We thank Christine Holt and Bill Harris for help with confocal microscopy. We thank Darren Gilmour for the fluorescent timer backbone constructs and Anna Huttenlocher for the Tol2-Lyz backbone vector. We thank Steve Renshaw for the Tg(mpx:GFP)i114 line. This work was supported by the MRC (MR/L019523/1), the Wellcome Trust [204845/Z/16/Z]; Isaac Newton Trust [12.21(a)i] and an Isaac Newton Trust grant 19.23(n). C.C. was supported by an MRC DTP studentship and partly by the Wellcome Trust [204845/Z/16/Z]; Isaac Newton Trust [12.21(a)i] grant. H.W. was supported by an MRC DTP Studentship. H.P. was supported by a Wellcome Trust PhD grant (105391/Z/14/Z) and partly by a Wellcome Trust [204845/Z/16/Z]; Isaac Newton Trust [12.21(a)i] grant and the MRC (MR/L019523/1).

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</ol>

The authors declare no conflict of interest

The method described allows live imaging of receptor dynamics in response to endogenous ligands in situ during an inflammatory response to tissue damage. The use of Cxcr1/Cxcr2 neutrophil reporters could be expanded to other physiological settings, such as infection, tumor models or other types of tissue damage14,25,26,27. In addition, transgenic rescue lines, in which the endogenous receptor is suppressed and rescued by an exogenous mutant receptor, could provide useful tools to dissect the importance of specific neutrophil migration patterns in immune responses. For example, Cxcr1 receptor mutants that have impaired desensitization cause more prominent neutrophil clustering at inflammatory sites14. This gain of function phenotype could be used to understand the role of neutrophil congregation in different physiological processes, e.g., wound repair, infectious disease, or tumor evolution, and complement receptor knockdown/knockout experiments. The methodology also provides a basis to expand the range of available reporters. The choice of fluorescent reporter is important to consider and depends on the biological question. We found that the constitutive turnover of these chemokine receptors in neutrophils was high, in comparison to epithelial cells, and that reporters with fast maturation (e.g., sfGFP) were required to report membrane levels at steady state and resolve differences upon neutrophil stimulation8,14. Thus, membrane ratios of sfGFP/tagRFP are not applicable for measuring ligand-induced internalization in this cell type, but the pattern of tagRFP allows tracking of the intracellular fates of the receptor, which could be useful in some studies. We also found that the more concentrated intracellular signal of tagRFP is useful for screening individual larvae. An alternative approach for measuring receptor levels at the plasma membrane would be to co-express a fluorescent membrane marker in neutrophils either in the same transgene9 or in an independent transgene14. In the former scenario the transgene would provide an additional means for screening the fish and expression levels would be comparable between the marker and the receptor. The latter approach would be more modular, in that a zebrafish line with a receptor reporter could be combined with different reporter lines. In either case, it is worth noting that membrane quantification of the receptor levels is challenging in clustered neutrophils (see below). Finally, we note that a possible extension of this protocol would be to follow up the live imaging by immunohistochemistry for more detailed localization analyses.
The Tol2 transgenesis system is well established7 and the lysozyme C promoter has been used extensively for neutrophil expression11,15. The transgenesis approach is, therefore, relatively straightforward and the expression level achieved with this promoter is high enough to provide sufficient contrast for analysis of receptor dynamics. A possible limitation is that the expression level does not recapitulate endogenous receptor expression levels. New CRISPR technologies could be deployed to establish knock-in lines for receptors of particular interest28. These technologies are still cumbersome and may not guarantee the required expression levels for subcellular imaging, but their successful development would be an important breakthrough for understanding endogenous signaling dynamics. Functional validations are important for interpreting data with transgenic receptor constructs. For example, ligand recognition assays can be used to establish that the fluorescent fusion protein is functional and rescue of knockout phenotypes could be used to establish that the transgenic neutrophil expression levels are compatible with functionality14. Finally, a more direct way to validate the receptor fusion would be to utilize an in vitro functional assay with labelled receptor alongside non-labeled versions14.
The quantification approach addresses specific difficulties in accurate membrane segmentation in neutrophils in vivo. In cells of epithelial nature, quantification of receptor levels can be executed automatically by normalizing membrane receptor levels to a control marker, which can be expressed in tandem or separately9. Indeed, we have applied such an approach, when using the ligand-recognition assay in gastrulating embryos14. However, neutrophils undergo complex, rapid changes in cell shape in vivo, making the membrane segmentation difficult both in 2D and 3D14. This is even more challenging when neutrophils cluster, which occurs in many physiological settings29. The contrast metric overcomes this limitation as it does not require membrane segmentation but instead reflects the overall state of receptor distribution in the cell (membranous/smooth vs vesicular/dotty). It is important to note that contrast metric can be affected by the overall contrast of the image, so normalization of individual cell values to an internal reference is required to account for variability of signal in different embryos/samples. For example, we used the mean cell contrast value of non-responsive neutrophils in the CHT (i.e., neutrophils that remain stationary and do not migrate into the ventral fin)14. An additional possibility would be to normalize with contrast values of a control marker in the same cell. This would provide a solution when an internal reference of non-responding cells is not available and may likely resolve finer quantitative differences in receptor dynamics between different conditions.
The location of imaging is another variable to consider. The reason for choosing the ventral fin wound here, as opposed to the more commonly used tail fin wound model16,30, is because the site of wounding is nearby the site of neutrophil residence/migratory origin. This accelerates the timeline of the assay, as it takes relatively little time for neutrophils to arrive. Additionally, it provides the opportunity to capture cell behavior both at the migration origin (CHT) and the target location of interest (wound). This is relevant here, because the spatial and temporal resolution required for subcellular imaging is difficult to combine with a large field of view or multi-position scanning. Thus, the ventral fin wound assay permits tracking of the evolution of the migratory response from the migration origin and simultaneous capturing of unspecific receptor fluctuations in cells that do not respond. As mentioned above, the latter is useful for quantification purposes as it provides an internal reference for unspecific dynamics. In other systems, it may not be possible to have such an internal reference, in which case the contrast values of a co-expressed membrane marker would provide an alternative control.
In summary, we anticipate that the methodology is applicable to other systems and can be deployed for a variety of purposes. For example, the same reporters could be utilized in other inflammatory settings, such as infection settings or other disease models. The repertoire of zebrafish receptor reporter lines could be expanded to other signaling receptors, to understand signaling mechanisms or report ligand dynamics in vivo. The approach can be combined with knockdown/knockout techniques to interrogate the mechanistic basis of observed dynamics. For example, perturbation of ligand expression can indicate the ligand dependency for observed receptor dynamics. In the future, we envisage that the system could be further refined to incorporate knock-in insertion of reporters. Ultimately, findings using this methodology would provide novel insights valuable beyond the zebrafish community, given the conservation of these signaling receptors in mammals and the relative challenge of conducting these studies in larger organisms.

