The low density lipoprotein receptor-related protein (LRP) is a novel beta-secretase (BACE1) substrate

Alzheimer's therapeutics: translation of preclinical science to clinical drug development

Over the past three decades, significant progress has been made in understanding the neurobiology of Alzheimer's disease. In recent years, the first attempts to implement novel mechanism-based treatments brought rather disappointing results, with low, if any, drug efficacy and significant side effects. A discrepancy between our expectations based on preclinical models and the results of clinical trials calls for a revision of our theoretical views and questions every stage of translation-from how we model the disease to how we run clinical trials. In the following sections, we will use some specific examples of the therapeutics from acetylcholinesterase inhibitors to recent anti-Aβ immunization and γ-secretase inhibition to discuss whether preclinical studies could predict the limitations in efficacy and side effects that we were so disappointed to observe in recent clinical trials. We discuss ways to improve both the predictive validity of mouse models and the translation of knowledge between preclinical and clinical stages of drug development.

Identification and biology of β-secretase

Our knowledge of the etiology of Alzheimer's disease (AD) has advanced tremendously since the discovery of amyloid beta (Aβ) aggregation in diseased brains. Accumulating evidence suggests that Aβ plays a causative role in AD. The β-secretase enzyme, beta-site APP cleaving enzyme-1 (BACE1), is also implicated in AD pathogenesis, given that BACE1 cleavage of amyloid precursor protein is the initiating step in the formation of Aβ. As a result, BACE1 inhibition has been branded as a potential AD therapy. In this study, we review the identification and basic characteristics of BACE1, as well as the progress in our understanding of BACE1 cell biology, substrates, and phenotypes of BACE1 knockout mice that are informative about the physiological functions of BACE1 beyond amyloid precursor protein cleavage. These data are crucial for predicting potential mechanism-based toxicity that would arise from inhibiting BACE1 for the treatment or prevention of AD.

FRET in Membrane Biophysics: An Overview

Förster resonance energy transfer (FRET), in most applications used as a “spectroscopic ruler,” allows an easy determination of the donor-acceptor intermolecular distance. However, the situation becomes complex in membranes, since around each donor there is an ensemble of acceptors at non-correlated distances. In this review, state-of-the-art methodologies for this situation are presented, usually involving time-resolved data and model fitting. This powerful approach can be used to study the occurrence of phase separation (“rafts” or other type of domains), allowing their detection as well as size evaluation. Formalisms for studying lipid–protein and protein–protein interactions according to specific topologies are also addressed. The advantages and added complexity of a specific type of FRET (energy homotransfer or energy migration) are described, as well as applications of FRET under the microscope.

Cathepsin D is partly endocytosed by the LRP1 receptor and inhibits LRP1-regulated intramembrane proteolysis

The aspartic protease cathepsin-D (cath-D) is a marker of poor prognosis in breast cancer that is overexpressed and hypersecreted by human breast cancer cells. Secreted pro-cath-D binds to the extracellular domain of the β-chain of the LDL receptor-related protein-1 (LRP1) in fibroblasts. The LRP1 receptor has an 85-kDa transmembrane β-chain and a noncovalently attached 515-kDa extracellular α-chain. LRP1 acts by (1) internalizing many ligands via its α-chain, (2) activating signaling pathways by phosphorylating the LRP1β-chain tyrosine and (3) modulating gene transcription by regulated intramembrane proteolysis (RIP) of its β-chain. LRP1 RIP involves two cleavages: the first liberates the LRP1 ectodomain to give a membrane-associated form, LRP1β-CTF, and the second generates the LRP1β-intracellular domain, LRP1β-ICD, that modulates gene transcription. Here, we investigated the endocytosis of pro-cath-D by LRP1 and the effect of pro-cath-D/LRP1β interaction on LRP1β tyrosine phosphorylation and/or LRP1β RIP. Our results indicate that pro-cath-D was partially endocytosed by LRP1 in fibroblasts. However, pro-cath-D and ectopic cath-D did not stimulate phosphorylation of the LRP1β-chain tyrosine. Interestingly, ectopic cath-D and its catalytically inactive (D231N)cath-D, and pro-(D231N)cath-D all significantly inhibited LRP1 RIP by preventing LRP1β-CTF production. Thus, cath-D inhibits LRP1 RIP independently of its catalytic activity by blocking the first cleavage. As cath-D triggers fibroblast outgrowth by LRP1, we propose that cath-D modulates the growth of fibroblasts by inhibiting LRP1 RIP in the breast tumor microenvironment.

The role of lipoprotein receptors on the physiological function of APP

In this review, we will primarily focus on the role of members of the low-density lipoprotein receptor (LDL-R) family that are involved in trafficking and processing of the amyloid precursor protein (APP). We will discuss the role of the LDL-receptor family members, low-density lipoprotein receptor-related protein 1 (LRP1), LRP1b, apolipoprotein E receptor 2, sortilin-related receptor (SorLA/LR11) and megalin/LRP2 on the physiological function of APP and its cellular localization. Additionally, we will focus on adaptor proteins that have been shown to influence the physiological function of LDL-R family members in combination with APP processing. The results in this review emphasize that the physiological function of APP cannot be explained by the focus on the APP protein alone but rather in combination with various direct or indirect interaction partners within the cellular environment.

Development of a specific ELISA to measure BACE1 levels in human tissues

The aspartyl protease BACE1 is the rate limiting enzyme in the synthesis of amyloid beta, which accumulation in the human brain is a hallmark of Alzheimer's disease (AD). BACE1 has been proposed as a surrogate marker of AD; however, very few BACE1 immunoassays have been reported. In the present study we have screened ten BACE1 antibodies by Western blot and several antibody pairs to develop a new BACE1 sandwich ELISA procedure. We identified one pair that showed little background and good reproducibility. Several dilution buffers and sample denaturation methods were tried to partially unfold BACE1 before capture. We found that dilution in PBS followed by 10 min incubation at 50°C critically improves the performance of the assay. Finally, we successfully measured BACE1 levels in a few human brain and platelet lysates as well as in plasma and AD CSF. We anticipate that this assay will lay the ground to accurately measure BACE1 levels in human tissues, which could facilitate the molecular diagnosis of AD in the near future.

