Aspartate mutations in presenilin and gamma-secretase inhibitors both impair notch1 proteolysis and nuclear translocation with relative preservation of notch1 signaling

TDP43 is a newly identified substrate for PS1, enhancing the expression of APP following cleavage

Alzheimer's disease (AD) has been comprehensively studied; however, most research has focused on Aβ plaque deposition and Tau protein phosphorylation. Emerging evidence suggests that TDP43 may be significantly involved AD and potentially worsening its pathology. To investigate the role of TDP43 in the pathological development of AD, we employed the STRING protein network interaction tool to identify potential relationships between TDP43 and other proteins, including PS1 and APP. Subsequent co-immunoprecipitation experiments were conducted, and the results indicated that TDP43 could interact with PS1. Further studies have shown that the interaction between the two would also lead to the loss of nuclear localization of TDP43. We also found that overexpression or knockdown of PS1 in both primary cells, HeLa and NSC34 cells indicated that TDP43 is likely to be a substrate of PS1. Subsequent use of the L685,458 and z-VAD, the PS1 mutant plasmids D257A and D385A, and bioinformatics approaches demonstrated that PS1 is dependent on γ-secretase and caspase activity to cleave TDP43, and that the cleavage site is at amino acid 315 of TDP43. Besides, our study demonstrated that the interaction of TDP43 with PS1 in primary cells, HeLa and NSC34 cells can promote APP expression, resulting in elevated Aβ levels. Finally, we investigated whether the interaction between TDP43 and PS1 affects the expression of other PS1 substrates, Notch and E-cadherin. Our results demonstrated that TDP43 cleaved by PS1 only promoted APP expression and had no effect on other PS1 substrates. In conclusion, these results suggest that TDP43 is a new substrate of PS1 and that TDP43 cleaved by PS1 promotes APP expression, which leads to increased Aβ content, which may explain why TDP43 promotes AD development. These insights enhance our understanding of TDP43's role in AD development.

Generation and Characterization of a Novel Knockin Mouse Model Expressing PSEN1 D385A: Implications for Investigating Herbal Drug Effects in γ-Secretase Activity

Background

Presenilin (PSEN, PS) is essential for γ-secretase function, and mutations can disrupt amyloid-β (Aβ) production in familial Alzheimer's disease. Targeting γ-secretase is complex due to its broad involvement in physiological processes.

Objective

Our aim was to create a novel knockin (KI) mouse model expressing PSEN1 D385A mutation and investigate the efficacy of a Geniposide and Ginsenoside Rg1 combination (NeuroProtect modified formula, NP-2) in restoring γ-secretase activity.

Methods

Using gene manipulation, we established the PS1 D385A KI mouse model and confirmed the mutation, mRNA, and protein levels using Southern blotting, northern blotting, and western blotting, respectively. In vitro γ-secretase assay was conducted to measure γ-secretase activity, while histological analyses examined neurogenesis effects. NP-2 administration evaluated its impact on γ-secretase activity.

Results

The PS1 D385A KI homozygotes displayed severe cerebral hemorrhage, postnatal lethality, developmental disorders, reduced proliferation of neural progenitor cells, and disrupted γ-secretase function. The mutation abolished PS1 protein self-shearing, leading to compromised γ-secretase activity. NP-2 intervention effectively restored γ-secretase activity in the heterozygous mice.

Conclusions

PS1 D385A mutant disrupted PS1 protein self-cleaving, impairing γ-secretase activity in KI mice. NP-2 restored γ-secretase function, offering potential for novel AD treatment strategies despite the challenges posed by γ-secretase's complex role in physiological processes.

Comprehensive analysis reveals the potential value of inflammatory response genes in the prognosis, immunity, and drug sensitivity of lung adenocarcinoma

Background

Although the relationship between inflammatory response and tumor has been gradually recognized, the potential implications of of inflammatory response genes in lung adenocarcinoma (LUAD) remains poorly investigated.

Methods

RNA sequencing and clinical data were obtained from multiple independent datasets (GSE29013, GSE30219, GSE31210, GSE37745, GSE42127, GSE50081, GSE68465, GSE72094, TCGA and GTEx). Unsupervised clustering analysis was used to identify different tumor subtypes, and LASSO and Cox regression analysis were applied to construct a novel scoring tool. We employed multiple algorithms (ssGSEA, CIBERSORT, MCP counter, and ESTIMATE) to better characterize the LUAD tumor microenvironment (TME) and immune landscapes. GSVA and Metascape analysis were performed to investigate the biological processes and pathway activity. Furthermore, ‘pRRophetic’ R package was used to evaluate the half inhibitory concentration (IC50) of each sample to infer drug sensitivity.

Results

We identified three distinct tumor subtypes, which were related to different clinical outcomes, biological pathways, and immune characteristics. A scoring tool called inflammatory response gene score (IRGS) was established and well validated in multiple independent cohorts, which could well divide patients into two subgroups with significantly different prognosis. High IRGS patients, characterized by increased genomic variants and mutation burden, presented a worse prognosis, and might show a more favorable response to immunotherapy and chemotherapy. Additionally, based on the cross-talk between TNM stage, IRGS and patients clinical outcomes, we redefined the LUAD stage, which was called ‘IRGS-Stage’. The novel staging system could distinguish patients with different prognosis, with better predictive ability than the conventional TNM staging.

Conclusions

Inflammatory response genes present important potential value in the prognosis, immunity and drug sensitivity of LUAD. The proposed IRGS and IRGS-Stage may be promising biomarkers for estimating clinical outcomes in LUAD patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12920-022-01340-7.

The GSM BPN-15606 as a Potential Candidate for Preventative Therapy in Alzheimer's Disease

BACKGROUND

In the amyloid hypothesis of Alzheimer's disease (AD), the dysregulation of amyloid-β protein (Aβ) production and clearance leads to amyloid deposits, tau tangles, neuronal loss, and cognitive dysfunction. Thus far, therapies targeting the enzymes responsible for Aβ production have been found ineffective or having significant side effects.

OBJECTIVE

To test whether a γ-secretase modulator, BPN-15606, is an effective disease-modifying or preventative treatment in the PSAPP mouse model of AD.

METHODS

We treated pre-plaque (3-month-old) and post-plaque (6-month-old) PSAPP AD transgenic mice for 3 months and examined behavioral, biochemical, and pathological end points.

RESULTS

BPN-15606 attenuated cognitive impairment and reduced amyloid plaque load, microgliosis, and astrogliosis associated with the AD phenotype of PSAPP mice when administered to pre-plaque (3-month-old) but was ineffective when administered to post-plaque (6-month-old) mice. No treatment-related toxicity was observed.

CONCLUSION

BPN-15606 appears efficacious when administered prior to significant pathology.

Advances in targeted therapy for malignant lymphoma

The incidence of lymphoma has gradually increased over previous decades, and it ranks among the ten most prevalent cancers worldwide. With the development of targeted therapeutic strategies, though a subset of lymphoma patients has become curable, the treatment of refractory and relapsed diseases remains challenging. Many efforts have been made to explore new targets and to develop corresponding therapies. In addition to novel antibodies targeting surface antigens and small molecular inhibitors targeting oncogenic signaling pathways and tumor suppressors, immune checkpoint inhibitors and chimeric antigen receptor T-cells have been rapidly developed to target the tumor microenvironment. Although these targeted agents have shown great success in treating lymphoma patients, adverse events should be noted. The selection of the most suitable candidates, optimal dosage, and effective combinations warrant further investigation. In this review, we systematically outlined the advances in targeted therapy for malignant lymphoma, providing a clinical rationale for mechanism-based lymphoma treatment in the era of precision medicine.

