Presenilin 1 is involved in maturation and trafficking of N-cadherin to the plasma membrane

Advances in the cell biology of the trafficking and processing of amyloid precursor protein: impact of familial Alzheimer's disease mutations

The production of neurotoxic amyloid-β peptides (Aβ) is central to the initiation and progression of Alzheimer's disease (AD) and involves sequential cleavage of the amyloid precursor protein (APP) by β- and γ-secretases. APP and the secretases are transmembrane proteins and their co-localisation in the same membrane-bound sub-compartment is necessary for APP cleavage. The intracellular trafficking of APP and the β-secretase, BACE1, is critical in regulating APP processing and Aβ production and has been studied in several cellular systems. Here, we summarise the intracellular distribution and transport of APP and its secretases, and the intracellular location for APP cleavage in non-polarised cells and neuronal models. In addition, we review recent advances on the potential impact of familial AD mutations on APP trafficking and processing. This is critical information in understanding the molecular mechanisms of AD progression and in supporting the development of novel strategies for clinical treatment.

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

Introduction

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

Methods

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

Result

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

Conclusion

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

Flying under the radar: CDH2 (N-cadherin), an important hub molecule in neurodevelopmental and neurodegenerative diseases

CDH2 belongs to the classic cadherin family of Ca2+-dependent cell adhesion molecules with a meticulously described dual role in cell adhesion and β-catenin signaling. During CNS development, CDH2 is involved in a wide range of processes including maintenance of neuroepithelial integrity, neural tube closure (neurulation), confinement of radial glia progenitor cells (RGPCs) to the ventricular zone and maintaining their proliferation-differentiation balance, postmitotic neural precursor migration, axon guidance, synaptic development and maintenance. In the past few years, direct and indirect evidence linked CDH2 to various neurological diseases, and in this review, we summarize recent developments regarding CDH2 function and its involvement in pathological alterations of the CNS.

β2-adrenergic Agonists Rescue Lysosome Acidification and Function in PSEN1 Deficiency by Reversing Defective ER-to-lysosome Delivery of ClC-7

Lysosomal dysfunction is considered pathogenic in Alzheimer disease (AD). Loss of presenilin-1 (PSEN1) function causing AD impedes acidification via defective vacuolar ATPase (vATPase) V0a1 subunit delivery to lysosomes. We report that isoproterenol (ISO) and related β2-adrenergic agonists reacidify lysosomes in PSEN1 Knock out (KO) cells and fibroblasts from PSEN1 familial AD patients, which restores lysosomal proteolysis, calcium homeostasis, and normal autophagy flux. We identify a novel rescue mechanism involving Portein Kinase A (PKA)-mediated facilitation of chloride channel-7 (ClC-7) delivery to lysosomes which reverses markedly lowered chloride (Cl^-) content in PSEN1 KO lysosomes. Notably, PSEN1 loss of function impedes Endoplasmic Reticulum (ER)-to-lysosome delivery of ClC-7. Transcriptomics of PSEN1-deficient cells reveals strongly downregulated ER-to-lysosome transport pathways and reversibility by ISO, thus accounting for lysosomal Cl^- deficits that compound pH elevation due to deficient vATPase and its rescue by β2-adrenergic agonists. Our findings uncover a broadened PSEN1 role in lysosomal ion homeostasis and novel pH modulation of lysosomes through β2-adrenergic regulation of ClC-7, which can potentially be modulated therapeutically.

Presenilins and γ-Secretase in Membrane Proteostasis

The presenilin (PS) proteins exert a crucial role in the pathogenesis of Alzheimer disease (AD) by mediating the intramembranous cleavage of amyloid precursor protein (APP) and the generation of amyloid β-protein (Aβ). The two homologous proteins PS1 and PS2 represent the catalytic subunits of distinct γ-secretase complexes that mediate a variety of cellular processes, including membrane protein metabolism, signal transduction, and cell differentiation. While the intramembrane cleavage of select proteins by γ-secretase is critical in the regulation of intracellular signaling pathways, the plethora of identified protein substrates could also indicate an important role of these enzyme complexes in membrane protein homeostasis. In line with this notion, PS proteins and/or γ-secretase has also been implicated in autophagy, a fundamental process for the maintenance of cellular functions and homeostasis. Dysfunction in the clearance of proteins in the lysosome and during autophagy has been shown to contribute to neurodegeneration. This review summarizes the recent knowledge about the role of PS proteins and γ-secretase in membrane protein metabolism and trafficking, and the functional relation to lysosomal activity and autophagy.

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.

Consequences of inhibiting amyloid precursor protein processing enzymes on synaptic function and plasticity

Alzheimer's disease (AD) is a neurodegenerative disease, one of whose major pathological hallmarks is the accumulation of amyloid plaques comprised of aggregated β-amyloid (Aβ) peptides. It is now recognized that soluble Aβ oligomers may lead to synaptic dysfunctions early in AD pathology preceding plaque deposition. Aβ is produced by a sequential cleavage of amyloid precursor protein (APP) by the activity of β- and γ-secretases, which have been identified as major candidate therapeutic targets of AD. This paper focuses on how Aβ alters synaptic function and the functional consequences of inhibiting the activity of the two secretases responsible for Aβ generation. Abnormalities in synaptic function resulting from the absence or inhibition of the Aβ-producing enzymes suggest that Aβ itself may have normal physiological functions which are disrupted by abnormal accumulation of Aβ during AD pathology. This interpretation suggests that AD therapeutics targeting the β- and γ-secretases should be developed to restore normal levels of Aβ or combined with measures to circumvent the associated synaptic dysfunction(s) in order to have minimal impact on normal synaptic function.

