An injury-induced serotonergic neuron subpopulation contributes to axon regrowth and function restoration after spinal cord injury in zebrafish

Pharmacological targeting cGAS/STING/NF-κB axis by tryptanthrin induces microglia polarization toward M2 phenotype and promotes functional recovery in a mouse model of spinal cord injury

JOURNAL/nrgr/04.03/01300535-202511000-00031/figure1/v/2024-12-20T164640Z/r/image-tiff The M1/M2 phenotypic shift of microglia after spinal cord injury plays an important role in the regulation of neuroinflammation during the secondary injury phase of spinal cord injury. Regulation of shifting microglia polarization from M1 (neurotoxic and proinflammatory type) to M2 (neuroprotective and anti-inflammatory type) after spinal cord injury appears to be crucial. Tryptanthrin possesses an anti-inflammatory biological function. However, its roles and the underlying molecular mechanisms in spinal cord injury remain unknown. In this study, we found that tryptanthrin inhibited microglia-derived inflammation by promoting polarization to the M2 phenotype in vitro . Tryptanthrin promoted M2 polarization through inactivating the cGAS/STING/NF-κB pathway. Additionally, we found that targeting the cGAS/STING/NF-κB pathway with tryptanthrin shifted microglia from the M1 to M2 phenotype after spinal cord injury, inhibited neuronal loss, and promoted tissue repair and functional recovery in a mouse model of spinal cord injury. Finally, using a conditional co-culture system, we found that microglia treated with tryptanthrin suppressed endoplasmic reticulum stress-related neuronal apoptosis. Taken together, these results suggest that by targeting the cGAS/STING/NF-κB axis, tryptanthrin attenuates microglia-derived neuroinflammation and promotes functional recovery after spinal cord injury through shifting microglia polarization to the M2 phenotype.

Blueprints for healing: central nervous system regeneration in zebrafish and neonatal mice

In adult mammals, including humans, neurons, and axons in the brain and spinal cord are inherently incapable of regenerating after injury. Studies of animals with innate capacity for regeneration are providing valuable insights into the mechanisms driving tissue healing. The aim of this review is to summarize recent data on regeneration mechanisms in the brain and spinal cord of zebrafish and neonatal mice. We infer that elucidating these mechanisms and understanding how and why they are lost in adult mammals will contribute to the development of strategies to promote central nervous system regeneration.

Comparison of Spinal Cord Regeneration Capacity in Zebrafish and Medaka

In mammals, spinal cord injury often results in permanent impairment of motor function owing to ineffective tissue regeneration. Unlike mammals, zebrafish have the remarkable ability to regenerate many tissues, including the spinal cord. Cross-species comparison is an attractive approach for revealing regeneration-specific mechanisms, but the large evolutionary distance between species sometimes hinders direct comparison. Recent studies have revealed that another model fish species, medaka, has a low regenerative ability in some tissues, making comparisons with them advantageous to revealing regeneration-specific mechanisms. However, their spinal cord regenerative ability has not been compared to other models. In this study, we functionally and histologically compared the spinal cord regeneration abilities of zebrafish and medaka. Swimming speed recovery was significantly lower in medaka than in zebrafish. Bridging of glia and neural tissue were thinner in medaka than in zebrafish. Axonal extension across the injured site was observed in zebrafish but not in medaka. Comparison of their gene expression profiles revealed genes involved in "Regeneration" were upregulated in zebrafish, whereas genes related to "Synaptic signaling" were downregulated in medaka. These results suggest that the ability to regenerate the spinal cord is lower in medaka than in zebrafish, making medaka an attractive model for revealing the mechanisms of spinal cord regeneration.

Harnessing spinal circuit reorganization for targeted functional recovery after spinal cord injury

Spinal cord injury (SCI) disrupts the communication between the brain and spinal cord, resulting in the loss of motor function below the injury site. However, spontaneous structural and functional plasticity occurs in neural circuits after SCI, with unaffected synaptic inputs forming new connections and detour pathways to support recovery. The review discusses various mechanisms of circuit reorganization post-SCI, including supraspinal pathways, spinal interneurons, and spinal central pattern generators. Functional recovery may rely on maintaining a balance between excitatory and inhibitory neural activity, as well as enhancing proprioceptive input, which plays a key role in limb stability. The review emphasizes the importance of endogenous neuronal regeneration, neuromodulation therapies (such as electrical stimulation) and proprioception in SCI treatment. Future research should integrate advanced technologies such as gene targeting, imaging, and single-cell mapping to better understand the mechanisms underpinning SCI recovery, aiming to identify key neuronal subpopulations for targeted reconstruction and enhanced functional recovery. By harnessing spinal circuit reorganization, these efforts hold the potential to pave the way for more precise and effective strategies for functional recovery after SCI.

