Effects of serum deprivation on the mechanical properties of adherent vascular smooth muscle cells

Ecto-5'-nucleotidase (Nt5e/CD73)-mediated adenosine signaling attenuates TGFβ-2 induced elastin and cellular contraction

Arterial calcification due to deficiency of CD73 (ACDC) is a rare genetic disease caused by a loss-of-function mutation in the NT5E gene encoding the ecto-5'-nucleotidase (cluster of differentiation 73, CD73) enzyme. Patients with ACDC develop vessel arteriomegaly, tortuosity, and vascular calcification in their lower extremity arteries. Histological analysis shows that patients with ACDC vessels exhibit fragmented elastin fibers similar to that seen in aneurysmal-like pathologies. It is known that alterations in transforming growth factor β (TGFβ) pathway signaling contribute to this elastin phenotype in several connective tissue diseases, as TGFβ regulates extracellular matrix (ECM) remodeling. Our study investigates whether CD73-derived adenosine modifies TGFβ signaling in vascular smooth muscle cells (SMCs). We show that Nt5e-/- SMCs have elevated contractile markers and elastin gene expression compared with Nt5e+/+ SMCs. Ecto-5'-nucleotidase (Nt5e)-deficient SMCs exhibit increased TGFβ-2 and activation of small mothers against decapentaplegic (SMAD) signaling, elevated elastin transcript and protein, and potentiate SMC contraction. These effects were diminished when the A2b adenosine receptor was activated. Our results identify a novel link between adenosine and TGFβ signaling, where adenosine signaling via the A2b adenosine receptor attenuates TGFβ signaling to regulate SMC homeostasis. We discuss how disruption in adenosine signaling is implicated in ACDC vessel tortuosity and could potentially contribute to other aneurysmal pathogenesis.

Fibrotic Signaling in Cardiac Fibroblasts and Vascular Smooth Muscle Cells: The Dual Roles of Fibrosis in HFpEF and CAD

Patients with heart failure with preserved ejection fraction (HFpEF) and atherosclerosis-driven coronary artery disease (CAD) will have ongoing fibrotic remodeling both in the myocardium and in atherosclerotic plaques. However, the functional consequences of fibrosis differ for each location. Thus, cardiac fibrosis leads to myocardial stiffening, thereby compromising cardiac function, while fibrotic remodeling stabilizes the atherosclerotic plaque, thereby reducing the risk of plaque rupture. Although there are currently no drugs targeting cardiac fibrosis, it is a field under intense investigation, and future drugs must take these considerations into account. To explore similarities and differences of fibrotic remodeling at these two locations of the heart, we review the signaling pathways that are activated in the main extracellular matrix (ECM)-producing cells, namely human cardiac fibroblasts (CFs) and vascular smooth muscle cells (VSMCs). Although these signaling pathways are highly overlapping and context-dependent, effects on ECM remodeling mainly act through two core signaling cascades: TGF-β and Angiotensin II. We complete this by summarizing the knowledge gained from clinical trials targeting these two central fibrotic pathways.

TRPM2 promotes autophagic degradation in vascular smooth muscle cells

Transient receptor potential channel M2 (TRPM2) is a Ca²⁺-permeable channel that is activated by reactive oxygen species (ROS). In many cell types, ROS activate TRPM2 to induce excessive Ca²⁺ influx, resulting in Ca²⁺ overload and consequent cell death. Recent studies suggest that TRPM2 may also regulate autophagy in pericytes and cancer cells by acting on the early step of autophagy, i.e. autophagic induction. However, there is no report on the role of TRPM2 in autophagic degradation, which is the late stage of autophagy. In the present study, we found abundant TRPM2 expression in lysosomes/autolysosomes in the primary cultured mouse aortic smooth muscle cells (mASMCs). Nutrient starvation stimulated autophagic flux in mASMCs mainly by promoting autophagic degradation. This starvation-induced autophagic degradation was reduced by TRPM2 knockout. Importantly, starvation-induced lysosomal/autolysosomal acidification and cell death were also substantially reduced by TRPM2 knockout. Taken together, the present study uncovered a novel mechanism that lysosomal TRPM2 facilitates lysosomal acidification to stimulate excessive autolysosome degradation and consequent cell death.

