Mechanical signaling in a pulmonary microvascular endothelial cell monolayer

Disturbed fluid flow reinforces endothelial tractions and intercellular stresses

Disturbed fluid flow is well understood to have significant ramifications on endothelial function, but the impact disturbed flow has on endothelial biomechanics is not well understood. In this study, we measured tractions, intercellular stresses, and cell velocity of endothelial cells exposed to disturbed flow using a custom-fabricated flow chamber. Our flow chamber exposed cells to disturbed fluid flow within the following spatial zones: zone 1 (inlet; length 0.676-2.027 cm): 0.0037 ± 0.0001 Pa; zone 2 (middle; length 2.027-3.716 cm): 0.0059 ± 0.0005 Pa; and zone 3 (outlet; length 3.716-5.405 cm): 0.0051 ± 0.0025 Pa. Tractions and intercellular stresses were observed to be highest in the middle of the chamber (zone 2) and lowest at the chamber outlet (zone 3), while cell velocity was highest near the chamber inlet (zone 1), and lowest near the middle of the chamber (zone 2). Our findings suggest endothelial biomechanical response to disturbed fluid flow to be dependent on not only shear stress magnitude, but the spatial shear stress gradient as well. We believe our results will be useful to a host of fields including endothelial cell biology, the cardiovascular field, and cellular biomechanics in general.

Human Placental Mesenchymal Stem Cells-Exosomes Alleviate Endothelial Barrier Dysfunction via Cytoskeletal Remodeling through hsa-miR-148a-3p/ROCK1 Pathway

Background

Endothelial barrier disruption of human pulmonary vascular endothelial cells (HPVECs) is an important pathogenic factor for acute lung injury (ALI)/acute respiratory distress syndrome (ARDS). Mesenchymal stem cells-exosome (MSCs-Exo) represents an ideal carrier for cell-free therapy. The therapeutic implication and underlying mechanism of human placental MSCs-Exo (HPMSCs-Exo) in ALI/ARDS need to be further explored.

Materials and Methods

HPMSCs-Exo was extracted from HPMSCs and characterized. Then, the therapeutic effects of exosomes were evaluated in ALI mice and HPVECs. RNA-sequencing was applied to reveal the miRNA profile of HPMSCs-Exo and differentially expressed genes (DEGs) in HPMSCs-Exo-pretreated HPVECs. The targets of miRNAs were predicted by bioinformatics methods and correlated to DEGs. Finally, the role of hsa-miR-148a-3p/ROCK1 pathway in HPVECs has been further discussed.

Results

The results showed that HPMSCs-Exo could downregulate Rho-associated coiled-coil-containing protein kinase 1 (ROCK1), upregulate the expression of zonula occludens-1 (ZO-1) and F-actin, promote HPVECs migration and tube formation, reduce cytoskeletal disorders and cell permeability, and thus improve ALI/ARDS. RNA-sequencing revealed the DEGs were mainly enriched in cell junction, angiogenesis, inflammation, and energy metabolism. HPMSCs-Exo contains multiple miRNAs which are associated with cytoskeletal function; the expression abundance of hsa-miR-148a-3p is the highest. Bioinformatic analysis identified ROCK1 as a target of hsa-miR-148a-3p. The overexpression of hsa-miR-148a-3p in HPMSCs-Exo promoted the migration and tube formation of HPVECs and reduced ROCK1 expression. However, the overexpression of ROCK1 on HPVECs reduced the therapeutic effect of HPMSCs-Exo.

Conclusions

HPMSCs-Exo represents a protective regimen against endothelial barrier disruption of HPVECs in ALI/ARDS, and the hsa-miR-148a-3p/ROCK1 pathway plays an important role in this therapeutics implication.

A machine learning approach to predict cellular mechanical stresses in response to chemical perturbation

Mechanical stresses generated at the cell-cell level and cell-substrate level have been suggested to be important in a host of physiological and pathological processes. However, the influence various chemical compounds have on the mechanical stresses mentioned above is poorly understood, hindering the discovery of novel therapeutics, and representing a barrier in the field. To overcome this barrier, we implemented two approaches: 1) monolayer boundary predictor and 2) discretized window predictor utilizing either stepwise linear regression or quadratic support vector machine machine learning model to predict the dose-dependent response of tractions and intercellular stresses to chemical perturbation. We used experimental traction and intercellular stress data gathered from samples subject to 0.2 or 2 μg/mL drug concentrations along with cell morphological properties extracted from the bright-field images as predictors to train our model. To demonstrate the predictive capability of our machine learning models, we predicted tractions and intercellular stresses in response to 0 and 1 μg/mL drug concentrations which were not utilized in the training sets. Results revealed the discretized window predictor trained just with four samples (292 images) to best predict both intercellular stresses and tractions using the quadratic support vector machine and stepwise linear regression models, respectively, for the unseen sample images.

