Presenilin 1 and cadherins: stabilization of cell-cell adhesion and proteolysis-dependent regulation of transcription

Presenilin: A Multi-Functional Molecule in the Pathogenesis of Alzheimer's Disease and Other Neurodegenerative Diseases

Presenilin, a transmembrane protein primarily known for its role in Alzheimer's disease (AD) as part of the γ-secretase complex, has garnered increased attention due to its multifaceted functions in various cellular processes. Recent investigations have unveiled a plethora of functions beyond its amyloidogenic role. This review aims to provide a comprehensive overview of presenilin's diverse roles in AD and other neurodegenerative disorders. It includes a summary of well-known substrates of presenilin, such as its involvement in amyloid precursor protein (APP) processing and Notch signaling, along with other functions. Additionally, it highlights newly discovered functions, such as trafficking function, regulation of ferritin expression, apolipoprotein E (ApoE) secretion, the interaction of ApoE and presenilin, and the Aβ42-to-Aβ40-converting activity of ACE. This updated perspective underscores the evolving landscape of presenilin research, emphasizing its broader impact beyond established pathways. The incorporation of these novel findings accentuates the dynamic nature of presenilin's involvement in cellular processes, further advancing our comprehension of its multifaceted roles in neurodegenerative disorders. By synthesizing evidence from a range of studies, this review sheds light on the intricate web of presenilin functions and their implications in health and disease.

Presenilin mutations and their impact on neuronal differentiation in Alzheimer's disease

The presenilin genes (PSEN1 and PSEN2) are mainly responsible for causing early-onset familial Alzheimer's disease, harboring ~300 causative mutations, and representing ~90% of all mutations associated with a very aggressive disease form. Presenilin 1 is the catalytic core of the γ-secretase complex that conducts the intramembranous proteolytic excision of multiple transmembrane proteins like the amyloid precursor protein, Notch-1, N- and E-cadherin, LRP, Syndecan, Delta, Jagged, CD44, ErbB4, and Nectin1a. Presenilin 1 plays an essential role in neural progenitor maintenance, neurogenesis, neurite outgrowth, synaptic function, neuronal function, myelination, and plasticity. Therefore, an imbalance caused by mutations in presenilin 1/γ-secretase might cause aberrant signaling, synaptic dysfunction, memory impairment, and increased Aβ₄₂/Aβ₄₀ ratio, contributing to neurodegeneration during the initial stages of Alzheimer's disease pathogenesis. This review focuses on the neuronal differentiation dysregulation mediated by PSEN1 mutations in Alzheimer's disease. Furthermore, we emphasize the importance of Alzheimer's disease-induced pluripotent stem cells models in analyzing PSEN1 mutations implication over the early stages of the Alzheimer's disease pathogenesis throughout neuronal differentiation impairment.

The ectodomain of cadherin-11 binds to erbB2 and stimulates Akt phosphorylation to promote cranial neural crest cell migration

During development, a multi-potent group of cells known as the cranial neural crest (CNC) migrate to form craniofacial structures. Proper migration of these cells requires proteolysis of cell adhesion molecules, such as cadherins. In Xenopus laevis, preventing extracellular cleavage of cadherin-11 impairs CNC migration. However, overexpression of the soluble cleavage product (EC1-3) is capable of rescuing this phenotype. The mechanism by which EC1-3 promotes CNC migration has not been investigated until now. Here we show that EC1-3 stimulates phosphorylation of Akt, a target of PI3K, in X.laevis CNC. Through immunoprecipitation experiments, we determined that EC1-3 interacts with all ErbB receptors, PDGFRα, and FGFR1. Of these receptors, only ErbB2 was able to produce an increase in Akt phosphorylation upon treatment with a recombinant EC1-3. This increase was abrogated by mubritinib, an inhibitor of ErbB2. We were able to recapitulate this decrease in Akt phosphorylation in vivo by knocking down ErbB2 in CNC cells. Knockdown of the receptor also significantly reduced CNC migration in vivo. We confirmed the importance of ErbB2 and ErbB receptor signaling in CNC migration using mubritinib and canertinib, respectively. Mubritinib and the PI3K inhibitor LY294002 significantly decreased cell migration while canertinib nearly prevented it altogether. These data show that ErbB2 and Akt are important for CNC migration and implicate other ErbB receptors and Akt-independent signaling pathways. Our findings provide the first example of a functional interaction between the extracellular domain of a type II classical cadherin and growth factor receptors.

