Regeneration of Dopaminergic Neurons in Adult Zebrafish Depends on Immune System Activation and Differs for Distinct Populations. J Neurosci. 2019;39:4694-4713. [PMID: 30948475 DOI: 10.1523/jneurosci.2706-18.2019]

Telencephalic stab wound injury induces regenerative angiogenesis and neurogenesis in zebrafish: unveiling the role of vascular endothelial growth factor signaling and microglia

JOURNAL/nrgr/04.03/01300535-202510000-00025/figure1/v/2024-11-26T163120Z/r/image-tiff After brain damage, regenerative angiogenesis and neurogenesis have been shown to occur simultaneously in mammals, suggesting a close link between these processes. However, the mechanisms by which these processes interact are not well understood. In this work, we aimed to study the correlation between angiogenesis and neurogenesis after a telencephalic stab wound injury. To this end, we used zebrafish as a relevant model of neuroplasticity and brain repair mechanisms. First, using the Tg( fli1:EGFP × mpeg1.1:mCherry ) zebrafish line, which enables visualization of blood vessels and microglia respectively, we analyzed regenerative angiogenesis from 1 to 21 days post-lesion. In parallel, we monitored brain cell proliferation in neurogenic niches localized in the ventricular zone by using immunohistochemistry. We found that after brain damage, the blood vessel area and width as well as expression of the fli1 transgene and vascular endothelial growth factor ( vegfaa and vegfbb ) were increased. At the same time, neural stem cell proliferation was also increased, peaking between 3 and 5 days post-lesion in a manner similar to angiogenesis, along with the recruitment of microglia. Then, through pharmacological manipulation by injecting an anti-angiogenic drug (Tivozanib) or Vegf at the lesion site, we demonstrated that blocking or activating Vegf signaling modulated both angiogenic and neurogenic processes, as well as microglial recruitment. Finally, we showed that inhibition of microglia by clodronate-containing liposome injection or dexamethasone treatment impairs regenerative neurogenesis, as previously described, as well as injury-induced angiogenesis. In conclusion, we have described regenerative angiogenesis in zebrafish for the first time and have highlighted the role of inflammation in this process. In addition, we have shown that both angiogenesis and neurogenesis are involved in brain repair and that microglia and inflammation-dependent mechanisms activated by Vegf signaling are important contributors to these processes. This study paves the way for a better understanding of the effect of Vegf on microglia and for studies aimed at promoting angiogenesis to improve brain plasticity after brain injury.

Chronic Exposure of Zebrafish to Iron and Aluminum: Evaluation of Reversal and Generational Transposition of Behavioral, Histopathological, and Genotoxic Changes

This study aimed to report the effects of chronic exposure of zebrafish exposed to environmentally relevant concentrations of 0.5, 2.4, and 5.0 mg L-1 iron (Fe) and 0.2, 0.4, and 2.0 mg L-1 aluminum (Al). We also evaluated the reversal and generational transposition (F1) of possible histopathological, behavioral, and genotoxic changes in the species. Locomotion changes that may have been caused by the increase in the number of apoptotic cells and in the telencephalic mitochondrial activity were observed especially after the 30 days exposure to Al and persisted after recovery (30 days). We also observed histopathological changes, such as an increase in the number of intestinal goblet cells, even after the recovery period in these animals. Our results also showed that the Fe concentrations used were insufficient to cause genotoxicity, behavioral and intestinal epithelium changes. The adult offspring (F1) of animals exposed to Al showed changes in locomotion and in the amount of goblet cells, demonstrating that even in low concentrations this pollutant can harm subsequent generations in the aquatic biota. Animals demonstrate, in general, greater tolerance to Fe which may be related to the physiological demand of this metal by the body. Even so, all concentrations of both metals that caused some change in the species represent Brazilian environmental occurrences or Brazilian legislation. It highlights the need for updating the guidelines and constant monitoring of aquatic environments, since even in the face of a hypothetical decontamination of the environment, some changes could persist and affect different trophic levels.

