Uptake and transepithelial transport of nerve growth factor in suckling rat ileum

Myosin 5b is required for proper localization of the intermicrovillar adhesion complex in the intestinal brush border

Intestinal enterocytes have an elaborate apical membrane of actin-rich protrusions known as microvilli. The organization of microvilli is orchestrated by the intermicrovillar adhesion complex (IMAC), which connects the distal tips of adjacent microvilli. The IMAC is composed of CDHR2 and CDHR5 as well as the scaffolding proteins USH1C, ANKS4B, and Myosin 7b (MYO7B). To create an IMAC, cells must transport the proteins to the apical membrane. Myosin 5b (MYO5B) is a molecular motor that traffics ion transporters to the apical membrane of enterocytes, and we hypothesized that MYO5B may also be responsible for the localization of IMAC proteins. To address this question, we used two different mouse models: 1) neonatal germline MYO5B knockout (MYO5B KO) mice and 2) adult intestinal-specific tamoxifen-inducible VillinCreERT2;MYO5Bflox/flox mice. In control mice, immunostaining revealed that CDHR2, CDHR5, USH1C, and MYO7B were highly enriched at the tips of the microvilli. In contrast, neonatal germline and adult MYO5B-deficient mice showed loss of apical CDHR2, CDHR5, and MYO7B in the brush border and accumulation in a subapical compartment. Colocalization analysis revealed decreased Mander's coefficients in adult inducible MYO5B-deficient mice compared with control mice for CDHR2, CDHR5, USH1C, and MYO7B. Scanning electron microscopy images further demonstrated aberrant microvilli packing in adult inducible MYO5B-deficient mouse small intestine. These data indicate that MYO5B is responsible for the delivery of IMAC components to the apical membrane.NEW & NOTEWORTHY The intestinal epithelium absorbs nutrients and water through an elaborate apical membrane of highly organized microvilli. Microvilli organization is regulated by the intermicrovillar adhesion complexes, which create links between neighboring microvilli and control microvilli packing and density. In this study, we report a new trafficking partner of the IMAC, Myosin 5b. Loss of Myosin 5b results in a disorganized brush border and failure of IMAC proteins to reach the distal tips of microvilli.

Assessment of intestinal macromolecular absorption in young piglets to pave the way to oral vaccination: preliminary results

The small intestine of the piglet has evolved to be permeable immediately after birth to facilitate the uptake of colostrum-derived immunoglobulins as well as other macromolecules, and cells. However, the precise timing of gut closure in today’s precocious pig is not known. We gavaged piglets immediately after birth and at 1-h after birth with Cy5-labeled Ovalbumin (Cy5-Ova) then harvested their small intestine’s 6–7 h later. To assess localization of Cy5-Ova in the small intestinal epithelial cells, we performed immunohistochemistry using a basolateral surface marker and a recycling endosome marker called pIgR, the late endosomal marker Rab7, and the lysosomal marker LAMP-1. Cy5-Ova co-localized with Rab7 and LAMP-1 in the duodenum and jejunum of 0-h old and 1-h old gavaged piglets, but only in the ileum of 0-h gavaged piglets. These data suggest that movement of Cy5-Ova through the late endosomes to the lysosomes was much reduced in the ileum of 1-h gavaged piglets. Cy5-Ova was largely present in epithelial cell digestive and transport vacuoles, but it did not colocalize with pIgR-positive endosomes in 0-h and 1-h gavaged piglets. Differences in macromolecular uptake across the different regions of the small intestine after only 1-h may be due to prior processing of colostral macromolecules, changes in the intestine due to initiation of colonization by microflora and/or the initiation of gut-closure. Understanding the relationship between the localization of Cy5-Ova and small intestinal permeability may contribute to establishing whether oral vaccination in the newborn can capitalize on the transient permeability before gut closure to promote immune protection.

Loss of myosin Vb promotes apical bulk endocytosis in neonatal enterocytes

In patients with inactivating mutations in myosin Vb (Myo5B), enterocytes show large inclusions lined by microvilli. The origin of inclusions in small-intestinal enterocytes in microvillus inclusion disease is currently unclear. We postulated that inclusions in Myo5b KO mouse enterocytes form through invagination of the apical brush border membrane. 70-kD FITC-dextran added apically to Myo5b KO intestinal explants accumulated in intracellular inclusions. Live imaging of Myo5b KO-derived enteroids confirmed the formation of inclusions from the apical membrane. Treatment of intestinal explants and enteroids with Dyngo resulted in accumulation of inclusions at the apical membrane. Inclusions in Myo5b KO enterocytes contained VAMP4 and Pacsin 2 (Syndapin 2). Myo5b;Pacsin 2 double-KO mice showed a significant decrease in inclusion formation. Our results suggest that apical bulk endocytosis in Myo5b KO enterocytes resembles activity-dependent bulk endocytosis, the primary mechanism for synaptic vesicle uptake during intense neuronal stimulation. Thus, apical bulk endocytosis mediates the formation of inclusions in neonatal Myo5b KO enterocytes.

