Lipoprotein receptors in the nervous system

Loss of gamma-secretase function impairs endocytosis of lipoprotein particles and membrane cholesterol homeostasis

Presenilins (PSs) are components of the gamma-secretase complex that mediates intramembranous cleavage of type I membrane proteins. We show that gamma-secretase is involved in the regulation of cellular lipoprotein uptake. Loss of gamma-secretase function decreased endocytosis of low-density lipoprotein (LDL) receptor. The decreased uptake of lipoproteins led to upregulation of cellular cholesterol biosynthesis by increased expression of CYP51 and enhanced metabolism of lanosterol. Genetic deletion of PS1 or transgenic expression of PS1 mutants that cause early-onset Alzheimer's disease led to accumulation of gamma-secretase substrates and mistargeting of adaptor proteins that regulate endocytosis of the LDL receptor. Consistent with decreased endocytosis of these receptors, PS1 mutant mice have elevated levels of apolipoprotein E in the brain. Thus, these data demonstrate a functional link between two major genetic factors that cause early-onset and late-onset Alzheimer's disease.

Loss of LR11/SORLA enhances early pathology in a mouse model of amyloidosis: evidence for a proximal role in Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia, resulting in progressive neuronal death and debilitating damage to brain loci that mediate memory and higher cognitive function. While pathogenic genetic mutations have been implicated in approximately 2% of AD cases, the proximal events that underlie the common, sporadic form of the disease are incompletely understood. Converging lines of evidence from human neuropathology, basic biology, and genetics have implicated loss of the multifunctional receptor LR11 (also known as SORLA and SORL1) in AD pathogenesis. Cell-based studies suggest that LR11 reduces the formation of beta-amyloid (Abeta), the molecule believed to be a primary toxic species in AD. Recently, mutant mice deficient in LR11 were shown to upregulate murine Abeta in mouse brain. In the current study, LR11-deficient mice were crossed with transgenic mice expressing autosomal-dominant human AD genes, presenilin-1 (PS1DeltaE9) and amyloid precursor protein (APPswe). Here, we show that LR11 deficiency in this AD mouse model significantly increases Abeta levels and exacerbates early amyloid pathology in brain, causing a forward shift in disease onset that is LR11 gene dose-dependent. Loss of LR11 increases the processing of the APP holo-molecule into alpha-, beta-, and gamma-secretase derived metabolites. We propose that LR11 regulates APP processing and Abeta accumulation in vivo and is of proximal importance to the cascade of pathological amyloidosis. The results of the current study support the hypothesis that control of LR11 expression may exert critical effects on Alzheimer's disease susceptibility in humans.

Circulatory lipid transport: lipoprotein assembly and function from an evolutionary perspective

Circulatory transport of neutral lipids (fat) in animals relies on members of the large lipid transfer protein (LLTP) superfamily, including mammalian apolipoprotein B (apoB) and insect apolipophorin II/I (apoLp-II/I). Latter proteins, which constitute the structural basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride transfer protein (MTP)--another LLTP family member--and bind them by means of amphipathic structures. Comparative research reveals that LLTPs have evolved from the earliest animals and additionally highlights the structural and functional adaptations in these lipid carriers. For instance, in contrast to mammalian apoB, the insect apoB homologue, apoLp-II/I, is post-translationally cleaved by a furin, resulting in their appearance of two non-exchangeable apolipoproteins in the insect low-density lipoprotein (LDL) homologue, high-density lipophorin (HDLp). An important difference between mammalian and insect lipoproteins relates to the mechanism of lipid delivery. Whereas in mammals, endocytic uptake of lipoprotein particles, mediated via members of the LDL receptor (LDLR) family, results in their degradation in lysosomes, the insect HDLp was shown to act as a reusable lipid shuttle which is capable of reloading lipid. Although the recent identification of a lipophorin receptor (LpR), a homologue of LDLR, reveals that endocytic uptake of HDLp may constitute an additional mechanism of lipid delivery, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. Binding studies indicate that the HDLp-LpR complex, in contrast to the LDL-LDLR complex, is resistant to dissociation at endosomal pH as well as by treatment with EDTA mimicking the drop in Ca(2+) concentration in the endosome. This remarkable stability of the ligand-receptor complex may provide a crucial key to the recycling mechanism. Based on the binding and dissociation capacities of mutant and hybrid receptors, the specific binding interaction of the ligand-binding domain of the receptor with HDLp was characterized. These structural similarities and functional adaptations of the lipid transport systems operative in mammals and insects are discussed from an evolutionary perspective.

LRP1 and cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA), which causes intracerebral hemorrhage in the elderly population, is a major hallmark of Alzheimer’s disease. In CAA, amyloid-β (Aβ) deposition is mainly detected in the cortex and leptomeningeal arteries along the interstitial fluid drainage pathway. Failure to eliminate Aβ leads to accumulation of Aβ in the perivascular region and CAA. Several clearance routes of Aβ have been described, including elimination along the perivascular interstitial fluid drainage pathway, elimination through the blood–brain barrier and uptake and degradation by glia and neurons. All of these routes express the low-density lipoprotein receptor-related protein (LRP)1, which plays a critical role in Aβ clearance. Consequently, an impairment of Aβ clearance through LRP1 likely contributes to CAA pathogenesis. This review summarizes what is known about LRP1 and CAA as well as providing insights into the possible roles of LRP1 in Aβ clearance.

Lrp4 modulates extracellular integration of cell signaling pathways in development

The extent to which cell signaling is integrated outside the cell is not currently appreciated. We show that a member of the low-density receptor-related protein family, Lrp4 modulates and integrates Bmp and canonical Wnt signalling during tooth morphogenesis by binding the secreted Bmp antagonist protein Wise. Mouse mutants of Lrp4 and Wise exhibit identical tooth phenotypes that include supernumerary incisors and molars, and fused molars. We propose that the Lrp4/Wise interaction acts as an extracellular integrator of epithelial-mesenchymal cell signaling. Wise, secreted from mesenchyme cells binds to BMP's and also to Lrp4 that is expressed on epithelial cells. This binding then results in the modulation of Wnt activity in the epithelial cells. Thus in this context Wise acts as an extracellular signaling molecule linking two signaling pathways. We further show that a downstream mediator of this integration is the Shh signaling pathway.

LRP4 serves as a coreceptor of agrin

Neuromuscular junction (NMJ) formation requires agrin, a factor released from motoneurons, and MuSK, a transmembrane tyrosine kinase that is activated by agrin. However, how signal is transduced from agrin to MuSK remains unclear. We report that LRP4, a low-density lipoprotein receptor (LDLR)-related protein, is expressed specifically in myotubes and binds to neuronal agrin. Its expression enables agrin binding and MuSK signaling in cells that otherwise do not respond to agrin. Suppression of LRP4 expression in muscle cells attenuates agrin binding, agrin-induced MuSK tyrosine phosphorylation, and AChR clustering. LRP4 also forms a complex with MuSK in a manner that is stimulated by agrin. Finally, we showed that LRP4 becomes tyrosine-phosphorylated in agrin-stimulated muscle cells. These observations indicate that LRP4 is a coreceptor of agrin that is necessary for MuSK signaling and AChR clustering and identify a potential target protein whose mutation and/or autoimmunization may cause muscular dystrophies.

