Toll‐like receptor 4 is involved in neuroprotection afforded by ischemic preconditioning

Abstract

1These authors contributed equally to this study.Abbreviations used: BCCAO, bilateral common carotid arteries occlusion; COX-2, cyclooxygenase-2; EAAT, excitatory amino acid transporter; IjB alpha, inhibitory kappa B alpha; iNOS, inducible nitric oxide synthase; IPC, ischemic preconditioning; IT, ischemic tolerance; LPS, lipopolysaccharide; MCA, middle cerebral artery; NF-jB, nuclear factor kappa B; pMCAO, permanent middle cerebral artery occlusion; TACE, TNF-alpha converting enzyme; TLR, toll-like receptor; TNF-a, tumor necrosis factor alpha. AbstractIt has been demonstrated that a short ischemic event (ischemic preconditioning, IPC) results in a subsequent resistance to severe ischemia (ischemic tolerance, IT). We have recently demonstrated the role of innate immunity and in particular of toll-like receptor (TLR) 4 in brain ischemia. Several evidences suggest that TLR4 might also be involved in IT. Therefore, we have now used an in vivo model of IPC to investigate whether TLR4 is involved in IT. A 6-min temporary bilateral common carotid arteries occlusion was used for focal IPC and it was performed on TLR4-deficient mice (C57BL/ 10ScNJ) and animals that express TLR4 normally (C57BL/ 10ScSn). To assess the ability of IPC to induce IT, permanent middle cerebral artery occlusion was performed 48 h after IPC. Stroke outcome was evaluated by determination of infarct volume and assessment of neurological scores. IPC caused neuroprotection as shown by a reduction in infarct volume and better outcome in mice expressing TLR4 normally. TLR4-deficient mice showed less IPC-induced neuroprotection than wild-type animals. Western blot analysis of tumor necrosis factor alpha (TNF-a), inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) showed an up-regulation in the expression of these proteins in both substrains of mice measured 18, 24 and 48 h after IPC, being higher in mice with TLR4. Similarly, nuclear factor-kappa B (NF-jB) activation was observed 18, 24 and 48 h after IPC, being more intense in TLR4-expressing mice. These data demonstrate that TLR4 signalling is involved in brain tolerance as shown by the difference in the percentage of neuroprotection produced by IPC between ScSn and ScNJ (60% vs. 18%). The higher expression of TNF-a, iNOS and cyclooxygenase-2 and NF-jB activation in mice expressing TLR4 is likely to participate in this endogenous neuroprotective effect.

Developmental Onset of Bilirubin-induced Neurotoxicity Involves Toll-like Receptor 2-dependent Signaling in Humanized UDP-glucuronosyltransferase1 Mice

“…The outcome of TLR activation may depend on the dose of the ligand, the etiology of CNS diseases, and the timing of activation (13). A recent study of a stroke event has shown that stimulating TLRs with a low dose of ligand prior to stroke leads to a decrease in infarct size and an improved neurological outcome in response to ischemic injury (40), supporting the role of neuroprotection of TLR signaling. The similarities of our results in a model of hyperbilirubinemia brain injury support a common theme of repair linked to TLR2 signaling.…”

Microglial Activation in Stroke: Therapeutic Targets

“…82 Toll-like receptors have also been implicated in the phenomenon of tolerance, whereby stimulation of one or more of these receptors with ligands such as lipopolysaccharide (TLR4) or CpG (TLR9) led to protection from subsequent lethal insults. [83][84][85][86] Mice lacking these receptors failed to achieve tolerance, 83 but the role for microglia in this process remains to be established.…”

Evaluating the role of Toll-like receptors in diseases of the central nervous system

“…In addition it was shown that Pam3CSK4 administration 24 hrs before cerebral ischaemia reduced infarct size [63] and pretreatment of TLR9 agonists CpG before stroke also conferred neuroprotection, at least partly due to the release of TNF alpha [67]. TLRs are also involved in ischaemic preconditioning where ischaemia induced of a short duration provides resistance to subsequent challenge thus conferring ischaemic tolerance [68]. This protective effect was shown to be dependent upon TLR4 to upregulate TNFα, inducible NO synthase, cyclooxygenase 2 and NFκB [68].…”

“…TLRs are also involved in ischaemic preconditioning where ischaemia induced of a short duration provides resistance to subsequent challenge thus conferring ischaemic tolerance [68]. This protective effect was shown to be dependent upon TLR4 to upregulate TNFα, inducible NO synthase, cyclooxygenase 2 and NFκB [68]. M a n u s c r i p t 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 …”