Regulated and constitutive secretion. Differential effects of protein synthesis arrest on transport of glycosaminoglycan chains to the two secretory pathways

Looking at time dependent differentiation of mesenchymal stem cells by culture media using MALDI-TOF-MS

Mesenchymal stem cells (MSCs) are multipotent cells which are popular in human regenerative medicine. These cells can renew themselves and differentiate into several specialized cell types including osteoblasts, adipocytes, and chondrocytes under physiological and experimental conditions. MSCs can secret a lot of components including proteins and metabolites. These components have significant effects on their surrounding cells and also can be used to characterize them. This characterization of multipotent MSCs plays a critical role in their therapeutic potential. The metabolic profile of culture media verified by applying matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF-MS) techniques. Also, the differentiation and development of MSCs have monitored through culture media metabolome or secretome (secreted metabolites). Totally, 24 potential metabolites were identified. Between them 12 metabolites are unique to BM-MSCs and 5 metabolites are unique to AD-MSCs. Trilineage differentiation including chondrocytes, osteoblasts, and adipocytes, as well as metabolites that are being differentiated, have been shown in different weeks. In the present study, the therapeutic effects of MSCs analyzed by decoding the metabolome for MSCs secretome via metabolic profiling using MALDI-TOF-MS techniques.

Application of Ru(II)-Catalyzed Enyne Cyclization in the Synthesis of Brefeldin A

The approach to brefeldin A described herein hinges on Ru(II)-catalyzed cycloisomerization of an enyne obtained by the reaction of an alkynylzinc reagent with an α-chloro sulfide. Other key steps include Mislow-Evans rearrangement, cross-metathesis, and macrocyclization using a Roush-Masamune protocol.

Engineered insulin secretion from neuroendocrine cells isolated from human thyroid

BACKGROUND

Insulin-secreting beta-like cells are vulnerable to diabetic autoimmunity. We hypothesized that human thyroid neuroendocrine (NE) cells could be engineered to secrete human insulin, be glucose-responsive, and avoid autoimmunity.

METHODS

Collagenase-digested thyroid tissue was cultured and subjected to size-based fluorescence-activated cell sorting. Insulin secretion and storage in NE cells transduced with viral vectors carrying an insulin sequence was assessed by enzyme-linked immunosorbent assay (ELISA) and immunogold transmission electron microscopy (TEM). Baseline mRNA expression was assessed by Illumina expression array analysis. Transduction with retrovirus expressing transcription factors PDX1, NGN3, MAFA, or HNF6 altered mRNA expression in a custom polymerase chain reaction (PCR) array. Gastrin-releasing peptide (GRP) in conditioned medium and cell lysates was determined by reverse transcription (RT)-PCR, ELISA, and immunohistochemistry.

RESULTS

Isolation yielded an average of 2.2 × 10(6) cells/g thyroid tissue, which stained for calcitonin/calcitonin gene-related protein, expressed genes consistent with NE origins, and secreted GRP. Transduced cells secreted 56 % and retained 48 % of total insulin produced. Immunogold TEM revealed insulin in secretory vesicles. PDX1, NGN3, and MAFA overexpression increased expression of genes typical for hepatocytes and beta cells. Overexpression of HNF6 also increased the message of genes critical for glucose sensing.

CONCLUSIONS

Human thyroid NE cells can produce human insulin, fractions of which are both secreted and retained in secretory granules. Overexpression of HNF6, PDX1, or NGN3 enhances expression of both hepatocyte and beta cell typical mRNAs, including the message of proteins critical for glucose sensing. These data suggest that reimplantation of engineered autologous NE cells may develop as a viable treatment for diabetes mellitus type 1.

Secretion of urocortin I by human glioblastoma cell lines, possibly via the constitutive pathway

Corticotropin-releasing factor (CRF) and its family of peptides, i.e., urocortins (UCNs), play a critical role in systemic and peripheral stress-response systems and are widely expressed not only in normal tissues but also in various types of cancer cells. Given limited understanding of the mechanism of UCN I secretion, we investigated the UCN I secretory pathway in human neural stem cells (HNSCs) and in two glioblastoma cell lines, e.g., A172 and U-138 MG. Immunoreactivities for CRF receptors were detected in A172 glioblastoma cells, but not in HNSCs or U-138 glioblastoma cells, while UCN I immunoreactivity was detected in A172 and U-138 MG glioblastoma cell lines by both light field and electron microscopy. Interestingly, electron microscopy revealed UCN I immunoreactivtiy in vesicle-like structures in the plasma membrane of the glioblastoma cells. Tracking of a hybrid fluorescent protein containing a UCN I signal peptide expressed in A172 human glioblastoma cells revealed that fluorescence in secretory granules could be decreased by cycloheximide (100μg/ml), indicating that the forward transport of secretory granules containing fluorescent protein was not altered by the inhibition of protein synthesis by cycloheximide. Retrograde transport and the fusion of fluorescent granules in A172 human glioblastoma cells was induced by brefeldin A (10μg/ml), indicating that UCN I secretory granules may be transported via the constitutive pathway. Based on these results, it appears that UCN I is secreted from human glioblastoma cells by exocytosis through constitutive secretory granules, indicating that transcription of UCN I mRNA may be correlated to secretion of UCN I protein.

