Cell-stiffness-induced mechanosignaling - a key driver of leukocyte transendothelial migration

Endothelial calcium firing mediates the extravasation of metastatic tumor cells

Metastatic dissemination is driven by genetic, biochemical, and biophysical cues that favor the distant colonization of organs and the formation of life-threatening secondary tumors. We have previously demonstrated that endothelial cells (ECs) actively remodel during extravasation by enwrapping arrested tumor cells (TCs) and extruding them from the vascular lumen while maintaining perfusion. In this work, we dissect the cellular and molecular mechanisms driving endothelial remodeling. Using high-resolution intravital imaging in zebrafish embryos, we demonstrate that the actomyosin network of ECs controls tissue remodeling and subsequent TC extravasation. Furthermore, we uncovered that this cytoskeletal remodeling is driven by altered endothelial-calcium (Ca2+) signaling caused by arrested TCs. Accordingly, we demonstrated that the inhibition of voltage-dependent calcium L-type channels impairs extravasation. Lastly, we identified P2X4, TRP, and Piezo1 mechano-gated Ca2+ channels as key mediators of the process. These results further highlight the central role of endothelial remodeling during the extravasation of TCs and open avenues for successful therapeutic targeting.

Tension at the gate: sensing mechanical forces at the blood-brain barrier in health and disease

Microvascular brain endothelial cells tightly limit the entry of blood components and peripheral cells into the brain by forming the blood-brain barrier (BBB). The BBB is regulated by a cascade of mechanical and chemical signals including shear stress and elasticity of the adjacent endothelial basement membrane (BM). During physiological aging, but especially in neurological diseases including multiple sclerosis (MS), stroke, small vessel disease, and Alzheimer's disease (AD), the BBB is exposed to inflammation, rigidity changes of the BM, and disturbed cerebral blood flow (CBF). These altered forces lead to increased vascular permeability, reduced endothelial reactivity to vasoactive mediators, and promote leukocyte transmigration. Whereas the molecular players involved in leukocyte infiltration have been described in detail, the importance of mechanical signalling throughout this process has only recently been recognized. Here, we review relevant features of mechanical forces acting on the BBB under healthy and pathological conditions, as well as the endothelial mechanosensory elements detecting and responding to altered forces. We demonstrate the underlying complexity by focussing on the family of transient receptor potential (TRP) ion channels. A better understanding of these processes will provide insights into the pathogenesis of several neurological disorders and new potential leads for treatment.

Amoeboid cells undergo durotaxis with soft end polarized NMIIA

Cell migration towards stiff substrates has been coined as durotaxis and implicated in development, wound healing, and cancer, where complex interplays between immune and non-immune cells are present. Compared to the emerging mechanisms underlying the strongly adhesive mesenchymal durotaxis, little is known about whether immune cells - migrating in amoeboid mode - could follow mechanical cues. Here, we develop an imaging-based confined migration device with a stiffness gradient. By tracking live cell trajectory and analyzing the directionality of T cells and neutrophils, we observe that amoeboid cells can durotax. We further delineate the underlying mechanism to involve non-muscle myosin IIA (NMIIA) polarization towards the soft-matrix-side but may not require differential actin flow up- or down-stiffness gradient. Using the protista Dictyostelium, we demonstrate the evolutionary conservation of amoeboid durotaxis. Finally, these experimental phenomena are theoretically captured by an active gel model capable of mechanosensing. Collectively, these results may shed new lights on immune surveillance and recently identified confined migration of cancer cells, within the mechanically inhomogeneous tumor microenvironment or the inflamed fibrotic tissues.

Extravasation of immune and tumor cells from an endothelial perspective

Crossing the vascular endothelium is a necessary stage for circulating cells aiming to reach distant organs. Leukocyte passage through the endothelium, known as transmigration, is a multistep process during which immune cells adhere to the vascular wall, migrate and crawl along the endothelium until they reach their exit site. Similarly, circulating tumor cells (CTCs), which originate from the primary tumor or reseed from early metastatic sites, disseminate using the blood circulation and also must cross the endothelial barrier to set new colonies in distant organs. CTCs are thought to mimic arrest and extravasation utilized by leukocytes; however, their extravasation also requires processes that, from an endothelial perspective, are specific to cancer cells. Although leukocyte extravasation relies on maintaining endothelial impermeability, it appears that cancer cells can indoctrinate endothelial cells into promoting their extravasation independently of their normal functions. In this Review, we summarize the common and divergent mechanisms of endothelial responses during extravasation of leukocytes (in inflammation) and CTCs (in metastasis), and highlight how these might be leveraged in the development of anti-metastatic treatments.

Tracer-based metabolomics for profiling nitric oxide metabolites in a 3D microvessels-on-chip model

Endothelial dysfunction, prevalent in cardiovascular diseases (CVDs) and linked to conditions like diabetes, hypertension, obesity, renal failure, or hypercholesterolemia, is characterized by diminished nitric oxide (NO) bioavailability-a key signaling molecule for vascular homeostasis. Current two-dimensional (2D) in vitro studies on NO synthesis by endothelial cells (ECs) lack the crucial laminar shear stress, a vital factor in modulating the NO-generating enzyme, endothelial nitric oxide synthase (eNOS), under physiological conditions. Here we developed a tracer-based metabolomics approach to measure NO-specific metabolites with mass spectrometry (MS) and show the impact of fluid flow on metabolic parameters associated with NO synthesis using 2D and 3D platforms. Specifically, we tracked the conversion of stable-isotope labeled NO substrate L-Arginine to L-Citrulline and L-Ornithine to determine eNOS activity. We demonstrated clear responses in human coronary artery endothelial cells (HCAECs) cultured with 13C6, 15N4-L-Arginine, and treated with eNOS stimulator, eNOS inhibitor, and arginase inhibitor. Analysis of downstream metabolites, 13C6, 15N3 L-Citrulline and 13C5, 15N2 L-Ornithine, revealed distinct outcomes. Additionally, we evaluated the NO metabolic status in static 2D culture and 3D microvessel models with bidirectional and unidirectional fluid flow. Our 3D model exhibited significant effects, particularly in microvessels exposed to the eNOS stimulator, as indicated by the 13C6, 15N3 L-Citrulline/13C5, 15N2 L-Ornithine ratio, compared to the 2D culture. The obtained results indicate that the 2D static culture mimics an endothelial dysfunction status, while the 3D model with a unidirectional fluid flow provides a more representative physiological environment that provides a better model to study endothelial dysfunction.

Conditions that promote transcellular neutrophil migration in vivo

Circulating leukocytes enter tissue either through endothelial junctions (paracellular) or via a pore through the body of endothelial cells (transcellular). We have previously shown that genetically replacing VE-cadherin with a VE-cadherin-α-catenin (VEC-αC) fusion construct-which binds constitutively to actin-obstructs junctions, and blocks leukocyte extravasation in lung, skin and postcapillary venules of cremaster muscle. However, neutrophil recruitment into the inflamed peritoneal cavity was unimpaired. Investigating reasons for this, here, we visualized neutrophil diapedesis by 3D intravital video microscopy in the cremaster muscle and omentum, the major site of neutrophil recruitment into the peritoneal cavity. We found that 80% of neutrophil-extravasation occurred through HEVs in the omentum, which was unimpaired by VEC-αC. In addition, in larger venules (60-85 µm) of both tissues, less than 15% of neutrophils extravasated transcellularly in WT mice. However, in VEC-α-C mice, transcellular diapedesis increased severalfold in the omentum, but not in the cremaster. In line with this, omental venules expressed higher levels of ICAM-1 and atypical chemokine receptor 1. Furthermore, only in the omentum, VEC-αC expression caused reduced elongation of venular endothelium in flow-direction, suggesting different biomechanical properties. Collectively, VEC-αC does not inhibit paracellular transmigration in all types of venules and can modulate the diapedesis route.

