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

Accumulation of APP C-terminal fragments causes endolysosomal dysfunction through the dysregulation of late endosome to lysosome-ER contact sites

Neuronal endosomal and lysosomal abnormalities are among the early changes observed in Alzheimer's disease (AD) before plaques appear. However, it is unclear whether distinct endolysosomal defects are temporally organized and how altered γ-secretase function or amyloid precursor protein (APP) metabolism contribute to these changes. Inhibiting γ-secretase chronically, in mouse embryonic fibroblast and hippocampal neurons, led to a gradual endolysosomal collapse initiated by decreased lysosomal calcium and increased cholesterol, causing downstream defects in endosomal recycling and maturation. This endolysosomal demise is γ-secretase dependent, requires membrane-tethered APP cytoplasmic domains, and is rescued by APP depletion. APP C-terminal fragments (CTFs) localized to late endosome/lysosome-endoplasmic reticulum contacts; an excess of APP-CTFs herein reduced lysosomal Ca2+ refilling from the endoplasmic reticulum, promoting cholesterol accretion. Tonic regulation by APP-CTFs provides a mechanistic explanation for their cellular toxicity: failure to timely degrade APP-CTFs sustains downstream signaling, instigating lysosomal dyshomeostasis, as observed in prodromal AD. This is the opposite of substrates such as Notch, which require intramembrane proteolysis to initiate signaling.

PI4P and BLOC-1 remodel endosomal membranes into tubules

Intracellular trafficking is mediated by transport carriers that originate by membrane remodeling from donor organelles. Tubular carriers contribute to the flux of membrane lipids and proteins to acceptor organelles, but how lipids and proteins impose a tubular geometry on the carriers is incompletely understood. Using imaging approaches on cells and in vitro membrane systems, we show that phosphatidylinositol-4-phosphate (PI4P) and biogenesis of lysosome-related organelles complex 1 (BLOC-1) govern the formation, stability, and functions of recycling endosomal tubules. In vitro, BLOC-1 binds and tubulates negatively charged membranes, including those containing PI4P. In cells, endosomal PI4P production by type II PI4-kinases is needed to form and stabilize BLOC-1-dependent recycling endosomal tubules. Decreased PI4KIIs expression impairs the recycling of endosomal cargoes and the life cycles of intracellular pathogens such as Chlamydia bacteria and influenza virus that exploit the membrane dynamics of recycling endosomes. This study demonstrates how a phospholipid and a protein complex coordinate the remodeling of cellular membranes into functional tubules.

Dictyostelium discoideum: A Model System for Neurological Disorders

Background: The incidence of neurological disorders is increasing due to population growth and extended life expectancy. Despite advances in the understanding of these disorders, curative strategies for treatment have not yet eventuated. In part, this is due to the complexities of the disorders and a lack of identification of their specific underlying pathologies. Dictyostelium discoideum has provided a useful, simple model to aid in unraveling the complex pathological characteristics of neurological disorders including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, neuronal ceroid lipofuscinoses and lissencephaly. In addition, D. discoideum has proven to be an innovative model for pharmaceutical research in the neurological field. Scope of review: This review describes the contributions of D. discoideum in the field of neurological research. The continued exploration of proteins implicated in neurological disorders in D. discoideum may elucidate their pathological roles and fast-track curative therapeutics.

Presenilin 1 Regulates Membrane Homeostatic Pathways that are Dysregulated in Alzheimer's Disease

Mutations in the PSEN1 gene, encoding presenilin 1 (PS1), are the most common cause of familial Alzheimer's disease (fAD). Since the first mutations in the PSEN1 gene were discovered more than 25 years ago, many postulated functions of PS1 have been investigated. The majority of earlier studies focused on its role as the catalytic component of the γ-secretase complex, which in concert with β site amyloid precursor protein cleaving enzyme 1 (BACE1), mediates the formation of Aβ from amyloid-β protein precursor (AβPP). Though mutant PS1 was originally considered to cause AD by promoting Aβ pathology through its protease function, it is now becoming clear that PS1 is a multifunctional protein involved in regulating membrane dynamics and protein trafficking. Therefore, through loss of these abilities, mutant PS1 has the potential to impair numerous cellular functions such as calcium flux, organization of proteins in different compartments, and protein turnover via vacuolar metabolism. Impaired calcium signaling, vacuolar dysfunction, mitochondrial dysfunction, and increased ER stress, among other related membrane-dependent disturbances, have been considered critical to the development and progression of AD. Given that PS1 plays a key regulatory role in all these processes, this review will describe the role of PS1 in different cellular compartments and provide an integrated view of how PS1 dysregulation (due to mutations or other causes) could result in impairment of various cellular processes and result in a "multi-hit", integrated pathological outcome that could contribute to the etiology of AD.

Nicastrin-Like, a Novel Transmembrane Protein from Trypanosoma cruzi Associated to the Flagellar Pocket

Nicastrin (NICT) is a transmembrane protein physically associated with the polytypical aspartyl protease presenilin that plays a vital role in the correct localization and stabilization of presenilin to the membrane-bound γ-secretase complex. This complex is involved in the regulation of a wide range of cellular events, including cell signaling and the regulation of endocytosed membrane proteins for their trafficking and protein processing. Methods: In Trypanosoma cruzi, the causal agent of the Chagas disease, a NICT-like protein (Tc/NICT) was identified with a short C-terminus orthologous to the human protein, a large ectodomain (ECD) with numerous glycosylation sites and a single-core transmembrane domain containing a putative TM-domain (457GSVGA461) important for the γ-secretase complex activity. Results: Using the Spot-synthesis strategy with Chagasic patient sera, five extracellular epitopes were identified and synthetic forms were used to generate rabbit anti-Tc/NICT polyclonal serum that recognized a ~72-kDa molecule in immunoblots of T. cruzi epimastigote extracts. Confocal microscopy suggests that Tc/NICT is localized in the flagellar pocket, which is consistent with data from our previous studies with a T. cruzi presenilin-like protein. Phylogenetically, Tc/NICT was localized within a subgroup with the T. rangeli protein that is clearly detached from the other Trypanosomatidae, such as T. brucei. These results, together with a comparative analysis of the selected peptide sequence regions between the T. cruzi and mammalian proteins, suggest a divergence from the human NICT that might be relevant to Chagas disease pathology. As a whole, our data show that a NICT-like protein is expressed in the infective and replicative stages of T. cruzi and may be considered further evidence for a γ-secretase complex in trypanosomatids.

