Temporal memory deficits in Alzheimer's mouse models: rescue by genetic deletion of BACE1

Impairments in remote memory stabilization precede hippocampal synaptic and cognitive failures in 5XFAD Alzheimer mouse model

Although animal models of Alzheimer's disease (AD) recapitulate beta-amyloid-dependent hippocampal synaptic and cognitive dysfunctions, it is poorly understood how cortex-dependent remote memory stabilization following initial hippocampal coding is affected. Here, we systematically analyzed biophysical and behavioral phenotypes, including remote memory functions, of 5XFAD APP/PS1 transgenic mice containing five familial AD mutations. We found that 5XFAD mice show hippocampal dysfunctions as observed by reduced levels of baseline transmission and long-term potentiation at Schaffer collateral-CA1 synapses. Hippocampus-dependent memory tested 1 day after contextual fear conditioning was also impaired age-dependently in 5XFAD mice, as correlated with the onset of hippocampal synaptic failures. Importantly, remote memory stabilization during 30 days after training significantly declined in 5XFAD mice at time well before the onset of hippocampal dysfunctions. Our results indicate that 5XFAD mice provide a useful model system to investigate the mechanisms and therapeutic interventions for multiple synaptic and memory dysfunctions associated with AD.

The role of amyloid precursor protein processing by BACE1, the beta-secretase, in Alzheimer disease pathophysiology

Amyloid plaques, composed of the amyloid beta-protein (Abeta), are hallmark neuropathological lesions in Alzheimer disease (AD) brain. Abeta fulfills a central role in AD pathogenesis, and reduction of Abeta levels should prove beneficial for AD treatment. Abeta generation is initiated by proteolysis of amyloid precursor protein (APP) by the beta-secretase enzyme BACE1. Bace1 knockout (Bace1(-/-)) mice have validated BACE1 as the authentic beta-secretase in vivo. BACE1 is essential for Abeta generation and represents a suitable drug target for AD therapy, especially because this enzyme is up-regulated in AD. However, although initial data indicated that Bace1(-/-) mice lack an overt phenotype, the BACE1-mediated processing of APP and other substrates may be important for specific biological processes. In this minireview, topics range from the initial identification of BACE1 to the fundamental knowledge gaps that remain in our understanding of this protease. We address pertinent questions such as putative causes of BACE1 elevation in AD and discuss why, nine years since the identification of BACE1, treatments that address the underlying pathological mechanisms of AD are still lacking.

Therapeutic strategies for Alzheimer's disease

Therapeutic approaches for Alzheimer's disease (AD) are guided by four disease characteristics: amyloid plaques, neurofibrillar tangles (NFT), neurodegeneration, and dementia. Amyloid plaques are composed largely of 4 kDa beta-amyloid (Abeta) peptides, with the more amyloidogenic, 42 amino acid form (Abeta42) as the primary species. Because multiple, rare mutations that cause early-onset, familial AD lead to increased production or aggregation of Abeta42, amyloid therapeutics aim to reduce the amount of toxic Abeta42 aggregates. Amyloid-based therapies include gamma-secretase inhibitors and modulators, BACE inhibitors, aggregation blockers, catabolism inducers, and anti-Abeta biologics. Tangles are composed of paired helical filaments of hyperphosphorylated tau protein. Tau-based therapeutics include kinase inhibitors, microtubule stabilizers, and catabolism inducers. Therapeutic strategies for neurodegeneration target multiple mechanisms, including excitotoxicity, mitochondrial dysfunction, oxidative damage, and inflammation or stimulation of neuronal viability. Although not disease modifying, cognition enhancers are important to treat the symptom of dementia. Strategies for cognition enhancement include cholinesterase inhibitors, and other approaches to enhance the signaling of cholinergic and glutamatergic neurons. In summary, plaques, tangles, neurodegeneration and dementia guide the development of multiple therapeutic approaches for AD and are the subject of this review.