Regulating the formation of Müller glia-derived progenitor cells in the retina

We summarize recent findings in different animal models regarding the different cell-signaling pathways and gene networks that influence the reprogramming of Müller glia into proliferating, neurogenic progenitor cells in the retina. Not surprisingly, most of the cell-signaling pathways that guide the proliferation and differentiation of embryonic retinal progenitors also influence the ability of Müller glia to become proliferating Müller glia-derived progenitor cells (MGPCs). Further, the neuronal differentiation of MGPC progeny is potently inhibited by networks of neurogenesis-suppressing genes in chick and mouse models but occurs freely in zebrafish. There are important differences between the model systems, particularly pro-inflammatory signals that are active in mature Müller glia in damaged rodent and chick retinas, but less so in fish retinas. These pro-inflammatory signals are required to initiate the process of reprogramming, but if sustained suppress the potential of Müller glia to become neurogenic MGPCs. Further, there are important differences in how activated Müller glia up- or downregulate pro-glial transcription factors in the different model systems. We review recent findings regarding regulatory cell signaling and gene networks that influence the activation of Müller glia and the transition of these glia into proliferating progenitor cells with neurogenic potential in fish, chick, and mouse model systems.

Socially induced plasticity of the posterior tuberculum and motor behavior in zebrafish (Danio rerio)

Social dominance is prevalent throughout the animal kingdom. It facilitates the stabilization of social relationships and allows animals to divide resources according to social rank. Zebrafish form stable dominance relationships that consist of dominants and subordinates. Although social status-dependent differences in behavior must arise as a result of neural plasticity, mechanisms by which neural circuits are reconfigured to cope with social dominance are poorly described. Here, we describe how the posterior tuberculum nucleus (PTN), which integrates sensory social information to modulate spinal motor circuits, is morphologically and functionally influenced by social status. We combined non-invasive behavioral monitoring of motor activity (startle escape and swim) and histological approaches to investigate how social dominance affects the morphological structure, axosomatic synaptic connectivity and functional activity of the PTN in relation to changes in motor behavior. We show that dopaminergic cell number significantly increases in dominants compared with subordinates, while PTN synaptic interconnectivity, demonstrated with PSD-95 expression, is higher in subordinates than in dominants. Secondly, these socially induced morphological differences emerge after 1 week of dominance formation and correlate with differences in cellular activities illustrated with higher phosphor-S6 ribosomal protein expression in dominants compared with subordinates. Thirdly, these morphological differences are reversible as the social environment evolves and correlate with adaptations in startle escape and swim behaviors. Our results provide new insights into the neural bases of social behavior that may be applicable to other social species with similar structural and functional organization.

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.

Different electrophysiological profiles of genetically labelled dopaminergic neurons in the mouse midbrain and olfactory bulb

Dopaminergic (DA) neurons play pivotal roles in diverse brain functions, spanning movement, reward processing and sensory perception. DA neurons are most abundant in the midbrain (Substantia Nigra pars compacta [SNC] and Ventral Tegmental Area [VTA]) and the olfactory bulb (OB) in the forebrain. Interestingly, a subtype of OB DA neurons is capable of regenerating throughout life, while a second class is exclusively born during embryonic development. Compelling evidence in SNC and VTA also indicates substantial heterogeneity in terms of morphology, connectivity and function. To further investigate this heterogeneity and directly compare form and function of midbrain and forebrain bulbar DA neurons, we performed immunohistochemistry and whole-cell patch-clamp recordings in ex vivo brain slices from juvenile DAT-tdTomato mice. After confirming the penetrance and specificity of the dopamine transporter (DAT) Cre line, we compared soma shape, passive membrane properties, voltage sags and action potential (AP) firing across midbrain and forebrain bulbar DA subtypes. We found that each DA subgroup within midbrain and OB was highly heterogeneous, and that DA neurons across the two brain areas are also substantially different. These findings complement previous work in rats as well as gene expression and in vivo datasets, further questioning the existence of a single "dopaminergic" neuronal phenotype.

