Early Life Exposure to Intermittent Hypoxia Induces Long‐Lasting Changes in Microglial Inflammatory Responses

Chronic intermittent hypoxia (IH), a hallmark of sleep‐disordered breathing, is prevalent in pediatric and pre‐term infant populations. IH increases reactive oxygen species formation, causes neuronal apoptosis, promotes peripheral and CNS inflammation, and results in long‐term neurocognitive deficits. However, little is known about cellular mechanisms underlying long‐term alterations in CNS function following early life IH exposure, nor if the activities of adult microglia, CNS resident innate immune cells, are also affected by early‐life IH. Previously, we observed that early life IH increased the microglial inflammatory response to a peripheral immune challenge the day following the last IH exposure, and that this occurred without changes in basal levels of microglial inflammation, suggesting that IH primes microglial responses acutely. It is unknown if these effects persist into adulthood, or if they are sex‐specific. To test the hypothesis that early life IH exposure will exert long‐lasting, sex‐specific effects on microglial inflammatory gene responses to an immune challenge in adulthood, we exposed male and female rat pups to 8 nights of IH (2 min cycles of 10.5% O2 for 8hrs/day) beginning on the first postnatal day (P1). We assessed microglial responses to a low‐dose peripheral lipopolysaccharide challenge (LPS; 0.1 mg/kg for 3hrs) either the day after the last IH exposure, or 6 weeks after. Using RNA sequencing (RNA‐Seq) and qRT‐PCR, we found in microglia from 9 day‐old neonates that IH alone downregulated multiple genes involved in metabolism, RNA processing and chromosome regulation, without changing basal expression of inflammatory genes such as IL‐1b and iNOS. Interestingly, microglial responses to acute LPS challenge in neonates and adults were different; microglial responses to an LPS challenge at 6 weeks of age were impaired in males. To begin to dissect potential chromatin changes (i.e. histone modifications) that may contribute to hypoxia‐induced alterations in microglial immune responses, we performed chromatin immunoprecipitation‐sequencing (ChIP‐Seq) in immortalized male microglia to evaluate genome‐wide histone 3 lysine 4 trimethylation (H3K4me3) changes, a mark of active gene transcription. We found increases in this mark at multiple genes involved in hypoxic and inflammatory responses, and decreases at genes downregulated by IH in vivo. Ongoing studies are aimed at resolving mechanisms whereby early life exposure to hypoxic challenges can exert epigenetic alterations that modulate the adult microglial inflammatory response. Understanding these mechanisms may enable us to identify and harness cellular mechanisms that can restore proper microglial immune responses in adult CNS disorders, many of which display sexual dimorphisms. Support or Funding Information This work was supported by NIH R01 NS085226