Alzheimer's disease: a dysfunction of the amyloid precursor protein(1)

Mitochondrial Complex I and β-Amyloid Peptide Interplay in Alzheimer's Disease: A Critical Review of New and Old Little Regarded Findings

Alzheimer's disease (AD) is the most common neurodegenerative disorder and the main cause of dementia which is characterized by a progressive cognitive decline that severely interferes with daily activities of personal life. At a pathological level, it is characterized by the accumulation of abnormal protein structures in the brain-β-amyloid (Aβ) plaques and Tau tangles-which interfere with communication between neurons and lead to their dysfunction and death. In recent years, research on AD has highlighted the critical involvement of mitochondria-the primary energy suppliers for our cells-in the onset and progression of the disease, since mitochondrial bioenergetic deficits precede the beginning of the disease and mitochondria are very sensitive to Aβ toxicity. On the other hand, if it is true that the accumulation of Aβ in the mitochondria leads to mitochondrial malfunctions, it is otherwise proven that mitochondrial dysfunction, through the generation of reactive oxygen species, causes an increase in Aβ production, by initiating a vicious cycle: there is therefore a bidirectional relationship between Aβ aggregation and mitochondrial dysfunction. Here, we focus on the latest news-but also on neglected evidence from the past-concerning the interplay between dysfunctional mitochondrial complex I, oxidative stress, and Aβ, in order to understand how their interplay is implicated in the pathogenesis of the disease.

The amyloid cascade hypothesis: an updated critical review

Results from recent clinical trials of antibodies that target amyloid-β (Aβ) for Alzheimer's disease have created excitement and have been heralded as corroboration of the amyloid cascade hypothesis. However, while Aβ may contribute to disease, genetic, clinical, imaging and biochemical data suggest a more complex aetiology. Here we review the history and weaknesses of the amyloid cascade hypothesis in view of the new evidence obtained from clinical trials of anti-amyloid antibodies. These trials indicate that the treatments have either no or uncertain clinical effect on cognition. Despite the importance of amyloid in the definition of Alzheimer's disease, we argue that the data point to Aβ playing a minor aetiological role. We also discuss data suggesting that the concerted activity of many pathogenic factors contribute to Alzheimer's disease and propose that evolving multi-factor disease models will better underpin the search for more effective strategies to treat the disease.

Mitophagy Failure in APP and Tau Overexpression Model of Alzheimer's Disease

Mitochondrial alterations and oxidative stress are common features of Alzheimer's disease brain and peripheral tissues. Moreover, mitochondrial recycling process by autophagy has been found altered in the sporadic form of the disease. However, the contribution of the main proteins involved in this pathology such as amyloid-β protein precursor (AβPP) and tau needs to be achieved. With this aim, human unmodified fibroblasts were transduced with lentivectors encoding APP and Tau and treated with CCCP to study the mitophagy process. Both AβPP and tau separately increased autophagy flux mainly by improving degradation phase. However, in the specific case of mitophagy, labeling of mitochondria by PINK1 and PARK2 to be degraded by autophagy seemed reduced, which correlates with the long-term accumulation of mitochondria. Nevertheless, the combination of tau and AβPP was necessary to cause a mitophagy functional impairment reflected in the accumulation of depolarized mitochondria labeled by PINK1. The overexpression of Tau and APP recapitulates the mitophagy failure previously found in sporadic Alzheimer's disease.

The Role of Stress-Induced O-GlcNAc Protein Modification in the Regulation of Membrane Transport

O-linked N-acetylglucosamine (O-GlcNAc) is a posttranslational modification that is increasingly recognized as a signal transduction mechanism. Unlike other glycans, O-GlcNAc is a highly dynamic and reversible process that involves the addition and removal of a single N-acetylglucosamine molecule to Ser/Thr residues of proteins. UDP-GlcNAc—the direct substrate for O-GlcNAc modification—is controlled by the rate of cellular metabolism, and thus O-GlcNAc is dependent on substrate availability. Serving as a feedback mechanism, O-GlcNAc influences the regulation of insulin signaling and glucose transport. Besides nutrient sensing, O-GlcNAc was also implicated in the regulation of various physiological and pathophysiological processes. Due to improvements of mass spectrometry techniques, more than one thousand proteins were detected to carry the O-GlcNAc moiety; many of them are known to participate in the regulation of metabolites, ions, or protein transport across biological membranes. Recent studies also indicated that O-GlcNAc is involved in stress adaptation; overwhelming evidences suggest that O-GlcNAc levels increase upon stress. O-GlcNAc elevation is generally considered to be beneficial during stress, although the exact nature of its protective effect is not understood. In this review, we summarize the current data regarding the oxidative stress-related changes of O-GlcNAc levels and discuss the implications related to membrane trafficking.

Metabolic Syndrome as a Risk Factor for Alzheimer's Disease: Is Aβ a Crucial Factor in Both Pathologies?

SIGNIFICANCE

Recently, chronic degenerative diseases have become one of the main health problems worldwide. That is the case of Alzheimer's disease (AD) and metabolic syndrome (MetS), whose expression can be influenced by different risk factors. Recent Advances: In recent decades, it has been widely described that MetS increases the risk of cognitive impairment and dementia. MetS pathogenesis involves several vascular risk factors such as diabetes, dyslipidemia, hypertension, and insulin resistance (I/R).

CRITICAL ISSUES

Reported evidence shows that vascular risk factors are associated with AD, particularly in the development of protein aggregation, inflammation, oxidative stress, neuronal dysfunction, and disturbances in signaling pathways, with insulin receptor signaling being a common alteration between MetS and AD.

FUTURE DIRECTIONS

Insulin signaling has been involved in tau phosphorylation and amyloid β (Aβ) metabolism. However, it has also been demonstrated that Aβ oligomers can bind to insulin receptors, triggering their internalization, decreasing neuron responsiveness to insulin, and promoting insulin I/R. Thus, it could be argued that Aβ could be a convergent factor in the development of both pathologies. Antioxid. Redox Signal. 26, 542-560.

Chronic Traumatic Encephalopathy: Known Causes, Unknown Effects

Chronic traumatic encephalopathy (CTE) is a neuropathologic diagnosis typically made in human brains with a history of repetitive traumatic brain injury (rTBI). It remains unknown whether CTE occurs exclusively after rTBI, or whether a single TBI (sTBI) can cause CTE. Similarly, it is unclear whether impact (eg, motor vehicle accidents) and non-impact (eg, blasts) types of energy transfer trigger divergent or common pathologies. While it is established that a history of rTBI increases the risk of multiple neurodegenerative diseases (eg, dementia, parkinsonism, and CTE), the possible pathophysiologic and molecular mechanisms underlying these risks have yet to be elucidated.

