Huaxi Xu

Apolipoprotein E and Alzheimer disease: risk, mechanisms and therapy

Apolipoprotein E (Apo-E) is a major cholesterol carrier that supports lipid transport and injury repair in the brain. APOE polymorphic alleles are the main genetic determinants of Alzheimer disease (AD) risk: individuals carrying the ε4 allele are at increased risk of AD compared with those carrying the more common ε3 allele, whereas the ε2 allele decreases risk. Presence of the APOE ε4 allele is also associated with increased risk of cerebral amyloid angiopathy and age-related cognitive decline during normal ageing. Apo-E–lipoproteins bind to several cell-surface receptors to deliver lipids, and also to hydrophobic amyloid-β (Aβ) peptide, which is thought to initiate toxic events that lead to synaptic dysfunction and neurodegeneration in AD. Apo-E isoforms differentially regulate Aβ aggregation and clearance in the brain, and have distinct functions in regulating brain lipid transport, glucose metabolism, neuronal signalling, neuroinflammation, and mitochondrial function. In this Review, we describe current knowledge on Apo-E in the CNS, with a particular emphasis on the clinical and pathological features associated with carriers of different Apo-E isoforms. We also discuss Aβ-dependent and Aβ-independent mechanisms that link Apo-E4 status with AD risk, and consider how to design effective strategies for AD therapy by targeting Apo-E.

Intraneuronal Aβ42 accumulation in human brain

Alzheimers disease (AD) is characterized by the deposition of senile plaques (SPs) and neurofibrillary tangles (NFTs) in vulnerable brain regions. SPs are composed of aggregated β-amyloid (Aβ) 40/42(43) peptides. Evidence implicates a central role for Aβ in the pathophysiology of AD. Mutations in βAPP and presenilin 1 (PS1) lead to elevated secretion of Aβ, especially the more amyloidogenic Aβ42. Immunohistochemical studies have also emphasized the importance of Aβ42 in initiating plaque pathology. Cell biological studies have demonstrated that Aβ is generated intracellularly. Recently, endogenous Aβ42 staining was demonstrated within cultured neurons by confocal immunofluorescence microscopy and within neurons of PS1 mutant transgenic mice. A central question about the role of Aβ in disease concerns whether extracellular Aβ deposition or intracellular Aβ accumulation initiates the disease process. Here we report that human neurons in AD-vulnerable brain regions specifically accumulate γ-cleaved Aβ42 and suggest that this intraneuronal Aβ42 immunoreactivity appears to precede both NFT and Aβ plaque deposition. This study suggests that intracellular Aβ42 accumulation is an early event in neuronal dysfunction and that preventing intraneuronal Aβ42 aggregation may be an important therapeutic direction for the treatment of AD.

Intraneuronal Alzheimer Aβ42 Accumulates in Multivesicular Bodies and Is Associated with Synaptic Pathology

A central question in Alzheimers disease concerns the mechanism by which beta-amyloid contributes to neuropathology, and in particular whether intracellular versus extracellular beta-amyloid plays a critical role. Alzheimer transgenic mouse studies demonstrate brain dysfunction, as beta-amyloid levels rise, months before the appearance of beta-amyloid plaques. We have now used immunoelectron microscopy to determine the subcellular site of neuronal beta-amyloid in normal and Alzheimer brains, and in brains from Alzheimer transgenic mice. We report that beta-amyloid 42 localized predominantly to multivesicular bodies of neurons in normal mouse, rat, and human brain. In transgenic mice and human Alzheimer brain, intraneuronal beta-amyloid 42 increased with aging and beta-amyloid 42 accumulated in multivesicular bodies within presynaptic and especially postsynaptic compartments. This accumulation was associated with abnormal synaptic morphology, before beta-amyloid plaque pathology, suggesting that intracellular accumulation of beta-amyloid plays a crucial role in Alzheimers disease.

