A unique macrophage subpopulation signals directly to progenitor cells to promote regenerative neurogenesis in the zebrafish spinal cord

Neutrophil immune profile guides spinal cord regeneration in zebrafish

Spinal cord injury triggers a strong innate inflammatory response in both non-regenerative mammals and regenerative zebrafish. Neutrophils are the first immune population to be recruited to the injury site. Yet, their role in the repair process, particularly in a regenerative context, remains largely unknown. Here, we show that, following rapid recruitment to the injured spinal cord, neutrophils mostly reverse migrate throughout the zebrafish body. In addition, promoting neutrophil inflammation resolution by inhibiting Cxcr4 boosts cellular and functional regeneration. Neutrophil-specific RNA-seq analysis reveals an enhanced activation state that correlates with a transient increase in tnf-α expression in macrophage/microglia populations. Conversely, blocking neutrophil recruitment through Cxcr1/2 inhibition diminishes the presence of macrophage/microglia at the injury site and impairs spinal cord regeneration. Altogether, these findings provide new insights into the role of neutrophils in spinal cord regeneration, emphasizing the significant impact of their immune profile on the outcome of the repair process.

Elevated phagocytic capacity directs innate spinal cord repair

Immune cells elicit a continuum of transcriptional and functional states after spinal cord injury (SCI). In mammals, inefficient debris clearance and chronic inflammation impede recovery and overshadow pro-regenerative immune functions. We found that, unlike mammals, zebrafish SCI elicits transient immune activation and efficient debris clearance, without causing chronic inflammation. Single-cell transcriptomics and inducible genetic ablation showed zebrafish macrophages are highly phagocytic and required for regeneration. Cross-species comparisons between zebrafish and mammalian macrophages identified transcription and immune response regulator ( tcim ) as a macrophage-enriched zebrafish gene. Genetic deletion of zebrafish tcim impairs phagocytosis and regeneration, causes aberrant and chronic immune activation, and can be rescued by transplanting wild-type immune precursors into tcim mutants. Conversely, genetic expression of human TCIM accelerates debris clearance and regeneration by reprogramming myeloid precursors into activated phagocytes. This study establishes a central requirement for elevated phagocytic capacity to achieve innate spinal cord repair.

Macrophages modulate fibrosis during newt lens regeneration

BACKGROUND

Previous studies have suggested that macrophages are present during lens regeneration in newts, but their role in the process is yet to be elucidated.

METHODS

Here we generated a transgenic reporter line using the newt, Pleurodeles waltl, that traces macrophages during lens regeneration. Furthermore, we assessed early changes in gene expression during lens regeneration using two newt species, Notophthalmus viridescens and Pleurodeles waltl. Finally, we used clodronate liposomes to deplete macrophages during lens regeneration in both species and tested the effect of a subsequent secondary injury after macrophage recovery.

RESULTS

Macrophage depletion abrogated lens regeneration, induced the formation of scar-like tissue, led to inflammation, decreased iris pigment epithelial cell (iPEC) proliferation, and increased rates of apoptosis in the eye. Some of these phenotypes persisted throughout the last observation period of 100 days and could be attenuated by exogenous FGF2 administration. A distinct transcript profile encoding acute inflammatory effectors was established for the dorsal iris. Reinjury of the newt eye alleviated the effects of macrophage depletion, including the resolution of scar-like tissue, and re-initiated the regeneration process.

CONCLUSIONS

Together, our findings highlight the importance of macrophages for facilitating a pro-regenerative environment in the newt eye by regulating fibrotic responses, modulating the overall inflammatory landscape, and maintaining the proper balance of early proliferation and late apoptosis of the iPECs.

What can we learn about fish neutrophil and macrophage response to immune challenge from studies in zebrafish

Fish rely, to a high degree, on the innate immune system to protect them against the constant exposure to potential pathogenic invasion from the surrounding water during homeostasis and injury. Zebrafish larvae have emerged as an outstanding model organism for immunity. The cellular component of zebrafish innate immunity is similar to the mammalian innate immune system and has a high degree of sophistication due to the needs of living in an aquatic environment from early embryonic stages of life. Innate immune cells (leukocytes), including neutrophils and macrophages, have major roles in protecting zebrafish against pathogens, as well as being essential for proper wound healing and regeneration. Zebrafish larvae are visually transparent, with unprecedented in vivo microscopy opportunities that, in combination with transgenic immune reporter lines, have permitted visualisation of the functions of these cells when zebrafish are exposed to bacterial, viral and parasitic infections, as well as during injury and healing. Recent findings indicate that leukocytes are even more complex than previously anticipated and are essential for inflammation, infection control, and subsequent wound healing and regeneration.

