Graduate School of Biomedical Sciences, Interdisciplinary Graduate Program
Dissertations, UMMS; Toll-Like Receptors; Myeloid Differentiation Factor 88; Membrane Glycoproteins; Receptors, Interleukin-1; Polymorphism, Single Nucleotide
The innate immune system is the first line of defense against invading pathogens. Recognition of microbial ligands by the innate immune system relies on germ-line encoded, evolutionarily conserved receptors called pattern recognition receptors (PRRs). Toll-like receptors (TLRs) are one such family of PRRs and are involved in innate defenses to a variety of microbes. At the core of TLR signaling pathways are Toll interleukin-1 receptor (TIR) domain containing adapter proteins. Much of the specificity of TLR pathways arise from the differential use of these adapter proteins.
The TLR signaling cascade that ensues upon ligand recognition is marked by finely orchestrated molecular interactions between the receptor and the TIR domain containing adapter proteins, as well as various downstream kinases and effector molecules. Conserving the structural integrity of the TLR components is thus essential for maintaining a robust host defense system. Sometimes, changes in a protein can be brought about by single nucleotide polymorphisms (SNPs). Studies carried out in this thesis focus on polymorphisms in MyD88 adapter-like (Mal) and myeloid differentiation protein 88 (MyD88), two TIR domain-containing adapter proteins, which incidentally are also highly polymorphic.
Mal is a 235 amino acid protein that is involved in TLR2 and TLR4 signaling. The known polymorphisms in the coding region of Mal were screened with an aim to identify SNPs with altered signaling potential. A TIR domain polymorphism, D96N, was found to be completely defective in TLR2 and TLR4 signaling. Immortalized macrophage-like cell lines expressing D96N have impaired cytokine production as well as NF-κB activation. The reason for this loss-of-function phenotype is the inability of Mal D96N to bind the downstream adapter MyD88, an event necessary for signaling to occur. Genotyping studies reveal a very low frequency of this polymorphism in the population.
Similar SNP analysis was carried out in myeloid differentiation protein 88 (MyD88). MyD88 is a key signaling adapter in TLR signaling; critical for all TLR pathways except TLR3. In reporter assays, a death domain variant, S34Y, was found to be inactive. Importantly, in reconstituted macrophage-like cell lines derived from knockout mice, MyD88 S34Y was severely compromised in its ability to respond to all MyD88-dependent TLR ligands. S34Y mutant has a dramatically different localization pattern as compared to wild type MyD88. Unlike wild type MyD88, S34Y is unable to form distinct foci in the cells but is present diffused in the cytoplasm. IRAK4, a downstream kinase, colocalizes with MyD88 in these aggregates or “Myddosomes”. S34Y MyD88, however, is unable to assemble into Myddosomes, thus demonstrating that proper cellular localization of MyD88 is a feature required for MyD88 function.
This thesis thus describes two loss‐of‐function polymorphisms in TLR adapter proteins Mal and MyD88. It sheds light not only on the structural aspects of signaling by these two proteins, but also has implications for the development of novel pharmaceutical agents.
Nagpal, K. A Tale of Two SNPS: Polymorphism Analysis of Toll-like Receptor (TLR) Adapter Proteins: A Dissertation. (2011). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 540. http://escholarship.umassmed.edu/gsbs_diss/540
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