to quantify structurally relevant parameters of bone in non-standard bone shapes. Here we present a new analysis approach based on open-source semi-automatic methods combining image processing, solid modeling, and numerical calculations to analyze bone tissue at a more granular level using μCT image data from a mouse digit model of bone regeneration. Examining interior architecture, growth patterning, spatial mineral content, and mineral density distribution, these methods are then applied to two types of 6-month old mouse digits - 1) those prior to amputation injury (unamputated) and 2) those 42 days after amputation when bone has regenerated. Results show regenerated digits exhibit increased inner void fraction, decreased patterning, different patterns of spatial mineral distribution, and increased mineral density values when compared to unamputated bone. Our approach demonstrates the utility of this new analysis technique in assessment of non-standard bone models, such as the regenerated bone of the digit, and aims to bring a deeper level of analysis with an open-source, integrative platform to the greater bone community.

Control of mesenchymal cell fate via application of FGF-8b in vitro

In order to develop strategies to regenerate complex tissues in mammals, understanding the role of signaling in regeneration competent species and mammalian development is of critical importance. Fibroblast growth factor 8 (FGF-8) signaling has an essential role in limb morphogenesis and blastema outgrowth. Therefore, we aimed to study the effect of FGF-8b on the proliferation and differentiation of mesenchymal stem cells (MSCs), which have tremendous potential for therapeutic use of cell-based therapy. Rat adipose derived stem cells (ADSCs) and muscle progenitor cells (MPCs) were isolated and cultured in growth medium and various types of differentiation medium (osteogenic, chondrogenic, adipogenic, tenogenic, and myogenic medium) with or without FGF-8b supplementation. We found that FGF-8b induced robust proliferation regardless of culture medium. Genes related to limb development were upregulated in ADSCs by FGF-8b supplementation. Moreover, FGF-8b enhanced chondrogenic differentiation and suppressed adipogenic and tenogenic differentiation in ADSCs. Osteogenic differentiation was not affected by FGF-8b supplementation. FGF-8b was found to enhance myofiber formation in rat MPCs. Overall, this study provides foundational knowledge on the effect of FGF-8b in the proliferation and fate determination of MSCs and provides insight in its potential efficacy for musculoskeletal therapies.

Heart regeneration: beyond new muscle and vessels

The most striking consequence of a heart attack is the loss of billions of heart muscle cells, alongside damage to the associated vasculature. The lost cardiovascular tissue is replaced by scar formation, which is non-functional and results in pathological remodelling of the heart and ultimately heart failure. It is, therefore, unsurprising that the heart regeneration field has centred efforts to generate new muscle and blood vessels through targeting cardiomyocyte proliferation and angiogenesis following injury. However, combined insights from embryological studies and regenerative models, alongside the adoption of -omics technology, highlight the extensive heterogeneity of cell types within the forming or re-forming heart and the significant crosstalk arising from non-muscle and non-vessel cells. In this review, we focus on the roles of fibroblasts, immune, conduction system, and nervous system cell populations during heart development and we consider the latest evidence supporting a function for these diverse lineages in contributing to regeneration following heart injury. We suggest that the emerging picture of neurologically, immunologically, and electrically coupled cell function calls for a wider-ranging combinatorial approach to heart regeneration.

Generation of metabolically functional hepatocyte-like cells from dedifferentiated fat cells by Foxa2, Hnf4a and Sall1 transduction

Mature adipocyte-derived dedifferentiated fat (DFAT) cells have been identified to possess similar multipotency to mesenchymal stem cells, but a method for converting DFAT cells into hepatocytes was previously unknown. Here, using comprehensive analysis of gene expression profiles, we have extracted three transcription factors, namely Foxa2, Hnf4a and Sall1 (FHS), that can convert DFAT cells into hepatocytes. Hepatogenic induction has converted FHS-infected DFAT cells into an epithelial-like morphological state and promoted the expression of hepatocyte-specific features. Furthermore, the DFAT-derived hepatocyte-like (D-Hep) cells catalyzed the detoxification of several compounds. These results indicate that the transduction of DFAT cells with three genes, which were extracted by comprehensive gene expression analysis, efficiently generated D-Hep cells with detoxification abilities similar to those of primary hepatocytes. Thus, D-Hep cells may be useful as a new cell source for surrogate hepatocytes and may be applied to drug discovery studies, such as hepatotoxicity screening and drug metabolism tests.

