Interplay of pu.1 and gata1 determines myelo-erythroid progenitor cell fate in zebrafish

3-Hydroxykynurenine targets kainate receptors to promote defense against infection

Bacterial infection involves a complex interaction between the pathogen and host where the outcome of infection is not solely determined by pathogen eradication. To identify small molecules that promote host survival by altering the host-pathogen dynamic, we conducted an in vivo chemical screen using zebrafish embryos and found that treatment with 3-hydroxykynurenine (3-HK) protects from lethal bacterial infection. 3-HK, a metabolite produced through host tryptophan metabolism, has no direct antibacterial activity but enhances host survival by restricting bacterial expansion in macrophages through a systemic mechanism that targets kainate-sensitive glutamate receptors. These findings reveal a new pathway by which tryptophan metabolism and kainate-sensitive glutamate receptors function and interact to modulate immunity, with important implications for the coordination between the immune and nervous systems in pathological conditions.

Macrophage depletion overcomes human hematopoietic cell engraftment failure in zebrafish embryo

Zebrafish is widely adopted as a grafting model for studying human development and diseases. Current zebrafish xenotransplantations are performed using embryo recipients, as the adaptive immune system, responsible for host versus graft rejection, only reaches maturity at juvenile stage. However, transplanted primary human hematopoietic stem/progenitor cells (HSC) rapidly disappear even in zebrafish embryos, suggesting that another barrier to transplantation exists before the onset of adaptive immunity. Here, using a labelled macrophage zebrafish line, we demonstrated that engraftment of human HSC induces a massive recruitment of macrophages which rapidly phagocyte transplanted cells. Macrophages depletion, by chemical or pharmacological treatments, significantly improved the uptake and survival of transplanted cells, demonstrating the crucial implication of these innate immune cells for the successful engraftment of human cells in zebrafish. Beyond identifying the reasons for human hematopoietic cell engraftment failure, this work images the fate of human cells in real time over several days in macrophage-depleted zebrafish embryos.

Zebrafish nampt-a mutants are viable despite perturbed primitive hematopoiesis

BACKGROUND

Nicotinamide phosphoribosyltransferase (Nampt) is required for recycling NAD+ in numerous cellular contexts. Morpholino-based knockdown of zebrafish nampt-a has been shown to cause abnormal development and defective hematopoiesis concomitant with decreased NAD+ levels. However, surprisingly, nampt-a mutant zebrafish were recently found to be viable, suggesting a discrepancy between the phenotypes in knockdown and knockout conditions. Here, we address this discrepancy by directly comparing loss-of-function approaches that result in identical defective transcripts in morphants and mutants.

RESULTS

Using CRISPR/Cas9-mediated mutagenesis, we generated nampt-a mutant lines that carry the same mis-spliced mRNA as nampt-a morphants. Despite reduced NAD+ levels and perturbed expression of specific blood markers, nampt-a mutants did not display obvious developmental defects and were found to be viable. In contrast, injection of nampt-a morpholinos into wild-type or mutant nampt-a embryos caused aberrant phenotypes. Moreover, nampt-a morpholinos caused additional reduction of blood-related markers in nampt-a mutants, suggesting that the defects observed in nampt-a morphants can be partially attributed to off-target effects of the morpholinos.

CONCLUSIONS

Our findings show that zebrafish nampt-a mutants are viable despite reduced NAD+ levels and a perturbed hematopoietic gene expression program, indicating strong robustness of primitive hematopoiesis during early embryogenesis.

Microglia Mitigate Neuronal Activation in a Zebrafish Model of Dravet Syndrome

It has been known for a long time that epileptic seizures provoke brain neuroinflammation involving the activation of microglial cells. However, the role of these cells in this disease context and the consequences of their inflammatory activation on subsequent neuron network activity remain poorly understood so far. To fill this gap of knowledge and gain a better understanding of the role of microglia in the pathophysiology of epilepsy, we used an established zebrafish Dravet syndrome epilepsy model based on Scn1Lab sodium channel loss-of-function, combined with live microglia and neuronal Ca2+ imaging, local field potential (LFP) recording, and genetic microglia ablation. Data showed that microglial cells in scn1Lab-deficient larvae experiencing epileptiform seizures displayed morphological and biochemical changes characteristic of M1-like pro-inflammatory activation; i.e., reduced branching, amoeboid-like morphology, and marked increase in the number of microglia expressing pro-inflammatory cytokine Il1β. More importantly, LFP recording, Ca2+ imaging, and swimming behavior analysis showed that microglia-depleted scn1Lab-KD larvae displayed an increase in epileptiform seizure-like neuron activation when compared to that seen in scn1Lab-KD individuals with microglia. These findings strongly suggest that despite microglia activation and the synthesis of pro-inflammatory cytokines, these cells provide neuroprotective activities to epileptic neuronal networks, making these cells a promising therapeutic target in epilepsy.

A tryptophan metabolite modulates the host response to bacterial infection via kainate receptors

Bacterial infection involves a complex interaction between the pathogen and host where the outcome of infection is not solely determined by pathogen eradication. To identify small molecules that promote host survival by altering the host-pathogen dynamic, we conducted an in vivo chemical screen using zebrafish embryos and found that treatment with 3-hydroxy-kynurenine protects from lethal gram-negative bacterial infection. 3-hydroxy-kynurenine, a metabolite produced through host tryptophan metabolism, has no direct antibacterial activity but enhances host survival by restricting bacterial expansion in macrophages by targeting kainate-sensitive glutamate receptors. These findings reveal new mechanisms by which tryptophan metabolism and kainate-sensitive glutamate receptors function and interact to modulate immunity, with significant implications for the coordination between the immune and nervous systems in pathological conditions.

Activation of lineage competence in hemogenic endothelium precedes the formation of hematopoietic stem cell heterogeneity

Hematopoietic stem and progenitor cells (HSPCs) are considered as a heterogeneous population, but precisely when, where and how HSPC heterogeneity arises remain largely unclear. Here, using a combination of single-cell multi-omics, lineage tracing and functional assays, we show that embryonic HSPCs originate from heterogeneous hemogenic endothelial cells (HECs) during zebrafish embryogenesis. Integrated single-cell transcriptome and chromatin accessibility analysis demonstrates transcriptional heterogeneity and regulatory programs that prime lymphoid/myeloid fates at the HEC level. Importantly, spi2+ HECs give rise to lymphoid/myeloid-primed HSPCs (L/M-HSPCs) and display a stress-responsive function under acute inflammation. Moreover, we uncover that Spi2 is required for the formation of L/M-HSPCs through tightly controlling the endothelial-to-hematopoietic transition program. Finally, single-cell transcriptional comparison of zebrafish and human HECs and human induced pluripotent stem cell-based hematopoietic differentiation results support the evolutionary conservation of L/M-HECs and a conserved role of SPI1 (spi2 homolog in mammals) in humans. These results unveil the lineage origin, biological function and molecular determinant of HSPC heterogeneity and lay the foundation for new strategies for induction of transplantable lineage-primed HSPCs in vitro.

Zebrafish as a Model to Study Retinoic Acid Signaling in Development and Disease

Retinoic acid (RA) is a metabolite of vitamin A (retinol) that plays various roles in development to influence differentiation, patterning, and organogenesis. RA also serves as a crucial homeostatic regulator in adult tissues. The role of RA and its associated pathways are well conserved from zebrafish to humans in both development and disease. This makes the zebrafish a natural model for further interrogation into the functions of RA and RA-associated maladies for the sake of basic research, as well as human health. In this review, we explore both foundational and recent studies using zebrafish as a translational model for investigating RA from the molecular to the organismal scale.

