Role of apolipoprotein E receptors in regulating the differential in vivo neurotrophic effects of apolipoprotein E.

Apolipoprotein E (apoE) is known to bind to at least five receptors, including the low-density lipoprotein (LDL) receptor-related protein (LRP), very low density LDL receptor (VLDL-R), LDL-R, apoE receptor 2 (apoER2), and megalin/gp330. In this context, the main objective of the present study was to better understand the contributions of LRP and LDL-R to the in vivo neurotrophic effects of apoE. For this purpose, apoE-deficient and receptor-associated protein (RAP)-deficient mice were infused with recombinant apoE3, RAP, or saline. Infusion of apoE3 into apoE-deficient mice resulted in amelioration of degenerative alterations of pyramidal neurons, but had no effect on somatostatin-producing interneurons. In contrast, infusion of apoE3 into RAP-deficient mice resulted in amelioration of degenerative alterations of somatostatin-producing interneurons. LRP and LDL-R levels were significantly reduced in RAP-deficient mice, but significantly increased in the apoE-deficient mice. In contrast, levels of apoE were reduced in the RAP-deficient mice compared to wildtype controls, suggesting that neurotrophic effects of apoE3 in the RAP-deficient mice were related to a combined deficit in endogenous apoE and selected apoE receptors. Furthermore, in apoE-deficient mice, infusion of apoE3 had a neurotrophic effect on somatostatin-producing interneurons only when combined with RAP, suggesting that increased expression of apoE receptors in apoE-deficient mice prevented apoE from rescuing somatostatin-producing neurons. This study supports the contention that some of the in vivo neurotrophic effects of apoE are mediated by LRP and LDL-R and that a critical balance between levels of apoE and its receptors is necessary for the differential neurotrophic effects to appear.

LDL receptor-related protein (LRP) in Alzheimer's disease: towards a unified theory of pathogenesis.

To date, mutations in three genes, beta-amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2), have been found to be causally related to familial Alzheimer's disease (AD). In addition, polymorphisms in three other genes (among others), apolipoprotein E (apoE), alpha2-macroglobulin (alpham), and the low density lipoprotein receptor-related protein (LRP), are implicated to contribute to AD pathogenesis. Interestingly, the encoded gene products are all functionally related in various ways to LRP. Specifically apoE, alpha2m, secreted APP, and amyloid beta-protein (Abeta) complexed to either apoE or alpha2m are ligands of LRP. Furthermore, over-expression of presenilin 1 results in decreased expression of LRP. Since levels of many LRP ligands are increased in Alzheimer's disease and LRP and its ligands are present in senile plaques, decreased LRP function may be a central component in AD pathogenesis. This review explores the current knowledge of LRP in AD and its relationship to the other known AD susceptibility markers.

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

In order to delineate the neuroprotective role of the low density lipoprotein receptor-related protein (LRP) against amyloid beta-protein toxicity, studies were performed in C6 cells challenged with amyloid beta-protein in the presence or absence of activated alpha(2)-macroglobulin. Toxicity was assessed via two cell viability assays. We found that this endocytic receptor conferred protection against amyloid beta-protein toxicity in the presence of activated alpha(2)-macroglobulin and its down-regulation via inhibition by receptor-associated protein or transfection of cells with presenilin 1, increased susceptibility to amyloid beta-protein toxicity. Increased surface LRP immunoreactivity in response to amyloid beta-protein challenge was associated with increased translocation of LRP from the endoplasmic reticulum to the surface, rather than from increased mRNA or protein expression. Furthermore, this translocation of LRP to the surface was mediated by a calcium/calmodulin protein kinase II-dependent signaling pathway. These studies provide evidence for a protective role of LRP against amyloid beta-protein toxicity and may explain the aggressive nature of presenilin-1 mutation in familial Alzheimer's disease.

A role for apoE in regulating the levels of alpha-1-antichymotrypsin in the aging mouse brain and in Alzheimer's disease.

This study was designed to explore the possible functional relationships between apolipoprotein E (apoE) and the protease inhibitor alpha-1-antichymotrypsin in the aging mouse brain and in Alzheimer's disease. For this purpose, levels of EB22/5 (the mouse homologue to human alpha-1-antichymotrypsin) mRNA expression was studied in apoE-deficient mice. These mice showed an age-dependent increase of EB22/5 mRNA expression in the brain. Furthermore, overexpression of allele 3 of human APOE gene in transgenic mice (in an apoE-deficient background) resulted in normalization of levels of EB22/5 mRNA expression compatible with levels found in control mice. In contrast, overexpression of human APOE4 allele or down-regulation of the apoE receptor low density lipoprotein receptor-related protein by deletion of the receptor-associated protein was associated with elevated levels of EB22/5 similar to apoE-deficient mice. Consistent with the findings in murine models, human alpha-1-antichymotrypsin protein was increased in brain homogenates from patients with Alzheimer's disease, and levels of this serpin were the highest in patients with the APOE4 allele. In summary, the present study showed evidence supporting a role for apoE in regulating alpha-1-antichymotrypsin expression. This is relevant to Alzheimer's disease because these two molecules appear to be closely associated with the pathogenesis of this disorder.

Aberrant presenilin-1 expression downregulates LDL receptor-related protein (LRP): is LRP central to Alzheimer's disease pathogenesis?

