Inhibition of nicotinic acetylcholine receptors by apolipoprotein E-derived peptides in rat hippocampal slices

Are apolipoprotein E fragments a promising new therapeutic target for Alzheimer’s disease?

Human apolipoprotein E (ApoE) is a 299-amino acid secreted glycoprotein that binds cholesterol and phospholipids. ApoE exists as three common isoforms (ApoE2, ApoE3, and ApoE4) and heterozygous carriers of the ε4 allele of the gene encoding ApoE (APOE) have a fourfold greater risk of developing Alzheimer’s disease (AD). The enzymes thrombin, cathepsin D, α-chymotrypsin-like serine protease, and high-temperature requirement serine protease A1 are responsible for ApoE proteolytic processing resulting in bioactive C-terminal-truncated fragments that vary depending on ApoE isoforms, brain region, aging, and neural injury. The objectives of the present narrative review were to describe ApoE processing, discussing current hypotheses about the potential role of various ApoE fragments in AD pathophysiology, and reviewing the current development status of different anti-ApoE drugs. The exact mechanism by which APOE gene variants increase/decrease AD risk and the role of ApoE fragments in the deposition are not fully understood, but APOE is known to directly affect tau-mediated neurodegeneration. ApoE fragments co-localize with neurofibrillary tangles and amyloid β (Aβ) plaques, and may cause neurodegeneration. Among anti-ApoE approaches, a fascinating strategy may be to therapeutically overexpress ApoE2 in APOE ε4/ε4 carriers through vector administration or liposomal delivery systems. Another approach involves reducing ApoE4 expression by intracerebroventricular antisense oligonucleotides that significantly decreased Aβ pathology in transgenic mice. Differences in the proteolytic processing of distinct ApoE isoforms and the use of ApoE fragments as mimetic peptides in AD treatment are also under investigation. Treatment with peptides that mimic the structural and biological properties of native ApoE may reduce Aβ deposition, tau hyperphosphorylation, and glial activation in mouse models of Aβ pathology. Alternative strategies involve the use of ApoE4 structure correctors, passive immunization to target a certain form of ApoE, conversion of the ApoE4 aminoacid sequence into that of ApoE3 or ApoE2, and inhibition of the ApoE-Aβ interaction.

Fragmentation of brain apolipoprotein E (ApoE) and its relevance in Alzheimer's disease

Alzheimer's disease (AD) is a very common cause of dementia in the elderly. It is characterized by progressive amnesia and accretions of neurofibrillary tangles (NFTs) of neurons and senile plaques in the neuropil. After aging, the inheritance of the apolipoprotein E (ApoE) epsilon 4 (ε4) allele is the greatest risk factor for late-onset AD. The ApoE protein is the translated product of the ApoE gene. This protein undergoes proteolysis, and the resulting fragments colocalize with neurofibrillary tangles and amyloid plaques, and for that matter may be involved in AD onset and/or progression. Previous studies have reported the pathogenic potential of various ApoE fragments in AD pathophysiology. However, the pathways activated by the fragments are not fully understood. In this review, ApoE fragments obtained from post-mortem brains and body fluids, cerebrospinal fluid (CSF) and plasma, are discussed. Additionally, current knowledge about the process of fragmentation is summarized. Finally, the mechanisms by which these fragments are involved in AD pathogenesis and pathophysiology are discussed.

Understanding the Role of ApoE Fragments in Alzheimer's Disease

Alzheimer's disease (AD) is one of the most devastating neurodegenerative diseases. It has been known for decades that the APOE ɛ4 allele is the most significant genetic risk factor for late-onset AD and yet its precise role in the disease remains unclear. The APOE gene encodes apolipoprotein E (apoE), a 35 kDa glycoprotein highly expressed in the brain. There are three different isoforms: apoE3 is the most common allele in the population, whilst apoE2 decreases, and apoE4 increases AD risk. ApoE has numerous functions that affect neuronal and non-neuronal cells, thus how it contributes to disease onset and progression is hotly debated. The apoE4 isoform has been linked to the accumulation of both of the major pathological hallmarks of AD, amyloid plaques containing amyloid β peptides, and neurofibrillary tangles containing hyperphosphorylated tau protein, as well as other hallmarks of the disease, including inflammation and oxidative stress. Numerous studies have shown that apoE undergoes fragmentation in the human brain, and that the fragmentation pattern varies between isoforms. It was previously shown that apoE4 has neurotoxic functions, however recent data has also identified a neuroprotective role for the apoE N-terminal 25 kDa fragment, which is more prevalent in apoE3 individuals. The ability of the apoE 25 kDa fragment to promote neurite outgrowth was recently demonstrated and this suggests there is a potential loss of neuroprotection in apoE4 individuals in addition to the previously described gain of toxic function for specific apoE4 fragments. Here we review the enzymes proposed to be responsible for apoE fragmentation, the specific functions of different apoE fragments and their possible links with AD.