Effects of blocking integrin β1 and N-cadherin cellular interactions on mechanical properties of vascular smooth muscle cells

Experimental measurements of cellular mechanical properties have shown large variability in whole-cell mechanical properties between cells from a single population. This heterogeneity has been observed in many cell populations and with several measurement techniques but the sources are not yet fully understood. Cell mechanical properties are directly related to the composition and organization of the cytoskeleton, which is physically coupled to neighboring cells through adherens junctions and to underlying matrix through focal adhesion complexes. This high level of heterogeneity may be attributed to varying cellular interactions throughout the sample. We tested the effect of cell-cell and cell-matrix interactions on the mechanical properties of vascular smooth muscle cells (VSMCs) in culture by using antibodies to block N-cadherin and integrin β1 interactions. VSMCs were cultured on substrates of varying stiffness with and without tension. Under each of these conditions, cellular mechanical properties were characterized by performing atomic force microscopy (AFM) and cellular structure was analyzed through immunofluorescence imaging. As expected, VSMC mechanical properties were greatly affected by the underlying culture substrate and applied tension. Interestingly, the cell-to-cell variation in mechanical properties within each sample decreased significantly in the antibody conditions. Thus, the cells grown with blocking antibodies were more homogeneous in their mechanical properties on both glass and soft substrates. This suggests that diversified adhesion binding between cells and the ECM is responsible for a significant amount of mechanical heterogeneity that is observed in 2D cell culture studies.

Sensitivity to Strain and Shear Stress of Isolated Mechanosensitive Enteric Neurons

Within the enteric nervous system, the neurons in charge to control motility of the gastrointestinal tract reside in a particular location nestled between two perpendicular muscle layers which contract and relax. We used primary cultured myenteric neurons of male guinea pigs to study mechanosensitivity of enteric neurons in isolation. Ultrafast Neuroimaging with a voltage-sensitive dye technique was used to record neuronal activity in response to shear stress and strain. Strain was induced by locally deforming the elastic cell culture substrate next to a neuron. Measurements showed that substrate strain was mostly elongating cells. Shear stress was exerted by hydrodynamic forces in a microchannel. Both stimuli induced excitatory responses. Strain activated 14% of the stimulated myenteric neurons that responded with a spike frequency of 1.9 (0.7/3.2) Hz, whereas shear stress excited only a few neurons (5.6%) with a very low spike frequency of 0 (0/0.6) Hz. Thus, shear stress does not seem to be an adequate stimulus for mechanosensitive enteric neurons (MEN) while strain activates enteric neurons in a relevant manner. Analyzing the adaptation behavior of MEN showed that shear stress activated rapidly/slowly/ultraslowly adapting MEN (2/62/36%) whereas strain only slowly (46%) and ultraslowly (54%) MEN. Paired experiments with strain and normal stress revealed three mechanosensitive enteric neuronal populations: one strain-sensitive (37%), one normal stress-sensitive (17%) and one strain- and stress-sensitive (46%). These results indicate that shear stress does not play a role in the neuronal control of motility but normal stress and strain.

The effect of the serum corona on interactions between a single nano-object and a living cell

Nanoparticles (NPs) which enter physiological fluids are rapidly coated by proteins, forming a so-called corona which may strongly modify their interaction with tissues and cells relative to the bare NPs. In this work the interactions between a living cell and a nano-object, and in particular the effect on this of the adsorption of serum proteins, are directly examined by measuring the forces arising as an Atomic Force Microscope tip (diameter 20 nm) - simulating a nano-object - approaches and contacts a cell. We find that the presence of a serum protein corona on the tip strongly modifies the interaction as indicated by pronounced increase in the indentation, hysteresis and work of adhesion compared to a bare tip. Classically one expects an AFM tip interacting with a cell surface to be repelled due to cell elastic distortion, offset by tip-cell adhesion, and indeed such a model fits the bare-tip/cell interaction, in agreement with earlier work. However, the force plots obtained with serum-modified tips are very different, indicating that the cell is much more compliant to the approaching tip. The insights obtained in this work may promote better design of NPs for drug delivery and other nano-medical applications.

Evaluating a novel dimensional reduction approach for mechanical fractionation of cells using a tandem flexible micro spring array (tFMSA)

We present a novel methodology to establish experimental models for the rational design of cell fractionation based on physical properties of cells. Label-free microfluidic separation of cells based on size is a widely employed technique. However, close observation reveals that cell capture results cannot be explained by cell sizes alone. This is particularly apparent with viable cell fractionation, where cells retain their native deformability. We have developed a principal size cutoff (PSC) model based on the analysis of size distribution and size-based filtration efficiency for cell populations. The goal of this analysis is to use an unbiased approach to achieve dimensional reduction of deformability and other mechanical properties that affect cell capture. The PSC model provides a single calibrated principal size component that may be compared directly to device gap width, which is the critical dimension for cell filtration. The PSC model was evaluated experimentally using a tandem flexible micro spring array (tFMSA) device made of parylene filtration elements applied within micro-molded polydimethylsiloxane (PDMS) chambers. In the tFMSA device, a mixture of cells is sequentially passed through individual filters with decreasing gap widths to allow size-based selection. We applied this method to demonstrate viable separation of subgroups of cells with different mechanical properties from complex mixtures, including fractionation according to cancer cell type, cell cycle stage, cell viability status, and leukocyte nuclear phenotype. The PSC methodology and tFMSA device can advance a better understanding of complex factors affecting mechanical cell fractionation and provide a miniature platform for obtaining rationally designed cell fractions for biomedical applications.

