The lh3 Glycosyltransferase Directs Target-Selective Peripheral Nerve Regeneration

Iron role paradox in nerve degeneration and regeneration

Iron accumulates in the neural tissue during peripheral nerve degeneration. Some studies have already been suggested that iron facilitates Wallerian degeneration (WD) events such as Schwann cell de-differentiation. On the other hand, intracellular iron levels remain elevated during nerve regeneration and gradually decrease. Iron enhances Schwann cell differentiation and axonal outgrowth. Therefore, there seems to be a paradox in the role of iron during nerve degeneration and regeneration. We explain this contradiction by suggesting that the increase in intracellular iron concentration during peripheral nerve degeneration is likely to prepare neural cells for the initiation of regeneration. Changes in iron levels are the result of changes in the expression of iron homeostasis proteins. In this review, we will first discuss the changes in the iron/iron homeostasis protein levels during peripheral nerve degeneration and regeneration and then explain how iron is related to nerve regeneration. This data may help better understand the mechanisms of peripheral nerve repair and find a solution to prevent or slow the progression of peripheral neuropathies.

Target-selective vertebrate motor axon regeneration depends on interaction with glial cells at a peripheral nerve plexus

A critical step for functional recovery from peripheral nerve injury is for regenerating axons to connect with their pre-injury targets. Reestablishing pre-injury target specificity is particularly challenging for limb-innervating axons as they encounter a plexus, a network where peripheral nerves converge, axons from different nerves intermingle, and then re-sort into target-specific bundles. Here, we examine this process at a plexus located at the base of the zebrafish pectoral fin, equivalent to tetrapod forelimbs. Using live cell imaging and sparse axon labeling, we find that regenerating motor axons from 3 nerves coalesce into the plexus. There, they intermingle and sort into distinct branches, and then navigate to their original muscle domains with high fidelity that restores functionality. We demonstrate that this regeneration process includes selective retraction of mistargeted axons, suggesting active correction mechanisms. Moreover, we find that Schwann cells are enriched and associate with axons at the plexus, and that Schwann cell ablation during regeneration causes profound axonal mistargeting. Our data provide the first real-time account of regenerating vertebrate motor axons navigating a nerve plexus and reveal a previously unappreciated role for Schwann cells to promote axon sorting at a plexus during regeneration.

Diosgenin restores memory function via SPARC-driven axonal growth from the hippocampus to the PFC in Alzheimer's disease model mice

Central nervous system axons have minimal capacity to regenerate in adult brains, hindering memory recovery in Alzheimer's disease (AD). Although recent studies have shown that damaged axons sprouted in adult and AD mouse brains, long-distance axonal re-innervation to their targets has not been achieved. We selectively visualized axon-growing neurons in the neural circuit for memory formation, from the hippocampus to the prefrontal cortex, and showed that damaged axons successfully extended to their native projecting area in mouse models of AD (5XFAD) by administration of an axonal regenerative agent, diosgenin. In vivo transcriptome analysis detected the expression profile of axon-growing neurons directly isolated from the hippocampus of 5XFAD mice. Secreted protein acidic and rich in cysteine (SPARC) was the most expressed gene in axon-growing neurons. Neuron-specific overexpression of SPARC via adeno-associated virus serotype 9 delivery in the hippocampus recovered memory deficits and axonal projection to the prefrontal cortex in 5XFAD mice. DREADDs (Designer receptors exclusively activated by designer drugs) analyses revealed that SPARC overexpression-induced axonal growth in the 5XFAD mouse brain directly contributes to memory recovery. Elevated levels of SPARC on axonal membranes interact with extracellular rail-like collagen type I to promote axonal remodeling along their original tracings in primary cultured hippocampal neurons. These findings suggest that SPARC-driven axonal growth in the brain may be a promising therapeutic strategy for AD and other neurodegenerative diseases.

Mechanism of Danggui Sini underlying the treatment of peripheral nerve injury based on network pharmacology and molecular docking: A review

Danggui Sini is a traditional Chinese medicine prescription for treating peripheral nerve injury (PNI). We studied the mechanisms of this decoction through network pharmacology analysis and molecular docking. Using R language and Perl software, the active components and predicted targets of Danggui Sini, as well as the related gene targets of PNI, were mined through TCMSP, GeneCards, OMIM, TTD, and DrugBank. The network diagram of active components and intersection targets was constructed using Cytoscape software and the STRING database. The CytoNCA plug-in was used to screen out the core compounds and key targets. The genes were analyzed for Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment. AutoDock was used to analyze the molecular docking of key targets and core compounds of diseases. The drug component disease target regulatory network showed that the key components included quercetin, kaempferol, naringenin, and licochalcone A, which play key roles in the whole network and may be the primary compounds associated with the action of Danggui Sini against PNI. PPI network topology analysis showed high degree values for RELA, JUN, MAPK1, RB1, and FOS. Enrichment analysis showed that the core targets of Danggui Sini participated in pathways associated with neurogenesis-multiple diseases. Molecular docking showed that the active ingredients in Danggui Sini had a good binding ability with key targets. We conclude that many active components of Danggui Sini play therapeutic roles in PNI treatment by regulating RELA, JUN, MAPK1, RB1, and FOS, and multiple other targets in inflammation, immunity, and lipid metabolism.

VEGFA-modified DPSCs combined with LC-YE-PLGA NGCs promote facial nerve injury repair in rats

Objective

The aim of this research was to investigate the effect of vascular endothelial growth factor A (VEGFA)-overexpressing rat dental pulp stem cells (rDPSCs) combined with laminin-coated and yarn-encapsulated poly(_l-lactide-co-glycolide) (PLGA) nerve guidance conduit (LC-YE-PLGA NGC) in repairing 10 mm facial nerve injury in rats.

Study Design

rDPSCs isolated from rat mandibular central incisor were cultured and identified in vitro and further transfected with the lentiviral vectors (Lv-VEGFA). To investigate the role and mechanisms of VEGFA in neurogenic differentiation in vitro, semaxanib (SU5416), Cell Counting Kit-8 (CCK-8), real-time quantitative polymerase chain reaction (qPCR) and Western blotting were performed. Ten-millimeter facial nerve defect models in rats were established and bridged by LC-YE-PLGA NGCs. The repair effects were detected by transmission electron microscopy (TEM), compound muscle action potential (CMAP), immunohistochemistry and immunofluorescence.

Results

Extracted cells exhibited spindle-shaped morphology, presented typical markers (CD44⁺CD90⁺CD34^-CD45^-), and presented multidirectional differentiation potential. The DPSCs with VEGFA overexpression were constructed successfully. VEGFA enhanced the proliferation and neural differentiation ability of rDPSCs, and the expression of neuron-specific enolase (NSE) and βIII-tubulin was increased. However, these trends were reversed with the addition of SU5416. This suggests that VEGFA mediates the above effects mainly through vascular endothelial growth factor receptor 2 (VEGFR2) binding. The LC-YE-NGC basically meet the requirements of facial nerve repair. For the in vivo experiment, the CMAP latency period was shorter in DPSCS-VEGFA-NGC group in comparison with other experimental groups, while the amplitude was increased. Such functional recovery correlated well with an increase in histological improvement. Further study suggested that VEGFA-modified DPSCs could increase the myelin number, thickness and axon diameter of facial nerve. NSE, βIII-tubulin and S100 fluorescence intensity and immunohistochemical staining intensity were significantly enhanced.