Presenilin-1 Targeted Morpholino Induces Cognitive Deficits, Increased Brain Aβ1-42 and Decreased Synaptic Marker PSD-95 in Zebrafish Larvae

Presenilins are transmembrane proteases required for the proteolytic cleavage of Notch and also act as the catalytic core of the γ-secretase complex, which is responsible for the final cleavage of the amyloid precursor protein into Amyloid-β (Aβ) peptides of varying lengths. Presenilin-1 gene (psen1) mutations are the main cause of early-onset autosomal-dominant Familial Alzheimer Disease. Elucidating the roles of Presenilin-1 and other hallmark proteins involved in Alzheimer's disease is crucial for understanding the disease etiology and underlying molecular mechanisms. In our study, we used a morpholino antisense nucleotide that targets exon 8 splicing site of psen1 resulting in a dominant negative protein previously validated to investigate behavioral and molecular effects in 5 days post fertilization (dpf) zebrafish larvae. Morphants showed specific cognitive deficits in two optomotor tasks and morphological phenotypes similar to those induced by suppression of Notch signaling pathway. They also had increased mRNA levels of neurog1 at 5 dpf, confirming the potential interaction of Presenilin-1 and Notch in our model. We also evaluated levels of apoptotic markers including p53, PAR-4, Caspase-8 and bax-alpha and found only bax-a decreased at 5dpf. Western Blot analysis showed an increase in Aβ_1-42 and a decrease in the selective post-synaptic marker PSD-95 at 5 dpf. Our data demonstrates that psen1 splicing interference induces phenotypes that resemble early-stage AD, including cognitive deficit, Aβ_1-42 accumulation and synaptic reduction, reinforcing the potential contribution of zebrafish larvae to studies of human brain diseases.

Integrating network, sequence and functional features using machine learning approaches towards identification of novel Alzheimer genes

Background

Alzheimer’s disease (AD) is a complex progressive neurodegenerative disorder commonly characterized by short term memory loss. Presently no effective therapeutic treatments exist that can completely cure this disease. The cause of Alzheimer’s is still unclear, however one of the other major factors involved in AD pathogenesis are the genetic factors and around 70 % risk of the disease is assumed to be due to the large number of genes involved. Although genetic association studies have revealed a number of potential AD susceptibility genes, there still exists a need for identification of unidentified AD-associated genes and therapeutic targets to have better understanding of the disease-causing mechanisms of Alzheimer’s towards development of effective AD therapeutics.

Results

In the present study, we have used machine learning approach to identify candidate AD associated genes by integrating topological properties of the genes from the protein-protein interaction networks, sequence features and functional annotations. We also used molecular docking approach and screened already known anti-Alzheimer drugs against the novel predicted probable targets of AD and observed that an investigational drug, AL-108, had high affinity for majority of the possible therapeutic targets. Furthermore, we performed molecular dynamics simulations and MM/GBSA calculations on the docked complexes to validate our preliminary findings.

Conclusions

To the best of our knowledge, this is the first comprehensive study of its kind for identification of putative Alzheimer-associated genes using machine learning approaches and we propose that such computational studies can improve our understanding on the core etiology of AD which could lead to the development of effective anti-Alzheimer drugs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3108-1) contains supplementary material, which is available to authorized users.