The presenilin loop region is essential for glycogen synthase kinase 3 β (GSK3β) mediated functions on motor proteins during axonal transport

Neurons require intracellular transport of essential components for function and viability and defects in transport has been implicated in many neurodegenerative diseases including Alzheimer's disease (AD). One possible mechanism by which transport defects could occur is by improper regulation of molecular motors. Previous work showed that reduction of presenilin (PS) or glycogen synthase kinase 3 beta (GSK3β) stimulated amyloid precursor protein vesicle motility. Excess GSK3β caused transport defects and increased motor binding to membranes, while reduction of PS decreased active GSK3β and motor binding to membranes. Here, we report that functional PS and the catalytic loop region of PS is essential for the rescue of GSK3β-mediated axonal transport defects. Disruption of PS loop (PSΔE9) or expression of the non-functional PS variant, PSD447A, failed to rescue axonal blockages in vivo. Further, active GSK3β associated with and phosphorylated kinesin-1 in vitro. Our observations together with previous work that showed that the loop region of PS interacts with GSK3β propose a scaffolding mechanism for PS in which the loop region sequesters GSK3β away from motors for the proper regulation of motor function. These findings are important to uncouple the complex regulatory mechanisms that likely exist for motor activity during axonal transport in vivo.

Presenilin 1 deficiency suppresses autophagy in human neural stem cells through reducing γ-secretase-independent ERK/CREB signaling

Autophagy impairment is commonly implicated in the pathological characteristic of Alzheimer’s disease (AD). Presenilin 1 (PS1) expression in human brain gradually decreases with age and its mutations account for the most common cases of early-onset familial Alzheimer’s disease (FAD). The dominant autophagy phenotypes occur in PS1-knockout and PS1 mutant neurons; it is still unknown whether PS1 deficiency causes serious autophagy impairment in neural stem cells (NSCs). Herein, we generated the heterozygote and homozygote of PS1 knockout in human induced pluripotent stem cells (iPSCs) via CRISPR/Cas9-based gene editing and differentiated them into human NSCs. In these human PS1-deficient NSCs, reduced autophagosome formation and downregulated expression of autophagy–lysosome pathway (ALP)-related mRNAs, as well as proteins were observed. Mechanistically, ERK/CREB inhibition and GSK3β activation had key roles in reducing TFEB expression in PS1-knockout NSCs. Pharmacological inhibition of GSK3β upregulated the expression of TFEB and ALP-related proteins in PS1-knockout NSCs, whereas this effect could be blocked by CREB inhibition. These findings demonstrate that PS1 deficiency causes autophagy suppression in human NSCs via downregulating ERK/CREB signaling.

Mutational re-modeling of di-aspartyl intramembrane proteases: uncoupling physiologically-relevant activities from those associated with Alzheimer's disease

The intramembrane proteolytic activities of presenilins (PSEN1/PS1 and PSEN2/PS2) underlie production of β-amyloid, the key process in Alzheimer’s disease (AD). Dysregulation of presenilin-mediated signaling is linked to cancers. Inhibition of the γ-cleavage activities of PSENs that produce Aβ, but not the ε-like cleavage activity that release physiologically essential transcription activators, is a potential approach for the development of rational therapies for AD. In order to identify whether different activities of PSEN1 can be dissociated, we designed multiple mutations in the evolutionary conserved sites of PSEN1. We tested them in vitro and in vivo assays and compared their activities with mutant isoforms of presenilin-related intramembrane di-aspartyl protease (IMPAS1 (IMP1)/signal peptide peptidase (SPP)). PSEN1 auto-cleavage was more resistant to the mutation remodeling than the ε-like proteolysis. PSEN1 with a G382A or a P433A mutation in evolutionary invariant sites retains functionally important APP ε- and Notch S3- cleavage activities, but G382A inhibits APP γ-cleavage and Aβ production and a P433A elevates Aβ. The G382A variant cannot restore the normal cellular ER Ca²⁺ leak in PSEN1/PSEN2 double knockout cells, but efficiently rescues the loss-of-function (Egl) phenotype of presenilin in C. elegans. We found that, unlike in PSEN1 knockout cells, endoplasmic reticulum (ER) Ca²⁺ leak is not changed in the absence of IMP1/SPP. IMP1/SPP with the analogous mutations retained efficiency in cleavage of transmembrane substrates and rescued the lethality of Ce-imp-2 knockouts. In summary, our data show that mutations near the active catalytic sites of intramembrane di-aspartyl proteases have different consequences on proteolytic and signaling functions.

The NOTCH1-MYC highway toward T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is a highly proliferative hematologic malignancy that results from the transformation of immature T-cell progenitors. Aberrant cell growth and proliferation in T-ALL lymphoblasts are sustained by activation of strong oncogenic drivers promoting cell anabolism and cell cycle progression. Oncogenic NOTCH signaling, which is activated in more than 65% of T-ALL patients by activating mutations in the NOTCH1 gene, has emerged as a major regulator of leukemia cell growth and metabolism. T-ALL NOTCH1 mutations result in ligand-independent and sustained NOTCH1-receptor signaling, which translates into activation of a broad transcriptional program dominated by upregulation of genes involved in anabolic pathways. Among these, the MYC oncogene plays a major role in NOTCH1-induced transformation. As result, the oncogenic activity of NOTCH1 in T-ALL is strictly dependent on MYC upregulation, which makes the NOTCH1-MYC regulatory circuit an attractive therapeutic target for the treatment of T-ALL.

Postnatal dysregulation of Notch signal disrupts dendrite development of adult-born neurons in the hippocampus and contributes to memory impairment

Deficits in the Notch pathway are involved in a number of neurologic diseases associated with mental retardation or/and dementia. The mechanisms by which Notch dysregulation are associated with mental retardation and dementia are poorly understood. We found that Notch1 is highly expressed in the adult-born immature neurons in the hippocampus of mice. Retrovirus mediated knockout of notch1 in single adult-born immature neurons decreases mTOR signaling and compromises their dendrite morphogenesis. In contrast, overexpression of Notch1 intracellular domain (NICD), to constitutively activate Notch signaling in single adult-born immature neurons, promotes mTOR signaling and increases their dendrite arborization. Using a unique genetic approach to conditionally and selectively knockout notch 1 in the postnatally born immature neurons in the hippocampus decreases mTOR signaling, compromises their dendrite morphogenesis, and impairs spatial learning and memory. Conditional overexpression of NICD in the postnatally born immature neurons in the hippocampus increases mTOR signaling and promotes dendrite arborization. These data indicate that Notch signaling plays a critical role in dendrite development of immature neurons in the postnatal brain, and dysregulation of Notch signaling in the postnatally born neurons disrupts their development and thus contributes to the cognitive deficits associated with neurological diseases.

The Small Molecule IMR-1 Inhibits the Notch Transcriptional Activation Complex to Suppress Tumorigenesis

In many cancers, aberrant Notch activity has been demonstrated to play a role in the initiation and maintenance of the neoplastic phenotype and in cancer stem cells, which may allude to its additional involvement in metastasis and resistance to therapy. Therefore, Notch is an exceedingly attractive therapeutic target in cancer, but the full range of potential targets within the pathway has been underexplored. To date, there are no small-molecule inhibitors that directly target the intracellular Notch pathway or the assembly of the transcriptional activation complex. Here, we describe an in vitro assay that quantitatively measures the assembly of the Notch transcriptional complex on DNA. Integrating this approach with computer-aided drug design, we explored potential ligand-binding sites and screened for compounds that could disrupt the assembly of the Notch transcriptional activation complex. We identified a small-molecule inhibitor, termed Inhibitor of Mastermind Recruitment-1 (IMR-1), that disrupted the recruitment of Mastermind-like 1 to the Notch transcriptional activation complex on chromatin, thereby attenuating Notch target gene transcription. Furthermore, IMR-1 inhibited the growth of Notch-dependent cell lines and significantly abrogated the growth of patient-derived tumor xenografts. Taken together, our findings suggest that a novel class of Notch inhibitors targeting the transcriptional activation complex may represent a new paradigm for Notch-based anticancer therapeutics, warranting further preclinical characterization. Cancer Res; 76(12); 3593-603. ©2016 AACR.

