Intestinal transport of antibodies in the newborn rat

Intracellular traffic of the MHC class I-like IgG Fc receptor, FcRn, expressed in epithelial MDCK cells

Transfer of passive immunity from mother to the fetus or newborn involves the transport of IgG across several epithelia. Depending on the species, IgG is transported prenatally across the placenta and yolk sac or is absorbed from colostrum and milk by the small intestine of the suckling newborn. In both cases apical to basolateral transepithelial transport of IgG is thought to be mediated by FcRn, an IgG Fc receptor with homology to MHC class I antigens. We have now expressed the human FcRn in polarized MDCK cells and analyzed the intracellular routing of the receptor. FcRn showed a predominant intracellular localization at steady state. Newly synthesized FcRn was delivered in a non-vectorial fashion to both the apical and basolateral surfaces of MDCK cell monolayers. Following internalization from the apical or basolateral domain, the receptor transcytosed to the opposite surface. These findings provide direct evidence for the transepithelial transport function of FcRn and indicate that the receptor undergoes multiple rounds of transcytosis.

Functional role of sialic acid in IgG binding to microvillus membranes in neonatal rat intestine

A close parallelism exists between sialylation and endocytotic activity of the small intestine during postnatal development in rats. Thus, the binding of 125I-labelled IgG to microvillus membranes and its relationship to membrane sialic acid has been studied in suckling rat intestine, during (a) postnatal development; (b) cortisone-induced precocious maturation, and (c) after desialylation of brush borders by neuraminidase treatment. Neuraminidase-treated membranes exhibited low (42%, p < 0.001) IgG binding. The observed decrease in IgG binding to desialylated membranes was associated with a decrease in the value of affinity constant, (-Ka = 0.4 x 10(6) M-1 in control and 0.23 x 10(6) M-1 in desialylated membranes). The number of IgG-binding sites (2.3 nmol/mg protein) was unchanged under these conditions. A similar decrease (50%) in IgG binding to brush borders was also observed in cortisone-injected pups. This was associated with reduced sialic acid (37%) content of the membranes compared to the controls. The value of -Ka was reduced from 0.4 x 10(6) M-1 in the control to 0.3 x 10(6) M-1 in the hormone-injected pups. The number of binding sites (n) was decreased from 2.2 to 1.4 nmol/mg protein under these conditions. Low concentrations of calcium (0.1-1.6 mM) in the incubation medium enhanced IgG binding (p < 0.001) to brush borders in pups but there was no change in binding of IgG to the membranes at 2 mM Ca2+ concentration compared to controls. Addition of Zn2+ or Mg2+ did not affect IgG binding under these conditions. These findings suggest a functional role of Ca2+ and sialic acid residues of the membrane glycans in IgG-receptor interactions in suckling rat intestine.

Nonvectorial surface transport, endocytosis via a Di-leucine-based motif, and bidirectional transcytosis of chimera encoding the cytosolic tail of rat FcRn expressed in Madin-Darby canine kidney cells

Transfer of passive immunity from the mother to the fetus or newborn involves the transport of IgG across several epithelia. Depending on the species, IgG is transported prenatally across the placenta and yolk sac or is absorbed from colostrum and milk by the small intestine of the suckling newborn. In both cases apical to basolateral transepithelial transport of IgG is thought to be mediated by FcRn, an IgG Fc receptor with homology to major histocompatibility class I antigens. Here, we analyzed the intracellular routing of chimera encoding the rat FcRn tail fused to the ecto- and transmembrane domain of the macrophage FcgammaRIIb. Newly synthesized chimera were delivered in a nonvectorial manner to the apical and basolateral cell surface, from where the chimera were able to internalize and transcytose. Apical to basolateral and basolateral to apical transcytosis were differently regulated. This intracellular routing of the chimera is similar to that of the native FcRn, indicating that the cytosolic tail of the receptor is necessary and sufficient to endow an unrelated FcR with the intracellular transport behavior of FcRn. Furthermore, the di-leucine motif in the cytosolic domain of FcRn was required for rapid and efficient endocytosis but not for basolateral sorting of the chimera.

Beta2-microglobulin-deficient mice are resistant to bullous pemphigoid

Recent understanding of the mechanism of immunoglobulin G (IgG) catabolism has yielded new insight into antibody-mediated diseases. We proposed that beta2-microglobulin (beta2m)-deficient mice have been protected from systemic lupus erythematosis (SLE)-like syndromes because they lack the beta2m-associated IgG protection receptor (FcRn) and therefore catabolize IgG, including pathogenic IgG autoantibodies, considerably more rapidly than normal mice. Such an hypothesis would predict that beta2m-deficient mice would also be resistant to experimental bullous pemphigoid, a disease with a pathogenesis thought to be much simpler than SLE, being the result of antibody directed toward a pathogenic epitope on the epidermal hemidesmosome that anchors basal keratinocytes to the basement membrane. To test this hypothesis, we administered pathogenic rabbit antibody directed toward the hemidesmosome to beta2m-deficient mice and to normal control mice, both intraperitoneally and intradermally, and assessed the mice clinically, histologically, and immunologically for manifestations of skin disease. We found that the beta2m-deficient mice were protected when the antibody was given intraperitoneally whereas intradermal administration resulted in blisters only slightly less severe than those seen in normal mice. These data would indicate that autoantibody-mediated inflammation might be prevented or controlled by appropriate modulation of FcRn function.

Ontogenetic development and distribution of antibody transport and Fc receptor mRNA expression in rat intestine

The intestine of the suckling rat has the unique capacity of absorbing immunoglobulins from maternal milk. We investigated intestinal Fc receptor mRNA expression and the absorption of orally administered antibodies to delineate the ontogeny and tissue specificity of this transport system. Duodenal expression of Fc receptor mRNA was at maximum levels between 1 and 19 days of age, but was not detectable during fetal life and in animals after weaning. Along the horizontal axis of the intestine, FcRn mRNA expression was maximum in the proximal duodenum and declined gradually in distal bowel. Similarly, absorption of orally administered antibody was low shortly after birth, but reached maximum levels at 14 days of age. By the time of weaning, antibody uptake had almost completely ceased. These data further delineate the temporal and spatial nature of the intestinal immunoglobulin transport system, and represent additional examples of how the intestinal Fc receptor is transcriptionally regulated.

Finally! The Brambell receptor (FcRB). Mediator of transmission of immunity and protection from catabolism for IgG

F. W. Rogers Brambell was the father of the field of transmission of immunity, which he entered 50 years before the present era. As part of his quantitative and temporal studies on transmission, he defined the first Fc receptor system for IgG, and furthermore recognized the link between transmission of passive immunity from mother to young and protection from catabolism for IgG. This article provides a historical overview of the efforts of Professor Brambell and summarizes the subsequent elaboration of the details of the physiology and molecular biology of this remarkable receptor system.

