Prohormone processing in the trans-Golgi network: endoproteolytic cleavage of prosomatostatin and formation of nascent secretory vesicles in permeabilized cells

Small GTPase control of pituitary hormone secretion: Evidence from studies in the goldfish (Carassius auratus) neuroendocrine model

The secretion of vertebrate pituitary hormones is regulated by multiple hypothalamic factors, which, while generally activating unique receptor systems, ultimately propagate signals through interacting intracellular regulatory elements to modulate hormone exocytosis. One important family of intracellular regulators is the monomeric small GTPases, a subset of which (Arf1/6, Rac, RhoA, and Ras) is highly conserved across vertebrates and regulates secretory vesicle exocytosis in many cell types. In this study, we investigated the roles of these small GTPases in basal and agonist-dependent hormone release from dispersed goldfish (Carassius auratus) pituitary cells in perifusion experiments. Inhibition of these small GTPases elevated basal LH and GH secretion, except for Ras inhibition which only increased basal LH release. However, variable responses were observed with regard to LH and GH responses to the two goldfish native gonadotropin-releasing hormones (GnRH2 and GnRH3). GnRH-dependent LH release, but not GH secretion, was mediated by Arf1/6 GTPases. In contrast, inhibition of Rac and RhoA GTPases selectively enhanced GnRH3- and GnRH2-dependent GH release, respectively, while Ras inhibition only enhanced GnRH3-evoked LH secretion. Together, our results reveal novel divergent cell-type- and ligand-specific roles for small GTPases in the control of goldfish pituitary hormone exocytosis in unstimulated and GnRH-evoked release.

WFS1 functions in ER export of vesicular cargo proteins in pancreatic β-cells

The sorting of soluble secretory proteins from the endoplasmic reticulum (ER) to the Golgi complex is mediated by coat protein complex II (COPII) vesicles and thought to required specific ER membrane cargo-receptor proteins. However, these receptors remain largely unknown. Herein, we show that ER to Golgi transfer of vesicular cargo proteins requires WFS1, an ER-associated membrane protein whose loss of function leads to Wolfram syndrome. Mechanistically, WFS1 directly binds to vesicular cargo proteins including proinsulin via its ER luminal C-terminal segment, whereas pathogenic mutations within this region disrupt the interaction. The specific ER export signal encoded in the cytosolic N-terminal segment of WFS1 is recognized by the COPII subunit SEC24, generating mature COPII vesicles that traffic to the Golgi complex. WFS1 deficiency leads to abnormal accumulation of proinsulin in the ER, impeding the proinsulin processing as well as insulin secretion. This work identifies a vesicular cargo receptor for ER export and suggests that impaired peptide hormone transport underlies diabetes resulting from pathogenic WFS1 mutations.

The role of cargo receptors in proinsulin export from the ER is unclear. Here, the authors identify the WFS1 protein, which is mutated in Wolfram syndrome and associated with diabetes, as an ER to Golgi cargo receptor required for normal insulin processing and secretion.

Janus sword actions of chloroquine and hydroxychloroquine against COVID-19

Chloroquine (CQ) and its analogue hydroxychloroquine (HCQ) have been thrust into our everyday vernacular because some believe, based on very limited basic and clinical data, that they might be helpful in preventing and/or lessening the severity of the pandemic coronavirus disease 2019 (COVID-19). However, lacking is a temperance in enthusiasm for their possible use as well as sufficient perspective on their effects and side-effects. CQ and HCQ have well-known properties of being diprotic weak bases that preferentially accumulate in acidic organelles (endolysosomes and Golgi apparatus) and neutralize luminal pH of acidic organelles. These primary actions of CQ and HCQ are responsible for their anti-malarial effects; malaria parasites rely on acidic digestive vacuoles for survival. Similarly, de-acidification of endolysosomes and Golgi by CQ and HCQ may block severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) integration into host cells because SARS-CoV-2 may require an acidic environment for its entry and for its ability to bud and infect bystander cells. Further, de-acidification of endolysosomes and Golgi may underly the immunosuppressive effects of these two drugs. However, modern cell biology studies have shown clearly that de-acidification results in profound changes in the structure, function and cellular positioning of endolysosomes and Golgi, in signaling between these organelles and other subcellular organelles, and in fundamental cellular functions. Thus, studying the possible therapeutic effects of CQ and HCQ against COVID-19 must occur concurrent with studies of the extent to which these drugs affect organellar and cell biology. When comprehensively examined, a better understanding of the Janus sword actions of these and other drugs might yield better decisions and better outcomes.

Cyclic Peptides that Govern Signal Transduction Pathways: From Prokaryotes to Multi-Cellular Organisms

Cyclic peptide scaffolds are key components of signal transduction pathways in both prokaryotic and eukaryotic organisms since they act as chemical messengers that activate or inhibit specific cognate receptors. In prokaryotic organisms these peptides are utilized in non-essential pathways, such as quorum sensing, that are responsible for virulence and pathogenicity. In the more evolved eukaryotic systems, cyclic peptide hormones play a key role in the regulation of the overall function of multicellular organisms, mainly through the endocrine system. This review will highlight several prokaryote and eukaryote systems that use cyclic peptides as their primary signals and the potential associated with utilizing these scaffolds for the discovery of novel therapeutics for a wide range of diseases and illnesses.

Golgi fragmentation in amyotrophic lateral sclerosis, an overview of possible triggers and consequences

Amyotrophic Lateral Sclerosis (ALS) is an invariably fatal neurodegenerative disorder, which specifically targets motor neurons in the brain, brain stem and spinal cord. Whilst the etiology of ALS remains unknown, fragmentation of the Golgi apparatus is detected in ALS patient motor neurons and in animal/cellular disease models. The Golgi is a highly dynamic organelle that acts as a dispatching station for the vesicular transport of secretory/transmembrane proteins. It also mediates autophagy and maintains endoplasmic reticulum (ER) and axonal homeostasis. Both the trigger for Golgi fragmentation and the functional consequences of a fragmented Golgi apparatus in ALS remain unclear. However, recent evidence has highlighted defects in vesicular trafficking as a pathogenic mechanism in ALS. This review summarizes the evidence describing Golgi fragmentation in ALS, with possible links to other disease processes including cellular trafficking, ER stress, defective autophagy, and axonal degeneration.

Analysis of the proteolysis of bioactive peptides using a peptidomics approach

Identifying the peptidases that inactivate bioactive peptides (e.g., peptide hormones and neuropeptides) in mammals is an important unmet challenge. This protocol describes a recent approach that uses liquid chromatography-mass spectrometry (LC-MS) peptidomics to identify endogenous cleavage sites of a bioactive peptide; it also addresses the subsequent biochemical purification of a candidate peptidase on the basis of these cleavage sites and the validation of the candidate peptidase's role in the physiological regulation of the bioactive peptide by examining a peptidase-knockout mouse. We highlight the successful application of this protocol in the discovery that insulin-degrading enzyme (IDE) regulates physiological calcitonin gene-related peptide (CGRP) levels, and we detail the key stages and steps in this approach. This protocol requires 7 d of work; however, the total time for this protocol is highly variable because of its dependence on the availability of biological reagents such as purified enzymes and knockout mice. The protocol is valuable because it expedites the characterization of mammalian peptidases, such as IDE, which in certain instances can be used to develop novel therapeutics.

