Readiness of zebrafish brain neurons to regenerate a spinal axon correlates with differential expression of specific cell recognition molecules

Thymosin β4 promotes zebrafish Mauthner axon regeneration by facilitating actin polymerization through binding to G-actin

BACKGROUND

Thymosin beta 4 (Tβ4) is a monomeric actin-binding protein that plays many roles in biological activities. However, some studies on the role of Tβ4 in central axon regeneration have yielded contradictory results. Previous research has focused primarily on cultured cells, leading to a deficiency in in vivo experimental evidence. Therefore, we used a single axon injury model of Mauthner cells in zebrafish larvae to investigate the role of Tβ4 in central axon regeneration in vivo.

RESULTS

Our results demonstrated that knockout of Tβ4 impaired axon regeneration, whereas overexpression of Tβ4 promoted axon regeneration. Moreover, this promotion is mediated through the interaction between Tβ4 and G-actin. Furthermore, our results suggest that the binding of Tβ4 to G-actin promotes actin polymerization rather than depolymerization. In the rapid escape behavior test, larvae with damaged axons presented impaired tail muscle control, resulting in a lack of normal tail bending, termed the straight tail phenomenon. The proportion of straight tails was significantly negatively correlated with axon regeneration length, suggesting that it is a new indicator for assessing rapid escape behavior recovery. Finally, the results showed that the overexpression of Tβ4 effectively restored the functionality of rapid escape behaviors mediated by Mauthner cells.

CONCLUSIONS

Our results provide evidence that Tβ4 promotes central axon regeneration in vivo through binding to G-actin and suggest that Tβ4 could serve as a potential polypeptide drug for clinical therapy.

C9ORF72 Deficiency Results in Neurodegeneration in the Zebrafish Retina

Hexanucleotide repeat expansions within the gene C9ORF72 are the most common cause of the neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). This disease-causing expansion leads to a reduction in C9ORF72 expression levels in patients, suggesting loss of C9ORF72 function could contribute to disease. To further understand the consequences of C9ORF72 deficiency in vivo, we generated a c9orf72 mutant zebrafish line. Analysis of the adult female spinal cords revealed no appreciable neurodegenerative pathology such as loss of motor neurons or increased levels of neuroinflammation. However, detailed examination of adult female c9orf72-/- retinas showed prominent neurodegenerative features, including a decrease in retinal thickness, gliosis, and an overall reduction in neurons of all subtypes. Analysis of rod and cone cells within the photoreceptor layer showed a disturbance in their outer segment structure and rhodopsin mislocalization from rod outer segments to their cell bodies and synaptic terminals. Thus, C9ORF72 may play a previously unappreciated role in retinal homeostasis and suggests C9ORF72 deficiency can induce tissue specific neuronal loss.

Myostatin is a negative regulator of adult neurogenesis after spinal cord injury in zebrafish

Intrinsic and extrinsic inhibition of neuronal regeneration obstruct spinal cord (SC) repair in mammals. In contrast, adult zebrafish achieve functional recovery after complete SC transection. While studies of innate SC regeneration have focused on axon regrowth as a primary repair mechanism, how local adult neurogenesis affects functional recovery is unknown. Here, we uncover dynamic expression of zebrafish myostatin b (mstnb) in a niche of dorsal SC progenitors after injury. mstnb mutants show impaired functional recovery, normal glial and axonal bridging across the lesion, and an increase in the profiles of newborn neurons. Molecularly, neuron differentiation genes are upregulated, while the neural stem cell maintenance gene fgf1b is downregulated in mstnb mutants. Finally, we show that human fibroblast growth factor 1 (FGF1) treatment rescues the molecular and cellular phenotypes of mstnb mutants. These studies uncover unanticipated neurogenic functions for mstnb and establish the importance of local adult neurogenesis for innate SC repair.

Intrinsic heterogeneity in axon regeneration

The nervous system is composed of a variety of neurons and glial cells with different morphology and functions. In the mammalian peripheral nervous system (PNS) or the lower vertebrate central nervous system (CNS), most neurons can regenerate extensively after axotomy, while the neurons in the mammalian CNS possess only limited regenerative ability. This heterogeneity is common within and across species. The studies about the transcriptomes after nerve injury in different animal models have revealed a series of molecular and cellular events that occurred in neurons after axotomy. However, responses of various types of neurons located in different positions of individuals were different remarkably. Thus, researchers aim to find the key factors that are conducive to regeneration, so as to provide the molecular basis for solving the regeneration difficulties after CNS injury. Here we review the heterogeneity of axonal regeneration among different cell subtypes in different animal models or the same organ, emphasizing the importance of comparative studies within and across species.

Mitochondrial function in spinal cord injury and regeneration

Many people around the world suffer from some form of paralysis caused by spinal cord injury (SCI), which has an impact on quality and life expectancy. The spinal cord is part of the central nervous system (CNS), which in mammals is unable to regenerate, and to date, there is a lack of full functional recovery therapies for SCI. These injuries start with a rapid and mechanical insult, followed by a secondary phase leading progressively to greater damage. This secondary phase can be potentially modifiable through targeted therapies. The growing literature, derived from mammalian and regenerative model studies, supports a leading role for mitochondria in every cellular response after SCI: mitochondrial dysfunction is the common event of different triggers leading to cell death, cellular metabolism regulates the immune response, mitochondrial number and localization correlate with axon regenerative capacity, while mitochondrial abundance and substrate utilization regulate neural stem progenitor cells self-renewal and differentiation. Herein, we present a comprehensive review of the cellular responses during the secondary phase of SCI, the mitochondrial contribution to each of them, as well as evidence of mitochondrial involvement in spinal cord regeneration, suggesting that a more in-depth study of mitochondrial function and regulation is needed to identify potential targets for SCI therapeutic intervention.

Survival and Axonal Outgrowth of the Mauthner Cell Following Spinal Cord Crush Does Not Drive Post-injury Startle Responses

A pair of Mauthner cells (M-cells) can be found in the hindbrain of most teleost fish, as well as amphibians and lamprey. The axons of these reticulospinal neurons cross the midline and synapse on interneurons and motoneurons as they descend the length of the spinal cord. The M-cell initiates fast C-type startle responses (fast C-starts) in goldfish and zebrafish triggered by abrupt acoustic/vibratory stimuli. Starting about 70 days after whole spinal cord crush, less robust startle responses with longer latencies manifest in adult goldfish, Carassius auratus. The morphological and electrophysiological identifiability of the M-cell provides a unique opportunity to study cellular responses to spinal cord injury and the relation of axonal regrowth to a defined behavior. After spinal cord crush at the spinomedullary junction about one-third of the damaged M-axons of adult goldfish send at least one sprout past the wound site between 56 and 85 days postoperatively. These caudally projecting sprouts follow a more lateral trajectory relative to their position in the fasciculus longitudinalis medialis of control fish. Other sprouts, some from the same axon, follow aberrant pathways that include rostral projections, reversal of direction, midline crossings, neuromas, and projection out the first ventral root. Stimulating M-axons in goldfish that had post-injury startle behavior between 198 and 468 days postoperatively resulted in no or minimal EMG activity in trunk and tail musculature as compared to control fish. Although M-cells can survive for at least 468 day (∼1.3 years) after spinal cord crush, maintain regrowth, and elicit putative trunk EMG responses, the cell does not appear to play a substantive role in the emergence of acoustic/vibratory-triggered responses. We speculate that aberrant pathway choice of this neuron may limit its role in the recovery of behavior and discuss structural and functional properties of alternative candidate neurons that may render them more supportive of post-injury startle behavior.

