BACE1 Accumulation Research Articles

Axonal BACE1 dynamics and targeting in hippocampal neurons: a role for Rab11 GTPase

BackgroundBACE1 is one of the two enzymes that cleave amyloid precursor protein to generate Alzheimer's disease (AD) beta amyloid peptides. It is widely believed that BACE1 initiates APP processing in endosomes, and in the brain this cleavage is known to occur during axonal transport of APP. In addition, BACE1 accumulates in dystrophic neurites surrounding brain senile plaques in individuals with AD, suggesting that abnormal accumulation of BACE1 at presynaptic terminals contributes to pathogenesis in AD. However, only limited information is available on BACE1 axonal transport and targeting.ResultsBy visualizing BACE1-YFP dynamics using live imaging, we demonstrate that BACE1 undergoes bi-directional transport in dynamic tubulo-vesicular carriers along axons in cultured hippocampal neurons and in acute hippocampal slices of transgenic mice. In addition, a subset of BACE1 is present in larger stationary structures, which are active presynaptic sites. In cultured neurons, BACE1-YFP is preferentially targeted to axons over time, consistent with predominant in vivo localization of BACE1 in presynaptic terminals. Confocal analysis and dual-color live imaging revealed a localization and dynamic transport of BACE1 along dendrites and axons in Rab11-positive recycling endosomes. Impairment of Rab11 function leads to a diminution of total and endocytosed BACE1 in axons, concomitant with an increase in the soma. Together, these results suggest that BACE1 is sorted to axons in endosomes in a Rab11-dependent manner.ConclusionOur results reveal novel information on dynamic BACE1 transport in neurons, and demonstrate that Rab11-GTPase function is critical for axonal sorting of BACE1. Thus, we suggest that BACE1 transcytosis in endosomes contributes to presynaptic BACE1 localization.

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.

BACE1 Protein Endocytosis and Trafficking Are Differentially Regulated by Ubiquitination at Lysine 501 and the Di-leucine Motif in the Carboxyl Terminus

β-Site amyloid precursor protein-cleaving enzyme (BACE1) is a membrane-tethered member of the aspartyl proteases that has been identified as β-secretase. BACE1 is targeted through the secretory pathway to the plasma membrane and then is internalized to endosomes. Sorting of membrane proteins to the endosomes and lysosomes is regulated by the interaction of signals present in their carboxyl-terminal fragment with specific trafficking molecules. The BACE1 carboxyl-terminal fragment contains a di-leucine sorting signal ((495)DDISLL(500)) and a ubiquitination site at Lys-501. Here, we report that lack of ubiquitination at Lys-501 (BACE1K501R) does not affect the rate of endocytosis but produces BACE1 stabilization and accumulation of BACE1 in early and late endosomes/lysosomes as well as at the cell membrane. In contrast, the disruption of the di-leucine motif (BACE1LLAA) greatly impairs BACE1 endocytosis and produces a delayed retrograde transport of BACE1 to the trans-Golgi network (TGN) and a delayed delivery of BACE1 to the lysosomes, thus decreasing its degradation. Moreover, the combination of the lack of ubiquitination at Lys-501 and the disruption of the di-leucine motif (BACE1LLAA/KR) produces additive effects on BACE1 stabilization and defective internalization. Finally, BACE1LLAA/KR accumulates in the TGN, while its levels are decreased in EEA1-positive compartments indicating that both ubiquitination at Lys-501 and the di-leucine motif are necessary for the trafficking of BACE1 from the TGN to early endosomes. Our studies have elucidated a differential role for the di-leucine motif and ubiquitination at Lys-501 in BACE1 endocytosis, trafficking, and degradation and suggest the involvement of multiple adaptor molecules.