Effects of simvastatin on nuclear receptors, drug metabolizing enzymes and transporters expression in Human Umbilical Vein Endothelial Cells

BACKGROUND

Vascular endothelial cells (EC) are constantly exposed to endo- and exogenous compounds, which may disturb EC function. One of the protecting mechanisms against chemicals consists of drug metabolizing enzymes and transporter proteins regulated by nuclear receptors and transcription factors. Therefore, the aim of the current study was to assess the regulation of nuclear receptors and their coordinated genes in Human Umbilical Vein Endothelial Cells (HUVEC).

METHODS

HUVEC were exposed to TCDD (10 nM), oltipraz (100 μM) and simvastatin (1 μM) for 24 h. Gene expressions were evaluated using quantitative real-time PCR. The protein expression levels were determined by Western blotting. Enzymatic activity of CYP1A1/CYP1B1 was assessed by luciferin-labelled CYPs substrate.

RESULTS

Our study confirmed that nuclear receptor AhR and nuclear factor Nrf2 are highly expressed in HUVECs. Treatment of HUVECs with TCDD (AhR inducer) resulted in a significant induction of AHR target genes CYP1A1, CYP1B1 and NQO1. Oltipraz (Nrf2 inducer) also markedly increased expression of NQO1 but did not affect Nrf2 mRNA nor protein levels. Under simvastatin stimulation PXR and NRF2 target transcripts were not altered, however AHR-regulated genes: CYP1A1, CYP1B1 and MDR1 were significantly induced. Western blot analysis confirmed CYP1B1 induction in TCDD-treated HUVECs, but not in the simvastatin group. Moreover, HUVEC exposure to TCDD resulted in induction of CYP1A1/CYP1B1 enzymatic activity.

CONCLUSIONS

This study revealed functional expression of AhR and Nrf2 in HUVECs. Moreover, it was defined that simvastatin induced AhR and its related genes.

The Endosomal Protein Endotubin Is Required for Enterocyte Differentiation

BACKGROUND & AIMS

During late embryonic development and through weaning, enterocytes of the ileum are highly endocytic. Defects in endocytosis and trafficking are implicated in neonatal disease, however, the mechanisms regulating trafficking during the developmental period are incompletely understood. The apical endosomal protein endotubin (EDTB) is highly expressed in the late embryonic and neonatal ileum. In epithelial cells in vitro, EDTB regulates both trafficking of tight junction proteins and proliferation through modulation of YAP activity. However, EDTB function during the endocytic stage of development of the intestine is unknown.

METHODS

By using Villin-CreERT2, we induced knockout of EDTB during late gestation and analyzed the impact on endocytic compartments and enterocyte structure in neonates using immunofluorescence, immunocytochemistry, and transmission electron microscopy.

RESULTS

Deletion of the apical endosomal protein EDTB in the small intestine during development impairs enterocyte morphogenesis, including loss of the apical endocytic complex, defective formation of the lysosomal compartment, and some cells had large microvillus-rich inclusions similar to those observed in microvillus inclusion disease. There also was a decrease in apical endocytosis and mislocalization of proteins involved in apical trafficking.

CONCLUSIONS

Our results show that EDTB-mediated trafficking within the epithelial cells of the developing ileum is important for maintenance of endocytic compartments and enterocyte integrity during early stages of gut development.

Long-term health outcomes and mechanisms associated with breastfeeding

Breastfeeding is superior to formula feeding because it has factors that have long term consequences for early metabolism and disease later in life. In this paper, the scientific evidence in support of why breast milk is beneficial for infants is summarized and the mechanisms in which breastfeeding impacts on disease are explored. Human milk may show a reduced occurrence of disease because mammalian evolution promotes survival, and because of specific factors in milk that promote active stimulation of the infant's immune system and gastrointestinal mucosal maturation decrease the incidence of infection and alter the gut microflora. Bioactive factors, including: hormones, growth factors, colony-stimulating factors and specific nutrients, may have such far-reaching effects on the infant's immune response that normal development depends heavily on its provision. All mothers should be encouraged and supported to continue breastfeeding for 6 months and beyond in order to promote the good health of their infants.

Activation of a Unique Population of CD8+T Cells by Intestinal Epithelial Cells

Intestinal epithelial cells may play a role in the regulation of immune responses toward luminal antigens. We show that a subset of CD8(+) T cells undergoes oligoclonal expansion in the intestinal mucosa, probably through interaction with a unique complex expressed on epithelial cells, formed by a CEA subfamily member (gp180) and CD1d. This subset, which is regulatory in vitro, may play a role in the control of intestinal immune responses toward luminal antigens. A lack of expansion of these CD8(+) regulatory T cells, probably related to the defective expression of the gp180/CD1d complex, is observed in inflammatory bowel disease.

Colonic epithelial expression of ErbB2 is required for postnatal maintenance of the enteric nervous system

We utilized the Cre-LoxP system to establish erbB2 conditional mutant mice in order to investigate the role of erbB2 in postnatal development of the enteric nervous system. The erbB2/nestin-Cre conditional mutants exhibit retarded growth, distended colons, and premature death, resembling human Hirschsprung's disease. Enteric neurons and glia are present at birth in the colon of erbB2/nestin-Cre mutants; however, a marked loss of multiple classes of enteric neurons and glia occurs by 3 weeks of age. Furthermore, we demonstrate that the requirement for erbB2 in maintaining the enteric nervous system is not cell autonomous, but rather erbB2 signaling in the colonic epithelia is required for the postnatal survival of enteric neurons and glia.

