Hereditary xerocytosis revisited

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.

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.

Pseudohyperkalemia: Three Cases and a Review of Literature

Hyperkalemia is a potentially fatal complication requiring prompt diagnosis and management. However, pseudohyperkalemia, defined as an artificial rise in serum potassium (S_k), is also an important diagnosis because management differs. Pseudohyperkalemia can result from multiple factors, including excessive potassium leakage from cells of the forearm during blood collection due to release from exercising the muscle during fist clenching, while washout is prevented by tourniquet application, hemolysis, problems with sample transport, preanalysis or contamination, cell damage and metabolic changes, familial conditions that permit excessive potassium ion (K⁺) leak from erythrocytes after blood sampling, and leukocytosis or thrombocytosis. In this review, we will discuss the major causes of pseudohyperkalemia, how to avoid certain diagnostic pitfalls, and comment on the clinical importance of recognizing these false readings. We will review three clinical cases seen in our nephrology and hypertension clinic that illustrate some of these problems.

Fine-Tuning of Piezo1 Expression and Activity Ensures Efficient Myoblast Fusion during Skeletal Myogenesis

Mechanical stimuli, such as stretch and resistance training, are essential in regulating the growth and functioning of skeletal muscles. However, the molecular mechanisms involved in sensing mechanical stress during muscle formation remain unclear. Here, we investigated the role of the mechanosensitive ion channel Piezo1 during myogenic progression of both fast and slow muscle satellite cells. We found that Piezo1 level increases during myogenic differentiation and direct manipulation of Piezo1 in muscle stem cells alters the myogenic progression. Indeed, Piezo1 knockdown suppresses myoblast fusion, leading to smaller myotubes. Such an event is accompanied by significant downregulation of the fusogenic protein Myomaker. In parallel, while Piezo1 knockdown also lowers Ca²⁺ influx in response to stretch, Piezo1 activation increases Ca²⁺ influx in response to stretch and enhances myoblasts fusion. These findings may help understand molecular defects present in some muscle diseases. Our study shows that Piezo1 is essential for terminal muscle differentiation acting on myoblast fusion, suggesting that Piezo1 deregulation may have implications in muscle aging and degenerative diseases, including muscular dystrophies and neuromuscular disorders.

Diagnosis and clinical management of red cell membrane disorders

Heterogeneous red blood cell (RBC) membrane disorders and hydration defects often present with the common clinical findings of hemolytic anemia, but they may require substantially different management, based on their pathophysiology. An accurate and timely diagnosis is essential to avoid inappropriate interventions and prevent complications. Advances in genetic testing availability within the last decade, combined with extensive foundational knowledge on RBC membrane structure and function, now facilitate the correct diagnosis in patients with a variety of hereditary hemolytic anemias (HHAs). Studies in patient cohorts with well-defined genetic diagnoses have revealed complications such as iron overload in hereditary xerocytosis, which is amenable to monitoring, prevention, and treatment, and demonstrated that splenectomy is not always an effective or safe treatment for any patient with HHA. However, a multitude of variants of unknown clinical significance have been discovered by genetic evaluation, requiring interpretation by thorough phenotypic assessment in clinical and/or research laboratories. Here we discuss genotype-phenotype correlations and corresponding clinical management in patients with RBC membranopathies and propose an algorithm for the laboratory workup of patients presenting with symptoms and signs of hemolytic anemia, with a clinical case that exemplifies such a workup.

Surface model of the human red blood cell simulating changes in membrane curvature under strain

We present mathematical simulations of shapes of red blood cells (RBCs) and their cytoskeleton when they are subjected to linear strain. The cell surface is described by a previously reported quartic equation in three dimensional (3D) Cartesian space. Using recently available functions in Mathematica to triangularize the surfaces we computed four types of curvature of the membrane. We also mapped changes in mesh-triangle area and curvatures as the RBCs were distorted. The highly deformable red blood cell (erythrocyte; RBC) responds to mechanically imposed shape changes with enhanced glycolytic flux and cation transport. Such morphological changes are produced experimentally by suspending the cells in a gelatin gel, which is then elongated or compressed in a custom apparatus inside an NMR spectrometer. A key observation is the extent to which the maximum and minimum Principal Curvatures are localized symmetrically in patches at the poles or equators and distributed in rings around the main axis of the strained RBC. Changes on the nanometre to micro-meter scale of curvature, suggest activation of only a subset of the intrinsic mechanosensitive cation channels, Piezo1, during experiments carried out with controlled distortions, which persist for many hours. This finding is relevant to a proposal for non-uniform distribution of Piezo1 molecules around the RBC membrane. However, if the curvature that gates Piezo1 is at a very fine length scale, then membrane tension will determine local curvature; so, curvatures as computed here (in contrast to much finer surface irregularities) may not influence Piezo1 activity. Nevertheless, our analytical methods can be extended address these new mechanistic proposals. The geometrical reorganization of the simulated cytoskeleton informs ideas about the mechanism of concerted metabolic and cation-flux responses of the RBC to mechanically imposed shape changes.

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.