Cell transplantation and secretome based approaches in spinal cord injury regenerative medicine

The axonal growth-restrictive character of traumatic spinal cord injury (SCI) makes finding a therapeutic strategy a very demanding task, due to the postinjury events impeditive to spontaneous axonal outgrowth and regeneration. Considering SCI pathophysiology complexity, it has been suggested that an effective therapy should tackle all the SCI-related aspects and provide sensory and motor improvement to SCI patients. Thus, the current aim of any therapeutic approach for SCI relies in providing neuroprotection and support neuroregeneration. Acknowledging the current SCI treatment paradigm, cell transplantation is one of the most explored approaches for SCI with mesenchymal stem cells (MSCs) being in the forefront of many of these. Studies showing the beneficial effects of MSC transplantation after SCI have been proposing a paracrine action of these cells on the injured tissues, through the secretion of protective and trophic factors, rather than attributing it to the action of cells itself. This manuscript provides detailed information on the most recent data regarding the neuroregenerative effect of the secretome of MSCs as a cell-free based therapy for SCI. The main challenge of any strategy proposed for SCI treatment relies in obtaining robust preclinical evidence from in vitro and in vivo models, before moving to the clinics, so we have specifically focused on the available vertebrate and mammal models of SCI currently used in research and how can SCI field benefit from them.

Know How to Regrow-Axon Regeneration in the Zebrafish Spinal Cord

The capacity for long-distance axon regeneration and functional recovery after spinal cord injury is poor in mammals but remarkable in some vertebrates, including fish and salamanders. The cellular and molecular basis of this interspecies difference is beginning to emerge. This includes the identification of target cells that react to the injury and the cues directing their pro-regenerative responses. Among existing models of successful spinal cord regeneration, the zebrafish is arguably the most understood at a mechanistic level to date. Here, we review the spinal cord injury paradigms used in zebrafish, and summarize the breadth of neuron-intrinsic and -extrinsic factors that have been identified to play pivotal roles in the ability of zebrafish to regenerate central nervous system axons and recover function.

Insulin-Like Growth Factor-1 Enhances Motoneuron Survival and Inhibits Neuroinflammation After Spinal Cord Transection in Zebrafish

Insulin-like growth factor-1 (IGF-1) is a neurotrophic factor produced locally in the central nervous system which can promote axonal regeneration, protect motoneurons, and inhibit neuroinflammation. In this study, we used the zebrafish spinal transection model to investigate whether IGF-1 plays an important role in the recovery of motor function. Unlike mammals, zebrafish can regenerate axons and restore mobility in remarkably short period after spinal cord transection. Quantitative real-time PCR and immunofluorescence showed decreased IGF-1 expression in the lesion site. Double immunostaining for IGF-1 and Islet-1 (motoneuron marker)/GFAP (astrocyte marker)/Iba-1 (microglia marker) showed that IGF-1 was mainly expressed in motoneurons and was surrounded by astrocyte and microglia. Following administration of IGF-1 morpholino at the lesion site of spinal-transected zebrafish, swimming test showed retarded recovery of mobility, the number of motoneurons was reduced, and increased immunofluorescence density of microglia was caused. Our data suggested that IGF-1 enhances motoneuron survival and inhibits neuroinflammation after spinal cord transection in zebrafish, which suggested that IGF-1 might be involved in the motor recovery.

Activating Transcription Factor 6 Contributes to Functional Recovery After Spinal Cord Injury in Adult Zebrafish

Spinal cord injury (SCI) is one of the most common devastating injuries, with little possibility of recovery in humans. However, zebrafish efficiently regenerate functional nervous system tissue after SCI. Therefore, the spinal cord transection model of adult zebrafish was applied to explore the role of ATF6 in neuro-recovery. Activating transcription factor 6 (ATF6) is a type-II transmembrane protein in the endoplasmic reticulum (ER). ATF6 target genes could improve ER homeostasis, which contributes to cytoprotection. Herein, we found that the ATF6 level increased at 12 h and 3 days post SCI, and returned to sham levels at 7 days post SCI. ATF6-expressing motor neurons were present in the central canal of the spinal cord and increased at 12 h post SCI. ATF6 morpholino treatment showed that inhibition of ATF6 delayed locomotor recovery and hindered neuron axon regrowth in SCI zebrafish. Furthermore, we investigated the role of both binding immunoglobulin protein (Bip) and C/EBP homologous transcription factor protein (CHOP), the two target genes of ATF6. We found that Bip expression significantly increased in the spinal cord at 7 days after SCI, which served as a pro-survival chaperone. Our results also showed that CHOP expression significantly decreased in the spinal cord at 7 days after SCI, which was identified as a protein involved in apoptosis. Taken together, our data demonstrate that ATF6 may contribute to the functional recovery after SCI in adult zebrafish, via up-regulation of Bip and down-regulation of CHOP to restore the homeostasis of ER.

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.

Heterogeneity in the regenerative abilities of central nervous system axons within species: why do some neurons regenerate better than others?

Some neurons, especially in mammalian peripheral nervous system or in lower vertebrate or in vertebrate central nervous system (CNS) regenerate after axotomy, while most mammalian CNS neurons fail to regenerate. There is an emerging consensus that neurons have different intrinsic regenerative capabilities, which theoretically could be manipulated therapeutically to improve regeneration. Population-based comparisons between “good regenerating” and “bad regenerating” neurons in the CNS and peripheral nervous system of most vertebrates yield results that are inconclusive or difficult to interpret. At least in part, this reflects the great diversity of cells in the mammalian CNS. Using mammalian nervous system imposes several methodical limitations. First, the small sizes and large numbers of neurons in the CNS make it very difficult to distinguish regenerating neurons from non-regenerating ones. Second, the lack of identifiable neurons makes it impossible to correlate biochemical changes in a neuron with axonal damage of the same neuron, and therefore, to dissect the molecular mechanisms of regeneration on the level of single neurons. This review will survey the reported responses to axon injury and the determinants of axon regeneration, emphasizing non-mammalian model organisms, which are often under-utilized, but in which the data are especially easy to interpret.

