Intestinal transport of antibodies in the newborn rat

Ultrastructural localization of calcium, by electron-probe X-ray microanalysis, in the small intestine of suckling rats

The subcellular distribution of calcium has been investigated in samples, from the intestinal mucosa of 10-day rats, prepared for X-ray microanalysis by various techniques designed to minimize the loss of this element. Calcium retention and its threshold of detection was most satisfactory in freeze-dried frozen thin sections. In resin-embedded samples the best retention of calcium was found in specimens fixed in absolute ethanol, embedded without osmication, and sectioned onto glycerol. The results of this investigation indicate the presence of calcium in the supranuclear vacuole of enterocytes in the distal intestine of the neonatal rat. This calcium is probably taken up during the endocytosis of material from the intestinal lumen. The same mechanism may also be important in the uptake of other metals by suckling animals.

Specific suppression of rat IgE responses with milk from immunized females and with feeds of serum antibody

Suckling by ovalbumin sensitized females suppressed the IgE anti-ovalbumin response of young rats. Suppression was also achieved after feeding rat pups with (i) immune adult rat serum and (ii) isolated serum IgG containing anti-ovalbumin antibody.

Absorption of peroxidase-conjugated immunoglobulin G by human placenta: an in vitro study

The uptake of IgG-HRP into human placental villi has been demonstrated. Initial uptake occurred via coated pits at the surface of the syncytium. It was subsequently localized in small vesicles and larger vacuoles (multivesicular bodies) within the syncytium. Reaction product was also localized in occasional cytotrophoblastic cells. The basal lamina contained some reaction product; protein was apparently capable of diffusing through the basal lamina, as evidenced by the presence of reaction product in and around fetal capillaries, and in placental macrophages (Hofbauer cells). Within the fetal capillary endothelium small vesicles were labelled, suggesting movement across this layer was vesicle-mediated and not by movement through the intercellular space.

Enzymes involved in protein transmission by the intestine of the newborn lamb

The intestine of lambs killed immediately after birth and at intervals after the first feed was studied by electron microscope cytochemistry. Ferritin, incorporated into this feed, was found within 2 h of feeding within vesicles throughout the cytoplasm of enterocytes lining the proximal and mid-intestine. Some of these vesicles had fused with the lateral and basal membranes of the enterocytes. Histochemical reaction products for alkaline phosphatase and a series of lysosomal enzymes were localized within the vesicles; the distribution of acid hydrolases, however, was not uniform within each cell. Biochemical estimations of the activity of these enzymes showed greatest activity in the distal intestine of the newborn lamb. The activity of only one of these enzymes, N-acetyl-beta-glucosaminidase, was maximal in the mid-intestine. These observations indicate that cytoplasmic vesicles, translocating proteins across the enterocyte, probably carry intestinal alkaline phosphatase activity in their limiting membrane. Lysosomal enzymes, particularly glucosaminidase, are introduced into these vesicles as they traverse the enterocytes of the mid-intestine. A less specialized complement of lysosomal enzymes is probably introduced into vesicles in the distal intestine where ingested protein may be digested, rather than transported across the cell.

Electron-microscopic and morphometric study of vesiculation in the epithelial cell layer of the toad urinary bladder. Effect of antidiuretic hormone

Basolateral vesicles are apparent in mitochondria-rich, granular and basal cells of the epithelium of unstimulated toad urinary bladder. Such vesicles are smooth-walled rather than coated. Because of the number of these vesicles, we decided to investigate the effect of arginine vasopressin (AVP) on this vesicle population during the hydroosmotic response. Both glutaraldehyde and glutaraldehyde-osmium tetroxide fixation procedures, as well as electron-microscopic morphometric techniques were utilized. Fifteen min after the administration of AVP in the presence of an osmotic gradient, the intracellular area occupied by basolateral vesicles was increased approximately threefold when compared to unstimulated control hemibladders (0.92 +/- 0.15% vs: 2.67 +/- 0.39%). The intracellular nature of the majority of these vesicles was confirmed by experiments utilizing lanthanum as an electron-dense tracer. It is hypothesized that this vesicular activity may be involved in water movement across the tissue after hormone treatment.

Effects of guanethidine-induced sympathectomy on morphology of the pre- and postclosure ileum of the neonatal rat

The disappearance of the characteristic supranuclear vacuole and extensive apical canalicular system from enterocytes of the ileal villi occurs during the third postnatal week in rats. This phenomenon is associated with loss of permeability of these cells to macromolecules and is therefore termed closure. The present study was designed to analyze the influence of neonatal guanethidine (GTN)-induced sympathectomy on the morphology of the pre- and postclosure ileum of the rat. Light and electron microscopy of control and GTN-sympathectomized rats demonstrated the retention of immature, vacuolated cells on ileal villi as late as 23 days postnatally in GTN-treated rats. Villi from control rats contained only adultlike nonpermeable cells. Electron microscopy further demonstrated no structural differences in the apical canalicular system or storage vacuoles of the delayed cells in GTN rats when compared to the ileal epithelium from preclosure time periods (7 and 15 days) in both GTN-sympathectomized and control rats. Goblet cells were counted on Periodic-Acid-Schiff-stained sections of ileum from 7, 15, and 23-day GTN and control rats. The percentage of goblet cells in the total epithelial cell population of the villus was significantly higher in control versus GTN rats at all time periods. The percentage of goblet cells increased in both groups from day 7 to 15. However, closure in the control group (approximately day 18) was coincident with a steep increase in the percentage of ileal goblet cells which was not evident in the goblet-cell population of the GTN villus. This pattern of change in control versus GTN goblet-cell production was correlated with a similar pattern of variation in the number of crypt cell mitoses between the two groups over the same time period.

