Hypobaric hypoxia (HH) is a major stress factor that is associated with physiological, biochemical, molecular and genomic alterations. Brain is the organ that reacts sensitively to oxygen deprivation, which leads to oxidative stress and cognitive function impairment. Our previous studies have reported that downregulation of brain derived neurotrophic factor (BDNF) leads to neurodegeneration and memory impairment. The aim of the present study was to investigate the effect of HH exposure on DNA methylation and its regulation in BDNF expression, neurodegeneration and spatial memory impairment. For this purpose, Sprague Dawley rats were exposed to HH at a simulated altitude of 25,000 feet for 14 days. Real-time polymerase chain reaction was used for transcriptional expression of DNA Methyltransferases (DNMTs) including DNMT1, DNMT3a and -DNMT3b, and immunoblotting was used for the translational expression of DNMT1, DNMT3a, DNMT3b, Methyl CpG binding protein 2 (MeCP2), pMeCP2 and BDNF in rat hippocampus. Additionally, neuronal morphology alteration and neurodegeneration in CA1 region of hippocampus were investigated though Cresyl violet (CV) staining and Fluoro-Jade C staining respectively. Results obtained suggested that HH exposure increased the expression of DNMT1 DNMT3b at the mRNA as well as protein level, whereas no significant change was observed in the level of DNMT3a. Furthermore, the level of pMeCP2 and BDNF were significantly decreased; however, the expression level of MeCP2 was significantly increased. The CV and Fluoro-Jade C-positive cells were significantly enhanced in the CA1 region of hippocampus in the HH exposed group as compared to unexposed rats. Thus, the present study concluded that HH decreases neuronal activation by the upregulation of DNA methylation and MeCP2 and decreased the expression of pMeCP2, which result in the downregulation of BDNF. The decreased BDNF expression is associated with neuronal loss and spatial memory impairment. This study highlights that DNMT inhibition could be an important therapeutic target for neurodegenerative diseases.

Keywords:
Hypobaric hypoxia, DNA methyltransferase, Methyl CpG binding protein 2, Brain-derived neurotrophic factor, Neurodegeneration
1.
Saugy JJ, Schmitt L, Fallet S, Faiss R, Vesin JM, Bertschi M, Heinzer R, Millet GP: Sleep disordered breathing during live high-train low in normobaric versus hypobaric hypoxia. High Alt Med Biol 2016; 17: 233–238.
2.
Jain V, Baitharu I, Barhwal K, Prasad D, Singh SB, Ilavazhagan G: Enriched environment prevents hypobaric hypoxia induced neurodegeneration and is independent of antioxidant signaling. Cell Mol Neurobiol 2012; 32: 599–611.
3.
Muthuraju S, Maiti P, Pati S, Solanki P, Sharma AK, Singh SB, Prasad D, Ilavazhagan G: Role of cholinergic markers on memory function of rats exposed to hypobaric hypoxia. Eur J Pharmacol 2011; 672: 96–105.
4.
Hota SK, Barhwal K, Ray K, Singh SB, Ilavazhagan G: Ceftriaxone rescues hippocampal neurons from excitotoxicity and enhances memory retrieval in chronic hypobaric hypoxia. Neurobiol Learn Mem 2008; 89: 522–532.
5.
Gulchina Y, Xu SJ, Snyder MA, Elefant F, Gao WJ: Epigenetic mechanisms underlying NMDA receptor hypofunction in the prefrontal cortex of juvenile animals in the MAM model for schizophrenia. J Neurochem 2017; 143: 320–333.
6.
Spiers H, Hannon E, Schalkwyk LC, Bray NJ, Mill J: 5-hydroxymethylcytosine is highly dynamic across human fetal brain development. BMC Genomics 2017; 18: 738.
7.
Griñán-Ferré C, Corpas R, Puigoriol-Illamola D, Palomera-Ávalos V, Sanfeliu C, Pallàs M: Understanding epigenetics in the neurodegeneration of Alzheimer’s disease: SAMP8 mouse model. J Alzheimers Dis 2018; 62: 943–963.
