Involvement of Dab1 in APP processing and β-amyloid deposition in sporadic Creutzfeldt–Jakob patients

Alzheimer's disease and prion pathologies (e.g., Creutzfeldt–Jakob disease (CJD)) display profound neural lesions associated with aberrant protein processing and extracellular amyloid deposits. Dab1 has been implicated in the regulation of amyloid precursor protein (APP), but a direct link between human prion diseases and Dab1/APP interactions has not been published. Here we examined this putative relationship in 17 cases of sporadic CJD (sCJD) post-mortem. Biochemical analyses of brain tissue revealed two groups, which also correlated with PrP^sc types 1 and 2. One group with PrP^sc type 1 showed increased Dab1 phosphorylation and lower βCTF production with an absence of Aβ deposition. The second sCJD group, which carried PrP^sc type 2, showed lower levels of Dab1 phosphorylation and βCTF production, and Aβ deposition. Thus, the present observations suggest a correlation between Dab1 phosphorylation, Aβ deposition and PrP^sc type in sCJD.     

Intraneuronal β-amyloid accumulation and synapse pathology in Alzheimer’s disease

The aberrant accumulation of aggregated β-amyloid peptides (Aβ) as plaques is a hallmark of Alzheimer’s disease (AD) neuropathology and reduction of Aβ has become a leading direction of emerging experimental therapies for the disease. The mechanism(s) whereby Aβ is involved in the pathophysiology of the disease remain(s) poorly understood. Initially fibrils, and subsequently oligomers of extracellular Aβ have been viewed as the most important pathogenic form of Aβ in AD. More recently, the intraneuronal accumulation of Aβ has been described in the brain, although technical considerations and its relevance in AD have made this a controversial topic. Here, we review the emerging evidence linking intraneuronal Aβ accumulation to the development of synaptic pathology and plaques in AD, and discuss the implications of intraneuronal β-amyloid for AD pathology, biology, diagnosis and therapy.     

Genes and mechanisms involved in β-amyloid generation and Alzheimer’s disease

Alzheimer's disease is characterized by the invariable accumulation of senile plaques that are predominantly composed of amyloid beta-peptide (Abeta). Abeta is generated by proteolytic processing of the beta-amyloid precursor protein (betaAPP) involving the combined action of beta- and gamma-secretase. Cleavage within the Abeta domain by alpha-secretase prevents Abeta generation. In some very rare cases of familial AD (FAD), mutations have been identified within the betaAPP gene. These mutations are located close to or at the cleavage sites of the secretases and pathologically effect betaAPP processing by increasing Abeta production, specifically its highly amyloidogenic 42 amino acid variant (Abeta42). Most of the mutations associated with FAD have been identified in the two presenilin (PS) genes, particularly the PS1 gene. Like the mutations identified within the betaAPP gene, mutations in PS1 and PS2 cause the increased generation of Abeta42. PS1 has been shown to be functionally involved in Notch signaling, a key process in cellular differentation, and in betaAPP processing. A gene knock out of PS1 in mice leads to an embryonic lethal phenotype similar to that of mice lacking Notch. In addition, absence of PS1 results in reduced gamma-secretase cleavage and leads to an accumulation of betaAPP C-terminal fragments and decreased amounts of Abeta. Recent work may suggest that PS1 could be the gamma-secretase itself, exhibiting the properties of a novel aspartyl protease. Mutagenesis of either of two highly conserved intramembraneous aspartate residues of PS1 leads to reduced Abeta production as observed in the PS1 knockout. A corresponding mutation in PS2 interfered with betaAPP processing and Notch signaling suggesting a functional redundancy of both presenilins. In this issue, some of the recent work on the molecular mechanisms involved in Alzheimer's disease (AD) as well as novel diagnostic approaches and risk factors for AD will be discussed. In the first article, we like to give an overview on mechanisms involved in the proteolytic generation of Amyloid beta-peptide (Abeta), the major pathological player of this devastating disease. In the second part of this article recent results will be described, which demonstrate an unexpected biological and pathological function of an AD associated gene.     

Sporadic Creutzfeldt–Jakob disease

To investigate a possible relationship between the severity of pathological and radiological lesions in diffusion–weighted MRI (DWI) we compared DWI findings from 6 sequential brain MRI scans with pathological features of numerous tissue blocks of different cortical and subcortical regions in a case of autopsy–proven sporadic CJD. Whereas DWI and pathological examination revealed multifocal, cortical and deep hyperintensities at corresponding localizations, no correlation between the degree of severity of radiologically visible and pathological damage was found.The characteristic focal involvement and extension of lesions of the cortex and the basal ganglia bilaterally shown by DWI may be an argument for the spreading of the disease per contiguitatem.     