Learning from Zebrafish Hematopoiesis

Hematopoiesis is a complex process that tightly regulates the generation, proliferation, differentiation, and maintenance of hematopoietic cells. Disruptions in hematopoiesis can lead to various diseases affecting both hematopoietic and non-hematopoietic systems, such as leukemia, anemia, thrombocytopenia, rheumatoid arthritis, and chronic granuloma. The zebrafish serves as a powerful vertebrate model for studying hematopoiesis, offering valuable insights into both hematopoietic regulation and hematopoietic diseases. In this chapter, we present a comprehensive overview of zebrafish hematopoiesis, highlighting its distinctive characteristics in hematopoietic processes. We discuss the ontogeny and modulation of both primitive and definitive hematopoiesis, as well as the microenvironment that supports hematopoietic stem/progenitor cells. Additionally, we explore the utility of zebrafish as a disease model and its potential in drug discovery, which not only advances our understanding of the regulatory mechanisms underlying hematopoiesis but also facilitates the exploration of novel therapeutic strategies for hematopoietic diseases.

Fluxapyroxad disrupt erythropoiesis in zebrafish (Danio rerio) embryos

Fluxapyroxad, a succinate dehydrogenase inhibitor (SDHI) fungicide, is commercialized worldwide to control a variety of fungal diseases. Growing evidence shows that fluxapyroxad is teratogenic to aquatic organisms. In this study, the influence of fluxapyroxad toward hematopoietic development was evaluated using zebrafish embryos which were exposed to fluxapyroxad (0.03 µM, 0.3 µM and 3 µM) from 3 h post fertilization (hpf) to 3 days post fertilization (dpf). Compared to the control groups, the hemoglobin was ectopic and decreased in response to fluxapyroxad treatment. The transcription levels of genes (hbbe1, hbbe2, and gata1a) involved in erythropoiesis were reduced after exposure to fluxapyroxad. In contrast, the distributions and expression of marker genes for myeloid lineage cells were unaffected by fluxapyroxad exposure. Our data suggested that fluxapyroxad might specifically affect erythropoiesis and hold great promise for the assessment of the toxicity of fluxapyroxad to aquatic organisms.

Neurotoxicity of diesel exhaust extracts in zebrafish and its implications for neurodegenerative disease

Long-term air pollution (AP) exposure, including diesel exhaust exposure, is increasingly being recognized as a major contributor to the development of neurodegenerative diseases such as Parkinson's and Alzheimer's disease. How AP increases the risk of neurodegeneration is not well understood but might include direct neurotoxicity and CNS inflammation. We investigated the impact of diesel exhaust particulate extract (DEPe) exposure on the brain and the mechanisms by which microglia and astroglia might mediate neuronal changes. Zebrafish (ZF) were utilized to determine neuronal toxicity of and microglial response to DEPe and single cell RNA sequencing was employed to study cell type-specific transcriptomic responses within the ZF brain. DEPe exposure induced neuronal injury and microglial activation in vivo. However, preventing the development of microglia did not attenuate DEPe-induced neuron loss, leading us to investigate microglial, astroglial, and neuronal response to DEPe exposure at single-cell resolution. Differentially expressed genes and disease-relevant pathways were identified within glial and neuronal clusters after DEPe exposure. Microglia and astroglia existed in multiple states, some of which appear toxic and others protective to neurons. Neuronal transcriptomic analysis revealed that DEPe exposure reduced expression of autophagy-related genes consistent with direct neurotoxicity. In summary, DEPe exposure was neurotoxic in developing ZF larvae and induced neuroinflammation. The microglial inflammatory response did not contribute to neurotoxicity of DEPe and in fact, some glial clusters upregulated transcriptional pathways that are likely protective. Furthermore, DEPe exposure led to reduced expression of autophagy-related genes in neurons that likely contribute to its toxicity.

The ETS transcription factor Spi2 regulates hematopoietic cell development in zebrafish

The E26 transformation-specific or E-twenty-six (ETS) genes encode a superfamily of transcription factors involved in diverse biological processes. Here, we report the identification and characterization of a previously unidentified member of the ETS transcription factors, Spi2, that is found exclusively in the ray-finned fish kingdom. We show that the expression of spi2 is restricted to hemogenic endothelial cells (HECs) and to hematopoietic stem and progenitor cells (HSPCs) in zebrafish. Using bacteria artificial chromosome transgenesis, we generate a spi2 reporter line, TgBAC(spi2:P2a-GFP), which manifests the GFP pattern recapitulating the endogenous spi2 expression. Genetic ablation of spi2 has little effect on HEC formation and the endothelial-to-hematopoietic transition, but results in compromised proliferation of HSPCs in the caudal hematopoietic tissue (CHT) during early development and in severe myeloid lineage defect in adulthood. Epistatic analysis shows that spi2 acts downstream of runx1 in regulating HSPC development in the CHT. Our study identifies Spi2 as an essential regulator for definitive hematopoietic cell development and creates a TgBAC(spi2:P2a-GFP) reporter line for tracking HECs, HSPCs, myeloid cells and thrombocytes from early development to adulthood.

Trim33 conditions the lifespan of primitive macrophages and onset of definitive macrophage production

Trim33 (Tif1γ) is a transcriptional regulator that is notably involved in several aspects of hematopoiesis. It is essential for the production of erythrocytes in zebrafish, and for the proper functioning and aging of hematopoietic stem and progenitor cells (HSPCs) in mice. Here, we have found that, in zebrafish development, Trim33 is essential cell-autonomously for the lifespan of the yolk sac-derived primitive macrophages, as well as for the initial production of definitive (HSPC-derived) macrophages in the first niche of definitive hematopoiesis, the caudal hematopoietic tissue. Moreover, Trim33 deficiency leads to an excess production of definitive neutrophils and thrombocytes. Our data indicate that Trim33 radically conditions the differentiation output of aorta-derived HSPCs in all four erythro-myeloid cell types, in a niche-specific manner.

Csf1rb regulates definitive hematopoiesis in zebrafish

In vertebrates, hematopoietic stem and progenitor cells (HSPCs) are capable of self-renewal and continuously replenishing all mature blood lineages throughout life. However, the molecular signaling regulating the maintenance and expansion of HSPCs remains incompletely understood. Colony-stimulating factor 1 receptor (CSF1R) is believed to be the primary regulator for the myeloid lineage but not HSPC development. Here, we show a surprising role of Csf1rb, a zebrafish homolog of mammalian CSF1R, in preserving the HSPC pool by maintaining the proliferation of HSPCs. Deficiency of csf1rb leads to a reduction in both HSPCs and their differentiated progenies, including myeloid, lymphoid and erythroid cells at early developmental stages. Likewise, the absence of csf1rb conferred similar defects upon HSPCs and leukocytes in adulthood. Furthermore, adult hematopoietic cells from csf1rb mutants failed to repopulate immunodeficient zebrafish. Interestingly, loss-of-function and gain-of-function assays suggested that the canonical ligands for Csf1r in zebrafish, including Csf1a, Csf1b and Il34, were unlikely to be ligands of Csf1rb. Thus, our data indicate a previously unappreciated role of Csf1r in maintaining HSPCs, independently of known ligands.

Endocardial identity is established during early somitogenesis by Bmp signalling acting upstream of npas4l and etv2

The endocardium plays important roles in the development and function of the vertebrate heart; however, few molecular markers of this tissue have been identified and little is known about what regulates its differentiation. Here, we describe the Gt(SAGFF27C); Tg(4xUAS:egfp) line as a marker of endocardial development in zebrafish. Transcriptomic comparison between endocardium and pan-endothelium confirms molecular distinction between these populations and time-course analysis suggests differentiation as early as eight somites. To investigate what regulates endocardial identity, we employed npas4l, etv2 and scl loss-of-function models. Endocardial expression is lost in npas4l mutants, significantly reduced in etv2 mutants and only modestly affected upon scl loss-of-function. Bmp signalling was also examined: overactivation of Bmp signalling increased endocardial expression, whereas Bmp inhibition decreased expression. Finally, epistasis experiments showed that overactivation of Bmp signalling was incapable of restoring endocardial expression in etv2 mutants. By contrast, overexpression of either npas4l or etv2 was sufficient to rescue endocardial expression upon Bmp inhibition. Together, these results describe the differentiation of the endocardium, distinct from vasculature, and place npas4l and etv2 downstream of Bmp signalling in regulating its differentiation.

