Binding sites of gamma-secretase inhibitors in rodent brain: distribution, postnatal development, and effect of deafferentation

Molecular imaging of Alzheimer's disease-related gamma-secretase in mice and nonhuman primates

The pathogenesis of Alzheimer’s disease (AD) is primarily driven by brain accumulation of the amyloid-β-42 (Aβ₄₂) peptide generated from the amyloid-β precursor protein (APP) via cleavages by β- and γ-secretase. γ-Secretase is a prime drug target for AD; however, its brain regional expression and distribution remain largely unknown. Here, we are aimed at developing molecular imaging tools for visualizing γ-secretase. We used our recently developed γ-secretase modulators (GSMs) and synthesized our GSM-based imaging agent, [¹¹C]SGSM-15606. We subsequently performed molecular imaging in rodents, including AD transgenic animals, and macaques, which revealed that our probe displayed good brain uptake and selectivity, stable metabolism, and appropriate kinetics and distribution for imaging γ-secretase in the brain. Interestingly, rodents and macaques shared certain brain areas with high γ-secretase expression, suggesting a functional conservation of γ-secretase. Collectively, we have provided the first molecular brain imaging of γ-secretase, which may not only accelerate our drug discovery for AD but also advance our understanding of AD.

Presynaptic failure in Alzheimer's disease

Synaptic loss is the best correlate of cognitive deficits in Alzheimer's disease (AD). Extensive experimental evidence also indicates alterations of synaptic properties at the early stages of disease progression, before synapse loss and neuronal degeneration. A majority of studies in mouse models of AD have focused on post-synaptic mechanisms, including impairment of long-term plasticity, spine structure and glutamate receptor-mediated transmission. Here we review the literature indicating that the synaptic pathology in AD includes a strong presynaptic component. We describe the evidence indicating presynaptic physiological functions of the major molecular players in AD. These include the amyloid precursor protein (APP) and the two presenilin (PS) paralogs PS1 or PS2, genetically linked to the early-onset form of AD, in addition to tau which accumulates in a pathological form in the AD brain. Three main mechanisms participating in presynaptic functions are highlighted. APP fragments bind to presynaptic receptors (e.g. nAChRs and GABA_B receptors), presenilins control Ca²⁺ homeostasis and Ca²⁺-sensors, and tau regulates the localization of presynaptic molecules and synaptic vesicles. We then discuss how impairment of these presynaptic physiological functions can explain or forecast the hallmarks of synaptic impairment and associated dysfunction of neuronal circuits in AD. Beyond the physiological roles of the AD-related proteins, studies in AD brains also support preferential presynaptic alteration. This review features presynaptic failure as a strong component of pathological mechanisms in AD.

Presenilin-mediated cleavage of APP regulates synaptotagmin-7 and presynaptic plasticity

Mutations of the intramembrane protease presenilin (PS) or of its main substrate, the amyloid precursor protein (APP), cause early-onset form of Alzheimer disease. PS and APP interact with proteins of the neurotransmitter release machinery without identified functional consequences. Here we report that genetic deletion of PS markedly decreases the presynaptic levels of the Ca²⁺ sensor synaptotagmin-7 (Syt7) leading to impaired synaptic facilitation and replenishment of synaptic vesicles. The regulation of Syt7 expression by PS occurs post-transcriptionally and depends on γ-secretase proteolytic activity. It requires the substrate APP as revealed by the combined genetic invalidation of APP and PS1, and in particular the APP-Cterminal fragments which interact with Syt7 and accumulate in synaptic terminals under pharmacological or genetic inhibition of γ-secretase. Thus, we uncover a role of PS in presynaptic mechanisms, through APP cleavage and regulation of Syt7, that highlights aberrant synaptic vesicle processing as a possible new pathway in AD.

Mutations in presenilin, which cleaves amyloid precursor protein, cause familial Alzheimer’s Disease. Here, the authors show that loss of presenilin leads to loss of synaptotagmin 7, leading to impaired presynaptic release.

Insulin-Like Growth Factor-1 Alleviates Expression of Aβ1-40 and α-, β-, and γ-Secretases in the Cortex and Hippocampus of APP/PS1 Double Transgenic Mice

To examine the effect of subcutaneous injection of insulin-like growth factor-1 (IGF-1) on the expression of the amyloid protein (Aβ_1-40), α-secretase (ADAM10), β-secretase (BACE1), and γ-secretase (PS1) in APP/PS1 double transgenic mice. APP/PS1 double transgenic mice and wild-type mice were divided into wild-type group, wild-type therapy group, transgenome group, and transgenic therapy group. Subcutaneous injection of IGF-1 (50 μg/kg day) was administered once daily to the wild-type therapy group and transgenic therapy group for 8 weeks, respectively. The expression of the Aβ_1-40 in the cortex and hippocampus was detected by immunohistochemistry 8 weeks after administration. The levels of Aβ_1-40, DAM10, BACE1, and PS1 were analysed by Western blot. The expression of the Aβ_1-40 in the cortex of the gene therapy group was significantly lower than that of the transgenome group (p < 0.05). In APP/PS1 double transgenic mice, BACE1 expression was markedly higher in both the hippocampus (p < 0.001, p = 0.00009) and the cortex (p = 0.001), compared to that of the wild-type mice. The treatment of IGF-1 markedly reduced ADAM10 expression in the hippocampus in both transgenic mice and wild-type mice (p < 0.05), whereas the treatment mainly decreased BACE1 expression in transgenic mice but not in the wild-type mice (p < 0.05). No significant differences in PS1 levels were detected in all groups. IGF decreased Aβ_1-40 over-expression in the cortex and hippocampus and might inhibit the damage induced by Aβ_1-40 in APP/PS1 double transgenic mice. Our study suggests that IGF-1 should inhibit Aβ production through α-secretase and β-secretase but not γ-secretase.