Plasmodium vivax infection compromises reticulocyte stability

The structural integrity of the host red blood cell (RBC) is crucial for propagation of Plasmodium spp. during the disease-causing blood stage of malaria infection. To assess the stability of Plasmodium vivax-infected reticulocytes, we developed a flow cytometry-based assay to measure osmotic stability within characteristically heterogeneous reticulocyte and P. vivax-infected samples. We find that erythroid osmotic stability decreases during erythropoiesis and reticulocyte maturation. Of enucleated RBCs, young reticulocytes which are preferentially infected by P. vivax, are the most osmotically stable. P. vivax infection however decreases reticulocyte stability to levels close to those of RBC disorders that cause hemolytic anemia, and to a significantly greater degree than P. falciparum destabilizes normocytes. Finally, we find that P. vivax new permeability pathways contribute to the decreased osmotic stability of infected-reticulocytes. These results reveal a vulnerability of P. vivax-infected reticulocytes that could be manipulated to allow in vitro culture and develop novel therapeutics.

During Plasmodium intra-erythrocytic developmental, parasites compromise the structural integrity of host red-blood cells. Here, Clark et al. develop a flow cytometric osmotic stability assay to show that P. vivax infection destabilizes host reticulocytes, which are less stable than P. falciparum-infected normocytes.

The Interplay between Drivers of Erythropoiesis and Iron Homeostasis in Rare Hereditary Anemias: Tipping the Balance

Rare hereditary anemias (RHA) represent a group of disorders characterized by either impaired production of erythrocytes or decreased survival (i.e., hemolysis). In RHA, the regulation of iron metabolism and erythropoiesis is often disturbed, leading to iron overload or worsening of chronic anemia due to unavailability of iron for erythropoiesis. Whereas iron overload generally is a well-recognized complication in patients requiring regular blood transfusions, it is also a significant problem in a large proportion of patients with RHA that are not transfusion dependent. This indicates that RHA share disease-specific defects in erythroid development that are linked to intrinsic defects in iron metabolism. In this review, we discuss the key regulators involved in the interplay between iron and erythropoiesis and their importance in the spectrum of RHA.

A common polymorphism in the mechanosensitive ion channel PIEZO1 is associated with protection from severe malaria in humans

Malaria caused by the apicomplexan parasite Plasmodium falciparum has served as a strong evolutionary force throughout human history, selecting for red blood cell polymorphisms that confer innate protection against severe disease. Recently, gain-of-function mutations in the mechanosensitive ion channel PIEZO1 were shown to ameliorate Plasmodium parasite growth, blood-brain barrier dysfunction, and mortality in a mouse model of malaria. In humans, the gain-of-function allele PIEZO1 E756del is highly prevalent and enriched in Africans, raising the possibility that it is under positive selection due to malaria. Here we used a case-control study design to test for an association between PIEZO1 E756del and malaria severity among children in Gabon. We found that the E756del variant is strongly associated with protection against severe malaria in heterozygotes. In subjects with sickle cell trait, heterozygosity for PIEZO1 E756del did not confer additive protection and homozygosity was associated with an elevated risk of severe disease, suggesting an epistatic relationship between hemoglobin S and PIEZO1 E756del. Using donor blood samples, we show that red cells heterozygous for PIEZO1 E756del are not dehydrated and can support the intracellular growth of P. falciparum similar to wild-type cells. However, surface expression of the P. falciparum virulence protein PfEMP-1 was significantly reduced in infected cells heterozygous for PIEZO1 756del, a phenomenon that has been observed with other protective polymorphisms, such as hemoglobin C. Our findings demonstrate that PIEZO1 is an important innate determinant of malaria susceptibility in humans and suggest that the mechanism of protection may be related to impaired export of P. falciparum virulence proteins.

Mechanosensitive Piezo1 ion channel protein (PIEZO1 gene): update and extended mutation analysis of hereditary xerocytosis in India

Hereditary xerocytosis (HX), also known as dehydrated stomatocytosis (DHSt) is a dominantly inherited genetic disorder exhibiting red cell membrane dehydration caused by the loss of the monovalent cation K⁺ and water. Variants in mechanosensitive Piezo ionic channels of the PIEZO1 gene are the primary cause of HX. We have utilized high throughput and highly precise next-generation sequencing (NGS) to make a diagnosis and examine the genotype-phenotype relationship in inflexible HX cases. Seven unrelated patients with unexplained hemolytic anemia were scrutinized with a panel probing 8000 genes related to congenital anemia. Targeted next-generation sequencing identified 8 missense variants in the PIEZO1 gene in 7 unrelated Indian patients. Three of the 8 variants are novel (c.1795G > C, c.2915G > A, c.7372 T > C) and the remaining five (c.4082A > G, c.6829C > A, c.7374C > G, c.7381G > A, c.7483_7488dup) are previously reported. The variants have been validated by Sanger sequencing. One patient with autosomal dominant mutation (c.7372 T > C) is associated with iron refractory iron deficiency anemia. Of the 7 patients, one has HX in combination with a novel homozygous variant (c.994G > A) in the PKLR gene causing PK deficiency resulting in severe clinical manifestations with phenotypic variability. In silico prediction using bioinformatics tools were used to study the possible damaging effects of the novel variants. Structural-functional analysis of the novel variants was investigated by molecular modeling software (PyMOL and Swiss PDB). These results encompass the heterogeneous behavior of mechano-sensitive Piezo1 protein observed in HX patients in India. Moreover, NGS imparted a subtle, economical, and quick tool for understanding the genetic cause of undiagnosed cases of congenital hemolytic anemia. NGS grants a potential technology integrating clinical history together with molecular report profiting in such patients and their families.

