GFAP transgenic zebrafish

Age-dependent glial heterogeneity and traumatic injury responses in a vertebrate brain structure

The progression of traumatic brain injury (TBI) pathology is significantly influenced by age and involves a complex interplay of glial cells. However, the influence of age on the glial dynamics and their TBI responses remains mostly unexplored. Here, we obtain a comprehensive single-cell transcriptome atlas of three major glial types under the physiological and TBI conditions across four post-embryonic life stages in the zebrafish midbrain optic tectum. We identify a library of glial subtypes and states with specific age-dependent patterns that respond distinctly to TBI. Combining the glial interactome analysis and CRISPR-Cas9-mediated gene disruption, we reveal the essential roles of dla-notch3 and cxcl12a-cxcr4b interactions in the early-larval-stage-specific unresponsiveness of radial astrocytes to TBI and the TBI-induced age-independent recruitment of microglia to injury sites, respectively. Overall, our findings provide the molecular and cellular framework of TBI-induced age-related glial dynamics in vertebrate brains.

Analysis of the senescence secretome during zebrafish retina regeneration

Introduction

Zebrafish possess the innate ability to regenerate any lost or damaged retinal cell type with Müller glia serving as resident stem cells. Recently, we discovered that this process is aided by a population of damage-induced senescent immune cells. As part of the Senescence Associated Secretory Phenotype (SASP), senescent cells secrete numerous factors that can play a role in the modulation of inflammation and remodeling of the retinal microenvironment during regeneration. However, the identity of specific SASP factors that drive initiation and progression of retina regeneration remains unclear.

Materials and Methods

We mined the SASP Atlas and publicly available RNAseq datasets to identify common, differentially expressed SASP factors after retina injury. These datasets included two distinct acute damage regimens, as well as two chronic, genetic models of retina degeneration. We identified overlapping factors between these models and used genetic knockdown experiments, qRT/PCR and immunohistochemical staining to test a role for one of these factors (npm1a).

Results

We discovered an overlapping set of 31 SASP-related regeneration factors across all data sets and damage paradigms. These factors are upregulated after damage with functions that span the innate immune system, autophagic processing, cell cycle regulation, and cellular stress responses. From among these, we show that depletion of Nucleophosmin 1 (npm1a) inhibits retina regeneration and decreases senescent cell detection after damage.

Discussion

Our data suggest that differential expression of SASP factors promotes initiation and progression of retina regeneration after both acute and chronic retinal damage. The existence of a common, overlapping set of 31 factors provides a group of novel therapeutic targets for retina regeneration studies.

Mechanistic Insights into 3-Isopropylphenol-Induced Neurotoxicity in Zebrafish: A Network Toxicology and Molecular Docking Approach

Endocrine-disrupting chemicals (EDCs) are exogenous substances discharged into the environment through human activities. 3-Isopropylphenol, a typical alkylphenol-based EDC, has been extensively studied due to its broad application and potential ecological impacts. However, the mechanism of its neurotoxicity remains unclear. In this study, the neurotoxic effects of 3-isopropylphenol were examined using the zebrafish model. We predicted its potential toxic mechanisms and action targets using network toxicology and molecular docking and verified them via RT-qPCR. Results showed that 3-isopropylphenol exposure inhibits the cAMP/PKA signaling pathway in zebrafish larvae, promoting apoptosis, impairing neural development, and suppressing locomotor behavior. These findings enhance our understanding of the toxic effects and mechanisms of 3-isopropylphenol on zebrafish larval neural development and aid in evaluating its potential ecological hazards.

A multi-omics approach reveals impaired lipid metabolism and oxidative stress in a zebrafish model of Alexander disease

Alexander disease (AxD) is a rare leukodystrophy caused by heterozygous mutations in the GFAP gene. To date, several in vitro and in vivo models have been generated in an attempt to unravel the main mechanisms underlying this complex disease. However, none of these models is suitable for investigating the global dysregulation caused by AxD. To address this shortcoming, we have generated a stable transgenic zebrafish line (zAxD) carrying the human GFAP p.R239C mutation, which is associated with severe phenotypes of AxD type I patients. We then performed transcriptomics and proteomics analyses on the whole larvae of our zAxD model, confirming the involvement of several pathways such as the immune system response and inflammation, oxidative stress, extracellular matrix, lipoxidation and lipid metabolism, which were previously reported in more limited omic studies. Interestingly, new pathways emerged as well, including tyrosine and butanoate metabolic processes. Biochemical assays confirmed alterations in cell respiration and lipid metabolism as well as elevated oxidative stress. These findings confirm the reliability of the zAxD model to apply a whole-organism approach to investigate the molecular basis of the disease.

Microglia-Derived IL-6 Promotes Müller Glia Reprogramming and Proliferation in Zebrafish Retina Regeneration

Purpose

Inflammation activates the Jak1-Stat3 signaling pathway in zebrafish Müller glia (MG), leading to their status transition and proliferation following retinal injury. However, the source of Stat3-activating molecules remains unclear. This study aims to explore the expression and function of a Stat3-activating cytokine IL-6 in zebrafish retina regeneration.

Methods

Mechanical retinal injury was induced in adult zebrafish by a needle-poke lesion. Single-cell RNA sequencing (scRNAseq) and PCR were used to determine gene expression. Microglia ablation was performed by using the mpeg1:nsfb-mcherry transgenic zebrafish. Morpholino oligonucleotides, a recombinant zebrafish IL-6 protein and drugs, were used to manipulate IL-6 or Stat3 signaling in the retina. The 5-Ethynyl-2'-deoxyuridine (EdU) labeling was used to evaluate MG proliferation and the formation of MG-derived progenitor cells (MGPCs). Neuronal regeneration in the retina was analyzed by lineage tracing and immunostaining.

Results

The scRNAseq reveals that IL-6 is mainly expressed by a subset of pro-inflammatory microglia in the injured retina. Loss- and gain-of-function experiments demonstrate that IL-6 signaling promotes MG proliferation and the formation of MGPCs following retinal injury. Additionally, IL-6 facilitates MG status transition by modulating Jak1-Stat3 signaling and the expression of regeneration-associated genes. Interestingly, IL-6 may also regulate MGPC formation via phase-dependent pro-inflammatory and anti-inflammatory mechanisms. Finally, IL-6 promotes the early differentiation of MGPCs and contributes to the regeneration of retinal neurons in the injured retina.

Conclusions

Our study unveils the critical role of microglia-derived IL-6 in zebrafish retina regeneration, with potential implications for mammalian MG reprogramming.

Defining the molecular identity and morphology of glia limitans superficialis astrocytes in vertebrates

Astrocytes are a highly abundant glial cell type and perform critical homeostatic functions in the central nervous system. Like neurons, astrocytes have many discrete heterogeneous subtypes. The subtype identity and functions are, at least in part, associated with their anatomical location and can be highly restricted to strategically important anatomical domains. Here, we report that astrocytes forming the glia limitans superficialis, the outermost border of the brain and spinal cord, are a highly specialized astrocyte subtype and can be identified by a single marker: myocilin (Myoc). We show that glia limitans superficialis astrocytes cover the entire brain and spinal cord surface, exhibit an atypical morphology, and are evolutionarily conserved from zebrafish, rodents, and non-human primates to humans. Identification of this highly specialized astrocyte subtype will advance our understanding of CNS homeostasis and potentially be targeted for therapeutic intervention to combat peripheral inflammatory effects on the CNS.

