Membrane bending and sphingomyelinase associated sulfatide dependent hypoxic adhesion of sickle mature erythrocytes.

Abnormal erythrocyte adhesion due to polymerization of sickle hemoglobin is central to the pathophysiology of sickle cell disease (SCD). Mature erythrocytes constitute >80% of all erythrocytes in SCD, and the relative contributions made by erythrocytes to acute and chronic vasculopathy in SCD are not well understood. Here, we show that the bending stress exerted on the erythrocyte plasma membrane by the sickle hemoglobin polymerization under hypoxia enhances sulfatide-mediated abnormal mature erythrocyte adhesion. We hypothesized that sphingomyelinase activity that is upregulated by accumulated bending energy leads to elevated membrane sulfatide availability and thus hypoxic mature erythrocyte adhesion. We found that mature erythrocyte adhesion to laminin in controlled microfluidic experiments is significantly greater under hypoxia than under normoxia (1856±481 vs. 78±23, mean±SEM), while sickle reticulocyte (early erythrocyte) adhesion, high to begin with, does not change (1281 ±299 vs. 1258±328, mean±SEM). We show that greater mean accumulated bending energy of adhered mature erythrocytes is associated with higher acid SMase activity and increased mature erythrocyte adhesion (p=0.022, for the acid SMase activity and p=0.002 for the increase in mature erythrocyte adhesion with hypoxia, N=5). In addition, hypoxia results in sulfatide exposure on the erythrocyte membrane, and sphingomyelinase increases while anti-sulfatide inhibits the enhanced adhesion of erythrocytes. These results suggest that lipid components of the plasma membrane contribute to the complications in SCD. Therefore, sulfatide and the components of its upregulation pathway, particularly sphingomyelinase should be further explored as potential therapeutic targets to inhibit sickle erythrocyte adhesion.