Neutrophils facilitate the epicardial regenerative response after zebrafish heart injury

Despite extensive studies on endogenous heart regeneration within the past 20 years, the players involved in initiating early regeneration events are far from clear. Here, we assessed the function of neutrophils, the first-responder cells to tissue damage, during zebrafish heart regeneration. We detected rapid neutrophil mobilization to the injury site after ventricular amputation, peaking at 1-day post-amputation (dpa) and resolving by 3 dpa. Further analyses indicated neutrophil mobilization coincides with peak epicardial cell proliferation, and recruited neutrophils associated with activated, expanding epicardial cells at 1 dpa. Neutrophil depletion inhibited myocardial regeneration and significantly reduced epicardial cell expansion, proliferation, and activation. To explore the molecular mechanism of neutrophils on the epicardial regenerative response, we performed scRNA-seq analysis of 1 dpa neutrophils and identified enrichment of the FGF and MAPK/ERK signaling pathways. Pharmacological inhibition of FGF signaling indicated its' requirement for epicardial expansion, while neutrophil depletion blocked MAPK/ERK signaling activation in epicardial cells. Ligand-receptor analysis indicated the EGF ligand, hbegfa, is released from neutrophils and synergizes with other FGF and MAPK/ERK factors for induction of epicardial regeneration. Altogether, our studies revealed that neutrophils quickly motivate epicardial cells, which later accumulate at the injury site and contribute to heart regeneration.

Understanding the pathophysiology of acute critical illness: translational lessons from zebrafish models

The models used to investigate the pathophysiological mechanisms of acute critical illness are not limited to mammalian species. The zebrafish (Danio rerio) is a popular model organism for studying diseases due to its transparency and rapid development. The genes and signaling pathways involved in acute critical illness appear highly conserved among zebrafish and humans. Forward genetics such as random mutagenesis by a chemical mutagen or reverse genetics methods represented by CRISPR/Cas9 allowed researchers to reveal multiple novel aspects of pathological processes in areas including infection, immunity, and regeneration. As a model of sepsis, transgenic zebrafish allowed the visualization of lipopolysaccharide (LPS)-induced vascular leakage in vivo and the demonstration of changes in the expression of cellular junction proteins. Other transgenic zebrafish visualizing the extravascular migration of neutrophils and macrophages have demonstrated a decrease in neutrophil numbers and an increased expression of an inflammatory gene, which replicates a phenomenon observed in humans in clinically encountered sepsis. The regenerative potential and the visibility of zebrafish organs also enabled clarification of important mechanisms in wound healing, angiogenesis, and neurogenesis. After spinal cord injury (SCI), a marker gene expressed in glial bridging was discovered. Furthermore, localized epithelial-to-mesenchymal transition (EMT) and molecular mechanisms leading to spinal cord repair were revealed. These translational studies using zebrafish show the potential of the model system for the treatment of acute critical illnesses such as sepsis, organ failure, and trauma.

Spinal Cord Injury and Assays for Regeneration

Due to their renowned regenerative capacity, adult zebrafish are a premier vertebrate model to interrogate mechanisms of innate spinal cord regeneration. Following complete transection to their spinal cord, zebrafish extend glial and axonal bridges across severed tissue, regenerate neurons proximal to the lesion, and regain swim capacity within 8 weeks of injury. Here, we describe methods to perform complete spinal cord transections and to assess functional and cellular recovery during regeneration. For spinal cord injury, a complete transection is performed 4 mm caudal to the brainstem. Swim endurance is quantified as a central readout of functional spinal cord repair. For swim endurance, zebrafish are subjected to a constantly increasing water current velocity until exhaustion, and time at exhaustion is reported. To assess cellular regeneration, histological examination is performed to analyze the extents of glial and axonal bridging across the lesion.

In toto imaging of glial JNK signaling during larval zebrafish spinal cord regeneration

Identification of signaling events that contribute to innate spinal cord regeneration in zebrafish can uncover new targets for modulating injury responses of the mammalian central nervous system. Using a chemical screen, we identify JNK signaling as a necessary regulator of glial cell cycling and tissue bridging during spinal cord regeneration in larval zebrafish. With a kinase translocation reporter, we visualize and quantify JNK signaling dynamics at single-cell resolution in glial cell populations in developing larvae and during injury-induced regeneration. Glial JNK signaling is patterned in time and space during development and regeneration, decreasing globally as the tissue matures and increasing in the rostral cord stump upon transection injury. Thus, dynamic and regional regulation of JNK signaling help to direct glial cell behaviors during innate spinal cord regeneration.

Macrophages modulate fibrosis during newt lens regeneration

Background

Previous studies indicated that macrophages play a role during lens regeneration in newts, but their function has not been tested experimentally.

