Regulation of Synaptic Amyloid-β Generation through BACE1 Retrograde Transport in a Mouse Model of Alzheimer's Disease.

Amyloid-β (Aβ) peptides play a key role in synaptic damage and memory deficits in the early pathogenesis of Alzheimer's disease (AD). Abnormal accumulation of Aβ at nerve terminals leads to synaptic pathology and ultimately to neurodegeneration. β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is the major neuronal β-secretase for Aβ generation. However, the mechanisms regulating BACE1 distribution in axons and β cleavage of APP at synapses remain largely unknown. Here, we reveal that dynein-Snapin-mediated retrograde transport regulates BACE1 trafficking in axons and APP processing at presynaptic terminals. BACE1 is predominantly accumulated within late endosomes at the synapses of AD-related mutant human APP (hAPP) transgenic (Tg) mice and patient brains. Defective retrograde transport by genetic ablation of snapin in mice recapitulates late endocytic retention of BACE1 and increased APP processing at presynaptic sites. Conversely, overexpressing Snapin facilitates BACE1 trafficking and reduces synaptic BACE1 accumulation by enhancing the removal of BACE1 from distal AD axons and presynaptic terminals. Moreover, elevated Snapin expression via stereotactic hippocampal injections of adeno-associated virus particles in mutant hAPP Tg mouse brains decreases synaptic Aβ levels and ameliorates synapse loss, thus rescuing cognitive impairments associated with hAPP mice. Altogether, our study provides new mechanistic insights into the complex regulation of BACE1 trafficking and presynaptic localization through Snapin-mediated dynein-driven retrograde axonal transport, thereby suggesting a potential approach of modulating Aβ levels and attenuating synaptic deficits in AD.SIGNIFICANCE STATEMENT β-Site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) trafficking and synaptic localization significantly influence its β secretase activity and amyloid-β (Aβ) production. In AD brains, BACE1 is accumulated within dystrophic neurites, which is thought to augment Aβ-induced synaptotoxicity by Aβ overproduction. However, it remains largely unknown whether axonal transport regulates synaptic APP processing. Here, we demonstrate that Snapin-mediated retrograde transport plays a critical role in removing BACE1 from presynaptic terminals toward the soma, thus reducing synaptic Aβ production. Adeno-associated virus-mediated Snapin overexpression in the hippocampus of mutant hAPP mice significantly decreases synaptic Aβ levels, attenuates synapse loss, and thus rescues cognitive deficits. Our study uncovers a new pathway that controls synaptic APP processing by enhancing axonal BACE1 trafficking, thereby advancing our fundamental knowledge critical for ameliorating Aβ-linked synaptic pathology.

Ye X ，Feng T ，Tammineni P ，Chang Q ，Jeong YY ，Margolis DJ ，Cai H ，Kusnecov A ，Cai Q ... -  《-》

Presynaptic dystrophic neurites surrounding amyloid plaques are sites of microtubule disruption, BACE1 elevation, and increased Aβ generation in Alzheimer's disease.

Sadleir KR ，Kandalepas PC ，Buggia-Prévot V ，Nicholson DA ，Thinakaran G ，Vassar R ... -  《-》

Snapin-mediated BACE1 retrograde transport is essential for its degradation in lysosomes and regulation of APP processing in neurons.

β site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1) is the major β secretase for generating β-amyloid (Aβ) peptides. The acidic environment of endosomes is optimal for β secretase activity. However, the mechanisms regulating BACE1 traffic from endosomes to lysosomes for degradation are largely unknown. Here, using snapin-deficient mice combined with gene rescue experiments, we reveal that Snapin, as a dynein motor adaptor for late endosomes, mediates BACE1 retrograde transport. hAPP mutant live neurons and mouse brains exhibited BACE1 accumulation within the altered late endocytic organelles and defective lysosomal targeting due to reduced Snapin-dynein coupling. Deleting snapin or disrupting Snapin-dynein coupling reduces BACE1 transport to lysosomes for degradation, thus enhancing APP processing. Overexpressing Snapin in hAPP neurons reduces β site cleavage of APP by enhancing BACE1 turnover. Altogether, our study provides mechanistic insights into the complex regulation of BACE1 level and activity and turnover through retrograde transport, thus controlling Aβ generation in neurons.

Ye X ，Cai Q 《Cell Reports》

Autophagy-mediated Regulation of BACE1 Protein Trafficking and Degradation.

β-Site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is the major neuronal β-secretase for amyloid-β generation and is degraded in lysosomes. The autophagy-lysosomal system plays a key role in the maintenance of cellular homeostasis in neurons. Recent studies established that nascent autophagosomes in distal axons move predominantly in the retrograde direction toward the soma, where mature lysosomes are mainly located. However, it remains unknown whether autophagy plays a critical role in regulation of BACE1 trafficking and degradation. Here, we report that induction of neuronal autophagy enhances BACE1 turnover, which is suppressed by lysosomal inhibition. A significant portion of BACE1 is recruited to the autophagy pathway and co-migrates robustly with autophagic vacuoles along axons. Moreover, we reveal that autophagic vacuole-associated BACE1 is accumulated in the distal axon of Alzheimer's disease-related mutant human APP transgenic neurons and mouse brains. Inducing autophagy in mutant human APP neurons augments autophagic retention of BACE1 in distal axons, leading to enhanced β-cleavage of APP. This phenotype can be reversed by Snapin-enhanced retrograde transport, which facilitates BACE1 trafficking to lysosomes for degradation. Therefore, our study provides new insights into autophagy-mediated regulation of BACE1 turnover and APP processing, thus building a foundation for future development of potential Alzheimer's disease therapeutic strategies.

Feng T ，Tammineni P ，Agrawal C ，Jeong YY ，Cai Q ... -  《-》

P38α-MAPK phosphorylates Snapin and reduces Snapin-mediated BACE1 transportation in APP-transgenic mice.

Amyloid β peptide (Aβ) is the major pathogenic molecule in Alzheimer's disease (AD). BACE1 enzyme is essential for the generation of Aβ. Deficiency of p38α-MAPK in neurons increases lysosomal degradation of BACE1 and decreases Aβ deposition in the brain of APP-transgenic mice. However, the mechanisms mediating effects of p38α-MAPK are largely unknown. In this study, we used APP-transgenic mice and cultured neurons and observed that deletion of p38α-MAPK specifically in neurons decreased phosphorylation of Snapin at serine, increased retrograde transportation of BACE1 in axons and reduced BACE1 at synaptic terminals, which suggests that p38α-MAPK deficiency promotes axonal transportation of BACE1 from its predominant locations, axonal terminals, to lysosomes in the cell body. In vitro kinase assay revealed that p38α-MAPK directly phosphorylates Snapin. By further performing mass spectrometry analysis and site-directed mutagenic experiments in SH-SY5Y cell lines, we identified serine residue 112 as a p38α-MAPK-phosphorylating site on Snapin. Replacement of serine 112 with alanine did abolish p38α-MAPK knockdown-induced reduction of BACE1 activity and protein level, and transportation to lysosomes in SH-SY5Y cells. Taken together, our study suggests that activation of p38α-MAPK phosphorylates Snapin and inhibits the retrograde transportation of BACE1 in axons, which might exaggerate amyloid pathology in AD brain.

Schnöder L ，Tomic I ，Schwindt L ，Helm D ，Rettel M ，Schulz-Schaeffer W ，Krause E ，Rettig J ，Fassbender K ，Liu Y ... -  《-》