Presenilin-1 binds cytoplasmic epithelial cadherin, inhibits cadherin/p120 association, and regulates stability and function of the cadherin/catenin adhesion complex

A core function for p120-catenin in cadherin turnover

p120-catenin stabilizes epithelial cadherin (E-cadherin) in SW48 cells, but the mechanism has not been established. Here, we show that p120 acts at the cell surface to control cadherin turnover, thereby regulating cadherin levels. p120 knockdown by siRNA expression resulted in dose-dependent elimination of epithelial, placental, neuronal, and vascular endothelial cadherins, and complete loss of cell–cell adhesion. ARVCF and δ-catenin were functionally redundant, suggesting that proper cadherin-dependent adhesion requires the presence of at least one p120 family member. The data reveal a core function of p120 in cadherin complexes, and strongly predict a dose-dependent loss of E-cadherin in tumors that partially or completely down-regulate p120.

In vivo functions of catenins

The adhesion of cells to neighbor cells determines cellular and tissue morphogenesis and regulates major cellular processes including motility, growth, survival, and differentiation. Regions of cell-cell adhesion are adherens junctions, desmosomes, and tight junctions. Cadherins are transmembrane molecules whose extracellular domains transmit the direct interaction of two cells. The intracellular cadherin domains bind directly or indirectly to the submembranous catenins, which are linked to the cytoskeleton. Four types of catenins, alpha-catenin, beta-catenin, gamma-catenin, and p120 catenin are known. Three of them, beta-, gamma-, and p120 catenin, are structurally related and possess similar protein interaction domains, the so-called armadillo repeats. These catenins are also parts of signal transduction pathways and play a role in phenotypical changes of cells, e.g., during switches from adherent to migratory cells. The function of catenins in such basic cellular processes also determines a role of catenins in embryogenesis, adult tissue homeostasis, and disease. In particular, beta-catenin is known to be an important oncoprotein in human cancer development.

Phosphorylation of Presenilin 1 at the Caspase Recognition Site Regulates Its Proteolytic Processing and the Progression of Apoptosis

The Alzheimer's disease-associated presenilin (PS) 1 is intimately involved in gamma-secretase cleavage of beta-amyloid precursor protein and other proteins. In addition, PS1 plays a role in beta-catenin signaling and in the regulation of apoptosis. Here we demonstrate that phosphorylation of PS1 is regulated by two independent signaling pathways involving protein kinase (PK) A and PKC and that both kinases can directly phosphorylate the large hydrophilic domain of PS1 in vitro and in cultured cells. A phosphorylation site at serine residue 346 was identified that is selectively phosphorylated by PKC but not by PKA. This site is localized within a recognition motif for caspases, and phosphorylation strongly inhibits proteolytic processing of PS1 by caspase activity during apoptosis. Moreover, PS1 phosphorylation reduces the progression of apoptosis. Our data indicate that phosphorylation/dephosphorylation at the caspase recognition site provides a mechanism to reversibly regulate properties of PS1 in apoptosis.

p120-independent modulation of E-cadherin adhesion activity by the membrane-proximal region of the cytoplasmic domain

Cadherins are transmembrane glycoproteins that function as Ca2+-dependent cell-cell adhesion molecules and are linked to the actin cytoskeleton via catenins. Previously, we showed that, although E-cadherin lacking its cytoplasmic tail is active in aggregation assays, partially truncated E-cadherin lacking the carboxyl-terminal catenin-binding site is not. Contrary to this observation, a similar N-cadherin construct is found to be functional. Chimeric constructs, in which the membrane-proximal region of the partially truncated E-cadherin was replaced by that of N-cadherin, are active in aggregation assays. N-cadherin constructs in the opposite manner are nonfunctional. Although deletion of the membrane-proximal region, which eliminates the binding site for p120, results in activation of the nonfunctional E-cadherin mutant polypeptides, amino acid substitutions in the membrane-proximal region, which uncouple p120 binding, do not. The p120 uncoupling could not activate a full-length E-cadherin construct, which was beta-catenin-uncoupled by amino acid substitutions in the catenin-binding site. These results indicate that the membrane-proximal region determines the activity of these cadherin constructs but that p120 does not seem directly involved in the modulation of E-cadherin activity.

Amyloid precursor protein (APP) and the biology of proteolytic processing: relevance to Alzheimer's disease

The processing of amyloid precursor protein (APP) generates amyloid-beta (Abeta) peptides 1-40 and 1-42. The latter is neurotoxic and its accumulation results in amyloid fibril formation and the generation of senile plaques, the hallmark of Alzheimer's disease (AD). Whilst there has been considerable progress made in understanding the generation of Abeta by alpha-, beta- and gamma-secretase activity on APP, recently enzymes involved in the degradation of Abeta have been identified including neprilysin and insulin-degrading enzyme (IDE). We review the pathways involved in proteolytic processing of APP and discuss the potential implications of aberrant proteolysis on neurodegeneration. It is conceivable that single nucleotide polymorphisms (SNPs) in the regulatory regions of genes in these proteolytic cascades, which alter their expression, could contribute to some of the age-related changes seen in AD.

