Apolipoprotein E isoforms in Alzheimer's disease pathology and etiology

The apolipoprotein E (apoE) epsilon4 allele increases risk of Alzheimer's disease (AD), perhaps by accelerating plaque formation, or by impairing neuron repair. Considerable evidence supports both mechanisms. AD patients with epsilon4 have more and earlier amyloid deposits than do patients without epsilon4. The same is true of non-demented control subjects. In vitro, all apoE isoforms inhibit amyloid beta protein (Abeta) aggregation, but apoE4 less effectively than apoE3. Transgenic amyloid-producing mice expressing apoE3 or apoE4 develop less Abeta deposition than apoE knockout mice. These observations are consistent with an effect of apoE isoforms on Abeta aggregation in AD. ApoE is important for neurite maintenance since apoE knockout mice lose neurites and suffer behavioral deficits with aging or treatment with excitotoxins. ApoE4 mice show similar defects, but apoE3 mice are normal. AD patients with epsilon4 show more neuritic deficits than epsilon3 carriers. ApoE epsilon4 also worsens neurological impairment in head injury, stroke, and multiple sclerosis. Thus, apoE4 is less effective at neurite maintenance. Perhaps epsilon4 increases AD risk by both mechanisms: allowing amyloid deposition and failing to repair neurites. In either case, introducing apoE3 or apoE2 into the brain, for example by gene therapy or cell grafts, might delay AD progression.     

Astrocyte lipoproteins, effects of apoE on neuronal function, and role of apoE in amyloid-beta deposition in vivo

The genetic association between the E4 isoform of apolipoprotein E (apoE) and increased risk for Alzheimer's disease (AD) has prompted interest in the neurobiology of apoE and the possible relationship between lipoprotein metabolism in the brain and neurodegenerative disease. ApoE, a product of astrocytes, is abundant in brain and in cerebrospinal fluid (CSF) where it is found in lipoproteins the size of large plasma high-density lipoproteins (HDL). Cultured astrocytes also secrete apoE/HDL, although the lipid and apoprotein composition of these nascent particles differs from that found in CSF, suggesting possible functional differences. In vitro studies have demonstrated isoform-specific effects of apoE on neurite outgrowth, neuronal plasticity, neurotoxicity, lipid peroxidation, oxidative injury, binding to cytoskeletal proteins, and interactions with amyloid-beta (Abeta), a primary component of senile plaques in AD. A number of these proposed functions have also been assessed in apoE -/- mice and transgenic mice expressing human apoE3 or apoE4. Importantly, analysis of transgenic mice overexpressing a mutant form of the human amyloid precursor protein (APP(V717F)) in the presence of mouse apoE, no apoE, or human apoE3 or E4 has demonstrated a critical and isoform-specific role for apoE in neuritic plaque formation, a pathologic hallmark of AD. Together, these data have provided important clues as to possible mechanism(s) by which apoE genotype modifies AD risk.     

Cerebrospinal fluid lipoproteins in Alzheimer's disease

Interest in cerebrospinal fluid (CSF) lipoproteins has been stimulated by the association of certain alleles of the human apolipoprotein E gene (APOE) with an increased risk of Alzheimer's disease (AD), and because apolipoprotein E (apoE) is one of the major apolipoproteins in CSF. CSF lipoproteins (d < 1.210 g/ml fraction) are distinct from their plasma counterparts, and in AD patients CSF may contain novel particles. The protein concentration of CSF lipoproteins is reduced in AD patients. Moreover, the molecular distribution of apoE- and apoAII-containing apolipoproteins in CSF is dictated by APOE. The lipid composition suggests that CSF lipoproteins from AD patients may have undergone increased free radical-mediated damage; experimental data support the possibility that this may occur both before and after lipoprotein assembly. Finally, human CSF lipoproteins oxidized ex vivo are neurotoxic to neuronal cells in culture and disrupt microtubule structure, an activity not observed with oxidized bovine CSF lipoproteins. CSF lipoproteins may represent a means whereby apoE influences the outcome of free radical-mediated damage to brain.     