Exocytosis of pinocytic contents by Chinese hamster ovary cells

The extent of exocytosis of pinocytic vesicle contents was studied in suspension-cultured Chinese hamster ovary (CHO) cells using horseradish peroxidase (HRP) as a pinocytic content marker. HRP was shown to be internalized via fluid-phase pinocytosis in CHO cells. After an HRP pulse of 2.5-10 min a rapid decrease of 30-50% in cell-associated HRP activity was observed within 10-20 min at 37 degrees C. During this time the loss of cell-associated HRP was accompanied by an equivalent increase in extracellular HRP. After this rapid exocytosis of HRP, the remaining peroxidase activity decreased with a t1/2 of 6-8 h, the known lysosomal half-life of HRP. In pulse-chase experiments HRP was chased into a nonexocytic compartment. Based on cell fractionation and electron microscopic experiments, this nonexocytic compartment was identified as a lysosome and the compartment from which exocytosis occurs as a pinosome. The occurrence of pinocytic content exocytosis in cultured fibroblasts suggests that exocytosis of pinocytic vesicle contents is a general phenomenon.

Coated vesicles from human placenta carry ferritin, transferrin, and immunoglobulin G

Coated vesicles prepared by using isotonic gradients retain their contents. Ferritin, transferrin, and immunoglobulin G have been detected in human placental coated vesicles. Triton X-100 extraction of crude coated vesicles removes contaminating uncoated membranes, leaving coated particles and their associated proteins apparently intact; this provides an efficient means of preparing large quantities of coated vesicle proteins. The clathrin cages surround "cores" of structural protein containing polypeptides of 100,000 and 50,000 molecular weight.

Restricted mobility and endocytosis of anionic sites on newborn rat jejunal brush border membranes

The distribution and mobility of anionic sites on the microvillous surface of newborn rat jejunal absorptive cells were studied using polycationic ferritin (PCF) as a visual probe and were compared with anionic sites previously described for adult jejunum. Segments from 5- to 26-day-old rats were incubated in PCF for 5 minutes either before or after fixation for electron microscopy. From days 5 to 20, anionic sites were distributed diffusely along the lengths of the microvilli and did not show random translational mobility. In contrast, microvilli examined from animals at weaning (2l to 26 days) resembled those from adults in which most binding sites were capable of lateral mobility and were induced by PCF to culster into discrete patches. The diffuse pattern was altered by cortisone administration, paralleling a premature reduction in the endocytic apparatus of the cell. The difference in mobility of anionic sites with age coincides with differences in absorptive function. Evidence is presented showing that in the neonate binding of PCF to the microvilli was followed with time by endocytosis into an apical system of tubules for intracellular transport, incorporation into coated vesicles, and release through the lateral cell surface. The results suggest that endocytosis is accomplished by a mechanism that includes a directionally controlled movement for the selective internalization of PCF binding sites from the membranes of the microvilli to those of the tubular cytoplasmic channels.

Formation of cytoskeletal elements during mouse embryogenesis. II. Epithelial differentiation and intermediate-sized filaments in early postimplantation embryos

Following our study on the expression of cytokeratin filaments in preimplantation mouse embryos [30], we have examined the organization of cytoskeletal elements in early postimplantation embryos up to day 8 of gestation, employing electron microscopy, immunofluorescence microscopy an two-dimensional gel electrophoresis of cytoskeletal proteins labelled by incorporation of 35S-nethionine. The characteristic epithelia formed by the embryonic ectoderm and proximal (visceral) endoderm present well-developed junctional complexes and various differentiated membrane structures. Several apical differentiations of the proximal endodermal cells, such as brush border-like microvilli, the endocytotic labyrinthum, and the supranuclear vacuoles resemble the organization of epithelial cells of the ileum of neonatal mammals. Both embryonic epithelia show typical desmosomes and attached intermediate sized filaments of the cytokeratin type. Other types of intermediate-sized filaments, such as vimentin and desmin filaments, have not been detected in any of the cells of embryos of days 6 and 7, but filaments of the vimentin type can be seen, by immunofluorescence microscopy, late in day 8 in certain cells located in the forming mesoderm. Gel electrophoresis has further revealed that the major cytoskeletal proteins synthesized during days 6-8 in both extraembryonic and embryonic tissue are similar to those characteristic of preimplantation blastocysts and include a major polypeptide corresponding to cytokeratin A described in some internal organs of adult rodents. By the same techniques, synthesis of another cytoskeletal proteins vimentin, has first been found late in day 8. It is concluded that early postimplantation embryonic development, up to mesoderm formation, is characterized by the exclusive presence, in both embryonic ectoderm and proximal endoderm, of differentiated epithelial cells containing desmosome-cytokeratin filament complexes and that other types of intermediate-sized filaments are not yet expressed.

