Identity and function of ?-secretase

Gamma secretase inhibition: Effects on fertility and embryo-fetal development in rats

Avagacestat inhibits γ-secretase, a protease that cleaves the amyloid precursor protein (APP) to produce amyloid beta (Aβ). Aβ plaques, a predominant lesion in Alzheimer's patient's brain, is considered a mechanism driving neurodegeneration. As part of the nonclinical reproductive safety assessment, avagacestat effects on fertility and early embryonic development and embryo-fetal development were evaluated in rats. In the embryo-fetal development study, avagacestat was a selective developmental toxicant with dose-related increased fetal mortality, decreased fetal growth, and increased fetal malformations and variations (primarily affecting the axial and appendicular skeletal system) at ≥3 mg/kg/day. In the female fertility and early embryonic development study, avagacestat-related reductions in female fecundity at ≥5 mg/kg/day were attributed to impaired ovarian follicular development that was reflected in dose-dependent reductions in implantation sites, litter size, and gravid uterine weights. In the male fertility and early embryonic development study, avagacestat-related effects on reproduction could not be fully assessed because of low systemic exposures achieved due to extensive metabolism and clearance of the drug. The results obtained in these studies were consistent with pharmacologically mediated inhibition of γ-secretase and resulting inhibition of Notch processing and signaling that are key for embryonic development and ovary folliculogenesis. These findings are not considered a risk for late-onset AD where the patient population is ≥65 years old most with women who are post-menopausal. However, for treatment of early onset AD with a younger patient population, there are risks for reproductive or developmental toxicities with treatment with gamma secretase inhibitors like avagacestat.

Probing the Mechanisms of Inhibitors Binding to Presenilin Homologue Using Molecular Dynamics Simulations

γ-secretase is an intramembrane proteolytic enzyme that is mainly involved in the cleavage and hydrolysis of the amyloid precursor (APP). The catalytic subunit presenilin 1 (PS1) is the catalytic subunit of γ-secretase. Since it was found that PS1 is responsible for Aβ-producing proteolytic activity, which is involved in Alzheimer's disease, it is believed that reducing the activity of PS1 and preventing or delaying the production of Aβ could help treat Alzheimer's disease. Consequently, in recent years, researchers have begun investigating the potential clinical efficacy of PS1 inhibitors. Currently, most PS1 inhibitors are only used as a tool to study the structure and function of PS1, and a few inhibitors with a high selectivity have been tested in clinics. Less-selective PS1 inhibitors were found to not only inhibit Aβ production but also inhibit Notch cleavage, which led to serious adverse events. The archaeal presenilin homologue (PSH) is a surrogate protease of presenilin that is useful for agent screening. In this study, we performed 200 ns molecular dynamics simulations (MD) of four systems to explore the conformational changes of different ligands binding to PSH. Our results indicated that the PSH-L679 system formed 3-10 helices in TM4, loosening up TM4 and allowing substrates to enter the catalytic pocket, thereby making it less inhibitory. Additionally, we found that III-31-C can bring TM4 and TM6 closer, resulting in the contraction of the PSH active pocket. Altogether, these results provide the basis for the potential design of newer PS1 inhibitors.

Presenilin 1 deficiency impairs Aβ42-to-Aβ40- and angiotensin-converting activities of ACE

Introduction

Alzheimer's disease (AD) is associated with amyloid β-protein 1-42 (Aβ42) accumulation in the brain. Aβ42 and Aβ40 are the major two species generated from amyloid precursor protein. We found that angiotensin-converting enzyme (ACE) converts neurotoxic Aβ42 to neuroprotective Aβ40 in an ACE domain- and glycosylation-dependent manner. Presenilin 1 (PS1) mutations account for most of cases of familial AD and lead to an increased Aβ42/40 ratio. However, the mechanism by which PSEN1 mutations induce a higher Aβ42/40 ratio is unclear.

Methods

We over expressed human ACE in mouse wild-type and PS1-deficient fibroblasts. The purified ACE protein was used to analysis the Aβ42-to-Aβ40- and angiotensin-converting activities. The distribution of ACE was determined by Immunofluorescence staining.

Result

We found that ACE purified from PS1-deficient fibroblasts exhibited altered glycosylation and significantly reduced Aβ42-to-Aβ40- and angiotensin-converting activities compared with ACE from wild-type fibroblasts. Overexpression of wild-type PS1 in PS1-deficient fibroblasts restored the Aβ42-to-Aβ40- and angiotensin-converting activities of ACE. Interestingly, PS1 mutants completely restored the angiotensin-converting activity in PS1-deficient fibroblasts, but some PS1 mutants did not restore the Aβ42-to-Aβ40-converting activity. We also found that the glycosylation of ACE in adult mouse brain differed from that of embryonic brain and that the Aβ42-to-Aβ40-converting activity in adult mouse brain was lower than that in embryonic brain.

Conclusion

PS1 deficiency altered ACE glycosylation and impaired its Aβ42-to-Aβ40- and angiotensin-converting activities. Our findings suggest that PS1 deficiency and PSEN1 mutations increase the Aβ42/40 ratio by reducing the Aβ42-to-Aβ40-converting activity of ACE.

The Implication of Glial Metabotropic Glutamate Receptors in Alzheimer's Disease

Alzheimer's disease (AD) was first identified more than 100 years ago, yet aspects pertaining to its origin and the mechanisms underlying disease progression are not well known. To this date, there is no therapeutic approach or disease-modifying drug that could halt or at least delay disease progression. Until recently, glial cells were seen as secondary actors in brain homeostasis. Although this view was gradually refuted and the relevance of glial cells for the most diverse brain functions such as synaptic plasticity and neurotransmission was vastly proved, many aspects of its functioning, as well as its role in pathological conditions, remain poorly understood. Metabotropic glutamate receptors (mGluRs) in glial cells were shown to be involved in neuroinflammation and neurotoxicity. Besides its relevance for glial function, glutamatergic receptors are also central in the pathology of AD, and recent studies have shown that glial mGluRs play a role in the establishment and progression of AD. AD-related alterations in Ca2+ signalling, APP processing, and Aβ load, as well as AD-related neurodegeneration, are influenced by glial mGluRs. However, different types of mGluRs play different roles, depending on the cell type and brain region that is being analysed. Therefore, in this review, we focus on the current understanding of glial mGluRs and their implication in AD, providing an insight for future therapeutics and identifying existing research gaps worth investigating.

DAPT Attenuates Cadmium-Induced Toxicity in Mice by Inhibiting Inflammation and the Notch/HES-1 Signaling Axis

The small molecule DAPT inhibits the Notch signaling pathway by blocking γ-secretase mediated Notch cleavage. Given the critical role of the Notch signaling axis in inflammation, we asked whether DAPT could block Notch-mediated inflammation and thus exert neuronal protection. We established a mouse model of chronic exposure to cadmium (Cd)-induced toxicity and treated it with DAPT. DAPT was effective in ameliorating Cd-induced multi-organ damage and cognitive impairment in mice, as DAPT restored abnormal performance in the Y-maze, forced swimming and Morris water maze (MWM) tests. DAPT also reversed Cd-induced neuronal loss and glial cell activation to normal as observed by immunofluorescence and immunohistochemistry of brain tissue sections. In addition, Cd-intoxicated mice showed significantly increased levels of the Notch/HES-1 signaling axis and NF-κB, as well as decreased levels of the inflammatory inhibitors C/EBPβ and COP1. However, DAPT down regulated the elevated Notch/HES-1 signaling axis to normal, eliminating inflammation and thus protecting the nervous system. Thus, DAPT effectively eliminated the neurotoxicity of Cd, and blocking γ-secretase as well as Notch signaling axis may be a potential target for the development of neuronal protective drugs.