Co-administration of L-Ascorbic Acid and α-Tocopherol Alleviates Arsenic-Induced Immunotoxicities in the Thymus and Spleen by Dwindling Oxidative Stress-Induced Inflammation

Herein, we investigated whether L-ascorbic acid (L-AA) and α-tocopherol (α-T) co-administration has the potential to alleviate arsenic-induced immunotoxicities in the thymus, spleen, and circulating leukocytes. Forty-eight adult male Wistar rats were randomly divided into four groups before the treatment: group I (control); group II (sodium arsenite, 3 mg/kg/day/rat); group III (sodium arsenite + L-AA (200 mg/kg/day/rat) and α-T (400 mg/kg/day/rat)); group IV (L-AA and α-T). The result showed that sodium arsenite exposure (consecutive 30 days) caused weight reduction, structural alterations in the thymus and spleen, accompanied by a decrease in thymocyte and splenocyte count. Decreased superoxide dismutase and catalase activity, increased malondialdehyde and protein-carbonyl content, reduced Nrf2 and Bcl2 expression, and increased p-ERK, NF-kβ, Bax, and cleaved-caspase-3 expression were also observed in the thymus and spleen of arsenic-exposed rats. Enhanced plasma ACTH and corticosterone, ROS-induced apoptosis of lymphocytes were also observed. L-AA and α-T co-administration has the potential to abrogate the deleterious impact of arsenic on the thymus, spleen, and circulating lymphocytes. Whole transcriptome analysis of leukocytes revealed that arsenic treatment augmented the expression of Itga4, Itgam, and MMP9 genes, which might help in transient migration of the leukocytes through the endothelial cell layer. Co-administration with L-AA and α-T maintained Itga4, Itgam, and MMP9 gene expression within leukocytes at a lower level.

Blood Group Antigen A Carriers Exhibit an Extended Progression-Free Survival with no more Immune-Related Adverse Events

Extensive investigations have been conducted regarding the potential correlation between blood type and the immune system, as well as cancer risk in the Southern Chinese population. However, the prognostic value of the blood group and its genetic determinants in the context of immune checkpoint inhibitor (ICI) treatment remains unclear. Therefore, the associations between the ABO blood group and its single nucleotide polymorphisms (SNPs) were examined in relation to ICI treatment outcomes in 370 eligible patients with cancer. This approach allowed us to derive the blood group from the SNPs responsible for blood group determination. In the discovery cohort (N = 168), antigen A carriers (blood types A and AB) exhibited an extended progression-free survival (PFS; hazard ratio (HR) = 0.58, 95% confidence interval (CI) = 0.34-0.98). The association results from the SNP-derived blood were consistent with those from the measured blood group. In the validation cohort (N = 202), Cox regression analysis revealed that the antigen A carriers (rs507666 AA+GA genotype carriers) experienced significantly extended PFS compared with the non-antigen A carriers (HR = 0.61, 95% CI = 0.40-0.93). Therefore, a longer PFS was observed in antigen A carriers (P value = 0.003, HR = 0.60, 95% CI = 0.44-0.84). Furthermore, haplotype 2 carriers (rs507666 GA and rs659104 GG) demonstrated both extended PFS and improved overall survival. Notably, the presence of antigen A was not associated with the occurrence of overall immune-related adverse events (irAEs) or organ-specific toxicity. In summary, our findings revealed that antigen A carriers did not experience a higher incidence of irAEs while exhibiting better immunotherapy efficacy.

Age-dependent loss of HAPLN1 erodes vascular integrity via indirect upregulation of endothelial ICAM1 in melanoma

Melanoma, the most lethal form of skin cancer, often has worse outcomes in older patients. We previously demonstrated that an age-related decrease in the secreted extracellular matrix (ECM) protein HAPLN1 has a role in slowing melanoma progression. Here we show that HAPLN1 in the dermal ECM is sufficient to maintain the integrity of melanoma-associated blood vessels, as indicated by increased collagen and VE-cadherin expression. Specifically, we show that HAPLN1 in the ECM increases hyaluronic acid and decreases endothelial cell expression of ICAM1. ICAM1 phosphorylates and internalizes VE-cadherin, a critical determinant of vascular integrity, resulting in permeable blood vessels. We found that blocking ICAM1 reduces tumor size and metastasis in older mice. These results suggest that HAPLN1 alters endothelial ICAM1expression in an indirect, matrix-dependent manner. Targeting ICAM1 could be a potential treatment strategy for older patients with melanoma, emphasizing the role of aging in tumorigenesis.

Soluble vascular endothelial glycocalyx proteoglycans as potential therapeutic targets in inflammatory diseases

Reducing the activity of cytokines and leukocyte extravasation is an emerging therapeutic strategy to limit tissue-damaging inflammatory responses and restore immune homeostasis in inflammatory diseases. Proteoglycans embedded in the vascular endothelial glycocalyx, which regulate the activity of cytokines to restrict the inflammatory response in physiological conditions, are proteolytically cleaved in inflammatory diseases. Here we critically review the potential of proteolytically shed, soluble vascular endothelial glycocalyx proteoglycans to modulate pathological inflammatory responses. Soluble forms of the proteoglycans syndecan-1, syndecan-3 and biglycan exert beneficial anti-inflammatory effects by the removal of chemokines, suppression of proinflammatory cytokine expression and leukocyte migration, and induction of autophagy of proinflammatory M1 macrophages. By contrast, soluble versikine and decorin enhance proinflammatory responses by increasing inflammatory cytokine synthesis and leukocyte migration. Endogenous syndecan-2 and mimecan exert proinflammatory effects, syndecan-4 and perlecan mediate beneficial anti-inflammatory effects and glypican regulates Hh and Wnt signaling pathways involved in systemic inflammatory responses. Taken together, targeting the vascular endothelial glycocalyx-derived, soluble syndecan-1, syndecan-2, syndecan-3, syndecan-4, biglycan, versikine, mimecan, perlecan, glypican and decorin might be a potential therapeutic strategy to suppress overstimulated cytokine and leukocyte responses in inflammatory diseases.

A Novel Approach to Reducing Lung Metastasis in Osteosarcoma: Increasing Cell Stiffness with Carbenoxolone

AIM

Primary malignant bone tumor osteosarcoma can metastasize to the lung. Diminishing lung metastasis would positively affect the prognosis of patients. Our previous studies demonstrated that highly metastatic osteosarcoma cell lines are significantly softer than low-metastasis cell lines. We therefore hypothesized that increasing cell stiffness would suppress metastasis by reducing cell motility. In this study, we tested whether carbenoxolone (CBX) increases the stiffness of LM8 osteosarcoma cells and prevents lung metastasis in vivo.

METHODS

We evaluated the actin cytoskeletal structure and polymerization of CBX-treated LM8 cells using actin staining. Cell stiffness was measured using atomic force microscopy. Metastasis-related cell functions were analyzed using cell proliferation, wound healing, invasion, and cell adhesion assays. Furthermore, lung metastasis was examined in LM8-bearing mice administered with CBX.

RESULTS

Treatment with CBX significantly increased actin staining intensity and stiffness of LM8 cells compared with vehicle-treated LM8 cells (p < 0.01). In Young's modulus images, compared with the control group, rigid fibrillate structures were observed in the CBX treatment group. CBX suppressed cell migration, invasion, and adhesion but not cell proliferation. The number of LM8 lung metastases were significantly reduced in the CBX administration group compared with the control group (p < 0.01).

CONCLUSION

In this study, we demonstrated that CBX increases tumor cell stiffness and significantly reduces lung metastasis. Our study is the first to provide evidence that reducing cell motility by increasing cell stiffness might be effective as a novel anti-metastasis approach in vivo.

Mechanisms of sodium-mediated injury in cardiovascular disease: old play, new scripts

There is a strong association between salt intake and cardiovascular diseases, particularly hypertension, on the population level. The mechanisms that explain this association remain incompletely understood and appear to extend beyond blood pressure. In this review, we describe some of the 'novel' roles of Na⁺ in cardiovascular health and disease: energetic implications of sodium handling in the kidneys; local accumulation in tissue; fluid dynamics; and the role of the microvasculature, with particular focus on the lymphatic system. We describe the interplay between these factors that involves body composition, metabolic signatures, inflammation and composition of the extracellular and intracellular milieus.

