Red Blood Cell Passage of Small Capillaries Is Associated with Transient Ca2+-mediated Adaptations

Piezo1, the new actor in cell volume regulation

All animal cells control their volume through a complex set of mechanisms, both to counteract osmotic perturbations of the environment and to enable numerous vital biological processes, such as proliferation, apoptosis, and migration. The ability of cells to adjust their volume depends on the activity of ion channels and transporters which, by moving K+, Na+, and Cl- ions across the plasma membrane, generate the osmotic gradient that drives water in and out of the cell. In 2010, Patapoutian's group identified a small family of evolutionarily conserved, Ca2+-permeable mechanosensitive channels, Piezo1 and Piezo2, as essential components of the mechanically activated current that mediates mechanotransduction in vertebrates. Piezo1 is expressed in several tissues and its opening is promoted by a wide range of mechanical stimuli, including membrane stretch/deformation and osmotic stress. Piezo1-mediated Ca2+ influx is used by the cell to convert mechanical forces into cytosolic Ca2+ signals that control diverse cellular functions such as migration and cell death, both dependent on changes in cell volume and shape. The crucial role of Piezo1 in the regulation of cell volume was first demonstrated in erythrocytes, which need to reduce their volume to pass through narrow capillaries. In HEK293 cells, increased expression of Piezo1 was found to enhance the regulatory volume decrease (RVD), the process whereby the cell re-establishes its original volume after osmotic shock-induced swelling, and it does so through Ca2+-dependent modulation of the volume-regulated anion channels. More recently we reported that Piezo1 controls the RVD in glioblastoma cells via the modulation of Ca2+-activated K+ channels. To date, however, the mechanisms through which this mechanosensitive channel controls cell volume and maintains its homeostasis have been poorly investigated and are still far from being understood. The present review aims to provide a broad overview of the literature discussing the recent advances on this topic.

Microscale geometrical modulation of PIEZO1 mediated mechanosensing through cytoskeletal redistribution

The microgeometry of the cellular microenvironment profoundly impacts cellular behaviors, yet the link between it and the ubiquitously expressed mechanosensitive ion channel PIEZO1 remains unclear. Herein, we describe a fluorescent micropipette aspiration assay that allows for simultaneous visualization of intracellular calcium dynamics and cytoskeletal architecture in real-time, under varied micropipette geometries. By integrating elastic shell finite element analysis with fluorescent lifetime imaging microscopy and employing PIEZO1-specific transgenic red blood cells and HEK cell lines, we demonstrate a direct correlation between the microscale geometry of aspiration and PIEZO1-mediated calcium signaling. We reveal that increased micropipette tip angles and physical constrictions lead to a significant reorganization of F-actin, accumulation at the aspirated cell neck, and subsequently amplify the tension stress at the dome of the cell to induce more PIEZO1's activity. Disruption of the F-actin network or inhibition of its mobility leads to a notable decline in PIEZO1 mediated calcium influx, underscoring its critical role in cellular mechanosensing amidst geometrical constraints.

Is post-hypertonic lysis of human red blood cells caused by excessive cell volume regulation?

Human red blood cells (RBC) exposed to hypertonic media are subject to post-hypertonic lysis - an injury that only develops during resuspension to an isotonic medium. The nature of post-hypertonic lysis was previously hypothesized to be osmotic when cation leaks were observed, and salt loading was suggested as a cause of the cell swelling upon resuspension in an isotonic medium. However, it was problematic to account for the salt loading since the plasma membrane of human RBCs was considered impermeable to cations. In this study, the hypertonicity-related behavior of human RBCs is revisited within the framework of modern cell physiology, considering current knowledge on membrane ion transport mechanisms - an account still missing. It is recognized here that the hypertonic behavior of human RBCs is consistent with the acute regulatory volume increase (RVI) response - a healthy physiological reaction initiated by cells to regulate their volume by salt accumulation. It is shown by reviewing the published studies that human RBCs can increase cation conductance considerably by activating cell volume-regulated ion transport pathways inactive under normal isotonic conditions and thus facilitate salt loading. A simplified physiological model accounting for transmembrane ion fluxes and membrane voltage predicts the isotonic cell swelling associated with increased cation conductance, eventually reaching hemolytic volume. The proposed involvement of cell volume regulation mechanisms shows the potential to explain the complex nature of the osmotic response of human RBCs and other cells. Cryobiological implications, including mechanisms of cryoprotection, are discussed.

PIEZO Ion Channels in Cardiovascular Functions and Diseases

The cardiovascular system provides blood supply throughout the body and as such is perpetually applying mechanical forces to cells and tissues. Thus, this system is primed with mechanosensory structures that respond and adapt to changes in mechanical stimuli. Since their discovery in 2010, PIEZO ion channels have dominated the field of mechanobiology. These have been proposed as the long-sought-after mechanosensitive excitatory channels involved in touch and proprioception in mammals. However, more and more pieces of evidence point to the importance of PIEZO channels in cardiovascular activities and disease development. PIEZO channel-related cardiac functions include transducing hemodynamic forces in endothelial and vascular cells, red blood cell homeostasis, platelet aggregation, and arterial blood pressure regulation, among others. PIEZO channels contribute to pathological conditions including cardiac hypertrophy and pulmonary hypertension and congenital syndromes such as generalized lymphatic dysplasia and xerocytosis. In this review, we highlight recent advances in understanding the role of PIEZO channels in cardiovascular functions and diseases. Achievements in this quickly expanding field should open a new road for efficient control of PIEZO-related diseases in cardiovascular functions.

Signaling mechanisms in red blood cells: A view through the protein phosphorylation and deformability

Intracellular signaling mechanisms in red blood cells (RBCs) involve various protein kinases and phosphatases and enable rapid adaptive responses to hypoxia, metabolic requirements, oxidative stress, or shear stress by regulating the physiological properties of the cell. Protein phosphorylation is a ubiquitous mechanism for intracellular signal transduction, volume regulation, and cytoskeletal organization in RBCs. Spectrin-based cytoskeleton connects integral membrane proteins, band 3 and glycophorin C to junctional proteins, ankyrin and Protein 4.1. Phosphorylation leads to a conformational change in the protein structure, weakening the interactions between proteins in the cytoskeletal network that confers a more flexible nature for the RBC membrane. The structural organization of the membrane and the cytoskeleton determines RBC deformability that allows cells to change their ability to deform under shear stress to pass through narrow capillaries. The shear stress sensing mechanisms and oxygenation-deoxygenation transitions regulate cell volume and mechanical properties of the membrane through the activation of ion transporters and specific phosphorylation events mediated by signal transduction. In this review, we summarize the roles of Protein kinase C, cAMP-Protein kinase A, cGMP-nitric oxide, RhoGTPase, and MAP/ERK pathways in the modulation of RBC deformability in both healthy and disease states. We emphasize that targeting signaling elements may be a therapeutic strategy for the treatment of hemoglobinopathies or channelopathies. We expect the present review will provide additional insights into RBC responses to shear stress and hypoxia via signaling mechanisms and shed light on the current and novel treatment options for pathophysiological conditions.

Deciphering and disrupting PIEZO1-TMEM16F interplay in hereditary xerocytosis

ABSTRACT

Cell-surface exposure of phosphatidylserine (PS) is essential for phagocytic clearance and blood clotting. Although a calcium-activated phospholipid scramblase (CaPLSase) has long been proposed to mediate PS exposure in red blood cells (RBCs), its identity, activation mechanism, and role in RBC biology and disease remain elusive. Here, we demonstrate that TMEM16F, the long-sought-after RBC CaPLSase, is activated by calcium influx through the mechanosensitive channel PIEZO1 in RBCs. PIEZO1-TMEM16F functional coupling is enhanced in RBCs from individuals with hereditary xerocytosis (HX), an RBC disorder caused by PIEZO1 gain-of-function channelopathy. Enhanced PIEZO1-TMEM16F coupling leads to an increased propensity to expose PS, which may serve as a key risk factor for HX clinical manifestations including anemia, splenomegaly, and postsplenectomy thrombosis. Spider toxin GsMTx-4 and antigout medication benzbromarone inhibit PIEZO1, preventing force-induced echinocytosis, hemolysis, and PS exposure in HX RBCs. Our study thus reveals an activation mechanism of TMEM16F CaPLSase and its pathophysiological function in HX, providing insights into potential treatment.