Contextual binding of p120ctn to E-cadherin at the basolateral plasma membrane in polarized epithelia

E-cadherin-catenin complexes mediate cell-cell adhesion on the basolateral membrane of epithelial cells. The cytoplasmic tail of E-cadherin supports multiple protein interactions, including binding of beta-catenin at the C terminus and of p120ctn to the juxtamembrane domain. The temporal assembly and polarized trafficking of the complex or its individual components to the basolateral membrane are not fully understood. In Madin-Darby canine kidney cells at steady state and after treatment with cycloheximide or temperature blocks, E-cadherin and beta-catenin localized to the Golgi complex, but p120ctn was found only at the basolateral plasma membrane. We previously identified a dileucine sorting motif (Leu586-Leu587, termed S1) in the juxtamembrane domain of E-cadherin and now show that it is required to target full-length E-cadherin to the basolateral membrane. Removal of S1 resulted in missorting of E-cadherin mutants (EcadDeltaS1) to the apical membrane; beta-catenin was simultaneously missorted and appeared at the apical membrane. p120ctn was not mistargeted with EcadDeltaS1, but could be recruited to the E-cadherin-catenin complex only at the basolateral membrane. These findings help define the temporal assembly and sorting of the E-cadherin-catenin complex and show that membrane recruitment of p120ctn in polarized cells is contextual and confined to the basolateral membrane.

A CBP binding transcriptional repressor produced by the PS1/epsilon-cleavage of N-cadherin is inhibited by PS1 FAD mutations

Presenilin1 (PS1), a protein implicated in Alzheimer's disease (AD), forms complexes with N-cadherin, a transmembrane protein with important neuronal and synaptic functions. Here, we show that a PS1-dependent gamma-secretase protease activity promotes an epsilon-like cleavage of N-cadherin to produce its intracellular domain peptide, N-Cad/CTF2. NMDA receptor agonists stimulate N-Cad/CTF2 production suggesting that this receptor regulates the epsilon-cleavage of N-cadherin. N-Cad/CTF2 binds the transcription factor CBP and promotes its proteasomal degradation, inhibiting CRE-dependent transactivation. Thus, the PS1-dependent epsilon-cleavage product N-Cad/CTF2 functions as a potent repressor of CBP/CREB-mediated transcription. Importantly, PS1 mutations associated with familial AD (FAD) and a gamma-secretase dominant-negative mutation inhibit N-Cad/CTF2 production and upregulate CREB-mediated transcription indicating that FAD mutations cause a gain of transcriptional function by inhibiting production of transcriptional repressor N-Cad/CTF2. These data raise the possibility that FAD mutation-induced transcriptional abnormalities maybe causally related to the dementia associated with FAD.

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

One pathological characteristic of Alzheimer's disease (AD) is extensive synapse loss. Presenilin 1 (PS1) is linked to the pathogenesis of early onset familial Alzheimer's disease (FAD) and is localized at the synapse, where it binds N-cadherin and modulates its adhesive activity. To elucidate the role of the PS1/N-cadherin interaction in synaptic contact, we established SH-SY5Y cells stably expressing wild-type (wt) PS1 and dominant-negative (D385A) PS1. We show that the formation of cadherin-based cell-cell contact among SH-SY5Y cells stably expressing D385A PS1 was suppressed. Conversely, wt PS1 cells exhibited enhanced cell-cell contact and colony formation. Suppression of cell-cell contact in D385A cells was accompanied by an alteration in N-cadherin subcellular localization; N-cadherin was retained mainly in the endoplasmic reticulum (ER) and cell surface expression was reduced. We conclude that PS1 is essential for efficient trafficking of N-cadherin from the ER to the plasma membrane. PS1-mediated delivery of N-cadherin to the plasma membrane is important for N-cadherin to exert its physiological function, and it may control the state of cell-cell contact.

Regulatory roles of presenilin-1 and nicastrin in neuronal differentiation during in vitro neurogenesis

Presenilin (PS) in association with nicastrin (NICA) forms a gamma-secretase complex that plays a crucial role in facilitating intramembranous processing of Notch, a signaling receptor that is essential for neuronal fate specification and differentiation. Loss of function studies have implicated a role for PS1 in regulating neuronal differentiation in association with the down-regulation of Notch signaling during neurogenesis. By using a system for stable, as well as tetracycline-inducible expression of interfering RNAs (RNAi), we studied the functions of PS1 during neuronal differentiation in the murine pluripotent p19 embryonic carcinoma cell line. After retinoic acid (RA) treatment and in the absence of doxycycline, neuronal progenitor cells in the p19 clone were found to extend their processes towards the neighboring colony to form network-like connections, as revealed by neuron-specific microtubule-associated protein 2 staining and laser scanning confocal microscopy. However, doxycycline-induced expression of PS1 small interfering RNA (siRNA) in the p19 clone resulted in a severe defect in the formation of network-like connections. Expression of the NICA and Notch down-stream effector genes Hes1 and Hes5 was unaffected in p19 cells expressing doxycycline-induced PS1 siRNA. In contrast to PS1, constitutive inactivation of NICA by siRNA in p19 cells resulted in premature and partial differentiation without RA treatment. In these NICA siRNA-expressing p19 cells the expression of the Notch1 down-stream effector Hes1 gene was substantially reduced. After RA treatment the NICA siRNA clone failed to differentiate completely into networks of neurons. These results taken together provide direct evidence that PS1 and NICA may participate in neuronal differentiation during neurogenesis in vitro.

RIPped out by presenilin-dependent gamma-secretase

Presenilins (PS) constitute a fascinating family of proteins that play crucial roles in several major signalling processes involved in key cellular functions and are also closely associated with dysfunction in Alzheimer's disease (AD). Presenilin-dependent intramembrane cleavage of transmembrane proteins such as amyloid beta precursor protein (AbetaPP) and Notch resides in a high-molecular-weight gamma-secretase protein complex, of which at least five core components have now been identified. Remarkably, it has now become evident that presenilin-dependent gamma-secretase activity extends beyond its role in AbetaPP and Notch cleavages to have a generic role in the regulated intramembranous cleavage of certain membrane proteins. Actually, a new picture is emerging in which cells can relay signals from the extracellular space to their interior through presenilin-dependent proteolysis within the membrane-spanning regions of type 1 integral membrane proteins to generate potential transcriptionally active intracellular fragments. This review deals with the complex biology of presenilins and focuses more specifically on recent developments regarding the composition, assembly and regulation of the gamma-secretase protein complex, its substrates and its implications for cellular signalling.