Summary: A zebrafish transgenic reporter of the endocardium is identified, permitting transcriptomic analysis and identification of new endocardial markers. Epistasis experiments demonstrate npas4l and etv2 act downstream of Bmp signalling to regulate endocardial differentiation.

Hematogenesis Adaptation to Long-Term Hypoxia Acclimation in Zebrafish (Danio rerio)

When fish live in the wild or are cultured artificially, they will inevitably suffer from hypoxia. At the same time, blood physiological indexes represent the physiological state of fish. In order to study the effect of long-term hypoxia acclimation on fish hematogenesis, we cultured zebrafish embryos into adulthood in a hypoxia incubator (1.5 ± 0.2 mg/L). Then we compared the hematological parameters of zebrafish cultured in normoxia and hypoxia conditions. Transcriptome sequencing analysis of the main hematopoietic tissue, the head kidney, was also compared between the two groups. Results showed that the number of erythrocytes increased significantly in the long-term hypoxia acclimated group, while the size of several cell types, such as red blood cells, eosinophils, basophils, small lymphocytes and thrombocytes, decreased significantly. The transcriptomic comparisons revealed that there were 6475 differentially expressed genes (DEGs) between the two groups. A Gene Ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that hematopoiesis and cell proliferation signaling were the most significantly enriched pathways in the head kidney of hypoxia acclimated zebrafish. In addition, many genes involved in the hematopoietic process showed significantly higher levels of expression in the hypoxia acclimated zebrafish, when compared to the normoxia zebrafish. When considered together, these data allowed us to conclude that long-term hypoxia can promote the hematopoiesis process and cell proliferation signaling in the zebrafish head kidney, which resulted in higher red blood cell production. Higher numbers of red blood cells allow for better adaptation to the hypoxic environment. In conclusion, this study provides a basis for the in-depth understanding of the effects of hypoxia on hematogenesis in fish species.

Negative Elongation Factor (NELF) Inhibits Premature Granulocytic Development in Zebrafish

Gene expression is tightly regulated during hematopoiesis. Recent studies have suggested that RNA polymerase II (Pol II) promoter proximal pausing, a temporary stalling downstream of the promoter region after initiation, plays a critical role in regulating the expression of various genes in metazoans. However, the function of proximal pausing in hematopoietic gene regulation remains largely unknown. The negative elongation factor (NELF) complex is a key factor important for this proximal pausing. Previous studies have suggested that NELF regulates granulocytic differentiation in vitro, but its in vivo function during hematopoiesis remains uncharacterized. Here, we generated the zebrafish mutant for one NELF complex subunit Nelfb using the CRISPR-Cas9 technology. We found that the loss of nelfb selectively induced excessive granulocytic development during primitive and definitive hematopoiesis. The loss of nelfb reduced hematopoietic progenitor cell formation and did not affect erythroid development. Moreover, the accelerated granulocytic differentiation and reduced progenitor cell development could be reversed by inhibiting Pol II elongation. Further experiments demonstrated that the other NELF complex subunits (Nelfa and Nelfe) played similar roles in controlling granulocytic development. Together, our studies suggested that NELF is critical in controlling the proper granulocytic development in vivo, and that promoter proximal pausing might help maintain the undifferentiated state of hematopoietic progenitor cells.

Zebrafish imaging reveals TP53 mutation switching oncogene-induced senescence from suppressor to driver in primary tumorigenesis

Most tumours are thought to arise through oncogenic cell generation followed by additional mutations. How a new oncogenic cell primes tumorigenesis by acquiring additional mutations remains unclear. We show that an additional TP53 mutation stimulates primary tumorigenesis by switching oncogene-induced senescence from a tumour suppressor to a driver. Zebrafish imaging reveals that a newly emerged oncogenic cell with the Ras^G12V mutation becomes senescent and is eliminated from the epithelia, which is prevented by adding a TP53 gain-of-function mutation (TP53^R175H) into Ras^G12V cells. Surviving Ras^G12V-TP53^R175H double-mutant cells senesce and secrete senescence-associated secretory phenotype (SASP)-related inflammatory molecules that convert neighbouring normal cells into SASP factor-secreting senescent cells, generating a heterogeneous tumour-like cell mass. We identify oncogenic cell behaviours that may control the initial human tumorigenesis step. Ras and TP53 mutations and cellular senescence are frequently detected in human tumours; similar switching may occur during the initial step of human tumorigenesis.

It is unclear how a single oncogenic cell primes tumorigenesis. Here the authors visualised this behaviour using a zebrafish larval skin as a model and show that RasG12V oncogenic cell is eliminated through oncogene-senescence while a gain of function mutation in p53 alters this behaviour from tumour suppressive to tumour promoting.

Cyclooxygenase production of PGE2 promotes phagocyte control of A. fumigatus hyphal growth in larval zebrafish

Invasive aspergillosis is a common opportunistic infection, causing >50% mortality in infected immunocompromised patients. The specific molecular mechanisms of the innate immune system that prevent pathogenesis of invasive aspergillosis in immunocompetent individuals are not fully understood. Here, we used a zebrafish larva-Aspergillus infection model to identify cyclooxygenase (COX) enzyme signaling as one mechanism that promotes host survival. Larvae exposed to the pan-COX inhibitor indomethacin succumb to infection at a significantly higher rate than control larvae. COX signaling is both macrophage- and neutrophil-mediated. However, indomethacin treatment has no effect on phagocyte recruitment. Instead, COX signaling promotes phagocyte-mediated inhibition of germination and invasive hyphal growth. Increased germination and invasive hyphal growth is also observed in infected F0 crispant larvae with mutations in genes encoding for COX enzymes (ptgs2a/b). Protective COX-mediated signaling requires the receptor EP2 and exogenous prostaglandin E2 (PGE2) rescues indomethacin-induced decreased immune control of fungal growth. Collectively, we find that COX signaling activates the PGE2-EP2 pathway to increase control A. fumigatus hyphal growth by phagocytes in zebrafish larvae.

Characterization of mouse Bmp5 regulatory injury element in zebrafish wound models

Many key signaling molecules used to build tissues during embryonic development are re-activated at injury sites to stimulate tissue regeneration and repair. Bone morphogenetic proteins provide a classic example, but the mechanisms that lead to reactivation of BMPs following injury are still unknown. Previous studies have mapped a large "injury response element" (IRE) in the mouse Bmp5 gene that drives gene expression following bone fractures and other types of injury. Here we show that the large mouse IRE region is also activated in both zebrafish tail resection and mechanosensory hair cell injury models. Using the ability to test multiple constructs and image temporal and spatial dynamics following injury responses, we have narrowed the original size of the mouse IRE region by over 100 fold and identified a small 142 bp minimal enhancer that is rapidly induced in both mesenchymal and epithelial tissues after injury. These studies identify a small sequence that responds to evolutionarily conserved local signals in wounded tissues and suggest candidate pathways that contribute to BMP reactivation after injury.