Appendage regeneration in anamniotes utilizes genes active during larval-metamorphic stages that have been lost or altered in amniotes: The case for studying lizard tail regeneration

This review elaborates the idea that organ regeneration derives from specific evolutionary histories of vertebrates. Regenerative ability depends on genomic regulation of genes specific to the life-cycles that have differentially evolved in anamniotes and amniotes. In aquatic environments, where fish and amphibians live, one or multiple metamorphic transitions occur before the adult stage is reached. Each transition involves the destruction and remodeling of larval organs that are replaced with adult organs. After organ injury or loss in adult anamniotes, regeneration uses similar genes and developmental process than those operating during larval growth and metamorphosis. Therefore, the broad presence of regenerative capability across anamniotes is possible because generating new organs is included in their life history at metamorphic stages. Soft hyaluronate-rich regenerative blastemas grow in submersed or in hydrated environments, that is, essential conditions for regeneration, like during development. In adult anamniotes, the ability to regenerate different organs decreases in comparison to larval stages and becomes limited during aging. Comparisons of genes activated during metamorphosis and regeneration in anamniotes identify key genes unique to these processes, and include thyroid, wnt and non-coding RNAs developmental pathways. In the terrestrial environment, some genes or developmental pathways for metamorphic transitions were lost during amniote evolution, determining loss of regeneration. Among amniotes, the formation of soft and hydrated blastemas only occurs in lizards, a morphogenetic process that evolved favoring their survival through tail autotomy, leading to a massive although imperfect regeneration of the tail. Deciphering genes activity during lizard tail regeneration would address future attempts to recreate in other amniotes regenerative blastemas that grow into variably completed organs.

Senescence in Wound Repair: Emerging Strategies to Target Chronic Healing Wounds

Cellular senescence is a fundamental stress response that restrains tumour formation. Yet, senescence cells are also present in non-cancerous states, accumulating exponentially with chronological age and contributing to age- and diabetes-related cellular dysfunction. The identification of hypersecretory and phagocytic behaviours in cells that were once believed to be non-functional has led to a recent explosion of senescence research. Here we discuss the profound, and often opposing, roles identified for short-lived vs. chronic tissue senescence. Transiently induced senescence is required for development, regeneration and acute wound repair, while chronic senescence is widely implicated in tissue pathology. We recently demonstrated that sustained senescence contributes to impaired diabetic healing via the CXCR2 receptor, which when blocked promotes repair. Further studies have highlighted the beneficial effects of targeting a range of senescence-linked processes to fight disease. Collectively, these findings hold promise for developing clinically viable strategies to tackle senescence in chronic wounds and other cutaneous pathologies.

Midkine-a functions as a universal regulator of proliferation during epimorphic regeneration in adult zebrafish

Zebrafish have the ability to regenerate damaged cells and tissues by activating quiescent stem and progenitor cells or reprogramming differentiated cells into regeneration-competent precursors. Proliferation among the cells that will functionally restore injured tissues is a fundamental biological process underlying regeneration. Midkine-a is a cytokine growth factor, whose expression is strongly induced by injury in a variety of tissues across a range of vertebrate classes. Using a zebrafish Midkine-a loss of function mutant, we evaluated regeneration of caudal fin, extraocular muscle and retinal neurons to investigate the function of Midkine-a during epimorphic regeneration. In wildtype zebrafish, injury among these tissues induces robust proliferation and rapid regeneration. In Midkine-a mutants, the initial proliferation in each of these tissues is significantly diminished or absent. Regeneration of the caudal fin and extraocular muscle is delayed; regeneration of the retina is nearly completely absent. These data demonstrate that Midkine-a is universally required in the signaling pathways that convert tissue injury into the initial burst of cell proliferation. Further, these data highlight differences in the molecular mechanisms that regulate epimorphic regeneration in zebrafish.