SEPT8 modulates β-amyloidogenic processing of APP by affecting the sorting and accumulation of BACE1

Dysfunction and loss of synapses are early pathogenic events in Alzheimer's disease. A central step in the generation of toxic amyloid-β (Aβ) peptides is the cleavage of amyloid precursor protein (APP) by β-site APP-cleaving enzyme (BACE1). Here, we have elucidated whether downregulation of septin (SEPT) protein family members, which are implicated in synaptic plasticity and vesicular trafficking, affects APP processing and Aβ generation. SEPT8 was found to reduce soluble APPβ and Aβ levels in neuronal cells through a post-translational mechanism leading to decreased levels of BACE1 protein. In the human temporal cortex, we identified alterations in the expression of specific SEPT8 transcript variants in a manner that correlated with Alzheimer's-disease-related neurofibrillary pathology. These changes were associated with altered β-secretase activity. We also discovered that the overexpression of a specific Alzheimer's-disease-associated SEPT8 transcript variant increased the levels of BACE1 and Aβ peptides in neuronal cells. These changes were related to an increased half-life of BACE1 and the localization of BACE1 in recycling endosomes. These data suggest that SEPT8 modulates β-amyloidogenic processing of APP through a mechanism affecting the intracellular sorting and accumulation of BACE1.

Inheritance of steroid-independent male sexual behavior in male offspring of B6D2F1 mice

The importance of gonadal steroids in modulating male sexual behavior is well established. Individual differences in male sexual behavior, independent of gonadal steroids, are prevalent across a wide range of species, including man. However, the genetic mechanisms underlying steroid-independent male sexual behavior are poorly understood. A high proportion of B6D2F1 hybrid male mice demonstrates steroid-independent male sexual behavior (identified as "maters"), providing a mouse model that opens up avenues of investigation into the mechanisms regulating male sexual behavior in the absence of gonadal hormones. Recent studies have revealed several proteins that play a significant factor in regulating steroid-independent male sexual behavior in B6D2F1 male mice, including amyloid precursor protein (APP), tau, and synaptophysin. The specific goals of our study were to determine whether steroid-independent male sexual behavior was a heritable trait by determining if it was dependent upon the behavioral phenotype of the B6D2F1 sire, and whether the differential expression of APP, tau, and synaptophysin in the medial preoptic area found in the B6D2F1 sires that did and did not mate after gonadectomy was similar to those found in their male offspring. After adult B6D2F1 male mice were bred with C57BL/6J female mice, they and their male offspring (BXB1) were orchidectomized and identified as either maters or "non-maters". A significant proportion of the BXB1 maters was sired only from B6D2F1 maters, indicating that the steroid-independent male sexual behavior behavioral phenotype of the B6D2F1 hybrid males, when crossed with C57BL/6J female mice, is inherited by their male offspring. Additionally, APP, tau, and synaptophysin were elevated in in the medial preoptic area in both the B6D2F1 and BXB1 maters relative to the B6D2F1 and BXB1 non-maters, respectively, suggesting a potential genetic mechanism for the inheritance of steroid-independent male sexual behavior.

N-Terminal Extensions Retard Aβ42 Fibril Formation but Allow Cross-Seeding and Coaggregation with Aβ42

Amyloid β-protein (Aβ) sequence length variants with varying aggregation propensity coexist in vivo, where coaggregation and cross-catalysis phenomena may affect the aggregation process. Until recently, naturally occurring amyloid β-protein (Aβ) variants were believed to begin at or after the canonical β-secretase cleavage site within the amyloid β-protein precursor. However, N-terminally extended forms of Aβ (NTE-Aβ) were recently discovered and may contribute to Alzheimer's disease. Here, we have used thioflavin T fluorescence to study the aggregation kinetics of Aβ42 variants with N-terminal extensions of 5-40 residues, and transmission electron microscopy to analyze the end states. We find that all variants form amyloid fibrils of similar morphology as Aβ42, but the half-time of aggregation (t1/2) increases exponentially with extension length. Monte Carlo simulations of model peptides suggest that the retardation is due to an underlying general physicochemical effect involving reduced frequency of productive molecular encounters. Indeed, global kinetic analyses reveal that NTE-Aβ42s form fibrils via the same mechanism as Aβ42, but all microscopic rate constants (primary and secondary nucleation, elongation) are reduced for the N-terminally extended variants. Still, Aβ42 and NTE-Aβ42 coaggregate to form mixed fibrils and fibrils of either Aβ42 or NTE-Aβ42 catalyze aggregation of all monomers. NTE-Aβ42 monomers display reduced aggregation rate with all kinds of seeds implying that extended termini interfere with the ability of monomers to nucleate or elongate. Cross-seeding or coaggregation may therefore represent an important contribution in the in vivo formation of assemblies believed to be important in disease.

Synaptic dysfunction and septin protein family members in neurodegenerative diseases

Cognitive decline and disease progression in different neurodegenerative diseases typically involves synaptic dysfunction preceding the neuronal loss. The synaptic dysfunction is suggested to be caused by imbalanced synaptic plasticity i.e. enhanced induction of long-term depression and concomitantly decreased long-term potentiation accompanied with excess stimulation of extrasynaptic N-Methyl-D-aspartate (NMDA) receptors due to various disturbances in pre- and postsynaptic sites. Recent research has identified neurodegenerative disease-related changes in protein accumulation and aggregation, gene expression, and protein functions, which may contribute to imbalanced synaptic function. Nevertheless, a comprehensive understanding of the mechanisms regulating synaptic plasticity in health and disease is still lacking and therefore characterization of new candidates involved in these mechanisms is needed. Septins, a highly conserved group of guanosine-5'-triphosphate (GTP)-binding proteins, show high neuronal expression and are implicated in the regulation of synaptic vesicle trafficking and neurotransmitter release. In this review, we first summarize the evidence how synaptic dysfunction is related to the pathogenesis of Alzheimer's, Parkinson's and Huntington's disease and frontotemporal lobar degeneration. Then, we discuss different aspects of the potential involvement of the septin family members in the regulation of synaptic function in relation to the pathogenesis of neurodegenerative diseases.

The full-length form of the Drosophila amyloid precursor protein is involved in memory formation

The APP plays a central role in AD, a pathology that first manifests as a memory decline. Understanding the role of APP in normal cognition is fundamental in understanding the progression of AD, and mammalian studies have pointed to a role of secreted APPα in memory. In Drosophila, we recently showed that APPL, the fly APP ortholog, is required for associative memory. In the present study, we aimed to characterize which form of APPL is involved in this process. We show that expression of a secreted-APPL form in the mushroom bodies, the center for olfactory memory, is able to rescue the memory deficit caused by APPL partial loss of function. We next assessed the impact on memory of the Drosophila α-secretase kuzbanian (KUZ), the enzyme initiating the nonamyloidogenic pathway that produces secreted APPLα. Strikingly, KUZ overexpression not only failed to rescue the memory deficit caused by APPL loss of function, it exacerbated this deficit. We further show that in addition to an increase in secreted-APPL forms, KUZ overexpression caused a decrease of membrane-bound full-length species that could explain the memory deficit. Indeed, we observed that transient expression of a constitutive membrane-bound mutant APPL form is sufficient to rescue the memory deficit caused by APPL reduction, revealing for the first time a role of full-length APPL in memory formation. Our data demonstrate that, in addition to secreted APPL, the noncleaved form is involved in memory, raising the possibility that secreted and full-length APPL act together in memory processes.