Revisiting the regulated secretory pathway: from frogs to human

The regulated secretory pathway is a hallmark of endocrine and neuroendocrine cells. This process comprises different sequential steps, including ER-associated protein synthesis, ER-to-Golgi protein transport, Golgi-associated posttranslational modification, sorting and packing of secretory proteins into carrier granules, cytoskeleton-based granule transport towards the plasma membrane and tethering, docking and fusion of granules with specialized releasing zones in the plasma membrane. Each one of these steps is tightly regulated by a large number of factors that function in a spatially and temporarily coordinated fashion. During the past three decades, much effort has been devoted to characterize the precise role of the yet-known proteins participating in the different steps of this process and to identify new regulatory factors in order to obtain a unifying picture of the secretory pathway. In spite of this and given the enormous complexity of the process, certain steps are not fully understood yet and many players remain to be identified. In this review, we offer a summary of the current knowledge on the main molecular mechanisms that govern and ensure the correct release of secretory proteins. In addition, we have integrated the advance on the field made possible by studies carried out in non-mammalian vertebrates, which, although not very numerous, have substantially contributed to acquire a mechanistic understanding of the regulated secretory pathway.

Nongenomic STAT5-dependent effects on Golgi apparatus and endoplasmic reticulum structure and function

We report unexpected nongenomic functions of signal transducer and activator of transcription (STAT) 5 species in the cytoplasm aimed at preserving the structure and function of the Golgi apparatus and rough endoplasmic reticulum (ER) in vascular cells. Immunoimaging and green fluorescent protein-tagged-STAT5a protein localization studies showed the constitutive association of nonphosphorylated STAT5a, and to a lesser extent STAT5b, with the Golgi apparatus and of STAT5a with centrosomes in human pulmonary arterial endothelial and smooth muscle cells. Acute knockdown of STAT5a/b species using small interfering RNAs (siRNAs), including in the presence of an mRNA synthesis inhibitor (5,6-dichloro-1-β-d-ribofuranosylbenzimidazole), produced a dramatic phenotype within 1 day, consisting of dilatation and fragmentation of Golgi cisternae, a marked tubule-to-cyst change in the ER, increased accumulation of reticulon-4 (RTN4)/Nogo-B and atlastin-3 (ATL3) at cyst-zone boundaries, cystic separation of the outer and inner nuclear membranes, accompanied by scalloped/lunate distortion of the nucleus, with accumulation of RTN4 on convex sides of distorted nuclei. These cells showed inhibition of vesicular stomatitis virus G protein glycoprotein trafficking, mitochondrial fragmentation, and reduced mitochondrial function. STAT5a/b(-/-) mouse embryo fibroblasts also showed altered ER/Golgi dynamics. RTN4 knockdown using siRNA did not affect development of the cystic phenotype; ATL3 siRNA led to effacement of cyst-zone boundaries. In magnetic-bead cross-immunopanning assays, ATL3 bound both STAT5a and STAT5b. Remarkably, this novel cystic ER/lunate nucleus phenotype was characteristic of vascular cells in arterial lesions of idiopathic pulmonary hypertension, an unrelentingly fatal human disease. These data provide evidence of a STAT-family protein regulating the structure of a cytoplasmic organelle and implicate this mechanism in the pathogenesis of a human disease.

Investigating endogenous peptides and peptidases using peptidomics

Rather than simply being protein degradation products, peptides have proven to be important bioactive molecules. Bioactive peptides act as hormones, neurotransmitters, and antimicrobial agents in vivo. The dysregulation of bioactive peptide signaling is also known to be involved in disease, and targeting peptide hormone pathways has been a successful strategy in the development of novel therapeutics. The importance of bioactive peptides in biology has spurred research to elucidate the function and regulation of these molecules. Classical methods for peptide analysis have relied on targeted immunoassays, but certain scientific questions necessitated a broader and more detailed view of the peptidome--all the peptides in a cell, tissue, or organism. In this review we discuss how peptidomics has emerged to fill this need through the application of advanced liquid chromatography--tandem mass spectrometry (LC-MS/MS) methods that provide unique insights into peptide activity and regulation.

Identification of a gain-of-function mutation in a Golgi P-type ATPase that enhances Mn2+ efflux and protects against toxicity

P-type ATPases transport a wide array of ions, regulate diverse cellular processes, and are implicated in a number of human diseases. However, mechanisms that increase ion transport by these ubiquitous proteins are not known. SPCA1 is a P-type pump that transports Mn(2+) from the cytosol into the Golgi. We developed an intra-Golgi Mn(2+) sensor and used it to screen for mutations introduced in SPCA1, on the basis of its predicted structure, which could increase its Mn(2+) pumping activity. Remarkably, a point mutation (Q747A) predicted to increase the size of its ion permeation cavity enhanced the sensor response and a compensatory mutation restoring the cavity to its original size abolished this effect. In vivo and in vitro Mn(2+) transport assays confirmed the hyperactivity of SPCA1-Q747A. Furthermore, increasing Golgi Mn(2+) transport by expression of SPCA1-Q747A increased cell viability upon Mn(2+) exposure, supporting the therapeutic potential of increased Mn(2+) uptake by the Golgi in the management of Mn(2+)-induced neurotoxicity.

Golgi, trafficking, and mitosis dysfunctions in pulmonary arterial endothelial cells exposed to monocrotaline pyrrole and NO scavenging

Although the administration of monocrotaline (MCT) into experimental animals is in widespread use today in investigations of pulmonary arterial hypertension (PAH), the underlying cellular and subcellular mechanisms that culminate in vascular remodeling are incompletely understood. Bovine pulmonary arterial endothelial cells (PAECs) in culture exposed to monocrotaline pyrrole (MCTP) develop "megalocytosis" 18-24 h later characterized by enlarged hyperploid cells with enlarged Golgi, mislocalization of endothelial nitric oxide synthase away from the plasma membrane, decreased cell-surface/caveolar nitric oxide (NO), and hypo-S-nitrosylation of caveolin-1, clathrin heavy chain, and N-ethylmaleimide-sensitive factor. We investigated whether MCTP did in fact affect functional intracellular trafficking. The NO scavenger (4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) and the NO donor diethylamine NONOate were used for comparison. Both MCTP and c-PTIO produced distinctive four- to fivefold enlarged PAECs within 24-48 h with markedly enlarged/dispersed Golgi, as visualized by immunostaining for the Golgi tethers/matrix proteins giantin, GM130, and p115. Live-cell uptake of the Golgi marker C(5) ceramide revealed a compact juxtanuclear Golgi in untreated PAECs, brightly labeled enlarged circumnuclear Golgi after MCTP, but minimally labeled Golgi elements after c-PTIO. These Golgi changes were reduced by NONOate. After an initial inhibition during the first day, both MCTP and c-PTIO markedly enhanced anterograde secretion of soluble cargo (exogenous vector-expressed recombinant horseradish peroxidase) over the next 4 days. Live-cell internalization assays using fluorescently tagged ligands showed that both MCTP and c-PTIO inhibited the retrograde uptake of acetylated low-density lipoprotein, transferrin, and cholera toxin B. Moreover, MCTP, and to a variable extent c-PTIO, reduced the cell-surface density of all receptors assayed (LDLR, TfnR, BMPR, Tie-2, and PECAM-1/CD31). In an important distinction, c-PTIO enhanced mitosis in PAECs but MCTP inhibited mitosis, even that due to c-PTIO, despite markedly exaggerated Golgi dispersal. Taken together, these data define a broad-spectrum Golgi and subcellular trafficking dysfunction syndrome in endothelial cells exposed to MCTP or NO scavenging.