Parkinson's disease model in zebrafish using intraperitoneal MPTP injection

Introduction

Parkinson's disease (PD) is the second most common neurodegenerative disease that severely affects the quality of life of patients and their family members. Exposure to 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) has been shown to reflect behavioral, molecular, and proteomic features of PD. This study aimed to assess the protocol for inducing PD following MPTP injection in adult zebrafish.

Methods

Fish were injected with 100 μg/g of MPTP intraperitoneally once or twice and then assessed on days 1 to 30 post-injection.

Results

Between one-time and two-time injections, there was no significant difference in most locomotor parameters, expressions of tyrosine hydroxylase-2 (th2) and dopamine transporter (dat) genes, and dopaminergic neurons (tyrosine hydroxylase positive, TH+ cells) counts. However, caspase-3 levels significantly differed between one- and two-time injections on the day 1 assessment.

Discussion

Over a 30-day period, the parameters showed significant differences in swimming speed, total distance traveled, tyrosine hydroxylase-1 (th1) and dat gene expressions, caspase-3 and glutathione protein levels, and TH+ cell counts. Days 3 and 5 showed the most changes compared to the control. In conclusion, a one-time injection of MPTP with delayed assessment on days 3 to 5 is a good PD model for animal studies.

Advances in Zebrafish as a Comprehensive Model of Mental Disorders

As an important part in international disease, mental disorders seriously damage human health and social stability, which show the complex pathogenesis and increasing incidence year by year. In order to analyze the pathogenesis of mental disorders as soon as possible and to look for the targeted drug treatment for psychiatric diseases, a more reasonable animal model is imperious demands. Benefiting from its high homology with the human genome, its brain tissue is highly similar to that of humans, and it is easy to realize whole-body optical visualization and high-throughput screening; zebrafish stands out among many animal models of mental disorders. Here, valuable qualified zebrafish mental disorders models could be established through behavioral test and sociological analysis, which are simulated to humans, and combined with molecular analyses and other detection methods. This review focuses on the advances in the zebrafish model to simulate the human mental disorders; summarizes the various behavioral characterization means, the use of equipment, and operation principle; sums up the various mental disorder zebrafish model modeling methods; puts forward the current challenges and future development trend, which is to contribute the theoretical supports for the exploration of the mechanisms and treatment strategies of mental disorders.

Newly regenerated dopaminergic neurons in 6-OHDA-lesioned adult zebrafish brain proliferate in the Olfactory bulb and telencephalon, but migrate to, differentiate and mature in the diencephalon

In order to understand the biological processes underlying dopaminergic neurons (DpN) regeneration in a 6-hydroxydopamine(6-OHDA)-induced adult zebrafish-based Parkinson's disease model, this study investigated the specific phases of neuroregeneration in a time-based manner. Bromodeoxyuridine (BrdU) was administered 24 h before the harvest of brain tissues at day three, five, seven, nine, 12 and 14 postlesion. Potential migration of proliferative cells was tracked over 14 days postlesion through double-pulse tracking [BrdU and 5-ethynyl-2'-deoxyuridine (EdU)] of cells and immunohistostaining of astrocytes [glial fibrillary acidic protein (GFAP)]. Gene expression of foxa2 and nurr1 (nr4a2a) at day three, nine, 14, 18, 22 and 30 postlesion was quantified using qPCR. Protein expression of foxa2 at day three, seven, 14 and 22 postlesion was validated using the western blot technique. Double labelling [EdU and tyrosine hydroxylase (TH)] of proliferative cells was performed to ascertain their fate after the neuroregeneration processes. It was found that whilst cell proliferation remained unchanged in the area of substantial DpN loss, the ventral diencephalon (vDn), there was a transient increase of cell proliferation in the olfactory bulb (OB) and telencephalon (Tel) seven days postlesion. BrdU-immunoreactive (ir)/ EdU-ir cells and activated astrocytes were later found to be significantly increased in the vDn and its nearby area (Tel) 14 days postlesion. There was a significant but transient downregulation of foxa2 at day three and nine postlesion, and nr4a2a at day three, nine and 14 postlesion. The expression of both genes remained unchanged in the OB and Tel. There was a transient downregulation of foxa2 protein expression at day three and seven postlesion. The significant increase of EdU-ir/ TH-ir cells in the vDn 30 days postlesion indicates maturation of proliferative cells (formed between day five-seven postlesion) into DpN. The present findings warrant future investigation of critical factors that govern the distinctive phases of DpN regeneration.