A Simplified Method for Calculating Surface Area of Mammalian Erythrocytes

Knowledge of the geometric quantities of the erythrocyte is useful in several physiological studies, both for zoologists and veterinarians. While the diameter and volume (MCV) are easily obtained from observations of blood smears and complete blood count, respectively, the thickness and surface area are instead much more difficult to measure. The precise description of the erythrocyte geometry is given by the equation of the oval of Cassini, but the formulas deriving from it are very complex, comprising elliptic integrals. In this article, three solids are proposed as models approximating the erythrocyte: sphere, cylinder and a spheroid with concave caps. The volumes and surface areas obtained with these models are compared to those effectively measured. The spheroid with concave caps gives the best approximation and can be used as a simple model to determine the erythrocyte surface area. With this model, a simple method that allows one to estimate the surface area by knowing only the diameter and MCV is proposed.

The Gárdos Channel and Piezo1 Revisited: Comparison between Reticulocytes and Mature Red Blood Cells

The Gárdos channel (KCNN4) and Piezo1 are the best-known ion channels in the red blood cell (RBC) membrane. Nevertheless, the quantitative electrophysiological behavior of RBCs and its heterogeneity are still not completely understood. Here, we use state-of-the-art biochemical methods to probe for the abundance of the channels in RBCs. Furthermore, we utilize automated patch clamp, based on planar chips, to compare the activity of the two channels in reticulocytes and mature RBCs. In addition to this characterization, we performed membrane potential measurements to demonstrate the effect of channel activity and interplay on the RBC properties. Both the Gárdos channel and Piezo1, albeit their average copy number of activatable channels per cell is in the single-digit range, can be detected through transcriptome analysis of reticulocytes. Proteomics analysis of reticulocytes and mature RBCs could only detect Piezo1 but not the Gárdos channel. Furthermore, they can be reliably measured in the whole-cell configuration of the patch clamp method. While for the Gárdos channel, the activity in terms of ion currents is higher in reticulocytes compared to mature RBCs, for Piezo1, the tendency is the opposite. While the interplay between Piezo1 and Gárdos channel cannot be followed using the patch clamp measurements, it could be proved based on membrane potential measurements in populations of intact RBCs. We discuss the Gárdos channel and Piezo1 abundance, interdependencies and interactions in the context of their proposed physiological and pathophysiological functions, which are the passing of small constrictions, e.g., in the spleen, and their active participation in blood clot formation and thrombosis.

Piezo1 Regulation Involves Lipid Domains and the Cytoskeleton and Is Favored by the Stomatocyte-Discocyte-Echinocyte Transformation

Piezo1 is a mechanosensitive ion channel required for various biological processes, but its regulation remains poorly understood. Here, we used erythrocytes to address this question since they display Piezo1 clusters, a strong and dynamic cytoskeleton and three types of submicrometric lipid domains, respectively enriched in cholesterol, GM1 ganglioside/cholesterol and sphingomyelin/cholesterol. We revealed that Piezo1 clusters were present in both the rim and the dimple erythrocyte regions. Upon Piezo1 chemical activation by Yoda1, the Piezo1 cluster proportion mainly increased in the dimple area. This increase was accompanied by Ca2+ influx and a rise in echinocytes, in GM1/cholesterol-enriched domains in the dimple and in cholesterol-enriched domains in the rim. Conversely, the effects of Piezo1 activation were abrogated upon membrane cholesterol depletion. Furthermore, upon Piezo1-independent Ca2+ influx, the above changes were not observed. In healthy donors with a high echinocyte proportion, Ca2+ influx, lipid domains and Piezo1 fluorescence were high even at resting state, whereas the cytoskeleton membrane occupancy was lower. Accordingly, upon decreases in cytoskeleton membrane occupancy and stiffness in erythrocytes from patients with hereditary spherocytosis, Piezo1 fluorescence was increased. Altogether, we showed that Piezo1 was differentially controlled by lipid domains and the cytoskeleton and was favored by the stomatocyte-discocyte-echinocyte transformation.

Physical mechanisms of red blood cell splenic filtration

The splenic interendothelial slits fulfill the essential function of continuously filtering red blood cells (RBCs) from the bloodstream to eliminate abnormal and aged cells. To date, the process by which 8 [Formula: see text]m RBCs pass through 0.3 [Formula: see text]m-wide slits remains enigmatic. Does the slit caliber increase during RBC passage as sometimes suggested? Here, we elucidated the mechanisms that govern the RBC retention or passage dynamics in slits by combining multiscale modeling, live imaging, and microfluidic experiments on an original device with submicron-wide physiologically calibrated slits. We observed that healthy RBCs pass through 0.28 [Formula: see text]m-wide rigid slits at 37 °C. To achieve this feat, they must meet two requirements. Geometrically, their surface area-to-volume ratio must be compatible with a shape in two tether-connected equal spheres. Mechanically, the cells with a low surface area-to-volume ratio (28% of RBCs in a 0.4 [Formula: see text]m-wide slit) must locally unfold their spectrin cytoskeleton inside the slit. In contrast, activation of the mechanosensitive PIEZO1 channel is not required. The RBC transit time through the slits follows a [Formula: see text]1 and [Formula: see text]3 power law with in-slit pressure drop and slip width, respectively. This law is similar to that of a Newtonian fluid in a two-dimensional Poiseuille flow, showing that the dynamics of RBCs is controlled by their cytoplasmic viscosity. Altogether, our results show that filtration through submicron-wide slits is possible without further slit opening. Furthermore, our approach addresses the critical need for in vitro evaluation of splenic clearance of diseased or engineered RBCs for transfusion and drug delivery.

Changes in Hematological and Hemorheological Parameters Following Mild COVID-19: A 4-Month Follow-Up Study

BACKGROUND

Coronavirus Disease 2019 (COVID-19) was described to affect red blood cells (RBC) in both severe and mild disease courses. The aim of this study was to investigate whether hematological and hemorheological changes that were previously described for COVID-19 patients after the acute infection state are still prominent after another 4 months to assess potential long-term effects.

METHODS

Hematological and RBC rheological parameters, including deformability and aggregation, were measured 41 days after infection in COVID-19 patients and non-COVID control (T0) and 4 months later in COVID-19 patients (T1).

RESULTS

The data confirm alterations in hematological parameters, mainly related to cell volume and hemoglobin concentration, but also reduced deformability and increased aggregation at T0 compared to control. While RBC deformability seems to have recovered, hemoglobin-related parameters and RBC aggregation were still impaired at T1. The changes were thus more pronounced in male COVID-19 patients.

CONCLUSION

COVID-19-related changes of the RBC partly consist of several months and might be related to persistent symptoms reported by many COVID-19 patients.