Amyloid precursor protein associates with a nicastrin-dependent docking site on the presenilin 1-gamma-secretase complex in cells demonstrated by fluorescence lifetime imaging

Gamma-secretase cleavage is the final enzymatic step generating beta-amyloid via intramembranous cleavage of the amyloid precursor protein (APP). Presenilin (PS), initially identified as a gene in which mutations account for the vast majority of early-onset autosomal dominant Alzheimer's disease, is a major component of gamma-secretase. Enzymatic activity also depends on nicastrin, Aph-1, and Pen-2. We propose a model in which gamma-secretase components assemble, interact with substrates initially at a docking site, and then cleave and release substrates. To test this model, we developed a novel morphological technique on the basis of advanced fluorescence microscopy methods, fluorescence lifetime imaging microscopy (FLIM). FLIM allows us to examine protein-protein "proximity" in intact cells. We show that, although the strongest colocalization of APP and PS1 is in the perinuclear area, the strongest interactions detected by FLIM are at or near the cell surface. We also found that APP-PS1 interactions occur even when gamma-secretase inhibitors or "dominant-negative" PS1 mutations are used to block gamma-secretase activity. Finally, using nicastrin RNA interference, we demonstrate that nicastrin is critical for APP association with PS1. We interpret these results to suggest that there is a noncatalytic docking site closely associated with PS1-gamma-secretase.

Minimal mutation of the cytoplasmic tail inhibits the ability of E-cadherin to activate Rac but not phosphatidylinositol 3-kinase: direct evidence of a role for cadherin-activated Rac signaling in adhesion and contact formation

Classic cadherins are adhesion-activated cell signaling receptors. In particular, homophilic cadherin ligation can directly activate Rho family GTPases and phosphatidylinositol 3-kinase (PI3-kinase), signaling molecules with the capacity to support the morphogenetic effects of these adhesion molecules during development and disease. However, the molecular basis for cadherin signaling has not been elucidated, nor is its precise contribution to cadherin function yet understood. One attractive hypothesis is that cadherin-activated signaling participates in stabilizing adhesive contacts (Yap, A. S., and Kovacs, E. M. (2003) J. Cell Biol. 160, 11-16). We now report that minimal mutation of the cadherin cytoplasmic tail to uncouple binding of p120-ctn ablated the ability of E-cadherin to activate Rac. This was accompanied by profound defects in the capacity of cells to establish stable adhesive contacts, defects that were rescued by sustained Rac signaling. These data provide direct evidence for a role of cadherin-activated Rac signaling in contact formation and adhesive stabilization. In contrast, cadherin-activated PI3-kinase signaling was not affected by loss of p120-ctn binding. The molecular requirements for E-cadherin to activate Rac signaling thus appear distinct from those that stimulate PI3-kinase, and we postulate that p120-ctn may play a central role in the E-cadherin-Rac signaling pathway.

Amyloid precursor protein associates with a nicastrin-dependent docking site on the presenilin 1-gamma-secretase complex in cells demonstrated by fluorescence lifetime imaging

Gamma-secretase cleavage is the final enzymatic step generating beta-amyloid via intramembranous cleavage of the amyloid precursor protein (APP). Presenilin (PS), initially identified as a gene in which mutations account for the vast majority of early-onset autosomal dominant Alzheimer's disease, is a major component of gamma-secretase. Enzymatic activity also depends on nicastrin, Aph-1, and Pen-2. We propose a model in which gamma-secretase components assemble, interact with substrates initially at a docking site, and then cleave and release substrates. To test this model, we developed a novel morphological technique on the basis of advanced fluorescence microscopy methods, fluorescence lifetime imaging microscopy (FLIM). FLIM allows us to examine protein-protein "proximity" in intact cells. We show that, although the strongest colocalization of APP and PS1 is in the perinuclear area, the strongest interactions detected by FLIM are at or near the cell surface. We also found that APP-PS1 interactions occur even when gamma-secretase inhibitors or "dominant-negative" PS1 mutations are used to block gamma-secretase activity. Finally, using nicastrin RNA interference, we demonstrate that nicastrin is critical for APP association with PS1. We interpret these results to suggest that there is a noncatalytic docking site closely associated with PS1-gamma-secretase.

Distribution of p120 catenin during rat brain development: potential role in regulation of cadherin-mediated adhesion and actin cytoskeleton organization

p120 catenin (p120ctn) is implicated in the regulation of cadherin-mediated adhesion and actin cytoskeleton remodeling. The interaction of cytoplasmic p120ctn with the guanine exchange factor Vav2 is one of the signaling pathways implicated in cytoskeleton dynamics. We show here that p120ctn is regulated during rat brain development and is distributed at the membrane and within the cytoplasm where it associates with N-cadherin and Vav2, respectively. p120ctn shifts progressively from an axonal expression to a punctuate staining localized to a subset of synapses. In cultured hippocampal neurons, p120ctn redistributes from growth cones to synapses, where it partly colocalizes with N-cadherin or Vav2 and filamentous actin. In the adult forebrain, we show that p120ctn and Vav2 are highly expressed by neuroblasts migrating from the lateral subventricular zone to the olfactory bulb. The dynamic expression pattern of p120ctn and the biochemical evidences of its association with N-cadherin and Vav2 strongly suggest that p120ctn plays a major role in neuronal migration, neurite outgrowth and synapse formation, and plasticity.