Caveolin-1 is a primary determinant of endothelial stiffening associated with dyslipidemia, disturbed flow, and ageing

Endothelial stiffness is emerging as a major determinant in endothelial function. Here, we analyzed the role of caveolin-1 (Cav-1) in determining the stiffness of endothelial cells (EC) exposed to oxidized low density lipoprotein (oxLDL) under static and hemodynamic conditions in vitro and of aortic endothelium in vivo in mouse models of dyslipidemia and ageing. Elastic moduli of cultured ECs and of the endothelial monolayer of freshly isolated mouse aortas were measured using atomic force microscopy (AFM). We found that a loss of Cav-1 abrogates the uptake of oxLDL and oxLDL-induced endothelial stiffening, as well as endothelial stiffening induced by disturbed flow (DF), which was also oxLDL dependent. Mechanistically, Cav-1 is required for the expression of CD36 (cluster of differentiation 36) scavenger receptor. Genetic deletion of Cav-1 abrogated endothelial stiffening observed in the DF region of the aortic arch, and induced by a high fat diet (4-6 weeks) and significantly blunted endothelial stiffening that develops with advanced age. This effect was independent of stiffening of the sub-endothelium layer. Additionally, Cav-1 expression significantly increased with age. No differences in elastic modulus were observed between the sexes in advanced aged wild type and Cav-1 knockout mice. Taken together, this study demonstrates that Cav-1 plays a critical role in endothelial stiffening induced by oxLDL in vitro and by dyslipidemia, disturbed flow and ageing in vivo.

BRAF increases endothelial cell stiffness through reorganization of the actin cytoskeleton

The dynamics of the actin cytoskeleton and its connection to endothelial cell-cell junctions determine the barrier function of endothelial cells. The proper regulation of barrier opening/closing is necessary for the normal function of vessels, and its dysregulation can result in chronic and acute inflammation leading to edema formation. By using atomic force microscopy, we show here that thrombin-induced permeability of human umbilical vein endothelial cells, associated with actin stress fiber formation, stiffens the cell center. The depletion of the MEK/ERK kinase BRAF reduces thrombin-induced permeability prevents stress fiber formation and cell stiffening. The peripheral actin ring becomes stabilized by phosphorylated myosin light chain, while cofilin is excluded from the cell periphery. All these changes can be reverted by the inhibition of ROCK, but not of the MEK/ERK module. We propose that the balance between the binding of cofilin and myosin to F-actin in the cell periphery, which is regulated by the activity of ROCK, determines the local dynamics of actin reorganization, ultimately driving or preventing stress fiber formation.

Toxoplasma effector-induced ICAM-1 expression by infected dendritic cells potentiates transmigration across polarised endothelium

The obligate intracellular parasite Toxoplasma gondii makes use of infected leukocytes for systemic dissemination. Yet, how infection impacts the processes of leukocyte diapedesis has remained unresolved. Here, we addressed the effects of T. gondii infection on the trans-endothelial migration (TEM) of dendritic cells (DCs) across polarised brain endothelial monolayers. We report that upregulated expression of leukocyte ICAM-1 is a feature of the enhanced TEM of parasitised DCs. The secreted parasite effector GRA15 induced an elevated expression of ICAM-1 in infected DCs that was associated with enhanced cell adhesion and TEM. Consequently, gene silencing of Icam-1 in primary DCs or deletion of parasite GRA15 reduced TEM. Further, the parasite effector TgWIP, which impacts the regulation of host actin dynamics, facilitated TEM across polarised endothelium. The data highlight that the concerted action of the secreted effectors GRA15 and TgWIP modulate the leukocyte-endothelial interactions of TEM in a parasite genotype-related fashion to promote dissemination. In addition to the canonical roles of endothelial ICAM-1, this study identifies a previously unappreciated role for leukocyte ICAM-1 in infection-related TEM.

Rapid shear stress-dependent ENaC membrane insertion is mediated by the endothelial glycocalyx and the mineralocorticoid receptor

The contribution of the shear stress-sensitive epithelial Na⁺ channel (ENaC) to the mechanical properties of the endothelial cell surface under (patho)physiological conditions is unclear. This issue was addressed in in vivo and in vitro models for endothelial dysfunction. Cultured human umbilical vein endothelial cells (HUVEC) were exposed to laminar (LSS) or non-laminar shear stress (NLSS). ENaC membrane insertion was quantified using Quantum-dot-based immunofluorescence staining and the mechanical properties of the cell surface were probed with the Atomic Force Microscope (AFM) in vitro and ex vivo in isolated aortae of C57BL/6 and ApoE/LDLR^-/- mice. Flow- and acetylcholine-mediated vasodilation was measured in vivo using magnetic resonance imaging. Acute LSS led to a rapid mineralocorticoid receptor (MR)-dependent membrane insertion of ENaC and subsequent stiffening of the endothelial cortex caused by actin polymerization. Of note, NLSS stress further augmented the cortical stiffness of the cells. These effects strongly depend on the presence of the endothelial glycocalyx (eGC) and could be prevented by functional inhibition of ENaC and MR in vitro endothelial cells and ex vivo endothelial cells derived from C57BL/6, but not ApoE/LDLR^-/- vessel. In vivo In C57BL/6 vessels, ENaC- and MR inhibition blunted flow- and acetylcholine-mediated vasodilation, while in the dysfunctional ApoE/LDLR^-/- vessels, this effect was absent. In conclusion, under physiological conditions, endothelial ENaC, together with the glycocalyx, was identified as an important shear stress sensor and mediator of endothelium-dependent vasodilation. In contrast, in pathophysiological conditions, ENaC-mediated mechanotransduction and endothelium-dependent vasodilation were lost, contributing to sustained endothelial stiffening and dysfunction.

Increased cell stiffness contributes to complement-mediated injury of choroidal endothelial cells in a monkey model of early age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness in the aging population. Yet, no therapies exist for approximately 85% of all AMD patients who have the dry form that is marked by degeneration of the retinal pigmented epithelium (RPE) and underlying choroidal vasculature. As the choroidal vessels are crucial for RPE development and maintenance, understanding how they degenerate may lead to effective therapies for dry AMD. One likely causative factor for choroidal vascular loss is the cytolytic membrane attack complex (MAC) of the complement pathway that is abundant on choroidal vessels of humans with early dry AMD. To examine this possibility, we studied the effect of complement activation on choroidal endothelial cells (ECs) isolated from a rhesus monkey model of early AMD that, we report, exhibits MAC deposition and choriocapillaris endothelial loss similar to that seen in human early AMD. Treatment of choroidal ECs from AMD eyes with complement-competent normal human serum caused extensive actin cytoskeletal injury that was significantly less pronounced in choroidal ECs from young normal monkey eyes. We further show that ECs from AMD eyes are significantly stiffer than their younger counterparts and exhibit peripheral actin organization that is distinct from the longitudinal stress fibers in young ECs. Finally, these differences in complement susceptibility and mechanostructural properties were found to be regulated by the differential activity of the small GTPases Rac and Rho, because Rac inhibition in AMD cells led to simultaneous reduction in stiffness and complement susceptibility while Rho inhibition in young cells exacerbated complement injury. Thus, by identifying cell stiffness and cytoskeletal regulators Rac and Rho as important determinants of complement susceptibility, the current findings offer a new mechanistic insight into choroidal vascular loss in early AMD that warrants further investigation for assessment of translational potential. This article is protected by copyright. All rights reserved.