Novel nerve regeneration assessment method using adult zebrafish with crush spinal cord injury

Zebrafish (Danio rerio), an alternative to rodents, are widely used in neurological, genetic, and toxicology research. The zebrafish larval spinal cord injury model has been used in neural mechanistic analyses owing to its high regenerative capacity and throughput; however, it also had several limitations in imitating rodents. Therefore, we investigated the use of adult zebrafish as an alternative model to rodents for evaluating nerve regeneration. Here, we established a novel spinal cord regeneration evaluation method, which was based on the maximum swimming speed of adult zebrafish in a custom-built hydrodynamic-based aquarium. The spinal cords of adult male zebrafish were crushed using forceps, and maximum swimming speed and histological spinal cord regeneration were evaluated. Spinal cord-injured zebrafish showed a significant decline in motor function, followed by recovery at 3 weeks postoperatively, accompanied by histological regeneration. Spinal cord regeneration can be indirectly assessed by monitoring maximum swimming speed. They were also fed a diet containing fig extract, which can promote peripheral nerve regeneration; they were fed daily starting 1 week before the operation. Maximum swimming speed was measured time-dependently until 3 weeks postoperatively. Fig-consuming fish showed improved recovery of maximum swimming speed compared to the controls, which was consistent with the histological analysis. In summary, we established a spinal cord regeneration assessment system using adult zebrafish in a customized aquarium, which enables researchers to evaluate spinal cord regeneration in adult zebrafish similar to that of rodent experiments, contributing to faster and easier screening of neuroregenerative technology.

MiR-124 Delivered by Extracellular Vesicles from Mesenchymal Stem Cell Exerts Neuroprotective Effects by Stabilizing the p62-Keap1-Nrf2 Pathway after Spinal Cord Injury in Rats

Spinal cord injury (SCI) can cause irreversible trauma to nervous tissue, leading to permanent damage to the patient's motor and sensory functions. Extracellular vesicles derived from mesenchymal stem cells (MSC-EVs) can simulate most of the functions of MSCs and are considered an ideal treatment option for SCI. However, the potential mechanism of MSC-EVs treatment for SCI still needs to be explored. We cultured neurons in vitro to investigate the effect of miR-124 on the p62-Keap1-Nrf2 pathway. Besides, MSC-EVs containing miR-124 were injected into a rat spinal cord injury model to observe their neural repair effect. The accumulation of p62 can be reversed by miR-124, which promotes autophagy and alleviates oxidative stress, thereby exerting neuroprotective effects. Rats who received injection of MSC-EVs overexpressing miR-124 after surgery showed higher BBB scores, lower levels of cell apoptosis, and better spinal cord tissue morphology. Our results indicated that miR-124 can stabilize the p62-Keap1-Nrf2 loop, thereby promoting autophagy and alleviating oxidative stress to exert neuroprotective effects. Our research proposes a novel potential target for treating SCI.

Exercise therapy facilitates neural remodeling and functional recovery post-spinal cord injury via PKA/CREB signaling pathway modulation in rats

Background

Neuronal structure is disrupted after spinal cord injury (SCI), causing functional impairment. The effectiveness of exercise therapy (ET) in clinical settings for nerve remodeling post-SCI and its underlying mechanisms remain unclear. This study aims to explore the effects and related mechanisms of ET on nerve remodeling in SCI rats.

Methods

We randomly assigned rats to various groups: sham-operated group, sham-operated + ET, SCI alone, SCI + H89, SCI + ET, and SCI + ET + H89. Techniques including motor-evoked potential (MEP), video capture and analysis, the Basso-Beattie-Bresnahan (BBB) scale, western blotting, transmission electron microscopy, hematoxylin and eosin staining, Nissl staining, glycine silver staining, immunofluorescence, and Golgi staining were utilized to assess signal conduction capabilities, neurological deficits, hindlimb performance, protein expression levels, neuron ultrastructure, and tissue morphology. H89-an inhibitor that targets the protein kinase A (PKA)/cAMP response element-binding (CREB) signaling pathway-was employed to investigate molecular mechanisms.

Results

This study found that ET can reduce neuronal damage in rats with SCI, protect residual tissue, promote the remodeling of motor neurons, neurofilaments, dendrites/axons, synapses, and myelin sheaths, reorganize neural circuits, and promote motor function recovery. In terms of mechanism, ET mainly works by mediating the PKA/CREB signaling pathway in neurons.

Conclusions

Our findings indicated that: (1) ET counteracted the H89-induced suppression of the PKA/CREB signaling pathway following SCI; (2) ET significantly alleviated neuronal injury and improved motor dysfunction; (3) ET facilitated neuronal regeneration by mediating the PKA/CREB signaling pathway; (4) ET enhanced synaptic and dendritic spine plasticity, as well as myelin sheath remodeling, post-SCI through the PKA/CREB signaling pathway.

