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The Escherichia coli chemoreceptors form an extensive array that achieves cooperative and adaptive sensing of extracellular signals. The receptors control the activity of histidine kinase CheA, which drives a nonequilibrium phosphorylation-dephosphorylation reaction cycle for response regulator CheY. Cooperativity and dissipation are both important aspects of chemotaxis signaling, yet their consequences have only been studied separately. Recent single-cell FRET measurements revealed that kinase activity of the array spontaneously switches between active and inactive states, with asymmetric switching times that signify time-reversal symmetry breaking in the underlying dynamics. Here, we present a nonequilibrium lattice model of the chemosensory array, which demonstrates that the observed asymmetric switching dynamics can only be explained by an interplay between the dissipative reactions within individual core units and the cooperative coupling between neighboring units. Microscopically, the switching time asymmetry originates from irreversible transition paths. The model shows that strong dissipation enables sensitive and rapid signaling response by relieving the speed-sensitivity trade-off, which can be tested by future single-cell experiments. Overall, our model provides a general framework for studying biological complexes composed of coupled subunits that are individually driven by dissipative cycles and the rich nonequilibrium physics within. The Escherichia coli chemoreceptors form a cooperative array that controls the activity of histidine kinase CheA, which drives chemotaxis signaling. The authors propose a nonequilibrium lattice model showing that the observed asymmetric switching dynamics result from an interplay between dissipation within core units and cooperative coupling, enabling sensitive and rapid signaling responses.

CheV is one of the least understood chemosensory signaling proteins. Our demonstration that CheV interacts only with certain chemoreceptors offers fundamental new insights. These findings, combined with the observation that CheV is present in bacteria with numerous chemoreceptors, suggest that CheV plays a role in coordinating chemotactic outputs in complex chemosensory systems. Understanding the mechanisms by which chemotactic responses are defined in bacteria with a high number of chemoreceptors is a major research priority in the field of chemotaxis. While previous studies, including this one, show that the ability to be phosphorylated is crucial for CheV function, the molecular consequences of CheV phosphorylation have remained unclear. Our discovery that phosphorylation is essential for CheV binding to certain chemoreceptors fills in this critical gap in understanding the molecular mechanism of CheV. This study is likely to inspire further research into CheV function in other bacteria using similar approaches.

Reversible switching of the bacterial flagellar motor between clockwise (CW) and counterclockwise (CCW) rotation is necessary for chemotaxis, which enables cells to swim towards favorable chemical habitats. Increase in the viscous resistance to the rotation of the motor (mechanical load) inhibits switching. However, cells must maintain homeostasis in switching to navigate within environments of different viscosities. The mechanism by which the cell maintains optimal chemotactic function under varying loads is not understood. Here, we show that the flagellar motor allosterically controls the binding affinity of the chemotaxis response regulator, CheY-P, to the flagellar switch complex by modulating the mechanical forces acting on the rotor. Mechanosensitive CheY-P binding compensates for the load-induced loss of switching by precisely adapting the switch response to a mechanical stimulus. The interplay between mechanical forces and CheY-P binding tunes the chemotactic function to match the load. This adaptive response of the chemotaxis output to mechanical stimuli resembles the proprioceptive feedback in the neuromuscular systems of insects and vertebrates. It is unclear how bacterial cells adapt the reversible switching of flagellar motor rotation to environments of different viscosities. Here, Antani et al. show that flagellar mechanosensors allosterically control the motor’s binding affinity for the chemotaxis response regulator, CheY-P, to adapt flagellar switching over varying viscous loads.

In the enteric pathogen Salmonella enterica serovar Typhimurium, invasion and motility are coordinated by the master regulator HilD, which induces expression of the type III secretion system 1 (T3SS1) and motility genes. Methyl-accepting chemotaxis proteins (MCPs) detect specific ligands and control the direction of the flagellar motor, promoting tumbling and changes in direction (if a repellent is detected) or smooth swimming (in the presence of an attractant). Here, we show that HilD induces smooth swimming by upregulating an uncharacterized MCP (McpC), and this is important for invasion of epithelial cells. Remarkably, in vitro assays show that McpC can suppress tumbling and increase smooth swimming in the absence of exogenous ligands. Expression of mcpC is repressed by the universal regulator H-NS, which can be displaced by HilD. Our results highlight the importance of smooth swimming for Salmonella Typhimurium invasiveness and indicate that McpC can act via a ligand-independent mechanism when incorporated into the chemotactic receptor array. Protein HilD of Salmonella Typhimurium coordinates motility and host cell invasion by upregulating flagellar genes and a secretion system. Here, Cooper et al. show that HilD also modulates swimming behaviour by upregulating a subunit of the chemotactic receptor array, and this is important for invasion of epithelial cells.