Methods

Here we generated a transgenic newt reporter line in which macrophages can be visualized in vivo. Using this new tool, we analyzed the location of macrophages during lens regeneration. We uncovered early gene expression changes using bulk RNAseq in two newt species, Notophthalmus viridescens and Pleurodeles waltl. Next, we used clodronate liposomes to deplete macrophages, which inhibited lens regeneration in both newt species.

Results

Macrophage depletion induced the formation of scar-like tissue, an increased and sustained inflammatory response, an early decrease in iris pigment epithelial cell (iPEC) proliferation and a late increase in apoptosis. Some of these phenotypes persisted for at least 100 days and could be rescued by exogenous FGF2. Re-injury alleviated the effects of macrophage depletion and re-started the regeneration process.

Conclusions

Together, our findings highlight the importance of macrophages in facilitating a pro-regenerative environment in the newt eye, helping to resolve fibrosis, modulating the overall inflammatory landscape and maintaining the proper balance of early proliferation and late apoptosis.

Targeting tumor-associated macrophage: an adjuvant strategy for lung cancer therapy

The emergence of immunotherapy has revolutionized the treatment landscape for various types of cancer. Nevertheless, lung cancer remains one of the leading causes of cancer-related mortality worldwide due to the development of resistance in most patients. As one of the most abundant groups of immune cells in the tumor microenvironment (TME), tumor-associated macrophages (TAMs) play crucial and complex roles in the development of lung cancer, including the regulation of immunosuppressive TME remodeling, metabolic reprogramming, neoangiogenesis, metastasis, and promotion of tumoral neurogenesis. Hence, relevant strategies for lung cancer therapy, such as inhibition of macrophage recruitment, TAM reprograming, depletion of TAMs, and engineering of TAMs for drug delivery, have been developed. Based on the satisfactory treatment effect of TAM-targeted therapy, recent studies also investigated its synergistic effect with current therapies for lung cancer, including immunotherapy, radiotherapy, chemotherapy, anti-epidermal growth factor receptor (anti-EGFR) treatment, or photodynamic therapy. Thus, in this article, we summarized the key mechanisms of TAMs contributing to lung cancer progression and elaborated on the novel therapeutic strategies against TAMs. We also discussed the therapeutic potential of TAM targeting as adjuvant therapy in the current treatment of lung cancer, particularly highlighting the TAM-centered strategies for improving the efficacy of anti-programmed cell death-1/programmed cell death-ligand 1 (anti-PD-1/PD-L1) treatment.

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.

Decoding pancreatic endocrine cell differentiation and β cell regeneration in zebrafish

In contrast to mice, zebrafish have an exceptional yet elusive ability to replenish lost β cells in adulthood. Understanding this framework would provide mechanistic insights for β cell regeneration, which may be extrapolated to humans. Here, we characterize a krt4-expressing ductal cell type, which is distinct from the putative Notch-responsive cells, showing neogenic competence and giving rise to the majority of endocrine cells during postembryonic development. Furthermore, we demonstrate a marked ductal remodeling process featuring a Notch-responsive to krt4+ luminal duct transformation during late development, indicating several origins of krt4+ ductal cells displaying similar transcriptional patterns. Single-cell transcriptomics upon a series of time points during β cell regeneration unveil a previously unrecognized dlb+ transitional endocrine precursor cell, distinct regulons, and a differentiation trajectory involving cellular shuffling through differentiation and dedifferentiation dynamics. These results establish a model of zebrafish pancreatic endocrinogenesis and highlight key values of zebrafish for translational studies of β cell regeneration.

Spinal cord repair is modulated by the neurogenic factor Hb-egf under direction of a regeneration-associated enhancer

Unlike adult mammals, zebrafish regenerate spinal cord tissue and recover locomotor ability after a paralyzing injury. Here, we find that ependymal cells in zebrafish spinal cords produce the neurogenic factor Hb-egfa upon transection injury. Animals with hb-egfa mutations display defective swim capacity, axon crossing, and tissue bridging after spinal cord transection, associated with disrupted indicators of neuron production. Local recombinant human HB-EGF delivery alters ependymal cell cycling and tissue bridging, enhancing functional regeneration. Epigenetic profiling reveals a tissue regeneration enhancer element (TREE) linked to hb-egfa that directs gene expression in spinal cord injuries. Systemically delivered recombinant AAVs containing this zebrafish TREE target gene expression to crush injuries of neonatal, but not adult, murine spinal cords. Moreover, enhancer-based HB-EGF delivery by AAV administration improves axon densities after crush injury in neonatal cords. Our results identify Hb-egf as a neurogenic factor necessary for innate spinal cord regeneration and suggest strategies to improve spinal cord repair in mammals.