Conclusion

VEGFA-modified rDPSCs combined with LC-YE-PLGA NGCs have certain advantages in the growth and functional recovery of facial nerves in rats.

Salidroside Alleviates Diabetic Cognitive Dysfunction Via B3galt2/F3/Contactin Signaling Pathway in Mice

Diabetes is frequently accompanied by cognitive impairment with insidious onset, and progressive cognitive and behavioral changes. β-1, 3-galactosyltransferase 2 (B3galt2) contributes to glycosylation, showing a clue for neuronal apoptosis, proliferation and differentiation. However, the role of B3galt2 in diabetic cognitive dysfunction (DCD) has not been investigated. In the present study, we aimed to explore the role of B3galt2 in DCD. Additionally, the potential therapeutic effects of salidroside on DCD was also explored. Diabetic C57BL/6J mice showed cognitive dysfunction together with down-regulated B3galt2. Overexpression of B3galt2 reversed the cognitive decline of diabetic C57BL/6J. Moreover, cognitive impairment was aggravated in B3galt2^+/- diabetic mice compared with C57BL/6J diabetic mice. Immunohistochemistry fluorescence indicated that B3galt2 and F3/Contactin were co-localized in the hippocampal regions. Importantly, the expression of F3/Contactin can be regulated by the manipulation of B3galt2, overexpression of which assuaged hippocampal neuronal damage, protected the synapsin, and reduced neuronal apoptosis in diabetic mice. Interestingly, SAL alleviated DCD and reversed the expression of B3galt2 in diabetic C57BL/6J mice. These findings indicate that inhibition of B3galt2/F3/Contactin pathway contributes to DCD, and participates in SAL reversed DCD.

A glia cell dependent mechanism at a peripheral nerve plexus critical for target-selective axon regeneration

A critical step for functional recovery from peripheral nerve injury is for regenerating axons to connect with their pre-injury targets. Reestablishing pre-injury target specificity is particularly challenging for limb-innervating axons as they encounter a plexus, a network where peripheral nerves converge, axons from different nerves intermingle, and then re-sort into target-specific bundles. Here, we examine this process at a plexus located at the base of the zebrafish pectoral fin, equivalent to tetrapod forelimbs. Using live cell imaging and sparse axon labeling, we find that regenerating motor axons from three nerves coalesce into the plexus. There, they intermingle and sort into distinct branches, and then navigate to their original muscle domains with high fidelity that restores functionality. We demonstrate that this regeneration process includes selective retraction of mistargeted axons, suggesting active correction mechanisms. Moreover, we find that Schwann cells are enriched and associate with axons at the plexus, and that Schwann cell ablation during regeneration causes profound axonal mistargeting. Our data provide the first real time account of regenerating vertebrate motor axons navigating a nerve plexus and reveal a previously unappreciated role for Schwann cells to promote axon sorting at a plexus during regeneration.

Schwann cell functions in peripheral nerve development and repair

The glial cell of the peripheral nervous system (PNS), the Schwann cell (SC), counts among the most multifaceted cells of the body. During development, SCs secure neuronal survival and participate in axonal path finding. Simultaneously, they orchestrate the architectural set up of the developing nerves, including the blood vessels and the endo-, peri- and epineurial layers. Perinatally, in rodents, SCs radially sort and subsequently myelinate individual axons larger than 1 μm in diameter, while small calibre axons become organised in non-myelinating Remak bundles. SCs have a vital role in maintaining axonal health throughout life and several specialized SC types perform essential functions at specific locations, such as terminal SC at the neuromuscular junction (NMJ) or SC within cutaneous sensory end organs. In addition, neural crest derived satellite glia maintain a tight communication with the soma of sensory, sympathetic, and parasympathetic neurons and neural crest derivatives are furthermore an indispensable part of the enteric nervous system. The remarkable plasticity of SCs becomes evident in the context of a nerve injury, where SC transdifferentiate into intriguing repair cells, which orchestrate a regenerative response that promotes nerve repair. Indeed, the multiple adaptations of SCs are captivating, but remain often ill-resolved on the molecular level. Here, we summarize and discuss the knowns and unknowns of the vast array of functions that this single cell type can cover in peripheral nervous system development, maintenance, and repair.

The application of collagen in the repair of peripheral nerve defect

Collagen is a natural polymer expressed in the extracellular matrix of the peripheral nervous system. It has become increasingly crucial in peripheral nerve reconstruction as it was involved in regulating Schwann cell behaviors, maintaining peripheral nerve functions during peripheral nerve development, and being strongly upregulated after nerve injury to promote peripheral nerve regeneration. Moreover, its biological properties, such as low immunogenicity, excellent biocompatibility, and biodegradability make it a suitable biomaterial for peripheral nerve repair. Collagen provides a suitable microenvironment to support Schwann cells’ growth, proliferation, and migration, thereby improving the regeneration and functional recovery of peripheral nerves. This review aims to summarize the characteristics of collagen as a biomaterial, analyze its role in peripheral nerve regeneration, and provide a detailed overview of the recent advances concerning the optimization of collagen nerve conduits in terms of physical properties and structure, as well as the application of the combination with the bioactive component in peripheral nerve regeneration.

Peripheral nerve development in zebrafish requires muscle patterning by tcf15/paraxis

The vertebrate peripheral nervous system (PNS) is an intricate network that conveys sensory and motor information throughout the body. During development, extracellular cues direct the migration of axons and glia through peripheral tissues. Currently, the suite of molecules that govern PNS axon-glial patterning is incompletely understood. To elucidate factors that are critical for peripheral nerve development, we characterized the novel zebrafish mutant, stl159, that exhibits abnormalities in PNS patterning. In these mutants, motor and sensory nerves that develop adjacent to axial muscle fail to extend normally, and neuromasts in the posterior lateral line system, as well as neural crest-derived melanocytes, are incorrectly positioned. The stl159 genetic lesion lies in the basic helix-loop-helix (bHLH) transcription factor tcf15, which has been previously implicated in proper development of axial muscles. We find that targeted loss of tcf15 via CRISPR-Cas9 genome editing results in the PNS patterning abnormalities observed in stl159 mutants. Because tcf15 is expressed in developing muscle prior to nerve extension, rather than in neurons or glia, we predict that tcf15 non-cell-autonomously promotes peripheral nerve patterning in zebrafish through regulation of extracellular patterning cues. Our work underscores the importance of muscle-derived factors in PNS development.

