CXCR3 promotes plaque formation and behavioral deficits in an Alzheimer's disease model.

Chemokines are important modulators of neuroinflammation and neurodegeneration. In the brains of Alzheimer's disease (AD) patients and in AD animal models, the chemokine CXCL10 is found in high concentrations, suggesting a pathogenic role for this chemokine and its receptor, CXCR3. Recent studies aimed at addressing the role of CXCR3 in neurological diseases indicate potent, but diverse, functions for CXCR3. Here, we examined the impact of CXCR3 in the amyloid precursor protein (APP)/presenilin 1 (PS1) transgenic mouse model of AD. We found that, compared with control APP/PSI animals, plaque burden and Aß levels were strongly reduced in CXCR3-deficient APP/PS1 mice. Analysis of microglial phagocytosis in vitro and in vivo demonstrated that CXCR3 deficiency increased the microglial uptake of Aß. Application of a CXCR3 antagonist increased microglial Aß phagocytosis, which was associated with reduced TNF-α secretion. Moreover, in CXCR3-deficient APP/PS1 mice, microglia exhibited morphological activation and reduced plaque association, and brain tissue from APP/PS1 animals lacking CXCR3 had reduced concentrations of proinflammatory cytokines compared with controls. Further, loss of CXCR3 attenuated the behavioral deficits observed in APP/PS1 mice. Together, our data indicate that CXCR3 signaling mediates development of AD-like pathology in APP/PS1 mice and suggest that CXCR3 has potential as a therapeutic target for AD.

Progress and developments in tau aggregation inhibitors for Alzheimer disease.

Pharmacological approaches directed toward Alzheimer disease are diversifying in parallel with a growing number of promising targets. Investigations on the microtubule-associated protein tau yielded innovative targets backed by recent findings about the central role of tau in numerous neurodegenerative diseases. In this review, we summarize the recent evolution in the development of nonpeptidic small molecules tau aggregation inhibitors (TAGIs) and their advancement toward clinical trials. The compounds are classified according to their chemical structures, providing correlative insights into their pharmacology. Overall, shared structure-activity traits are emerging, as well as specific binding modes related to their ability to engage in hydrogen bonding. Medicinal chemistry efforts on TAGIs together with encouraging in vivo data argue for successful translation to the clinic.

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.

Discovery of γ-secretase modulators with a novel activity profile by text-based virtual screening.

We present an integrated approach to identify and optimize a novel class of γ-secretase modulators (GSMs) with a unique pharmacological profile. Our strategy included (i) virtual screening through application of a recently developed protocol (PhAST), (ii) synthetic chemistry to discover structure-activity relationships, and (iii) detailed in vitro pharmacological characterization. GSMs are promising agents for treatment or prevention of Alzheimer's disease. They modulate the γ-secretase product spectrum (i.e., amyloid-ß (Aß) peptides of different length) and induce a shift from toxic Aß42 to shorter Aß species such as Aß38 with no or minimal effect on the overall rate of γ-secretase cleavage. We describe the identification of a series of 4-hydroxypyridin-2-one derivatives, which display a novel type of γ-secretase modulation with equipotent inhibition of Aß42 and Aß38 peptide species.

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.

No improvement after chronic ibuprofen treatment in the 5XFAD mouse model of Alzheimer's disease.

Ibuprofen is a nonsteroidal anti-inflammatory drug (NSAID) that has been reported to reduce the risk of developing Alzheimer's disease (AD). Its preventive effects in AD are likely pleiotropic as ibuprofen displays both anti-inflammatory activity by inhibition of cyclooxygenases and anti-amyloidogenic activity by modulation of γ-secretase. In order to study the anti-inflammatory properties of ibuprofen independent of its anti-amyloidogenic activity, we performed a long-term treatment study with ibuprofen in 5XFAD mice expressing a presenilin-1 mutation that renders this AD model resistant to γ-secretase modulation. As expected, ibuprofen treatment for 3 months resulted in a reduction of the inflammatory reaction in the 5XFAD mouse model. Importantly, an unchanged amyloid beta (Aß) plaque load, an increase in soluble Aß42 levels, and an aggravation of some behavioral parameters were noted, raising the question whether suppression of inflammation by nonsteroidal anti-inflammatory drug is beneficial in AD.

Chemical Biology, Molecular Mechanism and Clinical Perspective of γ-Secretase Modulators in Alzheimer's Disease.

