De novo establishment of circuit modules restores locomotion after spinal cord injury in adult zebrafish

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.

Sports training improves motor function after spinal cord injury by regulating microtubule dynamics

Spinal cord injury (SCI) often results in persistent disabilities, primarily due to deficient axon regeneration and irreversible neuronal loss. Sports training is a widely adopted intervention in clinical practice and research to promote axonal sprouting and synaptic plasticity, thereby improving motor function after SCI. However, the precise mechanisms by which sports training improves motor function after SCI remain incompletely understood. We established a rat model of T9 spinal cord contusion and initiated sports training 1 week after SCI, which continued for eight weeks. Using transcriptome sequencing validated through western blotting and immunostaining, we demonstrated that sports training effectively reduced neuroinflammation and prevented neuronal loss. Furthermore, we discovered that sports training changed neuronal microtubule dynamics, facilitating axon regeneration and synaptic plasticity and ultimately improving motor function. These findings indicate that the modulation of neuronal microtubule dynamics may represent a critical mechanism through which sports training improves motor function after SCI.

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.

Apelin regulates mitochondrial dynamics by inhibiting Mst1-JNK-Drp1 signaling pathway to reduce neuronal apoptosis after spinal cord injury

In the secondary injury stage of spinal cord injury, mitochondrial dysfunction leads to decreased ATP production, increased ROS production, and activation of the mitochondria-mediated apoptosis signaling pathway. This ultimately intensifies neuronal death and promotes the progression of the injury. Apelin, a peptide produced by the APLN gene, has demonstrated promise in the treatment of spinal cord injury. The aim of this study was to investigate how Apelin protects neurons after spinal cord injury by influencing the mitochondrial dynamics. The results showed that Apelin has the ability to reduce mitochondrial fission, enhance the mitochondrial membrane potential, improve antioxidant capacity, facilitate the clearance of excess ROS, and ultimately decrease apoptosis in PC12 cells. Moreover, Apelin is overexpressed in neurons in the damaged part of the spinal cord, contributing to reduce mitochondrial fission, improve antioxidant capacity, increase ATP production, decrease apoptosis, promote spinal cord morphological repair, maintain the number of nissl bodies, and enhance signal transduction in the descending spinal cord pathway. Apelin exerts its protective effect by inhibiting the Mst1-JNK-Drp1 signaling pathway. In summary, our study further improved the effect of Apelin in the treatment of spinal cord injury, revealed the mechanism of Apelin in protecting damaged neurons after spinal cord injury by maintaining mitochondrial homeostasis, and provided a new therapeutic mechanism for Apelin in spinal cord injury.

Neuroprotective gap-junction-mediated bystander transformations in the adult zebrafish spinal cord after injury

The adult zebrafish spinal cord displays an impressive innate ability to regenerate after traumatic insults, yet the underlying adaptive cellular mechanisms remain elusive. Here, we show that while the cellular and tissue responses after injury are largely conserved among vertebrates, the large-size fast spinal zebrafish motoneurons are remarkably resilient by remaining viable and functional. We also reveal the dynamic changes in motoneuron glutamatergic input, excitability, and calcium signaling, and we underscore the critical role of calretinin (CR) in binding and buffering the intracellular calcium after injury. Importantly, we demonstrate the presence and the dynamics of a neuron-to-neuron bystander neuroprotective biochemical cooperation mediated through gap junction channels. Our findings support a model in which the intimate and dynamic interplay between glutamate signaling, calcium buffering, gap junction channels, and intercellular cooperation upholds cell survival and promotes the initiation of regeneration.

Potential Roles of Specific Subclasses of Premotor Interneurons in Spinal Cord Function Recovery after Traumatic Spinal Cord Injury in Adults

The differential expression of transcription factors during embryonic development has been selected as the main feature to define the specific subclasses of spinal interneurons. However, recent studies based on single-cell RNA sequencing and transcriptomic experiments suggest that this approach might not be appropriate in the adult spinal cord, where interneurons show overlapping expression profiles, especially in the ventral region. This constitutes a major challenge for the identification and direct targeting of specific populations that could be involved in locomotor recovery after a traumatic spinal cord injury in adults. Current experimental therapies, including electrical stimulation, training, pharmacological treatments, or cell implantation, that have resulted in improvements in locomotor behavior rely on the modulation of the activity and connectivity of interneurons located in the surroundings of the lesion core for the formation of detour circuits. However, very few publications clarify the specific identity of these cells. In this work, we review the studies where premotor interneurons were able to create new intraspinal circuits after different kinds of traumatic spinal cord injury, highlighting the difficulties encountered by researchers, to classify these populations.

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.

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.

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.

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.