Increased Aβ42-α7-like nicotinic acetylcholine receptor complex level in lymphocytes is associated with apolipoprotein E4-driven Alzheimer's disease pathogenesis

BACKGROUND

The apolipoprotein E ε4 (APOE4) genotype is a prominent late-onset Alzheimer's disease (AD) risk factor. ApoE4 disrupts memory function in rodents and may contribute to both plaque and tangle formation.

METHODS

Coimmunoprecipitation and Western blot detection were used to determine: 1) the effects of select fragments from the apoE low-density lipoprotein (LDL) binding domain and recombinant apoE subtypes on amyloid beta (Aβ)₄₂-α7 nicotinic acetylcholine receptor (α7nAChR) interaction and tau phosphorylation in rodent brain synaptosomes; and 2) the level of Aβ₄₂-α7nAChR complexes in matched controls and patients with mild cognitive impairment (MCI) and dementia due to AD with known APOE genotypes.

RESULTS

In an ex vivo study using rodent synaptosomes, apoE_141-148 of the apoE promotes Aβ₄₂-α7nAChR association and Aβ₄₂-induced α7nAChR-dependent tau phosphorylation. In a single-blind study, we examined lymphocytes isolated from control subjects, patients with MCI and dementia due to AD with known APOE genotypes, sampled at two time points (1 year apart). APOE ε4 genotype was closely correlated with heightened Aβ₄₂-α7nAChR complex levels and with blunted exogenous Aβ₄₂ effects in lymphocytes derived from AD and MCI due to AD cases. Similarly, plasma from APOE ε4 carriers enhanced the Aβ₄₂-induced Aβ₄₂-α7nAChR association in rat cortical synaptosomes. The progression of cognitive decline in APOE ε4 carriers correlated with higher levels of Aβ₄₂-α7nAChR complexes in lymphocytes and greater enhancement by their plasma of Aβ₄₂-induced Aβ₄₂-α7nAChR association in rat cortical synaptosomes.

CONCLUSIONS

Our data suggest that increased lymphocyte Aβ₄₂-α7nAChR-like complexes may indicate the presence of AD pathology especially in APOE ε4 carriers. We show that apoE, especially apoE4, promotes Aβ₄₂-α7nAChR interaction and Aβ₄₂-induced α7nAChR-dependent tau phosphorylation via its apoE_141-148 domain_. These apoE-mediated effects may contribute to the APOE ε4-driven neurodysfunction and AD pathologies.

Co-activation of nAChR and mGluR induces γ oscillation in rat medial septum diagonal band of Broca slices

AIM

To examine whether co-activation of nAChR and mGluR1 induced γ oscillation (20-60 Hz) in rat medial septum diagonal band of Broca (MSDB) slices.

METHODS

Rat brain sagittal slices containing the MSDB were prepared. Extracellular field potentials were recorded with glass microelectrodes. The nAChR and mGluR1 agonists were applied to the slices to induce network activity. Data analysis was performed off-line using software Spike 2.

RESULTS

Co-application of the nAChR agonist nicotine (1 μmol/L) and the mGluR1 agonist dihydroxyphenylglycine (DHPG, 25 μmol/L) was able to induce γ oscillation in MSDB slices. The intensity of nAChR and mGluR1 activation was critical for induction of network oscillation at a low (θ oscillation) or high frequency (γ oscillation): co-application of low concentrations of the two agonists only increased the power and frequency of oscillation within the range of θ, whereas γ oscillation mostly appeared when high concentrations of the two agonists were applied.

CONCLUSION

Activation of mGluR1 and nAChR is able to program slow or fast network oscillation by altering the intensity of receptor activation, which may provide a mechanism for modulation of learning and memory.

