Identification of PS1/gamma-secretase and glutamate transporter GLT-1 interaction sites.

The recently discovered interaction between Presenilin 1 (PS1), a catalytic subunit of γ-secretase responsible for generating amyloid-ß peptides, and GLT-1, a major glutamate transporter in the brain (EAAT2), provides a mechanistic link between these two key factors involved in Alzheimer's disease (AD) pathology. Modulating this interaction can be crucial to understand the consequence of such crosstalk in AD context and beyond. However, the interaction sites between these two proteins are unknown. Herein, we utilized an alanine scanning approach coupled with FRET-based fluorescence lifetime imaging microscopy to identify the interaction sites between PS1 and GLT-1 in their native environment within intact cells. We found that GLT-1 residues at position 276 to 279 (TM5) and PS1 residues at position 249 to 252 (TM6) are crucial for GLT-1-PS1 interaction. These results have been cross validated using AlphaFold Multimer prediction. To further investigate whether this interaction of endogenously expressed GLT-1 and PS1 can be prevented in primary neurons, we designed PS1/GLT-1 cell-permeable peptides (CPPs) targeting the PS1 or GLT-1 binding site. We used HIV TAT domain to allow for cell penetration which was assayed in neurons. First, we assessed the toxicity and penetration of CPPs by confocal microscopy. Next, to ensure the efficiency of CPPs, we monitored the modulation of GLT-1-PS1 interaction in intact neurons by fluorescence lifetime imaging microscopy. We saw significantly less interaction between PS1 and GLT-1 with both CPPs. Our study establishes a new tool to study the functional aspect of GLT-1-PS1 interaction and its relevance in normal physiology and AD models.

PS1/gamma-secretase acts as rogue chaperone of glutamate transporter EAAT2/GLT-1 in Alzheimer's disease.

The recently discovered interaction between presenilin 1 (PS1), a catalytic subunit of γ-secretase responsible for the generation of amyloid-ß(Aß) peptides, and GLT-1, the major glutamate transporter in the brain (EAAT2 in the human) may provide a mechanistic link between two important pathological aspects of Alzheimer's disease (AD): abnormal Aßoccurrence and neuronal network hyperactivity. In the current study, we employed a FRET-based approach, fluorescence lifetime imaging microscopy (FLIM), to characterize the PS1/GLT-1 interaction in its native environment in the brain tissue of sporadic AD (sAD) patients. There was significantly less interaction between PS1 and GLT-1 in sAD brains, compared to tissue from patients with frontotemporal lobar degeneration (FTLD), or non-demented age-matched controls. Since PS1 has been shown to adopt pathogenic "closed" conformation in sAD but not in FTLD, we assessed the impact of changes in PS1 conformation on the interaction. Familial AD (fAD) PS1 mutations which induce a "closed" PS1 conformation similar to that in sAD brain and gamma-secretase modulators (GSMs) which induce a "relaxed" conformation, reduced and increased the interaction, respectively. This indicates that PS1 conformation seems to have a direct effect on the interaction with GLT-1. Furthermore, using biotinylation/streptavidin pull-down, western blotting, and cycloheximide chase assays, we determined that the presence of PS1 increased GLT-1 cell surface expression and GLT-1 homomultimer formation, but did not impact GLT-1 protein stability. Together, the current findings suggest that the newly described PS1/GLT-1 interaction endows PS1 with chaperone activity, modulating GLT-1 transport to the cell surface and stabilizing the dimeric-trimeric states of the protein. The diminished PS1/GLT-1 interaction suggests that these functions of the interaction may not work properly in AD.

Identification of PS1/gamma-secretase and glutamate transporter GLT-1 interaction sites.