Linking Abeta and tau in late-onset Alzheimer's disease: a dual pathway hypothesis

Alzheimer's disease is characterized by abnormal elevation of Abeta peptide and abnormal hyperphosphorylation of the tau protein. The "amyloid hypothesis," which is based on molecular defects observed in autosomal-dominant early-onset Alzheimer's disease (EOAD), suggests a serial model of causality, whereby elevation of Abeta drives other disease features including tau hyperphosphorylation. Here, we review recent evidence from drug trials, genetic studies, and experimental work in animal models that suggests that an alternative model might exist in late-onset AD (LOAD), the complex and more common form of the disease. Specifically, we hypothesize a "dual pathway" model of causality, whereby Abeta and tau can be linked by separate mechanisms driven by a common upstream driver. This model may account for the results of recent drug trials and, if confirmed, may guide future drug development.

Expanding functions of lipoprotein receptors

Lipoprotein receptors are evolutionarily ancient proteins that are expressed on the surface of many cell types. Beginning with the appearance of the first primitive multicellular organisms, several structurally and functionally distinct families of lipoprotein receptors evolved. Originally, these cell surface proteins were thought to merely mediate the traffic of lipids and nutrients between cells and, in some cases, by functioning as scavenger receptors, remove other kinds of macromolecules, such as proteases and protease inhibitors from the extracellular space and the cell surface. Over the last decade, this picture has fundamentally changed. We now appreciate that many of these receptors are not mere cargo transporters; they are deeply embedded in the machinery by which cells communicate with each other. By physically interacting and coevolving with fundamental signaling pathways, lipoprotein receptors have occupied essential and surprisingly diverse functions that are indispensable for integrating the complex web of cellular signal input during development and in differentiated tissues.

Polarized traffic of LRP1 involves AP1B and SNX17 operating on Y-dependent sorting motifs in different pathways

Low-density lipoprotein receptor-related protein 1 (LRP1) is an endocytic recycling receptor with two cytoplasmic tyrosine-based basolateral sorting signals. Here we show that during biosynthetic trafficking LRP1 uses AP1B adaptor complex to move from a post-TGN recycling endosome (RE) to the basolateral membrane. Then it recycles basolaterally from the basolateral sorting endosome (BSE) involving recognition by sorting nexin 17 (SNX17). In the biosynthetic pathway, Y(29) but not N(26) from a proximal NPXY directs LRP1 basolateral sorting from the TGN. A N(26)A mutant revealed that this NPXY motif recognized by SNX17 is required for the receptor's exit from BSE. An endocytic Y(63)ATL(66) motif also functions in basolateral recycling, in concert with an additional endocytic motif (LL(86,87)), by preventing LRP1 entry into the transcytotic apical pathway. All this sorting information operates similarly in hippocampal neurons to mediate LRP1 somatodendritic distribution regardless of the absence of AP1B in neurons. LRP1 basolateral distribution results then from spatially and temporally segregation steps mediated by recognition of distinct tyrosine-based motifs. We also demonstrate a novel function of SNX17 in basolateral/somatodendritic recycling from a different compartment than AP1B endosomes.

Lipoprotein receptors and cholesterol in APP trafficking and proteolytic processing, implications for Alzheimer's disease

Amyloid-beta (Abeta) peptide accumulation in the brain is central to the pathogenesis of Alzheimer's disease (AD). Abeta is produced through proteolytic processing of a transmembrane protein, beta-amyloid precursor protein (APP), by beta- and gamma-secretases. Mounting evidence has demonstrated that alterations in APP cellular trafficking and localization directly impact its processing to Abeta. Members of the low-density lipoprotein receptor family, including LRP, LRP1B, SorLA/LR11, and apoER2, interact with APP and regulate its endocytic trafficking. Additionally, APP trafficking and processing are greatly affected by cellular cholesterol content. In this review, we summarize the current understanding of the roles of lipoprotein receptors and cholesterol in APP trafficking and processing and their implication for AD pathogenesis and therapy.

The pathogenesis of Niemann-Pick type C disease: a role for autophagy?

Niemann-Pick type C disease (NPC) is a sphingolipid-storage disorder that results from inherited deficiencies of intracellular lipid-trafficking proteins, and is characterised by an accumulation of cholesterol and glycosphingolipids in late endosomes and lysosomes. Patients with this disorder develop progressive neurological impairment that often begins in childhood, is ultimately fatal and is currently untreatable. How impaired lipid trafficking leads to neurodegeneration is largely unknown. Here we review NPC clinical features and biochemical defects, and discuss model systems used to study this disorder. Recent studies have established that NPC is associated with an induction of autophagy, a regulated and evolutionarily conserved process by which cytoplasmic proteins are sequestered within autophagosomes and targeted for degradation. This pathway enables recycling of limited or damaged macromolecules to promote cell survival. However, in other instances, robust activation of autophagy leads to cell stress and programmed cell death. We summarise evidence showing that autophagy induction and flux are increased in NPC by signalling through a complex of the class III phosphoinositide 3-kinase and beclin-1. We propose that an imbalance between induction and flux through the autophagic pathway contributes to cell stress and neuronal loss in NPC and related sphingolipid-storage disorders, and discuss potential therapeutic strategies for modulating activity of this pathway.

Biphasic requirement for geranylgeraniol in hippocampal long-term potentiation

Mice deficient in cholesterol 24-hydroxylase exhibit reduced rates of cholesterol synthesis and other non-sterol isoprenoids that arise from the mevalonate pathway. These metabolic abnormalities, in turn, impair learning in the whole animal and hippocampal long-term potentiation (LTP) in vitro. Here, we report pharmacogenetic experiments in hippocampal slices from wild-type and mutant mice that characterize the dependence of LTP on the non-sterol isoprenoid, geranylgeraniol. Addition of geranylgeraniol to slices from 24-hydroxylase knockout mice restores LTP to wild-type levels; however, farnesol, a chemically related compound, does not substitute for geranylgeraniol nor does another animal model of impaired LTP (apolipoprotein E deficiency) respond to this isoprenoid. The requirement for geranylgeraniol is independent of acute protein isoprenylation as judged in experiments employing cell-permeable inhibitors of protein farnesyl transferase and geranylgeranyl transferase enzymes and in mutant mice hypomorphic for geranylgeranyltransferase II. Time course studies show that geranylgeraniol acts within 5 min and at 2 different times during the establishment of LTP: just before electrical stimulation and approximately 15 min thereafter. Localized delivery of geranylgeraniol to the dendritic trees of CA1 hippocampal neurons via the recording electrode is sufficient to restore LTP in slices from 24-hydroxylase knockout mice. We conclude that geranylgeraniol acts specifically and quickly to affect LTP in the Schaffer collaterals of the hippocampus.