Apolipoprotein E mimetics and cholesterol-lowering properties

Apolipoprotein E (apoE) is a ligand for clearance of lipoprotein remnants such as chylomicrons and very low-density lipoproteins. It has anti-atherogenic and anti-inflammatory properties. Therefore, there is extensive ongoing research to create peptides that can mimic properties of apoE. A number of synthetic peptides that encompass different regions of apoE have been studied for inhibiting inflammatory states, including Alzheimer disease. However, peptides that clear atherogenic lipoproteins, analogous to apoE, via enhanced hepatic uptake have not been previously reviewed. Toward this end, we describe the design and studies of a dual-domain apoE mimetic peptide, Ac-hE18A-NH(2). This peptide consists of residues 141-150, the putative receptor-binding region of human apoE, covalently linked to a well characterized class A amphipathic helix, 18A, which has no sequence homology to any other exchangeable apolipoprotein sequences. It demonstrates dramatic effects in reducing plasma cholesterol levels in dyslipidemic mouse and rabbit models. We discuss the scientific rationale and review the literature for the design and efficacy of the peptide. Analogous to apoE, this peptide bypasses the low-density lipoprotein receptor for the hepatic uptake of atherogenic lipoproteins via heparan sulfate proteoglycan (HSPG). ApoE mimetics such as Ac-hE18A-NH(2) may therefore restore or replace ligands in genetically induced hyperlipidemias to enable reduction in atherogenic lipoproteins via HSPG even in the absence of functional low-density lipoprotein receptors. Therefore, this and similar peptides may be useful in the treatment of dyslipidemic disorders such as familial hyperlipidemia and atherosclerosis.

Neuronal nicotinic acetylcholine receptor-cholesterol crosstalk in Alzheimer's disease

Alzheimer's disease (AD) is one of the most devastating diseases of the central nervous system (CNS). It is characterized by two neuropathological findings: amyloid plaques and neurofibrillary tangles. AD is also accompanied by an extensive functional deficit in the cholinergic system, involving the neuronal-type nicotinic acetylcholine receptor (AChR). Furthermore there is increasing evidence showing a misregulation of cholesterol metabolism in the development of the disease. Since cholesterol affects AChR protein at multiple levels, the cognitive impairment and other neurological correlates of AD might be partly associated with an abnormal crosstalk between the receptor protein and the sterol in this synaptopathy.

Hippocampal infusions of MARCKS peptides impair memory of rats on the radial-arm maze

In vitro hippocampal studies by Gay et al. (2008) demonstrated that a myristoylated alanine-rich C kinase substrate (MARCKS) peptide comprising the phosphorylation site or effector domain of the protein acts as a powerful inhibitor of alpha7 nicotinic acetylcholine receptors (nAChRs), which are known to be critically involved in memory function. However, behavioral consequences of hippocampal MARCKS peptide infusions have not been investigated. The purpose of the current study was to determine if local infusions in the rat ventral hippocampus of long (comprising amino acids 151-175) and short (amino acids 159-165) forms of MARCKS peptides could affect memory performance in the 16-arm radial maze. Our results demonstrated a dramatic impairment of both working (changing) and reference (constant) memory with MARCKS(151-175) only. The shorter MARCKS peptide did not affect memory performance. This is in line with in vitro results reported by Gay et al. (2008) that long, but not short, MARCKS peptides inhibit alpha7 nAChRs. We also found that the effect of the MARCKS(151-175) peptide was dose-dependent, with a robust memory impairment at 10 microg/side, and smaller inconsistent effects at lower doses. Our present behavioral study, together with the earlier in vitro study by Gay et al. (2008), suggests that effector domain MARCKS peptides could play a significant role in memory regulation and impairment.

Alpha7 neuronal nicotinic receptors: the missing link to understanding Alzheimer's etiopathology?

