Age-related modulation of γ-secretase activity in non-human primate brains

The Binding of Different Substrate Molecules at the Docking Site and the Active Site of γ-Secretase Can Trigger Toxic Events in Sporadic and Familial Alzheimer's Disease

Pathogenic changes in γ-secretase activity, along with its response to different drugs, can be affected by changes in the saturation of γ-secretase with its substrate. We analyze the saturation of γ-secretase with its substrate using multiscale molecular dynamics studies. We found that an increase in the saturation of γ-secretase with its substrate could result in the parallel binding of different substrate molecules at the docking site and the active site. The C-terminal domain of the substrate bound at the docking site can interact with the most dynamic presenilin sites at the cytosolic end of the active site tunnel. Such interactions can inhibit the ongoing catalytic activity and increase the production of the longer, more hydrophobic, and more toxic Aβ proteins. Similar disruptions in dynamic presenilin structures can be observed with different drugs and disease-causing mutations. Both, C99-βCTF-APP substrate and its different Aβ products, can support the toxic aggregation. The aggregation depends on the substrate N-terminal domain. Thus, the C99-βCTF-APP substrate and β-secretase path can be more toxic than the C83-αCTF-APP substrate and α-secretase path. Nicastrin can control the toxic aggregation in the closed conformation. The binding of the C99-βCTF-APP substrate to γ-secretase can be controlled by substrate channeling between the nicastrin and β-secretase. We conclude that the presented two-substrate mechanism could explain the pathogenic changes in γ-secretase activity and Aβ metabolism in different sporadic and familial cases of Alzheimer's disease. Future drug-development efforts should target different cellular mechanisms that regulate the optimal balance between γ-secretase activity and amyloid metabolism.

Identification of the Aβ37/42 peptide ratio in CSF as an improved Aβ biomarker for Alzheimer's disease

INTRODUCTION

Identifying CSF-based biomarkers for the β-amyloidosis that initiates Alzheimer's disease (AD) could provide inexpensive and dynamic tests to distinguish AD from normal aging and predict future cognitive decline.

METHODS

We developed immunoassays specifically detecting all C-terminal variants of secreted amyloid β-protein and identified a novel biomarker, the Aβ 37/42 ratio, that outperforms the canonical Aβ42/40 ratio as a means to evaluate the γ-secretase activity and brain Aβ accumulation.

RESULTS

We show that Aβ 37/42 can distinguish physiological and pathological status in (1) presenilin-1 mutant vs wild-type cultured cells, (2) AD vs control brain tissue, and (3) AD versus cognitively normal (CN) subjects in CSF, where 37/42 (AUC 0.9622) outperformed 42/40 (AUC 0.8651) in distinguishing CN from AD.

DISCUSSION

We conclude that the Aβ 37/42 ratio sensitively detects presenilin/γ-secretase dysfunction and better distinguishes CN from AD than Aβ42/40 in CSF. Measuring this novel ratio alongside promising phospho-tau analytes may provide highly discriminatory fluid biomarkers for AD.

γ-Secretase Activity Is Associated with Braak Senile Plaque Stages

Amyloid β-proteins (Aβs) Aβ1-42 and Aβ1-43 are converted via two product lines of γ-secretase to Aβ1-38 and Aβ1-40. This parallel stepwise processing model of γ-secretase predicts that Aβ1-42 and Aβ1-43, and Aβ1-38 and Aβ1-40 are proportional to each other, respectively. To obtain further insight into the mechanisms of parenchymal Aβ deposition, these four Aβ species were quantified in insoluble fractions of human brains (Brodmann areas 9 to 11) at various Braak senile plaque (SP) stages, using specific enzyme-linked immunosorbent assays. With advancing SP stages, the amounts of deposited Aβ1-43 in the brain increased proportionally to those of Aβ1-42. Similarly, the amounts of deposited Aβ1-38 correlated with those of Aβ1-40. Surprisingly, the ratios of deposited Aβ1-38/Aβ1-42 and Aβ1-40/Aβ1-43 were proportional and discriminated the Braak SP stages accurately. This result indicates that the generation of Aβ1-38 and Aβ1-40 decreased and the generation of Aβ1-42 and Aβ1-43 increased with advancing SP stages. Thus, Aβs deposition might depend on γ-secretase activity, as it does in the cerebrospinal fluid. Here, the extracted γ-secretase from Alzheimer disease brains generates an amount of Aβ1-42 and Aβ1-43 compared with cognitively normal brains. This refractory γ-secretase localized in detergent-solubilized fractions from brain cortices. But activity modulated γ-secretase, which decreases Aβ1-42 and Aβ1-43 in the cerebrospinal fluid, localized in detergent-insoluble fractions. These drastic alterations reflect Aβ situation in Alzheimer disease brains.

Lipid class composition of membrane and raft fractions from brains of individuals with Alzheimer's disease

Perturbation of the homeostasis of brain membrane lipids has been implicated in the pathomechanism of Alzheimer's disease (AD). The ε4 allele of the apolipoprotein E gene (APOE) confers an increased risk, in a dosage-dependent manner, for brain amyloid-β accumulation and the development of sporadic AD. An effect of the APOE genotype on brain lipid homeostasis may underlie the AD risk associated with the ε4 allele. In this research, we examined an effect of APOE ε4 on the lipid class composition of crude membranes and raft-enriched fractions of brains. We applied enzymatic reaction-based methods for the quantification of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidic acid, and sphingomyelin. Our results indicate that brain lipid class composition was neither significantly altered in AD subjects nor affected by the presence of the APOE ε4 allele.

