The role of APP and BACE1 trafficking in APP processing and amyloid-β generation

Design, synthesis and multitarget biological profiling of second-generation anti-Alzheimer rhein-huprine hybrids

AIM

Simultaneous modulation of several key targets of the pathological network of Alzheimer's disease (AD) is being increasingly pursued as a promising option to fill the critical gap of efficacious drugs against this condition.

MATERIALS & METHODS

A short series of compounds purported to hit multiple targets of relevance in AD has been designed, on the basis of their distinct basicities estimated from high-level quantum mechanical computations, synthesized, and subjected to assays of inhibition of cholinesterases, BACE-1, and Aβ42 and tau aggregation, of antioxidant activity, and of brain permeation.

RESULTS

Using, as a template, a lead rhein-huprine hybrid with an interesting multitarget profile, we have developed second-generation compounds, designed by the modification of the huprine aromatic ring. Replacement by [1,8]-naphthyridine or thieno[3,2-e]pyridine systems resulted in decreased, although still potent, acetylcholinesterase or BACE-1 inhibitory activities, which are more balanced relative to their Aβ42 and tau antiaggregating and antioxidant activities.

CONCLUSION

Second-generation naphthyridine- and thienopyridine-based rhein-huprine hybrids emerge as interesting brain permeable compounds that hit several crucial pathogenic factors of AD.

Unraveling the genes implicated in Alzheimer's disease

Alzheimer's disease (AD) is a heterogeneous neurodegenerative disorder and it is the most common form of dementia in the elderly. Early onset AD is caused by mutations in three genes: Amyloid-β precursor protein, presenilin 1 (PSEN1) and PSEN2. Late onset AD (LOAD) is complex and apolipoprotein E is the only unanimously accepted genetic risk factor for its development. Various genes implicated in AD have been identified using advanced genetic technologies, however, there are many additional genes that remain unidentified. The present review highlights the genetics of early and LOAD and summarizes the genes involved in different signaling pathways. This may provide insight into neurodegenerative disease research and will facilitate the development of effective strategies to combat AD.

Low-Density Lipoprotein Receptor-Related Protein-1 (LRP1) C4408R Mutant Promotes Amyloid Precursor Protein (APP) α-Cleavage in Vitro

Previous studies have demonstrated that the low-density lipoprotein receptor-related protein-1 (LRP1) plays conflicting roles in Alzheimer's disease (AD) pathogenesis, clearing β-amyloid (Aβ) from the brain while also enhancing APP endocytosis and resultant amyloidogenic processing. We have recently discovered that co-expression of mutant LRP1 C-terminal domain (LRP1-CT C4408R) with Swedish mutant amyloid precursor protein (APPswe) in Chinese hamster ovary (CHO) cells decreases Aβ production, while also increasing sAPPα and APP α-C-terminal fragment (α-CTF), compared with CHO cells expressing APPswe alone. Surprisingly, the location of this mutation on LRP1 corresponded with the α-secretase cleavage site of APP. Further experimentation confirmed that in CHO cells expressing APPswe or wild-type APP (APPwt), co-expression of LRP1-CT C4408R decreases Aβ and increases sAPPα and α-CTF compared with co-expression of wild-type LRP1-CT. In addition, LRP1-CT C4408R enhanced the unglycosylated form of LRP1-CT and reduced APP endocytosis as determined by flow cytometry. This finding identifies a point mutation in LRP1 which slows LRP1-CT-mediated APP endocytosis and amyloidogenic processing, while enhancing APP α-secretase cleavage, thus demonstrating a potential novel target for slowing AD pathogenesis.