The Basic Biology of BACE1: A Key Therapeutic Target for Alzheimer's Disease

Alzheimer’s disease (AD) is an intractable, neurodegenerative disease that appears to be brought about by both genetic and non-genetic factors. The neuropathology associated with AD is complex, although amyloid plaques composed of the β-amyloid peptide (Aβ) are hallmark neuropathological lesions of AD brain. Indeed, Aβ plays an early and central role in this disease. β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is the initiating enzyme in Aβ genesis and BACE1 levels are elevated under a variety of conditions. Given the strong correlation between Aβ and AD, and the elevation of BACE1 in this disease, this enzyme is a prime drug target for inhibiting Aβ production in AD. However, nine years on from the initial identification of BACE1, and despite intense research, a number of key questions regarding BACE1 remain unanswered. Indeed, drug discovery and development for AD continues to be challenging. While current AD therapies temporarily slow cognitive decline, treatments that address the underlying pathologic mechanisms of AD are completely lacking. Here we review the basic biology of BACE1. We pay special attention to recent research that has provided some answers to questions such as those involving the identification of novel BACE1 substrates, the potential causes of BACE1 elevation and the putative function of BACE1 in health and disease. Our increasing understanding of BACE1 biology should aid the development of compounds that interfere with BACE1 expression and activity and may lead to the generation of novel therapeutics for AD.

The β-secretase enzyme BACE1 as a therapeutic target for Alzheimer's disease

Amyloid plaques are defining histopathologic lesions in the brains of Alzheimer's disease (AD) patients and are composed of the amyloid-beta peptide, which is widely considered to play a critical role in the pathogenesis of AD. The β-secretase, or β-site amyloid precursor protein cleaving enzyme 1 (BACE1; also called Asp2, memapsin 2), is the enzyme that initiates the generation of amyloid beta. Consequently, BACE1 is an attractive drug target for lowering cerebral levels of amyloid beta for the treatment or prevention of AD. Much has been learned about BACE1 since its discovery over 10 years ago. In the present article, we review BACE1 properties and characteristics, cell biology, in vivo validation, substrates, therapeutic potential, and inhibitor drug development. Studies relating to the physiological functions of BACE1 and the promise of BACE1 inhibition for AD will also be discussed. We conclude that therapeutic inhibition of BACE1 should be efficacious for AD, although careful titration of the drug dose may be necessary to limit mechanism-based side effects.

Cell cholesterol modulates metalloproteinase-dependent shedding of low-density lipoprotein receptor-related protein-1 (LRP-1) and clearance function

Low-density lipoprotein receptor-related protein-1 (LRP-1) is a plasma membrane scavenger and signaling receptor, composed of a large ligand-binding subunit (515-kDa α-chain) linked to a shorter transmembrane subunit (85-kDa β-chain). LRP-1 cell-surface level and function are controlled by proteolytic shedding of its ectodomain. Here, we identified ectodomain sheddases in human HT1080 cells and demonstrated regulation of the cleavage by cholesterol by comparing the classical fibroblastoid type with a spontaneous epithelioid variant, enriched ∼ 2-fold in cholesterol. Two membrane-associated metalloproteinases were involved in LRP-1 shedding: a disintegrin and metalloproteinase-12 (ADAM-12) and membrane-type 1 matrix metalloproteinase (MT1-MMP). Although both variants expressed similar levels of LRP-1, ADAM-12, MT1-MMP, and specific tissue inhibitor of metalloproteinases-2 (TIMP-2), LRP-1 shedding from epithelioid cells was ∼4-fold lower than from fibroblastoid cells. Release of the ectodomain was triggered by cholesterol depletion in epithelioid cells and impaired by cholesterol overload in fibroblastoid cells. Modulation of LRP-1 shedding on clearance was reflected by accumulation of gelatinases (MMP-2 and MMP-9) in the medium. We conclude that cholesterol exerts an important control on LRP-1 levels and function at the plasma membrane by modulating shedding of its ectodomain, and therefore represents a novel regulator of extracellular proteolytic activities.

Regulated intramembrane proteolysis--lessons from amyloid precursor protein processing

Regulated intramembrane proteolysis (RIP) controls the communication between cells and the extracellular environment. RIP is essential in the nervous system, but also in other tissues. In the RIP process, a membrane protein typically undergoes two consecutive cleavages. The first one results in the shedding of its ectodomain. The second one occurs within its transmembrane domain, resulting in secretion of a small peptide and the release of the intracellular domain into the cytosol. The proteolytic cleavage fragments act as versatile signaling molecules or are further degraded. An increasing number of membrane proteins undergo RIP. These include growth factors, cytokines, cell adhesion proteins, receptors, viral proteins and signal peptides. A dysregulation of RIP is found in diseases, such as leukemia and Alzheimer's disease. One of the first RIP substrates discovered was the amyloid precursor protein (APP). RIP processing of APP controls the generation of the amyloid β-peptide, which is believed to cause Alzheimer's disease. Focusing on APP as the best-studied RIP substrate, this review describes the function and mechanism of the APP RIP proteases with the goal to elucidate cellular mechanisms and common principles of the RIP process in general.

Sink Hypothesis and Therapeutic Strategies for Attenuating Aβ Levels

Amyloid β (Aβ) plaque, comprised mainly by Aβ peptides, is an important pathology of Alzheimer’s brains. Major efforts have been devoted to targeting this neurotoxic Aβ peptide for discovering disease-modifying treatments for Alzheimer’s disease. Inasmuch as Aβ is found in both the brain and the periphery, it is hypothesized that there is some form of equilibrium for the Aβ in the brain and the periphery such that Aβ can be transported across the blood-brain barrier. By modulating the periphery Aβ levels, it is predicted that the brain Aβ levels will undergo concomitant changes, forming the basis of the “sink hypothesis” for Aβ lowering strategies. In this review, the significance and implication of this sink hypothesis as well as how the sink hypothesis may contribute to the recent Aβ-based drug discovery in AD are discussed. Ultimately, the validity of the sink hypothesis will be resolved when the appropriate Aβ agents are being tested in the clinic.

Predicting memapsin 2 (β-secretase) hydrolytic activity

Memapsin 2 (BACE1, β-secretase), a membrane aspartic protease, functions in the cleavage of brain β-amyloid precursor protein (APP) leading to the production of β-amyloid. Because the excess level of β-amyloid in the brain is a leading factor in Alzheimer's disease (AD), memapsin 2 is a major therapeutic target for inhibitor drugs. The substrate-binding cleft of memapsin 2 accommodates 12 subsite residues, from P(8) to P(4)'. We have determined the hydrolytic preference as relative k(cat)/K(M) (preference constant) in all 12 subsites and used these data to establish a predictive algorithm for substrate hydrolytic efficiency. Using the sequences from 12 reported memapsin 2 protein substrates, the predicted and experimentally determined preference constants have an excellent correlation coefficient of 0.97. The predictive model indicates that the hydrolytic preference of memapsin 2 is determined mainly by the interaction with six subsites (from P(4) to P(2)'), a conclusion supported by the crystal structure B-factors calculated for the various residues of transition-state analogs bound to different memapsin 2 subsites. The algorithm also predicted that the replacement of the P(3), P(2), and P(1) subsites of APP from Val, Lys, and Met, respectively, to Ile, Asp, and Phe, respectively, (APP(IDF)) would result in a highest hydrolytic rate for β-amyloid-generating APP variants. Because more β-amyloid was produced from cells expressing APP(IDF) than those expressing APP with Swedish mutations, this designed APP variant may be useful in new memapsin 2 substrates or transgenic mice for AD studies.