Sorting of internalized neurotrophins into an endocytic transcytosis pathway via the Golgi system: Ultrastructural analysis in retinal ganglion cells

Subcellular pathways and accumulation of internalized radiolabeled neurotrophins NGF, BDNF, and NT-3 were examined in retinal ganglion cells (RGCs) of chick embryos by using quantitative electron microscopic autoradiography. All three neurotrophins accumulated in endosomes and multivesicular bodies. BDNF and NGF also concentrated at the plasma membrane, whereas NT-3 accumulated transiently in the Golgi system. The enhanced targeting of NT-3 to the Golgi system correlated with the anterograde axonal transport of this neurotrophin. Anterograde transport of NT-3, but not its internalization, was significantly attenuated by the tyrosine kinase (trk) inhibitor K252a. Abolishment of trk activity with K252a shifted NT-3 (and BDNF) away from the Golgi system and into a lysosomal pathway, indicating that trk activity regulated sorting of the ligand-receptor complex. Cross-linking of neurotrophins and immunoprecipitation with antibodies to the neurotrophin receptors p75, trkA, trkB, and trkC showed that the large majority of exogenous, receptor-bound NT-3 was bound to trkC in RGC somata, but during anterograde transport in the optic nerve most receptor-bound NT-3 was associated with p75, and after arrival and release in the optic tectum transferred to presumably postsynaptic trkC. These results reveal remarkable and unexpected differences in the intracellular pathways and fates of different neurotrophins within the same cell type. They provide first evidence for an endocytic pathway of internalized neurotrophic factors via the Golgi system before anterograde transport and transcytosis. The results challenge the belief that after internalization all neurotrophins are rapidly degraded in lysosomes.

Sorting of internalized neurotrophins into an endocytic transcytosis pathway via the Golgi system: Ultrastructural analysis in retinal ganglion cells

Subcellular pathways and accumulation of internalized radiolabeled neurotrophins NGF, BDNF, and NT-3 were examined in retinal ganglion cells (RGCs) of chick embryos by using quantitative electron microscopic autoradiography. All three neurotrophins accumulated in endosomes and multivesicular bodies. BDNF and NGF also concentrated at the plasma membrane, whereas NT-3 accumulated transiently in the Golgi system. The enhanced targeting of NT-3 to the Golgi system correlated with the anterograde axonal transport of this neurotrophin. Anterograde transport of NT-3, but not its internalization, was significantly attenuated by the tyrosine kinase (trk) inhibitor K252a. Abolishment of trk activity with K252a shifted NT-3 (and BDNF) away from the Golgi system and into a lysosomal pathway, indicating that trk activity regulated sorting of the ligand-receptor complex. Cross-linking of neurotrophins and immunoprecipitation with antibodies to the neurotrophin receptors p75, trkA, trkB, and trkC showed that the large majority of exogenous, receptor-bound NT-3 was bound to trkC in RGC somata, but during anterograde transport in the optic nerve most receptor-bound NT-3 was associated with p75, and after arrival and release in the optic tectum transferred to presumably postsynaptic trkC. These results reveal remarkable and unexpected differences in the intracellular pathways and fates of different neurotrophins within the same cell type. They provide first evidence for an endocytic pathway of internalized neurotrophic factors via the Golgi system before anterograde transport and transcytosis. The results challenge the belief that after internalization all neurotrophins are rapidly degraded in lysosomes.

Anterograde axonal transport, transcytosis, and recycling of neurotrophic factors: the concept of trophic currencies in neural networks

Traditional views of neurotrophic factor biology held that trophic factors are released from target cells, retrogradely transported along their axons, and rapidly degraded upon arrival in cell bodies. Increasing evidence indicates that several trophic factors such as brain-derived neurotrophic factor (BDNF), fibroblast growth factor (FGF-2), glial cell-line derived neurotrophic factor (GDNF), insulin-like growth factor (IGF-I), and neurotrophin-3 (NT-3), can move anterogradely along axons. They can escape the degradative pathway upon internalization and are recycled for future uses. Internalized ligands can move through intermediary cells by transcytosis, presumably by endocytosis via endosomes to the Golgi system, by trafficking of the factor to dendrites or by sorting into anterograde axonal transport with subsequent release from axon terminals and uptake by second- or third-order target neurons. Such data suggest the existence of multiple "trophic currencies," which may be used over several steps in neural networks to enable nurturing relationships between connected neurons or glial cells, not unlike currency exchanges between trading partners in the world economy. Functions of multistep transfer of trophic material through neural networks may include regulation of neuronal survival, differentiation of phenotypes and dendritic morphology, synapse plasticity, as well as excitatory neurotransmission. The molecular mechanisms of sorting, trafficking, and release of trophic factors from distinct neuronal compartments are important for an understanding of neurotrophism, but they present challenging tasks owing to the low levels of the endogenous factors.

