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

Experimental Models to Study Immune Dysfunction in the Pathogenesis of Parkinson's Disease

Parkinson's disease (PD) is a chronic, age-related, progressive multisystem disease associated with neuroinflammation and immune dysfunction. This review discusses the methodological approaches used to study the changes in central and peripheral immunity in PD, the advantages and limitations of the techniques, and their applicability to humans. Although a single animal model cannot replicate all pathological features of the human disease, neuroinflammation is present in most animal models of PD and plays a critical role in understanding the involvement of the immune system (IS) in the pathogenesis of PD. The IS and its interactions with different cell types in the central nervous system (CNS) play an important role in the pathogenesis of PD. Even though culture models do not fully reflect the complexity of disease progression, they are limited in their ability to mimic long-term effects and need validation through in vivo studies. They are an indispensable tool for understanding the interplay between the IS and the pathogenesis of this disease. Understanding the immune-mediated mechanisms may lead to potential therapeutic targets for the treatment of PD. We believe that the development of methodological guidelines for experiments with animal models and PD patients is crucial to ensure the validity and consistency of the results.

Microglial depletion after brain injury prolongs inflammation and impairs brain repair, adult neurogenesis and pro-regenerative signaling

The adult zebrafish brain, unlike mammals, has a remarkable regenerative capacity. Although inflammation in part hinders regeneration in mammals, it is necessary for zebrafish brain repair. Microglia are resident brain immune cells that regulate the inflammatory response. To explore the microglial role in repair, we used liposomal clodronate or colony stimulating factor-1 receptor (csf1r) inhibitor to suppress microglia after brain injury, and also examined regeneration in two genetic mutant lines that lack microglia. We found that microglial ablation impaired telencephalic regeneration after injury. Microglial suppression attenuated cell proliferation at the intermediate progenitor cell amplification stage of neurogenesis. Notably, the loss of microglia impaired phospho-Stat3 (signal transducer and activator of transcription 3) and ß-Catenin signaling after injury. Furthermore, the ectopic activation of Stat3 and ß-Catenin rescued neurogenesis defects caused by microglial loss. Microglial suppression also prolonged the post-injury inflammatory phase characterized by neutrophil accumulation, likely hindering the resolution of inflammation. These findings reveal specific roles of microglia and inflammatory signaling during zebrafish telencephalic regeneration that should advance strategies to improve mammalian brain repair.

The multicellular interplay of microglia in health and disease: lessons from leukodystrophy

Microglia are highly dynamic cells crucial for developing and maintaining lifelong brain function and health through their many interactions with essentially all cellular components of the central nervous system. The frequent connection of microglia to leukodystrophies, genetic disorders of the white matter, has highlighted their involvement in the maintenance of white matter integrity. However, the mechanisms that underlie their putative roles in these processes remain largely uncharacterized. Microglia have also been gaining attention as possible therapeutic targets for many neurological conditions, increasing the demand to understand their broad spectrum of functions and the impact of their dysregulation. In this Review, we compare the pathological features of two groups of genetic leukodystrophies: those in which microglial dysfunction holds a central role, termed 'microgliopathies', and those in which lysosomal or peroxisomal defects are considered to be the primary driver. The latter are suspected to have notable microglia involvement, as some affected individuals benefit from microglia-replenishing therapy. Based on overlapping pathology, we discuss multiple ways through which aberrant microglia could lead to white matter defects and brain dysfunction. We propose that the study of leukodystrophies, and their extensively multicellular pathology, will benefit from complementing analyses of human patient material with the examination of cellular dynamics in vivo using animal models, such as zebrafish. Together, this will yield important insight into the cell biological mechanisms of microglial impact in the central nervous system, particularly in the development and maintenance of myelin, that will facilitate the development of new, and refinement of existing, therapeutic options for a range of brain diseases.

Comparison of anti-inflammatory effects of Mecasin and its constituents on lipopolysaccharide-stimulated BV2 cells

Mecasin, a traditional medicine, contains nine herbal constituents: Curcuma longa, Salvia miltio rhiza, Gastrodia elata, Chaenomeles sinensis, Polygala tenuifolia, Paeonia japonica, Glycyrrhiza uralensis, Atractylodes japonica and processed Aconitum carmichaeli. Several biological effects of mecasin have been described both in vivo and in vitro. Previous studies have demonstrated that mecasin has anti-inflammatory effects. The purpose of the present study was to determine anti-inflammatory effects of mecasin and its natural product constituents on lipopolysaccharide (LPS)-stimulated BV2 cells by measuring nitrite and nitric oxide contents. Nitrite production levels in LPS-stimulated BV2 cells incubated with mecasin and each individual constituent of mecasin were measured. The results suggested that C. longa, P. tenuifolia and P. japonica inhibited nitrite production in a pattern similar to that of mecasin. The effect of mecasin was likely a result of synergistic effects of its natural herb constituents.

The immune response is a critical regulator of zebrafish retinal pigment epithelium regeneration

Loss of the retinal pigment epithelium (RPE) because of dysfunction or disease can lead to blindness in humans. Harnessing the intrinsic ability of the RPE to self-repair is an attractive therapeutic strategy; however, mammalian RPE is limited in its regenerative capacity. Zebrafish possess tremendous intrinsic regenerative potential in ocular tissues, including the RPE, but little is known about the mechanisms driving RPE regeneration. Here, utilizing transgenic and mutant zebrafish lines, pharmacological manipulations, transcriptomics, and imaging analyses, we identified elements of the immune response as critical mediators of intrinsic RPE regeneration. After genetic ablation, the RPE express immune-related genes, including leukocyte recruitment factors such as interleukin 34 We demonstrate that macrophage/microglia cells are responsive to RPE damage and that their function is required for the timely progression of the regenerative response. These data identify the molecular and cellular underpinnings of RPE regeneration and hold significant potential for translational approaches aimed toward promoting a pro-regenerative environment in mammalian RPE.

Inflammation Regulates the Multi-Step Process of Retinal Regeneration in Zebrafish

The ability to regenerate tissues varies between species and between tissues within a species. Mammals have a limited ability to regenerate tissues, whereas zebrafish possess the ability to regenerate almost all tissues and organs, including fin, heart, kidney, brain, and retina. In the zebrafish brain, injury and cell death activate complex signaling networks that stimulate radial glia to reprogram into neural stem-like cells that repair the injury. In the retina, a popular model for investigating neuronal regeneration, Müller glia, radial glia unique to the retina, reprogram into stem-like cells and undergo a single asymmetric division to generate multi-potent retinal progenitors. Müller glia-derived progenitors then divide rapidly, numerically matching the magnitude of the cell death, and differentiate into the ablated neurons. Emerging evidence reveals that inflammation plays an essential role in this multi-step process of retinal regeneration. This review summarizes the current knowledge of the inflammatory events during retinal regeneration and highlights the mechanisms whereby inflammatory molecules regulate the quiescence and division of Müller glia, the proliferation of Müller glia-derived progenitors and the survival of regenerated neurons.

