The role of lipoprotein receptors on the physiological function of APP

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

ApoE isoforms affect neuronal N-methyl-D-aspartate calcium responses and toxicity via receptor-mediated processes

Apolipoprotein E (apoE) alters the pathophysiology of Alzheimer's disease, but its mechanism is not fully understood. We examined the effects of recombinant human apoE3 and apoE4 on the neuronal calcium response to N-methyl-D-aspartate (NMDA), and compared them to their toxicity. ApoE4 (100 nM) significantly increased the resting calcium (by 70%) and the calcium response to NMDA (by 185%), whereas similar changes were not obtained in apoE3-treated neurons. ApoE4, but not apoE3, also significantly increased neurotoxicity, as evidenced by enhanced lactate dehydrogenase release (by 53%) and reduced 3-(4,5-dimethylthiazol-2-yl)-2,5,diphenyltetrazolium bromide levels (by 32%). ApoE4-induced changes in the calcium response to NMDA and associated neurotoxicity were blocked by coincubation with MK-801. Both the receptor-associated protein, which inhibits interaction of apoE with members of the LDL receptor family, including the low-density lipoprotein receptor-related protein (LRP), and activated alpha2-macroglobulin, another LRP ligand, prevented apoE4-induced enhancement of the calcium response to NMDA, resting calcium levels, and neurotoxicity. A tandem apoE peptide (100 nM) containing only the receptor binding region residues also eliminated the enhanced calcium signaling and neurotoxicity by apoE4. Taken together, our data demonstrate that differential effects of apoE3 and apoE4 on the calcium signaling in neurons correlate with their effect on neurotoxicity, which are secondary to receptor binding.     

Effects of hormone therapy on estrogen synthesis from E1S in the mammary gland of postmenopausal women

OBJECTIVES: To investigate the effect of hormone therapy (HT) on the expression of hormone receptors and the ex vivo estrogen biosynthesis in the breast. METHODS: Comparative studies were carried out in breast tissue from 28 postmenopausal women undergoing breast surgery due to breast cancer (BC). Glandular breast tissue at least 1cm distant from the tumor was analyzed. Groups included patients having received HT (n=18), and non-user of HT (controls, n=10) prior to BC diagnosis. Steroid sulfatase (STS) activity was evaluated by incubating homogenized breast tissue with [3H]-estrone sulfate. Thin layer chromatography was performed to separate the products estrone (E1) and estradiol (E2). Histomorphometry for breast tissue composition and immunohistochemistry for expression of estrogen receptor (ER) alpha and beta as well as progesterone receptor (PR) were performed. RESULTS: In all groups, significantly more E2 than E1 was produced. Local E2 formation was higher in women having been treated with estrogen and progestogen (p< or =0.05). Local EZ formation was positively correlated with ER alpha expression (r(s)=0.5; p=0.03). Histomorphometrical and immunohistochemical outcomes did not differ between groups. However, the amount of vessels was higher in women having been treated with HT compared to controls (p=0.09). CONCLUSIONS: Long-term HT increases local estrogen formation in normal human breast tissue.     

The cholesterol-lowering drug probucol increases apolipoprotein E production in the hippocampus of aged rats: implications for Alzheimer's disease

Several recent epidemiological studies have proposed that cholesterol-lowering drug Statin may provide protection against Alzheimer's disease (AD). Probucol is a non-Statin cholesterol-lowering drug and a potent inducer of apolipoprotein E (apoE) production in peripheral circulation. A recent clinical study using Probucol in elderly AD subjects revealed a concomitant stabilisation of cognitive symptoms and significant increases in apoE levels in the cerebral spinal fluid in these patients. To gain insight into the mechanisms underlying these effects, we treated a cohort of aged male rats (26-month-old) with oral dose of Probucol for 30 days. Specifically, we examined the effects of Probucol on apoE production and its receptors (low density lipoprotein receptor [LDLr] and low density lipoprotein receptor-related protein [LRP]), astroglial marker of cell damage (glial fibrillary acidic protein [GFAP]), markers of neuronal synaptic plasticity and integrity (synaptosomal associated protein of 25 kDa [SNAP-25] and synaptophysin) as well as cholesterol biosynthesis (3-hydroxy-3-methylglutaryl coenzyme A reductase [HMGCoAr]) in the hippocampus. We report that Probucol induces the production of apoE and one of its main receptors, LRP, increases HMGCoAr (rate-limiting enzyme in cholesterol synthesis), substantially attenuates age-related increases in glial activation, and induces production of synaptic marker SNAP-25, a molecule commonly associated with synaptogenesis and dendritic remodeling.These findings suggest that Probucol could promote neural and synaptic plasticity to counteract the synaptic deterioration associated with brain aging through an apoE/LRP-mediated system. Consistent with the beneficial effects of other cholesterol-lowering drugs such as the Statin, Probucol could also offers additional benefits based on apoE neurobiology.     

