Eryptosis in health and disease: A paradigm shift towards understanding the (patho)physiological implications of programmed cell death of erythrocytes

Abstract

During the course of their natural ageing and upon injury, anucleate erythrocytes can undergo an unconventional apoptosis-like cell death, termed eryptosis. Eryptotic erythrocytes display a plethora of morphological alterations including volume reduction, membrane blebbing and breakdown of the membrane phospholipid asymmetry resulting in phosphatidylserine externalization which, in turn, mediates their phagocytic recognition and clearance from the circulation. Overall, the eryptosis machinery is tightly orchestrated by a wide array of endogenous mediators, ion channels, membrane receptors, and a host of intracellular signaling proteins. Enhanced eryptosis shortens the lifespan of circulating erythrocytes and confers a procoagulant phenotype; this phenomenon has been tangibly implicated in the pathogenesis of anemia, deranged microcirculation, and increased prothrombotic risk associated with a multitude of clinical conditions. Herein, we reviewed the molecular mechanisms dictating eryptosis and erythrophagocytosis and critically analyzed the current evidence leading to the pathophysiological ramifications of eryptotic cell death in the context of human disease.

Introduction

Under physiological conditions, the erythrocyte count in circulating blood is approximately 4 × 10¹²/L, accounting for nearly half of the total blood volume. Erythrocytes originate from erythroblastic progenitors of the myeloid stem cell line; although these progenitors are nucleated, the nuclei of erythrocytes are extruded before entering the reticulocyte stage, which allows for high hemoglobin and oxygen content. Erythropoiesis is a dynamic process that tightly regulates erythrocyte numbers in the circulation as an estimated 10¹¹ erythrocytes are generated and removed from the circulation each day [1], [2]. Normal erythrocytes have a morphologically biconcave shape which enables greater membrane deformability and durability as the cells pass through narrow capillaries. The most important and well known function of erythrocytes is the transport of oxygen from lungs to tissues [3]. Erythrocytes are also essential in blood pH control and carbon dioxide transport due to the action of carbonic anhydrase; they have also been implicated in vascular tone regulation by releasing ATP and nitric oxide (NO) during shear stress and hypoxia, respectively [4], [5], [6], [7], [8].

In healthy individuals, the lifespan of circulating erythrocytes in vivo has been promulgated to vary between 100 and 120 days, after which they undergo senescence and are removed from the circulation [1]. Erythrocyte ageing includes morphological alterations such as volume, density, and shape changes as well as quantitative and qualitative changes on their surface [1], [2], [9], [10], [11]. Molecular alterations on senescent erythrocytes encompass modification of the anion exchanger protein band 3 and their binding to hemichromes, which subsequently leads to band 3 clustering, complement C3 deposition and binding of autologous immunglobulins to band 3 [9], [12]. Together, this process disrupts the band 3-mediated cytoskeletal connections to the lipid bilayer and vesicle generation that consequently uncovers senescent erythrocyte antigens. Changes in protein-carbohydrate moieties, decreased membrane fluidity and phospholipid scrambling may also occur in the erythrocyte membranes [13].

During the course of their natural ageing and prior to their senescence, erythrocytes may experience injury which compromises their integrity, function and survival [14], [15], [16], [17]. Under these circumstances, erythrocytes may undergo an apoptosis-like suicidal cell death called eryptosis which is conspicuous by the absence of nuclear condensation and mitochondrial depolarization; this phenomenon enables the disposal of defective cells without breaching the cell membrane integrity and release of cytosolic material to the exterior [15], [16]. Eryptosis of erythrocytes is characterized by a breakdown of the cell membrane phospholipid asymmetry characterized by the translocation of the cell membrane phospholipid phosphatidylserine (PS) from the inner leaflet to the exterior regulated by flippases [18]; this phenomenon is analogous to the programmed cell death of nucleated cells where flippases serve in the maintenance of the phospholipid architecture of the membrane [19], [20]. Morphologically, injured eryptotic erythrocytes are also associated with a reduced cell volume and the formation of blebs on the cell surface [14], [16], [17]. Eryptotic erythrocytes may further display alterations in their membrane elasticity [21], [22]. Similar to apoptotic nucleated cells, eryptotic erythrocytes are recognized by macrophages, phagocytosed and degraded, and cleared from the circulation [14], [15], [16]. Along these lines, eryptosis ostensibly serves the same purpose as apoptosis, i.e. the elimination of defective cells.

While it is becoming increasingly clearer that eryptosis is a phenomenon mechanistically distinct from the senescence of aged erythrocytes, the molecular pathways orchestrating the erythrocyte suicide machinery remain largely ill-defined. The objective of the present review is to describe the biological mechanisms governing erythrocyte death and erythrophagocytosis. The review further aims to provide novel insights into the current understanding of the pathophysiological and clinical relevance of eryptosis by collating recent findings from in vitro studies, animal models as well as studies on human patients.