Spinal cord injury: molecular mechanisms and therapeutic interventions

Spinal cord injury (SCI) remains a severe condition with an extremely high disability rate. The challenges of SCI repair include its complex pathological mechanisms and the difficulties of neural regeneration in the central nervous system. In the past few decades, researchers have attempted to completely elucidate the pathological mechanism of SCI and identify effective strategies to promote axon regeneration and neural circuit remodeling, but the results have not been ideal. Recently, new pathological mechanisms of SCI, especially the interactions between immune and neural cell responses, have been revealed by single-cell sequencing and spatial transcriptome analysis. With the development of bioactive materials and stem cells, more attention has been focused on forming intermediate neural networks to promote neural regeneration and neural circuit reconstruction than on promoting axonal regeneration in the corticospinal tract. Furthermore, technologies to control physical parameters such as electricity, magnetism and ultrasound have been constantly innovated and applied in neural cell fate regulation. Among these advanced novel strategies and technologies, stem cell therapy, biomaterial transplantation, and electromagnetic stimulation have entered into the stage of clinical trials, and some of them have already been applied in clinical treatment. In this review, we outline the overall epidemiology and pathophysiology of SCI, expound on the latest research progress related to neural regeneration and circuit reconstruction in detail, and propose future directions for SCI repair and clinical applications.

Macrophages modulate fibrosis during newt lens regeneration

Previous studies indicated that macrophages play a role during lens regeneration in newts, but their function has not been tested experimentally. Here we generated a transgenic newt reporter line in which macrophages can be visualized in vivo. Using this new tool, we analyzed the location of macrophages during lens regeneration. We uncovered early gene expression changes using bulk RNAseq in two newt species, Notophthalmus viridescens and Pleurodeles waltl. Next, we used clodronate liposomes to deplete macrophages, which inhibited lens regeneration in both newt species. Macrophage depletion induced the formation of scar-like tissue, an increased and sustained inflammatory response, an early decrease in iris pigment epithelial cell (iPEC) proliferation and a late increase in apoptosis. Some of these phenotypes persisted for at least 100 days and could be rescued by exogenous FGF2. Re-injury alleviated the effects of macrophage depletion and re-started the regeneration process. Together, our findings highlight the importance of macrophages in facilitating a pro-regenerative environment in the newt eye, helping to resolve fibrosis, modulating the overall inflammatory landscape and maintaining the proper balance of early proliferation and late apoptosis.

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.

Nanoparticle-based targeting of microglia improves the neural regeneration enhancing effects of immunosuppression in the zebrafish retina

Retinal Müller glia function as injury-induced stem-like cells in zebrafish but not mammals. However, insights gleaned from zebrafish have been applied to stimulate nascent regenerative responses in the mammalian retina. For instance, microglia/macrophages regulate Müller glia stem cell activity in the chick, zebrafish, and mouse. We previously showed that post-injury immunosuppression by the glucocorticoid dexamethasone accelerated retinal regeneration kinetics in zebrafish. Similarly, microglia ablation enhances regenerative outcomes in the mouse retina. Targeted immunomodulation of microglia reactivity may therefore enhance the regenerative potential of Müller glia for therapeutic purposes. Here, we investigated potential mechanisms by which post-injury dexamethasone accelerates retinal regeneration kinetics, and the effects of dendrimer-based targeting of dexamethasone to reactive microglia. Intravital time-lapse imaging revealed that post-injury dexamethasone inhibited microglia reactivity. The dendrimer-conjugated formulation: (1) decreased dexamethasone-associated systemic toxicity, (2) targeted dexamethasone to reactive microglia, and (3) improved the regeneration enhancing effects of immunosuppression by increasing stem/progenitor proliferation rates. Lastly, we show that the gene rnf2 is required for the enhanced regeneration effect of D-Dex. These data support the use of dendrimer-based targeting of reactive immune cells to reduce toxicity and enhance the regeneration promoting effects of immunosuppressants in the retina.

Optimizing spinal cord injury in zebrafish larvae: effects of age on the injury response

Zebrafish are an increasingly popular model to study spinal cord injury (SCI) regeneration. The transparency of larval zebrafish makes them ideal to study cellular processes in real time. Standardized approaches, including age 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 ages (3-7 days post fertilization or dpf) to determine whether the developmental complexity of the central nervous system affects the overall response to SCI. We then used imaging and behavioral analysis to evaluate whether differences existed based on the age of injury. All ages of larval zebrafish upregulated the required genes for glial bridge formation, ctgfa and gfap, at the site of injury, consistent with studies from adult zebrafish. Though all larval ages upregulated factors required to promote glial bridging, young larval zebrafish (3 dpf) were better able to regenerate axons independent of the glial bridge, unlike older zebrafish (7 dpf). Consistent with this data, locomotor experiments demonstrated that some swimming behavior occurs independent of glial bridge formation, further highlighting the need for standardization of this model and recovery assays. Overall, we found subtle cellular differences based on the age of transection in zebrafish, underlining the importance of considering age when designing experiments aimed at understanding regeneration.

