Alzheimer beta/A4 amyloid precursor protein in human brain: aging-associated increases in holoprotein and in a proteolytic fragment

Hippocampal microvasculature changes in association with oxidative stress in Alzheimer's disease

Vascular endothelial dysfunction is a primary phenotype of aging, and microvascular (MV) lesion is mainly associated with Alzheimer's disease (AD). Here we have studied the correlation of MV wall thickness and CA1 pyramidal neuronal pathology in autopsy-confirmed AD brains. Both hyaline (h-MV) and increased cell number (c-MV) associated MV wall thickening was found in age-matched control (AC) hippocampus without significant change in Aβ level (Braak stages 0-III). AC neurons neighboring the h-MV showed lower levels of oxidative DNA/RNA damage and Aβ precursor protein (APP), while the neurons around c-MV showed higher oxidative DNA/RNA damage with increased APP expression. Neurons in AC hippocampus without MV wall thickening (thin wall) showed increased DNA/RNA damage and APP levels compared to AC cases with h-MV and c-MV walls. In the AD hippocampus neurons neighboring h-MV walls showed increased levels of Aβ and decreased number of dendritic spines (at Braak stages IV-VI). C-MV neighboring neurons in the AD cases showed higher levels of DNA/RNA damage with increased APP at stages II - III, followed by lower levels of oxidative DNA/RNA damage, decreased APP and increased Aβ levels with loss of dendritic spines at stages IV-VI. Prolonged treatment of primary human fetal hippocampal neurons with tert-butyl hydroperoxide (TBHP) induced oxidative DNA damage with a sustained increase in APP. Aβ increased rapidly and then decreased overtime. Short-term TBHP treated neurons showed lower levels of superoxide (O₂^{• -}) without significant DNA damage. Short-term TBHP treatment induced a gradual decrease in APP but an increase in Aβ levels over time. In conclusion this study indicates that AD hippocampus at Braak stages II-III are characterized by strong oxidative DNA/RNA damage with increased APP in neurons associated with c-MV, while stages IV-VI are characterized by a slow increase in Aβ in neurons neighboring both h-MV and c-MV.

Soluble pre-fibrillar tau and β-amyloid species emerge in early human Alzheimer's disease and track disease progression and cognitive decline

Post-mortem investigations of human Alzheimer’s disease (AD) have largely failed to provide unequivocal evidence in support of the original amyloid cascade hypothesis, which postulated deposition of β-amyloid (Aβ) aggregates to be the cause of a demented state as well as inductive to tau neurofibrillary tangles (NFTs). Conflicting evidence suggests, however, that Aβ plaques and NFTs, albeit to a lesser extent, are present in a substantial subset of non-demented individuals. Hence, a range of soluble tau and Aβ species has more recently been implicated as the disease-relevant toxic entities. Despite the incorporation of soluble proteins into a revised amyloid cascade hypothesis, a detailed characterization of these species in the context of human AD onset, progression and cognitive decline has been lacking. Here, lateral temporal lobe samples (Brodmann area 21) of 46 human cases were profiled via tau and Aβ Western blot and native state dot blot protocols. Elevations in phospho-tau (antibodies: CP13, AT8 and PHF-1), pathological tau conformations (MC-1) and oligomeric tau (TOC1) agreed with medical diagnosis (non-AD cf. AD) and Braak stage classification (low, intermediate and high), alongside elevations in soluble Aβ species (MOAB-2 and pyro-glu Aβ) and a decline in levels of the amyloid precursor protein. Strong correlations were observed between individual Braak stages and multiple cognitive measures with all tau markers as well as total soluble Aβ. In contrast to previous reports, SDS-stable Aβ oligomers (*56) were not found to be reliable for all classifications and appeared likely to be a technical artefact. Critically, the robust predictive value of total soluble Aβ was dependent on native state quantification. Elevations in tau and Aβ within soluble fractions (Braak stage 2–3 cf. 0) were evident earlier than previously established in fibril-focused disease progression scales. Together, these data provide strong evidence that soluble forms of tau and Aβ co-localise early in AD and are closely linked to disease progression and cognitive decline.

Upregulation of calpain activity precedes tau phosphorylation and loss of synaptic proteins in Alzheimer's disease brain

Alterations in calcium homeostasis are widely reported to contribute to synaptic degeneration and neuronal loss in Alzheimer’s disease. Elevated cytosolic calcium concentrations lead to activation of the calcium-sensitive cysteine protease, calpain, which has a number of substrates known to be abnormally regulated in disease. Analysis of human brain has shown that calpain activity is elevated in AD compared to controls, and that calpain-mediated proteolysis regulates the activity of important disease-associated proteins including the tau kinases cyclin-dependent kinase 5 and glycogen kinase synthase-3. Here, we sought to investigate the likely temporal association between these changes during the development of sporadic AD using Braak staged post-mortem brain. Quantification of protein amounts in these tissues showed increased activity of calpain-1 from Braak stage III onwards in comparison to controls, extending previous findings that calpain-1 is upregulated at end-stage disease, and suggesting that activation of calcium-sensitive signalling pathways are sustained from early stages of disease development. Increases in calpain-1 activity were associated with elevated activity of the endogenous calpain inhibitor, calpastatin, itself a known calpain substrate. Activation of the tau kinases, glycogen-kinase synthase-3 and cyclin-dependent kinase 5 were also found to occur in Braak stage II-III brain, and these preceded global elevations in tau phosphorylation and the loss of post-synaptic markers. In addition, we identified transient increases in total amyloid precursor protein and pre-synaptic markers in Braak stage II-III brain, that were lost by end stage Alzheimer's disease, that may be indicative of endogenous compensatory responses to the initial stages of neurodegeneration. These findings provide insight into the molecular events that underpin the progression of Alzheimer's disease, and further highlight the rationale for investigating novel treatment strategies that are based on preventing abnormal calcium homeostasis or blocking increases in the activity of calpain or important calpain substrates.

Aging, Alzheimer's, and APOE genotype influence the expression and neuronal distribution patterns of microtubule motor protein dynactin-P50

Reports from neural cell cultures and experimental animal studies provide evidence of age- and disease-related changes in retrograde transport of spent or misfolded proteins destined for degradation or recycling. However, few studies address these issues in human brain from those who either age without dementia and overt neuropathology, or succumb to Alzheimer's; especially as such propensity may be influenced by APOE genotype. We studied the expression and distribution of the dynein subunit dynactin-P50, the β amyloid precursor protein (βAPP), and hyperphosphorylated tau (P-tau) in tissues and tissue sections of brains from non-demented, neuropathology-free patients and from Alzheimer patients, with either APOE ε3,3 or APOE ε4,4. We found that advanced age in patients without dementia or neuropathological change was associated with coordinated increases in dynactin-P50 and βAPP in neurons in pyramidal layers of the hippocampus. In contrast, in Alzheimer's, βAPP and dynactin were significantly reduced. Furthermore, the dynactin-P50 and βAPP that was present was located primarily in dystrophic neurites in Aβ plaques. Tissues from Alzheimer patients with APOE ε3,3 had less P-tau, more βAPP, dynactin-P50, and synaptophysin than did tissues from Alzheimer patients carrying APOE ε4,4. It is logical to conclude, then, that as neurons age successfully, there is coordination between retrograde delivery and maintenance and repair, as well as between retrograde delivery and degradation and/or recycling of spent proteins. The buildup of proteins slated for repair, synaptic viability, transport, and re-cycling in neuron soma and dystrophic neurites suggest a loss of this coordination in Alzheimer neurons. Inheritance of APOE ε3,3 rather than APOE ε4,4, is associated with neuronal resilience, suggestive of better repair capabilities, more synapses, more efficient transport, and less hyperphosphorylation of tau. We conclude that even in disease the ε3 allele is neuroprotective.

