What regulates secretion of non-stored proteins by eukaryotic cells?

Optofluidic platform for real-time monitoring of live cell secretory activities using Fano resonance in gold nanoslits

An optofluidic platform for real-time monitoring of live cell secretory activities is constructed via Fano resonance in a gold nanoslit array. Large-area and highly sensitive gold nanoslits with a period of 500 nm are fabricated on polycarbonate films using the thermal-annealed template-stripping method. The coupling between gap plasmon resonance in the slits and surface plasmon polariton Bloch waves forms a sharp Fano resonance with intensity sensitivity greater than 11 000% per refractive index unit. The nanoslit array is integrated with a cell-trapping microfluidic device to monitor dynamic secretion of matrix metalloproteinase 9 (MMP-9) from human acute monocytic leukemia cells in situ. Upon continuous lipopolysaccharide (LPS) stimulation, MMP-9 secretion is detected within 2 h due to ultrahigh surface sensitivity and close proximity of the sensor to the target cells. In addition to the advantage of detecting early cell responses, the sensor also allows interrogation of cell secretion dynamics. Furthermore, the average secretion per cell measured using our system well matches previous reports while it requires orders of magnitude less cells. The optofluidic platform may find applications in fundamental studies of cell functions and diagnostics based on secretion signals.

Synthesis of vesicle cargo determines amplitude of Ca(2+)-sensitive exocytosis

Here we examine the potential coupling between the synthesis of secretory proteins and the sensitivity of exocytosis to the concentration of free Ca(2+) in the cytosol ([Ca(2+)](i)) in plant cell. We therefore monitor in tobacco protoplasts the excursion of the membrane capacitance in response to an elevation of [Ca(2+)](i) as a measure for exocytotic activity. The data show that a ramp like elevation of [Ca(2+)](i) generates in protoplasts from wild type plants and from transgenic plants, which overexpress the secreted α-amylase, an exocytotic burst with an initial steep and a subsequent slow phase. The largest capacitive burst is obtained in α-amylase producing plants and the amplitude of the [Ca(2+)](i) evoked C(m) excursion is a function of the amylase synthesis of the plants. The data support a model according to which plant cells have at least two serial [Ca(2+)](i) sensitive processes in the final steps of their exocytotic pathway. The overproduction of a secreted cargo does not affect the kinetics of this process but the number of vesicles in pools upstream of the [Ca(2+)](i) sensitive steps.

Oxidative stress and release of tissue plasminogen activator in isolated rat hearts

To evaluate the potential of tissue plasminogen activator (t-PA) as a marker of endothelial activation or injury, the dose-response relationship between reactive oxygen intermediates and t-PA release was investigated in isolated rat hearts. After stabilization the hearts were perfused for 10 minutes with different concentrations of hydrogen peroxide (H2O2) (0 (control perfusion), 20, 40, 80, 120, 160, or 200 microM) (n = 8 hearts/group), followed by 30 minutes recovery. Higher concentrations than 80 microM induced cardiac dysfunction and a dose-dependent release of lactate dehydrogenase, indicating myocyte injury. H2O2-concentrations of 80 microM and more caused a significant, but temporary t-PA release. Peak t-PA release occurred more rapidly with higher concentrations, but otherwise there was no difference dependent on the H2O2-dose. The effects of H2O2 (120 or 200 microM) on t-PA release were also compared to the effects of bradykinin. Both were given for 10 minutes as above, and the procedure was repeated after 10 minutes recovery. Bradykinin (50 or 500 nM) released t-PA with the same magnitude, but with peak values occurring earlier than t-PA release induced by H2O2. Bradykinin, but not H2O2, induced t-PA release during the second exposure, suggesting different mechanisms of release.

IN CONCLUSION

Perfusion with H2O2 leads to a dose-dependent myocardial injury in isolated rat hearts. H2O2 also causes an acute t-PA release without dose-dependency, suggesting an all or nothing response of the endothelium. t-PA may be used as an indicator of, but cannot quantify endothelial activation or injury.

Secretory pathway of vitellogenesis in the liver of the cockerel as revealed by immuno-gold and computer-assisted digitization techniques

The protein A-gold immunocytochemical technique was used to localize the secretory pathway of oestradiol-induced vitellogenin in hepatic parenchymal cells of the cockerel. Liver was removed from experimental birds on the 1st, 4th and 8th day following oestradiol-treatment, and embedded in Lowicryl K4M resin cured at -20 degrees C. In selected electron micrographs the fractional surface area of each of the intracellular compartments was measured by the computer-assisted digitization technique. Labelling was detected over the cisternae of the rough endoplasmic reticulum (RER), the Golgi apparatus, the immature secretory vacuoles (ISV) including condensing vacuoles and the mature secretory vacuoles (MSV). Counts of the gold particles demonstrated an increasing concentration which progressed in the order RER less than Golgi less than ISV less than MSV and identified the secretory pathway of the protein. The highest density of labelling was obtained on the 4th day, when vitellogenin reaches its peak activity. Autophagic activity (or crinophagy) was also found in lysosomes and its labelling intensity increased daily. A hypothesis concerning the secretory pathway of non-stored proteins by the liver is discussed further.

Protein secretion in plants

Protein secretion is an ubiquitous but poorly understood process in plants. Secreted proteins are synthesized on the membranes of the rough endoplasmic reticulum and transported to the cell surface by secretary vesicles formed at the Golgi apparatus. Whereas many of the structural details of this process are known the mechanisms underlying secretion are just beginning to be understood, in this article we review some of the recent developments in this field, and we compare the progress made with animal and plant cells. CONTENTS Summary 567 I. Introduction 568 II. Proteins secreted by plants 568 III. Synthesis and post-translational modification of secreted proteins 571 IV. Molecular requirements for secretion 576 V. Vehicles of secretory transport 581 VI. Regulation of secretion 585 VII. Conclusions and Perspective 587 Acknowledgements 588 References 588.

