Intestinal transport of antibodies in the newborn rat

Sorting and endocytosis of viral glycoproteins in transfected polarized epithelial cells

Previous studies (Rindler, M. J., I. E., Ivanov, H. Plesken, and D. D. Sabatini, 1985, J. Cell Biol., 100: 136-151; Rindler, M. J., I. E. Ivanov, H. Plesken, E. J. Rodriguez-Boulan, and D. D. Sabatini, 1984, J. Cell Biol., 98: 1304-1319) have demonstrated that in polarized Madin-Darby canine kidney cells infected with vesicular stomatitis virus (VSV) or influenza virus the viral envelope glycoproteins G and HA are segregated to the basolateral and apical plasma membrane domains, respectively, where budding of the corresponding viruses takes place. Furthermore, it has been shown that this segregation of the glycoproteins reflects the polarized delivery of the newly synthesized polypeptides to each surface domain. In transfection experiments using eukaryotic expression plasmids that contain cDNAs encoding the viral glycoproteins, it is now shown that even in the absence of other viral components, both proteins are effectively segregated to the appropriate cell surface domain. In transfected cells, the HA glycoprotein was almost exclusively localized in the apical cell surface, whereas the G protein, although preferentially localized in the basolateral domains, was also present in lower amounts, in the apical surfaces of many cells. Using transfected and infected cells, it was demonstrated that, after reaching the cell surface, the G protein, but not the HA protein, undergoes interiorization by endocytosis. Thus, in the presence of chloroquine, a drug that blocks return of interiorized plasma membrane proteins to the cell surface, the G protein was quantitatively trapped in endosome- or lysosome-like vesicles. The sequestration of G was a rapid process that was completed in many cells by 1-2 h after chloroquine treatment. The fact that in transfected cells the surface content of G protein was not noticeably reduced during a 5-h incubation with cycloheximide, a protein synthesis inhibitor that did not prevent the effect of chloroquine, implies that normally, G protein molecules are not only interiorized but are also recycled to the cell surface.

Secretion of endogenous and exogenous proteins from polarized MDCK cell monolayers

Confluent monolayers of MDCK (Madin-Darby canine kidney) cells provide a widely used system to study the biogenesis of epithelial cell polarity. We now report that these cells are also capable of the vectorial constitutive secretion of a major endogenous product, a glycoprotein of 81 kDa, which is released into the medium from the apical surface within 30 min of its synthesis. This release represents a bona fide exocytotic secretory process and is not the result of proteolytic cleavage of a plasma membrane-associated precursor since, in cells treated with chloroquine, a protein indistinguishable from the mature secretory product accumulated intracellularly. In contrast to the vectorial secretion of the endogenous product, a variety of exogenous exocrine and endocrine proteins synthesized in MDCK cells transfected with the corresponding genes were secreted from both the apical and basolateral surfaces. These included proteins such as rat growth hormone, chicken oviduct lysozyme, bovine gastric prochymosin, and rat salivary gland alpha 2u-globulin, which in their cells of origin are secreted via a regulated pathway, as well as the liver form of the alpha 2u-globulin and the immunoglobulin kappa chain, which are normally released constitutively. These results demonstrate the existence of secretory pathways that lead to both surfaces of MDCK cells and are accessible to the foreign secretory products. They are consistent with the operation of a sorting mechanism in which the polarized secretion of the endogenous product is effected through the recognition of signals that prevent its random distribution within the fluid phase in the cellular endomembrane system.

The development of the ovine small intestine

Intestinal development was noted from 27 days gestation. By 50 days gestation the ovine small intestine shows the beginning of villus formation. The lumen becomes patent by 70 days. Proximal regions mature earlier than distal regions. Vacuolation of enterocytes occurs in all regions of the small intestine at some stage during development. Mature-looking enterocytes can be found in proximal regions by 125 days gestation while in distal regions immature vacuolated cells can still be found just prior to birth. Enterocytes lining the villi are more mature at the villus tip than those at the base in all regions. This gradient of maturation is consistent with a suggestion of the early establishment of adult patterns of proliferation and migration of enterocytes in the sheep fetus.

Intestinal uptake and retention of copper in the suckling rat, Rattus rattus--III. Effects of closure

The ileum of suckling rats contains a high level of copper, most of which is concentrated within cytoplasmic vesicles of the enterocytes. Intestinal closure, 20-21 days after birth, results in the replacement of enterocytes by cells devoid of these vesicles and there is a concomitant fall in the level of copper in the ileum. Administration of cortisone acetate (0.5 mg/g body wt) to 5-day-old rats results in premature loss of copper-laden ileal enterocytes and an 80-90% decrease in the ileal copper concentration. The loss of copper is mainly from the soluble fraction of the tissue and is proportionally greater from the high molecular weight-protein fraction rather than from the copper complex.

Intestinal uptake and retention of copper in the suckling rat, Rattus rattus--II. Copper accumulation in the ileum and distal jejunum

High concentrations of copper were demonstrated histochemically in the enterocytes lining the ileum and distal jejunum of suckling rats. Copper was not detected in cells from the duodenum or proximal jejunum of these rats or from any region of the small intestine of rats in which "closure" had taken place. X-Ray microanalysis demonstrated copper, in equi-atomic association with sulphur, within discrete vesicles in the supranuclear cytoplasm of the enterocytes. Despite the high concentrations of these two elements no biochemical evidence was found for the presence of significant amounts of copper-binding metallothionein. The highest concentrations of copper (226 +/- 48 mg atoms/kg dry wt: +/- SD) were found in vesicles adjacent to the nucleus, which did not accumulate particulate tracers, or calcium from the lumen of the intestine. These vesicles probably result from the coalescence of Golgi-derived primary lysosomes followed by fusion with endocytic vacuoles. They may provide a mechanism of copper excretion from the neonatal rat.

