Mechanisms of sickle hemoglobin polymerization and gelation in the absence and in the presence of shear

Abstract

Sickle cell disease is caused by a single amino acid substitution ({dollar}\beta{dollar}6:glu{dollar}\to{dollar}val) in the sickle hemoglobin (HbS) molecule. Upon deoxygenation, HbS molecules aggregate into solid, rope-like fibers (210 A diameter) which increase the intraerythrocytic viscosity and decrease the deformability of the affected cells. Thus, an understanding of the mechanisms of nucleation and growth of HbS fibers and the nature of the resultant gelation may provide insights towards designing a suitable therapy. Video-enhanced differential interference contrast (DIC) and dark-field microscopic methods showed that HbS fibers are nucleated by the aggregation of HbS molecules homogeneously (in the absence of a nucleating surface) and heterogeneously (on the surface of existing fibers); and thus confirmed the predictions of the double nucleation mechanism of HbS polymerization (Ferrone, et al., J. Mol. Biol. (1985) 183:611-631). Video-enhanced microscopy also provided the first direct measurements of the elongation rates of individual HbS fibers and showed that the HbS gel is a cross-linked network of flexible fibers. Hence, the development of therapeutic agents which alter the rates of nucleation, elongation and cross-linking of HbS fibers may greatly improve the health of afflicted individuals.;Shear (agitation) accelerates the kinetics of HbS gelation (Harris & Bensusan, J. Lab. Clin. Med. (1975) 86:564-575). Intraerythrocytic shear forces may likewise affect the pathogenesis of sickle cell disease by increasing the rate of gelation and altering the viscoelastic properties of the gel. The concentration, shear rate and temperature dependencies of HbS gelation under steady and oscillatory shearing conditions suggest that shear may accelerate HbS gelation by (1) enhancing the rate of homogeneous nucleation secondary to cavitations, (2) breaking HbS fibers when they exceed a critical length and/or became cross-linked within the gel network, and (3) increasing the rate of cross-link formation. Further understanding of the basic mechanisms by which shear accelerates the gelation of HbS prior to the initiation of and during the growth of polymers may offer opportunities for therapeutic intervention in this disorder.