Dynamic self-guiding analysis of Alzheimer's disease

We applied a self-guiding evolutionary algorithm to initiate the synthesis of the Alzheimer's disease-related data and literature. A protein interaction network associated with amyloid-beta precursor protein (APP) and a seed model that treats Alzheimer's disease as progressive dysregulation of APP-associated signaling were used as dynamic “guides” and structural “filters” in the recursive search, analysis, and assimilation of data to drive the evolution of the seed model in size, detail, and complexity. Analysis of data and literature across sub-disciplines and system-scale discovery platforms suggests a key role of dynamic cytoskeletal connectivity in the stability, plasticity, and performance of multicellular networks and architectures. Chronic impairment and/or dysregulation of cell adhesions/synapses, cytoskeletal networks, and/or reversible epithelial-to-mesenchymal-like transitions, which enable and mediate the stable and coherent yet dynamic and reconfigurable multicellular architectures, may lead to the emergence and persistence of the disordered, wound-like pockets/microenvironments of chronically disconnected cells. Such wound-like microenvironments support and are supported by pro-inflammatory, pro-secretion, de-differentiated cellular phenotypes with altered metabolism and signaling. The co-evolution of wound-like microenvironments and their inhabitants may lead to the selection and stabilization of degenerated cellular phenotypes, via acquisition of epigenetic modifications and mutations, which eventually result in degenerative disorders such as cancer and Alzheimer's disease.

A fast growing spectrum of biological functions of γ-secretase in development and disease

γ-secretase, which assembles as a tetrameric complex, is an aspartyl protease that proteolytically cleaves substrate proteins within their membrane-spanning domain; a process also known as regulated intramembrane proteolysis (RIP). RIP regulates signaling pathways by abrogating or releasing signaling molecules. Since the discovery, already >15 years ago, of its catalytic component, presenilin, and even much earlier with the identification of amyloid precursor protein as its first substrate, γ-secretase has been commonly associated with Alzheimer's disease. However, starting with Notch and thereafter a continuously increasing number of novel substrates, γ-secretase is becoming linked to an equally broader range of biological processes. This review presents an updated overview of the current knowledge on the diverse molecular mechanisms and signaling pathways controlled by γ-secretase, with a focus on organ development, homeostasis and dysfunction. This article is part of a Special Issue entitled: Intramembrane Proteases.

Presenilin 1/γ-secretase modulates P-cadherin processing and influences cell adhesion in oral squamous cell carcinoma cell lines

P-cadherin belongs to a family of Ca(2+)-dependent homophilic cell-cell adhesion proteins that are important for correct cellular localization and tissue integrity in the oral epithelium. P-cadherin is only expressed in the basal and suprabasal cell layers of the oral epithelium, but in advanced oral squamous cell carcinoma (OSCC), a reduced membranous and an enhanced cytoplasmic truncated P-cadherin level is observed. In this study, we investigated the impact of presenilin (PS) 1/γ-secretase on P-cadherin processing in OSCC. Western blot analyses showed an enhanced PS1 expression in OSCC cell lines and in primary oral keratinocytes (POK) isolated from primary OSCC tissue (OSCC POK) compared with POKs isolated from normal oral mucosa. Immunocytochemical stainings and co-immunoprecipitation experiments revealed a cytoplasmic colocalization and a direct interaction of P-cadherin and PS1 in OSCC POKs. Blocking of PS1/γ-secretase activity by the PS1/γ-secretase inhibitors and N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester, another specific γ-secretase inhibitor yielded a 100 kDa P-cadherin band on western blots of OSCC cell line extracts. Small interfering RNA knockdown of PS1 equally generated a 100 kDa P-cadherin form in OSCC POKs. Mass spectrometry analyses and experiments with the glycosylation inhibitor tunicamycin characterized the appearing 100 kDa P-cadherin band as the unglycosylated full-length form of P-cadherin. On the functional level, cell attachment assays demonstrated an enhanced cell adhesion after PS1/γ-secretase inhibition only in the transiently P-cadherin expressing OSCC cell line PCI52 but not in the PCI52 control cells. In summary, our results show that PS1/γ-secretase contributes to P-cadherin processing and to reduced cell adhesion in OSCC.

Inhibition of presenilins attenuates proliferation and invasion in bladder cancer cells through multiple pathways

OBJECTIVE

Presenilin (PS)/γ-secretase is a key protease that initiates various biological processes. We investigated the effect of PS/γ-secretase on the expression and inhibition of urothelial cell carcinoma of bladder (UCB) as a potential alternative therapeutic target for UCB.

