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

Early and late phases of liver sinusoidal endothelial cell (LSEC) defenestration in mouse model of systemic inflammation

BACKGROUND

Liver sinusoidal endothelial cells (LSECs) have transcellular pores, called fenestrations, participating in the bidirectional transport between the vascular system and liver parenchyma. Fenestrated LSECs indicate a healthy phenotype of liver while loss of fenestrations (defenestration) in LSECs is associated with liver pathologies.

METHODS

We introduce a unique model of systemic inflammation triggered by the deletion of Mcpip1 in myeloid leukocytes (Mcpip1fl/flLysMCre) characterised by progressive alterations in LSEC phenotype. We implement multiparametric characterisation of LSECs by using novel real-time atomic force microscopy supported with scanning electron microscopy and quantitative fluorescence microscopy. In addition, we provide genetic profiling, searching for characteristic genes encoding proteins that might be connected with the structure of fenestrations.

RESULTS

We demonstrate that LSECs in Mcpip1fl/flLysMCre display two phases of defenestration: the early phase, with modest defenestration that was fully reversible using cytochalasin B and the late phase, with severe defenestration that is mostly irreversible. By thorough analysis of LSEC porosity, elastic modulus and actin abundance in Mcpip1fl/flLysMCre and in response to cytochalasin B, we demonstrate that proteins other than actin must be additionally responsible for inducing open fenestrations. We highlight several genes that were severely affected in the late but not in the early phase of LSEC defenestration shedding a light on complex structure of individual fenestrations.

CONCLUSIONS

The presented model of LSEC derived from Mcpip1fl/flLysMCre provides a valuable reference for developing novel strategies for LSEC refenestration in the early and late phases of liver pathology.

Atomic Force Microscopy-Based Measurements of Retinal Microvessel Stiffness in Mice with Endothelial-Specific Deletion of CCN1

Vascular stiffness is an independent predictor of human vascular diseases and is linked to ischemia, diabetes, high blood pressure, hyperlipidemia, and/or aging. Blood vessel stiffening increases owing to changes in the microscale architecture and/or content of extracellular, cytoskeletal, and nuclear matrix proteins. These alterations, while best appreciated in large blood vessels, also gradually occur in the microvasculature and play an important role in the initiation and progression of numerous microangiopathies including diabetic retinopathy. Although macroscopic measurements of arterial stiffness by pulse wave velocity are often used for clinical diagnosis, stiffness changes of intact microvessels and their causative factors have not been characterized. Herein, we describe the use of atomic force microscopy (AFM) to determine stiffness of mouse retinal capillaries and assess its regulation by the cellular communication network (CCN) 1, a stiffness-sensitive gene-encoded matricellular protein. AFM yields reproducible measurements of retinal capillary stiffness in lightly fixed freshly isolated retinal flat mounts. AFM measurements also show significant changes in compliance properties of the retinal microvasculature of mice with endothelial-specific deletion of CCN1, indicating that CCN1 expression, or lack thereof, affects the mechanical properties of microvascular cells in vivo. Thus, AFM has the force sensitivity and the spatial resolution necessary to measure the local modulus of retinal capillaries in situ and eventually to investigate microvascular compliance heterogeneities as key components of disease pathogenesis.

An explicit model to extract viscoelastic properties of cells from AFM force-indentation curves

Atomic force microscopy (AFM) is widely used for quantifying the mechanical properties of soft materials such as cells. AFM force-indentation curves are conventionally fitted with a Hertzian model to extract elastic properties. These properties solely are, however, insufficient to describe the mechanical properties of cells. Here, we expand the analysis capabilities to describe the viscoelastic behavior while using the same force-indentation curves. Our model gives an explicit relation of force and indentation and extracts physically meaningful mechanical parameters. We first validated the model on simulated force-indentation curves. Then, we applied the fitting model to the force-indentation curves of two hydrogels with different crosslinking mechanisms. Finally, we characterized HeLa cells in two cell cycle phases, interphase and mitosis, and showed that mitotic cells have a higher apparent elasticity and a lower apparent viscosity. Our study provides a simple method, which can be directly integrated into the standard AFM framework for extracting the viscoelastic properties of materials.