Hydrogen sulfide inhibits A2A adenosine receptor agonist induced β-amyloid production in SH-SY5Y neuroblastoma cells via a cAMP dependent pathway

Alzheimer's disease (AD) is the leading cause of senile dementia in today's society. Its debilitating symptoms are manifested by disturbances in many important brain functions, which are influenced by adenosine. Hence, adenosinergic system is considered as a potential therapeutic target in AD treatment. In the present study, we found that sodium hydrosulfide (NaHS, an H2S donor, 100 µM) attenuated HENECA (a selective A2A receptor agonist, 10-200 nM) induced β-amyloid (1-42) (Aβ42) production in SH-SY5Y cells. NaHS also interfered with HENECA-stimulated production and post-translational modification of amyloid precursor protein (APP) by inhibiting its maturation. Measurement of the C-terminal APP fragments generated from its enzymatic cleavage by β-site amyloid precursor protein cleaving enzyme 1 (BACE1) showed that NaHS did not have any significant effect on β-secretase activity. However, the direct measurements of HENECA-elevated γ-secretase activity and mRNA expressions of presenilins suggested that the suppression of Aβ42 production in NaHS pretreated cells was mediated by inhibiting γ-secretase. NaHS induced reductions were accompanied by similar decreases in intracellular cAMP levels and phosphorylation of cAMP responsive element binding protein (CREB). NaHS significantly reduced the elevated cAMP and Aβ42 production caused by forskolin (an adenylyl cyclase, AC agonist) alone or forskolin in combination with IBMX (a phosphodiesterase inhibitor), but had no effect on those caused by IBMX alone. Moreover, pretreatment with NaHS significantly attenuated HENECA-elevated AC activity and mRNA expressions of various AC isoforms. These data suggest that NaHS may preferentially suppress AC activity when it was stimulated. In conclusion, H2S attenuated HENECA induced Aβ42 production in SH-SY5Y neuroblastoma cells through inhibiting γ-secretase via a cAMP dependent pathway.

Role of hippocalcin in mediating Aβ toxicity

Alzheimer's disease (AD) is the most common cause of dementia, and amyloid-β (Aβ) plaques and tau-containing tangles are its histopathological hallmark lesions. These do not occur at random; rather, the neurodegenerative process is stereotyped in that it is initiated in the entorhinal cortex and hippocampal formation. Interestingly, it is the latter brain area where the calcium-sensing enzyme hippocalcin is highly expressed. Because calcium deregulation is a well-established pathomechanism in AD, we aimed to address the putative role of hippocalcin in human AD brain and transgenic mouse models. We found that hippocalcin levels are increased in human AD brain and in Aβ plaque-forming APP23 transgenic mice compared to controls. To determine the role of hippocalcin in Aβ toxicity, we treated primary cultures derived from hippocalcin knockout (HC KO) mice with Aβ and found them to be more susceptible to Aβ toxicity than controls. Likewise, treatment with either thapsigargin or ionomycin, both known to deregulate intracellular calcium levels, caused an increased toxicity in hippocampal neurons from HC KO mice compared to wild-type. We found further that mitochondrial complex I activity increased from 3 to 6months in hippocampal mitochondria from wild-type and HC KO mice, but that the latter exhibited a significantly stronger aging phenotype than wild-type. Aβ treatment induced significant toxicity on hippocampal mitochondria from HC KO mice already at 3months of age, while wild-type mitochondria were spared. Our data suggest that hippocalcin has a neuroprotective role in AD, presenting it as a putative biomarker.

Substance P activates ADAM9 mRNA expression and induces α-secretase-mediated amyloid precursor protein cleavage

Altered levels of Substance P (SP), a neuropeptide endowed with neuroprotective and anti-apoptotic properties, were found in brain areas and spinal fluid of Alzheimer's disease (AD) patients. One of the hallmarks of AD is the abnormal extracellular deposition of neurotoxic beta amyloid (Aβ) peptides, derived from the proteolytic processing of amyloid precursor protein (APP). In the present study, we confirmed, the neurotrophic action of SP in cultured rat cerebellar granule cells (CGCs) and investigated its effects on APP metabolism. Incubation with low (5 mM) potassium induced apoptotic cell death of CGCs and amyloidogenic processing of APP, whereas treatment with SP (200 nM) reverted these effects via NK1 receptors. The non-amyloidogenic effect of SP consisted of reduction of Aβ(1-42), increase of sAPPα and enhanced α-secretase activity, without a significant change in steady-state levels of cellular APP. The intracellular mechanisms whereby SP alters APP metabolism were further investigated by measuring mRNA and/or steady-state protein levels of key enzymes involved with α-, β- and γ-secretase activity. Among them, Adam9, both at the mRNA and protein level, was the only enzyme to be significantly down-regulated following the induction of apoptosis (K5) and up-regulated after SP treatment. In addition to its neuroprotective properties, this study shows that SP is able to stimulate non-amyloidogenic APP processing, thereby reducing the possibility of generation of toxic Aβ peptides in brain.

ST101 induces a novel 17 kDa APP cleavage that precludes Aβ generation in vivo

OBJECTIVE

Inhibiting Aβ generation is a prime therapeutic goal for preventing or treating Alzheimer disease. Here we sought to identify any disease-modifying properties of an azaindolizinone derivative, spiro[imidazo[1,2-a]pyridine-3,2-idan]-2(3H)-one (ST101 or ZSET1446).

METHODS

The effects of ST101 were studied in 3xTg-AD mice and young cynomolgus monkeys using a combination of biochemical and histological analyses.

RESULTS

Here we describe that ST101 induces cleavage of APP protein at a novel site, generating a 17 kDa C-terminal fragment. This 17 kDa APP cleavage product does not appear to be a substrate for either α- or β-secretase, and thus bypasses generation of Aβ. ST101 is orally active, efficacious at low doses, improves memory function, and robustly reduces brain Aβ in transgenic mice and nonhuman primates.

INTERPRETATION

Using rodent and nonhuman primate models, we show that ST101 represents a novel class of small molecules that reduce central nervous system levels of Aβ by inducing an alternate pathway of APP cleavage.