Cellular FLICE-like Inhibitory Protein (c-FLIP) and PS1-associated Protein (PSAP) Mediate Presenilin 1-induced γ-Secretase-dependent and -independent Apoptosis, Respectively

Presenilin 1 (PS1) has been implicated in apoptosis; however, its mechanism remains elusive. We report that PS1-induced apoptosis was associated with cellular FLICE-like inhibitory protein (c-FLIP) turnover and that γ-secretase inhibitor blocked c-FLIP turnover and also partially blocked PS1-induced apoptosis. A complete inhibition of PS1-induced apoptosis was achieved by knockdown of PS1-associated protein (PSAP), a mitochondrial proapoptotic protein that forms a complex with Bax upon induction of apoptosis, in the presence of γ-secretase inhibitor. PS1-induced apoptosis was partially inhibited by knockdown of caspase-8, Fas-associated protein with death domain (FADD), or Bid. However, knockdown of Bax or overexpression of Bcl-2 resulted in complete inhibition of PS1-induced apoptosis. These data suggest that PS1 induces apoptosis through two pathways: the γ-secretase-dependent pathway mediated by turnover of c-FLIP and the γ-secretase-independent pathway mediated by PSAP-Bax complex formation. These two pathways converge on Bax to activate mitochondria-dependent apoptosis. These findings provide new insight into the mechanisms by which PS1 is involved in apoptosis and the mechanism by which PS1 exerts its pathogenic effects. In addition, our results suggest that PS2 induces apoptosis through a pathway that is different from that of PS1.

Mouse models of Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting 35 million people today. The search for new treatments is made ever more urgent by prospects for increasing prevalence due to population aging. Mouse models are one of the most important research tools for finding new treatments for AD. Here, we review those models. We begin by briefly reviewing the AD genetics on which mouse models are based and then consider the most common mouse models of AD, including mice transgenic for human amyloid precursor protein (hAPP) and beta-amyloid (Aβ), mice expressing mutant presenilin genes, mice modeling tau's role in AD, and apolipoprotein E models. The discussion highlights key features and important differences between these mouse models. We conclude with a discussion about the role of AD mouse models in the translational pipeline.

Should Alzheimer's disease be equated with human brain ageing? A maladaptive interaction between brain evolution and senescence

In this review Alzheimer's disease is seen as a maladaptive interaction between human brain evolution and senescence. It is predicted to occur in everyone although does not necessarily lead to dementia. The pathological process is initiated in relation to a senescence mediated functional down-regulation in the posteromedial cortex (Initiation Phase). This leads to a loss of glutamatergic excitatory input to layer II entorhinal cortex neurons. A human specific maladaptive neuroplastic response is initiated in these neurons leading to neuronal dysfunction, NFT formation and death. This leads to further loss of glutamatergic excitatory input and propagation of the maladaptive response along excitatory pathways linking evolutionary progressed vulnerable neurons (Propagation Phase). Eventually neurons are affected in many brain areas resulting in dementia. Possible therapeutic approaches include enhancing glutamatergic transmission. The theory may have implications with regards to how Alzheimer's disease is classified.

Down regulation of CSL activity inhibits cell proliferation in prostate and breast cancer cells

The Notch receptor pathway provides a paradigm for juxtacrine signaling pathways and controls stem cell function, developmental cell fate decisions, and cellular differentiation. The Notch pathway is constitutively activated in human cancers by chromosomal rearrangements, activating point mutations, or altered expression patterns. Therefore, the Notch pathway is the subject of chemotherapeutic intervention in a variety of human cancers. Notch receptor activation results in the gamma-secretase dependent proteolytic cleavage of the receptor to liberate the Notch intracellular domain that acts to mediate co-activator recruitment to the DNA binding transcription factor, CSL (CBF-1/RBP-Jκ, Su(H), Lag-1). Therapeutic targeting of the Notch pathway by gamma-secretase inhibitors prevents NICD production and regulates CSL-dependent transcriptional activity. To interrogate the loss of CSL activity in breast and prostate cancer cells, we used lentiviral-based shRNA knockdown of CSL. Knockdown of CSL expression was assessed by decreased DNA binding activity and resulted in decreased cell proliferation. In contrast, gamma-secretase inhibitor (GSI) treatment of these prostate and breast cancer cell lines resulted in minimal growth effects. PCR profiling of Notch pathway genes identified expression changes in few genes (Delta-like-1, Deltex-1, LMO2, and SH2D1A) after CSL knockdown. Consistent with differential effects of GSI on cell survival, GSI treatment failed to recapitulate the gene expression changes observed after CSL knockdown. Thus, CSL inhibition may provide a more effective mechanism to inhibit Notch-pathway dependent cancer cell proliferation as compared to GSI treatment.

Amyloid precursor protein selective gamma-secretase inhibitors for treatment of Alzheimer's disease

INTRODUCTION

Inhibition of gamma-secretase presents a direct target for lowering Aβ production in the brain as a therapy for Alzheimer's disease (AD). However, gamma-secretase is known to process multiple substrates in addition to amyloid precursor protein (APP), most notably Notch, which has limited clinical development of inhibitors targeting this enzyme. It has been postulated that APP substrate selective inhibitors of gamma-secretase would be preferable to non-selective inhibitors from a safety perspective for AD therapy.

METHODS

In vitro assays monitoring inhibitor potencies at APP γ-site cleavage (equivalent to Aβ40), and Notch ε-site cleavage, in conjunction with a single cell assay to simultaneously monitor selectivity for inhibition of Aβ production vs. Notch signaling were developed to discover APP selective gamma-secretase inhibitors. In vivo efficacy for acute reduction of brain Aβ was determined in the PDAPP transgene model of AD, as well as in wild-type FVB strain mice. In vivo selectivity was determined following seven days x twice per day (b.i.d.) treatment with 15 mg/kg/dose to 1,000 mg/kg/dose ELN475516, and monitoring brain Aβ reduction vs. Notch signaling endpoints in periphery.

RESULTS

The APP selective gamma-secretase inhibitors ELN318463 and ELN475516 reported here behave as classic gamma-secretase inhibitors, demonstrate 75- to 120-fold selectivity for inhibiting Aβ production compared with Notch signaling in cells, and displace an active site directed inhibitor at very high concentrations only in the presence of substrate. ELN318463 demonstrated discordant efficacy for reduction of brain Aβ in the PDAPP compared with wild-type FVB, not observed with ELN475516. Improved in vivo safety of ELN475516 was demonstrated in the 7d repeat dose study in wild-type mice, where a 33% reduction of brain Aβ was observed in mice terminated three hours post last dose at the lowest dose of inhibitor tested. No overt in-life or post-mortem indications of systemic toxicity, nor RNA and histological end-points indicative of toxicity attributable to inhibition of Notch signaling were observed at any dose tested.

CONCLUSIONS

The discordant in vivo activity of ELN318463 suggests that the potency of gamma-secretase inhibitors in AD transgenic mice should be corroborated in wild-type mice. The discovery of ELN475516 demonstrates that it is possible to develop APP selective gamma-secretase inhibitors with potential for treatment for AD.

