Presenilins: molecular switches between proteolysis and signal transduction

Substrate-Enzyme Interactions in Intramembrane Proteolysis: γ-Secretase as the Prototype

Intramembrane-cleaving proteases (I-CLiPs) catalyze the hydrolysis of peptide bonds within the transmembrane regions of membrane protein substrates, releasing bioactive fragments that play roles in many physiological and pathological processes. Based on their catalytic mechanism and nucleophile, I-CLiPs are classified into metallo, serine, aspartyl, and glutamyl proteases. Presenilin is the most prominent among I-CLiPs, as the catalytic subunit of γ-secretase (GS) complex responsible for cleaving the amyloid precursor protein (APP) and Notch, as well as many other membrane substrates. Recent cryo-electron microscopy (cryo-EM) structures of GS provide new details on how presenilin recognizes and cleaves APP and Notch. First, presenilin transmembrane helix (TM) 2 and 6 are dynamic. Second, upon binding to GS, the substrate TM helix is unwound from the C-terminus, resulting in an intermolecular β-sheet between the substrate and presenilin. The transition of the substrate C-terminus from α-helix to β-sheet is proposed to expose the scissile peptide bond in an extended conformation, leaving it susceptible to protease cleavage. Despite the astounding new insights in recent years, many crucial questions remain unanswered regarding the inner workings of γ-secretase, however. Key unanswered questions include how the enzyme recognizes and recruits substrates, how substrates are translocated from an initial docking site to the active site, how active site aspartates recruit and coordinate catalytic water, and the nature of the mechanisms of processive trimming of the substrate and product release. Answering these questions will have important implications for drug discovery aimed at selectively reducing the amyloid load in Alzheimer's disease (AD) with minimal side effects.

Interactions of Mitochondria/Metabolism and Calcium Regulation in Alzheimer's Disease: A Calcinist Point of View

Decades of research suggest that alterations in calcium are central to the pathophysiology of Alzheimer's Disease (AD). Highly reproducible changes in calcium dynamics occur in cells from patients with both genetic and non-genetic forms of AD relative to controls. The most robust change is an exaggerated release of calcium from internal stores. Detailed analysis of these changes in animal and cell models of the AD-causing presenilin mutations reveal robust changes in ryanodine receptors, inositol tris-phosphate receptors, calcium leak channels and store activated calcium entry. Similar anomalies in calcium result when AD-like changes in mitochondrial enzymes or oxidative stress are induced experimentally. The calcium abnormalities can be directly linked to the altered tau phosphorylation, amyloid precursor protein processing and synaptic dysfunction that are defining features of AD. A better understanding of these changes is required before using calcium abnormalities as therapeutic targets.

Phosphatidylethanolamine Metabolism in Health and Disease

Phosphatidylethanolamine (PE) is the second most abundant glycerophospholipid in eukaryotic cells. The existence of four only partially redundant biochemical pathways that produce PE, highlights the importance of this essential phospholipid. The CDP-ethanolamine and phosphatidylserine decarboxylase pathways occur in different subcellular compartments and are the main sources of PE in cells. Mammalian development fails upon ablation of either pathway. Once made, PE has diverse cellular functions that include serving as a precursor for phosphatidylcholine and a substrate for important posttranslational modifications, influencing membrane topology, and promoting cell and organelle membrane fusion, oxidative phosphorylation, mitochondrial biogenesis, and autophagy. The importance of PE metabolism in mammalian health has recently emerged following its association with Alzheimer's disease, Parkinson's disease, nonalcoholic liver disease, and the virulence of certain pathogenic organisms.

The Distinct Role of ADAM17 in APP Proteolysis and Microglial Activation Related to Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease with the symptom of cognitive impairment. The deposition of amyloid β (Aβ) peptide is believed to be the primary cause to neuronal dystrophy and eventually dementia. Aβ is the proteolytic product from its precursor amyloid precursor protein (APP) by β- and γ- secretase. An optional cleavage by α-secretase happens inside the Aβ domain. ADAM17 is supposed to be the regulated α-secretase of APP. Enhanced activity of ADAM17 leads to the increasing secretion of neuroprotective soluble APP α fragment and reduction of Aβ generation, which may be benefit to the disease. ADAM17 is then considered the potential therapeutic target for AD. Microglia activation and neuroinflammation is another important event in AD pathogenesis. Interestingly, ADAM17 also participates in the cleavage of many other membrane-bound proteins, especially some inflammatory factors related to microglia activation. The facilitating role of ADAM17 in inflammation and further neuronal damage has also been illustrated. In results, the activation of ADAM17 as the solution to AD may be a tricky task. The comprehensive consideration and evaluation has to be carried out carefully before the final treatment. In the present review, the distinct role of ADAM17 in AD-related APP shedding and neuroinflammatory microglial activation will be carefully discussed.

The very many faces of presenilins and the γ-secretase complex

Presenilin is a central, catalytic component of the γ-secretase complex which conducts intramembrane cleavage of various protein substrates. Although identified and mainly studied through its role in the development of amyloid plaques in Alzheimer disease, γ-secretase has many other important functions. The complex seems to be evolutionary conserved throughout the Metazoa, but recent findings in plants and Dictyostelium discoideum as well as in archeons suggest that its evolution and functions might be much more diversified than previously expected. In this review, a selective survey of the multitude of functions of presenilins and the γ-secretase complex is presented. Following a brief overview of γ-secretase structure, assembly and maturation, three functional aspects are analyzed: (1) the role of γ-secretase in autophagy and phagocytosis; (2) involvement of the complex in signaling related to endocytosis; and (3) control of calcium fluxes by presenilins.