Different inflammation responses modulate Müller glia proliferation in the acute or chronically damaged zebrafish retina

Unlike mammals, zebrafish regenerate in response to retinal damage. Because microglia are activated by retinal damage, we investigated their role during regeneration following either acute or chronic damage. At three weeks post-fertilization (wpf), both wild-type fish exhibiting NMDA-induced acute ganglion and amacrine cell death and gold rush (gosh) mutant fish possessing chronic cone photoreceptor degeneration displayed reactive microglia/macrophages and Müller glia proliferation. Dexamethasone-treated retinas, to inhibit the immune response, lacked reactive microglia/macrophages and possessed fewer PCNA-positive cells, while LPS treatment increased microglia/macrophages and PCNA-labeled cells. NMDA-injured retinas upregulated expression of il-1β and tnfα pro-inflammatory cytokine genes, followed by increased expression of il-10 and arg1 anti-inflammatory/remodeling cytokine genes. A transient early TNFα pro-inflammatory microglia/macrophage population was visualized in NMDA-damaged retinas. In contrast, gosh mutant retinas exhibited a slight increase of pro-inflammatory cytokine gene expression concurrently with a greater increased anti-inflammatory/remodeling cytokine gene expression. Few TNFα pro-inflammatory microglia/macrophages were observed in the gosh retina. Understanding why acute and chronic damage results in different inflammation profiles and their effects on regulating zebrafish retinal regeneration would provide important clues toward improving therapeutic strategies for repairing injured mammalian tissues.

Uncovering the spectrum of adult zebrafish neural stem cell cycle regulators

Adult neural stem and progenitor cells (aNSPCs) persist lifelong in teleost models in diverse stem cell niches of the brain and spinal cord. Fish maintain developmental stem cell populations throughout life, including both neuro-epithelial cells (NECs) and radial-glial cells (RGCs). Within stem cell domains of the brain, RGCs persist in a cycling or quiescent state, whereas NECs continuously divide. Heterogeneous populations of RGCs also sit adjacent the central canal of the spinal cord, showing infrequent proliferative activity under homeostasis. With the rise of the zebrafish (Danio rerio) model to study adult neurogenesis and neuroregeneration in the central nervous system (CNS), it has become evident that aNSPC proliferation is regulated by a wealth of stimuli that may be coupled with biological function. Growing evidence suggests that aNSPCs are sensitive to environmental cues, social interactions, nutrient availability, and neurotrauma for example, and that distinct stem and progenitor cell populations alter their cell cycle activity accordingly. Such stimuli appear to act as triggers to either turn on normally dormant aNSPCs or modulate constitutive rates of niche-specific cell cycle behaviour. Defining the various forms of stimuli that influence RGC and NEC proliferation, and identifying the molecular regulators responsible, will strengthen our understanding of the connection between aNSPC activity and their biological significance. In this review, we aim to bring together the current state of knowledge on aNSPCs from studies investigating the zebrafish CNS, while highlighting emerging cell cycle regulators and outstanding questions that will help to advance this fascinating field of stem cell biology.

