The absence of myelin basic protein promotes neuroinflammation and reduces amyloid β-protein accumulation in Tg-5xFAD mice

BACKGROUND: Abnormal accumulation of amyloid β-protein (Aβ) in the brain plays an important role in the pathogenesis \of Alzheimer’s disease (AD). Aβ monomers assemble into oligomers and fibrils that promote neuronal dysfunction. This assembly pathway is influenced by naturally occurring brain molecules, the Aβ chaperone proteins, which bind to Aβ and modulate its aggregation. Myelin basic protein (MBP) was previously identified as a novel Aβ chaperone protein and a potent inhibitor for Aβ fibril assembly in vitro. METHODS: In this study, we determined whether the absence of MBP would influence Aβ pathology in vivo by breeding MBP knockout mice (MBP(-/-)) with Tg-5xFAD mice, a model of AD-like parenchymal Aβ pathology. RESULTS: Through biochemical and immunohistochemical experiments, we found that bigenic Tg-5xFAD/MBP(-/-) mice had a significant decrease of insoluble Aβ and parenchymal plaque deposition at an early age. The expression of transgene encoded human AβPP, the levels of C-terminal fragments generated during Aβ production and the intracellular Aβ were unaffected in the absence of MBP. Likewise, we did not find a significant difference in plasma Aβ or cerebrospinal fluid Aβ, suggesting these clearance routes were unaltered in bigenic Tg-5xFAD/MBP(-/-) mice. However, MBP(-/-) mice and bigenic Tg-5xFAD/MBP(-/-) mice exhibited elevated reactive astrocytes and activated microglia compared with Tg-5xFAD mice. The Aβ degrading enzyme matrix metalloproteinase 9 (MMP-9), which is expressed by activated glial cells, was significantly increased in the Tg-5xFAD/MBP(-/-) mice. CONCLUSIONS: These findings indicate that the absence of MBP decreases Aβ deposition in transgenic mice and that this consequence may result from increased glial activation and expression of MMP-9, an Aβ degrading enzyme.