Impaired lipoprotein receptor-mediated peripheral binding of plasma amyloid-β is an early biomarker for mild cognitive impairment preceding Alzheimer's disease

Soluble circulating low density lipoprotein receptor-related protein-1 (sLRP) provides key plasma binding activity for Alzheimer's disease (AD) amyloid-β peptide (Aβ). sLRP normally binds 70-90% of plasma Aβ preventing free Aβ access to the brain. In AD, Aβ binding to sLRP is compromised by increased levels of oxidized sLRP which does not bind Aβ. Here, we determined plasma oxidized sLRP and Aβ40/42 sLRP-bound, other proteins-bound and free plasma fractions, cerebrospinal fluid (CSF) tau/Aβ42 ratios, and mini-mental state examination (MMSE) scores in patients with mild cognitive impairment (MCI) who progressed to AD (MCI-AD, n = 14), AD (n = 14) and neurologically healthy controls (n = 14) recruited from the Göteborg MCI study. In MCI-AD patients prior to conversion to AD and AD patients, the respective increases in oxidized sLRP and free plasma Aβ40 and Aβ42 levels were 4.9 and 3.7-fold, 1.8, and 1.7-fold and 4.3 and 3.3-fold (p < 0.05, ANOVA with Tuckey post-hoc test). In MCI-AD and AD patients increases in oxidized sLRP and free plasma Aβ40 and Aβ42 correlated with increases in CSF tau/Aβ42 ratios and reductions in MMSE scores (p < 0.05, Pearson analysis). A heterogeneous group of 'stable' MCI patients that was followed over 2-4 years (n = 24) had normal CSF tau/Aβ42 ratios but increased oxidized sLRP levels (p < 0.05, Student's t test). Data suggests that a deficient sLRP-Aβ binding might precede and correlate later in disease with an increase in the tau/Aβ42 CSF ratio and global cognitive decline in MCI individuals converting into AD, and therefore is an early biomarker for AD-type dementia.

BACE2 plays a role in the insulin receptor trafficking in pancreatic ß-cells

BACE1 (β-site amyloidogenic cleavage of precursor protein-cleaving enzyme 1) is a β-secretase protein that plays a central role in the production of the β-amyloid peptide in the brain and is thought to be involved in the Alzheimer's pathogenesis. In type 2 diabetes, amyloid deposition within the pancreatic islets is a pathophysiological hallmark, making crucial the study in the pancreas of BACE1 and its homologous BACE2 to understand the pathological mechanisms of this disease. The objectives of the present study were to characterize the localization of BACE proteins in human pancreas and determine their function. High levels of BACE enzymatic activity were detected in human pancreas. In normal human pancreas, BACE1 was observed in endocrine as well as in exocrine pancreas, whereas BACE2 expression was restricted to β-cells. Intracellular analysis using immunofluorescence showed colocalization of BACE1 with insulin and BACE2 with clathrin-coated vesicles of the plasma membrane in MIN6 cells. When BACE1 and -2 were pharmacologically inhibited, BACE1 localization was not altered, whereas BACE2 content in clathrin-coated vesicles was increased. Insulin internalization rate was reduced, insulin receptor β-subunit (IRβ) expression was decreased at the plasma membrane and increased in the Golgi apparatus, and a significant reduction in insulin gene expression was detected. Similar results were obtained after specific BACE2 silencing in MIN6 cells. All these data point to a role for BACE2 in the IRβ trafficking and insulin signaling. In conclusion, BACE2 is hereby presented as an important enzyme in β-cell function.

Inhibiting β-secretase activity in Alzheimer's disease cell models with single-chain antibodies specifically targeting APP

The Amyloid-β (Aβ) peptide is produced from the amyloid precursor protein (APP) by sequential proteolytic cleavage of APP first by β-secretase and then by γ-secretase. β-Site APP cleaving enzyme-1 (BACE-1) is the predominant enzyme involved in β-secretase processing of APP and is a primary therapeutic target for treatment of Alzheimer's disease. While inhibiting BACE-1 activity has obvious therapeutic advantages, BACE-1 also cleaves numerous other substrates with important physiological activity. Thus, blanket inhibition of BACE-1 function may have adverse side effects. We isolated a single chain variable fragment (scFv) from a human-based scFv yeast display library that selectively inhibits BACE-1 activity toward APP by binding the APP substrate at the proteolytic site. We selected the iBSEC1 scFv, since it recognizes the BACE-1 cleavage site on APP but does not bind the adjacent highly antigenic N-terminal of Aβ, and thus it will target APP but not soluble Aβ. When added to 7PA2 cells, a mammalian cell line that overexpresses APP, the iBSEC1 scFv binds APP on the cell surface, reduces toxicity induced by APP overexpression, and reduces both intracellular and extracellular Aβ levels by around 50%. Since the iBSEC1 scFv does not contain the antibody F(c) region, this construct does not pose the risk of exacerbating inflammation in the brain as faced with full-length monoclonal antibodies for potential therapeutic applications.

Signaling through LRP1: Protection from atherosclerosis and beyond

The low-density lipoprotein receptor-related protein (LRP1) is a multifunctional cell surface receptor that belongs to the LDL receptor (LDLR) gene family and that is widely expressed in several tissues. LRP1 consists of an 85-kDa membrane-bound carboxyl fragment (β chain) and a non-covalently attached 515-kDa (α chain) amino-terminal fragment. Through its extracellular domain, LRP1 binds at least 40 different ligands ranging from lipoprotein and protease inhibitor complex to growth factors and extracellular matrix proteins. LRP-1 has also been shown to interact with scaffolding and signaling proteins via its intracellular domain in a phosphorylation-dependent manner and to function as a co-receptor partnering with other cell surface or integral membrane proteins. LRP-1 is thus implicated in two major physiological processes: endocytosis and regulation of signaling pathways, which are both involved in diverse biological roles including lipid metabolism, cell growth/differentiation processes, degradation of proteases, and tissue invasion. The embryonic lethal phenotype obtained after target disruption of the LRP-1 gene in the mouse highlights the biological importance of this receptor and revealed a critical, but yet undefined role in development. Tissue-specific gene deletion studies also reveal an important contribution of LRP1 in vascular remodeling, foam cell biology, the central nervous system, and in the molecular mechanisms of atherosclerosis.