A preliminary study of phosphodiesterases and adenylyl cyclase signaling pathway on red blood cell deformability of sickle cell patients

Sickle cell disease (SCD) is an inherited hemoglobinopathy characterized by chronic anemia, intravascular hemolysis, and the occurrence of vaso-occlusive crises due to the mechanical obstruction of the microcirculation by poorly deformable red blood cells (RBCs). RBC deformability is a key factor in the pathogenesis of SCD, and is affected by various factors. In this study, we investigated the effects of adenylyl cyclase (AC) signaling pathway modulation and different phosphodiesterase (PDE) modulatory molecules on the deformability and mechanical stress responses of RBC from SCD patients (HbSS genotype) by applying 5 Pa shear stress with an ektacytometer (LORRCA). We evaluated RBC deformability before and after the application of shear stress. AC stimulation with Forskolin had distinct effects on RBC deformability depending on the application of 5 Pa shear stress. RBC deformability was increased by Forskolin before shear stress application but decreased after 5 Pa shear stress. AC inhibition with SQ22536 and protein kinase A (PKA) inhibition with H89 increased RBC deformability before and after the shear stress application. Non-selective PDE inhibition with Pentoxifylline increased RBC deformability. However, modulation of the different PDE types had distinct effects on RBC deformability, with PDE1 inhibition by Vinpocetine increasing deformability while PDE4 inhibition by Rolipram decreased RBC deformability after the shear stress application. The effects of the drugs varied greatly between patients suggesting some could benefit from one drug while others not. Developing drugs targeting the AC signaling pathway could have clinical applications for SCD, but more researches with larger patient cohorts are needed to identify the differences in the responses of sickle RBCs.

Modulation of red blood cell nitric oxide synthase phosphorylation in the quiescent and exercising human forearm

In vitro investigations demonstrate that human erythrocytes synthesize nitric oxide via a functional isoform of endothelial nitric oxide synthase (NOS) (RBC-NOS). We tested the hypothesis that phosphorylation of RBC-NOS at serine residue 1177 (RBC-NOS1177) would be amplified in blood draining-active skeletal muscle. Furthermore, given hypoxemia modulates local blood flow and thus shear stress, and nitric oxide availability, we performed duplicate experiments under normoxia and hypoxia. Nine healthy volunteers performed rhythmic handgrip exercise at 60% of individualized maximal workload for 3.5 min while breathing room air (normoxia) and after being titrated to an arterial oxygen saturation ≈80% (hypoxemia). We measured brachial artery blood flow by high-resolution duplex ultrasound, while continuously monitoring vascular conductance and mean arterial pressure using finger photoplethysmography. Blood was sampled during the final 30 s of each stage from an indwelling cannula. Blood viscosity was measured to facilitate calculation of accurate shear stresses. Erythrocytes were assessed for levels of phosphorylated RBC-NOS1177 and cellular deformability from blood collected at rest and during exercise. Forearm exercise increased blood flow, vascular conductance, and vascular shear stress, which coincided with a 2.7 ± 0.6-fold increase in RBC-NOS1177 phosphorylation (P < 0.0001) and increased cellular deformability (P < 0.0001) under normoxia. When compared with normoxia, hypoxemia elevated vascular conductance and shear stress (P < 0.05) at rest, while cellular deformability (P < 0.01) and RBC-NOS1177 phosphorylation (P < 0.01) increased. Hypoxemic exercise elicited further increases in vascular conductance, shear stress, and cell deformability (P < 0.0001), although a subject-specific response in RBC-NOS1177 phosphorylation was observed. Our data yield novel insights into the manner that hemodynamic force and oxygen tension modulate RBC-NOS in vivo.

Mechanical stimuli such as shear stress and piezo1 stimulation generate red blood cell extracellular vesicles

Introduction: Generating physiologically relevant red blood cell extracellular vesicles (RBC-EVs) for mechanistic studies is challenging. Herein, we investigated how to generate and isolate high concentrations of RBC-EVs in vitro via shear stress and mechanosensitive piezo1 ion channel stimulation. Methods: RBC-EVs were generated by applying shear stress or the piezo1-agonist yoda1 to RBCs. We then investigated how piezo1 RBC-EV generation parameters (hematocrit, treatment time, treatment dose), isolation methods (membrane-based affinity, ultrafiltration, ultracentrifugation with and without size exclusion chromatography), and storage conditions impacted RBC-EV yield and purity. Lastly, we used pressure myography to determine how RBC-EVs isolated using different methods affected mouse carotid artery vasodilation. Results: Our results showed that treating RBCs at 6% hematocrit with 10 µM yoda1 for 30 min and isolating RBC-EVs via ultracentrifugation minimized hemolysis, maximized yield and purity, and produced the most consistent RBC-EV preparations. Co-isolated contaminants in impure samples, but not piezo1 RBC-EVs, induced mouse carotid artery vasodilation. Conclusion: This work shows that RBC-EVs can be generated through piezo1 stimulation and may be generated in vivo under physiologic flow conditions. Our studies further emphasize the importance of characterizing EV generation and isolation parameters before using EVs for mechanistic analysis since RBC-EV purity can impact functional outcomes.

Piezo1 activation augments sickling propensity and the adhesive properties of sickle red blood cells in a calcium-dependent manner

Haemoglobin S polymerization in the red blood cells (RBCs) of individuals with sickle cell anaemia (SCA) can cause RBC sickling and cellular alterations. Piezo1 is a mechanosensitive protein that modulates intracellular calcium (Ca²⁺ ) influx, and its activation has been associated with increased RBC surface membrane phosphatidylserine (PS) exposure. Hypothesizing that Piezo1 activation, and ensuing Gárdos channel activity, alter sickle RBC properties, RBCs from patients with SCA were incubated with the Piezo1 agonist, Yoda1 (0.1-10 μM). Oxygen-gradient ektacytometry and membrane potential measurement showed that Piezo1 activation significantly decreased sickle RBC deformability, augmented sickling propensity, and triggered pronounced membrane hyperpolarization, in association with Gárdos channel activation and Ca²⁺ influx. Yoda1 induced Ca²⁺ -dependent adhesion of sickle RBCs to laminin, in microfluidic assays, mediated by increased BCAM binding affinity. Furthermore, RBCs from SCA patients that were homo-/heterozygous for the rs59446030 gain-of-function Piezo1 variant demonstrated enhanced sickling under deoxygenation and increased PS exposure. Thus, Piezo1 stimulation decreases sickle RBC deformability, and increases the propensities of these cells to sickle upon deoxygenation and adhere to laminin. Results support a role of Piezo1 in some of the RBC properties that contribute to SCA vaso-occlusion, indicating that Piezo1 may represent a potential therapeutic target molecule for this disease.

Impact of Plasmonic Nanoparticles on Poikilocytosis and Microrheological Properties of Erythrocytes

Plasmonic nanoparticles (NP) possess great potential in photothermal therapy and diagnostics. However, novel NP require a detailed examination for potential toxicity and peculiarities of interaction with cells. Red blood cells (RBC) are important for NP distribution and the development of hybrid RBC-NP delivery systems. This research explored RBC alterations induced by noble (Au and Ag) and nitride-based (TiN and ZrN) laser-synthesized plasmonic NP. Optical tweezers and conventional microscopy modalities indicated the effects arising at non-hemolytic levels, such as RBC poikilocytosis, and alterations in RBC microrheological parameters, elasticity and intercellular interactions. Aggregation and deformability significantly decreased for echinocytes independently of NP type, while for intact RBC, all NP except Ag NP increased the interaction forces but had no effect on RBC deformability. RBC poikilocytosis promoted by NP at concentration 50 μg mL-1 was more pronounced for Au and Ag NP, compared to TiN and ZrN NP. Nitride-based NP demonstrated better biocompatibility towards RBC and higher photothermal efficiency than their noble metal counterparts.