Intracellular synthesis and processing of the structural glycoproteins of turkey enteric coronavirus

Pulse labeling of cells with [35S]methionine or [3H]glucosamine at different times after infection, followed by SDS-PAGE and Western immunoblotting analysis using rabbit anti-TCV hyperimmune serum, was used to resolve and identify TCV-induced intracellular proteins. The viral structural proteins (gp 200, gp 140/gp 66, gp 100/gp 120, p 52, and gp 24/p 20) were detected in radiolabeled cell extracts by 9 to 12 hours post-infection, as well as two possible non-structural proteins with apparent mol.wts. of 36,000 and 32,000. The predominant 52,000 nucleocapsid protein could be detected in cell lysates as soon as 6 to 8 hours after infection; it was initially resolved as a complex of 3 closely migrating species with mol.wts. ranging from 46,000 to 52,000. Pulse-chase and immunoprecipitation experiments indicated that gp 200 arose from a putative precursor with mol.wt. of 150,000 to 170,000, that underwent glycosylation. Proteolytic cleavage of gp 200, in turn, probably yielded the gp 100 and gp 120 species. The unique TCV hemagglutinin protein originated from a primary precursor with mol.wt. of 60,000, which underwent rapid dimerization by disulfide bond formation and glycosylation to yield gp 140. The peplomeric and matrix proteins were both shown to be N-glycosylated, as indicated by their sensitivity to tunicamycin (TM) and their resistance to sodium monensin (SM). In the presence of TM, proteins with mol.wts. of 90,000, 120-130,000, and 150,000 accumulated in TCV-infected cells rather than peplomeric glycoproteins, and the matrix protein E1 was only detected in its unglycosylated form. The addition of TM to the culture medium interfered with the maturation of progeny viral particles, as suggested by the absence of peplomers at the surface of the intravacuolar and extracellular virions, and the accumulation of amorphous material not found in the absence of the glycosylation inhibitor. High yields of virus replication were obtained, in the presence of SM, even at concentrations which greatly affected the cellular functions.

Glucocorticoid-regulated localization of cell surface glycoproteins in rat hepatoma cells is mediated within the Golgi complex

Glucocorticoid hormones regulate the post-translational maturation and sorting of cell surface and extracellular mouse mammary tumor virus (MMTV) glycoproteins in M1.54 cells, a stably infected rat hepatoma cell line. Exposure to monensin significantly reduced the proteolytic maturation and externalization of viral glycoproteins resulting in a stable cellular accumulation of a single 70,000-Mr glycosylated polyprotein (designated gp70). Cell surface- and intracellular-specific immunoprecipitations of monensin-treated cells revealed that gp70 can be localized to the cell surface only in the presence of 1 microM dexamethasone, while in uninduced cells gp70 is irreversibly sequestered in an intracellular compartment. Analysis of oligosaccharide processing kinetics demonstrated that gp70 acquired resistance to endoglycosidase H with a half-time of 65 min in the presence or absence of hormone. In contrast, gp70 was inefficiently galactosylated after a 60-min lag in uninduced cells while rapidly acquiring this carbohydrate modification in the presence of dexamethasone. Furthermore, in the absence or presence of monensin, MMTV glycoproteins failed to be galactosylated in hormone-induced CR4 cells, a complement-selected sorting variant defective in the glucocorticoid-regulated compartmentalization of viral glycoproteins to the cell surface. Since dexamethasone had no apparent global effects on organelle morphology or production of total cell surface-galactosylated species, we conclude that glucocorticoids induce the localization of cell surface MMTV glycoproteins by regulating a highly selective step within the Golgi apparatus after the acquisition of endoglycosidase H-resistant oligosaccharide side chains but before or at the site of galactose attachment.

A C-terminal signal prevents secretion of luminal ER proteins

Proteins that permanently reside in the lumen of the endoplasmic reticulum (ER) must somehow be distinguished from newly synthesized secretory proteins, which pass through this compartment on their way out of the cell. Three luminal ER proteins whose sequence is known, grp78 ("BiP"), grp94, and protein disulphide isomerase, share the carboxy-terminal sequence Lys-Asp-Glu-Leu (KDEL). We show that deletion (or extension) of the carboxyl terminus of grp78 results in secretion of this protein when it is expressed in COS cells. Conversely, a derivative of chicken lysozyme containing the last six amino acids of grp78 fails to be secreted and instead accumulates in the ER. We propose that the KDEL sequence marks proteins that are to be retained in the ER and discuss possible retention mechanisms.

Phosphorylation of the secreted, free alpha subunit of human chorionic gonadotropin

Phosphorylation of secretory proteins is an uncommon event. In this manuscript, the phosphorylation of human chorionic gonadotropin, a glycoprotein hormone secreted by the JAR choriocarcinoma cell line, is described. Labeling of JAR cells with 32PO4 indicates that both the intracellular and the secreted forms of the free alpha subunit are phosphorylated. Although the secreted alpha beta dimer also incorporates 32PO4, there is little detectable phosphorylation of the intracellular precursors of alpha beta dimer, suggesting that dimer phosphorylation occurs as a late event in post-translational processing. In addition, phorbol 12-myristate 13-acetate markedly stimulates the phosphorylation of both intracellular and secreted forms of free alpha subunit and to a lesser extent of secreted alpha beta dimer. In vitro assays, using homogenates of JAR cells as a source of protein kinase activity, indicate that the uncombined alpha subunit is preferentially phosphorylated. The phosphorylation sites are on serine and threonine residues in the alpha subunit.