Evidence supports the premise that alpha7 nicotinic acetylcholine receptors in the central nervous system, sometimes referred to as neuronal nicotinic receptors (NNRs), play a central role in the development of Alzheimer's disease (AD) pathophysiology. Moreover, these receptors may represent the key to unifying aspects of the cholinergic hypothesis of AD with many of the apparently disparate mechanisms such as beta-amyloid deposition, tau hyperphosphorylation, and ApoE4 abnormalities variously proposed to underlie the progression of the disease. We hypothesize that neuronal degeneration in incipient AD is the result of coincident events involving, at their core, deficits in alpha7 NNR function. The resulting hypocholinergic tone could potentially have serious consequences since alpha7 NNRs are known to modulate fundamental pathways involved in cell survival such as JAK2-STAT3. This hypothesis predicts that any factors that compromise alpha7 function have the potential to negatively impact neuronal viability. For example, such factors could include deficits in the primary neurotransmitter acetylcholine (ACh), underactivity of normal cognitive processes that stimulate alpha7 NNRs (i.e., use-dependency), or the reported binding of beta-amyloid and ApoE4 to alpha7 NNRs, all of which could effectively decouple the receptors from key pro-survival pathways. Since these pathways are known to negatively modulate GSK-3beta, which regulates tau phosphorylation, downstream effects such as tau hyperphosphorylation would be expected to arise. Conversely, the maintenance of normal alpha7 NNR activity by adequate levels of ACh or other NNR agonists would be expected to support normal cholinergic tone, prevent the binding of beta-amyloid and ApoE4 and preserve the integrity of the neurons. We therefore propose that decreased cholinergic tone is at the apex of AD pathophysiology, with factors such as beta-amyloid and ApoE4 playing a contributory role rather that a causal one and hyperphosphorylation of tau representing a detector of concomitant hypocholinergic tone and beta-amyloid deposition. Thus the convergence of beta-amyloid deposition and/or ApoE4 binding and co-localization with alpha7 NNRs, which are favored under conditions of low cholinergic tone, and the downstream cascade of tau hyperphosphorylation through disinhibition of GSK-3beta appear to explain and reconcile many of the discordant findings in this very active area of CNS research.

Nicotinic acetylcholine receptor signalling: roles in Alzheimer's disease and amyloid neuroprotection

Alzheimer's disease (AD), the major contributor to dementia in the elderly, involves accumulation in the brain of extracellular plaques containing the beta-amyloid protein (Abeta) and intracellular neurofibrillary tangles of hyperphosphorylated tau protein. AD is also characterized by a loss of neurons, particularly those expressing nicotinic acetylcholine receptors (nAChRs), thereby leading to a reduction in nAChR numbers. The Abeta(1-42) protein, which is toxic to neurons, is critical to the onset and progression of AD. The discovery of new drug therapies for AD is likely to be accelerated by an improved understanding of the mechanisms whereby Abeta causes neuronal death. We examine the evidence for a role in Abeta(1-42) toxicity of nAChRs; paradoxically, nAChRs can also protect neurons when activated by nicotinic ligands. Abeta peptides and nicotine differentially activate several intracellular signaling pathways, including the phosphatidylinositol 3-kinase/v-akt murine thymoma viral oncogene homolog pathway, the extracellular signal-regulated kinase/mitogen-activated protein kinase, and JAK-2/STAT-3 pathways. These pathways control cell death or survival and the secretion of Abeta peptides. We propose that understanding the differential activation of these pathways by nicotine and/or Abeta(1-42) may offer the prospect of new routes to therapy for AD.

Hippocampal infusions of apolipoprotein E peptides induce long-lasting cognitive impairment

The inheritance of the varepsilon4 allele of apolipoprotein E (ApoE4) and cholinergic system dysfunction have long been associated with the pathology of Alzheimer's disease (AD). Recently, in vitro studies have established a direct link between ApoE and cholinergic function in that synthetic peptides containing segments of the ApoE protein (ApoE(133-149) and ApoE(141-148)) interact with alpha7 nicotinic acetylcholine receptors (nAChRs) in the hippocampus. This raises the possibility that ApoE peptides may contribute to cognitive impairment in AD in that the hippocampus plays a key role in cognitive functioning. To test this, we acutely infused ApoE peptides into the ventral hippocampus of female Sprague-Dawley rats and assessed the resultant effects on radial-arm maze choice accuracy over a period of weeks after the infusion. Local ventral hippocampal infusion of ApoE peptides caused significant cognitive impairment in radial-arm maze learning that persisted several weeks after the acute infusion. This persisting deficit may be an important model for understanding the relationship between ApoE protein-induced neurotoxicity and cognitive impairment as well as serve as a platform for the development of new therapies to avoid neurotoxicity and cognitive decline.