Proteomic techniques for characterisation of mesenchymal stem cell secretome

Mesenchymal stem cells (MSCs) are multipotent cells with a substantial potential in human regenerative medicine due to their ability to migrate to sites of injury, capability to suppress immune response and accessibility in large amount from patient's own bone marrow or fat tissue. It has been increasingly observed that the transplanted MSCs did not necessarily engraft and differentiate at the site of injury but might exert their therapeutic effects through secreted trophic signals. The MSCs secrete a variety of autocrine/paracrine factors, called secretome, that support regenerative processes in the damaged tissue, induce angiogenesis, protect cells from apoptotic cell death and modulate immune system. The cell culture medium conditioned by MSCs or osteogenic, chondrogenic as well as adipogenic precursors derived from MSCs has become a subject of intensive proteomic profiling in the search for and identification of released factors and microvesicles that might be applicable in regenerative medicine. Jointly with the methods for MSC isolation, expansion and differentiation, proteomic analysis of MSC secretome was enabled recently mainly due to the extensive development in protein separation techniques, mass spectrometry, immunological methods and bioinformatics. This review describes proteomic techniques currently applied or prospectively applicable in MSC secretomics, with a particular focus on preparation of the secretome sample, protein/peptide separation, mass spectrometry and protein quantification techniques, analysis of posttranslational modifications, immunological techniques, isolation and characterisation of secreted vesicles and exosomes, analysis of cytokine-encoding mRNAs and bioinformatics.

Nitric oxide and guanylate cyclase signalling are differentially involved in gonadotrophin (LH) release responses to two endogenous GnRHs from goldfish pituitary cells

Nitric oxide synthase (NOS) immunoreactivity is present in goldfish gonadotrophs. The present study investigated whether two native goldfish gonadotrophin-releasing hormones (GnRHs), salmon (s)GnRH and chicken (c)GnRH-II, use NOS/nitric oxide (NO) and soluble guanylate cyclase (sGC)/cyclic (c)GMP/protein kinase G (PKG) signalling to stimulate maturational gonadotrophin [teleost gonadotrophin-II, luteinising hormone (LH)] release. In cell column perifusion experiments with dispersed goldfish pituitary cells, the application of three NOS inhibitors (aminoguanidine hemisulphate, 1400W and 7-nitroindazole) and two NO scavengers [2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) and rutin hydrate] reduced sGnRH-elicited, but not cGnRH-II-induced, LH increases. The NO donor sodium nitroprusside (SNP) increased NO production in goldfish pituitary cells in static incubation. SNP-stimulated LH release in column perifusion was attenuated by PTIO and the sGC inhibitor 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-oneon (ODQ), and additive to responses elicited by cGnRH-II, but not sGnRH. ODQ and the PKG inhibitor KT5823 decreased sGnRH- and cGnRH-II-stimulated LH release. Similarly, the LH response to dibutyryl cGMP was reduced by KT5823. These results indicate that, although only sGnRH uses the NOS/NO pathway to stimulate LH release, both GnRHs utilise sGC/PKG to increase LH secretion.

Cysteine Cathepsins in the secretory vesicle produce active peptides: Cathepsin L generates peptide neurotransmitters and cathepsin B produces beta-amyloid of Alzheimer's disease

Recent new findings indicate significant biological roles of cysteine cathepsin proteases in secretory vesicles for production of biologically active peptides. Notably, cathepsin L in secretory vesicles functions as a key protease for proteolytic processing of proneuropeptides (and prohormones) into active neuropeptides that are released to mediate cell-cell communication in the nervous system for neurotransmission. Moreover, cathepsin B in secretory vesicles has been recently identified as a β-secretase for production of neurotoxic β- amyloid (Aβ) peptides that accumulate in Alzheimer's disease (AD), participating as a notable factor in the severe memory loss in AD. These secretory vesicle functions of cathepsins L and B for production of biologically active peptides contrast with the well-known role of cathepsin proteases in lysosomes for the degradation of proteins to result in their inactivation. The unique secretory vesicle proteome indicates proteins of distinct functional categories that provide the intravesicular environment for support of cysteine cathepsin functions. Features of the secretory vesicle protein systems insure optimized intravesicular conditions that support the proteolytic activity of cathepsins. These new findings of recently discovered biological roles of cathepsins L and B indicate their significance in human health and disease. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Analysis of low abundance membrane-associated proteins from rat pancreatic zymogen granules

Zymogen granules (ZG) are specialized storage organelles in the exocrine pancreas that allow the sorting, packaging, and regulated apical secretion of digestive enzymes. As there is a critical need for further understanding of the key processes in regulated secretion to develop new therapeutic options in medicine, we applied a suborganellar proteomics approach to identify peripheral membrane-associated ZG proteins. We focused on the analysis of a "basic" group (pH range 6.2-11) with about 46 spots among which 44 were identified by tandem mass spectrometry. These spots corresponded to 16 unique proteins, including rat mast cell chymase (RMCP-1) and peptidyl-prolyl cis-trans isomerase B (PpiB; cyclophilin B), an ER-resident protein. To confirm that these proteins were specific to zymogen granules and not contaminants of the preparation, we conducted a series of validation experiments. Immunoblotting of ZG subfractions revealed that chymase and PpiB behaved like bona fide peripheral membrane proteins. Their expression in rat pancreas was regulated by feeding behavior. Ultrastructural and immunofluorescence studies confirmed their ZG localization. Furthermore, a chymase-YFP fusion protein was properly targeted to ZG in pancreatic AR42J cells. Interestingly, for both proteins, proteoglycan-binding properties have been reported. The importance of our findings for sorting and packaging during ZG formation is discussed.

