Abstract
Traumatic brain injury (TBI) activates multiple neuronal cell death mechanisms, leading to post-traumatic neuronal loss and neurological deficits. TBI-induced cell cycle activation (CCA) in post-mitotic neurons causes regulated cell death involving cyclin-dependent kinase (CDK) activation and initiation of an E2F transcription factor-mediated pro-apoptotic program. Here we examine the mechanisms of CCA-dependent neuronal apoptosis in primary neurons in vitro and in mice exposed to controlled cortical impact (CCI). In contrast to our prior work demonstrating robust neuroprotective effects by CDK inhibitors after TBI, examination of neuronal apoptotic mechanisms in E2F1−/−/E2F2−/− or E2F2−/− transgenic mice following CCI suggests that E2F1 and/or E2F2 likely play only a modest role in neuronal cell loss after brain trauma. To elucidate more critical CCA molecular pathways involved in post-traumatic neuronal cell death, we investigated the neuroprotective effects and mechanisms of the potent CDK inhibitor CR8 in a DNA damage model of cell death in primary cortical neurons. CR8 treatment significantly reduced caspase activation and cleavage of caspase substrates, attenuating neuronal cell death. CR8 neuroprotective effects appeared to reflect inhibition of multiple pathways converging on the mitochondrion, including injury-induced elevation of pro-apoptotic Bcl-2 homology region 3 (BH3)-only proteins Puma and Noxa, thereby attenuating mitochondrial permeabilization and release of cytochrome c and AIF, with reduction of both caspase-dependent and -independent apoptosis. CR8 administration also limited injury-induced deficits in mitochondrial respiration. These neuroprotective effects may be explained by CR8-mediated inhibition of key upstream injury responses, including attenuation of c-Jun phosphorylation/activation as well as inhibition of p53 transactivation of BH3-only targets.
Highlights
Traumatic brain injury (TBI) is a major public health problem, with 2.8 million cases reported in 20131
We observed an injury-induced increase in tumor necrosis factor alpha (TNFα) (H(3) = 27.63, p = 0.0001), cyclin-dependent kinase inhibitor 1A-CDK1A (p21Cip/ WAF1) (H(3) = 27.31, p = 0.0001), c-Jun (H(3) = 26.49, p = 0.0001), and myeloid cell leukemia sequence 1 (Mcl-1) (F(3,35) = 18.86, p = 0.0001) mRNA levels in each genotype
For all tested mRNAs no differences were observed in wild-type (FVB and B6129SF2/J) vs. E2F2−/− and wild-type vs. E2F1−/−/E2F2−/− mice after cortical impact (CCI) (p > 0.05); for Mcl-1 no differences were observed in E2F2−/− vs. wild-type but E2F1−/−/E2F2−/− was higher than wild-type after CCI (p > 0.05) (Fig. 1b)
Summary
Traumatic brain injury (TBI) is a major public health problem, with 2.8 million cases reported in 20131. We and others have shown that TBI triggers a chronically progressive neurodegeneration and tissue loss, which is associated with functional impairments and highlighted the importance of identifying the responsible neuronal cell death pathways[8,9,10,11]. Previous studies have indicated that activation of the cell cycle machinery in post-mitotic neurons results in cell death and have shown the importance of cell cycle activation (CCA) as a secondary injury mechanism after TBI12–14. CCA leads to apoptosis in post-mitotic cells such as neurons and oligodendroglia and may contribute to microglia proliferation, neuroinflammation, and secondary neurotoxicity[15,16]. Rb phosphorylation and E2F1 activation following experimental TBI in rats may play a role in post-traumatic neuronal apoptosis[24]. The role of E2F1 in neuronal apoptosis is suggested by studies showing that cortical neuronal cultures and hippocampal slices from E2F1−/− knockout mice are protected against neurotoxicity following oxygen and glucose deprivation compared to E2F1+/+ mice[25]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