Receptor-mediated endocytosis of asialoglycoproteins by rat liver hepatocytes: biochemical characterization of the endosomal compartments

The endocytic compartments of the asialoglycoprotein (ASGP) pathway in rat hepatocytes were studied using a combined morphological and biochemical approach in the isolated perfused liver. Use of electron microscopic tracers and a temperature-shift protocol to synchronize ligand entry confirmed the route of ASGP internalization observed in our previous in vivo studies (1) and established conditions under which we could label the contents of successive compartments in the pathway for subcellular fractionation studies. Three endosomal compartments were demonstrated in which ASGPs appear after they enter the cell via coated pits and vesicles but before they reach their site of degradation in lysosomes. These three compartments could be distinguished by their location within the hepatocyte, by their morphological appearance in situ, and by their density in sucrose gradients. The distributions of ASGP receptors, both accessible and latent (revealed by detergent permeabilization), were also examined and compared with that of ligand during subcellular fractionation. Most accessible ASGP receptors co-distributed with conventional plasma membrane markers. However, hepatocytes contain a substantial intracellular pool of latent ASGP binding sites that exceeds the number of cell surface receptors and whose presence is not dependent on ASGP exposure. The distribution of these latent ASGP receptors on sucrose gradients (detected either immunologically or by binding assays) was coincident with that of ligand sequestered within the early endosome compartments. In addition, both early endosomes and the membrane vesicles containing latent ASGP receptors had high cholesterol content, because both shifted markedly in density upon exposure to digitonin.

Biological and pathological aspects of the mammalian small intestinal permeability to macromolecules

The literature on the biology and pathology of mammalian small intestinal permeability to macromolecules is reviewed. In mammals, macromolecules may penetrate the epithelial layer of the small intestinal mucosa, especially in the neonatal period. The neonatal uptake and transport of immunoglobulins is important in the acquisition of passive immunity in the newborn. In the mature small intestine the uptake of macromolecules almost ceases, except in M-cells. Excessive uptake and transport of macromolecules has been demonstrated in several experimental and spontaneous gastrointestinal diseases, in which increased concentration of macromolecules in the small intestinal lumen and/or damage of one or more components of the small intestinal mucosal barrier is present. Finally, methods to study macromolecular permeation of the small intestine are discussed.

Natural infection of Swiss mice with mouse mammary tumor virus (MMTV): viral expression in milk and transmission of infection

Quantitative determinations of gp52, the main envelope glycoprotein, and p28, the main core protein, of MMTV, have been performed in about 1000 individual samples of milk of breeding females from our colony of MMTV-infected Swiss mice, a line characterized by a moderate incidence of mammary tumors. A computer analysis of the results showed: 1-- an important individual variation, ranging from 0 to 120 micrograms per ml of milk for p28, and from 0 to 320 micrograms per ml of milk for gp52; 2-- a variation of the release of both antigens during a single lactation, with a maximum on the 7--8th day of nursing; 3-- an increase of the release of both antigens with parity up to the 6th lactation, followed by a marked decrease during later lactations; 4-- a higher degree of infection in the offspring of 2nd and 3rd litters. The possible dependence of viral expression and transmission of infection upon factors such as cyclic activity of the mammary gland and progressive immunization of mice against MMTV is analyzed. The status of our laboratory line of MMTV infected Swiss mice is discussed in comparison with high and low tumor incidence strains.

Comparison of in vitro binding to enterocyte brush borders of rat IgG subclasses and their transmission in vivo in the rat

The nature of the Fc receptors for IgG subclasses displayed on the epithelium of the small intestine in neonatal rats was examined by comparing results gained from 125I-IgG binding studies using isolated enterocyte brush borders, and from experiments where in vivo transmission from the gut to the bloodstream was quantified for each subclass. These measurements were made in two strains of rat, Cob Wistar and CB Hooded Nu/+. Subclasses of IgG were transported from gut to blood in different amounts, and the in vitro binding also varied according to isotype. Comparison of the results between rat strains and between isotypes suggested that at least two IgG Fc receptors are present on the enterocytes of the young rat.

Transtubular transport of proteins in rabbit proximal tubules

The purpose of the present experiments was to study possible different pathways of intracellular transport of proteins after luminal and basolateral uptake in isolated rabbit proximal tubules. Tubules were exposed to cationized ferritin (CF) in the perfusion fluid and horseradish peroxidase (HRP) in the bath simultaneously or to HRP in the bath alone for 30 min. The peritubular fluid (bath) and perfusion fluid were then exchanged and the tubules either fixed immediately or allowed to function during chase-periods for 10, 20, 30, or 60 min before fixation to follow the migration of the proteins through the cells. The proteins were to a large extent found separated in different vacuoles and lysosomes at all time periods studied, indicating separate pathways after uptake via the luminal and basolateral membranes respectively. About 0.5% of the CF taken up by the cells was transported through the cells and became located in the intercellular spaces. HRP was transported from the peritubular fluid to the apical cytoplasm of the tubules indicated by a gradual accumulation of small HRP-containing vesicles, first in the basal part of the cells and then in the apical cytoplasm. In tubules perfused with both CF and HRP in the perfusate, the CF and HRP were found together in apical vacuoles and lysosomes. After perfusion with HRP alone, this tracer was found in similar large vacuoles and lysosomes in the apical cytoplasm, in contrast to the small HRP-filled vacuoles seen after uptake from the bath.

