A complex web of signal-dependent trafficking underlies the triorganellar distribution of P-selectin in neuroendocrine PC12 cells

Differential sorting behavior for soluble and transmembrane cargoes at the trans-Golgi network in endocrine cells

Regulated secretion of neuropeptides and peptide hormones by secretory granules (SGs) is central to physiology. Formation of SGs occurs at the trans-Golgi network (TGN) where their soluble cargo aggregates to form a dense core, but the mechanisms controlling the sorting of regulated secretory cargoes (soluble and transmembrane) away from constitutively secreted proteins remain unclear. Optimizing the use of the retention using selective hooks method in (neuro-)endocrine cells, we now quantify TGN budding kinetics of constitutive and regulated secretory cargoes. We further show that, by monitoring two cargoes simultaneously, it becomes possible to visualize sorting to the constitutive and regulated secretory pathways in real time. Further analysis of the localization of SG cargoes immediately after budding from the TGN revealed that, surprisingly, the bulk of two studied transmembrane SG cargoes (phogrin and VMAT2) does not sort directly onto SGs during budding, but rather exit the TGN into nonregulated vesicles to get incorporated to SGs at a later step. This differential behavior of soluble and transmembrane cargoes suggests a more complex model of SG biogenesis than anticipated.

Two Clathrin Adaptor Protein Complexes Instruct Axon-Dendrite Polarity

The cardinal feature of neuronal polarization is the establishment and maintenance of axons and dendrites. How axonal and dendritic proteins are sorted and targeted to different compartments is poorly understood. Here, we identified distinct dileucine motifs that are necessary and sufficient to target transmembrane proteins to either the axon or the dendrite through direct interactions with the clathrin-associated adaptor protein complexes (APs) in C. elegans. Axonal targeting requires AP-3, while dendritic targeting is mediated by AP-1. The axonal dileucine motif binds to AP-3 with higher efficiency than to AP-1. Both AP-3 and AP-1 are localized to the Golgi but occupy adjacent domains. We propose that AP-3 and AP-1 directly select transmembrane proteins and target them to axon and dendrite, respectively, by sorting them into distinct vesicle pools.

The redistribution of Drosophila vesicular monoamine transporter mutants from synaptic vesicles to large dense-core vesicles impairs amine-dependent behaviors

Monoamine neurotransmitters are stored in both synaptic vesicles (SVs), which are required for release at the synapse, and large dense-core vesicles (LDCVs), which mediate extrasynaptic release. The contributions of each type of vesicular release to specific behaviors are not known. To address this issue, we generated mutations in the C-terminal trafficking domain of the Drosophila vesicular monoamine transporter (DVMAT), which is required for the vesicular storage of monoamines in both SVs and LDCVs. Deletion of the terminal 23 aa (DVMAT-Δ3) reduced the rate of endocytosis and localization of DVMAT to SVs, but supported localization to LDCVs. An alanine substitution mutation in a tyrosine-based motif (DVMAT-Y600A) also reduced sorting to SVs and showed an endocytic deficit specific to aminergic nerve terminals. Redistribution of DVMAT-Y600A from SV to LDCV fractions was also enhanced in aminergic neurons. To determine how these changes might affect behavior, we expressed DVMAT-Δ3 and DVMAT-Y600A in a dVMAT null genetic background that lacks endogenous dVMAT activity. When expressed ubiquitously, DVMAT-Δ3 showed a specific deficit in female fertility, whereas DVMAT-Y600A rescued behavior similarly to DVMAT-wt. In contrast, when expressed more specifically in octopaminergic neurons, both DVMAT-Δ3 and DVMAT-Y600A failed to rescue female fertility, and DVMAT-Y600A showed deficits in larval locomotion. DVMAT-Y600A also showed more severe dominant effects than either DVMAT-wt or DVMAT-Δ3. We propose that these behavioral deficits result from the redistribution of DVMAT from SVs to LDCVs. By extension, our data suggest that the balance of amine release from SVs versus that from LDCVs is critical for the function of some aminergic circuits.

Self-assembly of VPS41 promotes sorting required for biogenesis of the regulated secretory pathway

The regulated release of polypeptides has a central role in physiology, behavior, and development, but the mechanisms responsible for production of the large dense core vesicles (LDCVs) capable of regulated release have remained poorly understood. Recent work has implicated cytosolic adaptor protein AP-3 in the recruitment of LDCV membrane proteins that confer regulated release. However, AP-3 in mammals has been considered to function in the endolysosomal pathway and in the biosynthetic pathway only in yeast. We now find that the mammalian homolog of yeast VPS41, a member of the homotypic fusion and vacuole protein sorting (HOPS) complex that delivers biosynthetic cargo to the endocytic pathway in yeast, promotes LDCV formation through a common mechanism with AP-3, indicating a conserved role for these proteins in the biosynthetic pathway. VPS41 also self-assembles into a lattice, suggesting that it acts as a coat protein for AP-3 in formation of the regulated secretory pathway.

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.

Protein sorting in the synaptic vesicle life cycle

At early stages of differentiation neurons already contain many of the components necessary for synaptic transmission. However, in order to establish fully functional synapses, both the pre- and postsynaptic partners must undergo a process of maturation. At the presynaptic level, synaptic vesicles (SVs) must acquire the highly specialized complement of proteins, which make them competent for efficient neurotransmitter release. Although several of these proteins have been characterized and linked to precise functions in the regulation of the SV life cycle, a systematic and unifying view of the mechanisms underlying selective protein sorting during SV biogenesis remains elusive. Since SV components do not share common sorting motifs, their targeting to SVs likely relies on a complex network of protein-protein and protein-lipid interactions, as well as on post-translational modifications. Pleiomorphic carriers containing SV proteins travel and recycle along the axon in developing neurons. Nevertheless, SV components appear to eventually undertake separate trafficking routes including recycling through the neuronal endomembrane system and the plasmalemma. Importantly, SV biogenesis does not appear to be limited to a precise stage during neuronal differentiation, but it rather continues throughout the entire neuronal lifespan and within synapses. At nerve terminals, remodeling of the SV membrane results from the use of alternative exocytotic pathways and possible passage through as yet poorly characterized vacuolar/endosomal compartments. As a result of both processes, SVs with heterogeneous molecular make-up, and hence displaying variable competence for exocytosis, may be generated and coexist within the same nerve terminal.

