Metabolic remodeling of the human red blood cell membrane

Red blood cell flickering activity locally controlled by holographic optical tweezers

Red blood cells possess a singular mechanobiology, enabling efficient navigation through capillaries smaller than their own size. Their plasma membrane exhibits non-equilibrium shape fluctuation, often reported as enhanced flickering activity. Such active membrane motion is propelled by motor proteins that mediate interactions between the spectrin skeleton and the lipid bilayer. However, modulating the flickering in living red blood cells without permanently altering their mechanical properties represents a significant challenge. In this study, we developed holographic optical tweezers to generate a force field distributed along the equatorial membrane contour of individual red blood cells. In free-standing red blood cells, we observed heterogeneous flickering activity, attributed to localized membrane kickers. By employing holographic optical forces, these active kickers can be selectively halted under minimal invasion. Our findings shed light on the dynamics of membrane flickering and established a manipulation tool that could open new avenues for investigating mechanotransduction processes in living cells.

Recent developments in the use of pyruvate kinase activators as a new approach for treating sickle cell disease

ABSTRACT

Pyruvate kinase (PK) is a key enzyme in glycolysis, the sole source of adenosine triphosphate, which is essential for all energy-dependent activities of red blood cells. Activating PK shows great potential for treating a broad range of hemolytic anemias beyond PK deficiency, because they also enhance activity of wild-type PK. Motivated by observations of sickle-cell complications in sickle-trait individuals with concomitant PK deficiency, activating endogenous PK offers a novel and promising approach for treating patients with sickle-cell disease.

Measuring sub-nanometer undulations at microsecond temporal resolution with metal- and graphene-induced energy transfer spectroscopy

Out-of-plane fluctuations, also known as stochastic displacements, of biological membranes play a crucial role in regulating many essential life processes within cells and organelles. Despite the availability of various methods for quantifying membrane dynamics, accurately quantifying complex membrane systems with rapid and tiny fluctuations, such as mitochondria, remains a challenge. In this work, we present a methodology that combines metal/graphene-induced energy transfer (MIET/GIET) with fluorescence correlation spectroscopy (FCS) to quantify out-of-plane fluctuations of membranes with simultaneous spatiotemporal resolution of approximately one nanometer and one microsecond. To validate the technique and spatiotemporal resolution, we measure bending undulations of model membranes. Furthermore, we demonstrate the versatility and applicability of MIET/GIET-FCS for studying diverse membrane systems, including the widely studied fluctuating membrane system of human red blood cells, as well as two unexplored membrane systems with tiny fluctuations, a pore-spanning membrane, and mitochondrial inner/outer membranes.

Phosphoproteomics and morphology of stored human red blood cells treated by protein tyrosine phosphatases inhibitor

ABSTRACT

The process of protein phosphorylation is involved in numerous cell functions. In particular, phosphotyrosine (pY) has been reported to play a role in red blood cell (RBC) functions, including the cytoskeleton organization. During their storage before transfusion, RBCs suffer from storage lesions that affect their energy metabolism and morphology. This study investigated the relationship between pY and the storage lesions. To do so, RBCs were treated (in the absence of calcium) with a protein tyrosine phosphatase inhibitor (orthovanadate [OV]) to stimulate phosphorylation and with 3 selective kinase inhibitors (KIs). Erythrocyte membrane proteins were studied by western blot analyses and phosphoproteomics (data are available via ProteomeXchange with identifier PXD039914) and cell morphology by digital holographic microscopy. The increase of pY triggered by OV treatment (inducing a global downregulation of pS and pT) disappeared during the storage. Phosphoproteomic analysis identified 609 phosphoproteins containing 1752 phosphosites, of which 41 pY were upregulated and 2 downregulated by OV. After these phosphorylation processes, the shape of RBCs shifted from discocytes to spherocytes, and the addition of KIs partially inhibited this transition. The KIs modulated either pY or pS and pT via diverse mechanisms related to cell shape, thereby affecting RBC morphology. The capacity of RBCs to maintain their function is central in transfusion medicine, and the presented results contribute to a better understanding of RBC biology.

Cell membrane biomimetic nanoparticles in drug delivery

Despite the efficiency of nanoparticle (NP) therapy, in vivo investigations have shown that it does not perform as well as in vitro. In this case, NP confronts many defensive hurdles once they enter the body. The delivery of NP to sick tissue is inhibited by these immune-mediated clearance mechanisms. Hence, using a cell membrane to hide NP for active distribution offers up a new path for focused treatment. These NPs are better able to reach the disease's target location, leading to enhanced therapeutic efficacy. In this emerging class of drug delivery vehicles, the inherent relation between the NPs and the biological components obtained from the human body was utilized, which mimic the properties and activities of native cells. This new technology has shown the viability of using biomimicry to evade immune system-provided biological barriers, with an emphasis on restricting clearance from the body before reaching its intended target. Furthermore, by providing signaling cues and transplanted biological components that favorably change the intrinsic immune response at the disease site, the NPs would be capable interacting with immune cells regarding the biomimetic method. Thus, we aimed to provide a current landscape and future trends of biomimetic NPs in drug delivery.

Non-Markovian Modeling of Nonequilibrium Fluctuations and Dissipation in Active Viscoelastic Biomatter

Based on a Hamiltonian that incorporates the elastic coupling between a tracer particle and the embedding active viscoelastic biomatter, we derive a generalized non-Markovian Langevin model for the nonequilibrium mechanical tracer response. Our analytical expressions for the frequency-dependent tracer response function and the tracer positional autocorrelation function agree quantitatively with experimental data for red blood cells and actomyosin networks with and without adenosine triphosphate over the entire frequency range and in particular reproduce the low-frequency violation of the fluctuation-dissipation theorem. The viscoelastic power laws, the elastic constants and effective friction coefficients extracted from the experimental data allow straightforward physical interpretation.

Effective cell membrane tension protects red blood cells against malaria invasion

A critical step in how malaria parasites invade red blood cells (RBCs) is the wrapping of the membrane around the egg-shaped merozoites. Recent experiments have revealed that RBCs can be protected from malaria invasion by high membrane tension. While cellular and biochemical aspects of parasite actomyosin motor forces during the malaria invasion have been well studied, the important role of the biophysical forces induced by the RBC membrane-cytoskeleton composite has not yet been fully understood. In this study, we use a theoretical model for lipid bilayer mechanics, cytoskeleton deformation, and membrane-merozoite interactions to systematically investigate the influence of effective RBC membrane tension, which includes contributions from the lipid bilayer tension, spontaneous tension, interfacial tension, and the resistance of cytoskeleton against shear deformation on the progression of membrane wrapping during the process of malaria invasion. Our model reveals that this effective membrane tension creates a wrapping energy barrier for a complete merozoite entry. We calculate the tension threshold required to impede the malaria invasion. We find that the tension threshold is a nonmonotonic function of spontaneous tension and undergoes a sharp transition from large to small values as the magnitude of interfacial tension increases. We also predict that the physical properties of the RBC cytoskeleton layer-particularly the resting length of the cytoskeleton-play key roles in specifying the degree of the membrane wrapping. We also found that the shear energy of cytoskeleton deformation diverges at the full wrapping state, suggesting the local disassembly of the cytoskeleton is required to complete the merozoite entry. Additionally, using our theoretical framework, we predict the landscape of myosin-mediated forces and the physical properties of the RBC membrane in regulating successful malaria invasion. Our findings on the crucial role of RBC membrane tension in inhibiting malaria invasion can have implications for developing novel antimalarial therapeutic or vaccine-based strategies.

