Neurogenesis of retinal ganglion cells is not essential to visual functional recovery after optic nerve injury in adult zebrafish

Oligodendroglial heterogeneity in health, disease, and recovery: deeper insights into myelin dynamics

Decades of research asserted that the oligodendroglial lineage comprises two cell types: oligodendrocyte precursor cells and oligodendrocytes. However, recent studies employing single-cell RNA sequencing techniques have uncovered novel cell states, prompting a revision of the existing terminology. Going forward, the oligodendroglial lineage should be delineated into five distinct cell states: oligodendrocyte precursor cells, committed oligodendrocyte precursor cells, newly formed oligodendrocytes, myelin-forming oligodendrocytes, and mature oligodendrocytes. This new classification system enables a deeper understanding of the oligodendroglia in both physiological and pathological contexts. Adopting this uniform terminology will facilitate comparison and integration of data across studies. This, including the consolidation of findings from various demyelinating models, is essential to better understand the pathogenesis of demyelinating diseases. Additionally, comparing injury models across species with varying regenerative capacities can provide insights that may lead to new therapeutic strategies to overcome remyelination failure. Thus, by standardizing terminology and synthesizing data from diverse studies across different animal models, we can enhance our understanding of myelin pathology in central nervous system disorders such as multiple sclerosis, Alzheimer's disease, and amyotrophic lateral sclerosis, all of which involve oligodendroglial and myelin dysfunction.

Polystyrene nanoplastics chronic exposure cause zebrafish visual neurobehavior toxicity through TGFβ-crystallin axis

The ubiquitous presence of micro-and nanoplastics (MNPs) in the environment and everyday products has attracted global attention for their hazardous risks. However, the effects and underling mechanisms of MNPs chronic exposure on behavioral/visual changes of the adult and offspring remain unclear. The present study investigated the impact of polystyrene (PS) nanoplastics of 80, 200 and 500 nm diameters on zebrafish visual behaviors at an environmentally relevant concentration of 0.1 mg/L. Exposure to PS resulted in zebrafish hyperactivity, enhanced aggression, compacted shoaling and less sociability, and especially suppressed the adult optokinetic response (OKR) and offspring larval phototactic behavior, with the 500 nm PS being the most detrimental. Histopathological analysis showed 500 nm PS caused significant structural damage to the retina's pigment epithelium (RPE), photoreceptor cells (PRC), and crystalline lens. Fluorescence observation found PS accumulation in retinal layers correlated with reduced oligodendrocyte transcription factor 2 (Olig2) in optic nerve. Further transcriptomic analysis of the adult eye tissue revealed that 500 nm PS affected the transforming growth factor β (TGFβ) and phototransduction signaling pathways, dysregulated visual perception and lens development, potentially leading to dysopia in zebrafish. Specifically, TGFβ and its regulated-extracellular matrix/inflammatory factors and crystallin genes were increased, but the visual perception genes were decreased, suggesting the TGFβ-crystallin axis disorders contribute to the eye dysfunction induced by PS exposure. Collectively, our results provide new evidence revealing the molecular mechanisms of PS-induced visual toxicity and neurobehavioral changes highlighting that MNPs may pose a risk to vision health.

The transcription factor Jun is necessary for optic nerve regeneration in larval zebrafish

Damage to the axons of the adult mammalian central nervous system (CNS) from traumatic injury or neurodegenerative diseases often results in permanent loss of function due to failure of axons to regenerate. Zebrafish, however, can express regeneration-associated genes to revert CNS neurons to a growth-competent state and regenerate damaged axons to functionality. An established model for CNS axon regeneration is optic nerve injury in zebrafish, where it was previously shown that thousands of genes are temporally expressed during the regeneration time course. It is likely that hubs of key transcription factors, rather than individual factors regulate the temporal clusters of expression after injury to facilitate cell survival, regrowth, and synaptic targeting in the brain. One transcription factor of interest in orchestrating CNS axon regeneration is jun. However, it remains unclear if CNS regeneration can progress without Jun. To test this, a transgenic zebrafish line was developed to express a heat-shock inducible dominant negative Jun. Induction of dominant negative Jun downregulated endogenous jun expression and larvae with functional jun knockdown demonstrated impaired retinal ganglion cell axon regeneration. Analysis of select putative Jun target genes, previously shown to be upregulated in adult zebrafish optic nerve regeneration, demonstrated that with functional Jun knockdown, atf3 and ascl1a were significantly downregulated, and sox11a was upregulated at distinct time points. These results position jun as a key regulator for successful optic nerve regeneration, further distinguish the regeneration program from development, and advance our knowledge for the formation of future therapies to treat CNS damage.

Srebf2 mediates successful optic nerve axon regeneration via the mevalonate synthesis pathway

BACKGROUND

Axon regeneration within the mammalian central nervous system is extremely limited. In optic neuropathy conditions like glaucoma, the inability of retinal ganglion cell (RGC) axons to regenerate is a major impediment to functional recovery. In contrast, adult teleost fish such as zebrafish can fully regenerate RGC axons enabling visual recovery from optic nerve (ON) injury making it an ideal model to probe the mechanisms of successful axon regeneration.

METHODS

Laser Capture Microdissection followed by RNA-sequencing (LCM-seq) was used to identify genes and pathways differentially expressed in RGCs during ON regeneration. We validate these findings by in situ hybridization and qRT-PCR. Using loss- and gain-of-function experiments we demonstrate the necessity of srebf2 for efficient ON regeneration and recovery of visual function. Finally, we use LCM-seq coupled with experimental manipulations to identify downstream srebf2 target genes and test the role of hmgcra/b and mevalonate in this process. Statistical analysis was performed using Student's t-test, two-way ANOVA, or repeated measures with appropriate post-hoc tests where applicable.

RESULTS

LCM-seq comparison of uninjured versus 3-day post ON injury RGCs identified significant upregulation of the cholesterol synthesis pathway during axon regeneration. The master regulator of this pathway, the transcription factor srebf2, is upregulated throughout the regeneration phase. Chemical inhibition or morpholino-based gene knockdown of srebf2 decreased axon regeneration into the ON and optic tectum and delayed recovery of visual behavior over the course of normal optic nerve regeneration without causing a significant loss of RGCs. Constitutively active srebf2 can fully rescue axon regeneration and visual behavior losses caused by inhibition of endogenous srebf2 but does not accelerate regeneration compared to the control group. LCM-seq confirms the expected regulation of predicted srebf2 target genes after loss- or gain-of-function in vivo. Downstream of srebf2, hmgcra/b knockdown or simvastatin treatment delayed axon regeneration and this effect was rescued by supplemental mevalonate. Mevalonate treatment alone was sufficient to accelerate ON regeneration.

