Annexin I is phosphorylated in the multivesicular body during the processing of the epidermal growth factor receptor

We have previously shown that an active epidermal growth factor receptor (EGF-R) kinase is necessary for efficient sorting of the EGF-R to the lysosome, and we have shown that this occurs in the multivesicular body (MVB), where EGF-R are sorted away from recycling receptors by being removed to the internal vesicles of the MVB. The aim of the present study was to identify substrates of the EGF-R kinase associated with MVBs which might play a role in this sorting process. We used a density shift technique to isolate MVBs and show that the major substrates phosphorylated in vitro within MVBs which contain an active EGF-R kinase are the EGF-R itself and annexin I. Annexin I is associated with both plasma membrane and MVBs in a calcium-independent manner but can be phosphorylated in vitro only in MVBs. Phosphorylation of calcium-independent annexin I in isolated MVBs converts it to a form that requires calcium for membrane association. In cells with an active EGF-R kinase the amount of calcium-independent annexin I in MVBs is reduced, suggesting that a phosphorylation-induced conversion of the calcium independent to the calcium-dependent form also occurs in vivo. Our observations, together with the known properties of annexin I in mediating membrane fusion, suggest that inward vesiculation in MVBs is induced by the EGF-R and is mediated by phosphorylated annexin I.

Selective membrane protein trafficking: vectorial flow and filter

Membrane proteins trafficking along cellular pathways encounter molecular filters. These filters can introduce them to new pathways and thus direct them towards new destinations. Many proteins carry molecular signals within their cytoplasmic domains that allow them to be selected by the filters. The detailed characterization of these signals is providing new insights into the pathways themselves and indicating the locations of other, as yet unidentified filters.

Transferrin receptors promote the formation of clathrin lattices

Gold conjugates have been used to quantitate human transferrin receptors (hTfnRs) on transfected chick embryo fibroblasts. No relationship could be found between the number of hTfnRs and the number of clathrin-coated pits. However, hTfnRs are also associated with flat clathrin lattices that lie outside invaginated pits. With increasing levels of receptor expression, the density of hTfnRs within flat lattices increases, and at the highest levels of expression the total area of flat lattice increases up to 3-fold. These results show that increased receptor numbers can promote clathrin lattice growth and suggest that the recruitment of receptors like hTfnRs is an essential step in lattice construction. We conclude that the process of invagination, which gives rise to coated pits, is regulated separately.

Exocytosis in living salivary glands: direct visualization by video-enhanced microscopy and confocal laser microscopy

Although exocytosis is widely believed to involve granule movement, membrane fusion and the emptying of granule content, direct study of these processes has been difficult in living cells because of the limited resolution of conventional light microscopy. Using video-enhanced microscopy and confocal laser microscopy, we have now studied these processes in living rat parotid and submandibular gland acinar cells. Under a differential interference contrast (DIC) microscope equipped with a CCD camera and a high speed image processor, secretory granules were in general stationary even after secretory stimulation with isoproterenol (IPR). Following IPR stimulation, however, there were abrupt changes in light intensity of secretory granules, and many granules disappeared. Confocal microscopy was then performed to confirm whether the observed changes in granules were related to membrane fusion and content release. For this, cells were perfused with the fluid-phase tracer Lucifer Yellow; confocal images thus obtained clearly demonstrated the appearance of fluorescence in omega-shaped invaginations of the apical plasma membrane which corresponded to the sites at which changes were observed in DIC images. The time sequence analyses of confocal images showed that there was a repetitive appearance and disappearance of omega-shaped fluorescent foci at the apical plasma membrane until most of the granules were depleted. During this time, there did not appear to be any significant expansion of the apical plasma membrane and if endocytic uptake of the tracer occurred, it was below the limit of detection. These observations provide new insights into the exocytotic process in salivary glands and are at variance in some respects with previous interpretations made from electron microscopy.

Isolation and preliminary characterization of the major membrane boundaries of the endocytic pathway in lymphocytes

Plasma membrane, coated pits, endosomes, and lysosomes were isolated from a mouse T lymphoma cell line using a density shift protocol in which these compartments were selectively loaded with gold conjugates. The plasma membrane was prepared after selective labeling for 1 h at 2 degrees C with gold-ricin and gave a yield of 40% according to enzymatic and antigenic markers. Endosomes were obtained by loading the cells for 2 h at 22 degrees C with gold complexed to an antimouse transferrin receptor mAb. Coated pits were isolated using a similar procedure, but after an incubation at 10 degrees C, which allowed deep invagination of the pits but prevented internalization. The yield (calculated using the recovery of [125I]transferrin) was 32% for endosomes and 10% for coated pits. Finally lysosomes were prepared by loading the cells for 18 h at 37 degrees C with gold low density lipoproteins (LDLs) followed by a 3-h chase at 37 degrees C with LDL alone. The final lysosome yield (based on the recovery of lysosomal enzymes) was 16%. Studies of the protein composition of these cellular compartments on two-dimensional gels showed that while some major proteins are present throughout the pathway, specific proteins can be identified in each of the isolated fractions. The greatest change in the pattern of protein constituents seen along the pathway was between endosomal and lysosomal preparations.

