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

Low-Density Lipoprotein Receptor-Related Protein 1 as a Potential Therapeutic Target in Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease impacting the lives of millions of people worldwide. The formation of amyloid β (Aβ) plagues in the brain is the main pathological hallmark of AD. The Aβ deposits are formed due to the imbalance between the production and Aβ clearance in the brain and across the blood-brain barrier (BBB). In this respect, low-density lipoprotein receptor-related protein 1 (LRP1) plays a significant role by mediating both brain Aβ production and clearance. Due to its important role in AD pathogenesis, LRP1 is considered an attractive drug target for AD therapies. In the present review, we summarize the current knowledge about the role of LRP1 in AD pathogenesis as well as recent findings on changes in LRP1 expression and function in AD. Finally, we discuss the advances in utilizing LRP1 as a drug target for AD treatments as well as future perspectives on LRP1 research.

Alzheimer's amyloid-β and tau protein accumulation is associated with decreased expression of the LDL receptor-associated protein in human brain tissue

INTRODUCTION

One of the major neuropathological features of Alzheimer's disease (AD) is the accumulation of amyloid-β (Aβ) protein in the brain. Evidence suggests that the low-density lipoprotein receptor-associated protein (RAP) binds strongly to Aβ and enhances its cellular uptake and that decreased RAP expression correlates with increased Aβ production in animal models of AD.

METHODS

The current study examined whether RAP levels change in AD human brain tissue and whether they are related to the amount of AD pathology. RAP and NeuN levels were determined by Western blot, while low-density lipoprotein receptor-related protein 1 (LRP1), tau and Aβ levels were determined by ELISA in the temporal cortex of 17 AD and 16 control cases.

RESULTS

An increase in total Aβ and insoluble and soluble tau protein was observed in AD brain tissue. In contrast, RAP levels were significantly decreased in AD brain tissue compared to controls. Correlation analysis revealed that levels of RAP correlated with both total Aβ and soluble and insoluble tau levels. Neither LRP1 nor NeuN levels were significantly altered in AD brain tissue homogenates and did not correlate with Aβ or tau protein levels.

CONCLUSION

Reduction in RAP may contribute to the accumulation and aggregation of Aβ in the AD brain.

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.

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

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

LRP1 expression in cerebral cortex, choroid plexus and meningeal blood vessels: relationship to cerebral amyloid angiopathy and APOE status

APOE genotype is a risk factor for Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). The risk and severity of CAA increase with possession of APOE ε4, whereas APOE ε2 increases the risk of vessel rupture. Uptake of Aβ by cerebrovascular smooth muscle cells (CVSMCs) is mediated by low-density lipoprotein receptor-related protein-1 (LRP1). To determine whether APOE influences CAA by altering LRP1 expression, particularly by CVSMCs, we analysed APOE genotype, CAA severity, and LRP1 levels in post-mortem cerebral cortex, choroid plexus and meningeal vessels. LRP1 mRNA and protein were not related to CAA severity and presence. LRP1 mRNA was increased in meningeal vessels, but not cortex or choroid plexus, in AD and in association with APOE ε4, and was decreased in association with APOE ε3. In brains with CAA, APOE ε2 was associated with decreased LRP1 protein in meningeal vessels, and ε3 with increased LRP1 in choroid plexus. These findings suggest that APOE may influence the severity of CAA through altered expression of LRP1.