Bioinformatic identification of key candidate genes and pathways in axon regeneration after spinal cord injury in zebrafish

Zebrafish and human genomes are highly homologous; however, despite this genomic similarity, adult zebrafish can achieve neuronal proliferation, regeneration and functional restoration within 6–8 weeks after spinal cord injury, whereas humans cannot. To analyze differentially expressed zebrafish genes between axon-regenerated neurons and axon-non-regenerated neurons after spinal cord injury, and to explore the key genes and pathways of axonal regeneration after spinal cord injury, microarray GSE56842 was analyzed using the online tool, GEO2R, in the Gene Expression Omnibus database. Gene ontology and protein-protein interaction networks were used to analyze the identified differentially expressed genes. Finally, we screened for genes and pathways that may play a role in spinal cord injury repair in zebrafish and mammals. A total of 636 differentially expressed genes were obtained, including 255 up-regulated and 381 down-regulated differentially expressed genes in axon-regenerated neurons. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment results were also obtained. A protein-protein interaction network contained 480 node genes and 1976 node connections. We also obtained the 10 hub genes with the highest correlation and the two modules with the highest score. The results showed that spectrin may promote axonal regeneration after spinal cord injury in zebrafish. Transforming growth factor beta signaling may inhibit repair after spinal cord injury in zebrafish. Focal adhesion or tight junctions may play an important role in the migration and proliferation of some cells, such as Schwann cells or neural progenitor cells, after spinal cord injury in zebrafish. Bioinformatic analysis identified key candidate genes and pathways in axonal regeneration after spinal cord injury in zebrafish, providing targets for treatment of spinal cord injury in mammals.

Invasion of microglia/macrophages and granulocytes into the Mauthner axon myelin sheath following spinal cord injury of the adult goldfish, Carassius auratus

Rapid activation of resident glia occurs after spinal cord injury. Somewhat later, innate and adaptive immune responses occur with the invasion of peripheral immune cells into the wound site. The activation of resident and peripheral immune cells has been postulated to play harmful as well as beneficial roles in the regenerative process. Mauthner cells, large identifiable neurons located in the hindbrain of most fish and amphibians, provided the opportunity to study the morphological relationship between reactive cells and Mauthner axons (M-axons) severed by spinal cord crush or by selective axotomy. After crossing in the hindbrain, the M-axons of adult goldfish, Carassius auratus, extend the length of the spinal cord. Following injury, the M-axon undergoes retrograde degeneration within its myelin sheath creating an axon-free zone (proximal dieback zone). Reactive cells invade the wound site, enter the axon-free dieback zone and are observed in the vicinity of the retracted M-axon tip as early as 3 hr postinjury. Transmission electron microscopy allowed the detection of microglia/macrophages and granulocytes, some of which appear to be neutrophil-like, at each of these locations. We believe that this is the first report of the invasion of such cells within the myelin sheath of an identifiable axon in the vertebrate central nervous system (CNS). We speculate that microglia/macrophages and granulocytes that are attracted within a few hours to the damaged M-axon are part of an inflammatory response that allows phagocytosis of debris and plays a role in the regenerative process. Our results provide the baseline from which to utilize immunohistochemical and genetic approaches to elucidate the role of non-neuronal cells in the regenerative process of a single axon in the vertebrate CNS.

The role of the immune system during regeneration of the central nervous system

Central nervous system damage in mammals leads to neuronal cell death, axonal degeneration, and formation of a glial scar resulting in functional and behavioral defects. Other vertebrates, like fish and salamanders, have retained the ability to functionally regenerate after central nervous system injury. To date research from many research organisms has led to a more concise understanding of the response of local neural cells to injury. However, it has become clear that non-neural cells of the immune system play an important role in determining the tissue response to injury. In this review we briefly consider the mammalian response to injury compared to organisms with the natural ability to regenerate. We then discuss similarities and differences in how cells of the innate and adaptive immune system respond and contribute to tissue repair in various species.

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.

Comprehensive therapeutics targeting the corticospinal tract following spinal cord injury

Spinal cord injury (SCI), which is much in the public eye, is still a refractory disease compromising the well-being of both patients and society. In spite of there being many methods dealing with the lesion, there is still a deficiency in comprehensive strategies covering all facets of this damage. Further, we should also mention the structure called the corticospinal tract (CST) which plays a crucial role in the motor responses of organisms, and it will be the focal point of our attention. In this review, we discuss a variety of strategies targeting different dimensions following SCI and some treatments that are especially efficacious to the CST are emphasized. Over recent decades, researchers have developed many effective tactics involving five approaches: (1) tackle more extensive regions; (2) provide a regenerative microenvironment; (3) provide a glial microenvironment; (4) transplantation; and (5) other auxiliary methods, for instance, rehabilitation training and electrical stimulation. We review the basic knowledge on this disease and correlative treatments. In addition, some well-formulated perspectives and hypotheses have been delineated. We emphasize that such a multifaceted problem needs combinatorial approaches, and we analyze some discrepancies in past studies. Finally, for the future, we present numerous brand-new latent tactics which have great promise for curbing SCI.

AP-1cFos/JunB/miR-200a regulate the pro-regenerative glial cell response during axolotl spinal cord regeneration

Salamanders have the remarkable ability to functionally regenerate after spinal cord transection. In response to injury, GFAP⁺ glial cells in the axolotl spinal cord proliferate and migrate to replace the missing neural tube and create a permissive environment for axon regeneration. Molecular pathways that regulate the pro-regenerative axolotl glial cell response are poorly understood. Here we show axolotl glial cells up-regulate AP-1^cFos/JunB after injury, which promotes a pro-regenerative glial cell response. Injury induced upregulation of miR-200a in glial cells supresses c-Jun expression in these cells. Inhibition of miR-200a during regeneration causes defects in axonal regrowth and transcriptomic analysis revealed that miR-200a inhibition leads to differential regulation of genes involved with reactive gliosis, the glial scar, extracellular matrix remodeling and axon guidance. This work identifies a unique role for miR-200a in inhibiting reactive gliosis in axolotl glial cells during spinal cord regeneration.

Keith Sabin et al. showed that upregulation of the AP-1 complex, composed of c-Fos and JunB, in the axolotl spinal cord promotes a pro-regenerative glial cell response. This response is impaired by inhibition of miR-200a; suggesting an important role for this microRNA in axolotl spinal cord regeneration.

Analysis of the neurotoxic effects of neuropathic organophosphorus compounds in adult zebrafish

Inhibition and aging of neuropathy target esterase (NTE) by exposure to neuropathic organophosphorus compounds (OPs) can result in OP-induced delayed neuropathy (OPIDN). In the present study we aimed to build a model of OPIDN in adult zebrafish. First, inhibition and aging of zebrafish NTE activity were characterized in the brain by using the prototypic neuropathic compounds cresyl saligenin phosphate (CBDP) and diisopropylphosphorofluoridate (DFP). Our results show that, as in other animal models, zebrafish NTE is inhibited and aged by both neuropathic OPs. Then, a neuropathic concentration inhibiting NTE activity by at least 70% for at least 24 h was selected for each compound to analyze changes in phosphatidylcholines (PCs), lysophosphatidylcholines (LPCs) and glycerolphosphocholine (GPC) profiles. In spite to the strong inhibition of the NTE activity found for both compounds, only a mild increase in the LPCs level was found after 48 h of the exposure to DFP, and no effect were observed by CBDP. Moreover, histopathological evaluation and motor function outcome analyses failed to find any neurological abnormalities in the exposed fish. Thus, our results strongly suggest that zebrafish is not a suitable species for the development of an experimental model of human OPIDN.

