Tensile force transmitted through LFA-1 bonds mechanoregulate neutrophil inflammatory response

The bidirectional effect of neutrophils on periodontitis model in mice: A systematic review

OBJECTIVE

To evaluate the regulatory role of neutrophils as the first line of host immune defense in the periodontal microenvironment of mice.

METHODS

A systematic search was performed using PubMed, Web of Science, and ScienceDirect databases for articles published between 2012 and 2023. In this review, articles investigating the effect of neutrophils on alveolar bone resorption in a mouse model of periodontitis were selected and evaluated according to eligibility criteria. Important variables that may influence outcomes were analyzed.

RESULTS

Eleven articles were included in this systematic review. The results showed that because of their immune defense functions, the functional homeostasis of local neutrophils is critical for periodontal health. Neutrophil deficiency aggravates alveolar bone loss. However, several studies have shown that excessive neutrophil infiltration is positively correlated with alveolar bone resorption caused by periodontitis in mice. Therefore, the homeostasis of neutrophil function needs to be considered in the treatment of periodontitis.

CONCLUSIONS

Pooled analysis suggests that neutrophils play a bidirectional role in periodontal tissue remodeling in mouse periodontitis models. Therefore, targeted regulation of local neutrophil function provides a novel strategy for the treatment of periodontitis.

Mechanosensitive traction force generation is regulated by the neutrophil activation state

The generation of traction forces by neutrophils regulates many crucial effector functions responsible for host defense, such as attachment, spreading, migration, phagocytosis, and NETosis. The activation state of the cell is a strong determinant of the functional efficacy of the neutrophil; however, the effect of activation on traction force production has not yet been determined experimentally. Previously, the mapping of cellular-generated forces produced by human neutrophils via a Traction Force Microscopy (TFM) method has required a three-dimensional imaging modality to capture out-of-plane forces, such as confocal or multiphoton techniques. A method newly developed in our laboratories can capture out-of-plane forces using only a two-dimensional imaging modality. This novel technique-combined with a topology-based single particle tracking algorithm and finite element method calculations-can construct high spatial frequency three-dimensional traction fields, allowing for traction forces in-plane and out-of-plane to the substrate to now be differentially visualized and quantified with a standard epifluorescence microscope. Here we apply this technology to determine the effect of neutrophil activation on force generation. Sepsis is a systemic inflammatory response that causes dysregulated neutrophil activation in vivo. We found that neutrophils from septic patients produced greater total forces than neutrophils from healthy donors and that the majority of this dysregulation occurred in-plane to the substrate. Ex vivo activation of neutrophils from healthy donors showed differential consequences depending on activation stimuli with mechanosensitive force decreases observed in some cases. These findings demonstrate the feasibility of epifluorescence-based microscopy in mapping traction forces to ask biologically significant questions regarding neutrophil function.

Receptor-Ligand Binding: Effect of Mechanical Factors

Gaining insight into the in situ receptor-ligand binding is pivotal for revealing the molecular mechanisms underlying the physiological and pathological processes and will contribute to drug discovery and biomedical application. An important issue involved is how the receptor-ligand binding responds to mechanical stimuli. This review aims to provide an overview of the current understanding of the effect of several representative mechanical factors, such as tension, shear stress, stretch, compression, and substrate stiffness on receptor-ligand binding, wherein the biomedical implications are focused. In addition, we highlight the importance of synergistic development of experimental and computational methods for fully understanding the in situ receptor-ligand binding, and further studies should focus on the coupling effects of these mechanical factors.

Force-Regulated Calcium Signaling of Lymphoid Cell RPMI 8226 Mediated by Integrin α4β7/MAdCAM-1 in Flow

MAdCAM-1 binds to integrin α₄β₇, which mediates the rolling and arrest of circulating lymphocytes upon the vascular endothelia during lymphocytic homing. The calcium response by adhered lymphocytes is a critical event for lymphocyte activation and subsequent arrest and migration under flow. However, whether the interaction of integrin α₄β₇ /MAdCAM-1 can effectively trigger the calcium response of lymphocytes remains unclear, as well as whether the fluid force affects the calcium response. In this study, we explore the mechanical regulation of integrin α₄β₇-induced calcium signaling under flow. Flou-4 AM was used to examine the calcium response under real-time fluorescence microscopy when cells were firmly adhered to a parallel plate flow chamber. The interaction between integrin α₄β₇ and MAdCAM-1 was found to effectively trigger calcium signaling in firmly adhered RPMI 8226 cells. Meanwhile, increasing fluid shear stress accelerated the cytosolic calcium response and enhanced signaling intensity. Additionally, the calcium signaling of RPMI 8226 activated by integrin α₄β₇ originated from extracellular calcium influx instead of cytoplasmic calcium release, and the signaling transduction of integrin α₄β₇ was involved in Kindlin-3. These findings shed new light on the mechano-chemical mechanism of calcium signaling in RPMI 8226 cells induced by integrin α₄β_7.