The recently discovered interaction between Presenilin 1 (PS1), a catalytic subunit of γ-secretase responsible for generating amyloid-ß (Aß) peptides, and GLT-1, a major glutamate transporter in the brain (EAAT2) provides a mechanistic link between these two key factors involved in Alzheimer's disease (AD) pathology. Modulating this interaction can be crucial to understand the consequence of such crosstalk in AD context and beyond. However, the interaction sites between these two proteins are unknown. Herein, we utilized an alanine scanning approach coupled with FRET-based fluorescence lifetime imaging microscopy (FLIM) to identify the interaction sites between PS1 and GLT-1 in their native environment within intact cells. We found that GLT-1 residues at position 276 to 279 (TM5) and PS1 residues at position 249 to 252 (TM6) are crucial for GLT-1/PS1 interaction. These results have been cross validated using AlphaFold Multimer prediction. To further investigate whether this interaction of endogenously expressed GLT-1 and PS1 can be prevented in primary neurons, we designed PS1/GLT-1 cell-permeable peptides (CPPs) targeting the PS1 or GLT-1 binding site. We used HIV TAT domain to allow for cell penetration which was assayed in neurons. First, we assessed the toxicity and penetration of CPPs by confocal microscopy. Next, to ensure the efficiency of CPPs, we monitored the modulation of GLT-1/PS1 interaction in intact neurons by FLIM. We saw significantly less interaction between PS1 and GLT-1 with both CPPs. Our study establishes a new tool to study the functional aspect of GLT-1/PS1 interaction and its relevance in normal physiology and AD models.

Structural Determinant of ß-Amyloid Formation: From Transmembrane Protein Dimerization to ß-Amyloid Aggregates.

Most neurodegenerative diseases have the characteristics of protein folding disorders, i.e., they cause lesions to appear in vulnerable regions of the nervous system, corresponding to protein aggregates that progressively spread through the neuronal network as the symptoms progress. Alzheimer's disease is one of these diseases. It is characterized by two types of lesions: neurofibrillary tangles (NFTs) composed of tau proteins and senile plaques, formed essentially of amyloid peptides (Aß). A combination of factors ranging from genetic mutations to age-related changes in the cellular context converge in this disease to accelerate Aß deposition. Over the last two decades, numerous studies have attempted to elucidate how structural determinants of its precursor (APP) modify Aß production, and to understand the processes leading to the formation of different Aß aggregates, e.g., fibrils and oligomers. The synthesis proposed in this review indicates that the same motifs can control APP function and Aß production essentially by regulating membrane protein dimerization, and subsequently Aß aggregation processes. The distinct properties of these motifs and the cellular context regulate the APP conformation to trigger the transition to the amyloid pathology. This concept is critical to better decipher the patterns switching APP protein conformation from physiological to pathological and improve our understanding of the mechanisms underpinning the formation of amyloid fibrils that devastate neuronal functions.

CRISPR-Cas9 treatment partially restores amyloid-ß 42/40 in human fibroblasts with the Alzheimer's disease PSEN 1 M146L mutation.

Presenilin 1 (PS1) is a central component of γ-secretase, an enzymatic complex involved in the generation of the amyloid-ß (Aß) peptide that deposits as plaques in the Alzheimer's disease (AD) brain. The M146L mutation in the PS1 gene (PSEN1) leads to an autosomal dominant form of early-onset AD by promoting a relative increase in the generation of the more aggregation-prone Aß42. This change is evident not only in the brain but also in peripheral cells of mutation carriers. In this study we used the CRISPR-Cas9 system from Streptococcus pyogenes to selectively disrupt the PSEN1 M146L allele in human fibroblasts. A disruption of more than 50% of mutant alleles was observed in all CRISPR-Cas9-treated samples, resulting in reduced extracellular Aß42/40 ratios. Fluorescence resonance energy transfer-based conformation and western blot analyses indicated that CRISPR-Cas9 treatment also affects the overall PS1 conformation and reduces PS1 levels. Moreover, our guide RNA did not lead to any detectable editing at the highest-ranking candidate off-target sites identified by ONE-seq and CIRCLE-seq. Overall, our data support the effectiveness of CRISPR-Cas9 in selectively targeting the PSEN1 M146L allele and counteracting the AD-associated phenotype. We believe that this system could be developed into a therapeutic strategy for patients with this and other dominant mutations leading to early-onset AD.

Towards a dynamic compression facility at the ESRF.

Results of the 2018 commissioning and experimental campaigns of the new High Power Laser Facility on the Energy-dispersive X-ray Absorption Spectroscopy (ED-XAS) beamline ID24 at the ESRF are presented. The front-end of the future laser, delivering 15 J in 10 ns, was interfaced to the beamline. Laser-driven dynamic compression experiments were performed on iron oxides, iron alloys and bismuth probed by online time-resolved XAS.