The synaptic maintenance problem: membrane recycling, Ca2+ homeostasis and late onset degeneration

Most neurons are born with the potential to live for the entire lifespan of the organism. In addition, neurons are highly polarized cells with often long axons, extensively branched dendritic trees and many synaptic contacts. Longevity together with morphological complexity results in a formidable challenge to maintain synapses healthy and functional. This challenge is often evoked to explain adult-onset degeneration in numerous neurodegenerative disorders that result from otherwise divergent causes. However, comparably little is known about the basic cell biological mechanisms that keep normal synapses alive and functional in the first place. How the basic maintenance mechanisms are related to slow adult-onset degeneration in different diseasesis largely unclear. In this review we focus on two basic and interconnected cell biological mechanisms that are required for synaptic maintenance: endomembrane recycling and calcium (Ca²⁺) homeostasis. We propose that subtle defects in these homeostatic processes can lead to late onset synaptic degeneration. Moreover, the same basic mechanisms are hijacked, impaired or overstimulated in numerous neurodegenerative disorders. Understanding the pathogenesis of these disorders requires an understanding of both the initial cause of the disease and the on-going changes in basic maintenance mechanisms. Here we discuss the mechanisms that keep synapses functional over long periods of time with the emphasis on their role in slow adult-onset neurodegeneration.

Genetic studies in Alzheimer's disease

Alzheimer's disease (AD), the most common cause of dementia in aged populations, is believed to be caused by both environmental factors and genetic variations. Extensive linkage and association studies have established that a broad range of loci are associated with AD, including both causative and susceptibility (risk factor) genes. So far, at least three genes, APP, PS1, and PS2, have been identified as causative genes. Mutations in these genes have been found to cause mainly early-onset AD. On the other hand, APOE has been identified to be the most common high genetic risk factor for late-onset AD. Polymorphisms in the coding region, intron, and promoter region of certain genes constitute another kind of genetic variation associated with AD. A number of other genes or loci have been reported to have linkage with AD, but many show only a weak linkage or the results are not well reproduced. Currently, the measurable genetic associations account for about 50% of the population risk for AD. It is believed that more new loci will be found to associate with AD, either as causative genes or genetic risk factors, and that eventually the understanding of genetic factors in the pathogenesis of AD will be important for our efforts to cure this illness.

Peptides based on the presenilin-APP binding domain inhibit APP processing and Aβ production through interfering with the APP transmembrane domain

Presenilins (PSENs) form the catalytic component of the γ-secretase complex, responsible for intramembrane proteolysis of amyloid precursor protein (APP) and Notch, among many other membrane proteins. Previously, we identified a PSEN1-binding domain in APP, encompassing half of the transmembrane domain following the amyloid β (Aβ) sequence. Based on this, we designed peptides mimicking this interaction domain with the aim to selectively block APP processing and Aβ generation through interfering with enzyme-substrate binding. We identified a peptide sequence that, when fused to a virally derived translocation peptide, significantly lowered Aβ production (IC(50): 317 nM) in cell-free and cell-based assays using APP-carboxy terminal fragment as a direct γ-secretase substrate. Being derived from the APP sequence, this inhibitory peptide did not affect NotchΔE γ-cleavage, illustrating specificity and potential therapeutic value. In cell-based assays, the peptide strongly suppressed APP shedding, demonstrating that it exerts the inhibitory effect already upstream of γ-secretase, most likely through steric hindrance.

Trafficking and proteolytic processing of APP

Accumulations of insoluble deposits of amyloid β-peptide are major pathological hallmarks of Alzheimer disease. Amyloid β-peptide is derived by sequential proteolytic processing from a large type I trans-membrane protein, the β-amyloid precursor protein. The proteolytic enzymes involved in its processing are named secretases. β- and γ-secretase liberate by sequential cleavage the neurotoxic amyloid β-peptide, whereas α-secretase prevents its generation by cleaving within the middle of the amyloid domain. In this chapter we describe the cell biological and biochemical characteristics of the three secretase activities involved in the proteolytic processing of the precursor protein. In addition we outline how the precursor protein maturates and traffics through the secretory pathway to reach the subcellular locations where the individual secretases are preferentially active. Furthermore, we illuminate how neuronal activity and mutations which cause familial Alzheimer disease affect amyloid β-peptide generation and therefore disease onset and progression.

Enhanced ROS generation mediated by Alzheimer's disease presenilin regulation of InsP3R Ca2+ signaling

Familial Alzheimer's disease (FAD) is caused by mutations in amyloid precursor protein and presenilins (PS1, PS2). Many FAD-linked PS mutations affect intracellular calcium (Ca(2+)) homeostasis by proximal mechanisms independent of amyloid production by dramatically enhancing gating of the inositol trisphosphate receptor (InsP(3)R) intracellular Ca(2+) release channel by a gain-of-function effect that mirrors genetics of FAD and is independent of secretase activity. Electrophysiological recordings of InsP(3)R in FAD patient B cells, cortical neurons of asymptomatic PS1-AD mice, and other cells revealed they have higher occupancy in a high open probability burst mode, resulting in enhanced Ca(2+) signaling. Exaggerated Ca(2+) signaling through this mechanism results in enhanced generation of reactive oxygen species, believed to be an important component in AD pathogenesis. Exaggerated Ca(2+) signaling through InsP(3)R-PS interaction is a disease specific and robust proximal mechanism in AD that may contribute to the pathology of AD by enhanced generation of reactive oxygen species.

Watching worms whither: modeling neurodegeneration in C. elegans

Caenorhabditis elegans is increasingly being used to study neurodegenerative diseases. Nematodes are translucent, which facilitates study of particular neurons in the living animal, and easy to manipulate genetically. Despite vast evolutionary divergence, human proteins are functionally active when expressed in C. elegans, and disease-linked mutations in these proteins also cause phenotypic changes in the nematode. In this chapter, we review use of C. elegans to investigate the pathophysiology of Alzheimer's disease, Parkinson's disease, and axonal degeneration. Studies of presenilin, β-amyloid, tau, α-synuclein, and LRRK2 all produce strong phenotypic effects in C. elegans, and in many cases reproduce selective neuronal vulnerability observed in humans. Disease-linked mutations enhance degeneration in the C. elegans models. These studies are increasingly leading to high-throughput screens that identify novel genes and novel pharmaceuticals that modify the disease course.

