Regulation of MyD88 Aggregation and the MyD88-dependent Signaling Pathway by Sequestosome 1 and Histone Deacetylase 6

Abstract

MyD88 is an essential adaptor molecule for Toll-like receptors (TLRs) and interleukin (IL)-1 receptor. MyD88 is thought to be present as condensed forms or aggregated structures in the cytoplasm, although the reason has not yet been clear. Here, we show that endogenous MyD88 is present as small speckle-like condensed structures, formation of which depends on MyD88 dimerization. In addition, formation of large aggregated structures is related to cytoplasmic accumulation of sequestosome 1 (SQSTM1; also known as p62) and histone deacetylase 6 (HDAC6), which are involved in accumulation of polyubiquitinated proteins. A gene knockdown study revealed that SQSTM1 and HDAC6 were required for MyD88 aggregation and exhibited a suppressive effect on TLR ligand-induced expression of IL-6 and NOS2 in RAW264.7 cells. SQSTM1 and HDAC6 were partially involved in suppression of several TLR4-mediated signaling events, including activation of p38 and JNK, but they hardly affected degradation of IB␣ (inhibitor of nuclear factor B). Biochemical induction of MyD88 oligomerization induced recruitment of SQSTM1 and HDAC6 to the MyD88-TRAF6 signaling complex. Repression of SQSTM1 and HDAC6 enhanced formation of the MyD88-TRAF6 complex and conversely decreased interaction of the ubiquitin-specific negative regulator CYLD with the complex. Furthermore, ubiquitin-binding regions on SQSTM1 and HDAC6 were essential for MyD88 aggregation but were not required for interaction with the MyD88 complex. Thus, our study reveals not only that SQSTM1 and HDAC6 are important determinants of aggregated localization of MyD88 but also that MyD88 activates a machinery of polyubiquitinated protein accumulation that has a modulatory effect on MyD88-dependent signal transduction. MyD88 was originally identified as an inducible protein during terminal differentiation of M1 myeloleukemic cells upon interleukin (IL)-6 stimulation (1). The essential function ofMyD88 was later revealed to be a universal adaptor molecule for type 1 IL-1 receptor (IL-1R) 2 and Toll-like receptors (TLRs) (2-4). MyD88 is composed of three distinct regions, an N-terminal death domain, an intermediary domain, and a Toll/IL-1 homology domain at the C terminus (5). After receptor ligation, MyD88 interacts with the Toll/IL-1 homology domain of IL-1R/TLRs and then activates the signaling pathway through dimerization and utilizing death domain-containing IL-1R-associated kinases (2, 6, 7). This pathway is further activated through the ubiquitin E3 ligase TRAF6 (tumor necrosis factor (TNF) receptor-associated factor 6) that works together with a ubiquitin-conjugating enzyme complex consisting of UBC13 and UEV1A to catalyze Lys 63 -linked polyubiquitination, which then activates the TAK1 kinase. TAK1 activates IB kinases and cascades of mitogen-activated protein kinases (MAPKs), ultimately leading to early phase activation of nuclear factor (NF)-B and AP-1 and the transcription of genes encoding various proinflammatory mediators, such as TNF, NOS2 (nitricoxide synthase 2), and IL-6 (6, 7).In ...

HDAC6 Regulates Mutant SOD1 Aggregation through Two SMIR Motifs and Tubulin Acetylation

“…This is remarkably similar to the p62-mutant SOD1 interaction that was also independent of the ubiquitin binding domain of p62 (15). Moreover, the ubiquitinbinding domain of HDAC6 was similarly dispensable in the HDAC6-tau and HDAC6-MyD88 interactions (27,38).…”

“…HDAC6 was reported to interact with tau and MyD88 and to regulate their aggregation. The HDAC6-tau and HDAC6-MyD88 interactions were also independent of the enzymatic activity and the ubiquitin-binding domain of HDAC6 (27,38). We also examined the SE14 domain of HDAC6, which was reported to be required for the HDAC6-tau interaction (27).…”

Histone Deacetylase Inhibitors for Treating a Spectrum of Diseases Not Related to Cancer

“…The study showed that HDAC-6 and sequestosome 1 (SQSTM1) were required for MyD88 aggregation and lead to inhibition of TLR ligand-induced expression of IL-6 and NOS2 in RAW264.7 cells. HDAC-6 and SQSTM1 partially suppressed the activation of p38 and JNK, but they had no effect on degradation of IκB (54). According to these findings, inhibition of HDAC-6 would lead to enhanced MyD88 signal transduction activity.…”

MyD88 Interacts with Interferon Regulatory Factor (IRF) 3 and IRF7 in Atlantic Salmon (Salmo salar)

“…The aggresomes are proteinaceous inclusion bodies involved in the sequestration of polyubiquitinated proteins, a defense mechanism that protects the cells from potentially cytotoxic aggregates (58). It has recently been shown that the aggregation of MyD88 depends on its interaction with p62 and HDAC6, which are involved in accumulation of polyubiquitinated proteins and autophagy (59). More specifically, p62 was found to deliver polyubiquitinated proteins to autophagosomes because it can directly bind and link polyubiquitinated proteins to the autophagosome marker LC3 (60).…”