Abstract
BackgroundSepsis-associated encephalopathy (SAE) is an early and frequent event of infection-induced systemic inflammatory response syndrome. Phosphoinositide 3-kinase γ (PI3Kγ) is linked to neuroinflammation and inflammation-related microglial activity. In homeotherms, variations in ambient temperature (Ta) outside the thermoneutral zone lead to thermoregulatory responses, mainly driven by a gradually increasing sympathetic activity, and may affect disease severity. We hypothesized that thermoregulatory response to hypothermia (reduced Ta) aggravates SAE in PI3Kγ-dependent manner.MethodsExperiments were performed in wild-type, PI3Kγ knockout, and PI3Kγ kinase-dead mice, which were kept at neutral (30 ± 0.5 °C) or moderately lowered (26 ± 0.5 °C) Ta. Mice were exposed to lipopolysaccharide (LPS, 10 μg/g, from Escherichia coli serotype 055:B5, single intraperitoneal injection)—evoked systemic inflammatory response (SIR) and monitored 24 h for thermoregulatory response and blood–brain barrier integrity. Primary microglial cells and brain tissue derived from treated mice were analyzed for inflammatory responses and related cell functions. Comparisons between groups were made with one-way or two-way analysis of variance, as appropriate. Post hoc comparisons were made with the Holm–Sidak test or t tests with Bonferroni’s correction for adjustments of multiple comparisons. Data not following normal distribution was tested with Kruskal-Wallis test followed by Dunn’s multiple comparisons test.ResultsWe show that a moderate reduction of ambient temperature triggers enhanced hypothermia of mice undergoing LPS-induced systemic inflammation by aggravated SAE. PI3Kγ deficiency enhances blood–brain barrier injury and upregulation of matrix metalloproteinases (MMPs) as well as an impaired microglial phagocytic activity.ConclusionsThermoregulatory adaptation in response to ambient temperatures below the thermoneutral range exacerbates LPS-induced blood–brain barrier injury and neuroinflammation. PI3Kγ serves a protective role in suppressing release of MMPs, maintaining microglial motility and reinforcing phagocytosis leading to improved brain tissue integrity. Thus, preclinical research targeting severe brain inflammation responses is seriously biased when basic physiological prerequisites of mammal species such as preferred ambient temperature are ignored.
Highlights
Sepsis-associated encephalopathy (SAE) is an early and frequent event of infection-induced systemic inflammatory response syndrome
Phosphoinositide 3-kinase γ (PI3Kγ) serves a protective role in suppressing release of matrix metalloproteinases (MMPs), maintaining microglial motility and reinforcing phagocytosis leading to improved brain tissue integrity
Whereas the early period of LPS-induced systemic inflammatory response syndrome (SIRS) was characterized by a rather similar heart rate (HR) elevation, HR remained elevated in the animals kept under neutral Ta, but not in the animals kept under reduced Ta, throughout the observation period
Summary
Sepsis-associated encephalopathy (SAE) is an early and frequent event of infection-induced systemic inflammatory response syndrome. Activation of cerebral microvascular endothelial cells as the primary constituent of the BBB is regarded as an early event, induced by interaction with a pathogen product such as lipopolysaccharide (LPS) via pattern recognition receptors and proinflammatory factors. The latter include activated complement components and cytokines and lead to an increased endothelial production of reactive oxygen and nitrogen species, accelerated transendothelial cytokine trafficking, and enhanced endothelial permeability [4]. Microglial proinflammatory response further reinforces BBB breakdown and modifies it via PI3Kγ-depended cAMP control [5]
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