A megalin-like receptor is involved in protein endocytosis in the midgut of an insect (Bombyx mori, Lepidoptera)

The mechanism responsible for fluorescein isothiocyanate (FITC)-albumin internalization by columnar cells in culture obtained from the midgut of Bombyx mori larvae was examined by confocal laser scanning microscopy. Protein uptake changed over time, and it appeared to be energy dependent, since it was strongly reduced by both low temperatures and metabolic inhibitors. Labeled albumin uptake as a function of increasing protein concentration showed a saturation kinetics with a Michaelis constant value of 2.0 +/- 0.6 microM. These data are compatible with the occurrence of receptor-mediated endocytosis. RT-PCR analysis and colocalization experiments with an anti-megalin primary antibody indicated that the receptor involved was a putative homolog of megalin, the multiligand endocytic receptor belonging to the low-density lipoprotein receptor family, responsible for the uptake of various molecules, albumin included, in many epithelial cells of mammals. This insect receptor, like the mammalian counterpart, required Ca(2+) for albumin internalization and was inhibited by gentamicin. FITC-albumin internalization was clathrin mediated, since two inhibitors of this process caused a significant reduction of the uptake, and clathrin and albumin colocalized in the intermicrovillar areas of the apical plasma membrane. The integrity of actin and microtubule organization was essential for the correct functioning of the endocytic machinery.

Regulated proteolysis of APP and ApoE receptors

The beta-amyloid precursor protein (APP) shares intracellular and extracellular-binding partners with the family of receptors for apolipoprotein E (apoE). Binding of APP and apoE receptors to specific extracellular matrix proteins (F-spondin and Reelin) promotes their presence on the cell surface and influences whether they will interact with specific cytoplasmic adaptor proteins. Cleavage of APP and apoE receptors at the cell surface occurs by alpha-secretase activities; thus, the processing of these proteins can be regulated by their trafficking either to or from the cell surface. Their cleavages can also be regulated by tissue inhibitor of metalloproteinase-3 (TIMP-3), a metalloprotease inhibitor in the extracellular matrix. ApoE receptors have functions in neuronal migration during development and in proper synaptic function in the adult. Thus, the functions of apoE receptors and by analogy of APP will be modified by the various extracellular and intracellular interactions reviewed in this paper.

Increased locomotor activity in mice lacking the low-density lipoprotein receptor

While the low-density lipoprotein receptor (LDLR) is best known for its role in regulating serum cholesterol, LDLR is expressed in brain, suggesting that it may play a role in CNS function as well. Here, using mice with a null mutation in LDLR (LDLR-/-), we investigated whether the absence of LDLR affects a series of behavioral functions. We also utilized the fact that plasma cholesterol levels can be regulated in LDLR-/- mice by manipulating dietary cholesterol to investigate whether elevated plasma cholesterol might independently affect behavioral performance. LDLR-/- mice showed no major deficits in general sensory or motor function. However, LDLR-/- mice exhibited increased locomotor activity in an open field test without evidence of altered anxiety in either an open field or a light/dark emergence test. By contrast, modulating dietary cholesterol produced only isolated effects. While both C57BL/6J and LDLR-/- mice fed a high cholesterol diet showed increased anxiety in a light/dark task, and LDLR-/- mice fed a high cholesterol diet exhibited longer target latencies in the probe trial of the Morris water maze, no other findings supported a general effect of cholesterol on anxiety or spatial memory. Collectively these studies suggest that while LDLR-/- mice exhibit no major developmental defects, LDLR nevertheless plays a significant role in modulating locomotor behavior in the adult.

Mutations in the very low-density lipoprotein receptor VLDLR cause cerebellar hypoplasia and quadrupedal locomotion in humans

Quadrupedal gait in humans, also known as Unertan syndrome, is a rare phenotype associated with dysarthric speech, mental retardation, and varying degrees of cerebrocerebellar hypoplasia. Four large consanguineous kindreds from Turkey manifest this phenotype. In two families (A and D), shared homozygosity among affected relatives mapped the trait to a 1.3-Mb region of chromosome 9p24. This genomic region includes the VLDLR gene, which encodes the very low-density lipoprotein receptor, a component of the reelin signaling pathway involved in neuroblast migration in the cerebral cortex and cerebellum. Sequence analysis of VLDLR revealed nonsense mutation R257X in family A and single-nucleotide deletion c2339delT in family D. Both these mutations are predicted to lead to truncated proteins lacking transmembrane and signaling domains. In two other families (B and C), the phenotype is not linked to chromosome 9p. Our data indicate that mutations in VLDLR impair cerebrocerebellar function, conferring in these families a dramatic influence on gait, and that hereditary disorders associated with quadrupedal gait in humans are genetically heterogeneous.

Recycling of apolipoprotein E is not associated with cholesterol efflux in neuronal cells

After receptor-mediated endocytosis of apolipoprotein E (apoE)-containing lipoproteins in hepatocytes, the isoform apoE3 is efficiently recycled in a process which is associated with cholesterol efflux. Recycling and cholesterol efflux are greatly reduced when apoE4 is the only isoform present. ApoE is the main apolipoprotein in cerebrospinal fluid, and it plays a pivotal role in maintaining cholesterol homeostasis in the brain. The isoform apoE4 is associated with an increased risk of Alzheimer's disease and it has been postulated that high intracellular cholesterol levels promote the amyloidogenic processing of amyloid precursor protein. Therefore we investigated the cellular processing of different apoE isoforms as well as the associated cholesterol efflux in the murine neuronal cell line HT-22. Uptake of apoE3-containing lipoproteins resulted in the expected recycling while, as seen in non-neuronal cells, recycling of apoE4 was significantly reduced. However, despite these differences in apoE recycling, there was no difference in rates of cholesterol efflux. Therefore we conclude that in this neuronal cell model the reduced recycling of apoE4 does not affect cellular cholesterol metabolism.

Role of liver and plasma lipoproteins in selective transport of n-3 fatty acids to tissues: a comparative study of 14C-DHA and 3H-oleic acid tracers

We conducted a study aimed at a direct comparison of the plasma dynamics and uptake of docosahexaenoic (DHA) and oleic (OA) fatty acids by various organs. 14C-DHA and 3H-OA were intravenously co-injected into mice. At 5 min after injection, more than 40% of the 14C-DHA, but less than 20% of the 3H-OA, labels was associated with the liver. Heart uptake of 14C-DHA was three to four times greater compared to the 3H-OA label. Brain incorporation of 14C-DHA slowly rose to 0.7% at 24 h, but it remained at the 1-1.5% level for 3H-OA. Total 14C activity in plasma reached 2% of the injected dose at 20 min and leveled off at 0.5% after 1.5 h. Fifteen percent of 14C-DHA plasma activity at 30 min was associated with non-esterified fatty acids, whereas about 85% was recovered in triglycerides in very low-density lipoprotein (VLDL) and LDL fractions. Only 30% of 3H-OA derived activity was found in the VLDL fraction at 30 min. All 3H activity in plasma at later time points was in catabolite fractions. These findings demonstrate that liver plays an important role in the initial selectivity for DHA. It is likely that DHA is specifically taken up by liver, esterified, loaded into lipoproteins, and then delivered to brain, heart, and other target tissues.