Inhibition of native and recombinant nicotinic acetylcholine receptors by the myristoylated alanine-rich C kinase substrate peptide

A variety of peptide ligands are known to inhibit the function of neuronal nicotinic acetylcholine receptors (nAChRs), including small toxins and brain-derived peptides such as beta-amyloid(1-42) and synthetic apolipoproteinE peptides. The myristoylated alanine-rich C kinase substrate (MARCKS) protein is a major substrate of protein kinase C and is highly expressed in the developing and adult brain. The ability of a 25-amino acid synthetic MARCKS peptide, derived from the effector domain (ED), to modulate nAChR activity was tested. To determine the effects of the MARCKS ED peptide on nAChR function, receptors were expressed in Xenopus laevis oocytes, and two-electrode voltage-clamp experiments were performed. The MARCKS ED peptide completely inhibited acetylcholine (ACh)-evoked responses from alpha7 nAChRs in a dose-dependent manner, yielding an IC(50) value of 16 nM. Inhibition of ACh-induced responses was both activity- and voltage-independent. The MARCKS ED peptide was unable to block alpha-bungarotoxin binding. A MARCKS ED peptide in which four serine residues were replaced with aspartate residues was unable to inhibit alpha7 nAChR-mediated currents. The MARCKS ED peptide inhibited ACh-induced alpha4beta2 and alpha2beta2 responses, although with decreased potency. The effects of the MARCKS ED peptide on native nAChRs were tested using acutely isolated rat hippocampal slices. In hippocampal interneurons, the MARCKS ED peptide was able to block native alpha7 nAChRs in a dose-dependent manner. The MARCKS ED peptide represents a novel antagonist of neuronal nAChRs that has considerable utility as a research tool.

N-methyl-D-aspartate receptor inhibition by an apolipoprotein E-derived peptide relies on low-density lipoprotein receptor-associated protein

The effects of a synthetic apoE peptide, viz., residues 133-149 (apoE[133-149]), a mimetic that comprises the apoE receptor binding domain, on N-methyl-D-aspartate (NMDA)/glycine-induced ion flow through NMDA receptor (NMDAR) channels, have been investigated. The activity of apoE[133-149] was found to depend on the low-density lipoprotein receptor-related protein (LRP). Competition experiments with receptor-associated protein (RAP) and activated alpha(2)-macroglobulin (alpha(2)M*), two proteins that compete for apoE binding to LRP, demonstrate that apoE[133-149] inhibition of NMDAR function is mediated at a locus in LRP that overlaps with the binding sites of RAP and alpha(2)M*. A coreceptor of LRP, cell surface heparin sulfate proteoglycan, did not function in this system. Additional electrophysiology experiments demonstrated that the inhibitory potency of apoE[133-149] was threefold greater for NMDAR-transfected wild-type Chinese hamster ovary (CHO) cells compared with NMDAR-transfected CHO cells deficient in LRP. Studies with truncation and replacement variants of the apoE peptide demonstrated that the NMDAR inhibitory properties of these peptides correlate with their binding affinities for LRP. These novel results indicate that apoE functions as an inhibitor of NMDAR ion channels indirectly via LRP, and are suggestive of a participatory role for LRP in NMDAR-based neuropathies.

Aromatic residues at position 55 of rat alpha7 nicotinic acetylcholine receptors are critical for maintaining rapid desensitization

The rat alpha7 nicotinic acetylcholine receptor (nAChR) can undergo rapid onset of desensitization; however, the mechanisms of desensitization are largely unknown. The contribution of a tryptophan (W) residue at position 55 of the rat alpha7 nAChR subunit, which lies within the beta2 strand, was studied by mutating it to other hydrophobic and/or aromatic amino acids, followed by voltage-clamp experiments in Xenopus oocytes. When mutated to alanine, the alpha7-W55A nAChR desensitized more slowly, and recovered from desensitization more rapidly, than wildtype alpha7 nAChRs. The contribution of desensitization was validated by kinetic modelling. Mutating W55 to other aromatic residues (phenylalanine or tyrosine) had no significant effect on the kinetics of desensitization, whereas mutation to various hydrophobic residues (alanine, cysteine or valine) significantly decreased the rate of onset and increased the rate of recovery from desensitization. To gain insight into possible structural rearrangements during desensitization, we probed the accessibility of W55 by mutating W55 to cysteine (alpha7-W55C) and testing the ability of various sulfhydryl reagents to react with this cysteine. Several positively charged sulfhydryl reagents blocked ACh-induced responses for alpha7-W55C nAChRs, whereas a neutral sulfhydryl reagent potentiated responses; residue C55 was not accessible for modification in the desensitized state. These data suggest that W55 plays an important role in both the onset and recovery from desensitization in the rat alpha7 nAChR, and that aromatic residues at position 55 are critical for maintaining rapid desensitization. Furthermore, these data suggest that W55 may be a potential target for modulatory agents operating via hydrophobic interactions.