Cells Isolated from Regenerating Caudal Fin of Sparus aurata Can Differentiate into Distinct Bone Cell Lineages

Teleosts have the ability to regenerate their caudal fin upon amputation. A highly proliferative mass of undifferentiated cells called blastema forms beneath wound epidermis and differentiates to regenerate all missing parts of the fin. To date, the origin and fate of the blastema is not completely understood. However, current hypotheses suggest that the blastema is comprised of lineage-restricted dedifferentiated cells. To investigate the differentiation capacity of regenerating fin-derived cells, primary cultures were initiated from the explants of 2-days post-amputation (dpa) regenerates of juvenile gilthead seabream (Sparus aurata). These cells were subcultured for over 30 passages and were named as BSa2. After 10 passages they were characterized for their ability to differentiate towards different bone cell lineages and mineralize their extracellular matrix, through immunocytochemistry, histology, and RT-PCR. Exogenous DNA was efficiently delivered into these cells by nucleofection. Assessment of lineage-specific markers revealed that BSa2 cells were capable of osteo/chondroblastic differentiation. BSa2 cells were also found to be capable of osteoclastic differentiation, as demonstrated through TRAP-specific staining and pit resorption assay. Here, we describe the development of the first successful cell line viz., BSa2, from S. aurata 2-dpa regenerating caudal fins, which has the ability of multilineage differentiation and is capable of in vitro mineralization. The availability of such in vitro cell systems has the potential to stimulate research on the mechanisms of cell differentiation during fin regeneration and provide new insights into the mechanisms of bone formation.

Functional ectodermal organ regeneration as the next generation of organ replacement therapy

In this decade, substantial progress in the fields of developmental biology and stem cell biology has ushered in a new era for three-dimensional organ regenerative therapy. The emergence of novel three-dimensional cell manipulation technologies enables the effective mimicking of embryonic organ germ formation using the fate-determined organ-inductive potential of epithelial and mesenchymal stem cells. This advance shows great potential for the regeneration of functional organs with substitution of complete original function in situ. Organoids generated from multipotent stem cells or tissue stem cells via establishment of an organ-forming field can only partially recover original organ function owing to the size limitation; they are considered ‘mini-organs’. Nevertheless, they hold great promise to realize regenerative medicine. In particular, regeneration of a functional salivary gland and an integumentary organ system by orthotopic and heterotopic implantation of organoids clearly points to the future direction of organ regeneration research. In this review, we describe multiple strategies and recent progress in regenerating functional three-dimensional organs, focusing on ectodermal organs, and discuss their potential and future directions to achieve organ replacement therapy as a next-generation regenerative medicine.

Deciphering Cell Fate Decision by Integrated Single-Cell Sequencing Analysis

Cellular differentiation is a common underlying feature of all multicellular organisms through which naïve cells progressively become fate restricted and develop into mature cells with specialized functions. A comprehensive understanding of the regulatory mechanisms of cell fate choices during de- velopment, regeneration, homeostasis, and disease is a central goal of mod- ern biology. Ongoing rapid advances in single-cell biology are enabling the exploration of cell fate specification at unprecedented resolution. Here, we review single-cell RNA sequencing and sequencing of other modalities as methods to elucidate the molecular underpinnings of lineage specification. We specifically discuss how the computational tools available to reconstruct lineage trajectories, quantify cell fate bias, and perform dimensionality re- duction for data visualization are providing new mechanistic insights into the process of cell fate decision. Studying cellular differentiation using single- cell genomic tools is paving the way for a detailed understanding of cellular behavior in health and disease.

Evolution of epimorphosis in mammals

Mammalian epimorphic regeneration is rare and digit tip regeneration in mice is the best-studied model for a multi-tissue regenerative event that involves blastema formation. Digit tip regeneration parallels human fingertip regeneration, thus understanding the details of this response can provide insight into developing strategies to expand the potential of human regeneration. Following amputation, the digit stump undergoes a strong histolytic response involving osteoclast-mediated bone degradation that is spatially and temporally linked to the expansion of blastema osteoprogenitor cells. Blastemal differentiation occurs via direct intramembranous ossification. Although robust, digit regeneration is imperfect: The amputated cortical bone is replaced with woven bone and there is excessive bone regeneration restricted to the dorsal-ventral axis. Ontogenetic and phylogenetic analysis of digit regeneration in amphibians and mammals raise the possibility that mammalian blastema is a product of convergent evolution and we hypothesize that digit tip regeneration evolved from a nonregenerative precondition. A model is proposed in which the mammalian blastema evolved in part from an adaptation of two bone repair strategies (the bone remodeling cycle and fracture healing) both of which are conserved across tetrapod vertebrates. The view that epimorphic regeneration evolved in mammals from a nonregenerative precondition is supported by recent studies demonstrating that complex regenerative responses can be induced from a number of different nonregenerative amputation wounds by specific modification of the healing response.

