Effects of membrane viscoelasticity on the red blood cell dynamics in a microcapillary

Actin instability alters red blood cell mechanics and Piezo1 channel activity

The organization and dynamics of the spectrin-actin membrane cytoskeleton play a crucial role in determining the mechanical properties of red blood cells (RBC). RBC are subjected to various forces that induce deformation during blood microcirculation. Such forces also regulate membrane tension, leading to Piezo1 channel activation, which is functionally linked to RBC dehydration through calcium influx and subsequent activation of Gardos channels, ultimately resulting in variations in RBC volume. In this study, we investigated how actin instability affects Piezo1 channel gating, in relation to RBC deformation and mechanical properties, using micropipette aspiration and optical tweezers. Actin instability, induced by 0.5 μM Cytochalasin-D (Cyt-D), led to a 22% reduction in the activation pressure. Additionally, we observed a decreasing trend in Young's modulus, membrane tension, and viscosity. By measuring the time required for cell shape recovery after deformation in an optical trap, we found that Cyt-D-treated RBC took approximately 14% longer to recover compared to untreated cells. The bimodal imaging feature of our experimental approach allowed us to simultaneously measure and correlate activation pressure with mechanical properties at the single-cell level. A significant correlation was found between these parameters in both treated and untreated RBC. Our findings demonstrate the influence of actin instability on both Piezo1 activation and RBC mechanics. These results offer new insights into the interplay between F-actin and Piezo1 in RBC mechanobiology.

Escherichia coli α-hemolysin induces red blood cell retention in a microfluidic spleen-like device

α-hemolysin (HlyA) is a major exotoxin secreted by uropathogenic Escherichia coli (UPEC), known for its ability to lyse red blood cells (RBCs). Although its lytic effects are well characterized, the nonlytic alterations on RBCs, such as increased permeability to Ca2+, osmotic imbalance, and morphological alterations, remain less understood and may be critical in UPEC pathogenesis. This study investigates the impact of these nonlytic alterations on the rheology and mechanics of RBCs using two biomimetic microfluidic devices that model key aspects of RBCs' circulation. In the first device, which mimics the mechanical deformation of RBCs in narrow capillaries, HlyA sublytic concentrations were found to significantly impair RBC deformability. These changes were accompanied by an increase in cytosolic Ca2+ and volume expansion. In contrast, the nonacylated protoxin ProHlyA neither impaired the deformability of RBCs nor triggered changes in cytosolic Ca2+ or cell volume. The second device, which simulates the RBCs' filtration by the spleen's red pulp, revealed that HlyA, but not ProHlyA, increased RBCs' retention in small gaps resembling splenic fenestrations. The extent of RBCs' retention was partially mitigated by blocking purinergic signaling, indicating a contribution of the HlyA-induced volume increase in this process. Our results suggest that the increase in cytosolic Ca2+ elicited by HlyA impacts RBCs' circulation by decreasing RBCs' deformability and increasing spleen retention. However, this impairment of RBCs' performance can function as a defense mechanism to aid in the retention of HlyA-bound RBCs, removing them from circulation, and potentially preventing vascular hemolysis.

Endothelial calcium dynamics elicited by ATP release from red blood cells

Red blood cells (RBCs) exhibit an interesting response to hydrodynamic flow, releasing adenosine triphosphate (ATP). Subsequently, these liberated ATP molecules initiate a crucial interaction with endothelial cells (ECs), thereby setting off a cascade involving the release of calcium ions (Ca2 + ). Ca2 + exerts control over a plethora of cellular functions, and acts as a mediator for dilation and contraction of blood vessel walls. This study focuses on the relationship between RBC dynamics and Ca2 + dynamics, based on numerical simulations under Poiseuille flow within a linear two-dimensional channel. It is found that the concentration of ATP depends upon a variety of factors, including RBC density, channel width, and the vigor of the flow. The results of our investigation reveals several features. Firstly, the peak amplitude of Ca2 + per EC escalates in direct proportion to the augmentation of RBC concentration. Secondly, increasing the flow strength induces a reduction in the time taken to reach the peak of Ca2 + concentration, under the condition of a constant channel width. Additionally, when flow strength remains constant, an increase in channel width corresponds to an elevation in calcium peak amplitude, coupled with a decrease in peak time. This implies that Ca2 + signals should transition from relatively unconstrained channels to more confined pathways within real vascular networks. This notion gains support from our examination of calcium propagation in a linear channel. In this scenario, the localized Ca2 + release initiates a propagating wave that gradually encompasses the entire channel. Notably, our computed propagation speed agrees with observations.