Murder by insulin: suspected, purported and proven-a review

Murder by insulin-whether attempted, suspected or proven-is rare. Only 66 cases worldwide could be found for this review. A conviction was secured in 31 cases and additional weapon was employed in 11. Differentiation of attempted homicide from Munchausen syndrome by proxy in the young and from 'mercy killing' in the elderly was not attempted. Most perpetrators were close relatives and most victims were alive when discovered and responded to treatment. Hypoglycaemia is the first clue to homicidal insulin use in living subjects and requires the demonstration of a plasma insulin concentration of generally more than 1000 pmol/L and undetectable plasma C-peptide concentration to establish the diagnosis. Serum glucose measurements are valueless in victims found dead. The presence near the body of insulin vials, syringes or needles, loose talk by the suspected perpetrator or their ready access to insulin may be the only clue. The demonstration of insulin in tissue around an injection site by immunohistopathology or by measuring it in an extract clinches the diagnosis. Immunoassays suitable for clinical use to detect and measure insulin and C-peptide are subject to random errors and cannot be relied upon unless special precautions including separation by gel filtration or HPLC are undertaken prior to analysis. They do not detect or measure accurately a new generation of synthetic insulin analogues. Mass spectrometry will be required to do this and to validate clinical immunoassays, upon which convictions have always had to rely in the past.

The trans-Golgi proteins SCLIP and SCG10 interact with chromogranin A to regulate neuroendocrine secretion

Secretion of proteins and peptides from eukaryotic cells takes place by both constitutive and regulated pathways. Regulated secretion may involve interplay of proteins that are currently unknown. Recent studies suggest an important role of chromogranin A (CHGA) in the regulated secretory pathway in neuroendocrine cells, but the mechanism by which CHGA enters the regulated pathway, or even triggers the formation of the pathway, remains unclear. In this study, we used a transcriptome/proteome-wide approach, to discover binding partners for CHGA, by employing a phage display cDNA library method. Several proteins within or adjacent to the secretory pathway were initially detected as binding partners of recombinant human CHGA. We then focused on the trans-Golgi protein SCLIP (STMN3) and its stathmin paralog SCG10 (STMN2) for functional study. Co-immunoprecipitation experiments confirmed the interaction of each of these two proteins with CHGA in vitro. SCLIP and SCG10 were colocalized to the Golgi apparatus of chromaffin cells in vivo and shared localization with CHGA as it transited the Golgi. Downregulation of either SCLIP or SCG10 by synthetic siRNAs virtually abolished chromaffin cell secretion of a transfected CHGA-EAP chimera (expressing CHGA fused to an enzymatic reporter, and trafficked to the regulated pathway). SCLIP siRNA also decreased the level of secretion of endogenous CHGA and SCG2, as well as transfected human growth hormone, while SCG10 siRNA decreased the level of regulated secretion of endogenous CHGB. Moreover, a dominant negative mutant of SCG10 (Cys 22,Cys 24-->Ala 22,Ala 24) significantly blocked secretion of the transfected CHGA-EAP chimera. A decrease in the buoyant density of chromaffin granules was observed after downregulation of SCG10 by siRNA, suggesting participation of these stathmins in granule formation or maturation. We conclude that SCLIP and SCG10 interact with CHGA, share partial colocalization in the Golgi apparatus, and may be necessary for typical transmitter storage and release from chromaffin cells.

Protease pathways in peptide neurotransmission and neurodegenerative diseases

1. Recent research demonstrates the critical importance of neuroproteases for the production of peptide neurotransmitters, and for the production of toxic peptides in major neurodegenerative diseases that include Alzheimer's (AD) and Huntington's diseases. This review describes the strategies utilized to identify the appropriate proteases responsible for producing active peptides for neurotransmission, with application of such approaches for defining protease mechanisms in neurodegenerative diseases. 2. Integration of multidisciplinary approaches in neurobiology, biochemistry, chemistry, proteomics, molecular biology, and genetics has been utilized for neuroprotease studies. These investigations have identified secretory vesicle cathepsin L for the production of the enkephalin opioid peptide neurotransmitter and other neuropeptides. Furthermore, new results using these strategies have identified secretory vesicle cathepsin B for the production of beta-amyloid (Abeta) in the major regulated secretory pathway that provides activity-dependent secretion of Abeta peptides, which accumulate in AD. 3. CNS neuroproteases that participate in peptide neurotransmission and in neurodegenerative diseases represent new candidate drug targets that may be explored in future research for the development of novel therapeutic agents for neurological conditions.

A Model System to Study Intracellular Trafficking and Processing of the Alzheimer’s Amyloid Precursor Protein

The occurrence of consensus phosphorylation sites in the intracellular domain of the Alzheimer's amyloid precursor protein (APP), coupled with observations of their in vivo phosphorylation, prompted several workers to investigate the effects that phosphorylation of such sites could have on APP metabolism and subsequent Abeta production. However, hitherto all attempts to dissect the role played by such phosphorylation events failed to reveal substantial effects. Having decided to revisit this problem, our new approach was based on the following vectors: (1) site-directed mutagenesis of the target amino acids to mimic a specific phosphorylation state, (2) expression of wild-type and mutant APP-GFP (green fluorescent protein) fusion proteins for ease of visualization, (3) controlled low level expression to avoid 'flooding' cellular pathways, and (4) the use of cycloheximide to inhibit de novo protein synthesis. Using this method we were able to detect specific differences in APP processing that were correlated with the mimicked phosphorylation state of several phosphorylation sites. New combined methodologies, like the one described here, allow for the detailed analysis of key control points in the cellular metabolism of specific proteins that are central to neurodegenerative diseases and may be under the control of specific posttranslational modifications, such as reversible phosphorylation.