Macrophage–Neuroglia Interactions in Promoting Neuronal Regeneration in Zebrafish

The human nervous system exhibits limited regenerative capabilities following damage to the central nervous system (CNS), leading to a scarcity of effective treatments for nerve function recovery. In contrast, zebrafish demonstrate remarkable regenerative abilities, making them an ideal model for studying the modulation of inflammatory processes after injury. Such research holds significant translational potential to enhance our understanding of recovery from damage and disease. Macrophages play a crucial role in tissue repair and regeneration, with their subpopulations indirectly promoting axonal regeneration through developmental signals. The AP-1 signaling pathway, mediated by TNF/Tnfrsf1a, can elevate HDAC1 expression and facilitate regeneration. Furthermore, following spinal cord injury (SCI), pMN progenitors have been observed to switch between oligodendrocyte and motor neuron fates, with macrophage-secreted TNF-α potentially regulating the differentiation of ependymal–radial glia progenitors and oligodendrocytes. Radial glial cells (RGs) are also essential for CNS regeneration in zebrafish, as they perform neurogenesis and gliogenesis, with specific RG subpopulations potentially existing for the generation of neurons and oligodendrocytes. This review article underscores the critical role of macrophages and their subpopulations in tissue repair and regeneration, focusing on their secretion of TNF-α, which promotes axonal regeneration in zebrafish. We also offer insights into the molecular mechanisms underlying TNF-α’s ability to facilitate axonal regeneration and explore the potential of pMN progenitor cells and RGs following SCI in zebrafish. The review concludes with a discussion of various unresolved questions in the field, and ideas are suggested for future research. Studying innate immune cell interactions with neuroglia following injury may lead to the development of novel strategies for treating the inflammatory processes associated with regenerative medicine, which are commonly observed in injury and disease.

Embracing the diversity of model systems to deconstruct the basis of regeneration and tissue repair

The eighth EMBO conference in the series 'The Molecular and Cellular Basis of Regeneration and Tissue Repair' took place in Barcelona (Spain) in September 2022. A total of 173 researchers from across the globe shared their latest advances in deciphering the molecular and cellular basis of wound healing, tissue repair and regeneration, as well as their implications for future clinical applications. The conference showcased an ever-expanding diversity of model organisms used to identify mechanisms that promote regeneration. Over 25 species were discussed, ranging from invertebrates to humans. Here, we provide an overview of the exciting topics presented at the conference, highlighting novel discoveries in regeneration and perspectives for regenerative medicine.

Construction of a searchable database for gene expression changes in spinal cord injury experiments

Spinal cord injury (SCI) is a debilitating disease resulting in an estimated 18,000 new cases in the United States on an annual basis. Significant behavioral research on animal models has led to a large amount of data, some of which has been catalogued in the Open Data Commons for Spinal Cord Injury (ODC-SCI). More recently, high throughput sequencing experiments have been utilized to understand molecular mechanisms associated with SCI, with nearly 6,000 samples from over 90 studies available in the Sequence Read Archive. However, to date, no resource is available for efficiently mining high throughput sequencing data from SCI experiments. Therefore, we have developed a protocol for processing RNA-Seq samples from high-throughput sequencing experiments related to SCI resulting in both raw and normalized data that can be efficiently mined for comparisons across studies as well as homologous discovery across species. We have processed 1,196 publicly available RNA-seq samples from 50 bulk RNA-Seq studies across nine different species, resulting in an SQLite database that can be used by the SCI research community for further discovery. We provide both the database as well as a web-based front-end that can be used to query the database for genes of interest, differential gene expression, genes with high variance, and gene set enrichments.

Molecular Actors of Inflammation and Their Signaling Pathways: Mechanistic Insights from Zebrafish

Inflammation is a hallmark of the physiological response to aggressions. It is orchestrated by a plethora of molecules that detect the danger, signal intracellularly, and activate immune mechanisms to fight the threat. Understanding these processes at a level that allows to modulate their fate in a pathological context strongly relies on in vivo studies, as these can capture the complexity of the whole process and integrate the intricate interplay between the cellular and molecular actors of inflammation. Over the years, zebrafish has proven to be a well-recognized model to study immune responses linked to human physiopathology. We here provide a systematic review of the molecular effectors of inflammation known in this vertebrate and recapitulate their modes of action, as inferred from sterile or infection-based inflammatory models. We present a comprehensive analysis of their sequence, expression, and tissue distribution and summarize the tools that have been developed to study their function. We further highlight how these tools helped gain insights into the mechanisms of immune cell activation, induction, or resolution of inflammation, by uncovering downstream receptors and signaling pathways. These progresses pave the way for more refined models of inflammation, mimicking human diseases and enabling drug development using zebrafish models.