P-selectin binds to the D′-D3 domains of von Willebrand factor in Weibel-Palade bodies

It has recently been shown that the ultralarge platelet–recruiting von Willebrand factor (VWF) strings formed immediately at exocytosis from endothelial cells may be anchored to the cell surface by interaction with the integral membrane protein P-selectin. This finding of a new binding partner for VWF immediately prompts the question which domains of VWF bind to P-selectin. We have exploited the fact that VWF expression in HEK293 cells triggers the formation of Weibel-Palade body–like structures that can recruit P-selectin. A suitably modified version of this assay using coexpressed truncations of VWF, together with P-selectin variants in HEK293 cells, allowed us to determine which domains of VWF would recruit P-selectin within a physiologically appropriate intracellular environment. Confirming the results of such a cellular assay by conventional coimmunoprecipitation, we concluded that the lumenal domain of P-selectin interacts with the D′-D3 domains of VWF.

P-Selectin and CD63 Use Different Mechanisms for Delivery to Weibel-Palade Bodies

The biogenesis of endothelial-specific Weibel-Palade bodies (WPB) is poorly understood, despite their key role in both haemostasis and inflammation. Biogenesis of specialized organelles of haemopoietic cells is often adaptor protein complex 3-dependent (AP-3-dependent), and AP-3 has previously been shown to play a role in the trafficking of both WPB membrane proteins, P-selectin and CD63. However, WPB are thought to form at the trans Golgi network (TGN), which is inconsistent with a role for AP-3, which operates in post-Golgi trafficking. We have therefore investigated in detail the mechanisms of delivery of these two membrane proteins to WPB. We find that P-selectin is recruited to forming WPB in the trans-Golgi by AP-3-independent mechanisms that use sorting information within both the cytoplasmic tail and the lumenal domain of the receptor. In contrast, CD63 is recruited to already-budded WPB by an AP-3-dependent route. These different mechanisms of recruitment lead to the presence of distinct immature and mature populations of WPB in human umbilical vein endothelial cells (HUVEC).

Sorting of vesicular monoamine transporter 2 to the regulated secretory pathway confers the somatodendritic exocytosis of monoamines

The release of monoamine neurotransmitters from cell bodies and dendrites has an important role in behavior, but the mechanism (vesicular or non vesicular) has remained unclear. Because the location of vesicular monoamine transporter 2 (VMAT2) defines the secretory vesicles capable of monoamine release, we have studied its trafficking to assess the potential for monoamine release by exocytosis. In neuroendocrine PC12 cells, VMAT2 localizes exclusively to large dense-core vesicles (LDCVs), and we now show that cytoplasmic signals target VMAT2 directly to LDCVs within the biosynthetic pathway. In neurons, VMAT2 localizes to a population of vesicles that we now find undergo regulated exocytosis in dendrites. Although hippocampal neurons do not express typical LDCV proteins, transfected chromogranins A, B, and brain-derived neurotrophic factor (BDNF) colocalize with VMAT2. VMAT2 thus defines a population of secretory vesicles that mediate the activity-dependent somatodendritic release of multiple retrograde signals involved in synaptic function, growth, and plasticity.

Cytoplasmic Transport Signal is Involved in Phogrin Targeting and Localization to Secretory Granules

Phogrin is an integral glycoprotein primarily expressed in neuroendocrine cells. The predominant localization of phogrin is on dense-core secretory granules, and the lumenal domain has been shown to be involved in its efficient sorting to the regulated secretory pathway. Here, we present data showing that a leucine-based sorting signal [EExxxIL] within the cytoplasmic tail contributes its steady-state localization to secretory granules. Deletion mutants in the tail region failed to represent granular distribution in pancreatic beta-cell line, MIN6, and anterior pituitary cell line, AtT-20. A sorting signal mutant with two glutamic acids substituted into alanines (EE/AA) is primarily accumulated in the Golgi area instead of secretory granules, and another mutant (IL/AA) is trapped at the plasma membrane due to a defect in endocytosis. We further demonstrate that the leucine-based sorting signal of phogrin specifically interacts with both adaptor protein (AP)-1 and AP-2 clathrin adaptor complexes in vitro. These observations, along with previous studies, suggest that distinct domains of phogrin mediate proper localization of this transmembrane protein on secretory granules.