8.
Dawson MA, Kouzarides T: Cancer epigenetics: from mechanism to therapy. Cell 2012; 150: 12–27.
9.
Baribault C, Ehrlich KC, Ponnaluri VKC, Pradhan S, Lacey M, Ehrlich M: Developmentally linked human DNA hypermethylation is associated with down-modulation, repression, and upregulation of transcription. Epigenetics 2018: 1–36.
10.
Modrek AS, Golub D, Khan T, Bready D, Prado J, Bowman C, Deng J, Zhang G, Rocha PP, Raviram R, Lazaris C, Stafford JM, LeRoy G, Kader M, Dhaliwal J, Bayin NS, Frenster JD, Serrano J, Chiriboga L, Baitalmal R, Nanjangud G, Chi AS, Golfinos JG, Wang J, Karajannis MA, Bonneau RA, Reinberg D, Tsirigos A, Zagzag D, Snuderl M, Skok JA, Neubert TA, Placantonakis DG: Low-grade astrocytoma mutations in IDH1, P53, and ATRX cooperate to block differentiation of human neural stem cells via repression of SOX2. Cell Rep 2017; 21: 1267–1280.
11.
Medina-Franco JL, Méndez-Lucio O, Due- ñas-González A, Yoo J: Discovery and development of DNA methyltransferase inhibitors using in silico approaches. Drug Discov Today 2015; 20: 569–577.
12.
Schmalbach S, Petri S: Histone deacetylation and motor neuron degeneration. CNS Neurol Disord Drug Targets 2010;9:279–284. Review 2010; 9: 279–284.
13.
Saha RN, Pahan K: HATs and HDACs in neurodegeneration: a tale of disconcerted acetylation homeostasis. Cell Death Differ 2006; 13: 539–550.
14.
Endres M, Meisel A, Biniszkiewicz D, et al: DNA methyltransferase contributes to delayed ischemic brain injury. J Neurosci 2000; 20: 3175–3181.
15.
Day JJ, Sweatt JD: DNA methylation and memory formation. Nat Neurosci 2010; 13: 1319–1323.
16.
Levenson JM, Roth TL, Lubin FD, Miller CA, Huang IC, Desai P, Malone LM, Sweatt JD: Evidence that DNA (cytosine-5) methyltransferase regulates synaptic plasticity in the hippocampus. J Biol Chem 2006; 281: 15763–15773.
17.
Webb WM, Sanchez RG, Perez G, Butler AA, Hauser RM, Rich MC, O'Bierne AL, Jarome TJ, Lubin FD: Dynamic association of epigenetic H3K4me3 and DNA 5hmC marks in the dorsal hippocampus and anterior cingulate cortex following reactivation of a fear memory. Neurobiol Learn Mem 2017; 142(Pt A): 66–78.
18.
Deng JH, Yan W, Han Y, Chen C, Meng SQ, Sun CY, Xu LZ, Xue YX, Gao XJ, Chen N, Zhang FL, Wang YM, Shi J, Lu L: Predictable chronic mild stress during adolescence promotes fear memory extinction in adulthood. Sci Rep 201710; 7: 7857.
19.
Kishi N, Macklis JD: MECP2 is progressively expressed in post-migratory neurons and is involved in neuronal maturation rather than cell fate decisions. Mol Cell Neurosci 2004; 27: 306–321.
20.
Cohen S, Gabel HW, Hemberg M, Hutchinson AN, Sadacca LA, Ebert DH, Harmin DA, Greenberg RS, Verdine VK, Zhou Z, Wetsel WC, West AE, Greenberg ME: Genome-wide activity-dependent MeCP2 phosphorylation regulates nervous system development and function. Neuron 2011; 72: 72–85.
21.
Han X, Zhang X, Xue Z: Sevoflurane induces long-term memory impairment and increases MeCP2 phasphorylation in developing mice. Int J Clin Exp Med 2017;10.