Amyloid precursor protein regulates brain apolipoprotein E and cholesterol metabolism through lipoprotein receptor LRP1

Mutations in the amyloid precursor protein (APP) cause early-onset Alzheimer's disease (AD), but the only genetic risk factor for late-onset AD is the varepsilon4 allele of apolipoprotein E (apoE), a major cholesterol carrier. Using Cre-lox conditional knockout mice, we demonstrate that lipoprotein receptor LRP1 expression regulates apoE and cholesterol levels within the CNS. We also found that deletion of APP and its homolog APLP2, or components of the gamma-secretase complex, significantly enhanced the expression and function of LRP1, which was reversed by forced expression of the APP intracellular domain (AICD). We further show that AICD, together with Fe65 and Tip60, interacts with the LRP1 promoter and suppresses its transcription. Together, our findings support that the gamma-secretase cleavage of APP plays a central role in regulating apoE and cholesterol metabolism in the CNS via LRP1 and establish a biological linkage between APP and apoE, the two major genetic determinants of AD.

Galpha12/Galpha13 deficiency causes localized overmigration of neurons in the developing cerebral and cerebellar cortices

The heterotrimeric G proteins G(12) and G(13) link G-protein-coupled receptors to the regulation of the actin cytoskeleton and the induction of actomyosin-based cellular contractility. Here we show that conditional ablation of the genes encoding the alpha-subunits of G(12) and G(13) in the nervous system results in neuronal ectopia of the cerebral and cerebellar cortices due to overmigration of cortical plate neurons and cerebellar Purkinje cells, respectively. The organization of the radial glia and the basal lamina was not disturbed, and the Cajal-Retzius cell layer had formed normally in mutant mice. Embryonic cortical neurons lacking G(12)/G(13) were unable to retract their neurites in response to lysophosphatidic acid and sphingosine-1-phosphate, indicating that they had lost the ability to respond to repulsive mediators acting via G-protein-coupled receptors. Our data indicate that G(12)/G(13)-coupled receptors mediate stop signals and are required for the proper positioning of migrating cortical plate neurons and Purkinje cells during development.

Beta1 integrins in radial glia but not in migrating neurons are essential for the formation of cell layers in the cerebral cortex

Radial glial cells in the cerebral cortex serve as progenitors for neurons and glia and guide the migration of cortical neurons. The integrin alpha3beta1 is thought to mediate interactions of migrating neurons with radial glial cells and to function as a receptor for the reelin signaling molecule. Here, we challenge this view and demonstrate that beta1 integrins in migrating neurons are not essential for the formation of cell layers in the cerebral cortex. Cortical cell layers also form normally in mice deficient in the integrin alpha3beta1. However, we provide evidence that beta1 integrins in radial glia control the morphological differentiation of both glia and neurons. We conclude that beta1 integrins in radial glia are required for the proper development of the cerebral cortex, whereas beta1 integrins in migrating neurons are not essential for glial-guided migration and reelin signaling.

Plasminogen activator induction facilitates recovery of respiratory function following spinal cord injury

The possibility that plasminogen activator (PA) plays a role in synaptic plasticity was explored in the spinal cord during the crossed phrenic phenomenon (CPP), where respiratory functional plasticity develops following spinal cord injury. Synaptic remodeling on phrenic motorneurons occurs during the characteristic delay period following spinal cord injury before CPP recovery of respiratory function. The molecular mechanisms underlying this plasticity are not well-defined. During the critical 1-2 h delay period required for this synaptic plasticity following a C2 hemisection in mice, uPA and tPA mRNAs are rapidly induced in C4-5 ventral spinal cord neurons in the ipsilateral phrenic motor nucleus (PMN), as are uPA and tPA protein levels. A role for uPA in CPP spinal cord plasticity is confirmed by the impaired ability of uPA knockout mice to acquire a good CPP response by 6 h post-hemisection and their lack of structural remodeling of PMN synapses that underlies development of the CPP response.

Neurodevelopment in Seizure-prone and Seizure-resistant Rat Strains: Recognizing Conflicts in Management

Cytoarchitectural alterations during central nervous system (CNS) development are believed to underlie aberrations in brain morphology that lead to epilepsy. We have recently reported marked reductions in hippocampal and white matter volumes along with relative ventriculomegaly in a rat strain bred to be seizure-prone (FAST) compared to a strain bred to be seizure-resistant (SLOW) (Gilby et al., 2002, American Epilepsy Society 56th Annual Meeting). This study was designed to investigate deviations in gene expression during late-phase embryogenesis within the brains of FAST and SLOW rats. In this way, we hoped to identify molecular mechanisms operating differentially during neurodevelopment that might ultimately create the observed differences in brain morphology and/or seizure susceptibility. Using Superarray technology, we compared the expression level of 112 genes, known to play a role in neurodevelopment, within whole brains of embryonic day 21 (E21) FAST and SLOW rats. Results revealed that while most genes investigated showed near equivalent expression levels, both Apolipoprotein E (APOE) and the beta2 subunit of the voltage-gated sodium channel (SCN2beta) were significantly underexpressed in brains of the seizure-prone embryos. Currently, these transcripts have no known interactions during embryogenesis; however, they have both been independently linked to seizure disposition and/or neurodevelopmental aberrations leading to epilepsy. Thus, alterations in the timing and/or degree of expression for APOE and SCN2beta may be important to developmental cascades that ultimately give rise to the differing brain morphologies, behaviors, and/or seizure vulnerabilities that characterize these strains.

Characterization of the adaptor protein ARH expression in the brain and ARH molecular interactions

Previously, pA134 was identified as one of the mRNAs present in the squid giant axon. Comparative sequence analyses revealed that the pA134 gene product manifested significant similarity to the mammalian lipoprotein receptor adaptor protein also known as ARH (autosomal recessive hypercholesterolemia). ARH mRNA and protein displayed very similar pattern of expression throughout the mouse brain. Significant levels of expression were observed in cells with a predominantly neuronal profile in the cerebellum, brainstem, olfactory bulb, hippocampus, and cortex. A yeast two hybrid screen for ARH protein interactions in mouse brain identified the following binders: amyloid precursor-like protein 1, low density lipoprotein receptor-related protein (LRP) 1, LRP8, and GABA receptor-associated protein-like 1. The interactions of ARH with LRP1 and GABA receptor-associated protein-like 1 were subsequently verified by co-immunoprecipitation of the protein complexes from transfected human embryonic kidney cells. The presence of ARH mRNA in axon of primary sympathetic neurons was established by RT-PCR analyses and confirmed by in situ hybridization. Taken together, our data suggest that ARH is a multifunctional protein whose spectrum of function in the brain goes beyond the traditionally known metabolism of lipoproteins, and that ARH may be locally synthesized in the axon.

