Subcellular localization of the low density lipoprotein receptor-related protein (alpha 2-macroglobulin receptor) in human brain

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.

HDL-like-Mediated Cell Cholesterol Trafficking in the Central Nervous System and Alzheimer's Disease Pathogenesis

The main aim of this work is to review the mechanisms via which high-density lipoprotein (HDL)-mediated cholesterol trafficking through the central nervous system (CNS) occurs in the context of Alzheimer’s disease (AD). Alzheimer’s disease is characterized by the accumulation of extracellular amyloid beta (Aβ) and abnormally hyperphosphorylated intracellular tau filaments in neurons. Cholesterol metabolism has been extensively implicated in the pathogenesis of AD through biological, epidemiological, and genetic studies, with the APOE gene being the most reproducible genetic risk factor for the development of AD. This manuscript explores how HDL-mediated cholesterol is transported in the CNS, with a special emphasis on its relationship to Aβ peptide accumulation and apolipoprotein E (ApoE)-mediated cholesterol transport. Indeed, we reviewed all existing works exploring HDL-like-mediated cholesterol efflux and cholesterol uptake in the context of AD pathogenesis. Existing data seem to point in the direction of decreased cholesterol efflux and the impaired entry of cholesterol into neurons among patients with AD, which could be related to impaired Aβ clearance and tau protein accumulation. However, most of the reviewed studies have been performed in cells that are not physiologically relevant for CNS pathology, representing a major flaw in this field. The ApoE4 genotype seems to be a disruptive element in HDL-like-mediated cholesterol transport through the brain. Overall, further investigations are needed to clarify the role of cholesterol trafficking in AD pathogenesis.

Polymeric Nanoparticles Properties and Brain Delivery

Safe and reliable entry to the brain is essential for successful diagnosis and treatment of diseases, but it still poses major challenges. As a result, many therapeutic approaches to treating disorders associated with the central nervous system (CNS) still only show limited success. Nano-sized systems are being explored as drug carriers and show great improvements in the delivery of many therapeutics. The systemic delivery of nanoparticles (NPs) or nanocarriers (NCs) to the brain involves reaching the neurovascular unit (NVU), being transported across the blood-brain barrier, (BBB) and accumulating in the brain. Each of these steps can benefit from specifically controlled properties of NPs. Here, we discuss how brain delivery by NPs can benefit from careful design of the NP properties. Properties such as size, charge, shape, and ligand functionalization are commonly addressed in the literature; however, properties such as ligand density, linker length, avidity, protein corona, and stiffness are insufficiently discussed. This is unfortunate since they present great value against multiple barriers encountered by the NPs before reaching the brain, particularly the BBB. We further highlight important examples utilizing targeting ligands and how functionalization parameters, e.g., ligand density and ligand properties, can affect the success of the nano-based delivery system.

Endothelial LRP1 transports amyloid-β(1-42) across the blood-brain barrier

According to the neurovascular hypothesis, impairment of low-density lipoprotein receptor-related protein-1 (LRP1) in brain capillaries of the blood-brain barrier (BBB) contributes to neurotoxic amyloid-β (Aβ) brain accumulation and drives Alzheimer's disease (AD) pathology. However, due to conflicting reports on the involvement of LRP1 in Aβ transport and the expression of LRP1 in brain endothelium, the role of LRP1 at the BBB is uncertain. As global Lrp1 deletion in mice is lethal, appropriate models to study the function of LRP1 are lacking. Moreover, the relevance of systemic Aβ clearance to AD pathology remains unclear, as no BBB-specific knockout models have been available. Here, we developed transgenic mouse strains that allow for tamoxifen-inducible deletion of Lrp1 specifically within brain endothelial cells (Slco1c1-CreER(T2) Lrp1(fl/fl) mice) and used these mice to accurately evaluate LRP1-mediated Aβ BBB clearance in vivo. Selective deletion of Lrp1 in the brain endothelium of C57BL/6 mice strongly reduced brain efflux of injected [125I] Aβ(1-42). Additionally, in the 5xFAD mouse model of AD, brain endothelial-specific Lrp1 deletion reduced plasma Aβ levels and elevated soluble brain Aβ, leading to aggravated spatial learning and memory deficits, thus emphasizing the importance of systemic Aβ elimination via the BBB. Together, our results suggest that receptor-mediated Aβ BBB clearance may be a potential target for treatment and prevention of Aβ brain accumulation in AD.

Neurite outgrowth stimulation by n-3 and n-6 PUFAs of phospholipids in apoE-containing lipoproteins secreted from glial cells

PUFAs, which account for 25-30% of the total fatty acids in the human brain, are important for normal brain development and cognitive function. However, it remains unclear how PUFAs are delivered to neurons and exert their effects. In this study, we demonstrated that n-3 and n-6 PUFAs added to the medium are incorporated into membrane phospholipids of primary glial cells from rat cortices, and then secreted as the fatty acid moiety of phospholipids in apoE-containing lipoproteins (LpEs). Tandem mass spectrometry analysis further showed that LpEs secreted from glial cells contain a variety of metabolites of PUFAs produced in glial cells by elongation and unsaturation. LpEs are absorbed by endocytosis into neurons via LDL receptor-related protein 1. LpE-containing n-3 and n-6 PUFAs exhibit a strong effect on neurite outgrowth of hippocampal neurons by increasing the number of branches. This study sheds light on the novel role of LpEs in the central nervous system and also a novel pathway in which PUFAs act on neurons.

The protective effect of lactoferrin on ventral mesencephalon neurons against MPP + is not connected with its iron binding ability

Lactoferrin (Lf) can bind to lactoferrin receptor (LfR), leading to iron transport through the plasma membrane. Besides iron transportation, Lf also has antioxidant and anti-inflammatory properties. In the brain, Lf is only synthesized by activated microglia. LfR is present in blood vessels and nigral dopaminergic neurons. Both nigral iron accumulation and microglia activation is believed to be involved in Parkinson’s disease (PD), moreover, increased Lf and LfR in dopaminergic neurons were found in PD cases and MPTP-intoxicated mice. How iron influences microglia to release Lf? Does Lf tend to transport iron to dopaminergic neurons leading to cell death or to protect dopaminergic neuron from neurotoxin? In this study, we observed that iron increased Lf synthesis in activated microglia. In ventral mesencephalon neurons, both iron-free Lf (apo-Lf) and iron-saturated Lf (holo-Lf) exerted neuroprotective effects against MPP⁺ by mechanisms, believed to enhance the mitochondrial transmembrane potential, improve Cu/Zn-superoxide dismutase activity, increase Bcl-2 expression. Although apo-Lf but not holo-Lf chelated cellular iron, there was no difference between the two types of Lf in the neuroprotection. Our data indicate that iron overload increases the activated microglia releasing Lf. Lf plays protective role on ventral mesencephalon neurons against MPP⁺, which is iron-chelating independent.