An extended tyrosine-targeting motif for endocytosis and recycling of the dense-core vesicle membrane protein phogrin

Integral membrane proteins of neuroendocine dense-core vesicles (DCV) appear to undergo multiple rounds of exocytosis; however, their trafficking and site of incorporation into nascent DCVs is unclear. Previous studies with phogrin (IA-2beta) identified sorting signals in the luminal domain that is cleaved post-translationally; we now describe an independent DCV targeting motif in the cytosolic domain that may function at the level of endocytosis and recycling. Pulse-chase radiolabeling and cell surface biotinylation experiments in the pituitary corticotroph cell line AtT20 showed that the mature 60/65 kDa form that resides in the DCV is generated by limited proteolysis in a post-trans Golgi network compartment with similar kinetics to the formation of the principal cargo, ACTH. Phogrin is exposed on the cell surface in response to stimuli and progressively internalized to a perinuclear compartment that overlaps with recycling endosomes marked by transferrin. Chimeric molecules of phogrin transmembrane and cytosolic sequences with the interleukin-2 receptor alpha chain (Tac) were sorted to DCVs through the action of an extended tyrosine-based motif Y(654)QELCRQRMA located in a 27aa sequence adjacent to the membrane-spanning domain. A 36aa domain terminating in this sequence conferred DCV localization to Tac in the absence of any other cytosolic or luminal phogrin components. The endocytosis and DCV targeting of phogrin Y(654) > A mutants correlated with the impaired binding of the phogrin cytosolic tail to the micro-subunit of the AP2 adaptor complex in vitro.

Subcellular localization of neural-specific NPDC-1 protein

NPDC-1 is a gene specifically expressed in neural cells when they stop to divide and begin to differentiate. Immunocytochemical study analysis of differentiated PC12 cells transfected with NPDC-tag vectors showed that NPDC-1 is transported in vesicles from the Golgi apparatus to the cell membrane and is then likely internalized into endosomes. The protein colocalized, at least partially, with synaptic vesicle proteins: synaptophysin, synaptobrevin 2, and Rab3 GEP (Rab3 GTP/GDP exchange protein). Moreover, subcellular fractionation of rat brain showed that crude synaptic membrane and crude synaptic vesicle fractions were enriched in NPDC-1. Although NPDC-1 bound Rab3 GEP in vitro, it seems unlikely to be involved in Ca2+-dependent exocytosis and, thus, in synaptic vesicle trafficking.

CLN6, which is associated with a lysosomal storage disease, is an endoplasmic reticulum protein

The neuronal ceroid lipofuscinoses (NCLs) are severe inherited neurodegenerative disorders affecting children. In this disease, lysosomes accumulate autofluorescent storage material and there is death of neurons. Five types of NCL are caused by mutations in lysosomal proteins (CTSD, CLN1/PPT1, CLN2/TTPI, CLN3 and CLN5), and one type is caused by mutations in a protein that recycles between the ER and ERGIC (CLN8). The CLN6 gene underlying a variant of late infantile NCL (vLINCL) was recently identified. It encodes a novel 311 amino acid transmembrane protein. Antisera raised against CLN6 peptides detected a protein of 30 kDa by Western blotting of human cells, which was missing in cells from some CLN6 deficient patients. Using immunofluorescence microscopy, CLN6 was shown to reside in the endoplasmic reticulum (ER). CLN6 protein tagged with GFP at the C-terminus and expressed in HEK293 cells was also found within the ER. Investigation of the effect of five CLN6 disease mutations that affect single amino acids showed that the mutant proteins were retained in the ER. These data suggest that CLN6 is an ER resident protein, the activity of which, despite this location, must contribute to lysosomal function.

Re-establishment of VWF-dependent Weibel-Palade bodies in VWD endothelial cells

Type 3 von Willebrand disease (VWD) is a severe hemorrhagic defect in humans. We now identify the homozygous mutation in the Chapel Hill strain of canine type 3 VWD that results in premature termination of von Willebrand factor (VWF) protein synthesis. We cultured endothelium from VWD and normal dogs to study intracellular VWF trafficking and Weibel-Palade body formation. Weibel-Palade bodies could not be identified in the canine VWD aortic endothelial cells (VWD-AECs) by P-selectin, VWFpp, or VWF immunostaining and confocal microscopy. We demonstrate the reestablishment of Weibel-Palade bodies that recruit endogenous P-selectin by expressing wild-type VWF in VWD-AECs. Expression of mutant VWF proteins confirmed that VWF multimerization is not necessary for Weibel-Palade body creation. Although the VWF propeptide is required for the formation of Weibel-Palade bodies, it cannot independently induce the formation of the granule. These VWF-null endothelial cells provide a unique opportunity to examine the biogenesis of Weibel-Palade bodies in endothelium from a canine model of type 3 VWD.

Sorting nexin 17 accelerates internalization yet retards degradation of P-selectin

The transient appearance of P-selectin on the surface of endothelial cells helps recruit leukocytes into sites of inflammation. The tight control of cell surface P-selectin on these cells depends on regulated exocytosis of Weibel-Palade bodies where the protein is stored and on its rapid endocytosis. After endocytosis, P-selectin is either sorted via endosomes and the Golgi apparatus for storage in Weibel-Palade bodies or targeted to lysosomes for degradation. A potential player in this complex endocytic itinerary is SNX17, a member of the sorting nexin family, which has been shown in a yeast two-hybrid assay to bind P-selectin. Here, we show that overexpression of SNX17 in mammalian cells can influence two key steps in the endocytic trafficking of P-selectin. First, it promotes the endocytosis of P-selectin from the plasma membrane. Second, it inhibits the movement of P-selectin into lysosomes, thereby reducing its degradation.

Peritoneal macrophages express both P-selectin and PSGL-1

Macrophages, phagocytic cells involved in an early phase of host defense, are known to express the P-selectin ligand, PSGL-1. Heretofore, P-selectin has only been found on platelets and endothelial cells. Here, we demonstrate that peritoneal macrophages isolated by peritoneal lavage of unchallenged mice express P-selectin on the plasma membrane. The peritoneal macrophages synthesize P-selectin, as indicated by metabolic labeling experiments. P-Selectin is constitutively expressed on the extracellular surface of macrophages but is only partially colocalized with PSGL-1. P-Selectin is rapidly translocated from the macrophage plasma membrane to intracellular vesicles and to lysosomes. Peritoneal macrophages assemble into cell strings under flow conditions based upon macrophage–macrophage interactions mediated by P-selectin and PSGL-1. This is the first description of a leukocyte shown to express both P-selectin and PSGL-1.

