NEU1 SIALIDASE AND MATRIX METALLOPROTEINASE-9 CROSS-TALK IS ESSENTIAL FOR TOLL-LIKE RECEPTOR ACTIVATION AND CELLULAR SIGNALING
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The molecular mechanism(s) by which Toll-like receptors become activated are not well understood. For the majority of TLR receptors, dimerization is a prerequisite to facilitate MyD88-TLR complex formation and subsequent cellular signaling to activate NF-κB. However, the parameters controlling interactions between the receptors and their ligands still remain poorly defined. Previous reports have identified that neuraminidase-1 (NEU1) is an important intermediate in the initial process of TLR ligand induced receptor activation and subsequent cell function. What we do not yet understand is how NEU1 is activated following TLR ligand binding. In this thesis, the findings disclose a receptor signaling paradigm involving a process of receptor ligand-induced GPCR-signaling via neuromedin-B (NMBR) Gα-proteins, matrix metalloproteinase-9 (MMP-9) activation, and the induction of Neu1 activation. Central to this process is that NEU1–MMP-9-NMBR complex is associated with TLR-4 receptors on the cell surface of naive primary macrophages and TLR-expressing cell lines. Ligand binding to the receptor initiate GPCR-signaling via GPCR Gα subunit proteins and MMP-9 activation to induce NEU1. Activated NEU1 targets and hydrolyzes sialyl α-2-3-linked to β-galactosyl residues at the ectodomain of TLRs, enabling the removal of steric hindrance to receptor association, activation of receptors and cellular signaling. Furthermore, a novel glycosylation model is uncovered for the activation of nucleic acid sensing intracellular TLR-7 and TLR-9 receptors. It discloses an identical signaling paradigm as described for the cell-surface TLRs. NEU1 and MMP9 cross-talk in alliance with neuromedin-B receptors tethered to TLR-7 and -9 receptors at the ectodomain is essential for ligand activation of the TLRs and pro-inflammatory responses. However, the mechanism(s) behind this GPCR and TLR cross-talk has not been fully defined. Here, GPCR agonists mediate GPCR-signaling via membrane Gα subunit proteins to induce NEU1 and MMP-9 cross-talk at the TLR ectodomain on the cell surface. This molecular organizational GPCR signaling platform is proposed to be an initial processing stage for GPCR agonist-induced transactivation of TLRs and subsequent cellular signaling. Collectively, these novel findings radically redefine the current dogma(s) governing the mechanism(s) of the interaction of TLRs and their ligands, which may provide important pioneering approaches to disease intervention strategies.