New therapeutic approaches for Alzheimer's disease and cerebral amyloid angiopathy

Accumulating evidence has shown a strong relationship between Alzheimer’s disease (AD), cerebral amyloid angiopathy (CAA), and cerebrovascular disease. Cognitive impairment in AD patients can result from cortical microinfarcts associated with CAA, as well as the synaptic and neuronal disturbances caused by cerebral accumulations of β-amyloid (Aβ) and tau proteins. The pathophysiology of AD may lead to a toxic chain of events consisting of Aβ overproduction, impaired Aβ clearance, and brain ischemia. Insufficient removal of Aβ leads to development of CAA and plays a crucial role in sporadic AD cases, implicating promotion of Aβ clearance as an important therapeutic strategy. Aβ is mainly eliminated by three mechanisms: (1) enzymatic/glial degradation, (2) transcytotic delivery, and (3) perivascular drainage (3-“d” mechanisms). Enzymatic degradation may be facilitated by activation of Aβ-degrading enzymes such as neprilysin, angiotensin-converting enzyme, and insulin-degrading enzyme. Transcytotic delivery can be promoted by inhibition of the receptor for advanced glycation end products (RAGE), which mediates transcytotic influx of circulating Aβ into brain. Successful use of the RAGE inhibitor TTP488 in Phase II testing has led to a Phase III clinical trial for AD patients. The perivascular drainage system seems to be driven by motive force generated by cerebral arterial pulsations, suggesting that vasoactive drugs can facilitate Aβ clearance. One of the drugs promoting this system is cilostazol, a selective inhibitor of type 3 phosphodiesterase. The clearance of fluorescent soluble Aβ tracers was significantly enhanced in cilostazol-treated CAA model mice. Given that the balance between Aβ synthesis and clearance determines brain Aβ accumulation, and that Aβ is cleared by several pathways stated above, multi-drugs combination therapy could provide a mainstream cure for sporadic AD.

Blood-brain barrier dysfunction as a cause and consequence of Alzheimer's disease

The blood-brain barrier (BBB) plays critical roles in the maintenance of central nervous system (CNS) homeostasis. Dysfunction of the BBB occurs in a number of CNS diseases, including Alzheimer's disease (AD). A prevailing hypothesis in the AD field is the amyloid cascade hypothesis that states that amyloid-β (Aβ) deposition in the CNS initiates a cascade of molecular events that cause neurodegeneration, leading to AD onset and progression. In this review, the participation of the BBB in the amyloid cascade and in other mechanisms of AD neurodegeneration will be discussed. We will specifically focus on three aspects of BBB dysfunction: disruption, perturbation of transporters, and secretion of neurotoxic substances by the BBB. We will also discuss the interaction of the BBB with components of the neurovascular unit in relation to AD and the potential contribution of AD risk factors to aspects of BBB dysfunction. From the results discussed herein, we conclude that BBB dysfunction contributes to AD through a number of mechanisms that could be initiated in the presence or absence of Aβ pathology.

Mitochondrial ferritin in neurodegenerative diseases

Mitochondrial ferritin (FtMt) is a novel protein encoded by an intronless gene mapped to chromosome 5q23.1. Ferritin is ubiquitously expressed; however, FtMt expression is restricted to specific tissues such as the testis and the brain. The distribution pattern of FtMt suggests a functional role for this protein in the brain; however, data concerning the roles of FtMt in neurodegenerative diseases remain scarce. In the human cerebral cortex, FtMt expression was increased in Alzheimer's disease patients compared to control cases. Cultured neuroblastoma cells showed low-level expression of FtMt, which was increased by H2O2 treatment. FtMt overexpression showed a neuroprotective effect against H2O2-induced oxidative stress and Aβ-induced neurotoxicity in neuroblastoma cells. FtMt expression was also detected in dopaminergic neurons in the substantia nigra and was increased in patients with restless legs syndrome, while FtMt had a protective effect against cell death in a neuroblastoma cell line model of Parkinson's disease. FtMt is involved in other neurodegenerative diseases such as age-related macular degeneration (AMD), with an FtMt gene mutation identified in AMD patients, and Friedreich's ataxia, which is caused by a deficiency in frataxin. FtMt overexpression in frataxin-deficient cells increased cell resistance to H2O2 damage. These results implicate a neuroprotective role of FtMt in neurodegenerative diseases.

Developing therapeutic antibodies for neurodegenerative disease

The central nervous system has been considered off-limits to antibody therapeutics. However, recent advances in preclinical and clinical drug development suggest that antibodies can cross the blood-brain barrier in limited quantities and act centrally to mediate their effects. In particular, immunotherapy for Alzheimer's disease has shown that targeting beta amyloid with antibodies can reduce pathology in both mouse models and the human brain, with strong evidence supporting a central mechanism of action. These findings have fueled substantial efforts to raise antibodies against other central nervous system targets, particularly neurodegenerative targets, such as tau, beta-secretase, and alpha-synuclein. Nevertheless, it is also apparent that antibody penetration across the blood-brain barrier is limited, with an estimated 0.1-0.2 % of circulating antibodies found in brain at steady-state concentrations. Thus, technologies designed to improve antibody uptake in brain are receiving increased attention and are likely going to represent the future of antibody therapy for neurologic diseases, if proven safe and effective. Herein we review briefly the progress and limitations of traditional antibody drug development for neurodegenerative diseases, with a focus on passive immunotherapy. We also take a more in-depth look at new technologies for improved delivery of antibodies to the brain.

Low density lipoprotein receptor-related protein 1 (LRP1) modulates N-methyl-D-aspartate (NMDA) receptor-dependent intracellular signaling and NMDA-induced regulation of postsynaptic protein complexes

The lipoprotein receptor LRP1 is essential in neurons of the central nervous system, as was revealed by the analysis of conditional Lrp1-deficient mouse models. The molecular basis of its neuronal functions, however, is still incompletely understood. Here we show by immunocytochemistry, electron microscopy, and postsynaptic density preparation that LRP1 is located postsynaptically. Basal and NMDA-induced phosphorylation of the transcription factor cAMP-response element-binding protein (CREB) as well as NMDA target gene transcription are reduced in LRP1-deficient neurons. In control neurons, NMDA promotes γ-secretase-dependent release of the LRP1 intracellular domain (LRP1-ICD). However, pull-down and chromatin immunoprecipitation (ChIP) assays showed no direct interaction between the LRP1-ICD and either CREB or target gene promoters. On the other hand, NMDA-induced degradation of the postsynaptic scaffold protein PSD-95 was impaired in the absence of LRP1, whereas its ubiquitination was increased, indicating that LRP1 influences the composition of postsynaptic protein complexes. Accordingly, NMDA-induced internalization of the AMPA receptor subunit GluA1 was impaired in LRP1-deficient neurons. These results show a role of LRP1 in the regulation and turnover of synaptic proteins, which may contribute to the reduced dendritic branching and to the neurological phenotype observed in the absence of LRP1.