Receptor-mediated and bulk-phase endocytosis cause macrophage and cholesterol accumulation in Niemann-Pick C disease

These studies explored the roles of receptor-mediated and bulk-phase endocytosis as well as macrophage infiltration in the accumulation of cholesterol in the mouse with Niemann-Pick type C (NPC) disease. Uptake of LDL-cholesterol varied from 514 microg/day in the liver to zero in the central nervous system. In animals lacking LDL receptors, liver uptake remained about the same (411 microg/day), but more cholesterol was taken up in extrahepatic organs. This uptake was unaffected by the reductive methylation of LDL and consistent with bulk-phase endocytosis. All tissues accumulated cholesterol in mice lacking NPC1 function, but this accumulation was decreased in adrenal, unchanged in liver, and increased in organs like spleen and lung when LDL receptor function was also deleted. Over 56 days, the spleen and lung accumulated amounts of cholesterol greater than predicted, and these organs were heavily infiltrated with macrophages. This accumulation of both cholesterol and macrophages was increased by deleting LDL receptor function. These observations indicate that both receptor-mediated and bulk-phase endocytosis of lipoproteins, as well as macrophage infiltration, contribute to the cholesterol accumulation seen in NPC disease. These macrophages may also play a role in parenchymal cell death in this syndrome.

Sorting by the cytoplasmic domain of the amyloid precursor protein binding receptor SorLA

SorLA/LR11 (250 kDa) is the largest and most composite member of the Vps10p-domain receptors, a family of type 1 proteins preferentially expressed in neuronal tissue. SorLA binds several ligands, including neurotensin, platelet-derived growth factor-bb, and lipoprotein lipase, and via complex-formation with the amyloid precursor protein it downregulates generation of Alzheimer's disease-associated Abeta-peptide. The receptor is mainly located in vesicles, suggesting a function in protein sorting and transport. Here we examined SorLA's trafficking using full-length and chimeric receptors and find that its cytoplasmic tail mediates efficient Golgi body-endosome transport, as well as AP-2 complex-dependent endocytosis. Functional sorting sites were mapped to an acidic cluster-dileucine-like motif and to a GGA binding site in the C terminus. Experiments in permanently or transiently AP-1 mu1-chain-deficient cells established that the AP-1 adaptor complex is essential to SorLA's transport between Golgi membranes and endosomes. Our results further implicate the GGA proteins in SorLA trafficking and provide evidence that SNX1 and Vps35, as parts of the retromer complex or possibly in a separate context, are engaged in retraction of the receptor from endosomes.

Deletion of apolipoprotein E receptor-2 in mice lowers brain selenium and causes severe neurological dysfunction and death when a low-selenium diet is fed

Selenoprotein P (Sepp1) is a plasma and extracellular protein that is rich in selenium. Deletion of Sepp1 results in sharp decreases of selenium levels in the brain and testis with dysfunction of those organs. Deletion of Sepp1 also causes increased urinary selenium excretion, leading to moderate depletion of whole-body selenium. The lipoprotein receptor apolipoprotein E receptor-2 (apoER2) binds Sepp1 and facilitates its uptake by Sertoli cells, thus providing selenium for spermatogenesis. Experiments were performed to assess the effect of apoER2 on the concentration and function of selenium in the brain and on whole-body selenium. ApoER2-/- and apoER2+/+ male mice were fed a semipurified diet with selenite added as the source of selenium. ApoER2-/- mice had depressed brain and testis selenium, but normal levels in liver, kidney, muscle, and the whole body. Feeding a selenium-deficient diet to apoER2-/- mice led to neurological dysfunction and death, with some of the characteristics exhibited by Sepp1-/- mice fed the same diet. Thus, although it does not affect whole-body selenium, apoER2 is necessary for maintenance of brain selenium and for prevention of neurological dysfunction and death under conditions of selenium deficiency, suggesting an interaction of apoER2 with Sepp1 in the brain.

Slow endocytosis of the LDL receptor-related protein 1B: implications for a novel cytoplasmic tail conformation

The LDL receptor-related protein 1B (LRP1B) is a putative tumor suppressor homologous to LRP1. Both LRP1 and LRP1B contain cytoplasmic tails with several potential endocytosis motifs. Although the positions of these endocytic motifs are similar in both receptors, LRP1B is internalized at a 15-fold slower rate than LRP1. To determine whether the slow endocytosis of LRP1B is due to the utilization of an endocytosis motif other than the YATL motif used by LRP1, we tested minireceptors with mutations in each of the five potential motifs in the LRP1B tail. Only mutation of both NPXY motifs together abolished LRP1B endocytosis, suggesting that LRP1B can use either of these motifs for internalization. LRP1B contains a unique insertion of 33 amino acids not present in LRP1 that could lead to altered recognition of trafficking motifs. Surprisingly, deletion of this insertion had no effect on the endocytosis rate of LRP1B. However, replacing either half of the LRP1B tail with the corresponding LRP1 sequence markedly accelerated LRP1B endocytosis. From these data, we propose that both halves of the LRP1B cytoplasmic tail contribute to a unique global conformation, which results in less efficient recognition by endocytic adaptors and a slow endocytosis rate.

A cytoplasmic PPPSP motif determines megalin's phosphorylation and regulates receptor's recycling and surface expression

Megalin is a large endocytic receptor expressed at the apical surface of several absorptive epithelia. It binds multiple ligands including apolipoproteins, vitamin and hormone carrier proteins and signaling molecules such as parathyroid hormone and the morphogen sonic hedgehog. An important characteristic of megalin is its high endocytic activity, which is mediated by tyrosine-based endocytic motifs within the receptor's cytoplasmic tail. This domain also harbors several putative consensus phosphorylation motifs for protein kinase (PK) C and casein kinase-II and one consensus motif for PKA and glycogen synthase kinase-3 (GSK3). Here we report that the cytoplasmic domain of megalin is constitutively phosphorylated depending on the integrity of a PPPSP motif, a putative GSK3 site, with a minor participation of the other phosphorylation motifs. Mutation of the serine residue within the PPPSP motif as well as blocking GSK3 activity, with two different inhibitors, significantly decreased the phosphorylation levels of the receptor. Both the megalin PPPAP mutant and the underphosphorylated wild-type receptor, by inhibition of GSK3 activity, were more expressed at the cell surface and more efficiently recycled, but they were not inhibited in their initial endocytosis rates. Altogether, these results show that the PPPSP motif and the GSK3 activity are critical to allow megalin phosphorylation and also negatively regulate the receptor's recycling.

Structure of a receptor-binding fragment of reelin and mutational analysis reveal a recognition mechanism similar to endocytic receptors

Reelin, a large secreted protein implicated in the cortical development of the mammalian brain, is composed of eight tandem concatenations of "reelin repeats" and binds to neuronal receptors belonging to the low-density lipoprotein receptor gene family. We found that both receptor-binding and subsequent Dab1 phosphorylation occur solely in the segment spanning the fifth and sixth reelin repeats (R5-6). Monomeric fragment exhibited a suboptimal level of signaling activity and artificial oligomerization resulted in a 10-fold increase in activity, indicating the critical importance of higher-order multimerization in physiological reelin. A 2.0-A crystal structure from the R5-6 fragment revealed not only a unique domain arrangement wherein two repeats were aligned side by side with the same orientation, but also the unexpected presence of bound Zn ions. Structure-guided alanine mutagenesis of R5-6 revealed that two Lys residues (Lys-2360 and Lys-2467) constitute a central binding site for the low-density lipoprotein receptor class A module in the receptor, indicating a strong similarity to the ligand recognition mode shared among the endocytic lipoprotein receptors.