Very low density lipoprotein receptor in Alzheimer disease

The apolipoprotein (APO) E4 isoform is associated with an accelerated rate of Alzheimer disease (AD) expression in sporadic as well as late-onset familial forms of the disease but the precise mechanism is unknown. In an attempt to approach the possible mechanisms involved, APOE receptors have been studied. They all belong to the low density lipoprotein (LDL) receptor family and share the same structural motifs. Some of them are preferentially expressed in the brain such as the LDL receptor related protein, the apolipoprotein E receptor 2, and the very low density lipoprotein (VLDL) receptor. These receptors have been suspected to be involved in Alzheimer disease at various levels. Among them, the VLDL receptor was extensively explored. Although genetic studies conducted on a polymorphism in the promoter of the VLDL receptor in Japanese and Caucasian populations gave divergent results, this does not exclude a possible involvement of the VLDL receptor in AD.     

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.     

Apolipoprotein E immunoreactivity in neurons and neurofibrillary degeneration of aged non-demented and Alzheimer's disease patients

Apolipoprotein E (ApoE) genotype is a risk factor for Alzheimer's disease (AD) but its relationship with neurofibrillary degeneration remains obscure. To further analyze this relationship, hippocampal, entorhinal, temporopolar, and insular cortices of 10 non-demented and 7 Alzheimer disease brains were studied with both light and electron microscopy. Focus was directed on pretangles and neurons starting to accumulate tangles. ApoE immunolabeling in neurons and tangles was independent of ApoE individual genotype. The majority of the neurons in all of the brains were ApoE-negative, but virtually every brain also contained groups of ApoE-immunoreactive neurons, with diffuse cytoplasmic labeling. Most of the ApoE-positive tangles were extracellular, but a few tangles were shown to be intraneuronal when studied ultrastructurally. No ApoE immunoreactivity was found in neuropil threads, as well as in neurites associated with senile plaques. Double protocols with both AT-8 and anti-ApoE antibodies, performed to determine whether ApoE-positive neurons were pretangle neurons, did not detect cytoplasmic AT-8 in ApoE-positive neurons. Though careful electron microscopy studies found ApoE reaction product in an occasional ApoE-positive pretangle-like neuron and a few intracellular tangles, these findings do not support that ApoE is necessary for the accumulation of hyperphosphorylated tau protein. The more consistent colocalization of anti-ApoE and AT-8 in extracellular tangles reveals that ApoE mainly binds to tangles once they are in the extracellular space, in a manner similar to that described for amyloid fibrils.     

Microglial interaction with beta-amyloid: implications for the pathogenesis of Alzheimer's disease.

The etiology of Alzheimer's disease (AD) involves a significant inflammatory component as evidenced by the presence of elevated levels of a diverse range of proinflammatory molecules in the AD brain. These inflammatory molecules are produced principally by activated microglia, which are found to be clustered within and adjacent to the senile plaque. Moreover, long-term treatment of patients with non-steroidal anti-inflammatory drugs has been shown to reduce risk and incidence of AD and delay disease progression. The microglia respond to beta-amyloid (Abeta) deposition in the brain through the interaction of fibrillar forms of amyloid with cell surface receptors, leading to the activation of intracellular signal transduction cascades. The activation of multiple independent signaling pathways ultimately leads to the induction of proinflammatory gene expression and production of reactive oxygen and nitrogen species. These microglial inflammatory products act in concert to produce neuronal toxicity and death. Therapeutic approaches focused on inhibition of the microglial-mediated local inflammatory response in the AD brain offer new opportunities to intervene in the disease.     

Apolipoprotein J (clusterin) and Alzheimer's disease

Apolipoprotein J (clusterin) is a ubiquitous multifunctional glycoprotein capable of interacting with a broad spectrum of molecules. In pathological conditions, it is an amyloid associated protein, co-localizing with fibrillar deposits in systemic and localized amyloid disorders. In Alzheimer's disease, the most frequent form of amyloidosis in humans and the major cause of dementia in the elderly, apoJ is present in amyloid plaques and cerebrovascular deposits but is rarely seen in NFT-containing neurons. ApoJ expression is up-regulated in a wide variety of insults and may represent a defense response against local damage to neurons. Four different mechanisms of action could be postulated to explain the role of apoJ as a neuroprotectant during cellular stress: (1) function as an anti-apoptotic signal, (2) protection against oxidative stress, (3) inhibition of the membrane attack complex of complement proteins locally activated as a result of inflammation, and (4) binding to hydrophobic regions of partially unfolded, stressed proteins, and therefore avoiding aggregation in a chaperone-like manner. This review focuses on the association of apoJ in biological fluids with Alzheimer's soluble Abeta. This interaction prevents Abeta aggregation and fibrillization and modulates its blood-brain barrier transport at the cerebrovascular endothelium.     