Mechanisms of pulmonary microvascular endothelial cells barrier dysfunction induced by LPS: The roles of ceramides and the Txnip/NLRP3 inflammasome

Acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) are characterized by pulmonary microvascular endothelial cells (PMVECs) barrier dysfunction and proinflammatory cytokine influx into lung tissue, resulting in pulmonary oedema. Ceramide overproduction is an important mediator of pulmonary hyperinflammation and pulmonary oedema in Acute lung injury (ALI). Ceramides induce NLRP3 inflammasome activation are essential for the hyperinflammatory response. However, the roles and specific mechanisms of ceramide-induced NLRP3 inflammasome activation, proinflammatory cytokine manufacturing and PMVECs barrier dysfunction in ALI are unclear. Herein, pretreatment with the acid sphingomyelinase (ASMase) inhibitor imipramine (but not a neutral sphingomyelinase (NSMase) inhibitor or de novo pathway inhibitor) significantly inhibited ceramide early production in rats with lipopolysaccharide (LPS)-induced ALI; Furthermore, the Txnip/NLRP3 inflammasome activation, proinflammatory cytokine release, increased PMVECs permeability and lung injury were significantly decreased. Verapamil, a Txnip inhibitor, substantially inhibited Txnip/NLRP3 inflammasome activation, proinflammatory cytokine release, increased PMVECs permeability and lung injury in rats with C8-ceramide-induced ALI. In vitro, short hairpin RNA-mediated Txnip silencing significantly inhibited C8-ceramide-induced Txnip/NLRP3 inflammasome activation in NR8383 alveolar macrophages (AMs) and early secretion of the proinflammatory cytokines IL-1β (4-12 h) as well as IL-6 and TNF-α at subsequent times (later than 12 h). However, C8-ceramide significantly increased the early secretion (within 8 h) of the proinflammatory cytokines IL-1β, IL-6 and TNF-α in a co-culture model of NR8383 AMs and PMVECs, and Txnip silencing of NR8383 AMs inhibited the secretion of pro-inflammatory cytokines and reduced cytoskeletal rearrangements, intercellular connection breakage and hyperpermeability in PMVECs. Overall, our results suggest that in LPS-induced ALI, ceramide-mediated Txnip/NLRP3 inflammasome activation in NR8383 AMs leads to early IL-1β release, subsequently inducing PMVECs injury and release of the proinflammatory cytokines IL-6 and TNF-α, ultimately leading to PMVECs barrier dysfunction and ALI.

Extracellular Matrix Composition Alters Endothelial Force Transmission

ECM composition is important in a host of pathophysiological processes such as angiogenesis, atherosclerosis, and diabetes, for example and during each of these processes ECM composition has been reported to change over time. However, the impact ECM composition has on the endothelium’s ability to respond mechanically is currently unknown. Therefore, in this study we seeded human umbilical vein endothelial cells (HUVECs) onto soft hydrogels coated with an ECM concentration of 0.1 mg/mL at the following collagen I (Col-I) and fibronectin (FN) ratios: 100%Col-I, 75%Col-I-25%FN, 50%Col-I-50%FN, 25%Col-I-75%FN, and 100%FN. We subsequently measured tractions, intercellular stresses, strain energy, cell morphology, and cell velocity. Our results revealed huvecs seeded on gels coated with 50% Col-I - 50% FN to have the highest intercellular stresses, tractions, strain energies, but the lowest velocities and cell circularity. Huvecs seeded on 100% Col-I had the lowest tractions, cell area while havingthe highest velocities and cell circularity. In addition, cells cultured on 25% Col-I and 75% FN had the lowest intercellular stresses, but the highest cell area. Huvecs cultured on 100% FN yielded the lowest strain energies. We believe these results will be of great importance to the cardiovascular field, biomedical field, and cell mechanics. Summary: Study the influence of different Col-I - FN ECM compositions on endothelial cell mechanics and morphology.

Combined hyperspectral imaging, monolayer stress microscopy, and S8 image analysis approaches for simultaneously interrogating cellular signals and biomechanics

Second messenger signals, e.g., Ca2+ and cyclic nucleotides, orchestrate a wide range of cellular events. The methods by which second messenger signals determine specific physiological responses are complex. Recent studies point to the importance of temporal and spatial encoding in determining signal specificity. Studies also indicate the importance of mechanical stimuli, substrate stiffness, and mechanical responses - the "mechanosome" - in regulating physiology. Hence, approaches that probe both chemical and mechanical signals are needed. Here, we report preliminary efforts to combine hyperspectral imaging for second messenger signal measurements, monolayer stress microscopy for mechanical force measurements, and S8 analysis software for quantifying localized signals - specifically, Ca2+ dynamics and mechanical forces in human airway smooth muscle cells (HASMCs). HASMCs were prepared as confluent monolayers on 11 kPa gels with embedded fluorescent microparticles that serve as fiducial markers as well as smaller microparticles to measure deformation (strain). Imaging was performed using a custom excitation-scanning hyperspectral microscope. Hyperspectral images were unmixed to identify signals from cellular fluorescent labels (e.g., CAL 590-AM) and fluorescent microparticles. Images were analyzed to quantify localized force dynamics through monolayer stress microscopy. S8 software was used to identify, track, and quantify spatially-localized Ca2+ activity. Results indicate that localized and transient cellular signals and forces can be quantified and mapped within cell populations. Importantly, these results establish a method for simultaneous interrogation of cellular signals and mechanical forces that may play synergistic roles in regulating downstream cellular physiology in confluent monolayers. This work was supported by NIH P01HL066299, R01HL137030, R01HL058506, and NSF MRI1725937. Drs. Leavesley and Rich disclose financial interest in a university start-up company, SpectraCyte LLC, to commercialize spectral imaging technologies.

