Intraventricular infusions of antibodies to amyloid-beta-protein precursor impair the acquisition of a passive avoidance response in the rat

The interplay of aryl hydrocarbon receptor/WNT/CTNNB1/Notch signaling pathways regulate amyloid beta precursor mRNA/protein expression and effected the learning and memory of mice

The amyloid beta precursor protein (APP) plays a pathophysiological role in the development of Alzheimer's disease as well as a physiological role in neuronal growth and synaptogenesis. The aryl hydrocarbon receptor (AhR)/WNT/Catenin Beta 1 (CTNNB1)/Notch signaling pathways stamp in many functions, including development and growth of neurons. However, the regulatory role of AhR-/WNT-/CTNNB1-/Notch-induced APP expression and its influence on hippocampal-dependent learning and memory deficits is not clear. Male BALB/C mice received 6-formylindolo[3,2-b]carbazole (an AhR agonist), CH223191(an AhR antagonist), DAPT (an inhibitor of Notch signaling), and XAV-939 (a WNT pathway inhibitor) at a single dose of 100 μg/kg, 1, 5 , and 5 mg/kg of body weight, respectively, via intraperitoneal injection alone or in combination. Gene expression analyses and protein assay were performed on the 7th and 29th days. To assess the hippocampal-dependent memory, all six mice also underwent contextual fear conditioning on the 28th day after treatments. Our results showed that endogenous ligand of AhR has a regulatory effect on APP gene. Also, the interaction of AhR/WNT/CTNNB1 has a positive regulatory effect, but Notch has a negative regulatory effect on the mRNA and protein expression of APP, which have a correlation with mice's learning skills and memory.

Inhibition of protease-activated receptor 1 (PAR1) ameliorates cognitive performance and synaptic plasticity impairments in animal model of Alzheimer's diseases

INTRODUCTION

Alzheimer's disease (AD) is a progressive brain disorder accompanied with synaptic failures and decline in cognitive and learning processes. Protease-activated receptor 1 (PAR1) is the major thrombin receptor in the brain that is implicated in synaptic plasticity and memory formation. In the current study, we hypothesized that inhibition of PAR1 would theoretically prevent amyloid beta (Aβ) accumulation in the brain and then contribute to reduce risk of AD. The aim of the present study was to evaluate the effect of PAR1 inhibition by using SCH (as an inhibitor of PAR1) on spatial learning, memory, and synaptic plasticity in the CA1 region of the hippocampus in rat model of Alzheimer's disease.

METHODS

For the induction of Alzheimer's disease, amyloid beta (Aβ) 1-42 was injected in the CA1 region of the hippocampus. The rats were divided into four groups: group I (surgical sham); group II rat mode of Alzheimer's disease (AD); group III (SCH) (25 μg/kg) intraperitoneally (i.p.), and group IV (AD + SCH). After 14 days of protocol, the rats in group III received SCH and 30 min after injection behavioral and electrophysiological tests were performed. Learning and memory ability was assessed by Morris water maze and novel object recognition tests. Extracellular evoked field excitatory postsynaptic potentials (fEPSP) were recorded in the stratum radiatum of the CA1 area.

RESULTS

Our results showed that AD rats showed impairments in learning and memory, and long-term potentiation (LTP) was not induced in these rats. However, injection of SCH overcame the AD-induced impairment in LTP generation in the CA1 area of the hippocampus and improved learning and memory impairment.

Interactions between amyloid precursor protein-like (APPL) and MAGUK scaffolding proteins contribute to appetitive long-term memory in Drosophila melanogaster

Amyloid precursor protein (APP), the precursor of amyloid beta peptide, plays a central role in Alzheimer's disease (AD), a pathology characterized by memory decline and synaptic loss upon aging. Understanding the physiological role of APP is fundamental in deciphering the progression of AD, and several studies suggest a synaptic function via protein-protein interactions. Nevertheless, it remains unclear whether and how these interactions contribute to memory. In Drosophila, we previously showed that APP-like (APPL), the fly APP homolog, is required for aversive associative memory in the olfactory memory center, the mushroom body (MB). In the present study, we show that APPL is required for appetitive long-term memory (LTM), another form of associative memory, in a specific neuronal subpopulation of the MB, the α'/β' Kenyon cells. Using a biochemical approach, we identify the synaptic MAGUK (membrane-associated guanylate kinase) proteins X11, CASK, Dlgh2 and Dlgh4 as interactants of the APP intracellular domain (AICD). Next, we show that the Drosophila homologs CASK and Dlg are also required for appetitive LTM in the α'/β' neurons. Finally, using a double RNAi approach, we demonstrate that genetic interactions between APPL and CASK, as well as between APPL and Dlg, are critical for appetitive LTM. In summary, our results suggest that APPL contributes to associative long-term memory through its interactions with the main synaptic scaffolding proteins CASK and Dlg. This function should be conserved across species.

Human Brain-Derived Aβ Oligomers Bind to Synapses and Disrupt Synaptic Activity in a Manner That Requires APP

Compelling genetic evidence links the amyloid precursor protein (APP) to Alzheimer's disease (AD) and several theories have been advanced to explain the relationship. A leading hypothesis proposes that a small amphipathic fragment of APP, the amyloid β-protein (Aβ), self-associates to form soluble aggregates that impair synaptic and network activity. Here, we used the most disease-relevant form of Aβ, protein isolated from AD brain. Using this material, we show that the synaptotoxic effects of Aβ depend on expression of APP and that the Aβ-mediated impairment of synaptic plasticity is accompanied by presynaptic effects that disrupt the excitatory/inhibitory (E/I) balance. The net increase in the E/I ratio and inhibition of plasticity are associated with Aβ localizing to synapses and binding of soluble Aβ aggregates to synapses requires the expression of APP. Our findings indicate a role for APP in AD pathogenesis beyond the generation of Aβ and suggest modulation of APP expression as a therapy for AD.SIGNIFICANCE STATEMENT Here, we report on the plasticity-disrupting effects of amyloid β-protein (Aβ) isolated from Alzheimer's disease (AD) brain and the requirement of amyloid precursor protein (APP) for these effects. We show that Aβ-containing AD brain extracts block hippocampal LTP, augment glutamate release probability, and disrupt the excitatory/inhibitory balance. These effects are associated with Aβ localizing to synapses and genetic ablation of APP prevents both Aβ binding and Aβ-mediated synaptic dysfunctions. Our results emphasize the importance of APP in AD and should stimulate new studies to elucidate APP-related targets suitable for pharmacological manipulation.

Therapeutic Potential of Secreted Amyloid Precursor Protein APPsα

Cleavage of the amyloid precursor protein (APP) by α-secretase generates an extracellularly released fragment termed secreted APP-alpha (APPsα). Not only is this process of interest due to the cleavage of APP within the amyloid-beta sequence, but APPsα itself has many physiological properties that suggest its great potential as a therapeutic target. For example, APPsα is neurotrophic, neuroprotective, neurogenic, a stimulator of protein synthesis and gene expression, and enhances long-term potentiation (LTP) and memory. While most early studies have been conducted in vitro, effectiveness in animal models is now being confirmed. These studies have revealed that either upregulating α-secretase activity, acutely administering APPsα or chronic delivery of APPsα via a gene therapy approach can effectively treat mouse models of Alzheimer's disease (AD) and other disorders such as traumatic head injury. Together these findings suggest the need for intensifying research efforts to harness the therapeutic potential of this multifunctional protein.