Physiologic and pathologic events mediated by intramembranous and juxtamembranous proteolysis

Intramembranous proteolysis (IP) is a recently recognized mechanism for transmembrane signal transduction that involves proteolysis of transmembrane proteins within their membrane-spanning domains. Juxtamembranous proteolysis (JP) is similar, but proteolytic cleavage of a transmembrane protein occurs at a site close to, but not within, the transmembrane domain of the target protein. In both IP and JP, a soluble domain of a transmembrane protein is released from its membrane tether. This domain can then transmit a signal either locally or at some distance from the site of cleavage. In certain signaling pathways, JP and IP are linked. JP on one side of the membrane results in secondary IP, which then releases a signaling domain from the membrane. Whereas well-characterized proteases such as caspases, the proteasome, and metalloprotease disintegrins, have been implicated in JP, three families of multipass membrane proteases (MpMPs) have now been shown to carry out IP. Recent studies of events mediated by IP and JP indicate that they regulate key cellular signaling events including pathways involved in sterol regulation, cell fate selection, and growth regulation. Moreover, IP and JP have important roles in certain diseases such as Alzheimer's disease. Because some of the proteases mediating IP and JP can be selectivity inhibited, inhibitors targeting these proteases are likely to alter both physiologic and pathologic events triggered by IP and JP.

Sticky business: orchestrating cellular signals at adherens junctions

Cohesive sheets of epithelial cells are a fundamental feature of multicellular organisms and are largely a product of the varied functions of adherens junctions. These junctions and their cytoskeletal associations contribute heavily to the distinct shapes, polarity, spatially oriented mitotic spindle planes, and cellular movements of developing tissues. Deciphering the underlying mechanisms that govern these conserved cellular rearrangements is a prerequisite to understanding vertebrate morphogenesis.

Drosophila p120catenin plays a supporting role in cell adhesion but is not an essential adherens junction component

Cadherin-catenin complexes, localized to adherens junctions, are essential for cell-cell adhesion. One means of regulating adhesion is through the juxtamembrane domain of the cadherin cytoplasmic tail. This region is the binding site for p120, leading to the hypothesis that p120 is a key regulator of cell adhesion. p120 has also been suggested to regulate the GTPase Rho and to regulate transcription via its binding partner Kaiso. To test these hypothesized functions, we turned to Drosophila, which has only a single p120 family member. It localizes to adherens junctions and binds the juxtamembrane region of DE-cadherin (DE-cad). We generated null alleles of p120 and found that mutants are viable and fertile and have no substantial changes in junction structure or function. However, p120 mutations strongly enhance mutations in the genes encoding DE-cadherin or Armadillo, the beta-catenin homologue. Finally, we examined the localization of p120 during embryogenesis. p120 localizes to adherens junctions, but its localization there is less universal than that of core adherens junction proteins. Together, these data suggest that p120 is an important positive modulator of adhesion but that it is not an essential core component of adherens junctions.

Presenilins and APP in neuritic and synaptic plasticity: implications for the pathogenesis of Alzheimer's disease

A key neuropathological hallmark of Alzheimer's disease (AD) is the loss of neocortical and hippocampal synapses, which is closely correlated with the degree of memory impairment. Mutations in the genes encoding the amyloid precursorprotein (APP) and presenilins are responsible from some cases of early-onset autosomal-dominant AD. This article reviews the current understanding of how alterations in the cellular functions of APP and presenilins may result in the dysfunction and degeneration of synapses in AD. APP mutations result in increased production/aggregation of amyloid beta-peptide (Abeta), which induces oxidative stress, resulting in the impairment of synaptic membrane ion, glutamate, and glucose transporters. APP mutations may also compromise the production and/or function of secreted forms of APP that are believed to play important roles in learning and memory processes. Presenilin (PS1) mutations result in a major defect in endoplasmic reticulum (ER) calcium regulation, which may perturb synaptic function in ways that lead to impaired synaptic plasticity and neuronal degeneration. Studies in transgenic mice that express APP and PS1 mutations have provided evidence that the mutations result in altered cellular calcium homeostasis and synaptic plasticity, and impaired learning and memory. This article provides a brief review of the pathophysiological interactions of APP and presenilins with synaptic proteins, and discusses how AD-linked mutations in APP and PS1 may disrupt synaptic processes that contribute to memory formation.

Nectin-1alpha, an immunoglobulin-like receptor involved in the formation of synapses, is a substrate for presenilin/gamma-secretase-like cleavage

Nectin-1 is a member of the immunoglobulin superfamily and a Ca(2+)-independent adherens junction protein involved in synapse formation. Here we show that nectin-1alpha undergoes intramembrane proteolytic processing analogous to that of the Alzheimer's disease amyloid precursor protein, mediated by a presenilin (PS)-dependent gamma-secretase-like activity. 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment of Chinese hamster ovary cells activated a first proteolytic event, resulting in ectodomain shedding of nectin-1alpha. Subsequent cleavage of the remaining 26-kDa membrane-anchored C-terminal fragment (CTF) was inhibited independently by three specific gamma-secretase inhibitors and by expression of the dominant negative form of PS1. The PS/gamma-secretase-like cleavage product was detected in vivo following proteasome inhibitor treatment of cells. An in vitro gamma-secretase assay confirmed the generation of a 24-kDa nectin-1alpha intracellular domain, peripherally associated with the membrane fraction. We also found nectin-1alpha to interact with the N-terminal fragment of PS1. Finally, gamma-secretase inhibition resulted in beta-catenin release from cell junctions, concomitantly with the accumulation of the 26-kDa nectin-1alpha CTF, suggesting that high levels of nectin-1alpha CTF interfere with TPA-induced remodeling of cell-cell junctions. Our results are consistent with a previously reported role for PS/gamma-secretase in adherens junction function involving cleavage of cadherins. Similar to nectin-1, other members of the immunoglobulin superfamily involved in synapse formation may also serve as substrates for PS/gamma-secretase-like intramembrane proteolytic activity.

