Nuclear SUN1 stabilizes endothelial cell junctions via microtubules to regulate blood vessel formation

Endothelial cells line all blood vessels, where they coordinate blood vessel formation and the blood-tissue barrier via regulation of cell-cell junctions. The nucleus also regulates endothelial cell behaviors, but it is unclear how the nucleus contributes to endothelial cell activities at the cell periphery. Here, we show that the nuclear-localized linker of the nucleoskeleton and cytoskeleton (LINC) complex protein SUN1 regulates vascular sprouting and endothelial cell-cell junction morphology and function. Loss of murine endothelial Sun1 impaired blood vessel formation and destabilized junctions, angiogenic sprouts formed but retracted in SUN1-depleted sprouts, and zebrafish vessels lacking Sun1b had aberrant junctions and defective cell-cell connections. At the cellular level, SUN1 stabilized endothelial cell-cell junctions, promoted junction function, and regulated contractility. Mechanistically, SUN1 depletion altered cell behaviors via the cytoskeleton without changing transcriptional profiles. Reduced peripheral microtubule density, fewer junction contacts, and increased catastrophes accompanied SUN1 loss, and microtubule depolymerization phenocopied effects on junctions. Depletion of GEF-H1, a microtubule-regulated Rho activator, or the LINC complex protein nesprin-1 rescued defective junctions of SUN1-depleted endothelial cells. Thus, endothelial SUN1 regulates peripheral cell-cell junctions from the nucleus via LINC complex-based microtubule interactions that affect peripheral microtubule dynamics and Rho-regulated contractility, and this long-range regulation is important for proper blood vessel sprouting and junction integrity.

Peripheral NOD-like receptor deficient inflammatory macrophages trigger neutrophil infiltration into the brain disrupting daytime locomotion

Inflammation is known to disrupt normal behavior, yet the underlying neuroimmune interactions remain elusive. Here, we investigated whether inappropriate macrophage-evoked inflammation alters CNS control of daily-life animal locomotion using a set of zebrafish mutants selected for specific macrophage dysfunction and microglia deficiency. Large-scale genetic and computational analyses revealed that NOD-like receptor nlrc3l mutants are capable of normal motility and visuomotor response, but preferentially swim less in the daytime, suggesting possible low motivation rather than physical impairment. Examining their brain activities and structures implicates impaired dopaminergic descending circuits, where neutrophils abnormally infiltrate. Furthermore, neutrophil depletion recovered daytime locomotion. Restoring wild-type macrophages reversed behavioral and neutrophil aberrations, while three other microglia-lacking mutants failed to phenocopy nlrc3l mutants. Overall, we reveal how peripheral inflammatory macrophages with elevated pro-inflammatory cues (including il1β, tnfα, cxcl8a) in the absence of microglia co-opt neutrophils to infiltrate the brain, thereby potentially enabling local circuitry modulation affecting daytime locomotion.

Zebrafish harbor diverse intestinal macrophage populations including a subset intimately associated with enteric neural processes

Intestinal macrophages are essential for gut health but remain understudied outside of human and mouse systems. Here, we establish zebrafish as a powerful model that provides superior imaging capabilities for whole-gut analysis along all dimensions (anterior-posterior and center-outer axes) for dissecting macrophage biology in gastrointestinal health and disease. We utilized high-resolution imaging to show that the zebrafish gut contains bona fide muscularis and mucosal macrophages, as well as surprisingly large subsets intimately associated with enteric neural processes. Interestingly, most muscularis macrophages span multiple gut layers in stark contrast to their mammalian counterparts typically restricted to a single layer. Using macrophage-deficient irf8 zebrafish, we found a depletion of muscularis but not mucosal macrophages, and that they may be dispensable for gross intestinal transit in adults but not during development. These characterizations provide first insights into intestinal macrophages and their association with the enteric nervous system from development to adulthood in teleosts.