Gain-of-function mutations in PIEZO1 directly impair hepatic iron metabolism via the inhibition of the BMP/SMADs pathway

Dehydrated hereditary stomatocytosis (DHS), or xerocytosis, is an autosomal dominant hemolytic anemia. Most patients with DHS carry mutations in the PIEZO1 gene encoding a mechanosensitive cation channel. We here demonstrate that patients with DHS have low levels of hepcidin and only a slight increase of ERFE, the erythroid negative regulator of hepcidin. We demonstrated that at the physiological level, PIEZO1 activation induced Ca²⁺ influx and suppression of HAMP expression in primary hepatocytes. In two hepatic cellular models expressing PIEZO1 WT and two PIEZO1 gain-of-function mutants (R2456H and R2488Q), we highlight altered expression of a few genes/proteins involved in iron metabolism. Mutant cells showed increased intracellular Ca²⁺ compared to WT, which was correlated to increased phosphorylation of ERK1/2, inhibition of the BMP-SMADs pathway, and suppression of HAMP transcription. Moreover, the HuH7 cells, treated with PD0325901, a potent inhibitor of ERK1/2 phosphorylation, reduced the phosphorylation of ERK1/2 with the consequent increased phosphorylation of SMAD1/5/8, confirming the link between the two pathways. Another "proof of concept" for the mechanism that links PIEZO1 to HAMP regulation was obtained by mimicking PIEZO1 activation by cell Ca²⁺ overload, by the Ca²⁺ ionophore A23187. There was strong down-regulation of HAMP gene expression after this Ca²⁺ overload. Finally, the inhibition of PIEZO1 by GsMTx4 leads to phenotype rescue. This is the first demonstration of a direct link between PIEZO1 and iron metabolism, which defines the channel as a new hepatic iron metabolism regulator and as a possible therapeutic target of iron overload in DHS and other iron-loading anemias.

Calcium Channels and Calcium-Regulated Channels in Human Red Blood Cells

Free Calcium (Ca²⁺) is an important and universal signalling entity in all cells, red blood cells included. Although mature mammalian red blood cells are believed to not contain organelles as Ca²⁺ stores such as the endoplasmic reticulum or mitochondria, a 20,000-fold gradient based on a intracellular Ca²⁺ concentration of approximately 60 nM vs. an extracellular concentration of 1.2 mM makes Ca²⁺-permeable channels a major signalling tool of red blood cells. However, the internal Ca²⁺ concentration is tightly controlled, regulated and maintained primarily by the Ca²⁺ pumps PMCA1 and PMCA4. Within the last two decades it became evident that an increased intracellular Ca²⁺ is associated with red blood cell clearance in the spleen and promotes red blood cell aggregability and clot formation. In contrast to this rather uncontrolled deadly Ca²⁺ signals only recently it became evident, that a temporal increase in intracellular Ca²⁺ can also have positive effects such as the modulation of the red blood cells O₂ binding properties or even be vital for brief transient cellular volume adaptation when passing constrictions like small capillaries or slits in the spleen. Here we give an overview of Ca²⁺ channels and Ca²⁺-regulated channels in red blood cells, namely the Gárdos channel, the non-selective voltage dependent cation channel, Piezo1, the NMDA receptor, VDAC, TRPC channels, Ca_V2.1, a Ca²⁺-inhibited channel novel to red blood cells and i.a. relate these channels to the molecular unknown sickle cell disease conductance P_sickle. Particular attention is given to correlation of functional measurements with molecular entities as well as the physiological and pathophysiological function of these channels. This view is in constant progress and in particular the understanding of the interaction of several ion channels in a physiological context just started. This includes on the one hand channelopathies, where a mutation of the ion channel is the direct cause of the disease, like Hereditary Xerocytosis and the Gárdos Channelopathy. On the other hand it applies to red blood cell related diseases where an altered channel activity is a secondary effect like in sickle cell disease or thalassemia. Also these secondary effects should receive medical and pharmacologic attention because they can be crucial when it comes to the life-threatening symptoms of the disease.