Epigenetics and Communication Mechanisms in Microglia Activation with a View on Technological Approaches

Microglial cells, the immune cells of the central nervous system (CNS), play a crucial role for the proper brain development and function and in CNS homeostasis. While in physiological conditions, microglia continuously check the state of brain parenchyma, in pathological conditions, microglia can show different activated phenotypes: In the early phases, microglia acquire the M2 phenotype, increasing phagocytosis and releasing neurotrophic and neuroprotective factors. In advanced phases, they acquire the M1 phenotype, becoming neurotoxic and contributing to neurodegeneration. Underlying this phenotypic change, there is a switch in the expression of specific microglial genes, in turn modulated by epigenetic changes, such as DNA methylation, histones post-translational modifications and activity of miRNAs. New roles are attributed to microglial cells, including specific communication with neurons, both through direct cell–cell contact and by release of many different molecules, either directly or indirectly, through extracellular vesicles. In this review, recent findings on the bidirectional interaction between neurons and microglia, in both physiological and pathological conditions, are highlighted, with a focus on the complex field of microglia immunomodulation through epigenetic mechanisms and/or released factors. In addition, advanced technologies used to study these mechanisms, such as microfluidic, 3D culture and in vivo imaging, are presented.

Inflammation induces zebrafish regeneration

Tissue or organ regeneration is a complex process with successful outcomes depending on the type of tissue and organism. Upon damage, mammals can only efficiently restore a few tissues including the liver, skin, epithelia of the lung, kidney, and gut. In contrast, lower vertebrates such as zebrafish possess an extraordinary regeneration ability, which restores the normal function of a broad spectrum of tissues including heart, fin, brain, spinal cord, and retina. This regeneration process is either mediated by the proliferation of resident stem cells, or cells that dedifferentiate into a stem cell-like. In recent years, evidence has suggested that the innate immune system can modulate stem cell activity to initiate the regenerative response to damage. This review will explore some of the newer concepts of inflammation in zebrafish regeneration in different tissues. Understanding how inflammation regulates regeneration in zebrafish would provide important clues to improve the therapeutic strategies for repairing injured mammalian tissues that do not have an inherent regenerative capacity.

Noninvasive Imaging of Cone Ablation and Regeneration in Zebrafish

Purpose

To observe and characterize cone degeneration and regeneration in a selective metronidazole-mediated ablation model of ultraviolet-sensitive (UV) cones in zebrafish using in vivo optical coherence tomography (OCT) imaging.

Methods

Twenty-six sws1:nfsB-mCherry;sws2:eGFP zebrafish were imaged with OCT, treated with metronidazole to selectively kill UV cones, and imaged at 1, 3, 7, 14, 28, or 56 days after ablation. Regions 200 × 200 µm were cropped from volume OCT scans to count individual UV cones before and after ablation. Fish eyes were fixed, and immunofluorescence staining was used to corroborate cone density measured from OCT and to track monocyte response.

Results

Histology shows significant loss of UV cones after metronidazole treatment with a slight increase in observable blue cone density one day after treatment (Kruskal, Wallis, P = 0.0061) and no significant change in blue cones at all other timepoints. Regenerated UV cones measured from OCT show significantly lower density than pre-cone-ablation at 14, 28, and 56 days after ablation (analysis of variance, P < 0.01, P < 0.0001, P < 0.0001, respectively, 15.9% of expected nonablated levels). Histology shows significant changes to monocyte morphology (mixed-effects analysis, P < 0.0001) and retinal position (mixed-effects analysis, P < 0.0001).

Conclusions

OCT can be used to observe loss of individual cones selectively ablated by metronidazole prodrug activation and to quantify UV cone loss and regeneration in zebrafish. OCT images also show transient changes to the blue cone mosaic and inner retinal layers that occur concomitantly with selective UV cone ablation.

Translational Relevance

Profiling cone degeneration and regeneration using in vivo imaging enables experiments that may lead to a better understanding of cone regeneration in vertebrates.

Role of Macrophages and Microglia in Zebrafish Regeneration

Currently, there is no treatment for recovery of human nerve function after damage to the central nervous system (CNS), and there are limited regenerative capabilities in the peripheral nervous system. Since fish are known for their regenerative abilities, understanding how these species modulate inflammatory processes following injury has potential translational importance for recovery from damage and disease. Many diseases and injuries involve the activation of innate immune cells to clear damaged cells. The resident immune cells of the CNS are microglia, the primary cells that respond to infection and injury, and their peripheral counterparts, macrophages. These cells serve as key modulators of development and plasticity and have been shown to be important in the repair and regeneration of structure and function after injury. Zebrafish are an emerging model for studying macrophages in regeneration after injury and microglia in neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease. These fish possess a high degree of neuroanatomical, neurochemical, and emotional/social behavioral resemblance with humans, serving as an ideal simulator for many pathologies. This review explores literature on macrophage and microglial involvement in facilitating regeneration. Understanding innate immune cell behavior following damage may help to develop novel methods for treating toxic and chronic inflammatory processes that are seen in trauma and disease.

Zebrafish macrophage developmental arrest underlies depletion of microglia and reveals Csf1r-independent metaphocytes

Macrophages derive from multiple sources of hematopoietic progenitors. Most macrophages require colony-stimulating factor 1 receptor (CSF1R), but some macrophages persist in the absence of CSF1R. Here, we analyzed mpeg1:GFP-expressing macrophages in csf1r-deficient zebrafish and report that embryonic macrophages emerge followed by their developmental arrest. In larvae, mpeg1+ cell numbers then increased showing two distinct types in the skin: branched, putative Langerhans cells, and amoeboid cells. In contrast, although numbers also increased in csf1r-mutants, exclusively amoeboid mpeg1+ cells were present, which we showed by genetic lineage tracing to have a non-hematopoietic origin. They expressed macrophage-associated genes, but also showed decreased phagocytic gene expression and increased epithelial-associated gene expression, characteristic of metaphocytes, recently discovered ectoderm-derived cells. We further demonstrated that juvenile csf1r-deficient zebrafish exhibit systemic macrophage depletion. Thus, csf1r deficiency disrupts embryonic to adult macrophage development. Zebrafish deficient for csf1r are viable and permit analyzing the consequences of macrophage loss throughout life.