Brain region-specific immunolocalization of the lipolysis-stimulated lipoprotein receptor (LSR) and altered cholesterol distribution in aged LSR+/- mice

Brain lipid homeostasis is important for maintenance of brain cell function and synaptic communications, and is intimately linked to age-related cognitive decline. Because of the blood-brain barrier's limiting nature, this tissue relies on a complex system for the synthesis and receptor-mediated uptake of lipids between the different networks of neurons and glial cells. Using immunofluorescence, we describe the region-specific expression of the lipolysis-stimulated lipoprotein receptor (LSR), in the mouse hippocampus, cerebellum Purkinje cells, the ependymal cell interface between brain parenchyma and cerebrospinal fluid, and the choroid plexus. Colocalization with cell-specific markers revealed that LSR was expressed in neurons, but not astrocytes. Latency in arms of the Y-maze exhibited by young heterozygote LSR(+/-) mice was significantly different as compared to control LSR(+/+), and increased in older LSR(+/-) mice. Filipin and Nile red staining revealed membrane cholesterol content accumulation accompanied by significantly altered distribution of LSR in the membrane, and decreased intracellular lipid droplets in the cerebellum and hippocampus of old LSR(+/-) mice, as compared to control littermates as well as young LSR(+/-) animals. These data therefore suggest a potential role of LSR in brain cholesterol distribution, which is particularly important in preserving neuronal integrity and thereby cognitive functions during aging.

Desmoteplase: discovery, insights and opportunities for ischaemic stroke

Nature has provided a vast array of bioactive compounds that have been exploited for either diagnostic or therapeutic use. The field of thrombosis and haemostasis in particular has enjoyed much benefit from compounds derived from nature, notably from snakes and blood-feeding animals. Indeed, the likelihood that blood-feeding animals would harbour reagents with relevant pharmacology and with potential pharmaceutical benefit in haemostasis was not too far-fetched. Blood-feeding animals including leeches and ticks have evolved a means to keep blood from clotting or to at least maintain the liquid state, and some of these have been the subject of clinical development. A more recent example of this has been the saliva of the common vampire bat Desmodus rotundus, which has proven to harbour a veritable treasure trove of novel regulatory molecules. Among the bioactive compounds present is a fibrinolytic compound that was shown over 40 years ago to be a potent plasminogen activator. Studies of this vampire bat-derived plasminogen activator, more recently referred to as desmoteplase, revealed that this protease shared a number of structural and functional similarities to the human fibrinolytic protease, tissue-type plasminogen activator (t-PA) yet harboured critically important differences that have rendered this molecule attractive for clinical development for patients with ischaemic stroke.

Low-density lipoprotein receptor-related protein is decreased in optic neuropathy of Alzheimer disease

BACKGROUND

Alzheimer disease (AD) is associated with optic nerve degeneration, yet the underlying pathophysiology of this disease and the optic nerve disorder remain poorly understood. Low-density lipoprotein receptor-related protein (LRP) is implicated in the pathogenesis of AD by mediating the transport of amyloid-β (Aβ) out of the brain into the systemic circulation. As a key player in the reaction to central nervous system injury, astrocytes associate with LRP in AD. This study investigates the role of LRP and astrocytes in the pathogenesis of AD optic neuropathy.

METHODS

To investigate the role of LRP and astrocytes in the pathogenesis of AD optic neuropathy, we conducted immunohistochemical studies on postmortem optic nerves in AD patients (n = 11) and age-matched controls (n = 10) to examine the presence of LRP. Quantitative analyses using imaging software were used to document the extent of LRP in neural tissues. Axonal integrity was assessed by performing immunohistochemistry on the subjects' optic nerves with an antibody to neurofilament (NF) protein. Double-immunofluorescence labeling was performed to investigate whether LRP colocalized with astrocytes, expressing glial fibrillary acidic protein.

RESULTS

LRP expression was decreased in AD optic nerves compared to that in controls (P < 0.001). LRP immunoreactivity was observed in the microvasculature and perivascularly in close proximity to the astrocytic processes. Colocalization of LRP in the astrocytes of optic nerves was also demonstrated. The presence of optic neuropathy was confirmed in the AD optic nerves by demonstrating greatly reduced immunostaining for NF protein as compared to controls.

CONCLUSIONS

The reduction of LRP in the AD degenerative optic nerves supports the hypothesis that LRP may play a role in the pathophysiology of AD optic neuropathy.

Cholesterol metabolism in neurons and astrocytes

Cells in the mammalian body must accurately maintain their content of cholesterol, which is an essential membrane component and precursor for vital signalling molecules. Outside the brain, cholesterol homeostasis is guaranteed by a lipoprotein shuttle between the liver, intestine and other organs via the blood circulation. Cells inside the brain are cut off from this circuit by the blood-brain barrier and must regulate their cholesterol content in a different manner. Here, we review how this is accomplished by neurons and astrocytes, two cell types of the central nervous system, whose cooperation is essential for normal brain development and function. The key observation is a remarkable cell-specific distribution of proteins that mediate different steps of cholesterol metabolism. This form of metabolic compartmentalization identifies astrocytes as net producers of cholesterol and neurons as consumers with unique means to prevent cholesterol overload. The idea that cholesterol turnover in neurons depends on close cooperation with astrocytes raises new questions that need to be addressed by new experimental approaches to monitor and manipulate cholesterol homeostasis in a cell-specific manner. We conclude that an understanding of cholesterol metabolism in the brain and its role in disease requires a close look at individual cell types.

Transport characteristics of a novel peptide platform for CNS therapeutics

New and effective therapeutics that cross the blood-brain barrier (BBB) are critically needed for treatment of many brain diseases. We characterize here a novel drug development platform that is broadly applicable for the development of new therapeutics with increased brain penetration. The platform is based on the Angiopep-2 peptide, a sequence derived from ligands that bind to low-density lipoprotein receptor-related protein-1 (LRP-1), a receptor expressed on the BBB. Fluorescent imaging studies of a Cy5.5Angiopep-2 conjugate and immunohistochemical studies of injected Angiopep-2 in mice demonstrated efficient transport across the BBB into brain parenchyma and subsequent co-localization with the neuronal nuclei-selective marker NeuN and the glial marker glial fibrillary acidic protein (GFAP). Uptake of [(¹²⁵I]-Angiopep-2 into brain endothelial cells occurred by a saturable mechanism involving LRP-1. The primary sequence and charge of Angiopep-2 were crucial for its passage across the BBB. Overall, the results demonstrate the significant potential of this platform for the development of novel neurotherapeutics.

