Loss of white matter tracts and persistent microglial activation in the chronic phase of ischemic stroke in female rats and the effect of miR-20a-3p treatment

Our previous studies showed that intravenous injections of the small non-coding RNA mir-20a-3p is neuroprotective for stroke in the acute phase and attenuates long-term cognitive impairment in middle-aged female rats. In this study, we evaluated postmortem brain pathology at 100+d after stroke in a set of behaviorally characterized animals. This included Sham (no stroke) controls or stroke animals that received either mir20a-3p at 4h, 24h and 70d iv post stroke (MCAo+mir20a-3p) or a scrambled oligo (MCAo+Scr). Brain volumetric features were analyzed with T2 weighted and Diffusion Tensor magnetic resonance imaging (MRI) followed by histological analysis. Principal component analysis of Fractional Anisotropy (FA)-diffusion tensor MRI measures showed that MCAo+Scr and MCAo+mir20a-3p groups differed significantly in the volume of white matter but not gray matter. Weil myelin-stained sections confirmed decreased volume of the corpus callosum, internal capsule and the anterior commissure in the ischemic hemisphere of MCAo+Scr animals compared to the non-ischemic hemisphere, while sham and MCAo+Mir-20a-3p showed no hemispheric asymmetries. The MCAo+Scr group also exhibited asymmetry in hemisphere and lateral ventricle volumes, with ventricular enlargement in the ischemic hemisphere as compared to the non-ischemic hemisphere. The numbers of microglia were significantly elevated in white matter tracts in the MCAo+Scr group, with a trend towards increased myelin phagocytic microglia in these tracts. Regression analysis indicated that performance on an episodic memory test (novel object recognition test; NORT) was associated with decreased white matter volume and increased microglial numbers. These data support the hypothesis that stroke-induced cognitive impairment is accompanied by white matter attrition and persistent microglial activation and is consistent with reports that cognitive deterioration resulting from vascular diseases, such as stroke, is associated with secondary neurodegeneration in regions distal from the initial infarction.