Amyloidosis is associated with thicker myelin and increased oligodendrogenesis in the adult mouse brain

In Alzheimer's disease, amyloid plaque formation is associated with the focal death of oligodendrocytes and soluble amyloid β impairs the survival of oligodendrocytes in vitro. However, the response of oligodendrocyte progenitor cells (OPCs) to early amyloid pathology remains unclear. To explore this, we performed a histological, electrophysiological, and behavioral characterization of transgenic mice expressing a pathological form of human amyloid precursor protein (APP), containing three single point mutations associated with the development of familial Alzheimer's disease (PDGFB‐APPSw.Ind, also known as J20 mice). PDGFB‐APPSw.Ind transgenic mice had impaired survival from weaning, were hyperactive by 2 months of age, and developed amyloid plaques by 6 months of age, however, their spatial memory remained intact over this time course. Hippocampal OPC density was normal in P60‐P180 PDGFB‐APPSw.Ind transgenic mice and, by performing whole‐cell patch‐clamp electrophysiology, we found that their membrane properties, including their response to kainate (100 µM), were largely normal. However, by P100, the response of hippocampal OPCs to GABA was elevated in PDGFB‐APPSw.Ind transgenic mice. We also found that the nodes of Ranvier were shorter, the paranodes longer, and the myelin thicker for hippocampal axons in young adult PDGFB‐APPSw.Ind transgenic mice compared with wildtype littermates. Additionally, oligodendrogenesis was normal in young adulthood, but increased in the hippocampus, entorhinal cortex, and fimbria of PDGFB‐APPSw.Ind transgenic mice as pathology developed. As the new oligodendrocytes were not associated with a change in total oligodendrocyte number, these cells are likely required for cell replacement. Myelin structure is altered in the hippocampus of mice overexpressing a pathological variant of the human amyloid precursor protein (PDGFB‐APPSw.Ind; APP mice). APP mice have longer paranodes, thicker myelin, and shorter nodes of Ranvier than wildtype mice by 3 months of age, however, total axon density and the proportion of axons that were myelinated was normal. Between 4 and 6 months of age, APP mice have more new oligodendrocytes added to the hippocampus, entorhinal cortex, and fimbria, while total oligodendrocyte and oligodendrocyte progenitor cell density remained stable, suggesting that new oligodendrocytes may be produced to replace oligodendrocytes lost due to amyloid pathology.