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Chapter 101. Hemolytic Anemias and Anemia Due to Acute Blood Loss (Part 5)

Hemolytic Anemias Due to Abnormalities of the MembraneCytoskeleton Complex The detailed architecture of the red cell membrane is complex, but its basic design is relatively simple (Fig. 101-2). The lipid bilayer, which incorporates phospholipids and cholesterol, is spanned by a number of proteins that have their hydrophobic transmembrane domains embedded in the membrane. Most of these proteins have hydrophilic domains extending toward both the outside and the inside of the cell. Other proteins are tethered to the membrane through a glycosylphosphatidylinositol (GPI) anchor, and they have only an extracellular domain. These proteins are arranged roughly perpendicular to or lying across. | Chapter 101. Hemolytic Anemias and Anemia Due to Acute Blood Loss Part 5 Hemolytic Anemias Due to Abnormalities of the MembraneCytoskeleton Complex The detailed architecture of the red cell membrane is complex but its basic design is relatively simple Fig. 101-2 . The lipid bilayer which incorporates phospholipids and cholesterol is spanned by a number of proteins that have their hydrophobic transmembrane domains embedded in the membrane. Most of these proteins have hydrophilic domains extending toward both the outside and the inside of the cell. Other proteins are tethered to the membrane through a glycosylphosphatidylinositol GPI anchor and they have only an extracellular domain. These proteins are arranged roughly perpendicular to or lying across the membrane they include ion channels receptors for complement components receptors for other ligands and some of unknown function. The most abundant of these proteins are glycophorins and the so-called band 3 an anion transporter. The extracellular domains of many of these proteins are heavily glycosylated and they carry antigenic determinants that correspond to blood groups. Underneath the membrane and tangential to it is a network of other proteins that make up the cytoskeleton. The main cytoskeletal protein is spectrin the basic unit of which is a dimer of a-spectrin and 0-spectrin. The membrane is physically linked to the cytoskeleton by a third set of proteins including ankyrin and the so-called band 4.1 and band 4.2 which thus connect these two structures intimately. Figure 101-2 Diagram of red cell membrane cytoskeleton. For explanation see text. From N Young et al Clinical Hematology. Copyright Elsevier 2006 with permission. The membrane-cytoskeleton complex is indeed so integrated that not surprisingly an abnormality of almost any of its components will be disturbing or disruptive causing structural failure which results ultimately in hemolysis. These abnormalities are almost invariably inherited mutations .