The circulatory dynamics of human red blood cell homeostasis: Oxy-deoxy and PIEZO1-triggered changes

The vital function of red blood cells (RBCs) is to mediate the transport of oxygen from lungs to tissues and of CO2 from tissues to lungs. The gas exchanges occur during capillary transits within fractions of a second. Each oxygenation-deoxygenation and deoxygenation-reoxygenation transition on hemoglobin triggers sharp changes in RBC pH, leading to downstream changes in ion fluxes, membrane potential, and cell volume. The dynamics of these changes during the variable periods between capillary transits in vivo remains a mystery inaccessible to study by current methodologies, a knowledge gap on a fundamental physiological process that is the focus of the present study. The use of a computational model of human RBC homeostasis of tested accreditation enabled a detailed investigation of the expected RBC changes during intercapillary transits, with results advancing novel insights and predictions. The predicted rates of relative RBC volume change on oxygenation-deoxygenation (oxy-deoxy) and deoxygenation-reoxygenation transitions were about 1.5%/min and -0.9%/min, respectively, far too slow to allow the cells to reach steady states in the intervals between capillary transits. The amplitude of the oxy-deoxy-reoxygenation volume fluctuations varied in proportion with the duration of the intercapillary transit intervals. Upon capillary entry, oxy-deoxy-induced changes occur concurrently with deformation-induced PIEZO1 channel activation, both processes affecting cell pH, membrane potential, and cell volume during intertransit periods. The model showed that the effects were strictly additive as expected from processes operating independently on the cell's homeostatic fabric. Analysis of the mechanisms behind these predictions revealed, for the first time, the complex interactions between oxy-deoxy and ion transport processes that ensure the long-term homeostatic stability of RBCs for optimal gas transport in physiological conditions and how these may become altered in diseased states. Possible designs of microfluidic devices to test the model predictions are discussed.

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

The mechanical properties of red blood cells (RBCs) play key roles in their biological functions in microcirculation. In particular, RBCs must deform significantly to travel through microcapillaries with sizes comparable with or even smaller than their own. Although the dynamics of RBCs in microcapillaries have received considerable attention, the effect of membrane viscoelasticity has been largely overlooked. In this work, we present a computational study based on the boundary integral method and thin-shell mechanics to examine how membrane viscoelasticity influences the dynamics of RBCs flowing through straight and constricted microcapillaries. Our results reveal that the cell with a viscoelastic membrane undergoes substantially different motion and deformation compared with results based on a purely elastic membrane model. Comparisons with experimental data also suggest the importance of accounting for membrane viscoelasticity to properly capture the transient dynamics of an RBC flowing through a microcapillary. Taken together, these findings demonstrate the significant effects of membrane viscoelasticity on RBC dynamics in different microcapillary environments. The computational framework also lays the groundwork for more accurate quantitative modeling of the mechanical response of RBCs in their mechanotransduction process in subsequent investigations.

Precision 1070 nm Ultrafast Laser-Induced Photothrombosis of Depth-Targeted Vessels In Vivo

The cerebrovasculature plays an essential role in neurovascular and homeostatic functions in health and disease conditions. Many efforts have been made for developing vascular thrombosis methods to study vascular dysfunction in vivo, while technical challenges remain, such as accuracy and depth-selectivity to target a single vessel in the cerebral cortex. Herein, this paper first demonstrates the evaluation and quantification of the feasibility and effects of Rose Bengal (RB)-induced photothrombosis with 720-1070 nm ultrafast lasers in a raster scan. A flexible and reproducible approach is then proposed to employ a 1070 nm ultrafast laser with a spiral scan for producing RB-induced occlusion, which is described as precision ultrafast laser-induced photothrombosis (PLP). Combine with two-photon microscopy imaging, this PLP displays highly precise and fast occlusion induction of various vessel types, sizes, and depths, which enhances the precision and power of the photothrombosis protocol. Overall, the PLP method provides a real-time, practical, precise, and depth-selected single-vessel photothrombosis technology in the cerebral cortex with commercially available optical equipment, which is crucial for exploring brain vascular function with high spatial-temporal resolution in the brain.

Unidirectional fluxes of monovalent ions in human erythrocytes compared with lymphoid U937 cells: Transient processes after stopping the sodium pump and in response to osmotic challenge

Recently, we have developed software that allows, using a minimum of required experimental data, to find the characteristics of ion homeostasis and a list of all unidirectional fluxes of monovalent ions through the main pathways in the cell membrane both in a balanced state and during the transient processes. Our approach has been successfully validated in human proliferating lymphoid U937 cells during transient processes after stopping the Na/K pump by ouabain and for staurosporine-induced apoptosis. In present study, we used this approach to find the characteristics of ion homeostasis and the monovalent ion fluxes through the cell membrane of human erythrocytes in a resting state and during the transient processes after stopping the Na/K pump with ouabain and in response to osmotic challenge. Due to their physiological significance, erythrocytes remain the object of numerous studies, both experimental and computational methods. Calculations showed that, under physiological conditions, the K+ fluxes through electrodiffusion channels in the entire erythrocyte ion balance is small compared to the fluxes through the Na/K pump and cation-chloride cotransporters. The proposed computer program well predicts the dynamics of the erythrocyte ion balance disorders after stopping the Na/K pump with ouabain. In full accordance with predictions, transient processes in human erythrocytes are much slower than in proliferating cells such as lymphoid U937 cells. Comparison of real changes in the distribution of monovalent ions under osmotic challenge with the calculated ones indicates a change in the parameters of the ion transport pathways through the plasma membrane of erythrocytes in this case. The proposed approach may be useful in studying the mechanisms of various erythrocyte dysfunctions.

Piezo1 as a force-through-membrane sensor in red blood cells

Piezo1 is the stretch activated Ca2+ channel in red blood cells that mediates homeostatic volume control. Here, we study the organization of Piezo1 in red blood cells using a combination of super-resolution microscopy techniques and electron microscopy. Piezo1 adopts a non-uniform distribution on the red blood cell surface, with a bias toward the biconcave 'dimple'. Trajectories of diffusing Piezo1 molecules, which exhibit confined Brownian diffusion on short timescales and hopping on long timescales, also reflect a bias toward the dimple. This bias can be explained by 'curvature coupling' between the intrinsic curvature of the Piezo dome and the curvature of the red blood cell membrane. Piezo1 does not form clusters with itself, nor does it colocalize with F-actin, Spectrin, or the Gardos channel. Thus, Piezo1 exhibits the properties of a force-through-membrane sensor of curvature and lateral tension in the red blood cell.

The coronary capillary bed and its role in blood flow and oxygen delivery: A review

The assumption that the coronary capillary blood flow is exclusively regulated by precapillary vessels is not supported by recent data. Rather, the complex coronary capillary bed has unique structural and geometric characteristics that invalidate many assumptions regarding red blood cell (RBC) transport, for example, data based on a single capillary or that increases in flow are the result of capillary recruitment. It is now recognized that all coronary capillaries are open and that their variations in flow are due to structural differences, local O₂ demand and delivery, and variations in hematocrit. Recent data reveal that local mechanisms within the capillary bed regulate flow via signaling mechanisms involving RBC signaling and endothelial-associated pericytes that contract and relax in response to humoral and neural signaling. The discovery that pericytes respond to vasoactive signals (e.g., nitric oxide, phenylephrine, and adenosine) underscores the role of these cells in regulating capillary diameter and consequently RBC flux and oxygen delivery. RBCs also affect blood flow by sensing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>P</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> and releasing nitric oxide to facilitate relaxation of pericytes and a consequential capillary dilation. New data indicate that these signaling mechanisms allow control of blood flow in specific coronary capillaries according to their oxygen requirements. In conclusion, mechanisms in the coronary capillary bed facilitate RBC density and transit time, hematocrit, blood flow and O₂ delivery, factors that decrease capillary heterogeneity. These findings have important clinical implications for myocardial ischemia and infarction, as well as other vascular diseases.

