Acute inflammation initiates the regenerative response in the adult zebrafish brain

Microglia processes associate with diffusely injured axons following mild traumatic brain injury in the micro pig

Background

Mild traumatic brain injury (mTBI) is an all too common occurrence that exacts significant personal and societal costs. The pathophysiology of mTBI is complex, with reports routinely correlating diffuse axonal injury (DAI) with prolonged morbidity. Progressive chronic neuroinflammation has also recently been correlated to morbidity, however, the potential association between neuroinflammatory microglia and DAI is not well understood. The majority of studies exploring neuroinflammatory responses to TBI have focused on more chronic phases of injury involving phagocytosis associated with Wallerian change. Little, however, is known regarding the neuroinflammatory response seen acutely following diffuse mTBI and its potential relationship to early DAI. Additionally, while inflammation is drastically different in rodents compared to humans, pigs and humans share very similar inflammatory profiles and responses.

Methods

In the current study, we employed a modified central fluid percussion model in micro pigs. Using this model of diffuse mTBI, paired with various immunohistological endpoints, we assessed the potential association between acute thalamic DAI and neuroinflammation 6 h following injury.

Results

Injured micro pigs displayed substantial axonal damage reflected in the presence of APP+ proximal axonal swellings, which were particularly prominent in the thalamus. In companion, the same thalamic sites displayed extensive neuroinflammation, which was observed using Iba-1 immunohistochemistry. The physical relationship between microglia and DAI, assessed via confocal 3D analysis, revealed a dramatic increase in the number of Iba-1+ microglial processes that contacted APP+ proximal axonal swellings compared to uninjured myelinated thalamic axons in sham animals.

Conclusions

In aggregate, these studies reveal acute microglial process convergence on proximal axonal swellings undergoing DAI, an interaction not previously recognized in the literature. These findings transform our understanding of acute neuroinflammation following mTBI and may suggest its potential as a diagnostic and/or a therapeutic target.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0405-6) contains supplementary material, which is available to authorized users.

In vivo three-dimensional characterization of the adult zebrafish brain using a 1325 nm spectral-domain optical coherence tomography system with the 27 frame/s video rate

In this study, a spectral-domain optical coherence tomography (SD-OCT) system was used for noninvasive imaging of the adult zebrafish brain. Based on a 1325 nm light source and two high-speed galvo mirrors, our SD-OCT system can offer a large field of view of brain morphology with high resolution (12 μm axial and 13 μm lateral) at video rate (27 frame/s). In vivo imaging of both the control and injured brain was performed using adult zebrafish model. The recovered results revealed that olfactory bulb, optic commissure, telencephalon, tectum opticum, cerebellum, medulla, preglomerular complex and posterior tuberculum could be clearly identified in the cross-sectional SD-OCT images of the adult zebrafish brain. The reconstructed results also suggested that SD-OCT can be used for diagnosis and monitoring of traumatic brain injury. In particular, we found the reconstructed volumetric SD-OCT images enable a comprehensive three-dimensional characterization of the control or injured brain in the intact zebrafish.

MicroRNA dysregulation in response to RARβ2 inhibition reveals a negative feedback loop between MicroRNAs 1, 133a, and RARβ2 during tail and spinal cord regeneration in the adult newt

BACKGROUND

The molecular events underlying epimorphic regeneration of the adult urodele amphibian tail and caudal spinal cord are undetermined. Given the dynamic nature of gene expression control by retinoic acid (RA) signaling and the pleiotropic effects of microRNAs (miRNAs) on multiple mRNA targets in this complex system, we examined whether RA signaling through a specific receptor, RARβ2, alters expression of select miRNAs during spinal cord regeneration.

RESULTS

An initial screen identified 18 highly conserved miRNAs dysregulated in regenerating tail and spinal cord tissues after inhibition of RARβ2 signaling with a selective antagonist, LE135. miRNAs let-7c, miR-1, and miR-223 were expressed within the ependymoglial cells, coincident spatially with the expression of RARβ2. Altering the expression pattern of these three miRNAs led to a significant inhibition of caudal ependymal tube outgrowth by 21 days post tail amputation. We demonstrated that miR-1 targets the 3'-untranslated region of RARβ2 mRNA in vitro; and in vivo, up-regulation of miR-1 led to a significant decrease in RARβ2 protein.

CONCLUSIONS

These and previous data suggest that miR-1 and miR-133a, both members of the same miRNA gene cluster, may participate with RARβ2 in a negative feedback loop contributing to the regulation of the ependymal response after tail amputation.

Structural and functional evolution of 2',3'-cyclic nucleotide 3'-phosphodiesterase

2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) is an abundant membrane-associated enzyme within the vertebrate myelin sheath. While the physiological function of CNPase still remains to be characterized in detail, it is known - in addition to its in vitro enzymatic activity - to interact with other proteins, small molecules, and membrane surfaces. From an evolutionary point of view, it can be deduced that CNPase is not restricted to myelin-forming cells or vertebrate tissues. Its evolution has involved gene fusion, addition of other small segments with distinct functions, such as membrane attachment, and possibly loss of function at the polynucleotide kinase-like domain. Currently, it is unclear whether the enzymatic function of the conserved phosphodiesterase domain in vertebrate myelin has a physiological role, or if CNPase could actually function - like many other classical myelin proteins - in a more structural role. This article is part of a Special Issue entitled SI: Myelin Evolution.

Regeneration, Plasticity, and Induced Molecular Programs in Adult Zebrafish Brain

Regenerative capacity of the brain is a variable trait within animals. Aquatic vertebrates such as zebrafish have widespread ability to renew their brains upon damage, while mammals have—if not none—very limited overall regenerative competence. Underlying cause of such a disparity is not fully evident; however, one of the reasons could be activation of peculiar molecular programs, which might have specific roles after injury or damage, by the organisms that regenerate. If this hypothesis is correct, then there must be genes and pathways that (a) are expressed only after injury or damage in tissues, (b) are biologically and functionally relevant to restoration of neural tissue, and (c) are not detected in regenerating organisms. Presence of such programs might circumvent the initial detrimental effects of the damage and subsequently set up the stage for tissue redevelopment to take place by modulating the plasticity of the neural stem/progenitor cells. Additionally, if transferable, those “molecular mechanisms of regeneration” could open up new avenues for regenerative therapies of humans in clinical settings. This review focuses on the recent studies addressing injury/damage-induced molecular programs in zebrafish brain, underscoring the possibility of the presence of genes that could be used as biomarkers of neural plasticity and regeneration.