Gamma-secretase activity is not involved in presenilin-mediated regulation of beta-catenin

Presenilins (PS) are involved in gamma-secretase-mediated processing of beta-amyloid precursor protein (APP) and the Notch family of proteins. In addition, presenilin 1 (PS-1) binds to members of the armadillo family of proteins. In this study the relationship between PS-1-mediated proteolytic activity and PS-1-mediated regulation of beta-catenin function was investigated. Incubation of cells with a potent, small molecule gamma-secretase inhibitor did not affect PS-1/beta-catenin interaction as determined by co-immunoprecipitation, or affect the regulation of beta-catenin turnover, as determined by pulse-chase analysis, even at inhibitor concentrations that completely blocked PS-mediated APP processing. Moreover, inhibition of PS-1-mediated proteolytic activity did not affect beta-catenin trafficking, as determined by immunolocalization and immunoblotting, or beta-catenin-mediated transcription. These results indicate that PS-1-mediated regulation of gamma-secretase activity and PS-1-mediated regulation of beta-catenin function can be pharmacologically separated and support the idea that these are distinct functions.

A novel role for p120 catenin in E-cadherin function

Indirect evidence suggests that p120-catenin (p120) can both positively and negatively affect cadherin adhesiveness. Here we show that the p120 gene is mutated in SW48 cells, and that the cadherin adhesion system is impaired as a direct consequence of p120 insufficiency. Restoring normal levels of p120 caused a striking reversion from poorly differentiated to cobblestone-like epithelial morphology, indicating a crucial role for p120 in reactivation of E-cadherin function. The rescue efficiency was enhanced by increased levels of p120, and reduced by the presence of the phosphorylation domain, a region previously postulated to confer negative regulation. Surprisingly, the rescue was associated with substantially increased levels of E-cadherin. E-cadherin mRNA levels were unaffected by p120 expression, but E-cadherin half-life was more than doubled. Direct p120-E-cadherin interaction was crucial, as p120 deletion analysis revealed a perfect correlation between E-cadherin binding and rescue of epithelial morphology. Interestingly, the epithelial morphology could also be rescued by forced expression of either WT E-cadherin or a p120-uncoupled mutant. Thus, the effects of uncoupling p120 from E-cadherin can be at least partially overcome by artificially maintaining high levels of cadherin expression. These data reveal a cooperative interaction between p120 and E-cadherin and a novel role for p120 that is likely indispensable in normal cells.

Gene expression during the life cycle of Drosophila melanogaster

Molecular genetic studies of Drosophila melanogaster have led to profound advances in understanding the regulation of development. Here we report gene expression patterns for nearly one-third of all Drosophila genes during a complete time course of development. Mutations that eliminate eye or germline tissue were used to further analyze tissue-specific gene expression programs. These studies define major characteristics of the transcriptional programs that underlie the life cycle, compare development in males and females, and show that large-scale gene expression data collected from whole animals can be used to identify genes expressed in particular tissues and organs or genes involved in specific biological and biochemical processes.

The presenilin 1 deltaE9 mutation gives enhanced basal phospholipase C activity and a resultant increase in intracellular calcium concentrations

We studied effects of the familial Alzheimer's disease presenilin 1 (PS1) exon 9 deletion (PS1-DeltaE9) mutation on basal and carbachol-stimulated phosphoinositide (PI) hydrolysis and intracellular Ca(2+) concentrations ([Ca(2+)](i)) in human SH-SY5Y neuroblastoma cells. We demonstrate that PS1-DeltaE9 cells have an enhanced basal PI hydrolysis and [Ca(2+)](i) as compared with both wild type PS1 (PS1-WT) and nontransfected (NT) cells. Both were reversed by the phospholipase C (PLC) inhibitor neomycin. The PS1-DeltaE9-related high basal [Ca(2+)](i) was also reversed by xestospongin C confirming that this effect was inositol trisphosphate receptor-mediated. Carbachol gave a greater stimulation of [Ca(2+)](i) in PS1-DeltaE9 cells that took longer to return to basal as compared with responses seen in NT and PS1-WT cells. This long tail-off effect seen in PS1-DeltaE9 cells after carbachol stimulation was reversed by xestospongin C and dantrolene, suggesting that it was mediated by inositol trisphosphate receptor and ryanodine receptor amplification of Ca(2+). Ruthenium red only reduced carbachol peak elevations of [Ca(2+)](i) in NT and PS1-WT cells and not in PS1-DeltaE9 cells. No significant between cell type differences were seen for basal and carbachol-stimulated [Ca(2+)](i) with either ryanodine or the endoplasmic reticulum Ca(2+) ATPase inhibitor cyclopiazonic acid. Immunostaining experiments revealed that for all the cell types PS1 is present at the plasma membrane and co-localizes with N-cadherin, a component of the cell-cell adhesion complex. Immunoblotting of cell extracts for PLC-beta1 showed that, compared with NT and PS1-WT cells, the PS1-DeltaE9 transfectants gave a relative increase in levels of the calpain generated N-terminal fragment (100 kDa) over full-length (150 kDa) PLC-beta1. Our results suggest that the PS1-DeltaE9 mutation causes upstream changes in PI signaling with enhanced basal PLC activity as a primary effect that leads to a higher [Ca(2+)](i). This may provide a novel mechanism by which the PS1-DeltaE9 mutation sensitizes cells to apoptotic stimuli and enhanced amyloid beta generation.

