Dynamics of Abeta turnover and deposition in different beta-amyloid precursor protein transgenic mouse models following gamma-secretase inhibition

β-amyloid monomer scavenging by an anticalin protein prevents neuronal hyperactivity in mouse models of Alzheimer's Disease

Hyperactivity mediated by synaptotoxic β-amyloid (Aβ) oligomers is one of the earliest forms of neuronal dysfunction in Alzheimer's disease. In the search for a preventive treatment strategy, we tested the effect of scavenging Aβ peptides before Aβ plaque formation. Using in vivo two-photon calcium imaging and SF-iGluSnFR-based glutamate imaging in hippocampal slices, we demonstrate that an Aβ binding anticalin protein (Aβ-anticalin) can suppress early neuronal hyperactivity and synaptic glutamate accumulation in the APP23xPS45 mouse model of β-amyloidosis. Our results suggest that the sole targeting of Aβ monomers is sufficient for the hyperactivity-suppressing effect of the Aβ-anticalin at early disease stages. Biochemical and neurophysiological analyses indicate that the Aβ-anticalin-dependent depletion of naturally secreted Aβ monomers interrupts their aggregation to neurotoxic oligomers and, thereby, reverses early neuronal and synaptic dysfunctions. Thus, our results suggest that Aβ monomer scavenging plays a key role in the repair of neuronal function at early stages of AD.

Somatostatin and the pathophysiology of Alzheimer's disease

Among the central features of Alzheimer's disease (AD) progression are altered levels of the neuropeptide somatostatin (SST), and the colocalisation of SST-positive interneurons (SST-INs) with amyloid-β plaques, leading to cell death. In this theoretical review, I propose a molecular model for the pathogenesis of AD based on SST-IN hypofunction and hyperactivity. Namely, hypofunctional and hyperactive SST-INs struggle to control hyperactivity in medial regions in early stages, leading to axonal Aβ production through excessive presynaptic GABAB inhibition, GABAB1a/APP complex downregulation and internalisation. Concomitantly, excessive SST-14 release accumulates near SST-INs in the form of amyloids, which bind to Aβ to form toxic mixed oligomers. This leads to differential SST-IN death through excitotoxicity, further disinhibition, SST deficits, and increased Aβ release, fibrillation and plaque formation. Aβ plaques, hyperactive networks and SST-IN distributions thereby tightly overlap in the brain. Conversely, chronic stimulation of postsynaptic SST2/4 on gulutamatergic neurons by hyperactive SST-INs promotes intense Mitogen-Activated Protein Kinase (MAPK) p38 activity, leading to somatodendritic p-tau staining and apoptosis/neurodegeneration - in agreement with a near complete overlap between p38 and neurofibrillary tangles. This model is suitable to explain some of the principal risk factors and markers of AD progression, including mitochondrial dysfunction, APOE4 genotype, sex-dependent vulnerability, overactive glial cells, dystrophic neurites, synaptic/spine losses, inter alia. Finally, the model can also shed light on qualitative aspects of AD neuropsychology, especially within the domains of spatial and declarative (episodic, semantic) memory, under an overlying pattern of contextual indiscrimination, ensemble instability, interference and generalisation.

Retinal Alterations Predict Early Prodromal Signs of Neurodegenerative Disease

Neurodegenerative diseases are an increasingly common group of diseases that occur late in life with a significant impact on personal, family, and economic life. Among these, Alzheimer's disease (AD) and Parkinson's disease (PD) are the major disorders that lead to mild to severe cognitive and physical impairment and dementia. Interestingly, those diseases may show onset of prodromal symptoms early after middle age. Commonly, the evaluation of these neurodegenerative diseases is based on the detection of biomarkers, where functional and structural magnetic resonance imaging (MRI) have shown a central role in revealing early or prodromal phases, although it can be expensive, time-consuming, and not always available. The aforementioned diseases have a common impact on the visual system due to the pathophysiological mechanisms shared between the eye and the brain. In Parkinson's disease, α-synuclein deposition in the retinal cells, as well as in dopaminergic neurons of the substantia nigra, alters the visual cortex and retinal function, resulting in modifications to the visual field. Similarly, the visual cortex is modified by the neurofibrillary tangles and neuritic amyloid β plaques typically seen in the Alzheimer's disease brain, and this may reflect the accumulation of these biomarkers in the retina during the early stages of the disease, as seen in postmortem retinas of AD patients. In this light, the ophthalmic evaluation of retinal neurodegeneration could become a cost-effective method for the early diagnosis of those diseases, overcoming the limitations of functional and structural imaging of the deep brain. This analysis is commonly used in ophthalmic practice, and interest in it has risen in recent years. This review will discuss the relationship between Alzheimer's disease and Parkinson's disease with retinal degeneration, highlighting how retinal analysis may represent a noninvasive and straightforward method for the early diagnosis of these neurodegenerative diseases.

Circadian modulation by time-restricted feeding rescues brain pathology and improves memory in mouse models of Alzheimer's disease

Circadian disruptions impact nearly all people with Alzheimer's disease (AD), emphasizing both their potential role in pathology and the critical need to investigate the therapeutic potential of circadian-modulating interventions. Here, we show that time-restricted feeding (TRF) without caloric restriction improved key disease components including behavioral timing, disease pathology, hippocampal transcription, and memory in two transgenic (TG) mouse models of AD. We found that TRF had the remarkable capability of simultaneously reducing amyloid deposition, increasing Aβ42 clearance, improving sleep and memory, and normalizing daily transcription patterns of multiple genes, including those associated with AD and neuroinflammation. Thus, our study unveils for the first time the pleiotropic nature of timed feeding on AD, which has far-reaching effects beyond metabolism, ameliorating neurodegeneration and the misalignment of circadian rhythmicity. Since TRF can substantially modify disease trajectory, this intervention has immediate translational potential, addressing the urgent demand for accessible approaches to reduce or halt AD progression.

Sustained Hippocampal Neural Plasticity Questions the Reproducibility of an Amyloid-β-Induced Alzheimer's Disease Model

BACKGROUND

The use of Alzheimer's disease (AD) models obtained by intracerebral infusion of amyloid-β (Aβ) has been increasingly reported in recent years. Nonetheless, these models may present important challenges.

OBJECTIVE

We have focused on canonical mechanisms of hippocampal-related neural plasticity to characterize a rat model obtained by an intracerebroventricular (icv) injection of soluble amyloid-β42 (Aβ42).

METHODS

Animal behavior was evaluated in the elevated plus maze, Y-Maze spontaneous or forced alternation, Morris water maze, and open field, starting 2 weeks post-Aβ42 infusion. Hippocampal neurogenesis was assessed 3 weeks after Aβ42 injection. Aβ deposition, tropomyosin receptor kinase B levels, and neuroinflammation were appraised at 3 and 14 days post-Aβ42 administration.

RESULTS

We found that immature neuronal dendritic morphology was abnormally enhanced, but proliferation and neuronal differentiation in the dentate gyrus was conserved one month after Aβ42 injection. Surprisingly, animal behavior did not reveal changes in cognitive performance nor in locomotor and anxious-related activity. Brain-derived neurotrophic factor related-signaling was also unchanged at 3 and 14 days post-Aβ icv injection. Likewise, astrocytic and microglial markers of neuroinflammation in the hippocampus were unaltered in these time points.

CONCLUSION

Taken together, our data emphasize a high variability and lack of behavioral reproducibility associated with these Aβ injection-based models, as well as the need for its further optimization, aiming at addressing the gap between preclinical AD models and the human disorder.