Mild erythrocytosis as a presenting manifestation of PIEZO1 associated erythrocyte volume disorders

Piezo1, encoded by the gene PIEZO1, is an erythrocytic cellular membrane mechanoactivated cation channel. Mutations have been implicated in erythrocyte volume disorders (EVDs)-especially hereditary xerocytosis (HX)/dehydrated stomatocytosis (DHS). We identified three patients, all with novel PIEZO1 mutations, but only one displaying the HX/DHS phenotype. Retrospective review of three cases. Osmotic gradient red cell deformability (Osmoscan) was assessed via the Technicon Ektacytometer. Red cell band 3 content was estimated using Eosin-5'-Maleimide staining. Patient 1 was evaluated for polycythemia. Osmoscans suggested mild spherocytosis; a novel PIEZO1 mutation (p.Thr1589Ile, exon 35) was identified, causing mild erythrocytosis without hemolytic anemia. Patient 2 was evaluated for macrocytosis/reticulocytosis, normal-to-high hemoglobin, and indirect hyperbilirubinemia. Osmoscans suggested increased cellular hydration; a second novel PIEZO1 mutation (p.Arg1728Cys, exon 37) was identified, resulting in overhydrated stomatocytosis with well-compensated hemolysis. Patient 3 was evaluated for indirect hyperbilirubinemia only. Osmoscans suggested dehydrated stomatocytosis (DHS, xerocytosis); a third novel PIEZO1 mutation (p.Arg2279Cys, exon 47) was identified. All three patients' blood smears demonstrated stomatocytes and spherocytes. EVDs may be underdiagnosed due to the lack of "expected" anemia in a hemolytic disorder; two of three patients had high hemoglobin and red cell counts and one had high normal values for both parameters and the presence of stomatocytes/dehydrated cells lead to identification of causative PIEZO1 mutations. PIEZO1-associated EVDs may be more common than previously suspected and should be included in the diagnostic algorithms for mild erythrocytosis/unexplained jaundice.

Red Blood Cells: Chasing Interactions

Human red blood cells (RBC) are highly differentiated cells that have lost all organelles and most intracellular machineries during their maturation process. RBC are fundamental for the nearly all basic physiologic dynamics and they are key cells in the body's respiratory system by being responsible for the oxygen transport to all cells and tissues, and delivery of carbon dioxide to the lungs. With their flexible structure RBC are capable to deform in order to travel through all blood vessels including very small capillaries. Throughout their in average 120 days lifespan, human RBC travel in the bloodstream and come in contact with a broad range of different cell types. In fact, RBC are able to interact and communicate with endothelial cells (ECs), platelets, macrophages, and bacteria. Additionally, they are involved in the maintenance of thrombosis and hemostasis and play an important role in the immune response against pathogens. To clarify the mechanisms of interaction of RBC and these other cells both in health and disease as well as to highlight the role of important key players, we focused our interest on RBC membrane components such as ion channels, proteins, and phospholipids.

Dehydrated hereditary stomatocytosis: clinical perspectives

Dehydrated hereditary stomatocytosis (DHSt) is a nonimmune congenital hemolytic disorder characterized by red blood cell (RBC) dehydration and lysis. It has been a recognized diagnostic entity for almost 50 years, and autosomal dominant inheritance has long been suspected, but it was not until 2011 that the first genetic alterations were identified. The current study reviews 73 articles published during 1971–2019 and focuses on clinical perspectives of the disease. All but one of the published clinical data in DHSt were either single case reports or case series. From these, it can be seen that patients with DHSt often have fully or partially compensated hemolysis with few symptoms. Despite this, iron overload is an almost universal finding even in patients without or with only sporadic blood transfusions, and this may lead to organ dysfunction. Other severe complications, such as thrombosis and perinatal fluid effusions unrelated to fetal hemoglobin concentration, may also occur. No specific treatment for symptomatic hemolysis exists, and splenectomy should be avoided as it seems to aggravate the risk of thrombosis. Recently, treatment with senicapoc has shown activity against RBC dehydration in vitro; however, it is not known if this translates into relevant clinical effects. In conclusion, despite recent advances in the understanding of pathophysiology in DHSt, options for clinical management have not improved. Entering data into international registries has the potential to fill gaps in knowledge and eventually care of these rare patients.

Erythrocyte ion content and dehydration modulate maximal Gardos channel activity in KCNN4 V282M/+ hereditary xerocytosis red cells

Hereditary xerocytosis (HX) is caused by missense mutations in either the mechanosensitive cation channel PIEZO1 or the Ca²⁺-activated K⁺ channel KCNN4. All HX-associated KCNN4 mutants studied to date have revealed increased current magnitude and red cell dehydration. Baseline KCNN4 activity was increased in HX red cells heterozygous for KCNN4 mutant V282M. However, HX red cells maximally stimulated by Ca²⁺ ionophore A23187 or by PMCA Ca²⁺-ATPase inhibitor orthovanadate displayed paradoxically reduced KCNN4 activity. This reduced Ca²⁺-stimulated mutant KCNN4 activity in HX red cells was associated with unchanged sensitivity to KCNN4 inhibitor senicapoc and KCNN4 activator Ca²⁺, with slightly elevated Ca²⁺ uptake and reduced PMCA activity, and with decreased KCNN4 activation by calpain inhibitor PD150606. The altered intracellular monovalent cation content of HX red cells prompted experimental nystatin manipulation of red cell Na and K contents. Nystatin-mediated reduction of intracellular K⁺ with corresponding increase in intracellular Na⁺ in wild-type cells to mimic conditions of HX greatly suppressed vanadate-stimulated and A23187-stimulated KCNN4 activity in those wild-type cells. However, conferral of wild-type cation contents on HX red cells failed to restore wild-type-stimulated KCNN4 activity to those HX cells. The phenotype of reduced, maximally stimulated KCNN4 activity was shared by HX erythrocytes expressing heterozygous PIEZO1 mutants R2488Q and V598M, but not by HX erythrocytes expressing heterozygous KCNN4 mutant R352H or PIEZO1 mutant R2456H. Our data suggest that chronic KCNN4-driven red cell dehydration and intracellular cation imbalance can lead to reduced KCNN4 activity in HX and wild-type red cells.