Brain-localized and Intravenous Microinjections in the Larval Zebrafish to Assess Innate Immune Response

Creating a robust and controlled infection model is imperative for studying the innate immune response. Leveraging the particular strengths of the zebrafish model system, such as optical transparency, ex utero development, and large clutch size, allows for the development of methods that yield consistent and reproducible results. We created a robust model for activation of innate immunity by microinjecting bacterial particles or live bacteria into larval zebrafish, unlike previous studies which largely restricted such manipulations to embryonic stages of zebrafish. The ability to introduce stimuli locally or systemically at larval stages provides significant advantages to examine host response in more mature tissues as well as the possibility to interrogate adaptive immunity at older larval stages. This protocol describes two distinct modes of microinjection to introduce lipopolysaccharide (LPS) or bacteria into the living larval zebrafish: one localized to the brain, and another into the bloodstream via the caudal vein plexus. Graphic abstract: Schematic shows the two distinct modes of larval zebrafish microinjection, either in the brain parenchyma or in the blood stream intravenously. Reagents introduced into the zebrafish to assess immune response are depicted in the "injection components" as described in the protocol.

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.

Drainage of inflammatory macromolecules from the brain to periphery targets the liver for macrophage infiltration

Many brain pathologies are associated with liver damage, but a direct link has long remained elusive. Here, we establish a new paradigm for interrogating brain-periphery interactions by leveraging zebrafish for its unparalleled access to the intact whole animal for in vivo analysis in real time after triggering focal brain inflammation. Using traceable lipopolysaccharides (LPS), we reveal that drainage of these inflammatory macromolecules from the brain led to a strikingly robust peripheral infiltration of macrophages into the liver independent of Kupffer cells. We further demonstrate that this macrophage recruitment requires signaling from the cytokine IL-34 and Toll-like receptor adaptor MyD88, and occurs in coordination with neutrophils. These results highlight the possibility for circulation of brain-derived substances to serve as a rapid mode of communication from brain to the liver. Understanding how the brain engages the periphery at times of danger may offer new perspectives for detecting and treating brain pathologies.

Critical Role for a Subset of Intestinal Macrophages in Shaping Gut Microbiota in Adult Zebrafish

The gut microbiota is strongly influenced by environmental factors, although host contribution is far less understood. We leveraged macrophage-deficient interferon regulatory factor irf8 zebrafish mutants to investigate the role of macrophages in this process. In conventionally raised adult irf8-deficient mutants, we found a significant loss of intestinal macrophages associated with a strikingly altered gut microbiota when compared to co-housed siblings. The destabilization of the gut commensal microbiota was associated with a severe reduction in complement C1q genes and outgrowth of a rare bacterial species. Consistent with a critical function of irf8 in adult intestinal macrophages, irf8 is abundantly expressed in these cells normally, and restoring macrophage irf8 expression in irf8 mutants was sufficient to recover commensal microbes and C1q genes expression. This study reports an important subpopulation of intestinal macrophages that requires irf8 to establish in the gut, ensure normal colonization of gut microbes, and prevent immune dysregulation.

Genetic analysis of zebrafish homologs of human FOXQ1, foxq1a and foxq1b, in innate immune cell development and bacterial host response

FOXQ1 is a member of the forkhead-box transcription factor family that has important functions in development, cancer, aging, and many cellular processes. The role of FOXQ1 in cancer biology has raised intense interest, yet much remains poorly understood. We investigated the possible function of the two zebrafish orthologs (foxq1a and foxq1b) of human FOXQ1 in innate immune cell development and function. We employed CRISPR-Cas9 targeted mutagenesis to create null mutations of foxq1a and foxq1b in zebrafish. Using a combination of molecular, cellular, and embryological approaches, we characterized single and double foxq1a bcz11 and foxq1b bcz18 mutants. This study provides the first genetic mutant analyses of zebrafish foxq1a and foxq1b. Interestingly, we found that foxq1a, but not foxq1b, was transcriptionally regulated during a bacterial response, while the expression of foxq1a was detected in sorted macrophages and upregulated in foxq1a-deficient mutants. However, the transcriptional response to E. coli challenge of foxq1a and foxq1b mutants was not significantly different from that of their wildtype control siblings. Our data shows that foxq1a may have a role in modulating bacterial response, while both foxq1a and foxq1b are not required for the development of macrophages, neutrophils, and microglia. Considering the implicated role of FOXQ1 in a vast number of cancers and biological processes, the foxq1a and foxq1b null mutants from this study provide useful genetic models to further investigate FOXQ1 functions.

