A protective role of the low density lipoprotein receptor-related protein against amyloid beta-protein toxicity

A multitude of signaling pathways associated with Alzheimer's disease and their roles in AD pathogenesis and therapy

The exact molecular mechanisms associated with Alzheimer's disease (AD) pathology continue to represent a mystery. In the past decades, comprehensive data were generated on the involvement of different signaling pathways in the AD pathogenesis. However, the utilization of signaling pathways as potential targets for the development of drugs against AD is rather limited due to the immense complexity of the brain and intricate molecular links between these pathways. Therefore, finding a correlation and cross-talk between these signaling pathways and establishing different therapeutic targets within and between those pathways are needed for better understanding of the biological events responsible for the AD-related neurodegeneration. For example, autophagy is a conservative cellular process that shows link with many other AD-related pathways and is crucial for maintenance of the correct cellular balance by degrading AD-associated pathogenic proteins. Considering the central role of autophagy in AD and its interplay with many other pathways, the finest therapeutic strategy to fight against AD is the use of autophagy as a target. As an essential step in this direction, this comprehensive review represents recent findings on the individual AD-related signaling pathways, describes key features of these pathways and their cross-talk with autophagy, represents current drug development, and introduces some of the multitarget beneficial approaches and strategies for the therapeutic intervention of AD.

Role of LRP1 in the pathogenesis of Alzheimer's disease: evidence from clinical and preclinical studies

Among the LDL receptor (LDLR) family members, the roles of LDLR-related protein (LRP)1 in the pathogenesis of Alzheimer's disease (AD), especially late-onset AD, have been the most studied by genetic, neuropathological, and biomarker analyses (clinical studies) or cellular and animal model systems (preclinical studies) over the last 25 years. Although there are some conflicting reports, accumulating evidence from preclinical studies indicates that LRP1 not only regulates the metabolism of amyloid-β peptides (Aβs) in the brain and periphery, but also maintains brain homeostasis, impairment of which likely contributes to AD development in Aβ-independent manners. Several preclinical studies have also demonstrated an involvement of LRP1 in regulating the pathogenic role of apoE, whose gene is the strongest genetic risk factor for AD. Nonetheless, evidence from clinical studies is not sufficient to conclude how LRP1 contributes to AD development. Thus, despite very promising results from preclinical studies, the role of LRP1 in AD pathogenesis remains to be further clarified. In this review, we discuss the potential mechanisms underlying how LRP1 affects AD pathogenesis through Aβ-dependent and -independent pathways by reviewing both clinical and preclinical studies. We also discuss potential therapeutic strategies for AD by targeting LRP1.

Functional Roles of the Interaction of APP and Lipoprotein Receptors

The biological fates of the key initiator of Alzheimer’s disease (AD), the amyloid precursor protein (APP), and a family of lipoprotein receptors, the low-density lipoprotein (LDL) receptor-related proteins (LRPs) and their molecular roles in the neurodegenerative disease process are inseparably interwoven. Not only does APP bind tightly to the extracellular domains (ECDs) of several members of the LRP group, their intracellular portions are also connected through scaffolds like the one established by FE65 proteins and through interactions with adaptor proteins such as X11/Mint and Dab1. Moreover, the ECDs of APP and LRPs share common ligands, most notably Reelin, a regulator of neuronal migration during embryonic development and modulator of synaptic transmission in the adult brain, and Agrin, another signaling protein which is essential for the formation and maintenance of the neuromuscular junction (NMJ) and which likely also has critical, though at this time less well defined, roles for the regulation of central synapses. Furthermore, the major independent risk factors for AD, Apolipoprotein (Apo) E and ApoJ/Clusterin, are lipoprotein ligands for LRPs. Receptors and ligands mutually influence their intracellular trafficking and thereby the functions and abilities of neurons and the blood-brain-barrier to turn over and remove the pathological product of APP, the amyloid-β peptide. This article will review and summarize the molecular mechanisms that are shared by APP and LRPs and discuss their relative contributions to AD.

Role of ROS and RNS Sources in Physiological and Pathological Conditions

There is significant evidence that, in living systems, free radicals and other reactive oxygen and nitrogen species play a double role, because they can cause oxidative damage and tissue dysfunction and serve as molecular signals activating stress responses that are beneficial to the organism. Mitochondria have been thought to both play a major role in tissue oxidative damage and dysfunction and provide protection against excessive tissue dysfunction through several mechanisms, including stimulation of opening of permeability transition pores. Until recently, the functional significance of ROS sources different from mitochondria has received lesser attention. However, the most recent data, besides confirming the mitochondrial role in tissue oxidative stress and protection, show interplay between mitochondria and other ROS cellular sources, so that activation of one can lead to activation of other sources. Thus, it is currently accepted that in various conditions all cellular sources of ROS provide significant contribution to processes that oxidatively damage tissues and assure their survival, through mechanisms such as autophagy and apoptosis.

Targeting α-synuclein oligomers by protein-fragment complementation for drug discovery in synucleinopathies

OBJECTIVE

Reducing the burden of α-synuclein oligomeric species represents a promising approach for disease-modifying therapies against synucleinopathies such as Parkinson's disease and dementia with Lewy bodies. However, the lack of efficient drug discovery strategies that specifically target α-synuclein oligomers has been a limitation to drug discovery programs.

