Computational models of the primary cilium and endothelial mechanotransmission

Wall Shear Stress Alteration: a Local Risk Factor of Atherosclerosis

PURPOSE OF REVIEW

Wall shear stress describes the mechanical influence of blood flow on the arterial wall. In this review, we discuss the role of the wall shear stress in the development of atherosclerosis and its complications.

RECENT FINDINGS

Areas with chronically low, oscillating wall shear stress are most prone to plaque development and include outer bifurcation walls and inner walls of arches. In some diseases, patients have lower wall shear stress even in straight arterial segments; also, these findings were associated with atherosclerosis. High wall shear stress develops in the distal part (shoulder) of a stenosis and contributes to plaque destabilization. Wall shear stress changes are involved in the development of atherosclerosis. They are not fully understood yet and act in concert with tangential wall stress.

Primary cilium: a paradigm for integrating mathematical modeling with experiments and numerical simulations in mechanobiology

Primary cilia are non-motile, solitary (one per cell) microtubule-based organelles that emerge from the mother centriole after cells have exited the mitotic cycle. Identified as a mechanosensing organelle that responds to both mechanical and chemical stimuli, the primary cilium provides a fertile ground for integrative investigations of mathematical modeling, numerical simulations, and experiments. Recent experimental findings revealed considerable complexity to the underlying mechanosensory mechanisms that transmit extracellular stimuli to intracellular signaling many of which include primary cilia. In this invited review, we provide a brief survey of experimental findings on primary cilia and how these results lead to various mathematical models of the mechanics of the primary cilium bent under an external forcing such as a fluid flow or a trap. Mathematical modeling of the primary cilium as a fluid-structure interaction problem highlights the importance of basal anchorage and the anisotropic moduli of the microtubules. As theoretical modeling and numerical simulations progress, along with improved state-of-the-art experiments on primary cilia, we hope that details of ciliary regulated mechano-chemical signaling dynamics in cellular physiology will be understood in the near future.

Fluid Shear Stress Sensing by the Endothelial Layer

Blood flow produces mechanical frictional forces, parallel to the blood flow exerted on the endothelial wall of the vessel, the so-called wall shear stress (WSS). WSS sensing is associated with several vascular pathologies, but it is first a physiological phenomenon. Endothelial cell sensitivity to WSS is involved in several developmental and physiological vascular processes such as angiogenesis and vascular morphogenesis, vascular remodeling, and vascular tone. Local conditions of blood flow determine the characteristics of WSS, i.e., intensity, direction, pulsatility, sensed by the endothelial cells that, through their effect of the vascular network, impact WSS. All these processes generate a local-global retroactive loop that determines the ability of the vascular system to ensure the perfusion of the tissues. In order to account for the physiological role of WSS, the so-called shear stress set point theory has been proposed, according to which WSS sensing acts locally on vessel remodeling so that WSS is maintained close to a set point value, with local and distant effects of vascular blood flow. The aim of this article is (1) to review the existing literature on WSS sensing involvement on the behavior of endothelial cells and its short-term (vasoreactivity) and long-term (vascular morphogenesis and remodeling) effects on vascular functioning in physiological condition; (2) to present the various hypotheses about WSS sensors and analyze the conceptual background of these representations, in particular the concept of tensional prestress or biotensegrity; and (3) to analyze the relevance, explanatory value, and limitations of the WSS set point theory, that should be viewed as dynamical, and not algorithmic, processes, acting in a self-organized way. We conclude that this dynamic set point theory and the biotensegrity concept provide a relevant explanatory framework to analyze the physiological mechanisms of WSS sensing and their possible shift toward pathological situations.

Primary cilia respond to intermittent low-magnitude, high-frequency vibration and mediate vibration-induced effects in osteoblasts

Our objective was to investigate the role of primary cilia in low-magnitude, high-frequency vibration (LMHFV) treatment of MC3T3-E1 osteoblasts (OBs). We used chloral hydrate (CH), which has a well-characterized function in chemically removing primary cilia, to elucidate the role of primary cilia in LMHFV-induced OB osteogenic responses through cell viability assay, Western blot analysis, real-time quantitative RT-PCR, and histochemical staining methods. We observed a significant, 30% decrease in the number of MC3T3-E1 OBs with primary cilia (reduced from 64.3 ± 5%) and an approximately 50% reduction in length of primary cilia (reduced from 3 ± 0.8 μm) after LMHFV stimulation. LMHFV stimulation upregulated protein expression of the bone matrix markers collagen 1 (COL-1), osteopontin (OPN), and osteoclacin(OCN) in MC3T3-E1 OBs, indicating that LMHFV induces osteogenesis. High-concentration or long-duration CH exposure resulted in inhibition of MC3T3-E1 OB survival. In addition, Western blot analysis and RT-PCR revealed that CH treatment prevented LMHFV-induced osteogenesis. Furthermore, decreased alkaline phosphate activity, reduced OB differentiation, mineralization, and maturation were observed in CH-pretreated and LMHFV-treated OBs. We showed that LMHFV induces morphological changes in primary cilia that may fine-tune their mechanosensitivity. In addition, we demonstrated the significant inhibition by CH of LMHFV-induced OB mineralization, maturation, and differentiation, which might reveal the critical role of primary cilia in the process.

Understanding the Causes and Implications of Endothelial Metabolic Variation in Cardiovascular Disease through Genome-Scale Metabolic Modeling

High-throughput biochemical profiling has led to a requirement for advanced data interpretation techniques capable of integrating the analysis of gene, protein, and metabolic profiles to shed light on genotype-phenotype relationships. Herein, we consider the current state of knowledge of endothelial cell (EC) metabolism and its connections to cardiovascular disease (CVD) and explore the use of genome-scale metabolic models (GEMs) for integrating metabolic and genomic data. GEMs combine gene expression and metabolic data acting as frameworks for their analysis and, ultimately, afford mechanistic understanding of how genetic variation impacts metabolism. We demonstrate how GEMs can be used to investigate CVD-related genetic variation, drug resistance mechanisms, and novel metabolic pathways in ECs. The application of GEMs in personalized medicine is also highlighted. Particularly, we focus on the potential of GEMs to identify metabolic biomarkers of endothelial dysfunction and to discover methods of stratifying treatments for CVDs based on individual genetic markers. Recent advances in systems biology methodology, and how these methodologies can be applied to understand EC metabolism in both health and disease, are thus highlighted.