Failure of red blood cell maturation in mice with defects in the high-density lipoprotein receptor SR-BI.

Mammalian erythrocytes undergo a unique maturation process in which they discard their nuclei and organelles and assume a flexible biconcave shape. We found that altered plasma lipoprotein metabolism can profoundly influence these events. Abnormal erythrocyte morphology was observed in hypercholesterolemic mice lacking the high-density lipoprotein receptor SR-BI. This was exacerbated by feeding mice a high-cholesterol diet or, more dramatically, by inactivating the apolipoprotein E gene. Erythrocytes from SR-BI(-/-)/apolipoprotein E(-/-) mice and SR-BI(-/-) mice that were fed cholesterol had markedly increased membrane cholesterol. Their morphology appeared immature, with macrocytosis, irregular shape, and large autophagolysosomes. Autophagolysosomes from SR-BI(-/-)/apolipoprotein E(-/-) erythrocytes were expelled when the erythrocytes were transfused into wild-type animals or incubated in vitro with normolipidemic serum or the cholesterol-sequestering agent methyl cyclodextrin. We propose that autophagocytosis and phagolysosome expulsion are essential steps in erythroid maturation and that expulsion is inhibited in the presence of markedly increased cellular cholesterol.