Synthesis of the Potent, Selective, and Efficacious β-Secretase (BACE1) Inhibitor NB-360

Starting from lead compound 4, the 1,4-oxazine headgroup was optimized to improve potency and brain penetration. Focusing at the 6-position of the 5-amino-1,4-oxazine, the insertion of a Me and a CF₃ group delivered an excellent pharmacological profile with a pK_a of 7.1 and a very low P-gp efflux ratio enabling high central nervous system (CNS) penetration and exposure. Various synthetic routes to access BACE1 inhibitors bearing a 5-amino-6-methyl-6-(trifluoromethyl)-1,4-oxazine headgroup were investigated. Subsequent optimization of the P3 fragment provided the highly potent N-(3-((3R,6R)-5-amino-3,6-dimethyl-6-(trifluoromethyl)-3,6-dihydro-2H-1,4-oxazin-3-yl)-4-fluorophenyl)-5-cyano-3-methylpicolinamide 54 (NB-360), able to reduce significantly Aβ levels in mice, rats, and dogs in acute and chronic treatment regimens.

The C99 domain of the amyloid precursor protein resides in the disordered membrane phase

Processing of the amyloid precursor protein (APP) via the amyloidogenic pathway is associated with the etiology of Alzheimer's disease. The cleavage of APP by β-secretase to generate the transmembrane 99-residue C-terminal fragment (C99) and subsequent processing of C99 by γ-secretase to yield amyloid-β (Aβ) peptides are essential steps in this pathway. Biochemical evidence suggests that amyloidogenic processing of C99 occurs in cholesterol- and sphingolipid-enriched liquid-ordered phase membrane rafts. However, direct evidence that C99 preferentially associates with these rafts has remained elusive. Here, we tested this by quantifying the affinity of C99-GFP for raft domains in cell-derived giant plasma membrane vesicles (GPMVs). We found that C99 was essentially excluded from ordered domains in vesicles from HeLa cells, undifferentiated SH-SY5Y cells, or SH-SY5Y-derived neurons; instead, ∼90% of C99 partitioned into disordered domains. The strong association of C99 with disordered domains occurred independently of its cholesterol-binding activity or homodimerization, or of the presence of the familial Alzheimer disease Arctic mutation (APP E693G). Finally, through biochemical studies we confirmed previous results, which showed that C99 is processed in the plasma membrane by α-secretase, in addition to the well-known γ-secretase. These findings suggest that C99 itself lacks an intrinsic affinity for raft domains, implying that either i) amyloidogenic processing of the protein occurs in disordered regions of the membrane, ii) processing involves a marginal subpopulation of C99 found in rafts, or iii) as-yet-unidentified protein-protein interactions with C99 in living cells drive this protein into membrane rafts to promote its cleavage therein.

Novel Anti-Alzheimer's Therapeutic Molecules Targeting Amyloid Precursor Protein Processing

Alzheimer's disease (AD) is the most common cause of dementia among older people, and the prevalence of this disease is estimated to rise quickly in the upcoming years. Unfortunately, almost all of the drug candidates tested for AD until now have failed to exhibit any efficacy. Henceforth, there is an increased necessity to avert and/or slow down the advancement of AD. It is known that one of the major pathological characteristics of AD is the presence of senile plaques (SPs) in the brain. These SPs are composed of aggregated amyloid beta (Aβ), derived from the amyloid precursor protein (APP). Pharmaceutical companies have conducted a number of studies in order to identify safe and effective anti-Aβ drugs to combat AD. It is known that α-, β-, and γ-secretases are the three proteases that are involved in APP processing. Furthermore, there is a growing interest in these proteases, as they have a contribution to the modulation and production of Aβ. It has been observed that small compounds can be used to target these important proteases. Indeed, these compounds must satisfy the common strict requirements of a drug candidate targeted for brain penetration and selectivity toward different proteases. In this article, we have focused on the auspicious molecules which are under development for targeting APP-processing enzymes. We have also presented several anti-AD molecules targeting Aβ accumulation and phosphorylation signaling in APP processing. This review highlights the structure-activity relationship and other physicochemical features of several pharmacological candidates in order to successfully develop new anti-AD drugs.

Amyloid Beta Secreted during Consolidation Prevents Memory Malleability

Memory allows organisms to predict future events based on their prior sampling of the world. Rather than faithfully encoding each detail of related episodes, the brain is thought to incrementally construct probabilistic estimates of environmental statistics that are re-evaluated each time relevant events are encountered [1]. When faced with evidence that does not adequately fit mnemonic predictions, a process called reconsolidation can alter relevant memories to better recapitulate ongoing experience [2]. Conversely, when an ongoing event matches well-established predictions, reactivated memories tend to remain stable [3, 4]. In part, the brain may confer selective mnemonic stability by shifting cell-intrinsic mechanisms of plasticity induction [5], which could serve to constrain maladaptive updating of reliably predictive representations during anomalous events. Based on evidence of decreased cognitive flexibility and restricted synaptic plasticity in later life [6], we hypothesized that some prevalent age-associated neurobiological changes might in fact contribute to mnemonic stability [7]. Specifically, we predicted that amyloid beta (Aβ)-a peptide that often accumulates in the brains of individuals expressing senescent dementia [8-10]-is required for memory stabilization. Indeed, we observe elevated soluble Aβ_x-42 concentrations in the amygdala shortly after young adult rats form reconsolidation-resistant auditory fear memories. Suppressing secretases required for Aβ production immediately after learning prevents mnemonic stabilization, rendering these memories vulnerable to disruption by post-reactivation amnestic treatments. Thus, the seemingly pathogenic Aβ₄₂ peptide may serve an adaptive physiological function during memory consolidation by engaging mechanisms that protect reliably predictive representations against subsequent modification.

Elevation of plasma soluble amyloid precursor protein beta in Alzheimer's disease

INTRODUCTION

Beta-amyloid is considered to be a pathophysiological marker in Alzheimer's disease (AD). Soluble amyloid precursor proteins (sAPPs) -α (sAPPα) and -β (sAPPβ), which are the byproducts of non-amyloidogenic and amyloidogenic process of APP, respectively, have been repeatedly observed in the cerebrospinal fluids (CSF) of AD patients. The present study focused on the determination of sAPP levels in peripheral blood.

METHODS

The plasma protein levels of sAPPα and sAPPβ were measured with ELISA. Plasma from 52 AD patients, 98 amnestic mild cognitive impairment (MCI) patients, and 114 cognitively normal controls were compared.

RESULTS

The plasma level of sAPPβ was significantly increased in AD patients than in cognitively healthy controls. However, no significant change in plasma sAPPα was observed among the three groups. Furthermore, the plasma sAPPβ levels significantly correlated with cognitive assessment scales, such as clinical dementia rating (CDR), and mini-mental status examination (MMSE). Interestingly, sAPPα and sAPPβ had a positive correlation with each other in blood plasma, similar to previous studies on CSF sAPP. This correlation was stronger in the MCI and AD groups than in the cognitively healthy controls.

CONCLUSIONS

These results suggest that individuals with elevated plasma sAPPβ levels are at an increased risk of AD; elevation in these levels may reflect the progression of disease.