Can brain impermeable BACE1 inhibitors serve as anti-CAA medicine?

Background

Cerebral amyloid angiopathy (CAA) is characterized by the deposition of ß-amyloid peptides (Aß) in and surrounding the wall of microvasculature in the central nervous system, together with parenchymal amyloid plaques collectively referred to as cerebral amyloidosis, which occurs in the brain commonly among the elderly and more frequently in patients with Alzheimer’s disease (AD). CAA is associated with vascular injury and may cause devastating neurological outcomes. No therapeutic approach is available for this lesion to date.

Main body

ß-Secretase 1 (BACE1) is the enzyme initiating Aß production. Brain permeable BACE1 inhibitors targeting primarily at the parenchymal plaque pathology are currently evaluated in clinical trials. This article presents findings in support of a role of BACE1 elevation in the development of CAA, in addition to plaque pathogenesis. The rationale, feasibility, benefit and strategic issues for developing BACE1 inhibitors against CAA are discussed. Brain impermeable compounds are considered preferable as they might exhibit sufficient anti-CAA efficacy without causing significant neuronal/synaptic side effects.

Conclusion

Early pharmacological intervention to the pathogenesis of CAA is expected to provide significant protection for cerebral vascular health and hence brain health. Brain impermeable BACE1 inhibitors should be optimized and tested as potential anti-CAA therapeutics.

Inverse correlation between Alzheimer's disease and cancer: implication for a strong impact of regenerative propensity on neurodegeneration?

BACKGROUND

Recent studies have revealed an inverse epidemiological correlation between Alzheimer's disease (AD) and cancer - patients with AD show a reduced risk of cancer, while cancer survivors are less likely to develop AD. These late discoveries in human subjects call for explorative studies to unlock the underlying biological mechanism, but also may shed new light on conceptual interrogation of the principal pathogenic players in AD etiology.

DISCUSSION

Here we hypothesize that this negative correlation reflects a rebalance of biosynthetic propensity between body systems under the two disease statuses. In normal condition the body cellular systems are maintained homeostatically under a balanced cell degenerative vs. surviving/regenerative propensities, determined by biosynthetic resources for anabolic processing. AD pathogenesis involves neurodegeneration but also aberrant regenerative, or reactive anabolic, burden, while cancer development is driving by uncontrolled proliferation inherent with excessive anabolic activity. The aberrant neural regenerative propensity in AD pathogenesis and the uncontrolled cellular proliferative propensity in cancer pathogeneses can manifest as competitive processes, which could result in the inverse epidemiological correlation seen among the elderly.

SUMMARY

The reduced prevalence of AD in cancer survivors may implicate a strong impact of aberrant neural regenerative burden in neurodegeneration. Further explorative studies into the inverse correlation between AD and cancer should include examinations of the proliferative propensity of tumor cells in AD models, and the development of AD-like neuropathology in cancer models as well as following anti-proliferative drug treatment.

Can BACE1 inhibition mitigate early axonal pathology in neurological diseases?

β-Secretase-1 (BACE1) is the rate-limiting enzyme for the genesis of amyloid-β (Aβ) peptides, the main constituents of the amyloid plaques in the brains of Alzheimer's disease (AD) patients. BACE1 is being evaluated as an anti-Aβ target for AD therapy. Recent studies indicate that BACE1 elevation is associated with axonal and presynaptic pathology during plaque development. Evidence also points to a biological role for BACE1 in axonal outgrowth and synapse formation during development. Axonal, including presynaptic, pathology exists in AD as well as many other neurological disorders such as Parkinson's disease, epilepsy, stroke, and trauma. In this review, we discuss pharmaceutical BACE1 inhibition as a therapeutic option for axonal pathogenesis, in addition to amyloid pathology. We first introduce the amyloidogenic processing of amyloid-β protein precursor and describe the normal expression pattern of the amyloidogenic proteins in the brain, with an emphasis on BACE1. We then address BACE1 elevation relative to amyloid plaque development, followed by updating recent understanding of a neurotrophic role of BACE1 in axon and synapse development. We further elaborate the occurrence of axonal pathology in some other neurological conditions. Finally, we propose pharmacological inhibition of excessive BACE1 activity as an option to mitigate early axonal pathology occurring in AD and other neurological disorders.

