Progenitor-derived glia are required for spinal cord regeneration in zebrafish

A single-cell landscape of the regenerating spinal cord of zebrafish

JOURNAL/nrgr/04.03/01300535-202602000-00046/figure1/v/2025-05-05T160104Z/r/image-tiff Unlike mammals, zebrafish possess a remarkable ability to regenerate their spinal cord after injury, making them an ideal vertebrate model for studying regeneration. While previous research has identified key cell types involved in this process, the underlying molecular and cellular mechanisms remain largely unexplored. In this study, we used single-cell RNA sequencing to profile distinct cell populations at different stages of spinal cord injury in zebrafish. Our analysis revealed that multiple subpopulations of neurons showed persistent activation of genes associated with axonal regeneration post injury, while molecular signals promoting growth cone collapse were inhibited. Radial glial cells exhibited significant proliferation and differentiation potential post injury, indicating their intrinsic roles in promoting neurogenesis and axonal regeneration, respectively. Additionally, we found that inflammatory factors rapidly decreased in the early stages following spinal cord injury, creating a microenvironment permissive for tissue repair and regeneration. Furthermore, oligodendrocytes lost maturity markers while exhibiting increased proliferation following injury. These findings demonstrated that the rapid and orderly regulation of inflammation, as well as the efficient proliferation and redifferentiation of new neurons and glial cells, enabled zebrafish to reconstruct the spinal cord. This research provides new insights into the cellular transitions and molecular programs that drive spinal cord regeneration, offering promising avenues for future research and therapeutic strategies.

Zebrafish optic nerve regeneration involves resident and retinal oligodendrocytes

JOURNAL/nrgr/04.03/01300535-202602000-00049/figure1/v/2025-05-05T160104Z/r/image-tiff The visual system of teleost fish grows continuously, which is a useful model for studying regeneration of the central nervous system. Glial cells are key for this process, but their contribution is still not well defined. We followed oligodendrocytes in the visual system of adult zebrafish during regeneration of the optic nerve at 6, 24, and 72 hours post-lesion and at 7 and 14 days post-lesion via the sox10:tagRFP transgenic line and confocal microscopy. To understand the changes that these oligodendrocytes undergo during regeneration, we used Sox2 immunohistochemistry, a stem cell marker involved in oligodendrocyte differentiation. We also used the Click-iT™ Plus TUNEL assay to study cell death and a BrdU assay to determine cell proliferation. Before optic nerve crush, sox10:tagRFP oligodendrocytes are located in the retina, in the optic nerve head, and through all the entire optic nerve. Sox2-positive cells are present in the peripheral germinal zone, the mature retina, and the optic nerve. After optic nerve crush, sox10:tagRFP cells disappeared from the optic nerve crush zone, suggesting that they died, although they were not TUNEL positive. Concomitantly, the number of Sox2-positive cells increased around the crushed area, the optic nerve head, and the retina. Then, between 24 hours post-lesion and 14 days post-lesion, double sox10:tagRFP /Sox2-positive cells were detected in the retina, optic nerve head, and whole optic nerve, together with a proliferation response at 72 hours post-lesion. Our results confirm that a degenerating process may occur prior to regeneration. First, sox10:tagRFP oligodendrocytes that surround the degenerated axons stop wrapping them, change their "myelinating oligodendrocyte" morphology to a "nonmyelinating oligodendrocyte" morphology, and die. Then, residual oligodendrocyte progenitor cells in the optic nerve and retina proliferate and differentiate for the purpose of remyelination. As new axons arise from the surviving retinal ganglion cells, new sox10:tagRFP oligodendrocytes arise from residual oligodendrocyte progenitor cells to guide, nourish and myelinate them. Thus, oligodendrocytes play an active role in zebrafish axon regeneration and remyelination.

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.

Protocol for whole-mount preparation, clearing, and visualization of the adult zebrafish spinal cord structures

The vascular repair, a pivotal element of the spinal cord (SC) injury response, has been notably neglected in zebrafish research. Here, we detail the extraction and dissection of whole-mount zebrafish SCs and the optimized whole-mount immunofluorescence staining and clearing protocols for the visualization of SC structures, such as the vascular network. Additional downstream applications of these samples, from gelatin embedding to cryosectioning and tissue staining, can be done after clearing reversion and are also described in this work. For complete details on the use and execution of this protocol, please refer to Ribeiro et al.1.

