Although aging and apolipoprotein E (APOE) ε4 allele have been documented as two major risk factors for late‐onset Alzheimer's disease (LOAD), their interaction and potential underlying mechanisms remain unelucidated. Using humanized ApoE4‐ and ApoE3‐ target replacement mice, we found the accumulation of senescent neurons and the activation of mTOR and endosome‐lysosome‐autophagy (ELA) system in the hippocampus of aged ApoE4 mice. Further analyses revealed that ApoE4 aggravated the profile change of hippocampal transcription and metabolism in an age‐dependent manner, accompanying with an disruption of metabolism, which is presented with the downregulating activity of citrate synthase, the level of ATP and, most importantly, the level of acetyl coenzyme A (Ac‐CoA); GTA supplement, an Ac‐CoA substrate, reversed the senescent characteristics, decreased the activation of mTOR and ELA system, and enhanced the synaptic structure and increasing level of pre‐/post‐synaptic plasticity‐related protein, leading to cognitive improvement in aged ApoE4 mice. These data suggest that ApoE4 exacerbates neuronal senescence due to a deficiency of acetyl‐CoA, which can be ameliorated by GTA supplement. The findings provide novel insights into the potential therapeutic value of GTA supplement for the cognitive improvement in aged APOE4 carriers.

Background: Defective autophagic flux is implicated in Alzheimer's disease (AD), but the molecular mechanisms underlying this process are not fully understood. Salt-inducible kinase 2 (SIK2) is associated with autophagic function. However, its specific involvement in autophagic flux regulation and AD pathogenesis remains unclear. Methods: We evaluated hippocampal SIK2 expression and its age-related changes in postmortem AD patients and 5 × FAD mice by bioinformatics analysis, immunofluorescence, qPCR, and Western blotting. To investigate the functional role of SIK2, we employed adeno-associated virus-mediated SIK2 knockdown and overexpression in combination with behavioral tests (Morris water maze), electrophysiological recordings (long-term potentiation, LTP), and ultrastructural analysis (electron microscopy) to evaluate cognitive function and synaptic plasticity. Autophagic flux was measured using LC3B/p62 turnover assays, mRFP-GFP-LC3 tandem fluorescence assay, and transmission electron microscopy. Mechanistic insights were gained through co-immunoprecipitation assay, GST-pull down assay, phosphoproteomics, and site-directed mutagenesis. Additionally, phosphorylation-mimetic (S72E) and non-phosphorylatable (S72A) mutants of GABA type A receptor-associated protein-like 2 (GABARAPL2) were intrahippocampally delivered to 5 × FAD mice to explore their effects. Results: Our study identified SIK2 as a critical regulator that is progressively downregulated in hippocampal neurons of AD patients and 5 × FAD mice, correlating with spatial memory deficits. Reducing SIK2 levels exacerbates cognitive impairment and amyloid-β (Aβ) plaque burden in mice, whereas restoring SIK2 levels mitigates these deficits, restores LTP amplitude, reverses synaptic ultrastructural pathology, and reduces Aβ deposition. Mechanistically, SIK2 enhances autophagic flux by phosphorylating GABARAPL2 at Ser72, a modification essential for autophagosome–lysosome fusion. Remarkably, hippocampal delivery of the phosphorylation-mimetic GABARAPL2-S72E mutant replicated the beneficial effects of SIK2, alleviating Aβ pathology and synaptic dysfunction in 5 × FAD mice. In contrast, the non-phosphorylatable S72A mutant failed to show any protective effects. Conclusions: These findings establish the SIK2–GABARAPL2 axis as a novel signaling cascade governing autophagic flux through lysosomal fusion competence. Dysfunction in this axis contributes to Aβ deposition in AD, offering new insights into the pathogenic mechanisms underlying autophagosome–lysosome fusion in AD and highlighting its potential as a therapeutic target. Graphical

Objectives: This study attempted to investigate whether early life stress (ELS) induces lipid peroxidation in the prefrontal cortex (PFC) of mice and its correlation with depression-like behavioral changes. Methods: A mouse ELS model was established via maternal separation. Depressive and anxiety-like behaviors were assessed using the forced swim test, sucrose preference test, tail suspension test, and open field test. The expression levels of GPX4, SLC3A2, SLC7A11, TFR1, and lipid peroxidation markers in the PFC of mice were measured and correlated with depression-like behavioral changes. Results: ELS induced depressive and anxiety-like behaviors in mice. The mRNA and protein expressions of GPX4, SLC3A2, and SLC7A11 were downregulated in the PFC of ELS mice; the mRNA level of TFR1 was upregulated but its protein level remained unchanged. In the PFC of ELS mice, the product of lipid peroxidation, malondialdehyde, was significantly increased, while the antioxidants, glutathione and superoxide dismutase, were significantly decreased. These markers were significantly correlated with sucrose preference or immobility time of the ELS mice. Conclusions: The study evidences that early life stress can induce lipid peroxidation in the PFC of mice and that the latter is closely associated with depression-like behaviors, hinting that ELS may induce depression via lipid peroxidation in the PFC. These findings may suggest a potential strategy for the treatment of early-stage stress-related depression.