Pro-cathepsin D interacts with the extracellular domain of the beta chain of LRP1 and promotes LRP1-dependent fibroblast outgrowth

Interactions between cancer cells and fibroblasts are crucial in cancer progression. We have previously shown that the aspartic protease cathepsin D (cath-D), a marker of poor prognosis in breast cancer that is overexpressed and highly secreted by breast cancer cells, triggers mouse embryonic fibroblast outgrowth via a paracrine loop. Here, we show the requirement of secreted cath-D for human mammary fibroblast outgrowth using a three-dimensional co-culture assay with breast cancer cells that do or do not secrete pro-cath-D. Interestingly, proteolytically-inactive pro-cath-D remains mitogenic, indicating a mechanism involving protein-protein interaction. We identify the low-density lipoprotein (LDL) receptor-related protein-1, LRP1, as a novel binding partner for pro-cath-D in fibroblasts. Pro-cath-D binds to residues 349-394 of the β chain of LRP1, and is the first ligand of the extracellular domain of LRP1β to be identified. We show that pro-cath-D interacts with LRP1β in cellulo. Interaction occurs at the cell surface, and overexpressed LRP1β directs pro-cath-D to the lipid rafts. Our results reveal that the ability of secreted pro-cath-D to promote human mammary fibroblast outgrowth depends on LRP1 expression, suggesting that pro-cath-D-LRP1β interaction plays a functional role in the outgrowth of fibroblasts. Overall, our findings strongly suggest that pro-cath-D secreted by epithelial cancer cells promotes fibroblast outgrowth in a paracrine LRP1-dependent manner in the breast tumor microenvironment.

BACE1 gene promoter single-nucleotide polymorphisms in Alzheimer's disease

Alzheimer's disease (AD) is the most neurodegenerative disorder leading to dementia. Neuritic plaque formation in brains is a hallmark of AD pathogenesis. Amyloid beta protein (Abeta) is the central component of neuritic plaques. Processing beta-amyloid precursor protein (APP) at the beta-secretase site by the beta-site APP cleaving enzyme 1 (BACE1) is essential for generation of Abeta. Elevation of BACE1 activity and expression has been reported in AD brains. However, no mutation in the BACE1 coding sequence has been identified in AD cases. Human BACE1 expression is tightly regulated at the transcription and translation level. To determine whether there is any single-nucleotide polymorphisms in the BACE1 gene promoter region affecting BACE1 expression in AD pathogenesis, in this study, we screened 2.6 kb of the human BACE1 gene promoter region from late-onset AD patients and found that there was no significant association between single-nucleotide polymorphisms and AD cases.

BACE1-/- mice exhibit seizure activity that does not correlate with sodium channel level or axonal localization

Background

BACE1 is a key enzyme in the generation of the Aβ peptide that plays a central role in the pathogenesis of Alzheimer's disease. While BACE1 is an attractive therapeutic target, its normal physiological function remains largely unknown. Examination of BACE1^-/- mice can provide insight into this function and also help anticipate consequences of BACE1 inhibition. Here we report a seizure-susceptibility phenotype that we have identified and characterized in BACE1^-/- mice.

Results

We find that electroencephalographic recordings reveal epileptiform abnormalities in some BACE1^-/- mice, occasionally including generalized tonic-clonic and absence seizures. In addition, we find that kainic acid injection induces seizures of greater severity in BACE1^-/- mice relative to BACE1^+/+ littermates, and causes excitotoxic cell death in a subset of BACE1^-/- mice. This hyperexcitability phenotype is variable and appears to be manifest in approximately 30% of BACE1^-/- mice. Finally, examination of the expression and localization of the voltage-gated sodium channel α-subunit Na_v1.2 reveals no correlation with BACE1 genotype or any measure of seizure susceptibility.

Conclusions

Our data indicate that BACE1 deficiency predisposes mice to spontaneous and pharmacologically-induced seizure activity. This finding has implications for the development of safe therapeutic strategies for reducing Aβ levels in Alzheimer's disease. Further, we demonstrate that altered sodium channel expression and axonal localization are insufficient to account for the observed effect, warranting investigation of alternative mechanisms.

Is BACE1 a suitable therapeutic target for the treatment of Alzheimer's disease? Current strategies and future directions

Alzheimer's disease (AD) is characterized by the extracellular deposition of the β-amyloid protein (Aβ). Aβ is a fragment of a much larger precursor protein, the amyloid precursor protein (APP). Sequential proteolytic cleavage of APP by β-secretase and γ-secretase liberates Aβ from APP. The aspartyl protease BACE1 (β-site APP-cleaving enzyme 1) catalyses the rate-limiting step in the production of Aβ, and as such it is considered to be a major target for drug development in Alzheimer's disease. However, the development of a BACE1 inhibitor therapy is problematic for two reasons. First, BACE1 has been found to have important physiological roles. Therefore, inhibition of the enzyme could have toxic consequences. Second, the active site of BACE1 is relatively large, and many of the bulky compounds that are needed to inhibit BACE1 activity are unlikely to cross the blood-brain barrier. This review focuses on the structure BACE1, current therapeutic strategies based on developing active-site inhibitors, and new approaches to therapy involving targeting the expression or post-translational regulation of BACE1.

BACE1 deficiency causes altered neuronal activity and neurodegeneration

BACE1 is required for the release of beta-amyloid (Abeta) in vivo, and inhibition of BACE1 activity is targeted for reducing Abeta generation in Alzheimer's patients. To further our understanding of the safe use of BACE1 inhibitors in human patients, we aimed to study the physiological functions of BACE1 by characterizing BACE1-null mice. Here, we report the finding of spontaneous behavioral seizures in BACE1-null mice. Electroencephalographic recordings revealed abnormal spike-wave discharges in BACE1-null mice, and kainic acid-induced seizures also occurred more frequently in BACE1-null mice compared with their wild-type littermates. Biochemical and morphological studies showed that axonal and surface levels of Na(v)1.2 were significantly elevated in BACE1-null mice, consistent with the increased fast sodium channel current recorded from BACE1-null hippocampal neurons. Patch-clamp recording also showed altered intrinsic firing properties of isolated BACE1-null hippocampal neurons. Furthermore, population spikes were significantly increased in BACE1-null brain slices, indicating hyperexcitability of BACE1-null neurons. Together, our results suggest that increased sodium channel activity contributes to the epileptic behaviors observed in BACE1-null mice. The knowledge from this study is crucial for the development of BACE1 inhibitors for Alzheimer's therapy and to the applicative study of epilepsy.