BACE1 gene deletion: Impact on behavioral function in a model of Alzheimer's disease

Accumulation of cerebral amyloid-beta (Abeta) has been implicated as a putative causal factor in the development of Alzheimer's disease (AD). Transgenic mice like the PDAPP line overexpress human mutant Amyloid Precursor Protein (hAPP) and recapitulate many features of AD, including amyloid neuropathology and cognitive deficits. Inhibition of the beta-site aspartyl cleaving enzyme (BACE1) enzyme responsible for the first proteolytic cleavage that ultimately generates Abeta has been proposed as a strategy for AD therapy. To assess the theoretical repercussions of beta-secretase activity reduction in an in vivo model of AD, BACE1(-/-) mice bred to the PDAPP line were examined in a series of behavioral tasks. Although BACE1 gene ablation abolished hAbeta accumulation, BACE1(-/-) mice had unexpected sensorimotor impairments, spatial memory deficits, and displayed seizures, phenotypes which were severe on the PDAPP background. These results suggest that while excess Abeta is functionally pathological, BACE1-mediated processing of APP and other substrates play a role in "normal" learning, memory and sensorimotor processes.

P-glycoprotein efflux and other factors limit brain amyloid beta reduction by beta-site amyloid precursor protein-cleaving enzyme 1 inhibitors in mice

Alzheimer's disease (AD) is a progressive neurodegenerative disease. Amyloid beta (Abeta) peptides are hypothesized to cause the initiation and progression of AD based on pathologic data from AD patients, genetic analysis of mutations that cause early onset forms of AD, and preclinical studies. Based on this hypothesis, beta-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1) inhibitors are an attractive therapeutic approach for AD because cleavage of the APP by BACE1 is required to form Abeta. In this study, three potent BACE1 inhibitors are characterized. All three inhibitors decrease Abeta formation in cultured cells with IC(50) values less than 10 nM. Analysis of APP C-terminal fragments by immunoblotting and Abeta peptides by mass spectrometry showed that these inhibitors decreased Abeta by inhibiting BACE1. An assay for Abeta1-40 in mice was developed and used to show that these BACE1 inhibitors decreased plasma Abeta1-40, but not brain Abeta1-40, in wild-type mice. Because these BACE1 inhibitors were substrates for P-glycoprotein (P-gp), a member of the ATP-binding cassette superfamily of efflux transporters, these inhibitors were administered to P-gp knockout (KO) mice. These studies showed that all three BACE1 inhibitors decreased brain Abeta1-40 in P-gp KO mice, demonstrating that P-gp is a major limitation for development of BACE1 inhibitors to test the amyloid hypothesis. A comparison of plasma Abeta1-40 and brain Abeta1-40 dose responses for these three compounds revealed differences in relative ED(50) values, indicating that factors other than P-gp can also contribute to poor brain activity by BACE1 inhibitors.

Neurosecretases provide strategies to treat sporadic and familial Alzheimer disorders

Recent discoveries on neurosecretases and their trafficking to release fibril-forming neuropeptides or other products, are of interest to pathology, cell signaling and drug discovery. Nomenclature arose from the use of amyloid precursor protein (APP) as a prototypic type-1 substrate leading to the isolation of beta-secretase (BACE), multimeric complexes (gamma-secretase, gamma-SC) for intramembranal cleavage, and attributing a new function to well-characterized metalloproteases of the ADAM family (alpha-secretase) for normal APP turnover. While purified alpha/beta-secretases facilitate drug discovery, gamma-SC presents greater challenges for characterization and mechanisms of catalysis. The review comments on links between mutation or polymorphisms in relation to enzyme mechanisms and disease. The association between lipoprotein receptor LRP11 variants and sporadic Alzheimer's disease (SAD) offers scope to integrate components of pre- and post-Golgi membranes, or brain clathrin-coated vesicles within pathways for trafficking as targets for intervention. The presence of APP and metabolites in brain clathrin-coated vesicles as significant cargo with lipoproteins and adaptors focuses attention as targets for therapeutic intervention. This overview emphasizes the importance to develop new therapies targeting neurosecretases to treat a major neurological disorder that has vast economic and social implications.

