Kaempferol Inhibits Extra-synaptic NMDAR-Mediated Downregulation of TRkβ in Rat Hippocampus During Hypoxia

Abstract

Cognitive dysfunction on chronic exposure to hypobaric hypoxia has been attributed to a myriad of survival and degenerative factors. Downregulation of Trkβ and compromised survival signaling has been ascribed as a major contributing factor for hypoxic neurodegeneration. The mechanisms leading to downregulation of Trkβ in hypoxia, however, remain to be elucidated. The present study aimed at investigating the upstream signaling mechanisms leading to Trkβ downregulation in hypoxia and the potential of Kaempferol in ameliorating these changes. Our results showed a duration-dependent increase in hypoxic neurodegeneration as measured by Fluoro-Jade C staining of hippocampal CA3 neurons. Protein expression studies revealed strong correlation of Trkβ with NR1 and NR2b expression on exposure to hypoxic stress. Administration of Kaempferol during hypoxic stress revealed its neuroprotective effect and Morris Water Maze test also highlighted its efficacy in improving spatial learning and memory. Further elucidation of the signaling mechanisms using specific inhibitors and in vitro silencing experiments confirmed involvement of extra-synaptic N-methyl-d-aspartate receptor (NMDAR) i.e. NR2b receptor subunit in downregulation of Trkβ under hypoxic conditions. ChIP assay showed involvement of E47 transcription factor in NR2b mediated Trkβ downregulation. Selective inhibition of signaling intermediate MLK2 by CEP11004 and inhibition of extra-synaptic NMDAR during hypoxic stress prevented Trkβ downregulation in the hippocampus of hypoxic rats. Administration of Kaempferol also inhibited phosphorylation of E47 and hypoxia-induced downregulation of Trkβ. The present study establishes the role of extra-synaptic NMDAR in hypoxia-induced downregulation of Trkβ and the efficacy of Kaempferol in inhibiting extra-synaptic NMDAR-mediated signaling.