Inflammatory mediators promote production of shed LRP1/CD91, which regulates cell signaling and cytokine expression by macrophages

LRP1 is a type-1 transmembrane receptor that mediates the endocytosis of diverse ligands. LRP1 β-chain proteolysis results in release of sLRP1 that is present in human plasma. In this study, we show that LPS and IFN-γ induce shedding of LRP1 from RAW 264.7 cells and BMMs in vitro. ADAM17 was principally responsible for the increase in LRP1 shedding. sLRP1 was also increased in vivo in mouse plasma following injection of LPS and in plasma from human patients with RA or SLE. sLRP1, which was purified from human plasma, and full-length LRP1, purified from mouse liver, activated cell signaling when added to cultures of RAW 264.7 cells and BMMs. Robust activation of p38 MAPK and JNK was observed. The IKK-NF-κB pathway was transiently activated. Proteins that bind to the ligand-binding clusters in LRP1 failed to inhibit sLRP1-initiated cell signaling, however an antibody that targets the sLRP1 N terminus was effective. sLRP1 induced expression of regulatory cytokines by RAW 264.7 cells, including TNF-α, MCP-1/CCL2, and IL-10. These results demonstrate that sLRP1 is generated in inflammation and may regulate inflammation by its effects on macrophage physiology.

The role of low-density receptor-related protein 1 (LRP1) as a competitive substrate of the amyloid precursor protein (APP) for BACE1

Cleavage of APP by BACE1 is the first proteolytic step in the production of amyloid-beta (Abeta), which accumulates in senile plaques in Alzheimer's disease. Through its interaction with APP, the low-density receptor-related protein 1 (LRP1) enhances APP internalization. Recently, BACE1 has been shown to interact with and cleave the light chain (lc) of LRP1. Since LRP1 is known to compete with APP for cleavage by gamma-secretase, we tested the hypothesis that LRP1 also acts as a competitive substrate for beta-secretase. We found that the increase in secreted APP (sAPP) mediated by over-expression of BACE1 in APP-transfected cells could be decreased by simultaneous LRP1 over-expression. Analysis by multi-spot ELISA revealed that this is due to a decrease in sAPPbeta, but not sAPPalpha. Interaction between APP and BACE1, as measured by immunoprecipitation and fluorescence lifetime assays, was impaired by LRP1 over-expression. We also demonstrate that APP over-expression leads to decreased LRP1 association with and cleavage by BACE1. In conclusion, our data suggest that--in addition to its role in APP trafficking--LRP1 affects APP processing by competing for cleavage by BACE1.

Protein aggregation diseases: pathogenicity and therapeutic perspectives

A growing number of diseases seem to be associated with inappropriate deposition of protein aggregates. Some of these diseases--such as Alzheimer's disease and systemic amyloidoses--have been recognized for a long time. However, it is now clear that ordered aggregation of pathogenic proteins does not only occur in the extracellular space, but in the cytoplasm and nucleus as well, indicating that many other diseases may also qualify as amyloidoses. The common structural and pathogenic features of these diverse protein aggregation diseases is only now being fully understood, and may provide novel opportunities for overarching therapeutic approaches such as depleting the monomeric precursor protein, inhibiting aggregation, enhancing aggregate clearance or blocking common aggregation-induced cellular toxicity pathways.

The secretases: enzymes with therapeutic potential in Alzheimer disease

The amyloid hypothesis has yielded a series of well-validated candidate drug targets with potential for the treatment of Alzheimer disease (AD). Three proteases that are involved in the processing of amyloid precursor protein-alpha-secretase, beta-secretase and gamma-secretase-are of particular interest as they are central to the generation and modulation of amyloid-beta peptide and can be targeted by small compounds in vitro and in vivo. Given that these proteases also fulfill other important biological roles, inhibiting their activity will clearly be inherently associated with mechanism-based toxicity. Carefully determining a suitable therapeutic window and optimizing the selectivity of the drug treatments towards amyloid precursor protein processing might be ways of overcoming this potential complication. Secretase inhibitors are likely to be the first small-molecule therapies aimed at AD modification that will be fully tested in the clinic. Success or failure of these first-generation AD therapies will have enormous consequences for further drug development efforts for AD and possibly other neurodegenerative conditions.

Proteases and Proteolysis in Alzheimer Disease: A Multifactorial View on the Disease Process

Alzheimer disease is characterized by the accumulation of abnormally folded protein fragments, i.e., amyloid beta peptide (Aβ) and tau that precipitate in amyloid plaques and neuronal tangles, respectively. In this review we discuss the complicated proteolytic pathways that are responsible for the generation and clearance of these fragments, and how disturbances in these pathways interact and provide a background for a novel understanding of Alzheimer disease as a multifactorial disorder. Recent insights evolve from the static view that the morphologically defined plaques and tangles are disease driving towards a more dynamic, biochemical view in which the intermediary soluble Aβ oligomers and soluble tau fragments are considered as the main mediators of neurotoxicity. The relevance of proteolytic pathways, centered on the generation and clearance of toxic Aβ, on the cleavage and nucleation of tau, and on the general proteostasis of the neurons, then becomes obvious. Blocking or stimulating these pathways provide, or have the potential to provide, interesting drug targets, which raises the hope that we will be able to provide a cure for this dreadful disorder.

Membrane rafts in Alzheimer's disease beta-amyloid production

Alzheimer's disease (AD), the most common age-associated dementing disorder, is pathologically manifested by progressive cognitive dysfunction concomitant with the accumulation of senile plaques consisting of amyloid-beta (Abeta) peptide aggregates in the brain of affected individuals. Abeta is derived from a type I transmembrane protein, amyloid precursor protein (APP), by the sequential proteolytic events mediated by beta-site APP cleaving enzyme 1 (BACE1) and gamma-secretase. Multiple lines of evidence have implicated cholesterol and cholesterol-rich membrane microdomains, termed lipid rafts in the amyloidogenic processing of APP. In this review, we summarize the cell biology of APP, beta- and gamma-secretases and the data on their association with lipid rafts. Then, we will discuss potential raft targeting signals identified in the secretases and their importance on amyloidogenic processing of APP.

Extensive proteomic screening identifies the obesity-related NYGGF4 protein as a novel LRP1-interactor, showing reduced expression in early Alzheimer's disease

BACKGROUND

The low-density lipoprotein receptor related protein 1 (LRP1) has been implicated in Alzheimer's disease (AD) but its signalling has not been fully evaluated. There is good evidence that the cytoplasmic domain of LRP1 is involved in protein-protein interactions, important in the cell biology of LRP1.

