Synthesis of a potent photoreactive acidic γ-secretase modulator for target identification in cells.

Supramolecular self-assembly of amyloidogenic peptides is closely associated with numerous pathological conditions. For instance, Alzheimer´s disease (AD) is characterized by abundant amyloid plaques originating from the proteolytic cleavage of the amyloid precursor protein (APP) by ß- and γ-secretases. Compounds named γ-secretase modulators (GSMs) can shift the substrate cleavage specificity of γ-secretase toward the production of non-amyloidogenic, shorter Aß fragments. Herein, we describe the synthesis of highly potent acidic GSMs, equipped with a photoreactive diazirine moiety for photoaffinity labeling. The probes labeled the N-terminal fragment of presenilin (the catalytic subunit of γ-secretase), supporting a mode of action involving binding to γ-secretase. This fundamental step toward the elucidation of the molecular mechanism governing the GSM-induced shift in γ-secretase proteolytic specificity should pave the way for the development of improved drugs against AD.

Presenilin-1 but not amyloid precursor protein mutations present in mouse models of Alzheimer's disease attenuate the response of cultured cells to γ-secretase modulators regardless of their potency and structure.

γ-Secretase modulators (GSMs) inhibit the generation of amyloidogenic Aß42 peptides and are promising agents for treatment or prevention of Alzheimer's disease (AD). Recently, a second generation of GSMs with favorable pharmacological properties has emerged, but preclinical studies to assess their efficacy in vivo are lacking. Such studies rely on transgenic mouse models that express amyloid precursor protein (APP) and presenilin (PSEN) mutations associated with early-onset familial AD. Previously, we have shown that certain PSEN1 mutations attenuated the response of cultured cells to GSMs and potentially confound in vivo studies in AD mouse models. However, different combinations of familial AD mutations might have synergistic or opposing effects, and we have now systematically determined the response of APP and PSEN1 mutations present in current AD models. Using a potent acidic GSM, we found that APP mutations, either single mutations or in combination, did not affect the potency of GSMs. In contrast, all PSEN1 mutations that have been used to accelerate pathological changes in AD models strongly attenuated the Aß42-lowering activity of GSMs with two exceptions (M146L, A246E). Similar results were obtained with potent non-acidic GSMs indicating that the attenuating effect of PSEN1 mutations cannot simply be overcome by increased potency or structural changes. Notably, two non-acidic compounds fully compensated the attenuating effect of the PSEN1-G384A mutation. Taken together, our findings indicate that most AD models with rapid pathology and advanced phenotypes are unsuitable for preclinical GSM studies. However, we also provide evidence that additional compound screens could discover GSMs that are able to break the attenuating effects of PSEN mutations.

Targeting glucocorticoid receptors prevents the effects of early life stress on amyloid pathology and cognitive performance in APP/PS1 mice.

Exposure to chronic stress or elevated glucocorticoid hormone levels in adult life has been associated with cognitive deficits and an increased risk for Alzheimer's disease (AD). Since exposure to stress during early life enhances stress-responsiveness and lastingly affects cognition in adult life, we here investigated; (i) whether chronic early life stress (ELS) affects AD pathology and cognition in middle-aged APPswe/PS1dE9 mice, and (ii) whether it is still possible to rescue these late effects by briefly blocking glucocorticoid receptors (GRs) at a translationally relevant, middle age. Transgenic APPswe/PS1dE9 mice were subjected to ELS by housing dams and pups with limited nesting and bedding material from postnatal days 2-9 only. In 6- and 12-month-old offspring, this resulted in enhanced hippocampal amyloid-ß (Aß)-40 and -42 levels, and in reduced cognitive flexibility, that correlated well with the Aß42 levels. In parallel, CORT levels and BACE1 levels were significantly elevated. Surprisingly, blocking GRs for only 3 days at 12 months of age reduced CORT levels, reduced hippocampal Aß40 and -42, and ß-site APP-cleaving enzyme 1 (BACE1) levels, and notably rescued the cognitive deficits in 12-month-old APPswe/PS1dE9 mice. These mouse data demonstrate that exposure to stress during the sensitive period early in life influences later amyloid pathology and cognition in genetically predisposed, mutant mice, and as such, may increase AD vulnerability. The fact that a short treatment with a GR antagonist at middle age lastingly reduced Aß levels and rescued the cognitive deficits after ELS, highlights the therapeutic potential of this drug for reducing amyloid pathology.

Endothelial LRP1 transports amyloid-ß(1-42) across the blood-brain barrier.