Movement of internalized ligand-receptor complexes along a continuous endosomal reticulum

Complexes of cell-surface receptors and their ligands are commonly internalized by endocytosis and enter a prelysosomal endosomal pathway for further processing. Fluorescence microscopy and video recording of living cells to trace the passage of ligand-receptor complexes has identified the endosomal compartment as an extensive network of tubular cisternae. Endocytosed material entering this reticulum enters discrete swellings, identified as multivesicular bodies by electron microscopy, which move along the reticulum towards the pericentriolar area.

Kinase activity controls the sorting of the epidermal growth factor receptor within the multivesicular body

We compared the internalization and intracellular sorting of epidermal growth factor receptor (EGF-R) and point mutant kinase-negative EGF-R separately expressed in NIH 3T3 cells lacking endogenous receptor. Both EGF-Rs internalized rapidly, but kinase-negative receptor was surface down-regulated only with monensin or at 20 degrees C. Furthermore, EGF internalized by mutant receptor alone was, in significant proportion, returned to the cell surface undegraded. Hence unlike wild-type receptor, kinase-negative EGF-R recycles. By electron microscopy the early pathways of endocytosis for the two receptors were identical; however, after 10-20 min the pathways diverged at the multivesicular body (MVB). Wild-type EGF-R, destined for degradation, localized to internal vesicles, while kinase-negative EGF-R, destined for recycling, localized to surface membranes of the MVBs and moved to small tubulovesicles. We conclude that sorting of internalized receptor for degradation or recycling can occur through spatial segregation within the MVB, and sorting of EGF-R is controlled by tyrosine kinase activity.

Endocytic pathways in polarized Caco-2 cells: identification of an endosomal compartment accessible from both apical and basolateral surfaces

The enterocyte-like cell line Caco-2 forms a polarized epithelium when grown on filters. We have investigated the interaction of endocytic pathways from the apical and basolateral surfaces. The transferrin receptor was an appropriate marker for the basolateral route; uptake of radiolabeled transferrin was highly polarized, and recycling of this ligand back to the basolateral surface occurred with an efficiency of 95%, even after prolonged incubations with transferrin. Using a transferrin-peroxidase conjugate to delineate the morphological pathway, we have identified an early endocytic compartment in the basolateral cytoplasm of the cells. Longer incubations revealed a deeper endocytic compartment in the apical cytoplasm. Concanavalin A complexed to gold was used to simultaneously label the apical endocytic route. After 60 min, extensive mixing of the two labels was seen in endocytic elements throughout the apical cytoplasm, including in the Golgi area, but never in the basal cytoplasm. Using a second double labeling procedure in which antitransferrin receptor antibody complexed to gold was applied to the basolateral surface for up to 2 h and free peroxidase applied to the apical surface for shorter periods, we demonstrated that this apical marker rapidly (within 5 min) reached endosomes containing antibody-gold. Our results indicate that, in Caco-2 cells, the endocytic pathways from the apical and basolateral surfaces meet in an endosomal compartment from which transferrin can still be recycled.

Subfractionation of the endocytic pathway: isolation of compartments involved in the processing of internalised epidermal growth factor-receptor complexes

The aim of the present study was to isolate different parts of the endocytic pathway in order to examine the role of epidermal growth factor (EGF)-receptor internalisation in mediating the biological effects of EGF. We have used an antibody to the transferrin receptor complexed with colloidal gold to modify the density of the endocytic compartments so that they can be purified by sucrose density centrifugation. Using this technique, we have been able to isolate a highly purified preparation of endocytic vesicles from H.Ep.2 cells that contain internalised EGF. By employing pulse--chase protocols, it is possible to isolate the different parts of the endocytic pathway and show that they are temporally distinct with regard to the processing of EGF. It should now be possible to examine interactions between the EGF receptor and intracellular substrates in different parts of the endocytic pathway.