Translocation of amyloid precursor protein C-terminal fragment(s) to the nucleus precedes neuronal death due to thiamine deficiency-induced mild impairment of oxidative metabolism

Thiamine deficiency (TD) is a model of neurodegeneration induced by mild impairment of oxidative metabolism. TD produces time-dependent glial activation, inflammation, oxidative stress, altered metabolism of amyloid precursor protein (APP), exacerbation of plaque formation from APP, and finally, selective neuron death in specific brain regions. The sub-medial thalamic nucleus (SmTN) is the most sensitive region to TD. Alteration in APP metabolism and nuclear translocation of carboxy-terminal fragments (CTF) of APP has been implicated in neuron death in other models of neurodegeneration. These experiments tested whether TD causes translocation of CTF into the nucleus of neurons in the SmTN that are destined to die after 9 days of TD by examining overlapping immunoreactivity (IR) of antibody APP 369 with either Alz90, 6E10 or 4G8 epitopes in the nuclei of the neurons in the SmTN. TD caused the accumulation of the CTF of APP in nuclei of SmTN neurons within 3 days of TD. These changes did not occur in the cortex which is spared in TD. Western blot analysis of nuclear fractions revealed a significant (61%; P < 0.026) increase in CTF 12 levels in TD SmTN (2.08 +/- 0.56) compared to control SmTN (1.29 +/- 0.41). Although TD increased CTF 15 levels in TD SmTN (1.95 +/- 0.73) compared to control SmTN (0.62 +/- 0.52) by 214%; P < 0.665 and decreased the full-length holo-APP levels in TD SmTN (0.32 +/- 0.30) compared to control SmTN (0.47 +/- 0.18) by 34%; P < 0.753, the differences were statistically insignificant. TD did not alter CTF 15 or CTF 12 levels in cortex. These findings demonstrate that changes in APP metabolism occur in early stages of TD, and they may play an important role in TD-induced selective neuronal loss.

Studies on brain volume, Alzheimer-related proteins and cytokines in mice with chronic overexpression of IL-1 receptor antagonist

Inflammation is associated with both acute and chronic neurological disorders, including stroke and Alzheimer's disease (AD). Cytokines such as interleukin (IL)-1 have several activities in the brain both under physiological and pathophysiological conditions. The objective of this study was to evaluate consequences of the central blockade of IL-1 transmission in a previously developed transgenic mouse strain with brain-directed overexpression of human soluble IL-1 receptor antagonist (Tg hsIL-1ra). Effects on brain morphology and brain levels of the AD-related proteins beta-amyloid precursor protein (APP) and presenilin 1(PS1), as well as the levels of IL-1beta, IL-6 and tumour necrosis factor-alpha (TNF-alpha) were analysed in homozygotic and heterozygotic mice and wild type (WT) controls, of both genders and of young (30-40 days) and adult (13-14 months) age. A marked reduction in brain volume was observed in transgenic mice as determined by volumetry. Western blot analysis showed higher levels of APP, but lower levels of PS1, in adult animals than in young ones. In the cerebellum, heterozygotic (Tg hsIL-1ra(+/-)) mice had lower levels of APP and PS1 than WT mice. With one exception, there were no genotypic differences in the levels of IL-1beta, IL-6 and TNF-alpha. The cytokine levels were generally higher in adult than in young mice. In conclusion, the chronic blockade of IL-1 signalling in the brain was associated with an atrophic phenotype of the brain, and with modified levels of APP and PS1. Brain-directed overexpression of hsIL-1ra was not followed by major compensatory changes in the levels of pro-inflammatory cytokines.

Amyloid-β in Alzheimer Disease: The Null versus the Alternate Hypotheses

For nearly 20 years, the primary focus for researchers studying Alzheimer disease has been centered on amyloid-beta, such that the amyloid cascade hypothesis has become the "null hypothesis." Indeed, amyloid-beta is, by the current definition of the disease, an obligate player in pathophysiology, is toxic to neurons in vitro, and, perhaps most compelling, is increased by all of the human genetic influences on the disease. Therefore, targeting amyloid-beta is the focus of considerable basic and therapeutic interest. However, an increasingly vocal group of investigators are arriving at an "alternate hypothesis" stating that amyloid-beta, while certainly involved in the disease, is not an initiating event but rather is secondary to other pathogenic events. Furthermore and perhaps most contrary to current thinking, the alternate hypothesis proposes that the role of amyloid-beta is not as a harbinger of death but rather a protective response to neuronal insult. To determine which hypothesis relates best to Alzheimer disease requires a broader view of disease pathogenesis and is discussed herein.

Alpha- and beta-secretase activity as a function of age and beta-amyloid in Down syndrome and normal brain

Aged individuals with Down syndrome (DS) develop Alzheimer's disease (AD) neuropathology by the age of 40 years. The purpose of the current study was to measure age-associated changes in APP processing in 36 individuals with DS (5 months-69 years) and in 26 controls (5 months-100 years). Alpha-secretase significantly decreased with age in DS, particularly in cases over the age of 40 years and was stable in controls. The levels of C-terminal fragments of APP reflecting alpha-secretase processing (CTF-alpha) decreased with age in both groups. In both groups, there was significant increase in beta-secretase activity with age. CTF-beta remained constant with age in controls suggesting compensatory increases in turnover/clearance mechanisms. In DS, young individuals had the lowest CTF-beta levels that may reflect rapid conversion of beta-amyloid (Abeta) to soluble pools or efficient CTF-beta clearance mechanisms. Treatments to slow or prevent AD in the general population targeting secretase activity may be more efficacious in adults with DS if combined with approaches that enhance Abeta degradation and clearance.

siRNA targeted against amyloid precursor protein impairs synaptic activity in vivo

The amyloid precursor protein (APP) plays a central role in Alzheimer's disease (AD) pathogenesis through its cleavage leading to the accumulation of the peptide betaA4. Diffusible oligomeric assemblies of amyloid beta peptide are thought to induce synaptic dysfunction, an early change in AD. We tested the hypothesis that a reduction in presynaptic APP could itself lead to a decrease in synaptic efficacy in vivo. Twenty-four hours after intraocular injection, siRNA targeted against APP accumulated in retinal cells and the APP in retinal terminals in the superior colliculus was significantly reduced. Surprisingly, the amyloid precursor-like protein 2 (APLP2) was reduced as well. Functional imaging experiments in rats during visual stimulation showed that knockdown of presynaptic APP/APLP2 significantly reduced the stimulation-induced glucose utilization in the superior colliculus. Our results suggest that perturbations in the amount of APP/APLP2 axonally transported to, and/or in their turnover in the nerve terminal alter synaptic function and could be a pathogenic mechanism in AD.

Pin1 promotes production of Alzheimer's amyloid beta from beta-cleaved amyloid precursor protein

Here we show that prolyl isomerase Pin1 is involved in the Abeta production central to the pathogenesis of Alzheimer's disease. Enzyme immunoassay of brains of the Pin1-deficient mice revealed that production of Abeta40 and Abeta42 was lower than that of the wild-type mice, indicating that Pin1 promotes Abeta production in the brain. GST-Pin1 pull-down and immunoprecipitation assay revealed that Pin1 binds phosphorylated Thr668-Pro of C99. In the Pin1-/- MEF transfected with C99, Pin1 co-transfection enhanced the levels of Abeta40 and Abeta42 compared to that without Pin1 co-transfection. In COS7 cells transfected with C99, Pin1 co-transfection enhanced the generation of Abeta40 and Abeta42, and reduced the expression level of C99, facilitating the C99 turnover. Thus, Pin1 interacts with C99 and promotes its gamma-cleavage, generating Abeta40 and Abeta42. Further, GSK3 inhibitor lithium blocked Pin1 binding to C99 by decreasing Thr668 phosphorylation and attenuated Abeta generation, explaining the inhibitory effect of lithium on Abeta generation.