Distinct Spontaneous Brain Activity Patterns in Different Biologically-Defined Alzheimer's Disease Cognitive Stage: A Preliminary Study

Background: The National Institute on Aging-Alzheimer's Association (NIA-AA) has proposed a biological definition of Alzheimer's disease (AD): individuals with both abnormal amyloid and tau biomarkers (A+T+) would be defined as AD. It remains unclear why different cognitive status is present in subjects with biological AD. Resting-state functional magnetic resonance imaging (rsfMRI) has provided an opportunity to reveal the brain activity patterns in a biologically-defined AD cohort. Accordingly, we aimed to investigate distinct brain activity patterns in subjects with existed AD pathology but in the different cognitive stages. Method: We selected individuals with AD pathology (A+T+) and healthy controls (HC, A-T-) based on the cerebrospinal fluid (CSF) biomarkers. According to the cognitive stage, we divided the A+T+ cohort into three groups: (1) preclinical AD; (2) prodromal AD; and (3) AD with dementia (d-AD). We compared spontaneous brain activity measured by a fractional amplitude of low-frequency fluctuation (fALFF) approach among four groups. Results: The analysis of covariance (ANCOVA) results showed significant differences in fALFF in the posterior cingulate cortex/precuneus (PCC/PCu). Further, compared to HC, we found increased fALFF values in the right inferior frontal gyrus (IFG) in the preclinical AD stage, whereas prodromal AD patients showed reduced fALFF in the bilateral precuneus, right middle frontal gyrus (MFG), right precentral gyrus, and postcentral gyrus. Within the d-AD group, both hyperactivity (right fusiform gyrus, right parahippocampal gyrus (PHG)/hippocampus, and inferior temporal gyrus) and hypoactivity (bilateral precuneus, left posterior cingulate cortex, left cuneus and superior occipital gyrus) were detected. Conclusion: We found the distinct brain activity patterns in different cognitive stages among the subjects defined as AD biologically. Our findings may be helpful in understanding mechanisms leading to cognitive changes in the AD pathophysiological process.

Characterization of the selective in vitro and in vivo binding properties of crenezumab to oligomeric Aβ

Background

Accumulation of amyloid β (Aβ) in the brain is proposed as a cause of Alzheimer’s disease (AD), with Aβ oligomers hypothesized to be the primary mediators of neurotoxicity. Crenezumab is a humanized immunoglobulin G4 monoclonal antibody that has been shown to bind to synthetic monomeric and aggregated Aβ in vitro; however, less is known about the binding characteristic in vivo. In this study, we evaluated the binding patterns of crenezumab to synthetic and native forms of Aβ both in vitro and in vivo.

Methods

Crenezumab was used to immunoprecipitate Aβ from synthetic Aβ preparations or brain homogenates from a PS2APP mouse model of AD to determine the forms of Aβ that crenezumab interacts with. Following systemic dosing in PS2APP or nontransgenic control mice, immunohistochemistry was used to localize crenezumab and assess its relative distribution in the brain, compared with amyloid plaques and markers of neuritic dystrophies (BACE1; LAMP1). Pharmacodynamic correlations were performed to investigate the relationship between peripheral and central target engagement.

Results

In vitro, crenezumab immunoprecipitated Aβ oligomers from both synthetic Aβ preparations and endogenous brain homogenates from PS2APP mice. In vivo studies in the PS2APP mouse showed that crenezumab localizes to regions surrounding the periphery of amyloid plaques in addition to the hippocampal mossy fibers. These regions around the plaques are reported to be enriched in oligomeric Aβ, actively incorporate soluble Aβ, and contribute to Aβ-induced neurotoxicity and axonal dystrophy. In addition, crenezumab did not appear to bind to the dense core region of plaques or vascular amyloid.

Conclusions

Crenezumab binds to multiple forms of amyloid β (Aβ), particularly oligomeric forms, and localizes to brain areas rich in Aβ oligomers, including the halo around plaques and hippocampal mossy fibers, but not to vascular Aβ. These insights highlight a unique mechanism of action for crenezumab of engaging Aβ oligomers.

Tau Accumulation in Clinically Normal Older Adults Is Associated with Hippocampal Hyperactivity

Animal studies demonstrate that hyperactive neurons facilitate early accumulation and spread of tau and amyloid-β proteins in the pathological cascade of Alzheimer's disease (AD). Human neuroimaging studies have linked hippocampal hyperactivity to amyloid-β accumulation, apolipoprotein ε4 (APOE4) and clinical progression from prodromal AD to clinical dementia. The relationship between hippocampal hyperactivity and early AD molecular pathology (amyloid-β and tau accumulation) before clinical symptoms remains to be elucidated. Here, we studied 120 clinically normal older humans (80 females/40 males) enrolled in the Harvard Aging Brain Study. We measured functional magnetic resonance imaging (fMRI) activity during successful memory encoding and amyloid-β accumulation with PiB-positron emission tomography imaging. Additionally, we measured tau accumulation using AV1451 PET imaging in a subset of 87 participants. In this subset, we found that inferior temporal tau accumulation was associated with increased fMRI activity in the hippocampus, but showed no clear association with amyloid. Together, the findings support a hypothetical model of the evolution of preclinical AD that place hippocampal hyperactivity concurrent with spread of tau pathology to neocortical regions before clinical impairment.SIGNIFICANCE STATEMENT The circumstances under which the hippocampus becomes hyperactive in preclinical stages of Alzheimer's disease (AD) have thus far remained elusive. Recent advances in positron emission tomography (PET) tracers now enable in vivo characterization of amyloid-β and tau accumulation. Here, we combine amyloid and tau PET with functional magnetic resonance imaging (fMRI) to examine the association between Alzheimer's disease pathology and memory-related brain activity in clinically normal older adults. We found an association between increased hippocampal activity and tau accumulation in the inferior temporal cortex. These data suggest that the pathogenesis of hippocampal hyperactivity occurs concurrent with the spread of tau pathology from the entorhinal cortex to the neocortex, before the clinical manifestations of Alzheimer's disease.

Cerebral Aβ40 and systemic hypertension

Mid-life hypertension and cerebral hypoperfusion may be preclinical abnormalities in people who later develop Alzheimer's disease. Although accumulation of amyloid-beta (Aβ) is characteristic of Alzheimer's disease and is associated with upregulation of the vasoconstrictor peptide endothelin-1 within the brain, it is unclear how this affects systemic arterial pressure. We have investigated whether infusion of Aβ₄₀ into ventricular cerebrospinal fluid modulates blood pressure in the Dahl salt-sensitive rat. The Dahl salt-sensitive rat develops hypertension if given a high-salt diet. Intracerebroventricular infusion of Aβ induced a progressive rise in blood pressure in rats with pre-existing hypertension produced by a high-salt diet ( p < 0.0001), but no change in blood pressure in normotensive rats. The elevation in arterial pressure in high-salt rats was associated with an increase in low frequency spectral density in systolic blood pressure, suggesting autonomic imbalance, and reduced cardiac baroreflex gain. Our results demonstrate the potential for intracerebral Aβ to exacerbate hypertension, through modulation of autonomic activity. Present findings raise the possibility that mid-life hypertension in people who subsequently develop Alzheimer's disease may in some cases be a physiological response to reduced cerebral perfusion complicating the accumulation of Aβ within the brain.

The Physiological Roles of Amyloid-β Peptide Hint at New Ways to Treat Alzheimer's Disease

Amyloid-ß (Aß) is best known as the misfolded peptide that is involved in the pathogenesis of Alzheimer's disease (AD), and it is currently the primary therapeutic target in attempts to arrest the course of this disease. This notoriety has overshadowed evidence that Aß serves several important physiological functions. Aß is present throughout the lifespan, it has been found in all vertebrates examined thus far, and its molecular sequence shows a high degree of conservation. These features are typical of a factor that contributes significantly to biological fitness, and this suggestion has been supported by evidence of functions that are beneficial for the brain. The putative roles of Aß include protecting the body from infections, repairing leaks in the blood-brain barrier, promoting recovery from injury, and regulating synaptic function. Evidence for these beneficial roles comes from in vitro and in vivo studies, which have shown that the cellular production of Aß rapidly increases in response to a physiological challenge and often diminishes upon recovery. These roles are further supported by the adverse outcomes of clinical trials that have attempted to deplete Aß in order to treat AD. We suggest that anti-Aß therapies will produce fewer adverse effects if the known triggers of Aß deposition (e.g., pathogens, hypertension, and diabetes) are addressed first.

