Phospholipid asymmetry in health and disease.

The asymmetric distribution of membrane phospholipids was first described more than two decades ago. The selective localization across the bilayer indicated that asymmetric biomembranes are assembled and maintained by specific mechanisms that control transbilayer lipid sidedness. The loss of phospholipid asymmetry and in particular the appearance of phosphatidylserine (PS) on the outer surface is associated with many physiologic and pathologic phenomena. Understanding the mechanisms that govern membrane lipid sidedness, including those that promote a collapse of phospholipid asymmetry, seems essential to understanding the disease states in which this unwanted PS exposure, or lack of PS exposure, is observed. Recent studies have identified some of the keyplayers in these regulatory systems. Their role, as well as the consequences of PS exposure in blood cell pathology are summarized in this chapter.