Regulation of the formation and trafficking of vesicles from Golgi by PCH family proteins during chemotaxis

Previous study demonstrated that WASP localizes on vesicles during Dictyostelium chemotaxis and these vesicles appear to be preferentially distributed at the leading and trailing edge of migrating cells. In this study, we have examined the role of PCH family proteins, Nwk/Bzz1p-like protein (NLP) and Syndapin-like protein (SLP), in the regulation of the formation and trafficking of WASP-vesicles during chemotaxis. NLP and SLP appear to be functionally redundant and deletion of both nlp and slp genes causes the loss of polarized F-actin organization and significant defects in chemotaxis. WASP and NLP are colocalized on vesicles and interactions between two molecules via the SH3 domain of NLP/SLP and the proline-rich repeats of WASP are required for vesicle formation from Golgi. Microtubules are required for polarized trafficking of these vesicles as vesicles showing high directed mobility are absent in cells treated with nocodazole. Our results suggest that interaction of WASP with NLP/SLP is required for the formation and trafficking of vesicles from Golgi to the membrane, which might play a central role in the establishment of cell polarity during chemotaxis.

Depletion of the ATPase NSF from Golgi membranes with hypo-S-nitrosylation of vasorelevant proteins in endothelial cells exposed to monocrotaline pyrrole

Investigations of regulated S-nitrosylation and denitrosylation of vasorelevant proteins are a newly emergent area in vascular biology. We previously showed that monocrotaline pyrrole (MCTP)-induced megalocytosis of pulmonary arterial endothelial cells (PAECs), which underlies the development of pulmonary arterial hypertension, was associated with a Golgi blockade characterized by the trapping of diverse vesicle tethers, soluble N-ethylmaleimide-sensitive factor (NSF)-attachment protein receptors (SNAREs), and soluble NSF-attachment proteins (SNAPs) in the Golgi; reduced trafficking of caveolin-1 (cav-1) and endotheial nitric oxide (NO) synthase (eNOS) from the Golgi to the plasma membrane; and decreased caveolar NO. We have investigated whether NSF, the ATPase involved in all SNARE disassembly, might be the upstream target of MCTP and whether MCTP might regulate NSF by S-nitrosylation. Immunofluorescence microscopy and Golgi purification techniques revealed the discordant decrease of NSF by approximately 50% in Golgi membranes after MCTP despite increases in alpha-SNAP, cav-1, eNOS, and syntaxin-6. The NO scavenger (4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide failed to affect the initiation or progression of MCTP megalocytosis despite a reduction of 4,5-diaminofluorescein diacetate fluorescence and inhibition of S-nitrosylation of eNOS as assayed using the biotin-switch method. Moreover, the latter assay not only revealed constitutive S-nitrosylation of NSF, eNOS, cav-1, and clathrin heavy chain (CHC) in PAECs but also a dramatic 70-95% decrease in the S-nitrosylation of NSF, eNOS, cav-1, and CHC after MCTP. These data point to depletion of NSF from Golgi membranes as a mechanism for Golgi blockade after MCTP and to denitrosylation of vasorelevant proteins as critical to the development of endothelial cell megalocytosis.

Spontaneous fibrillation of the native neuropeptide hormone Somatostatin-14

Natural Somatostatin-14 is a small cyclic neuropeptide hormone with broad inhibitory effects on endocrine secretions. Here we show that natural Somatostatin-14 spontaneously self-assembles in water and in 150 mM NaCl into liquid crystalline nanofibrils, which follow characteristic structural features of amyloid fibrils. These non-covalent highly stable structures are based on the Somatostatin native backbone conformation and are formed under non-denaturing conditions. Our results support the hypothesis that self-assembly into amyloid fibrils is a generic property of the polypeptide chain under appropriate conditions. Given recent advances on the mechanisms of biological storage and sorting modes of peptide/protein hormones into secretory granules, we propose that Somatostatin-14 fibrillation could be relevant to the regulated secretion pathway of this neuropeptide hormone. Such a hypothesis is consistent with the emerging concept of the existence of non-disease related but functional amyloids.

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.

Processing and intracellular targeting of prosomatostatin-derived peptides: the role of mammalian endoproteases

Prosomatostatin is cleaved at dibasic and monobasic sites to produce somatostatin-14 and somatostatin-28 respectively. The mammalian pro-protein convertases comprising furin, PACE4 and PC1-6 have recently been identified and are believed to mediate endoproteolysis of prohormone precursors such as prosomatostatin. Furin is membrane bound, localized to the Golgi and mediates constitutive processing. PC1 and PC2 are soluble and are expressed solely in endocrine and neuroendocrine tissues suggesting a key role in prohormone processing. We have investigated the endogenous and heterologous synthesis and processing of rat prosomatostatin in 1027B2 rat islet somatostatinoma cells and in constitutive (COS-7, PC-12) and regulated (AtT-20, GH3/GH4C1) secretory cells. We have correlated processing efficiency with: secretion through the constitutive or regulated pathways; endogenous expression of furin, PC1 and PC2; and expression or overexpression of furin, PC1 and PC2. Pulse-chase studies showed that prosomatostatin is rapidly and independently processed to somatostatin-14 and somatostatin-28. Furin is capable of monobasic processing of prosomatostatin and is a candidate somatostatin-28 convertase. PC1 and PC2 both effect dibasic processing of prosomatostatin and qualify as putative somatostatin-14 convertases. PC1 is active in constitutive and regulated secretory cells, has a broader specificity and is overall more potent than PC2. Efficient processing of prosomatostatin begins in a Golgi or pre Golgi compartment. It requires the milieu of the secretory cell but not the secretory granule.

Dysfunction of Golgi tethers, SNAREs, and SNAPs in monocrotaline-induced pulmonary hypertension

Monocrotaline (MCT)-induced pulmonary hypertension (PH) in the rat is a widely used experimental model. We have previously shown that MCT pyrrole (MCTP) produces loss of caveolin-1 (cav-1) and endothelial nitric oxide synthase from plasma membrane raft microdomains in pulmonary arterial endothelial cells (PAEC) with the trapping of these proteins in the Golgi organelle (the Golgi blockade hypothesis). In the present study, we investigated the mechanisms underlying this intracellular trafficking block in experiments in cell culture and in the MCT-treated rat. In cell culture, PAEC showed trapping of cav-1 in Golgi membranes as early as 6 h after exposure to MCTP. Phenotypic megalocytosis and a reduction in anterograde trafficking (assayed in terms of the secretion of horseradish peroxidase derived from exogenously transfected expression constructs) were evident within 12 h after MCTP. Cell fractionation and immunofluorescence techniques revealed the marked accumulation of diverse Golgi tethers, soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptors (SNAREs), and soluble NSF attachment proteins (SNAPs), which mediate membrane fusion during vesicular trafficking (GM130, p115, giantin, golgin 84, clathrin heavy chain, syntaxin-4, -6, Vti1a, Vti1b, GS15, GS27, GS28, SNAP23, and alpha-SNAP) in the enlarged/circumnuclear Golgi in MCTP-treated PAEC and A549 lung epithelial cells. Moreover, NSF, an ATPase required for the "disassembly" of SNARE complexes subsequent to membrane fusion, was increasingly sequestered in non-Golgi membranes. Immunofluorescence studies of lung tissue from MCT-treated rats confirmed enlargement of perinuclear Golgi elements in lung arterial endothelial and parenchymal cells as early as 4 days after MCT. Thus MCT-induced PH represents a disease state characterized by dysfunction of Golgi tethers, SNAREs, and SNAPs and of intracellular vesicular trafficking.