Noncholinesterase effects induced by organophosphate pesticides and their relationship to cognitive processes: implication for the action of acylpeptide hydrolase

Organophosphate pesticides have been classically described as inhibitors of acetylcholinesterase (AChE) activity in insects and invertebrates. However, there is now more evidence supporting the hypothesis that these compounds also act through noncholinergic pathways, especially those related to cognitive processes. The enzyme acylpeptide hydrolase was identified as a new target for organophosphate pesticides. This enzyme is more sensitive than AChE to some organophosphates (OP), including dichlorvos, which is the parent compound for metrifonate, a therapeutic agent used in the treatment of cognitive impairment associated to Alzheimer's disease. Therefore, there is some doubt as to whether the mechanism of action of this drug is mediated by a potentiation of cholinergic transmission. However, the direct action of acylpeptide hydrolase in cognitive processes and the physiological and molecular mechanisms underlying subacute exposure to OP have yet to be demonstrated. This review deals with evidence demonstrating the existence of mechanisms of actions of OP, which are independent of cholinergic pathway potentiation and which have an effect on cognitive processes. In addition, the possible participation of the enzyme acylpeptide hydrolase in these processes is also discussed. Finally, the possibility of using this enzyme activity as a new biomarker for exposure to OP is considered.

Structural determinates for apolipoprotein E-derived peptide interaction with the alpha7 nicotinic acetylcholine receptor

Neuronal nicotinic acetylcholine receptor (nAChR) signaling has been implicated in a variety of normal central nervous system (CNS) functions as well as an array of neuropathologies. Previous studies have demonstrated both neurotoxic and neuroprotective actions of peptides derived from apolipoprotein E (apoE). It has been discovered that apoE-derived peptides inhibit native and recombinant alpha7-containing nAChRs, indicating a direct interaction between apoE peptides and nAChRs. To probe the structure/function interaction between alpha7 nAChRs and the apoE peptide apoE(141-148), experiments were conducted in Xenopus laevis oocytes expressing wild-type and mutated nAChRs. Mutation of Trp55 to alanine blocks apoE peptide-induced inhibition of acetylcholine (ACh)-mediated alpha7 nAChR responses. Additional mutations at Trp55 suggest that hydrophobic interactions between the receptor and apoE(141-148) are essential for inhibition of alpha7 nAChR function. A mutated apoE peptide also demonstrated decreased inhibition at alpha7-W55A nAChRs as well as activity-dependent inhibition of both wild-type alpha7 nAChRs and alpha7-W55A receptors. Finally, a three-dimensional model of the alpha7 nAChR was developed based on the recently refined Torpedo marmorata nACh receptor. A structural model is proposed for the binding of apoE(141-148) to the alpha7 nAChR where the peptide binds at the interface between two subunits, near the ACh binding site. Similar to the functional data, the computational docking suggests the importance of hydrophobic interactions between the alpha7 nAChR and the apoE peptide for inhibition of receptor function. The current data suggest a mode for apoE peptide binding that directly blocks alpha7 nAChR activity and consequently may disrupt nAChR signaling.

Effects of APOE and CHRNA4 genotypes on retinal nerve fibre layer thickness at the optic disc and on risk for developing exfoliation syndrome

PURPOSE

To investigate whether CHRNA4 and APOE genotypes influence retinal nerve fibre layer (RNFL) thickness at the optic disc, intraocular pressure (IOP) and the development of exfoliation syndrome (XFS).

METHODS

A sample of 88 healthy adults (aged 50-75 years) genotyped for polymorphisms of APOE and CHRNA4 underwent an eye examination including slit-lamp examination and fundus photography, as well as measurements of visual acuity, refraction, IOP and RNFL thickness at the optic disc by optical coherence tomography. The Fisher-Boschloo unconditional full multinomial test and two sample t-tests were used in the statistical analyses.