Key Points The current view of the neutrophil as a short-lived, homogeneous cell type with a role limited to the elimination of pathogens during the innate immune response has begun to change. Recent studies have revealed that the lifespan of a neutrophil in circulation might be much longer, and that differential subpopulations of neutrophils and their reservoirs (marginal pools) might exist (although it still remains to be determined whether these subpopulations are functional or lineage-restricted). The classical cascade of neutrophil recruitment has been updated recently to reflect our better understanding of how this process occurs in the blood under shear stress conditions (for example, neutrophils have been found to form tethers and slings to anchor themselves to the vasculature). In addition, our understanding has improved regarding what are preferable sites of neutrophil extravasation. It is also now clear that there are exceptions to this classical cascade in a number of organs, such as the liver, lung and brain, where some steps of the cascade do not occur and/or different molecules are used by neutrophils. Furthermore, we recognize there might be differences between sterile and infectious inflammation. Once extravasated, neutrophils follow a hierarchy of chemotactic molecules to reach the site of inflammation, following first 'intermediate' chemoattractants (endogenous chemokines) and then later 'end-target' chemoattractants (bacterial peptides or complement components). The process of chemotaxis is controlled by multiple intracellular signalling pathways (mitogen-activated protein kinase-dependent) controlling 'go' and 'stop' signals. Despite the pre-existing dogma that neutrophils leave the vasculature and die, it has been revealed that some extravasated neutrophils might re-enter circulation, leading to the dissemination of inflammation to other organs and subsequent tissue injury. In other cases, transmigrating cells may play an important part in the resolution of inflammation. In fact, neutrophils were shown to participate in wound healing and to actively limit self-recruitment through the release of endogenous molecules that inhibit integrin activation or cytoskeletal changes. Newly described roles of neutrophils cover their involvement in adaptive immunity by controlling the activation of T and B cells, and through the presentation of antigens to professional antigen-presenting cells in lymph nodes. Neutrophil extracellular trap (NET) formation, a strategy of pathogen eradication discovered less than a decade ago, has now been described to occur in vivo not only during acute (bacterial or viral) inflammation but also in numerous pathological conditions, such as autoimmune diseases, vascular diseases and cancer. Recently described mechanisms of NET formation indicate that neutrophils releasing NETs in vivo do not immediately die but rather keep performing functions such as chemotaxis and phagocytosis. It is becoming clear that the immune functions of neutrophils are more complex than once thought. Here, the authors provide an updated version of the classical neutrophil recruitment cascade and discuss the pro-inflammatory and anti-inflammatory roles of these cells in different immune settings. Neutrophils have traditionally been thought of as simple foot soldiers of the innate immune system with a restricted set of pro-inflammatory functions. More recently, it has become apparent that neutrophils are, in fact, complex cells capable of a vast array of specialized functions. Although neutrophils are undoubtedly major effectors of acute inflammation, several lines of evidence indicate that they also contribute to chronic inflammatory conditions and adaptive immune responses. Here, we discuss the key features of the life of a neutrophil, from its release from bone marrow to its death. We discuss the possible existence of different neutrophil subsets and their putative anti-inflammatory roles. We focus on how neutrophils are recruited to infected or injured tissues and describe differences in neutrophil recruitment between different tissues. Finally, we explain the mechanisms that are used by neutrophils to promote protective or pathological immune responses at different sites.

Allostery in proteins influences various biological processes such as regulation of gene transcription and activities of enzymes and cell signaling. Computational approaches for analysis of allosteric coupling provide inexpensive opportunities to predict mutations and to design small-molecule agents to control protein function and cellular activity. We develop a computationally efficient network-based method, Ohm, to identify and characterize allosteric communication networks within proteins. Unlike previously developed simulation-based approaches, Ohm relies solely on the structure of the protein of interest. We use Ohm to map allosteric networks in a dataset composed of 20 proteins experimentally identified to be allosterically regulated. Further, the Ohm allostery prediction for the protein CheY correlates well with NMR CHESCA studies. Our webserver, Ohm.dokhlab.org, automatically determines allosteric network architecture and identifies critical coupled residues within this network. The computational prediction of protein allostery can guide experimental studies of protein function and cellular activity. Here, the authors develop a network-based method to detect allosteric coupling within proteins solely based on their structures, and set up a webserver for allostery prediction.

