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Jan Holm - One of the best experts on this subject based on the ideXlab platform.
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Li+-induced release of the Folate Binding Protein from human granulocytes
Scandinavian journal of haematology, 2009Co-Authors: Jan Holm, Steen Ingemann Hansen, Jørgen LyngbyeAbstract:Release of Folate Binding Protein from human leucocytes suspended in isotonic NaCl was studied in experiments where Folate Binding activity of the suspending medium was determined by equilibrium dialysis (pH 7.4, 37 degrees C) against 3H-Folate. Release from leucocytes containing particularly high concentrations of Folate Binding Protein was significantly increased in the presence of 70 mmol/l Li+. The stimulated release was apparently not an osmotic phenomenon since no effect was observed with isomolar Na+/K+. The released Protein resembled the Folate Binding Protein in leucocyte lysate as to characteristics of radioligand Binding, ionic properties and molecular size. Hence no transformation of the Protein seemed to occur during Li+-induced release.
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A high-affinity Folate Binding Protein in human cerebrospinal fluid.
Acta neurologica Scandinavica, 2009Co-Authors: Steen Ingemann Hansen, Jan Holm, Jørgen LyngbyeAbstract:Binding of 3H-folic acid to a Protein in human cerebrospinal fluid (CSF) was studied in equilibrium dialysis experiments at 37 degrees C and pH 7.4. The Binding was of a high-affinity type (10(10) l/mol), and the relative molecular mass of the Protein was 25,000 as determined by gel filtration. The concentration of Folate Binding Protein in CSF, expressed as maximum Folate bound per 1, was 0.3 nmol/l. This value was of the same order of magnitude as that of serum.
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Stopped-Flow Kinetic Studies of the Interaction of Bovine Folate Binding Protein (FBP) and Folate
Bioscience Reports, 2006Co-Authors: Ulla Christensen, Jan Holm, Steen Ingemann HansenAbstract:The kinetics of the interaction of bovine Folate Binding Protein and Folate at pH 7.4 and 5.0 were followed by measuring the changes of the intrinsic Protein fluorescence intensity using the stopped-flow technique, which enables the study of reactions from the millisecond time-range. Our results immediately reject a simple one-step Binding model, which requires a linear dependence of the observed rate constant on the concentration of the ligand. Thus, we are able to conclude that at pH 5.0 the interaction occurs in two steps and at pH 7.4 in three steps. Changes of fluorescence spectra at equilibrium were used to estimate the overall Binding constants. Comparative studies on the Binding of Folate to human albumin are also reported.
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Megalin binds and mediates cellular internalization of Folate Binding Protein
FEBS Journal, 2005Co-Authors: Henrik Birn, Steen Ingemann Hansen, Jan Holm, Erik Ilsø Christensen, Xiao-yue Zhai, Christian Jacobsen, Søren K. MoestrupAbstract:Folate is an essential vitamin involved in a number of biological processes. High affinity Folate Binding Proteins (FBPs) exist both as glycosylphosphatidylinositol-linked, membrane associated Folate Binding Proteins and as soluble FBPs in plasma and some secretory fluids such as milk, saliva and semen. The function and significance of FBPs are unresolved, however, it has been suggested that they may facilitate Folate uptake, e.g. during suckling. The present study shows that megalin, a large, multiligand endocytic receptor and member of the low-density lipoProtein-receptor family, is able to bind and mediate cellular uptake of FBP. Surface plasmon resonance analysis shows Binding of bovine and human milk FBP to immobilized megalin, but not to low density lipoProtein receptor related Protein. Binding of 125I-labeled Folate Binding Protein (FBP) to sections of kidney proximal tubule, known to express high levels of megalin, is inhibitable by excess unlabeled FBP and by receptor associated Protein, a known inhibitor of Binding to megalin. Immortalized rat yolk sac cells, representing an established model for studying megalin-mediated uptake, reveal 125I-labeled FBP uptake which is inhibited by receptor associated Protein and by antimegalin antibodies. Microinjection of 125I-labeled FBP into renal tubules in vivo shows proximal tubular uptake by endocytosis. Megalin is expressed in several absorptive epithelia, including intestine and kidney proximal tubule, and thus the present findings provide a mechanism for intestinal and renal endocytic uptake of soluble FBP.
