Time-Course Alterations of Toll-Like Receptor 4 and NF-κB p65, and Their Co-Expression in the Gerbil Hippocampal CA1 Region After Transient Cerebral Ischemia

Abstract

Innate immune system is very important to modulate the host defense against a large variety of pathogens. Toll-like receptors (TLRs) play a key role in controlling innate immune response. Among TLRs, TLR4 is a specific receptor for lipopolysaccharide and associated with the release of pro-inflammatory cytokines. In the present study, we investigated ischemia-related changes of TLR4 immunoreactivity and its protein level, and nuclear factor κB (NF-κB) p65 immunoreactivity regarding inflammatory responses in the hippocampal CA1 region after 5 min of transient cerebral ischemia to identify the correlation between transient ischemia and inflammation. In the sham-operated group, TLR4 immunoreactivity was easily detected in pyramidal neurons of the hippocampal CA1 region (CA1). TLR4 immunoreactivity in pyramidal neurons was distinctively decreased after ischemia/reperfusion (I/R); instead, based on double immunofluorescence study, TLR4 immunoreactivity was expressed in non-pyramidal neurons and astrocytes from 2 days postischemia. In addition, TLR4 protein level was lowest at 1 day postischemia and highest 4 days after I/R. On the other hand, NF-κB p65 immunoreactivity was not detected in the CA1 of the sham-operated group, and NF-κB p65 immunoreactivity was not observed until 1 day after I/R. However, NF-κB p65 immunoreactivity began to be expressed in astrocytes at 2 days postischemia, and the immunoreactivity was strong 4 days postischemia. Our results indicate that TLR4 and NF-κB p65 immunoreactivity are changed in CA1 pyramidal neurons and newly expressed in astrocytes, not in microglia, in the CA1 region after transient cerebral ischemia.

Adolescent Morphine Exposure Affects Long-Term Microglial Function and Later-Life Relapse Liability in a Model of Addiction

“…Others report that TLR4 is also present on astrocytes; however, we found virtually no protein expression of TLR4 on other cell types rapidly isolated from these two brain regions. The reports indicating TLR4 expression on astrocytes and sometimes neurons use either models of disease or stroke (Zurolo et al, 2011; Yoo et al, 2011; Casula et al, 2011; Saito et al, 2010), collect tissue from the spinal cord (Saito et al, 2010), or measure expression in cultured astrocytes (Santos-Galindo et al, 2011; Gorina et al, 2011) suggesting that TLR4 expression may be specific to microglia except perhaps in pathological or artificial conditions. In our model we hypothesize that adolescent morphine pre-exposure increases TLR4 expression on microglia within the NAcc, which results in exaggerated activation of the TLR4 signaling pathway (Figure 9) and exaggerated activation of microglia (Figure 9) following morphine treatment in adulthood.…”

Neuronal toll-like receptor 4 signaling induces brain endothelial activation and neutrophil transmigration in vitro

“…Indeed, constitutive expression of TLR4 has been detected in hippocampal neurons [21], sensory neurons [13], neural stem cells [12] and photoreceptor cells [14]. These observations taken together with our data suggest that neurons can, therefore, sense bacterial infection (and possibly endogenous DAMPs), and we demonstrated here that neurons synthesize chemokines (that is, RANTES and KC) as well as the cytokines TNFα and IL-6 (albeit at much lower levels) in response to LPS.…”

Neuroimmune signaling: a key component of alcohol abuse

“…Microglial activation of TLR4 is required for astrocyte pro-inflammatory responses [7*]. Neurons have also been shown to express TLR4 [4,8–10] and propagate LPS-induced signaling [11], indicating an unexpected role for neurons in innate immunity and eluding to significant cross-communication among microglia, astrocytes, and neurons that likely characterizes innate immune signaling in the CNS. Brain endothelial cells also express TLR4 and are able to receive neuroimmune stimulation from the brain side and secrete cytokines into the blood or receive stimulation from the blood and secrete cytokines into the brain, suggesting that the blood brain barrier (BBB) may be a fourth component involved in the cross-talk between neurons, microglia, and astrocytes [12].…”