Lipopolysaccharide Sensitizes Neonatal Hypoxic-Ischemic Brain Injury in a MyD88-Dependent Manner

Abstract

Neurological deficits in children, including cerebral palsy, are associated with prior infection during the perinatal period. Experimentally, we have shown that pre-exposure to the Gram-negative component LPS potentiates hypoxic-ischemic (HI) brain injury in newborn animals. LPS effects are mediated by binding to TLR4, which requires recruitment of the MyD88 adaptor protein or Toll/IL-1R domain-containing adapter inducing IFN-β for signal transduction. In this study, we investigated the role of MyD88 in neonatal brain injury. MyD88 knockout (MyD88 KO) and wild-type mice were subjected to left carotid artery ligation and 10% O2 for 50 min on postnatal day 9. LPS or saline were administered i.p. at 14 h before HI. At 5 days after HI in wild-type mice, LPS in combination with HI caused a significant increase in gray and white matter tissue loss compared with the saline-HI group. By contrast, in the MyD88 KO mice there was no potentiation of brain injury with LPS-HI. MyD88 KO mice exhibited reduced NFκB activation and proinflammatory cytokine-chemokine expression in response to LPS. The number of microglia and caspase-3 activation was increased in the brain of MyD88 KO mice after LPS exposure. Collectively, these findings indicate that MyD88 plays an essential role in LPS-sensitized HI neonatal brain injury, which involves both inflammatory and caspase-dependent pathways.

Inflammation‐induced sensitization of the brain in term infants

“…Sensitization to hypoxic-ischaemic damage following neonatal administration of LPS appears to last at least up to 3 days in rats 62 and 14 hours in mice. 59 In additional experiments, we showed that intrauterine exposure of pregnant mice to LPS resulted in increased cerebral vulnerability of pups to hypoxia-ischaemia up to 14 days postnatally. 16 However, it has become evident that within a certain time of LPS exposure the brain becomes less vulnerable or, so-called, preconditioned.…”

“…21,58 Administering an agent with anti-inflammatory properties, or disabling the TLR inflammatory pathway, reduced the severity of the LPS sensitization. 59,60 These studies demonstrate that exposure to inflammation prior to a moderate hypoxic-ischaemic insult is sufficient, through a sensitization process, to make the newborn brain extremely susceptible to damage.…”

“…LPS binds to TLR4, which triggers Myd88-dependent induction of NFjB. 21,59,78 In addition to inducing multiple cytokines (including some of those discussed above), NFjB evokes p53 production. p53 is a tumour suppressor that triggers apoptosis (by transactivating proapoptotic and repressing anti-apoptotic genes), causes cell cycle arrest, and inhibits or stimulates autophagy.…”

“…21,58 Administering an agent with anti-inflammatory properties, or disabling the TLR inflammatory pathway, reduced the severity of the LPS sensitization. 59,60 These studies demonstrate that exposure to inflammation prior to a moderate hypoxic-ischaemic insult is sufficient, through a sensitization process, to make the newborn brain extremely susceptible to damage.In the excitotoxic model, systemic injection of IL-1b, IL-6, TNFa, or IL-9 in newborn mice or rats, between postnatal days 1 and 5, makes the brain much more sensitive to brain insults induced by ibotenate on postnatal day 5, with up to twice the level of brain damage observed in these rodents compared with that observed in non-sensitized animals. 20,61 Similarly, delivery of LPS to pregnant rats on the last 2 days of gestation exacerbated ibotenate-induced brain damage in postnatal day 4 pups, with the highest dose of LPS doubling the lesion size.…”

Staphylococcus epidermidisBacteremia Induces Brain Injury in Neonatal Mice via Toll-like Receptor 2-Dependent and -Independent Pathways

“…Each hemisphere (approximately 50 mg) was sonicated in ice-cold homogenization buffer (50 mM Tris, pH 7.3, 2 mM ethylenediaminetetraacetic acid, 1% protease inhibitor cocktail [P8340; Sigma]). The homogenate was centrifuged at 10 000g for 10 minutes at 4°C, and the supernatant (S2; cytosolic fraction) was used in the determination of caspase-3 activity as described previously [26,27]. Staphylococcus epidermidis bacteremia induces systemic cytokine and brain CCL2 chemokine production 6 hours after injection.…”