Quantitative Measurement of γ-Secretase-mediated Amyloid Precursor Protein and Notch Cleavage in Cell-based Luciferase Reporter Assay Platforms
Summary
We have successfully generated two substrate-specific γ-secretase assays. Both cell-based assays presented here are designed to quantify γ-secretase enzymatic activities via the output of firefly luciferase reporters.
Abstract
We have developed a pair of cell-based reporter gene assays to quantitatively measure γ-secretase cleavage of distinct substrates. This manuscript describes procedures that may be used to monitor γ-secretase-mediated cleavage of either APP-C99 or Notch, using a Gal4 promoter-driven firefly luciferase reporter system. These assays were established by stably co-transfecting HEK293 cells with the Gal4-driven luciferase reporter gene and either the Gal4/VP16-tagged C-terminal fragment of APP (APP-C99; CG cells), or the Gal4/VP16-tagged Notch-ΔE (NΔE; NG cells). Using these reporter assays in parallel, we have demonstrated that an ErbB2 inhibitor, CL-387,785, can preferentially suppress γ-secretase cleavage of APP-C99 in CG cells, but not NΔE in NG cells. The differential responses exhibited by the CG and NG cells, when treated with CL-387,785, represent a preferred characteristic for γ-secretase modulators, and these responses are in stark contrast to the pan-inhibition of γ-secretase induced by DAPT. Our studies provide direct evidence that γ-secretase activities toward different substrates can be differentiated in a cellular context. These new assays may therefore be useful tools in drug discovery for improved AD therapies.
Introduction
Oligomeric forms of amyloid-β (Aβ) are believed to be the primary cause of neurodegeneration in the brains of patients suffering from Alzheimer's disease (AD)1. Aβ peptides are produced by the stepwise cleavage of amyloid precursor protein (APP), first by β-secretase and then by γ-secretase2. In the past decade, therapeutic approaches toward treatment of AD have focused on the prevention of Aβ production3. The majority of studies have focused on either the augmentation of α-secretase activity, which can preclude γ-secretase cleavage of APP and thereby decrease the production of Aβ, or the inhibition of β-and/or γ-secretase activities4. Unfortunately, non-selective inhibition of β- or γ-secretases results in unavoidable side effects that are due to interference with the functioning of other physiological substrates of β- and γ-secretase5,6. With regard to γ-secretase inhibitors, recent studies have reported the discovery of a number of chemical and genetic modulators that can regulate Aβ production while exerting negligible effects on the essential γ-secretase-mediated processing of Notch7,8,9,10,11,12, however, successful therapeutics have not yet been developed. Thus, further systematic screens are warranted to discover novel genetic and chemical modifiers that may selectively modulate γ-secretase-mediated APP processing.
γ-Secretase is known to cleave more than 90 different membrane-anchored proteins. Among these substrates is Notch, whose activity is critical for cell fate determination and differentiation during development13. Selectively modulating γ-secretase-catalyzed APP processing in the absence of affecting Notch processing will be essential for minimizing Notch-related side effects of γ-secretase inhibitors, and this selectivity is thought to be a primary factor that will dictate the biological efficacy of potential γ-secretase modulators for the chronic treatment of AD. There have been a number of arylsulfonamide derivatives, such as GSI-953 (begacestat) and BMS-708163 (avagacestat), that have been found to exhibit potent and selective inhibition of γ-secretase14,15. A previous study has demonstrated that the differential inhibition of γ-secretase cleavage of APP and Notch can be observed using a quantitative ELISA-based assay in combination with in vivo validation using zebrafish16. Furthermore, cell-based assay paradigms have been established to address the potential substrate selectivity of γ-secretase toward APP and Notch17,18. Using protocols similar to those described herein, we have recently discovered a novel class of (D)-leucinamides that potently modulate γ-secretase cleavage of APP with Notch-sparing selectivity19. Together, this collection of studies provides a proof-of-concept supporting the notion that the substrate selectivity/availability of γ-secretase can be subject to chemical modulation and experimental verification. However, these assay platforms often rely on relatively low-throughput readouts and labor-intensive approaches that might not meet industrial standards for drug discovery programs.
