Structure and function of gamma-secretase

Effects and mechanisms of APP and its cleavage product Aβ in the comorbidity of sarcopenia and Alzheimer's disease

Sarcopenia and AD are both classic degenerative diseases, and there is growing epidemiological evidence of their comorbidity with aging; however, the mechanisms underlying the biology of their commonality have not yet been thoroughly investigated. APP is a membrane protein that is expressed in tissues and is expressed not only in the nervous system but also in the NMJ and muscle. Deposition of its proteolytic cleavage product, Aβ, has been described as a central component of AD pathogenesis. Recent studies have shown that excessive accumulation and aberrant expression of APP in muscle lead to pathological muscle lesions, but the pathogenic mechanism by which APP and its proteolytic cleavage products act in skeletal muscle is less well understood. By summarizing and analyzing the literature concerning the role, pathogenicity and pathological mechanisms of APP and its cleavage products in the nervous system and muscles, we aimed to explore the intrinsic pathological mechanisms of myocerebral comorbidities and to provide new perspectives and theoretical foundations for the prevention and treatment of AD and sarcopenia comorbidities.

Functional and topological analysis of PSENEN, the fourth subunit of the γ-secretase complex

The γ-secretase complexes are intramembrane cleaving proteases involved in the generation of the Aβ peptides in Alzheimer's disease. The complex consists of four subunits, with Presenilin harboring the catalytic site. Here, we study the role of the smallest subunit, PSENEN or Presenilin enhancer 2, encoded by the gene Psenen, in vivo and in vitro. We find a profound Notch deficiency phenotype in Psenen-/- embryos confirming the essential role of PSENEN in the γ-secretase complex. We used Psenen-/- fibroblasts to explore the structure-function of PSENEN by the scanning cysteine accessibility method. Glycine 22 and proline 27, which border the membrane domains 1 and 2 of PSENEN, are involved in complex formation and stabilization of γ-secretase. The hairpin structured hydrophobic membrane domains 1 and 2 are exposed to a water-containing cavity in the complex, while transmembrane domain 3 is not water exposed. We finally demonstrate the essential role of PSENEN for the cleavage activity of the complex. PSENEN is more than a structural component of the γ-secretase complex and might contribute to the catalytic mechanism of the enzyme.

Oxidative damage in neurodegeneration: roles in the pathogenesis and progression of Alzheimer disease

Alzheimer disease (AD) is associated with multiple etiologies and pathological mechanisms, among which oxidative stress (OS) appears as a major determinant. Intriguingly, OS arises in various pathways regulating brain functions, and it seems to link different hypotheses and mechanisms of AD neuropathology with high fidelity. The brain is particularly vulnerable to oxidative damage, mainly because of its unique lipid composition, resulting in an amplified cascade of redox reactions that target several cellular components/functions ultimately leading to neurodegeneration. The present review highlights the "OS hypothesis of AD," including amyloid beta-peptide-associated mechanisms, the role of lipid and protein oxidation unraveled by redox proteomics, and the antioxidant strategies that have been investigated to modulate the progression of AD. Collected studies from our groups and others have contributed to unraveling the close relationships between perturbation of redox homeostasis in the brain and AD neuropathology by elucidating redox-regulated events potentially involved in both the pathogenesis and progression of AD. However, the complexity of AD pathological mechanisms requires an in-depth understanding of several major intracellular pathways affecting redox homeostasis and relevant for brain functions. This understanding is crucial to developing pharmacological strategies targeting OS-mediated toxicity that may potentially contribute to slow AD progression as well as improve the quality of life of persons with this severe dementing disorder.

Inhibition of Notch Signaling Enhances Antitumor Activity of Histone Deacetylase Inhibitor LAQ824

As a novel histone deacetylase inhibitor (HDACi), LAQ824 (LAQ) effectively inhibits the proliferation of hematological malignancies and solid tumors. However, phase II trials of LAQ in solid tumors were terminated due to dose-dependent toxicity. Furthermore, LAQ has been shown to induce the activation of the Notch signaling pathway in hematopoietic stem cells, which is associated with tumor progression and drug resistance in colon and breast cancers. Therefore, in this study, we investigated the strategy of LAQ combined with a Notch signaling pathway inhibitor to treat solid tumors. We used RT-PCR and Western blot methods to demonstrate that LAQ upregulated the Notch signaling pathway in solid tumor cell lines at the molecular level. The combination of LAQ and a Notch signaling pathway inhibitor was shown by a Chou-Talalay assay to have a synergistic effect in inhibiting solid tumor cell line proliferation in vitro. We also demonstrated that the combination of LAQ and a Notch signaling pathway inhibitor significantly inhibited the growth of tumor cells in vivo using an allograft tumor model. This study indicates that inhibition of the Notch signaling pathway provides a valuable strategy for enhancing solid tumor sensitivity to LAQ.

Analysis of Non-Amyloidogenic Mutations in APP Supports Loss of Function Hypothesis of Alzheimer's Disease

Proteolytic processing of amyloid precursor protein (APP) plays a critical role in pathogenesis of Azheimer's disease (AD). Sequential cleavage of APP by β- and γ-secretases leads to generation of Aβ40 (non-amyloidogenic) and Aβ42 (amyloidogenic) peptides. Presenilin-1 (PS1) or presenilin-2 (PS2) act as catalytic subunits of γ-secretase. Multiple familial AD (FAD) mutations in APP, PS1, or PS2 affect APP proteolysis by γ-secretase and influence levels of generated Aβ40 and Aβ42 peptides. The predominant idea in the field is the "amyloid hypothesis" that states that the resulting increase in Aβ42:Aβ40 ratio leads to "toxic gain of function" due to the accumulation of toxic Aβ42 plaques and oligomers. An alternative hypothesis based on analysis of PS1 conditional knockout mice is that "loss of function" of γ-secretase plays an important role in AD pathogenesis. In the present paper, we propose a mechanistic hypothesis that may potentially reconcile these divergent ideas and observations. We propose that the presence of soluble Aβ peptides in endosomal lumen (and secreted to the extracellular space) is essential for synaptic and neuronal function. Based on structural modeling of Aβ peptides, we concluded that Aβ42 peptides and Aβ40 peptides containing non-amyloidogenic FAD mutations in APP have increased the energy of association with the membranes, resulting in reduced levels of soluble Aβ in endosomal compartments. Analysis of PS1-FAD mutations also revealed that all of these mutations lead to significant reduction in both total levels of Aβ produced and in the Aβ40/Aβ42 ratio, suggesting that the concentration of soluble Aβ in the endosomal compartments is reduced as a result of these mutations. We further reasoned that similar changes in Aβ production may also occur as a result of age-related accumulation of cholesterol and lipid oxidation products in postsynaptic spines. Our analysis more easily reconciled with the "loss of γ-secretase function" hypothesis than with the "toxic gain of Aβ42 function" idea. These results may also explain why inhibitors of β- and γ- secretase failed in clinical trials, as these compounds are also expected to significantly reduce soluble Aβ levels in the endosomal compartments.