Vascular smooth muscle cell glycocalyx mediates shear stress-induced contractile responses via a Rho kinase (ROCK)-myosin light chain phosphatase (MLCP) pathway

The vascular smooth muscle cells (VSMCs) are exposed to interstitial flow induced shear stress that may be sensed by the surface glycocalyx, a surface layer composed primarily of proteoglycans and glycoproteins, to mediate cell contraction during the myogenic response. We, therefore, attempted to elucidate the signal pathway of the glycocalyx mechanotransduction in shear stress regulated SMC contraction. Human umbilical vein SMCs (HUVSMCs) deprived of serum for 3–4 days were exposed to a step increase (0 to 20 dyn/cm²) in shear stress in a parallel plate flow chamber, and reduction in the cell area was quantified as contraction. The expressions of Rho kinase (ROCK) and its downstream signal molecules, the myosin-binding subunit of myosin phosphatase (MYPT) and the myosin light chain 2 (MLC2), were evaluated. Results showed that the exposure of HUVSMCs to shear stress for 30 min induced cell contraction significantly, which was accompanied by ROCK1 up-regulation, re-distribution, as well as MYPT1 and MLC activation. However, these shear induced phenomenon could be completely abolished by heparinase III or Y-27632 pre-treatment. These results indicate shear stress induced VSMC contraction was mediated by cell surface glycocalyx via a ROCK-MLC phosphatase (MLCP) pathway, providing evidence of the glycocalyx mechanotransduction in myogenic response.

Discrimination Between Cervical Cancer Cells and Normal Cervical Cells Based on Longitudinal Elasticity Using Atomic Force Microscopy

The mechanical properties of cells are considered promising biomarkers for the early diagnosis of cancer. Recently, atomic force microscopy (AFM)-based nanoindentation technology has been utilized for the examination of cell cortex mechanics in order to distinguish malignant cells from normal cells. However, few attempts to evaluate the biomechanical properties of cells have focused on the quantification of the non-homogeneous longitudinal elasticity of cellular structures. In the present study, we applied a variation of the method of Carl and Schillers to investigate the differences between longitudinal elasticity of human cervical squamous carcinoma cells (CaSki) and normal cervical epithelial cells (CRL2614) using AFM. The results reveal a three-layer heterogeneous structure in the probing volume of both cell types studied. CaSki cells exhibited a lower whole-cell stiffness and a softer nuclei zone compared to the normal counterpart cells. Moreover, a better differentiated cytoskeleton was found in the inner cytoplasm/nuclei zone of the normal CRL2614 cells, whereas a deeper cytoskeletal distribution was observed in the probing volume of the cancerous counterparts. The sensitive cortical panel of CaSki cells, with a modulus of 0.35~0.47 kPa, was located at 237~225 nm; in normal cells, the elasticity was 1.20~1.32 kPa at 113~128 nm. The present improved method may be validated using the conventional Hertz-Sneddon method, which is widely reported in the literature. In conclusion, our results enable the quantification of the heterogeneous longitudinal elasticity of cancer cells, in particular the correlation with the corresponding depth. Preliminary results indicate that our method may potentially be applied to improve the detection of cancerous cells and provide insights into the pathophysiology of the disease.

Altered phenotypic gene expression of 10T1/2 mesenchymal cells in nonuniformly stretched PEGDA hydrogels