PIEZO1 Hypomorphic Variants in Congenital Lymphatic Dysplasia Cause Shape and Hydration Alterations of Red Blood Cells

PIEZO1 is a cation channel activated by mechanical force. It plays an important physiological role in several biological processes such as cardiovascular, renal, endothelial and hematopoietic systems. Two different diseases are associated with alteration in the DNA sequence of PIEZO1: (i) dehydrated hereditary stomatocytosis (DHS1, #194380), an autosomal dominant hemolytic anemia caused by gain-of-function mutations; (ii) lymphatic dysplasia with non-immune fetal hydrops (LMPH3, #616843), an autosomal recessive condition caused by biallelic loss-of-function mutations. We analyzed a 14-year-old boy affected by severe lymphatic dysplasia already present prenatally, with peripheral edema, hydrocele, and chylothoraces. By whole exome sequencing, we identified compound heterozygosity for PIEZO1, with one splicing and one deletion mutation, the latter causing the formation of a premature stop codon that leads to mRNA decay. The functional analysis of the erythrocytes of the patient highlighted altered hydration with the intracellular loss of the potassium content and structural abnormalities with anisopoikolocytosis and presence of both spherocytes and stomatocytes. This novel erythrocyte trait, sharing features with both hereditary spherocytosis and overhydrated hereditary stomatocytosis, complements the clinical features associated with loss-of-function mutations of PIEZO1 in the context of the generalized lymphatic dysplasia of LMPH3 type.

Genotype-phenotype correlation and risk stratification in a cohort of 123 hereditary stomatocytosis patients

Hereditary stomatocytoses (HSts) are a wide spectrum of hemolytic anemias in which the erythrocyte membrane cation permeability is increased. Dehydrated hereditary stomatocytosis is the most frequent among HSts. It is caused by missense mutations in PIEZO1 and KCNN4 genes. We described 123 patients enrolled in our Genetic Unit from 2013 to 2017. Overall HSt subjects exhibit macrocytic mild anemia. We found that PIEZO1 is the most frequent mutated gene within our families (47% of pedigrees). In 59.1% of cases the mutations localized in the nonpore protein domain, while in 40.9% of patients they localized in the central pore region. The genotype-phenotype correlation analysis on 29 PIEZO1-patients demonstrated that most of severely affected patients carried mutations in the pore domain, suggesting that the severity of this condition is related to the pore properties and intracellular domain that could be responsible of interactions with intracellular components. This is the first cohort study on a large set of hereditary stomatocytosis patients, stratified according to their causative gene useful for diagnosis, prognosis, and management of these patients.

The Shape Shifting Story of Reticulocyte Maturation

The final steps of erythropoiesis involve unique cellular processes including enucleation and reorganization of membrane proteins and the cytoskeleton to produce biconcave erythrocytes. Surprisingly this process is still poorly understood. In vitro erythropoiesis protocols currently produce reticulocytes rather than biconcave erythrocytes. In addition, immortalized lines and iPSC-derived erythroid cell suffer from low enucleation and suboptimal final maturation potential. In light of the increasing prospect to use in vitro produced erythrocytes as (personalized) transfusion products or as therapeutic delivery agents, the mechanisms driving this last step of erythropoiesis are in dire need of resolving. Here we review the elusive last steps of reticulocyte maturation with an emphasis on protein sorting during the defining steps of reticulocyte formation during enucleation and maturation.

The Molecular Basis for Altered Cation Permeability in Hereditary Stomatocytic Human Red Blood Cells

Normal human RBCs have a very low basal permeability (leak) to cations, which is continuously corrected by the Na,K-ATPase. The leak is temperature-dependent, and this temperature dependence has been evaluated in the presence of inhibitors to exclude the activity of the Na,K-ATPase and NaK2Cl transporter. The severity of the RBC cation leak is altered in various conditions, most notably the hereditary stomatocytosis group of conditions. Pedigrees within this group have been classified into distinct phenotypes according to various factors, including the severity and temperature-dependence of the cation leak. As recent breakthroughs have provided more information regarding the molecular basis of hereditary stomatocytosis, it has become clear that these phenotypes elegantly segregate with distinct genetic backgrounds. The cryohydrocytosis phenotype, including South-east Asian Ovalocytosis, results from mutations in SLC4A1, and the very rare condition, stomatin-deficient cryohydrocytosis, is caused by mutations in SLC2A1. Mutations in RHAG cause the very leaky condition over-hydrated stomatocytosis, and mutations in ABCB6 result in familial pseudohyperkalemia. All of the above are large multi-spanning membrane proteins and the mutations may either modify the structure of these proteins, resulting in formation of a cation pore, or otherwise disrupt the membrane to allow unregulated cation movement across the membrane. More recently mutations have been found in two RBC cation channels, PIEZO1 and KCNN4, which result in dehydrated stomatocytosis. These mutations alter the activation and deactivation kinetics of these channels, leading to increased opening and allowing greater cation fluxes than in wild type.