Vulnerability of primary neurons derived from Tg2576 Alzheimer mice to oxygen and glucose deprivation: role of intraneuronal amyloid-β accumulation and astrocytes

Microvascular dysfunction is considered an integral part of Alzheimer disease (AD) pathogenesis, but the possible relationship between amyloid pathology, microvascular dysfunction and cell death is still unclear. In order to investigate the influence of intraneuronal amyloid-β (Aβ) accumulation on vulnerability to hypoxia, we isolated primary cortical neurons from Tg2576 (carrying the amyloid precursor protein APPSwe mutation) and wild-type fetal mice. We first demonstrated that neurons isolated from Tg2576 newborn mice show an increase in VEGFa mRNA expression and a decrease in the expression of the two VEGF receptors, Flt1 and Kdr, compared with wild-type cells. Moreover, APPSwe primary neurons displayed higher spontaneous and glutamate-induced cell death. We then deprived the cultures of oxygen and glucose (OGD) as an in vitro model of hypoxia. After OGD, APPSwe neurons display higher levels of cell death in terms of percentage of pyknotic/fragmented nuclei and mitochondrial depolarization, accompanied by an increase in the intraneuronal Aβ content. To explore the influence of intraneuronal Aβ peptide accumulation, we used the γ-secretase inhibitor LY450139, which showed that the reduction of the intracellular amyloid fully protects APPSwe neurons from OGD-induced degeneration. Conditioned medium from OGD-exposed APPSwe or wild-type astrocytes protected APPswe neurons but not wild-type neurons, during OGD. In conclusion, the presence of the mutated human APP gene, leading to the intracellular accumulation of APP and Aβ fragments, worsens OGD toxicity. Protection of APPSwe neurons can be obtained either using a γ-secretase inhibitor or astrocyte conditioned medium.

Summary:In vitro systems derived from AD mice can be used to investigate the vulnerability of AD neurons to different neurotoxic challenges, including oxygen glucose deprivation.

Robust Translation of γ-Secretase Modulator Pharmacology across Preclinical Species and Human Subjects

The amyloid-β peptide (Aβ)—in particular, the 42–amino acid form, Aβ1-42—is thought to play a key role in the pathogenesis of Alzheimer’s disease (AD). Thus, several therapeutic modalities aiming to inhibit Aβ synthesis or increase the clearance of Aβ have entered clinical trials, including γ-secretase inhibitors, anti-Aβ antibodies, and amyloid-β precursor protein cleaving enzyme inhibitors. A unique class of small molecules, γ-secretase modulators (GSMs), selectively reduce Aβ1-42 production, and may also decrease Aβ1-40 while simultaneously increasing one or more shorter Aβ peptides, such as Aβ1-38 and Aβ1-37. GSMs are particularly attractive because they do not alter the total amount of Aβ peptides produced by γ-secretase activity; they spare the processing of other γ-secretase substrates, such as Notch; and they do not cause accumulation of the potentially toxic processing intermediate, β-C-terminal fragment. This report describes the translation of pharmacological activity across species for two novel GSMs, (S)-7-(4-fluorophenyl)-N2-(3-methoxy-4-(3-methyl-1H-1,2,4-triazol-1-yl)phenyl)-N4-methyl-6,7-dihydro-5H-cyclopenta[d]pyrimidine-2,4-diamine (BMS-932481) and (S,Z)-17-(4-chloro-2-fluorophenyl)-34-(3-methyl-1H-1,2,4-triazol-1-yl)-16,17-dihydro-15H-4-oxa-2,9-diaza-1(2,4)-cyclopenta[d]pyrimidina-3(1,3)-benzenacyclononaphan-6-ene (BMS-986133). These GSMs are highly potent in vitro, exhibit dose- and time-dependent activity in vivo, and have consistent levels of pharmacological effect across rats, dogs, monkeys, and human subjects. In rats, the two GSMs exhibit similar pharmacokinetics/pharmacodynamics between the brain and cerebrospinal fluid. In all species, GSM treatment decreased Aβ1-42 and Aβ1-40 levels while increasing Aβ1-38 and Aβ1-37 by a corresponding amount. Thus, the GSM mechanism and central activity translate across preclinical species and humans, thereby validating this therapeutic modality for potential utility in AD.

Increased CSF Aβ during the very early phase of cerebral Aβ deposition in mouse models

Abnormalities in brains of Alzheimer's disease (AD) patients are thought to start long before the first clinical symptoms emerge. The identification of affected individuals at this ‘preclinical AD’ stage relies on biomarkers such as decreased levels of the amyloid-β peptide (Aβ) in the cerebrospinal fluid (CSF) and positive amyloid positron emission tomography scans. However, there is little information on the longitudinal dynamics of CSF biomarkers, especially in the earliest disease stages when therapeutic interventions are likely most effective. To this end, we have studied CSF Aβ changes in three Aβ precursor protein transgenic mouse models, focusing our analysis on the initial Aβ deposition, which differs significantly among the models studied. Remarkably, while we confirmed the CSF Aβ decrease during the extended course of brain Aβ deposition, a 20–30% increase in CSF Aβ40 and Aβ42 was found around the time of the first Aβ plaque appearance in all models. The biphasic nature of this observed biomarker changes stresses the need for longitudinal biomarker studies in the clinical setting and the search for new ‘preclinical AD’ biomarkers at even earlier disease stages, by using both mice and human samples. Ultimately, our findings may open new perspectives in identifying subjects at risk for AD significantly earlier, and in improving the stratification of patients for preventive treatment strategies.

A novel BACE inhibitor NB-360 shows a superior pharmacological profile and robust reduction of amyloid-β and neuroinflammation in APP transgenic mice

Background

Alzheimer’s disease (AD) is the most common form of dementia, the number of affected individuals is rising, with significant impacts for healthcare systems. Current symptomatic treatments delay, but do not halt, disease progression. Genetic evidence points to aggregation and deposition of amyloid-β (Aβ) in the brain being causal for the neurodegeneration and dementia typical of AD. Approaches to target Aβ via inhibition of γ-secretase or passive antibody therapy have not yet resulted in substantial clinical benefits. Inhibition of BACE1 (β-secretase) has proven a challenging concept, but recent BACE1inhibitors can enter the brain sufficiently well to lower Aβ. However, failures with the first clinical BACE1 inhibitors have highlighted the need to generate compounds with appropriate efficacy and safety profiles, since long treatment periods are expected to be necessary in humans.

Results

Treatment with NB-360, a potent and brain penetrable BACE-1 inhibitor can completely block the progression of Aβ deposition in the brains of APP transgenic mice, a model for amyloid pathology. We furthermore show that almost complete reduction of Aβ was achieved also in rats and in dogs, suggesting that these findings are translational across species and can be extrapolated to humans. Amyloid pathology may be an initial step in a complex pathological cascade; therefore we investigated the effect of BACE-1 inhibition on neuroinflammation, a prominent downstream feature of the disease. NB-360 stopped accumulation of activated inflammatory cells in the brains of APP transgenic mice. Upon chronic treatment of APP transgenic mice, patches of grey hairs appeared.

Conclusions

In a rapidly developing field, the data on NB-360 broaden the chemical space and expand knowledge on the properties that are needed to make a BACE-1 inhibitor potent and safe enough for long-term use in patients. Due to its excellent brain penetration, reasonable oral doses of NB-360 were sufficient to completely block amyloid-β deposition in an APP transgenic mouse model. Data across species suggest similar treatment effects can possibly be achieved in humans. The reduced neuroinflammation upon amyloid reduction by NB-360 treatment supports the notion that targeting amyloid-β pathology can have beneficial downstream effects on the progression of Alzheimer’s disease.