Dictyostelium possesses highly diverged presenilin/gamma-secretase that regulates growth and cell-fate specification and can accurately process human APP: a system for functional studies of the presenilin/gamma-secretase complex

Presenilin (PS) is the catalytic moiety of the gamma-secretase complex. PS and other gamma-secretase components are well conserved among metazoa, but their presence and function in more-distant species are not resolved. Because inappropriate gamma-secretase processing of amyloid precursor protein (APP) in humans is associated with familial Alzheimer's disease, understanding essential elements within each gamma-secretase component is crucial to functional studies. Diverged proteins have been identified in primitive plants but experiments have failed to demonstrate gamma-secretase activity. We have identified highly diverged orthologs for each gamma-secretase component in the ancient eukaryote Dictyostelium, which lacks equivalents of APP, Notch and other characterized PS/gamma-secretase substrates. We show that wild-type (WT) Dictyostelium is capable of amyloidogenic processing of ectopically expressed human APP to generate amyloid-beta peptides Abeta(40) and Abeta(42); strains deficient in gamma-secretase cannot produce Abeta peptides but accumulate processed intermediates of APP that co-migrate with the C-terminal fragments alpha- and beta-CTF of APP that are found in mammalian cells. We further demonstrate that Dictyostelium requires PS for phagocytosis and cell-fate specification in a cell-autonomous manner, and show that regulation of phagocytosis requires an active gamma-secretase, a pathway suggested, but not proven, to occur in mammalian and Drosophila cells. Our results indicate that PS signaling is an ancient process that arose prior to metazoan radiation, perhaps independently of Notch. Dictyostelium might serve to identify novel PS/gamma-secretase signaling targets and provide a unique system for high-throughput screening of small-molecule libraries to select new therapeutic targets for diseases associated with this pathway.

Inhibition of KSHV-infected primary effusion lymphomas in NOD/SCID mice by gamma-secretase inhibitor

Primary effusion lymphoma (PEL) is a common cancer in AIDS patients closely associated with Kaposi's sarcoma-associated herpesvirus (KSHV). Previously, we showed that KSHV latency associated nuclear antigen (LANA) stabilizes intracellular activated Notch1 (ICN) involved in maintenance of the malignant phenotype of KSHV infected PEL cells in vitro. The gamma-secretase inhibitor (GSI) which specifically blocks the production of ICN slows down the proliferation of the KSHV infected PEL cell lines BCBL1, BC3 as well as JSC1 in vitro. In this study, we extended these studies to explore the possibility that manipulation of the Notch signaling by GSI would prevent the growth of the PEL tumors in vivo. We observed that the onset of tumorigenesis of KSHV infected PELs was significantly delayed in GSI treated SCID mice harboring the PEL cell lines. We also found that GSI treatment resulted in necrosis as well as apoptosis in tumors generated by the xenotransplanted KSHV positive PEL cell lines. In contrast, GSI had no effect on mice harboring BJAB cells, a KSHV negative Burkitt's lymphoma cell line where ICN levels were negligible. Our study provides further evidence to suggest that targeted downregulation of abnormal Notch signaling has therapeutic potential for KSHV related primary effusion lymphomas.

Beta-catenin up-regulates Atoh1 expression in neural progenitor cells by interaction with an Atoh1 3' enhancer

Atoh1, a basic helix-loop-helix transcription factor, plays a critical role in the differentiation of several epithelial and neural cell types. We found that beta-catenin, the key mediator of the canonical Wnt pathway, increased expression of Atoh1 in mouse neuroblastoma cells and neural progenitor cells, and baseline Atoh1 expression was decreased by siRNA directed at beta-catenin. The up-regulation of Atoh1 was caused by an interaction of beta-catenin with the Atoh1 enhancer that could be demonstrated by chromatin immunoprecipitation. We found that two putative Tcf-Lef sites in the 3' enhancer of the Atoh1 gene displayed an affinity for beta-catenin and were critical for the activation of Atoh1 transcription because mutation of either site decreased expression of a reporter gene downstream of the enhancer. Tcf-Lef co-activators were found in the complex that bound to these sites in the DNA together with beta-catenin. Inhibition of Notch signaling, which has previously been shown to induce bHLH transcription factor expression, increased beta-catenin expression in progenitor cells of the nervous system. Because this could be a mechanism for up-regulation of Atoh1 after inhibition of Notch, we tested whether siRNA to beta-catenin prevented the increase in Atoh1 and found that beta-catenin expression was required for increased expression of Atoh1 after Notch inhibition.

Development of a high-throughput assay for screening of gamma-secretase inhibitor with endogenous human, mouse or Drosophila gamma-secretase

Selective lowering of amyloid-β levels with small-molecule γ-secretase inhibitors is a promising therapeutic approach for Alzheimer’s disease. In this work, we developed a high throughput assay for screening of γ-secretase inhibitors with endogenous γ-secretase and a fluorogenic substrate. The IC₅₀ values of known γ-secretase inhibitors generated with this method were comparable with reported values obtained by other methods. The assay was optimized and applied to a small-scale screening of 1,280 compounds. The discovery of several new inhibitors warrants further investigation. This assay was also proven to be easily adopted to test compounds for drosophila and mouse γ-secretase, which could be very useful to assess compounds activity against γ-secretase from different species before the in vivo test in animal models.

Evidence for the Presence of a Free C-Terminal Fragment of Cx43 in Cultured Cells

Migration of the gap junction protein connexin 43 (Cx43) in SDS-PAGE yields 2 to 4 distinct bands, detectable in the 40-47 kDa range. Here, we show that antibodies against the carboxy-terminal domain of Cx43 recognized an additional 20-kDa product. This protein was detected in some culture cell lysates. The presence of the 20-kDa band was not prevented by the use of protease inhibitors (Complete(R) and phenylmethylsulfonyl fluoride (PMSF), 1-5 mM). The band was absent from cells treated with Cx43-specific RNAi, and from those derived from Cx43-deficient mice, indicating that this Cx43-immunoreactive protein is a product of the Cx43 gene. Treatment of CHO cells with cyclosporin A caused a reduction in the amount of full-length Cx43 and a concomitant increase in the amount of the 20-kDa band. Overall, our data show that a fraction of the Cx43-immunoreactive protein pool within a given cell may correspond to a C-terminal fragment of the protein.

Opposing actions of Notch1 and VEGF in post-natal cardiac valve endothelial cells

The endothelium of the cardiac valves is unique compared the rest of the vasculature in its ability to undergo an endothelial-to-mesenchymal transformation (EMT) in vitro in response to transforming growth factor-beta (TGF-beta). EMT is a critical event during embryonic valve development, and both VEGF-A and Notch1 have been shown to function in this process. Here we investigate the effects of VEGF-A and Notch1 on EMT in clonal endothelial cell (EC) populations isolated from adult aortic valve leaflets. VEGF-A inhibited TGF-beta-induced EMT. Endothelial growth, however, was not affected by VEGF-A or TGF-beta. A positive role for Notch1 was revealed in three experiments: (1) TGF-beta induced Notch1 mRNA in valve ECs, (2) a gamma-secretase inhibitor of Notch1 signaling blocked EMT, and (3) overexpression of a ligand-independent form of Notch1 induced EMT. These results demonstrate, for the first time, that VEGF-A and Notch1 play opposing roles in regulating EMT in post-natal valve endothelium.

Repression of chondrogenesis through binding of notch signaling proteins HES-1 and HEY-1 to N-box domains in the COL2A1 enhancer site

OBJECTIVE

Notch signaling is implicated in the repression of mesenchymal stem cell (MSC) chondrogenic differentiation. The purpose of this study was to examine the mechanism of this repression and how it is modulated to permit chondrogenesis.