γ-Secretase in Alzheimer's disease

Alzheimer's disease (AD) is caused by synaptic and neuronal loss in the brain. One of the characteristic hallmarks of AD is senile plaques containing amyloid β-peptide (Aβ). Aβ is produced from amyloid precursor protein (APP) by sequential proteolytic cleavages by β-secretase and γ-secretase, and the polymerization of Aβ into amyloid plaques is thought to be a key pathogenic event in AD. Since γ-secretase mediates the final cleavage that liberates Aβ, γ-secretase has been widely studied as a potential drug target for the treatment of AD. γ-Secretase is a transmembrane protein complex containing presenilin, nicastrin, Aph-1, and Pen-2, which are sufficient for γ-secretase activity. γ-Secretase cleaves >140 substrates, including APP and Notch. Previously, γ-secretase inhibitors (GSIs) were shown to cause side effects in clinical trials due to the inhibition of Notch signaling. Therefore, more specific regulation or modulation of γ-secretase is needed. In recent years, γ-secretase modulators (GSMs) have been developed. To modulate γ-secretase and to understand its complex biology, finding the binding sites of GSIs and GSMs on γ-secretase as well as identifying transiently binding γ-secretase modulatory proteins have been of great interest. In this review, decades of findings on γ-secretase in AD are discussed.

Unraveling the Gordian knot: genetics and the troubled road to effective therapeutics for Alzheimer's disease

In the late 20th century, identification of the major protein components of amyloid plaques and neurofibrillary tangles provided a window into the molecular pathology of Alzheimer’s disease, ushering in an era of optimism that targeted therapeutics would soon follow. The amyloid-cascade hypothesis took hold very early, supported by discoveries that dominant mutations in APP, PSEN1, and PSEN2 cause the very rare, early-onset, familial forms of the disease. However, in the past decade, a stunning series of failed Phase-3 clinical trials, testing anti-amyloid antibodies or processing-enzyme inhibitors, prompts the question, What went wrong? The FDA’s recent controversial approval of aducanumab, despite widespread concerns about efficacy and safety, only amplifies the question. The assumption that common, late-onset Alzheimer’s is a milder form of familial disease was not adequately questioned. The differential timing of discoveries, including blood–brain–barrier-penetrant tracers for imaging of plaques and tangles, made it easy to focus on amyloid. Furthermore, the neuropathology community initially implemented Alzheimer’s diagnostic criteria based on plaques only. The discovery that MAPT mutations cause frontotemporal dementia with tauopathy made it even easier to overlook the tangles in Alzheimer’s. Many important findings were simply ignored. The accepted mouse models did not predict the human clinical trials data. Given this lack of pharmacological validity, input from geneticists in collaboration with neuroscientists is needed to establish criteria for valid models of Alzheimer’s disease. More generally, scientists using genetic model organisms as whole-animal bioassays can contribute to building the pathogenesis network map of Alzheimer’s disease.

Synaptic Elimination in Neurological Disorders

Synapses are well known as the main structures responsible for transmitting information through the release and recognition of neurotransmitters by pre- and post-synaptic neurons. These structures are widely formed and eliminated throughout the whole lifespan via processes termed synaptogenesis and synaptic pruning, respectively. Whilst the first pro-cess is needed for ensuring proper connectivity between brain regions and also with the periphery, the second phenomenon is important for their refinement by eliminating weaker and unnecessary synapses and, at the same time, maintaining and fa-voring the stronger ones, thus ensuring proper synaptic transmission. It is well-known that synaptic elimination is modulated by neuronal activity. However, only recently the role of the classical complement cascade in promoting this phenomenon has been demonstrated. Specifically, microglial cells recognize activated complement component 3 (C3) bound to synapses tar-geted for elimination, triggering their engulfment. As this is a highly relevant process for adequate neuronal functioning, dis-ruptions or exacerbations in synaptic pruning could lead to severe circuitry alterations that could underlie neuropathological alterations typical of neurological and neuropsychiatric disorders. In this review, we focus on discussing the possible in-volvement of excessive synaptic elimination in Alzheimer’s disease, as it has already been reported dendritic spine loss in post-synaptic neurons, increased association of complement proteins with its synapses and, hence, augmented microglia-mediated pruning in animal models of this disorder. In addition, we briefly discuss how this phenomenon could be related to other neurological disorders, including multiple sclerosis and schizophrenia.

Single administration of a novel γ-secretase modulator ameliorates cognitive dysfunction in aged C57BL/6J mice

Mutations in presenilin 1 (PS1) and presenilin 2 (PS2) are known to cause early onset of Alzheimer's disease (AD). These proteins comprise the catalytic domain of γ-secretase, which catalyzes the cleavage of β-amyloid (Aβ) from amyloid precursor protein (APP). In recent reports, PS1 and PS2 were linked to the modulation of intracellular calcium ion (Ca(2+)) dynamics, a key regulator of synaptic function. Ca(2+) dysregulation and synaptic dysfunction are leading hypothesis of cognitive dysfunctions during aging and AD progression. Accordingly, manipulations of presenilins by small molecules may have therapeutic potential for the treatment of cognitive dysfunction. In an accompanying report, we showed that chronic treatment with compound-1, a novel γ-secretase modulator (GSM), reduced Aβ production and ameliorated cognitive dysfunction in Tg2576 APP transgenic mice. Accordingly, in the present study we showed that single oral administration of compound-1 at 1 and 3mg/kg ameliorated cognitive dysfunction in aged non-transgenic mice. Moreover, compound-1 enhanced synaptic plasticity in hippocampal slices from aged C57BL/6J mice and increased messenger RNA (mRNA) expression of the immediate early gene c-fos, which has been shown to be related to synaptic plasticity in vivo. Finally, compound-1 modulated Ca(2+) signals through PS1 in mouse embryonic fibroblast cells. Taken together, compound-1 ameliorates both Aβ pathology and age-related cognitive dysfunctions. Hence, compound-1 may have potential as an early intervention for the cognitive declines that are commonly diagnosed in aged subjects, such as mild cognitive impairment (MCI) and prodromal AD.

The APP Proteolytic System and Its Interactions with Dynamic Networks in Alzheimer's Disease

Diseases of aging are often complex and multifactorial, involving many genetic and life course modifiers. Systems biology is becoming an essential tool to investigate disease initiation and disease progression. Alzheimer's disease (AD) can be used as a case study to investigate the application of systems biology to complex disease. Here we describe approaches to capturing biological data, representing data in terms of networks and interpreting their meaning in relation to the human population. We highlight issues that remain to be addressed both in terms of modeling disease progression and in relating findings to the current understanding of human disease.