The C99 domain of the amyloid precursor protein resides in the disordered membrane phase

Processing of the amyloid precursor protein (APP) via the amyloidogenic pathway is associated with the etiology of Alzheimer's disease. The cleavage of APP by β-secretase to generate the transmembrane 99-residue C-terminal fragment (C99) and subsequent processing of C99 by γ-secretase to yield amyloid-β (Aβ) peptides are essential steps in this pathway. Biochemical evidence suggests that amyloidogenic processing of C99 occurs in cholesterol- and sphingolipid-enriched liquid-ordered phase membrane rafts. However, direct evidence that C99 preferentially associates with these rafts has remained elusive. Here, we tested this by quantifying the affinity of C99-GFP for raft domains in cell-derived giant plasma membrane vesicles (GPMVs). We found that C99 was essentially excluded from ordered domains in vesicles from HeLa cells, undifferentiated SH-SY5Y cells, or SH-SY5Y-derived neurons; instead, ∼90% of C99 partitioned into disordered domains. The strong association of C99 with disordered domains occurred independently of its cholesterol-binding activity or homodimerization, or of the presence of the familial Alzheimer disease Arctic mutation (APP E693G). Finally, through biochemical studies we confirmed previous results, which showed that C99 is processed in the plasma membrane by α-secretase, in addition to the well-known γ-secretase. These findings suggest that C99 itself lacks an intrinsic affinity for raft domains, implying that either i) amyloidogenic processing of the protein occurs in disordered regions of the membrane, ii) processing involves a marginal subpopulation of C99 found in rafts, or iii) as-yet-unidentified protein-protein interactions with C99 in living cells drive this protein into membrane rafts to promote its cleavage therein.

Trypanosoma cruzi Presenilin-Like Transmembrane Aspartyl Protease: Characterization and Cellular Localization

The increasing detection of infections of Trypanosoma cruzi, the etiological agent of Chagas disease, in non-endemic regions beyond Latin America has risen to be a major public health issue. With an impact in the millions of people, current treatments rely on antiquated drugs that produce severe side effects and are considered nearly ineffective for the chronic phase. The minimal progress in the development of new drugs highlights the need for advances in basic research on crucial biochemical pathways in T. cruzi to identify new targets. Here, we report on the T. cruzi presenilin-like transmembrane aspartyl enzyme, a protease of the aspartic class in a unique phylogenetic subgroup with T. vivax separate from protozoans. Computational analyses suggest it contains nine transmembrane domains and an active site with the characteristic PALP motif of the A22 family. Multiple linear B-cell epitopes were identified by SPOT-synthesis analysis with Chagasic patient sera. Two were chosen to generate rabbit antisera, whose signal was primarily localized to the flagellar pocket, intracellular vesicles, and endoplasmic reticulum in parasites by whole-cell immunofluorescence. The results suggest that the parasitic presenilin-like enzyme could have a role in the secretory pathway and serve as a target for the generation of new therapeutics specific to the T. cruzi.

Differential Profile of Systemic Extracellular Vesicles From Sporadic and Familial Alzheimer's Disease Leads to Neuroglial and Endothelial Cell Degeneration

Evidence suggests that extracellular vesicles (EVs) act as mediators and biomarkers of neurodegenerative diseases. Two distinct forms of Alzheimer disease (AD) are known: a late-onset sporadic form (SAD) and an early-onset familial form (FAD). Recently, neurovascular dysfunction and altered systemic immunological components have been linked to AD neurodegeneration. Therefore, we characterized systemic-EVs from postmortem SAD and FAD patients and evaluated their effects on neuroglial and endothelial cells. We found increase CLN-5 spots with vesicular morphology in the abluminal portion of vessels from SAD patients. Both forms of AD were associated with larger and more numerous systemic EVs. Specifically, SAD patients showed an increase in endothelial- and leukocyte-derived EVs containing mitochondria; in contrast, FAD patients showed an increase in platelet-derived EVs. We detected a differential protein composition for SAD- and FAD-EVs associated with the coagulation cascade, inflammation, and lipid-carbohydrate metabolism. Using mono- and cocultures (endothelium-astrocytes-neurons) and human cortical organoids, we showed that AD-EVs induced cytotoxicity. Both forms of AD featured decreased neuronal branches area and astrocytic hyperreactivity, but SAD-EVs led to greater endothelial detrimental effects than FAD-EVs. In addition, FAD- and SAD-EVs affected calcium dynamics in a cortical organoid model. Our findings indicate that the phenotype of systemic AD-EVs is differentially defined by the etiopathology of the disease (SAD or FAD), which results in a differential alteration of the NVU cells implied in neurodegeneration.

Importance of γ-secretase in the regulation of liver X receptor and cellular lipid metabolism

Inhibition of the Alzheimer associated γ-secretase impairs the regulation of cellular lipid droplet homeostasis.

Presenilins (PS) are the catalytic components of γ-secretase complexes that mediate intramembrane proteolysis. Mutations in the PS genes are a major cause of familial early-onset Alzheimer disease and affect the cleavage of the amyloid precursor protein, thereby altering the production of the amyloid β-peptide. However, multiple additional protein substrates have been identified, suggesting pleiotropic functions of γ-secretase. Here, we demonstrate that inhibition of γ-secretase causes dysregulation of cellular lipid homeostasis, including up-regulation of liver X receptors, and complex changes in the cellular lipid composition. Genetic and pharmacological inhibition of γsecretase leads to strong accumulation of cytoplasmic lipid droplets, associated with increased levels of acylglycerols, but lowered cholesteryl esters. Furthermore, accumulation of lipid droplets was augmented by increasing levels of amyloid precursor protein C-terminal fragments, indicating a critical involvement of this γ-secretase substrate. Together, these data provide a mechanism that functionally connects γ-secretase activity to cellular lipid metabolism. These effects were also observed in human astrocytic cells, indicating an important function of γ-secretase in cells critical for lipid homeostasis in the brain.

