The Hypothalamus and Brainstem of Sudden Infant Death Syndrome (SIDS); the Orexinergic System & Proteomic Analysis

Abstract

Introduction: SIDS is the leading cause of post-neonatal death in Australia. SIDS is defined as the sudden death of an infant less than 1 year of age that cannot be explained after a thorough investigation is conducted, including a complete autopsy, examination of the death scene, and a review of the clinical history. Orexin (Ox) is produced in the hypothalamus and plays a role in maintaining wakeful/sleeping states, arousal and respiration. This thesis examined if the orexinergic system is impaired in SIDS and the possible causes for impairment. The studies performed aimed to examine: (i) the developmental change in Ox expression, (ii) changes in Ox expression in SIDS, (iii) effects of SIDS risk factors: prone sleeping and cigarette exposure on Ox expression in SIDS piglet models (intermittent hypercapnic hypoxia (IHH) and postnatal nicotine), (iv) mechanisms of decreased Ox expression in SIDS and (v) examine abnormal protein expression within the medulla of SIDS infants using imaging proteomics. Methods: Tissue from the hypothalamus, pons and medulla were collected from the Department of Forensic Medicine, Australia, for non-SIDS (n=19) and SIDS (n=27) infants, and children (n=7). Young (n=4) and older (n=7) adult tissue was collected from the NSW Brain Bank, University of Sydney. Piglet (n=32) hypothalamus and pons tissue were from our laboratory repository. Experimental methods used include immunohistochemistry (all studies), immunofluorescence (aims: (i), (ii), (iv)) and imaging mass spectrometry (aim: (v)). Results: A 23% decrease in Ox expression between infants and older adults was observed indicating that maturation contributes to a gradual decline in Ox neurons. SIDS infants were found to have a 21% decrease in Ox expression in the hypothalamus and a 40-50% decrease in Ox fibre staining in the pons. IHH piglets had 25% and 40% decreases in Ox expression in the hypothalamus and pons, being comparable to that seen in SIDS, nicotine exposure increased expression. The decreased Ox expression in SIDS was associated with an accumulation of unfolded protein response marker phosphorylated pancreatic endoplasmic reticulum kinase (pPERK) and activation transcription factor 4. The proteomics data showed abnormal expression patterns for 9 proteins localised to the raphe nuclei, hypoglossal and pyramids, suggesting abnormal neurological development in these regions. Conclusion: SIDS infants have decreased Ox expression during a critical sleep-wake cycle development stage; this likely is due to an upregulation of pPERK with IHH exposure potentially contributing to this pathway.