Dscamb regulates cone mosaic formation in zebrafish via filopodium-mediated homotypic recognition

Cone photoreceptors assemble to form a regular mosaic pattern in vertebrate retinas. In zebrafish, four distinct spectral cone types (red, green, blue, and ultraviolet), form a lattice-like pattern. However, the mechanism of cone mosaic formation has been unknown. Here we show that Down Syndrome Cell Adhesion Molecule b (Dscamb) regulates the cone mosaic pattern in zebrafish, especially via red-cone spacing. During photoreceptor differentiation, newly formed cones extend filopodium-like processes laterally to apical surfaces of neighboring cones. Interestingly, red cones extend filopodia, but promptly retract them when they meet their own cone type, suggesting filopodium-mediated, homotypic recognition and self-avoidance. This self-avoidance is compromised in zebrafish dscamb mutants, leading to abnormal clustering of red cones and subsequent disruption of regular cone spacing. Thus, apical filopodium-mediated spacing of the same cone type depends on Dscamb and is essential for cone mosaic formation in zebrafish.

An ultrastructural map of a spinal sensorimotor circuit reveals the potential of astroglial modulation

Information flow through circuits is dictated by the precise connectivity of neurons and glia. While a single astrocyte can contact many synapses, how glial-synaptic interactions are arranged within a single circuit to impact information flow remains understudied. Here, we use the local spinal sensorimotor circuit in zebrafish as a model to understand how neurons and astroglia are connected in a vertebrate circuit. With semi-automated cellular reconstructions and automated approaches to map all the synaptic connections, we identified the precise synaptic connections of the local sensorimotor circuit, from dorsal root ganglia neurons to spinal interneurons and finally to motor neurons. This revealed a complex network of interneurons that interact in the local sensorimotor circuit. We then mapped the glial processes within tripartite synapses in the circuit. We demonstrate that tripartite synapses are equally distributed across the circuit, supporting the idea that glia can modulate information flow through the circuit at different levels. We show that multiple astroglia, including bona fide astrocytes, contact synapses within a single sensory neuron's circuit and that each of these astroglia can contact multiple parts of the circuit. This detailed map reveals an extensive network of connected neurons and astroglia that process sensory stimuli in a vertebrate. We then utilized this ultrastructural map to model how synaptic thresholding and glial modulation could alter information flow in circuits. We validated this circuit map with GCaMP6s imaging of dorsal root ganglia, spinal neurons and astroglia. This work provides a foundational resource detailing the ultrastructural organization of neurons and glia in a vertebrate circuit, offering insights in how glia could influence information flow in complex neural networks.

Phenyl salicylate induces neurotoxicity and early Alzheimer's disease-like symptoms through ndrg1-regulated myelin damage, increasing bace1 in zebrafish

Phenyl salicylate, an important industrial raw material, is widely used in plastics, cosmetics, and pharmaceuticals. However, little is known about its neurotoxicity on wildlife. Here, we exposed zebrafish embryos at 4 hours post-fertilization (hpf) to 0.025, 0.05, 0.1, 0.25, 0.5, and 1.0 mg/L of phenyl salicylate up to 144 hpf and found its developmental- and neuro-toxicity. Specifically, a dose-dependent increase in mortality and malformation in zebrafish were revealed. Phenyl salicylate also adversely affected the development of monoaminergic neurons, cerebral blood vessels, and the blood-brain barrier (BBB), as well as induced cerebral hemorrhages and locomotion change. RNA-sequencing results combined with verification data showed that phenyl salicylate downregulated the expression of the N-myc downstream regulated gene-1 (ndrg1), caused myelin damage in zebrafish, and then increased expression of beta-secretase 1 (bace1), which ultimately led to early Alzheimer's disease (AD)-like symptoms, including BBB leakage, bleeding in the brain, and upregulation of the glial fibrillary acidic protein gene (gfap) and cholinergic system-related gene (chrna7a). In conclusion, phenyl salicylate exposure triggered developmental toxicity and neurotoxicity in zebrafish, which has a potential risk for the development of AD. Given the effects of phenyl salicylate exposure to ecosystem, the safety usage limit should be treated with caution.

Activation of Wnt/β-catenin in neural progenitor cells regulates blood-brain barrier development and promotes neuroinflammation

The central nervous system (CNS) requires specialized blood vessels to support neural function within specific microenvironments. During neurovascular development, endothelial Wnt/β-catenin signaling is required for BBB development within the brain parenchyma, whereas fenestrated blood vessels that lack BBB properties do not require Wnt/β-catenin signaling. Here, we used zebrafish to further characterize this phenotypic heterogeneity of the CNS vasculature. Using transgenic reporters of Wnt/β-catenin transcriptional activity, we found an inverse correlation between activated Wnt/β-catenin signaling in endothelial cells (ECs) versus non-ECs within these distinct microenvironments. Our results indicated that the level of Wnt/β-catenin signaling in non-ECs may regulate Wnt/β-catenin activity in adjacent ECs. To further test this concept, we generated a transgenic Tet-On inducible system to drive constitutively active β-catenin expression in neural progenitor cells (NPCs). We found that dose-dependent activation of Wnt/β-catenin in NPCs caused severe deficiency in CNS angiogenesis and BBB development. Additionally, we discovered a significant increase in the proliferation of microglia and infiltration of peripheral neutrophils indicative of a stereotypical neuroinflammatory response. In conclusion, our results demonstrate the importance of proper Wnt/β-catenin signaling within specific CNS microenvironments and highlights the potentially deleterious consequences of aberrant Wnt activation.

Zebrafish glial-vascular interactions progressively expand over the course of brain development

Glial-vascular interactions are critical for the formation and maintenance of brain blood vessels and the blood-brain barrier (BBB) in mammals, but their role in the zebrafish BBB remains unclear. Using three glial gene promoters-gfap, glast, and glastini (a truncated glast)-we explored glial-vascular development in zebrafish. Sparse labeling showed fewer glial-vascular interactions at early stages, with glial coverage and contact area increasing with age. Stable transgenic lines for glast and glastini revealed similar developmental increases, starting at ∼30% coverage at 3 days post-fertilization (dpf) and peaking at ∼60% by 10 dpf, and consistently higher glial coverage in the forebrain and midbrain than in the hindbrain. Electron microscopy analyses showed similar progressive increases in glial-vascular interactions, with maximal coverage of ∼70% in adults-significantly lower than the ∼100% seen in mammals. These findings define the temporal and regional maturation of glial-vascular interactions in zebrafish and highlight differences from mammalian systems.

tet2 and tet3 regulate cell fate specification and differentiation events during retinal development

Tet enzymes are epigenetic modifiers that impact gene expression via 5mC to 5hmC oxidation. Previous work demonstrated the requirement for Tet and 5hmC during zebrafish retinogenesis. tet2 -/- ;tet3 -/- mutants possessed defects in the formation of differentiated retinal neurons, but the mechanisms underlying these defects are unknown. Here, we leveraged scRNAseq technologies to better understand cell type-specific deficits and molecular signatures underlying the tet2 -/- ;tet3 -/- retinal phenotype. Our results identified defects in the tet2 -/- ;tet3 -/- retinae that included delayed specification of several retinal cell types, reduced maturity across late-stage cones, expansions of immature subpopulations of horizontal and bipolar cells, and altered biases of bipolar cell subtype fates at late differentiation stages. Together, these data highlight the critical role that tet2 and tet3 play as regulators of cell fate specification and terminal differentiation events during retinal development.