Investigations on the Relationship between Ovarian, Endocrine, and Renal Findings in Nonclinical Safety Studies of the γ-secretase Inhibitor Avagacestat

Avagacestat, a gamma (γ)-secretase inhibitor that was in development for treatment of Alzheimer's disease, produced ovarian granulosa-thecal cell tumors in rats and dogs and a glomerulopathy with profound proteinuria in female rats. This report describes the results of follow-up investigative studies, including the use of ovariectomized (OVX) rats, to further characterize these findings and determine their mechanism(s). Ovarian proliferative changes in rats likely resulted from: 1) inhibition of Notch signaling pathways regulating ovarian follicular differentiation/development, characterized microscopically as altered ovarian cyclicity and/or ovarian follicular degeneration; 2) subsequent disruption of the hypothalamic-pituitary-ovarian axis due to ovarian atrophy with decreases in serum estrogen and progesterone (as low as 0.45× and 0.21× controls, respectively); and 3) chronic gonadotropin stimulation and pituitary hypertrophy/hyperplasia in response to the absence of negative feedback. Gonadotropin stimulation in rats was confirmed by increases in serum follicle-stimulating hormone (FSH; up to 7.75× controls) and luteinizing hormone (LH; up to 5.84×). A similar non-genotoxic mechanism was likely responsible for the ovarian findings in dogs although changes in serum hormone levels were not detected. The dose- and time-dependent glomerulopathy with progression to chronic progressive nephropathy in female rats appears to be a direct effect of avagacestat and was not ameliorated with co-administration of 17β-estradiol or an antihypertensive (enalapril) and was not present in control OVX rats. In contrast, adrenocortical hypertrophy in female rats was considered secondary to ovarian changes based on the absence of this finding in avagacestat-treated OVX rats and no increase in ACTH staining in the pituitary.

Gamma secretase orthologs are required for lysosomal activity and autophagic degradation in Dictyostelium discoideum, independent of PSEN (presenilin) proteolytic function

Mutations in the γ-secretase complex are strongly associated with familial Alzheimer disease. Both proteolytic and non-proteolytic functions for the γ-secretase complex have been previously described in mammalian model organisms, but their relative contributions to disease pathology remain unclear. Here, we dissect the roles of orthologs of the γ-secretase components in the model system Dictyostelium, focusing on endocytosis, lysosomal activity and autophagy. In this model, we show that the orthologs of PSEN (psenA and psenB), Ncstn (nicastrin) and Aph-1 (gamma-secretase subunit Aph-1), are necessary for optimal fluid-phase uptake by macropinocytosis and in multicellular development under basic pH conditions. Disruption of either psenA/B or Aph-1 proteins also leads to disrupted phagosomal proteolysis as well as decreased autophagosomal acidification and autophagic flux. This indicates a general defect in lysosomal trafficking and degradation, which we show leads to the accumulation of ubiquitinated protein aggregates in cells lacking psenA/B and Aph-1 proteins. Importantly, we find that all the endocytic defects observed in Dictyostelium PSEN ortholog mutants can be fully rescued by proteolytically inactive Dictyostelium psenB and human PSEN1 proteins. Our data therefore demonstrates an evolutionarily conserved non-proteolytic role for presenilin, and γ-secretase component orthologs, in maintaining Dictyostelium lysosomal trafficking and autophagy.

Abbreviations: Atg8: autophagy protein 8a; Aph-1: gamma-secretase subunit Aph-1; crtA: calreticulin; ER: endoplasmic reticulum; GFP: green fluorescent protein; GSK3B: glycogen synthase kinase 3 beta; Ncstn: nicastrin; PSEN1: presenilin 1; psenA and psenB: Dictyostelium presenilin A and B; TRITC; tetramethylrhodamine isothiocyanate.

Presenilins and γ-Secretase in Membrane Proteostasis

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

A Review of Notch Processing With New Insights Into Ligand-Independent Notch Signaling in T-Cells

The Notch receptor is an evolutionarily highly conserved transmembrane protein essential to a wide spectrum of cellular systems, and its deregulation has been linked to a vast number of developmental disorders and malignancies. Regulated Notch function is critical for the generation of T-cells, in which abnormal Notch signaling results in leukemia. Notch activation through trans-activation of the receptor by one of its ligands expressed on adjacent cells has been well defined. In this canonical ligand-dependent pathway, Notch receptor undergoes conformational changes upon ligand engagement, stimulated by a pulling-force on the extracellular fragment of Notch that results from endocytosis of the receptor-bound ligand into the ligand-expressing cell. These conformational changes in the receptor allow for two consecutive proteolytic cleavage events to occur, which release the intracellular region of the receptor into the cytoplasm. It can then travel to the nucleus, where it induces gene transcription. However, there is accumulating evidence that other pathways may induce Notch signaling. A ligand-independent mechanism of Notch activation has been described in which receptor processing is initiated via cell-internal signals. These signals result in the internalization of Notch into endosomal compartments, where chemical changes existing in this microenvironment result in the conformational modifications required for receptor processing. This review will present mechanisms underlying both canonical ligand-dependent and non-canonical ligand-independent Notch activation pathways and discuss the latter in the context of Notch signaling in T-cells.

Drosophila melanogaster Models of Metal-Related Human Diseases and Metal Toxicity

Iron, copper and zinc are transition metals essential for life because they are required in a multitude of biological processes. Organisms have evolved to acquire metals from nutrition and to maintain adequate levels of each metal to avoid damaging effects associated with its deficiency, excess or misplacement. Interestingly, the main components of metal homeostatic pathways are conserved, with many orthologues of the human metal-related genes having been identified and characterized in Drosophila melanogaster. Drosophila has gained appreciation as a useful model for studying human diseases, including those caused by mutations in pathways controlling cellular metal homeostasis. Flies have many advantages in the laboratory, such as a short life cycle, easy handling and inexpensive maintenance. Furthermore, they can be raised in a large number. In addition, flies are greatly appreciated because they offer a considerable number of genetic tools to address some of the unresolved questions concerning disease pathology, which in turn could contribute to our understanding of the metal metabolism and homeostasis. This review recapitulates the metabolism of the principal transition metals, namely iron, zinc and copper, in Drosophila and the utility of this organism as an experimental model to explore the role of metal dyshomeostasis in different human diseases. Finally, a summary of the contribution of Drosophila as a model for testing metal toxicity is provided.

Emerging roles of extracellular vesicles in neurodegenerative disorders: focus on HIV-associated neurological complications

Exosomes are membrane-enriched extracellular vesicles with a proposed diameter in the range of 30-100 nm. They are released during both normal homeostasis as well as under pathological conditions by most cell types. In recent years, there has been robust interest in the study of these vesicles as conduits for the delivery of information between cells in both analogous as well as disparate tissues. Their ability to transport specialized cargo including signaling mediators, proteins, messenger RNA and miRNAs characterizes these vesicles as primary facilitators of cell-to-cell communication and regulation. Exosomes have also been demonstrated to have important roles in the field of cancer biology and metastasis. More recently, their role in several neurodegenerative disorders has been gaining increased momentum as these particles have been shown to promote the spread of toxic factors such as amyloid beta and prions, adding further validity to their role as important regulators of disease pathogenesis. This review briefly summarizes current findings and thoughts on exosome biology in the context of neurodegenerative disorders and the manipulation of these particles for the development of potential therapeutic strategies.