Skeletal callus formation is a nerve-independent regenerative response to limb amputation in mice and Xenopus

To clarify the mechanism of limb regeneration that differs between mammals (non-regenerative) and amphibians (regenerative), responses to limb amputation and the accessory limb inducible surgery (accessory limb model, ALM) were compared between mice and Xenopus, focusing on the events leading to blastema formation. In both animals, cartilaginous calluses were formed around the cut edge of bones after limb amputation. They not only are morphologically similar but show other similarities, such as growth driven by undifferentiated cell proliferation and macrophage-dependent and nerve-independent induction. It appears that amputation callus formation is a common nerve-independent regenerative response in mice and Xenopus. In contrast, the ALM revealed that the wound epithelium (WE) in Xenopus was innervated by many regenerating axons when a severed nerve ending was placed underneath it, whereas only a few axons were found within the WE in mice. Since nerves are involved in induction of the regeneration-permissive WE in amphibians, whether or not nerves can interact with the WE might be one of the key processes separating successful nerve-dependent blastema formation in Xenopus and failure in mice.

New neurons for injured brains? The emergence of new genetic model organisms to study brain regeneration

Neuronal circuits in the adult brain have long been viewed as static and stable. However, research in the past 20 years has shown that specialized regions of the adult brain, which harbor adult neural stem cells, continue to produce new neurons in a wide range of species. Brain plasticity is also observed after injury. Depending on the extent and permissive environment of neurogenic regions, different organisms show great variability in their capacity to replace lost neurons by endogenous neurogenesis. In Zebrafish and Drosophila, the formation of new neurons from progenitor cells in the adult brain was only discovered recently. Here, we compare properties of adult neural stem cells, their niches and regenerative responses from mammals to flies. Current models of brain injury have revealed that specific injury-induced genetic programs and comparison of neuronal fitness are implicated in brain repair. We highlight the potential of these recently implemented models of brain regeneration to identify novel regulators of stem cell activation and regenerative neurogenesis.

Tendon regeneration and scar formation: The concept of scarless healing

Tendon healing is characterized by the formation of fibrovascular scar tissue, as tendon has very little intrinsic regenerative capacity. This creates a substantial clinical challenge in the setting of large, chronic tears seen clinically. Interest in regenerative healing seen in amphibians and certain strains of mice has arisen in response to the biological behavior of tendon tissue. Bone is also a model of tissue regeneration as healing bone will achieve the mechanical and histologic characteristics of the original tissue. The ultimate goal of the study of genes and mechanisms that contribute to true tissue regeneration is to ultimately attempt to manipulate the expression of those genes and activate these mechanisms in the setting of tendon injury and repair. Clearly, further research is needed to bring this to the forefront, however, study of scarless healing has potential to have meaningful application to tendon healing.

Combination of inflammation-related cytokines promotes long-term muscle stem cell expansion

Muscle stem cells (MuSCs, satellite cells) are the major contributor to muscle regeneration. Like most adult stem cells, long-term expansion of MuSCs in vitro is difficult. The in vivo muscle regeneration abilities of MuSCs are quickly lost after culturing in vitro, which prevents the potential applications of MuSCs in cell-based therapies. Here, we establish a system to serially expand MuSCs in vitro for over 20 passages by mimicking the endogenous microenvironment. We identified that the combination of four pro-inflammatory cytokines, IL-1α, IL-13, TNF-α, and IFN-γ, secreted by T cells was able to stimulate MuSC proliferation in vivo upon injury and promote serial expansion of MuSCs in vitro. The expanded MuSCs can replenish the endogenous stem cell pool and are capable of repairing multiple rounds of muscle injuries in vivo after a single transplantation. The establishment of the in vitro system provides us a powerful method to expand functional MuSCs to repair muscle injuries.

Ozone promotes regeneration by regulating the inflammatory response in zebrafish

Ozone is thought to advance wound healing by inhibiting inflammation, but the mechanism of this phenomenon has not been determined. Although the zebrafish is often used in regeneration experiments, there has been no report of zebrafish treated with ozonated water. We successfully established a zebrafish model of ozonated water treatment and demonstrate that ozonated water stimulates the regeneration of the zebrafish caudal fin, its mechanism, and time dependence. The growth rate of the caudal fin and the number of neutrophils migrating to the caudal fin wound after resection were higher in the experimental (ozonated) group than in the control group, preliminarily confirming that ozone-promoted regeneration is related to the stimulation of an early inflammatory response by ozone. Ozone modulated the expression of tumor necrosis factor-α (TNF-α) in two ways by regulating interleukin 10 (IL-10) expression. Therefore, ozone promotes tissue regeneration by regulating the inflammatory pathways. This effect of ozone in an experimental zebrafish model is demonstrated for the first time, confirming its promotion of wound healing and the mechanism of its effect in tissue regeneration. These results will open up new directions for ozone and regeneration research.

Immune modulation of stem cells and regeneration

The immune system, best known as the first line of defense against invading pathogens, is integral to tissue development, homeostasis, and wound repair. In recent years, there has been a growing appreciation that cellular and humoral components of the immune system also contribute to regeneration of damaged tissues, including limbs, skeletal muscle, heart, and the nervous system. Here, we discuss key findings that implicate inflammatory cells and their secreted factors in tissue replacement after injury via stem cells and other reparative mechanisms. We highlight clinical conditions that are amenable to immune-mediated regeneration and suggest immune targeting strategies for tissue regeneration.

Signaling networks organizing regenerative growth of the zebrafish fin

In contrast to mammals, adult salamanders and fish can completely regenerate their appendages after amputation. The cellular and molecular mechanisms underlying this fascinating phenomenon are beginning to emerge, including substantial progress in the identification of signals that control regenerative growth of the zebrafish caudal fin. Despite the fairly simple architecture of the fin, the regulation of its regeneration is complex. Many signals, including fibroblast growth factor (FGF), Wnt, Hedgehog (Hh), retinoic acid (RA), Notch, bone morphogenic protein (BMP), activin, and insulin-like growth factor (IGF), are required for regeneration. Much work needs to be done to dissect tissue-specific functions of these pathways and how they interact, but Wnt/β-catenin signaling is already emerging as a central player. Surprisingly, Wnt/β-catenin signaling appears to largely indirectly control epidermal patterning, progenitor cell proliferation, and osteoblast maturation via regulation of a multitude of secondary signals.