High-yield isolation of functionally competent endosomes from mouse lymphocytes

A discontinuous-sucrose-gradient procedure for isolating endosomes from mouse lymphoma cells has been developed. After centrifugation, most organelles (especially mitochondria and lysosomes) are recovered in the denser fractions of the gradient, whereas a mixture of plasma membrane and endosomes is present at lighter densities. The endosome recovery in this fraction can be increased (by 100%) by (a) a mild trypsin treatment of the postnuclear supernatant and (b) loading the cell endosomes with a saturating concentration of low-density lipoproteins. Removal of the plasma-membrane contamination was achieved by preincubating the cells with a gold-ricin complex at 4 degrees C. On centrifugation, the gold-loaded membranes sediment to the bottom of the gradient. The endosome preparation isolated by these procedures is less than 6% contaminated by other organelles and contains 42% of internalized 125I-transferrin. We show that these isolated endosomes are functional, as displayed by their ability to fuse and to acidify in a cell-free system. Endosome fusion was studied by a new assay based on the use of fluorescence resonance energy transfer. This fusion is dependent on ATP and on a cytosolic, thermoresistant but trypsin- and N-ethylmaleimide-sensitive, protein factor. Early endosomes fuse more actively among themselves than with late-endocytic vesicles, and they fuse only slowly with plasma-membrane vesicles.

Basolateral secretion of kappa light chain in the polarised epithelial cell line, Caco-2

The immunoglobulin kappa light chain is constitutively secreted in non-polarised cells. It is therefore unlikely to display any of the signals thought to be required for the selective delivery of proteins to the apical or basolateral borders of polarised epithelial cells. We have transfected the gene for the kappa light chain into a polarised epithelial cell line (Caco-2) and shown that it is secreted predominantly from the basolateral surface. Metabolically labelled endogenous secretory products show the same polarity and we conclude, therefore, that in Caco-2 cells there is a major intracellular trafficking route to the basolateral border that requires no sorting signal.

Cellular origins of different forms of gastrin. The specific immunocytochemical localization of related peptides

We have localized the antigenic determinants for the main forms of gastrin (big gastrin, G34, and little gastrin, G17) in hog antral mucosa using sequence specific antibodies and an indirect immunofluorescence technique. Populations of monospecific antibodies were obtained after affinity immunoadsorption to remove populations of unwanted specificity. The specificity of the purified antisera was established by direct binding of 125I labeled peptides to antisera at the same dilutions as those used in immunocytochemistry. The results indicate that in hog antral mucosa there is a single population of cells with the antigenic determinants of the C-terminal region of G17 and G34, the N-terminal region of G17, the N-terminal region of G34, and the intact G17 molecule. In duodenum there are cells with only C-terminal reactivity; since gastrin and CCK share a common C-terminal sequence it is concluded that this cell type contains CCK-like peptides rather than gastrin.

Dynamic behavior of the transferrin receptor followed in living epidermoid carcinoma (A431) cells with nanovid microscopy

Transferrin receptors labeled with the B3/25 monoclonal antibody-gold complexes were followed in living A431 cells by using video-enhanced contrast microscopy. Initially, the antibody-gold complexes bind to receptors which are freely mobile on the upper cell surface; they then become trapped at the inner margins of the peripheral lamellae and internalize. During endocytosis discrete gold-loaded vesicular elements first appear, and then, as they fuse, a heterogenous peripheral endosomal compartment forms. The endosomes from this compartment then begin to migrate centripetally through the cytoplasm in a saltatory way so that within 15 min gold label accumulates in a juxtanuclear endosome compartment. This compartment, which consists mainly of multivesicular bodies, is thus formed by the influx and retention of peripheral endosomal elements and their continued fusion in the juxtanuclear area. Although their overall migration is inward, saltating endosomes frequently reverse their direction of movement. As label builds up in the juxtanuclear area, small vesicles containing gold label continuously pinch off from the larger elements and migrate toward the cell periphery. Experiments with nocodazole and sodium azide show that the saltatory movements, the accumulation and retention of endosomes in the juxtanuclear area, and the separation of vesicles from endosomes are driven by a microtubule-associated, ATP-dependent, motility-generating mechanism. Analysis of the movements shows that although each individual vesicle saltation can occur unpredictably toward the centre or the periphery of the cell, a net centripetal flux is observed. Moreover, it is evident that the probability of migration toward and maintenance in the juxtanuclear area is related to the diameter of the vesicles. We propose a mechanism by which bidirectional saltation along microtubules forming a radial network may be instrumental in the selective concentration of large endosomes in the juxtanuclear area while small vesicles are left free to return to the periphery. This process may be responsible for the sorting of receptors and ligands destined either for intracellular degradation in juxtanuclear lysosomes or, alternatively, for recycling to the plasma membrane.