Limb apraxia and verb processing in Alzheimer’s disease

Objective: The present research investigates language and praxis abilities in patients with Alzheimer’s disease in order to study the relationship between these two cognitive domains. Method: The experimental evaluation of patients and control group performance was designed to permit a direct comparison of linguistic abilities (i.e., verb and noun naming and sentence comprehension) and praxic abilities (i.e., gesture execution for complex movements). Moreover, for the first time, action comprehension was explored using the Action Sequence Comprehension. Results and conclusion: Analyses of variance (ANOVAs) and correlational analyses showed that a direct relationship may exist between language impairment and apraxia in patients with Alzheimer’s disease. In addition, the production and comprehension of both language and action were equally impaired in patients, providing further evidence for a spectrum of concomitant linguistic and praxis deficits in Alzheimer’s disease. Finally, the ability to correctly comprehend action semantics was related more directly to verb production ability than to noun production.     

Reverse relationship between β-amyloid precursor protein and β-amyloid peptide plaques in Down’s syndrome versus sporadic/familial Alzheimer’s disease

Strong genetic evidence has been accumulated in favor of a central role of β-amyloid precursor protein (APP) and β-amyloid peptide (βA4) in the pathogenesis of Alzheimer’s disease (AD). We employed four newly developed APP and βA4 antibodies and performed a comparative neuropathological study of patients with Down’s syndrome (DS), early-onset familial AD and sporadic AD to investigate the distribution of APP and βA4 plaque densities in the cerebral cortex of these disorders. Quantitative analysis of APP versus βA4 plaques revealed that brains with early-onset familial AD and sporadic AD showed significantly more βA4 plaques than brains with DS (P < 0.05). In contrast, APP plaques were more abundant in DS cerebral cortex (P < 0.02). These observations suggest that the development of pathological changes in DS brains does not parallel that observed in AD, which might be attributable to different causes in the pathogenesis of βA4 formation. A comparison of these disorders may be useful to further complement our knowledge of the mechanisms leading to plaque development. Received: 26 March 1998 / Revised: 25 May 1998 / Accepted: 30 July 1998     

MRI sequence findings in sporadic Creutzfeldt–Jakob disease

MRI has had an important role in the diagnosis of Creutzfeldt–Jakob disease (CJD). The aim of our study was to compare the efficacy of different MRI sequences among six biopsy-proven patients with sporadic CJD (sCJD) and seven patients with probable sCJD. These 13 patients with CJD aged from 36 years to 75 years (mean age: 55.5 years) were evaluated with T1-weighted, T2-weighted, and fluid-attenuated inversion recovery (FLAIR) MRI and diffusion-weighted imaging (DWI). The characteristic MRI lesion pattern was found to be bilateral, symmetric and hyperintense signal changes in the basal ganglia and cortical regions. Two major lesion patterns were identified in all patients involving the cortex and basal ganglia. No signal abnormality was found in the thalamus. We found lesions in the cortex and basal ganglia in 7/13 patients (54%), isolated cortical involvement in 2/13 patients (15%), and isolated basal ganglia lesions in 4/13 patients (31%). The cortical involvement was widespread (in at least two regions) and usually included the frontal or occipital lobes (9/13, 69%) on DWI. Only one patient showed moderate high-signal intensity in the basal ganglia on T2-weighted MRI. T1-weighted MRI revealed no signal intensity abnormalities. We conclude that high signal changes in the basal ganglia and cerebral cortex on FLAIR and DWI are useful in the diagnosis of sCJD. Isolated cortical involvement on DWI and FLAIR should lead to a suspicion of CJD. DWI is the most sensitive MRI technique in the diagnosis of CJD, which supports an amendment to the clinical diagnostic criteria for sCJD to include findings from MRI.     