Fc receptors and their interactions with immunoglobulins

Receptors for the Fc domain of immunoglobulins play an important role in immune defense. There are two well-defined functional classes of mammalian receptors. One class of receptors transports immunoglobulins across epithelial tissues to their main sites of action. This class includes the neonatal Fc receptor (FcRn), which transports immunoglobulin G (IgG), and the polymeric immunoglobulin receptor (pIgR), which transports immunoglobulin A (IgA) and immunoglobulin M (IgM). Another class of receptors present on the surfaces of effector cells triggers various biological responses upon binding antibody-antigen complexes. Of these, the IgG receptors (Fc gamma R) and immunoglobulin E (IgE) receptors (Fc epsilon R) are the best characterized. The biological responses elicited include antibody-dependent, cell-mediated cytotoxicity, phagocytosis, release of inflammatory mediators, and regulation of lymphocyte proliferation and differentiation. We summarize the current knowledge of the structures and functions of FcRn, pIgR, and the Fc gamma R and Fc epsilon RI proteins, concentrating on the interactions of the extracellular portions of these receptors with immunoglobulins.

Abnormally short serum half-lives of IgG in beta 2-microglobulin-deficient mice

The MHC class I-related receptor, FcRn, mediates the transfer of maternal gamma globulin (IgG) to young rodents, primarily via intestinal transcytosis, and this provides humoral immunity for the first few weeks after birth. In a previous study, the site of mouse IgG1 (mIgG1) with which FcRn interacts has been mapped using recombinant wild-type and mutated Fc-hinge fragments. The site encompasses residues at the CH2-CH3 domain interface of Fc (Ile253, His310, Gln311, His433 and Asn434) and the same amino acids are involved in regulating the pharmacokinetics of the Fc-hinge fragments. This suggests that in addition to its known function, FcRn might also play a role in IgG homeostasis. Consistent with this hypothesis, in this study, we demonstrate that FcRn alpha-chain mRNA is present not only in neonatal brush border but also in other tissues of adult animals (liver, lung, spleen and endothelial cells). In addition, analysis of the pharmacokinetics of mouse Ig/Fc-hinge fragments in genetically manipulated mice that are deficient in the expression of FcRn demonstrates that the beta-phase half-lives are abnormally short. These findings suggest that FcRn is involved in IgG homeostasis.

Effects of receptor dimerization on the interaction between the class I major histocompatibility complex-related Fc receptor and IgG

The neonatal Fc receptor (FcRn) transports maternal IgG from ingested milk in the gut to the bloodstream of newborn mammals. An FcRn dimer was observed in crystals of the receptor alone and of an FcRn-Fc complex, but its biological relevance was unknown. Here we use surface plasmon resonance-based biosensor assays to assess the role of FcRn dimerization in IgG binding. We find high-affinity IgG binding when FcRn is immobilized on a biosensor chip in an orientation facilitating dimerization but not when its orientation disrupts dimerization. This result supports a model in which IgG-induced dimerization of FcRn is relevant for signaling the cell to initiate endocytosis of the IgG-FcRn complex.

Localization and biosynthesis of aminopeptidase N in pig fetal small intestine

BACKGROUND & AIMS

Little is known about the expression of brush border enzymes in fetal enterocytes. The aim of this study was to describe the localization and biosynthesis of porcine fetal aminopeptidase N.

METHODS

This study was performed using histochemistry and immunoelectron microscopy and [35S]methionine labeling of cultured mucosal explants.

RESULTS

Enzyme activity was present in the brush border membrane and extended into the apical cytoplasm. The protein was colocalized with cationized ferritin at the surface of endocytic structures including coated pits, vesicles, tubules, and large vacuoles in the apical cytoplasm. The transient high mannose-glycosylated form of fetal aminopeptidase N was processed to the mature complex-glycosylated form at a markedly slower rate than the enzyme in adult intestine. Likewise, dimerization occurred slowly compared with the adult form of aminopeptidase N, and it took place mainly after the Golgi-associated complex glycosylation. The enzyme had a biphasic appearance in the Mg(2+)-precipitated and microvillar fractions, indicating that the bulk of newly made aminopeptidase N is transported to the brush border membrane before appearing in the apical endocytic structures.

CONCLUSIONS

In comparison with the adult enzyme, fetal aminopeptidase N has a more widespread subcellular distribution with substantial amounts present in apical endocytic compartments characteristic of the fetal enterocyte.

Comparison of the gastrointestinal anatomy, physiology, and biochemistry of humans and commonly used laboratory animals

In addition to metabolic differences, the anatomical, physiological, and biochemical differences in the gastrointestinal (G.I.) tract of the human and common laboratory animals can cause significant variation in drug absorption from the oral route. Among the physiological factors, pH, bile, pancreatic juice, and mucus and fluid volume and content can modify dissolution rates, solubility, transit times, and membrane transport of drug molecules. The microbial content of the G.I. tract can significantly affect the reductive metabolism and enterohepatic circulation of drugs and colonic delivery of formulations. The transit time of dosage forms can be significantly different between species due to different dimensions and propulsive activities of the G.I. tract. The lipid/protein composition of the enterocyte membrane along the G.I. tract can alter binding and passive, active, and carrier-mediated transport of drugs. The location and number of Peyer's patches can also be important in the absorption of large molecules and particulate matter. While small animals, rats, mice, guinea pigs, and rabbits, are most suitable for determining the mechanism of drug absorption and bioavailability values from powder or solution formulations, larger animals, dogs, pigs, and monkeys, are used to assess absorption from formulations. The understanding of physiological, anatomical, and biochemical differences between the G.I. tracts of different animal species can lead to the selection of the correct animal model to mimic the bioavailability of compounds in the human. This article reviews the anatomical, physiological, and biochemical differences between the G.I. tracts of humans and commonly used laboratory animals.

Endocytosis and transepithelial transport of endolymph in the endolymphatic sac

The fate of cationized ferritin (CF) injected into the endolymphatic space of the endolymphatic sac was observed by transmission electron microscopy. At 10 min after the injection, CF particles bound to the apical plasma membrane of epithelial cells of the sac and were then endocytosed with coated pits. However, they never passed through the junctional complexes between the epithelial cells. At 30 min after the injection, the CF particles were transferred to endosomes and lysosomes by small vesicles of 100-150 nm in diameter. CF particles were also found in small vesicles close to Golgi cisternae and in multivesicular bodies. Acid phosphatase positive lysosomes were found close to endosomes containing CF particles. In addition, a small fraction of the small vesicles containing CF particles became inserted into the basolateral plasma membrane. At 60 min after the injection, many CF particles were found in acid phosphatase positive secondary lysosomes. These observations suggest that endocytosis of endolymph is actively performed by the epithelial cells of the sac, and transepithelial vesicular transport across the epithelial cells occurs.