CONCLUSIONS

These results demonstrate that srebf2 and the downstream mevalonate synthesis pathway plays an important role in regulating efficient axon regeneration in the zebrafish visual system. Involvement of this pathway should be closely examined in failed mammalian ON regeneration.

Noninvasive Light Flicker Stimulation Promotes Optic Nerve Regeneration by Activating Microglia and Enhancing Neural Plasticity in Zebrafish

Purpose

Forty-hertz light flicker stimulation has been proven to reduce neurodegeneration, but its effect on optic nerve regeneration is unclear. This study explores the effect of 40-Hz light flicker in promoting optic nerve regeneration in zebrafish and investigates the underlying mechanisms.

Methods

Wild-type and mpeg1:EGFP zebrafish were used to establish a model of optic nerve crush. Biocytin tracing and hematoxylin and eosin staining were employed to observe whether 40-Hz light flicker promotes regeneration of retinal ganglion cell axons and dendrites. Optomotor and optokinetic responses were evaluated to assess recovery of visual function. Immunofluorescence staining of mpeg1:EGFP zebrafish was performed to observe changes in microglia. Differentially expressed genes that promote optic nerve regeneration following 40-Hz light flicker stimulation were identified and validated through RNA-sequencing analysis and quantitative real-time PCR (qRT-PCR).

Results

Zebrafish exhibited spontaneous optic nerve regeneration after optic nerve injury and restored visual function. We observed that 40-Hz light flicker significantly activated microglia following optic nerve injury and promoted regeneration of retinal ganglion cell axons and dendrites, as well as recovery of visual function. Transcriptomics and qRT-PCR analyses revealed that 40-Hz light flicker increased the expression of genes associated with neuronal plasticity, including bdnf, npas4a, fosab, fosb, egr4, and ier2a.

Conclusions

To our knowledge, this study is the first to demonstrate that 40-Hz light flicker stimulation promotes regeneration of retinal ganglion cell axons and dendrites and recovery of visual function in zebrafish, which is associated with microglial activation and enhancement of neural plasticity.

Xenopus retinal ganglion cell axon extension is unaffected by 5-HT 1B/D receptor activation during visual system development

Activating 5-HT 1B/D receptors with the agonist Zolmitriptan was previously shown to facilitate Xenopus retinal ganglion cell (RGC) axon extension from ectopic eye primordia transplanted to the ventral fin. To determine if 5-HT 1B/D receptor activation influenced entopic RGC axonal outgrowth toward the optic tectum during typical visual system development, we reared embryos in 50 μΜ Zolmitriptan then visualized optic tracts with anterograde HRP labeling. Zolmitriptan did not significantly alter entopic RGC extension in the contralateral brain. Consequently, RGC axon extension in ectopic but not entopic locations is influenced by altering serotonergic signaling .

Retinal ganglion cell repopulation for vision restoration in optic neuropathy: a roadmap from the RReSTORe Consortium

Retinal ganglion cell (RGC) death in glaucoma and other optic neuropathies results in irreversible vision loss due to the mammalian central nervous system's limited regenerative capacity. RGC repopulation is a promising therapeutic approach to reverse vision loss from optic neuropathies if the newly introduced neurons can reestablish functional retinal and thalamic circuits. In theory, RGCs might be repopulated through the transplantation of stem cell-derived neurons or via the induction of endogenous transdifferentiation. The RGC Repopulation, Stem Cell Transplantation, and Optic Nerve Regeneration (RReSTORe) Consortium was established to address the challenges associated with the therapeutic repair of the visual pathway in optic neuropathy. In 2022, the RReSTORe Consortium initiated ongoing international collaborative discussions to advance the RGC repopulation field and has identified five critical areas of focus: (1) RGC development and differentiation, (2) Transplantation methods and models, (3) RGC survival, maturation, and host interactions, (4) Inner retinal wiring, and (5) Eye-to-brain connectivity. Here, we discuss the most pertinent questions and challenges that exist on the path to clinical translation and suggest experimental directions to propel this work going forward. Using these five subtopic discussion groups (SDGs) as a framework, we suggest multidisciplinary approaches to restore the diseased visual pathway by leveraging groundbreaking insights from developmental neuroscience, stem cell biology, molecular biology, optical imaging, animal models of optic neuropathy, immunology & immunotolerance, neuropathology & neuroprotection, materials science & biomedical engineering, and regenerative neuroscience. While significant hurdles remain, the RReSTORe Consortium's efforts provide a comprehensive roadmap for advancing the RGC repopulation field and hold potential for transformative progress in restoring vision in patients suffering from optic neuropathies.

Dual leucine zipper kinase is necessary for retinal ganglion cell axonal regeneration in Xenopus laevis

Retinal ganglion cell (RGC) axons of the African clawed frog, Xenopus laevis, unlike those of mammals, are capable of regeneration and functional reinnervation of central brain targets following injury. Here, we describe a tadpole optic nerve crush (ONC) procedure and assessments of brain reinnervation based on live imaging of RGC-specific transgenes which, when paired with CRISPR/Cas9 injections at the one-cell stage, can be used to assess the function of regeneration-associated genes in vivo in F0 animals. Using this assay, we find that map3k12, also known as dual leucine zipper kinase (Dlk), is necessary for RGC axonal regeneration and acts in a dose-dependent manner. Loss of Dlk does not affect RGC innervation of the brain during development or visually driven behavior but does block both axonal regeneration and functional vision restoration after ONC. Dlk loss does not alter the acute changes in mitochondrial movement that occur within RGC axons hours after ONC but does completely block the phosphorylation and nuclear translocation of the transcription factor Jun within RGCs days after ONC; yet, Jun is dispensable for reinnervation. These results demonstrate that in a species fully capable of regenerating its RGC axons, Dlk is essential for the axonal injury signal to reach the nucleus but may affect regeneration through a different pathway than by which it signals in mammalian RGCs.

