Possession of the ε4 allele of apolipoprotein E (APOE) is the major genetic risk factor for late-onset Alzheimer's disease (AD). Although numerous hypotheses have been proposed, the precise cause of this increased AD risk is not yet known. In order to gain a more comprehensive understanding of APOE4's role in AD, we performed RNA-sequencing on an AD-vulnerable vs. an AD-resistant brain region from aged APOE targeted replacement mice. This transcriptomics analysis revealed a significant enrichment of genes involved in endosomal–lysosomal processing, suggesting an APOE4-specific endosomal–lysosomal pathway dysregulation in the brains of APOE4 mice. Further analysis revealed clear differences in the morphology of endosomal–lysosomal compartments, including an age-dependent increase in the number and size of early endosomes in APOE4 mice. These findings directly link the APOE4 genotype to endosomal–lysosomal dysregulation in an in vivo, AD pathology-free setting, which may play a causative role in the increased incidence of AD among APOE4 carriers.

APOE4 is a genetic risk factor for ageing-related cognitive decline and Alzheimer's disease. Using human and mouse tissue, Peng et al. report that APOE4 reduces brain exosome production and release. Exosome pathway dysfunction may be the initial event in endosomal-lysosomal-exosomal alterations contributing to neuronal vulnerability and neurodegenerative risk in APOE4-carriers.

While apolipoprotein (Apo)E4 is linked to increased incidence of Alzheimer’s disease (AD), there is growing evidence that it plays a role in functional brain irregularities that are independent of AD pathology. However, ApoE4-driven functional differences within olfactory processing regions have yet to be examined. Utilizing knock-in mice humanized to ApoE4 versus the more common ApoE3, we examined a simple olfactory perceptual memory that relies on the transfer of information from the olfactory bulb (OB) to the piriform cortex (PCX), the primary cortical region involved in higher order olfaction. In addition, we have recorded in vivo resting and odor-evoked local field potentials (LPF) from both brain regions and measured corresponding odor response magnitudes in anesthetized young (6-month-old) and middle-aged (12-month-old) ApoE mice. Young ApoE4 compared to ApoE3 mice exhibited a behavioral olfactory deficit coinciding with hyperactive odor-evoked response magnitudes within the OB that were not observed in older ApoE4 mice. Meanwhile, middle-aged ApoE4 compared to ApoE3 mice exhibited heightened response magnitudes in the PCX without a corresponding olfactory deficit, suggesting a shift with aging in ApoE4-driven effects from OB to PCX. Interestingly, the increased ApoE4-specific response in the PCX at middle-age was primarily due to a dampening of baseline spontaneous activity rather than an increase in evoked response power. Our findings indicate that early ApoE4-driven olfactory memory impairments and OB network abnormalities may be a precursor to later network dysfunction in the PCX, a region that not only is targeted early in AD, but may be selectively vulnerable to ApoE4 genotype.

Variation in environmental factors such as day length and social context greatly affects reproductive behavior and the brain areas that regulate these behaviors. One such behavior is song in songbirds, which males use to attract a mate during the breeding season. In these species the absence of a potential mate leads to an increase in the number of songs produced, while the presence of a mate greatly diminishes singing. Interestingly, although long days promote song behavior, producing song itself can promote the incorporation of new neurons in brain regions controlling song output. Social context can also affect such neuroplasticity in these song control nuclei. The goal of the present study was to investigate in canaries (Serinus canaria), a songbird species, how photoperiod and social context affect song and the incorporation of new neurons, as measured by the microtubule-associated protein, doublecortin (DCX), in HVC, a key vocal production brain region of the song control system. We show that long days increased HVC size and singing activity. In addition, male canaries paired with a female for two weeks showed enhanced DCX-immunoreactivity in HVC relative to birds housed alone. Strikingly, however, paired males sang fewer songs that exhibited a reduction in acoustic features such as song complexity and energy, compared to birds housed alone, which sang prolifically. These results show that social presence plays a significant role in the regulation of neural and behavioral plasticity in songbirds and can exert these effects in opposition to what might be expected based on activity-induced neurogenesis.

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