Abstract
BackgroundCerebral intraventricular hemorrhage (IVH) is a major cause of severe neurodevelopmental impairment in preterm infants. To date, no therapy is available that prevents infants from developing serious neurological disability following IVH. Thus, to develop treatment strategies for IVH, it is essential to characterize the initial sequence of molecular events that leads to brain damage. In this study, we investigated extracellular hemoglobin (Hb) as a causal initiator of inflammation in preterm IVH.MethodsUsing a preterm rabbit pup model, we investigated the molecular mechanisms and events following IVH. We also characterized the concentrations of cell-free Hb metabolites and pro-inflammatory mediators in the cerebrospinal fluid (CSF) of preterm human infants and rabbit pups. Finally, Hb metabolites were evaluated as causal initiators of inflammation in primary rabbit astrocyte cell cultures.ResultsFollowing IVH in preterm rabbit pups, the intraventricular CSF concentration of cell-free methemoglobin (metHb) increased from 24 to 72 hours and was strongly correlated with the concentration of TNFα at 72 hours (r2 = 0.896, P <0.001). Also, the mRNA expression of TNFα, IL-1β, and Toll-like receptor-4 and TNFα protein levels were significantly increased in periventricular tissue at 72 hours, which was accompanied by extensive astrocyte activation (that is, glial fibrillary acidic protein (GFAP)staining). Furthermore, exposure of primary rabbit astrocyte cell cultures to metHb caused a dose-dependent increase in TNFα mRNA and protein levels, which was not observed following exposure to oxyhemoglobin (oxyHb) or hemin. Finally, a positive correlation (r2 = 0.237, P <0.03) between metHb and TNFα concentrations was observed in the CSF of preterm human infants following IVH.ConclusionsFollowing preterm IVH, increased metHb formation in the intraventricular space induces expression of pro-inflammatory cytokines. Thus, the formation of metHb might be a crucial initial event in the development of brain damage following preterm IVH. Accordingly, removal, scavenging, or neutralization of Hb could present a therapeutic opportunity and plausible approach to decreasing the damage in the immature brain following preterm IVH.
Highlights
Cerebral intraventricular hemorrhage (IVH) is a major cause of severe neurodevelopmental impairment in preterm infants
We found that levels of extracellular oxyHb were relatively unchanged in intraventricular cerebrospinal fluid (CSF) during the first three days following hemorrhage, suggesting a constant rate of hemolysis and a proportionate degradation into Hb metabolites, including auto-oxidation to metHb
Our results show that extracellular metHb accumulates in the intraventricular space during the first 72 hours following preterm IVH and induces expression of the proinflammatory cytokine TNFα
Summary
Cerebral intraventricular hemorrhage (IVH) is a major cause of severe neurodevelopmental impairment in preterm infants. No therapy is available that prevents infants from developing serious neurological disability following IVH. To develop treatment strategies for IVH, it is essential to characterize the initial sequence of molecular events that leads to brain damage. No therapy is known to prevent infants from developing serious neurological disability following IVH. IVH in the immature brain is followed by microglial and astrocytic activation and increased expression of the proinflammatory cytokines TNFα and IL-1β in periventricular brain tissue [10]. Intervention aimed at blocking the effect of TNFα following IVH has been associated with decreased periventricular cell death [12]
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