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Characterization of a high affinity Folate Binding Protein in porcine serum: ionic charge, concentration-dependent polymerization and ligand Binding mechanism.
Bioscience Reports, 2003Co-Authors: Jan Holm, Steen Ingemann HansenAbstract:The Folate Binding Protein in porcine serum, present at concentrations of 50-100 nM, is cationic at near neutral pH as evidenced by ion exchange chromatography. The gel filtration profile of the Protein isolated from porcine serum by methotrexate affinity chromatography exhibited one peak at 48 kDa and an additional peak of 91 kDa at higher Protein concentrations. This could suggest the involvement of concentration-dependent polymerization phenomena. Binding of [3H] Folate was of a high-af.nity type with upward convex Scatchard plots and Hill coefficients >1.0 indicative of apparent positive cooperativity. However, Binding to Protein isolated from porcine serum after affinity chromatography was biphasic (high/low-affinity) in the absence of Triton X-100, 1 g/1. These findings which are similar to those reported for purified milk Folate Binding Proteins are consistent with a model predicting association between unliganded and liganded monomers to weak-ligand affinity heterodimers. Amphiphatic substances, e.g. Triton X-100, form micelles which could separate hydrophobic unliganded monomers from hydrophilic liganded monomers (monomers are hydrophilic in the liganded state) thereby preventing heterodimerization. The Folate analogue N10 methyl Folate was a potent and competitive inhibitor of [3H] Folate Binding to the Folate Binding Protein, and moreover changed the Binding type to apparent negative cooperativity.
Steen Ingemann Hansen - One of the best experts on this subject based on the ideXlab platform.
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Li+-induced release of the Folate Binding Protein from human granulocytes
Scandinavian journal of haematology, 2009Co-Authors: Jan Holm, Steen Ingemann Hansen, Jørgen LyngbyeAbstract:Release of Folate Binding Protein from human leucocytes suspended in isotonic NaCl was studied in experiments where Folate Binding activity of the suspending medium was determined by equilibrium dialysis (pH 7.4, 37 degrees C) against 3H-Folate. Release from leucocytes containing particularly high concentrations of Folate Binding Protein was significantly increased in the presence of 70 mmol/l Li+. The stimulated release was apparently not an osmotic phenomenon since no effect was observed with isomolar Na+/K+. The released Protein resembled the Folate Binding Protein in leucocyte lysate as to characteristics of radioligand Binding, ionic properties and molecular size. Hence no transformation of the Protein seemed to occur during Li+-induced release.
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A high-affinity Folate Binding Protein in human cerebrospinal fluid.
Acta neurologica Scandinavica, 2009Co-Authors: Steen Ingemann Hansen, Jan Holm, Jørgen LyngbyeAbstract:Binding of 3H-folic acid to a Protein in human cerebrospinal fluid (CSF) was studied in equilibrium dialysis experiments at 37 degrees C and pH 7.4. The Binding was of a high-affinity type (10(10) l/mol), and the relative molecular mass of the Protein was 25,000 as determined by gel filtration. The concentration of Folate Binding Protein in CSF, expressed as maximum Folate bound per 1, was 0.3 nmol/l. This value was of the same order of magnitude as that of serum.
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Stopped-Flow Kinetic Studies of the Interaction of Bovine Folate Binding Protein (FBP) and Folate
Bioscience Reports, 2006Co-Authors: Ulla Christensen, Jan Holm, Steen Ingemann HansenAbstract:The kinetics of the interaction of bovine Folate Binding Protein and Folate at pH 7.4 and 5.0 were followed by measuring the changes of the intrinsic Protein fluorescence intensity using the stopped-flow technique, which enables the study of reactions from the millisecond time-range. Our results immediately reject a simple one-step Binding model, which requires a linear dependence of the observed rate constant on the concentration of the ligand. Thus, we are able to conclude that at pH 5.0 the interaction occurs in two steps and at pH 7.4 in three steps. Changes of fluorescence spectra at equilibrium were used to estimate the overall Binding constants. Comparative studies on the Binding of Folate to human albumin are also reported.
