Chronic Low Dose Rate Radiation-Induced Persistent DNA Damage and miRNAs/mRNAs Expression Changes in the Mouse Hippocampus

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Abstract
Our previous study demonstrated that the acute high-dose rate (3.3 Gy/minute) gamma irradiation of postnatal day 3 (P3) mice with 5Gy induced depression and drastic neuropathological changes in the dentate gyrus of the hippocampus of the adult mice. The present study investigated the effects of chronic low-dose-rate (1.2mGy/h) gamma irradiation from P30 to P180 with a cumulative dose of 5Gy on animal behaviour, hippocampal cellular change, miRNA and mRNA expression in the hippocampus and blood in female mice. The radiation exposure did not significantly affect the animal's body weight and neuropsychiatric changes such as anxiety and depression examined by neurobehavioural tests including open field, light-dark box, elevated plus maze, tail suspension test and forced swim test. Immunohistochemical staining did not detect any obvious loss of mature and immature neurons (NeuN and DCX) or any inflammatory glial response (IBA1, GFAP, and PDGFR1). Nevertheless, r-H2AX foci in the stratum granulosum of the dentate gyrus were significantly increased, suggesting the chronic low dose rate irradiation induced persistent DNA damage foci in mice. miRNA sequencing and qRT-PCR indicated an increased expression of miR-448-3p and miR-361-5p, but decreased expression of miR-193a-3p in the mouse hippocampus. Meanwhile, mRNA sequencing and qRT-PCR showed a reduced expression of Ccn1, Fli1, Fosb, Ets1, Hs3st5, and Eif4ebp1, but enhanced expression of Cort, Foxh1, and Opalin. Database searching by miRDB and TargetScan predicted that FLI1 and HS3ST5 are the targets of miR-448-3p, and Eif4ebp2 is the target of miR-361-5p. miRNA/mRNA sequencing and qRT-PCR results in blood showed the increased expression of miR-6967-3p and the decreased expression of its target S1pr5. The interactions of these miRNAs and mRNAs may contribute to the chronic low dose rate radiation-induced persistent DNA damage.
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