Neuroinflammation and central nervous system regeneration in vertebrates

How neurobehavior and brain development in alternative whole-organism models can contribute to prediction of developmental neurotoxicity

Developmental neurotoxicity (DNT) is not routinely evaluated in chemical risk assessment because current test paradigms for DNT require the use of mammalian models which are ethically controversial, expensive, and resource demanding. Consequently, efforts have focused on revolutionizing DNT testing through affordable novel alternative methods for risk assessment. The goal is to develop a DNT in vitro test battery amenable to high-throughput screening (HTS). Currently, the DNT in vitro test battery consists primarily of human cell-based assays because of their immediate relevance to human health. However, such cell-based assays alone are unable to capture the complexity of a developing nervous system. Whole organismal systems that qualify as 3 R (Replace, Reduce and Refine) models are urgently needed to complement cell-based DNT testing. These models can provide the necessary organismal context and be used to explore the impact of chemicals on brain function by linking molecular and/or cellular changes to behavioural readouts. The nematode Caenorhabditis elegans, the planarian Dugesia japonica, and embryos of the zebrafish Danio rerio are all suited to low-cost HTS and each has unique strengths for DNT testing. Here, we review the strengths and the complementarity of these organisms in a novel, integrative context and highlight how they can augment current cell-based assays for more comprehensive and robust DNT screening of chemicals. Considering the limitations of all in vitro test systems, we discuss how a smart combinatory use of these systems will contribute to a better human relevant risk assessment of chemicals that considers the complexity of the developing brain.

Short-term lipopolysaccharide treatment leads to astrocyte activation in LRRK2 G2019S knock-in mice without loss of dopaminergic neurons

Background

The G2019S mutation of LRRK2, which enhances kinase activity of the protein, confers a substantial risk of developing Parkinson's disease (PD). However, the mutation demonstrates incomplete penetrance, suggesting the involvement of other genetic or environmental modulating factors. Here, we investigated whether LRRK2 G2019S knock-in (KI) mice treated with the inflammogen lipopolysaccharide (LPS) could model LRRK2 PD.

Results

We found that short-term (2 weeks) treatment with LPS did not result in the loss of dopaminergic neurons in either LRRK2 G2019S KI or wild-type (WT) mice. Compared with WT mice, LRRK2 G2019S-KI mice showed incomplete recovery from LPS-induced weight loss. In LRRK2 G2019S KI mice, LPS treatment led to upregulated phosphorylation of LRRK2 at the autophosphorylation site Serine 1292, which is known as a direct readout of LRRK2 kinase activity. LPS treatment caused a greater increase in the activated astrocyte marker glial fibrillary acidic protein (GFAP) in the striatum and substantia nigra of LRRK2 G2019S mice than in those of WT mice. The administration of caffeine, which was recently identified as a biomarker of resistance to developing PD in individuals with LRRK2 mutations, attenuated LPS-induced astrocyte activation specifically in LRRK2 G2019S KI mice.

Conclusions

Our findings suggest that 2 weeks of exposure to LPS is not sufficient to cause dopaminergic neuronal loss in LRRK2 G2019S KI mice but rather results in increased astrocyte activation, which can be ameliorated by caffeine.

The Vestibular Nuclei: A Cerebral Reservoir of Stem Cells Involved in Balance Function in Normal and Pathological Conditions

In this review, we explore the intriguing realm of neurogenesis in the vestibular nuclei-a critical brainstem region governing balance and spatial orientation. We retrace almost 20 years of research into vestibular neurogenesis, from its discovery in the feline model in 2007 to the recent discovery of a vestibular neural stem cell niche. We explore the reasons why neurogenesis is important in the vestibular nuclei and the triggers for activating the vestibular neurogenic niche. We develop the symbiotic relationship between neurogenesis and gliogenesis to promote vestibular compensation. Finally, we examine the potential impact of reactive neurogenesis on vestibular compensation, highlighting its role in restoring balance through various mechanisms.

Double crosslinked biomimetic composite hydrogels containing topographical cues and WAY-316606 induce neural tissue regeneration and functional recovery after spinal cord injury

Spinal cord injury (SCI) is an overwhelming and incurable disabling condition, for which increasing forms of multifunctional biomaterials are being tested, but with limited progression. The promising material should be able to fill SCI-induced cavities and direct the growth of new neurons, with effective drug loading to improve the local micro-organism environment and promote neural tissue regeneration. In this study, a double crosslinked biomimetic composite hydrogel comprised of acellularized spinal cord matrix (ASCM) and gelatin-acrylated-β-cyclodextrin-polyethene glycol diacrylate (designated G-CD-PEGDA) hydrogel, loaded with WAY-316606 to activate canonical Wnt/β-catenin signaling, and reinforced by a bundle of three-dimensionally printed aligned polycaprolactone (PCL) microfibers, was constructed. The G-CD-PEGDA component endowed the composite hydrogel with a dynamic structure with a self-healing capability which enabled cell migration, while the ASCM component promoted neural cell affinity and proliferation. The diffusion of WAY-316606 could recruit endogenous neural stem cells and improve neuronal differentiation. The aligned PCL microfibers guided neurite elongation in the longitudinal direction. Animal behavior studies further showed that the composite hydrogel could significantly recover the motor function of rats after SCI. This study provides a proficient approach to produce a multifunctional system with desirable physiological, chemical, and topographical cues for treating patients with SCI.

Chrysophanol facilitates long-term neurological recovery through limiting microglia-mediated neuroinflammation after ischemic stroke in mice

BACKGROUND

Inflammation plays an important role in ischemic brain injury and affects brain recovery and neuroplasticity. Chrysophanol (CHR), has attracted attention for its protective effects through immunomodulatory and anti-inflammatory properties. However, the effect of CHR for brain recovery and neuroplasticity is not clear. The current study aimed to investigate the effect of CHR in the chronic phase of stroke in mice, and to elucidate the underlying mechanisms.

METHODS

C57BL/6 mice were subjected to treatment with Vehicle or CHR immediately through intraperitoneal injection daily for 14 d after distal middle cerebral artery occlusion (dMCAO). Neurological deficits were monitored up to 28 days after stroke. Nissl and Golgi stain, neural plasticity, and microglia-associated inflammatory cytokines were detected. Primary cortical neuron and BV2 microglia cell lines were employed to explore the underlying mechanism in vitro.

RESULTS

Compared with Vehicle group, CHR mitigated the histological damage, facilitated the neural plasticity and improved the neurological function up to 4 weeks after stroke. In vitro, CHR promoted the complexity of neurons and the spine density by modulating microglial polarization and reducing the expression of microglia-associated inflammatory cytokines, especially IL-6. In vivo, microglia activation and inflammatory cytokines were significantly increased after dMCAO and downregulated by CHR. Further investigation showed STAT3 is the major downstream effector of IL-6 signaling.

CONCLUSIONS

CHR ameliorated microenvironment for neural plasticity and exhibited neuroprotection via arresting microglia toward pro-inflammatory phenotype and downregulation of the expressions of pro-inflammatory cytokines, especially of IL-6. IL-6-STAT3 signaling might be CHR's therapeutic target for neuroinflammatory responses after stroke.