Amyloid-modifying therapies for Alzheimer's disease: therapeutic progress and its implications

Alzheimer's disease (AD) is the most prevalent form of dementia, affecting an estimated 4.8 million people in North America. For the past decade, the amyloid cascade hypothesis has dominated the field of AD research. This theory posits that the deposition of amyloid-beta protein (Abeta) in the brain is the key pathologic event in AD, which induces a series of neuropathological changes that manifest as cognitive decline and eventual dementia. Based on this theory, interventions that reduce Abeta burden in the brain would be expected to alleviate both the neuropathological changes and dementia, which characterize AD. Several diverse pharmacological strategies have been developed to accomplish this. These include inhibiting the formation of Abeta, preventing the aggregation of Abeta into insoluble aggregates, preventing the entry of Abeta into the brain from the periphery and enhancing the clearance of Abeta from the central nervous system. To date, no amyloid-modifying therapy has yet been successful in phase 3 clinical trials; however, several trials are currently underway. This article provides a review of the status of amyloid-modifying therapies and the implications for the amyloid cascade hypothesis.

Neurons generated from APP/APLP1/APLP2 triple knockout embryonic stem cells behave normally in vitro and in vivo: lack of evidence for a cell autonomous role of the amyloid precursor protein in neuronal differentiation

Alzheimer's disease amyloid precursor protein (APP) has been implicated in many neurobiologic processes, but supporting evidence remains indirect. Studies are confounded by the existence of two partially redundant APP homologues, APLP1 and APLP2. APP/APLP1/APLP2 triple knockout (APP tKO) mice display cobblestone lissencephaly and are perinatally lethal. To circumvent this problem, we generated APP triple knockout embryonic stem (ES) cells and differentiated these to APP triple knockout neurons in vitro and in vivo. In comparison with wild-type (WT) ES cell-derived neurons, APP tKO neurons formed equally pure neuronal cultures, had unaltered in vitro migratory capacities, had a similar acquisition of polarity, and were capable of extending long neurites and forming active excitatory synapses. These data were confirmed in vivo in chimeric mice with APP tKO neurons expressing the enhanced green fluorescent protein (eGFP) present in a WT background brain. The results suggest that the loss of the APP family of proteins has no major effect on these critical neuronal processes and that the apparent multitude of functions in which APP has been implicated might be characterized by molecular redundancy. Our stem cell culture provides an excellent tool to circumvent the problem of lack of viability of APP/APLP triple knockout mice and will help to explore the function of this intriguing protein further in vitro and in vivo.

Proteases and Proteolysis in Alzheimer Disease: A Multifactorial View on the Disease Process

Alzheimer disease is characterized by the accumulation of abnormally folded protein fragments, i.e., amyloid beta peptide (Aβ) and tau that precipitate in amyloid plaques and neuronal tangles, respectively. In this review we discuss the complicated proteolytic pathways that are responsible for the generation and clearance of these fragments, and how disturbances in these pathways interact and provide a background for a novel understanding of Alzheimer disease as a multifactorial disorder. Recent insights evolve from the static view that the morphologically defined plaques and tangles are disease driving towards a more dynamic, biochemical view in which the intermediary soluble Aβ oligomers and soluble tau fragments are considered as the main mediators of neurotoxicity. The relevance of proteolytic pathways, centered on the generation and clearance of toxic Aβ, on the cleavage and nucleation of tau, and on the general proteostasis of the neurons, then becomes obvious. Blocking or stimulating these pathways provide, or have the potential to provide, interesting drug targets, which raises the hope that we will be able to provide a cure for this dreadful disorder.

Epigenetic control of aquaporin 1 expression by the amyloid precursor protein

Cellular processing of the amyloid precursor protein (APP) has been extensively studied, but its precise function remains elusive. The intracellular domain of APP has been proposed to regulate expression of several genes by mechanisms that are largely unknown. We report that APP regulates expression of the aquaporin 1 (AQP1) gene in mouse embryonic fibroblasts and in transgenic mice. AQP1 mRNA and protein were down-regulated in fibroblasts lacking APP or presenilin 2 in which AQP1 expression was restored by stable expression of full-length APP or presenilin 2 but not by APP deleted from its carboxy-terminal domain. The transcriptional activity of the AQP1 gene promoter and the stability of AQP1 mRNA were identical in fibroblasts expressing or not expressing APP. Control of AQP1 expression by APP was sensitive to trichostatin A, an histone deacetylase inhibitor, and histone deacetylase activity coimmunoprecipitated with APP. Altogether, these data show that a presenilin-2-dependent gamma-secretase activity releases the intracellular domain of APP involved in the epigenetic control of AQP1 expression. Since AQP1 is found in astrocytes surrounding senile plaques, this epigenetic control of AQP1 expression could have important implications in Alzheimer disease.

The interactome of the amyloid beta precursor protein family members is shaped by phosphorylation of their intracellular domains

BACKGROUND

Brain tissue from patients with Alzheimer's disease has shown an increase of phosphorylation of Tyr-682, located on the conserved Y682ENPTY motif, and Thr-668 residues, both in the intracellular domain (AID) of amyloid beta precursor protein (APP), although the role of these two residues is not yet known.

RESULTS

Here, we report that the phosphorylation status of Tyr-682, and in some cases Thr-668, shapes the APP interactome. It creates a docking site for SH2-domain containing proteins, such as ShcA, ShcB, ShcC, Grb7, Grb2, as well as adapter proteins, such as Crk and Nck, that regulate important biological processes, cytosolic tyrosine kinases, such as Abl, Lyn and Src, which regulate signal transduction pathways, and enzymes that control phosphatidylinositols levels and signaling, such as PLC-gamma. At the same time, it either reduces (like for JIP1, NUMB, NUMBL and ARH) or abolishes (like for Fe65, Fe65L1 and Fe65L2) binding of other APP interactors. Phosphorylation of Thr-668, unlike Tyr-682, does not seem to affect APP's ability to interact with the various proteins, with Pin1 and X11 being the exclusions. We also found that there are some differences between the interactions to AID and to ALID1 and ALID2, its two homologues.