Disease-related phenotype modulation of many cell types has been shown to be closely related to mechanical loading conditions; for example, vascular smooth muscle cell (SMC) phenotype shift from a mature, contractile state to a proliferative, synthetic state contributes to the formation of neointimal tissue during atherosclerosis and restenosis development and is related to SMC mechanical loading in vivo. The majority of past in vitro cell-stretching experiments have employed simplistic (uniform, uniaxial or biaxial) stretching environments to elucidate mechanobiological pathways involved in phenotypic shifts. However, the in vivo mechanics of the vascular wall consists of highly nonuniform stretch. Here we subjected 10T1/2 murine mesenchymal cells (an SMC precursor) to two- and three-dimensional nonuniform stretch environments. After 24 h of stretch, cells on an elastomeric membrane demonstrated varied proliferation [assessed by 5-bromo-2'-deoxyuridine (BrdU) incorporation] depending on location upon the membrane, with maximal proliferation occurring in a region of high, uniaxial stretch. Cells subjected to a nonuniform stretching regimen within three-dimensional polyethylene glycol diacrylate (PEGDA) hydrogel constructs demonstrated marked changes in mRNA expression of several phenotype-related proteins, indicating a sort of "hybrid" phenotype with contractile and synthetic markers being both upregulated and downregulated. Furthermore, expression levels of mRNAs were significantly different between various locations within the stretched gel. With the proliferation results, these data exhibit the capability of nonuniform stretching devices to induce heterogeneous cell responses, potentially indicative of spatial distributions of disease-related behaviors in vivo.

A computational approach to understand phenotypic structure and constitutive mechanics relationships of single cells

The goal of this study is to construct a representative 3D finite element model (FEM) of individual cells based on their sub-cellular structures that predicts cell mechanical behavior. The FEM simulations replicate atomic force microscopy (AFM) nanoindentation experiments on live vascular smooth muscle cells. Individual cells are characterized mechanically with AFM and then imaged in 3D using a spinning disc confocal microscope. Using these images, geometries for the FEM are automatically generated via image segmentation and linear programming algorithms. The geometries consist of independent structures representing the nucleus, actin stress fiber network, and cytoplasm. These are imported into commercial software for mesh refinement and analysis. The FEM presented here is capable of predicting AFM results well for 500 nm indentations. The FEM results are relatively insensitive to both the exact number and diameter of fibers used. Despite the localized nature of AFM nanoindentation, the model predicts that stresses are distributed in an anisotropic manner throughout the cell body via the actin stress fibers. This pattern of stress distribution is likely a result of the geometric arrangement of the actin network.

Contractility dominates adhesive ligand density in regulating cellular de-adhesion and retraction kinetics

Cells that are enzymatically detached from a solid substrate rapidly round up as the tensile prestress in the cytoskeleton is suddenly unopposed by cell-ECM adhesions. We recently showed that this retraction follows sigmoidal kinetics with time constants that correlate closely with cortical stiffness values. This raises the promising prospect that these de-adhesion measurements may be used for high-throughput screening of cell mechanical properties; however, an important limitation to doing so is the possibility that the retraction kinetics may also be influenced and potentially rate-limited by the time needed to sever matrix adhesions. In this study, we address this open question by separating contributions of contractility and adhesion to cellular de-adhesion and retraction kinetics. We first develop serum-free conditions under which U373 MG glioma cells can be cultured on substrates of fixed fibronectin density without direct matrix contributions from the medium. We show that while spreading area increases with ECM protein density, cortical stiffness and the time constants of retraction do not. Conversely, addition of lysophosphatidic acid (LPA) to stimulate cell contractility strongly speeds retraction, independent of the initial matrix protein density and LPA's contributions to spreading area. All of these trends hold in serum-rich medium commonly used in tissue culture, with the time constants of retraction much more closely tracking cortical stiffness than adhesive ligand density or cell spreading. These results support the use of cellular de-adhesion measurements to track cellular mechanical properties.

Frictional Behavior of Individual Vascular Smooth Muscle Cells Assessed By Lateral Force Microscopy

With the advancement of the field of biotribology, considerable interest has arisen in the study of cell and tissue frictional properties. From the perspective of medical device development, the frictional properties between a rigid surface and underlying cells and tissues are of a particular clinical interest. As with many bearing surfaces, it is likely that contact asperities exist at the size scale of single cells and below. Thus, a technique to measure cellular frictional properties directly would be beneficial from both a clinical and a basic science perspective. In the current study, an atomic force microscope (AFM) with a 5 µm diameter borosilicate spherical probe simulating endovascular metallic stent asperities was used to characterize the surface frictional properties of vascular smooth muscle cells (VSMCs) in contact with a metallic endovascular stent. Various treatments were used to alter cell structure, in order to better understand the cellular components and mechanisms responsible for governing frictional properties. The frictional coefficient of the probe on VSMCs was found to be approximately 0.06. This frictional coefficient was significantly affected by cellular crosslinking and cytoskeletal depolymerization agents. These results demonstrate that AFM-based lateral force microscopy is a valuable technique to assess the friction properties of individual single cells on the micro-scale.