Oral delivery of synthetic eel calcitonin, elcatonin, in rats

This study was designed to develop an oral dosage form of elcatonin (EC), a hypocalcemic peptide. The EC absorption was estimated by the reduction in plasma calcium concentrations. When EC was orally coadministered with nitroso-N-acetyl-D,L-penicillamine (SNAP, 4.0 mg) and 0.02% Carbopol solution or with taurocholate (20 mM) and 0.02% Carbopol solution, the lowering effect was increased compared with that after EC alone, but the F values (0.32 and 0.30%) were extremely small. The oral administration of the mucoadhesive emulsion, which was prepared by coating the W/O/W emulsion with 0.1% Carbopol, enhanced the calcium lowering effect, with the F value of 0.43%. The strong mucoadhesion of the mucoadhesive emulsion to the gastrointestinal mucosa was observed. A capsule containing EC (500 microg), taurocholate (6 mg) and lyophilized Carbopol (3.5 mg) administered orally gave a sustained but comparatively small calcium lowering effect. In the in vitro enzymatic hydrolysis experiment, EC was more rapidly hydrolyzed in the intestinal fluid than in the mucosal extract. The combination of 20 mM taurocholate with 0.02% Carbopol showed the greatest inhibitory effect in both fluid and extract. These data indicated that EC was effectively absorbed through the intestinal wall, but the peptide was dominantly degraded by proteolytic enzymes in the GI tract. These results will offer a potential approach to the oral delivery of EC.

Delivery systems for penetration enhancement of peptide and protein drugs: design considerations

This paper discusses the challenges to be met in designing delivery systems that maximize the absorption of peptide and protein drugs from the gastrointestinal and respiratory tracts. The ideal delivery system for either route of administration is one that will release its contents only at a favorable region of absorption, where the delivery system attaches by virtue of specific interaction with surface determinants unique to that region and where the delivery system travels at a rate independent of the transitory constraints inherent of the route of administration. Such a delivery system, which is as yet unavailable, will benefit not only peptide and protein drugs, but other poorly absorbed drugs.

Development of intestinal transport function in mammals

Considerable progress has been made over the last decade in the understanding of mechanisms responsible for the ontogenetic changes of mammalian intestine. This review presents the current knowledge about the development of intestinal transport function in the context of intestinal mucosa ontogeny. The review predominantly focuses on signals that trigger and/or modulate the developmental changes of intestinal transport. After an overview of the proliferation and differentiation of intestinal mucosa, data about the bidirectional traffic (absorption and secretion) across the developing intestinal epithelium are presented. The largest part of the review is devoted to the description of developmental patterns concerning the absorption of nutrients, ions, water, vitamins, trace elements, and milk-borne biologically active substances. Furthermore, the review examines the development of intestinal secretion that has a variety of functions including maintenance of the fluidity of the intestinal content, lubrication of mucosal surface, and mucosal protection. The age-dependent shifts of absorption and secretion are the subject of integrated regulatory mechanisms, and hence, the input of hormonal, nervous, immune, and dietary signals is reviewed. Finally, the utilization of energy for transport processes in the developing intestine is highlighted, and the interactions between various sources of energy are discussed. The review ends with suggestions concerning possible directions of future research.

Milk-borne insulin with trypsin inhibitor in milk induces pancreatic amylase development at the onset of weaning in rats

BACKGROUND

The physiologic significance of milk-borne hormones and growth factors for internal organs of suckling animals is poorly understood. In this study the significance of milk-borne insulin was evaluated, as well as its combination with trypsin inhibitor, and its role in the development of pancreatic digestive capacity at the time of weaning was investigated.

METHODS

Experiments were performed using insulin-deficient milk formula (standard formula), insulin (20 ng/ml) formula, or insulin with trypsin inhibitor (1 U/ml) formula by a rat artificial-rearing technique.

RESULTS

In 17-day-old rats administered standard formula, the plasma insulin level was as low as that in 14-day-old rats. When insulin-trypsin inhibitor formula was administered to rat pups, the plasma insulin level was significantly higher than those in rats given standard or insulin formula. In rats artificially reared on standard formula, the usual developmental increases in pancreatic amylase activity and plasma insulin concentration at the beginning of weaning did not occur. Insulin formula elevated the pups' plasma insulin concentration and amylase activity at the onset of weaning but not to the levels observed in mother-reared rats. In rats reared on insulin-trypsin inhibitor formula, the developmental increases in the plasma insulin concentration and amylase activity observed in mother-reared rats were induced.

CONCLUSIONS

The present study demonstrates the necessity of milk-borne insulin for the development of pancreatic amylase during the weaning period.

Human milk as a carrier of biochemical messages

Not only does breast milk provides an ideal nutrient composition for the newborn, but it also contains a variety of substances that may actively influence growth and development of the infant and stimulate neonatal protection against gastrointestinal diseases. Hormones, growth factors, cytokines and even whole cells are present in breast milk and act to establish biochemical and immunological communication between mother and child. In addition, milk nutrients such as nucleotides, glutamine and lactoferrin have been shown to influence gastrointestinal development and host defense. The unique properties of milk as a mediator of biochemical messages will be presented and the clinical significance of breastfeeding in the prevention of neonatal gastrointestinal diseases will be discussed.