Flickering of fetal erythrocytes membrane under gestational diabetes observed with dual time resolved membrane fluctuation spectroscopy

The membrane flickering of human fetal red blood cells (RBCs) affected by gestational diabetes mellitus (GDM) was studied with dual time resolved membrane fluctuation spectroscopy (D-TRMFS). This new technique is a modified version of the dual optical tweezers method that has been adapted to measure the mechanical properties of RBCs at two distant membrane points simultaneously. The micro-rheological parameters were obtained from direct membrane flickering measurements, followed by Fourier decomposition and cell membrane model adjustment. Our results show a significant decrease of 6.01 ± 1.19 nm in membrane fluctuations amplitude in healthy fetal, compared with healthy adult RBCs, meanwhile the amplitude in GDM cells increased 3.22 ± 1.10 nm compared with healthy fetal RBCs. Between GDM and healthy fetal RBCs, there are significant differences, especially in the bending modulus. Considering the mean of the two membrane points measured, the tension for GDM RBCs increased by 6.431 ± 3.57 (10-7 [N/m]) compared with healthy fetal RBCs, meanwhile, the bending was increased by 2.483 ± 0.58 (10-19 [J]) in GDM compared with healthy fetal RBCs. These results showed significant increment of 1.23 ± 0.07-fold and 3.29 ± 0.36-fold in tension and bending modulus in GDM, respectively. The strong impact of GDM on bending modulus could be associated with oxidative stress and lipid peroxidation, previously reported in fetal plasma of GDM cases.

Ligand Binding-Induced Cellular Membrane Deformation is Correlated with the Changes in Membrane Stiffness

Study interaction between ligands and protein receptors is a key step for biomarker research and drug discovery. In situ measurement of cell surface membrane protein binding on whole cells eliminates the cost and pitfalls associated with membrane protein purification. Ligand binding to membrane protein was recently found to induce nanometer-scale cell membrane deformations, which can be monitored with real-time optical imaging to quantify ligand/protein binding kinetics. However, the insight into this phenomenon has still not been fully understood. We hypothesize that ligand binding can change membrane stiffness, which induces membrane deformation. To investigate this, cell height and membrane stiffness changes upon ligand binding are measured using atomic force microscopy (AFM). Wheat germ agglutinin (WGA) is used as a model ligand that binds to the cell surface glycoprotein. The changes in cell membrane stiffness and cell height upon ligand bindings are determined for three different cell lines (human A431, HeLa, and rat RBL-2H3) on two different substrates. AFM results show that cells become stiffer with increased height after WGA modification for all cases studied. The increase in cell membrane stiffness is further confirmed by plasmonic scattering microscopy, which shows an increased cell spring constant upon WGA binding. Therefore, this study provides direct experimental evidence that the membrane stiffness changes are directly correlated with ligand binding-induced cell membrane deformation.

Trapped tracer in a non-equilibrium bath: dynamics and energetics

We study the dynamics of a tracer that is elastically coupled to active particles being kept at two different temperatures, as a prototype of tracer dynamics in a non-equilibrium bath. Employing analytical techniques, we find the exact solution of the probability density function for the effective motion of the tracer. The analytical results are supported by numerical simulations. By combining the experimentally accessible quantities such as the response function and the power spectrum, we measure the non-equilibrium fluctuations in terms of the effective temperature. In addition, we compute the energy dissipation rate to find the precise effects of activity. Our study is relevant in understanding athermal fluctuations arising in cytoskeletal networks or inside a chromosome.

Splenectomy improves erythrocyte functionality in spherocytosis based on septin abundance, but not maturation defects

Splenectomy improves the clinical parameters of patients with hereditary spherocytosis, but its potential benefit to red blood cell (RBC) functionality and the mechanism behind this benefit remain largely overlooked. Here, we compared 7 nonsplenectomized and 13 splenectomized patients with mutations in the β-spectrin or the ankyrin gene. We showed that hematological parameters, spherocyte abundance, osmotic fragility, intracellular calcium, and extracellular vesicle release were largely but not completely restored by splenectomy, whereas cryohemolysis was not. Affected RBCs exhibited decreases in β-spectrin and/or ankyrin contents and slight alterations in spectrin membrane distribution, depending on the mutation. These modifications were found in both splenectomized and nonsplenectomized patients and poorly correlated with RBC functionality alteration, suggesting additional impairments. Accordingly, we found an increased abundance of septins, small guanosine triphosphate-binding cytoskeletal proteins. Septins-2, -7, and -8 but not -11 were less abundant upon splenectomy and correlated with the disease severity. Septin-2 membrane association was confirmed by immunolabeling. Except for cryohemolysis, all parameters of RBC morphology and functionality correlated with septin abundance. The increased septin content might result from RBC maturation defects, as evidenced by (1) the decreased protein 4.2 and Rh-associated glycoprotein content in all patient RBCs, (2) increased endoplasmic reticulum remnants and endocytosis proteins in nonsplenectomized patients, and (3) increased lysosomal and mitochondrial remnants in splenectomized patients. Our study paves the way for a better understanding of the involvement of septins in RBC membrane biophysical properties. In addition, the lack of restoration of septin-independent cryohemolysis by splenectomy may call into question its recommendation in specific cases.

Dynamic photodamage of red blood cell induced by CisDiMPyP porphyrin

It is well-known that oxidative damage in red blood cell (RBC) usually causes morphological changes and increased membrane rigidity. Although many studies have focused on investigating how RBC responds to a photodynamic stimulus, the intermediate steps between membrane damage and hemolysis are not reported. To give a comprehensive insight into changes of RBC membrane property under different oxidative damage levels, we employed the photoactivation of CisDiMPyP porphyrin that primarily generates singlet oxygen ¹O₂ as oxidant species. We found that there were distinguishable characteristic damages depending on the ¹O₂ flux over the membrane, in a way that each impact of photooxidative damage was categorized under three damage levels: mild (maintaining the membrane morphology and elasticity), moderate (membrane elongation and increased membrane elasticity) and severe (wrinkle-like deformation and hemolysis). When sodium azide (NaN₃) was used as a singlet oxygen quencher, delayed cell membrane alterations and hemolysis were detected. The delay times showed that ¹O₂ indeed plays a key role that causes RBC photooxidation by CisDiMPyP. We suggest that the sequence of morphological changes (RBC discoid area expansion, wrinkle-like patterns, and hemolysis) under photooxidative damage occurs due to damage to the lipid membrane and cytoskeletal network proteins.

Membrane Lesions and Reduced Life Span of Red Blood Cells in Preeclampsia as Evidenced by Atomic Force Microscopy

Preeclampsia (PE) presents with maternal de novo hypertension and significant proteinuria and is one of the leading causes of maternal and perinatal morbidity and mortality with unknown etiology. The disease is associated with inflammatory vascular response and severe red blood cell (RBC) morphology changes. This study examined the nanoscopic morphological changes of RBCs from PE women versus normotensive healthy pregnant controls (PCs) and non-pregnant controls (NPCs) applying atomic force microscopy (AFM) imaging. The results revealed that the membrane of fresh PE RBCs differed significantly from healthy ones by the presence of invaginations and protrusions and an increased roughness value (R_rms) (4.7 ± 0.8 nm for PE vs. 3.8 ± 0.5 nm and 2.9 ± 0.4 nm for PCs and NPCs, respectively). PE-cells aging resulted in more pronounced protrusions and concavities, with exponentially increasing R_rms values, in contrast to the controls, where the R_rms parameter decreased linearly with time. The R_rms, evaluated on a 2 × 2 µm² scanned area, for senescent PE cells (13 ± 2.0 nm) was significantly higher (p < 0.01) than that of PCs (1.5 ± 0.2 nm) and NPCs (1.9 ± 0.2 nm). Furthermore, the RBCs from PE patients appeared fragile, and often only ghosts were observed instead of intact cells at 20-30 days of aging. Oxidative-stress simulation on healthy cells led to RBC membrane features similar to those observed for PE cells. The results demonstrate that the most pronounced effects on RBCs in PE patients are related to impaired membrane homogeneity and strongly altered roughness values, as well as to vesiculation and ghost formation in the course of cell aging.