Meserine, a novel carbamate AChE inhibitor, ameliorates scopolamine-induced dementia and alleviates amyloidogenesis of APP/PS1 transgenic mice

AIMS

To investigate whether Meserine, a novel phenylcarbamate derivative of (-)-meptazinol, possesses beneficial activities against cholinergic deficiency and amyloidogenesis, the two major pathological characteristics of Alzheimer's disease (AD).

METHODS

Ellman's assay and Morris water maze were used to detect acetylcholinesterase (AChE) activity and evaluate spatial learning and memory ability, respectively. Both high content screening and Western blotting were carried out to detect β-amyloid precursor protein (APP), while RT-PCR and ELISA were conducted to detect APP-mRNA and β-amyloid peptide (Aβ).

RESULTS

In scopolamine-induced dementia mice, Meserine (1 mg/kg, i.p.) significantly ameliorated spatial learning and memory deficits, which was consistent with its in vitro inhibitory ability against AChE (recombinant human AChE, IC50 = 274 ± 49 nM). Furthermore, Meserine (7.5 mg/kg) injected intraperitoneally once daily for 3 weeks lowered APP level by 28% and Aβ42 level by 42% in APP/PS1 transgenic mouse cerebrum. This APP modulation action might be posttranscriptional, as Meserine reduced APP by about 30% in SH-SY5Y-APP695 cells but did not alter APP-mRNA level. And both APP and Aβ42 lowering action of Meserine maintained longer than that of rivastigmine.

CONCLUSION

Meserine executes dual actions against cholinergic deficiency and amyloidogenesis and provides a promising lead compound for symptomatic and modifying therapy of AD.

The Drosophila homologue of the amyloid precursor protein is a conserved modulator of Wnt PCP signaling

Wnt Planar Cell Polarity (PCP) signaling is a universal regulator of polarity in epithelial cells, but it regulates axon outgrowth in neurons, suggesting the existence of axonal modulators of Wnt-PCP activity. The Amyloid precursor proteins (APPs) are intensely investigated because of their link to Alzheimer's disease (AD). APP's in vivo function in the brain and the mechanisms underlying it remain unclear and controversial. Drosophila possesses a single APP homologue called APP Like, or APPL. APPL is expressed in all neurons throughout development, but has no established function in neuronal development. We therefore investigated the role of Drosophila APPL during brain development. We find that APPL is involved in the development of the Mushroom Body αβ neurons and, in particular, is required cell-autonomously for the β-axons and non-cell autonomously for the α-axons growth. Moreover, we find that APPL is a modulator of the Wnt-PCP pathway required for axonal outgrowth, but not cell polarity. Molecularly, both human APP and fly APPL form complexes with PCP receptors, thus suggesting that APPs are part of the membrane protein complex upstream of PCP signaling. Moreover, we show that APPL regulates PCP pathway activation by modulating the phosphorylation of the Wnt adaptor protein Dishevelled (Dsh) by Abelson kinase (Abl). Taken together our data suggest that APPL is the first example of a modulator of the Wnt-PCP pathway specifically required for axon outgrowth.

Amyloid beta precursor protein and prion protein have a conserved interaction affecting cell adhesion and CNS development

Genetic and biochemical mechanisms linking onset or progression of Alzheimer Disease and prion diseases have been lacking and/or controversial, and their etiologies are often considered independent. Here we document a novel, conserved and specific genetic interaction between the proteins that underlie these diseases, amyloid-β precursor protein and prion protein, APP and PRP, respectively. Knockdown of APP and/or PRNP homologs in the zebrafish (appa, appb, prp1, and prp2) produces a dose-dependent phenotype characterized by systemic morphological defects, reduced cell adhesion and CNS cell death. This genetic interaction is surprisingly exclusive in that prp1 genetically interacts with zebrafish appa, but not with appb, and the zebrafish paralog prp2 fails to interact with appa. Intriguingly, appa & appb are largely redundant in early zebrafish development yet their abilities to rescue CNS cell death are differentially contingent on prp1 abundance. Delivery of human APP or mouse Prnp mRNAs rescue the phenotypes observed in app-prp-depleted zebrafish, highlighting the conserved nature of this interaction. Immunoprecipitation revealed that human APP and PrP(C) proteins can have a physical interaction. Our study reports a unique in vivo interdependence between APP and PRP loss-of-function, detailing a biochemical interaction that considerably expands the hypothesized roles of PRP in Alzheimer Disease.

Different clinical phenotypes in siblings with a presenilin-1 P264L mutation

BACKGROUND

Mutations in the presenilin-1 gene (PSEN1) have been identified in autosomal dominant early-onset cases of Alzheimer's disease (AD).

AIMS

To investigate different clinical phenotypes of siblings possessing the same heterozygous P264L mutation in the PSEN1 gene.

METHODS

We evaluated clinical features, neuroimaging results, and neuropsychological examinations. The PSEN1 gene and other dementia-related gene mutations were screened.

RESULTS

We clinically diagnosed the proband as atypical AD with frontotemporal dementia features and diagnosed the elder brother of the proband as typical AD, based on neuropsychological symptoms and a brain imaging examination including amyloid imaging data. A heterozygous P264L mutation in the PSEN1 gene was identified in both siblings.

CONCLUSION

This study is one of few reports of AD siblings possessing the same mutation but exhibiting different clinical phenotypes in a Japanese family possessing a P264L mutation in the PSEN1 gene. The current results suggest that unknown modifiers, including both genetic and epigenetic factors, may alter the pathological and clinical phenotypes of a genetically predetermined disease.

In vitro screening on amyloid precursor protein modulation of plants used in Ayurvedic and traditional Chinese medicine for memory improvement

ETHNOPHARMACOLOGICAL RELEVANCE

The 15 herbs for the screening have been traditionally used in Ayurvedic medicine or in Traditional Chinese medicine (TCM) for the treatment of cognitive disorders clinically.

AIM OF THE STUDY

Fifteen plant species were investigated for the inhibition of amyloid peptide (Aβ) production and modulation of amyloid precursor protein (APP) processing.

