Nano-mechanical model of endothelial dysfunction for AFM-based diagnostics at the cellular level

Design of Liquid Formulation Based on F127-Loaded Natural Dimeric Flavonoids as a New Perspective Treatment for Leishmaniasis

Infectious and Parasitic Diseases (IPD) remain a challenge for medicine due to several interconnected reasons, such as antimicrobial resistance (AMR). American tegumentary leishmaniasis (ATL) is an overlooked IPD causing persistent skin ulcers that are challenging to heal, resulting in disfiguring scars. Moreover, it has the potential to extend from the skin to the mucous membranes of the nose, mouth, and throat in both humans and various animals. Given the limited effectiveness and AMR of current drugs, the exploration of new substances has emerged as a promising alternative for ATL treatment. Arrabidaea brachypoda (DC). Bureau is a native Brazilian plant rich in dimeric flavonoids, including Brachydin (BRA), which displays antimicrobial activity, but still little has been explored regarding the development of therapeutic formulations. In this work, we present the design of a low-cost liquid formulation based on the use of Pluronic F127 for encapsulation of high BRA concentration (LF-B500). The characterization techniques revealed that BRA-loaded F127 micelles are well-stabilized in an unusual worm-like form. The in vitro cytotoxicity assay demonstrated that LF-B500 was non-toxic to macrophages but efficient in the inactivation of forms of Leishmania amazonensis promastigotes with IC50 of 16.06 µg/mL. The results demonstrated that LF-B500 opened a new perspective on the use of liquid formulation-based natural products for ATL treatment.

Universal contact stiffness of elastic solids covered with tensed membranes and its application in indentation tests of biological materials

The inherent membrane tension of biological materials could vitally affect their responses to contact loading but is generally ignored in existing indentation analysis. In this paper, the authors theoretically investigate the contact stiffness of axisymmetric indentations of elastic solids covered with thin tensed membranes. When the indentation size decreases to the same order as the ratio of membrane tension to elastic modulus, the contact stiffness accounting for the effect of membrane tension becomes much higher than the prediction of conventional contact theory. An explicit expression is derived for the contact stiffness, which is universal for axisymmetric indentations using indenters of arbitrary convex profiles. On this basis, a simple method of analysis is proposed to estimate the membrane tension and elastic modulus of biological materials from the indentation load-depth data, which is successfully applied to analyze the indentation experiments of cells and lungs. This study might be helpful for the comprehensive assessment of the mechanical properties of soft biological systems. STATEMENT OF SIGNIFICANCE: This paper highlights the crucial effect of the inherent membrane tension on the indentation response of soft biomaterials, which has been generally ignored in existing analysis of experiments. For typical indentation tests on cells and organs, the contact stiffness can be twice or higher than the prediction of conventional contact model. A universal expression of the contact stiffness accounting for the membrane tension effect is derived. On this basis, a simple method of analysis is proposed to abstract the membrane tension of biomaterials from the experimentally recorded indentation load-depth data. With this method, the elasticity of soft biomaterials can be characterized more comprehensively.

AFM-based nanoindentation indicates an impaired cortical stiffness in the AAV-PCSK9DY atherosclerosis mouse model