Capsule self-oscillating gels showing cell-like nonthermal membrane/shape fluctuations

A primary interest in cell membrane and shape fluctuations is establishing experimental models reflecting only nonthermal active contributions. Here we report a millimeter-scaled capsule self-oscillating gel model mirroring the active contribution effect on cell fluctuations. In the capsule self-oscillating gels, the propagating chemical signals during a Belousov-Zhabotinsky (BZ) reaction induce simultaneous local deformations in the various regions, showing cell-like shape fluctuations. The capsule self-oscillating gels do not fluctuate without the BZ reaction, implying that only the active chemical parameter induces the gel fluctuations. The period and amplitude depend on the gel layer thickness and the concentration of the chemical substrate for the BZ reaction. Our results allow for a solid experimental platform showing actively driven cell-like fluctuations, which can potentially contribute to investigating the active parameter effect on cell fluctuations.

Optical trapping with holographically structured light for single-cell studies

A groundbreaking work in 1970 by Arthur Ashkin paved the way for developing various optical trapping techniques. Optical tweezers have become an established method for the manipulation of biological objects, due to their noninvasiveness and precise controllability. Recent innovations are accelerating and now enable single-cell manipulation through holographic light structuring. In this review, we provide an overview of recent advances in optical tweezer techniques for studies at the individual cell level. Our review focuses on holographic optical tweezers that utilize active spatial light modulators to noninvasively manipulate live cells. The versatility of the technology has led to valuable integrations with microscopy, microfluidics, and biotechnological techniques for various single-cell studies. We aim to recapitulate the basic principles of holographic optical tweezers, highlight trends in their biophysical applications, and discuss challenges and future prospects.

Recent updates of stem cell-based erythropoiesis

Blood transfusions are now an essential part of modern medicine. Transfusable red blood cells (RBCs) are employed in various therapeutic strategies; however, the processes of blood donation, collection, and administration still involve many limitations. Notably, a lack of donors, the risk of transfusion-transmitted disease, and recent pandemics such as COVID-19 have prompted us to search for alternative therapeutics to replace this resource. Originally, RBC production was attempted via the ex vivo differentiation of stem cells. However, a more approachable and effective cell source is now required for broader applications. As a viable alternative, pluripotent stem cells have been actively used in recent research. In this review, we discuss the basic concepts related to erythropoiesis, as well as early research using hematopoietic stem cells ex vivo, and discuss the current trend of in vitro erythropoiesis using human-induced pluripotent stem cells.

Effects of urban particulate matter on the quality of erythrocytes

With the acceleration of industrialisation and urbanisation, air pollution has become a serious global concern as a hazard to human health, with urban particulate matter (UPM) accounting for the largest share. UPM can rapidly pass into and persist within systemic circulation. However, few studies exist on whether UPM may have any impact on blood components. In this study, UPM standards (SRM1648a) were used to assess the influence of UPM on erythrocyte quality in terms of oxidative and metabolic damage as well as phagocytosis by macrophages in vitro and clearance in vivo. Our results showed that UPM had weak haemolytic properties. It can oxidise haemoglobin and influence the oxygen-carrying function, redox balance, and metabolism of erythrocytes. UPM increases the content of reactive oxygen species (ROS) and decreases antioxidant function according to the data of malonaldehyde (MDA), glutathione (GSH), and glucose 6 phosphate dehydrogenase (G6PDH). UPM can adhere to or be internalised by erythrocytes at higher concentrations, which can alter their morphology. Superoxide radicals produced in the co-incubation system further disrupted the structure of red blood cell membranes, thereby lowering the resistance to the hypotonic solution, as reflected by the osmotic fragility test. Moreover, UPM leads to an increase in phosphatidylserine exposure in erythrocytes and subsequent clearance by the mononuclear phagocytic system in vivo. Altogether, this study suggests that the primary function of erythrocytes may be affected by UPM, providing a warning for erythrocyte quality in severely polluted areas. For critically ill patients, transfusion of erythrocytes with lesions in morphology and function will have serious clinical consequences, suggesting that potential risks should be considered during blood donation screening. The current work expands the scope of blood safety studies.

Influence of storage and buffer composition on the mechanical behavior of flowing red blood cells

On-chip study of blood flow has emerged as a powerful tool to assess the contribution of each component of blood to its overall function. Blood has indeed many functions, from gas and nutrient transport to immune response and thermal regulation. Red blood cells play a central role therein, in particular through their specific mechanical properties, which directly influence pressure regulation, oxygen perfusion, or platelet and white cell segregation toward endothelial walls. As the bloom of in-vitro studies has led to the apparition of various storage and sample preparation protocols, we address the question of the robustness of the results involving cell mechanical behavior against this diversity. The effects of three conservation media (EDTA, citrate, and glucose-albumin-sodium-phosphate) and storage time on the red blood cell mechanical behavior are assessed under different flow conditions: cell deformability by ektacytometry, shape recovery of cells flowing out of a microfluidic constriction, and cell-flipping dynamics under shear flow. The impact of buffer solutions (phosphate-buffered saline and density-matched suspension using iodixanol/Optiprep) are also studied by investigating individual cell-flipping dynamics, relative viscosity of cell suspensions, and cell structuration under Poiseuille flow. Our results reveal that storing blood samples up to 7 days after withdrawal and suspending them in adequate density-matched buffer solutions has, in most experiments, a moderate effect on the overall mechanical response, with a possible rapid evolution in the first 3 days after sample collection.

Quantifying Shear-induced Margination and Adhesion of Platelets in Microvascular Blood Flow

Platelet margination and adhesion are two critical and closely related steps in thrombus formation. Using dissipative particle dynamics (DPD) method that seamlessly models blood cells, blood plasma, and vessel walls with functionalized surfaces, we quantify the shear-induced margination and adhesion of platelets in microvascular blood flow. The results show that the occurrence of shear-induced RBC-platelet collisions has a remarkable influence on the degree of platelet margination. We characterize the lateral motion of individual platelets by a mean square displacement analysis of platelet trajectories, and find that the wall-induced lift force and the shear-induced displacement in wall-bounded flow cause the variation in near-wall platelet distribution. We then investigate the platelet adhesive dynamics under different flow conditions, by conducting DPD simulations of blood flow in a microtube with fibrinogen-coated wall surfaces. We find that the platelet adhesion is enhanced with the increase of fibrinogen concentration level but decreased with the increase of shear rate. These results are consistent with available experimental results. In addition, we demonstrate that the adherent platelets have a negative impact on the margination dynamics of the circulating platelets, which is mainly due to the climbing effect induced by the adherent ones. Taken together, these findings provide useful insights into the platelet margination and adhesion dynamics, which may facilitate the understanding of the predominant processes governing the initial stage of thrombus formation.

Hypoxic storage of donor red cells preserves deformability after exposure to plasma from adults with sickle cell disease

BACKGROUND

Red cell (RBC) transfusions are beneficial for patients with sickle cell disease (SCD), but ex vivo studies suggest that inflamed plasma from patients with SCD during crises may damage these RBCs, diminishing their potential efficacy. The hypoxic storage of RBCs may improve transfusion efficacy by minimizing the storage lesion. We tested the hypotheses that (1) The donor RBCs exposed to the plasma of patients in crisis would have lower deformability and higher hemolysis than those exposed to non-crisis plasma, and (2) hypoxic storage, compared to standard storage, of donor RBCs could preserve deformability and reduce hemolysis.