Beta 2-microglobulin co-distributes with the heavy chain of the intestinal IgG-Fc receptor throughout the transepithelial transport pathway of the neonatal rat

Maternal IgG crosses the proximal small intestine of the suckling rat by receptor-mediated endocytosis and transepithelial transport. The Fc receptor resembles the major histocompatibility complex class I antigens in that it consists of two subunits: a transmembrane glycoprotein (gp50) in association with beta 2-microglobulin. We used immunofluorescence microscopy and quantitative immunogold cytochemistry to study the subcellular distribution of the two subunits. In mature absorptive cells both subunits were colocalized in each of the membrane compartments that mediate transcytosis of IgG. IgG administered in situ apparently caused both subunits to concentrate within endocytic pits of the apical plasma membrane, suggesting that ligand causes redistribution of receptors at this site. These results support a model for transport in which IgG is transferred across the cell as a complex with both subunits. During absorptive cell differentiation, gp50 and beta 2-microglobulin showed nearly identical patterns of increased expression that accompanied the development of the apical endocytic apparatus and terminal web. However, absorptive cells in weanling rats expressed no detectable gp50 and only low levels of beta 2-microglobulin in the Golgi region and on the basolateral plasma membrane where class I antigens would likely reside. Thus, beta 2-microglobulin has a novel distribution unrelated to its function as a subunit of the class I antigens. The co-expression of the two receptor subunits is restricted to neonatal epithelial cells engaged in IgG transport and is coordinately regulated during absorptive cell differentiation and during postnatal intestinal development.

Crystal structure of the complex of rat neonatal Fc receptor with Fc

The neonatal Fc receptor (FcRn) transports maternal immunoglobulin G (IgG) to the bloodstream of the newborn. FcRn is structurally similar to class I major histocompatibility complex (MHC) molecules, despite differences in the ligands they bind (the Fc portion of IgG and antigenic peptides, respectively). A low-resolution crystal structure of the complex between FcRn and Fc localizes the binding site for Fc to the side of FcRn, distinct from the tops of the alpha 1 and alpha 2 domains which serve as the peptide and T-cell receptor binding sites in class I molecules. FcRn binds to Fc at the interface between the Fc CH2 and CH3 domains, which contains several histidine residues that could account for the sharply pH-dependent FcRn/IgG interaction. A dimer of FcRn heterodimers observed in the co-crystals and in the crystals of FcRn alone could be involved in binding Fc, correlating with the 2:1 binding stoichiometry between FcRn and IgG (ref. 4) and suggesting an unusual orientation of FcRn on the membrane.

Immunolocalization of a mesenchymal antigen specific to the gastrointestinal tract

BACKGROUND

The aim of the present study was to localize, at the fine structural level, a protein found by indirect immunofluorescence to be associated with the mesenchymal tissue 1) closely applied to the intervillus epithelium before the formation of intestinal crypts in the mouse fetus and 2) around intestinal crypts during and after their formation.

METHODS

We used a pre-embedding immunolabeling technique for extracellular matrix molecules, and a monoclonal antibody (Mab) directed against antigen MIM-1/130.

RESULTS

Immunofluorescence disclosed the presence of antigen 1/130 in the connective tissue closely applied to the epithelium of the gallbladder, pyloric glands, and intestinal and colonic crypts in adult mice. The antigen was absent in all salivary glands, kidney, liver, lung, spleen, and pancreas. At the fine structural level, gold particles in positive organs were associated with the interstitial matrix around collagen fibrils underneath the epithelia; gold particles were completely absent in the basement membranes. In the small intestine, labeling was seen only around crypts from cell position 1 up to the crypt-villus junction; it was totally absent under the villus epithelium. In order to confirm this particular localization in vivo, Mab 1/130 was administered orogastrically to 9-day-old mice: after 3 hours the antibody was found lining the immediate periphery of duodenal crypts as seen by indirect immunofluorescence. In control animals, an anti-mouse laminin Mab of the same subclass as Mab 1/130 was orogastrically fed using the same protocol: basal laminae were labeled under the epithelium of duodenal villi and crypts and also in the lamina propria, with a decreasing gradient from the top of the villi to the bottom of the crypts.

CONCLUSION

These observations indicate that the extracellular matrix associated with the epithelium of pyloric glands, of intestinal and colonic crypts, and of gallbladder contains a new antigen whose function remains to be determined. The neonatal mouse hence constitutes a good model to study the role of extracellular matrix components in determining organ differentiation in vivo.

Immunocytochemical study of the ontogeny of Peyer's patches in the Brazilian marsupial Didelphis albiventris

A detailed ontogenetic immunocytochemical study is reported on gut-associated lymphoid development in the Brazilian marsupial Didelphis albiventris. This employed antibody probes raised to evolutionarily conserved peptides which have been shown to detect HLA-DR-like (class II MHC) antigens and T and B cell markers in a wide range of animal species. Cells with macrophage and dendritic morphology expressing class II MHC and a few cells expressing the T cell marker CD3 were found in the lamina propria of duodenal villi in early (approximately 24 mm crown-rump length) latent opossum. Cells with B cell markers were not detected until lactent animals reached > 60 mm. Development of Peyer's patches (PP) was seen first in the duodenum in 45-60 mm lactent animals, progressing to well developed PP in the duodenum and ileum in lactent animals > 80 mm. These PP, like those in weanling and juvenile animals, consisted of follicles with a network of class II MHC positive dendritic cells and round cells lacking T and B markers, but lacking well defined mantle zones. B cells were present mainly in the lymphatic sinuses, with CD3 T cells present between follicles in the PP and intraepithelially in the villi. The study reveals the sequential development of class II MHC positive dendritic cells, T cells and B cells in the intestinal ontogeny of the opossum PP. These features occurred initially exclusively in the duodenum and subsequently in the ileum, paralleling the physiological maturation of the gut in eutheria.

Immunoelectron microscopical demonstration of the absorption of colostral IgG by small intestinal enterocytes in newborn rats

The protein A-gold technique was used in the cranial, transitional and caudal segments of the small intestine of 12 newborn rats to demonstrate the process of absorption of gold-labelled IgG through the enterocytes. The observation of the attachment of labelled IgG molecules to the wall of coated vesicles suggested a receptor-mediated transport of colostral IgG in the cranial segment of the small intestine. However, intracellular micropinocytotic transport predominated in the transitional and caudal segments of the small intestine. There was no evidence for paracellular transport. Lysosomal structures in the enterocytes did not appear to impede the absorptive activity during the absorption period, which lasted 20 days.

