Dynamic Imaging Reveals Coordinate Effects of Cyclic Stretch and Substrate Stiffness on Endothelial Integrity

Mechanical resilience of the sessile tunicate Botryllus schlosseri

We demonstrate that the sessile tunicate Botryllus schlosseri is remarkably resilient to applied loads by attaching the animals to an extensile substrate subjected to quasistatic equiradial loads. Animals can withstand radial extension of the substrate to strain values as high as 20% before they spontaneously detach. In the small to moderate strain regime, we found no relationship between the dynamic size of the external vascular bed and the magnitude of applied stretch, despite known force sensitivities of the vascular tissue at the cellular level. We attribute this resilience to the presence and mechanical properties of the tunic, the cellulose-enriched gel-like substance that encases the animal bodies and surrounding vasculature.

Engineering tools for quantifying and manipulating forces in epithelia

The integrity of epithelia is maintained within dynamic mechanical environments during tissue development and homeostasis. Understanding how epithelial cells mechanosignal and respond collectively or individually is critical to providing insight into developmental and (patho)physiological processes. Yet, inferring or mimicking mechanical forces and downstream mechanical signaling as they occur in epithelia presents unique challenges. A variety of in vitro approaches have been used to dissect the role of mechanics in regulating epithelia organization. Here, we review approaches and results from research into how epithelial cells communicate through mechanical cues to maintain tissue organization and integrity. We summarize the unique advantages and disadvantages of various reduced-order model systems to guide researchers in choosing appropriate experimental systems. These model systems include 3D, 2D, and 1D micromanipulation methods, single cell studies, and noninvasive force inference and measurement techniques. We also highlight a number of in silico biophysical models that are informed by in vitro and in vivo observations. Together, a combination of theoretical and experimental models will aid future experiment designs and provide predictive insight into mechanically driven behaviors of epithelial dynamics.

Extracellular matrix regulates force transduction at VE-cadherin junctions

Increased tension on VE-cadherin (VE-cad) complexes activates adaptive cell stiffening and local cytoskeletal reinforcement--two key signatures of intercellular mechanotransduction. Here we demonstrate that tugging on VE-cad receptors initiates a cascade that results in downstream integrin activation. The formation of new integrin adhesions potentiates vinculin and actin recruitment to mechanically reinforce stressed cadherin adhesions. This cascade differs from documented antagonistic effects of integrins on intercellular junctions. We identify focal adhesion kinase, Abl kinase, and RhoA GTPase as key components of the positive feedback loop. Results further show that a consequence of integrin involvement is the sensitization of intercellular force transduction to the extracellular matrix (ECM) not by regulating junctional tension but by altering signal cascades that reinforce cell-cell adhesions. On type 1 collagen or fibronectin substrates, integrin subtypes α2β1 and α5β1, respectively, differentially control actin remodeling at VE-cad adhesions. Specifically, ECM-dependent differences in VE-cad force transduction mirror differences in the rigidity sensing mechanisms of α2β1 and α5β1 integrins. The findings verify the role of integrins in VE-cad force transduction and uncover a previously unappreciated mechanism by which the ECM impacts the mechanical reinforcement of interendothelial junctions.

Mechanical disruption of E-cadherin complexes with epidermal growth factor receptor actuates growth factor-dependent signaling

Increased intercellular tension is associated with enhanced cell proliferation and tissue growth. Here, we present evidence for a force-transduction mechanism that links mechanical perturbations of epithelial (E)-cadherin (CDH1) receptors to the force-dependent activation of epidermal growth factor receptor (EGFR, ERBB1)-a key regulator of cell proliferation. Here, coimmunoprecipitation studies first show that E-cadherin and EGFR form complexes at the plasma membrane that are disrupted by either epidermal growth factor (EGF) or increased tension on homophilic E-cadherin bonds. Although force on E-cadherin bonds disrupts the complex in the absence of EGF, soluble EGF is required to mechanically activate EGFR at cadherin adhesions. Fully quantified spectral imaging fluorescence resonance energy transfer further revealed that E-cadherin and EGFR directly associate to form a heterotrimeric complex of two cadherins and one EGFR protein. Together, these results support a model in which the tugging forces on homophilic E-cadherin bonds trigger force-activated signaling by releasing EGFR monomers to dimerize, bind EGF ligand, and signal. These findings reveal the initial steps in E-cadherin-mediated force transduction that directly link intercellular force fluctuations to the activation of growth regulatory signaling cascades.

Integration of substrate- and flow-derived stresses in endothelial cell mechanobiology

Endothelial cells (ECs) lining all blood vessels are subjected to large mechanical stresses that regulate their structure and function in health and disease. Here, we review EC responses to substrate-derived biophysical cues, namely topography, curvature, and stiffness, as well as to flow-derived stresses, notably shear stress, pressure, and tensile stresses. Because these mechanical cues in vivo are coupled and are exerted simultaneously on ECs, we also review the effects of multiple cues and describe burgeoning in vitro approaches for elucidating how ECs integrate and interpret various mechanical stimuli. We conclude by highlighting key open questions and upcoming challenges in the field of EC mechanobiology.