Pharmacological properties of a novel and potent γ-secretase modulator as a therapeutic option for the treatment of Alzheimer's disease

Previous studies of γ-secretase inhibitors (GSIs) and Notch-sparing GSIs have shown reduced amyloid-β (Aβ) peptide levels but increased Notch-related and -unrelated adverse effects. In this study, we examined the effects of compound-1 on Aβ processing and cognitive function and assessed Notch-related and -unrelated adverse effects. Compound-1 reduced Aβ40 and Aβ42 levels but inversely increased Aβ37 in Neuro2a cells, leading to no net changes in total Aβ levels, indicating that compound-1 is a γ-secretase modulator (GSM). In time-course experiments, compound-1 reduced Aβ40 and Aβ42 levels in tris-soluble fractions, with peak reduction at approximately 3h after oral administration in C57BL mice. Moreover, at >1mg/kg, compound-1 dose dependently reduced Aβ40 and Aβ42 levels in Tg2576 mice. Chronic treatment with compound-1 in Tg2576 mice for 4 months significantly reduced both soluble and insoluble Aβ42 levels and ameliorated cognitive impairments, even after drug withdrawal for 10 days following oral administration for 2 months. In contrast with compound-1, at 100-fold higher doses (100mg/kg), the GSI LY450139 decreased HES1 mRNA expression in thymus tissues and increased the intensity of periodic acid-Schiff (PAS)-positive areas in the intestine. Moreover, the Notch-sparing GSI BMS708163 led to amyloid precursor protein (APP)-β-C-terminal fragment accumulation in mouse primary neurons. BMS708163 significantly hampered cognitive function in normal mice 1 month after administration, whereas compound-1 significantly improved cognitive function. Taken together, the present novel and orally active GSM is a promising molecule for the treatment of pathologies associated with Aβ42 and Aβ40.

Presenilin-1 regulates the expression of p62 to govern p62-dependent tau degradation

Mutations in presenilin-1 (PS1) are tightly associated with early-onset familial Alzheimer's disease (FAD), which is characterized by extracellular amyloid plaques and the accumulation of intracellular Tau. In addition to being the catalytic subunit of γ-secretase, PS1 has been shown to regulate diverse cellular functions independent of its proteolytic activity. We found that cells deficient in PS1 exhibit reduced levels of p62 protein, a cargo-receptor shuttling Tau for degradation. The downregulation of PS1 led to a significant decrease in both the protein and mRNA transcript of p62, concomitant with attenuated p62 promoter activity. This PS1-dependent regulation of p62 expression was mediated through an Akt/AP-1 pathway independent of the proteolytic activity of PS1/γ-secretase. This p62-mediated Tau degradation was significantly impaired in PS1-deficient cells, which can be rescued by ectopic expression of either p62 or wild-type PS1 but not mutant PS1 containing FAD-linked mutations. Our study suggests a novel function for PS1 in modulating p62 expression to control the proteostasis of Tau.

Betaine attenuates Alzheimer-like pathological changes and memory deficits induced by homocysteine

Hyperhomocysteinemia (Hhcy) may induce memory deficits with β-amyloid (Aβ) accumulation and tau hyperphosphorylation. Simultaneous supplement of folate and vitamin B12 partially restored the plasma homocysteine level and attenuated tau hyperphosphorylation, Aβ accumulation and memory impairments induced by Hhcy. However, folate and vitamin B12 treatment have no effects on Hhcy which has the methylenetetrahydrofolate reductase genotype mutation. In this study, we investigated the effects of simultaneous supplement of betaine on Alzheimer-like pathological changes and memory deficits in hyperhomocysteinemic rats after a 2-week induction by vena caudalis injection of homocysteine (Hcy). We found that supplementation of betaine could ameliorate the Hcy-induced memory deficits, enhance long-term potentiation (LTP) and increase dendritic branches numbers and the density of the dendritic spines, with up-regulation of NR1, NR2A, synaptotagmin, synaptophysin, and phosphorylated synapsin I protein levels. Supplementation of betaine also attenuated the Hcy-induced tau hyperphosphorylation at multiple AD-related sites through activation protein phosphatase-2A (PP2A) with decreased inhibitory demethylated PP2A(C) at Leu309 and phosphorylated PP2A(C) at Tyr307. In addition, supplementation of betaine also decreased Aβ production with decreased presenilin-1 protein levels. Our data suggest that betaine could be a promising candidate for arresting Hcy-induced AD-like pathological changes and memory deficits.

Morphologic and functional effects of gamma secretase inhibition on splenic marginal zone B cells

The γ-secretase complex is a promising target in Alzheimer's disease because of its role in the amyloidogenic processing of β-amyloid precursor protein. This enzyme also catalyzes the cleavage of Notch receptor, resulting in the nuclear translocation of intracellular Notch where it modulates gene transcription. Notch signaling is essential in cell fate decisions during embryogenesis, neuronal differentiation, hematopoiesis, and development of T and B cells, including splenic marginal zone (MZ) B cells. This B cell compartment participates in the early phases of the immune response to blood-borne bacteria and viruses. Chronic treatment with the oral γ-secretase inhibitor RO4929097 resulted in dose-dependent decreased cellularity (atrophy) of the MZ of rats and mice. Significant decreases in relative MZ B-cell numbers of RO4929097-treated animals were confirmed by flow cytometry. Numbers of MZ B cells reverted to normal after a sufficient RO4929097-free recovery period. Functional characterization of the immune response in relation to RO4929097-related MZ B cell decrease was assessed in mice vaccinated with inactivated vesicular stomatitis virus (VSV). Compared with the immunosuppressant cyclosporin A, RO4929097 caused only mild and reversible delayed early neutralizing IgM and IgG responses to VSV. Thus, the functional consequence of MZ B cell decrease on host defense is comparatively mild.

Identification of novel γ-secretase-associated proteins in detergent-resistant membranes from brain

In Alzheimer disease, oligomeric amyloid β-peptide (Aβ) species lead to synapse loss and neuronal death. γ-Secretase, the transmembrane protease complex that mediates the final catalytic step that liberates Aβ from its precursor protein (APP), has a multitude of substrates, and therapeutics aimed at reducing Aβ production should ideally be specific for APP cleavage. It has been shown that APP can be processed in lipid rafts, and γ-secretase-associated proteins can affect Aβ production. Here, we use a biotinylated inhibitor for affinity purification of γ-secretase and associated proteins and mass spectrometry for identification of the purified proteins, and we identify novel γ-secretase-associated proteins in detergent-resistant membranes from brain. Furthermore, we show by small interfering RNA-mediated knockdown of gene expression that a subset of the γ-secretase-associated proteins, in particular voltage-dependent anion channel 1 (VDAC1) and contactin-associated protein 1 (CNTNAP1), reduced Aβ production (Aβ40 and Aβ42) by around 70%, whereas knockdown of presenilin 1, one of the essential γ-secretase complex components, reduced Aβ production by 50%. Importantly, these proteins had a less pronounced effect on Notch processing. We conclude that VDAC1 and CNTNAP1 associate with γ-secretase in detergent-resistant membranes and affect APP processing and suggest that molecules that interfere with this interaction could be of therapeutic use for Alzheimer disease.

