The water channel aquaporin-1 partitions into exosomes during reticulocyte maturation: implication for the regulation of cell volume

Abstract

Aquaporin-1 (AQP-1), the universal water channel, is responsible for rapid response of cell volume to changes in plasma tonicity. In the membrane of the red cell the concentration of the protein is tightly controlled. Here, we show that AQP-1 is partially lost during in vitro maturation of mouse reticulocytes and that it is associated with exosomes, released throughout this process. AQP-1 in young reticulocytes localizes to the plasma membrane and also in endosomal compartments and exosomes, formed both in vitro and in vivo. During maturation a part of the total pool of AQP-1 is differentially sorted and released via the exosomal pathway. A proteasome inhibitor, MG132, suppresses secretion of AQP-1, implying that ubiquitination is a sorting signal for its release. We further show that modulation of medium tonicity in vitro regulates the secretion of AQP-1, thus showing that extracellular osmotic conditions can drive sorting of selected proteins by the exosomal pathway. These results lead us to suggest that AQP-1 sorting into exosomes may be the mechanism by which the reticulocyte adapts to environmental changes during its maturation. (Blood. 2009;114:3928-3934) IntroductionThe final stage of erythropoiesis consists of maturation of reticulocytes into erythrocytes. Young immature reticulocytes are generated through the process of enucleation, which occurs within bone marrow erythroid niches called erythroblastic islands. During steady state, reticulocyte maturation occurs over a period of approximately 72 hours, two-thirds of this time in the marrow, one-third in the circulation. 1,2 This maturation process involves complete loss of intracellular organelles, including mitochondria, endoplasmic reticulum, Golgi apparatus, and endocytic vesicles. 3 Equally important is the extensive remodelling of the plasma membrane, with progressive loss of various membrane proteins. 4 These include the transferrin receptor (TfR), because iron for heme synthesis is no longer required, and adhesion receptors such as ␤1-integrin, 5 the presence of which could otherwise cause mature circulating red cells to adhere to vascular endothelial cells. To date the best studied of these is the TfR, which is completely shed from the reticulocyte in exosomes, small vesicles released into the circulation. 6 During maturation, the TfR is rerouted from a recycling pathway into a secretion pathway. 7 The plasma membranebound receptors are sorted into endocytic vesicles, which are internalized and assembled into multivesicular bodies. These fuse with the plasma membrane and release their contents into the plasma in the form of exosomes. Although the molecular mechanisms of TfR sorting into exosomes have been explored, 8 they are far from completely understood. Nevertheless, the identification of the components involved in the formation of multivesicular bodies affords some insight into the biogenesis of exosomes. 9 Four protein complexes, ESCRT-0, -1, -2, and -3 (referred to as endosomal sorting complexes, required for transport), were shown to pla...

Mature erythrocyte membrane homeostasis is compromised by loss of the GATA1-FOG1 interaction

“…by guest www.bloodjournal.org From reticulocytes into mature erythrocytes and this process is dependent on the osmotic condition of the environment. 44 We surmise that targeting-deficiency of aquaporin-1 may worsen the hemolysis event in V205GR pups, which may be triggered by the environmental changes during delivery.…”

Homozygosity for aquaporin 7 G264V in three unrelated children with hyperglyceroluria and a mild platelet secretion defect

“…AQP7 could interfere with platelet-dense granule formation and functioning, as previously described with respect to AQP1, which was found to be associated with exosomes during reticulocyte maturation. 41 A largely variable neurologic phenotype was observed among the homozygous G264V carriers, ranging from severe hypotonia, psychomotor retardation, and/or epilepsy needing medication in the index cases to normal functioning in their healthy siblings. More studies are needed to define whether mental disability is also related to the AQP7 G264V variant, or whether a separate genetic factor is responsible for the neurological phenotype in the affected homozygous carriers.…”

Aquaporin-1 Expression in Canine Peripheral Erythrocytes and Its Relation to Cell Volume

“…Although AQP1 is known to be a major water channel in RBCs, there was no significant difference in AQP expression between large (light) and small (dense) fractions of both HK and LK RBCs. Recently, it was reported that AQP1 was partially lost during maturation by the release of exosomes containing AQP1 protein from reticulocytes [1]. Moreover, high tonicity suppressed the secretion of AQP1, implying that modulation of medium tonicity regulated the secretion of AQP1 [1].…”

“…Recently, it was reported that AQP1 was partially lost during maturation by the release of exosomes containing AQP1 protein from reticulocytes [1]. Moreover, high tonicity suppressed the secretion of AQP1, implying that modulation of medium tonicity regulated the secretion of AQP1 [1]. In general, reticulocytes possess larger cell volumes than mature erythrocytes, and peripheral RBCs gradually reduce their cell volumes during circulation [18].…”