The zinc transporter ZnT3 interacts with AP-3 and it is preferentially targeted to a distinct synaptic vesicle subpopulation

Synaptic vesicles (SV) are generated by two different mechanisms, one AP-2 dependent and one AP-3 dependent. It has been uncertain, however, whether these mechanisms generate SV that differ in molecular composition. We explored this hypothesis by analyzing the targeting of ZnT3 and synaptophysin both to PC12 synaptic-like microvesicles (SLMV) as well as SV isolated from wild-type and AP-3-deficient mocha brains. ZnT3 cytosolic tail interacted selectively with AP-3 in cell-free assays. Accordingly, pharmacological disruption of either AP-2- or AP-3-dependent SLMV biogenesis preferentially reduced synaptophysin or ZnT3 targeting, respectively; suggesting that these antigens were concentrated in different vesicles. As predicted, immuno-isolated SLMV revealed that ZnT3 and synaptophysin were enriched in different vesicle populations. Likewise, morphological and biochemical analyses in hippocampal neurons indicated that these two antigens were also present in distinct but overlapping domains. ZnT3 SV content was reduced in AP-3-deficient neurons, but synaptophysin was not altered in the AP-3 null background. Our evidence indicates that neuroendocrine cells assemble molecularly heterogeneous SV and suggests that this diversity could contribute to the functional variety of synapses.

Cellugyrin and synaptogyrin facilitate targeting of synaptophysin to a ubiquitous synaptic vesicle-sized compartment in PC12 cells

Cellugyrin represents a ubiquitously expressed four-transmembrane domain protein that is closely related to synaptic vesicle protein synaptogyrin and, more remotely, to synaptophysin. We report here that, in PC12 cells, cellugyrin is localized in synaptic-like microvesicles (SLMVs), along with synaptogyrin and synaptophysin. Upon overexpression of synaptophysin in PC12 cells, it is localized in rapidly sedimenting membranes and practically is not delivered to the SLMVs. On the contrary, the efficiency of the SLMV targeting of exogenously expressed cellugyrin and synaptogyrin is high. Moreover, expression of cellugyrin (or synaptogyrin) in PC12 cells dramatically and specifically increases SLMV targeting of endogenous synaptophysin. Finally, we utilized the SLMV purification scheme on a series of non-neuroendocrine cell types including the mouse fibroblast cell line 3T3-L1, the Chinese hamster ovary cell line CHO-K1, and the monkey kidney epithelial cell line COS7 and found that a cellugyrin-positive microvesicular compartment was present in all cell types tested. We suggest that synaptic vesicles have evolved from cellugyrin-positive ubiquitous microvesicles and that neuroendocrine SLMVs represent a step along that pathway of evolution.

Ligand-induced internalization of the p75 neurotrophin receptor: a slow route to the signaling endosome

The nerve growth factor (NGF) family of neurotrophins binds two classes of cell-surface receptors, trk receptor tyrosine kinases and the shared p75 receptor. Rapid internalization and retrograde trafficking of neurotrophin-trk complexes have been demonstrated in a number of systems and are thought to transmit trophic signals from terminals to neuronal cell bodies. In contrast, the internalization and trafficking of neurotrophin-p75 complexes are not well understood. In this study, we used biotinylated NGF and a fluorescent-labeled anti-p75 antibody to follow the kinetics and route of ligand-induced internalization of the p75 receptor in cycling and differentiated PC12 cells. Binding of neurotrophins to p75 induced internalization at a rate approximately three times slower than that of transferrin and NGF-TrkA complexes in the same cells. The ligand-p75 complex was internalized via clathrin-coated pits into early endosomes and eventually accumulated in recycling endosomes in the cell body and vesicles colabeled by the cholera toxin B-subunit in the growth cones. Both internalized ligand and p75 were protected from proteolytic degradation and accumulated in vesicles that did not undergo acidification. Finally, NGF induced endosomal association of p75 and its MAGE interactors, necdin and NRAGE. These data suggest that signaling endosomes containing activated p75 are involved in neurotrophin signaling, and that such endosomes may be temporally and spatially distinct from those containing trk receptors.

Ligand-induced internalization of the p75 neurotrophin receptor: a slow route to the signaling endosome

The nerve growth factor (NGF) family of neurotrophins binds two classes of cell-surface receptors, trk receptor tyrosine kinases and the shared p75 receptor. Rapid internalization and retrograde trafficking of neurotrophin-trk complexes have been demonstrated in a number of systems and are thought to transmit trophic signals from terminals to neuronal cell bodies. In contrast, the internalization and trafficking of neurotrophin-p75 complexes are not well understood. In this study, we used biotinylated NGF and a fluorescent-labeled anti-p75 antibody to follow the kinetics and route of ligand-induced internalization of the p75 receptor in cycling and differentiated PC12 cells. Binding of neurotrophins to p75 induced internalization at a rate approximately three times slower than that of transferrin and NGF-TrkA complexes in the same cells. The ligand-p75 complex was internalized via clathrin-coated pits into early endosomes and eventually accumulated in recycling endosomes in the cell body and vesicles colabeled by the cholera toxin B-subunit in the growth cones. Both internalized ligand and p75 were protected from proteolytic degradation and accumulated in vesicles that did not undergo acidification. Finally, NGF induced endosomal association of p75 and its MAGE interactors, necdin and NRAGE. These data suggest that signaling endosomes containing activated p75 are involved in neurotrophin signaling, and that such endosomes may be temporally and spatially distinct from those containing trk receptors.

