Interaction with telencephalin and the amyloid precursor protein predicts a ring structure for presenilins

The carboxyl terminus of presenilin 1 and 2 (PS1 and PS2) binds to the neuron-specific cell adhesion molecule telencephalin (TLN) in the brain. PS1 deficiency results in the abnormal accumulation of TLN in a yet unidentified intracellular compartment. The first transmembrane domain and carboxyl terminus of PS1 form a binding pocket with the transmembrane domain of TLN. Remarkably, APP binds to the same regions via part of its transmembrane domain encompassing the critical residues mutated in familial Alzheimer's disease. Our data surprisingly indicate a spatial dissociation between the binding site and the proposed catalytic site near the critical aspartates in PSs. They provide important experimental evidence to support a ring structure model for PS.

Elevation of beta-amyloid peptide 2-42 in sporadic and familial Alzheimer's disease and its generation in PS1 knockout cells

Urea-based beta-amyloid (Abeta) SDS-polyacrylamide gel electrophoresis and immunoblots were used to analyze the generation of Abeta peptides in conditioned medium from primary mouse neurons and a neuroglioma cell line, as well as in human cerebrospinal fluid. A comparable and highly conserved pattern of Abeta peptides, namely, 1-40/42 and carboxyl-terminal-truncated 1-37, 1-38, and 1-39, was found. Besides Abeta1-42, we also observed a consistent elevation of amino-terminal-truncated Abeta2-42 in a detergent-soluble pool in brains of subjects with Alzheimer's disease. Abeta2-42 was also specifically elevated in cerebrospinal fluid samples of Alzheimer's disease patients. To decipher the contribution of potential different gamma-secretases (presenilins (PSs)) in generating the amino-terminal- and carboxyl-terminal-truncated Abeta peptides, we overexpressed beta-amyloid precursor protein (APP)-trafficking mutants in PS1+/+ and PS1-/- neurons. As compared with APP-WT (primary neurons from control or PS1-deficient mice infected with Semliki Forest virus), PS1-/- neurons and PS1+/+ neurons overexpressing APP-Deltact (a slow-internalizing mutant) show a decrease of all secreted Abeta peptide species, as expected, because this mutant is processed mainly by alpha-secretase. This drop is even more pronounced for the APP-KK construct (APP mutant carrying an endoplasmic reticulum retention motif). Surprisingly, Abeta2-42 is significantly less affected in PS1-/- neurons and in neurons transfected with the endocytosis-deficient APP-Deltact construct. Our data confirm that PS1 is closely involved in the production of Abeta1-40/42 and the carboxyl-terminal-truncated Abeta1-37, Abeta1-38, and Abeta1-39, but the amino-terminal-truncated and carboxyl-terminal-elongated Abeta2-42 seems to be less affected by PS1 deficiency. Moreover, our results indicate that the latter Abeta peptide species could be generated by a beta(Asp/Ala)-secretase activity.

The amyloid precursor protein (APP)-cytoplasmic fragment generated by gamma-secretase is rapidly degraded but distributes partially in a nuclear fraction of neurones in culture

The gamma-secretase cleavage is the last step in the generation of the beta-amyloid peptide (Abeta) from the amyloid precursor protein (APP). The Abeta precipitates in the amyloid plaques in the brain of Alzheimer's disease patients. The fate of the intracellular APP carboxy-terminal stub generated together with Abeta has been, in contrast, only poorly documented. The analogies between the processing of APP and other transmembrane proteins like SREBP and Notch suggests that this intracellular fragment could have important signalling functions. We demonstrate here that APP-C59 is rapidly degraded (half-life approximately 5 min) when overexpressed in baby hamster kidney cells or primary cultures of neurones by a mechanism that is not inhibited by endosomal/lysosomal or proteasome inhibitors. Furthermore, APP-C59 binds to the DNA binding protein Fe65, although this does not increase the half-life of APP-C59. Finally, we demonstrate that a fraction of APP-C59 becomes redistributed to the nuclear detergent-insoluble pellet, in which the transcription factor SP1 is also present. Overall our results reinforce the analogy between Notch and APP processing, and suggest that the APP intracellular domain, like the Notch intracellular domain, could have a role in signalling events from the plasma membrane to the nucleus.