STUDY DESIGN AND METHODS

18 SCD plasma samples from patients who had severe acute-phase symptoms (A-plasma; n = 9) or were at a steady-state (S = plasma; n = 9) were incubated with 16 RBC samples from eight units that were stored either under conventional(CRBC) or hypoxic(HRBC) conditions. Hemolysis and microcapillary deformability assays of these RBCs were analyzed using linear mixed-effect models after each sample was incubated in patient plasma overnight at 37°C RESULTS: Relative deformability was 0.036 higher (p < 0.0001) in HRBC pairs compared to CRBC pairs regardless of plasma type. Mean donor RBC hemolysis was 0.33% higher after incubation with A-plasma compared to S-plasma either with HRBC or CRBC (p = 0.04). HRBCs incubated with steady-state patient plasma demonstrated the highest deformability and lowest hemolysis.

CONCLUSION

Hypoxic storage significantly influenced RBC deformability. Patient condition significantly influenced post-incubation hemolysis. Together, HRBCs in steady-state plasma maximized donor red cell ex vivo function and survival.

Ring stage classification of Babesia microti and Plasmodium falciparum using optical diffraction 3D tomographic technique

Background

Babesia is an intraerythrocytic parasite often misdiagnosed as a malaria parasite, leading to inappropriate treatment of the disease especially in co-endemic areas. In recent years, optical diffraction tomography (ODT) has shown great potential in the field of pathogen detection by quantification of three-dimensional (3D) imaging tomograms. The 3D imaging of biological cells is crucial to investigate and provide valuable information about the mechanisms behind the pathophysiology of cells and tissues.

Methods

The early ring stage of P. falciparum were obtained from stored stock of infected RBCs and of B. microti were obtained from infected patients during diagnosis. The ODT technique was applied to analyze and characterize detailed differences between P. falciparum and B. microti ring stage at the single cell level. Based on 3D quantitative information, accurate measurement was performed of morphological, biochemical, and biophysical parameters.

Results

Accurate measurements of morphological parameters indicated that the host cell surface area at the ring stage in B. microti was significantly smaller (140.2 ± 17.1 µm2) than that in P. falciparum (159.0 ± 15.2 µm2), and sphericities showed higher levels in B. microti-parasitized cells (0.66 ± 0.05) than in P. falciparum (0.60 ± 0.04). Based on biochemical parameters, host cell hemoglobin level was significantly higher and membrane fluctuations were respectively more active in P. falciparum-infected cells (30.25 ± 2.96 pg; 141.3 ± 24.68 nm) than in B. microti (27.28 ± 3.52 pg; 110.1 ± 38.83 nm). The result indicates that P. falciparum more actively altered host RBCs than B. microti.

Conclusion

Although P. falciparum and B. microti often show confusable characteristics under the microscope, and the actual three-dimensional properties are different. These differences could be used in differential clinical diagnosis of erythrocytes infected with B. microti and P. falciparum.

Graphical Abstract

Time-Resolved Four-Channel Jones Matrix Measurement of Birefringent Materials Using an Ultrafast Laser

A method for ultrafast time-resolved four-channel Jones matrix measurement of birefringent materials using an ultrafast laser is investigated. This facilitated the acquisition of a four-channel angular multiplexing hologram in a single shot. The Jones matrix information of a birefringent sample was retrieved from the spatial spectrum of a hologram. The feasibility of this approach was established by measuring the Jones matrix of starch granules in microfluidic chips and the complex amplitude distribution and phase delay distribution of liquid crystal cell at different voltages. Moreover, when the picosecond laser was switched to a femtosecond laser, ultrafast measurements were possible provided that the time interval between two detection pulses was larger than the pulse width.

Non-equilibrium shapes and dynamics of active vesicles

Active vesicles, constructed through the confinement of self-propelled particles (SPPs) inside a lipid membrane shell, exhibit a large variety of non-equilibrium shapes, ranging from the formation of local tethers and dendritic conformations, to prolate and bola-like structures. To better understand the behavior of active vesicles, we perform simulations of membranes modelled as dynamically triangulated surfaces enclosing active Brownian particles. A systematic analysis of membrane deformations and SPP clustering, as a function of SPP activity and volume fraction inside the vesicle is carried out. Distributions of membrane local curvature, and the clustering and mobility of SPPs obtained from simulations of active vesicles are analysed. There exists a feedback mechanism between the enhancement of membrane curvature, the formation of clusters of active particles, and local or global changes in vesicle shape. The emergence of active tension due to the activity of SPPs can well be captured by the Young-Laplace equation. Furthermore, a simple numerical method for tether detection is presented and used to determine correlations between the number of tethers, their length, and local curvature. We also provide several geometrical arguments to explain different tether characteristics for various conditions. These results contribute to the future development of steerable active vesicles or soft micro-robots whose behaviour can be controlled and used for potential applications.

Structural and mechanical properties of the red blood cell’s cytoplasmic membrane seen through the lens of biophysics

Red blood cells (RBCs) are the most abundant cell type in the human body and critical suppliers of oxygen. The cells are characterized by a simple structure with no internal organelles. Their two-layered outer shell is composed of a cytoplasmic membrane (RBC_cm) tethered to a spectrin cytoskeleton allowing the cell to be both flexible yet resistant against shear stress. These mechanical properties are intrinsically linked to the molecular composition and organization of their shell. The cytoplasmic membrane is expected to dominate the elastic behavior on small, nanometer length scales, which are most relevant for cellular processes that take place between the fibrils of the cytoskeleton. Several pathologies have been linked to structural and compositional changes within the RBC_cm and the cell’s mechanical properties. We review current findings in terms of RBC lipidomics, lipid organization and elastic properties with a focus on biophysical techniques, such as X-ray and neutron scattering, and Molecular Dynamics simulations, and their biological relevance. In our current understanding, the RBC_cm’s structure is patchy, with nanometer sized liquid ordered and disordered lipid, and peptide domains. At the same time, it is surprisingly soft, with bending rigidities κ of 2–4 k_BT. This is in strong contrast to the current belief that a high concentration of cholesterol results in stiff membranes. This extreme softness is likely the result of an interaction between polyunsaturated lipids and cholesterol, which may also occur in other biological membranes. There is strong evidence in the literature that there is no length scale dependence of κ of whole RBCs.

Biomechanical properties of native and cultured red blood cells–Interplay of shape, structure and biomechanics

Modern medicine increases the demand for safe blood products. Ex vivo cultured red blood cells (cRBC) are eagerly awaited as a standardized, safe source of RBC. Established culture models still lack the terminal cytoskeletal remodeling from reticulocyte to erythrocyte with changes in the biomechanical properties and interacts with membrane stiffness, viscosity of the cytoplasm and the cytoskeletal network. Comprehensive data on the biomechanical properties of cRBC are needed to take the last step towards translation into clinical use in transfusion medicine. Aim of the study was the comparative analysis of topographical and biomechanical properties of cRBC, generated from human CD34⁺ adult hematopoietic stem/progenitor cells, with native reticulocytes (nRET) and erythrocytes (nRBC) using cell biological and biomechanical technologies. To gain the desired all-encompassing information, a single method was unsatisfactory and only the combination of different methods could lead to the goal. Topographical information was matched with biomechanical data from optical tweezers (OT), atomic force microscopy (AFM) and digital holographic microscopy (DHM). Underlying structures were investigated in detail. Imaging, deformability and recovery time showed a high similarity between cRBC and nRBC. Young’s modulus and plasticity index also confirmed this similarity. No significant differences in membrane and cytoskeletal proteins were found, while lipid deficiency resulted in spherical, vesiculated cells with impaired biomechanical functionality. The combination of techniques has proven successful and experiments underscore a close relationship between lipid content, shape and biomechanical functionality of RBC.