A novel GFAP frameshift variant identified in a family with optico-retinal dysplasia and vision impairment

Gain-of-function variants in GFAP leads to protein aggregation and is the cause of the severe neurodegenerative disorder Alexander Disease (AxD), while loss of GFAP function has been considered benign. Here, we investigated a six-generation family, where multiple individuals presented with gliosis of the optic nerve head and visual impairment. Whole genome sequencing (WGS) revealed a frameshift variant in GFAP (c.928dup, p.(Met310Asnfs*113)) segregating with disease. Analysis of human embryonic tissues revealed strong expression of GFAP in retinal neural progenitors. A zebrafish model verified that c.928dup does not result in extensive GFAP protein aggregation and zebrafish gfap loss-of-function mutants showed vision impairment and retinal dysplasia, characterized by a significant loss of Müller glia cells and photoreceptor cells. Our findings show how different mutational mechanisms can cause diverging phenotypes and reveal a novel function of GFAP in vertebrate eye development.

A zebrafish-based in vivo model of Zika virus infection unveils alterations of the glutamatergic neuronal development and NS4A as a key viral determinant of neuropathogenesis

Infection of pregnant women by Zika virus (ZIKV) is associated with severe neurodevelopmental defects in newborns through poorly defined mechanisms. Here, we established a zebrafish in vivo model of ZIKV infection to circumvent limitations of existing mammalian models. Leveraging the unique tractability of this system, we gained unprecedented access to the ZIKV-infected brain at early developmental stages. The infection of zebrafish larvae with ZIKV phenocopied the disease in mammals including a reduced head area and neural progenitor cells (NPC) infection and depletion. Moreover, transcriptomic analyses of NPCs isolated from ZIKV-infected embryos revealed a distinct dysregulation of genes involved in survival and neuronal differentiation, including downregulation of the expression of the glutamate transporter vglut1, resulting in an altered glutamatergic network in the brain. Mechanistically, ectopic expression of ZIKV protein NS4A in the larvae recapitulated the morphological defects observed in infected animals, identifying NS4A as a key determinant of neurovirulence and a promising antiviral target for developing therapies.

'Iterative Bleaching Extends Multiplexity' facilitates simultaneous identification of all major retinal cell types

To understand the multicellular composition of tissues, and how it is altered during development, ageing and/or disease, we must visualise the complete cellular landscape. Currently, this is hindered by our limited ability to combine multiple cellular markers. To overcome this, we adapted a highly multiplexed immunofluorescence (IF) technique called 'Iterative Bleaching Extends Multiplexity' (IBEX) to the zebrafish retina. We optimised fluorescent antibody micro-conjugation to perform sequential rounds of labelling on a single tissue to simultaneously visualise all major retinal cell types with 11 cell-specific antibodies. We further adapted IBEX to be compatible with fluorescent transgenic reporter lines, in situ hybridisation chain reaction (HCR), and whole-mount immunofluorescence (WMIF). We applied IBEX at multiple stages to study the spatial and temporal relationships between glia and neurons during retinal development. Finally, we demonstrate the utility of IBEX across species by testing it on the turquoise killifish (Nothobranchius furzeri) and African clawed frog (Xenopus laevis) to glean large amounts of information from precious tissues. These techniques will revolutionise our ability to visualise multiple cell types in any organism where antibodies are readily available.

Non-apoptotic caspase events and Atf3 expression underlie direct neuronal differentiation of adult neural stem cells

Neural stem cells (NSCs) generate neurons over a lifetime in adult vertebrate brains. In the adult zebrafish pallium, NSCs persist long term through balanced fate decisions. These decisions include direct neuronal conversions, i.e. delamination and neurogenesis without a division. To characterize this process, we reanalyze intravital imaging data of adult pallial NSCs, and observe shared delamination dynamics between NSCs and committed neuronal progenitors. Searching for mechanisms predicting direct NSC conversions, we build an NSC-specific genetic tracer of Caspase3/7 activation (Cas3*/Cas7*) in vivo. We show that non-apoptotic Cas3*/7* events occur in adult NSCs and are biased towards lineage termination under physiological conditions, with a predominant generation of single neurons. We further identify the transcription factor Atf3 as necessary for this bias. Finally, we show that the Cas3*/7* pathway is engaged by NSCs upon parenchymal lesion and correlates with NSCs more prone to lineage termination and neuron formation. These results provide evidence for non-apoptotic caspase events occurring in vertebrate adult NSCs and link these events with the NSC fate decision of direct conversion, which is important for long-term NSC population homeostasis.

Protocol for tissue-specific mutagenesis with fluorescent labeling in zebrafish

Here, we present a protocol for tissue-specific mutagenesis in zebrafish. We describe the preparation of the Tol2 transposase donor vector containing a U6 promoter that drives the transcription of single-guide RNAs (sgRNAs) and Cas9 under the control of a tissue-specific promoter. We then detail the establishment, identification, and phenotypic analysis of the stable tissue-specific mutagenesis zebrafish line. This protocol is useful for generating stable tissue-specific knockout lines to analyze mosaic loss-of-function phenotypes. For complete details on the use and execution of this protocol, please refer to Luo et al.1.

Endolysosomal dysfunction in radial glia progenitor cells leads to defective cerebral angiogenesis and compromised blood-brain barrier integrity

The neurovascular unit (NVU) is a complex multicellular structure that helps maintain cerebral homeostasis and blood-brain barrier (BBB) integrity. While extensive evidence links NVU alterations to cerebrovascular diseases and neurodegeneration, the underlying molecular mechanisms remain unclear. Here, we use zebrafish embryos carrying a mutation in Scavenger Receptor B2, a highly conserved endolysosomal protein expressed predominantly in Radial Glia Cells (RGCs), to investigate the interplay among different NVU components. Through live imaging and genetic manipulations, we demonstrate that compromised acidification of the endolysosomal compartment in mutant RGCs leads to impaired Notch3 signaling, thereby inducing excessive neurogenesis and reduced glial differentiation. We further demonstrate that alterations to the neuron/glia balance result in impaired VEGF and Wnt signaling, leading to severe vascular defects, hemorrhages, and a leaky BBB. Altogether, our findings provide insights into NVU formation and function and offer avenues for investigating diseases involving white matter defects and vascular abnormalities.

The potential toxic effects of estrogen exposure on neural and vascular development in zebrafish

Estrogens and estrogenic chemicals are endocrine-disrupting chemicals (EDCs). The potential toxicity of EDCs to humans and aquatic organisms has become increasingly concerning. However, at present, the potential toxic mechanisms of EDCs on neural and vascular development are still being fully investigated. During the study, we utilized zebrafish to assess the developmental neural and vascular toxicity of different estrogens. The results indicated that zebrafish treated with different estrogens, especially E2, exhibit developmental malformations, including increased mortality, decreased body length, decreased heart rate, aberrant swimming behavior, and increased developmental malformations, including spinal curvature (SC), yolk edema (YE) and pericaidial edema (PE), in a dose-dependent manner with 72 h-treated. Further morphological evaluation revealed that E2 exposure significantly induced motor neural abnormalities in zebrafish embryos. In addition, treated with these three estrogens also impaired the vascular development in the early stage of zebrafish embryos. Mechanistically, the identification of downstream factors revealed that several key neural and vascular development-related genes, including syn2a, gfap, gap43, shha, kdr, flt1 and flt4, were transcriptionally downregulated after estrogen exposure in zebrafish, suggesting that estrogen exposure might cause neural and vascular toxicity by interfering the mRNA levels of genes relevant to neural and vascular development.