Comparative iTRAQ proteomics revealed proteins associated with lobed fin regeneration in Bichirs

Background

Polypterus senegalus can fully regenerate its pectoral lobed fins, including a complex endoskeleton, with remarkable precision. However, despite the enormous potential of this species for use in medical research, its regeneration mechanisms remain largely unknown.

Methods

To identify the differentially expressed proteins (DEPs) during the early stages of lobed fin regeneration in P. senegalus, we performed a differential proteomic analysis using isobaric tag for relative and absolute quantitation (iTRAQ) approach based quantitative proteome from the pectoral lobed fins at 3 time points. Furthermore, we validated the changes in protein expression with multiple-reaction monitoring (MRM) analysis.

Results

The experiment yielded a total of 3177 proteins and 15,091 unique peptides including 1006 non-redundant (nr) DEPs. Of these, 592 were upregulated while 349 were downregulated after lobed fin amputation when compared to the original tissue. Bioinformatics analyses showed that the DEPs were mainly associated with Ribosome and RNA transport, metabolic, ECM-receptor interaction, Golgi and endoplasmic reticulum, DNA replication, and Regulation of actin cytoskeleton.

Conclusions

To our knowledge, this is the first proteomic research to investigate alterations in protein levels and affected pathways in bichirs’ lobe-fin/limb regeneration. In addition, our study demonstrated a highly dynamic regulation during lobed fin regeneration in P. senegalus. These results not only provide a comprehensive dataset on differentially expressed proteins during the early stages of lobe-fin/limb regeneration but also advance our understanding of the molecular mechanisms underlying lobe-fin/limb regeneration.

Sirtuin 3 deficiency does not impede digit regeneration in mice

The mitochondrial deacetylase sirtuin 3 (SIRT3) is thought to be one of the main contributors to metabolic flexibility-promoting mitochondrial energy production and maintaining homeostasis. In bone, metabolic profiles are tightly regulated and the loss of SIRT3 has deleterious effects on bone volume in vivo and on osteoblast differentiation in vitro. Despite the prominent role of this protein in bone stem cell proliferation, metabolic activity, and differentiation, the importance of SIRT3 for regeneration after bone injury has never been reported. We show here, using the mouse digit amputation model, that SIRT3 deficiency has no impact on the regenerative capacity and architecture of bone and soft tissue. Regeneration occurs in SIRT3 deficient mice in spite of the reduced oxidative metabolic profile of the periosteal cells. These data suggest that bone regeneration, in contrast to homeostatic bone turnover, is not reliant upon active SIRT3, and our results highlight the need to examine known roles of SIRT3 in the context of injury.

Tail regeneration in Lepidosauria as an exception to the generalized lack of organ regeneration in amniotes

The present review hypothesizes that during the transition from water to land, amniotes lost part of the genetic program for metamorphosis utilized in larvae of their amphibian ancestors, a program that in extant fish and amphibians allows organ regeneration. The direct development of amniotes, with their growth from embryos to adults, occurred with the elimination of larval stages, increases the efficiency of immune responses and the complexity of nervous circuits. In amniotes, T-cells and macrophages likely eliminate embryonic-larval antigens that are replaced with the definitive antigens of adult organs. Among lepidosaurians numerous lizard families during the Permian and Triassic evolved the process of tail autotomy to escape predation, followed by tail regeneration. Autotomy limits inflammation allowing the formation of a regenerative blastema rich in the immunosuppressant and hygroscopic hyaluronic acid. Expression loss of developmental genes for metamorphosis and segmentation in addition to an effective immune system, determined an imperfect regeneration of the tail. Genes involved in somitogenesis were likely lost or are inactivated and the axial skeleton and muscles of the original tail are replaced with a nonsegmented cartilaginous tube and segmental myotomes. Lack of neural genes, negative influence of immune system, and isolation of the regenerating spinal cord within the cartilaginous tube impede the production of nerve and glial cells, and a stratified spinal cord with ganglia. Tissue and organ regeneration in other body regions of lizards and other reptiles is relatively limited, like in the other amniotes, although the cartilage shows a higher regenerative capability than in mammals.