Advances on the understanding of the origins of synaptic pathology in AD

Although Alzheimer's disease (AD) was first discovered a century ago, we are still facing a lack of definitive diagnosis during the patient's lifetime and are unable to prescribe a curative treatment. However, the past 10 years have seen a "revamping" of the main hypothesis about AD pathogenesis and the hope to foresee possible treatment. AD is no longer considered an irreversible disease. A major refinement of the classic beta-amyloid cascade describing amyloid fibrils as neurotoxins has been made to integrate the key scientific evidences demonstrating that the first pathological event occurring in AD early stages affects synaptic function and maintenance. A concept fully compatible with synapse loss being the best pathological correlate of AD rather than other described neuropathological hallmarks (amyloid plaques, neurofibrillary tangles or neuronal death). The notion that synaptic alterations might be reverted, thus offering a potential curability, was confirmed by immunotherapy experiments targeting beta-amyloid protein in transgenic AD mice in which cognitive functions were improved despite no reduction in the amyloid plaques burden. The updated amyloid cascade now integrates the synapse failure triggered by soluble Abeta-oligomers. Still no consensus has been reached on the most toxic Abeta conformations, neither on their site of production nor on their extra- versus intra-cellular actions. Evidence shows that soluble Abeta oligomers or ADDLs bind selectively to neurons at their synaptic loci, and trigger major changes in synapse composition and morphology, which ultimately leads to dendritic spine loss. However, the exact mechanism is not yet fully understood but is suspected to involve some membrane receptor(s).

Repeated novel cage exposure-induced improvement of early Alzheimer's-like cognitive and amyloid changes in TASTPM mice is unrelated to changes in brain endocannabinoids levels

Environmental factors (e.g. stress, exercise, enrichment) are thought to play a role in the development of Alzheimer's disease later in life. We investigated the influence of repeated novel cage exposure on the development of early Alzheimer's-like pathology in adult (4 months old) double transgenic mice over-expressing the amyloid precursor protein and presenilin-1 genes (TASTPM mouse line). The procedure involves the repeated placement of the animal into a novel clean cage, a manipulation which induces a stress response and exploratory activity and, as such, can also be seen as a mild form of enrichment. Before and after exposure to the novel cage procedure, separate groups of mice were evaluated for locomotor performance and short-term contextual memory in the fear-conditioning test. Repeated novel cage exposure prevented the onset of a short-term memory deficit that was apparent in 5.5- but not 4-month-old TASTPM mice, without reversing the deficit in extinction already evident at 4 months of age. Brain regional levels of soluble and insoluble amyloid and of endocannabinoids were quantified. Novel cage exposure attenuated soluble and insoluble amyloid accumulation in the hippocampus and frontal cortex, without affecting the age-related increases in regional brain endocannabinoids levels. These beneficial effects are likely to be the consequence of the increase in physical and exploratory activity induced by novel cage exposure and suggest that the impact of environmental factors on Alzheimer's-like changes may be dependent on the degree of activation of stress pathways.

The Alzheimer's disease beta-secretase enzyme, BACE1

The pathogenesis of Alzheimer's disease is highly complex. While several pathologies characterize this disease, amyloid plaques, composed of the β-amyloid peptide are hallmark neuropathological lesions in Alzheimer's disease brain. Indeed, a wealth of evidence suggests that β-amyloid is central to the pathophysiology of AD and is likely to play an early role in this intractable neurodegenerative disorder. The BACE1 enzyme is essential for the generation of β-amyloid. BACE1 knockout mice do not produce β-amyloid and are free from Alzheimer's associated pathologies including neuronal loss and certain memory deficits. The fact that BACE1 initiates the formation of β-amyloid, and the observation that BACE1 levels are elevated in this disease provide direct and compelling reasons to develop therapies directed at BACE1 inhibition thus reducing β-amyloid and its associated toxicities. However, new data indicates that complete abolishment of BACE1 may be associated with specific behavioral and physiological alterations. Recently a number of non-APP BACE1 substrates have been identified. It is plausible that failure to process certain BACE1 substrates may underlie some of the reported abnormalities in the BACE1-deficient mice. Here we review BACE1 biology, covering aspects ranging from the initial identification and characterization of this enzyme to recent data detailing the apparent dysregulation of BACE1 in Alzheimer's disease. We pay special attention to the putative function of BACE1 during healthy conditions and discuss in detail the relationship that exists between key risk factors for AD, such as vascular disease (and downstream cellular consequences), and the pathogenic alterations in BACE1 that are observed in the diseased state.