RESEARCH DESIGN AND METHODS

Here we describe an innovative strategy that harnesses the power of bimolecular protein-fragment complementation to monitor synuclein-synuclein interactions. We have developed two robust models to monitor α-synuclein oligomerization by generating novel stable cell lines expressing α-synuclein fusion proteins for either fluorescent or bioluminescent protein-fragment complementation under the tetracycline-controlled transcriptional activation system.

MAIN OUTCOME MEASURES

A pilot screen was performed resulting in the identification of two potential hits, a p38 MAPK inhibitor and a casein kinase 2 inhibitor, thereby demonstrating the suitability of our protein-fragment complementation assay for the measurement of α-synuclein oligomerization in living cells at high throughput.

CONCLUSIONS

The application of the strategy described herein to monitor α-synuclein oligomer formation in living cells with high throughput will facilitate drug discovery efforts for disease-modifying therapies against synucleinopathies and other proteinopathies.

Apolipoprotein E and lipid homeostasis in the etiology and treatment of sporadic Alzheimer's disease

The discovery that the apolipoprotein E (apoE) ε4 allele is genetically linked to both sporadic and familial late-onset Alzheimer's disease (AD) raises the possibility that a dysfunction of the lipid transport system could seriously affect lipid homeostasis in the brain of AD subjects. The presence of the ε4 allele has been associated with lower levels of apoE in both serum and brain tissues of normal and AD subjects. In an attempt to reverse the apoE deficit in AD, we identified and characterized several apoE inducer agents using a low-throughput in vitro screening assay. The most promising of these compounds is called probucol. Administration of probucol, an old cholesterol-lowering drug, in a pilot trial in mild-to-moderate sporadic AD led to a significant increase in cerebrospinal fluid (CSF) apoE levels and a decrease in CSF in both phosphorylated tau 181 and beta-amyloid 1-42 concentrations without significant modifications of lipid hydroperoxide levels.

The Nrf2-ARE pathway: a valuable therapeutic target for the treatment of neurodegenerative diseases

Modulation of NF-E2 related factor 2 (Nrf2) has been shown in several neurodegenerative disorders. The overexpression of Nrf2 has become a potential therapeutic avenue for various neurodegenerative disorders such as Parkinson, Amyotrophic lateral sclerosis, and Alzheimer's disease. The expression of phase II detoxification enzymes is governed by the cis-acting regulatory element known as antioxidant response element (ARE). The transcription factor Nrf2 binds to ARE thereby transcribing multitude of antioxidant genes. Keap1, a culin 3-based E3 ligase that targets Nrf2 for degradation, sequesters Nrf2 in cytoplasm. Disruption of Keap1-Nrf2 interaction or genetic overexpression of Nrf2 can increase the endogenous antioxidant capacity of the brain thereby rendering protection against oxidative stress in neurodegenerative disorders. This review primarily focuses on recent patents that target Nrf2 overexpression as a promising therapeutic strategy for the treatment of neurodegenerative disorders.

Protease-activated alpha-2-macroglobulin can inhibit amyloid formation via two distinct mechanisms

α₂-Macroglobulin (α₂M) is an extracellular chaperone that inhibits amorphous and fibrillar protein aggregation. The reaction of α₂M with proteases results in an ‘activated’ conformation, where the proteases become covalently-linked within the interior of a cage-like structure formed by α₂M. This study investigates, the effect of activation on the ability of α₂M to inhibit amyloid formation by Aβ_1–42 and I59T human lysozyme and shows that protease-activated α₂M can act via two distinct mechanisms: (i) by trapping proteases that remain able to degrade polypeptide chains and (ii) by a chaperone action that prevents misfolded clients from continuing along the amyloid forming pathway.

Structured summary of protein interactions

Aβ_1–42 and Aβ_1–42 bind by fluorescence technology (View interaction)I59T lysozyme and I59T lysozyme bind by light scattering (View interaction)I59T lysozyme and I59T lysozyme bind by fluorescence technology (View interaction)Alpha-lactalbumin and Alpha-lactalbumin bind by fluorescence technology (View interaction)I59T lysozyme and I59T lysozyme bind by electron microscopy (View interaction)Aβ_1–42 and Aβ_1–42 bind by electron microscopy (View interaction)

Serum levels of pregnancy zone protein are elevated in presymptomatic Alzheimer's disease

We have sought for disease-related proteins that could predict the onset of Alzheimer's disease (AD) in a study population derived from the Rotterdam Scan Study, a population-based prospective cohort study designed to investigate the etiology and natural history of age-related brain changes in the elderly. The serum proteome of 43 persons who developed AD, after an average of 4.2 years (±2.6 years SD) after blood sampling, and 43 gender- and age-matched controls who remained dementia-free during follow-up was investigated by liquid chromatography mass spectrometry. We identified 61 differentially expressed peptides between presymptomatic AD and controls, 9 of which were derived from pregnancy zone protein (PZP). Quantitative measurements using a multiple reaction monitoring assay showed a significant increase in concentration of PZP in presymptomatic AD (34.3 ± 20.6 mg/L) compared with controls (23.6 ± 13.6 mg/L) (p = 0.006). The difference in PZP was significant in women. Immunohistochemical validation of the findings on brain tissue sections showed strong PZP expression in senile plaques and in microglial and glial cells in AD with only low expression in some scattered glial cells in controls.