Characterisation of an evolutionary conserved protein interacting with the putative guanine nucleotide exchange factor DelGEF and modulating secretion

A human cDNA library was screened for proteins interacting with the deafness locus putative guanine nucleotide exchange factor (DelGEF) using a yeast two-hybrid system. A protein with a predicted size of 9 kDa was identified as a binding partner, this protein was designated DelGEF interacting protein 1 (DelGIP1). The interaction between DelGEF and DelGIP1 was verified by co-immunoprecipitation of a DelGEF-DelGIP1 complex from cell lysates. Highly conserved homologues of DelGIP1 were identified in higher and lower eukaryotes by database searching. The human DelGIP1 gene is ubiquitously expressed as judged by human multiple tissue Northern blot analysis. DelGEF was recently shown to interact with Sec5, a protein involved in secretion, and to regulate secretion of proteoglycans. Downregulation of endogenous DelGIP1 in HeLa cells induced increased extracellular secretion of proteoglycans indicating a possible role for DelGIP1 in the secretion process.

Membrane targeting in secretion

Regulated secretion and exocytosis require the selective packaging of regulated secretory proteins in secretory storage organelles and the controlled docking and fusion of these organelles with the plasma membrane. Secretory granule biogenesis involves sorting of secretory proteins and membrane components both at the level of the trans-Golgi network and the immature secretory granule. Sorting is thought to be mediated by selective protein aggregation and the interaction of these proteins with specific membrane domains. There is now considerable interest in the understanding of the complex lipid-protein and protein-protein interactions at the trans-Golgi network and the granule membrane. A role for lipid microdomains and associated sorting receptors in membrane targeting and granule formation is vividly discussed for (neuro)endocrine cells. In exocrine cells, however, little has been known of granule membrane composition and membrane protein function. With the cloning and characterization of granule membrane proteins and their interactions at the inner leaflet of zymogen granules of pancreatic acinar cells, it is now possible to elucidate their function in membrane targeting and sorting of zymogens at the molecular level.

Cysteine proteases are the major ?-secretase in the regulated secretory pathway that provides most of the ?-amyloid in Alzheimer's disease: Role of BACE 1 in the constitutive secretory pathway

This article focuses on beta-amyloid (Abeta) peptide production and secretion in the regulated secretory pathway and how this process relates to accumulation of toxic Abeta in Alzheimer's disease. New findings are presented demonstrating that most of the Abeta is produced and secreted, in an activity-dependent manner, through the regulated secretory pathway in neurons. Only a minor portion of cellular Abeta is secreted via the basal, constitutive secretory pathway. Therefore, regulated secretory vesicles contain the primary beta-secretases that are responsible for producing the majority of secreted Abeta. Investigation of beta-secretase activity in regulated secretory vesicles of neuronal chromaffin cells demonstrated that cysteine proteases account for the majority of the beta-secretase activity. BACE 1 is present in regulated secretory vesicles but provides only a small percentage of the beta-secretase activity. Moreover, the cysteine protease activities prefer to cleave the wild-type beta-secretase site, which is relevant to the majority of AD cases. In contrast, BACE 1 prefers to cleave the Swedish mutant beta-secretase site that is expressed in a minor percentage of the AD population. These new findings lead to a unifying hypothesis in which cysteine proteases are the major beta-secretases for the production of Abeta in the major regulated secretory pathway and BACE 1 is the beta-secretase responsible for Abeta production in the minor constitutive secretory pathway. These results indicate that inhibition of multiple proteases may be needed to decrease Abeta production as a therapeutic strategy for Alzheimer's disease.

DelGEF, a homologue of the Ran guanine nucleotide exchange factor RanGEF, binds to the exocyst component Sec5 and modulates secretion

In order to identify the function of deafness locus putative guanine nucleotide exchange factor (DelGEF), a protein homologous to the nucleotide exchange factor for the small GTPase Ran, a cDNA library was screened for interacting proteins using a yeast two-hybrid system. The human homologue of Sec5, a protein involved in vesicle transport and secretion, was identified as a binding partner. The interaction between DelGEF and Sec5 was found to be dependent on Mg2+ and stimulated by guanosine triphosphate (GTP) or deoxycytidine triphosphate (dCTP). Downregulation of endogenous DelGEF in HeLa cells induced increased extracellular secretion of proteoglycans indicating a possible role for DelGEF in the secretion process.

Activation of mucin exocytosis and upregulation of MUC genes in polarized human intestinal mucin-secreting cells by the thiol-activated exotoxin listeriolysin O

The secreted thiol-activated cytolysin listeriolysin O (LLO) was responsible for L. monocytogenes-induced high-molecular glycoproteins (HMGs) exocytosis in cultured human mucosecreting HT29-MTX cells. By biochemical analysis we demonstrate that the majority of secreted HMGs in LLO-stimulated cells are of mucin origin. In parallel, analysis of the expression of MUCs genes showed that the transcription of the MUC3, MUC4 and MUC12 genes encoding for membrane-bound mucins was increased in LLO-stimulated cells. Upregulation of the MUC3 gene correlates with an increased expression of the membrane-bound MUC3 mucin. In contrast, increase in secretion of the gel-forming MUC5AC mucin develops without upregulation of the MUC5AC gene. Finally, results showed that NF-kappaB and AP-1 transcription factors were not involved in LLO-induced upregulation of MUCs genes in HT29-MTX cells, whereas L. monocytogenes infection was able to promote the degradation of IkappaB proteins in the cells.