MATERIALS AND METHODS

PS-1 and PS-2 were identified in normal and malignant human bladder transitional cells by immunohistochemistry. We blocked PSs using a PS/γ-secretase inhibitor N-(N-[3,5-difluorophenacetyl]-L-alanyl)-S-phenylglycine-t-butylester (DAPT), and the proliferative and invasive potential of UCB cells SW780, BIU-87, 5637, and T24, and human normal urothelial cell line SV-HUC-1 were analyzed using Western blot, cell viability test, flow cytometry, and transwell assay. All experiments were repeated at least 3 times.

RESULTS

Human bladder samples of UCB, SW780, BIU-87, 5637, and T24 cells expressed higher PS-1 compared with normal ones. Cell vitality test demonstrated that DAPT attenuated UCB cell proliferation more than SV-HUC-1. Flow cytometry and transwell assay showed that T24 cells were arrested at G1/S checkpoint and its invasive ability was impaired. Western blot assay markedly showed that protein levels of CD44-intracellular domain, insulinlike growth factor-1Rβ, extracellular regulated protein kinase 1/2, cyclin D1, proliferating cell nuclear antigen, and matrix metalloproteinase-9 were downregulated by DAPT, whereas vascular endothelial growth factor receptor-2 and vascular endothelial growth factor-165 were upregulated.

CONCLUSIONS

Our study revealed that PS-1 might be implicated in the proliferation and invasion of UCB, and that it may serve as a potential therapeutic target for UCB, but further studies are warranted to verify the effects of inhibition of PS/γ-secretase on angiogenesis.

Interactome analyses of mature γ-secretase complexes reveal distinct molecular environments of presenilin (PS) paralogs and preferential binding of signal peptide peptidase to PS2

γ-Secretase plays a pivotal role in the production of neurotoxic amyloid β-peptides (Aβ) in Alzheimer disease (AD) and consists of a heterotetrameric core complex that includes the aspartyl intramembrane protease presenilin (PS). The human genome codes for two presenilin paralogs. To understand the causes for distinct phenotypes of PS paralog-deficient mice and elucidate whether PS mutations associated with early-onset AD affect the molecular environment of mature γ-secretase complexes, quantitative interactome comparisons were undertaken. Brains of mice engineered to express wild-type or mutant PS1, or HEK293 cells stably expressing PS paralogs with N-terminal tandem-affinity purification tags served as biological source materials. The analyses revealed novel interactions of the γ-secretase core complex with a molecular machinery that targets and fuses synaptic vesicles to cellular membranes and with the H⁺-transporting lysosomal ATPase macrocomplex but uncovered no differences in the interactomes of wild-type and mutant PS1. The catenin/cadherin network was almost exclusively found associated with PS1. Another intramembrane protease, signal peptide peptidase, predominantly co-purified with PS2-containing γ-secretase complexes and was observed to influence Aβ production.

Arginylation-dependent regulation of a proteolytic product of talin is essential for cell-cell adhesion

Talin is a large scaffolding molecule that plays a major role in integrin-dependent cell-matrix adhesion. A role for talin in cell-cell attachment through cadherin has never been demonstrated, however. Here, we identify a novel calpain-dependent proteolytic cleavage of talin that results in the release of a 70-kD C-terminal fragment, which serves as a substrate of posttranslational arginylation. The intracellular levels of this fragment closely correlated with the formation of cell-cell adhesions, and this fragment localized to cadherin-containing cell-cell contacts. Moreover, reintroduction of this fragment rescued the cell-cell adhesion defects in arginyltransferase (Ate1) knockout cells, which normally have a very low level of this fragment. Arginylation of this fragment further enhanced its ability to rescue cell-cell adhesion formation. In addition, arginylation facilitated its turnover, suggesting a dual role of arginylation in its intracellular regulation. Thus, our work identifies a novel proteolytic product of talin that is regulated by arginylation and a new role of talin in cadherin-dependent cell-cell adhesion.