Investigating atherosclerosis and endothelial dysfunction has mainly become established in genetically modified ApoE^−/− or LDL-R^−/− mice transgenic models. A new AAV-PCSK9DY^DY mouse model with no genetic modification has now been reported as an alternative atherosclerosis model. Here, we aimed to employ this AAV-PCSK9^DY mouse model to quantify the mechanical stiffness of the endothelial surface, an accepted hallmark for endothelial dysfunction and forerunner for atherosclerosis. Ten-week-old male C57BL/6 N mice were injected with AAV-PCSK9^DY (0.5, 1 or 5 × 10¹¹ VG) or saline as controls and fed with Western diet (1.25% cholesterol) for 3 months. Total cholesterol (TC) and triglycerides (TG) were measured after 6 and 12 weeks. Aortic sections were used for atomic force microscopy (AFM) measurements or histological analysis using Oil-Red-O staining. Mechanical properties of in situ endothelial cells derived from ex vivo aorta preparations were quantified using AFM-based nanoindentation. Compared to controls, an increase in plasma TC and TG and extent of atherosclerosis was demonstrated in all groups of mice in a viral load-dependent manner. Cortical stiffness of controls was 1.305 pN/nm and increased (10%) in response to viral load (≥ 0.5 × 10¹¹ VG) and positively correlated with the aortic plaque content and plasma TC and TG. For the first time, we show changes in the mechanical properties of the endothelial surface and thus the development of endothelial dysfunction in the AAV-PCSK9^DY mouse model. Our results demonstrate that this model is highly suitable and represents a good alternative to the commonly used transgenic mouse models for studying atherosclerosis and other vascular pathologies.

Rac1 regulates lipid droplets formation, nanomechanical, and nanostructural changes induced by TNF in vascular endothelium in the isolated murine aorta

Endothelial inflammation is recognized as a critical condition in the development of cardiovascular diseases. TNF-induced inflammation of endothelial cells is linked to the formation of lipid droplets, augmented cortical stiffness, and nanostructural endothelial plasma membrane remodelling, but the insight into the mechanism linking these responses is missing. In the present work, we determined the formation of lipid droplets (LDs), nanomechanical, and nanostructural responses in the model of TNF-activated vascular inflammation in the isolated murine aorta using Raman spectroscopy, fluorescence imaging, atomic force microscopy (AFM), and scanning electron microscopy (SEM). We analysed the possible role of Rac1, a major regulator of cytoskeletal organization, in TNF-induced vascular inflammation. We demonstrated that the formation of LDs, polymerization of F-actin, alterations in cortical stiffness, and nanostructural protuberances in endothelial plasma membrane were mediated by the Rac1. In particular, we revealed a significant role for Rac1 in the regulation of the formation of highly unsaturated LDs formed in response to TNF. Inhibition of Rac1 also downregulated the overexpression of ICAM-1 induced by TNF, supporting the role of Rac1 in vascular inflammation. Altogether, our results demonstrate that LDs formation, an integral component of vascular inflammation, is activated by Rac1 that also regulates nanomechanical and nanostructural alterations linked to vascular inflammation.

Lipid Droplets Formation Represents an Integral Component of Endothelial Inflammation Induced by LPS

Endothelial inflammation is the hallmark of vascular pathology often proceeding with cardiovascular diseases. Here, we adopted a multiparameter approach combining various imaging techniques at the nano- and microscale (Raman, AFM and fluorescence) to investigate endothelial inflammation in response to lipopolysaccharides (LPS) in vitro in human microvascular endothelial cells (HMEC-1) with a focus on lipid droplets (LDs) formation. Our results show that LPS-induced LDs in HMEC-1 have a composition depending on LPS-incubation time and their formation requires the presence of serum. Robust endothelial inflammation induced by LPS was linked to LDs composed of highly unsaturated lipids, as well as prostacyclin release. LPS-induced LDs were spatially associated with nanostructural changes in the cell membrane architecture. In summary, LDs formation represents an integral component of endothelial inflammation induced by LPS.

On the Determination of Mechanical Properties of Aqueous Microgels-Towards High-Throughput Characterization

Aqueous microgels are distinct entities of soft matter with mechanical signatures that can be different from their macroscopic counterparts due to confinement effects in the preparation, inherently made to consist of more than one domain (Janus particles) or further processing by coating and change in the extent of crosslinking of the core. Motivated by the importance of the mechanical properties of such microgels from a fundamental point, but also related to numerous applications, we provide a perspective on the experimental strategies currently available and emerging tools being explored. Albeit all techniques in principle exploit enforcing stress and observing strain, the realization differs from directly, as, e.g., by atomic force microscope, to less evident in a fluid field combined with imaging by a high-speed camera in high-throughput strategies. Moreover, the accompanying analysis strategies also reflect such differences, and the level of detail that would be preferred for a comprehensive understanding of the microgel mechanical properties are not always implemented. Overall, the perspective is that current technologies have the capacity to provide detailed, nanoscopic mechanical characterization of microgels over an extended size range, to the high-throughput approaches providing distributions over the mechanical signatures, a feature not readily accessible by atomic force microscopy and micropipette aspiration.