Intraperitoneal injection of JNK-specific inhibitor SP600125 inhibits the expression of presenilin-1 and Notch signaling in mouse brain without induction of apoptosis

Presenilin-1 (PS1) is a multifunctional protein involved in many cellular functions including the processing of type 1 membrane proteins such as β-amyloid precursor protein (APP) and Notch 1 receptor. PS1 acts as the catalytic subunit of the γ-secretase complex, and participates in Notch 1 processing to release Notch intracellular domain (NICD) in the cytoplasm. NICD subsequently migrates to the nucleus and causes Notch signaling by increasing the expression of the Hes1 gene. We have previously shown that inhibition of basal activity of c-jun-NH2-terminal kinase (JNK) with JNK-specific inhibitor SP600125 represses the expression of PS1 and γ-secretase activity by increasing p53 level in SK-N-SH cell line in vitro (Lee and Das, 2008, 2010). However, it is largely unknown whether PS1 can be effectively suppressed in vivo in adult mouse brains. In this report we showed that intraperitoneal (i.p) injection of JNK-specific inhibitor SP600125 decreased p-JNK level, and reduced PS1 expression by increasing p53 level in adult mouse brains. We also showed that suppression of PS1 expression by SP600125 reduced γ-secretase activity which decreased Notch 1 processing to reduce NICD in mouse brains. Furthermore, inhibition of Notch 1 processing by SP600125 decreased Notch 1 signaling by reducing the expression of the NICD target Hes1 gene in mouse brains without induction of apoptosis. These results provide insights for further study on PS1-mediated reduction of Notch 1 and APP processing for the treatment of Alzheimer's disease.

Studies of protein aggregation in A53T α-synuclein transgenic, Tg2576 transgenic, and P246L presenilin-1 knock-in cross bred mice

Synucleinopathies are a group of neurodegenerative disorders, including Parkinson disease, associated with neuronal amyloid inclusions comprised of the presynaptic protein α-synuclein (α-syn); however the biological events that initiate and lead to the formation of these inclusions are still poorly understood. There is mounting evidence that intracellular α-syn aggregation may proceed via a seeding mechanism and could spread between neurons through a prion-like mechanism that may involve other amyloidogenic proteins. Several lines of evidence suggest that Aβ peptides and/or extracellular Aβ deposits may directly or indirectly promote intracellular α-syn aggregation. To assess the effects of Aβ peptides and extracellular Aβ deposits on α-syn aggregate formation, transgenic mice (line M83) expressing A53T human α-syn that are sensitive to developing α-syn pathological inclusions were cross bred to Tg2576 transgenic mice that generated elevated levels of Aβ peptides and develop abundant Aβ plaques. In addition these mice were bred to mice with the P264L presenilin-1 knock-in mutation that further promotes Aβ plaque formation. These mice demonstrated the expected formation of Aβ plaques; however despite the accumulation of hyperphosphorylated α-syn dystrophic neurites within or surrounding Aβ plaques, no additional α-syn pathologies were observed. These studies show that Aβ amyloid deposits can cause the local aggregation of α-syn, but these did not lead to more extensive α-syn pathology.

Mitochondria and Alzheimer's disease: amyloid-beta peptide uptake and degradation by the presequence protease, hPreP

Several lines of evidence suggest mitochondrial dysfunction as a possible underlying mechanism of Alzheimer's disease (AD). Accumulation of the amyloid-beta peptide (Abeta), a neurotoxic peptide implicated in the pathogenesis of AD, has been detected in brain mitochondria of AD patients and AD transgenic mouse models. In vitro evidence suggests that the Abeta causes mitochondrial dysfunction e.g. oxidative stress, mitochondrial fragmentation and decreased activity of cytochrome c oxidase and TCA cycle enzymes. Here we review the link between mitochondrial dysfunctions and AD. In particular we focus on the mechanism for Abeta uptake by mitochondria and on the recently identified Abeta degrading protease in human brain mitochondria.

Hyperhomocysteinemia increases beta-amyloid by enhancing expression of gamma-secretase and phosphorylation of amyloid precursor protein in rat brain

Hyperhomocysteinemia and beta-amyloid (Abeta) overproduction are critical etiological and pathological factors in Alzheimer disease, respectively; however, the intrinsic link between them is still missing. Here, we found that Abeta levels increased and amyloid precursor protein (APP) levels simultaneously decreased in hyperhomocysteinemic rats after a 2-week induction by vena caudalis injection of homocysteine. Concurrently, both the mRNA and protein levels of presenilin-1, a component of gamma-secretase, were elevated, whereas the expression levels of beta-secretase and presenilin-2 were not altered. We also observed that levels of phosphorylated APP at threonine-668, a crucial site facilitating the amyloidogenic cleavage of APP, increased in rats with hyperhomocysteinemia, although the phosphorylation per se did not increase the binding capacity of pT668-APP to the secretases. The enhanced phosphorylation of APP in these rats was not relevant to either c-Jun N-terminal kinase or cyclin-dependent kinase-5. A prominent spatial memory deficit was detected in rats with hyperhomocysteinemia. Simultaneous supplementation of folate and vitamin-B12 attenuated the hyperhomocysteinemia-induced abnormal processing of APP and improved memory. Our data revealed that hyperhomocysteinemia could increase Abeta production through the enhanced expression of gamma-secretase and APP phosphorylation, causing memory deficits that could be rescued by folate and vitamin-B12 treatment in these rats. It is suggested that hyperhomocysteinemia may serve as an upstream factor for increased Abeta production as seen in patients with Alzheimer disease.

gamma-Secretase activity modulates store-operated Ca2+ entry into rat sensory neurons

Presenilin-1 is required for gamma-secretase activity, which participates in Notch receptor processing, the pathogenesis of Alzheimer's disease and the modulation of Ca(2+) signaling. We tested the hypothesis that gamma-secretase proteolytic activity modulates store-operated Ca(2+) entry (SOCE) in rat dorsal root ganglion (DRG) neurons. Depletion of intracellular Ca(2+) stores by blocking the endoplasmic reticulum (ER) Ca(2+) pump with cyclopiazonic acid (CPA) evoked a transient increase in [Ca(2+)](i) but no sustained Ca(2+) influx. However, in cells expressing a dominant negative presenilin-1 mutant (PS1-D257A), gamma-secretase activity was inhibited and treatment with CPA evoked sustained Ca(2+) influx. Similarly, pharmacologic inhibition of gamma-secretase with DAPT for 48h enhanced SOCE. SKF96365, an inhibitor of store-operated channels, blocked SOCE in cells expressing PS1-D257A. Thus, gamma-secretase proteolytic activity regulates a SOCE pathway in sensory neurons.

Tumor necrosis factor-alpha-elicited stimulation of gamma-secretase is mediated by c-Jun N-terminal kinase-dependent phosphorylation of presenilin and nicastrin

Gamma-secretase is a multiprotein complex composed of presenilin (PS), nicastrin (NCT), Aph-1, and Pen-2, and it catalyzes the final proteolytic step in the processing of amyloid precursor protein to generate amyloid-beta. Our previous results showed that tumor necrosis factor-alpha (TNF-alpha) can potently stimulate gamma-secretase activity through a c-Jun N-terminal kinase (JNK)-dependent pathway. Here, we demonstrate that TNF-alpha triggers JNK-dependent serine/threonine phosphorylation of PS1 and NCT to stimulate gamma-secretase activity. Blocking of JNK activity with a potent JNK inhibitor (SP600125) reduces TNF-alpha-triggered phosphorylation of PS1 and NCT. Consistent with this, we show that activated JNKs can be copurified with gamma-secretase complexes and that active recombinant JNK2 can promote the phosphorylation of PS1 and NCT in vitro. Using site-directed mutagenesis and a synthetic peptide, we clearly show that the Ser(319)Thr(320) motif in PS1 is an important JNK phosphorylation site that is critical for the TNF-alpha-elicited regulation of gamma-secretase. This JNK phosphorylation of PS1 at Ser(319)Thr(320) enhances the stability of the PS1 C-terminal fragment that is necessary for gamma-secretase activity. Together, our findings strongly suggest that JNK is a critical intracellular mediator of TNF-alpha-elicited regulation of gamma-secretase and governs the pivotal step in the assembly of functional gamma-secretase.