Hormonal control of intestinal Fc receptor gene expression and immunoglobulin transport in suckling rats

Hormonal control of immunoglobulin (Ig) absorption and of intestinal Fc receptor mRNA expression were investigated in rats to assess its potential role in the normal postsuckling inhibition of this transport system. Corticosterone and L-thyroxine therapy caused premature inhibition of the absorption of orally administered murine monoclonal antibody and of Fc receptor mRNA expression in a dose- and time-dependent manner. Low-dose corticosterone had no effect on Fc receptor mRNA synthesis after 3 d but decreased Ig transport fivefold after 7 d. High dose corticosterone resulted in a threefold reduction in Fc receptor after 3 d, and there was almost complete inhibition (> 30-fold) of transport and of Fc receptor transcript levels after 7 d. Similarly, 7 d of high-dose thyroxine decreased both serum Ig transport and Fc receptor (> 30-fold). However, adrenalectomy did not prevent the normal post-suckling declines in Ig transport or receptor synthesis. This study demonstrates that exogenous corticosteroids and thyroxine hormone inhibit Ig transport and steady-state duodenal Fc receptor mRNA levels in suckling rats. Endogenous adrenal steroids however, do not appear to be entirely responsible for the age-dependent decline in this transport system.

Transcytosis of protein through the mammalian cerebral epithelium and endothelium. II. Adsorptive transcytosis of WGA-HRP and the blood-brain and brain-blood barriers

Morphological evidence of the potential for adsorptive transcytosis of protein through the mammalian blood-brain fluid barriers, first reported from this laboratory in the mouse, has been confirmed and expanded upon in rats injected intravenously or into the lateral cerebral ventricle/subarachnoid space with with exogenous lectin wheatgerm agglutinin (WGA) conjugated to horseradish peroxidase (HRP). Blood-borne WGA-HRP rapidly enters cerebral endothelia by the process of adsorptive endocytosis and labels the vascular tree throughout the CNS. At 3 h post-injection and longer, WGA-HRP occupies the perivascular clefts and labels perivascular cells and basal lamina; this suspected transendothelial transfer of the lectin conjugate from blood to brain involves specific constituents of the endothelial endomembrane system of organelles (e.g., plasmalemma, vesicles, endosomes, Golgi complex). Within 6 h, reaction product is evident in extracellular clefts beyond the perivascular basal lamina and labels endocytic vesicles, endosomes, and dense bodies within cells and processes of the neuropil. Exposure of the abluminal surface of blood-brain barrier endothelia for 1-18 h to WGA-HRP delivered into the cerebral ventricles or subarachnoid space indicates blood-brain barrier endothelia do not engage in demonstrable adsorptive endocytosis at the abluminal surface. In this preparation, no endothelial organelles comparable to those sequestering blood-borne WGA-HRP are labelled with the lectin conjugate; hence, significant adsorptive transcytosis of WGA-HRP through cerebral endothelia from brain to blood is unlikely. The demonstrable difference in membrane internalization of the luminal versus abluminal plasmalemma of blood-brain barrier endothelia suggests the blood-brain barrier is polarized regarding adsorptive endocytosis of WGA-HRP. If adsorptive transcytosis of macromolecules through the blood-brain barrier does occur, the process appears unidirectional, from blood to brain but not from brain to blood. Absence of demonstrable endocytosis at the abluminal front is an enigma in the scheme of transcytosis through the blood-brain barrier from blood to brain insofar as exocytosis and endocytosis are complementary events in the cellular secretory process. This unconventional membrane behavior associated with the abluminal plasmalemma argues against a significant transcytosis of blood-borne protein through blood-brain barrier endothelia. The potential for transcytosis of macromolecules through the blood-cerebrospinal fluid barrier of choroid plexus epithelia is not as problemmatic as that through blood-brain barrier endothelia; additional evidence is provided to suggest choroid plexus epithelia participate in adsorptive endocytosis circumferentially and adsorptive transcytosis of WGA-HRP bidirectionally between the blood and cerebrospinal fluid.

Formation of crystalloid inclusions in the small intestine of neonatal pigs: an immunocytochemical study using colloidal gold

Enterocytes of the small intestine in 1-day-old suckling piglets contain numerous vesicles in the apical cytoplasm and a large granule located beneath the nucleus. Within the next 3 days, these granules transform into electron-dense crystalloid inclusions. These membrane-bound inclusions are up to 10 microns in length and 1-2 microns in diameter, and they are composed of electron-dense lamellae 3.9 nm apart. Postembedding immunocytochemistry, using rabbit anti-porcine IgG and goat anti-rabbit IgG conjugated to 10 nm colloidal gold, revealed that both the granules and the crystalloid inclusions contained a high concentration of maternal IgG. Although the IgG content of the crystalloid inclusions was detected on epoxy-embedded sections, the use of LR White resin resulted in a much higher density of labelling. Quantification of the labelling density showed that the concentration of IgG in the crystalloid inclusions was approximately ten times higher than that in the lumenal colostrum and approximately three times higher than that in the granules. These observations showed that there are at least three compartments involved in the accretion of IgG in the small intestine of neonatal piglets: smaller apical endocytotic vesicles, large subnuclear granules and crystalloid inclusions. The role of these compartments in maternal immunoglobulin absorption and in the acquisition of passive immunity has yet to be explored.

Is fetal enteral nutrition important for normal gastrointestinal growth?: a discussion

Long-term total parenteral nutrition results in atrophy of small intestinal structure and function. Maintenance or re-establishment of enteral nutrition can prevent or redress this loss. Paradoxically, the fetus develops in a total parenteral nutrition environment, but at the same time must achieve appropriate levels of gastrointestinal maturation in readiness for enteral feeding soon after birth. The fetus swallows large amounts of fluid during life in utero and growth is arrested if fetal ingestion is impaired. It is possible therefore that enteral nutrition provided by fetal swallowing is just as important in ensuring normal gastrointestinal homeostasis and growth in the fetus as it is in the adult.

Feeding neonatal rats with IgG antibodies leads to humoral hyporesponsiveness in the adult

Feeding monoclonal IgG2a or IgG1 anti-horseradish peroxidase (HRP) antibodies to 12-16-day-old neonatal rats caused a profound suppression of the humoral anti-HRP response in these rats as adults. The hyporesponsiveness to HRP was specific and long-lasting (up to 5 months). It was shown to be dose dependent, requiring relatively large doses of IgG (100-600 micrograms) for maximum effect. Secondary IgG (IgG1, IgG2a and IgG2b) responses were most depressed. The effect could be reproduced by i.p. injection of antibody. Hyporesponsiveness was not attributable to circulating antiidiotype antibodies directed against the monoclonal IgG, nor to the continued presence of the monoclonal anti-HRP since rats receiving antibody at or some weeks after the time of weaning and gut 'closure' responded well to subsequent HRP challenge. The effect was thus dependent on IgG administered over the identical period during which the neonatal circulation is rich in maternal IgG supplied via the milk. A direct function for maternal IgG in moulding the immune repertoire of the offspring, as well as providing passive protection, is suggested by these results.