Effects of long non-coding RNA uc.245 on cardiomyocyte-like differentiation in P19 cells via FOG2

Each year, cardiac diseases may cause a high morbidity and mortality worldwide. Long non-coding RNAs (lncRNAs) that contained ultra-conserved elements (UCEs) may play important roles on cardiomyocytes differentiation. Further investigations underlying mechanisms of lncRNA-UC regulating embryonic heart development are necessary. In this study, we investigated the effects of lnc-uc.245 on proliferation, migration, apoptosis, and cardiomyocyte-like differentiation in P19 cells with DMSO stimulation, and hypothesized that lnc-uc.245 would influence cardiomyocytes differentiation via FOG2. Lentiviral vectors of pGPU6/GFP/Neo-uc.245 and pGPU6/GFP/Neo-shRNA-uc.245 were respectively transfected into P19 cells to overexpress or silence uc.245. MTT assay, Annexin V-FITC/PI double-staining, scratch test and transwell assay were performed and the results showed that uc.245 overexpression could significantly suppress P19 cell proliferation, migration, cardiomyocyte-like differentiation but promote cell apoptosis. Contrarily, sh-uc.245 treatment caused the opposite changes. Uc.245 overexpression obviously downregulated the expression of cardiomyogenic-specific molecular markers (cTnI, ANP, α-MHC, Nkx2.5, GATA4, MEF2C) but remarkably upregulated the expression of FOG2. Subsequently, we transfected the recombinant vectors loaded FOG2 or shRRNA-FOG2 into P19 cells to further address the functional significance of FOG2 in uc.245-regulated cardiomyocyte-like differentiation. Interestingly, we found that overexpressing of FOG2 promoted cell proliferation, migration, and inhibited apoptosis both in uc.245 overexpressed and silenced P19 cells, especially in uc.245 silenced cell line. In addition, sh-FOG2 promoted cardiomyocyte-like differentiation and upregulated the expression of cardiomyogenic-specific markers at the gene and protein levels both in uc.245 overexpressed and silenced P19 cells. Similarly, this upregulation effect of sh-FOG2 was more obvious after uc.245 silencing. These findings suggest that FOG2 is a key mediator during uc.245-regulated differentiation of P19 cells into cardiomyocytes. It is expected that lnc-uc.245/FOG2 will become a promising therapeutic target for cardiac diseases.

Ablation of BMP signaling hampers the blastema formation in Poecilia latipinna by dysregulating the extracellular matrix remodeling and cell cycle turnover

Bone morphogenetic proteins play a pivotal role in the epimorphic regeneration in vertebrates. Blastema formation is central to the epimorphic regeneration and crucially determines its fate. Despite an elaborate understanding of importance of Bone morphogenetic protein signaling in regeneration, its specific role during the blastema formation remains to be addressed. Regulatory role of BMP signaling during blastema formation was investigated using LDN193189, a potent inhibitor of BMP receptors. The study involved morphological observation, in vivo proliferation assay by incorporation of BrdU, comet assay, qRT-PCR and western blot. Blastemal outgrowth was seen reduced due to LDN193189 treatment, typified by dimensional differences, reduced number of proliferating cells and decreased levels of PCNA. Additionally, proapoptotic markers were found to be upregulated signifying a skewed cellular turnover. Further, the cell migration was seen obstructed and ECM remodeling was disturbed as well. These findings were marked by differential transcript as well as protein expressions of the key signaling and regulatory components, their altered enzymatic activities and other microscopic as well as molecular characterizations. Our results signify, for the first time, that BMP signaling manifests its effect on blastema formation by controlling the pivotal cellular processes possibly via PI3K/AKT. Our results indicate the pleiotropic role of BMPs specifically during blastema formation in regulating cell migration, cell proliferation and apoptosis, and lead to the generation of a molecular regulatory map of determinative molecules.