22.
Chen WG, Chang Q, Lin Y, Meissner A, West AE, Griffith EC, Jaenisch R, Greenberg ME: Derepression of BDNF transcription involves calcium-dependent phosphorylation of MeCP2. Science 2003; 302: 885–889.
23.
Zhou Z, Hong EJ, Cohen S, Zhao WN, Ho HY, Schmidt L, Chen WG, Lin Y, Savner E, Griffith EC, Hu L, Steen JA, Weitz CJ, Greenberg ME: Brain-specific phosphorylation of MeCP2 regulates activity-dependent BDNF transcription, dendritic growth, and spine maturation. Neuron 2006; 52: 255–269.
24.
Geranton SM, Morenilla-Palao C, Hunt SP: A role for transcriptional repressor methyl-CpG-binding protein 2 and plasticity-related gene serum- and glucocorticoid-inducible kinase 1 in the induction of inflammatory pain states. J Neurosci 2007; 27: 6163–6173.
25.
Geranton SM, Fratto V, Tochiki KK, Hunt SP: Descending serotonergic controls regulate inflammation-induced mechanical sensitivity and methyl-CpG-binding protein 2 phosphorylation in the rat superficial dorsal horn. Mol Pain 2008; 4: 35.
26.
Deng JV, Rodriguiz RM, Hutchinson AN, Kim IH, Wetsel WC, West AE: MeCP2 in the nucleus accumbens contributes to neural and behavioural responses to psychostimulants. Nat Neurosci 2010; 13: 1128–1136.
27.
Li H, Zhong X, Chau KF, Williams EC, Chang Q: Loss of activity-induced phosphorylation of MeCP2 enhances synaptogenesis, LTP and spatial memory. Nat Neurosci 2011; 14: 1001–1008.
28.
Mao LM, Horton E, Guo ML, Xue B, Jin DZ, Fibuch EE, Wang JQ: Cocaine increases phosphorylation of MeCP2 in the rat striatum in vivo: a differential role of NMDA receptors. Neurochem Int 59: 610–617.
29.
Jain V, Baitharu I, Prasad D, Ilavazhagan G: Enriched environment prevents hypobaric hypoxia induced memory impairment and neurodegeneration: role of BDNF/PI3K/GSK3β pathway coupled with CREB activation. PLoS One 2013; 8:e62235.
30.
Yang L, Mao L, Chen H, Catavsan M, Kozinn J, Arora A, Liu X, Wang JQ: A signaling mechanism from G alpha q-protein-coupled metabotropic glutamate receptors to gene expression: role of the c-Jun N-terminal kinase pathway. J Neurosci 2006; 26: 971–980.
31.
Zhang GC, Mao LM, Liu XY, Parelkar NK, Arora A, Yang L, Hains M, Fibuch EE, Wang JQ: In vivo regulation of Homer1a expression in the striatum by cocaine. Mol Pharmacol 2007; 71: 1148–1158.
32.
Xie Z, Enkhjargal B, Wu L, Zhou K, Sun C, Hu X, Gospodarev V, Tang J, You C, Zhang JH: Exendin-4 attenuates neuronal death via GLP-1R/PI3K/Akt pathway in early brain injury after subarachnoid hemorrhage in rats. Neuropharmacology 2018; 128: 142–151.
33.
Wang YC, Sanchez-Mendoza EH, Doeppner TR, Hermann DM: Post-acute delivery of memantine promotes post-ischemic neurological recovery, peri-infarct tissue remodelling, and contralesional brain plasticity. J Cereb Blood Flow Metab 2017; 37: 980–993.
34.
Dandi E, Kalamari A, Touloumi O, Lagoudaki R, Nousiopoulou E, Simeonidou C, Spandou E, Tata DA: Beneficial effects of environmental enrichment on behavior, stress reactivity and synaptophysin/BDNF expression in hippocampus following early life stress. Int J Dev Neurosci 2018: pii: S0736-5748(17)30320-9.