Her4.3+ radial glial cells maintain the brain vascular network through activation of Wnt signaling

During vascular development, radial glial cells (RGCs) regulate vascular patterning in the trunk and contribute to the early differentiation of the blood-brain barrier. Ablation of RGCs results in excessive sprouting vessels or the absence of bilateral vertebral arteries. However, interactions of RGCs with later brain vascular networks after pattern formation remain unknown. Here, we generated a her4.3 transgenic line to label RGCs and applied the metronidazole/nitroreductase system to ablate her4.3+ RGCs. The ablation of her4.3+ RGCs led to the collapse of the cerebral vascular network, disruption of the blood-brain barrier, and downregulation of Wnt signaling. The inhibition of Wnt signaling resulted in the collapse of cerebral vasculature, similar to that caused by her4.3+ RGC ablation. The defects in the maintenance of brain vasculature resulting from the absence of her4.3+ RGCs were partially rescued by the activation of Wnt signaling or overexpression of Wnt7aa or Wnt7bb. Together, our study suggests that her4.3+ RGCs maintain the cerebral vascular network through Wnt signaling.

Neutrophil immune profile guides spinal cord regeneration in zebrafish

Spinal cord injury triggers a strong innate inflammatory response in both non-regenerative mammals and regenerative zebrafish. Neutrophils are the first immune population to be recruited to the injury site. Yet, their role in the repair process, particularly in a regenerative context, remains largely unknown. Here, we show that, following rapid recruitment to the injured spinal cord, neutrophils mostly reverse migrate throughout the zebrafish body. In addition, promoting neutrophil inflammation resolution by inhibiting Cxcr4 boosts cellular and functional regeneration. Neutrophil-specific RNA-seq analysis reveals an enhanced activation state that correlates with a transient increase in tnf-α expression in macrophage/microglia populations. Conversely, blocking neutrophil recruitment through Cxcr1/2 inhibition diminishes the presence of macrophage/microglia at the injury site and impairs spinal cord regeneration. Altogether, these findings provide new insights into the role of neutrophils in spinal cord regeneration, emphasizing the significant impact of their immune profile on the outcome of the repair process.

Clair RM, Lasseigne AM, Ballif BA, Ebert AM. The intracellular domain of Sema6A is essential for development of the zebrafish retina

Semaphorin6A (Sema6A) is a repulsive guidance molecule that plays many roles in central nervous system, heart and bone development, as well as immune system responses and cell signaling in cancer. Loss of Sema6A or its receptor PlexinA2 in zebrafish leads to smaller eyes and improper retinal patterning. Here, we investigate a potential role for the Sema6A intracellular domain in zebrafish eye development and dissect which phenotypes rely on forward signaling and which rely on reverse signaling. We performed rescue experiments on zebrafish Sema6A morphants with either full-length Sema6A (Sema6A-FL) or Sema6A lacking its intracellular domain (Sema6A-ΔC). We identified that the intracellular domain is not required for eye size and retinal patterning, however it is required for retinal integrity, the number and end feet strength of Müller glia and protecting against retinal cell death. This novel function for the intracellular domain suggests a role for Sema6A reverse signaling in zebrafish eye development.

Mycb and Mych stimulate Müller glial cell reprogramming and proliferation in the uninjured and injured zebrafish retina

In the injured zebrafish retina, Müller glial cells (MG) reprogram to adopt retinal stem cell properties and regenerate damaged neurons. The strongest zebrafish reprogramming factors might be good candidates for stimulating a similar regenerative response by mammalian MG. Myc proteins are potent reprogramming factors that can stimulate cellular plasticity in differentiated cells; however, their role in MG reprogramming and retina regeneration remains poorly explored. Here, we report that retinal injury stimulates mycb and mych expression and that, although both Mycb and Mych stimulate MG reprogramming and proliferation, only Mych enhances retinal neuron apoptosis. RNA-sequencing analysis of wild-type, mychmut and mycbmut fish revealed that Mycb and Mych regulate ∼40% and ∼16%, respectively, of the genes contributing to the regeneration-associated transcriptome of MG. Of these genes, those that are induced are biased towards regulation of ribosome biogenesis, protein synthesis, DNA synthesis, and cell division, which are the top cellular processes affected by retinal injury, suggesting that Mycb and Mych are potent MG reprogramming factors. Consistent with this, forced expression of either of these proteins is sufficient to stimulate MG proliferation in the uninjured retina.

In Vivo Monitoring of Fabp7 Expression in Transgenic Zebrafish

In zebrafish, like in mammals, radial glial cells (RGCs) can act as neural progenitors during development and regeneration in adults. However, the heterogeneity of glia subpopulations entails the need for different specific markers of zebrafish glia. Currently, fluorescent protein expression mediated by a regulatory element from the glial fibrillary acidic protein (gfap) gene is used as a prominent glia reporter. We now expand this tool by demonstrating that a regulatory element from the mouse Fatty acid binding protein 7 (Fabp7) gene drives reliable expression in fabp7-expressing zebrafish glial cells. By using three different Fabp7 regulatory element-mediated fluorescent protein reporter strains, we reveal in double transgenic zebrafish that progenitor cells expressing fluorescent proteins driven by the Fabp7 regulatory element give rise to radial glia, oligodendrocyte progenitors, and some neuronal precursors. Furthermore, Bergmann glia represent the almost only glial population of the zebrafish cerebellum (besides a few oligodendrocytes), and the radial glia also remain in the mature cerebellum. Fabp7 regulatory element-mediated reporter protein expression in Bergmann glia progenitors suggests their origin from the ventral cerebellar proliferation zone, the ventricular zone, but not from the dorsally positioned upper rhombic lip. These new Fabp7 reporters will be valuable for functional studies during development and regeneration.

IL-1β disrupts the initiation of blood-brain barrier development by inhibiting endothelial Wnt/β-catenin signaling

During neuroinflammation, the proinflammatory cytokine interleukin-1β (IL-1β) impacts blood-brain barrier (BBB) function by disrupting brain endothelial tight junctions, promoting vascular permeability, and increasing transmigration of immune cells. Here, we examined the effects of Il-1β on the in vivo initiation of BBB development. We generated doxycycline-inducible transgenic zebrafish to secrete Il-1β in the CNS. To validate the utility of our model, we showed Il-1β dose-dependent mortality, recruitment of neutrophils, and expansion of microglia. Using live imaging, we discovered that Il-1β causes a significant reduction in CNS angiogenesis and barriergenesis. To demonstrate specificity, we rescued the Il-1β induced phenotypes by targeting the zebrafish il1r1 gene using CRISPR-Cas9. Mechanistically, we determined that Il-1β disrupts the initiation of BBB development by decreasing Wnt/β-catenin transcriptional activation in brain endothelial cells. Given that several neurodevelopmental disorders are associated with inflammation, our findings support further investigation into the connections between proinflammatory cytokines, neuroinflammation, and neurovascular development.