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Megalin binds and mediates cellular internalization of Folate Binding Protein
FEBS Journal, 2005Co-Authors: Henrik Birn, Steen Ingemann Hansen, Jan Holm, Erik Ilsø Christensen, Xiao-yue Zhai, Christian Jacobsen, Søren K. MoestrupAbstract:Folate is an essential vitamin involved in a number of biological processes. High affinity Folate Binding Proteins (FBPs) exist both as glycosylphosphatidylinositol-linked, membrane associated Folate Binding Proteins and as soluble FBPs in plasma and some secretory fluids such as milk, saliva and semen. The function and significance of FBPs are unresolved, however, it has been suggested that they may facilitate Folate uptake, e.g. during suckling. The present study shows that megalin, a large, multiligand endocytic receptor and member of the low-density lipoProtein-receptor family, is able to bind and mediate cellular uptake of FBP. Surface plasmon resonance analysis shows Binding of bovine and human milk FBP to immobilized megalin, but not to low density lipoProtein receptor related Protein. Binding of 125I-labeled Folate Binding Protein (FBP) to sections of kidney proximal tubule, known to express high levels of megalin, is inhibitable by excess unlabeled FBP and by receptor associated Protein, a known inhibitor of Binding to megalin. Immortalized rat yolk sac cells, representing an established model for studying megalin-mediated uptake, reveal 125I-labeled FBP uptake which is inhibited by receptor associated Protein and by antimegalin antibodies. Microinjection of 125I-labeled FBP into renal tubules in vivo shows proximal tubular uptake by endocytosis. Megalin is expressed in several absorptive epithelia, including intestine and kidney proximal tubule, and thus the present findings provide a mechanism for intestinal and renal endocytic uptake of soluble FBP.
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Characterization of a high affinity Folate Binding Protein in porcine serum: ionic charge, concentration-dependent polymerization and ligand Binding mechanism.
Bioscience Reports, 2003Co-Authors: Jan Holm, Steen Ingemann HansenAbstract:The Folate Binding Protein in porcine serum, present at concentrations of 50-100 nM, is cationic at near neutral pH as evidenced by ion exchange chromatography. The gel filtration profile of the Protein isolated from porcine serum by methotrexate affinity chromatography exhibited one peak at 48 kDa and an additional peak of 91 kDa at higher Protein concentrations. This could suggest the involvement of concentration-dependent polymerization phenomena. Binding of [3H] Folate was of a high-af.nity type with upward convex Scatchard plots and Hill coefficients >1.0 indicative of apparent positive cooperativity. However, Binding to Protein isolated from porcine serum after affinity chromatography was biphasic (high/low-affinity) in the absence of Triton X-100, 1 g/1. These findings which are similar to those reported for purified milk Folate Binding Proteins are consistent with a model predicting association between unliganded and liganded monomers to weak-ligand affinity heterodimers. Amphiphatic substances, e.g. Triton X-100, form micelles which could separate hydrophobic unliganded monomers from hydrophilic liganded monomers (monomers are hydrophilic in the liganded state) thereby preventing heterodimerization. The Folate analogue N10 methyl Folate was a potent and competitive inhibitor of [3H] Folate Binding to the Folate Binding Protein, and moreover changed the Binding type to apparent negative cooperativity.
Erik Ilsø Christensen - One of the best experts on this subject based on the ideXlab platform.