Defective Transcytosis of APP and Lipoproteins in Human iPSC-Derived Neurons with Familial Alzheimer's Disease Mutations

We investigated early phenotypes caused by familial Alzheimer's disease (fAD) mutations in isogenic human iPSC-derived neurons. Analysis of neurons carrying fAD PS1 or APP mutations introduced using genome editing technology at the endogenous loci revealed that fAD mutant neurons had previously unreported defects in the recycling state of endocytosis and soma-to-axon transcytosis of APP and lipoproteins. The endocytosis reduction could be rescued through treatment with a β-secretase inhibitor. Our data suggest that accumulation of β-CTFs of APP, but not Aβ, slow vesicle formation from an endocytic recycling compartment marked by the transcytotic GTPase Rab11. We confirm previous results that endocytosis is affected in AD and extend these to uncover a neuron-specific defect. Decreased lipoprotein endocytosis and transcytosis to the axon suggest that a neuron-specific impairment in endocytic axonal delivery of lipoproteins and other key materials might compromise synaptic maintenance in fAD.

Presenilin 1 and Presenilin 2 Target γ-Secretase Complexes to Distinct Cellular Compartments

γ-Secretase complexes achieve the production of amyloid peptides playing a key role in Alzheimer disease. These proteases have many substrates involved in important physiological functions. They are composed of two constant subunits, nicastrin and PEN2, and two variable ones, presenilin (PS1 or PS2) and APH1 (APH1aL, APH1aS, or APH1b). Whether the composition of a given γ-secretase complex determines a specific cellular targeting remains unsolved. Here we combined a bidirectional inducible promoter and 2A peptide technology to generate constructs for the temporary, stoichiometric co-expression of six different combinations of the four γ-secretase subunits including EGFP-tagged nicastrin. These plasmids allow for the formation of functional γ-secretase complexes displaying specific activities and maturations. We show that PS1-containing γ-secretase complexes were targeted to the plasma membrane, whereas PS2-containing ones were addressed to the trans-Golgi network, to recycling endosomes, and, depending on the APH1-variant, to late endocytic compartments. Overall, these novel constructs unravel a presenilin-dependent subcellular targeting of γ-secretase complexes. These tools should prove useful to determine whether the cellular distribution of γ-secretase complexes contributes to substrate selectivity and to delineate regulations of their trafficking.

Beyond γ-secretase activity: The multifunctional nature of presenilins in cell signalling pathways

The presenilins are the catalytic subunit of the membrane-embedded tetrameric γ-secretase protease complexes. More that 90 transmembrane proteins have been reported to be γ-secretase substrates, including the widely studied amyloid precursor protein (APP) and the Notch receptor, which are precursors for the generation of amyloid-β peptides and biologically active APP intracellular domain (AICD) and Notch intracellular domain (NICD). The diversity of γ-secretase substrates highlights the importance of presenilin-dependent γ-secretase protease activities as a regulatory mechanism in a range of biological systems. However, there is also a growing body of evidence that supports the existence of γ-secretase-independent functions for the presenilins in the regulation and progression of an array of cell signalling pathways. In this review, we will present an overview of current literature that proposes evolutionarily conserved presenilin functions outside of the γ-secretase complex, with a focus on the suggested role of the presenilins in the regulation of Wnt/β-catenin signalling, protein trafficking and degradation, calcium homeostasis and apoptosis.

Sphingosine derivatives inhibit cell signaling by electrostatically neutralizing polyphosphoinositides at the plasma membrane

Mast cell stimulation via IgE receptors causes activation of multiple processes, including Ca(2+) mobilization, granule exocytosis, and outward trafficking of recycling endosomes to the plasma membrane. We used fluorescein-conjugated cholera toxin B (FITC-CTxB) to label GM(1) in recycling endsomes and to monitor antigen-stimulated trafficking to the plasma membrane in both fluorimeter and imaging-based assays. We find that the sphingosine derivatives D-sphingosine and N,N'-dimethylsphingosine effectively inhibit this outward trafficking response, whereas a quarternary ammonium derivative, N,N',N″-trimethylsphingosine, does not inhibit. This pattern of inhibition is also found for Ca(2+) mobilization and secretory lysosomal exocytosis, indicating a general effect on Ca(2+)-dependent signaling processes. This inhibition correlates with the capacity of sphingosine derivatives to flip to the inner leaflet of the plasma membrane that is manifested as changes in plasma membrane-associated FITC-CTxB fluorescence and cytoplasmic pH. Using a fluorescently labeled MARCKS effector domain to monitor plasma membrane-associated polyphosphoinositides, we find that these sphingosine derivatives displace the electrostatic binding of this MARCKS effector domain to the plasma membrane in parallel with their capacity to inhibit Ca(2+)-dependent signaling. Our results support roles for plasma membrane polyphosphoinositides in Ca(2+) signaling and stimulated exocytosis, and they illuminate a mechanism by which D-sphingosine regulates signaling responses in mammalian cells.

Gamma-secretase subunits associate in intracellular membrane compartments in Arabidopsis thaliana

Gamma-secretase is a multisubunit complex with intramembrane proteolytic activity. In humans it was identified in genetic screens of patients suffering from familial forms of Alzheimer's disease, and since then it was shown to mediate cleavage of more than 80 substrates, including amyloid precursor protein or Notch receptor. Moreover, in animals, γ-secretase was shown to be involved in regulation of a wide range of cellular events, including cell signalling, regulation of endocytosis of membrane proteins, their trafficking, and degradation. Here we show that genes coding for γ-secretase homologues are present in plant genomes. Also, amino acid motifs crucial for γ-secretase activity are conserved in plants. Moreover, all γ-secretase subunits: PS1/PS2, APH-1, PEN-2, and NCT colocalize and interact with each other in Arabidopsis thaliana protoplasts. The intracellular localization of γ-secretase subunits in Arabidopsis protoplasts revealed a distribution in endomembrane system compartments that is consistent with data from animal studies. Together, our data may be considered as a starting point for analysis of γ-secretase in plants.

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.

ARH directs megalin to the endocytic recycling compartment to regulate its proteolysis and gene expression

ARH is required for the trafficking of megalin from early endosomes to the endocytic recycling compartment, where megalin undergoes intramembrane proteolysis, releasing a tail fragment that represses megalin transcription.