The chemotaxis machinery of Escherichia coli has served as a model for exploring the molecular signaling mechanisms of transmembrane chemoreceptors known as methyl-accepting chemotaxis proteins (MCPs). Yet, fundamental questions about signal transmission through MCP molecules remain unanswered. Our work with the E. coli serine chemoreceptor Tsr has developed in vivo reporters that distinguish kinase-OFF and kinase-ON structures in the cytoplasmic methylation helix (MH) cap, which receives stimulus signals from an adjoining, membrane-proximal histidine kinase, adenylyl cyclases, MCPs, and phosphatases (HAMP) domain. The cytoplasmic helices of the Tsr homodimer interact mainly through packing interactions of hydrophobic residues at a and d heptad positions. We investigated the in vivo crosslinking properties of Tsr molecules bearing cysteine replacements at functionally tolerant g heptad positions in the N-terminal and C-terminal cap helices. Upon treatment of cells with bismaleimidoethane (BMOE), a bifunctional thiol-reagent, Tsr-G273C/Q504C readily formed a doubly crosslinked product in the presence of serine but not in its absence. Moreover, a serine stimulus combined with BMOE treatment during in vivo Förster resonance energy transfer–based kinase assays locked Tsr-G273C/Q504C in kinase-OFF output. An OFF-shifting lesion in MH1 (D269P) promoted the formation of the doubly crosslinked species in the absence of serine, whereas an ON-shifting lesion (G268P) suppressed the formation of the doubly crosslinked species. Tsr-G273C/Q504C also showed output-dependent crosslinking patterns in combination with ON-shifting and OFF-shifting adaptational modifications. Our results are consistent with a helix breathing-axial rotation-bundle repacking signaling mechanism and imply that in vivo crosslinking tools could serve to probe helix-packing transitions and their output consequences in other regions of the receptor molecule.

Chemotaxis is the directed motility by means of which microbes sense chemical cues and relocate towards more favorable environments. Methyl-accepting chemotaxis proteins (MCPs) are the most common receptors in bacteria and archaea. They are arranged as trimers of dimers that, in turn, form hexagonal arrays in the cytoplasmic membrane or in the cytoplasm. Several different classes of MCPs have been identified according to their ligand binding region and membrane topology. MCPs have been further classified based on the length and sequence conservation of their cytoplasmic domains. Clusters of membrane-embedded MCPs often localize to the poles of the cell, whereas cytoplasmic MCPs can be targeted to the poles or distributed throughout the cell body. MCPs play an important role in cell survival, pathogenesis, and biodegradation. Bacterial adaptation to diverse environmental conditions promotes diversity among the MCPs. This review summarizes structure, classification, and structure–activity relationship of the known MCP receptors, with a brief overview of the signal transduction mechanisms in bacteria and archaea.

Neutrophil migration is essential for inflammatory responses to kill pathogens; however, excessive neutrophilic inflammation also leads to tissue injury and adverse effects. To discover novel therapeutic targets that modulate neutrophil migration, we performed a neutrophil-specific microRNA (miRNA) overexpression screen in zebrafish and identified 8 miRNAs as potent suppressors of neutrophil migration. Among those, miR-199 decreases neutrophil chemotaxis in zebrafish and human neutrophil-like cells. Intriguingly, in terminally differentiated neutrophils, miR-199 alters the cell cycle-related pathways and directly suppresses cyclin-dependent kinase 2 (Cdk2), whose known activity is restricted to cell cycle progression and cell differentiation. Inhibiting Cdk2, but not DNA replication, disrupts cell polarity and chemotaxis of zebrafish neutrophils without inducing cell death. Human neutrophil-like cells deficient in CDK2 fail to polarize and display altered signaling downstream of the formyl peptide receptor. Chemotaxis of primary human neutrophils is also reduced upon CDK2 inhibition. Furthermore, miR-199 overexpression or CDK2 inhibition significantly improves the outcome of lethal systemic inflammation challenges in zebrafish. Our results therefore reveal previously unknown functions of miR-199 and CDK2 in regulating neutrophil migration and provide directions in alleviating systemic inflammation.

Various microorganisms and some mammalian cells are able to swim in viscous fluids by performing nonreciprocal body deformations, such as rotating attached flagella or by distorting their entire body. In order to perform chemotaxis (i.e., to move toward and to stay at high concentrations of nutrients), they adapt their swimming gaits in a nontrivial manner. Here, we propose a computational model, which features autonomous shape adaptation of microswimmers moving in one dimension toward high field concentrations. As an internal decision-making machinery, we use artificial neural networks, which control the motion of the microswimmer. We present two methods to measure chemical gradients, spatial and temporal sensing, as known for swimming mammalian cells and bacteria, respectively. Using the genetic algorithm NeuroEvolution of Augmenting Topologies, surprisingly simple neural networks evolve. These networks control the shape deformations of the microswimmers and allow them to navigate in static and complex time-dependent chemical environments. By introducing noisy signal transmission in the neural network, the well-known biased run-and-tumble motion emerges. Our work demonstrates that the evolution of a simple and interpretable internal decision-making machinery coupled to the environment allows navigation in diverse chemical landscapes. These findings are of relevance for intracellular biochemical sensing mechanisms of single cells or for the simple nervous system of small multicellular organisms such as Caenorhabditis elegans.