Aging Activates the Immune System and Alters the Regenerative Capacity in the Zebrafish Heart

Age-associated organ failure and degenerative diseases have a major impact on human health. Cardiovascular dysfunction has an increasing prevalence with age and is one of the leading causes of death. In contrast to humans, zebrafish have extraordinary regeneration capacities of complex organs including the heart. In addition, zebrafish has recently become a model organism in research on aging. Here, we have compared the ventricular transcriptome as well as the regenerative capacity after cryoinjury of old and young zebrafish hearts. We identified the immune system as activated in old ventricles and found muscle organization to deteriorate upon aging. Our data show an accumulation of immune cells, mostly macrophages, in the old zebrafish ventricle. Those immune cells not only increased in numbers but also showed morphological and behavioral changes with age. Our data further suggest that the regenerative response to cardiac injury is generally impaired and much more variable in old fish. Collagen in the wound area was already significantly enriched in old fish at 7 days post injury. Taken together, these data indicate an ‘inflammaging’-like process in the zebrafish heart and suggest a change in regenerative response in the old.

Cavefish cope with environmental hypoxia by developing more erythrocytes and overexpression of hypoxia-inducible genes

Dark caves lacking primary productivity can expose subterranean animals to hypoxia. We used the surface-dwelling (surface fish) and cave-dwelling (cavefish) morphs of Astyanax mexicanus as a model for understanding the mechanisms of hypoxia tolerance in the cave environment. Primitive hematopoiesis, which is restricted to the posterior lateral mesoderm in other teleosts, also occurs in the anterior lateral mesoderm in Astyanax, potentially pre-adapting surface fish for hypoxic cave colonization. Cavefish have enlarged both hematopoietic domains and develop more erythrocytes than surface fish, which are required for normal development in both morphs. Laboratory-induced hypoxia suppresses growth in surface fish but not in cavefish. Both morphs respond to hypoxia by overexpressing hypoxia-inducible factor 1 (hif1) pathway genes, and some hif1 genes are constitutively upregulated in normoxic cavefish to similar levels as in hypoxic surface fish. We conclude that cavefish cope with hypoxia by increasing erythrocyte development and constitutive hif1 gene overexpression.

Methylmercury-induced hair cell loss requires hydrogen peroxide production and leukocytes in zebrafish embryos

Hearing impairment and deafness is frequently observed as one of the neurological signs in patients with Minamata disease caused by methylmercury (MeHg) poisoning. Loss of hair cells in humans and animals is a consequence of MeHg poisoning. However, it is still not clear how MeHg causes hearing deficits. We employed the hair cells of the lateral line system of zebrafish embryos as a model to explore this question. We exposed transgenic zebrafish embryos to MeHg (30-360 μg/L) at the different stages, and scored the numbers of hair cells. We find that MeHg-induced reduction of hair cells is in a concentration dependent manner. By employing antisense morpholino against to pu.1, we confirm that loss of hair cells involves the action of leukocytes. Moreover, hair cell loss is attenuated by co-treating MeHg-exposed embryos with pharmacological inhibitors of NADPH oxidases named diphenyleneiodonium (DPI) and VAS2870. In situ gene expression analysis showed that genes encoding the SQSTM1-Keap1-Nrf2 systems involved in combating oxidative stress and immune responses are highly expressed in the lateral line organs of embryos exposed to MeHg. This suggests that induction of hydrogen peroxide (H₂O₂) is the primary effect of MeHg on the hair cells. Genes induced by MeHg are also involved in regeneration of the hair cells. These features are likely related to the capacity of the zebrafish to regenerate the lost hair cells.

Mechanisms underlying microglial colonization of developing neural retina in zebrafish

Microglia are brain-resident macrophages that function as the first line of defense in brain. Embryonic microglial precursors originate in peripheral mesoderm and migrate into the brain during development. However, the mechanism by which they colonize the brain is incompletely understood. The retina is one of the first brain regions to accommodate microglia. In zebrafish, embryonic microglial precursors use intraocular hyaloid blood vessels as a pathway to migrate into the optic cup via the choroid fissure. Once retinal progenitor cells exit the cell cycle, microglial precursors associated with hyaloid blood vessels start to infiltrate the retina preferentially through neurogenic regions, suggesting that colonization of retinal tissue depends upon the neurogenic state. Along with blood vessels and retinal neurogenesis, IL34 also participates in microglial precursor colonization of the retina. Altogether, CSF receptor signaling, blood vessels, and neuronal differentiation function as cues to create an essential path for microglial migration into developing retina.

The immune system is comprised of many different cells which protect our bodies from infection and other illnesses. The brain contains its own population of immune cells called microglia. Unlike neurons, these cells form outside the brain during development. They then travel to the brain and colonize specific regions like the retina, the light-sensing part of the eye in vertebrates.

It is poorly understood how newly formed microglia migrate to the retina and whether their entry depends on the developmental state of nerve cells (also known as neurons) in this region. To help answer these questions, Ranawat and Masai attached fluorescent labels that can be seen under a microscope to microglia in the embryos of zebrafish. Developing zebrafish are transparent, making it easy to trace the fluorescent microglia as they travel to the retina and insert themselves among its neurons.

Ranawat and Masai found that blood vessels around the retina act as a pathway that microglia move along. Once they reach the retina, the microglia remain attached and only enter the retina at sites where brain cells are starting to mature in to adult neurons. Further experiments showed that microglia fail to infiltrate and colonize the retina when blood vessels are damaged or neuron maturation is blocked.

These findings reveal some of the key elements that guide microglia to the retina during development. However, further work is needed to establish the molecular and biochemical processes that allow microglia to attach to blood vessels and detect when cells in the retina are starting to mature.

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

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

TLR7 ligation augments hematopoiesis in Rps14 (uS11) deficiency via paradoxical suppression of inflammatory signaling

Myelodysplastic syndrome (MDS) is a hematological malignancy characterized by blood cytopenias and predisposition to acute myeloid leukemia (AML). Therapies for MDS are lacking, particularly those that have an impact in the early stages of disease. We developed a model of MDS in zebrafish with knockout of Rps14, the primary mediator of the anemia associated with del(5q) MDS. These mutant animals display dose- and age-dependent abnormalities in hematopoiesis, culminating in bone marrow failure with dysplastic features. We used Rps14 knockdown to undertake an in vivo small-molecule screening, to identify compounds that ameliorate the MDS phenotype, and we identified imiquimod, an agonist of Toll-like receptor-7 (TLR7) and TLR8. Imiquimod alleviates anemia by promoting hematopoietic stem and progenitor cell expansion and erythroid differentiation, the mechanism of which is dependent on TLR7 ligation and Myd88. TLR7 activation in this setting paradoxically promoted an anti-inflammatory gene signature, indicating cross talk via TLR7 between proinflammatory pathways endogenous to Rps14 loss and the NF-κB pathway. Finally, in highly purified human bone marrow samples from anemic patients, imiquimod led to an increase in erythroid output from myeloerythroid progenitors and common myeloid progenitors. Our findings have both specific implications for the development of targeted therapeutics for del(5q) MDS and wider significance identifying a potential role for TLR7 ligation in modifying anemia.

IL-6 Signaling Protects Zebrafish Larvae during Staphylococcus epidermidis Infection in a Bath Immersion Model

The host immune responses to Staphylococcus epidermidis, a frequent cause of nosocomial infections, are not well understood. We have established a bath immersion model of this infection in zebrafish (Danio rerio) larvae. Macrophages play a primary role in the host immune response and are involved in clearance of infection in the larvae. S. epidermidis infection results in upregulation of tlr-2 There is marked inflammation characterized by heightened NF-κB signaling and elevation of several proinflammatory cytokines. There is rapid upregulation of il-1b and tnf-a transcripts, whereas an increase in il-6 levels is relatively more delayed. The IL-6 signaling pathway is further amplified by elevation of IL-6 signal transducer (il-6st) levels, which negatively correlates with miRNA dre-miR-142a-5p. Enhanced IL-6 signaling is protective to the host in this model as inhibition of the signaling pathway resulted in increased mortality upon S. epidermidis infection. Our study describes the host immune responses to S. epidermidis infection, establishes the importance of IL-6 signaling, and identifies a potential role of miR-142-5p-il-6st interaction in this infection model.