Immunohistochemical expression of two members of the GDNF family of growth factors and their receptors in the olfactory system

The glial cell line-derived (GDNF) family of trophic factors, GDNF, neurturin, persephin and artemin, are known to support the survival and regulate differentiation of many neuronal populations, including peripheral autonomic, enteric and sensory neurons. Members of this family of related ligands bind to specific GDNF family receptor (GFR) proteins, which complex and signal through the Ret receptor tyrosine kinase. We showed previously that GDNF protein was detectable in olfactory sensory neurons (OSNs) in the olfactory neuroepithelium (ON). In this immunohistochemical study, we localized GDNF, neurturin, GFRα1, GFRα2 and Ret in the adult rat ON and olfactory bulb. We found that GDNF and Ret were widely expressed by immature and mature OSNs, while neurturin was selectively expressed in a subpopulation of OSNs zonally restricted in the ON. The GFRs had differential expression, with mature OSNs and their axons preferentially expressing GFRα1, whereas progenitors and immature neurons more avidly expressed GFRα2. In the bulb, GDNF was highly expressed by the mitral and tufted cells, and by periglomerular cells, and its distribution generally resembled that of Ret, with the exception that Ret was far more predominant on fibers than cell bodies. Neurturin, in contrast, was present at lower levels and was more restricted in its expression to the axonal compartment. GFRα2 appeared to be the dominant accessory protein in the bulb. These data are supportive of two members of this neurotrophic family, GDNF and neurturin, playing different physiological roles in the olfactory neuronal system.     

Apolipoprotein E polymorphism and lipid levels differ by gender and family history of diabetes: the Rancho Bernardo Study

Association between apolipoprotein E (apoE) gene polymorphism and lipid levels was studied in 164 nondiabetic first-degree relatives of persons with diabetes and 962 nondiabetic persons with no family history of diabetes. Sex-specific genotypic distribution of apoE polymorphism did not differ between persons with and without a family history of diabetes. In first-degree relatives, lipid levels did not differ among persons with apoE2 (E2/2, E2/3), apoE3/3, and apoE4 (E4/4, E3/4) after adjusting for age, waist circumference, smoking, and alcohol and estrogen use. In persons without a family history of diabetes, both men (p<0.01) and women (p<0.001) with apoE2 showed lower levels of total and low density lipoprotein cholesterol compared with persons with apoE3/3 and apoE4. In women with a family history of diabetes, persons with apoE4 had larger waist circumference (p<0.05). ApoE2 allele is associated with more favorable levels of total and low density lipoprotein cholesterol in men and women without a family history of diabetes. ApoE4 allele is associated with obesity independent of dyslipidemia in women but not men with a family history of diabetes. ApoE polymorphism is not associated with lipids in men or women with a family history of diabetes.     