Increased beta-amyloid levels in the choroid plexus following lead exposure and the involvement of low-density lipoprotein receptor protein-1

The choroid plexus, a barrier between the blood and cerebrospinal fluid (CSF), is known to accumulate lead (Pb) and also possibly function to maintain brain's homeostasis of Abeta, an important peptide in the etiology of Alzheimer's disease. This study was designed to investigate if Pb exposure altered Abeta levels at the blood-CSF barrier in the choroid plexus. Rats received ip injection of 27 mg Pb/kg. Twenty-four hours later, a FAM-labeled Abeta (200 pmol) was infused into the lateral ventricle and the plexus tissues were removed to quantify Abeta accumulation. Results revealed a significant increase in intracellular Abeta accumulation in the Pb-exposed animals compared to controls (p<0.001). When choroidal epithelial Z310 cells were treated with 10 microM Pb for 24 h and 48 h, Abeta (2 microM in culture medium) accumulation was significantly increased by 1.5 fold (p<0.05) and 1.8 fold (p<0.05), respectively. To explore the mechanism, we examined the effect of Pb on low-density lipoprotein receptor protein-1 (LRP1), an intracellular Abeta transport protein. Following acute Pb exposure with the aforementioned dose regimen, levels of LRP1 mRNA and proteins in the choroid plexus were decreased by 35% (p<0.05) and 31.8% (p<0.05), respectively, in comparison to those of controls. In Z310 cells exposed to 10 microM Pb for 24 h and 48 h, a 33.1% and 33.4% decrease in the protein expression of LRP1 was observed (p<0.05), respectively. Knocking down LRP1 resulted in even more substantial increases of cellular accumulation of Abeta, from 31% in cells without knockdown to 72% in cells with LRP1 knockdown (p<0.05). Taken together, these results suggest that the acute exposure to Pb results in an increased accumulation of intracellular Abeta in the choroid plexus; the effect appears to be mediated, at least in part, via suppression of LRP1 production following Pb exposure.

TGF beta2-induced changes in LRP-1/T beta R-V and the impact on lysosomal A beta uptake and neurotoxicity

Numerous studies suggest a central role for the low-density lipoprotein receptor-related protein/transforming growth factor beta receptor V in Alzheimer's Disease. We continue our investigation of a ligand for this receptor, transforming growth factor beta2, which is also implicated in Alzheimer Disease pathogenesis, but whose mechanism(s) remain elusive. Confocal imaging reveals that transforming growth factor beta2 rapidly targets amyloid beta peptide to the lysosomal compartment in cortical neurons and induces cell death. Low-density lipoprotein receptor-related protein/transforming growth factor beta receptor V is known as an endocytic receptor, delivering proteins to the lysosomal compartment for degradation. Transforming growth factor beta2 may alter this pathway resulting in increased uptake, intracellular accumulation and toxicity of amyloid beta peptide. RT-PCR and Western blot analysis of transforming growth factor beta2-treated cells demonstrate that transforming growth factor beta2 modestly increases the mRNA and protein levels of low-density lipoprotein receptor-related protein/transforming growth factor beta receptor V as well as increases the uptake activity. Furthermore, transforming growth factor beta2 alters the morphology and numbers of lysosomes in neurons. Lucifer Yellow and lysosomal hydrolase analysis show that transforming growth factor beta2 makes lysosomal membranes unstable and leaky and this effect is exacerbated with the addition of amyloid beta protein. Our data support a key role for low-density lipoprotein receptor-related protein/transforming growth factor beta receptor V in mediating transforming growth factor beta2 enhancement of amyloid beta peptide uptake and neurotoxicity.

Long-term soluble Aβ1–40 activates CaM kinase II in organotypic hippocampal cultures

Recent findings suggested a role for soluble amyloid-beta (Abeta) peptides in Alzheimer's disease associated cognitive decline. We investigated the action of soluble, monomeric Abeta(1-40) on CaM kinase II, a kinase involved in neuroplasticity and cognition. We treated organotypic hippocampal cultures short-term (up to 4h) and long-term (5 days) with Abeta(1-40) (1nM-5microM). Abeta did not induce cell damage, apoptosis or synaptic loss. Short-term treatment down-regulated enzymatic activity of the kinase, by reducing its Thr(286) phosphorylation. In contrast, long-term treatment (1nM-microM) markedly and significantly up-regulated enzymatic activity, with peak stimulation at 10nM (three-fold). Up-regulation of activity was associated with increased expression of the alpha-isoform of CaM kinase II, increased phosphorylation at Thr(286) (activator residue) and decreased phosphorylation at Thr(305-306) (inhibitory residues). We investigated the effect of glutamate on CaM kinase II following exposure to 1 or 10nM Abeta(1-40). As previously reported, glutamate increased CaM kinase II activity. However, the glutamate effect was not altered by pretreatment of slices with Abeta. Short- and long-term Abeta treatment showed opposite effects on CaM kinase II, suggesting that long-term changes are an adaptation to the kinase early down-regulation. The marked effect of Abeta(1-40) on the kinase suggests that semi-physiological and slowly raising peptide concentrations may have a significant impact on synaptic plasticity in the absence of synaptic loss or neuronal cell death.