Receptors internalized by endocytosis can return to the plasma membrane (PM) directly from early endosomes (EE; fast recycling) or they can traffic from EE to the endocytic recycling compartment (ERC) and recycle from there (slow recycling). How receptors are sorted for trafficking along these two pathways remains unclear. Here we show that autosomal recessive hypercholesterolemia (ARH) is required for trafficking of megalin, a member of the LDL receptor family, from EE to the ERC by coupling it to dynein; in the absence of ARH, megalin returns directly to the PM from EE via the connecdenn2/Rab35 fast recycling pathway. Binding of ARH to the endocytic adaptor AP-2 prevents fast recycling of megalin. ARH-mediated trafficking of megalin to the ERC is necessary for γ-secretase mediated cleavage of megalin and release of a tail fragment that mediates transcriptional repression. These results identify a novel mechanism for sorting receptors for trafficking to the ERC and link ERC trafficking to regulated intramembrane proteolysis (RIP) and expression of megalin.

Small molecule suppressors of Drosophila kinesin deficiency rescue motor axon development in a zebrafish model of spinal muscular atrophy

Proximal spinal muscular atrophy (SMA) is the most common inherited motor neuropathy and the leading hereditary cause of infant mortality. Currently there is no effective treatment for the disease, reflecting a need for pharmacologic interventions that restore performance of dysfunctional motor neurons or suppress the consequences of their dysfunction. In a series of assays relevant to motor neuron biology, we explored the activities of a collection of tetrahydroindoles that were reported to alter the metabolism of amyloid precursor protein (APP). In Drosophila larvae the compounds suppressed aberrant larval locomotion due to mutations in the Khc and Klc genes, which respectively encode the heavy and light chains of kinesin-1. A representative compound of this class also suppressed the appearance of axonal swellings (alternatively termed axonal spheroids or neuritic beads) in the segmental nerves of the kinesin-deficient Drosophila larvae. Given the importance of kinesin-dependent transport for extension and maintenance of axons and their growth cones, three members of the class were tested for neurotrophic effects on isolated rat spinal motor neurons. Each compound stimulated neurite outgrowth. In addition, consistent with SMA being an axonopathy of motor neurons, the three axonotrophic compounds rescued motor axon development in a zebrafish model of SMA. The results introduce a collection of small molecules as pharmacologic suppressors of SMA-associated phenotypes and nominate specific members of the collection for development as candidate SMA therapeutics. More generally, the results reinforce the perception of SMA as an axonopathy and suggest novel approaches to treating the disease.

Presenilin controls kinesin-1 and dynein function during APP-vesicle transport in vivo

Neurons and other cells require intracellular transport of essential components for viability and function. Previous work has shown that while net amyloid precursor protein (APP) transport is generally anterograde, individual vesicles containing APP move bi-directionally. This discrepancy highlights our poor understanding of the in vivo regulation of APP-vesicle transport. Here, we show that reduction of presenilin (PS) or suppression of gamma-secretase activity substantially increases anterograde and retrograde velocities for APP vesicles. Strikingly, PS deficiency has no effect on an unrelated cargo vesicle class containing synaptotagmin, which is powered by a different kinesin motor. Increased velocities caused by PS or gamma-secretase reduction require functional kinesin-1 and dynein motors. Together, our findings suggest that a normal function of PS is to repress kinesin-1 and dynein motor activity during axonal transport of APP vesicles. Furthermore, our data suggest that axonal transport defects induced by loss of PS-mediated regulatory effects on APP-vesicle motility could be a major cause of neuronal and synaptic defects observed in Alzheimer's Disease (AD) pathogenesis. Thus, perturbations of APP/PS transport could contribute to early neuropathology observed in AD, and highlight a potential novel therapeutic pathway for early intervention, prior to neuronal loss and clinical manifestation of disease.

Presenilin promotes dietary copper uptake

Dietary copper is essential for multicellular organisms. Copper is redox active and required as a cofactor for enzymes such as the antioxidant Superoxide Dismutase 1 (SOD1). Copper dyshomeostasis has been implicated in Alzheimer's disease. Mutations in the presenilin genes encoding PS1 and PS2 are major causes of early-onset familial Alzheimer's disease. PS1 and PS2 are required for efficient copper uptake in mammalian systems. Here we demonstrate a conserved role for presenilin in dietary copper uptake in the fly Drosophila melanogaster. Ubiquitous RNA interference-mediated knockdown of the single Drosophila presenilin (PSN) gene is lethal. However, PSN knockdown in the midgut produces viable flies. These flies have reduced copper levels and are more tolerant to excess dietary copper. Expression of a copper-responsive EYFP construct was also lower in the midgut of these larvae, indicative of reduced dietary copper uptake. SOD activity was reduced by midgut PSN knockdown, and these flies were sensitive to the superoxide-inducing chemical paraquat. These data support presenilin being needed for dietary copper uptake in the gut and so impacting on SOD activity and tolerance to oxidative stress. These results are consistent with previous studies of mammalian presenilins, supporting a conserved role for these proteins in mediating copper uptake.

The γ-secretase inhibitor DAPT increases the levels of gangliosides at neuritic terminals of differentiating PC12 cells

Mutations in presenilins are the major cause of early onset familial Alzheimer disease. It has recently been argued that clinical presenilin mutations work as loss-of-function but not toxic gain-of-function. To investigate whether presenilins are involved in the regulation of the distribution of neuronal membrane lipids, we treated neuronally differentiated PC12 cells with DAPT, an inhibitor of presenilin-dependent γ-secretase, and performed lipid analyses of neuritic terminals, which is an initial site of Aβ deposition in brains, using liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) in combination with multiple reaction monitoring (MRM). With DAPT treatment, levels of sphingomyelin, phosphatidylcholine, and cholesterol remained unchanged. However, DAPT treatment increased the ganglioside levels in PC12 neuritic terminals. Together with a previous finding that accumulation of gangliosides at neuritic terminals facilitates Aβ assembly and deposition, the present data suggest that the loss-of-function of presenilins, i.e., a decrease in γ-secretase activity, has an impact on neuronal membrane architecture in a way that eventually exacerbates Alzheimer pathology.

Amyloid and Alzheimer's disease: inside and out

Alzheimer's disease (AD) is poised to become the most serious healthcare issue of our generation. The leading theory of AD pathophysiology is the Amyloid Cascade Hypothesis, and clinical trials are now proceeding based on this hypothesis. Here, we review the original evidence for the Amyloid Hypothesis, which was originally focused on the extracellular deposition of beta amyloid peptides (Aβ) in large fibrillar aggregates, as well as how this theory has been extended in recent years to focus on highly toxic small soluble amyloid oligomers. We will also examine emerging evidence that Aβ may actually begin to accumulate intracellularly in lysosomes, and the role for intracellular Aβ and lysosomal dysfunction may play in AD pathophysiology. Finally, we will review the clinical implications of these findings.

