Effects of Nitric Oxide in Sickle Cell Blood

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Sickle cell disease is caused by a nnutant form of hemoglobin that polymerizes when exposed to low oxygen tension. Polymerization makes the red blood cells rigid so that they cannot traverse some blood vessels leading to blockage of these vessels which then leads to significant morbidity and mortality. Although polymerization can be thought of as the primary cause of the disease, clinical presentation includes symptoms that result from a host of pathological states including acute and chronic inflammation, stasis, reperfusion injury, endothelial activation and dysfunction, coagulation activation, and excess oxidant stress. The link between polymerization and these pathologies is not clear. However, part ofthe link may be tied to the function of Nitric Oxide (NO), an important signaling molecule. NO is synthesized in endothelial cells of blood vessels and diffuses to neighboring smooth muscle cells where it acts as a signaling molecule, causing muscle relaxation and vasodilation. It also decreases endothelial and platelet activation, acts as an anti-oxidant, and is anti-inflammatory. Sickle red blood cells are fragile, rupturing during transit in the circulation (hemolysis). Several groups have hypothesized that cell-free hemoglobin that is released upon hemolysis efficiently scavenges NO, leading to an NO-related deficiency associated with sickle cell disease. The roles of hemolysis and NO in sickle cell disease remain controversial. This project aims to explore those roles while examining the links between polymerization, hemolysis, and other pathological consequences in sickle cell disease. This project will also examine aspects of therapeutic interventions that involve increasing nitric oxide bioavailability via nitrite therapy. The laboratories participating in this project have recently shown that, contrary to the existing paradigm, nitrite acts as a vasodilator in human circulation that is preferentially released under low oxygen conditions. Results from this project will elucidate mechanistic pathways in the pathology of sickle cell disease and nitric oxide biology and extend these insights to translational studies. As NO is important in many diseases, information gained herein will have wide application.