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 f...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 Aβdeposition.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 Aβpathology and synaptic dysfunction in 5×FAD mice.In contrast,the nonphosphorylatable 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 Aβdeposition in AD,offering new insights into the pathogenic mechanisms underlying autophagosome–lysosome fusion in AD and highlighting its potential as a therapeutic target.展开更多
基金supported in part by National Natural Science Foundation of China(82101481,U21A20362,U22A20298)National Science and Technology Innovation 2030 Major Projects(2022ZD0211600)+1 种基金Excellent Young Scholars Cultivation Project of Fujian Medical University Union Hospital(2022XH032)Joint Funds for the Innovation of Science and Technology,Fujian province(2024Y9236).
文摘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 Aβdeposition.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 Aβpathology and synaptic dysfunction in 5×FAD mice.In contrast,the nonphosphorylatable 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 Aβdeposition in AD,offering new insights into the pathogenic mechanisms underlying autophagosome–lysosome fusion in AD and highlighting its potential as a therapeutic target.