The many substrates of presenilin/γ-secretase

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

Discoveries in Down syndrome: moving basic science to clinical care

This review describes recent discoveries in neurobiology of Down syndrome (DS) achieved with use of mouse genetic models and provides an overview of experimental approaches aimed at development of pharmacological restoration of cognitive function in people with this developmental disorder. Changes in structure and function of synaptic connections within the hippocampal formation of DS model mice, as well as alterations in innervations of the hippocampus by noradrenergic and cholinergic neuromodulatory systems, provided important clues for potential pharmacological treatments of cognitive disabilities in DS. Possible molecular and cellular mechanisms underlying this genetic disorder have been addressed. We discuss novel mechanisms engaging misprocessing of amyloid precursor protein (App) and other proteins, through their affect on axonal transport and endosomal dysfunction, to "Alzheimer-type" neurodegenerative processes that affect cognition later in life. In conclusion, a number of therapeutic strategies have been defined that may restore cognitive function in mouse models of DS. In the juvenile and young animals, these strategists focus on restoration of synaptic plasticity, rate of adult neurogenesis, and functions of the neuromodulatory subcortical systems. Later in life, the major focus is on recuperation of misprocessed App and related proteins. It is hoped that the identification of an increasing number of potential targets for pharmacotherapy of cognitive deficits in DS will add to the momentum for creating and completing clinical trials.

The recycling endosome and its role in neurological disorders

The recycling endosome (RE) is an organelle in the endocytic pathway where plasma membranes (proteins and lipids) internalized by endocytosis are processed back to the cell surface for reuse. Endocytic recycling is the primary way for the cell to maintain constituents of the plasma membrane (Griffiths et al., 1989), i.e., to maintain the abundance of receptors and transporters on cell surfaces. Membrane traffic through the RE is crucial for several key cellular processes including cytokinesis and cell migration. In polarized cells, including neurons, the RE is vital for the generation and maintenance of the polarity of the plasma membrane. Many RE dependent cargo molecules are known to be important for neuronal function and there is evidence that improper function of key proteins in RE-associated pathways may contribute to the pathogenesis of neurological disorders, including Huntington's disease. The function of the RE in neurons is poorly understood. Therefore, there is need to understand how membrane dynamics in RE-associated pathways are affected or participate in the development or progression of neurological diseases. This review summarizes advances in understanding endocytic recycling associated with the RE, challenges in elucidating molecular mechanisms underlying RE function, and evidence for RE dysfunction in neurological disorders.

The Basic Biology of BACE1: A Key Therapeutic Target for Alzheimer's Disease

Alzheimer’s disease (AD) is an intractable, neurodegenerative disease that appears to be brought about by both genetic and non-genetic factors. The neuropathology associated with AD is complex, although amyloid plaques composed of the β-amyloid peptide (Aβ) are hallmark neuropathological lesions of AD brain. Indeed, Aβ plays an early and central role in this disease. β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is the initiating enzyme in Aβ genesis and BACE1 levels are elevated under a variety of conditions. Given the strong correlation between Aβ and AD, and the elevation of BACE1 in this disease, this enzyme is a prime drug target for inhibiting Aβ production in AD. However, nine years on from the initial identification of BACE1, and despite intense research, a number of key questions regarding BACE1 remain unanswered. Indeed, drug discovery and development for AD continues to be challenging. While current AD therapies temporarily slow cognitive decline, treatments that address the underlying pathologic mechanisms of AD are completely lacking. Here we review the basic biology of BACE1. We pay special attention to recent research that has provided some answers to questions such as those involving the identification of novel BACE1 substrates, the potential causes of BACE1 elevation and the putative function of BACE1 in health and disease. Our increasing understanding of BACE1 biology should aid the development of compounds that interfere with BACE1 expression and activity and may lead to the generation of novel therapeutics for AD.

Trafficking of amyloid β-precursor protein products C83 and C99 on the endocytic pathway

Amyloid β-precursor protein (APP) proteolytic products C83 and C99 are substrates for γ-secretase as well as products, respectively, of α- or β-secretase. In contrast to APP, C83 and C99 were derivatized by a water soluble biotinylation reagent to a much greater extent at 18°C than at 0°C in CHO cells expressing the Swedish mutant form of APP750. Intracellular C99 and C83 cycle to the cell surface when maintained in buffered saline at 18°C thus identifying proteins derivatized at 18°C as residing in recycling compartments. More than 80% of C99 and C83 biotinylated at 18°C is associated with detergent resistant membrane (DRM). There thus appears to be no differential distribution of α- or β-secretase products into the DRM fraction that would be expected if localization to DRM determines alternative secretase pathways. γ-Secretase inhibitors increased the fraction of C99 but not C83 in the 18°C pool by >50% and doubled the half-life of C99 in that compartment, showing that a substantial amount of C99 is proteolyzed by γ-secretase in a compartment rich in recycling proteins. The temporal appearance of APP on the cell surface preceded that of C99 in the recycling compartment, further supporting the cleavage of APP in recycling endosomes.

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

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

Secretory granule membrane protein recycles through multivesicular bodies

The recycling of secretory granule membrane proteins that reach the plasma membrane following exocytosis is poorly understood. As a model, peptidylglycine alpha-amidating monooxygenase (PAM), a granule membrane protein that catalyzes a final step in peptide processing was examined. Ultrastructural analysis of antibody internalized by PAM and surface biotinylation showed efficient return of plasma membrane PAM to secretory granules. Electron microscopy revealed the rapid movement of PAM from early endosomes to the limiting membranes of multivesicular bodies and then into intralumenal vesicles. Wheat germ agglutinin and PAM antibody internalized simultaneously were largely segregated when they reached multivesicular bodies. Mutation of basally phosphorylated residues (Thr(946), Ser(949)) in the cytoplasmic domain of PAM to Asp (TS/DD) substantially slowed its entry into intralumenal vesicles. Mutation of the same sites to Ala (TS/AA) facilitated the entry of internalized PAM into intralumenal vesicles and its subsequent return to secretory granules. Entry of PAM into intralumenal vesicles is also associated with a juxtamembrane endoproteolytic cleavage that releases a 100-kDa soluble PAM fragment that can be returned to secretory granules. Controlled entry into the intralumenal vesicles of multivesicular bodies plays a key role in the recycling of secretory granule membrane proteins.

