Mitochondria as cellular hub in neonatal hypoxic-ischemic encephalopathy: identification of melatonin-responsive biomarkers predicting brain damage severity

Mitochondrial dysfunction is considered one of the hallmarks of ischemia/reperfusion brain injury, and maintaining mitochondrial dynamics is crucial to cell function and survival. In our lab, we have been studying the neuroprotective effects of melatonin in different experimental models of ischemia/reperfusion brain damage, and, more recently, we used HT22 cells exposed to oxygen/glucose deprivation (OGD). Using this cell model, we found that melatonin improved cell viability, reduced mitochondrial reactive oxygen species production and oxidative cell damage, and preserved the mitochondrial respiratory chain activity. Melatonin also promoted mitochondrial fusion/fission dynamics. Transmission electron microscopy showed that, in the presence of melatonin, mitochondrial morphology was better preserved, and the number of apoptotic mitochondria significantly decreased. Melatonin also fostered mitochondrial transfer between injured HT22 cells through tunnelling nanotubes (TNTs) connections. In OGD-exposed HT22 cells, melatonin significantly reduced the release of mitochondrial DNA (mtDNA) into the cytosol - where it acts as a powerful DAMP (Damage-Associated Molecular Pattern) - and of HMGB-1, a nuclear protein released during cellular stress. The DAMP-activated cGAS-STING pathway was reduced in the presence of melatonin with a consequent decrease of IFNβ and IL-6. In addition, melatonin increased the release of FGF-21, a mitokine involved in mitochondria protection and cell survival. Our results demonstrate that melatonin reduces the oxidative damage induced by OGD and fosters the mitochondrial transfer between cells through TNTs formation. This can represent a novel mechanism mediating neuroprotection in ischemia/reperfusion injury and can be also relevant for mesenchymal cell transplantation. Melatonin also reduces the release of DAMP proteins, resulting in decreased mito-inflammatory response. The possibility that changes in circulating mtDNA and mitokines may represent putative diagnostic/prognostic tools for neonates affected by hypoxicischemic brain damage is under investigation.