Differential effects of estrogen in the injured forebrain of young adult and reproductive senescent animals

Estrogen attenuates neural damage resulting from a variety of experimental injury models in adult female rats. To determine whether estrogens neuroprotective actions are age-specific, the present study compared the effects of estrogen on young adult and reproductive senescent animals subject to excitotoxic injury to the forebrain. NMDA was injected bilaterally into the olfactory bulbs of estrogen and placebo-replaced young adult and reproductive senescent animals. Lysates of the olfactory bulb and its basal forebrain afferent, the horizontal limb of the diagonal band of Broca (hlDBB), harvested 24h later were analyzed for expression of IL-1beta, IL-10, and nerve growth factor (NGF). NMDA injections resulted in local activation of microglia and an increase in IL-1beta. Estrogen replacement decreased IL-1beta expression in young adult females, but paradoxically enhanced its expression in reproductive senescent females. Furthermore, bulb injury increased IL-1beta production in the hlDBB of reproductive senescent animals although estrogen replacement was able to suppress lesion-induced expression of this cytokine. In both, the olfactory bulb and hlDBB, constitutive expression of the anti-inflammatory cytokine IL-10 was significantly higher while that of NGF was almost 50% lower in senescent animals as compared to young adults, indicating that aging preferentially altered Th2-type secretions. The present findings are consistent with our earlier observations that estrogen does not exert trophic effects in the aging forebrain and supports the hypothesis that estrogen treatment to reproductive senescent females may exacerbate neural injury.     

Tissue-type plasminogen activator and the low-density lipoprotein receptor-related protein mediate cerebral ischemia-induced nuclear factor-kappaB pathway activation

Tissue-type plasminogen activator (tPA) is a serine proteinase found in the intravascular space and the central nervous system. The low-density lipoprotein receptor-related protein (LRP) is a member of the low-density lipoprotein receptor gene family found in neurons and astrocytes. Cerebral ischemia induces activation of the nuclear factor (NF)-kappaB pathway. The present study investigated the role that the interaction between tPA and LRP plays on middle cerebral artery occlusion (MCAO)-induced NF-kappaB-mediated inflammatory response. We found that MCAO increased LRP expression primarily in astrocytes and that this effect was significantly decreased in the absence of tPA. The onset of the ischemic insult induced activation of the NF-kappaB pathway in wild-type and plasminogen (Plg(-/-))-deficient mice, and this effect was attenuated after inhibition of LRP or genetic deficiency of tPA. Moreover, administration of tPA to tPA(-/-) mice resulted in activation of the NF-kappaB pathway comparable with that observed in wild-type and Plg(-/-) mice. We also report that inhibition of either tPA activity or LRP or genetic deficiency of tPA resulted in a significant decrease in MCAO-induced nitric oxide production and inducible nitric-oxide synthase expression. In conclusion, our results demonstrate that after MCAO the interaction between tPA and LRP results in NF-kappaB activation in astrocytes and induction of inducible nitric-oxide synthase expression in the ischemic tissue, suggesting a cytokine-like plasminogen-independent role for tPA during cerebral ischemia.     

Neurotrophin expression in the reproductively senescent forebrain is refractory to estrogen stimulation

The present studies compared the regulation of the neurotrophin ligands and receptors by estrogen in young adult and reproductively senescent rats. Both groups of animals were ovariectomized and replaced with 17beta-estradiol or placebo pellets for 4 weeks. Protein expression of specific neurotrophins and their receptors were measured in the olfactory bulb and its basal forebrain afferent, the horizontal limb of the diagonal band of Broca (hlDBB). Young-adult rats responded to estrogen with an increase in the expression of brain-derived neurotrophic factor (BDNF) in the olfactory bulb and hlDBB, as well as bulbar trkA and trkB receptors. Older rats did not respond to estrogen in this manner. Additionally, estrogen treatment decreased the expression of the universal neurotrophin receptor p75 in young adult animals, but increased expression of this receptor in reproductively senescent rats. The latter group, however, had significantly greater estrogen receptor alpha (ERalpha) expression in the olfactory bulb as compared to their younger counterparts, but very low expression of the steroid receptor coactivator, SRC-1. Changes in the proportion or ratio of steroid receptor/coactivator systems in the aging forebrain may contribute to the refractory response to estrogen in the reproductively senescent animals.     