Discovery of a novel series of Notch-sparing gamma-secretase inhibitors

Using a cell-based assay, we have identified a new series of Notch-sparing gamma-secretase inhibitors from HTS screening leads 2a and 2e. Lead optimization studies led to the discovery of analog 8e with improved gamma-secretase inhibitory potency and Notch-sparing selectivity.

Regulated intramembrane proteolysis of the low-density lipoprotein receptor-related protein mediates ischemic cell death

The low-density lipoprotein receptor-related protein (LRP), a member of the low-density lipoprotein receptor gene family, mediates cellular signal transduction pathways. In this study we investigated the role of LRP in cell death. We found that incubation of mouse embryonic fibroblasts in serum-free media induces caspase-3 activation, an effect that is attenuated in LRP-deficient (LRP(-/-)) mouse embryonic fibroblasts. Since we previously demonstrated that middle cerebral artery occlusion (MCAO) in mice induces shedding of the LRP ectodomain, we investigated here whether cerebral ischemia induces regulated intramembrane proteolysis of LRP and whether this process is related to cell death. We found that MCAO induces an increase in gamma-secretase activity in the ischemic hemisphere and that treatment with the gamma-secretase inhibitor L-685,458 improves the neurological outcome and results in a 50% decrease in the volume of the ischemic lesion. Furthermore, MCAO caused nuclear translocation of the intracellular domain of LRP in neurons within the area of ischemic penumbra, and this effect was attenuated in mice treated with L-685,458. Finally, inhibition of either LRP or gamma-secretase attenuated cerebral ischemia-induced caspase-3 cleavage and apoptotic cell death. In summary, our results indicate that gamma-secretase-mediated regulated intramembrane proteolysis of LRP results in cell death under ischemic conditions.

The benefit of docosahexanoic acid on the migration of vascular smooth muscle cells is partially dependent on Notch regulation of MMP-2/-9

The Notch pathway is involved in the regulation of the migratory/proliferative phenotype acquired by vascular smooth muscle cells (VSMCs) in the pro-inflammatory context of vascular diseases. Here, we investigated whether docosahexaenoic acid (DHA), a polyunsaturated, omega-3 fatty acid, could reduce fibrinolytic/matrix-metalloproteinase (MMP) activity and whether this reduction occurs through the modulation of Notch signaling. Rat VSMCs were transdifferentiated with interleukin-1beta and then treated with DHA. Migration/proliferation was determined by performing a wound healing assay and measuring MMP-2/-9 activity, type 1 plasminogen activator inhibitor levels, and the expression of these proteins. The involvement of Notch in regulating the fibrinolytic/MMP system was evidenced using Notch pathway inhibitors and the forced expression of Notch1 and Notch3 intracellular domains. DHA significantly decreased VSMC migration/proliferation induced by interleukin-1beta as well as fibrinolytic/MMP activity. Prevention of Notch1 target gene transcription enhanced the interleukin-1beta effects on MMPs and on migration, whereas Notch3 intracellular domain overexpression reduced these effects. Finally, DHA increased Notch3 expression, Hes-1 transcription (a Notch target gene), and enhanced gamma-secretase complex activity. These results suggest that inhibition of the Notch pathway participates in the transition of VSMCs toward a migratory phenotype. These results also suggest that the beneficial inhibitory effects of DHA on fibrinolytic/MMP activity are related in part to the effects of DHA on the expression of Notch pathway components, providing new insight into the mechanisms by which omega-3 fatty acids prevent cardiovascular diseases.

Moonlighting activity of presenilin in plants is independent of gamma-secretase and evolutionarily conserved

Presenilins (PS) provide the catalytic activity for gamma-secretase, which cleaves physiologically relevant substrates including Notch, ErbB4, and APP. Recent genetic studies indicated that the contribution of PS1 to mouse development includes gamma-secretase-independent functions that cannot be easily explained by any of the demonstrated or hypothesized functions of this protein. To begin a nonbiased analysis of PS1 activity unencumbered by the dominant effect stemming from loss of Notch function, we characterized PS functions in the early land plant Physcomitrella patens, which lacks Notch, ErbB4, and APP. Removal of P. patens PS resulted in phenotypic abnormalities. Further assays performed to delineate the defective pathways in PS-deficient P. patens implicated improper function of the cytoskeletal network. Importantly, this characterization of a nonmetazoan PS uncovered a previously undescribed, evolutionarily conserved function (human PS1 can rescue the growth and light responses) that is gamma-secretase-independent (mutants with substitutions of the catalytic aspartyl residues retain the activity). Introduction of PpPS into PS-deficient mouse embryonic fibroblasts rescues normal growth rates, demonstrating that at least some metazoan functions of PS are evolutionarily conserved.

Notch signaling in blood vessels: who is talking to whom about what?

It has become increasingly clear that the Notch signaling pathway plays a critical role in the development and homeostasis of the cardiovascular system. This notion has emerged from loss- and gain-of-function analysis and from the realization that several hereditary cardiovascular disorders originate from gene mutations that have a direct impact on Notch signaling. Current research efforts are focused on determining the specific cellular and molecular effects of Notch signaling. The rationale for this has stemmed from the clinical importance and therapeutic potential of modulating vascular formation during various disease states. A more complete appreciation of Notch signaling, as it relates to vascular morphogenesis, requires an in-depth knowledge of expression patterns of the various signaling components and a comprehensive understanding of downstream targets. The goal of this review is to summarize current knowledge regarding Notch signaling during vascular development and within the adult vascular wall. Our focus is on the genetic analysis and cellular experiments that have been performed with Notch ligands, receptors, and downstream targets. We also highlight questions and controversies regarding the contribution of this pathway to vascular development.

Modulation of Abeta generation by small ubiquitin-like modifiers does not require conjugation to target proteins

The sequential processing of the APP (amyloid precursor protein) by the beta- and gamma-secretase and generation of the Abeta (amyloid-beta) peptide is a primary pathological factor in AD (Alzheimer's disease). Regulation of the processing or turnover of these proteins represents potential targets for the development of AD therapies. Sumoylation is a process by which SUMOs (small ubiquitin-like modifiers) are covalently conjugated to target proteins, resulting in a number of functional consequences. These include regulation of protein-protein interactions, intracellular trafficking and protein stability, which all have the potential to impact on several aspects of the amyloidogenic pathway. The present study examines the effects of overexpression and knockdown of the major SUMO isoforms (SUMO1, 2 and 3) on APP processing and the production of Abeta peptides. SUMO3 overexpression significantly increased Abeta40 and Abeta42 secretion, which was accompanied by an increase in full-length APP and its C-terminal fragments. These effects of SUMO3 were independent of its covalent attachment or chain formation, as mutants lacking the motifs responsible for SUMO chain formation or SUMO conjugation led to similar changes in Abeta. SUMO3 overexpression also up-regulated the expression of the transmembrane protease BACE (beta-amyloid-cleaving enzyme), but failed to affect levels of several other unrelated proteins. Suppression of SUMO1 or combined SUMO2+3 by RNA interference did not affect APP levels or Abeta production. These findings confirm a specific effect of SUMO3 overexpression on APP processing and the production of Abeta peptides but also suggest that endogenous sumoylation is not essential and likely plays an indirect role in modulating the amyloid processing pathway.