Repetitive Transcranial Magnetic Stimulation for the Treatment of Spinal Cord Injury: Current Status and Perspective

Spinal cord injury (SCI) can lead to devastating dysfunctions and complications, significantly impacting patients' quality of life and aggravating the burden of disease. Since the main pathological mechanism of SCI is the disruption of neuronal circuits, the primary therapeutic strategy for SCI involves reconstructing and activating circuits to restore neural signal transmission. Repetitive transcranial magnetic stimulation (rTMS), a noninvasive brain stimulation technique, can modulate the function or state of the nervous system by pulsed magnetic fields. Here, we discuss the basic principles and potential mechanisms of rTMS for treating SCI, including promoting the reconstruction of damaged circuits in the spinal cord, activating reorganization of the cerebral cortex and circuits, modulating the balance of inputs to motoneurons, improving the microenvironment and intrinsic regeneration ability in SCI. Based on these mechanisms, we provide an overview of the therapeutic effects of rTMS in SCI patients with motor dysfunction, spasticity and neuropathic pain. We also discuss the challenges and prospectives of rTMS, especially the potential of combination therapy of rTMS and neural progenitor cell transplantation, and the synergistic effects on promoting regeneration, relay formation and functional connectivity. This review is expected to offer a relatively comprehensive understanding and new perspectives of rTMS for SCI treatment.

Can minor trauma cause asymptomatic cervical spinal cord compression leading to severe cervical spinal cord injury in rats?

The study aimed to determine whether asymptomatic rats with cervical spinal cord compression (CSCC) experience more severe cervical spinal cord injury (SCI) compared with rats without CSCC under the same degree of minor trauma. Four weeks after the polyvinyl alcohol-polyacrylamide hydrogel was implanted into the C5 vertebral canal, asymptomatic rats were selected based on locomotor function score. Mild cervical SCI was subsequently established based on CSCC. The motor function, morphology, neuron loss, myelin destruction, nerve cell apoptosis, microglia activation, and neuroinflammation were evaluated after SCI. Under the same injury conditions, rats in the CSCC group exhibited more severe motor dysfunction compared with those without CSCC. Similarly, asymptomatic CSCC rats showed significant damage to spinal cord tissue, neurons, and myelin. Finally, compared with rats without CSCC, asymptomatic CSCC rats experienced increased nerve cell apoptosis, microglial activation, and neuroinflammation following the same SCI. In asymptomatic CSCC rats, the same degree of minor trauma resulted in more severe cervical SCI compared with rats without CSCC. This was evidenced by increased nerve cell apoptosis, microglial activation, neuron death, myelin destruction, and a strong neuroinflammatory response, leading to severe motor dysfunction.

Effects of age on the response to spinal cord injury: optimizing the larval zebrafish model

Zebrafish are an increasingly popular model to study regeneration after spinal cord injury (SCI). The transparency of larval zebrafish makes them ideal to study cellular processes in real time. Standardized approaches, including age at the time of injury, are not readily available making comparisons of the results with other models challenging. In this study, we systematically examined the response to spinal cord transection of larval zebrafish at three different larval ages (3-, 5-, or 7-days post fertilization (dpf)) to determine whether the developmental complexity of the larvae affects the overall response to SCI. We then used imaging and behavioral analysis to evaluate whether differences existed based on the age of injury. Injury led to increased expression of cytokines associated with the immune response; however, we found that the timing of specific inflammatory markers changed with the age of the injury. We also observed changes in glial and axonal bridging with age. Young larvae (3 dpf) were better able to regenerate axons independent of the glial bridge, unlike older larvae (7 dpf), consistent with results seen in adult zebrafish. Finally, locomotor experiments demonstrated that some swimming behavior occurs independent of glial bridge formation, further highlighting the need for standardization of this model and functional recovery assays. Overall, we found differences based on the age of transection in larval zebrafish, underlining the importance of considering age when designing experiments aimed at understanding regeneration.

Identifying signature genes and their associations with immune cell infiltration in spinal cord injury

Background

Early detection of spinal cord injury (SCI) is conducive to improving patient outcomes. In addition, many studies have revealed the role of immune cells in the progression or treatment of SCI. The objective of this study was to identify the early signature genes and clarify how they are related to immune cell infiltration in SCI.

Methods

We analysed and identified early signature genes associated with SCI via bioinformatics analysis of the GSE151371 dataset from the GEO database. These genes were subsequently verified in the GSE33886 dataset and qRT-PCR. Finally, the CIBERSORT algorithm was used to examine the immune cell infiltration in SCI and its relationship with signature genes.

Results

Seven SCI-related signature genes, including ARG1, RETN, BPI, GGH, CCNB1, HIST1H2AC, and HIST1H2BJ, were identified, and their expression was verified via an external validation cohort and qRT-PCR. Moreover, the ROC curves revealed the diagnostic value of these genes. In addition, on the basis of immune cell infiltration analysis, plasma cells, M0 macrophages, activated CD4+ memory T cells, γδ T cells, naive CD4+ T cells, and resting CD4+ memory T cells may participate in the progression of SCI.