Fluid Biomarkers in Clinical Trials of Alzheimer's Disease Therapeutics

With the demographic shift of the global population toward longer life expectancy, the number of people living with Alzheimer’s disease (AD) has rapidly expanded and is projected to triple by the year 2050. Current treatments provide symptomatic relief but do not affect the underlying pathology of the disease. Therapies that prevent or slow the progression of the disease are urgently needed to avoid this growing public health emergency. Insights gained from decades of research have begun to unlock the pathophysiology of this complex disease and have provided targets for disease-modifying therapies. In the last decade, few therapeutic agents designed to modify the underlying disease process have progressed to clinical trials and none have been brought to market. With the focus on disease modification, biomarkers promise to play an increasingly important role in clinical trials. Six biomarkers have now been included in diagnostic criteria for AD and are regularly incorporated into clinical trials. Three biomarkers are neuroimaging measures – hippocampal atrophy measured by magnetic resonance imaging (MRI), amyloid uptake as measured by Pittsburg compound B positron emission tomography (PiB-PET), and decreased fluorodeoxyglucose (18F) uptake as measured by PET (FDG-PET) – and three are sampled from fluid sources – cerebrospinal fluid levels of amyloid β42 (Aβ42), total tau, and phosphorylated tau. Fluid biomarkers are important because they can provide information regarding the underlying biochemical processes that are occurring in the brain. The purpose of this paper is to review the literature regarding the existing and emerging fluid biomarkers and to examine how fluid biomarkers have been incorporated into clinical trials.

Amyloid cascade hypothesis: Pathogenesis and therapeutic strategies in Alzheimer's disease

Alzheimer's disease is an irreversible, progressive neurodegenerative disorder. Various therapeutic approaches are being used to improve the cholinergic neurotransmission, but their role in AD pathogenesis is still unknown. Although, an increase in tau protein concentration in CSF has been described in AD, but several issues remains unclear. Extensive and accurate analysis of CSF could be helpful to define presence of tau proteins in physiological conditions, or released during the progression of neurodegenerative disease. The amyloid cascade hypothesis postulates that the neurodegeneration in AD caused by abnormal accumulation of amyloid beta (Aβ) plaques in various areas of the brain. The amyloid hypothesis has continued to gain support over the last two decades, particularly from genetic studies. Therefore, current research progress in several areas of therapies shall provide an effective treatment to cure this devastating disease. This review critically evaluates general biochemical and physiological functions of Aβ directed therapeutics and their relevance.

Can insulin signaling pathways be targeted to transport Aβ out of the brain?

Although the causal role of Amyloid-β (Aβ) in Alzheimer’s disease (AD) is unclear, it is still reasonable to expect that lowering concentrations of Aβ in the brain may decrease the risk of developing the neurocognitive symptoms of the disease. Brain capillary endothelial cells forming the blood-brain barrier (BBB) express transporters regulating the efflux of Aβ out of the cerebral tissue. Age-related BBB dysfunctions, that have been identified in AD patients, might impair Aβ clearance from the brain. Thus, targeting BBB outward transport systems has been suggested as a way to stimulate the clearance of Aβ from the brain. Recent data indicate that the increase in soluble brain Aβ and behavioral impairments in 3×Tg-AD mice generated by months of intake of a high-fat diet can be acutely reversed by the administration of a single dose of insulin. A concomitant increase in plasma Aβ suggests that clearance from the brain through the BBB is a likely mechanism for this rapid effect of insulin. Here, we review how BBB insulin response pathways could be stimulated to decrease brain Aβ concentrations and improve cognitive performance, at least on the short term.

Prion-Protein-interacting Amyloid-β Oligomers of High Molecular Weight Are Tightly Correlated with Memory Impairment in Multiple Alzheimer Mouse Models

Alzheimer disease (AD) is characterized by amyloid-β accumulation, with soluble oligomers (Aβo) being the most synaptotoxic. However, the multivalent and unstable nature of Aβo limits molecular characterization and hinders research reproducibility. Here, we characterized multiple Aβo forms throughout the life span of various AD mice and in post-mortem human brain. Aβo exists in several populations, where prion protein (PrP(C))-interacting Aβo is a high molecular weight Aβ assembly present in multiple mice and humans with AD. Levels of PrP(C)-interacting Aβo match closely with mouse memory and are equal or superior to other Aβ measures in predicting behavioral impairment. However, Aβo metrics vary considerably between mouse strains. Deleting PrP(C) expression in mice with relatively low PrP(C)-interacting Aβo (Tg2576) results in partial rescue of cognitive performance as opposed to complete recovery in animals with a high percentage of PrP(C)-interacting Aβo (APP/PSEN1). These findings highlight the relative contributions and interplay of Aβo forms in AD.

Alzheimer's disease pathology is attenuated in a CD38-deficient mouse model

OBJECTIVE

Alzheimer's disease (AD)-associated dementia is due to tissue damage caused by amyloid β (Aβ) deposition within the brain and by accompanying neuroinflammation. The nicotinamide adenine dinucleotide (NAD) glycohydrolase CD38, which is expressed by neurons, astrocytes, and microglial cells, regulates inflammatory and repair processes in the brain and other tissues by degrading NAD and repressing the activity of other NAD-consuming enzymes and by producing NAD-derived metabolites that regulate calcium signaling and migration of inflammatory cells. Given the role of CD38 in neuroinflammation and repair, we examined the effect of CD38 deletion on AD pathology.

METHODS

We crossed APPswePS1ΔE9 (APP.PS) mice with Cd38(-) (/) (-) mice to generate AD-prone CD38-deficient animals (APP.PS.Cd38(-) (/) (-) ) and examined AD-related phenotypes in both groups.

RESULTS

APP.PS.Cd38(-) (/) (-) mice exhibited significant reductions in Aβ plaque load and soluble Aβ levels compared to APP.PS mice, and this correlated with improved spatial learning. Although CD38 deficiency resulted in decreased microglia/macrophage (MM) accumulation, the transcription profile of the Cd38(-) (/) (-) and Cd38(+/) (+) MM was similar, suggesting that the decreased Aβ burden in APP.PS.Cd38(-) (/) (-) mice was not due to alterations in MM activation/function. Instead, APP.PS.Cd38(-) (/) (-) neuronal cultures secreted less Aβ and this reduction was mimicked when APP.PS neuronal cultures were treated with inhibitors that blocked CD38 enzyme activity or the signaling pathways controlled by CD38-derived metabolites. Furthermore, β- and γ-secretase activity was decreased in APP.PS.Cd38(-) (/) (-) mice, which correlated with decreased Aβ production.

INTERPRETATION

CD38 regulates AD pathology in the APP.PS model of AD, suggesting that CD38 may be a novel target for AD treatment.

Amyloid-beta and Alzheimer's disease: the role of neprilysin-2 in amyloid-beta clearance

Accumulation of the amyloid-beta (Aβ) peptide is a central factor in Alzheimer's disease (AD) pathogenesis as supported by continuing evidence. This review concisely summarizes this evidence supporting a critical role for Aβ in AD before discussing the clearance of this peptide. Mechanisms of clearance of Aβ are critical for preventing pathological elevations in Aβ concentration. Direct degradation of Aβ by endopeptidases has emerged as one important pathway for clearance. Of particular interest are endopeptidases that are sensitive to the neprilysin (NEP) inhibitors thiorphan and phosphoramidon (i.e., are "NEP-like") as these inhibitors induce a dramatic increase in Aβ levels in rodents. This review will focus on neprilysin-2 (NEP2), a NEP-like endopeptidase which cooperates with NEP to control Aβ levels in the brain. The evidence for the involvement of NEP2 in AD is discussed as well as the therapeutic relevance with regards to gene therapy and the development of molecular markers for the disease.