Isolation of an endocytic compartment from A431 cells using a density modification procedure employing a receptor-specific monoclonal antibody complexed with colloidal gold

Our objective was to isolate a prelysosomal compartment involved in receptor-mediated endocytosis in human epidermoid carcinoma (A431) cells. The isolation protocol involves density modification of endosome elements in A431 cells, caused by the receptor-dependent binding and internalization at 20 degrees C of colloidal gold-transferrin receptor antibody (B3/25) particles. The use of 125I-labelled gold-B3/25 provides a radioactive marker for the endosome compartment, the major peak being recovered at the bottom of a continuous sucrose gradient at a density of 1.23g ml-1. Enzyme markers characteristic of other cytoplasmic compartments are present only in negligible amounts in this fraction and L-[35S]methionine-labelling of the cells indicates approximately a 200-fold enrichment of 125I-labelled gold-B3/25 versus protein. Electron microscopy of the endosome-rich fraction reveals that we have isolated a highly purified population of small gold-containing vesicles and tubules from which the transferrin receptor can be immunoprecipitated using the B3/25 antibody. Gel electrophoresis and fluorography of L-[35S]-methionine-labelled cells suggests that these elements contain a characteristic profile of approximately 10 major proteins of which three appear to be specifically enriched. In cells incubated with [125I]transferrin, 12% of the ligand sediments with the gold-labelled elements. We conclude, therefore, that the components we have isolated play a role in the intracellular processing of the transferrin-transferrin receptor complexes.

Epinephrine, glucose, and lactate infusion in exercising adrenodemedullated rats

The purpose of this study was to determine the metabolic function of the marked increase in plasma epinephrine which occurs in fasted rats during treadmill exercise. Fasted adrenodemedullated (ADM) and sham-operated (SHAM) rats were run on a rodent treadmill (21 m/min, 15% grade) for 30 min or until exhaustion. ADM rats were infused with saline, epinephrine, glucose, or lactate during the exercise bouts. ADM saline-infused rats showed markedly reduced endurance, hypoglycemia, elevated plasma insulin, reduced blood lactate, and reduced muscle glycogenolysis compared with exercising SHAM's. Epinephrine infusion corrected all deficiencies. Glucose infusion restored endurance run times and blood glucose to normal without correcting the deficiencies in blood lactate and muscle glycogenolysis. Infusion of lactate partially corrected the hypoglycemia at 30 min of exercise, but endurance was not restored to normal and rats were hypoglycemic at exhaustion. We conclude that in the fasted exercising rat, actions of epinephrine in addition to provision of gluconeogenic substrate are essential for preventing hypoglycemia and allowing the rat to run for long periods of time.

Localization of the epidermal growth factor (EGF) receptor within the endosome of EGF-stimulated epidermoid carcinoma (A431) cells

We have followed the internalization pathway of both epidermal growth factor (EGF) and its receptor in human epidermoid carcinoma (A431) cells. Using EGF conjugated with horseradish peroxidase and anti-receptor monoclonal antibodies (TL5 and EGFR1) coupled either directly or indirectly to colloidal gold we have identified an extensive elaboration of endosomal compartments, consisting of a peripheral branching network of tubular cisternae connected to vacuolar elements that contain small vesicles and a pericentriolar compartment consisting of a tubular cisternal network connected to multivesicular bodies. Immunocytochemistry on frozen thin sections using receptor-specific antibody-gold revealed that at 4 degrees C in the presence of EGF, receptors were mainly on the plasma membrane and, to a lesser extent, within some elements of both the peripheral and pericentriolar endosomal compartments. Upon warming to 37 degrees C there was an EGF-dependent redistribution of most binding sites, first to the peripheral endosome compartment and then to the pericentriolar compartment and lysosomes. Upon warming only to 20 degrees C the ligand-receptor complex accumulated in the pericentriolar compartment. Acid phosphatase cytochemistry identifies hydrolytic activity only within secondary lysosomes and trans cisternae of the Golgi stacks. Together these observations suggest that the prelysosomal endosome compartment extends to the pericentriolar complex and that the transfer of EGF receptor complexes to the acid phosphatase-positive lysosome involves a discontinuous, temperature-dependent step.

Receptor-mediated endocytosis of transferrin and epidermal growth factor receptors: a comparison of constitutive and ligand-induced uptake

The distribution of cell surface receptors for transferrin-iron and epidermal growth factor (EGF) on the surface of cultured epithelioid (A431) cells has been identified by immunocytochemical electron microscopy. The patterns of movement displayed by these two receptor populations as they transfer to their sites of internalization on the cell surface are different. The movement of recycling transferrin receptors over the surface is ligand-independent whereas EGF receptors are more stable residents and remain monodisperse until they bind ligand. Prior to uptake transferrin receptors cluster, predominantly within existing clathrin-coated pits while the aggregates formed by EGF ligand-receptor complexes induce new membrane invaginations. These results are discussed in relation to receptor populations concerned with constitutive, high capacity uptake processes and receptors involved in signal transduction.

