A Mechanistic Model of Early FcεRI Signaling: Lipid Rafts and the Question of Protection from Dephosphorylation

Dipak Barua,Byron Goldstein,David Holowka

doi:10.1371/journal.pone.0051669

Abstract

We present a model of the early events in mast cell signaling mediated by FcεRI where the plasma membrane is composed of many small ordered lipid domains (rafts), surrounded by a non-order region of lipids consisting of the remaining plasma membrane. The model treats the rafts as transient structures that constantly form and breakup, but that maintain a fixed average number per cell. The rafts have a high propensity for harboring Lyn kinase, aggregated, but not unaggregated receptors, and the linker for the activation of T cells (LAT). Phosphatase activity in the rafts is substantially reduced compared to the nonraft region. We use the model to analyze published experiments on the rat basophilic leukemia (RBL)-2H3 cell line that seem to contradict the notion that rafts offer protection. In these experiments IgE was cross-linked with a multivalent antigen and then excess monovalent hapten was added to break-up cross-links. The dephosphorylation of the unaggregated receptor (nonraft associated) and of LAT (raft associated) were then monitored in time and found to decay at similar rates, leading to the conclusion that rafts offer no protection from dephosphorylation. In the model, because the rafts are transient, a protein that is protected while in a raft will be subject to dephosphorylation when the raft breaks up and the protein finds itself in the nonraft region of the membrane. We show that the model is consistent with the receptor and LAT dephosphorylation experiments while still allowing rafts to enhance signaling by providing substantial protection from phosphatases.

Highlights

Lipid rafts are ordered regions of the plasma membrane enriched in cholesterol and glycosphingolipids [1,2,3,4] that are thought to play important roles in immune recognition receptormediated signaling [5,6,7]
The steady-state protein phosphorylation in the model is shown in Figure 3 as a function of the ligand concentration at various levels of lipid raft protections. (Because internalization of receptors and down regulation of signaling molecules is not in the model, at long times phosphorylation goes to an elevated steady state value rather than returning to a basal level.) The parameter z, that accounts for a lipid raft’s ability to exclude the protein tyrosine phosphatases (PTPases), represents the level of protection of phosphorylated proteins inside the lipid rafts
In this study we have extended our previous model of mast cell signaling [41] to include LAT, Grb2 and lipid rafts and used the model to answer the following question: in rat basophilic leukemia (RBL) cells can lipid rafts offer protection from PTPases that results in enhanced signaling

Summary

Introduction

Lipid rafts are ordered regions of the plasma membrane enriched in cholesterol and glycosphingolipids [1,2,3,4] that are thought to play important roles in immune recognition receptormediated signaling [5,6,7]. Lipid rafts have been extensively studied in the context of signaling mediated by FceRI, the high-affinity receptor for IgE, in rat basophilic leukemia (RBL) cells. These studies have revealed details as to how the different proteins in this signaling pathway are partitioned by lipid rafts in the cell membrane [21,22,23,24,25,26]. When we present our model of FceRI cell signaling, we consider a simplified picture of a cell surface and take the surface to be composed of a set of rafts of identical composition with the remainder of the surface composed of a homogeneous nonraft region

Methods

Results

Conclusion