Distribution of Fc gamma receptors on isolated gut enterocytes of the suckling rat

An erythrocyte-antibody rosette assay has been used to study the presence and distribution of Fc gamma receptors on enterocytes isolated from 12-day-old suckling rat gut by means of a buffer medium containing EGTA. Such receptors were found to be restricted to enterocytes in the proximal region (duodenum and jejunum) of the small intestine and to be acid-pH dependent. For the majority of enterocytes indicator red cells bound in high density to the abluminal plasmalemma but not to the apical microvillous brush border. Since immunofluorescence studies revealed strong binding of added IgG to the microvillous region, a likely explanation is that there is a paucity of Fc gamma receptors from the tips of microvilli (at least under the conditions of the rosette assay) and that receptors more deeply situated as inaccessible to indicator red cells. Binding of indicator red cells was readily inhibited by rabbit, human, guinea pig and rat IgG but less so by bovine IgG, and of the two sub-classes, bovine IgG2 inhibited much more readily than bovine IgG1. Cortisone acetate injection virtually abolished Fc gamma receptor expression on isolated enterocytes within three days. These findings correlate both with selective transport of IgG of different species in vivo and the known effect of cortisone acetate to terminate such transport.

Evidence for the sorting of endocytic vesicle contents during the receptor-mediated transport of IgG across the newborn rat intestine

Fc receptors on the luminal membranes of intestinal epithelial cells in the neonatal rat mediate the vesicular transfer of functionally intact IgG from the intestinal lumen to the circulation. In addition, there is a low level of nonselective protein uptake, but in this case transfer does not occur. To determine whether a specialized class of endocytic vesicles could account for the selective transfer of IgG, mixtures of IgG conjugated to ferritin (IgG-Ft) and unconjugated horseradish peroxidase (HRP) were injected together into the proximal intestine of 10-d-old rats, and the cellular distribution of these two different tracers was determined by electron microscopy. Virtually all apical endocytic vesicles contained both tracers, indicating simultaneous uptake of both proteins within the same vesicle. However, only IgG-Ft bound to the apical plasma membrane, appeared within coated vesicles at the lateral cell surface, and was released from cells. HRP did not bind to the luminal membrane and was not transferred across cells but was confined to apical lysosomes as identified by acid phosphatase and aryl sulfatase activities. To test the possibility that the binding of IgG to its receptor stimulated endocytosis, HRP was used as a fluid volume tracer, and the amount of HRP taken up by cells in the presence and absence of IgG was measured morphologically and biochemically. The results demonstrate that endocytosis in these cells is constitutive and occurs at the same level in the absence of IgG. The evidence presented indicates that the principal selective mechanism for IgG transfer is the binding of IgG to its receptor during endocytosis. Continued binding to vesicle membranes appears to be required for successful transfer because unbound proteins are removed from the transport pathway before exocytosis. These results favor the proposal that IgG is transferred across cells as an IgG-receptor complex.

Retro-endocytosis of low density lipoprotein by cultured bovine aortic smooth muscle cells

Cultured bovine aortic smooth muscle cells, pretreated with 125I-labelled low density lipoprotein (LDL), rapidly released significant amounts of the lipoprotein as trichloroacetic acid-precipitable material during a subsequent chase period. The time and temperature dependence of this release process and its insensitivity to heparin-pretreatment of equilibrated cells suggest that LDL was regurgitated from cells by a rapid process that we have termed 'retro-endocytosis'. The total amount of lipoprotein released from cells equilibrated at 37 degrees C with 125I-labelled LDL was approximately 20% of the amount degraded, pointing to the existence of a small pool of material which was distinct from the lysosomal pathway. To quantify the flux of LDL through retro-endocytosis, the fate of surface-bound lipoproteins was analyzed. Cells, pretreated with 125I-labelled LDL at 4 degrees C, regurgitated about 50% of the initial surface-bound LDL during a chase period at 37 degrees C and degraded the remainder more slowly through the lysosomal pathway. The involvement of LDL-receptors was implicated because retro-endocytosis was a saturable process and was affected by up- and down-regulation. The apolipoprotein of the released LDL showed little proteolytic modification as analyzed by gel filtration. We conclude that in a steady-state situation the fraction of LDL that passes through the retro-endocytosis pathway is of the same order as that which is directed through the lysosomal system.