Re-expression of N-cadherin in remyelinating lesions of experimental inflammatory demyelination

The cell adhesion molecule N-cadherin is involved in several processes during central nervous system development, but also in certain pathologic conditions in the adult brain, including tumorigenesis and Alzheimer's disease. N-cadherin function in inflammatory demyelinating disease has so far not been investigated. In vitro studies suggest a role of N-cadherin in myelination; on the other hand N-cadherin has been implicated in the formation of the glial scar, which is believed to impede remyelination. The aim of our study was to investigate the expression pattern of N-cadherin immunoreactivity in experimental autoimmune encephalomyelitis induced by myelin oligodendrocyte glycoprotein (MOG-EAE), an animal model closely mimicking multiple sclerosis. It allows a detailed evaluation of all stages of de- and remyelination during lesion development. Immunopathological evaluation was performed on paraffin-embedded CNS sections sampled at days 20 to 120 post immunization. We found a predominant expression of N-cadherin on oligodendrocytes in early remyelinating lesions, while in fully remyelinated shadow plaques there was no detectable immunoreactivity for N-cadherin. This expression pattern indicates a role of N-cadherin in the initiation of remyelination, most likely by providing a guidance between myelin lamellae and oligodendrocytes. Once the initial contact is made N-cadherin is then rapidly downregulated and virtually absent after completion of the repair process, analog to its known role in developmental myelination. Our results show that N-cadherin plays an important role in creating a remyelination-facilitating environment.

Neurodegeneration in Alzheimer disease: role of amyloid precursor protein and presenilin 1 intracellular signaling

Alzheimer disease (AD) is a heterogeneous neurodegenerative disorder characterized by (1) progressive loss of synapses and neurons, (2) intracellular neurofibrillary tangles, composed of hyperphosphorylated Tau protein, and (3) amyloid plaques. Genetically, AD is linked to mutations in few proteins amyloid precursor protein (APP) and presenilin 1 and 2 (PS1 and PS2). The molecular mechanisms underlying neurodegeneration in AD as well as the physiological function of APP are not yet known. A recent theory has proposed that APP and PS1 modulate intracellular signals to induce cell-cycle abnormalities responsible for neuronal death and possibly amyloid deposition. This hypothesis is supported by the presence of a complex network of proteins, clearly involved in the regulation of signal transduction mechanisms that interact with both APP and PS1. In this review we discuss the significance of novel finding related to cell-signaling events modulated by APP and PS1 in the development of neurodegeneration.

Neuroprotection by donepezil against glutamate excitotoxicity involves stimulation of alpha7 nicotinic receptors and internalization of NMDA receptors

BACKGROUND AND PURPOSE

Glutamate excitotoxicity may be involved in ischaemic injury to the CNS and some neurodegenerative diseases, such as Alzheimer's disease. Donepezil, an acetylcholinesterase (AChE) inhibitor, exerts neuroprotective effects. Here we demonstrated a novel mechanism underlying the neuroprotection induced by donepezil.

EXPERIMENTAL APPROACH

Cell damage in primary rat neuron cultures was quantified by lactate dehydrogenase release. Morphological changes associated with neuroprotective effects of nicotine and AChE inhibitors were assessed by immunostaining. Cell surface levels of the glutamate receptor sub-units, NR1 and NR2A, were analyzed using biotinylation. Immunoblot was used to measure protein levels of cleaved caspase-3, total NR1, total NR2A and phosphorylated NR1. Immunoprecipitation was used to measure association of NR1 with the post-synaptic protein, PSD-95. Intracellular Ca(2+) concentrations were measured with fura 2-acetoxymethylester. Caspase 3-like activity was measured using enzyme substrate, 7-amino-4-methylcoumarin (AMC)-DEVD.

KEY RESULTS

Levels of NR1, a core subunit of the NMDA receptor, on the cell surface were significantly reduced by donepezil. In addition, glutamate-mediated Ca(2+) entry was significantly attenuated by donepezil. Methyllycaconitine, an inhibitor of alpha7 nicotinic acetylcholine receptors (nAChR), inhibited the donepezil-induced attenuation of glutamate-mediated Ca(2+) entry. LY294002, a phosphatidyl inositol 3-kinase (PI3K) inhibitor, had no effect on attenuation of glutamate-mediated Ca(2+) entry induced by donepezil.

CONCLUSIONS AND IMPLICATIONS

Decreased glutamate toxicity through down-regulation of NMDA receptors, following stimulation of alpha7 nAChRs, could be another mechanism underlying neuroprotection by donepezil, in addition to up-regulating the PI3K-Akt cascade or defensive system.

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.