METHODS

Notch intracellular domain (NICD) protein levels were monitored via Western blotting throughout chondrogenic differentiation of human MSCs in pellet cultures. Overexpression of Notch signaling components and their effect on chondrogenesis was achieved by transfecting plasmids coding for NICD, HES-1, and HERP-2/HEY-1. COL2A1 and AGGRECAN expression was monitored via quantitative polymerase chain reaction analysis. Chromatin immunoprecipitation (ChIP) was used to test whether HES-1 and HEY-1 bind putative N-box domains in intron 1 of COL2A1.

RESULTS

High levels of NICD proteins were reduced during chondrogenesis of human MSCs, and this was mediated by transforming growth factor beta3 (TGFbeta3). COL2A1 gene expression was repressed following overexpression of NICD (2-fold) and HES-1 (3-fold) and was markedly repressed by overexpression of HEY-1 (80-fold). HEY-1 repressed AGGRECAN expression 10-fold, while NICD and HES-1 had no effect. We identified 2 putative N-box domains adjacent to, and part of, the SOX9 enhancer binding site located in intron 1 of COL2A1. ChIP studies showed that endogenous HES-1 and HEY-1 bound to these sites. Transducin-like enhancer, the HES-1 corepressor protein, was displaced during chondrogenic differentiation and following TGFbeta3 treatment.

CONCLUSION

These results reveal novel mechanisms by which Notch signaling represses gene expression. Notch signaling proteins act on the SOX9 binding site in the COL2A1 enhancer and prevent SOX9-mediated transcriptional activation of COL2A1 and, thus, chondrogenic differentiation.

Tumor necrosis factor-alpha and endothelial cells modulate Notch signaling in the bone marrow microenvironment during inflammation

OBJECTIVE

Homeostasis of the hematopoietic compartment is challenged and maintained during conditions of stress by mechanisms that are poorly defined. To understand how the bone marrow (BM) microenvironment influences hematopoiesis, we explored the role of Notch signaling and BM endothelial cells in providing microenvironmental cues to hematopoietic cells in the presence of inflammatory stimuli.

MATERIALS AND METHODS

The human BM endothelial cell line (BMEC) and primary human BM endothelial cells were analyzed for expression of Notch ligands and the ability to expand hematopoietic progenitors in an in vitro coculture system. In vivo experiments were carried out to identify modulation of Notch signaling in BM endothelial and hematopoietic cells in mice challenged with tumor necrosis factor-alpha (TNF-alpha) or lipopolysaccharide (LPS), or in Tie2-tmTNF-alpha transgenic mice characterized by constitutive TNF-alpha activation.

RESULTS

BM endothelial cells were found to express Jagged ligands and to greatly support progenitor's colony-forming ability. This effect was markedly decreased by Notch antagonists and augmented by increasing levels of Jagged2. Physiologic upregulation of Jagged2 expression on BMEC was observed upon TNF-alpha activation. Injection of TNF-alpha or LPS upregulated three- to fourfold Jagged2 expression on murine BM endothelial cells in vivo and resulted in increased Notch activation on murine hematopoietic stem/progenitor cells. Similarly, constitutive activation of endothelial cells in Tie2-tmTNF-alpha mice was characterized by increased expression of Jagged2 and by augmented Notch activation on hematopoietic stem/progenitor cells.

CONCLUSIONS

Our results provide the first evidence that BM endothelial cells promote expansion of hematopoietic progenitor cells by a Notch-dependent mechanism and that TNF-alpha and LPS can modulate the levels of Notch ligand expression and Notch activation in the BM microenvironment in vivo.

Blocking Notch1 signaling by RNA interference can induce growth inhibition in HeLa cells

The Notch proteins constitute a family of transmembrane receptors that play a pivotal role in cellular differentiation, proliferation, and apoptosis. RNA interference of Presenilin1 (PS1) and Notch1 was carried out in this research to determine whether it could block Notch signaling and induce growth inhibition in HeLa cells. We transfected synthesized target small interfering RNA (siRNA) into HeLa cells, and blocking of Notch signaling was detected by C-promoter binding factor-1 (CBF1) reporter. We then conducted cell proliferation assay. Cells transfected with PS1 and Notch1 siRNA showed great inhibition in proliferation compared to the controls in vitro and in vivo. We conclude that RNA interference of PS1 or Notch1 can block Notch signaling and consequently induce growth inhibition of HeLa cells.

The Notch-1 intracellular domain is found in sub-nuclear bodies in SH-SY5Y neuroblastomas and in primary cortical neurons

Notch signalling affects most aspects of development, not least the determination of neural stem cell fate. Here, we describe the presence of the Notch-1 intracellular domain (N1(ICD)) in sub-nuclear bodies in SH-SY5Y neuroblastomas and in primary rat cortical neurons as well as several other mammalian cell lines. We also demonstrate that these N1(ICD)-positive sub-nuclear bodies are distinct from premyelocytic leukaemia (PML) and SC35 bodies. Furthermore, using Notch deletion constructs we determined that a region C-terminal of amino acid 2094 is involved in targeting the N1(ICD) into sub-nuclear bodies. These findings have ramifications for nuclear architecture and gene transcription.

High levels of Notch signaling down-regulate Numb and Numblike

Inhibition of Notch signaling by Numb is critical for many cell fate decisions. In this study, we demonstrate a more complex relationship between Notch and the two vertebrate Numb homologues Numb and Numblike. Although Numb and Numblike at low levels of Notch signaling negatively regulated Notch, high levels of Notch signaling conversely led to a reduction of Numb and Numblike protein levels in cultured cells and in the developing chick central nervous system. The Notch intracellular domain but not the canonical Notch downstream proteins Hes 1 and Hey 1 caused a reduction of Numb and Numblike. The Notch-mediated reduction of Numblike required the PEST domain in the Numblike protein and was blocked by the proteasome inhibitor MG132. Collectively, these observations reveal a reciprocal negative regulation between Notch and Numb/Numblike, which may be of relevance for stabilizing asymmetric cell fate switches and for tumor development.

Pathogenic mechanisms in Alzheimer's disease

Alzheimer's disease is a progressive neurodegenerative disorder associated with aging and characterized by neurofibrillary tangles and amyloid plaques that deposit in the brain, triggering the neurodegenerative phenomena and leading to neuronal death. Amyloid plaques are primarily composed of beta-amyloid peptides, which derive from the Amyloid Precursor Protein (APP) upon the consequential action of beta- and gamma-secretase. This review discusses recent literature on beta- and gamma-secretase, and on those cellular factors, like cholesterol and phosphorylation of APP, that are involved in aging and may affect the function of both beta- and gamma-secretase.

Intracellular activated Notch1 is critical for proliferation of Kaposi's sarcoma-associated herpesvirus-associated B-lymphoma cell lines in vitro

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human tumor virus expressing latent antigens critical for pathogenesis. The mechanism by which KSHV mediates oncogenesis has not been fully elucidated. Notch signaling is an evolutionarily conserved pathway controlling diverse events related to development, proliferation, and tissue homeostasis. Deregulation of Notch signaling has also been shown to be highly correlated with oncogenesis. Here we show that the activated intracellular domain of Notch1 (ICN) is aberrantly accumulated in latently KSHV-infected pleural effusion lymphoma cells and results in increased proliferation. Specifically, growth of the infected cells was dramatically inhibited at the G(1) phase by treatment with a gamma-secretase inhibitor which specifically blocks the production of ICN. Increased ICN also up-regulated the cyclin D1 cell cycle regulator. Taken together, these studies define an important mechanism directly linking latent KSHV infection to induction of oncogenesis through dysregulation of the conserved Notch signaling pathway.