Restoring Soluble Amyloid Precursor Protein α Functions as a Potential Treatment for Alzheimer's Disease

Soluble amyloid precursor protein α (sAPPα), a secreted proteolytic fragment of nonamyloidogenic amyloid precursor protein (APP) processing, is known for numerous neuroprotective functions. These functions include but are not limited to proliferation, neuroprotection, synaptic plasticity, memory formation, neurogenesis, and neuritogenesis in cell culture and animal models. In addition, sAPPα influences amyloid-β (Aβ) production by direct modulation of APP β-secretase proteolysis as well as Aβ-related or unrelated tau pathology, hallmark pathologies of Alzheimer's disease (AD). Thus, the restoration of sAPPα levels and functions in the brain by increasing nonamyloidogenic APP processing and/or manipulation of its signaling could reduce AD pathology and cognitive impairment. It is likely that identification and characterization of sAPPα receptors in the brain, downstream effectors, and signaling pathways will pave the way for an attractive therapeutic target for AD prevention or intervention. © 2016 Wiley Periodicals, Inc.

Impaired theta-gamma coupling in APP-deficient mice

Amyloid precursor protein (APP) is critically involved in the pathophysiology of Alzheimer's disease, but its physiological functions remain elusive. Importantly, APP knockout (APP-KO) mice exhibit cognitive deficits, suggesting that APP plays a role at the neuronal network level. To investigate this possibility, we recorded local field potentials (LFPs) from the posterior parietal cortex, dorsal hippocampus and lateral prefrontal cortex of freely moving APP-KO mice. Spectral analyses showed that network oscillations within the theta- and gamma-frequency bands were not different between APP-KO and wild-type mice. Surprisingly, however, while gamma amplitude coupled to theta phase in all recorded regions of wild-type animals, in APP-KO mice theta-gamma coupling was strongly diminished in recordings from the parietal cortex and hippocampus, but not in LFPs recorded from the prefrontal cortex. Thus, lack of APP reduces oscillatory coupling in LFP recordings from specific brain regions, despite not affecting the amplitude of the oscillations. Together, our findings reveal reduced cross-frequency coupling as a functional marker of APP deficiency at the network level.

Alzheimer's disease-like APP processing in wild-type mice identifies synaptic defects as initial steps of disease progression

Background

Alzheimer’s disease (AD) is the most frequent form of dementia in the elderly and no effective treatment is currently available. The mechanisms triggering AD onset and progression are still imperfectly dissected. We aimed at deciphering the modifications occurring in vivo during the very early stages of AD, before the development of amyloid deposits, neurofibrillary tangles, neuronal death and inflammation. Most current AD models based on Amyloid Precursor Protein (APP) overproduction beginning from in utero, to rapidly reproduce the histological and behavioral features of the disease within a few months, are not appropriate to study the early steps of AD development. As a means to mimic in vivo amyloid APP processing closer to the human situation in AD, we used an adeno-associated virus (AAV)-based transfer of human mutant APP and Presenilin 1 (PS1) genes to the hippocampi of two-month-old C57Bl/6 J mice to express human APP, without significant overexpression and to specifically induce its amyloid processing.

Results

The human APP, βCTF and Aβ42/40 ratio were similar to those in hippocampal tissues from AD patients. Three months after injection the murine Tau protein was hyperphosphorylated and rapid synaptic failure occurred characterized by decreased levels of both PSD-95 and metabolites related to neuromodulation, on proton magnetic resonance spectroscopy (¹H-MRS). Astrocytic GLT-1 transporter levels were lower and the tonic glutamatergic current was stronger on electrophysiological recordings of CA1 hippocampal region, revealing the overstimulation of extrasynaptic N-methyl D-aspartate receptor (NMDAR) which precedes the loss of long-term potentiation (LTP). These modifications were associated with early behavioral impairments in the Open-field, Y-maze and Morris Mater Maze tasks.

Conclusions

Altogether, this demonstrates that an AD-like APP processing, yielding to levels of APP, βCTF and Aβ42/Aβ40 ratio similar to those observed in AD patients, are sufficient to rapidly trigger early steps of the amyloidogenic and Tau pathways in vivo. With this strategy, we identified a sequence of early events likely to account for disease onset and described a model that may facilitate efforts to decipher the factors triggering AD and to evaluate early neuroprotective strategies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0070-y) contains supplementary material, which is available to authorized users.

Amyloid β precursor protein as a molecular target for amyloid β--induced neuronal degeneration in Alzheimer's disease

A role of amyloid β (Aβ) peptide aggregation and deposition in Alzheimer's disease (AD) pathogenesis is widely accepted. Significantly, abnormalities induced by aggregated Aβ have been linked to synaptic and neuritic degeneration, consistent with the "dying-back" pattern of degeneration that characterizes neurons affected in AD. However, molecular mechanisms underlying the toxic effect of aggregated Aβ remain elusive. In the last 2 decades, a variety of aggregated Aβ species have been identified and their toxic properties demonstrated in diverse experimental systems. Concurrently, specific Aβ assemblies have been shown to interact and misregulate a growing number of molecular effectors with diverse physiological functions. Such pleiotropic effects of aggregated Aβ posit a mayor challenge for the identification of the most cardinal Aβ effectors relevant to AD pathology. In this review, we discuss recent experimental evidence implicating amyloid β precursor protein (APP) as a molecular target for toxic Aβ assemblies. Based on a significant body of pathologic observations and experimental evidence, we propose a novel pathologic feed-forward mechanism linking Aβ aggregation to abnormalities in APP processing and function, which in turn would trigger the progressive loss of neuronal connectivity observed early in AD.

Characterization of the beta amyloid precursor protein-like gene in the central nervous system of the crab Chasmagnathus. Expression during memory consolidation

Background

Human β-amyloid, the main component in the neuritic plaques found in patients with Alzheimer's disease, is generated by cleavage of the β-amyloid precursor protein. Beyond the role in pathology, members of this protein family are synaptic proteins and have been associated with synaptogenesis, neuronal plasticity and memory, both in vertebrates and in invertebrates. Consolidation is necessary to convert a short-term labile memory to a long-term and stable form. During consolidation, gene expression and de novo protein synthesis are regulated in order to produce key proteins for the maintenance of plastic changes produced during the acquisition of new information.

Results

Here we partially cloned and sequenced the beta-amyloid precursor protein like gene homologue in the crab Chasmagnathus (cappl), showing a 37% of identity with the fruit fly Drosophila melanogaster homologue and 23% with Homo sapiens but with much higher degree of sequence similarity in certain regions. We observed a wide distribution of cappl mRNA in the nervous system as well as in muscle and gills. The protein localized in all tissues analyzed with the exception of muscle. Immunofluorescence revealed localization of cAPPL in associative and sensory brain areas. We studied gene and protein expression during long-term memory consolidation using a well characterized memory model: the context-signal associative memory in this crab species. mRNA levels varied at different time points during long-term memory consolidation and correlated with cAPPL protein levels

Conclusions

cAPPL mRNA and protein is widely distributed in the central nervous system of the crab and the time course of expression suggests a role of cAPPL during long-term memory formation.