Dissecting the spatiotemporal diversity of adult neural stem cells

Adult stem cells are important for tissue turnover and regeneration. However, in most adult systems it remains elusive how stem cells assume different functional states and support spatially patterned tissue architecture. Here, we dissected the diversity of neural stem cells in the adult zebrafish brain, an organ that is characterized by pronounced zonation and high regenerative capacity. We combined single-cell transcriptomics of dissected brain regions with massively parallel lineage tracing and in vivo RNA metabolic labeling to analyze the regulation of neural stem cells in space and time. We detected a large diversity of neural stem cells, with some subtypes being restricted to a single brain region, while others were found globally across the brain. Global stem cell states are linked to neurogenic differentiation, with different states being involved in proliferative and non-proliferative differentiation. Our work reveals principles of adult stem cell organization and establishes a resource for the functional manipulation of neural stem cell subtypes.

Photoreceptor calyceal processes accompany the developing outer segment, adopting a stable length despite a dynamic core

Vertebrate photoreceptors detect light through a large cilium-based outer segment, which is filled with photopigment-laden membranous discs. Surrounding the base of the outer segment are microvilli-like calyceal processes (CPs). Although CP disruption has been associated with altered outer segment morphology and photoreceptor degeneration, the role of the CPs remains elusive. Here, we used zebrafish as a model to characterize CPs. We quantified CP parameters and report a strong disparity in outer segment coverage between photoreceptor subtypes. CP length is stable across light and dark conditions, yet heat-shock inducible expression of tagged actin revealed rapid turnover of the CP actin core. Detailed imaging of the embryonic retina uncovered substantial remodeling of the developing photoreceptor apical surface, including a transition from dynamic tangential processes to vertically oriented CPs immediately prior to outer segment formation. Remarkably, we also found a direct connection between apical extensions of the Müller glia and retinal pigment epithelium, arranged as bundles around the ultraviolet sensitive cones. In summary, our data characterize the structure, development and surrounding environment of photoreceptor microvilli in the zebrafish retina.

IL-1β disrupts blood-brain barrier development by inhibiting endothelial Wnt/β-catenin signaling

During neuroinflammation, the proinflammatory cytokine Interleukin-1β (IL-1β) impacts blood-brain barrier (BBB) function by disrupting brain endothelial tight junctions, promoting vascular permeability, and increasing transmigration of immune cells. Here, we examined the effects of Il-1β on the in vivo development of the BBB. We generated a doxycycline-inducible transgenic zebrafish model that drives secretion of Il-1β in the CNS. To validate the utility of our model, we showed Il-1β dose-dependent mortality, recruitment of neutrophils, and expansion of microglia. Using live imaging, we discovered that Il-1β causes a significant reduction in CNS angiogenesis and barriergenesis. To demonstrate specificity, we rescued the Il-1β induced phenotypes by targeting the zebrafish il1r1 gene using CRISPR/Cas9. Mechanistically, we determined that Il-1β disrupts BBB development by decreasing Wnt/β-catenin transcriptional activation in brain endothelial cells. Given that several neurodevelopmental disorders are associated with inflammation, our findings support further investigation into the connections between proinflammatory cytokines, neuroinflammation, and neurovascular development.

In toto imaging of glial JNK signaling during larval zebrafish spinal cord regeneration

Identification of signaling events that contribute to innate spinal cord regeneration in zebrafish can uncover new targets for modulating injury responses of the mammalian central nervous system. Using a chemical screen, we identify JNK signaling as a necessary regulator of glial cell cycling and tissue bridging during spinal cord regeneration in larval zebrafish. With a kinase translocation reporter, we visualize and quantify JNK signaling dynamics at single-cell resolution in glial cell populations in developing larvae and during injury-induced regeneration. Glial JNK signaling is patterned in time and space during development and regeneration, decreasing globally as the tissue matures and increasing in the rostral cord stump upon transection injury. Thus, dynamic and regional regulation of JNK signaling help to direct glial cell behaviors during innate spinal cord regeneration.

Brd activity regulates Müller glia-dependent retinal regeneration in zebrafish

The zebrafish retina possesses tremendous regenerative potential. Müller glia underlie retinal regeneration through their ability to reprogram and generate multipotent neuronal progenitors that re-differentiate into lost neurons. Many factors required for Müller glia reprogramming and proliferation have been identified; however, we know little about the epigenetic and transcriptional regulation of these genes during regeneration. Here, we determined whether transcriptional regulation by members of the Bromodomain (Brd) family is required for Müller glia-dependent retinal regeneration. Our data demonstrate that three brd genes were expressed in Müller glia upon injury. brd2a and brd2b were expressed in all Müller glia and brd4 was expressed only in reprogramming Müller glia. Utilizing (+)-JQ1, a pharmacological inhibitor of Brd function, we demonstrate that transcriptional regulation by Brds plays a critical role in Müller glia reprogramming and regeneration. (+)-JQ1 treatment prevented cell cycle re-entry of Müller glia and the generation of neurogenic progenitors. Modulating the (+)-JQ1 exposure window, we identified the first 48 h post-injury as the time-period during which Müller glia reprogramming occurs. (+)-JQ1 treatments after 48 h post-injury had no effect on the re-differentiation of UV cones, indicating that Brd function is required only for Müller glia reprogramming and not subsequent specification/differentiation events. Brd inhibition also prevented the expression of reprogramming genes like ascl1a and lepb in Müller glia, but not effector genes like mmp9, nor did it affect microglial recruitment after injury. These results demonstrate that transcriptional regulation by Brds plays a critical role during Müller glia-dependent retinal regeneration in zebrafish.

Single-cell RNA sequencing unravels the transcriptional network underlying zebrafish retina regeneration

In the lesioned zebrafish retina, Müller glia produce multipotent retinal progenitors that generate all retinal neurons, replacing lost cell types. To study the molecular mechanisms linking Müller glia reactivity to progenitor production and neuronal differentiation, we used single-cell RNA sequencing of Müller glia, progenitors and regenerated progeny from uninjured and light-lesioned retinae. We discover an injury-induced Müller glia differentiation trajectory that leads into a cell population with a hybrid identity expressing marker genes of Müller glia and progenitors. A glial self-renewal and a neurogenic trajectory depart from the hybrid cell population. We further observe that neurogenic progenitors progressively differentiate to generate retinal ganglion cells first and bipolar cells last, similar to the events observed during retinal development. Our work provides a comprehensive description of Müller glia and progenitor transcriptional changes and fate decisions in the regenerating retina, which are key to tailor cell differentiation and replacement therapies for retinal dystrophies in humans.

Cellular and molecular mechanisms of aflatoxin B1-mediated neurotoxicity: The therapeutic role of natural bioactive compounds

Aflatoxin B1 (AFB1), a dietary toxin from the mold Aspergillus species, is well acknowledged to elicit extra-hepatic toxicity in both animals and humans. The neurotoxicity of AFB1 has become a global public health concern. Contemporary research on how AFB1 enters the brain to elicit neuronal dysregulation leading to noxious neurological outcomes has increased greatly in recent years. The current review discusses several neurotoxic outcomes and susceptible targets of AFB1 toxicity at cellular, molecular and genetic levels. Specifically, neurotoxicity studies involving the use of brain homogenates, neuroblastoma cell line IMR-32, human brain microvascular endothelial cells, microglial cells, and astrocytes, as well as mammalian and non-mammalian models to unravel the mechanisms associated with AFB1 exposure are highlighted. Further, some naturally occurring bioactive compounds with compelling therapeutic effects on AFB1-induced neurotoxicity are reviewed. In conclusion, available data from literature highlight AFB1 as a neurotoxin and its possible pathological contribution to neurological disorders. Further mechanistic studies aimed at discovering and developing effective therapeutics for AFB1 neurotoxicity is warranted.