Axonal regeneration in zebrafish spinal cord

In the present review we discuss two interrelated events-axonal damage and repair-known to occur after spinal cord injury (SCI) in the zebrafish. Adult zebrafish are capable of regenerating axonal tracts and can restore full functionality after SCI. Unlike fish, axon regeneration in the adult mammalian central nervous system is extremely limited. As a consequence of an injury there is very little repair of disengaged axons and therefore functional deficit persists after SCI in adult mammals. In contrast, peripheral nervous system axons readily regenerate following injury and hence allow functional recovery both in mammals and fish. A better mechanistic understanding of these three scenarios could provide a more comprehensive insight into the success or failure of axonal regeneration after SCI. This review summarizes the present understanding of the cellular and molecular basis of axonal regeneration, in both the peripheral nervous system and the central nervous system, and large scale gene expression analysis is used to focus on different events during regeneration. The discovery and identification of genes involved in zebrafish spinal cord regeneration and subsequent functional experimentation will provide more insight into the endogenous mechanism of myelination and remyelination. Furthermore, precise knowledge of the mechanism underlying the extraordinary axonal regeneration process in zebrafish will also allow us to unravel the potential therapeutic strategies to be implemented for enhancing regrowth and remyelination of axons in mammals.

A Small Organic Compound Mimicking the L1 Cell Adhesion Molecule Promotes Functional Recovery after Spinal Cord Injury in Zebrafish

Tacrine is a small organic compound that was discovered to mimic the functions of the neural cell adhesion molecule L1 by promoting the cognate functions of L1 in vitro, such as neuronal survival, neuronal migration, neurite outgrowth, and myelination. Based on studies indicating that L1 enhances functional recovery in different central and peripheral nervous system disease paradigms of rodents, it deemed interesting to investigate the beneficial role of tacrine in the attractive zebrafish animal model, by evaluating functional recovery after spinal cord injury. To this aim, larval and adult zebrafish were exposed to tacrine treatment after spinal cord injury and monitored for locomotor recovery and axonal regrowth. Tacrine promoted the rapid recovery of locomotor activities in both larval and adult zebrafish, enhanced regrowth of severed axons and myelination, and reduced astrogliosis in the spinal cords. Tacrine treatment upregulated the expression of L1.1 (a homolog of the mammalian recognition molecule L1) and enhanced the L1.1-mediated intracellular signaling cascades in the injured spinal cords. These observations lead to the hope that, in combination with other therapeutic approaches, this old drug may become a useful reagent to ameliorate the deficits resulting from acute and chronic injuries of the mammalian nervous system.

Timing of neuronal plasticity in development and aging

Molecular oscillators are well known for their roles in temporal control of some biological processes like cell proliferation, but molecular mechanisms that provide temporal control of differentiation and postdifferentiation events in cells are less understood. In the nervous system, establishment of neuronal connectivity during development and decline in neuronal plasticity during aging are regulated with temporal precision, but the timing mechanisms are largely unknown. Caenorhabditis elegans has been a preferred model for aging research and recently emerges as a new model for the study of developmental and postdevelopmental plasticity in neurons. In this review we discuss the emerging mechanisms in timing of developmental lineage progression, axon growth and pathfinding, synapse formation, and reorganization, and neuronal plasticity in development and aging. We also provide a current view on the conserved core axon regeneration molecules with the intention to point out potential regulatory points of temporal controls. We highlight recent progress in understanding timing mechanisms that regulate decline in regenerative capacity, including progressive changes of intrinsic timers and co-opting the aging pathway molecules. WIREs Dev Biol 2018, 7:e305. doi: 10.1002/wdev.305 This article is categorized under: Invertebrate Organogenesis > Worms Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Nervous System Development > Worms Gene Expression and Transcriptional Hierarchies > Regulatory RNA.

In vivo imaging of Mauthner axon regeneration, remyelination and synapses re-establishment after laser axotomy in zebrafish larvae

Zebrafish is an excellent model to study central nervous system (CNS) axonal degeneration and regeneration since we can observe these processes in vivo and in real time in transparent larvae. Previous studies have shown that Mauthner cell (M-cell) axon regenerates poorly after mechanical spinal cord injury. Inconsistent with this result, however, we have found that M-cell possesses a great capacity for axon regeneration after two-photon laser ablation. By using ZEISS LSM 710 two-photon microscope, we performed specific unilateral axotomy of GFP labeled M-cells in the Tol-056 enhancer trap line larvae. Our results showed that distal axons almost degenerated completely at 24h after laser axotomy. After that, the proximal axons initiated a robust regeneration and many of the M-cell axons almost regenerated fully at 4days post axotomy. Furthermore, we also visualized that regenerated axons were remyelinated when we severed fluorescent dye labeled M-cells in the Tg (mbp:EGFP-CAAX) line larvae. Moreover, by single M-cell co-electroporation with Syp:EGFP and DsRed2 plasmids we observed synapses re-establishment in vivo during laser injury-induced axon re-extension which suggested re-innervation of denervated pathways. In addition, we further demonstrated that nocodazole administration could completely abolish this regeneration capacity. These results together suggested that in vivo time-lapse imaging of M-cell axon laser injury may provide a powerful analytical model for understanding the underlying cellular and molecular mechanisms of the CNS axon regeneration.

The Adhesion Molecule-Characteristic HNK-1 Carbohydrate Contributes to Functional Recovery After Spinal Cord Injury in Adult Zebrafish

The human natural killer cell antigen-1 (HNK-1) is functionally important in development, synaptic activity, and regeneration after injury in the nervous system of several mammalian species. It contains a sulfated glucuronic acid which is carried by neural adhesion molecules and expressed in nonmammalian species, including zebrafish, which, as opposed to mammals, spontaneously regenerate after injury in the adult. To evaluate HNK-1's role in recovery of function after spinal cord injury (SCI) of adult zebrafish, we assessed the effects of the two HNK-1 synthesizing enzymes, glucuronyl transferase and HNK-1 sulfotransferase. Expression of these two enzymes was increased at the messenger RNA (mRNA) level 11 days after injury in the brainstem nuclei that are capable of regrowth of severed axons, namely, the nucleus of medial longitudinal fascicle and intermediate reticular formation, but not at earlier time points after SCI. mRNA levels of glucuronyl transferase and sulfotransferase were increased in neurons, not only of these nuclei but also in the spinal cord caudal to the injury site at 11 days. Mauthner neurons which are not capable of regeneration did not show increased levels of enzyme mRNAs after injury. Reducing protein levels of the enzymes by application of anti-sense morpholinos resulted in reduction of locomotor recovery for glucuronyl transferase, but not for HNK-1 sulfotransferase. The combined results indicate that HNK-1 is upregulated in expression only in those neurons that are intrinsically capable of regeneration and contributes to regeneration after spinal cord injury in adult zebrafish in the absence of its sulfate moiety.

