Hemolysis in the spleen drives erythrocyte turnover.

Red pulp macrophages (RPMs) of the spleen mediate turnover of billions of senescent erythrocytes per day. However, the molecular mechanisms involved in sequestration of senescent erythrocytes, their recognition, and their subsequent degradation by RPMs remain unclear. In this study, we provide evidence that the splenic environment is of substantial importance in facilitating erythrocyte turnover through induction of hemolysis. Upon isolating human spleen RPMs, we noted a substantial lack of macrophages that were in the process of phagocytosing intact erythrocytes. Detailed characterization of erythrocyte and macrophage subpopulations from human spleen tissue led to the identification of erythrocytes that are devoid of hemoglobin, so-called erythrocyte ghosts. By using in vivo imaging and transfusion experiments, we further confirmed that senescent erythrocytes that are retained in the spleen are subject to hemolysis. In addition, we showed that erythrocyte adhesion molecules, which are specifically activated on aged erythrocytes, cause senescent erythrocytes to interact with extracellular matrix proteins that are exposed within the splenic architecture. Such adhesion molecule-driven retention of senescent erythrocytes under low shear conditions was found to result in steady shrinkage of the cell and ultimately resulted in hemolysis. In contrast to intact senescent erythrocytes, the remnant erythrocyte ghost shells were prone to recognition and breakdown by RPMs. These data identify hemolysis as a key event in the turnover of senescent erythrocytes, which alters our current understanding of how erythrocyte degradation is regulated.

CD36 as a multiple-ligand signaling receptor in atherothrombosis.

The glycoprotein CD36, also known as glycoprotein IIIb/IV or FAT, is expressed on the surface of platelets, monocytes, microvascular endothelial cell, smooth muscle cells, cardiomyocytes and other cells of the cardiovascular system. In spite of its abundant presence, CD36 has remained for long a mysterious protein with a poorly understood role. In this paper, we review how CD36 can affect cellular responses by interaction with a variety of ligands, in particular thrombospondin-1, oxidized lipoproteins and fatty acids. Furthermore, given the structure of CD36 with two transmembrane domains and short cytoplasmic tails, we consider how this receptor can induce intracellular signaling, likely in junction with other cellular receptors or associated proteins in the membrane. Current literature points to activation of Src-family and mitogen-activated protein kinases, as well as to activation of the NFκB and Rho pathways. The new insights make CD36 attractive as a therapeutic target to suppress platelet and monocyte/macrophage function and thereby atherothrombosis.