Typical and atypical properties of peripheral nerve allografts enable novel strategies to repair segmental-loss injuries

We review data showing that peripheral nerve injuries (PNIs) that involve the loss of a nerve segment are the most common type of traumatic injury to nervous systems. Segmental-loss PNIs have a poor prognosis compared to other injuries, especially when one or more mixed motor/sensory nerves are involved and are typically the major source of disability associated with extremities that have sustained other injuries. Relatively little progress has been made, since the treatment of segmental loss PNIs with cable autografts that are currently the gold standard for repair has slow and incomplete (often non-existent) functional recovery. Viable peripheral nerve allografts (PNAs) to repair segmental-loss PNIs have not been experimentally or clinically useful due to their immunological rejection, Wallerian degeneration (WD) of anucleate donor graft and distal host axons, and slow regeneration of host axons, leading to delayed re-innervation and producing atrophy or degeneration of distal target tissues. However, two significant advances have recently been made using viable PNAs to repair segmental-loss PNIs: (1) hydrogel release of Treg cells that reduce the immunological response and (2) PEG-fusion of donor PNAs that reduce the immune response, reduce and/or suppress much WD, immediately restore axonal conduction across the donor graft and re-innervate many target tissues, and restore much voluntary behavioral functions within weeks, sometimes to levels approaching that of uninjured nerves. We review the rather sparse cellular/biochemical data for rejection of conventional PNAs and their acceptance following Treg hydrogel and PEG-fusion of PNAs, as well as cellular and systemic data for their acceptance and remarkable behavioral recovery in the absence of tissue matching or immune suppression. We also review typical and atypical characteristics of PNAs compared with other types of tissue or organ allografts, problems and potential solutions for PNA use and storage, clinical implications and commercial availability of PNAs, and future possibilities for PNAs to repair segmental-loss PNIs.

Neuromechanobiology: An Expanding Field Driven by the Force of Greater Focus

The brain processes information by transmitting signals through highly connected and dynamic networks of neurons. Neurons use specific cellular structures, including axons, dendrites and synapses, and specific molecules, including cell adhesion molecules, ion channels and chemical receptors to form, maintain and communicate among cells in the networks. These cellular and molecular processes take place in environments rich of mechanical cues, thus offering ample opportunities for mechanical regulation of neural development and function. Recent studies have suggested the importance of mechanical cues and their potential regulatory roles in the development and maintenance of these neuronal structures. Also suggested are the importance of mechanical cues and their potential regulatory roles in the interaction and function of molecules mediating the interneuronal communications. In this review, the current understanding is integrated and promising future directions of neuromechanobiology are suggested at the cellular and molecular levels. Several neuronal processes where mechanics likely plays a role are examined and how forces affect ligand binding, conformational change, and signal induction of molecules key to these neuronal processes are indicated, especially at the synapse. The disease relevance of neuromechanobiology as well as therapies and engineering solutions to neurological disorders stemmed from this emergent field of study are also discussed.

Integrin cross-talk modulates stiffness-independent motility of CD4+ T lymphocytes

To carry out their physiological responsibilities, CD4+ T lymphocytes interact with various tissues of different mechanical properties. Recent studies suggest that T cells migrate upstream on surfaces expressing intracellular adhesion molecule-1 (ICAM-1) through interaction with leukocyte function-associated antigen-1 (α_Lβ₂) (LFA-1) integrins. LFA-1 likely behaves as a mechanosensor, and thus we hypothesized that substrate mechanics might affect the ability of LFA-1 to support upstream migration of T cells under flow. Here we measured motility of CD4+ T lymphocytes on polyacrylamide gels with predetermined stiffnesses containing ICAM-1, vascular cell adhesion molecule-1 (VCAM-1), or a 1:1 mixture of VCAM-1/ICAM-1. Under static conditions, we found that CD4+ T cells exhibit an increase in motility on ICAM-1, but not on VCAM-1 or VCAM-1/ICAM-1 mixed, surfaces as a function of matrix stiffness. The mechanosensitivity of T-cell motility on ICAM-1 is overcome when VLA-4 (very late antigen-4 [α₄β₁]) is ligated with soluble VCAM-1. Last, we observed that CD4+ T cells migrate upstream under flow on ICAM-1-functionalized hydrogels, independent of substrate stiffness. In summary, we show that CD4+ T cells under no flow respond to matrix stiffness through LFA-1, and that the cross-talk of VLA-4 and LFA-1 can compensate for deformable substrates. Interestingly, CD4+ T lymphocytes migrated upstream on ICAM-1 regardless of the substrate stiffness, suggesting that flow can compensate for substrate stiffness.

Impairing flow-mediated endothelial remodeling reduces extravasation of tumor cells

Tumor progression and metastatic dissemination are driven by cell-intrinsic and biomechanical cues that favor the growth of life-threatening secondary tumors. We recently identified pro-metastatic vascular regions with blood flow profiles that are permissive for the arrest of circulating tumor cells. We have further established that such flow profiles also control endothelial remodeling, which favors extravasation of arrested CTCs. Yet, how shear forces control endothelial remodeling is unknown. In the present work, we aimed at dissecting the cellular and molecular mechanisms driving blood flow-dependent endothelial remodeling. Transcriptomic analysis of endothelial cells revealed that blood flow enhanced VEGFR signaling, among others. Using a combination of in vitro microfluidics and intravital imaging in zebrafish embryos, we now demonstrate that the early flow-driven endothelial response can be prevented upon specific inhibition of VEGFR tyrosine kinase and subsequent signaling. Inhibitory targeting of VEGFRs reduced endothelial remodeling and subsequent metastatic extravasation. These results confirm the importance of VEGFR-dependent endothelial remodeling as a driving force of CTC extravasation and metastatic dissemination. Furthermore, the present work suggests that therapies targeting endothelial remodeling might be a relevant clinical strategy in order to impede metastatic progression.

Shear forces induce ICAM-1 nanoclustering on endothelial cells that impact on T-cell migration

The leukocyte-specific β₂-integrin LFA-1 and its ligand ICAM-1, expressed on endothelial cells (ECs), are involved in the arrest, adhesion, and transendothelial migration of leukocytes. Although the role of mechanical forces on LFA-1 activation is well established, the impact of forces on its major ligand ICAM-1 has received less attention. Using a parallel-plate flow chamber combined with confocal and super-resolution microscopy, we show that prolonged shear flow induces global translocation of ICAM-1 on ECs upstream of flow direction. Interestingly, shear forces caused actin rearrangements and promoted actin-dependent ICAM-1 nanoclustering before LFA-1 engagement. T cells adhered to mechanically prestimulated ECs or nanoclustered ICAM-1 substrates developed a promigratory phenotype, migrated faster, and exhibited shorter-lived interactions with ECs than when adhered to non mechanically stimulated ECs or to monomeric ICAM-1 substrates. Together, our results indicate that shear forces increase ICAM-1/LFA-1 bonds because of ICAM-1 nanoclustering, strengthening adhesion and allowing cells to exert higher traction forces required for faster migration. Our data also underscore the importance of mechanical forces regulating the nanoscale organization of membrane receptors and their contribution to cell adhesion regulation.

Engineering T cells to enhance 3D migration through structurally and mechanically complex tumor microenvironments

Defining the principles of T cell migration in structurally and mechanically complex tumor microenvironments is critical to understanding escape from antitumor immunity and optimizing T cell-related therapeutic strategies. Here, we engineered nanotextured elastic platforms to study and enhance T cell migration through complex microenvironments and define how the balance between contractility localization-dependent T cell phenotypes influences migration in response to tumor-mimetic structural and mechanical cues. Using these platforms, we characterize a mechanical optimum for migration that can be perturbed by manipulating an axis between microtubule stability and force generation. In 3D environments and live tumors, we demonstrate that microtubule instability, leading to increased Rho pathway-dependent cortical contractility, promotes migration whereas clinically used microtubule-stabilizing chemotherapies profoundly decrease effective migration. We show that rational manipulation of the microtubule-contractility axis, either pharmacologically or through genome engineering, results in engineered T cells that more effectively move through and interrogate 3D matrix and tumor volumes. Thus, engineering cells to better navigate through 3D microenvironments could be part of an effective strategy to enhance efficacy of immune therapeutics.

The mechanics of the migration of T cells into tumours is an important aspect of tumour immunity. Here the authors engineer complex 3D environments to explore functions of microtubules and cell contractility as strategies to enhance T cell migration in tumour microenvironments.