Assessing the Neurotoxicity of a Sub-Optimal Dose of Rotenone in Zebrafish (Danio rerio) and the Possible Neuroactive Potential of Valproic Acid, Combination of Levodopa and Carbidopa, and Lactic Acid Bacteria Strains

Parkinson’s disease (PD) is an enigmatic neurodegenerative disorder that is currently the subject of extensive research approaches aiming at deepening the understanding of its etiopathophysiology. Recent data suggest that distinct compounds used either as anticonvulsants or agents usually used as dopaminergic agonists or supplements consisting of live active lactic acid bacteria strains might alleviate and improve PD-related phenotypes. This is why we aimed to elucidate how the administration of rotenone (ROT) disrupts homeostasis and the possible neuroactive potential of valproic acid (VPA), antiparkinsonian agents (levodopa and carbidopa – LEV+CARB), and a mixture of six Lactobacillus and three Bifidobacterium species (PROBIO) might re-establish the optimal internal parameters. ROT causes significant changes in the central nervous system (CNS), notably reduced neurogenesis and angiogenesis, by triggering apoptosis, reflected by the increased expression of PARKIN and PINK1 gene(s), low brain dopamine (DA) levels, and as opposed to LRRK2 and SNCA compared with healthy zebrafish. VPA, LEV/CARB, and PROBIO sustain neurogenesis and angiogenesis, manifesting a neuroprotective role in diminishing the effect of ROT in zebrafish. Interestingly, none of the tested compounds influenced oxidative stress (OS), as reflected by the level of malondialdehyde (MDA) level and superoxide dismutase (SOD) enzymatic activity revealed in non-ROT-exposed zebrafish. Overall, the selected concentrations were enough to trigger particular behavioral patterns as reflected by our parameters of interest (swimming distance (mm), velocity (mm/s), and freezing episodes (s)), but sequential testing is mandatory to decipher whether they exert an inhibitory role following ROT exposure. In this way, we further offer data into how ROT may trigger a PD-related phenotype and the possible beneficial role of VPA, LEV+CARB, and PROBIO in re-establishing homeostasis in Danio rerio.

How to Heal the Gut's Brain: Regeneration of the Enteric Nervous System

The neural-crest-derived enteric nervous system (ENS) is the intrinsic nervous system of the gastrointestinal (GI) tract and controls all gut functions, including motility. Lack of ENS neurons causes various ENS disorders such as Hirschsprung Disease. One treatment option for ENS disorders includes the activation of resident stem cells to regenerate ENS neurons. Regeneration in the ENS has mainly been studied in mammalian species using surgical or chemically induced injury methods. These mammalian studies showed a variety of regenerative responses with generally limited regeneration of ENS neurons but (partial) regrowth and functional recovery of nerve fibers. Several aspects might contribute to the variety in regenerative responses, including observation time after injury, species, and gut region targeted. Zebrafish have recently emerged as a promising model system to study ENS regeneration as larvae possess the ability to generate new neurons after ablation. As the next steps in ENS regeneration research, we need a detailed understanding of how regeneration is regulated on a cellular and molecular level in animal models with both high and low regenerative capacity. Understanding the regulatory programs necessary for robust ENS regeneration will pave the way for using neural regeneration as a therapeutic approach to treating ENS disorders.

Incomplete Recovery of Zebrafish Retina Following Cryoinjury

Zebrafish show an extraordinary potential for regeneration in several organs from fins to central nervous system. Most impressively, the outcome of an injury results in a near perfect regeneration and a full functional recovery. Indeed, among the various injury paradigms previously tested in the field of zebrafish retina regeneration, a perfect layered structure is observed after one month of recovery in most of the reported cases. In this study, we applied cryoinjury to the zebrafish eye. We show that retina exposed to this treatment for one second undergoes an acute damage affecting all retinal cell types, followed by a phase of limited tissue remodeling and regrowth. Surprisingly, zebrafish developed a persistent retinal dysplasia observable through 300 days post-injury. There is no indication of fibrosis during the regeneration period, contrary to the regeneration process after cryoinjury to the zebrafish cardiac ventricle. RNA sequencing analysis of injured retinas at different time points has uncovered enriched processes and a number of potential candidate genes. By means of this simple, time and cost-effective technique, we propose a zebrafish injury model that displays a unique inability to completely recover following focal retinal damage; an outcome that is unreported to our knowledge. Furthermore, RNA sequencing proved to be useful in identifying pathways, which may play a crucial role not only in the regeneration of the retina, but in the first initial step of regeneration, degeneration. We propose that this model may prove useful in comparative and translational studies to examine critical pathways for successful regeneration.

Central nervous system regeneration: the roles of glial cells in the potential molecular mechanism underlying remyelination

Glial cells play crucial roles in brain homeostasis and pathogenesis of central nervous system (CNS) injuries and diseases. However, the roles of these cells and the molecular mechanisms toward regeneration in the CNS have not been fully understood, especially the capacity of them toward demyelinating diseases. Therefore, there are still very limited therapeutic strategies to restore the function of adult CNS in diseases such as multiple sclerosis (MS). Remyelination, a spontaneous regeneration process in the CNS, requires the involvement of multiple cellular and extracellular components. Promoting remyelination by therapeutic interventions is a promising novel approach to restore the CNS function. Herein, we review the role of glial cells in CNS diseases and injuries. Particularly, we discuss the roles of glia and their functional interactions and regulatory mechanisms in remyelination, as well as the current therapeutic strategies for MS.

Profile of the RNA in exosomes from astrocytes and microglia using deep sequencing: implications for neurodegeneration mechanisms

Glial cells play an important role in signal transduction, energy metabolism, extracellular ion homeostasis and neuroprotection of the central nervous system. However, few studies have explained the potential effects of exosomes from glial cells on central nervous system health and disease. In this study, the genes expressed in exosomes from astrocytes and microglia were identified by deep RNA sequencing. Kyoto Encyclopedia of Genes and Genomes analysis indicated that several pathways in these exosomes are responsible for promoting neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease and Huntington’s disease. Gene ontology analysis showed that extracellular exosome, mitochondrion and growth factor activity were enriched in exosomes from the unique astrocyte group, while extracellular exosome and mitochondrion were enriched in exosomes from the unique microglia group. Next, combined with the screening of hub genes, the protein-protein interaction network analysis showed that exosomes from astrocytes influence neurodegenerative diseases through metabolic balance and ubiquitin-dependent protein balance, whereas exosomes from microglia influence neurodegenerative diseases through immune inflammation and oxidative stress. Although there were differences in RNA expression between exosomes from astrocytes and microglia, the groups were related by the hub genes, ubiquitin B and heat shock protein family A (Hsp70) member 8. Ubiquitin B appeared to be involved in pleiotropic regulatory functions, including immune regulation, inflammation inhibition, protein catabolism, intracellular protein transport, exosomes and oxidative stress. The results revealed the clinical significance of exosomes from glia in neurodegenerative diseases. This study was approved by the Animal Ethics Committee of Nantong University, China (approval No. S20180102-152) on January 2, 2018.

