Plan your trip before you leave: The neutrophils' search-and-run journey.

Reading, interpreting and crawling along gradients of chemotactic cues is one of the most complex questions in cell biology. In this issue, Georgantzoglou et al. (2022. J. Cell. Biol.https://doi.org/10.1083/jcb.202103207) use in vivo models to map the temporal sequence of how neutrophils respond to an acutely arising gradient of chemoattractant.

Steering yourself by the bootstraps: how cells create their own gradients for chemotaxis.

Chemotaxis, where cell movement is steered by chemical gradients, is a widespread and essential way of organising cell behaviour. But where do the instructions come from - who makes gradients, and how are they controlled? We discuss the emerging concept that chemotactic cells often create attractant gradients at the same time as responding to them. This self-guidance is more robust, works across greater distances, and is more informative about the local environment than passive responses. Several mechanisms can establish autonomous gradients. Best known are self-generated gradients, in which the cells degrade a widespread attractant, but cells also produce repellents and 'relay' by secreting fresh attractant after stimulation. Understanding how cells make and interpret their own chemoattractant gradients is fundamental to understanding the spatial patterns seen in all organisms.

Antibiotic-chemoattractants enhance neutrophil clearance of Staphylococcus aureus.

The pathogen Staphylococcus aureus can readily develop antibiotic resistance and evade the human immune system, which is associated with reduced levels of neutrophil recruitment. Here, we present a class of antibacterial peptides with potential to act both as antibiotics and as neutrophil chemoattractants. The compounds, which we term 'antibiotic-chemoattractants', consist of a formylated peptide (known to act as chemoattractant for neutrophil recruitment) that is covalently linked to the antibiotic vancomycin (known to bind to the bacterial cell wall). We use a combination of in vitro assays, cellular assays, infection-on-a-chip and in vivo mouse models to show that the compounds improve the recruitment, engulfment and killing of S. aureus by neutrophils. Furthermore, optimizing the formyl peptide sequence can enhance neutrophil activity through differential activation of formyl peptide receptors. Thus, we propose antibiotic-chemoattractants as an alternate approach for antibiotic development.

Characterization of a composite with enhanced attraction to savannah tsetse flies from constituents or analogues of tsetse refractory waterbuck (Kobus defassa) body odor.

Savannah tsetse flies avoid flying toward tsetse fly-refractory waterbuck (Kobus defassa) mediated by a repellent blend of volatile compounds in their body odor comprised of δ-octalactone, geranyl acetone, phenols (guaiacol and carvacrol), and homologues of carboxylic acids (C5-C10) and 2-alkanones (C8-C13). However, although the blends of carboxylic acids and that of 2-alkanones contributed incrementally to the repellency of the waterbuck odor to savannah tsetse flies, some waterbuck constituents (particularly, nonanoic acid and 2-nonanone) showed significant attractive properties. In another study, increasing the ring size of δ-octalactone from six to seven membered ring changed the activity of the resulting molecule (ε-nonalactone) on the savannah tsetse flies from repellency to attraction. In the present study, we first compared the effect of blending ε-nonalactone, nonanoic acid and 2-nonanone in 1:1 binary and 1:1:1 ternary combination on responses of Glossina pallidipes and Glossina morsitans morsitans tsetse flies in a two-choice wind tunnel. The compounds showed clear synergistic effects in the blends, with the ternary blend demonstrating higher attraction than the binary blends and individual compounds. Our follow up laboratory comparisons of tsetse fly responses to ternary combinations with different relative proportions of the three components showed that the blend in 1:3:2 proportion was most attractive relative to fermented cow urine (FCU) to both tsetse species. In our field experiments at Shimba Hills game reserve in Kenya, where G. pallidipes are dominant, the pattern of tsetse catches we obtained with different proportions of the three compounds were similar to those we observed in the laboratory. Interestingly, the three-component blend in 1:3:2 proportion when released at optimized rate of 13.71mg/h was 235% more attractive to G. pallidipes than a combination of POCA (3-n-Propylphenol, 1-Octen-3-ol, 4-Cresol, and Acetone) and fermented cattle urine (FCU). This constitutes a novel finding with potential for downstream deployment in bait technologies for more effective control of G. pallidipes, G. m. morsitans, and perhaps other savannah tsetse fly species, in 'pull' and 'pull-push' tactics.

Chemical Composition and Attractant Activity of Volatiles from Rhus potaninii to The Spring Aphid Kaburagia rhusicola.