Cofilin-mediated neurodegeneration in Alzheimer's disease and other amyloidopathies

Transport defects may arise in various neurodegenerative diseases from failures in molecular motors, microtubule abnormalities, and the chaperone/proteasomal degradation pathway leading to aggresomal-lysosomal accumulations. These defects represent important steps in the neurodegenerative cascade, although in many cases, a clear consensus has yet to be reached regarding their causal relationship to the disease. A growing body of evidence lends support to a link between neurite transport defects in the very early stages of many neurodegenerative diseases and alterations in the organization and dynamics of the actin cytoskeleton initiated by filament dynamizing proteins in the ADF/cofilin family. This article focuses on cofilin, which in neurons under stress, including stress induced by the amyloid-beta (Abeta) 1-42 peptide, undergoes dephosphorylation (activation) and forms rod-shaped actin bundles (rods). Rods inhibit transport, are sites of amyloid precursor protein accumulation, and contribute to the pathology of Alzheimer's disease. Because rods form rapidly in response to anoxia, they could also contribute to synaptic deficits associated with ischemic brain injury (e.g., stroke). Surprisingly, cofilin undergoes phosphorylation (inactivation) in hippocampal neurons treated with Abeta1-40 at high concentrations, and these neurons undergo dystrophic morphological changes, including accumulation of pretangle phosphorylated-tau. Therefore, extremes in phosphoregulation of cofilin by different forms of Abeta may explain much of the Alzheimer's disease pathology and provide mechanisms for synaptic loss and plaque expansion.

Neuropathological quantification of dtg APP/PS1: neuroimaging, stereology, and biochemistry

Murine models that mimic the neuropathology of Alzheimer's disease (AD) have the potential to provide insight into the pathogenesis of the disease and lead to new strategies for the therapeutic management of afflicted patients. We used magnetic resonance imaging (MRI), design-based stereology, and high performance liquid chromatography (HPLC) to assess the age-related neuropathology in double transgenic mice that overexpress two AD-related proteins--amyloid precursor protein (APP) and presenilin 1 (PS1)--and age- and gender-matched wild-type (WT) controls. In mice ranging in age from 4-28 months, total volumes of the hippocampal formation (V (HF)) and whole brain (V (brain)) were quantified by the Cavalieri-point counting method on a systematic-random sample of coronal T2-weighted MRI images; the same stereological methods were used to quantify V (HF) and V (brain) after perfusion and histological processing. To assess changes in AD-type beta-amyloid (A beta) plaques, sections from the hippocampal formation and amylgdaloid complex of mice aged 5, 12, and 15 months were stained by Congo Red histochemistry. In aged mice with large numbers of amyloid plaques, systematic-random samples of sections were stained by GFAP immunocytochemistry to assess gender and genotype effects on total numbers of astrocytes. In addition, levels of norepinephrine (NE), dopamine (DA), serotonin (5-HT) and 5-HT metabolites were assayed by HPLC in fresh-frozen samples from neocortex, striatum, hippocampus, and brainstem. We confirmed age-related increases in amyloid plaques, beginning with a few plaques at 5 months of age and increasing densities by 12 and 15 months. At 15 months of age, there were robust genotype effects, but no gender effects, on GFAP-immunopositive astrocytes in the amygdaloid complex and hippocampus. There were no effects on monoamine levels in all brain regions examined, and no volume changes in hippocampal formation or whole brain as quantified on either neuroimages or tissue sections. Strong correlations were present between volume estimates from MRI images and histological sections, with about 85% reduction in mean V (HF) or mean V (brain) between MRI and processed histological sections. In summary, these findings show that the double transgenic expression of AD-type mutations is associated with age-related increases in amyloid plaques and astrocytosis; however, this model does not recapitulate the cortical atrophy or neurochemical changes that are characteristic of AD.