Her6 and Prox1a are novel regulators of photoreceptor regeneration in the zebrafish retina

Damage to light-sensing photoreceptors (PRs) occurs in highly prevalent retinal diseases. As humans cannot regenerate new PRs, these diseases often lead to irreversible blindness. Intriguingly, animals, such as the zebrafish, can regenerate PRs efficiently and restore functional vision. Upon injury, mature Müller glia (MG) undergo reprogramming to adopt a stem cell-like state. This process is similar to cellular dedifferentiation, and results in the generation of progenitor cells, which, in turn, proliferate and differentiate to replace lost retinal neurons. In this study, we tested whether factors involved in dedifferentiation of Drosophila CNS are implicated in the regenerative response in the zebrafish retina. We found that hairy-related 6 (her6) negatively regulates of PR production by regulating the rate of cell divisions in the MG-derived progenitors. prospero homeobox 1a (prox1a) is expressed in differentiated PRs and may promote PR differentiation through phase separation. Interestingly, upon Her6 downregulation, Prox1a is precociously upregulated in the PRs, to promote PR differentiation; conversely, loss of Prox1a also induces a downregulation of Her6. Together, we identified two novel candidates of PR regeneration that cross regulate each other; these may be exploited to promote human retinal regeneration and vision recovery.

Apical size and deltaA expression predict adult neural stem cell decisions along lineage progression

The maintenance of neural stem cells (NSCs) in the adult brain depends on their activation frequency and division mode. Using long-term intravital imaging of NSCs in the zebrafish adult telencephalon, we reveal that apical surface area and expression of the Notch ligand DeltaA predict these NSC decisions. deltaA-negative NSCs constitute a bona fide self-renewing NSC pool and systematically engage in asymmetric divisions generating a self-renewing deltaAneg daughter, which regains the size and behavior of its mother, and a neurogenic deltaApos daughter, eventually engaged in neuronal production following further quiescence-division phases. Pharmacological and genetic manipulations of Notch, DeltaA, and apical size further show that the prediction of activation frequency by apical size and the asymmetric divisions of deltaAneg NSCs are functionally independent of Notch. These results provide dynamic qualitative and quantitative readouts of NSC lineage progression in vivo and support a hierarchical organization of NSCs in differently fated subpopulations.

Spinal cord repair is modulated by the neurogenic factor Hb-egf under direction of a regeneration-associated enhancer

Unlike adult mammals, zebrafish regenerate spinal cord tissue and recover locomotor ability after a paralyzing injury. Here, we find that ependymal cells in zebrafish spinal cords produce the neurogenic factor Hb-egfa upon transection injury. Animals with hb-egfa mutations display defective swim capacity, axon crossing, and tissue bridging after spinal cord transection, associated with disrupted indicators of neuron production. Local recombinant human HB-EGF delivery alters ependymal cell cycling and tissue bridging, enhancing functional regeneration. Epigenetic profiling reveals a tissue regeneration enhancer element (TREE) linked to hb-egfa that directs gene expression in spinal cord injuries. Systemically delivered recombinant AAVs containing this zebrafish TREE target gene expression to crush injuries of neonatal, but not adult, murine spinal cords. Moreover, enhancer-based HB-EGF delivery by AAV administration improves axon densities after crush injury in neonatal cords. Our results identify Hb-egf as a neurogenic factor necessary for innate spinal cord regeneration and suggest strategies to improve spinal cord repair in mammals.

Development and repair of blood vessels in the zebrafish spinal cord

The vascular system is inefficiently repaired after spinal cord injury (SCI) in mammals, resulting in secondary tissue damage and immune deregulation that contribute to the limited functional recovery. Unlike mammals, zebrafish can repair the spinal cord (SC) and restore motility, but the vascular response to injury has not been investigated. Here, we describe the zebrafish SC blood vasculature, starting in development with the initial vessel ingression in a body size-dependent manner, the acquisition of perivascular support and the establishment of ventral to dorsal blood circulation. The vascular organization grows in complexity and displays multiple barrier specializations in adulthood. After injury, vessels rapidly regrow into the lesion, preceding the glial bridge and axons. Vascular repair involves an early burst of angiogenesis that creates dysmorphic and leaky vessels. Dysfunctional vessels are later removed, as pericytes are recruited and the blood-SC barrier is re-established. This study demonstrates that zebrafish can successfully re-vascularize the spinal tissue, reinforcing the value of this organism as a regenerative model for SCI.

Heterogeneity in quiescent Müller glia in the uninjured zebrafish retina drive differential responses following photoreceptor ablation

Introduction

Loss of neurons in the neural retina is a leading cause of vision loss. While humans do not possess the capacity for retinal regeneration, zebrafish can achieve this through activation of resident Müller glia. Remarkably, despite the presence of Müller glia in humans and other mammalian vertebrates, these cells lack an intrinsic ability to contribute to regeneration. Upon activation, zebrafish Müller glia can adopt a stem cell-like state, undergo proliferation and generate new neurons. However, the underlying molecular mechanisms of this activation subsequent retinal regeneration remains unclear.

Methods/Results

To address this, we performed single-cell RNA sequencing (scRNA-seq) and report remarkable heterogeneity in gene expression within quiescent Müller glia across distinct dorsal, central and ventral retina pools of such cells. Next, we utilized a genetically driven, chemically inducible nitroreductase approach to study Müller glia activation following selective ablation of three distinct photoreceptor subtypes: long wavelength sensitive cones, short wavelength sensitive cones, and rods. There, our data revealed that a region-specific bias in activation of Müller glia exists in the zebrafish retina, and this is independent of the distribution of the ablated cell type across retinal regions. Notably, gene ontology analysis revealed that injury-responsive dorsal and central Müller glia express genes related to dorsal/ventral pattern formation, growth factor activity, and regulation of developmental process. Through scRNA-seq analysis, we identify a shared genetic program underlying initial Müller glia activation and cell cycle entry, followed by differences that drive the fate of regenerating neurons. We observed an initial expression of AP-1 and injury-responsive transcription factors, followed by genes involved in Notch signaling, ribosome biogenesis and gliogenesis, and finally expression of cell cycle, chromatin remodeling and microtubule-associated genes.

Discussion

Taken together, our findings document the regional specificity of gene expression within quiescent Müller glia and demonstrate unique Müller glia activation and regeneration features following neural ablation. These findings will improve our understanding of the molecular pathways relevant to neural regeneration in the retina.

Loss of ndrg2 Function Is Involved in Notch Activation in Neuromast Hair Cell Regeneration in Zebrafish

The regeneration of hair cells in zebrafish is a complex process involving the precise regulation of multiple signaling pathways, but this complicated regulatory network is not fully understood. Current research has primarily focused on finding molecules and pathways that can regulate hair cell regeneration and restore hair cell functions. Here, we show the role of N-Myc downstream regulated gene 2 (ndrg2) in zebrafish hair cell regeneration. We first found that ndrg2 was dynamically expressed in neuromasts of the developing zebrafish, and this expression was increased after neomycin-induced hair cell damage. Then, ndrg2 loss-of-function larvae showed reduced numbers of regenerated hair cells but increased numbers of supporting cells after neomycin exposure. By in situ hybridization, we further observed that ndrg2 loss of function resulted in the activation of Notch signaling and downregulation of atoh1a during hair cell regeneration in vivo. Additionally, blocking Notch signaling rescued the number of regenerated hair cells in ndrg2-deficient larvae. Together, this study provides evidence for the role of ndrg2 in regulating hair cell regeneration in zebrafish neuromasts.