Colony-Stimulating Factor 1 Receptor (CSF1R) Regulates Microglia Density and Distribution, but Not Microglia Differentiation In Vivo

Microglia are brain-resident macrophages with trophic and phagocytic functions. Dominant loss-of-function mutations in a key microglia regulator, colony-stimulating factor 1 receptor (CSF1R), cause adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP), a progressive white matter disorder. Because it remains unclear precisely how CSF1R mutations affect microglia, we generated an allelic series of csf1r mutants in zebrafish to identify csf1r-dependent microglia changes. We found that csf1r mutations led to aberrant microglia density and distribution and regional loss of microglia. The remaining microglia still had a microglia-specific gene expression signature, indicating that they had differentiated normally. Strikingly, we also observed lower microglia numbers and widespread microglia depletion in postmortem brain tissue of ALSP patients. Both in zebrafish and in human disease, local microglia loss also presented in regions without obvious pathology. Together, this implies that CSF1R mainly regulates microglia density and that early loss of microglia may contribute to ALSP pathogenesis.

MCP1-CCR2 and neuroinflammation in the ALS motor cortex with TDP-43 pathology

BACKGROUND

The involvement of non-neuronal cells and the cells of innate immunity has been attributed to the initiation and progression of ALS. TDP-43 pathology is observed in a broad spectrum of ALS cases and is one of the most commonly shared pathologies. The potential involvement of the neuroimmune axis in the motor cortex of ALS patients with TDP-43 pathology needs to be revealed. This information is vital for building effective treatment strategies.

METHODS

We investigated the presence of astrogliosis and microgliosis in the motor cortex of ALS patients with TDP-43 pathology. prpTDP-43^A315T-UeGFP mice, corticospinal motor neuron (CSMN) reporter line with TDP-43 pathology, are utilized to reveal the timing and extent of neuroimmune interactions and the involvement of non-neuronal cells to neurodegeneration. Electron microscopy and immunolabeling techniques are used to mark and monitor cells of interest.

RESULTS

We detected both activated astrocytes and microglia, especially rod-like microglia, in the motor cortex of patients and TDP-43 mouse model. Besides, CCR2+ TMEM119- infiltrating monocytes were detected as they penetrate the brain parenchyma. Interestingly, Betz cells, which normally do not express MCP1, were marked with high levels of MCP1 expression when diseased.

CONCLUSIONS

There is an early contribution of a neuroinflammatory response for upper motor neuron (UMN) degeneration with respect to TDP-43 pathology, and MCP1-CCR2 signaling is important for the recognition of diseased upper motor neurons by infiltrating monocytes. The findings are conserved among species and are observed in both ALS and ALS-FTLD patients.

Microglial response patterns following damage to the zebrafish olfactory bulb

The inherent plasticity of the zebrafish olfactory system serves as a useful model for examining immune cell responses after injury. Microglia are the resident immune cells of the CNS that respond to damage by migrating to the site of injury and phagocytizing neuronal debris. While the olfactory system is renowned for its ability to recover from damage, the specific mechanisms of microglial involvement in olfactory system plasticity are unknown. To approach the potentially time-dependent effects of microglial activation after injury, we performed a time course analysis of microglial response profiles and patterns following different forms of damage: deafferentation by cautery ablation of the olfactory organ, deafferentation by chemical ablation of the olfactory epithelium, and direct lesioning of the olfactory bulb. Our aim was to demonstrate that immunocytochemistry and microscopy methods in zebrafish can be used to determine the timing of distinct microglial response patterns following various forms of injury. We found that permanent and temporary forms of damage to the olfactory bulb resulted in different microglial response profiles from 1 to 72 h after injury, suggesting that there may be critical timepoints in which microglia are activated that contribute to tissue and neuronal repair with a regenerative outcome versus a degenerative outcome. These distinctions between the different forms of damage suggest temporal changes relative to the potential for regeneration, since cautery deafferentation is permanent and unrecoverable while chemical ablation deafferentation and direct lesioning is reversible and can be used to observe the microglial relationship in neural regeneration and functional recovery in future studies.

Let's get small (and smaller): Combining zebrafish and nanomedicine to advance neuroregenerative therapeutics

Several key attributes of zebrafish make them an ideal model system for the discovery and development of regeneration promoting therapeutics; most notably their robust capacity for self-repair which extends to the central nervous system. Further, by enabling large-scale drug discovery directly in living vertebrate disease models, zebrafish circumvent critical bottlenecks which have driven drug development costs up. This review summarizes currently available zebrafish phenotypic screening platforms, HTS-ready neurodegenerative disease modeling strategies, zebrafish small molecule screens which have succeeded in identifying regeneration promoting compounds and explores how intravital imaging in zebrafish can facilitate comprehensive analysis of nanocarrier biodistribution and pharmacokinetics. Finally, we discuss the benefits and challenges attending the combination of zebrafish and nanoparticle-based drug optimization, highlighting inspiring proof-of-concept studies and looking toward implementation across the drug development community.

Homozygous Mutations in CSF1R Cause a Pediatric-Onset Leukoencephalopathy and Can Result in Congenital Absence of Microglia

Microglia are CNS-resident macrophages that scavenge debris and regulate immune responses. Proliferation and development of macrophages, including microglia, requires Colony Stimulating Factor 1 Receptor (CSF1R), a gene previously associated with a dominant adult-onset neurological condition (adult-onset leukoencephalopathy with axonal spheroids and pigmented glia). Here, we report two unrelated individuals with homozygous CSF1R mutations whose presentation was distinct from ALSP. Post-mortem examination of an individual with a homozygous splice mutation (c.1754-1G>C) demonstrated several structural brain anomalies, including agenesis of corpus callosum. Immunostaining demonstrated almost complete absence of microglia within this brain, suggesting that it developed in the absence of microglia. The second individual had a homozygous missense mutation (c.1929C>A [p.His643Gln]) and presented with developmental delay and epilepsy in childhood. We analyzed a zebrafish model (csf1r^DM) lacking Csf1r function and found that their brains also lacked microglia and had reduced levels of CUX1, a neuronal transcription factor. CUX1⁺ neurons were also reduced in sections of homozygous CSF1R mutant human brain, identifying an evolutionarily conserved role for CSF1R signaling in production or maintenance of CUX1⁺ neurons. Since a large fraction of CUX1⁺ neurons project callosal axons, we speculate that microglia deficiency may contribute to agenesis of the corpus callosum via reduction in CUX1⁺ neurons. Our results suggest that CSF1R is required for human brain development and establish the csf1r^DM fish as a model for microgliopathies. In addition, our results exemplify an under-recognized form of phenotypic expansion, in which genes associated with well-recognized, dominant conditions produce different phenotypes when biallelically mutated.

