To be or not to be toxic: aggregations in Huntington and Alzheimer disease

Insoluble aggregated proteins in Alzheimer disease and Huntington disease might not be pathogenic. Human studies have poor correlations between aggregates and clinical disease or pathology in these disorders, whereas mouse models have demonstrated that neuronal loss can occur in the absence of detectable aggregates. Furthermore, aggregates can exist in the absence of disease pathology in mice or symptoms in humans. Recent research suggests that soluble protein fragments, not insoluble aggregated proteins, are the toxic species in these disorders.     

Oxysterols, cholesterol homeostasis, and Alzheimer disease

Aberrant cholesterol metabolism has been implicated in Alzheimer disease (AD) and other neurological disorders. Oxysterols and other cholesterol oxidation products are effective ligands of liver X activated receptor (LXR) nuclear receptors, major regulators of genes subserving cholesterol homeostasis. LXR receptors act as molecular sensors of cellular cholesterol concentrations and effectors of tissue cholesterol reduction. Following their interaction with oxysterols, activation of LXRs induces the expression of ATP-binding cassette, sub-family A member 1, a pivotal modulator of cholesterol efflux. The relative solubility of oxysterols facilitates lipid flux among brain compartments and egress across the blood-brain barrier. Oxysterol-mediated LXR activation induces local apoE biosynthesis (predominantly in astrocytes) further enhancing cholesterol re-distribution and removal. Activated LXRs invoke additional neuroprotective mechanisms, including induction of genes governing bile acid synthesis (sterol elimination pathway), apolipoprotein elaboration, and amyloid precursor protein processing. The latter translates into attenuated beta-amyloid production that may ameliorate amyloidogenic neurotoxicity in AD brain. Stress-induced up-regulation of the heme-degrading enzyme, heme oxygenase-1 in AD-affected astroglia may impact central lipid homeostasis by promoting the oxidation of cholesterol to a host of oxysterol intermediates. Synthetic oxysterol-mimetic drugs that activate LXR receptors within the CNS may provide novel therapeutics for management of AD and other neurological afflictions characterized by deranged tissue cholesterol homeostasis.     

Epigenetics,oxidative stress,and Alzheimer disease

Alzheimer disease (AD) is a progressive neurodegenerative disorder whose clinical manifestations appear in old age. The sporadic nature of 90% of AD cases, the differential susceptibility to and course of the illness, as well as the late age onset of the disease suggest that epigenetic and environmental components play a role in the etiology of late-onset AD. Animal exposure studies demonstrated that AD may begin early in life and may involve an interplay between the environment, epigenetics, and oxidative stress. Early life exposure of rodents and primates to the xenobiotic metal lead (Pb) enhanced the expression of genes associated with AD, repressed the expression of others, and increased the burden of oxidative DNA damage in the aged brain. Epigenetic mechanisms that control gene expression and promote the accumulation of oxidative DNA damage are mediated through alterations in the methylation or oxidation of CpG dinucleotides. We found that environmental influences occurring during brain development inhibit DNA-methyltransferases, thus hypomethylating promoters of genes associated with AD such as the β-amyloid precursor protein (APP). This early life imprint was sustained and triggered later in life to increase the levels of APP and amyloid-β (Aβ). Increased Aβ levels promoted the production of reactive oxygen species, which damage DNA and accelerate neurodegenerative events. Whereas AD-associated genes were overexpressed late in life, others were repressed, suggesting that these early life perturbations result in hypomethylation as well as hypermethylation of genes. The hypermethylated genes are rendered susceptible to Aβ-enhanced oxidative DNA damage because methylcytosines restrict repair of adjacent hydroxyguanosines. Although the conditions leading to early life hypo- or hypermethylation of specific genes are not known, these changes can have an impact on gene expression and imprint susceptibility to oxidative DNA damage in the aged brain.     

Apolipoprotein E, amyloid, and Alzheimer disease

Despite important inroads into the molecular pathology of Alzheimer disease, effective long-term treatment for the condition remains elusive. Among the many gene products that are recognized as factors in the disease is apolipoprotein ( (apoE). The risk that specific isoforms of apoE pose with regard to Alzheimer Disease clearly varies, and so the roles that apoE plays in the brain will be crucial to a full understanding of the disease and to efforts to develop effective therapies.     

Alzheimer disease

Alzheimer disease (AD) has traditionally been thought to involve the misfolding and aggregation of two different factors that contribute in parallel to pathogenesis: amyloid-β (Aβ) peptides, which represent proteolytic fragments of the transmembrane amyloid precursor protein, and tau, which normally functions as a neuronally enriched, microtubule-associated protein that predominantly accumulates in axons. Recent evidence has challenged this model, however, by revealing numerous functional interactions between Aβ and tau in the context of pathogenic mechanisms for AD. Moreover, the propagation of toxic, misfolded Aβ and tau bears a striking resemblance to the propagation of toxic, misfolded forms of the canonical prion protein, PrP, and misfolded Aβ has been shown to induce tau misfolding in vitro through direct, intermolecular interaction. In this review we discuss evidence for the prion-like properties of both Aβ and tau individually, as well as the intriguing possibility that misfolded Aβ acts as a template for tau misfolding in vivo.     