Amyloid-beta in Alzheimer's disease: the horse or the cart? Pathogenic or protective?

While the pathogenesis of Alzheimer's disease (AD) is unclear, amyloid-beta plaques remain major lesions in the brain of individuals with AD. Likewise, amyloid-beta is one of the best-studied proteins relating to the pathogenesis of AD. Indeed, the pathological diagnosis of AD tends to be congruous with the quantity of amyloid-beta. However, it is important to recognize that pathological diagnosis merely represents the association of a pattern of pathological changes with a clinical phenotype. Therefore, it should be acknowledged that, although amyloid-beta detection and semiquantification have some diagnostic utility, the simple presence of amyloid plaques, as with proteinaceous accumulations in essentially all neurodegenerative diseases, does not presume aetiology. Thus, in this review, we discuss the role of amyloid-beta in the pathogenesis of AD and provide an alternative view to the widely accepted dogma.

Pathophysiological roles of amyloidogenic carboxy-terminal fragments of the beta-amyloid precursor protein in Alzheimer's disease

Several lines of evidence suggest that some of the neurotoxicity in Alzheimer's disease (AD) is attributed to proteolytic fragments of amyloid precursor protein (APP) and beta-amyloid (Abeta) may not be the sole active component involved in the pathogenesis of AD. The potential effects of other cleavage products of APP need to be explored. The CTFs, carboxy-terminal fragments of APP, have been found in AD patients' brain and reported to exhibit much higher neurotoxicity in a variety of preparations than Abeta. Furthermore CTFs are known to impair calcium homeostasis and learning and memory through blocking LTP, triggering a strong inflammatory reaction through MAPKs- and NF-kappaB-dependent astrocytosis and iNOS induction. Recently, it was reported that CTF translocated into the nucleus, binding with Fe65 and CP2, and in turn, affected transcription of genes including glycogen synthase kinase-3beta, which results in the induction of tau-rich neurofibrillary tangles and subsequently cell death. Spatial memory of transgenic (Tg) mice overexpressing CT100 was significantly impaired and CTFs were detected in the neurons as well as in plaques of the Tg mice and double Tg mice carrying CT100 and mutant tau. In this review, we summarize observations indicating that both CTF and Abeta may participate in the neuronal degeneration in the progress of AD by differential mechanisms.

Statins cause intracellular accumulation of amyloid precursor protein, beta-secretase-cleaved fragments, and amyloid beta-peptide via an isoprenoid-dependent mechanism

The use of statins, 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors that block the synthesis of mevalonate (and downstream products such as cholesterol and nonsterol isoprenoids), as a therapy for Alzheimer disease is currently the subject of intense debate. It has been reported that statins reduce the risk of developing the disorder, and a link between cholesterol and Alzheimer disease pathophysiology has been proposed. Moreover, experimental studies focusing on the cholesterol-dependent effects of statins have demonstrated a close association between cellular cholesterol levels and amyloid production. However, evidence suggests that statins are pleiotropic, and the potential cholesterol-independent effects of statins on amyloid precursor protein (APP) metabolism and amyloid beta-peptide (A beta) genesis are unknown. In this study, we developed a novel in vitro system that enabled the discrete analysis of cholesterol-dependent and -independent (i.e. isoprenoid-dependent) statin effects on APP cleavage and A beta formation. Given the recent interest in the role that intracellular A beta may play in Alzheimer disease, we analyzed statin effects on both secreted and cell-associated A beta. As reported previously, low cellular cholesterol levels favored the alpha-secretase pathway and decreased A beta secretion presumably within the endocytic pathway. In contrast, low isoprenoid levels resulted in the accumulation of APP, amyloidogenic fragments, and A beta likely within biosynthetic compartments. Importantly, low cholesterol and low isoprenoid levels appeared to have completely independent effects on APP metabolism and A beta formation. Although the implications of these effects for Alzheimer disease pathophysiology have yet to be investigated, to our knowledge, these results provide the first evidence that isoprenylation is involved in determining levels of intracellular A beta.

Functional and morphological alterations in compound transgenic mice overexpreszing Cu/Zn superoxide dismutaze and amyloid precursor protein

Down's syndrome (DS), the phenotypic manifestation of trisomy 21, involves overexpression of chromosome 21-encoded genes. The gene for amyloid precursor protein (APP), known to be involved in AD pathology, resides on chromosome 21 along with the gene for Cu/Zn superoxide dismutase (SOD1), a key enzyme in the metabolism of oxygen free radicals. We investigated the consequences of a combined increase in APP and SOD1, in a double-transgenic (tg)-APP-SOD1 mouse. These mice expressed severe impairment in learning, working and long-term memory. Expression of long-term potentiation in hippocampal slices was impaired in both tg-SOD and tg-APP-SOD mice, but not in tg-APP mice, indicating that increased APP by itself did not affect in vitro synaptic plasticity. In tg-APP-SOD mice, membrane-bound high molecular weight APP species accumulated while APP cleavage products did not increase and levels of secreted APP were unchanged. Severe morphological damage, including lipofuscin accumulation and mitochondria abnormalities, were found in aged tg-APP-SOD but not in the other mice. Thus, a combined elevation of the two chromosome 21 genes in tg-APP-SOD mice induced age-dependent alterations in morphological and behavioural functions.

Will preventing protein aggregates live up to its promise as prophylaxis against neurodegenerative diseases?

Protein aggregation and misfolding characterize most age-related neurodegenerative diseases including Alzheimer, Parkinson and Huntington diseases. Protein aggregation has generally been assumed to be responsible for neurodegeneration in these disorders due to association and genetics. However, protein aggregation may, in fact, be an attempt to protect neurons from the stress resulting from the disease etiology. In this review, we weigh the evidence of whether removal of amyloids, aggregates and neuronal inclusions represent a reasonable strategy for protecting neurons.

Proteolytic processing of the Alzheimer's disease amyloid precursor protein in brain and platelets

Proteolytic processing of the amyloid precursor protein by beta -and gamma-secretases results in the production of Alzheimer's disease (AD) Abeta amyloid peptides. Modulation of secretase activity is being investigated as a potential therapeutic approach. Recent studies with human brain have revealed that the beta-secretase protein, BACE, is increased in cortex of AD patients. Analysis of betaCTF (or C99), the amyloid precursor protein (APP) product of BACE cleavage that is the direct precursor to Abeta, shows it is also elevated in AD, underlying the importance of beta-secretase cleavage in AD pathogenesis. The C-terminal product of gamma-secretase cleavage of APP, epsilonCTF (or AICD), is enriched in human brain cortical nuclear fractions, a subcellular distribution appropriate for a putative involvement of APP cytosolic domain in signal transduction. Analysis of AD cortex samples, particularly that of a carrier of a familial APP mutation, suggests that processing of APP transmembrane domain generates an alternative CTF product. All these particularities observed in the AD brain demonstrate that APP processing is altered in AD. The transgenic mouse model Tg2576 seems to be a promising laboratory tool to test potential modulators of Abeta formation. Indeed, C-terminal products of alpha-, beta-, and gamma-secretase cleavage are readily detectable in the brain of these transgenic mice. Finally, the finding of the same secretase products in platelets and neurons make platelets a potentially useful and easily accessible clinical tool to monitor effects of novel therapies based on inhibition of beta- or gamma-secretase.