CONCLUSION

Our data indicates that APP can regulate diverse cellular processes and that, vice versa, a network of signaling events can impact APP processing. Our results also suggest that phosphorylation of the APP Intracellular Domain will dramatically shape the APP interactome and, consequently, will regulate APP processing, APP transport and APP/AID-mediated functions.

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.

Lipoprotein receptors and cholesterol in APP trafficking and proteolytic processing, implications for Alzheimer's disease

Amyloid-beta (Abeta) peptide accumulation in the brain is central to the pathogenesis of Alzheimer's disease (AD). Abeta is produced through proteolytic processing of a transmembrane protein, beta-amyloid precursor protein (APP), by beta- and gamma-secretases. Mounting evidence has demonstrated that alterations in APP cellular trafficking and localization directly impact its processing to Abeta. Members of the low-density lipoprotein receptor family, including LRP, LRP1B, SorLA/LR11, and apoER2, interact with APP and regulate its endocytic trafficking. Additionally, APP trafficking and processing are greatly affected by cellular cholesterol content. In this review, we summarize the current understanding of the roles of lipoprotein receptors and cholesterol in APP trafficking and processing and their implication for AD pathogenesis and therapy.

BRI2 inhibits amyloid beta-peptide precursor protein processing by interfering with the docking of secretases to the substrate

Genetic alterations of amyloid beta-peptide (Abeta) production caused by mutations in the Abeta precursor protein (APP) cause familial Alzheimer's disease (AD). Mutations in BRI2, a gene of undefined function, are linked to familial British and Danish dementias, which are pathologically and clinically similar to Alzheimer's disease. We report that BRI2 is a physiological suppressor of Abeta production. BRI2 restrict docking of gamma-secretase to APP and access of alpha- and beta-secretases to their cleavage APP sequences. Alterations of BRI2 by gene targeting or transgenic expression regulate Abeta levels and AD pathology in mouse models of AD. Competitive inhibition of APP processing by BRI2 may provide a new approach to AD therapy and prevention.

Evidence that the Amyloid beta Precursor Protein-intracellular domain lowers the stress threshold of neurons and has a "regulated" transcriptional role

Background

Regulated intramembrane proteolysis of the β-amyloid precursor protein by the γ-secretase yields two peptides. One, amyloid-β, is the major component of the amyloid plaques found in Alzheimer's disease patients. The other, APP IntraCellular Domain, has been involved in regulation of apoptosis, calcium flux and gene transcription. To date, a few potential target genes transcriptionally controlled by AID, alone or complexed with Fe65/Tip60, have been described. Although the reports are controversial: these include KAI1, Neprilysin, p53, EGFR, LRP and APP itself. Furthermore, p53 has been implicated in AID mediated susceptibility to apoptosis. To extend these findings, and assess their in vivo relevance, we have analyzed the expression of the putative target genes and of the total brain basal transriptoma in transgenic mice expressing AID in the forebrain. Also, we have studied the susceptibility of primary neurons from such mice to stress and pro-apoptotic agents.

Results

We found that AID-target genes and the mouse brain basal transcriptoma are not influenced by transgenic expression of AID alone, in the absence of Fe65 over-expression. Also, experiments conducted on primary neurons from AID transgenic mice, suggest a role for AID in sensitizing these cells to toxic stimuli. Overall, these findings hint that a role for AID, in regulating gene transcription, could be induced by yet undefined, and possibly stressful, stimuli in vivo.

Conclusion

Overall, these data suggest that the release of the APP intracellular domain may modulate the sensitivity of neuronal cells to toxic stimuli, and that a transcriptional role of AID could be inscribed in signaling pathways thatare not activated in basal conditions.

Abeta(25-35) attenuated SREBP level in nuclear extracts of serum-deprived human neuroblastoma cells

Disturbance in cholesterol homeostasis appears to be an important factor in the pathogenesis of neurodegenerative disorders. The aim of the present study was to investigate sterol regulatory element binding protein (SREBP) levels in the nuclear extracts of human neuroblastoma cells and the possible interaction of beta-amyloid peptide (Abeta) and cholesterol with this transcription factor. In this study, cultured human neuroblastoma cells (SHSY-5Y) were incubated in serum-deprived media in the presence or absence of Abeta((25-35)) (1 microM) or cholesterol (300 microM) for 24 h. Nuclear extracts were subjected to SDS-PAGE, and SREBP cleavage product (68 kDa) was detected by immunoblotting. SREBP levels were elevated in the cells incubated 24 h in serum-deprived experimental media and were attenuated by Abeta or cholesterol-supplementation. It is likely that the ability of Abeta to release cholesterol into the medium and downregulate SREBP is due to a feedback mechanism.

Amyloidogenic processing but not amyloid precursor protein (APP) intracellular C-terminal domain production requires a precisely oriented APP dimer assembled by transmembrane GXXXG motifs

The beta-amyloid peptide (Abeta) is the major constituent of the amyloid core of senile plaques found in the brain of patients with Alzheimer disease. Abeta is produced by the sequential cleavage of the amyloid precursor protein (APP) by beta- and gamma-secretases. Cleavage of APP by gamma-secretase also generates the APP intracellular C-terminal domain (AICD) peptide, which might be involved in regulation of gene transcription. APP contains three Gly-XXX-Gly (GXXXG) motifs in its juxtamembrane and transmembrane (TM) regions. Such motifs are known to promote dimerization via close apposition of TM sequences. We demonstrate that pairwise replacement of glycines by leucines or isoleucines, but not alanines, in a GXXXG motif led to a drastic reduction of Abeta40 and Abeta42 secretion. beta-Cleavage of mutant APP was not inhibited, and reduction of Abeta secretion resulted from inhibition of gamma-cleavage. It was anticipated that decreased gamma-cleavage of mutant APP would result from inhibition of its dimerization. Surprisingly, mutations of the GXXXG motif actually enhanced dimerization of the APP C-terminal fragments, possibly via a different TM alpha-helical interface. Increased dimerization of the TM APP C-terminal domain did not affect AICD production.