[(3) H]-L685,458 binding sites are abundant in multiple peripheral organs in rats: implications for safety assessment of putative γ-secretase targeting drugs

γ-Secretase is a multimeric enzyme complex that carries out proteolytic processing to a variety of cellular proteins. It is currently explored as a therapeutic target for Alzheimer's disease (AD) and cancer. Mechanism-based toxicity needs to be thoroughly evaluated for γ-secretase inhibitory and/or modulatory drugs. This study comparatively assessed putative γ-secretase catalytic sites in rat peripheral tissues relative to brain and explored an effort of its pharmacological inhibition on hair regeneration. Using [(3) H]-labelled L685,458, a potent γ-secretase inhibitor, as probe, we found more abundant presence of γ-secretase binding sites in the liver, gastrointestinal tract, hair follicle, pituitary gland, ovary and testis, as compared to the brain. Local application of L658,458 delayed vibrissal regrowth following whisker removal. These results suggest that γ-secretase may execute important biological functions in many peripheral systems, as in the brain. The development of γ-secretase inhibitors/modulators for AD and cancer therapy should include close monitoring of toxicological panels for hepatic, gastrointestinal, endocrinal and reproductive functions.

γ-secretase binding sites in aged and Alzheimer's disease human cerebrum: the choroid plexus as a putative origin of CSF Aβ

Deposition of β -amyloid (Aβ) peptides, cleavage products of β-amyloid precursor protein (APP) by β-secretase-1 (BACE1) and γ-secretase, is a neuropathological hallmark of Alzheimer's disease (AD). γ-Secretase inhibition is a therapeutical anti-Aβ approach, although changes in the enzyme's activity in AD brain are unclear. Cerebrospinal fluid (CSF) Aβ peptides are thought to derive from brain parenchyma and thus may serve as biomarkers for assessing cerebral amyloidosis and anti-Aβ efficacy. The present study compared active γ-secretase binding sites with Aβ deposition in aged and AD human cerebrum, and explored the possibility of Aβ production and secretion by the choroid plexus (CP). The specific binding density of [(3) H]-L-685,458, a radiolabeled high-affinity γ-secretase inhibitor, in the temporal neocortex and hippocampal formation was similar for AD and control cases with similar ages and post-mortem delays. The CP in post-mortem samples exhibited exceptionally high [(3) H]-L-685,458 binding density, with the estimated maximal binding sites (Bmax) reduced in the AD relative to control groups. Surgically resected human CP exhibited APP, BACE1 and presenilin-1 immunoreactivity, and β-site APP cleavage enzymatic activity. In primary culture, human CP cells also expressed these amyloidogenic proteins and released Aβ40 and Aβ42 into the medium. Overall, our results suggest that γ-secretase activity appears unaltered in the cerebrum in AD and is not correlated with regional amyloid plaque pathology. The CP appears to be a previously unrecognised non-neuronal contributor to CSF Aβ, probably at reduced levels in AD.

Differential effects between γ-secretase inhibitors and modulators on cognitive function in amyloid precursor protein-transgenic and nontransgenic mice

γ-Secretase inhibitors (GSIs) reduce amyloid-β (Aβ) peptides but inevitably increase the β-C-terminal fragment (β-CTF) of amyloid precursor protein (APP), potentially having undesirable effects on synapses. In contrast, γ-secretase modulators (GSMs) reduce Aβ42 without increasing β-CTF. Although the Aβ-lowering effects of these compounds have been extensively studied, little effort has been made to investigate their effects on cognition. Here, we compared the effects of two GSIs--(2S)-2-hydroxy-3-methyl-N-[(2S)-1-{[(1S)-3-methyl-2-oxo-2,3,4,5-tetrahydro-1H-3-benzazepin-1-yl]amino}-1-oxopropan-2-yl]butanamide (LY450139, semagacestat) and (2R)-2-[[(4-chlorophenyl)sulfonyl][[2-fluoro-4-(1,2,4-oxazol-3-yl)phenyl]methyl]amino-5,5,5-trifluoropentanamide (BMS-708163)--and a second-generation GSM [{(2S,4R)-1-[(4R)-1,1,1-trifluoro-7-methyloctan-4-yl]-2-[4-(trifluoromethyl)phenyl]piperidin-4-yl}acetic acid (GSM-2)] on spatial working memory in APP-transgenic (Tg2576) and nontransgenic mice using the Y-maze task. While acute dosing with either GSI ameliorated memory deficits in 5.5-month-old Tg2576 mice, these effects disappeared after 8 d subchronic dosing. Subchronic dosing with either GSI rather impaired normal cognition in 3-month-old Tg2576 mice, with no inhibition on the processing of other γ-secretase substrates, such as Notch, N-cadherin, or EphA4, in the brain. LY450139 also impaired normal cognition in wild-type mice; however, the potency was 10-fold lower than that in Tg2576 mice, indicating an APP-dependent mechanism likely with β-CTF accumulation. Immunofluorescence studies revealed that the β-CTF accumulation was localized in the presynaptic terminals of the hippocampal stratum lucidum and dentate hilus, implying an effect on presynaptic function in the mossy fibers. In contrast, both acute and subchronic dosing with GSM-2 significantly ameliorated memory deficits in Tg2576 mice and did not affect normal cognition in wild-type mice. We demonstrated a clear difference between GSI and GSM in effects on functional consequences, providing new insights into strategies for developing these drugs against Alzheimer's disease.