Erythrocyte oxidative stress and thrombosis

Thrombosis is a common disorder with a relevant burden of morbidity and mortality worldwide, particularly among elderly patients. Growing evidence demonstrated a direct role of oxidative stress in thrombosis, with various cell types contributing to this process. Among them, erythrocytes produce high quantities of intracellular reactive oxygen species (ROS) by NADPH oxidase activation and haemoglobin autoxidation. Concomitantly, extracellular ROS released by other cells in the blood flow can be uptaken and accumulate within erythrocytes. This oxidative milieu can alter erythrocyte membrane structure, leading to an impaired erythrocyte function, and promoting erythrocytes lysis, binding to endothelial cells, activation of platelet and of coagulation factors, phosphatidylserine exposure and release of microvesicles. Moreover, these abnormal erythrocytes are able to adhere to the vessel wall, contributing to thrombin generation within the thrombus. This process results in accelerated haemolysis and in a hypercoagulable state, in which structurally impaired erythrocytes contribute to increase thrombus size, to reduce its permeability and susceptibility to lysis. However, the wide plethora of mechanisms by which oxidised erythrocytes contribute to thrombosis is not completely elucidated. This review discusses the main biochemical aspects linking erythrocytes, oxidative stress and thrombosis, addressing their potential implication for clinical and therapeutic management.

New KCNN4 Variants Associated With Anemia: Stomatocytosis Without Erythrocyte Dehydration

The K+ channel activated by the Ca2+, KCNN4, has been shown to contribute to red blood cell dehydration in the rare hereditary hemolytic anemia, the dehydrated hereditary stomatocytosis. We report two de novo mutations on KCNN4, We reported two de novo mutations on KCNN4, V222L and H340N, characterized at the molecular, cellular and clinical levels. Whereas both mutations were shown to increase the calcium sensitivity of the K+ channel, leading to channel opening for lower calcium concentrations compared to WT KCNN4 channel, there was no obvious red blood cell dehydration in patients carrying one or the other mutation. The clinical phenotype was greatly different between carriers of the mutated gene ranging from severe anemia for one patient to a single episode of anemia for the other patient or no documented sign of anemia for the parents who also carried the mutation. These data compared to already published KCNN4 mutations question the role of KCNN4 gain-of-function mutations in hydration status and viability of red blood cells in bloodstream.

Fire-Shaped Nozzles to Produce a Stress Peak for Deformability Studies

Fire-shaped nozzles can be used to study the deformability of microcapsules, particles, or cells traveling in a flow. Though their geometry depends on the dimensions of the original glass capillary and the heating conditions, they all produce a strain rate peak approximately at the section where the diameter is 1.5 times the minimum. The intensity of this peak and the time from its position to the neck can be easily estimated from the flow rate and three geometrical parameters, without the need for any simulation. In the convergent region of these nozzles, it is possible to observe the evolution of the deformation. It is necessary to use a sufficiently long nozzle to produce the maximum deformation before the neck.

Cyclic Mechanical Stresses Alter Erythrocyte Membrane Composition and Microstructure and Trigger Macrophage Phagocytosis

Red blood cells (RBCs) are cleared from the circulation when they become damaged or display aging signals targeted by macrophages. This process occurs mainly in the spleen, where blood flows through submicrometric constrictions called inter-endothelial slits (IES), subjecting RBCs to large-amplitude deformations. In this work, RBCs are circulated through microfluidic devices containing microchannels that replicate the IES. The cyclic mechanical stresses experienced by the cells affect their biophysical properties and molecular composition, accelerating cell aging. Specifically, RBCs quickly transition to a more spherical, less deformable phenotype that hinders microchannel passage, causing hemolysis. This transition is associated with the release of membrane vesicles, which self-extinguishes as the spacing between membrane-cytoskeleton linkers becomes tighter. Proteomics analysis of the mechanically aged RBCs reveals significant losses of essential proteins involved in antioxidant protection, gas transport, and cell metabolism. Finally, it is shown that these changes make mechanically aged RBCs more susceptible to macrophage phagocytosis. These results provide a comprehensive model explaining how physical stress induces RBC clearance in the spleen. The data also suggest new biomarkers of early "hemodamage" and inflammation preceding hemolysis in RBCs subjected to mechanical stress.

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.

Piezo1 regulates shear-dependent nitric oxide production in human erythrocytes

Mature circulating red blood cells (RBC) are classically viewed as passive participants in circulatory function, given erythroblasts eject their organelles during maturation. Endogenous production of nitric oxide (NO) and its effects are of particular significance; however, the integration between RBC sensation of the local environment and subsequent activation of mechano-sensitive signaling networks that generate NO remain poorly understood. The present study investigated endogenous NO-production via the RBC-specific nitric oxide synthase-isoform (RBC-NOS), connecting membrane strain with intracellular enzymatic processes. Isolated RBC were obtained from apparently healthy humans. Intracellular NO was compared at rest and following shear (cellular deformation) using semi-quantitative fluorescent imaging. Concurrently, RBC-NOS phosphorylation at its Serine¹¹⁷⁷ (ser¹¹⁷⁷) residue was measured. The contribution of cellular deformation to shear-induced NO-production in RBC was determined by rigidifying RBC with the thiol-oxidizing agent diamide; rigid RBC exhibited significantly impaired (up to 80%) capacity to generate NO via RBC-NOS during shear. Standardizing membrane strain of rigid RBC by applying increased shear did not normalize NO-production, or RBC-NOS activation. Calcium-imaging with Fluo-4 revealed that diamide-treated RBC exhibited a 42%-impairment in Piezo1-mediated calcium-movement when compared with untreated RBC. Pharmacological inhibition of Piezo1 with GsMTx4 during shear inhibited RBC-NOS activation in untreated RBC, while Piezo1-activation with Yoda1 in the absence of shear stimulated RBC-NOS activation. Collectively, a novel, mechanically-activated signaling pathway in mature RBC is described. Opening of Piezo1 and subsequent influx of calcium appears to be required for endogenous production of NO in response to mechanical shear, which is accompanied by phosphorylation of RBC-NOS at ser¹¹⁷⁷.

Piezo1 Is a Mechanosensor Channel in Central Nervous System Capillaries

Capillaries are equipped to sense neurovascular coupling agents released onto the outer wall of a capillary, translating these external signals into electrical/Ca2+ changes that play a crucial role in blood flow regulation and ensuring that neuronal demands are met. However, control mechanisms attributable to forces imposed onto the lumen are less clear. Here, we show that Piezo1 channels act as mechanosensors in central nervous system capillaries. Electrophysiological analyses confirmed expression and function of Piezo1 channels in brain cortical and retinal capillaries. Activation of Piezo1 channels evoked currents that were sensitive to endothelial cell-specific Piezo1 deletion. Using genetically encoded Ca2+ indicator mice and an ex vivo pressurized retina preparation, we found that activation of Piezo1 channels by mechanical forces triggered Ca2+ signals in capillary endothelial cells. Collectively, these findings indicate that Piezo1 channels are capillary mechanosensors that initiate crucial Ca2+ signals and could, therefore, have a profound impact on central nervous system blood flow control.

Erysense, a Lab-on-a-Chip-Based Point-of-Care Device to Evaluate Red Blood Cell Flow Properties With Multiple Clinical Applications

In many medical disciplines, red blood cells are discovered to be biomarkers since they "experience" various conditions in basically all organs of the body. Classical examples are diabetes and hypercholesterolemia. However, recently the red blood cell distribution width (RDW), is often referred to, as an unspecific parameter/marker (e.g., for cardiac events or in oncological studies). The measurement of RDW requires venous blood samples to perform the complete blood cell count (CBC). Here, we introduce Erysense, a lab-on-a-chip-based point-of-care device, to evaluate red blood cell flow properties. The capillary chip technology in combination with algorithms based on artificial neural networks allows the detection of very subtle changes in the red blood cell morphology. This flow-based method closely resembles in vivo conditions and blood sample volumes in the sub-microliter range are sufficient. We provide clinical examples for potential applications of Erysense as a diagnostic tool [here: neuroacanthocytosis syndromes (NAS)] and as cellular quality control for red blood cells [here: hemodiafiltration (HDF) and erythrocyte concentrate (EC) storage]. Due to the wide range of the applicable flow velocities (0.1-10 mm/s) different mechanical properties of the red blood cells can be addressed with Erysense providing the opportunity for differential diagnosis/judgments. Due to these versatile properties, we anticipate the value of Erysense for further diagnostic, prognostic, and theragnostic applications including but not limited to diabetes, iron deficiency, COVID-19, rheumatism, various red blood cell disorders and anemia, as well as inflammation-based diseases including sepsis.

