Graduate School of Biomedical Sciences, Immunology & Virology
Immunity, Natural; Signal Transduction; Adaptor Proteins, Signal Transducing; Isoenzyme; Toll-Like Receptor 4; Antigens, Surface; Carrier Proteins; Lipopolysaccharides; Academic Dissertations; Dissertations, UMMS
Over the last decade, the innate immune system has been the subject of extensive research. Often overlooked by the robustness and specificity of the adaptive immune system, the innate immune system is proving to be just as complex. The identification of several families of pattern recognition receptors (PRRs) has revealed an ancient yet multifaceted system of proteins that are responsible for initiating host defense. A wide array of pathogens, from virus to bacteria, is detected using this assortment of receptors. One such family, the Toll-like receptors (TLRs), has been at the forefront of this research. To date, 10 TLRs have been described in the human genome. Activation of TLRs leads to the induction of immune-related genes that ultimately control the response of the host. However, the signaling pathways emanating from activated TLRs and other PRRs are not fully understood. In particular, the pathway leading to the activation of interferon regulatory factor 3 (IRF3), a transcription factor crucial for the induction of type I interferon, remains undefined. IRF3 activation occurs as the consequence of viral infection and through the activation of TLRs 3 and 4 by dsRNA and lipopolysaccharide (LPS), respectively. The focus of this research is to describe components of the IRF3 activation pathway, partly through the analysis of TLR signal transduction.
IRF3 normally resides in the cytoplasm of cells. Upon infection with certain viruses and bacteria, IRF3 is activated though phosphorylation at its C-terminus. Phosphorylated IRF3 homodimerizes and associates with co-activators CBP-p300. After translocating to the nucleus, the activate IRF3 complex induces the activation of type 1 interferon and interferon related genes. Little is known about the pathways that lead to the activation of IRF3, especially the kinases involved. In this study we report that the non-canonical IкB kinase homologues, IкB kinase epsilon (IKKε) and TANK-binding kinase-1 (TBK1), which were previously implicated in NF-кB activation, are also essential components of the IRF3 signaling pathway. In particular, mouse embryonic fibroblasts from TBK1 deficient mice fail to activate IRF3 in response to both viral infection and stimulation with LPS or poly (IC), a dsRNA analog. Thus, both IKKε and TBK1 play a critical role in innate immunity and host defense.
In addition to viral infection, IRF3 activation also occurs via the activation of TLR3 and 4. TLRs signal through a subfamily of Toll-IL-1-Resistance (TIR) domain containing adapter molecules. One such adapter, MyD88, is crucial for all TLRs, with the exception of TLR3. MyD88 participates in a signal transduction pathway culminating in the activation of the transcription factor NF-кB. Studies from MyD88-deficient mice reveal that both TLR3 and 4 still are capable of activating NF-кB, although with slightly delayed kinetics. Another aspect of the MyD88-independent signal transduction pathway is the activation of IRF3. A second TIR domain containing adapter molecule called Mal/Tirap was discovered and originally thought to mediate the MyD88-independent pathway. However, Mal-deficient mice were found to be defective in both TLR2 and 4 mediated NF-кB activation. We hypothesized that other TIR domain containing adapters could mediate this MyD88-independent pathway of TLR3 and 4 leading to the activation of IRF3. Two additional TIR adapters were discovered, TRIF and TRAM. TRIF was shown to mediate TLR3 signal transduction. In this study, we report that both TRIF and TRAM mediate the activation of the MyD88-independent pathway in response to LPS/TLR4 activation. Unlike any of the other known TIR domain containing adapters, TRAM appears to be restricted to the LPS/TLR4 activation pathway while TRIF plays a role in both TLR3 and TLR4 pathways leading to IRF3 target gene expression.
Our studies revealed that TRAM could be acting upstream of TRIF in the LPS/TLR4 pathway. To this end, we sought to determine the localization of TRAM within the cell. We found that TRAM localizes to the plasma membrane. TRAM localization is the result of myristoylation since mutation of the predicted myristoylation site (G2A) resulted in the re-distribution of TRAM from the membrane into the cytoplasm. Reconstitution of TRAM-deficient macrophages with TRAM G2A is unable to rescue LPS/TLR4 signal transduction. Thus, myristoylation and membrane association of TRAM are critical for LPS/TLR4 signal transduction.
The data generated in this dissertation extends our understanding of the signaling pathways of the innate immune system. Indeed, the molecules and pathways described herein could prove to be beneficial targets for ameliorating symptoms of disease, both autoimmune and pathogen-associated. Finally, the research described here will spur further insight into the complex signaling pathways of a once ignored arm of the immune system.
Rowe, DC. Analysis of Toll-Like Receptor 4 Signal Transduction and IRF3 Activation in the Innate Immune Response: A Dissertation. (2006). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 163. http://escholarship.umassmed.edu/gsbs_diss/163
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