Replenishing our mind orchards: Enhancing myelin renewal to rescue cognition in Alzheimer's disease

Temporal Alterations in White Matter in An App Knock-In Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia and results in neurodegeneration and cognitive impairment. White matter (WM) is affected in AD and has implications for neural circuitry and cognitive function. The trajectory of these changes across age, however, is still not well understood, especially at earlier stages in life. To address this, we used the AppNL-G-F/NL-G-F knock-in (APPKI) mouse model that harbors a single copy knock-in of the human amyloid precursor protein (APP) gene with three familial AD mutations. We performed in vivo diffusion tensor imaging (DTI) to study how the structural properties of the brain change across age in the context of AD. In late age APPKI mice, we observed reduced fractional anisotropy (FA), a proxy of WM integrity, in multiple brain regions, including the hippocampus, anterior commissure (AC), neocortex, and hypothalamus. At the cellular level, we observed greater numbers of oligodendrocytes in middle age (prior to observations in DTI) in both the AC, a major interhemispheric WM tract, and the hippocampus, which is involved in memory and heavily affected in AD, prior to observations in DTI. Proteomics analysis of the hippocampus also revealed altered expression of oligodendrocyte-related proteins with age and in APPKI mice. Together, these results help to improve our understanding of the development of AD pathology with age, and imply that middle age may be an important temporal window for potential therapeutic intervention.

Brain region-specific myelinogenesis is not directly linked to amyloid-β in APP/PS1 transgenic mice

Alzheimer's disease (AD) is characterized by aggregating amyloid beta-protein (Aβ). Recent evidence has shown that insufficient myelinogenesis contributes to AD-related functional deficits. However, it remains unclear whether Aβ, in either plaque or soluble form, could alter myelinogenesis in AD brains. By cell-lineage tracing and labeling, we found both myelinogenesis and Aβ deposits displayed a region-specific pattern in the 13-month-old APP/PS1 transgenic mouse brains. Aβ plaques cause focal demyelination, but only about 15% Aβ plaques are closely associated with newly formed myelin in the APP/PS1 brains. Further, the Aβ plaque total area and the amount of new myelin are not linearly correlated across different cortical regions, suggesting that Aβ plaques induce demyelination but may not exclusively trigger remyelination. To understand the role of soluble Aβ in regulating myelinogenesis, we chose to observe the visual system, wherein soluble Aβ is detectable but without the presence of Aβ plaques in the APP/PS1 retina, optic nerve, and optic tract. Interestingly, newly-formed myelin density was not significantly altered in the APP/PS1 optic nerves and optic tracts as compared to the wildtype controls, suggesting soluble Aβ probably does not change myelinogenesis. Further, treatment of purified oligodendrocyte precursor cells (OPCs) with soluble Aβ (oligomers) for 48 h did not change the cell densities of MBP positive cells and PDGFRα positive OPCs in vitro. Consistently, injection of soluble Aβ into the lateral ventricles did not alter myelinogenesis in the corpus callosum of NG2-CreErt; Tau-mGFP mice significantly. Together, these findings indicate that the region-dependent myelinogenesis in AD brains is not directly linked to Aβ, but rather probably a synergic result in adapting to AD pathology.

Unlocking the Memory Component of Alzheimer’s Disease:Biological Processes and Pathways across Brain Regions

Alzheimer’s Disease (AD) is a neurodegenerative disorder characterized by a progressive loss of memory and a general cognitive decline leading to dementia. AD is characterized by changes in the behavior of the genome and can be traced across multiple brain regions and cell types. It is mainly associated with β-amyloid deposits and tau protein misfolding, leading to neurofibrillary tangles. In recent years, however, research has shown that there is a high complexity of mechanisms involved in AD neurophysiology and functional decline enabling its diverse presentation and allowing more questions to arise. In this study, we present a computational approach to facilitate brain region-specific analysis of genes and biological processes involved in the memory process in AD. Utilizing current genetic knowledge we provide a gene set of 265 memory-associated genes in AD, combinations of which can be found co-expressed in 11 different brain regions along with their functional role. The identified genes participate in a spectrum of biological processes ranging from structural and neuronal communication to epigenetic alterations and immune system responses. These findings provide new insights into the molecular background of AD and can be used to bridge the genotype–phenotype gap and allow for new therapeutic hypotheses.