Elucidating the Biomechanics of Leukocyte Transendothelial Migration by Quantitative Imaging

Leukocyte transendothelial migration is crucial for innate immunity and inflammation. Upon tissue damage or infection, leukocytes exit blood vessels by adhering to and probing vascular endothelial cells (VECs), breaching endothelial cell-cell junctions, and transmigrating across the endothelium. Transendothelial migration is a critical rate-limiting step in this process. Thus, leukocytes must quickly identify the most efficient route through VEC monolayers to facilitate a prompt innate immune response. Biomechanics play a decisive role in transendothelial migration, which involves intimate physical contact and force transmission between the leukocytes and the VECs. While quantifying these forces is still challenging, recent advances in imaging, microfabrication, and computation now make it possible to study how cellular forces regulate VEC monolayer integrity, enable efficient pathfinding, and drive leukocyte transmigration. Here we review these recent advances, paying particular attention to leukocyte adhesion to the VEC monolayer, leukocyte probing of endothelial barrier gaps, and transmigration itself. To offer a practical perspective, we will discuss the current views on how biomechanics govern these processes and the force microscopy technologies that have enabled their quantitative analysis, thus contributing to an improved understanding of leukocyte migration in inflammatory diseases.

Micropatterned soft hydrogels to study the interplay of receptors and forces in T cell activation

The analysis of T cell responses to mechanical properties of antigen presenting cells (APC) is experimentally challenging at T cell-APC interfaces. Soft hydrogels with adjustable mechanical properties and biofunctionalization are useful reductionist models to address this problem. Here, we report a methodology to fabricate micropatterned soft hydrogels with defined stiffness to form spatially confined T cell/hydrogel contact interfaces at micrometer scale. Using automatized microcontact printing we prepared arrays of anti-CD3 microdots on poly(acrylamide) hydrogels with Young's Modulus in the range of 2 to 50 kPa. We optimized the printing process to obtain anti-CD3 microdots with constant area (50 µm², corresponding to 8 µm diameter) and comparable anti-CD3 density on hydrogels of different stiffness. The anti-CD3 arrays were recognized by T cells and restricted cell attachment to the printed areas. To test functionality of the hydrogel-T cell contact, we analyzed several key events downstream of T cell receptor (TCR) activation. Anti-CD3 arrays on hydrogels activated calcium influx, induced rearrangement of the actin cytoskeleton, and led to Zeta-chain-associated protein kinase 70 (ZAP70) phosphorylation. Interestingly, upon increase in the stiffness, ZAP70 phosphorylation was enhanced, whereas the rearrangements of F-actin (F-actin clearance) and phosphorylated ZAP70 (ZAP70/pY centralization) were unaffected. Our results show that micropatterned hydrogels allow tuning of stiffness and receptor presentation to analyze TCR mediated T cell activation as function of mechanical, biochemical, and geometrical parameters.

Polyethylene glycol-fusion repair of sciatic allografts in female rats achieves immunotolerance via attenuated innate and adaptive responses

Ablation/segmental loss peripheral nerve injuries (PNIs) exhibit poor functional recovery due to slow and inaccurate outgrowth of regenerating axons. Viable peripheral nerve allografts (PNAs) as growth-guide conduits are immunologically rejected and all anucleated donor/host axonal segments undergo Wallerian degeneration. In contrast, we report that ablation-type sciatic PNIs repaired by neurorrhaphy of viable sciatic PNAs and a polyethylene glycol (PEG)-fusion protocol using PEG immediately restored axonal continuity for many axons, reinnervated/maintained their neuromuscular junctions, and prevented much Wallerian degeneration. PEG-fused PNAs permanently restored many sciatic-mediated behaviors within 2-6 weeks. PEG-fused PNAs were not rejected even though host/donors were neither immunosuppressed nor tissue-matched in outbred female Sprague Dawley rats. Innate and adaptive immune responses to PEG-fused sciatic PNAs were analyzed using electron microscopy, immunohistochemistry, and quantitative reverse transcription polymerase chain reaction for morphological features, T cell and macrophage infiltration, major histocompatibility complex (MHC) expression, apoptosis, expression of cytokines, chemokines, and cytotoxic effectors. PEG-fused PNAs exhibited attenuated innate and adaptive immune responses by 14-21 days postoperatively, as evidenced by (a) many axons and cells remaining viable, (b) significantly reduced infiltration of cytotoxic and total T cells and macrophages, (c) significantly reduced expression of inflammatory cytokines, chemokines, and MHC proteins, (d) consistently low apoptotic response. Morphologically and/or biochemically, PEG-fused sciatic PNAs often resembled sciatic autografts or intact sciatic nerves. In brief, PEG-fused PNAs are an unstudied, perhaps unique, example of immune tolerance of viable allograft tissue in a nonimmune-privileged environment and could greatly improve the clinical outcomes for PNIs relative to current protocols.

Mechanisms of endothelial stiffening in dyslipidemia and aging: Oxidized lipids and shear stress

Vascular stiffening of the arterial walls is well-known as a key factor in aging and the development of cardiovascular disease; however, the role of endothelial stiffness in vascular dysfunction is still an emerging topic. In this review, the authors discuss the impact of dyslipidemia, oxidized lipids, substrate stiffness, age and pro-atherogenic disturbed flow have on endothelial stiffness. Furthermore, we investigate several mechanistic pathways that are key contributors in endothelial stiffness and discuss their physiological effects in the onset of atherogenesis in the disturbed flow regions of the aortic vasculature. The findings in this chapter describe a novel paradigm of synergistic interaction of plasma dyslipidemia/oxidized lipids and pro-atherogenic disturbed shear stress, as well as aging has on endothelial stiffness and vascular dysfunction.

Modulating T Cell Activation Using Depth Sensing Topographic Cues

This report examines how sensing of substrate topography can be used to modulate T cell activation, a key coordinating step in the adaptive immune response. Inspired by the native T cell-antigen presenting cell interface, micrometer scale pits with varying depth are fabricated into planar substrates. Primary CD4⁺ T cells extend actin-rich protrusions into the micropits. T cell activation, reflected in secretion of cytokines interleukin-2 and interferon gamma, is sensitive to the micropit depth. Surprisingly, arrays of micropits with 4 μm depth enhance activation compared to flat substrates but deeper micropits are less effective at increasing cell response, revealing a biphasic dependence in activation as a function of feature dimensions. Inhibition of cell contractility abrogates the enhanced activation associated with the micropits. In conclusion, this report demonstrates that the 3D, microscale topography can be used to enhance T cell activation, an ability that most directly can be used to improve production of these cells for immunotherapy.

Recombinant Human Soluble Thrombomodulin Suppresses Monocyte Adhesion by Reducing Lipopolysaccharide-Induced Endothelial Cellular Stiffening

Endothelial cellular stiffening has been observed not only in inflamed cultured endothelial cells but also in the endothelium of atherosclerotic regions, which is an underlying cause of monocyte adhesion and accumulation. Although recombinant soluble thrombomodulin (rsTM) has been reported to suppress the inflammatory response of endothelial cells, its role in regulating endothelial cellular stiffness remains unclear. The purpose of this study was to investigate the impact of anticoagulant rsTM on lipopolysaccharide (LPS)-induced endothelial cellular stiffening. We show that LPS increases endothelial cellular stiffness by using atomic force microscopy and that rsTM reduces LPS-induced cellular stiffening not only through the attenuation of actin fiber and focal adhesion formation but also via the improvement of gap junction functionality. Moreover, post-administration of rsTM, after LPS stimulation, attenuated LPS-induced cellular stiffening. We also found that endothelial cells regulate leukocyte adhesion in a substrate- and cellular stiffness-dependent manner. Our result show that LPS-induced cellular stiffening enhances monocytic THP-1 cell line adhesion, whereas rsTM suppresses THP-1 cell adhesion to inflamed endothelial cells by reducing cellular stiffness. Endothelial cells increase cellular stiffness in reaction to inflammation, thereby promoting monocyte adhesion. Treatment of rsTM reduced LPS-induced cellular stiffening and suppressed monocyte adhesion in a cellular stiffness-dependent manner.

Alterations of aorta intima and media transcriptome in swine fed high-fat diet over 1-year follow-up period and of the switch to normal diet

BACKGROUND AND AIM

We previously showed that 12-month high-fat diet (HFD) in pigs led to fattening and increased artery intima-media-thickness, which were partly reversed after 3-month return to control diet (CD). The aim of this study was to decipher underlying mechanism of action by using transcriptomic analyses of intima and media of aorta.