Leukocyte migration is of paramount importance for immune responses. Immune cells are prototypical migratory cells, which are remarkably capable of traversing tissues and blood vessels and sensing a range of chemical guidance cues to migrate directionally towards microbes or other host cells of importance. Correct guidance relies on the recognition of chemoattractants, among which chemokines represent the most prominent category1. Chemokines are recognized by highly specific seven-transmembrane G protein coupled receptors. Upon chemokine binding, chemokine receptors change conformation leading to the activation of associated trimeric G proteins and their dissociation into functional signaling subunits that promote cytoskeletal changes and directed migration1. Secondarily, chemokine receptors are phosphorylated, and this modification leads to desensitization to attractant, which can be followed by rapid re-sensitization/recycling or intracellular degradation and down-regulation from the cell surface2. These receptor dynamics influence the duration and dose of signaling received by the cells but how they affect leukocyte migration behavior has been difficult to elucidate in vivo.
Tracking receptor dynamics in live leukocytes in traditional mammalian systems faces several challenges. For live studies, receptor fusions with fluorescent proteins must be expressed in the cells. This is challenging in primary leukocytes, particularly in neutrophils, and studies so far have used surrogate neutrophil cell lines to express chemokine receptors3,4. Generation of transgenic mouse models, in which leukocytes express a fluorescent receptor or mutant receptors with informative trafficking defects5,6, entails considerable investment of time and resources. Even in these instances, the imaging resolution and contrast for imaging receptor dynamics in the live animal can be limited and studies have used immunohistochemistry on fixed tissue sections5. Given these technical challenges, our understanding of how chemoattractant receptors dynamics affect cell behavior in a live tissue setting is currently limited.
Here we provide a methodology to monitor receptor trafficking in zebrafish neutrophils. Zebrafish are genetically tractable, like mice, but transgenesis is relatively more straightforward through the use of efficient transposon systems and direct zygote manipulation7. The transparent larva is ideally amenable to imaging. The chemokine receptor dynamics have been visualized in primordial germ cells and the lateral line primordium by expression of corresponding fusions with fluorescent reporters8,9,10. Zebrafish larvae are equipped with mature neutrophils that have highly conserved genetic and cellular properties with respect to mammalian neutrophils. Subcellular signaling dynamics such as cytoskeletal dynamics and polarity regulators have been visualized in these cells by the generation of corresponding transgenic lines11,12,13. Recently, we visualized and functionally analyzed chemokine receptor dynamics in neutrophils during the course of inflammatory responses to tissue damage14. Here, we describe the generation of transgenic reporter lines for chemokine signaling in neutrophils, preparation of embryos for live imaging, a wound assay for studying neutrophil signaling and the protocol for acquisition and analysis of images. We also provide a side-protocol to test chemokine receptor responses to candidate ligands, which is useful when trying to establish ligand recognition patterns in uncharacterized receptors. These techniques can be used in combination with further genetic manipulations, such as inhibition of endogenous chemokine expression or generation of mutant receptors with altered ligand-induced trafficking, to interrogate how specific signaling dynamics affect leukocyte behavior in vivo. The transgenic lines expressing fluorescently tagged chemokine receptors can also be used as reporters for endogenous chemokine gradients, which are otherwise difficult to detect by direct antibody staining. The described methodology provides scope for expanding the generation of reporters to other immuno-signaling receptors.