Telomerase RNA recruits RNA polymerase II to target gene promoters to enhance myelopoiesis

Dyskeratosis congenita (DC) is a rare inherited bone marrow failure and cancer predisposition syndrome caused by mutations in telomerase or telomeric proteins. Here, we report that zebrafish telomerase RNA (terc) binds to specific DNA sequences of master myeloid genes and controls their expression by recruiting RNA Polymerase II (Pol II). Zebrafish terc harboring the CR4-CR5 domain mutation found in DC patients hardly interacted with Pol II and failed to regulate myeloid gene expression in vivo and to increase their transcription rates in vitro. Similarly, TERC regulated myeloid gene expression and Pol II promoter occupancy in human myeloid progenitor cells. Strikingly, induced pluripotent stem cells derived from DC patients with a TERC mutation in the CR4-CR5 domain showed impaired myelopoiesis, while those with mutated telomerase catalytic subunit differentiated normally. Our findings show that TERC acts as a transcription factor, revealing a target for therapeutic intervention in DC patients.

Transient, flexible gene editing in zebrafish neutrophils and macrophages for determination of cell-autonomous functions

Zebrafish are an important model for studying phagocyte function, but rigorous experimental systems to distinguish whether phagocyte-dependent effects are neutrophil or macrophage specific have been lacking. We have developed and validated transgenic lines that enable superior demonstration of cell-autonomous neutrophil and macrophage genetic requirements. We coupled well-characterized neutrophil- and macrophage-specific Gal4 driver lines with UAS:Cas9 transgenes for selective expression of Cas9 in either neutrophils or macrophages. Efficient gene editing, confirmed by both Sanger and next-generation sequencing, occurred in both lineages following microinjection of efficacious synthetic guide RNAs into zebrafish embryos. In proof-of-principle experiments, we demonstrated molecular and/or functional evidence of on-target gene editing for several genes (mCherry, lamin B receptor, trim33) in either neutrophils or macrophages as intended. These new UAS:Cas9 tools provide an improved resource for assessing individual contributions of neutrophil- and macrophage-expressed genes to the many physiological processes and diseases modelled in zebrafish. Furthermore, this gene-editing functionality can be exploited in any cell lineage for which a lineage-specific Gal4 driver is available.

This article has an associated First Person interview with the first author of the paper.

Summary: We developed new tools for lineage-specific gene editing in neutrophils or macrophages based on leukocyte-specific Cas9 expression, that can be used with injected synthetic gRNAs.

A zebrafish model of granulin deficiency reveals essential roles in myeloid cell differentiation

Granulin is a pleiotropic protein involved in inflammation, wound healing, neurodegenerative disease, and tumorigenesis. These roles in human health have prompted research efforts to use granulin to treat rheumatoid arthritis and frontotemporal dementia and to enhance wound healing. But how granulin contributes to each of these diverse biological functions remains largely unknown. Here, we have uncovered a new role for granulin during myeloid cell differentiation. We have taken advantage of the tissue-specific segregation of the zebrafish granulin paralogues to assess the functional role of granulin in hematopoiesis without perturbing other tissues. By using our zebrafish model of granulin deficiency, we revealed that during normal and emergency myelopoiesis, myeloid progenitors are unable to terminally differentiate into neutrophils and macrophages in the absence of granulin a (grna), failing to express the myeloid-specific genes cebpa, rgs2, lyz, mpx, mpeg1, mfap4, and apoeb. Functionally, macrophages fail to recruit to the wound, resulting in abnormal healing. Our CUT&RUN experiments identify Pu.1, which together with Irf8, positively regulates grna expression. In vivo imaging and RNA sequencing experiments show that grna inhibits the expression of gata1, leading to the repression of the erythroid program. Importantly, we demonstrated functional conservation between the mammalian granulin and the zebrafish ortholog grna. Our findings uncover a previously unrecognized role for granulin during myeloid cell differentiation, which opens a new field of study that can potentially have an impact on different aspects of human health and expand the therapeutic options for treating myeloid disorders such as neutropenia or myeloid leukemia.

Live-imaging of endothelial Erk activity reveals dynamic and sequential signalling events during regenerative angiogenesis

The formation of new blood vessel networks occurs via angiogenesis during development, tissue repair, and disease. Angiogenesis is regulated by intracellular endothelial signalling pathways, induced downstream of vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs). A major challenge in understanding angiogenesis is interpreting how signalling events occur dynamically within endothelial cell populations during sprouting, proliferation, and migration. Extracellular signal-regulated kinase (Erk) is a central downstream effector of Vegf-signalling and reports the signalling that drives angiogenesis. We generated a vascular Erk biosensor transgenic line in zebrafish using a kinase translocation reporter that allows live-imaging of Erk-signalling dynamics. We demonstrate the utility of this line to live-image Erk activity during physiologically relevant angiogenic events. Further, we reveal dynamic and sequential endothelial cell Erk-signalling events following blood vessel wounding. Initial signalling is dependent upon Ca²⁺ in the earliest responding endothelial cells, but is independent of Vegfr-signalling and local inflammation. The sustained regenerative response, however, involves a Vegfr-dependent mechanism that initiates concomitantly with the wound inflammatory response. This work reveals a highly dynamic sequence of signalling events in regenerative angiogenesis and validates a new resource for the study of vascular Erk-signalling in real-time.

Liposomal Clodronate-mediated Macrophage Depletion in the Zebrafish Model

The ability to conduct in vivo macrophage-specific depletion remains an effective means to uncover functions of macrophages in a wide range of physiological contexts. Compared to the murine model, zebrafish offer superior imaging capabilities due to their optical transparency starting from a single-cell stage to throughout larval development. These qualities become important for in vivo cell specific depletions so that the elimination of the targeted cells can be tracked and validated in real time through microscopy. Multiple methods to deplete macrophages in zebrafish are available, including genetic (such as an irf8 knockout), chemogenetic (such as the nitroreductase/metronidazole system), and toxin-based depletions (such as using clodronate liposomes). The use of clodronate-containing liposomes to induce macrophage apoptosis after phagocytosing the liposomes is effective in depleting macrophages as well as testing their ability to phagocytose. Here we describe a detailed protocol for the systemic depletion of macrophages in zebrafish larvae by intravenous injection of liposomal clodronate supplemented with fluorescent dextran conjugates. Co-injection with the fluorescent dextran allows tracking of macrophage depletion in real time starting with verifying the successful intravenous injection to macrophage uptake of molecules and their eventual death. To verify a high degree of macrophage depletion, the level of brain macrophage (microglia) elimination can be determined by a rapid neutral red vital dye staining when clodronate injection is performed at early larval stages. Graphical abstract: Experimental workflow for in vivo macrophage-specific depletion by liposomal clodronate in larval zebrafish.