RNA Profiling of Mouse Ependymal Cells after Spinal Cord Injury Identifies the Oncostatin Pathway as a Potential Key Regulator of Spinal Cord Stem Cell Fate

Ependymal cells reside in the adult spinal cord and display stem cell properties in vitro. They proliferate after spinal cord injury and produce neurons in lower vertebrates but predominantly astrocytes in mammals. The mechanisms underlying this glial-biased differentiation remain ill-defined. We addressed this issue by generating a molecular resource through RNA profiling of ependymal cells before and after injury. We found that these cells activate STAT3 and ERK/MAPK signaling post injury and downregulate cilia-associated genes and FOXJ1, a central transcription factor in ciliogenesis. Conversely, they upregulate 510 genes, seven of them more than 20-fold, namely Crym, Ecm1, Ifi202b, Nupr1, Rbp1, Thbs2 and Osmr—the receptor for oncostatin, a microglia-specific cytokine which too is strongly upregulated after injury. We studied the regulation and role of Osmr using neurospheres derived from the adult spinal cord. We found that oncostatin induced strong Osmr and p-STAT3 expression in these cells which is associated with reduction of proliferation and promotion of astrocytic versus oligodendrocytic differentiation. Microglial cells are apposed to ependymal cells in vivo and co-culture experiments showed that these cells upregulate Osmr in neurosphere cultures. Collectively, these results support the notion that microglial cells and Osmr/Oncostatin pathway may regulate the astrocytic fate of ependymal cells in spinal cord injury.

Neuron-Radial Glial Cell Communication via BMP/Id1 Signaling Is Key to Long-Term Maintenance of the Regenerative Capacity of the Adult Zebrafish Telencephalon

The central nervous system of adult zebrafish displays an extraordinary neurogenic and regenerative capacity. In the zebrafish adult brain, this regenerative capacity relies on neural stem cells (NSCs) and the careful management of the NSC pool. However, the mechanisms controlling NSC pool maintenance are not yet fully understood. Recently, Bone Morphogenetic Proteins (BMPs) and their downstream effector Id1 (Inhibitor of differentiation 1) were suggested to act as key players in NSC maintenance under constitutive and regenerative conditions. Here, we further investigated the role of BMP/Id1 signaling in these processes, using different genetic and pharmacological approaches. Our data show that BMPs are mainly expressed by neurons in the adult telencephalon, while id1 is expressed in NSCs, suggesting a neuron-NSC communication via the BMP/Id1 signaling axis. Furthermore, manipulation of BMP signaling by conditionally inducing or repressing BMP signaling via heat-shock, lead to an increase or a decrease of id1 expression in the NSCs, respectively. Induction of id1 was followed by an increase in the number of quiescent NSCs, while knocking down id1 expression caused an increase in NSC proliferation. In agreement, genetic ablation of id1 function lead to increased proliferation of NSCs, followed by depletion of the stem cell pool with concomitant failure to heal injuries in repeatedly injured mutant telencephala. Moreover, pharmacological inhibition of BMP and Notch signaling suggests that the two signaling systems cooperate and converge onto the transcriptional regulator her4.1. Interestingly, brain injury lead to a depletion of NSCs in animals lacking BMP/Id1 signaling despite an intact Notch pathway. Taken together, our data demonstrate how neurons feedback on NSC proliferation and that BMP1/Id1 signaling acts as a safeguard of the NSC pool under regenerative conditions.

Lycopus lucidus Turcz Exerts Neuroprotective Effects Against H2O2-Induced Neuroinflammation by Inhibiting NLRP3 Inflammasome Activation in Cortical Neurons

Purpose

Lycopus lucidus Turcz (LLT) is a potent traditional medicinal herb that exerts therapeutic effects, regulating inflammatory disorders. However, the precise mechanisms by which LLT plays a potent role as an anti-inflammatory agent are still unknown, and in particular, the effects of LLT on cortical neurons and related mechanisms of neuroinflammation have not been studied. The NLRP3 inflammasome pathway is one of the most well known as an important driver of inflammation. We therefore hypothesized that LLT, as an effective anti-inflammatory agent, might have neurotherapeutic potential by inhibiting the NLRP3 inflammasome pathway in cortical neurons.

Materials and Methods

Primary cortical neurons were isolated from the embryonic rat cerebral cortex, and H₂O₂ was used to stimulate neuron damage in vitro. After treatment with LLT at three concentrations (10, 25, and 50 µg/mL), the expression of iNOS, NLRP3, ASC, caspase-1, IL-1β, IL-18, IL-6, and IL-10 was determined by immunocytochemistry, qPCR, and ELISA. Neuron apoptosis was also evaluated using Annexin V-FITC/PI double staining FACS analysis. Neural regeneration-related factors (BDNF, NGF, synaptophysin, NT3, AKT, and mTOR) were analyzed by immunocytochemistry and qPCR.

Results

LLT effectively protected cultured rat cortical neurons from H₂O₂-induced neuronal injury by significantly inhibiting NLRP3 inflammasome activation. In addition, it significantly reduced caspase-1 activation, which is induced by inflammasome formation and regulated the secretion of IL-1β/IL-18. We demonstrated that LLT enhances axonal elongation and synaptic connectivity upon H₂O₂-induced neuronal injury in rat primary cortical neurons.

Conclusion

It was first demonstrated in vitro that LLT suppresses NLRP3 inflammasome activation, attenuates inflammation and apoptosis, and consequently promotes neuroprotection and the stimulation of neuron repair, suggesting that it is a promising therapeutic for neurological diseases.

Mechanical Brain Injury Increases Cells' Production of Cystathionine β-Synthase and Glutamine Synthetase, but Reduces Pax2 Expression in the Telencephalon of Juvenile Chum Salmon, Oncorhynchus keta

The considerable post-traumatic brain recovery in fishes makes them a useful model for studying the mechanisms that provide reparative neurogenesis, which is poorly represented in mammals. After a mechanical injury to the telencephalon in adult fish, lost neurons are actively replaced due to the proliferative activity of neuroepithelial cells and radial glia in the neurogenic periventricular zone. However, it is not enough clear which signaling mechanisms are involved in the activation of adult neural stem cells (aNSC) after the injury (reactive proliferation) and in the production of new neurons (regenerative neurogenesis) from progenitor cells (NPC). In juvenile Pacific salmon, the predominant type of NSCs in the telencephalon are neuroepithelial cells corresponding to embryonic NSCs. Expression of glutamine synthetase (GS), a NSC molecular marker, was detected in the neuroepithelial cells of the pallium and subpallium of juvenile chum salmon, Oncorhynchus keta. At 3 days after a traumatic brain injury (TBI) in juvenile chum salmon, the GS expression was detected in the radial glia corresponding to aNSC in the pallium and subpallium. The maximum density of distribution of GS⁺ radial glia was found in the dorsal pallial region. Hydrogen sulfide (H₂S) is a proneurogenic factor that reduces oxidative stress and excitotoxicity effects, along with the increased GS production in the brain cells of juvenile chum salmon. In the fish brain, H₂S producing by cystathionine β-synthase in neurogenic zones may be involved in maintaining the microenvironment that provides optimal conditions for the functioning of neurogenic niches during constitutive neurogenesis. After injury, H₂S can determine cell survivability, providing a neuroprotective effect in the area of injury and reducing the process of glutamate excitotoxicity, acting as a signaling molecule involved in changing the neurogenic environment, which leads to the reactivation of neurogenic niches and cell regeneration programs. The results of studies on the control of the expression of regulatory Sonic Hedgehog genes (Shh) and the transcription factors Paired Box2 (Pax2) regulated by them are still insufficient. A comparative analysis of Pax2 expression in the telencephalon of intact chum salmon showed the presence of constitutive patterns of Pax2 expression in neurogenic areas and non-neurogenic parenchymal zones of the pallium and subpallium. After mechanical injury, the patterns of Pax2 expression changed, and the amount of Pax2⁺ decreased (p < 0.05) in lateral (Dl), medial (Dm) zones of the pallium, and the lateral zone (Vl) of the subpallium compared to the control. We believe that the decrease in the expression of Pax2 may be caused by the inhibitory effect of the Pax6 transcription factor, whose expression in the juvenile salmon brain increases upon injury.