Elevated plasma cholesterol does not affect brain Abeta in mice lacking the low-density lipoprotein receptor

Epidemiological studies support an association between vascular risk factors, including hypercholesterolemia, and Alzheimer's disease (AD). Recently, there has been much interest in the possibility that hypercholesterolemia might directly promote beta-amyloid (Abeta) production. Indeed, in vitro studies have shown that increasing cellular cholesterol levels enhances Abeta production. However, studies in AD transgenic mouse models have not consistently found that elevated plasma cholesterol leads to increased Abeta production or deposition in vivo. In this study, we determined whether elevated peripheral cholesterol influences Abeta production in mice with a null mutation of the low-density lipoprotein receptor (LDLR). We show that dramatically elevated plasma cholesterol levels, whether induced by high cholesterol, high fat, or high fat/high cholesterol diets, did not affect either levels of brain Abeta40, Abeta42, or APP, or the Abeta42/40 or APP-CTF/APP ratios, nor substantially alter brain cholesterol levels. ApoE protein levels in brain were, however, elevated, in LDLR-/- mice by post-transcriptional mechanisms. Collectively, these studies argue that plasma cholesterol levels do not normally regulate production of brain Abeta.

Apolipoprotein E-containing lipoproteins protect neurons from apoptosis via a signaling pathway involving low-density lipoprotein receptor-related protein-1

Apolipoprotein E (apoE)-containing lipoproteins (LPs) are secreted by glia and play important roles in lipid homeostasis in the CNS. Glia-derived LPs also promote synaptogenesis and stimulate axon growth of CNS neurons. Here, we provide evidence that glia-derived LPs protect CNS neurons from apoptosis by a receptor-mediated signaling pathway. The protective effect was greater for apolipoprotein E3 than for apolipoprotein E4, the expression of which is a risk factor for Alzheimer's disease. The anti-apoptotic effect of LPs required the association of apolipoprotein E with lipids but did not require cholesterol. Apoptosis was not prevented by lipids alone or by apoA1- or apoJ-containing lipoproteins. The prevention of neuronal apoptosis was initiated after the binding of LPs to the low-density lipoprotein receptor-related protein (LRP), a multifunctional receptor of the low-density lipoprotein receptor family. We showed that inhibition of LRP activation, by treatment of neurons with receptor-associated protein or anti-LRP antibodies, or by LRP gene-silencing experiments, reduced the protective effect of LPs. Furthermore, another LRP ligand, alpha2-macroglobulin, also protected the neurons from apoptosis. After binding to LRP, LPs initiate a signaling pathway that involves activation of protein kinase Cdelta and inactivation of glycogen synthase kinase-3beta. These findings indicate the potential for using glial lipoproteins or an activator of the LRP signaling pathway for treatment for neurodegenerative disorders such as Alzheimer's disease.

ApoE receptor 2 controls neuronal survival in the adult brain

A central pathogenic feature of neurodegenerative diseases and neurotrauma is the death of neurons. A mechanistic understanding of the factors and conditions that induce the dysfunction and death of neurons is essential for devising effective treatment strategies against neuronal loss after trauma or during aging. Because Apolipoprotein E (ApoE) is a major risk factor for several neurodegenerative diseases, including Alzheimer's disease , a direct or indirect role of ApoE receptors in the disease process is likely. Here we have used gene targeting in mice to investigate possible roles of ApoE receptors in the regulation of neuronal survival. We demonstrate that a differentially spliced isoform of an ApoE receptor, ApoE receptor 2 (Apoer2), is essential for protection against neuronal cell loss during normal aging. Furthermore, the same splice form selectively promotes neuronal cell death after injury through mechanisms that may involve serine/threonine kinases of the Jun N-terminal kinase (JNK) family. These findings raise the possibility that ApoE and its receptors cooperatively regulate common mechanisms that are essential to neuronal survival in the adult brain.

The development of an aquatic bivalve model: evaluating the toxic effects on gametogenesis following 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) exposure in the eastern oyster (Crassostrea virginica)

The objective of this study is to develop a gametogenesis protocol to serve as a model for evaluating the toxic effects of chemicals on oogenesis and spermatogenesis in the eastern oyster (Crassostrea virginica). The compound 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) was selected as a "proof of principle" toxicant to examine developmental toxicity in this invertebrate system. The studies were designed to: (1) test the model using 2,3,7,8-TCDD and (2) to use histopathological evaluations to characterize the effects on oocyte and sperm development during stages of gametogenesis. 2,3,7,8-TCDD at 10 pg/g resulted in significant histopathological gonadal lesions by day 14 of gametogenesis in both female and male oysters. These lesions resulted in complete inhibition of gonadogenesis. Studies also showed that a total body dose of 2 and 10 pg/g 2,3,7,8-TCDD caused adverse responses resulting in abnormal gametogenesis in female and male oysters, respectively, such as: (1) incomplete oocyte division, (2) inhibition of oocyte growth and maturation, (3) unsynchronized sperm development, and (4) inhibition of spermatogenesis. The eastern oyster is one of the most responsive invertebrate models tested to date for reproductive effects of chemicals. Therefore, the eastern oyster can be used as a sensitive toxicological model for examining the effects of dioxin-like compounds and other xenobiotics on gametogenesis. The reported studies show that environmentally relevant concentrations of 2,3,7,8-TCDD (2-10 pg/g) have a significant adverse effect on oyster gametogenesis.

Estradiol replacement increases the low-density lipoprotein receptor related protein (LRP) in the mouse brain

Numerous epidemiology studies have shown protective effects of hormone therapy (HT) on chronic neurological diseases. We have proposed that some of the neuroprotective effects of estrogen are mediated by apolipoprotein E (apoE). Polymorphisms of receptors for apoE modify the risk for dementia. To our knowledge, no reports exist showing CNS effects of estrogen replacement on members of the low-density lipoprotein receptor family. The current study focused on the effect of estradiol-17beta (E2) replacement on protein expression of two members of the receptor family, the low-density lipoprotein receptor (LDL-r) and low-density lipoprotein receptor related protein (LRP) in ovariectomized mice. Five days of E2 replacement significantly increased LRP expression in the hippocampus, olfactory bulb and neocortex but not in cerebellum. In contrast, E2 treatment decreased LDL-r protein expression in olfactory bulb. HT modification of both apoE and LRP could have wide-spread effects on cellular function given LRP's manifold signaling functions.

Conditional animal models for the study of lipid metabolism and lipid disorders

The advent of technologies that allow conditional mutagenesis has revolutionized our ability to explore gene functions and to establish animal models of human diseases. Both aspects have proven to be of particular importance in the study of lipid-related disorders. Classical approaches to gene inactivation by conventional gene targeting strategies have been successfully applied to generate animal models like the LDL receptor- and the apolipoprotein E-knockout mice, which are still widely used to study diverse aspects of atherosclerosis, lipid transport, and neurodegenerative disease. In many cases, however, simply inactivating the gene of interest has resulted in early lethal or complex phenotypes which are difficult to interpret. In recent years, additional tools have therefore been developed that allow the spatiotemporally controlled manipulation of the genome, as described in detail in Part I of this volume. Our aim is to provide an exemplary survey of the application of different conditional mutagenesis techniques in lipid research in order to illustrate their potential to unravel physiological functions of a broad range of genes involved in lipid homeostasis.

