Increased A20-E3 ubiquitin ligase interactions in bid-deficient glia attenuate TLR3- and TLR4-induced inflammation

Chronic pro-inflammatory signaling propagates damage to neural tissue and affects the rate of disease progression. Increased activation of Toll-like receptors (TLRs), master regulators of the innate immune response, is implicated in the etiology of several neuropathologies including amyotrophic lateral sclerosis, Alzheimer's disease, and Parkinson's disease. Previously, we identified that the Bcl-2 family protein BH3-interacting domain death agonist (Bid) potentiates the TLR4-NF-κB pro-inflammatory response in glia, and specifically characterized an interaction between Bid and TNF receptor associated factor 6 (TRAF6) in microglia in response to TLR4 activation.

We assessed the activation of mitogen-activated protein kinase (MAPK) and interferon regulatory factor 3 (IRF3) inflammatory pathways in response to TLR3 and TLR4 agonists in wild-type (wt) and bid-deficient microglia and macrophages, using Western blot and qPCR, focusing on the response of the E3 ubiquitin ligases Pellino 1 (Peli1) and TRAF3 in the absence of microglial and astrocytic Bid. Additionally, by Western blot, we investigated the Bid-dependent turnover of Peli1 and TRAF3 in wt and bid^-/- microglia using the proteasome inhibitor Bortezomib. Interactions between the de-ubiquitinating Smad6-A20 and the E3 ubiquitin ligases, TRAF3 and TRAF6, were determined by FLAG pull-down in TRAF6-FLAG or Smad6-FLAG overexpressing wt and bid-deficient mixed glia.

We elucidated a positive role of Bid in both TIR-domain-containing adapter-inducing interferon-β (TRIF)- and myeloid differentiation primary response 88 (MyD88)-dependent pathways downstream of TLR4, concurrently implicating TLR3-induced inflammation. We identified that Peli1 mRNA levels were significantly reduced in PolyI:C- and lipopolysaccharide (LPS)-stimulated bid-deficient microglia, suggesting disturbed IRF3 activation. Differential regulation of TRAF3 and Peli1, both essential E3 ubiquitin ligases facilitating TRIF-dependent signaling, was observed between wt and bid ^-/- microglia and astrocytes. bid deficiency resulted in increased A20-E3 ubiquitin ligase protein interactions in glia, specifically A20-TRAF6 and A20-TRAF3, implicating enhanced de-ubiquitination as the mechanism of action by which E3 ligase activity is perturbed. Furthermore, Smad6-facilitated recruitment of the de-ubiquitinase A20 to E3-ligases occurred in a bid-dependent manner.

This study demonstrates that Bid promotes E3 ubiquitin ligase-mediated signaling downstream of TLR3 and TLR4 and provides further evidence for the potential of Bid inhibition as a therapeutic for the attenuation of the robust pro-inflammatory response culminating in TLR activation.