Notch signalling and voltage-gated Na+ channel activity in human prostate cancer cells: independent modulation of in vitro motility

This study tested the possible functional relationship of two signalling mechanisms shown previously to be involved in human prostate cancer (PCa), Notch and voltage-gated sodium channel. Notch1 and Notch2 were differentially expressed in PCa cell lines of varying metastatic potential (LNCaP, PC-3, PC-3M) in comparison to a normal prostate cell line (PNT2), whereas Notch3 and Notch4 were not expressed. The Notch ligand Jagged1, but not Jagged2, was increased in all cell lines, whereas the Notch downstream target Deltex was not expressed. In comparison to the LNCaP cell line, Hes1, another downstream target, showed elevated expression in the metastatic PC-3 and PC-3M cells and promoted lateral motility. In contrast, the Notch ligand Delta-like1 (Dll1) levels were higher in LNCaP compared with PC-3 and PC-3M cells. Importantly, decreasing Dll1 expression increased the lateral motility of PC-3 cells, whereas blocking voltage-gated Na(+) channel activity with tetrodotoxin decreased motility. However, the effect of Dll1 was independent of Notch signalling through Hes1 and voltage-gated Na(+) channel expression/activity.

Familial Alzheimer's disease presenilin 1 mutations cause alterations in the conformation of presenilin and interactions with amyloid precursor protein

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, an enzymatic activity that cleaves amyloid beta (Abeta) from the amyloid precursor protein (APP). More than 100 mutations spread throughout the PS1 molecule are linked to autosomal dominant familial Alzheimer's disease (FAD). All of these mutations lead to a similar phenotype: an increased ratio of Abeta42 to Abeta40, increased plaque deposition, and early age of onset. We use a recently developed microscopy approach, fluorescence lifetime imaging microscopy, to monitor the relative molecular distance between PS1 N and C termini in intact cells. We show that FAD-linked missense mutations located near the N and C termini, in the mid-region of PS1, and the exon 9 deletion mutation all change the spatial relationship between PS1 N and C termini in a similar way, increasing proximity of the two epitopes. This effect is opposite of that observed by treatment with Abeta42-lowering nonsteroidal anti-inflammatory drugs (NSAIDs) (Lleo et al., 2004b). Accordingly, treatment of M146L PS1-overexpressing neurons with high-dose NSAIDs somewhat offsets the conformational change associated with the mutation. Moreover, by monitoring the relative distance between a PS1 loop epitope and the APP C terminus, we demonstrate that the FAD PS1 mutations are also associated with a consistent change in the configuration of the PS1-APP complex. The nonpathogenic E318G PS1 polymorphism had no effect on PS1 N terminus-C terminus proximity or PS1-APP interactions. We propose that the conformational change we observed may therefore provide a shared molecular mechanism for FAD pathogenesis caused by a wide range of PS1 mutations.

A Two Decade Contribution of Molecular Cell Biology to the Centennial of Alzheimer's Disease: Are We Progressing Toward Therapy?

Alzheimer's disease (AD), described for the first time 100 years ago, is a neurodegenerative disease characterized by two neuropathological hallmarks: neurofibrillary tangles containing hyperphosphorylated tau and senile plaques. These lesions are likely initiated by an imbalance between production and clearance of amyloid beta, leading to increased oligomerization of these peptides, formation of amyloid plaques in the brain of the patient, and final dementia. Amyloid beta is generated from amyloid precursor protein (APP) by subsequent beta- and gamma-secretase cleavage, the latter being a multiprotein complex consisting of presenilin-1 or -2, nicastrin, APH-1, and PEN-2. Alternatively, APP can be cleaved by alpha- and gamma-secretase, precluding the production of Abeta. In this review, we discuss the major breakthroughs during the past two decades of molecular cell biology and the current genetic and cell biological state of the art on APP proteolysis, including structure-function relationships and subcellular localization. Finally, potential directions for cell biological research toward the development of AD therapies are briefly discussed.

Notch1 modulates timing of G1-S progression by inducing SKP2 transcription and p27 Kip1 degradation

Cyclin-dependent kinase inhibitors (CKIs) and Notch receptor activation have been shown to influence adult stem cells and progenitors by altering stem cell self-renewal and proliferation. Yet, no interaction between these molecular pathways has been defined. Here we show that ligand-independent and ligand-dependent activation of Notch1 induces transcription of the S phase kinase–associated protein 2 (SKP2), the F-box subunit of the ubiquitin-ligase complex SCF^SKP2 that targets proteins for degradation. Up-regulation of SKP2 by Notch signaling enhances proteasome-mediated degradation of the CKIs, p27^Kip1 and p21^Cip1, and causes premature entry into S phase. Silencing of SKP2 by RNA interference in G₁ stabilizes p27^Kip1 and p21^Cip1 and abolishes Notch effect on G₁-S progression. Thus, SKP2 serves to link Notch1 activation with the cell cycle machinery. This novel pathway involving Notch/SKP2/CKIs connects a cell surface receptor with proximate mediators of cell cycle activity, and suggests a mechanism by which a known physiologic mediator of cell fate determination interfaces with cell cycle control.

Two domains within the first putative transmembrane domain of presenilin 1 differentially influence presenilinase and gamma-secretase activity

Presenilins (PS) are thought to contain the active site for presenilinase endoproteolysis of PS and gamma-secretase cleavage of substrates. The structural requirements for PS incorporation into the gamma-secretase enzyme complex, complex stability and maturation, and appropriate presenilinase and gamma-secretase activity are poorly understood. We used rescue assays to identify sequences in transmembrane domain one (TM1) of PS1 required to support presenilinase and gamma-secretase activities. Swap mutations identified an N-terminal TM1 domain that is important for gamma-secretase activity only and a C-terminal TM1 domain that is essential for both presenilinase and gamma-secretase activities. Exchange of residues 95-98 of PS1 (sw95-98) completely abolishes both activities while the familial Alzheimer's disease mutation V96F significantly inhibits both activities. Reversion of residue 96 back to valine in the sw95-98 mutant rescues PS function, identifying V96 as the critical residue in this region. The TM1 mutants do not bind to an aspartyl protease transition state analog gamma-secretase inhibitor, indicating a conformational change induced by the mutations that abrogates catalytic activity. TM1 mutant PS1 molecules retain the ability to interact with gamma-secretase substrates and gamma-secretase complex members, although Nicastrin stability is decreased by the presence of these mutants. gamma-Secretase complexes that contain V96F mutant PS1 molecules display a partial loss of function for gamma-secretase that alters the ratio of amyloid-beta peptide species produced, leading to the amyloid-beta peptide aggregation that causes familial Alzheimer's disease.

Presenilin function and gamma-secretase activity

Alzheimer's disease (AD) is the most common form of dementia and is characterized pathologically by the accumulation of beta-amyloid (Abeta) plaques and neurofibrillary tangles in the brain. Genetic studies of AD first highlighted the importance of the presenilins (PS). Subsequent functional studies have demonstrated that PS form the catalytic subunit of the gamma-secretase complex that produces the Abeta peptide, confirming the central role of PS in AD biology. Here, we review the studies that have characterized PS function in the gamma-secretase complex in Caenorhabditis elegans, mice and in in vitro cell culture systems, including studies of PS structure, PS interactions with substrates and other gamma-secretase complex members, and the evidence supporting the hypothesis that PS are aspartyl proteases that are active in intramembranous proteolysis. A thorough knowledge of the mechanism of PS cleavage in the context of the gamma-secretase complex will further our understanding of the molecular mechanisms that cause AD, and may allow the development of therapeutics that can alter Abeta production and modify the risk for AD.