Müller glia-myeloid cell crosstalk accelerates optic nerve regeneration in the adult zebrafish

Neurodegenerative disorders, characterized by progressive neuronal loss, eventually lead to functional impairment in the adult mammalian central nervous system (CNS). Importantly, these deteriorations are irreversible, due to the very limited regenerative potential of these CNS neurons. Stimulating and redirecting neuroinflammation was recently put forward as an important approach to induce axonal regeneration, but it remains elusive how inflammatory processes and CNS repair are intertwined. To gain more insight into these interactions, we investigated how immunomodulation affects the regenerative outcome after optic nerve crush (ONC) in the spontaneously regenerating zebrafish. First, inducing intraocular inflammation using zymosan resulted in an acute inflammatory response, characterized by an increased infiltration and proliferation of innate blood-borne immune cells, reactivation of Müller glia, and altered retinal cytokine expression. Strikingly, inflammatory stimulation also accelerated axonal regrowth after optic nerve injury. Second, we demonstrated that acute depletion of both microglia and macrophages in the retina, using pharmacological treatments with both the CSF1R inhibitor PLX3397 and clodronate liposomes, compromised optic nerve regeneration. Moreover, we observed that csf1ra/b double mutant fish, lacking microglia in both retina and brain, displayed accelerated RGC axonal regrowth after ONC, which was accompanied with unusual Müller glia proliferative gliosis. Altogether, our results highlight the importance of altered glial cell interactions in the axonal regeneration process after ONC in adult zebrafish. Unraveling the relative contribution of the different cell types, as well as the signaling pathways involved, may pinpoint new targets to stimulate repair in the vertebrate CNS.

Past, present and future of zebrafish in epilepsy research

Animal models contribute greatly to our understanding of brain development and function as well as its dysfunction in neurological diseases. Epilepsy research is a very good example of how animal models can provide us with a mechanistic understanding of the genes, molecules, and pathophysiological processes involved in disease. Over the course of the last two decades, zebrafish came in as a new player in epilepsy research, with an expanding number of laboratories using this animal to understand epilepsy and to discover new strategies for preventing seizures. Yet, zebrafish as a model offers a lot more for epilepsy research. In this viewpoint, we aim to highlight some key contributions of zebrafish to epilepsy research, and we want to emphasize the great untapped potential of this animal model for expanding these contributions. We hope that our suggestions will trigger further discussions between clinicians and researchers with a common goal to understand and cure epilepsy.

Altered expression of genes regulating inflammation and synaptogenesis during regrowth of afferent neurons to cochlear hair cells

The spiral ganglion neurons constitute the primary connection between auditory hair cells and the brain. The spiral ganglion afferent fibers and their synapse with hair cells do not regenerate to any significant degree in adult mammalian ears after damage. We have investigated gene expression changes after kainate-induced disruption of the synapses in a neonatal cochlear explant model in which peripheral fibers and the afferent synapse do regenerate. We compared gene expression early after damage, during regeneration of the fibers and synapses, and after completion of in vitro regeneration. These analyses revealed a total of 2.5% differentially regulated transcripts (588 out of 24,000) based on a threshold of p<0.005. Inflammatory response genes as well as genes involved in regeneration of neural circuits were upregulated in the spiral ganglion neurons and organ of Corti, where the hair cells reside. Prominent genes upregulated at several time points included genes with roles in neurogenesis (Elavl4 and Sox21), neural outgrowth (Ntrk3 and Ppp1r1c), axonal guidance (Rgmb and Sema7a), synaptogenesis (Nlgn2 and Psd2), and synaptic vesicular function (Syt8 and Syn1). Immunohistochemical and in situ hybridization analysis of genes that had not previously been described in the cochlea confirmed their cochlear expression. The time course of expression of these genes suggests that kainate treatment resulted in a two-phase response in spiral ganglion neurons: an acute response consistent with inflammation, followed by an upregulation of neural regeneration genes. Identification of the genes activated during regeneration of these fibers suggests candidates that could be targeted to enhance regeneration in adult ears.

Walking with Salamanders: From Molecules to Biorobotics

How do four-legged animals adapt their locomotion to the environment? How do central and peripheral mechanisms interact within the spinal cord to produce adaptive locomotion and how is locomotion recovered when spinal circuits are perturbed? Salamanders are the only tetrapods that regenerate voluntary locomotion after full spinal transection. Given their evolutionary position, they provide a unique opportunity to bridge discoveries made in fish and mammalian models. Genetic dissection of salamander neural circuits is becoming feasible with new methods for precise manipulation, elimination, and visualisation of cells. These approaches can be combined with classical tools in neuroscience and with modelling and a robotic environment. We propose that salamanders provide a blueprint of the function, evolution, and regeneration of tetrapod locomotor circuits.

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.

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.

Markers of Regenerative Processes in Patients with Bipolar Disorder: A Case-control Study

Progress in medical science has allowed the discovery of many factors affecting the pathogenesis of bipolar disorder, and among the most recent research directions are found regenerative and inflammatory processes. The role of regenerative processes remains particularly poorly explored, but available data encourage further research, which may explain the pathogenesis of bipolar disorder (BD). The aim of this study was to evaluate the mobilization of stem cells into peripheral blood, in patients with bipolar disorder during stable phase, not treated with lithium salts. The study included 30 unrelated individuals with the diagnosis of bipolar disorder, with disease duration of at least 10 years, not treated with lithium salts for at least five years prior to the study. The control group consisted of 30 healthy subjects, matched for age, sex, body mass index (BMI), origin, socio-demographic factors and nicotine use. Blood samples underwent cytometric analyses to assess concentrations of: Very Small Embryonic Like (VSEL) CD34+, VSEL AC133+, HSC CD34+, HSC AC133+. There were no significant differences in stem cell levels between patients with BD and healthy controls. However, the level of VSEL cells AC133 + was significantly higher in type I BD patients compared to healthy controls. Our results indicate a disturbance in regenerative processes in patients with bipolar disorder.

High-resolution mapping of injury-site dependent functional recovery in a single axon in zebrafish

In non-mammalian vertebrates, some neurons can regenerate after spinal cord injury. One of these, the giant Mauthner (M-) neuron shows a uniquely direct link to a robust survival-critical escape behavior but appears to regenerate poorly. Here we use two-photon microscopy in parallel with behavioral assays in zebrafish to show that the M-axon can regenerate very rapidly and that the recovery of functionality lags by just days. However, we also find that the site of the injury is critical: While regeneration is poor both close and far from the soma, rapid regeneration and recovery of function occurs for injuries between 10% and 50% of total axon length. Our findings show that rapid regeneration and the recovery of function can be studied at remarkable temporal resolution after targeted injury of one single M-axon and that the decision between poor and rapid regeneration can be studied in this one axon.

Alexander Hecker et al. study the regeneration potential of the axon of the giant Mauthner (M) neuron in zebrafish. Using two-photon microscopy and behavioral assays, they show that the M-axon can recover rapidly days after injury. They also characterize the optimal injury site that enables rapid regeneration and functional recovery.