<table><tbody><tr><td>1-phenyl-2-thiourea</td><td>Sigma-Aldrich</td><td>P7629-25G</td><td></td></tr><tr><td>50mL CellStar Centrifuge Tube</td><td>Grenier Bio-One Limited</td><td>210270</td><td></td></tr><tr><td>Clark capillary glass</td><td>Harvard Apparatus</td><td>30-0020</td><td></td></tr><tr><td>Cleanline Thin Bleach</td><td>Scientific Laboratory Supplies</td><td>CLE0301</td><td></td></tr><tr><td>Dissecting scope</td><td>Nikon</td><td>SMZ645</td><td></td></tr><tr><td>Dri Block heater</td><td>Techne</td><td>FDB02AD</td><td></td></tr><tr><td>Fiji</td><td>National Institutes of Health, Bethesda, MD</td><td></td><td></td></tr><tr><td>Forceps, Jewelers Dumont No. 5</td><td>Sigma-Aldrich</td><td>F6521</td><td></td></tr><tr><td>Glass bottom microwell dishes</td><td>MatTek</td><td>P35G-1.5-14-C</td><td></td></tr><tr><td>Glass pasteur pipette</td><td>Fisherbrand</td><td>11795098</td><td></td></tr><tr><td>Invitrogen Ambion mMESSAGE mMACHINE SP6 Transcription Kit</td><td>Invitrogen</td><td>10391175</td><td></td></tr><tr><td>Leica SP8 confocal microscope</td><td>Leica</td><td>N/A</td><td></td></tr><tr><td>Low melting point agarose</td><td>Invitrogen</td><td>16520-100</td><td></td></tr><tr><td>Magnetic stand</td><td>Narishige</td><td>GJ-1</td><td></td></tr><tr><td>MATLAB</td><td>The MathWorks, Inc., Natick, MA</td><td></td><td></td></tr><tr><td>Methylene blue</td><td>Sigma-Aldrich</td><td>M9140-25G</td><td></td></tr><tr><td>MgSO4</td><td>Sigma-Aldrich</td><td>M-2643</td><td></td></tr><tr><td>Microinjection system</td><td>Parker</td><td>Picospritzer III</td><td></td></tr><tr><td>Microloader pipette tips</td><td>Eppendorf</td><td>5242956003</td><td></td></tr><tr><td>Micromanipulator</td><td>Narishige</td><td>M-152</td><td></td></tr><tr><td>Micropipette Puller</td><td>Sutter Instrument Company</td><td>Flaming/Brown P-97</td><td></td></tr><tr><td>Microscope slide</td><td>Thermo-Scientific</td><td>J1800AMNZ</td><td></td></tr><tr><td>PerkinElmer Spinning Disk UltraVIEW ERS, Olympus IX81 spinnng disk confocal microscope</td><td>Olympus</td><td>N/A</td><td></td></tr><tr><td>Petri dish (120mm)</td><td>Grenier Bio-One Limited</td><td>632180</td><td></td></tr><tr><td>Phenol red</td><td>Sigma-Aldrich</td><td>P0290-100ML</td><td></td></tr><tr><td>Poly(A) Tailing Kit</td><td>Invitrogen</td><td>AM1350</td><td></td></tr><tr><td>Prism</td><td>GraphPad Software</td><td></td><td></td></tr><tr><td>Small paintbrush</td><td>N/A</td><td></td><td></td></tr><tr><td>Spectral Applied Research LMM5 laser module</td><td></td><td>N/A</td><td></td></tr><tr><td>Surgical Scalpel Blade No. 23</td><td>Swann-Morton</td><td>233-5528</td><td></td></tr><tr><td>Tricaine MS222</td><td>Sigma-Aldrich</td><td>E10521-50G</td><td></td></tr><tr><td>Volocity software</td><td></td><td>N/A</td><td></td></tr><tr><td>Yokogawa CSU10 spinning disc confocal scanner</td><td>Yokogawa</td><td>N/A</td><td></td></tr></tbody></table>

live imaging of chemokine receptors in zebrafish neutrophils during wound responses

Leukocyte guidance by chemical gradients is essential for immune responses. Neutrophils are the first cells to be recruited to sites of tissue damage where they execute crucial antimicrobial functions. Their trafficking to these loci is orchestrated by several inflammatory chemoattractants, including chemokines. At the molecular level, chemoattractant signaling is regulated by the intracellular trafficking of the corresponding receptors. However, it remains unclear how subcellular changes in chemokine receptors affect leukocyte migration dynamics at the cell and tissue level. Here we describe a methodology for live imaging and quantitative analysis of chemokine receptor dynamics in neutrophils during inflammatory responses to tissue damage. These tools have revealed that differential chemokine receptor trafficking in zebrafish neutrophils coordinates neutrophil clustering and dispersal at sites of tissue damage. This has implications for our understanding of how inflammatory responses are self-resolved. The described tools could be used to understand neutrophil migration patterns in a variety of physiological and pathological settings and the methodology could be expanded to other signaling receptors.