The many substrates of presenilin/γ-secretase

The Alzheimer's disease (AD)-associated amyloid-β protein precursor (AβPP) is cleaved by α-, β-, and presenilin (PS)/γ-secretases through sequential regulated proteolysis. These proteolytic events control the generation of the pathogenic amyloid-β (Aβ) peptide, which excessively accumulates in the brains of individuals afflicted by AD. A growing number of additional proteins cleaved by PS/γ-secretase continue to be discovered. Similarly to AβPP, most of these proteins are type-I transmembrane proteins involved in vital signaling functions regulating cell fate, adhesion, migration, neurite outgrowth, or synaptogenesis. All the identified proteins share common structural features, which are typical for their proteolysis. The consequences of the PS/γ-secretase-mediated cleavage on the function of many of these proteins are largely unknown. Here, we review the current literature on the proteolytic processing mediated by the versatile PS/γ-secretase complex. We begin by discussing the steps of AβPP processing and PS/γ-secretase complex composition and localization, which give clues to how and where the processing of other PS/γ-secretase substrates may take place. Then we summarize the typical features of PS/γ-secretase-mediated protein processing. Finally, we recapitulate the current knowledge on the possible physiological function of PS/γ-secretase-mediated cleavage of specific substrate proteins.

Notch inhibition as a promising new approach to cancer therapy

The Notch pathway powerfully influences stem cell maintenance, development and cell fate and is increasingly recognized for the key roles it plays in cancer. Notch promotes cell survival, angiogenesis and treatment resistance in numerous cancers, making it a promising target for cancer therapy. It also crosstalks with other critical oncogenes, providing a means to affect numerous signaling pathways with one intervention. While the gamma-secretase inhibitors are the only form of Notch inhibitors in clinical trials, other forms of Notch inhibition have been developed or are theoretically feasible. In this chapter we review the rationales for Notch inhibition in cancer and then discuss in detail the various modalities for Notch inhibition, both current and speculative.

Caspase-3 cleavage links delta-catenin to the novel nuclear protein ZIFCAT

δ-Catenin is an Armadillo protein of the p120-catenin subfamily capable of modulating cadherin stability, small GTPase activity, and nuclear transcription. From yeast two-hybrid screening of a human embryonic stem cell cDNA library, we identified δ-catenin as a potential interacting partner of the caspase-3 protease, which plays essential roles in apoptotic as well as non-apoptotic processes. Interaction of δ-catenin with caspase-3 was confirmed using cleavage assays conducted in vitro, in Xenopus apoptotic extracts, and in cell line chemically induced contexts. The cleavage site, a highly conserved caspase consensus motif (DELD) within Armadillo repeat 6 of δ-catenin, was identified through peptide sequencing. Cleavage thus generates an amino-terminal (residues 1-816) and carboxyl-terminal (residues 817-1314) fragment, each containing about half of the central Armadillo domain. We found that cleavage of δ-catenin both abolishes its association with cadherins and impairs its ability to modulate small GTPases. Interestingly, 817-1314 possesses a conserved putative nuclear localization signal that may facilitate the nuclear targeting of δ-catenin in defined contexts. To probe for novel nuclear roles of δ-catenin, we performed yeast two-hybrid screening of a mouse brain cDNA library, resolving and then validating interaction with an uncharacterized KRAB family zinc finger protein, ZIFCAT. Our results indicate that ZIFCAT is nuclear and suggest that it may associate with DNA as a transcriptional repressor. We further determined that other p120 subfamily catenins are similarly cleaved by caspase-3 and likewise bind ZIFCAT. Our findings potentially reveal a simple yet novel signaling pathway based upon caspase-3 cleavage of p120-catenin subfamily members, facilitating the coordinate modulation of cadherins, small GTPases, and nuclear functions.