The P-selectin cytoplasmic domain directs the cellular storage of a recombinant chimeric factor IX

Hemophilia B was recognized as a good candidate for gene therapy. Several strategies have been attempted and gave promising results in hemophilic animals but failed to achieve corrective levels in humans. To overcome this inconvenience we aimed to generate intracellular pools of factor (F)IX in cells that are implicated in the hemostatic response, e.g. endothelial cells and platelets. Upon stimulation, these cells release their granule content, which in this case would result in an increase in local FIX concentration, and could locally produce an effective hemostasis. In an attempt to produce an intracellular pool of releasable coagulation FIX, the cytoplasmic domain of the P-selectin (pselCT) molecule was fused to the carboxy-terminal extremity of the human FIX protein. The properties of this chimeric molecule (FIX-pselCT) were studied in AtT20, a cell line which possesses storage granules. As previously shown for transmembrane molecules but not for a soluble protein such as FIX, the pselCT fragment induces the storage of FIX-pselCT. The coagulant activity of FIX-pselCT was not affected by the addition of the pselCT tail. The treatment of AtT20 cells with different inhibitors revealed that FIX-pselCT was not submitted to intracellular degradation and that the half-life of the chimeric molecule was at least two times longer than that of FIX-WT. An immunoelectron microscopic analysis demonstrated a specific localization of FIX-pselCT within the ACTH-containing granules. Cell stimulation using Phorbol Myristrate Acetate (PMA), ionophore A-23187 or 8-Br-cAMP induced efficient release of an active FIX-pselCT. These data demonstrate that the addition of the cytoplasmic domain of P-selectin to FIX modifies the cellular fate of the FIX molecule by directing the recombinant protein toward regulated-secretory granules without altering its coagulant activity.

The lumenal domain of the integral membrane protein phogrin mediates targeting to secretory granules

Phogrin, a transmembrane glycoprotein of neuroendocrine cells, is localized to dense-core secretory granules. We have investigated the subcellular targeting of phogrin by analyzing the sorting of a series of deletion mutants to the regulated pathway of secretion in AtT20 cells. The lumenal domain as a soluble protein was efficiently routed to granules, based on a combination of morphological analysis and secretion studies. Sorting was not dependent on a candidate targeting signal consisting of an N-terminal conserved cysteine-rich motif. Both the pro-region and the lumenal domain of mature, post-translationally processed phogrin independently reached the granule, although the pro-region was sorted more efficiently. Once within the regulated secretory pathway, all phogrin lumenal domain proteins were stored in functional granules for extended periods of time. Thus, phogrin possesses several domains contributing to its targeting to the secretory granule. Our findings support a model of granule biogenesis where proteins are sorted on the basis of their biochemical properties rather than via signal-dependent binding to a targeting receptor. Sorting of integral membrane proteins mediated by the lumenal domain may ensure that functionally important transmembrane molecules are included in the forming granule.

Biogenesis of Weibel–Palade bodies

Weibel-Palade bodies (WPBs) are the lysosome-related secretory organelles of endothelial cells. Their content protein von Willebrand factor, plays a key role in haemostasis, whilst P-selectin in the membranes is critical in the initiation of inflammation. Biogenesis of these rod-shaped structures is driven by von Willebrand factor, since its heterologous expression leads to formation of organelles morphologically indistinguishable from bona fide WPBs. The two main membrane proteins of WPBs, CD63 and P-selectin, have complex itineraries controlled largely by cytoplasmic targeting signals. We are only just beginning to understand the way in which these three proteins come together to form mature WPBs.

Selective and signal-dependent recruitment of membrane proteins to secretory granules formed by heterologously expressed von Willebrand factor

von Willebrand factor (vWF) is a large, multimeric protein secreted by endothelial cells and involved in hemostasis. When expressed in AtT-20 cells, vWF leads to the de novo formation of cigar-shaped organelles similar in appearance to the Weibel-Palade bodies of endothelial cells in which vWF is normally stored before regulated secretion. The membranes of this vWF-induced organelle, termed the pseudogranule, are uncharacterized. We have examined the ability of these pseudogranules, which we show are secretagogue responsive, to recruit membrane proteins. Coexpression experiments show that the Weibel-Palade body proteins P-selectin and CD63, as well as the secretory organelle membrane proteins vesicle-associated membrane protein-2 and synaptotagmin I are diverted away from the endogenous adrenocorticotropic hormone-containing secretory granules to the vWF-containing pseudogranules. However, transferrin receptor, lysosomal-associated membrane protein 1, and sialyl transferase are not recruited. The recruitment of P-selectin is dependent on a tyrosine-based motif within its cytoplasmic domain. Our data show that vWF pseudogranules specifically recruit a subset of membrane proteins, and that in a process explicitly driven by the pseudogranule content (i.e., vWF), the active recruitment of at least one component of the pseudogranule membrane (i.e., P-selectin) is dependent on residues of P-selectin that are cytosolic and therefore unable to directly interact with vWF.

P-selectin targeting to secretory lysosomes of Rbl-2H3 cells

The biogenesis of secretory lysosomes, which combine characteristics of both lysosomes and secretory granules, is currently of high interest. In particular, it is not clear whether delivery of membrane proteins to the secretory lysosome requires lysosomal, secretory granule, or some novel targeting determinants. Heterologous expression of P-selectin has established that this membrane protein contains targeting signals for both secretory granules and lysosomes. P-selectin is therefore an ideal probe with which to determine the signals required for targeting to secretory lysosomes. We have exploited subcellular fractionation and immunofluorescence microscopy to monitor targeting of transiently expressed wild-type and mutant horseradish peroxidase (HRP)-P-selectin chimeras to secretory lysosomes of Rbl-2H3 cells. The exposure of the HRP chimeras to intracellular proteolysis was also determined as a third monitor of secretory lysosome targeting. Our data show that HRP-P-selectin accumulates in secretory lysosomes of Rbl-2H3 cells using those cytoplasmic sequences previously found to be sufficient for targeting to conventional lysosomes. This work highlights the similar sorting signals used for targeting of membrane proteins to conventional lysosomes and secretory lysosomes.