Intracellular routing of transferrin and transferrin receptors in epidermoid carcinoma A431 cells

Using transferrin peroxidase (Tfn-HRP) and a transferrin receptor-specific antibody complexed to colloidal gold (ATR) we have identified the intracellular compartments concerned with processing internalized transferrin-receptor complexes. We have identified major membrane-bound systems in the peripheral cytoplasm and in the juxtanuclear area, from which components of these complexes are returned to the cell surface. Time course studies indicate that the peripheral system is concerned with a "short circuit," recycling ligand and receptor complexes back to the upper surface of the cell. The juxtanuclear compartment is part of a longer circuit that routes some receptors to the basal surface and others, along with ligand, to the lysosome.

Internalization and processing of transferrin and the transferrin receptor in human carcinoma A431 cells

The binding and subsequent intracellular processing of transferrin and transferrin receptors was studied in A431 cells using 125I-transferrin and a monoclonal antibody to the receptor (ATR) labeled with 125I and gold colloid. Using 125I-transferrin we have shown that, whereas at 37 degrees C uptake proceeded linearly for up to 60 min, most of the ligand that was bound was internalized and then rapidly returned to the incubation medium undegraded. At 37 degrees C, the intracellular half-life of the most rapidly recycled transferrin was 7.5 min. 125I-ATR displayed the same kinetics of uptake but following its internalization at 37 degrees C, it was partially degraded. At 22 degrees C and below, the intracellular degradation of 125I-ATR was selectively inhibited and as a result it accumulated intracellularly. Electron microscopy of conventional thin sections and of whole-cell mounts was used to follow the uptake and processing of transferrin receptors labeled with ATR-gold colloid complexes. Using a pulse-chase protocol, the intracellular pathway followed by internalized ATR gold-receptor complexes was outlined in detail. Within 5 min at 22 degrees C the internalized complexes were transferred from coated pits on the cell surface to a system of narrow, branching cisternae within the peripheral cytoplasm. By 15 min they reached larger, more dilated elements that, in thin section, appeared as irregular profiles containing small (30-50-nm diam) vesicles. By 30 min, the gold complexes were located predominantly within typical spherical multivesicular bodies lying in the peripheral cytoplasm, and by 40-60 min, they reached a system of cisternal and multivesicular body elements in the juxtanuclear area. At 22 degrees C, no other compartments became labeled but if they were warmed to 37 degrees C the gold complexes were transferred to lysosome-like elements. Extracting ATR-gold complexes with Triton X after a 30-min chase at 22 degrees C and purifying them on Sepharose-transferrin indicated that the internalized complexes remained bound to the transferrin receptor during their intracellular processing.

Localization of fibronectin, laminin-entactin, and entactin in Reichert's membrane by immunoelectron microscopy

Immunoelectron microscopy using protein A-colloidal gold complexes of different sizes was used to study the relative distribution of extracellular matrix glycoproteins within Reichert's membrane (RM) of 13.5-day mouse embryos. Labelling for fibronectin was distributed asymmetrically; the highest concentration occurring in the outermost layer adjacent to the trophoblast cells and negligible labelling in the inner matrix, beneath the parietal endoderm cells. Within the main body of the membrane, fibronectin was concentrated in discrete electron-opaque deposits. Antibodies raised against the native complex between laminin and entactin , and against entactin alone labelled the RM more uniformly.

Studies on the kinetics of dopamine-regulated prolactin secretion

The regulation of prolactin (PRL) release by dopamine has been studied using short-term dissociated rat pituitary cell cultures. Concentrations of dopamine between 10(-12) and 10(-10) M stimulate PRL release, whereas concentrations between 10(-9) and 10(-7) M inhibit. By studying the kinetics of PRL release it was found that (1) the rate of PRL release increases only after a post-stimulus delay of 20 min, and (2) the response is phasic, stimulatory intervals being separated by intervals in which the rate of PRL release falls below the level of basal release. Inhibition of PRL release by dopamine occurs within 10 min after adding dopamine and is maintained tonically for a least 2 h. Cells treated with non-specific secretogogue (57 mM K+) during the inhibitory phases of dopamine-regulated release, are stimulated to secrete significant amounts of hormone.