Measurement of alpha- and beta-secretase cleaved amyloid precursor protein in cerebrospinal fluid from Alzheimer patients

One of the major histopathological hallmarks of Alzheimer's disease (AD) is redundant senile plaques mainly composed of beta-amyloid (Abeta) aggregates. Alternative cleavage of the amyloid precursor protein (APP), occurring in both normal and AD subjects, results in the generation and secretion of soluble APP (sAPP) and Abeta. We examined the cerebrospinal fluid (CSF) for alpha- and beta-secretase cleaved sAPP (alpha-sAPP and beta-sAPP) in 81 sporadic AD patients, 19 patients with mild cognitive impairment, and 42 healthy controls by using newly developed sandwich enzyme-linked immunosorbent assay methods. We found that neither the level of CSF-alpha-sAPP nor CSF-beta-sAPP differed between sporadic AD patients and healthy controls. These findings further support the conclusion that there is no change in APP expression in sporadic AD. However, the level of CSF-beta-sAPP was significantly increased in patients with mild cognitive impairment compared to controls. We also investigated the relationship between the CSF level of alpha/beta-sAPP and Abeta(42) and the apoE epsilon 4 (apoE4) allele. Significantly lower levels of CSF-alpha-sAPP were found in AD patients possessing one or two apoE4 alleles than in those not possessing the apoE4 allele. Neither the levels of CSF-beta-sAPP nor CSF-Abeta(42) differed when comparing ApoE4 allele-positive with allele-negative individuals.

The state versus amyloid-beta: the trial of the most wanted criminal in Alzheimer disease

Investigators studying the primary culprit responsible for Alzheimer disease have, for the past two decades, primarily focused on amyloid-beta (Abeta). Here, we put Abeta on trial and review evidence amassed by the prosecution that implicate Abeta and also consider arguments and evidence gathered by the defense team who are convinced of the innocence of their client. As in all trials, the arguments provided by the prosecution and defense revolve around the same evidence, with opposing interpretations. Below, we present a brief synopsis of the trial for you, the jury, to decide the verdict. Amyloid-beta: guilty or not-guilty?

Increasing neurite outgrowth capacity of beta-amyloid precursor protein proteoglycan in Alzheimer's disease

Progressive cerebral deposition of beta-amyloid peptide either in blood vessels or around neurites is one of the most important features of Alzheimer's disease (AD). The beta-peptide, known as Abeta or A4, is produced by proteolytic cleavage of the amyloid precursor protein (APP). Two APP processing pathways have been proposed as physiological alternatives; only one of which leads to the production of Abeta or amyloidogenic peptides. However, we have little information regarding these processing pathways in the brain, or on whether posttranslational modifications such as glycosylation affect APP processing in vivo. Furthermore, the physiological function(s) of this protein in nervous tissue remains unclear, although modulatory roles in cell adhesion and neuritic extension have been suggested. It has been reported that APP may be glycosylated as a proteoglycan. We purified this APP population from human brain, and our data indicate that PG-APP supports neurite extension of hippocampal neurons. Neurons grown on this substratum showed an increased capacity to elongate neurites and increased neuritic "branching" compared to culture on laminin. These effects were enhanced with PG-APP samples obtained from AD brains. Our results suggest that this APP population may act as a neurite outgrowth and branching promoter and may thus play a role in some pathological conditions. These findings may have significant implications in understanding normal brain development and pathological situations (such as AD).

Biochemical assay for amyloid beta deposits to distinguish Alzheimer's disease from other dementias

Biochemical markers for Alzheimer's disease (AD) are of great value for precise diagnosis and in studies of the pathogenetic processes of this disease. A new biochemical assay allowing to differentiate AD from other forms of dementia is described. The assay is based on the extraction of amyloid beta (A beta) from milligram amounts of brain tissue by using 20% acetonitrile in 0.1% trifluoroacetic acid and its detection by Western blotting. The presence of the 4 kDa A beta was demonstrated in all cases of AD (n = 8) that were diagnosed by the independent histopathological examination of the postmortem tissues. No A beta was found in tissue extracts from seven out of eight cases of other forms of dementia. In contrast to other biochemical techniques of A beta detection in brain, the developed assay is simple; it does not require any special equipment and allows detection of A beta using milligram amounts of brain tissue.

Subcellular localization of the Alzheimer's disease amyloid precursor protein and derived polypeptides expressed in a recombinant yeast system

Different isoforms and derived polypeptides of the Alzheimer's disease amyloid protein precursor (A beta PP) have been expressed in the yeast Pichia pastoris. The expression characteristics of the different A beta PP polypeptides were studied by post-embedding immunogold electron microscopy with various A beta PP antibodies. The site of intracellular expression could be readily identified with specific antibodies. Full length A beta PP was expressed in association with the nuclear membrane and the endoplasmic reticulum. Secretory derivatives of A beta PP were localized in membrane-bound secretory vesicles. A construct encoding two copies of beta A4[1-42] linked head-to-tail (beta A4duplex) accumulated as irregular dense cytoplasmic and intranuclear inclusions which reacted with all beta A4 antibodies tested. A beta A4-C-terminal construct accumulated into membranous structures in the cytoplasm and nucleus and reacted with most antibodies to beta A4 and the cytoplasmic domain of A beta PP. The two shorter constructs containing the beta A4 sequence formed similar intranuclear aggregates to those reported for intranuclear inclusions of polyglutamine peptides from huntingtin (in Huntington's disease) and ataxin protein fragments (in spinocerebellar ataxia). This is of interest because intracellular aggregation of the polyglutamine and beta A4 peptides may affect cells by similar toxic mechanisms. These studies demonstrate clear differences in the expression properties of different A beta PP polypeptides.

Novel neuritic clusters with accumulations of amyloid precursor protein and amyloid precursor-like protein 2 immunoreactivity in brain regions damaged by thiamine deficiency

Experimental thiamine deficiency (TD) is a classical model of a nutritional deficit associated with a generalized impairment of oxidative metabolism and selective cell loss in the brain. In rats, TD-induced cell degeneration is accompanied by an accumulation of amyloid precursor protein (APP)/amyloid precursor-like protein 2 (APLP2) immunoreactivity in abnormal neurites and perikarya along the periphery of, or scattered within, the lesion. Prompted by these data and our previous findings of a genetic variation in the development of TD symptoms, we extended our studies to mice. C57BL/6, ApoE knockout, and APP YAC transgenic mice received thiamine-deficient diet and pyrithiamine injections. Unlike rats, APP/APLP2-immunoreactive neurites in all strains of mice were sparsely scattered within damaged areas and did not delimit the thalamic lesion. In addition, abnormal clusters of intensely immunoreactive neurites occurred only in areas of damage including the thalamus, mammillary body, and inferior colliculus. The clusters appeared as either irregular clumps or round or oval rosettes that strikingly resembled the neuritic component of Alzheimer amyloid plaques. However, immunostaining using various antisera to synthetic amyloid beta-protein (A beta 1-40) and thioflavine S histochemistry failed to show evidence of a component of A beta Neither APP/APLP2-immunoreactive clusters nor amyloid plaques were observed in the brain from patients with Wernicke-Korsakoff syndrome, the clinical manifestation of TD in man. Our results demonstrate species (i.e., genetic) differences in the response to TD-induced damage and support a role for APP and APLP2 in the response to brain injury. This is the first report that chronic oxidative deficits can lead to this novel pathology.