RESULTS

We carried out three yeast two-hybrid screens to identify proteins that interact with the cytoplasmic domain of LRP1. The screens included both conventional screens as well as a novel, split-ubiquitin-based screen in which an LRP1 construct was expressed and screened as a transmembrane protein. The split-ubiquitin screen was validated in a screen using full-length amyloid protein precursor (APP), which successfully identified FE65 and FE65L2, as well as novel interactors (Rab3a, Napg, and ubiquitin b). Using both a conventional screen as well as the split-ubiquitin screen, we identified NYGGF4 as a novel LRP1 interactor. The interaction between LRP1 and NYGGF4 was validated using two-hybrid assays, coprecipitation and colocalization in mammalian cells. Mutation analysis demonstrated a specific interaction of NYGGF4 with an NPXY motif that required an intact tyrosine residue. Interestingly, while we confirmed that other LRP1 interactors we identified, including JIP1B and EB-1, were also able to bind to APP, NYGGF4 was unique in that it showed specific binding with LRP1. Expression of NYGGF4 decreased significantly in patients with AD as compared to age-matched controls, and showed decreasing expression with AD disease progression. Examination of Nyggf4 expression in mice with different alleles of the human APOE4 gene showed significant differences in Nyggf4 expression.

CONCLUSIONS

These results implicate NYGGF4 as a novel and specific interactor of LRP1. Decreased expression of LRP1 and NYGGF4 over disease, evident with the presence of even moderate numbers of neuritic plaques, suggests that LRP1-NYGGF4 is a system altered early in disease. Genetic and functional studies have implicated both LRP1 and NYGGF4 in obesity and cardiovascular disease and the physical association of these proteins may reflect a common mechanism. This is particularly interesting in light of the dual role of ApoE in both cardiovascular risk and AD. The results support further studies on the functional relationship between NYGGF4 and LRP1.

Cholesterol metabolism and transport in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder, affecting millions of people worldwide. Apart from age, the major risk factor identified so far for the sporadic form of AD is possession of the epsilon4 allele of apolipoprotein E (APOE), which is also a risk factor for coronary artery disease (CAD). Other apolipoproteins known to play an important role in CAD such as apolipoprotein B are now gaining attention for their role in AD as well. AD and CAD share other risk factors, such as altered cholesterol levels, particularly high levels of low density lipoproteins together with low levels of high density lipoproteins. Statins--drugs that have been used to lower cholesterol levels in CAD, have been shown to protect against AD, although the protective mechanism(s) involved are still under debate. Enzymatic production of the beta amyloid peptide, the peptide thought to play a major role in AD pathogenesis, is affected by membrane cholesterol levels. In addition, polymorphisms in several proteins and enzymes involved in cholesterol and lipoprotein transport and metabolism have been linked to risk of AD. Taken together, these findings provide strong evidence that changes in cholesterol metabolism are intimately involved in AD pathogenic processes. This paper reviews cholesterol metabolism and transport, as well as those aspects of cholesterol metabolism that have been linked with AD.

Cholesterol interaction with proteins that partition into membrane domains: an overview

Biological membranes are complex structures composed largely of proteins and lipids. These components have very different structural and physical properties and consequently they do not form a single homogeneous mixture. Rather components of the mixture are more enriched in some regions than in others. This can be demonstrated with simple lipid mixtures that spontaneously segregate components so as to form different lipid phases that are immiscible with one another. The segregation of molecular components of biological membranes also involves proteins. One driving force that would promote the segregation of membrane components is the preferential interaction between a protein and certain lipid components. Among the varied lipid components of mammalian membranes, the structure and physical properties of cholesterol is quite different from that of other major membrane lipids. It would therefore be expected that in many cases proteins would have very different energies of interaction with cholesterol vs. those of other membrane lipids. This would be sufficient to cause segregation of components in membranes. The factors that facilitate the interaction of proteins with cholesterol are varied and are not yet completely understood. However, there are certain groups that are present in some proteins that facilitate interaction of the protein with cholesterol. These groups include saturated acyl chains of lipidated proteins, as well as certain amino acid sequences. Although there is some understanding as to why these particular groups favour interaction with cholesterol, our knowledge of these molecular features is not sufficiently developed to allow for the design of agents that will modify such binding.

Identification of beta-secretase (BACE1) substrates using quantitative proteomics

β-site APP cleaving enzyme 1 (BACE1) is a transmembrane aspartyl protease with a lumenal active site that sheds the ectodomains of membrane proteins through juxtamembrane proteolysis. BACE1 has been studied principally for its role in Alzheimer's disease as the β-secretase responsible for generating the amyloid-β protein. Emerging evidence from mouse models has identified the importance of BACE1 in myelination and cognitive performance. However, the substrates that BACE1 processes to regulate these functions are unknown, and to date only a few β-secretase substrates have been identified through candidate-based studies. Using an unbiased approach to substrate identification, we performed quantitative proteomic analysis of two human epithelial cell lines stably expressing BACE1 and identified 68 putative β-secretase substrates, a number of which we validated in a cell culture system. The vast majority were of type I transmembrane topology, although one was type II and three were GPI-linked proteins. Intriguingly, a preponderance of these proteins are involved in contact-dependent intercellular communication or serve as receptors and have recognized roles in the nervous system and other organs. No consistent sequence motif predicting BACE1 cleavage was identified in substrates versus non-substrates. These findings expand our understanding of the proteins and cellular processes that BACE1 may regulate, and suggest possible mechanisms of toxicity arising from chronic BACE1 inhibition.

The beta-secretase enzyme BACE in health and Alzheimer's disease: regulation, cell biology, function, and therapeutic potential

The beta-amyloid (Abeta) peptide is the major constituent of amyloid plaques in Alzheimer's disease (AD) brain and is likely to play a central role in the pathogenesis of this devastating neurodegenerative disorder. The beta-secretase, beta-site amyloid precursor protein cleaving enzyme (BACE1; also called Asp2, memapsin 2), is the enzyme responsible for initiating Abeta generation. Thus, BACE is a prime drug target for the therapeutic inhibition of Abeta production in AD. Since its discovery 10 years ago, much has been learned about BACE. This review summarizes BACE properties, describes BACE translation dysregulation in AD, and discusses BACE physiological functions in sodium current, synaptic transmission, myelination, and schizophrenia. The therapeutic potential of BACE will also be considered. This is a summary of topics covered at a symposium held at the 39th annual meeting of the Society for Neuroscience and is not meant to be a comprehensive review of BACE.

Linking vascular disorders and Alzheimer's disease: potential involvement of BACE1

The etiology of Alzheimer's disease (AD) remains unknown. However, specific risk factors have been identified, and aging is the strongest AD risk factor. The majority of cardiovascular events occur in older people and a close relationship between vascular disorders and AD exists. Amyloid plaques, composed of the beta amyloid peptide (Abeta), are hallmark lesions in AD and evidence indicates that Abeta plays a central role in AD pathophysiology. The BACE1 enzyme is essential for Abeta generation, and BACE1 levels are elevated in AD brain. The cause(s) of this BACE1 elevation remains undetermined. Here we review the potential contribution of vascular disease to AD pathogenesis. We examine the putative vasoactive properties of Abeta and how the cellular changes associated with vascular disease may elevate BACE1 levels. Despite increasing evidence, the exact role(s) vascular disorders play in AD remains to be determined. However, given that vascular diseases can be addressed by lifestyle and pharmacologic interventions, the potential benefits of these therapies in delaying the clinical appearance and progression of AD may warrant investigation.