First and second generation γ-secretase modulators (GSMs) modulate amyloid-β (Aβ) peptide production through different mechanisms

γ-Secretase-mediated cleavage of amyloid precursor protein (APP) results in the production of Alzheimer disease-related amyloid-β (Aβ) peptides. The Aβ42 peptide in particular plays a pivotal role in Alzheimer disease pathogenesis and represents a major drug target. Several γ-secretase modulators (GSMs), such as the nonsteroidal anti-inflammatory drugs (R)-flurbiprofen and sulindac sulfide, have been suggested to modulate the Alzheimer-related Aβ production by targeting the APP. Here, we describe novel GSMs that are selective for Aβ modulation and do not impair processing of Notch, EphB2, or EphA4. The GSMs modulate Aβ both in cell and cell-free systems as well as lower amyloidogenic Aβ42 levels in the mouse brain. Both radioligand binding and cellular cross-competition experiments reveal a competitive relationship between the AstraZeneca (AZ) GSMs and the established second generation GSM, E2012, but a noncompetitive interaction between AZ GSMs and the first generation GSMs (R)-flurbiprofen and sulindac sulfide. The binding of a (3)H-labeled AZ GSM analog does not co-localize with APP but overlaps anatomically with a γ-secretase targeting inhibitor in rodent brains. Combined, these data provide compelling evidence of a growing class of in vivo active GSMs, which are selective for Aβ modulation and have a different mechanism of action compared with the original class of GSMs described.

BACE1 elevation is associated with aberrant limbic axonal sprouting in epileptic CD1 mice

The brain is capable of remarkable synaptic reorganization following stress and injury, often using the same molecular machinery that governs neurodevelopment. This form of plasticity is crucial for restoring and maintaining network function. However, neurodegeneration and subsequent reorganization can also play a role in disease pathogenesis, as is seen in temporal lobe epilepsy and Alzheimer's disease. β-Secretase-1 (BACE1) is a protease known for cleaving β-amyloid precursor protein into β-amyloid (Aβ), a major constituent in amyloid plaques. Emerging evidence suggests that BACE1 is also involved with synaptic plasticity and nerve regeneration. Here we examined whether BACE1 immunoreactivity (IR) was altered in pilocarpine-induced epileptic CD1 mice in a manner consistent with the synaptic reorganization seen during epileptogenesis. BACE1-IR increased in the CA3 mossy fiber field and dentate inner molecular layer in pilocarpine-induced epileptic mice, relative to controls (saline-treated mice and mice 24-48 h after pilocarpine-status), and paralleled aberrant expression of neuropeptide Y. Regionally increased BACE1-IR also occurred in neuropil in hippocampal area CA1 and in subregions of the amygdala and temporal cortex in epileptic mice, colocalizing with increased IR for growth associated protein 43 (GAP43) and polysialylated-neural cell adhesion molecule (PSA-NCAM), but reduced IR for microtubule-associated protein 2 (MAP2). These findings suggest that BACE1 is involved in aberrant limbic axonal sprouting in a model of temporal lobe epilepsy, warranting further investigation into the role of BACE1 in physiological vs. pathological neuronal plasticity.

γ-Secretase as a target for Alzheimer's disease

γ-Secretase is a protease complex responsible for cutting the transmembrane domain of the amyloid β-protein precursor (APP) to form the amyloid β-protein (Aβ), an aggregation-prone product that accumulates in the brain in Alzheimer's disease. As evidence suggests that Aβ is critical to Alzheimer pathogenesis, γ-secretase is considered a key target for the development of disease-modifying therapeutics. The protease complex cuts many other substrates, and some of these proteolytic events are part of signaling pathways or other important cellular functions. Among these, proteolysis of the Notch receptor is essential for signaling that is involved in a number of cell-fate determinations. Many inhibitors of γ-secretase have been identified, but it is clear that drug candidates for Alzheimer's disease should have minimal effects on the Notch signaling pathway, as serious safety issues have arisen with nonselective inhibitors. Two types of promising candidates that target this protease complex have emerged: the so-called "Notch-sparing" γ-secretase inhibitors, which block cleavage of APP selectively over that of Notch, and γ-secretase modulators, which shift the proportion of Aβ peptides produced in favor of shorter, less aggregation-prone species. The current status and prospects for these two general types of candidates will be discussed.

γ-Secretase inhibitors and modulators for Alzheimer's disease

γ-Secretase is a membrane embedded aspartyl protease complex with presenilin as the catalytic component. Along with β-secretase, this enzyme produces the amyloid β-protein of Alzheimer's disease (AD) from the amyloid β-protein precursor. Because of its key role in the pathogenesis of AD, γ-secretase has been a prime target for drug discovery, and many inhibitors of this protease have been developed. The therapeutic potential of these inhibitors is virtually negated by the fact that γ-secretase is an essential part of the Notch signaling pathway, rendering the compounds unacceptably toxic upon chronic exposure. However, these compounds have served as useful chemical tools for biological investigations. In contrast, γ-secretase modulators continue to be of keen interest as possible AD therapeutics. These modulators either shift amyloid β-protein production to shorter, less pathogenic peptides or inhibit the proteolysis of amyloid β-protein precursor selectively compared to that of Notch. The various chemical types of inhibitors and modulators will be discussed, along with their use as probes for basic biology and their potential as AD therapeutics.