Involvement of low-density lipoprotein receptor-related protein and ABCG1 in stimulation of axonal extension by apoE-containing lipoproteins

Apolipoprotein E (apoE)-containing lipoproteins (LpE) are produced by glial cells in the central nervous system (CNS). When LpE are supplied to distal axons, but not cell bodies, of CNS neurons (retinal ganglion cells) the rate of axonal extension is increased. In this study we have investigated the molecular requirements underlying the stimulatory effect of LpE on axonal extension. We show that enhancement of axonal growth by LpE requires the presence of the low-density lipoprotein receptor-related protein-1 (LRP1) in neurons since RNA silencing of LRP1 in neurons, or antibodies directed against LRP, suppressed the LpE-induced axonal extension. In contrast, an alternative LRP1 ligand, α2-macroglobulin, failed to stimulate axonal extension, suggesting that LpE do not exert their growth-stimulatory effect solely by activation of a LRP1-mediated signaling pathway. In addition, although apoE3-containing LpE enhanced axonal extension, apoE4-containing LpE did not. Over-expression of ABCG1 in rat cortical glial cells resulted in production of LpE that increased the rate of axonal extension to a greater extent than did expression of an inactive, mutant form of ABGC1. Furthermore, reconstituted lipoprotein particles containing apoE3, phosphatidylcholine and sphingomyelin, but not cholesterol, stimulated axonal extension, suggesting that sphingomyelin, but not cholesterol, is involved in the stimulatory effect of LpE. These observations demonstrate that LpE and LRP1 promote axonal extension, and suggest that lipids exported to LpE by ABCG1 are important for the enhancement of axonal extension mediated by LpE.

Low-density lipoprotein receptor-related protein 1 (LRP1) mediates neuronal Abeta42 uptake and lysosomal trafficking

BACKGROUND

Alzheimer's disease (AD) is characterized by the presence of early intraneuronal deposits of amyloid-beta 42 (Abeta42) that precede extracellular amyloid deposition in vulnerable brain regions. It has been hypothesized that endosomal/lysosomal dysfunction might be associated with the pathological accumulation of intracellular Abeta42 in the brain. Our previous findings suggest that the LDL receptor-related protein 1 (LRP1), a major receptor for apolipoprotein E, facilitates intraneuronal Abeta42 accumulation in mouse brain. However, direct evidence of neuronal endocytosis of Abeta42 through LRP1 is lacking.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that LRP1 endocytic function is required for neuronal Abeta42 uptake. Overexpression of a functional LRP1 minireceptor, mLRP4, increases Abeta42 uptake and accumulation in neuronal lysosomes. Conversely, knockdown of LRP1 expression significantly decreases neuronal Abeta42 uptake. Disruptions of LRP1 endocytic function by either clathrin knockdown or by removal of its cytoplasmic tail decreased both uptake and accumulation of Abeta42 in neurons. Finally, we show that LRP1-mediated neuronal accumulation of Abeta42 is associated with increased cellular toxicity.

CONCLUSIONS/SIGNIFICANCE

These results demonstrate that LRP1 endocytic function plays an important role in the uptake and accumulation of Abeta42 in neuronal lysosomes. These findings emphasize the central function of LRP1 in neuronal Abeta metabolism.

Immunohistochemical localization of low density lipoprotein receptor-related protein 1 and alpha(2)-Macroglobulin in retinal and choroidal tissue of proliferative retinopathies

The immunolocalization of the low density lipoprotein receptor-related protein 1 (LRP1) and its ligand alpha 2-Macroglobulin (alpha(2)M) was examined in tissues from human donor eyes of normal, diabetic and sickle cell disease subjects. Streptavidin alkaline phosphatase immunohistochemistry was performed with a mouse anti-human LRP1 and rabbit anti-human alpha(2)M antibodies. Retinal and choroidal blood vessels were labeled with mouse anti-human CD34 antibody in adjacent tissue sections. Mean scores for immunostaining from the pathological and control eyes were statistically compared. LRP1 immunoreactivity was very weak to negative in the neural retina of normal subjects except in scattered astrocytes. LRP1 expression in diabetic eyes was detected in the internal limiting membrane (ILM), astrocytes, inner photoreceptor matrix, choriocapillaris and choroidal stroma. The ligand alpha(2)M, however, was limited mainly to blood vessel walls, some areas of the inner nuclear layer (INL), photoreceptors, RPE-Bruch's membrane-choriocapillaris complex, intercapillary septa, and choroidal stroma. In sickle cell eyes, avascular and vascular retina as well as choroidal neovascularization (CNV) were analyzed. In avascular areas, LRP1 immunoreactivity was in innermost retina (presumably ILM, astrocytes, and Muller cells) and INL as well as RPE-Bruch's membrane-choriocapillaris complex and choroidal stroma. alpha(2)M was very weak in avascular peripheral retina compared to vascularized areas and limited to stroma in choroid. In contrast, in areas with CNV, LRP1 immunoreactivity was significantly decreased in overlying retina and in RPE-Bruch's membrane and choroidal stroma compared to the controls, while alpha(2)M was elevated in RPE-Bruch's membrane near CNV compared to normal areas in sickle cell choroid. The mean scores revealed that LRP1 and alpha(2)M in neural retina were significantly elevated in astrocytes and ILM in diabetic eyes (p < or = 0.05), whereas in sickle cell eyes scores were elevated in ILM and INL (p < or = 0.05). In addition, alpha(2)M immunoreactivity was in photoreceptors in both ischemic retinopathies. In choroid, the patterns of LRP1 and alpha(2)M expression were different and not coincident. This is the first demonstration of the presence of LRP1 and alpha(2)M in human proliferative retinopathies. Elevated LRP1 expression in sickle cell neural retina and diabetic inner retina and choroid suggests that LRP1 plays an important role in ischemic neovascular diseases.

Low density lipoprotein receptor-related protein 1 promotes anti-apoptotic signaling in neurons by activating Akt survival pathway

The low density lipoprotein receptor-related protein 1 (LRP1) is a multi-ligand receptor abundantly expressed in neurons. Previous work has shown that brain LRP1 levels are decreased during aging and in Alzheimer disease. Although mounting evidence has demonstrated a role for LRP1 in the metabolism of apolipoprotein E/lipoprotein and amyloid-beta peptide, whether LRP1 also plays a direct role in neuronal survival is not clear. Here, we show that LRP1 expression is critical for the survival of primary neurons under stress conditions including trophic withdrawal, the presence of apoptosis inducers, or amyloid-beta-induced neurotoxicity. Using lentiviral short hairpin RNA to knock down endogenous LRP1 expression, we showed that a depletion of LRP1 leads to an activation of caspase-3 and increased neuronal apoptosis, an effect that was rescued by a caspase-3 inhibitor. A correlation between decreased Akt phosphorylation and the activation of caspase-3 was demonstrated in LRP1 knocked down neurons. Notably, LRP1 knockdown decreased insulin receptor levels in primary neurons, suggesting that decreased neuronal survival might be a consequence of an impaired insulin receptor signaling pathway. Correspondingly, both insulin receptor and phospho-Akt levels were decreased in LRP1 forebrain knock-out mice. These results demonstrate that LRP1 mediates anti-apoptotic function in neurons by regulating insulin receptor and the Akt survival pathway and suggest that restoring LRP1 expression in Alzheimer disease brain might be beneficial to inhibiting neurodegeneration.