Common PIEZO1 Allele in African Populations Causes RBC Dehydration and Attenuates Plasmodium Infection

Hereditary xerocytosis is thought to be a rare genetic condition characterized by red blood cell (RBC) dehydration with mild hemolysis. RBC dehydration is linked to reduced Plasmodium infection in vitro; however, the role of RBC dehydration in protection against malaria in vivo is unknown. Most cases of hereditary xerocytosis are associated with gain-of-function mutations in PIEZO1, a mechanically activated ion channel. We engineered a mouse model of hereditary xerocytosis and show that Plasmodium infection fails to cause experimental cerebral malaria in these mice due to the action of Piezo1 in RBCs and in T cells. Remarkably, we identified a novel human gain-of-function PIEZO1 allele, E756del, present in a third of the African population. RBCs from individuals carrying this allele are dehydrated and display reduced Plasmodium infection in vitro. The existence of a gain-of-function PIEZO1 at such high frequencies is surprising and suggests an association with malaria resistance.

Sickle cell dehydration: Pathophysiology and therapeutic applications

Cell dehydration is a distinguishing characteristic of sickle cell disease and an important contributor to disease pathophysiology. Due to the unique dependence of Hb S polymerization on cellular Hb S concentration, cell dehydration promotes polymerization and sickling. In double heterozygosis for Hb S and C (SC disease) dehydration is the determining factor in disease pathophysiology. Three major ion transport pathways are involved in sickle cell dehydration: the K-Cl cotransport (KCC), the Gardos channel (KCNN4) and Psickle, the polymerization induced membrane permeability, most likely mediated by the mechano-sensitive ion channel PIEZO1. Each of these pathways exhibit unique characteristics in regulation by oxygen tension, intracellular and extracellular environment, and functional expression in reticulocytes and mature red cells. The unique dependence of K-Cl cotransport on intracellular Mg and the abnormal reduction of erythrocyte Mg content in SS and SC cells had led to clinical studies assessing the effect of oral Mg supplementation. Inhibition of Gardos channel by clotrimazole and senicapoc has led to Phase 1,2,3 trials in patients with sickle cell disease. While none of these studies has resulted in the approval of a novel therapy for SS disease, they have highlighted the key role played by these pathways in disease pathophysiology.

Hereditary stomatocytosis: An underdiagnosed condition

Hereditary stomatocytoses are a wide class of hemolytic anemias characterized by alterations of ionic flux with increased cation permeability that results in inappropriate shrinkage or swelling of the erythrocytes, and water lost or gained osmotically. The last few years have been crucial for new acquisitions in this field in terms of identifying new causative genes and of studying their pathogenetic mechanisms. This review summarizes the main features of erythrocyte membrane transport diseases, dividing them into forms with either isolated erythroid phenotype (nonsyndromic) or extra-hematological manifestations (syndromic), and focusing particularly on the most recent advances regarding dehydrated forms of hereditary stomatocytosis and familial pseudohyperkalemia.

Hemoglobin C trait accentuates erythrocyte dehydration in hereditary xerocytosis

A 17-year-old male presented with acute hemolysis with stomatocytosis, elevated mean corpuscular hemoglobin concentration (MCHC), and osmotic gradient ektacytometry consistent with marked erythrocyte dehydration. Erythrocytes from both parents also demonstrated evidence of dehydration with elevated MCHC and abnormal ektacytometry, but neither to the degree of the patient. Genetic studies revealed the patient had hereditary xerocytosis (HX) due to a novel PIEZO1 mutation inherited from his mother and hemoglobin C (HbC) trait inherited from his father. HbC trait accentuated the erythrocyte dehydration of HX. Coinheritance of interrelated disorders and/or modifier alleles should be considered whenever severe erythrocyte dehydration is observed.