Regulation of gonadotropin secretion in the anterior pituitary

Studies on the regulation of gonadotropin secretion in dissociated pituitary cell cultures are described. Initial studies employing a ferritin-labelled analogue of gonadotropin hormone releasing hormone (GnRH) to localize its receptor sites on the gonadotropin cell surface that while these receptor sites initially have a random monodisperse distribution, binding of the ligand causes coarse aggregation and internalization of the GnRH receptor. These events are not due to the multivalency of the ligand and probably reflect redistributive events in vivo. By using an octapeptide analogue GnRH that binds to the GnRH receptor but lacks gonadotropin releasing activity in conjunction with sequence-specific antisera it is shown that antibodies that bind the octapeptide can induce the octapeptide to release gonadotropin. These data suggest that receptor aggregation is important in GnRH stimulation. Finally immunocytochemical studies are described in which golg-protein-A-antibody complexes are used to identify gonadotropins on ultrathin frozen sections of porcine pituitary cells. These studies indicate that in porcine gonadotropin cells the majority of the secretory granules contain both luteinizing hormone and follicle-stimulating hormone.

Transmembrane linkage between surface glycoproteins and components of the cytoplasm in neutrophil leukocytes

An experimental approach is described that enables the analysis of interactions between exogenous surface ligands and components of the cytoplasm in neutrophil leukocytes. Neutrophils treated with the nonionic detergent Lubrol PX, under controlled conditions, yield intact detergent-insoluble ghosts. Morphological analysis of neutrophil ghosts shows that they retain the original dimensions of the cell and consist almost entirely of a peripheral filamentous network, representing the submembranous cortical web, concentric to nuclear remnants. All intracellular membrane-bounded organelles, plasma membrane, and background cytoplasmic electron density are absent. Biochemical analysis of the ghosts shows that less than 10% of enzyme markers for the soluble and granule fractions remain, and that greater than 90% of total cell phospholipid is removed during detergent extraction. The major proteins remaining in the ghosts comigrate, on polyacrylamide gels in the presence of SDS, with chicken gizzard actin, myosin, filamin, and a 110-kdalton protein. Patches and caps induced on neutrophils with either fluorescein isothiocyanate-concanavalin A or ferritin-concanavalin A retain their original location and morphology on ghosts after lysis, as determined by both fluorescence and electron microscopy. In similar experiments, but using 125I-labeled lectins, 37% of total cell bound concanavalin A (Con A) and 25% succinylated Con A remain attached to the ghosts. A major 125I-labeled membrane glycoprotein (80 kdaltons) is associated with ghosts prepared from intact neutrophils iodinated in the presence of exogenous lactoperoxidase. Further 125I-labeled membrane glycoproteins (217, 170, and 147 kdaltons) become associated with ghosts prepared from iodinated cells treated before lysis with Con A, but not with succinylated Con A. These data taken together suggest that linkages exist in neutrophils between proteins exposed on the outer surface of the plasma membrane and the peripheral filamentous network independent of the presence of lipid bilayer. The implications of these findings for surface motile phenomena will be discussed.

Cell surface labelling with gold colloid particulates: the use of avidin and staphylococcal protein A-coated gold in conjunction with biotin and fc-bearing ligands

Procedures for preparing gold colloid particles stabilized with either avidin or protein A are described. Methods of using these general utility tracers for localizing biotinylated and fc bearing immunoglobulins are outlined and, as examples of the way in which these methods can be applied, procedures for identifying epidermal growth factor receptors and surface fibronectin on ovarian granulosa cells are described.

Early events following the binding of epidermal growth factor to surface receptors on ovarian granulosa cells

Receptors for epidermal growth factor (EGF) have been localized on the surface of ovarian granulosa cells using a colloidal gold avidin complex in conjunction with biotinylated EGF. On cells incubated at low temperature the receptors are predominantly monodisperse and have a random distribution. When cells are warmed to 37 degrees C the receptors aggregate into clusters.

The identification of urogastrone in serum, saliva, and gastric juice

Urogastrone, a peptide isolated from human urine, is known to cause inhibition of gastric acid secretion and proliferation of fibroblasts in culture; furthermore immunofluorescent localization techniques show it to be present in submandibular and Brunner's glands. Serum, saliva, and gastric juice samples have now been fractionated upon Sephadex G-200 and G-50 and the immunoreactive urogastrone located using a specific radioimmunoassay. Biologic activity was shown by mitogenic studies with 3T6 fibroblasts. In serum, the major immunoreactive component was ca. 1-2 X 10(5) daltons, but trypsin treatment then gave a smaller biologically active species in the same position as pure urogastrone on Sephadex G-50. Both saliva and gastric juice showed major components at the position defined by urogastrone, and these also stimulated the uptake of [H]thymidine into the fibroblasts. It is concluded that a urogastrone-like molecule can be released enzymically from a high molecular weight serum precursor and that the small biologically active peptide is also a normal component of saliva and gastric juice.