Rhus potaninii Maxim, a type of sumac, is an economically important tree widely cultivated in mountainous areas of western and central China. A gall, called the bellied gallnut, induced by the aphid, Kaburagia rhusicola Takagi, is important in the food, medical, and chemical industries in China. Volatiles from R. potaninii were found to attract K. rhusicola, but little is known about them. The chemical composition of these volatiles was investigated using GC-MS analysis and Y-tube olfactometer methods. Twenty-five compounds accounting for 55.3% of the volatiles were identified, with the highest proportion of 1-(4-ethylphenyl)ethanone (11.8%), followed by 1-(4-hydroxy-3-methylphenyl)ethanone (11.2%) and p-cymen-7-ol (7.1%). These findings provide a theoretical basis for the preparation of attractants and could eventually lead to increased bellied gallnut yield.

Leptospira spp. Toolbox for Chemotaxis Assay.

A toolbox for chemotaxis assay adapted to Leptospira spp. has emerged in the recent years: soft agar assay, capillary assay, and videomicroscopy tracking. Those methods allow to demonstrate chemotaxis defect, identify diverse chemoattractants, or decipher motile behavior quantitatively. These experiments have demonstrated a role of motility and potentially chemotaxis in leptospirosis pathogenesis. We describe extensively the methods and provide the key steps to use this toolbox.

A novel chemotactic factor derived from the extracellular matrix protein decorin recruits mesenchymal stromal cells in vitro and in vivo.

Soft tissue is composed of cells surrounded by an extracellular matrix that is made up of a diverse array of intricately organized proteins. These distinct components work in concert to maintain homeostasis and respond to tissue damage. During tissue repair, extracellular matrix proteins and their degradation products are known to influence physiological processes such as angiogenesis and inflammation. In this study we developed a discovery platform using a decellularized extracellular matrix biomaterial to identify new chemotrophic factors derived from the extracellular matrix. An in vitro culture of RAW.264 macrophage cells with the biomaterial ovine forestomach matrix led to the identification of a novel ~12 kDa chemotactic factor, termed 'MayDay', derived from the N-terminal 31-188 sequence of decorin. The recombinant MayDay protein was shown to be a chemotactic agent for mesenchymal stromal cells in vitro and in vivo. We hypothesize that the macrophage-induced cleavage of decorin, via MMP-12, leads to the release of the chemotactic molecule MayDay, that in turn recruits cells to the site of damaged tissue.

Is Graphene Oxide a Chemoattractant?

The use of nanomaterials to regulate cell surface receptors is considered a novel strategy to manipulate cell behaviors. However, recognition is important to drive nanoparticle-cell complex formation. Here, we report a novel approach that uses graphene oxide (GO) as a chemoattractant to lure bacteria to nanosurface, facilitating complex formation. The amount of Escherichia coli (E. coli) cells attracted into capillaries containing 20 mg/L GO was more than 8.6-fold higher than that attracted into capillaries containing 20 mg/L glucose. The inherent mechanism involved interference with transmembrane chemoreceptors and activation of the chemotactic system via GO attachment and a subsequent increase in cell aggregation and migration via self-secreted quorum sensing molecules. The key feature of this strategy is the potential to improve the efficiency of the nanoparticle-cell recognition pattern and to expedite the development of surface-contact-related nanotechnology.

Onion-like Multifunctional Microtrap Vehicles for Attraction-Trapping-Destruction of Biological Threats.

A multifunctional motile microtrap is developed that is capable of autonomously attracting, trapping, and destroying pathogens by controlled chemoattractant and therapeutic agent release. The onion-inspired multi-layer structure contains a magnesium engine core and inner chemoattractant and therapeutic layers. Upon chemical propulsion, the magnesium core is depleted, resulting in a hollow structure that exposes the inner layers and serves as structural trap. The sequential dissolution and autonomous release of the chemoattractant and killing agents result in long-range chemotactic attraction, trapping, and destruction of motile pathogens. The dissolved chemoattractant (l-serine) significantly increases the accumulation and capture of motile pathogens (E. coli) within the microtrap structure, while the internal release of silver ions (Ag+ ) leads to lysis of the pathogen accumulated within the microtrap cavity.

A Biased Agonist at Immunometabolic Receptor GPR84 Causes Distinct Functional Effects in Macrophages.

GPR84 is an orphan G-protein-coupled receptor that is expressed on immune cells and implicated in several inflammatory diseases. The validation of GPR84 as a therapeutic target is hindered by the narrow range of available chemical tools and consequent poor understanding of GPR84 pathophysiology. Here we describe the discovery and characterization of DL-175, a potent, selective, and structurally novel GPR84 agonist and the first to display significantly biased signaling across GPR84-overexpressing cells, primary murine macrophages, and human U937 cells. By comparing DL-175 with reported GPR84 ligands, we show for the first time that biased GPR84 agonists have markedly different abilities to induce chemotaxis in human myeloid cells, while causing similar levels of phagocytosis enhancement. This work demonstrates that biased agonism at GPR84 enables the selective activation of functional responses in immune cells and delivers a high-quality chemical probe for further investigation.

