The Role of Myeloid miR-155 in Neonatal Hypoxia Induced Seizures

Hypoxic Ischemic Encephalopathy (HIE) is the 5th most common cause of death in children under the age of 5 worldwide. Therapeutics used to manage HIE are extremely limited. Most work by minimising local inflammation, however there are no treatments that target specific inflammatory mediators or pathways driven by hypoxia.

Hypoxic stimuli potently induce the transcription and induction of multiple microRNAs, most prominently miR-210 and miR-374a. One such microRNA, miR-155 is a master regulator of inflammatory pathways and therefore plays a crucial role in multiple disease settings. Furthermore, miR-155 has been well documented in hypoxic diseases. Induction of miR-155 has been noted in numerous different models of hypoxic disease including cerebral ischaemia-reperfusion and stroke [1-3]. Hypoxic induction of miR-155 has also been seen in a number of different cell types and tissues such as intestinal tissue, human umbilical vein endothelial cells and renal tubular cells [4-6]. Hypoxia also potently activates myeloid cells, including monocytes, macrophages and microglia, triggering their recruitment, activation and polarisation. Specifically, induction of miR-155 in macrophages and microglia transforms their transcriptional signatures, upregulating inflammatory pathway signalling, consequentially increasing the production and secretion of inflammatory mediators by these cells. Hypoxia induces both miR-155 and myeloid cell activity, and the miR-155/myeloid cell axis has been identified to be important to driving inflammation in disease.

While the profile and function of myeloid cells has been explored in HIE there is a gap in the literature with regards to the functions of miR-155 in these cells following hypoxic ischemic brain injury in newborns. The aim of this project is to investigate the role of myeloid miR-155 in a murine model of neonatal hypoxia induced seizures (Hypoxia-Sz) induced by global cerebral hypoxia, which is comparable to the more localised brain injury that occurs with mixed hypoxic-ischaemic insults. To investigate the specific pathophysiological changes mediated by myeloid miR-155, we made use of miR-155fl/fl x LysMCre C57Bl/6 mice where miR-155 is selectively knocked down in cells of a myeloid lineage, including monocytes, macrophages and microglia.

In vivo findings support the potential for myeloid miR-155 knockdown to ameliorate hypoxic encephalopathy in neonates. Knockdown increased transcription of miR-155 targets SHIP-1, SOCS-1, ARG-2 and BDNF and protein levels of BDNF. Knockdown also ameliorated hypoxic induction of proinflammatory IL-1 and IL-6 transcripts and IL-1 protein expression in the brain. Most significantly, mice deficient in myeloid miR-155 had less severe behavioural seizures and lower seizure frequency and burden as measured by electroencephalography (EEG).

Overall, the data presented in this research project demonstrates a role for myeloid miR-155 in driving hypoxia-induced inflammatory pathways and epileptogenesis in neonates. Therefore, targeting myeloid miR-155 may provide a novel strategy for the treatment of hypoxic-brain injury through the regulation of neuroinflammation.