Expression of APP pathway mRNAs and proteins in Alzheimer's disease

In both trisomy 21 and rare cases of triplication of amyloid precursor protein (APP) Alzheimer's disease (AD) pathological changes are believed to be secondary to increased expression of APP. We hypothesized that sporadic AD may also be associated with changes in transcription of APP or its metabolic partners. To address this issue, temporal neocortex of 27 AD and 21 non-demented control brains was examined to assess mRNA levels of APP isoforms (total APP, APP containing the Kunitz protease inhibitor domain [APP-KPI] and APP770) and APP metabolic enzymatic partners (the APP cleaving enzymes beta-secretase [BACE] and presenilin-1 [PS-1], and putative clearance molecules, low-density lipoprotein receptor protein [LRP] and apolipoprotein E [apoE]). Furthermore, we evaluated how changes in APP at the mRNA level affect the amount of Tris buffer extractable APP protein and Abeta40 and 42 peptides in AD and control brains. As assessed by quantitative PCR, APP-KPI (p=0.007), APP770 (p=0.004), PS-1 (p=0.004), LRP (p=0.003), apoE (p=0.0002) and GFAP (p<0.0001) mRNA levels all increased in AD, and there was a shift from APP695 (a neuronal isoform) towards KPI containing isoforms that are present in glia as well. APP-KPI mRNA levels correlated with soluble APPalpha-KPI protein (sAPPalpha-KPI) levels measured by ELISA (tau=0.33, p=0.015 by Kendall's rank correlation); in turn, soluble APPalpha-KPI protein levels positively correlated with Tris-extractable, soluble Abeta40 (p=0.046) and 42 levels (p=0.007). The ratio of soluble APPalpha-KPI protein levels to total APP protein increased in AD, and also correlated with GFAP protein levels in AD. These results suggest that altered transcription of APP in AD is proportionately associated with Abeta peptide, may occur in the context of gliosis, and may contribute to Abeta deposition in sporadic AD.

Dementia epidemiology research: it is time to modify the focus of research

The incidence and prevalence of dementia are increasing. There is an urgent need to develop a preventive strategy. The identification of modifiable risk factors must therefore be a high priority. Newer imaging techniques provide an opportunity to identify subclinical manifestations of "dementias" that can be limited to the risk factors and subsequent clinical disease. The contribution of vascular disease to dementia and Alzheimer's disease (AD) should be a high priority as it offers a potential preventive strategy. Study designs need to be modified, including a greater emphasis on geographic variations in AD and dementia based on imaging studies, longitudinal studies of successful aging without cardiovascular disease (CVD) or AD, gene-environment interactions, and studies of unique populations that may be at lower risk because of specific lifestyles. Primary prevention trials for vascular disease should include a dementia component. Most, if not all, studies should include newer imaging studies.

Lipoprotein receptors in Alzheimer's disease

Lipoprotein receptors have important roles in pathological processes that lead to Alzheimer's disease (AD). Previously, they were believed to act mainly by modulating the neuronal metabolism of cholesterol and apolipoprotein E, major risk factors for spontaneous AD. However, recent findings point towards an unexpected new function for lipoprotein receptors in regulation of intracellular transport and processing of the amyloid precursor protein (APP) to give amyloid-beta peptide, the principal component of senile plaques. Here, we will discuss how lipoprotein receptors might modulate distinct steps in neuronal trafficking of APP, and how an intricate balance between opposing receptor activities might be a crucial determinant of APP processing, and of onset and progression of neurodegeneration.

Loss of nicastrin elicits an apoptotic phenotype in mouse embryos

Nicastrin is a member of the high molecular weight presenilin complex that plays a central role in gamma-secretase cleavage of numerous type-1 membrane-associated proteins required for cell signaling, proliferation and lineage development. We have generated a nicastrin-null mouse line by disruption of exon 3. Similar to previously described nicastrin-null mice, these animals demonstrate severe growth retardation, mortality beginning at embryonic age 10.5 days, and marked developmental abnormalities indicative of a severe Notch phenotype. Preceding their mortality, 10.5-day-old nicastrin-null embryos were found to also exhibit specific apoptosis within regions showing profound deformities, particularly in the developing heart and brain. This result suggests that complete disruption of presenilin complexes elicits programmed cell death, in addition to a Notch phenotype, which may contribute to the developmental abnormalities and embryonic mortality of nicastrin-null mice and possibly neurodegeneration in Alzheimer's disease.

A chemically modified preparation of alpha2-macroglobulin binds beta-amyloid peptide with increased affinity and inhibits Abeta cytotoxicity