The bending rigidity of the red blood cell cytoplasmic membrane

An important mechanical property of cells is the membrane bending modulus, κ. In the case of red blood cells (RBCs) there is a composite membrane consisting of a cytoplasmic membrane and an underlying spectrin network. Literature values of κ are puzzling, as they are reported over a wide range, from 5 kBT to 230 kBT. To disentangle the contribution of the cytoplasmic membrane from the spectrin network, we investigated the bending of red blood cell cytoplasmic membranes (RBCcm) in the absence of spectrin and adenosine triphosphate (ATP). We used a combination of X-ray diffuse scattering (XDS), neutron spin-echo (NSE) spectrometry and Molecular Dynamics (MD) simulations. Our results indicate values of κ of order 4 kBT to 6 kBT, relatively small compared to literature values for most single component lipid bilayers. We suggest two ways this relative softness might confer biological advantage.

The Shape of Human Red Blood Cells Suspended in Autologous Plasma and Serum

(1) Background: In almost all studies of the shape of the human red blood cell (RBC), the suspending medium was a salt solution supplemented with albumin. However, the ratio of thickness across the dimple region to the thickness of the rim (THR) depends on the albumin concentration. Values of the THR in the literature range from 0.27 to 0.627 whereas in the present work it was 0.550 or 0.601 whether measured in plasma or serum. (2) Methods: 9911 RBCs of eight donors were suspended in autologous plasma or serum. Sedimented RBCs were observed under bright field illumination at 416 nm. From the profiles of gray value, the THR was determined. (3) Results: The THR displays a wide distribution within a single blood sample. A direct correlation of THR and spontaneous curvature of the membrane is likely. The variation of the mean THR between different donors is large. The aspect ratio of RBCs viewed face-on ranged on average from 1 to 1.48. In oval RBCs, the rim is thicker along the major axis than along the minor axis, an effect increasing with increasing aspect ratio. Remodeling of the membrane skeleton occurs in vivo with a characteristic time (τ) on the order of 1 h. (4) Conclusions: Consideration of these data in models of RBC behavior might improve the agreement with observations. τ≈1 h suggests a more general type of reference configuration of the membrane skeleton than a stress free shape.

A particle-based computational model to analyse remodelling of the red blood cell cytoskeleton during malaria infections

Red blood cells can withstand the harsh mechanical conditions in the vasculature only because the bending rigidity of their plasma membrane is complemented by the shear elasticity of the underlying spectrin-actin network. During an infection by the malaria parasite Plasmodium falciparum, the parasite mines host actin from the junctional complexes and establishes a system of adhesive knobs, whose main structural component is the knob-associated histidine rich protein (KAHRP) secreted by the parasite. Here we aim at a mechanistic understanding of this dramatic transformation process. We have developed a particle-based computational model for the cytoskeleton of red blood cells and simulated it with Brownian dynamics to predict the mechanical changes resulting from actin mining and KAHRP-clustering. Our simulations include the three-dimensional conformations of the semi-flexible spectrin chains, the capping of the actin protofilaments and several established binding sites for KAHRP. For the healthy red blood cell, we find that incorporation of actin protofilaments leads to two regimes in the shear response. Actin mining decreases the shear modulus, but knob formation increases it. We show that dynamical changes in KAHRP binding affinities can explain the experimentally observed relocalization of KAHRP from ankyrin to actin complexes and demonstrate good qualitative agreement with experiments by measuring pair cross-correlations both in the computer simulations and in super-resolution imaging experiments.

Malaria is one of the deadliest infectious diseases and its symptoms are related to the blood stage, when the parasite multiplies within red blood cells. In order to avoid clearance by the spleen, the parasite produces specific factors like the adhesion receptor PfEMP1 and the multifunctional protein KAHRP that lead to the formation of adhesive knobs on the surface of the red blood cells and thus increase residence time in the vasculature. We have developed a computational model for the parasite-induced remodelling of the actin-spectrin network to quantitatively predict the dynamical changes in the mechanical properties of the infected red blood cells and the spatial distribution of the different protein components of the membrane skeleton. Our simulations show that KAHRP can relocate to actin junctions due to dynamical changes in binding affinities, in good qualitative agreement with super-resolution imaging experiments. In the future, our simulation framework can be used to gain further mechanistic insight into the way malaria parasites attack the red blood cell cytoskeleton.

Technologies for measuring red blood cell deformability

Human red blood cells (RBCs) are approximately 8 μm in diameter, but must repeatedly deform through capillaries as small as 2 μm in order to deliver oxygen to all parts of the body. The loss of this capability is associated with the pathology of many diseases, and is therefore a potential biomarker for disease status and treatment efficacy. Measuring RBC deformability is a difficult problem because of the minute forces (∼pN) that must be exerted on these cells, as well as the requirements for throughput and multiplexing. The development of technologies for measuring RBC deformability date back to the 1960s with the development of micropipette aspiration, ektacytometry, and the cell transit analyzer. In the past 10 years, significant progress has been made using microfluidics by leveraging the ability to precisely control fluid flow through microstructures at the size scale of individual RBCs. These technologies have now surpassed traditional methods in terms of sensitivity, throughput, consistency, and ease of use. As a result, these efforts are beginning to move beyond feasibility studies and into applications to enable biomedical discoveries. In this review, we provide an overview of both traditional and microfluidic techniques for measuring RBC deformability. We discuss the capabilities of each technique and compare their sensitivity, throughput, and robustness in measuring bulk and single-cell RBC deformability. Finally, we discuss how these tools could be used to measure changes in RBC deformability in the context of various applications including pathologies caused by malaria and hemoglobinopathies, as well as degradation during storage in blood bags prior to blood transfusions.

Sex-Specific Differences of Adenosine Triphosphate Levels in Red Blood Cells Isolated From ApoE/LDLR Double-Deficient Mice

In this study for the first time, we investigated the correlation between sex-specific differences in adenosine triphosphate (ATP) levels in red blood cells (RBCs) and their mechanical, biochemical, and morphological alterations during the progression of atherosclerosis in ApoE/LDLR double-deficient (ApoE/LDLR^−/−) mice. Our results indicate that both sex and age affect alterations in RBCs of both ApoE/LDLR^−/− and C57BL/6J mice. When compared with male RBCs, female RBCs were characterized by lower basal ATP and mean corpuscular hemoglobin concentration (MCHC), higher hemoglobin concentration (HGB), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), deformability, and phosphatidylserine (PS) exposure levels, regardless of age in both, ApoE/LDLR^−/− and C57BL/6J mice. ApoE/LDLR^−/− mice compared with age-matched controls showed lower basal ATP levels regardless of age and sex. Intracellular ATP level of RBCs was decreased solely in senescent female C57BL/6J mice, while it was elevated in males. Basal extracellular ATP levels were 400 times lower than corresponding intracellular level. In conclusion, basal ATP levels, RBC morphology, deformability, PS exposure levels alterations are sex-dependent in mice. Changes in basal ATP levels were correlated with PS exposure and trends of changes in MCV. Trends of changes of the most RBC parameters were similar in both sexes of ApoE/LDLR^−/− mice compared with age-matched controls; however, their kinetics and levels vary greatly between different stages of disease progression.

How the spleen reshapes and retains young and old red blood cells: A computational investigation

The spleen, the largest secondary lymphoid organ in humans, not only fulfils a broad range of immune functions, but also plays an important role in red blood cell’s (RBC) life cycle. Although much progress has been made to elucidate the critical biological processes involved in the maturation of young RBCs (reticulocytes) as well as removal of senescent RBCs in the spleen, the underlying mechanisms driving these processes are still obscure. Herein, we perform a computational study to simulate the passage of RBCs through interendothelial slits (IES) in the spleen at different stages of their lifespan and investigate the role of the spleen in facilitating the maturation of reticulocytes and in clearing the senescent RBCs. Our simulations reveal that at the beginning of the RBC life cycle, intracellular non-deformable particles in reticulocytes can be biomechanically expelled from the cell upon passage through IES, an insightful explanation of why this peculiar “pitting” process is spleen-specific. Our results also show that immature RBCs shed surface area by releasing vesicles after crossing IES and progressively acquire the biconcave shape of mature RBCs. These findings likely explain why RBCs from splenectomized patients are significantly larger than those from nonsplenectomized subjects. Finally, we show that at the end of their life span, senescent RBCs are not only retained by IES due to reduced deformability but also become susceptible to mechanical lysis under shear stress. This finding supports the recent hypothesis that transformation into a hemolyzed ghost is a prerequisite for phagocytosis of senescent RBCs. Altogether, our computational investigation illustrates critical biological processes in the spleen that cannot be observed in vivo or in vitro and offer insights into the role of the spleen in the RBC physiology.