Regenerative neurogenesis: the integration of developmental, physiological and immune signals

In fishes and salamanders, but not mammals, neural stem cells switch back to neurogenesis after injury. The signalling environment of neural stem cells is strongly altered by the presence of damaged cells and an influx of immune, as well as other, cells. Here, we summarise our recently expanded knowledge of developmental, physiological and immune signals that act on neural stem cells in the zebrafish central nervous system to directly, or indirectly, influence their neurogenic state. These signals act on several intracellular pathways, which leads to changes in chromatin accessibility and gene expression, ultimately resulting in regenerative neurogenesis. Translational approaches in non-regenerating mammals indicate that central nervous system stem cells can be reprogrammed for neurogenesis. Understanding signalling mechanisms in naturally regenerating species show the path to experimentally promoting neurogenesis in mammals.

Comparative Study in Zebrafish and Medaka Unravels the Mechanisms of Tissue Regeneration

Tissue regeneration has been in the spotlight of research for its fascinating nature and potential applications in human diseases. The trait of regenerative capacity occurs diversely across species and tissue contexts, while it seems to decline over evolution. Organisms with variable regenerative capacity are usually distinct in phylogeny, anatomy, and physiology. This phenomenon hinders the feasibility of studying tissue regeneration by directly comparing regenerative with non-regenerative animals, such as zebrafish (Danio rerio) and mice (Mus musculus). Medaka (Oryzias latipes) is a fish model with a complete reference genome and shares a common ancestor with zebrafish approximately 110–200 million years ago (compared to 650 million years with mice). Medaka shares similar features with zebrafish, including size, diet, organ system, gross anatomy, and living environment. However, while zebrafish regenerate almost every organ upon experimental injury, medaka shows uneven regenerative capacity. Their common and distinct biological features make them a unique platform for reciprocal analyses to understand the mechanisms of tissue regeneration. Here we summarize current knowledge about tissue regeneration in these fish models in terms of injured tissues, repairing mechanisms, available materials, and established technologies. We further highlight the concept of inter-species and inter-organ comparisons, which may reveal mechanistic insights and hint at therapeutic strategies for human diseases.

Attractin Participates in Schizophrenia by Affecting Testosterone Levels

Attractin (ATRN) is a widely expressed glycoprotein that is involved in energy homeostasis, neurodevelopment, and immune response. It is encoded by a gene spanning 180 kb on chromosome 20p13, a region previously implicated in schizophrenia by linkage studies. To address a possible role of ATRN in disorders of the central nervous system, we created an atrn knockout zebrafish line and performed behavioral tests. Adult atrn^–/– zebrafish exhibited more pronounced attack behavior relative to wild-type control zebrafish in a tracking analysis. Biochemical analysis revealed elevated testosterone levels in atrn^–/– zebrafish. At the gene expression level, we noted an upregulation of cyp51 and hsd17b7, key proteins in testosterone synthesis in the brains of both adult and larvae of atrn^–/– zebrafish. In order to further elucidate the relationship between testosterone and behavioral syndromes, we then compared testosterone levels of 9,008 psychiatric patients and 247 healthy controls from the same catchment area. Of all subjects examined, male subjects with schizophrenia exhibited lower testosterone levels compared with controls. In contrast, female subjects with a diagnosis of schizophrenia or bipolar disorder featured higher testosterone levels than did same sex controls. Purposeful sampling of extreme groups showed reduced ATRN expression in a subset of these subjects. Finally, we identified 14 subjects with ATRN mutations. All of whom displayed abnormal testosterone levels. In summary, the interplay of ATRN and testosterone may help to explain sexual dimorphisms in selected behavioral phenotypes.