PEDF regulates vascular permeability by a γ-secretase-mediated pathway

Increased vascular permeability is an inciting event in many vascular complications including diabetic retinopathy. We have previously reported that pigment epithelium-derived factor (PEDF) is able to inhibit vascular endothelial growth factor (VEGF)-induced angiogenesis through a novel γ-secretase-dependent pathway. In this study, we asked whether inhibition of VEGF-induced permeability by PEDF is also γ-secretase-mediated and to dissect the potential mechanisms involved. Vascular permeability was assessed in vitro by measuring transendothelial resistance and paracellular permeability to dextran and in vivo by following leakage of intravenous FITC-labelled albumin into the retina in the presence or absence of VEGF and PEDF in varying combinations. Experiments were undertaken in the presence or absence of a γ-secretase inhibitor. To assess junctional integrity immunohistochemistry for the adherens junction (AJ) proteins, VE-cadherin and β-catenin, and the tight junction (TJ) protein, claudin-5 was undertaken using cultured cells and flat mount retinas. Protein expression and the association between AJ proteins, VEGF receptors (VEGFRs) and γ-secretase constituents were determined by immunoprecipitation and Western Blot analysis. In selected experiments the effect of hypoxia on junctional integrity was also assessed. PEDF inhibition of VEGF-induced permeability, both in cultured microvascular endothelial cell monolayers and in vivo in the mouse retinal vasculature, is mediated by γ-secretase. PEDF acted by a) preventing dissociation of AJ and TJ proteins and b) regulating both the association of VEGF receptors with AJ proteins and the subsequent phosphorylation of the AJ proteins, VE-cadherin and β-catenin. Association of γ-secretase with AJ proteins appears to be critical in the regulation of vascular permeability. Although hypoxia increased VEGFR expression there was a significant dissociation of VEGFR from AJ proteins. In conclusion, PEDF regulates VEGF-induced vascular permeability via a novel γ-secretase dependent pathway and targeting downstream effectors of PEDF action may represent a promising therapeutic strategy for preventing or ameliorating increased vascular permeability.

Ca2+ signaling in airway epithelial cells facilitates leukocyte recruitment and transepithelial migration

In airway cells, TLR2 stimulation by bacterial products activates Ca2+ fluxes that signal leukocyte recruitment to the lung and facilitates transepithelial migration into the airway lumen. TLR2 is apically displayed on airway cells, where it senses bacterial stimuli. Biochemical and genetic approaches demonstrate that TLR2 ligands stimulate release of Ca2+ from intracellular stores by activating TLR2 phosphorylation by c-Src and recruiting PI3K and PLCgamma to affect Ca2+ release through IP3Rs. This Ca2+ release plays a pivotal role in signaling TLR2-dependent NF-kappaB activation and chemokine expression to recruit PMNs to the lung. In addition, TLR2-initiated Ca2+ release activates Ca2+-dependent proteases, calpains, which cleave the transmembrane proteins occludin and E-cadherin to promote PMN transmigration. This review highlights recent findings that demonstrate a central role for Ca2+ signaling in airway epithelial cells to induce proinflammatory gene transcription and to initiate junctional changes that accommodate transmigration of recruited PMNs.

Calpain-mediated N-cadherin proteolytic processing in brain injury

Neural-cadherin (N-cadherin), a member of the classical cadherin family of transmembrane glycoproteins, mediates cellular recognition and cell-cell adhesion through calcium-dependent homophilic interactions and plays important roles in the development and maintenance of the nervous system. Metalloproteinase is known to cleave N-cadherin, which is further cleaved by gamma-secretase. The intracellular domain of N-cadherin interacts with beta-catenin, and beta-catenin stability is critical for cell-cell adhesion and cell survival. In the present study, we showed that N-cadherin is cleaved specifically by calpain, resulting in the generation of a novel 110 kDa fragment. The cleavage occurred in ischemic brain lesions and in vitro neural cells in the presence of NMDA and ionomycin, and was restored by calpain inhibitors but not matrix metalloproteinase or gamma-secretase inhibitors. Calpain directly cleaved N-cadherin in in vitro calpain assays, and calpain inhibitors prevented its cleavage in a dose-dependent manner. Using N-cadherin deletion mutants, we found that calpain cleavage sites exist in at least four regions of the cytoplasmic domain. Treatment with NMDA induced neuronal death, and it suppressed the expression of surface N-cadherin and the N-cadherin/beta-catenin interaction, effects that were prevented by calpain inhibitor. Furthermore, calpain-mediated N-cadherin cleavage significantly affected cell-cell adhesion, AKT signaling, the N-cadherin/beta-catenin interaction and the Wnt target gene expressions through the accumulation of nuclear beta-catenin.