Efficient Cargo Delivery into Adult Brain Tissue Using Short Cell-Penetrating Peptides

Zebrafish brains can regenerate lost neurons upon neurogenic activity of the radial glial progenitor cells (RGCs) that reside at the ventricular region. Understanding the molecular events underlying this ability is of great interest for translational studies of regenerative medicine. Therefore, functional analyses of gene function in RGCs and neurons are essential. Using cerebroventricular microinjection (CVMI), RGCs can be targeted efficiently but the penetration capacity of the injected molecules reduces dramatically in deeper parts of the brain tissue, such as the parenchymal regions that contain the neurons. In this report, we tested the penetration efficiency of five known cell-penetrating peptides (CPPs) and identified two- polyR and Trans - that efficiently penetrate the brain tissue without overt toxicity in a dose-dependent manner as determined by TUNEL staining and L-Plastin immunohistochemistry. We also found that polyR peptide can help carry plasmid DNA several cell diameters into the brain tissue after a series of coupling reactions using DBCO-PEG4-maleimide-based Michael's addition and azide-mediated copper-free click reaction. Combined with the advantages of CVMI, such as rapidness, reproducibility, and ability to be used in adult animals, CPPs improve the applicability of the CVMI technique to deeper parts of the central nervous system tissues.

Wnt/ß-catenin signaling is required for radial glial neurogenesis following spinal cord injury

Spinal cord injury results in permanent sensorimotor loss in mammals, in part due to a lack of injury-induced neurogenesis. The regeneration of neurons depends upon resident neural progenitors, which in zebrafish persist throughout the central nervous system as radial glia. However the molecular mechanisms regulating spinal cord progenitors remain uncharacterized. Wnt/ß-catenin signaling is necessary for the regenerative response of multiple tissues in zebrafish as well as other vertebrates, but it is not known whether the pathway has a role in spinal cord regeneration. Here we show that spinal radial glia exhibit Wnt/ß-catenin activity as they undergo neurogenesis following transection. We then use Cre-mediated lineage tracing to label the progeny of radial glia and show that Wnt/ß-catenin signaling is required for progenitors to differentiate into neurons. Finally, we show that axonal regrowth after injury also requires Wnt/ß-catenin signaling, suggesting coordinated roles for the pathway in functional recovery. Our data thus establish Wnt/ß-catenin pathway activation as a necessary step in spinal cord regeneration.

Using zebrafish as the model organism to understand organ regeneration

The limited regenerative capacity of several organs, such as central nervous system (CNS), heart and limb in mammals makes related major diseases quite difficult to recover. Therefore, dissection of the cellular and molecular mechanisms underlying organ regeneration is of great scientific and clinical interests. Tremendous progression has already been made after extensive investigations using several model organisms for decades. Unfortunately, distance to the final achievement of the goal still remains. Recently, zebrafish became a popular model organism for the deep understanding of regeneration based on its powerful regenerative capacity, in particular the organs that are limitedly regenerated in mammals. Additionally, zebrafish are endowed with other advantages good for the study of organ regeneration. This review summarizes the recent progress in the study of zebrafish organ regeneration, in particular regeneration of fin, heart, CNS, and liver as the representatives. We also discuss reasons of the reduced regenerative capacity in higher vertebrate, the roles of inflammation during regeneration, and the difference between organogenesis and regeneration.

Effects of inflammation on stem cells: together they strive?

Inflammation entails a complex set of defense mechanisms acting in concert to restore the homeostatic balance in organisms after damage or pathogen invasion. This immune response consists of the activity of various immune cells in a highly complex manner. Inflammation is a double-edged sword as it is reported to have both detrimental and beneficial consequences. In this review, we discuss the effects of inflammation on stem cell activity, focusing primarily on neural stem/progenitor cells in mammals and zebrafish. We also give a brief overview of the effects of inflammation on other stem cell compartments, exemplifying the positive and negative role of inflammation on stemness. The majority of the chronic diseases involve an unremitting phase of inflammation due to improper resolution of the initial pro-inflammatory response that impinges on the stem cell behavior. Thus, understanding the mechanisms of crosstalk between the inflammatory milieu and tissue-resident stem cells is an important basis for clinical efforts. Not only is it important to understand the effect of inflammation on stem cell activity for further defining the etiology of the diseases, but also better mechanistic understanding is essential to design regenerative therapies that aim at micromanipulating the inflammatory milieu to offset the negative effects and maximize the beneficial outcomes.

Emerging roles of gut microbiota and the immune system in the development of the enteric nervous system

The enteric nervous system (ENS) consists of neurons and glial cells that differentiate from neural crest progenitors. During embryogenesis, development of the ENS is controlled by the interplay of neural crest cell-intrinsic factors and instructive cues from the surrounding gut mesenchyme. However, postnatal ENS development occurs in a different context, which is characterized by the presence of microbiota and an extensive immune system, suggesting an important role of these factors on enteric neural circuit formation and function. Initial reports confirm this idea while further studies in this area promise new insights into ENS physiology and pathophysiology.

Dedifferentiation, Transdifferentiation, and Proliferation: Mechanisms Underlying Cardiac Muscle Regeneration in Zebrafish

The adult mammalian heart is increasingly recognized as a regenerative organ with a measurable capacity to replenish cardiomyocytes throughout its lifetime, illuminating the possibility of stimulating endogenous regenerative capacity to treat heart diseases. Unlike mammals, certain vertebrates possess robust capacity for regenerating a damaged heart, providing a model to understand how regeneration could be augmented in injured human hearts. Facilitated by its rich history in the study of heart development, the teleost zebrafish Danio rerio has been established as a robust model to investigate the underlying mechanism of cardiac regeneration. This review discusses the current understanding of the endogenous mechanisms behind cardiac regeneration in zebrafish, with a particular focus on cardiomyocyte dedifferentiation, transdifferentiation, and proliferation.

Intravital correlated microscopy reveals differential macrophage and microglial dynamics during resolution of neuroinflammation

Many brain diseases involve activation of resident and peripheral immune cells to clear damaged and dying neurons. Which immune cells respond in what way to cues related to brain disease, however, remains poorly understood. To elucidate these in vivo immunological events in response to brain cell death we used genetically targeted cell ablation in zebrafish. Using intravital microscopy and large-scale electron microscopy, we defined the kinetics and nature of immune responses immediately following injury. Initially, clearance of dead cells occurs by mononuclear phagocytes, including resident microglia and macrophages of peripheral origin, whereas amoeboid microglia are exclusively involved at a later stage. Granulocytes, on the other hand, do not migrate towards the injury. Remarkably, following clearance, phagocyte numbers decrease, partly by phagocyte cell death and subsequent engulfment of phagocyte corpses by microglia. Here, we identify differential temporal involvement of microglia and peripheral macrophages in clearance of dead cells in the brain, revealing the chronological sequence of events in neuroinflammatory resolution. Remarkably, recruited phagocytes undergo cell death and are engulfed by microglia. Because adult zebrafish treated at the larval stage lack signs of pathology, it is likely that this mode of resolving immune responses in brain contributes to full tissue recovery. Therefore, these findings suggest that control of such immune cell behavior could benefit recovery from neuronal damage.