Got RIP? Presenilin-dependent intramembrane proteolysis in growth factor receptor signaling

A number of cell surface growth factor receptors are subject to presenilin-dependent regulated intramembrane proteolysis (PS-RIP) after ligand binding and/or ectodomain cleavage. PS-RIP is mediated by a highly conserved multi-component membrane-bound protease, termed gamma-secretase, responsible for generating Alzheimer's disease (AD)-associated Abeta peptide from its membrane-bound beta-amyloid precursor protein (APP), as well as for cleaving a number of other type-I membrane receptors. PS-RIP is a conserved cellular process by which cells transmit signals from one compartment to another, including the liberation of membrane-bound transcription factors. Recent studies indicate that PS-RIP also mediates the proteolytic inactivation of heteromeric receptor complexes by removing the transmembrane domains required for receptor-receptor interaction. Thus, PS-RIP appears to regulate diverse cellular pathways either by generating soluble effectors from membrane-bound precursors, or by removing the transmembrane domain of a membrane-tethered signaling component.

Targeted deletion of numb and numblike in sensory neurons reveals their essential functions in axon arborization

Mouse Numb homologs antagonize Notch1 signaling pathways through largely unknown mechanisms. Here we demonstrate that conditional mouse mutants with deletion of numb and numblike in developing sensory ganglia show a severe reduction in axonal arborization in afferent fibers, but no deficit in neurogenesis. Consistent with these results, expression of Cre recombinase in sensory neurons from numb conditional mutants results in reduced endocytosis, a significant increase in nuclear Notch1, and severe reductions in axon branch points and total axon length. Conversely, overexpression of Numb, but not mutant Numb lacking alpha-adaptin-interacting domain, leads to accumulation of Notch1 in markedly enlarged endocytic-lysosomal vesicles, reduced nuclear Notch1, and dramatic increases in axonal length and branch points. Taken together, our data provide evidence for previously unidentified functions of Numb and Numblike in sensory axon arborization by regulating Notch1 via the endocytic-lysosomal pathways.

Notch1 and notch2 have opposite effects on embryonal brain tumor growth

The role of Notch signaling in tumorigenesis can vary; Notch1 acts as an oncogene in some neoplasms, and a tumor suppressor in others. Here, we show that different Notch receptors can have opposite effects in a single tumor type. Expression of truncated, constitutively active Notch1 or Notch2 in embryonal brain tumor cell lines caused antagonistic effects on tumor growth. Cell proliferation, soft agar colony formation, and xenograft growth were all promoted by Notch2 and inhibited by Notch1. We also found that Notch2 receptor transcripts are highly expressed in progenitor cell-derived brain tumors such as medulloblastomas, whereas Notch1 is scarce or undetectable. This parallels normal cerebellar development, during which Notch2 is predominantly expressed in proliferating progenitors and Notch1 in postmitotic differentiating cells. Given the oncogenic effects of Notch2, we analyzed its gene dosage in 40 embryonal brain tumors, detecting an increased copy number in 15% of cases. Notch2 gene amplification was confirmed by fluorescence in situ hybridization in one case with extremely high Notch2 mRNA levels. In addition, expression of the Notch pathway target gene Hes1 in medulloblastomas was associated with significantly shorter patient survival (P = 0.01). Finally, pharmacological inhibition of Notch signaling suppresses growth of medulloblastoma cells. Our data indicate that Notch1 and Notch2 can have opposite effects on the growth of a single tumor type, and show that Notch2 can be overexpressed after gene amplification in human tumors.

Mutations in APP have independent effects on Aβ and CTFγ generation

Understanding the molecular mechanism of beta-amyloid (Abeta) generation is crucial for Alzheimer's disease pathogenesis as well as for normal APP function. The transmembrane domain (TM) of APP appears to undergo presenilin-dependent gamma-secretase cleavage at two topologically distinct sites: a site in the middle of the TM domain that is crucial for the generation of Abeta-peptides, and a site close to the cytoplasmic border (S3-like/epsilon site) of the TM domain that leads to production of the APP intracellular domain (CTFgamma/AICD). We demonstrate that, in contrast to the unique effect of familial Alzheimer's disease (FAD) mutations in APP on Abeta42 production, some but not all FAD mutations also affect CTFgamma generation. Furthermore, changes in total CTFgamma levels do not correlate with either an increase or a decrease of any Abeta species, and inhibition of Abeta-peptide formation starting from position +1 (Abeta1-x) does not affect CTFgamma production. These results suggest that cleavage at the gamma40/42- and the S3-like sites can be dissociated, and that APP signaling and Abeta production are not tightly linked.

Notch Oncoproteins Depend on γ-Secretase/Presenilin Activity for Processing and Function

During normal development Notch receptor signaling is important in regulating numerous cell fate decisions. Mutations that truncate the extracellular domain of Notch receptors can cause aberrant signaling and promote unregulated cell growth. We have examined two types of truncated Notch oncoproteins that arise from proviral insertion into the Notch4 gene (Notch4/int-3) or a chromosomal translocation involving the Notch1 gene (TAN-1). Both Notch4/int-3 and TAN-1 oncoproteins lack most or all of their ectodomain. Normal Notch signaling requires gamma-secretase/presenilin-mediated proteolytic processing, but whether Notch oncoproteins are also dependent on gamma-secretase/presenilin activity is not known. We demonstrate that Notch4/int-3-induced activation of the downstream transcription factor, CSL, is abrogated in cells deficient in presenilins or treated with a pharmacological inhibitor of gamma-secretase/presenilins. Furthermore, we find that both Notch4/int-3 and TAN-1 accumulate at the cell surface, where presenilin-dependent cleavage occurs, when gamma-secretase/presenilin activity is inhibited. gamma-Secretase/presenilin inhibition effectively blocks cellular responses to Notch4/int-3, but not TAN-1, apparently because some TAN-1 polypeptides lack transmembrane domains and do not require gamma-secretase/presenilin activity for nuclear access. These studies highlight potential uses and limitations of gamma-secretase/presenilin inhibitors in targeted therapy of Notch-related neoplasms.

Dual roles for the Notch target gene Hes-1 in the differentiation of 3T3-L1 preadipocytes

The process of adipogenesis involves a complex program of gene expression that includes down-regulation of the gene encoding Hes-1, a target of the Notch signaling pathway. To determine if Notch signaling affects adipogenesis, we exposed 3T3-L1 preadipocytes to the Notch ligand Jagged1 and found that differentiation was significantly reduced. This effect could be mimicked by constitutive expression of Hes-1. The block was associated with a complete loss of C/EBPalpha and peroxisome proliferator-activated receptor gamma (PPARgamma) induction and could be overcome by retroviral expression of either C/EBPalpha or PPARgamma2. Surprisingly, small interfering RNA (siRNA)-mediated reduction of Hes-1 mRNA in 3T3-L1 cells also inhibited differentiation, suggesting an additional, obligatory role for Hes-1 in adipogenesis. This role may be related to our observation that both Notch signaling and Hes-1 down-regulate transcription of the gene encoding DLK/Pref-1, a protein known to inhibit differentiation of 3T3-L1 cells. The results presented in this study establish a new target downstream of the Notch-Hes-1 pathway and suggest a dual role for Hes-1 in adipocyte development.

