Quantifying Endothelial Mechanotransmission

Abstract

Mechanotransduction of haemodynamic forces by endothelial cells is a focal determinant of atherosclerosis. Endothelial cells may be a potential target for treatment and diagnosis of atherosclerosis if more was understood about how flow-induced wall shear stress is transmitted throughout the cell. We have quantified mechanotransmission using a computational modelling approach that integrates haemodynamic boundary conditions, material properties of the cell, and cell morphology informed by experimental measurement. Special emphasis has been placed on the effect on mechanotransmission of both 1) endothelial cell morphology and its variation at a sub-cellular, cellular and population level; and, 2) the primary cilium, a sub-cellular organelle that is involved in the earlier stages of atherosclerosis. We considered the effect of morphological variation across a population on mechanotransmission using a generative approach: morphology was quantified using custom spatial descriptors with sufficient sophistication to recreate specific cell morphology. These descriptors were used to generate virtual cells with morphology characteristic of the trends seen in the whole population. The relationship between morphological variation and mechanotransmission was quantified: we found a six-fold increase in estimates of intracellular stress. This effect is of the same order of magnitude as inclusion of other subcellular components and organelle, hence population morphology substantially affects mechanotransmission. Because the primary cilium is associated with inflammatory cell responses to wall shear stress and the pathogenesis of atherosclerosis, particular emphasis was placed on this organelle. The role of the primary cilium as a force amplifier was investigated using a cell-specific image-informed ciliated model. Primary cilia substantially amplify mechanotransmission of wall shear stress, in the local region of the cilia. A smaller force amplification effect also occurs in the region more than 5 mm away from the base of the cilium. We report the presence of cilia for the first time in the human microvascular endothelial cell (HMEC-1) type. Additionally, we identified optimal in vitro models for studying endothelial primary cilia in the micro-vasculature (HMEC-1s grown for 5 days post confluence in high serum) and in large vessels (human umbilical vein endothelial cells (HUVECs), grown in high serum). Alternatively, low serum HUVECs grown to confluence are an ideal cell model for non-ciliated endothelial studies: ciliogenesis is suppressed without the need for chemical agents or mutant knock-out models. We quantified the morphological variation across the population of the cytoskeleton and primary cilia using a generative approach to enable inclusion of these components into future mechanical analyses. EC mechanotransmission and mechanotransduction of wall shear stress are complex processes due to the interplay of mechanical forces, biochemical signalling and gene and protein network activation. We present a framework for integrating a wide range experimental data with multi-scale modelling to understand EC mechanotransmission and mechanotransduction of wall shear stress. Future modelling directions are also discussed, as are the major challenges needed to integrate primary cilium models into a multicomponent EC mechanical model. These comprehensive computational and experimental studies have provided new insight on the role of EC subcellular components in mechanotransmission and the processes of mechanotransmission and mechanotransduction of wall shear stress. In doing so, this research has contributed to the long term goal of understanding the cellular basis of atherosclerosis.