Assaying Optic Nerve Regeneration in Larval Zebrafish

Zebrafish have a remarkable capacity for spontaneously regenerating their central nervous system. Larval zebrafish are optically transparent and therefore are widely used to dynamically visualize cellular processes in vivo, such as nerve regeneration. Regeneration of retinal ganglion cell (RGC) axons within the optic nerve has been previously studied in adult zebrafish. In contrast, assays of optic nerve regeneration have previously not been established in larval zebrafish. In order to take advantage of the imaging capabilities in the larval zebrafish model, we recently developed an assay to physically transect RGC axons and monitor optic nerve regeneration in larval zebrafish. We found that RGC axons rapidly and robustly regrow to the optic tectum. Here, we describe the methods for performing the optic nerve transections, as well as methods for visualizing RGC regeneration in larval zebrafish.

Use of gene therapy for optic nerve protection: Current concepts

Gene therapy has become an essential treatment for optic nerve injury (ONI) in recent years, and great strides have been made using animal models. ONI, which is characterized by the loss of retinal ganglion cells (RGCs) and axons, can induce abnormalities in the pupil light reflex, visual field defects, and even vision loss. The eye is a natural organ to target with gene therapy because of its high accessibility and certain immune privilege. As such, numerous gene therapy trials are underway for treating eye diseases such as glaucoma. The aim of this review was to cover research progress made in gene therapy for ONI. Specifically, we focus on the potential of gene therapy to prevent the progression of neurodegenerative diseases and protect both RGCs and axons. We cover the basic information of gene therapy, including the classification of gene therapy, especially focusing on genome editing therapy, and then we introduce common editing tools and vector tools such as Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) -Cas9 and adeno-associated virus (AAV). We also summarize the progress made on understanding the roles of brain derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), phosphatase-tensin homolog (PTEN), suppressor of cytokine signal transduction 3 (SOCS3), histone acetyltransferases (HATs), and other important molecules in optic nerve protection. However, gene therapy still has many challenges, such as misalignment and mutations, immunogenicity of AAV, time it takes and economic cost involved, which means that these issues need to be addressed before clinical trials can be considered.

Epigenetic Regulation of Optic Nerve Development, Protection, and Repair

Epigenetic factors are known to influence tissue development, functionality, and their response to pathophysiology. This review will focus on different types of epigenetic regulators and their associated molecular apparatus that affect the optic nerve. A comprehensive understanding of epigenetic regulation in optic nerve development and homeostasis will help us unravel novel molecular pathways and pave the way to design blueprints for effective therapeutics to address optic nerve protection, repair, and regeneration.

Differential Retinal Ganglion Cell Vulnerability, A Critical Clue for the Identification of Neuroprotective Genes in Glaucoma

Retinal ganglion cells (RGCs) are the neurons in the retina which directly project to the brain and transmit visual information along the optic nerve. Glaucoma, one of the leading causes of blindness, is characterized by elevated intraocular pressure (IOP) and degeneration of the optic nerve, which is followed by RGC death. Currently, there are no clinical therapeutic drugs or molecular interventions that prevent RGC death outside of IOP reduction. In order to overcome these major barriers, an increased number of studies have utilized the following combined analytical methods: well-established rodent models of glaucoma including optic nerve injury models and transcriptomic gene expression profiling, resulting in the successful identification of molecules and signaling pathways relevant to RGC protection. In this review, we present a comprehensive overview of pathological features in a variety of animal models of glaucoma and top differentially expressed genes (DEGs) depending on disease progression, RGC subtypes, retinal regions or animal species. By comparing top DEGs among those different transcriptome profiles, we discuss whether commonly listed DEGs could be defined as potential novel therapeutic targets in glaucoma, which will facilitate development of future therapeutic neuroprotective strategies for treatments of human patients in glaucoma.

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.

Retinal ganglion cell survival after severe optic nerve injury is modulated by crosstalk between Jak/Stat signaling and innate immune responses in the zebrafish retina

Visual information is transmitted from the eye to the brain along the optic nerve, a structure composed of retinal ganglion cell (RGC) axons. The optic nerve is highly vulnerable to damage in neurodegenerative diseases, such as glaucoma, and there are currently no FDA-approved drugs or therapies to protect RGCs from death. Zebrafish possess remarkable neuroprotective and regenerative abilities. Here, utilizing an optic nerve transection (ONT) injury and an RNA-seq-based approach, we identify genes and pathways active in RGCs that may modulate their survival. Through pharmacological perturbation, we demonstrate that Jak/Stat pathway activity is required for RGC survival after ONT. Furthermore, we show that immune responses directly contribute to RGC death after ONT; macrophages/microglia are recruited to the retina and blocking neuroinflammation or depleting these cells after ONT rescues survival of RGCs. Taken together, these data support a model in which crosstalk between macrophages/microglia and RGCs, mediated by Jak/Stat pathway activity, regulates RGC survival after optic nerve injury.

Strip1 regulates retinal ganglion cell survival by suppressing Jun-mediated apoptosis to promote retinal neural circuit formation

In the vertebrate retina, an interplay between retinal ganglion cells (RGCs), amacrine (AC), and bipolar (BP) cells establishes a synaptic layer called the inner plexiform layer (IPL). This circuit conveys signals from photoreceptors to visual centers in the brain. However, the molecular mechanisms involved in its development remain poorly understood. Striatin-interacting protein 1 (Strip1) is a core component of the striatin-interacting phosphatases and kinases (STRIPAK) complex, and it has shown emerging roles in embryonic morphogenesis. Here, we uncover the importance of Strip1 in inner retina development. Using zebrafish, we show that loss of Strip1 causes defects in IPL formation. In strip1 mutants, RGCs undergo dramatic cell death shortly after birth. AC and BP cells subsequently invade the degenerating RGC layer, leading to a disorganized IPL. Mechanistically, zebrafish Strip1 interacts with its STRIPAK partner, Striatin 3 (Strn3), and both show overlapping functions in RGC survival. Furthermore, loss of Strip1 or Strn3 leads to activation of the proapoptotic marker, Jun, within RGCs, and Jun knockdown rescues RGC survival in strip1 mutants. In addition to its function in RGC maintenance, Strip1 is required for RGC dendritic patterning, which likely contributes to proper IPL formation. Taken together, we propose that a series of Strip1-mediated regulatory events coordinates inner retinal circuit formation by maintaining RGCs during development, which ensures proper positioning and neurite patterning of inner retinal neurons.