Use of protein A-coated colloidal gold particles for immunoelectronmicroscopic localization of ACTH on ultrathin sections

ACTH was localized in dissociated porcine adenohypophysial cells using a novel indirect EM immunocytochemical technique. Incubation of ultrathin resin sections in anti-ACTH was followed by incubation with protein A-coated colloidal gold particles. Protein A binds specifically to the Fc part of the IgG molecule, and thus the ACTH-containing secretory granules became labelled with electron-dense gold particles. With this method, the dissociated porcine ACTH cells was identified as containing numerous round or ovoid 170--300 nm secretory granules.

Calcium as a second messenger in the stimulation of luteinizing hormone secretion

In cells dissociated from porcine anterior pituitary glands and maintained in culture for 48 h the specific secretagogue luteinizing hormone-releasing hormone (LH-RH) induces a biphasic pattern of luteinizing hormone (LH) release. A biphasic pattern of release is also induced by 57 X 10(-3) M K+ and the ionophore A-23187. By reducing the availability of Ca2+, either by omission from the medium, chelation or interfering with Ca2+ transport across the plasma membrane, it is shown that LH release stimulated by LH-RH is much less dependent upon the availability of extracellular Ca2+ than that stimulated by either high K+ or A-23187. Nevertheless, by using a lanthanum displacement protocol to follow the influx of 45Ca2+ it is shown that LH-RH stimulation does induce an influx of extracellular Ca2+. Parallel experiments in which the stimulated 45Ca2+ efflux from preloaded cells is followed confirm the influx data but suggest, in addition, that when the influx of extracellular Ca2+ is inhibited, the peptide is able to mobilize Ca2+ from an intracellular location. It is thus concluded that while LH release can be initiated by an increase in the intracellular level of Ca2+, and although LH-RH stimulation does increase the permeability of the plasma membrane to Ca2+, the stimulation of LH release by LH-RH is not dependent upon extracellular Ca2+.

Dissociation of the porcine anterior pituitary: the kinetics of luteinizing hormone release in response to luteinizing hormone-releasing hormone

A method has been developed for disaggregating porcine anterior pituitary tissue and for providing dissociated preparations which have good, stable viability in culture. The secretory capacity for these preparations in terms of their ability to release luteinizing hormone in response to luteinizing hormone-releasing hormone has been documented in detail. In following the short-term kinetics of LH secretion a biphasic pattern of release has been demonstrated in which a sharp initial peak, maximal at 2 min, is followed by a second, more prolonged phase of release reaching a maximum between 10 and 20 min.

Discrimination of LH, FSH, TSH and ACTH in dissociated porcine anterior pituitary cells by light and electron microscope immunocytochemistry

Using the PAP unlabelled antibody method, LH, FSH, TSH and ACTH were localized at light microscope level in cultured cells dissociated from the porcine adenohypophysis. Antisera were shown to be specific for the beta-subunits of the porcine glycoprotein hormones by radioimmunoassay and absorption studies. Using these antisera, it was found that LH and FSH were contained within the same cell, with TSH in a separate cell. When absorbed with LH, anti-porcine ACTH stained a separate distinct population of ACTH cells. Adjacent ultra-thin sections stained with anti-pLH beta and anti-pFSH beta, and examined at electron microscope level, showed that the ovoid, 150--400nm secretory granules of the LH/FSH gonadotrophs contained both LH and FSH.

Topographical localization of the receptors for luteinizing hormone-releasing hormone on the surface of dissociated pituitary cells

A derivative of the hypothalamic peptide luteinizing hormone-releasing hormone (LHRH) has been coupled to ferritin and the conjugate purified by gel chromatography. In its ability to stimulate the secretion of luteinizing hormone from pituitary cells in vitro, the conjugate has the same potency and specificity as the native peptide. When dissociated pituitary cells maintained in short-term culture are lightly fixed with formaldehyde and then incubated with the conjugate, examination in the electron microscope shows an even distribution of ferritin particles over the free cell surface of the gonadotrophin cells. This binding appears to be specific for the LHRH receptor since it is prevented by a 10-fold excess of native peptide. In addition to the gonadotrophin cells, some somatotrophin and thyrotrophin cells bind conjugate on their free surfaces under similar conditions. If living cells are incubated with the conjugate for 15 min, the bound conjugate becomes aggregated and then concentrated in one localized area of the cell surface. In this area, which lies immediately above the juxtanuclear Golgi complex, the plasma membrane is frequently invaginated in a manner which suggests that the bound, aggregated conjugate is internalized by endocytosis.