N-cadherin-based adhesion enhances Abeta release and decreases Abeta42/40 ratio

In neurons, Presenilin 1(PS1)/gamma-secretase is located at the synapses, bound to N-cadherin. We have previously reported that N-cadherin-mediated cell-cell contact promotes cell-surface expression of PS1/gamma-secretase. We postulated that N-cadherin-mediated trafficking of PS1 might impact synaptic PS1-amyloid precursor protein interactions and Abeta generation. In the present report, we evaluate the effect of N-cadherin-based contacts on Abeta production. We demonstrate that stable expression of N-cadherin in Chinese hamster ovary cells, expressing the Swedish mutant of human amyloid precursor protein leads to enhanced secretion of Abeta in the medium. Moreover, N-cadherin expression decreased Abeta(42/40) ratio. The effect of N-cadherin expression on Abeta production was accompanied by the enhanced accessibility of PS1/gamma-secretase to amyloid precursor protein as well as a conformational change of PS1, as demonstrated by the fluorescence lifetime imaging technique. These results indicate that N-cadherin-mediated synaptic adhesion may modulate Abeta secretion as well as the Abeta(42/40) ratio via PS1/N-cadherin interactions.

Novel role of presenilins in maturation and transport of integrin beta 1

Presenilins (PSs) play important roles in modulating the trafficking and maturation of several membrane proteins. However, the target membrane proteins whose trafficking and maturation are regulated by PS are largely unknown. By characterizing PS-deficient fibroblasts, we found that integrin beta1 maturation is promoted markedly in PS1 and PS2 double-deficient fibroblasts and moderately in PS1- or PS2-deficient fibroblasts; in contrast, nicastrin maturation is completely inhibited in PS1 and PS2 double-deficient fibroblasts. Subcellular fractionation analysis demonstrated that integrin beta1 maturation is promoted in the Golgi apparatus. The mature integrin beta1 with an increased expression level was delivered to the cell surface, which resulted in an increased cell surface expression level of mature integrin beta1 in PS1 and PS2 double-deficient fibroblasts. PS1 and PS2 double-deficient fibroblasts exhibited an enhanced ability to adhere to culture dishes coated with integrin beta1 ligands, namely, fibronectin and laminin. The inhibition of gamma-secretase activity enhances neither integrin beta1 maturation nor the adhesion of wild-type cells. Moreover, PS deficiency also promoted the maturation of integrins alpha3 and alpha5 and the cell surface expression of integrin alpha3. Integrins alpha3 and alpha5 were coimmunoprecipitated with integrin beta1, suggesting the formation of the functional heterodimers integrins alpha3beta1 and alpha5beta1. Note that integrin beta1 exhibited features opposite those of nicastrin in terms of maturation and trafficking from the endoplasmic reticulum (ER) to the Golgi apparatus in PS1 and PS2 double-deficient fibroblasts. Our results therefore suggest that PS regulates the maturation of membrane proteins in opposite directions and cell adhesion by modulating integrin maturation.

Pharmacogenomics in Alzheimer's disease

Pharmacological treatment in Alzheimer's disease (AD) accounts for 10-20% of direct costs, and fewer than 20% of AD patients are moderate responders to conventional drugs (donepezil, rivastigmine, galantamine, memantine), with doubtful cost-effectiveness. Both AD pathogenesis and drug metabolism are genetically regulated complex traits in which hundreds of genes cooperatively participate. Structural genomics studies demonstrated that more than 200 genes might be involved in AD pathogenesis regulating dysfunctional genetic networks leading to premature neuronal death. The AD population exhibits a higher genetic variation rate than the control population, with absolute and relative genetic variations of 40-60% and 0.85-1.89%, respectively. AD patients also differ in their genomic architecture from patients with other forms of dementia. Functional genomics studies in AD revealed that age of onset, brain atrophy, cerebrovascular hemodynamics, brain bioelectrical activity, cognitive decline, apoptosis, immune function, lipid metabolism dyshomeostasis, and amyloid deposition are associated with AD-related genes. Pioneering pharmacogenomics studies also demonstrated that the therapeutic response in AD is genotype-specific, with apolipoprotein E (APOE) 4/4 carriers the worst responders to conventional treatments. About 10-20% of Caucasians are carriers of defective cytochrome P450 (CYP) 2D6 polymorphic variants that alter the metabolism and effects of AD drugs and many psychotropic agents currently administered to patients with dementia. There is a moderate accumulation of AD-related genetic variants of risk in CYP2D6 poor metabolizers (PMs) and ultrarapid metabolizers (UMs), who are the worst responders to conventional drugs. The association of the APOE-4 allele with specific genetic variants of other genes (e.g., CYP2D6, angiotensin-converting enzyme [ACE]) negatively modulates the therapeutic response to multifactorial treatments affecting cognition, mood, and behavior. Pharmacogenetic and pharmacogenomic factors may account for 60-90% of drug variability in drug disposition and pharmacodynamics. The incorporation of pharmacogenetic/pharmacogenomic protocols to AD research and clinical practice can foster therapeutics optimization by helping to develop cost-effective pharmaceuticals and improving drug efficacy and safety.