Loss of gamma-secretase function impairs endocytosis of lipoprotein particles and membrane cholesterol homeostasis

Presenilins (PSs) are components of the gamma-secretase complex that mediates intramembranous cleavage of type I membrane proteins. We show that gamma-secretase is involved in the regulation of cellular lipoprotein uptake. Loss of gamma-secretase function decreased endocytosis of low-density lipoprotein (LDL) receptor. The decreased uptake of lipoproteins led to upregulation of cellular cholesterol biosynthesis by increased expression of CYP51 and enhanced metabolism of lanosterol. Genetic deletion of PS1 or transgenic expression of PS1 mutants that cause early-onset Alzheimer's disease led to accumulation of gamma-secretase substrates and mistargeting of adaptor proteins that regulate endocytosis of the LDL receptor. Consistent with decreased endocytosis of these receptors, PS1 mutant mice have elevated levels of apolipoprotein E in the brain. Thus, these data demonstrate a functional link between two major genetic factors that cause early-onset and late-onset Alzheimer's disease.

Neuronal protein trafficking associated with Alzheimer disease: from APP and BACE1 to glutamate receptors

Aberrant and/or cumulative amyloid-beta (Abeta) production, resulting from proteolytic processing of the amyloid precursor protein (APP) by beta and gamma-secretases, have been postulated to be a main etiological basis of Alzheimer disease (AD). A number of proteins influence the subcellular trafficking itinerary of APP and the beta-site APP-cleaving enzyme (BACE1) between the cell surface, endosomes and the trans-Golgi network (TGN). Available evidence suggests that co-residence of APP and BACE1 in the endosomal compartments promotes amyloidogenesis. Retrograde transport of APP out of the endosome to the TGN reduces Abeta production, while APP routed to and kept at the cell surface enhances its non-amyloidogenic, alpha-secretase-mediated processing. Changes in post-Golgi membrane trafficking in aging neurons that may influence APP processing is particularly relevant to late-onset, idiopathic AD. Dystrophic axons are key features of AD pathology, and impaired axonal transport could play crucial roles in the pathogenesis of idiopathic AD. Recent evidence has also indicated that Abeta-induced synaptic defects and memory impairment could be explained by a loss of both AMPA and NMDA receptors through endocytosis. Detail understanding of factors that influence these neuronal trafficking processes will open up novel therapeutic avenues for preventing or delaying the onset of symptomatic AD.

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.

The amyloid-beta rise and gamma-secretase inhibitor potency depend on the level of substrate expression

The amyloid-beta (Abeta) peptide, which likely plays a key role in Alzheimer disease, is derived from the amyloid-beta precursor protein (APP) through consecutive proteolytic cleavages by beta-site APP-cleaving enzyme and gamma-secretase. Unexpectedly gamma-secretase inhibitors can increase the secretion of Abeta peptides under some circumstances. This "Abeta rise" phenomenon, the same inhibitor causing an increase in Abeta at low concentrations but inhibition at higher concentrations, has been widely observed. Here we show that the Abeta rise depends on the beta-secretase-derived C-terminal fragment of APP (betaCTF) or C99 levels with low levels causing rises. In contrast, the N-terminally truncated form of Abeta, known as "p3," formed by alpha-secretase cleavage, did not exhibit a rise. In addition to the Abeta rise, low betaCTF or C99 expression decreased gamma-secretase inhibitor potency. This "potency shift" may be explained by the relatively high enzyme to substrate ratio under conditions of low substrate because increased concentrations of inhibitor would be necessary to affect substrate turnover. Consistent with this hypothesis, gamma-secretase inhibitor radioligand occupancy studies showed that a high level of occupancy was correlated with inhibition of Abeta under conditions of low substrate expression. The Abeta rise was also observed in rat brain after dosing with the gamma-secretase inhibitor BMS-299897. The Abeta rise and potency shift are therefore relevant factors in the development of gamma-secretase inhibitors and can be evaluated using appropriate choices of animal and cell culture models. Hypothetical mechanisms for the Abeta rise, including the "incomplete processing" and endocytic models, are discussed.

Gamma-secretase and metalloproteinase activity regulate the distribution of endoplasmic reticulum to hippocampal neuron dendritic spines

The neuronal endoplasmic reticulum (ER) contributes to many physiological and pathological processes in the brain. A subset of dendritic spines on hippocampal neurons contains ER that may contribute to synapse-specific intracellular signaling. Distribution of ER to spines is dynamic, but knowledge of the regulatory mechanisms is lacking. In live cell imaging experiments we now show that cultured hippocampal neurons rapidly lost ER from spines after phorbol ester treatment. ER loss was reduced by inhibiting gamma-secretase (DAPT at 2 microM) and metalloproteinase (TAPI-0 and GM6001 at 4 microM) activity. Inhibition of protein kinase C also diminished loss of ER by preventing exit of ER from spines. Furthermore, gamma-secretase and metalloproteinase inhibition, in the absence of phorbol ester, triggered a dramatic increase in spine ER content. Metalloproteinases and gamma-secretase cleave several transmembrane proteins. Many of these substrates are known to localize to adherens junctions, a structural specialization with which spine ER interacts. One interesting possibility is thus that ER content within spines may be regulated by proteolytic activity affecting adherens junctions. Our data demonstrate a hitherto unknown role for these two proteolytic activities in regulating dynamic aspects of cellular ultrastructure, which is potentially important for cellular calcium homeostasis and several intracellular signaling pathways.

Metalloproteases and gamma-secretase: new membrane partners regulating p75 neurotrophin receptor signaling?

Signaling by the p75 neurotrophin receptor (p75) has been implicated in diverse neuronal responses, including the control of neuronal survival versus death and axonal regeneration and growth cone collapse, involving p75 in different neuropathological conditions. There are different levels of complexity regulating p75-mediated signaling. First, p75 can interact with different ligands and co-receptors in the plasma membrane, forming tripartite complexes, whose activation result in different cellular outcomes. Moreover, it was recently described that trafficking capacities of p75 in neurons are regulating, in addition to p75 downstream interactions, also the sequential cleavage of p75. The proteolytical processing of p75 involves, first, a shedding event that releases a membrane-bound carboxiterminal fragment (p75-CTF), followed by a gamma-secretase mediated cleavage, generating a soluble intracellular domain (p75-ICD) with signaling capabilities. The first shedding event, generating a p75-CTF, is the key step to regulating the production of p75-ICD, and although the generation of p75-ICD is important for both p75-mediated control of neuronal survival and the control of neurite outgrowth, little is known how both cleavage events are regulated. In this review, we argue that both sheddases and gamma-secretase are key membrane components regulating p75-mediated signaling transduction; therefore, further attention should be paid to their roles as p75 signaling regulators.