METHODS AND RESULTS

Thirty-two pigs were divided into three groups: CD for 12 months; HFD for 12 months; switch diet group (regression diet; RD): HFD for 9 months followed by CD for 3 months. After 12 months, RNA was isolated from aorta intima and media for nutrigenomic analyses. HFD significantly affected gene expression in intima, while RD gene expression profile was distinct from the CD group. This suggests that switch to CD is not sufficient to correct gene expression alterations induced by HFD but counteracted expression of a group of genes. HFD also affected gene expression in media and as for intima, the expression profile of media of pigs on RD differed from that of these on CD.

CONCLUSIONS

This study revealed nutrigenomic modifications induced by long-term HFD consumption on arterial intima and media. The return to CD was not sufficient to counteract the genomic effect of HFD.

Bioinformatics analysis of key biomarkers and potential molecular mechanisms in hepatocellular carcinoma induced by hepatitis B virus

BACKGROUND

Hepatocellular carcinoma (HCC) accounts for up to 90% of all primary hepatic malignancies; it is the sixth most common cancer and the second most common cause of cancer mortality worldwide. Numerous studies have shown that hepatitis B virus and its products, HBV integration, and mutation can induce HCC. However, the molecular mechanisms underpinning the regulation of HCC induced by HBV remain unclear.

METHODS

We downloaded 2 gene expression profiling datasets, of HBV and of HCC induced by HBV, from the gene expression omnibus (GEO) database. Differentially expressed genes (DEGs) between HCC and HBV were identified to explore any predisposing changes in gene expression associated with HCC. DEGs between HCC and adjacent healthy tissues were investigated to identify genes that may play a key role in HCC. Any overlapping genes among these DEGs were included in our bioinformatics analysis. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses of overlapping genes were performed using the Metascape online database; the protein-protein interaction (PPI) network was analyzed using the STRING online database; and we obtained the hub genes of the PPI network using Cytoscape software. An overall survival (OS) analysis of hub genes was performed using km-plotter and the gene expression profiling interactive analysis (GEPIA) online database. The expression levels of hub genes were determined using the TCGA and GEPIA databases. Finally, the relationships between hub genes and tumors were analyzed using the comparative toxicogenomics database (CTD).

RESULTS

We identified 113 overlapping genes from the 2 datasets. Using functional and pathway analyses, we found that the overlapping genes were mainly related to the AMPK signaling pathway and cellular responses to cadmium ions. C8A, SPP2, KLKB1, PROZ, C6, FETUB, MBL2, HGFAC, C8B, and ANGPTL3 were identified as hub genes and C8A, SPP2, PROZ, C6, HGFAC, and C8B were found to be significant for survival.

CONCLUSION

The DEGs re-analyzed between HCC and hepatitis B enable a systematic understanding of the molecular mechanisms of HCC reliant on hepatitis B virus.

Syndecans in Inflammation at a Glance

Syndecans are transmembrane proteoglycans with heparan and chondroitin sulfate chains attached to their extracellular domain. Like many proteoglycans, they interact with a large number of ligands, such as growth factors, adhesion receptors, soluble small molecules, proteinases, and other extracellular matrix proteins to initiate downstream signaling pathways. Syndecans play a major role in inflammation, mainly by regulating leukocyte extravasation and cytokine function. At the same time, syndecans can undergo cytokine mediated changes in their expression levels during inflammation. The function of syndecans during inflammation appears to depend on the stage of inflammation, sulfation of heparan/chondroitin sulfate chains, the rate of ectodomain shedding and the solubility of the ectodomains. From the current literature, it is clear that syndecans are not only involved in the initial recruitment of pro-inflammatory molecules but also in establishing a balanced progression of inflammation. This review will summarize how cell surface and soluble syndecans regulate multiple aspects of inflammation.

Fluids and their mechanics in tumour transit: shaping metastasis

Metastasis is a dynamic succession of events involving the dissemination of tumour cells to distant sites within the body, ultimately reducing the survival of patients with cancer. To colonize distant organs and, therefore, systemically disseminate within the organism, cancer cells and associated factors exploit several bodily fluid systems, which provide a natural transportation route. Indeed, the flow mechanics of the blood and lymphatic circulatory systems can be co-opted to improve the efficiency of cancer cell transit from the primary tumour, extravasation and metastatic seeding. Flow rates, vessel size and shear stress can all influence the survival of cancer cells in the circulation and control organotropic seeding patterns. Thus, in addition to using these fluids as a means to travel throughout the body, cancer cells exploit the underlying physical forces within these fluids to successfully seed distant metastases. In this Review, we describe how circulating tumour cells and tumour-associated factors leverage bodily fluids, their underlying forces and imposed stresses during metastasis. As the contribution of bodily fluids and their mechanics raises interesting questions about the biology of the metastatic cascade, an improved understanding of this process might provide a new avenue for targeting cancer cells in transit.

Global mapping of live cell mechanical features using PeakForce QNM AFM

Atomic force microscopy (AFM) has been widely used to acquire surface topography upon different scanning modes and to quantify mechanical properties of a cell using single-point ramp force mode. However, these traditional measurements need massive force curves originating from multiple points of a cell to exclude the potential errors resulted from limited and factitious selections of testing points, making the measurements time-consuming and highly localized. PeakForce Quantitative NanoMechanics (PF QNM) is a high-speed (faster by 3–4 order of magnitude) and global surface mechanical property mapping method with high spatial resolution, overcoming the drawbacks of traditional ramp mode especially used for a live cell with high heterogeneity. In this protocol, we elaborated how to run PF QNM measurements for live cells and relevant modification may be needed when extending this method to other cell-like soft materials.

Cells with Higher Cortical Membrane Tension Are More Sensitive to Lysis by Biosurfactant Di-rhamnolipids

Tissue and cellular stiffening is associated with pathologies including fibrosis and cancer. Healthy cells also exhibit a wide range of membrane cortical tensions, which have been studied in the field of mechanobiology. Here, we quantify the mechanosensitivity of the lysis agent the di-rhamnolipid (RHA), which is a bacterially produced biosurfactant. RHA exhibited selective lysis correlated strongly with cortical membrane tension in osteoblasts, smooth muscle cells, fibroblasts, epithelial cells, and erythrocytes. Reducing cortical membrane tension by cytoskeleton inhibitors (cytochalasin D and nocodazole) or osmotic regulators (sucrose, polyethylene glycol, and nonionic surfactants) attenuated the RHA toxicity. The selective toxicity of RHA toward human chronic myeloid leukemia K562 cells over healthy blood cells suggests a potential therapy for blood cancer. Targeted killing of myofibroblasts transformed from either fibroblasts or epithelial cells indicates its antifibrotic effect. Combined, these studies showed the potential for specific targeting of cells with differential mechanical properties rather than chemical or biological pathways.

The interplay of signaling pathway in endothelial cells-matrix stiffness dependency with targeted-therapeutic drugs

Cardiovascular diseases (CVDs) have been one of the major causes of human deaths in the world. The study of CVDs has focused on cell chemotaxis for decades. With the advances in mechanobiology, accumulating evidence has demonstrated the influence of mechanical stimuli on arterial pathophysiology and endothelial dysfunction that is a hallmark of atherosclerosis development. An increasing number of drugs have been exploited to decrease the stiffness of vascular tissue for CVDs therapy. However, the underlying mechanisms have yet to be explored. This review aims to summarize how matrix stiffness mediates atherogenesis through various important signaling pathways in endothelial cells and cellular mechanophenotype, including RhoA/Rho-associated protein kinase (ROCK), mitogen-activated protein kinase (MAPK), and Hippo pathways. We also highlight the roles of putative mechanosensitive non-coding RNAs in matrix stiffness-mediated atherogenesis. Finally, we describe the usage of tunable hydrogel and its future strategy to improve our knowledge underlying matrix stiffness-mediated CVDs mechanism.