Radial glia in the zebrafish brain: Functional, structural, and physiological comparison with the mammalian glia

The neuroscience community has witnessed a tremendous expansion of glia research. Glial cells are now on center stage with leading roles in the development, maturation, and physiology of brain circuits. Over the course of evolution, glia have highly diversified and include the radial glia, astroglia or astrocytes, microglia, oligodendrocytes, and ependymal cells, each having dedicated functions in the brain. The zebrafish, a small teleost fish, is no exception to this and recent evidences point to evolutionarily conserved roles for glia in the development and physiology of its nervous system. Due to its small size, transparency, and genetic amenability, the zebrafish has become an increasingly prominent animal model for brain research. It has enabled the study of neural circuits from individual cells to entire brains, with a precision unmatched in other vertebrate models. Moreover, its high neurogenic and regenerative potential has attracted a lot of attention from the research community focusing on neural stem cells and neurodegenerative diseases. Hence, studies using zebrafish have the potential to provide fundamental insights about brain development and function, and also elucidate neural and molecular mechanisms of neurological diseases. We will discuss here recent discoveries on the diverse roles of radial glia and astroglia in neurogenesis, in modulating neuronal activity and in regulating brain homeostasis at the brain barriers. By comparing insights made in various animal models, particularly mammals and zebrafish, our goal is to highlight the similarities and differences in glia biology among species, which could set new paradigms relevant to humans.

Growth Hormone Neuroprotection Against Kainate Excitotoxicity in the Retina is Mediated by Notch/PTEN/Akt Signaling

Purpose

In the retina, growth hormone (GH) promotes axonal growth, synaptic restoration, and protective actions against excitotoxicity. Notch signaling pathway is critical for neural development and participates in the retinal neuroregenerative process. We investigated the interaction of GH with Notch signaling pathway during its neuroprotective effect against excitotoxic damage in the chicken retina.

Methods

Kainate (KA) was used as excitotoxic agent and changes in the mRNA expression of several signaling markers were determined by qPCR. Also, changes in phosphorylation and immunoreactivity were determined by Western blotting. Histology and immunohistochemistry were performed for morphometric analysis. Overexpression of GH was performed in the quail neuroretinal-derived immortalized cell line (QNR/D) cell line. Exogenous GH was administered to retinal primary cell cultures to study the activation of signaling pathways.

Results

KA disrupted the retinal cytoarchitecture and induced significant cell loss in several retinal layers, but the coaddition of GH effectively prevented these adverse effects. We showed that GH upregulates the Notch signaling pathway during neuroprotection leading to phosphorylation of the PI3K/Akt signaling pathways through downregulation of PTEN. In contrast, cotreatment of GH with the Notch signaling inhibitor, DAPT, prevented its neuroprotective effect against KA. We identified binding sites in Notch1 and Notch2 genes for STAT5. Also, GH prevented Müller cell transdifferentiation and downregulated Sox2, FGF2, and PCNA after cotreatment with KA. Additionally, GH modified TNF receptors immunoreactivity suggesting anti-inflammatory actions.

Conclusions

Our data indicate that the neuroprotective effects of GH against KA injury in the retina are mediated through the regulation of Notch signaling. Additionally, anti-inflammatory and antiproliferative effects were observed.

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.

Neuroinflammation and Optic Nerve Regeneration: Where Do We Stand in Elucidating Underlying Cellular and Molecular Players?

Neurodegenerative diseases and central nervous system (CNS) trauma are highly irreversible, in part because adult mammals lack a robust regenerative capacity. A multifactorial problem underlies the limited axonal regeneration potential. Strikingly, neuroinflammation seems able to induce axonal regrowth in the adult mammalian CNS. It is increasingly clear that both blood-borne and resident inflammatory cells as well as reactivated glial cells affect axonal regeneration. The scope of this review is to give a comprehensive overview of the knowledge that links inflammation (with a focus on the innate immune system) to axonal regeneration and to critically reflect on the controversy that still prevails about the cells, molecules and pathways that are dominating the scene. Also, a brief overview is given of what is already known about the crosstalk between and the heterogeneity of cell types that might play a role in axonal regeneration. Recent research indicates that inflammation-induced axonal regrowth is not solely driven by a single-cell population but probably relies on the crosstalk between multiple cell types and the strong regulation of these cell populations in time and space. Moreover, there is growing evidence that the different cell populations are highly heterogeneous and as such can react differently upon injury. This could explain the controversial results that have been obtained over the past years. The primary focus of this manuscript is the retinofugal system of adult mammals, however, when relevant, insights or examples of the spontaneous regenerating zebrafish model and spinal cord research are added.

Zebrafish as a translational regeneration model to study the activation of neural stem cells and role of their environment

The review is an overview of the current knowledge of neuronal regeneration properties in mammals and fish. The ability to regenerate the damaged parts of the nervous tissue has been demonstrated in all vertebrates. Notably, fish and amphibians have the highest capacity for neurogenesis, whereas reptiles and birds are able to only regenerate specific regions of the brain, while mammals have reduced capacity for neurogenesis. Zebrafish (Danio rerio) is a promising model of study because lesions in the brain or complete cross-section of the spinal cord are followed by an effective neuro-regeneration that successfully restores the motor function. In the brain and the spinal cord of zebrafish, stem cell activity is always able to re-activate the molecular programs required for central nervous system regeneration. In mammals, traumatic brain injuries are followed by reduced neurogenesis and poor axonal regeneration, often insufficient to functionally restore the nervous tissue, while spinal injuries are not repaired at all. The environment that surrounds the stem cell niche constituted by connective tissue and stimulating factors, including pro-inflammation molecules, seems to be a determinant in triggering stem cell proliferation and/or the trans-differentiation of connective elements (mainly fibroblasts). Investigating and comparing the neuronal regeneration in zebrafish and mammals may lead to a better understanding of the mechanisms behind neurogenesis, and the failure of the regenerative response in mammals, first of all, the role of inflammation, considered the main inhibitor of the neuronal regeneration.

Malva sylvestris extract alleviates the astrogliosis and inflammatory stress in LPS-induced depression mice

Neuro-inflammation is widely regarded as the inflammation occurred in the central nervous system (CNS) tissue, which authentically involved in the pathogenesis such as depression although the underlying mechanism remains to be elucidated. Malva sylvestris (MS), a plant widely used in traditional medicine to mitigate urological, respiratory and oral diseases, exhibits excellent anti-oxidative and anti-inflammatory properties. In the present study, we first used LPS-induced depression-like mice to evaluate the neuro-protective effect of MS extract. We found that, after 7 days' administration of MS extract, the cognitive impairment of LPS-induced depression-like mice was efficiently alleviated, evaluated by behavioral test including the Open field, Morris water maze (MWM), Elevated plus-maze (EPM) and Rota-rod test. Furthermore, we found that MS extract also inhibited the LPS-induced neuron apoptosis and astrogliosis both in the cortex and the CA1 region of hippocampus. Finally, our findings showed that the extract of MS relieved inflammatory stress induced by LPS injury, indicated by the down-regulation of IL-1β/6 and TNF-α, and up-regulation of IL-4 level both in vitro and in vivo. Collectively, MS extract exhibits neuro-protective activity in vivo, and therefore, it may be widely used for food to relieve the symptoms of neuro-inflammation associated disorders such as depression.

Astrocytic N-Myc Downstream-regulated Gene-2 Is Involved in Nuclear Transcription Factor κB-mediated Inflammation Induced by Global Cerebral Ischemia

BACKGROUND

Inflammation is a key element in the pathophysiology of cerebral ischemia. This study investigated the role of N-Myc downstream-regulated gene-2 in nuclear transcription factor κB-mediated inflammation in ischemia models.

