Membrane cholesterol content accounts for developmental differences in surface B cell receptor compartmentalization, actin reorganization and signaling

Abstract

B cells undergo different antigen receptor induced signaling responses and cell fate decisions depending on their stage of development. Transitional immature B cells contain less cholesterol than mature B cells, and antigen receptor stimulated transitional immature B cells fail to compartmentalize their B cell receptor (BCR) with cholesterol-enriched plasma membrane. Cholesterol is a key component in the formation of the membrane microdomain termed the lipid raft, and lipid rafts have been proposed to function as platforms for antigen receptor signal transduction. Augmentation of cholesterol levels in transitional immature B cells such that the plasma membrane has equal levels of cholesterol compared to mature B cells results in antigen receptor stimulated association of the BCR with cholesterol-enriched plasma membrane. Association of the BCR with lipid rafts correlates with sustained signaling through the PLCγ2/NFκB/c-myc pathway. In addition, mature B cells undergo directed actin polymerization following BCR stimulation, while transitional immature B cells are deficient in this response. Interestingly, Vav phosphorylation in mature B cells is stronger than transitional immature B cells, and this correlates with increased GTP-loading of Rac1. Also, BCR stimulation of mature B cells leads to membrane ruffling, a Rac1-dependent process. Transitional immature B cells do not undergo membrane ruffling following BCR stimulation consistent with the defect in Rac1-GTP loading. Cholesterol addition to transitional immature B cells rescues Rac1-GTP loading and directed actin polymerization and membrane ruffling. These studies provide a link between B cell cholesterol levels and downstream cellular signaling processes