Differential requirement for irf8 in formation of embryonic and adult macrophages in zebrafish

Interferon regulatory factor 8 (Irf8) is critical for mammalian macrophage development and innate immunity, but its role in teleost myelopoiesis remains incompletely understood. In particular, genetic tools to analyze the role of Irf8 in zebrafish macrophage development at larval and adult stages are lacking. We generated irf8 null mutants in zebrafish using TALEN-mediated targeting. Our analysis defines different requirements for irf8 at different stages. irf8 is required for formation of all macrophages during primitive and transient definitive hematopoiesis, but not during adult-phase definitive hematopoiesis starting at 5-6 days postfertilization. At early stages, irf8 mutants have excess neutrophils and excess cell death in pu.1-expressing myeloid cells. Macrophage fates were recovered in irf8 mutants after wildtype irf8 expression in neutrophil and macrophage lineages, suggesting that irf8 regulates macrophage specification and survival. In juvenile irf8 mutant fish, mature macrophages are present, but at numbers significantly reduced compared to wildtype, indicating an ongoing requirement for irf8 after embryogenesis. As development progresses, tissue macrophages become apparent in zebrafish irf8 mutants, with the possible exception of microglia. Our study defines distinct requirement for irf8 in myelopoiesis before and after transition to the adult hematopoietic system.

An anti-inflammatory NOD-like receptor is required for microglia development

Microglia are phagocytic cells that form the basis of the brain's immune system. They derive from primitive macrophages that migrate into the brain during embryogenesis, but the genetic control of microglial development remains elusive. Starting with a genetic screen in zebrafish, we show that the noncanonical NOD-like receptor (NLR) nlrc3-like is essential for microglial formation. Although most NLRs trigger inflammatory signaling, nlrc3-like acts cell autonomously in microglia precursor cells to suppress unwarranted inflammation in the absence of overt immune challenge. In nlrc3-like mutants, primitive macrophages initiate a systemic inflammatory response with increased proinflammatory cytokines and actively aggregate instead of migrating into the brain to form microglia. NLRC3-like requires both its pyrin and NACHT domains, and it can bind the inflammasome component apoptosis-associated speck-like protein. Our studies suggest that NLRC3-like may regulate the inflammasome and other inflammatory pathways. Together, these results demonstrate that NLRC3-like prevents inappropriate macrophage activation, thereby allowing normal microglial development.

An effective assay for high cellular resolution time-lapse imaging of sensory placode formation and morphogenesis

Background

The vertebrate peripheral nervous system contains sensory neurons that arise from ectodermal placodes. Placodal cells ingress to move inside the head to form sensory neurons of the cranial ganglia. To date, however, the process of placodal cell ingression and underlying cellular behavior are poorly understood as studies have relied upon static analyses on fixed tissues. Visualizing placodal cell behavior requires an ability to distinguish the surface ectoderm from the underlying mesenchyme. This necessitates high resolution imaging along the z-plane which is difficult to accomplish in whole embryos. To address this issue, we have developed an imaging system using cranial slices that allows direct visualization of placode formation.

Results

We demonstrate an effective imaging assay for capturing placode development at single cell resolution using chick embryonic tissue ex vivo. This provides the first time-lapse imaging of mitoses in the trigeminal placodal ectoderm, ingression, and intercellular contacts of placodal cells. Cell divisions with varied orientations were found in the placodal ectoderm all along the apical-basal axis. Placodal cells initially have short cytoplasmic processes during ingression as young neurons and mature over time to elaborate long axonal processes in the mesenchyme. Interestingly, the time-lapse imaging data reveal that these delaminating placodal neurons begin ingression early on from within the ectoderm, where they start to move and continue on to exit as individual or strings of neurons through common openings on the basal side of the epithelium. Furthermore, dynamic intercellular contacts are abundant among the delaminating placodal neurons, between these and the already delaminated cells, as well as among cells in the forming ganglion.

Conclusions

This new imaging assay provides a powerful method to analyze directly development of placode-derived sensory neurons and subsequent ganglia formation for the first time in amniotes. Viewing placode development in a head cross-section provides a vantage point from which it is possible to study comprehensive events in placode formation, from differentiation, cell ingression to ganglion assembly. Understanding how placodal neurons form may reveal a new mechanism of neurogenesis distinct from that in the central nervous system and provide new insight into how cells acquire motility from a stationary epithelial cell type.