Dibasic cleavage site is required for sorting to the regulated secretory pathway for both pro- and neuropeptide Y

To investigate the signals governing routing of biologically active peptides to the regulated secretory pathway, we have expressed mutated and non-mutated proneuropeptide Y (ProNPY) in pituitary-derived AtT20 cells. The mutations were carried out on dibasic cleavage site and or ProNPY C-terminal sequence. Targeting to the regulated secretory pathway was studied using protein kinase A (8-BrcAMP), protein kinase C (phorbol myristate acetate) specific activators and protein synthesis inhibitor cycloheximide, and by pulse chase. The analysis of expressed peptides in cells and culture media indicated that: neuropeptide Y (NPY) and ProNPY were differently secreted, whilst NPY was exclusively secreted via regulatory pathway; ProNPY was secreted via regulated and constitutive-like secretory pathways. ProNPY secretion behaviour was not Proteolytic cleavage efficiency-dependent. The dibasic cleavage was essential for ProNPY and NPY cAMP-dependent regulated secretion and may have function as a retention signal.

Beta-amyloid peptide in regulated secretory vesicles of chromaffin cells: evidence for multiple cysteine proteolytic activities in distinct pathways for beta-secretase activity in chromaffin vesicles

A key factor in Alzheimer's disease (AD) is the beta-secretase activity that is required for the production of beta-amyloid (Abeta) peptide from its amyloid precursor protein (APP) precursor. In this study, the majority of Abeta secretion from neuronal chromaffin cells was found to occur via the regulated secretory pathway, compared with the constitutive secretory pathway; therefore, beta-secretase activity in the regulated secretory pathway was examined for the production and secretion of Abeta in chromaffin cells obtained from in vivo adrenal medullary tissue. The presence of Abeta(1-40) in APP-containing chromaffin vesicles, which represent regulated secretory vesicles, was demonstrated by radioimmunoassay (RIA) and reverse-phase high-performance liquid chromatography. These vesicles also contain Abeta(1-42), measured by RIA. Significantly, regulated secretion of Abeta(1-40) from chromaffin cells represented the majority of secreted Abeta (> 95% of total secreted Abeta), compared with low levels of constitutively secreted Abeta(1-40). These results indicate the importance of Abeta production and secretion in the regulated secretory pathway as a major source of extracellular Abeta. Beta-secretase activity in isolated chromaffin vesicles was detected with the substrate Z-Val-Lys-Met-/MCA (methylcoumarinamide) that contains the beta-secretase cleavage site. Optimum beta-secretase activity in these vesicles required reducing conditions and acidic pH (pH 5-6), consistent with the in vivo intravesicular environment. Evidence for cysteine protease activity was shown by E64c inhibition of Z-Val-Lys-Met-MCA-cleaving activity, and E64c inhibition of Abeta(1-40) production in isolated chromaffin vesicles. Chromatography resolved the beta-secretase activity into two distinct proteolytic pathways consisting of: (i) direct cleavage of the beta-secretase site at Met-/Asp by two cysteine proteolytic activities represented by peaks Il-A and Il-B, and (ii) an aminopeptidase-dependent pathway represented by peak I cysteine protease activity that cleaves between Lys-/Met, followed by Met-aminopeptidase that would generate the beta-secretase cleavage site. Treatment of chromaffin cells in primary culture with the cysteine protease inhibitor E64d reduced the production of the beta-secretase product, a 12-14 kDa C-terminal APP fragment. In addition, BACE 1 and BACE 2 were detected in chromaffin vesicles; BACE 1 represented a small fraction of total beta-secretase activity in these vesicles. These results illustrate that multiple cysteine proteases, in combination with BACE 1, contribute to beta-secretase activity in the regulated secretory pathway. These results complement earlier findings for BACE 1 as beta3-secretase for Abeta production in the constitutive secretory pathway that provides basal secretion of Abeta into conditioned media. These findings suggest that drug inhibition of several proteases may be required for reducing Abeta levels as a potential therapeutic approach for AD.

Signal transduction mechanisms mediating secretion in goldfish gonadotropes and somatotropes

The intracellular signal transduction mechanisms mediating maturational gonadotropin and somatotropin secretion in goldfish are reviewed. Several major signaling mechanisms, including changes in intracellular [Ca2+], arachidonic acid cascades, protein kinase C, cyclic AMP/protein kinase A, calmodulin, nitric oxide, and Na+/H+ antiport, are functional in both cell types. However, their relative importance in mediating basal secretion and neuroendocrine-factor-regulated hormone release differs according to cell type. Similarly, agonist- and cell-type-specificity are also present in the transduction pathways leading to neuroendocrine factor-modulated maturational gonadotropin and somatotropin release. Specificity is present not only in the actions of different regulators within the same cell type and with the same ligand in the two cell types, but this also exists between isoforms of the same neuroendocrine factor within a single cell type. Other evidence suggests that function-selectivity of signaling may also result from differential modulation of Ca2+ fluxes from different sources. The interaction of different second messenger systems provide the basis by which regulation of maturational gonadotropin and somatotropin release by multiple neuroendocrine factors can be integrated at the target cell level.

Key words: Ca2+ signaling, cAMP, PKC, arachidonic acid, NO.

Regulation of protein sorting at the TGN by plasma membrane receptor activation

We show that in the rat basophilic leukemia cell line RBL, the physiological stimulation of the IgE receptor or direct activation of PKC leads to the missorting of proteins to the plasma membrane, diverting them from their normal intracellular destination. This is demonstrated for two classes of proteins that are normally targeted to the secretory lysosomes via completely different mechanisms, i.e. proteoglycans and the aspartic protease cathepsin D. In the latter case, normal processing of the enzyme is also affected, leading to secretion of the immature form of cathepsin. The present study shows how completely different sorting mechanisms, such as those for delivering proteoglycans and cathepsin D to secretory lysosomes, might share common regulatory signals and are similarly affected when the levels of these signals are perturbed. Finally, protein kinase C appears to be a major player in the signal transduction pathways, leading to proteoglycan and cathepsin D missorting.