Novel mechanisms of PIEZO1 dysfunction in hereditary xerocytosis

Mutations in PIEZO1 are the primary cause of hereditary xerocytosis, a clinically heterogeneous, dominantly inherited disorder of erythrocyte dehydration. We used next-generation sequencing-based techniques to identify PIEZO1 mutations in individuals from 9 kindreds referred with suspected hereditary xerocytosis (HX) and/or undiagnosed congenital hemolytic anemia. Mutations were primarily found in the highly conserved, COOH-terminal pore-region domain. Several mutations were novel and demonstrated ethnic specificity. We characterized these mutations using genomic-, bioinformatic-, cell biology-, and physiology-based functional assays. For these studies, we created a novel, cell-based in vivo system for study of wild-type and variant PIEZO1 membrane protein expression, trafficking, and electrophysiology in a rigorous manner. Previous reports have indicated HX-associated PIEZO1 variants exhibit a partial gain-of-function phenotype with generation of mechanically activated currents that inactivate more slowly than wild type, indicating that increased cation permeability may lead to dehydration of PIEZO1-mutant HX erythrocytes. In addition to delayed channel inactivation, we found additional alterations in mutant PIEZO1 channel kinetics, differences in response to osmotic stress, and altered membrane protein trafficking, predicting variant alleles that worsen or ameliorate erythrocyte hydration. These results extend the genetic heterogeneity observed in HX and indicate that various pathophysiologic mechanisms contribute to the HX phenotype.

Hereditary Xerocytosis due to Mutations in PIEZO1 Gene Associated with Heterozygous Pyruvate Kinase Deficiency and Beta-Thalassemia Trait in Two Unrelated Families

Hereditary xerocytosis (HX) is a rare disorder caused by defects of RBC permeability, associated with haemolytic anaemia of variable degree and iron overload. It is sometimes misdiagnosed as hereditary spherocytosis or other congenital haemolytic anaemia. Splenectomy is contraindicated due to increased risk of thromboembolic complications. We report the clinical, haematological, and molecular characteristics of four patients from two unrelated Italian families affected by HX, associated with beta-thalassemia trait and heterozygous pyruvate kinase deficiency, respectively. Two patients had been splenectomised and displayed thrombotic episodes. All patients had iron overload in the absence of transfusion, two of them requiring iron chelation. The diagnosis of HX was confirmed by LoRRca Osmoscan analysis showing a left-shifted curve. PIEZO1 gene sequencing revealed the presence of mutation p.E2496ELE, showing that this is one of the most frequent mutations in this disease. The concomitant defects did not aggravate the clinical phenotype; however, in one patient, the initial diagnosis of pyruvate kinase deficiency delayed the correct diagnosis of HX for many years and resulted in splenectomy followed by thrombotic complications. The study underlines the importance of a precise diagnosis in HX, particularly in view of splenectomy, and the need of a molecular confirmation of suspected RBC enzymopathy.

Genetic Diseases of PIEZO1 and PIEZO2 Dysfunction

Mutations in the genes encoding the mechanosensitive cation channels PIEZO1 and PIEZO2 are responsible for multiple hereditary human diseases. Loss-of-function mutations in the human PIEZO1 gene cause autosomal recessive congenital lymphatic dysplasia. Gain-of-function mutations in the human PIEZO1 gene cause the autosomal dominant hemolytic anemia, hereditary xerocytosis (also known as dehydrated stomatocytosis). Loss-of-function mutations in the human PIEZO2 gene cause an autosomal recessive syndrome of muscular atrophy with perinatal respiratory distress, arthrogryposis, and scoliosis. Gain-of-function mutations in the human PIEZO2 gene cause three clinical types of autosomal dominant distal arthrogryposis. This chapter will review the hereditary diseases caused by mutations in the PIEZO genes and will discuss additional physiological systems in which PIEZO channel dysfunction may contribute to human disease pathophysiology.

The Kinetics and the Permeation Properties of Piezo Channels

Piezo channels are eukaryotic, cation-selective mechanosensitive channels (MSCs), which show rapid activation and voltage-dependent inactivation. The kinetics of these channels are largely consistent across multiple cell types and different stimulation paradigms with some minor variability. No accessory subunits that associate with Piezo channels have been reported. They are homotrimers and each ∼300kD monomer has an N-terminal propeller blade-like mechanosensing module, which can confer mechanosensing capabilities on ASIC-1 (the trimeric non-MSC, acid-sensing ion channel-1) and a C-terminal pore module, which influences conductance, selectivity, and channel inactivation. Repeated stimulation can cause domain fracture and diffusion of these channels leading to synchronous loss of inactivation. The reconstituted channels spontaneously open only in asymmetric bilayers but lack inactivation. Mutations that cause hereditary xerocytosis alter PIEZO1 kinetics. The kinetics of the wild-type PIEZO1 and alterations thereof in mutants (M2225R, R2456K, and DhPIEZO1) are summarized in the form of a quantitative model and hosted online. The pore is permeable to alkali ions although Li⁺ permeates poorly. Divalent cations, notably Ca²⁺, traverse the channel and inhibit the flux of monovalents. The large monovalent organic cations such as tetramethyl ammonium and tetraethyl ammonium can traverse the channel, but slowly, suggesting a pore diameter of ∼8Å, and the estimated in-plane area change upon opening is around 6-20nm². Ruthenium red can enter the channel only from the extracellular side and seems to bind in a pocket close to residue 2496.