A conserved mechanism of synaptogyrin localization

We have studied the localization of synaptogyrin family members in vivo. Both native and green fluorescent protein (GFP)-tagged Caenorhabditis elegans synaptogyrin (SNG-1) are expressed in neurons and synaptically localized. Deletion and mutational analysis with the use of GFP-tagged SNG-1 has defined a 38 amino acid sequence within the C terminus of SNG-1 and a single arginine in the cytoplasmic loop between transmembrane domain 2 and 3 that are required for SNG-1 localization. These domains may represent components of signals that target synaptogyrin for endocytosis from the plasma membrane and direct synaptogyrin to synaptic vesicles, respectively. In chimeric studies, these regions were sufficient to relocalize cellugyrin, a nonneuronal form of synaptogyrin, from nonsynaptic regions such as the sensory dendrites and the cell body to synaptic vesicles. Furthermore, GFP-tagged rat synaptogyrin is synaptically localized in neurons of C. elegans and in cultured hippocampal neurons. Similarly, the C-terminal domain of rat synaptogyrin is necessary for localization in hippocampal neurons. Our study suggests that the mechanisms for synaptogyrin localization are likely to be conserved from C. elegans to vertebrates.

A Glut4-vesicle marker protein, insulin-responsive aminopeptidase, is localized in a novel vesicular compartment in PC12 cells

Glut4-containing vesicles represent a regulated recycling compartment in insulin-sensitive fat and skeletal muscle cells, the nature and origin of which are not fully understood. In addition to Glut4 itself, these vesicles compartmentalize a number of proteins, at least one of which, insulin-responsive aminopeptidase, or IRAP, is completely colocalized with Glut4 in insulin-sensitive tissues. However, unlike Glut4, IRAP is expressed in a variety of other tissues and cell lines. Here, we explored the intracellular localization of IRAP in the rat pheochromocytoma cell line PC12. We found that this protein is present in a distinct population of slowly recycling light vesicles. By gradient centrifugations, immunoadsorption and double immunofluorescent staining, these vesicles are different from transferrin-containing endosomes, small synaptic vesicles and secretory granules and may thus represent a novel compartment in PC12 cells. Glut4-GFP chimera transiently expressed in PC12 cells is targeted to IRAP-containing vesicles indicating that cotargeting of Glut4 and IRAP is not specific for adipocytes and myocytes, but is faithful in a foreign cell type. We suggest that PC12 cells may possess a novel type of a vesicular carrier that may represent the homolog of Glut4-vesicles.

AP-3 adaptor functions in targeting P-selectin to secretory granules in endothelial cells

P-selectin, a cell adhesion protein participating in the early stages of inflammation, contains multiple sorting signals that regulate its cell surface expression. Targeting to secretory granules regulates delivery of P-selectin to the cell surface. Internalization followed by sorting from early to late endosomes mediates rapid removal of P-selectin from the surface. We show here that the P-selectin cytoplasmic domain bound AP-2 and AP-3 adaptor complexes in vitro. The amino acid substitution L768A, which abolishes endosomal sorting and impairs granule targeting of P-selectin, reduced binding of AP-3 adaptors but not AP-2 adaptors. Turnover of P-selectin was 2.4-fold faster than turnover of transferrin receptor in AP-3-deficient mocha fibroblasts, similar to turnover of these two proteins in AP-3-competent cells, demonstrating that AP-3 function is not required for endosomal sorting. However, sorting P-selectin to secretory granules was defective in endothelial cells from AP-3-deficient pearl mice, demonstrating a role for AP-3 adaptors in granule assembly in endothelial cells. P-selectin sorting to platelet alpha-granules was normal in pearl mice, consistent with earlier evidence that granule targeting of P-selectin is mechanistically distinct in endothelial cells and platelets. These observations establish that AP-3 adaptor functions in assembly of conventional secretory granules, in addition to lysosomes and the 'lysosome-like' secretory granules of platelets and melanocytes.

An acidic motif retains vesicular monoamine transporter 2 on large dense core vesicles

The release of biogenic amines from large dense core vesicles (LDCVs) depends on localization of the vesicular monoamine transporter VMAT2 to LDCVs. We now find that a cluster of acidic residues including two serines phosphorylated by casein kinase 2 is required for the localization of VMAT2 to LDCVs. Deletion of the acidic cluster promotes the removal of VMAT2 from LDCVs during their maturation. The motif thus acts as a signal for retention on LDCVs. In addition, replacement of the serines by glutamate to mimic phosphorylation promotes the removal of VMAT2 from LDCVs, whereas replacement by alanine to prevent phosphorylation decreases removal. Phosphorylation of the acidic cluster thus appears to reduce the localization of VMAT2 to LDCVs by inactivating a retention mechanism.