Partial reduction of BACE1 has dramatic effects on Alzheimer plaque and synaptic pathology in APP Transgenic Mice

The aspartyl protease beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) initiates processing of amyloid precursor protein (APP) into amyloid beta (Abeta) peptide, the major component of Alzheimer disease (AD) plaques. To determine the role that BACE1 plays in the development of Abeta-driven AD-like pathology, we have crossed PDAPP mice, a transgenic mouse model of AD overexpressing human mutated APP, onto mice with either a homozygous or heterozygous BACE1 gene knockout. Analysis of PDAPP/BACE(-/-) mice demonstrated that BACE1 is absolutely required for both Abeta generation and the development of age-associated plaque pathology. Furthermore, synaptic deficits, a neurodegenerative pathology characteristic of AD, were also reversed in the bigenic mice. To determine the extent of BACE1 reduction required to significantly inhibit pathology, PDAPP mice having a heterozygous BACE1 gene knock-out were evaluated for Abeta generation and for the development of pathology. Although the 50% reduction in BACE1 enzyme levels caused only a 12% decrease in Abeta levels in young mice, it nonetheless resulted in a dramatic reduction in Abeta plaques, neuritic burden, and synaptic deficits in older mice. Quantitative analyses indicate that brain Abeta levels in young APP transgenic mice are not the sole determinant for the changes in plaque pathology mediated by reduced BACE1. These observations demonstrate that partial reductions of BACE1 enzyme activity and concomitant Abeta levels lead to dramatic inhibition of Abeta-driven AD-like pathology, making BACE1 an excellent target for therapeutic intervention in AD.

Alterations in intrinsic neuronal excitability during normal aging

Normal aging subjects, including humans, have difficulty learning hippocampus-dependent tasks. For example, at least 50% of normal aging rabbits and rats fail to meet a learning criterion in trace eyeblink conditioning. Many factors may contribute to this age-related learning impairment. An important cause is the reduced intrinsic excitability observed in hippocampal pyramidal neurons from normal aging subjects, as reflected by an enlarged postburst afterhyperpolarization (AHP) and an increased spike-frequency adaptation (accommodation). In this review, we will focus on the alterations in the AHP and accommodation during learning and normal aging. We propose that age-related increases in the postburst AHP and accommodation in hippocampal pyramidal neurons play an integral role in the learning impairment observed in normal aging subjects.

Beta-site amyloid precursor protein cleaving enzyme 1 levels become elevated in neurons around amyloid plaques: implications for Alzheimer's disease pathogenesis

Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) (beta-secretase) initiates generation of beta-amyloid (Abeta), which plays an early role in Alzheimer's disease (AD). BACE1 levels are increased in postmortem AD brain, suggesting BACE1 elevation promotes Abeta production and AD. Alternatively, the BACE1 increase may be an epiphenomenon of late-stage AD. To distinguish between these possibilities, we analyzed BACE1 elevation using a highly specific BACE1 antibody, BACE-Cat1, made in BACE1-/- mice, which mount a robust anti-BACE1 immune response. Previous BACE1 immunohistochemical studies lack consistent results because typical BACE1 antibodies produce nonspecific background, but BACE-Cat1 immunolabels BACE1 only. BACE1 elevation was recapitulated in two amyloid precursor protein (APP) transgenic mouse lines. 5XFAD mice form amyloid plaques at young ages and exhibit neuron loss. In contrast, Tg2576 form plaques at a more advanced age and do not show cell death. These two mouse lines allow differentiation between early Abeta-induced events and late phenomena related to neuron death. BACE1 levels became elevated in parallel with amyloid burden in each APP transgenic, starting early in 5XFAD and late in Tg2576. The increase in BACE1 protein occurred without any change in BACE1 mRNA level, indicating a posttranscriptional mechanism. In APP transgenic and AD brains, high BACE1 levels were observed in an annulus around Abeta42-positive plaque cores and colocalized with neuronal proteins. These results demonstrate that amyloid plaques induce BACE1 in surrounding neurons at early stages of pathology before neuron death occurs. We conclude that BACE1 elevation is most likely triggered by the amyloid pathway and may drive a positive-feedback loop in AD.