MATERIALS AND METHODS

The selected plants were extracted successively with 70% ethyl alcohol and absolute alcohol concentrated with rotary evaporation then lyophilized. Using a mouse neuroblastoma cells expressing Swedish APP (N2a-SweAPP), MTT assay was performed to determine the toxicity concentration of each herbal extract. In order to evaluate the activity of ethanol extracts on Aβ inhibition, the N2a-SweAPP cells were treated with a high and low dosage of different extracts for 24h, Aβs levels in the supernatant of conditioned media were assessed by ELISA. The most active extracts were then subjected to test the effect on APP modulation in N2a-SweAPP cells by determining their effect on sAPPα and sAPPβ through western blot analysis.

RESULTS

Among the screened herbal extracts, only Polygonum multiflorum Thunb. (root) and Convolvulus pluricaulis Choisy. (leaves) showed profound inhibition of Aβ production. MTT assay demonstrated that the anti-Aβ effect of these extracts was not a sequential consequence of their cytotoxicity. The extract of Polygonum multiflorum Thunb. (root) could reduce Aβ production only through APP modulation, which was exhibited together with the up-regulation of sAPPα and down-regulation of sAPPβ.

CONCLUSION

The results show that extract of Polygonum multiflorum Thunb. (root) can lower Aβ generation by modulating APP processing in the N2a-SwedAPP cell line. These results corroborate the traditional use of Polygonum multiflorum Thunb. (root) for the treatment of cognitive disorders including Alzheimer's disease (AD).

Is TNF a link between aging-related reproductive endocrine dyscrasia and Alzheimer's disease?

This commentary addresses a novel mechanism by which aging-related changes in reproductive hormones could mediate their action in the brain. It presents the evidence that dyotic endocrine signals modulate the expression of tumor necrosis factor (TNF) and related cytokines, and that these cytokines are a functionally important downstream link mediating neurodegeneration and dysfunction. This convergence of dyotic signaling on TNF-mediated degeneration and dysfunction has important implications for understanding the pathophysiology of AD, stroke, and traumatic brain disease, and also for the treatment of these diseases.

Ageing in people with autistic spectrum disorder

BACKGROUND

Although autism in children and in adults attracts attention with respect to clinical and research needs, autism in the older individuals has not been considered to any degree. We review the evidence for urgently addressing the question of ageing in people with autistic spectrum disorder (ASD), focusing on those with disability.

METHODS

Perspectives are reviewed in relation to demographics, experiences of relatives or carers, anticipated residential care needs, requirement for specifically designed cognitive assessment tools and importance of initiating new brain ageing research initiatives in this area.

RESULTS

With escalating numbers of ASD individuals with disability reaching old age, provision of care is the paramount issue that is only beginning to be addressed in a few European communities and in the USA. How ageing affects cognition in such individuals as they reach an age no longer consistent with parental care is unknown, lacking any published evidence, and there is a clear need to design cognitive and behavioural assessment tools appropriate to ageing in ASD individuals with disability, as was the case with respect to dementia as a whole. Although there is a growing body of evidence on pathological, imaging, neuropharmacological and other key brain abnormalities in ASD, these are, to date, confined to children and young (only rarely to middle aged) adults.

CONCLUSIONS

The need for new initiatives in research into ageing in ASD is urgent. Apart from a growing care crisis, the prospect of understanding brain ageing in this population may bring potential rewards beyond immediate clinical need given the precedent of Down syndrome.

Can cytogenetics explain the possible association between exposure to extreme low-frequency magnetic fields and Alzheimer's disease?

Recently, a number of epidemiological studies have suggested that occupational as well as residential exposure to extreme low-frequency electromagnetic fields (ELF-EMFs) may be a risk factor for Alzheimer's disease. This is not proven yet and there are no known biological mechanisms to explain this alleged association. Alzheimer's disease is characterized by a number of events that have, at least partially, a genetic origin. In particular, trisomy of chromosomes 17 and 21 seems to be involved. Overall ELF-EMFs have not been identified as genotoxic agents, but there are some papers in the scientific literature that indicate that they may enhance the effects of agents that are known to induce mutations or tumors. There are also some indications that ELF-EMFs may induce aneuploïdy. This opens some perspectives for investigating the alleged association between ELF-EMFs and Alzheimer's. This paper reviews the possibility of a cytogenetic association between the electromagnetic fields and Alzheimer's disease.

Surface expression and limited proteolysis of ADAM10 are increased by a dominant negative inhibitor of dynamin

Background

The amyloid precursor protein (APP) is cleaved by β- and γ-secretases to generate toxic amyloid β (Aβ) peptides. Alternatively, α-secretases cleave APP within the Aβ domain, precluding Aβ formation and releasing the soluble ectodomain, sAPPα. We previously showed that inhibition of the GTPase dynamin reduced APP internalization and increased release of sAPPα, apparently by prolonging the interaction between APP and α-secretases at the plasma membrane. This was accompanied by a reduction in Aβ generation. In the present study, we investigated whether surface expression of the α-secretase ADAM (a disintegrin and metalloprotease)10 is also regulated by dynamin-dependent endocytosis.

Results

Transfection of human embryonic kidney (HEK) cells stably expressing M3 muscarinic receptors with a dominant negative dynamin I mutant (dyn I K44A), increased surface expression of both immature, and mature, catalytically active forms of co-expressed ADAM10. Surface levels of ADAM10 were unaffected by activation of protein kinase C (PKC) or M3 receptors, indicating that receptor-coupled shedding of the ADAM substrate APP is unlikely to be mediated by inhibition of ADAM10 endocytosis in this cell line. Dyn I K44A strongly increased the formation of a C-terminal fragment of ADAM10, consistent with earlier reports that the ADAM10 ectodomain is itself a target for sheddases. The abundance of this fragment was increased in the presence of a γ-secretase inhibitor, but was not affected by M3 receptor activation. The dynamin mutant did not affect the distribution of ADAM10 and its C-terminal fragment between raft and non-raft membrane compartments.

Conclusions

Surface expression and limited proteolysis of ADAM10 are regulated by dynamin-dependent endocytosis, but are unaffected by activation of signaling pathways that upregulate shedding of ADAM substrates such as APP. Modulation of ADAM10 internalization could affect cellular behavior in two ways: by altering the putative signaling activity of the ADAM10 C-terminal fragment, and by regulating the biological function of ADAM10 substrates such as APP and N-cadherin.