Genetics, Functions, and Clinical Impact of Presenilin-1 (PSEN1) Gene

Presenilin-1 (PSEN1) has been verified as an important causative factor for early onset Alzheimer's disease (EOAD). PSEN1 is a part of γ-secretase, and in addition to amyloid precursor protein (APP) cleavage, it can also affect other processes, such as Notch signaling, β-cadherin processing, and calcium metabolism. Several motifs and residues have been identified in PSEN1, which may play a significant role in γ-secretase mechanisms, such as the WNF, GxGD, and PALP motifs. More than 300 mutations have been described in PSEN1; however, the clinical phenotypes related to these mutations may be diverse. In addition to classical EOAD, patients with PSEN1 mutations regularly present with atypical phenotypic symptoms, such as spasticity, seizures, and visual impairment. In vivo and in vitro studies were performed to verify the effect of PSEN1 mutations on EOAD. The pathogenic nature of PSEN1 mutations can be categorized according to the ACMG-AMP guidelines; however, some mutations could not be categorized because they were detected only in a single case, and their presence could not be confirmed in family members. Genetic modifiers, therefore, may play a critical role in the age of disease onset and clinical phenotypes of PSEN1 mutations. This review introduces the role of PSEN1 in γ-secretase, the clinical phenotypes related to its mutations, and possible significant residues of the protein.

A Novel NCSTN Mutation in a Three-Generation Chinese Family with Hidradenitis Suppurative

Objective

Hidradenitis suppurativa (HS) is a rare autosomal dominant condition characterized by inflamed nodules, cysts, deep abscesses, draining sinuses in the axillae, inguinal, and anogenital regions. Mutations in the NCSTN gene have been perceived to be responsible for the major underlying changes in the disorder. The purpose of this study is to identify a novel gene mutation in a Chinese family with HS.

Methods

A Chinese family with HS present was investigated. The proband had manifested with multiple draining sinuses on the posterior neck, chest, bilateral axillae, and perineal regions. DNA was isolated from the peripheral blood of the family members. The encoding exons with introns of the NCSTN gene were analyzed by polymerase chain reactions (PCR) and direct DNA sequencing. Sanger sequencing was performed to confirm the next-generation sequencing results and to analyze each mutation's familial segregation. Furthermore, the identified mutation was localized onto a 3D structure model using the DeepView Swiss-PdbViewer 4.1 software.

Results

In this family comprising 10 HS patients, one novel mutation of the NCSTN gene was identified, involving a deletion mutation (c.447delC(p.N150Ifs∗52)) in the NCSTN gene resulting in a frameshift and the new formation of a hydrogen bond.

Conclusion

Our study reports the identification of a novel mutation that causes familial HS and could expand the spectrum of mutations in the γ-secretase genes underlying HS.

Conformational Models of APP Processing by Gamma Secretase Based on Analysis of Pathogenic Mutations

Proteolytic processing of amyloid precursor protein (APP) plays a critical role in the pathogenesis of Alzheimer's disease (AD). Sequential cleavage of APP by β and γ secretases leads to the generation of Aβ40 (non-amyloidogenic) and Aβ42 (amyloidogenic) peptides. Presenilin-1 (PS1) or presenilin-2 (PS2) play the role of a catalytic subunit of γ-secretase. Multiple familial AD (FAD) mutations in APP, PS1, or PS2 result in an increased Aβ42:Aβ40 ratio and the accumulation of toxic Aβ42 oligomers and plaques in patient brains. In this study, we perform molecular modeling of the APP complex with γ-secretase and analyze potential effects of FAD mutations in APP and PS1. We noticed that all FAD mutations in the APP transmembrane domain are predicted to cause an increase in the local disorder of its secondary structure. Based on structural analysis of known γ-secretase structures, we propose that APP can form a complex with γ-secretase in 2 potential conformations-M1 and M2. In conformation, the M1 transmembrane domain of APP forms a contact with the perimembrane domain that follows transmembrane domain 6 (TM6) in the PS1 structure. In conformation, the M2 transmembrane domain of APP forms a contact with transmembrane domain 7 (TM7) in the PS1 structure. By analyzing the effects of PS1-FAD mutations on the local protein disorder index, we discovered that these mutations increase the conformational flexibility of M2 and reduce the conformational flexibility of M1. Based on these results, we propose that M2 conformation, but not M1 conformation, of the γ secretase complex with APP leads to the amyloidogenic (Aβ42-generating) processing of APP. Our model predicts that APP processing in M1 conformation is favored by curved membranes, such as the membranes of early endosomes. In contrast, APP processing in M2 conformation is likely to be favored by relatively flat membranes, such as membranes of late endosomes and plasma membranes. These predictions are consistent with published biochemical analyses of APP processing at different subcellular locations. Our results also suggest that specific inhibitors of Aβ42 production could be potentially developed by selectively targeting the M2 conformation of the γ secretase complex with APP.

γ-Secretase Genetics of Hidradenitis Suppurativa: A Systematic Literature Review

BACKGROUND

Acne inversa/hidradenitis suppurativa (HS) is a chronic, recurrent inflammatory disease of the skin that can significantly affect patients' quality of life. The etiology and pathogenesis of HS are unclear and gene mutations might play a role.

SUMMARY

The primary focus of the review is on aggregating the gene mutations reported, summarizing the structure of γ-secretase and analyzing and speculating about the mechanism and the underlying relations between gene mutation and functional changes of protein. The systematic literature review was done by searching the PubMed, Embase, and Web of Science databases. γ-Secretase is an intramembrane protease complex responsible for the intramembranous cleavage of more than 30 type-1 transmembrane proteins including amyloid precursor protein and Notch receptors. The protein complex consists of four hydrophobic proteins: presenilin, presenilin enhancer-2 (PSENEN), nicastrin, and anterior pharynx defective 1 (APH1). To date, 57 mutations of γ-secretase genes have been reported in 70 patients or families worldwide, including 39 in NCSTN, 14 in PSENEN, and 4 in PSEN1, of which 17 are frameshifts, 15 result in nonsense mutations, 13 in missense mutations, and 12 are splice site mutations. Given the structure of γ-secretase and analysis of related mutation loci of NCSTN, PSENEN, and PSEN1, mutations in γ-secretase genes could affect activation of presenilin, prevent substrate binding, and hinder intramembrane cleavage of select proteins.