RESULTS

There was no correlation between CHRNA4 and APOE genotypes and average RNFL thickness at the optic disc in the two eyes. Mean IOP in the eyes of APOE2 carriers was 13.18 mmHg, whereas that in the eyes of non-APOE2 carriers, at 14.82 mmHg, was significantly higher (p = 0.014). Exfoliation syndrome was found in one or both eyes in 15 persons. The presence of XFS was less likely in CC carriers of the CHRNA4 gene than in TT and TC carriers (p = 0.049).

CONCLUSIONS

We found no significant difference in RNFL thickness at the optic disc in the different genotype carriers of the APOE and CHRNA4 genes, and thereby no evidence for increased loss of ganglion cells in the retina as an effect of these genes. APOE2 carriers had significantly lower IOP than non-APOE2 carriers. However, both values are within the normal range and RNFL thickness measurements at the optic disc showed no difference between these two groups. In our study population, CC carriers of the CHRNA4 gene were less likely to develop XFS.

Interactive effects of APOE and CHRNA4 on attention and white matter volume in healthy middle-aged and older adults

In the present study, we investigated age-related changes in interactions between efficiency of neuronal repair mechanisms and efficiency of cholinergic neurotransmission in the context of attentional orienting. In addition, we explored white matter volume changes as possible neuronal underpinnings. A sample of 230 healthy middle-aged (53-64 years) and older (65-75 years) adults was genotyped for polymorphisms of APOE and CHRNA4, a nicotinic receptor subunit gene. Participants were administered a visuospatial attention task involving letter discrimination with location cues of varying validity. Genotype effects on white matter volume were also investigated in a subset of participants who received MRI scans. APOE interacted with CHRNA4, such that APOE-epsilon4 carriers who were also CHRNA4 TT homozygotes showed disproportionately slowed reaction time (RT) following invalid location cues. The interaction was stronger in the middle-aged participants than in the older participants. There was also a trend for individuals with combined APOE-epsilon4/CHRNA4 TT genotypes to show both lower white matter volume and slower overall RT on the attention task The interaction of a neurotransmission gene (CHRNA4) and a susceptibility gene (APOE) suggests that the efficiency of neuronal repair mechanisms may modulate the cholinergic system to influence attentional function.

Functional somato-dendritic alpha7-containing nicotinic acetylcholine receptors in the rat basolateral amygdala complex

Multiple subtypes of nicotinic acetylcholine receptors (nAChRs) are expressed in the CNS. The amygdala complex, the limbic structure important for emotional memory formation, receives cholinergic innervation from the basal forebrain. Although cholinergic drugs have been shown to regulate passive avoidance performance via the amygdala, the neuronal subtypes and circuits involved in this regulation are unknown. In the present study, whole-cell patch-clamp electrophysiological techniques were used to identify and characterize the presence of functional somato-dendritic nAChRs within the basolateral complex of the amygdala. Pressure-application of acetylcholine (ACh; 2 mm) evoked inward current responses in a subset of neurons from both the lateral (49%) and basolateral nuclei (72%). All responses displayed rapid activation kinetics, and were blocked by the alpha7-selective antagonist methyllycaconitine. In addition, the alpha7-selective agonist choline induced inward current responses that were similar to ACh-evoked responses. Spiking patterns were consistent with pyramidal class I neurons (the major neuronal type in the basolateral complex); however, there was no correlation between firing frequency and the response to ACh. The local photolysis of caged carbachol demonstrated that the functional expression of nAChRs is located both on the soma and dendrites. This is the first report demonstrating the presence of functional nAChR-mediated current responses from rat amygdala slices, where they may be playing a significant role in fear and aversively motivated memory.

Blockade of nicotinic acetylcholine receptors suppresses hippocampal long-term potentiation in wild-type but not ApoE4 targeted replacement mice

