Alternative ZAP70-p38 signals prime a classical p38 pathway through LAT and SOS to support regulatory T cell differentiation.

T cell receptor (TCR) stimulation activates diverse kinase pathways, which include the mitogen-activated protein kinases (MAPKs) ERK and p38, the phosphoinositide 3-kinases (PI3Ks), and the kinase mTOR. Although TCR stimulation activates the p38 pathway through a "classical" MAPK cascade that is mediated by the adaptor protein LAT, it also stimulates an "alternative" pathway in which p38 is activated by the kinase ZAP70. Here, we used dual-parameter, phosphoflow cytometry and in silico computation to investigate how both classical and alternative p38 pathways contribute to T cell activation.

One-way membrane trafficking of SOS in receptor-triggered Ras activation.

SOS is a key activator of the small GTPase Ras. In cells, SOS-Ras signaling is thought to be initiated predominantly by membrane recruitment of SOS via the adaptor Grb2 and balanced by rapidly reversible Grb2-SOS binding kinetics. However, SOS has multiple protein and lipid interactions that provide linkage to the membrane.

Regulation of ras exchange factors and cellular localization of ras activation by lipid messengers in T cells.

The Ras-MAPK signaling pathway is highly conserved throughout evolution and is activated downstream of a wide range of receptor stimuli. Ras guanine nucleotide exchange factors (RasGEFs) catalyze GTP loading of Ras and play a pivotal role in regulating receptor-ligand induced Ras activity. In T cells, three families of functionally important RasGEFs are expressed: RasGRF, RasGRP, and Son of Sevenless (SOS)-family GEFs.

Activation of extracellular signal-regulated kinase but not of p38 mitogen-activated protein kinase pathways in lymphocytes requires allosteric activation of SOS.

Thymocytes convert graded T cell receptor (TCR) signals into positive selection or deletion, and activation of extracellular signal-related kinase (ERK), p38, and Jun N-terminal protein kinase (JNK) mitogen-activated protein kinases (MAPKs) has been postulated to play a discriminatory role. Two families of Ras guanine nucleotide exchange factors (RasGEFs), SOS and RasGRP, activate Ras and the downstream RAF-MEK-ERK pathway. The pathways leading to lymphocyte p38 and JNK activation are less well defined.

Essential role of membrane cholesterol in accelerated BCR internalization and uncoupling from NF-kappa B in B cell clonal anergy.

Divergent hypotheses exist to explain how signaling by the B cell receptor (BCR) is initiated after antigen binding and how it is qualitatively altered in anergic B cells to selectively uncouple from nuclear factor kappaB and c-Jun N-terminal kinase pathways while continuing to activate extracellular signal-regulated kinase and calcium-nuclear factor of activated T cell pathways. Here we find that BCRs on anergic cells are endocytosed at a very enhanced rate upon binding antigen, resulting in a large steady-state pool of intracellularly sequestered receptors that appear to be continuously cycling between surface and intracellular compartments. This endocytic mechanism is exquisitely sensitive to the lowering of plasma membrane cholesterol by methyl-beta-cyclodextrin, and, when blocked in this way, the sequestered BCRs return to the cell surface and RelA nuclear accumulation is stimulated.

Scaffolding of antigen receptors for immunogenic versus tolerogenic signaling.

Lymphocyte antigen receptors are responsible for inducing the opposite responses of immunity or tolerance. How the correct polarity of antigen receptor signaling is encoded has been an enduring enigma. Here we summarize recent advances defining key scaffolding molecules, CARMA1 (also known as CARD11) and the Cbl family of ubiquitin ligases, required for either immunogenic or tolerogenic signaling by antigen receptors.

Identifying the MAGUK protein Carma-1 as a central regulator of humoral immune responses and atopy by genome-wide mouse mutagenesis.

In a genome-wide ENU mouse mutagenesis screen a recessive mouse mutation, unmodulated, was isolated with profound defects in humoral immune responses, selective deficits in B cell activation by antigen receptors and T cell costimulation by CD28, and gradual development of atopic dermatitis with hyper-IgE. Mutant B cells are specifically defective in forming connections between antigen receptors and two key signaling pathways for immunogenic responses, NF-kappaB and JNK, but signal normally to calcium, NFAT, and ERK. The mutation alters a conserved leucine in the coiled-coil domain of CARMA-1/CARD11, a member of the MAGUK protein family implicated in organizing multimolecular signaling complexes.