Bioscaffolds embedded with regulatory modules for cell growth and tissue formation: A review

The demand for artificial organs has greatly increased because of various aging-associated diseases and the wide need for organ transplants. A recent trend in tissue engineering is the precise reconstruction of tissues by the growth of cells adhering to bioscaffolds, which are three-dimensional (3D) structures that guide tissue and organ formation. Bioscaffolds used to fabricate bionic tissues should be able to not only guide cell growth but also regulate cell behaviors. Common regulation methods include biophysical and biochemical stimulations. Biophysical stimulation cues include matrix hardness, external stress and strain, surface topology, and electromagnetic field and concentration, whereas biochemical stimulation cues include growth factors, proteins, kinases, and magnetic nanoparticles. This review discusses bioink preparation, 3D bioprinting (including extrusion-based, inkjet, and ultraviolet-assisted 3D bioprinting), and regulation of cell behaviors. In particular, it provides an overview of state-of-the-art methods and devices for regulating cell growth and tissue formation and the effects of biophysical and biochemical stimulations on cell behaviors. In addition, the fabrication of bioscaffolds embedded with regulatory modules for biomimetic tissue preparation is explained. Finally, challenges in cell growth regulation and future research directions are presented.

3D-printable cell crowding device enables imaging of live cells in compression

We designed and fabricated, using low-cost 3D printing technologies, a device that enables direct control of cell density in epithelial monolayers. The device operates by varying the tension of a silicone substrate upon which the cells are adhered. Multiple devices can be manufactured easily and placed in any standard incubator. This allows long-term culturing of cells on pretensioned substrates until the user decreases the tension, thereby inducing compressive forces in plane and subsequent instantaneous cell crowding. Moreover, the low-profile device is completely portable and can be mounted directly onto an inverted optical microscope. This enables visualization of the morphology and dynamics of living cells in stretched or compressed conditions using a wide range of high-resolution microscopy techniques.

In Vitro Model of Stretch-Induced Lung Injury to Study Different Lung Ventilation Regimens and the Role of Sedatives

BACKGROUND

Currently lung injury is managed conservatively through supportive care including mechanical ventilation. However, mechanical ventilation can also cause additional lung injury due to over-stretch along with atelectasis and cytokine release. Here we developed an in vitro mechanical ventilation model using cyclic stretch of lung epithelial cells to mimic high and low tidal volume (TV) ventilation strategy, so that we could use this platform for pathophysiology analysis and screening for therapeutic drugs.

METHOD

We subjected MLE-15 cells to the following treatments. 1) No treatment, 2) lipopolysaccharide (100 ng/mL) stimulation for 24 hours, 3) mechanical stretch initiated at 6-hour time point for 18 hours, 4) LPS stimulation at time point 0 hour, and mechanical stretch was added at 6-hour time point for 18 hours. Biaxial cyclic stretch with a triangular wave was given via the Flexcell FX-6000 tension system to mimic low and high TV. Anesthetics dexmedetomidine and propofol were also tested.

RESULT

Our high TV mimic stretch increased cell death, while low TV mimic stretch did not affect the degree of cell death. Using this system, we examined the effect of sedatives commonly used in intensive care units on cell death and found that dexmedetomidine attenuated necrosis associated with stretch.

CONCLUSION

We described the in vitro cyclic stretch system mimicking high and low TV ventilation. High TV mimetic was associated with increased cell death. Dexmedetomidine attenuated the degree of cell death.

Vascular Endothelial Cell Behavior in Complex Mechanical Microenvironments

The vascular mechanical microenvironment consists of a mixture of spatially and temporally changing mechanical forces. This exposes vascular endothelial cells to both hemodynamic forces (fluid flow, cyclic stretching, lateral pressure) and vessel forces (basement membrane mechanical and topographical properties). The vascular mechanical microenvironment is "complex" because these forces are dynamic and interrelated. Endothelial cells sense these forces through mechanosensory structures and transduce them into functional responses via mechanotransduction pathways, culminating in behavior directly affecting vascular health. Recent in vitro studies have shown that endothelial cells respond in nuanced and unique ways to combinations of hemodynamic and vessel forces as compared to any single mechanical force. Understanding the interactive effects of the complex mechanical microenvironment on vascular endothelial behavior offers the opportunity to design future biomaterials and biomedical devices from the bottom-up by engineering for the cellular response. This review describes and defines (1) the blood vessel structure, (2) the complex mechanical microenvironment of the vascular endothelium, (3) the process in which vascular endothelial cells sense mechanical forces, and (4) the effect of mechanical forces on vascular endothelial cells with specific attention to recent works investigating the influence of combinations of mechanical forces. We conclude this review by providing our perspective on how the field can move forward to elucidate the effects of the complex mechanical microenvironment on vascular endothelial cell behavior.