Biophysical nanocharacterization of liver sinusoidal endothelial cells through atomic force microscopy

The structural-functional hallmark of the liver sinusoidal endothelium is the presence of fenestrae grouped in sieve plates. Fenestrae are open membrane bound pores supported by a (sub)membranous cytoskeletal lattice. Changes in number and diameter of fenestrae alter bidirectional transport between the sinusoidal blood and the hepatocytes. Their physiological relevance has been shown in different liver disease models. Although the structural organization of fenestrae has been well documented using different electron microscopy approaches, the dynamic nature of those pores remained an enigma until the recent developments in the research field of four dimensional (4-D) AFM. In this contribution we highlight how AFM as a biophysical nanocharacterization tool enhanced our understanding in the dynamic behaviour of liver sinusoidal endothelial fenestrae. Different AFM probing approaches, including spectroscopy, enabled mapping of topography and nanomechanical properties at unprecedented resolution under live cell imaging conditions. This dynamic biophysical characterization approach provided us with novel information on the 'short' life-span, formation, disappearance and closure of hepatic fenestrae. These observations are briefly reviewed against the existing literature.

Actin-spectrin scaffold supports open fenestrae in liver sinusoidal endothelial cells

Fenestrae are open transmembrane pores that are a structural hallmark of healthy liver sinusoidal endothelial cells (LSECs). Their key role is the transport of solutes and macromolecular complexes between the sinusoidal lumen and the space of Disse. To date, the biochemical nature of the cytoskeleton elements that surround the fenestrae and sieve plates in LSECs remain largely elusive. Herein, we took advantage of the latest developments in atomic force imaging and super‐resolution fluorescence nanoscopy to define the organization of the supramolecular complex(es) that surround the fenestrae. Our data revealed that spectrin, together with actin, lines the inner cell membrane and provided direct structural support to the membrane‐bound pores. We conclusively demonstrated that diamide and iodoacetic acid (IAA) affect fenestrae number by destabilizing the LSEC actin‐spectrin scaffold. Furthermore, IAA induces rapid and repeatable switching between the open vs closed state of the fenestrae, indicating that the spectrin‐actin complex could play an important role in controlling the pore number. Our results suggest that spectrin functions as a key regulator in the structural preservation of the fenestrae, and as such, it might serve as a molecular target for altering transendothelial permeability.

Effect of selected B-ring-substituted oxysterols on artificial model erythrocyte membrane and isolated red blood cells

In this paper, systematic studies concerning the influence of selected oxysterols on the structure and fluidity of human erythrocyte membrane modeled as Langmuir monolayers have been performed. Three oxidized cholesterol derivatives, namely 7α-hydroxycholesterol (7α-OH) 7β-hydroxycholesterol (7β-OH) and 7-ketocholesterol (7-K) have been incorporated in two different proportions (10 and 50%) into artificial erythrocyte membrane, modeled as two-component (cholesterol:POPC) Langmuir monolayer. All the studied oxysterols were found to alter membrane fluidity and the effect was more pronounced for higher oxysterol content. 7α-OH increased membrane fluidity while opposite effect was observed for 7β-OH and 7-K. Experiments performed on model systems have been verified in biological studies on red blood cells (RBC). Consistent results have been found, i.e. under the influence of 7α-OH, the elasticity of erythrocytes increased, and in the presence of other investigated oxysterols - decreased. The strongest effect was noticed for 7-K. Change of membrane elasticity was associated with the change of erythrocytes shape, being most noticeable under the influence of 7-K.