The back of the eye is lined with an intricate tissue known as the retina, which consists of carefully stacked neurons connecting to each other in well-defined ‘synaptic’ layers. Near the surface, photoreceptors cells detect changes in light levels, before passing this information through the inner plexiform layer to retinal ganglion cells (or RGCs) below. These neurons will then relay the visual signals to the brain. Despite the importance of this inner retinal circuit, little is known about how it is created as an organism develops.

As a response, Ahmed et al. sought to identify which genes are essential to establish the inner retinal circuit, and how their absence affects retinal structure. To do this, they introduced random errors in the genetic code of zebrafish and visualised the resulting retinal circuits in these fast-growing, translucent fish. Initial screening studies found fish with mutations in a gene encoding a protein called Strip1 had irregular layering of the inner retina.

Further imaging experiments to pinpoint the individual neurons affected showed that in zebrafish without Strip1, RGCs died in the first few days of development. Consequently, other neurons moved into the RGC layer to replace the lost cells, leading to layering defects. Ahmed et al. concluded that Strip1 promotes RGC survival and thereby coordinates proper positioning of neurons in the inner retina.

In summary, these findings help to understand how the inner retina is wired; they could also shed light on the way other layered structures are established in the nervous system. Moreover, this study paves the way for future research investigating Strip1 as a potential therapeutic target to slow down the death of RGCs in conditions such as glaucoma.

The basic science of optic nerve regeneration

Diverse insults to the optic nerve result in partial to total vision loss as the axons of retinal ganglion cells are destroyed. In glaucoma, axons are injured at the optic nerve head; in other optic neuropathies, axons can be damaged along the entire visual pathway. In all cases, as mammals cannot regenerate injured central nervous system cells, once the axons are lost, vision loss is irreversible. However, much has been learned about how retinal ganglion cells respond to axon injuries, and many of these crucial discoveries offer hope for future regenerative therapies. Here we review the current understanding regarding the temporal progression of axonal degeneration. We summarize known survival and regenerative mechanisms in mammals, including specific signaling pathways, key transcription factors, and reprogramming genes. We cover mechanisms intrinsic to retinal ganglion cells as well as their interactions with myeloid and glial cell populations in the retina and optic nerve that affect survival and regeneration. Finally, we highlight some non-mammalian species that are able to regenerate their retinal ganglion cell axons after injury, as understanding these successful regenerative responses may be essential to the rational design of future clinical interventions to regrow the optic nerve. In the end, a combination of many different molecular and cellular interventions will likely be the only way to achieve functional recovery of vision and restore quality of life to millions of patients around the world.

In vivo imaging reveals mature Oligodendrocyte division in adult Zebrafish

Whether mature oligodendrocytes (mOLs) participate in remyelination has been disputed for several decades. Recently, some studies have shown that mOLs participate in remyelination by producing new sheaths. However, whether mOLs can produce new oligodendrocytes by asymmetric division has not been proven. Zebrafish is a perfect model to research remyelination compared to other species. In this study, optic nerve crushing did not induce local mOLs death. After optic nerve transplantation from olig2:eGFP fish to AB/WT fish, olig2⁺ cells from the donor settled and rewrapped axons in the recipient. After identifying these rewrapping olig2⁺ cells as mOLs at 3 months posttransplantation, in vivo imaging showed that olig2⁺ cells proliferated. Additionally, in vivo imaging of new olig2⁺ cell division from mOLs was also captured within the retina. Finally, fine visual function was renewed after the remyelination program was completed. In conclusion, our in vivo imaging results showed that new olig2⁺ cells were born from mOLs by asymmetric division in adult zebrafish, which highlights the role of mOLs in the progression of remyelination in the mammalian CNS.

Ablation of Lrp4 in Schwann Cells Promotes Peripheral Nerve Regeneration in Mice

Low-density lipoprotein receptor-related protein 4 (Lrp4) is a critical protein involved in the Agrin-Lrp4-MuSK signaling pathway that drives the clustering of acetylcholine receptors (AChRs) at the neuromuscular junction (NMJ). Many studies have shown that Lrp4 also functions in kidney development, bone formation, nervous system development, etc. However, whether Lrp4 participates in nerve regeneration in mammals remains unknown. Herein, we show that Lrp4 is expressed in SCs and that conditional knockout (cKO) of Lrp4 in SCs promotes peripheral nerve regeneration. In Lrp4 cKO mice, the demyelination of SCs was accelerated, and the proliferation of SCs was increased in the injured nerve. Furthermore, we identified that two myelination-related genes, Krox-20 and Mpz, were downregulated more dramatically in the cKO group than in the control group. Our results elucidate a novel role of Lrp4 in peripheral nerve regeneration and thereby provide a potential therapeutic target for peripheral nerve recovery.

Transcriptome Analyses Reveal IL6/Stat3 Signaling Involvement in Radial Glia Proliferation After Stab Wound Injury in the Adult Zebrafish Optic Tectum

Adult zebrafish have many neurogenic niches and a high capacity for central nervous system regeneration compared to mammals, including humans and rodents. The majority of radial glia (RG) in the zebrafish optic tectum are quiescent under physiological conditions; however, stab wound injury induces their proliferation and differentiation into newborn neurons. Although previous studies have functionally analyzed the molecular mechanisms of RG proliferation and differentiation and have performed single-cell transcriptomic analyses around the peak of RG proliferation, the cellular response and changes in global gene expression during the early stages of tectum regeneration remain poorly understood. In this study, we performed histological analyses which revealed an increase in isolectin B4+ macrophages prior to the induction of RG proliferation. Moreover, transcriptome and pathway analyses based on differentially expressed genes identified various enriched pathways, including apoptosis, the innate immune system, cell proliferation, cytokine signaling, p53 signaling, and IL6/Jak-Stat signaling. In particular, we found that Stat3 inhibition suppressed RG proliferation after stab wound injury and that IL6 administration into cerebroventricular fluid activates RG proliferation without causing injury. Together, the findings of these transcriptomic and functional analyses reveal that IL6/Stat3 signaling is an initial trigger of RG activation during optic tectum regeneration.

