Red blood cell aquaporin-1 expression is decreased in hereditary spherocytosis

Aquaporin-1 (AQP1) is the membrane water channel responsible for changes in erythrocyte volume in response to the tonicity of the medium. As the aberrant distribution of proteins in hereditary spherocytosis (HS) generates deficiencies of proteins other than those codified by the mutated gene, we postulated that AQP1 expression might be impaired in spherocytes. AQP1 expression was evaluated through flow cytometry in 5 normal controls, 1 autoimmune hemolytic anemia, 10 HS (2 mild, 3 moderate, 2 severe, and 3 splenectomized), and 3 silent carriers. The effect of AQP1 inhibitors was evaluated through water flow-based tests: osmotic fragility and hypertonic cryohemolysis. Serum osmolality was measured in 20 normal controls and 13 HS. The effect of erythropoietin (Epo) on AQP1 expression was determined in cultures of erythroleukemia UT-7 cells, dependent on Epo to survive. Independent of erythrocyte size, HS patients showed a lower content of AQP1 in erythrocyte membranes which correlated with the severity of the disease. Accordingly, red blood cells from HS subjects were less sensitive to cryohemolysis than normal erythrocytes after inhibition of the AQP1 water channel. A lower serum osmolality in HS with respect to normal controls suggests alterations during reticulocyte remodeling. The decreased AQP1 expression could contribute to explain variable degrees of anemia in hereditary spherocytosis. The finding of AQP1 expression induced by Epo in a model of erythroid cells may be interpreted as a mechanism to restore the balance of red cell water fluxes.