Impact of Na+ permeation on collective migration of pulmonary arterial endothelial cells

Collective migration of endothelial cells is important for wound healing and angiogenesis. During such migration, each constituent endothelial cell coordinates its magnitude and direction of migration with its neighbors while retaining intercellular adhesion. Ensuring coordination and cohesion involves a variety of intra- and inter-cellular signaling processes. However, the role of permeation of extracellular Na⁺ in collective cell migration remains unclear. Here, we examined the effect of Na⁺ permeation in collective migration of pulmonary artery endothelial cell (PAEC) monolayers triggered by either a scratch injury or a barrier removal over 24 hours. In the scratch assay, PAEC monolayers migrated in two approximately linear phases. In the first phase, wound closure started with fast speed which then rapidly reduced within 5 hours after scratching. In the second phase, wound closure maintained at slow and stable speed from 6 to 24 hours. In the absence of extracellular Na⁺, the wound closure distance was reduced by >50%. Fewer cells at the leading edge protruded prominent lamellipodia. Beside transient gaps, some sustained interendothelial gaps also formed and progressively increased in size over time, and some fused with adjacent gaps. In the absence of both Na⁺ and scratch injury, PAEC monolayer migrated even more slowly, and interendothelial gaps obviously increased in size towards the end. Pharmacological inhibition of the epithelial Na⁺ channel (ENaC) using amiloride reduced wound closure distance by 30%. Inhibition of both the ENaC and the Na⁺/Ca²⁺ exchanger (NCX) using benzamil further reduced wound closure distance in the second phase and caused accumulation of floating particles in the media. Surprisingly, pharmacological inhibition of the Ca²⁺ release-activated Ca²⁺ (CRAC) channel protein 1 (Orai1) using GSK-7975A, the transient receptor potential channel protein 1 and 4 (TRPC1/4) using Pico145, or both Orai1 and TRPC1/4 using combined GSK-7975A and Pico145 treatment did not affect wound closure distance dramatically. Nevertheless, the combined treatment appeared to cause accumulation of floating particles. Note that GSK-7975A also inhibits small inward Ca²⁺ currents via Orai2 and Orai3 channels, whereas Pico145 also blocks TRPC4, TRPC5, and TRPC1/5 channels. By contrast, gene silence of Orai1 by shRNAs led to a 25% reduction of wound closure in the first 6 hours but had no effect afterwards. However, in the absence of extracellular Na⁺ or cellular injury, Orai1 did not affect PAEC collective migration. Overall, the data reveal that Na⁺ permeation into cells contributes to PAEC monolayer collective migration by increasing lamellipodial formation, reducing accumulation of floating particles, and improving intercellular adhesion.

Unleashing shear: Role of intercellular traction and cellular moments in collective cell migration

In the field of endothelial biology, the term "shear forces" is tied to the forces exerted by the flowing blood on the quiescent cells. But endothelial cells themselves also exert physical forces on their immediate and distant neighbors. Specific factors of such intrinsic mechanical signals most relevant to immediate neighbors include normal (Fn) and shear (Fs) components of intercellular tractions, and those factors most relevant to distant neighbors include contractile or dilatational (Mc) and shear (Ms) components of the moments of cytoskeletal forces. However, for cells within a monolayer, Fn, Fs, Mc, and Ms remain inaccessible to experimental evaluation. Here, we present an approach that enables quantitative assessment of these properties. Remarkably, across a collectively migrating sheet of pulmonary microvascular endothelial cells, Fs was of the same order of magnitude as Fn. Moreover, compared to the normal components (Fn, Mc) of the mechanical signals, the shear components (Fs, Ms) were more distinctive in the cells closer to the migration front. Individual cells had an innately collective tendency to migrate along the axis of maximum contractile moment - a collective migratory process we referred to as cellular plithotaxis. Notably, larger Fs and Ms were associated with stronger plithotaxis, but dilatational moment appeared to disengage plithotactic guidance. Overall, cellular plithotaxis was more strongly associated with the "shear forces" (Fs, Ms) than with the "normal forces" (Fn, Mc). Finally, the mechanical state of the cells with fast migration speed and those with highly circular shape were reminiscent of fluid-like and solid-like matter, respectively. The results repeatedly pointed to neighbors imposing shear forces on a cell as a highly significant event, and hence, the term "shear forces" must include not just the forces from flowing fluid but also the forces from the substrate and neighbors. Collectively, these advances set the stage for deeper understanding of mechanical signaling in cellular monolayers.