Neurodegeneration, Neuroprotection and Regeneration in the Zebrafish Retina

Neurodegenerative retinal diseases, such as glaucoma and diabetic retinopathy, involve a gradual loss of neurons in the retina as the disease progresses. Central nervous system neurons are not able to regenerate in mammals, therefore, an often sought after course of treatment for neuronal loss follows a neuroprotective or regenerative strategy. Neuroprotection is the process of preserving the structure and function of the neurons that have survived a harmful insult; while regenerative approaches aim to replace or rewire the neurons and synaptic connections that were lost, or induce regrowth of damaged axons or dendrites. In order to test the neuroprotective effectiveness or the regenerative capacity of a particular agent, a robust experimental model of retinal neuronal damage is essential. Zebrafish are being used more often in this type of study because their eye structure and development is well-conserved between zebrafish and mammals. Zebrafish are robust genetic tools and are relatively inexpensive to maintain. The large array of functional and behavioral tests available in zebrafish makes them an attractive model for neuroprotection studies. Some common insults used to model retinal disease and study neuroprotection in zebrafish include intense light, chemical toxicity and mechanical damage. This review covers the existing retinal neuroprotection and regeneration literature in the zebrafish and highlights their potential for future studies.

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.

Noninvasive Imaging of Cone Ablation and Regeneration in Zebrafish

Purpose

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

Methods

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

Results

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

Conclusions

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

Translational Relevance

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

Optic nerve regeneration: A long view

The optic nerve conveys information about the outside world from the retina to multiple subcortical relay centers. Until recently, the optic nerve was widely believed to be incapable of re-growing if injured, with dire consequences for victims of traumatic, ischemic, or neurodegenerative diseases of this pathway. Over the past 10-20 years, research from our lab and others has made considerable progress in defining factors that normally suppress axon regeneration and the ability of retinal ganglion cells, the projection neurons of the retina, to survive after nerve injury. Here we describe research from our lab on the role of inflammation-derived growth factors, suppression of inter-cellular signals among diverse retinal cell types, and combinatorial therapies, along with related studies from other labs, that enable animals with optic nerve injury to regenerate damaged retinal axons back to the brain. These studies raise the possibility that vision might one day be restored to people with optic nerve damage.

P2X7 and A2A receptor endogenous activation protects against neuronal death caused by CoCl2 -induced photoreceptor toxicity in the zebrafish retina

Injured retinas in mammals do not regenerate and heal with loss of function. The adult retina of zebrafish self-repairs after damage by activating cell-intrinsic mechanisms, which are regulated by extrinsic signal interactions. Among relevant regulatory extrinsic systems, purinergic signaling regulates progenitor proliferation during retinogenesis and regeneration and glia proliferation in proliferative retinopathies. ATP-activated P2X7 (P2RX7) and adenosine (P1R) receptors are involved in the progression of almost all retinopathies leading to blindness. Here, we examined P2RX7 and P1R participation in the retina regenerative response induced by photoreceptor damage caused by a specific dose of CoCl₂ . First, we found that treatment of uninjured retinas with a potent agonist of P2RX7 (BzATP) provoked photoreceptor damage and mitotic activation of multipotent progenitors. In CoCl₂ -injured retinas, blockade of endogenous extracellular ATP activity on P2RX7 caused further neurodegeneration, Müller cell gliosis, progenitor proliferation, and microglia reactivity. P2RX7 inhibition in injured retinas also increased the expression of lin28a and tnfα genes, which are related to multipotent progenitor proliferation. Levels of hif1α, vegf3r, and vegfaa mRNA were enhanced by blockade of P2RX7 immediately after injury, indicating hypoxic like damage and endothelial cell growth and proliferation. Complete depletion of extracellular nucleotides with an apyrase treatment strongly potentiated cell death and progenitor proliferation induced with CoCl₂ . Blockade of adenosine P1 and A_2A receptors (A_2A R) had deleterious effects and deregulated normal timing for progenitor and precursor cell proliferation following photoreceptor damage. ATP via P2RX7 and adenosine via A_2A R are survival extracellular signals key for retina regeneration in zebrafish.

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.

Target-Derived Neurotrophic Factor Deprivation Puts Retinal Ganglion Cells on Death Row: Cold Hard Evidence and Caveats

Glaucoma and other optic neuropathies are characterized by axonal transport deficits. Axonal cargo travels back and forth between the soma and the axon terminus, a mechanism ensuring homeostasis and the viability of a neuron. An example of vital molecules in the axonal cargo are neurotrophic factors (NTFs). Hindered retrograde transport can cause a scarcity of those factors in the retina, which in turn can tilt the fate of retinal ganglion cells (RGCs) towards apoptosis. This postulation is one of the most widely recognized theories to explain RGC death in the disease progression of glaucoma and is known as the NTF deprivation theory. For several decades, research has been focused on the use of NTFs as a novel neuroprotective glaucoma treatment. Until now, results in animal models have been promising, but translation to the clinic has been highly disappointing. Are we lacking important knowledge to lever NTF therapies towards the therapeutic armamentarium? Or did we get the wrong end of the stick regarding the NTF deprivation theory? In this review, we will tackle the existing evidence and caveats advocating for and against the target-derived NTF deprivation theory in glaucoma, whilst digging into associated therapy efforts.

Inflammation-induced mammalian target of rapamycin signaling is essential for retina regeneration

Upon retina injury, Müller glia in the zebrafish retina respond by generating multipotent progenitors to repair the retina. However, the complete mechanisms underlying retina regeneration remain elusive. Here we report inflammation-induced mammalian target of rapamycin (mTOR) signaling in the Müller glia is essential for retina regeneration in adult zebrafish. We show after a stab injury, mTOR is rapidly activated in Müller glia and later Müller glia-derived progenitor cells (MGPCs). Importantly, mTOR is required for Müller glia dedifferentiation, as well as the proliferation of Müller glia and MGPCs. Interestingly, transient mTOR inhibition by rapamycin only reversibly suppresses MGPC proliferation, while its longer suppression by knocking down Raptor significantly inhibits the regeneration of retinal neurons. We further show mTOR promotes retina regeneration by regulating the mRNA expression of key reprogramming factors ascl1a and lin-28a, cell cycle-related genes and critical cytokines. Surprisingly, we identify microglia/macrophage-mediated inflammation as an important upstream regulator of mTOR in the Müller glia and it promotes retina regeneration through mTOR. Our study not only demonstrates the important functions of mTOR but also reveals an interesting link between inflammation and the mTOR signaling during retina regeneration.