Interaction of aglycosyl immunoglobulins with the IgG Fc transport receptor from neonatal rat gut: comparison of deglycosylation by tunicamycin treatment and genetic engineering

The role of carbohydrate in the structure and function of immuno-globulin Fc regions has been studied using the interaction of a monoclonal mouse IgG2b anti-NIP antibody with the IgG Fc transport receptor from neonatal rat gut. An aglycosyl variant of this immunoglobulin, in which site-directed mutagenesis had been used to eliminate the carbohydrate attachment site in the CH2 domain by changing Asn297 to Ala, was compared in this system to aglycosyl immunoglobulin prepared from immunoglobulin-secreting cells treated with tunicamycin to inhibit N-linked glycosylation. Loss of carbohydrate from the heavy chain was confirmed for both methods by Western blotting of the separated chains with Concanavalin A, and no significant differences in circular dichroism spectra were found between glycosylated and non-glycosylated mutants. Removal of carbohydrate by site-directed mutagenesis had no effect on binding of the immunoglobulin to the Fc transport receptor (FcTR) in vitro or transport from the gut to blood in vivo. Short-term clearance from circulation and degradation by gut contents in vitro were similarly unaffected. Mutation of Glu235 to Leu, an alteration that allows binding to human monocyte Fc gamma RI, did not alter the interaction with FcTR. However, treatment of wild-type or aglycosyl mutant cells with tunicamycin resulted in immunoglobulin which was less stable, cleared more rapidly and was transported slightly less efficiently. These findings indicate that the binding site for the FcTR may be unique among Fc-binding ligands, and that tunicamycin treatment may cause alterations in the immunoglobulin molecule in addition to loss of N-linked carbohydrate.

Discoordinate hormonal and ontogenetic regulation of four rat serpin genes

To understand the roles of four highly homologous rat hepatic serine protease inhibitor genes (Spi 2.1, Spi 2.2, Spi 2.3, and alpha 1-antitrypsin), we measured the hepatic content of their specific mRNAs under several physiological conditions. Spi 2.1 and 2.3 mRNAs, which are regulated by growth hormone, paralleled serum growth hormone levels developmentally. Only Spi 2.1 mRNA decreased with starvation, while Spi 2.2, 2.3, and alpha 1-antitrypsin mRNAs did not change. Despite the close homology of the Spi genes to mouse contrapsin, which is regulated by testosterone, none of the serine protease inhibitor mRNAs examined here was dependent on androgens for expression. Spi 2.2 mRNA displayed a unique ontogenetic regulation, with a rise in hepatic content at day 19 to levels five times that of any other age group. These studies confirm the importance of growth hormone in the regulation of Spi 2.1 and 2.3 mRNAs and suggest that Spi 2.2 mRNA may be regulated by metabolic alterations occurring in the weaning period.

Transcytosis in thyroid follicle cells: regulation and implications for thyroglobulin transport

In order to analyze quantitatively the translocation of plasma membrane during endocytosis and transcytosis and the regulation of these processes in thyroid follicle cells, the apical cell surfaces of resting and TSH-stimulated inside-out follicles were labeled with cationized ferritin. Morphometric analyses showed that the rates of endocytosis and transcytosis are TSH-dependent. More interestingly, whereas the effect of TSH on endocytosis was transient (with a maximum at 16 min), the effect on transcytosis continued to increase until the end of the experiment (i.e, 70 min). During 1 h of endocytosis, the fraction of membrane involved in transcytosis increased by a factor 4 upon TSH stimulation, corresponding to about 12% of the internalized apical plasma membrane area. Cooling to 15 degrees C slowed down, but did not block endocytosis entirely, whereas transcytosis and transfer to lysosomes were totally inhibited In order to quantitate transcytosis of thyroglobulin (TG) and to ascertain whether this molecule undergoes cleavage during transcytosis, inside-out follicles were incubated in a medium containing 3H-labeled TG in the presence of TSH; upon washing and reopening of follicles, the luminal fluid containing TG after transcytosis was found to contain about 10% of the total radioactivity taken up by follicle cells. Transcytosed TG proved to be unmodified with respect to its electrophoretic mobility. We conclude that (i) the fraction of transcytosed TG corresponds approximately to the fraction of membrane involved in this process, (ii) TG does not undergo cleavage during transcytosis, (iii) endocytosis and transcytosis are regulated by TSH but differ in their kinetics after stimulation, and (iv) transcytosis is affected by temperature in a similar way as transfer to lysosomes, suggesting the existence of a common gating step for both pathways.

Biologically active peptides in the gastrointestinal lumen

The release of a variety of biologically active peptides into the gastrointestinal lumen via gastric, duodenal and intestinal secretions, as well as in the saliva, pancreatic juice and bile, has been explored. The key features of luminal secretion of peptides such as secretion at high concentrations, neurohormonal regulation, luminal orientation of stimulated secretion, stability of peptides in the gastrointestinal lumen, altered secretion under pathophysiological conditions, and biological activity of luminally administered peptides are discussed. This review develops a detailed picture of the current understanding of luminal secretion of peptides and their possible biological functions under normal and pathophysiological conditions.

Uptake and transport of epidermal growth factor by the small intestinal epithelium of the fetal rat

Epidermal growth factor may play an important part in postnatal gastrointestinal development. However, little is known of its role prenatally. The aim of this study was to detect epidermal growth factor in amniotic fluid and to study its uptake and transfer across the epithelium of fetal rat small intestine. Anesthetized 20-day gestation rats underwent caesarean section. Three fetuses were exteriorized, their abdomens were opened, and ligated loops of proximal and distal small intestine were infused with 100 micrograms epidermal growth factor. Infused segments were removed 30 min later and processed for electron microscopy, and tissue was embedded in LR gold resin. Sections were treated with rabbit anti-rat epidermal growth factor antibody, followed by 5 nm gold-labeled goat anti-rabbit immunoglobulin before staining. Epidermal growth factor was measured in amniotic fluid by radioimmunoassay. In the proximal and distal small intestine epidermal growth factor was found membrane-associated along the luminal surface of microvilli, within apical invaginations and endosomal compartments, free from the membrane in multivesicular bodies, within large clear vesicles, basal vesicles and in association with the basolateral membrane and beyond. Epidermal growth factor was found in amniotic fluid at a concentration of 0.38 +/- 0.07 (SD) ng/ml. This study shows that epidermal growth factor is present in amniotic fluid and is transported across the epithelium of fetal rats by an endocytotic process in both the upper and lower small intestine. It is likely that amniotic fluid epidermal growth factor plays a part in intestinal mucosal development, and may be active systemically after transepithelial passage in utero.

Transcytosis of macromolecules through the blood-brain barrier: a cell biological perspective and critical appraisal

A critical appraisal is presented of nearly two decades of research publications and review articles advocating the bidirectional transcytosis of fluid-phase molecules, most notably native horseradish peroxidase (HRP), through the normal and experimentally modified blood-brain barrier (BBB). Extracellular routes circumventing the BBB in normal and pathological states and artifact introduced in histological preparation of CNS tissue exposed to blood-borne peroxidase are emphasized. The potential for transcytosis of macromolecules entering the nonfenestrated cerebral endothelium by the processes of non-specific fluid phase endocytosis (e.g., HRP), adsorptive endocytosis (e.g., lectins) and receptor-mediated endocytosis (e.g., ligands) is analyzed in the context of the cellular secretory process and the complimentary events of endocytosis and exocytosis at the luminal and abluminal plasma membranes. Available data suggest that the cerebral endothelium is polarized with regard to endocytosis and the internalization of cell surface membrane; hence, the transcytosis of specific macromolecules through the BBB may be vectorial. If these data are correct, the blood-brain barrier is not absolute, whereas its counterpart, the brain-blood barrier, may be.