Altering Glypican-1 levels modulates canonical Wnt signaling during trigeminal placode development

Glypicans are conserved cell surface heparan sulfate proteoglycans expressed in a spatiotemporally regulated manner in many developing tissues including the nervous system. Here, we show that Glypican-1 (GPC1) is expressed by trigeminal placode cells as they ingress and contribute to trigeminal sensory neurons in the chick embryo. Either expression of full-length or truncated GPC1 in vivo causes defects in trigeminal gangliogenesis in a manner that requires heparan sulfate side chains. This leads to either abnormal placodal differentiation or organization, respectively, with near complete loss of the ophthalmic (OpV) trigeminal ganglion in the most severe cases after overexpression of full-length GPC1. Interestingly, modulating GPC1 alters levels of endogenous Wnt signaling activity in the forming trigeminal ganglion, as indicated by Wnt reporter expression. Accordingly, GPC1 overexpression phenocopies Wnt inhibition in causing loss of OpV placodal neurons. Furthermore, increased Wnt activity rescues the effects of GPC1 overexpression. Taken together, these results suggest that appropriate levels of GPC1 are essential for proper regulation of canonical Wnt signaling during differentiation and organization of trigeminal placodal cells into ganglia.

N-cadherin acts in concert with Slit1-Robo2 signaling in regulating aggregation of placode-derived cranial sensory neurons

Vertebrate cranial sensory ganglia have a dual origin from the neural crest and ectodermal placodes. In the largest of these, the trigeminal ganglion, Slit1-Robo2 signaling is essential for proper ganglion assembly. Here, we demonstrate a crucial role for the cell adhesion molecule N-cadherin and its interaction with Slit1-Robo2 during gangliogenesis in vivo. A common feature of chick trigeminal and epibranchial ganglia is the expression of N-cadherin and Robo2 on placodal neurons and Slit1 on neural crest cells. Interestingly, N-cadherin localizes to intercellular adherens junctions between placodal neurons during ganglion assembly. Depletion of N-cadherin causes loss of proper ganglion coalescence, similar to that observed after loss of Robo2, suggesting that the two pathways might intersect. Consistent with this possibility, blocking or augmenting Slit-Robo signaling modulates N-cadherin protein expression on the placodal cell surface concomitant with alteration in placodal adhesion. Lack of an apparent change in total N-cadherin mRNA or protein levels suggests post-translational regulation. Co-expression of N-cadherin with dominant-negative Robo abrogates the Robo2 loss-of-function phenotype of dispersed ganglia, whereas loss of N-cadherin reverses the aberrant aggregation induced by increased Slit-Robo expression. Our study suggests a novel mechanism whereby N-cadherin acts in concert with Slit-Robo signaling in mediating the placodal cell adhesion required for proper gangliogenesis.

Robo2-Slit1 dependent cell-cell interactions mediate assembly of the trigeminal ganglion

Vertebrate cranial sensory ganglia, responsible for sensation of touch, taste and pain in the face and viscera, are composed of both ectodermal placode and neural crest cells. The cellular and molecular interactions allowing generation of complex ganglia remain unknown. Here, we show that proper formation of the trigeminal ganglion, the largest of the cranial ganglia, relies on reciprocal interactions between placode and neural crest cells in chick, as removal of either population resulted in severe defects. We demonstrate that ingressing placode cells express the Robo2 receptor and early migrating cranial neural crest cells express its cognate ligand Slit1. Perturbation of this receptor-ligand interaction by blocking Robo2 function or depleting either Robo2 or Slit1 using RNA interference disrupted proper ganglion formation. The resultant disorganization mimics the effects of neural crest ablation. Thus, our data reveal a novel and essential role for Robo2-Slit1 signaling in mediating neural crest-placode interactions during trigeminal gangliogenesis.

Identification of candidate secreted factors involved in trigeminal placode induction

Cranial ectodermal placodes are critical for normal development of the peripheral nervous system of the head. However, many aspects of the molecular and tissue interactions involved in their induction have yet to be elucidated. The trigeminal placode is induced by an unidentified secreted factor(s) from the dorsal neural tube. To determine candidates that may be involved in this induction process, we have performed reverse transcriptase-polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization to screen for receptors expressed by uninduced presumptive trigeminal level ectoderm. We have found that receptors for fibroblast growth factors, insulin-like growth factors, platelet-derived growth factors, Sonic hedgehog, the transforming growth factor-beta superfamily, and Wnts all are expressed in patterns consistent with a role in trigeminal placode formation. This RT-PCR screen for candidate receptors expressed in presumptive trigeminal ectoderm is the first systematic screen to identify potential interactions underlying induction of the trigeminal placode and represents a critical step for understanding this complex process.