Icariside II Effectively Reduces Spatial Learning and Memory Impairments in Alzheimer's Disease Model Mice Targeting Beta-Amyloid Production

Icariside II (ICS II) is a broad-spectrum anti-cancer natural compound extracted from Herba Epimedii Maxim. Recently, the role of ICS II has been investigated in central nervous system, especially have a neuroprotective effect in Alzheimer’s disease (AD). In this study, we attempted to investigate the effects of ICS II, on cognitive deficits and beta-amyloid (Aβ) production in APPswe/PS1dE9 (APP/PS1) double transgenic mice. It was found that chronic ICS II administrated not only effectively ameliorated cognitive function deficits, but also inhibited neuronal degeneration and reduced the formation of plaque burden. ICS II significantly suppressed Aβ production via promoting non-amyloidogenic APP cleavage process by up-regulating a disintegrin and metalloproteinase domain 10 (ADAM10) expression, inhibited amyloidogenic APP processing pathway by down-regulating amyloid precursor protein (APP) and β-site amyloid precursor protein cleavage enzyme 1 (BACE1) expression in APP/PS1 transgenic mice. Meanwhile, ICS II attenuated peroxisome proliferator-activated receptor-γ (PPARγ) degradation as well as inhibition of eukaryotic initiation factor α phosphorylation (p-eIF2α) and PKR endoplasmic reticulum regulating kinase phosphorylation (p-PERK). Moreover, phosphodiesterase type 5 inhibitors (PDE5-Is) have recently emerged as a possible therapeutic target for cognitive enhancement via inhibiting Aβ levels, and we also found that ICS II markedly decreased phosphodiesterase-5A (PDE5A) expression. In conclusion, the present study demonstrates that ICS II could attenuate spatial learning and memory impairments in APP/PS1 transgenic mice. This protection appears to be due to the increased ADAM10 expression and decreased expression of both APP and BACE1, resulting in inhibition of Aβ production in the hippocampus and cortex. Inhibition of PPARγ degradation and PERK/eIF2α phosphorylation are involved in the course, therefore suggesting that ICS II might be a promising potential compound for the treatment of AD.

Memantine reduces the production of amyloid-β peptides through modulation of amyloid precursor protein trafficking

Memantine, an uncompetitive glutamatergic N-methyl-D-aspartate (NMDA) receptor antagonist, is widely used as medication for the treatment of Alzheimer's disease (AD). It has been reported that memantine reduces amyloid-β peptide (Aβ) levels in both neuronal cultures and in brains of animal models of AD. However, the underlying mechanism of these effects is unclear. Here we examined the effect of memantine on Aβ production. Memantine was administered to 9-month-old Tg2576 mice, a transgenic mouse model of AD, at 10 or 20mg/kg/day in drinking water for 1 month. Memantine significantly reduced the amounts of both CHAPS-soluble and CHAPS-insoluble Aβ in the brains of Tg2576 mice. Memantine at 10mg/kg/day for 1 month also reduced the levels of insoluble Aβ42 in the brains of aged F344 rats. Moreover, memantine reduced Aβ and sAPPβ levels in conditioned media from rat primary cortical cultures without affecting the enzymatic activities of α-secretase, β-secretase, or γ-secretase. Notably, in a cell-surface biotinylation assay, memantine increased the amount of amyloid precursor protein (APP) at the cell surface without changing the total amount of APP. Collectively, our results indicate that chronic treatment with memantine reduces the levels of Aβ both in AD models and in aged animals, and that memantine affects the endocytosis pathway of APP, which is required for β-secretase-mediated cleavage. This leads to a reduction in Aβ production. These results suggest that memantine reduces Aβ production and plaque deposition through the regulation of intracellular trafficking of APP.

Sex Hormone Decline and Amyloid β Synthesis, Transport and Clearance in the Brain

Sex hormones (SH) are essential regulators of the central nervous system. The decline in SH levels along with ageing may contribute to compromised neuroprotection and set the grounds for neurodegeneration and cognitive impairments. In Alzheimer's disease, besides other pathological features, there is an imbalance between amyloid β (Aβ) production and clearance, leading to its accumulation in the brain of older subjects. Aβ accumulation is a primary cause for brain inflammation and degeneration, as well as concomitant cognitive decline. There is mounting evidence that SH modulate Aβ production, transport and clearance. Importantly, SH regulate most of the molecules involved in the amyloidogenic pathway, their transport across brain barriers for elimination, and their degradation in the brain interstitial fluid. This review brings together data on the regulation of Aβ production, metabolism, degradation and clearance by SH.