Macromolecular transport in the fetal rat intestine

Macromolecular barrier function of the fetal rat small intestine and colon was analyzed from 16 to 22 days gestation (birth). During this period the epithelium is converted from stratified to simple columnar. To assess permeability, horseradish peroxidase (HRP) was introduced by microinjection into the lumen or into the umbilical circulation. Proximal small intestine, distal small intestine, and colon were examined after 10-20 min. Paracellular passage of HRP through occluding junctions was not observed after either intraluminal or intravascular injection. After intraluminal injection, transepithelial transport of HRP from lumen to blood occurred in all regions at all ages studied. Horseradish peroxidase was present in cytoplasmic vesicles of most cells in the primitive stratified epithelia, during epithelial conversion, and in simple columnar epithelia. After intravascular injection, HRP was present in the lamina propria and in intercellular spaces of the epithelium, but HRP did not enter tight junctions. Tracer was taken up into cytoplasmic vesicles of both stratified and simple columnar epithelial cells, but was only rarely seen in the lumen. We conclude that there is rapid transcellular, vesicle-mediated transport from lumen to blood across both stratified and simple columnar epithelia of fetal rat small intestine and colon; after intravascular injection, macromolecules may be taken up into vesicles at basolateral epithelial cell surfaces but are not rapidly transported into the lumen; paracellular passage does not occur in the fetal ages studied.

Maternally derived immunity in young mice to infection with Trypanosoma brucei and its potentiation by Berenil chemotherapy

Young mice which were allowed to suckle, from birth, a mother infected with Trypanosoma brucei, or a mother whose infection had been cured before parturition with Berenil chemotherapy, were themselves immune to homologous trypanosome challenge. This immunity extended until approximately 25 days of age, and was transmitted in the colostrum/milk of the mother. Mice born of infected mothers, but transferred at birth to normal foster mothers, were susceptible to trypanosome infection. Drug prophylaxis in normal newborn mice was also effective for approximately 25 days, but in mice which, in addition, received colostral antibody from the mother, combined immunochemoprophylaxis protected the offspring for 40-50 days. Since the combination of protective strategies continued to resist challenge beyond the stage when, on its own, each component's efficacy had decayed, it may be of practical value as an approach to improved disease control under certain field conditions where trypanosomiasis prevails.

Development of fetal rat intestine in organ and monolayer culture

Maturation and differentiation of intestinal epithelial cells was demonstrated in segments of fetal rat small intestine, maintained for more than a month in suspension organ culture, by ultrastructural, biochemical, and immunological criteria. Over a 5-7 d period, fragments of fetal intestine evolved into globular structures covered with a single columnar epithelium ultrastructurally similar to suckling villus cells. Loose mesenchymal cells, cellular debris, and collagen were present inside the structures. After 6 d in culture, goblet cells, not present in the fetal intestine at day 18, were numerous and well developed. Intestinal endocrine cells were also observed. Immunofluorescence studies employing monoclonal antibodies specific for villus and crypt cells in vivo, and various enzyme assays, have demonstrated a level of differentiation and maturation of the cultured epithelial cells similar but not identical to that of suckling intestinal mucosa in vivo. Crypts and crypt cell markers were not observed in the the cultures. Addition of glucocorticoids to the culture medium resulted in the induction of sucrase-isomaltase but failed to promote most of the functional changes characteristic of the intestinal epithelium at weaning in vivo. Epithelial cells were identified in explants derived from the organ cultures by their specific expression of intestinal cytokeratin. Differentiation-specific markers, present in the epithelial cells in primary cultures, were lost upon selection and subculturing of pure epithelial cell populations. These results suggest a requirement for mesenchymal and/or extracellular matrix components in the maintenance of the differentiated state of the epithelial cells. The fetal intestinal organ cultures described here present significant advantages over traditional organ and monolayer culture techniques for the study of the cellular and molecular interactions involved in the development and differentiation of the intestinal epithelium.

Uptake and transport of intact macromolecules in the intestinal epithelium of carp (Cyprinus carpio L.) and the possible immunological implications

Two protein antigens, horseradish peroxidase (HRP) and ferritin, have been administered to the digestive tract of carp. Electron-microscopical observations reveal considerable absorption of both antigens in the second segment of the gut (from 70 to 95% of the total length) and also, although to a lesser extent, in the first segment (from 0 to 70% of the total length). Even when administered physiologically with food, a large amount of ferritin is absorbed by enterocytes in the second gut segment. HRP and ferritin are processed by enterocytes in different ways. HRP seems to adhere to the apical cell membrane, probably by binding to receptors, and is transported in vesicles to branched endings of lamellar infoldings of the lateral and basal cell membrane. Consequently, most of the HRP is released in the intercellular space where it contacts intra-epithelial lymphoid cells. Only small amounts of HRP become localized in secondary lysosomes of enterocytes. Ferritin does not bind to the apical cell membrane; after uptake by pinocytosis, it is present in small vesicles or vacuoles that appear to fuse with lysosome-like-bodies. In the second segment, intact ferritin ends up in the large supranuclear vacuoles (after 8 h), where it is digested slowly. Although no ferritin is found in the intercellular space, ferritin-containing macrophages are present between the epithelial cells, in the lamina propria and also to a small extent in the spleen. The transport of antigens from the intestinal lumen, through enterocytes, to intra-epithelial lymphoid cells or macrophages may have immunological implications, such as induction of a local immune response and prospectives for oral vaccination.

Fluid-phase and adsorptive endocytosis in ciliated epithelial cells of the rat ductuli efferentes

Ciliated cells of the ductuli efferentes show at their apex a discrete endocytic apparatus composed of small vesicles connected to or subjacent to the apical plasma membrane, small apical membranous tubules, and pale multivesicular bodies. Deeper in the cytoplasm, there are acid phosphatase-positive denser, multivesicular bodies and secondary lysosomes showing an electron-dense cortex and a crystalline, paler stained core. Cationic ferritin and concanavalin A-ferritin used to demonstrate adsorptive endocytosis, when injected into the rete testis, rapidly reached the lumen of the ductuli efferentes. At 1 min after injection, these tracers were seen bound to the apical plasma membrane of ciliated cells and within small endocytic vesicles and by 5 min in narrow apical tubules. At 15 and 30 min after injection, the tracers appeared in pale multivesicular bodies while at 1 hr they were found within dense multivesicular bodies. At 2 hr and longer time intervals these tracers accumulated within secondary lysosomes. Native ferritin, concanavalin A-ferritin in the presence of alpha-methyl-D-mannoside, and horseradish peroxidase or albumin-colloidal gold complexes were used to analyze fluid-phase endocytosis. At various intervals following their injection into the rete testis, these tracers presented a distribution identical in all respects to that described for cationic ferritin and concanavalin A-ferritin. In the present work, none of the above tracers were transported to the abluminal aspect of the ciliated cells. These cells, like the nonciliated epithelial cells of the ductuli efferentes are thus involved in adsorptive as well as in fluid-phase endocytosis.

