Repeated mild traumatic brain injury induces persistent variations in mitochondrial DNA copy number in mesocorticolimbic neurocircuitry of the rat

DNA Methylation-Mediated Mfn2 Gene Regulation in the Brain: A Role in Brain Trauma-Induced Mitochondrial Dysfunction and Memory Deficits

Repeated mild traumatic brain injuries (rMTBI) affect mitochondrial homeostasis in the brain. However, mechanisms of long-lasting neurobehavioral effects of rMTBI are largely unknown. Mitofusin 2 (Mfn2) is a critical component of tethering complexes in mitochondria-associated membranes (MAMs) and thereby plays a pivotal role in mitochondrial functions. Herein, we investigated the implications of DNA methylation in the Mfn2 gene regulation, and its consequences on mitochondrial dysfunction in the hippocampus after rMTBI. rMTBI dramatically reduced the mitochondrial mass, which was concomitant with decrease in Mfn2 mRNA and protein levels. DNA hypermethylation at the Mfn2 gene promoter was observed post 30 days of rMTBI. The treatment of 5-Azacytidine, a pan DNA methyltransferase inhibitor, normalized DNA methylation levels at Mfn2 promoter, which further resulted into restoration of Mfn2 function. The normalization of Mfn2 function was well correlated with recovery in memory deficits in rMTBI-exposed rats. Since, glutamate excitotoxicity serves as a primary insult after TBI, we employed in vitro model of glutamate excitotoxicity in human neuronal cell line SH-SY5Y to investigate the causal epigenetic mechanisms of Mfn2 gene regulation. The glutamate excitotoxicity reduced Mfn2 levels via DNA hypermethylation at Mfn2 promoter. Loss of Mfn2 caused significant surge in cellular and mitochondrial ROS levels with lowered mitochondrial membrane potential in cultured SH-SY5Y cells. Like rMTBI, these consequences of glutamate excitotoxicity were also prevented by 5-AzaC pre-treatment. Therefore, DNA methylation serves as a vital epigenetic mechanism involved in Mfn2 expression in the brain; and this Mfn2 gene regulation may play a pivotal role in rMTBI-induced persistent cognitive deficits. Closed head weight drop injury method was employed to induce repeated mild traumatic brain (rMTBI) in jury in adult, male Wistar rats. rMTBI causes hyper DNA methylation at the Mfn2 promoter and lowers the Mfn2 expression triggering mitochondrial dysfunction. However, the treatment of 5-azacytidine normalizes DNA methylation at the Mfn2 promoter and restores mitochondrial function.

Repeated mild traumatic brain injuries perturb the mitochondrial biogenesis via DNA methylation in the hippocampus of rat

Mitochondrial biogenesis in the brain is impaired in various neurological disorders including traumatic brain injury (TBI). The long-lasting effects of TBI may be, in part, attributed to epigenetic mechanisms such as DNA methylation. However, the role of DNA methylation on regulatory elements of nuclear and mitochondrial genome in mitochondrial biogenesis is not known. We examined the epigenetic regulation of mitochondrial transcription factor A (TFAM), and further probed its implications in mitochondrial dysfunction in the hippocampus of rats subjected to repeated mild TBI (rMTBI) using weight drop injury paradigm. rMTBI-induced hypermethylation at TFAM promoter resulted in deficits in its protein levels in mitochondria after immediate (48 h) and protracted (30 d) time points. Further, rMTBI also caused hypomethylation of mitochondrial DNA (mtDNA) promoters (HSP1 and HSP2), which further culminated into low binding of TFAM. rMTBI-induced changes weakened mitochondrial biogenesis in terms of reduced mtDNA-encoded rRNA, mRNA, and protein levels leading to shortages of ATP. To verify the potential role of mtDNA methylation in rMTBI-induced persistent mitochondrial dysfunction, rMTBI-induced rats were treated with methionine, a methyl donor. Methionine treatment restored the methylation levels on HSP1 and HSP2 resulting in efficient binding of TFAM and normalized the rRNA, mRNA, and protein levels. These findings suggest the crucial role of DNA methylation at nuclear and mitochondrial promoter regions in mitochondrial gene expression and ATP activity in the hippocampus after rMTBI.

Epigenetic Blockade of Hippocampal SOD2 Via DNMT3b-Mediated DNA Methylation: Implications in Mild Traumatic Brain Injury-Induced Persistent Oxidative Damage

The recurrent events of mild trauma exacerbate the vulnerability for post-traumatic stress disorder; however, the underlying molecular mechanisms are scarcely known. The repeated mild traumatic brain injury (rMTBI) perturbs redox homeostasis which is primarily managed by superoxide dismutase 2 (SOD2). The current study investigates the role of DNA methylation in SOD2 gene regulation and its involvement in rMTBI-induced persistent neuropathology inflicted by weight drop injury paradigm. The oxidative damage, neurodegenerative indicators, and SOD2 function and its regulation in the hippocampus were analyzed after 48 h and 30 days of rMTBI. The temporal and episodic increase in ROS levels (oxidative stress) heightened 8-hydroxyguanosine levels indicating oxidative damage after rMTBI that was concomitant with decline in SOD2 function. In parallel, occupancy of DNMT3b at SOD2 promoter was higher post 30 days of the first episode of rMTBI causing hypermethylation at SOD2 promoter. This epigenetic silencing of SOD2 promoter was sustained after the second episode of rMTBI causing permanent blockade in SOD2 response. The resultant oxidative stress further culminated into the increasing number of degenerating neurons. The treatment with 5-azacytidine, a pan DNMT inhibitor, normalized DNA methylation levels and revived SOD2 function after the second episode of rMTBI. The release of blockade in SOD2 expression by DNMT inhibition also normalized the post-traumatic oxidative consequences and relieved the neurodegeneration and deficits in learning and memory as measured by novel object recognition test. In conclusion, DNMT3b-mediated DNA methylation plays a critical role in SOD2 gene regulation in the hippocampus, and the perturbations therein post rMTBI are detrimental to redox homeostasis manifesting into neurological consequences.

DNA damage and repair following traumatic brain injury

Traumatic brain injury (TBI) is known to promote significant DNA damage irrespective of age, sex, and species. Chemical as well as structural DNA modification start within minutes and persist for days after TBI. Although several DNA repair pathways are induced following TBI, the simultaneous downregulation of some of the genes and proteins of these pathways leads to an aberrant overall DNA repair process. In many instances, DNA damages escape even the most robust repair mechanisms, especially when the repair process becomes overwhelmed or becomes inefficient by severe or repeated injuries. The persisting DNA damage and/or lack of DNA repair contributes to long-term functional deficits. In this review, we discuss the mechanisms of TBI-induced DNA damage and repair. We further discussed the putative experimental therapies that target the members of the DNA repair process for improved outcome following TBI.