Molecular characterization of human clathrin

Clathrin extracted from coated vesicles at pH 8.0 sediments as a single boundary with 8.1S sedimentation constant (s020,w) of 8.1 +/- 0.1 S. Sedimentation equilibrium gave a molecular weight (Mr) of 610 000 +/- 30 000. The clathrin frictional ratio (pH 7.5) computed from s020,w and Mr is very large, i.e., 3.06 +/- 0.18. Analysis of the circular dichroic spectrum in the far-ultraviolet showed that about half of the peptide residues are in a alpha-helical conformation. The molecular weight of a preparation of clathrin purified to homogeneity on a Sepharose CL-4B column in 6 M guanidine hydrochloride was 170 000 +/- 26 000 by sedimentation equilibrium, which is in agreement with the values we and others obtained by sodium dodecyl sulfate gel electrophoresis. The 8.1S clathrin species may be regarded as the "native" promoter since (1) it is extracted from coated vesicles by an extremely mild procedure, (2) it is stable over considerable ranges of pH, temperature, and ionic strength, and (3) it readily polymerizes into characteristic closed lattice structures resembling those observed in coated vesicles in the electron microscope. The 8.1S clathrin molecule self-associates at pH 6.3 to form two very high molecular weight species with average sedimentation coefficients of 150 and 300 S. The sedimenting boundaries of both of these species have been analyzed to reveal their molecular heterogeneity. The two species observed by sedimentation velocity may correspond to the two sizes of coated vesicles previously reported to be present in some cells when observed by electron microscopy. Analysis of the sedimentation pattern in the ultracentrifuge also gives the amount of unreacted 8.1S clathrin from which the yield of polymerizable clathrin is obtainable. This methodology can therefore be employed to estimate the quality of the 8.1S preparation of clathrin and thereby affords an assay of its activity.

Sprouting in the hippocampus is accompanied by an increase in coated vesicles

Coated vesicles within dentate gyrus granule cell dendrites were found free in the cytoplasm and also attached to the smooth endoplasmic reticulum and plasmalemma. Coated vesicles began increasing in number 40-48 h following partial dendritic deafferentation. This increase continued up to 15-30 days postlesion, after which the population steadily declined to control values. These results combined with the findings of previous studies suggest an involvement of coated vesicles in synaptic reinnervation.

Transport across endodermal cells of the chick yolk sac during early stages of development

The endoderm of the chick yolk sac mediates the transfer of materials from the yolk mass to the embryonic circulation. There is little evidence of endocytotic activity in the area pellucida, but the endodermal cells of the area vasculosa possess many microvilli and bristle-coated pits and vesicles, as well as a canalicular system and vacuoles in the apical cytoplasm. Three tracers, horseradish peroxidase, ferritin, and latex spheres, were injected beneath the endoderm of both cultured embryos and embryos in ovo to study the pathway of uptake of extracellular materials. All tracers were sequestered in bristle-coated pits (200-500 nm in diameter) of the endodermal cells of the area vasculosa, but not those in the area pellucida. Both horseradish peroxidase and latex spheres (and probably ferritin) were incorporated into pleomorphic intracellular yolk drops through bristle-coated pits and vesicles, and then into apical vacuoles, which fuse with the intracellular yolk drops. Horseradish peroxidase and ferritin were also found within apical canaliculi. The apical junctions between endodermal cells prevented the intercellular passage of the tracers. A "topping-up" hypothesis is opposed whereby endodermal cells of the area vasculosa continually sequester extracellular yolk material, which is incorporated into a digesting intracellular yolk drop while, at the same time, digested yolk products are being transported across the vascular pole of the endoderm to the extraembryonic circulation and thence to the embryo.

Rapid cell surface appearance of endocytic membrane proteins in Chinese hamster ovary cells

Lactoperoxidase was used to selectively radiolabel endocytic membrane. CHO cells were incubated with enzyme at 37 degrees C for 10 min to permit lactoperoxidase internalization. Radioiodination was done at 4 degrees C. About 90% of the radioiodinated products pelleted at 100,000 X g. From 12 to 15 different electrophoretic species were detected by one-dimensional gel electrophoresis. When cells labeled by internalized lactoperoxidase were warmed to 37 degrees C, the incorporated radioactivity was lost in a biphasic manner with an overall t1/2 of approximately 20 h. Upon warming cells to 37 degrees C, the labeled species became sensitive to pronase or trypsin digestion. The increase in protease sensitivity was progressive over a 10- to 20-min period. Maximally 45% of the initially intracellular radiolabel could be released. A digest of exterior-radioiodinated cells released 50% of the incorporated radioiodine. These observations strongly suggest a rapid shuttling of approximately 90% of the radioiodinated membrane species initially present within the cell to the cell surface.

Endocytosis in absorptive cells of cultured human small-intestinal tissue: horseradish peroxidase, lactoperoxidase, and ferritin as markers

The occurrence of endocytotic mechanisms in human small-intestinal absorptive cells was investigated by culturing biopsy specimens in the presence of horseradish peroxidase (HRP), lactoperoxidase (LPO), and ferritin. The results indicate that both HRP and LPO entered the cells by apical endocytosis, after which they were transported via apical vesicles and tubules to the lysosome-like bodies. Ferritin, which showed a distinct affinity for the cell-coat glycoproteins, was not interiorized by the absorptive cells. These findings suggest that although human absorptive cells have an endocytotic mechanism, possibly fluid-phase endocytosis, cell-coat glycoproteins are not taken up by the cells, as indicated by the absence of ferritin in the apical vesicles and tubules, as well as the lysosome-like bodies. These findings provide indirect support for our hypothesis that the lysosome-like bodies have a function in the regulation of cell-coat glycoprotein transport via a crinophagic mechanism (fusion of apical vesicles and tubules with lysosome-like bodies) rather than via an exocytotic-endocytotic mechanism.