The possible place of cathepsins and cystatins in the puzzle of Alzheimer disease: a review

Lysosomal proteinases (cathepsins) and their endogenous inhibitors (cystatins) have been found to be closely associated with senile plaques, cerebrovascular amyloid deposits, and neurofibrillary tangles in Alzheimer disease (AD). Further, profound changes in the lysosomal system seem to be an early event in "at-risk" neurons of AD brains. There is an ongoing controversy as to whether lysosome-associated proteolytic mechanisms are causally related to the development and/or further progression of the disease. The present article deals with some arguments "pro" and "contra" an involvement of the endosomal/lysosomal pathway in amyloidogenesis as a cardinal process in AD. Other putative targets of acidic proteinases and their natural inhibitors in the pathogenesis of AD (such as formation of neurofibrillary tangles and regulation of apolipoprotein E) are also discussed.

Generation of amyloidogenic C-terminal fragments during rapid axonal transport in vivo of beta-amyloid precursor protein in the optic nerve

The amyloid beta-protein (A beta) is a major component of extracellular deposits that are characteristic features of Alzheimer's disease. A beta is derived from the large transmembrane beta-amyloid precursor protein (beta APP). In the rabbit optic nerve/optic tract (ON), beta APP is synthesized in vivo in retinal ganglion cell perikarya, rapidly transported into the ON axons in small transport vesicles and is subsequently transferred to the axonal plasma membrane as well as to the presynaptic nerve terminals (Morin, P. J., Abraham, C. R., Amaratunga, A., Johnson, R.J., Huber, G., Sandell, J. H., and Fine, R. E. (1993) J. Neurochem. 61, 464-473). Present results indicate that there is rapid processing of beta APP in the ON to generate a 14-kDa C-terminal membrane-associated fragment that contains the A beta sequence. By using equilibrium sucrose density gradient fractionation, this fragment, as well as non-amyloidogenic C-terminal fragments and intact beta APP, are detected in at least two classes of transport vesicles destined for the plasma membrane and the presynaptic nerve terminal. The two classes of transported vesicles are distinguished by labeling kinetics as well as by density. In contrast to the ON, only nonamyloidogenic C-terminal fragments are generated in the retina, which contains the perikarya of retinal ganglion cells and glial (Muller) cells which also produce beta APP.

Intracellular A beta 1-42 aggregates stimulate the accumulation of stable, insoluble amyloidogenic fragments of the amyloid precursor protein in transfected cells

We have analyzed the effect of internalized amyloid beta-protein (A beta) 1-42 aggregates on the metabolism of the amyloid precursor protein (APP) in stably transfected 293 cells. The amount of potentially amyloidogenic fragments of APP immunoprecipitated by anti-carboxyl-terminal APP and anti-A beta antibodies is dramatically enhanced by the treatment of the cells with A beta 1-42, which is resistant to degradation, but not A beta 1-28, which does not accumulate in cells. This accumulation of amyloidogenic carboxyl-terminal fragments is specific, since there is relatively little effect of A beta 1-42 on the amount of the nonamyloidogenic alpha-secretase carboxyl-terminal fragment. The amyloidogenic fragments accumulate in the same nonionic detergent-insoluble fraction of the cell that contains the internalized A beta 1-42. Western analysis indicates that a subset of the amyloidogenic fragments react with antibodies that recognize a conformation of A beta that is specifically associated with aggregated forms of A beta, suggesting that the adoption of this aggregation-related conformation may be an early event which precedes the final processing that produces A beta. Pulse-chase analysis of the [35S]Met-labeled 16-kDa amyloidogenic fragment indicates that it is relatively stable in A beta 1-42-treated cells, with a half-life of approximately 50 h. This fragment is degraded with a half-life of 30 min in control cells treated with A beta 1-28. In contrast, the turnover of the nonamyloidogenic alpha-secretase product is not significantly altered by the presence of A beta 1-42. The continuous uptake of A beta 1-42 from the medium is not required for the stimulation of amyloidogenic fragment accumulation, suggesting that the presence of intracellular A beta 1-42 aggregates establishes a new pathway for APP catabolism in cells which leads to the long term stability of the fragments. If these amyloidogenic fragments of APP ultimately give rise to A beta, then the production of A beta may be an autocatalytic, "runaway" process in cells containing A beta 1-42 nuclei. It is conceivable that the accumulation of insoluble APP and amyloidogenic fragments of APP in response to A beta 1-42 aggregates may mimic the pathophysiology of dystrophic neurites, where the accumulation of intracellular APP and APP fragments has been documented by immunohistochemistry.

Proteolytic processing of Alzheimer's disease beta A4 amyloid precursor protein in human platelets

The processing of amyloid precursor protein (APP) and production of beta A4 amyloid are events likely to influence the development and progression of Alzheimer's disease, since beta A4 is the major constituent of amyloid deposited in this disorder. Our previous studies showed that human platelets contain full-length APP (APPFL) and are a suitable substrate to study normal APP processing. In the present study, we show that a 22-kDa beta A4-containing carboxyl-terminal fragment (22-CTF) of APP is present in unstimulated platelets. Both APPFL and 22-CTF are proteolytically degraded when platelets are activated with thrombin, collagen, or calcium ionophore A23187. Complete cleavage of APPFL and 22-CTF require the presence of extracellular calcium. Following stimulation in the presence of calcium, a new CTF of 17 kDa is generated, and the NH2-terminal epitope of beta A4 amyloid is lost. Preincubation of platelets with the cell-permeable cysteine protease inhibitors calpeptin, (2S,3S)-trans-epoxysuccinyl-L-leucyl-amido-3-methylbutane ethyl ester (E64d), Na alpha-p-tosyl-L-lysine chloromethyl ketone, or calcium chelator EGTA before platelet stimulation inhibits the degradation of both APPFL and 22-CTF. Divalent metal ions including zinc, copper, and cobalt inhibit the degradation of APPFL and 22-CTF. This study suggests that a calcium-dependent neutral cysteine protease is involved in the proteolytic processing of an amyloidogenic species of APP in human platelets.

beta-Amyloid precursor protein isoforms show correlations with neurones but not with glia of demented subjects

Post-mortem cerebral cortex from 15 demented patients was specially collected to minimise autolysis and two membrane fractions and one soluble fraction were quantitatively examined for the major species of beta-amyloid precursor protein (APP) of high apparent molecular mass (> or = 80 kDa) together with the major mRNA species encoding APP isoforms. The number of pyramidal neurones and astrocytes, putative biochemical indices of interneurones and pyramidal neurones, and choline acetyl transferase activity were also determined. Multiple regression analysis has been used to investigate intercorrelations of APP species with biochemical and morphometric measures, free of any effects of confounding demographic variables. Subjects with Alzheimer's disease showed a loss of cholinergic activity and D-aspartate uptake compared with patients with other causes of dementia. The major finding of the study is that measures of neurones rather than astrocytes most closely correlate with the concentration of APP. Pyramidal cell numbers were positively correlated with mRNA for APP695. APP in the soluble fraction showed a negative correlation with pyramidal cell numbers and cholinergic activity. These results indicate that neurones within the cerebral cortex are the major source of APP, and that secretion of APP is dependent upon cortical pyramidal neuronal activity and cholinergic activity.