Hox gene expression determines cell fate of adult periosteal stem/progenitor cells

Hox genes are evolutionarily conserved transcription factors that during embryonic development function as master regulators of positional identity. In postnatal life, the function of Hox proteins is less clear: Hox genes are expressed during tissue repair, but in this context their function(s) are largely unknown. Here we show that Hox genes are expressed in periosteal stem/progenitor cells in a distribution similar to that during embryonic development. Using unbiased sequencing, we established that periosteal stem/progenitor cells from distinct anatomic sites within the skeleton significantly differ in their transcriptome, and that Hox expression status best defines these differences. Lastly, we provide evidence that Hox gene expression is one potential mechanism that maintains periosteal stem/progenitor cells in a more primitive, tripotent state, while suppression of Hox genes leads to fate changes with loss of tripotency. Together, our data describe an adult role of Hox genes other than positional identity, and the modulatory role of Hox genes in fate decisions may offer potential druggable targets for the treatment of fractures, non-unions and bone defects.

Potential Clinical Risk of Inflammation and Toxicity from Rare-Earth Nanoparticles in Mice

Background:

Nanotechnology is emerging as a promising tool to perform noninvasive therapy and optical imaging. However, nanomedicine may pose a potential risk of toxicity during in vivo applications. In this study, we aimed to investigate the potential toxicity of rare-earth nanoparticles (RENPs) using mice as models.

Methods:

We synthesized RENPs through a typical co-precipitation method. Institute of Cancer Research (ICR) mice were randomly divided into seven groups including a control group and six experimental groups (10 mice per group). ICR mice were intravenously injected with bare RENPs at a daily dose of 0, 0.5, 1.0, and 1.5 mg/kg for 7 days. To evaluate the toxicity of these nanoparticles in mice, magnetic resonance imaging (MRI) was performed to assess their uptake in mice. In addition, hematological and biochemical analyses were conducted to evaluate any impairment in the organ functions of ICR mice. The analysis of variance (ANOVA) followed by a one-way ANOVA test was used in this study. A repeated measures' analysis was used to determine any significant differences in white blood cell (WBC), alanine aminotransferase (ALT), and creatinine (CREA) levels at different evaluation times in each group.

Results:

We demonstrated the successful synthesis of two different sizes (10 nm and 100 nm) of RENPs. Their physical properties were characterized by transmission electron microscopy and a 980 nm laser diode. Results of MRI study revealed the distribution and circulation of the RENPs in the liver. In addition, the hematological analysis found an increase of WBCs to (8.69 ± 0.85) × 10⁹/L at the 28^th day, which is indicative of inflammation in the mouse treated with 1.5 mg/kg NaYbF₄:Er nanoparticles. Furthermore, the biochemical analysis indicated increased levels of ALT ([64.20 ± 15.50] U/L) and CREA ([27.80 ± 3.56] μmol/L) at the 28^th day, particularly those injected with 1.5 mg/kg NaYbF₄:Er nanoparticles. These results suggested the physiological and pathological damage caused by these nanoparticles to the organs and tissues of mice, especially to liver and kidney.

Conclusion:

The use of bare RENPs may cause possible hepatotoxicity and nephritictoxicity in mice.

The Bacterial Metabolite Indole Inhibits Regeneration of the Planarian Flatworm Dugesia japonica

Planarian flatworms have been used for over a century as models for regeneration. Planarians live in aquatic environments with constant exposure to microbes, but the mechanisms by which bacteria may mediate planarian regeneration are largely unknown. We characterized the microbiome of laboratory populations of the planarian Dugesia japonica and determined how individual bacteria impact D. japonica regeneration. Eight to ten taxa in the phyla Bacteroidetes and Proteobacteria consistently occur across planarian colonies housed in different research laboratories. Individual members of the D. japonica microbiome can delay regeneration including the development of eye spots and blastema formation. The microbial metabolite indole is produced in significant quantities by two bacteria that are consistently found in the D. japonica microbiome and contributes to delays in regeneration. Collectively, these results provide a baseline understanding of the bacteria associated with the planarian D. japonica and demonstrate how metabolite production by host-associated microbes can affect regeneration.

