A novel myelin protein zero transgenic zebrafish designed for rapid readout of in vivo myelination

Flubendazole exposure disrupts neural development and function of zebrafish embryos (Danio rerio)

Flubendazole (FBZ) is a benzimidazole anthelmintic drug widely used for treating parasitic infections by disrupting microtubule formation and function through tubulin binding. Recently, its use has extended to include anticancer applications, leading to increased environmental exposure to benzimidazole drugs. However, the impact of FBZ on neural development in aquatic organisms, particularly in aquatic vertebrates, remains poorly understood. This study aimed to investigate the potential developmental toxicity of FBZ during neural development using zebrafish model. Various assessments, including analysis of overall developmental changes, morphological abnormalities, apoptosis, gene expression alterations, axon length measurements, and electrophysiological neural function, were performed. FBZ exposure resulted in concentration-dependent effects on survival rate, hatching rate, heartbeat, and the occurrence of developmental abnormalities. Notably, FBZ-induced changes included reductions in body length, head size, and eye size, as well as the detection of apoptotic cells in the central nervous system. Gene expression analysis revealed upregulation of apoptosis-related genes (p53, casp3, and casp8), downregulation of neural differentiation-related genes (shha, nrd, ngn1, and elavl3), and alterations in neural maturation and axon growth-related genes (gap43, mbp, and syn2a). Additionally, shortened motor neuron axon length and impaired electrophysiological neural function were observed. These findings provide novel insights into the potential risks of FBZ on the neural development of zebrafish embryos, emphasizing the need for risk prevention strategies and therapeutic approaches to address the environmental toxicity of benzimidazole anthelmintics.

Lysophosphatidic acid signaling via LPA6 : A negative modulator of developmental oligodendrocyte maturation

The developmental process of central nervous system (CNS) myelin sheath formation is characterized by well-coordinated cellular activities ultimately ensuring rapid and synchronized neural communication. During this process, myelinating CNS cells, namely oligodendrocytes (OLGs), undergo distinct steps of differentiation, whereby the progression of earlier maturation stages of OLGs represents a critical step toward the timely establishment of myelinated axonal circuits. Given the complexity of functional integration, it is not surprising that OLG maturation is controlled by a yet fully to be defined set of both negative and positive modulators. In this context, we provide here first evidence for a role of lysophosphatidic acid (LPA) signaling via the G protein-coupled receptor LPA6 as a negative modulatory regulator of myelination-associated gene expression in OLGs. More specifically, the cell surface accessibility of LPA6 was found to be restricted to the earlier maturation stages of differentiating OLGs, and OLG maturation was found to occur precociously in Lpar6 knockout mice. To further substantiate these findings, a novel small molecule ligand with selectivity for preferentially LPA6 and LPA6 agonist characteristics was functionally characterized in vitro in primary cultures of rat OLGs and in vivo in the developing zebrafish. Utilizing this approach, a negative modulatory role of LPA6 signaling in OLG maturation could be corroborated. During development, such a functional role of LPA6 signaling likely serves to ensure timely coordination of circuit formation and myelination. Under pathological conditions as seen in the major human demyelinating disease multiple sclerosis (MS), however, persistent LPA6 expression and signaling in OLGs can be seen as an inhibitor of myelin repair. Thus, it is of interest that LPA6 protein levels appear elevated in MS brain samples, thereby suggesting that LPA6 signaling may represent a potential new druggable pathway suitable to promote myelin repair in MS.

Developmental Neurotoxicity and Behavioral Screening in Larval Zebrafish with a Comparison to Other Published Results

With the abundance of chemicals in the environment that could potentially cause neurodevelopmental deficits, there is a need for rapid testing and chemical screening assays. This study evaluated the developmental toxicity and behavioral effects of 61 chemicals in zebrafish (Danio rerio) larvae using a behavioral Light/Dark assay. Larvae (n = 16–24 per concentration) were exposed to each chemical (0.0001–120 μM) during development and locomotor activity was assessed. Approximately half of the chemicals (n = 30) did not show any gross developmental toxicity (i.e., mortality, dysmorphology or non-hatching) at the highest concentration tested. Twelve of the 31 chemicals that did elicit developmental toxicity were toxic at the highest concentration only, and thirteen chemicals were developmentally toxic at concentrations of 10 µM or lower. Eleven chemicals caused behavioral effects; four chemicals (6-aminonicotinamide, cyclophosphamide, paraquat, phenobarbital) altered behavior in the absence of developmental toxicity. In addition to screening a library of chemicals for developmental neurotoxicity, we also compared our findings with previously published results for those chemicals. Our comparison revealed a general lack of standardized reporting of experimental details, and it also helped identify some chemicals that appear to be consistent positives and negatives across multiple laboratories.

