Nuclear signaling by the APP intracellular domain occurs predominantly through the amyloidogenic processing pathway

Proteolytic processing of the amyloid precursor protein (APP) occurs via two alternative pathways, localized to different subcellular compartments, which result in functionally distinct outcomes. Cleavage by a beta-gamma sequence generates the Abeta peptide that plays a central role in Alzheimer's disease. In the case of alpha-gamma cleavage, a secreted neurotrophic molecule is generated and the Abeta peptide cleaved and destroyed. In both cases, a cytosolic APP intracellular domain (AICD) is generated. We have previously shown that coexpression of APP with the APP-binding protein Fe65 and the histone acetyltransferase Tip60 results in the formation of nuclear complexes (termed AFT complexes), which localize to transcription sites. We now show that blocking endocytosis or the pharmacological or genetic inhibition of the endosomal beta-cleavage pathway reduces translocation of AICD to these nuclear AFT complexes. AICD signaling further depends on active transport along microtubules and can be modulated by interference with both anterograde and retrograde transport systems. Nuclear signaling by endogenous AICD in primary neurons could similarly be blocked by inhibiting beta-cleavage but not by alpha-cleavage inhibition. This suggests that amyloidogenic cleavage, despite representing the minor cleavage pathway of APP, is predominantly responsible for AICD-mediated nuclear signaling.

Age-dependent increase in desmosterol restores DRM formation and membrane-related functions in cholesterol-free DHCR24-/- mice

Cholesterol is a prominent modulator of the integrity and functional activity of physiological membranes and the most abundant sterol in the mammalian brain. DHCR24-knock-out mice lack cholesterol and accumulate desmosterol with age. Here we demonstrate that brain cholesterol deficiency in 3-week-old DHCR24^−/− mice was associated with altered membrane composition including disrupted detergent-resistant membrane domain (DRM) structure. Furthermore, membrane-related functions differed extensively in the brains of these mice, resulting in lower plasmin activity, decreased β-secretase activity and diminished Aβ generation. Age-dependent accumulation and integration of desmosterol in brain membranes of 16-week-old DHCR24^−/− mice led to the formation of desmosterol-containing DRMs and rescued the observed membrane-related functional deficits. Our data provide evidence that an alternate sterol, desmosterol, can facilitate processes that are normally cholesterol-dependent including formation of DRMs from mouse brain extracts, membrane receptor ligand binding and activation, and regulation of membrane protein proteolytic activity. These data indicate that desmosterol can replace cholesterol in membrane-related functions in the DHCR24^−/− mouse.

Targeting beta-amyloid pathology in Alzheimer's disease with Abeta immunotherapy

More than 10 clinical trials of Abeta immunotherapy are currently underway in patients with Alzheimer's disease (AD). The aim is to identify safe approaches for the efficacious antibody-mediated removal of brain beta-amyloid or its neurotoxic oligomeric precursors consisting of aggregated amyloid beta-peptide (Abeta). Initial experimental and neuro-pathological evidence for clearance of brain beta-amyloid in response to Abeta immunotherapy is associated with structural and functional rescue of neurons, as well as initial signs of clinical stabilization and reduced rates of dementia progression. For the next steps in the future improvement of Abeta immunotherapy, major challenges in pharmacokinetics, safety, and tolerability need to be addressed. These include the low penetrations rates of IgG molecules through the blood-brain barrier, possible reductions in brain volume, the possibility of autoimmune disease related to unwanted cross-reactivity with endogenous antigens on physiological structures, micro-hemorrhages related to cross-reaction with pre-existing vascular amyloid pathology, possible relocalization of Abeta from beta-amyloid plaques to brain blood vessels resulting in increased amyloid angiopathy, and the lacking activity of Abeta antibodies on pre-existing neurofibrillary tangle pathology, as well as the lacking molecular identification of the forms of Abeta to be therapeutically targeted. The solutions to these problems will be guided by the fine lines between tolerance and immunity against physiological and pathological structures, respectively, as well as by the understanding of the pathogenic transition of soluble Abeta into toxic oligomeric aggregation intermediates in the dynamic equilibrium of beta-amyloid fibril assembly. Provided that the ongoing and planned clinical trials address these issues in a timely manner, there is a good chance for Abeta immunotherapy to be one of the first disease-modifying therapies of Alzheimer's disease to be introduced into clinical practice.

