MyD88 adaptor-like (Mal) functions in the epithelial barrier and contributes to intestinal integrity via protein kinase C.

MyD88 adapter-like (Mal)-deficient mice displayed increased susceptibility to oral but not intraperitoneal infection with Salmonella Typhimurium. Bone marrow chimeras demonstrated that mice with Mal-deficient non-hematopoietic cells were more susceptible to infection, indicating a role for Mal in non-myeloid cells. We observed perturbed barrier function in Mal(-/-) mice, as indicated by reduced electrical resistance and increased mucosa blood permeability following infection. Altered expression of occludin, Zonula occludens-1, and claudin-3 in intestinal epithelia from Mal(-/-) mice suggest that Mal regulates tight junction formation, which may in part contribute to intestinal integrity. Mal interacted with several protein kinase C (PKC) isoforms in a Caco-2 model of intestinal epithelia and inhibition of Mal or PKC increased permeability and bacterial invasion via a paracellular route, while a pan-PKC inhibitor increased susceptibility to oral infection in mice. Mal signaling is therefore beneficial to the integrity of the intestinal barrier during infection.

Succinate is an inflammatory signal that induces IL-1ß through HIF-1α.

Macrophages activated by the Gram-negative bacterial product lipopolysaccharide switch their core metabolism from oxidative phosphorylation to glycolysis. Here we show that inhibition of glycolysis with 2-deoxyglucose suppresses lipopolysaccharide-induced interleukin-1ß but not tumour-necrosis factor-α in mouse macrophages. A comprehensive metabolic map of lipopolysaccharide-activated macrophages shows upregulation of glycolytic and downregulation of mitochondrial genes, which correlates directly with the expression profiles of altered metabolites. Lipopolysaccharide strongly increases the levels of the tricarboxylic-acid cycle intermediate succinate. Glutamine-dependent anerplerosis is the principal source of succinate, although the 'GABA (γ-aminobutyric acid) shunt' pathway also has a role. Lipopolysaccharide-induced succinate stabilizes hypoxia-inducible factor-1α, an effect that is inhibited by 2-deoxyglucose, with interleukin-1ß as an important target. Lipopolysaccharide also increases succinylation of several proteins. We therefore identify succinate as a metabolite in innate immune signalling, which enhances interleukin-1ß production during inflammation.

Building an immune system from nine domains.

Four families of PRRs (pattern-recognition receptors) have been identified as important components of innate immunity, participating in the sensory system for host defence against the invasion of infectious agents. The TLRs (Toll-like receptors) recognize a variety of conserved microbial PAMPs (pathogen-associated molecular patterns) derived from bacteria, viruses, protozoa and fungi. They work in synergy with the cytosolic NLRs [NOD (nucleotide binding and oligomerization domain)-like receptors] (which sense bacteria), RLRs [RIG-I (retinoic acid-inducible gene 1)-like receptors] (which sense viruses) and CLRs (C-type lectin receptors) (which sense fungi). All of these receptor families signal an increase in the expression of a range of immune and inflammatory genes. The structural architecture of these receptors is conserved, involving seven distinct domains: the LRR (leucine-rich repeat) domain, the TIR [Toll/IL (interleukin)-1 receptor] domain, the NBS (nucleotide-binding site), the CARD (caspase recruitment domain), the PYD (pyrin domain), the helicase domain and the CTLD (C-type lectin domain). Two other domains, the Ig domain and the ITAM (immunoreceptor tyrosine-based activation motif) domain also participate and are also found in antibodies and TCRs (T-cell receptors), key proteins in adaptive immunity. This total of nine domains can therefore be used to construct immune systems which are common to many, if not all, species, allowing us to speculate on the minimum requirement for a complex immune system in structural terms. These insights are important for our overall understanding of the regulation of immunity in health and disease.

The potential of targeting Toll-like receptor 2 in autoimmune and inflammatory diseases.

The last decade has revealed interesting insights into the initiation and pathophysiology of the innate immune system. Toll-like receptors are of key importance for this process and they are a family of receptors expressed mainly on leukocytes that recognize a variety of microbial products derived from bacteria, viruses, protozoa and fungi. As key players of innate immunity, TLRs and downstream signalling components are important target candidates for drug development. In this review, we focus on TLR2, which recognizes bacterial lipopeptide. TLR2 forms dimers with TLR1 or TLR6. The TLR2/TLR1 dimer recognizes triacylated lipopeptides, whilst the TLR2/TLR6 dimer recognizes diacylated lipopeptides. TLR2 has been implicated in several auto-immune and inflammatory conditions, and its role in disease pathogenesis has been supported by numerous reports of TLR2 polymorphisms in humans linked to disease. Here we discuss the potential of TLR2 as a drug target in autoimmune and inflammatory disease.

Mal (MyD88-adapter-like) is required for Toll-like receptor-4 signal transduction.

The recognition of microbial pathogens by the innate immune system involves Toll-like receptors (TLRs), which recognize pathogen-associated molecular patterns. Different TLRs recognize different pathogen-associated molecular patterns, with TLR-4 mediating the response to lipopolysaccharide from Gram-negative bacteria. All TLRs have a Toll/IL-1 receptor (TIR) domain, which is responsible for signal transduction. MyD88 is one such protein that contains a TIR domain. It acts as an adapter, being involved in TLR-2, TLR-4 and TLR-9 signalling; however, our understanding of how TLR-4 signals is incomplete. Here we describe a protein, Mal (MyD88-adapter-like), which joins MyD88 as a cytoplasmic TIR-domain-containing protein in the human genome. Mal activates NF-kappaB, Jun amino-terminal kinase and extracellular signal-regulated kinase-1 and -2. Mal can form homodimers and can also form heterodimers with MyD88. Activation of NF-kappaB by Mal requires IRAK-2, but not IRAK, whereas MyD88 requires both IRAKs. Mal associates with IRAK-2 by means of its TIR domain. A dominant negative form of Mal inhibits NF-kappaB, which is activated by TLR-4 or lipopolysaccharide, but it does not inhibit NF-kappaB activation by IL-1RI or IL-18R. Mal associates with TLR-4. Mal is therefore an adapter in TLR-4 signal transduction.