The rise and fall of insulin signaling in Alzheimer's disease

The prevalence of both diabetes and Alzheimer's disease (AD) are reaching epidemic proportions worldwide. Alarmingly, diabetes is also a risk factor for Alzheimer's disease. The AD brain is characterised by the accumulation of peptides called Aβ as plaques in the neuropil and hyperphosphorylated tau protein in the form of neurofibrillary tangles within neurons. How diabetes confers risk is unknown but a simple linear relationship has been proposed whereby the hyperinsulinemia associated with type 2 diabetes leads to decreased insulin signaling in the brain, with downregulation of the PI3K/AKT signalling pathway and its inhibition of the major tau kinase, glycogen synthase kinase 3β. The earliest studies of post mortem AD brain tissue largely confirmed this cascade of events but subsequent studies have generally found either an upregulation of AKT activity, or that the relationship between insulin signaling and AD is independent of glycogen synthase kinase 3β altogether. Given the lack of success of beta-amyloid-reducing therapies in clinical trials, there is intense interest in finding alternative or adjunctive therapeutic targets for AD. Insulin signaling is a neuroprotective pathway and represents an attractive therapeutic option. However, this incredibly complex signaling pathway is not fully understood in the human brain and particularly in the context of AD. Here, we review the ups and downs of the research efforts aimed at understanding how diabetes modifies AD risk.

BIN1 regulates BACE1 intracellular trafficking and amyloid-β production

BIN1 is a genetic risk factor of late-onset Alzheimer disease (AD), which was identified in multiple genome-wide association studies. BIN1 is a member of the amphiphysin family of proteins, and contains N-terminal Bin-Amphiphysin-Rvs and C-terminal Src homology 3 domains. BIN1 is widely expressed in the mouse and human brains, and has been reported to function in the endocytosis and the endosomal sorting of membrane proteins. BACE1 is a type 1 transmembrane aspartyl protease expressed predominantly in neurons of the brain and responsible for the production of amyloid-β peptide (Aβ). Here we report that the depletion of BIN1 increases cellular BACE1 levels through impaired endosomal trafficking and reduces BACE1 lysosomal degradation, resulting in increased Aβ production. Our findings provide a mechanistic role of BIN1 in the pathogenesis of AD as a novel genetic regulator of BACE1 levels and Aβ production.

APP controls the formation of PI(3,5)P(2) vesicles through its binding of the PIKfyve complex

Phosphoinositides are signalling lipids that are crucial for major signalling events as well as established regulators of membrane trafficking. Control of endosomal sorting and endosomal homeostasis requires phosphatidylinositol-3-phosphate (PI(3)P) and phosphatidylinositol-3,5-bisphosphate (PI(3,5)P2), the latter a lipid of low abundance but significant physiological relevance. PI(3,5)P2 is formed by phosphorylation of PI(3)P by the PIKfyve complex which is crucial for maintaining endosomal homeostasis. Interestingly, loss of PIKfyve function results in dramatic neurodegeneration. Despite the significance of PIKfyve, its regulation is still poorly understood. Here we show that the Amyloid Precursor Protein (APP), a central molecule in Alzheimer's disease, associates with the PIKfyve complex (consisting of Vac14, PIKfyve and Fig4) and that the APP intracellular domain directly binds purified Vac14. We also show that the closely related APP paralogues, APLP1 and 2 associate with the PIKfyve complex. Whether APP family proteins can additionally form direct protein-protein interaction with PIKfyve or Fig4 remains to be explored. We show that APP binding to the PIKfyve complex drives formation of PI(3,5)P2 positive vesicles and that APP gene family members are required for supporting PIKfyve function. Interestingly, the PIKfyve complex is required for APP trafficking, suggesting a feedback loop in which APP, by binding to and stimulating PI(3,5)P2 vesicle formation may control its own trafficking. These data suggest that altered APP processing, as observed in Alzheimer's disease, may disrupt PI(3,5)P2 metabolism, endosomal sorting and homeostasis with important implications for our understanding of the mechanism of neurodegeneration in Alzheimer's disease.