Development of BCR-ABL1 Transgenic Zebrafish Model Reproducing Chronic Myeloid Leukemia (CML) Like-Disease and Providing a New Insight into CML Mechanisms

Zebrafish has proven to be a versatile and reliable experimental in vivo tool to study human hematopoiesis and model hematological malignancies. Transgenic technologies enable the generation of specific leukemia types by the expression of human oncogenes under specific promoters. Using this technology, a variety of myeloid and lymphoid malignancies zebrafish models have been described. Chronic myeloid leukemia (CML) is a clonal myeloproliferative neoplasia characterized by the BCR-ABL1 fusion gene, derived from the t (9;22) translocation causing the Philadelphia Chromosome (Ph). The BCR-ABL1 protein is a constitutively activated tyrosine kinas inducing the leukemogenesis and resulting in an accumulation of immature leukemic cells into bone marrow and peripheral blood. To model Ph+ CML, a transgenic zebrafish line expressing the human BCR-ABL1 was generated by the Gal4/UAS system, and then crossed with the hsp70-Gal4 transgenic line. The new line named (BCR-ABL1pUAS:CFP/hsp70-Gal4), presented altered expression of hematopoietic markers during embryonic development compared to controls and transgenic larvae showed proliferating hematopoietic cells in the caudal hematopoietic tissue (CHT). The present transgenic zebrafish would be a robust CML model and a high-throughput drug screening tool.

Transformed notochordal cells trigger chronic wounds destabilizing the vertebral column and bone homeostasis

Notochordal cells play a pivotal role in vertebral column patterning, contributing to the formation of the inner architecture of intervertebral discs (IVDs). Their disappearance during development has been associated with reduced repair capacity and IVD degeneration. Notochord cells can give rise to chordomas, a highly invasive bone cancer associated with late diagnosis. Understanding the impact of neoplastic cells during development and on the surrounding vertebral column could open avenues for earlier intervention and therapeutics. We investigated the impact of transformed notochord cells in the zebrafish skeleton using a line expressing RAS in the notochord under the control of the kita promoter, with the advantage of adulthood endurance. Transformed cells caused damage in the notochord and destabilised the sheath layer, triggering a wound repair mechanism, with enrolment of sheath cells (col9a2⁺) and expression of wt1b, similar to induced notochord wounds. Moreover, increased recruitment of neutrophils and macrophages, displaying abnormal behaviour in proximity to the notochord sheath and transformed cells, supported parallels between chordomas, wound and inflammation. Cancerous notochordal cells interfere with differentiation of sheath cells to form chordacentra domains, leading to fusions and vertebral clefts during development. Adults displayed IVD irregularities reminiscent of degeneration, including reduced bone mineral density and increased osteoclast activity, along with disorganised osteoblasts and collagen, indicating impaired bone homeostasis. By depleting inflammatory cells, we abrogated chordoma development and rescued the skeletal features of the vertebral column. Therefore, we showed that transformed notochord cells alter the skeleton during life, causing a wound-like phenotype and activating chronic wound response, suggesting parallels between chordoma, wound, IVD degeneration and inflammation, highlighting inflammation as a promising target for future therapeutics.

This article has an associated First Person interview with the first author of the paper.

Summary: Analyses using a zebrafish line expressing RAS in the notochord, under the control of the kita promoter, revealed that transformed notochord cells alter the skeleton during life, causing a wound-like phenotype and activating chronic wound response.

Inflammation induces zebrafish regeneration

Tissue or organ regeneration is a complex process with successful outcomes depending on the type of tissue and organism. Upon damage, mammals can only efficiently restore a few tissues including the liver, skin, epithelia of the lung, kidney, and gut. In contrast, lower vertebrates such as zebrafish possess an extraordinary regeneration ability, which restores the normal function of a broad spectrum of tissues including heart, fin, brain, spinal cord, and retina. This regeneration process is either mediated by the proliferation of resident stem cells, or cells that dedifferentiate into a stem cell-like. In recent years, evidence has suggested that the innate immune system can modulate stem cell activity to initiate the regenerative response to damage. This review will explore some of the newer concepts of inflammation in zebrafish regeneration in different tissues. Understanding how inflammation regulates regeneration in zebrafish would provide important clues to improve the therapeutic strategies for repairing injured mammalian tissues that do not have an inherent regenerative capacity.

Inhibition of red blood cell development by arsenic-induced disruption of GATA-1

Anemia is a hematological disorder that adversely affects the health of millions of people worldwide. Although many variables influence the development and exacerbation of anemia, one major contributing factor is the impairment of erythropoiesis. Normal erythropoiesis is highly regulated by the zinc finger transcription factor GATA-1. Disruption of the zinc finger motifs in GATA-1, such as produced by germline mutations, compromises the function of this critical transcription factor and causes dyserythropoietic anemia. Herein, we utilize a combination of in vitro and in vivo studies to provide evidence that arsenic, a widespread environmental toxicant, inhibits erythropoiesis likely through replacing zinc within the zinc fingers of the critical transcription factor GATA-1. We found that arsenic interacts with the N- and C-terminal zinc finger motifs of GATA-1, causing zinc loss and inhibition of DNA and protein binding activities, leading to dyserythropoiesis and an imbalance of hematopoietic differentiation. For the first time, we show that exposures to a prevalent environmental contaminant compromises the function of a key regulatory factor in erythropoiesis, producing effects functionally similar to inherited GATA-1 mutations. These findings highlight a novel molecular mechanism by which arsenic exposure may cause anemia and provide critical insights into potential prevention and intervention for arsenic-related anemias.

Role of thrombopoiesis in leishmaniasis

The blood vascular system of mammals is unique in nature; inhabited with a pool of tiny small cell fragments called platelets; attributed with the most important patrolling tasks to check integrity of the entire endothelial landscape. Their production is tightly coupled with hematopoietic system where everything starts from self renewable multipotent hematopoietic stem cells (HSCs) which eventually undergo dual step (megakaryopoiesis-thrombopoiesis) thrombocytes production. Several cytokines tune the fate of every progenitor cells during hematopoiesis through temporal activation of specific transcription factors. Though platelets generated through steady state hematopoiesis are involved in the regulation of vascular homeostasis, these cells can sense pathogens through its innate immune sensors and can mount crucial responses against the invading pathogen. For this, the primary aim of many infections including Leishmania is to induce thrombocytopenia within infected host. But the underlying mechanism of this induced thrombocytopenia in Leishmania infection has not been evaluated. Elucidation of these mechanisms will be fruitful to design new chemotherapeutic strategies.

Entosis and apical cell extrusion constitute a tumor-suppressive mechanism downstream of Matriptase

Armistead et al. show that in a bilayered epithelium in vivo, apical cell extrusion of basal cells is achieved via their engulfment by surface cells. In zebrafish hai1a mutants, this constitutes a tumor-suppressive mechanism, revealing a double face of Matriptase.

The type II transmembrane serine protease Matriptase 1 (ST14) is commonly known as an oncogene, yet it also plays an understudied role in suppressing carcinogenesis. This double face is evident in the embryonic epidermis of zebrafish loss-of-function mutants in the cognate Matriptase inhibitor Hai1a (Spint1a). Mutant embryos display epidermal hyperplasia, but also apical cell extrusions, during which extruding outer keratinocytes carry out an entosis-like engulfment and entrainment of underlying basal cells, constituting a tumor-suppressive effect. These counteracting Matriptase effects depend on EGFR and the newly identified mediator phospholipase D (PLD), which promotes both mTORC1-dependent cell proliferation and sphingosine-1-phosphate (S1P)–dependent entosis and apical cell extrusion. Accordingly, hypomorphic hai1a mutants heal spontaneously, while otherwise lethal hai1a amorphs are efficiently rescued upon cotreatment with PLD inhibitors and S1P. Together, our data elucidate the mechanisms underlying the double face of Matriptase function in vivo and reveal the potential use of combinatorial carcinoma treatments when such double-face mechanisms are involved.