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Simple mechanical model to describe viscoelastic properties of soft matter

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A model fitted directly to force-indentation curves

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Capturing the distinct nature of hydrogels crosslinked in different mechanisms

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Comparing viscoelastic properties of cells in interphase and mitotic states

Endothelial glycocalyx detection and characterization by means of atomic force spectroscopy: Comparison of various data analysis approaches

In recent years, atomic force spectroscopy (AFS) has been used to detect and characterize the endothelial glycocalyx (eGlx) in in vitro and ex vivo experiments. Several analysis methods were proposed, which differ not only in the numerical implementations, but also in physical models of glycocalyx description. Therefore, it is difficult to directly relate the experiments performed by different groups. In this work, we compared different models used for quantitative analysis of atomic force spectroscopy datasets recorded for eGlx. To capture glycocalyx at various structural conditions, we used basic enzymatic protocols for glycocalyx removal and restoration in human aortal endothelial cells (HAEC). Nanoindentation experiments for this model system were performed for (i) untreated cells, (ii) for cells after heparinase incubation, which enzymatically removes glycocalyx, (iii) for cells with successive heparin treatment, which partially restores the glycocalyx layer. Analysis of nanoindentation data was performed using different models: (a) a single-layer contact mechanics, (b) a double-layer model contact mechanics, (c) a polymer "brush" two-layer model based on the Alexander - de Gennes theory and (d) a simple single-layer "mechanical spring" model. Although different physical parameters are evaluated in methods (a-d), we show that all approaches revealed similar qualitative changes of the glycocalyx layer, which reflected the processes of glycocalyx degradation and its partial restoration. This paper may facilitate a direct comparison of past and future glycocalyx oriented AFS experiments that are analysed with different approaches.

Passive myocardial mechanical properties: meaning, measurement, models

Passive mechanical tissue properties are major determinants of myocardial contraction and relaxation and, thus, shape cardiac function. Tightly regulated, dynamically adapting throughout life, and affecting a host of cellular functions, passive tissue mechanics also contribute to cardiac dysfunction. Development of treatments and early identification of diseases requires better spatio-temporal characterisation of tissue mechanical properties and their underlying mechanisms. With this understanding, key regulators may be identified, providing pathways with potential to control and limit pathological development. Methodologies and models used to assess and mimic tissue mechanical properties are diverse, and available data are in part mutually contradictory. In this review, we define important concepts useful for characterising passive mechanical tissue properties, and compare a variety of in vitro and in vivo techniques that allow one to assess tissue mechanics. We give definitions of key terms, and summarise insight into determinants of myocardial stiffness in situ. We then provide an overview of common experimental models utilised to assess the role of environmental stiffness and composition, and its effects on cardiac cell and tissue function. Finally, promising future directions are outlined.

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.

Sensing and Modelling Mechanical Response in Large Deformation Indentation of Adherent Cell Using Atomic Force Microscopy

The mechanical behaviour of adherent cells when subjected to the local indentation can be modelled via various approaches. Specifically, the tensegrity structure has been widely used in describing the organization of discrete intracellular cytoskeletal components, including microtubules (MTs) and microfilaments. The establishment of a tensegrity model for adherent cells has generally been done empirically, without a mathematically demonstrated methodology. In this study, a rotationally symmetric prism-shaped tensegrity structure is introduced, and it forms the basis of the proposed multi-level tensegrity model. The modelling approach utilizes the force density method to mathematically assure self-equilibrium. The proposed multi-level tensegrity model was developed by densely distributing the fundamental tensegrity structure in the intracellular space. In order to characterize the mechanical behaviour of the adherent cell during the atomic force microscopy (AFM) indentation with large deformation, an integrated model coupling the multi-level tensegrity model with a hyperelastic model was also established and applied. The coefficient of determination between the computational force-distance (F-D) curve and the experimental F-D curve was found to be at 0.977 in the integrated model on average. In the simulation range, along with the increase in the overall deformation, the local stiffness contributed by the cytoskeletal components decreased from 75% to 45%, while the contribution from the hyperelastic components increased correspondingly.