Regimes of Flow over Complex Structures of Endothelial Glycocalyx: A Molecular Dynamics Simulation Study

Role of blood flow in endothelial functionality: a review

Endothelial cells, located on the surface of blood vessel walls, are constantly stimulated by mechanical forces from the blood flow. The mechanical forces, i.e., fluid shear stress, induced by the blood flow play a pivotal role in controlling multiple physiological processes at the endothelium and in regulating various pathways that maintain homeostasis and vascular function. In this review, research looking at different blood fluid patterns and fluid shear stress in the circulation system is summarized, together with the interactions between the blood flow and the endothelial cells. This review also highlights the flow profile as a response to the configurational changes of the endothelial glycocalyx, which is less revisited in previous reviews. The role of endothelial glycocalyx in maintaining endothelium health and the strategies for the restoration of damaged endothelial glycocalyx are discussed from the perspective of the fluid shear stress. This review provides a new perspective regarding our understanding of the role that blood flow plays in regulating endothelial functionality.

Iatrogenic air embolism: pathoanatomy, thromboinflammation, endotheliopathy, and therapies

Iatrogenic vascular air embolism is a relatively infrequent event but is associated with significant morbidity and mortality. These emboli can arise in many clinical settings such as neurosurgery, cardiac surgery, and liver transplantation, but more recently, endoscopy, hemodialysis, thoracentesis, tissue biopsy, angiography, and central and peripheral venous access and removal have overtaken surgery and trauma as significant causes of vascular air embolism. The true incidence may be greater since many of these air emboli are asymptomatic and frequently go undiagnosed or unreported. Due to the rarity of vascular air embolism and because of the many manifestations, diagnoses can be difficult and require immediate therapeutic intervention. An iatrogenic air embolism can result in both venous and arterial emboli whose anatomic locations dictate the clinical course. Most clinically significant iatrogenic air emboli are caused by arterial obstruction of small vessels because the pulmonary gas exchange filters the more frequent, smaller volume bubbles that gain access to the venous circulation. However, there is a subset of patients with venous air emboli caused by larger volumes of air who present with more protean manifestations. There have been significant gains in the understanding of the interactions of fluid dynamics, hemostasis, and inflammation caused by air emboli due to in vitro and in vivo studies on flow dynamics of bubbles in small vessels. Intensive research regarding the thromboinflammatory changes at the level of the endothelium has been described recently. The obstruction of vessels by air emboli causes immediate pathoanatomic and immunologic and thromboinflammatory responses at the level of the endothelium. In this review, we describe those immunologic and thromboinflammatory responses at the level of the endothelium as well as evaluate traditional and novel forms of therapy for this rare and often unrecognized clinical condition.

Understanding the Role of Endothelial Glycocalyx in Mechanotransduction via Computational Simulation: A Mini Review

Endothelial glycocalyx (EG) is a forest-like structure, covering the lumen side of blood vessel walls. EG is exposed to the mechanical forces of blood flow, mainly shear, and closely associated with vascular regulation, health, diseases, and therapies. One hallmark function of the EG is mechanotransduction, which means the EG senses the mechanical signals from the blood flow and then transmits the signals into the cells. Using numerical modelling methods or in silico experiments to investigate EG-related topics has gained increasing momentum in recent years, thanks to tremendous progress in supercomputing. Numerical modelling and simulation allows certain very specific or even extreme conditions to be fulfilled, which provides new insights and complements experimental observations. This mini review examines the application of numerical methods in EG-related studies, focusing on how computer simulation contributes to the understanding of EG as a mechanotransducer. The numerical methods covered in this review include macroscopic (i.e., continuum-based), mesoscopic [e.g., lattice Boltzmann method (LBM) and dissipative particle dynamics (DPD)] and microscopic [e.g., molecular dynamics (MD) and Monte Carlo (MC) methods]. Accounting for the emerging trends in artificial intelligence and the advent of exascale computing, the future of numerical simulation for EG-related problems is also contemplated.

Cross talk between endothelial and red blood cell glycocalyces via near-field flow