Macromolecules that bind beta-amyloid peptide (Abeta) and neutralize Abeta cytotoxicity offer a promising new approach for treating Alzheimer's disease. When the plasma protein, alpha2-macroglobulin (alpha2M), is treated with methylamine (alpha2M-MA), it undergoes conformational change and acquires Abeta-binding activity. In this study, we demonstrate that a chemically stabilized preparation of human alpha2M conformational intermediates (alpha2M-cis-Pt/MA) binds Abeta with greatly increased affinity, compared with alpha2M-MA. alpha2M-cis-Pt/MA was generated by reacting alpha2M with the protein cross-linking reagent, cis-Pt, followed by methylamine. Increased Abeta-binding to alpha2M-cis-Pt/MA was demonstrated by co-migration of radio-iodinated proteins in non-denaturing PAGE, chemical cross-linking, and co-immunoprecipitation. The apparent K(D) for Abeta-binding to alpha2M-cis-Pt/MA was decreased 10-fold, compared with alpha2M-MA, to 29 nm. Native alpha2M demonstrated negligible Abeta-binding, as anticipated. alpha2M-cis-Pt/MA markedly counteracted Abeta-induced C6 cell apoptosis. Essentially complete inhibition of apoptosis was observed even when the Abeta was present at fourfold molar excess to alpha2M-cis-Pt/MA. Under equivalent conditions, alpha2M-MA inhibited apoptosis by 25 +/- 6%. When Abeta and alpha2M-cis-Pt/MA were added to human plasma in vitro, significant binding was detected. No binding was observed when an equivalent concentration of native alpha2M or alpha2M-MA was added to plasma. We propose that alpha2M-cis-Pt/MA is a novel alternative to Abeta-specific antibodies, for studying the efficacy of Abeta-binding agents in vitro and in vivo.

The expression of calcium/calmodulin-dependent protein kinase II-α in the hippocampus of patients with Alzheimer's disease and its links with AD-related pathology

Alzheimer's disease (AD) is characterized pathologically by selective neuronal loss and by the formation of neurofibrillary tangles (NFTs) and senile plaques (SPs). Since calcium/calmodulin-dependent protein kinase II-alpha (CaMKII-alpha), one of the most abundant kinases in the brain, is involved in the phosphorylation of tau and amyloid precursor protein (APP), we examined the expression of CaMKII-alpha and its relationships with the neuropathology in the hippocampus of AD patients using immunohistochemistry and double-labeling immunofluorescence methods. The results showed that CaMKII-alpha containing neurons were selectively lost in the CA1 subfield of AD hippocampus and accompanied with enhanced immunoreactivity in the remaining neurons. About 33% hyperphosphorylated tau-containing neurons labeled by monoclonal antibody AT-8 were also immunoreactive for CaMKII-alpha. Moreover, we found for the first time that the immunoreactivity of CaMKII-alpha was largely deposited in the SPs of the AD hippocampus. The pattern of the co-localization of CaMKII-alpha with beta amyloid depended on the type of SPs. Since the co-localization of CaMKII-alpha with hyperphosphorylated tau is relatively rare, we concluded that CaMKII-alpha may be related with beta-amyloid more closely than being involved in tau hyperphosphorylation.

LRP and Alzheimer's Disease

The low-density lipoprotein receptor (LDLR)-related protein, LRP, is a unique member of the LDLR family. Frequently referred to as a scavenger receptor, LRP is a large transmembrane endocytic receptor that can bind and internalize many functionally distinct ligands. Besides its role as a cargo-receptor, LRP has also been implicated in many signaling pathways. LRP knockout mice die at early embryonic age, which strongly suggests that LRP's functions are essential for normal development. Within the CNS, LRP is highly expressed in neuronal cell bodies and dendritic processes. In vitro, neurite outgrowth is stimulated by apolipoprotein E (apoE)-containing lipoprotein particles via binding to LRP. ApoE is the major cholesterol transporter in the brain and human carriers of one or two copies of the e4 allele of apoE are at a higher risk of developing Alzheimer's disease (AD). LRP also binds the amyloid precursor protein (APP) and its proteolytic fragment, the amyloid-beta peptide (Abeta), which are major players in the pathogenesis of AD. Finally, LRP has been linked to AD by genetic evidence. In this review we discuss the potential mechanisms by which LRP can affect APP and Abeta metabolism, and therefore contribute to the pathogenesis of AD.

Midkine and pleiotrophin in neural development and cancer

The midkine (MK) family consists of only two members, namely heparin-binding growth factors MK and pleiotrophin (PTN). During embryogenesis, MK is highly expressed in the mid-gestational period, whereas PTN expression reaches the maximum level around birth. Both proteins are localized in the radial glial processes of the embryonic brain, along which neural stem cells migrate and differentiate. Zebrafish and Xenopus MK can induce neural tissues. In addition, deposits of MK and/or PTN are found in neurodegenerative diseases, such as Alzheimer's disease and multiple system atrophy. Both molecules are induced in reactive astrocytes by ischemic insults. In this context, it is interesting that LDL receptor-related protein is a receptor for MK and PTN, and this receptor has been implicated in the pathogenesis of Alzheimer's disease. MK and PTN share receptors, and show similar biological activities that include fibrinolytic, anti-apoptotic, mitogenic, transforming, angiogenic, and chemotactic ones. These activities explain how these molecules are involved in carcinogenesis. MK is detected in human carcinoma specimens from pre-cancerous stages to advanced stages. Strong expression of PTN is also detected in several carcinomas, although, in general, MK is expressed more intensely and in a wide range of carcinomas than PTN. The blood MK level is frequently elevated in advanced human carcinomas, decreases after surgical removal of the tumors, and is correlated with prognostic factors. Thus, it is a good market for evaluating the progress of carcinomas. Furthermore, antisense oligonucleotides for MK and ribozymes for PTN show anti-tumor activity. Therefore, MK and PTN are candidate molecular targets for therapy for human carcinomas.