Ventral fin wounding is followed by rapid neutrophil mobilization from the CHT into the ventral fin and clustering at the wound margin, within 30-60 min (Figure 1). We visualized the distribution of two chemokine receptors, Cxcr1 and Cxcr2, which are expressed by zebrafish neutrophils24 and recognize Cxcl8a and Cxcl8b14, using spinning-disk confocal microscopy. We generated two corresponding transgenic lines, Tg(lyz:Cxcr1-FT) and Tg(lyz:Cxcr2-FT), in which neutrophils express a fluorescent timer (FT) construct of the receptor, i.e. a fusion with a tandem of sfGFP and tagRFP (Figure 2 and reference14). The use of the two fluorophores was intended to allow monitoring of a broad range of receptor fates and provide estimates of protein turnover time at the plasma membrane, as newly synthesized receptors would fluoresce in green and progressively become red as they age8,14. However, these receptors were found to have fast constitutive turnover at the neutrophil plasma membrane and that the residence time was shorter than the maturation time of tagRFP, with sfGFP showing membrane localisation and tagRFP showing vesicular localization at steady state (Supplementary Video 1 and ref14). Therefore, we focused on the distribution of sfGFP to monitor ligand-induced internalization at sites of tissue damage. The pattern of receptor distribution was quantified using the contrast metric, which reports differences in intensity between neighboring pixels. The rationale is that when the receptor distribution is membranous and smooth, the contrast value is low. When the receptor distribution is vesicular and more punctate, then the contrast value is high (Figure 3).
An alternative method is to quantify the ratio of receptor levels (sfGFP intensity) over the levels of a control membrane marker e.g. membrane CFP (mCFP) (Figure 3). Both methods could detect receptor internalization, as indicated by more vesicular receptor distribution pattern globally in the cell (higher contrast value) or lower receptor levels at the membrane (lower sfGFP/mCFP ratio). However, the contrast metric could also detect receptor internalization in neutrophil clusters at the wound, in which membrane segmentation was less accurate and not applicable (Figure 3). Using this metric, we were able to quantify visible differences between Cxcr1 and Cxcr2 trafficking in neutrophils at wounds (Figure 4 and Supplementary Video 2). Cxcr1-FT internalized in cells located at the wound whereas Cxcr2-FT remained membranous in neutrophils at the wound (Figure 4A-C, Supplementary Video 2 and Supplementary Video 3). Suppression of Cxcl8a and Cxcl8b, through morpholino treatment, differentially affected Cxcr1-FT internalization at wounds (Figure 4C,D). To further validate that Cxcr1-FT responds to Cxcl8a, we performed chemokine response assays in early embryos. We found that Cxcr1-FT markedly internalized in embryos in which Cxcl8a was co-expressed (Figure 5). Altogether these results indicate that the described methods can be deployed to measure chemokine-induced receptor internalization in neutrophils and establish the identity of the ligand mediating these effects.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/61679/61679fig01.jpg" /> 
Figure 1: Neutrophil migration to ventral fin wounds.&#160;(A) (Left) Cartoon of 3 dpf larva showing the location of the caudal hematopoietic tissue (CHT), the venus circulation (VC, blue), the ventral fin (VF) and the wound site. (Right) Cartoon depicting the area of the wound (W) with neutrophils getting mobilized from the CHT and clustering at the wound. The caudal vein plexus (CVP) of the CHT tissue is drawn in blue. (B) Bright field image (left) and confocal projection (right) showing the ventral fin wound and the distribution of neutrophils in Tg(mpx:GFP) larvae at 2 h post-wounding. Dashed lines show VF and CHT outlines. Scale bar = 25 &#181;m. Cartoon and fluorescent image modified from ref.14 (http://creativecommons.org/licenses/by/4.0/). <a href="https://www.jove.com/files/ftp_upload/61679/61679fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/61679/61679fig02.jpg" /> 
Figure 2: Live imaging of chemokine receptor trafficking in neutrophils.&#160;(A) Constructs used for neutrophil-specific transgenic expression of Cxcr1-FT (Fluorescent Timer) and Cxcr2-FT. Confocal projections of neutrophils in the head of a 3 dpf transgenic larva (Tg(lyz:Cxcr1-FT), top; Tg(lyz:Cxcr2-FT), bottom) showing tRFP (magenta) and sfGFP (green) channels. Scale bar = 20 &#181;m. (B) Anatomical scheme of 3 dpf larva as in Figure 1A. Below the larva are schemes depicting the area of the wound (W) with neutrophils getting mobilized from the CHT (top) or performing chemotaxis upon entering the ventral fin (bottom). Dashed square indicates area imaged in snapshots on the right. (C) Neutrophils in Tg(lyz:Cxcr1-FT) larvae (sfGFP is shown) upon mobilization from the CHT (top panels) or chemotaxis towards the wound (bottom panels). Arrows show the same cells over time. Time points on the right image are minutes elapsed after image on the left. Scale bar = 10 &#181;m. (D) Schematic representation of experimental approach for live imaging of chemokine receptor trafficking. Panels A-C modified from ref.14 (http://creativecommons.org/licenses/by/4.0/). <a href="https://www.jove.com/files/ftp_upload/61679/61679fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/61679/61679fig03.jpg" /> 
Figure 3: Quantification examples of receptor dynamics.&#160;Single (blue) or clustered neutrophils (green) at wounds or non-mobilized neutrophils in the CHT (red, orange) were segmented and analyzed by different methods to compare results. The same example cells shown were analyzed with two methods to relate what is seen in the image with the range of values extracted. (A) The surface of the selected, example cells were segmented based on contour definition in the sfGFP channel. (B) Contrast was computed from the example cells shown in A. (C) The membrane of the selected, example cells were segmented based on contour definition in the CFP channel. Ratiometric analysis of sfGFP/CFP followed. (D) The ratio of sfGFP/CFP was computed on the example cells shown in C. Error bars represent S.E.M. from individual cells, in cases of n&#62;1, values here were not used for statistical analysis but merely to exemplify measurements obtained with the different quantification methods. Scale bar = 10 &#181;m. Figure modified from ref.14 (http://creativecommons.org/licenses/by/4.0/). <a href="https://www.jove.com/files/ftp_upload/61679/61679fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/61679/61679fig04v2.jpg" /> 
Figure 4: Differential dynamics of Cxcr1 and Cxcr2 in response to wounding.&#160;(A) Confocal projection of neutrophils in Tg(lyz:Cxcr1-FT) or Tg(lyz:Cxcr2-FT) larvae at the wound at 80 min post-wounding (sfGFP channel shown). Scale bar = 10 &#181;m.&#160;&#160;(B) Magnified Cxcr1-FT neutrophil (left) and Cxcr2-FT (right) at the wound. Green receptor is shown in gray. Scale bar = 5 &#181;m. (C) Normalized contrast (contrast per individual neutrophil normalized to the mean contrast of non-mobilized cells in the CHT). cxcl8a refers to injection of a splice-blocking together with a translation-blocking morpholino for cxcl8a. cxcl8b refers to injection with a splice-blocking morpholino for cxcl8b. For Tg(lyz:Cxcr1-FT): n=24 cells (CHT), n=47 cells (wound) from 8 larvae. For Tg(lyz:Cxcr1-FT) with morpholinos: n=28 cells (Cxcl8a-MO) from 5 larvae, n=16 cells (Cxcl8b-MO) from 5 larvae. For Tg(lyz:Cxcr2-FT): n=10 cells (CHT) and n=20 cells (wound) from 3 larvae. Data were pooled from independent larvae acquired in 1-5 imaging sessions. Kruskal-Wallis test with Dunn&#8217;s multiple comparisons test for Tg(lyz:Cxcr1-FT), two-tailed unpaired Mann-Whitney test for Tg(lyz:Cxcr2-FT). (D) Confocal projection of neutrophils in Tg(lyz:Cxcr1-FT) transgenic larvae treated with cxcl8a morpholino (MO) (left) and cxcl8b MO (right) responding to fin wounds (sfGFP channel shown in green). Snapshot taken at timepoints of equivalent neutrophil accumulation (85 min post-wounding&#160;in left image and 45 min post-wounding&#160;in right image). Scale bar = 10 &#181;m. Figure modified from ref.14 (http://creativecommons.org/licenses/by/4.0/). <a href="https://www.jove.com/files/ftp_upload/61679/61679fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/61679/61679fig05.jpg" /> 
Figure 5: Chemokine response assay in early embryos.&#160;Laser-scanning confocal slices of gastrulating embryos showing expression and distribution of Cxcr1-FT. 100 pg of Cxcr1-FT mRNA was injected into one cell-stage eggs with or without 150 pg Cxcl8a mRNA. Green and red receptors are shown in separate channels. Control membrane CFP marker (mCFP) is shown in the cyan channel. Scale bar = 20 &#956;m. Figure modified from ref.14 (http://creativecommons.org/licenses/by/4.0/). <a href="https://www.jove.com/files/ftp_upload/61679/61679fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Supplementary Movie 1: Transgenic neutrophils in the head of a Tg(lyz:Cxcr1-FT) (left) and Tg(lyz:Cxcr2-FT) (right) larva at 3 dpf. sfGFP(green), tagRFP (magenta). Frame interval is 30 sec and frame rate is 5 fps. Scale bar = 20 &#956;m. Video originates from ref.14 (http://creativecommons.org/licenses/by/4.0/). <a href="https://cloudflare.jove.com/files/ftp_upload/61679/Supplementary_Movie1.avi" target="_blank">Please click here to download this video.</a>
Supplementary Movie 2: Neutrophils in Tg(lyz:Cxcr1-FT) (left) and Tg(lyz:Cxcr2-FT) (right) transgenic larvae responding to fin wounds. Movie starts within 10 min post-wounding&#160;and lasts 60 min. sfGFP (green), tagRFP (magenta). Frame interval is 30 sec and frame rate is 10 fps. CHT = caudal hematopoietic tissue. VF = ventral fin. Scale bar = 25 &#956;m. Video originates from ref.14 (http://creativecommons.org/licenses/by/4.0/). <a href="https://cloudflare.jove.com/files/ftp_upload/61679/Supplementary_Movie2.avi" target="_blank">Please click here to download this video.</a>
Supplementary Movie 3. Additional examples of neutrophils from a wounded Tg(lyz:Cxcr1- FT) transgenic larva (different larva to that shown in Video 2), acquired at higher resolution, showing receptor internalization (sfGFP channel shown in green) upon mobilization in the CHT or upon entry and chemotaxis in the ventral fin. Frame interval is 30 sec and frame rate is 2 fps. Scale bar = 10 &#956;m. Video originates from ref.14 (http://creativecommons.org/licenses/by/4.0/). <a href="https://cloudflare.jove.com/files/ftp_upload/61679/Supplementary_Movie3.avi" target="_blank">Please click here to download this video.</a>
Supplementary File 1: <a href="https://cloudflare.jove.com/files/ftp_upload/61679/Supplementary file 1.m" target="_blank">Please click here to download this file.</a>
Supplementary File 2: <a href="https://cloudflare.jove.com/files/ftp_upload/61679/Supplementary file 2.m" target="_blank">Please click here to download this file.</a>
Supplementary File 3: <a href="https://cloudflare.jove.com/files/ftp_upload/61679/Supplementary file 3.m" target="_blank">Please click here to download this file.</a>
Supplementary File 4: <a href="https://cloudflare.jove.com/files/ftp_upload/61679/Supplementary file 4.m" target="_blank">Please click here to download this file.</a>