Optic nerve regeneration in larval zebrafish exhibits spontaneous capacity for retinotopic but not tectum specific axon targeting

In contrast to mammals, retinal ganglion cells (RGC) axons of the optic nerve even in mature zebrafish exhibit a remarkable capacity for spontaneous regeneration. One constraint of using adult zebrafish is the limited ability to visualize the regeneration process in live animals. To dynamically visualize and trace the degree of target specific optic nerve regeneration, we took advantage of the optical transparency still preserved in post developmental larval zebrafish. We developed a rapid and robust assay to physically transect the larval optic nerve and find that by 96 hours post injury RGC axons have robustly regrown onto the optic tectum. We observe functional regeneration by 8 days post injury, and demonstrate that similar to adult zebrafish, optic nerve transection in larval zebrafish does not prominently induce cell death or proliferation of RGC neurons. Furthermore, we find that partial optic nerve transection results in axonal growth predominantly to the original, contralateral tectum, while complete transection results in innervation of both the correct contralateral and ‘incorrect’ ipsilateral tectum. Axonal tracing reveals that although regenerating axons innervate the ‘incorrect’ ipsilateral tectum, they successfully target their topographically appropriate synaptic areas. Combined, our results validate post developmental larval zebrafish as a powerful model for optic nerve regeneration, and reveal intricate mechanistic differences between axonal growth, midline guidance and synaptic targeting during zebrafish optic nerve regeneration.

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.

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

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

Tail regeneration and other phenomena of wound healing and tissue restoration in lizards

Wound healing is a fundamental evolutionary adaptation with two possible outcomes: scar formation or reparative regeneration. Scars participate in re-forming the barrier with the external environment and restoring homeostasis to injured tissues, but are well understood to represent dysfunctional replacements. In contrast, reparative regeneration is a tissue-specific program that near-perfectly replicates that which was lost or damaged. Although regeneration is best known from salamanders (including newts and axolotls) and zebrafish, it is unexpectedly widespread among vertebrates. For example, mice and humans can replace their digit tips, while many lizards can spontaneously regenerate almost their entire tail. Whereas the phenomenon of lizard tail regeneration has long been recognized, many details of this process remain poorly understood. All of this is beginning to change. This Review provides a comparative perspective on mechanisms of wound healing and regeneration, with a focus on lizards as an emerging model. Not only are lizards able to regrow cartilage and the spinal cord following tail loss, some species can also regenerate tissues after full-thickness skin wounds to the body, transections of the optic nerve and even lesions to parts of the brain. Current investigations are advancing our understanding of the biological requirements for successful tissue and organ repair, with obvious implications for biomedical sciences and regenerative medicine.

Translational profiling of retinal ganglion cell optic nerve regeneration in Xenopus laevis

Unlike adult mammals, adult frogs regrow their optic nerve following a crush injury, making Xenopus laevis a compelling model for studying the molecular mechanisms that underlie neuronal regeneration. Using Translational Ribosome Affinity Purification (TRAP), a method to isolate ribosome-associated mRNAs from a target cell population, we have generated a transcriptional profile by RNA-Seq for retinal ganglion cells (RGC) during the period of recovery following an optic nerve injury. Based on bioinformatic analysis using the Xenopus laevis 9.1 genome assembly, our results reveal a profound shift in the composition of ribosome-associated mRNAs during the early stages of RGC regeneration. As factors involved in cell signaling are rapidly down-regulated, those involved in protein biosynthesis are up-regulated alongside key initiators of axon development. Using the new genome assembly, we were also able to analyze gene expression profiles of homeologous gene pairs arising from a whole-genome duplication in the Xenopus lineage. Here we see evidence of divergence in regulatory control among a significant proportion of pairs. Our data should provide a valuable resource for identifying genes involved in the regeneration process to target for future functional studies, in both naturally regenerative and non-regenerative vertebrates.

Neuroinflammation as Fuel for Axonal Regeneration in the Injured Vertebrate Central Nervous System

Damage to the central nervous system (CNS) is one of the leading causes of morbidity and mortality in elderly, as repair after lesions or neurodegenerative disease usually fails because of the limited capacity of CNS regeneration. The causes underlying this limited regenerative potential are multifactorial, but one critical aspect is neuroinflammation. Although classically considered as harmful, it is now becoming increasingly clear that inflammation can also promote regeneration, if the appropriate context is provided. Here, we review the current knowledge on how acute inflammation is intertwined with axonal regeneration, an important component of CNS repair. After optic nerve or spinal cord injury, inflammatory stimulation and/or modification greatly improve the regenerative outcome in rodents. Moreover, the hypothesis of a beneficial role of inflammation is further supported by evidence from adult zebrafish, which possess the remarkable capability to repair CNS lesions and even restore functionality. Lastly, we shed light on the impact of aging processes on the regenerative capacity in the CNS of mammals and zebrafish. As aging not only affects the CNS, but also the immune system, the regeneration potential is expected to further decline in aged individuals, an element that should definitely be considered in the search for novel therapeutic strategies.

BAX to basics: How the BCL2 gene family controls the death of retinal ganglion cells

Retinal ganglion cell (RGC) death is the principal consequence of injury to the optic nerve. For several decades, we have understood that the RGC death process was executed by apoptosis, suggesting that there may be ways to therapeutically intervene in this cell death program and provide a more direct treatment to the cells and tissues affected in diseases like glaucoma. A major part of this endeavor has been to elucidate the molecular biological pathways active in RGCs from the point of axonal injury to the point of irreversible cell death. A major component of this process is the complex interaction of members of the BCL2 gene family. Three distinct family members of proteins orchestrate the most critical junction in the apoptotic program of RGCs, culminating in the activation of pro-apoptotic BAX. Once active, BAX causes irreparable damage to mitochondria, while precipitating downstream events that finish off a dying ganglion cell. This review is divided into two major parts. First, we summarize the extent of knowledge of how BCL2 gene family proteins interact to facilitate the activation and function of BAX. This area of investigation has rapidly changed over the last few years and has yielded a dramatically different mechanistic understanding of how the intrinsic apoptotic program is run in mammalian cells. Second, we provided a comprehensive analysis of nearly two decades of investigation of the role of BAX in the process of RGC death, much of which has provided many important insights into the overall pathophysiology of diseases like glaucoma.

