Inflammatory stressors in serotonergic brainstem dysfunction and SIDS

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Project Summary Sudden infant death syndrome (SIDS) remains the leading cause of post-neonatal mortality in the U.S.– an unchanging and devastating fact despite implementation of safe sleep practices (extrinsic risk reduction). Addressing this 21st century health crisis now requires discovery of intrinsic biological vulnerabilities and plausible molecular pathways that might lead to biomarkers and preventative interventions. In multiple independent SIDS tissue datasets, serotonergic (5-HTergic) abnormalities in the brainstem were consistently identified; in animal models of reduced brainstem 5-HTergic activity, compromised autoresuscitation (AR) was observed – the ability of mouse pups to recover from cycles of asphyxial apneas and bradycardia (resembling the cycles of apnea and bradycardia observed in some SIDS cases) was significantly diminished. Such 5- HTergic system dysfunction, as an intrinsic vulnerability, may be caused or exacerbated by extrinsic stressors such as pre- and/or postnatal hypoxia (e.g., placental insufficiency, parental smoking) and/or antemortem infections. Hypoxia and infection are each risk factors for SIDS and can increase neuroinflammation, which can impair AR. Notable new findings in some SIDS cases as compared to controls are elevations of the neuroinflammatory markers IL-1β, IL-2, IL-4, IL-17, and GM-CSF and/or in neopterin (a marker of Th1 (proinflammatory) cellular activation) in the cerebrospinal fluid. We postulate that neuroinflammation, triggered by hypoxia and/or antemortem infections (bacterial or viral), interact to create a vulnerable 5- HTergic system, reduce AR effectiveness, and increase the risk for sudden death, and may underlie some SIDS cases. We propose: 1) To quantitate inflammatory mediators within SIDS brains and determine whether a profile of mediators associates with 5-HTergic brainstem abnormalities. We will test the hypothesis that specific inflammatory profiles associate with low 5-HT1A and 5-HT2A receptor binding and low 5-HT levels. 2) To map at single-cell resolution, differences in gene expression profiles and overall cell-type composition/states of brainstem tissue across SIDS cases (the SIDS subsets identified through Aim 1) and controls. We hypothesize that SIDS subsets will be distinguished by specific inflammatory profiles in glia, neurons, and/or endothelial cells, and gene expression differences will identify novel, previously unrecognized SIDS-related pathways for mechanistic testing in cell and animal models. 3) Assess the interaction between chronic intermittent hypoxia (gestational to P8) and postnatal antemortem infection on molecular, cellular, inflammatory, and physiological readouts, including the autoresuscitation response (AR). We will test the hypothesis that the combined effects of antemortem hypoxia and infection interact to create greater neuroinflammation, more severe 5-HTergic deficits, and increased likelihood of AR failure, compared to either hypoxia or infection alone. SIDS research must address the missing mechanistic links between risk factors and postmortem pathology to develop life- saving interventions.