Insights Into Central Nervous System Glial Cell Formation and Function From Zebrafish

The term glia describes a heterogenous collection of distinct cell types that make up a large proportion of our nervous system. Although once considered the glue of the nervous system, the study of glial cells has evolved significantly in recent years, with a large body of literature now highlighting their complex and diverse roles in development and throughout life. This progress is due, in part, to advances in animal models in which the molecular and cellular mechanisms of glial cell development and function as well as neuron-glial cell interactions can be directly studied in vivo in real time, in intact neural circuits. In this review we highlight the instrumental role that zebrafish have played as a vertebrate model system for the study of glial cells, and discuss how the experimental advantages of the zebrafish lend themselves to investigate glial cell interactions and diversity. We focus in particular on recent studies that have provided insight into the formation and function of the major glial cell types in the central nervous system in zebrafish.

Small molecule screening as an approach to encounter inefficient myelin repair

While current multiple sclerosis therapies are focused on immunomodulation, thereby slowing down disease progression, scientific interest has nowadays been shifted toward regenerative therapies aiming at reversing already existing deficits. The application of chemical compounds was proven to be valuable for the understanding of oligodendrogenesis and for exposing mechanisms that can boost remyelination. However, sufficient myelin repair has not been achieved yet, thus underscoring the need for more studies toward this unmet clinical goal. In this regard, many research groups have significantly contributed to the field via developing compound screening approaches or using single substances. We, here, present an overview of recent studies addressing the identification of myelin repair drugs and provide insights into technical aspects and identified substances.

Application and advantages of zebrafish model in the study of neurovascular unit

The concept of "Neurovascular Unit" (NVU) was put forward, so that the research goal of Central Nervous System (CNS) diseases gradually transitioned from a single neuron to the structural and functional integrity of the NVU. Zebrafish has the advantages of high homology with human genes, strong reproductive capacity and visualization of neural circuits, so it has become an emerging model organism for NVU research and has been applied to a variety of CNS diseases. Based on CNKI (https://www.cnki.net/) and PubMed (https://pubmed.ncbi.nlm.nih.gov/about/) databases, the author of this article sorted out the relevant literature, analyzed the construction of a zebrafish model of various CNS diseases，and the use of diagrams showed the application of zebrafish in the NVU, revealed its relationship, which would provide new methods and references for the treatment and research of CNS diseases.

The Akt-mTOR Pathway Drives Myelin Sheath Growth by Regulating Cap-Dependent Translation

In the vertebrate CNS, oligodendrocytes produce myelin, a specialized membrane, to insulate and support axons. Individual oligodendrocytes wrap multiple axons with myelin sheaths of variable lengths and thicknesses. Myelin grows at the distal ends of oligodendrocyte processes, and multiple lines of work have provided evidence that mRNAs and RNA binding proteins localize to myelin, together supporting a model where local translation controls myelin sheath growth. What signal transduction mechanisms could control this? One strong candidate is the Akt-mTOR pathway, a major cellular signaling hub that coordinates transcription, translation, metabolism, and cytoskeletal organization. Here, using zebrafish as a model system, we found that Akt-mTOR signaling promotes myelin sheath growth and stability during development. Through cell-specific manipulations to oligodendrocytes, we show that the Akt-mTOR pathway drives cap-dependent translation to promote myelination and that restoration of cap-dependent translation is sufficient to rescue myelin deficits in mTOR loss-of-function animals. Moreover, an mTOR-dependent translational regulator was phosphorylated and colocalized with mRNA encoding a canonically myelin-translated protein in vivo, and bioinformatic investigation revealed numerous putative translational targets in the myelin transcriptome. Together, these data raise the possibility that Akt-mTOR signaling in nascent myelin sheaths promotes sheath growth via translation of myelin-resident mRNAs during development.SIGNIFICANCE STATEMENT In the brain and spinal cord, oligodendrocytes extend processes that tightly wrap axons with myelin, a protein- and lipid-rich membrane that increases electrical impulses and provides trophic support. Myelin membrane grows dramatically following initial axon wrapping in a process that demands protein and lipid synthesis. How protein and lipid synthesis is coordinated with the need for myelin to be generated in certain locations remains unknown. Our study reveals that the Akt-mTOR signaling pathway promotes myelin sheath growth by regulating protein translation. Because we found translational regulators of the Akt-mTOR pathway in myelin, our data raise the possibility that Akt-mTOR activity regulates translation in myelin sheaths to deliver myelin on demand to the places it is needed.