Purification and characterization of clathrin from bovine brain

Clathrin has been purified to electrophoretic homogeneity by initial extraction of clathrin from purified coated vesicle fraction, followed by column chromatographies with gel filtration, DEAE-cellulose, and hydroxylapatite and finally by preparative sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Antibody specific to clathrin has also been obtained. Two forms of native clathrin, fast and slow components, have been prepared to about 95% purity by hydroxylapatite column chromatography. Both fast and slow components are believed to represent two different aggregates of clathrin subunit because they comigrate in agarose electrophoresis, pH 7.4, and also migrate as clathrin subunit on SDS-PAGE with a molecular weight of 175,000. Furthermore, both components cross-react with antibody against purified clathrin and compete for antibody binding site with labeled fast component. The fast component can also be converted to the slow component. In addition to clathrin, two proteins of about 38,000 and 35,000 M.W. that consistently copurified with native clathrin are probably also intrinsic to coated vesicle.

The uptake and transmission of protein by neonatal rat enterocytes

1. Proximal enterocytes transmitted intact immunoglobulin G (IgG) preferentially in the order rat, human, sheep and bovine; the removal from the vascular compartment of these transmitted molecules occurred at about the same rate. 2. Heterologous IgGs are processed similarly to rat IgG: they are either transmitted intact or broken down to less than 1000 mol. wt. fragments. 3. All of the human transferrin removed from the intestine was broken down to less than 1000 mol. wt. fragments, but a small amount of bovine serum albumin (BSA) was transmitted intact. 4. The IgGs and BSA are relatively indigestible molecules whereas human transferrin is relatively digestible. 5. These observations are discussed in the context of receptor-mediated transmission.

Molluscan visceral muscle fine structure. General structure and sarcolemmal organization in the smooth muscle of the intestinal wall of Buccinum undatum L

Fine structure of intestinal muscle in the gastropod Buccinum undatum is described. Myofibrillar organization is typical of non-pseudostriated molluscan muscles. The dense body system is poorly developed but there are extensive attachments plaques. The sarcolemma is elaborately modified. Deep infoldings of the membrane give the cells an irregular outline. Such infoldings enclose extracellular matrix and are associated with attachment plaques. Arising from these and from the general sarcolemma are numerous tubular membranous invaginations ending blindly at varying depth in the sarcoplasm. These structures have a helical coat of particles on the cytoplasmic face. Associated with both types of invagination are subsarcolemmal vesicles. The possibility that the tubular invaginations are analogues of vertebrate smooth muscle caveolae or striated muscle T-tubules and that the vesicles are the corresponding sarcoplasmic reticulum is discussed. The occurrence of such structures in molluscan muscle and elsewhere is reviewed.

Immune complexes facilitate uptake of antigen from the gut

Radiolabeled DNP-ovalbumin was injected with and without DNP-specific antibody into ligated gut segments of neonatal rats. The transfer of this labeled protein to mesenteric lymph nodes and binding to gut tissue was examined. When the antigen, DNP-ovalbumin, was present in the gut in an immune complex from a selective localization of antigen in the mesenteric lymph node was observed. These data suggest that immune complexes in milk could potentially modulate neonatal immunological development.

An electron-histochemical study on protein uptake and distribution in the colonic mucosa of the neonatal rat

The uptake of exogenous protein by the colonic mucosa in neonatal rats was studied after enema infusions of horseradish peroxidase, of immunoglobulin conjugated with horseradish peroxidase, of unconjugated immunoglobulin, and of normal saline. Specimens of colonic mucosa were processed for the histochemical demonstration of peroxidase activity and then prepared for electron microscopy. Enzyme activity and, by inference, exogenous protein were located on the luminal plasma membrane of epithelial lining cells, in pinocytotic vesicles in their apical cytoplasm, in Golgi-associated sacs and in their secondary lysosomes. Protein was also located within lymphocytes which had invaded the epithelium. Transepithelial passage of protein was proven by its demonstration within the lysosomal systems of macrophages and nerves in the lamina propria. The demonstration of exogenous protein within the macrophage system of the colon is of interest in view of the putative allergic basis of certain bowel conditions.

The antibody-induced clustering and endocytosis of HLA antigens on cultured human fibroblasts

It has previously been shown by immunofluorescence experiments that the cross-linking of HLA antigens into patches (by antibody reagents directed to human beta 2--microglobulin) on the surfaces of cultured human fibroblasts leads to the lining up of the patches over the actomyosin-containing stress fibers lying immediately under the surface membrane. These experiments have now been extended to the resolution of the electron microscope by the use of ferritin-conjugated antibody. The results show that a substantial part of the HLA surface clusters that form by 5 min after the addition of the antibody reagents is found in small uncoated surface invaginations which are subsequently endocytosed and ultimately fuse with lysosomal bodies. At no stage in this process is there any indication that coated pits or coated vesicles participate. These and other results suggest, therefore, that there are at least two distinct mechanisms for the ligand-induced endocytosis and lysosomal processing of membrane components, one involving coated pits and the other the noncoated invaginations described in this paper. Transmembrane associations of clusters with intracellular actomyosin-containing structures may have a role in the endocytosis of these noncoated invaginations.