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.

RAGE, LRP-1, and amyloid-beta protein in Alzheimer's disease

The receptor for advanced glycation end products (RAGE) is thought to be a primary transporter of beta-amyloid across the blood-brain barrier (BBB) into the brain from the systemic circulation, while the low-density lipoprotein receptor-related protein (LRP)-1 mediates transport of beta-amyloid out of the brain. To determine whether there are Alzheimer's disease (AD)-related changes in these BBB-associated beta-amyloid receptors, we studied RAGE, LRP-1, and beta-amyloid in human elderly control and AD hippocampi. In control hippocampi, there was robust RAGE immunoreactivity in neurons, whereas microvascular staining was barely detectable. LRP-1 staining, in contrast, was clearly evident within microvessels but only weakly stained neurons. In AD cases, neuronal RAGE immunoreactivity was significantly decreased. An unexpected finding was the strongly positive microvascular RAGE immunoreactivity. No evidence for colocalization of RAGE and beta-amyloid was seen within either microvessels or senile plaques. A reversed pattern was evident for LRP-1 in AD. There was very strong staining for LRP-1 in neurons, with minimal microvascular staining. Unlike RAGE, colocalization of LRP-1 and beta-amyloid was clearly present within senile plaques but not microvessels. Western blot analysis revealed a much higher concentration of RAGE protein in AD hippocampi as compared with controls. Concentration of LRP-1 was increased in AD hippocampi, likely secondary to its colocalization with senile plaques. These data confirm that AD is associated with changes in the relative distribution of RAGE and LRP-1 receptors in human hippocampus. They also suggest that the proportion of amyloid within the brains of AD patients that is derived from the systemic circulation may be significant.

tPA receptors and the fibrinolytic response in multiple sclerosis lesions

Axonal damage in multiple sclerosis (MS) lesions is associated with failure of fibrinolysis because of the inhibition of the plasminogen activator system. Plasma membrane receptors for tissue plasminogen activator (tPA) and plasminogen concentrate proteolytic activity on the cell surface and provide protection from inhibitors that in turn may locally enhance the fibrinolytic response. Therefore, we have investigated expression of two of these receptors in MS lesions, annexin II tetramer (AIIt) and low-density lipoprotein receptor-related protein (LRP). In acute MS lesions both AIIt and LRP were immunolocalized on macrophages and astrocytes while LRP was additionally found on neuronal cells in cortical gray matter. Western blot analysis confirmed a significant increase in AIIt in MS lesions and in a proportion of normal-appearing white matter samples, with a highly significant correlation between annexin II levels and factors associated with impeded fibrinolysis, such as plasminogen activator inhibitor-1. Immunoblotting analysis of plasmin(ogen) revealed increased levels of lysine-plasminogen in samples expressing high AIIt protein levels. Our results suggest that limited availability of tPA in MS lesions because of formation of tPA-plasminogen activator inhibitor-1 complexes reduces capability of tPA receptors to generate plasmin, which further diminishes fibrinolytic capacity in active MS lesions and possibly leads to axonal damage.

Association study and meta-analysis of low-density lipoprotein receptor related protein in Alzheimer's disease

Genome scans in sporadic Alzheimer's disease (AD) have revealed a possible susceptibility locus on chromosome 12. The low density lipoprotein receptor related protein (LRP1) gene lies within this area of linkage. Eighteen previous AD case-control studies have investigated the C766T polymorphism in LRP1 with conflicting results, including a protective effect on AD of the T allele, an increased susceptibility towards AD with both the C and T alleles, or no association at all. We have now performed a case-control study based on a large UK cohort of 477 AD patients and 466 matched controls, and have included these data, with those drawn from the 18 previous studies, into in a meta-analysis of 4668 AD patients and 4473 controls. We find no evidence for influence on the risk for AD in either our own present cohort or in the combined data set. Furthermore, we investigated whether the C766T polymorphism might modify the clinical and pathological phenotype in our cohort. We found no association with AD when the cohort was stratified into those with early (<65 years) or late (>65 years) onset, or when split into Apolipoprotein E (APOE) epsilon4 bearers and epsilon4 non-bearers. In addition, the C766T polymorphism was shown not to influence the age onset of AD. In a separate autopsy-confirmed cohort of 130 AD cases, no association with genotype or allele was observed for tissue levels of beta-amyloid 40, beta-amyloid 42, total beta-amyloid, pathological tau proteins, microglial cells or extent of astrocytic activity. Therefore, in this present study, we find no evidence for the involvement of this polymorphism either in increasing the susceptibility to AD, or by acting as a phenotypic modifier.

The transferrin homologue, melanotransferrin (p97), is rapidly catabolized by the liver of the rat and does not effectively donate iron to the brain

Melanotransferrin (MTf) or melanoma tumor antigen p97 is a membrane-bound transferrin (Tf) homologue that binds iron (Fe). This protein is also found as a soluble form in the plasma (sMTf) and was suggested to be an Alzheimer's disease marker. In addition, sMTf has been recently suggested to cross the blood-brain barrier (BBB) and accumulate in the brain of the mouse following intravenous infusion. Considering the importance of this observation to the physiology and pathophysiology of the BBB and the function of sMTf in vivo, we investigated the uptake and distribution of 59Fe-125I-sMTf and compared it to 59Fe-125I-Tf that were injected intravenously in rats. Studies were also performed to measure 59Fe and 125I-protein uptake by reticulocytes using these radiolabelled proteins. The results showed that sMTf was rapidly catabolized, mainly in the liver and to a lesser extent by the kidneys. The 59Fe was largely retained by these organs but the 125I was released into the plasma. Only a small amount of 125I-sMTf or its bound 59Fe was taken up by the brain, less than that from 59Fe-125I-Tf. There was much less 59Fe uptake by erythropoietic organs (spleen and femurs) from 59Fe-sMTf than from 59Fe-Tf, and no evidence of receptor-mediated uptake of sMTf was obtained using reticulocytes. It is concluded that compared to Tf, sMTf plays little or no role in Fe supply to the brain and erythropoietic tissue. However, a small amount of sMTf was taken up from the plasma by the brain and a far greater amount by the liver.

