Novel Role for the Innate Immune Receptor Toll-Like Receptor 4 (TLR4) in the Regulation of the Wnt Signaling Pathway and Photoreceptor Apoptosis
Figure 8
Preconditioning by LPS does not require canonical Wnt signaling.
(A) Wnt signaling was measured in preconditioned Muller glia-photoreceptor cultures. The cultures were treated with LPS with or without Wnt3a, 2 hr prior to injury by H2O2 (0.4 mM). Wnt signaling was measured using a luciferase reporter assay. Wnt3a increased Wnt signaling (p<0.05, n = 5, compared with H2O2 only) but preconditioning with LPS did not induce Wnt signaling or change the level of Wnt3a-dependent Wnt signaling. Therefore, LPS-induced preconditioning is independent of the canonical Wnt pathway. (B) The Muller glia-photoreceptor cultures were incubated with LPS with or without the Wnt pathway inhibitor Dkk1 (200 ng/ml), followed by exposure to H2O2. The protection by LPS was not altered by Dkk1, indicating that endogenous Wnt pathway activation is not required. Mean ± SD, n = 5, compared with PBS treated. Viability was measured by Cell Titer Blue. (C) Proposed model of regulation of photoreceptor neuroprotection by cross-talk between the TLR4 and Wnt signaling pathways. (Left) During oxidative stress injury, TLR4 signaling suppresses Wnt signaling at the level of LRP6 receptor activation, leading to reduced Wnt3a-mediated photoreceptor protection. (Right) In the presence of LPS preconditioning prior to oxidative stress, activation of TLR4 reduces TNFα, leading to increased neuroprotection. TLR4 does not regulate the Wnt signaling pathway during preconditioning.