Aberrant cytoplasmic sequestration of eNOS in endothelial cells after monocrotaline, hypoxia, and senescence: live-cell caveolar and cytoplasmic NO imaging

We previously reported the disruption of caveolae/rafts, dysfunction of Golgi tethers, N-ethylmaleimide-sensitive factor-attachment protein (SNAP) receptor proteins (SNAREs), and SNAPs, and inhibition of anterograde trafficking in endothelial cells in culture and rat lung exposed to monocrotaline pyrrole (MCTP) as a prelude to the development of pulmonary hypertension. We have now investigated 1) whether this trafficking block affects subcellular localization and function of endothelial nitric oxide (NO) synthase (eNOS) and 2) whether Golgi blockade and eNOS sequestration are observed after hypoxia and senescence. Immunofluorescence data revealed that MCTP-induced "megalocytosis" of pulmonary arterial endothelial cells (PAEC) was accompanied by a loss of eNOS from the plasma membrane, with increased accumulation in the cytoplasm. This cytoplasmic eNOS was sequestered in heterogeneous compartments and partially colocalized with Golgi and endoplasmic reticulum (ER) markers, caveolin-1, NOSTRIN, and ER Tracker, but not Lyso Tracker. Hypoxia and senescence also produced enlarged PAEC, with dysfunctional Golgi and loss of eNOS from the plasma membrane, with sequestration in the cytoplasm. Live-cell imaging of caveolar and cytoplasmic NO with 4,5-diaminofluorescein diacetate (DAF-2DA) as probe showed a marked loss of caveolar NO after MCTP, hypoxia, and senescence. Although ionomycin stimulated DAF-2DA fluorescence in control PAEC, this ionophore decreased DAF-2DA fluorescence in MCTP-treated and senescent PAEC, suggesting localization of eNOS in an aberrant cytoplasmic compartment that was readily discharged by Ca(2+)-induced exocytosis. Thus monocrotaline, hypoxia, and senescence produce a Golgi blockade in PAEC, leading to sequestration of eNOS away from its functional caveolar location and providing a mechanism for the often-reported reduction in pulmonary arterial NO levels in experimental pulmonary hypertension, despite sustained eNOS protein levels.

Prodynorphin storage and processing in axon terminals and dendrites

The classical view postulates that neuropeptide precursors in neurons are processed into mature neuropeptides in the somatic trans-Golgi network (TGN) and in secretory vesicles during axonal transport. Here we show that prodynorphin (PDYN), precursor to dynorphin opioid peptides, is predominantly located in axon terminals and dendrites in hippocampal and striatal neurons. The molar content of unprocessed PDYN was much greater than that of dynorphin peptides in axon terminals of PDYN-containing neurons projecting to the CA3 region of the hippocampus and in the striatal projections to the ventral tegmental area. Electron microscopy showed coexistence of PDYN and dynorphins in the same axon terminals with occasional codistribution in individual dense core vesicles. Thus, the precursor protein is apparently stored at presynaptic sites. In comparison with the hippocampus and striatum, PDYN and dynorphins were more equally distributed between neuronal somata and processes in the amygdala and cerebral cortex, suggesting regional differences in the regulation of trafficking and processing of the precursor protein. Potassium-induced depolarization activated PDYN processing and secretion of opioid peptides in neuronal cultures and in a model cell line. Regulation of PDYN storage and processing at synapses by neuronal activity or extracellular stimuli may provide a local mechanism for regulation of synaptic transmission.

Two distinct chloride ion requirements in the constitutive protein secretory pathway

The role of chloride ions in regulated secretion is well described but remains poorly characterised in the constitutive system. In the liver, newly synthesised proalbumin is transported to the trans Golgi network where it is converted to albumin by a furin protease and then immediately secreted. We used this acid-dependent hydrolysis and the measurement of specific protein secretion rates to examine the H+ and Cl- ion dependence of albumin synthesis and secretion, a major constitutive protein secretory event in all mammals. Using permeabilised primary rat hepatocytes we show that ordinarily chloride ions are essential for the processing of proalbumin to albumin. However Cl- is not required for transport which continues but releases solely proalbumin. Prior treatment of the cells with Tris (used as a membrane-permeable weak base to neutralise Golgi luminal pH) both eliminated the formation of albumin and very greatly reduced secretion. After washing out Tris, both authentic secretion and processing could be restarted if Cl-, ATP, GTP, cAMP, Ca2+ and cytosolic proteins were added. Hence a requirement for chloride in transport, in addition to processing, can be uncovered by first neutralising pH gradients. Furthermore, the chloride channel blocker DIDS (4,4-diisothiocyanostilbene 2,2-disulphonic acid) reversibly inhibited the constitutive secretory pathway. However, the total mass of proalbumin detectable in DIDS-treated cells fell to 36% of control while the fraction processed to albumin remained almost constant. This clearly dissociates a large part of the Cl- requirement of the constitutive protein secretory pathway from the function of known liver Golgi Cl- channels.

Elevated zinc transporter-6 in mild cognitive impairment, Alzheimer disease, and pick disease

Filamentous cytoplasmic inclusions are hallmarks of Alzheimer disease (AD) and Pick disease (PD). Although previous studies show elevated zinc (Zn) in AD brain, there has been little study of zinc transporter (ZnT) proteins that are critical in the maintenance of Zn homeostasis. Using Western blot analysis, we show significantly elevated ZnT-6, the protein responsible for sequestration of Zn in the trans-Golgi network, in the hippocampus/parahippocampal gyrus (HPG) of AD subjects compared to age-matched controls and a trend toward elevated ZnT-6 in subjects with amnestic mild cognitive impairment (MCI). Based on these data, we used immunohistochemistry to investigate the cellular distribution of ZnT-6 in the HPG of control subjects and subjects with MCI, AD, and PD. Comparison of immediately adjacent serial sections stained using the modified Bielschowsky method and immunostained for ZnT-6 showed elevated ZnT-6 in 89 +/- 7% of neurofibrillary tangle (NFT)-bearing neurons in AD and 100 +/- 19% of Pick bodies in PD specimens. Confocal microscopy of HPG from MCI subjects double labeled for ZnT-6 and MC-1, a marker of early NFT formation, showed 85 +/- 4% of MC-1-positive cells were strongly ZnT-6-positive. Increased ZnT-6 immunostaining in neurons containing cytoplasmic inclusions in MCI, AD, and PD suggests a role for ZnT-6 in the pathogenesis of these lesions.

Allelic variation on chromosome 5 controls beta-cell mass expansion during hyperglycemia in leptin receptor-deficient diabetes mice

Leptin signaling is a critical component of normal insulin sensitivity. Overt hyperglycemia and type 2 diabetes mellitus can be manifested in states of leptin signaling deficiencies by the additional effects of other genetic factors. We have previously described the contrasting insulin sensitivities and glycemic states of two congenic diabetes (db/db) mouse strains. C57BL/6J db/db mice have mild insulin resistance and achieve euglycemia with mild hyperinsulinemia. FVB db/db mice have severe insulin resistance and are hyperglycemic despite escalating hyperinsulinemia with expanded pancreatic beta-cell mass. Analysis of obese progeny from the two reciprocal backcrosses suggests that genetic modifiers for insulin sensitivity are separable from loci that modulate beta-cell mass. A genome scan of the backcross to FVB suggests that one or more modifier genes are present on chromosome 5. This evidence is supported by the phenotypes of multiple incipient congenic strains wherein the hyperglycemia observed in obese FVB mice is reproduced. With similar degrees of hyperglycemia in obese mice of these strains, the haplotype at chromosome 5 is associated with beta-cell mass and circulating insulin concentrations. Finally, we offer arguments that production of multiple incipient congenic lines is an economical alternative to the production of speed congenic strains.

