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MicroRNA‐21 protects neurons from ischemic death

MicroRNAs are small RNAs that attenuate protein expression by complementary binding to the 3′‐UTR of a target mRNA. Currently, very little is known about microRNAs after cerebral ischemia. In particular, microRNA‐21 (miR‐21) is a strong antiapoptotic factor in some biological systems. We investigated the role of miR‐21 after stroke in the rat. We employed in situ hybridization and laser capture microdissection in combination with real‐time RT‐PCR to investigate the expression of miR‐21 after stroke. In situ hybridization revealed that miR‐21 expression was upregulated in neurons of the ischemic boundary zone, and quantitative real‐time RT‐PCR analysis revealed that stroke increased mature miR‐21 levels by approximately threefold in neurons isolated from the ischemic boundary zone by laser capture microdissection as compared with homologous contralateral neurons 2 days (n = 4; P < 0.05) and 7 days (n = 3; P < 0.05) after stroke. In vitro, overexpression of miR‐21 in cultured cortical neurons substantially suppressed oxygen and glucose deprivation‐induced apoptotic cell death, whereas attenuation of endogenous miR‐21 by antisense inhibition exacerbated cell death after oxygen and glucose deprivation. Moreover, overexpression of miR‐21 in neurons significantly reduced FASLG levels, and introduction of an miR‐21 mimic into 293‐HEK cells substantially reduced luciferase activity in a reporter system containing the 3′‐UTR of Faslg. Our data indicate that overexpression of miR‐21 protects against ischemic neuronal death, and that downregulation of FASLG, a tumor necrosis factor‐α family member and an important cell death‐inducing ligand whose gene is targeted by miR‐21, probably mediates the neuroprotective effect. These novel findings suggest that miR‐21 may be an attractive therapeutic molecule for treatment of stroke.