The chick as a model for the study of the cellular mechanisms and potential therapies for Alzheimer's disease

While animal experiments have contributed much to our understanding of the mechanisms of Alzheimer's disease (AD), their value in predicting the effectiveness of treatment strategies in clinical trials has remained controversial. The disparity between the results obtained in animal models and clinical trials may in part be explained by limitations of the models and species-specific differences. We propose that one trial passive avoidance in the day-old chick is a useful system to study AD because of the close sequence homologies of chick and human amyloid precursor protein (APP). In the chick, APP is essential for memory consolidation, and disrupting its synthesis or structure results in amnesia. RER, a tripeptide sequence corresponding to part of the growth domain of APP, can restore memory loss and act as a cognitive enhancer. We suggest that RER and its homologues may form the basis for potential pharmacological protection against memory loss in AD.

Behavioral and neurochemical characterization of mice deficient in the N-type Ca2+ channel alpha1B subunit

N-type voltage-dependent calcium channels (VDCCs) play an important role in neurotransmission, synaptic plasticity, and brain development. They are composed of several subunits named alpha(1), alpha(2), delta, beta and gamma. The alpha(1) subunit is essential for channel functions and determines fundamental channel properties. Since N-type VDCC are critically involved in the release of neurotransmitters and clinical relevance, we predicted that alpha(1) subunit KO mice would show several alterations in behavior. In the present study, we investigated neuronal functions in mice lacking the alpha(1B) (Ca(V)2.2) subunit of the N-type calcium channels. Ca(V)2.2(-/-) mice exhibited a significant increase in locomotion on an activity wheel during the dark phase. Furthermore, when challenged with apomorphine, mutant mice showed enhanced locomotor activity. Cognitive functions were examined using a Y-maze task for short-term memory and a passive avoidance task for long-term memory. The Y-maze revealed no differences in spontaneous alternation behavior between mutant and wild-type mice. The passive avoidance test revealed that the latency time in mutant mice was significantly decreased. The mutant mice showed prepulse inhibition deficits reminiscent of the sensorimotor gating deficits observed in a large majority of schizophrenic patients. Decreases in baseline levels of dopamine and serotonin within the striata and frontal cortices of mutant mice were also observed. These results suggest that Ca(2+) in the central nervous system modulates various neurophysiological functions, such as locomotor activity, long-term memory, and sensorimotor gating through the alpha(1B) subunit of the N-type calcium channels.

Amyloid precursor protein and its homologues: a family of proteolysis-dependent receptors

The Alzheimer's amyloid precursor protein (APP) belongs to a conserved gene family that also includes the mammalian APLP1 and APLP2, the Drosophila APPL, and the C. elegans APL-1. The biological function of APP is still not fully clear. However, it is known that the APP family proteins have redundant and partly overlapping functions, which demonstrates the importance of studying all APP family members to gain a more complete picture. When APP was first cloned, it was speculated that it could function as a receptor. This theory has been further substantiated by studies showing that APP and its homologues bind both extracellular ligands and intracellular adaptor proteins. The APP family proteins undergo regulated intramembrane proteolysis (RIP), generating secreted and cytoplasmic fragments that have been ascribed different functions. In this review, we will discuss the APP family with focus on biological functions, binding partners, and regulated processing.

Amyloid beta mediates memory formation

The amyloid precursor protein (APP) undergoes sequential cleavages to generate various polypeptides, including the amyloid beta (1-42) peptide (Abeta[1-42]), which is believed to play a major role in amyloid plaque formation in Alzheimer's disease (AD). Here we provide evidence that, in contrast with its pathological role when accumulated, endogenous Abeta in normal hippocampi mediates learning and memory formation. Furthermore, hippocampal injection of picomolar concentrations of exogenous Abeta(1-42) enhances memory consolidation. Correlative data suggest that Abeta peptides may exert their function via nicotinic acethylcoline receptors. Hence, Abeta peptides, including Abeta(1-42), play an important physiological role in hippocampal memory formation.

Endogenous secreted amyloid precursor protein-alpha regulates hippocampal NMDA receptor function, long-term potentiation and spatial memory

Secreted amyloid precursor protein-alpha (sAPP alpha) levels are reduced during the pathogenesis of Alzheimer's disease, but the significance of this for neural function is not well understood. Here, we show that intrahippocampal infusion of antibodies targeted to endogenous sAPP alpha reduced long-term potentiation (LTP) in the dentate gyrus of adult rats by approximately 50%. Conversely, infusion of recombinant sAPP alpha dose-dependently increased LTP and facilitated in vitro tetanically evoked NMDA receptor-mediated currents. Pharmacological inhibition of alpha-secretase and other a-disintegrin-and-metalloproteases by TAPI-1 reduced both LTP and tetanus-evoked NMDA receptor-mediated currents in dentate granule cells. Both effects were prevented by co-application of exogenous recombinant sAPP alpha. Similarly, spatial memory was inhibited by intrahippocampal TAPI-1, an effect that was prevented by co-application of recombinant sAPP alpha. Together these findings indicate that endogenous sAPP alpha is a key contributor to synaptic plasticity and spatial memory. Its reduced production in Alzheimer's disease may thus contribute to the clinical memory deficits.

Amyloid precursor protein knockout mice show age-dependent deficits in passive avoidance learning

Amyloid precursor protein (APP) is involved in the pathogenesis of Alzheimer's disease (AD), but its role in cognition has been relatively little studied. APP knockout (KO) animals have been described previously and show deficits in grip strength, reduced locomotor activity and impaired learning and memory in a conditioned avoidance test and the Morris water-maze. In order to further investigate the in vivo function of APP and its proteolytic derivatives, we tested APP KO mice and age-matched wild type controls at two different ages, 3 and 8 months, in a range of behavioural tests measuring neuromuscular, locomotor and cognitive functions. These tests included the acquisition of a passive avoidance response as a measure of long-term memory of an aversive experience, and spontaneous alternation in a Y-maze, regarded as a measure of spatial short-term memory. The absence of APP expression in APP KO mice was confirmed at the protein level using hippocampal tissue in Western blotting. APP KO mice displayed deficits in forelimb grip strength and locomotor activity in agreement with previous studies. In the Y-maze test used for spontaneous alternation behaviour, APP KO animals did not exhibit reduced alternation rates. On the other hand, in the passive avoidance test, APP KO mice showed an age-related deficit in retention of memory for an aversive experience. The results suggest that APP and/or its proteolytic derivatives may play a role in long-term memory in adult brain and/or may be required during the development and maintenance of neuronal networks involved in this type of memory.