Presenilin 1 regulates epidermal growth factor receptor turnover and signaling in the endosomal-lysosomal pathway

Mutations in the gene encoding presenilin 1 (PS1) cause the most aggressive form of early-onset familial Alzheimer disease. In addition to its well established role in Abeta production and Notch proteolysis, PS1 has been shown to mediate other physiological activities, such as regulation of the Wnt/beta-catenin signaling pathway, modulation of phosphatidylinositol 3-kinase/Akt and MEK/ERK signaling, and trafficking of select membrane proteins and/or intracellular vesicles. In this study, we present evidence that PS1 is a critical regulator of a key signaling receptor tyrosine kinase, epidermal growth factor receptor (EGFR). Specifically, EGFR levels were robustly increased in fibroblasts deficient in both PS1 and PS2 (PS(-/-)) due to delayed turnover of EGFR protein. Stable transfection of wild-type PS1 but not PS2 corrected EGFR to levels comparable to PS(+/+) cells, while FAD PS1 mutations showed partial loss of activity. The C-terminal fragment of PS1 was sufficient to fully reduce EGFR levels. In addition, the rapid ligand-induced degradation of EGFR was markedly delayed in PS(-/-) cells, resulting in prolonged signal activation. Despite the defective turnover of EGFR, ligand-induced autophosphorylation, ubiquitination, and endocytosis of EGFR were not affected by the lack of PS1. Instead, the trafficking of EGFR from early endosomes to lysosomes was severely delayed by PS1 deficiency. Elevation of EGFR was also seen in brains of adult mice conditionally ablated in PS1 and in skin tumors associated with the loss of PS1. These findings demonstrate a critical role of PS1 in the trafficking and turnover of EGFR and suggest potential pathogenic effects of elevated EGFR as well as perturbed endosomal-lysosomal trafficking in cell cycle control and Alzheimer disease.

Biogenesis and processing of the amyloid precursor protein in the early secretory pathway

The beta-amyloid peptide is an aggregation-prone peptide that is released from the amyloid precursor protein (APP) after cleavage by the beta- and gamma-secretase. A number of studies have suggested that processing of APP by beta- and gamma-secretase occurs not only at the cell surface and in the endosomal compartments but also in the endoplasmic reticulum (ER) and Golgi complex. Here, we studied the role of the early secretory pathway in processing of APP. For this purpose, APP was in vitro translated in semi-permeabilized cells, which have a functionally intact ER and Golgi complex but lack a functional plasma membrane. We show that the beta-secretase cleavage product C99 is generated in the early secretory pathway. Moreover, nicastrin and presenilin, two components of the gamma-secretase complex, interacted with newly synthesized APP. Administration of the gamma-secretase inhibitor L685,458 caused accumulation of full length APP and C99. Full length APP but not C99 interacted with several protein quality control ER chaperones including the thiol oxidoreductase ERp57. Our in vitro study suggests that newly synthesized APP is subject to amyloidogenic processing during the initial phases of the secretory pathway.

GSK3beta activity modifies the localization and function of presenilin 1

Presenilin 1, a causative gene product of familial Alzheimer disease, has been reported to be localized mainly in the endoplasmic reticulum and Golgi membranes. However, endogenous Presenilin 1 also localizes at the plasma membrane as a biologically active molecule. Presenilin 1 interacts with N-cadherin/beta-catenin to form a trimeric complex at the synaptic site through its loop domain, whose serine residues (serine 353 and 357) can be phosphorylated by glycogen synthase kinase 3beta. Here, we demonstrate that cell-surface expression of Presenilin 1/gamma-secretase is enhanced by N-cadherin-based cell-cell contact. Physical interaction between Presenilin 1 and N-cadherin/beta-catenin plays an important role in this process. Glycogen synthase kinase 3beta-mediated phosphorylation of Presenilin 1 reduces its binding to N-cadherin, thereby down-regulating its cell-surface expression. Moreover, reduction of the Presenilin 1.N-cadherin.beta-catenin complex formation leads to an impaired activation of contact-mediated phosphatidylinositol 3-kinase/Akt cell survival signaling. Furthermore, phosphorylation of Presenilin 1 hinders epsilon-cleavage of N-cadherin, whereas epsilon-cleavage of APP remained unchanged. This is the first report that clarifies the regulatory mechanism of Presenilin 1/gamma-secretase with respect to its subcellular distribution and its differential substrate cleavage. Because the cleavage of various membrane proteins by Presenilin 1/gamma-cleavage is involved in cellular signaling, glycogen synthase kinase 3beta-mediated phosphorylation of Presenilin 1 should be deeply associated with signaling functions. Our findings indicate that the abnormal activation of glycogen synthase kinase 3beta can reduce neuronal viability and synaptic plasticity via modulating Presenilin 1/N-cadherin/beta-catenin interaction and thus have important implications in the pathophysiology of Alzheimer disease.

Amyloid β inhibits ectodomain shedding of N-cadherin via down-regulation of cell-surface NMDA receptor