Comprehensive expression map of transcription regulators in the adult zebrafish telencephalon reveals distinct neurogenic niches

The zebrafish has become a model to study adult vertebrate neurogenesis. In particular, the adult telencephalon has been an intensely studied structure in the zebrafish brain. Differential expression of transcriptional regulators (TRs) is a key feature of development and tissue homeostasis. Here we report an expression map of 1,202 TR genes in the telencephalon of adult zebrafish. Our results are summarized in a database with search and clustering functions to identify genes expressed in particular regions of the telencephalon. We classified 562 genes into 13 distinct patterns, including genes expressed in the proliferative zone. The remaining 640 genes displayed unique and complex patterns of expression and could thus not be grouped into distinct classes. The neurogenic ventricular regions express overlapping but distinct sets of TR genes, suggesting regional differences in the neurogenic niches in the telencephalon. In summary, the small telencephalon of the zebrafish shows a remarkable complexity in TR gene expression.

The adult zebrafish telencephalon has become a model to study neurogenesis. We established the expression pattern of more than 1200 transcription regulators (TR) in the adult telencephalon. The neurogenic regions express overlapping but distinct sets of TR genes suggesting regional differences in the neurogenic potential. J. Comp. Neurol. 523:1202–1221, 2015. © 2015 Wiley Periodicals, Inc.

Circulating cells contribute to cardiomyocyte regeneration after injury

RATIONALE

The contribution of bone marrow-borne hematopoietic cells to the ischemic myocardium has been documented. However, a pivotal study reported no evidence of myocardial regeneration from hematopoietic-derived cells. The study did not take into account the possible effect of early injury-induced signaling as the test mice were parabiotically paired to partners immediately after surgery-induced myocardial injury when cross-circulation has not yet developed.

OBJECTIVE

To re-evaluate the role of circulating cells in the injured myocardium.

METHODS AND RESULTS

By combining pulse-chase labeling and parabiosis model, we show that circulating cells derived from the parabiont expressed cardiac-specific markers in the injured myocardium. Genetic fate mapping also revealed that circulating hematopoietic cells acquired cardiac cell fate by means of cell fusion and transdifferentiation.

CONCLUSIONS

These results suggest that circulating cells participate in cardiomyocyte regeneration in a mouse model of parabiosis when the circulatory system is fully developed before surgery-induced heart injury.

Age-related changes in tissue macrophages precede cardiac functional impairment

Cardiac tissue macrophages (cTMs) are abundant in the murine heart but the extent to which the cTM phenotype changes with age is unknown. This study characterizes aging-dependent phenotypic changes in cTM subsets. Using the Cx₃cr1^GFP/+ mouse reporter line where GFP marks cTMs, and the tissue macrophage marker Mrc1, we show that two major cardiac tissue macrophage subsets, Mrc1⁻GFP^hi and Mrc1⁺GFP^hi cTMs, are present in the young (<10 week old) mouse heart, and a third subset, Mrc1⁺GFP^lo, comprises ~50% of total Mrc1⁺ cTMs from 30 weeks of age. Immunostaining and functional assays show that Mrc1⁺ cTMs are the principal myeloid sentinels in the mouse heart and that they retain proliferative capacity throughout life. Gene expression profiles of the two Mrc1⁺ subsets also reveal that Mrc1⁺GFP^lo cTMs have a decreased number of immune response genes (Cx₃cr1, Lpar6, CD9, Cxcr4, Itga6 and Tgfβr1), and an increased number of fibrogenic genes (Ltc4s, Retnla, Fgfr1, Mmp9 and Ccl24), consistent with a potential role for cTMs in cardiac fibrosis. These findings identify early age-dependent gene expression changes in cTMs, with significant implications for cardiac tissue injury responses and aging-associated cardiac fibrosis.

Differential requirement for irf8 in formation of embryonic and adult macrophages in zebrafish

Interferon regulatory factor 8 (Irf8) is critical for mammalian macrophage development and innate immunity, but its role in teleost myelopoiesis remains incompletely understood. In particular, genetic tools to analyze the role of Irf8 in zebrafish macrophage development at larval and adult stages are lacking. We generated irf8 null mutants in zebrafish using TALEN-mediated targeting. Our analysis defines different requirements for irf8 at different stages. irf8 is required for formation of all macrophages during primitive and transient definitive hematopoiesis, but not during adult-phase definitive hematopoiesis starting at 5-6 days postfertilization. At early stages, irf8 mutants have excess neutrophils and excess cell death in pu.1-expressing myeloid cells. Macrophage fates were recovered in irf8 mutants after wildtype irf8 expression in neutrophil and macrophage lineages, suggesting that irf8 regulates macrophage specification and survival. In juvenile irf8 mutant fish, mature macrophages are present, but at numbers significantly reduced compared to wildtype, indicating an ongoing requirement for irf8 after embryogenesis. As development progresses, tissue macrophages become apparent in zebrafish irf8 mutants, with the possible exception of microglia. Our study defines distinct requirement for irf8 in myelopoiesis before and after transition to the adult hematopoietic system.

Magnetic-activated cell sorting (MACS) can be used as a large-scale method for establishing zebrafish neuronal cell cultures

Neuronal cell cultures offer a crucial tool to mechanistically analyse regeneration in the nervous system. Despite the increasing importance of zebrafish (Danio rerio) as an in vivo model in neurobiological and biomedical research, in vitro approaches to the nervous system are lagging far behind and no method is currently available for establishing enriched neuronal cell cultures. Here we show that magnetic-activated cell sorting (MACS) can be used for the large-scale generation of neuronal-restricted progenitor (NRP) cultures from embryonic zebrafish. Our findings provide a simple and semi-automated method that is likely to boost the use of neuronal cell cultures as a tool for the mechanistic dissection of key processes in neuronal regeneration and development.