Synthesis and characterization of hierarchically macroporous and mesoporous CaO-MO-SiO(2)-P(2)O(5) (M=Mg, Zn, Sr) bioactive glass scaffolds

Mg-, Zn- and Sr-doped hierarchically macroporous and mesoporous CaO-MO-SiO(2)-P(2)O(5) (M=Mg, Zn or Sr) bioactive glass (HMMBG) scaffolds were synthesized using the non-ionic block copolymer EO(20)PO(70)EO(20) and polyurethane sponges as cotemplates. The Mg-, Zn- or Sr-doped HMMBG scaffolds showed no distinct difference in phase composition, macroporous structure or pore volume from the HMMBG scaffolds without Mg, Zn or Sr. The Mg-, Zn- and Sr-doped HMMBG scaffolds showed no cytotoxicity. The gradual release of Ca, P, Si, Mg, Zn and Sr into the culture medium from these scaffolds contributed to the enhancement of the proliferation and ALP activity of mesenchymal stem cells (MSCs). The Mg-, Zn- and Sr-doped HMMBG scaffolds may be used as bone substitute materials.

BACE1 elevation is involved in amyloid plaque development in the triple transgenic model of Alzheimer's disease: differential Aβ antibody labeling of early-onset axon terminal pathology

β-amyloid precursor protein (APP) and presenilins mutations cause early-onset familial Alzheimer's disease (FAD). Some FAD-based mouse models produce amyloid plaques, others do not. β-Amyloid (Aβ) deposition can manifest as compact and diffuse plaques; it is unclear why the same Aβ molecules aggregate in different patterns. Is there a basic cellular process governing Aβ plaque pathogenesis? We showed in some FAD mouse models that compact plaque formation is associated with a progressive axonal pathology inherent with increased expression of β-secretase (BACE1), the enzyme initiating the amyloidogenic processing of APP. A monoclonal Aβ antibody, 3D6, visualized distinct axon terminal labeling before plaque onset. The present study was set to understand BACE1 and axonal changes relative to diffuse plaque development and to further characterize the novel axonal Aβ antibody immunoreactivity (IR), using triple transgenic AD (3xTg-AD) mice as experimental model. Diffuse-like plaques existed in the forebrain in aged transgenics and were regionally associated with increased BACE1 labeled swollen/sprouting axon terminals. Increased BACE1/3D6 IR at axon terminals occurred in young animals before plaque onset. These axonal elements were also co-labeled by other antibodies targeting the N-terminal and mid-region of Aβ domain and the C-terminal of APP, but not co-labeled by antibodies against the Aβ C-terminal and APP N-terminal. The results suggest that amyloidogenic axonal pathology precedes diffuse plaque formation in the 3xTg-AD mice, and that the early-onset axonal Aβ antibody IR in transgenic models of AD might relate to a cross-reactivity of putative APP β-carboxyl terminal fragments.

Modeling presenilin-dependent familial Alzheimer's disease: emphasis on presenilin substrate-mediated signaling and synaptic function

Mutations in PSEN genes, which encode presenilin proteins, cause familial early-onset Alzheimer's disease (AD). Transgenic mouse models based on coexpression of familial AD-associated presenilin and amyloid precursor protein variants successfully mimic characteristic pathological features of AD, including plaque formation, synaptic dysfunction, and loss of memory. Presenilins function as the catalytic subunit of gamma-secretase, the enzyme that catalyzes intramembraneous proteolysis of amyloid precursor protein to release beta-amyloid peptides. Familial AD-associated mutations in presenilins alter the site of gamma-secretase cleavage in a manner that increases the generation of longer and highly fibrillogenic beta-amyloid peptides. In addition to amyloid precursor protein, gamma-secretase catalyzes intramembrane proteolysis of many other substrates known to be important for synaptic function. This paper focuses on how various animal models have enabled us to elucidate the physiological importance of diverse gamma-secretase substrates, including amyloid precursor protein and discusses their roles in the context of cellular signaling and synaptic function.

gamma-Secretase dependent production of intracellular domains is reduced in adult compared to embryonic rat brain membranes

Background

γ-Secretase is an intramembrane aspartyl protease whose cleavage of the amyloid precursor protein (APP) generates the amyloid β-peptide (Aβ) and the APP intracellular domain. Aβ is widely believed to have a causative role in Alzheimer's disease pathogenesis, and therefore modulation of γ-secretase activity has become a therapeutic goal. Besides APP, more than 50 substrates of γ-secretase with different cellular functions during embryogenesis as well as adulthood have been revealed. Prior to γ-secretase cleavage, substrates are ectodomain shedded, producing membrane bound C-terminal fragments (CTFs).

Principal Findings

Here, we investigated γ-secretase cleavage of five substrates; APP, Notch1, N-cadherin, ephrinB and p75 neurotrophin receptor (p75-NTR) in membranes isolated from embryonic, young or old adult rat brain by analyzing the release of the corresponding intracellular domains (ICDs) or Aβ40 by western blot analysis and ELISA respectively. The highest levels of all ICDs and Aβ were produced by embryonic membranes. In adult rat brain only cleavage of APP and Notch1 could be detected and the Aβ40 and ICD production from these substrates was similar in young and old adult rat brain. The CTF levels of Notch1, N-cadherin, ephrinB and p75-NTR were also clearly decreased in the adult brain compared to embryonic brain, whereas the APP CTF levels were only slightly decreased.