Both impaired nicotinic neurotransmission and the inheritance of apoE4 are associated with increased risk for Alzheimer disease (AD) as well as other deficiencies in memory-related behavior. Long-term potentiation (LTP), a cellular model of memory, is known to be altered by nicotinic agents. Recent studies also support an emergent role for apoE in LTP. We compared the effects of mecamylamine, a nonspecific antagonist of nicotinic acetylcholine receptors (nAChRs), on basal synaptic transmission and LTP in hippocampal slices from wild-type (wt) mice and targeted replacement mice expressing human apoE4 (apoE4-TR). Field excitatory postsynaptic potentials (EPSPs) were recorded in the dentate gyrus (DG) in response to medial perforant path activation, and theta burst stimulation was used to induce LTP. Bath application of mecamylamine (3 microM) did not alter input-output relationships or paired pulse depression in either mouse strain. Under control conditions, apoE4-TR mice showed significantly less LTP than wt mice (17.5% +/- 3.2%, n = 9, vs. 30.1% +/- 3.9%, n = 11, P < 0.02). Mecamylamine reduced LTP in wt mice to a level that was similar to control levels for apoE4-TR mice (15.7% +/- 3.4%, n = 9), whereas apoE4-TR showed no further reduction of LTP (16.6% +/- 3.7%, n = 8) by mecamylamine. Thus mice expressing human apoE4 differ from wt mice both in their capacity for LTP and in the effect on LTP of nicotinic cholinergic blockade. It is possible that nicotinic neurotransmission is already compromised in apoE4-TR mice and, hence, that interference with the integrity of this cholinergic system represents a mechanism by which inheritance of the apoE4 allele contributes to cognitive risk.

Apolipoprotein E-derived peptides block alpha7 neuronal nicotinic acetylcholine receptors expressed in xenopus oocytes

For decades, the pathology of Alzheimer's disease has been associated with dysfunction of cholinergic signaling; however, the cellular mechanisms by which nicotinic acetylcholine receptor (nAChR) function is impaired in Alzheimer's disease are as yet unknown. The most significant genetic risk factor for the development of Alzheimer's disease is inheritance of the epsilon4 allele of apolipoprotein E (apoE). Recent data have demonstrated the ability of apoE-derived peptides to inhibit nAChRs in rat hippocampus. In the current study, the functional interaction between nAChRs and apoE-derived peptides was investigated in Xenopus oocytes expressing selected nAChRs. Both a 17-amino acid peptide fragment, apoE(133-149), and an eight-amino acid peptide, apoE(141-148), were able to maximally block acetylcholine (ACh)-mediated peak current responses for homomeric alpha7 nAChRs. ApoE peptide inhibition was dose-dependent and voltage- and activity-independent. The current findings suggest that apoE peptides are noncompetitive for acetylcholine and do not block functional alpha-bungarotoxin binding. ApoE peptides had a significantly decreased ability to inhibit ACh-mediated peak current responses for alpha4beta2 and alpha2beta2 nAChRs. Amino acid substitutions in the apoE peptide sequence suggest that the arginines are critical for peptide blockade of the alpha7 nAChR. The current data suggest that apoE fragments can disrupt nAChR signaling through a direct blockade of alpha7 nAChRs. These results may be useful in elucidating the mechanisms underlying memory loss and cognitive decline seen in Alzheimer's disease as well as aid in the development of novel therapeutics using apoE-derived peptides.

Paired-pulse potentiation of alpha7-containing nAChRs in rat hippocampal CA1 stratum radiatum interneurones

Diverse subtypes of nicotinic acetylcholine receptors (nAChRs), including fast-desensitizing alpha7-containing receptors, are expressed in the CNS. While nAChRs appear to regulate cognitive processing and synaptic plasticity, little is known to date about how this regulation occurs, particularly in brain regions known to be important for cognition. By combining patch-clamp electrophysiology with local photolysis of caged carbachol to rapidly activate the alpha7-containing nAChRs in rat hippocampal CA1 stratum radiatum interneurones in slices, we describe a novel transient up-regulation of channel function. The nAChRs were activated using a paired-pulse uncaging protocol, where the duration of the UV laser pulses (5-25 ms) and the interval between pulses (200 ms to 30 s) were varied. At relatively long interpulse intervals, we observed a strong (> 75%) decrease in the amplitude of the second response due to desensitization. However, when two pulses were applied at a 200 ms interval, a > 3-fold increase in the amplitude of the second response was observed, a phenomenon referred to here as paired-pulse potentiation. Interestingly, this potentiation appeared to be regulated by [Ca2+]i, and/or Ca2+-dependent processes, as it was significantly enhanced by dialysing cells with either the Ca2+ chelator BAPTA, or with peptide inhibitors of either calcineurin or PKC, and was attenuated by dialysing cells with the CaMKII inhibitor KN-93. No potentiation was observed using caged GABA or glutamate, indicating some specificity for nAChRs. Thus, rat hippocampal alpha7-containing nAChRs possess a newly described phenomenon of paired-pulse potentiation that may be involved in regulating synaptic plasticity in the hippocampus.