Application of a layered model for determination of the elasticity of biological systems

Elasticity of biological systems is considered to be an important property that might be related to functional or pathological changes. Therefore, careful study and detailed understanding of cell and tissue elasticity is crucial for correct description of their functioning. Atomic Force Microscopy (AFM) is a powerful technique, which allows for determination of the physical properties, such as elasticity, of soft-matter systems in nano-scale. An important step in AFM elasticity studies is a proper interpretation of experimental data. Two most frequently used theoretical schemes applied to determine elasticity are due to Hertz and Sneddon, which are effectively one-parameter models. In this work, we go beyond this approach. Firstly, as elasticity is a local property, we extract from the slope of experimental force-indentation curve an elasticity parameter, which varies with indentation depth. Then secondly, we find best approximation of this parameter by applying the two-layer model with four effective parameters, as proposed by Kovalev. This method is employed to the experimental data taken on murine liver sinusoidal endothelial cells in non-alcoholic fatty liver disease model. The obtained results show additional effects, not seen within the traditional, simplified scheme. Namely, the elasticity of the first layer does not change its value in the model of non-alcoholic fatty liver disease, but the increase of stiffness is noticed in second layer. The second goal of this article is to reveal and discuss the differences between traditional approaches and the one being presented. The deviations from the original assumptions are analysed and the corresponding restrictions on utility of theoretical models are presented.

Biomechanical Characterization at the Cell Scale: Present and Prospects

The rapidly growing field of mechanobiology demands for robust and reproducible characterization of cell mechanical properties. Recent achievements in understanding the mechanical regulation of cell fate largely rely on technological platforms capable of probing the mechanical response of living cells and their physico–chemical interaction with the microenvironment. Besides the established family of atomic force microscopy (AFM) based methods, other approaches include optical, magnetic, and acoustic tweezers, as well as sensing substrates that take advantage of biomaterials chemistry and microfabrication techniques. In this review, we introduce the available methods with an emphasis on the most recent advances, and we discuss the challenges associated with their implementation.

Are elastic moduli of biological cells depth dependent or not? Another explanation using a contact mechanics model with surface tension

Atomic force microscopy (AFM) has become the most commonly used tool to measure the mechanical properties of biological cells. In AFM indentation experiments, the Hertz and Sneddon models of contact mechanics are usually adopted to extract the elastic modulus by analyzing the load-indent depth curves for spherical and conical tips, respectively. However, the effects of surface tension, neglected in existing contact models, become more significant in indentation responses due to the lower elastic moduli of living cells. Here, we present two simple yet robust relations between load and indent depth considering surface tension effects for spherical and conical indentations, through dimensional analysis and finite element simulations. When the indent depth is smaller than the intrinsic length defined as the ratio of surface tension to elastic modulus, the elastic modulus obtained by classical contact mechanics theories would be overestimated. Contrary to the majority of reported results, we find that the elastic modulus of a cell could be independent of indent depths if surface tension is taken into account. Our model seems to be in agreement with experimental data available. A comprehensive comparison will be done in the future.

Structure and elasticity of bush and brush-like models of the endothelial glycocalyx

The endothelial glycocalyx (EG), a sugar-rich layer that lines the luminal surface of blood vessels, is an important constituent of the vascular system. Although the chemical composition of the EG is fairly well known, there is no consensus regarding its ultrastructure. While previous experiments probed the properties of the layer at the continuum level, they did not provide sufficient insight into its molecular organisation. In this work, we investigate the EG mechanics using two simple brush and bush-like simulation models, and use these models to describe its molecular structure and elastic response to indentation. We analyse the relationship between the mechanical properties of the EG layer and several molecular parameters, including the filament bending rigidity, grafting density, and the type of ultrastructure . We show that variations in the glycan density determine the elasticity of the EG for small deformations, and that the normal stress may be effectively dampened by the EG layer, preventing the stress from being transferred to the cell membrane. Furthermore, our bush-like model allows us to evaluate the forces and energies required to overcome the mechanical resistance of the EG.