The spleen, the largest secondary lymphoid organ in humans, not only fulfils a broad range of immune functions, but also plays an important role in red blood cell (RBC) life cycle. In this study, we perform a computational study to simulate the passage of RBCs through interendothelial slits (IES) in the spleen at different stages of their lifespan, a critical biological process that cannot be observed in humans. Our simulation results illustrate a specific role of spleen in shaping young RBCs, which points to a probable missing step in current in vitro RBC culture protocols that fail to generate a majority of typical biconcave RBCs. Our results also reveal that intra-splenic mechanical constraints likely contribute to the final clearance and elimination of aged RBCs. Altogether, we demonstrate that our computational model can provide mechanistic rationales for experimental studies, offer insights into the role of the spleen in the RBC physiology and help the optimization of in vitro RBC culture techniques.

High-speed Fourier ptychographic microscopy for quantitative phase imaging

Fourier ptychographic microscopy (FPM), as an emerging computational imaging method, has been applied to quantitative phase imaging with resolution bypassing the physical limit of the detection objective. Due to the weak illumination intensity and long image acquisition time, the achieved imaging speed in current FPM methods is still low, making them unsuitable for real-time imaging applications. We propose and demonstrate a high-speed FPM method based on using laser illumination and digital micro-mirror devices for illumination angle scanning. In this new, to the best of our knowledge, FPM method, we realized quantitative phase imaging and intensity imaging at over 42 frames per second (fps) with around 1 µm lateral resolution. The quantitative phase images have revealed membrane height fluctuations of red blood cells with nanometer-scale sensitivity, while the intensity images have resolved subcellular features in stained cancer tissue slices.

In vitro assay for single-cell characterization of impaired deformability in red blood cells under recurrent episodes of hypoxia

Red blood cells (RBCs) are subjected to recurrent changes in shear stress and oxygen tension during blood circulation. The cyclic shear stress has been identified as an important factor that alone can weaken cell mechanical deformability. The effects of cyclic hypoxia on cellular biomechanics have yet to be fully investigated. As the oxygen affinity of hemoglobin plays a key role in the biological function and mechanical performance of RBCs, the repeated transitions of hemoglobin between its R (high oxygen tension) and T (low oxygen tension) states may impact their mechanical behavior. The present study focuses on developing a novel microfluidic-based assay for characterization of the effects of cyclic hypoxia on cell biomechanics. The capability of this assay is demonstrated by a longitudinal study of individual RBCs in health and sickle cell disease subjected to cyclic hypoxia conditions of various durations and levels of low oxygen tension. The viscoelastic properties of cell membranes are extracted from tensile stretching and relaxation processes of RBCs induced by the electrodeformation technique. Results demonstrate that cyclic hypoxia alone can significantly reduce cell deformability, similar to the fatigue damage accumulated through cyclic mechanical loading. RBCs affected by sickle cell disease are less deformable (significantly higher membrane shear modulus and viscosity) than normal RBCs. The fatigue resistance of sickle RBCs to the cyclic hypoxia challenge is significantly inferior to that of normal RBCs, and this trend is more significant in mature erythrocytes of sickle cells. When the oxygen affinity of sickle hemoglobin is enhanced by anti-sickling drug treatment of 5-hydroxymethyl-2-furfural (5-HMF), sickle RBCs show ameliorated resistance to fatigue damage induced by cyclic hypoxia. These results indicate an important biophysical mechanism underlying RBC senescence in which the cyclic hypoxia challenge alone can lead to mechanical degradation of the RBC membrane. We envision that the application of this assay can be further extended to RBCs in other blood diseases and other cell types.

A Novel Fragmentation Sensitivity Index Determines the Susceptibility of Red Blood Cells to Mechanical Trauma

Supraphysiological shear stresses (SSs) induce irreversible impairments of red blood cell (RBC) deformability, overstretching of RBC membrane, or fragmentation of RBCs that causes free hemoglobin to be released into plasma, which may lead to anemia. The magnitude and exposure tisme of the SSs are two critical parameters that determine the hemolytic threshold of a healthy RBC. However, impairments in the membrane stability of damaged cells reduce the hemolytic threshold and increase the susceptibility of the cell membrane to supraphysiological SSs, leading to cell fragmentation. The severity of the RBC fragmentation as a response to the mechanical damage and the critical SS levels causing fragmentation are not previously defined. In this study, we investigated the RBC mechanical damage in oxidative stress (OS) and metabolic depletion (MD) models by applying supraphysiological SSs up to 100 Pa by an ektacytometer (LORRCA MaxSis) and then assessed RBC deformability. Next, we examined hemolysis and measured RBC volume and count by Multisizer 3 Coulter Counter to evaluate RBC fragmentation. RBC deformability was significantly impaired in the range of 20-50 Pa in OS compared with healthy controls (p < 0.05). Hemolysis was detected at 90-100 Pa SS levels in MD and all applied SS levels in OS. Supraphysiological SSs increased RBC volume in both the damage models and the control group. The number of fragmented cells increased at 100 Pa SS in the control and MD and at all SS levels in OS, which was accompanied by hemolysis. Fragmentation sensitivity index increased at 50-100 Pa SS in the control, 100 Pa SS in MD, and at all SS levels in OS. Therefore, we propose RBC fragmentation as a novel sensitivity index for damaged RBCs experiencing a mechanical trauma before they undergo fragmentation. Our approach for the assessment of mechanical risk sensitivity by RBC fragmentation could facilitate the close monitoring of shear-mediated RBC response and provide an effective and accurate method for detecting RBC damage in mechanical circulatory assist devices used in routine clinical procedures.

Lateral Deformation of Human Red Blood Cells by Optical Tweezers

In this paper, we studied the lateral deformation of human red blood cells (RBCs) during lateral indentation by an optically trapped silica bead with a diameter of 4.5 µm (Bangs Laboratories, Inc. Fishers, IN, USA). The images were captured using a CCD camera and the Boltzmann statistics method was used for force calibration. Using the Hertz model, we calculated and compared the elastic stiffness resulting from the lateral force, showing that the differences are important and that the force should be considered. Besides the lateral component, this setup also allowed us to examine the lateral cell–bead interaction. The mean values of the cell shear stiffness measured during indentation were 3.37 ± 0.40 µN/m for biconcave RBCs, 3.48 ± 0.23 µN/m for spherical RBCs, and 3.80 ± 0.22 µN/m for crenelated RBCs, respectively. These results show that this approach can be used as a routine method for RBC study, because it enabled us to manipulate the cell without contact with the wall.

Effects of mobile phone emissions on human red blood cells

Raman spectroscopy was performed on GSM 900 and 1800 MHz mobile phone signal exposed red blood cells (RBCs). The observed changes in the Raman spectra of mobile signal exposed RBCs compared to unexposed control suggest reduced hemoglobin-oxygen affinity for the exposed cells. The possible mechanism may involve activation of the voltage gated membrane Ca²⁺ channels by the mobile phone emissions resulting in an increase in the levels of adenosine triphosphate (ATP) and 2,3-diphosphoglycerate (2,3-DPG) in cells via altered metabolic activities. Further studies carried out with fluorescent Ca²⁺ indicator confirmed increased intracellular Ca²⁺ level in the exposed cells. Since intracellular ATP level influences the shape and mechanics of RBCs, exposed cells were studied using diffraction phase microscopy and optical tweezers. Detectable changes in shape and mechanical properties were observed due to mobile signal exposure.