Hyaluronic acid and fibrin from L-PRP form semi-IPNs with tunable properties suitable for use in regenerative medicine

Autologous leukocyte- and platelet-rich plasma (L-PRP) combined with hyaluronic acid (HA) has been widely used in local applications for cartilage and bone regeneration. The association between L-PRP and HA confers structural and rheological changes that differ among individual biomaterials but has not been investigated. Therefore, the standardization and characterization of L-PRP-HA are important to consider when comparing performance results to improve future clinical applications. To this end, we prepared semi-interpenetrating polymer networks (semi-IPNs) of L-PRP and HA and characterized their polymerization kinetics, morphology, swelling ratio, stability and rheological behavior, which we found to be tunable according to the HA molar mass (MM). Mesenchymal stem cells derived from human adipose tissue (h-AdMSCs) seeded in the semi-IPNs had superior viability and chondrogenesis and osteogenesis capabilities compared to the viability and capabilities of fibrin. We have demonstrated that the preparation of the semi-IPNs under controlled mixing ensured the formation of cell-friendly hydrogels rich in soluble factors and with tunable properties according to the HA MM, rendering them suitable for clinical applications in regenerative medicine.

The role of endothelial MERTK during the inflammatory response in lungs

As a key homeostasis regulator in mammals, the MERTK receptor tyrosine kinase is crucial for efferocytosis, a process that requires remodeling of the cell membrane and adjacent actin cytoskeleton. Membrane and cytoskeletal reorganization also occur in endothelial cells during inflammation, particularly during neutrophil transendothelial migration (TEM) and during changes in permeability. However, MERTK’s function in endothelial cells remains unclear. This study evaluated the contribution of endothelial MERTK to neutrophil TEM and endothelial barrier function. In vitro experiments using primary human pulmonary microvascular endothelial cells found that neutrophil TEM across the endothelial monolayers was enhanced when MERTK expression in endothelial cells was reduced by siRNA knockdown. Examination of endothelial barrier function revealed increased passage of dextran across the MERTK-depleted monolayers, suggesting that MERTK helps maintain endothelial barrier function. MERTK knockdown also altered adherens junction structure, decreased junction protein levels, and reduced basal Rac1 activity in endothelial cells, providing potential mechanisms of how MERTK regulates endothelial barrier function. To study MERTK’s function in vivo, inflammation in the lungs of global Mertk^-/- mice was examined during acute pneumonia. In response to P. aeruginosa, more neutrophils were recruited to the lungs of Mertk^-/- than wildtype mice. Vascular leakage of Evans blue dye into the lung tissue was also greater in Mertk^-/- mice. To analyze endothelial MERTK’s involvement in these processes, we generated inducible endothelial cell-specific (iEC) Mertk^-/- mice. When similarly challenged with P. aeruginosa, iEC Mertk^-/- mice demonstrated no difference in neutrophil TEM into the inflamed lungs or in vascular permeability compared to control mice. These results suggest that deletion of MERTK in human pulmonary microvascular endothelial cells in vitro and in all cells in vivo aggravates the inflammatory response. However, selective MERTK deletion in endothelial cells in vivo failed to replicate this response.

The cell adhesion molecule IGPR-1 is activated by and regulates responses of endothelial cells to shear stress

Vascular endothelial cells respond to blood flow-induced shear stress. However, the mechanisms through which endothelial cells transduce mechanical signals to cellular responses remain poorly understood. In this report, using tensile-force assays, immunofluorescence and atomic force microscopy, we demonstrate that immunoglobulin and proline-rich receptor-1 (IGPR-1) responds to mechanical stimulation and increases the stiffness of endothelial cells. We observed that IGPR-1 is activated by shear stress and tensile force and that flow shear stress-mediated IGPR-1 activation modulates remodeling of endothelial cells. We found that under static conditions, IGPR-1 is present at the cell-cell contacts; however, under shear stress, it redistributes along the cell borders into the flow direction. IGPR-1 activation stimulated actin stress fiber assembly and cross-linking with vinculin. Moreover, we noted that IGPR-1 stabilizes cell-cell junctions of endothelial cells as determined by staining of cells with ZO1. Mechanistically, shear stress stimulated activation of AKT Ser/Thr kinase 1 (AKT1), leading to phosphorylation of IGPR-1 at Ser-220. Inhibition of this phosphorylation prevented shear stress-induced actin fiber assembly and endothelial cell remodeling. Our findings indicate that IGPR-1 is an important player in endothelial cell mechanosensing, insights that have important implications for the pathogenesis of common maladies, including ischemic heart diseases and inflammation.

Endothelial Cell Mechano-Metabolomic Coupling to Disease States in the Lung Microvasculature

Lungs are the most vascular part of humans, accepting the totality of cardiac output in a volume much smaller than the body itself. Due to this cardiac output, the lung microvasculature is subject to mechanical forces including shear stress and cyclic stretch that vary with the cardiac and breathing cycle. Vessels are surrounded by extracellular matrix which dictates the stiffness which endothelial cells also sense and respond to. Shear stress, stiffness, and cyclic stretch are known to influence endothelial cell state. At high shear stress, endothelial cells exhibit cell quiescence marked by low inflammatory markers and high nitric oxide synthesis, whereas at low shear stress, endothelial cells are thought to "activate" into a pro-inflammatory state and have low nitric oxide. Shear stress' profound effect on vascular phenotype is most apparent in the arterial vasculature and in the pathophysiology of vascular inflammation. To conduct the flow of blood from the right heart, the lung microvasculature must be rigid yet compliant. It turns out that excessive substrate rigidity or stiffness is important in the development of pulmonary hypertension and chronic fibrosing lung diseases via excessive cell proliferation or the endothelial-mesenchymal transition. Recently, a new body of literature has evolved that couples mechanical sensing to endothelial phenotypic changes through metabolic signaling in clinically relevant contexts such as pulmonary hypertension, lung injury syndromes, as well as fibrosis, which is the focus of this review. Stretch, like flow, has profound effect on endothelial phenotype; metabolism studies due to stretch are in their infancy.

ZIPK mediates endothelial cell contraction through myosin light chain phosphorylation and is required for ischemic-reperfusion injury

The paracellular gap formed by endothelial cell (EC) contraction is fundamental for endothelium permeability, but the mechanism underlying EC contraction has yet to be determined. Here, we identified the zipper-interacting protein kinase (ZIPK) as the kinase for EC contraction and myosin light chain (MLC) phosphorylation. Inhibition of ZIPK activity by pharmacological inhibitors and small interfering RNAs led to a significant decrease in the mono- and diphosphorylation of MLCs along with a contractile response to thrombin, suggesting an essential role of ZIPK in EC paracellular permeability. To assess the role of ZIPK in vivo, we established mouse lines with conditional deletion of Zipk gene. The endothelium-specific deletion of Zipk led to embryonic lethality, whereas the UBC-Cre^ERT2-mediated deletion of Zipk by tamoxifen induction at adulthood caused no apparent phenotype. The induced deletion of Zipk significantly inhibited ischemia-reperfusion-induced blood-brain barrier dysfunction and neuronal injuries from middle cerebral artery occlusion and reperfusion, as evidenced by reduced infarct and edema volume, attenuated Evans blue dye leakage, and improved neuronal behavior. We thus concluded that ZIPK and its phosphorylation of MLC were required for EC contraction and ischemic neuronal injuries. ZIPK may be a prospective therapeutic target for stroke.-Zhang, Y., Zhang, C., Zhang, H., Zeng, W., Li, S., Chen, C., Song, X., Sun, J., Sun, Z., Cui, C., Cao, X., Zheng, L., Wang, P., Zhao, W., Zhang, Z., Xu, Y., Zhu, M., Chen, H. ZIPK mediates endothelial cell contraction through myosin light chain phosphorylation and is required for ischemic-reperfusion injury.