Inhibition of gamma-secretase causes increased secretion of amyloid precursor protein C-terminal fragments in association with exosomes

Alzheimer's disease (AD) is the most common form of dementia and is associated with the deposition of the 39- to 43-amino acid beta-amyloid peptide (Abeta) in the brain. C-terminal fragments (CTFs) of amyloid precursor protein (APP) can accumulate in endosomally derived multivesicular bodies (MVBs). These intracellular structures contain intraluminal vesicles that are released from the cell as exosomes when the MVB fuses with the plasma membrane. Here we have investigated the role of exosomes in the processing of APP and show that these vesicles contain APP-CTFs, as well as Abeta. In addition, inhibition of gamma-secretase results in a significant increase in the amount of alpha- and beta-secretase cleavage, further increasing the amount of APP-CTFs contained within these exosomes. We identify several key members of the secretase family of proteases (BACE, PS1, PS2, and ADAM10) to be localized in exosomes, suggesting they may be a previously unidentified site of APP cleavage. These results provide further evidence for a novel pathway in which APP fragments are released from cells and have implications for the analysis of APP processing and diagnostics for Alzheimer's disease.

Alzheimer's presenilin 1 modulates sorting of APP and its carboxyl-terminal fragments in cerebral neurons in vivo

Studies in continuously cultured cells have established that familial Alzheimer's disease (FAD) mutant presenilin 1 (PS1) delays exit of the amyloid precursor protein (APP) from the trans-Golgi network (TGN). Here we report the first description of PS1-regulated APP trafficking in cerebral neurons in culture and in vivo. Using neurons from transgenic mice or a cell-free APP transport vesicle biogenesis system derived from the TGN of those neurons, we demonstrated that knocking-in an FAD-associated mutant PS1 transgene was associated with delayed kinetics of APP arrival at the cell surface. Apparently, this delay was at least partially attributable to impaired exit of APP from the TGN, which was documented in the cell-free APP transport vesicle biogenesis assay. To extend the study to APP and carboxyl terminal fragment (CTF) trafficking to cerebral neurons in vivo, we performed subcellular fractionation of brains from APP transgenic mice, some of which carried a second transgene encoding an FAD-associated mutant form of PS1. The presence of the FAD mutant PS1 was associated with a slight shift in the subcellular localization of both holoAPP and APP CTFs toward iodixanol density gradient fractions that were enriched in a marker for the TGN. In a parallel set of experiments, we used an APP : furin chimeric protein strategy to test the effect of artificially forcing TGN concentration of an APP : furin chimera that could be a substrate for beta- and gamma-cleavage. This chimeric substrate generated excess Abeta42 when compared with wildtype APP. These data indicate that the presence of an FAD-associated mutant human PS1 transgene is associated with redistribution of the APP and APP CTFs in brain neurons toward TGN-enriched fractions. The chimera experiment suggests that TGN-enrichment of a beta-/gamma-secretase substrate may play an integral role in the action of mutant PS1 to elevate brain levels of Abeta42.

Endosomal transport of neurotrophins: roles in signaling and neurodegenerative diseases

The internalization and retrograde axonal transport of neurotrophin receptors is important for their retrograde signal transduction supporting neuronal differentiation, plasticity, and survival. To influence transcription, neurotrophin signals initiated at synapses have to be conveyed retrogradely to the cell body. Signaling endosomes containing neurotrophin receptor signaling complexes mediate retrograde neurotrophin signaling from synapses to the nucleus. Interestingly, many neurodegenerative diseases, including Alzheimer's disease, Niemann Pick disease Type C, and Charcot-Marie-Tooth neuropathies, show alterations of vesicular transport, suggesting that traffic jams within neuronal processes may cause neurodegeneration. Although most of these diseases are complex and may be modulated by diverse pathways contributing to neuronal death, altered neurotrophin transport is emerging as a strong candidate influence on neurodegeneration. In this article, we review the mechanisms of internalization and endocytic trafficking of neurotrophin receptors, and discuss the potential roles of perturbations in neurotrophin trafficking in a number of neurodegenerative diseases.

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

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

Presenilin diversifies its portfolio

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

Abeta40 inhibits amyloid deposition in vivo

Numerous studies have established a pivotal role for Abeta42 in Alzheimer's disease (AD) pathogenesis. In contrast, although Abeta40 is the predominant form of amyloid beta (Abeta) produced and accumulates to a variable degree in the human AD brain, its role in AD pathogenesis has not been established. It has generally been assumed that an increase in Abeta40 would accelerate amyloid plaque formation in vivo. We have crossed BRI-Abeta40 mice that selectively express high levels of Abeta40 with both Tg2576 (APPswe, K670N+M671L) mice and BRI-Abeta42A mice expressing Abeta42 selectively and analyzed parenchymal and cerebrovascular Abeta deposition in the bitransgenic mice compared with their singly transgenic littermates. In the bitransgenic mice, the increased steady-state levels of Abeta40 decreased Abeta deposition by 60-90%. These results demonstrate that Abeta42 and Abeta40 have opposing effects on amyloid deposition: Abeta42 promotes amyloid deposition but Abeta40 inhibits it. In addition, increasing Abeta40 levels protected BRI-Abeta40/Tg2576 mice from the premature-death phenotype observed in Tg2576 mice. The protective properties of Abeta40 with respect to amyloid deposition suggest that strategies that preferentially target Abeta40 may actually worsen the disease course and that selective increases in Abeta40 levels may actually reduce the risk for development of AD.

TrkA receptor activation by nerve growth factor induces shedding of the p75 neurotrophin receptor followed by endosomal gamma-secretase-mediated release of the p75 intracellular domain

Neurotrophins are trophic factors that regulate important neuronal functions. They bind two unrelated receptors, the Trk family of receptor-tyrosine kinases and the p75 neurotrophin receptor (p75). p75 was recently identified as a new substrate for gamma-secretase-mediated intramembrane proteolysis, generating a p75-derived intracellular domain (p75-ICD) with signaling capabilities. Using PC12 cells as a model, we studied how neurotrophins activate p75 processing and where these events occur in the cell. We demonstrate that activation of the TrkA receptor upon binding of nerve growth factor (NGF) regulates the metalloprotease-mediated shedding of p75 leaving a membrane-bound p75 C-terminal fragment (p75-CTF). Using subcellular fractionation to isolate a highly purified endosomal fraction, we demonstrate that p75-CTF ends up in endosomes where gamma-secretase-mediated p75-CTF cleavage occurs, resulting in the release of a p75-ICD. Moreover, we show similar structural requirements for gamma-secretase processing of p75 and amyloid precursor protein-derived CTFs. Thus, NGF-induced endocytosis regulates both signaling and proteolytic processing of p75.