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Megalin binds and mediates cellular internalization of Folate Binding Protein
FEBS Journal, 2005Co-Authors: Henrik Birn, Steen Ingemann Hansen, Jan Holm, Erik Ilsø Christensen, Xiao-yue Zhai, Christian Jacobsen, Søren K. MoestrupAbstract:Folate is an essential vitamin involved in a number of biological processes. High affinity Folate Binding Proteins (FBPs) exist both as glycosylphosphatidylinositol-linked, membrane associated Folate Binding Proteins and as soluble FBPs in plasma and some secretory fluids such as milk, saliva and semen. The function and significance of FBPs are unresolved, however, it has been suggested that they may facilitate Folate uptake, e.g. during suckling. The present study shows that megalin, a large, multiligand endocytic receptor and member of the low-density lipoProtein-receptor family, is able to bind and mediate cellular uptake of FBP. Surface plasmon resonance analysis shows Binding of bovine and human milk FBP to immobilized megalin, but not to low density lipoProtein receptor related Protein. Binding of 125I-labeled Folate Binding Protein (FBP) to sections of kidney proximal tubule, known to express high levels of megalin, is inhibitable by excess unlabeled FBP and by receptor associated Protein, a known inhibitor of Binding to megalin. Immortalized rat yolk sac cells, representing an established model for studying megalin-mediated uptake, reveal 125I-labeled FBP uptake which is inhibited by receptor associated Protein and by antimegalin antibodies. Microinjection of 125I-labeled FBP into renal tubules in vivo shows proximal tubular uptake by endocytosis. Megalin is expressed in several absorptive epithelia, including intestine and kidney proximal tubule, and thus the present findings provide a mechanism for intestinal and renal endocytic uptake of soluble FBP.
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Renal Tubular Reabsorption of Folate Mediated by Folate Binding Protein 1
Journal of The American Society of Nephrology, 2005Co-Authors: Henrik Birn, Ofer Spiegelstein, Erik Ilsø Christensen, Richard H. FinnellAbstract:Renal tubular reabsorption of filtered Folate is essential for the conservation and normal homeostasis of this important vitamin. Different molecular mechanisms have been implicated in epithelial Folate transport, including Folate receptors. Defective expression or antibody inactivation of these is associated with embryonic defects also correlated with low Folate intake; however, their contribution to renal tubular Folate reabsorption has not been established. With the use of targeted inactivation of the Folate Binding Protein 1 (folbp1) and Folate Binding Protein 2 (folbp2) genes in mice, the role of Folate receptors in renal epithelial Folate reabsorption was evaluated during low and normal Folate intake. Inactivation of folbp1 was associated with ( 1 ) loss of 3 H-folic acid Binding to crude kidney membranes, ( 2 ) increase in renal Folate clearance, and ( 3 ) increase in urinary excretion and decrease in renal uptake of injected 3 H-methyltetrahydroFolate. No changes in renal Folate handling were observed as a result of folbp2 inactivation. Thus, folbp1 is essential for normal renal tubular Folate reabsorption, preventing excessive urinary Folate loss. Folbp1 is heavily expressed in choroid plexus, yolk sac, and placenta, supporting a role of folbp1 in Folate transport in other tissues. The greatest significance of folbp1 for renal Folate uptake was observed at conditions of low Folate intake, providing a possible explanation for the ability of Folate supplementation to prevent developmental defects associated with folbp1 inactivation.
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internalization and intracellular transport of Folate Binding Protein in rat kidney proximal tubule
American Journal of Physiology-cell Physiology, 1993Co-Authors: Henrik Birn, J Selhub, Erik Ilsø ChristensenAbstract:Folate-Binding Protein (FBP) is involved in Folate reabsorption in the renal proximal tubule. Immunocytochemical studies have located FBP to the brush-border membrane, endocytic vacuoles, and dense apical tubules. We applied the same polyclonal antibody (anti-FBP) against FBP to investigate the dynamic relationship between FBP in the different compartments by microinjecting the antibody into rat kidney proximal tubules in situ. Specific Binding of anti-FBP in vivo to the brush-border membrane was followed by fixation at various times. Protein A-gold labeling shows that anti-FBP is transported from endocytic invaginations into vacuoles followed by transport into dense apical tubules within 15 s. Thus FBP is rapidly internalized, and together with previous studies this study strongly suggests recycling of FBP back to the luminal plasma membrane through dense apical tubules. The results are consistent with reabsorption of Folate through endocytosis of the FBP-Folate complex followed by dissociation and recycling of FBP. When time is allowed there is a steady accumulation of FBP in dense apical tubules combined with an increase in surface density of the same compartment. A possible explanation involves partial inhibition of the fusion between dense apical tubules and plasma membrane because of the anti-FBP labeling of the receptor.