Intradiencephalon injection of histamine inhibited the recovery of locomotor function of spinal cord injured zebrafish

Human spinal cord injury (SCI) usually causes irreversible disability beneath the injured site due to poor neural regeneration. On the contrary, zebrafish show significant regenerative ability after SCI, thus is usually worked as an animal model for studying neuroregeneration. Most of the previous SCI studies focused on the local site of SCI, the supraspinal-derived signals were rarely mentioned. Here we showed that intradiencephalon injection of histamine (HA) inhibited the locomotor recovery in adult zebrafish post-SCI. Immunofluorescence results showed that intradiencephalon HA administration increased the activated microglia 3 days post injury (dpi), promoted the proliferation of radial glial cells at 7 dpi and affected the morphology of radial glial cells at 11 dpi. Furthermore, quantitative real-time polymerase chain reaction (qRT-PCR) results showed that intradiencephalon HA administration also reduced the expression of neurotrophic factors including brain-derived neurotrophic factor (BDNF) and insulin-like growth factor1 (IGF-1) at the lesion site, however, had no effect on the expression of pro-inflammatory factors such as TNF-alpha and IL-1 beta. Hence, our data suggested that exogenous intradiencephalon HA retarded locomotor recovery in spinal cord injured zebrafish via modulating the repair microenvironment.

Spinal cord regeneration in Xenopus laevis

Here we present a protocol for the husbandry of Xenopus laevis tadpoles and froglets, and procedures to study spinal cord regeneration. This includes methods to induce spinal cord injury (SCI); DNA and morpholino electroporation for genetic studies; in vivo imaging for cell analysis; a swimming test to measure functional recovery; and a convenient model for screening for new compounds that promote neural regeneration. These protocols establish X. laevis as a unique model organism for understanding spinal cord regeneration by comparing regenerative and nonregenerative stages. This protocol can be used to understand the molecular and cellular mechanisms involved in nervous system regeneration, including neural stem and progenitor cell (NSPC) proliferation and neurogenesis, extrinsic and intrinsic mechanisms involved in axon regeneration, glial response and scar formation, and trophic factors. For experienced personnel, husbandry takes 1-2 months; SCI can be achieved in 5-15 min; and swimming recovery takes 20-30 d.

L1.2, the zebrafish paralog of L1.1 and ortholog of the mammalian cell adhesion molecule L1 contributes to spinal cord regeneration in adult zebrafish

PURPOSE

The aim of the study was to investigate the functional role of L1.2, the zebrafish paralog of L1.1 and ortholog of mammalian L1CAM in adult zebrafish spinal cord regeneration after injury. L1CAM and L1.1 have shown beneficial features in ameliorating nervous system dysfunctions in different experimental paradigms. It thus deemed important to characterize the L1.2 member of the L1CAM family, the functions of which are unknown.

METHODS

Spinal cord transection of adult zebrafish, application of anti-sense morpholino to reduce L1.2 expression, qPCR, immunohistology, immunoblotting, in situ hybridization, retrograde tracing, anterograde tracing.

RESULTS

Similar to L1.1, L1.2 expression in adult zebrafish is upregulated after spinal cord transection. By co-localization of in situ hybridization and immunohistology, L1.2 is expressed in neurons and, in contrast to L1.1, it is also expressed in GFAP-immunoreactive glia. Reducing L1.2 protein levels leads to impaired locomotor recovery and reduction of regrowth of severed descending axons from a brain stem nucleus which is composed of neurons innately capable of axonal regrowth.

CONCLUSIONS

Our findings support the speculation that paralogs of duplicated genes can exert similar functions and may thus represent an advantage over other species that do not carry duplicated genes.

Beneficial roles of melanoma cell adhesion molecule in spinal cord transection recovery in adult zebrafish

Melanoma cell adhesion molecule (MCAM) is a multifunctional protein involved in miscellaneous processes, including development and tumor angiogenesis. Here, spinal cord transection in adult zebrafish was used to investigate the effects of MCAM on spinal cord injury (SCI) and subsequent recovery. Expression of MCAM mRNA increased and co-localized with motoneurons in the spinal cord after SCI. With MCAM morpholino treatment, inhibition of MCAM retarded both axon regrowth and locomotor recovery in the spinal cord injured zebrafish. Furthermore, MCAM mRNA expression was also observed in fli1a:EGFP transgenic zebrafish, which specifically show labeled blood vessels. Inhibition of MCAM down-regulated the expression of angiogenesis-related factors, such as VEGFR-2, p-p38 and p-AKT, and the inflammatory factors TNF-α, IL-1β and IL-8. Taken together, these data suggest that MCAM may have a beneficial role in the recovery from SCI, via the promotion of neurogenesis and angiogenesis. In the context of adult zebrafish spinal cord injury, we proved that Melanoma cell adhesion molecule (MCAM) is beneficial to the recovery, possibly via mechanisms of angiogenensis and inflammation. MCAM promotes angiogenesis by adjusting VEGFR-2, p-p38 and p-AKT. MCAM affects inflammatory factors such as TNF-α, IL-1β and IL-8. Our results extend the beneficial role of MCAM in the regeneration of central nervous system.

Neural stem/progenitor cells in Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease and a worldwide health challenge. Different therapeutic approaches are being developed to reverse or slow the loss of affected neurons. Another plausible therapeutic way that may complement the studies is to increase the survival of existing neurons by mobilizing the existing neural stem/progenitor cells (NSPCs) - i.e. "induce their plasticity" - to regenerate lost neurons despite the existing pathology and unfavorable environment. However, there is controversy about how NSPCs are affected by the unfavorable toxic environment during AD. In this review, we will discuss the use of stem cells in neurodegenerative diseases and in particular how NSPCs affect the AD pathology and how neurodegeneration affects NSPCs. In the end of this review, we will discuss how zebrafish as a useful model organism with extensive regenerative ability in the brain might help to address the molecular programs needed for NSPCs to respond to neurodegeneration by enhanced neurogenesis.