Mechanism of Cellular Formation and In Vivo Seeding Effects of Hexameric ß-Amyloid Assemblies.

The ß-amyloid peptide (Aß) is found as amyloid fibrils in senile plaques, a typical hallmark of Alzheimer's disease (AD). However, intermediate soluble oligomers of Aß are now recognized as initiators of the pathogenic cascade leading to AD. Studies using recombinant Aß have shown that hexameric Aß in particular acts as a critical nucleus for Aß self-assembly. We recently isolated hexameric Aß assemblies from a cellular model, and demonstrated their ability to enhance Aß aggregation in vitro. Here, we report the presence of similar hexameric-like Aß assemblies across several cellular models, including neuronal-like cell lines. In order to better understand how they are produced in a cellular context, we investigated the role of presenilin-1 (PS1) and presenilin-2 (PS2) in their formation. PS1 and PS2 are the catalytic subunits of the γ-secretase complex that generates Aß. Using CRISPR-Cas9 to knockdown each of the two presenilins in neuronal-like cell lines, we observed a direct link between the PS2-dependent processing pathway and the release of hexameric-like Aß assemblies in extracellular vesicles. Further, we assessed the contribution of hexameric Aß to the development of amyloid pathology. We report the early presence of hexameric-like Aß assemblies in both transgenic mice brains exhibiting human Aß pathology and in the cerebrospinal fluid of AD patients, suggesting hexameric Aß as a potential early AD biomarker. Finally, cell-derived hexameric Aß was found to seed other human Aß forms, resulting in the aggravation of amyloid deposition in vivo and neuronal toxicity in vitro.

Limited Substrate Specificity of PS/γ-Secretase Is Supported by Novel Multiplexed FRET Analysis in Live Cells.

Presenilin (PS)/γ-secretase is an aspartyl protease that processes a wide range of transmembrane proteins such as the amyloid precursor protein (APP) and Notch1, playing essential roles in normal biological events and diseases. However, whether there is a substrate preference for PS/γ-secretase processing in cells is not fully understood. Structural studies of PS/γ-secretase enfolding a fragment of APP or Notch1 showed that the two substrates engage the protease in broadly similar ways, suggesting the limited substrate specificity of PS/γ-secretase. In the present study, we developed a new multiplexed imaging platform that, for the first time, allowed us to quantitatively monitor how PS/γ-secretase processes two different substrates (e.g., APP vs. Notch1) in the same cell. In this assay, we utilized the recently reported, spectrally compatible visible and near-infrared (NIR)-range Förster resonance energy transfer (FRET) biosensors that permit quantitative recording of PS/γ-secretase activity in live cells. Here, we show that, overall, PS/γ-secretase similarly cleaves Notch1 N100, wild-type APP C99, and familial Alzheimer's disease (FAD)-linked APP C99 mutants in Chinese hamster ovary (CHO) cells, which further supports the limited PS/γ-secretase substrate specificity. On the other hand, a cell-by-cell basis analysis demonstrates a certain degree of variability in substrate recognition and processing by PS/γ-secretase among different cells. Our new multiplexed FRET assay could be a useful tool to better understand how PS/γ-secretase processes its multiple substrates in normal and disease conditions in live, intact cells.

Erratum: Dimeric Transmembrane Orientations of APP/C99 Regulate γ-Secretase Processing Line Impacting Signaling and Oligomerization.

[This corrects the article DOI: 10.1016/j.isci.2020.101887.].

Dimeric Transmembrane Orientations of APP/C99 Regulate γ-Secretase Processing Line Impacting Signaling and Oligomerization.

Amyloid precursor protein (APP) cleavage by the ß-secretase produces the C99 transmembrane (TM) protein, which contains three dimerization-inducing Gly-x-x-x-Gly motifs. We demonstrate that dimeric C99 TM orientations regulate the precise cleavage lines by γ-secretase. Of all possible dimeric orientations imposed by a coiled-coil to the C99 TM domain, the dimer containing the 33Gly-x-x-x-Gly37 motif in the interface promoted the Aß42 processing line and APP intracellular domain-dependent gene transcription, including the induction of BACE1 mRNA, enhancing amyloidogenic processing and signaling. Another orientation exhibiting the 25Gly-x-x-x-Gly29 motif in the interface favored processing to Aß43/40. It induced significantly less gene transcription, while promoting formation of SDS-resistant "Aß-like" oligomers, reminiscent of Aß peptide oligomers. These required both Val24 of a pro-ß motif and the 25Gly-x-x-x-Gly29 interface. Thus, crossing angles imposed by precise dimeric orientations control γ-secretase initial cleavage at Aß48 or Aß49, linking the former to enhanced signaling and Aß42 production.