An insight on established retinal injury mechanisms and prevalent retinal stem cell activation pathways in vertebrate models

Implementing different tools and injury mechanisms in multiple animal models of retina regeneration, researchers have discovered the existence of retinal stem/progenitor cells. Although they appear to be distributed uniformly across the vertebrate lineage, the reparative potential of the retina is mainly restricted to lower vertebrates. Regenerative repair post-injury requires the creation of a proliferative niche, vital for proper stem cell activation, propagation, and lineage differentiation. This seems to be lacking in mammals. Hence, in this review, we first discuss the many forms of retinal injuries that have been generated using animal models. Next, we discuss how they are utilized to stimulate regeneration and mimic eye disease pathologies. The key to driving stem cell activation in mammals relies on the information we can gather from these models. Lastly, we present a brief update about the genes, growth factors, and signaling pathways that have been brought to light using these models.

Gonadotropin Releasing Hormone (Gnrh) Triggers Neurogenesis in the Hypothalamus of Adult Zebrafish

Recently, it has been shown in adult mammals that the hypothalamus can generate new cells in response to metabolic changes, and tanycytes, putative descendants of radial glia, can give rise to neurons. Previously we have shown in vitro that neurospheres generated from the hypothalamus of adult zebrafish show increased neurogenesis in response to exogenously applied hormones. To determine whether adult zebrafish have a hormone-responsive tanycyte-like population in the hypothalamus, we characterized proliferative domains within this region. Here we show that the parvocellular nucleus of the preoptic region (POA) labels with neurogenic/tanycyte markers vimentin, GFAP/Zrf1, and Sox2, but these cells are generally non-proliferative. In contrast, Sox2+ proliferative cells in the ventral POA did not express vimentin and GFAP/Zrf1. A subset of the Sox2+ cells co-localized with Fezf2:GFP, a transcription factor important for neuroendocrine cell specification. Exogenous treatments of GnRH and testosterone were assayed in vivo. While the testosterone-treated animals showed no significant changes in proliferation, the GnRH-treated animals showed significant increases in the number of BrdU-labeled cells and Sox2+ cells. Thus, cells in the proliferative domains of the zebrafish POA do not express radial glia (tanycyte) markers vimentin and GFAP/Zrf1, and yet, are responsive to exogenously applied GnRH treatment.

A unique macrophage subpopulation signals directly to progenitor cells to promote regenerative neurogenesis in the zebrafish spinal cord

Central nervous system injury re-initiates neurogenesis in anamniotes (amphibians and fishes), but not in mammals. Activation of the innate immune system promotes regenerative neurogenesis, but it is fundamentally unknown whether this is indirect through the activation of known developmental signaling pathways or whether immune cells directly signal to progenitor cells using mechanisms that are unique to regeneration. Using single-cell RNA-seq of progenitor cells and macrophages, as well as cell-type-specific manipulations, we provide evidence for a direct signaling axis from specific lesion-activated macrophages to spinal progenitor cells to promote regenerative neurogenesis in zebrafish. Mechanistically, TNFa from pro-regenerative macrophages induces Tnfrsf1a-mediated AP-1 activity in progenitors to increase regeneration-promoting expression of hdac1 and neurogenesis. This establishes the principle that macrophages directly communicate to spinal progenitor cells via non-developmental signals after injury, providing potential targets for future interventions in the regeneration-deficient spinal cord of mammals.

Modeling Neuroregeneration and Neurorepair in an Aging Context: The Power of a Teleost Model

Aging increases the risk for neurodegenerative disease and brain trauma, both leading to irreversible and multifaceted deficits that impose a clear societal and economic burden onto the growing world population. Despite tremendous research efforts, there are still no treatments available that can fully restore brain function, which would imply neuroregeneration. In the adult mammalian brain, neuroregeneration is naturally limited, even more so in an aging context. In view of the significant influence of aging on (late-onset) neurological disease, it is a critical factor in future research. This review discusses the use of a non-standard gerontology model, the teleost brain, for studying the impact of aging on neurorepair. Teleost fish share a vertebrate physiology with mammals, including mammalian-like aging, but in contrast to mammals have a high capacity for regeneration. Moreover, access to large mutagenesis screens empowers these teleost species to fill the gap between established invertebrate and rodent models. As such, we here highlight opportunities to decode the factor age in relation to neurorepair, and we propose the use of teleost fish, and in particular killifish, to fuel new research in the neuro-gerontology field.