Presenilin-1 uses phospholipase D1 as a negative regulator of beta-amyloid formation

Presenilin (PS1/PS2) is a major component of gamma-secretase, the activity that mediates proteolysis of beta-amyloid precursor protein to generate beta-amyloid (Abeta). Here we demonstrate that PS1, through its loop region, binds to phospholipase D1 (PLD1), thereby recruiting it to the Golgi/trans-Golgi network. Overexpression of wild-type PLD1 reduces Abeta generation. Conversely, down-regulation of endogenous PLD1 by small hairpin RNA elevates Abeta production. The Abeta-lowering effect of PLD1 is independent of its ability to promote vesicular budding of beta-amyloid precursor protein. The data indicate that overexpression of PLD1 decreases, and down-regulation of PLD1 increases, the catalytic activity, and the association of the subunits, of gamma-secretase.

Phospholipase D1 corrects impaired betaAPP trafficking and neurite outgrowth in familial Alzheimer's disease-linked presenilin-1 mutant neurons

Presenilins (PS1/PS2) regulate proteolysis of beta-amyloid precursor protein (betaAPP) and affect its intracellular trafficking. Here, we demonstrate that a PS1-interacting protein, phospholipase D1 (PLD1), affects intracellular trafficking of betaAPP. Overexpression of PLD1 in PS1wt cells promotes generation of betaAPP-containing vesicles from the trans-Golgi network. Conversely, inhibition of PLD1 activity by 1-butanol decreases betaAPP trafficking in both wt and PS1-deficient cells. The subcellular localization of PLD1 is altered, and PLD enzymatic activity is reduced in cells expressing familial Alzheimer's disease (FAD) PS1 mutations compared with PS1wt cells. Overexpression of wt, but not catalytically inactive, PLD1 increases budding of betaAPP-containing vesicles from the trans-Golgi network in FAD mutant cells. Surface delivery of betaAPP is also increased by PLD1 in these cells. The impaired neurite outgrowth capacity in FAD mutant neurons was corrected by introducing PLD1 into these cells. The results indicate that PLD1 may represent a therapeutic target for rescuing compromised neuronal function in AD.

Multiple signals are required for alpha2,6-sialyltransferase (ST6Gal I) oligomerization and Golgi localization

A single amino acid difference in the catalytic domain of two isoforms of the alpha2,6-sialyltransferase (ST6Gal I) leads to differences in their trafficking, processing, and oligomerization. The STtyr isoform is transiently localized in the Golgi and is ultimately cleaved and secreted, whereas the STcys isoform is stably localized in the Golgi and is not cleaved and secreted. The stable localization of STcys is correlated with its enhanced ability to oligomerize. To test the hypothesis that multiple signals can mediate Golgi localization and further evaluate the role of oligomerization in the localization process, we evaluated the effects of individually and simultaneously altering the cytosolic tail and transmembrane region of the STcys isoform. We found that the localization, processing, and oligomerization of STcys were not substantially changed when either the core amino acids of the cytosolic tail were deleted or the sequence and length of the transmembrane region were altered. In contrast, when these changes were made simultaneously, the STcys isoform was converted into a form that was processed, secreted, and weakly oligomerized like STtyr. We propose that STcys oligomerization is a secondary event resulting from its concentration in the Golgi via mechanisms independently mediated by its cytosolic tail and transmembrane region.

Gleevec inhibits beta-amyloid production but not Notch cleavage

Amyloid-beta (Abeta) peptides, consisting mainly of 40 and 42 aa (Abeta40 and Abeta42, respectively), are metabolites of the amyloid precursor protein and are believed to be major pathological determinants of Alzheimer's disease. The proteolytic cleavages that form the Abeta N and C termini are catalyzed by beta-secretase and gamma-secretase, respectively. Here we demonstrate that gamma-secretase generation of Abeta in an N2a cell-free system is ATP dependent. In addition, the Abl kinase inhibitor imatinib mesylate (Gleevec, or STI571), which targets the ATP-binding site of Abl and several other tyrosine kinases, potently reduces Abeta production in the N2a cell-free system and in intact N2a cells. Both STI571 and a related compound, inhibitor 2, also reduce Abeta production in rat primary neuronal cultures and in vivo in guinea pig brain. STI571 does not inhibit the gamma-secretase-catalyzed S3 cleavage of Notch-1. Furthermore, production of Abeta and its inhibition by STI571 were demonstrated to occur to similar extents in both Abl-/- and WT mouse fibroblasts, indicating that the effect of STI571 on Abeta production does not involve Abl kinase. The efficacy of STI571 in reducing Abeta without affecting Notch-1 cleavage may prove useful as a basis for developing novel therapies for Alzheimer's disease.

Generation of a bioactive neuropeptide in a cell-free system

We have developed an in vitro assay for pre-pro-neuropeptide synthesis and processing. Mouse proopiomelanocortin (POMC) cDNA was cloned into a vector containing T7 promoter. In vitro transcription and translation were carried out to produce the proopiomelanocortin peptide. The pro-peptide was processed by incubating with cell extract of mouse and rat pituitary cell lines. The activity of processed mature peptide was tested in a cell line expressing human melanocortin 4 receptor (MC4R). Using this approach, we produced biologically active alpha-melanocyte-stimulating hormone (alpha-MSH). Furthermore, we developed a universal functional assay for G-protein-coupled receptors (GPCRs) using a reporter gene assay. More than 20 different GPCRs were examined using this functional assay. Our results demonstrated that the activity of all GPCRs could be measured by the functional assay. It should be possible to identify novel bioactive peptides for orphan GPCRs by the combination of in vitro processing and GPCR functional assays.

Morphology and dynamics of clathrin/GGA1-coated carriers budding from the trans-Golgi network

Sorting of transmembrane proteins and their ligands at various compartments of the endocytic and secretory pathways is mediated by selective incorporation into clathrin-coated intermediates. Previous morphological and biochemical studies have shown that these clathrin-coated intermediates consist of spherical vesicles with a diameter of 60-100 nm. Herein, we report the use of fluorescent imaging of live cells to demonstrate the existence of a different type of transport intermediate containing associated clathrin coats. Clathrin and the adaptors GGA1 and adaptor protein-1, labeled with different spectral variants of the green fluorescent protein, are shown to colocalize to the trans-Golgi network and to a population of vesicles and tubules budding from it. These intermediates are highly pleiomorphic and move toward the peripheral cytoplasm for distances of up to 10 microm with average speeds of approximately 1 microm/s. The labeled clathrin and GGA1 cycle on and off membranes with half-times of 10-20 s, independently of vesicle budding. Our observations indicate the existence of a novel type of trans-Golgi network-derived carriers containing associated clathrin, GGA1 and adaptor protein-1 that are larger than conventional clathrin-coated vesicles, and that undergo long-range translocation in the cytoplasm before losing their coats.

Presenilin-1 regulates intracellular trafficking and cell surface delivery of beta-amyloid precursor protein

Presenilins (PS1/PS2) play a critical role in proteolysis of beta-amyloid precursor protein (beta APP) to generate beta-amyloid, a peptide important in the pathogenesis of Alzheimer's disease. Nevertheless, several regulatory functions of PS1 have also been reported. Here we demonstrate, in neuroblastoma cells, that PS1 regulates the biogenesis of beta APP-containing vesicles from the trans-Golgi network and the endoplasmic reticulum. PS1 deficiency or the expression of loss-of-function variants leads to robust vesicle formation, concomitant with increased maturation and/or cell surface accumulation of beta APP. In contrast, release of vesicles containing beta APP is impaired in familial Alzheimer's disease (FAD)-linked PS1 mutant cells, resulting in reduced beta APP delivery to the cell surface. Moreover, diminution of surface beta APP is profound at axonal terminals in neurons expressing a PS1 FAD variant. These results suggest that PS1 regulation of beta APP trafficking may represent an alternative mechanism by which FAD-linked PS1 variants modulate beta APP processing.