Visceral mesoderm signaling regulates assembly position and function of the Drosophila testis niche

Tissue homeostasis often requires a properly placed niche to support stem cells. Morphogenetic processes that position a niche are just being described. For the Drosophila testis, we recently showed that pro-niche cells, specified at disparate positions during early gonadogenesis, must assemble into one collective at the anterior of the gonad. We now find that Slit and FGF signals emanating from adjacent visceral mesoderm regulate assembly. In response to signaling, niche cells express islet, which we find is also required for niche assembly. Without signaling, niche cells specified furthest from the anterior are unable to migrate, remaining dispersed. The function of such niches is severely disrupted, with niche cells evading cell cycle quiescence, compromised in their ability to signal the incipient stem cell pool, and failing to orient stem cell divisions properly. Our work identifies both extrinsic signaling and intrinsic responses required for proper assembly and placement of the testis niche.

Mature but not developing Schwann cells promote axon regeneration after peripheral nerve injury

Since Schwann cells (SCs) support axonal growth at development as well as after peripheral nerve injury (PNI), developing SCs might be able to promote axon regeneration after PNI. The purpose of the current study was to elucidate the capability of developing SCs to induce axon regeneration after PNI. SC precursors (SCPs), immature SCs (ISCs), repair SCs (RSCs) from injured nerves, and non-RSCs from intact nerves were tested by grafting into acellular region of rat sciatic nerve with crush injury. Both of developing SCs completely failed to support axon regeneration, whereas both of mature SCs, especially RSCs, induced axon regeneration. Further, RSCs but not SCPs promoted neurite outgrowth of adult dorsal root ganglion neurons. Transcriptome analysis revealed that the gene expression profiles were distinctly different between RSCs and SCPs. These findings indicate that developing SCs are markedly different from mature SCs in terms of functional and molecular aspects and that RSC is a viable candidate for regenerative cell therapy for PNI.

Lessons from Injury: How Nerve Injury Studies Reveal Basic Biological Mechanisms and Therapeutic Opportunities for Peripheral Nerve Diseases

Since Waller and Cajal in the nineteenth and early twentieth centuries, laboratory traumatic peripheral nerve injury studies have provided great insight into cellular and molecular mechanisms governing axon degeneration and the responses of Schwann cells, the major glial cell type of peripheral nerves. It is now evident that pathways underlying injury-induced axon degeneration and the Schwann cell injury-specific state, the repair Schwann cell, are relevant to many inherited and acquired disorders of peripheral nerves. This review provides a timely update on the molecular understanding of axon degeneration and formation of the repair Schwann cell. We discuss how nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) and sterile alpha TIR motif containing protein 1 (SARM1) are required for axon survival and degeneration, respectively, how transcription factor c-JUN is essential for the Schwann cell response to nerve injury and what each tells us about disease mechanisms and potential therapies. Human genetic association with NMNAT2 and SARM1 strongly suggests aberrant activation of programmed axon death in polyneuropathies and motor neuron disorders, respectively, and animal studies suggest wider involvement including in chemotherapy-induced and diabetic neuropathies. In repair Schwann cells, cJUN is aberrantly expressed in a wide variety of human acquired and inherited neuropathies. Animal models suggest it limits axon loss in both genetic and traumatic neuropathies, whereas in contrast, Schwann cell secreted Neuregulin-1 type 1 drives onion bulb pathology in CMT1A. Finally, we discuss opportunities for drug-based and gene therapies to prevent axon loss or manipulate the repair Schwann cell state to treat acquired and inherited neuropathies and neuronopathies.

Intrinsic positional memory guides target-specific axon regeneration in the zebrafish vagus nerve

Regeneration after peripheral nerve damage requires that axons re-grow to the correct target tissues in a process called target-specific regeneration. Although much is known about the mechanisms that promote axon re-growth, re-growing axons often fail to reach the correct targets, resulting in impaired nerve function. We know very little about how axons achieve target-specific regeneration, particularly in branched nerves that require distinct targeting decisions at branch points. The zebrafish vagus motor nerve is a branched nerve with a well-defined topographic organization. Here, we track regeneration of individual vagus axons after whole-nerve laser severing and find a robust capacity for target-specific, functional re-growth. We then develop a new single-cell chimera injury model for precise manipulation of axon-environment interactions and find that (1) the guidance mechanism used during regeneration is distinct from the nerve's developmental guidance mechanism, (2) target selection is specified by neurons' intrinsic memory of their position within the brain, and (3) targeting to a branch requires its pre-existing innervation. This work establishes the zebrafish vagus nerve as a tractable regeneration model and reveals the mechanistic basis of target-specific regeneration.

Unraveling Axon Guidance during Axotomy and Regeneration

During neuronal development and regeneration axons extend a cytoskeletal-rich structure known as the growth cone, which detects and integrates signals to reach its final destination. The guidance cues “signals” bind their receptors, activating signaling cascades that result in the regulation of the growth cone cytoskeleton, defining growth cone advance, pausing, turning, or collapse. Even though much is known about guidance cues and their isolated mechanisms during nervous system development, there is still a gap in the understanding of the crosstalk between them, and about what happens after nervous system injuries. After neuronal injuries in mammals, only axons in the peripheral nervous system are able to regenerate, while the ones from the central nervous system fail to do so. Therefore, untangling the guidance cues mechanisms, as well as their behavior and characterization after axotomy and regeneration, are of special interest for understanding and treating neuronal injuries. In this review, we present findings on growth cone guidance and canonical guidance cues mechanisms, followed by a description and comparison of growth cone pathfinding mechanisms after axotomy, in regenerative and non-regenerative animal models.

Central versus peripheral nervous system regeneration: is there an exception for cranial nerves?

There exists a dichotomy in regenerative capacity between the PNS and CNS, which poses the question - where do cranial nerves fall? Through the discussion of the various cells and processes involved in axonal regeneration, we will evaluate whether the assumption that cranial nerve regeneration is analogous to peripheral nerve regeneration is valid. It is evident from this review that much remains to be clarified regarding both PNS and CNS regeneration. Furthermore, it is not clear if cranial nerves follow the PNS model, CNS model or possess an alternative novel regenerative process altogether. Future research should continue to focus on elucidating how cranial nerves regenerate; and the various cellular interactions, molecules and pathways involved.