A Novel NIR-FRET Biosensor for Reporting PS/γ-Secretase Activity in Live Cells.

Presenilin (PS)/γ-secretase plays a pivotal role in essential cellular events via proteolytic processing of transmembrane proteins that include APP and Notch receptors. However, how PS/γ-secretase activity is spatiotemporally regulated by other molecular and cellular factors and how the changes in PS/γ-secretase activity influence signaling pathways in live cells are poorly understood. These questions could be addressed by engineering a new tool that enables multiplexed imaging of PS/γ-secretase activity and additional cellular events in real-time. Here, we report the development of a near-infrared (NIR) FRET-based PS/γ-secretase biosensor, C99 720-670 probe, which incorporates an immediate PS/γ-secretase substrate APP C99 with miRFP670 and miRFP720 as the donor and acceptor fluorescent proteins, respectively. Extensive validation demonstrates that the C99 720-670 biosensor enables quantitative monitoring of endogenous PS/γ-secretase activity on a cell-by-cell basis in live cells (720/670 ratio: 2.47 ± 0.66 (vehicle) vs. 3.02 ± 1.17 (DAPT), ** p < 0.01). Importantly, the C99 720-670 and the previously developed APP C99 YPet-Turquoise-GL (C99 Y-T) biosensors simultaneously report PS/γ-secretase activity. This evidences the compatibility of the C99 720-670 biosensor with cyan (CFP)-yellow fluorescent protein (YFP)-based FRET biosensors for reporting other essential cellular events. Multiplexed imaging using the novel NIR biosensor C99 720-670 would open a new avenue to better understand the regulation and consequences of changes in PS/γ-secretase activity.

Amyloid Precursor Protein (APP) Controls the Expression of the Transcriptional Activator Neuronal PAS Domain Protein 4 (NPAS4) and Synaptic GABA Release.

The amyloid precursor protein (APP) has been extensively studied as the precursor of the ß-amyloid (Aß) peptide, the major component of the senile plaques found in the brain of Alzheimer's disease (AD) patients. However, the function of APP per se in neuronal physiology remains to be fully elucidated. APP is expressed at high levels in the brain. It resembles a cell adhesion molecule or a membrane receptor, suggesting that its function relies on cell-cell interaction and/or activation of intracellular signaling pathways. In this respect, the APP intracellular domain (AICD) was reported to act as a transcriptional regulator. Here, we used a transcriptome-based approach to identify the genes transcriptionally regulated by APP in the rodent embryonic cortex and on maturation of primary cortical neurons. Surprisingly, the overall transcriptional changes were subtle, but a more detailed analysis pointed to genes clustered in neuronal-activity dependent pathways. In particular, we observed a decreased transcription of neuronal PAS domain protein 4 (NPAS4) in APP-/- neurons. NPAS4 is an inducible transcription factor (ITF) regulated by neuronal depolarization. The downregulation of NPAS4 co-occurs with an increased production of the inhibitory neurotransmitter GABA and a reduced expression of the GABAA receptors α1. CRISPR-Cas-mediated silencing of NPAS4 in neurons led to similar observations. Patch-clamp investigation did not reveal any functional decrease of GABAA receptors activity, but long-term potentiation (LTP) measurement supported an increased GABA component in synaptic transmission of APP-/- mice. Together, NPAS4 appears to be a downstream target involved in APP-dependent regulation of inhibitory synaptic transmission.

Divergent Synthesis of Densely Substituted Arenes and Pyridines via Cyclotrimerization Reactions of Alkynyl Triazenes.