Targeting of membrane proteins to the regulated secretory pathway in anterior pituitary endocrine cells

Unlike the neuroendocrine cell lines widely used to study trafficking of soluble and membrane proteins to secretory granules, the endocrine cells of the anterior pituitary are highly specialized for the production of mature secretory granules. Therefore, we investigated the trafficking of three membrane proteins in primary anterior pituitary endocrine cells. Peptidylglycine alpha-amidating monooxygenase (PAM), an integral membrane protein essential to the production of many bioactive peptides, is cleaved and enters the regulated secretory pathway even when expressed at levels 40-fold higher than endogenous levels. Myc-TMD/CD, a membrane protein lacking the lumenal, catalytic domains of PAM, is still stored in granules. Secretory granules are not the default pathway for all membrane proteins, because Tac accumulates on the surface of pituitary endocrine cells. Overexpression of PAM is accompanied by a diminution in its endoproteolytic cleavage and in its BaCl(2)-stimulated release from mature granules. Because internalized PAM/PAM-antibody complexes are returned to secretory granules, the endocytic machinery of the pituitary endocrine cells is not saturated. As in corticotrope tumor cells, expression of PAM or Myc-TMD/CD alters the organization of the actin cytoskeleton. PAM-mediated alterations in the cytoskeleton may limit maturation of PAM and storage in mature granules.

Rapid transport of internalized P-selectin to late endosomes and the TGN: roles in regulating cell surface expression and recycling to secretory granules

Prior studies on receptor recycling through late endosomes and the TGN have suggested that such traffic may be largely limited to specialized proteins that reside in these organelles. We present evidence that efficient recycling along this pathway is functionally important for nonresident proteins. P-selectin, a transmembrane cell adhesion protein involved in inflammation, is sorted from recycling cell surface receptors (e.g., low density lipoprotein [LDL] receptor) in endosomes, and is transported from the cell surface to the TGN with a half-time of 20-25 min, six to seven times faster than LDL receptor. Native P-selectin colocalizes with LDL, which is efficiently transported to lysosomes, for 20 min after internalization, but a deletion mutant deficient in endosomal sorting activity rapidly separates from the LDL pathway. Thus, P-selectin is sorted from LDL receptor in early endosomes, driving P-selectin rapidly into late endosomes. P-selectin then recycles to the TGN as efficiently as other receptors. Thus, the primary effect of early endosomal sorting of P-selectin is its rapid delivery to the TGN, with rapid turnover in lysosomes a secondary effect of frequent passage through late endosomes. This endosomal sorting event provides a mechanism for efficiently recycling secretory granule membrane proteins and, more generally, for downregulating cell surface receptors.

Weibel-Palade body membrane proteins exhibit differential trafficking after exocytosis in endothelial cells

Weibel-Palade bodies, the secretory granules of endothelial cells, possess two different membrane proteins. However, P-selectin is seen only in Weibel-Palade bodies in HUVECs, whereas CD63 is also seen in late endosomes/lysosomes. Since P-selectin is targeted to lysosomes in heterologous expression studies, we have determined whether a lysosomal targeting signal also operates within HUVECs. We have also examined the trafficking of CD63 to its two different intracellular locations. By following antibodies bound at the plasma membrane during stimulation, we have discovered that while half of the P-selectin recycles to the WPBs, 50% is rapidly delivered to a lamp-1-positive compartment. Thus, the lysosomal targeting signal of this protein also operates in HUVECs. CD63 is found constitutively at the cell surface of HUVECs and most of it is delivered to the late endosomes/lysosomes after internalisation. However, stimulation causes both a rise in the CD63 plasma membrane level and in the amount that recycles to the WPBs. Our data strongly suggest that the CD63 that originates in the WPB preferentially recycles to the granule rather than being delivered to the late endosome/lysosome, and that there are, therefore, two separate pools of this protein within HUVECs. Our findings indicate that although P-selectin and CD63 are both targeted to the same compartments from the PM, the kinetics and the ratio of their targeting to Weibel-Palade bodies versus lysosomes are very different.

Selective inhibition of adaptor complex-mediated vesiculation

A short while ago, we could only inhibit post-Golgi membrane traffic with crude, unselective tools, such as low temperature or high extracellular sucrose. Molecular dissection of vesiculation steps has revealed unexpected complexity in the coating machinery that has initiated a search for more specific inhibitors. We have learned that membrane vesiculation is driven by a tightly regulated multicomponent, membrane-associated protein machine held together by carefully specified interaction domains. An experimental advantage of such complex interacting machinery is that it is very susceptible to disruption by dominant negative inhibitors or by overexpression. As a result, we now have much more specific inhibitors of post-Golgi membrane traffic. Some, such as dynamin K44A, may be general inhibitors, whereas others can distinguish classes of endocytotic events (10), binding events that require clathrin from those that do not (42), or specific steps of endocytosis (62). Ligand-mediated uptake of EGF and numerous, but not all, GPCRs can be inhibited by overexpression of an ARF GTPase-activating protein that has no effect on transferrin uptake (67). We can look forward to increasingly powerful and selective inhibitors that should help us to navigate successfully the complex routes of post-Golgi membrane traffic.

Sorting to synaptic-like microvesicles from early and late endosomes requires overlapping but not identical targeting signals

In PC12 neuroendocrine cells, synaptic-like microvesicles (SLMV) are thought to be formed by two pathways. One pathway sorts the proteins to SLMV directly from the plasma membrane (or a specialized domain thereof) in an adaptor protein complex 2-dependent, brefeldin A (BFA)-insensitive manner. Another pathway operates via an endosomal intermediate, involves adaptor protein complex 3, and is BFA sensitive. We have previously shown that when expressed in PC12 cells, HRP-P-selectin chimeras are directed to SLMV mostly via the endosomal, BFA-sensitive route. We have now found that two endosomal intermediates are involved in targeting of HRP-P-selectin chimeras to SLMV. The first intermediate is the early, transferrin-positive, epidermal growth factor-positive endosome, from which exit to SLMV is controlled by the targeting determinants YGVF and KCPL, located within the cytoplasmic domain of P-selectin. The second intermediate is the late, transferrin-negative, epidermal growth factor-positive late endosome, from where HRP-P-selectin chimeras are sorted to SLMV in a YGVF- and DPSP-dependent manner. Both sorting steps, early endosomes to SLMV and late endosomes to SLMV, are affected by BFA. In addition, analysis of double mutants with alanine substitutions of KCPL and YGVF or KCPL and DPSP indicated that chimeras pass sequentially through these intermediates en route both to lysosomes and to SLMV. We conclude that a third site of formation for SLMV, the late endosomes, exists in PC12 cells.