Association of ApoE and LRP mRNA levels with dementia and AD neuropathology

Inheritance of the ε4 allele of apolipoprotein E (ApoE) is the only confirmed and consistently replicated risk factor for late onset Alzheimer's disease (AD). ApoE is also a key ligand for low-density lipoprotein (LDL) receptor-related protein (LRP), a major neuronal low-density lipoprotein receptor. Despite the considerable converging evidence that implicates ApoE and LRP in the pathogenesis of AD, the precise mechanism by which ApoE and LRP modulate the risk for AD remains elusive. Moreover, studies investigating expression of ApoE and LRP in AD brain have reported variable and contradictory results. To overcome these inconsistencies, we studied the mRNA expression of ApoE and LRP in the postmortem brain of persons who died at different stages of dementia and AD-associated neuropathology relative to controls by quantitative polymerase chain reaction (qPCR) and Western blotting analyses. Clinical dementia rating scores were used as a measure of dementia severity, whereas, Braak neuropathological staging and neuritic plaque density were used as indexes of the neuropathological progression of AD. ApoE and LRP mRNA expression was significantly elevated in the postmortem inferior temporal gyrus (area 20) and the hippocampus from individuals with dementia compared with those with intact cognition. In addition to their strong association with the progression of cognitive dysfunction, LRP and ApoE mRNA levels were also positively correlated with increasing neuropathological hallmarks of AD. Additionally, Western blot analysis of ApoE protein expression in the hippocampus showed that the differential expression observed at the transcriptional level is also reflected at the protein level. Given the critical role played by LRP and ApoE in amyloid beta (Aβ) and cholesterol trafficking, increased expression of LRP and ApoE may not only disrupt cholesterol homeostasis but may also contribute to some of the neurobiological features of AD, including plaque deposition.

Receptor-associated protein (RAP) plays a central role in modulating Abeta deposition in APP/PS1 transgenic mice

Background

Receptor associated protein (RAP) functions in the endoplasmic reticulum (ER) to assist in the maturation of several membrane receptor proteins, including low density lipoprotein receptor-related protein (LRP) and lipoprotein receptor 11 (SorLA/LR11). Previous studies in cell and mouse model systems have demonstrated that these proteins play roles in the metabolism of the amyloid precursor protein (APP), including processes involved in the generation, catabolism and deposition of β-amyloid (Aβ) peptides.

Methodology/Principal Findings

Mice transgenic for mutant APPswe and mutant presenilin 1 (PS1dE9) were mated to mice with homozygous deletion of RAP. Unexpectedly, mice that were homozygous null for RAP and transgenic for APPswe/PS1dE9 showed high post-natal mortality, necessitating a shift in focus to examine the levels of amyloid deposition in APPswe/PS1dE9 that were hemizygous null for RAP. Immunoblot analysis confirmed 50% reductions in the levels of RAP with modest reductions in the levels of proteins dependent upon RAP for maturation [LRP trend towards a 20% reduction ; SorLA/LR11 statistically significant 15% reduction (p<0.05)]. Changes in the levels of these proteins in the brains of [APPswe/PS1dE9](+/−)/RAP(+/−) mice correlated with 30–40% increases in amyloid deposition by 9 months of age.

Conclusions/Significance

Partial reductions in the ER chaperone RAP enhance amyloid deposition in the APPswe/PS1dE9 model of Alzheimer amyloidosis. Partial reductions in RAP also affect the maturation of LRP and SorLA/LR11, which are each involved in several different aspects of APP processing and Aβ catabolism. Together, these findings suggest a central role for RAP in Alzheimer amyloidogenesis.

LDL receptor-related protein 1: unique tissue-specific functions revealed by selective gene knockout studies

The LDL receptor-related protein (originally called LRP, but now referred to as LRP1) is a large endocytic receptor that is widely expressed in several tissues. LRP1 is a member of the LDL receptor family that plays diverse roles in various biological processes including lipoprotein metabolism, degradation of proteases, activation of lysosomal enzymes, and cellular entry of bacterial toxins and viruses. Deletion of the LRP1 gene leads to lethality in mice, revealing a critical, but as of yet, undefined role in development. Tissue-specific gene deletion studies reveal an important contribution of LRP1 in the vasculature, central nervous system, macrophages, and adipocytes. Three important properties of LRP1 dictate its diverse role in physiology: 1) its ability to recognize more than 30 distinct ligands, 2) its ability to bind a large number of cytoplasmic adaptor proteins via determinants located on its cytoplasmic domain in a phosphorylation-specific manner, and 3) its ability to associate with and modulate the activity of other transmembrane receptors such as integrins and receptor tyrosine kinases.