Densely substituted fused aromatic triazenes can be prepared by [2 + 2 + 2] cyclotrimerization reactions of 1-alkynyl triazenes. The Cp*Ru-catalyzed cyclization proceeds well with both simple alkynyl triazenes and tethered 1-diynyl triazenes. Attractively, the methodology can be extended to pyridine synthesis by replacing an alkyne with a nitrile. The reaction is regioselective and yields the sterically more hindered product. The triazene group precisely installed on the synthesized aryl and pyridyl ring is a highly versatile moiety, which is effortlessly converted into the most important and frequently used functional aryl substituents, including fluorides. It is also suited for intramolecular transformations to afford a variety of valuable heterocycles. The coordination chemistry of alkynyl triazenes and Cp*RuCl was studied and led to the structural characterization of a Cp*RuCl(η2-alkyne) complex, a Cp*RuCl(η4-cyclobutadiene) complex, and an unusual dinuclear Ru complex with a bridging tetramethylfulvene ligand. Complexes of this type are potentially involved in catalyst deactivation pathways.

Influence of the familial Alzheimer's disease-associated T43I mutation on the transmembrane structure and γ-secretase processing of the C99 peptide.

Extracellular deposition of ß-amyloid (Aß) peptides in the brain is a hallmark of Alzheimer's disease (AD). Upon ß-secretase-mediated cleavage of the ß C-terminal fragment (ß-CTF) from the Aß precursor protein, the γ-secretase complex produces the Aß peptides associated with AD. The familial T43I mutation within the transmembrane domain of the ß-CTF (also referred to as C99) increases the ratio between the Aß42 and Aß40 peptides largely due to a decrease in Aß40 formation. Aß42 is the principal component of amyloid deposits within the brain parenchyma, and an increase in the Aß42/Aß40 ratio is correlated with early-onset AD. Using NMR and FTIR spectroscopy, here we addressed how the T43I substitution influences the structure of C55, the minimal sequence containing the entire extracellular and transmembrane (TM) domains of C99 needed for γ-secretase processing. 13C NMR chemical shifts indicated that the T43I substitution increases helical structure within the TM domain of C55. These structural changes were associated with a shift of the C55 dimer to the monomer and an increase in the tilt of the TM helix relative to the membrane normal in the T43I mutant compared with that of WT C55. The A21G (Flemish) mutation was previously found to increase secreted Aß40 levels; here, we combined this mutation in the extracellular domain of C99 with T43I and observed that the T43I/A21G double mutant decreases Aß40 formation. We discuss how the observed structural changes in the T43I mutant may decrease Aß40 formation and increase the Aß42/Aß40 ratio.

ß-Sheet Structure within the Extracellular Domain of C99 Regulates Amyloidogenic Processing.

Familial mutations in C99 can increase the total level of the soluble Aß peptides produced by proteolysis, as well as the Aß42/Aß40 ratio, both of which are linked to the progression of Alzheimer's disease. We show that the extracellular sequence of C99 forms ß-sheet structure upon interaction with membrane bilayers. Mutations that disrupt this structure result in a significant increase in Aß production and, in specific cases, result in an increase in the amount of Aß42 relative to Aß40. Fourier transform infrared and solid-state NMR spectroscopic studies reveal a central ß-hairpin within the extracellular sequence comprising Y10-E11-V12 and L17-V18-F19 connected by a loop involving H13-H14-Q15. These results suggest how familial mutations in the extracellular sequence influence C99 processing and provide a structural basis for the development of small molecule modulators that would reduce Aß production.

Divergent Asymmetric Synthesis of Polycyclic Compounds via Vinyl Triazenes.

Vinyl triazenes were obtained by enantioselective [2+2] cycloaddition reactions of bicyclic alkenes with 1-alkynyl triazenes in the presence of a RuII catalyst with a chiral cyclopentadienyl ligand. These triazenes serve as unique vinyl cation surrogates. Under acidic conditions, the triazene functionality can be replaced with a variety of groups, including halides, alkoxides, sulfoxides, amides, arenes, and heteroarenes, thus providing efficient access to a pool of chiral polycyclic compounds.

VIP and PACAP analogs regulate therapeutic targets in high-risk neuroblastoma cells.

