Previous publications from this laboratory demonstrated that administration of leukemia inhibitory factor (LIF) (125 µg/kg) to young, male Sprague-Dawley rats at 6, 24, and 48 h after middle cerebral artery occlusion (MCAO) reduced infract volume, improved sensimotor skills, and alleviated damage to white matter at 72 h after the injury. In vitro studies using cultured oligodendrocytes (OLs) showed that LIF (200 ng/ml) also protects against 24 h of oxygen-glucose deprivation through activation of Akt signaling and upregulation of the antioxidant enzymes peroxiredoxin IV and metallothionein III. Other groups have demonstrated that LIF reduces neurodegeneration in animal models of disease, but the neuroprotective mechanisms of LIF during permanent ischemia have not yet been examined. The overall hypothesis to be tested in this project is whether LIF exerts similar protective mechanisms against neurons during ischemia through increased antioxidant enzyme expression in neurons.
In the first set of experiments, superoxide dismutase (SOD) activity was significantly increased in the ipsilateral hemisphere of LIF-treated rats compared to rats that received PBS treatment at 72 h after MCAO. Western blot and immunohistochemical analysis revealed that SOD3 was upregulated in brain tissue and induced specifically in cortical neurons tissue at this time point. Neurons that expressed high levels of SOD3 at 72 h after MCAO also showed high levels of phosphorylated Akt (Ser473). LIF (200 ng/ml) reduced necrotic and apoptotic cell death against 24 h of OGD as measured by lactate dehydrogenase (LDH) release and caspase-3 activation. Quantitative real-time PCR analysis showed that LIF treatment upregulated SOD3 gene expression in vitro during OGD. Treatment with 10 µM Akt Inhibitor IV and transfection with SOD3 siRNA counteracted the neuroprotective effects of LIF in vitro, showing that upregulation of SOD3 and activation of Akt signaling are necessary for LIF-mediated neuroprotection.
Several transcription factors that regulated Akt-inducible genes were previously identified by this lab, including myeloid zinc finger-1 (MZF-1) and specificity protein-1 (Sp1). The goal of the second set of experiments was to determine whether LIF exerted protective actions through MZF-1 and Sp1. According to analysis with Genomatix, MZF-1 and Sp1 have multiple binding sites in the promoter for the rat SOD3 gene. Western blot analysis showed that there was a trend towards increased MZF-1 protein expression in the brains of LIF-treated rats that approached significance. Immunohistochemical analysis and quantitative real-time PCR showed a significant in vitro upregulation in MZF-1 expression among LIF-treated neurons compared to PBS-treated neurons. Sp1 gene expression was not changed by LIF treatment, but there was a trend towards increased protein expression. In addition, there was a significant correlation between Sp1 and MZF-1 among brain samples from LIF-treated rats but not PBS-treated or sham rats at 72 h after MCAO. Immunohistochemical analysis revealed that Sp1 and MZF-1 co-localized with neuronal nuclei and SOD3 at 72 h after MCAO. Neurons that were transfected with MZF-1 or Sp1 siRNA following isolation did not show a significant decrease in LDH release after 24 h OGD that was observed among neurons transfected with scrambled siRNA. These data demonstrate that Sp1 and MZF-1 are involved with the neuroprotective signaling of LIF under ischemia.
This laboratory has demonstrated that LIF activates transcription of protective genes and increases the activity of transcription factors through modulation of intracellular signaling. However, the upstream signaling mechanisms of LIF during ischemia had not previously been investigated. Previous investigators found that the LIF-specific subunit of the heterodimeric LIF receptor (LIFR) is induced by CNS injury. Western blot analysis was used to determine whether LIFR was induced in the brain and the spleen, which plays a role in the peripheral immune response, after MCAO. According to these results, LIF treatment significantly upregulates LIF in the brain compared to PBS treatment or sham injury at 72 h after MCAO. Genomatix analysis of the LIFR promoter region revealed a binding site for Sp1, which is one of the transcription factors responsible for neuroprotection by LIF. At this same time point, splenic LIFR expression is significantly reduced after MCAO compared to sham injury. LIF treatment did not significantly increase LIFR expression, but did significantly increase spleen size compared to PBS treatment at 72 h after MCAO. Although there was a trend towards increased LIFR expression in the spleen from 24 h to 72 h after MCAO, this increase was not statistically significant. However, there was a significant positive correlation between spleen weight and LIFR expression among rats euthanized 24-72 h after MCAO/sham injury. In addition, there was a significant negative correlation between LIFR expression in the brain and the spleen weight, thus showing that LIFR is upregulated following the splenic response.
According to findings from other groups, JAK1 has been shown to associate with the heterodimeric LIF receptor (LIFR/gp130) and directly activate PI3K/Akt signaling. To test whether JAK1 contributes neuroprotection during ischemia, cultured neurons were treated with several concentrations (2.5-50 nM) of GLPG0634, a JAK1-specific inhibitor prior to 24 h of OGD. With the exception of the 2.5 nM concentration, all concentrations of GLPG0634 significantly decreased LDH release compared to DMSO treatment, with the 5 nM concentration having the most potent effect on reducing cytotoxicity. However, the 5 nM concentration had no significant did not significantly reduce LDH release compared to DMSO treatment under 24 h of normoxic conditions. These results indicate that JAK1 activity is primarily detrimental to neurons during ischemia. Although it is possible that LIF signaling activates JAK1, it is unlikely that JAK1 is responsible for LIF-mediated neuroprotection during ischemia.
The results of these experiments allowed us to determine several molecular mechanisms for LIF-mediated neuroprotection. LIF, which binds to its heterodimeric receptor, activates Akt signaling during ischemia. The transcription factors Sp1 and MZF-1, which are located downstream of Akt, bind to the promoter of the SOD3 gene. In addition, Sp1 also regulates the LIFR gene. SOD3 upregulation increases total SOD activity, which decreases apoptotic and necrotic cell death during apoptosis. Due to its ability to promote antioxidant expression and survival signaling in multiple neural cell types, LIF shows promise as a novel treatment for permanent focal cerebral ischemia.
| Identifer | oai:union.ndltd.org:USF/oai:scholarcommons.usf.edu:etd-7414 |
| Date | 04 May 2016 |
| Creators | Davis, Stephanie |
| Publisher | Scholar Commons |
| Source Sets | University of South Flordia |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Graduate Theses and Dissertations |
| Rights | default |
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