Continuous Percoll Gradient Centrifugation of Erythrocytes-Explanation of Cellular Bands and Compromised Age Separation

(1) Background: When red blood cells are centrifuged in a continuous Percoll-based density gradient, they form discrete bands. While this is a popular approach for red blood cell age separation, the mechanisms involved in banding were unknown. (2) Methods: Percoll centrifugations of red blood cells were performed under various experimental conditions and the resulting distributions analyzed. The age of the red blood cells was measured by determining the protein band 4.1a to 4.1b ratio based on western blots. Red blood cell aggregates, so-called rouleaux, were monitored microscopically. A mathematical model for the centrifugation process was developed. (3) Results: The red blood cell band pattern is reproducible but re-centrifugation of sub-bands reveals a new set of bands. This is caused by red blood cell aggregation. Based on the aggregation, our mathematical model predicts the band formation. Suppression of red blood cell aggregation reduces the band formation. (4) Conclusions: The red blood cell band formation in continuous Percoll density gradients could be explained physically by red blood cell aggregate formation. This aggregate formation distorts the density-based red blood cell age separation. Suppressing aggregation by osmotic swelling has a more severe effect on compromising the RBC age separation to a higher degree.

Proteomic Analysis of the Role of the Adenylyl Cyclase-cAMP Pathway in Red Blood Cell Mechanical Responses

Red blood cell (RBC) deformability is modulated by the phosphorylation status of the cytoskeletal proteins that regulate the interactions of integral transmembrane complexes. Proteomic studies have revealed that receptor-related signaling molecules and regulatory proteins involved in signaling cascades are present in RBCs. In this study, we investigated the roles of the cAMP signaling mechanism in modulating shear-induced RBC deformability and examined changes in the phosphorylation of the RBC proteome. We implemented the inhibitors of adenylyl cyclase (SQ22536), protein kinase A (H89), and phosphodiesterase (PDE) (pentoxifylline) to whole blood samples, applied 5 Pa shear stress (SS) for 300 s with a capillary tubing system, and evaluated RBC deformability using a LORRCA MaxSis. The inhibition of signaling molecules significantly deteriorated shear-induced RBC deformability (p < 0.05). Capillary SS slightly increased the phosphorylation of RBC cytoskeletal proteins. Tyrosine phosphorylation was significantly elevated by the modulation of the cAMP/PKA pathway (p < 0.05), while serine phosphorylation significantly decreased as a result of the inhibition of PDE (p < 0.05). AC is the core element of this signaling pathway, and PDE works as a negative feedback mechanism that could have potential roles in SS-induced RBC deformability. The cAMP/PKA pathway could regulate RBC deformability during capillary transit by triggering significant alterations in the phosphorylation state of RBCs.

Fluorescence-coupled micropipette aspiration assay to examine calcium mobilization caused by red blood cell mechanosensing

Mechanical stimuli such as tension, compression, and shear stress play critical roles in the physiological functions of red blood cells (RBCs) and their homeostasis, ATP release, and rheological properties. Intracellular calcium (Ca²⁺) mobilization reflects RBC mechanosensing as they transverse the complex vasculature. Emerging studies have demonstrated the presence of mechanosensitive Ca²⁺ permeable ion channels and their function has been implicated in the regulation of RBC volume and deformability. However, how these mechanoreceptors trigger Ca²⁺ influx and subsequent cellular responses are still unclear. Here, we introduce a fluorescence-coupled micropipette aspiration assay to examine RBC mechanosensing at the single-cell level. To achieve a wide range of cell aspirations, we implemented and compared two negative pressure adjusting apparatuses: a homemade water manometer (- 2.94 to 0 mmH₂O) and a pneumatic high-speed pressure clamp (- 25 to 0 mmHg). To visualize Ca²⁺ influx, RBCs were pre-loaded with an intensiometric probe Cal-520 AM, then imaged under a confocal microscope with concurrent bright-field and fluorescent imaging at acquisition rates of 10 frames per second. Remarkably, we observed the related changes in intracellular Ca²⁺ levels immediately after aspirating individual RBCs in a pressure-dependent manner. The RBC aspirated by the water manometer only displayed 1.1-fold increase in fluorescence intensity, whereas the RBC aspirated by the pneumatic clamp showed up to threefold increase. These results demonstrated the water manometer as a gentle tool for cell manipulation with minimal pre-activation, while the high-speed pneumatic clamp as a much stronger pressure actuator to examine cell mechanosensing directly. Together, this multimodal platform enables us to precisely control aspiration and membrane tension, and subsequently correlate this with intracellular calcium concentration dynamics in a robust and reproducible manner.

The Function of Ion Channels and Membrane Potential in Red Blood Cells: Toward a Systematic Analysis of the Erythroid Channelome

Erythrocytes represent at least 60% of all cells in the human body. During circulation, they experience a huge variety of physical and chemical stimulations, such as pressure, shear stress, hormones or osmolarity changes. These signals are translated into cellular responses through ion channels that modulate erythrocyte function. Ion channels in erythrocytes are only recently recognized as utmost important players in physiology and pathophysiology. Despite this awareness, their signaling, interactions and concerted regulation, such as the generation and effects of “pseudo action potentials”, remain elusive. We propose a systematic, conjoined approach using molecular biology, in vitro erythropoiesis, state-of-the-art electrophysiological techniques, and channelopathy patient samples to decipher the role of ion channel functions in health and disease. We need to overcome challenges such as the heterogeneity of the cell population (120 days lifespan without protein renewal) or the access to large cohorts of patients. Thereto we will use genetic manipulation of progenitors, cell differentiation into erythrocytes, and statistically efficient electrophysiological recordings of ion channel activity.

PIEZO1 mediates a mechanothrombotic pathway in diabetes

[Figure: see text].

Deregulated calcium signaling in blood cancer: Underlying mechanisms and therapeutic potential

Intracellular calcium signaling regulates diverse physiological and pathological processes. In solid tumors, changes to calcium channels and effectors via mutations or changes in expression affect all cancer hallmarks. Such changes often disrupt transport of calcium ions (Ca²⁺) in the endoplasmic reticulum (ER) or mitochondria, impacting apoptosis. Evidence rapidly accumulates that this is similar in blood cancer. Principles of intracellular Ca²⁺ signaling are outlined in the introduction. We describe different Ca²⁺-toolkit components and summarize the unique relationship between extracellular Ca²⁺ in the endosteal niche and hematopoietic stem cells. The foundational data on Ca²⁺ homeostasis in red blood cells is discussed, with the demonstration of changes in red blood cell disorders. This leads to the role of Ca²⁺ in neoplastic erythropoiesis. Then we expand onto the neoplastic impact of deregulated plasma membrane Ca²⁺ channels, ER Ca²⁺ channels, Ca²⁺ pumps and exchangers, as well as Ca²⁺ sensor and effector proteins across all types of hematologic neoplasms. This includes an overview of genetic variants in the Ca²⁺-toolkit encoding genes in lymphoid and myeloid cancers as recorded in publically available cancer databases. The data we compiled demonstrate that multiple Ca²⁺ homeostatic mechanisms and Ca²⁺ responsive pathways are altered in hematologic cancers. Some of these alterations may have genetic basis but this requires further investigation. Most changes in the Ca²⁺-toolkit do not appear to define/associate with specific disease entities but may influence disease grade, prognosis, treatment response, and certain complications. Further elucidation of the underlying mechanisms may lead to novel treatments, with the aim to tailor drugs to different patterns of deregulation. To our knowledge this is the first review of its type in the published literature. We hope that the evidence we compiled increases awareness of the calcium signaling deregulation in hematologic neoplasms and triggers more clinical studies to help advance this field.