Rassf7a promotes spinal cord regeneration and controls spindle orientation in neural progenitor cells

Spinal cord injury (SCI) can cause long-lasting disability in mammals due to the lack of axonal regrowth together with the inability to reinitiate spinal neurogenesis at the injury site. Deciphering the mechanisms that regulate the proliferation and differentiation of neural progenitor cells is critical for understanding spinal neurogenesis after injury. Compared with mammals, zebrafish show a remarkable capability of spinal cord regeneration. Here, we show that Rassf7a, a member of the Ras-association domain family, promotes spinal cord regeneration after injury. Zebrafish larvae harboring a rassf7a mutation show spinal cord regeneration and spinal neurogenesis defects. Live imaging shows abnormal asymmetric neurogenic divisions and spindle orientation defects in mutant neural progenitor cells. In line with this, the expression of rassf7a is enriched in neural progenitor cells. Subcellular analysis shows that Rassf7a localizes to the centrosome and is essential for cell cycle progression. Our data indicate a role for Rassf7a in modulating spindle orientation and the proliferation of neural progenitor cells after spinal cord injury.

Isolation and Characterization of Peritoneal Macrophages from Salamanders

Salamanders have been used as research models for centuries. While they exhibit a wide range of biological features not seen in mammals, none has captivated scientists like their ability to regenerate. Interestingly, axolotl macrophages have emerged as an essential cell population for tissue regeneration. Whether the same is true in other salamanders such as newt species Notophthalmus viridescens, Cynops pyrrhogaster, or Pleurodeles waltl remains to be seen. Unfortunately, regardless of the species, molecular tools to study macrophage function in salamanders are lacking. We propose that the readily available, terminally differentiated peritoneal macrophages from newts or axolotls could be used to validate molecular reagents in the study of macrophage function during tissue regeneration in salamanders.

An anti-inflammatory activation sequence governs macrophage transcriptional dynamics during tissue injury in zebrafish

Macrophages are essential for tissue repair and regeneration. Yet, the molecular programs, as well as the timing of their activation during and after tissue injury are poorly defined. Using a high spatio-temporal resolution single cell analysis of macrophages coupled with live imaging after sensory hair cell death in zebrafish, we find that the same population of macrophages transitions through a sequence of three major anti-inflammatory activation states. Macrophages first show a signature of glucocorticoid activation, then IL-10 signaling and finally the induction of oxidative phosphorylation by IL-4/Polyamine signaling. Importantly, loss-of-function of glucocorticoid and IL-10 signaling shows that each step of the sequence is independently activated. Lastly, we show that IL-10 and IL-4 signaling act synergistically to promote synaptogenesis between hair cells and efferent neurons during regeneration. Our results show that macrophages, in addition to a switch from M1 to M2, sequentially and independently transition though three anti-inflammatory pathways in vivo during tissue injury in a regenerating organ.

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.

Different inflammation responses modulate Müller glia proliferation in the acute or chronically damaged zebrafish retina

Unlike mammals, zebrafish regenerate in response to retinal damage. Because microglia are activated by retinal damage, we investigated their role during regeneration following either acute or chronic damage. At three weeks post-fertilization (wpf), both wild-type fish exhibiting NMDA-induced acute ganglion and amacrine cell death and gold rush (gosh) mutant fish possessing chronic cone photoreceptor degeneration displayed reactive microglia/macrophages and Müller glia proliferation. Dexamethasone-treated retinas, to inhibit the immune response, lacked reactive microglia/macrophages and possessed fewer PCNA-positive cells, while LPS treatment increased microglia/macrophages and PCNA-labeled cells. NMDA-injured retinas upregulated expression of il-1β and tnfα pro-inflammatory cytokine genes, followed by increased expression of il-10 and arg1 anti-inflammatory/remodeling cytokine genes. A transient early TNFα pro-inflammatory microglia/macrophage population was visualized in NMDA-damaged retinas. In contrast, gosh mutant retinas exhibited a slight increase of pro-inflammatory cytokine gene expression concurrently with a greater increased anti-inflammatory/remodeling cytokine gene expression. Few TNFα pro-inflammatory microglia/macrophages were observed in the gosh retina. Understanding why acute and chronic damage results in different inflammation profiles and their effects on regulating zebrafish retinal regeneration would provide important clues toward improving therapeutic strategies for repairing injured mammalian tissues.