METHODS

Mice (n = 6 to 12) with or without nuclear transcription factor κB inhibitor pyrrolidinedithiocarbamate pretreatment were subjected to global cerebral ischemia for 20 min. Pure astrocyte cultures or astrocyte-neuron cocultures (n = 6) with or without pyrrolidinedithiocarbamate pretreatment were exposed to oxygen-glucose deprivation for 4 h or 2 h. Astrocytic nuclear transcription factor κB and N-Myc downstream-regulated gene-2 expression, proinflammatory cytokine secretion, neuronal apoptosis and survival, and memory function were analyzed at different time points after reperfusion or reoxygenation. Proinflammatory cytokine secretion was also studied in lentivirus-transfected astrocyte lines after reoxygenation.

RESULTS

Astrocytic nuclear transcription factor κB and N-Myc downstream-regulated gene-2 expression and proinflammatory cytokine secretion increased after reperfusion or reoxygenation. Pyrrolidinedithiocarbamate pretreatment significantly reduced N-Myc downstream-regulated gene-2 expression and proinflammatory cytokine secretion in vivo and in vitro, reduced neuronal apoptosis induced by global cerebral ischemia/reperfusion (from 65 ± 4% to 47 ± 4%, P = 0.0375) and oxygen-glucose deprivation/reoxygenation (from 45.6 ± 0.2% to 22.0 ± 4.0%, P < 0.001), and improved memory function in comparison to vehicle-treated control animals subjected to global cerebral ischemia/reperfusion. N-Myc downstream-regulated gene-2 lentiviral knockdown reduced the oxygen-glucose deprivation-induced secretion of proinflammatory cytokines.

CONCLUSIONS

Astrocytic N-Myc downstream-regulated gene-2 is up-regulated after cerebral ischemia and is involved in nuclear transcription factor κB-mediated inflammation. Pyrrolidinedithiocarbamate alleviates ischemia-induced neuronal injury and hippocampal-dependent cognitive impairment by inhibiting increases in N-Myc downstream-regulated gene-2 expression and N-Myc downstream-regulated gene-2-mediated inflammation.

Prior Exposure to Immunosuppressors Sensitizes Retinal Microglia and Accelerates Optic Nerve Regeneration in Zebrafish

As adult mammals lack the capacity to replace or repair damaged neurons, degeneration and trauma (and subsequent dysfunction) of the central nervous system (CNS) seriously constrains the patient's life quality. Recent work has shown that appropriate modulation of acute neuroinflammation upon CNS injury can trigger a regenerative response; yet, the underlying cellular and molecular mechanisms remain largely elusive. In contrast to mammals, zebrafish retain high regenerative capacities into adulthood and thus form a powerful model to study the contribution of neuroinflammation to successful regeneration. Here, we used pharmacological immunosuppression methods to study the role of microglia/macrophages during optic nerve regeneration in adult zebrafish. We first demonstrated that systemic immunosuppression with dexamethasone (dex) impedes regeneration after optic nerve injury. Secondly, and strikingly, local intravitreal application of dex or clodronate liposomes prior to injury was found to sensitize retinal microglia. Consequently, we observed an exaggerated inflammatory response to subsequent optic nerve damage, along with enhanced tectal reinnervation. In conclusion, we found a strong positive correlation between the acute inflammatory response in the retina and the regenerative capacity of the optic nerve in adult zebrafish subjected to nerve injury.

Engineering advanced neural tissue constructs to mitigate acute cerebral inflammation after brain transplantation in rats

Stroke, traumatic brain injuries, and other similar conditions often lead to significant loss of functional brain tissue and associated disruption of neuronal signaling. A common strategy for replacing lost neurons is the injection of dissociated neural stem cells or differentiated neurons. However, this method is unlikely to be suitable for replacing large brain cavities, and the resulting distribution of neurons may lack the necessary architecture to support appropriate brain function. Engineered neural tissues may be a viable alternative. Cell death is a prominent concern in neuronal grafting studies, a problem that could be magnified with the transplantation of engineered neural tissues. Here, we examined the effect of one contributor to cell death, acute cerebral inflammation, on neuronal survival after the transplantation of bioengineered constructs based on silk scaffolds. We found evidence of a high degree of inflammation and poor neuronal survival after introducing engineered constructs into the motor cortex of rats. Integrating a corticosteroid (methylprednisolone) into the constructs resulted in significantly improved neuron survival during the acute phase of inflammation. The improved construct survival was associated with decreased markers of inflammation and an anti-inflammatory state of the immune system due to the steroid treatment.

Injectable Nanoreinforced Shape-Memory Hydrogel System for Regenerating Spinal Cord Tissue from Traumatic Injury

Traumatic injury in the central nervous system can lead to loss of functional neurons. Transplantation of neural progenitors is a promising therapeutic strategy. However, infusion of dissociated cells often suffers from low viability, uneven cell distribution, and poor in vivo engraftment that could be reinforced by a better cell delivery system. Here, we develop an injectable composite hydrogel system for use as a minimally invasive treatment of spinal cord injury (SCI) using motor neurons (MNs) derived from embryonic stem cells (ESCs). The composite hydrogel is based on a modified gelatin matrix integrated with shape-memory polymer fibers. The gelatin matrix creates a local microenvironment for cell assembly and also acts as a lubricant during injection through a fine catheter. Notably, shape-memory fiber scaffolds are able to recover to maintain the microstructures even after dramatic deformation from injection operation, providing the necessary support and guidance for motor neuron differentiation. We find that the composite hydrogel with an aligned fiber scaffold greatly improves the viability of ESCs and their differentiation toward MNs both in vitro and in vivo. When transplanted to SCI animals by injection, the ESC-loaded composite hydrogels are identified to significantly enhance tissue regeneration and motor function recovery in mice. With this proof-of-concept study, we believe that the injectable composite hydrogel system provides a promising solution for in vivo cell delivery with minimum invasiveness and can be readily extended to other stem-cell-based regenerative treatments.

Involvement of sonic hedgehog and notch signaling in regenerative neurogenesis in adult zebrafish optic tectum after stab injury

Unlike humans and other mammals, adult zebrafish have the superior capability to recover from central nervous system (CNS) injury. We previously found that proliferation of radial glia (RG) is induced in response to stab injury in optic tectum and that new neurons are generated from RG after stab injury. However, molecular mechanisms which regulate proliferation and differentiation of RG are not well known. In the present study, we investigated Shh and Notch signaling as potential mechanisms regulating regeneration in the optic tectum of adult zebrafish. We used Shh reporter fish and confirmed that canonical Shh signaling is activated specifically in RG after stab injury. Moreover, we have shown that Shh signaling promotes RG proliferation and suppresses their differentiation into neurons after stab injury. In contrast, Notch signaling was down-regulated after stab injury, indicated by the decrease in the expression level of her4 and her6, a target gene of Notch signaling. We also found that inhibition of Notch signaling after stab injury induced more proliferative RG, but that inhibition of Notch signaling inhibited generation of newborn neurons from RG after stab injury. These results suggest that high level of Notch signaling keeps RG quiescent and that appropriate level of Notch signaling is required for generation of newborn neurons from RG. Under physiological condition, activation of Shh signaling or inhibition of Notch signaling also induced RG proliferation. In adult optic tectum of zebrafish, canonical Shh signaling and Notch signaling play important roles in proliferation and differentiation of RG in physiological and regenerative conditions.