Amyloid beta protein levels in cerebrospinal fluid are elevated in early-onset Alzheimer's disease

The 4-kd amyloid beta protein (A beta) deposited as amyloid in Alzheimer's disease (AD) is produced and released by normal proteolytic processing of the amyloid beta protein precursor (beta APP) and is readily detected in cerebrospinal fluid (CSF). Here, we present the levels of A beta in CSF from a total of 95 subjects, including 38 patients with AD, 14 with early-onset AD and 24 with late-onset AD, 25 normal control subjects, and 32 patients with other neurological diseases. The level of A beta decreased with normal aging, and there was a significant elevation in the level of A beta in the CSF of early-onset AD patients (4.14 +/- 1.37 pmol/ml, p < 0.01). Neither Mini-Mental State nor Functional Assessment Staging were correlated with the amount of A beta in the CSF. The A beta/secreted form of beta APP ratio was elevated, but the level of alpha 1-antichymotrypsin in the CSF did not correlate with the level of CSF A beta in early-onset AD patients. Thus, the level of A beta in the CSF is elevated in early-onset AD patients and is suggested to be correlated with the pathology in the brain that characterizes AD.

Familial Alzheimer's disease cells abnormally accumulate beta-amyloid-harbouring peptides preferentially in cytosol but not in extracellular fluid

The proteolytic processing of beta-amyloid precursor protein (APP) was analyzed and compared for familial Alzheimer's disease and normal lymphoid cells, focusing on beta-amyloid-harbouring peptides and the extracellular fragments released into the medium. Tris/tricine gel electrophoresis of anti-beta A4-(8-17)-immunoprecipitated peptides and subsequent N-terminal amino acid sequencing revealed previously unidentified peptides; the 14-kDa peptide with a beta A4 N-terminus and the 12-kDa peptide with an Esch's-site N-terminus in the cytosols, and the same 12-kDa peptide predominating in the media. Moreover, some early onset familial Alzheimer's disease cells, but not normal cells, express a 4-kDa peptide with a beta-amyloid N-terminus in the cytosol. Two-dimensional gel electrophoresis of the extracellular APP peptides immunoprecipitated with anti-APP-(144-654)-peptide showed that familial Alzheimer's disease cells are deficient in processing, especially the 50-53-kDa peptides with the Kunitz-protease-inhibitor domain. This may reflect their unique expression of a serine protease identified as cleaving APP at the beta-amyloid N-terminus.

Immunocytochemical localization of amyloid precursor protein in rat brain

The localization of amyloid precursor protein (APP) in rat brain was studied with a cytoplasmic domain-specific antibody. Light microscopic immunocytochemistry demonstrated that APP is present in most neurons, in some oligodendrocytes, and in a population of cells with diameters less than 10 microns that may be glial. Marked differences in immunoreactivity among neurons were observed, and the strongest immunoreactivity was contained in larger neurons. Neurons with scant cytoplasm, such as granule cells in the olfactory bulb, dentate gyrus, and cerebellum, were weakly immunoreactive. Differences in neuropil immunoreactivity were also observed; this type of staining was strongest in the caudatoputamen, lateral septum, medial habenula, nucleus reticularis of the dorsal thalamus, and the lateral portion of the ventroposterior nucleus. Neuropil immunostaining was weakest in layer IV of cortex and in areas containing granule cells. The fact that APP seems to be present in the vast majority of neurons suggests that this protein plays a role common to all neurons. The fact that there is a great difference in the steady-state amount of APP among different types of neurons suggests that APP may play a specific role in the function of certain classes of neurons.

Induction of gene expression of amyloid precursor protein (APP) in activated human lymphoblastoid cells and lymphocytes

To understand the possible role of amyloid precursor protein (APP) in human lymphocytes, and the regulation of APP gene expression in this cell type, we determined levels of cellular APP protein and of mRNA in human T-cell-derived Jurkat cells that were treated with lectin, phorbol ester, and calcium ionophore. We also related these levels to cell aggregation and adhesion. Cell-cell aggregation and cell-plastic adhesion were observed over a 24-h period after incubating cells for 2 h with phytohemagglutinin or phorbol myristate acetate. Cells treated with a calcium ionophore showed no aggregation or adhesion. Western blots indicated no obvious alteration in the level of cellular APP with different treatments. Northern blots showed a significant transient increase of APP mRNA after incubation with the calcium ionophore, whereas phorbol ester treatment showed a slight increase of APP mRNA. We analyzed the level of APP mRNA in human peripheral T cells which had been separated from peripheral lymphocytes. The level increased transiently by up to threefold after treatment with calcium ionophore plus phorbol esters. These data suggest that cell-cell aggregation and cell-matrix adhesion by human lymphocytes are not associated with an increased level of cellular APP protein or of mRNA.

Senile plaques in temporal lobe epilepsy

Senile plaques (SP) are one of the characteristic pathological lesions of Alzheimer's disease (AD). They are also seen in the brains of some non-demented individuals as an age-related change. Identification of clinical conditions associated with these "incidental" SP could provide insight into AD pathogenesis. We have examined the presence of SP in lobectomy specimens (n = 101) removed in the surgical treatment of temporal lobe epilepsy (TLE). SP were present in 10 specimens from epileptic patients aged 36 to 61 years and the presence of SP correlated positively with patient age. No other significant AD-related pathology was identified and no patients showed any evidence of dementia on neuropsychological testing. When compared with temporal lobe tissue from non-demented, non-epileptic autopsy controls (n = 406), the density and distribution of SP was the same. The age-related incidence of SP however, was significantly greater in the epileptics. This suggests that some aspects of TLE has a positive influence on the formation of SP.

Senile plaques do not progressively accumulate with normal aging

Senile plaques (SP) are one of the pathologic hallmarks of Alzheimer's disease (AD). Models of SP formation, particularly the early stages, could provide valuable insight into AD pathogenesis. One such model may be provided by non-demented elderly individuals in whom some SP are a common incidental finding. This study has examined post-mortem brain tissue from a large number of such neurologically normal patients in an attempt to better understand the temporal sequence of SP formation. SP were identified in modified Bielschowsky-stained sections of mesial temporal lobe in 122 (30%) of 402 cases. The prevalence of SP in the temporal neocortex correlated strongly with patient age. Surprisingly, however, neither the mean nor maximum SP density showed any increase with age. This suggests that SP do not progressively accumulate in normal aging but develop over a limited time period after which their number stabilizes at a constant level. In most cases, all SP were of the diffuse type. In 37 cases (9%), however, some neuritic SP (NP) were also seen. Although the NP density did not show a significant increase with age either, the proportion of SP which were neuritic (NP/SP), did. This suggests that changes in SP morphology may be more important than total SP numbers in normal aging.