Targeting Fibronectin to Overcome Remyelination Failure in Multiple Sclerosis: The Need for Brain- and Lesion-Targeted Drug Delivery

Multiple sclerosis (MS) is a neuroinflammatory and neurodegenerative disease with unknown etiology that can be characterized by the presence of demyelinated lesions. Prevailing treatment protocols in MS rely on the modulation of the inflammatory process but do not impact disease progression. Remyelination is an essential factor for both axonal survival and functional neurological recovery but is often insufficient. The extracellular matrix protein fibronectin contributes to the inhibitory environment created in MS lesions and likely plays a causative role in remyelination failure. The presence of the blood–brain barrier (BBB) hinders the delivery of remyelination therapeutics to lesions. Therefore, therapeutic interventions to normalize the pathogenic MS lesion environment need to be able to cross the BBB. In this review, we outline the multifaceted roles of fibronectin in MS pathogenesis and discuss promising therapeutic targets and agents to overcome fibronectin-mediated inhibition of remyelination. In addition, to pave the way for clinical use, we reflect on opportunities to deliver MS therapeutics to lesions through the utilization of nanomedicine and discuss strategies to deliver fibronectin-directed therapeutics across the BBB. The use of well-designed nanocarriers with appropriate surface functionalization to cross the BBB and target the lesion sites is recommended.

Spatiotemporal Dynamics of the Molecular Expression Pattern and Intercellular Interactions in the Glial Scar Response to Spinal Cord Injury

Nerve regeneration in adult mammalian spinal cord is poor because of the lack of intrinsic regeneration of neurons and extrinsic factors - the glial scar is triggered by injury and inhibits or promotes regeneration. Recent technological advances in spatial transcriptomics (ST) provide a unique opportunity to decipher most genes systematically throughout scar formation, which remains poorly understood. Here, we first constructed the tissue-wide gene expression patterns of mouse spinal cords over the course of scar formation using ST after spinal cord injury from 32 samples. Locally, we profiled gene expression gradients from the leading edge to the core of the scar areas to further understand the scar microenvironment, such as neurotransmitter disorders, activation of the pro-inflammatory response, neurotoxic saturated lipids, angiogenesis, obstructed axon extension, and extracellular structure re-organization. In addition, we described 21 cell transcriptional states during scar formation and delineated the origins, functional diversity, and possible trajectories of subpopulations of fibroblasts, glia, and immune cells. Specifically, we found some regulators in special cell types, such as Thbs1 and Col1a2 in macrophages, CD36 and Postn in fibroblasts, Plxnb2 and Nxpe3 in microglia, Clu in astrocytes, and CD74 in oligodendrocytes. Furthermore, salvianolic acid B, a blood-brain barrier permeation and CD36 inhibitor, was administered after surgery and found to remedy fibrosis. Subsequently, we described the extent of the scar boundary and profiled the bidirectional ligand-receptor interactions at the neighboring cluster boundary, contributing to maintain scar architecture during gliosis and fibrosis, and found that GPR37L1_PSAP, and GPR37_PSAP were the most significant gene-pairs among microglia, fibroblasts, and astrocytes. Last, we quantified the fraction of scar-resident cells and proposed four possible phases of scar formation: macrophage infiltration, proliferation and differentiation of scar-resident cells, scar emergence, and scar stationary. Together, these profiles delineated the spatial heterogeneity of the scar, confirmed the previous concepts about scar architecture, provided some new clues for scar formation, and served as a valuable resource for the treatment of central nervous system injury.

Endoplasmic Reticulum in Metaplasticity: From Information Processing to Synaptic Proteostasis

The ER (endoplasmic reticulum) is a Ca²⁺ reservoir and the unique protein-synthesizing machinery which is distributed throughout the neuron and composed of multiple different structural domains. One such domain is called EMC (endoplasmic reticulum membrane protein complex), pleiotropic nature in cellular functions. The ER/EMC position inside the neurons unmasks its contribution to synaptic plasticity via regulating various cellular processes from protein synthesis to Ca²⁺ signaling. Since presynaptic Ca²⁺ channels and postsynaptic ionotropic receptors are organized into the nanodomains, thus ER can be a crucial player in establishing TMNCs (transsynaptic molecular nanocolumns) to shape efficient neural communications. This review hypothesized that ER is not only involved in stress-mediated neurodegeneration but also axon regrowth, remyelination, neurotransmitter switching, information processing, and regulation of pre- and post-synaptic functions. Thus ER might not only be a protein-synthesizing and quality control machinery but also orchestrates plasticity of plasticity (metaplasticity) within the neuron to execute higher-order brain functions and neural repair.