Enterocyte ultrastructure and uptake of immunoglobulins in the small intestine of the neonatal lamb

Although the small intestine of the sheep is relatively mature at birth, there are still vacuolated enterocytes present for at least 2 days in distal regions. In the distal regions, vacuolated cells possess a range of vesicle morphology which might be indicative of at least 2 separate routes for enterocyte handling of proteins taken up from the lumen. The localisation of immunoreactive immunoglobulins within the enterocytes, presumably of colostral or milk origin, in both proximal (non-vacuolated) and distal (vacuolated) regions, does not follow patterns which suggest orderly renewal at closure. It is suggested that closure is not solely brought about by epithelial cell replacement.

Purification and characterization of clathrin-coated vesicles from Chlamydomonas

Clathrin-coated vesicles, identified by negative staining with uranyl acetate, were purified from Chlamydomonas reinhardtii. Isolated coated vesicles had diameters ranging from 70 to 140 nm (mean diameter +/- SD of 95 +/- 17 nm, n = 300). These vesicles were markedly heterogeneous in both density and surface charge, as indicated by equilibrium density sedimentation and elution from anion-exchange columns. Highly-purified coated-vesicle fractions contained 2 major polypeptides, identified as the clathrin heavy chain (185 kDa) and the clathrin light chain (40 kDa). Chlamydomonas clathrin heavy chain cross-reacts weakly with an antibody against bovine brain clathrin heavy chain. Coat stability in several buffers was compared to that of bovine brain coated vesicles. Stability was similar, except for a greater stability of Chlamydomonas coated vesicles in 0.5 M Tris at pH 7.0.

Membranous intermediates in endocytosis are labile, as shown in a temperature-sensitive mutant

Membranous tubules, especially prevalent in mammalian absorptive epithelia and insect oocytes, are one of the pleomorphic endocytic compartments that have a role in receptor-mediated endocytosis. To determine whether these tubules are evanescent, and to investigate their temporal relationships with other endocytic intermediates, we studied these tubules during oocyte vitellogenesis in the temperature-sensitive mutant Drosophila melanogaster, shibiretsl. Raising the temperature of shibire oocytes for 1 min from 19 degrees C to 29 degrees C caused a loss of these membranous tubules. The percentage of membrane in tubules decreased from 36% at 19 degrees C to 1.5% after 5 min at 29 degrees C. Concomitantly, the amount of surface membrane increased from 64% at 19 degrees C to 98% after 5 min at 29 degrees C, causing surface membrane invaginations to extend deeper into the cortex. At 29 degrees C the cytoplasmic face of the plasma membrane was studded with coated pits, and the extracellular face was coated with electron-dense material. Return from 29 degrees C to either 19 degrees C or 26 degrees C for 1-2 min produced a rapid reappearance of tubules containing extracellular horseradish peroxidase in the cortex. These data suggest that tubular intermediates are evanescent structures, and that temperature shock (i) rapidly blocks their formation from the plasma membrane, (ii) causes existing tubules to rapidly recycle to the plasma membrane, and (iii) is rapidly reversed, as newly formed tubules derive their membrane and content from the cell surface.

Role of carbohydrate in binding of IgG to the Fc receptor of neonatal rat enterocytes

Monoclonal IgG1, IgG2a, IgG2b and IgG2c were prepared from rat hybridoma cells treated with tunicamycin in order to inhibit N-linked glycosylation. The IgG produced by these cells was about 70% lower in carbohydrate content compared to IgG from equivalent untreated cells, but was similar to the corresponding normal IgG in terms of antigen binding. However, the ability of carbohydrate deficient (CHO-) IgG to bind in vitro to Fc receptor extracted from jejunum of neonatal rats was impaired in most cases and, in all but one case, the amount of CHO- IgG transported from gut lumen to blood in vivo was markedly reduced. No reduction in binding of normal IgG to extracted receptor was observed in the presence of various sugars. It is postulated that N-linked carbohydrate acts to stabilize the structure within the IgG molecule which is responsible for binding to this Fc receptor, possibly in the CH2 domain.

Transcytosis of protein through the mammalian cerebral epithelium and endothelium. I. Choroid plexus and the blood-cerebrospinal fluid barrier

The potential for transcytosis (endocytosis----intracellular transport----exocytosis) of protein and membrane events associated with fluid phase and adsorptive endocytic processes within epithelia of the choroid plexus [blood-cerebrospinal fluid (CSF) barrier] were investigated in mice injected intravenously or into the lateral cerebral ventricle with native horseradish peroxidase (HRP) or the lectin wheatgerm agglutinin (WGA) conjugated to HRP. WGA binds to specific cell surface oligosaccharides and enters cells by the process of adsorptive endocytosis; native HRP is taken into cells non-specifically by fluid phase endocytosis. The lysosomal system of organelles and the endoplasmic reticulum, identified by enzyme cytochemical markers applied to choroid epithelia, were analysed for possible participation in transcytosis and compared to epithelial organelles harbouring the exogenous tracer proteins. Blood-borne native HRP was endocytosed readily by choroid epithelia whereas WGA-HRP was not, perhaps because WGA-HRP does not escape fenestrated endothelia as easily as native HRP. The blood-borne proteins incorporated within endocytic vesicles by choroid epithelia were directed to endosomes (prelysosomes) and secondary lysosomes (e.g. tubules, multivesicular/dense bodies) for eventual degradation and did not reach the apical/microvillus surface. Both CSF-borne native HRP and WGA-HRP entered choroid epithelia within endocytic vesicles derived from the microvillus border. Native HRP, ultimately sequestered within endosomes and secondary lysosomes, failed to undergo transcytosis through the epithelia into the basolateral clefts. Conversely, CSF-borne WGA-HRP was transported through the epithelia and released into the basolateral clefts within 10 min post-injection. The lectin conjugate labelled epithelial vesicles, endosomes, secondary lysosomes and, at 30 min post-injection, the transmost saccule of the Golgi complex which exhibits acid hydrolase activity. Tubular profiles, related either to the endosome apparatus or to the lysosomal system, and the endoplasmic reticulum did not appear involved in the transcytotic pathway. The data suggest that CSF-borne protein entering the choroid epithelium by adsorptive endocytosis can undergo rapid transcytosis through the cell. The results provide insight to transcytotic pathways utilizing vesicles, the endosomal apparatus, and the Golgi complex within the choroid epithelium for circumventing the blood-CSF barrier. Hypothesized membrane events and morphological associations among constitutents of the endomembrane system within the choroid epithelium are summarized diagrammatically.