A phosphorylation site regulates sorting of the vesicular acetylcholine transporter to dense core vesicles

Vesicular transport proteins package classical neurotransmitters for regulated exocytotic release, and localize to at least two distinct types of secretory vesicles. In PC12 cells, the vesicular acetylcholine transporter (VAChT) localizes preferentially to synaptic-like microvesicles (SLMVs), whereas the closely related vesicular monoamine transporters (VMATs) localize preferentially to large dense core vesicles (LDCVs). VAChT and the VMATs contain COOH-terminal, cytoplasmic dileucine motifs required for internalization from the plasma membrane. We now show that VAChT undergoes regulated phosphorylation by protein kinase C on a serine (Ser-480) five residues upstream of the dileucine motif. Replacement of Ser-480 by glutamate, to mimic the phosphorylation event, increases the localization of VAChT to LDCVs. Conversely, the VMATs contain two glutamates upstream of their dileucine-like motif, and replacement of these residues by alanine conversely reduces sorting to LDCVs. The results provide some of the first information about sequences involved in sorting to LDCVs. Since the location of the transporters determines which vesicles store classical neurotransmitters, a change in VAChT trafficking due to phosphorylation may also influence the mode of transmitter release.

Evidence for a primary endocytic vesicle involved in synaptic vesicle biogenesis

The regulated release of neurotransmitters at synapses is mediated by the fusion of neurotransmitter-filled synaptic vesicles with the plasma membrane. Continuous synaptic activity relies on the constant recycling of synaptic vesicle proteins into newly formed synaptic vesicles. At least two different mechanisms are presumed to mediate synaptic vesicle biogenesis at the synapse as follows: direct retrieval of synaptic vesicle proteins and lipids from the plasma membrane, and indirect passage of synaptic vesicle proteins through an endosomal intermediate. We have identified a vesicle population with the characteristics of a primary endocytic vesicle responsible for the recycling of synaptic vesicle proteins through the indirect pathway. We find that synaptic vesicle proteins colocalize in this vesicle with a variety of proteins known to recycle from the plasma membrane through the endocytic pathway, including three different glucose transporters, GLUT1, GLUT3, and GLUT4, and the transferrin receptor. These vesicles differ from "classical" synaptic vesicles in their size and their generic protein content, indicating that they do not discriminate between synaptic vesicle-specific proteins and other recycling proteins. We propose that these vesicles deliver synaptic vesicle proteins that have escaped internalization by the direct pathway to endosomes, where they are sorted from other recycling proteins and packaged into synaptic vesicles.

Di-leucine signals mediate targeting of tyrosinase and synaptotagmin to synaptic-like microvesicles within PC12 cells

One pathway in forming synaptic-like microvesicles (SLMV) involves direct budding from the plasma membrane, requires adaptor protein 2 (AP2) and is brefeldin A (BFA) resistant. A second route leads from the plasma membrane to an endosomal intermediate from which SLMV bud in a BFA-sensitive, AP3-dependent manner. Because AP3 has been shown to bind to a di-leucine targeting signal in vitro, we have investigated whether this major class of targeting signals is capable of directing protein traffic to SLMV in vivo. We have found that a di-leucine signal within the cytoplasmic tail of human tyrosinase is responsible for the majority of the targeting of HRP-tyrosinase chimeras to SLMV in PC12 cells. Furthermore, we have discovered that a Met-Leu di-hydrophobic motif within the extreme C terminus of synaptotagmin I supports 20% of the SLMV targeting of a CD4-synaptotagmin chimera. All of the traffic to the SLMV mediated by either di-Leu or Met-Leu is BFA sensitive, strongly suggesting a role for AP3 and possibly for an endosomal intermediate in this process. The differential reduction in SLMV targeting for HRP-tyrosinase and CD4-synaptotagmin chimeras by di-alanine substitutions or BFA treatment implies that different proteins use the two routes to the SLMV to differing extents.

Secretagogue-triggered transfer of membrane proteins from neuroendocrine secretory granules to synaptic-like microvesicles

The membrane proteins of all regulated secretory organelles (RSOs) recycle after exocytosis. However, the recycling of those membrane proteins that are targeted to both dense core granules (DCGs) and synaptic-like microvesicles (SLMVs) has not been addressed. Since neuroendocrine cells contain both RSOs, and the recycling routes that lead to either organelle overlap, transfer between the two pools of membrane proteins could occur during recycling. We have previously demonstrated that a chimeric protein containing the cytosolic and transmembrane domains of P-selectin coupled to horseradish peroxidase is targeted to both the DCG and the SLMV in PC12 cells. Using this chimera, we have characterized secretagogue-induced traffic in PC12 cells. After stimulation, this chimeric protein traffics from DCGs to the cell surface, internalizes into transferrin receptor (TFnR)-positive endosomes and thence to a population of secretagogue-responsive SLMVs. We therefore find a secretagogue-dependent rise in levels of HRP within SLMVs. In addition, the levels within SLMVs of the endogenous membrane protein, synaptotagmin, as well as a green fluorescent protein-tagged version of vesicle-associated membrane protein (VAMP)/synaptobrevin, also show a secretagogue-dependent increase.