Cenani–Lenz syndrome restricted to limb and kidney anomalies associated with a novel LRP4 missense mutation

Cenani–Lenz syndrome (CLS) is a rare autosomal recessive developmental disorder of the limbs. The disorder is characterized by complete syndactyly with metacarpal fusions and/or oligodactyly sometimes accompanied by radioulnar synostosis. The clinical expression is variable and kidney agenesis/hypoplasia, craniofacial dysmorphism and teeth abnormalities are frequent features as well as lower limb involvement. CLS was recently associated with mutations in the low-density lipoprotein receptor-related protein 4 (LRP4) gene and dysregulated canonical WNT signaling. We have identified a large consanguineous Pakistani pedigree with 9 members affected by CLS. The affected individuals present with a consistent expression of the syndrome restricted to the limbs and kidneys. Symptoms from the lower limb are mild or absent and there were no radioulnar synostosis or craniofacial involvement. Genetic analysis using autozygosity mapping and sequencing revealed homozygosity for a novel missense mutation c.2858T > C (p.L953P) in the LRP4 gene. The mutation is located in a region encoding the highly conserved low-density lipoprotein receptor repeat class B domain of LRP4. Our findings add to the genotype–phenotype correlations in CLS and support kidney anomalies as a frequent associated feature.     

Estradiol prevents ozone-induced increases in brain lipid peroxidation and impaired social recognition memory in female rats

There is increasing concern about the neurodegenerative and behavioral consequences of ozone pollution in industrialized urban centers throughout the world and that women may be more susceptible to brain neurodegenerative disorders. In the present study we have investigated the effects of chronic (30 or 60 days) exposure to ozone on olfactory perception and memory and on levels of lipid peroxidation, α and β estrogen receptors and dopamine β-hydroxylase in the olfactory bulb in ovariectomized female rats. The ability of 17β-estradiol to prevent these effects was then assessed. Results showed that ozone exposure for 30 or 60 days impaired formation/retention of a selective olfactory recognition memory 120 min after exposure to a juvenile stimulus animal with the effect at 60 days being significantly greater than at 30 days. They also showed impaired speed in locating a buried chocolate reward after 60 days of ozone exposure indicating some loss of olfactory perception. These functional impairments could all be prevented by coincident estradiol treatment. In the olfactory bulb, levels of lipid peroxidation were increased at both 30- and 60-day time-points and numbers of cells with immunohistochemical staining for α and β estrogen receptors, and dopamine β-hydroxylase were reduced as were α and β estrogen receptor protein levels. These effects were prevented by estradiol treatment. Oxidative stress damage caused by chronic exposure to ozone does therefore impair olfactory perception and social recognition memory and may do so by reducing noradrenergic and estrogen receptor activity in the olfactory bulb. That these effects can be prevented by estradiol treatment suggests increased susceptibility to neurodegenerative disorders in aging women may be contributed to by reduced estrogen levels post-menopause.     

Enhanced Macrophage Resistance to Pseudomonas Exotoxin A Is Correlated with Decreased Expression of the Low-Density Lipoprotein Receptor-Related Protein

Cellular intoxification by exotoxin A of Pseudomonas aeruginosa (PEA) begins when PEA binds to its cellular receptor, the low-density lipoprotein receptor-related protein (LRP). This receptor is particularly abundant on macrophages. We hypothesize here that inducible changes in cellular expression levels of the LRP represent an important mechanism by which macrophage susceptibility to PEA is regulated by the host. We have examined the effect of lipopolysaccharide (LPS) on LRP expression and PEA sensitivity in the macrophage-like cell line HS-P. Using a [(3)H]leucine incorporation assay to measure inhibition of protein synthesis, we have demonstrated that HS-P macrophages are highly sensitive to PEA and that PEA toxicity is decreased by the LRP antagonist receptor-associated protein. LPS pretreatment decreases HS-P PEA sensitivity in a time- and dose-dependent manner. The dose of toxin required to inhibit protein synthesis by 50% increased from 11.3 +/- 1.2 ng/ml in untreated cells to 25.7 +/- 2.0 ng/ml in cells treated with LPS. In pulse experiments, involving brief exposure to saturating concentrations of PEA, [(3)H]leucine incorporation was more than threefold higher in cells pretreated with LPS than in untreated macrophages. These changes in HS-P PEA sensitivity following LPS treatment were consistently associated with a fivefold decrease in HS-P LRP mRNA expression as measured by Northern blot analysis and a three-and-a-half-fold decrease in HS-P LRP-specific ligand internalization as determined by activated alpha(2)-macroglobulin internalization studies. These data demonstrate for the first time that modulation of LRP levels by extracellular signaling molecules can alter cellular PEA sensitivity.     

