MyD88 is the central signaling adapter of innate immunity, mediating signaling through Toll-like receptors and IL-1R. MyD88 is regulated at several different levels. Mutations within the TIR domain of MyD88 are associated with a DLBCL lymphoma. Dimerization of TIR domains of MyD88 is the rate limiting step for the formation of the Myddosomal signaling platform. Investigation of the TIR domain proteins and mutations found in B-cell lymphoma provide an insight into the molecular mechanism of signaling in inflammation. MyD88 is composed of a death domain and TIR domain linked by an intermediary domain that also plays an important role for mediation of interactions with receptors and downstream kinases. In addition to the previously known properties and regulatory mechanisms of MyD88 we found that MyD88 can be specifically cleaved within the intermediary domain by caspase-1, which can negatively regulate signaling via MyD88 and that this process is triggered by activation of the inflammasome. Further we found that the MyD88 and other components of the myddosome are found in the extracellular vesicles (EVs) shed by cells that express the constitutively active form of MyD88. EVs are able to transfer the myddosome to the target cells and trigger activation of the signaling pathway. Particularly the form of MyD88 associated to the certain types of the diffuse large B-cell lymphoma and Waldenstrom’s macroglobulinaemia are found in the EVs, which represents a new type of the transfer of the inflammatory triggers, which contributes to the shaping of the tumor microenvironment. New features of MyD88 thus expand our understanding of the molecular mechanisms of MyD88-mediated signaling and may open new therapeutic approaches.
B.04 Guest lecture
COBISS.SI-ID: 6147354MyD88 is the central signaling adapter of innate immunity, mediating signaling through Toll-like receptors and IL-1R. MyD88 is regulated at several different levels. Mutations within the TIR domain of MyD88 which trigger the constitutive activation of signaling pathway are associated with a DLBCL lymphoma. Investigation of the TIR domain proteins and mutations found in B-cell lymphoma provide an insight into the molecular mechanism of signaling in inflammation. MyD88 is composed of a death domain and TIR domain linked by an intermediary domain that also plays an important role in mediating interactions with receptors and downstream kinases. Here we report that MyD88 can be cleaved within the intermediary domain by caspase-1. This process is triggered by activation of the inflammasome and negatively regulates MyD88 signaling. Further we found that the MyD88 and other components of the myddosome can be transmitted between cells through extracellular vesicles (EVs) shed by cells that express the constitutively active form of MyD88. Myddosome loaded EVs trigger activation of the signaling pathway. The MyD88L256P associated the diffuse large B-cell lymphoma and Waldenstrom’s macroglobulinaemia are found in the EVs, which represents a new type of the transfer of the inflammatory triggers, which contributes to the shaping of the tumor microenvironment. New features of MyD88 thus expand our understanding of the molecular mechanisms of MyD88-mediated signaling and may open new therapeutic approaches.
B.04 Guest lecture
COBISS.SI-ID: 6299418MyD88 is the central mediator of innate immunity whose deficiency leads to strong sensitivity to infection while its autoactivation leads to the constitutive cell activation observed in Waldenstrom’s macroglobulinemia (WM). We have previously shown that autoactivation is a consequence of augmented oligomerisation due to a point mutantion L265P in the TIR domain of MyD88. Understanding the regulation of MyD88 is therefore very important for a large number of physiological processes. Most of Toll-like receptors (TLRs) activate signal transduction mediated by MyD88 and initiate production of proinflammatory cytokines such as TNFalpha, IL-6 or pro-IL-1beta. Recently, IL-beta has been shown to play an important role in pathology of several diseases, such as atherosclerosis, arthritis, Crohn’s disease, cancer and diabetes. Formation of the mature IL-1beta is highly regulated as it requires two signals, first via the activation of TLRs or other activators of the NF-kappaB signaling pathway that induces the production of pro-IL-beta and the second that activates the Nod-like receptors (NLRs) leading to the assembly of inflammasome and subsequent caspase-1 activation, which is required for the processing of pro-IL-1beta. A precise regulation of innate immune signaling can be achieved by feed-back mechanisms. We show that inflammasome activation negatively affects TLR signaling. Inflammasome dependent caspase-1 cleaves MyD88 and generates a short C-terminal fragment of MyD88 that acts inhibitory on the TLR signaling. Our data demonstrate that activation of the inflammasome in THP-1 derived macrophages lowers the amount of MyD88-dependent cytokines such as TNFalphaor IL-6 due to the cleavage of MyD88. Further, we were interested if a cleaved form of MyD88 is present in diseases displaying constitutive cell activation. We detected a cleaved fragment of MyD88 in WM cell line MWCL1, where we also showed the elevated expression of several caspases in contrast to the B lymphocytes. We propose that activation of caspases is responsible for the cleavage of MyD88, since the addition of pancaspase inhibitors elevates the amounts of TNFalphaor IL-6 in MWCL1 cell line. Further work will elucidate the relevance of this discovery for the diagnostics or treatment of WM.
B.04 Guest lecture
COBISS.SI-ID: 6042650