Neuronal labeling patterns in the spinal cord of adult transgenic Zebrafish

We describe neuronal patterns in the spinal cord of adult zebrafish. We studied the distribution of cells and processes in the three spinal regions reported in the literature: the 8th vertebra used as a transection injury site, the 15th vertebra mainly used for motor cell recordings and also for crush injury, and the 24th vertebra used to record motor nerve activity. We used well-known transgenic lines in which expression of green fluorescent protein (GFP) is driven by promoters to hb9 and isl1 in motoneurons, alx/chx10 and evx1 interneurons, ngn1 in sensory neurons and olig2 in oligodendrocytes, as well as antibodies for neurons (HuC/D, NF and SV2) and glia (GFAP). In isl1:GFP fish, GFP-positive processes are retained in the upper part of ventral horns and two subsets of cell bodies are observed. The pattern of the transgene in hb9:GFP adults is more diffuse and fibers are present broadly through the adult spinal cord. In alx/chx10 and evx1 lines we respectively observed two and three different GFP-positive populations. Finally, the ngn1:GFP transgene identifies dorsal root ganglion and some cells in dorsal horns. Interestingly some GFP positive fibers in ngn1:GFP fish are located around Mauthner axons and their density seems to be related to a rostrocaudal gradient. Many other cell types have been described in embryos and need to be studied in adults. Our findings provide a reference for further studies on spinal cytoarchitecture. Combined with physiological, histological and pathological/traumatic approaches, these studies will help clarify the operation of spinal locomotor circuits of adult zebrafish.

Regeneration, Plasticity, and Induced Molecular Programs in Adult Zebrafish Brain

Regenerative capacity of the brain is a variable trait within animals. Aquatic vertebrates such as zebrafish have widespread ability to renew their brains upon damage, while mammals have—if not none—very limited overall regenerative competence. Underlying cause of such a disparity is not fully evident; however, one of the reasons could be activation of peculiar molecular programs, which might have specific roles after injury or damage, by the organisms that regenerate. If this hypothesis is correct, then there must be genes and pathways that (a) are expressed only after injury or damage in tissues, (b) are biologically and functionally relevant to restoration of neural tissue, and (c) are not detected in regenerating organisms. Presence of such programs might circumvent the initial detrimental effects of the damage and subsequently set up the stage for tissue redevelopment to take place by modulating the plasticity of the neural stem/progenitor cells. Additionally, if transferable, those “molecular mechanisms of regeneration” could open up new avenues for regenerative therapies of humans in clinical settings. This review focuses on the recent studies addressing injury/damage-induced molecular programs in zebrafish brain, underscoring the possibility of the presence of genes that could be used as biomarkers of neural plasticity and regeneration.

Ptena, but not Ptenb, reduces regeneration after spinal cord injury in adult zebrafish

Based on the observation that the tumor suppressor gene PTEN (phosphatase and tensin homolog) reduces regeneration after spinal cord injury (SCI) as evidenced in the PTEN knockout mouse, we have investigated the function of Ptena and Ptenb, the two zebrafish homologs of mammalian PTEN, in adult zebrafish after spinal cord injury with the aim to assess the contribution of the two zebrafish genes to functional recovery in an animal species that spontaneously recovers from central nervous system injury. The inhibition of Ptena expression by antisense morpholino (MO) application improved spinal cord regeneration through 4 to 5weeks after injury. Retrograde tracing showed regrowth of axons from neurons of the regeneration-competent nucleus of the medial longitudinal fascicle in the brainstem in the Ptena MO-treated fish. Ptenb MO-treated fish recovered as well as control MO-treated fish at 4 and 5weeks after SCI, with their locomotion being similar to that of sham-injured and non-injured fish. The mRNA levels of Ptena were upregulated after SCI at the early stage after injury (12h and 6days) caudal to the lesion site, compared to the non-injured control, while the levels of Ptenb were upregulated only at 12h after injury. In situ hybridization experiments were in agreement with the qPCR measurements. At the protein level, Ptena was found to be expressed in spinal motoneurons and immature neurons. These results indicate that Ptena, but not Ptenb, inhibits regeneration in zebrafish, thus sharing this feature with PTEN in mammals. The fact that zebrafish regenerate better than mammals despite the inhibitory presence of Ptena is likely due to regeneration-conducive molecules that tip the balance from inhibition to enhancement. Interestingly, although Ptena and Ptenb have been shown to be functionally redundant in promoting the development of the fish larval central nervous system, they are not functionally redundant in the adult, suggesting that regeneration in fish is not predominantly due to the overall recapitulation of development.

Non-mammalian model systems for studying neuro-immune interactions after spinal cord injury

Mammals exhibit poor recovery after injury to the spinal cord, where the loss of neurons and neuronal connections can be functionally devastating. In contrast, it has long been appreciated that many non-mammalian vertebrate species exhibit significant spontaneous functional recovery after spinal cord injury (SCI). Identifying the biological responses that support an organism's inability or ability to recover function after SCI is an important scientific and medical question. While recent advances have been made in understanding the responses to SCI in mammals, we remain without an effective clinical therapy for SCI. A comparative biological approach to understanding responses to SCI in non-mammalian vertebrates will yield important insights into mechanisms that promote recovery after SCI. Presently, mechanistic studies aimed at elucidating responses, both intrinsic and extrinsic to neurons, that result in different regenerative capacities after SCI across vertebrates are just in their early stages. There are several inhibitory mechanisms proposed to impede recovery from SCI in mammals, including reactive gliosis and scarring, myelin associated proteins, and a suboptimal immune response. One hypothesis to explain the robust regenerative capacity of several non-mammalian vertebrates is a lack of some or all of these inhibitory signals. This review presents the current knowledge of immune responses to SCI in several non-mammalian species that achieve anatomical and functional recovery after SCI. This subject is of growing interest, as studies increasingly show both beneficial and detrimental roles of the immune response following SCI in mammals. A long-term goal of biomedical research in all experimental models of SCI is to understand how to promote functional recovery after SCI in humans. Therefore, understanding immune responses to SCI in non-mammalian vertebrates that achieve functional recovery spontaneously may identify novel strategies to modulate immune responses in less regenerative species and promote recovery after SCI.

Inflammation and wound repair

Wound repair requires the integration of complex cellular networks to restore tissue homeostasis. Defects in wound repair are associated with human disease including pyoderma gangrenosum, a heterogeneous disorder that is characterized by unhealed wounds and chronic inflammation of unclear etiology. Despite its clinical importance, there remain significant gaps in understanding how different types of cells communicate to integrate inflammation and wound repair. Recent progress in wound and regenerative biology has been gained by studying genetically tractable model organisms, like zebrafish, that retain the ability to regenerate. The optical transparency and ease of genetic manipulation make zebrafish an ideal model system to dissect multi-cellular and tissue level interactions during wound repair. The focus of this review is on recent advances in understanding how inflammation and wound repair are orchestrated and integrated to achieve wound resolution and tissue regeneration using zebrafish.

gdf6a is required for cone photoreceptor subtype differentiation and for the actions of tbx2b in determining rod versus cone photoreceptor fate