Dysregulation of Ca2+ signaling in astrocytes from mice lacking amyloid precursor protein

The relationship between altered metabolism of the amyloid-β precursor protein (APP) and Alzheimer's disease is well established but the physiological roles of APP still remain unclear. Here, we studied Ca(2+) signaling in primary cultured and freshly dissociated cortical astrocytes from APP knockout (KO) mice and from Tg5469 mice overproducing by five- to sixfold wild-type APP. Resting cytosolic Ca(2+) (measured with fura-2) was not altered in cultured astrocytes from APP KO mice. The stored Ca(2+) evaluated by measuring peak amplitude of cyclopiazonic acid [CPA, endoplasmic reticulum (ER) Ca(2+) ATPase inhibitor]-induced Ca(2+) transients in Ca(2+)-free medium was significantly smaller in APP KO astrocytes than in wild-type cells. Store-operated Ca(2+) entry (SOCE) activated by ER Ca(2+) store depletion with CPA was also greatly reduced in APP KO astrocytes. This reflected a downregulated expression in APP KO astrocytes of TRPC1 (C-type transient receptor potential) and Orai1 proteins, essential components of store-operated channels (SOCs). Indeed, silencer RNA (siRNA) knockdown of Orai1 protein expression in wild-type astrocytes significantly attenuated SOCE. SOCE was also essentially reduced in freshly dissociated APP KO astrocytes. Importantly, knockdown of APP with siRNA in cultured wild-type astrocytes markedly attenuated ATP- and CPA-induced ER Ca(2+) release and extracellular Ca(2+) influx. The latter correlated with downregulation of TRPC1. Overproduction of APP in Tg5469 mice did not alter, however, the stored Ca(2+) level, SOCE, and expression of TRPC1/4/5 in cultured astrocytes from these mice. The data demonstrate that the functional role of APP in astrocytes involves the regulation of TRPC1/Orai1-encoded SOCs critical for Ca(2+) signaling.

Drosophila amyloid precursor protein-like is required for long-term memory

The amyloid precursor protein (APP) plays an important role in Alzheimer's disease (AD), a progressive neurodegenerative pathology that first manifests as a decline of memory. While the main hypothesis for AD pathology centers on the proteolytic processing of APP, very little is known about the physiological function of the APP protein in the adult brain. Likewise, whether APP loss of function contributes to AD remains unclear. Drosophila has been used extensively as a model organism to study neuronal function and pathology. In addition, many of the molecular mechanisms underlying memory are thought to be conserved from flies to mammals, prompting us to study the function of APPL, the fly APP ortholog, during associative memory. It was previously shown that APPL expression is highly enriched in the mushroom bodies (MBs), a specialized brain structure involved in olfactory memory. We analyzed memory in flies in which APPL expression has been silenced specifically and transiently in the adult MBs. Our results show that in adult flies, APPL is not required for learning but is specifically involved in long-term memory, a long lasting memory whose formation requires de novo protein synthesis and is thought to require synaptic structural plasticity. These data support the hypothesis that disruption of normal APP function may contribute to early AD cognitive impairment.

The hAPP-YAC transgenic model has elevated UPS activity in the frontal cortex similar to Alzheimer's disease and Down's syndrome

The ubiquitin-proteasome system (UPS) is critical for handling the intra-cellular load of abnormal and misfolded proteins in several neurodegenerative diseases. First, to determine the effects of the over-expression of human amyloid precursor protein (hAPP) on UPS, we measured proteasome activities using fluorescent substrates in the frontal cortex of hAPP-yeast artificial chromosome (YAC) transgenic (tg) mice (R1.40, hemizygous; Lamb, Nat Genet, 9, 4; 1995). Chymotrypsin and PGPH-like activities of proteasome were increased in frontal cortex of hAPP-YAC tg mice. These proteasome activities (both chymotrypsin and PGPH-like) were further increased by cholinergic stimulation in littermate control mice, but not in hAPP-YAC tg mice. Nerve growth factor (NGF) levels were decreased by hAPP over-expression in the frontal cortex and hippocampus of hAPP-YAC tg mice, and further decreased by M1 agonist treatment in the hippocampus of littermate control and hAPP-YAC tg mice. Interestingly, the frontal cortex (BA9 area) of Alzheimer's disease (AD) patients (Stage 3, n=11) and Down's syndrome (DS) patients (n=9) showed similar up-regulation of the UPS activities to those seen in hAPP-YAC tg mice. M1 agonist stimulation increased the activities of α-secretase, which were down-regulated by hAPP over-expression in the frontal cortex of hAPP-YAC tg mice. These results demonstrate that (i) hAPP-YAC tg mice have an up-regulation in the frontal cortex of the UPS similar to AD and DS patients; (ii) muscarinic stimulation increase UPS activities, increase secreted APP (APPs) levels, and decrease amyloid beta 42/40 ratio only in littermate controls, but not in hAPP-YAC tg mice. Taken together, these results suggest that both the adaptive reactions in the proteostatic network and pathological changes in AD and DS need to be considered in the future potential therapeutics.

Aspects of beta-amyloid as a biomarker for Alzheimer's disease

Alzheimer's disease is an age-related neurodegenerative disorder that results in progressive cognitive impairment and death. The accumulation of beta-amyloid (Abeta) in specific brain regions is believed by many to represent the earliest event in the pathogenesis of the disease. Here, we review the key aspects of Abeta as a biomarker for Alzheimer's disease, including the pathogenicity of Abeta, the possible biological functions of its precursor protein, the Abeta metabolism and homeostasis, the diagnostic performance of different Abeta assays in different settings and the potential usefulness of Abeta as a surrogate marker for treatment efficacy in clinical trials of novel Abeta-targeting drugs against Alzheimer's disease.

The APOE-491 A/T promoter polymorphism effect on cognitive profile of Alzheimer's patients

Alzheimer's Disease (AD) is a neurodegenerative disorder with a complex aetiology displayed by multiple pathogenic factors. The APOE varepsilon4 allele represents the only established genetic risk factor for sporadic AD; in addition, previous findings on three single nucleotide polymorphisms (SNPs) located on the APOE promoter region, have led to a growing interest in their potential role in AD pathogenesis. The -491 A/T promoter polymorphism has been the one most frequently shown to be associated with AD, as it influences the APOE coding region transcription. The aim of this study was to evaluate the possible effect of the -491 A/T polymorphism on the cognitive profile of sporadic AD patients with a disease severity ranging from mild to moderate. Our results showed that patients carrying the -491 AA genotype had poorer cognitive performances than the -491 AT ones, statistically significant in demanding tests of visual attention, especially for the late-onset AD (LOAD). No further differences on cognitive profile were observed when stratifying AA and AT patients according to their APOE genotype. These results suggest a possible functional effect of the -491 A/T promoter on the neuropsychological performances of AD. This role seems to be independent of APOE genotype. In fact the effect of -491 A/T occurs predominantly on attention while the APOE varepsilon4 allele mainly affects memory performances. According to the biological effect exerted on APOE transcription, the -491 A/T polymorphism could be considered a disease modifier more than a risk factor for sporadic AD.

APP transgenic mice: the effect of active and passive immunotherapy in cognitive tasks

Various immunotherapy strategies for APP transgenic mice have emerged in recent years. Specifically, active immunization with beta-amyloid (A beta) or passive immunization with anti-A beta-antibodies in APP transgenic mice has appeared most promising. Recent studies have shown that treatment of APP transgenic mice either with A beta(40/42) or A beta-specific antibodies can have beneficial effects in cognitive tasks. Active as well as passive immunization have been shown to affect spatial, non-spatial, emotional and object-related learning and memory. Such effects can be observed when treatments are applied prophylactically (before apparent A beta pathology) or therapeutically (after the development of A beta pathology) in APP transgenic mice. This review focuses on such cognitive outcomes of different active and passive immunization strategies in APP transgenic mice.