Adult Endogenous Dopaminergic Neuroregeneration Against Parkinson's Disease: Ideal Animal Models?

Parkinson's disease (PD) is the second most common neurodegenerative disease. The etiology of PD remains an enigma with no available disease modifying treatment or cure. Pharmacological compensation is the only quality of life improving treatments available. Endogenous dopaminergic neuroregeneration has recently been considered a plausible therapeutic strategy for PD. However, researchers have to first decipher the complexity of adult endogenous neuroregeneration. This raises the need of animal models to understand the underlying molecular basis. Mammalian models with highly conserved genetic homology might aid researchers to identify specific molecular mechanisms. However, the scarcity of adult neuroregeneration potential in mammals obfuscates such investigations. Nowadays, non-mammalian models are gaining popularity due to their explicit ability to neuroregenerate naturally without the need of external enhancements, yet these non-mammals have a much diverse gene homology that critical molecular signals might not be conserved across species. The present review highlights the advantages and disadvantages of both mammalian and non-mammalian animal models that can be essentially used to study the potential of endogenous DpN regeneration against PD.

Vertebrate brain regeneration - a community effort of fate-restricted precursor cell types

The process of regeneration describes the full restoration of tissue after destruction from injury or disease. Most mammals show very limited ability for regeneration of adult organs, while vertebrate models of regeneration such as fish and salamanders, allow to study regeneration mechanism of the brain, heart, limbs, retina, and other organs in adults. The regenerative abilities of teleost fish are well documented, but the cellular sources for regeneration, the specificity of source cells for restored cell types, as well as the extent and fidelity of cell replacement are only beginning to be revealed for many regeneration paradigms. Here, we highlight recent analyses of adult neurogenesis and regeneration after injury in teleost fish that address these issues, and we discuss how such analyses can help to evaluate the role of different cells in tissues in the regeneration process.

Dopaminergic neurons regenerate following chemogenetic ablation in the olfactory bulb of adult Zebrafish (Danio rerio)

Adult zebrafish have the ability to regenerate cells of the central nervous system. However, few neuronal regeneration studies in adult zebrafish addressed their ability to regenerate specific types of neurons following cell specific ablation. We show here that treatment of transgenic Tg(dat:CFP-NTR) adult zebrafish with the prodrug metronidazole (Mtz) according to our administration regimen predominantly ablates dopamine (DA) neurons within the olfactory bulb (OB) of adult fish. Loss of DA neurons was accompanied by an impaired olfaction phenotype, as early as 1-week post-treatment, in which fish were unable to sense the presence of the repulsive stimulus cadaverine. The olfactory impairment was reversed within 45 days and coincided with the recovery of DA neuron counts in the OB. A multi-label pulse-chase analysis with BrdU and EdU over the first seventeen days-post Mtz exposure showed that newly formed DA neurons were recruited within the first nine days following exposure and led to functional and morphological recovery of the OB.

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.

Inflammation and matrix metalloproteinase 9 (Mmp-9) regulate photoreceptor regeneration in adult zebrafish

Brain injury activates complex inflammatory signals in dying neurons, surviving neurons, and glia. Here, we establish that inflammation regulates the regeneration of photoreceptors in the zebrafish retina and determine the cellular expression and function of the inflammatory protease, matrix metalloproteinase 9 (Mmp-9), during this regenerative neurogenesis. Following photoreceptor ablation, anti-inflammatory treatment suppresses the number of injury-induced progenitors and regenerated photoreceptors. Upon photoreceptor injury, mmp-9 is induced in Müller glia and Müller glia-derived photoreceptor progenitors. Deleting mmp-9 results in over production of injury-induced progenitors and regenerated photoreceptors, but over time the absence of Mmp-9 compromises the survival of the regenerated cones. At all time-points studied, the levels of tnf-α are significantly elevated in mutant retinas. Anti-inflammatory treatment in mutants rescues the defects in cone survival. These data provide a link between injury-induced inflammation in the vertebrate CNS, Mmp-9 function during neuronal regeneration and the requirement of Mmp-9 for the survival of regenerated cones.