A tuned gelatin methacryloyl (GelMA) hydrogel facilitates myelination of dorsal root ganglia neurons in vitro

Investigating axonal myelination by Schwann cells (SCs) is crucial for understanding mechanisms underlying demyelination and remyelination, which may help gain insights into incurable disorders like neurodegenerative diseases. In this study, a gelatin-based hydrogel, gelatin methacryloyl (GelMA), was optimized to achieve the biocompatibility, porosity, mechanical stability, and degradability needed to provide high cell viability for dorsal root ganglia (DRG) neurons and SCs, and to enable their long-term coculture needed for myelination studies. The results of cell viability, neurite elongation, SC function and maturation, SC-axon interaction, and myelination were compared with two other commonly used substrates, namely collagen and Poly-d Lysine (PDL). The tuned GelMA constructs (Young's modulus of 32.6 ± 1.9 kPa and the median value of pore size of 10.3 μm) enhanced single axon generation (unlike collagen) and promoted the interaction of DRG neurons and SCs (unlike PDL). While DRG cells exhibited relatively higher viability on PDL after 48 h, i.e., 83.8%, the cells had similar survival rate on GelMA and collagen substrates, 66.7% and 61.5%, respectively. Further adjusting the hydrogel properties to achieve two distinct ranges of relatively small and large pores supported SCs to extend their processes freely and enabled physical contact with and wrapping around their corresponding axons. Staining the cells with myelin basic protein (MBA) and myelin-associated glycoprotein (MAG) revealed enhanced myelination on GelMA hydrogel compared to PDL and collagen. Moreover, the engineered porosity enhanced DRGs and SCs attachments and flexibility of movement across the substrate. This engineered hydrogel structure can now be further explored to model demyelination in neurodegenerative diseases, as well as to study the effects of various compounds on myelin regeneration.

Jinmaitong ameliorates diabetes-induced peripheral neuropathy in rats through Wnt/β-catenin signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Jinmaitong (JMT) is a prescription of Traditional Chinese Medicine, which is composed of ten herbal drugs and two animal drugs. It has long been used for the treatment of diabetic peripheral neuropathy (DPN).

AIM OF STUDY

Wnt/β-catenin pathway is considered as an essential and direct driver of myelinogenesis. This study aims to evaluate the protective effect of JMT against DPN dynamically during a 16-weeks' treatment, and to investigate the underlying mechanism in which the Wnt/β-catenin pathway is involved.

MATERIALS AND METHODS

Diabetic model was induced by single intraperitoneal injection of Streptozotocin (STZ) using male Sprague-Dawley rats. The model rats were divided into five groups and administrated with JMT at three doses (0.437, 0.875, and 1.75 g/kg per day), neurotropin (positive drug, 2.67 NU/kg per day), and placebo (deionized water), respectively, for continuous 8 weeks (n = 9-10), 12 weeks (n = 8-10), or 16 weeks (n = 7-9). Meanwhile, rats in control group were administrated with placebo (n = 10 for 8 weeks, n = 9 for 12 and 16 weeks, respectively). Blood glucose and body weight were monitored every four weeks. Mechanical allodynia was assessed using mechanical withdrawal threshold (MWT) test. The morphological change of sciatic nerves were observed by transmission electron microscope (TEM) and hematoxylin and eosin (HE) stain. The mRNA and protein levels of targeted genes were evaluated by quantitative real time-PCR and western bolt, respectively. Myelin protein zero (MPZ) and mediators involved in Wnt/β-catenin pathway, such as β-catenin, glycogen synthase kinase 3β (GSK-3β), and WNT inhibitory factor-1 (WIF-1), were compared among different groups after treatment of 8, 12, and 16 weeks, respectively.