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No change was found in the composition of lipid classes of brains with Alzheimer's disease.

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The APOE ε4 allele did not affect lipid class composition of the brain membrane or rafts.

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The enzymatic measurement of phospholipids is applicable to brain tissues.

Pro- and Anti-inflammatory Effects of High Cholesterol Diet on Aged Brain

Both hypercholesterolemia and aging are related to cognitive decline or Alzheimer's disease. However, their interactive influence on the neurodegenerative progress remains unclear. To address this issue, 6-month-old and 16-month-old female mice were fed a 3% cholesterol diet for 8 weeks, followed by hippocampus-related functional, pathological, biochemical and molecular analyses. The high cholesterol diet did not exacerbate age-dependent cognitive decline and hippocampal neuronal death, and even greatly mitigated decreases of synaptophysin and growth associated protein 43 expression in the hippocampus of aged mice. Compared with young controls, aged mice fed normal diet showed mild activation of hippocampal microglia with increased expression of CD68, a marker of the microglial M1 phenotype, and decreased expression of CD206, a marker of the microglial M2 phenotype. More interestingly, the high cholesterol diet not only improved NLRP3 inflammasome activation and IL-1β expression, but also increased levels of anti-inflammatory cytokines IL-4 and IL-6 in the hippocampus of old mice, suggesting playing pro- and anti-neuroinflammatory effects. In addition, the cholesterol rich diet resulted in a defect of the blood-brain barrier of aged hippocampus, as revealed by increased brain albumin content. These results have revealed both harmful and protective effects of high cholesterol diet on aged brain, which helps us to understand that hypercholesterolemia in the aged population is not associated with dementia and cognitive impairment.

Pre-synaptic localization of the γ-secretase-inhibiting protein p24α2 in the mammalian brain

Dysregulated metabolism and consequent extracellular accumulation of amyloid-β (Aβ) peptides in the brain underlie the pathogenesis of Alzheimer's disease. Extracellular Aβ in the brain parenchyma is mainly secreted from the pre-synaptic terminals of neuronal cells in a synaptic activity-dependent manner. The p24 family member p24α2 reportedly attenuates Aβ generation by inhibiting γ-secretase processing of amyloid precursor protein; however, the pattern of expression and localization of p24α2 in the brain remains unknown. We performed immunohistochemical staining and subcellular fractionation for p24α2 in the mouse brain. Immunostaining showed that p24α2 is broadly distributed in the gray matter of the central nervous system and is predominantly localized to synapses. Subcellular fractionation revealed prominent localization of p24α2 in the pre-synaptic terminals. Immunoisolation of synaptic vesicles (SV) indicated that p24α2 is condensed at active zone-docked SV. During development, p24α2 expression is highest in the post-natal period and gradually decreases with age. We also confirmed that amyloid precursor protein and γ-secretase components are localized at active zone-docked SV. Our results suggest a novel functional role for p24α2 in the regulation of synaptic transmission and synaptogenesis, and provide evidence for the participation of p24α2 in the regulation of Aβ generation and secretion in the brain. The p24 family member p24α2 attenuates amyloid-β (Aβ) generation by inhibiting the γ-secretase processing. We report that p24α2 is condensed at active zone-docked synaptic vesicles in the brain. p24α2 expression is highest in the post-natal period and gradually decreases with age. Our results suggest a novel function for p24α2 at the synapse, including the regulation of brain Aβ generation.

Amyloid Beta1-42 and the Phoshorylated Tau Threonine 231 in Brains of Aged Cynomolgus Monkeys (Macaca fascicularis)

Pathological hallmarks indicative of Alzheimer’s disease (AD), which are the plaques of amyloid beta_1–42 and neurofibrillary tangles, were found in brain of aged cynomolgus monkey. The aim of this study was to investigate if aged monkeys exhibiting spatial memory impairment and levels of biomarkers indicative of AD, had brain lesions similar to human patients suffering from senile dementia. Generating immunohistochemistry technique to biomarkers of amyloid beta_1–42 and the phosphorylated tau 231, our study assessed the amyloidopathy, such as indicative to the senile plaques and cerebral amyloid angiopathy, and the tauopathy, to possible neurofibrillary tangles. Six aged monkeys were selected based on their spatial memory performance and profile of biomarkers of AD, divided equally to affected aged subject – with Memory-affected and low amyloid level, and aged with higher performance in memory and amyloid, as the age-matched subjects. Using immunohistochemistry, plaques of amyloid beta_1–42 were observed in two out of three brains of aged subjects with memory impairment and biomarkers indicative of AD. The cerebral amyloid angiopathy was observed in both aged monkey groups, and unlike in the human, the amyloids were found to deposit in the small veins and capillaries. In one of the affected individuals, phosphorylated tau was positively stained intracellularly of the neurons, indicating a possibility of an early stage of the formation of tangles. These findings add to the body of evidence of the utility of the aged cynomolgus monkeys as a spontaneous model for Alzheimer-related disease.