Dysfunction in the synapse is recognized as an early and the primary pathological process in Alzheimer's disease (AD). N-cadherin, an essential adhesion molecule for excitatory synaptic contact, forms a complex with presenilin 1 (PS1) and beta-catenin in the synaptic membrane. N-cadherin is sequentially cleaved by ADAM10 and PS1/gamma-secretase, producing a cytoplasmic fragment, N-cadherin C-terminal fragment (Ncad/CTF2) after NMDA receptor stimulation [Marambaud P, Wen PH, Dutt A, Shioi J, Takashima A, Siman R, Robakis NK (2003) A CBP binding transcriptional repressor produced by the PS1/epsilon-cleavage of N-cadherin is inhibited by PS1 FAD mutations. Cell 114:635-645; Reiss K, Maretzky T, Ludwig A, Tousseyn T, de Strooper B, Hartmann D, Saftig P (2005) ADAM10 cleavage of N-cadherin and regulation of cell-cell adhesion and beta-catenin nuclear signalling. EMBO J 24:1762]. Ncad/CTF2 translocates to the nucleus together with beta-catenin to enhance beta-catenin nuclear signaling [Uemura K, Kihara T, Kuzuya A, Okawa K, Nishimoto T, Bito H, Ninomiya H, Sugimoto H, Kinoshita A, Shimohama S (2006a) Activity-dependent regulation of beta-catenin via epsilon-cleavage of N-cadherin. Biochem Biophys Res Commun 345:951-958]. To examine whether an impairment of N-cadherin metabolism is involved in AD pathogenesis, we investigated the effect of amyloid beta peptide (Abeta) treatment on sequential N-cadherin cleavage. Here, we demonstrate that both synthetic and cell-derived Abeta species inhibit ectodomain shedding of mouse N-cadherin. Inhibition of N-cadherin cleavage by Abeta treatment was suggested to be mediated by the enhanced endocytosis of NMDA receptor, resulting in reduced turnover of N-cadherin. Since both N-cadherin and beta-catenin are essential for synaptic plasticity, impairment of N-cadherin cleavage caused by Abeta may underlie the synapse toxicity involved in AD pathogenesis.

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/gamma-secretase-mediated cleavage regulates association of leukocyte-common antigen-related (LAR) receptor tyrosine phosphatase with beta-catenin

Leukocyte-common antigen-related (LAR) receptor tyrosine phosphatase regulates cell adhesion and formation of functional synapses and neuronal networks. Here we report that LAR is sequentially cleaved by alpha- and presenilin (PS)/gamma-secretases, which also affect signaling and/or degradation of type-I membrane proteins including the Alzheimer disease-related beta-amyloid precursor protein. Similar to the previously characterized PS/gamma-secretase substrates, inhibition of gamma-secretase activity resulted in elevated LAR C-terminal fragment (LAR-CTF) levels in stably LAR-overexpressing Chinese hamster ovary (CHO) cells, human neuroglioma cells, and mouse cortical neurons endogenously expressing LAR. Furthermore, LAR-CTF levels increased in cells lacking functional PS, indicating that gamma-secretase-mediated cleavage of LAR was PS-dependent. Inhibition of alpha-secretase activity by TAPI-1 treatment blocked LAR-CTF accumulation, demonstrating that prior ectodomain shedding was prerequisite for PS/gamma-secretase-mediated cleavage of LAR. Moreover, we identified the product of PS/gamma-secretase cleavage, LAR intracellular domain (LICD), both in vitro and in cells overexpressing full-length (FL) LAR or LAR-CTFs. LAR localizes to cadherin-beta-catenin-based cellular junctions. Assembly and disassembly of these junctions are regulated by tyrosine phosphorylation. We found that endogenous tyrosine-phosphorylated beta-catenin coimmunoprecipitated with LAR in CHO cells. However, when PS/gamma-secretase activity was inhibited, the association between LAR and beta-catenin significantly diminished. In addition to cell adhesion, beta-catenin is involved in transcriptional regulation. We observed that LICD significantly decreased transcription of cyclin D1, one of the beta-catenin target genes. Thus, our results show that PS/gamma-secretase-mediated cleavage of LAR controls LAR-beta-catenin interaction, suggesting an essential role for PS/gamma-secretase in the regulation of LAR signaling.

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

Presenilin-1 (PS1) has gained intensive attention in relation to Alzheimer's disease, since it has been shown that PS1 mutations are linked to familial Alzheimer's disease (FAD), and that PS1 is a member of the high molecular weight complex of gamma-secretase, which generates the carboxyl end of beta-amyloid peptide (gamma-cleavage). A parallel line of evidence suggests that upon formation of cell-cell contacts, presenilin colocalizes with cadherins at the cell surface and stabilizes the cadherin-based adhesion complex. Under conditions stimulating cell-cell dissociation, cadherins are processed by a PS1/gamma-secretase activity, promoting disassembly of adherens junctions, and resulting in the increase of cytosolic beta-catenin, which is an important regulator of the Wnt/Wingless signaling pathway. PS1 also controls the cleavage of a number of transmembrane proteins at the interface of their transmembrane and cytosolic domains (epsilon-cleavage), producing intracellular fragments with a putative transcriptional role. Remarkably, cleavage of N-cadherin by PS1 produces an intracellular fragment that downregulates CREB-mediated transcription, indicating a role of PS1 in gene expression. PS1 mutations associated with FAD abolish production of the N-cadherin intracellular fragment and thus fail to suppress CREB-dependent transcription. These findings suggest an alternative explanation for FAD that is separate from the widely accepted 'amyloid hypothesis': dysfunction in transcription regulatory mechanisms.

Targeting the role of the endosome in the pathophysiology of Alzheimer's disease: a strategy for treatment

Membrane-bound endosomal vesicles play an integral role in multiple cellular events, including protein processing and turnover, and often critically regulate the cell-surface availability of receptors and other plasma membrane proteins in many different cell types. Neurons are no exception, being dependent on endosomal function for housekeeping and synaptic events. Growing evidence suggests a link between neuronal endosomal function and Alzheimer's disease (AD) pathophysiology. Endosomal abnormalities invariably occur within neurons in AD brains, and endocytic compartments are one likely site for the production of the pathogenic beta-amyloid peptide (Abeta), which accumulates within the brain during the disease and is generated by proteolytic processing of the amyloid precursor protein (APP). The enzymes and events involved in APP processing are appealing targets for therapeutic agents aimed at slowing or reversing the pathogenesis of AD. The neuronal endosome may well prove to be the intracellular site of action for inhibitors of beta-amyloidogenic APP processing. We present here the view that knowledge of the endosomal system in the disease can guide drug discovery of AD therapeutic agents.