Conclusion

This study identified seven early signature genes of SCI that may serve as biomarkers for the early diagnosis of SCI and contribute to our understanding of immune changes during the pathology of SCI.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Single-cell analysis of innate spinal cord regeneration identifies intersecting modes of neuronal repair

Adult zebrafish have an innate ability to recover from severe spinal cord injury. Here, we report a comprehensive single nuclear RNA sequencing atlas that spans 6 weeks of regeneration. We identify cooperative roles for adult neurogenesis and neuronal plasticity during spinal cord repair. Neurogenesis of glutamatergic and GABAergic neurons restores the excitatory/inhibitory balance after injury. In addition, a transient population of injury-responsive neurons (iNeurons) show elevated plasticity 1 week post-injury. We found iNeurons are injury-surviving neurons that acquire a neuroblast-like gene expression signature after injury. CRISPR/Cas9 mutagenesis showed iNeurons are required for functional recovery and employ vesicular trafficking as an essential mechanism that underlies neuronal plasticity. This study provides a comprehensive resource of the cells and mechanisms that direct spinal cord regeneration and establishes zebrafish as a model of plasticity-driven neural repair.

Serotonin neuromodulation directs optic nerve regeneration

Optic nerve (ON) regeneration in mammalian systems is limited by an overshadowing dominance of inhibitory factors. This has severely hampered the identification of pro-regenerative pathways. Here, we take advantage of the regenerative capacity of larval zebrafish to identify pathways that promote ON regeneration. From a small molecule screen, we identified modulators of serotonin (5-HT) signaling that inhibit ON regeneration. We find several serotonin type-1 receptor genes are expressed in RGC neurons during regeneration and that inhibiting 5-HT1 receptors or components of the 5-HT pathway selectively impedes ON regeneration. We show that 5-HT1 receptor signaling is dispensable during ON development yet is critical for regenerating axons to emerge from the injury site. Blocking 5-HT receptors once ON axons have crossed the chiasm does not inhibit regeneration, suggesting a selective role for 5-HT receptor signaling early during ON regeneration. Finally, we show that agonist-mediated activation of 5-HT1 receptors leads to enhanced and ectopic axonal regrowth. Combined, our results provide evidence for mechanisms through which serotonin-dependent neuromodulation directs ON regeneration in vivo.

AAV6 mediated Gsx1 expression in neural stem progenitor cells promotes neurogenesis and restores locomotor function after contusion spinal cord injury

Genomic screened homeobox 1 (Gsx1 or Gsh1) is a neurogenic transcription factor required for the generation of excitatory and inhibitory interneurons during spinal cord development. In the adult, lentivirus (LV) mediated Gsx1 expression promotes neural regeneration and functional locomotor recovery in a mouse model of lateral hemisection spinal cord injury (SCI). The LV delivery method is clinically unsafe due to insertional mutations to the host DNA. In addition, the most common clinical case of SCI is contusion/compression. In this study, we identify that adeno-associated virus serotype 6 (AAV6) preferentially infects neural stem/progenitor cells (NSPCs) in the injured spinal cord. Using a rat model of contusion SCI, we demonstrate that AAV6 mediated Gsx1 expression promotes neurogenesis, increases the number of neuroblasts/immature neurons, restores excitatory/inhibitory neuron balance and serotonergic neuronal activity through the lesion core, and promotes locomotor functional recovery. Our findings support that AAV6 preferentially targets NSPCs for gene delivery and confirmed Gsx1 efficacy in clinically relevant rat model of contusion SCI.

The raphe nuclei are the early lesion site of gastric α-synuclein propagation to the substantia nigra

Parkinson's disease (PD) is a neurodegeneration disease with α-synuclein accumulated in the substantia nigra pars compacta (SNpc) and most of the dopaminergic neurons are lost in SNpc while patients are diagnosed with PD. Exploring the pathology at an early stage contributes to the development of the disease-modifying strategy. Although the "gut-brain" hypothesis is proposed to explain the underlying mechanism, where the earlier lesioned site in the brain of gastric α-synuclein and how α-synuclein further spreads are not fully understood. Here we report that caudal raphe nuclei (CRN) are the early lesion site of gastric α-synuclein propagating through the spinal cord, while locus coeruleus (LC) and substantia nigra pars compacta (SNpc) were further affected over a time frame of 7 months. Pathological α-synuclein propagation via CRN leads to neuron loss and disordered neuron activity, accompanied by abnormal motor and non-motor behavior. Potential neuron circuits are observed among CRN, LC, and SNpc, which contribute to the venerability of dopaminergic neurons in SNpc. These results show that CRN is the key region for the gastric α-synuclein spread to the midbrain. Our study provides valuable details for the "gut-brain" hypothesis and proposes a valuable PD model for future research on early PD intervention.

Serotonergic modulation of vigilance states in zebrafish and mice

Vigilance refers to being alertly watchful or paying sustained attention to avoid potential threats. Animals in vigilance states reduce locomotion and have an enhanced sensitivity to aversive stimuli so as to react quickly to dangers. Here we report that an unconventional 5-HT driven mechanism operating at neural circuit level which shapes the internal state underlying vigilance behavior in zebrafish and male mice. The neural signature of internal vigilance state was characterized by persistent low-frequency high-amplitude neuronal synchrony in zebrafish dorsal pallium and mice prefrontal cortex. The neuronal synchronization underlying vigilance was dependent on intense release of 5-HT induced by persistent activation of either DRN 5-HT neuron or local 5-HT axon terminals in related brain regions via activation of 5-HTR7. Thus, we identify a mechanism of vigilance behavior across species that illustrates the interplay between neuromodulators and neural circuits necessary to shape behavior states.