Watch this Scientific Journal Video about Live Imaging of Chemokine Receptors in Zebrafish Neutrophils During Wound Responses at JoVE.com

Live Imaging of Chemokine Receptors in Zebrafish Neutrophils During Wound Responses

Here we describe protocols to perform live imaging and quantitative analysis of chemoattractant receptor dynamics in zebrafish neutrophils

Leukocyte guidance by chemical gradients is essential for immune responses. Neutrophils are the first cells to be recruited to sites of tissue damage ...

live-imaging-chemokine-receptors-zebrafish-neutrophils-during-wound

Research

JoVE Journal

Immunology and Infection

2.4K Views.  University of Cambridge. Here we describe protocols to perform live imaging and quantitative analysis of chemoattractant receptor dynamics in zebrafish neutrophils

Video: Live Imaging of Chemokine Receptors in Zebrafish Neutrophils During Wound Responses

Live Imaging of Innate Immune and Preneoplastic Cell Interactions Using an Inducible Gal4/UAS Expression System in Larval Zebrafish Skin

Here we describe a method to conditionally induce epithelial cell transformation by the use of the 4-Hydroxytamoxifen (4-OHT) inducible KalTA4-ERT2/UAS expression system1 in zebrafish larvae, and the subsequent live imaging of innate immune cell interaction with HRASG12V expressing skin cells. The KalTA4-ERT2/UAS system is both inducible and reversible which allows us to induce cell transformation with precise temporal/spatial resolution in vivo. This provides us with a unique opportunity to live image how individual preneoplastic cells interact with host tissues as soon as they emerge, then follow their progression as well as regression. Recent studies in zebrafish larvae have shown a trophic function of innate immunity in the earliest stages of tumorigenesis2,3. Our inducible system would allow us to live image the onset of cellular transformation and the subsequent host response, which may lead to important insights on the underlying mechanisms for the regulation of oncogenic trophic inflammatory responses. We also discuss how one might adapt our protocol to achieve temporal and spatial control of ectopic gene expression in any tissue of interest.

