Hyperoxia-induced immature brain injury through the TLR4 signaling pathway in newborn mice.

Toll-like receptor 4 (TLR4), a pathogen-associated molecular pattern receptor, is known to initiate an inflammatory cascade in response to certain stimuli within the central nervous system (CNS). Although TLR4 activation is known to be a first-line response of the innate immune system, whether and how hyperoxia influences TLR4 signaling in an immature brain remains unclear. In this study, TLR4 wild-type (W) and TLR4 knock-out(M) mice were exposed to 100% oxygen (the WO 2 and MO 2 groups, respectively), and control groups were exposed to ambient air (the WA and MA groups, respectively) for 48 h after postnatal-day (PND) 3. Next, neuronal apoptosis was quantified, and Morris water maze assays were conducted. The WO 2 mice showed increased TLR4 expression compared with the WA mice, additionally, the expression level of Tumor Necrosis Factor- α (TNF- α ) in the WO 2 mice was significantly increased compared with the levels in the WA, MA and MO 2 mice. Electron microscopy and terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) assays showed a significant increase, compared to the WO 2 mice, in neuronal apoptosis within the prefrontal cortex and hippocampal CA1 region in the WO 2 mice. In contrast, there were no obvious differences in neuronal apoptosis between the MO 2 and MA groups. The results of the Morris water maze tests demonstrated marked deficits in learning and memory in the WO 2 mice but much milder deficits in the MO 2 mice compared to the WA and MA groups, respectively. Moreover, cultured N9 (TLR4 wild-type, derived from ICR/CD1 mice) microglia exposed to hyperoxia showed an immediate increase in the expression of TLR4 mRNA, followed by an increase in the expression of both TNF- α and reactive oxygen species (ROS), but this increase was abrogated by the loss of TLR4 signaling in TLR4-knockout microglia (primary cells from a C3H/HeJ strain defective in TLR4). Taken together, these data suggest that 1) TLR4 signaling is involved in hyperoxia-induced immune responses in the immature brain and 2) the loss of TLR4 activation may abrogate the neuronal apoptosis and cognitive deficits following hyperoxia exposure in newborn mice. [ABSTRACT FROM AUTHOR]