Transport of horseradish peroxidase across monkey trophoblastic epithelium in coated and uncoated vesicles

This study used membranous chorion of the macaque monkey placenta to examine uptake and processing of exogenous proteins. Tissue was incubated with either cationic or anionic horseradish peroxidase. Incubation time was varied between 5-25 min to follow the endocytic pathways. In spite of some differences in binding, uptake and processing of the isozymes was similar. In the presence of tracers at 37 degrees C both horseradish peroxidases were taken up in large (150-175) nm diameter) coated vesicles. In addition, coated tubules 300-400 nm in length and 50-100 nm in diameter were seen in the apical cytoplasm. Studies using ruthenium red indicated that the coated tubules were derived from long coated invaginations of the free surface that pinch off into the apical cytoplasm. Often, the tubules bud off small (85-105 nm diameter) protein-filled coated vesicles which traversed the cytoplasm and fused with the basal-lateral plasma membrane. In other cases, the tubules or vesicles lost their clathrin coats and fused to form larger endocytic vesicles which later fused with phagolysosomes. After long incubation, larger uncoated vesicles (endosomes) were seen releasing their contents at the basal-lateral membrane. These results suggest that multiple transport pathways exist in this epithelium. The first, involving only coated structures, may function to sort and concentrate specific ligands important for embryonic development. The second, involving the formation and translocation of large uncoated vesicles to the basal-lateral membrane, may also provide nutrients to the embryo. A third pathway directs the protein to phagolysosomes where it is presumably degraded.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor mechanisms of the neonatal intestine and their relationship to immunoglobulin absorption and disease

Immunoglobulin absorption by the calf has been the subject of considerable research. Despite these efforts little is known about the cytological events that occur at the level of the intestinal epithelial cell. These events have been studied extensively and characterized in the laboratory rodent; however, there have been few attempts to make corollaries between the two species. All neonatal animals display certain similarities in their intestinal morphology that may be correlated, with immunoglobulin absorption. Selectivity in absorption appears to be variable among neonatal animal species; however, all demonstrate some selectivity. Selectivity in absorption implies that receptors are a necessary component in the transport of immunoglobulins. Selectivity further requires binding of immunoglobulins to an endocytic vesicle membrane to ensure transport through the cell, circumvention of intracellular digestion, and release at the basolateral cell membrane. A decrease of immunoglobulin absorption may be accomplished in a variety of ways such as competition between intestinal microbes and immunoglobulins for a common receptor on the intestinal epithelial cell. An additional consideration is aberrant synthesis or recycling of the cell membrane receptor, as induced by metabolic decelerators such as cortisol. Failure to recycle immunoglobulin receptors also would decrease efficiency of absorption.

Endocytic uptake, transport, and catabolism of proteins by epithelial cells

Adsorptive and/or receptor-mediated endocytosis of proteins is a universal cell property, which is highly expressed in epithelial cells. Some absorbed proteins are transported intact across cells and in this manner subserve specialized functions such as the transference of immunity from mother to child. Mainly, however, absorbed proteins are transported to lysosomes, where they undergo complete hydrolysis to amino acids. This process is essential for the homeostasis of circulating proteins. This brief review considers the intracellular pathways taken by endocytosed proteins and the quantitative aspects and determinants of protein uptake and catabolism by epithelial cells. The topics to be briefly discussed are initial internalization sites, transport organelles (endosomes), and lysosomal and nonlysosomal pathways of transport; intracellular sorting of internalized proteins, membranes, and receptors; kinetics and selectivity of renal cell uptake of low-molecular-weight proteins and proteohormones; receptor-mediated endocytosis of larger proteins (e.g., glycoproteins) by hepatocytes; and lysosomal catabolism of absorbed proteins and its dependence on protein load and endosomal-lysosomal pH and function. The perspectives of the field and some of the outstanding unsolved problems are briefly discussed.

Fenestrations of the basal lamina of intestinal villi of the rat. Scanning and transmission electron microscopy

Fenestrations of the basal lamina of rat intestinal villi were revealed by scanning electron microscopy after removal of the overlying epithelial cells by osmic acid maceration. These fenestrations are circular to oval in shape and are 0.5 micron to 5 microns in diameter. They are richly distributed at a density of 1-2 X 10(4)/mm2 in the upper two thirds of the villi, except at the very tips. Roughly 500 fenestrations are found on each side of an average sized tongue-shaped villus. Transmission electron-microscopic observations showed that these fenestrations were passages for migrating cells of the immune system such as lymphocytes, eosinophils and macrophages. Protrusions from the basal parts of epithelial cells were also observed passing through these fenestrations. These findings are discussed with respect to their immunological implications and to the passage of nutrients.