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.

Inhibition of APP intracellular domain (AICD) transcriptional activity via covalent conjugation with Nedd8

The processing of amyloid precursor protein (APP) by gamma-secretase generates the APP intracellular domain (AICD), which functions as a transcriptional factor for target gene activation following localization into the nucleus. In this study, we demonstrate that AICD could be modified via covalent conjugation with Nedd8, a ubiquitin-like protein. Domain analysis and site-directed substitution of neddylation sites showed that multiple lysine residues of the APP C-terminal C99 fragment including AICD were acceptor sequences for Nedd8 conjugation. AICD-mediated transcriptional activation was inhibited by Nedd8 conjugation. Furthermore, the transcriptional activity of the neddylation-defective AICD mutant was not altered by Nedd8 expression. Nedd8 conjugation of AICD inhibited its interaction with Fe65, and consequently resulted in the impairment of AICD-Fe65-Tip60 complex formation for the transcriptional activation of the target gene. These results illustrate the regulatory mechanisms by which AICD transcriptional activity might be regulated via covalent conjugation with Nedd8.

Fe65 does not stabilize AICD during activation of transcription in a luciferase assay

The APP intracellular domain (AICD) could be involved in signaling via interaction with the adaptor protein Fe65, and with the histone acetyl transferase Tip60. However, the real function of AICD and Fe65 in regulation of transcription remains controversial. In this study, the human APPGal4 fusion protein was expressed in CHO cells and the transcriptional activity of AICDGal4 was measured in a luciferase-based reporter assay. AICDGal4 was stabilized by expression of Fe65 and levels of AICDGal4 controlled luciferase activity. On the contrary, when human APP was expressed in CHO cells, coexpression of Fe65 increased luciferase activity without affecting the amount of AICD fragment. AICD produced from APP was protected from degradation by orthophenanthroline, but not by lactacystine, indicating that AICD is not a substrate of the chymotryptic activity of the proteasome. It is concluded that Fe65 can control luciferase activity without stabilizing the labile AICD fragment.

Rer1p competes with APH-1 for binding to nicastrin and regulates gamma-secretase complex assembly in the early secretory pathway

The γ-secretase complex, consisting of presenilin, nicastrin, presenilin enhancer-2 (PEN-2), and anterior pharynx defective-1 (APH-1) cleaves type I integral membrane proteins like amyloid precursor protein and Notch in a process of regulated intramembrane proteolysis. The regulatory mechanisms governing the multistep assembly of this “proteasome of the membrane” are unknown. We characterize a new interaction partner of nicastrin, the retrieval receptor Rer1p. Rer1p binds preferentially immature nicastrin via polar residues within its transmembrane domain that are also critical for interaction with APH-1. Absence of APH-1 substantially increased binding of nicastrin to Rer1p, demonstrating the competitive nature of these interactions. Moreover, Rer1p expression levels control the formation of γ-secretase subcomplexes and, concomitantly, total cellular γ-secretase activity. We identify Rer1p as a novel limiting factor that negatively regulates γ-secretase complex assembly by competing with APH-1 during active recycling between the endoplasmic reticulum (ER) and Golgi. We conclude that total cellular γ-secretase activity is restrained by a secondary ER control system that provides a potential therapeutic value.

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.

An antibody to the beta-secretase cleavage site on amyloid-beta-protein precursor inhibits amyloid-beta production

Proteolytic cleavage of amyloid-beta-protein precursor (AbetaPP) by beta- and gamma-secretases results in production of the amyloid-beta peptide (Abeta) that accumulates in the brains of sufferers of Alzheimer's disease (AD). We have developed a monoclonal antibody, 2B12, which binds in the vicinity of the beta-secretase cleavage site on AbetaPP but does not bind within the Abeta region. We hypothesised that this antibody, directed against the substrate rather than the enzyme, could inhibit cleavage of AbetaPP by beta-secretase via steric hindrance and thus reduce downstream production of Abeta. The antibody would enter cells by binding to AbetaPP when it is at the cell surface and then be internalised with the protein. We subsequently demonstrated that, after addition of 2B12 to standard growth media, this antibody was indeed capable of inhibiting Abeta40 production in neuroblastoma and astrocytoma cells expressing native AbetaPP, as measured by an ELISA. This inhibition was both concentration- and time-dependent and was specific to 2B12. We were only able to inhibit approximately 50% of Abeta40 production suggesting that not all AbetaPP is trafficked to the cell surface. We propose that this antibody could be used as a novel, putative therapy for the treatment of AD.

Assembly, trafficking and function of gamma-secretase

Gamma-secretase catalyzes the final cleavage of the beta-amyloid precursor protein to generate amyloid-beta peptide, the principal component of amyloid plaques in the brains of patients suffering from Alzheimer's disease. Here, we review the identification of gamma-secretase as a protease complex and its assembly and trafficking to its site(s) of cellular function. In reconstitution experiments, gamma-secretase was found to be composed of four integral membrane proteins, presenilin (PS), nicastrin (NCT), PEN-2 and APH-1 that are essential and sufficient for gamma-secretase activity. PS, which serves as a catalytic subunit of gamma-secretase, was identified as a prototypic member of novel aspartyl proteases of the GxGD type. In human cells, gamma-secretase could be further defined as a heterogeneous activity consisting of distinct complexes that are composed of PS1 or PS2 and APH-1a or APH-1b homologues together with NCT and PEN-2. Using green fluorescent protein as a reporter we localized PS and gamma-secretase activity at the plasma membrane and endosomes. Investigation of gamma-secretase complex assembly in knockdown and knockout cells of the individual subunits allowed us to develop a model of complex assembly in which NCT and APH-1 first stabilize PS before PEN-2 assembles as the last component. Furthermore, we could map domains in PS and PEN-2 that govern assembly and trafficking of the complex. Finally, Rer1 was identified as a PEN-2-binding protein that serves a role as an auxiliary factor for gamma-secretase complex assembly.