A new splice variant of glial fibrillary acidic protein, GFAP epsilon, interacts with the presenilin proteins

We describe a new human isoform, GFAP epsilon, of the intermediary filament protein GFAP (glial fibrillary acidic protein). GFAP epsilon mRNA is the result of alternative splicing and a new polyadenylation signal, and thus GFAP epsilon has a new C-terminal protein sequence. This provides GFAP epsilon with the capacity for specific binding of presenilin proteins in yeast and in vitro. Our observations suggest a direct link between the presenilins and the cytoskeleton where GFAP epsilon is incorporated. Mutations in GFAP and presenilins are associated with Alexander disease and Alzheimer's disease, respectively. Accordingly, GFAP epsilon should be taken into consideration when studying neurodegenerative diseases.

Gamma-secretase: substrates and inhibitors

The amyloid beta-protein (Abeta) deposited in Alzheimer's disease (AD), the most common form of dementia in the elderly, is a secreted proteolytic product of the amyloid beta-protein precursor (APP). Generation of Abeta from the APP requires two sequential proteolytic events, beta-secretase cleavage to generate the amino terminus, followed by gamma-secretase cleavage to generate the carboxyl terminus. Because this process is a central event in the pathogenesis of AD, gamma-secretase is believed to be an excellent therapeutic target. Gamma-secretase activity has been demonstrated to be membrane-associated, with the cleavage site primarily determined by the location of the substrate with respect to the membrane. It has also been shown that this unusual proteolytic activity not only occurs for APP, but also for proteins involved in morphogenic processes or cell proliferation and differentiation such as Notch and ErbB4. Thus far, all gamma-secretase substrates are involved in some form of nuclear signaling. These recent findings have important implications for the development of pharmacological interventions that target gamma-secretase.

The aspartate-257 of presenilin 1 is indispensable for mouse development and production of beta-amyloid peptides through beta-catenin-independent mechanisms

To differentiate multiple activities of presenilin 1 (PS1), we generated transgenic mice expressing two human PS1 alleles: one with the aspartate to alanine mutation at residue 257 (hPS1D257A) that impairs the proteolytic activity of PS1, and the other deleting amino acids 340-371 of the hydrophilic loop sequence (hPS1Deltacat) essential for beta-catenin interaction. We show here that although hPS1Deltacat is fully competent in rescuing the PS1-null lethal phenotype, hPS1D257A does not exhibit developmental activity. hPS1D257A also leads to the concurrent loss of the proteolytic processing of Notch and beta-amyloid precursor protein (APP) and the generation of beta-amyloid peptides (Abeta). Further, by measuring the levels of endogenous Abeta(X-40) and Abeta(X-42) in primary neuronal cultures, we confirmed the concept that PS1 is indispensable for the production of secreted Abeta.

Advances in the cellular and molecular biology of the beta-amyloid protein in Alzheimer's disease

Alzheimer's disease (AD) is a progressive senile dementia characterized by deposition of a 4 kDa peptide of 39-42 residues known as amyloid beta-peptide (Abeta) in the form of senile plaques and the microtubule associated protein tau as paired helical filaments. Genetic studies have identified mutations in the Abeta precursor protein (APP) as the key triggers for the pathogenesis of AD. Other genes such as presenilins 1 and 2 (PS1/2) and apolipoprotein E (APOE) also play a critical role in increased Abeta deposition. Several biochemical and molecular studies using transfected cells and transgenic animals point to mechanisms by which Abeta is generated and aggregated to trigger the neurodegeneration that may cause AD. Three important enzymes collectively known as "secretases" participate in APP processing. An enzymatic activity, beta-secretase, cleaves APP on the amino side of Abeta producing a large secreted derivative, sAPPbeta, and an Abeta-bearing membrane-associated C-terminal derivative, CTFbeta, which is subsequently cleaved by the second activity, gamma-secretase, to release Abeta. Alternatively, a third activity, alpha-secretase, cleaves APP within Abeta to the secreted derivative sAPPalpha and membrane-associated CTFalpha. The predominant secreted APP derivative is sAPPalpha in most cell-types. Most of the secreted Abeta is 40 residues long (Abeta40) although a small percentage is 42 residues in length (Abeta42). However, the longer Abeta42 aggregates more readily and was therefore considered to be the pathologically important form. Advances in our understanding of APP processing, trafficking, and turnover will pave the way for better drug discovery for the eventual treatment of AD. In addition, APP gene regulation and its interaction with other proteins may provide useful drug targets for AD. The emerging knowledge related to the normal function of APP will help in determining whether or not the AD associated changes in APP metabolism affect its function. The present review summarizes our current understanding of APP metabolism and function and their relationship to other proteins involved in AD.

Overexpression of wild type but not an FAD mutant presenilin-1 promotes neurogenesis in the hippocampus of adult mice

Mutations in the presenilin-1 (PS-1) gene are one cause of familial Alzheimer's disease (FAD). However, the functions of the PS-1 protein as well as how PS-1 mutations cause FAD are incompletely understood. Here we investigated if neuronal overexpression of wild-type or FAD mutant PS-1 in transgenic mice affects neurogenesis in the hippocampus of adult animals. We show that either a wild-type or an FAD mutant PS-1 transgene reduces the number of neural progenitors in the dentate gyrus. However, the wild-type, but not the FAD mutant PS-1 promoted the survival and differentiation of progenitors leading to more immature granule cell neurons being generated in PS-1 wild type expressing animals. These studies suggest that PS-1 plays a role in regulating neurogenesis in adult hippocampus and that FAD mutants may have deleterious properties independent of their effects on amyloid deposition.