Neuroblastoma (NB) is a pediatric cancer. New therapies for high-risk NB aim to induce cell differentiation and to inhibit MYCN and ALK signaling in NB. The vasoactive intestinal peptide (VIP) and the pituitary adenylate cyclase-activating polypeptide (PACAP) are 2 related neuropeptides sharing common receptors. The level of VIP increases with NB differentiation. Here, the effects of VIP and PACAP analogs developed for therapeutic use were studied in MYCN-amplified NB SK-N-DZ and IMR-32 cells and in Kelly cells that in addition present the F1174L ALK mutation. As previously reported by our group in IMR-32 cells, VIP induced neuritogenesis in SK-N-DZ and Kelly cells and reduced MYCN expression in Kelly but not in SK-N-DZ cells. VIP decreased AKT activity in the ALK-mutated Kelly cells. These effects were PKA-dependent. IMR-32, SK-NDZ and Kelly cells expressed the genes encoding the 3 subtypes of VIP and PACAP receptors, VPAC1, VPAC2 and PAC1. In parallel to its effect on MYCN expression, VIP inhibited invasion in IMR-32 and Kelly cells. Among the 3 PACAP analogs tested, [Hyp(2)]PACAP-27 showed higher efficiency than VIP in Kelly cells. These results indicate that VIP and PACAP analogs act on molecular and cellular processes that could reduce aggressiveness of high-risk NB.

A Novel Platinum-Maurocalcine Conjugate Induces Apoptosis of Human Glioblastoma Cells by Acting through the ROS-ERK/AKT-p53 Pathway.

Glioblastoma multiforme (GBM) is a highly malignant and aggressive primary brain tumor. In spite of an arsenal of therapeutic interventions, the prognosis of glioblastoma remains very poor. Cisplatin-based therapy is one of the most important chemotherapy treatments for GBM, although its efficacy is limited by drug resistance and undesirable side effects. In the present study, we designed a chimera molecule containing the platinum binding moiety MBL-III-7 (1) attached N-terminal to the sequence of d-maurocalcine (D-MCa), a protease-resistant and highly efficient cell-penetrating peptide (CPP) derived from the Tunisian chactid scorpion toxin, L-MCa. The concept behind this design is that MCa, through its cell retention properties, should reduce cell expulsion of the platinum complex and increase its efficiency. The anti-cancer properties of the synthesized platinum analogue Pt-MBL-III_7-D_MCa (Pt-1-DMCa) were assessed in human glioblastoma cells (U87) by assaying cell viability and apoptosis. The new molecule exhibited enhanced anti-cancer efficacy compared to cisplatin, especially at low doses. By inducing intracellular oxidative stress, Pt-1-DMCa potentiated platinum-induced DNA damage and led to enhanced p53 phosphorylation, followed by increased activation of both mitochondrial and death receptor pathways. Decreased phosphorylated AKT and ERK levels were associated with the apoptosis induced by the novel synthesized cisplatin analogue. Our results suggested that a chimera between platinum and a maurocalcine-derived CPP is a highly successful anti-cancer compound that works by targeting the intracellular redox system. Pt-1-DMCa is an interesting candidate for a preclinical assessment of platinum-based therapy in GBM treatments and possibly other cancer types.

1-Alkynyltriazenes as Functional Analogues of Ynamides.

The chemical reactivity of 1-alkynyltriazenes has been investigated and is found to parallel the reactivity of ynamides. The similarity in reactivity of these two classes of compounds is demonstrated by addition reactions with acids, by cycloaddition reactions with ketenes, tetracyanoethene, and cyclopropanes, as well as by intramolecular cyclization reactions. The presence of reactive triazene groups in the products enables subsequent transformations. Overall, our results suggest that 1-alkynyltriazenes should become valuable reagents in synthetic organic chemistry.

Multi-frame acquisition scheme for efficient energy-dispersive X-ray magnetic circular dichroism in pulsed high magnetic fields at the Fe K-edge.

Using a fast silicon strip detector, a multi-frame acquisition scheme was implemented to perform energy-dispersive X-ray magnetic circular dichroism at the iron K-edge in pulsed high magnetic fields. The acquisition scheme makes use of the entire field pulse. The quality of the signal obtained from samples of ferrimagnetic erbium iron garnet allows for quantitative evaluation of the signal amplitude. Below the compensation point, two successive field-induced phase transitions and the reversal of the net magnetization of the iron sublattices in the intermediate phase were observed.