Synthesis and sorting of proteoglycans

Proteoglycans are widely expressed in animal cells. Interactions between negatively charged glycosaminoglycan chains and molecules such as growth factors are essential for differentiation of cells during development and maintenance of tissue organisation. We propose that glycosaminoglycan chains play a role in targeting of proteoglycans to their proper cellular or extracellular location. The variability seen in glycosaminoglycan chain structure from cell type to cell type, which is acquired by use of particular Ser-Gly sites in the protein core, might therefore be important for post-synthesis sorting. This links regulation of glycosaminoglycan synthesis to the post-Golgi fate of proteoglycans.

Externalization of tropomyosin isoform 5 in colon epithelial cells

Ulcerative colitis (UC) is associated with autoantibody response to a cytoskeletal protein, human tropomyosin (hTM) isoform-5 (hTM5). Because hTM5 is an intracellular protein, it may remain inaccessible to the autoantibodies. Therefore, we have investigated the possibility of externalization of hTM5 in colon epithelial cells. Freshly isolated colonic and small intestinal epithelial cells and LS-180 colon cancer cell line were examined for surface expression of hTM5 by flow cytometric analysis using hTM isoform-specific MoAbs. The extracellular release of hTM5 was determined by Western blot and radioimmunoprecipitation analyses. Physical association of hTM5 with a membrane-associated colon epithelial protein (CEP) was examined by co-immunoprecipitation of hTM5 with anti-CEP MoAb, and CEP with anti-hTM5 MoAb. Cell surface expression of hTM5 was observed in colonic epithelial and LS-180 cells but not in small intestinal epithelial cells. LS-180 cells spontaneously released hTM5 as well as CEP into the culture medium that was significantly stimulated by a calcium ionophore, A23187, but inhibited by phorbol-12-myristate-13-acetate, monensin and methylamine. Co-immunoprecipitation experiments revealed that hTM5 forms a complex with CEP. We conclude that hTM5 is externalized in colon but not in small intestinal epithelial cells. The physical association of hTM5 with CEP suggests a possible chaperone function of CEP in the transport of hTM5, a putative target autoantigen in UC.

Morphological and biochemical analysis of the secretory pathway in melanoma cells with distinct metastatic potential

In this report, we have investigated whether alterations of the morphological and functional aspects of the biosecretory membrane system are associated with the metastatic potential of tumor cells. To this end, we have analyzed the morphology of the Golgi complex, the cytoskeleton organization and membrane trafficking steps of the secretory pathway in two human melanoma A375 cell line variants with low (A375-P) and high metastatic (A375-MM) potential. Immunofluorescence analysis showed that in A375-P cells, the Golgi complex showed a collapsed morphology. Conversely, in A375-MM cells, the Golgi complex presented a reticular and extended morphology. At the ultrastructural level, the Golgi complex of A375-P cells was fragmented and cisternae were swollen. When the cytoskeleton was analyzed, the microtubular network appeared normal in both cell variants, whereas actin stress fibers were largely absent in A375-P, but not in A375-MM cells. In addition, the F-actin content in A375-P cells was significantly lower than in A375-MM cells. These morphological differences in A375-P cells were accompanied by acceleration and an increase in the endoplasmic reticulum to Golgi and the trans-Golgi network to cell surface membrane transport, respectively. Our results indicate that in human A375 melanoma cells, metastatic potential correlates with a well-structured morphofunctional organization of the Golgi complex and actin cytoskeleton.

Kalirin, a multifunctional PAM COOH-terminal domain interactor protein, affects cytoskeletal organization and ACTH secretion from AtT-20 cells

The production and regulated secretion of bioactive peptides require a series of lumenal enzymes to convert inactive precursors into bioactive peptides plus several cytosolic proteins to govern granule formation, maturation, translocation, and exocytosis. Peptidylglycine alpha-amidating monooxygenase (PAM), an enzyme essential for biosynthesis of many peptides, is an integral membrane protein with trafficking information in both its lumenal and cytosolic domains. Kalirin, a PAM cytosolic domain interactor protein with spectrin-like repeats and GDP/GTP exchange factor activity for Rac1, is expressed with PAM in neurons but is not expressed in the anterior pituitary or AtT-20 corticotrope cells. Expression of Kalirin alters the cytoskeletal organization of Chinese hamster ovary and AtT-20 cells expressing membrane PAM. Expression of membrane PAM also alters cytoskeletal organization, demonstrating the presence of endogenous proteins that can mediate this effect. Significant amounts of both PAM and Kalirin fractionate with cytoskeletal elements. Since cytoskeletal organization is critical for exocytosis, constitutive-like and regulated secretions were evaluated. Whereas the constitutive-like secretion of adrenocorticotropic hormone (ACTH) is increased by expression of membrane PAM, regulated secretion is eliminated. Expression of Kalirin in AtT-20 cells expressing membrane PAM restores stimulated secretion of ACTH. Thus, Kalirin or its homologue may be essential for regulated secretion, and the PAM-Kalirin interaction may coordinate intragranular with cytosolic events.