Studying the earliest events of preneoplastic cell progression and innate immune cell interaction is pivotal to understand and treat cancer. Here we describe a method to conditionally induce epithelial cell transformations and the subsequent live imaging of innate immune cell interaction with HRASG12V expressing skin cells in zebrafish larvae.

Here we describe a method to conditionally induce epithelial cell transformation by the use of the 4-Hydroxytamoxifen (4-OHT) inducible KalTA4-ERT2/UAS ...

Developmental Biology

Non-invasive Imaging of Disseminated Candidiasis in Zebrafish Larvae

Disseminated candidiasis caused by the pathogen Candida albicans is a clinically important problem in hospitalized individuals and is associated with a 30 to 40% attributable mortality6. Systemic candidiasis is normally controlled by innate immunity, and individuals with genetic defects in innate immune cell components such as phagocyte NADPH oxidase are more susceptible to candidemia7-9. Very little is known about the dynamics of C. albicans interaction with innate immune cells in vivo. Extensive in vitro studies have established that outside of the host C. albicans germinates inside of macrophages, and is quickly destroyed by neutrophils10-14. In vitro studies, though useful, cannot recapitulate the complex in vivo environment, which includes time-dependent dynamics of cytokine levels, extracellular matrix attachments, and intercellular contacts10, 15-18. To probe the contribution of these factors in host-pathogen interaction, it is critical to find a model organism to visualize these aspects of infection non-invasively in a live intact host. 
The zebrafish larva offers a unique and versatile vertebrate host for the study of infection. For the first 30 days of development zebrafish larvae have only innate immune defenses2, 19-21, simplifying the study of diseases such as disseminated candidiasis that are highly dependent on innate immunity. The small size and transparency of zebrafish larvae enable imaging of infection dynamics at the cellular level for both host and pathogen. Transgenic larvae with fluorescing innate immune cells can be used to identify specific cells types involved in infection22-24. Modified anti-sense oligonucleotides (Morpholinos) can be used to knock down various immune components such as phagocyte NADPH oxidase and study the changes in response to fungal infection5. In addition to the ethical and practical advantages of using a small lower vertebrate, the zebrafish larvae offers the unique possibility to image the pitched battle between pathogen and host both intravitally and in color. 
The zebrafish has been used to model infection for a number of human pathogenic bacteria, and has been instrumental in major advances in our understanding of mycobacterial infection3, 25. However, only recently have much larger pathogens such as fungi been used to infect larva5, 23, 26, and to date there has not been a detailed visual description of the infection methodology. Here we present our techniques for hindbrain ventricle microinjection of prim25 zebrafish, including our modifications to previous protocols. Our findings using the larval zebrafish model for fungal infection diverge from in vitro studies and reinforce the need to examine the host-pathogen interaction in the complex environment of the host rather than the simplified system of the Petri dish5.

The rapid development, small size and transparency of zebrafish are tremendous advantages for the study of innate immune control of infection1-4. Here we demonstrate techniques for infecting zebrafish larvae using the fungal pathogen Candida albicans by microinjection, methodology recently used to implicate phagocyte NADPH oxidase activity in control of fungal dimorphism5.

Disseminated candidiasis caused by the pathogen Candida albicans is a clinically important problem in hospitalized individuals and is associated with a 30 ...

Facilitating Drug Discovery: An Automated High-content Inflammation Assay in Zebrafish

Zebrafish larvae are particularly amenable to whole animal small molecule screens1,2 due to their small size and relative ease of manipulation and observation, as well as the fact that compounds can simply be added to the bathing water and are readily absorbed when administered in a &lt;1% DMSO solution. Due to the optical clarity of zebrafish larvae and the availability of transgenic lines expressing fluorescent proteins in leukocytes, zebrafish offer the unique advantage of monitoring an acute inflammatory response in vivo. Consequently, utilizing the zebrafish for high-content small molecule screens aiming at the identification of immune-modulatory compounds with high throughput has been proposed3-6, suggesting inflammation induction scenarios e.g. localized nicks in fin tissue, laser damage directed to the yolk surface of embryos7 or tailfin amputation3,5,6. The major drawback of these methods however was the requirement of manual larva manipulation to induce wounding, thus preventing high-throughput screening. Introduction of the chemically induced inflammation (ChIn) assay8 eliminated these obstacles. Since wounding is inflicted chemically the number of embryos that can be treated simultaneously is virtually unlimited. Temporary treatment of zebrafish larvae with copper sulfate selectively induces cell death in hair cells of the lateral line system and results in rapid granulocyte recruitment to injured neuromasts. The inflammatory response can be followed in real-time by using compound transgenic cldnB::GFP/lysC::DsRED26,9 zebrafish larvae that express a green fluorescent protein in neuromast cells, as well as a red fluorescent protein labeling granulocytes.
In order to devise a screening strategy that would allow both high-content and high-throughput analyses we introduced robotic liquid handling and combined automated microscopy with a custom developed software script. This script enables automated quantification of the inflammatory response by scoring the percent area occupied by red fluorescent leukocytes within an empirically defined area surrounding injured green fluorescent neuromasts. Furthermore, we automated data processing, handling, visualization, and storage all based on custom developed MATLAB and Python scripts.
In brief, we introduce an automated HC/HT screen that allows testing of chemical compounds for their effect on initiation, progression or resolution of a granulocytic inflammatory response. This protocol serves a good starting point for more in-depth analyses of drug mechanisms and pathways involved in the orchestration of an innate immune response. In the future, it may help identifying intolerable toxic or off-target effects at earlier phases of drug discovery and thereby reduce procedural risks and costs for drug development.

Here we describe a novel high-content chemically induced inflammation assay aiming at the identification of immune-modulatory bioactives. We have successfully combined automated microscopy with custom developed software scripts enabling automated quantification of the inflammatory response as well as further data processing, analysis, mining, and storage.