Changes in readthrough acetylcholinesterase expression modulate amyloid-beta pathology

Alzheimer's disease has long been known to involve cholinergic deficits, but the linkage between cholinergic gene expression and the Alzheimer's disease amyloid pathology has remained incompletely understood. One known link involves synaptic acetylcholinesterase (AChE-S), shown to accelerate amyloid fibrils formation. Here, we report that the 'Readthrough' AChE-R splice variant, which differs from AChE-S in its 26 C-terminal residues, inversely exerts neuroprotective effects from amyloid beta (Abeta) induced toxicity. In vitro, highly purified AChE-R dose-dependently suppressed the formation of insoluble Abeta oligomers and fibrils and abolished Abeta toxicity to cultured cells, competing with the prevalent AChE-S protein which facilitates these processes. In vivo, double transgenic APPsw/AChE-R mice showed lower plaque burden, fewer reactive astrocytes and less dendritic damage than single APPsw mice, inverse to reported acceleration of these features in double APPsw/AChE-S mice. In hippocampi from Alzheimer's disease patients (n = 10), dentate gyrus neurons showed significantly elevated AChE-R mRNA and reduced AChE-S mRNA. However, immunoblot analyses revealed drastic reductions in the levels of intact AChE-R protein, suggesting that its selective loss in the Alzheimer's disease brain exacerbates the Abeta-induced damages and revealing a previously unforeseen linkage between cholinergic and amyloidogenic events.

Abeta oligomer-mediated long-term potentiation impairment involves protein phosphatase 1-dependent mechanisms

Amyloid beta (Abeta) oligomers are derived from proteolytic cleavage of amyloid precursor protein (APP) and can impair memory and hippocampal long-term potentiation (LTP) in vivo and in vitro. They are recognized as the primary neurotoxic agents in Alzheimer's disease. The mechanisms underlying such toxicity on synaptic functions are complex and not fully understood. Here, we provide the first evidence that these mechanisms involve protein phosphatase 1 (PP1). Using a novel transgenic mouse model expressing human APP with the Swedish and Arctic mutations that render Abeta more prone to form oligomers (arcAbeta mice), we show that the LTP impairment induced by Abeta oligomers can be fully reversed by PP1 inhibition in vitro. We further demonstrate that the genetic inhibition of endogenous PP1 in vivo confers resistance to Abeta oligomer-mediated toxicity and preserves LTP. Overall, these results reveal that PP1 is a key player in the mechanisms of AD pathology.

Common genetic variation within the low-density lipoprotein receptor-related protein 6 and late-onset Alzheimer's disease

Genome-wide linkage studies have defined a broad susceptibility region for late-onset Alzheimer's disease on chromosome 12, which contains the Low-Density Lipoprotein Receptor-Related Protein 6 (LRP6) gene, a coreceptor for Wnt signaling. Here, we report the association between common LRP6 variants and late-onset Alzheimer's disease in a multicenter case-control series as well as in a large family-based series ascertained by the National Institute of Mental Health-National Institute on Aging Genetics Initiative. As shown in the genome-wide linkage studies, our association depends mainly on apolipoprotein E-epsilon4 (APOE-epsilon4) carrier status. Haplotype tagging single-nucleotide polymorphisms (SNPs) with a set of seven allelic variants of LRP6 identified a putative risk haplotype, which includes a highly conserved coding sequence SNP: Ile-1062 --> Val. Functional analyses revealed that the associated allele Val-1062, an allele previously linked to low bone mass, has decreased beta-catenin signaling in HEK293T cells. Our study unveils a genetic relationship between LRP6 and APOE and supports the hypothesis that altered Wnt/beta-catenin signaling may be involved in this neurodegenerative disease.