Ranking migration cue contributions to guiding individual fibroblasts faced with a directional decision in simple microfluidic bifurcations.

Directed cell migration in complex micro-environments, such as in vivo pores, is important for predicting locations of artificial tissue growth and optimizing scaffold architectures. Yet, the directional decisions of cells facing multiple physiochemical cues have not been characterized. Hence, we aim to provide a ranking of the relative importance of the following cues to the decision-making of individual fibroblast cells: chemoattractant concentration gradient, channel width, mitosis, and contact-guidance. In this study, bifurcated micro-channels with branches of different widths were created. Fibroblasts were then allowed to travel across these geometries by following a gradient of platelet-derived growth factor-BB (PDGF-BB) established inside the channels. Subsequently, a combination of statistical analysis and image-based diffusion modeling was used to report how the presence of multiple complex migration cues, including cell-cell influences, affect the fibroblast decision-making. It was found that the cells prefer wider channels over a higher chemoattractant gradient when choosing between asymmetric bifurcated branches. Only when the branches were symmetric in width did the gradient become predominant in directing which path the cell will take. Furthermore, when both the gradient and the channels were symmetric, contact guidance became important for guiding the cells in making directional choices. Based on these results we were able to rank these directional cues from most influential to the least as follows: mitosis > channel width asymmetry > chemoattractant gradient difference > and contact-guidance. It is expected that these results will benefit the fields of regenerative medicine, wound healing and developmental biology.

Kinetic and photonic techniques to study chemotactic signaling in sea urchin sperm.

Sperm from sea urchins are attracted by chemical cues released by the egg-a mechanism called chemotaxis. We describe here the signaling pathway and molecular components endowing sperm with single-molecule sensitivity. Chemotactic signaling and behavioral responses occur on a timescale of a few milliseconds to seconds. We describe the techniques and chemical tools used to resolve the signaling events in time. The techniques include rapid-mixing devices, rapid stroboscopic microscopy, and photolysis of caged second messengers and chemoattractants.

The influence of serum-supplemented culture media in a transwell migration assay.

Our work cautions against the use of serum-supplemented culture media in a transwell migration assay when using chemoattractants other than FBS. At 24 h, a 5% foetal bovine serum (FBS) gradient caused BV2 microglia to migrate toward the lower compartment of the transwell apparatus. Interestingly, FBS-supplemented media in the absence of a gradient also resulted in notable microglia migration. Serum can therefore confound the interpretation of a transwell migration assay when another chemoattractant is used.

Structural basis of Gip1 for cytosolic sequestration of G protein in wide-range chemotaxis.

G protein interacting protein 1 (Gip1) binds and sequesters heterotrimeric G proteins in the cytosolic pool, thus regulating G protein-coupled receptor (GPCR) signalling for eukaryotic chemotaxis. Here, we report the underlying structural basis of Gip1 function. The crystal structure reveals that the region of Gip1 that binds to the G protein has a cylinder-like fold with a central hydrophobic cavity composed of six α-helices. Mutagenesis and biochemical analyses indicate that the hydrophobic cavity and the hydrogen bond network at the entrance of the cavity are essential for complex formation with the geranylgeranyl modification on the Gγ subunit. Mutations of the cavity impair G protein sequestration and translocation to the membrane from the cytosol upon receptor stimulation, leading to defects in chemotaxis at higher chemoattractant concentrations. These results demonstrate that the Gip1-dependent regulation of G protein shuttling ensures wide-range gradient sensing in eukaryotic chemotaxis.

Shape Effects of Peptide Amphiphile Micelles for Targeting Monocytes.

Peptide amphiphile micelles (PAMs) are a nanoparticle platform that have gained popularity for their targeting versatility in a wide range of disease models. An important aspect of micelle design is considering the type of hydrophobic moiety used to synthesize the PAM, which can act as a contributing factor regarding their morphology and targeting capabilities. To delineate and compare the characteristics of spherical and cylindrical micelles, we incorporated the monocyte-targeting chemokine, monocyte chemoattractant protein-1 (MCP-1), into our micelles (MCP-1 PAMs). We report that both shapes of nanoparticles were biocompatible with monocytes and enhanced the secondary structure of the MCP-1 peptide, thereby improving the ability of the micelles to mimic the native MCP-1 protein structure. As a result, both shapes of MCP-1 PAMs effectively targeted monocytes in an in vitro binding assay with murine monocytes. Interestingly, cylindrical PAMs showed a greater ability to attract monocytes compared to spherical PAMs in a chemotaxis assay. However, the surface area, the multivalent display of peptides, and the zeta potential of PAMs may also influence their biomimetic properties. Herein, we introduce variations in the methods of PAM synthesis and discuss the differences in PAM characteristics that can impact the recruitment of monocytes, a process associated with disease and cancer progression.