Stem cells from human apical papilla decrease neuro-inflammation and stimulate oligodendrocyte progenitor differentiation via activin-A secretion

Secondary damage following spinal cord injury leads to non-reversible lesions and hampering of the reparative process. The local production of pro-inflammatory cytokines such as TNF-α can exacerbate these events. Oligodendrocyte death also occurs, followed by progressive demyelination leading to significant tissue degeneration. Dental stem cells from human apical papilla (SCAP) can be easily obtained at the removal of an adult immature tooth. This offers a minimally invasive approach to re-use this tissue as a source of stem cells, as compared to biopsying neural tissue from a patient with a spinal cord injury. We assessed the potential of SCAP to exert neuroprotective effects by investigating two possible modes of action: modulation of neuro-inflammation and oligodendrocyte progenitor cell (OPC) differentiation. SCAP were co-cultured with LPS-activated microglia, LPS-activated rat spinal cord organotypic sections (SCOS), and LPS-activated co-cultures of SCOS and spinal cord adult OPC. We showed for the first time that SCAP can induce a reduction of TNF-α expression and secretion in inflamed spinal cord tissues and can stimulate OPC differentiation via activin-A secretion. This work underlines the potential therapeutic benefits of SCAP for spinal cord injury repair.

The role of neuro-epithelial-like and radial-glial stem and progenitor cells in development, plasticity, and repair

Neural stem and progenitor cells (NSPCs) are the primary source of new neurons in the brain and serve critical roles in tissue homeostasis and plasticity throughout life. Within the vertebrate brain, NSPCs are located within distinct neurogenic niches differing in their location, cellular composition, and proliferative behaviour. Heterogeneity in the NSPC population is hypothesized to reflect varying capacities for neurogenesis, plasticity and repair between different neurogenic zones. Since the discovery of adult neurogenesis, studies have predominantly focused on the behaviour and biological significance of adult NSPCs (aNSPCs) in rodents. However, compared to rodents, who show lifelong neurogenesis in only two restricted neurogenic niches, zebrafish exhibit constitutive neurogenesis across multiple stem cell niches that provide new neurons to every major brain division. Accordingly, zebrafish are a powerful model to probe the unique cellular and molecular profiles of NSPCs and investigate how these profiles govern tissue homeostasis and regenerative plasticity within distinct stem cell populations over time. Amongst the NSPC populations residing in the zebrafish central nervous system (CNS), proliferating radial-glia, quiescent radial-glia and neuro-epithelial-like cells comprise the majority. Here, we provide insight into the extent to which these distinct NSPC populations function and mature during development, respond to experience, and contribute to successful CNS regeneration in teleost fish. Together, our review brings to light the dynamic biological roles of these individual NSPC populations and showcases their diverse regenerative modes to achieve vertebrate brain repair later in life.

Dynamic changes in microglial and macrophage characteristics during degeneration and regeneration of the zebrafish retina

BACKGROUND

In contrast to mammals, zebrafish have the capacity to regenerate retinal neurons following a variety of injuries. Two types of glial cells, Müller glia (MG) and microglia, are known to exist in the zebrafish retina. Recent work has shown that MG give rise to regenerated retinal neurons, but the role of resident microglia, and the innate immune system more generally, during retinal regeneration is not well defined. Specifically, characteristics of the immune system and microglia following substantial neuron death and a successful regenerative response have not been documented.

METHODS

The neurotoxin ouabain was used to induce a substantial retinal lesion of the inner retina in zebrafish. This lesion results in a regenerative response that largely restores retinal architecture, neuronal morphologies, and connectivities, as well as recovery of visual function. We analyzed cryosections from damaged eyes following immunofluorescence and H&E staining to characterize the initial immune response to the lesion. Whole retinas were analyzed by confocal microscopy to characterize microglia morphology and distribution. Statistical analysis was performed using a two-tailed Student's t test comparing damaged to control samples.

RESULTS

We find evidence of early leukocyte infiltration to the retina in response to ouabain injection followed by a period of immune cell proliferation that likely includes both resident microglia and substantial numbers of proliferating, extra-retinally derived macrophages, leading to rapid accumulation upon retinal damage. Following immune cell proliferation, Müller glia re-enter the cell cycle. In retinas that have regenerated the layers lost to the initial injury (histologically regenerated), microglia retain morphological features of activation, suggesting ongoing functions that are likely essential to restoration of retinal function.

CONCLUSIONS

Collectively, these results indicate that microglia and the immune system are dynamic during a successful regenerative response in the retina. This study provides an important framework to probe inflammation in the initiation of, and functional roles of microglia during retinal regeneration.

Mechanisms of Pathology-Induced Neural Stem Cell Plasticity and Neural Regeneration in Adult Zebrafish Brain

Purpose of the Review

The purpose of this study is to review the current knowledge on the damage-induced molecular programs that underlie the regenerative ability in zebrafish brain.

Recent Findings

Neural stem cells are the reservoir for new neurons during development and regeneration of the vertebrate brains. Pathological conditions such as neurodegenerative diseases hamper neural stem cell plasticity and neurogenic outcome in humans, whereas adult zebrafish brain can enhance proliferation and neurogenic capacity of its neural stem cells despite the incipient pathology. Evidence suggests that zebrafish uses damage-induced molecular programs to enable neural stem cells to efficiently initiate regeneration. Since this aptitude may be harnessed for regenerative therapies in human brain, understanding the molecular programs regulating neural stem cell proliferation and quiescence in zebrafish is of utmost importance for clinical efforts.

Summary

Specific molecular programs that are different than those in the homeostatic conditions regulate adult zebrafish neural stem cell plasticity and the regenerative capacity after injury and neurodegeneration. These programs can serve as candidates for stem cell-based regenerative therapies in humans.

BEX1 is an RNA-dependent mediator of cardiomyopathy

Regulation of mRNA splicing, processing and stability is increasingly recognized as a critical control point in dynamically altering gene expression during stress or disease. Very little is understood of this process in heart failure. Here, we show that BEX1 is a heart failure-induced gene functioning as an mRNA-associated protein that enhances expression of a subset of cardiac disease-promoting genes. Modeling the increase in BEX1 that occurs in disease, cardiac-specific BEX1 transgenic mice show worse cardiac disease with stress stimulation, whereas Bex1 gene-deleted mice are protected from heart failure-promoting insults. Proteomic and interactive screening assays show that BEX1 is part of a large ribonucleoprotein processing complex involved in regulating proinflammatory mRNA expression in the heart. Specifically, induction of BEX1 augments the stability and expression of AU-rich element containing mRNAs typically found within proinflammatory genes. Thus, BEX1 functions as an mRNA-dependent effector that augments pathology-promoting gene expression during heart failure.

Little is known about the changes in mRNA splicing, processing and stability that can alter gene expression during heart failure. Here, the authors show that BEX1 is induced during heart failure and is part of a ribonucleoprotein complex enhancing the expression and stability of proinflammatory genes.