Immune cell dynamics in the CNS: Learning from the zebrafish

A major question in research on immune responses in the brain is how the timing and nature of these responses influence physiology, pathogenesis or recovery from pathogenic processes. Proper understanding of the immune regulation of the human brain requires a detailed description of the function and activities of the immune cells in the brain. Zebrafish larvae allow long-term, noninvasive imaging inside the brain at high-spatiotemporal resolution using fluorescent transgenic reporters labeling specific cell populations. Together with recent additional technical advances this allows an unprecedented versatility and scope of future studies. Modeling of human physiology and pathology in zebrafish has already yielded relevant insights into cellular dynamics and function that can be translated to the human clinical situation. For instance, in vivo studies in the zebrafish have provided new insight into immune cell dynamics in granuloma formation in tuberculosis and the mechanisms involving treatment resistance. In this review, we highlight recent findings and novel tools paving the way for basic neuroimmunology research in the zebrafish. GLIA 2015;63:719-735.

Neurogenesis in the embryonic and adult brain: same regulators, different roles

Neurogenesis persists in adult mammals in specific brain areas, known as neurogenic niches. Adult neurogenesis is highly dynamic and is modulated by multiple physiological stimuli and pathological states. There is a strong interest in understanding how this process is regulated, particularly since active neuronal production has been demonstrated in both the hippocampus and the subventricular zone (SVZ) of adult humans. The molecular mechanisms that control neurogenesis have been extensively studied during embryonic development. Therefore, we have a broad knowledge of the intrinsic factors and extracellular signaling pathways driving proliferation and differentiation of embryonic neural precursors. Many of these factors also play important roles during adult neurogenesis, but essential differences exist in the biological responses of neural precursors in the embryonic and adult contexts. Because adult neural stem cells (NSCs) are normally found in a quiescent state, regulatory pathways can affect adult neurogenesis in ways that have no clear counterpart during embryogenesis. BMP signaling, for instance, regulates NSC behavior both during embryonic and adult neurogenesis. However, this pathway maintains stem cell proliferation in the embryo, while it promotes quiescence to prevent stem cell exhaustion in the adult brain. In this review, we will compare and contrast the functions of transcription factors (TFs) and other regulatory molecules in the embryonic brain and in adult neurogenic regions of the adult brain in the mouse, with a special focus on the hippocampal niche and on the regulation of the balance between quiescence and activation of adult NSCs in this region.

Molecular magnetic resonance imaging of brain-immune interactions

Although the blood-brain barrier (BBB) was thought to protect the brain from the effects of the immune system, immune cells can nevertheless migrate from the blood to the brain, either as a cause or as a consequence of central nervous system (CNS) diseases, thus contributing to their evolution and outcome. Accordingly, as the interface between the CNS and the peripheral immune system, the BBB is critical during neuroinflammatory processes. In particular, endothelial cells are involved in the brain response to systemic or local inflammatory stimuli by regulating the cellular movement between the circulation and the brain parenchyma. While neuropathological conditions differ in etiology and in the way in which the inflammatory response is mounted and resolved, cellular mechanisms of neuroinflammation are probably similar. Accordingly, neuroinflammation is a hallmark and a decisive player of many CNS diseases. Thus, molecular magnetic resonance imaging (MRI) of inflammatory processes is a central theme of research in several neurological disorders focusing on a set of molecules expressed by endothelial cells, such as adhesion molecules (VCAM-1, ICAM-1, P-selectin, E-selectin, …), which emerge as therapeutic targets and biomarkers for neurological diseases. In this review, we will present the most recent advances in the field of preclinical molecular MRI. Moreover, we will discuss the possible translation of molecular MRI to the clinical setting with a particular emphasis on myeloperoxidase imaging, autologous cell tracking, and targeted iron oxide particles (USPIO, MPIO).

How to make neurons--thoughts on the molecular logic of neurogenesis in the central nervous system

Neuronal differentiation relies on a set of interconnected molecular events to achieve the differentiation of pan-neuronal hallmarks, together with neuronal subtype-specific features. Here, we propose a conceptual framework for these events, based on recent findings. This framework encompasses a dimension in time during development, progressing from early master regulators to later expressed effector genes and terminal selector genes. As a horizontal intersection, we propose the action of permissive fate determinants that are critical in allowing progression through the above transcriptional phases. Typically, these are widely expressed and often interact with the chromatin remodeling machinery. We conclude by discussing this model in the context of the direct fate conversion of various somatic cells into neurons.

In vivo high-content evaluation of three-dimensional scaffolds biocompatibility

While developing tissue engineering strategies, inflammatory response caused by biomaterials is an unavoidable aspect to be taken into consideration, as it may be an early limiting step of tissue regeneration approaches. We demonstrate the application of flat and flexible films exhibiting patterned high-contrast wettability regions as implantable platforms for the high-content in vivo study of inflammatory response caused by biomaterials. Screening biomaterials by using high-throughput platforms is a powerful method to detect hit spots with promising properties and to exclude uninteresting conditions for targeted applications. High-content analysis of biomaterials has been mostly restricted to in vitro tests where crucial information is lost, as in vivo environment is highly complex. Conventional biomaterials implantation requires the use of high numbers of animals, leading to ethical questions and costly experimentation. Inflammatory response of biomaterials has also been highly neglected in high-throughput studies. We designed an array of 36 combinations of biomaterials based on an initial library of four polysaccharides. Biomaterials were dispensed onto biomimetic superhydrophobic platforms with wettable regions and processed as freeze-dried three-dimensional scaffolds with a high control of the array configuration. These chips were afterward implanted subcutaneously in Wistar rats. Lymphocyte recruitment and activated macrophages were studied on-chip, by performing immunocytochemistry in the miniaturized biomaterials after 24 h and 7 days of implantation. Histological cuts of the surrounding tissue of the implants were also analyzed. Localized and independent inflammatory responses were detected. The integration of these data with control data proved that these chips are robust platforms for the rapid screening of early-stage in vivo biomaterials' response.

Axon-Schwann cell interactions during peripheral nerve regeneration in zebrafish larvae

Background

Peripheral nerve injuries can severely affect the way that animals perceive signals from the surrounding environment. While damage to peripheral axons generally has a better outcome than injuries to central nervous system axons, it is currently unknown how neurons re-establish their target innervations to recover function after injury, and how accessory cells contribute to this task. Here we use a simple technique to create reproducible and localized injury in the posterior lateral line (pLL) nerve of zebrafish and follow the fate of both neurons and Schwann cells.