Amyloid precursor protein knockdown by siRNA impairs spontaneous alternation in adult mice

The cleavage-product of amyloid precursor protein (APP) constitutes the core component of plaques found in the brains of Alzheimer's disease (AD) patients. APP is ubiquitously expressed and its precise physiological functions remain unclear. This protein has been proposed to regulate synaptic function and processes underlying learning and memory. While APP knockout mice show behavioral impairments, these may occur due to early changes during development and/or due to abolition of APP function in adult. To investigate the acute effects of APP knockdown without involving developmental processes, APP expression was reduced using RNA interference in adult mouse brain. Small interfering RNAs (siRNAs) that down-regulated mouse APP protein levels (APP-siRNA) were identified using an APP plasmid-siRNA co-transfection assay in mouse NIH/3T3 fibroblast cells. Infusion of APP-siRNAs into the ventricular system for 2 weeks also down-regulated APP mRNA in mouse brain. Highest knockdown of APP mRNA levels was found in the CA2-CA3 regions of the hippocampus. Mice treated with the most active APP-siRNA showed a significant reduction in spontaneous alternation rate in the Y-maze, without effects on forelimb grip strength or locomotor activity. These data suggest that acute knockdown of APP in adult mouse brain impairs hippocampus-dependent spatial working memory.

Protection against Aβ-induced memory loss by tripeptide D-Arg-L-Glu-L-Arg

The molecular and cellular mechanisms underlying the memory deficits in Alzheimer's disease are increasingly thought to be associated with faulty processing of amyloid precursor protein. Following our earlier findings that it is possible to use the tripeptide RER (NH2-D-Arg-L-Glu-L-Arg-COOH, derived from the external domain of amyloid precursor protein) to rescue memory in animal models, we report here that the diasteromeric (D/L) form of the acetylated tripeptide RER protects against Abeta-induced memory loss for a passive avoidance task in young chicks and enhances retention for a weak version of the task when injected peripherally up to 12 h before training. The tripeptide readily crosses the blood-brain barrier, binds to membrane receptor sites in the brain and is without adverse effects on general behaviour. We discuss this finding in the context of other studies of the importance of peptides containing D-amino acids, and conclude that these RER-related peptides may form the basis for a potential therapeutic agent in the early stages of Alzheimer's disease.

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.

Amyloid precursor protein: from synaptic plasticity to Alzheimer's disease

The amyloid precursor protein (APP) has been shown to be implicated in age-associated plastic changes at synapses that might contribute to memory loss in Alzheimer's disease. As APP has previously been reported to have multiple functions during normal development, and as human and avian APP share 95% homology in amino acid sequence, we have employed a one-trial passive avoidance task in day-old chicks to study its role in the process of memory formation. Administration of anti-APP antibodies, raised against human APP, APP-antisense, and Abeta during pre-training, prevented memory formation without effects on general behavior or initial acquisition. Amnesia is apparent by 30 min post-training and lasts for at least 24 hours. Injection of APP-derived peptides RERMS (APP(328-332)) and RER (APP(328-330)) homologous to the short stretches of amino acids in the Kang sequence (APP(319-335)), rescue the memory in animals rendered amnestic by previous (anti-APP antibody, antisense, and Abeta pretreatments. The protected form of RER, with a prolonged half-life (acetylated RER), proved to be effective when injected intracranially and peripherally. The tripeptide RER exerts its biological activity by binding to two neuronal plasma membrane proteins (60 and 110 kDa). The results obtained in this study suggest that RER alleviates memory deficits via receptor-mediated events, and that short APP-derived peptides might represent a novel group of therapeutically active molecules for the alleviation of memory deficits in age-related dementias.

Neural cell adhesion molecule (NCAM-/-) null mice show impaired sensitization of the startle response

The neural cell adhesion molecule (NCAM) plays important roles in development of the nervous system and in synaptic plasticity and memory formation in the adult. The present study sought to further investigate the role of NCAM in learning by testing habituation and footshock sensitization learning of the startle response (SR) in NCAM null mutant (NCAM-/-) and wildtype littermate (NCAM+/+) mice. Whereas habituation is a form of non-associative learning, footshock sensitization is induced by rapid contextual fear conditioning. Habituation was tested by repetitive presentation of acoustic and tactile startle stimuli. Although NCAM-/- mice showed differences in sensitivity in both stimulus modalities, habituation learning was intact in NCAM-/- mice, suggesting that NCAM does not play a role in the mechanisms underlying synaptic plasticity in the startle pathway. Footshock sensitization was elicited by presentation of electric footshocks between two series of acoustic stimuli. In contrast to habituation, footshock sensitization learning was attenuated in NCAM-/- mice: the acoustic SR increase after the footshocks was lower in the mutant than in wildtype mice, indicating that NCAM plays an important role in the relevant brain areas, such as amygdala and/or the hippocampus.

Development of beta-amyloid-induced neurodegeneration in Alzheimer's disease and novel neuroprotective strategies

Alzheimer's disease (AD) is a form of dementia in which people develop rapid neurodegeneration, complete loss of cognitive abilities, and are likely to die prematurely. At present, no treatment for AD is known. One of the hallmarks in the development of AD is the aggregation of amyloid protein fragments in the brain, and much evidence points towards beta-amyloid fragments being one of the main causes of the neurodegenerative processes. This review summarises the present concepts and theories on how AD develops, and lists the evidence that supports them. A cascade of biochemical events is initiated that ultimately leads to neuronal death involving an imbalance of intracellular calcium homeostasis via activation of calcium channels, intracellular calcium stores, and subsequent production of free radicals by calcium-sensitive enzymes. Secondary processes include inflammatory responses that produce more free radicals and the induction of apoptosis. Recently, several new strategies have been proposed to try to ameliorate the neurodegenerative developments associated with AD. These include the activation of neuronal growth factor receptors and insulin-like receptors, both of which have neuroprotective properties. Furthermore, the role of cholesterol and potential protective properties of cholesterol-lowering drugs are under intense investigation. Other promising strategies include the inhibition of beta- and gamma-secretases which produce beta-amyloid, activation of proteases that degrade beta-amyloid, glutamate receptor selective drugs, antioxidants, and metal chelating agents, all of which prevent formation of plaques. Novel drugs that act at different levels of the neurodegenerative processes show great promise to reduce neurodegeneration. They could help to prolong the time of unimpaired cognitive abilities of people who develop AD, allowing them to lead an independent life.

Amyloid precursor protein expression in the rat hippocampal dentate gyrus modulates during memory consolidation

Despite advances in our understanding of the basic biology of amyloid precursor protein (APP), the normal physiological function(s) of APP in learning and memory remains unclear. Here we show increased APP degradation in the hippocampus to be associated with the consolidation of a passive avoidance response. Neurone-specific APP695 expression became transiently reduced 2-4 h post-training through association with endosomal adaptin proteins and enhanced internalization. By contrast, internalization of glial-associated APP containing a Kunitz protease inhibitor-like domain (APP-KPI) was dependent on the low-density lipoprotein receptor-related protein (LRP). In addition, LRP expression and association with apolipoprotein E increased in the 2-4 h post-training period. The LRP antagonist receptor-associated protein prevented the APP-KPI internalization and LRP-apolipoprotein E association and this resulted in amnesia. Degradation of APP695 and APP-KPI did not appear to be related to alpha-secretase activity, as no learning-associated increase of secreted APP was observed in the CSF. Moreover, as internalization of APP isoforms was observed only in dentate gyrus, it probably relates to the learning-associated restructuring of the perforant path terminals. Memory-associated APP processing in both neuronal and glial compartments points to a role for glial unsheathing of synaptic connections, an event required for the synaptic restructuring that accompanies memory consolidation. These observations may have a direct relevance to understanding the pathophysiology of Alzheimer's disease as beta/gamma-secretase-derived beta-amyloid is formed following internalization of cell surface APP into the endosomal compartment.