The discrepancy between presenilin subcellular localization and gamma-secretase processing of amyloid precursor protein

We investigated the relationship between PS1 and gamma-secretase processing of amyloid precursor protein (APP) in primary cultures of neurons. Increasing the amount of APP at the cell surface or towards endosomes did not significantly affect PS1-dependent gamma-secretase cleavage, although little PS1 is present in those subcellular compartments. In contrast, almost no gamma-secretase processing was observed when holo-APP or APP-C99, a direct substrate for gamma-secretase, were specifically retained in the endoplasmic reticulum (ER) by a double lysine retention motif. Nevertheless, APP-C99-dilysine (KK) colocalized with PS1 in the ER. In contrast, APP-C99 did not colocalize with PS1, but was efficiently processed by PS1-dependent gamma-secretase. APP-C99 resides in a compartment that is negative for ER, intermediate compartment, and Golgi marker proteins. We conclude that gamma-secretase cleavage of APP-C99 occurs in a specialized subcellular compartment where little or no PS1 is detected. This suggests that at least one other factor than PS1, located downstream of the ER, is required for the gamma-cleavage of APP-C99. In agreement, we found that intracellular gamma-secretase processing of APP-C99-KK both at the gamma40 and the gamma42 site could be restored partially after brefeldin A treatment. Our data confirm the "spatial paradox" and raise several questions regarding the PS1 is gamma-secretase hypothesis.

Presenilin function in APP processing

Familial Alzheimer's disease (FAD) is now linked to at least three genes encoding the amyloid precursor protein (APP) on chromosome 21, and presenilin 1 and 2 on chromosome 14 and 1, respectively. FAD cases in whom presenilin mutations occur are more frequent than those with APP mutations. However, altogether they only account for approximately 0.1% of all the people suffering from Alzheimer's disease (AD), and the causes of the remaining 99.9% of the sporadic form of AD or senile dementia remain unknown. Since FAD presents with the same neuropathological features as sporadic AD, i.e., cognitive impairments and the amyloid plaques and tangles in the brain, our working hypothesis is that similar molecular pathogenic mechanisms underly both sporadic and familial AD. It follows that APP and the presenilins must be key players in the disease. Detailed knowledge about the cell biology of these proteins will be a rich source of insight into the pathology of AD, but will also shed light on the fundamental neurobiology of these proteins.

Presenilin 2 deficiency causes a mild pulmonary phenotype and no changes in amyloid precursor protein processing but enhances the embryonic lethal phenotype of presenilin 1 deficiency

Mutations in the homologous presenilin 1 (PS1) and presenilin 2 (PS2) genes cause the most common and aggressive form of familial Alzheimer's disease. Although PS1 function and dysfunction have been extensively studied, little is known about the function of PS2 in vivo. To delineate the relationships of PS2 and PS1 activities and whether PS2 mutations involve gain or loss of function, we generated PS2 homozygous deficient (-/-) and PS1/PS2 double homozygous deficient mice. In contrast to PS1(-/-) mice, PS2(-/-) mice are viable and fertile and develop only mild pulmonary fibrosis and hemorrhage with age. Absence of PS2 does not detectably alter processing of amyloid precursor protein and has little or no effect on physiologically important apoptotic processes, indicating that Alzheimer's disease-causing mutations in PS2, as in PS1, result in gain of function. Although PS1(+/-) PS2( -/-) mice survive in relatively good health, complete deletion of both PS2 and PS1 genes causes a phenotype closely resembling full Notch-1 deficiency. These results demonstrate in vivo that PS1 and PS2 have partially overlapping functions and that PS1 is essential and PS2 is redundant for normal Notch signaling during mammalian embryological development.

A presenilin-1-dependent gamma-secretase-like protease mediates release of Notch intracellular domain

Signalling through the receptor protein Notch, which is involved in crucial cell-fate decisions during development, requires ligand-induced cleavage of Notch. This cleavage occurs within the predicted transmembrane domain, releasing the Notch intracellular domain (NICD), and is reminiscent of gamma-secretase-mediated cleavage of beta-amyloid precursor protein (APP), a critical event in the pathogenesis of Alzheimer's disease. A deficiency in presenilin-1 (PS1) inhibits processing of APP by gamma-secretase in mammalian cells, and genetic interactions between Notch and PS1 homologues in Caenorhabditis elegans indicate that the presenilins may modulate the Notch signalling pathway. Here we report that, in mammalian cells, PS1 deficiency also reduces the proteolytic release of NICD from a truncated Notch construct, thus identifying the specific biochemical step of the Notch signalling pathway that is affected by PS1. Moreover, several gamma-secretase inhibitors block this same step in Notch processing, indicating that related protease activities are responsible for cleavage within the predicted transmembrane domains of Notch and APP. Thus the targeting of gamma-secretase for the treatment of Alzheimer's disease may risk toxicity caused by reduced Notch signalling.