Roles for lipoprotein lipase in Alzheimer's disease: an association study

Lipoprotein lipase (LPL) assists lipid transport by transferring lipids between lipoprotein particles and cells. LPL binds apolipoprotein E (apoE) lipoprotein particles and a major apoE receptor, low density lipoprotein receptor related protein (LRP). Because apoE and LRP polymorphisms alter Alzheimer's disease (AD) risk, and LPL itself is found in AD amyloid plaques, we examined whether LPL variants also affect AD risk. In case-control studies in the United States and Canada, the frequencies of two LPL alleles known to affect LPL enzymatic activity were measured in Caucasian AD or elderly normal (N) subjects. Pathologically confirmed subjects in both studies exhibited similar trends toward fewer 447Ter and more 291Ser alleles in AD. Combining results from both countries gave allele frequencies for 447Ter of 13.7% (26/190) in N and 9.4% (80/852) in AD (P = 0.10), and for 291Ser of 0.0% (0/184) in N and 1. 3% (8/636) in AD (P = 0.21). The trend appeared even greater for homozygous 447Ter subjects: 4.2% (4/95) of N vs. 1.4% (6/426) of AD (P = 0.09). These trends are consistent with a putative protective effect of 447Ter and causative effect of 291Ser on AD. Furthermore, brains of AD patients with 447Ter showed trends toward fewer plaques, tangles, and glia, and more neurons and cortical thickness than AD patients without 447Ter. Hippocampal plaques were significantly reduced. LPL might affect hippocampal function and thus dementia via its role as supplier of membrane components or antioxidants to neurons. Alternatively, LPL may play a part in plaque formation through its interaction with apoE and LRP.     

Phagocytic properties of microglia in vitro: implications for a role in multiple sclerosis and EAE.

The microglial cell, after many years of neglect, has become recognized as the sole representative cell of the immune system that resides in the normal central nervous system. While normally dormant, microglia can be activated by secretory substances or signals associated with disease or injury, and becomes a phagocytic cell, which also produces its own injurious molecules. In the activating process, its morphology is changed from a resting process-bearing cell, into a rounded amoebic form, and displays new or increased amounts of functional markers, such as receptors and Class I and Class II MHC molecules. Microglia prepared from newborn mice or rats for tissue culture are already activated, and can be used for studies of their phagocytic properties. Although they can phagocytize foreign substances, their uptake and metabolism of myelin are emphasized here, in keeping with their role in demyelinating diseases. A number of receptors have been implicated and appear to be important in the attachment to, and ingestion of, myelin particles in vitro, including the Fc, complement, macrophage scavenger, and the Galectin-3/MAC-2 receptors, although the alpha2-macroglobulin/low-density lipoprotein receptor and mannose receptors have also been suggested as participants in myelin phagocytosis. Certain cytokines and adhesion molecules also regulate the phagocytic activity of microglia. Comparative in vitro studies of phagocytosis by peritoneal macrophages and microglia have shown that the two kinds of cells respond differently to regulatory molecules, and it is concluded that they have different innate properties. The role of microglia in the demyelinative diseases experimental autoimmune encephalomyelitis and multiple sclerosis is emphasized here, and the possible means of intervention in the process leading to myelin destruction is discussed. Published 2001 Wiley-Liss, Inc.     

Amyloid-beta aggregates formed at polar-nonpolar interfaces differ from amyloid-beta protofibrils produced in aqueous buffers

The deposition of aggregated amyloid-beta (Abeta) peptides in the brain as senile plaques is a pathological hallmark of Alzheimer's disease (AD). Several lines of evidence indicate that fibrillar and, in particular, soluble aggregates of these 40- and 42-residue peptides are important in the etiology of AD. Recent studies also stress that amyloid aggregates are polymorphic and that a single polypeptide can fold into multiple amyloid conformations. Here we review our recent reports that Abeta(1-40) in vitro can form soluble aggregates with predominant beta-structures that differ in stability and morphology. One class of aggregates involved soluble Abeta protofibrils, prepared by vigorous overnight agitation of monomeric Abeta(1-40) in low ionic strength buffers. These aggregates were quite stable and disaggregated to only a limited extent on dilution. A second class of soluble Abeta aggregates was generated at polar-nonpolar interfaces. Aggregation in a two-phase system of buffer over chloroform occurred more rapidly than in buffer alone. In buffered 2% hexafluoroisopropanol (HFIP), microdroplets of HFIP were formed and the half-time for aggregation was less than 10 minutes. Like Abeta protofibrils, these interfacial aggregates showed increased thioflavin T fluorescence and were rich in beta-structure by circular dichroism. However, electron microscopy and atomic force microscopy revealed very different morphologies. The HFIP aggregates formed initial globular clusters that progressed over several days to soluble fibrous aggregates. When diluted out of HFIP these aggregates initially were very unstable and disaggregated completely within 2 minutes. However, their stability increased as they progressed to fibers. It is important to determine whether similar interfacial Abeta aggregates are produced in vivo.     