Modulation of calcium/calmodulin kinase-II provides partial neuroprotection against beta-amyloid peptide toxicity

Beta-amyloid (Abeta) peptide-induced neurotoxicity has been implicated in the pathogenesis of Alzheimer's disease (AD). The exact mechanism by which Abeta peptides trigger neuronal death is not well defined and may be related to an abrupt increase in intracellular calcium, leading to the activation of many pro-apoptotic pathways. While modulation of intracellular calcium increase receives much attention for pharmaceutical intervention, Ca2+-mediated pro-apoptotic signalling pathways have not been systematically studied. We have reported our study on the roles of calcium/calmodulin-dependent protein kinase II (CaMKII) in Abeta peptide neurotoxicity. By treating the primary cortical neurons exposed to Abeta peptides (Abeta(25-35) and Abeta(1-42)) with two selective CaMKII inhibitors, autocamtide-related inhibitory peptide (AIP) and KN93, Abeta peptide neurotoxicity was significantly reduced. Release of LDH and DNA fragmentation/condensation (by DAPI staining) in neurons exposed to Abeta peptides were significantly decreased in the presence of AIP and KN93. While these inhibitors significantly attenuated Abeta peptide-triggered activation of caspase-2 and caspase-3, and AIP significantly decreased the degree of tau phosphorylation of the Abeta peptide-treated neurons at early time, they could elicit partial neuroprotection only. Pharmacological inhibitor targeting calmodulin, W7, did not provide neuroprotection. Morphine, which activates CaMKII via micro receptors, augments Abeta-induced LDH release, caspase-2 and caspase-3 activities and neuronal apoptosis. Taken together, although CaMKII plays a role in Abeta peptide neurotoxicity, pharmacological inhibition cannot afford complete neuroprotection.

The low-density lipoprotein receptor-related protein associates with calnexin, calreticulin, and protein disulfide isomerase in receptor-associated-protein-deficient fibroblasts

The low-density lipoprotein receptor-related protein (LRP) is a large (>600 kDa) multi-ligand-binding cell surface receptor that is now known to participate in a diverse range of cellular events. To accomplish this diverse role, LRP is composed of repetitive amino acid motifs consisting of complement-type and EGF precursor-type repeats. Within these repeats are six conserved cysteine residues that form the core disulfide bond structure of each repeat. To accommodate the intricate folding that such a complex structure dictates, a specialized chaperone is present in the endoplasmic reticulum (ER) called the receptor-associated protein (RAP) that binds to LRP immediately following its biosynthesis and assists in its exocytic transport. Interestingly, RAP -/- mice show reduced LRP expression in certain cell types, but not a more global affect on LRP expression that was expected. Such a tissue-restricted effect by RAP prompted an investigation if other ER chaperones associate with LRP to assist in its complex folding requirements and compensate for the absence of RAP in RAP -/- cells. Fibroblasts obtained from RAP -/- mice demonstrate similar LRP expression levels and subcellular distribution as RAP +/+ fibroblasts. Moreover, RAP -/- cells show an identical exocytic trafficking rate for LRP as RAP +/+ cells and comparable cell surface internalization kinetics. In RAP -/- cells, three well-known ER chaperones, calnexin, calreticulin, and protein disulfide isomerase (PDI), associate with LRP and likely compensate for the absence of RAP.

Increased soluble amyloid-beta peptide and memory deficits in amyloid model mice overexpressing the low-density lipoprotein receptor-related protein

Amyloid-beta peptide (Abeta) is central to the pathogenesis of Alzheimer's disease, and the low-density lipoprotein receptor-related protein (LRP) has been shown to alter Abeta metabolism in vitro. Here, we show that overexpression of a functional LRP minireceptor in the brain of PDAPP mice results in age-dependent increase of soluble brain Abeta, with no changes in Abeta plaque burden. Importantly, soluble brain Abeta was found to be primarily in the form of monomers/dimers and to be highly correlated with deficits in spatial learning and memory. These results provide in vivo evidence that LRP may contribute to memory deficits typical of Alzheimer's disease by modulating the pool of small soluble forms of Abeta.

Functional analysis of plasma α2-macroglobulin from Alzheimer's disease patients with the A2M intronic deletion

alpha(2)-Macroglobulin (alpha(2)M) is an abundant plasma/extracellular space protein implicated in clearance of amyloid beta (Abeta), a key constituent of Alzheimer's disease (AD) plaques. alpha(2)M also regulates proteinase and growth factor activities. In recent years, there have been >30 genetic studies debating the controversial role of a five-base-pair intronic deletion in the A2M gene in late-onset AD. However, little is known about potential effects of the deletion upon alpha(2)M function. In this study, we examined the subunit and conformational structure of alpha(2)M in AD plasma samples, and its capacity to bind trypsin, transforming growth factor-beta1, and Abeta. Plasma from patients homozygous for the deletion (DD) showed normal alpha(2)M subunit size, conformation, and proteinase inhibitory activity. Interestingly, plasma alpha(2)M from two DD patients showed markedly increased TGF-beta1 binding. Moreover, methylamine-treated DD plasma samples showed modest, but significant, elevations in Abeta binding to alpha(2)M* compared with samples from patients lacking the deletion. These observations suggest a possible functional basis by which the A2M deletion may influence multifactorial AD pathogenesis.

