Tyrosine phosphorylation-independent regulation of lipopolysaccharide-mediated response by the transmembrane adaptor protein LAB

The transmembrane adaptor protein NTAL limits mast cell chemotaxis toward prostaglandin E2

Chemotaxis of mast cells is one of the crucial steps in their development and function. Non–T cell activation linker (NTAL) is a transmembrane adaptor protein that inhibits the activation of mast cells and B cells in a phosphorylation-dependent manner. Here, we studied the role of NTAL in the migration of mouse mast cells stimulated by prostaglandin E2 (PGE2). Although PGE2 does not induce the tyrosine phosphorylation of NTAL, unlike IgE immune complex antigens, we found that loss of NTAL increased the chemotaxis of mast cells toward PGE2. Stimulation of mast cells that lacked NTAL with PGE2 enhanced the phosphorylation of AKT and the production of phosphatidylinositol 3,4,5-trisphosphate. In resting NTAL-deficient mast cells, phosphorylation of an inhibitory threonine in ERM family proteins accompanied increased activation of β1-containing integrins, which are features often associated with increased invasiveness in tumors. Rescue experiments indicated that only full-length, wild-type NTAL restored the chemotaxis of NTAL-deficient cells toward PGE2. Together, these data suggest that NTAL is a key inhibitor of mast cell chemotaxis toward PGE2, which may act through the RHOA/ERM/β1-integrin and PI3K/AKT axes.

pTRAPs: Transmembrane adaptors in innate immune signaling

Transmembrane adaptor proteins (TRAPs) are protein scaffolds and signaling regulators with established roles in signal-induced activation of lymphocytes. A subset of the TRAP family, the palmitoylated TRAPs (pTRAPs), are increasingly emerging with additional roles in innate immune cells. Targeted to lipid rafts, tetraspannin-enriched microdomains, and protein microclusters in membranes, pTRAP scaffolds exert spatiotemporal regulation by recruiting signaling kinases, particularly Src and Syk family members, as well as Csk, and other effectors. In this way, pTRAPs modulate signaling and influence resulting cell responses, including the selective output of inflammatory cytokines and other mediators. Here, we review studies revealing that different pTRAPs work together, often with overlapping or redundant roles, for positive and negative regulation of key innate immune pathways, including Fc receptor and pattern recognition receptor signaling. Recent findings show that pTRAPs can bind directly to innate immune receptors, in addition to other transmembrane binding partners. Thus, pTRAPs are important, multifunctional scaffolds in pathways that are fundamental to diverse innate immune responses.

LAB/NTAL facilitates fungal/PAMP-induced IL-12 and IFN-γ production by repressing β-catenin activation in dendritic cells

Fungal pathogens elicit cytokine responses downstream of immunoreceptor tyrosine-based activation motif (ITAM)-coupled or hemiITAM-containing receptors and TLRs. The Linker for Activation of B cells/Non-T cell Activating Linker (LAB/NTAL) encoded by Lat2, is a known regulator of ITAM-coupled receptors and TLR-associated cytokine responses. Here we demonstrate that LAB is involved in anti-fungal immunity. We show that Lat2-/- mice are more susceptible to C. albicans infection than wild type (WT) mice. Dendritic cells (DCs) express LAB and we show that it is basally phosphorylated by the growth factor M-CSF or following engagement of Dectin-2, but not Dectin-1. Our data revealed a unique mechanism whereby LAB controls basal and fungal/pathogen-associated molecular patterns (PAMP)-induced nuclear β-catenin levels. This in turn is important for controlling fungal/PAMP-induced cytokine production in DCs. C. albicans- and LPS-induced IL-12 and IL-23 production was blunted in Lat2-/- DCs. Accordingly, Lat2-/- DCs directed reduced Th1 polarization in vitro and Lat2-/- mice displayed reduced Natural Killer (NK) and T cell-mediated IFN-γ production in vivo/ex vivo. Thus our data define a novel link between LAB and β-catenin nuclear accumulation in DCs that facilitates IFN-γ responses during anti-fungal immunity. In addition, these findings are likely to be relevant to other infectious diseases that require IL-12 family cytokines and an IFN-γ response for pathogen clearance.