Humanin inhibits neuronal cell death by interacting with a cytokine receptor complex or complexes involving CNTF receptor alpha/WSX-1/gp130

Humanin (HN) inhibits neuronal death induced by various Alzheimer's disease (AD)-related insults via an unknown receptor on cell membranes. Our earlier study indicated that the activation of STAT3 was essential for HN-induced neuroprotection, suggesting that the HN receptor may belong to the cytokine receptor family. In this study, a series of loss-of-function tests indicated that gp130, the common subunit of receptors belonging to the IL-6 receptor family, was essential for HN-induced neuroprotection. Overexpression of ciliary neurotrophic factor receptor alpha (CNTFR) and/or the IL-27 receptor subunit, WSX-1, but not that of any other tested gp130-related receptor subunit, up-regulated HN binding to neuronal cells, whereas siRNA-mediated knockdown of endogenous CNTFR and/or WSX-1 reduced it. These results suggest that both CNTFR and WSX-1 may be also involved in HN binding to cells. Consistent with these results, loss-of-functions of CNTFR or WSX-1 in neuronal cells nullified their responsiveness to HN-mediated protection. In vitro-reconstituted binding assays showed that HN, but not the other control peptide, induced the hetero-oligomerization of CNTFR, WSX-1, and gp130. Together, these results indicate that HN protects neurons by binding to a complex or complexes involving CNTFR/WSX-1/gp130.

Overexpression of wild-type human APP in mice causes cognitive deficits and pathological features unrelated to Abeta levels

Transgenic mice expressing mutant human amyloid precursor protein (APP) develop an age-dependent amyloid pathology and memory deficits, but no overt neuronal loss. Here, in mice overexpressing wild-type human APP (hAPP(wt)) we found an early memory impairment, particularly in the water maze and to a lesser extent in the object recognition task, but beta-amyloid peptide (Abeta(42)) was barely detectable in the hippocampus. In these mice, hAPP processing was basically non-amyloidogenic, with high levels of APP carboxy-terminal fragments, C83 and APP intracellular domain. A tau pathology with an early increase in the levels of phosphorylated tau in the hippocampus, a likely consequence of enhanced ERK1/2 activation, was also observed. Furthermore, these mice presented a loss of synapse-associated proteins: PSD95, AMPA and NMDA receptor subunits and phosphorylated CaMKII. Importantly, signs of neurodegeneration were found in the hippocampal CA1 subfield and in the entorhinal cortex that were associated to a marked loss of MAP2 immunoreactivity. Conversely, in mice expressing mutant hAPP, high levels of Abeta(42) were found in the hippocampus, but no signs of neurodegeneration were apparent. The results support the notion of Abeta-independent pathogenic pathways in Alzheimer's disease.

A capillary electrophoresis method for evaluation of Abeta proteolysis in vitro

According to the amyloid hypothesis, Abeta peptides are neurotoxic and underlie development and progression of Alzheimer's disease (AD). Multiple Abeta clearance mechanisms, including destruction of the peptides by proteolytic enzymes, are hypothesized to regulate physiological Abeta peptide levels. The insulin-degrading enzyme (IDE) is considered one of the predominant enzymes having Abeta degrading activity. Despite its putative role in protecting against AD, relatively few methods exist for studying IDE activity in vitro. We report the application of capillary electrophoresis (CE) as a novel method for evaluating IDE-mediated Abeta 1-40 proteolysis. This method employs chemically unmodified substrates that are readily obtained from commercial sources. It involves minimal sample preparation, and requires no specialized equipment beyond a CE instrument equipped with a standard fused silica capillary. In the present analysis, we demonstrate that this CE-based method is amenable to kinetic analysis, and show that IDE-mediated Abeta proteolysis is significantly and disproportionately inhibited in the presence of insulin, an alternative IDE substrate.

Targeted proteomics in Alzheimer's disease: focus on amyloid-beta

Diagnosis and monitoring of sporadic Alzheimer's disease (AD) have long depended on clinical examination of individuals with end-stage disease. However, upcoming anti-AD therapies are optimally initiated when individuals show very mild signs of neurodegeneration. There is a developing consensus for cerebrospinal fluid amyloid-beta (Abeta) as a core biomarker for the mild cognitive impairment stage of AD. Abeta is directly involved in the pathogenesis of AD or tightly correlated with other primary pathogenic factors. It is produced from amyloid precursor protein (APP) by proteolytic processing that depends on the beta-site APP-cleaving enzyme 1 and the gamma-secretase complex, and is degraded by a broad range of proteases. This review summarizes targeted proteomic studies of Abeta in biological fluids and identifies clinically useful markers of disrupted Abeta homeostasis in AD. The next 5 years will see a range of novel assays developed on the basis of these results. From a longer perspective, establishment of the most effective combinations of different biomarkers and other diagnostic modalities may be foreseen.

Protein expression of BACE1, BACE2 and APP in Down syndrome brains

Down syndrome (DS) is the most common human chromosomal abnormality caused by an extra copy of chromosome 21. The phenotype of DS is thought to result from overexpression of a gene or genes located on the triplicated chromosome or chromosome region. Several reports have shown that the neuropathology of DS comprises developmental abnormalities and Alzheimer-like lesions such as senile plaques. A key component of senile plaques is amyloid beta-peptide which is generated from the amyloid precursor protein (APP) by sequential action of beta-secretases (BACE1 and BACE2) and gamma-secretase. While BACE1 maps to chromosome 11, APP and BACE2 are located on chromosome 21. To challenge the gene dosage effect and gain insight into the expressional relation between beta-secretases and APP in DS brain, we evaluated protein expression levels of BACE1, BACE2 and APP in fetal and adult DS brain compared to controls. In fetal brain, protein expression levels of BACE2 and APP were comparable between DS and controls. BACE1 was increased, but did not reach statistical significance. In adult brain, BACE1 and BACE2 were comparable between DS and controls, but APP was significantly increased. We conclude that APP overexpression seems to be absent during the development of DS brain up to 18-19 weeks of gestational age. However, its overexpression in adult DS brain could lead to disturbance of normal function of APP contributing to neurodegeneration. Comparable expression of BACE1 and BACE2 speaks against the hypothesis that increased beta-secretase results in (or even underlies) increased production of amyloidogenic A beta fragments. Furthermore, current data indicate that the DS phenotype cannot be fully explained by simple gene dosage effect.

Neuronal binucleation in Alzheimer disease hippocampus

AIMS

The literature and teachings instruct that neurones in the adult brain are fully differentiated, quiescent cells that never divide. Somewhat surprisingly, and counter to such dogma, susceptible neurones in Alzheimer disease display an activated cell cycle phenotype. However, whether this leads to a coordinated procession through the cell cycle is unclear, particularly whether neurones enter anaphase and beyond. To begin to address this issue, in this study we sought to determine whether nuclear division occurs in these neurones.