Mimi Høier-madsen - One of the best experts on this subject based on the ideXlab platform.
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Ionic charge, hydrophobicity and tryptophan fluorescence of the Folate Binding Protein isolated from cow's milk.
Bioscience reports, 2001Co-Authors: Jan Holm, Steen Ingemann Hansen, Mimi Høier-madsenAbstract:A high-affinity Folate Binding Protein was isolated and purified from cow's milk by a combination of cation exchange chromatography and methotrexate affinity chromatography. Chromatofocusing studies revealed that the Protein possessed isoelectric points in the pH-interval 8–7. Polymers of the Protein prevailing at pH values close to the isoelectric points seemed to be more hydrophobic than monomers present at pH 5.0 as evidenced by hydrophobic interaction chromatography and turbidity (absorbance at 340 nm) in aqueous buffer solutions (pH 5–8). Ligand Binding seemed to induce a conformation change that decreased the hydrophobicity of the Protein. In addition, Ligand Binding quenched the tryptophan fluorescence of Folate Binding Protein suggesting that tryptophan is present at the Binding site and/or ligand Binding induces a conformation change that affects tryptophan environment in the Protein. There was a noticeable discordance between the ability of individual Folate analogues to compete with Folate for Binding and the quenching effect.
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Characterization of a Folate receptor in parotid gland and a Folate Binding Protein in saliva from humans: Epitope relatedness to human milk Folate Binding Protein
APMIS : acta pathologica microbiologica et immunologica Scandinavica, 2000Co-Authors: Jan Holm, Steen Ingemann Hansen, Mimi Høier-madsen, Carl W. NicholsAbstract:The present study was performed to establish the antigenic identity and origin of the Folate Binding Protein in human saliva. We identified a Folate receptor in human parotid and submandibular gland which immunoreacted with antibodies against human milk Folate Binding Protein, as evidenced by ELISA and immunostaining of ductal epithelium and secretory glandular material. The receptor concentration was 0.4-1.4 nmol 3 H-Folate bound/g Protein. Ligand Binding was of a high-affinity (K=10 10 M -1 ) type, exhibited positive cooperativity, a slow radioligand dissociation at pH 7.4, and inhibition by Folate analogues. The concentration of immunoreactive Folate Binding Protein in saliva as determined by ELISA with antibodies against human milk Folate Binding Protein was several fold higher than that determined by radioligand Binding (nil - 1 nM). This indicates that a major fraction of the immunoreactive material does not bind 3 H-Folate, and could represent a precursor form of the Protein. In conclusion, the Folate Binding Protein in human saliva seems to be a secretory product of the salivary glands. The Protein is also epitope-related to Folate Binding Proteins in other human mucosal secretions.
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A Folate Binding Protein in Ascitic Fluid, Serum and Ovarian Tissue of Patients with Ovarian Adenocarcinoma Immunoreacts with Antibodies Against Human Milk Folate Binding Protein
Bioscience Reports, 1998Co-Authors: Jan Holm, Steen Ingemann Hansen, Mimi Høier-madsen, Poul-erik HelkjærAbstract:The presence of a Folate Binding Protein which immunoreacts with antibodies against human milk Folate Binding Protein was demonstrated in ascitic fluids from seven patients with ovarian adenocarcinoma. Ascitic fluids collected from two patients with other malignancies contained non-immunoreactive FBP. Tumor tissue specimens from five patients with ovarian carcinoma contained immunoreactive FBP. By contrast to normal ovaries ovarian carcinoma tissue showed positive immunostaining on immunohistochemistry. Ascitic fluids from two patients with ovarian carcinoma exhibited single distinct bands on SDS-PAGE immunoblotting. The gel filtration profile of ovarian carcinoma tissue homogenate from two patients contained 25 and 100 kDa peaks of radioligand-bound and immunoreactive Folate Binding Protein, while ascitic fluid from one of the patients exhibited a large 100 kDa immunoreactive peak with no radioligand Binding activity. The immunoreactive non-functional 100 kDa FBP could represent unprocessed precursor FBP. Future studies are necessary to evaluate whether determination of immunoreactive FBP in ovarian adenocarcinomatosis is of any diagnostic value.
