Endothelial Nitric Oxide Synthase Regulates White Matter Changes via the BDNF/TrkB Pathway after Stroke in Mice

Stroke induced white matter (WM) damage is associated with neurological functional deficits, but the underlying mechanisms are not well understood. In this study, we investigate whether endothelial nitric oxide synthase (eNOS) affects WM-damage post-stroke. Adult male wild-type (WT) and eNOS knockout (eNOS(-/-)) mice were subjected to middle cerebral artery occlusion. Functional evaluation, infarct volume measurement, immunostaining and primary cortical cell culture were performed. To obtain insight into the mechanisms underlying the effects of eNOS(-/-) on WM-damage, measurement of eNOS, brain-derived neurotrophic factor (BDNF) and its receptor TrkB in vivo and in vitro were also performed. No significant differences were detected in the infarction volume, myelin density in the ipsilateral striatal WM-bundles and myelin-based protein expression in the cerebral ischemic border between WT and eNOS(-/-) mice. However, eNOS(-/-) mice showed significantly: 1) decreased functional outcome, concurrent with decreases of total axon density and phosphorylated high-molecular weight neurofilament density in the ipsilateral striatal WM-bundles. Correlation analysis showed that axon density is significantly positive correlated with neurological functional outcome; 2) decreased numbers of oligodendrocytes / oligodendrocyte progenitor cells in the ipsilateral striatum; 3) decreased synaptophysin, BDNF and TrkB expression in the ischemic border compared with WT mice after stroke (n = 12/group, p<0.05). Primary cortical cell culture confirmed that the decrease of neuronal neurite outgrowth in the neurons derived from eNOS(-/-) mice is mediated by the reduction of BDNF/TrkB (n = 6/group, p<0.05). Our data show that eNOS plays a critical role in WM-damage after stroke, and eNOS(-/-)-induced decreases in the BDNF/TrkB pathway may contribute to increased WM-damage, and thereby decrease functional outcome.