Polarized delivery of viral glycoproteins to the apical and basolateral plasma membranes of Madin-Darby canine kidney cells infected with temperature-sensitive viruses

The intracellular route followed by viral envelope glycoproteins in polarized Madin-Darby canine kidney cells was studied by using temperature-sensitive mutants of vesicular stomatitis virus (VSV) and influenza, in which, at the nonpermissive temperature (39.5 degrees C), the newly synthesized glycoproteins (G proteins) and hemagglutinin (HA), respectively, are not transported out of the endoplasmic reticulum. After infection with VSV and incubation at 39.5 degrees C for 4-5 h, synchronous transfer of G protein to the plasma membrane was initiated by shifting to the permissive temperature (32.5 degrees C). Immunoelectron microscopy showed that under these conditions the protein moved to the Golgi apparatus and from there directly to a region of the lateral plasma membrane near this organelle. G protein then seemed to diffuse progressively to basal regions of the cell surface and, only after it had accumulated in the basolateral domain, it began to appear on the apical surface near the intercellular junctions. The results of these experiments indicate that the VSV G protein must be sorted before its arrival at the cell surface, and suggest that passage to the apical domain occurs only late in infection when tight junctions are no longer an effective barrier. In complementary experiments, using the temperature-sensitive mutant of influenza, cultures were first shifted from the nonpermissive temperature (39.5 degrees C) to 18.5 degrees C, to allow entrance of the glycoprotein into the Golgi apparatus (see Matlin, K.S., and K. Simons, 1983, Cell, 34:233-243). Under these conditions HA accumulated in Golgi stacks and vesicles but did not reach the plasma membrane. When the temperature was subsequently shifted to 32.5 degrees C, HA rapidly appeared in discrete regions of the apical surface near, and often directly above, the Golgi elements, and later diffused throughout this surface. To ensure that the anti-HA antibodies had access to lateral domains, monolayers were treated with a hypertonic medium to dilate the intercellular spaces. Some labeling was then observed in the lateral plasma membranes soon after the shift, but this never increased beyond 1.0 gold particle/micron, whereas characteristic densities of labeling in apical surfaces soon became much higher (approximately 10 particles/micron). Our results suggest that the bulk of HA follows a direct pathway leading from the Golgi to regions of the apical surface close to trans-Golgi cisternae.

Biogenesis of epithelial cell polarity: intracellular sorting and vectorial exocytosis of an apical plasma membrane glycoprotein

To study the biogenetic pathway of influenza hemagglutinin (HA), a model apical glycoprotein, in polarized epithelial MDCK cells, anti-HA antibodies were added to the basolateral surface during influenza infection. In monolayers grown on collagen gels influenza and VSV plaque development was blocked only when the antibodies were added to the respective budding surface. Addition of anti-HA antibodies to the basal medium of monolayers grown on nitrocellulose filter chambers neither resulted in HA-coupled transport of antibody nor inhibited HA migration to the apical surface. These results indicate that the bulk of HA is vectorially inserted into the apical surface of MDCK cells by polarized exocytosis. Other apical proteins in epithelia may use a similar mechanism during biogenesis.

Adrenergic effects on exocrine secretion of rat submandibular epidermal growth factor

The present study was undertaken to investigate the effect of alpha- and beta-adrenergic agonists on secretion of epidermal growth factor (EGF) from the rat submandibular glands and to test the possibility of intestinal absorption of EGF. Alpha-adrenergic agonists increased the concentration of salivary EGF by approximately a hundred times, while the serum concentration of EGF was unchanged. The contents of EGF in the submandibular glands decreased upon administration of the alpha-adrenergic agonist noradrenaline, and this was confirmed on immunohistochemical investigation of the glands. Beta-adrenergic agonists had no effect on secretion of EGF from the submandibular glands. Intestinal absorption of EGF could not be confirmed, as stimulation by noradrenaline with free passage of saliva to the gastrointestinal tract and intrajejunal infusion of EGF had no influence on the concentration of EGF in serum. This study shows that alpha-adrenergic agonists stimulate exocrine secretion of submandibular EGF and that EGF in physiological amounts are not absorbed in the gastrointestinal tract.

Transcytosis of the G protein of vesicular stomatitis virus after implantation into the apical plasma membrane of Madin-Darby canine kidney cells. I. Involvement of endosomes and lysosomes

The G protein of vesicular stomatitis virus, implanted into the apical plasma membrane of Madin-Darby canine kidney cells, is rapidly transcytosed to the basolateral membrane. In this and the accompanying paper (Pesonen, M., R. Bravo, and K. Simons, 1984, J. Cell Biol. 99:803-809.) we have studied the intracellular route by which the G protein traverses during transcytosis. Using Percoll density gradient centrifugation and free flow electrophoresis we could demonstrate that the G protein is endocytosed into a nonlysosomal compartment with a density of approximately 1.05 g/cm3, which has many of the characteristics of endosomes. Transcytosis to the basolateral membrane appeared to occur from this compartment. No direct evidence for the involvement of lysosomes in the transcytotic route could be obtained. No G protein was detected in the lysosomes when transcytosis of G protein was occurring. Moreover, at 21 degrees C when passage of G protein to the lysosomes was shown to be arrested, transcytosis of G protein could still be demonstrated.

High-voltage electron microscopy of capillary endothelial vesicles

Conventional electron microscopy of thin sections through capillary walls is inadequate to discern the relationships between endothelial vesicles and their association with the cell surface. High-voltage electron microscopy of thick sections through diaphragm muscle capillaries has been employed to visualize the three-dimensional structure of the vesicular system. Stereopairs of thick sections provide for direct three-dimensional observations of samples several vesicle diameters deep. A variety of simple and compound vesicular forms are present but not all are conjoined or maintain connections with the endothelial cell surface. This contributes to the concept of a dynamic interaction between free and attached vesicular structures where fission and fusion events compartmentalize and reconnect repeatedly. Such interactions would provide for a discontinuous pathway across the capillary wall but with a higher degree of complexity than a simple shuttling mechanism.