MicroRNA-9 promotes axon regeneration of mauthner-cell in zebrafish via her6/ calcium activity pathway

MicroRNA (miRNA), functioning as a post-transcriptional regulatory element, plays a significant role in numerous regulatory mechanisms and serves as a crucial intrinsic factor influencing axon regeneration. Prior investigations have elucidated the involvement of miRNA-9 in various processes, however, its specific contribution to axon regeneration in the central nervous system (CNS) remains uncertain. Hence, the zebrafish Mauthner axon regeneration model was employed to manipulate the expression of miRNA-9 in single cells, revealing that upregulation of miRNA-9 facilitated axon regeneration. Additionally, her6, a downstream target gene of miRNA-9, was identified as a novel gene associated with axon regeneration. Suppression of her6 resulted in enhanced Mauthner axon regeneration, as evidenced by the significantly improved regenerative capacity observed in her6 knockout zebrafish. In addition, modulation of her6 expression affects intracellular calcium levels in neurons and promoting her6 expression leads to a decrease in calcium levels in vivo using the new NEMOf calcium indicator. Moreover, the administration of the neural activity activator, pentylenetetrazol (PTZ) partially compensated for the inhibitory effect of her6 overexpression on the calcium level and promoted axon regeneration. Taken together, our study revealed a role for miRNA-9 in the process of axon regeneration in the CNS, which improved intracellular calcium activity and promoted axon regeneration by inhibiting the expression of downstream target gene her6. In our study, miRNA-9 emerged as a novel and intriguing target in the intricate regulation of axon regeneration and offered compelling evidence for the intricate relationship between calcium activity and the facilitation of axon regeneration.

Insights into the mechanisms of neuron generation and specification in the zebrafish ventral spinal cord

The vertebrate nervous system is composed of a wide range of neurons and complex synaptic connections, raising the intriguing question of how neuronal diversity is generated. The spinal cord provides an excellent model for exploring the mechanisms governing neuronal diversity due to its simple neural network and the conserved molecular processes involved in neuron formation and specification during evolution. This review specifically examines two distinct progenitor domains present in the zebrafish ventral spinal cord: the lateral floor plate (LFP) and the p2 progenitor domain. The LFP is responsible for the production of GABAergic Kolmer-Agduhr neurons (KA″), glutamatergic V3 neurons, and intraspinal serotonergic neurons, while the p2 domain generates V2 precursors that subsequently differentiate into three unique subpopulations of V2 neurons, namely glutamatergic V2a, GABAergic V2b, and glycinergic V2s. Based on recent findings, we will examine the fundamental signaling pathways and transcription factors that play a key role in the specification of these diverse neurons and neuronal subtypes derived from the LFP and p2 progenitor domains.

Single-cell analysis of innate spinal cord regeneration identifies intersecting modes of neuronal repair

Adult zebrafish have an innate ability to recover from severe spinal cord injury. Here, we report a comprehensive single nuclear RNA sequencing atlas that spans 6 weeks of regeneration. We identify cooperative roles for adult neurogenesis and neuronal plasticity during spinal cord repair. Neurogenesis of glutamatergic and GABAergic neurons restores the excitatory/inhibitory balance after injury. In addition, transient populations of injury-responsive neurons (iNeurons) show elevated plasticity between 1 and 3 weeks post-injury. Using cross-species transcriptomics and CRISPR/Cas9 mutagenesis, we found iNeurons are injury-surviving neurons that share transcriptional similarities with a rare population of spontaneously plastic mouse neurons. iNeurons are required for functional recovery and employ vesicular trafficking as an essential mechanism that underlies neuronal plasticity. This study provides a comprehensive resource of the cells and mechanisms that direct spinal cord regeneration and establishes zebrafish as a model of plasticity-driven neural repair.

An experimental platform for stochastic analyses of single serotonergic fibers in the mouse brain

The self-organization of the serotonergic matrix, a massive axon meshwork in all vertebrate brains, is driven by the structural and dynamical properties of its constitutive elements. Each of these elements, a single serotonergic axon (fiber), has a unique trajectory and can be supported by a soma that executes one of the many available transcriptional programs. This "individuality" of serotonergic neurons necessitates the development of specialized methods for single-fiber analyses, both at the experimental and theoretical levels. We developed an integrated platform that facilitates experimental isolation of single serotonergic fibers in brain tissue, including regions with high fiber densities, and demonstrated the potential of their quantitative analyses based on stochastic modeling. Single fibers were visualized using two transgenic mouse models, one of which is the first implementation of the Brainbow toolbox in this system. The trajectories of serotonergic fibers were automatically traced in the three spatial dimensions with a novel algorithm, and their properties were captured with a single parameter associated with the directional von Mises-Fisher probability distribution. The system represents an end-to-end workflow that can be imported into various studies, including those investigating serotonergic dysfunction in brain disorders. It also supports new research directions inspired by single-fiber analyses in the serotonergic matrix, including supercomputing simulations and modeling in physics.