Glial lipoproteins stimulate axon growth of central nervous system neurons in compartmented cultures

The role of lipoproteins secreted by cortical glial cells in axon growth of central nervous system (CNS) neurons was investigated. We first established compartmented cultures of CNS neurons (retinal ganglion cells). Addition of glial cell-conditioned medium (GCM) to distal axons increased the rate of axon extension by approximately 50%. Inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase in glial cells diminished the secretion of cholesterol and apolipoprotein E, and prevented the growth stimulatory effect of GCM. When glia-derived lipoproteins containing apolipoprotein E were provided to distal axons, axon extension was stimulated to the same extent as by GCM. In contrast, addition of lipoproteins to cell bodies failed to enhance growth. The growth stimulatory effect of glial lipoproteins was abrogated in the presence of receptor-associated protein, RAP, indicating involvement of receptor(s) of the low density lipoprotein receptor family in stimulation of axonal extension. These observations suggest that glial cells stimulate axon growth of CNS neurons by providing lipoproteins containing cholesterol and apolipoprotein E to distal axons.

Association of the C766T polymorphism of the low-density lipoprotein receptor-related protein gene with Alzheimer's disease

The low-density lipoprotein receptor-related protein (LRP) is one of the most important cholesterol receptors in the brain. Gene variation of its ligand, apolipoprotein E, is a major genetic risk-factor for Alzheimer's disease (AD). The C-allele of the silent C766T polymorphism in exon 3 of the LRP gene might be associated with AD, however, results are conflicting and thus discussed controversially. Consequently, we compared the prevalence of this polymorphism in a homogenous cohort of patients with AD and control subjects. We found that carriers of a C-allele were at lower risk of AD; in agreement with this observation, AD patients who were carriers of a C-allele presented with a later age at onset of the disease than carriers of the TT genotype. These data suggest that LRP polymorphism influences the risk as well as the age at onset of AD. Our results contrast with other studies which described the C-allele to be a risk-factor for AD, but are in line with a recent publication on the effect of LRP polymorphism on longevity and on the risk for coronary artery disease. Further research on LRP polymorphisms is needed to evaluate their effects on the risk of AD, on coronary artery disease and on longevity.

Lipoprotein receptors in the nervous system

The low-density-lipoprotein (LDL) receptor family is an evolutionarily ancient gene family of structurally closely related cell-surface receptors. Members of the family are involved in the cellular uptake of extracellular ligands and regulate diverse biological processes including lipid and vitamin metabolism and cell-surface protease activity. Some members of the family also participate in cellular signaling and regulate the development and functional maintenance of the nervous system. Here we review the roles of this family of multifunctional receptors in the nervous system and focus on recent advances toward the understanding of the mechanisms by which lipoprotein receptors and their ligands transmit and modulate signals in the brain.

LR11, a mosaic LDL receptor family member, mediates the uptake of ApoE-rich lipoproteins in vitro

Since the molecular identification of the low density lipoprotein receptor (LDLR), an ever increasing number of related proteins have been discovered. These receptors belonging to the LDLR family are thought to play key roles in lipoprotein metabolism in a variety of tissues, including the arterial wall. We have discovered that the expression of a 250-kDa mosaic LDLR-related protein, which we termed LR11 for the presence of 11 LDLR ligand-binding repeats, is markedly induced in smooth muscle cells in the hyperplastic intima of animal models used for the study of atherosclerosis. Here, we demonstrate that the human LR11, when overexpressed in hamster cells, binds and internalizes 39-kDa receptor-associated protein (RAP), an in vitro ligand for all receptors belonging to the LDLR family. Furthermore, LR11 binds the apolipoprotein E (apoE)-rich lipoproteins, beta-very low density lipoproteins (VLDLs), with a high affinity similar to that of other members, such as the LDLR and VLDL receptor. RAP and beta-VLDL compete with each other; however, other serum lipoproteins are not able to inhibit their binding. LR11 shows specific binding of apoE-enriched HDL prepared from human cerebrospinal fluid as well as of beta-VLDL, suggesting that the apoE content of lipoproteins is most likely important for mediating the high-affinity binding to the receptor. LR11-overexpressing cells are able to internalize and degrade the bound beta-VLDL; these cells also show increased accumulation of cholesteryl esters when incubated with beta-VLDL. Incubation for 48 hours with beta-VLDL of LR11-overexpressing cells, but not of control cells, promotes the appearance of numerous intracellular lipid droplets. Taken together, LR11, a mosaic LDLR family member whose expression in smooth muscle cells is markedly induced in atheroma, has all the properties of a receptor for the endocytosis of lipoproteins, particularly for the incorporation of apoE-rich lipoproteins.

Elevation of LDL receptor-related protein levels via ligand interactions in Alzheimer disease and in vitro

The low-density lipoprotein (LDL) receptor-related protein (LRP) is a multifunctional receptor in the CNS that binds both apolipoprotein E (apoE) and activated alpha2-macroglobulin (alpha2M*); all 3 proteins are genetically associated with Alzheimer disease (AD). In this study we found an 85% increase in LRP levels in human AD brain frontal cortex, along with an increased level of the LRP ligands, apoE, and alpha2M. We speculated that LRP levels might be increased in response to the increased levels of its ligands, apoE, and alpha2M*. To test this hypothesis we examined the effects of alpha2M* on LRP in primary cultures. Treatment of neurons with alpha2M* significantly increased LRP levels (by 92%). This increase was prevented by coculture with receptor-associated protein (RAP), which blocks binding of LRP ligands to LRP Native alpha2M or RAP alone did not change LRP levels in vitro. We also found that alpha2M* stimulated activation of astrocytes in vitro and promoted the levels of LRP by 65%. These data indicate 1) the LRP ligand alpha2M* increases levels of LRP in primary neuronal and astrocytic cultures, 2) alpha2M*-induction of LRP levels in vitro depends on binding to LRP, and 3) LRP levels are increased in AD brain, perhaps in response to the increased levels of alpha2M.

t-Butyl hydroperoxide and oxidized low density lipoprotein enhance phospholipid hydrolysis in lipopolysaccharide-stimulated retinal pericytes