Subcellular fractionation, electromigration analysis and mapping of organelles

Subcellular fractionation has provided the means required to analyze the composition and properties of purified cellular elements. In particular, subcellular fractionation has helped to define membrane boundaries and became necessary for the development of cell-free assays that reconstitute complicated cellular processes. Although cell fractionation techniques have improved over the last decades the purification of organelles to homogeneity is still a barely accessible goal in cell biology. In this article, we will first briefly review the basic principles of subcellular fractionation, and the establishment of different organelle fractions by density centrifugation, using tissue culture cells as a paradigm. Then we will discuss some of the intrinsic problems and will compare gradient purification of cellular extracts with electromigration analysis. Finally, we will describe alternative approaches, such as immunoisolation and flow cytometry to purify organelles from tissue culture cells.

Crucial role of milk-borne insulin in the development of pancreatic amylase at the onset of weaning in rats

The development of pancreatic amylase activity was examined in rats fed in regular cages or in special cages, designed so the pups could not reach solid food to prevent weaning. In both groups, the amylase activity in zymogen granules increased in rat pups aged 14 days, peaked at 18 days, and thereafter remained at a 1.6-fold higher level than at 14 days of age. An increase in the plasma concentration of immunoreactive insulin preceded the increase of amylase activity, whereas the plasma concentration of C-peptide, indicating the secretion rate of endogenous insulin, remained unchanged. The administration of insulin at 20 ng/ml (the physiological concentration) in the milk formula caused an increase in the plasma insulin concentration of 17-day-old pups. In addition, increased pancreatic amylase activity was observed in 17-day-old rats raised on milk formula to which insulin was added. We propose that the increase of amylase activity at the beginning of weaning is dependent on the milk-borne insulin and not on the dietary change in rats.

Biosynthesis of endotubin: an apical early endosomal glycoprotein from developing rat intestinal epithelial cells

Endosomes are the site of sorting of internalized receptors and ligands in all cell types and, in polarized cells, the apical endosomal compartment is involved in the selective transepithelial transport of immunoglobulins and growth factors. The biochemical composition of this specialized compartment remains largely unresolved. We have characterized a glycoprotein, called endotubin, that is located in the apical endosomal tubules of developing rat intestinal epithelial cells. A monoclonal antibody against endotubin recognizes a broad band of 55-60kDa, which upon isoelectric focusing can be resolved into two bands, and a faint band of 140kDa. Metabolic labelling followed by immunoprecipitation indicates that endotubin is synthesized as a 140kDa precursor that is cleaved to the 55-60kDa forms. High pH washing of endosomal membranes removes the 55-60kDa forms from the membrane, whereas the high-molecular-mass form remains membrane associated and appears to be an integral membrane protein. Immunoblotting with a polyclonal antibody against the putative cytoplasmic tail of the protein identifies a 140kDa band and a band of 74kDa, presumably the cleavage product. Immunoprecipitation with antibodies against the 55-60kDa form results in coprecipitation of a 74kDa protein, and immunoprecipitation with antibody against the 74kDa protein results in coprecipitation of the 55-60kDa form. Epitope mapping of the monoclonal antibody binding site supports a proposed type I membrane protein orientation. We propose that endotubin is proteolytically processed into a heterodimer with the 55-60kDa fragment remaining membrane-associated through a non-covalent association with the membrane-bound 74kDa portion of the molecule.

Dendroaxonal transcytosis of transferrin in cultured hippocampal and sympathetic neurons

Previous studies using overexpressed polymeric immunoglobulin receptor in cultured neurons have suggested that these cells may use a dendroaxonal transcytotic pathway (Ikonen et al., 1993; de Hoop et al., 1995). By using a combination of semiquantitative light microscopy, video microscopy, and a biochemical assay, we show that this pathway is used by the endogenous ligand transferrin (Tf) and its receptor. Labeled Tf added to fully mature hippocampal neurons changes the intracellular distribution of its receptor from preferentially dendritic shortly after addition to dendritic and axonal at longer times. Incubation of living neurons with (caged)FITC-Tf followed by uncaging in the dendrites results in the later appearance of fluorescence in the axon of the same cell. In "chambered" sympathetic neurons in culture, 125I-Tf or iron as 55Fe-Tf added to the cell body/dendrite chamber is recovered in the axonal chamber, showing that internalized ligand from the cell body-dendrite area is released at the axonal end. Finally, we show that excitatory neurotransmitters increase Tf receptor transcytosis, whereas inhibitory neurotransmitters reduce it. The dendritic uptake, transcytotic transport, and axonal release of physiologically active Tf demonstrated here could be envisioned for other trophic factors and therefore have important consequences for neuronal anterograde target maturation. Moreover, the changes in transcytosis after neurotransmitter addition may be important in the cellular responses that follow electrical activation.