Reverse genetic screen reveals that Il34 facilitates yolk sac macrophage distribution and seeding of the brain

Microglia are brain-resident macrophages, which have specialized functions important in brain development and in disease. They colonize the brain in early embryonic stages, but few factors that drive the migration of yolk sac macrophages (YSMs) into the embryonic brain, or regulate their acquisition of specialized properties, are currently known. Here, we present a CRISPR/Cas9-based in vivo reverse genetic screening pipeline to identify new microglia regulators using zebrafish. Zebrafish larvae are particularly suitable due to their external development, transparency and conserved microglia features. We targeted putative microglia regulators, by Cas9/gRNA complex injections, followed by Neutral-Red-based visualization of microglia. Microglia were quantified automatically in 3-day-old larvae using a software tool we called SpotNGlia. We identified that loss of zebrafish colony-stimulating factor 1 receptor (Csf1r) ligand, Il34, caused reduced microglia numbers. Previous studies on the role of IL34 in microglia development in vivo were ambiguous. Our data, and a concurrent paper, show that, in zebrafish, il34 is required during the earliest seeding of the brain by microglia. Our data also indicate that Il34 is required for YSM distribution to other organs. Disruption of the other Csf1r ligand, Csf1, did not reduce microglia numbers in mutants, whereas overexpression increased the number of microglia. This shows that Csf1 can influence microglia numbers, but might not be essential for the early seeding of the brain. In all, we identified il34 as a modifier of microglia colonization, by affecting distribution of YSMs to target organs, validating our reverse genetic screening pipeline in zebrafish.This article has an associated First Person interview with the joint first authors of the paper.

Retinal microglia signaling affects Müller cell behavior in the zebrafish following laser injury induction

Microglia are the resident tissue macrophages of the central nervous system including the retina. Under pathophysiological conditions, microglia can signal to Müller cells, the major glial component of the retina, affecting their morphological, molecular, and functional responses. Microglia-Müller cell interactions appear to be bidirectional shaping the overall injury response in the retina. Hence, microglia and Müller cell responses to disease and injury have been ascribed both positive and negative outcomes. However, Müller cell reactivity and survival in the absence of immune cells after injury have not been investigated in detail in adult zebrafish. Here, we develop a model of focal retinal injury combined with pharmacological treatments for immune cell depletion in zebrafish. The retinal injury was induced by a diode laser to damage photoreceptors. Two pharmacological treatments were used to deplete either macrophage-microglia (PLX3397) or selectively eliminate peripheral macrophages (clodronate liposomes). We show that PLX3397 treatment hinders retinal regeneration in zebrafish, which is reversed by microglial repopulation. On the other hand, selective macrophage elimination did not affect the kinetics of retinal regeneration. The absence of retinal microglia and macrophages leads to dysregulated Müller cell behavior. In the untreated fish, Müller cells react after injury induction showing glial fibrillary acidic protein (GFAP), Phospho-p44/42 MAPK (Erk1/2), and PCNA upregulation. However, in the immunosuppressed animals, GFAP and phospho-p44/42 MAPK (Erk1/2) expression was not upregulated overtime and the reentry in the cell cycle was not affected. Thus, microglia and Müller cell signaling is pivotal to unlock the regenerative potential of Müller cells in order to repair the damaged retina.

Resident Immunity in Tissue Repair and Maintenance: The Zebrafish Model Coming of Age

The zebrafish has emerged as an exciting vertebrate model to study different aspects of immune system development, particularly due to its transparent embryonic development, the availability of multiple fluorescent reporter lines, efficient genetic tools and live imaging capabilities. However, the study of immunity in zebrafish has largely been limited to early larval stages due to an incomplete knowledge of the full repertoire of immune cells and their specific markers, in particular, a lack of cell surface antibodies to detect and isolate such cells in living tissues. Here we focus on tissue resident or associated immunity beyond development, in the adult zebrafish. It is our view that, with our increasing knowledge and the development of improved tools and protocols, the adult zebrafish will be increasingly appreciated for offering valuable insights into the role of immunity in tissue repair and maintenance, in both health and disease throughout the lifecourse.

Activation of BDNF-TrkB signaling pathway-regulated brain inflammation in pentylenetetrazole-induced seizures in zebrafish

Seizures are sustained neuronal hyperexcitability in brain that result in loss of consciousness and injury. Understanding how the brain responds to seizures is critical to help developing new therapeutic strategies for epilepsy, a neurological disorder characterized by recurrent and unprovoked seizures. However, the mechanisms underlying seizure-dependent alterations of biological properties are poorly understood. In this study, we analyzed gene expression profiles of the zebrafish heads that were undergoing seizures and identified 1776 differentially expressed genes. Gene-regulatory network analysis revealed that BDNF-TrkB signaling pathway positively regulated brain inflammation in zebrafish during seizures. Using K252a, a TrkB inhibitor to block BDNF-TrkB signaling pathway, attenuated pentylenetetrazole (PTZ)-induced seizures, which also confirmed BDNF-TrkB mediated inflammatory responses including regulation of il1β and nfκb, and neutrophil and macrophage infiltration of brain. Our results have provided novel insights into seizure-induced brain inflammation in zebrafish and anti-inflammatory related therapy for epilepsy.

Using zebrafish larval models to study brain injury, locomotor and neuroinflammatory outcomes following intracerebral haemorrhage

Intracerebral haemorrhage (ICH) is a devastating condition with limited treatment options, and current understanding of pathophysiology is incomplete. Spontaneous cerebral bleeding is a characteristic of the human condition that has proven difficult to recapitulate in existing pre-clinical rodent models. Zebrafish larvae are frequently used as vertebrate disease models and are associated with several advantages, including high fecundity, optical translucency and non-protected status prior to 5 days post-fertilisation. Furthermore, other groups have shown that zebrafish larvae can exhibit spontaneous ICH. The aim of this study was to investigate whether such models can be utilised to study the pathological consequences of bleeding in the brain, in the context of pre-clinical ICH research.