PIEZO1 gene mutation in a Japanese family with hereditary high phosphatidylcholine hemolytic anemia and hemochromatosis-induced diabetes mellitus

Hereditary xerocytosis (HX) or dehydrated hereditary stomatocytosis (DHS) [OMIM 194380], in which PIEZO1 gene mutation has recently been identified, is difficult to diagnose. We report here the discovery of a PIEZO1 gene mutation in a Japanese family (father, daughter, and son) who were previously diagnosed with hereditary high phosphatidylcholine hemolytic anemia (HPCHA). All of the affected family members had non-spherocytic hemolytic anemia associated with severe hemochromatosis-related diabetes mellitus. Although the causative correlation between HPCHA and PIEZO1-gene mutated HX/DHS remains to be clarified, our findings raise an important question as to whether any of the HPCHA cases previously diagnosed in Japan may have in fact been the form of hemolytic anemia known as HX/DHS with PIEZO1 gene mutation.

Novel Gardos channel mutations linked to dehydrated hereditary stomatocytosis (xerocytosis)

Dehydrated hereditary stomatocytosis (DHSt) is an autosomal dominant congenital hemolytic anemia with moderate splenomegaly and often compensated hemolysis. Affected red cells are characterized by a nonspecific cation leak of the red cell membrane, reflected in elevated sodium content, decreased potassium content, elevated MCHC and MCV, and decreased osmotic fragility. The majority of symptomatic DHSt cases reported to date have been associated with gain-of-function mutations in the mechanosensitive cation channel gene, PIEZO1. A recent study has identified two families with DHSt associated with a single mutation in the KCNN4 gene encoding the Gardos channel (KCa3.1), the erythroid Ca(2+) -sensitive K(+) channel of intermediate conductance, also expressed in many other cell types. We present here, in the second report of DHSt associated with KCNN4 mutations, two previously undiagnosed DHSt families. Family NA exhibited the same de novo missense mutation as that recently described, suggesting a hot spot codon for DHSt mutations. Family WO carried a novel, inherited missense mutation in the ion transport domain of the channel. The patients' mild hemolytic anemia did not improve post-splenectomy, but splenectomy led to no serious thromboembolic events. We further characterized the expression of KCNN4 in the mutated patients and during erythroid differentiation of CD34+ cells and K562 cells. We also analyzed KCNN4 expression during mouse embryonic development.

A mutation in the Gardos channel is associated with hereditary xerocytosis

The Gardos channel is a Ca(2+)-sensitive, intermediate conductance, potassium selective channel expressed in several tissues including erythrocytes and pancreas. In normal erythrocytes, it is involved in cell volume modification. Here, we report the identification of a dominantly inherited mutation in the Gardos channel in 2 unrelated families and its association with chronic hemolysis and dehydrated cells, also referred to as hereditary xerocytosis (HX). The affected individuals present chronic anemia that varies in severity. Their red cells exhibit a panel of various shape abnormalities such as elliptocytes, hemighosts, schizocytes, and very rare stomatocytic cells. The missense mutation concerns a highly conserved residue among species, located in the region interacting with Calmodulin and responsible for the channel opening and the K(+) efflux. Using 2-microelectrode experiments on Xenopus oocytes and patch-clamp electrophysiology on HEK293 cells, we demonstrated that the mutated channel exhibits a higher activity and a higher Ca(2+) sensitivity compared with the wild-type (WT) channel. The mutated channel remains sensitive to inhibition suggesting that treatment of this type of HX by a specific inhibitor of the Gardos channel could be considered. The identification of a KCNN4 mutation associated with chronic hemolysis constitutes the first report of a human disease caused by a defect of the Gardos channel.

Ionic Selectivity and Permeation Properties of Human PIEZO1 Channels

Members of the eukaryotic PIEZO family (the human orthologs are noted hPIEZO1 and hPIEZO2) form cation-selective mechanically-gated channels. We characterized the selectivity of human PIEZO1 (hPIEZO1) for alkali ions: K⁺, Na⁺, Cs⁺ and Li⁺; organic cations: TMA and TEA, and divalents: Ba²⁺, Ca²⁺, Mg²⁺ and Mn²⁺. All monovalent ions permeated the channel. At a membrane potential of -100 mV, Cs⁺, Na⁺ and K⁺ had chord conductances in the range of 35–55 pS with the exception of Li⁺, which had a significantly lower conductance of ~ 23 pS. The divalents decreased the single-channel permeability of K⁺, presumably because the divalents permeated slowly and occupied the open channel for a significant fraction of the time. In cell-attached mode, 90 mM extracellular divalents had a conductance for inward currents carried by the divalents of: 25 pS for Ba²⁺ and 15 pS for Ca²⁺ at -80 mV and 10 pS for Mg²⁺ at -50 mV. The organic cations, TMA and TEA, permeated slowly and attenuated K⁺ currents much like the divalents. As expected, the channel K⁺ conductance increased with K⁺ concentration saturating at ~ 45 pS and the K_D of K⁺ for the channel was 32 mM. Pure divalent ion currents were of lower amplitude than those with alkali ions and the channel opening rate was lower in the presence of divalents than in the presence of monovalents. Exposing cells to the actin disrupting reagent cytochalasin D increased the frequency of openings in cell-attached patches probably by reducing mechanoprotection.