Essential role of the disulfide-bonded loop of chromogranin B for sorting to secretory granules is revealed by expression of a deletion mutant in the absence of endogenous granin synthesis

Sorting of regulated secretory proteins in the TGN to immature secretory granules (ISG) is thought to involve at least two steps: their selective aggregation and their interaction with membrane components destined to ISG. Here, we have investigated the sorting of chromogranin B (CgB), a member of the granin family present in the secretory granules of many endocrine cells and neurons. Specifically, we have studied the role of a candidate structural motif implicated in the sorting of CgB, the highly conserved NH2-terminal disulfide- bonded loop. Sorting to ISG of full-length human CgB and a deletion mutant of human CgB (Deltacys-hCgB) lacking the 22-amino acid residues comprising the disulfide-bonded loop was compared in the rat neuroendocrine cell line PC12. Upon transfection, i.e., with ongoing synthesis of endogenous granins, the sorting of the deletion mutant was only slightly impaired compared to full-length CgB. To investigate whether this sorting was due to coaggregation of the deletion mutant with endogenous granins, we expressed human CgB using recombinant vaccinia viruses, under conditions in which the synthesis of endogenous granins in the infected PC12 cells was shut off. In these conditions, Deltacys-hCgB, in contrast to full-length hCgB, was no longer sorted to ISG, but exited from the TGN in constitutive secretory vesicles. Coexpression of full-length hCgB together with Deltacys-hCgB by double infection, using the respective recombinant vaccinia viruses, rescued the sorting of the deletion mutant to ISG. In conclusion, our data show that (a) the disulfide-bonded loop is essential for sorting of CgB to ISG and (b) the lack of this structural motif can be compensated by coexpression of loop-bearing CgB. Furthermore, comparison of the two expression systems, transfection and vaccinia virus-mediated expression, reveals that analyses under conditions in which host cell secretory protein synthesis is blocked greatly facilitate the identification of sequence motifs required for sorting of regulated secretory proteins to secretory granules.

Sorting of the Specific Granule Protein, NGAL, During Granulocytic Maturation of HL-60 Cells

The different types of human neutrophil granules (azurophil, specific, and gelatinase granules) are formed sequentially during maturation of neutrophils from the promyelocyte stage to the band cell stage. The promyelocytic HL-60 cells can maturate to segmented granulocytes but are incapable of activating the transcription of any known intragranular protein, normally located in specific or gelatinase granules. To study the sorting of granule proteins during maturation, we transfected HL-60 cells with the specific granule protein NGAL, inserted under control of a cytomegalovirus promoter. We previously showed that NGAL is sorted to azurophil granules and colocalizes with myeloperoxidase in undifferentiated HL-60 cells. We show here that, when such transfected HL-60 cells differentiate into granulocytes, newly synthesized NGAL is not retained in granules but is constitutively secreted. This indicates that highly specific mechanisms must exist that are responsible for diverting transport vesicles into storage granules, and that HL-60 cells not only lack the ability to activate transcription of specific granule proteins, but also lose the ability to form storage granules during maturation.

The regulated secretion and vectorial targeting of neurotrophins in neuroendocrine and epithelial cells

The varied roles that neurotrophins play in the development and activity-dependent plasticity of the nervous system presumably require that the sites and quantity of neurotrophin release be precisely regulated. As a step toward understanding how different neurotrophins are sorted and secreted by neurons, we expressed nerve growth factor (NGF), brain-derived neurotrophic factor, and neurotrophin-3 in cell lines used as models for neuronal protein sorting. All three neurotrophins were secreted by a regulated pathway in transfected AtT-20 and PC12 neuroendocrine cells, with a 3-6-fold increase in neurotrophin release in response to 8-bromo-cAMP or depolarization, respectively. To determine if the propeptide directs the intracellular sorting of mature NGF, we examined mutants in which regions spanning the propeptide were deleted. These mutants underwent regulated release in every case in which expression could be detected. Similarly, NGF sorting was not significantly altered by mutations which specifically abolished N-glycosylation or proteolytic processing sites within the NGF precursor. Finally, we found that all three neurotrophins were secreted 65-75% basolaterally by polarized Madin-Darby canine kidney epithelial cells. These findings suggest that the determinants of regulated neurotrophin secretion lie within the mature neurotrophin moiety and that NGF, brain-derived neurotrophic factor, and neurotrophin-3 are likely to be sorted similarly and released in a regulated manner by neurons.

Coat proteins and vesicle budding

The trafficking of proteins within eukaryotic cells is achieved by the capture of cargo and targeting molecules into vesicles that bud from a donor membrane and deliver their contents to a receiving department. This process is bidirectional and may involve multiple organelles within a cell. Distinct coat proteins mediate each budding event, serving both to shape the transport vesicle and to select by direct or indirect interaction the desired set of cargo molecules. Secretion, which has been viewed as a default pathway, may require sorting and packaging signals on transported molecules to ensure their rapid delivery to the cell surface.

Regulation of constitutive exocytic transport by membrane receptors. A biochemical and morphometric study

Biochemical and morphometric approaches were combined to examine whether constitutive secretory transport might be controlled by plasma membrane receptors, as this possibility would have significant physiological implications. Indeed, IgE receptor stimulation in rat basophilic leukemia cells potently increased the rate of transport of soluble pulse-labeled 35S-sulfated glycosaminoglycans from distal Golgi compartments to the cell surface. This effect was largely protein kinase C (PKC)-dependent. Direct activation of PKC also stimulated constitutive transport of glycosaminoglycans, as indicated by the use of agonistic and antagonistic PKC ligands. PKC ligands also had potent, but different, effects on the exocytic transport from distal Golgi compartments to the plasma membrane of a membrane-bound protein (vesicular stomatitis virus glycoprotein), which was slightly stimulated by activators and profoundly suppressed by inhibitors of PKC. Morphological analysis showed impressive changes of the organelles of the secretory pathway in response to IgE receptor stimulation and to direct PKC activation (enhanced number of buds and vesicles originating from the endoplasmic reticulum and Golgi and increase in surface and volume of Golgi compartments), suggestive of an overall activation of exocytic movements. These results show that rapid and large changes in constitutive transport fluxes and in the morphology of the exocytic apparatus can be induced by membrane receptors (as well as by direct PKC stimulation).

