Clustered Ligands Beget Clustered Receptors

Abstract

Clustering of receptors in sphingolipid-rich membrane rafts can promote receptor activation. Grassmé et al. explored the possibility that membrane-bound ligands might themselves become clustered, thereby contributing to receptor activation. They analyzed CD40, a receptor that mediates B cell activation. The CD40 ligand is membrane bound, and itself has some properties of a receptor, mediating "outside-to-inside" signals when it binds CD40. When mouse EL4 cells, which express CD40 ligand, were exposed to antibody to the CD40 ligand or to B lymphocytes expressing CD40, the CD40 ligand formed clusters on the cell surface. Such clustering failed to occur in T cells deficient in acid sphingomyelinase (ASM), an enzyme that helps form the sphingolipid-rich rafts. A search for proteins associated with the CD40 ligand turned up p53. Although p53 is better known as a nuclear protein regulating transcription, endogenous p53 was associated with CD40 immunoprecipitated from peripheral blood T lymphocytes. Furthermore, T lymphocytes from p53 knock-out mice failed to cluster CD40 ligand when treated with antibody to the CD40 ligand. In assays for activation of CD40 itself (in which T cells expressing CD40 ligand activated CD40 on B cells), conditions that prevented clustering of CD40 ligand also prevented clustering of CD40. Thus T cells lacking ASM or p53 failed to cause clustering of CD40 as did EL4 cells treated with agents that prevent metabolism of cholesterol and formation of sphingolipid-rich rafts. Given that clustering of CD40 is a prerequisite for signaling by that receptor, the results indicate that at "synapses" between cells bearing CD40 and its ligand, clustering of the CD40 ligand may first be required for proper clustering and signaling by CD40 on target cells. H. Grassmé, M. Bock, J. Kun, E. Gulbins, Clustering of CD40 ligand is required to form a functional contact with CD40. J. Biol. Chem. 277 , 30289-30299 (2002). [Abstract] [Full Text]