Chaperones increase association of tau protein with microtubules

Molecular chaperones and their functions in protein folding have been implicated in several neurodegenerative diseases, including Parkinsons disease and Huntingtons disease, which are characterized by accumulation of protein aggregates (e.g., α-synuclein and huntingtin, respectively). These aggregates have been shown in various experimental systems to respond to changes in levels of molecular chaperones suggesting the possibility of therapeutic intervention and a role for chaperones in disease pathogenesis. It remains unclear whether chaperones also play a role in Alzheimers disease, a neurodegenerative disorder characterized by β-amyloid and tau protein aggregates. Here, we report an inverse relationship between aggregated tau and the levels of heat shock protein (Hsp)70/90 in tau transgenic mouse and Alzheimers disease brains. In various cellular models, increased levels of Hsp70 and Hsp90 promote tau solubility and tau binding to microtubules, reduce insoluble tau and cause reduced tau phosphorylation. Conversely, lowered levels of Hsp70 and Hsp90 result in the opposite effects. We have also demonstrated a direct association of the chaperones with tau proteins. Our results suggest that up-regulation of molecular chaperones may suppress formation of neurofibrillary tangles by partitioning tau into a productive folding pathway and thereby preventing tau aggregation.

APP processing in Alzheimer's disease

An important pathological feature of Alzheimers disease (AD) is the presence of extracellular senile plaques in the brain. Senile plaques are composed of aggregations of small peptides called β-amyloid (Aβ). Multiple lines of evidence demonstrate that overproduction/aggregation of Aβ in the brain is a primary cause of AD and inhibition of Aβ generation has become a hot topic in AD research. Aβ is generated from β-amyloid precursor protein (APP) through sequential cleavages first by β-secretase and then by γ-secretase complex. Alternatively, APP can be cleaved by α-secretase within the Aβ domain to release soluble APPα and preclude Aβ generation. Cleavage of APP by caspases may also contribute to AD pathologies. Therefore, understanding the metabolism/processing of APP is crucial for AD therapeutics. Here we review current knowledge of APP processing regulation as well as the patho/physiological functions of APP and its metabolites.

Inhibition of amyloid-β aggregation and caspase-3 activation by the Ginkgo biloba extract EGb761

Standardized extract from the leaves of the Ginkgo biloba tree, labeled EGb761, has been used in clinical trials for its beneficial effects on brain functions, particularly in connection with age-related dementias and Alzheimers disease (AD). Substantial experimental evidence indicates that EGb761 protects against neuronal damage from a variety of insults, but its cellular and molecular mechanisms remain unknown. Using a neuroblastoma cell line stably expressing an AD-associated double mutation, we report that EGb761 inhibits formation of amyloid-β (Aβ) fibrils, which are the diagnostic, and possibly causative, feature of AD. The decreased Aβ fibrillogenesis in the presence of EGb761 was observed both in the conditioned medium of this Aβ-secreting cell line and in solution in vitro. In the cells, EGb761 significantly attenuated mitochondrion-initiated apoptosis and decreased the activity of caspase 3, a key enzyme in the apoptosis cell-signaling cascade. These results suggest that (i) neuronal damage in AD might be due to two factors: a direct Aβ toxicity and the apoptosis initiated by the mitochondria; and (ii) multiple cellular and molecular neuroprotective mechanisms, including attenuation of apoptosis and direct inhibition of Aβ aggregation, underlie the neuroprotective effects of EGb761.

Hypoxia-inducible Factor 1α (HIF-1α)-mediated Hypoxia Increases BACE1 Expression and β-Amyloid Generation