Cell proliferation and apoptosis in optic nerve and brain integration centers of adult trout Oncorhynchus mykiss after optic nerve injury

Fishes have remarkable ability to effectively rebuild the structure of nerve cells and nerve fibers after central nervous system injury. However, the underlying mechanism is poorly understood. In order to address this issue, we investigated the proliferation and apoptosis of cells in contralateral and ipsilateral optic nerves, after stab wound injury to the eye of an adult trout Oncorhynchus mykiss. Heterogenous population of proliferating cells was investigated at 1 week after injury. TUNEL labeling gave a qualitative and quantitative assessment of apoptosis in the cells of optic nerve of trout 2 days after injury. After optic nerve injury, apoptotic response was investigated, and mass patterns of cell migration were found. The maximal concentration of apoptotic bodies was detected in the areas of mass clumps of cells. It is probably indicative of massive cell death in the area of high phagocytic activity of macrophages/microglia. At 1 week after optic nerve injury, we observed nerve cell proliferation in the trout brain integration centers: the cerebellum and the optic tectum. In the optic tectum, proliferating cell nuclear antigen (PCNA)-immunopositive radial glia-like cells were identified. Proliferative activity of nerve cells was detected in the dorsal proliferative (matrix) area of the cerebellum and in parenchymal cells of the molecular and granular layers whereas local clusters of undifferentiated cells which formed neurogenic niches were observed in both the optic tectum and cerebellum after optic nerve injury. In vitro analysis of brain cells of trout showed that suspension cells compared with monolayer cells retain higher proliferative activity, as evidenced by PCNA immunolabeling. Phase contrast observation showed mitosis in individual cells and the formation of neurospheres which gradually increased during 1-4 days of culture. The present findings suggest that trout can be used as a novel model for studying neuronal regeneration.

Matrix metalloproteinases as promising regulators of axonal regrowth in the injured adult zebrafish retinotectal system

Overcoming the failure of axon regeneration in the mammalian central nervous system (CNS) after injury remains a major challenge, which makes the search for proregenerative molecules essential. Matrix metalloproteinases (MMPs) have been implicated in axonal outgrowth during CNS development and show increased expression levels during vertebrate CNS repair. In mammals, MMPs are believed to alter the suppressive extracellular matrix to become more permissive for axon regrowth. We investigated the role of MMPs in axonal regeneration following optic nerve crush (ONC) in adult zebrafish, which fully recover from such injuries due to a high intrinsic axon growth capacity and a less inhibitory environment. Lowering general retinal MMP activity through intravitreal injections of GM6001 after ONC strongly reduced retinal ganglion cell (RGC) axonal regrowth, without influencing RGC survival. Based on a recently performed transcriptome profiling study, the expression pattern of four MMPs after ONC was determined via combined use of western blotting and immunostainings. Mmp-2 and -13a were increasingly present in RGC somata during axonal regrowth. Moreover, Mmp-2 and -9 became upregulated in regrowing RGC axons and inner plexiform layer (IPL) synapses, respectively. In contrast, after an initial rise in IPL neurites and RGC axons during the injury response, Mmp-14 expression decreased during regeneration. Altogether, a phase-dependent expression pattern for each specific MMP was observed, implicating them in axonal regrowth and inner retina remodeling after injury. In conclusion, these data suggest a novel, neuron-intrinsic function for multiple MMPs in axon regrowth that is distinct from breaking down environmental barriers. J. Comp. Neurol. 524:1472-1493, 2016. © 2015 Wiley Periodicals, Inc.

Integrated analyses of zebrafish miRNA and mRNA expression profiles identify miR-29b and miR-223 as potential regulators of optic nerve regeneration

Background

Unlike mammals, zebrafish have the ability to regenerate damaged parts of their central nervous system (CNS) and regain functionality of the affected area. A better understanding of the molecular mechanisms involved in zebrafish regeneration may therefore provide insight into how CNS repair might be induced in mammals. Although many studies have described differences in gene expression in zebrafish during CNS regeneration, the regulatory mechanisms underpinning the differential expression of these genes have not been examined.

Results

We used microarrays to analyse and integrate the mRNA and microRNA (miRNA) expression profiles of zebrafish retina after optic nerve crush to identify potential regulatory mechanisms that underpin central nerve regeneration. Bioinformatic analysis identified 3 miRNAs and 657 mRNAs that were differentially expressed after injury. We then combined inverse correlations between our miRNA expression and mRNA expression, and integrated these findings with target predictions from TargetScan Fish to identify putative miRNA-gene target pairs. We focused on two over-expressed miRNAs (miR-29b and miR-223), and functionally validated seven of their predicted gene targets using RT-qPCR and luciferase assays to confirm miRNA-mRNA binding. Gene ontology analysis placed the miRNA-regulated genes (eva1a, layna, nefmb, ina, si:ch211-51a6.2, smoc1, sb:cb252) in key biological processes that included cell survival/apoptosis, ECM-cytoskeleton signaling, and heparan sulfate proteoglycan binding,

Conclusion

Our results suggest a key role for miR-29b and miR-223 in zebrafish regeneration. The identification of miRNA regulation in a zebrafish injury model provides a framework for future studies in which to investigate not only the cellular processes required for CNS regeneration, but also how these mechanisms might be regulated to promote successful repair and return of function in the injured mammalian brain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1772-1) contains supplementary material, which is available to authorized users.

Methods for studying the zebrafish brain: past, present and future

The zebrafish (Danio rerio) is one of the most promising new model organisms. The increasing popularity of this amazing small vertebrate is evident from the exponentially growing numbers of research articles, funded projects and new discoveries associated with the use of zebrafish for studying development, brain function, human diseases and screening for new drugs. Thanks to the development of novel technologies, the range of zebrafish research is constantly expanding with new tools synergistically enhancing traditional techniques. In this review we will highlight the past and present techniques which have made, and continue to make, zebrafish an attractive model organism for various fields of biology, with a specific focus on neuroscience.