Brain clathrin: immunofluorescent patterns in cultured cells and tissues

High-purity bovine brain clathrin elicited antibodies, which were separated by affinity chromatography on clathrin conjugated with CNBr-activated Sepharose 4B. Indirect immunofluorescent labeling on cultured rat fibroblasts showed the characteristic dotted fluorescence throughout the entire cytoplasm and concentrated patches of fluorescence in areas where the Golgi apparatus and GERL (Golgi apparatus-endoplasmic reticulum-lysosome complex) exist next to the nucleus. Cerebellar tissue sections exhibited profuse immunofluorescence in the granular layer; dissociated cultures of neonatal rat cerebellar cells showed intense fluorescence in the cytoplasm and processes of granule cell neurons. The data suggest that clathrin is an abundant protein distinctly localized in the cytoplasm of cells and clearly correlated in its distribution and arrangement with actin stress fibers.

Coated pits act as molecular filters

Two proteins--theta and H63 antigens--are found to be excluded from coated pits on fibroblast plasma membranes. Coated pits thus act as molecular filters, pinching off, into the cell, lipid vesicles containing a limited number of specific receptors but excluding other plasma membrane proteins. This constitutes part of a lipid flow cycle that was proposed earlier to explain the capping of crosslinked surface antigens.

An immunofluorescence study of IgG receptors in rodent enterocytes

Locations of IgG receptors in the small intestine of suckling rats were visualized by treating sections of the gut with IgG and FITC-labelled anti-IgG antibodies. Receptors, operative optimally at acid pH, occurred only in younger rats whose intestines were still permeable to antibodies; they were located on the brush borders of the enterocytes on the apical parts of the villi in the duodenum and jejunum. Premature disappearance of the receptors from the gut was achieved by cortisone treatment. In rats and mice, IgG from different species each competed for attachment to the receptors in vitro in the same way as it inhibited the transmission of other IgG across the intestine in vivo, thus reflecting the involvement of the receptors in the transport mechanism.

Studies on the immunoglobulin-G Fc-fragment receptor from neonatal rat small intestine

1. A method for preparing the small-intestinal brush-border membrane of neonatal rats is described in which enzymic methods are used to remove associated polysaccharide and cell nuclei. 2. 125I-labelled IgG (immunoglobulin G) and 125I-labelled IgG Fc fragment have high specific binding and low non-specific binding to brush borders prepared in this way. F(ab)'2 fragment however, does not bind, indicating the existence of a specific receptor for the Fc fragment of IgG. The receptor system is saturable, and the affinity (KA) for the binding of rat IgG was determined by both equilibrium and kinetic methods. 3. The binding of heterologous IgG species (human and bovine) was compared and demonstrated a close similarity between human IgG and rat IgG in their receptor affinities. 4. Kinetic results are presented that are consistent with previously proposed models of ligand-induced receptor aggregation.

The uptake of IgG by human placental chorionic villi: a correlated autoradiographic and wide aperture counting study

The uptake of 3H-IgG into human placental chorionic villi in vitro takes place for at least 1 h at 37 degrees C and at 4 degrees C. The rate of uptake is lower at the latter temperature, but still about 20 per cent of the 37 degrees C total. Measurement of cell-associated redioactivity at 4 degrees C cannot therefore be used as a measure of binding: genuine uptake, probably as a result of endocytosis, appears to occur at this temperature. Some proportion of the uptake of IgG at 37 degrees C can be inhibited by colchicine and by cytochalasin B, but some is refractory to these treatments. A coated vesicle-enriched fraction isolated from placenta previously incubated with 3H-IgG was found to be associated with radioisotope.

Immunoglobulin production and transport by the mammary gland

In origin immunoglobulins in mammary secretions are both humoral, arising from the blood stream, and local, arising from production by plasmacytes in the mammary gland. The relative importance of each of these sources varies between species. In some species (human, rabbit, etc.), the transfer of maternal immunoglobulins to the blood stream of the neonate occurs in utero across the placenta or yolk sac membrane. In other species, including ruminants, transfer of maternal immunoglobulins to the neonate occurs exclusively via the colostrum. Both in utero and colostral routes of transfer are operative in other species. The concentration and class of immunoglobulins in the colostrum and milk of a species reflect the route and origin of the immunoglobulins. Immunoglobulins transferred in quantity in utero or via the colostrum are mainly of the IgG class. Immunoglobulins locally produced by plasmacytes located adjacent to the secretory epithelium and in the mammary secretions are largely of the IgA and IgM classes. The bovine transfers large amounts of IgG immunoglobulins, and IgG1 in particular, from the blood stream across the mammary barrier into colostrum (and milk) by a specific transport mechanism. Bovine colostrum and milk also contain much smaller amounts of locally produced IgA and IgM.