Microvesicles from Schwann-Like Cells as a New Biomaterial Promote Axonal Growth

Schwann cells promote axonal regeneration following peripheral nerve injury. However, in terms of clinical treatment, the therapeutic effects of Schwann cells are limited by their source. The transmission of microvesicles from neuroglia cells to axons is a novel communication mechanism in axon regeneration.To evaluate the effect of microvesicles released from Schwann-like cells on axonal regeneration, neural stem cells derived from human embryonic stem cells differentiated into Schwann-like cells, which presented a typical morphology and characteristics similar to those of schwann cells. The glial markers like MBP, P0, P75^NTR, PMP-22, GFAP, HNK-1 and S100 were upregulated, whereas the neural stem markers like NESTIN, SOX1 and SOX2 were significantly downregulated in schwann-like cells. Microvesicles enhanced axonal growth in dorsal root ganglia neurons and regulated GAP43 expression in neuron-like cells (N2A and PC12) through the PTEN/PI3 K/Akt signaling pathway. A 5 mm section of sciatic nerve was transected in Sprague-Dawley rats. With microvesicles transplantation, regenerative nerves were evaluated after 6 weeks. Microvesicles increased sciatic function index scores, delayed gastrocnemius muscle atrophy and elevated βIII-tubulin-labeled axons in vivo. Schwann-like cells serve as a convenient source and promote axonal growth by secreting microvesicles, which may potentially be used as bioengineering materials for nerve tissue repair.

Current Advances in Comprehending Dynamics of Regenerating Axons and Axon-Glia Interactions after Peripheral Nerve Injury in Zebrafish

Following an injury, axons of both the central nervous system (CNS) and peripheral nervous system (PNS) degenerate through a coordinated and genetically conserved mechanism known as Wallerian degeneration (WD). Unlike central axons, severed peripheral axons have a higher capacity to regenerate and reinnervate their original targets, mainly because of the favorable environment that they inhabit and the presence of different cell types. Even though many aspects of regeneration in peripheral nerves have been studied, there is still a lack of understanding regarding the dynamics of axonal degeneration and regeneration, mostly due to the inherent limitations of most animal models. In this scenario, the use of zebrafish (Danio rerio) larvae combined with time-lapse microscopy currently offers a unique experimental opportunity to monitor the dynamics of the regenerative process in the PNS in vivo. This review summarizes the current knowledge and advances made in understanding the dynamics of the regenerative process of PNS axons. By using different tools available in zebrafish such as electroablation of the posterior lateral line nerve (pLLn), and laser-mediated transection of motor and sensory axons followed by time-lapse microscopy, researchers are beginning to unravel the complexity of the spatiotemporal interactions among different cell types during the regenerative process. Thus, understanding the cellular and molecular mechanisms underlying the degeneration and regeneration of peripheral nerves will open new avenues in the treatment of acute nerve trauma or chronic conditions such as neurodegenerative diseases.

Repurposing developmental mechanisms in the adult nervous system

Molecules and cellular processes important for nervous system development can be repurposed in adulthood for the regulation of adult neurogenesis, synaptic plasticity, and neural regeneration following injury or degeneration. Efforts to recapitulate neural development in order to ameliorate injury or disease are promising, but these often fall short of functional restoration due in part to our incomplete understanding of how these damaged circuits initially developed. Despite these limitations, such strategies provide hope that harnessing developmental mechanisms can restore nervous system functions following damage or disease.

Migrating Schwann cells direct axon regeneration within the peripheral nerve bridge

Schwann cells within the peripheral nervous system possess a remarkable regenerative potential. Current research shows that peripheral nerve-associated Schwann cells possess the capacity to promote repair of multiple tissues including peripheral nerve gap bridging, skin wound healing, digit tip repair as well as tooth regeneration. One of the key features of the specialized repair Schwann cells is that they become highly motile. They not only migrate into the area of damaged tissue and become a key component of regenerating tissue but also secrete signaling molecules to attract macrophages, support neuronal survival, promote axonal regrowth, activate local mesenchymal stem cells, and interact with other cell types. Currently, the importance of migratory Schwann cells in tissue regeneration is most evident in the case of a peripheral nerve transection injury. Following nerve transection, Schwann cells from both proximal and distal nerve stumps migrate into the nerve bridge and form Schwann cell cords to guide axon regeneration. The formation of Schwann cell cords in the nerve bridge is key to successful peripheral nerve repair following transection injury. In this review, we first examine nerve bridge formation and the behavior of Schwann cell migration in the nerve bridge, and then discuss how migrating Schwann cells direct regenerating axons into the distal nerve. We also review the current understanding of signals that could activate Schwann cell migration and signals that Schwann cells utilize to direct axon regeneration. Understanding the molecular mechanism of Schwann cell migration could potentially offer new therapeutic strategies for peripheral nerve repair.

Axo-glial interaction in the injured PNS

Axons share a close relationship with Schwann cells, their glial partners in peripheral nerves. An intricate axo-glia network of signals and bioactive molecules regulates the major aspects of nerve development and normal functioning of the peripheral nervous system. Disruptions to these complex axo-glial interactions can have serious neurological consequences, as typically seen in injured nerves. Recent studies in inherited neuropathies have demonstrated that damage to one of the partners in this symbiotic unit ultimately leads to impairment of the other partner, emphasizing the bidirectional influence of axon to glia and glia to axon signaling in these diseases. After physical trauma to nerves, dramatic alterations in the architecture and signaling environment of peripheral nerves take place. Here, axons and Schwann cells respond adaptively to these perturbations and change the nature of their reciprocal interactions, thereby driving the remodeling and regeneration of peripheral nerves. In this review, we focus on the nature and importance of axon-glia interactions in injured nerves, both for the reshaping and repair of nerves after trauma, and in driving pathology in inherited peripheral neuropathies.

Recent Findings on the Effects of Pharmacological Agents on the Nerve Regeneration after Peripheral Nerve Injury

Peripheral nerve injuries (PNIs) are accompanied with neuropathic pain and functional disability. Despite improvements in surgical repair techniques in recent years, the functional recovery is yet unsatisfied. Indeed a successful nerve repair depends not only on the surgical strategy but also on the cellular and molecular mechanisms involved in traumatic nerve injury. In contrast to all strategies suggested for nerve repair, pharmacotherapy is a cheap, accessible and non-invasive treatment that can be used immediately after nerve injury. This study aimed to review the effects of some pharmacological agents on the nerve regeneration after traumatic PNI evaluated by functional, histological and electrophysiological assessments. In addition, some cellular and molecular mechanisms responsible for their therapeutic actions, restricted to neural tissue, are suggested. These findings can not only help to find better strategies for peripheral nerve repair, but also to identify the neuropathic effects of various medications and their mechanisms of action.

Functions of histone modifications and histone modifiers in Schwann cells

Schwann cells (SCs) are the main glial cells present in the peripheral nervous system (PNS). Their primary functions are to insulate peripheral axons to protect them from the environment and to enable fast conduction of electric signals along big caliber axons by enwrapping them in a thick myelin sheath rich in lipids. In addition, SCs have the peculiar ability to foster axonal regrowth after a lesion by demyelinating and converting into repair cells that secrete neurotrophic factors and guide axons back to their former target to finally remyelinate regenerated axons. The different steps of SC development and their role in the maintenance of PNS integrity and regeneration after lesion are controlled by various factors among which transcription factors and chromatin-remodeling enzymes hold major functions. In this review, we discussed how histone modifications and histone-modifying enzymes control SC development, maintenance of PNS integrity and response to injury. The functions of histone modifiers as part of chromatin-remodeling complexes are discussed in another review published in the same issue of Glia.