Clinical, pathological, and biochemical spectrum of Alzheimer disease associated with PS-1 mutations

Three genes have been implicated in the etiology of early-onset autosomal-dominant Alzheimer disease (AD): the amyloid precursor protein, the presenilin-1, and presenilin-2 genes. Approximately half of autosomal-dominant AD cases are associated with mutations in the presenilin-1 (PS-1) gene on the long arm of Chromosome 14. Marked allelic heterogeneity characterizes families with PS-1 gene mutations; more than 100 different mutations have been found in independent families thus far. With the exception of age at onset, the clinical phenotype is similar to late-onset AD, although some rare specific phenotypes have been described. These mutations lead to enhanced deposition of total Abeta and Abeta42 (but not Abeta40) in the brain, compared with sporadic AD. There is a considerable heterogeneity in the histological profiles among brains from patients with different mutations, and although some lead to predominantly parenchymal deposition of Abeta in the form of diffuse and cored plaques, others show predominantly vascular deposition, with severe amyloid angiopathy. Only some mutations are associated with enhanced neurofibrillary tangle formation and increased neuronal loss compared with sporadic AD. However, there is an important clinical and pathological variability even among family members with the same mutation, which suggests the involvement of other genetic or environmental factors that modulate the clinical expression of the disease. This represents a valuable model for identifying such factors and has potential implications for the development of new therapeutic strategies for delaying disease onset.

Notch1 competes with the amyloid precursor protein for gamma-secretase and down-regulates presenilin-1 gene expression

Presenilin 1 (PS1) is a critical component of the gamma-secretase complex, which is involved in the cleavage of several substrates including the amyloid precursor protein (APP) and Notch1. Based on the fact that APP and Notch are processed by the same gamma-secretase, we postulated that APP and Notch compete for the enzyme activity. In this report, we examined the interactions between APP, Notch, and PS1 using the direct gamma-secretase substrates, Notch 1 Delta extracellular domain (N1DeltaEC) and APP carboxyl-terminal fragment of 99 amino acids, and measured the effects on amyloid-beta protein production and Notch signaling, respectively. Additionally, we tested the hypothesis that downstream effects on PS1 expression may coexist with the competition phenomenon. We observed significant competition between Notch and APP for gamma-secretase activity; transfection with either of two direct substrates of gamma-secretase led to a reduction in the gamma-cleaved products, Notch intracellular domain or amyloid-beta protein. In addition, however, we found that activation of the Notch signaling pathway, by either N1 Delta EC or Notch intracellular domain, induced down-regulation of PS1 gene expression. This finding suggests that Notch activation directly engages gamma-secretase and subsequently leads to diminished PS1 expression, suggesting a complex set of feedback interactions following Notch activation.

Human neurodegenerative disease modeling using Drosophila

A number of approaches have been taken to recreate and to study the role of genes associated with human neurodegenerative diseases in the model organism Drosophila. These studies encompass the polyglutamine diseases, Parkinson's disease, Alzheimer's disease, and tau-associated pathologies. The findings highlight Drosophila as an important model system in which to study the fundamental pathways influenced by these genes and have led to new insights into aspects of pathogenesis and modifier mechanisms.

Notch and Presenilin: regulated intramembrane proteolysis links development and degeneration

Intensive studies of three proteins--Presenilin, Notch, and the amyloid precursor protein (APP)--have led to the recognition of a direct intersection between early development and late-life neurodegeneration. Notch signaling mediates many different intercellular communication events that are essential for determining the fates of neural and nonneural cells during development and in the adult. The Notch receptor acts in a core pathway as a membrane-bound transcription factor that is released to the nucleus by a two-step cleavage mechanism called regulated intramembrane proteolysis (RIP). The second cleavage is effected by Presenilin, an unusual polytopic aspartyl protease that apparently cleaves Notch and numerous other single-transmembrane substrates within the lipid bilayer. Another Presenilin substrate, APP, releases the amyloid ss-protein that can accumulate over time in limbic and association cortices and help initiate Alzheimer's disease. Elucidating the detailed mechanism of Presenilin processing of membrane proteins is important for understanding diverse signal transduction pathways and potentially for treating and preventing Alzheimer's disease.

Presenilins mutated at Asp-257 or Asp-385 restore Pen-2 expression and Nicastrin glycosylation but remain catalytically inactive in the absence of wild type Presenilin

The Presenilins are part of the gamma-secretase complex that is involved in the regulated intramembrane proteolysis of amyloid precursor protein and other type I integral membrane proteins. Nicastrin, Pen-2, and Aph1 are the other proteins of this complex. The Presenilins probably contribute the catalytic activity to the protease complex. However, several investigators reported normal Abeta-peptide generation in cells expressing Presenilins mutated at the putative catalytic site residue Asp-257, contradicting this hypothesis. Because endogenously expressed wild type Presenilin could contribute to residual gamma-secretase activity in these experiments, we have reinvestigated the problem by expressing mutated Presenilins in a Presenilin-negative cell line. We confirm that Presenilins with mutated Asp residues are catalytically inactive. Unexpectedly, these mutated Presenilins are still partially processed into amino- and carboxyl-terminal fragments by a "Presenilinase"-like activity. They are also able to rescue Pen-2 expression and Nicastrin glycosylation in Presenilin-negative cells and become incorporated into large approximately 440-kDa complexes as assessed by blue native gel electrophoresis. Our study demonstrates that the catalytic activity of Presenilin and its other functions in the generation, stabilization, and transport of the gamma-secretase complex can be separated and extends the concept that Presenilins are multifunctional proteins.

Regulated hyperaccumulation of presenilin-1 and the "gamma-secretase" complex. Evidence for differential intramembranous processing of transmembrane subatrates

Intramembranous "gamma-secretase" processing of beta-amyloid precursor protein (APP) and other transmembrane proteins, including Notch, is mediated by a macromolecular complex consisting of presenilins (PSs), nicastrin (NCT), APH-1, and PEN-2. We now demonstrate that in cells coexpressing PS1, APH-1, and NCT, full-length PS1 accumulates to high levels and is fairly stable. Upon expression of PEN-2, the levels of PS1 holoprotein are significantly reduced, commensurate with an elevation in levels of PS1 fragments. These findings suggest that APH-1 and NCT are necessary for stabilization of full-length PS1 and that PEN-2 is critical for the proteolysis of stabilized PS1. In N2a and 293 cell lines that stably overexpress PS1, APH-1, NCT, and PEN-2, PS1 fragment levels are elevated by up to 10-fold over endogenous levels. In these cells, we find a marked accumulation of the APP-CTF gamma (AICD) fragment and a concomitant reduction in levels of both APP-CTF beta and CTF alpha. Moreover, the production of the gamma-secretase-generated Notch S3/NICD derivative is modestly elevated. However, we failed to observe a corresponding increase in levels of secreted A beta peptides in the medium of these cells. These results lead us to conclude that, although the PS1, APH-1, NCT, and PEN-2 are essential for gamma-secretase activity, the proteolysis of APP-CTF and Notch S2/NEXT are differentially regulated and require the activity of additional cofactors that promote production of AICD, NICD, and A beta.

Learning and memory deficits in Notch mutant mice

Notch is a critical component of evolutionarily conserved signaling mechanisms that regulate development and may contribute to plasticity-related processes, including changes in neurite structure and maintenance of neural stem cells. Deficits in the Notch pathway are responsible for Alagille and Cadasil syndromes, which are associated with mental retardation and dementia. Additionally, in postmitotic neurons, Notch proteins interact with presenilins and with beta-amyloid precursor protein and could therefore have a role in the memory deficits associated with familial and sporadic Alzheimer's disease. To test if alterations in Notch signaling can lead to learning and memory deficits, we studied mice with mutations in this pathway. Here, we show that null heterozygous mutations in Notch1 result in deficits in spatial learning and memory without affecting other forms of learning, motor control, or exploratory activity. We also show that null heterozygous mutations in the downstream cofactor RBP-J result in similarly specific spatial learning and memory deficits. These data indicate that a constitutive decrease in Notch signaling can result in specific learning and memory deficits and suggest that abnormalities in Notch-dependent transcription may contribute to the cognitive deficits associated with Alzheimer's disease and Alagille and Cadasil syndromes.