Genomics and expression profiles of the Hedgehog and Notch signaling pathways in sea urchin development

The Hedgehog (Hh) and Notch signal transduction pathways control a variety of developmental processes including cell fate choice, differentiation, proliferation, patterning and boundary formation. Because many components of these pathways are conserved, it was predicted and confirmed that pathway components are largely intact in the sea urchin genome. Spatial and temporal location of these pathways in the embryo, and their function in development offer added insight into their mechanistic contributions. Accordingly, all major components of both pathways were identified and annotated in the sea urchin Strongylocentrotus purpuratus genome and the embryonic expression of key components was explored. Relationships of the pathway components, and modifiers predicted from the annotation of S. purpuratus, were compared against cnidarians, arthropods, urochordates, and vertebrates. These analyses support the prediction that the pathways are highly conserved through metazoan evolution. Further, the location of these two pathways appears to be conserved among deuterostomes, and in the case of Notch at least, display similar capacities in endomesoderm gene regulatory networks. RNA expression profiles by quantitative PCR and RNA in situ hybridization reveal that Hedgehog is produced by the endoderm beginning just prior to invagination, and signals to the secondary mesenchyme-derived tissues at least until the pluteus larva stage. RNA in situ hybridization of Notch pathway members confirms that Notch functions sequentially in the vegetal-most secondary mesenchyme cells and later in the endoderm. Functional analyses in future studies will embed these pathways into the growing knowledge of gene regulatory networks that govern early specification and morphogenesis.

Regulated intramembrane proteolysis of amyloid precursor protein and regulation of expression of putative target genes

gamma-Secretase-dependent regulated intramembrane proteolysis of amyloid precursor protein (APP) releases the APP intracellular domain (AICD). The question of whether this domain, like the Notch intracellular domain, is involved in nuclear signalling is highly controversial. Although some reports suggest that AICD regulates the expression of KAI1, glycogen synthase kinase-3beta, Neprilysin and APP, we found no consistent effects of gamma-secretase inhibitors or of genetic deficiencies in the gamma-secretase complex or the APP family on the expression levels of these genes in cells and tissues. Finally, we demonstrate that Fe65, an important AICD-binding protein, transactivates a wide variety of different promoters, including the viral simian virus 40 promoter, independent of AICD coexpression. Overall, the four currently proposed target genes are at best indirectly and weakly influenced by APP processing. Therefore, inhibition of APP processing to decrease Abeta generation in Alzheimer's disease will not interfere significantly with the function of these genes.

Feedback Regulation of SREBP and Aromatase in A β(25-35)-Supplemented Human Neuroblastoma Cells

1. The analogies between the processing of amyloid precursor protein (APP) and other transmembrane sterol regulatory element binding proteins (SREBPs) inspired us to conduct further studies on whether beta-amyloid (Abeta) affects aromatase by interacting with APP and SREBP. 2. In this study, cultured human neuroblastoma cells (SHSY-5Y) were incubated in experimental media (media without FBS, the main cholesterol source) in the presence or absence of Abeta (1 microM) for 24 h. 3. Cellular extracts were subjected to immunoblot analysis using anti-APP, anti-aromatase and anti-SREBP-1. In these cell lines, we detected aromatase (55 kDa), SREBP cleavage product (68 kDa) and APP precursor (100-95 kDa) and cleavage product (60 kDa) by immunoblotting. Aromatase and SREBP levels were elevated in the cells incubated 24 h in experimental media and were attenuated in Abeta-supplemented experimental media. 4. The disturbance of cholesterol homeostasis appears to be an important factor in the pathogenesis of Alzheimer's disease. These findings may have important implications for understanding the mechanisms of the aromatase enzyme gene in disease states such as Alzheimer's.

Presenilin-1-mediated Retention of APP Derivatives in Early Biosynthetic Compartments

Processing of the amyloid precursor protein (APP) leads to the production of amyloid-beta (Abeta), the major component of extracellular plaques in the brains of Alzheimer's disease (AD) patients. Presenilin-1 (PS-1) plays a key role in the final step of Abeta formation, the gamma-secretase cleavage. Previously, we showed that PS-1 is retained in pre-Golgi compartments by incorporation into COPI-coated membranes of the vesicular tubular clusters (VTCs) between endoplasmic reticulum (ER) and Golgi complex. Here, we show that PS-1 also mediates the retention of the beta-cleavage-derived APP-C-terminal fragment (CTFbeta) and/or Abeta in pre-Golgi membranes. Overexpression of PS-1 increased the percentage of CTFbeta and/or Abeta in VTCs as well as their distribution to COPI-coated VTC membranes. By contrast, overexpression of the dominant-negative aspartate mutant PS-1(D257A) or PS-knockout decreased incorporation of these APP derivatives into COPI-coated membranes. Sorting of APP derivatives to COPI-coated VTC membranes was not depending on the APP cytosolic tail. In post-Golgi compartments, PS-1 expression enhanced the association of full-length APP/APPs with endosomal compartments at the expense of plasma membrane-bound APP. We conclude that PS-1, in addition to its role in gamma-secretase cleavage, is also required for the subcellular routing of APP and its derivatives. Malfunctioning of PS-1 in this role may have important consequences for the progress of AD.

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

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

A dominant role for FE65 (APBB1) in nuclear signaling

FE65 has been described as an adaptor protein; its partners include the beta-amyloid precursor protein (APP) and Tip60 (a histone acetyltransferase). Recent evidence suggests that APP may function in a nuclear signaling pathway via formation of APP-FE65-Tip60 complexes. The evidence is largely based on experiments in which APP/Tip60 is fused to the DNA binding domain of a yeast transcriptional factor Gal4 (Gal4DB) that can activate a reporter gene only when FE65 is coexpressed. One interpretation of published experiments has not yet been tested; however, there is the possibility that FE65 itself is the dominant transcriptional activator, whereas APP and Tip60 serve merely as positive/negative modulators or bridges for connecting FE65 to Gal4DB. To test this possibility, we fused Gal4DB directly to either end of FE65 and assessed their nuclear signaling in the presence or absence of exogenous APP/Tip60 or after knockdown of endogenous APP/Tip60. We found that FE65-Gal4DB by itself was able to trigger robust reporter activities. Increasing levels of APP could not further augment the reporter activity, but knocking down endogenous APP or interrupting FE65-APP binding reduced the signaling by up to 2-fold. The magnitudes of the reporter activities did not correlate with relative FE65 affinities for APP. Both overexpression and knockdown experiments showed that Tip60 plays a negative role. The results are consistent with the notion that FE65 is the key agent of Gal4DB-mediated transcriptional transactivation, whereas Tip60 is an FE65-associated repressor. Although APP may have modest stimulating effects, apparently there is no absolute requirement for that molecule for the nuclear signaling pathway.