Lipid rafts, flotillin-1 and Alzheimer disease

A beta peptide accumulates in the extracellular space during Alzheimer's disease. It is the cleavage product of APP (Amyloid Precursor Protein), a large transmembrane protein. After ultracentrifugation, APP is found in a low-density fraction, enriched in cholesterol. These properties are characteristic of lipid rafts, which are microdomains that "float" like rafts on the plasma membrane. We have confirmed the presence of cholesterol in the core of the senile plaque, using the fluorescent probe filipin. In addition, we have shown that flotillin-1, a marker of rafts, accumulated in lysosomes of neurons in Alzheimer's disease. In most cases (76% of the flotillin-1 positive neurons), the accumulation was associated with the presence of neurofibrillary tangles. Our data suggest that the A beta peptide, which is poorly soluble in water, is actually linked with cholesterol, possibly from cellular membranes, in the extracellular space.     

Prions,prionoids and pathogenic proteins in Alzheimer disease

Like patients with prion disease, Alzheimer patients suffer from a fatal, progressive form of dementia. There is growing evidence that amyloid-β (Aβ) aggregates may be transmissible similar to prions, at least under extreme experimental conditions. However, unlike mice infected with prion protein (PrP) prions, those inoculated with Aβ do not die. The transmission of Aβ and PrP thus differs conspicuously in the neurological effects they induce in their hosts, the difference being no less than a matter of life and death. Far from being a mere academic nuance, this distinction between Aβ and PrP begs the crucial questions of what, exactly, controls prion toxicity and how prion toxicity relates to prion infectivity.     

Presenilin structure, function and role in Alzheimer disease

Numerous missense mutations in the presenilins are associated with the autosomal dominant form of familial Alzheimer disease. Presenilin genes encode polytopic transmembrane proteins, which are processed by proteolytic cleavage and form high-molecular-weight complexes under physiological conditions. The presenilins have been suggested to be functionally involved in developmental morphogenesis, unfolded protein responses and processing of selected proteins including the beta-amyloid precursor protein. Although the underlying mechanism by which presenilin mutations lead to development of Alzheimer disease remains elusive, one consistent mutational effect is an overproduction of long-tailed amyloid beta-peptides. Furthermore, presenilins interact with beta-catenin to form presenilin complexes, and the physiological and mutational effects are also observed in the catenin signal transduction pathway.     

Prion protein and Alzheimer disease

Alzheimer and prion diseases are neurodegenerative disorders characterised by the abnormal processing of amyloid-β (Aβ) peptide and prion protein (PrP^C), respectively. Recent evidence indicates that PrP^C may play a critical role in the pathogenesis of Alzheimer disease. PrP^C interacts with and inhibits the β-secretase BACE1, the rate-limiting enzyme in the production of Aβ. More recently PrP^C was identified as a receptor for Aβ oligomers and the expression of PrP^C appears to be controlled by the amyloid intracellular domain (AICD). Here we review these observations and propose a feedback loop in the normal brain where PrP^C exerts an inhibitory effect on BACE1 to decrease both Aβ and AICD production. In turn, the AICD upregulates PrP^C expression, thus maintaining the inhibitory effect of PrP^C on BACE1. In Alzheimer disease, this feedback loop is disrupted, and the increased level of Aβ oligomers bind to PrP^C and prevent it from regulating BACE1 activity.Key words: alzheimer disease, amyloid-β, Aβ oligomers, amyloid intracellular domain, BACE1, presenilin, prion protein     

Prion protein and Alzheimer disease

Alzheimer and prion diseases are neurodegenerative disorders characterised by the abnormal processing of amyloid-b (Ab) peptide and prion protein (PrP^C), respectively. Recent evidence indicates that PrP^C may play a critical role in the pathogenesis of Alzheimer disease. PrP^C interacts with and inhibits the b-secretase BACE1, the rate-limiting enzyme in the production of Ab. More recently PrP^C was identified as a receptor for Ab oligomers and the expression of PrP^C appears to be controlled by the amyloid intracellular domain (AICD). Here we review these observations and propose a feedback loop in the normal brain where PrP^C exerts an inhibitory effect on BACE1 to decrease both Ab and AICD production. In turn, the AICD upregulates PrP^C expression, thus maintaining the inhibitory effect of PrP^C on BACE1. In Alzheimer disease, this feedback loop is disrupted, and the increased level of Ab oligomers bind to PrP^C and prevent it from regulating BACE1 activity.     