Characterization of sequential N-cadherin cleavage by ADAM10 and PS1

N-cadherin is essential for excitatory synaptic contact in the hippocampus. At the sites of synaptic contact, it forms a complex with Presenilin 1(PS1) and beta-catenin. N-cadherin is cleaved by ADAM10 in response to NMDA receptor stimulation, producing a membrane fragment Ncad/CTF1 in neurons. NMDA receptor stimulation also enhances PS1/gamma-secretase-mediated cleavage of N-cadherin. To characterize the regulatory mechanisms of the ADAM10 and PS1-mediated cleavages, we first identified the precise cleavage sites of N-cadherin by ADAM10 and PS1/gamma-secretase by producing cleavage-deficient N-cadherin mutants. Next, we found that ectodomain shedding of N-cadherin by ADAM10 is a primary regulatory step in response to calcium influx, and that it is required for the subsequent PS1/gamma-secretase-mediated epsilon-cleavage of N-cadherin, which is a constitutive process to yield a cytoplasmic fragment, Ncad/CTF2. Since N-cadherin is essential for the structure and function of synapses including the long-term potentiation, those proteolytic events of N-cadherin should affect the adhesive behavior of the synapses, thereby taking part in learning and memory.

Activity-dependent regulation of beta-catenin via epsilon-cleavage of N-cadherin

N-cadherin is essential for excitatory synaptic contact in the hippocampus. Presenilin 1 (PS1) is located at sites of synaptic contact, forming a complex with N-cadherin and beta-catenin. Here, we report that human N-cadherin is cleaved by PS1/gamma-secretase in response to physiological concentration of glutamate (Glu) stimulation, yielding a fragment Ncad/CTF2. The expression of Ncad/CTF2 in neuronal cells led to its translocation to the nucleus, and caused a prominent enhancement of cytoplasmic and nuclear beta-catenin levels in a cell-cell contact dependent manner, via following mechanisms: 1, inhibition of beta-catenin phosphorylation; 2, transactivation of beta-catenin; and 3, inhibition of N-cadherin transcription, and finally enhanced beta-catenin nuclear signaling. Since the regulation of cellular beta-catenin level is essential for synaptic function, disruption in the cleavage of N-cadherin may be causally linked to the synaptic dysfunction associated with Alzheimer's disease (AD).

Presenilin/γ‐secretase activity regulates protein clearance from the endocytic recycling compartment

The presenilin (PS)/gamma-secretase complex proteolytically cleaves more than 20 different proteins in addition to the amyloid precursor protein (APP). These substrates are almost exclusively type I membrane proteins. Many undergo internalization from the cell surface followed by degradation or recycling back to the plasma membrane through the endocytic recycling compartment (ERC). Evidence shows that the PSs also regulate intracellular trafficking of APP and its C-terminal fragments (CTFs). To investigate whether PS/gamma-secretase activity is required for normal endosomal recycling, we performed live cell imaging experiments with fluorescently labeled transferrin, reported to specifically traffic through the ERC. By using pharmacological gamma-secretase inhibitors or cell lines lacking functional PS/gamma-secretase, here we show that PS/gamma-secretase activity is required for clearance of transferrin from the ERC. Interestingly, lack of PS/gamma-secretase function also resulted in the accumulation of APP and APP-CTFs in the ERC in addition to the cell surface. Familial Alzheimer's disease mutations in APP-CTFs did not affect endocytic recycling of these proteins. Our results suggest that PS/gamma-secretase activity is required for normal endosomal recycling of soluble and membrane-associated proteins through the ERC and propose a new mechanism by which impaired PS/gamma-secretase function may eventually contribute to neurodegeneration.

Linking molecular motors to Alzheimer's disease

It is currently thought that Alzheimer's disease develops due to aberrant generation of amyloid-beta peptides. However, the mechanisms underlying the aberrant generation of amyloid-beta peptides remain unknown. An emerging concept suggests that impaired axonal transport may play a pivotal role in the aberrant generation of amyloid-beta peptides. Here we review and discuss advances in understanding AD with the primary focus on the possible role of molecular motors and axonal transport in its pathogenesis.