Abeta oligomer-induced aberrations in synapse composition, shape, and density provide a molecular basis for loss of connectivity in Alzheimer's disease

The basis for memory loss in early Alzheimer's disease (AD) seems likely to involve synaptic damage caused by soluble Abeta-derived oligomers (ADDLs). ADDLs have been shown to build up in the brain and CSF of AD patients and are known to interfere with mechanisms of synaptic plasticity, acting as gain-of-function ligands that attach to synapses. Because of the correlation between AD dementia and synaptic degeneration, we investigated here the ability of ADDLs to affect synapse composition, structure, and abundance. Using highly differentiated cultures of hippocampal neurons, a preferred model for studies of synapse cell biology, we found that ADDLs bound to neurons with specificity, attaching to presumed excitatory pyramidal neurons but not GABAergic neurons. Fractionation of ADDLs bound to forebrain synaptosomes showed association with postsynaptic density complexes containing NMDA receptors, consistent with observed attachment of ADDLs to dendritic spines. During binding to hippocampal neurons, ADDLs promoted a rapid decrease in membrane expression of memory-related receptors (NMDA and EphB2). Continued exposure resulted in abnormal spine morphology, with induction of long thin spines reminiscent of the morphology found in mental retardation, deafferentation, and prionoses. Ultimately, ADDLs caused a significant decrease in spine density. Synaptic deterioration, which was accompanied by decreased levels of the spine cytoskeletal protein drebrin, was blocked by the Alzheimer's therapeutic drug Namenda. The observed disruption of dendritic spines links ADDLs to a major facet of AD pathology, providing strong evidence that ADDLs in AD brain cause neuropil damage believed to underlie dementia.

BACE1 gene deletion prevents neuron loss and memory deficits in 5XFAD APP/PS1 transgenic mice

Evidence suggests that beta-amyloid (Abeta) peptide triggers a pathogenic cascade leading to neuronal loss in Alzheimer's disease (AD). However, the causal link between Abeta and neuron death in vivo remains unclear since most animal models fail to recapitulate the dramatic cell loss observed in AD. We have recently developed transgenic mice that overexpress human APP and PS1 with five familial AD mutations (5XFAD mice) and exhibit robust neuron death. Here, we demonstrate that genetic deletion of the beta-secretase (BACE1) not only abrogates Abeta generation and blocks amyloid deposition but also prevents neuron loss found in the cerebral cortex and subiculum, brain regions manifesting the most severe amyloidosis in 5XFAD mice. Importantly, BACE1 gene deletion also rescues memory deficits in 5XFAD mice. Our findings provide strong evidence that Abeta ultimately is responsible for neuron death in AD and validate the therapeutic potential of BACE1-inhibiting approaches for the treatment of AD.

A ganglioside-induced toxic soluble Abeta assembly. Its enhanced formation from Abeta bearing the Arctic mutation

The mechanism underlying plaque-independent neuronal death in Alzheimer disease (AD), which is probably responsible for early cognitive decline in AD patients, remains unclarified. Here, we show that a toxic soluble Abeta assembly (TAbeta) is formed in the presence of liposomes containing GM1 ganglioside more rapidly and to a greater extent from a hereditary variant-type ("Arctic") Abeta than from wild-type Abeta. TAbeta is also formed from soluble Abeta through incubation with natural neuronal membranes prepared from aged mouse brains in a GM1 ganglioside-dependent manner. An oligomer-specific antibody (anti-Oligo) significantly suppresses TAbeta toxicity. Biophysical and structural analyses by atomic force microscopy and size exclusion chromatography revealed that TAbeta is spherical with diameters of 10-20 nm and molecular masses of 200-300 kDa. TAbeta induces neuronal death, which is abrogated by the small interfering RNA-mediated knockdown of nerve growth factor receptors, including TrkA and p75 neurotrophin receptor. Our results suggest that soluble Abeta assemblies, such as TAbeta, can cause plaque-independent neuronal death that favorably occurs in nerve growth factor-dependent neurons in the cholinergic basal forebrain in AD.

100 years and counting: prospects for defeating Alzheimer's disease

This week marks a century since the first description of Alzheimer's disease (AD). Despite approval of several drugs for AD, the disease continues to rob millions of their memories and their lives. Fortunately, many new therapies directly targeting the mechanisms underlying AD are now in the pipeline. Among the investigative AD therapies in clinical trials are several strategies to block pathogenic amyloid-beta peptides and to rescue vulnerable neurons from degeneration. Complementary but less mature strategies aim to prevent the copathogenic effects of apolipoprotein E and the microtubule-associated protein tau. New insights into selective neuronal vulnerability and the link between aging and AD may provide additional entry points for therapeutic interventions. The predicted increase in AD cases over the next few decades makes the development of better treatments a matter of utmost importance and urgency.