RESULTS

The mechanical allodynia and peripheral nerve morphology were degenerated in DPN rats over time, and notably improved after JMT-treatment of 12 and 16 weeks. The decreased MPZ level in DPN rats were also significantly amended by JMT. More importantly, we found that the suppressed Wnt/β-catenin pathway in sciatic nerves of DPN rats was overtly up-regulated by JMT in a time-dependent manner. Among the three doses, JMT at the middle dose showed the best effect.

CONCLUSIONS

JMT effectively ameliorated diabetic-induced peripheral neuropathy, which was mediated by the activation of Wnt/β-catenin signaling pathway. This study provided new perspective to understand the neuroprotective mechanism of JMT.

Spinal cord precursors utilize neural crest cell mechanisms to generate hybrid peripheral myelinating glia

During development, oligodendrocytes and Schwann cells myelinate central and peripheral nervous system axons, respectively, while motor exit point (MEP) glia are neural tube-derived, peripheral glia that myelinate axonal territory between these populations at MEP transition zones. From which specific neural tube precursors MEP glia are specified, and how they exit the neural tube to migrate onto peripheral motor axons, remain largely unknown. Here, using zebrafish, we found that MEP glia arise from lateral floor plate precursors and require foxd3 to delaminate and exit the spinal cord. Additionally, we show that similar to Schwann cells, MEP glial development depends on axonally derived neuregulin1. Finally, our data demonstrate that overexpressing axonal cues is sufficient to generate additional MEP glia in the spinal cord. Overall, these studies provide new insight into how a novel population of hybrid, peripheral myelinating glia are generated from neural tube precursors and migrate into the periphery.

Celecoxib Exerts a Therapeutic Effect Against Demyelination by Improving the Immune and Inflammatory Microenvironments

Background

The myelin sheath can be damaged by genetic and/or environmental factors, leading to demyelinating diseases, for which effective treatments are lacking. Recently, cyclooxygenase-2 (COX-2) overexpression was detected in demyelinating lesions both in patients and animal models, opening an avenue for promoting endogenous remyelination. The aim of this study was to investigate the therapeutic effect of celecoxib, a selective COX-2 inhibitor, against demyelination in a zebrafish model.

Methods

The biotoxicity of celecoxib was evaluated on zebrafish embryos. Metronidazole was used to deplete the oligodendrocytes in Tg (mbp:nfsB-egfp) transgenic fish. Celecoxib was then administered both in larvae and adults. The regeneration of the myelin sheath and the underlying mechanisms were explored by immunohistochemistry, flow cytometry, Western blot analysis, quantitative real-time polymerase chain reaction, and behavioral test.

Results

Celecoxib had low in vivo toxicity. A stable and practical demyelination model was established by metronidazole induction. Following celecoxib treatment, the number of oligodendrocytes was increased significantly and the concentric structure of the myelin sheath reappeared. The locomotor ability was notably improved and was close to its physiological levels. The expression of arg1, mrc1, il-10, and il-4 was upregulated, while that of il-1β, il-12, tnf-α, il-6, caspase-3 and caspase-7 was downregulated.

Conclusion

Inhibition of COX-2 contributed to the transformation of microglia/macrophages from the M1 to the M2 phenotype, improved the inflammatory microenvironment, and suppressed caspase-dependent apoptosis, thus exerting a therapeutic effect against demyelination.

Myelination during fracture healing in vivo in myelin protein zero (p0) transgenic medaka line

During the fracture healing process, osteoblasts and osteoclasts, as well as the nervous system are known to play important roles for signaling in the body. Glia cells contribute to the healing process by myelination, which can increase the speed of signals transmitted between neurons. However, the behavior of myelinating cells at a fracture site remains unclear. We developed a myelin protein zero (mpz)-EGFP transgenic medaka line for tracing myelinating cells. Mpz-enhanced green fluorescence protein (EGFP)-positive (mpz⁺) cells are driven by the 2.9-kb promoter of the medaka mpz gene, which is distributed throughout the nervous system, such as the brain, spinal cord, lateral line, and peripheral nerves. In the caudal fin region, mpz⁺ cells were found localized parallel with the fin ray (bone) in the adult stage. mpz⁺ cells were not distributed with fli-DsRed positive (fli⁺) blood vessels, but with some nerve fibers, and were dyed with the anti-acetylated tubulin antibody. We then fractured one side of the caudal lepidotrichia in a caudal fin of mpz-EGFP medaka and found a unique phenomenon, in that mpz⁺ cells were accumulated at 1 bone away from the fracture site. This mpz⁺ cell accumulation phenomenon started from 4 days after fracture of the proximal bone. Thereafter, mpz⁺ cells became elongated from the proximal bone to the distal bone and finally showed a crosslink connection crossing the fracture site to the distal bone at 28 days after fracture. Finally, the effects of rapamycin, known as a mTOR inhibitor, on myelination was examined. Rapamycin treatment of mpz-EGFP/osterix-DsRed double transgenic medaka inhibited not only the crosslink connection of mpz⁺ cells but also osterix⁺ osteoblast accumulation at the fracture site, accompanied with a fracture healing defect. These findings indicated that mTOR signaling plays important roles in bone formation and neural networking during fracture healing. Taken together, the present results are the first to show the dynamics of myelinating cells in vivo.