Vascular cognitive impairment: Modeling a critical neurologic disease in vitro and in vivo

BACKGROUND

Vascular contributions to cognitive impairment and dementia (VCID) is a complex form of dementia, combining aspects of vascular disease and other forms of dementia, such as Alzheimer's disease. VCID encompasses a wide spectrum of cerebrovascular-driven cognitive impairment, from mild cognitive impairment to fully developed dementia. This disease state is further complicated by metabolic disorders, such as type 2 diabetes and hypertension, and lifestyle factors, like obesity and high fat diets.

SCOPE OF REVIEW

This manuscript is meant to both define VCID and review the in vitro and in vivo models of the disease state. This includes in vitro models of the neurovascular unit, models of chronic cerebral hypoperfusion, animals with NOTCH3 mutations as a model of small vessel disease, large animals with cerebral amyloid angiopathy (CAA), and animal models of mixed dementia.

MAJOR CONCLUSIONS

Synthetic microvessels are a promising technique to study the neurovascular unit and canines, despite the cost, are an excellent model to study CAA. While there are several good models of individual aspects of VCID, the heterogeneity of the disease states prevents them from being a model of all aspects of the disease. Therefore, VCID needs to be further defined into disease states that exist within this umbrella term. This includes specific guidelines for stroke counts and stroke locations and further categorization of overlapping cerebrovascular and AD pathologies that contribute to dementia. This will allow for better models and a more thorough understanding of how vascular disease contributes to dementia.

GENERAL SIGNIFICANCE

VCID is the second most common form of dementia and is expected to increase in coming years. The heterogeneity of VCID makes it difficult to study, but without better definitions and models, VCID presents a major public health problem for our aging population. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia, edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Neuroproteomic profiling of human body fluids

Analysis of protein expression and abundance provides a possibility to extend the current knowledge on disease-associated processes and pathways. The human brain is a complex organ and dysfunction or damage can give rise to a variety of neurological diseases. Although many proteins potentially reflecting disease progress are originating from brain, the scarce availability of human tissue material has lead to utilization of body fluids such as cerebrospinal fluid and blood in disease-related research. Within the most common neurological disorders, much effort has been spent on studying the role of a few hallmark proteins in disease pathogenesis but despite extensive investigation, the signatures they provide seem insufficient to fully understand and predict disease progress. In order to expand the view the field of neuroproteomics has lately emerged alongside developing technologies, such as affinity proteomics and mass spectrometry, for multiplexed and high-throughput protein profiling. Here, we provide an overview of how such technologies have been applied to study neurological disease and we also discuss some important considerations concerning discovery of disease-associated profiles.

Significance of transcytosis in Alzheimer's disease: BACE1 takes the scenic route to axons

Neurons have developed elaborate mechanisms for sorting of proteins to their destination in dendrites and axons as well as dynamic local trafficking. Recent evidence suggests that polarized axonal sorting of β-site converting enzyme 1 (BACE1), a type I transmembrane aspartyl protease involved in Alzheimer's disease (AD) pathogenesis, entails an unusual journey. In hippocampal neurons, BACE1 internalized from dendrites is conveyed in recycling endosomes via unidirectional retrograde transport towards the soma and sorted to axons where BACE1 becomes enriched. In comparison to other transmembrane proteins that undergo transcytosis or elimination in somatodendritic compartment, vectorial transport of internalized BACE1 in dendrites is unique and intriguing. Dysfunction of protein transport contributes to pathogenesis of AD and other neurodegenerative diseases. Therefore, characterization of BACE1 transcytosis is an important addition to the multiple lines of evidence that highlight the crucial role played by endosomal trafficking pathway as well as axonal sorting mechanisms in AD pathogenesis.