Plaques, tangles, and memory loss in mouse models of neurodegeneration

Within the past decade, our understanding of the pathogenic mechanisms in Alzheimer's disease (AD) has dramatically advanced because of the development of transgenic mouse models that recapitulate the key pathological and behavioral phenotypes of the disease. These mouse models have allowed investigators to test detailed questions about how pathology develops and to evaluate potential therapeutic approaches that could slow down the development of this disease. In this review, we discuss the status of transgenic mouse models and review the complex relationship between pathology and behavior in the development of neuropathological syndromes in AD.

Intraneuronal beta-amyloid aggregates, neurodegeneration, and neuron loss in transgenic mice with five familial Alzheimer's disease mutations: potential factors in amyloid plaque formation

Mutations in the genes for amyloid precursor protein (APP) and presenilins (PS1, PS2) increase production of beta-amyloid 42 (Abeta42) and cause familial Alzheimer's disease (FAD). Transgenic mice that express FAD mutant APP and PS1 overproduce Abeta42 and exhibit amyloid plaque pathology similar to that found in AD, but most transgenic models develop plaques slowly. To accelerate plaque development and investigate the effects of very high cerebral Abeta42 levels, we generated APP/PS1 double transgenic mice that coexpress five FAD mutations (5XFAD mice) and additively increase Abeta42 production. 5XFAD mice generate Abeta42 almost exclusively and rapidly accumulate massive cerebral Abeta42 levels. Amyloid deposition (and gliosis) begins at 2 months and reaches a very large burden, especially in subiculum and deep cortical layers. Intraneuronal Abeta42 accumulates in 5XFAD brain starting at 1.5 months of age (before plaques form), is aggregated (as determined by thioflavin S staining), and occurs within neuron soma and neurites. Some amyloid deposits originate within morphologically abnormal neuron soma that contain intraneuronal Abeta. Synaptic markers synaptophysin, syntaxin, and postsynaptic density-95 decrease with age in 5XFAD brain, and large pyramidal neurons in cortical layer 5 and subiculum are lost. In addition, levels of the activation subunit of cyclin-dependent kinase 5, p25, are elevated significantly at 9 months in 5XFAD brain, although an upward trend is observed by 3 months of age, before significant neurodegeneration or neuron loss. Finally, 5XFAD mice have impaired memory in the Y-maze. Thus, 5XFAD mice rapidly recapitulate major features of AD amyloid pathology and may be useful models of intraneuronal Abeta42-induced neurodegeneration and amyloid plaque formation.

Acylguanidines as Small-Molecule β-Secretase Inhibitors

BACE1 is an aspartyl protease responsible for cleaving amyloid precursor protein to liberate Abeta, which aggregates leading to plaque deposits implicated in Alzheimer's disease. We have identified small-molecule acylguanidine inhibitors of BACE1. Crystallographic studies show that these compounds form unique hydrogen-bonding interactions with the catalytic site aspartic acids and stabilize the protein in a flap-open conformation. Structure-based optimization led to the identification of potent analogs, such as 10d (BACE1 IC(50) = 110 nM).

A Two Decade Contribution of Molecular Cell Biology to the Centennial of Alzheimer's Disease: Are We Progressing Toward Therapy?

Alzheimer's disease (AD), described for the first time 100 years ago, is a neurodegenerative disease characterized by two neuropathological hallmarks: neurofibrillary tangles containing hyperphosphorylated tau and senile plaques. These lesions are likely initiated by an imbalance between production and clearance of amyloid beta, leading to increased oligomerization of these peptides, formation of amyloid plaques in the brain of the patient, and final dementia. Amyloid beta is generated from amyloid precursor protein (APP) by subsequent beta- and gamma-secretase cleavage, the latter being a multiprotein complex consisting of presenilin-1 or -2, nicastrin, APH-1, and PEN-2. Alternatively, APP can be cleaved by alpha- and gamma-secretase, precluding the production of Abeta. In this review, we discuss the major breakthroughs during the past two decades of molecular cell biology and the current genetic and cell biological state of the art on APP proteolysis, including structure-function relationships and subcellular localization. Finally, potential directions for cell biological research toward the development of AD therapies are briefly discussed.