Fast, In Vivo Model for Drug-Response Prediction in Patients with B-Cell Precursor Acute Lymphoblastic Leukemia

Only half of patients with relapsed B-cell precursor (BCP) acute lymphoblastic leukemia (ALL) currently survive with standard treatment protocols. Predicting individual patient responses to defined drugs prior to application would help therapy stratification and could improve survival. With the purpose to aid personalized targeted treatment approaches, we developed a human–zebrafish xenograft (ALL-ZeFiX) assay to predict drug response in a patient in 5 days. Leukemia blast cells were pericardially engrafted into transiently immunosuppressed Danio rerio embryos, and engrafted embryos treated for the test case, venetoclax, before single-cell dissolution for quantitative whole blast cell analysis. Bone marrow blasts from patients with newly diagnosed or relapsed BCP-ALL were successfully expanded in 60% of transplants in immunosuppressed zebrafish embryos. The response of BCP-ALL cell lines to venetoclax in ALL-ZeFiX assays mirrored responses in 2D cultures. Venetoclax produced varied responses in patient-derived BCP-ALL grafts, including two results mirroring treatment responses in two refractory BCP-ALL patients treated with venetoclax. Here we demonstrate proof-of-concept for our 5-day ALL-ZeFiX assay with primary patient blasts and the test case, venetoclax, which after expanded testing for further targeted drugs could support personalized treatment decisions within the clinical time window for decision-making.

A fish cytokine related to human IL-3, IL-5, and GM-CSF, induces development of eosinophil/basophil/mast-cell type (EBM) granulocytes

Interleukin-3 (IL-3), IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF) are related cytokines that signal through receptors possessing the β common (βc) chain. As a family, these cytokines combine rather non-specific hematopoietic growth factor properties with a special importance for eosinophils, basophils, and mast cells. In fish the cytokines of this family are called IL-5fam, and the present study, using carp, constitutes their first functional analysis. Carp il-5fam expression was enhanced by stimulation with phytohemagglutinin and killed bacteria. Reminiscent of mammalian IL-3/IL-5/GM-CSF family members, recombinant carp IL-5fam (rcIL-5fam) induced activation of transcription factor STAT5 and efficiently promoted proliferation and colony-formation of eosinophil/basophil/mast-cell type (EBM) granulocytes. Upon addition of recombinant carp βc the growth effect of rcIL-5fam was reduced, suggesting βc participation in the signaling route. In summary, despite differences in individual cytokines and cell populations, fish and mammalian IL-3/IL-5/GM-CSF family members share growth factor functions for non-neutrophil granulocytes.

Zebrafish (Danio rerio) as an animal model for bath infection by Flavobacterium psychrophilum

Flavobacterium psychrophilum is the causative agent of bacterial cold-water disease and rainbow trout syndrome in freshwater salmonid fish worldwide, generating injuries and high mortality rates. Despite several studies on this bacterium, the infection mechanism remains unknown due to limitations in the employed animal models. In this work, we propose using zebrafish (Danio rerio) as a model for studying bacterial pathogenicity. To substantiate this proposal, zebrafish infection by F. psychrophilum strain JIP 02/86 was characterized. Zebrafish larvae were infected using the bath method, and morphological changes and innate immune system activation were monitored using transgenic fish. Salmonid-like infection phenotypes were observed in 4.74% of treated larvae, as manifested by fin, muscle and caudal peduncle damage. Symptomatic and dead larvae accounted for 1.35% of all challenged larvae. Interestingly, infected larvae with no infection phenotypes showed stronger innate immune system activation than specimens with phenotypes. A failure of function assay for myeloid factor pu.1 resulted in more infected larvae (up to 43.5%), suggesting that low infection rates by F. psychrophilum would be due to the protective actions of the innate immune system against this bacterium in zebrafish larvae. Our results support the use of zebrafish as an infection model for studying F. psychrophilum. Furthermore, the percentage of infected fish can be modulated by disturbing, to varying extents, the differentiation of myeloid cells. Using this evidence as a starting point, different aspects of the infection mechanism of F. psychrophilum could be studied in vivo.

Dysregulation of NIPBL leads to impaired RUNX1 expression and haematopoietic defects

The transcription factor RUNX1, a pivotal regulator of HSCs and haematopoiesis, is a frequent target of chromosomal translocations, point mutations or altered gene/protein dosage. These modifications lead or contribute to the development of myelodysplasia, leukaemia or platelet disorders. A better understanding of how regulatory elements contribute to fine‐tune the RUNX1 expression in haematopoietic tissues could improve our knowledge of the mechanisms responsible for normal haematopoiesis and malignancy insurgence. The cohesin RAD21 was reported to be a regulator of RUNX1 expression in the human myeloid HL60 cell line and during primitive haematopoiesis in zebrafish. In our study, we demonstrate that another cohesin, NIPBL, exerts positive regulation of RUNX1 in three different contexts in which RUNX1 displays important functions: in megakaryocytes derived from healthy donors, in bone marrow samples obtained from adult patients with acute myeloid leukaemia and during zebrafish haematopoiesis. In this model, we demonstrate that alterations in the zebrafish orthologue nipblb reduce runx1 expression with consequent defects in its erythroid and myeloid targets such as gata1a and spi1b in an opposite way to rad21. Thus, also in the absence of RUNX1 translocation or mutations, additional factors such as defects in the expression of NIPBL might induce haematological diseases.

Efficacy of Voriconazole against Aspergillus fumigatus Infection Depends on Host Immune Function

Antifungal therapy can fail in a remarkable number of patients with invasive fungal disease, resulting in significant morbidity worldwide. A major contributor to this failure is that while these drugs have high potency in vitro, we do not fully understand how they work inside infected hosts. Here, we used a transparent larval zebrafish model of Aspergillus fumigatus infection amenable to real-time imaging of invasive disease as an in vivo intermediate vertebrate model to investigate the efficacy and mechanism of the antifungal drug voriconazole. We found that the ability of voriconazole to protect against A. fumigatus infection depends on host innate immune cells and, specifically, on the presence of macrophages. While voriconazole inhibits fungal spore germination and growth in vitro, it does not do so in larval zebrafish. Instead, live imaging of whole, intact larvae over a multiday course of infection revealed that macrophages slow down initial fungal growth, allowing voriconazole time to target and kill A. fumigatus hyphae postgermination. These findings shed light on how antifungal drugs such as voriconazole may synergize with the immune response in living hosts.

Determining macrophage versus neutrophil contributions to innate immunity using larval zebrafish

The specific roles of the two major innate immune cell types – neutrophils and macrophages – in response to infection and sterile inflammation are areas of great interest. The larval zebrafish model of innate immunity, and the imaging capabilities it provides, is a source of new research and discoveries in this field. Multiple methods have been developed in larval zebrafish to specifically deplete functional macrophages or neutrophils. Each of these has pros and cons, as well as caveats, that often make it difficult to directly compare results from different studies. The purpose of this Review is to (1) explore the pros, cons and caveats of each of these immune cell-depleted models; (2) highlight and place into a broader context recent key findings on the specific functions of innate immune cells using these models; and (3) explore future directions in which immune cell depletion methods are being expanded.

Summary: Macrophages and neutrophils are distinct innate immune cells with diverse roles in diverse inflammatory contexts. Recent research in larval zebrafish using cell-specific depletion methods has revealed new insights into these cells' functions.

Elmo1 function, linked to Rac1 activity, regulates peripheral neuronal numbers and myelination in zebrafish

Peripheral nervous system development involves a tight coordination of neuronal birth and death and a substantial remodelling of the myelinating glia cytoskeleton to achieve myelin wrapping of its projecting axons. However, how these processes are coordinated through time is still not understood. We have identified engulfment and cell motility 1, Elmo1, as a novel component that regulates (i) neuronal numbers within the Posterior Lateral Line ganglion and (ii) radial sorting of axons by Schwann cells (SC) and myelination in the PLL system in zebrafish. Our results show that neuronal and myelination defects observed in elmo1 mutant are rescued through small GTPase Rac1 activation. Inhibiting macrophage development leads to a decrease in neuronal numbers, while peripheral myelination is intact. However, elmo1 mutants do not show defective macrophage activity, suggesting a role for Elmo1 in PLLg neuronal development and SC myelination independent of macrophages. Forcing early Elmo1 and Rac1 expression specifically within SCs rescues elmo1-/- myelination defects, highlighting an autonomous role for Elmo1 and Rac1 in radial sorting of axons by SCs and myelination. This uncovers a previously unknown function of Elmo1 that regulates fundamental aspects of PNS development.