Amyloid β: one of three danger-associated molecules that are secondary inducers of the proinflammatory cytokines that mediate Alzheimer's disease

This review concerns how the primary inflammation preceding the generation of certain key damage-associated molecular patterns (DAMPs) arises in Alzheimer's disease (AD). In doing so, it places soluble amyloid β (Aβ), a protein hitherto considered as a primary initiator of AD, in a novel perspective. We note here that increased soluble Aβ is one of the proinflammatory cytokine-induced DAMPs recognized by at least one of the toll-like receptors on and in various cell types. Moreover, Aβ is best regarded as belonging to a class of DAMPs, as do the S100 proteins and HMBG1, that further exacerbate production of these same proinflammatory cytokines, which are already enhanced, and induces them further. Moreover, variation in levels of other DAMPs of this same class in AD may explain why normal elderly patients can exhibit high Aβ plaque levels, and why removing Aβ or its plaque does not retard disease progression. It may also explain why mouse transgenic models, having been designed to generate high Aβ, can be treated successfully by this approach.

LRP1 Downregulates the Alzheimer's β-Secretase BACE1 by Modulating Its Intraneuronal Trafficking

The β-secretase called BACE1 is a membrane-associated protease that initiates the generation of amyloid β-protein (Aβ), a key event in Alzheimer's disease (AD). However, the mechanism of intraneuronal regulation of BACE1 is poorly understood. Here, we present evidence that low-density lipoprotein receptor-related protein 1 (LRP1), a multi-functional receptor, has a previously unrecognized function to regulate BACE1 in neurons. We show that deficiency of LRP1 exerts promotive effects on the protein expression and function of BACE1, whereas expression of LRP-L4, a functional LRP1 mini-receptor, specifically decreases BACE1 levels in both human embryonic kidney (HEK) 293 cells and rat primary neurons, leading to reduced Aβ production. Our subsequent analyses further demonstrate that (1) both endogenous and exogenous BACE1 and LRP1 interact with each other and are colocalized in soma and neurites of primary neurons, (2) LRP1 reduces the protein stability and cell-surface expression of BACE1, and (3) LRP1 facilitates the shift in intracellular localization of BACE1 from early to late endosomes, thereby promoting lysosomal degradation. These findings establish that LRP1 specifically downregulates BACE1 by modulating its intraneuronal trafficking and stability through protein interaction and highlight LRP1 as a potential therapeutic target in AD.

3-methyladenine, an autophagic inhibitor, attenuates therapeutic effects of sirolimus on scopolamine-induced cognitive dysfunction in a rat model

Previous studies have demonstrated that sirolimus has therapeutic effects for Alzheimer's disease which characterized by cognitive dysfunction. However, its underlying mechanisms have not been fully elucidated. In the present study, we aimed to investigate the mechanisms of therapeutic effects of sirolimus for cognitive dysfunction rat model which induced by chronic administration of scopolamine. Forty Wistar rats were randomly divided into 4 groups (n=10 each): saline group and scopolamine group, sirolimus plus scopolamine group and 3-methyladenine pretreatment group. Morris water maze test was applied to measure the cognitive function of rat. After behavioral test, rats were sacrificed and prefrontal cortex and hippocampus were harvested for measuring amyloid-β (Aβ), Beclin-1 and mammalian target of rapamycin (mTOR). Compared with saline group, scopolamine administered significantly decreased the cognitive performance of rats during the Morris water maze test and changed Aβ, Beclin-1 and mTOR levels in rat prefrontal cortex and hippocampus (P<0.05); In addition, rats in sirolimus plus scopolamine group significantly reversed scopolamine-induced effects (P<0.05). Most importantly, 3-methyladenine abrogated the effects of sirolimus on scopolamine-induced cognitive dysfunction (P<0.05). In conclusion, the mechanism of sirolimus exerting therapeutic effects for scopolamine-induced cognitive dysfunction is likely related to the activation of autophagy.