METHODS

We examined a series of 101 archived, routinely stained hippocampal sections collected at post mortem for neuropathological evaluation for evidence of neuronal binucleation.

RESULTS

We report for the first time, binucleated neurones within the hippocampus in cases of Alzheimer disease but not in control cases (P < 0.05).

CONCLUSIONS

While a relatively rare event, occurring once every 20,000 neurones, this morphological evidence that neuronal cells within the cortical regions of the adult human brain in Alzheimer disease contain two nuclei supports the hypothesis that neuronal cells can re-enter into a coordinated cell cycle that culminates in nuclear division.

Protocols for studying adult neurogenesis: insights and recent developments

The first evidence that neurogenesis occurs in the adult brain was reported in rodents in the early 1960s, using [(3)H]-thymidine autoradiography. In the 1980s and 90s, the advent of new techniques and protocols for studying cell proliferation in situ, and particularly bromodeoxyuridine labeling, helped to confirm that neurogenesis occurs in the adult brain and neural stem cells reside in the adult CNS, including in humans. Bromodeoxyuridine labeling is currently the method most commonly used for studying neurogenesis in the adult brain. However, this procedure is not without limitations, and controversies. In this article, I will review recent protocols for studying adult neurogenesis, particularly new protocols for studying cell kinetics and cell proliferative history, using halopyrimidines. I will review these techniques, and discuss their implications for the field of adult neurogenesis.

Heparan sulfate mediates neuroprotection from degeneration in experimental glutaric aciduria

Glutaric aciduria type 1 (GA1) is a childhood metabolic disorder associated with crises that lead to striatal necrosis. Although the disorder can be controlled with diet, there is no current treatment to ameliorate the neurodegeneration following a metabolic crisis. We hypothesized that heparan sulfate (HS) administration would stimulate neural stem cell proliferation by dimerizing with FGF-2 and binding to the FGF-2 receptor on neural stem cells, thus enhancing the number of newly generated neurons to repair damage following a metabolic crisis. In addition, FGF-2 is known to exert neuroprotective effects independent of neurogenesis, so HS may also have neuroprotective activities. To test these hypotheses, ibotenic acid was injected into the striatum of adult mice, mimicking the metabolic crisis and damage caused by glutaric aciduria. Daily doses of HS and bromodeoxyuridine (BrdU) or BrdU alone were administered starting 1 day after the ibotenic acid lesion. BrdU was used to label dividing cells. Fluorescent immunohistochemistry was used to quantify the lesion size and evaluate the phenotype of BrdU-positive cells. Intrastriatal administration of ibotenic acid resulted in a substantial striatal lesion that occupied 18.5% of the ipsilateral brain hemisphere. In contrast, animals treated with HS exhibited a lesion volume representing <1% of the ipsilateral brain hemisphere (ANOVA; p < 0.0001). Increased neurogenesis, however, was not observed in this group. These results suggest that HS administration 2 days after a "metabolic crisis" can ameliorate brain injury in an animal model of GA1. The neuroprotective mechanisms of HS, however, remain to be elucidated but may exert their actions indirectly through binding with FGF-2.

Calnuc binds to Alzheimer's beta-amyloid precursor protein and affects its biogenesis

Calnuc, a Golgi calcium binding protein, plays a key role in the constitution of calcium storage. Abnormal calcium homeostasis has been linked to Alzheimer's disease (AD). Excessive production and/or accumulation of beta-amyloid (Abeta) peptides that are proteolytically derived from the beta-amyloid precursor protein (APP) have been linked to the pathogenesis of AD. APP has also been indicated to play multiple physiological functions. In this study, we demonstrate that calnuc interacts with APP through direct binding to the carboxyl-terminal region of APP, possibly in a calcium-sensitive manner. Immunofluorescence study revealed that the two proteins co-localize in the Golgi in both cultured cells and mouse brains. Over-expression of calnuc in neuroblastoma cells significantly reduces the level of endogenous APP. Conversely, down-regulation of calnuc by siRNA increases cellular levels of APP. Additionally, we show that over-expression of calnuc down-regulates the APP mRNA level and inhibits APP biosynthesis, which in turn results in a parallel reduction of APP proteolytic metabolites, sAPP, CTFs and Abeta. Furthermore, we found that the level of calnuc was significantly decreased in the brain of AD patients as compared with that of age-matched non-AD controls. Our results suggest a novel function of calnuc in modulating the levels of APP and its proteolytic metabolites, which may further affect the patho/physiological functions of APP including AD pathogenesis.

Estrogen, beta-amyloid metabolism/trafficking, and Alzheimer's disease

Estrogen plays key regulatory roles in a variety of biological actions besides its classic function as a sex hormone. Recently, estrogen has been linked to neurodegenerative diseases including Alzheimer's disease (AD) and Parkinson's disease (PD). Several lines of evidence support the notion that brain estrogen exerts neuroprotective effects against various types of neurotoxicity in different cellular and animal models. Despite some controversies, estrogen replacement therapy (ERT) at an early stage, especially when given prior to menopause, has been shown to reduce the risk of AD in postmenopausal women. In addition, multiple lines of evidence have proven the neuroprotective effects of estrogen, such as enhancing neurotrophin signaling and synaptic activities pertinent to memory functions and protecting neurons against oxidative injuries and beta-amyloid toxicity; the latter is widely accepted as the prime culprit known to trigger the pathogenesis of AD. Here we will summarize our findings that estrogen decreased generation and secretion of beta-amyloid peptides in cultured cells and primary neurons and that administration of estrogen in estrogen-deprived mice reversed the elevated levels of brain Abeta. We will also discuss the molecular and cellular mechanisms underlying estrogen's effects on Abeta metabolism, which is highlighted by our demonstration that estrogen increases intracellular trafficking of beta-amyloid precursor protein (betaAPP) and hence reduces maximal Abeta generation within the trans-Golgi network (TGN), a subcellular compartment in which APP is known to be cleaved by the secretase enzymes to generate Abeta.

