Measurement of cellular beta-site of APP cleaving enzyme 1 activity and its modulation in neuronal assay systems

A Novel Cell-based β-secretase Enzymatic Assay for Alzheimer's Disease

BACKGROUND

Deposition of the amyloid β protein (Aβ) into neuritic plaques is the neuropathological hallmark of Alzheimer's Disease (AD). Aβ is generated through the cleavage of the Amyloid Precursor Protein (APP) by β-secretase and γ-secretase. Currently, the evaluation of APP cleavage by β-secretase in experimental settings has largely depended on models that do not replicate the physiological conditions of this process.

OBJECTIVE

To establish a novel live cell-based β-secretase enzymatic assay utilizing a novel chimeric protein that incorporates the natural sequence of APP and more closely replicates its cleavage by β-secretase under physiological conditions.

METHODS

We have developed a chimeric protein construct, ASGβ, incorporating the β-site cleavage sequence of APP targeted by β-secretase and its intracellular trafficking signal into a Phosphatase-eGFP secreted reporter system. Upon cleavage by β-secretase, ASGβ releases a phosphatase-containing portion that can be measured in the culture medium, and an intracellular fraction that can be detected through Western Blot. Subsequently, we have generated a cell line stably expressing ASGβ that can be utilized to assay β-secretase in real time.

RESULTS

ASGβ is specifically targeted by β-secretase, being cleaved exclusively at the site responsible for the generation of Aβ. Dosage response to β-secretase inhibitors shows that β-secretase activity can be positively correlated to phosphatase activity in culture media.

CONCLUSION

Our findings suggest this system could be a high-throughput tool to screen compounds that aim to modulate β-secretase activity and Aβ production under physiological conditions, as well as evaluating factors that regulate this cleavage.

Highly specific and selective anti-pS396-tau antibody C10.2 targets seeding-competent tau

Introduction

The abnormal hyperphosphorylation of the microtubule-associated protein tau plays a crucial role in neurodegeneration in Alzheimer's disease (AD) and other tauopathies.

Methods

Highly specific and selective anti-pS396-tau antibodies have been generated using peptide immunization with screening against pathologic hyperphosphorylated tau from rTg4510 mouse and AD brains and selection in in vitro and in vivo tau seeding assays.

Results

The antibody C10.2 bound specifically to pS396-tau with an IC₅₀ of 104 pM and detected preferentially hyperphosphorylated tau aggregates from AD brain with an IC₅₀ of 1.2 nM. C10.2 significantly reduced tau seeding of P301L human tau in HEK293 cells, murine cortical neurons, and mice. AD brain extracts depleted with C10.2 were not able to seed tau in vitro and in vivo, demonstrating that C10.2 specifically recognized pathologic seeding-competent tau.

Discussion

Targeting pS396-tau with an antibody like C10.2 may provide therapeutic benefit in AD and other tauopathies.

Reduced Aβ secretion by human neurons under conditions of strongly increased BACE activity

The initial step in the amyloidogenic cascade of amyloid precursor protein (APP) processing is catalyzed by beta-site APP-cleaving enzyme (BACE), and this protease has increased activities in affected areas of Alzheimer's disease brains. We hypothesized that altered APP processing, because of augmented BACE activity, would affect the actions of direct and indirect BACE inhibitors. We therefore compared post-mitotic human neurons (LUHMES) with their BACE-overexpressing counterparts (BLUHMES). Although β-cleavage of APP was strongly increased in BLUHMES, they produced less full-length and truncated amyloid beta (Aβ) than LUHMES. Moreover, low concentrations of BACE inhibitors decreased cellular BACE activity as expected, but increased Aβ_1-40 levels. Several other approaches to modulate BACE activity led to a similar, apparently paradoxical, behavior. For instance, reduction in intracellular acidification by bepridil increased Aβ production in parallel with decreased BACE activity. In contrast to BLUHMES, the respective control cells (LUHMES or BLUHMES with catalytically inactive BACE) showed conventional pharmacological responses. Other non-canonical neurochemical responses (so-called 'rebound effects') are well-documented for the Aβ pathway, especially for γ-secretase: a partial block of its activity leads to an increased Aβ secretion by some cell types. We therefore compared LUHMES and BLUHMES regarding rebound effects of γ-secretase inhibitors and found an Aβ rise in LUHMES but not in BLUHMES. Thus, different cellular factors are responsible for the γ-secretase- versus BACE-related Aβ rebound. We conclude that increased BACE activity, possibly accompanied by an altered cellular localization pattern, can dramatically influence Aβ generation in human neurons and affect pharmacological responses to secretase inhibitors. OPEN PRACTICES: Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/.

Control of Aβ release from human neurons by differentiation status and RET signaling

Few studies have compared the processing of endogenous human amyloid precursor protein (APP) in younger and older neurons. Here, we characterized LUHMES cells as a human model to study Alzheimer's disease-related processes during neuronal maturation and aging. Differentiated LUHMES expressed and spontaneously processed APP via the secretase pathways, and they secreted amyloid β (Aβ) peptide. This was inhibited by cholesterol depletion or secretase inhibition, but not by block of tau phosphorylation. In vitro aged cells increased Aβ secretion without upregulation of APP or secretases. We identified the medium constituent glial cell line-derived neurotrophic factor (GDNF) as responsible for this effect. GDNF-triggered Aβ release was associated with rapid upregulation of the GDNF coreceptor "rearranged during transfection" (RET). Other direct (neurturin) or indirect (nerve growth factor) RET activators also increased Aβ, whereas different neurotrophins were ineffective. Downstream of RET, we found activation of protein kinase B (AKT) to be involved. Accordingly, inhibitors of the AKT regulator phosphatidylinositol-3-kinase completely blocked GDNF-triggered AKT phosphorylation and Aβ increase. This suggests that RET signaling affects Aβ release from aging neurons.

GFAP-independent inflammatory competence and trophic functions of astrocytes generated from murine embryonic stem cells

The directed generation of pure astrocyte cultures from pluripotent stem cells has proven difficult. Generation of defined pluripotent-stem-cell derived astrocytes would allow new approaches to the investigation of plasticity and heterogeneity of astrocytes. We here describe a two-step differentiation scheme resulting in the generation of murine embryonic stem cell (mESC) derived astrocytes (MEDA), as characterized by the upregulation of 19 astrocyte-associated mRNAs, and positive staining of most cells for GFAP (glial fibrillary acidic protein), aquaporin-4 or glutamine synthetase. The MEDA cultures could be cryopreserved, and they neither contained neuronal, nor microglial cells. They also did not react to the microglial stimulus lipopolysaccharide, while inflammatory activation by a complete cytokine mix (CCM) or its individual components (TNF-α, IL1-β, IFN-γ) was readily observed. MEDA, stimulated by CCM, became susceptible to CD95 ligand-induced apoptosis and produced NO and IL-6. This was preceded by NF-kB activation, and up-regulation of relevant mRNAs. Also GFAP-negative astrocytes were fully inflammation-competent. Neurotrophic support by MEDA was found to be independent of GFAP expression. In summary, we described here the generation and functional characterization of microglia-free murine astrocytes, displaying phenotypic heterogeneity as is commonly observed in brain astrocytes.