RNAscope™ Multiplex Detection in Zebrafish

The RNAscope methodology is a powerful tool to detect RNA expression patterns with high subcellular resolution and possibility for RNA-protein colocalization studies. Presented here is a two-day protocol for robust multiplex detection of up to three different RNAs in zebrafish whole-mount embryos using the RNAscope procedure. Application of the protocol offers the simultaneous detection of multiple RNAs with a high signal-to-noise ratio in an intact embryo.

Dynamic expression of Slit1-3 and Robo1-2 in the mouse peripheral nervous system after injury

The Slit family of axon guidance cues act as repulsive molecules for precise axon pathfinding and neuronal migration during nervous system development through interactions with specific Robo receptors. Although we previously reported that Slit1-3 and their receptors Robo1 and Robo2 are highly expressed in the adult mouse peripheral nervous system, how this expression changes after injury has not been well studied. Herein, we constructed a peripheral nerve injury mouse model by transecting the right sciatic nerve. At 14 days after injury, quantitative real-time polymerase chain reaction was used to detect mRNA expression of Slit1-3 and Robo1-2 in L4-5 spinal cord and dorsal root ganglia, as well as the sciatic nerve. Immunohistochemical analysis was performed to examine Slit1-3, Robo1-2, neurofilament heavy chain, F4/80, and vimentin in L4-5 spinal cord, L4 dorsal root ganglia, and the sciatic nerve. Co-expression of Slit1-3 and Robo1-2 in L4 dorsal root ganglia was detected by in situ hybridization. In addition, Slit1-3 and Robo1-2 protein expression in L4-5 spinal cord, L4 dorsal root ganglia, and sciatic nerve were detected by western blot assay. The results showed no significant changes of Slit1-3 or Robo1-2 mRNA expression in the spinal cord within 14 days after injury. In the dorsal root ganglion, Slit1-3 and Robo1-2 mRNA expression were initially downregulated within 4 days after injury; however, Robo1-2 mRNA expression returned to the control level, while Slit1-3 mRNA expression remained upregulated during regeneration from 4-14 days after injury. In the sciatic nerve, Slit1-3 and their receptors Robo1-2 were all expressed in the proximal nerve stump; however, Slit1, Slit2, and Robo2 were barely detectable in the nerve bridge and distal nerve stump within 14 days after injury. Slit3 was highly ex-pressed in macrophages surrounding the nerve bridge and slightly downregulated in the distal nerve stump within 14 days after injury. Robo1 was upregulated in vimentin-positive cells and migrating Schwann cells inside the nerve bridge. Robo1 was also upregulated in Schwann cells of the distal nerve stump within 14 days after injury. Our findings indicate that Slit3 is the major ligand expressed in the nerve bridge and distal nerve stump during peripheral nerve regeneration, and Slit3/Robo signaling could play a key role in peripheral nerve repair after injury. This study was approved by Plymouth University Animal Welfare Ethical Review Board (approval No. 30/3203) on April 12, 2014.

Construction and reconstruction of brain circuits: normal and pathological axon guidance

Perception of our environment entirely depends on the close interaction between the central and peripheral nervous system. In order to communicate each other, both systems must develop in parallel and in coordination. During development, axonal projections from the CNS as well as the PNS must extend over large distances to reach their appropriate target cells. To do so, they read and follow a series of axon guidance molecules. Interestingly, while these molecules play critical roles in guiding developing axons, they have also been shown to be critical in other major neurodevelopmental processes, such as the migration of cortical progenitors. Currently, a major hurdle for brain repair after injury or neurodegeneration is the absence of axonal regeneration in the mammalian CNS. By contrasts, PNS axons can regenerate. Many hypotheses have been put forward to explain this paradox but recent studies suggest that hacking neurodevelopmental mechanisms may be the key to promote CNS regeneration. Here we provide a seminar report written by trainees attending the second Flagship school held in Alpbach, Austria in September 2018 organized by the International Society for Neurochemistry (ISN) together with the Journal of Neurochemistry (JCN). This advanced school has brought together leaders in the fields of neurodevelopment and regeneration in order to discuss major keystones and future challenges in these respective fields.

Lysyl hydroxylase 3 is required for normal lens capsule formation and maintenance of lens epithelium integrity and fate

Lens abnormalities are a major cause of reduced vision and blindness. One mechanism that can lead to reduced lens transparency, i.e. cataract, is abnormal behavior of lens epithelial cells (LECs), the precursors of the transparent lens fiber cells. Here we describe a zebrafish mutation causing the embryonic lens epithelium to generate cellular masses comprising partially differentiated lens fiber cells. We identify the mutant gene as plod3, which encodes for Lysyl hydroxylase 3 (Lh3), an enzyme essential for modification of collagens, including Collagen IV, a main component of the lens capsule. We show that plod3-deficient lenses have abnormal lens epithelium from an early developmental stage, as well as abnormal lens capsules. Subsequently, upregulation of TGFβ signaling takes place, which drives the formation of lens epithelial cellular masses. We identify a similar phenotype in Collagen IVα5-deficient embryos, suggesting a key role for the defective lens capsule in the pathogenesis. We propose that plod3 and col4a5 mutant zebrafish can serve as useful models for better understanding the biology of LECs during embryonic development and in formation of lens epithelium-derived cataract.

N-Glycosylation of the Discoidin Domain Receptor Is Required for Axon Regeneration in Caenorhabditis elegans

Axon regeneration following neuronal injury is an important repair mechanism that is not well understood at present. In Caenorhabditis elegans, axon regeneration is regulated by DDR-2, a receptor tyrosine kinase (RTK) that contains a discoidin domain and modulates the Met-like SVH-2 RTK-JNK MAP kinase signaling pathway. Here, we describe the svh-10/sqv-3 and svh-11 genes, which encode components of a conserved glycosylation pathway, and show that they modulate axon regeneration in C. elegans Overexpression of svh-2, but not of ddr-2, can suppress the axon regeneration defect observed in svh-11 mutants, suggesting that SVH-11 functions between DDR-2 and SVH-2 in this glycosylation pathway. Furthermore, we found that DDR-2 is N-glycosylated at the Asn-141 residue located in its discoidin domain, and mutation of this residue caused an axon regeneration defect. These findings indicate that N-linked glycosylation plays an important role in axon regeneration in C. elegans.

