Astrocytic Toll-Like Receptor 3 Is Associated with Ischemic Preconditioning- Induced Protection against Brain Ischemia in Rodents

Abstract

BackgroundCerebral ischemic preconditioning (IPC) protects brain against ischemic injury. Activation of Toll-like receptor 3 (TLR3) signaling can induce neuroprotective mediators, but whether astrocytic TLR3 signaling is involved in IPC-induced ischemic tolerance is not known.MethodsIPC was modeled in mice with three brief episodes of bilateral carotid occlusion. In vitro, IPC was modeled in astrocytes by 1-h oxygen-glucose deprivation (OGD). Injury and components of the TLR3 signaling pathway were measured after a subsequent protracted ischemic event. A neutralizing antibody against TLR3 was used to evaluate the role of TLR3 signaling in ischemic tolerance.ResultsIPC in vivo reduced brain damage from permanent middle cerebral artery occlusion in mice and increased expression of TLR3 in cortical astrocytes. IPC also reduced damage in isolated astrocytes after 12-h OGD. In astrocytes, IPC or 12-h OGD alone increased TLR3 expression, and 12-h OGD alone increased expression of phosphorylated NFκB (pNFκB). However, IPC or 12-h OGD alone did not alter the expression of Toll/interleukin receptor domain-containing adaptor-inducing IFNβ (TRIF) or phosphorylated interferon regulatory factor 3 (pIRF3). Exposure to IPC before OGD increased TRIF and pIRF3 expression but decreased pNFκB expression. Analysis of cytokines showed that 12-h OGD alone increased IFNβ and IL-6 secretion; 12-h OGD preceded by IPC further increased IFNβ secretion but decreased IL-6 secretion. Preconditioning with TLR3 ligand Poly I:C increased pIRF3 expression and protected astrocytes against ischemic injury; however, cells treated with a neutralizing antibody against TLR3 lacked the IPC- and Poly I:C-induced ischemic protection and augmentation of IFNβ.ConclusionsThe results suggest that IPC-induced ischemic tolerance is mediated by astrocytic TLR3 signaling. This reprogramming of TLR3 signaling by IPC in astrocytes may play an important role in suppression of the post-ischemic inflammatory response and thereby protect against ischemic damage. The mechanism may be via activation of the TLR3/TRIF/IRF3 signaling pathway.

Neuroimmune Response in Ischemic Preconditioning

“…The large overlap between molecular pathways activated in response to LPS and ischemia is, in part, responsible for the phenomenon of crosstolerance (discussed previously), suggesting that IPC may modulate microglia/astrocyte interplay, with both cells playing important roles in conferring IPC-mediated neuroprotection. Astrocytes express TLR4 [108], a critical component in IPC-mediated neuroprotection [11,12]. Activation of astrocytic TLR4 results in activation of nuclear factor kB and signal transducer and activator of transcription 3 in astrocytes [109], and downstream of this pathway is growth-associated protein (GAP)43, which is a protein kinase C-activated phosphoprotein implicated in axonal plasticity and regeneration [109].…”

The Toll-like receptors mRNA expression profile in the pineal gland of sheep during long and short days

“…Notwithstanding, the role of TLR3 in the pineal gland may be more complex. Recent studies carried out on mice (Pan et al, 2014) showed that astrocytic TLR3 signaling plays an important role in suppression of the post-ischaemic inflammatory response and thereby protects against ischaemic damage. Therefore, the expression of TLR3 in the pineal gland may be involved in the protection of this tissue against strong inflammatory stimulation.…”

Hind limb perconditioning renoprotection by modulation of inflammatory cytokines after renal ischemia/reperfusion

“…25 Recently, in an animal study of Pan et al ischemic preconditioning reduced brain injury and the systemic inflammatory response, reported by decreased IL-6 levels. 26 Altogether, it could be concluded that reduced IL-6 levels may be an important consequence of ischemic conditioning in the ischemic kidney. Several molecular pathways are implicated in remote ischemic condition models which predominantly implicates systemic multifactorial anti-inflammatory, neuronal, and humoral signaling pathways.…”