Immunotherapy for Alzheimer disease

Immunotherapy approaches for Alzheimer's disease currently are among the leading therapeutic directions for the disease. Active and passive immunotherapy against the β-amyloid peptides that aggregate and accumulate in the brain of those afflicted by the disease have been shown by numerous groups to reduce plaque pathology and improve behavior in transgenic mouse models of the disease. Several ongoing immunotherapy clinical trials for Alzheimer’s disease are in progress. The background and ongoing challenges for these immunological approaches for the treatment of Alzheimer's disease are discussed.     

Immunotherapy for Alzheimer disease

Immunotherapy approaches for Alzheimer disease currently are among the leading therapeutic directions for the disease. Active and passive immunotherapy against the β-amyloid peptides that aggregate and accumulate in the brain of those afflicted by the disease have been shown by numerous groups to reduce plaque pathology and improve behavior in transgenic mouse models of the disease. Several ongoing immunotherapy clinical trials for Alzheimer disease are in progress. The background and ongoing challenges for these immunological approaches for the treatment of Alzheimer disease are discussed.Key words: Alzheimer disease, amyloid, tau, immunotherapy, vaccineThe publication in Nature on a vaccine approach for Alzheimer disease (AD) by Schenk and colleagues in 1999 initiated a push for treatment for this major disease of aging. AD neuropathology is characterized by the progressive loss of synapses and neurons, and the aberrant accumulation in the brain of β-amyloid peptides in plaques and the microtubule associated protein tau in neurofibrillary tangles. Mutations in familial forms of AD have been associated with elevated β-amyloid levels, whereas mutations in tau have been linked to familial forms of frontotemporal dementia. Remarkably, injection of β-amyloid peptides with Freund''s adjuvant into transgenic mice harboring a human AD mutation that develop AD-like neuropathology and progressive cognitive decline led to reduced β-amyloid plaque pathology.¹ This study was subsequently confirmed and extended by multiple groups to show also behavioral improvement in AD transgenic mice with active β-amyloid immunization.^2,3 Passive immunotherapy with antibodies directed at β-amyloid were similarly effective in reducing plaques and improving behavior in AD transgenic mice.⁴ A temporary setback occurred when the first clinical trial with β-amyloid vaccination was halted after 6% of patients developed an inflammatory reaction in the brain (chemical meningoencephalitis). A subsequent study supported clinical benefits among patients in this active vaccination trial.⁵ A more recent postmortem study on a subset of patients who had participated in the aborted trial supported active removal of β-amyloid plaques by inflammatory cells, but also indicated that 7 of the 8 patients who were studied at autopsy continued to have progressive cognitive decline despite the removal of amyloid plaques.⁶The critical mechanisms whereby active or passive vaccination against β-amyloid can affect the disease process remain uncertain. Recruitment and activation of microglia, the macrophage of the central nervous system, by β-amyloid antibodies is thought to lead to β-amyloid plaque removal. At the same time, fibrillar β-amyloid containing plaques, formerly viewed as the major toxic entities in AD, are increasingly viewed as potentially only pathological remnants of the disease. Smaller assemblies, particularly of two to twelve β-amyloid peptides (oligomers), are considered pathogenic, although the site of pathogenesis remains controversial. Secreted, extracellular β-amyloid oligomers have been shown to damage synapses.⁷ Some groups stress the aberrant accumulation of β-amyloid within neurons and synapses leading to subsequent extracellular localization following destruction of neurites and synapses.⁸ Evidence has been presented that antibodies targeting β-amyloid peptides up to 42–43 amino acids can block the toxic effects of extracellular β-amyloid oligomers on synapses.⁷ Interestingly, β-amyloid immunotherapy was also shown to clear intraneuronal β-amyloid in an AD transgenic mouse; the intraneuronal variety is a pool of β-amyloid that correlates with the onset of cognitive decline prior to plaques and tangles in these mice.⁹ Intriguingly, antibodies directed at the β-amyloid domain exposed to the extracellular space within the amyloid precursor protein (APP) were shown to be internalized by neurons, where they reduced the intraneuronal pool of β-amyloid and protected against synaptic damage in neurons cultured from AD transgenic mice.^10,11 It is possible that inefficient clearance of the intracellular pool of β-amyloid played a role in the continued cognitive decline in the seven of eight patients in the aborted active vaccination clinical trial studied at autopsy who showed clearance of β-amyloid plaques.Work on β-amyloid immunotherapy in AD contributed to a reevaluation of the role of the immune system in the brain. Previously, it was considered that the brain was immune privileged, and that antibodies entered the brain only with the breakdown of the blood brain barrier. Rare neuroimmunological disorders had suggested more complex interactions. Pathological antibodies directed at neuronal proteins could be found localizing to specific types of neurons in paraneoplastic diseases linked to diverse systemic cancers^12,13 or collagen-vascular diseases such as lupus.