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The planarian worm Dugesia japonica is colonized by Bacteroidetes and Proteobacteria

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Many of these bacteria can be cultured and experimentally manipulated

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Some bacteria can inhibit regeneration, including eye and blastema formation

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Indole produced by planarian-associated bacteria contributes to regeneration delays

Advances in Functional Restoration of the Lacrimal Glands

The lacrimal glands produce tears to support a healthy homeostatic environment on the ocular surface. The lacrimal gland dysfunction characteristic of dry eye disease causes ocular discomfort and visual disturbances and in severe cases can result in a loss of vision. The demand for adequate restoration of lacrimal gland function has been intensified due to advances in stem cell biology, developmental biology, and bioengineering technologies. In addition to conventional therapies, including artificial tears, tear alternatives (such as autologous serum eye drops) and salivary gland transplantation, a regenerative medicine approach has been identified as a novel strategy to restore the function of the lacrimal gland. Recent studies have demonstrated the potential of progenitor cell injection therapy to repair the tissue of the lacrimal glands. A current three-dimensional (3D) tissue engineering technique has been shown to regenerate a secretory gland structure by reproducing reciprocal epithelial-mesenchymal interactions during ontogenesis in vitro and in vivo. A novel direct reprogramming method has suggested a possibility to induce markers in the lacrimal gland developmental process from human pluripotent stem cells. The development of this method is supported by advances in our understanding of gene expression and regulatory networks involved in the development and differentiation of the lacrimal glands. Engineering science has proposed a medical device to stimulate tearing and a bio-hybrid scaffold to reconstruct the 3D lacrimal gland structure. In this review, we will summarize recent bioengineering advances in lacrimal gland regeneration toward the functional restoration of the lacrimal glands as a future dry eye therapy.

Hedgehog and Wnt Signaling Pathways Regulate Tail Regeneration

Urodele amphibians have a tremendous capacity for the regeneration of appendages, including limb and tail, following injury. While studies have focused on the cellular and morphological changes during appendicular regeneration, the signaling mechanisms that govern these cytoarchitectural changes during the regenerative response are unclear. In this study, we describe the essential role of hedgehog (Hh) and Wnt signaling pathways following tail amputation in the newt. Quantitative PCR studies revealed that members of both the Hh and Wnt signaling pathways, including the following: shh, ihh, ptc-1, wnt-3a, β-catenin, axin2, frizzled (frzd)-1, and frzd-2 transcripts, were induced following injury. Continuous pharmacological-mediated inhibition of Hh signaling resulted in spike-like regenerates with no evidence of tissue patterning, whereas activation of Hh signaling enhanced the regenerative process. Pharmacological-mediated temporal inhibition experiments demonstrated that the Hh-mediated patterning of the regenerating tail occurs early during regeneration and Hh signals are continuously required for proliferation of the blastemal progenitors. BrdU incorporation and PCNA immunohistochemical studies demonstrated that Hh signaling regulates the cellular proliferation of the blastemal cells following amputation. Similarly, Wnt inhibition resulted in perturbed regeneration, whereas its activation promoted tail regeneration. Using an inhibitor-activator strategy, we demonstrated that the Wnt pathway is likely to be upstream of the Hh pathway and together these signaling pathways function in a coordinated manner to facilitate tail regeneration. Mechanistically, the Wnt signaling pathway activated the Hh signaling pathway that included ihh and ptc-1 during the tail regenerative process. Collectively, our results demonstrate the absolute requirement of signaling pathways that are essential in the regulation of tail regeneration.

Niche signals and transcription factors involved in tissue-resident macrophage development

Tissue-resident macrophages form an essential part of the first line of defense in all tissues of the body. Next to their immunological role, they play an important role in maintaining tissue homeostasis. Recently, it was shown that they are primarily of embryonic origin. During embryogenesis, precursors originating in the yolk sac and fetal liver colonize the embryonal tissues where they develop into mature tissue-resident macrophages. Their development is governed by two distinct sets of transcription factors. First, in the pre-macrophage stage, a core macrophage program is established by lineage-determining transcription factors. Under the influence of tissue-specific signals, this core program is refined by signal-dependent transcription factors. This nurturing by the niche allows the macrophages to perform tissue-specific functions. In the last 15 years, some of these niche signals and transcription factors have been identified. However, detailed insight in the exact mechanism of development is still lacking.