Conclusions

In summary our data suggests that γ-secretase dependent ICD production is down-regulated in the adult brain compared to embryonic brain. In addition, the present approach may be useful for evaluating the specificity of γ-secretase inhibitors.

Neuronal migration and neurodegeneration: 2 sides of the same coin

The human genome contains only about double the number of genes in comparison to the fruit fly. This necessitates efficient recurrent usage of the same molecular components to participate in different processes. When the same proteins are used for different signaling pathways, it may be conceivable that if they go awry the phenotypic consequences may vary to a great extent. The involvement of amyloid beta precursor protein, Presenilin-1, and Tau in the pathogenesis of Alzheimer's disease is well established. Here we are highlighting a second facet of their function, their participation in developmental and adult neuronal migration. We propose that the prevalent and early Anosmia found in Alzheimer's patients may be due in part to malfunctioning of the above-mentioned proteins.

Ex vivo occupancy of gamma-secretase inhibitors correlates with brain beta-amyloid peptide reduction in Tg2576 mice

Reduction of brain beta-amyloid peptide (Abeta) synthesis by gamma-secretase inhibitors is a promising approach for the treatment of Alzheimer's disease. However, measurement of central pharmacodynamic effects in the Alzheimer's disease patient will be a challenge. Determination of drug occupancy may facilitate the analysis of efficacy of gamma-secretase inhibitors in a clinical setting. In this study, the relationship of gamma-secretase site occupancy and brain Abeta40 reduction by gamma-secretase inhibitors was examined in Tg2576 mice. [3H](2R,3S)-2-Isobutyl-N1-((S)-1-methyl-2-oxo-5-phenyl-2,3-dihydro-1H-benzo[e][1,4]diazepin-3-yl)-3-propylsuccinamide (IN973) was used as a gamma-secretase radioligand, since it has been shown to bind to gamma-secretase in rat, rhesus, and human brains with high affinity and specificity. We extended these findings by showing that [3H]IN973 bound to gamma-secretase in Tg2576 brains with an affinity, specificity, and regional localization very similar to the other species. To quantify gamma-secretase occupancy by gamma-secretase inhibitors, an ex vivo binding assay was developed using [3H]IN973 and frozen brain sections from drug-treated mice. Gamma-secretase occupancy and brain Abeta40 reduction were found to be highly correlated in animals dosed with either 2-[(1R)-1-[[4-chlorophenyl)-sulfonyl](2,5-difluorophenyl)amino] ethyl]-5-fluoro-benzenepropanoic acid (BMS-299897) or (S)-2-((S)-2-(3,5-difluorophenyl)-2-hydroxyacetamido)-N-((S,Z)-3-methyl-4-oxo-4,5-dihydro-3H-benzo[d][1,2]diazepin-5-yl)propanamide (BMS-433796) over a wide range of doses and times postdose, with the exception of the earliest times postdose. This lag in Abeta40 response to gamma-secretase inhibition is probably related to the delayed clearance of previously produced Abeta40. The excellent correlation between brain Abeta40 and gamma-secretase occupancy suggests that a positron emission tomography ligand for gamma-secretase could be a valuable biomarker to determine whether gamma-secretase inhibitors bind to their target in humans.

[3H]-L-685,458 as a radiotracer that maps γ-secretase complex in the rat brain: Relevance to Aβ genesis and presence of active presenilin-1 components

Gamma-secretase is a multimeric enzyme important for normal cell/neuronal proliferation, differentiation and plasticity. Determining in vivo gamma-secretase expression and activity remains a challenge because its subunit proteins can exist in immature and preassembled forms, but may execute cellular roles irrelevant to gamma-site cleavage. In this study, we characterized [3H]-L-685,458 as a radiotracer for the detection of active gamma-secretase in adult rat brain. In vitro autoradiography indicated that [3H]-L-685,458 binding was saturatable, displaceable by peptidomimetic and small molecule gamma-secretase inhibitors, and exhibited rapid association and dissociation kinetics. In cultured hippocampal slices, [3H]-L-685,458 binding density correlated with Abeta reduction following in-dish dosing of this radioligand or a non-radioactive gamma-secretase inhibitor. [3H]-L-685,458 binding sites in the adult brain were differentially distributed across regions and laminas, with heavy binding localized to the olfactory glomeruli, hippocampal CA3 and cerebellar molecular layer, and moderate binding in the cerebral cortex, amygdala and selected subcortical regions. All of these regions showed labeling for presenilin-1 N-terminal fragments (PS1-NTFs). A distinct correlation of dense binding sites with abundant presence of PS1-NTFs was verified in hippocampal mossy fiber terminals and olfactory bulb glomeruli, suggestive of a rich expression of gamma-secretase in the synapses at these locations that are characteristic of dynamic plasticity. Together, [3H]-L-685,458 is an excellent radiotracer for mapping active gamma-secretase complex, and may serve as a useful tool for studying the enzyme in vivo and in vitro.