Mechanistic Target of Rapamycin Regulates the Oligodendrocyte Cytoskeleton during Myelination

During differentiation, oligodendrocyte precursor cells (OPCs) extend a network of processes that make contact with axons and initiate myelination. Recent studies revealed that actin polymerization is required for initiation of myelination whereas actin depolymerization promotes myelin wrapping. Here, we used primary OPCs in culture isolated from neonatal rat cortices of both sexes and young male and female mice with oligodendrocyte-specific deletion of mechanistic target of rapamycin (mTOR) to demonstrate that mTOR regulates expression of specific cytoskeletal targets and actin reorganization in oligodendrocytes during developmental myelination. Loss or inhibition of mTOR reduced expression of profilin2 and ARPC3, actin polymerizing factors, and elevated levels of active cofilin, which mediates actin depolymerization. The deficits in actin polymerization were revealed in reduced phalloidin and deficits in oligodendrocyte cellular branching complexity at the peak of morphologic differentiation and a delay in initiation of myelination. We further show a critical role for mTOR in expression and localization of myelin basic protein (Mbp) mRNA and MBP protein to the cellular processes where it is necessary at the myelin membrane for axon wrapping. Mbp mRNA transport deficits were confirmed by single molecule RNA FISH. Moreover, expression of the kinesin family member 1B, an Mbp mRNA transport protein, was reduced in CC1+ cells in the mTOR cKO and in mTOR inhibited oligodendrocytes undergoing differentiation in vitro These data support the conclusion that mTOR regulates both initiation of myelination and axon wrapping by targeting cytoskeletal reorganization and MBP localization to oligodendrocyte processes.SIGNIFICANCE STATEMENT Myelination is essential for normal CNS development and adult axon preservation and function. The mechanistic target of rapamycin (mTOR) signaling pathway has been implicated in promoting CNS myelination; however, there is a gap in our understanding of the mechanisms by which mTOR promotes developmental myelination through regulating specific downstream targets. Here, we present evidence that mTOR promotes the initiation of myelination through regulating specific cytoskeletal targets and cellular process expansion by oligodendrocyte precursor cells as well as expression and cellular localization of myelin basic protein.

In vivo identification of small molecules mediating Gpr126/Adgrg6 signaling during Schwann cell development

Gpr126/Adgrg6, an adhesion family G protein-coupled receptor (aGPCR), is required for the development of myelinating Schwann cells in the peripheral nervous system. Myelin supports and insulates vertebrate axons to permit rapid signal propagation throughout the nervous system. In mammals and zebrafish, mutations in Gpr126 arrest Schwann cells at early developmental stages. We exploited the optical and pharmacological tractability of larval zebrafish to uncover drugs that mediate myelination by activating Gpr126 or functioning in parallel. Using a fluorescent marker of mature myelinating glia (Tg[mbp:EGFP-CAAX]), we screened hypomorphic gpr126 mutant larvae for restoration of myelin basic protein (mbp) expression along peripheral nerves following small molecule treatment. Our screens identified five compounds sufficient to promote mbp expression in gpr126 hypomorphs. Using an allelic series of gpr126 mutants, we parsed the ability of small molecules to restore mbp, suggesting differences in drug efficacy dependent on Schwann cell developmental state. Finally, we identify apomorphine hydrochloride as a direct small molecule activator of Gpr126 using combined in vivo/in vitro assays and show that aporphine class compounds promote Schwann cell development in vivo. Our results demonstrate the utility of in vivo screening for aGPCR modulators and identify small molecules that interact with the gpr126-mediated myelination program.