Neural circuit reorganisation after spinal cord injury in zebrafish

Spinal cord injuries disrupt signalling from the brain leading to loss of limb, locomotion, sexual and bladder function, usually irreversible in humans. In zebrafish, recovery of function occurs in a few days for larvae or a few weeks for adults due to regrowth of axons and de novo neurogenesis. Together with its genetic amenability and optical clarity, this makes zebrafish a powerful animal model to study circuit reorganisation after spinal cord injuries. With the fast evolution of techniques, we can forecast significative improvements of our knowledge of the mechanisms leading to successful or failed recovery of spinal cord function. We review here the present knowledge on the subject, the new technological approaches and we propose future directions of research.

Regeneration of the central nervous system-principles from brain regeneration in adult zebrafish

Poor recovery of neuronal functions is one of the most common healthcare challenges for patients with different types of brain injuries and/or neurodegenerative diseases. Therapeutic interventions face two major challenges: (1) How to generate neurons de novo to replenish the neuronal loss caused by injuries or neurodegeneration (restorative neurogenesis) and (2) How to prevent or limit the secondary tissue damage caused by long-term accumulation of glial cells, including microglia, at injury site (glial scar). In contrast to mammals, zebrafish have extensive regenerative capacity in numerous vital organs, including the brain, thus making them a valuable model to improve the existing therapeutic approaches for human brain repair. In response to injuries to the central nervous system (CNS), zebrafish have developed specific mechanisms to promote the recovery of the lost tissue architecture and functionality of the damaged CNS. These mechanisms include the activation of a restorative neurogenic program in a specific set of glial cells (ependymoglia) and the resolution of both the glial scar and inflammation, thus enabling proper neuronal specification and survival. In this review, we discuss the cellular and molecular mechanisms underlying the regenerative ability in the adult zebrafish brain and conclude with the potential applicability of these mechanisms in repair of the mammalian CNS.

Hexb enzyme deficiency leads to lysosomal abnormalities in radial glia and microglia in zebrafish brain development

Sphingolipidoses are severe, mostly infantile lysosomal storage disorders (LSDs) caused by defective glycosphingolipid degradation. Two of these sphingolipidoses, Tay Sachs and Sandhoff diseases, are caused by β-Hexosaminidase (HEXB) enzyme deficiency, resulting in ganglioside (GM2) accumulation and neuronal loss. The precise sequence of cellular events preceding, and leading to, neuropathology remains unclear, but likely involves inflammation and lysosomal accumulation of GM2 in multiple cell types. We aimed to determine the consequences of Hexb activity loss for different brain cell types using zebrafish. Hexb deficient zebrafish (hexb-/- ) showed lysosomal abnormalities already early in development both in radial glia, which are the neuronal and glial progenitors, and in microglia. Additionally, at 5 days postfertilization, hexb-/- zebrafish showed reduced locomotor activity. Although specific oligosaccharides accumulate in the adult brain, hexb-/- ) zebrafish are viable and apparently resistant to Hexb deficiency. In all, we identified cellular consequences of loss of Hexb enzyme activity during embryonic brain development, showing early effects on glia, which possibly underlie the behavioral aberrations. Hereby, we identified clues into the contribution of non-neuronal lysosomal abnormalities in LSDs affecting the brain and provide a tool to further study what underlies the relative resistance to Hexb deficiency in vivo.