The ATP-P2X7 Signaling Pathway Participates in the Regulation of Slit1 Expression in Satellite Glial Cells

Slit1 is one of the known signaling factors of the slit family and can promote neurite growth by binding to its receptor, Robo2. Upregulation of Slit1 expression in dorsal root ganglia (DRG) after peripheral nerve injury plays an important role in nerve regeneration. Each sensory neuronal soma in the DRG is encapsulated by several surrounding satellite glial cells (SGCs) to form a neural structural unit. However, the temporal and spatial patterns of Slit1 upregulation in SGCs in DRG and its molecular mechanisms are not well understood. This study examined the spatial and temporal patterns of Slit1 expression in DRG after sciatic nerve crush by immunohistochemistry and western blotting. The effect of neuronal damage signaling on the expression of Slit1 in SGCs was studied in vivo by fluorescent gold retrograde tracing and double immunofluorescence staining. The relationship between the expression of Slit1 in SGCs and neuronal somas was also observed by culturing DRG cells and double immunofluorescence labeling. The molecular mechanism of Slit1 was further explored by immunohistochemistry and western blotting after intraperitoneal injection of Bright Blue G (BBG, P2X7R inhibitor). The results showed that after peripheral nerve injury, the expression of Slit1 in the neurons and SGCs of DRG increased. The expression of Slit1 was presented with a time lag in SGCs than in neurons. The expression of Slit1 in SGCs was induced by contact with surrounding neuronal somas. Through injured cell localization, it was found that the expression of Slit1 was stronger in SGCs surrounding injured neurons than in SGCs surrounding non-injured neurons. The expression of vesicular nucleotide transporter (VNUT) in DRG neurons was increased by injury signaling. After the inhibition of P2X7R, the expression of Slit1 in SGCs was downregulated, and the expression of VNUT in DRG neurons was upregulated. These results indicate that the ATP-P2X7R pathway is involved in signal transduction from peripheral nerve injury to SGCs, leading to the upregulation of Slit1 expression.

Zebrafish is a central model to dissect the peripheral neuropathy

The peripheral nervous system (PNS) is composed with all nerves extended from the brain and spinal cord, which are the central nervous system to other organs of the body. Dysfunctional peripheral motion resulting from the regressive neuronal axons in the defected PNS leads to several peripheral neuropathies including both inherited and non-inherited disorders. Because of poor understanding of cellular and molecular mechanisms involved in the peripheral neuropathy, there is currently non-targeted treatment of the disorder. Basic researches have paid attention to dissect roles of causative genes, identified from the inherited peripheral neuropathies, in PNS development. However, recent studies focusing on investigation of therapeutic targets have suggested that successful regeneration of the impaired peripheral nerves may be most effective treatment. The regeneration studies have been limited in the rodents system due to some of practical and physiological disadvantages until zebrafish model has emerged as an ideal system. Hence, this review aims to provide a comprehensive overview of the advantages of zebrafish as a model for the peripheral neuropathy researches and to suggest the disease genes-involved potential mechanisms targeting the PNS regeneration that may be demonstrated in zebrafish.

The ubiquitin ligase PHR promotes directional regrowth of spinal zebrafish axons

To reconnect with their synaptic targets, severed axons need to regrow robustly and directionally along the pre-lesional trajectory. While mechanisms directing axonal regrowth are poorly understood, several proteins direct developmental axon outgrowth, including the ubiquitin ligase PHR (Mycbp2). Invertebrate PHR also limits regrowth of injured axons, whereas its role in vertebrate axonal regrowth remains elusive. Here we took advantage of the high regrowth capacity of spinal zebrafish axons and observed robust and directional regrowth following laser transection of spinal Mauthner axons. We found that PHR directs regrowing axons along the pre-lesional trajectory and across the transection site. At the transection site, initial regrowth of wild-type axons was multidirectional. Over time, misdirected sprouts were corrected in a PHR-dependent manner. Ablation of cyfip2, known to promote F-actin-polymerization and pharmacological inhibition of JNK reduced misdirected regrowth of PHR-deficient axons, suggesting that PHR controls directional Mauthner axonal regrowth through cyfip2- and JNK-dependent pathways.

Repair Schwann cell update: Adaptive reprogramming, EMT, and stemness in regenerating nerves

Schwann cells respond to nerve injury by cellular reprogramming that generates cells specialized for promoting regeneration and repair. These repair cells clear redundant myelin, attract macrophages, support survival of damaged neurons, encourage axonal growth, and guide axons back to their targets. There are interesting parallels between this response and that found in other tissues. At the cellular level, many other tissues also react to injury by cellular reprogramming, generating cells specialized to promote tissue homeostasis and repair. And at the molecular level, a common feature possessed by Schwann cells and many other cells is the injury-induced activation of genes associated with epithelial-mesenchymal transitions and stemness, differentiation states that are linked to cellular plasticity and that help injury-induced tissue remodeling. The number of signaling systems regulating Schwann cell plasticity is rapidly increasing. Importantly, this includes mechanisms that are crucial for the generation of functional repair Schwann cells and nerve regeneration, although they have no or a minor role elsewhere in the Schwann cell lineage. This encourages the view that selective tools can be developed to control these particular cells, amplify their repair supportive functions and prevent their deterioration. In this review, we discuss the emerging similarities between the injury response seen in nerves and in other tissues and survey the transcription factors, epigenetic mechanisms, and signaling cascades that control repair Schwann cells, with emphasis on systems that selectively regulate the Schwann cell injury response.

Dynein promotes sustained axonal growth and Schwann cell remodeling early during peripheral nerve regeneration

Following injury, axons of the peripheral nervous system have retained the capacity for regeneration. While it is well established that injury signals require molecular motors for their transport from the injury site to the nucleus, whether kinesin and dynein motors play additional roles in peripheral nerve regeneration is not well understood. Here we use genetic mutants of motor proteins in a zebrafish peripheral nerve regeneration model to visualize and define in vivo roles for kinesin and dynein. We find that both kinesin-1 and dynein are required for zebrafish peripheral nerve regeneration. While loss of kinesin-1 reduced the overall robustness of axonal regrowth, loss of dynein dramatically impaired axonal regeneration and also reduced injury-induced Schwann cell remodeling. Chimeras between wild type and dynein mutant embryos demonstrate that dynein function in neurons is sufficient to promote axonal regrowth. Finally, by simultaneously monitoring actin and microtubule dynamics in regenerating axons we find that dynein appears dispensable to initiate axonal regrowth, but is critical to stabilize microtubules, thereby sustaining axonal regeneration. These results reveal two previously unappreciated roles for dynein during peripheral nerve regeneration, initiating injury induced Schwann cell remodeling and stabilizing axonal microtubules to sustain axonal regrowth.

Nerve regeneration requires coordinated responses from multiple cell types after injury. Axons must extend from the neuronal cell body back towards their targets, while surrounding Schwann cells enter a repair cell state in which they promote regeneration. While nerves of the peripheral nervous system can regrow, it is estimated that fewer than 10 percent of patients fully recover function after nerve injury. In order to understand the mechanisms by which peripheral nerves regrow, we used live cell imaging in the zebrafish to observe the process of nerve regeneration, monitoring axons and Schwann cells simultaneously during this process. Using genetic mutants, we identified a role for the molecular motors kinesin-1 and dynein in promoting axonal regrowth. Furthermore, we found that dynein plays an additional role in Schwann cell response to injury. Thus, we demonstrate that molecular motors are required in multiple cell types to promote nerve regeneration.