Vascular endothelial cells and circulating red blood cell (RBC) surfaces are both covered by a layer of bushy glycocalyx. The interplay between these glycocalyx layers is hardly measurable and insufficiently understood. This study aims to investigate and qualify the possible interactions between the glycocalyces of RBCs and endothelial cells using mathematical modeling and numerical simulation. Dissipative particle dynamics (DPD) simulations are conducted to investigate the response of the endothelial glycocalyx (EG) to varying ambient conditions. A two-compartment model including EG and flow and a three-compartment model comprising EG, RBC glycocalyx, and flow are established. The two-compartment analysis shows that a relatively fast flow is associated with a predominantly bending motion of the EG, whereas oscillatory motions are predominant in a relatively slow flow. Results show that circulating RBCs cause the contactless deformation of EG. Its deformation is dependent on the chain layout, chain length, bending stiffness, RBC-to-EG distance, and RBC velocities. Specifically, shorter EG chains or RBC-to-EG distance leads to greater relative deflections of EG. Deformation of EG is enhanced when the EG chains are rarefied or RBCs move faster. The bending stiffness maintains stretching conformation of EG. Moreover, a compact EG chain layout and shedding EG chains disturb the neighboring flow field, causing disordered flow velocity distributions. In contrast, the movement of EG chains on RBC surfaces exerts a marginal driving force on RBCs. The DPD method is used for the first time, to our knowledge, in the three-compartment system to explore the cross talk between EG and RBC glycocalyx. This study suggests that RBCs drive the EG deformation via the near-field flow, whereas marginal propulsion of RBCs by the EG is observed. These new, to our knowledge, findings provide a new angle to understand the roles of glycocalyx in mechanotransduction and microvascular permeability and their perturbations under idealized pathophysiologic conditions associated with EG degradation.

Strategy to Chemically Decorate Nanopores of a Carbon Membrane for Filtrating Polyphenolics from Ethanol

This study invents a post-pyrolysis modification approach to render the resulting carbon membrane (CM) competent for organic solvent nanofiltration (OSN). A bitumen coating on a porous stainless-steel disk (PSD) serves as the precursor for the intended carbon membrane (CM), which is attained through pyrolysis in Ar. The bitumen coating casts dual-pore networks in the CM because of the dominant asphaltene constituent in bitumen. The subsequent chemical decoration of CM was pursued through the following protocol: dopamine (DA) was deployed in the nanopores of CM via pressurized infiltration and followed by Tris buffer passes through to trigger in situ conversion of DA to polydopamine (PDA), which was affixed over the pore walls to furnish chemical affinity (termed as CM_PDA). Additionally, the catechol moiety of PDA was arranged to chelate with the Zn²⁺ ion, aiming to trim the -OH anchor (termed as CM_PDA-Zn) to probe the effect of chelate on separation. The three membranes (CM, CM_PDA, and CM_PDA-Zn) were thereafter assessed by the separation of ethanol or isopropanol from phenolics [tannic acid (TA)/tetracycline (TC)]. A significantly improved OSN performance [rejection (%) ↔ permeance (L/(m²·h·bar))] of CM vs CM_PDA was observed: (i) for TA feed, 13% ↔ 85 L/(m²·h·bar) vs 83% ↔ 12 L/(m²·h·bar); and (ii) for TC feed, 20% ↔ 78 L/(m²·h·bar) vs 78% ↔ 12 L/(m²·h·bar). Compared to CM_PDA, CM_PDA-Zn further advances the rejection performance (∼89% for TA and ∼80% for TC) over 50 h separation. They are benchmarked by the latest literature results. The performance enhancements can be attributed to the spreading of PDA or PDA-Zn sites in the dual-pore networks, so that they are able to offer H-bonding and steric blocking roles, a chemicomechanical effect, to seize solute molecules over pore walls. It is this interfacial drag effect that sustains the solute rejection.

Liquid and Droplet Transport in Brush-Coated Cylindrical Nanochannels: Brush-Assisted Droplet Formation

We study, by coarse-grained molecular dynamics simulations, equilibrium and flow properties of a liquid in cylindrical nanochannels, coated with polymer brushes. The parameters of the interaction potential model confer a chemical incompatibility between brush monomers and liquid particles. First, we study cylindrical channels whose radii are larger than the brush height and a continuous column of liquid forms at the center of the channel. These results are contrasted to the limiting case in which the radius of the cylinder is comparable to the brush height. In this second case, the grafted polymers interact across the channel and "close" it. We observe a train of droplets as the stable liquid morphology. The droplet size is comparable to the cylinder radius. By applying a constant body force onto the liquid, we induce a Poiseuille-like flow and investigate the morphology and flow rate as a function of driving force. Upon increasing the driving force, we encounter a nonequilibrium transition from a closed channel with slowly moving droplets to a flowing liquid thread at the center. The switching between these two states is reversible.

Principal mode of Syndecan-4 mechanotransduction for the endothelial glycocalyx is a scissor-like dimer motion

AIM

Endothelial glycocalyx (EG) plays a pivotal role in a plethora of diseases, like cardiovascular and renal diseases. One hallmark function of the EG as a mechanotransducer which transmits mechanical signals into cytoplasm has been documented for decades. However, the basic question - how the glycocalyx transmits the flow shear stress- is unanswered so far. Our aim is to shed light on the fundamental mode of signal transmission from flow to the endothelial cytoskeleton.