Design and synthesis of benzoazepinone-derived cyclic malonamides and aminoamides as potent γ-secretase inhibitors

We report the synthesis of benzoazepine-derived cyclic malonamides (2) and aminoamides (3) as gamma-secretase inhibitors for the potential treatment of Alzheimer's disease. The in vitro structure-activity relationships of 2 and 3 along with dog pharmacokinetic results are described.

beta-Secretase expression in normal and functionally deprived rat olfactory bulbs: inverse correlation with oxidative metabolic activity

Cerebral hypometabolism, mitochondrial dysfunction, and beta-amyloid peptide (Abeta) accumulation are well-characterized manifestations of Alzheimer's disease (AD). beta-Secretase (BACE) is a prerequisite for amyloidogenesis, and it is up-regulated in sporadic AD. To explore a potential in vivo mechanism by which Abeta production is modulated by neuronal activity and/or oxidative metabolism, we compared BACE expression with cytochrome c oxidase (CO) or succinic dehydrogenase (SDH) activity in normal and functionally deprived adult rat olfactory bulb. In normal bulb, BACE was expressed predominantly in the glomerular layer, but labeling intensity within individual glomeruli varied substantially. A strong negative correlation existed between BACE labeling intensity and CO or SDH activity among individual glomeruli. Unilateral naris occlusion resulted in elevated glomerular BACE labeling in the deprived bulbs relative to the nondeprived counterparts, which was correlated with decreased CO activity in the same anatomic location. Enhanced BACE labeling was confirmed by measurements of elevated protein levels, enzymatic activity, and beta-site cleavage products of amyloid precursor protein in bulb extracts. Our findings reveal a negative regulation of BACE expression by physiological neuronal activity and an intrinsic inverse correlation between BACE expression and oxidative metabolism at the first synapse on the olfactory pathway. The results point to a biological role of BACE in synapse function and plasticity as well as a potential mechanism whereby reduced neuronal activity or metabolism could lead to amyloid overproduction in synaptic terminals.

Mitochondrial respiratory inhibition and oxidative stress elevate beta-secretase (BACE1) proteins and activity in vivo in the rat retina

Cerebral hypometabolism, oxidative stress and beta-amyloid peptide (Abeta) accumulation are key pathological events in Alzheimer's disease (AD). Beta-secretase (BACE, i.e., BACE1), a prerequisite for Abeta genesis, is elevated in sporadic AD. Recent studies show BACE upregulation in experimental conditions likely associated with energy insufficiency and/or oxidative stress. We investigated the effect of sublethal doses of mitochondrial respiratory inhibitors and potential endogenous oxidative substances on BACE expression in vivo using the retina as a model. Retinas were analyzed biochemically and anatomically 48 h following intraocular applications of mitochondrial complex I, II and IV inhibitors including rotenone, 3-nitropropionic acid and sodium azide, and plaque-containing oxidants including Fe(3+) and Abeta42 fibrils (Abeta42f). All agents caused elevations of BACE proteins and beta-site amyloid precursor protein (APP) cleavage product, beta-CTF, in retinal lysates in a dose-dependant manner. BACE activity and Abeta40 levels were also increased in agent-treated retinas relative to vehicle controls. BACE immunoreactivity in normal adult rat retina was present mostly in the plexiform layers, indicating a localization of the enzyme to synaptic terminals. No apparent change in laminar or cellular distribution of BACE labeling was detected in the experimental retinas. However, signs of neuronal stress including glial activation were observed in agent-treated retinas especially in high dosage groups. Our data suggest that mitochondrial respiratory inhibition and oxidative stress facilitate BACE expression in vivo. In addition, plaque constituents such as Fe(3+) and Abeta42f may participate in a self-enforcing cycle of amyloidogenesis via BACE upregulation.

Cellular distribution of gamma-secretase subunit nicastrin in the developing and adult rat brains

Nicastrin and presenilin 1 are integral components of the high molecular weight gamma-secretase complexes that regulate proteolytic processing of various type I membrane proteins including amyloid precursor protein and Notch. At present, there is little information regarding the cellular distribution of nicastrin in the developing or adult rat brain. We report here, using immunoblotting and immunohistochemical methods, that nicastrin in the adult rat brain is widely expressed and co-localized with presenilin 1 in select neuronal populations within all major areas, including the basal forebrain, striatum, cortex, hippocampus, amygdala, thalamus, hypothalamus, cerebellum and brainstem. We also observed dense neuropil labeling in many regions in the brain, suggesting that nicastrin gets transported to dendrites and/or axon terminals in the central nervous system. The levels of nicastrin are found to be relatively high at the early stages of postnatal development and then declined gradually to reach the adult profile. At the cellular level, nicastrin is localized predominantly in neuronal cell bodies at early postnatal stages, but is apparent both in cell bodies and dendrites/neuropil in all brain regions at the later stages. The regulation of nicastrin expression and localization during development and its distribution in a wide spectrum of neurons in the postnatal and adult rat brains provide an anatomical basis to suggest a multifunctional role for the gamma-secretase complex in the developing and adult rat brains.