Sorting Out the Role of the Sortilin-Related Receptor 1 in Alzheimer's Disease

Sortilin-related receptor 1 (SORL1) encodes a large, multi-domain containing, membrane-bound receptor involved in endosomal sorting of proteins between the trans-Golgi network, endosomes and the plasma membrane. It is genetically associated with Alzheimer's disease (AD), the most common form of dementia. SORL1 is a unique gene in AD, as it appears to show strong associations with the common, late-onset, sporadic form of AD and the rare, early-onset familial form of AD. Here, we review the genetics of SORL1 in AD and discuss potential roles it could play in AD pathogenesis.

A pathogenic PSEN1 Trp165Cys mutation associated with early-onset Alzheimer's disease

Background

Presenilin-1 (PSEN1) is one of the causative genes for early onset Alzheimer’s disease (EOAD). Recently, emerging studies reported several novel PSEN1 mutations among Asian. We describe a male with EOAD had a pathogenic PSEN1 mutation.

Case presentation

A 53-year-old male presented with memory decline, followed by difficulty in finding ways. Patient had positive family history, since his mother and one of his brother was also affected with dementia. Brain magnetic resonance imaging (MRI) scan showed mild degree of atrophy of bilateral hippocampus and parietal lobe. ¹⁸F-Florbetaben-PET (FBB-PET) revealed increased amyloid deposition in bilateral frontal, parietal, temporal lobe and precuneus. Whole exome analysis revealed a heterozygous, probably pathogenic PSEN1 (c.695G > T, p.W165C) mutation. Interestingly, Trp165Cys mutation is located in trans membrane (TM)-III region, which is conserved between PSEN1/PSEN2. In vitro studies revealed that PSEN1 Trp165Cys could result in disturbances in amyloid metabolism. This prediction was confirmed by structure predictions and previous in vitro studies that the p.Trp165Cys could result in decreased Aβ42/Aβ40 ratios.

Conclusion

We report a case of EOAD having a pathogenic PSEN1 (Trp165Cys) confirmed with in silico and in vitro predictions.

Dual role of DMXL2 in olfactory information transmission and the first wave of spermatogenesis

Gonad differentiation is a crucial step conditioning the future fertility of individuals and most of the master genes involved in this process have been investigated in detail. However, transcriptomic analyses of developing gonads from different animal models have revealed that hundreds of genes present sexually dimorphic expression patterns. DMXL2 was one of these genes and its function in mammalian gonads was unknown. We therefore investigated the phenotypes of total and gonad-specific Dmxl2 knockout mouse lines. The total loss-of-function of Dmxl2 was lethal in neonates, with death occurring within 12 hours of birth. Dmxl2-knockout neonates were weak and did not feed. They also presented defects of olfactory information transmission and severe hypoglycemia, suggesting that their premature death might be due to global neuronal and/or metabolic deficiencies. Dmxl2 expression in the gonads increased after birth, during follicle formation in females and spermatogenesis in males. DMXL2 was detected in both the supporting and germinal cells of both sexes. As Dmxl2 loss-of-function was lethal, only limited investigations of the gonads of Dmxl2 KO pups were possible. They revealed no major defects at birth. The gonadal function of Dmxl2 was then assessed by conditional deletions of the gene in gonadal supporting cells, germinal cells, or both. Conditional Dmxl2 ablation in the gonads did not impair fertility in males or females. By contrast, male mice with Dmxl2 deletions, either throughout the testes or exclusively in germ cells, presented a subtle testicular phenotype during the first wave of spermatogenesis that was clearly detectable at puberty. Indeed, Dmxl2 loss-of-function throughout the testes or in germ cells only, led to sperm counts more than 60% lower than normal and defective seminiferous tubule architecture. Transcriptomic and immunohistochemichal analyses on these abnormal testes revealed a deregulation of Sertoli cell phagocytic activity related to germ cell apoptosis augmentation. In conclusion, we show that Dmxl2 exerts its principal function in the testes at the onset of puberty, although its absence does not compromise male fertility in mice.

Reduced Cytosolic Calcium as an Early Decisive Cellular State in Parkinson's Disease and Synucleinopathies

The more than 30-year-old Calcium hypothesis postulates that dysregulation in calcium dependent processes in the aging brain contributes to its increased vulnerability and this concept has been extended to Alzheimer’s disease and Parkinson’s disease. Central to the hypothesis is that increased levels of intracellular calcium develop and contributes to neuronal demise. We have studied the impact on cells encountering a gradual build-up of aggregated α-synuclein, which is a central process to Parkinson’s disease and other synucleinopathies. Surprisingly, we observed a yet unrecognized phase characterized by a reduced cytosolic calcium in cellular and neuronal models of Parkinson’s disease, caused by α-synuclein aggregates activating the endoplasmic calcium ATPase, SERCA. Counteracting the initial phase with low calcium rescues the subsequent degenerative phase with increased calcium and cell death – and demonstrates this early phase initiates decisive degenerative signals. In this review, we discuss our findings in relation to literature on calcium dysregulation in Parkinson’s disease and dementia.

Irreversible modifications of receptor tyrosine kinases

Each group of the 56 receptor tyrosine kinases (RTK) binds with one or more soluble growth factors and coordinates a vast array of cellular functions. These outcomes are tightly regulated by inducible post-translational events, such as tyrosine phosphorylation, ubiquitination, ectodomain shedding, and regulated intramembrane proteolysis. Because of the delicate balance required for appropriate RTK function, cells may become pathogenic upon dysregulation of RTKs themselves or their post-translational covalent modifications. For example, reduced ectodomain shedding and decreased ubiquitination of the cytoplasmic region, both of which enhance growth factor signals, characterize malignant cells. Whereas receptor phosphorylation and ubiquitination are reversible, proteolytic cleavage events are irreversible, and either modification might alter the subcellular localization of RTKs. Herein, we focus on ectodomain shedding by metalloproteinases (including ADAM family proteases), cleavage within the membrane or cytoplasmic regions of RTKs (by gamma-secretases and caspases, respectively), and complete receptor proteolysis in lysosomes and proteasomes. Roles of irreversible modifications in RTK signaling, pathogenesis, and pharmacology are highlighted.