Free radicals induced by organic peroxides or oxidized low density lipoprotein (oxLDL) play a critical role in the development of atherosclerosis. In investigating this process, and the concomitant inflammatory response, the role of pericytes, cells supporting the endothelial ones in blood vessels, has received little attention. In this study we tested the hypothesis that tert-butyl hydroperoxide (t-BuOOH) and oxLDL, administered in sublethal doses to the culture medium of retinal pericytes, function as prooxidant signals to increase the stimulation of the peroxidation process induced by lipopolysaccharide (LPS). Confluent cell monolayers were exposed to t-BuOOH (25-400 microM), native LDL or oxLDL (3.4-340 nmol hydroperoxides/mg protein, 1-100 micro). LPS (1 microg/ml), t-BuOOH (200 microM), and oxLDL (100 microM), but not native LDL, incubated for 24 h with cells, markedly increased lipid peroxidation, cytosolic phospholipase A2 (cPLA2) activity and arachidonic acid (AA) release in a time- and dose-dependent manner. AACOCF(3), a potent cPLA2 inhibitor, and the antioxidant alpha-tocopherol strongly inhibited the prooxidant-stimulated AA release. Long-term exposure to maximal concentrations of t-BuOOH (400 microM) or oxLDL (100 microM) had a sharp cytotoxic effect on the cells, described by morphological and biochemical indices. The presence of t-BuOOH or oxLDL at the same time, synergistically increased phospholipid hydrolysis induced by LPS alone. 400 microM t-BuOOH or 100 microM oxLDL had no significant effect on the stimulation of an apoptosis process estimated by DNA laddering and light and electron microscopy. The results indicate that (i) pericytes may be the target of extensive oxidative damage; (ii) activation of cPLA2 mediates AA liberation; (iii) as long-term regulatory signals, organic peroxide and specific constituents of oxLDL increase the pericyte ability to degrade membrane phospholipids mediated by LPS which was used, in the present study, to simulate in vitro an inflammatory burst in the retinal capillaries.

Neuronal apoptosis by apolipoprotein E4 through low-density lipoprotein receptor-related protein and heterotrimeric GTPases

The epsilon4 genotype of apolipoprotein E (apoE4) is the most established predisposing factor in Alzheimer's disease (AD); however, it remains unclear how apoE4 contributes to the pathophysiology. Here, we report that the apoE4 protein (ApoE4) evokes apoptosis in neuronal cells through the low-density lipoprotein receptor-related protein (LRP) and heterotrimeric GTPases. We examined neuron/neuroblastoma hybrid F11 cells and found that these cells were killed by 30 microg/ml ApoE4, but not by 30 microg/ml ApoE3. ApoE4-induced death occurred with typical features for apoptosis in time- and dose-dependent manners, and was observed in SH-SY5Y neuroblastomas, but not in glioblastomas or non-neuronal Chinese hamster ovary cells. Activated, but not native, alpha2-macroglobulin suppressed this ApoE4 toxicity. Suppression by the antisense oligonucleotide to LRP and inhibition by low nanomolar concentrations of LRP-associated protein RAP provided evidence for the involvement of LRP. The involvement of heterotrimeric GTPases was demonstrated by the findings that (1) ApoE4-induced death was suppressed by pertussis toxin (PTX), but not by heat-inactivated PTX; and (2) transfection with PTX-resistant mutant cDNAs of Galpha(i) restored the toxicity of ApoE4 restricted by PTX. We thus conclude that one of the neurotoxic mechanisms triggered by ApoE4 is to activate a cell type-specific apoptogenic program involving LRP and the G(i) class of GTPases and that the apoE4 gene may play a direct role in the pathogenesis of AD and other forms of dementia.

Alpha2-macroglobulin enhances the clearance of endogenous soluble beta-amyloid peptide via low-density lipoprotein receptor-related protein in cortical neurons

Apolipoprotein E and alpha2-macroglobulin (alpha2M) are genetic risk factors for late-onset Alzheimer's disease, and both bind a cell surface receptor, the low-density lipoprotein receptor-related protein (LRP). To investigate the role of LRP on preventing the accumulation of beta-amyloid peptide (A beta), we examined the effects of alpha2M on the clearance of endogenous A beta. Studies were performed in primary Tg2576 transgenic mouse cortical neuronal cultures expressing human mutant amyloid precursor protein (APP) 695. This system allowed us to follow endogenous A beta using immunoblots to detect monomeric forms of the peptide. A beta and APP levels were measured in conditioned media. We found that activated alpha2M (alpha2M*) substantially decreased soluble A beta levels and had no effect on secreted or full-length APP levels. Native alpha2M, which is not a ligand for LRP, did not affect A beta levels. The receptor-associated protein, which inhibits interaction of all ligands with LRP in vitro, prevented alpha2M*-induced decreases of soluble A beta levels. These data suggest that alpha2M* affects soluble A beta clearance rather than A beta production. Further studies showed that similar A beta clearance via an LRP-mediated pathway was observed after treatment with another LRP ligand, lactoferrin. Taken together, these data demonstrate that alpha2M* enhances the clearance of soluble A beta via LRP in cortical neurons.

Receptor-mediated transcytosis of lactoferrin through the blood-brain barrier

Lactoferrin (Lf) is an iron-binding protein involved in host defense against infection and severe inflammation; it accumulates in the brain during neurodegenerative disorders. Before determining Lf function in brain tissue, we investigated its origin and demonstrate here that it crosses the blood-brain barrier. An in vitro model of the blood-brain barrier was used to examine the mechanism of Lf transport to the brain. We report that differentiated bovine brain capillary endothelial cells exhibited specific high (Kd = 37.5 nM; n = 90,000/cell) and low (Kd = 2 microM; n = 900,000 sites/cell) affinity binding sites. Only the latter were present on nondifferentiated cells. The surface-bound Lf was internalized only by the differentiated cell population leading to the conclusion that Lf receptors were acquired during cell differentiation. A specific unidirectional transport then occurred via a receptor-mediated process with no apparent intraendothelial degradation. We further report that iron may cross the bovine brain capillary endothelial cells as a complex with Lf. Finally, we show that the low density lipoprotein receptor-related protein might be involved in this process because its specific antagonist, the receptor-associated protein, inhibits 70% of Lf transport.

The expression of the LDL receptor-related protein (LRP) correlates with the differentiation of human neuroblastoma cells

The low density lipoprotein receptor-related protein (LRP) has been localized in human brain at the level of neurons, astrocytes and along capillary membranes. It is a multifunctional receptor responsible for binding and internalization of lipoproteins enriched with apoliprotein E, lipoprotein lipase, protease-alpha 2 macroglobulin complexes and plasminogen activator-inhibitor complexes. LRP expression is observed in cells involved in Alzheimer's disease, neoplastic transformation and tissue repair. Moreover, its synthesis is modulated during brain development. In this study we used the SK-N-AS human neuroblastoma cell line as a model system to study LRP expression during cellular differentiation induced by phorbol esters, retinoic acid and interferon gamma. Since LRP plays a major role in the regulation of lipid metabolism, the decreased levels of LRP measured by immunofluorescence, western blot and PCR on differentiated neuroblastoma cells may be the consequence of the lower requirements of cholesterol and lipids of differentiated cells in relation to their reduced mitotic index.