Melatonin regulates the rhythmic migration of neutrophils in live zebrafish

The circadian clock plays a vital role in physiology and behavior such as the sleep-wake cycle and blood pressure and hormone levels. Immune responses also display circadian rhythmicity and particularly pineal melatonin contributes to immunological processes. Little attention, however, is given to mechanisms underlying rhythmic neutrophil responses to the injury. Here, we used a transgenic Tg(lyz:EGFP) zebrafish tail fin transection model to investigate whether the recruitment of neutrophils toward the injured site is regulated by the circadian clock. We found that migrating neutrophils display robust rhythmicity, peaking at darkness. Melatonin positively regulates rhythmic neutrophil migration, as evidenced that treatment with melatonin at low dosage can significantly enhance neutrophil recruitment toward the injured site, which is attenuated by luzindole treatment and in pinealectomized fish. Furthermore, using a transgenic zebrafish eyeball model, we observed that melatonin enhances secretion of two cytokines, TNF-α and IL-8, both of which markedly enhance neutrophil migration. Taken together, these results highlight a positive role of melatonin in rhythmic neutrophil migration and help obtain a better understanding of circadian regulation in immunology.

Decreased thyroid hormone signaling accelerates the reinnervation of the optic tectum following optic nerve crush in adult zebrafish

The regenerative capacity of the adult mammalian central nervous system (CNS) is poor and finding ways to stimulate long distance axonal regeneration in humans remains a challenge for neuroscientists. Thyroid hormones, well known for their key function in CNS development and maturation, more recently also emerged as molecules influencing regeneration. While several studies investigated their influence on peripheral nerve regeneration, in vivo studies on their role in adult CNS regeneration remain scarce. We therefore investigated the effect of lowering T3 signaling on the regeneration of the optic nerve (ON) following crush in zebrafish, a species where full recovery occurs spontaneously. Adult zebrafish were exposed to iopanoic acid (IOP), which lowered intracellular 3,5,3'-triiodothyronine (T3) availability, or to the thyroid hormone receptor β antagonist methylsulfonylnitrobenzoate (C1). Both treatments accelerated optic tectum (OT) reinnervation. At 7days post injury (7dpi) there was a clear increase in the biocytin labeled area in the OT following anterograde tracing as well as an increased immunostaining of Gap43, a protein expressed in outgrowing axons. This effect was attenuated by T3 supplementation to IOP-treated fish. ON crush induced very limited cell death and proliferation at the level of the retina in control, IOP- and C1-treated fish. The treatments also had no effect on the mRNA upregulation of the regeneration markers gap43, tub1a, and socs3b at the level of the retina at 4 and 7dpi. We did, however, find a correlation between the accelerated OT reinnervation and a more rapid resolution of microglia/macrophages in the ON and the OT of IOP-treated fish. Taken together these data indicate that lowering T3 signaling accelerates OT reinnervation following ON crush in zebrafish and that this is accompanied by a more rapid resolution of the inflammatory response.

Using zebrafish as the model organism to understand organ regeneration

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

Characterization of optic nerve regeneration using transgenic zebrafish

In contrast to the adult mammalian central nervous system (CNS), fish are able to functionally regenerate severed axons upon injury. Although the zebrafish is a well-established model vertebrate for genetic and developmental studies, its use for anatomical studies of axon regeneration has been hampered by the paucity of appropriate tools to visualize re-growing axons in the adult CNS. On this account, we used transgenic zebrafish that express enhanced green fluorescent protein (GFP) under the control of a GAP-43 promoter. In adult, naïve retinae, GFP was restricted to young retinal ganglion cells (RGCs) and their axons. Within the optic nerve, these fluorescent axons congregated in a distinct strand at the nerve periphery, indicating age-related order. Upon optic nerve crush, GFP expression was markedly induced in RGC somata and intra-retinal axons at 4 to at least 14 days post injury. Moreover, individual axons were visualized in their natural environment of the optic nerve using wholemount tissue clearing and confocal microscopy. With this novel approach, regenerating axons were clearly detectable beyond the injury site as early as 2 days after injury and grew past the optic chiasm by 4 days. Regenerating axons in the entire optic nerve were labeled from 6 to at least 14 days after injury, thereby allowing detailed visualization of the complete regeneration process. Therefore, this new approach could now be used in combination with expression knockdown or pharmacological manipulations to analyze the relevance of specific proteins and signaling cascades for axonal regeneration in vivo. In addition, the RGC-specific GFP expression facilitated accurate evaluation of neurite growth in dissociated retinal cultures. This fast in vitro assay now enables the screening of compound and expression libraries. Overall, the presented methodologies provide exciting possibilities to investigate the molecular mechanisms underlying successful CNS regeneration in zebrafish.

MASH1/Ascl1a leads to GAP43 expression and axon regeneration in the adult CNS

Unlike CNS neurons in adult mammals, neurons in fish and embryonic mammals can regenerate their axons after injury. These divergent regenerative responses are in part mediated by the growth-associated expression of select transcription factors. The basic helix-loop-helix (bHLH) transcription factor, MASH1/Ascl1a, is transiently expressed during the development of many neuronal subtypes and regulates the expression of genes that mediate cell fate determination and differentiation. In the adult zebrafish (Danio rerio), Ascl1a is also transiently expressed in retinal ganglion cells (RGCs) that regenerate axons after optic nerve crush. Utilizing transgenic zebrafish with a 3.6 kb GAP43 promoter that drives expression of an enhanced green fluorescent protein (EGFP), we observed that knock-down of Ascl1a expression reduces both regenerative gap43 gene expression and axonal growth after injury compared to controls. In mammals, the development of noradrenergic brainstem neurons requires MASH1 expression. In contrast to zebrafish RGCs, however, MASH1 is not expressed in the mammalian brainstem after spinal cord injury (SCI). Therefore, we utilized adeno-associated viral (AAV) vectors to overexpress MASH1 in four month old rat (Rattus norvegicus) brainstem neurons in an attempt to promote axon regeneration after SCI. We discovered that after complete transection of the thoracic spinal cord and implantation of a Schwann cell bridge, animals that express MASH1 exhibit increased noradrenergic axon regeneration and improvement in hindlimb joint movements compared to controls. Together these data demonstrate that MASH1/Ascl1a is a fundamental regulator of axonal growth across vertebrates and can induce modifications to the intrinsic state of neurons to promote functional regeneration in response to CNS injury.