Phenotypic Variability in Autosomal Dominant Familial Alzheimer Disease due to the S170F Mutation of Presenilin-1

BACKGROUND

In rare cases, patients with Alzheimer disease (AD) present at an early age and with a family history suggestive of an autosomal dominant mode of inheritance. Mutations of the presenilin-1 (PSEN1) gene are the most common causes of dementia in these patients. Early-onset and particularly familial AD patients frequently present with variable non-amnestic cognitive symptoms such as visual, language or behavioural changes as well as non-cognitive, e.g. motor, symptoms.

OBJECTIVE

To investigate the phenotypic variability in carriers of the PSEN1 S170F mutation.

METHODS

We report a family with 4 patients carrying the S170F mutation of whom 2 underwent detailed clinical examinations. We discuss our current findings in the context of previously reported S170F cases.

RESULTS

The clinical phenotype was consistent regarding initial memory impairment and early onset in the late twenties found in all S170F patients. There were frequent non-amnestic cognitive changes and, at early stages of the disease, indications of a more pronounced disturbance of visuospatial abilities as compared to face and object recognition. Non-cognitive symptoms most often included myoclonus and cerebellar ataxia. A review of the available case reports indicates some phenotypic variability associated with the S170F mutation including different constellations of symptoms such as parkinsonism and delusions.

CONCLUSION

The variable clinical findings associated with the S170F mutation highlight the relevance of atypical phenotypes in the context of research and under a clinical perspective. CSF sampling and detection of Aβ species may be essential to indicate AD pathology in unclear cases presenting with cognitive and motor symptoms at a younger age.

Pharmacological Agents Targeting γ-Secretase Increase Risk of Cancer and Cognitive Decline in Alzheimer's Disease Patients: A Systematic Review and Meta-Analysis

BACKGROUND

Drugs targeting γ-secretase in Alzheimer's disease (AD) have failed to demonstrate efficacy in clinical trials.

OBJECTIVE

To perform a meta-analysis of randomized controlled trials (RCTs) to evaluate the efficacy and safety of drugs targeting γ-secretase in AD.

METHODS

Ten trials were identified involving 5,227 patients using electronic databases and manual review of reference lists. RCTs of at least two weeks duration involving a drug targeting γ-secretase were eligible. The main outcomes examined were adverse events and cognitive measures (ADAS-cog, MMSE, ADCS-ADL, and CDR-sb). A sub-group analysis was performed, excluding the γ-secretase modulator tarenflurbil, to evaluate the safety and efficacy of γ-secretase inhibitors only.

RESULTS

There was an increased risk of adverse events (Odds Ratio (OR) 1.38, 95% CI 1.09-1.73; p = 0.01), serious adverse events (OR 1.50, 95% CI 1.22-1.84; p < 0.001), and skin cancers (OR 4.77, 95% CI 2.83-8.06; p < 0.001). There was significantly increased risk of infections (OR 1.36, 95% CI 1.13-1.63; p < 0.001) in the subgroup analysis excluding tarenflurbil. Pooled results also revealed a worsening in ADAS-cog (difference in means 1.33, 95% CI 0.58-2.08; p < 0.001) and MMSE (difference in means -0.66, 95% CI -0.96 to 0.35; p < 0.001), but not ADCS-ADL or CDR-sb.

CONCLUSION

The use of γ-secretase inhibitors is associated with significantly increased risk of serious adverse events including skin cancers, and worsening in cognitive indicators. This evidence indicates that γ-secretase may not be an appropriate target for clinical treatment of AD.

Regulated intramembrane proteolysis: emergent role in cell signalling pathways

Receptor signalling events including those initiated following activation of cytokine and growth factor receptors and the well-characterised death receptors (tumour necrosis factor receptor, type 1, FasR and TRAIL-R1/2) are initiated at the cell surface through the recruitment and formation of intracellular multiprotein signalling complexes that activate divergent signalling pathways. Over the past decade, research studies reveal that many of these receptor-initiated signalling events involve the sequential proteolysis of specific receptors by membrane-bound proteases and the γ-secretase protease complexes. Proteolysis enables the liberation of soluble receptor ectodomains and the generation of intracellular receptor cytoplasmic domain fragments. The combined and sequential enzymatic activity has been defined as regulated intramembrane proteolysis and is now a fundamental signal transduction process involved in the termination or propagation of receptor signalling events. In this review, we discuss emerging evidence for a role of the γ-secretase protease complexes and regulated intramembrane proteolysis in cell- and immune-signalling pathways.

The role of Ca2+ signaling in Parkinson's disease

Across all kingdoms in the tree of life, calcium (Ca2+) is an essential element used by cells to respond and adapt to constantly changing environments. In multicellular organisms, it plays fundamental roles during fertilization, development and adulthood. The inability of cells to regulate Ca2+ can lead to pathological conditions that ultimately culminate in cell death. One such pathological condition is manifested in Parkinson's disease, the second most common neurological disorder in humans, which is characterized by the aggregation of the protein, α-synuclein. This Review discusses current evidence that implicates Ca2+ in the pathogenesis of Parkinson's disease. Understanding the mechanisms by which Ca2+ signaling contributes to the progression of this disease will be crucial for the development of effective therapies to combat this devastating neurological condition.

The lipidome associated with the γ-secretase complex is required for its integrity and activity

γ-Secretase is a multi-subunit membrane protease complex that catalyses the final intramembrane cleavage of the β-amyloid precursor protein (APP) during the neuronal production of amyloid-β peptides (Aβ), which are implicated as the causative agents of Alzheimer's disease (AD). In the present study, we report the reconstitution of a highly purified, active γ-secretase complex into proteoliposomes without exogenous lipids and provide the first direct evidence for the existence of a microenvironment of 53 molecular species from 11 major lipid classes specifically associated with the γ-secretase complex, including phosphatidylcholine and cholesterol. Importantly, we demonstrate that the pharmacological modulation of certain phospholipids abolishes both the integrity and the enzymatic activity of the intramembrane protease. Together, our findings highlight the importance of a specific lipid microenvironment for the structure and function of γ-secretase.