Decreased passage of the nonapeptide dDAVP over the intestinal epithelium during development in the young rat

The nonapeptide 1-deamino-cysteine-8-D-arginine vasopressin (dDAVP) was gavage-fed together with cow's milk whey protein to young, developing rats. The transepithelial passage of dDAVP in the gastrointestinal (GI) tract was assessed by a specific RIA as immunoreactive levels in blood serum extracts and as urinary excretion of dDAVP 0.5-8 h after feeding. In 14-day-old rats the passage of dDAVP was higher than in 30-day-old rats, since the 14-day-old rats had significantly higher serum levels (5-10 times) 0.5-2 h after feeding and a urinary excretion approaching 0.15% of the administered amount after 8 h. In the 30-day-old rats urinary excretion increased up to 0.05% after 2 h and then levelled off. It was also clear that 30-day-old rats had a slower transfer to and faster elimination from serum than 14-day-old rats. dDAVP appeared to be passed over the GI tract mucosa independently of intestinal proteolysis since feeding it to 30-day-old rats together with the proteinase inhibitors, soya-bean trypsin inhibitor and pepstatin did not influence the serum or urinary levels. Thus, dDAVP was taken up from the GI tract into the blood circulation and excreted in the urine of young rats. The decrease in the passage of dDAVP found around weaning appears to be related to developmental processes affecting the permeability of the intestinal epithelium rather than intestinal proteolysis.

Role of apical tubules in endocytosis in nonciliated cells of the ductuli efferentes of the rat: a kinetic analysis

The apical region of nonciliated cells of the ductuli efferentes of the rat contains tubular coated pits (TCP) connected to the apical plasma membrane, apical tubules (AT) which occasionally show a partial coat, and endosomes which are often continuous with one or more apical tubules. To investigate the formation and fate of TCP and AT, a quantitative analysis was performed on the labeling indices of these structures at various time intervals (0.5-120 min) after a single injection of a tracer, cationic ferritin (CF), into the lumen of the rete testis. The labeling indices of both TCP and AT exhibited similar cyclical patterns, first reaching a peak at 25 min, then dropping to a minimum at 35 min, then rising to a second peak at 60 min. Since TCP were well labeled at 30 sec while AT were not, the tracer must rapidly enter TCP and thence AT. However, since tracer was virtually absent from the lumen by 30 min, it was not possible to reconcile the second peak of labeling index of TCP and AT by this mechanism. In another experiment, rats were injected once as before, injected again at 30 min, and then sacrificed at 30 min following the second injection. The results from this experiment showed that the labeling index of TCP and AT did not drop but was similar to that of the 60-min peak after a single injection. The interpretation is that there was recycling of tracer, which had already migrated from TCP to AT to endosomes, back to the apical plasma membrane via apical tubules. Moreover, when rats were injected once, injected again at 30 min, and sacrificed 3 min following the second injection, the labeling index for TCP and AT was significantly higher (P less than .05) than at the 30-min time interval after a single injection, indicating that recycled apical tubules were functionally capable of binding further CF. Morphological observations on images of transition between TCP and AT and the fact that AT were often found connected to endosomes suggest that TCP detach from the cell surface to give rise to AT, which in turn fuse to form endosomes. The kinetic analysis demonstrates in quantitative terms that a portion of the AT, which fuses to form endosomes, recycles back to the apical plasma membrane and contributes to the formation of new TCP.

Uptake and digestion of 125I-labelled bovine serum albumin by the rat visceral yolk sac cultured in vitro as a closed vesicle

A system for culturing the rat visceral yolk sac in vitro as a closed vesicle--the 'giant' yolk sac--has been employed to investigate the vectorial nature of the uptake and digestion of exogenous protein substrates. Uptake of 125I-labelled formaldehyde-denatured bovine serum albumin by such yolk sacs was found to be similar to that observed in yolk sacs removed directly from the mother at 17.5 days' gestation, provided that homologous serum was used as a culture medium. However, unlike the control yolk sacs, giant yolk sacs tended to accumulate substrate within the tissue with increasing culture time. The concentration of digestion products released to the inside of the closed vesicle was found to be greater than that released to the surrounding culture medium at time intervals up to five hours. Giant yolk sacs preloaded with 125I-labelled bovine serum albumin were found to release material to the culture medium or the inside of the vesicle almost entirely in the acid-soluble (digested) form. This system is a useful model for studying the polar nature of epithelia, particularly those involved in the uptake and transport of nutritional and/or informational macromolecules.

Study on the origin of apical tubules in ileal absorptive cells of suckling rats using concanavalin-A as a membrane-bound tracer

The ileal absorptive cells of suckling rats exhibit high levels of endocytic activity being engaged in nonselective uptake of macromolecules from the intestinal lumen. The apical cytoplasm usually contains an extensive network of small, membrane-limited tubules (apical tubules: AT), in addition to newly formed endocytic vesicles and large endocytic vacuoles. To determine whether the AT are directly involved in the endocytic process by carrying the tracer into the cell, we have analysed movements of the apical cell membrane of the ileal absorptive cells by using a membrane-bound tracer (horseradish peroxidase-labelled concanavalin-A: Con-A HRP). The ileal absorptive cells were exposed in vitro to Con-A HRP for 10 min at 4 degrees C, incubated for different times in Con-A free medium at 37 degrees C, and prepared for electron microscopy. After 1 min incubation at 37 degrees C, invaginations of the apical cell membrane, including coated pits, and endocytic vesicles were labelled with HRP-reaction product, whereas the AT and large endocytic vacuoles were negative. After 2.5 min, almost all the large endocytic vacuoles were labelled with reaction product, which was seen in their vacuolar lumen and along the luminal surface of their limiting membrane. A few AT with reaction product were seen in the apical cytoplasm; they were in frequent connection with the reaction-positive large endocytic vacuoles. With increasing incubation time, the number of the labelled AT increased. Thus, after 15 min at 37 degrees C, the apical cytoplasm was fully occupied by the reaction-positive AT. The ends of these AT were often continuous with small spherical coated vesicles. No reaction product was detected in the Golgi complex at any time after incubation. These observations indicate that the AT located in the apical cytoplasm probably originate by budding off from the large endocytic vacuoles, rather than being involved in the process of endocytosis.

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.

