Midkine as a factor to counteract the deposition of amyloid β-peptide plaques: in vitro analysis and examination in knockout mice

Midkine is a heparin-binding cytokine involved in cell survival and various inflammatory processes. Midkine accumulates in senile plaques of patients with Alzheimer's disease, while it counteracts the cytotoxic effects of amyloid β-peptide and inhibits its oligomerization. The present study was conducted to understand the role of midkine upon plaque formation of amyloid β-peptide. A surface plasmon assay was performed to determine the affinity of midkine for amyloid β-peptide. The deposition of amyloid β-peptide was compared in the brain of wild-type and midkine-deficient mice. An effect of midkine to microglias was examined by cell migration assay. Midkine bound to amyloid β-peptide with the affinity of 160 nM. The C-terminal half bound to the peptide more strongly than the N-terminal half, and heparin inhibited midkine from binding to the peptide. Pleiotrophin, which has about 50% sequence identity with midkine also bound to amyloid β-peptide. The deposition of amyloid β-peptide plaques in the cortex and hippocampus was more intense in 15-month-old midkine-deficient mice, compared to the corresponding wild-type mice. Midkine promoted migration of microglias in culture. These results are consistent with the view that midkine attenuates the deposition of amyloid β-peptide plaques, and thus progression of Alzheimer's disease, by direct binding and also by promoting migration of microglias.