BACE and gamma-secretase characterization and their sorting as therapeutic targets to reduce amyloidogenesis

Secretases are named for enzymes processing amyloid precursor protein (APP), a prototypic type-1 membrane protein. This led directly to discovery of novel Aspartyl proteases (beta-secretases or BACE), a tetramer complex gamma-secretase (gamma-SC) containing presenilins, nicastrin, aph-1 and pen-2, and a new role for metalloprotease(s) of the ADAM family as a alpha-secretases. Recent advances in defining pathways that mediate endosomal-lysosomal-autophagic-exosomal trafficking now provide targets for new drugs to attenuate abnormal production of fibril forming products characteristic of AD. A key to success includes not only characterization of relevant secretases but mechanisms for sorting and transport of key metabolites to abnormal vesicles or sites for assembly of fibrils. New developments we highlight include an important role for an 'early recycling endosome' coated in retromer complex containing lipoprotein receptor LRP-II (SorLA) for switching APP to a non-amyloidogenic pathway for alpha-secretases processing, or to shuttle APP to a 'late endosome compartment' to form Abeta or AICD. LRP11 (SorLA) is of particular importance since it decreases in sporadic AD whose etiology otherwise is unknown.

Metalloproteinase-dependent shedding of low-density lipoprotein receptor-related protein-1 ectodomain decreases endocytic clearance of endometrial matrix metalloproteinase-2 and -9 at menstruation

Cyclic elimination of the endometrium functional layer through menstrual bleeding results from intense tissue breakdown by proteolytic enzymes, mainly members of the matrix metalloproteinase (MMP) family. In contrast to menstrual-restricted MMPs, e.g. interstitial collagenase (MMP-1), gelatinases A (MMP-2) and B (MMP-9) mRNAs are abundant throughout the cycle without detectable tissue degradation at proliferative and secretory phases, implying a tight posttranslational control of both gelatinases. This paper addresses the role of low-density lipoprotein receptor-related protein (LRP)-1 in the endocytic clearance of endometrial gelatinases. LRP-1 mRNA and protein were studied using RT-PCR, Western blotting, and immunolabeling. Posttranslational control of LRP-1 was analyzed in explant culture. The receptor-associated protein (RAP), used as LRP antagonist, strongly increased (pro)gelatinase accumulation in medium conditioned by endometrial explants, suggesting a role for LRP-1 in their clearance. Although LRP-1 mRNA remained constant throughout the cycle, the protein ectodomain vanished at menses. LRP-1 immunolabeling selectively disappeared in areas of extracellular matrix breakdown in menstrual samples. It also disappeared from explants cultured without estrogen and progesterone (EP) due to ectodomain shedding in the medium. The shedding was inhibited by metalloproteinase inhibitors, including a disintegrin and metalloproteinase (ADAM) inhibitor, and by tissue inhibitors of MMPs (TIMP)-3 and -2, but barely by TIMP-1, pointing to ADAM-12 as the putative sheddase. In good agreement, ADAM-12 mRNA expression was repressed by EP. In conclusion, the efficient LRP-1-mediated clearance of gelatinase activity in nonbleeding endometrium is abrogated upon EP withdrawal, due to shedding of LRP-1 ectodomain by a metalloproteinase, presumably ADAM-12, itself regulated by EP.

Neurovascular mechanisms and blood-brain barrier disorder in Alzheimer's disease

Vascular dysfunction has a critical role in Alzheimer's disease (AD). Recent data from brain imaging studies in humans and animal models suggest that cerebrovascular dysfunction may precede cognitive decline and onset of neurodegenerative changes in AD and AD models. Cerebral hypoperfusion and impaired amyloid beta-peptide (Abeta) clearance across the blood-brain barrier (BBB) may contribute to the onset and progression of dementia AD type. Decreased cerebral blood flow (CBF) negatively affects the synthesis of proteins required for memory and learning, and may eventually lead to neuritic injury and neuronal death. Impaired clearance of Abeta from the brain by the cells of the neurovascular unit may lead to its accumulation on blood vessels and in brain parenchyma. The accumulation of Abeta on the cerebral blood vessels, known as cerebral amyloid angiopathy (CAA), is associated with cognitive decline and is one of the hallmarks of AD pathology. CAA can severely disrupt the integrity of the blood vessel wall resulting in micro or macro intracerebral bleedings that exacerbates neurodegenerative process and inflammatory response and may lead to hemorrhagic stroke, respectively. Here, we review the role of the neurovascular unit and molecular mechanisms in vascular cells behind AD and CAA pathogenesis. First, we discuss apparent vascular changes, including the cerebral hypoperfusion and vascular degeneration that contribute to different stages of the disease process in AD individuals. We next discuss the role of the low-density lipoprotein receptor related protein-1 (LRP), a key Abeta clearance receptor at the BBB and along the cerebrovascular system, whose expression is suppressed early in AD. We also discuss how brain-derived apolipoprotein E isoforms may influence Abeta clearance across the BBB. We then review the role of two interacting transcription factors, myocardin and serum response factor, in cerebral vascular cells in controlling CBF responses and LRP-mediated Abeta clearance. Finally, we discuss the role of microglia and perivascular macrophages in Abeta clearance from the brain. The data reviewed here support an essential role of neurovascular and BBB mechanisms in contributing to both, onset and progression of AD.

LRP1 shedding in human brain: roles of ADAM10 and ADAM17

Background

The low-density lipoprotein receptor-related protein 1 (LRP1) plays critical roles in lipid metabolism, cell survival, and the clearance of amyloid-β (Aβ) peptide. Functional soluble LRP1 (sLRP1) has been detected in circulating human placenta; however, whether sLRP1 is also present in the central nervous system is unclear.

Results

Here we show that abundant sLRP1 capable of binding its ligands is present in human brain tissue and cerebral spinal fluid (CSF). Interestingly, the levels of sLRP1 in CSF are significantly increased in older individuals, suggesting that either LRP1 shedding is increased or sLRP1 clearance is decreased during aging. To examine potential effects of pathological ligands on LRP1 shedding, we treated MEF cells with Aβ peptide and found that LRP1 shedding was increased. ADAM10 and ADAM17 are key members of the ADAM family that process membrane-associated proteins including amyloid precursor protein and Notch. We found that LRP1 shedding was significantly decreased in MEF cells lacking ADAM10 and/or ADAM17. Furthermore, forced expression of ADAM10 increased LRP1 shedding, which was inhibited by ADAM-specific inhibitor TIMP-3.