Chaperones increase association of tau protein with microtubules

Molecular chaperones and their functions in protein folding have been implicated in several neurodegenerative diseases, including Parkinson's disease and Huntington's disease, which are characterized by accumulation of protein aggregates (e.g., alpha-synuclein and huntingtin, respectively). These aggregates have been shown in various experimental systems to respond to changes in levels of molecular chaperones suggesting the possibility of therapeutic intervention and a role for chaperones in disease pathogenesis. It remains unclear whether chaperones also play a role in Alzheimer's disease, a neurodegenerative disorder characterized by beta-amyloid and tau protein aggregates. Here, we report an inverse relationship between aggregated tau and the levels of heat shock protein (Hsp)7090 in tau transgenic mouse and Alzheimer's disease brains. In various cellular models, increased levels of Hsp70 and Hsp90 promote tau solubility and tau binding to microtubules, reduce insoluble tau and cause reduced tau phosphorylation. Conversely, lowered levels of Hsp70 and Hsp90 result in the opposite effects. We have also demonstrated a direct association of the chaperones with tau proteins. Our results suggest that up-regulation of molecular chaperones may suppress formation of neurofibrillary tangles by partitioning tau into a productive folding pathway and thereby preventing tau aggregation.

Evidence of a novel dipeptidyl aminopeptidase in mammalian GH(3) cells: new insights into the processing of peptide hormone precursors

We investigated whether yeast signals could regulate hormone processing in mammalian cells. Chmeric genes coding for the prepro region of yeast alpha-factor and the functional hormone region of anglerfish somatostatin was expressed in rat pituitary GH(3) cells. The nascent prepro-alpha-factor-somatostatin peptides disappeared from cells with a half-life of 30 min, and about 20% of unprocessed precursors remained intracellular after a 2 h chase period. Disappearance of propeptide was insensitive to lysosomotropic agents, but was inhibited at 15 degrees C or 20 degrees C, suggesting that the hybrid propeptides were not degraded in the secretory pathway to the trans Golgi network or in lysosomes. It appeared that while most unprocessed precursors were constitutively secreted into the medium, a small portion were processed at their paired dibasic sites by prohormone-processing enzymes located in trans Golgi network/secretory vesicles, resulting in the production of mature somatostatin peptides. To test this hypothesis, we investigated the processing pattern of two different hybrid precursors: the 52-1 hybrid precursor, which has a Glu-Ala spacer between the prepro region of alpha-factor and somatostatin, and the 58-1 hybrid precursor, which lacks the Glu-Ala spacer. Processing of metabolically labeled hybrid propeptides to smaller somatostatin peptides was assessed by HPLC. When pulse-labeled cells were chased for up to 2 h, 68% of the initially synthesized propeptides were secreted constitutively. About 22% of somatostatin-related products were proteolytically processed to mature somatostatin, of which 38.7% were detected intracellularly after 2 h. From N-terminal peptide sequence determination of somatostatin-related products in GH(3)-52 and GH(3)-58 cells, we found that both hybrid precursors were accurately cleaved at their dibasic amino acid sites. Notably, we also observed that the Glu-Ala spacer sequence was removed from 52-1 hybrid precursors. The latter result strongly suggests that a novel dipeptidyl aminopeptidase activity - a yeast STE13-like enzyme - is present in the post-trans Golgi network compartment of GH(3) cells. The data from these studies indicate that mechanisms which control protein secretion are conserved between yeast and mammalian cells.

Estrogen lowers Alzheimer beta-amyloid generation by stimulating trans-Golgi network vesicle biogenesis

Estrogen reduces the risk of Alzheimer's disease in post-menopausal women, beta-amyloid (Abeta) burden in animal models of Alzheimer's disease, and secretion of Abeta from neuronal cultures. The biological basis for these effects remains unknown. Here, utilizing cell-free systems derived from both neuroblastoma cells and primary neurons, we demonstrate that 17beta-estradiol (17beta-E2) stimulates formation of vesicles containing the beta-amyloid precursor protein (betaAPP) from the trans-Golgi network (TGN). Accelerated betaAPP trafficking precludes maximal Abeta generation within the TGN. 17beta-E2 appears to modulate TGN phospholipid levels, particularly those of phosphatidylinositol, and to recruit soluble trafficking factors, such as Rab11, to the TGN. Together, these results suggest that estrogen may exert its anti-Abeta effects by regulating betaAPP trafficking within the late secretory pathway. These results suggest a novel mechanism through which 17beta-E2 may act in estrogen-responsive tissues and illustrate how altering the kinetics of the transport of a protein can influence its metabolic fate.

Fragmentation and re-assembly of the Golgi apparatus in vitro. A requirement for phosphatidic acid and phosphatidylinositol 4,5-bisphosphate synthesis

Recent work from our laboratory demonstrated that phosphatidic acid (PA) and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)), are required to maintain the structural integrity of the Golgi apparatus. To investigate the role of these lipids in regulating Golgi structure and function, we developed a novel assay to follow the release of post-Golgi vesicles. Isolated rat liver Golgi membranes were incubated with [(3)H]CMP sialic acid to radiolabel endogenous soluble and membrane glycoproteins present in the late Golgi and trans-Golgi network. The release of post-Golgi secretory vesicles was determined by measuring incorporation of (3)H-labeled proteins into a medium speed supernatant. Vesicle budding was dependent on temperature, cytosol, energy and time. Electron microscopy of Golgi fractions prior to and after incubation demonstrated that the stacked Golgi cisternae generated a heterogeneous population of vesicles (50- to 350-nm diameter). Inhibition of phospholipase D-mediated PA synthesis, by incubation with 1-butanol, resulted in the complete fragmentation of the Golgi membranes in vitro into 50- to 100-nm vesicles; this correlated with diminished PtdIns(4,5)P(2) synthesis. Following alcohol washout, PA synthesis resumed and in the presence of cytosol PtdIns(4,5)P(2) synthesis was restored. Most significantly, under these conditions the fragmented Golgi elements reformed into flattened cisternae and the re-assembled Golgi supported vesicle release. These data demonstrate that inositol phospholipid synthesis is essential for the structure and function of the Golgi apparatus.

The transmembrane domain of the Alzheimer's beta-secretase (BACE1) determines its late Golgi localization and access to beta -amyloid precursor protein (APP) substrate

Release of Abeta peptides from beta-amyloid precursor protein (APP) requires sequential cleavage by two endopeptidases, beta- and gamma-secretases. beta-Secretase was recently identified as a novel membrane-bound aspartyl protease, named BACE1, Asp2, or memapsin 2. Employing confocal microscopy and subcellular fractionation, we have found that BACE1 is largely situated in the distal Golgi membrane with a minor presence in the endoplasmic reticulum, endosomes, and plasma membrane in human neuroblastoma SHEP cells and in mouse Neuro-2a cell lines expressing either endogenous mouse BACE1 or additional exogenous human BACE1. The major cellular beta-secretase activity is located in the late Golgi apparatus, consistent with its cellular localization. Furthermore, we demonstrate that the single transmembrane domain of BACE1 alone determines the retention of BACE1 to the Golgi compartments, through examination of recombinant proteins of various BACE1 fragments fused to a reporter green fluorescence protein. In addition, we show that the transmembrane domain of BACE1 is required for the access of BACE1 enzymatic activity to the cellular APP substrate and hence for the optimal generation of the C-terminal fragment of APP (CTF99). The results suggest a molecular and cell biological mechanism for the regulation of beta-secretase activity in vivo.