CSF-contacting neurons respond to Streptococcus pneumoniae and promote host survival during central nervous system infection

The pathogenic bacterium Streptococcus pneumoniae (S. pneumoniae) can invade the cerebrospinal fluid (CSF) and cause meningitis with devastating consequences. Whether and how sensory cells in the central nervous system (CNS) become activated during bacterial infection, as recently reported for the peripheral nervous system, is not known. We find that CSF infection by S. pneumoniae in larval zebrafish leads to changes in posture and behavior that are reminiscent of pneumococcal meningitis, including dorsal arching and epileptic-like seizures. We show that during infection, invasion of the CSF by S. pneumoniae massively activates in vivo sensory neurons contacting the CSF, referred to as "CSF-cNs" and previously shown to detect spinal curvature and to control posture, locomotion, and spine morphogenesis. We find that CSF-cNs express orphan bitter taste receptors and respond in vitro to bacterial supernatant and metabolites via massive calcium transients, similar to the ones observed in vivo during infection. Upon infection, CSF-cNs also upregulate the expression of numerous cytokines and complement components involved in innate immunity. Accordingly, we demonstrate, using cell-specific ablation and blockade of neurotransmission, that CSF-cN neurosecretion enhances survival of the host during S. pneumoniae infection. Finally, we show that CSF-cNs respond to various pathogenic bacteria causing meningitis in humans, as well as to the supernatant of cells infected by a neurotropic virus. Altogether, our work uncovers that central sensory neurons in the spinal cord, previously involved in postural control and morphogenesis, contribute as well to host survival by responding to the invasion of the CSF by pathogenic bacteria during meningitis.

Research Progress on the Construction and Application of a Diabetic Zebrafish Model

Diabetes is a metabolic disease characterized by high blood glucose levels. With economic development and lifestyle changes, the prevalence of diabetes is increasing yearly. Thus, it has become an increasingly serious public health problem in countries around the world. The etiology of diabetes is complex, and its pathogenic mechanisms are not completely clear. The use of diabetic animal models is helpful in the study of the pathogenesis of diabetes and the development of drugs. The emerging vertebrate model of zebrafish has many advantages, such as its small size, large number of eggs, short growth cycle, simple cultivation of adult fish, and effective improvement of experimental efficiency. Thus, this model is highly suitable for research as an animal model of diabetes. This review not only summarizes the advantages of zebrafish as a diabetes model, but also summarizes the construction methods and challenges of zebrafish models of type 1 diabetes, type 2 diabetes, and diabetes complications. This study provides valuable reference information for further study of the pathological mechanisms of diabetes and the research and development of new related therapeutic drugs.

Cone photoreceptors transfer damaged mitochondria to Müller glia

Mitochondria are vital organelles that require sophisticated homeostatic mechanisms for maintenance. Intercellular transfer of damaged mitochondria is a recently identified strategy broadly used to improve cellular health and viability. Here, we investigate mitochondrial homeostasis in the vertebrate cone photoreceptor, the specialized neuron that initiates our daytime and color vision. We find a generalizable response to mitochondrial stress that leads to loss of cristae, displacement of damaged mitochondria from their normal cellular location, initiation of degradation, and transfer to Müller glia cells, a key non-neuronal support cell in the retina. Our findings show transmitophagy from cones to Müller glia as a response to mitochondrial damage. Intercellular transfer of damaged mitochondria represents an outsourcing mechanism that photoreceptors use to support their specialized function.

Pten associates with important gene regulatory network to fine-tune Müller glia-mediated zebrafish retina regeneration

Unlike mammals, zebrafish possess a remarkable ability to regenerate damaged retina after an acute injury. Retina regeneration in zebrafish involves the induction of Müller glia-derived progenitor cells (MGPCs) exhibiting stem cell-like characteristics, which are capable of restoring all retinal cell-types. The induction of MGPC through Müller glia-reprograming involves several cellular, genetic and biochemical events soon after a retinal injury. Despite the knowledge on the importance of Phosphatase and tensin homolog (Pten), which is a dual-specificity phosphatase and tumor suppressor in the maintaining of cellular homeostasis, its importance during retina regeneration remains unknown. Here, we explored the importance of Pten during zebrafish retina regeneration. The Pten gets downregulated upon retinal injury and is absent from the MGPCs, which is essential to trigger Akt-mediated cellular proliferation essential for retina regeneration. We found that the downregulation of Pten in the post-injury retina accelerates MGPCs formation, while its overexpression restricts the regenerative response. We observed that Pten regulates the proliferation of MGPCs not only through Akt pathway but also by Mmp9/Notch signaling. Mmp9-activity is essential to induce the proliferation of MGPCs in the absence of Pten. Lastly, we show that expression of Pten is fine-tuned through Mycb/histone deacetylase1 and Tgf-β signaling. The present study emphasizes on the stringent regulation of Pten and its crucial involvement during the zebrafish retina regeneration.

Evolutionarily conserved concepts in glial cell biology

The evolutionary conservation of glial cells has been appreciated since Ramon y Cajal and Del Rio Hortega first described the morphological features of cells in the nervous system. We now appreciate that glial cells have essential roles throughout life in most nervous systems. The field of glial cell biology has grown exponentially in the last ten years. This new wealth of knowledge has been aided by seminal findings in non-mammalian model systems. Ultimately, such concepts help us to understand glia in mammalian nervous systems. Rather than summarizing the field of glial biology, I will first briefly introduce glia in non-mammalian models systems. Then, highlight seminal findings across the glial field that utilized non-mammalian model systems to advance our understanding of the mammalian nervous system. Finally, I will call attention to some recent findings that introduce new questions about glial cell biology that will be investigated for years to come.

Anp32a Promotes Neuronal Regeneration after Spinal Cord Injury of Zebrafish Embryos

After spinal cord injury (SCI) in mammals, neuronal regeneration is limited; in contrast, such regeneration occurs quickly in zebrafish. Member A of the acidic nuclear phosphoprotein 32 (ANP32a) family is involved in neuronal development, but its function is controversial, and its involvement in zebrafish SCI remains unknown. To determine the role of zebrafish ANP32a in the neuronal regeneration of SCI embryos, we microinjected ANP32a mRNA into embryos from zebrafish transgenic line Tg(mnx1:GFP) prior to SCI. Compared to control SCI embryos, the results showed that the regeneration of spinal cord and resumption of swimming capability were promoted by the overexpression of ANP32a mRNA but reduced by its knockdown. We next combined fluorescence-activated cell sorting with immunochemical staining of anti-GFAP and immunofluorescence staining against anti-PH3 on Tg(gfap:GFP) SCI embryos. The results showed that ANP32a promoted the proliferation and cell number of radial glial cells at the injury epicenter at 24 h post-injury (hpi). Moreover, when we applied BrdU labeling to SCI embryos derived from crossing the Tg(gfap:GFP) and Tg(mnx1:TagRFP) lines, we found that both radial glial cells and motor neurons had proliferated, along with their increased cell numbers in Anp32a-overexpression SCI-embryos. On this basis, we conclude that ANP32a plays a positive role in the regeneration of zebrafish SCI embryos.