Ageing research in Greece

Ageing research in Greece is well established. Research groups located in universities, research institutes or public hospitals are studying various and complementary aspects of ageing. These research activities include (a) functional analysis of Clusterin/Apolipoprotein J, studies in healthy centenarians and work on protein degradation and the role of proteasome during senescence at the National Hellenic Research Foundation; (b) regulation of cell proliferation and tissue formation, a nationwide study of determinants and markers of successful ageing in Greek centenarians and studies of histone gene expression and acetylation at the National Center for Scientific Research, Demokritos; (c) work on amyloid precursor protein and Presenilin 1 at the University of Athens; (d) oxidative stress-induced DNA damage and the role of oncogenes in senescence at the University of Ioannina; (e) studies in the connective tissue at the University of Patras; (f) proteomic studies at the Biomedical Sciences Research Center Alexander Fleming; (g) work on Caenorhabditis elegans at the Foundation for Research and Technology; (h) the role of ultraviolet radiation in skin ageing at Andreas Sygros Hospital; (i) follow-up studies in healthy elderly at the Athens Home for the Aged; and (j) socio-cultural aspects of ageing at the National School of Public Health. These research activities are well recognized by the international scientific community as it is evident by the group's very good publication records as well as by their direct funding from both European Union and USA. This article summarizes these research activities and discuss future directions and efforts towards the further development of the ageing field in Greece.

The cadherin family of cell adhesion molecules: multiple roles in synaptic plasticity

Cadherins are cell adhesion molecules that are critically important for establishing brain structure and connectivity during early development. They are enriched at synapses and, by virtue of a number of properties including homophilic recognition and molecular diversity, have been implicated in the generation of synaptic specificity. Cadherins also participate in remodeling synaptic architecture and modifying the strength of the synaptic signal, thereby retaining an active role in synaptic structure, function, and plasticity, which extends beyond initial development. Cadherins have been implicated in the induction of long-term potentiation (LTP) of hippocampal synaptic strength, a cellular model for learning and memory. LTP is associated with the synthesis and recruitment of N-cadherin to newly forming synaptic junctions, induces molecular changes to N-cadherin indicative of augmented adhesive force, and can be prevented when cadherin adhesion is blocked. NMDA receptor activation, which is critically required for synaptic plasticity, may provide a signal that regulates the molecular configuration of synaptic N-cadherin, and therefore the strength of adhesion across the synaptic cleft. Additionally, there exists at the synapse a pool of surface cadherins that is untethered to the actin cytoskeleton and capable of a rapid and reversible dispersion along the plasmalemma under conditions of strong activity. These observations suggest that synaptic activity dynamically regulates both the strength and the localization of cadherin-cadherin bonds across the synaptic junctional interface, changes that may be crucial for regulating synaptic plasticity.

Turn-off, drop-out: functional state switching of cadherins

The classic cadherins are a group of calcium dependent, homophilic cell-cell adhesion molecules that drive morphogenetic rearrangements and maintain the integrity of cell groups through the formation of adherens junctions. The formation and maintenance of cadherin-mediated adhesions is a multistep process and mechanisms have evolved to regulate each step. This suggests that functional state switching plays an important role in development. Among the many challenges ahead is to determine the developmental role that functional state switching plays in tissue morphogenesis and to define the roles of each of the several regulatory interactions that participate in switching. One correlate of the loss of cadherin-mediated adhesion, the "turn-off" of cadherin function, is the exit, or "drop-out" of cells from neural and epithelial layers and their conversion to a motile phenotype. We suggest that epithelial mesenchymal conversions may be initiated by signaling pathways that result in the loss of cadherin function. Tyrosine phosphorylation of beta-catenin is one such mechanism. Enhanced phosphorylation of tyrosine residues on beta-catenin is almost invariably associated with loss of the cadherin-actin connection concomitant with loss of adhesive function. There are several tyrosine kinases and phosphatases that have been shown to have the potential to alter the phosphorylation state of beta-catenin and thus the function of cadherins. Our laboratory has focused on the role of the nonreceptor tyrosine phosphatase PTP1B in regulating the phosphorylation of beta-catenin on tyrosine residues. Our data suggest that PTP1B is crucial for maintenance of N-cadherin-mediated adhesions in embryonic neural retina cells. By using an L-cell model system constitutively expressing N-cadherin, we have worked out many of the molecular interactions essential for this regulatory interaction. Extracellular cues that bias this critical regulatory interaction toward increased phosphorylation of beta-catenin may be a critical component of many developmental events.

A presenilin-1/gamma-secretase cleavage releases the E-cadherin intracellular domain and regulates disassembly of adherens junctions

E-cadherin controls a wide array of cellular behaviors including cell-cell adhesion, differentiation and tissue development. Here we show that presenilin-1 (PS1), a protein involved in Alzheimer's disease, controls a gamma-secretase-like cleavage of E-cadherin. This cleavage is stimulated by apoptosis or calcium influx and occurs between human E-cadherin residues Leu731 and Arg732 at the membrane-cytoplasm interface. The PS1/gamma-secretase system cleaves both the full-length E-cadherin and a transmembrane C-terminal fragment, derived from a metalloproteinase cleavage after the E-cadherin ectodomain residue Pro700. The PS1/gamma-secretase cleavage dissociates E-cadherins, beta-catenin and alpha-catenin from the cytoskeleton, thus promoting disassembly of the E-cadherin-catenin adhesion complex. Furthermore, this cleavage releases the cytoplasmic E-cadherin to the cytosol and increases the levels of soluble beta- and alpha-catenins. Thus, the PS1/gamma-secretase system stimulates disassembly of the E-cadherin- catenin complex and increases the cytosolic pool of beta-catenin, a key regulator of the Wnt signaling pathway.