Cerebrospinal fluid β-amyloid1–42 correlates with rate of progression in Alzheimer’s disease

Emerging treatment options targeting the pathogenetic mechanisms in Alzheimer's disease (AD) and the need to monitor efficacy during treatment trials necessitate the use of biomarkers, which not only may facilitate early and reliable diagnosis, but may also assist in the stratification of patient populations according to their rate of progression. The objective of the present study is to examine whether demographic and cerebrospinal fluid (CSF) parameters at initial evaluation [total tau, tau phosphorylated at threonine-181 and amyloid-beta(1-42) (Aβ42)] can be used to discriminate between slow and rapid progressors in patients with AD. A total of 74 AD patients were included in the study. Patients recruited were divided into slow and rapid progressors according to their Mini-Mental Status Examination (MMSE) score decline before evaluation. Patients with a drop rate of >4/year were considered rapid progressors. Commercially available ELISA kits were used for measuring CSF biomarkers. Comparisons were performed using analysis of covariance. Significantly lower Aβ42 levels in the CSF were found in rapid (mean 392 pg/ml) as compared to slow progressors (mean 453 pg/ml), with a p value of 0.042. The results of the present study suggest that levels of the Aβ42 peptide may be related to the rate of disease progression. Further studies with a prospective design are needed in order to test the possible predictive value of CSF Aβ42 analysis.     

APOE gene polymorphisms and susceptibility to Creutzfeldt–Jakob disease

Associations between apolipoprotein E (APOE) gene polymorphisms and Creutzfeldt–Jakob disease (CJD) have been reported, but the results from many of these studies are conflicting. To investigate the association between APOE polymorphisms and CJD risk, we performed a meta-analysis. We used odds ratios (OR) with 95% confidence intervals (CI) to assess the strength of the association. The frequency of putative risk alleles in control subjects was estimated with the Mantel-Haenszel method. Cochran’s Q statistic and the inconsistency index (I²) were used to test heterogeneity. Egger’s test and an inverted funnel plot were used to assess bias. Our study included 11 published case–control studies with APOE genotyping, involving a total of 1001 CJD patients and 1211 controls. Overall, the APOE 34 (OR 1.37, 95% CI: 1.09–1.72), and APOE 44 (OR 3.16, 95% CI: 1.37–7.26) genotypes and the APOE 4 (OR 1.41, 95% CI: 1.08–1.85) allele were associated with an increased risk of CJD, and the APOE 33 (OR 0.81, 95% CI: 0.67–0.97) genotype tended to protect against CJD. However, we did not find significant evidence supporting associations of the APOE 22 (OR 1.15, 95% CI: 0.45–2.93), APOE 23 (OR 0.84, 95% CI: 0.64–1.09), or APOE 24 (OR 1.40, 95% CI: 0.70–2.77) genotypes, nor the APOE 2 (OR 1.02, 95% CI: 0.73–1.42) or APOE 3 (OR 0.82, 95% CI: 0.65–1.02) alleles with CJD using a fixed-effects model. Our results support a genetic association between APOE polymorphisms and CJD.     

Cerebrovascular P-glycoprotein expression is decreased in Creutzfeldt–Jakob disease

The abnormal conformation and assembly of proteins in the central nervous system is increasingly thought to be a critical pathogenic mechanism in neurodegenerative disorders such as Creutzfeldt–Jakob disease (CJD) and Alzheimer’s disease (AD). CJD is marked primarily by the buildup of misfolded prion protein (PrP^Sc) in brain, whereas the accrual of β-amyloid protein (Aβ) and tau protein are characteristic for AD. Prior studies have shown that the ATP-binding cassette transporter P-glycoprotein (P-gp) is a cellular efflux pump for Aβ, and that age-associated deficits in P-gp may be involved in the pathogenesis of Alzheimer’s disease. In the present study, we investigated the relationship between P-gp and idiopathic CJD, and found that CJD, like AD, is associated with a decrease in the expression of cerebrovascular P-gp. In some instances, Aβ and PrP deposits coexist in cases of CJD, suggesting the possibility of pathogenic interactions. Since there is, to date, no evidence that PrP itself is a substrate for P-gp, we hypothesize that the age-related deficits in P-gp could promote the accumulation of PrP^Sc either by promoting the buildup of Aβ (which could act as a seed for the aggregation of PrP^Sc), or by overloading the ubiquitin-proteasomal catabolic system, and thereby facilitating the accumulation of PrP. Alternatively, the loss of P-gp could be a non-specific response to neurodegenerative changes in the central nervous system. In either case, dysfunction of this critical toxin-elimination pathway in CJD and AD suggests that selectively increasing cerebrovascular P-gp function could open new therapeutic pathways for the prevention and/or treatment of a number of proteopathic disorders of the central nervous system.