The Promising Role of a Zebrafish Model Employed in Neural Regeneration Following a Spinal Cord Injury

Spinal cord injury (SCI) is a devastating event that results in a wide range of physical impairments and disabilities. Despite the advances in our understanding of the biological response to injured tissue, no effective treatments are available for SCIs at present. Some studies have addressed this issue by exploring the potential of cell transplantation therapy. However, because of the abnormal microenvironment in injured tissue, the survival rate of transplanted cells is often low, thus limiting the efficacy of such treatments. Many studies have attempted to overcome these obstacles using a variety of cell types and animal models. Recent studies have shown the utility of zebrafish as a model of neural regeneration following SCIs, including the proliferation and migration of various cell types and the involvement of various progenitor cells. In this review, we discuss some of the current challenges in SCI research, including the accurate identification of cell types involved in neural regeneration, the adverse microenvironment created by SCIs, attenuated immune responses that inhibit nerve regeneration, and glial scar formation that prevents axonal regeneration. More in-depth studies are needed to fully understand the neural regeneration mechanisms, proteins, and signaling pathways involved in the complex interactions between the SCI microenvironment and transplanted cells in non-mammals, particularly in the zebrafish model, which could, in turn, lead to new therapeutic approaches to treat SCIs in humans and other mammals.

Distribution of 5HT receptors during the regeneration process after spinal cord transection in goldfish

Spinal cord injury in teleosts leads to a fibrous scar, but axons sometimes spontaneously regenerate beyond the scar. In goldfish, regenerating axons enter the scar through tubular structures and enlargement of the tubular diameter is proportional to the increase in the number of regenerating axons. During the regeneration process, mast cells containing 5-hydroxytryptamine (5HT) are recruited to the injury site, and 5HT neurons are newly generated. Here, we investigated the distribution of 5HT receptors during this process to determine their role in remodeling the fibrous scar and tubular structures. At 2 weeks after spinal cord transection (SCT) in goldfish, expression of the 5HT2A and 5HT2C receptor subtypes was observed in the ependymo-radial glial cells lining the central canal of the spinal cord. 5HT2A was expressed at the luminal surface, suggesting that it is receptive to 5HT in the cerebrospinal fluid. 5HT2C, on the other hand, was expressed around the nuclei and in the radial processes protruding from the basal surface, suggesting that it is receptive to 5HT released from nearby nerve endings. 5HT2C was also expressed in the fibrous scar where mast cells containing 5HT were abundant. 5HT1B expression was coincident with the basement membrane bordering the fibrous scar and the surrounding nervous tissue, and with the basement membrane of the tubular structure through which axons pass during regeneration. Our findings suggest that multiple 5HT receptors are involved in remodeling the injured site during the regenerative process following SCT. Ependymo-radial glial cells expressing 5HT2A and 5HT2C are involved in neurogenesis and gliogenesis, which might contribute to remodeling the fibrous scar in coordination with 5HT-containing mast cells. Coincident expression of 5HT1B with the basement membrane might be involved in remodeling the tubular structures, thereby promoting axonal regeneration.

sox1a:eGFP transgenic line and single-cell transcriptomics reveal the origin of zebrafish intraspinal serotonergic neurons

Sox transcription factors are crucial for vertebrate nervous system development. In zebrafish embryo, sox1 genes are expressed in neural progenitor cells and neurons of ventral spinal cord. Our recent study revealed that the loss of sox1a and sox1b function results in a significant decline of V2 subtype neurons (V2s). Using single-cell RNA sequencing, we analyzed the transcriptome of sox1a lineage progenitors and neurons in the zebrafish spinal cord at four time points during embryonic development, employing the Tg(sox1a:eGFP) line. In addition to previously characterized sox1a-expressing neurons, we discovered the expression of sox1a in late-developing intraspinal serotonergic neurons (ISNs). Developmental trajectory analysis suggests that ISNs arise from lateral floor plate (LFP) progenitor cells. Pharmacological inhibition of the Notch signaling pathway revealed its role in negatively regulating LFP progenitor cell differentiation into ISNs. Our findings highlight the zebrafish LFP as a progenitor domain for ISNs, alongside known Kolmer-Agduhr (KA) and V3 interneurons.

Functional trajectories during innate spinal cord repair

Adult zebrafish are capable of anatomical and functional recovery following severe spinal cord injury. Axon growth, glial bridging and adult neurogenesis are hallmarks of cellular regeneration during spinal cord repair. However, the correlation between these cellular regenerative processes and functional recovery remains to be elucidated. Whereas the majority of established functional regeneration metrics measure swim capacity, we hypothesize that gait quality is more directly related to neurological health. Here, we performed a longitudinal swim tracking study for 60 individual zebrafish spanning 8 weeks of spinal cord regeneration. Multiple swim parameters as well as axonal and glial bridging were integrated. We established rostral compensation as a new gait quality metric that highly correlates with functional recovery. Tensor component analysis of longitudinal data supports a correspondence between functional recovery trajectories and neurological outcomes. Moreover, our studies predicted and validated that a subset of functional regeneration parameters measured 1 to 2 weeks post-injury is sufficient to predict the regenerative outcomes of individual animals at 8 weeks post-injury. Our findings established new functional regeneration parameters and generated a comprehensive correlative database between various functional and cellular regeneration outputs.