Regulation of macropinocytosis in v-Src-transformed fibroblasts: cyclic AMP selectively promotes regurgitation of macropinosomes

Stable transformation of Rat-1 fibroblasts by the v-Src oncoprotein results into the constitutive formation of macropinosomes. In the present report, we found that macropinosomes do not fuse with transferrin-containing endosomes and investigated the effects of cyclic AMP as a regulator of macropinocytosis in this cell system. The permeant analogs dibutyryl cyclic AMP and 8-bromo-cyclic AMP, as well as the pharmacological activator of adenylate cyclase forskolin, similarly decreased by about 35% the net endocytic accumulation of the fluid-phase tracer horseradish peroxidase at intervals >5 minutes in v-Src-transformed cells but not in the non-transformed parental Rat-1 cell line. However, and in contrast to the phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N, N-diphenylcarbamate or the phosphatidylinositol 3-kinase inhibitor wortmannin, dibutyryl cyclic AMP neither returned the peroxidase accumulation rate of v-Src-transformed cells to that of parental Rat-1/control cells, nor prevented macropinosome formation, as shown by confocal microscopy. Detailed analysis of the kinetics of tracer entry and efflux in transformed cells revealed that dibutyryl cyclic AMP inhibited peroxidase accumulation only after intervals >5 minutes, due to accelerated peroxidase regurgitation, but did not alter the rate of transferrin recycling. Taken together, these data indicate that, in v-Src-transformed fibroblasts, macropinocytosis and micropinocytosis serve different pathways and that cyclic AMP affects neither micropinocytosis nor the formation of macropinosomes, but selectively promotes regurgitation therefrom.

Dileucine-based sorting signals bind to the beta chain of AP-1 at a site distinct and regulated differently from the tyrosine-based motif-binding site

In previous work, we showed that peptides from endocytosed proteins containing the tyrosine YXXphi sorting motif are recognized by the mu 2 subunit of AP-2, the plasma membrane clathrin adaptor protein complex. This interaction is activated by phosphoinositide lipids that are phosphorylated at the D-3 position of the inositol ring, and is also enhanced by the formation of clathrin-AP-2 coats. Here, we describe the detection of a specific interaction between peptides containing a second sorting motif, the dileucine motif, and AP-1, the clathrin adaptor complex responsible for sorting proteins at the trans-Golgi network (TGN). Surprisingly, the site of dileucine binding is the beta1 subunit, not mu 1. A YXXphi-containing peptide from a protein trafficked within the TGN does bind to mu 1, however. Phosphatidylinositol 3,4-diphosphate and 3,4, 5-triphosphate did not activate the interaction between dileucine-containing peptides and AP-1 but instead inhibited it, and clathrin-AP-1 coat formation did not alter the interaction. Thus, there are at least two physically separate binding sites for sorting signals on APs, which are also regulated independently.

Assembly of clathrin coats disrupts the association between Eps15 and AP-2 adaptors

Eps15 is a phosphorylation substrate of the epidermal growth factor receptor kinase. In vivo, it is largely found in complex with AP-2, the plasma membrane clathrin adaptor protein complex. Although AP-2 is uniformly distributed across the surface of clathrin-coated pits and vesicles, Eps15 is preferentially found in the rims of endocytic clathrin-coated pits (1). This observation suggests that Eps15 may disengage from AP-2 during coat formation. Here we use two new anti-Eps15 antibodies to show that, contrary to our own earlier suggestion, coated vesicles isolated from brain do not contain detectable amounts of Eps15. Furthermore, when AP-2 complexes that are saturated with Eps15 are used for in vitro assembly of clathrin-AP-2 coats, normal structures are formed that contain the expected amounts of clathrin and AP-2, but the amount of Eps15 present is dramatically lower than that of AP-2. We propose that during coated pit formation, addition of clathrin to the growing edge at the rim of the pit releases Eps15 from AP-2.

Parallel dimers and anti-parallel tetramers formed by epidermal growth factor receptor pathway substrate clone 15

The recently discovered localization of epidermal growth factor receptor pathway substrate clone 15 (Eps15) to plasma membrane clathrin-coated pits and its constitutive association with the endocytic clathrin adaptor protein complex, AP-2, strongly suggest that Eps15 has an important role in the pathway of clathrin-dependent endocytic traffic. We report here that Eps15 forms dimers and tetramers of distinct shape. The Eps15 dimer is an elongated molecule, 32 nm in length. There is a globular "head" at one end of the molecule and an extended "stalk" of 25 nm which is kinked at about 17 nm away from the head. In the Eps15 dimer, two subunits are arranged parallel to each other, so that the head corresponds to two side by side copies of the N-terminal region I, which contains the three Eps15 homology domains. The proximal part of the stalk is the coiled-coil central region II containing 20 heptad repeats. The kink is at the boundary between region II and the C-terminal region III, which contains the AP-2 binding site, 15 aspartic-proline-phenylalanine repeats, and proline-rich Src homology domain ligand sites. The Eps15 tetramer has a "dumbbell" shape, approximately 31 nm in length; it is formed by the anti-parallel association of two Eps15 dimers. Formation of these Eps15 tetramers appears to require contacts between regions I of one dimer and regions III of a second apposing dimer. The extended shapes of the Eps15 dimers and tetramers suggest how Eps15 oligomers are located in the clathrin coat. We discuss the implications for accessibility to partners and for proposed functions of Eps15.