Nicotine effects on the regulation of amyloid precursor protein splicing, neurotrophin and glucose transporter RNA levels in aged rats

It has been reported that an inverse relationship exists between nicotine intake and the incidence of Alzheimer's Disease (AD). Although nicotine has been reported to induce c-fos, in the present study it was shown that this induction does not alter the accumulation of a number of transcripts associated with AD. Altered splicing patterns of Amyloid Precursor Protein (APP) and changes in neurotrophin and glucose transporter expression have been implicated in AD and behavioral deficits in rats. The effects of subacute administration of nicotine (12 mg/ml at 2.3 microliters/hr for 14 days) on the abundance levels of APP, glucose transporter (GLUT) and neurotrophin transcripts were determined by rtPCR in the hippocampus, cortex, and striatum of aged (22-24 months) male Wistar rats. No significant differences between saline and nicotine infused rats were detected for APP abundance levels or ratio of the various isoforms. However, both groups had a higher level of APP transcripts containing the Kunitz Protease Inhibitor (KPI) domain in the hippocampus than in either the cortex or striatum. The mean percentages of APP 695 for the two groups were 75% in the hippocampus and 82 and 81% in the cortex and striatum, respectively (P < 0.01). No changes in the abundance of GLUT1, GLUT3, nerve growth factor (NGF) or brain derived neurotrophic factor (BDNF) transcripts were detected. However, since both APP and GLUT1 are thought to be regulated post-transcriptionally, the present results do not rule out a change at the protein level. Further work will be required to determine whether nicotine can influence the expression of these proteins which affect neuronal function.

Amyloid precursor protein is enriched in radial glia: implications for neuronal development

The amyloid precursor protein (APP) is a transmembrane glycoprotein and the source of the amyloid that accumulates in brains of individuals with Alzheimer's and other neurodegenerative diseases. Although its function is unknown, previous in vitro studies indicate that APP can facilitate neuronal cell adhesion and neuronal survival. If APP has a similar function in vivo, it should be expressed in fetal brain. To investigate this possibility, the present study determined the cellular distribution of APP in fetal and postnatal mouse brain. Immunocytochemistry and confocal microscopy detected abundant APP immunoreactivity in radial glial cells in fetal and early postnatal mouse brain. In mature mouse brain, APP was detected in neuronal perikarya and dendrites but not in astrocytes. These results support a functional role for APP during radial glia-mediated neuronal histogenesis.

The Alzheimer beta-amyloid protein precursor/protease nexin-II is cleaved by secretase in a trans-Golgi secretory compartment in human neuroglioma cells

Alzheimer beta-amyloid protein precursor (beta APP) is expressed endogenously and abundantly by human neuroglioma (H4) cells. Its secretory processing has been shown to involve discrete proteolysis within the beta A4 region, thus preventing beta-amyloid formation, by an enzyme which has been referred to as 'beta APP secretase'. This cleavage results in secretion of a soluble N-terminal 135 kDa protein and retention of an integral membrane C-terminal fragment within the cell. The membrane-associated C-terminal fragment is sorted to lysosomes where it undergoes limited degradation. We show here that most newly synthesized beta APP is degraded via a non-lysosomal pathway before maturation in H4 cells, and most mature beta APP is processed predominantly by the so-called secretase. The rapid kinetics of appearance/disappearance of a cleaved 135 kDa protein within a microsomal fraction and the slow accumulation of this form in the extracellular medium indicated that secretase cleaves beta APP in an intracellular compartment. Low-temperature block (20 degrees C) was used to demonstrate that beta APP is cleaved within a late Golgi compartment after sulphation which occurs in the trans-Golgi network (TGN). This is consistent with (1) the immunolocalization of most of the beta APP within a Golgi compartment that reacts with wheat germ agglutinin, (2) the fact that less than 1.5% of the total mature full-length beta APP is present at the plasma membrane and (3) subcellular fractionation studies which showed that the mature full-length and intracellular cleaved beta APPs co-sediment with a membrane fraction that is slightly more dense than the plasma membrane. This study provides evidence that most of the beta APP secretase in H4 cells is intracellular, and confirms that the resulting C-terminal fragment is delivered to lysosomes immediately after cleavage. These results are discussed with regard to the possibility that mature full-length beta APP escapes secretase cleavage and is delivered directly from the TGN to the lysosome without passing through the plasma membrane. Either pathway will result in the generation of amyloidogenic fragments.

The beta A4 amyloid protein precursor in human circulation

beta A4, the principal constituent of the brain amyloid collections in Alzheimer's disease, is derived from a much larger precursor, the amyloid protein precursor (APP). APP exists in the blood as full-length, potentially amyloidogenic forms in platelets, and as an attenuated species in plasma and T-lymphocytes. Studies of circulating APP facilitate the elaboration of the function of this protein, as well as the elucidation of its processing in health and disease.

High-level expression and in vitro mutagenesis of a fibrillogenic 109-amino-acid C-terminal fragment of Alzheimer's-disease amyloid precursor protein

We amplified DNA encoding the 3' 109 codons of Alzheimer's-disease amyloid precursor protein (APP) inclusive of the beta protein (A beta) and cytoplasmic domains from cDNA using oligonucleotide primers designed to facilitate cloning into the T7 expression vector pT7Ad23K13. We also modified this construct to generate recombinant molecules incorporating two recently described APP mutants by site-directed mutagenesis. Both native C109 (deletion construct inclusive of the C-terminal 109 residues of APP) and constructs with a single mutation at codon 642 (T-->G, resulting in a substitution of glycine for valine) or a double mutation at codons 595 (G-->T, substituting asparagine for lysine) and 596 (A-->C, substituting leucine for methionine) were expressed in Escherichia coli to levels of 5-20% of total bacterial protein after induction. The major constituent of expressed C109 protein had an apparent molecular mass of 16-18 kDa by SDS/PAGE and appeared to be the full-length construct by size and N-terminal microsequencing. Also present was a 4-5 kDa species that co-purified with C109, constituting only approximately 1% of expressed protein, which was revealed by Western-blot analysis with antibodies specific for A beta epitopes and after biotinylation of purified recombinant C109. This fragment shared N-terminal sequence with, and appeared to arise by proteolysis of, full-length C109 in biosynthetic labelling experiments. C109 spontaneously precipitated after dialysis against NaCl or water, and with prolonged (> 20 weeks) standing was found by electron microscopy to contain a minor (< 5%) fibrillar component that was reactive with antibodies to a C-terminal epitope of APP. Recombinant C109 appears to duplicate some of the biochemical and physicochemical properties of C-terminal A beta-inclusive fragments of APP that have been found in transfected cells, brain cortex and cerebral microvessels.