Accumulation of triosephosphate isomerase, with sequence homology to Beta amyloid peptides, in vessel walls of the newborn piglet hippocampus

We investigated whether beta-amyloid (Abeta)-like immunoreactivity was seen in the brains of newborn piglets. The immunoreactivity for Abeta(1-42) and Abeta(1-40) proteins, but not Abeta precursor protein, was present in CD68-positive perivascular cells of the hippocampus and in parts of the meninges. It was colocalized with immunoreactivity for receptor for advanced glycation end product and tumor necrosis factor-alpha. The protein with a molecular mass of 27 kDa, which was recognized by the Abeta antibodies, was identified as triosephosphate isomerase (TPI) with sequence homology to Abeta peptides by N-terminal amino acid sequencing, mass fingerprint analysis using matrix-associated laser desorption/ionization mass spectrometry, and Western blotting. Western blotting assay also revealed that detectable expression of Abeta proteins were not seen in the piglet brains. These findings indicate that TPI with sequence homology to Abeta peptides accumulates in perivascular cells of the microglia/macrophage lineage located around arterial vessels of the newborn piglet hippocampus.     

Choroid plexus: target for polypeptides and site of their synthesis

Choroid plexus (CP) is an important target organ for polypeptides. The fenestrated phenotype of choroidal endothelium facilitates the penetration of blood-borne polypeptides across the capillary walls. Thus, both circulating and cerebrospinal fluid (CSF)-borne polypeptides can reach their receptors on choroidal epithelium. Several polypeptides have been demonstrated to regulate CSF formation by controlling blood flow to choroid plexus and/or the activity of ion transport in choroidal epithelium. However, many ligand-receptor interactions occurring in the CP are not involved in the regulation of fluid secretion. Increasing evidence suggests that the choroidal epithelium plays an important role in hormonal signaling via a receptor-mediated transport into the brain (e.g., leptin) and helps to clear certain CSF-borne polypeptides (e.g., soluble amyloid beta-protein). Thus, impaired choroidal transport or insufficient clearance of polypeptides may contribute to pathogenesis of systemic or central nervous system (CNS) disorders, such as obesity or Alzheimer's disease. CP epithelium is not only a target but is also a source of neuropeptides, growth factors, and cytokines in the CNS. These polypeptides following their release into the CSF may exert distal, endocrine-like effects on target cells in the brain due to bulk flow of this fluid. Distinct temporal patterns of choroidal expression of several polypeptides are observed during brain development and in various CNS disorders, including traumatic brain injury and ischemia. Therefore, it is proposed that the CP plays an integral role not only in normal brain functioning, but also in the recovery from the injury. This review attempts to critically analyze the available data to support the above hypothesis.     

Calcium signalling in cells of oligodendroglial lineage

Intracellular Ca2+ is the key signal that regulates the efficacy of neurotransmitter release and synaptic plasticity in neurons but is also an important second messenger involved in the signal transduction and modulation of gene expression in both excitable and non-excitable cells. Glial cells, including cells of oligodendroglial (OLG) lineage, are capable of responding to extracellular stimuli via changes in the intracellular Ca2+. This review article focuses on the mechanisms of Ca2+ signalling in cells of OLG lineage with the goal of providing the basis for understanding the relevance of receptor- and non-receptor-mediated signalling to oligodendroglial development, myelination, and demyelination. Conclusions to date indicate that cells of OLG lineage exhibit remarkable plasticity with regard to the expression of ion channels and receptors linked to Ca2+ signalling and that perturbation of [Ca2](i) homeostasis contributes to the pathogenesis of demyelinating diseases.     