Functional vision restoration is within reach via stem cell therapy, but one of the largest obstacles is the derivation of colour-sensitive cone photoreceptors that are required for high-acuity daytime vision. To enhance progress made using nocturnal murine models, we instead utilize cone-rich zebrafish and herein investigate relationships between gdf6a and tbx2b in cone photoreceptor development. Growth/differentiation factor 6a (gdf6a), a bone morphogenetic protein family ligand, is an emerging factor in photoreceptor degenerative diseases. The T-box transcription factor tbx2b is required to specify UV cone photoreceptor fate instead of rod photoreceptor fate. Interactions between these factors in cone development would be unanticipated, considering the discrete phenotypes in their respective mutants. However, gdf6a positively modulates the abundance of tbx2b transcript during early eye morphogenesis, and we extended this conclusion to later stages of retinal development comprising the times when photoreceptors differentiate. Despite this, gdf6a-/- larvae possess a normal relative number of UV cones and instead present with a low abundance of blue cone photoreceptors, approximately half that of siblings (p<0.001), supporting a differential role for gdf6a amongst the spectral subtypes of cone photoreceptors. Further, gdf6a-/- larvae from breeding of compound heterozygous gdf6a+/-;tbx2b+/- mutants exhibit the recessive lots-of-rods phenotype (which also shows a paucity of UV cones) at significantly elevated rates (44% or 48% for each of two tbx2b alleles, χ2 p≤0.007 for each compared to expected Mendelian 25%). Thus the gdf6a-/- background sensitizes fish such that the recessive lots-of-rods phenotype can appear in heterozygous tbx2b+/- fish. Overall, this work establishes a novel link between tbx2b and gdf6a in determining photoreceptor fates, defining the nexus of an intricate pathway influencing the abundance of cone spectral subtypes and specifying rod vs. cone photoreceptors. Understanding this interaction is a necessary step in the refinement of stem cell-based restoration of daytime vision in humans.

The asparaginyl endopeptidase legumain is essential for functional recovery after spinal cord injury in adult zebrafish

Unlike mammals, adult zebrafish are capable of regenerating severed axons and regaining locomotor function after spinal cord injury. A key factor for this regenerative capacity is the innate ability of neurons to re-express growth-associated genes and regrow their axons after injury in a permissive environment. By microarray analysis, we have previously shown that the expression of legumain (also known as asparaginyl endopeptidase) is upregulated after complete transection of the spinal cord. In situ hybridization showed upregulation of legumain expression in neurons of regenerative nuclei during the phase of axon regrowth/sprouting after spinal cord injury. Upregulation of Legumain protein expression was confirmed by immunohistochemistry. Interestingly, upregulation of legumain expression was also observed in macrophages/microglia and neurons in the spinal cord caudal to the lesion site after injury. The role of legumain in locomotor function after spinal cord injury was tested by reducing Legumain expression by application of anti-sense morpholino oligonucleotides. Using two independent anti-sense morpholinos, locomotor recovery and axonal regrowth were impaired when compared with a standard control morpholino. We conclude that upregulation of legumain expression after spinal cord injury in the adult zebrafish is an essential component of the capacity of injured neurons to regrow their axons. Another feature contributing to functional recovery implicates upregulation of legumain expression in the spinal cord caudal to the injury site. In conclusion, we established for the first time a function for an unusual protease, the asparaginyl endopeptidase, in the nervous system. This study is also the first to demonstrate the importance of legumain for repair of an injured adult central nervous system of a spontaneously regenerating vertebrate and is expected to yield insights into its potential in nervous system regeneration in mammals.

Axonal regeneration after spinal cord injury in zebrafish and mammals: differences, similarities, translation

Spinal cord injury (SCI) in mammals results in functional deficits that are mostly permanent due in part to the inability of severed axons to regenerate. Several types of growth-inhibitory molecules expressed at the injury site contribute to this regeneration failure. The responses of axons to these inhibitors vary greatly within and between organisms, reflecting axons' characteristic intrinsic propensity for regeneration. In the zebrafish (Danio rerio) many but not all axons exhibit successful regeneration after SCI. This review presents and compares the intrinsic and extrinsic determinants of axonal regeneration in the injured spinal cord in mammals and zebrafish. A better understanding of the molecules and molecular pathways underlying the remarkable individualism among neurons in mature zebrafish may support the development of therapies for SCI and their translation to the clinic.

Syntenin-a promotes spinal cord regeneration following injury in adult zebrafish

In contrast to mammals, adult zebrafish recover locomotor function after spinal cord injury, in part due to the capacity of the central nervous system to repair severed connections. To identify molecular cues that underlie regeneration, we conducted mRNA expression profiling and found that syntenin-a expression is upregulated in the adult zebrafish spinal cord caudal to the lesion site after injury. Syntenin is a scaffolding protein involved in mammalian cell adhesion and movement, axonal outgrowth, establishment of cell polarity, and protein trafficking. It could thus be expected to be involved in supporting regeneration in fish. Syntenin-a mRNA and protein are expressed in neurons, glia and newly generated neural cells, and upregulated caudal to the lesion site on days 6 and 11 following spinal cord injury. Treatment of spinal cord-injured fish with two different antisense morpholinos to knock down syntenin-a expression resulted in significant inhibition of locomotor recovery at 5 and 6 weeks after injury, when compared to control morpholino-treated fish. Knock-down of syntenin-a reduced regrowth of descending axons from brainstem neurons into the spinal cord caudal to the lesion site. These observations indicate that syntenin-a is involved in regeneration after traumatic insult to the central nervous system of adult zebrafish, potentially leading to novel insights into the cellular and molecular mechanisms that require activation in the regeneration-deficient mammalian central nervous system.

Major vault protein promotes locomotor recovery and regeneration after spinal cord injury in adult zebrafish

In contrast to mammals, adult zebrafish recover locomotor functions after spinal cord injury (SCI), in part due to axonal regrowth and regeneration permissivity of the central nervous system. Upregulation of major vault protein (MVP) expression after spinal cord injury in the brainstem of the adult zebrafish prompted us to probe for its contribution to recovery after SCI. MVP is a multifunctional protein expressed not only in many types of tumours but also in the nervous system, where its importance for regeneration is, however, unclear. Using an established zebrafish SCI model, we found that MVP mRNA and protein expression levels were increased in ependymal cells in the spinal cord caudal to the lesion site at 6 and 11 days after SCI. Double immunolabelling showed that MVP was co-localised with Islet-1 or tyrosine hydroxylase around the central canal of the spinal cord in sham-injured control fish and injured fish 11 days after surgery. MVP co-localised with the neural stem cell marker nestin in ependymal cells after injury. By using an in vivo morpholino-based knock-down approach, we found that the distance moved by MVP morpholino-treated fish was reduced at 4, 5 and 6 weeks after SCI when compared to fish treated with standard control morpholino. Knock-down of MVP resulted in reduced regrowth of axons from brainstem neurons into the spinal cord caudal to the lesion site. These results indicate that MVP supports locomotor recovery and axonal regrowth after SCI in adult zebrafish.