Distribution of immunoglobulin G receptors in the small intestine of the young rat

Conjugates of horseradish peroxidase (HRP) and immunoglobulin G (IgG) were used to map the distribution of cell surface receptors that can bind IgG at 0 degrees C within the small intestine of 10-12-d-old rats. Luminal receptors are present only within the duodenum and proximal jejunum. In these locations, receptors are limited to absorptive cells that line the upper portion of individual villi. Near villus tips, receptors are relatively evenly distributed over the entire luminal plasmalemma. In the midregion of villi, receptors are unevenly distributed over the luminal surface. Receptors (a) specifically bind rat and rabbit IgG, (b) recognize the Fc portion of the immunoglobulins, and (c) bind at pH 6.0 but not pH 7.4. To determine whether IgG receptors are confined to the luminal portion of the plasmalemma, intact epithelial cells were isolated from the proximal intestine of 10-12-d-old rats and incubated with HRP conjugates at 0 degree C. The specific binding of rat IgG-HRP to cells at pH 6.0 indicates that IgG receptors, which are functionally similar to those found on the luminal surface, are also present over the entire abluminal surface of absorptive cells. These results are consistent with the transport of IgG to the abluminal plasma membrane in the form of IgG-receptor complexes on the surface of vesicles. Exposure of these complexes to the serosal plasma, which is presumably at pH 7.4, would cause release of IgG from the receptors. To assess possible inward movement of vesicles from the abluminal surface after discharge of IgG, intravenously injected HRP was used as a space-filling tracer in the serosal plasma. HRP could be visualized within the coated and tubular vesicles responsible for transport of IgG in the opposite direction. These vesicles may, therefore, provide a pathway whereby receptors shuttle between the luminal and abluminal surfaces of cells.

Three-dimensional visualization of coated vesicle formation in fibroblasts

Fibroblasts apparently ingest low density lipoproteins (LDL) by a selective mechanism of receptor-mediated endocytosis involving the formation of coated vesicles from the plasma membrane. However, it is not known exactly how coated vesicles collect LDL receptors and pinch off from the plasma membrane. In this report, the quick-freeze, deep-etch, rotary-replication method has been applied to fibroblasts; it displays with unusual clarity the coats that appear under the plasma membrane at the start of receptor-mediated endocytosis. These coats appear to be polygonal networks of 7-nm strands or struts arranged into 30-nm polygons, most of which are hexagons but some of which are 5- and 7-sided rings. The proportion of pentagons in each network increases as the coated area of the plasma membrane puckers up from its planar configuration (where the network is mostly hexagons) to its most sharply curved condition as a pinched-off coated vesicle. Coats around the smallest vesicles (which are icosahedrons of hexagons and pentagons) appear only slightly different from "empty coats" purified from homogenized brain, which are less symmetrical baskets containing more pentagons than hexagons. A search for structural intermediates in this coat transformation allows a test of T. Kanaseki and K. Kadota's (1969. J. Cell Biol. 42:202--220.) original idea that an internal rearrangement in this basketwork from hexagons to pentagons could "power" coated vesicle formation. The most noteworthy variations in the typical hexagonal honeycomb are focal juxtapositions of 5- and 7-sided polygons at points of partial contraction and curvature in the basketwork. These appear to precede complete contraction into individual pentagons completely surrounded by hexagons, which is the pattern that characterizes the final spherical baskets around coated vesicles.

Mechanisms of uptake and the fate of serum proteins and horseradish peroxidase in cultured human glioma cells. A light- and electron-immunocytochemical study

Previous studies have shown that serum proteins are taken up from extracellular oedema fluid by reactive astrocytes and by tumour astrocytes. The present investigation was designed to define the mechanism of this protein uptake. Two or 3-week-old explant cultures from 26 astrocytic gliomas, one anaplastic ependymoma, and five non-glial intracranial tumours were treated with either human IgG (12 mg/ml), human serum albumin, (44 mg/ml) or horseradish peroxidase (0.1--4.0 mg/ml) for 4--24 h. Human IgG and albumin were subsequently detected in cultured cells by the indirect peroxidase-antiperoxidase (PAP) method for light microscopy or by direct peroxidase conjugate technique for electron microscopy. Horseradish peroxidase activity was localised by treatment with diaminobenzidine and hydrogen peroxide. Results of the study show that human serum proteins and horseradish peroxidase are taken up by tumour astrocytes and ependymal cells, and by macrophages, but not by non-glial tumour cells nor by mesenchymal elements in the glioma cultures. Electron immunocytochemistry suggests that the serum proteins are taken up by smooth walled micropinocytic vesicles (approximately 80 nm in diameter) which fuse to form larger endocytic vesicles (200--300 nm); these vacuoles in turn fuse with secondary lysosomes to form cytoplasmic bodies 1.2--3 mum in diameter.

Selective protein transport: identity of the solubilized phosvitin receptor from chicken oocytes

By two independent methods, the solubilized receptor for phosvitin (PV) has a subunit MW of 116K. Affinity chromatography, showed that only 2 of the more than 25 proteins present in the total detergent solubilized oocyte membrane extract were retained on a PV-agarose column. These proteins of MW of 116K and 100K could be eluted from PV-agarose with free PV. By gel exclusion chromatography, the receptor-125I-PV complexes elute in the void volume of a Biogel A-1.5 column. When these void fractions were assayed gy SDS-PAGE only a single protein of MW of 116K was observed in addition to 125I-PV.