Chronic γ-secretase inhibition reduces amyloid plaque-associated instability of pre- and postsynaptic structures

The loss of synapses is a strong histological correlate of the cognitive decline in Alzheimer's disease (AD). Amyloid β-peptide (Aβ), a cleavage product of the amyloid precursor protein (APP), exerts detrimental effects on synapses, a process thought to be causally related to the cognitive deficits in AD. Here, we used in vivo two-photon microscopy to characterize the dynamics of axonal boutons and dendritic spines in APP/Presenilin 1 (APP(swe)/PS1(L166P))-green fluorescent protein (GFP) transgenic mice. Time-lapse imaging over 4 weeks revealed a pronounced, concerted instability of pre- and postsynaptic structures within the vicinity of amyloid plaques. Treatment with a novel sulfonamide-type γ-secretase inhibitor (GSI) attenuated the formation and growth of new plaques and, most importantly, led to a normalization of the enhanced dynamics of synaptic structures close to plaques. GSI treatment did neither affect spines and boutons distant from plaques in amyloid precursor protein/presenilin 1-GFP (APPPS1-GFP) nor those in GFP-control mice, suggesting no obvious neuropathological side effects of the drug.

Targeting the proper amyloid-beta neuronal toxins: a path forward for Alzheimer's disease immunotherapeutics

Levels of amyloid-beta monomer and deposited amyloid-beta in the Alzheimer’s disease brain are orders of magnitude greater than soluble amyloid-beta oligomer levels. Monomeric amyloid-beta has no known direct toxicity. Insoluble fibrillar amyloid-beta has been proposed to be an in vivo mechanism for removal of soluble amyloid-beta and exhibits relatively low toxicity. In contrast, soluble amyloid-beta oligomers are widely reported to be the most toxic amyloid-beta form, both causing acute synaptotoxicity and inducing neurodegenerative processes. None of the amyloid-beta immunotherapies currently in clinical development selectively target soluble amyloid-beta oligomers, and their lack of efficacy is not unexpected considering their selectivity for monomeric or fibrillar amyloid-beta (or both) rather than soluble amyloid-beta oligomers. Because they exhibit acute, memory-compromising synaptic toxicity and induce chronic neurodegenerative toxicity and because they exist at very low in vivo levels in the Alzheimer’s disease brain, soluble amyloid-beta oligomers constitute an optimal immunotherapeutic target that should be pursued more aggressively.

γ-Secretase inhibitors and modulators

γ-Secretase is a fascinating, multi-subunit, intramembrane cleaving protease that is now being considered as a therapeutic target for a number of diseases. Potent, orally bioavailable γ-secretase inhibitors (GSIs) have been developed and tested in humans with Alzheimer's disease (AD) and cancer. Preclinical studies also suggest the therapeutic potential for GSIs in other disease conditions. However, due to inherent mechanism based-toxicity of non-selective inhibition of γ-secretase, clinical development of GSIs will require empirical testing with careful evaluation of benefit versus risk. In addition to GSIs, compounds referred to as γ-secretase modulators (GSMs) remain in development as AD therapeutics. GSMs do not inhibit γ-secretase, but modulate γ-secretase processivity and thereby shift the profile of the secreted amyloid β peptides (Aβ) peptides produced. Although GSMs are thought to have an inherently safe mechanism of action, their effects on substrates other than the amyloid β protein precursor (APP) have not been extensively investigated. Herein, we will review the current state of development of GSIs and GSMs and explore pertinent biological and pharmacological questions pertaining to the use of these agents for select indications. This article is part of a Special Issue entitled: Intramembrane Proteases.

Competition between homodimerization and cholesterol binding to the C99 domain of the amyloid precursor protein

The 99-residue transmembrane C-terminal domain (C99, also known as β-CTF) of the amyloid precursor protein (APP) is the product of the β-secretase cleavage of the full-length APP and is the substrate for γ-secretase cleavage. The latter cleavage releases the amyloid-β polypeptides that are closely associated with Alzheimer's disease. C99 is thought to form homodimers; however, the free energy in favor of dimerization has not previously been quantitated. It was also recently documented that cholesterol forms a 1:1 complex with monomeric C99 in bicelles. Here, the affinities for both homodimerization and cholesterol binding to C99 were measured in bilayered lipid vesicles using both electron paramagnetic resonance (EPR) and Förster resonance energy transfer (FRET) methods. Homodimerization and cholesterol binding were seen to be competitive processes that center on the transmembrane G₇₀₀XXXG₇₀₄XXXG₇₀₈ glycine-zipper motif and adjacent Gly709. On one hand, the observed Kd for cholesterol binding (Kd = 2.7 ± 0.3 mol %) is on the low end of the physiological cholesterol concentration range in mammalian cell membranes. On the other hand, the observed K(d) for homodimerization (K(d) = 0.47 ± 0.15 mol %) likely exceeds the physiological concentration range for C99. These results suggest that the 1:1 cholesterol/C99 complex will be more highly populated than C99 homodimers under most physiological conditions. These observations are of relevance for understanding the γ-secretase cleavage of C99.

Transgenic expression of β1 antibody in brain neurons impairs age-dependent amyloid deposition in APP23 mice

Heterologous expression of the functional amyloid beta (Aβ) antibody β1 in the central nervous system was engineered to maximize antibody exposure in the brain and assess the effects on Aβ production and accumulation in these conditions. A single open reading frame encoding the heavy and light chains of β1 linked by the mouth and foot virus peptide 2A was expressed in brain neurons of transgenic mice. Two of the resulting BIN66 transgenic lines were crossed with APP23 mice, which develop severe central amyloidosis. Brain concentrations at steady-state 5 times greater than those found after peripheral β1 administration were obtained. Similar brain and plasma β1 concentrations indicated robust antibody efflux from the brain. In preplaque mice, β1 formed a complex with Aβ that caused a modest Aβ increase in brain and plasma. At 11 months of age, β1 expression reduced amyloid by 97% compared with age-matched APP23 mice. Interference of β1 with β-secretase cleavage of amyloid precursor protein was relatively small. Our data suggest that severely impaired amyloid formation was primarily mediated by a complex of β1 with soluble Aβ, which might have prevented Aβ aggregation or favored transport out of the brain.

The neurotrophic compound J147 reverses cognitive impairment in aged Alzheimer's disease mice

Introduction

Despite years of research, there are no disease-modifying drugs for Alzheimer's disease (AD), a fatal, age-related neurodegenerative disorder. Screening for potential therapeutics in rodent models of AD has generally relied on testing compounds before pathology is present, thereby modeling disease prevention rather than disease modification. Furthermore, this approach to screening does not reflect the clinical presentation of AD patients which could explain the failure to translate compounds identified as beneficial in animal models to disease modifying compounds in clinical trials. Clearly a better approach to pre-clinical drug screening for AD is required.

Methods

To more accurately reflect the clinical setting, we used an alternative screening strategy involving the treatment of AD mice at a stage in the disease when pathology is already advanced. Aged (20-month-old) transgenic AD mice (APP/swePS1ΔE9) were fed an exceptionally potent, orally active, memory enhancing and neurotrophic molecule called J147. Cognitive behavioral assays, histology, ELISA and Western blotting were used to assay the effect of J147 on memory, amyloid metabolism and neuroprotective pathways. J147 was also investigated in a scopolamine-induced model of memory impairment in C57Bl/6J mice and compared to donepezil. Details on the pharmacology and safety of J147 are also included.