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The high-affinity Folate Binding Protein in normal and malignant mammary gland tissue.
Advances in experimental medicine and biology, 1993Co-Authors: Jan Holm, Steen Ingemann Hansen, Knud Søndergaard, Mimi Høier-madsenAbstract:Human milk contains a soluble 25 kDa Folate Binding Protein (FBP) as well as a membrane-derived FBP with a hydrophobic glycosyl-phosphatidylinositol tail; the enzyme phosphatidylinositol specific phospholipase C reduced the apparent size of the latter FBP from 100 to 25 kDa1. The two FBPs from human milk had identical N-terminal amino acid sequence for 39 cycles and shared antigenic determinants2,3. A high correlation between the concentrations of FBP and Folate in milk suggested the secretion of a Folate mammary FBP complex into milk4. In the present study we have characterized and compared FBP in the normal mammary gland and mammary tumors.
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A high-affinity Folate Binding Protein in proximal tubule cells of human kidney.
Kidney international, 1992Co-Authors: Jan Holm, Steen Ingemann Hansen, Mimi Høier-madsen, Leif BostadAbstract:A high-affinity Folate Binding Protein in proximal tubule cells of human kidney. High-affinity 3 H-Folate Binding in solubilized brush border membranes of human kidney cortex display characteristics such as apparent positive cooperativity typical of specific Folate Binding. The Folate Binding activity and the activity of the brush border membrane-marker enzyme, gammaglutamyltransferase, were eightfold higher in brush border membranes compared to crude kidney homogenate. Ultrogel R AcA 44 chromatography revealed a major (M r ∼ 100 kDa) and a minor (M r ∼ 25 kDa) Folate Binding Protein in brush border membranes. The large molecular size form may represent a membrane-derived hydrophobic Folate Binding Protein inserted in Triton X-100 micelles. This notion was supported by the identical molecular weights of the 100 kDa and 25 kDa Folate Binding peaks determined by sodium dodecylsulfate polyacrylamide gel electrophoresis and immunoblotting. The Folate Binding Protein in renal brush border membranes cross reacted with rabbit antibodies against 25 kDa human milk Folate Binding Protein, and showed immunoprecipitation in the presence of these antibodies. Paraffin embedded sections of kidney cortex showed immu-nostaining of cells in the convoluted proximal tubules after exposure to rabbit antiserum (1:8000 dilution). Glomeruli and distal tubules showed no immunostaining.
Henrik Birn - One of the best experts on this subject based on the ideXlab platform.