The Chloride Conductance Inhibitor NS3623 Enhances the Activity of a Non-selective Cation Channel in Hyperpolarizing Conditions

Handbooks of physiology state that the strategy adopted by red blood cells (RBCs) to preserve cell volume is to maintain membrane permeability for cations at its minimum. However, enhanced cation permeability can be measured and observed in specific physiological and pathophysiological situations such as in vivo senescence, storage at low temperature, sickle cell anemia and many other genetic defects affecting transporters, membrane or cytoskeletal proteins. Among cation pathways, cation channels are able to dissipate rapidly the gradients that are built and maintained by the sodium and calcium pumps. These situations are very well-documented but a mechanistic understanding of complex electrophysiological events underlying ion transports is still lacking. In addition, non-selective cation (NSC) channels present in the RBC membrane have proven difficult to molecular identification and functional characterization. For instance, NSC channel activity can be elicited by Low Ionic Strength conditions (LIS): the associated change in membrane potential triggers its opening in a voltage dependent manner. But, whereas this depolarizing media produces a spectacular activation of NSC channel, Gárdos channel-evoked hyperpolarization's have been shown to induce sodium entry through a pathway thought to be conductive and termed P_cat. Using the CCCP method, which allows to follow fast changes in membrane potential, we show here (i) that hyperpolarization elicited by Gárdos channel activation triggers sodium entry through a conductive pathway, (ii) that chloride conductance inhibition unveils such conductive cationic conductance, (iii) that the use of the specific chloride conductance inhibitor NS3623 (a derivative of Neurosearch compound NS1652), at concentrations above what is needed for full anion channel block, potentiates the non-selective cation conductance. These results indicate that a non-selective cation channel is likely activated by the changes in the driving force for cations rather than a voltage dependence mechanism per se.

Mechanism of megaloblastic anemia combined with hemolysis

Megaloblastic anemia (MA) patients often exhibit hemolysis, but it is not clear whether there are other hemolytic mechanisms in addition to intramedullary hemolysis. We retrospectively analyzed the clinical characteristics of 124 MA patients, measured erythrocyte physical parameters in two patients with hemolysis and one healthy volunteer by atomic force microscopy, and measured 18F-FDG uptake in one MA patient with hemolysis. In multivariate analysis, hemolysis was associated with mean corpuscular volume (MCV) and indirect bilirubin. A receiver operating characteristic curve analysis, with sensitivity of 83.1% and specificity of 68.7%, suggested that the MCV cutoff value that predicts hemolysis is 116.4 fL. Hb was negatively correlated with MCV in the hemolysis group (r = −0.317, P = 0.007) but not in the nonhemolysis group. The erythrocyte peak-valley value, average cell surface roughness and surface area in the MA patients with hemolysis were significantly lower than those in controls (P < 0.05). 18F-FDG uptake by the liver and spleen was diffuse and increased in MA patients undergoing hemolysis. MA combined with extramedullary hemolysis could be caused by macrophages removing mechanically damaged erythrocytes and the retention of erythrocytes with decreased deformability when blood circulates through narrow spaces in the liver and spleen.

Mechanistic ion channel interactions in red cells of patients with Gárdos channelopathy

In patients with Gárdos channelopathy (p.R352H), an increased concentration of intracellular Ca2+ was previously reported. This is a surprising finding because the Gárdos channel (KCa3.1) is a K+ channel. Here, we confirm the increased intracellular Ca2+ for patients with the KCa3.1 mutation p.S314P. Furthermore, we provide the concept of KCa3.1 activity resulting in a flickering of red blood cell (RBC) membranepotential, which activates the CaV2.1 channel allowing Ca2+ to enter the RBC. Activity of the nonselective cation channel Piezo1 modulates the aforementioned interplay in away that a closed Piezo1 is in favor of the KCa3.1-CaV2.1 interaction. In contrast, Piezo1 openings compromise the membrane potential flickering, thus limiting the activity of CaV2.1. With the compound NS309, we mimic a gain-of-function mutation of KCa3.1. Assessing the RBC Ca2+ response by Fluo-4-based flow cytometry and by measuring the membrane potential using the Macey-Bennekou-Egée method, we provide data that support the concept of the KCa3.1/CaV2.1/Piezo1 interplay as a partial explanation for an increased number of high Ca2+ RBCs. With the pharmacological inhibition of KCa3.1 (TRAM34 and Senicapoc), CaV2.1 (ω-agatoxin TK), and Piezo1 (GsMTx-4), we could project the NS309 behavior of healthy RBCs to the RBCs of Gárdos channelopathy patients.

Gardos channelopathy: functional analysis of a novel KCNN4 variant

We show that the novel KCNN4 variant p.S314P is a gain-of-function mutation but is less severe than the previously reported p.R352H variant. The clinical heterogeneity, blurred symptoms, and absence of specific diagnostic markers make the diagnosis of Gardos channelopathy challenging.

Red cell membrane disorders: structure meets function

The mature red blood cell (RBC) lacks a nucleus and organelles characteristic of most cells, but it is elegantly structured to perform the essential function of delivering oxygen and removing carbon dioxide from all other cells while enduring the shear stress imposed by navigating small vessels and sinusoids. Over the past several decades, the efforts of biochemists, cell and molecular biologists, and hematologists have provided an appreciation of the complexity of RBC membrane structure, while studies of the RBC membrane disorders have offered valuable insights into structure-function relationships. Within the last decade, advances in genetic testing and its increased availability have made it possible to substantially build upon this foundational knowledge. Although disorders of the RBC membrane due to altered structural organization or altered transport function are heterogeneous, they often present with common clinical findings of hemolytic anemia. However, they may require substantially different management depending on the underlying pathophysiology. Accurate diagnosis is essential to avoid emergence of complications or inappropriate interventions. We propose an algorithm for laboratory evaluation of patients presenting with symptoms and signs of hemolytic anemia with a focus on RBC membrane disorders. Here, we review the genotypic and phenotypic variability of the RBC membrane disorders in order to raise the index of suspicion and highlight the need for correct and timely diagnosis.

An Insight into the Stages of Ion Leakage during Red Blood Cell Storage

Packed red blood cells (pRBCs), the most commonly transfused blood product, are exposed to environmental disruptions during storage in blood banks. In this study, temporal sequence of changes in the ion exchange in pRBCs was analyzed. Standard techniques commonly used in electrolyte measurements were implemented. The relationship between ion exchange and red blood cells (RBCs) morphology was assessed with use of atomic force microscopy with reference to morphological parameters. Variations observed in the Na⁺, K⁺, Cl⁻, H⁺, HCO₃⁻, and lactate ions concentration show a complete picture of singly-charged ion changes in pRBCs during storage. Correlation between the rate of ion changes and blood group type, regarding the limitations of our research, suggested, that group 0 is the most sensitive to the time-dependent ionic changes. Additionally, the impact of irreversible changes in ion exchange on the RBCs membrane was observed in nanoscale. Results demonstrate that the level of ion leakage that leads to destructive alterations in biochemical and morphological properties of pRBCs depend on the storage timepoint.