Plasticity of tyrosine hydroxylase and serotonergic systems in the regenerating spinal cord of adult zebrafish

Monoaminergic innervation of the spinal cord has important modulatory functions for locomotion. Here we performed a quantitative study to determine the plastic changes of tyrosine hydroxylase-positive (TH1(+); mainly dopaminergic), and serotonergic (5-HT(+)) terminals and cells during successful spinal cord regeneration in adult zebrafish. TH1(+) innervation in the spinal cord is derived from the brain. After spinal cord transection, TH1(+) immunoreactivity is completely lost from the caudal spinal cord. Terminal varicosities increase in density rostral to the lesion site compared with unlesioned controls and are re-established in the caudal spinal cord at 6 weeks post lesion. Interestingly, axons mostly fail to re-innervate more caudal levels of the spinal cord even after prolonged survival times. However, densities of terminal varicosities correlate with recovery of swimming behavior, which is completely lost again after re-lesion of the spinal cord. Similar observations were made for terminals derived from descending 5-HT(+) axons from the brain. In addition, spinal 5-HT(+) neurons were newly generated after a lesion and transiently increased in number up to fivefold, which depended in part on hedgehog signaling. Overall, TH1(+) and 5-HT(+) innervation is massively altered in the successfully regenerated spinal cord of adult zebrafish. Despite these changes in TH and 5-HT systems, a remarkable recovery of swimming capability is achieved, suggesting significant plasticity of the adult spinal network during regeneration.

A surgery protocol for adult zebrafish spinal cord injury

Adult zebrafish has a remarkable capability to recover from spinal cord injury, providing an excellent model for studying neuroregeneration. Here we list equipment and reagents, and give a detailed protocol for complete transection of the adult zebrafish spinal cord. In this protocol, potential problems and their solutions are described so that the zebrafish spinal cord injury model can be more easily and reproducibly performed. In addition, two assessments are introduced to monitor the success of the surgery and functional recovery: one test to assess free swimming capability and the other test to assess extent of neuroregeneration by in vivo anterograde axonal tracing. In the swimming behavior test, successful complete spinal cord transection is monitored by the inability of zebrafish to swim freely for 1 week after spinal cord injury, followed by the gradual reacquisition of full locomotor ability within 6 weeks after injury. As a morphometric correlate, anterograde axonal tracing allows the investigator to monitor the ability of regenerated axons to cross the lesion site and increasingly extend into the gray and white matter with time after injury, confirming functional recovery. This zebrafish model provides a paradigm for recovery from spinal cord injury, enabling the identification of pathways and components of neuroregeneration.

Cysteine- and glycine-rich protein 1a is involved in spinal cord regeneration in adult zebrafish

In contrast to mammals, adult zebrafish have the ability to regrow descending axons and gain locomotor recovery after spinal cord injury (SCI). In zebrafish, a decisive factor for successful spinal cord regeneration is the inherent ability of some neurons to regrow their axons via (re)expressing growth-associated genes during the regeneration period. The nucleus of the medial longitudinal fascicle (NMLF) is one of the nuclei capable of regenerative response after SCI. Using microarray analysis with laser capture microdissected NMLF, we show that cysteine- and glycine-rich protein (CRP)1a (encoded by the csrp1a gene in zebrafish), the function of which is largely unknown in the nervous system, was upregulated after SCI. In situ hybridization confirmed the upregulation of csrp1a expression in neurons during the axon growth phase after SCI, not only in the NMLF, but also in other nuclei capable of regeneration, such as the intermediate reticular formation and superior reticular formation. The upregulation of csrp1a expression in regenerating nuclei started at 3 days after SCI and continued to 21 days post-injury, the longest time point studied. In vivo knockdown of CRP1a expression using two different antisense morpholino oligonucleotides impaired axon regeneration and locomotor recovery when compared with a control morpholino, demonstrating that CRP1a upregulation is an important part of the innate regeneration capability in injured neurons of adult zebrafish. This study is the first to demonstrate the requirement of CRP1a for zebrafish spinal cord regeneration.

Retracted: Overexpression of αCP2, a translational repressor of GAP-43, inhibited axon outgrowth during development in Xenopus laevis

The 3'-untranslated regions (3'-UTRs) of growth-associated protein 43 (GAP-43), which is crucial for neural development and axonal regeneration, are highly conserved among vertebrates. Previous studies in mammals have identified one U-rich cis element within GAP-43 3'-UTR and several trans factors that regulate its mRNA stability. However, much less is known in lower vertebrates. The Xenopus GAP-43 3'-UTR, despite its high similarity with those in higher vertebrates, contains unique CU-rich sequences, suggesting the existence of novel cis elements and trans factors. In current study, we isolated four proteins bound to GAP-43 3'-UTR from juvenile frog brain using affinity purification. Mass spectrometry identified Hu antigen D (HuD) and poly(C) binding protein 2 (αCP2) as the proteins forming 48- and 44-kDa ribonucleoprotein complexes, respectively. We validated the association between αCP2 and GAP-43 3'-UTR in vivo. After confirming the post-transcriptional effects of αCP2 on GAP-43 expression, we demonstrated that αCP2 directly inhibited the translation of GAP-43 gene, without affecting its mRNA stability. αCP2 overexpression led to decreased level of GAP-43 protein and significantly inhibited axonal outgrowth in primarily cultured neurons. Our study therefore provided insights on novel functions of αCP2 in vertebrate nervous system during development and new mechanisms of post-transcriptional regulation for GAP-43 gene.

Teleost fish as a model system to study successful regeneration of the central nervous system

Traumatic brain injury and spinal cord injury are devastating conditions that may result in death or long-term disability. A promising strategy for the development of effective cell replacement therapies involves the study of regeneration-competent organisms. Among this group, teleost fish are distinguished by their excellent potential to regenerate nervous tissue and to regain function after injury to the central nervous system. In this chapter, we summarize our current understanding of the cellular processes that mediate this regenerative potential, and we show that several of these processes are shared with the normal development of the intact central nervous system; we describe how the spontaneous self-repair of the teleostean central nervous system leads to functional recovery, at physiological and behavioral levels; we discuss the possible function of molecular factors associated with the degenerative and regenerative processes after injury; and, finally, we speculate on evolutionary aspects of adult neurogenesis and neuronal regeneration, and on how a better understanding of these aspects could catalyze the development of therapeutic strategies to overcome the regenerative limits of the mammalian CNS.

Molecular control of axon growth: insights from comparative gene profiling and high-throughput screening

Axon regeneration in the mammalian adult central nervous system (CNS) is limited by an intrinsically low capacity for axon growth in many CNS neurons. In contrast, embryonic, peripheral, and many nonmammalian neurons are capable of successful regeneration. Numerous studies have compared mammalian CNS neurons to their counterparts in regenerating systems in an effort to identify candidate genes that control regenerative ability. This review summarizes work using this comparative strategy and examines our current understanding of gene function in axon growth, highlighting the emergence of genome-wide expression profiling and high-throughput screening strategies to identify novel regulators of axon growth.