Zebrafish larvae are particularly amenable to whole animal small molecule screens1,2 due to their small size and relative ease of manipulation and ...

Microstructured Devices for Optimized Microinjection and Imaging of Zebrafish Larvae

Zebrafish have emerged as a powerful model of various human diseases and a useful tool for an increasing range of experimental studies, spanning fundamental developmental biology through to large-scale genetic and chemical screens. However, many experiments, especially those related to infection and xenograft models, rely on microinjection and imaging of embryos and larvae, which are laborious techniques that require skill and expertise. To improve the precision and throughput of current microinjection techniques, we developed a series of microstructured devices to orient and stabilize zebrafish embryos at 2 days post fertilization (dpf) in ventral, dorsal, or lateral orientation prior to the procedure. To aid in the imaging of embryos, we also designed a simple device with channels that orient 4 zebrafish laterally in parallel against a glass cover slip. Together, the tools that we present here demonstrate the effectiveness of photolithographic approaches to generate useful devices for the optimization of zebrafish techniques.

Microinjection of zebrafish embryos and larvae is a crucial but challenging technique used in many zebrafish models. Here, we present a range of microscale tools to aid in the stabilization and orientation of zebrafish for both microinjection and imaging.

Zebrafish have emerged as a powerful model of various human diseases and a useful tool for an increasing range of experimental studies, spanning ...

Non-invasive Imaging of the Innate Immune Response in a Zebrafish Larval Model of Streptococcus iniae Infection

The aquatic pathogen, Streptococcus iniae, is responsible for over 100 million dollars in annual losses for the aquaculture industry and is capable of causing systemic disease in both fish and humans. A better understanding of S. iniae disease pathogenesis requires an appropriate model system. The genetic tractability and the optical transparency of the early developmental stages of zebrafish allow for the generation and non-invasive imaging of transgenic lines with fluorescently tagged immune cells. The adaptive immune system is not fully functional until several weeks post fertilization, but zebrafish larvae have a conserved vertebrate innate immune system with both neutrophils and macrophages. Thus, the generation of a larval infection model allows the study of the specific contribution of innate immunity in controlling S. iniae infection.
The site of microinjection will determine whether an infection is systemic or initially localized. Here, we present our protocols for otic vesicle injection of zebrafish aged 2-3 days post fertilization as well as our techniques for fluorescent confocal imaging of infection. A localized infection site allows observation of initial microbe invasion, recruitment of host cells and dissemination of infection. Our findings using the zebrafish larval model of S. iniae infection indicate that zebrafish can be used to examine the differing contributions of host neutrophils and macrophages in localized bacterial infections. In addition, we describe how photolabeling of immune cells can be used to track individual host cell fate during the course of infection.

Non-invasive Imaging of the Innate Immune Response in a Zebrafish Larval Model of Streptococcus iniae Infection

Here, we present a protocol for the generation and imaging of a localized bacterial infection in the zebrafish otic vesicle.

The aquatic pathogen, Streptococcus iniae, is responsible for over 100 million dollars in annual losses for the aquaculture industry and is capable of ...

An Ectopic Chemokine Expression Model for Testing Macrophage Recruitment In Vivo

Zebrafish are widely used in basic and biomedical research. Many zebrafish transgenic lines are currently available to label various types of cells. Owing to the transparent embryonic body of zebrafish, it is convenient for us to study the effect of one chemokine on the behavior of a certain type of cells in vivo. Here we provided a workflow to investigate the function of a chemokine on macrophage migration in vivo. We constructed a tissue-specific overexpression plasmid to overexpress IL-34 and injected the plasmid into one-cell stage transgenic fish embryos whose macrophages were specifically labeled by a fluorescent protein. We then used whole mount fluorescent in situ hybridization and immunostaining to detect the pattern of the chemokine expression and the number or location of macrophages. The injected WT embryos were raised to generate a stable transgenic line. Finally, we used confocal live imaging to directly observe macrophage behavior in the stable transgenic fish to study the function of IL-34 on macrophages in vivo.

To test the effect of a chemokine on macrophage recruitment in vivo, the whole mount in situ hybridization was used to detect the ectopic expression of the chemokine, and immunostaining was used to label macrophages. Live imaging was used for real-time observation of macrophage migration.

Zebrafish are widely used in basic and biomedical research. Many zebrafish transgenic lines are currently available to label various types of cells. Owing ...

Using Zebrafish Models of Human Influenza A Virus Infections to Screen Antiviral Drugs and Characterize Host Immune Cell Responses

Each year, seasonal influenza outbreaks profoundly affect societies worldwide. In spite of global efforts, influenza remains an intractable healthcare burden. The principle strategy to curtail infections is yearly vaccination. In individuals who have contracted influenza, antiviral drugs can mitigate symptoms. There is a clear and unmet need to develop alternative strategies to combat influenza. Several animal models have been created to model host-influenza interactions. Here, protocols for generating zebrafish models for systemic and localized human influenza A virus (IAV) infection are described. Using a systemic IAV infection model, small molecules with potential antiviral activity can be screened. As a proof-of-principle, a protocol that demonstrates the efficacy of the antiviral drug Zanamivir in IAV-infected zebrafish is described. It shows how disease phenotypes can be quantified to score the relative efficacy of potential antivirals in IAV-infected zebrafish. In recent years, there has been increased appreciation for the critical role neutrophils play in the human host response to influenza infection. The zebrafish has proven to be an indispensable model for the study of neutrophil biology, with direct impacts on human medicine. A protocol to generate a localized IAV infection in the Tg(mpx:mCherry) zebrafish line to study neutrophil biology in the context of a localized viral infection is described. Neutrophil recruitment to localized infection sites provides an additional quantifiable phenotype for assessing experimental manipulations that may have therapeutic applications. Both zebrafish protocols described faithfully recapitulate aspects of human IAV infection. The zebrafish model possesses numerous inherent advantages, including high fecundity, optical clarity, amenability to drug screening, and availability of transgenic lines, including those in which immune cells such as neutrophils are labeled with fluorescent proteins. The protocols detailed here exploit these advantages and have the potential to reveal critical insights into host-IAV interactions that may ultimately translate into the clinic.