Endocytic apparatus and transcytosis in epithelial cells of the vas deferens in the rat

The apex of the principal epithelial cells lining the vas deferens of the rat contains coated pits in continuity with the apical plasma membrane and large subsurface-coated vesicles (100-125 nm). In the apical cytoplasm, large, pale, uncoated vesicles (150-300 nm), small coated and uncoated vesicles (50-60 nm), uncoated vesicles about 75-90 nm, and membranous apical tubules are present, in addition to large, vacuolar, pale, multivesicular bodies, dense multivesicular bodies, and secondary lysosomes seen deeper in the cytoplasm amongst numerous ER cisternae, saccules of the Golgi apparatus, and mitochondria. The endocytic activity of these cells was investigated by using cationic ferritin (CF) as a marker of adsorptive endocytosis and native ferritin (NF) for demonstrating fluid-phase endocytosis. These tracers were injected separately into the lumen of the vas deferens, and the animals were killed at various time intervals thereafter from 2 to 90 minutes. At 2 minutes CF was seen bound predominantly to microvilli and to areas of the apical plasma membrane delimiting coated pits as well as in large, coated vesicles. At 5 and 15 minutes the tracers were seen in apical tubules and pale multivesicular bodies; at 30 minutes moderately dense multivesicular bodies were labeled. At 1 hour and longer time intervals dense multivesicular bodies and secondary lysosomes were labeled. NF followed the same pathway as CF; however, no binding to microvilli or areas delimiting coated pits was observed. The numerous other vesicular structures, i.e., the large uncoated vesicles (150-300 nm) and the small coated and uncoated vesicles (50-60 nm), never became labeled with the tracers and therefore were not involved in the endocytic process. There was, however, an exception in the case of several small (75-90 nm) uncoated vesicles seen deeper in the apical cytoplasm of these cells which were labeled exclusively with CF. With time such vesicles appeared along the lateral and basal surfaces of these cells and discharged their content of CF into the lateral intercellular space or the connective tissue space at the base of these cells. Thus the principal epithelial cells in addition to sequestering the endocytosed tracers within secondary lysosomes where they are presumably degraded also appear to be involved in the transcytosis of material from the lumen of the vas deferens to the underlying lamina propria.

Human amnion as a model for IgG transport

The ability of fresh human amnion to bind and internalize horseradish peroxidase-labeled IgG (IgG-HRP) was examined in an in vitro Ussing chamber system. The amnion demonstrated unique cell membrane receptors for the Fc portion of IgG molecules (Fc gamma R). The Fc gamma R exhibit exquisite specificity and affinity for IgG monomers as demonstrated by staining with labeled IgG. Labeled IgA, IgM, F(ab')2 fragments of IgG, aggregated IgG, and antigen-antibody complexes all failed to bind to the amnionic epithelial cells. Binding was only minimally affected by changes in ionic strength or pH when viewed at the light microscopic level. The Fc gamma R are located on both the apical and basal cell membranes. The binding of IgG-HRP to the amnion cell membrane was detectable within 1 min, and internalization of the ligand occurred within 5 min. No binding of IgG-HRP was observed following treatment of the membrane with 0.25% trypsin for 30 min at room temperature. Incubation of the amnion at 4 degrees C or in the presence of colchicine or cytochalasin D prevented internalization of the IgG-HRP. These experiments demonstrate Fc gamma R on human amnionic epithelial cells that both bind and internalize IgG, thus allowing the amnion to be used as a model system for studying IgG transport.

Age influences on intestinal sugar absorption

Intestinal absorption of sugars show differences depending on animals age. This is demonstrated using in vivo and in vitro techniques. The age dependence relationship is present in animals of different species such as avian, rodents and ruminants. In chicken the intestinal sugar transport increases after hatching and attains its maximum capacity by the first week of life. The D-glucose and D-galactose uptake is greater in young rats, maximum at 21 days, while it decreases thereafter. The total capacity of the small intestine of adult sheep for sugar absorption was approx. 25% of that for lambs less than 1 week of age. The differences observed in intestinal absorption of sugars at different ages could be attributed to differences in sodium and calcium transport. Other authors assume that it is induced by morphological differentiation during intestinal development.

Multivesicular bodies play a key role in vitellogenin endocytosis by Xenopus oocytes

A combination of electron microscopic tracers and subcellular fractionation has been used to examine the endocytic pathway of the yolk protein precursor, vitellogenin (VG), in Xenopus oocytes. VG was adsorbed to colloidal gold, and the organelles traversed by newly internalized ligand were examined at various time intervals after endocytosis. VG-Au enters oocytes via coated pits and vesicles and then appears rapidly in tubular endosomes and multivesicular bodies (MVBs). MVBs play a central role in VG processing for storage; the large majority of newly internalized VG enters this compartment, remaining there for up to several hours. Condensation of VG into crystalline bodies begins in MVBs, and continues with growth of the crystals until typical platelets are formed. When oocytes are exposed to high [VG], MVBs containing large amounts of internalized VG are morphologically indistinguishable from the primordial yolk platelets described earlier (Dumont, 1978). The use of VG-Au particles of two sizes demonstrates that gold particles in early MVBs were generally associated with the limiting membrane of these organelles, while older MVB compartments have gold particles well separated from the limiting membranes, suggesting that dissociation of VG from its receptor occurs in this compartment. Newly internalized ligand preferentially forms a new MVB, rather than fusing and mixing with previously formed MVBs. Progressive yolk protein condensation gradually transforms MVBs into yolk platelets over a period of several hours. Analysis of 125I-VG-Au behavior after sucrose gradient fractionation of oocytes allowed correlation of biochemical compartments with those observed in the electron microscope. MVBs containing yolk in progressive stages of condensation were found at densities from 1.16 up to 1.21 g/cc. The final, rate-limiting step in VG transport is a shift of ligand from light (1.21 g/cc) to heavy (1.23 g/cc) platelet compartments (Wall and Meleka, 1985). The morphological correlate of this process is movement of VG-Au from small (less than 3-4 microns diameter) to large (greater than 4 microns diameter) platelets.

Intestinal uptake and retention of copper in the suckling rat, Rattus rattus--IV. Mechanisms of intestinal copper accumulation

Copper-67, administered either parenterally or via the maternal milk, accumulates principally in the intestine and liver of the 6-day-old pup. Most of the 67Cu in the soluble fraction of the intestine is associated with the heterogeneous Cu-complex, which is located predominantly in the ileum. The rates of uptake and loss of 67Cu in the liver and intestine indicate that enterohepatic circulation of Cu in the neonate is appreciable. Whilst the concentration of Cu in the bile of the 13-day-old pup is high (16-fold greater than that in the adult male rat), translocation of Cu from both the liver and duodenum to the ileum probably occurs via the blood, rather than by the reabsorption of biliary Cu. Although the Cu-complex normally seems to be retained within the distal intestine until the enterocytes are desquamated, Cu in this form is utilized when the Cu-intake of the neonate is restricted.

Postnatal changes in the distribution and elemental composition of Paneth cells in normal and corticosteroid-treated rats

Paneth cells containing zinc-rich granules were found in the small intestine of 6-day-old rats. These cells were more numerous in older animals and were consistently most common in the distal ileum. The zinc content of granules from 10-day-old rats was similar to that found in adults (ca 300 mg atoms/kg dry weight) but no calcium could be detected. An injection of cortisone acetate at 5 days resulted in a premature increase in the numbers of Paneth cells in 10-day-old rats. The cell granules contained normal, adult levels of zinc, a calcium concentration of ca 400 mg atoms/kg dry weight and also an increased concentration of phosphorus.