Dendroaxonal transcytosis of transferrin in cultured hippocampal and sympathetic neurons

Previous studies using overexpressed polymeric immunoglobulin receptor in cultured neurons have suggested that these cells may use a dendroaxonal transcytotic pathway (Ikonen et al., 1993; de Hoop et al., 1995). By using a combination of semiquantitative light microscopy, video microscopy, and a biochemical assay, we show that this pathway is used by the endogenous ligand transferrin (Tf) and its receptor. Labeled Tf added to fully mature hippocampal neurons changes the intracellular distribution of its receptor from preferentially dendritic shortly after addition to dendritic and axonal at longer times. Incubation of living neurons with (caged)FITC-Tf followed by uncaging in the dendrites results in the later appearance of fluorescence in the axon of the same cell. In "chambered" sympathetic neurons in culture, 125I-Tf or iron as 55Fe-Tf added to the cell body/dendrite chamber is recovered in the axonal chamber, showing that internalized ligand from the cell body-dendrite area is released at the axonal end. Finally, we show that excitatory neurotransmitters increase Tf receptor transcytosis, whereas inhibitory neurotransmitters reduce it. The dendritic uptake, transcytotic transport, and axonal release of physiologically active Tf demonstrated here could be envisioned for other trophic factors and therefore have important consequences for neuronal anterograde target maturation. Moreover, the changes in transcytosis after neurotransmitter addition may be important in the cellular responses that follow electrical activation.

Prosaposin, a neurotrophic factor: presence and properties in milk

The presence of prosaposin, the precursor of the sphingolipid activator proteins (saposins A, B, C, and D), was investigated in bovine milk. The milk proteins were resolved by SDS-PAGE, blotted onto nitrocellulose sheets, and immunostained. Each of three appropriate antibodies defined a band from milk that matched in mobility the reference prosaposin from human milk at a relative molecular mass of 66,000. Evidence of mature saposins was not found. Prosaposin was detected in milk of other species chimpanzee, rhesus, goat, and rat) and was consistently observed in samples of retail milk and from individual cows. Prosaposin was not associated with particulate matter (fat globules, casein micelles, membrane fragments, and somatic cells) in either human or bovine milk. Rather, prosaposin was located exclusively in the milk serum (whey), existing in monomeric form, as revealed by nondenaturing PAGE. A commercial whey protein concentrate (75% protein) appeared to retain milk prosaposin quantitatively. Properties that were useful in the isolation of prosaposin from milk were its binding to concanavalin A, retention by anion-exchange cellulose, and resistance to precipitation by heating. The possibility that bovine milk prosaposin nutritionally benefits the humans who consume it is enhanced by the fact that only part of its saposin C segment is required for neurotrophic activity.

Targeting of an intestinal apical endosomal protein to endosomes in nonpolarized cells

Polarized cells such as epithelial cells and neurons have distinct endosomal compartments associated with different plasma membrane domains. The endosomes of the neuronal cell body and the basolateral cytoplasm of epithelial cells are thought to perform cellular "housekeeping" functions such as the uptake of nutrients and metabolites, while the endosomes in the apical cytoplasm or axons are thought to be specialized for the sorting and transcytosis of cell type-specific ligands and receptors. However, it is not known if nonpolarized cells such as fibroblasts contain a specialized endosomal compartment analogous to the specialized endosomes found in neurons and epithelia. We have expressed a protein that is normally found in the apical early endosomes of developing intestinal epithelial cells in normal rat kidney fibroblasts. This apical endosomal marker, called endotubin, is targeted to early endosomes in transfected fibroblasts, and is present in peripheral as well as perinuclear endosomes. The peripheral endosomes that contain endotubin appear to exclude transferrin, fluid phase markers, and the mannose-6-phosphate receptor, although in the perinuclear region colocalization of endotubin and these markers is present. In addition, endotubin positive structures do not tubulate in response to brefeldin A and instead redistribute to a diffuse perinuclear location. Since this endosomal compartment has many of the characteristics of an apical or axonal endosomal compartment, our results indicate that nonpolarized cells also contain a specialized early endosomal compartment.

Intracellular routing of wild-type and mutated polymeric immunoglobulin receptor in hippocampal neurons in culture

Certain epithelial cells synthesize the polymeric immunoglobulin receptor (pIgR) to transport immunoglobulins (Igs) A and M into external secretions. In polarized epithelia, newly synthesized receptor is first delivered to the basolateral plasma membrane and is then, after binding the Ig, transcytosed to the apical plasma membrane, where the receptor-ligand complex is released by proteolytic cleavage. In a previous work (Ikonen et al., 1993), we implied the existence of a dendro-axonal transcytotic pathway for the rabbit pIgR expressed in hippocampal neurons via the Semliki Forest Virus (SFV) expression system. By labeling surface-exposed pIgR in live neuronal cells, we now show (a) internalization of the receptor from the dendritic plasma membrane to the dendritic early endosomes, (b) redistribution of the receptor from the dendritic to the axonal domain, (c) inhibition of this movement by brefeldin A (BFA) and (d) stimulation by the activation of protein kinase C (PKC) via phorbol myristate acetate (PMA). In addition, we show that a mutant form of the receptor lacking the epithelial basolateral sorting signal is directly delivered to the axonal domain of hippocampal neurons. Although this mutant is internalized into early endosomes, no transcytosis to the dendrites could be observed. In epithelial Madin-Darby Canine Kidney (MDCK) cells, the mutant receptor could also be internalized into basolaterally derived early endosomes. These results suggest the existence of a dendro-axonal transcytotic pathway in neuronal cells which shares similarities with the basolateral to apical transcytosis in epithelial cells and constitute the basis for the future analysis of its physiological role.