Comprehensive evidence supports that oligomerization and accumulation of amyloidogenic Aß42 peptides in brain is crucial in the pathogenesis of both familial and sporadic forms of Alzheimer's disease. Imaging studies indicate that the buildup of Aß begins many years before the onset of clinical symptoms, and that subsequent neurodegeneration and cognitive decline may proceed independently of Aß. This implies the necessity for early intervention in cognitively normal individuals with therapeutic strategies that prioritize safety. The aspartyl protease γ-secretase catalyses the last step in the cellular generation of Aß42 peptides, and is a principal target for anti-amyloidogenic intervention strategies. Due to the essential role of γ-secretase in the NOTCH signaling pathway, overt mechanism-based toxicity has been observed with the first generation of γ-secretase inhibitors, and safety of this approach has been questioned. However, two new classes of small molecules, γ-secretase modulators (GSMs) and NOTCH-sparing γ-secretase inhibitors, have revitalized γ-secretase as a drug target in AD. GSMs are small molecules that cause a product shift from Aß42 towards shorter and less toxic Ab peptides. Importantly, GSMs spare other physiologically important substrates of the γ-secretase complex like NOTCH. Recently, GSMs with nanomolar potency and favorable in vivo properties have been described. In this review, we summarize the knowledge about the unusual proteolytic activity of γ-secretase, and the chemical biology, molecular mechanisms and clinical perspective of compounds that target the γ-secretase complex, with a particular focus on GSMs.

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.

Amyloid beta 42 peptide (Abeta42)-lowering compounds directly bind to Abeta and interfere with amyloid precursor protein (APP) transmembrane dimerization.

Following ectodomain shedding by beta-secretase, successive proteolytic cleavages within the transmembrane sequence (TMS) of the amyloid precursor protein (APP) catalyzed by gamma-secretase result in the release of amyloid-beta (Abeta) peptides of variable length. Abeta peptides with 42 amino acids appear to be the key pathogenic species in Alzheimer's disease, as they are believed to initiate neuronal degeneration. Sulindac sulfide, which is known as a potent gamma-secretase modulator (GSM), selectively reduces Abeta42 production in favor of shorter Abeta species, such as Abeta38. By studying APP-TMS dimerization we previously showed that an attenuated interaction similarly decreased Abeta42 levels and concomitantly increased Abeta38 levels. However, the precise molecular mechanism by which GSMs modulate Abeta production is still unclear. In this study, using a reporter gene-based dimerization assay, we found that APP-TMS dimers are destabilized by sulindac sulfide and related Abeta42-lowering compounds in a concentration-dependent manner. By surface plasmon resonance analysis and NMR spectroscopy, we show that sulindac sulfide and novel sulindac-derived compounds directly bind to the Abeta sequence. Strikingly, the attenuated APP-TMS interaction by GSMs correlated strongly with Abeta42-lowering activity and binding strength to the Abeta sequence. Molecular docking analyses suggest that certain GSMs bind to the GxxxG dimerization motif in the APP-TMS. We conclude that these GSMs decrease Abeta42 levels by modulating APP-TMS interactions. This effect specifically emphasizes the importance of the dimeric APP-TMS as a promising drug target in Alzheimer's disease.

Tau protein and tau aggregation inhibitors.

Alzheimer disease is characterized by pathological aggregation of two proteins, tau and Abeta-amyloid, both of which are considered to be toxic to neurons. In this review we summarize recent advances on small molecule inhibitors of protein aggregation with emphasis on tau, with activities mediated by the direct interference of self-assembly. The inhibitors can be clustered in several compound classes according to their chemical structure, with subsequent description of the structure-activity relationships, showing that hydrophobic interactions are prevailing. The description is extended to the pharmacological profile of the compounds in order to evaluate their drug-likeness, with special attention to toxicity and bioavailability. The collected data indicate that following the improvements of the in vitro inhibitory potencies, the consideration of the in vivo pharmacokinetics is an absolute prerequisite for the development of compounds suitable for a transfer from bench to bedside.

Development of tau aggregation inhibitors for Alzheimer's disease.

A variety of human diseases are suspected to be directly linked to protein misfolding. Highly organized protein aggregates, called amyloid fibrils, and aggregation intermediates are observed; these are considered to be mediators of cellular toxicity and thus attract a great deal of attention from investigators. Neurodegenerative pathologies such as Alzheimer's disease account for a major part of these protein misfolding diseases. The last decade has witnessed a renaissance of interest in inhibitors of tau aggregation as potential disease-modifying drugs for Alzheimer's disease and other "tauopathies". The recent report of a phase II clinical trial with the tau aggregation inhibitor MTC could hold promise for the validation of the concept. This Review summarizes the available data concerning small-molecule inhibitors of tau aggregation from a medicinal chemistry point of view.