Conclusion

Our results demonstrate that LRP1 is shed by ADAM10 and ADAM17 and functional sLRP1 is abundantly present in human brain and CSF. Dysregulated LRP1 shedding during aging could alter its function and may contribute to the pathogenesis of AD.

Apolipoprotein E and its receptors in Alzheimer's disease: pathways, pathogenesis and therapy

The vast majority of Alzheimer's disease (AD) cases are late-onset and their development is probably influenced by both genetic and environmental risk factors. A strong genetic risk factor for late-onset AD is the presence of the epsilon4 allele of the apolipoprotein E (APOE) gene, which encodes a protein with crucial roles in cholesterol metabolism. There is mounting evidence that APOE4 contributes to AD pathogenesis by modulating the metabolism and aggregation of amyloid-beta peptide and by directly regulating brain lipid metabolism and synaptic functions through APOE receptors. Emerging knowledge of the contribution of APOE to the pathophysiology of AD presents new opportunities for AD therapy.

Interaction of the apolipoprotein E receptors low density lipoprotein receptor-related protein and sorLA/LR11

In this study, we examined protein-protein interactions between two neuronal receptors, low density lipoprotein receptor-related protein (LRP) and sorLA/LR11, and found that these receptors interact, as indicated by three independent lines of evidence: co-immunoprecipitation experiments on mouse brain extracts and mouse neuronal cells, surface plasmon resonance analysis with purified human LRP and sorLA, and fluorescence lifetime imaging microscopy (FLIM) on rat primary cortical neurons. Immunocytochemistry experiments revealed widespread co-localization of LRP and sorLA within perinuclear compartments of rat primary neurons, while FLIM analysis showed that LRP-sorLA interactions take place within a subset of these compartments.

Lipoprotein receptors and cholesterol in APP trafficking and proteolytic processing, implications for Alzheimer's disease

Amyloid-beta (Abeta) peptide accumulation in the brain is central to the pathogenesis of Alzheimer's disease (AD). Abeta is produced through proteolytic processing of a transmembrane protein, beta-amyloid precursor protein (APP), by beta- and gamma-secretases. Mounting evidence has demonstrated that alterations in APP cellular trafficking and localization directly impact its processing to Abeta. Members of the low-density lipoprotein receptor family, including LRP, LRP1B, SorLA/LR11, and apoER2, interact with APP and regulate its endocytic trafficking. Additionally, APP trafficking and processing are greatly affected by cellular cholesterol content. In this review, we summarize the current understanding of the roles of lipoprotein receptors and cholesterol in APP trafficking and processing and their implication for AD pathogenesis and therapy.

The role of the cell surface LRP and soluble LRP in blood-brain barrier Abeta clearance in Alzheimer's disease

Low-density lipoprotein receptor related protein-1 (LRP) is a member of the low-density lipoprotein (LDL) receptor family which has been linked to Alzheimer's disease (AD) by biochemical and genetic evidence. Levels of neurotoxic amyloid beta-peptide (Abeta) in the brain are elevated in AD contributing to the disease process and neuropathology. Faulty Abeta clearance from the brain appears to mediate focal Abeta accumulations in AD. Central and peripheral production of Abeta from Abeta-precursor protein (APP), transport of peripheral Abeta into the brain across the blood-brain barrier (BBB) via receptor for advanced glycation end products (RAGE), enzymatic Abeta degradation, Abeta oligomerization and aggregation, neuroinflammatory changes and microglia activation, and Abeta elimination from brain across the BBB by cell surface LRP; all may control brain Abeta levels. Recently, we have shown that a soluble form of LRP (sLRP) binds 70 to 90 % of plasma Abeta, preventing its access to the brain. In AD individuals, the levels of LRP at the BBB are reduced, as are levels of Abeta binding to sLRP in plasma. This, in turn, may increase Abeta brain levels through a decreased efflux of brain Abeta at the BBB and/or reduced sequestration of plasma Abeta associated with re-entry of free Abeta into the brain via RAGE. Thus, therapies which increase LRP expression at the BBB and/or enhance the peripheral Abeta "sink" activity of sLRP, hold potential to control brain Abeta accumulations, neuroinflammation and cerebral blood flow reductions in AD.

New therapeutic targets in the neurovascular pathway in Alzheimer's disease

Recent findings indicate that neurovascular dysfunction is an integral part of Alzheimer's disease (AD). Changes in the vascular system of the brain may significantly contribute to the onset and progression of dementia and to the development of a chronic neurodegenerative process. In contrast to the neurocentric view, which proposes that changes in chronic neurodegenerative disorders, including AD, can be attributed solely to neuronal disorder and neuronal dysfunction, the neurovascular concept proposes that dysfunction of non-neuronal neighboring cells and disintegration of neurovascular unit function may contribute to the pathogenesis of dementias in the elderly population, and understanding these processes will be crucial for the development of new therapeutic approaches to normalize both vascular and neuronal dysfunction. In this review, I discuss briefly the role of vascular factors and vascular disorder in AD, the link between cerebrovascular disorder and AD, the clearance hypothesis for AD, the role of RAGE (receptor for advanced glycation end products) and LRP (low density lipoprotein receptor related protein 1) in maintaining the levels of amyloid beta-peptide (Abeta) in the brain by controlling its transport across the blood-brain barrier (BBB), and the role of impaired vascular remodeling and cerebral blood flow dysregulation in the disease process. The therapeutic strategies based on new targets in the AD neurovascular pathway, such as RAGE and LRP receptors, and on a few selected genes implicated in AD neurovascular dysfunction (e.g., mesenchyme homeobox gene 2 and myocardin) are also discussed.

Visualizing interaction of proteins relevant to Alzheimer's disease in intact cells

To understand normal function of memory studying models of pathological memory decline is essential. The most common form of dementia leading to memory decline is Alzheimer's disease (AD), which is characterized by the presence of neurofibrillary tangles and amyloid plaques in the affected brain regions. Altered production of amyloid beta (Abeta) through sequential cleavage of amyloid precursor protein (APP) by beta- and gamma-secretases seems to be a central event in the molecular pathogenesis of the disease. Thus, the study of the complex interplay of proteins that are involved in or modify Abeta production is very important to gain insight into the pathogenesis of AD. Here, we describe the use of Fluorescence lifetime imaging microscopy (FLIM), a Fluorescence resonance energy transfer (FRET)-based method, to visualize protein-protein-interaction in intact cells, which has proven to be a valuable method in AD research.