Notch inhibitors as a new tool in the war on cancer: a pathway to watch

Notch was first recognized as an important developmental pathway in Drosophila in the first half of the 20th century. Many decades later, this pathway has been found to play central roles in humans in stem cell maintenance, cell fate decisions, and in cancer as well. Notch family members are being revealed as oncogenes in an ever-increasing number of cancers. Though significant progress has been made in dissecting the complex workings of this signaling pathway, there are very limited options available for Notch inhibitors. However, the pioneering class of Notch inhibitors is already in clinical trials for two cancer types. This review will address the current state-of-the-art, agents in the pipeline, and potential strategies for future Notch inhibitors. Successful development of Notch inhibitors in the clinic holds great promise as a new anti-cancer strategy.

Presenilin: RIP and beyond

Over the years the presenilins (PSENs), a family of multi-transmembrane domain proteins, have been ascribed a number of diverse potential functions. Recent in vivo evidence has supported the existence of PSEN functions beyond its well-established role in regulated intramembrane proteolysis. In this review, we will briefly discuss the ability of PSEN to modulate cellular signaling pathways through gamma-secretase cleavage of transmembrane proteins. Additionally, we will critically examine the proposed roles of PSEN in the regulation of beta-catenin function, protein trafficking, calcium regulation, and apoptosis.

Homer2 and Homer3 interact with amyloid precursor protein and inhibit Abeta production

The study of Amyloid Precursor Protein (APP) processing has been the focus of considerable interest, since it leads to Abeta peptide generation, the main constituent of neuritic plaques found in brains of Alzheimer's disease patients. Therefore, the identification of novel APP binding partners that regulate Abeta peptide production represents a pharmaceutical target aiming at reducing Alphabeta pathology. In this study, we provide evidence that Homer2 and Homer3 but not Homer1 proteins interact specifically with APP. Their expression inhibits APP processing and reduces secretion of Abeta peptides. In addition, they decrease the levels of cell surface APP and inhibit maturation of APP and beta-secretase (BACE1). The effects of Homer2 and Homer3 on APP trafficking to the cell surface and/or on APP and BACE1 maturation could be part of the mechanism by which the expression of these proteins leads to the significant reduction of Abeta peptide production.

gamma-Secretase: a multifaceted regulator of angiogenesis

Physiological angiogenesis is essential for development, homeostasis and tissue repair but pathological neovascularization is a major feature of tumours, rheumatoid arthritis and ocular complications. Studies over the last decade have identified γ-secretase, a presenilin-dependent protease, as a key regulator of angiogenesis through: (i) regulated intramembrane proteolysis and transmembrane cleavage of receptors (e.g. VEGFR-1, Notch, ErbB-4, IGFI-R) followed by translocation of the intracellular domain to the nucleus, (ii) translocation of full length membrane-bound receptors to the nucleus (VEGFR-1), (iii) phosphorylation of membrane bound proteins (VEGFR-1 and ErbB-4), (iv) modulation of adherens junctions (cadherin) and regulation of permeability and (v) cleavage of amyloid precursor protein to amyloid-β which is able to regulate the angiogenic process. The γ-secretase-induced translocation of receptors to the nucleus provides an alternative intracellular signalling pathway, which acts as a potent regulator of transcription. γ-secretase is a complex composed of four different integral proteins (presenilin, nicastrin, Aph-1 and Pen-2), which determine the stability, substrate binding, substrate specificity and proteolytic activity of γ-secretase. This seeming complexity allows numerous possibilities for the development of targeted γ-secretase agonists/antagonists, which can specifically regulate the angiogenic process. This review will consider the structure and function of γ-secretase, the growing evidence for its role in angiogenesis and the substrates involved, γ-secretase as a therapeutic target and future challenges in this area.