Results

Data presented here demonstrate that J147 has the ability to rescue cognitive deficits when administered at a late stage in the disease. The ability of J147 to improve memory in aged AD mice is correlated with its induction of the neurotrophic factors NGF (nerve growth factor) and BDNF (brain derived neurotrophic factor) as well as several BDNF-responsive proteins which are important for learning and memory. The comparison between J147 and donepezil in the scopolamine model showed that while both compounds were comparable at rescuing short term memory, J147 was superior at rescuing spatial memory and a combination of the two worked best for contextual and cued memory.

Conclusion

J147 is an exciting new compound that is extremely potent, safe in animal studies and orally active. J147 is a potential AD therapeutic due to its ability to provide immediate cognition benefits, and it also has the potential to halt and perhaps reverse disease progression in symptomatic animals as demonstrated in these studies.

Cellular and animal models for high-throughput screening of therapeutic agents for the treatment of the diseases of the elderly in general and Alzheimer's disease in particular(†)

It is currently thought that the dementia of Alzheimer's disease is due to the neurotoxicity of the deposits or aggregates of amyloid-β (Aβ) in the extracellular space of the cerebral cortex. This model has been widely criticized because there is a poor correlation between deposits and dementia. Others have questioned whether Aβ is truly neurotoxic. Yet, in spite of these concerns, the search for therapeutic agents has been based on the development of mouse models transfected with mutant genes associated in humans with early onset Alzheimer's disease. A major limitation of these models is that although they exhibit many of the pathological and clinical manifestation of the human disease, the bulk of individuals who develop the dementia of Alzheimer's disease have none of these mutant genes. Furthermore, nine clinical trials of drugs that were effective in transgenic mice failed to show any benefit in patients. Finally, a major unresolved issue with the Aβ model is that since Aβ is produced in everyone, why are deposits only seen in the elderly? This issue must be resolved if we are to understand the etiology of the disease and develop test systems for both diagnosis and drug discovery. Published studies from my laboratory demonstrate that in human cerebrospinal fluid immunoreactive Aβ is only present as a complex with two chaperones, ERp57 and calreticulin and is N-glycosylated. This complex formation is catalyzed by the posttranslational protein processing system of the endoplasmic reticulum (ER). Others have reported that in plaque Aβ is present only as the naked peptide. Together these results suggest that both plaque and dementia are secondary to an age related decline in the capacity of the ER to catalyze protein, posttranslational processing. Since the synaptic membrane proteins necessary for a functioning memory are also processed in the ER, these findings would suggest that the loss of cognition is due to a decline in the capacity of the neuron to produce and maintain functioning synapses. Work from my laboratory and from others further indicate that the components of the ER, posttranslational, protein processing pathway do dramatically decline with age. These data suggest that this decline may be found in all cells and could account not only for the dementia of Alzheimer's disease, but also for many of the other manifestations of the aging process. These observations also suggest that declining ER function has a role in two well-recognized phenomena associated with aging: a loss of mitochondrial function and a decrease in myelin. Finally, based on this paradigm I propose new cellular and animals models for high-throughput screening for drug discovery.

In Vivo Characterization of a Novel γ-Secretase Inhibitor SCH 697466 in Rodents and Investigation of Strategies for Managing Notch-Related Side Effects

Substantial evidence implicates β-amyloid (Aβ) peptides in the etiology of Alzheimer's disease (AD). Aβ is produced by the proteolytic cleavage of the amyloid precursor protein by β- and γ-secretase suggesting that γ-secretase inhibition may provide therapeutic benefit for AD. Although many γ-secretase inhibitors have been shown to be potent at lowering Aβ, some have also been shown to have side effects following repeated administration. All of these side effects can be attributed to altered Notch signaling, another γ-secretase substrate. Here we describe the in vivo characterization of the novel γ-secretase inhibitor SCH 697466 in rodents. Although SCH 697466 was effective at lowering Aβ, Notch-related side effects in the intestine and thymus were observed following subchronic administration at doses that provided sustained and complete lowering of Aβ. However, additional studies revealed that both partial but sustained lowering of Aβand complete but less sustained lowering of Aβ were successful approaches for managing Notch-related side effects. Further, changes in several Notch-related biomarkers paralleled the side effect observations. Taken together, these studies demonstrated that, by carefully varying the extent and duration of Aβ lowering by γ-secretase inhibitors, it is possible to obtain robust and sustained lowering of Aβ without evidence of Notch-related side effects.

Reducing available soluble β-amyloid prevents progression of cerebral amyloid angiopathy in transgenic mice

Cerebral amyloid angiopathy (CAA), the accumulation of β-amyloid (Aβ) in the walls of leptomeningeal and cortical blood vessels of the brain, is a major cause of intracerebral hemorrhage and cognitive impairment and is commonly associated with Alzheimer disease. The progression of CAA, as measured in transgenic mice by longitudinal imaging with multiphoton microscopy, occurs in a predictable linear manner. The dynamics of Aβ deposition in and clearance from vascular walls and their relationship to the concentration of Aβ in the brain are poorly understood. We manipulated Aβ levels in the brain using 2 approaches: peripheral clearance via administration of the amyloid binding "peripheral sink" protein gelsolin and direct inhibition of its formation via administration of LY-411575, a small-molecule γ-secretase inhibitor. We found that gelsolin and LY-411575 both reduced the rate of CAA progression in Tg2576 mice from untreated rates of 0.58% ± 0.15% and 0.52% ± 0.09% to 0.11% ± 0.18% (p = 0.04) and -0.17% ± 0.09% (p < 0.001) of affected vessel per day, respectively, in the absence of an immune response. The progression of CAA was also halted when gelsolin was combined with LY-411575 (-0.004% ± 0.10%, p < 0.003). These data suggest that CAA progression can be prevented with non-immune approaches that may reduce the availability of soluble Aβ but without evidence of substantial amyloid clearance from vessels.

Transient pharmacologic lowering of Aβ production prior to deposition results in sustained reduction of amyloid plaque pathology

Background

Alzheimer’s disease (AD) is the leading cause of dementia among the elderly. Disease modifying therapies targeting Aβ that are in development have been proposed to be more effective if treatment was initiated prior to significant accumulation of Aβ in the brain, but optimal timing of treatment initiation has not been clearly established in the clinic. We compared the efficacy of transient pharmacologic reduction of brain Aβ with a γ-secretase inhibitor (GSI ) for 1–3 months (M) treatment windows in APP Tg2576 mice and subsequent aging of the mice to either 15M or 18M.

Results

These data show that reducing Aβ production in a 2-3M windows both initiated and discontinued before detectable Aβ deposition has the most significant impact on Aβ loads up to 11M after treatment discontinuation. In contrast, initiation of treatment for 3M windows from 7-10M or 12-15M shows progressively decreasing efficacy.

Conclusions

These data have major implications for clinical testing of therapeutics aimed at lowering Aβ production, indicating that; i) these therapies may have little efficacy unless tested as prophylactics or in the earliest preclinical stage of AD where there is no or minimal Aβ accumulation and ii) lowering Aβ production transiently during a critical pre-deposition window potentially provides long-lasting efficacy after discontinuation of the treatment.

Critical role of soluble amyloid-β for early hippocampal hyperactivity in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by a progressive dysfunction of central neurons. Recent experimental evidence indicates that in the cortex, in addition to the silencing of a fraction of neurons, other neurons are hyperactive in amyloid-β (Aβ) plaque-enriched regions. However, it has remained unknown what comes first, neuronal silencing or hyperactivity, and what mechanisms might underlie the primary neuronal dysfunction. Here we examine the activity patterns of hippocampal CA1 neurons in a mouse model of AD in vivo using two-photon Ca(2+) imaging. We found that neuronal activity in the plaque-bearing CA1 region of older mice is profoundly altered. There was a marked increase in the fractions of both silent and hyperactive neurons, as previously also found in the cortex. Remarkably, in the hippocampus of young mice, we observed a selective increase in hyperactive neurons already before the formation of plaques, suggesting that soluble species of Aβ may underlie this impairment. Indeed, we found that acute treatment with the γ-secretase inhibitor LY-411575 reduces soluble Aβ levels and rescues the neuronal dysfunction. Furthermore, we demonstrate that direct application of soluble Aβ can induce neuronal hyperactivity in wild-type mice. Thus, our study identifies hippocampal hyperactivity as a very early functional impairment in AD transgenic mice and provides direct evidence that soluble Aβ is crucial for hippocampal hyperactivity.