Here, we compared existing genetic (bubblehead) and chemically inducible (atorvastatin) zebrafish larval models of spontaneous ICH and studied the subsequent disease processes. Through live, non-invasive imaging of transgenic fluorescent reporter lines and behavioural assessment we quantified brain injury, locomotor function and neuroinflammation following ICH. We show that ICH in both zebrafish larval models is comparable in timing, frequency and location. ICH results in increased brain cell death and a persistent locomotor deficit. Additionally, in haemorrhaged larvae we observed a significant increase in macrophage recruitment to the site of injury. Live in vivo imaging allowed us to track active macrophage-based phagocytosis of dying brain cells 24 hours after haemorrhage. Morphological analyses and quantification indicated that an increase in overall macrophage activation occurs in the haemorrhaged brain.

Our study shows that in zebrafish larvae, bleeding in the brain induces quantifiable phenotypic outcomes that mimic key features of human ICH. We hope that this methodology will enable the pre-clinical ICH community to adopt the zebrafish larval model as an alternative to rodents, supporting future high throughput drug screening and as a complementary approach to elucidating crucial mechanisms associated with ICH pathophysiology.

The Kaleidoscope of Microglial Phenotypes

Gene expression analyses of microglia, the tissue-resident macrophages of the central nervous system (CNS), led to the identification of homeostatic as well as neurological disease-specific gene signatures of microglial phenotypes. Upon alterations in the neural microenvironment, either caused by local insults from within the CNS (during neurodegenerative diseases) or by macroenvironmental incidents, such as social stress, microglia can switch phenotypes-generally referred to as "microglial activation." The interplay between the microenvironment and its influence on microglial phenotypes, regulated by (epi)genetic mechanisms, can be imagined as the different colorful crystal formations (microglial phenotypes) that change upon rotation (microenvironmental changes) of a kaleidoscope. In this review, we will discuss microglial phenotypes in relation to neurodevelopment, homeostasis, in vitro conditions, aging, and neurodegenerative diseases based on transcriptome studies. By overlaying these disease-specific microglial signatures, recent publications have identified a specific set of genes that is differentially expressed in all investigated diseases, called a microglial core gene signature with multiple diseases. We will conclude this review with a discussion about the complexity of this microglial core gene signature associated with multiple diseases.

Psychoneuroimmunology and immunopsychiatry of zebrafish

Despite the high prevalence of neural and immune disorders, their etiology and molecular mechanisms remain poorly understood. As the zebrafish (Danio rerio) is increasingly utilized as a powerful model organism in biomedical research, mounting evidence suggests these fish as a useful tool to study neural and immune mechanisms and their interplay. Here, we discuss zebrafish neuro-immune mechanisms and their pharmacological and genetic modulation, the effect of stress on cytokines, as well as relevant models of microbiota-brain interplay. As many human brain diseases are based on complex interplay between the neural and the immune system, here we discuss zebrafish models, as well as recent successes and challenges, in this rapidly expanding field. We particularly emphasize the growing utility of zebrafish models in translational immunopsychiatry research, as they improve our understanding of pathogenetic neuro-immune interactions, thereby fostering future discovery of potential therapeutic agents.

Nifurpirinol: A more potent and reliable substrate compared to metronidazole for nitroreductase-mediated cell ablations

The zebrafish is a popular animal model with well-known regenerative capabilities. To study regeneration in this fish, the nitroreductase/metronidazole-mediated system is widely used for targeted ablation of various cell types. Nevertheless, we highlight here some variability in ablation efficiencies with the metronidazole prodrug that led us to search for a more efficient and reliable compound. Herein, we present nifurpirinol, another nitroaromatic antibiotic, as a more potent prodrug compared to metronidazole to trigger cell-ablation in nitroreductase expressing transgenic models. We show that nifurpirinol induces robust and reliable ablations at concentrations 2,000 fold lower than metronidazole and three times below its own toxic concentration. We confirmed the efficiency of nifurpirinol in triggering massive ablation of three different cell types: the pancreatic beta cells, osteoblasts, and dopaminergic neurons. Our results identify nifurpirinol as a very potent prodrug for the nitroreductase-mediated ablation system and suggest that its use could be extended to many other cell types, especially if difficult to ablate, or when combined pharmacological treatments are desired.

A Brief History of Microglial Ultrastructure: Distinctive Features, Phenotypes, and Functions Discovered Over the Past 60 Years by Electron Microscopy

The first electron microscope was constructed in 1931. Several decades later, techniques were developed to allow the first ultrastructural analysis of microglia by transmission electron microscopy (EM). In the 50 years that followed, important roles of microglia have been identified, specifically due to the ultrastructural resolution currently available only with EM. In particular, the addition of electron-dense staining using immunohistochemical EM methods has allowed the identification of microglial cell bodies, as well as processes, which are difficult to recognize in EM, and to uncover their complex interactions with neurons and synapses. The ability to recognize neuronal, astrocytic, and oligodendrocytic compartments in the neuropil without any staining is another invaluable advantage of EM over light microscopy for studying intimate cell-cell contacts. The technique has been essential in defining microglial interactions with neurons and synapses, thus providing, among other discoveries, important insights into their roles in synaptic stripping and pruning via phagocytosis of extraneous synapses. Recent technological advances in EM including serial block-face imaging and focused-ion beam scanning EM have also facilitated automated acquisition of large tissue volumes required to reconstruct neuronal circuits in 3D at nanometer-resolution. These cutting-edge techniques which are now becoming increasingly available will further revolutionize the study of microglia across stages of the lifespan, brain regions, and contexts of health and disease. In this mini-review, we will focus on defining the distinctive ultrastructural features of microglia and the unique insights into their function that were provided by EM.

New Insights into the Role of Neuron-Specific Enolase in Neuro-Inflammation, Neurodegeneration, and Neuroprotection

Neurodegeneration is a complex process that leads to irreversible neuronal damage and death in spinal cord injury (SCI) and various neurodegenerative diseases, which are serious, debilitating conditions. Despite exhaustive research, the cause of neuronal damage in these degenerative disorders is not completely understood. Elevation of cell surface α-enolase activates various inflammatory pathways, including the production of pro-inflammatory cytokines, chemokines, and some growth factors that are detrimental to neuronal cells. While α-enolase is present in all neurological tissues, it can also be converted to neuron specific enolase (NSE). NSE is a glycolytic enzyme found in neuronal and neuroendocrine tissues that may play a dual role in promoting both neuroinflammation and neuroprotection in SCI and other neurodegenerative events. Elevated NSE can promote ECM degradation, inflammatory glial cell proliferation, and actin remodeling, thereby affecting migration of activated macrophages and microglia to the injury site and promoting neuronal cell death. Thus, NSE could be a reliable, quantitative, and specific marker of neuronal injury. Depending on the injury, disease, and microenvironment, NSE may also show neurotrophic function as it controls neuronal survival, differentiation, and neurite regeneration via activation of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signaling pathways. This review discusses possible implications of NSE expression and activity in neuroinflammation, neurodegeneration, and neuroprotection in SCI and various neurodegenerative diseases for prognostic and therapeutic potential.