Lysyl hydroxylases are transcription targets for GATA3 driving lung cancer cell metastasis

Metastasis associates with late stages of lung cancer progression and remains the main cause of patient death due to the lack of clinically effective therapeutics. Here we report that the transcription factor GATA3 and its co-factor FOG2 commonly promote the expression of the lysyl hydroxylase (LH) family members, including LH2 and LH3, which in turn drive lung adenocarcinoma cell migration, invasion, and metastasis. We show evidence that both LH2 and LH3 are direct transcription targets for GATA3. Knockdown of either LH2 or LH3 suppresses migration and invasion; on the contrary, forced expression of LH2 or LH3 promotes growth and migration, suggesting that the two LHs exert redundant oncogenic functions. Importantly, re-expression of LH2 is sufficient to restore the metastatic capacity of GATA3-depleted cells, suggesting a role for LHs as the downstream mediators of GATA3. Collectively, our data reveal a pro-metastatic GATA3-LHs axis for lung cancer, supporting the notion that targeting LHs may be useful for treating lung cancer.

Transcriptomic Landscapes of Immune Response and Axonal Regeneration by Integrative Analysis of Molecular Pathways and Interactive Networks Post-sciatic Nerve Transection

Potential interaction between immune response and axonal regeneration has recently attracted much attention in peripheral nervous system (PNS). Previously, global mRNA expression changes in proximal nerve segments were profiled and merely focused on the differentially change of the key biological processes. To further uncover molecular mechanisms of peripheral nerve regeneration, here we focused on the interaction between immune response and axonal regeneration that associated with specific molecular pathways and interactive networks following sciatic nerve transection. To offer an outline of the specific molecular pathways elaborating axonal regeneration and immune response, and to figure out the molecular interaction between immune response and axonal regeneration post-sciatic nerve transection, we carried out comprehensive approaches, including gene expression profiling plus multi-level bioinformatics analysis and then further experimental validation. Alcam, Nrp1, Nrp2, Rac1, Creb1, and Runx3 were firstly considered as the key or hub genes of the protein-protein interaction (PPI) network in rat models of sciatic nerve transection, which are highly correlated with immune response and axonal regeneration. Our work provide a new way to figure out molecular mechanism of peripheral nerve regeneration and valuable resources to figure out the molecular courses which outline neural injury-induced micro-environmental variation to discover novel therapeutic targets for axonal regeneration.

The synaptic receptor Lrp4 promotes peripheral nerve regeneration

Early during PNS regeneration, regenerating axons emerge from the proximal nerve stump, yet whether they extend simultaneously or whether pioneering axons establish a path for follower axons remains unknown. Moreover, the molecular mechanisms underlying robust regeneration are incompletely understood. Using live imaging, we demonstrate that in zebrafish pioneering axons establish a regenerative path for follower axons. We find this process requires the synaptic receptor lrp4, and in lrp4 mutants pioneers are unaffected while follower axons frequently stall at the injury gap, providing evidence for molecular diversity between pioneering and follower axons in regeneration. We demonstrate that Lrp4 promotes regeneration through an axon extrinsic mechanism and independent of membrane anchoring and MuSK co-receptor signaling essential for synaptic development. Finally, we show that Lrp4 coordinates the realignment of denervated Schwann cells with regenerating axons, consistent with a model by which Lrp4 is repurposed to promote sustained peripheral nerve regeneration via axon-glia interactions.

Design and Applications of Cell-Selective Surfaces and Interfaces

Tissue regeneration involves versatile types of cells. The accumulation and disorganized behaviors of undesired cells impair the natural healing process, leading to uncontrolled immune response, restenosis, and/or fibrosis. Cell-selective surfaces and interfaces can have specific and positive effects on desired types of cells, allowing tissue regeneration with restored structures and functions. This review outlines the importance of surfaces and interfaces of biomaterials with cell-selective properties. The chemical and biological cues including peptides, antibodies, and other molecules, physical cues such as topography and elasticity, and physiological cues referring mainly to interactions between cells-cells and cell-chemokines or cytokines are effective modulators for achieving cell selectivity upon being applied into the design of biomaterials. Cell-selective biomaterials have also shown practical significance in tissue regeneration, in particular for endothelialization, nerve regeneration, capture of stem cells, and regeneration of tissues of multiple structures and functions.

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.

It is all about the support - The role of the extracellular matrix in regenerating axon guidance

Although it is known for long time that the peripheral nervous system has the capacity for self-regeneration, the molecular mechanisms by which Schwann cells and extracellular matrix (ECM) guide the injured axons to regrow along their original path, remains a poorly understood process. Due to the importance of ECM molecules during development, constitutive mutant organisms display increased lethality, therefore, conditional or inducible strategies have been used to increase the survival of the organisms and allow the study of the role of ECM proteins. In a recent report published in Neuron, Isaacman-Beck and colleagues (2015) used these pioneering genetic studies on zebrafish combined with in vivo fluorescent imaging, to investigate the micro-environmental conditions required for targeted regeneration of the dorsal motor nerve of zebrafish larvae after laser-transection. A candidate gene approach targeting lh3 basal laminar collagen substrates revealed that the lh3 substrate col4α5 regulates dorsal nerve regeneration by destabilizing misdirected axons. Col4α5 was upregulated in a small population of lh3 expressing Schwann cells located ventrally and ventro-laterally to the injury site and found to co-localize with the molecule slit guidance ligand 1 (slit1a). Capitalizing on the crucial observations of mistargeted regeneration of dorsal nerves in mutant larvae, they put forward a model in which Schwann cells shape an environment that allows and directs axonal regeneration to their original synaptic target. In the light of Isaacman-Beck and colleagues (2015) findings, we will review how their study contributes to the research field, and comment on its potential implications for promoting nerve regeneration after injury.

Epigenetic Control of Schwann Cells

The journey of Schwann cells from their origin in the neural crest to their ensheathment and myelination of peripheral nerves is a remarkable one. Their apparent static function in enabling saltatory conduction of mature nerve is not only vital for long-term health of peripheral nerve but also belies an innate capacity of terminally differentiated Schwann cells to radically alter their differentiation status in the face of nerve injury. The transition from migrating neural crest cells to nerve ensheathment, and then myelination of large diameter axons has been characterized extensively and several of the transcriptional networks have been identified. However, transcription factors must also modify chromatin structure during Schwann cell maturation and this review will focus on chromatin modification machinery that is involved in promoting the transition to, and maintenance of, myelinating Schwann cells. In addition, Schwann cells are known to play important regenerative roles after peripheral nerve injury, and information on epigenomic reprogramming of the Schwann cell genome has emerged. Characterization of epigenomic requirements for myelin maintenance and Schwann cell responses to injury will be vital in understanding how the various Schwann cell functions can be optimized to maintain and repair peripheral nerve function.