Lack of association between the levels of the low-density lipoprotein receptor-related protein (LRP) and either Alzheimer dementia or LRP exon 3 genotype

The low-density lipoprotein receptor-related protein (LRP), which interacts with the Alzheimer disease (AD) beta-amyloid precursor protein (APP), represents an important pathway in AD pathology. LRP-mediated receptor pathways appear to regulate both the production and the clearance of amyloid beta-protein (Abeta), a principal neuropathological product in AD. Several conflicting studies have examined levels of LRP in AD brains, as well as the relationship between the LRP exon 3 (C766T) polymorphism and LRP levels and/or disease susceptibility. In order to further investigate the role of LRP in AD, we examined well-characterized brain samples collected from subjects with varying degrees of cognitive impairment for LRP protein expression levels as well as for the presence of the LRP exon 3 polymorphism. We found no correlation between LRP levels and either presence of the disease or cognitive decline. In addition, we found no correlation between the LRP exon 3 polymorphism and either AD or LRP levels.

Alzheimer's disease: the cholesterol connection

A hallmark of all forms of Alzheimer's disease (AD) is an abnormal accumulation of the beta-amyloid protein (Abeta) in specific brain regions. Both the generation and clearance of Abeta are regulated by cholesterol. Elevated cholesterol levels increase Abeta in cellular and most animals models of AD, and drugs that inhibit cholesterol synthesis lower Abeta in these models. Recent studies show that not only the total amount, but also the distribution of cholesterol within neurons, impacts Abeta biogenesis. The identification of a variant of the apolipoprotein E (APOE) gene as a major genetic risk factor for AD is also consistent with a role for cholesterol in the pathogenesis of AD. Clinical trials have recently been initiated to test whether lowering plasma and/or neuronal cholesterol levels is a viable strategy for treating and preventing AD. In this review, we describe recent findings concerning the molecular mechanisms underlying the cholesterol-AD connection.

Increased extracellular amyloid deposition and neurodegeneration in human amyloid precursor protein transgenic mice deficient in receptor-associated protein

The low-density lipoprotein receptor-related protein (LRP) is an abundant neuronal cell surface receptor that regulates amyloid beta-protein (Abeta) trafficking into the cell. Specifically, LRP binds secreted Abeta complexes and mediates its degradation. Previously, we have shown in vitro that the uptake of Abeta mediated by LRP is protective and that blocking this receptor significantly enhances neurotoxicity. To further characterize the effects of LRP and other lipoprotein receptors on Abeta deposition, an in vivo model of decreased LRP expression, receptor-associated protein (RAP)-deficient (RAP-/-) mice was crossed with human amyloid protein precursor transgenic (hAPP tg) mice, and plaque formation and neurodegeneration were analyzed. We found that, although the age of onset for plaque formation was the same in hAPP tg and hAPP tg/RAP-/- mice, the amount of amyloid deposited doubled in the hAPP tg/RAP-/- background. Moreover, these mice displayed increased neuronal damage and astrogliosis. Together, these results further support the contention that LRP and other lipoprotein receptors might be neuroprotective against Abeta toxicity and that this receptor might play an integral role in Abeta clearance.

Serum deprivation increases the expression of low density lipoprotein receptor-related protein in primary cultured rat astrocytes

The low density lipoprotein receptor (LDLR)-related protein (LRP) is a multifunctional receptor which mediates the endocytic uptake of several ligands implicated in Alzheimer's disease pathophysiology. Although LRP, as a member of the LDLR family, is likely to be regulated in response to various cellular stresses, this regulation has not been fully understood yet. In the present study we studied the regulation of LRP expression in primary cultured rat astrocytes in response to serum deprivation as a general cellular stress. A significant increase in LRP expression was detected after serum deprivation and this increase was blocked by treatment of U0126, an inhibitor of MAP kinase. This serum deprivation action was partially reversed by either serum or D-glucose supplementation, but further augmented by glutamine. This result contrasted with a finding that glutamine suppressed gadd153 protein induced by serum deprivation. Taken together, the present data suggest that serum deprivation induces dramatically LRP expression in astrocytes partly by MAPK signaling pathways and by signaling pathways apparently distinct from gadd153 induction.

alpha-Synuclein protects against oxidative stress via inactivation of the c-Jun N-terminal kinase stress-signaling pathway in neuronal cells

The expression of alpha-synuclein, a synaptic molecule implicated in the pathogenesis of neurodegenerative disorders such as Parkinson's disease and Lewy body disease is increased upon injury to the nervous system, indicating that it might play a role in regeneration and plasticity; however, the mechanisms are unclear. Because c-Jun N-terminal kinase (JNK), a member of the mitogen-activated protein kinase family, plays an important role in stress response, the main objective of the present study was to better understand the involvement of this pathway in the signaling responses associated with resistance to injury in cells expressing alpha-synuclein. For this purpose, the JNK-signaling pathway was investigated in alpha-synuclein-transfected neuronal cell line glucose transporter (GT) 1-7 under oxidative stress conditions. Although hydrogen peroxide challenge resulted in JNK activation and cell death in cells transfected with vector control or beta-synuclein, alpha-synuclein-transfected cells were resistant to hydrogen peroxide, and JNK was not activated. The inactivation of JNK in the alpha-synuclein-transfected cells was associated with increased expression and activity of JNK-interacting protein (JIP)-1b/islet-brain (IB)1, the scaffold protein for the JNK pathway. Similarly, cells transfected with JIP-1b/IB1 were resistant to hydrogen peroxide associated with inactivation of the JNK pathway. In these cells, expression of endogenous alpha-synuclein was significantly increased at the protein level. Furthermore, alpha-synuclein was co-localized with JIP-1b/IB1 in the growth cones. Taken together, these results suggest that increased alpha-synuclein expression might protect cells from oxidative stress by inactivation of JNK via increased expression of JIP-1b/IB1. Furthermore, interactions between alpha-synuclein and JIP-1b/IB1 may play a mutual role in the neuronal response to injury and neurodegeneration.