Synapsin III Regulates Dopaminergic Neuron Development in Vertebrates

Attention deficit and hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by alterations in the mesocorticolimbic and nigrostriatal dopaminergic pathways. Polymorphisms in the Synapsin III (Syn III) gene can associate with ADHD onset and even affect the therapeutic response to the gold standard ADHD medication, methylphenidate (MPH), a monoamine transporter inhibitor whose efficacy appears related with the stimulation of brain-derived neurotrophic factor (BDNF). Interestingly, we previously showed that MPH can bind Syn III, which can regulate neuronal development. These observations suggest that Syn III polymorphism may impinge on ADHD onset and response to therapy by affecting BDNF-dependent dopaminergic neuron development. Here, by studying zebrafish embryos exposed to Syn III gene knock-down (KD), Syn III knock-out (ko) mice and human induced pluripotent stem cells (iPSCs)-derived neurons subjected to Syn III RNA interference, we found that Syn III governs the earliest stages of dopaminergic neurons development and that this function is conserved in vertebrates. We also observed that in mammals Syn III exerts this function acting upstream of brain-derived neurotrophic factor (BDNF)- and cAMP-dependent protein kinase 5 (Cdk5)-stimulated dendrite development. Collectively, these findings own significant implications for deciphering the biological basis of ADHD.

Embryonic hyperglycemia perturbs the development of specific retinal cell types, including photoreceptors

Diabetes is linked to various long-term complications in adults, such as neuropathy, nephropathy and diabetic retinopathy. Diabetes poses additional risks for pregnant women, because glucose passes across the placenta, and excess maternal glucose can result in diabetic embryopathy. While many studies have examined the teratogenic effects of maternal diabetes on fetal heart development, little is known about the consequences of maternal hyperglycemia on the development of the embryonic retina. To address this question, we investigated retinal development in two models of embryonic hyperglycemia in zebrafish. Strikingly, we found that hyperglycemic larvae displayed a significant reduction in photoreceptors and horizontal cells, whereas other retinal neurons were not affected. We also observed reactive gliosis and abnormal optokinetic responses in hyperglycemic larvae. Further analysis revealed delayed retinal cell differentiation in hyperglycemic embryos that coincided with increased reactive oxygen species (ROS). Our results suggest that embryonic hyperglycemia causes abnormal retinal development via altered timing of cell differentiation and ROS production, which is accompanied by visual defects. Further studies using zebrafish models of hyperglycemia will allow us to understand the molecular mechanisms underlying these effects.

Radial glia and radial glia-like cells: Their role in neurogenesis and regeneration

Radial glia is a cell type traditionally associated with the developing nervous system, particularly with the formation of cortical layers in the mammalian brain. Nonetheless, some of these cells, or closely related types, called radial glia-like cells are found in adult central nervous system structures, functioning as neurogenic progenitors in normal homeostatic maintenance and in response to injury. The heterogeneity of radial glia-like cells is nowadays being probed with molecular tools, primarily by the expression of specific genes that define cell types. Similar markers have identified radial glia-like cells in the nervous system of non-vertebrate organisms. In this review, we focus on adult radial glia-like cells in neurogenic processes during homeostasis and in response to injury. We highlight our results using a non-vertebrate model system, the echinoderm Holothuria glaberrima where we have described a radial glia-like cell that plays a prominent role in the regeneration of the holothurian central nervous system.

Jag2b-Notch3/1b-mediated neuron-to-glia crosstalk controls retinal gliogenesis

In the developing central nervous systems (CNS), neural progenitor cells generate neurons and glia in sequential order. However, the influence of neurons on glia generation remains elusive. Here, we report that photoreceptor cell-derived Jag2b is required for Notch-dependent Müller glia (MG) generation in the developing zebrafish retina. In jab2b-/- mutants, differentiating MGs are re-specified into lineage-related bipolar neuron fate at the expense of mature MG. Single-cell transcriptome analysis and knock-in animals reveal that jab2b is specifically expressed in crx+ -photoreceptor cells during MG generation. Crx promoter-driven jag2b, but not other Notch ligands, is sufficient to rescue the loss of MGs observed in jag2b-/- mutants. Furthermore, we observe a severe and moderate decrease in the number of MGs in notch3-/- and notch1b-/- mutants, respectively, and the activation of Notch3 or Notch1b rescues the MG loss in jag2b-/- mutants. Together, our findings reveal that the interaction of Jag2b and Notch3/Notch1b mediates the crosstalk between neurons and glial cells to ensure the irreversible differentiation of MG, providing novel mechanistic insights into the temporal specification of glial cell fate in a developing vertebrate CNS structure.

Teleost Fish and Organoids: Alternative Windows Into the Development of Healthy and Diseased Brains

The variety in the display of animals' cognition, emotions, and behaviors, typical of humans, has its roots within the anterior-most part of the brain: the forebrain, giving rise to the neocortex in mammals. Our understanding of cellular and molecular events instructing the development of this domain and its multiple adaptations within the vertebrate lineage has progressed in the last decade. Expanding and detailing the available knowledge on regionalization, progenitors' behavior and functional sophistication of the forebrain derivatives is also key to generating informative models to improve our characterization of heterogeneous and mechanistically unexplored cortical malformations. Classical and emerging mammalian models are irreplaceable to accurately elucidate mechanisms of stem cells expansion and impairments of cortex development. Nevertheless, alternative systems, allowing a considerable reduction of the burden associated with animal experimentation, are gaining popularity to dissect basic strategies of neural stem cells biology and morphogenesis in health and disease and to speed up preclinical drug testing. Teleost vertebrates such as zebrafish, showing conserved core programs of forebrain development, together with patients-derived in vitro 2D and 3D models, recapitulating more accurately human neurogenesis, are now accepted within translational workflows spanning from genetic analysis to functional investigation. Here, we review the current knowledge of common and divergent mechanisms shaping the forebrain in vertebrates, and causing cortical malformations in humans. We next address the utility, benefits and limitations of whole-brain/organism-based fish models or neuronal ensembles in vitro for translational research to unravel key genes and pathological mechanisms involved in neurodevelopmental diseases.

Amoeboid-like migration ensures correct horizontal cell layer formation in the developing vertebrate retina

Migration of cells in the developing brain is integral for the establishment of neural circuits and function of the central nervous system. While migration modes during which neurons employ predetermined directional guidance of either preexisting neuronal processes or underlying cells have been well explored, less is known about how cells featuring multipolar morphology migrate in the dense environment of the developing brain. To address this, we here investigated multipolar migration of horizontal cells in the zebrafish retina. We found that these cells feature several hallmarks of amoeboid-like migration that enable them to tailor their movements to the spatial constraints of the crowded retina. These hallmarks include cell and nuclear shape changes, as well as persistent rearward polarization of stable F-actin. Interference with the organization of the developing retina by changing nuclear properties or overall tissue architecture hampers efficient horizontal cell migration and layer formation showing that cell-tissue interplay is crucial for this process. In view of the high proportion of multipolar migration phenomena observed in brain development, the here uncovered amoeboid-like migration mode might be conserved in other areas of the developing nervous system.