35.
Zhang S, Zhang Y, Jiang S, Liu Y, Huang L, Zhang T, Lu G, Gong K, Ji X, Shao G: The effect of hypoxia preconditioning on DNA methyltransferase and PP1γ in hippocampus of hypoxia preconditioned mice. High Alt Med Biol 2014; 15: 483–490.
36.
Shao G, Zhang R, Zhang S, Jiang S, Liu Y, Zhang W, Zhang Y, Li J, Gong K, Hu XR, Jiang SW: Splice variants DNMT3B4 and DNMT3B7 overexpression inhibit cell proliferation in 293A cell line. In Vitro Cell Dev Biol Anim 2013; 49: 386–394.
37.
Liu PK, Hsu CY, Dizdaroglu M, et al: Damage, repair, and mutagenesis in nuclear genes after mouse forebrain ischemia-reperfusion. J Neurosci 1996; 16: 6795–6806. Daetstidar et al., (2012).
38.
Brooks PJ, Marietta C, Goldman D: DNA mismatch repair and DNA methylation in adult brain neurons. J Neurosci 1996; 16: 939–945.
39.
Xin X, Dang H, Zhao X, Wang H: Effects of hypobaric hypoxia on rat retina and protective response of resveratrol to the stress. Int J Med Sci 2017; 14: 943–950.
40.
Feng J, Zhou Y, Campbell SL, Le T, Li E, Sweatt JD, Silva AJ, Fan G: Dnmt1 and Dnmt3a maintain DNA methylation and regulate synaptic function in adult forebrain neurons. Nat Neurosci 2010; 13: 423–430.
41.
Betz A, Jayatilaka S, Joshi J, Ramanan S, Debartolo D, Pylypiw H, Franke E: Chronic exposure to benzyl butyl phthalate (BBP) alters social interaction and fear conditioning in male adult rats: alterations in amygdalar MeCP2, ERK1/2 and ERα. Neuro Endocrinol Lett 2013; 34: 347–358.
42.
Subbanna S, Nagre NN, Shivakumar M, Umapathy NS, Psychoyos D, Basavarajappa BS: Ethanol induced acetylation of histone at G9a exon1 and G9a-mediated histone H3 dimethylation leads to neurodegeneration in neonatal mice. Neuroscience 2014; 258: 422–432.
43.
Dastidar SG, Bardai FH, Ma C, Price V, Rawat V, Verma P, Narayanan V, D’Mello SR: Isoform-specific toxicity of Mecp2 in postmitotic neurons: suppression of neurotoxicity by FoxG1. J Neurosci 2012; 32: 2846–2855.
44.
Lubin FD, Roth TL, Sweatt JD: Epigenetic regulation of BDNF gene transcription in the consolidation of fear memory. J Neurosci 2008; 28: 10576–10586.
45.
Mizuno K, Dempster E, Mill J, Giese KP: Long-lasting regulation of hippocampal BDNF gene transcription after contextual fear conditioning. Genes Brain Behav 2012; 11: 651–659.
46.
Kuga GK, Muñoz VR, Gaspar RC, Nakandakari SCBR, da Silva ASR, Botezelli JD, Leme JACA, Gomes RJ, de Moura LP, Cintra DE, Ropelle ER, Pauli JR: Impaired insulin signaling and spatial learning in middle-aged rats: The role of PTP1B. Exp Gerontol 2018; 104: 66–71.
47.
Nelson ED, Kavalali ET, Monteggia LM: Activity-dependent suppression of miniature neurotransmission through the regulation of DNA methylation. J Neurosci 2008; 28: 395–406.
48.
Neeper SA, Gomez-Pinilla F, Choi J, Cotman C: Exercise and brain neurotrophins. Nature 1995; 373: 109.
49.
Zhong X, Li H, Chang Q: MeCP2 phasphorylation is required for modulating synaptic scaling through mGluR5. J Neurosci 2012; 32: 12841–1284.
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