Results

Using pLL single axon labeling by transient transgene expression, as well as transplantation of glial precursor cells in zebrafish larvae, we individualize different components in this system and characterize their cellular behaviors during the regenerative process. Neurectomy is followed by loss of Schwann cell differentiation markers that is reverted after nerve regrowth. We show that reinnervation of lateral line hair cells in neuromasts during pLL nerve regeneration is a highly dynamic process with promiscuous yet non-random target recognition. Furthermore, Schwann cells are required for directional extension and fasciculation of the regenerating nerve. We provide evidence that these cells and regrowing axons are mutually dependant during early stages of nerve regeneration in the pLL. The role of ErbB signaling in this context is also explored.

Conclusion

The accessibility of the pLL nerve and the availability of transgenic lines that label this structure and their synaptic targets provides an outstanding in vivo model to study the different events associated with axonal extension, target reinnervation, and the complex cellular interactions between glial cells and injured axons during nerve regeneration.

Transcriptional response to cardiac injury in the zebrafish: systematic identification of genes with highly concordant activity across in vivo models

Background

Zebrafish is a clinically-relevant model of heart regeneration. Unlike mammals, it has a remarkable heart repair capacity after injury, and promises novel translational applications. Amputation and cryoinjury models are key research tools for understanding injury response and regeneration in vivo. An understanding of the transcriptional responses following injury is needed to identify key players of heart tissue repair, as well as potential targets for boosting this property in humans.

Results

We investigated amputation and cryoinjury in vivo models of heart damage in the zebrafish through unbiased, integrative analyses of independent molecular datasets. To detect genes with potential biological roles, we derived computational prediction models with microarray data from heart amputation experiments. We focused on a top-ranked set of genes highly activated in the early post-injury stage, whose activity was further verified in independent microarray datasets. Next, we performed independent validations of expression responses with qPCR in a cryoinjury model. Across in vivo models, the top candidates showed highly concordant responses at 1 and 3 days post-injury, which highlights the predictive power of our analysis strategies and the possible biological relevance of these genes. Top candidates are significantly involved in cell fate specification and differentiation, and include heart failure markers such as periostin, as well as potential new targets for heart regeneration. For example, ptgis and ca2 were overexpressed, while usp2a, a regulator of the p53 pathway, was down-regulated in our in vivo models. Interestingly, a high activity of ptgis and ca2 has been previously observed in failing hearts from rats and humans.

Conclusions

We identified genes with potential critical roles in the response to cardiac damage in the zebrafish. Their transcriptional activities are reproducible in different in vivo models of cardiac injury.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-852) contains supplementary material, which is available to authorized users.

Neurogenesis and inflammation after ischemic stroke: what is known and where we go from here

This review covers the pathogenesis of ischemic stroke and future directions regarding therapeutic options after injury. Ischemic stroke is a devastating disease process affecting millions of people worldwide every year. The mechanisms underlying the pathophysiology of stroke are not fully understood but there is increasing evidence demonstrating the contribution of inflammation to the drastic changes after cerebral ischemia. This inflammation not only immediately affects the infarcted tissue but also causes long-term damage in the ischemic penumbra. Furthermore, the interaction between inflammation and subsequent neurogenesis is not well understood but the close relationship between these two processes has garnered significant interest in the last decade or so. Current approved therapy for stroke involving pharmacological thrombolysis is limited in its efficacy and new treatment strategies need to be investigated. Research aimed at new therapies is largely about transplantation of neural stem cells and using endogenous progenitor cells to promote brain repair. By understanding the interaction between inflammation and neurogenesis, new potential therapies could be developed to further establish brain repair mechanisms.

Glial cells as progenitors and stem cells: new roles in the healthy and diseased brain

The diverse functions of glial cells prompt the question to which extent specific subtypes may be devoted to a specific function. We discuss this by reviewing one of the most recently discovered roles of glial cells, their function as neural stem cells (NSCs) and progenitor cells. First we give an overview of glial stem and progenitor cells during development; these are the radial glial cells that act as NSCs and other glial progenitors, highlighting the distinction between the lineage of cells in vivo and their potential when exposed to a different environment, e.g., in vitro. We then proceed to the adult stage and discuss the glial cells that continue to act as NSCs across vertebrates and others that are more lineage-restricted, such as the adult NG2-glia, the most frequent progenitor type in the adult mammalian brain, that remain within the oligodendrocyte lineage. Upon certain injury conditions, a distinct subset of quiescent astrocytes reactivates proliferation and a larger potential, clearly demonstrating the concept of heterogeneity with distinct subtypes of, e.g., astrocytes or NG2-glia performing rather different roles after brain injury. These new insights not only highlight the importance of glial cells for brain repair but also their great potential in various aspects of regeneration.

Inflammation and Its Correlates in Regenerative Wound Healing: An Alternate Perspective

Objective: The wound healing response may be viewed as partially overlapping sets of two physiological processes, regeneration and wound repair with the former overrepresented in some lower species such as newts and the latter more typical of mammals. A robust and quantitative model of regenerative healing has been described in Murphy Roths Large (MRL) mice in which through-and-through ear hole wounds in the ear pinna leads to scarless healing and replacement of all tissue through blastema formation and including cartilage. Since these mice are naturally autoimmune and display many aspects of an enhanced inflammatory response, we chose to examine the inflammatory status during regenerative ear hole closure and observed that inflammation has a clear positive effect on regenerative healing. Approach: The inflammatory gene expression patterns (Illumina microarrays) of early healing ear tissue from regenerative MRL and nonregenerative C57BL/6 (B6) strains are presented along with a survey of innate inflammatory cells found in this tissue type pre and postinjury. The role of inflammation on healing is tested using a COX-2 inhibitor. Innovation and Conclusion: We conclude that (1) enhanced inflammation is consistent with, and probably necessary, for a full regenerative response and (2) the inflammatory gene expression and cell distribution patterns suggest a novel mast cell population with markers found in both immature and mature mast cells that may be a key component of regeneration.