An octapeptide in the juxtamembrane domain of VE-cadherin is important for p120ctn binding and cell proliferation

The cadherins are a family of adhesive proteins involved in cell-cell homophilic interactions. VE-cadherin, expressed in endothelial cells, is involved in morphogenesis, regulation of permeability, and cellular proliferation. The cytoplasmic tails of cadherins contain two major domains, the juxtamembrane domain that plays a role in the intercellular localization of the protein and also serves for binding of p120ctn, and a C-terminal domain that associates with beta- or gamma-catenin. A highly conserved region present in the juxtamembrane domain of the cadherins has been shown to be necessary for p120ctn binding in E-cadherin. Using a mutant VE-cadherin lacking a highly conserved octapeptide, we demonstrated that it is required for p120ctn binding to VE-cadherin as determined by immunoprecipitation and colocalization studies. By immunofluorescence, this mutant protein has a topographical distribution similar to that of the wild-type VE-cadherin and, therefore, we conclude that the topographical distribution of VE-cadherin is independent of this motif. In addition, although cell-cell association is present in cells expressing this mutant form of VE-cadherin, we found that the strength of adhesion is decreased. Finally, our results for the first time demonstrate that the interaction of VE-cadherin with p120 catenin plays an important role in cellular growth, suggesting that the binding of p120 catenin to cadherins may regulate cell proliferation.

Requirement of the juxtamembrane domain of the cadherin cytoplasmic tail for morphogenetic cell rearrangement during myotome development

During development, the activity of cadherin cell adhesion molecules is assumed to be regulated to allow for cell rearrangement or translocation. Previous studies suggest that the juxtamembrane (JM) domain of the cadherin cytoplasmic tail, which contains the site for binding to p120ctn, has a regulatory function in this adhesion system. To study the possible role of JM domain-dependent cadherin regulation in embryonic cell rearrangement, we ectopically expressed a series of N-cadherin mutants in developing somites of chicken embryos. When a JM domain-deficient N-cadherin was expressed, the morphogenetic expansion of the myotome was strongly suppressed. However, a triple alanine substitution in the JM domain, which specifically inhibited the p120ctn binding, had no effect on myotome development. Furthermore, a dominant negative N-cadherin, which had a deletion at the extracellular domain but maintained the normal cytoplasmic tail, did not affect myotome expansion; although it disrupted intersomite boundaries. Overexpression of p120ctn also did not affect myotome expansion, but it did perturb myofiber orientation. These and other observations suggest that the JM domain of N-cadherin has a regulatory role in myotome cell rearrangement in which molecules other than p120ctn are involved. The p120ctn molecule itself seems to play a critical role in the arrangement of myofibers.

Presenilin 1 independently regulates beta-catenin stability and transcriptional activity

Presenilin 1 (PS1) regulates beta-catenin stability; however, published data regarding the direction of the effect are contradictory. We examined the effects of wild-type and mutant forms of PS1 on the membrane, cytoplasmic, nuclear, and signaling pools of endogenous and exogenous beta-catenin by immunofluorescence microscopy, subcellular fractionation, and in a transcription assay. We found that PS1 destabilizes the cytoplasmic and nuclear pools of beta-catenin when stabilized by Wnt or Dvl but not when stabilized at lower levels of the Wnt pathway. The PS1 mutants examined were less able to reduce the stability of beta-catenin. PS1 also inhibited the transcriptional activity of endogenous beta-catenin, and the PS1 mutants were again less inhibitory at the level of Dvl but showed a different pattern of inhibition toward transcription below Dvl. The transcriptional activity of exogenously expressed wild-type beta-catenin and two mutants, DeltaN89beta-catenin and DeltaSTbeta-catenin, were also inhibited by wild-type and mutant PS1. We conclude that PS1 negatively regulates the stability and transcriptional activity of beta-catenin at different levels in the Wnt pathway, that the effect on transcriptional activity appears to be independent of the GSK-3beta mediated degradation of beta-catenin, and that mutations in PS1 differentially affect the stability and transcriptional activity of beta-catenin.

Loss of presenilin 1 is associated with enhanced beta-catenin signaling and skin tumorigenesis

Presenilin 1 (PS1) is required for the proteolytic processing of Notch and the beta-amyloid precursor protein (APP), molecules that play pivotal roles in cell-fate determination during development and Alzheimer's disease pathogenesis, respectively. In addition, PS1 interacts with beta-catenin and promotes its turnover through independent mechanisms. Consistent with this activity, we report here that PS1 is important in controlling epidermal cell proliferation in vivo. PS1 knockout mice that are rescued through neuronal expression of human PS1 transgene develop spontaneous skin cancers. PS1-null keratinocytes exhibit higher cytosolic beta-catenin and beta-catenin/lymphoid enhancer factor-1/T cell factor (beta-catenin/LEF)-mediated signaling. This effect can be reversed by reintroducing wild-type PS1, but not a PS1 mutant active in Notch processing but defective in beta-catenin binding. Nuclear beta-catenin protein can be detected in tumors. Elevated beta-catenin/LEF signaling is correlated with activation of its downstream target cyclin D1 and accelerated entry from G(1) into S phase of the cell cycle. This report demonstrates a function of PS1 in adult tissues, and our analysis suggests that deregulation of beta-catenin pathway contributes to the skin tumor phenotype.