Confinement effect on the microcapillary flow and shape of red blood cells

The ability to change shape is essential for the proper functioning of red blood cells (RBCs) within the microvasculature. The shape of RBCs significantly influences blood flow and has been employed in microfluidic lab-on-a-chip devices, serving as a diagnostic biomarker for specific pathologies and enabling the assessment of RBC deformability. While external flow conditions, such as the vessel size and the flow velocity, are known to impact microscale RBC flow, our comprehensive understanding of how their shape-adapting ability is influenced by channel confinement in biomedical applications remains incomplete. This study explores the impact of various rectangular and square channels, each with different confinement and aspect ratios, on the in vitro RBC flow behavior and characteristic shapes. We demonstrate that rectangular microchannels, with a height similar to the RBC diameter in combination with a confinement ratio exceeding 0.9, are required to generate distinctive well-defined croissant and slipper-like RBC shapes. These shapes are characterized by their equilibrium positions in the channel cross section, and we observe a strong elongation of both stable shapes in response to the shear rate across the different channels. Less confined channel configurations lead to the emergence of unstable other shape types that display rich shape dynamics. Our work establishes an experimental framework to understand the influence of channel size on the single-cell flow behavior of RBCs, providing valuable insights for the design of biomicrofluidic single-cell analysis applications.

Hydrodynamics of a multicomponent vesicle under strong confinement

We numerically investigate the hydrodynamics and membrane dynamics of a multicomponent vesicle in two strongly confined geometries. This serves as a simplified model for red blood cells undergoing large deformations while traversing narrow constrictions. We propose a new parameterization for the bending modulus that remains positive for all lipid phase parameter values. For a multicomponent vesicle passing through a stenosis, we establish connections between various properties: lipid phase coarsening, size and flow profile of the lubrication layers, excess pressure, and the tank-treading velocity of the membrane. For a multicomponent vesicle passing through a contracting channel, we find that the lipid always phase separates so that the vesicle is stiffer in the front as it passes through the constriction. For both cases of confinement we find that lipid coarsening is arrested under strong confinement, and resumes at a high rate upon relief from extreme confinement. The results may be useful for efficient sorting lipid domains using microfluidic flows by controlled release of vesicles passing through strong confinement.

Effect of Cell Age and Membrane Rigidity on Red Blood Cell Shape in Capillary Flow

Blood flow in the microcirculatory system is crucially affected by intrinsic red blood cell (RBC) properties, such as their deformability. In the smallest vessels of this network, RBCs adapt their shapes to the flow conditions. Although it is known that the age of RBCs modifies their physical properties, such as increased cytosol viscosity and altered viscoelastic membrane properties, the evolution of their shape-adapting abilities during senescence remains unclear. In this study, we investigated the effect of RBC properties on the microcapillary in vitro flow behavior and their characteristic shapes in microfluidic channels. For this, we fractioned RBCs from healthy donors according to their age. Moreover, the membranes of fresh RBCs were chemically rigidified using diamide to study the effect of isolated graded-membrane rigidity. Our results show that a fraction of stable, asymmetric, off-centered slipper-like cells at high velocities decreases with increasing age or diamide concentration. However, while old cells form an enhanced number of stable symmetric croissants at the channel centerline, this shape class is suppressed for purely rigidified cells with diamide. Our study provides further knowledge about the distinct effects of age-related changes of intrinsic cell properties on the single-cell flow behavior of RBCs in confined flows due to inter-cellular age-related cell heterogeneity.