A comparative assessment of γ-secretase activity in transgenic and non-transgenic rodent brain

Amyloid-beta (Abeta) deposits are one of the hallmarks of the neuropathological degeneration observed in Alzheimer's disease (AD) and Abeta concentrations have been reported to vary in different brain regions of AD patients. Abeta is produced by the sequential cleavage of amyloid precursor protein (APP) by beta-secretase and gamma-secretase, respectively. Previous studies have shown that over-expression of the gamma-secretase complex leads to increased gamma-secretase proteolytic activity increasing Abeta production. However, it is not known whether brain regions with highest Abeta concentration also express relatively higher levels of gamma-secretase activity. Accordingly, the relationship between Abeta levels and gamma-secretase activity across brain regions was investigated and correlated in the brains of transgenic and non-transgenic rodents commonly used in AD research. The data demonstrated that Abeta levels do vary in different brain regions in both transgenic and non-transgenic mice but are not correlated with regional gamma-secretase activity. Furthermore, this study demonstrated that while mutations in the APP and PS1 sequences affect the absolute Abeta levels this is not reflected in an increase in gamma-secretase proteolytic activity. The data in the current paper indicate that this assay is able to measure the level of gamma-secretase activity in rodent species. Using this methodology will aid our understanding of physiological gamma-secretase function.

Transient abundance of presenilin 1 fragments/nicastrin complex associated with synaptogenesis during development in rat cerebellum

Immunolocalization and expression of endogenous nicastrin (NCT) and presenilin 1 (PS1) fragments during postnatal development of rat cerebellum were investigated with fragment-specific antibodies. Immunoblotting for NCT revealed the expected mature and immature species, which gradually declined during development. In contrast, the expression of PS1 N-terminal fragment exhibited a peak at postnatal day 14 (P14) and declined thereafter. This chronological change was similarly observed with PS1 C-terminal fragment. Immunoprecipitation of NCT indicated its physical association with PS1 fragments. Colocalization of these molecules to the endoplasmic reticulum in cerebellar Purkinje cells indicates that they are organized into a complex in developing neurons. In addition, active sites of synaptogenesis, the base of the external granular layer and glomeruli, contained PS1 fragments and smaller amount of NCT. Isolated synaptic fraction contained both PS1 and NCT, suggesting their functional association within synapses. Transient abundance of NCT and PS1 fragments as a complex, when (P14) and where synaptogenesis is active, is consistent with intracellular trafficking of this complex in developing neurons and suggests its role as gamma-secretase in synaptogenesis.

In vitro characterization of a gamma-secretase radiotracer in mammalian brain

Inhibition of gamma-secretase is a potential therapeutic target for Alzheimer's disease (AD). The present studies have characterized the in vitro properties of a radiolabeled small molecule gamma-secretase inhibitor, [3H]compound D (Yan et al., 2004, J. Neurosci.24, 2942-2952) in mammalian brain. [3H]Compound D was shown to bind with nanomolar affinity (Kd = 0.32-1.5 nM) to a single population of saturable sites in rat, rhesus and human brain cortex homogenates, the density of binding sites ranging from 4 to 7 nM across the species. Competition studies with a structurally diverse group of gamma-secretase inhibitors with a wide range of binding affinities showed that the binding affinities of these compounds correlated well with their ability to inhibit gamma-secretase in vitro. Autoradiographic studies showed that the specific binding of [3H]compound D was widely distributed throughout adult rat, rhesus and normal human brain. There did not appear to be any difference in distribution of [3H]compound D specific binding sites in AD cortex compared with control human cortex as measured using tissue section autoradiography, nor any correlation between gamma-secretase binding and plaque burden as measured immunohistochemically. [3H]compound D is a useful tool to probe the expression and pharmacology of gamma-secretase in mammalian brain.

Gamma-secretase subunit composition and distribution in the presenilin wild-type and mutant mouse brain

Studies conducted in cell culture indicate that the gamma-secretase involved in amyloid beta-formation and Notch signaling is a multisubunit aspartic protease. Little is known, however, of the structure, function, or localization of gamma-secretase in the adult brain, or possible effects of familial Alzheimer's disease (FAD)-causing mutations on the brain protease. We report here that mouse brain contains a complex composed of gamma-secretase subunits presenilin-1 N-terminal fragment, presenilin-1 C-terminal fragment, Nicastrin, Aph-1a and Pen-2. A homozygous FAD-linked Presenilin-1 knock-in mutation does not alter relative subunit levels. Immunocytochemical localization of gamma-secretase subunits revealed overlapping but distinct regional and subcellular distributions. All subunits are expressed throughout the neuraxis predominantly in neurons, and are present in axons. Their distributions and levels of expression are unaffected by mutant presenilin-1. In a presenilin-1/amyloid precursor protein double knock-in mouse, subunits are associated with plaques, but are expressed at similar levels in amyloid-rich and -poor regions. gamma-Secretase subunits are distributed much more extensively than circumscribed amyloid deposits, suggesting the importance of other factors for localized amyloid deposition. These results indicate a widespread neuronal function for gamma-secretase in the adult brain, and suggest the pathogenic mechanism of FAD-linked mutations does not involve alterations in the composition, expression or brain distribution of the protease. The subcellular localization of gamma-secretase subunits is consistent with a nerve terminal source for amyloid aggregates.