Identification and characterization of a novel protein (p137) which transcytoses bidirectionally in Caco-2 cells

Antisera raised against detergent-extracted membrane fractions from the human intestinal epithelial cell line Caco-2 were used to screen a human colon cDNA library in a bacteriophage expression vector. This led to the identification, molecular cloning, and sequencing of a novel plasma membrane protein (p137) which was present in approximately equal amounts on the basolateral and apical surfaces of the cell. The pattern of extraction of p137 from membranes by Triton X-114 and its release from membranes after incubation with phosphatidylinositol-specific phospholipase C were consistent with it being a glycosylphosphatidylinositol-anchored membrane protein. Using antibodies raised against bacterial fusion proteins, it was shown that p137 was present on the cell surface as a reducible homodimer of 137 kDa subunits. There was constitutive release of p137 into the culture medium as a non-reducible 280-kDa entity. Pulse-chase experiments showed that newly synthesized p137 appeared at the basolateral side of a Caco-2 cell layer before appearing at the apical domain. Domain-specific surface biotinylation of Caco-2 cells at 4 degrees C, followed by chasing at 37 degrees C, demonstrated that p137 is capable of transcytosing in both directions across Caco-2 cells. The unusual plasma membrane domain distribution of this glycosylphosphatidylinositol-linked protein and its transcytosis characteristics demonstrate the existence of a previously uncharacterized apical to basolateral transcytotic pathway in Caco-2 cells.

Molecular characterization of an apical early endosomal glycoprotein from developing rat intestinal epithelial cells

The apical endosomal compartment is thought to be involved in the sorting and selective transport of receptors and ligands across polarized epithelia. To learn about the protein components of this compartment, we have isolated and sequenced a cDNA that encodes a glycoprotein that is located in the apical endosomal tubules of developing rat intestinal epithelial cells. The deduced amino acid sequence predicts a protein of 1216 amino acids with a molecular mass of 133,769 Da. The deduced amino acid sequence together with amino-terminal amino acid sequencing indicate that there is a cleaved 21-amino acid signal sequence at the NH2-terminal portion of the molecule. There is a single hydrophobic region near the carboxyl terminus that has the characteristics of a membrane-spanning domain and a 36-amino acid cytoplasmic tail. We have found that the major form of this protein in intestinal epithelial cells has a molecular mass of 55-60 kDa, which is significantly smaller than the size predicted from the cDNA sequence, suggesting that the protein is synthesized as a large precursor and processed to the smaller form. The smaller form remains associated with the membrane, however, possibly through noncovalent association with the transmembrane portion of the molecule or with another membrane protein. The extracytoplasmic domain is cysteine-rich, with three cysteine-rich repeats that are similar to cysteine repeats present in several receptor proteins. However, there is no other significant similarity to other proteins in the GenBank. The cytoplasmic tail contains a possible internalization motif and several consensus motifs for serine/threonine kinases. Northern blot analysis suggests a single abundant message, and Southern blot analysis is consistent with a single gene and the absence of pseudogenes for this unique endosomal protein.

Hormonally active peptides in human milk

Human milk contains many hormone and hormone-like peptides. The gastrointestinal tract of newborn infants exhibits lower proteolytic activity than in adults and higher "permeability" for macromolecules. Studies in experimental animals demonstrate that several peptides (epidermal growth factors, insulin-like growth factor I and bombesin) after orogastric administration exhibit effects on the small intestine and other organs (liver or pancreas). Few studies performed in human neonates suggest a "survival" of epidermal growth factor in their gastric content. Further studies are needed to evaluate the role of milk-borne hormonally active peptides. This need is stressed by the fact that several of those known to be present in human milk were found to be low or not detectable in infant formulae (epidermal growth factor, insulin-like growth factor I, insulin, parathyroid hormone-related peptide.

How protein hormones reach their target cells. Receptor-mediated transcytosis of hCG through endothelial cells

In many organs the vascular endothelium forms a barrier which impedes the free diffusion of large molecules. The mechanism by which protein hormones are transported through the endothelial cells to reach their target cells is unknown. We have examined the transport of human chorionic gonadotropin (hCG) in rat testicular microvasculature by electron microscopy and by analysing the transfer of radiolabeled hormone and antibodies. Surprisingly, we have observed that the same receptor molecule which is present in target Leydig cells is also involved in transcytosis through the endothelial cells. The hormone was internalized by coated pits and vesicles on the luminal side of the endothelium. It was then localized in the endosomal compartment and subsequently appeared to be delivered by smooth vesicles into the subendothelial space. Moreover, anti-LH/hCG receptor antibodies were efficiently transported via the same system and delivered into the interstitial space. If generalized, these observations may define a new level of modulation of hormone action and may be of importance for drug targeting into the numerous organs which are responsive to the various protein hormones.