Three unrelated perturbations similarly uncouple fluid, bulk-membrane, and receptor endosomal flow in rat fetal fibroblasts

We have compared the effects of three perturbations (treatment with 2 microM monensin, potassium depletion, and incubation in 0.35 M NaCl) on recycling of internalized fluid-phase, bulk-membrane, and receptor-mediated tracers in rat fetal fibroblasts. Monensin accelerated 2-fold the regurgitation of the fluid-phase tracer horseradish peroxidase (HRP), as previously described in these cells after potassium depletion or upon incubation in hypertonic medium (1), and all treatments severely inhibited transfer of HRP from endosomes to lysosomes. In comparison, recycling of (6-[N-(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]hexanoyl glucosylsphingosine (C6-NBD-GlcCer), a fluorescent lipid used as a bulk-membrane probe, was not significantly affected while transferrin recycling was slowed down 2-fold. The striking similarities of these unrelated perturbations in their distinct effects on fluid, bulk-membrane, and receptor addressing point to common targets regulating these mechanisms.

v-Src induces constitutive macropinocytosis in rat fibroblasts

The role of v-Src as regulator of fluid-phase pinocytosis was investigated in Rat-1 cells expressing a stable (Rat-1/BB16) or a thermosensitive (Rat-1/tsLA29) v-Src protein. In the second cell line, this protein is inactive when cells are cultured at 40 degrees C but recovers its tyrosine kinase activity upon transfer to 34 degrees C, resulting into a transformed phenotype. The rate of fluid-phase pinocytosis of the tracer horseradish peroxidase was 2-fold higher in v-Src-transformed fibroblasts (Rat-1/BB16, Rat-1/tsLA29 cultured at 34 degrees C) as compared to non-transformed cells (Rat-1, Rat-1/tsLA29 kept at 40 degrees C). In contrast, receptor-mediated endocytosis of transferrin was poorly affected, suggesting that structures distinct from clathrin-coated pits are involved in pinocytosis stimulation. By light and electron microscopy, transformed cells frequently contained large peroxidase-labeled pinocytic vesicles located near to membrane ruffles, demonstrating that stimulation of pinocytosis corresponds to induction of constitutive macropinocytosis. Stimulation of pinocytosis occurred more than 8 hours after transfer to the permissive temperature, whereas transfer to the non-permissive temperature partially reversed the stimulation within 2 hours. Protein synthesis inhibition for 6 hours abrogated pinocytosis stimulation in transformed cells, indicating that constitutive macropinocytosis induced by v-Src depends on continuous synthesis of a short-lived regulatory machinery.

Clathrin polymerization is not required for bulk-phase endocytosis in rat fetal fibroblasts

To assess the role of clathrin in the bulk endocytic flow of rat foetal fibroblasts, the rate of internalization of fluid-phase and membrane-lipid tracers were compared, under control conditions and after inhibition of endocytic clathrin-coated pit formation. After intracellular potassium depletion or upon cell transfer into 0.35 M NaCl, the rate of internalization of receptor-bound transferrin and the residual membrane area of plasmalemmal clathrin-coated pits and vesicles were similarly decreased by approximately 90%. In contrast, the initial rate (< 5 min) of intracellular accumulation of the fluid-phase tracer HRP was not affected. Both in control and treated cells, the rate of HRP accumulation declined after approximately 5 min, and was twofold lower in treated cells, due to enhanced regurgitation. After correction for regurgitation, the endocytic rate constant was similar to measurements at shorter intervals and identical in control and treated cells. Similarly, the rate of internalization and the steady-state level of intracellular accumulation of two fluorescent lipid derivatives, 6-[N-(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]hexanoylglucosylsp hingosine (C6-NBD-GlcCer) and 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH), were not affected by potassium depletion, indicating that the endocytic membrane traffic was equally preserved. Finally, the size distribution of primary endocytic particles that were accessible to HRP within 15 s before glutaraldehyde fixation was also indistinguishable in control and potassium-depleted cells. The simplest explanation is that clathrin polymerization is necessary to concentrate receptor-bound ligands in primary endocytic vesicles, but superfluous to the basic endocytic machinery in rat foetal fibroblasts.