Association study of cholesterol-related genes in Alzheimer's disease

Alzheimer's disease (AD) is a genetically complex disorder, and several genes related to cholesterol metabolism have been reported to contribute to AD risk. To identify further AD susceptibility genes, we have screened genes that map to chromosomal regions with high logarithm of the odds scores for AD in full genome scans and are related to cholesterol metabolism. In a European screening sample of 115 sporadic AD patients and 191 healthy control subjects, we analyzed single nucleotide polymorphisms in 28 cholesterol-related genes for association with AD. The genes HMGCS2, FDPS, RAFTLIN, ACAD8, NPC2, and ABCG1 were associated with AD at a significance level of P < or = 0.05 in this sample. Replication trials in five independent European samples detected associations of variants within HMGCS2, FDPS, NPC2, or ABCG1 with AD in some samples (P = 0.05 to P = 0.005). We did not identify a marker that was significantly associated with AD in the pooled sample (n = 2864). Stratification of this sample revealed an APOE-dependent association of HMGCS2 with AD (P = 0.004). We conclude that genetic variants investigated in this study may be associated with a moderate modification of the risk for AD in some samples.

Lack of neprilysin suffices to generate murine amyloid-like deposits in the brain and behavioral deficit in vivo

Accumulation of the beta-amyloid peptide (Abeta) in the brain is a major pathological hallmark of Alzheimer's disease (AD), leading to synaptic dysfunction, neuronal death, and memory impairment. The levels of neprilysin, a major Abeta-degrading enzyme, are decreased in AD brains and during aging. Because neprilysin cleaves Abeta in vivo, its down-regulation may contribute to the pathophysiology of AD. The aim of this study was to assess the consequences of neprilysin deficiency on accumulation of murine Abeta in brains and associated pathologies in vivo by investigating neprilysin-deficient mice on biochemical, morphological, and behavioral levels. Aged neprilysin-deficient mice expressed physiological amyloid precursor protein (APP) levels and exhibited elevated brain Abeta concentrations and amyloid-like deposits in addition to signs of neuronal degeneration in their brains. Behaviorally, neprilysin-deficient mice acquired a significantly weaker conditioned taste aversion that extinguished faster than the aversion of age-matched controls. Our data establish that, under physiological APP expression levels, neprilysin deficiency is associated with increased Abeta accumulation in the brain and leads to deposition of amyloid-like structures in vivo as well as with signs of AD-like pathology and with behavioral deficits.

Genetic association study on colony-stimulating factor 1 in Alzheimer's disease

BACKGROUND

Colony-stimulating factor 1 (CSF1) regulates the proliferation and differentiation of myelomonocytic cells. Microglial cells of CSF1-deficient mice are reduced in number and are functionally impaired. CSF1-deficient mice exhibit subtle neurodevelopmental defects, enhanced neuronal vulnerability. Moreover, it has been reported that these mice may have amyloid-plaque-like depositions in the brain at an early age. The human CSF1 gene maps to chromosome 1p21-p13, a region previously linked to Alzheimer's disease (AD). Thus, CSF1 is a functional and positional candidate gene for AD.

OBJECTIVE

We assessed if genetic variability of CSF1 may influence the risk for AD.

METHODS

We conducted a population-based case-control association study with 3 single nucleotide polymorphisms (SNPs) across the CSF1 locus in a sample of n = 185 (rs3093054, rs756325) and n = 327 (rs1058885) individuals.

RESULTS

None of the 3 investigated SNPs was associated with the risk for AD in our sample.

CONCLUSION

These data do not support the hypothesis that genetic variability of CSF1 influences the risk for AD.