Regulated intramembrane proteolysis of the low-density lipoprotein receptor-related protein mediates ischemic cell death

The low-density lipoprotein receptor-related protein (LRP), a member of the low-density lipoprotein receptor gene family, mediates cellular signal transduction pathways. In this study we investigated the role of LRP in cell death. We found that incubation of mouse embryonic fibroblasts in serum-free media induces caspase-3 activation, an effect that is attenuated in LRP-deficient (LRP(-/-)) mouse embryonic fibroblasts. Since we previously demonstrated that middle cerebral artery occlusion (MCAO) in mice induces shedding of the LRP ectodomain, we investigated here whether cerebral ischemia induces regulated intramembrane proteolysis of LRP and whether this process is related to cell death. We found that MCAO induces an increase in gamma-secretase activity in the ischemic hemisphere and that treatment with the gamma-secretase inhibitor L-685,458 improves the neurological outcome and results in a 50% decrease in the volume of the ischemic lesion. Furthermore, MCAO caused nuclear translocation of the intracellular domain of LRP in neurons within the area of ischemic penumbra, and this effect was attenuated in mice treated with L-685,458. Finally, inhibition of either LRP or gamma-secretase attenuated cerebral ischemia-induced caspase-3 cleavage and apoptotic cell death. In summary, our results indicate that gamma-secretase-mediated regulated intramembrane proteolysis of LRP results in cell death under ischemic conditions.     

5-Hydroxytryptamine action in the rat olfactory bulb: in vitro electrophysiological patch-clamp recordings of juxtaglomerular and mitral cells

The olfactory bulb, first relay of olfactory pathways, is densely innervated by serotoninergic centrifugal fibers originating from the raphe nuclei. Although serotonin innervation was reported to be involved in olfactory learning in mammals, the action of this neurotransmitter on its putative cellular targets has been never described through unitary recordings. This lack of data initiated the present study where the effects of 5HT on juxtaglomerular and mitral cells are analyzed using whole-cell recordings on olfactory bulb slices. Serotonin depolarizes 34% of 525 JG cells. A multivariate statistical analysis of juxtaglomerular cells characteristics shows that the serotonin responsive cell group can be individualized regarding their tonic discharge-mode in response to a direct current injection, their lower expression of hyperpolarization-activated cation current and their low membrane capacities. The use of ion channel blockers and ramp voltage protocol indicate that serotoninergic depolarization of juxtaglomerular cells may be due to a nonselective cation current with a reversal potential of -44 mV. Pharmacological tests with serotonin receptor antagonists and agonists reveal that 5HT action on juxtaglomerular cells would be mainly mediated by 5HT2C receptors. In mitral cells, serotonin acts on 49.1% of the 242 tested cells, inducing two types of responses. A first subset of mitral cells (26.8%, n=65) were hyperpolarized by serotonin. This response would be indirect and mediated by action of GABA on GABAA receptors since it was antagonized by bicuculline. The involved GABAergic neurons are hypothesized to be juxtaglomerular and granular cells, on which serotonin would act mainly via 5HT2C and via 5HT2A receptors respectively. The second subset of mitral cells (22.3%, n=54) were directly depolarized by serotonin acting through 5HT2A receptors. Our data on serotonin action on juxtaglomerular cells and mitral cells reveal a part of functional mechanisms whereby serotonin can act on olfactory bulb network. This is expected to enrich the understanding of its determining role in olfactory learning.     