Differential activation of the calcium/protein kinase C and the canonical beta-catenin pathway by Wnt1 and Wnt7a produces opposite effects on cell proliferation in PC12 cells

We examined the effect of Wnt1 and Wnt7a on cell proliferation using undifferentiated PC12 cells, which originate from the neural crest and are widely employed as a neuronal cell model. Heterologous expression of Wnt1 enhanced [3H]thymidine incorporation and expression of cyclin D1 and cylin E in PC12 cells. Opposite effects were observed in PC12 cells expressing Wnt7a. Searching for the mechanisms underlying the opposite effects of Wnt1 and Wnt7a on PC12 cell proliferation, we examined the activation of the canonical beta-catenin/T-cell-lymphoid enhancer-binding protein transcription factor pathway and the 'calcium pathway' by co-transfecting the cells with a reporter gene controlled by either T-cell-lymphoid enhancer-binding protein transcription factor or the calcium-activated transcription factor, NFAT. Wnt1 and Wnt7a activated both pathways, but to a different extent. While Wnt1 preferentially activated the calcium pathway, Wnt7a mainly activated the canonical pathway. Pharmacological inhibition of protein kinase C, which is a component of the calcium pathway, abrogated the increase in cell proliferation induced by Wnt1 without affecting the antiproliferative action of Wnt7a. The action of Wnt7a was instead occluded by lithium ions, which mimic the activation of the canonical pathway, and was largely reduced by Dickkopf-1, which acts as an inhibitor of the canonical pathway. In addition, expression of a constitutively active mutant of beta-catenin potently activated the canonical Wnt pathway and reduced [3H]thymidine incorporation. These data challenge the view that the canonical Wnt pathway invariably supports cell growth and suggest that, at least in PC12 cells, cell proliferation is regulated by the balance between the calcium/protein kinase C pathway and the canonical pathway.

Amyloid Precursor Protein and Presenilin1 Interact with the Adaptor GRB2 and Modulate ERK 1,2 Signaling

The amyloid precursor protein (APP) and the presenilins 1 and 2 are genetically linked to the development of familial Alzheimer disease. APP is a single-pass transmembrane protein and precursor of fibrillar and toxic amyloid-beta peptides, which are considered responsible for Alzheimer disease neurodegeneration. Presenilins are multipass membrane proteins, involved in the enzymatic cleavage of APP and other signaling receptors and transducers. The role of APP and presenilins in Alzheimer disease development seems to be related to the formation of amyloid-beta peptides; however, their physiological function, reciprocal interaction, and molecular mechanisms leading to neurodegeneration are unclear. APP and presenilins are also involved in multiple interactions with intracellular proteins, the significance of which is under investigation. Among the different APP-interacting proteins, we focused our interest on the GRB2 adaptor protein, which connects cell surface receptors to intracellular signaling pathways. In this study we provide evidence by co-immunoprecipitation experiments, confocal and electron microscopy, and by fluorescence resonance energy transfer experiments that both APP and presenilin1 interact with GRB2 in vesicular structures at the centrosome of the cell. The final target for these interactions is ERK1,2, which is activated in mitotic centrosomes in a PS1- and APP-dependent manner. These data suggest that both APP and presenilin1 can be part of a common signaling pathway that regulates ERK1,2 and the cell cycle.

Presenilin diversifies its portfolio

Presenilin, the catalytic member of the gamma-secretase proteolytic complex, was discovered through its roles in generating Alzheimer's-disease-associated amyloid-beta peptides from the amyloid-beta precursor protein and in releasing the transcriptionally active domain of the receptor Notch. Recent work has revealed many additional cleavage substrates and interacting proteins, suggesting a diversity of roles for presenilin during development and adult life, some of which might contribute to Alzheimer's disease progression. Although many of these functions depend on the proteolytic activity of gamma-secretase, others are independent of its role as a protease. Here, we review recent data on candidate functions for presenilin and its interactors and on their potential significance in disease.

Presenilin 2 is secreted in mouse primary neurons: a release enhanced by apoptosis

Presenilin 1 (PS1) and presenilin 2 (PS2) are homologous transmembrane proteins genetically associated with Alzheimer disease. As previously reported by our group for PS1, here we demonstrated that, in mouse primary neurons and microglial cells, PS2 C-terminal fragment (CTF) is released by shedding into the extracellular compartment as a soluble form and that this release is 4.07-fold increased during apoptosis. When compared with PS1, PS2 seems to be more susceptible to apoptosis since its secretion is increased 2.8-fold more than PS1. During apoptosis either proteins were colocalized especially within shedded vesicles. The present study suggest an active role for the presenilins CTF on putative target cells.