Neuronal neprilysin overexpression is associated with attenuation of Aβ-related spatial memory deficit

Converging evidence links abnormally high brain concentrations of amyloid-beta peptides (Abeta) to the pathology of Alzheimer's disease (AD). Lowering brain Abeta levels, therefore, is a therapeutic strategy for the treatment of AD. Neuronal neprilysin upregulation led to increased degradation of Abeta, reduced the formation of Abeta-plaques and the associated cytopathology, but whether overexpression of neprilysin can improve cognition is unknown. We show that neuronal overexpression of neprilysin improved the Morris water maze memory performance in mice with memory deficits resulting from overexpression of the AD-causing mutated human amyloid precursor protein (APP). This improvement was associated with decreased brain levels of Abeta and with unchanged endoproteolytic processing of APP. These results provide the evidence that lowering of brain Abeta levels by increasing its degradation can improve cognitive functions in vivo, and suggest that increasing the activity of neprilysin in brain may be effective in preventing cognitive decline in AD.

Better memory and neural efficiency in young apolipoprotein E epsilon4 carriers

The apolipoprotein E (APOE) epsilon4 allele is the major genetic risk factor for Alzheimer's disease, but an APOE effect on memory performance and memory-related neurophysiology in young, healthy subjects is unknown. We found an association of APOE epsilon4 with better episodic memory compared with APOE epsilon2 and epsilon3 in 340 young, healthy persons. Neuroimaging was performed in a subset of 34 memory-matched individuals to study genetic effects on memory-related brain activity independently of differential performance. E4 carriers decreased brain activity over 3 learning runs, whereas epsilon2 and epsilon3 carriers increased activity. This smaller neural investment of epsilon4 carriers into learning reappeared during retrieval: epsilon4 carriers exhibited reduced retrieval-related activity with equal retrieval performance. APOE isoforms had no differential effects on cognitive measures other than memory, brain volumes, and brain activity related to working memory. We suggest that APOE epsilon4 is associated with good episodic memory and an economic use of memory-related neural resources in young, healthy humans.

Impaired spatial reference memory and increased exploratory behavior in P301L tau transgenic mice

The neuropathological hallmark shared between Alzheimer's disease (AD) and familial frontotemporal dementia (FTDP-17) are neurofibrillary tangles (NFT) which are composed of filamentous aggregates of the microtubule-associated protein tau. Their formation has been reproduced in transgenic mice, which express the FTDP-17-associated mutation P301L of tau. In these mice, tau aggregates are found in many brain areas including the hippocampus and the amygdala, both of which are characterized by NFT formation in AD. Previous studies using an amygdala-specific test battery revealed an increase in exploratory behavior and an accelerated extinction of conditioned taste aversion in these mice. Here, we assessed P301L mice in behavioral tests known to depend on an intact hippocampus. Morris water maze and Y-maze revealed intact spatial working memory but impairment in spatial reference memory at 6 and 11 months of age. In addition, a modest disinhibition of exploratory behavior at 6 months of age was confirmed in the open field and the elevated O-maze and was more pronounced during aging.

Enhanced brain activity may precede the diagnosis of Alzheimer's disease by 30 years

Presenilin 1 (PSEN1) mutations cause autosomal dominant familial Alzheimer's disease (FAD). PSEN1 mutation carriers undergo the course of cognitive deterioration, which is typical for sporadic Alzheimer's disease but disease onset is earlier and disease progression is faster. Here, we sought to detect signs of FAD in presymptomatic carriers of the PSEN1 mutation (C410Y) by use of a neuropsychological examination, functional MRI during learning and memory tasks and MRI volumetry. We examined five non-demented members of a FAD family and 21 non-related controls. Two of the five family members were carrying the mutation; one was 20 years old and the other 45 years old. The age of clinical manifestation of FAD in the family studied here is approximately 48 years. Neuropsychological assessments suggested subtle problems with episodic memory in the 20-year-old mutation carrier. The middle-aged mutation carrier fulfilled criteria for amnestic mild cognitive impairment. The 20-year-old mutation carrier exhibited increased, while the middle-aged mutation carrier exhibited decreased brain activity compared to controls within memory-related neural networks during episodic learning and retrieval, but not during a working-memory task. The increased memory-related brain activity in the young mutation carrier might reflect a compensatory effort to overcome preclinical neural dysfunction caused by first pathological changes. The activity reductions in the middle-aged mutation carrier might reflect gross neural dysfunction in a more advanced stage of neuropathology. These data suggest that functional neuroimaging along with tasks that challenge specifically those brain areas which are initial targets of Alzheimer's disease pathology may reveal activity alterations on a single-subject level decades before the clinical manifestation of Alzheimer's disease.