Low-density lipoprotein receptor-related protein 8 gene polymorphisms and dementia

The sole known genetic risk factor for sporadic Alzheimer's disease (AD) is the gene encoding apolipoprotein E (APOE), but the underlying mechanism is still under debate. One hypothesis relies on an interaction between APOE and its receptors. Previous studies have shown association of LDL receptor-related protein (LRP1) with AD and we previously reported a modulation by LRP1 of the risk of AD conferred by the -499A>G promoter polymorphism of the MAPK8IP1, a gene encoding Islet-brain-1 (IB1), the human counterpart of c-Jun NH(2) terminal kinase interacting protein-1 (JIP-1). Here we tested in two independent population samples a possible impact of another receptor for APOE, namely the low-density lipoprotein receptor-related protein 8 (LRP8), on the risk of dementia. Our results did not reveal any direct impact of a LRP8 coding (Arg952Gln) mutation on the risk of AD. However, this polymorphism increased the risk of AD conferred by the MAPK8IP1 G allele.     

Apolipoprotein E on hepatitis C virion facilitates infection through interaction with low-density lipoprotein receptor

Hepatitis C virus (HCV) infection is a major cause of liver disease. HCV associates with host apolipoproteins and enters hepatocytes through complex processes involving some combination of CD81, claudin-1, occludin, and scavenger receptor BI. Here we show that infectious HCV resembles very low density lipoprotein (VLDL) and that entry involves co-receptor function of the low-density lipoprotein receptor (LDL-R). Blocking experiments demonstrate that β-VLDL itself or anti-apolipoprotein E (apoE) antibody can block HCV entry. Knockdown of the LDL-R by treatment with 25-hydroxycholesterol or siRNA ablated ligand uptake and reduced HCV infection of cells, whereas infection was rescued upon cell ectopic LDL-R expression. Analyses of gradient-fractionated HCV demonstrate that apoE is associated with HCV virions exhibiting peak infectivity and dependence upon the LDL-R for cell entry. Our results define the LDL-R as a cooperative HCV co-receptor that supports viral entry and infectivity through interaction with apoE ligand present in an infectious HCV/lipoprotein complex comprising the virion. Disruption of HCV/LDL-R interactions by altering lipoprotein metabolism may therefore represent a focus for future therapy.     

ApoE genotype does not affect plasma tPA and PAI-1 antigen levels

The presence of one or two apoliprotein E4 (apoE4) alleles constitutes a major risk factor for Alzheimer's disease (AD) and coronary heart disease (CHD). Numerous observations have suggested that misregulation of proteases may be instrumental in both diseases. Tissue-type plasminogen activator (tPA) has been recently demonstrated to play a key role in neuronal plasticity and in experimental neurodegeneration. One receptor for the ApoE protein is the LRP/α2 macroglobulin receptor, which also binds to and endocytoses tPA and plasminogen activator inhibitor I (PAI-1). Here we tested whether the apoE genotype has an influence on the plasma levels of these proteins. We demonstrate that there is no difference in plasma levels of tPA- and PAI-1-antigens between middled-aged individuals with one apoE4 allele and those having none. This suggests that the impact of apoE4 on Alzheimer's disease is not the result of altered clearance of tPA or PAI-1 by the LRP receptor. Am. J. Med. Genet. 74:172–175, 1997. © 1997 Wiley-Liss, Inc.     

Attenuated amplitude of circadian and sleep-dependent modulation of electroencephalographic sleep spindle characteristics in elderly human subjects

The low density lipoprotein receptor-related protein (LRP) gene is a candidate gene for Alzheimer's disease (AD) due to its role as a receptor for apolipoprotein E (apoE), a major genetic risk factor for late-onset familial and sporadic AD. Recently, several studies have reported a correlation between a polymorphism (C766T) in exon 3 of LRP and AD. We examined this polymorphism in a Caucasian population of 225 neuropathologically confirmed cases with AD and 187 elderly cases without any AD neuropathological changes. We found that the exon 3 LRP C/C genotype was slightly but not significantly higher in the AD group when compared to the control group. A meta-analysis of previous studies revealed only a weak correlation of this polymorphism with AD (odds ratio 1.34, [95% CI 1.16-1.54], P < 0.0001). These data indicate that the polymorphism in exon 3 of LRP is only a minor risk factor for AD and that another locus on chromosome 12 is likely responsible for the associations observed in other studies.