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Megalin binds and mediates cellular internalization of Folate Binding Protein
FEBS Journal, 2005Co-Authors: Henrik Birn, Steen Ingemann Hansen, Jan Holm, Erik Ilsø Christensen, Xiao-yue Zhai, Christian Jacobsen, Søren K. MoestrupAbstract:Folate is an essential vitamin involved in a number of biological processes. High affinity Folate Binding Proteins (FBPs) exist both as glycosylphosphatidylinositol-linked, membrane associated Folate Binding Proteins and as soluble FBPs in plasma and some secretory fluids such as milk, saliva and semen. The function and significance of FBPs are unresolved, however, it has been suggested that they may facilitate Folate uptake, e.g. during suckling. The present study shows that megalin, a large, multiligand endocytic receptor and member of the low-density lipoProtein-receptor family, is able to bind and mediate cellular uptake of FBP. Surface plasmon resonance analysis shows Binding of bovine and human milk FBP to immobilized megalin, but not to low density lipoProtein receptor related Protein. Binding of 125I-labeled Folate Binding Protein (FBP) to sections of kidney proximal tubule, known to express high levels of megalin, is inhibitable by excess unlabeled FBP and by receptor associated Protein, a known inhibitor of Binding to megalin. Immortalized rat yolk sac cells, representing an established model for studying megalin-mediated uptake, reveal 125I-labeled FBP uptake which is inhibited by receptor associated Protein and by antimegalin antibodies. Microinjection of 125I-labeled FBP into renal tubules in vivo shows proximal tubular uptake by endocytosis. Megalin is expressed in several absorptive epithelia, including intestine and kidney proximal tubule, and thus the present findings provide a mechanism for intestinal and renal endocytic uptake of soluble FBP.
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Renal Tubular Reabsorption of Folate Mediated by Folate Binding Protein 1
Journal of The American Society of Nephrology, 2005Co-Authors: Henrik Birn, Ofer Spiegelstein, Erik Ilsø Christensen, Richard H. FinnellAbstract:Renal tubular reabsorption of filtered Folate is essential for the conservation and normal homeostasis of this important vitamin. Different molecular mechanisms have been implicated in epithelial Folate transport, including Folate receptors. Defective expression or antibody inactivation of these is associated with embryonic defects also correlated with low Folate intake; however, their contribution to renal tubular Folate reabsorption has not been established. With the use of targeted inactivation of the Folate Binding Protein 1 (folbp1) and Folate Binding Protein 2 (folbp2) genes in mice, the role of Folate receptors in renal epithelial Folate reabsorption was evaluated during low and normal Folate intake. Inactivation of folbp1 was associated with ( 1 ) loss of 3 H-folic acid Binding to crude kidney membranes, ( 2 ) increase in renal Folate clearance, and ( 3 ) increase in urinary excretion and decrease in renal uptake of injected 3 H-methyltetrahydroFolate. No changes in renal Folate handling were observed as a result of folbp2 inactivation. Thus, folbp1 is essential for normal renal tubular Folate reabsorption, preventing excessive urinary Folate loss. Folbp1 is heavily expressed in choroid plexus, yolk sac, and placenta, supporting a role of folbp1 in Folate transport in other tissues. The greatest significance of folbp1 for renal Folate uptake was observed at conditions of low Folate intake, providing a possible explanation for the ability of Folate supplementation to prevent developmental defects associated with folbp1 inactivation.
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internalization and intracellular transport of Folate Binding Protein in rat kidney proximal tubule
American Journal of Physiology-cell Physiology, 1993Co-Authors: Henrik Birn, J Selhub, Erik Ilsø ChristensenAbstract:Folate-Binding Protein (FBP) is involved in Folate reabsorption in the renal proximal tubule. Immunocytochemical studies have located FBP to the brush-border membrane, endocytic vacuoles, and dense apical tubules. We applied the same polyclonal antibody (anti-FBP) against FBP to investigate the dynamic relationship between FBP in the different compartments by microinjecting the antibody into rat kidney proximal tubules in situ. Specific Binding of anti-FBP in vivo to the brush-border membrane was followed by fixation at various times. Protein A-gold labeling shows that anti-FBP is transported from endocytic invaginations into vacuoles followed by transport into dense apical tubules within 15 s. Thus FBP is rapidly internalized, and together with previous studies this study strongly suggests recycling of FBP back to the luminal plasma membrane through dense apical tubules. The results are consistent with reabsorption of Folate through endocytosis of the FBP-Folate complex followed by dissociation and recycling of FBP. When time is allowed there is a steady accumulation of FBP in dense apical tubules combined with an increase in surface density of the same compartment. A possible explanation involves partial inhibition of the fusion between dense apical tubules and plasma membrane because of the anti-FBP labeling of the receptor.