Early Upregulation of NLRP3 in the Brain of Neonatal Mice Exposed to Hypoxia-Ischemia: No Early Neuroprotective Effects of NLRP3 Deficiency

Background: The NLRP3 inflammasome acts as an early mediator of inflammation by cleaving and releasing IL-1β and IL-18 from their proforms. Objective: The aim of this study was to describe NLRP3 activation and evaluate whether deficiency of NLRP3 protects against neonatal hypoxic ischemic brain damage. Methods: C57BL/6 and NLRP3-/- mice at P9 were subjected to unilateral common carotid ligation followed by hypoxia. RT-PCR was used on mRNA in five different subregions of the brain. Brain infarction was evaluated by histopathology and 2,3,5-triphenyltetrazolium chloride staining. Plasma levels of IL-18 were measured by ELISA. Double labeling immunohistochemistry was used to examine cell-specific NLRP3 expression. Results: NLRP3 was upregulated 24 h after hypoxia-ischemia (HI) in the hippocampus (2.6-fold), striatum (2.2-fold) and thalamus (2.3-fold). Brain infarction volumes were not statistically significantly different in NLRP3-/- mice compared to WT mice 24 h after HI, accompanied by no significant changes in plasma IL-18 levels. Three hours after HI, NLRP3 expression occurred in astrocytes located in the hippocampus and habenular nucleus of the thalamus. Microglia only showed scarce expression at this time point, but prominent NLRP3 expression 72 h after HI. Conclusion: Astrocytes are early mediators of NLRP3 activity. No early neuroprotective effect of NLRP3 deficiency in neonatal HI brain damage was shown.