Hijacking of host cell IKK signalosomes by the transforming parasite Theileria

Parasites have evolved a plethora of mechanisms to ensure their propagation and evade antagonistic host responses. The intracellular protozoan parasite Theileria is the only eukaryote known to induce uncontrolled host cell proliferation. Survival of Theileria-transformed leukocytes depends strictly on constitutive nuclear factor kappa B (NF-kappaB) activity. We found that this was mediated by recruitment of the multisubunit IkappaB kinase (IKK) into large, activated foci on the parasite surface. IKK signalosome assembly was specific for the transforming schizont stage of the parasite and was down-regulated upon differentiation into the nontransforming merozoite stage. Our findings provide insights into IKK activation and how pathogens subvert host-cell signaling pathways.     

Activation by IKKalpha of a second, evolutionary conserved, NF-kappa B signaling pathway

In mammals, the canonical nuclear factor kappaB (NF-kappaB) signaling pathway activated in response to infections is based on degradation of IkappaB inhibitors. This pathway depends on the IkappaB kinase (IKK), which contains two catalytic subunits, IKKalpha and IKKbeta. IKKbeta is essential for inducible IkappaB phosphorylation and degradation, whereas IKKalpha is not. Here we show that IKKalpha is required for B cell maturation, formation of secondary lymphoid organs, increased expression of certain NF-kappaB target genes, and processing of the NF-kappaB2 (p100) precursor. IKKalpha preferentially phosphorylates NF-kappaB2, and this activity requires its phosphorylation by upstream kinases, one of which may be NF-kappaB-inducing kinase (NIK). IKKalpha is therefore a pivotal component of a second NF-kappaB activation pathway based on regulated NF-kappaB2 processing rather than IkappaB degradation.     

Limb and skin abnormalities in mice lacking IKKalpha

The gene encoding inhibitor of kappa B (IkappaB) kinase alpha (IKKalpha; also called IKK1) was disrupted by gene targeting. IKKalpha-deficient mice died perinatally. In IKKalpha-deficient fetuses, limb outgrowth was severely impaired despite unaffected skeletal development. The epidermal cells in IKKalpha-deficient fetuses were highly proliferative with dysregulated epidermal differentiation. In the basal layer, degradation of IkappaB and nuclear localization of nuclear factor kappa B (NF-kappaB) were not observed. Thus, IKKalpha is essential for NF-kappaB activation in the limb and skin during embryogenesis. In contrast, there was no impairment of NF-kappaB activation induced by either interleukin-1 or tumor necrosis factor-alpha in IKKalpha-deficient embryonic fibroblasts and thymocytes, indicating that IKKalpha is not essential for cytokine-induced activation of NF-kappaB.     

Prokaryotic regulation of epithelial responses by inhibition of IkappaB-alpha ubiquitination

Epithelia of the vertebrate intestinal tract characteristically maintain an inflammatory hyporesponsiveness toward the lumenal prokaryotic microflora. We report the identification of enteric organisms (nonvirulent Salmonella strains) whose direct interaction with model human epithelia attenuate synthesis of inflammatory effector molecules elicited by diverse proinflammatory stimuli. This immunosuppressive effect involves inhibition of the inhibitor kappaB/nuclear factor kappaB (IkappaB/NF-kappaB) pathway by blockade of IkappaB-alpha degradation, which prevents subsequent nuclear translocation of active NF-kappaB dimer. Although phosphorylation of IkappaB-alpha occurs, subsequent polyubiquitination necessary for regulated IkappaB-alpha degradation is completely abrogated. These data suggest that prokaryotic determinants could be responsible for the unique tolerance of the gastrointestinal mucosa to proinflammatory stimuli.     

Essential cytoplasmic translocation of a cytokine receptor-assembled signaling complex

Cytokine signaling is thought to require assembly of multicomponent signaling complexes at cytoplasmic segments of membrane-embedded receptors, in which receptor-proximal protein kinases are activated. Indeed, CD40, a tumor necrosis factor receptor (TNFR) family member, forms a complex containing adaptor molecules TRAF2 and TRAF3, ubiquitin-conjugating enzyme Ubc13, cellular inhibitor of apoptosis proteins 1 and 2 (c-IAP1/2), IkappaB kinase regulatory subunit IKKgamma (also called NEMO), and mitogen-activated protein kinase (MAPK) kinase kinase MEKK1 upon ligation. TRAF2, Ubc13, and IKKgamma were required for complex assembly and activation of MEKK1 and MAPK cascades. However, these kinases were not activated unless the multicomponent signaling complex translocated from CD40 to the cytosol upon c-IAP1/2-induced degradation of TRAF3. This two-stage signaling mechanism may apply to other innate immune receptors, accounting for spatial and temporal separation of MAPK and IKK signaling.     

The vaccine adjuvant monophosphoryl lipid A as a TRIF-biased agonist of TLR4

The inflammatory toxicity of lipopolysaccharide (LPS), a component of bacterial cell walls, is driven by the adaptor proteins myeloid differentiation factor 88 (MyD88) and Toll-interleukin 1 receptor domain-containing adapter inducing interferon-beta (TRIF), which together mediate signaling by the endotoxin receptor Toll-like receptor 4 (TLR4). Monophosphoryl lipid A (MPLA) is a low-toxicity derivative of LPS with useful immunostimulatory properties, which is nearing regulatory approval for use as a human vaccine adjuvant. We report here that, in mice, the low toxicity of MPLA's adjuvant function is associated with a bias toward TRIF signaling, which we suggest is likely caused by the active suppression, rather than passive loss, of proinflammatory activity of this LPS derivative. This finding may have important implications for the development of future vaccine adjuvants.     

Feedback loops shape cellular signals in space and time

Positive and negative feedback loops are common regulatory elements in biological signaling systems. We discuss core feedback motifs that have distinct roles in shaping signaling responses in space and time. We also discuss approaches to experimentally investigate feedback loops in signaling systems.