Location and mechanism of alpha 2,6-sialyltransferase dimer formation. Role of cysteine residues in enzyme dimerization, localization, activity, and processing

A significant proportion of the alpha2,6-sialyltransferase of protein Asn-linked glycosylation (ST6Gal I) forms disulfide-bonded dimers that exhibit decreased activity, but retain the ability to bind asialoglycoprotein substrates. Here, we have investigated the subcellular location and mechanism of ST6Gal I dimer formation, as well as the role of Cys residues in the enzyme's trafficking, localization, and catalytic activity. Pulse-chase analysis demonstrated that the ST6Gal I disulfide-bonded dimer forms in the endoplasmic reticulum. Mutagenesis experiments showed that Cys-24 in the transmembrane region is required for dimerization, while catalytic domain Cys residues are required for trafficking and catalytic activity. Replacement of Cys-181 and Cys-332 generated proteins that are largely retained in the endoplasmic reticulum and minimally active or inactive, respectively. Replacement of Cys-350 or Cys-361 inactivated the enzyme without compromising its localization or processing, suggesting that these amino acids are part of the enzyme's active site. Replacement of Cys-139 or Cys-403 generated proteins that are catalytically active and appear to be more stably localized in the Golgi, since they exhibited decreased cleavage and secretion. The Cys-139 mutant also exhibited increased dimer formation suggesting that ST6Gal I dimers may be critical in the oligomerization process involved in stable ST6Gal I Golgi localization.

Intracellular localization of phospholipase D1 in mammalian cells

Phospholipase D (PLD) hydrolyzes phosphatidylcholine to generate phosphatidic acid. In mammalian cells this reaction has been implicated in the recruitment of coatomer to Golgi membranes and release of nascent secretory vesicles from the trans-Golgi network. These observations suggest that PLD is associated with the Golgi complex; however, to date, because of its low abundance, the intracellular localization of PLD has been characterized only indirectly through overexpression of chimeric proteins. We have used highly sensitive antibodies to PLD1 together with immunofluorescence and immunogold electron microscopy as well as cell fractionation to identify the intracellular localization of endogenous PLD1 in several cell types. Although PLD1 had a diffuse staining pattern, it was enriched significantly in the Golgi apparatus and was also present in cell nuclei. On fragmentation of the Golgi apparatus by treatment with nocodazole, PLD1 closely associated with membrane fragments, whereas after inhibition of PA synthesis, PLD1 dissociated from the membranes. Overexpression of an hemagglutinin-tagged form of PLD1 resulted in displacement of the endogenous enzyme from its perinuclear localization to large vesicular structures. Surprisingly, when the Golgi apparatus collapsed in response to brefeldin A, the nuclear localization of PLD1 was enhanced significantly. Our data show that the intracellular localization of PLD1 is consistent with a role in vesicle trafficking from the Golgi apparatus and suggest that it also functions in the cell nucleus.

In vitro assays of vesicular transport

Movement of proteins and lipids between the various compartments of eukaryotic cells is fundamental to the maintenance of cellular homeostasis, and an understanding of the molecular mechanisms that govern these processes remains a key goal of cell biological research. This aim has been greatly facilitated by the development of assays that recapitulate specific events in vitro. In the following article we provide an overview of some of the currently used assays that measure the movement of proteins within the exocytic and endocytic pathways, and provide a starting point for those wishing to establish their own systems to study other vesicular transport steps.

Processing of pro-Muclin and divergent trafficking of its products to zymogen granules and the apical plasma membrane of pancreatic acinar cells

Proteins are sorted and packaged into regulated secretory granules at the trans Golgi network but how such granules form is poorly understood. We are studying Muclin, the major sulfated protein of the mouse pancreatic acinar cell, and what its role may be in zymogen granule formation. Muclin behaves as a peripheral membrane protein localized to the lumen of the zymogen granule but the cDNA for this protein predicts it is a type I membrane protein with a short, 16-amino-acid, cytosolic tail (C-Tail). Using domain-specific antibodies, we demonstrate that Muclin is derived from a precursor, pro-Muclin, which is cleaved to produce Muclin and an approximately 80-kDa membrane glycoprotein (p80). Incubation of pulse-labeled cells at < or = 22 degrees C to block exit from the trans Golgi network also blocks cleavage of pro-Muclin but not sulfation, a trans Golgi network event, suggesting that cleavage occurs in a post-Golgi compartment. After cleavage the two products of pro-Muclin diverge with Muclin remaining in the regulated secretory pathway and p80 trafficking to the apical plasma membrane, presumably via the constitutive-like pathway. When transfected into exocrine AR42J cells, Muclin labeling is perinuclear and in large sub-plasma membrane puncta. Transiently transfected AR42J cells have greater immunolabeling for amylase than nontransfected cells, suggesting a role for Muclin in cargo accumulation in the regulated secretory pathway. A construct with the C-Tail deleted targets to small diffusely-distributed puncta and without the large sub-plasma membrane structures. Thus, the C-Tail is required for proper Muclin targeting. When transfected into neuroendocrine AtT-20 cells Muclin is not colocalized with ACTH in cell processes, and it appears to be constitutively trafficked to the plasma membrane, suggesting that Muclin has exocrine-specific information. We present a working model for pro-Muclin as a Golgi cargo receptor for exocrine secretory granule formation at the trans Golgi network.

Formation of insoluble oligomers correlates with ST6Gal I stable localization in the golgi

The ST6Gal I is a sialyltransferase that functions in the late Golgi to modify the N-linked oligosaccharides of glycoproteins. The ST6Gal I is expressed as two isoforms with a single amino acid difference in their catalytic domains. The STcys isoform is stably retained in the cell and is predominantly found in the Golgi, whereas the STtyr isoform is only transiently localized in the Golgi and is cleaved and secreted from a post-Golgi compartment. These two ST6Gal I isoforms were used to explore the role of the bilayer thickness mechanism and oligomerization in Golgi localization. Analysis of STcys and STtyr proteins with longer transmembrane regions suggested that the bilayer thickness mechanism is not the predominant mechanism used for ST6Gal I Golgi localization. In contrast, the formation and quantity of Triton X-100-insoluble oligomers was correlated with the stable or transient localization of the ST6Gal I isoforms in the Golgi. Nearly 100% of the STcys and only 13% of the STtyr were found as Triton-insoluble oligomers when Golgi membranes of COS-1 cells expressing these proteins were solubilized at pH 6.3, the pH of the late Golgi. In contrast, both proteins were found in the soluble fraction when these membranes were solubilized at pH 8.0. Analysis of other mutants suggested that a conformational change in the catalytic domain rather than increased disulfide bond-based cross-linking is the basis for the increased ability of STcys protein to form oligomers and the stable localization of STcys protein in the Golgi.