Systemic and localized zebrafish infection models for human influenza A virus are demonstrated. Using a systemic infection model, zebrafish can be used to screen antiviral drugs. Using a localized infection model, zebrafish can be used to characterize host immune cell responses.

Each year, seasonal influenza outbreaks profoundly affect societies worldwide. In spite of global efforts, influenza remains an intractable healthcare ...

Long-term In Vivo Tracking of Inflammatory Cell Dynamics Within Drosophila Pupae

During the rapid inflammatory response to tissue damage, cells of the innate immune system are quickly recruited to the injury site. Once at the wound, innate immune cells perform a number of essential functions, such as fighting infection, clearing necrotic debris, and stimulating matrix deposition. In order to fully understand the diverse signaling events that regulate this immune response, it is crucial to observe the complex behaviors of (and interactions that occur between) multiple cell lineages in vivo, and in real-time, with the high spatio-temporal resolution. The optical translucency and the genetic tractability of Drosophila embryos have established Drosophila as an invaluable model to live-image and dissect fundamental aspects of inflammatory cell behavior, including mechanisms of developmental dispersal, clearance of apoptotic corpses&#160;and/or microbial pathogens, and recruitment to wounds. However, more recent work has now demonstrated that employing a much later stage in the Drosophila lifecycle &#8211; the Drosophila pupa &#8211; offers a number of distinct advantages, including improved RNAi efficiency, longer imaging periods, and significantly greater immune cell numbers. Here we describe a protocol for imaging wound repair and the associated inflammatory response at the high spatio-temporal resolution in live Drosophila pupae. To follow the dynamics of both re-epithelialization and inflammation, we use a number of specific in vivo fluorescent markers for both the epithelium and innate immune cells. We also demonstrate the effectiveness of photo-convertible fluorophores, such as Kaede, for following the specific immune cell subsets, to track their behavior as they migrate to, and resolve from, the injury site.

Long-term In Vivo Tracking of Inflammatory Cell Dynamics Within Drosophila Pupae

Here we present a protocol for live-imaging wound repair and the associated inflammatory response at high spatio-temporal resolution in vivo. This method utilizes the pupal stage of Drosophila development to enable long-term imaging and tracking of specific cell populations over time and is compatible with efficient RNAi-mediated gene inactivation.

During the rapid inflammatory response to tissue damage, cells of the innate immune system are quickly recruited to the injury site. Once at the wound, ...

Monitoring On-Target Signaling Responses in Larval Zebrafish - Z-REX Unmasks Precise Mechanisms of Electrophilic Drugs and Metabolites

Reactive metabolites and related electrophilic drugs are among the most challenging small molecules to study. Conventional approaches to deconstruct the mode of action (MOA) of such molecules leverage bulk treatment of experimental specimens with an excess of a specific reactive species. In this approach, the high reactivity of electrophiles renders non-discriminate labeling of the proteome in a time- and context-dependent manner; redox-sensitive proteins and processes can also be indirectly and often irreversibly affected. Against such a backdrop of innumerable potential targets and indirect secondary effects, linking phenotype to specific target engagement remains a complex task. Zebrafish targeting reactive electrophiles and oxidants (Z-REX)-an on-demand reactive-electrophile delivery platform adapted for use in larval zebrafish-is designed to deliver electrophiles to a specific protein of interest (POI) in otherwise unperturbed live fish embryos. Key features of this technique include a low level of invasiveness, along with dosage-, chemotype-, and spatiotemporally-controlled precision electrophile delivery. Thus, in conjunction with a unique suite of controls, this technique sidesteps off-target effects and systemic toxicity, otherwise observed following uncontrolled bulk exposure of animals to reactive electrophiles and pleiotropic electrophilic drugs. Leveraging Z-REX, researchers can establish a foothold in the understanding of how individual stress responses and signaling outputs are altered as a result of specific reactive ligand engagement with a specific POI, under near-physiologic conditions in intact living animals.

Zebrafish targeting reactive electrophiles and oxidants (Z-REX) is a chemical biology-based method for the investigation of reactive small-molecule signaling. This technique can be applied to live fish of different developmental stages. Here, we couple standard assays in zebrafish with Z-REX for signaling pathway analysis.

Reactive metabolites and related electrophilic drugs are among the most challenging small molecules to study. Conventional approaches to deconstruct the ...

Biology

Photoconversion-Based Leukocyte Tracking in Bacterial-Infected Transgenic Zebrafish

Source: Harvie, E. A. et al., <a href="https://app.jove.com/v/52788">Non-invasive Imaging of the Innate Immune Response in a Zebrafish Larval Model of Streptococcus iniae Infection.</a>&#160;J. Vis. Exp. (2015)
This video demonstrates the tracking of leukocyte recruitment using transgenic zebrafish larvae expressing photoconvertible green fluorescent protein. Pre-injecting red fluorescent dye-labeled bacteria into the otic vesicle and inducing photoconversion enables observation of leukocytes&#39; migration, phagocytosis, and dispersal throughout the body, providing valuable insights into immune response dynamics.

1. Preparing Microinjection Needles

	Prepare thin wall glass capillary injection needles (1.0 OD/0.75 ID) using a micropipette puller device with the ...

Live Imaging of Chemokine Receptors in Zebrafish Neutrophils During Wound Responses

Summary

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