Endocytosis in nonciliated epithelial cells of the ductuli efferentes in the rat

The nonciliated cells lining the ductuli efferentes presented three distinct cytoplasmic regions. The apical region contained, in addition to cisternae of endoplasmic reticulum and mitochondria, two distinct membranous elements. The tubulovesicular system consisted of dilated tubules connected to the apical plasma membrane and subjacent distended vesicular profiles. The apical tubules, not connected to the cell surface, consisted of numerous densely stained tubules of small size which contain a compact, finely granulated material. The supranuclear region, in addition to a Golgi apparatus and ER cisternae, contained dilated vacuoles, pale and dense multivesicular bodies, as well as numerous dense granules identified cytochemically as lysosomes. The basal region contained the nucleus and many lipid droplets. The endocytic activity of these cells was investigated using cationic ferritin (CF) and concanavalin-A-ferritin (Con-A-ferritin) as markers of adsorptive endocytosis; and native ferritin (NF), concanavalin-A-ferritin in the presence of alpha-methyl mannoside, and horseradish peroxidase or albumin bound to colloidal gold for demonstrating fluid-phase endocytosis. These tracers were injected separately into the rete testis, and animals were sacrificed at various time intervals after injection. At 1 min, CF or Con-A-ferritin were seen bound to the apical plasma membrane, to the membrane of microvilli, and to the membrane delimiting elements of the tubulovesicular system. Between 2 and 5 min, these tracers accumulated in the densely stained apical tubules and at 15 min in the dilated vacuoles. Between 30 min and 1 hr, the tracers appeared in multivesicular bodies of progressively increasing density, whereas at 2 hr and later time intervals, many dense lysosomal elements became labeled. The tracers for fluid-phase endocytosis showed a distribution similar to that for CF or Con-A-ferritin except that they did not bind to the apical plasma membrane, microvilli, or membrane delimiting the tubulovesicular system. At no time interval were any of the tracers observed in the abluminal spaces. Thus, the nonciliated epithelial cells of the ductuli efferentes are actively involved in fluid-phase and adsorptive endocytosis, both of which result in the sequestration of endocytosed material within the lysosomal apparatus of the cell.

Changes in the fine structure of rat visceral yolk-sac placenta during prolonged pregnancy

Visceral yolk-sac membranes were obtained surgically from viable rat fetuses which had been retained experimentally in utero in lactating animals for as long as 3 days beyond delivery of their littermates. They were examined with the electron microscope. The fine structure of the yolk-sac placental membrane (i.e., the tissues separating vitelline capillary lumina from the uterine cavity) remained unimpaired during the delay period. Progressive changes included a decrease in the height of the endodermal epithelium, but a thickening of the basement membrane underlying it by repeated replication of the subepithelial basal lamina and an increase in collagen fiber content in the reticular lamina. Transcytosis across the endothelium of the capillaries of the peripheral vitelline circulation remained unchanged throughout the delay period; endocytosis of materials, presumably maternal serum proteins, from the uterine cavity into the apices of the endodermal epithelium, however, decreased with time. At the oldest stage (26 days post-coitum) most endocytosed materials had been stored for hydrolysis in large subapical vacuoles which were identified as secondary lysosomes; and evidence of uptake from the uterine lumen was essentially absent. Accordingly, placental transport of protein by the visceral yolk sac of the rat can be regarded as a perinatal process. The action of an elaborately developed and well-preserved Golgi apparatus in both cellular mechanisms, viz., digestion of maternal proteins and protein transfer, was inferred.

Receptor-mediated transport of IgG

The intestinal epithelium of the neonatal rat is a model system for the study of receptor-mediated endocytosis in which large amounts of IgG are transferred intact across polarized cells. This review summarizes the ultrastructural pathway followed by IgG during cellular transit and several important properties of the membrane receptor that recognizes the IgG.

Endocytosis in epithelial cells lining the rete testis of the rat

The endocytic activity of the low cuboidal cells lining the rete testis was analyzed by electron microscopy following injection of various tracers into the lumen of these anastomotic channels. At 1 and 5 minutes after injection, cationic ferritin (CF) and concanavalin A-ferritin (Con A) were seen bound to the apical plasma membrane and to the membrane of subjacent vesicles or invaginations connected to this apical membrane. At 30 and 60 minutes, these tracers were found in intracytoplasmic vesicles and in vesicles connected to the lateral or basal plasma membrane as well as in the lateral intercellular space and in the lamina lucida of basal lamina. At 30 minutes, CF and Con A also appeared in the matrix of pale multivesicular bodies while at 1 hour dense multivesicular bodies were labeled. At 2 hours and later time intervals, the tracers accumulated in dense granules identified as lysosomes. Native ferritin (NF), concanavalin A-ferritin in presence of alpha-methyl-D-mannoside, and horseradish peroxidase or albumin bound to colloidal gold were all to be incorporated by the lysosomal system of these epithelial cells, as just described for CF and Con A, but these various tracers were not bound to the apical plasma membrane or to the membrane of cytoplasmic vesicles, nor were they found in the intercellular spaces or the lamina lucida at the base of the cells. Thus, the epithelial cells of the rete testis do not appear to be only involved in the uptake of substances from the lumen and their disposal by the lysosomal system, but also appear to contribute to the transport of certain macromolecules from the lumen to the laterobasal surfaces of the cells. These cells may thus play a role in determining the composition of the rete testis fluid.

Endocytosis: relation to capping and cell locomotion

Most mammalian cells, such as fibroblasts, continuously internalize part of their surface membrane by endocytosis, and then later return it to the cell surface. This cyclical process is initiated by coated pits in the plasma membrane. These pits collect specific receptors plus lipid for internalization, but exclude other proteins. On a motile cell, the sites of endocytosis (randomly located on the cell) and those of membrane return (located at the front of the cell) are not coincident. This causes a bulk flow of lipid plus receptors in the plasma membrane, away from the front of the cell. Large objects on the cell surface are swept to the rear of the cell by this flow, a process called capping. Cells may use this polarized endocytic cycle to move.