Lysophospholipid acyltransferases and eicosanoid biosynthesis in zebrafish myeloid cells

Eicosanoids derived from the enzymatic oxidation of arachidonic acid play important roles in a large number of physiological and pathological processes in humans. Many animal and cellular models have been used to investigate the intricate mechanisms regulating their biosynthesis and actions. Zebrafish is a widely used model to study the embryonic development of vertebrates. It expresses homologs of the key enzymes involved in eicosanoid production, and eicosanoids have been detected in extracts from adult or embryonic fish. In this study we prepared cell suspensions from kidney marrow, the main hematopoietic organ in fish. Upon stimulation with calcium ionophore, these cells produced eicosanoids including PGE2, LTB4, 5-HETE and, most abundantly, 12-HETE. They also produced small amounts of LTB5 derived from eicosapentaenoic acid. These eicosanoids were also produced in kidney marrow cells stimulated with ATP, and this production was greatly enhanced by preincubation with thimerosal, an inhibitor of arachidonate reacylation into phospholipids. Microsomes from these cells exhibited acyltransferase activities consistent with expression of MBOAT5/LPCAT3 and MBOAT7/LPIAT1, the main arachidonoyl-CoA:lysophospholipid acyltransferases. In summary, this work introduces a new cellular model to study the regulation of eicosanoid production through a phospholipid deacylation-reacylation cycle from a well-established, versatile vertebrate model species.

Macrophages modulate adult zebrafish tail fin regeneration

Neutrophils and macrophages, as key mediators of inflammation, have defined functionally important roles in mammalian tissue repair. Although recent evidence suggests that similar cells exist in zebrafish and also migrate to sites of injury in larvae, whether these cells are functionally important for wound healing or regeneration in adult zebrafish is unknown. To begin to address these questions, we first tracked neutrophils (lyzC⁺, mpo⁺) and macrophages (mpeg1⁺) in adult zebrafish following amputation of the tail fin, and detailed a migratory timecourse that revealed conserved elements of the inflammatory cell response with mammals. Next, we used transgenic zebrafish in which we could selectively ablate macrophages, which allowed us to investigate whether macrophages were required for tail fin regeneration. We identified stage-dependent functional roles of macrophages in mediating fin tissue outgrowth and bony ray patterning, in part through modulating levels of blastema proliferation. Moreover, we also sought to detail molecular regulators of inflammation in adult zebrafish and identified Wnt/β-catenin as a signaling pathway that regulates the injury microenvironment, inflammatory cell migration and macrophage phenotype. These results provide a cellular and molecular link between components of the inflammation response and regeneration in adult zebrafish.

Sensory hair cell regeneration in the zebrafish lateral line

BACKGROUND

Damage or destruction of sensory hair cells in the inner ear leads to hearing or balance deficits that can be debilitating, especially in older adults. Unfortunately, the damage is permanent, as regeneration of the inner ear sensory epithelia does not occur in mammals.

RESULTS

Zebrafish and other non-mammalian vertebrates have the remarkable ability to regenerate sensory hair cells and understanding the molecular and cellular basis for this regenerative ability will hopefully aid us in designing therapies to induce regeneration in mammals. Zebrafish not only possess hair cells in the ear but also in the sensory lateral line system. Hair cells in both organs are functionally analogous to hair cells in the inner ear of mammals. The lateral line is a mechanosensory system found in most aquatic vertebrates that detects water motion and aids in predator avoidance, prey capture, schooling, and mating. Although hair cell regeneration occurs in both the ear and lateral line, most research to date has focused on the lateral line due to its relatively simple structure and accessibility.

CONCLUSIONS

Here we review the recent discoveries made during the characterization of hair cell regeneration in zebrafish.

Axolotls with an under- or oversupply of neural crest can regulate the sizes of their dorsal root ganglia to normal levels

How animals adjust the size of their organs is a fundamental, enduring question in biology. Here we manipulate the amount of neural crest (NC) precursors for the dorsal root ganglia (DRG) in axolotl. We produce embryos with an under- or over-supply of pre-migratory NC in order to find out if DRG can regulate their sizes during development. Axolotl embryos are perfectly suitable for this research. Firstly, they are optimal for microsurgical manipulations and tissue repair. Secondly, they possess, unlike most other vertebrates, only one neural crest string located on top of the neural tube. This condition and position enables NC cells to migrate to either side of the embryo and participate in the regulation of NC cell distribution. We show that size compensation of DRG in axolotl occurs in 2 cm juveniles after undersupply of NC (up-regulation) and in 5 cm juveniles after oversupply of NC (down-regulation). The size of DRG is likely to be regulated locally within the DRG and not via adaptations of the pre-migratory NC or during NC cell migration. Ipsi- and contralateral NC cell migration occurs both in embryos with one and two neural folds, and contralateral migration of NC is the only source for contralateral DRG formation in embryos with only one neural fold. Compensatory size increase is accompanied by an increase in cell division of a DRG precursor pool (PCNA+/SOX2-), rather than by DRG neurons or glial cells. During compensatory size decrease, increased apoptosis and reduced proliferation of DRG cells are observed.

Stab wound injury of the zebrafish adult telencephalon: a method to investigate vertebrate brain neurogenesis and regeneration

Adult zebrafish have an amazing capacity to regenerate their central nervous system after injury. To investigate the cellular response and the molecular mechanisms involved in zebrafish adult central nervous system (CNS) regeneration and repair, we developed a zebrafish model of adult telencephalic injury. In this approach, we manually generate an injury by pushing an insulin syringe needle into the zebrafish adult telencephalon. At different post injury days, fish are sacrificed, their brains are dissected out and stained by immunohistochemistry and/or in situ hybridization (ISH) with appropriate markers to observe cell proliferation, gliogenesis, and neurogenesis. The contralateral unlesioned hemisphere serves as an internal control. This method combined for example with RNA deep sequencing can help to screen for new genes with a role in zebrafish adult telencephalon neurogenesis, regeneration, and repair.

Non-mammalian model systems for studying neuro-immune interactions after spinal cord injury

Mammals exhibit poor recovery after injury to the spinal cord, where the loss of neurons and neuronal connections can be functionally devastating. In contrast, it has long been appreciated that many non-mammalian vertebrate species exhibit significant spontaneous functional recovery after spinal cord injury (SCI). Identifying the biological responses that support an organism's inability or ability to recover function after SCI is an important scientific and medical question. While recent advances have been made in understanding the responses to SCI in mammals, we remain without an effective clinical therapy for SCI. A comparative biological approach to understanding responses to SCI in non-mammalian vertebrates will yield important insights into mechanisms that promote recovery after SCI. Presently, mechanistic studies aimed at elucidating responses, both intrinsic and extrinsic to neurons, that result in different regenerative capacities after SCI across vertebrates are just in their early stages. There are several inhibitory mechanisms proposed to impede recovery from SCI in mammals, including reactive gliosis and scarring, myelin associated proteins, and a suboptimal immune response. One hypothesis to explain the robust regenerative capacity of several non-mammalian vertebrates is a lack of some or all of these inhibitory signals. This review presents the current knowledge of immune responses to SCI in several non-mammalian species that achieve anatomical and functional recovery after SCI. This subject is of growing interest, as studies increasingly show both beneficial and detrimental roles of the immune response following SCI in mammals. A long-term goal of biomedical research in all experimental models of SCI is to understand how to promote functional recovery after SCI in humans. Therefore, understanding immune responses to SCI in non-mammalian vertebrates that achieve functional recovery spontaneously may identify novel strategies to modulate immune responses in less regenerative species and promote recovery after SCI.