Neurotoxicity of the 22 kDa thrombin-cleavage fragment of apolipoprotein E and related synthetic peptides is receptor-mediated

Potent neurotoxicity is associated with both apolipoprotein E (apoE)-related synthetic peptides and the 22 kDa N-terminal thrombin-cleavage fragment of apoE. Furthermore, the E4 isoform of the 22 kDa fragment is significantly more toxic than the same fragment derived from the E3 isoform, suggesting the possibility of a direct role of apoE-associated neurotoxicity in the pathophysiology of Alzheimer's disease. In the present study, the potential role of cell surface receptors in mediating neurotoxicity was assessed by using a variety of agents that should block the heparin-binding and receptor-binding activity of apoE. Effective inhibitors of neurotoxicity of both the apoE peptides and the apoE fragment include heparin, heparan sulfate, sodium chlorate and heparinase, the low-density lipoprotein (LDL) receptor-related protein receptor-associated protein, and a polyclonal anti-LDL receptor-related protein antibody. These results suggest that the neurotoxicity of the 22 kDa thrombin cleavage fragment of apoE and related peptides is receptor-mediated, and that the most likely candidate receptor is a heparan sulfate proteoglycan-LDL receptor-related protein complex.

Apolipoprotein E and Alzheimer's disease: a review of recent studies

There are three isoforms of the 33-kDa protein apolipoprotein E (apoE), termed apoE2, apoE3, and apoE4, each encoded by distinct genes APOE2, APOE3 and APOE4, respectively. In 1993, the APOE genotype was identified as a risk factor for Alzheimer's disease (AD) and was subsequently acknowledged to account for approximately 60% of all cases. The influence of the APOE genotype in AD is clearly isoform dependent, APOE4 imparting susceptibility and APOE2 protection. Thus, patients homozygous for the E4 allele show a very strong likelihood of developing the disease by age 75, whereas patients carrying at least one E2 allele are unlikely to develop symptoms of AD by this age. A major issue in AD research is therefore to understand the functional differences between the ApoE isoforms, with the ultimate aim of designing the next generation of drugs to treat this disease. The purpose of the present article is to summarise some of this work. This review encompasses the rapidly developing molecular, cellular and behavioural research into ApoE, and attempts to highlight those findings we consider to be of particular significance.

Low density lipoprotein receptor-related protein is expressed early and becomes restricted to a somatodendritic domain during neuronal differentiation in culture

Low density lipoprotein receptor-related protein (LRP) is a multi-functional receptor which mediates the endocytotic uptake of several ligands implicated in neuronal pathophysiology. In this study, LRP expression and localization, in cultured hippocampal neurons from 18-day-old rats, were examined by immunofluorescence microscopy. LRP was restricted to the cell bodies and dendrites of mature neurons, where it was uniformly distributed on both dendritic shafts and spines. Immunoreactive protein was detected within the first 24 h of culture and acquired a polarized distribution by the end of the first week. Expression of LRP mRNA by the cultured neurons was demonstrated by Northern blot analysis. Binding studies with the LRP ligand, activated alpha2-macroglobulin, confirmed that LRP was present and functional on the hippocampal neuron cell surface. These studies demonstrate that neuronal LRP undergoes selective compartmentation during neuronal maturation and suggest that LRP-mediated endocytosis is largely restricted to the somatodendritic compartment.

Alzheimer's beta-amyloid precursor protein is expressed on the surface of immediately ex vivo brain cells: a flow cytometric study

Beta-amyloid precursor protein (beta APP) is ubiquitously expressed, but deposition of the beta APP proteolytic fragment A beta is virtually restricted to the brain, suggesting cell-specific processing of this molecule. Our laboratory has investigated expression of beta APP in mechanically dissociated, unfixed, immediately ex vivo cells from various mouse and rat organs by flow cytometry. Epitopes of predicted extracellular domains of beta APP recognized by the N-terminal 22C11 monoclonal antibody (mAb) and the juxtamembrane 4G8 mAb were not detectable on the surface of lymphoid cells, hepatocytes, or kidney cells. In contrast, surface 22C11 and 4G8 beta APP immunoreactivity was abundant on intact (propidium iodide-excluding) dissociated brain cells. The predicted C-terminal intracellular beta APP determinant recognized by the mAb Jonas was not detectable on the surface of intact brain cells, but was present in ethanol-permeabilized cells, consistent with a transmembrane configuration of beta APP in brain cells. Trypsinization of intact brain cells abolished cell surface immunoreactivity for 22C11, which was then reestablished by short-term culture. Augmentation of 22C11 and 4G8 surface immunoreactivity occurred when brain cells were cultured short-term in phenylarsine oxide, a general endocytosis inhibitor. By double staining protocols of brain cells with mAbs directed against beta APP ectodomain epitopes and the neuronal surface proteins Thy-1 or neural cell adhesion molecule (NCAM), we observed that all Thy-1+ and NCAM+ cells (approximately 50%) were immunoreactive for surface beta APP, but that some beta APP+ cells (approximately 20%) were negative for these neuronal markers. Our data suggest that neurons and a subpopulation of other brain cells, unlike peripheral cells, can support beta APP as a type 1 intrinsic membrane molecule with an intact ectodomain, and that beta APP surface abundance is regulated by an equilibrium between membranes vesicle insertion and endocytotic internalization. Transmembrane beta APP holoprotein may be a critical determinant of brain-predominant processing of beta APP to A beta, and may participate in a receptor/transducer function unique to brain cells.

The inflammatory response system of brain: implications for therapy of Alzheimer and other neurodegenerative diseases

Cultured brain cells are capable of generating many molecules associated with inflammatory and immune functions. They constitute the endogenous immune response system of brain. They include complement proteins and their regulators, inflammatory cytokines, acute phase reactants and many proteases and protease inhibitors. Most of the proteins are made by microglia and astrocytes, but even neurons are producers. Many appear in association with Alzheimer disease lesions, indicating a state of chronic inflammation in Alzheimer disease brain. Such a state can apparently exist without stimulation by peripheral inflammatory mediators or the peripheral immune system. A strong inflammatory response may be autotoxic to neurons, exacerbating the fundamental pathology in Alzheimer disease and perhaps other neurological disorders. Autotoxic processes may contribute to cellular death in chronic inflammatory diseases affecting other parts of the body, suggesting the general therapeutic value of anti-inflammatory agents. With respect to Alzheimer disease, multiple epidemiological studies indicate that patients taking anti-inflammatory drugs or suffering from conditions in which such drugs are routinely used, have a decreased risk of developing Alzheimer disease. In one very preliminary clinical trial, the anti-inflammatory drug indomethacin arrested progress of the disease. New agents directed against the inflammatory processes revealed in studies of Alzheimer disease lesions may have broad therapeutic applications.