Comparative efficacy of five most common traditional Chinese medicine monomers for promoting recovery of motor function in rats with blunt spinal cord injury: a network meta-analysis

Objective

This research employed a network meta-analysis (NMA) to examine the effectiveness of five traditional Chinese medicine (TCM) monomers for promoting motor function recovery in rats with blunt spinal cord injury (SCI).

Methods

Wangfang, China National Knowledge Infrastructure, Web of Science, Embase, Chinese Scientific Journal Database, PubMed, and the Chinese Biomedical Literature Databases were searched for retrieving relevant articles published from their inception to December 2022. Two reviewers performed screening of search results, data extraction, and literature quality assessment independently.

Results

For this meta-analysis, 59 publications were included. Based on the recovery of motor function at weeks 1, 2, 3, and 4 in NMA, almost all TCM groups had significantly increased positive effects than the negative control animals. In terms of cumulative probability, the tanshinone IIA (TIIA) group ranked first in restoring motor function in the first week after blunt SCI, and the resveratrol (RSV) group ranked first during the last 3 weeks.

Conclusion

The NMA revealed that TCM monomers could effectively restore motor function in the rat model of blunt SCI. In rats with blunt SCI, TIIA may be the most effective TCM monomer during the first week, whereas RSV may be the most effective TCM monomer during the last 3 weeks in promoting motor function recovery. For better evidence reliability in preclinical investigations and safer extrapolation of those findings into clinical settings, further research standardizing the implementation and reporting of animal experiments is required.

Systematic Review Registration

https://inplasy.com/, identifier INPLASY202310070.

Functional Trajectories during innate spinal cord repair

Adult zebrafish are capable of anatomical and functional recovery following severe spinal cord injury. Axon growth, glial bridging and adult neurogenesis are hallmarks of cellular regeneration during spinal cord repair. However, the correlation between these cellular regenerative processes and functional recovery remains to be elucidated. Whereas the majority of established functional regeneration metrics measure swim capacity, we hypothesize that gait quality is more directly related to neurological health. Here, we performed a longitudinal swim tracking study for sixty individual zebrafish spanning eight weeks of spinal cord regeneration. Multiple swim parameters as well as axonal and glial bridging were integrated. We established rostral compensation as a new gait quality metric that highly correlates with functional recovery. Tensor component analysis of longitudinal data supports a correspondence between functional recovery trajectories and neurological outcomes. Moreover, our studies predicted and validated that a subset of functional regeneration parameters measured 1 to 2 weeks post-injury is sufficient to predict the regenerative outcomes of individual animals at 8 weeks post-injury. Our findings established new functional regeneration parameters and generated a comprehensive correlative database between various functional and cellular regeneration outputs.

High-resolution spatiotemporal analysis of single serotonergic axons in an in vitro system

Vertebrate brains have a dual structure, composed of (i) axons that can be well-captured with graph-theoretical methods and (ii) axons that form a dense matrix in which neurons with precise connections operate. A core part of this matrix is formed by axons (fibers) that store and release 5-hydroxytryptamine (5-HT, serotonin), an ancient neurotransmitter that supports neuroplasticity and has profound implications for mental health. The self-organization of the serotonergic matrix is not well understood, despite recent advances in experimental and theoretical approaches. In particular, individual serotonergic axons produce highly stochastic trajectories, fundamental to the construction of regional fiber densities, but further advances in predictive computer simulations require more accurate experimental information. This study examined single serotonergic axons in culture systems (co-cultures and monolayers), by using a set of complementary high-resolution methods: confocal microscopy, holotomography (refractive index-based live imaging), and super-resolution (STED) microscopy. It shows that serotonergic axon walks in neural tissue may strongly reflect the stochastic geometry of this tissue and it also provides new insights into the morphology and branching properties of serotonergic axons. The proposed experimental platform can support next-generation analyses of the serotonergic matrix, including seamless integration with supercomputing approaches.

Generation of a TPH2-EGFP reporter cell line for purification and monitoring of human serotonin neurons in vitro and in vivo

Generation of serotonin neurons (SNs) from human pluripotent stem cells (hPSCs) provides a promising platform to explore the mechanisms of serotonin-associated neuropsychiatric disorders. However, neural differentiation always yields heterogeneous cell populations, making it difficult to identify and purify SNs in vitro or track them in vivo following transplantation. Herein, we generated a TPH2-EGFP reporter hPSC line with insertion of EGFP into the endogenous tryptophan hydroxylase 2 (TPH2) locus using CRISPR-Cas9-mediated gene editing technology. This TPH2-reporter, which faithfully indicated TPH2 expression during differentiation, enabled us to obtain purified SNs for subsequent transcriptional analysis and study of pharmacological responses to antidepressants. In addition, the reporter system showed strong EGFP expression to indicate SNs, which enabled us to explore in vitro and ex vivo electrophysiological properties of SNs. In conclusion, this TPH2-EGFP reporter cell line might be of great significance for studies on human SN-related development and differentiation, drug screening, disease modeling, and cell replacement therapies.