Regenerative neurogenesis: the integration of developmental, physiological and immune signals

In fishes and salamanders, but not mammals, neural stem cells switch back to neurogenesis after injury. The signalling environment of neural stem cells is strongly altered by the presence of damaged cells and an influx of immune, as well as other, cells. Here, we summarise our recently expanded knowledge of developmental, physiological and immune signals that act on neural stem cells in the zebrafish central nervous system to directly, or indirectly, influence their neurogenic state. These signals act on several intracellular pathways, which leads to changes in chromatin accessibility and gene expression, ultimately resulting in regenerative neurogenesis. Translational approaches in non-regenerating mammals indicate that central nervous system stem cells can be reprogrammed for neurogenesis. Understanding signalling mechanisms in naturally regenerating species show the path to experimentally promoting neurogenesis in mammals.

Selective Cdk9 inhibition resolves neutrophilic inflammation and enhances cardiac regeneration in larval zebrafish

Sustained neutrophilic inflammation is detrimental for cardiac repair and associated with adverse outcomes following myocardial infarction (MI). An attractive therapeutic strategy to treat MI is to reduce or remove infiltrating neutrophils to promote downstream reparative mechanisms. CDK9 inhibitor compounds enhance the resolution of neutrophilic inflammation; however, their effects on cardiac repair/regeneration are unknown. We have devised a cardiac injury model to investigate inflammatory and regenerative responses in larval zebrafish using heartbeat-synchronised light-sheet fluorescence microscopy. We used this model to test two clinically approved CDK9 inhibitors, AT7519 and flavopiridol, examining their effects on neutrophils, macrophages and cardiomyocyte regeneration. We found that AT7519 and flavopiridol resolve neutrophil infiltration by inducing reverse migration from the cardiac lesion. Although continuous exposure to AT7519 or flavopiridol caused adverse phenotypes, transient treatment accelerated neutrophil resolution while avoiding these effects. Transient treatment with AT7519, but not flavopiridol, augmented wound-associated macrophage polarisation, which enhanced macrophage-dependent cardiomyocyte number expansion and the rate of myocardial wound closure. Using cdk9^−/− knockout mutants, we showed that AT7519 is a selective CDK9 inhibitor, revealing the potential of such treatments to promote cardiac repair/regeneration.

Summary: This study is the first to show that resolving neutrophilic inflammation using a clinically approved immunomodulatory drug (AT7519) improves heart regeneration in zebrafish.

RNA-Seq of Single Fish Cells – Seeking Out the Leukocytes Mediating Immunity in Teleost Fishes

The immune system is a complex and sophisticated biological system, spanning multiple levels of complexity, from the molecular level to that of tissue. Our current understanding of its function and complexity, of the heterogeneity of leukocytes, is a result of decades of concentrated efforts to delineate cellular markers using conventional methods of antibody screening and antigen identification. In mammalian models, this led to in-depth understanding of individual leukocyte subsets, their phenotypes, and their roles in health and disease. The field was further propelled forward by the development of single-cell (sc) RNA-seq technologies, offering an even broader and more integrated view of how cells work together to generate a particular response. Consequently, the adoption of scRNA-seq revealed the unexpected plasticity and heterogeneity of leukocyte populations and shifted several long-standing paradigms of immunology. This review article highlights the unprecedented opportunities offered by scRNA-seq technology to unveil the individual contributions of leukocyte subsets and their crosstalk in generating the overall immune responses in bony fishes. Single-cell transcriptomics allow identifying unseen relationships, and formulating novel hypotheses tailored for teleost species, without the need to rely on the limited number of fish-specific antibodies and pre-selected markers. Several recent studies on single-cell transcriptomes of fish have already identified previously unnoticed expression signatures and provided astonishing insights into the diversity of teleost leukocytes and the evolution of vertebrate immunity. Without a doubt, scRNA-seq in tandem with bioinformatics tools and state-of-the-art methods, will facilitate studying the teleost immune system by not only defining key markers, but also teaching us about lymphoid tissue organization, development/differentiation, cell-cell interactions, antigen receptor repertoires, states of health and disease, all across time and space in fishes. These advances will invite more researchers to develop the tools necessary to explore the immunology of fishes, which remain non-conventional animal models from which we have much to learn.

Know How to Regrow-Axon Regeneration in the Zebrafish Spinal Cord

The capacity for long-distance axon regeneration and functional recovery after spinal cord injury is poor in mammals but remarkable in some vertebrates, including fish and salamanders. The cellular and molecular basis of this interspecies difference is beginning to emerge. This includes the identification of target cells that react to the injury and the cues directing their pro-regenerative responses. Among existing models of successful spinal cord regeneration, the zebrafish is arguably the most understood at a mechanistic level to date. Here, we review the spinal cord injury paradigms used in zebrafish, and summarize the breadth of neuron-intrinsic and -extrinsic factors that have been identified to play pivotal roles in the ability of zebrafish to regenerate central nervous system axons and recover function.