Inhibition of phosphatidic acid synthesis alters the structure of the Golgi apparatus and inhibits secretion in endocrine cells

In mammalian cells, activation of a Golgi-associated phospholipase D by ADP-ribosylation factor results in the hydrolysis of phosphatidylcholine to form phosphatidic acid (PA). This reaction stimulates the release of nascent secretory vesicles from the trans-Golgi network of endocrine cells. To understand the role of PA in mediating secretion, we have exploited the transphosphatidylation activity of phospholipase D. Rat anterior pituitary GH3 cells, which secrete growth hormone and prolactin, were treated with 1-butanol resulting in the synthesis of phosphatidylbutanol rather than PA. Under these conditions transport from the ER through the Golgi apparatus and secretion of polypeptide hormones were inhibited quantitatively. Furthermore, the in vitro synthesis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) by Golgi membranes was inhibited quantitatively. Most significantly, in the presence of 1-butanol the architecture of the Golgi apparatus was disrupted, resulting in its disassembly and fragmentation. Removal of the alcohol resulted in the rapid restoration of Golgi structure and secretion of growth hormone and prolactin. Our results suggest that PA stimulation of PtdIns(4,5)P(2) synthesis is required for maintaining the structural integrity and function of the Golgi apparatus.

The cell biology of the prohormone convertases PC1 and PC2

Mature peptide hormones and neuropeptides are typically synthesized from much larger precursors and require several posttranslational processing steps--including proteolytic cleavage--for the formation of the bioactive species. The subtilisin-related proteolytic enzymes that accomplish neuroendocrine-specific cleavages are known as prohormone convertases 1 and 2 (PC1 and PC2). The cell biology of these proteases within the regulated secretory pathway of neuroendocrine cells is complex, and they are themselves initially synthesized as inactive precursor molecules. ProPC1 propeptide cleavage occurs rapidly in the endoplasmic reticulum, yet its major site of action on prohormones takes place later in the secretory pathway. PC1 undergoes an interesting carboxyl terminal processing event whose function appears to be to activate the enzyme. ProPC2, on the other hand, exhibits comparatively long initial folding times and exits the endoplasmic reticulum without propeptide cleavage, in association with the neuroendocrine-specific protein 7B2. Once the proPC2/7B2 complex arrives at the trans-Golgi network, 7B2 is internally cleaved into two domains, the 21-kDa fragment and a carboxy-terminal 31 residue peptide. PC2 propeptide removal occurs in the maturing secretory granule, most likely through autocatalysis, and 7B2 association does not appear to be directly required for this cleavage event. However, if proPC2 has not encountered 7B2 intracellularly, it cannot generate a catalytically active mature species. The molecular mechanism behind the intriguing intracellular association of 7B2 and proPC2 is still unknown, but may involve conformational rearrangement or stabilization of a proPC2 conformer mediated by a 36-residue internal segment of 21-kDa 7B2.

Neuroregulation of ProTRH biosynthesis and processing

This review presents an overview of the current knowledge on proTRH biosynthesis, its processing, its tissue distribution, and the role of known processing enzymes in proTRH maturation. The neuroendocrine regulation of TRH biosynthesis, the biological actions of its products, and the signal transduction and catabolic pathways used by those products are also reviewed. The widespread expression of proTRH, PC1, and PC2 rnRNAs in hypophysiotropic and extrahypophysiotropic areas of the brain, with their overlapping distribution in many areas, indicates the striking versatility provided by tissue-specific processing in generating quantitative and qualitative differences in nonTRH peptide products as well as TRH. Evidence is presented suggesting that differential processing for proTRH at the intracellular level is physiologically relevant. It is clear that control over the diverse range of proTRH-derived peptides within a specific cell is accomplished most from the regulation at the posttranslational level rather than the translational or transcriptional levels. Several examples supporting this hypothesis are presented in this review. A better understanding of proTRH-derived peptides role represents an exciting new frontier in proTRH research. These connecting sequences in between TRH molecules to form the precursor protein may function as structural or targeting elements that guide the folding and sorting of proTRH and its larger intermediates so that subsequent processing and secretion are properly regulated. The particular anatomical distribution of the proTRH end products, as well as regulation of their levels by neuroendocrine or pharmacological manipulations, supports a unique potential biologic role for these peptides.

Biosynthesis and packaging of carboxypeptidase D into nascent secretory vesicles in pituitary cell lines

Metallocarboxypeptidase D (CPD) is a membrane-bound trans-Golgi network (TGN) protein. In AtT-20 cells, CPD is initially produced as a 170-kDa endoglycosidase H-sensitive glycoprotein. Within 30 min of chase, the CPD increases to 180 kDa and is resistant to endoglycosidase H as a result of carbohydrate maturation. CPD also undergoes an activation step required for binding to a substrate affinity resin. Blocking the protein exit from the endoplasmic reticulum inhibits the increase in molecular mass but not the step required for affinity column binding, suggesting that enzyme activation precedes carbohydrate maturation and that these reactions occur in distinct intracellular compartments. Only the higher molecular weight mature CPD enters nascent secretory vesicles, which bud from the TGN of permeabilized AtT-20 and GH3 cells. The budding efficiency of CPD into vesicles is 2-3-fold lower than that of endogenous proopiomelanocortin in AtT-20 cells or prolactin in GH3 cells. In contrast, the packaging of a truncated form of CPD, which lacks the cytoplasmic tail and transmembrane domain, was similar to that of proopiomelanocortin. Taken together, the results support the proposal that CPD functions in the TGN in the processing of proteins that transit the secretory pathway and that the C-terminal region plays a major role in TGN retention.

Endoplasmic reticulum and trans-Golgi network generate distinct populations of Alzheimer beta-amyloid peptides

The excessive generation and accumulation of 40- and 42-aa beta-amyloid peptides (Abeta40/Abeta42) in selectively vulnerable brain regions is a major neuropathological feature of Alzheimer's disease. Abeta, derived by proteolytic cleavage from the beta-amyloid precursor protein (betaAPP), is normally secreted. However, recent evidence suggests that significant levels of Abeta also may remain inside cells. Here, we have investigated the subcellular compartments within which distinct amyloid species are generated and the compartments from which they are secreted. Three experimental approaches were used: (i) immunofluorescence performed in intact cortical neurons; (ii) sucrose gradient fractionation performed with mouse neuroblastoma cells stably expressing wild-type betaAPP695 (N2a695); and (iii) cell-free reconstitution of Abeta generation and trafficking from N2a695 cells. These studies demonstrate that: (i) Abeta40 (Abeta1-40 plus Abetax-40, where x is an NH2-terminal truncation) is generated exclusively within the trans-Golgi Network (TGN) and packaged into post-TGN secretory vesicles; (ii) Abetax-42 is made and retained within the endoplasmic reticulum in an insoluble state; (iii) Abeta42 (Abeta1-42 plus Abetax-42) is made in the TGN and packaged into secretory vesicles; and (iv) the amyloid peptides formed in the TGN consist of two pools (a soluble population extractable with detergents and a detergent-insoluble form). The identification of the organelles in which distinct forms of Abeta are generated and from which they are secreted should facilitate the identification of the proteolytic enzymes responsible for their formation.

The use of permeabilized cells to investigate secretory granule biogenesis

To investigate the mechanism of secretory granule biogenesis in endocrine cells, our laboratory used rat anterior pituitary GH3 cells which secrete growth hormone and prolactin. Here we describe a simple and rapid procedure for generating permeabilized cells to dissect molecular mechanisms involved in nascent secretory vesicle budding from the trans-Golgi network (TGN). Using this system, we demonstrate that vesicle budding is temperature, energy, and cytosol dependent; in addition, cytosol from a variety of cells, including yeast (Saccharomyces cerevisiae), can support vesicle release. The budding of nascent secretory vesicles from the TGN is stimulated by a phospholipase D activity that is associated with Golgi membranes. Our results suggest that phospholipid metabolism plays an important role in the release of nascent secretory vesicles from the TGN.