According to the neurovascular hypothesis, impairment of low-density lipoprotein receptor-related protein-1 (LRP1) in brain capillaries of the blood-brain barrier (BBB) contributes to neurotoxic amyloid-ß (Aß) brain accumulation and drives Alzheimer's disease (AD) pathology. However, due to conflicting reports on the involvement of LRP1 in Aß transport and the expression of LRP1 in brain endothelium, the role of LRP1 at the BBB is uncertain. As global Lrp1 deletion in mice is lethal, appropriate models to study the function of LRP1 are lacking. Moreover, the relevance of systemic Aß clearance to AD pathology remains unclear, as no BBB-specific knockout models have been available. Here, we developed transgenic mouse strains that allow for tamoxifen-inducible deletion of Lrp1 specifically within brain endothelial cells (Slco1c1-CreER(T2) Lrp1(fl/fl) mice) and used these mice to accurately evaluate LRP1-mediated Aß BBB clearance in vivo. Selective deletion of Lrp1 in the brain endothelium of C57BL/6 mice strongly reduced brain efflux of injected [125I] Aß(1-42). Additionally, in the 5xFAD mouse model of AD, brain endothelial-specific Lrp1 deletion reduced plasma Aß levels and elevated soluble brain Aß, leading to aggravated spatial learning and memory deficits, thus emphasizing the importance of systemic Aß elimination via the BBB. Together, our results suggest that receptor-mediated Aß BBB clearance may be a potential target for treatment and prevention of Aß brain accumulation in AD.

Neprilysin deficiency alters the neuropathological and behavioral phenotype in the 5XFAD mouse model of Alzheimer's disease.

The deposition of amyloid-ß (Aß) is one of the major neuropathological hallmarks of Alzheimer's disease (AD). In the case of sporadic AD, an imbalance in Aß in production and clearance seems to be the reason for an enhanced Aß accumulation. Besides a systematic clearance through the blood-brain barrier, Aß is cleared from the brain by Aß-degrading enzymes. The metalloprotease neprilysin (NEP) is an important Aß-degrading enzyme as shown by numerous in vitro, in vivo and reverse genetics studies. 5XFAD mice represent an early-onset AD mouse model which develops plaque pathology starting with 2 months of age in addition to robust behavioral deficits at later time points. By crossing 5XFAD mice with homozygous NEP-knock-out mice (NEP-/-), we show that hemizygous NEP deficiency aggravates the behavioral and neuropathological phenotype of 5XFAD mice. We found that 5XFAD mice per se showed strongly decreased NEP expression levels compared to wildtype mice, which was aggravated by NEP reduction. 5XFAD/NEP+/- mice demonstrated impairment in spatial working memory and increased astrocytosis in all studied brain areas, in addition to an overall increased level of soluble Aß42 as well as region-specific increases in extracellular Aß deposition. Surprisingly, in young mice, a more abundant cortical Aß plaque pathology was observed in 5XFAD compared to 5XFAD/NEP+/- mice. Additionally, young 5XFAD/NEP+/- as well as hemi- and homozygous NEP knockout mice showed elevated levels of endothelin-converting enzyme 1 (ECE1), suggesting a mutual regulation of ECE1 and NEP at young ages. The present data indicate that NEP mainly degrades soluble Aß peptides, which confirms previous observations. Increased ECE1 levels correlated well with the strongly reduced extracellular plaque load in young 5XFAD/NEP+/- mice and might suggest a reciprocal effect between ECE and NEP activities in Aß degradation.

Gene Dosage Dependent Aggravation of the Neurological Phenotype in the 5XFAD Mouse Model of Alzheimer's Disease.

In the present report, we extend previous findings in the 5XFAD mouse model with regard to a characterization of behavioral deficits and neuropathological alterations. We demonstrate that these mice develop a robust age-dependent motor phenotype and spatial reference memory deficits when bred to homozygosity, leading to a strongly reduced age of onset of behavioral symptoms. At postnatal day sixteen, abundant AßPP was detected in subiculum and cortical pyramidal neurons. From six weeks on, intraneuronal Aß could be detected which was much more abundant in homozygous mice. The same gene-dosage effect was seen on memory and motor deficits. While at 2 months of age neither heterozygous nor homozygous 5XFAD mice show any neurological phenotype except for alterations in anxiety behavior, at 5 months they were clearly evident. Interestingly, despite abundant motor deficiencies, homozygous 5XFAD mice were able to perform the acquisition training of the Morris water maze task with no difference in the swimming performance between the groups. Therefore the aggravated spatial memory and spatial reference memory deficits of the homozygous mice correlated with the elevated soluble and insoluble Aß levels. Homozygous 5XFAD mice represent a model with several advantages in comparison to the heterozygous mice, developing amyloid pathology much more rapidly together with a neurological phenotype. These advantages allow reducing the number of animals for Alzheimer's disease research.

Nanoparticulate flurbiprofen reduces amyloid-ß42 generation in an in vitro blood-brain barrier model.