¹⁴ Such pathological antibodies can be directed at synaptic or even intracellular proteins in selective neurons in the brain, leading to localized neurological symptoms. For paraneoplastic diseases it is hypothesized that antibodies directed at the cancer cells cross-react with neuronal antigens. Since titers of antibodies can be higher in brain than in the blood, intrathecal synthesis of antibodies from sequestration of B cells has been proposed to occur in the brain.¹⁵ The interaction between the immune system and the brain is therefore viewed as increasingly complex, with antibodies not only gaining access to the brain but also nerve cells, where they can even alter intracellular biology.¹⁰ These findings open up new possibilities for antibody-directed therapies for diseases of the nervous system.Currently, leading concerns for β-amyloid immunotherapy are the potential development of chemical meningoencephalitis and micro-hemorrhages in the brain. Involvement of T cells in damage to the brain vasculature is considered to contribute to these potential side effects. In addition, the β-amyloid released upon antibody-induced removal of plaques may damage blood vessels as β-amyloid is cleared from the brain via the vasculature.¹⁶ Recently, a phase 2 Elan/Wyeth study using passive β-amyloid immunotherapy with a humanized monoclonal antibody described (at the 2008 International Conference on Alzheimer''s disease) significant benefits in patients not harboring the apolipoprotein E4 (apoE4) allele genetic risk factor for late onset AD. In contrast, no clear therapeutic benefit and more cases with brain inflammation occurred in those with the apoE4 allele linked with an increased risk for AD. Why apoE4 carriers did not benefit in this β-amyloid immunotherapy trial is unknown, but has prompted separation of patients into E4 negative and positive groups in subsequent clinical trials. The less robust than hoped for effects even in the apoE4 negative patients has further dampened expectations. The reason for why the human studies are not showing the protection seen in the transgenic mouse studies could relate to β-amyloid playing less of a role in the more typical late onset AD than it does in the rare autosomal dominant familial forms used to generate the AD transgenic mice. It is also not clear which β-amyloid epitope(s) should be targeted by antibodies to maximize potential benefits while minimizing side effects in AD patients. Optimizing antibody specificity for immunotherapy is further complicating by the varied conformations of different β-amyloid aggregation states. In addition, β-amyloid immunotherapy may be more challenging in patients with AD because it is not effective in reducing tau tangle pathology.⁶ Most immunotherapy studies were done on transgenic AD mouse models that deposit β-amyloid plaques, but not tau tangles. In the more recently generated triple transgenic AD mouse that develops both plaques and tangles, β-amyloid antibodies reversed β-amyloid pathology and early pre-tangle tau pathology, but not hyperphosphorylated tau aggregates.⁸ Recent evidence supports that β-amyloid neurotoxicity acts synergistic with tau,¹⁷ and that both pathologies begin at synapses.¹⁸ Interestingly, tau immunotherapy was reported to protect against tau pathology in transgenic mice harboring mutant tau.¹⁹ Thus, dual immunotherapy targeting of both β-amyloid and tau can be considered. Finally, immunotherapy at earlier stages of the disease process may be more effective.In summary, the β-amyloid vaccine heralded a new era of therapeutic research for AD and despite some setbacks is actively being pursued in several ongoing clinical trials. It continues to be among the leading hopes in the AD research community. Another major effort to specifically block the generation of β-amyloid is also progressing, although not without setbacks along the way. For example, the protease involved in the final cleavage to liberate β-amyloid was found to be involved in multiple other important activities, such as cleavage of Notch. Antibody approaches are also being applied in efforts to block secretase cleavage to generate β-amyloid.²⁰ Finally, there remains some worry that β-amyloid peptides have an as yet unknown normal biological function, although cumulative immunotherapy and other therapeutic studies in animal models have provided sufficient support for the continued pursuit of β-amyloid lowering as a treatment for AD.     

Galanin in Alzheimer disease

Galanin (GAL) and GAL receptors (GALR) are overexpressed in limbic brain regions associated with cognition in Alzheimer disease (AD). The functional consequences of this overexpression are unclear. Because GAL inhibits cholinergic transmission and restricts long-term potentiation in the hippocampus, GAL overexpression may exacerbate clinical features of AD. In contrast, GAL expression increases in response to neuronal injury, and galaninergic hyperinnervation prevents the decreased production of protein phosphatase 1 subtype mRNAs in cholinergic basal forebrain neurons in AD. Thus, GAL may also be neuroprotective for AD. Further elucidation of GAL activity in selectively vulnerable brain regions will help gauge the therapeutic potential of GALR ligands for the treatment of AD.