Knockout of Toll-like receptor 2 attenuates Aβ25–35-induced neurotoxicity in organotypic hippocampal slice cultures

Abstract

Toll-like receptors (TLRs), which have been implicated in various neuroinflammatory responses, are thought to act in defense mechanisms by inhibiting neuronal cell death in Alzheimer's disease. In this study, we evaluated the effects of TLR2 on amyloid beta peptide 25-35 (Aβ25-35)-induced neuronal cell death, synaptic dysfunction, and microglial activation in organotypic hippocampal slice cultures (OHSCs) from wild-type (WT) C57BL/6 mice and TLR2-knockout (KO) mice. In WT mice, Aβ25-35 induced β-amyloid aggregation and surrounding TLR2 expression. And, propidium iodide (PI) uptake, which is a measure of cell death, increased in a dose-dependent manner in slices with Aβ25-35 treatment. In the Aβ25-35-treated TLR2-KO OHSCs, the PI uptake was significantly attenuated to the control level, indicating that the cells were less susceptible to Aβ25-35-induced neuronal toxicity. In the ultrastructural analysis, nuclear shrinkage, slightly swollen mitochondria, and degraded organelles were detected in the Aβ25-35-treated slices from WT mice but not in the Aβ25-35-treated slices from TLR2-KO, suggesting the resistance of TLR2-KO to Aβ25-35-induced neurotoxicity. In Aβ25-35-treated OHSCs of WT mice, the levels of phosphorylated tau were increased and the levels of synaptophysin were decreased in a dose-dependent manner, but they were not changed in OHSCs of TLR2-KO mice. In WT mice, Aβ25-35 increased total protein level and immunoreactivity of Iba-1, which was colocalized with TLR2. However, there were no significant changes in the slices of Aβ25-35-treated TLR2-KO mice. These results suggested that TLR2 may play a role in Aβ25-35-induced neuronal cell loss and synaptic dysfunction through the activation of microglia in OHSCs.

Hyperactivation of monocytes and macrophages in MCI patients contributes to the progression of Alzheimer's disease

“…This could be a protective mechanism (e.g., shedding or internalization) for decreasing pro-inflammatory cytokine production during overstimulation [ 61 ]. Similar results were obtained for TLR2 expression, except at the basal state of SMC subjects [ 165 , 166 ]. This is in line with data which found that the genomic deletion of TLR2 markedly deteriorated the neurobehavioral functions in a mouse model of AD [ 167 ].…”

Phagocytic Roles of Glial Cells in Healthy and Diseased Brains

“…Therefore, synapse elimination is critical in the shaping of neural circuits during development and the regulation of synaptic plasticity in response to experience and memory. Certain neurodegenerative diseases, such as Alzheimer’s disease (AD), are associated with profound synapse loss early in the disease state ( Jana et al ., 2008 ; Meyer-Luehmann et al ., 2008 ; Pihlaja et al ., 2008 ; Suh et al ., 2013 ; Sollvander et al ., 2016 ), which underscores the importance of understanding the molecular mechanisms of synapse loss.…”

“…Microglia constitutively express TLR2 ( Olson and Miller, 2004 ; Hanke and Kielian, 2011 ), and TLRs play a role in Aβ-induced microglial activation ( Chen et al ., 2006 ; Tahara et al ., 2006 ; Tang et al ., 2007 ; Liu et al ., 2012 ). Aβ-triggered inflammatory activation is reduced in TLR2-deficient microglia ( Jana et al ., 2008 ; Suh et al ., 2013 ), and TLR2 deficiency reduces Aβ-triggered inflammatory activation in cultured microglia, which suggests a beneficial effect of TLR2 inhibition in AD pathogenesis ( Liu et al ., 2012 ).…”

Genomic deletion of TLR2 induces aggravated white matter damage and deteriorated neurobehavioral functions in mouse models of Alzheimer’s disease

“…Recent studies demonstrated that activated bone-marrow-derived microglia (BMDM) could uptake Aβ and help to clear Aβ deposition [23], while TLR2 deficit BMDM could not perform its Aβ clearance function; moreover, TLR2 deficiency aggregated cognitive dysfunction in APP/PS1 transgenic mice [24, 25]. In contrast, other studies have reported that long-term administration of the TLR2 inhibitor in AD mice could reduce Aβ aggregation and glial activation [26], and that TLR2 gene knockout and the blocking of the interaction between TLR2 and MyD88 could attenuate the neurotoxicity and pathological changes of AD [27, 28]. While these contradictory phenomena could be interpreted as the consequence of different experimental conditions, such as differences in animal models, observed time points, and cell types, it is nonetheless apparent that TLR2 does play a role in the process of AD, although the exact effect remains to be elucidated.…”