Temporal Imaging of Live Cells by High-Speed Confocal Raman Microscopy

Label-free live cell imaging was performed using a custom-built high-speed confocal Raman microscopy system. For various cell types, cell-intrinsic Raman bands were monitored. The high-resolution temporal Raman images clearly delineated the intracellular distribution of biologically important molecules such as protein, lipid, and DNA. Furthermore, optical phase delay measured using quantitative phase microscopy shows similarity with the image reconstructed from the protein Raman peak. This reported work demonstrates that Raman imaging is a powerful label-free technique for studying various biomedical problems in vitro with minimal sample preparation and external perturbation to the cellular system.

More than just a barrier: using physical models to couple membrane shape to cell function

The correct execution of many cellular processes, such as division and motility, requires the cell to adopt a specific shape. Physically, these shapes are determined by the interplay of the plasma membrane and internal cellular driving factors. While the plasma membrane defines the boundary of the cell, processes inside the cell can result in the generation of forces that deform the membrane. These processes include protein binding, the assembly of protein superstructures, and the growth and contraction of cytoskeletal networks. Due to the complexity of the cell, relating observed membrane deformations back to internal processes is a challenging problem. Here, we review cell shape changes in endocytosis, cell adhesion, cell migration and cell division and discuss how by modeling membrane deformations we can investigate the inner working principles of the cell.

The nanomorphology of cell surfaces of adhered osteoblasts

The functionality of living cells is inherently linked to subunits with dimensions ranging from several micrometers down to the nanometer scale. The cell surface plays a particularly important role. Electric signaling, including information processing, takes place at the membrane, as well as adhesion and contact. For osteoblasts, adhesion and spreading are crucial processes with regard to bone implants. Here we present a comprehensive characterization of the 3D nanomorphology of living, as well as fixed, osteoblastic cells using scanning ion conductance microscopy (SICM), which is a nanoprobing method that largely avoids mechanical perturbations. Dynamic ruffles are observed, manifesting themselves in characteristic membrane protrusions. They contribute to the overall surface corrugation, which we systematically study by introducing the relative 3D excess area as a function of the projected adhesion area. A clear anticorrelation between the two parameters is found upon analysis of ca. 40 different cells on glass and on amine-covered surfaces. At the rim of lamellipodia, characteristic edge heights between 100 and 300 nm are observed. Power spectral densities of membrane fluctuations show frequency-dependent decay exponents with absolute values greater than 2 on living osteoblasts. We discuss the capability of apical membrane features and fluctuation dynamics in aiding the assessment of adhesion and migration properties on a single-cell basis.

Biomechanics of Ex Vivo-Generated Red Blood Cells Investigated by Optical Tweezers and Digital Holographic Microscopy

Ex vivo-generated red blood cells are a promising resource for future safe blood products, manufactured independently of voluntary blood donations. The physiological process of terminal maturation from spheroid reticulocytes to biconcave erythrocytes has not been accomplished yet. A better biomechanical characterization of cultured red blood cells (cRBCs) will be of utmost interest for manufacturer approval and therapeutic application. Here, we introduce a novel optical tweezer (OT) approach to measure the deformation and elasticity of single cells trapped away from the coverslip. To investigate membrane properties dependent on membrane lipid content, two culture conditions of cRBCs were investigated, cRBCPlasma with plasma and cRBCHPL supplemented with human platelet lysate. Biomechanical characterization of cells under optical forces proves the similar features of native RBCs and cRBCHPL, and different characteristics for cRBCPlasma. To confirm these results, we also applied a second technique, digital holographic microscopy (DHM), for cells laid on the surface. OT and DHM provided related results in terms of cell deformation and membrane fluctuations, allowing a reliable discrimination between cultured and native red blood cells. The two techniques are compared and discussed in terms of application and complementarity.

On the Role of Curved Membrane Nanodomains, and Passive and Active Skeleton Forces in the Determination of Cell Shape and Membrane Budding

Biological membranes are composed of isotropic and anisotropic curved nanodomains. Anisotropic membrane components, such as Bin/Amphiphysin/Rvs (BAR) superfamily protein domains, could trigger/facilitate the growth of membrane tubular protrusions, while isotropic curved nanodomains may induce undulated (necklace-like) membrane protrusions. We review the role of isotropic and anisotropic membrane nanodomains in stability of tubular and undulated membrane structures generated or stabilized by cyto- or membrane-skeleton. We also describe the theory of spontaneous self-assembly of isotropic curved membrane nanodomains and derive the critical concentration above which the spontaneous necklace-like membrane protrusion growth is favorable. We show that the actin cytoskeleton growth inside the vesicle or cell can change its equilibrium shape, induce higher degree of segregation of membrane nanodomains or even alter the average orientation angle of anisotropic nanodomains such as BAR domains. These effects may indicate whether the actin cytoskeleton role is only to stabilize membrane protrusions or to generate them by stretching the vesicle membrane. Furthermore, we demonstrate that by taking into account the in-plane orientational ordering of anisotropic membrane nanodomains, direct interactions between them and the extrinsic (deviatoric) curvature elasticity, it is possible to explain the experimentally observed stability of oblate (discocyte) shapes of red blood cells in a broad interval of cell reduced volume. Finally, we present results of numerical calculations and Monte-Carlo simulations which indicate that the active forces of membrane skeleton and cytoskeleton applied to plasma membrane may considerably influence cell shape and membrane budding.

Correlated flickering of erythrocytes membrane observed with dual time resolved membrane fluctuation spectroscopy under different D-glucose concentrations

A correlated human red blood cell membrane fluctuation dependent on D-glucose concentration was found with dual time resolved membrane fluctuation spectroscopy (D-TRMFS). This new technique is a modified version of the dual optical tweezers method that has been adapted to measure the mechanical properties of red blood cells (RBCs) at distant membrane points simultaneously, enabling correlation analysis. Mechanical parameters under different D-glucose concentrations were obtained from direct membrane flickering measurements, complemented with membrane fluidity measurements using Laurdan Generalized Polarization (GP) Microscopy. Our results show an increase in the fluctuation amplitude of the lipid bilayer, and a decline in tension value, bending modulus and fluidity as D-glucose concentration increases. Metabolic mechanisms are proposed as explanations for the results.

Holotomography: Refractive Index as an Intrinsic Imaging Contrast for 3-D Label-Free Live Cell Imaging

Live cell imaging provides essential information in the investigation of cell biology and related pathophysiology. Refractive index (RI) can serve as intrinsic optical imaging contrast for 3-D label-free and quantitative live cell imaging, and provide invaluable information to understand various dynamics of cells and tissues for the study of numerous fields. Recently significant advances have been made in imaging methods and analysis approaches utilizing RI, which are now being transferred to biological and medical research fields, providing novel approaches to investigate the pathophysiology of cells. To provide insight into how RI can be used as an imaging contrast for imaging of biological specimens, here we provide the basic principle of RI-based imaging techniques and summarize recent progress on applications, ranging from microbiology, hematology, infectious diseases, hematology, and histopathology.