Cholesterol in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia and affects up to 15 million people worldwide. Although no single cause of AD has been identified, recent research has suggested that several pathogenetic factors influence risk and expression. A growing amount of evidence underscores a mechanistic link between cholesterol metabolism in the brain and the formation of amyloid plaques. Excess brain cholesterol has been associated with increased formation and deposition of amyloid-beta peptide from amyloid precursor protein. Cholesterol-lowering statins have become a focus of research in AD. Genetic polymorphisms associated with pivotal points in cholesterol metabolism in brain tissues may contribute to the risk and pathogenesis of AD. In this review, we summarise current knowledge of the role of cholesterol metabolism in the pathogenesis of AD and examine the potential of statins in the prevention and treatment of AD.

Candidate genes for late-onset Alzheimer's disease: Focus on chromosome 12

In recent years, there was an increasing interest on candidate genes may play an important role in the development of Alzheimer's disease (AD). Several genome wide screens have undertaken so far or expanded recently, and suggested a number of genomic areas that may contain novel susceptibility genes for AD, in particular most compelling have been the findings on chromosome 12. Polymorphisms in different susceptibility genes on chromosome 12 (A2M, LRP1, CP2 and OLR1) are now being suggested as possible genetic markers for increased risk of developing AD. However, many of these studies are controversial and have shown conflicting results. Thus far, the search for the chromosome 12 Alzheimer's gene must continue and there are several other genes in this region that we are looking at. In this article, we focused on the current knowledge of the genetics of familial late-onset and sporadic AD linked to the chromosome 12, and the future search for other candidate genes.

Association study of polymorphisms in LRP1, tau and 5-HTT genes and Alzheimer’s disease in a sample of Colombian patients

Analysis of genetic susceptibility factors for Alzheimer's disease (AD) in populations with different genetic and environmental background may be useful to understand AD etiology. There are few genetic association studies of AD in Latin America. In the present work, we analyzed polymorphisms in 3 candidate genes; the LDL receptor related protein-1, the microtubule-associated protein Tau and the serotonin transporter genes in a sample of 106 Colombian AD patients and 97 control subjects. We did not find a significant allelic or genotypic association with any of the three polymorphisms analyzed using different statistical analysis, including a neural network model or different sample stratifications. To date, APOE polymorphisms are the only genetic risk factors identified for AD in the Colombian population. It may be factible that future combination of high-throughput genotyping platforms and multivariate analysis models may lead to the identification of other genetic susceptibility factors for AD in the Colombian population.

Current knowledge of chromosome 12 susceptibility genes for late-onset Alzheimer's disease

In the last decade, it has become more apparent the important role genes play in the development of late-onset Alzheimer's disease (AD). Great efforts, involving human genome scans and candidate gene studies, have been given towards identifying susceptibility genes for AD. A number of regions on different chromosomes have been reported to demonstrate linkage for AD. Of these, findings on chromosome 12 are some of the most compelling. Worldwide genetic association studies pre-dating and subsequent to recent linkage studies have identified and focused upon a number of genes that map to the areas of reported linkage on chromosome 12, however, analyses of those genes studied to date, on the whole, remain inconclusive and ambiguous. This paper reviews studies that have provided evidence of linkage for AD on chromosome 12 and in turn discusses the work conducted to date on candidate genes that have been identified and map to the chromosome 12 regions of interest.

