p38MAPK/MK2-dependent phosphorylation controls cytotoxic RIPK1 signalling in inflammation and infection.

Receptor-interacting protein kinase-1 (RIPK1), a master regulator of cell fate decisions, was identified as a direct substrate of MAPKAP kinase-2 (MK2) by phosphoproteomic screens using LPS-treated macrophages and stress-stimulated embryonic fibroblasts. p38MAPK/MK2 interact with RIPK1 in a cytoplasmic complex and MK2 phosphorylates mouse RIPK1 at Ser321/336 in response to pro-inflammatory stimuli, such as TNF and LPS, and infection with the pathogen Yersinia enterocolitica. MK2 phosphorylation inhibits RIPK1 autophosphorylation, curtails RIPK1 integration into cytoplasmic cytotoxic complexes, and suppresses RIPK1-dependent apoptosis and necroptosis. In Yersinia-infected macrophages, RIPK1 phosphorylation by MK2 protects against infection-induced apoptosis, a process targeted by Yersinia outer protein P (YopP). YopP suppresses p38MAPK/MK2 activation to increase Yersinia-driven apoptosis. Hence, MK2 phosphorylation of RIPK1 is a crucial checkpoint for cell fate in inflammation and infection that determines the outcome of bacteria-host cell interaction.

Cell death triggered by Yersinia enterocolitica identifies processing of the proinflammatory signal adapter MyD88 as a general event in the execution of apoptosis.

Many pathogenic microorganisms have evolved tactics to modulate host cell death or survival pathways for establishing infection. The enteropathogenic bacterium Yersinia enterocolitica deactivates TLR-induced signaling pathways, which triggers apoptosis in macrophages. In this article, we show that Yersinia-induced apoptosis of human macrophages involves caspase-dependent cleavage of the TLR adapter protein MyD88. MyD88 was also cleaved when apoptosis was mediated by overexpression of the Toll-IL-1R domain-containing adapter inducing IFN-ß in epithelial cells. The caspase-processing site was mapped to aspartate-135 in the central region of MyD88. MyD88 is consequently split by caspases in two fragments, one harboring the death domain and the other the Toll-IL-1R domain. Caspase-3 was identified as the protease that conferred the cleavage of MyD88 in in vitro caspase assays. In line with a broad role of caspase-3 in the execution of apoptosis, the processing of MyD88 was not restricted to Yersinia infection and to proapoptotic Toll-IL-1R domain-containing adapter inducing IFN-ß signaling, but was also triggered by staurosporine treatment. The cleavage of MyD88 therefore seems to be a common event in the advanced stages of apoptosis, when caspase-3 is active. We propose that the processing of MyD88 disrupts its scaffolding function and uncouples the activation of TLR and IL-1Rs from the initiation of proinflammatory signaling events. The disruption of MyD88 may consequently render dying cells less sensitive to proinflammatory stimuli in the execution phase of apoptosis. The cleavage of MyD88 could therefore be a means of conferring immunogenic tolerance to apoptotic cells to ensure silent, noninflammatory cell demise.