SUMO1 modulates Aβ generation via BACE1 accumulation

Accumulation of disease-related proteins is a characteristic event observed in the pathogenesis of neurodegenerative diseases. β-secretase (BACE)-1, which initiates generation of β-amyloid (Aβ), is increased in the Alzheimer's diseased brain. However, the mechanisms of BACE1 accumulation in Alzheimer's disease are largely unknown. In this report, we found that small ubiquitin-like modifier (SUMO)-1 interacts with the dileucine motif of BACE1 and regulates the level of BACE1 protein. This was proved by the coimmunoprecipitation, and gain or loss of function experiments. Altering 3 SUMO isoforms affects BACE1 protein levels, and consequently results in altered amyloid precursor protein processing and Aβ generation. BACE1 levels were increased in response to Aβ or apoptosis, but not in cells lacking SUMO1. Aβ increased SUMO1 protein levels in rat cortical neurons. Moreover, SUMO1 immunoreactivity was increased in the amyloid precursor protein transgenic mice. Furthermore, the C-terminus fragments of BACE1 containing dileucine motif reduced Aβ generation by SUMO1 overexpression. Our study indicates SUMO1 is not only a novel and potent regulator of BACE1 accumulation and Aβ generation but also a potential therapeutic target for Alzheimer's disease.

Aβ1‐42‐mediated down‐regulation of Uch‐L1 is dependent on NF‐κB activation and impaired BACE1 lysosomal degradation

Amyloid-β 1-42 accumulation is the major pathogenetic event in Alzheimer's disease (AD), believed to be responsible for synaptic dysfunction and neuronal cell death. However, the physiologic activity of Aβ peptides remains elusive: Aβ might not only play a toxic role, but also act as a functional signaling intermediate. We recently reported that Aβ1-42 promotes BACE1 transcription through the activation of the JNK-c-jun pathway. Here, we show that the Aβ1-42-mediated increase in BACE1 expression is accompanied by a decrease in ubiquitin C-terminal hydrolase L1 (Uch-L1) expression and activity in different cellular models such as neuroblastoma SH-SY5Y as well as NT(2) neuronal cells. We also found that the increase in BACE1 and the decrease in Uch-L1 are related events and depend on NF-κB pathway; thus, Aβ1-42 is able to activate NF-κB pathway and the pretreatment with a pharmacological inhibitor, able to block the nuclear translocation of the transactivating unit p65, almost completely prevents both the decrease in Uch-L1 and the increase in BACE1 expression. In addition, the decrease in Uch-L1 activity interferes with the lysosomal degradation of BACE1, as demonstrated by the decrease in Cathepsin D activity and the partial accumulation of BACE1 in lysosomes after Aβ1-42 treatment as well after Uch-L1 inhibition. In support of the in vitro data, we observed low protein levels of Uch-L1 associated with high protein levels of BACE1 in sporadic AD brains. Our data suggest that Uch-L1 could be an attractive target for the development of new therapeutic approaches for AD.

Autophagy: a common road to perdition in acute brain injuries and Alzheimer’s disease

Autophagy: a common road to perdition in acute brain injuries and Alzheimer’s disease

Palmitic acid-treated astrocytes induce BACE1 upregulation and accumulation of C-terminal fragment of APP in primary cortical neurons

High-fat diet is a significant risk factor for the development of Alzheimer's disease (AD). In addition, the AD brain is characterized by elevated levels of fatty acids as compared to that of healthy controls. Despite this, it is unclear how elevated levels of fatty acids are related to the pathogenesis of AD. The present study examines the role of saturated fatty acid, palmitic acid (PA), in causing BACE1 upregulation and consequent amyloidogenic processing of β-amyloid precursor protein (APP), one of the main characteristic signatures of AD pathology. Here, primary rat cortical neurons and astrocytes were treated with pathological concentration of PA. There was no change in the BACE1 levels in the rat cortical neurons treated directly with PA as compared to controls. The conditioned medium from PA-treated astrocytes, however, caused BACE1 upregulation in the cortical neurons. Moreover, there was a consequent increase in the cleavage of APP leading to the accumulation of the C-terminal fragment of APP (C99) in the cortical neurons. Co-treatment of neurons with 1,3-dimethyl urea (DMU), an antioxidant, decreased PA-induced upregulation in the levels of BACE1 and C99. The present results establish an important role of saturated fatty acids in AD-associated amyloidogenesis through astroglia-mediated oxidative stress.