Biosynthesis and transport of glycoproteins in the small intestinal epithelium of rats. I. Developmental change and effect of hypophysectomy

The biosynthesis and intracellular transport of glycoproteins in duodenal absorptive cells of intact rats at 6 and 24 days and hypophysectomized rats at 24 days of age were studied after 20 min intralumenal pulse-labeling of D-[3H]galactose, L-[3H]fucose, or D-[3H]mannose. Autoradiographic studies showed that the incorporation of sugars increased significantly in intact rats between 6 and 24 days. When rats were hypophysectomized at 6 days of age, the intestinal epithelium at 24 days incorporated D-[3H]galactose at a level significantly lower than that of intact rats at 24 days. Hypophysectomy also interfered with the developmental increase in D-[3H]mannose, but not in L-[3H]fucose, incorporation. Biochemical study indicated that the radioactivity in the lipid-free acid-precipitable glycoproteins in the intestine of 24-day-old intact rats at 20 min after D-[3H]galactose injection was 129% and 97% higher than that in 6-day-old rats and in 24-day-old hypophysectomized rats, respectively. The patterns of intracellular transport of newly synthesized galactosylated or fucosylated glycoproteins in all animal groups were similar; the labeled glycoproteins were initially present in the Golgi and were transported through the smooth endoplasmic reticulum to either the lateral membrane or the brush-border membrane within 60 min after the injection of labeled sugars. The proportion of labeled glycoproteins that migrated to the brush-border membrane, however, increased about twofold in the intact rats between 6 and 24 days of age at 60-240 min after D-[3H]galactose injection. Hypophysectomy interfered with developmental increase in the transport of glycoproteins from the apical cytoplasm to the brush-border membrane. It was concluded that the incorporation of monosaccharide precursors into glycoproteins and the proportion of newly synthesized galactosylated or fucosylated glycoproteins transported to the brush-border membrane increase during postnatal development. The developmental changes are regulated, at least partially, by the pituitary gland.

Immunofluorescent patterns of clathrin and dopamine beta-hydroxylase in chromaffin cells in culture

Bovine chromaffin cells maintained in culture for eight days were loaded with [3H]noradrenaline and then stimulated by a depolarizing concentration (56 mM) of K+. Control and stimulated cells were fixed in 3.7% formaldehyde, treated with acetone or Triton X-100, and then exposed to antibodies raised against dopamine beta-hydroxylase (a secretory granule marker) and clathrin, and purified by affinity chromatography. The cellular distribution of the correspondent antigens was investigated by indirect immunofluorescence. Cells treated with anti-dopamine beta-hydroxylase exhibited a granular pattern of fluorescence in the cytosol of the cell body, neurites, and terminal cones. Chromaffin cells exposed to anti-clathrin also showed a punctate pattern of fluorescence staining. However, in this case, the fluorescent dots were smaller than those observed with anti-dopamine beta-hydroxylase, and they were differently distributed. The speckled anti-clathrin fluorescence was preferentially condensed in the juxtanuclear region of the cell bodies, suggesting the possibility that clathrin was concentrated at the level of the Golgi apparatus. The stimulation of cultured chromaffin cells by 10 pulses of 56 mM K+ produced 91 +/- 2% (n = 5) depletion in the [3H]noradrenaline cell content and a concomitant displacement of the dopamine beta-hydroxylase fluorescence to the periphery of the cells. Four days after cell stimulation the dopamine beta-hydroxylase fluorescence was similar to that observed in control cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Endocytosis and exocytosis: current concepts of vesicle traffic in animal cells

Animal cells have specific pathways to transport macromolecules from their surrounding environment to their interior, and from internal compartments to the cell surface or other intracellular locations. Many of these movements appear to be receptor-dependent processes in which specific membrane receptors bind macromolecules, segregate them into discrete membrane-limited compartments, and move the molecules to new locations. Such processes include the clustering and internalization of receptor-bound ligands at the cell surface in clathrin-coated pits, the formation of endocytic vesicles (receptosomes) from coated pits, the movement of receptosomes by saltatory motion to the Golgi system, the concentration of materials in the coated pits of the Golgi system that are destined for delivery to lysosomes, and the directed traffic of materials destined for exocytosis out of the Golgi to the cell surface. This review describes some of the experiments which have led to our current understanding of the various organelles involved in this traffic and some of the biochemical mechanisms involved.

Maturation of the ileal epithelium in the young rat

Normal maturation in the rat ileal epithelium (ileal 'closure') occurs between 18 and 21 days, probably by a process of cellular replacement. A single dose of cortisone acetate induced a precocious ileal 'closure' within 6 days. The morphological changes observed were similar to those seen during normal 'closure'. Morphometric measurements made of ileal villi during both normal and induced 'closure' suggest that there was a gradual change in the type of cells produced by the crypt. This was perhaps related to the duration of exposure of the cryptal cells to the 'closure' stimulus. Ten days after cortisone acetate treatment vacuolated cells were again seen on the villus in some animals. The variation in their distribution on the villus was perhaps due to differing rates at which the villi had recommenced pinocytosis because of the physiological state of the animal. A single dose of corticosterone failed to promote any morphological changes indicative of ileal 'closure' when observed at the electron microscope level.