METHODS

We conduct a series of large-scale molecular dynamics computational experiments to investigate the dynamics of glycocalyx under varying conditions (changing blood flow velocities and shedding of glycocalyx sugar chains).

RESULTS

We have identified that the main pathway of signal transmission in this system manifests as a scissors-like motion of the Syndecan-4 core protein. Results have suggested that the force transmitted into the cytoskeleton with an order of 10 ~ 100 pN, and the main function of sugar chains of a glycocalyx element is to protect the core proteins from severe conformational changes thereby maintaining the functionality of the EG.

CONCLUSION

This research provides a reconciling explanation for a longstanding debate about the force transmission threshold based on our findings. A new explanation has also been provided to relate the role of the EG as a mechanotransducer to its function as a microvascular barrier: the EG regulates the mechanotransduction by altering the median value and variation range of the scissor angle, and the EG governs the microvascular barrier via controlling the scissor angle which will affect the intercellular cleft.

Membrane Deformation of Endothelial Surface Layer Interspersed with Syndecan-4: A Molecular Dynamics Study

The lipid membrane of endothelial cells plays a pivotal role in maintaining normal circulatory system functions. To investigate the response of the endothelial cell membrane to changes in vascular conditions, an atomistic model of the lipid membrane interspersed with Syndecan-4 core protein was established based on experimental observations and a series of molecular dynamics simulations were undertaken. The results show that flow results in continuous deformation of the lipid membrane, and the degree of membrane deformation is not in monotonic relationship with the environmental changes (either the changes in blood velocity or the alteration of the core protein configuration). An explanation for such non-monotonic relationship is provided, which agrees with previous experimental results. The elevation of the lipid membrane surface around the core protein of the endothelial glycocalyx was also observed, which can be mainly attributed to the Coulombic interactions between the biomolecules therein. The present study demonstrates that the blood flow can deform the lipid membrane directly via the interactions between water molecules and lipid membrane atoms thereby affecting mechanosensing; it also presents an additional force transmission pathway from the flow to the lipid membrane via the glycocalyx core protein, which complements previous mechanotransduction hypothesis.

Microvascular ion transport through endothelial glycocalyx layer: new mechanism and improved Starling principle

Ion transport through the endothelial glycocalyx layer is closely associated with many vascular diseases. Clarification of ion behaviors around the endothelial glycocalyx layer under varying circumstances will benefit pathologies related to cardiovascular and renal diseases. In this research, a series of large-scale molecular dynamics simulations are conducted to study the response of ion transport to the changing blood flow velocity and the shedding of endothelial glycocalyx sugar chains. Results indicate that blood flow promotes the outward Na+ transport from the near-membrane region to the lumen via the endothelial glycocalyx layer. Scrutiny of sugar-chain dynamics and their interactions with Na+ suggests that corner conformation of endothelial glycocalyx sugar chains confines the movement of the Na+, whereas stretching conformation facilitates the motion of Na+ ions. The flow impact on ion transport of Na+ is nonlinear. Based on the findings, the Starling principle and its revised version, which are prevailingly used to predict the ion transport of the endothelial glycocalyx layer, are further improved. An estimation based on the further revised Starling principle indicates that physiological flow changes the osmotic part of transendothelial water flux by 8% compared with the stationary situation.

NEW & NOTEWORTHY The biophysical roles of negatively charged oligosaccharides of the endothelial glycocalyx have gained increasing attention due to their importance in regulating microvascular fluid exchange. The Starling principle and its revisions are at the heart of the understanding of fluid homeostasis in the periphery. Here, the blood flow changes the conformations of glycocalyx sugar chains, thereby influencing availability of Na+ for transport. Based on the findings, the Starling principle and its revision are further improved.

Reducing Salt Intake and Exercising Regularly: Implications From Molecular Dynamics Simulations of Endothelial Glycocalyx

It is widely accepted that salt intake reduction and regular exercise is a healthy lifestyle, which can prevent cardiovascular diseases (CVD). Meanwhile, there is evidence that the endothelial glycocalyx layer (EGL) is related to CVD. However, how such a healthy lifestyle helps to prevent CVD via the function of the EGL has not been scientifically established. In this research, a series of large-scale molecular dynamics simulations have been conducted to study ion transport inside the EGL under varying flow velocities. Results show that a fast blood flow velocity favors the Na⁺ transport out of the EGL, which can explain the increase in the thickness of an exclusion layer between red blood cells and the EGL under fast blood flow situations, as witnessed in some previous experiments. Based on findings from this fundamental research, a theory is proposed, which can answer the open-ended question “Why do we need to reduce salt intake and exercise regularly”. The findings may also have implications for other therapies to combat cardiovascular diseases.