Transcytosis of the polymeric immunoglobulin receptor in cultured hippocampal neurons

BACKGROUND

A wide variety of proteins are transported across epithelial cells by vesicular carriers. This process, transcytosis, is used to generate cell surface polarity and to transport macromolecules between the luminal and serosal sides of the epithelial layer. The polymeric immunoglobulin receptor is a well-characterized transcytotic molecule in epithelia. It binds to its ligand, polymeric immunoglobulin, at the basolateral surface, and the receptor-ligand complex is transcytosed to the apical surface, where the ligand is released. Our previous studies have shown that hippocampal neurons may employ mechanisms similar to those of epithelial cells to sort proteins to two plasma membrane domains. The machinery used for axonal delivery recognizes proteins that are targeted apically in epithelia, whereas basolaterally destined proteins are delivered to the dendrites. It has not been clear, however, whether transcytosis occurs in neurons.

RESULTS

We report expression of the polymeric immunoglobulin receptor in cultured hippocampal neurons, using a Semliki Forest Virus expression system, and show by immunofluorescence microscopy that the newly synthesized receptor is targeted from the Golgi complex predominantly to the dendrites - only about 20% of the infected neurons display axonal immunofluorescence. Addition of ligand leads to significant redistribution of the receptor to the axons, shown by an approximately three-fold increase in axonal immunoreactivity with the anti-receptor antibodies.

CONCLUSIONS

Our results suggest that a transcytotic route, analogous to that in epithelia, exists in neurons, where it transports proteins from the somatodendritic to the axonal domain. Cultured neurons expressing the polymeric immunoglobulin receptor offer an experimental system that should be useful for further characterization of this novel neuronal pathway at the molecular and functional level.

Uptake and transport of macromolecules by the intestine: possible role in clinical disorders (an update)

The intestine is exposed to a wide variety of macromolecules. Because macromolecules are antigenic, mechanisms have evolved in the gastrointestinal tract to regulate their absorption. Macromolecular uptake can be beneficial in delivering essential factors for growth and in sampling the antigenic milieu of the gastrointestinal tract. Specific transport mechanisms exist to execute this physiological absorption. However, inappropriate and uncontrolled antigen transport may occur in disease states or as a prelude to disease states in the gastrointestinal tract. Such transport may result in immune responses that are harmful. This review examines physiological transport of macromolecules through epithelia and through M cells. It also considers uncontrolled transport and its relation to disease states. The review concludes with an examination of the interrelationship between antigen transport and an altered immune system in the establishment of gastrointestinal disease.

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.

Ultrastructural anomalies in the fetal small intestine indicate that fetal swallowing is important for normal development: an experimental study

Fetal swallowing is established early in development and if fetal ingestion is prevented, the gastrointestinal (GI) tract fails to grow normally. In this article we describe the ultrastructural features of GI tissues developing in the absence of swallowing, in the fetal sheep. We have noted a number of defects in enterocyte morphology. These include abnormal or absence of microvilli, inappropriate cell extrusion, glycogen accumulation and altered lysosomal morphology. Many of these changes resemble those seen in malnourished infants. It is possible that fetal ingestion provides a significant source of nutrients, ensuring adequate GI tract growth in utero, in addition to specific growth factors which may be present in ingested fluid.

An internal deletion in the cytoplasmic tail reverses the apical localization of human NGF receptor in transfected MDCK cells

A cDNA encoding the full-length 75-kD human nerve growth factor receptor was transfected into MDCK cells and its product was found to be expressed predominantly (80%) on the apical membrane, as a result of vectorial targeting from an intracellular site. Apical hNGFR bound NGF with low affinity and internalized it inefficiently (6% of surface bound NGF per hour). Several mutant hNGFRs were analyzed, after transfection in MDCK cells, for polarized surface expression, ligand binding, and endocytosis. Deletionof juxta-membrane attachment sites for a cluster of O-linked sugars did not alter apical localization. A mutant receptor lacking the entire cytoplasmic tail (except for the five proximal amino acids) was also expressed on the apical membrane, suggesting that information for apical sorting was contained in the ectoplasmic or transmembrane domains. However, a 58 amino acid deletion in the hNGFR tail that moved a cytoplasmic tyrosine (Tyr 308) closer to the membrane into a more charged environment resulted in a basolateral distribution of the mutant receptor and reversed vectorial (basolateral) targeting. The basolateral mutant receptor also internalized 125I-NGF rapidly (90% of surface bound NGF per hour), exhibited a larger intracellular fraction and displayed a considerably shortened half-life (approximately 3 h). We suggest that hNGFR with the internal cytoplasmic deletion expresses a basolateral targeting signal, related to endocytic signals, that is dominant over apical targeting information in the ecto/transmembrane domains. These results apparently contradict a current model that postulates that basolateral targeting is a default mechanism.