Membrane topology of human presenilin-1 in SK-N-SH cells determined by fluorescence correlation spectroscopy and fluorescent energy transfer

Presenilin-1 (PS1) protein acts as passive ER Ca(2+) leak channels that facilitate passive Ca(2+) leak across ER membrane. Mutations in the gene encoding PS1 protein cause neurodegeneration in the brains of patients with familial Alzheimer's disease (FAD). FADPS1 mutations abrogate the function of ER Ca(2+) leak channel activity in human neuroblastoma SK-N-SH cells in vitro (Das et al., J Neurochem 122(3):487-500, 2012) and in mouse embryonic fibroblasts. Consequently, genetic deletion or mutations of the PS1 gene cause calcium (Ca(2+)) signaling abnormalities leading to neurodegeneration in FAD patients. By analogy with other known ion channels it has been proposed that the functional PS1 channels in ER may be multimers of several PS1 subunits. To test this hypothesis, we conjugated the human PS1 protein with an NH2-terminal YFP-tag and a COOH-terminal CFP-tag. As expected YFP-PS1, and PS1-CFP were found to be expressed on the plasma membranes by TIRF microscopy, and both these fusion proteins increased ER Ca(2+) leak channel activity similar to PS1 (WT) in SK-N-SH cells, as determined by functional calcium imaging. PS1-CFP was either expressed alone or together with YFP-PS1 into SK-N-SH cell line and the interaction between YFP-PS1 and PS1-CFP was determined by Förster resonance energy transfer analysis. Our results suggest interaction between YFP-PS1 and PS1-CFP confirming the presence of a dimeric or multimeric form of PS1 in SK-N-SH cells. Lateral diffusion of PS1-CFP and YFP-PS1 in the plasma membrane of SK-N-SH cells was measured in the absence or in the presence of glycerol by fluorescence correlation spectroscopy to show that both COOH-terminal and NH2-terminal of human PS1 are located on the cytoplasmic side of the plasma membrane. Therefore, we conclude that both COOH-terminal and NH2-terminal of human PS1 may also be oriented on the cytosolic side of ER membrane.

Characterization of Pterocarpus erinaceus kino extract and its gamma-secretase inhibitory properties

ETHNOPHARMACOLOGICAL RELEVANCE

The aqueous decoction of Pterocarpus erinaceus has been traditionally used in Benin against memory troubles.

AIM OF THE STUDY

New strategies are needed against Alzheimer׳s disease (AD), for, to date, AD treatment is symptomatic and consists in drugs treating the cognitive decline. An interesting target is the β-amyloid peptide (Aβ), whose accumulation and progressive deposition into amyloid plaques are key events in AD aetiology. Identifying new and more selective γ-secretase inhibitors or modulators (none of the existing has proven so far to be selective or fully efficient) appears in this respect of particular interest. We studied the activity and mechanisms of action of Pterocarpus erinaceus kino aqueous extract, after the removal of catechic tannins (KAST).

METHODS AND RESULTS

We tested KAST at non-toxic concentrations on cells expressing the human Amyloid Precursor Protein (APP695), as well as on primary neurons. Pterocarpus erinaceus extract was found to inhibit Aβ release in both models. We further showed that KAST inhibited γ-secretase activity in cell-free and in vitro assays, strongly suggesting that KAST is a natural γ-secretase inhibitor. Importantly, this extract did not inhibit the cleavage of Notch, another γ-secretase substrate responsible for major detrimental side effects observed with γ-secretase inhibitors. Epicatechin was further identified in KAST by HPLC-MS.

CONCLUSION

Pterocarpus erinaceus kino extract appears therefore as a new γ-secretase inhibitor selective towards APP processing.

Notch signaling in serous ovarian cancer

Ovarian cancer is the most lethal of all gynecologic malignancies because women commonly present with advanced stage disease and develop chemotherapy refractory tumors. While cytoreductive surgery followed by platinum based chemotherapy are initially effective, ovarian tumors have a high propensity to recur highlighting the distinct need for novel therapeutics to improve outcomes for affected women. The Notch signaling pathway plays an established role in embryologic development and deregulation of this signaling cascade has been linked to many cancers. Recent genomic profiling of serous ovarian carcinoma revealed that Notch pathway alterations are among the most prevalent detected genomic changes. A growing body of scientific literature has confirmed heightened Notch signaling activity in ovarian carcinoma, and has utilized in vitro and in vivo models to suggest that targeting this pathway with gamma secretase inhibitors (GSIs) leads to anti-tumor effects. While it is currently unknown if Notch pathway inhibition can offer clinical benefit to women with ovarian cancer, several GSIs are currently in phase I and II trials across many disease sites including ovary. This review will provide background on Notch pathway function and will focus on the pre-clinical literature that links altered Notch signaling to ovarian cancer progression.

A synthetic antibody fragment targeting nicastrin affects assembly and trafficking of γ-secretase

The γ-secretase complex, composed of presenilin, nicastrin (NCT), anterior pharynx-defective 1 (APH-1), and presenilin enhancer 2 (PEN-2), is assembled in a highly regulated manner and catalyzes the intramembranous proteolysis of many type I membrane proteins, including Notch and amyloid precursor protein. The Notch family of receptors plays important roles in cell fate specification during development and in adult tissues, and aberrant hyperactive Notch signaling causes some forms of cancer. γ-Secretase-mediated processing of Notch at the cell surface results in the generation of the Notch intracellular domain, which associates with several transcriptional coactivators involved in nuclear signaling events. On the other hand, γ-secretase-mediated processing of amyloid precursor protein leads to the production of amyloid β (Aβ) peptides that play an important role in the pathogenesis of Alzheimer disease. We used a phage display approach to identify synthetic antibodies that specifically target NCT and expressed them in the single-chain variable fragment (scFv) format in mammalian cells. We show that expression of a NCT-specific scFv clone, G9, in HEK293 cells decreased the production of the Notch intracellular domain but not the production of amyloid β peptides that occurs in endosomal and recycling compartments. Biochemical studies revealed that scFvG9 impairs the maturation of NCT by associating with immature forms of NCT and, consequently, prevents its association with the other components of the γ-secretase complex, leading to degradation of these molecules. The reduced cell surface levels of mature γ-secretase complexes, in turn, compromise the intramembranous processing of Notch.