Innervation of cerebral blood vessels: morphology, plasticity, age-related, and Alzheimer's disease-related neurodegeneration

The light microscopical and ultrastructural morphology of the innervation of the major cerebral arteries and pial vessels is described, including the origins of the different groups of nerve fibres and their characteristic neurotransmitter phenotype. Species and region specific variations are described and novel data regarding the parasympathetic innervation of cerebral vessels are presented. The dynamic nature, or plasticity, of cerebrovascular innervation is emphasized in describing changes affecting particular subpopulations of neurons during normal ageing and in Alzheimer's disease. The molecular controls on plasticity are discussed with particular reference to target-associated factors such as the neurotrophins and their neuronal receptors, as well as extracellular matrix related factors such as laminin. Hypotheses are presented regarding the principal extrinsic and intrinsic influences on plasticity of the cerebrovascular innervation.     

Link between heart disease, cholesterol, and Alzheimer's disease: a review

Increased prevalence of Alzheimer's disease-like beta-amyloid deposits in the neuropil and within neurons occurs in the brains of non-demented individuals with heart disease. Heart disease is a prevalent finding in Alzheimer's disease, and may be a forerunner to the dementing disorder. In the cholesterol-fed rabbit model of human coronary heart disease there is production and accumulation of beta-amyloid in the brain. This accumulation of beta-amyloid can be reversed by removing cholesterol from the rabbits' diet. In culture cells, a cholesterol challenge has been shown to increase production of beta-amyloid, and dramatic reductions of cholesterol produced by HMG Co-A reductase inhibitors decrease production of beta-amyloid. Increased beta-amyloid production is also produced by dietary cholesterol in a number of transgenic mouse models of Alzheimer's disease. Administration of HMG Co-A reductase inhibitors may block beta-amyloid production caused by dietary cholesterol in rabbits. Clinical trials testing the benefit of HMG Co-A reductase inhibitors in the treatment of Alzheimer's disease are underway.     

Integrins as receptors for laminins

Laminins are a family of trimeric glycoproteins present in the extracellular matrix and the major constituents of basement membranes. Integrins are alpha beta transmembrane receptors that play critical roles in both cell-matrix and cell-cell adhesion. Several members of the integrin family, including alpha 1 beta 1, alpha 2 beta 1, alpha 3 beta 1, alpha 6 beta 1, alpha 7 beta 1 and alpha 6 beta 4 heterodimers serve as laminin receptors on a variety of cell types. This review summarizes recent advances in understanding the involvement of individual integrins in cell interactions with laminins and the roles of laminin-binding integrins in adhesion-mediated events in vertebrates, including embryonic development, cell migration and tumor cell invasiveness, cell proliferation and differentiation, as well as basement membrane assembly. We discuss the regulation of integrin function via alternative splicing of cytoplasmic domains of alpha and beta subunits of the integrin receptors for laminins and present examples of functional collaboration between laminin-binding integrins and non-integrin laminin receptors. Advances in our understanding of the laminin-binding integrins continue to demonstrate the essential roles these receptors play in maintaining cell polarity and tissue architecture.     

Distribution of GABA and glycine receptors on bipolar and ganglion cells in the mammalian retina

The amino acids GABA and glycine mediate synaptic transmission via specific neurotransmitter receptors. Molecular cloning studies have shown that there is a great diversity of GABA and glycine receptors. In the present article, the distribution of GABA and glycine receptors on identified bipolar and ganglion cell types in the mammalian retina is reviewed. Immunofluorescence obtained with antibodies against GABA and glycine receptors is punctate. Electron microscopy shows that the puncta represent a cluster of receptors at synaptic sites. Bipolar cell types were identified with immunohistochemical markers. Double immunofluorescence with subunit-specific antibodies was used to analyze the distribution of receptor clusters on bipolar axon terminals. The OFF cone bipolar cells seem to be dominated by glycinergic input, whereas the ON cone bipolar and rod bipolar cells are dominated by GABAergic input. Ganglion cells were intracellularly injected with Neurobiotin, visualized with Streptavidin coupled to FITC, and subsequently stained with subunit specific antibodies. The distribution and density of receptor clusters containing the alpha1 subunit of the GABA(A) receptor and the alpha1 subunit of the glycine receptor, respectively, were analyzed on midget and parasol cells in the marmoset (a New World monkey). Both GABA(A) and glycine receptors are distributed uniformly along the dendrites of ON and OFF types of parasol and midget ganglion cells, indicating that functional differences between these subtypes of ganglion cells are not determined by GABA or glycinergic input.