Transcytosis in thyroid follicle cells

Inside-out follicles prepared from pig thyroid glands were used for studies on endocytosis. endocytosis. In this in vitro system, only the apical plasma membranes of follicle cells were exposed to tracers added to the culture medium. Cationized ferritin (CF) bound to the apical plasma membrane and was transferred first to endosomes and to lysosomes (within 5 min). Later, after approximately 30 min, CF was also found in stacked Golgi cisternae. In addition, a small fraction of endocytic vesicles carrying CF particles became inserted into the lateral (at approximately 11 min) and the basal (at approximately 16 min) plasma membranes. Morphometric evaluation of CF adhering to the basolateral cell surfaces showed that the vesicular transport across thyroid follicle cells (transcytosis) was temperature-sensitive; it ceased at 15 degrees C but increased about ninefold in follicles stimulated with thyrotropin (TSH). Thyroglobulin-gold conjugates and [3H]thyroglobulin (synthesized in separate follicle preparations in the presence of [3H]leucine) were absorbed to the apical plasma membrane and detected mainly in lysosomes. A small fraction was also transported to the basolateral cell surfaces where the thyroglobulin preparations detached and accumulated in the newly formed central cavity. As in the case of CF, transcytosis of thyroglobulin depended on the stimulation of follicles with TSH. The observations showed that a transepithelial vesicular transport operates in thyroid follicle cells. This transport is regulated by TSH and includes the transfer of thyroglobulin from the apical to the basolateral plasma membranes. Transcytosis of thyroglobulin could explain the occurrence of intact thyroglobulin in the circulation of man and several mammalian species.

Analysis of maternal IgG subpopulations which are transported into the chicken oocyte

Chicken serum and oocyte IgG were compared. Purified intact IgG and mercuripapain-produced Fc fragments of yolk and serum IgG were analysed by isoelectric focusing. All IgG bands were identical, indicating that all subpopulations of serum IgG were present in the yolk. Upon papain hydrolysis of both serum and yolk IgG, four identical Fc bands were produced from all serum and yolk samples. Sialic acid measurements showed that there was no significant difference in sialic acid content between serum and oocyte IgG. From these results we conclude that: (i) ovarian IgG receptor(s) selectively transports all subpopulations of maternal IgG; (ii) there is no selective destruction of IgG during transport; and (iii) yolk IgG has the same amount of sialic acid as the serum IgG.

A new experimental method for the dynamic study of the antibody transfer mechanism from mother to fetus in the rat

The yolk-sac is known to be a route for the transport of passive immunity from mother to fetus in the rat. The main purpose of the present paper is to describe an experimental system for ultrastructurally studying the kinetics of the uptake and transport of immunoglobulin by rat yolk-sac. This system has the advantage of enabling the membrane to be externalized and then exposed to protein under controlled environmental conditions whilst at the same time maintaining the conceptus in connection with the in situ placenta. Preliminary investigations have utilized homologous anti-horseradish peroxidase (HRP) IgG (detected as antibody by application of HRP) or HRP alone. Comparison has been made with the localization of endogenous IgG transmitted in vivo after immunization of the female rat with HRP. The results show the rapid binding of IgG to membrane since only 30 sec were sufficient for this attachment to occur. Moreover, the endocytic process also appears to be very fast as localization of IgG in clusters or patches, caveolae or pits and even rare microvesicles is observed within 8 min. On the other hand, no binding of HRP to microvilli was observed and, unlike IgG, HRP became located in the apico-tubulocanalicular system.

Receptor-mediated endocytosis and exocytosis of transferrin in Concanavalin A-stimulated rat lymphoblasts

Iron-loaded transferrin has been shown to be necessary for the support of cell proliferation in culture. This function depends upon interaction of transferrin with a specific high-affinity cell surface receptor. The present report is directed toward determining the consequences of the interaction of transferrin with this receptor on Concanavalin A-stimulated rat lymphocytes. Three specific questions have been posed: a) Is transferrin endocytosed following binding to its specific receptor in a temperature-dependent fashion? b) Following endocytosis, is the carrier protein released from the cell in a structurally and functionally intact form? and c) Is the cell surface transferrin receptor also endocytosed following ligand binding? The results provide affirmative answers to all questions. Using two independent probes of the cell surface versus intracellular location of transferrin we observed that cell-bound transferrin moved from the cell surface to the inside of the cell and subsequently back to the medium. This process occurred in a temperature-dependent fashion. When cells containing only intracellular transferrin were further incubated at 37 degrees C approximately 80% of cell-bound transferrin was released to the medium. Nearly all of this material retained reactivity with antibody to transferrin. In addition, endocytosed transferrin exhibited qualitatively and quantitatively equivalent binding reactivity with the transferrin receptor and showed identical electrophoretic mobility on SDS gel electrophoresis. Finally, using similar methodology to that employed with transferrin itself, we provide evidence that the specific receptor is also endocytosed.

Uptake of cationized ferritin by colonic epithelium

Guinea-pig, rat and mouse were used in this electron microscopic study to demonstrate endocytosis in absorptive and goblet cells of the surface epithelium of the colon. Cationised ferritin was used as the electron dense tracer which attached in vivo to negatively charged membrane components. Both coated and uncoated vesicles entered the cells. The major pathway for the vesicles was to the secondary lysosomes and occurred within 10-30 min. This demonstrates a new pathway of absorption in the adult rodent colon with potential for the uptake of macromolecules. It may provide new clues to the immunology, physiology and pathology of the tissue.

A morphological, tracer and cytochemical study of the role of the papillary layer of the rat-incisor enamel organ during enamel maturation

Thin-section, freeze-fracture, tracer and cytochemical methods showed that papillary cells were characterized by the presence of numerous mitochondria and microvilli. The cytoplasms contained Golgi apparatus, multivesicular bodies (MVB), tubular structures and coated vesicles. In freeze-fracture replicas, some tubular structures and vesicles were continuous with the cell surfaces. Intravenously-injected horseradish peroxidase (HRP) was rapidly incorporated into the papillary cells via coated vesicles and tubular structures. Horseradish peroxidase-labelled coated vesicles and tubular structures then fused with endocytic vacuoles and MVB. These endocytic vacuoles and MVB showed HRP-labelling of various degrees. Acid-phosphatase activity was demonstrated in the Golgi apparatus, endocytic vacuoles and MVB of papillary cells. Enzymic activity sometimes took place in tubular structures but never occurred in coated vesicles. These results indicate that absorptive and digestive activities of papillary cells are closely related to loss of organic materials from the enamel matrix during enamel maturation.