Tuning immunity through tissue mechanotransduction

Immune responses are governed by signals from the tissue microenvironment, and in addition to biochemical signals, mechanical cues and forces arising from the tissue, its extracellular matrix and its constituent cells shape immune cell function. Indeed, changes in biophysical properties of tissue alter the mechanical signals experienced by cells in many disease conditions, in inflammatory states and in the context of ageing. These mechanical cues are converted into biochemical signals through the process of mechanotransduction, and multiple pathways of mechanotransduction have been identified in immune cells. Such pathways impact important cellular functions including cell activation, cytokine production, metabolism, proliferation and trafficking. Changes in tissue mechanics may also represent a new form of 'danger signal' that alerts the innate and adaptive immune systems to the possibility of injury or infection. Tissue mechanics can change temporally during an infection or inflammatory response, offering a novel layer of dynamic immune regulation. Here, we review the emerging field of mechanoimmunology, focusing on how mechanical cues at the scale of the tissue environment regulate immune cell behaviours to initiate, propagate and resolve the immune response.

β2-Integrin Adhesive Bond Tension under Shear Stress Modulates Cytosolic Calcium Flux and Neutrophil Inflammatory Response

On arrested neutrophils a focal adhesive cluster of ~200 high affinity (HA) β₂-integrin bonds under tension is sufficient to trigger Ca²⁺ flux that signals an increase in activation in direct proportion to increments in shear stress. We reasoned that a threshold tension acting on individual β₂-integrin bonds provides a mechanical means of transducing the magnitude of fluid drag force into signals that enhance the efficiency of neutrophil recruitment and effector function. Tension gauge tethers (TGT) are a duplex of DNA nucleotides that rupture at a precise shear force, which increases with the extent of nucleotide overlap, ranging from a tolerance of 54pN to 12pN. TGT annealed to a substrate captures neutrophils via allosteric antibodies that stabilize LFA-1 in a high- or low-affinity conformation. Neutrophils sheared on TGT substrates were recorded in real time to form HA β₂-integrin bonds and flux cytosolic Ca²⁺, which elicited shape change and downstream production of reactive oxygen species. A threshold force of 33pN triggered consolidation of HA β₂-integrin bonds and triggered membrane influx of Ca²⁺, whereas an optimum tension of 54pN efficiently transduced activation at a level equivalent to chemotactic stimulation on ICAM-1. We conclude that neutrophils sense the level of fluid drag transduced through individual β₂-integrin bonds, providing an intrinsic means to modulate inflammatory response in the microcirculation.

Engineering physical microenvironments to study innate immune cell biophysics

Innate immunity forms the core of the human body's defense system against infection, injury, and foreign objects. It aims to maintain homeostasis by promoting inflammation and then initiating tissue repair, but it can also lead to disease when dysregulated. Although innate immune cells respond to their physical microenvironment and carry out intrinsically mechanical actions such as migration and phagocytosis, we still do not have a complete biophysical description of innate immunity. Here, we review how engineering tools can be used to study innate immune cell biophysics. We first provide an overview of innate immunity from a biophysical perspective, review the biophysical factors that affect the innate immune system, and then explore innate immune cell biophysics in the context of migration, phagocytosis, and phenotype polarization. Throughout the review, we highlight how physical microenvironments can be designed to probe the innate immune system, discuss how biophysical insight gained from these studies can be used to generate a more comprehensive description of innate immunity, and briefly comment on how this insight could be used to develop mechanical immune biomarkers and immunomodulatory therapies.