Hyperammonemia alters membrane expression of GluA1 and GluA2 subunits of AMPA receptors in hippocampus by enhancing activation of the IL-1 receptor: underlying mechanisms

Background

Hyperammonemic rats reproduce the cognitive alterations of patients with hepatic encephalopathy, including altered spatial memory, attributed to altered membrane expression of AMPA receptor subunits in hippocampus. Neuroinflammation mediates these cognitive alterations. We hypothesized that hyperammonemia-induced increase in IL-1β in hippocampus would be responsible for the altered GluA1 and GluA2 membrane expression. The aims of this work were to (1) assess if increased IL-1β levels and activation of its receptor are responsible for the changes in GluA1 and/or GluA2 membrane expression in hyperammonemia and (2) identify the mechanisms by which activation of IL-1 receptor leads to altered membrane expression of GluA1 and GluA2.

Methods

We analyzed in hippocampal slices from control and hyperammonemic rat membrane expression of AMPA receptors using the BS3 cross-linker and phosphorylation of the GluA1 and GluA2 subunits using phosphor-specific antibodies. The IL-1 receptor was blocked with IL-Ra, and the signal transduction pathways involved in modulation of membrane expression of GluA1 and GluA2 were analyzed using inhibitors of key steps.

Results

Hyperammonemia reduces GluA1 and increases GluA2 membrane expression and reduces phosphorylation of GluA1 at Ser831 and of GluA2 at Ser880. Hyperammonemia increases IL-1β, enhancing activation of IL-1 receptor. This leads to activation of Src. The changes in membrane expression of GluA1 and GluA2 are reversed by blocking the IL-1 receptor with IL-1Ra or by inhibiting Src with PP2.

After Src activation, the pathways for GluA2 and GluA1 diverge. Src increases phosphorylation of GluN2B at Tyr14721 and membrane expression of GluN2B in hyperammonemic rats, leading to activation of MAP kinase p38, which binds to and reduces phosphorylation at Thr560 and activity of PKCζ, resulting in reduced phosphorylation at Ser880 and enhanced membrane expression of GluA2.

Increased Src activity in hyperammonemic rats also activates PKCδ which enhances phosphorylation of GluN2B at Ser1303, reducing membrane expression of CaMKII and phosphorylation at Ser831 and membrane expression of GluA1.

Conclusions

This work identifies two pathways by which neuroinflammation alters glutamatergic neurotransmission in hippocampus. The steps of the pathways identified could be targets to normalize neurotransmission in hyperammonemia and other pathologies associated with increased IL-1β by acting, for example, on p38 or PKCδ.

Graphical abstract

IL-1β alters membrane expression of GluA1 and GluA2 AMPA receptor subunits by two difrerent mechanisms in the hippocampus of hyperammonemic rats.

Abscopal Activation of Microglia in Embryonic Fish Brain Following Targeted Irradiation with Heavy-Ion Microbeam

Microglia remove apoptotic cells by phagocytosis when the central nervous system is injured in vertebrates. Ionizing irradiation (IR) induces apoptosis and microglial activation in embryonic midbrain of medaka (Oryzias latipes), where apolipoprotein E (ApoE) is upregulated in the later phase of activation of microglia In this study, we found that another microglial marker, l-plastin (lymphocyte cytosolic protein 1), was upregulated at the initial phase of the IR-induced phagocytosis when activated microglia changed their morphology and increased motility to migrate. We further conducted targeted irradiation to the embryonic midbrain using a collimated microbeam of carbon ions (250 μm diameter) and found that the l-plastin upregulation was induced only in the microglia located in the irradiated area. Then, the activated microglia might migrate outside of the irradiated area and spread through over the embryonic brain, expressing ApoE and with activated morphology, for longer than 3 days after the irradiation. These findings suggest that l-plastin and ApoE can be the biomarkers of the activated microglia in the initial and later phase, respectively, in the medaka embryonic brain and that the abscopal and persisted activation of microglia by IR irradiation could be a cause of the abscopal and/or adverse effects following irradiation.

Immunomodulation-accelerated neuronal regeneration following selective rod photoreceptor cell ablation in the zebrafish retina

Müller glia (MG) function as inducible retinal stem cells in zebrafish, completely repairing the eye after damage. The innate immune system has recently been shown to promote tissue regeneration in which classic wound-healing responses predominate. However, regulatory roles for leukocytes during cellular regeneration-i.e., selective cell-loss paradigms akin to degenerative disease-are less well defined. To investigate possible roles innate immune cells play during retinal cell regeneration, we used intravital microscopy to visualize neutrophil, macrophage, and retinal microglia responses to induced rod photoreceptor apoptosis. Neutrophils displayed no reactivity to rod cell loss. Peripheral macrophage cells responded to rod cell loss, as evidenced by morphological transitions and increased migration, but did not enter the retina. Retinal microglia displayed multiple hallmarks of immune cell activation: increased migration, translocation to the photoreceptor cell layer, proliferation, and phagocytosis of dying cells. To test function during rod cell regeneration, we coablated microglia and rod cells or applied immune suppression and quantified the kinetics of (i) rod cell clearance, (ii) MG/progenitor cell proliferation, and (iii) rod cell replacement. Coablation and immune suppressants applied before cell loss caused delays in MG/progenitor proliferation rates and slowed the rate of rod cell replacement. Conversely, immune suppressants applied after cell loss had been initiated led to accelerated photoreceptor regeneration kinetics, possibly by promoting rapid resolution of an acute immune response. Our findings suggest that microglia control MG responsiveness to photoreceptor loss and support the development of immune-targeted therapeutic strategies for reversing cell loss associated with degenerative retinal conditions.