Identification of Candidate Genes for Skeletal Muscle Injury Prevention in Two Different Types

The study aims to uncover mechanisms on repair process of two different types of skeletal muscle injuries, freezing injury (FI) and contraction-induced injury (CI). GSE5413 was utilized, including 11 eccentric CI, 11 FI, and 3 control samples at 4 time points (6 hours, 1 day, 3 days, and 7 days after injury). Differentially expressed genes (DEGs) separately were selected in FI and CI. Correlation analysis of samples at different time points was performed. Clustering analysis was conducted for DEGs in FI and CI, respectively. Moreover, enrichment analysis and protein/protein interaction network analysis were performed for the specific DEGs. There were 616 and 465 DEGs separately in FI and CI samples. For both FI and CI, samples between 6 hours and 1 day, and between 3 and 7 days, had a close distance. DEGs in FI and CI separately were enriched in leukocyte transendothelial migration (e.g., ICAM1 [intercellular adhesion molecule 1], ITGAM, MMP9) and protein processing in endoplasmic reticulum pathway (e.g., HSPH1, HSP90AA1). In addition, MMP9 (matrix metallopeptidase 9) and ITGAM, and MYC, HSPA1B, and HSPA1A were hub nodes in the networks in FI and CI, respectively. ICAM1, ITGAM, and MMP9 in FI, and MYC and HSP70 family members in CI were biomarkers for injury prevention.

Actin Cytoskeleton Straddling the Immunological Synapse between Cytotoxic Lymphocytes and Cancer Cells

The immune system is a fundamental part of the tumor microenvironment. In particular, cytotoxic lymphocytes, such as cytolytic T cells and natural killer cells, control tumor growth and disease progression by interacting and eliminating tumor cells. The actin cytoskeleton of cytotoxic lymphocytes engaged in an immunological synapse has received considerable research attention. It has been recognized as a central mediator of the formation and maturation of the immunological synapse, and its signaling and cytolytic activities. In comparison, fewer studies have explored the organization and function of actin filaments on the target cancer cell side of the immunological synapse. However, there is growing evidence that the actin cytoskeleton of cancer cells also undergoes extensive remodeling upon cytotoxic lymphocyte attack, and that such remodeling can alter physical and functional interactions at the immunological synapse. In this article, we review the current knowledge of actin organization and functions at both sides of the immunological synapse between cytotoxic lymphocytes and cancer cells, with particular focus on synapse formation, signaling and cytolytic activity, and immune evasion.

Balance of mechanical forces drives endothelial gap formation and may facilitate cancer and immune-cell extravasation

The formation of gaps in the endothelium is a crucial process underlying both cancer and immune cell extravasation, contributing to the functioning of the immune system during infection, the unfavorable development of chronic inflammation and tumor metastasis. Here, we present a stochastic-mechanical multiscale model of an endothelial cell monolayer and show that the dynamic nature of the endothelium leads to spontaneous gap formation, even without intervention from the transmigrating cells. These gaps preferentially appear at the vertices between three endothelial cells, as opposed to the border between two cells. We quantify the frequency and lifetime of these gaps, and validate our predictions experimentally. Interestingly, we find experimentally that cancer cells also preferentially extravasate at vertices, even when they first arrest on borders. This suggests that extravasating cells, rather than initially signaling to the endothelium, might exploit the autonomously forming gaps in the endothelium to initiate transmigration.

Endothelial Nanomechanics in the Context of Endothelial (Dys)function and Inflammation

SIGNIFICANCE

Stiffness of endothelial cells is closely linked to the function of the vasculature as it regulates the release of vasoactive substances such as nitric oxide (NO) and reactive oxygen species. The outer layer of endothelial cells, consisting of the glycocalyx above and the cortical zone beneath the plasma membrane, is a vulnerable compartment able to adapt its nanomechanical properties to any changes of forces exerted by the adjacent blood stream. Sustained stiffening of this layer contributes to the development of endothelial dysfunction and vascular pathologies. Recent Advances: The development of specific techniques to quantify the mechanical properties of cells enables the detailed investigation of the mechanistic link between structure and function of cells.

CRITICAL ISSUES

Challenging the mechanical stiffness of cells, for instance, by inflammatory mediators can lead to the development of endothelial dysfunction. Prevention of sustained stiffening of the outer layer of endothelial cells in turn improves endothelial function.

FUTURE DIRECTIONS

The mechanical properties of cells can be used as critical marker and test system for the proper function of the vascular system. Pharmacological substances, which are able to improve endothelial nanomechanics and function, could take a new importance in the prevention and treatment of vascular diseases. Thus, detailed knowledge acquisition about the structure/function relationship of endothelial cells and the underlying signaling pathways should be promoted.

The Functional Implications of Endothelial Gap Junctions and Cellular Mechanics in Vascular Angiogenesis

Angiogenesis—the sprouting and growth of new blood vessels from the existing vasculature—is an important contributor to tumor development, since it facilitates the supply of oxygen and nutrients to cancer cells. Endothelial cells are critically affected during the angiogenic process as their proliferation, motility, and morphology are modulated by pro-angiogenic and environmental factors associated with tumor tissues and cancer cells. Recent in vivo and in vitro studies have revealed that the gap junctions of endothelial cells also participate in the promotion of angiogenesis. Pro-angiogenic factors modulate gap junction function and connexin expression in endothelial cells, whereas endothelial connexins are involved in angiogenic tube formation and in the cell migration of endothelial cells. Several mechanisms, including gap junction function-dependent or -independent pathways, have been proposed. In particular, connexins might have the potential to regulate cell mechanics such as cell morphology, cell migration, and cellular stiffness that are dynamically changed during the angiogenic processes. Here, we review the implication for endothelial gap junctions and cellular mechanics in vascular angiogenesis.

Deconstructing Immune Microenvironments of Lymphoid Tissues for Reverse Engineering

The immune microenvironment presents a diverse panel of cues that impacts immune cell migration, organization, differentiation, and the immune response. Uniquely, both the liquid and solid phases of every specific immune niche within the body play an important role in defining cellular functions in immunity at that particular location. The in vivo immune microenvironment consists of biomechanical and biochemical signals including their gradients, surface topography, dimensionality, modes of ligand presentation, and cell-cell interactions, and the ability to recreate these immune biointerfaces in vitro can provide valuable insights into the immune system. This manuscript reviews the critical roles played by different immune cells and surveys the current progress of model systems for reverse engineering of immune microenvironments with a focus on lymphoid tissues.

Reconstitution of immune cell interactions in free-standing membranes

The spatiotemporal regulation of signalling proteins at the contacts formed between immune cells and their targets determines how and when immune responses begin and end. Therapeutic control of immune responses therefore relies on thorough elucidation of the molecular processes occurring at these interfaces. However, the detailed investigation of each component's contribution to the formation and regulation of the contact is hampered by the complexities of cell composition and architecture. Moreover, the transient nature of these interactions creates additional challenges, especially in the use of advanced imaging technology. One approach that circumvents these problems is to establish in vitro systems that faithfully mimic immune cell interactions, but allow complexity to be 'dialled-in' as needed. Here, we present an in vitro system that makes use of synthetic vesicles that mimic important aspects of immune cell surfaces. Using this system, we began to explore the spatial distribution of signalling molecules (receptors, kinases and phosphatases) and how this changes during the initiation of signalling. The GUV/cell system presented here is expected to be widely applicable.

Vascular Endothelial (VE)-Cadherin, Endothelial Adherens Junctions, and Vascular Disease

Endothelial cell-cell adherens junctions (AJs) supervise fundamental vascular functions, such as the control of permeability and transmigration of circulating leukocytes, and the maintenance of existing vessels and formation of new ones. These processes are often dysregulated in pathologies. However, the evidence that links dysfunction of endothelial AJs to human pathologies is mostly correlative. In this review, we present an update of the molecular organization of AJ complexes in endothelial cells (ECs) that is mainly based on observations from experimental models. Furthermore, we report in detail on a human pathology, cerebral cavernous malformation (CCM), which is initiated by loss-of-function mutations in the genes that encode the three cytoplasmic components of AJs (CCM1, CCM2, and CCM3). At present, these represent a unique example of mutations in components of endothelial AJs that cause human disease. We describe also how studies into the defects of AJs in CCM are shedding light on the crucial regulatory mechanisms and signaling activities of these endothelial structures. Although these observations are specific for CCM, they support the concept that dysfunction of endothelial AJs can directly contribute to human pathologies.