A role for presenilin in post-stress regulation: effects of presenilin mutations on Ca2+ currents in Drosophila

It has been shown that presenilin is involved in maintaining Ca2+ homeostasis in neurons, including regulating endoplasmic reticulum (ER) Ca2+ storage. From studies of primary cultures and cell lines, however, its role in stress-induced responses is still controversial. In the present study we analyzed the effects of presenilin mutations on membrane currents and synaptic functions in response to stress using an in vivo preparation. We examined voltage-gated K+ and Ca2+ currents at the Drosophila larval neuromuscular junction (NMJ) with voltage-clamp recordings. Our data showed that both currents were generally unaffected by loss-of-function or Alzheimer's disease (AD) -associated presenilin mutations under normal or stress conditions induced by heat shock (HS) or ER stress. In larvae expressing the mutant presenilins, prolonged Ca2+ tail current, reflecting slower deactivation kinetics of Ca2+ channels, was observed 1 day after stress treatments were terminated. It was further demonstrated that the L-type Ca2+ channel was specifically affected under these conditions. Moreover, synaptic plasticity at the NMJ was reduced in larvae expressing the mutant presenilins. At the behavioral level, memory in adult flies was impaired in the presenilin mutants 1 day after HS. The results show that presenilin function is important during the poststress period and its impairment contributes to memory dysfunction observed during adaptation to normal conditions after stress. Our findings suggest a new stress-related mechanism by which presenilin may be implicated in the neuropathology of AD.

Cleavage of E-cadherin: a mechanism for disruption of the intestinal epithelial barrier by Candida albicans

To investigate how intestinal epithelial cells respond to contact with Candida albicans, an organism able to invade the bloodstream via the gastrointestinal tract, we focused on the junction proteins occludin, E-cadherin, and desmoglein-2. The levels of these 3 junction proteins were reduced in lysates of human intestinal epithelial monolayers (Caco-2) after a 24-h inoculation with C. albicans, compared with lysates from Saccharomyces cerevisiae-inoculated monolayers. Treatment with pepstatin A did not change the effect of C. albicans on full-length occludin, desmoglein-2, and E-cadherin; however, pepstatin A enhanced the accumulation of a 35-kDa fragment derived from the intracellular portion of E-cadherin. This 35-kDa fragment also accumulated in the presence of gamma-secretase inhibitors. These observations suggest that enhancement of E-cadherin cleavage by C. albicans generates an intracellular E-cadherin fragment that can serve as a substrate for gamma-secretase. An 89-kDa extracellular fragment of E-cadherin was detected in supernatants of C. albicans-inoculated monolayers; this cleavage event was insensitive to both pepstatin A and gamma-secretase inhibitors. Transepithelial electrical resistance, a measure of monolayer integrity, decreased significantly and synchronously with increased generation of the 89-kDa extracellular E-cadherin fragment. Cleavage of E-cadherin may destabilize the homotypic interactions between adjacent epithelial cells and could contribute to loss of monolayer integrity. These experiments identify 2 E-cadherin cleavage events that are enhanced by contact with C. albicans: an intracellular cleavage event that generates a substrate for gamma-secretase and an extracellular cleavage event that is temporally associated with an increase in monolayer permeability.

Cholesterol-like effects of selective cyclooxygenase inhibitors and fibrates on cellular membranes and amyloid-beta production

Strong evidence suggests a mechanistic link between cholesterol metabolism and the formation of amyloid-beta peptides, the principal constituents of senile plaques found in the brains of patients with Alzheimer's disease. Here, we show that several fibrates and diaryl heterocycle cyclooxygenase inhibitors, among them the commonly used drugs fenofibrate and celecoxib, exhibit effects similar to those of cholesterol on cellular membranes and amyloid precursor protein (APP) processing. These drugs have the same effects on membrane rigidity as cholesterol, monitored here by an increase in fluorescence anisotropy. The effect of the drugs on cellular membranes was also reflected in the inhibitory action on the sarco(endo)plasmic reticulum Ca(2+)-ATPase, which is known to be inhibited by excess ordering of membrane lipids. The drug-induced decrease of membrane fluidity correlated with an increased association of APP and its beta-site cleaving enzyme BACE1 with detergent-resistant membranes (DRMs), which represent membrane clusters of substantial rigidity. DRMs are hypothesized to serve as platforms for the amyloidogenic processing of APP. According to this hypothesis, both cholesterol and the examined compounds stimulated the beta-secretase cleavage of APP, resulting in a massive increase of secreted amyloid-beta peptides. The membrane-ordering potential of the drugs was observed in a cell-free assay, suggesting that the amyloid-beta promoting effect was analog to cholesterol due to primary effect on membrane rigidity. Because fenofibrate and celecoxib are widely used in humans as hypolipidemic drugs for prevention of atherosclerosis and as anti-inflammatory drugs against arthritis, possible side effects should be considered upon long-term clinical application.

APL-1, a Caenorhabditis elegans protein related to the human beta-amyloid precursor protein, is essential for viability

Dominant mutations in the amyloid precursor protein (APP) gene are associated with rare cases of familial Alzheimer's disease; however, the normal functions of APP and related proteins remain unclear. The nematode Caenorhabditis elegans has a single APP-related gene, apl-1, that is expressed in multiple tissues. Loss of apl-1 disrupts several developmental processes, including molting and morphogenesis, and results in larval lethality. The apl-1 lethality can be rescued by neuronal expression of the extracellular domain of APL-1. These data highlight the importance of the extracellular domain of an APP family member and suggest that APL-1 acts noncell-autonomously during development. Overexpression of APL-1 also causes several defects, including a high level of larval lethality. Decreased activity of sel-12, a C. elegans homologue of the human gamma-secretase component presenilin 1, partially rescues the lethality associated with APL-1 overexpression, suggesting that SEL-12 activity regulates APL-1 activity either directly or indirectly.

Unique neuronal functions of cathepsin L and cathepsin B in secretory vesicles: biosynthesis of peptides in neurotransmission and neurodegenerative disease

Proteases are required for the production of peptide neurotransmitters and toxic peptides in neurodegenerative diseases. Unique roles of the cysteine proteases cathepsin L and cathepsin B in secretory vesicles for the production of biologically active peptides have been demonstrated in recent studies. Secretory vesicle cathepsin L participates in the proteolytic conversion of proenkephalin into the active enkephalin, an opioid peptide neurotransmitter that mediates pain relief. Moreover, recent findings provide evidence that cathepsin B in regulated secretory vesicles participates in the production of toxic beta-amyloid peptides that are known to accumulate extracellularly in Alzheimer's disease brains. The neurobiological functions of cathepsins L and B demonstrate that these secretory vesicle cysteine proteases produce biologically active peptides. These results demonstrate newly identified roles for cathepsins L and B in neurosecretory vesicles in the production of biologically active peptides.