Neuronal expression of brain derived neurotrophic factor in the injured telencephalon of adult zebrafish

The reparative ability of the central nervous system varies widely in the animal kingdom. In the mammalian brain, the regenerative mechanisms are very limited and newly formed neurons do not survive longer, probably due to a non-suitable local environment. On the opposite, fish can repair the brain after injury, with fast and complete recovery of damaged area. The brain of zebrafish, a teleost fish widely used as vertebrate model, also possesses high regenerative properties after injury. Taking advantage of this relevant model, the aim of the present study was to investigate the role of brain-derived neurotrophic factor (BDNF) in the regenerative ability of adult brain, after stab wound telencephalic injury. BDNF is involved in many brain functions and plays key roles in the repair process after traumatic brain lesions. It has been reported that BDNF strengthens the proliferative activity of neuronal precursor cells, facilitates the neuronal migration toward injured areas, and shows survival properties due to its anti-apoptotic effects. BDNF mRNA levels, assessed by quantitative PCR and in situ hybridization at 1, 4, 7, and 15 days after the lesion, were increased in the damaged telencephalon, mostly suddenly after the lesion. Double staining using in situ hybridization and immunocytochemistry revealed that BDNF mRNA was restricted to cells identified as mature neurons. BDNF mRNA expressing neurons mostly increased in the area around the lesion, showing a peak 1 day after the lesion. Taken together, these results highlight the role of BDNF in brain repair processes and reinforce the value of zebrafish for the study of regenerative neurogenesis.

IL4/STAT6 Signaling Activates Neural Stem Cell Proliferation and Neurogenesis upon Amyloid-β42 Aggregation in Adult Zebrafish Brain

Human brains are prone to neurodegeneration, given that endogenous neural stem/progenitor cells (NSPCs) fail to support neurogenesis. To investigate the molecular programs potentially mediating neurodegeneration-induced NSPC plasticity in regenerating organisms, we generated an Amyloid-β42 (Aβ42)-dependent neurotoxic model in adult zebrafish brain through cerebroventricular microinjection of cell-penetrating Aβ42 derivatives. Aβ42 deposits in neurons and causes phenotypes reminiscent of amyloid pathophysiology: apoptosis, microglial activation, synaptic degeneration, and learning deficits. Aβ42 also induces NSPC proliferation and enhanced neurogenesis. Interleukin-4 (IL4) is activated primarily in neurons and microglia/macrophages in response to Aβ42 and is sufficient to increase NSPC proliferation and neurogenesis via STAT6 phosphorylation through the IL4 receptor in NSPCs. Our results reveal a crosstalk between neurons and immune cells mediated by IL4/STAT6 signaling, which induces NSPC plasticity in zebrafish brains.

Human TAUP301L overexpression results in TAU hyperphosphorylation without neurofibrillary tangles in adult zebrafish brain

Microtubule-associated TAU protein is a pathological hallmark in Alzheimer's disease (AD), where hyperphosphorylation of TAU generates neurofibrillary tangles. To investigate the effects of TAU in a regenerative adult vertebrate brain system, we generated a cre/lox-based transgenic model of zebrafish that chronically expresses human TAUP301L, which is a variant of human TAU protein that forms neurofibrillary tangles in mouse models and humans. Interestingly, we found that although chronic and abundant expression of TAUP301L starting from early embryonic development led to hyperphosphorylation, TAUP301L did not form oligomers and neurofibrillary tangles, and did not cause elevated apoptosis and microglial activation, which are classical symptoms of tauopathies in mammals. Additionally, TAUP301L neither increased neural stem cell proliferation nor activated the expression of regenerative factor Interleukin-4, indicating that TAUP301L toxicity is prevented in the adult zebrafish brain. By combining TAUP301L expression with our established Aβ42 toxicity model, we found that Aβ42 ceases to initiate neurofibrillary tangle formation by TAUP301L, and TAUP301L does not exacerbate the toxicity of Aβ42. Therefore, our results propose a cellular mechanism that protects the adult zebrafish brain against tauopathies, and our model can be used to understand how TAU toxicity can be prevented in humans.

The role of jab1, a putative downstream effector of the neurotrophic cytokine macrophage migration inhibitory factor (MIF) in zebrafish inner ear hair cell development

Macrophage migration inhibitory factor (MIF) is a neurotrophic cytokine essential for inner ear hair cell (HC) development and statoacoustic ganglion (SAG) neurite outgrowth, and SAG survival in mouse, chick and zebrafish. Another neurotrophic cytokine, Monocyte chemoattractant protein 1 (MCP1) is known to synergize with MIF; but MCP1 alone is insufficient to support mouse/chick SAG neurite outgrowth or neuronal survival. Because of the relatively short time over which the zebrafish inner ear develops (~30hpf), the living zebrafish embryo is an ideal system to examine mif and mcp1 cytokine pathways and interactions. We used a novel technique: direct delivery of antisense oligonucleotide morpholinos (MOs) into the embryonic zebrafish otocyst to discover downstream effectors of mif as well as to clarify the relationship between mif and mcp1 in inner ear development. MOs for mif, mcp1 and the presumptive mif and mcp1 effector, c-Jun activation domain-binding protein-1 (jab1), were injected and then electroporated into the zebrafish otocyst 25-48hours post fertilization (hpf). We found that although mif is important at early stages (before 30hpf) for auditory macular HC development, jab1 is more critical for vestibular macular HC development before 30hpf. After 30hpf, mcp1 becomes important for HC development in both maculae.

Inflammation and metabolism in tissue repair and regeneration

Tissue repair after injury is a complex, metabolically demanding process. Depending on the tissue's regenerative capacity and the quality of the inflammatory response, the outcome is generally imperfect, with some degree of fibrosis, which is defined by aberrant accumulation of collagenous connective tissue. Inflammatory cells multitask at the wound site by facilitating wound debridement and producing chemokines, metabolites, and growth factors. If this well-orchestrated response becomes dysregulated, the wound can become chronic or progressively fibrotic, with both outcomes impairing tissue function, which can ultimately lead to organ failure and death. Here we review the current understanding of the role of inflammation and cell metabolism in tissue-regenerative responses, highlight emerging concepts that may expand therapeutic perspectives, and briefly discuss where important knowledge gaps remain.

The effects of aging on Amyloid-β42-induced neurodegeneration and regeneration in adult zebrafish brain

Alzheimer disease is the most prevalent neurodegenerative disease and is associated with aggregation of Amyloid-β42 peptides. In mammals, Amyloid-β42 causes impaired neural stem/progenitor cell (NSPC) proliferation and neurogenesis, which exacerbate with aging. The molecular programs necessary to enhance NSPC proliferation and neurogenesis in our brains to mount successful regeneration are largely unknown. Therefore, to identify the molecular basis of effective brain regeneration, we previously established an Amyloid-β42 model in adult zebrafish that displayed Alzheimer-like phenotypes reminiscent of humans. Interestingly, zebrafish exhibited enhanced NSPC proliferation and neurogenesis after microinjection of Amyloid-β42 peptide. Here, we compare old and young fish to address the effects of aging on regenerative ability after Amyloid-β42 deposition. We found that aging does not affect the rate of NSPC proliferation but reduces the neurogenic response and microglia/macrophage activation after microinjection of Amyloid-β42 in zebrafish, suggesting an important link between aging, neuroinflammation, regenerative neurogenesis and neural stem cell plasticity.

Optical Projection Tomography as a Novel Method to Visualize and Quantitate Whole-Brain Patterns of Cell Proliferation in the Adult Zebrafish Brain

How distinct cell populations are distributed in three-dimensional space under homeostasis or following injury, neurodegeneration, or with senescence can teach us much about brain-wide patterns and signaling along the neuroaxis. Visualizing individual cell populations in the mature vertebrate central nervous system (CNS) has remained a challenge as a result of difficulty clearing adult brain tissue or limitations in imaging depth or resolution. We have developed a simple clearing and imaging pipeline optimally suited for the adult zebrafish brain to investigate changes in patterns of cell proliferation in wild-type and transgenic backgrounds that can easily be quantified and represented using FIJI and IMARIS software.