Intravital calcium imaging in myeloid leukocytes identifies calcium frequency spectra as indicators of functional states

The assessment of leukocyte activation in vivo is mainly based on surrogate parameters, such as cell shape changes and migration patterns. Consequently, additional parameters are required to dissect the complex spatiotemporal activation of leukocytes during inflammation. Here, we showed that intravital microscopy of myeloid leukocyte Ca²⁺ signals with Ca²⁺ reporter mouse strains combined with bioinformatic signal analysis provided a tool to assess their activation in vivo. We demonstrated by two-photon microscopy that tissue-resident macrophages reacted to sterile inflammation in the cremaster muscle with Ca²⁺ transients in a distinct spatiotemporal pattern. Moreover, through high-resolution, intravital spinning disk confocal microscopy, we identified the intracellular Ca²⁺ signaling patterns of neutrophils during the migration cascade in vivo. These patterns were modulated by the Ca²⁺ channel Orai1 and Gα_i-coupled GPCRs, whose effects were evident through analysis of the range of frequencies of the Ca²⁺ signal (frequency spectra), which provided insights into the complex patterns of leukocyte Ca²⁺ oscillations. Together, these findings establish Ca²⁺ frequency spectra as an additional dimension to assess leukocyte activation and migration during inflammation in vivo.

Multiscale biomechanics and mechanotransduction from liver fibrosis to cancer

A growing body of multiscale biomechanical studies has been proposed to highlight the mechanical cues in the development of hepatic fibrosis and cancer. At the cellular level, changes in mechanical microenvironment induce phenotypic and functional alterations of hepatic cells, initiating a positive feedback loop that promotes liver fibrogenesis and hepatocarcinogenesis. Tumor mechanical microenvironment of hepatocellular carcinoma facilitates tumor cell growth and metastasis, and hinders the drug delivery and immunotherapy. At the molecular level, mechanical forces are sensed and transmitted into hepatic cells via allosteric activation of mechanoreceptors on the cell membrane, leading to the activation of various mechanotransduction pathways including integrin and YAP signaling and then regulating cell function. Thus, the application of mechanomedicine concept in the treatment of liver diseases is promising for rational design and cell-specific delivery of therapeutic drugs. This review mainly discusses the correlation between biomechanical cues and liver diseases from the viewpoint of mechanobiology.

The voltage-gated potassium channel KV1.3 regulates neutrophil recruitment during inflammation

AIMS

Neutrophil trafficking within the vasculature strongly relies on intracellular calcium signalling. Sustained Ca2+ influx into the cell requires a compensatory efflux of potassium to maintain membrane potential. Here, we aimed to investigate whether the voltage-gated potassium channel KV1.3 regulates neutrophil function during the acute inflammatory process by affecting sustained Ca2+ signalling.

METHODS AND RESULTS

Using in vitro assays and electrophysiological techniques, we show that KV1.3 is functionally expressed in human neutrophils regulating sustained store-operated Ca2+ entry through membrane potential stabilizing K+ efflux. Inhibition of KV1.3 on neutrophils by the specific inhibitor 5-(4-Phenoxybutoxy)psoralen (PAP-1) impaired intracellular Ca2+ signalling, thereby preventing cellular spreading, adhesion strengthening, and appropriate crawling under flow conditions in vitro. Using intravital microscopy, we show that pharmacological blockade or genetic deletion of KV1.3 in mice decreased neutrophil adhesion in a blood flow dependent fashion in inflamed cremaster muscle venules. Furthermore, we identified KV1.3 as a critical component for neutrophil extravasation into the inflamed peritoneal cavity. Finally, we also revealed impaired phagocytosis of Escherichia coli particles by neutrophils in the absence of KV1.3.

CONCLUSION

We show that the voltage-gated potassium channel KV1.3 is critical for Ca2+ signalling and neutrophil trafficking during acute inflammatory processes. Our findings do not only provide evidence for a role of KV1.3 for sustained calcium signalling in neutrophils affecting key functions of these cells, they also open up new therapeutic approaches to treat inflammatory disorders characterized by overwhelming neutrophil infiltration.

Kindlin-3 disrupts an intersubunit association in the integrin LFA1 to trigger positive feedback activation by Rap1 and talin1