Signature amyloid β profiles are produced by different γ-secretase complexes

γ-Secretase complexes are involved in the generation of amyloid-β (Aβ) in the brain. Therefore, γ-secretase has been proposed as a potential therapeutic target in Alzheimer disease (AD). Targeting γ-secretase activity in AD requires the pharmacological dissociation of the processing of physiological relevant substrates and the generation of "toxic" Aβ. Previous reports suggest the differential targeting of γ-secretase complexes, based on their subunit composition, as a valid strategy. However, little is known about the biochemical properties of the different complexes, and key questions regarding their Aβ product profiles should be first addressed. Here, we expressed, purified, and analyzed, under the same conditions, the endopeptidase and carboxypeptidase-like activities of the four γ-secretase complexes present in humans. We find that the nature of the catalytic subunit in the complex affects both activities. Interestingly, PSEN2 complexes discriminate between the Aβ40 and Aβ38 production lines, indicating that Aβ generation in one or the other pathway can be dissociated. In contrast, the APH1 subunit mainly affects the carboxypeptidase-like activity, with APH1B complexes favoring the generation of longer Aβ peptides. In addition, we determined that expression of a single human γ-secretase complex in cell lines retains the intrinsic attributes of the protease while present in the membrane, providing validation for the in vitro studies. In conclusion, our data show that each γ-secretase complex produces a characteristic Aβ signature. The qualitative and quantitative differences between different γ-secretase complexes could be used to advance drug development in AD and other disorders.

Cadherin-6B is proteolytically processed during epithelial-to-mesenchymal transitions of the cranial neural crest

The epithelial-to-mesenchymal transition (EMT) is a highly coordinated process underlying both development and disease. Premigratory neural crest cells undergo EMT, migrate away from the neural tube, and differentiate into diverse cell types during vertebrate embryogenesis. Adherens junction disassembly within premigratory neural crest cells is one component of EMT and, in chick cranial neural crest cells, involves cadherin-6B (Cad6B) down-regulation. Whereas Cad6B transcription is repressed by Snail2, the rapid loss of Cad6B protein during EMT is suggestive of posttranslational mechanisms that promote Cad6B turnover. For the first time in vivo, we demonstrate Cad6B proteolysis during neural crest cell EMT, which generates a Cad6B N-terminal fragment (NTF) and two C-terminal fragments (CTF1/2). Coexpression of relevant proteases with Cad6B in vitro shows that a disintegrin and metalloproteinases (ADAMs) ADAM10 and ADAM19, together with γ-secretase, cleave Cad6B to produce the NTF and CTFs previously observed in vivo. Of importance, both ADAMs and γ-secretase are expressed in the appropriate spatiotemporal pattern in vivo to proteolytically process Cad6B. Overexpression or depletion of either ADAM within premigratory neural crest cells prematurely reduces or maintains Cad6B, respectively. Collectively these results suggest a dual mechanism for Cad6B proteolysis involving two ADAMs, along with γ-secretase, during cranial neural crest cell EMT.

Design and synthesis of bicyclic heterocycles as potent γ-secretase modulators

The evolution of amide 3 into conformationally restricted bicyclic triazolo-piperidine 14-S as a γ-secretase modulator is described. This is a potential disease modifying anti-Alzheimer's drug which demonstrated high in vitro and in vivo potency against Aβ42 peptide, reduced lipophilicity and enhanced brain free fraction compared to the previous series.

Presenilins are novel substrates for TRAF6-mediated ubiquitination

Mutations in presenilins (PS1 and PS2) have been linked to the pathogenesis of early onset familial Alzheimer's disease. Presenilins function as the catalytic component of the γ-secretase protease complexes responsible for the cleavage of the amyloid precursor protein (APP), subsequent generation of amyloid-β and associated amyloid plaques in Alzheimer's disease. Biochemical and genetic studies have revealed that through interactions with several proteins, the presenilins are functionally involved in a range of cellular processes, including the regulation of intracellular calcium homeostasis. Our group has previously reported an association between presenilins and members of the tumour necrosis factor receptor-associated factor (TRAF) family of proteins. In this study we further investigated the association between TRAF6, an E3 ubiquitin ligase, and the presenilins. Here we show that the presenilin full-length holoproteins are novel substrates of TRAF6-mediated Lysine-63-linked ubiquitination. Interestingly, co-expression of catalytically active TRAF6 with the presenilins leads to decreased turnover of PS1 full-length holoprotein accompanying elevated presenilin protein levels. Similarly, while overexpression of TRAF6 increases presenilin holoprotein levels and ubiquitination in HEK293 cells, expression of catalytically deficient TRAF6 or TRAF6-deficiency leads to a reduction in presenilin protein levels and reduced PS1 ubiquitination. We also demonstrate that TRAF6 induces PS1 gene transcription in a JNK-dependent manner. Notably, we reveal that TRAF6-mediated ubiquitination of presenilin does not affect γ-secretase enzyme activity, but may regulate presenilin function in calcium signalling. Taken together, we propose that presenilins are novel substrates for TRAF6-mediated K63-linked ubiquitination and that ubiquitination of presenilins by TRAF6 increases presenilin holoprotein levels and in conditions in which TRAF6 ubiquitination of presenilins is reduced results in reduction of calcium release from the endoplasmic reticulum.

Previously not recognized deletion in presenilin-1 (p.Leu174del.) in a patient with early-onset familial Alzheimer's disease

We report on a previously not recognized mutation in exon 6 of presenilin-1 (PSEN1) (c.520_522delCTG) in a male patient with early onset familial Alzheimer disease. The mutation results in the deletion of a leucine at amino acid position 174 of the protein. The index patient presented with progressive memory loss at 50 years of age. Initially, depression was the only ancillary symptom. At age 53 clinical diagnosis of early Alzheimer disease was made based on neuropsychological, neuroimaging, and CSF findings. The patient's father and his paternal grandmother also suffered from memory loss and cognitive decline. The clinical findings in the patient are similar to signs and symptoms in previously reported patients with missense mutations at codon 174 of PSEN1.

Design, synthesis and evaluation of tacrine-flurbiprofen-nitrate trihybrids as novel anti-Alzheimer's disease agents

To search for multifunctional anti-Alzheimer's disease (AD) agents with good safety, the previously synthesized tacrine-flurbiprofen hybrids 1a and 1b were modified into tacrine-flurbiprofen-nitrate trihybrids 3a-h. These compounds displayed comparable or higher cholinesterase inhibitory activity relative to the bivalent hybrids. Compound 3a was the most potent, which released moderate NO, exerted blood vessel relaxative activity, and showed significant Aβ inhibitory effects whereas tacrine and flurbiprofen did not exhibit any Aβ inhibitory activity at the same dose. In addition, 3a was active in improving memory impairment in vivo. More importantly, the hepatotoxicity study showed that 3a was much safer than tacrine, suggesting it might be a promising anti-AD agent for further investigation.