The peptide sequence Arg-Glu-Arg, present in the amyloid precursor protein, protects against memory loss caused by A beta and acts as a cognitive enhancer

Amino acid sequences containing the palindromic tripeptide RER, matching amino acids 328-330 of the amyloid precursor protein APP, when injected intracerebrally prior to or just after training, protect against memory loss induced by amyloid-beta (A beta) in a one-trial passive avoidance task in the young chick. RER also acts as a cognitive enhancer, strengthening memory for a weak version of the task. N-terminal acylation of RER protects it against rapid degradation, and AcRER is effective in restoring memory if administered peripherally. Biotinylated RER binds to chick neuronal perikarya in an APP-displaceable manner via 66 and approximately 110 kDa neuronal cell membrane proteins. We suggest that RER binding is likely to exert effects on memory retention via receptor-mediated events that include activation of second messenger pathways. These findings suggest that RER and its derivatives may offer a novel approach to enhancing the neuroprotective effects of APP and alleviating the effects of memory loss in the early stages of Alzheimer's disease.

Genetic programming by the proteolytic fragments of the amyloid precursor protein: somewhere between confusion and clarity

Mice engineered to overexpress disease-causing mutant amyloid precursor proteins (APP) display plaque deposition, but lack the hyperphosphorylated tau and massive neuronal loss characteristic of Alzheimer's disease (AD). Global gene expression profiles of brain regions from AD patients show upregulation of proapoptotic and inflammatory genes and down-regulation of neurotrophic, MAPK, phosphatase, and synaptic genes, while a profile of mice overexpressing a mutant APP shows the opposite trends in apoptotic and neurotrophic genes. The proteolytic fragments of the amyloid precursor protein have distinct biological actions. Both the gamma-secretase cleaved COOH-terminal fragment (CTFgamma) and the alpha-secretase cleaved NH2-terminal of APP (sAPPalpha) can regulate gene expression. While Abeta and CTFgamma can lead to toxicity and cell death, sAPPalpha promotes neurite outgrowth, enhances memory, and protects against a variety of insults, including Abeta toxicity. In AD, Abeta levels increase while sAPPalpha levels decrease. These subtleties in the levels of APP cleavage products are not reproduced in mice overexpressing mutant APP. In fact, the gene expression changes driven by sAPPalpha, such as increases in transthyretin and insulin-like growth factor 2, may protect these mice from high levels of Abeta.

Roles of amyloid precursor protein and its fragments in regulating neural activity, plasticity and memory

Amyloid-beta precursor protein (APP) is a membrane-spanning protein with a large extracellular domain and a much smaller intracellular domain. It is the source of the amyloid-beta (Abeta) peptide found in neuritic plaques of Alzheimer's disease (AD) patients. Because Abeta shows neurotoxic properties, and because familial forms of AD promote Abeta accumulation, a massive international research effort has been aimed at understanding the mechanisms of Abeta generation, catabolism and toxicity. APP, however, is an extremely complex molecule that may be a functionally important molecule in its full-length configuration, as well as being the source of numerous fragments with varying effects on neural function. For example, one fragment derived from the non-amyloidogenic processing pathway, secreted APPalpha (sAPPalpha), is neuroprotective, neurotrophic and regulates cell excitability and synaptic plasticity, while Abeta appears to exert opposing effects. Less is known about the neural functions of other fragments, but there is a growing interest in understanding the basic biology of APP as it has become recognized that alterations in the functional activity of the APP fragments during disease states will have complex effects on cell function. Indeed, it has been proposed that reductions in the level or activity of certain APP fragments, in addition to accumulation of Abeta, may play a critical role in the cognitive dysfunction associated with AD, particularly early in the course of the disease. To test and modify this hypothesis, it is important to understand the roles that full-length APP and its fragments normally play in neuronal structure and function. Here we review evidence addressing these fundamental questions, paying particular attention to the contributions that APP fragments play in synaptic transmission and neural plasticity, as these may be key to understanding their effects on learning and memory. It is clear from this literature that APP fragments, including Abeta, can exert a powerful regulation of key neural functions including cell excitability, synaptic transmission and long-term potentiation, both acutely and over the long-term. Furthermore, there is a small but growing literature confirming that these fragments correspondingly regulate behavioral learning and memory. These data indicate that a full account of cognitive dysfunction in AD will need to incorporate the actions of the full complement of APP fragments. To this end, there is an urgent need for a dedicated research effort aimed at understanding the behavioral consequences of altered levels and activity of the different APP fragments as a result of experience and disease.

Analysis of differential gene expression supports a role for amyloid precursor protein and a protein kinase C substrate (MARCKS) in long-term memory

Previous work has identified the intermediate and medial part of the hyperstriatum ventrale (IMHV) as a region of the chick brain storing information acquired through the learning process of imprinting. We have examined in this brain region changes in expression of candidate genes involved in memory. Chicks were exposed to a rotating red box and the strength of their preference for it, a measure of learning, determined. Brain samples were removed approximately 24 h after training. Candidate genes whose expressions were different in IMHV samples derived from strongly imprinted chicks relative to those from chicks showing little or no learning were identified using subtractive hybridization. The translation products of two candidate genes were investigated further in samples from the left and right IMHV and from two other brain regions not previously implicated in imprinting, the left and right posterior neostriatum. One of the proteins was the amyloid precursor protein (APP), the other was myristoylated alanine rich C kinase substrate (MARCKS). In the left IMHV the levels of the two proteins increased with the strength of learning. The effects in the right IMHV were not significantly different from those in the left. There were no effects of learning in the posterior neostriatum. This is the first study to relate changes in the amounts of MARCKS and APP proteins to the strength of learning in a brain region known to be a memory store and demonstrates that the systematic identification of protein molecules involved in memory formation is possible.

