Youth-associated platelet-derived chemokine reverses brain aging through neuroimmune mechanisms

Age-related loss of brain function has been seen as inevitable, yet recent work leveraging the systemic environment challenges this notion. Schroer et al. demonstrate that youth-associated platelet factor 4 (PF4) partially restores brain function in aged mice while reducing peripheral immune dysfunction, supporting periphery-based approaches to treat age-associated brain disorders.

Neuronal TIMP2 regulates hippocampus-dependent plasticity and extracellular matrix complexity

Functional output of the hippocampus, a brain region subserving memory function, depends on highly orchestrated cellular and molecular processes that regulate synaptic plasticity throughout life. The structural requirements of such plasticity and molecular events involved in this regulation are poorly understood. Specific molecules, including tissue inhibitor of metalloproteinases-2 (TIMP2) have been implicated in plasticity processes in the hippocampus, a role that decreases with brain aging as expression is lost. Here, we report that TIMP2 is highly expressed by neurons within the hippocampus and its loss drives changes in cellular programs related to adult neurogenesis and dendritic spine turnover with corresponding impairments in hippocampus-dependent memory. Consistent with the accumulation of extracellular matrix (ECM) in the hippocampus we observe with aging, we find that TIMP2 acts to reduce accumulation of ECM around synapses in the hippocampus. Moreover, its deletion results in hindrance of newborn neuron migration through a denser ECM network. A novel conditional TIMP2 knockout (KO) model reveals that neuronal TIMP2 regulates adult neurogenesis, accumulation of ECM, and ultimately hippocampus-dependent memory. Our results define a mechanism whereby hippocampus-dependent function is regulated by TIMP2 and its interactions with the ECM to regulate diverse processes associated with synaptic plasticity.

APOE4 homozygote females are resistant to the beneficial effects of 17β-estradiol on memory and CA1 dendritic spine density in the EFAD mouse model of Alzheimer's disease

Female APOE4 carriers are at greatest risk of Alzheimer's disease (AD). The potent estrogen 17β-estradiol (E2) may mediate AD risk, as the onset of memory decline coincides with the menopausal transition. Whether APOE genotype mediates E2's effects on memory and neuronal morphology is poorly understood. We used the APOE+/+/5xFAD+/- (EFAD) mouse model to examine how APOE3 homozygote (E3FAD), APOE3/4 heterozygote (E3/4FAD), and APOE4 homozygote (E4FAD) genotypes modulate effects of E2 on object and spatial memory consolidation, dendritic spine density, and dorsal hippocampal estrogen receptor expression in 6-month-old ovariectomized EFAD mice. Dorsal hippocampal E2 infusion enhanced memory consolidation and increased CA1 apical spine density in E3FAD and E3/4FAD, but not E4FAD, mice. CA1 basal mushroom spines were also increased by E2 in E3FADs. E4FAD mice exhibited reduced CA1 and mPFC basal spine density, and increased dorsal hippocampal ERα protein, independent of E2. Overall, E2 benefitted hippocampal memory and structural plasticity in females bearing one or no APOE4 allele, whereas two APOE4 alleles impeded the memory-enhancing and spinogenic effects of E2.

The detrimental effects of APOE4 on risk for Alzheimer's disease may result from altered dendritic spine density, synaptic proteins, and estrogen receptor alpha

Women carriers of APOE4, the greatest genetic risk factor for late-onset Alzheimer's disease (AD), are at highest risk of developing AD, yet factors underlying interactions between APOE4 and sex are not well characterized. Here, we examined how sex and APOE3 or APOE4 genotypes modulate object and spatial memory, dendritic spine density and branching, and protein expression in 6-month-old male and female E3FAD and E4FAD mice (APOE+/+/5xFAD+/-). APOE4 negatively impacted object recognition and spatial memory, with male E3FADs exhibiting the best memory across 2 object-based tasks. In both sexes, APOE4 reduced basal dendritic spine density in the medial prefrontal cortex and dorsal hippocampus. APOE4 reduced dorsal hippocampal levels of PDS-95, synaptophysin, and phospho-CREB, yet increased levels of ERα. E4FAD females exhibited strikingly increased GFAP levels, in addition to the lowest levels of PSD-95 and pCREB. Overall, our results suggest that APOE4 negatively impacts object memory, dendritic spine density, and levels of hippocampal synaptic proteins and ERα. However, the general lack of sex differences or sex by genotype interactions suggests that the sex-specific effects of APOE4 on AD risk may be related to factors unexplored in the present study.

APOE4 genotype or ovarian hormone loss influence open field exploration in an EFAD mouse model of Alzheimer's disease

Anxiety is a prominent and debilitating symptom in Alzheimer's disease (AD) patients. Carriers of APOE4, the greatest genetic risk factor for late-onset AD, may experience increased anxiety relative to carriers of other APOE genotypes. However, whether APOE4 genotype interacts with other AD risk factors to promote anxiety-like behaviors is less clear. Here, we used open field exploration to assess anxiety-like behavior in an EFAD mouse model of AD that expresses five familial AD mutations (5xFAD) and human APOE3 or APOE4. We first examined whether APOE4 genotype exacerbates anxiety-like exploratory behavior in the open field relative to APOE3 genotype in a sex-specific manner among six-month-old male and female E3FAD (APOE3^+/+/5xFAD^+/-) and E4FAD mice (APOE4^+/+/5xFAD^+/-). Next, we determined whether circulating ovarian hormone loss influences exploratory behavior in the open field among female E3FAD and E4FADs. APOE4 genotype was associated with decreased time in the center of the open field, particularly among female EFADs. Furthermore, ovariectomy (OVX) decreased time in the center of the open field among female E3FADs to levels similar to intact and OVXed E4FAD females. Our results suggest that APOE4 genotype increased anxiety-like behavior in the open field, and that ovarian hormones may protect against an anxiety-like phenotype in female E3FAD, but not E4FAD mice.

Dickkopf-1 blocks 17β-estradiol-enhanced object memory consolidation in ovariectomized female mice

The memory-enhancing effects of 17β-estradiol (E₂) depend upon rapid activation of several cell-signaling cascades within the dorsal hippocampus (DH). Among the many cell-signaling pathways that mediate memory processes, Wnt/β-catenin signaling has emerged as a potential key player because of its importance to hippocampal development and synaptic plasticity. However, whether E₂ interacts with Wnt/β-catenin signaling to promote memory consolidation is unknown. Therefore, the present study examined whether Wnt/β-catenin signaling within the DH is necessary for E₂-induced memory consolidation in ovariectomized mice tested in the object recognition and object placement tasks. Ovariectomized C57BL/6 mice received immediate post-training infusions of E₂ or vehicle into the dorsal third ventricle plus the endogenous Wnt/β-catenin antagonist Dickkopf-1 (Dkk-1) or vehicle into the DH to assess whether the memory-enhancing effects of E₂ depend on activation of Wnt/β-catenin signaling. Our results suggest that Dkk-1 blocks E₂-induced memory enhancement as hypothesized, but may do so by only moderately blunting Wnt/β-catenin signaling while concurrently activating Wnt/JNK signaling. The current study provides novel insights into the mechanisms through which E₂ enhances memory consolidation in the DH, as well as critical information about the mechanistic actions of Dkk-1.