Neurons Expressing Pathological Tau Protein Trigger Dramatic Changes in Microglial Morphology and Dynamics

Microglial cells, the resident macrophages of the brain, are important players in the pathological process of numerous neurodegenerative disorders, including tauopathies, a heterogeneous class of diseases characterized by intraneuronal Tau aggregates. However, microglia response in Tau pathologies remains poorly understood. Here, we exploit a genetic zebrafish model of tauopathy, combined with live microglia imaging, to investigate the behavior of microglia in vivo in the disease context. Results show that while microglia were almost immobile and displayed long and highly dynamic branches in a wild-type context, in presence of diseased neurons, cells became highly mobile and displayed morphological changes, with highly mobile cell bodies together with fewer and shorter processes. We also imaged, for the first time to our knowledge, the phagocytosis of apoptotic tauopathic neurons by microglia in vivo and observed that microglia engulfed about as twice materials as in controls. Finally, genetic ablation of microglia in zebrafish tauopathy model significantly increased Tau hyperphosphorylation, suggesting that microglia provide neuroprotection to diseased neurons. Our findings demonstrate for the first time the dynamics of microglia in contact with tauopathic neurons in vivo and open perspectives for the real-time study of microglia in many neuronal diseases.

Interleukin-6 signaling regulates hematopoietic stem cell emergence

Hematopoietic stem cells (HSCs) produce all lineages of mature blood cells for the lifetime of an organism. In vertebrates, HSCs derive from the transition of the hemogenic endothelium (HE) in the floor of the embryonic dorsal aorta. Most recently, a series of proinflammatory factors, such as tumor necrosis factor-α, interferon-γ, and Toll-like receptor 4, have been confirmed to play a key role in HSC specification. However, the full complement of necessary signaling inputs remains unknown to date. Here, we show that interleukin-6R (IL6R) via IL6 is required and sufficient for HSC generation. We found that Notch activates IL6R by regulating its expression in the HE and in HSCs. The secretion of IL6 mainly originates from HSC-independent myeloid cells, but not from HSCs and their adjacent vascular endothelial cells. In addition, blocking IL6 signaling does not affect vascular development or the production of primitive erythrocytes. Taken together, our results uncover a previously obscure relationship between IL6 signaling and HSC production and provide new insights into HSC regeneration using proinflammatory factors in vitro.

A molecule that triggers inflammation, interleukin-6 (IL6), is crucial for development of blood stem cells, known as hematopoetic stem cells (HSCs). HSCs can differentiate into any type of blood or immune cell, and have enormous therapeutic potential. Although some of the signaling molecules that trigger their development are known, HSCs have yet to be grown under laboratory conditions, so knowledge gaps remain. Based on clues that IL6 might be one of the missing signals, He Huang at the Zhejiang University School of Medicine, Hangzhou, China, and coworkers investigated how IL6 affects HSC development in zebrafish, a common model of development. Zebrafish embryos lacking IL6 were deficient in HSCs, but showed normal blood vessel growth. Treating with IL6 restored HSC production. These results represent another step toward being able to grow HSCs for transplantation and therapy.

Protective Effect and Mechanisms of New Gelatin on Chemotherapy-Induced Hematopoietic Injury Zebrafish Model

The aim of the study is to explore the protective effect of new gelatin (NG, Xin'ejiao in China) on hematopoietic injury caused by chemotherapy. Zebrafish, at 48 hours post fertilization (hpf), was treated with different chemotherapeutic drugs to establish the zebrafish hematopoietic damage model with reduced thrombocytes and erythrocytes. The protecting effects of NG on the thrombocytes and erythrocytes were observed, respectively, on zebrafish models. Then, the RT-PCR method was used to detect the change of mRNA level of the hematopoiesis-related cytokines scl1, c-myb, pu.1, GATA1, and runx1 genes. The results showed that 50 μg·mL⁻¹ and 100 μg·mL⁻¹ NG rescued and increased the thrombocytes numbers induced by vinorelbine (NVB) and chloramphenicol (CHL) and the erythrocytes numbers induced by methotrexate (MTX), doxorubicin (ADM), and mechlorethamine hydrochloride (MH) in zebrafish models. Meanwhile, the mRNA expression of scl1, c-myb, and GATA1 genes in the NG treatment group was raised compared with the MTX treatment group. Also, the mRNA expression of pu.1 and Runx1 in the NG treatment group was reduced compared with the MTX treatment group. In consequence, traditional Chinese medicine NG showed a certain degree protective effect on hematopoiesis injury induced by chemotherapy in this study, which may depend on the promotion of erythrocytes proliferation and the regulation of the hematopoietic genes level.

Aspergillus fumigatus establishes infection in zebrafish by germination of phagocytized conidia, while Aspergillus niger relies on extracellular germination

Among opportunistically pathogenic filamentous fungi of the Aspergillus genus, Aspergillus fumigatus stands out as a drastically more prevalent cause of infection than others. Utilizing the zebrafish embryo model, we applied a combination of non-invasive real-time imaging and genetic approaches to compare the infectious development of A. fumigatus with that of the less pathogenic A. niger. We found that both species evoke similar immune cell migratory responses, but A. fumigatus is more efficiently phagocytized than A. niger. Though efficiently phagocytized, A. fumigatus conidia retains the ability to germinate and form hyphae from inside macrophages leading to serious infection even at relatively low infectious burdens. By contrast, A. niger appears to rely on extracellular germination, and rapid hyphal growth to establish infection. Despite these differences in the mechanism of infection between the species, galactofuranose mutant strains of both A. fumigatus and A. niger display attenuated pathogenesis. However, deficiency in this cell wall component has a stronger impact on A. niger, which is dependent on rapid extracellular hyphal growth. In conclusion, we uncover differences in the interaction of the two fungal species with innate immune cells, noticeable from very early stages of infection, which drive a divergence in their route to establishing infections.

EvpP inhibits neutrophils recruitment via Jnk-caspy inflammasome signaling in vivo

Innate immunity is regulated by phagocytic cells and is critical for host control of bacterial infection. In many bacteria, the type VI secretion system (T6SS) can affect bacterial virulence in certain environments, but little is known about the mechanisms underlying T6SS regulation of innate immune responses during infection in vivo. Here, we developed an infection model by microinjecting bacteria into the tail vein muscle of 3-day-post-fertilized zebrafish larvae, and found that both macrophages and neutrophils are essential for bacterial clearance. Further study revealed that EvpP plays a critical role in promoting the pathogenesis of Edwardsiella piscicida (E. piscicida) via inhibiting the phosphorylation of Jnk signaling to reduce the expression of chemokine (CXC motif) ligand 8 (cxcl8a), matrix metallopeptidase 13 (mmp13) and interleukin-1β (IL-1β) in vivo. Subsequently, by utilizing Tg (mpo:eGFP^+/+) zebrafish larvae for E. piscicida infection, we found that the EvpP-inhibited Jnk-caspy (caspase-1 homolog) inflammasome signaling axis significantly suppressed the recruitment of neutrophils to infection sites, and the caspy- or IL-1β-morpholino (MO) knockdown larvae were more susceptible to infection and failed to restrict bacterial colonization in vivo. taken together, this interaction improves our understanding about the complex and contextual role of a bacterial T6SS effector in modulating the action of neutrophils during infection, and offers new insights into the warfare between bacterial weapons and host immunological surveillance.