Environmental conditions influence hippocampus-dependent behaviours and brain levels of amyloid precursor protein in rats

Sprague-Dawley rats were reared in enriched (EC; group housing, exposure to stimulating objects, frequent handling) or restricted (RC; individual housing, no exposure to stimulating objects, minimal handling) environments starting on day 23 of life. At six months of age, they underwent behavioural tests to assess 'cognitive' and 'stimulus-response' memory, selective attention, and inflammatory pain processing. Alterations in synapses and cell survival may occur as a result of environment differences; therefore we assessed the brain levels of several proteins implicated in neurite outgrowth, synaptogenesis, and cell survival. Brains were dissected and analysed for amyloid precursor protein (APP) and other synaptic and cytoskeletal proteins using Western blotting. The performance of EC animals in a hidden platform water maze task, and in a test of selective attention (both of which are thought to involve the hippocampus) was superior to that of RC animals. In contrast, performance of RC animals on two stimulus-response tasks, the visible platform water maze test and simple visual discrimination (both of which are thought to be hippocampal independent) was indistinguishable from that of EC animals. Male EC rats displayed a different behavioural response to formalin during the inflammatory phase of nociception--the phase affected by hippocampal processing; a similar trend was observed in females. Female but not male RC rats exhibited elevated plasma corticosterone levels; adrenal weights were unaffected by environmental conditions. Region-specific increases in brain levels of APP, neurofilament-70 (NF-70), and platelet-activating factor receptor (PAF-R) were found in EC rats. These data suggest that enriched animals manifest enhanced functioning of certain hippocampus-mediated behaviours when compared with that of their restricted counterparts; and that brain levels of various synaptic and structural proteins involved in neurite outgrowth, cell survival, and synaptogenesis, are affected by environmental factors.

Age-dependent differential regulation of genes encoding APP and alpha-synuclein in hippocampal synaptic plasticity

We investigated the modulation of the messenger RNA encoding the amyloid precursor protein (APP) and alpha-synuclein following induction of long-term potentiation (LTP) in the dentate gyrus of young and aged rats. Three hours after tetanic stimulation, LTP induced in the young rats was maintained; the aged rats, however, fell into two subgroups: those in which LTP was maintained, and those in which LTP had declined to basal levels. In young rats, the global expression of mRNAs of all isoforms of APP and in particular that of the isoform lacking the KPI domain were significantly upregulated. In aged rats, the global expression of mRNAs of all isoforms of APP was not modified, regardless of whether LTP was maintained or not. The level of mRNA encoding the Kunitz protease-inhibitory (KPI)-minus isoform of APP, however, was increased in aged rats in which LTP was maintained, suggesting that the gene of this isoform may be more specifically regulated by synaptic plasticity. In contrast, we found that the gene encoding alpha-synuclein showed a trend towards being downregulated at the mRNA level in young rats following LTP, and significantly so in aged rats in which LTP was maintained, whereas it was not downregulated in aged rats with decremental LTP. These data suggest that the regulated expression of APP isoforms is part of the tanscriptional response associated with the enduring forms of synaptic plasticity and is altered with age. Whereas the level of alpha-synuclein mRNA is not apparently modified in normal LTP, it may reflect a mechanism of apoptotic cell death in aging that is in part responsible for decremental synaptic plasticity.

Phosphatidylinositol-3-kinase-Akt kinase and p42/p44 mitogen-activated protein kinases mediate neurotrophic and excitoprotective actions of a secreted form of amyloid precursor protein

The alpha-secretase-derived form of the amyloid precursor protein (sAPPalpha), which is released from neurons in an activity-dependent manner, has been shown to promote long-term survival of hippocampal and cortical neurons in culture and can protect those neurons against excitotoxic and ischemic injury in culture and in vivo. The signal transduction pathway(s) activated by sAPPalpha has not been established. We now report that sAPPalpha activates the phosphatidylinositol-3-kinase (PI(3)K)-Akt kinase signaling pathway in cultured hippocampal neurons. sAPPalpha also stimulates phosphorylation of p42 (ERK1) and p44 (ERK2) mitogen-activated protein (MAP) kinases by a PI(3)K-independent pathway. Treatment of neurons with sAPPalpha protects them against death induced by trophic factor deprivation and exposure to glutamate, and these survival-promoting effects of sAPPalpha are abolished or attenuated when either PI(3)K or p42/p44 MAP kinases are selectively blocked. Exposure of neurons to sAPPalpha resulted in a decrease in the level of IkappaBbeta and an increase in NF-kappaB DNA binding activity, both of which were blocked by wortmannin, suggesting that the transcription factor NF-kappaB may be a downstream target of the PI(3)K-Akt pathway that may play a role in the cell survival-promoting action of sAPPalpha. These findings suggest that the PI(3)K-Akt pathway and p42/p44 MAP kinases mediate responses of neurons to sAPPalpha in physiological and pathological settings, with implications for synaptic plasticity and the pathogenesis of Alzheimer's disease.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

The beta-amyloid precursor protein and its derivatives: from biology to learning and memory processes

Intensive investigation towards the understanding of the biology and physiological functions of the beta-amyloid precursor protein (APP) have been supported since it is known that a 39-43 amino acid fragment of APP, called the beta-amyloid protein (Abeta), accumulates in the brain parenchyma to form the typical lesions associated with Alzheimer's disease (AD). It emerges from extensive data that APP and its derivatives show a wide range of contrasting physiological properties and therefore might be involved in distinct physiological functions. Abeta has been shown to disrupt neuronal activity and to demonstrate neurotoxic properties in a wide range of experimental procedures. In contrast, both in vitro and in vivo studies suggest that APP and/or its secreted forms are important factors involved in the viability, growth and morphological and functional plasticity of nerve cells. Furthermore, several recent studies suggest that APP and its derivatives have an important role in learning and memory processes. Memory impairments can be induced in animals by intracerebral treatment with Abeta. Altered expression of the APP gene in aged animals or in genetically-modified animals also leads to memory deficits. By contrast, secreted forms of APP have recently been shown to facilitate learning and memory processes in mice. These interesting findings open novel perspectives to understand the involvement of APP in the development of cognitive deficits associated with AD. In this review, we summarize the current data concerning the biology and the behavioral effects of APP and its derivatives which may be relevant to the roles of these proteins in memory and in AD pathology.

Reduced cerebrospinal fluid levels of alpha-secretase-cleaved amyloid precursor protein in aged rats: correlation with spatial memory deficits

The amyloid precursor protein undergoes proteolysis at several sites to yield a number of functionally relevant peptides, including beta-amyloid and the soluble amyloid precursor protein derivatives alpha-soluble amyloid precursor protein and beta-soluble amyloid precursor protein. beta-Amyloid is the primary constituent of senile plaques associated with Alzheimer's disease, while a-soluble amyloid precursor protein promotes synaptogenesis and plays a role in neuroprotective processes. We tested for age-related alterations in these amyloid precursor protein proteolytically derived peptides by measuring the levels of alpha-soluble amyloid precursor protein, total soluble amyloid precursor proteins (alpha- and beta-soluble amyloid precursor protein combined) and beta-amyloid in cerebrospinal fluid from three-, 13- and 23-month-old Fischer-344 rats. Western blot analysis using selective antibodies revealed 50% less total soluble amyloid precursor protein and a-soluble amyloid precursor protein in cisternal cerebrospinal fluid from 23-month-old rats compared with three- and 13-month-old animals. Mass spectrometric analysis indicated, however, that beta-amyloid in cerebrospinal fluid was not different between the three age groups. In a second group of young (five to six months of age) and aged (24-25 months of age) rats, spatial working and reference memory were assessed in a water maze followed by collection of cerebrospinal fluid. As a group, the aged rats consistently performed below the young rats in both working and reference memory tests. The aged rats also had 49% less cerebrospinal fluid alpha-soluble amyloid precursor protein than did their younger counterparts. There was a positive correlation (r= 0.52-0.57, P < 0.001) between performance in spatial memory tasks and cerebrospinal fluid alpha-soluble amyloid precursor protein in these young and aged rats. These results suggest that there is a positive association between cerebrospinal fluid levels of alpha-soluble amyloid precursor protein and cognitive performance in rats, and that alpha-soluble amyloid precursor protein may be involved in the spatial learning and memory changes that accompany ageing.