Cholesterol 25-hydroxylase on chromosome 10q is a susceptibility gene for sporadic Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia. It is characterized by beta-amyloid (A beta) plaques, neurofibrillary tangles and the degeneration of specifically vulnerable brain neurons. We observed high expression of the cholesterol 25-hydroxylase (CH25H) gene in specifically vulnerable brain regions of AD patients. CH25H maps to a region within 10q23 that has been previously linked to sporadic AD. Sequencing of the 5' region of CH25H revealed three common haplotypes, CH25Hchi2, CH25Hchi3 and CH25Hchi4; CSF levels of the cholesterol precursor lathosterol were higher in carriers of the CH25Hchi4 haplotype. In 1,282 patients with AD and 1,312 healthy control subjects from five independent populations, a common variation in the vicinity of CH25H was significantly associated with the risk for sporadic AD (p = 0.006). Quantitative neuropathology of brains from elderly non-demented subjects showed brain A beta deposits in carriers of CH25Hchi4 and CH25Hchi3 haplotypes, whereas no A beta deposits were present in CH25Hchi2 carriers. Together, these results are compatible with a role of CH25Hchi4 as a putative susceptibility factor for sporadic AD; they may explain part of the linkage of chromosome 10 markers with sporadic AD, and they suggest the possibility that CH25H polymorphisms are associated with different rates of brain A beta deposition.

Intracellular Abeta and cognitive deficits precede beta-amyloid deposition in transgenic arcAbeta mice

The brain pathology of Alzheimer's disease is characterized by abnormally aggregated Abeta in extracellular beta-amyloid plaques and along blood vessel walls, but the relation to intracellular Abeta remains unclear. To address the role of intracellular Abeta deposition in vivo, we expressed human APP with the combined Swedish and Arctic mutations in mice (arcAbeta mice). Intracellular punctate deposits of Abeta occurred concomitantly with robust cognitive impairments at the age of 6 months before the onset of beta-amyloid plaque formation and cerebral beta-amyloid angiopathy. beta-Amyloid plaques from arcAbeta mice had distinct dense-core morphologies with blood vessels appearing as seeding origins, suggesting reduced clearance of Abeta across blood vessels in arcAbeta mice. The co-incidence of intracellular Abeta deposits with behavioral deficits support an early role of intracellular Abeta in the pathophysiological cascade leading to beta-amyloid formation and functional impairment.

Ethnicity-dependent genetic association of ABCA2 with sporadic Alzheimer's disease

A recent study demonstrated a significant genetic association between the ATP-binding cassette transporter A2 (ABCA2) and the risk for Alzheimer's disease (AD) in a large Caucasian sample. The rare T allele of the synonymous exonic single nucleotide polymorphism (SNP) rs908832 was overrepresented in early-onset AD patients as compared to cognitively healthy controls. Here we confirm the association of rs908832 with AD in a Western European population (n = 291, P = 0.008). In a second sample from Southern Europe, rs908832 was not associated with AD. Interestingly, rs908832 was not polymorphic in a Japanese sample. Furthermore, rs908832 was not associated with either serum cholesterol levels or with the risk for coronary artery disease, but seemed to be related to cholesterol levels in the cerebrospinal fluid. These data suggest that ABCA2 may exert population-dependent effects on the genetic risk for sporadic AD and support a role of ABC lipid transporters in the pathogenesis of this disease.