Estradiol treatment decreases cell proliferation in the neurogenic zones of adult female zebrafish (Danio rerio) brain

While estrogens are known to play a crucial role in the neurogenesis of the mammalian and avian brain, their role in teleost adult proliferation pattern is not yet fully understood. The present study aimed to determine the estrogen effects in adult brain proliferation zones, using zebrafish, as a model organism. Indeed, teleost fish brain provides a unique adult neurogenesis model, based on its extensive proliferation, contrasting the restricted adult telencephalic neurogenesis observed in birds and mammals. To determine the effect of estrogens, 17-β estradiol was administrated for 7 days in adult female zebrafish, followed by bromodeoxyuridine (BrdU)-immunohistochemistry and double immunofluorescence. Stereological analyses of the BrdU-positive cells within the neurogenic zones, showed region-specific decreases of actively proliferating cells in the estrogen-treated animals, compared to matched controls. Interestingly, the most prominent estradiol effects were found in the number of cycling cells of the ventral nucleus of ventral telencephalic area (Vv) and cerebellar areas. Significant decreases were also determined in the dorso-lateral telencephalic, preoptic and dorsal hypothalamic areas. In contrast, medial dorsal telencephalic, caudal (Hc) and ventral (Hv) hypothalamic areas were unaffected by estrogen treatment. The majority of the BrdU-labeled cells were found to co-express PCNA proliferating marker in Hc, Hv and Vv. Additionally, a population of proliferating cells co-expressed the early neuronal marker TOAD in all areas studied. These results provide significant evidence on the 17-β estradiol impact on adult neurogenesis, down-regulating the fast-cycling and post-mitotic cells within the female zebrafish brain neurogenetic zones.

Early activation of STAT3 regulates reactive astrogliosis induced by diverse forms of neurotoxicity

Astrogliosis, a cellular response characterized by astrocytic hypertrophy and accumulation of GFAP, is a hallmark of all types of central nervous system (CNS) injuries. Potential signaling mechanisms driving the conversion of astrocytes into “reactive” phenotypes differ with respect to the injury models employed and can be complicated by factors such as disruption of the blood-brain barrier (BBB). As denervation tools, neurotoxicants have the advantage of selective targeting of brain regions and cell types, often with sparing of the BBB. Previously, we found that neuroinflammation and activation of the JAK2-STAT3 pathway in astrocytes precedes up regulation of GFAP in the MPTP mouse model of dopaminergic neurotoxicity. Here we show that multiple mechanistically distinct mouse models of neurotoxicity (MPTP, AMP, METH, MDA, MDMA, KA, TMT) engender the same neuroinflammatory and STAT3 activation responses in specific regions of the brain targeted by each neurotoxicant. The STAT3 effects seen for TMT in the mouse could be generalized to the rat, demonstrating cross-species validity for STAT3 activation. Pharmacological antagonists of the neurotoxic effects blocked neuroinflammatory responses, pSTAT3^tyr705 and GFAP induction, indicating that damage to neuronal targets instigated astrogliosis. Selective deletion of STAT3 from astrocytes in STAT3 conditional knockout mice markedly attenuated MPTP-induced astrogliosis. Monitoring STAT3 translocation in GFAP-positive cells indicated that effects of MPTP, METH and KA on pSTAT3^tyr705 were localized to astrocytes. These findings strongly implicate the STAT3 pathway in astrocytes as a broadly triggered signaling pathway for astrogliosis. We also observed, however, that the acute neuroinflammatory response to the known inflammogen, LPS, can activate STAT3 in CNS tissue without inducing classical signs of astrogliosis. Thus, acute phase neuroinflammatory responses and neurotoxicity-induced astrogliosis both signal through STAT3 but appear to do so through different modules, perhaps localized to different cell types.

Excitotoxic brain injury in adult zebrafish stimulates neurogenesis and long-distance neuronal integration

Zebrafish maintain a greater capacity than mammals for central nervous system repair after injury. Understanding differences in regenerative responses between different vertebrate species may shed light on mechanisms to improve repair in humans. Quinolinic acid is an excitotoxin that has been used to induce brain injury in rodents for modeling Huntington's disease and stroke. When injected into the adult rodent striatum, this toxin stimulates subventricular zone neurogenesis and neuroblast migration to injury. However, most new neurons fail to survive and lesion repair is minimal. We used quinolinic acid to lesion the adult zebrafish telencephalon to study reparative processes. We also used conditional transgenic lineage mapping of adult radial glial stem cells to explore survival and integration of neurons generated after injury. Telencephalic lesioning with quinolinic acid, and to a lesser extent vehicle injection, produced cell death, microglial infiltration, increased cell proliferation, and enhanced neurogenesis in the injured hemisphere. Lesion repair was more complete with quinolinic acid injection than after vehicle injection. Fate mapping of her4-expressing radial glia showed injury-induced expansion of radial glial stem cells that gave rise to neurons which migrated to injury, survived at least 8 weeks and formed long-distance projections that crossed the anterior commissure and synapsed in the contralateral hemisphere. These findings suggest that quinolinic acid lesioning of the zebrafish brain stimulates adult neural stem cells to produce robust regeneration with long-distance integration of new neurons. This model should prove useful for elucidating reparative mechanisms that can be applied to restorative therapies for mammalian brain injury.

A neonatal blueprint for cardiac regeneration

Adult mammals undergo minimal regeneration following cardiac injury, which severely compromises cardiac function and contributes to the ongoing burden of heart failure. In contrast, the mammalian heart retains a transient capacity for cardiac regeneration during fetal and early neonatal life. Recent studies have established the importance of several evolutionarily conserved mechanisms for heart regeneration in lower vertebrates and neonatal mammals including induction of cardiomyocyte proliferation, epicardial cell activation, angiogenesis, extracellular matrix deposition and immune cell infiltration. In this review, we provide an up-to-date account of the molecular and cellular basis for cardiac regeneration in lower vertebrates and neonatal mammals. The historical context for these recent findings and their ramifications for the future development of cardiac regenerative therapies are also discussed.