Secreted form of amyloid precursor protein enhances basal glucose and glutamate transport and protects against oxidative impairment of glucose and glutamate transport in synaptosomes by a cyclic GMP-mediated mechanism

Synaptic dysfunction and degeneration are believed to underlie the cognitive deficits that characterize Alzheimer's disease, and overactivation of glutamate receptors under conditions of increased oxidative stress and metabolic compromise may contribute to the neurodegenerative process in many different disorders. The secreted form of amyloid precursor protein (sAPPalpha), which is released from neurons in an activity-dependent manner, can modulate neurite outgrowth, synaptic plasticity, and neuron survival. We now report that sAPPalpha can enhance glucose and glutamate transport in synaptic compartments. Treatment of cortical synaptosomes with nanomolar concentrations of sAPPalpha resulted in an attenuation of impairment of glutamate and glucose transport induced by exposure to amyloid beta-peptide and Fe2+. The protective effect of sAPPalpha was mimicked by treatment with 8-bromo-cyclic GMP and blocked by a cyclic GMP-dependent protein kinase inhibitor, suggesting that protective action of sAPPalpha is mediated by cyclic GMP. Our data suggest that glucose and glutamate transport can be regulated locally at the level of the synapse and further suggest important roles for sAPPalpha and cyclic GMP in modulating synaptic physiology under normal and pathophysiological conditions.

No hippocampal neuron or synaptic bouton loss in learning-impaired aged beta-amyloid precursor protein-null mice

Aged beta-amyloid precursor protein-null mice were used to investigate the relationship between beta-amyloid precursor protein, hippocampal neuron and synaptic bouton number, and cognitive function. Learning and memory performance of aged beta-amyloid precursor protein-null mice and age-matched controls were assessed in the Morris water maze. Beta-amyloid precursor protein-null mice demonstrated impaired task acquisition as measured by significantly longer swim path lengths, a higher percentage of failed trials, and more frequent thigmotaxis behavior than controls. In a subsequent probe trial, beta-amyloid precursor protein-null mice spent significantly less time in the old goal quadrant, and made fewer crossings over the old platform location than did controls. No differences in motor or visual skills were observed which could account for the performance differences. In light of these findings and previous evidence for a role of beta-amyloid precursor protein in neuronal maintenance and synaptogenesis, we pursued the hypothesis that the learning impairment of beta-amyloid precursor protein-null mice may be a reflection of differences in neuron or synaptophysin-positive presynaptic bouton number. Thus, unbiased stereological analysis was used to estimate neuron and synaptic bouton number in dentate gyrus and hippocampal CA1 of the behaviorally characterized mice. No difference in neuron or synaptophysin-positive presynaptic bouton number was found between the beta-amyloid precursor protein-null mice and age-matched controls. Our results suggest that the learning impairment of beta-amyloid precursor protein-null mice is not mediated by a loss of hippocampal neurons or synaptic boutons.

Novel domain-specific actions of amyloid precursor protein on developing synapses

The effect of the secretory form of amyloid precursor protein (sAPP) on synaptic transmission was examined by using developing neuromuscular synapses in Xenopus cell cultures. The frequency of spontaneous postsynaptic currents (SSCs) was reduced by the addition of sAPP, whereas the amplitude of impulse-evoked postsynaptic currents (ESCs) was increased by sAPP. These opposing effects on spontaneous versus evoked release were separated by using the specific domain of APP. The C-terminal fragment of sAPP (CAPP) only reduced SSC frequency and did not affect ESCs. By contrast, the N-terminal fragment of sAPP (NAPP) did not affect SSC frequency but did increase ESC amplitude. The reduction of SSC frequency by sAPP appears to be mediated by activation of potassium channels through a cGMP-dependent pathway, whereas the increase of ESC amplitude is mediated by a different pathway involving activation of protein kinase(s). These results suggest the potential role of sAPP as a modulator of synaptic activity by two specific domains.

Memory-enhancing effects of secreted forms of the beta-amyloid precursor protein in normal and amnestic mice

When administered intracerebroventricularly to mice performing various learning tasks involving either short-term or long-term memory, secreted forms of the beta-amyloid precursor protein (APPs751 and APPs695) have potent memory-enhancing effects and block learning deficits induced by scopolamine. The memory-enhancing effects of APPs were observed over a wide range of extremely low doses (0.05-5,000 pg intracerebroventricularly), blocked by anti-APPs antisera, and observed when APPs was administered either after the first training session in a visual discrimination or a lever-press learning task or before the acquisition trial in an object recognition task. APPs had no effect on motor performance or exploratory activity. APPs695 and APPs751 were equally effective in the object recognition task, suggesting that the memory-enhancing effect of APPs does not require the Kunitz protease inhibitor domain. These data suggest an important role for APPss on memory processes.

Neurobehavioral development, adult openfield exploration and swimming navigation learning in mice with a modified beta-amyloid precursor protein gene

The processing of beta-amyloid precursor protein (betaAPP) and its metabolites plays an important role in the pathogenesis of Alzheimer's disease (AD) and Down's syndrome. The authors have reported elsewhere that a targeted mutation resulting in low expression of a shortened betaAPP protein (betaAPP(delta/delta)) entails reduced learning abilities. Here the authors investigate whether these effects were caused by postnatal developmental actions of the altered protein. The authors examined 35 mice carrying the betaAPP(delta/delta) mutation for somatic growth and sensorimotor development during the first 4 postnatal weeks (pw) and compared them with 31 wildtype litter-mates. Thereafter, the same mice were tested at about 10 weeks of age for openfield behavior and for swimming navigation learning. Mutant mice showed both transient and long-lasting deficits in development. Body weight deficit started to emerge at postnatal day (pd) 12, peaked with a 15.1% deficit at pd 27 and lasted until pw 33-37. Significant transient deficits in mutant mice during sensorimotor development were observed in three time windows (pd 3-10, pd 11-19 and pd 20-27), long-lasting effects, manifest at pw 8-12 and pw 33-37, emerged at any of the three periods. In the adult mice, exploratory activity of betaAPP mutants in the openfield arena was severely reduced. In the Morris water maze task, mutant mice showed moderate escape performance deficits during the acquisition period but no impairment in spatial memory. The authors conclude that a defective betaAPP gene impairs postnatal somatic development, associated with transient as well as long-lasting neurobehavioral retardation and muscular weakness. Comparison with earlier data suggests that early postnatal handling may attenuate some of the non-cognitive performance deficits in the water maze. Further, the manifestation and time course of behavioral yet not neuropathological symptoms in betaAPP mutant mice resemble in some aspects those of the human Down's syndrome.