The amyloid precursor protein potentiates CHOP induction and cell death in response to ER Ca2+ depletion

Here we investigated the role of the amyloid precursor protein (APP) in regulation of Ca(2+) store depletion-induced neural cell death. Ca(2+) store depletion from the endoplasmic reticulum (ER) was induced by the SERCA (Sarco/Endoplasmic Reticulum Calcium ATPase) inhibitor thapsigargin which led to a rapid induction of the unfolded protein response (UPR) and a delayed activation of executioner caspases in the cultures. Overexpression of APP potently enhanced cytosolic Ca(2+) levels and cell death after ER Ca(2+) store depletion in comparison to vector-transfected controls. GeneChip and RT-PCR analysis revealed that the expression of classical UPR chaperone genes was not altered by overexpression of APP. Interestingly, the induction of the ER stress-responsive pro-apoptotic transcription factor CHOP was significantly upregulated in APP-overexpressing cells in comparison to vector-transfected controls. Chelation of intracellular Ca(2+) with BAPTA-AM revealed that enhanced CHOP expression after store depletion occurred in a Ca(2+)-dependent manner in APP-overexpressing cells. Prevention of CHOP induction by BAPTA-AM and by RNA interference was also able to abrogate the potentiating effect of APP on thapsigargin-induced apoptosis. Application of the store-operated channel (SOC)-inhibitors SK & F96365 and 2-APB downmodulated APP-triggered potentiation of cytosolic Ca(2+) levels and apoptosis after treatment with thapsigargin. Our data demonstrate that APP significantly modulates Ca(2+) store depletion-induced cell death in a SOC- and CHOP-dependent manner, but independent of the UPR.

Unraveling in vivo functions of amyloid precursor protein: insights from knockout and knockdown studies

The amyloid precursor protein (APP) is a widely expressed transmembrane protein that is cleaved to generate Abeta peptides in the central nervous system and is a key player in the pathogenesis of Alzheimer's disease. The precise biological functions of APP still remain unclear although various roles have been proposed. While a commonly accepted model argues that Abeta peptides are the cause of onset and early pathogenesis of Alzheimer's disease, recent discussions challenge this 'Abeta hypothesis' and suggest a direct role for APP in this neurodegenerative disease. Loss-of-function studies are an efficient way to elucidate the role of proteins and concurrently a variety of in vitro and in vivo studies has been performed for APP where protein levels have been downregulated and functional consequences monitored. Complete disruption of APP gene expression has been achieved by the generation of APP knockout animal models. Further knockdown studies using antisense and RNA interference have allowed scientists to reduce APP expression levels and have opened new avenues to explore the physiological roles of APP. In the present review, we focus on knockout and knockdown approaches that have provided insights into the physiological functions of APP and discuss their advantages and drawbacks.

beta-Amyloid infusion results in delayed and age-dependent learning deficits without role of inflammation or beta-amyloid deposits

beta-Amyloid (Abeta) polypeptide plays a critical role in the pathogenesis of Alzheimer's disease (AD), which is characterized by progressive decline of cognitive functions, formation of Abeta deposits and neurofibrillary tangles, and loss of neurons. Increased genetic production or direct intracerebral administration of Abeta in animal models results in Abeta deposition, gliosis, and impaired cognitive functions. Whether aging renders the brain prone to Abeta and whether inflammation is required for Abeta-induced learning deficits is unclear. We show that intraventricular infusion of Abeta1-42 results in learning deficits in 9-month-old but not 2.5-month-old mice. Deficits that become detectable 12 weeks after the infusion are associated with a slight reduction in Cu,Zn superoxide dismutase activity but do not correlate with Abeta deposition and are not associated with gliosis. In rats, Abeta infusion induced learning deficits that were detectable 6 months after the infusion. Approximately 20% of the Abeta immunoreactivity in rats was associated with astrocytes. NMR spectrum analysis of the animals cerebrospinal fluid revealed a strong reduction trend in several metabolites in Abeta-infused rats, including lactate and myo-inositol, supporting the idea of dysfunctional astrocytes. Even a subtle increase in brain Abeta1-42 concentration may disrupt normal metabolism of astrocytes, resulting in altered neuronal functions and age-related development of learning deficits independent of Abeta deposition and inflammation.

Suppression of cyclin-dependent kinase 5 activation by amyloid precursor protein: a novel excitoprotective mechanism involving modulation of tau phosphorylation

Alzheimer's disease is cytopathologically characterized by loss of synapses and neurons, neuritic amyloid plaques consisting of beta-amyloid (Abeta) peptides, and neurofibrillary tangles consisting of hyperphosphorylated tau protein in susceptible brain regions. Abeta, which triggers a cascade of pathogenic events including tau phosphorylation and neuronal excitotoxicity, is proteolytically derived from beta-amyloid precursor protein (APP); the pathological and physiological functions of APP, however, remain undefined. Here we demonstrate that the level of tau phosphorylation in cells and brains deficient in APP is significantly higher than that in wild-type controls, resulting from activation of cyclin-dependent kinase 5 (CDK5) but not glycogen synthase kinase 3, the two major tau kinases. In addition, we show that overexpression of APP or its non-amyloidogenic homolog amyloid precursor-like protein 1 suppresses both basal and stress-induced CDK5 activation. The ectodomain of APP, sAPPalpha, is responsible for inhibiting CDK5 activation. Furthermore, neurons derived from APP-deficient mice exhibit reduced metabolism and survival rates and are more susceptible to excitotoxic glutamate-induced apoptosis. These neurons also manifest significant defects in neurite outgrowth compared with neurons from the wild-type littermates. The observed neuronal excitotoxicity/apoptosis is mediated through a mechanism involving CDK5 activation. Our study defines a novel neuroprotective function for APP in preventing tau hyperphosphorylation via suppressing overactivation of CDK5. We suggest that CDK5 activation, through a calcium/calpain/p25 pathway, plays a key role in neuronal excitotoxicity and represents an underlying mechanism for the physiological functions of APP.

Caspase-cleaved amyloid precursor protein in Alzheimer's disease

Caspase-3 mediated cleavage of the amyloid precursor protein (APP) has been proposed as a putative mechanism underlying amyloidosis and neuronal cell death in Alzheimer's disease (AD). We utilized an antibody that selectively recognizes the neo epitope generated by caspase-3 mediated cleavage of APP (alphadeltaC(csp)-APP) to determine if this proteolytic event occurs in senile plaques in the inferior frontal gyrus and superior temporal gyrus of autopsied AD and age-matched control brains. Consistent with a role for caspase-3 activation in AD pathology, alphadeltaC(csp)-APP immunoreactivity colocalized with a subset of TUNEL-positive pyramidal neurons in AD brains. AlphadeltaC(csp)-APP immunoreactivity was found in neurons and glial cells, as well as in small- and medium-size particulate elements, resembling dystrophic terminals and condensed nuclei, respectively, in AD and age-matched control brains. There were a larger number of alphadeltaC(csp)-APP immunoreactive elements in the inferior frontal gyrus and superior temporal gyrus of subjects with AD pathology than age-matched controls. AlphadeltaC(csp)-APP immunoreactivity in small and medium size particulate elements were the main component colocalized with 30% of senile plaques in the inferior frontal gyrus and superior temporal gyrus of AD brains. In some control brains, alphadeltaC(csp)-APP immunoreactivity appeared to be associated with a clinical history of metabolic encephalopathy. Our results suggest that apoptosis contributes to cell death resulting from amyloidosis and plaque deposition in AD.