The amyloid-beta precursor protein: integrating structure with biological function

Proteolytic processing of the amyloid-beta precursor protein (APP) generates the Abeta amyloid peptide of Alzheimer's disease. The biological function of APP itself remains, however, unclear. In the current review, we study in detail the different subdomains of APP and try to assign functional significance to particular structures identified in the protein.

Concepts for the treatment of Alzheimer's disease: molecular mechanisms and clinical application

To date, various strategies have been developed in order to prevent or to slow down the progression of Alzheimer's disease (AD). Despite the medical need for an effective therapeutic treatment of AD, progress towards this goal is painstakingly slow. Although AD is the most common neurodegenerative disorder and a large amount of primary basic and clinical research has been performed already, it appears very difficult to identify appropriate targets, which would promise fast, effective and safe strategies to combat the disease onset and progression. In this review, we present some of clinically applied treatment options, which may improve AD symptoms for a short period but so far lack the ability to prevent or halt this devastating disease. Additionally, we summarize some of the experimental approaches in AD therapy, which might lead to the development of more promising drugs in the future.

Mind bomb-2 is an E3 ligase for Notch ligand

The zebrafish gene, mind bomb (mib), encodes a protein that positively regulates of the Delta-mediated Notch signaling. It interacts with the intracellular domain of Delta to promote its ubiquitination and endocytosis. In our search for the mouse homologue of zebrafish mind bomb, we cloned two homologues in the mouse genome: a mouse orthologue (mouse mib1) and a paralogue, named mind bomb-2 (mib2), which is evolutionarily conserved from Drosophila to human. Both Mib1 and Mib2 have an E3 ubiquitin ligase activity in their C-terminal RING domain and interact with Xenopus Delta (XD) via their N-terminal region. Mib2 is also able to ligate ubiquitin to XD and shift the membrane localization of Delta to intracellular vesicles. Importantly, Mib2 rescues both the neuronal and vascular defects in the zebrafish mib(ta52b) mutants. In contrast to the functional similarities between Mib1 and Mib2, mib2 is highly expressed in adult tissues, but almost not at all in embryos, whereas mib1 is abundantly expressed in both embryos and adult tissues. These data suggest that Mib2 has functional similarities to Mib1, but might have distinct roles in Notch signaling as an E3 ubiquitin ligase.

Presenilin-dependent processing and nuclear function of gamma-protocadherins

The recently described protocadherin gene clusters encode cadherin-related proteins, which are highly expressed in the vertebrate nervous system. Here, we report biochemical studies addressing proteolytic processing of gamma-protocadherins. These type-I transmembrane proteins are cleaved by a metalloproteinase in vivo, generating a soluble extracellular fragment and a carboxyl-terminal fragment associated with the cellular membrane. In addition, we show that the carboxyl-terminal fragment is a substrate for further cleavage mediated by presenilin. Consequently, accumulation of the fragment is found when gamma-secretase is inactivated either by the specific presenilin-inhibitor L685,458 or in double mutant murine embryonic fibroblasts lacking both presenilin genes. The gamma-secretase-generated carboxyl-terminal fragment is largely unstable but accumulates when proteasomal degradation is inhibited. Interestingly, the proteolytic fragment generated by gamma-secretase can localize to the nucleus. This is the first report providing experimental evidence for a cell surface receptor signaling function of protocadherins regulated by proteolytic events.

Processing of Notch and amyloid precursor protein by gamma-secretase is spatially distinct

gamma-Secretase activity is associated with a presenilin (PS)-containing macromolecular complex. Whether PS contains the active site of gamma-secretase has been controversial. One challenge is to find PS that is engaged in the active gamma-secretase complex at the cell surface, where some substrates appear to be processed. In this study, we developed an intact cell photolabeling technique that allows the direct visualization of active gamma-secretase at the cell surface. We demonstrated that active gamma-secretase is present in the plasma membrane. Moreover, the PS1 heterodimer is specifically photolabeled at the cell surface by a potent inhibitor that binds to only the active gamma-secretase. We also explored the cellular processing sites of gamma-secretase for amyloid precursor protein (APP) and Notch by using small molecular probes. MRL631, a gamma-secretase inhibitor that is unable to penetrate the cell membrane, significantly blocks gamma-secretase-mediated Notch cleavage but has little effect on APP processing. These results indicate that Notch is processed at the cell surface and that the majority of APP is processed by intracellular gamma-secretase. Furthermore, the fact that inhibitors first target gamma-secretase in the plasma membrane for Notch processing, and not for APP, will have important implications for drug development to treat Alzheimer's disease and cancer.

Release of a membrane-bound death domain by gamma-secretase processing of the p75NTR homolog NRADD

Neurotrophin receptor alike death domain protein (NRADD) is a death-receptor-like protein with a unique ectodomain and an intracellular domain homologous to p75(NTR). Expression of NRADD results in apoptosis, but only in certain cell types. This paper characterizes the expression and proteolytic processing of the mature 55 kDa glycoprotein. N-terminally truncated NRADD is processed by a gamma-secretase activity that requires presenilins and has the same susceptibility to gamma-secretase inhibitors as the secretion of amyloid beta (A beta). The ectodomain of endogenous NRADD is shed by activation of metalloproteinases. Inhibitor studies provide evidence that NRADD is cleaved in two steps typical of regulated intramembrane proteolysis (RIP). Inhibition of gamma-secretase abrogates both the production of the soluble intracellular domain of NRADD and the appearance of NRADD in subnuclear structures. Thus, solubilized death domains with close homology to p75(NTR) might have a nuclear function. Furthermore, presenilin deficiency leads to abnormally glycosylated NRADD and overexpression of presenilin 2 inhibits NRADD maturation, which is dependent on the putative active site residue D366 but not on gamma-secretase activity. Our results demonstrate that NRADD is an additional gamma-secretase substrate and suggest that drugs against Alzheimer's disease will need to target gamma-secretase in a substrate-specific manner.