INTRODUCTION: The amyloid-ß42 (Aß42) peptide plays a crucial role in the pathogenesis of Alzheimer's disease (AD), the most common neurodegenerative disorder affecting the elderly. Over the past years, several approaches and compounds developed for the treatment of AD have failed in clinical studies, likely in part due to their low penetration of the blood-brain barrier (BBB). Since nanotechnology-based strategies offer new possibilities for the delivery of drugs to the brain, this technique is studied intensively for the treatment of AD and other neurological disorders. METHODS: The Aß42 lowering drug flurbiprofen was embedded in polylactide (PLA) nanoparticles by emulsification-diffusion technique and their potential as drug carriers in an in vitro BBB model was examined. First, the cytotoxic potential of the PLA-flurbiprofen nanoparticles on endothelial cells and the cellular binding and uptake by endothelial cells was studied. Furthermore, the biological activity of the nanoparticulate flurbiprofen on γ-secretase modulation as well as its in vitro release was examined. Furthermore, the protein corona of the nanoparticles was studied as well as their ability to transport flurbiprofen across an in vitro BBB model. RESULTS: PLA-flurbiprofen nanoparticles were endocytosed by endothelial cells and neither affected the vitality nor barrier function of the endothelial cell monolayer. The exposure of the PLA-flurbiprofen nanoparticles to human plasma occurred in a rapid protein corona formation, resulting in their decoration with bioactive proteins, including apolipoprotein E. Furthermore, luminally administered PLA-flurbiprofen nanoparticles in contrast to free flurbiprofen were able to modulate γ-secretase activity by selectively decreasing Aß42 levels in the abluminal compartment of the BBB model. CONCLUSIONS: In this study, we were able to show that flurbiprofen can be transported by PLA nanoparticles across an in vitro BBB model and most importantly, the transported flurbiprofen modulated γ-secretase activity by selectively decreasing Aß42 levels. These results demonstrate that the modification of drugs via embedding in nanoparticles is a promising tool to facilitate drug delivery to the brain, which enables future development for the treatment of neurodegenerative disorders like AD.

Presenilin is the molecular target of acidic γ-secretase modulators in living cells.

The intramembrane-cleaving protease γ-secretase catalyzes the last step in the generation of toxic amyloid-ß (Aß) peptides and is a principal therapeutic target in Alzheimer's disease. Both preclinical and clinical studies have demonstrated that inhibition of γ-secretase is associated with prohibitive side effects due to suppression of Notch processing and signaling. Potentially safer are γ-secretase modulators (GSMs), which are small molecules that selectively lower generation of the highly amyloidogenic Aß42 peptides but spare Notch processing. GSMs with nanomolar potency and favorable pharmacological properties have been described, but the molecular mechanism of GSMs remains uncertain and both the substrate amyloid precursor protein (APP) and subunits of the γ-secretase complex have been proposed as the molecular target of GSMs. We have generated a potent photo-probe based on an acidic GSM that lowers Aß42 generation with an IC(50) of 290 nM in cellular assays. By combining in vivo photo-crosslinking with affinity purification, we demonstrated that this probe binds the N-terminal fragment of presenilin (PSEN), the catalytic subunit of the γ-secretase complex, in living cells. Labeling was not observed for APP or any of the other γ-secretase subunits. Binding was readily competed by structurally divergent acidic and non-acidic GSMs suggesting a shared mode of action. These findings indicate that potent acidic GSMs target presenilin to modulate the enzymatic activity of the γ-secretase complex.

Formation of Pmel17 amyloid is regulated by juxtamembrane metalloproteinase cleavage, and the resulting C-terminal fragment is a substrate for gamma-secretase.

The formation of insoluble cross beta-sheet amyloid is pathologically associated with disorders such as Alzheimer, Parkinson, and Huntington diseases. One exception is the nonpathological amyloid derived from the protein Pmel17 within melanosomes to generate melanin pigment. Here we show that the formation of insoluble MalphaC intracellular fragments of Pmel17, which are the direct precursors to Pmel17 amyloid, depends on a novel juxtamembrane cleavage at amino acid position 583 between the furin-like proprotein convertase cleavage site and the transmembrane domain. The resulting Pmel17 C-terminal fragment is then processed by the gamma-secretase complex to release a short-lived intracellular domain fragment. Thus, by analogy to the Notch receptor, we designate this cleavage the S2 cleavage site, whereas gamma-secretase mediates proteolysis at the intramembrane S3 site. Substitutions or deletions at this S2 cleavage site, the use of the metalloproteinase inhibitor TAPI-2, as well as small interfering RNA-mediated knock-down of the metalloproteinases ADAM10 and 17 reduced the formation of insoluble Pmel17 fragments. These results demonstrate that the release of the Pmel17 ectodomain, which is critical for melanin amyloidogenesis, is initiated by S2 cleavage at a juxtamembrane position.