N‐Methyl D‐Aspartate (NMDA) Receptors in Human Red Blood Cells in Health and Disease

Abstract

In search for the molecular identity of the Ca2+ uptake pathways in red blood cells (RBCs) we have recently discovered glutamate/homocysteic acid-sensitive Ca2+ transporter which appeared to be the erythroid NMDA receptor. Using ex-vivo erythropoiesis system and RBCs of healthy human subjects and sickle cell disease patients we were able to characterize subunit composition, function and regulation of these receptors in erythroid precursor cells and in the circulating RBCs. The number of the NMDA receptors per cell changed from ~300 000 in proeythroblasts to ~30 in young RBCs decreasing further to ~4 in senescent cells of healthy humans. Activation of these receptors with glutamate and glycine resulted in transient increase in the intracellular calcium which could be blocked by the NMDA receptor antagonists. Activation of the NMDA receptors in RBC triggered transient shrinkage due to the activation of Gardos channels, stimulation of NO synthase and induction of oxidative stress. Calcium uptake was furthermore inducing transient decrease in hemoglobin oxygen affinity. Abnormally high numbers of NMDA receptor copies found in RBCs of patients with sickle cell disease were most likely caused by the disturbed clearance of the receptors from reticulocytes. In RBCs of patients receptor-mediated calcium uptake was high during asymptomatic periods and further up-regulated during vaso-occlusive haemolytic crises. Hyper-activation of the NMDA receptors caused oxidation, dehydration and promoted polymerisation of deoxygenated haemoglobin S. Sickle cell transformation was largely prevented by treatment of RBCs with the NMDA receptor antagonist memantine.