Intestinal absorption of immune complexes by neonatal rats: a route of antigen transfer from mother to young

Horseradish peroxidase (HRP) in the presence of specific immunoglobulin G antibody to HRP is selectively absorbed from the gut lumen and transferred by intestinal epithelial cells to the lamina propria in newborn rats. The HRP is not transferred in detectable amounts in the absence of the antibody. Transport of maternally derived antigen via antigen-antibody complexes may have important influences on the developing immune system in young mammals.

Development of villus absorptive cells in the human fetal small intestine: a morphological and morphometric study

We describe the sequential ultrastructural changes in villus absorptive cells of human fetal small intestine between 9 and 22 weeks of gestation. In concert with villus formation at 9 to 10 weeks, a complex membranous system designated the apical tubular system appeared in the apical cytonous system designated the apical tubular system appeared in the apical cytoplasm of absorptive cells. The apical tubular system consisted of deep invaginations of plasma membrane and membrane-bounded vesicles and tubules. Some elements of this system were characterized by linear arrays of particles on the inner (luminal) membrane leaflet. After villus formation, many lysosomal elements designated "meconium corpuscles" also appeared in the apical cytoplasm. Modified morphometric studies suggested that both the apical tubular system and the lysosomal elements were more extensively developed in the distal than in the proximal intestine, were most abundant at 15 to 17 weeks, and decreased by 18 to 22 weeks. Morhpometry also showed an inverse relationship between the relative surface density of the apical tubular system and microvillus membrane, suggesting the possible derivation of elements of the former from the apical plasma membrane. Exposure of intestine to ferritin for 8 to 40 minutes in vitro revealed ferritin in elements of the apical tubular system of 12- to 20-week fetuses. There was no evidence of transport of ferritin across absorptive cells. Distinctive membranous bodies composed of convoluted membrane-bound cisternae separated by narrow channels of cytoplasmic matrix were seen in the Golgi region and apical cytoplasm of fetal absorptive cells between 14 and 22 weeks. In a single 22-week fetus, there was marked proliferation of smooth endoplasmic reticulum, a decrease in cytoplasmic glycogen and loss of most lysosomal and apical tubular elements in the proximal but not the distal intestine. Thus, by the end of the second trimester, the structure of absorptive cells in proximal intestine was remarkably similar to absorptive cells in adult intestine.

The development of immunoglobulin-containing cell populations in intestine, spleen and mesenteric lymphnode of the young rat

Immunohistological and immunochemical techniques were used to study the absorption, synthesis and secretion of immunoglobulins A, G and M in rats from 0-51 days of age. Intestinal epithelial cell-staining with IgA and IgG occurred during the first two weeks of the suckling period. Immunoglobulin G was detected in serum at all ages studied. IgA was first detected in animals aged one week, the levels rising gradually in succeeding weeks; there was a concomitant increase in the number of IgA-containing cells in the lamina propria of the intestine. No evidence was obtained to indicate the route of intestinal secretion of this class of immunoglobulin. Immunoglobulin M-containing cells were present in only small numbers.

The effect of anti-microvillus membrane antibodies on the transport of IgG across the suckling rat intestine

Rabbits were immunized with a preparation of microvillus membranes derived from the absorptive cells of the small intestine of suckling rats. The resulting immune serum contained antibodies which reacted specifically with the brush borders and walls of cytoplasmic vesicles of the intestinal cells, and was more effective than normal serum in preventing the transport of IgG across the intestinal cells. These results reflect the participation of microvillus membrane components, or receptors, in the transport mechanism.

Lipoprotein particles in the jejunal mucosa of postnatal developing rats

The jejunal mucosa of neonatal rats contains lipid particles of the same size, electron density and intracellular and extracellular distribution as particles identified by others in adult jejunum as lipoprotein particles. As in fetal jejunum obtained during the last three days of gestation, the jejunal mucosa of unsuckled newborn rats contains exclusively lipoprotein particles the size of very low density lipoproteins (VLDL). Within one day after initiation of suckling, there is in the mucosa a spectrum of lipoprotein particles ranging widely in size from those of VLDL particles to those of chylomicrons. These particles are seen in the endoplasmic reticulum and Golgi material of absorptive cells and within interepithelial cell spaces, the extracellular spaces of the lamina propria and lymphatic lacteals. VLDL-sized and chylomicron-sized particles are also seen, although in decreasing number, in the jejunal mucosa of 18-day-old suckling rats. However, in rats of comparable age, fasted for 48 or 72 hours, only VLDL-sized particles are seen in the jejunal mucosa. Ligation and transection of bile duct followed by fasting in rats of this age results in a marked decrease in the number of lipoprotein particles in absorptive cells. The results indicate that endogenous lipid contributes to the formation of VLDL particles whereas dietary triglycerides are needed for formation of chylomicrons.