Phosphorylation of vimentin is an intermediate step in protein kinase C-mediated glycoconjugate secretion

We have previously shown that fibroblasts from patients with cystic fibrosis (CF) display a higher response to 4 beta-phorbol 12-myristate 13-acetate (PMA) than control fibroblasts for stimulation of both protein kinase C (PKC) cytosol-to-membrane translocation and glycoconjugate secretion. In this study we took advantage of these cells with differential responsiveness to PMA to investigate the endogenous substrate(s) involved in PKC stimulation of glycoconjugate secretion after verification of cystic fibrosis transmembrane conductance regulator gene expression in control and CF fibroblasts. We show that a 57-kDa protein that was associated with cytoskeleton and was identified as vimentin by immunoblotting emerged as a good candidate for mediating PKC stimulation of glycoconjugate secretion. 1) Its phosphorylation by PMA was abolished by PKC inhibition or depletion. 2) In both control and CF fibroblasts, the PMA-induced increase in its phosphorylation preceded the phorbol ester stimulation of glycoconjugate secretion. 3) For both processes, the concentration-response curves were superimposable, with higher maximal levels for CF fibroblasts relative to controls. 4) PMA-stimulated 57-kDa protein phosphorylation, like PMA-stimulated glycoconjugate secretion, was significantly increased by Ca2+. 5) Increased PMA phosphorylation of the 57-kDa protein as a result of okadaic acid inhibition of intracellular phosphatases was reflected in increased PMA stimulation of glycoconjugate secretion. In conclusion, 1) PMA phosphorylation of a cytoskeletal 57-kDa protein, identified as vimentin, appears to be an intermediate step in PKC stimulation of constitutive glycoconjugate secretion in human skin fibroblasts; and 2) this process is impaired in CF disease.

Molecular cloning and analysis of the protein modules of aggrecans

The large aggregating chondroitin sulfate proteoglycan of cartilage, aggrecan, has served as a prototype of proteoglycan structure. Molecular cloning has elucidated its primary structure and revealed both known and unknown domains. To date the complete structures of chicken, rat and human aggrecans have been deduced, while partial sequences have been reported for bovine aggrecan. A related proteoglycan, human versican, has also been cloned and sequenced. Both aggrecan and versican have two lectin domains, one at the amino-terminus which binds hyaluronic acid and one at the carboxyl-terminus whose physiological ligand is unknown. Both lectins have homologous counterparts in other types of proteins. Within the aggrecans the keratan sulfate domain may be variably present and also has a prominent repeat in some species. The chondroitin sulfate domain has three distinct regions which vary in their prominence in different species. The complex molecular structure of aggrecans is consistent with the concept of exon shuffling and aggrecans serve as suitable prototypes for comprehending the evolution of multi-domain proteins.

Inhibition of constitutive protein secretion from lactating mouse mammary epithelial cells by FIL (feedback inhibitor of lactation), a secreted milk protein

The effect of a protein feedback inhibitor of lactation (FIL) on casein synthesis and secretion was examined using isolated acini from lactating mouse mammary gland. As previously found, FIL partially inhibited protein synthesis but produced an additional inhibition of constitutive casein secretion. The inhibition of synthesis and secretion showed similar dose-dependency and the inhibition was fully reversible. Constitutive secretion of pre-formed protein was inhibited by FIL in a pulse-chase protocol, indicating that the inhibitor regulated protein secretion by reducing protein movement through the secretory pathway independently of any initial inhibition of synthesis. Regulated exocytosis was not inhibited since casein release due to elevation of cytosolic Ca2+ concentration by the ionophore ionomycin was unaffected. Brefeldin A, which is known to block ER-to-Golgi transport, also inhibited both protein synthesis and secretion in mammary cells. The action of FIL on synthesis and secretion and previously described actions on casein degradation would be consistent with a block at an early stage in the secretory pathway. In support of this idea FIL treatment was found to result in vesiculation and swelling of the endoplasmic reticulum. These data provide evidence for a novel control of a constitutive secretory pathway by a physiological extracellular regulatory protein.

Molecular cloning and analysis of the protein modules of aggrecans

The large aggregating chondroitin sulfate proteoglycan of cartilage, aggrecan, has served as a prototype of proteoglycan structure. Molecular cloning has elucidated its primary structure and revealed both known and unknown domains. To date the complete structures of chicken, rat and human aggrecans have been deduced, while partial sequences have been reported for bovine aggrecan. A related proteoglycan, human versican, has also been cloned and sequenced. Both aggrecan and versican have two lectin domains, one at the amino-terminus which binds hyaluronic acid and one at the carboxyl-terminus whose physiological ligand is unknown. Both lectins have homologous counterparts in other types of proteins. Within the aggrecans the keratan sulfate domain may be variably present and also has a prominent repeat in some species. The chondroitin sulfate domain has three distinct regions which vary in their prominence in different species. The complex molecular structure of aggrecans is consistent with the concept of exon shuffling and aggrecans serve as suitable prototypes for comprehending the evolution of multi-domain proteins.