We have recently generated cell-based luciferase reporter gene assays that can quantitatively determine the catalytic activity of γ-secretase toward two distinct substrates, the 99-amino acid C-terminal fragment of APP (APP-C99) and the extracellular domain-deleted Notch peptide (NΔE). Both APP-C99 and NΔE have been widely used as direct substrates of γ-secretase in various assays. To generate homogeneous and consistent luciferase reporter gene assays for the γ-secretase cleavage of either APP-C99 or NΔE, we generated one HEK-derived stable cell line (CG) that constitutively expresses a Gal4 promoter-driven firefly luciferase reporter (Gal4-Luc) and Gal4/VP16-tagged APP-C99 fusion protein (C99-GV). In addition, we generated another HEK-derived stable cell line (NG) that constitutively expresses Gal4-Luc and Gal4/VP16-tagged NΔE (NΔE-GV). Using these novel substrate-specific γ-secretase assays, we now provide a method to quantify and distinguish the catalytic activity of γ-secretase toward distinct substrates (APP-C99 in CG cells, or NΔE in NG cells). Moreover, the substrate-specific γ-secretase assays were designed to be conducive to high-content screening platforms. These newly generated γ-secretase assays could pave the way for the discovery of novel γ-secretase modulators that could improve cognitive function by suppressing Aβ production without eliciting undesirable Notch inhibition for the next-generation treatment of AD.
Protocol
Representative Results
Discussion
The promising outcomes from a phase 1b trial of aducanumab immunotherapy for AD have firmly established the critical role of Aβ in the pathogenesis of AD22 and suggest that anti-Aβ approaches are still a viable strategy for the development of anti-AD drugs. Here, we describe cell-based assays for quantifying γ-secretase-catalyzed proteolysis of APP-C99 and NΔE using firefly luciferase reporter systems. APP-C99 and NΔE are the two most well-studied γ-secretase substrat…
Disclosures
The authors have nothing to disclose.
Acknowledgements
The authors thank the Core Facility of the Institute of Cellular and Organismic Biology, Academia Sinica, for technical support. This study was supported by the Ministry of Science and Technology, Taiwan (MOST 103-2320-B-001-016-MY3 to Y.-F. L.), the Program for Translational Innovation of Biopharmaceutical Development – Technology Supporting Platform Axis Scheme (NP7 to Y.-F.L.), and Academia Sinica (to Y.-F.L.).
Materials
| VictorLight luminescence plate reader | PerkinElmer | 2030-0010 | |
| Class II, Type 2 Biological Safety Cabinet | Thermo Scientific | 1300 Series A2 | |
| CL-387,785 | EMD Calbiochem | 233100-1MG | |
| DAPT | Merck | 565770 | |
| Dulbecco’s Modified Eagle Medium (DMEM) | Thermo Fisher | 12100046 | main compnent of growth medium |
| Fetal bovine serum (FBS) | Thermo Fisher | 16000044 | |
| Blasticidin | Thermo Fisher | A1113903 | |
| Zeocin | Thermo Fisher | R25005 | |
| Hygromycin B | Gibco | 10687010 | |
| Hemocytomer | Sigma-Aldrich | BR717805 Aldrich | |
| 96-well microplate | Nunc | 156545 | |
| Tetracycline | Sigma | T7660 | |
| Dimethyl sulfoxide (DMSO) | Sigma | D2650 | |
| Steady-Glo luciferase assay reagent | Promega | E2510 | |
| Humidified CO2 incubator | Revco | Ultima II | |
| T-REx293 cell line | Invitrogen | R71007 | |
| Wallac 1420 software version 3.0 | PerkinElmer | instrument control software of the luminescence microplate reader |
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