Apolipoprotein E3- and Nitric Oxide–Dependent Modulation of Endothelial Cell Inflammatory Responses

Objective— Although apolipoprotein E3 (apoE3) is known to be atheroprotective, its mechanisms of protection in endothelial cells remain unclear.

Methods and Results— Cultured human aortic endothelial cells were stimulated with tumor necrosis factor (TNF)-α in the presence of human recombinant apoE3 solubilized in dimyristoyl phosphatidylcholine liposomes. Using flow cytometry and real-time polymerase chain reaction, a significant increase of inflammatory cell adhesion proteins (vascular cell adhesion molecule-1 and E-Selectin), and MCP-1, interleukin-8, and intercellular adhesion molecule-1 gene expression was observed within 5 hours of TNF-α exposure, which was markedly attenuated in cells coincubated with apoE3. Treatment with apoE4 resulted in increased inflammatory gene expression relative to either TNF treatment alone or TNF + apoE3 treatment. NO synthase inhibition experiments demonstrated NO to be an active participant in the actions of both TNF and apoE. To clarify the role of NO, dose-response experiments were performed with 0.03 to 300 μmol/L DEA-NONOate. Using flow cytometry and real-time polymerase chain reaction, a modulatory role of NO in TNF-induced endothelial cell activation was observed.

Conclusions— These data suggest a role of vascular wall apoE3 to balance the intracellular redox state in injured endothelial cells via NO-dependent pathways.

Hitch-hiking between cells on lipoprotein particles

Cell surface proteins containing covalently linked lipids associate with specialized membrane domains. Morphogens like Hedgehog and Wnt use their lipid anchors to bind to lipoprotein particles and employ lipoproteins to travel through tissues. Removal of their lipid anchors or decreasing lipoprotein levels give rise to adverse Hedgehog and Wnt signaling. Some parasites can also transfer their glycosylphosphatidylinositol-anchored surface proteins to host lipoprotein particles. These antigen-loaded lipoproteins spread throughout the circulation, and probably hamper an adequate immune response by killing neutrophils. Together, these findings imply a widespread role for lipoproteins in intercellular transfer of lipid-anchored surface proteins, and may have various physiological consequences. Here, we discuss how lipid-modified proteins may be transferred to and from lipoproteins at the cellular level.

Reelin, lipoprotein receptors and synaptic plasticity

Apolipoprotein E (APOE) is a cholesterol transport protein and an isoform-specific major risk factor for neurodegenerative diseases. The lipoprotein receptors that bind APOE have recently been recognized as pivotal components of the neuronal signalling machinery. The interaction between APOE receptors and one of their ligands, reelin, allows them to function directly as signal transduction receptors at the plasma membrane to control not only neuronal positioning during brain development, but also synaptic plasticity in the adult brain. Here, we review the molecular mechanisms through which APOE, cholesterol, reelin and APOE receptors control synaptic functions that are essential for cognition, learning, memory, behaviour and neuronal survival.

Statins induce differentiation and cell death in neurons and astroglia

Statins are potent inhibitors of the hydroxy-methyl-glutaryl-coenzyme A reductase, the rate limiting enzyme for cholesterol biosynthesis. Experimental and clinical studies with statins suggest that they have beneficial effects on neurodegenerative disorders. Thus, it was of interest to characterize the direct effects of statins on CNS neurons and glial cells. We have treated defined cultures of neurons and astrocytes of newborn rats with two lipophilic statins, atorvastatin and simvastatin, and analyzed their effects on morphology and survival. Treatment of astrocytes with statins induced a time- and dose-dependent stellation, followed by apoptosis. Similarly, statins elicited programmed cell death of cerebellar granule neurons but with a higher sensitivity. Analysis of different signaling cascades revealed that statins fail to influence classical pathways such as Akt or MAP kinases, known to be activated in CNS cells. In addition, astrocyte stellation triggered by statins resembled dibutryl-cyclic AMP (db-cAMP) induced morphological differentiation. However, in contrast to db-cAMP, statins induced upregulation of low-density lipoprotein receptors, without affecting GFAP expression, indicating separate underlying mechanisms. Analysis of the cholesterol biosynthetic pathway revealed that lack of mevalonate and of its downstream metabolites, mainly geranylgeranyl-pyrophosphate (GGPP), is responsible for the statin-induced apoptosis of neurons and astrocytes. Moreover, astrocytic stellation triggered by statins was inhibited by mevalonate and GGPP. Interestingly, neuronal cell death was significantly reduced in astrocyte/neuron co-cultures treated with statins. We postulate that under these conditions signals provided by astrocytes, e.g., isoprenoids play a key role in neuronal survival.

Selective expression ofLDLR andVLDLR in myelinating oligodendrocytes

Oligodendrocytes form myelin sheaths around axons in the central nervous system. Although cholesterol is one of the major lipid components in myelin sheath, the source of cholesterol for myelination remains to be defined. In this study, we report that low-density lipoprotein receptor (LDLR) and very low-density lipoprotein receptor (VLDLR) are selectively expressed in mature myelinating oligodendrocytes in the postnatal CNS. Both receptors are specifically expressed in differentiated oligodendrocytes in P7 spinal cord, but progressively down-regulated after P15. In adult animals, only LDLR expression can be detected in a small number of oligodendrocytes throughout the entire spinal cord. In the brain region, LDLR is expressed by the white matter oligodendrocytes of both cerebellum and cerebral cortex, whereas VLDLR has a weak expression in cerebellar oligodendrocytes. Together, our expression studies suggest that cholesterol uptake by LDLR and VLDLR may play an important role in the formation of myelin sheath.

Secreted PCSK9 decreases the number of LDL receptors in hepatocytes and in livers of parabiotic mice

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a member of the proteinase K subfamily of subtilases that reduces the number of LDL receptors (LDLRs) in liver through an undefined posttranscriptional mechanism. We show that purified PCSK9 added to the medium of HepG2 cells reduces the number of cell-surface LDLRs in a dose- and time-dependent manner. This activity was approximately 10-fold greater for a gain-of-function mutant, PCSK9(D374Y), that causes hypercholesterolemia. Binding and uptake of PCSK9 were largely dependent on the presence of LDLRs. Coimmunoprecipitation and ligand blotting studies indicated that PCSK9 and LDLR directly associate; both proteins colocalized to late endocytic compartments. Purified PCSK9 had no effect on cell-surface LDLRs in hepatocytes lacking autosomal recessive hypercholesterolemia (ARH), an adaptor protein required for endocytosis of the receptor. Transgenic mice overexpressing human PCSK9 in liver secreted large amounts of the protein into plasma, which increased plasma LDL cholesterol concentrations to levels similar to those of LDLR-knockout mice. To determine whether PCSK9 was active in plasma, transgenic PCSK9 mice were parabiosed with wild-type littermates. After parabiosis, secreted PCSK9 was transferred to the circulation of wild-type mice and reduced the number of hepatic LDLRs to nearly undetectable levels. We conclude that secreted PCSK9 associates with the LDLR and reduces hepatic LDLR protein levels.