Microglial Activation by Genetically Targeted Conditional Neuronal Ablation in the Zebrafish

In neurodegenerative diseases activation of immune cells is thought to play a major role. Microglia are the main immune cells of the central nervous system. When encountering disease related stimuli microglia adopt an activated phenotype that typically includes a rounded morphology. The exact role of microglia or other potentially infiltrating myeloid cells in different brain diseases is not fully understood. In this chapter we present techniques in zebrafish to induce degeneration of neurons, to activate the microglia, and to study activation phenotypes by immunohistochemistry and in vivo by fluorescence microscopic imaging.

Microglia in Physiology and Disease

As the immune-competent cells of the brain, microglia play an increasingly important role in maintaining normal brain function. They invade the brain early in development, transform into a highly ramified phenotype, and constantly screen their environment. Microglia are activated by any type of pathologic event or change in brain homeostasis. This activation process is highly diverse and depends on the context and type of the stressor or pathology. Microglia can strongly influence the pathologic outcome or response to a stressor due to the release of a plethora of substances, including cytokines, chemokines, and growth factors. They are the professional phagocytes of the brain and help orchestrate the immunological response by interacting with infiltrating immune cells. We describe here the diversity of microglia phenotypes and their responses in health, aging, and disease. We also review the current literature about the impact of lifestyle on microglia responses and discuss treatment options that modulate microglial phenotypes.

Identification of a conserved and acute neurodegeneration-specific microglial transcriptome in the zebrafish

Microglia are brain resident macrophages important for brain development, connectivity, homeostasis and disease. However, it is still largely unclear how microglia functions and their identity are regulated at the molecular level. Although recent transcriptomic studies have identified genes specifically expressed in microglia, the function of most of these genes in microglia is still unknown. Here, we performed RNA sequencing on microglia acutely isolated from healthy and neurodegenerative zebrafish brains. We found that a large fraction of the mouse microglial signature is conserved in the zebrafish, corroborating the use of zebrafish to help understand microglial genetics in mammals in addition to studying basic microglia biology. Second, our transcriptome analysis of microglia following neuronal ablation suggested primarily a proliferative response of microglia, which we confirmed by immunohistochemistry and in vivo imaging. Together with the recent improvements in genome editing technology in zebrafish, these data offer opportunities to facilitate functional genetic research on microglia in vivo in the healthy as well as in the diseased brain. GLIA 2016;65:138–149

The role of enterocyte defects in the pathogenesis of congenital diarrheal disorders

Congenital diarrheal disorders are rare, often fatal, diseases that are difficult to diagnose (often requiring biopsies) and that manifest in the first few weeks of life as chronic diarrhea and the malabsorption of nutrients. The etiology of congenital diarrheal disorders is diverse, but several are associated with defects in the predominant intestinal epithelial cell type, enterocytes. These particular congenital diarrheal disorders (CDD^ENT) include microvillus inclusion disease and congenital tufting enteropathy, and can feature in other diseases, such as hemophagocytic lymphohistiocytosis type 5 and trichohepatoenteric syndrome. Treatment options for most of these disorders are limited and an improved understanding of their molecular bases could help to drive the development of better therapies. Recently, mutations in genes that are involved in normal intestinal epithelial physiology have been associated with different CDD^ENT. Here, we review recent progress in understanding the cellular mechanisms of CDD^ENT. We highlight the potential of animal models and patient-specific stem-cell-based organoid cultures, as well as patient registries, to integrate basic and clinical research, with the aim of clarifying the pathogenesis of CDD^ENT and expediting the discovery of novel therapeutic strategies.

Summary: Overview of the recent progress in our understanding of congenital diarrheal disorders, and the available models to study these diseases.

Caspase-mediated apoptosis induction in zebrafish cerebellar Purkinje neurons

The zebrafish is a well-established model organism in which to study in vivo mechanisms of cell communication, differentiation and function. Existing cell ablation methods are either invasive or they rely on the cellular expression of prokaryotic enzymes and the use of antibiotic drugs as cell death-inducing compounds. We have recently established a novel inducible genetic cell ablation system based on tamoxifen-inducible Caspase 8 activity, thereby exploiting mechanisms of cell death intrinsic to most cell types. Here, we prove its suitability in vivo by monitoring the ablation of cerebellar Purkinje cells (PCs) in transgenic zebrafish that co-express the inducible caspase and a fluorescent reporter. Incubation of larvae in tamoxifen for 8 h activated endogenous Caspase 3 and cell death, whereas incubation for 16 h led to the near-complete loss of PCs by apoptosis. We observed synchronous cell death autonomous to the PC population and phagocytosing microglia in the cerebellum, reminiscent of developmental apoptosis in the forebrain. Thus, induction of apoptosis through targeted activation of caspase by tamoxifen (ATTAC^TM) further expands the repertoire of genetic tools for conditional interrogation of cellular functions.

Monocyte-Derived Macrophages Contribute to Spontaneous Long-Term Functional Recovery after Stroke in Mice

Stroke is a leading cause of disability and currently lacks effective therapy enabling long-term functional recovery. Ischemic brain injury causes local inflammation, which involves both activated resident microglia and infiltrating immune cells, including monocytes. Monocyte-derived macrophages (MDMs) exhibit a high degree of functional plasticity. Here, we determined the role of MDMs in long-term spontaneous functional recovery after middle cerebral artery occlusion in mice. Analyses by flow cytometry and immunocytochemistry revealed that monocytes home to the stroke-injured hemisphere., and that infiltration peaks 3 d after stroke. At day 7, half of the infiltrating MDMs exhibited a bias toward a proinflammatory phenotype and the other half toward an anti-inflammatory phenotype, but during the subsequent 2 weeks, MDMs with an anti-inflammatory phenotype dominated. Blocking monocyte recruitment using the anti-CCR2 antibody MC-21 during the first week after stroke abolished long-term behavioral recovery, as determined in corridor and staircase tests, and drastically decreased tissue expression of anti-inflammatory genes, including TGFβ, CD163, and Ym1. Our results show that spontaneously recruited monocytes to the injured brain early after the insult contribute to long-term functional recovery after stroke.

SIGNIFICANCE STATEMENT

For decades, any involvement of circulating immune cells in CNS repair was completely denied. Only over the past few years has involvement of monocyte-derived macrophages (MDMs) in CNS repair received appreciation. We show here, for the first time, that MDMs recruited to the injured brain early after ischemic stroke contribute to long-term spontaneous functional recovery through inflammation-resolving activity. Our data raise the possibility that inadequate recruitment of MDMs to the brain after stroke underlies the incomplete functional recovery seen in patients and that boosting homing of MDMs with an anti-inflammatory bias to the injured brain tissue may be a new therapeutic approach to promote long-term improvement after stroke.