Wnt signaling controls pro-regenerative Collagen XII in functional spinal cord regeneration in zebrafish

The inhibitory extracellular matrix in a spinal lesion site is a major impediment to axonal regeneration in mammals. In contrast, the extracellular matrix in zebrafish allows substantial axon re-growth, leading to recovery of movement. However, little is known about regulation and composition of the growth-promoting extracellular matrix. Here we demonstrate that activity of the Wnt/β-catenin pathway in fibroblast-like cells in the lesion site is pivotal for axon re-growth and functional recovery. Wnt/β-catenin signaling induces expression of col12a1a/b and deposition of Collagen XII, which is necessary for axons to actively navigate the non-neural lesion site environment. Overexpression of col12a1a rescues the effects of Wnt/β-catenin pathway inhibition and is sufficient to accelerate regeneration. We demonstrate that in a vertebrate of high regenerative capacity, Wnt/β-catenin signaling controls the composition of the lesion site extracellular matrix and we identify Collagen XII as a promoter of axonal regeneration. These findings imply that the Wnt/β-catenin pathway and Collagen XII may be targets for extracellular matrix manipulations in non-regenerating species.

Following spinal injury in zebrafish, non-neural cells establish an extracellular matrix to promote axon re-growth but how this is regulated is unclear. Here, the authors show that Wnt/β-catenin signaling in fibroblast-like cells at a lesion activates axon re-growth via deposition of Collagen XII.

Reconnecting Eye to Brain

Although much is known about the regenerative capacity of retinal ganglion cells, very significant barriers remain in our ability to restore visual function following traumatic injury or disease-induced degeneration. Here we summarize our current understanding of the factors regulating axon guidance and target engagement in regenerating axons, and review the state of the field of neural regeneration, focusing on the visual system and highlighting studies using other model systems that can inform analysis of visual system regeneration. This overview is motivated by a Society for Neuroscience Satellite meeting, "Reconnecting Neurons in the Visual System," held in October 2015 sponsored by the National Eye Institute as part of their "Audacious Goals Initiative" and co-organized by Carol Mason (Columbia University) and Michael Crair (Yale University). The collective wisdom of the conference participants pointed to important gaps in our knowledge and barriers to progress in promoting the restoration of visual system function. This article is thus a summary of our existing understanding of visual system regeneration and provides a blueprint for future progress in the field.

A small molecule screen identifies in vivo modulators of peripheral nerve regeneration in zebrafish

Adult vertebrates have retained the ability to regenerate peripheral nerves after injury, although regeneration is frequently incomplete, often leading to functional impairments. Small molecule screens using whole organisms have high potential to identify biologically relevant targets, yet currently available assays for in vivo peripheral nerve regeneration are either very laborious and/or require complex technology. Here we take advantage of the optical transparency of larval zebrafish to develop a simple and fast pectoral fin removal assay that measures peripheral nerve regeneration in vivo. Twenty-four hours after fin amputation we observe robust and stereotyped nerve regrowth at the fin base. Similar to laser mediated nerve transection, nerve regrowth after fin amputation requires Schwann cells and FGF signaling, confirming that the fin amputation assay identifies pathways relevant for peripheral nerve regeneration. From a library of small molecules with known targets, we identified 21 compounds that impair peripheral nerve regeneration. Several of these compounds target known regulators of nerve regeneration, further validating the fin removal assay. Twelve of the identified compounds affect targets not previously known to control peripheral nerve regeneration. Using a laser-mediated nerve transection assay we tested ten of those compounds and confirmed six of these compounds to impair peripheral nerve regeneration: an EGFR inhibitor, a glucocorticoid, prostaglandin D2, a retinoic acid agonist, an inhibitor of calcium channels and a topoisomerase I inhibitor. Thus, we established a technically simple assay to rapidly identify valuable entry points into pathways critical for vertebrate peripheral nerve regeneration.

Expression patterns of Slit and Robo family members in adult mouse spinal cord and peripheral nervous system

The secreted glycoproteins, Slit1-3, are classic axon guidance molecules that act as repulsive cues through their well characterised receptors Robo1-2 to allow precise axon pathfinding and neuronal migration. The expression patterns of Slit1-3 and Robo1-2 have been most characterized in the rodent developing nervous system and the adult brain, but little is known about their expression patterns in the adult rodent peripheral nervous system. Here, we report a detailed expression analysis of Slit1-3 and Robo1-2 in the adult mouse sciatic nerve as well as their expression in the nerve cell bodies within the ventral spinal cord (motor neurons) and dorsal root ganglion (sensory neurons). Our results show that, in the adult mouse peripheral nervous system, Slit1-3 and Robo1-2 are expressed in the cell bodies and axons of both motor and sensory neurons. While Slit1 and Robo2 are only expressed in peripheral axons and their cell bodies, Slit2, Slit3 and Robo1 are also expressed in satellite cells of the dorsal root ganglion, Schwann cells and fibroblasts of peripheral nerves. In addition to these expression patterns, we also demonstrate the expression of Robo1 in blood vessels of the peripheral nerves. Our work gives important new data on the expression patterns of Slit and Robo family members within the peripheral nervous system that may relate both to nerve homeostasis and the reaction of the peripheral nerves to injury.

Analysis of myelinated axon formation in zebrafish

Myelin is a lipid-rich sheath formed by the spiral wrapping of specialized glial cells around axon segments. Myelinating glia allow for rapid transmission of nerve impulses and metabolic support of axons, and the absence of or disruption to myelin results in debilitating motor, cognitive, and emotional deficits in humans. Because myelin is a jawed vertebrate innovation, zebrafish are one of the simplest vertebrate model systems to study the genetics and development of myelinating glia. The morphogenetic cellular movements and genetic program that drive myelination are conserved between zebrafish and mammals, and myelin develops rapidly in zebrafish larvae, within 3-5days postfertilization. Myelin ultrastructure can be visualized in the zebrafish from larval to adult stages via transmission electron microscopy, and the dynamic development of myelinating glial cells may be observed in vivo via transgenic reporter lines in zebrafish larvae. Zebrafish are amenable to genetic and pharmacological screens, and screens for myelinating glial phenotypes have revealed both genes and drugs that promote myelin development, many of which are conserved in mammalian glia. Recently, zebrafish have been employed as a model to understand the complex dynamics of myelinating glia during development and regeneration. In this chapter, we describe these key methodologies and recent insights into mechanisms that regulate myelination using the zebrafish model.

The multicellular complexity of peripheral nerve regeneration

Peripheral nerves show a remarkable ability to regenerate following a transection injury. Downstream of the cut, the axons degenerate and so to regenerate the nerve, the severed axons need to regrow back to their targets and regain function. This requires the axons to navigate through two different environments. (1) The bridge of new tissue that forms between the two nerve stumps and (2) the distal stump of the nerve that remains associated with the target tissues. This involves distinct, complex multicellular responses that guide and sustain axonal regrowth. These processes have important implications for our understanding of the regeneration of an adult tissue and have parallels to aspects of tumour formation and spread.