Presenilin 1 is involved in the maturation of β-site amyloid precursor protein-cleaving enzyme 1 (BACE1)

One of the pathologic hallmarks of Alzheimer's disease is the excessive deposition of beta-amyloid peptides (Abeta) in senile plaques. Abeta is generated when beta-amyloid precursor protein (APP) is cleaved sequentially by beta-secretase, identified as beta-site APP-cleaving enzyme 1 (BACE1), and gamma-secretase, a putative enzymatic complex containing presenilin 1 (PS1). However, functional interaction between PS1 and BACE1 has never been known. In addition to this classical role in the generation of Abeta peptides, it has also been proposed that PS1 affects the intracellular trafficking and maturation of selected membrane proteins. We show that the levels of exogenous and endogenous mature BACE1 expressed in presenilin-deficient mouse embryonic fibroblasts (PS-/-MEFs) were reduced significantly compared to those in wild-type MEFs. Moreover, the levels of mature BACE1 were increased in human neuroblastoma cell line, SH-SY5Y, stably expressing wild-type PS1, compared to native cells. Conversely, the maturation of BACE1 was compromised under the stable expression of dominant-negative mutant PS1 overexpression. Immunoprecipitation assay showed that PS1 preferably interacts with proBACE1 rather than mature BACE1, indicating that PS1 can be directly involved in the maturation process of BACE1. Further, endogenous PS1 was immunoprecipitated with endogenous BACE1 in SH-SY5Y cells and mouse brain tissue. We conclude that PS1 is directly involved in the maturation of BACE1, thus possibly functioning as a regulator of both beta- and gamma-secretase in Abeta generation.

Assembly of the N-cadherin complex during synapse formation involves uncoupling of p120-catenin and association with presenilin 1

N-cadherin is an adhesion receptor that participates in both interaction between immature pre- and postsynaptic neurons and in the stabilization and function of matured neuron-neuron synapses. To better understand how the N-cadherin complex contributes to synapse formation, we examined its distribution and composition during synapse formation in the chick ciliary neurons. It was found that at early phases of synaptogenesis, N-cadherin is distributed in small clusters on the cell surface and primarily associates with p120-catenin and beta-catenin. In contrast, as synaptic contacts matured, larger N-cadherin clusters were found localized adjacent to the active zone and associated with PS1 and gamma-catenin, while p120- and beta-catenin were dispersed among other cell regions, including axons. As it is known that PS1 binds gamma-catenin and that uncoupled p120-catenin can alter the cytoskeleton via its effect on Rho GTPases, these changes in the molecular composition of the N-cadherin complex (represented by the uncoupling of p120-catenin and association with PS1) may correspond to distinct functional states of the complex involved in synaptic maturation.

Presenilins mediate phosphatidylinositol 3-kinase/AKT and ERK activation via select signaling receptors. Selectivity of PS2 in platelet-derived growth factor signaling

The Alzheimer's disease-linked genes, PS1 and PS2, are required for intramembrane proteolysis of multiple type I proteins, including Notch and amyloid precursor protein. In addition, it has been documented that PS1 positively regulates, whereas PS1 familial Alzheimer disease mutations suppress, phosphatidylinositol 3-kinase (PI3K)/Akt activation, a pathway known to inactivate glycogen synthase kinase-3 and reduce tau phosphorylation. In this study, we show that the loss of presenilins not only inhibits PI3K/Akt signaling and increases tau phosphorylation but also suppresses the MEK/ERK pathway. The deficits in Akt and ERK activation in cells deficient in both PS1 and PS2 (PS-/-) are evident after serum withdrawal and stimulation with fetal bovine serum or ligands of select receptor tyrosine kinases, platelet-derived growth factor receptor beta (PDGFR beta) and PDGFR alpha, but not insulin-like growth factor-1R and epidermal growth factor receptor. The defects in PDGF signaling in PS-/- cells are due to reduced expression of PDGF receptors. Whereas fetal bovine serum-induced Akt activation is reconstituted by both PS1 and PS2 in PS-/- cells, PDGF signaling is selectively restored by PS2 but not PS1 and is dependent on the N-terminal fragment of PS2 but not gamma-secretase activity or the hydrophilic loop of PS2. The rescue of PDGF receptor expression and activation by PS2 is facilitated by FHL2, a PS2-interacting transcriptional co-activator. Finally, we present evidence that PS1 mutations interfere with this PS2-mediated activity by reducing PS2 fragments. These findings highlight important roles of both presenilins in Akt and ERK signaling via select signaling receptors.

Aph-1 interacts at the cell surface with proteins in the active γ-secretase complex and membrane-tethered Notch

The activity of the gamma-secretase complex is critical for the processing of a number of transmembrane proteins, including Notch. Functional gamma-secretase activity can be reconstituted from four proteins--presenilin, nicastrin, Pen-2 and Aph-1--but the role of the individual proteins remains unclear. In this report we describe the cellular localization and protein interactions of Aph-1, with particular regard to Notch receptor processing. We found that Aph-1 is present at the cell surface, where it interacts with Pen-2, the mature forms of presenilin and nicastrin, and full-length Notch. Aph-1 also interacts with a truncated form of Notch, which is a direct substrate for gamma-secretase, but not with the Notch intracellular domain. Immunoprecipitation data for Notch and Aph-1 showed that the Notch-containing gamma-secretase complexes most likely form a small subset of the total number of gamma-secretase complexes. In conclusion, these data demonstrate that Aph-1 is present at the cell surface, presumably in active gamma-secretase complexes, and interacts with the Notch receptor, both before and after ligand activation.

The ins and outs of E-cadherin trafficking

One way of controlling the activity of E-cadherin--a protein that is, simultaneously, a major cell-adhesion molecule, a powerful tumour suppressor, a determinant of cell polarity and a partner to the potent catenin signalling molecules--is to keep it on the move. During the past two decades, many insights into the fundamental role of E-cadherin in these processes have been garnered. Studies during the past five years have begun to reveal the importance of intracellular trafficking as a means of regulating the functions of E-cadherin. E-cadherin is trafficked to and from the cell surface by exocytic and multiple endocytic pathways. In this article, we survey the vesicle-trafficking machinery that is responsible for the sorting, transport, actin association and vesicle targeting of E-cadherin to regulate its movement and function during growth and development and, possibly, in cancer.

