Pharmacology and Therapeutics - Theses
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ItemA novel role for STING-mediated type-I IFN response in traumatic brain injury( 2018)Traumatic brain injury (TBI) represents a major cause of disability and death worldwide with sustained neuro-inflammation and autophagy dysfunction contributes to cellular damage. Stimulator of interferon genes (STING)-induced type-I interferon (IFN) signaling is known to be essential in mounting the innate immune response against infections and cell injury in the periphery, but its role in the CNS remains unclear. Recent studies also implicated the STING and type-I interferon pathways in autophagy activation. We have previously identified the type-I IFN pathway as a key mediator of neuro-inflammation and neuronal cell death in TBI (Karve et al., 2016). This thesis has explored the modulation of the type-I IFN and neuro-inflammatory responses by STING and its contribution to neuronal cell death and autophagy activity after TBI in vivo. Additionally, a role for STING in regulating autophagy activity and cellular viability in response to H2O2-induced oxidative stress has also been investigated in vitro. To investigate this, C57BL/6J wildtype (WT) and STING-/- mice (8-10-week-old male) were subjected to controlled-cortical impact (CCI) surgery. It was found that STING expression was upregulated by 1.24 + 0.20 and 4.45 + 0.93 fold at 2h and 24h, respectively in WT mice after CCI as determined by qPCR with increased expression confirmed by Western blot and immunohistochemistry. This correlated with an elevated pro-inflammatory cytokine profile with an upregulation in TNF-α, IL-1β and type-I IFN (IFN-α and IFN-β) levels and heightened glial reactivity. Significantly, this expression was suppressed in the STING-/- mice with a smaller lesion volume in the knockout animals at 24h post CCI (WT = 4.16 + 0.27mm3, STING-/- = 3.20 + 0.17 mm3; p< 0.05) as assessed by triphenyl tetrazolium chloride (TTC) staining. Supporting a role for STING in human TBI, a significant upregulation in STING expression (2.25 + 0.50 fold; p< 0.0001) was detected in late trauma human brain samples as compared to the control group. Further, impaired autophagy activation with concurrent increased levels of LC3-II, p62 and LAMP2 were detected in WT mice 24h after TBI. However, STING-/- mice showed reduced LAMP2 expression as compared with LC3-II and p62 levels 24h after TBI suggesting a role for STING in driving dysfunctional autophagy seen in TBI. Interestingly, ablation of the STING pathway in vitro revealed a differential role for STING in promoting cellular survival following H2O2-induced oxidative stress evidenced by higher cellular viability in the WT MEF SV40 cells as compared to STING-/- MEF SV40 cells line. Supporting a role for STING in promoting cellular survival after H2O2 treatment, it was found that STING-/- MEF SV40 cells show impaired autophagy activity as compared to the WT cells following H2O2 insult. This thesis uncovers a novel role for STING-mediated type-I IFN signalling in regulating neuro- inflammatory processes and autophagy activity after TBI. More importantly, this thesis demonstrates a multi-faceted role for STING in the CCI animal model of TBI and in the cell- based model of oxidative stress induced by H2O2. Taken together, these findings implicate for the first time, a detrimental role for STING in mediating the TBI-induced neuro- inflammatory response and autophagy dysfunction and has potentially identified a new therapeutic target for reducing the cellular damage in TBI.