Prothymosin‐alpha preconditioning activates TLR4–TRIF signaling to induce protection of ischemic retina

Abstract

Prothymosin-alpha protects the brain and retina from ischemic damage. Although prothymosin-alpha contributes to toll-like receptor (TLR4)-mediated immnunopotentiation against viral infection, the beneficial effects of prothymosin-alpha-TLR4 signaling in protecting against ischemia remain to be elucidated. In this study, intravitreal administration of prothymosinalpha 48 h before induction of retinal ischemia prevented retinal cellular damage as evaluated by histology, and retinal functional deficits as evaluated by electroretinography. Prothymosin-alpha preconditioning completely prevented the ischemia-induced loss of ganglion cells with partial survival of bipolar and photoreceptor cells, but not amacrine cells, in immunohistochemistry experiments. Prothymosin-alpha treatment in the absence of ischemia caused mild activation, proliferation, and migration of retinal microglia, whereas the ischemia-induced microglial activation was inhibited by prothymosin-alpha preconditioning. All these preventive effects of prothymosin-alpha preconditioning were abolished in TLR4 knock-out mice and by pre-treatments with anti-TLR4 antibodies or minocycline, a microglial inhibitor. Prothymosinalpha preconditioning inhibited the retinal ischemia-induced up-regulation of TLR4-related injury genes, and increased expression of TLR4-related protective genes. Furthermore, the prothymosin-alpha preconditioning-induced prevention of retinal ischemic damage was abolished in TIR-domain-containing adapter-inducing interferon-b knock-out mice, but not in myeloid differentiation primary response gene 88 knock-out mice. Taken together, the results of this study suggest that prothymosin-alpha preconditioning selectively drives TLR4-TIR-domain-containing adapter-inducing interferon-b signaling and microglia in the prevention of retinal ischemic damage.

Neuroprotective impact of prothymosin alpha-derived hexapeptide against retinal ischemia–reperfusion

“…Fluorescence immunostaining revealed that Iba-1-positive microglia were distributed in the retinal GCL and IPL of control mice (Fig. 6A), which is consistent with previous reports (Lin et al, 2012;Halder et al, 2013c;Halder et al, 2015). Retinal ischemia caused microglial activation in terms of morphological change, migration, increased population and Iba-1 intensity throughout different retinal layers seven days after the ischemic stress in vehicle-treated mice, compared with the retina of untreated mice (Fig.…”

“…Immunohistochemical data revealed that the distribution of GFAP-positive astrocytes was mainly in the retinal GCL of untreated mice (Fig. 5D), consistent with previous studies (Lin et al, 2012;Halder et al, 2013c;Halder et al, 2015). Following retinal ischemia, GFAP-positive astrocytes were activated in terms of morphological change, increased number and GFAP intensity throughout the GCL to ONL seven days after the ischemic stress ( Fig.…”

Prothymosin Alpha and Immune Responses

“…As for the mechanism underlying proTα's neuroprotective role, it has been shown that the polypeptide is released via a nonclassical manner under ischemic stress conditions and its extracellular release is facilitated by its interaction with a cargo molecule, namely S100A13 (Halder et al, 2012;Matsunaga & Ueda, 2010). Upon release, proTα activates the TLR4-TRIF-signaling pathway, inducing the expression of neuroprotective factors (Halder, Matsunaga, Ishii, & Ueda, 2015).…”

“…In 2010, Mosoian and colleagues confirmed that proTα ligates TLR4 and signals through both the TRIF-dependent and the MyD88 pathways for induction of IFN-β and proinflammatory cytokines, respectively (Mosoian et al, 2010). Most recently, proTα was shown to bind to the TLR4/MD-2 complex and activate the TRIF-IRF3-signaling pathway downstream TLR4 (Halder et al, 2015;Omotuyi et al, 2015).…”

Wogonin prevents TLR4-NF-κB-medicated neuro-inflammation and improves retinal ganglion cells survival in retina after optic nerve crush

“…A leading theory is that the chronic neuro-inflammation generated by the activation of neuroglial cells (microglia, astrocytes, and Müller cells) which is involved in a number of complex signaling pathways [10, 11] in the retina after glaucoma [7, 8]. Recent studies have shown that activated neuroglial cells dramatically induced the activation of toll-like receptor 4-nuclear factor-kappa B (TLR4-NF-κB) pathways, which may play a vital role in promoting the release of pro-inflammatory cytokines, causing RGCs death in retina after optic nerve (ON) injury [11, 12]. There was evidence to show a reduced inflammatory responses and improved RGCs preservation following optic nerve crush (ONC) in TLR4-deficient mice, which indicated that the presence of TLR4-dependent pathway induced the RGCs death [13].…”