Gamma-secretase exists on the plasma membrane as an intact complex that accepts substrates and effects intramembrane cleavage

Research on Alzheimer's disease led to the identification of a novel proteolytic mechanism in all metazoans, the presenilin/gamma-secretase complex. This unique intramembrane-cleaving aspartyl protease is required for the normal processing of Notch, Jagged, beta-amyloid precursor protein (APP), E-cadherin, and many other receptor-like proteins. We recently provided indirect evidence of gamma-secretase activity at the cell surface in HeLa cells following inhibition of receptor-mediated endocytosis. Here, we directly identify and isolate gamma-secretase as an intact complex (Presenilin, Nicastrin, Aph-1, and Pen-2) from the plasma membrane, both in overexpressing cell lines and endogenously. Inhibition of its proteolytic activity allowed cell surface gamma-secretase to be captured in association with its plasma membrane-localized APP substrates (C83 and C99). Moreover, non-denaturing isolation of the intact enzyme complex revealed that cell surface gamma-secretase can specifically generate amyloid beta-protein from an APP substrate and similarly cleave a Notch substrate. These data directly establish the proteolytic function of gamma-secretase on the plasma membrane, independent of a hypothesized substrate trafficking role. We conclude that presenilin/gamma-secretase exists as a mature complex at the cell surface, where it interacts with and can cleave its substrates, consistent with an essential function in processing many adhesion molecules and receptors required for cell-cell interaction or intercellular signaling.

p120 catenin is required for morphogenetic movements involved in the formation of the eyes and the craniofacial skeleton in Xenopus

During Xenopus development, p120 transcripts are enriched in highly morphogenetic tissues. We addressed the developmental function of p120 by knockdown experiments and by expressing E-cadherin mutants unable to bind p120. This resulted in defective eye formation and provoked malformations in the craniofacial cartilage structures, derivatives of the cranial neural crest cells. Closer inspection showed that p120 depletion impaired evagination of the optic vesicles and migration of cranial neural crest cells from the neural tube into the branchial arches. These morphogenetic processes were also affected by p120-uncoupled cadherins or E-cadherin containing a deletion of the juxtamembrane domain. Irrespective of the manipulation that caused the malformations, coexpression of dominant-negative forms of either Rac1 or LIM kinase rescued the phenotypes. Wild-type RhoA and constitutively active Rho kinase caused partial rescue. Our results indicate that, in contrast to invertebrates, p120 is an essential factor for vertebrate development and an adequate balance between cadherin activity and cytoskeletal condition is critical for correct morphogenetic movements.

Presenilin 1 stabilizes the C-terminal fragment of the amyloid precursor protein independently of gamma-secretase activity

The cleavage of the transmembrane amyloid precursor protein (APP) by beta-secretase leaves the C-terminal fragment of APP, C99, anchored in the plasma membrane. C99 is subsequently processed by gamma-secretase, an unusual aspartyl protease activity largely dependent on presenilin (PS), generating the amyloid beta-peptide (Abeta) that accumulates in the brain of patients with Alzheimer's disease. It has been suggested that PS proteins are the catalytic core of this proteolytic activity, but a number of other proteins mandatory for gamma-secretase cleavage have also been discovered. The exact role of PS in the gamma-secretase activity remains a matter of debate, because cells devoid of PS still produce some forms of Abeta. Here, we used insect cells expressing C99 to demonstrate that the expression of presenilin 1 (PS1), which binds C99, not only increases the production of Abeta by these cells but also increases the intracellular levels of C99 to the same extent. Using pulse-chase experiments, we established that this results from an increased half-life of C99 in cells expressing PS1. In Chinese hamster ovary cells producing C99 from full-length human APP, similar results were observed. Finally, we show that a functional inhibitor of gamma-secretase does not alter the ability of PS1 to increase the intracellular levels of C99. This finding suggests that the binding of PS1 to C99 does not necessarily lead to its immediate cleavage by gamma-secretase, which could be a spatio-temporally regulated or an induced event, and provides biochemical evidence for the existence of a substrate-docking site on PS1.

Loss of presenilin function causes impairments of memory and synaptic plasticity followed by age-dependent neurodegeneration

Mutations in presenilins are the major cause of familial Alzheimer's disease, but the pathogenic mechanism by which presenilin mutations cause memory loss and neurodegeneration remains unclear. Here we demonstrate that conditional double knockout mice lacking both presenilins in the postnatal forebrain exhibit impairments in hippocampal memory and synaptic plasticity. These deficits are associated with specific reductions in NMDA receptor-mediated responses and synaptic levels of NMDA receptors and alphaCaMKII. Furthermore, loss of presenilins causes reduced expression of CBP and CREB/CBP target genes, such as c-fos and BDNF. With increasing age, mutant mice develop striking neurodegeneration of the cerebral cortex and worsening impairments of memory and synaptic function. Neurodegeneration is accompanied by increased levels of the Cdk5 activator p25 and hyperphosphorylated tau. These results define essential roles and molecular targets of presenilins in synaptic plasticity, learning and memory, and neuronal survival in the adult cerebral cortex.