Leucine/glutamine and v-ATPase/lysosomal acidification via mTORC1 activation are required for position-dependent regeneration

In animal regeneration, control of position-dependent cell proliferation is crucial for the complete restoration of patterned appendages in terms of both, shape and size. However, detailed mechanisms of this process are largely unknown. In this study, we identified leucine/glutamine and v-ATPase/lysosomal acidification, via mechanistic target of rapamycin complex 1 (mTORC1) activation, as effectors of amputation plane-dependent zebrafish caudal fin regeneration. mTORC1 activation, which functions in cell proliferation, was regulated by lysosomal acidification possibly via v-ATPase activity at 3 h post amputation (hpa). Inhibition of lysosomal acidification resulted in reduced growth factor-related gene expression and suppression of blastema formation at 24 and 48 hpa, respectively. Along the proximal-distal axis, position-dependent lysosomal acidification and mTORC1 activation were observed from 3 hpa. We also report that Slc7a5 (L-type amino acid transporter), whose gene expression is position-dependent, is necessary for mTORC1 activation upstream of lysosomal acidification during fin regeneration. Furthermore, treatment with leucine and glutamine, for both proximal and distal fin stumps, led to an up-regulation in cell proliferation via mTORC1 activation, indicating that leucine/glutamine signaling possesses the ability to change the position-dependent regeneration. Our findings reveal that leucine/glutamine and v-ATPase/lysosomal acidification via mTORC1 activation are required for position-dependent zebrafish fin regeneration.

Newt cells secrete extracellular vesicles with therapeutic bioactivity in mammalian cardiomyocytes

Newts can regenerate amputated limbs and cardiac tissue, unlike mammals which lack broad regenerative capacity. Several signaling pathways involved in cell proliferation, differentiation and survival during newt tissue regeneration have been elucidated, however the factors that coordinate signaling between cells, as well as the conservation of these factors in other animals, are not well defined. Here we report that media conditioned by newt limb explant cells (A1 cells) protect mammalian cardiomyocytes from oxidative stress-induced apoptosis. The cytoprotective effect of A1-conditioned media was negated by exposing A1 cells to GW4869, which suppresses the generation of extracellular vesicles (EVs). A1-EVs are similar in diameter (~100–150 nm), structure, and share several membrane surface and cargo proteins with mammalian exosomes. However, isolated A1-EVs contain significantly higher levels of both RNA and protein per particle than mammalian EVs. Additionally, numerous cargo RNAs and proteins are unique to A1-EVs. Of particular note, A1-EVs contain numerous mRNAs encoding nuclear receptors, membrane ligands, as well as transcription factors. Mammalian cardiomyocytes treated with A1-EVs showed increased expression of genes in the PI3K/AKT pathway, a pivotal player in survival signaling. We conclude that newt cells secrete EVs with diverse, distinctive RNA and protein contents. Despite ~300 million years of evolutionary divergence between newts and mammals, newt EVs confer cytoprotective effects on mammalian cardiomyocytes.

Homeostatic and regenerative neurogenesis in salamanders

Large-scale regeneration in the adult central nervous system is a unique capacity of salamanders among tetrapods. Salamanders can replace neuronal populations, repair damaged nerve fibers and restore tissue architecture in retina, brain and spinal cord, leading to functional recovery. The underlying mechanisms have long been difficult to study due to the paucity of available genomic tools. Recent technological progress, such as genome sequencing, transgenesis and genome editing provide new momentum for systematic interrogation of regenerative processes in the salamander central nervous system. Understanding central nervous system regeneration also entails designing the appropriate molecular, cellular, and behavioral assays. Here we outline the organization of salamander brain structures. With special focus on ependymoglial cells, we integrate cellular and molecular processes of neurogenesis during developmental and adult homeostasis as well as in various injury models. Wherever possible, we correlate developmental and regenerative neurogenesis to the acquisition and recovery of behaviors. Throughout the review we place the findings into an evolutionary context for inter-species comparisons.