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Comparison of the cellular expression of Let7b microRNA in the white matter in two rat models of immature brain injury

Presented at the Neonatal Society 2016 Autumn Meeting.

Mohamed M1,2, Ponnusamy V3,4, Schwendimann L5, Fleiss B5,6, Michael-Titus AT1, Gressens P5,6, Shah DK3,7, Yip PK1

1 Centre for Neuroscience & Trauma, Blizard Institute, UK
2 Neurosurgery, The Royal London Hospital, UK
3 Centre for Genomics & Child Health, Blizard Institute, UK
4 NICU Ashford & St Peter’s Hospitals, UK
5 INSERM U1141, France
6 King’s College London, London, UK
7 NICU, The Royal London Hospital, UK

Background: The underlying cause of brain white matter injury in newborn babies is not always known, but hypoxia-ischaemia (HI) and/or maternal inflammation have been implicated. Let7b microRNA is a non-coding RNA involved in the regulation of gene expression and it has been associated with many pathologies. Hypotheses: [a] injury induces astrogliosis and microgliosis, and [b] increased cellular Let7b expression in astrocytes and microglia in the corpus callosum.

Methods: Two rat models of immature brain injury were studied. The hypoxia-ischaemia (HI) injury, Rice-Vannucci model was utilised at P8. A model of maternal exposure to lipopolysaccharide was carried out at gestational day 20. All animals were perfused at P9 and brains sectioned coronally using a cryostat. Immunostained astrocytes and microglia in the corpus callosum were double-labelled with Let7b using fluorescence in situ hybridization. The levels of expression and markers co-localisation were analysed.

Results: In comparison to the corpus callosum of the un-injured group, no significant change in GFAP (astrocytes) expression level was detected, in both the HI (p=0.859) and intrauterine inflammation (p=0.564) models (n=3 per group). A significant reduction in Iba1 (microglia) expression was observed in the intrauterine inflammation group (p=0.0002) but not in the HI group (p=0.520). A significant decrease in Let7b expression in the Iba1-positive cells (p=0.034) but not in Let7b expression in the GFAP-positive cells (p=0.411) was observed after intrauterine inflammation. No significant change in Let7b expression was observed in both GFAP-positive (p=0.412) and Iba1-positive (p=0.615) cells of the HI group.

Conclusion: We demonstrated a decrease in microglia and Let7b expression within microglial cells in the corpus callosum of pups after maternal exposure to lipopolysaccharide, but not in pups with HI, at 24 h. This decrease in microglia in the corpus callosum may contribute to the transient motor dysfunction observed in this rat model (Roussett et al., 2012)

Corresponding author: mouminakm@gmail.com

References
Rousset CI, Kassem J., Aubert A., Planchenault D., Gressens P., Chalon S., Belzung C. & Saliba E (2012). Maternal exposure to lipopolysaccharide leads to transient motor dysfunction in neonatal rats. Dev. Neurosci., 1-10.

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