Quantitative absorption imaging of red blood cells to determine physical and mechanical properties

Red blood cells or erythrocytes, constituting 40 to 45 percent of the total volume of human blood are vesicles filled with hemoglobin with a fluid-like lipid bilayer membrane connected to a 2D spectrin network. The shape, volume, hemoglobin mass, and membrane stiffness of RBCs are important characteristics that influence their ability to circulate through the body and transport oxygen to tissues. In this study, we show that a simple two-LED set up in conjunction with standard microscope imaging can accurately determine the physical and mechanical properties of single RBCs. The Beer–Lambert law and undulatory motion dynamics of the membrane have been used to measure the total volume, hemoglobin mass, membrane tension coefficient, and bending modulus of RBCs. We also show that this method is sensitive enough to distinguish between the mechanical properties of RBCs during morphological changes from a typical discocyte to echinocytes and spherocytes. Measured values of the tension coefficient and bending modulus are 1.27 × 10⁻⁶ J m⁻² and 7.09 × 10⁻²⁰ J for discocytes, 4.80 × 10⁻⁶ J m⁻² and 7.70 × 10⁻²⁰ J for echinocytes, and 9.85 × 10⁻⁶ J m⁻² and 9.69 × 10⁻²⁰ J for spherocytes, respectively. This quantitative light absorption imaging reduces the complexity related to the quantitative imaging of the biophysical and mechanical properties of a single RBC that may lead to enhanced yet simplified point of care devices for analyzing blood cells.

The constant thickness in the microfluidic channel is used for controlled absorption of red and blue light to measure red blood cell hemoglobin and height mapping. High speed recording of the height mapping provides us the membrane fluctuation.

A Multifunctional Microfluidic Device for Blood Typing and Primary Screening of Blood Diseases

In this work, we demonstrate a multifunctional, portable, and disposable microfluidic device for blood typing and primary screening of blood diseases. Preloaded antibodies (anti-A, anti-B, and anti-D) interact with injected whole blood cells to cause an agglutination reaction that blocks a microslit in the microfluidic channel to accumulate red blood cells and form a visible red line that can be easily read to determine the blood type. Moreover, the different blood density and agglutination properties of normal and subtype blood groups, as well as different blood diseases, including anemia and polycythemia vera, generate different lengths of blood agglutination within the channels, which allows us to successfully screen these various conditions in as little as 2 min. The required blood volume for each test is just 1 μL, which can be obtained by minimally invasive finger pricking. This novel method of observing agglutinated red blood cells to distinguish blood types and diseases is both feasible and affordable, suggesting its promise for use in areas with limited resources.

Active particles induce large shape deformations in giant lipid vesicles

Biological cells generate intricate structures by sculpting their membrane from within to actively sense and respond to external stimuli or to explore their environment^1-4. Several pathogenic bacteria also provide examples of how localized forces strongly deform cell membranes from inside, leading to the invasion of neighbouring healthy mammalian cells⁵. Giant unilamellar vesicles have been successfully used as a minimal model system with which to mimic biological cells^6-11, but the realization of a minimal system with localized active internal forces that can strongly deform lipid membranes from within and lead to dramatic shape changes remains challenging. Here we present a combined experimental and simulation study that demonstrates how self-propelled particles enclosed in giant unilamellar vesicles can induce a plethora of non-equilibrium shapes and active membrane fluctuations. Using confocal microscopy, in the experiments we explore the membrane response to local forces exerted by self-phoretic Janus microswimmers. To quantify dynamic membrane changes, we perform Langevin dynamics simulations of active Brownian particles enclosed in thin membrane shells modelled by dynamically triangulated surfaces. The most pronounced shape changes are observed at low and moderate particle loadings, with the formation of tether-like protrusions and highly branched, dendritic structures, whereas at high volume fractions globally deformed vesicle shapes are observed. The resulting state diagram predicts the conditions under which local internal forces generate various membrane shapes. A controlled realization of such distorted vesicle morphologies could improve the design of artificial systems such as small-scale soft robots and synthetic cells.

Improved Erythrocyte Deformability Induced by Sodium-Glucose Cotransporter 2 Inhibitors in Type 2 Diabetic Patients

PURPOSE

Sodium-glucose cotransporter 2 (SGLT-2) inhibitors are antidiabetic drugs that improve cardiovascular outcomes. Hemoglobin and hematocrit values increase after SGLT-2 inhibitor administration. Although these factors increase blood viscosity and the risk of cardiovascular disease, SGLT-2 inhibitors have protective effects on the cardiovascular system. The mechanisms for this paradoxical phenomenon remain unclear, and the effect of SGLT-2 inhibitors on hemorheology has not been studied.

METHODS

We evaluated the hemorheological parameters of 63 patients of whom 38 received metformin with a dipeptidyl peptidase 4 (DPP-4) inhibitor, while 25 received metformin with SGLT-2 inhibitor. Blood viscosity was measured using a cone-and-plate viscometer, erythrocyte aggregation was measured using a modified erythrocyte sedimentation rate method, and erythrocyte membrane fluctuation was measured as deformability, using a diffraction optical tomography.

RESULTS

Both blood viscosity and erythrocyte aggregation increased in the SGLT-2 inhibitor group, although erythrocyte deformability was significantly improved compared with that of the DPP-4 inhibitor group (DPP-4 inhibitor 43.71 ± 5.13 nm; SGLT-2 inhibitor 53.88 ± 4.88 nm; p < 0.001). When the two groups were compared after propensity score matching, no differences in blood viscosity at high shear rates and erythrocyte aggregation were observed, although erythrocyte deformability was significantly improved in the SGLT-2 inhibitor group (DPP-4 inhibitor 45.01 ± 5.28 nm; SGLT-2 inhibitor 53.14 ± 4.72 nm; p = 0.001).

CONCLUSION

This study demonstrates that erythrocyte deformability was improved in the SGLT-2 inhibitor group compared with that in the DPP-4 inhibitor group. This improvement in erythrocyte deformability is expected to have a protective effect on the cardiovascular system.

Non-uniform distribution of myosin-mediated forces governs red blood cell membrane curvature through tension modulation

The biconcave disk shape of the mammalian red blood cell (RBC) is unique to the RBC and is vital for its circulatory function. Due to the absence of a transcellular cytoskeleton, RBC shape is determined by the membrane skeleton, a network of actin filaments cross-linked by spectrin and attached to membrane proteins. While the physical properties of a uniformly distributed actin network interacting with the lipid bilayer membrane have been assumed to control RBC shape, recent experiments reveal that RBC biconcave shape also depends on the contractile activity of nonmuscle myosin IIA (NMIIA) motor proteins. Here, we use the classical Helfrich-Canham model for the RBC membrane to test the role of heterogeneous force distributions along the membrane and mimic the contractile activity of sparsely distributed NMIIA filaments. By incorporating this additional contribution to the Helfrich-Canham energy, we find that the RBC biconcave shape depends on the ratio of forces per unit volume in the dimple and rim regions of the RBC. Experimental measurements of NMIIA densities at the dimple and rim validate our prediction that (a) membrane forces must be non-uniform along the RBC membrane and (b) the force density must be larger in the dimple than the rim to produce the observed membrane curvatures. Furthermore, we predict that RBC membrane tension and the orientation of the applied forces play important roles in regulating this force-shape landscape. Our findings of heterogeneous force distributions on the plasma membrane for RBC shape maintenance may also have implications for shape maintenance in different cell types.

The spectrin-actin network of the membrane skeleton plays an important role in controlling specialized cell membrane morphology. In the paradigmatic red blood cell (RBC), where actin filaments are present exclusively in the membrane skeleton, recent experiments reveal that nonmuscle myosin IIA (NMIIA) motor contractility maintains the RBC biconcave disk shape. In this study, we have identified criteria for micron-scale distributions of NMIIA forces at the membrane required to maintain the biconcave disk shape of an RBC in the resting condition. Supported by experimental measurements of RBC NMIIA distribution, we showed that a heterogeneous force distribution with a larger force density at the dimple is able to capture the experimentally observed biconcave morphology of an RBC with better accuracy compared to previous models that did not consider the heterogeneity in the force distribution. Furthermore, we showed that the biconcave geometry of the RBC is closely regulated by the effective membrane tension and the direction of applied forces on the membrane. These findings can be generalized to any force-mediated membrane shape, providing insight into the role of actomyosin forces in prescribing and maintaining the morphology of different cell types.