Functional role of the low-density lipoprotein receptor-related protein in Alzheimer's disease

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder, characterized by neuronal loss, neurofibrillary tangle formation and the extracellular deposition of amyloid-beta (Abeta) plaques. The amyloid precursor protein (APP) and the enzymes responsible for Abeta generation seem to be the base elements triggering the destructive processes. Initially, the low-density lipoprotein receptor-related protein (LRP) was genetically linked to AD and later it emerged to impact on many fundamental events related to this disease. LRP is not only involved in Abeta clearance but is also the major receptor of several AD-associated ligands, e.g. apolipoprotein E and alpha2-macroglobulin. APP processing is mediated by LRP on many levels. Enhanced APP internalization through LRP decreases cell surface APP levels and thereby reduces APP shedding. As a consequence of increased APP internalization LRP enhances Abeta secretion. These effects could be attributed to the cytoplasmic tails of LRP and APP. The receptors bind via their NPXY motifs to the two PID domains of FE65 and form a tripartite complex. However, it appears that the second NPVY motif of LRP is the one responsible for the observed influence over APP metabolism. A more in-depth knowledge of the mechanisms regulating APP cleavage may offer additional targets for therapeutic intervention.

Multiplicity of the interactions of Wnt proteins and their receptors

Wnts are secreted proteins that are essential for a wide array of developmental and physiological processes. They signal across the plasma membranes by interacting with serpentine receptors of the Frizzled (Fz) family and members of the low-density-lipoprotein receptor-related protein (LRP) family. Recent advances in the Wnt signaling field have revealed that Wnt-unrelated proteins activate or suppress Wnt signaling by binding to Fzs or LRP5/6 and that atypical receptor tyrosine kinases mediate Wnt signaling independently of Fz and/or function as a Fz co-receptor. This review highlights recent progress in our understanding of the multiplicity of Wnts and their receptors. We discuss how the interaction between the ligands and receptors activate distinct intracellular signaling pathways. We also discuss how intracellular trafficking of Wnt signaling components can regulate the sensitivity of cells to Wnts.

Identification of Apolipoprotein A-I as a “STOP” Signal for Myopia

Good visual acuity requires that the axial length of the ocular globe is matched to the refractive power of the cornea and lens to focus the images of distant objects onto the retina. During the growth of the juvenile eye, this is achieved through the emmetropization process that adjusts the ocular axial length to compensate for the refractive changes that occur in the anterior segment. A failure of the emmetropization process can result in either excessive or insufficient axial growth, leading to myopia or hyperopia, respectively. Emmetropization is mainly regulated by the retina, which generates two opposite signals: "GO/GROW" signals to increase axial growth and "STOP" signals to block it. The presence of GO/GROW and STOP signals was investigated by a proteomics analysis of the retinas from chicken with experimental myopia and hyperopia. Of 18 differentially expressed proteins that were identified, five displayed an expression profile corresponding to GO/GROW signals, and two corresponded to STOP signals. Western blotting confirmed that apolipoprotein A-I (apoA-I) has the characteristics of a STOP signal both in the retina as well as in the fibrous sclera. In accordance with this, intraocular application of the peroxisome proliferator-activated receptor alpha agonist GW7647 resulted in up-regulation of apoA-I levels and in a significant reduction of experimental myopia. In conclusion, using a comprehensive functional proteomics analysis of chicken ocular growth models we identified targets for ocular growth control. The correlation of elevated apoA-I levels with reduced ocular axial growth points toward a functional relationship with the observed morphological changes of the eye.

Lipid-induced extension of apolipoprotein E helix 4 correlates with low density lipoprotein receptor binding ability

Apolipoprotein E (apoE) serves as a ligand for the low density lipoprotein receptor (LDLR) only when bound to lipid. The N-terminal domain of lipid-free apoE exists as globular 4-helix bundle that is conferred with LDLR recognition ability after undergoing a lipid binding-induced conformational change. To investigate the structural basis for this phenomenon, site-directed spin label electron paramagnetic resonance spectroscopy experiments were conducted, focusing on the region near the C-terminal end of helix 4 (Ala-164). Using C112S apoE-N-terminal as template, a series of single cysteine substitution variants (at sequence positions 161, 165, 169, 173, 176, and 181) were produced, isolated, and labeled with the nitroxide probe, methane thiosulfonate. Electron paramagnetic resonance analysis revealed that lipid association induced fixed secondary structure in a region of the molecule known to exist as random coil in the lipid-free state. In a complementary approach, site-directed fluorescence analysis using an environmentally sensitive probe indicated that the lipid-induced transition of this region of the protein to alpha helix was accompanied by relocation to a more hydrophobic environment. In studies with full-length apoE single Cys variants, a similar random coil to stable backbone transition was observed, consistent with the concept that lipid interaction induced an extension of helix 4 beyond the boundary defining its lipid-free conformation. This structural transition likely represents a key conformational change necessary for manifestation of the LDLR recognition properties of apoE.

The generation and function of soluble apoE receptors in the CNS

More than a decade has passed since apolipoprotein E4 (APOE-ε4) was identified as a primary risk factor for Alzheimer 's disease (AD), yet researchers are even now struggling to understand how the apolipoprotein system integrates into the puzzle of AD etiology. The specific pathological actions of apoE4, methods of modulating apolipoprotein E4-associated risk, and possible roles of apoE in normal synaptic function are still being debated. These critical questions will never be fully answered without a complete understanding of the life cycle of the apolipoprotein receptors that mediate the uptake, signaling, and degradation of apoE. The present review will focus on apoE receptors as modulators of apoE actions and, in particular, explore the functions of soluble apoE receptors, a field almost entirely overlooked until now.

LR11/SorLA expression is reduced in sporadic Alzheimer disease but not in familial Alzheimer disease

LR11 is an ApoE receptor that is enriched in the brain. We have shown that LR11 is markedly downregulated in patients with sporadic Alzheimer disease (AD). This finding led us to explore whether reduced LR11 expression reflects a primary mechanism of disease or merely a secondary consequence of other AD-associated changes. Therefore, LR11 expression was assessed in a transgenic mouse model of AD and familial AD (FAD) brains. Immunohistochemistry and immunoblotting of LR11 in PS1/APP transgenic and wild-type mice indicated that LR11 levels are not affected by genotype or accumulation of amyloid pathology. LR11 expression was also evaluated based on immunoblotting and LR11 immunostaining intensity in human frontal cortex in controls, sporadic AD, and FAD, including cases with presenilin-1 (PS1) and presenilin-2 (PS2) mutations. Although LR11 was reduced in sporadic AD, there was no difference in protein level or staining intensity between control and FAD cases. The finding that LR11 expression is unaffected in both a mouse model of AD and autosomal-dominant forms of AD suggests that LR11 is not regulated by amyloid accumulation or other AD neuropathologic changes. We hypothesize that LR11 loss may be specific to sporadic AD and influence amyloid pathology through mechanisms independent of substrate-enzyme interactions regulated by FAD mutations.