Low density lipoprotein receptor-related protein (LRP) polymorphisms have recently been associated with an increased susceptibility of Alzheimer's disease (AD). Furthermore, LRP has been linked to molecules that confer susceptibility to AD (apolipoprotein E, alpha-2-macroglobulin, amyloid precursor protein), previously with the exception of the presenilins. Here we report that aberrant presenilin-1 expression in vivo and in vitro downregulates LRP. Specifically, transgenic mice overexpressing the M146L or L286V presenilin-1 mutation show decreased levels of LRP expression in neuronal populations where presenilin-1 and LRP are closely colocalized or coexpressed. Moreover, cell lines transfected with presenilin-1 also expressed decreased levels of LRP. These findings suggest that LRP may be central to AD pathogenesis since all proteins genetically associated with AD can now be linked via a single pathway to LRP.

Synaptotagmin and synaptic transmission alterations in apolipoprotein E-deficient mice.

1. Aged apoE-deficient mice and age-matched controls were tested for cognitive alterations in the Morris water maze. 2. Water maze results were correlated with in vivo electrophysiology and expression of the synaptic protein synaptotagmin (p65). 3. Compared to age-matched controls, apolipoprotein E-deficient mice displayed significant performance impairment accompanied by in vivo electrophysiological alterations in the dentate gyrus. 4. Apolipoprotein E-deficient mice also showed a significant increase in the synaptic protein, synaptotagmin, a synaptic calcium sensor involved in neurotransmitter release. 5. Cognitive impairments in these animals may be associated with decreased synaptic excitability in hippocampal neurons and the regulatory role of apolipoprotein E in synaptic function might be mediated by modulation of the expression of calcium sensor proteins.

A novel role for receptor-associated protein in somatostatin modulation: implications for Alzheimer's disease.

Receptor-associated protein appears to play an important role in low-density lipoprotein receptor-related protein trafficking. Since ligands for the low-density lipoprotein receptor-related protein have been implicated in Alzheimer's disease and normal functioning of this protein is indispensable for central nervous system development, deficient receptor-associated protein expression may result in central nervous system alterations. In this study, receptor-associated protein knockout mice were behaviorally tested and nervous system integrity was assessed via in situ hybridization and immunocytochemical/laser confocal microscopy methods. Receptor-associated protein knockout mice were found to be cognitively impaired in the Morris water maze compared to controls. In wild-type mice, the receptor-associated protein was found to be highly co-expressed with somatostatin in hippocampal and neocortical inhibitory neurons. Receptor-associated protein knockout mice, however, showed a significant decrease in number of somatostatin-expressing neurons of the CA1 region and somatostatin expression within these neurons. The decreased number of somatostatin neurons significantly correlated with cognitive impairment observed in the receptor-associated protein knockout mice. These results suggest a novel role for receptor-associated protein in modulating the functioning of somatostatin-producing neurons. Furthermore, this has implications for Alzheimer's disease pathogenesis, in which altered regulation of both somatostatin and the known low-density lipoprotein receptor-related protein ligands are a consistent finding.

Altered long-term potentiation in the hippocampus of apolipoprotein E-deficient mice.

Recent studies suggest that apolipoprotein E (apoE) plays a neurotrophic role in the central nervous system and that an aberrant function of this molecule might result in neurodegeneration. Supporting this notion, apoE-deficient mice show neurodegenerative and cognitive alterations. To characterize physiological changes associated with synaptic damage and cognitive impairment in apoE-deficient mice, we investigated synaptic plasticity in the hippocampus of urethane anesthetized mice. Electrical stimulation was delivered to the perforant pathway and the resulting evoked field excitatory postsynaptic potential (EPSP) and population spike were recorded in the hilus. Long-term potentiation, as measured in the population spike, was reduced by 50% in apoE-deficient mice when compared to wild-type controls. In contrast, there were no significant differences in the evoked field EPSP between wild-type and apoE-deficient mice following high-frequency stimulation. These results support the notion that cognitive impairment and synaptic loss in the hippocampus of apoE-deficient mice might be associated with impaired long-term potentiation.

Anxiolytic activity of NPY receptor agonists in the conflict test.

This investigation examined receptor subtype specificity and possible modulation by GABAa receptor ligands of NPY-induced behavioral responses to stressful stimuli. First, a series of NPY receptor agonists were examined for their potential effects on punished responding in a conflict test modified for incremental shock. NPY, peptide YY (PYY) and NPY Y1 receptor agonists [Leu31,Pro34]-NPY and [Gly6, Glu26,Lys26,Pro34]-NPY produced increases in punished responding in the conflict test. No significant effects on unpunished responding were noted. The pattern of responding was similar to that observed with the benzodiazepine agonist chlordiazepoxide. Neither pancreatic peptide (PP) nor the Y2 agonists NPY13-36 or [Glu2,32,Ala6,Dpr27,Lys28]-NPY significantly altered punished or unpunished responding. Of significance, the atypical Y1 agonist [Cys7,21,Pro34]-NPY produced negligible effects on punished responding, consistent with the presence of a subclass of Y1 receptors. Second, the anxiolytic effects of NPY were subjected to treatments that block actions at the GABAa receptor complex. The increase in punished responding produced by NPY was not altered by administration of the benzodiazepine antagonist flumazenil and only partially blocked by the picrotoxinin receptor ligand isopropylbicyclophosphate (10 and 15 microg/kg). These findings further support the hypothesis that the pharmacologic substrates for the anxiolytic-like actions of NPY may be mediated by the Y1 receptor subtype and suggest that these actions are independent of either the benzodiazepine or picrotoxinin binding sites of the GABA/benzodiazepine receptor complex.