Integrin activation by the intracellular adaptor proteins talin1 and kindlin-3 is essential for lymphocyte adhesion. These adaptors cooperatively control integrin activation through bidirectional (inside-out and outside-in) activation signals. Using single-molecule measurements, we revealed the distinct dynamics of talin1 and kindlin-3 interactions with the integrin LFA1 (αLβ2) and their functions in LFA1 activation and LFA1-mediated adhesion. The kinetics of talin1 binding to the tail of the β2 subunit corresponded to those of LFA1 binding to its ligand ICAM1. ICAM1 binding induced transient interactions between the membrane-proximal cytoplasmic region of the β2 subunit with an N-terminal domain of kindlin-3, leading to disruption of the association between the integrin subunits (the α/β clasp) and unbending of the ectodomains of the α/β heterodimer. These conformational changes promoted high-affinity talin1 binding to the β2 tail that required the talin rod domain and the actomyosin cytoskeleton. Inside-out signaling induced by the GTPase Rap1 did not markedly stabilize the binding of talin1 and kindlin-3 to LFA1. In contrast, ligand-induced outside-in signaling, the stabilization of open LFA1 conformers, or shear force substantially altered the dynamics of talin1 and kindlin-3 association with LFA1 and enhanced both Rap1 and LFA1 activation. In migrating lymphocytes, asymmetrical distribution of talin1 and kindlin-3 correlated with the maturation of LFA1 from a low-affinity conformation at the leading edge to a high-affinity conformation in the adherent mid-body. Our results suggest that kindlin-3 spatiotemporally mediates a positive feedback circuit of LFA1 activation to control dynamic adhesion and migration of lymphocytes.

Emerging evidence for kindlin oligomerization and its role in regulating kindlin function

Integrin-mediated cell-extracellular matrix (ECM) interactions play crucial roles in a broad range of physiological and pathological processes. Kindlins are important positive regulators of integrin activation. The FERM-domain-containing kindlin family comprises three members, kindlin-1, kindlin-2 and kindlin-3 (also known as FERMT1, FERMT2 and FERMT3), which share high sequence similarity (identity >50%), as well as domain organization, but exhibit diverse tissue-specific expression patterns and cellular functions. Given the significance of kindlins, analysis of their atomic structures has been an attractive field for decades. Recently, the structures of kindlin and its β-integrin-bound form have been obtained, which greatly advance our understanding of the molecular functions that involve kindlins. In particular, emerging evidence indicates that oligomerization of kindlins might affect their integrin binding and focal adhesion localization, positively or negatively. In this Review, we presented an update on the recent progress of obtaining kindlin structures, and discuss the implication for integrin activation based on kindlin oligomerization, as well as the possible regulation of this process.

Visualization of integrin molecules by fluorescence imaging and techniques

Integrin molecules are transmembrane αβ heterodimers involved in cell adhesion, trafficking, and signaling. Upon activation, integrins undergo dynamic conformational changes that regulate their affinity to ligands. The physiological functions and activation mechanisms of integrins have been heavily discussed in previous studies and reviews, but the fluorescence imaging techniques -which are powerful tools for biological studies- have not. Here we review the fluorescence labeling methods, imaging techniques, as well as Förster resonance energy transfer assays used to study integrin expression, localization, activation, and functions.

Case Report: A Case of Leukocyte Adhesion Deficiency, Type III Presenting With Impaired Platelet Function, Lymphocytosis and Granulocytosis

Fermitin family homolog 3 (FERMT3), alternatively kindlin-3 (KIND3), is an integrin binding protein (of 667 residues) encoded by the FERMT3 gene. The molecule is essential for activating integrin α_IIbβ₃ (the fibrinogen receptor) on platelets and for the integrin-mediated hematopoietic cell (including platelets, T lymphocytes, B lymphocytes, and granulocytes) adhesion. Its defects are associated with impaired primary hemostasis, described as "Glanzmann's thrombasthenia (MIM#273800)-like bleeding problem." The defects are also associated with infections, designated as "LAD1 (leukocyte adhesion deficiency, type I; MIM#116920)-like immune deficiency." The entity that joins the impaired primary hemostasis with the leukocyte malfunction has been termed "leukocyte adhesion deficiency, type III" (LAD3, autosomal recessive, MIM#612840), representing a defective activation of the integrins β₁, β₂, and β3 on leukocytes and platelets. Here, we report a male toddler with novel compound heterozygous variants, NM_178443.2(FERMT3):c.1800G>A, p.Trp600^* (a non-sense variant) and NM_178443.2(FERMT3):c.2001del p.^*668Glufs^*106 (a non-stop variant). His umbilical cord separated at about 3 weeks of age. A skin rash (mainly petechiae and purpura) and recurrent episodes of severe epistaxis required blood transfusions in early infancy. His hemostatic work-up was remarkable for a normal platelet count, but abnormal platelet function screen with markedly prolonged collagen-epinephrine and collagen-ADP closure times. The impaired platelet function was associated with reduced platelet aggregation with all agonists. The expression of platelet receptors was normal. Other remarkable findings were persistent lymphocytosis and granulocytosis, representing defects in diapedesis due to the integrin dysfunction. The natural history of his condition, structure and sequence analysis of the variations, and comparison with other LAD3 cases reported in the literature are presented.