Gfap-positive radial glial cells are an essential progenitor population for later-born neurons and glia in the zebrafish spinal cord

Radial glial cells are presumptive neural stem cells (NSCs) in the developing nervous system. The direct requirement of radial glia for the generation of a diverse array of neuronal and glial subtypes, however, has not been tested. We employed two novel transgenic zebrafish lines and endogenous markers of NSCs and radial glia to show for the first time that radial glia are essential for neurogenesis during development. By using the gfap promoter to drive expression of nuclear localized mCherry we discerned two distinct radial glial-derived cell types: a major nestin+/Sox2+ subtype with strong gfap promoter activity and a minor Sox2+ subtype lacking this activity. Fate mapping studies in this line indicate that gfap+ radial glia generate later-born CoSA interneurons, secondary motorneurons, and oligodendroglia. In another transgenic line using the gfap promoter-driven expression of the nitroreductase enzyme, we induced cell autonomous ablation of gfap+ radial glia and observed a reduction in their specific derived lineages, but not Blbp+ and Sox2+/gfap-negative NSCs, which were retained and expanded at later larval stages. Moreover, we provide evidence supporting classical roles of radial glial in axon patterning, blood-brain barrier formation, and locomotion. Our results suggest that gfap+ radial glia represent the major NSC during late neurogenesis for specific lineages, and possess diverse roles to sustain the structure and function of the spinal cord. These new tools will both corroborate the predicted roles of astroglia and reveal novel roles related to development, physiology, and regeneration in the vertebrate nervous system. GLIA 2016;64:1170-1189.

Large-scale Scanning Transmission Electron Microscopy (Nanotomy) of Healthy and Injured Zebrafish Brain

Large-scale 2D electron microscopy (EM), or nanotomy, is the tissue-wide application of nanoscale resolution electron microscopy. Others and we previously applied large scale EM to human skin pancreatic islets, tissue culture and whole zebrafish larvae^1-7. Here we describe a universally applicable method for tissue-scale scanning EM for unbiased detection of sub-cellular and molecular features. Nanotomy was applied to investigate the healthy and a neurodegenerative zebrafish brain. Our method is based on standardized EM sample preparation protocols: Fixation with glutaraldehyde and osmium, followed by epoxy-resin embedding, ultrathin sectioning and mounting of ultrathin-sections on one-hole grids, followed by post staining with uranyl and lead. Large-scale 2D EM mosaic images are acquired using a scanning EM connected to an external large area scan generator using scanning transmission EM (STEM). Large scale EM images are typically ~ 5 - 50 G pixels in size, and best viewed using zoomable HTML files, which can be opened in any web browser, similar to online geographical HTML maps. This method can be applied to (human) tissue, cross sections of whole animals as well as tissue culture^1-5. Here, zebrafish brains were analyzed in a non-invasive neuronal ablation model. We visualize within a single dataset tissue, cellular and subcellular changes which can be quantified in various cell types including neurons and microglia, the brain's macrophages. In addition, nanotomy facilitates the correlation of EM with light microscopy (CLEM)⁸ on the same tissue, as large surface areas previously imaged using fluorescent microscopy, can subsequently be subjected to large area EM, resulting in the nano-anatomy (nanotomy) of tissues. In all, nanotomy allows unbiased detection of features at EM level in a tissue-wide quantifiable manner.

Correlated light and electron microscopy: ultrastructure lights up!

Microscopy has gone hand in hand with the study of living systems since van Leeuwenhoek observed living microorganisms and cells in 1674 using his light microscope. A spectrum of dyes and probes now enable the localization of molecules of interest within living cells by fluorescence microscopy. With electron microscopy (EM), cellular ultrastructure has been revealed. Bridging these two modalities, correlated light microscopy and EM (CLEM) opens new avenues. Studies of protein dynamics with fluorescent proteins (FPs), which leave the investigator 'in the dark' concerning cellular context, can be followed by EM examination. Rare events can be preselected at the light microscopy level before EM analysis. Ongoing development-including of dedicated probes, integrated microscopes, large-scale and three-dimensional EM and super-resolution fluorescence microscopy-now paves the way for broad CLEM implementation in biology.

Spotlight on zebrafish: translational impact

In recent years, the zebrafish has emerged as an increasingly prominent model in biomedical research. To showcase the translational impact of the model across multiple disease areas, Disease Models & Mechanisms has compiled a Special Issue that includes thought-provoking reviews, original research reporting new and important insights into disease mechanisms, and novel resources that expand the zebrafish toolkit. This Editorial provides a summary of the issue’s contents, highlighting the diversity of zebrafish disease models and their clinical applications.

Ontogeny and polarization of macrophages in inflammation: blood monocytes versus tissue macrophages

The explosion of new information in recent years on the origin of macrophages in the steady-state and in the context of inflammation has opened up numerous new avenues of investigation and possibilities for therapeutic intervention. In contrast to the classical model of macrophage development, it is clear that tissue-resident macrophages can develop from yolk sac-derived erythro-myeloid progenitors, fetal liver progenitors, and bone marrow-derived monocytes. Under both homeostatic conditions and in response to pathophysiological insult, the contribution of these distinct sources of macrophages varies significantly between tissues. Furthermore, while all of these populations of macrophages appear to be capable of adopting the polarized M1/M2 phenotypes, their respective contribution to inflammation, resolution of inflammation, and tissue repair remains poorly understood and is likely to be tissue- and disease-dependent. A better understanding of the ontology and polarization capacity of macrophages in homeostasis and disease will be essential for the development of novel therapies that target the inherent plasticity of macrophages in the treatment of acute and chronic inflammatory disease.

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.

Glial cell development and function in zebrafish

The zebrafish is a premier vertebrate model system that offers many experimental advantages for in vivo imaging and genetic studies. This review provides an overview of glial cell types in the central and peripheral nervous system of zebrafish. We highlight some recent work that exploited the strengths of the zebrafish system to increase the understanding of the role of Gpr126 in Schwann cell myelination and illuminate the mechanisms controlling oligodendrocyte development and myelination. We also summarize similarities and differences between zebrafish radial glia and mammalian astrocytes and consider the possibility that their distinct characteristics may represent extremes in a continuum of cell identity. Finally, we focus on the emergence of zebrafish as a model for elucidating the development and function of microglia. These recent studies have highlighted the power of the zebrafish system for analyzing important aspects of glial development and function.