Endothelial cells on an aged subendothelial matrix display heterogeneous strain profiles in silico

Within the artery intima, endothelial cells respond to mechanical cues and changes in subendothelial matrix stiffness. Recently, we found that the aging subendothelial matrix stiffens heterogeneously and that stiffness heterogeneities are present on the scale of one cell length. However, the impacts of these complex mechanical micro-heterogeneities on endothelial cells have not been fully understood. Here, we simulate the effects of matrices that mimic young and aged vessels on single- and multi-cell endothelial cell models and examine the resulting cell basal strain profiles. Although there are limitations to the model which prohibit the prediction of intracellular strain distributions in alive cells, this model does introduce mechanical complexities to the subendothelial matrix material. More heterogeneous basal strain distributions are present in the single- and multi-cell models on the matrix mimicking an aged artery over those exhibited on the young artery. Overall, our data indicate that increased heterogeneous strain profiles in endothelial cells are displayed in silico when there is an increased presence of microscale arterial mechanical heterogeneities in the matrix.

Direct Imaging of Protein Stability and Folding Kinetics in Hydrogels

We apply fast relaxation imaging (FReI) as a novel technique for investigating the folding stability and dynamics of proteins within polyacrylamide hydrogels, which have diverse and widespread uses in biotechnology. FReI detects protein unfolding in situ by imaging changes in fluorescence resonance energy transfer (FRET) after temperature jump perturbations. Unlike bulk measurements, diffraction-limited epifluorescence imaging combined with fast temperature perturbations reveals the impact of local environment effects on protein-biomaterial compatibility. Our experiments investigated a crowding sensor protein (CrH2) and phosphoglycerate kinase (PGK), which undergoes cooperative unfolding. The crowding sensor quantifies the confinement effect of the cross-linked hydrogel: the 4% polyacrylamide hydrogel is similar to aqueous solution (no confinement), while the 10% hydrogel is strongly confining. FRAP measurements and protein concentration gradients in the 4% and 10% hydrogels further support this observation. PGK reveals that noncovalent interactions of the protein with the polymer surface are more important than confinement for determining protein properties in the gel: the mere presence of hydrogel increases protein stability, speeds up folding relaxation, and promotes irreversible binding to the polymer even at the solution-gel interface, whereas the difference between the 4% and the 10% hydrogels is negligible despite their large difference in confinement. The imaging capabilities of FReI, demonstrated to be diffraction limited, further revealed spatially homogeneous protein unfolding across the hydrogels at 500 nm length scales and revealed differences in protein properties at the gel-solution boundary.

Substrate stiffness and VE-cadherin mechano-transduction coordinate to regulate endothelial monolayer integrity

The vascular endothelium is subject to diverse mechanical cues that regulate vascular endothelial barrier function. In addition to rigidity sensing through integrin adhesions, mechanical perturbations such as changes in fluid shear stress can also activate force transduction signals at intercellular junctions. This study investigated how extracellular matrix rigidity and intercellular force transduction, activated by vascular endothelial cadherin, coordinate to regulate the integrity of endothelial monolayers. Studies used complementary mechanical measurements of endothelial monolayers grown on patterned substrates of variable stiffness. Specifically perturbing VE-cadherin receptors activated intercellular force transduction signals that increased integrin-dependent cell contractility and disrupted cell-cell and cell-matrix adhesions. Further investigations of the impact of substrate rigidity on force transduction signaling demonstrated how cells integrate extracellular mechanics cues and intercellular force transduction signals, to regulate endothelial integrity and global tissue mechanics. VE-cadherin specific signaling increased focal adhesion remodeling and cell contractility, while sustaining the overall mechanical equilibrium at the mesoscale. Conversely, increased substrate rigidity exacerbates the disruptive effects of intercellular force transduction signals, by increasing heterogeneity in monolayer stress distributions. The results provide new insights into how substrate stiffness and intercellular force transduction coordinate to regulate endothelial monolayer integrity.

The cell-stretcher: A novel device for the mechanical stimulation of cell populations

Mechanical stimulation appears to be a critical modulator for many aspects of biology, both of living tissue and cells. The cell-stretcher, a novel device for the mechanical uniaxial stimulation of populations of cells, is described. The system is based on a variable stroke cam-lever-tappet mechanism which allows the delivery of cyclic stimuli with frequencies of up to 10 Hz and deformation between 1% and 20%. The kinematics is presented and a simulation of the dynamics of the system is shown, in order to compute the contact forces in the mechanism. The cells, following cultivation and preparation, are plated on an ad hoc polydimethylsiloxane membrane which is then loaded on the clamps of the cell-stretcher via force-adjustable magnetic couplings. In order to show the viability of the experimentation and biocompatibility of the cell-stretcher, a set of two in vitro tests were performed. Human epithelial carcinoma cell line A431 and Adult Mouse Ventricular Fibroblasts (AMVFs) from a dual reporter mouse were subject to 0.5 Hz, 24 h cyclic stretching at 15% strain, and to 48 h stimulation at 0.5 Hz and 15% strain, respectively. Visual analysis was performed on A431, showing definite morphological changes in the form of cellular extroflections in the direction of stimulation compared to an unstimulated control. A cytometric analysis was performed on the AMVF population. Results show a post-stimulation live-dead ratio deviance of less than 6% compared to control, which proves that the environment created by the cell-stretcher is suitable for in vitro experimentation.