Measurement of apolipoprotein E and amyloid β clearance rates in the mouse brain using bolus stable isotope labeling

Background

Abnormal proteostasis due to alterations in protein turnover has been postulated to play a central role in several neurodegenerative diseases. Therefore, the development of techniques to quantify protein turnover in the brain is critical for understanding the pathogenic mechanisms of these diseases. We have developed a bolus stable isotope-labeling kinetics (SILK) technique coupled with multiple reaction monitoring mass spectrometry to measure the clearance of proteins in the mouse brain.

Results

Cohorts of mice were pulse labeled with ¹³ C₆-leucine and the brains were isolated after pre-determined time points. The extent of label incorporation was measured over time using mass spectrometry to measure the ratio of labeled to unlabeled apolipoprotein E (apoE) and amyloid β (Aβ). The fractional clearance rate (FCR) was then calculated by analyzing the time course of disappearance for the labeled protein species. To validate the technique, apoE clearance was measured in mice that overexpress the low-density lipoprotein receptor (LDLR). The FCR in these mice was 2.7-fold faster than wild-type mice. To demonstrate the potential of this technique for understanding the pathogenesis of neurodegenerative disease, we applied our SILK technique to determine the effect of ATP binding cassette A1 (ABCA1) on both apoE and Aβ clearance. ABCA1 had previously been shown to regulate both the amount of apoE in the brain, along with the extent of Aβ deposition, and represents a potential molecular target for lowering brain amyloid levels in Alzheimer's disease patients. The FCR of apoE was increased by 1.9- and 1.5-fold in mice that either lacked or overexpressed ABCA1, respectively. However, ABCA1 had no effect on the FCR of Aβ, suggesting that ABCA1 does not regulate Aβ metabolism in the brain.

Conclusions

Our SILK strategy represents a straightforward, cost-effective, and efficient method to measure the clearance of proteins in the mouse brain. We expect that this technique will be applicable to the study of protein dynamics in the pathogenesis of several neurodegenerative diseases, and could aid in the evaluation of novel therapeutic agents.

The dynamics of Aβ distribution after γ-secretase inhibitor treatment, as determined by experimental and modelling approaches in a wild type rat

Inhibition of the enzyme(s) that produce the Amyloid beta (Aβ) peptide, namely BACE and γ-secretase, is considered an attractive target for Alzheimer's disease therapy. However, the optimal pharmacokinetic-pharmacodynamic modelling method to describe the changes in Aβ levels after drug treatment is unclear. In this study, turnover models were employed to describe Aβ levels following treatment with the γ-secretase inhibitor RO5036450, in the wild type rat. Initially, Aβ level changes in the brain, cerebral spinal fluid (CSF) and plasma were modeled as separate biological compartments, which allowed the estimation of a compound IC₅₀ and Aβ turnover. While the data were well described, the model did not take into consideration that the CSF pool of Aβ most likely originates from the brain via the CSF drainage pathway. Therefore, a separate model was carried out, with the assumption that CSF Aβ levels originated from the brain. The optimal model that described the data involved two brain Aβ 40 sub-compartments, one with a rapid turnover, from which CSF Aβ 40 is derived, and a second quasi-static pool of ~20%. Importantly, the estimated in vivo brain IC₅₀ was in a good range of the in vitro IC₅₀ (ratio, 1.4). In conclusion, the PK/PD models presented here are well suited for describing the temporal changes in Aβ levels that occur after treatment with an Aβ lowering drug, and identifying physiological parameters.

Transgenic expression of intraneuronal Aβ42 but not Aβ40 leads to cellular Aβ lesions, degeneration, and functional impairment without typical Alzheimer's disease pathology

An early role of amyloid-β peptide (Aβ) aggregation in Alzheimer's disease pathogenesis is well established. However, the contribution of intracellular or extracellular forms of Aβ to the neurodegenerative process is a subject of considerable debate. We here describe transgenic mice expressing Aβ1-40 (APP47) and Aβ1-42 (APP48) with a cleaved signal sequence to insert both peptides during synthesis into the endoplasmic reticulum. Although lower in transgene mRNA, APP48 mice reach a higher brain Aβ concentration. The reduced solubility and increased aggregation of Aβ1-42 may impair its degradation. APP48 mice develop intracellular Aβ lesions in dendrites and lysosomes. The hippocampal neuron number is reduced already at young age. The brain weight decreases during aging in conjunction with severe white matter atrophy. The mice show a motor impairment. Only very few Aβ1-40 lesions are found in APP47 mice. Neither APP47 nor APP48 nor the bigenic mice develop extracellular amyloid plaques. While intracellular membrane expression of Aβ1-42 in APP48 mice does not lead to the AD-typical lesions, Aβ aggregates develop within cells accompanied by considerable neurodegeneration.

Phosphorylation of amyloid-β peptide at serine 8 attenuates its clearance via insulin-degrading and angiotensin-converting enzymes

Accumulation of amyloid-β peptides (Aβ) in the brain is a common pathological feature of Alzheimer disease (AD). Aggregates of Aβ are neurotoxic and appear to be critically involved in the neurodegeneration during AD pathogenesis. Accumulation of Aβ could be caused by increased production, as indicated by several mutations in the amyloid precursor protein or the γ-secretase components presenilin-1 and presenilin-2 that cause familial early-onset AD. However, recent data also indicate a decreased clearance rate of Aβ in AD brains. We recently demonstrated that Aβ undergoes phosphorylation by extracellular or cell surface-localized protein kinase A, leading to increased aggregation. Here, we provide evidence that phosphorylation of monomeric Aβ at Ser-8 also decreases its clearance by microglial cells. By using mass spectrometry, we demonstrate that phosphorylation at Ser-8 inhibited the proteolytic degradation of monomeric Aβ by the insulin-degrading enzyme, a major Aβ-degrading enzyme released from microglial cells. Phosphorylation also decreased the degradation of Aβ by the angiotensin-converting enzyme. In contrast, Aβ degradation by plasmin was largely unaffected by phosphorylation. Thus, phosphorylation of Aβ could play a dual role in Aβ metabolism. It decreases its proteolytic clearance and also promotes its aggregation. The inhibition of extracellular Aβ phosphorylation, stimulation of protease expression and/or their proteolytic activity could be explored to promote Aβ degradation in AD therapy or prevention.

Alzheimer's therapeutics: translation of preclinical science to clinical drug development

Over the past three decades, significant progress has been made in understanding the neurobiology of Alzheimer's disease. In recent years, the first attempts to implement novel mechanism-based treatments brought rather disappointing results, with low, if any, drug efficacy and significant side effects. A discrepancy between our expectations based on preclinical models and the results of clinical trials calls for a revision of our theoretical views and questions every stage of translation-from how we model the disease to how we run clinical trials. In the following sections, we will use some specific examples of the therapeutics from acetylcholinesterase inhibitors to recent anti-Aβ immunization and γ-secretase inhibition to discuss whether preclinical studies could predict the limitations in efficacy and side effects that we were so disappointed to observe in recent clinical trials. We discuss ways to improve both the predictive validity of mouse models and the translation of knowledge between preclinical and clinical stages of drug development.