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

Comprehensive evidence supports that oligomerization and accumulation of amyloidogenic Aβ42 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 Aβ begins many years before the onset of clinical symptoms, and that subsequent neurodegeneration and cognitive decline may proceed independently of Aβ. 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 Aβ42 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 Aβ42 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.

Activation of the γ-secretase complex and presence of γ-secretase-activating protein may contribute to Aβ42 production in sporadic inclusion-body myositis muscle fibers

The muscle-fiber phenotype of sporadic inclusion-body myositis (s-IBM), the most common muscle disease associated with aging, shares several pathological abnormalities with Alzheimer disease (AD) brain, including accumulation of amyloid-β 42 (Aβ42) and its cytotoxic oligomers. The exact mechanisms leading to Aβ42 production within s-IBM muscle fibers are not known. Aβ42 and Aβ40 are generated after the amyloid-precursor protein (AβPP) is cleaved by β-secretase and the γ-secretase complex. Aβ42 is considered more cytotoxic than Aβ40, and it has a higher propensity to oligomerize, form amyloid fibrils, and aggregate. Recently, we have demonstrated in cultured human muscle fibers that experimental inhibition of lysosomal enzyme activities leads to Aβ42 oligomerization. In s-IBM muscle, we here demonstrate prominent abnormalities of the γ-secretase complex, as evidenced by: a) increase of γ-secretase components, namely active presenilin 1, presenilin enhancer 2, nicastrin, and presence of its mature, glycosylated form; b) increase of mRNAs of these γ-secretase components; c) increase of γ-secretase activity; d) presence of an active form of a newly-discovered γ-secretase activating protein (GSAP); and e) increase of GSAP mRNA. Furthermore, we demonstrate that experimental inhibition of lysosomal autophagic enzymes in cultured human muscle fibers a) activates γ-secretase, and b) leads to posttranslational modifications of AβPP and increase of Aβ42. Since autophagy is impaired in biopsied s-IBM muscle, the same mechanism might be responsible for its having increased γ-secretase activity and Aβ42 production. Accordingly, improving lysosomal function might be a therapeutic strategy for s-IBM patients.

Tailoring of membrane proteins by alternative splicing of pre-mRNA

Alternative splicing (AS) of RNA is a key mechanism for diversification of the eukaryotic proteome. In this process, different mRNA transcripts can be produced through altered excision and/or inclusion of exons during processing of the pre-mRNA molecule. Since its discovery, AS has been shown to play roles in protein structure, function, and localization. Dysregulation of this process can result in disease phenotypes. Moreover, AS pathways are promising therapeutic targets for a number of diseases. Integral membrane proteins (MPs) represent a class of proteins that may be particularly amenable to regulation by alternative splicing because of the distinctive topological restraints associated with their folding, structure, trafficking, and function. Here, we review the impact of AS on MP form and function and the roles of AS in MP-related disorders such as Alzheimer's disease.

Identification of a tetratricopeptide repeat-like domain in the nicastrin subunit of γ-secretase using synthetic antibodies

The γ-secretase complex, composed of presenilin, anterior-pharynx-defective 1, nicastrin, and presenilin enhancer 2, catalyzes the intramembranous processing of a wide variety of type I membrane proteins, including amyloid precursor protein (APP) and Notch. Earlier studies have revealed that nicastrin, a type I membrane-anchored glycoprotein, plays a role in γ-secretase assembly and trafficking and has been proposed to bind substrates. To gain more insights regarding nicastrin structure and function, we generated a conformation-specific synthetic antibody and used it as a molecular probe to map functional domains within nicastrin ectodomain. The antibody bound to a conformational epitope within a nicastrin segment encompassing residues 245-630 and inhibited the processing of APP and Notch substrates in in vitro γ-secretase activity assays, suggesting that a functional domain pertinent to γ-secretase activity resides within this region. Epitope mapping and database searches revealed the presence of a structured segment, located downstream of the previously identified DAP domain (DYIGS and peptidase; residues 261-502), that is homologous to a tetratricopeptide repeat (TPR) domain commonly involved in peptide recognition. Mutagenesis analyses within the predicted TPR-like domain showed that disruption of the signature helical structure resulted in the loss of γ-secretase activity but not the assembly of the γ-secretase and that Leu571 within the TPR-like domain plays an important role in mediating substrate binding. Taken together, these studies offer provocative insights pertaining to the structural basis for nicastrin function as a "substrate receptor" within the γ-secretase complex.

The mechanism of γ-Secretase dysfunction in familial Alzheimer disease

Mutations in presenilin (PSEN) and amyloid precursor protein (APP) cause dominant early-onset Alzheimer's disease (AD), but the mechanism involved is debated. Here, such mutations are shown to alter γ-secretase activity, leading to changes in Aβ peptide cleavage patterns.

The mechanisms by which mutations in the presenilins (PSEN) or the amyloid precursor protein (APP) genes cause familial Alzheimer disease (FAD) are controversial. FAD mutations increase the release of amyloid β (Aβ)42 relative to Aβ40 by an unknown, possibly gain-of-toxic-function, mechanism. However, many PSEN mutations paradoxically impair γ-secretase and ‘loss-of-function' mechanisms have also been postulated. Here, we use kinetic studies to demonstrate that FAD mutations affect Aβ generation via three different mechanisms, resulting in qualitative changes in the Aβ profiles, which are not limited to Aβ42. Loss of ɛ-cleavage function is not generally observed among FAD mutants. On the other hand, γ-secretase inhibitors used in the clinic appear to block the initial ɛ-cleavage step, but unexpectedly affect more selectively Notch than APP processing, while modulators act as activators of the carboxypeptidase-like (γ) activity. Overall, we provide a coherent explanation for the effect of different FAD mutations, demonstrating the importance of qualitative rather than quantitative changes in the Aβ products, and suggest fundamental improvements for current drug development efforts.