Identification of neuronal plasma membrane microdomains that colocalize beta-amyloid and presenilin: implications for beta-amyloid precursor protein processing

Alzheimer's disease (AD) is associated with the accumulation of extracellular deposits of the beta-amyloid protein (Abeta). Abeta is a result of misprocessing of the beta-amyloid precursor protein (APP). Gamma-secretase is involved in APP misprocessing and one hypothesis holds that this secretase is identical to PS1. We tested this hypothesis by determining whether PS is co-localised with Abeta in situ. Using confocal analyses and a sensitive immunogold procedure we show that PS and Abeta are co-localised within discrete microdomains of neuronal plasma membranes in AD patients and in aged dogs, an established model of human brain aging. Our data indicate that APP misprocessing occurs in discrete plasma membrane domains of neurons and provide evidence that PS1 is critically involved in Abeta formation.

Presenilin-1 exists in both pre- and post-Golgi compartments and recycles via COPI-coated membranes

Presenilin-1 is involved in intramembrane proteolysis of various proteins, but its intracellular site of action has remained elusive. Here, we determined by quantitative immunogold-electron microscopy that presenilin-1 in Chinese hamster ovary cells is present in pre-Golgi compartments as well as at the plasma membrane and endosomes. Notably, a high percentage of presenilin-1 resides in COPI-coated membranes between the endoplasmic reticulum and the Golgi complex, indicating significant recycling to the endoplasmic reticulum. By contrast, the inactive aspartate mutant presenilin-1D257A is relatively excluded from COPI-coated membranes, concomitant with increased post-Golgi levels. These data provide critical evidence for the scenario that the complex containing presenilin-1 can serve as gamma-secretase at the plasma membrane or endosomes and suggest a role for COPI-mediated retrograde transport in regulating post-Golgi levels of presenilin-1.

Presenilin-1, nicastrin, amyloid precursor protein, and gamma-secretase activity are co-localized in the lysosomal membrane

Alzheimer's disease (AD) is caused by the cerebral deposition of beta-amyloid (Abeta), a 38-43-amino acid peptide derived by proteolytic cleavage of the amyloid precursor protein (APP). Initial studies indicated that final cleavage of APP by the gamma-secretase (a complex containing presenilin and nicastrin) to produce Abeta occurred in the endosomal/lysosomal system. However, other studies showing a predominant endoplasmic reticulum localization of the gamma-secretase proteins and a neutral pH optimum of in vitro gamma-secretase assays have challenged this conclusion. We have recently identified nicastrin as a major lysosomal membrane protein. In the present work, we use Western blotting and immunogold electron microscopy to demonstrate that significant amounts of mature nicastrin, presenilin-1, and APP are co-localized with lysosomal associated membrane protein-1 (cAMP-1) in the outer membranes of lysosomes. Furthermore, we demonstrate that these membranes contain an acidic gamma-secretase activity, which is immunoprecipitable with an antibody to nicastrin. These experiments establish APP, nicastrin, and presenilin-1 as resident lysosomal membrane proteins and indicate that gamma-secretase is a lysosomal protease. These data reassert the importance of the lysosomal/endosomal system in the generation of Abeta and suggest a role for lysosomes in the pathophysiology of AD.

The intracellular domain of teneurin-2 has a nuclear function and represses zic-1-mediated transcription

Teneurin-2, a vertebrate homologue of the Drosophila pair-rule gene ten-m/odz, is revealed to be a membrane-bound transcription regulator. In the nucleus, the intracellular domain of teneurin-2 colocalizes with promyelocytic leukemia (PML) protein in nuclear bodies implicated in transcription control. Since Drosophila ten-m acts epistatically to another pair-rule gene opa, we investigated whether gene regulation by the mammalian opa homologue zic-1 was influenced by the intracellular domain of teneurin-2. We found that zic-mediated transcription from the apolipoprotein E promoter was inhibited. Release of the intracellular domain of teneurin-2 could be stimulated by homophilic interaction of the extracellular domain, and the intracellular domain was stabilized by proteasome inhibitors. We have previously shown that teneurin-2 is expressed by neurons belonging to the same functional circuit. Therefore, we hypothesize that homophilic interaction enables neurons to identify their targets and that the release of the intracellular domain of teneurin-2 provides them with a signal to switch their gene expression program from growth towards differentiation once the proper contact has been made.

Monitoring proteins in intact cells

In the past decade, tremendous progress has been made in understanding the molecular mechanisms that underlie many neurodegenerative disorders, and common aspects of the pathological processes that lead to neuronal cell death have emerged. Nearly all of these advances have come from genetic, molecular, and biochemical studies that point to alterations in protein folding or protein-protein interactions as the fundamental mechanism behind these disorders. New microscopy/imaging technologies for detecting protein-protein interactions are now poised to contribute to progress in this field. Here we describe a novel technique based on fluorescence resonance energy transfer, called fluorescence lifetime imaging microscopy (FLIM), which allows monitoring of protein conformation and protein-protein interactions in intact cells. For example, using FLIM, we have studied the interaction of two proteins related to Alzheimer's disease: amyloid precursor protein (APP) and presenilin (PS-1). By measuring the lifetime of a donor fluorophore linked to PS-1, we show, with high subcellular resolution, localization of the PS-1-APP interaction within neurons. This approach may have widespread applicability in studies of neurodegenerative disease mechanisms.