Transgenerational epigenetic imprinting of the male germline by endocrine disruptor exposure during gonadal sex determination

Embryonic exposure to the endocrine disruptor vinclozolin at the time of gonadal sex determination was previously found to promote transgenerational disease states. The actions of vinclozolin appear to be due to epigenetic alterations in the male germline that are transmitted to subsequent generations. Analysis of the transgenerational epigenetic effects on the male germline (i.e. sperm) identified 25 candidate DNA sequences with altered methylation patterns in the vinclozolin generation sperm. These sequences were identified and mapped to specific genes and noncoding DNA regions. Bisulfite sequencing was used to confirm the altered methylation pattern of 15 of the candidate DNA sequences. Alterations in the epigenetic pattern (i.e. methylation) of these genes/DNA sequences were found in the F2 and F3 generation germline. Therefore, the reprogramming of the male germline involves the induction of new imprinted-like genes/DNA sequences that acquire an apparent permanent DNA methylation pattern that is passed at least through the paternal allele. The expression pattern of several of the genes during embryonic development were found to be altered in the vinclozolin F1 and F2 generation testis. A number of the imprinted-like genes/DNA sequences identified are associated with epigenetic linked diseases. In summary, an endocrine disruptor exposure during embryonic gonadal sex determination was found to promote an alteration in the epigenetic (i.e. induction of imprinted-like genes/DNA sequences) programming of the male germline, and this is associated with the development of transgenerational disease states.

2,6-Disubstituted N-arylsulfonyl piperidines as gamma-secretase inhibitors

A novel piperidine series of gamma-secretase inhibitors, potentially useful for the treatment of Alzheimer's disease, is disclosed. SAR investigation revealed the requirement for cis-stereochemistry of the substituents attached to the core, which resulted in the chair-like diaxial conformation of the piperidine ring. The series was optimized to provide inhibitors with IC(50)'s in the single-digit nanomolar range. Absolute stereochemistry of the active enantiomer was assigned.

Mapping of Interaction Domains Mediating Binding between BACE1 and RTN/Nogo Proteins

BACE1 is a membrane-bound aspartyl protease that specifically cleaves amyloid precursor protein (APP) at the beta-secretase site. Membrane bound reticulon (RTN) family proteins interact with BACE1 and negatively modulate BACE1 activity through preventing access of BACE1 to its cellular APP substrate. Here, we focused our study on RTN3 and further show that a C-terminal QID triplet conserved among mammalian RTN members is required for the binding of RTN to BACE1. Although RTN3 can form homo- or heterodimers in cells, BACE1 mainly binds to the RTN monomer and disruption of the QID triplet does not interfere with the dimerization. Correspondingly, the C-terminal region of BACE1 is required for the binding of BACE1 to RTNs. Furthermore, we show that the negative modulation of BACE1 by RTN3 relies on the binding of RTN3 to BACE1. The knowledge from this study may potentially guide discovery of small molecules that can mimic the effect of RTN3 on the inhibition of BACE1 activity.

Convergence of genes implicated in Alzheimer's disease on the cerebral cholesterol shuttle: APP, cholesterol, lipoproteins, and atherosclerosis

Polymorphic genes associated with Alzheimer's disease (see ) delineate a clearly defined pathway related to cerebral and peripheral cholesterol and lipoprotein homoeostasis. They include all of the key components of a glia/neurone cholesterol shuttle including cholesterol binding lipoproteins APOA1, APOA4, APOC1, APOC2, APOC3, APOD, APOE and LPA, cholesterol transporters ABCA1, ABCA2, lipoprotein receptors LDLR, LRP1, LRP8 and VLDLR, and the cholesterol metabolising enzymes CYP46A1 and CH25H, whose oxysterol products activate the liver X receptor NR1H2 and are metabolised to esters by SOAT1. LIPA metabolises cholesterol esters, which are transported by the cholesteryl ester transport protein CETP. The transcription factor SREBF1 controls the expression of most enzymes of cholesterol synthesis. APP is involved in this shuttle as it metabolises cholesterol to 7-betahydroxycholesterol, a substrate of SOAT1 and HSD11B1, binds to APOE and is tethered to LRP1 via APPB1, APBB2 and APBB3 at the cytoplasmic domain and via LRPAP1 at the extracellular domain. APP cleavage products are also able to prevent cholesterol binding to APOE. BACE cleaves both APP and LRP1. Gamma-secretase (PSEN1, PSEN2, NCSTN) cleaves LRP1 and LRP8 as well as APP and their degradation products control transcription factor TFCP2, which regulates thymidylate synthase (TS) and GSK3B expression. GSK3B is known to phosphorylate the microtubule protein tau (MAPT). Dysfunction of this cascade, carved out by genes implicated in Alzheimer's disease, may play a major role in its pathology. Many other genes associated with Alzheimer's disease affect cholesterol or lipoprotein function and/or have also been implicated in atherosclerosis, a feature of Alzheimer's disease, and this duality may well explain the close links between vascular and cerebral pathology in Alzheimer's disease. The definition of many of these genes as risk factors is highly contested. However, when polymorphic susceptibility genes belong to the same signaling pathway, the risk associated with multigenic disease is better related to the integrated effects of multiple polymorphisms of genes within the same pathway than to variants in any single gene [Wu, X., Gu, J., Grossman, H.B., Amos, C.I., Etzel, C., Huang, M., Zhang, Q., Millikan, R.E., Lerner, S., Dinney, C.P., Spitz, M.R., 2006. Bladder cancer predisposition: a multigenic approach to DNA-repair and cell-cycle-control genes. Am. J. Hum. Genet. 78, 464-479.]. Thus, the fact that Alzheimer's disease susceptibility genes converge on a clearly defined signaling network has important implications for genetic association studies.

Studies to investigate the in vivo therapeutic window of the gamma-secretase inhibitor N2-[(2S)-2-(3,5-difluorophenyl)-2-hydroxyethanoyl]-N1-[(7S)-5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]azepin-7-yl]-L-alaninamide (LY411,575) in the CRND8 mouse

Accumulation of amyloid beta-peptide (Abeta) is considered a key step in the etiology of Alzheimer's disease. Abeta is produced by sequential cleavage of the amyloid precursor protein by beta- and gamma-secretase enzymes. Consequently, inhibition of gamma-secretase provides a promising therapeutic approach to treat Alzheimer's disease. Preclinically, several gamma-secretase inhibitors have been shown to reduce plasma and brain Abeta, although they also produce mechanism-based side effects, including thymus atrophy and intestinal goblet cell hyperplasia. The present studies sought to establish an efficient screen for determining the therapeutic window of gamma-secretase inhibitors and to test various means of maximizing this window. Six-day oral administration of the gamma-secretase inhibitor N(2)-[(2S)-2-(3,5-difluorophenyl)-2-hydroxyethanoyl]-N(1)-[(7S)-5-methyl-6-oxo-6,7-dihydro-5H-dibenzo[b,d]azepin-7-yl]-l-alaninamide (LY411,575) reduced cortical Abeta(40) in young (preplaque) transgenic CRND8 mice (ED(50) approximately 0.6 mg/kg) and produced significant thymus atrophy and intestinal goblet cell hyperplasia at higher doses (>3 mg/kg). The therapeutic window was similar after oral and subcutaneous administration and in young and aged CRND8 mice. Both the thymus and intestinal side effects were reversible after a 2-week washout period. Three-week treatment with 1 mg/kg LY411,575 reduced cortical Abeta(40) by 69% without inducing intestinal effects, although a previously unreported change in coat color was observed. These studies demonstrate that the 3- to 5-fold therapeutic window for LY411,575 can be exploited to obtain reduction in Abeta levels without induction of intestinal side effects, that intermittent treatment could be used to mitigate side effects, and that a 6-day dosing paradigm can be used to rapidly determine the therapeutic window of novel gamma-secretase inhibitors.