Retinoic acid, an essential component of the roof plate organizer, promotes the spatiotemporal segregation of dorsal neural fates

Dorsal neural tube-derived retinoic acid promotes the end of neural crest production and transition into a definitive roof plate. Here, we analyze how this impacts the segregation of central and peripheral lineages, a process essential for tissue patterning and function. Localized in ovo inhibition in quail embryos of retinoic acid activity followed by single-cell transcriptomics unraveled a comprehensive list of differentially expressed genes relevant to these processes. Importantly, progenitors co-expressed neural crest, roof plate and dI1 interneuron markers, indicating a failure in proper lineage segregation. Furthermore, separation between roof plate and dI1 interneurons is mediated by Notch activity downstream of retinoic acid, highlighting their crucial role in establishing the roof plate-dI1 boundary. Within the peripheral branch, where absence of retinoic acid resulted in neural crest production and emigration extending into the roof plate stage, sensory progenitors failed to separate from melanocytes, leading to formation of a common glia-melanocyte cell with aberrant migratory patterns. In summary, the implementation of single-cell RNA sequencing facilitated the discovery and characterization of a molecular mechanism responsible for the segregation of dorsal neural fates during development.

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.

Effect of Voltage-Gated K+ Channel Inhibition by 4-aminopyridine in Spinal Cord Injury Recovery in Zebrafish

Spinal cord injury (SCI) affects between 250,000 to 500,000 individuals annually. After the initial injury, a delayed secondary cascade of cellular responses occurs causing progressive degeneration and permanent disability. One part of this secondary process is disturbance of ionic homeostasis. The K+ channel blocker, 4-aminopyridine (4-AP), is used clinically to alleviate symptoms of multiple sclerosis (MS). Several ongoing studies are being conducted to explore additional areas where 4-AP may have an effect, including stroke, traumatic brain injury, and nervous system recovery after SCI. The goal of our study was to determine whether 4-AP affects recovery from SCI in zebrafish (Danio rerio). Using the transgenic line Tg(gfap:EGFP), we created a spinal transection and tracked swim recovery. We found that constant treatment with 10 μM 4-AP increases swimming distance 40%. Live imaging demonstrated that treatment with 4-AP increases radial glial cells bridging at the site of injury in the presence of 4-AP. We conclude that 10 μM 4-AP is pro-regenerative after SCI.

Differential Roles of Diet on Development and Spinal Cord Regeneration in Larval Zebrafish

The zebrafish is a powerful model organism for studying development and regeneration. However, there is a lack of a standardized reference diet for developmental and regeneration experiments. Most studies evaluate the rate of growth, survival, and fecundity. In this study, we compare three diets and their effects on growth and regeneration after a spinal cord injury (SCI). Fish were fed daily for 1 week with daily measurements of overall length and width of spinal injury. Fish fed a live rotifer diet grew 32%, whereas a commercially available diet only led to a 4% increase in body length. Similarly, differences in rate of regeneration were observed with over 80% of rotifer-fed larvae forming a glial bridge after injury compared to <10% of zebrafish fed with the commercial diet. Our data highlight the need for establishing a standardized diet for regeneration studies to improve research reproducibility.

Astrocyte-Neuron Interactions in Spinal Cord Injury

Spinal cord injuries cause irreversible loss of sensory and motor functions. In mammals, intrinsic and extrinsic inhibitions of neuronal regeneration obstruct neural repair after spinal cord injury. Although astrocytes have been involved in a growing list of vital homeostatic functions in the nervous system, their roles after injury have fascinated and puzzled scientists for decades. Astrocytes undergo long-lasting morphological and functional changes after injury, referred to as reactive astrogliosis. Although reactive astrogliosis is required to contain spinal cord lesions and restore the blood-spinal cord barrier, reactive astrocytes have detrimental effects that inhibit neuronal repair and remyelination. Intriguingly, elevated regenerative capacity is preserved in some non-mammalian vertebrates, where astrocyte-like glial cells display exclusively pro-regenerative effects after injury. A detailed molecular and phenotypic catalog of the continuum of astrocyte reactivity states is an essential first step toward the development of glial cell manipulations for spinal cord repair.

REDOX Balance in Oligodendrocytes Is Important for Zebrafish Visual System Regeneration

Zebrafish (Danio rerio) present continuous growth and regenerate many parts of their body after an injury. Fish oligodendrocytes, microglia and astrocytes support the formation of new connections producing effective regeneration of the central nervous system after a lesion. To understand the role of oligodendrocytes and the signals that mediate regeneration, we use the well-established optic nerve (ON) crush model. We also used sox10 fluorescent transgenic lines to label fully differentiated oligodendrocytes. To quench the effect of reactive oxygen species (ROS), we used the endogenous antioxidant melatonin. Using these tools, we measured ROS production by flow cytometry and explored the regeneration of the optic tectum (OT), the response of oligodendrocytes and their mitochondria by confocal microscopy and Western blot. ROS are produced by oligodendrocytes 3 h after injury and JNK activity is triggered. Concomitantly, there is a decrease in the number of fully differentiated oligodendrocytes in the OT and in their mitochondrial population. By 24 h, oligodendrocytes partially recover. Exposure to melatonin blocks the changes observed in these oligodendrocytes at 3 h and increases their number and their mitochondrial populations after 24 h. Melatonin also blocks JNK upregulation and induces aberrant neuronal differentiation in the OT. In conclusion, a proper balance of ROS is necessary during visual system regeneration and exposure to melatonin has a detrimental impact.