Apolipoprotein E modulates Alzheimer's Abeta(1-42)-induced oxidative damage to synaptosomes in an allele-specific manner

Several functional differences have been reported among the three human e2, e3, and e4 alleles of apolipoprotein E (apoE). One functional difference lies in the antioxidant potential of these alleles; e4 has the poorest potential. Interestingly, e4 also correlates with increased oxidative damage in the Alzheimer's disease (AD) brain, which may explain why the inheritance of the e4 allele is a risk factor for the onset of AD. Beta-amyloid (Abeta) is also intimately involved in AD and promotes oxidative damage in vitro; therefore, we have examined the role of the different apoE alleles in modulating Abeta(1-42)-induced oxidation to synaptosomes. Measurement of specific markers of oxidation in synaptosomes isolated from mice that express one of the human apoE alleles indicates that Abeta-induced increases of these markers can be modulated by apoE in an allele-dependent manner (e2>e3>e4). Increases in reactive oxygen species formation and protein and lipid oxidation were always greatest in e4 synaptosomes as compared to e2 and e3 synaptosomes. Our data support the role of apoE as a modulator of Abeta toxicity and, consistent with the antioxidant potentials of the three alleles, suggest that the e4 allele may not be as effective in this role as the e2 or e3 alleles of apoE. These results are discussed with reference to mechanistic implications for neurodegeneration in the AD brain.

Case-control study and meta-analysis of low density lipoprotein receptor-related protein gene exon 3 polymorphism in Alzheimer's disease

The low density lipoprotein receptor-related protein (LRP) may influence both the clearance and the production of beta-amyloid peptide and thus plays a role in Alzheimer's disease (AD) pathogenesis. Previous studies, although inconsistent, have suggested that the LRP exon 3 CC genotype contributes to the risk of AD. A case-control study utilizing a clinically well-defined group of 305 sporadic AD patients and 304 control subjects was performed to test this association in an ethnically homogeneous population from Spain. In the current study, the LRP CC genotype was not over-represented in AD patients compared to non-demented controls. A meta-analysis of previous studies revealed a weak correlation of LRP CC genotype with AD (odds ratio of 1.35, P=0.01).

Role of alpha2-macroglobulin in regulating amyloid beta-protein neurotoxicity: protective or detrimental factor?

alpha2-Macroglobulin (alpha2M) has been identified as a carrier protein for beta-amyloid (Abeta) decreasing fibril formation and affecting the neurotoxicity of this peptide. The alpha2-macroglobulin receptor/low density lipoprotein receptor related protein (LRP) is involved in the internalization and degradation of the alpha2M/Abeta complexes and its impairment has been reported to occur in Alzheimer's disease. Previous studies have shown alpha2M to determine an enhancement or a reduction of Abeta toxicity in different culture systems. In order to clarify the role of alpha2M in Abeta neurotoxicity, we challenged human neuroblastoma cell lines with activated alpha2M in combination with Abeta. Our results show that in neuroblastoma cells expressing high levels of LRP, the administration of activated alpha2M protects the cells from Abeta neurotoxicity. Conversely, when this receptor is not present alpha2M determines an increase in Abeta toxicity as evaluated by MTT and TUNEL assays. In LRP-negative cells transfected with the full-length human LRP, the addition of activated alpha2M resulted to be protective against Abeta-induced neurotoxicity. By means of recombinant proteins we ascribed the neurotoxic activity of alpha2M to its FP3 fragment which has been previously shown to bind and neutralize transforming growth factor-beta. These studies provide evidence for both a neuroprotective and neurotoxic role of alpha2M regulated by the expression of its receptor LRP.

Low-density lipoprotein receptor family: endocytosis and signal transduction

The low-density lipoprotein receptor (LDLR) family is composed of a class of single transmembrane glycoproteins, generally recognized as cell surface endocytic receptors, which bind and internalize extracellular ligands for degradation by lysosomes. Structurally, members of the LDLR family share homology within their extracellular domains, which are highlighted by the presence of clusters of ligand-binding repeats. Recently, information regarding the structural and functional elements within their cytoplasmic tails has begun to emerge, which suggests that members of the LDLR family function not only in receptor-mediated endocytosis, but also in transducing signals that are important during embryonic development and the pathogenesis of Alzheimer's disease. This review focuses on recent knowledge of the structural and functional aspects of LDLR family members in endocytosis and signal transduction. The relationship of these functions to the development of the neuronal system and in the pathogenesis of Alzheimer's disease is specifically discussed.