Short term kinetics of luteinizing hormone secretion studied in dissociated pituitary cells attached to manipulable substrates

With the use of poly-L-lysine, a method has been developed which induces acutely dissociated rat anterior pituitary cells to attach to glass and polyacrylamide surfaces. In these attached cells the recovery of the secretory response, which is impaired in acutely dissociated cells, has been followed, and it has been established that, in terms of their ability to secrete luteinizing hormone (LH) in response to the specific secretogogue luteinizing-hormone-releasing hormone (LHRH), the cells become maximally responsive after 48 h. The attached cells also allow the short-term kinetics of LH secretion to be followed with great facility; and, when cells allowed to recover for 48 h are used, it is shown that in response to LHRH the pattern of LH release is biphasic.

Synthesis and secretion of a large glycoprotein in the pars intermedia

The biosynthesis, intracellular transport and release of [3-H]-leucine-containing secretory product has been followed in the pars intermedia of Xenopus laevis and, in particular, the synthesis and secretion of a large molecular weight glycoprotein secretory product was demonstrated. However, if Xenopus adrenocorticotrophin does contain a leucine residue the results obtained provided no support for the view that it serves as a precursor for melanocytestimulating hormone in this species.

Caerulein secretion by dermal glands in Xenopus laevis

Since gastrin and its related peptides are secreted by a minority population of widely dispersed cells in mamamalian tissues it has, in the past, been difficult to study the subcellular aspects of their secretion. From published reports (1, 2) it seemed possible that a satisfactory system for such studies might be provided by the skin of certain amphibians such as Xenopus laevis since in these tissues high concentrations of peptides such as caerulein exist, and there is some indication (3) that this, or a similar gastrin-like peptide, may be a dermal gland secretory product. We have therefore explored this possibility by studying the structure, secretory process, and secretory product of the most prominent non mucous type of gland in the skin of X. laevis. These studies clearly demonstrate that most, if not all, of the caerulein in which the skin is contained in secretion granules within the dermal glands and that its release can be specifically evoked by adrenergic stimulation. The release process by a holocrine mechanism expels all of the stored secretion onto the skin surface and thus for biosynthetic studies it should now be possible to synchronize the processes which lead to the replenishment of the peptide.

Hormone secretion by cells dissociated from rat anterior pituitaries

A new procedure has been developed for dissociating anterior pituitary tissue and producing a viable suspension of single cells. The procedure involves incubation of small tissue blocks in 1 mg/ml trypsin (15 min), followed by incubation in 8 microg/ml neuraminidase and 1 mM EDTA (15 min), followed by mechanical dispersion. Cell yields are approximately 55%, based on recovered DNA. By electron microscopy five types of secretory cells (somatotrophs, mammotrophs, thyrotrophs, gonadotrophs, and corticotrophs) plus endothelial and follicular cells can be identified and are morphologically well preserved up to 20 h after dissociation. Throughout this period, the cells incorporate linearly [(3)H]leucine into protein for up to 4 h at a rate 90% greater than hemipituitaries, and they synthesize, transport intracellularly, and release the two major pituitary secretory products, growth hormone and prolactin. Immediately after dissociation the cells' ability to respond to secretogogues (high K(+) and dibutyryl cyclic AMP) is impaired, but after a 6-12-h culture period, the cells apparently recover and discharge 24% and 52%, respectively, of their content of prelabeled growth hormone over a 3-h period in response to these two secretogogues. This represents a stimulation of 109% and 470% over that released by cells incubated in control medium. The results demonstrate that function and morphologic integrity are preserved in this cell system. Therefore it is suitable for the study of various aspects of pituitary secretion and its control.

The biosynthesis, intracellular transport, and packaging of melanocyte-stimulating peptides in the amphibian pars intermedia

Experiments in which glycine-(3)H has been introduced into excised neurointermediate lobes of Xenopus laevis incubated in a modified Krebs-Ringer bicarbonate medium have shown that approximately 50% of the incorporated radioactivity is present in small peptides which have an electrophoretic mobility characteristic of the melanocyte-stimulating (MSH) peptides shown to be elaborated within the tissue. Based on these results and the demonstration that a discrete approximately 7 min pulse of the label can be introduced into the tissue, electron microscope radioautography has been employed to follow the subcellular events concerned with the synthesis, intracellular transport, and packaging of the labeled secretory product. Together, these studies indicate that the newly synthesized material arises in peptide form, rather than as part of a larger prohormone molecule, on the ribosomes of the rough endoplasmic reticulum within the parenchymal cells of the intermediate portion of the lobe. A proportion is then incorporated into and remains for an extended period within the intracisternal granules which are a feature of the rough endoplasmic reticulum within these cells in vitro Most ( approximately 60%) of the labeled secretory product, however, is transferred to the Golgi complex within 30 min and, within a further 10 min, becomes packaged into small ( approximately 200 mmicro) electron-opaque secretory granules. It is probable that under the conditions employed these granules represent the final intracellular location of secretory product before it is released