Therapeutic approaches to modulating Notch signaling: current challenges and future prospects

Dysregulated Notch signaling has been implicated in numerous human diseases, including a broad spectrum of cancers. Mutations in Notch1 are prevalent in T-cell acute lymphoblastic leukemia, and abnormal expression of different human Notch receptors contributes to B-cell tumors as well as cancers of the breast, lung, pancreas, skin, prostate, colon, brain and other tissues. Several γ-secretase inhibitors, small chemical compounds that were initially developed to inhibit the activity of the γ-secretase aspartyl protease in Alzheimer's disease, are now being explored for their potential chemotherapeutic applications in Notch-associated cancers. An alternative approach involves the development of antibodies to inhibit specific Notch receptors, their activating ligands, or other components of the Notch pathway in tumors. Here we review recent progress and current challenges in the use of these strategies to modulate Notch signaling for cancer therapy.

Notch signaling modulates proliferation and differentiation of intestinal crypt base columnar stem cells

Notch signaling is known to regulate the proliferation and differentiation of intestinal stem and progenitor cells; however, direct cellular targets and specific functions of Notch signals had not been identified. We show here in mice that Notch directly targets the crypt base columnar (CBC) cell to maintain stem cell activity. Notch inhibition induced rapid CBC cell loss, with reduced proliferation, apoptotic cell death and reduced efficiency of organoid initiation. Furthermore, expression of the CBC stem cell-specific marker Olfm4 was directly dependent on Notch signaling, with transcription activated through RBP-Jκ binding sites in the promoter. Notch inhibition also led to precocious differentiation of epithelial progenitors into secretory cell types, including large numbers of cells that expressed both Paneth and goblet cell markers. Analysis of Notch function in Atoh1-deficient intestine demonstrated that the cellular changes were dependent on Atoh1, whereas Notch regulation of Olfm4 gene expression was Atoh1 independent. Our findings suggest that Notch targets distinct progenitor cell populations to maintain adult intestinal stem cells and to regulate cell fate choice to control epithelial cell homeostasis.

DC2 and keratinocyte-associated protein 2 (KCP2), subunits of the oligosaccharyltransferase complex, are regulators of the gamma-secretase-directed processing of amyloid precursor protein (APP)

The oligosaccharyltransferase complex catalyzes the transfer of oligosaccharide from a dolichol pyrophosphate donor en bloc onto a free asparagine residue of a newly synthesized nascent chain during the translocation in the endoplasmic reticulum lumen. The role of the less known oligosaccharyltransferase (OST) subunits, DC2 and KCP2, recently identified still remains to be determined. Here, we have studied DC2 and KCP2, and we have established that DC2 and KCP2 are substrate-specific, affecting amyloid precursor protein (APP), indicating that they are not core components required for N-glycosylation and OST activity per se. We show for the first time that DC2 and KCP2 depletion affects APP processing, leading to an accumulation of C-terminal fragments, both C99 and C83, and a reduction in full-length mature APP. This reduction in mature APP levels was not due to a block in secretion because the levels of sAPPα secreted into the media were unaffected. We discover that DC2 and KCP2 depletion affects only the γ-secretase complex, resulting in a reduction of the PS1 active fragment blocking Aβ production. Conversely, we show that the overexpression of DC2 and KCP2 causes an increase in the active γ-secretase complex, particularly the N-terminal fragment of PS1 that is generated by endoproteolysis, leading to a stimulation of Aβ production upon overexpression of DC2 and KCP2. Our findings reveal that components of the OST complex for the first time can interact with the γ-secretase and affect the APP processing pathway.

What the halted phase III γ-secretase inhibitor trial may (or may not) be telling us

The aborted trial of semagacestat has led some to invoke unanticipated effects of γ-secretase inhibition on formation of amyloid β. However, the many substrates for γ-secretases and the varied biological effects of each of the resultant cleavage products make ascribing causality much more complex than that.

Mutagenesis mapping of the presenilin 1 calcium leak conductance pore

Missense mutations in presenilin 1 (PS1) and presenilin 2 (PS2) proteins are a major cause of familial Alzheimer disease. Presenilins are proteins with nine transmembrane (TM) domains that function as catalytic subunits of the γ-secretase complex responsible for the cleavage of the amyloid precursor protein and other type I transmembrane proteins. The water-filled cavity within presenilin is necessary to mediate the intramembrane proteolysis reaction. Consistent with this idea, cysteine-scanning mutagenesis and NMR studies revealed a number of water-accessible residues within TM7 and TM9 of mouse PS1. In addition to γ-secretase function, presenilins also demonstrate a low conductance endoplasmic reticulum Ca(2+) leak function, and many familial Alzheimer disease presenilin mutations impair this function. To map the potential Ca(2+) conductance pore in PS1, we systematically evaluated endoplasmic reticulum Ca(2+) leak activity supported by a series of cysteine point mutants in TM6, TM7, and TM9 of mouse PS1. The results indicate that TM7 and TM9, but not TM6, could play an important role in forming the conductance pore of PS1. These results are consistent with previous cysteine-scanning mutagenesis and NMR analyses of PS1 and provide further support for our hypothesis that the hydrophilic catalytic cavity of presenilins may also constitute a Ca(2+) conductance pore.

Attenuated Abeta42 responses to low potency gamma-secretase modulators can be overcome for many pathogenic presenilin mutants by second-generation compounds

Sequential processing of the β-amyloid precursor protein by β- and γ-secretase generates the amyloid β-peptide (Aβ), which is widely believed to play a causative role in Alzheimer disease. Selective lowering of the pathogenic 42-amino acid variant of Aβ by γ-secretase modulators (GSMs) is a promising therapeutic strategy. Here we report that mutations in presenilin (PS), the catalytic subunit of γ-secretase, display differential responses to non-steroidal anti-inflammatory drug (NSAID)-type GSMs and more potent second-generation compounds. Although many pathogenic PS mutations resisted lowering of Aβ(42) generation by the NSAID sulindac sulfide, the potent NSAID-like second-generation compound GSM-1 was capable of lowering Aβ(42) for many but not all mutants. We further found that mutations at homologous positions in PS1 and PS2 can elicit differential Aβ(42) responses to GSM-1, suggesting that a positive GSM-1 response depends on the spatial environment in γ-secretase. The aggressive pathogenic PS1 L166P mutation was one of the few pathogenic mutations that resisted GSM-1, and Leu-166 was identified as a critical residue with respect to the Aβ(42)-lowering response of GSM-1. Finally, we found that GSM-1-responsive and -resistant PS mutants behave very similarly toward other potent second-generation compounds of different structural classes than GSM-1. Taken together, our data show that a positive Aβ(42) response for PS mutants depends both on the particular mutation and the GSM used and that attenuated Aβ(42) responses to low potency GSMs can be overcome for many PS mutants by second generation GSMs.