Possible causes of Alzheimer's disease: amyloid fragments, free radicals, and calcium homeostasis

Alzheimer's disease (AD) is a form of dementia in which patients develop neurodegeneration and complete loss of cognitive abilities and die prematurely. No treatment is known for this condition. Evidence points toward beta-amyloid as one of the main causes for cytotoxic processes. The cascade of biochemical events that lead to neuronal death appears to be interference with intracellular calcium homeostasis via activation of calcium channels, intracellular calcium stores, and subsequent production of free radicals by calcium-sensitive enzymes. The glutamatergic system seems to be implicated in mediating the toxic processes. Several strategies promise amelioration of neurodegenerative developments as judging from in vitro experiments. Glutamate receptor-selective drugs, antioxidants, inhibitors of nitric oxide synthase, calcium channel antagonists, receptor or enzyme inhibitors, and growth factors promise help. Especially combinations of drugs that act at different levels might prolong patients' health.

The interaction between chronic low-level lead and the amyloid beta precursor protein

Chronic low-level lead exposure is toxic to the developing nervous system. The amyloid beta precursor protein (A beta PP) plays a pivotal role in this developmental process, both as a neurotrophic/neuroprotective factor and as a mediator of cell adhesion. In this study, we have used an in vitro system to examine the interaction between chronic low-level lead and the expression and function of A beta PP. Chronic exposure of the HN9 mouse hippocampal cell line to lead chloride (10(-14) M to 10(-6) M) for 96 hours resulted in a 50% increase in the levels of the particulate form of the protein with a parallel decrease in the soluble form (A beta PP). This effect of lead was reversible following the removal of the toxin. This increase in membrane-bound A beta PP was also paralleled by an increase in cell adhesivity to a fibronectin substrate. In addition, A beta PP also acted to attenuate lead toxicity. Cells which secreted high levels of the protein were resistant to lead toxicity when compared with control cells suggesting that the protein may be acting to chelate the metal and thus attenuating its toxic action within the cell.

The role of the amyloid protein precursor (APP) in Alzheimer's disease: does the normal function of APP explain the topography of neurodegeneration?

Alzheimer's disease (AD) is the most common form of dementia in the aged population. Early-onset familial AD (FAD) involves mutations in a gene on chromosome 21 encoding the amyloid protein precursor or on chromosomes 14 or 1 encoding genes known as presenilins. All mutations examined have been found to increase the production of amyloidogenic forms of the amyloid protein (A beta), a 4 kDa peptide derived from APP. Despite the remarkable progress in elucidating the biochemical mechanisms responsible for AD, little is known about the normal function of APP. A model of how APP and A beta are involved in pathogenesis is presented. This model may explain why certain neuronal populations are selectively vulnerable in AD. It is suggested that those neurons which more readily undergo neuritic sprouting and synaptic remodelling are more vulnerable to A beta neurotoxicity.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

Secreted amyloid precursor protein alpha selectively suppresses N-methyl-D-aspartate currents in hippocampal neurons: involvement of cyclic GMP

The secreted form of beta-amyloid precursor protein (sAPP alpha) is released from neurons in an activity-dependent manner; data suggest sAPP alpha may play roles in regulating neuronal excitability, plasticity, and survival. In cultured hippocampal neurons sAPP alpha can suppress elevation of [Ca2+]i induced by glutamate and can protect neurons against excitotoxicity. We now report whole-cell patch-clamp data from studies of cultured embryonic rat hippocampal neurons which demonstrate that sAPP alpha selectively suppresses N-methyl-D-aspartate currents without affecting currents induced by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate or kainate. sAPP alpha suppressed N-methyl-D-aspartate current rapidly and reversibly at concentrations of 0.011 nM. Suppression of N-methyl-D-aspartate current by sAPP alpha is apparently mediated by cyclic guanosine monophosphate because 8-bromo-cyclic guanosine monophosphate suppressed N-methyl-D-aspartate current in a manner similar to sAPP alpha, and two different inhibitors of cyclic guanosine monophosphate-dependent protein kinase prevented sAPP alpha-induced suppression of N-methyl-D-aspartate current. In addition, okadaic acid prevented suppression of N-methyl-D-aspartate-induced current suggesting the involvement of a protein phosphatase in modulation of N-methyl-D-aspartate current by sAPP alpha. These data identify a mechanism whereby sAPP alpha can modulate cellular responses to glutamate, and suggest important roles for sAPP alpha in the various physiological and pathophysiological processes in which N-methyl-D-aspartate receptors participate.

Involvement of amyloid precursor protein in functional synapse formation in cultured hippocampal neurons

Amyloid precursor protein (APP) is known to be widely expressed in neuronal cells, and enriched in the central and peripheral synaptic sites. Although it has been proposed that APP functions in synaptogenesis, no direct evidence has yet been reported. In this study we investigated the involvement of APP in functional synapse formation by monitoring spontaneous oscillations of intracellular Ca2+ concentration ([Ca2+]i) in cultured hippocampal neurons. As more and more neurons form synapses with each other during the culture period, increasing numbers of neuronal cells show synchronized spontaneous oscillations of [Ca2+]i. The number of neurons that showed synchronized spontaneous oscillations of [Ca2+]i was significantly lower when cultured in the presence of monoclonal antibody 22C11 against the N-terminal portion of APP. Moreover, incubation with excess amounts of the secretory form of APP or the N-terminal fragment of APP also inhibited the increase in number of neurons with synchronized spontaneous oscillations of [Ca2+]i. The addition of monoclonal antibody 22C11 or secretory form of APP did not, however, affect MAP-2-positive neurite outgrowth. These findings suggest that APP play a role in functional synapse formation during CNS development.

Individual isoforms of the amyloid beta precursor protein demonstrate differential adhesive potentials to constituents of the extracellular matrix

The amyloid beta precursor protein (AbetaPP) can exist as a membrane-bound glycoprotein which modulates neural cell adhesion. The adhesion of clones of the AtT20 mouse pituitary cell line, transfected with cDNA coding for the 695 (AbetaPP695) and 751 (AbetaPP751) amino acid forms of the protein, to individual components of the extracellular matrix was determined using a centrifugal shear assay. On laminin, poly-L-lysine, fibronectin, and uncoated glass substrata, the cells transfected with AbetaPP695 (6A1 cells) demonstrated a 50% increase in adhesivity over nontransfected cells, while those transfected with AbetaPP751 (7A1 cells) showed a significant decrease in adhesion. There was, however, a significant increase in the adhesive strength of the 7A1 cells to collagen type IV with no change in the adhesivity of the 6A1 cells when compared with control. These changes in adhesivity could be attributed to changes in the levels of the membrane-bound protein and were not due to the interaction of soluble AbetaPP with elements of the extracellular matrix. These studies provide evidence for differential adhesivities of the constituent AbetaPP isoforms and the possible role of the Kunitz protease inhibitor (KPI) domain in influencing the adhesive properties of the protein backbone.