Ethyl ß-d-glucoside: a novel chemoattractant of Ralstonia solanacearum isolated from tomato root exudates by a bioassay-guided fractionation.

A chemoattractant of Ralstonia solanacearum isolated from the activated charcoal-adsorbed fraction of tomato root exudates was identified as ethyl ß-d-glucopyranoside by instrumental analyses and comparison with synthetic preparations. Ethyl ß-D-glucopyranoside showed unambiguous activity at above 1 µmol/disc. Its stereoisomers and D-glucose were inactive.

The Chemoattractant Glorin Is Inactivated by Ester Cleavage during Early Multicellular Development of Polysphondylium pallidum.

Among the amoebozoan species capable of forming fruiting bodies, the dictyostelid social amoebae stand out since they form true multicellular organisms by means of single cell aggregation. Upon food depletion, cells migrate across gradients of extracellular signals initiated by cells in aggregation centers. The model species that is widely used to study multicellular development of social amoebae, Dictyostelium discoideum, uses cyclic adenosine monophosphate (cAMP) as a chemoattractant to coordinate aggregation. Molecular phylogeny studies suggested that social amoebae evolved in four major groups, of which groups 1 and 2 are paraphyletic to groups 3 and 4. During early development, intercellular communication with cAMP appears to be restricted to group 4 species. Cells of group 1 and 2 taxa do not respond chemotactically to extracellular cAMP and likely use a dipeptide chemoattractant known as glorin ( N-propionyl-γ-L-glutamyl-L-ornithin-δ-lactam-ethylester) to regulate aggregation. Directional migration of glorin-responsive cells requires the periodic breakdown of the chemoattractant. Here, we identified an extracellular enzymatic activity (glorinase) in the glorin-responsive group 2 taxon Polysphondylium pallidum leading to the inactivation of glorin. We determined the inactivation mechanism to proceed via hydrolytic ethyl ester cleavage of the γ-glutamyl moiety of glorin. Synthetic glorinamide, in which the ethyl ester group was substituted by an ethyl amide group, had glorin-like biological activity but was resistant to degradation by glorinase. Our observations pave the way for future investigations toward an ancient eukaryotic chemotaxis system.

Use of Cryo-EM to Study the Structure of Chemoreceptor Arrays In Vivo.

Cryo-electron microscopy (cryo-EM) allows the imaging of intact macromolecular complexes in the context of whole cells. The biological samples for cryo-EM are kept in a near-native state by flash freezing, without the need for any additional sample preparation or fixation steps. Since transmission electron microscopy only generates 2D projections of the samples, the specimen has to be tilted in order to recover its 3D structural information. This is done by collecting images of the sample with various tilt angles in respect to the electron beam. The acquired tilt series can then be computationally back-projected. This technique is called electron cryotomography (ECT), and has been instrumental in unraveling the architecture of chemoreceptor arrays. Here we describe the method of visualizing in vivo bacterial chemoreceptor arrays in three main steps: immobilization of bacterial cells on EM grids by plunge-freezing; 2D image acquisition in tilt series; and 3D tomogram reconstruction.

Visualizing Chemoreceptor Arrays in Bacterial Minicells by Cryo-Electron Tomography and Subtomogram Analysis.

Bacterial chemoreceptors form a highly ordered array in concert with the CheA kinase and the CheW coupling protein. The precise architecture of the array is responsible for high sensitivity, high dynamic range, and strong amplification of chemotaxis signaling. Cryo-electron tomography (cryo-ET) has emerged as a unique tool to visualize bacterial chemotaxis arrays at molecular level. Here we describe a detailed cryo-ET and subtomogram averaging procedure to determine in situ structure of the chemoreceptor arrays in Salmonella minicells. The procedure should be readily applicable to visualize other large macromolecular assemblies in their cellular context.

Fluorescence Anisotropy to Detect In Vivo Stimulus-Induced Changes in Chemoreceptor Packing.

The anisotropy of the fluorescence emitted from fluorescent proteins, such as yellow fluorescent protein (YFP), is sensitive to Homo-FRET between the proteins. This effect can be used to detect in vivo ligand-induced changes in packing or conformation of tagged chemoreceptors. Such measurements of clustered or dispersed core-signaling units revealed quantitative dose-dependent responses of these sensors.