PIEZO1 and the mechanism of the long circulatory longevity of human red blood cells

Human red blood cells (RBCs) have a circulatory lifespan of about four months. Under constant oxidative and mechanical stress, but devoid of organelles and deprived of biosynthetic capacity for protein renewal, RBCs undergo substantial homeostatic changes, progressive densification followed by late density reversal among others, changes assumed to have been harnessed by evolution to sustain the rheological competence of the RBCs for as long as possible. The unknown mechanisms by which this is achieved are the subject of this investigation. Each RBC traverses capillaries between 1000 and 2000 times per day, roughly one transit per minute. A dedicated Lifespan model of RBC homeostasis was developed as an extension of the RCM introduced in the previous paper to explore the cumulative patterns predicted for repetitive capillary transits over a standardized lifespan period of 120 days, using experimental data to constrain the range of acceptable model outcomes. Capillary transits were simulated by periods of elevated cell/medium volume ratios and by transient deformation-induced permeability changes attributed to PIEZO1 channel mediation as outlined in the previous paper. The first unexpected finding was that quantal density changes generated during single capillary transits cease accumulating after a few days and cannot account for the observed progressive densification of RBCs on their own, thus ruling out the quantal hypothesis. The second unexpected finding was that the documented patterns of RBC densification and late reversal could only be emulated by the implementation of a strict time-course of decay in the activities of the calcium and Na/K pumps, suggestive of a selective mechanism enabling the extended longevity of RBCs. The densification pattern over most of the circulatory lifespan was determined by calcium pump decay whereas late density reversal was shaped by the pattern of Na/K pump decay. A third finding was that both quantal changes and pump-decay regimes were necessary to account for the documented lifespan pattern, neither sufficient on their own. A fourth new finding revealed that RBCs exposed to levels of PIEZO1-medited calcium permeation above certain thresholds in the circulation could develop a pattern of early or late hyperdense collapse followed by delayed density reversal. When tested over much reduced lifespan periods the results reproduced the known circulatory fate of irreversible sickle cells, the cell subpopulation responsible for vaso-occlusion and for most of the clinical manifestations of sickle cell disease. Analysis of the results provided an insightful new understanding of the mechanisms driving the changes in RBC homeostasis during circulatory aging in health and disease.

The average circulatory lifespan of human red blood cells is about four months, amounting to about 200000 capillary transits. Among the many documented age-related changes red cells experience during this long sojourn the most relevant to homeostasis control comprise progressive densification with late density reversal, decline in the activities of calcium and sodium-potassium pumps, and slow inverse changes in their original sodium and potassium contents. Early experimental results have long established the view that these changes result from the cumulative effects of myriad capillary transits. However, many aspects of this process remain inaccessible to in vivo investigation. This prompted us to attempt a modelling approach applying a dedicated extension to our original red cell model. The results relegated the cumulative mechanism to a secondary role and exposed surprising critical roles for the declining patterns of the calcium and sodium-potassium pumps, as if harnessed by evolution to extend the circulatory longevity of cells within volume ranges that enable optimal rheological performance. The mechanism the model revealed implicated complex interactions between PIEZO1, the calcium-activated potassium channel KCNN4, the anion exchanger AE1, and the calcium and sodium-potassium pumps. These studies proved the model potential for exploring red cell homeostasis in health and disease.

Up-down biphasic volume response of human red blood cells to PIEZO1 activation during capillary transits

In this paper we apply a novel JAVA version of a model on the homeostasis of human red blood cells (RBCs) to investigate the changes RBCs experience during single capillary transits. In the companion paper we apply a model extension to investigate the changes in RBC homeostasis over the approximately 200000 capillary transits during the ~120 days lifespan of the cells. These are topics inaccessible to direct experimentation but rendered mature for a computational modelling approach by the large body of recent and early experimental results which robustly constrain the range of parameter values and model outcomes, offering a unique opportunity for an in depth study of the mechanisms involved. Capillary transit times vary between 0.5 and 1.5s during which the red blood cells squeeze and deform in the capillary stream transiently opening stress-gated PIEZO1 channels allowing ion gradient dissipation and creating minuscule quantal changes in RBC ion contents and volume. Widely accepted views, based on the effects of experimental shear stress on human RBCs, suggested that quantal changes generated during capillary transits add up over time to develop the documented changes in RBC density and composition during their long circulatory lifespan, the quantal hypothesis. Applying the new red cell model (RCM) we investigated here the changes in homeostatic variables that may be expected during single capillary transits resulting from transient PIEZO1 channel activation. The predicted quantal volume changes were infinitesimal in magnitude, biphasic in nature, and essentially irreversible within inter-transit periods. A sub-second transient PIEZO1 activation triggered a sharp swelling peak followed by a much slower recovery period towards lower-than-baseline volumes. The peak response was caused by net CaCl2 and fluid gain via PIEZO1 channels driven by the steep electrochemical inward Ca2+ gradient. The ensuing dehydration followed a complex time-course with sequential, but partially overlapping contributions by KCl loss via Ca2+-activated Gardos channels, restorative Ca2+ extrusion by the plasma membrane calcium pump, and chloride efflux by the Jacobs-Steward mechanism. The change in relative cell volume predicted for single capillary transits was around 10-5, an infinitesimal volume change incompatible with a functional role in capillary flow. The biphasic response predicted by the RCM appears to conform to the quantal hypothesis, but whether its cumulative effects could account for the documented changes in density during RBC senescence required an investigation of the effects of myriad transits over the full four months circulatory lifespan of the cells, the subject of the next paper.

Lysophosphatidic Acid-Activated Calcium Signaling Is Elevated in Red Cells from Sickle Cell Disease Patients

(1) Background: It is known that sickle cells contain a higher amount of Ca2+ compared to healthy red blood cells (RBCs). The increased Ca2+ is associated with the most severe symptom of sickle cell disease (SCD), the vaso-occlusive crisis (VOC). The Ca2+ entry pathway received the name of Psickle but its molecular identity remains only partly resolved. We aimed to map the involved Ca2+ signaling to provide putative pharmacological targets for treatment. (2) Methods: The main technique applied was Ca2+ imaging of RBCs from healthy donors, SCD patients and a number of transgenic mouse models in comparison to wild-type mice. Life-cell Ca2+ imaging was applied to monitor responses to pharmacological targeting of the elements of signaling cascades. Infection as a trigger of VOC was imitated by stimulation of RBCs with lysophosphatidic acid (LPA). These measurements were complemented with biochemical assays. (3) Results: Ca2+ entry into SCD RBCs in response to LPA stimulation exceeded that of healthy donors. LPA receptor 4 levels were increased in SCD RBCs. Their activation was followed by the activation of Gi protein, which in turn triggered opening of TRPC6 and CaV2.1 channels via a protein kinase Cα and a MAP kinase pathway, respectively. (4) Conclusions: We found a new Ca2+ signaling cascade that is increased in SCD patients and identified new pharmacological targets that might be promising in addressing the most severe symptom of SCD, the VOC.

Mechanical Stress Induces Ca2+-Dependent Signal Transduction in Erythroblasts and Modulates Erythropoiesis

Bioreactors are increasingly implemented for large scale cultures of various mammalian cells, which requires optimization of culture conditions. Such upscaling is also required to produce red blood cells (RBC) for transfusion and therapy purposes. However, the physiological suitability of RBC cultures to be transferred to stirred bioreactors is not well understood. PIEZO1 is the most abundantly expressed known mechanosensor on erythroid cells. It is a cation channel that translates mechanical forces directly into a physiological response. We investigated signaling cascades downstream of PIEZO1 activated upon transitioning stationary cultures to orbital shaking associated with mechanical stress, and compared the results to direct activation of PIEZO1 by the chemical agonist Yoda1. Erythroblasts subjected to orbital shaking displayed decreased proliferation, comparable to incubation in the presence of a low dose of Yoda1. Epo (Erythropoietin)-dependent STAT5 phosphorylation, and Calcineurin-dependent NFAT dephosphorylation was enhanced. Phosphorylation of ERK was also induced by both orbital shaking and Yoda1 treatment. Activation of these pathways was inhibited by intracellular Ca²⁺ chelation (BAPTA-AM) in the orbital shaker. Our results suggest that PIEZO1 is functional and could be activated by the mechanical forces in a bioreactor setup, and results in the induction of Ca²⁺-dependent signaling cascades regulating various aspects of erythropoiesis. With this study, we showed that Yoda1 treatment and mechanical stress induced via orbital shaking results in comparable activation of some Ca²⁺-dependent pathways, exhibiting that there are direct physiological outcomes of mechanical stress on erythroblasts.