Therapeutic mechanism of transcranial iTBS on nerve regeneration and functional recovery in rats with complete spinal cord transection.. [DOI: 10.21203/rs.3.rs-2026215/v1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Background: After spinal cord transection injury, the inflammatory microenvironment formed in the injury site and the cascade of secondary injury results in limited regeneration of injured axons and the apoptosis of neurons in the sensorimotor cortex (SMC). It is crucial to reverse these adverse processes for the recovery of voluntary movement. In this study, transcranial intermittent theta-burst stimulation (iTBS) was used for the treatment of complete spinal cord transection in rats. The mechanism of transcranial iTBS as a new non-invasive neural regulation paradigm in promoting axonal regeneration and motor function repair was explored. Methods: Rats from the iTBS group were treated with transcranial iTBS 72h after spinal cord injury (SCI). Each rat was received behavioral testing. Inflammation, neuronal apoptosis, neuroprotective effect, regeneration and synaptic plasticity were measured by immunofluorescence staining, western blotting and mRNA sequencing 2 or 4w after SCI. Each rat was received anterograde tracings in the SMC or the long descending propriospinal neurons and tested for motor evoked potentials. Regeneration of corticospinal tract (CST) and 5-hydroxytryptamine (5-HT) nerve fibers were detected eight weeks after SCI. Results: Compared with the control group and the sham iTBS group, rats of the iTBS group showed reduced inflammatory responses and neuronal apoptosis in the SMC two weeks after treatment. After four weeks, the neuroimmune microenvironment at the injury site was improved, and neuroprotective effects were seen to promote axonal regeneration and synaptic plasticity. Significantly, eight weeks after treatment, transcranial iTBS also increased the regeneration of CST, 5-HT nerve fibers, and the long descending propriospinal tract (LDPT). Moreover, motor evoked potentials and hindlimb motor function were significantly improved at eight weeks. Conclusions: Collectively, our results verified that iTBS has the potential to provide neuroprotective effects at early injury stages and pro-regeneration effects related to the 1) CST–5-HT; 2) CST–LDPT; and 3) CST–5-HT–LDPT descending motor pathways and revealed the relationships among neural pathway activation, neuroimmune regulation, neuroprotection, and axonal regeneration, as well as the interaction network of key genes. The proposed non-invasive transcranial iTBS treatment is expected to provide a serviceable practical and theoretical support for spinal cord injury.

Unique advantages of zebrafish larvae as a model for spinal cord regeneration

The regenerative capacity of the spinal cord in mammals ends at birth. In contrast, teleost fish and amphibians retain this capacity throughout life, leading to the use of the powerful zebrafish model system to identify novel mechanisms that promote spinal cord regeneration. While adult zebrafish offer an effective comparison with non-regenerating mammals, they lack the complete array of experimental approaches that have made this animal model so successful. In contrast, the optical transparency, simple anatomy and complex behavior of zebrafish larvae, combined with the known conservation of pro-regenerative signals and cell types between larval and adult stages, suggest that they may hold even more promise as a system for investigating spinal cord regeneration. In this review, we highlight characteristics and advantages of the larval model that underlie its potential to provide future therapeutic approaches for treating human spinal cord injury.

The Consistency of Gastropod Identified Neurons Distinguishes Intra-Individual Plasticity From Inter-Individual Variability in Neural Circuits

Gastropod mollusks are known for their large, individually identifiable neurons, which are amenable to long-term intracellular recordings that can be repeated from animal to animal. The constancy of individual neurons can help distinguish state-dependent or temporal variation within an individual from actual variability between individual animals. Investigations into the circuitry underlying rhythmic swimming movements of the gastropod species, Tritonia exsulans and Pleurobranchaea californica have uncovered intra- and inter-individual variability in synaptic connectivity and serotonergic neuromodulation. Tritonia has a reliably evoked escape swim behavior that is produced by a central pattern generator (CPG) composed of a small number of identifiable neurons. There is apparent individual variability in some of the connections between neurons that is inconsequential for the production of the swim behavior under normal conditions, but determines whether that individual can swim following a neural lesion. Serotonergic neuromodulation of synaptic strength intrinsic to the CPG creates neural circuit plasticity within an individual and contributes to reorganization of the network during recovery from injury and during learning. In Pleurobranchaea, variability over time in the modulatory actions of serotonin and in expression of serotonin receptor genes in an identified neuron directly reflects variation in swimming behavior. Tracking behavior and electrophysiology over hours to days was necessary to identify the functional consequences of these intra-individual, time-dependent variations. This work demonstrates the importance of unambiguous neuron identification, properly assessing the animal and network states, and tracking behavior and physiology over time to distinguish plasticity within the same animal at different times from variability across individual animals.