Quantification of apolipoprotein E receptors in human brain-derived cell lines by real-time polymerase chain reaction

Apolipoprotein (apo) E4 is a risk factor for Alzheimer's disease (AD) and other neurodegenerative diseases, compared to wild-type apoE3. The mechanism(s) is unknown. One possibility, demonstrated in peripheral tissue cell lines, is that apoE stimulates nitric oxide synthase (NOS) via a receptor-dependent signalling pathway and that apoE4 generates inappropriate amounts of nitric oxide (NO) compared to apoE3. Prior to biochemical investigations, we have quantified the expression of several candidate receptor genes, including low-density lipoprotein-receptor (LDL-r) family members and scavenger receptor class B, types I and II (SR-BI/II), as well as the three NOS isoenzymes and protein kinase B (Akt), in 38 human cell lines, of which 12 derive from brain. Expression of apoE receptor 2 (apoER2), a known signalling receptor in brain, was readily detected in SH-SY-5Y and CCF-STTG1 cells, common models of neurons and astrocytes, respectively, and was highest in H4 neuroglioma, NT-2 precursor cells and IMR-32 neuroblastoma cells. Transcripts of the other lipoprotein receptors were widely, but variably, distributed across the different cell types. Of particular note was the predominant expression of SR-BII over SR-BI in many of the brain-derived cells. As the C-terminus of SR-BII, like apoER2, contains potential SH3 signalling motifs, we suggest that in brain SR-BII functions as a signal transducer receptor.

Role of LRP in TGF?2-mediated neuronal uptake of A? and effects on memory

There is increasing evidence that soluble amyloid-beta peptide (Abeta) uptake into neurons is an early event in the pathogenesis of Alzheimer's disease (AD). Identification of the early events leading to neuronal dysfunction is key to developing therapeutic strategies, but relative roles of receptors and factors modulating uptake are poorly understood. Studies have shown that transforming growth factor beta (TGFbeta), particularly TGFbeta2, can influence the targeting of Abeta to cells in vitro. TGFbeta2 can target Abeta to neurons in organotypic hippocampal slice cultures (OHSC). We examine a specific mechanism for TGFbeta2-mediated targeting of Abeta to neurons. The receptor-associated protein (RAP), a low-density lipoprotein receptor-related protein (LRP) antagonist, can attenuate the cellular targeting of Abeta both in vitro and in vivo and prevent Abeta/TGFbeta2-induced memory retention deficits. Using both in vitro and in vivo methods, we identify LRP as playing a role in TGFbeta2-mediated Abeta uptake, neurodegeneration, and spatial memory impairment.

Molecular genetics of Alzheimer's disease

Alzheimer disease (AD) is the most common cause of dementia. In the past decade, many advances in the understanding of the etiology of AD have been reported. Familial early onset AD is a heterogeneous disorder that can be caused by mutations in at least three different genes. Current studies are focused on identifying genetic risk factors for late onset AD. In this article, the authors will review the progress in understanding the pathogenic implications of the genes mutated in familial early onset AD and the mapping studies to identify additional genes involved in late-onset AD.

Commonalities between genetics of cardiovascular disease and neurodegenerative disorders

PURPOSE OF REVIEW

Numerous epidemiological and clinical data suggest that neurodegenerative disorders, such as Alzheimer's disease, may be related directly or indirectly to cardiovascular risk. Genetic studies have demonstrated that they share at least one common susceptibility gene, encoding apolipoprotein E, a modulator of cardiac risk and of cognitive impairment. Several studies have suggested that other genes involved in the development of cardiovascular diseases may be involved. Previous studies indicated that additional genes contribute to Alzheimer's disease, in particular to the sporadic, more common late-onset form. In this review, the authors focus on recent findings concerning the modulation of the risk of Alzheimer's disease by genes also involved in the development of cardiovascular diseases.

RECENT FINDINGS

The intensive search conducted in the past year gave rise to many publications, more than half of which were related to genes common to cardiovascular and neurodegenerative disorders. The majority of the genes studied are involved in cholesterol metabolism, hypertension, lipid oxidation and detoxication, or inflammatory processes.

SUMMARY

In the past year, approximately 100 studies concerning the genetics of Alzheimer's disease were published around the world. Results suggest that the risk of Alzheimer's disease is modulated by various genes encoding proteins involved in cholesterol metabolism, in the detoxication of lipoprotein oxidation or encoding cytokines.