The incidence of Alzheimer disease (AD) and vascular dementia is greatly increased following cerebral ischemia and stroke in which hypoxic conditions occur in affected brain areas. β-Amyloid peptide (Aβ), which is derived from the β-amyloid precursor protein (APP) by sequential proteolytic cleavages from β-secretase (BACE1) and presenilin-1 (PS1)/γ-secretase, is widely believed to trigger a cascade of pathological events culminating in AD and vascular dementia. However, a direct molecular link between hypoxic insults and APP processing has yet to be established. Here, we demonstrate that acute hypoxia increases the expression and the enzymatic activity of BACE1 by up-regulating the level of BACE1 mRNA, resulting in increases in the APP C-terminal fragment-β (βCTF) and Aβ. Hypoxia has no effect on the level of PS1, APP, and tumor necrosis factor-α-converting enzyme (TACE, an enzyme known to cleave APP at the α-secretase cleavage site). Sequence analysis, mutagenesis, and gel shift studies revealed binding of HIF-1 to the BACE1 promoter. Overexpression of HIF-1α increases BACE1 mRNA and protein level, whereas down-regulation of HIF-1α reduced the level of BACE1. Hypoxic treatment fails to further potentiate the stimulatory effect of HIF-1α overexpression on BACE1 expression, suggesting that hypoxic induction of BACE1 expression is primarily mediated by HIF-1α. Finally, we observed significant reduction in BACE1 protein levels in the hippocampus and the cortex of HIF-1α conditional knock-out mice. Our results demonstrate an important role for hypoxia/HIF-1α in modulating the amyloidogenic processing of APP and provide a molecular mechanism for increased incidence of AD following cerebral ischemic and stroke injuries.

Does insulin dysfunction play a role in Alzheimer's disease?

Age-related changes in hormone levels are determinants of a variety of human diseases. Insulin is known to affect numerous brain functions including cognition and memory, and several clinical studies have established links between Alzheimers disease (AD), insulin resistance and diabetes mellitus. These are reinforced by biological studies that reveal the effects of insulin on the molecular and cellular mechanisms that underlie the pathology of AD. For example, insulin regulates phosphorylation of tau protein, which underlies neurofibrillary lesions in the brains of AD patients. Insulin also affects the metabolism of beta-amyloid, the main constituent of AD amyloid pathology. Here, we discuss clinical and biological data that highlight potential targets for therapeutic intervention.

Antidiabetic drug metformin (GlucophageR) increases biogenesis of Alzheimer's amyloid peptides via up-regulating BACE1 transcription

Epidemiological, clinical and experimental evidence suggests a link between type 2 diabetes and Alzheimers disease (AD). Insulin modulates metabolism of β-amyloid precursor protein (APP) in neurons, decreasing the intracellular accumulation of β-amyloid (Aβ) peptides, which are pivotal in AD pathogenesis. The present study investigates whether the widely prescribed insulin-sensitizing drug, metformin (GlucophageR), affects APP metabolism and Aβ generation in various cell models. We demonstrate that metformin, at doses that lead to activation of the AMP-activated protein kinase (AMPK), significantly increases the generation of both intracellular and extracellular Aβ species. Furthermore, the effect of metformin on Aβ generation is mediated by transcriptional up-regulation of β-secretase (BACE1), which results in an elevated protein level and increased enzymatic activity. Unlike insulin, metformin exerts no effect on Aβ degradation. In addition, we found that glucose deprivation and various tyrphostins, known inhibitors of insulin-like growth factors/insulin receptor tyrosine kinases, do not modulate the effect of metformin on Aβ. Finally, inhibition of AMP-activated protein kinase (AMPK) by the pharmacological inhibitor Compound C largely suppresses metformins effect on Aβ generation and BACE1 transcription, suggesting an AMPK-dependent mechanism. Although insulin and metformin display opposing effects on Aβ generation, in combined use, metformin enhances insulins effect in reducing Aβ levels. Our findings suggest a potentially harmful consequence of this widely prescribed antidiabetic drug when used as a monotherapy in elderly diabetic patients.

Potential roles of insulin and IGF-1 in Alzheimer's disease.

Aging is characterized by a significant decline of metabolic and hormonal functions, which often facilitates the onset of severe age-associated pathologies. One outstanding example of this is the reported association of deranged signaling by insulin and insulin-like-growth-factor 1 (IGF-1) with Alzheimers disease (AD). Recent compelling biological data reveal effects of insulin and IGF-1 on molecular and cellular mechanisms underlying the pathology of AD. This review discusses available biological data that highlight the therapeutic potential of the insulin-IGF-1 signaling pathway in AD.