COUP-TFI controls Notch regulation of hair cell and support cell differentiation

The orphan nuclear receptor COUP-TFI (Nr2f1) regulates many aspects of mammalian development, but little is known about its role in cochlear hair cell and Deiter's support cell development. The COUP-TFI knockout (COUP-TFI(-/-)) has a significant increase in hair cell (HC) number in the mid-to-apical turns. The total number of hair cells is not increased over wild type, perhaps because of displaced hair cells and a shortened cochlear duct. This implicates a defect of convergent-extension in the COUP-TFI(-/-) duct. In addition, excess proliferation in the COUP-TFI(-/-) sensory epithelium indicates that the origin of the extra HCs in the apex is complex. Because loss-of-function studies of Notch signaling components have similar phenotypes, we investigated Notch regulation of hair cell differentiation in COUP-TFI(-/-) mice and confirmed misregulation of Notch signaling components, including Jag1, Hes5 and in a manner consistent with reduced Notch signaling, and correlated with increases in hair cell and support cell differentiation. The disruption of Notch signaling by a gamma-secretase inhibitor in an in vitro organ culture system of wild-type cochleae resulted in a reduction in expression of the Notch target gene Hes5 and an increase in hair cell differentiation. Importantly, inhibition of Notch activity resulted in a greater increase in hair cell differentiation in COUP-TFI(-/-) cochlear cultures than in wild-type cultures, suggesting a hypersensitivity to Notch inactivation in COUP-TFI(-/-) cochlea, particularly at the apical turn. Thus, we present evidence that reduced Notch signaling contributes to increases in hair cell and support cell differentiation in COUP-TFI(-/-) mice, and suggest that COUP-TFI is required for Notch regulation of hair cell and support cell differentiation.

Ontogenic reduction of Aph-1b mRNA and gamma-secretase activity in rats with a complex neurodevelopmental phenotype

Selectively bred apomorphine susceptible (APO-SUS) rats display a complex behavioral phenotype remarkably similar to that of human neurodevelopmental disorders, such as schizophrenia. We recently found that the APO-SUS rats have only one or two Aph-1b gene copies (I/I and II/II rats, respectively), whereas their phenotypic counterpart has three copies (III/III). Aph-1b is a component of the gamma-secretase enzyme complex that is involved in multiple (neuro)developmental signaling pathways. Nevertheless, surprisingly little is known about gamma-secretase expression during development. Here, we performed a longitudinal quantitative PCR study in embryos and the hippocampus of I/I, II/II and III/III rats, and found gene-dosage dependent differences in Aph-1b, but not Aph-1a, mRNA expression throughout pre- and post-natal development. On the basis of the developmental mRNA profiles, we assigned relative activities to the various Aph-1a and -1b gene promoters. Furthermore, in the three rat lines, we observed both tissue-specific and temporal alterations in gamma-secretase cleavage activity towards one of its best-known substrates, the amyloid-beta precursor protein APP. We conclude that the low levels of Aph-1b mRNA and gamma-secretase activity observed in the I/I and II/II rats during the entire developmental period may well underlie their complex phenotype.

Modulation of neutrophil apoptosis by β-amyloid proteins

This study examined the effect of amyloid beta peptide (Abeta) and the secretase inhibitors of amyloid precursor proteins (APP) on the spontaneous apoptosis of neutrophils. Abeta(1-40) decreased the apoptotic rate of neutrophils. The delayed apoptosis by Abeta was not blocked by pertussis toxin and N-formyl peptide receptor-like 1 antagonistic peptide, WRWWWW. The inhibitors of phoshoinositide 3-kinase (LY294002), phospholipase C (U73122), or Ca++-dependent protein kinase C (Go6976) abrogated the anti-apoptotic effect of Abeta on neutrophils. Moreover, the Abeta-induced delay of apoptosis was inhibited by a calcium chelator, BAPTA/AM. The amount of the APP protein was reduced in the cultured neutrophils and the APP level in the Abeta or pancaspase-treated neutrophils was lower than that in the cultured neutrophils. However, the reduction in APP level was recovered after treating them with the secretase inhibitors or anti-Fas antibody. The exogenous addition of cell permeable beta- and gamma-secretase inhibitors resulted in an increase in the rate of the apoptosis. The regulation of neutrophil apoptosis by the addition of Abeta and secretase inhibitors occurred via the caspase -8, -9, -3, and mitochondrial-dependent pathways. This suggests that the intracellular beta-amyloid proteins play a role as regulating factor of neutrophil survival and that Abeta-induced delay of apoptosis is mediated by other receptors rather than a seven-transmembrane G protein-coupled receptor(s).

Notch signaling regulates midline cell specification and proliferation in zebrafish

Notochord and floor plate cells are sources of molecules that pattern tissues near the midline, including the spinal cord. Hypochord cells are also found at the midline of anamniote embryos and are important for aorta development. Delta-Notch signaling regulates midline patterning in the dorsal organizer by inhibiting notochord formation and promoting hypochord and possibly floor plate development, but the precise mechanisms by which this regulation occurs are unknown. We demonstrate here that floor plate and hypochord cells arise from distinct regions of the zebrafish shield. Blocking Notch signaling during gastrulation entirely prevented hypochord specification but only reduced the number of floor plate cells that developed compared to control embryos. In contrast, elevation of Notch signaling at the beginning of gastrulation caused expansion of hypochord at the expense of notochord, but floor plate was not affected. A cell proliferation assay revealed that Notch signaling maintains dividing floor plate progenitors. Together, our results indicate that Notch signaling regulates allocation of appropriate numbers of different midline cells by different mechanisms.

Presenilin-dependent gamma-secretase on plasma membrane and endosomes is functionally distinct

The presenilin (PS)/gamma-secretase complex, which contains not only PS but also Aph-1, PEN-2, and nicastrin, mediates proteolysis of the transmembrane domain of beta-amyloid protein precursor (betaAPP). Intramembrane proteolysis occurs at the interface between the membrane and cytosol (epsilon-site) and near the middle of the transmembrane domain (gamma-site), generating the betaAPP intracellular domain (AICD) and Alzheimer disease-associated Abeta, respectively. Both cleavage sites exhibit some diversity. Changes in the precision of gamma-cleavage, which potentially results in secretion of pathogenic Abeta42, have been intensively studied, while those of epsilon-cleavage have not. Although a number of PS-associated factors have been identified, it is unclear whether any of them physiologically regulate the precision of cleavage by PS/gamma-secretase. Moreover, there is currently no clear evidence of whether PS/gamma-secretase function differs according to the subcellular site. Here, we show that endocytosis affects the precision of PS-dependent epsilon-cleavage in cell culture. Relative production of longer AICDepsilon49 increases on the plasma membrane, whereas that of shorter AICDepsilon51 increases on endosomes; however, this occurs without a concomitant major change in the precision of cleavage at gamma-sites. Moreover, very similar changes in the precision of epsilon-cleavage are induced by alteration of the pH. Our findings demonstrate that the precision of epsilon-cleavage by PS/gamma-secretase changes depending upon the conditions and the subcellular location. These results suggest that the precision of cleavage by the PS/gamma-secretase complex may be physiologically regulated by the subcellular location and conditions.